physical vapor deposition of zinc phthalocyanine
TRANSCRIPT
The Pennsylvania State University
The Graduate School
PHYSICAL VAPOR DEPOSITION OF ZINC PHTHALOCYANINE
NANOSTRUCTURES ON GRAPHENE TEMPLATED SUBSTRATES:
A STUDY OF REACTOR DESIGN, GROWTH KINETICS, AND SURFACE
MORPHOLOGY
A Thesis in
Materials Science and Engineering
by
Timothy J. Mirabito
©2020 Timothy J. Mirabito
Submitted in Partial Fulfillment
of the Requirements
for the Degree of
Master of Science
May 2020
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The thesis of Timothy J. Mirabito was reviewed and approved by the following:
David W. Snyder
Adjunct Professor of Chemical Engineering
Head of Electronic Materials and Devices Department
Thesis Advisor
Joan M. Redwing
Professor of Materials Science and Engineering
Suzanne E. Mohney
Professor of Materials Science and Engineering
John C. Mauro
Professor of Materials Science and Engineering
Chair, Intercollege Graduate Degree Program
Associate Head for Graduate Education, Materials Science and Engineering
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Abstract
Zinc phthalocyanine (ZnPc) is an important member of the organic semiconducting
phthalocyanine family, well known for its electrical properties and chemical stability. With the
recent push for new solar technology and energy efficient electronics, the interest in small
molecule organic semiconductors has accelerated.
Like many organic materials, high electrical and optical performance is contingent upon
the crystalline quality, control over the morphology, control over the orientation, and a selective
growth area. Of these challenges, one is dependent on the growth conditions and three are affected
by both the substrate interaction and growth conditions. Therefore, a growth process is required
that produces high quality ZnPc structures with immense control over the orientation and
morphology in a specific deposition area.
This thesis studies the effect of physical vapor deposition (PVD) reactor design, process
conditions, and graphene-based substrates on the quality, surface morphology and interfacial
properties of single crystal ZnPc. A vertical PVD reactor was designed and built to take advantage
of the growth kinetics of ZnPc. By scaling down the reactor geometry from traditional thermal
evaporators, control over the source material temperature, thermal gradient, and substrate
temperature, for a wide range of growth conditions, could be attained without the use of a turbo
pump. As part of the reactor study, a new pressure-based deposition method was developed,
allowing for consistent results and stable growth conditions. This combination of reactor design
and process method produced flexible morphologies from thin films to nanostructures verified
using scanning electron microscopy (SEM).
The effects of graphene layers and substrate material on the surface morphology were also
studied. Here, the number of graphene layers is shown to produce a change in structure from
nanowires to island clusters. Substrate material underlying single layer graphene is also shown to
produce a change in morphology between high aspect ratio nanowires on single crystal substrates,
to island clusters on amorphous silicon dioxide. Raman spectroscopy and kelvin probe atomic
force microscopy are implemented to characterize a novel AB stacked isolated graphene domain
and the work function of each distinct graphene layer, showing the 100 meV variation between
layers that may be controlling the growth morphology of ZnPc.
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The flexibility of the reactor and process is also demonstrated by growing both common
polymorphs, α-ZnPc and β-ZnPc. The structure and controlled orientation of each polymorph is
carried out using SEM, and X-ray diffraction. In each case, the introduction of graphene and a
defined substrate temperature of 200 ⁰C were shown to change the orientation between a horizontal
and vertical configuration while maintaining the desired polytype. Post-growth annealing of α-
ZnPc is demonstrated to transition to β-ZnPc at a temperature of 300 ⁰C.
Finally, an initial study of the influence of graphene defects is reported. Using a series
plasma treated graphene-based substrates, the growth morphology is characterized using both
SEM and Raman spectroscopy. The results show no correlation between graphene defect density
and growth morphology. Additionally, Raman characterization of ZnPc growth upon single and
multi-layered graphene areas are shown to cause no defects in the graphene as part of the
nucleation or growth process.
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Table of Contents
LIST OF FIGURES .................................................................................................................
LIST OF TABLES ...................................................................................................................
ACKNOWLEDGEMENTS .....................................................................................................
DEDICATION………………………………………………………………………………..
Chapter 1: Introduction / Literature Review ............................................................................
Motivation…………………………………………………………………………….
Metal Phthalocyanines …….………………………………………………………....
Structure of Zinc Phthalocyanine ................................................................................
Electrical & Optical Properties of Zinc Phthalocyanine ..............................................
Zinc Phthalocyanine Device Integration ......................................................................
Focus of this Study ……………………………………..............................................
Chapter 2: Experimental …………………..............................................................................
CVD Graphene Reactor & Growth Kinetics ..……………………………………….
Growth Conditions & Substrate Preparation ...……………………………………....
PVD Mk I Reactor Design …………………………………………………………...
PVD Mk II Reactor Design ...………………………………………………………..
PVD Mk III Reactor Design ……….…………..…………………………………….
Characterization Techniques …………………………................................................
Chapter 3: Systematic Study of Zinc Phthalocyanine Growth ................................................
Mk I Reactor Process Flow ..…………………………………………………………
Mk II Reactor Process Flow ...…………………………………….............................
Mk III Reactor Process Flow ………………………………………………………...
Comparison of Silicon and Graphene Substrates ..…………………………………..
Effect of Reactor Pressure ……….…………..………………………………………
Morphology and Temperature ………………………….............................................
Nanowire Growth Time ………………………….......................................................
XRD Characterization of ZnPc Growth ………………...............................................
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Chapter 4: Transport & Growth Kinetics .................................................................................
Sublimation Process .…………………………………………………………………
Vapor Transport ………………………………...........................................................
Deposition ……………………………………………………………………………
ZnPc-Graphene Interface .…………………………………………………………....
Nucleation and Growth ………………………………………………………………
Chapter 5: Substrates, Layers, Polymorphism & Defects.........................................................
Substrate Effects ..…….……………………………………………………………...
Multilayer Graphene ..…………………………..........................................................
Polymorphism …..……………………………………………………………………
Defects………………………………………………………………………………..
Chapter 6: Conclusions & Future Work...................................................................................
Conclusions ..…….…………………………………………………………………...
Future Work ..………………………….......................................................................
REFERENCES ........................................................................................................................
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List of Figures
Figure 1-1. Structures of (a) metallopoyphyrin and (b) metallophthalocyanine………………
Figure 1-2. Synthesis of ZnPc (c) from phthalic anhydride (a) and zinc acetate dihydrate
(b)…………………………………………………………………………………………………………
Figure 1-3. Monoclinic crystal structure of ZnPc shown from the y-axis of the unit cell……
Figure 1-4. (a) Triclinic crystal structure of α-ZnPc (b) Monoclinic crystal structure of β-
ZnPc……………………………………………………………………………………………………...
Figure 1-5. The herringbone molecular packing of a ZnPc thin film structure……………….
Figure 1-6. XRD spectra of a ZnPc thin film, at different thicknesses, deposited onto a
room temperature substrate [25]…………………………………………………………………….
Figure 1-7: ZnPc thin film XRD showing the effect of substrate temperature on its
identifying 2ϴ=6.86⁰ peak [25]………………………………………………………………………
Figure 1-8. XRD derived orientations of ZnPc at different substrate temperatures: (a)
Room Temperature (b) 373 K (c) 473 K…………………………………………………………….
Figure 1-9. XRD patterns of (a) ZnPc layers with different layer nominal thicknesses
deposited at room temperature and (b) 25 nm thick ZnPc layers deposited at different
substrate temperatures [26]…………………………………………………………………………..
Figure 1-10. UV-VIS absorption spectra of a MPc molecule. Strong absorption in the near
IR is characteristic of the MPc family. [27]………………………………………………………...
Figure 1-11. Phthalocyanine electronic energy levels [27]………………………………………
Figure 2-1. CVD equipment setup for the synthesis of large area graphene domains. The
copper substrate acts to catalyze the carbon into single atomic layer grains. [36]…………..
Figure 2-2. The many kinetic processes occurring simultaneously at the copper surface to
form graphene through the CVD process.[37]……………………………………………………..
Figure 2-3. The growth procedure for CVD graphene segmented by relevant kinetic
process step.[39]……………………………………………………………………………………….
Figure 2-4. The growth procedure for AB stacked multilayer isolated domains with the
common kinetic processes highlighted………………………………………………………………
Figure 2-5. Chemical etching transfer of CVD grown graphene film.[36]…………………….
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Figure 2-6. Representative Raman spectra of graphene as grown on Cu (Bottom) and after
the transfer on silicon (Top). The 2D-to-G peak intensity ratio and the symmetrical 2D
band with a full-width at half-maximum of about 30 cm-1 are indicative of the single-layer
nature of graphene……………………………………………………………………………………..
Figure 2-7. Post-transfer annealing furnace with mass flow controller for Ar vapor flow
and rotary pump for sub-atmosphere conditions…………………………………………………..
Figure 2-8. Mk I reactor showing improved vacuum lines and metrology……………………..
Figure 2-9. Mk I reactor showing the defining single heating coil furnace design…………...
Figure 2-10. Cross-sectional diagram of the initial single-zone PVD furnace and growth
process used to deposit ZnPc onto graphene/Si substrates……………………………………….
Figure 2-11. Mk II reactor showing the dual heating coil furnace design and argon inlet….
Figure 2-12. Cross-sectional diagram of the modified two-zone PVD furnace and growth
process used to deposit ZnPc onto graphene/Si substrates……………………………………….
Figure 2-13. Mk III reactor showing the water-cooling loop and heat sink…………………...
Figure 2-14. Dark field optical image of a millimeter-size single-crystalline single-layer
graphene domain as grown on Cu substrate………………………………………………………..
Figure 2-15. MERLIN FESEM from Carl Zeiss [42]……………………………………………..
Figure 2-16. Experimental and predicted Raman spectra of ZnPc [43]……………………….
Figure 2-17. ICDD reference XRD diffraction pattern and values for ZnPc powder………...
Figure 2-18. (a) Energy levels of the AFM tip and sample when separated (b) VCPD
formation from electrical contact (c) An applied bias equal to VCPD re-equalizing the
vacuum level and revealing the work function difference of the two materials [44]………….
Figure 3-1. Temperature profile of the Mk I reactor during a 350⁰C, 300 second
deposition. Note that the temperature does not reach the setpoint………………………………
Figure 3-2. Temperature profile of the Mk I reactor during a 450⁰C, 300 second
deposition. Note that the temperature does not reach the setpoint………………………………
Figure 3-3. ⁓2x increase in size from a) 300 second growth at 450⁰C b) 360 second growth
at 450⁰C………………………………………………………………………………………………….
Figure 3-4. Temperature profile of single stage and dual stage temperature tuning…………
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Figure 3-5. Temperature and pressure profiles for the ZnPc source and substrate
measured in-situ during the single-zone, constant pressure, temperature controlled ZnPc
growth process with a source temperature set to 400°C and a substrate temperature of
250°C…………………………………………………………………………………………
Figure 3-6. Temperature and pressure profiles for the ZnPc source and substrate
measured in-situ using the two-zone, pressure-controlled, constant temperature ZnPc
growth process using a source temperature set to 400°C and a substrate temperature of
250°C…………………………………………………………………………………………
Figure 3-7. SEM image showing growth morphology as a function of the substrate for
ZnPc nanowires grown at a source temperature of 400C and a substrate temperature of
250C. Left region shows growth on bare (unetched) silicon while the right shows growth
on single layer CVD graphene. ZnPc nanowires lay nearly flat on the silicon surface while
nanowires on the graphene show an inclined orientation nearly orthogonal to the
graphene…………………………………………………………………………………………………
Figure 3-8. SEM images of the graphene/Si substrates after depositing ZnPc for 1 min
using a Tsource of 450°C and a Tsubstrate of 250°C in the two-zone configuration under
different reactor pressures: (A) 17 mTorr, (B) 125 mTorr and, (C) 1000 mTorr. Scale bars
are 2 μm for each image……………………………………………………………………………….
Figure 3-9. SEM images of vertically oriented nanostructures grown on
Graphene/Si<100> for 5 minutes at 17mTorr: Images A,D,G show island clusters, while
images B,E demonstrate nanopillars and images C,E,F,H,I show nanowire formations.
Scale bars are 2μm for each image…………………………………………………………………..
Figure 3-10. (A) Cross-sectional FESEM images showing the ZnPc nanostructure
evolution on graphene-coated Si<100> as a function of growth times between 30 seconds
and 5 minutes, for the Tsource 450°C, Tsubstrate 250°C, 17mTorr condition. (B) Plot of the
nanowire dimensions as a function of growth time………………………………………………..
Figure 3-11. XRD measurement of the starting ZnPc bulk powder compared against the
PCPDS-ICDD database peak positions for β-ZnPc. The peak position agreement confirms
the starting material to be in the beta phase………………………………………………………..
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Figure 3-12. XRD measurement of ZnPc deposited on a native oxide silicon substrate and
compared against the PCPDS-ICDD database peak positions for β-ZnPc. The peak
position agreement confirms the deposited nanowires to be in the beta phase………………..
Figure 3-13. XRD measurement of ZnPc deposited on CVD graphene transferred onto a
native oxide silicon substrate. The peak positions at (1) 7.07° (2) 9.37° (3) 25.14° (4) 26.7°
(5) 27.88° correspond to β-ZnPc with some multi-layer graphene domains present………….
Figure 4-1. Typical commercial physical vapor deposition equipment setup………………….
Figure 4-2. Vapor pressure-temperature relationship for ZnPc. Operating source
temperature was held between 350-450°C for these experiments……………………………….
Figure 4-3. Flux-temperature relationship for ZnPc. Ideal evaporation condition is
represented by the a=1 series, with a very low efficiency evaporation represented by the
a=0.1 series……………………………………………………………………………………………..
Figure 4.4. JSmol model of the metal free phthalocyanine structure. The electrostatic
surface is shown to aid in measuring the kinetic diameter of 1.5 nm……………………………
Figure 4-5. The calculated mean-free path vs temperature. The log scale on the vertical
axis emphasizes the significant decrease in MFP over the deposition range in this system…
Figure 4-6. Mass diffusion of material through the temperature gradient followed by
crystal nucleation and growth………………………………………………………………………..
Figure 4-7. The calculated deposition rate vs temperature. Comparisons to cross-sectional
SEM measurements of thickness, the sticking coefficient is between 0.1 and 0.5……………...
Figure 4-8. Molecular orientation of ZnPc when deposited on silicon vs single-layer
graphene/silicon. (a) Edge-on molecular packing (b) Face-on molecular packing…………..
Figure 4-9. Preferential nucleation of ZnPc at the grain boundary of single-layer
graphene/silicon………………………………………………………………………………………..
Figure 4-10. ZnPc nucleation along a step edge, showing an apparent 2D film and 3D
island transition………………………………………………………………………………………...
Figure 5-1. ZnPc on single layer graphene/C-plane sapphire shows vertical orientation
while on bare sapphire the crystals are parallel to the
surface…………………………………..
Figure 5-2. ZnPc on single layer graphene/copper shows vertical orientation while on bare
copper the crystals are parallel to the surface……………………………………………………..
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Figure 5-3. ZnPc on bare copper shows vertical orientation due to a strong molecule-
substrate interaction………………...…………………………………………………………………
Figure 5-4. ZnPc on single layer graphene/SiO2 shows vertical orientation while on bare
SiO2 the crystals are parallel to the surface……………………………………………………….
Figure 5-5. ZnPc on single layer graphene/90 nm SiO2 shows vertical orientation while on
bare SiO2 the crystals are parallel to the surface…………………………………………………
Figure 5-6. ZnPc shows preferential nucleation on SLG at low substrate temperatures…...
Figure 5-7. SEM image of an isolated concentric domain of AB stacked graphene
highlighting the linear progression of layers……………………………………………………..
Figure 5-8. 2D Raman mapping of an isolated concentric domain of AB stacked graphene
highlighting the FWHM of the 2D Band………………………………………………………….
Figure 5-9. 2D Raman mapping of an isolated concentric domain of AB stacked graphene
highlighting the 2D/G peak intensity ratio……………………………………………………….
Figure 5-10. Growth on bi-layered and multilayered graphene shows the clustered island
morphology, while on single layer the nanopillar morphology is seen……………………….
Figure 5-11. Growth on layered graphene copper. SLG shows expected nanowire
morphology, bi-layered shows nanopillar morphology and multilayered graphene shows
the clustered island morphology…………………………………………………………………….
Figure 5-12. KPFM imaging of a SLG region in between two bilayer regions. (a) surface
profile showing minimal height delta (b) adhesion showing the entire region in uniform
contact with the substrate (c) work function measurement showing small variation between
layers……………………………………………………………………………………………………
Figure 5-13. XRD spectrum of α-ZnPc grown on silicon using a 460⁰C Tsrc and a 75⁰C
Tsub……………………………………………………………………………………………………..
Figure 5-14. XRD spectrum of α-ZnPc grown on graphene/silicon using a 325⁰C Tsrc and
a 160⁰C Tsub. The peak at 2θ = 27.9144⁰ suggests a face-on orientation on graphene……..
Figure 5-15. SEM image of α-ZnPc grown on graphene/silicon using a 325⁰C Tsrc and a
160⁰C Tsub……………………………………………………………………………………………..
Figure 5-16. SEM image of α-ZnPc isomorph α-CuPc grown on graphene/silicon from
Xiao et al.[52]…………………………………………………………………………………………
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Figure 5-17. SEM image of α-ZnPc grown on graphene/SiO2 using a 325⁰C Tsrc and a
160⁰C Tsub…………………………………………………………………………………………….
Figure 5-18. SEM image of a ZnPc domain on graphene/SiO2 post thermal treatment. The
appearance of nanowire structures suggests β-ZnPc……………………………………………
Figure 5-19. Comparison of defect density and growth morphology at different plasma
etching times. (a,b) Reference (c,d) 1 second (e,f) 3 seconds (g,h) 5 seconds……………….
Figure 5-20. Pre-growth characterization of a multilayer graphene domain. (a) optical
micrograph (b) 2D:G ratio demonstrating a layered structure (c) Raman spectra showing
no defects……………………………………………………………………………………………..
Figure 5-21. Post-growth characterization of a multilayer graphene domain. (a) SEM
micrograph (b) SLG spectrum (c)Bilayer spectrum (d) tri-layer spectrum (e) quad-layer
spectrum. The band at 1340cm-1 is characteristic of ZnPc and no significant D band
appears at 1350cm-1 ………………………………………………………………………………..
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List of Tables
Table 2-1. Vector description of predicted Raman modes [43]………………………………….
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Acknowledgements
First, a special thanks to Dr. David Snyder, my research advisor, for giving me an opportunity to
further my education. I would especially like to recognize him for the guidance and support he
has shown me.
I would like to thank Dr. Alejandro Briseno for reaching out to me and encouraging me to start
my graduate school journey and inspiring my research goals.
I’d like to sincerely thank Dr. Benjamin Huet for his unwavering support and mentorship
throughout my graduate career.
I’d like to thank Robert Lavelle for his vast knowledge and training on many characterization
methods employed in this project.
I would also like to thank everyone at the EMDD and the MCL including: Dr. Benjamin Huet,
Robert Lavelle, Josh Fox, Jeff Kronz, Maxwell Wetherington, for their research advice, support,
and assistance.
I’d like to thank Dr. Joan Redwing for her support and especially her patience at the conclusion
of this project.
I’d like to thank Dr. Suzanne Mohney and Dr. John Mauro who both went out of their way to
help me and provide feedback.
Finally, I’d like to thank my fellow classmates and friends for the endless laughs and their vast
insights into navigating the graduate life.
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Dedication
I dedicate this thesis to my wife and family: my wife Maryam; my parents Don and
Sharon; and my brother Chris, for their constant encouragement and love during my studies.
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Chapter 1 - Introduction
Motivation
As a member of the highly studied family of metal-phthalocyanines (MPcs), the organic
semiconductor Zinc phthalocyanine (ZnPc) has been used in various electronic applications
including solar cells [1], organic thin-film transistors (OTFT) [2], light emitting diodes (LED) [3],
and gas sensors [4]. The ability to chemically tailor properties has resulted in this application
flexibility [5]. Recently, there has been a growing interest in pairing MPcs with graphene for novel
device architectures such as flexible electronics as graphene provides unrivaled mechanical
flexibility and conductivity while also being transparent, a key factor for contacts in
optoelectronics. To date, various methods have been employed to deposit ZnPc on arbitrary
surfaces including Langmuir Blodgett [6], layer-by-layer (LbL) self-assembly [7], spin-coating
[8], and thermal evaporation [9, 10]. Thermal evaporation is an attractive method as it lacks
hazardous and tedious ZnPc preparation, functionalization, or dissolution. In addition, thermal
evaporation enables the deposition of thicker films with easy scalability to larger samples.
Research efforts have focused on understanding how the ZnPc/substrate interactions play a role in
the determination of the structural arrangement of ZnPc molecules. Such an arrangement is of
central importance for technological applications as ZnPc crystals exhibit an anisotropic electronic
transport induced by charge propagation through π -> π transitions. One such study showed that
ZnPc molecules will adopt a face-on orientation on graphene or metals while they prefer an edge-
on orientation on SiO2 [11]. This planar orientation at the interface is essential to improving the
quality of the charge transfer at the graphene/ZnPc interface [12]. Using graphene as template for
ZnPc crystals has been found to overcome limitations in carrier lifetimes and sub 25nm diffusion
lengths in optoelectronic applications [13].
In addition to ZnPc crystal orientation, it is also essential to control the film morphology
when considering technological applications. While conventional ZnPc evaporation studies
successfully deposited relatively uniform and smooth films [10], more recent works achieved
arrays of vertical crystalline nanopillars [14]. While thin films can be easily processed by
conventional microfabrication techniques for transistors, nanopillars exhibit a high surface-to-
volume ratio which is of great interest for gas sensors, batteries or bactericidal surfaces [15].
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Previous growth approaches have been operated at constant pressure while increasing the
temperature of both the ZnPc source and substrate [14,15,16]. While this approach did result in
the growth of ZnPc nanowires, it results in significant thermal transients, typically on the order of
100-200 seconds, and varying flux and substrate temperature during the nucleation and growth
stages. This is a critical control issue as the nanostructure polymorph can change from the α-ZnPc
phase to the β-ZnPc phase at 200°C and may occur in an uncontrolled manner during the
temperature transients.
In order to overcome these limitations, a new physical vapor deposition system (PVD)
system capable of reliably depositing both thin films and high aspect ratio nanopillars under
constant source and substrate temperatures needs to be implemented. With such a system, it is then
possible to systematically investigate the growth mechanisms of ZnPc nanostructures upon
graphene templated substrates.
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Metal Phthalocyanines
Phthalocyanines are a family of synthetic molecules that were originally prepared to mimic
the visual properties of naturally occurring metalloporphyrins. Since both compounds are
heterocyclic macrocycle organic compounds with conjugated bonds, absorption is strong in the
visible region of the electromagnetic spectrum. This can be seen in the porphyrin complex heme,
which gives the pigment of red blood cells, as well as copper phthalocyanine, which is responsible
for the blue pigment in cyan colored printing toner [17].
Figure 1-1. Structures of (a) metallopoyphyrin and (b) metallophthalocyanine
Historically, the metal phthalocyanine (MPc) family of molecules was first discovered in
1907, with iron phthalocyanine [18]. Over the next several decades, additional variants of the MPc
family were discovered. Initially, the interest in the molecule was for applications in dyes [19]. As
time progressed, however, significant research into MPcs would see their application space include
photovoltaics, photodynamic therapy, and catalysis [20].
As a consequence of this large application space, a significant effort has been undertaken
to amend the MPc kernel with ligands. This has produced many different processes that can be
implemented to synthesize MPcs, with the choice of method generally contingent on cost. The
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industry’s standard method is the heating of phthalic anhydride with urea and a metal salt, using
ammonium molybdate or aluminum oxide as a catalyst. [21,22]. In the specific case of Zinc
Phthalocyanine, Zinc acetate dihydrate is used as the metal salt for Zinc substitution.
Figure 1-2. Synthesis of ZnPc (c) from phthalic anhydride (a) and zinc acetate dihydrate (b).
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Structure of Zinc Phthalocyanine
As shown in figure 1-2c, zinc phthalocyanine is a large aromatic molecule with the
chemical formula C32 H16 N8 Zn. Like the rest of its class, ZnPc has a characteristic flat and
symmetrical molecular structure. This two-dimensional molecule contains a ring system consisting
of 18 π-electrons. The delocalization of the π-electrons lends itself to useful conductive properties
and plays a central role in its optical properties.
The first crystalline structure study of phthalocyanines was undertaken in 1936 by J.M
Robertson using x-ray diffraction [23]. In this inaugural work, the structure of free phthalocyanine,
C32 H18 N8, was determined to belong to the P21/a space group. The lattice parameters were then
determined to be a=19.85 Å, b=4.72 Å, c=14.8 Å with β=122.25⁰. As with all the phthalocyanines,
ZnPc shares the same P21/a space group, but with slightly different lattice parameters to account
for the metal atom. The molecular packing within the unit cell can be seen in figure 1-3.
Figure 1-3. Monoclinic crystal structure of ZnPc shown from the y-axis of the unit cell
Polymorphism is also prevalent within the phthalocyanine family and has been observed
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for both bulk and vacuum deposited thin films. In analyzing thin films, phthalocyanines have been
reported to contain several different polymorphic structures named α, β, δ, ε, χ, and R-Pc [24]. For
the vacuum deposition of ZnPc, the predominant phases are the metastable α-ZnPc and the
thermodynamically stable β-ZnPc. In XRD studies, the α-ZnPc is formed at lower substrate
temperatures, while the β-ZnPc is formed above 200 °C substrate temperatures or via a post
deposition annealing. Visually, the α-ZnPc grows as clustered strips while the β-ZnPc grows as
acicular crystals. In literature, there is some disagreement about the structure of the α-ZnPc, as it
is reported to be either triclinic or monoclinic. The structure of the β-ZnPc however has been
agreed upon as monoclinic. Each of these two structures can be seen in figure 1-4.
Figure 1-4. (a) Triclinic crystal structure of α-ZnPc (b) Monoclinic crystal structure of β-ZnPc
Several studies have been carried out to investigate the structural characterization of ZnPc
thin films by vacuum deposition which serve as a good starting point prior to the inclusion of a
graphene interlayer. As is common with other aromatic hydrocarbons, ZnPc will pack in a
herringbone pattern with the growth axis parallel to the substrate, when deposited as a thin film.
This packing formation is shown in figure 1-5.
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Figure 1-5. The herringbone molecular packing of a ZnPc thin film structure
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Using common X-ray diffraction methods, initial studies were carried out by Senthilarasu
et al. that revealed ZnPc films, grown upon glass substrates, below 250nm were amorphous while
thicker films were found to be polycrystalline. This transformation in crystallinity was shown as a
strong increase in the characteristic peak intensity at 2ϴ = 7.32⁰ and a reduction in the spectrum
between 20⁰ and 40⁰. This characteristic peak has been assigned to the (200) plane, which coincides
with the growth front of the crystal. Additionally, this peak helps reveal the molecular orientation
of ZnPc relative to the substrate. It can therefore be said that the 2ϴ = 7.32⁰ peak corresponds to
ZnPc packing in an edge-on configuration. The XRD spectra in figure 1-6 shows the increase in
the (200) peak intensity with an increase in film thickness.
Figure 1-6. XRD spectra of a ZnPc thin film, at different thicknesses, deposited onto a room
temperature substrate [25]
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Additional x-ray diffraction studies showed that the effect of substrate temperature on
orientation was notable. Eschewing traditional thermal evaporation processes where the substrate
is kept at room temperature, this experiment varied the temperature between 323 K and 473 K.
For the sample prepared at 323 K the predominant peak was at 2ϴ=6.86⁰ with a secondary peak at
2ϴ =25.92°. As the substrate temperature was increased to 373 K the intensity of the 2ϴ=6.86⁰
peak decreased. Further heating to 473 K, drastically increased the intensity of the 2ϴ=6.86⁰ peak
but also showed a decrease in the other indexed peaks. It was determined that the d-spacing for
the 2ϴ=6.86⁰ peaks were between 12.22 Å and 13.31 Å.
Figure 1-7: ZnPc thin film XRD showing the effect of substrate temperature on its identifying
2ϴ=6.86⁰ peak [25].
10
As the substrate temperature is increased, it should be noted that the peak at 2ϴ=25.92°
was found, which was correlated to some ZnPc molecules having a parallel orientation to the
substrate plane. The temperature study ultimately concluded that the substrate temperature causes
the stacking axis to tilt towards the substrate normal as seen in figure 1-8.
Figure 1-8. XRD derived orientations of ZnPc at different substrate temperatures: (a) Room
Temperature (b) 373 K (c) 473 K
Several years later, C. Schünemann et al. published a study refuting some of the results of
Senthilarasu et al. and providing more refined crystallization and structural details with XRD. One
of the important discoveries was that for a comparable film thickness, ZnPc thin films could be
crystalline rather than the previously reported amorphous structure. From the XRD spectra shown
in figure 1-9, the reflections at 2θ=6.89°, 13.76°, and 27.66° were indexed as the h00 reflections
of different orders of the triclinic (h=1, 2 and 4) or monoclinic (h=2, 4 and 8) phase.
The reason for the different results were attributed to the lower purity of the ZnPc powder
used. As a straightforward conclusion, the higher purity led to an enhanced crystalline growth from
limiting incorporated impurities.
11
Figure 1-9. XRD patterns of (a) ZnPc layers with different layer nominal thicknesses deposited
at room temperature and (b) 25 nm thick ZnPc layers deposited at different substrate
temperatures [26]
12
Electrical & Optical Properties of Zinc Phthalocyanine
Upon inspection of ZnPc, it is quickly observed that it has a deep blue/purple color. One
of the prominent features of the phthalocyanine family is a strong absorption of light between 600
and 700 nm. It is directly because of this, that phthalocyanines are widely used in organic-based
photovoltaic devices as the active semiconducting material.
Starting with an analysis of the optical absorption spectra, we can start to understand the
band structure of the system. Figure 1-10 shows a typical absorbance spectrum of a vacuum
deposited MPc thin film.
Figure 1-10. UV-VIS absorption spectra of a MPc molecule. Strong absorption in the near IR is
characteristic of the MPc family. [27]
Phthalocyanine spectra contain two distinct energy bands. One of them is called the Q
band, derived from the α band in porphyrins, and the other one is called the B band, which is
equivalent to the γ or Soret band in porphyrins [28]. The Q band and the B band are defined by the
a1u to eg transition, and an a2u to eg and a b2u to eg transition [29]. These two bands can be found
in the absorption spectrum at approximately 360 nm and 690 nm wavelengths, for the B band and
Q band respectively.
13
For ZnPc, its characteristic color can be attributed to the presence of the Q band at 690nm.
Towards the blue end of the electromagnetic spectrum, these transitions are more energetic, but
have a smaller magnitude. This is due to the transitions from the bonding to anti-bonding orbitals,
π→π* states. In phthalocyanines the highest occupied molecular orbital (HOMO) of the
phthalocyanine ring is a1u (π) and the next lowest orbital is a2u (π) [30]. The lowest unoccupied
molecular orbital (LUMO) is the doubly degenerate eg (π*) followed by b1u (π*) [30]. The first π-
π* transition involving the a1u to eg transition results in the higher magnitude Q-band near 690 nm.
The second π-π* transition involving the a2u to eg transition yields the lower magnitude B band
near 360 nm. These transitions are shown in figure 1-11.
Figure 1-11. Phthalocyanine electronic energy levels [27]
14
Zinc Phthalocyanine Device Integration
Since the first copper phthalocyanine/perylene derivative-based planar heterojunction
organic solar cell (OSC) created by Tang [31], OSCs have attracted considerable interest. To date,
the MPcs tested in these P-N junctions have led to excellent conversion efficiencies [32].
Unfortunately, a major pitfall of the planar heterojunction OSC is that the active layer thickness is
limited by the exciton diffusion length. The exciton diffusion length, being the average distance
separated charge carriers can travel before recombination, are much more limited in organic
molecules than inorganic materials. The typical value for an organic molecule is on the order of
30nm [33]. This presents a tradeoff limiting absorption due to active layer thickness.
An initial solution to this dilemma was the bulk heterojunction. This was a P-N junction formed
by the mixture of donor-acceptor molecules. With the intermixing, the limiting exciton diffusion
length can be mitigated slightly, but problems still persist.
The bulk heterojunction is fundamentally a random mixture, created during a co-deposition
of active material species. As such, it is possible that the bulk heterojunction contains segmented
regions of donor or acceptor molecules which have no charge carrier pathway towards an
electrode. In effect, these regions behave as charge trapping sites where recombination will occur.
Overall this will reduce the photocurrent and subsequently the fill factor. Furthermore, the charge
carrier mobilities are also limited due to higher molecular disorder in an amorphous BHJ compared
to a pristine crystalline layer [34].
It is therefore prudent to have a P-N junction architecture, that can overcome the small
exciton diffusion lengths, while simultaneously not inhibiting charge carrier percolation. One
possible solution has been to implement vertically oriented nanowire arrays.
The ability to organize organic molecules via self-assembly to fabricate precise patterns at
the nanoscale would allow for this type of array, and it has been demonstrated that the coupling
strength between MPcs and the substrate material, can control the orientation of the adsorbed
molecules [35]. The coupling strength between MPcs and graphene have allowed for a face-on
packing orientation optimal to take advantage charge pathways developed through the pi-stacking
of MPc molecules.
15
Focus of this Study
The literature review has shown the key properties of Zinc Phthalocyanine and its
application as an organic semiconductor. The effects of temperature during the growth on the
crystal quality and surface morphology were also presented. This thesis will use the current
fundamental knowledge of physical vapor deposition techniques to establish a new growth
methodology that is highly controlled and facile.
The focus is directed toward an understanding of this new growth methodology, and the
effect of growth parameters on the surface morphology of ZnPc thin films. It will be shown that
precision control over the growth parameters can allow for flexibility in deposition. In addition,
substrate effects, graphene layered screening, and graphene synthesis methods were shown to have
varying effects on the ZnPc microstructure. Therefore, exploration of these peripheral effects was
studied to further improve the understanding of the growth mode of this molecule at the substrate
interface.
16
Chapter 2 - Experimental
Overview
This project has two main thrusts, the construction of a new thermal evaporation furnace
with a corresponding growth methodology and the systematic study of the formation of Zinc
Phthalocyanine nanostructures on graphene coated substrates. There are several benefits to this
new system as it has the capability of a much faster vacuum evacuation time, and the impingement
flux is directed, by design. This allows the system to conduct the evaporation of ZnPc at much
higher pressures than tradition commercial thermal evaporation equipment. As a result, the cost of
manufacturing the system is lower, as it doesn’t require vacuum equipment such as a turbo pump.
The second facet of this work describes the synthesis of single crystal ZnPc nanostructures
grown using a pressure-controlled vertical physical vapor transport method. The process begins
with the substrate preparation, where graphene grown on copper through chemical vapor
deposition is then transferred onto the desired substrate. These samples are then grown upon using
a physical vapor transport method to be expanded upon later.
Several characterization techniques are utilized at varying points in the process to evaluate
the graphene, prior to the ZnPc growth. Raman spectroscopy is the technique utilized to evaluate
the condition of the CVD grown graphene. This has been performed both post-growth and post-
transfer to maintain consistency of the substrate samples. A Field Emission Scanning Electron
Microscope (FESEM) was also used post transfer to evaluate the surface quality and to identify
whether the graphene was a continuous film or an array of isolated domains.
Beginning with the substrate preparation, this chapter will detail the graphene growth
process and the technique used to transfer it. This will be followed by a discussion on the vertical
physical vapor deposition system broken down into its three iterative designs revolutions.
17
CVD Graphene Reactor & Growth Kinetics
Graphene’s unique properties have provided significant motivation for its integration into
2-dimensional heteroepitaxial growth. As described in chapter 1, it provides the template from
which the ZnPc nanostructures are nucleated and grown. It is therefore critical to understand the
surface kinetics and chemistry of graphene to encapsulate the big picture.
One method for the synthesis of graphene is by chemical vapor deposition (CVD). The
CVD process consists of depositing a solid material using a vapor precursor which chemically
reacts on a substrate surface at high temperature. In the case of graphene, methane (CH4) is
commonly used as a carbon source, hydrogen (H2) is used as a reducing agent, and argon (Ar) is
used to create an inert environment during the high temperature process. The catalyzing material
used depends on the characteristics demanded of the end graphene. In literature, extensive work
has been carried out looking into a vast range of substrates. Starting with transition metals such as
cobalt, copper, nickel, and rubidium, semiconductors such as germanium, and finally insulators
such as sapphire. For the synthesis of single layer graphene (SLG), a copper catalyst is used. An
elaboration of copper’s preference will be addressed during the discussion on the kinetic growth
process. Figure 2-1 demonstrates the system level overview of the CVD process.
Figure 2-1. CVD equipment setup for the synthesis of large area graphene domains. The copper
substrate acts to catalyze the carbon into single atomic layer grains. [36]
18
The growth parameters for the CVD process revolve around controlling the environment
near the Cu surface. This means controlling the vapor flow rate, and thus pressure, of the methane
and hydrogen. The ratio of these two vapors, as well as the overall system pressure also play a
role. Finally, the furnace temperature can be altered. The volume condition here is maintained as
a constant as the quartz tube in which the reaction occurs is sealed from the outside environment.
Each of these conditions can affect the density, quality and number of layers of the produced
graphene.
The chemical vapor deposition is commonly catalyzed using specific materials. Their role
in this process is to provide a thermodynamically preferable means for the decomposition of the
methane precursor into the C adatoms and to stabilize the deposited graphene structure. The use
of copper as a catalyzing agent, stems from its ability to do both previous processes in a goldilocks
range, in order to generate a single layer of graphene.
A key property of utilizing copper for this process, is that the low carbon solubility of the
material effectively confines the C adatom diffusion along the surface. Ultimately, this allows for
the preferential growth of larger single domains, as well as eliminating the bulk diffusion of carbon
to the surface when the substrate is cooled from its higher deposition temperature.
Ensuring the stabilization of the deposited graphene on the copper surface can be
understood by examining the bond orientation of the benzene ring. For each carbon atom, the
vertically oriented Pz orbital overlaps with the 4s orbital of copper at the surface forming a weak
bond.
The decomposition process of methane into graphene is shown in figure 2-2 and proceeds in the
following steps [36]:
1. Breakdown of the methane on the catalytic surface through dissociative adsorption; a
process whereby the hydrogen atoms of CH4 steadily dissociate in a stepwise manner (CH4-
>CH3->CH2->CH->C) until C adatoms are remaining on the surface.
2. Simultaneous adsorption, desorption, and surface diffusion of the CHx species.
3. Nucleation of CHx clusters
4. Growth of individual graphene domains through the preferential adsorption of additional
CHx species.
19
Figure 2-2. The many kinetic processes occurring simultaneously at the copper surface to form
graphene through the CVD process.[37]
20
Growth Conditions & Substrate Preparation
Aside from the widely held view of CVD graphene as the most mature synthesis method,
it is particularly useful towards achieving large-area growths with a controlled number of layers.
This is an important aspect of the project as several different types of graphene were required
including a coalesced film, isolated domains, and multilayer domains. In each case, the graphene
variants were produced using CVD of methane on Cu substrates.
Starting with the procedure for isolated domains, highly crystalline graphene with
millimeter-size domains have been obtained by using a CVD protocol similar to previously
reported works with a growth temperature of 1050°C and a hydrogen-to-methane ratio of about
800 [38]. Isolated graphene domains were preferred over the growth of a coalesced film as it allows
for the comparison of the ZnPc deposition of graphene and on the underlying substrate
simultaneously, without any patterning/etching processes. This process flow is outlined in figure
2-3.
Figure 2-3. The growth procedure for CVD graphene segmented by relevant kinetic process
step.[39]
21
As the project progressed, the effects of multiple graphene layers on the microstructure of
deposited ZnPc became of interest. A novel approach to producing multilayer graphene was then
found by amending the existing procedure to obtain isolated millimeter-size multilayer domains
with AB stacking. This process flow is shown in figure 2-4.
Figure 2-4. The growth procedure for AB stacked multilayer isolated domains with the common
kinetic processes highlighted.
Following the CVD synthesis, the graphene needed to be transferred onto a variety of
substrates. To accomplish this, a widely implemented transfer process using a polymer support
and copper etchant was used. Specifically, graphene was transferred onto several different
substrate materials using poly(methyl methacrylate) (MicroChem, 3% in anisole) as a mechanical
support and FeCl3 (Sigma-Aldrich, 45% FeCl3 basis) as a copper etchant. This transfer flow is
illustrated in figure 2-5.
22
Figure 2-5. Chemical etching transfer of CVD grown graphene film.[36]
This procedure starts with a copper substrate with the synthesized graphene on top. This
sample is then secured to a substrate using polyimide tape around the edges. Careful attention is
paid to make sure that the tape covers the edges, which will prevent the PMMA from encapsulating
the sample. Next a layer of the PMMA polymer is drop casted onto the surface and placed in a
spin coater for 1 minute at 2000 rpm. This PMMA/Graphene/Cu substrate is then left overnight to
dry.
With the top PMMA layer solidified, the sample was placed in a copper etchant bath
(FeCl3) mixed at 1:2 ratio with deionized water. Depending on the sample size, the copper
substrate will be etched away in approximately 1 hour. The copper etchant was then be replaced
with DI water, and the sample was transferred to a large 4-liter DI water bath to rinse any remaining
copper etchant from the surface.
After the sample has been thoroughly rinsed, it was placed onto a pre-cleaned 15mm x
15mm substrate. This substrate was manually scored from a larger wafer and cleaned via a standard
triple rinse process, using acetone, isopropyl alcohol, and DI water. This sample was then left to
dry overnight.
23
Finally, when the graphene is adhered to the substrate surface, the sample was placed into
an acetone bath heated to 50⁰C. The heated acetone will assist in removing the PMMA from the
sample surface. The substrate was then triple rinsed again, and dried using a N2 gun. At this point,
the sample was ready for deposition.
Characterization of the graphene quality was performed using Raman spectroscopy in
order to ensure that the transfer process did not result in graphene degradation or contamination
[40]. These Raman scans are shown in figure 2-6, illustrating the high quality single layer nature
of the graphene.
Figure 2-6. Representative Raman spectra of graphene as grown on Cu (Bottom) and after the
transfer on silicon (Top). The 2D-to-G peak intensity ratio and the symmetrical 2D band with a
full-width at half-maximum of about 30 cm-1 are indicative of the single-layer nature of
graphene.
24
During the course of this project an additional post-transfer anneal step was explored as
another means of eliminating PMMA residue not removed during the acetone bath. During this
step, the substrate was placed into an annealing furnace for 4 hours at 300⁰C under sub-atmosphere
pressures and a 50 sccm Ar flow. This setup is shown in figure 2-7.
Figure 2-7. Post-transfer annealing furnace with mass flow controller for Ar vapor flow and
rotary pump for sub-atmosphere conditions.
The sample was then characterized using pre and post Raman spectroscopy measurements
to analyze possible defect mitigation. Pre and post annealed samples were also compared following
ZnPc deposition. As a result, no significant changes in neither defects nor growth morphologies
were observed. This step was therefore removed from the overall process flow to streamline the
substrate preparation process.
25
PVD Mk I Reactor Design
To date, a limited number of studies have demonstrated the growth of MPc nanopillars
[14,15,16]. These initial experiments utilized a scaled down thermal deposition process with
limited metrology which led to inconsistent performance. Therefore, the first design iteration built
needed to provide appropriate information feedback to establish both control and consistent
performance.
The key design principles of the Mk I reactor were establishing an optimized low-pressure
environment and a real time feedback of the furnace temperature and the source material
temperature.
To optimize the low-pressure environment, the system was built around an Edwards rotary
pump with an ultimate base pressure of 10 mTorr. This pump was then routed to the scaled down
deposition chamber using a series of KF-25 tubing and crosses and a compression fitting for the
chamber. Leak checking was performed upon completion.
Figure 2-8. Mk I reactor showing improved vacuum lines and metrology
26
As shown in figure 2-8, the system also added several metrology elements including two
thermocouples to detect both the source material and substrate temperature, as well as a pressure
gauge. The pressure relief valve and the pump isolation valve were added to control the pressure
during the growth initiation and termination.
As with the vacuum aspects of the system, the heating elements of the system required
alteration. The single heating coil is maintained but a k-type thermocouple is placed between the
heating coil and the outside of the quartz furnace. This thermocouple couple serves as a reference
value for the PID controller which controls the heating coil based off the source thermocouple
feedback inside the chamber. Naturally, transitioning to a PID controller over a simple switch
provided the level of automated stability required for more extensive studies. The furnace setup
with the encapsulated deposition chamber is shown in figure 2-9.
Figure 2-9. Mk I reactor showing the defining single heating coil furnace design
27
ZnPc deposition utilizing the Mk I reactor followed a straightforward thermal evaporator
process flow. For this approach a constant pressure was maintained, and a transient temperature
increase was implemented for the source and substrate to initiate growth and then a transient
temperature decrease was used to end growth. A vertically standing cuvette held the MPc solid
source at the bottom and the substrate separated from the source by a few-centimeter long quartz
spacer. The cuvette was translated downward into a single zone furnace to heat the source and start
the vapor deposition process. As the source temperature increased the vapor pressure increased
and ZnPc was transported from the source to the substrate. The system was held for several
minutes until the desire growth time was reached. The cuvette was then translated upward to cool
the source material and end the vapor deposition. Figure 2-10 illustrates this approach.
Figure 2-10. Cross-sectional diagram of the initial single-zone PVD furnace and growth process
used to deposit ZnPc onto graphene/Si substrates.
28
PVD Mk II Reactor Design
Unfortunately, a single-zone furnace does not allow for the independent control of the
source temperature (Tsource) and the substrate temperature (Tsubstrate). Controlling the substrate
temperature is necessary to control the kinetics at the substrate surface and the source-substrate
temperature gradient which determines the ZnPc vapor flux. Also, due to the system thermal
inertia, the source and substrate both experience a significant gradual temperature increase and
decrease during the start and end of the growth process, respectively. Since the reactor is
maintained at a low pressure throughout the entire process, uncontrollable ZnPc deposition takes
place during these transient stages. Finally, having to manually move the reactor into the hot zone
can lead to reproducibility issues.
To overcome these limitations, the Mk II reactor was developed, as shown in figure 2-11.
This new system was able to provide i) independent, stable control of the source temperature, ii)
independent, stable control of the substrate temperature, and iii) rapid initiation and termination
of growth through a rapid reactor pressure change.
Figure 2-11. Mk II reactor showing the dual heating coil furnace design and argon inlet
29
The key elements of this new reactor include an additional heating coil positioned to
encircle the substrate, and an additional furnace thermocouple setup as a reference for a second
PID controller, as well as the incorporation of an argon gas line to initiate rapid pressure changes
while under an inert environment. These changes supplement a new process flow based on
constant, independent temperature control of the source and substrate followed by a rapid
evacuation to the growth pressure to start and then stop deposition.
The new process flow proceeds as shown in figure 2-12, where the reactor pressure was
stabilized at 20 Torr under Argon and the source and substrate temperatures were set using
independent PID controllers using thermocouples in direct contact with either the source or
substrate backside inside the cuvette. This approach does not require the translation of the cuvette
and the external furnace elements are simply ramped to their respective temperatures and allowed
to stabilize. ZnPc mass transport is suppressed during the temperature ramp up the argon
background pressure being significantly higher than the vapor pressure of ZnPc which is
approximately 7.1e-3 torr at 450°C [41].
The start of the growth is controlled by evacuating the reactor using a rotary pump from
20 torr to approximately 17 millitorr which takes only approximately 5 seconds. After the growth,
the reactor is backfilled to 20 Torr of argon in order to prevent any further deposition during the
cooling stage. The result is that each parameter is now independently controlled, and the system
reaches an equilibrium growth condition significantly faster.
Figure 2-12. Cross-sectional diagram of the modified two-zone PVD furnace and growth
process used to deposit ZnPc onto graphene/Si substrates.
30
PVD Mk III Reactor Design
In the previous section, part of the motivation for a second heating coil was controlling the
substrate temperature as it is necessary to control the kinetics at the substrate surface. This
provided a means to stabilize the substrate temperature above 200°C, but to achieve cooler
temperatures, the residual heat from the source needed to be extracted from the substrate region.
Therefore, a chilled water-cooling loop coupled with a copper heat sink have been added to the
system. These designs are shown in figure 2-13.
These additions have provided the necessary low substrate temperatures to achieve
depositions conditions consistent with α-ZnPc thin films. Moreover, exploration of this growth
space is unique in its ability to maintain a high mass flux without a high substrate temperature, a
trade off usually made with commercial systems.
Figure 2-13. Mk III reactor showing the water-cooling loop and heat sink
31
Characterization Techniques
Throughout this work, the synthesized graphene and ZnPc crystals were characterized by
several analytical techniques. The surface morphology was analyzed using optical microscopy and
scanning electron microscopy. Graphene defects and layer thickness were investigated using
Raman spectroscopy. ZnPc crystallographic structures were confirmed using x-ray diffraction.
Surface energy measurements and adhesion were carried out using AFM. This work was carried
out at the materials research institute at Penn State University.
Optical Microscopy
Optical microscopy was carried out using the Nikon L200ND, which is a high-resolution
optical microscope with complimentary high-resolution image capture. This microscope can
operate in both episcopic and diascopic modes, and all of the lenses can operate in brightfield,
darkfield, and DIC modes.
This instrument was useful for large scale characterization of as grown graphene on copper
particularly when utilizing darkfield imaging. Figure 2-14 shows an example of this use case.
Figure 2-14. Dark field optical image of a millimeter-size single-crystalline single-layer
graphene domain as grown on Cu substrate
32
Scanning Electron Microscopy
Nanoscale characterization of graphene domains pre and post transfer as well as ZnPc
morphology characterization were carried out using the MERLIN Field Emission Scanning
Electron Microscope (FE-SEM) from Carl Zeiss. Contrasting it with other FE-SEM systems
available, the double condenser system incorporated into the GEMINI II column helps achieve an
image resolution of 0.8 nanometers. This proves to be critical for effective characterization as
ZnPc feature sizes can be less than 100nm.
The MERLIN also boasts a unique charge compensation system which allows the high-
resolution imaging of non-conductive samples. This is another useful characteristic of the tool
specifically because ZnPc deposited upon insulating substrates, has the tendency to exhibit
charging. Since the SEM imaging and contrast of thin films have a dependence on the emitted
secondary electrons, thin conductive layers (~1-2nm) of gold are often sputtered. Performing
electron microscopy without the need for this additional gold sputtering step is beneficial to
looking at the graphene/ZnPc interface.
In the work presented here, high resolution images were obtained using an in-lens detector,
an acceleration voltage of 3kV, and a working distance of about 5 mm.
Figure 2-15. MERLIN FESEM from Carl Zeiss [42]
33
Raman Spectroscopy
Raman spectroscopy is a non-destructive vibrational spectroscopic technique that was
employed to analyze both the graphene and the ZnPc. Raman spectroscopy relies on a
monochromatic light source incident on a sample, typically a laser, which will interact with the
optical phonons on the surface of the sample. These excited vibrational states in the molecules will
then relax back to the ground state, emitting the light which will then be analyzed. The energy
change as a result of the interaction is called Raman-shifted scattering.
Focusing first on the prominent features of graphene, the observed spectra reveals three
bands, the D-band, the G-band, and the 2D-band at 1350, 1580, and 2700 cm-1 respectively. The
differentiation between graphene layer thickness can be observed first as a ratio of the G to 2D
peaks. Additionally, single layer graphene can be characterized by a ~30cm-1 full-width half-max
(FWHM). The role of the D-band is commonly referenced as the “defect band” as its intensity
correlates with the defect density of the sample. In addition to single point measurements, a 2D
view of the quality and layer structure can be obtained through the use of Raman mapping. Setup
appropriately, Raman mapping is able to show the ratio of the 2D-band intensity to the G-band
intensity, and display it as a topological map. This makes identifying uniformity over the sample
an easier task.
Due to it’s organic nature, Raman spectroscopy is also possible for ZnPc. However, the
electronic absorption B and Q bands fall within the visible region of the light spectrum at ~460nm
and ~630nm respectively. Therefore, Raman performed with typical excitation sources will exhibit
resonance. The experimental and predicted Raman spectrum are shown in figure 2-16.
Figure 2-16. Experimental and predicted Raman spectra of ZnPc [43]
34
Symmetry Modes Frequency (cm
-1) Vector Description
A1 Modes 252
symmetric stretching of the four Zn-N bonds (phthalocyanine ring breathing)
666
symmetric stretching of the four C-N-C bridges coupled with deformations in both the indole and benzene rings
1005 in-phase expansion of the benzene rings
1113
in-phase stretching of the Zn-N bonds coupled with indole ring deformations and benzene ring expansions.
1332
in-phase Zn-N stretches coupled with both indole and benzene ring deformations
1417
C-N-C bridge stretching coupled with indole expansions and benzene deformations.
1483
in-phase C-N-C bridge stretching coupled to indole ring deformations
B1 Modes 735 out-of-phase deformation of the indole groups
1004 out-of-phase expansion of the benzene rings
1134
out-of-phase expansions of both the indole and benzene rings
1278 out-of-phase deformations of the indole rings
1362
out-of-phase indole ring expansions and benzene ring deformations
1438 out-of-phase benzene ring deformations
1517 out-of-phase stretching of the C-N-C bridges
Table 2-1. Vector description of predicted Raman modes [43]
The major bands in the experimental Raman spectrum are assigned to A1 and B1 symmetry
modes. Of these, the B1 vibrations have the greatest intensities. Vector descriptions of these
vibrational modes are listed in table 2-1.
35
Xray Diffraction
To study the crystallographic structure of ZnPc in both the powder form and the deposited
form, x-ray diffraction (XRD) was implemented. These patterns were taken with a Malvern
Panalytical Empyrean x-ray diffractometer equipped with a Cu Kα x-ray source (λ = 0.154056
nm). Subsequently, these spectra were analyzed with MDI JADE software to identify the diffracted
peaks.
During an XRD experiment, generated x-rays incident to the sample surface interact
elastically with the crystallographic planes of the sample. These x-rays are subsequently diffracted
at an angle θ to the plane according to Bragg’s law, nλ=2dsinθ, where the x-ray wavelength is λ,
the inter-planar spacing is d, the incident angle is θ, and the order of reflection is n. The diffracted
x-rays are then aggregated at the detector. This procedure is performed over a range of incident
angles to produce the observed spectra. This material specific data can then be used to determine
crystal structures, phase composition, stress, and crystallinity.
The generated diffraction spectrum is traditionally analyzed using two distinct methods.
The first method identifies a crystalline material by comparing the diffraction angles of high
intensity peaks with a diffraction reference from a database. The full width half max of these peaks
can also be compared to the reference spectra for interplanar spacing derivations.
Alternatively, the experimental spectrum may be compared against a predicted spectrum,
produced from a computational model of the crystalline structure. The model is then fit against the
measured data, from which the fit accuracy will determine the confidence of the model.
The work presented here takes advantage of the information available in the reference data
base to quickly validate the ZnPc crystallinity and phase. Figure 2-16 presents the reference
spectrum of ZnPc.
36
Figure 2-17. ICDD reference XRD diffraction pattern and values for ZnPc powder
37
Kelvin Probe Force Microscopy
Kelvin probe force microscopy (KPFM) is a characterization method used to topologically
image the surface potential of materials. Understanding the details of both the instrument setup
and the sample is required to gain insight from the measurements as KPFM measures the contact
potential difference (VCPD) between an AFM tip and the sample. VCPD is defined as the difference
in work functions of the sample and tip relative to the electronic charge.
When the AFM tip and the sample surface are separated, the Fermi level of each are
different while the vacuum level of each is the same. As the two are brought into proximity, the
Fermi levels of each must align, and thus an electric potential is generated to reach equilibrium.
The vacuum level of the AFM tip and sample changes in this process and the difference between
the two is the physical definition of VCPD. Electrically biasing the AFM tip and sample surface will
then counteract the VCPD and re-equalize the vacuum levels while in electrical contact.
As the work function is defined as the potential difference between the Fermi level and the
vacuum, the bias necessary to counteract the VCPD is the difference in work function between the
AFM tip and the sample. The sample work function can thus be derived by knowing the work
function of the AFM tip. Figure 2-17 shows the energy level description of the KPFM process.
Figure 2-18. (a) Energy levels of the AFM tip and sample when separated (b) VCPD formation
from electrical contact (c) An applied bias equal to VCPD re-equalizing the vacuum level and
revealing the work function difference of the two materials [44].
38
In this study, KPFM measurements were performed using a Bruker Dimension Icon. The
measurements were taken in air within the quiet enclosure of the AFM system. The measurements
were based on the Peak Force Tapping mode with a silicon tip coated on the backside with aluminum
(PFQNE-AL). The KPFM scans were performed in a standard 2-pass modality, with the first scan
collecting topography, and second scan collecting the VCPD in the same line but with the tip lifted from
the surface. The acquired map was constructed using 512 lines/images.
39
Chapter 3 – Systematic Study of Zinc Phthalocyanine Growth
Overview
The first two chapters gave a background on the material properties of ZnPc and graphene,
the reactor designs and the characterization methods used throughout the project. The second phase
of this project concentrates on understanding the growth mechanisms of ZnPc, starting with how
the different reactor controls effect the morphology of ZnPc under a uniform graphene substrate
configuration.
The Mk I reactor design and process flow was touched upon in the equipment section
however this chapter will expand upon the results obtained. A discussion of these results will
provide the catalyst for the development of the Mk II reactor design and the reason for the process
change.
Once the Mk II design was implemented, the enhanced stability and control laid the
foundation necessary to properly couple each reactor parameter to a ZnPc growth effect. This
process was carried out in a systematic study, and the results presented in this thesis expand upon
the recently published work referenced here:
Mirabito, T., et al. "Physical Vapor Deposition of Zinc Phthalocyanine Nanostructures on Oxidized Silicon
and Graphene Substrates." Journal of Crystal Growth (2020): 125484.
Supplemental information regarding XRD characterization of the growth structures
obtained from the systematic study are presented in the last section. This information helps provide
additional details regarding crystallinity, molecular orientation, and phase that were unable to be
included in the published work above.
40
Mk I Reactor Process Flow
When the Mk I reactor was designed, the objective was to provide a physical vapor
deposition technique that could produce organic thin film results without the need for an expensive
commercial evaporation system. The primary challenge of scaling down a commercial system was
counteracting the draw backs of working in a higher pressure regime. Since most commercial
systems utilize large volume bell jar chambers, the 1st design choice was to scale down this
chamber to a 25mm diameter size by 300mm length. This allowed the chamber to be evacuated to
the millitorr regime using a rotary pump fairly effectively.
As was previously mentioned, ZnPc deposition utilizing the Mk I reactor followed a
straightforward thermal evaporator process flow of evacuating the chamber to the desired pressure,
and then heating the source material until deposition temperature is achieved. An example of this
procedure is illustrated in figure 3-1, in the temperature profile of the Mk I during a deposition.
Figure 3-1. Temperature profile of the Mk I reactor during a 350⁰C, 300 second deposition.
Note that the temperature does not reach the setpoint.
41
In a commercial system, a shutter is traditionally opened to start the deposition of the
source material onto the target substrate. Unfortunately, as a consequence of drastically scaling
down the size of the evaporation chamber, it wasn’t possible to include a shutter in the Mk I design.
Therefore, the start of the deposition occurs when there is significant enough vapor pressure at a
particular temperature for the pressure of the reactor. For a timed growth process, this introduces
uncontrolled variability into the growth procedure.
A second notable issue was that for this temperature setting and time duration, the source
temperature never reaches the set point. For a 300 second growth time, the temperature only
reached 300⁰C instead of the 350⁰C set point. In another deposition, also performed for 300
seconds at a 450⁰C set point, the same phenomenon occurs of not reaching the set point
temperature. This can be seen in figure 3-2.
Figure 3-2. Temperature profile of the Mk I reactor during a 450⁰C, 300 second deposition.
Note that the temperature does not reach the setpoint.
42
A potential solution to the uncertainty in the start of the deposition is to elongate the growth
time such that the system is able to reach the desired set point and have a semi-controlled system
given a common temperature-vapor pressure start point. While this would potentially work if the
set point was at the low temperature end of the growth window, at the higher end of the growth
window, the impingement flux is so large that increasing the growth time creates significant
variability in the thickness of the film deposition. As shown in figure 3-3, the SEM micrograph of
a 300 second growth and a 360 second growth at 450⁰C results in a 600nm increase in size.
Figure 3-3. ⁓2x increase in size from a) 300 second growth at 450⁰C b) 360 second growth at
450⁰C
43
An attempt to increase the ramp rate, thereby reaching the desired set point and establishing
temperate stability faster was also investigated. The initial temperature controller was an older
omega model without the ability to tune the PID settings. However, it was possible to manually
change the set point in multiple timed stages to force the system from a critically damped state to
a near overdamped state. This careful balance made sure to avoid the dramatic overshoot and
setpoint oscillations commonly associated with overdamped control system. The temperate profile
for this dual stage process is shown in figure 3-4.
Figure 3-4. Temperature profile of single stage and dual stage temperature tuning.
From this method the ramp time was improved from 300 seconds to 180 seconds. However,
the issue still persists that while the effects of a variable temperate may be mitigated, they cannot
be removed from the Mk I process.
44
Mk II Reactor Process Flow
While the Mk I reactor design was a successful starting point for the scale down of a
commercial PVD process, the issues present suggested a different approach would be needed to
be able to conduct a robust systematic study of the process parameters. Amongst all the issues
presented, the Mk I process had a general problem controlling the start and stop of a growth.
Without the option of using a quartz crystal oscillator to measure the film thickness, the process is
necessarily time dependent, exacerbating the aforementioned issues.
Figure 3-5. Temperature and pressure profiles for the ZnPc source and substrate measured in-
situ during the single-zone, constant pressure, temperature controlled ZnPc growth process with
a source temperature set to 400°C and a substrate temperature of 250°C.
45
Summarized in figure 3-5, dramatic temperature transients were observed with the source
and substrate temperatures continuing to rise throughout the growth. Initially, the source
temperature rises rapidly with the substrate temperature showing a significant lag. The
temperature gradient takes approximately 180 seconds to stabilize, however, the absolute values
of both source and substrate have not stabilized at 300 seconds. Furthermore, source temperatures
and the gradient dramatically change over several hundred seconds during the cooldown phase
indicating the growth can continue throughout this process step. Based on these measurements and
the 300 second temperature transient, it is clearly not possible to control the onset of growth,
substrate temperature or the flux due to the continuously changing temperatures during the process.
In scaling down the growth chamber to a 25mm diameter size by 300mm length, a much
smaller volume could be kept at a lower pressure. Another benefit is that a rotary pump could also
evacuate this smaller volume much quicker.
Figure 3-6. Temperature and pressure profiles for the ZnPc source and substrate measured in-
situ using the two-zone, pressure-controlled, constant temperature ZnPc growth process using a
source temperature set to 400°C and a substrate temperature of 250°C.
46
Figure 3-6 shows the corresponding measurements for temperatures and pressure during
growth for the two-zone configuration for a 300 second experiment. Two immediate observations
are that the pressure transient which is used to start and stop growth is very rapid, on the order of
5 seconds and that the source temperature, substrate temperature and temperature gradient show
are very stable with only approximately 3-4°C change over the 300 seconds. A very slight increase
in source temperature was observed at the end of the growth corresponding to the pressure increase
to terminate growth. This increase is attributed to a potential change in contact of the control
thermocouple with the ZnPc source.
Mk III Reactor Process Flow
The Mk III reactor process flow doesn’t deviate significantly from the Mk II flow. The
alterations done to the system allow for an increased temperature range of the substrate. The copper
heat sink added into the deposition chamber is done during the loading phase, prior to starting the
system.
Depending on the growth time and substrate temperature necessary, the copper heat sink
can be pre-conditioned at low temperatures prior to its insertion. If this was done, careful attention
not to introduce additional moisture into the growth chamber was undertaken.
47
Comparison of Silicon and Graphene Substrates
A great challenge for the development of organic semiconductor devices is control over
the nanoscale morphology and the orientation of the active materials. For example, it is known
that molecules from the metal phthalocyanine family exhibit an edge-on orientation on amorphous
substrates [45]. It has also been shown that the molecular orientation can be altered with the
introduction of structural interlayers [46]. As one such interlayer, graphene, has been shown to
induce several different morphologies in phthalocyanines, dependent upon the substrate from
which they were grown [47,48].
While several studies have investigated the growth of ZnPc, the scope of growth conditions
have been limited. A detailed characterization of the growth and orientation of ZnPc molecules on
transferred graphene is therefore required. A systematic investigation was performed that
combined Xray scattering and diffraction, and scanning electron microscopy (SEM).
Starting with graphene transferred onto a Si substrate, it is shown to act as templates to
orient ZnPc molecules to a face-on orientation in thin films. In contrast, the edge-on orientation of
ZnPc is normally induced by surfaces such as oxidized silicon. Figure 3-7 shows the growth using
the two-zone configuration on substrates with single domains of CVD graphene and the growth
on silicon simulataneously.
This was achieved by transferring partial layers of graphene onto the silicon. The boundary
of the graphene region is illustrated by the dashed line. This image shows that the ZnPc nanowires
lay nearly flat on the silicon surface while nanowires grown on the graphene show an inclined
orientation nearly orthogonal to the graphene surface. Based on these results the role of graphene
in the onset of nucleation is apparent and this will serve as the basis for future nucleation and
substrate surface energy studies.
48
Figure 3-7. SEM image showing growth morphology as a function of the substrate for ZnPc
nanowires grown at a source temperature of 400C and a substrate temperature of 250C. Left
region shows growth on bare (unetched) silicon while the right shows growth on single layer
CVD graphene. ZnPc nanowires lay nearly flat on the silicon surface while nanowires on the
graphene show an inclined orientation nearly orthogonal to the graphene.
49
Effect of Reactor Pressure
In order to understand the role of pressure on nucleation and growth of nanowires on
graphene, a series of experiments were conducted at pressures from 17mTorr up to 1000 mTorr as
shown in figure 3-8. These growths used substrate temperatures of 250C and source temperatures
of 400C in the two-zone configuration. At 17 mTorr high density nanowires were observed. As
the pressure was increased the morphology transitioned to a nanopillar structure with features
preferentially aligned. The alignment is believed to be an artifact of the rolling marks in the copper
foils used for graphene growth imparting texture into the graphene film. At the higher pressure
similar nanopillar growth was observed with a significantly lower density of smaller nanopillars.
At the conditions used for these experiments, the flux of ZnPc to the surface decreases with
increasing reactor pressure resulting in the change in density and size of features. The observation
of nanowires at low pressures (highest flux condition) suggests that the high flux of ZnPc relative
to surface diffusion leads to nanowire formation whereas at high pressures (lowest flux condition)
the structure forms nanopillars due to the increased surface diffusion distance at low flux. For
subsequent experiments a reactor pressure of 17 mTorr was used to promote nanowire growth.
Figure 3-8. SEM images of the graphene/Si substrates after depositing ZnPc for 1 min using a
Tsource of 450°C and a Tsubstrate of 250°C in the two-zone configuration under different reactor
pressures: (A) 17 mTorr, (B) 125 mTorr and, (C) 1000 mTorr. Scale bars are 2 μm for each
image.
50
Morphology and Temperature
The nanostructure of the ZnPc growth has been systematically investigated as a function of
ZnPc powder temperature, substrate temperature and evaporation duration. The boundary limits
of the parameter space are mainly based on the physical properties of ZnPc and the capabilities
performance of the rotary pump which possess an ultimate base pressure of 17 mTorr.
The temperature effects on morphology were studied for source temperature between 350°C
and 450°C. The substrate temperature was varied from 200-300°C in order to form the stable β-
ZnPc and avoid the metastable α-ZnPc which typically forms for temperatures lower than 200°C
[25] and to limit the desorption of ZnPc from the substrate surface.
Figure 3-9. SEM images of vertically oriented nanostructures grown on Graphene/Si<100> for
5 minutes at 17mTorr: Images A,D,G show island clusters, while images B,E demonstrate
nanopillars and images C,E,F,H,I show nanowire formations. Scale bars are 2μm for each
image.
51
Figure 3-9 shows ZnPc nanostructures obtained by varying Tsource and Tsubstrate at 50°C
intervals. At source temperatures of 350°C unevenly deposited ZnPc islands with only partial
coverage over the graphene surface were observed at all substrate temperatures. For higher
substrate temperature (300°C) the size of the islands increased, however, the coverage did not
change. Increasing Tsource leads to the formation of a more uniform film exhibiting high aspect ratio
nanopillars.
For source temperatures of 400°C, a coarse film surface with complete coverage was
observed. Increasing the substrate temperature to 250°C resulted in a high density of nanowires
and at a substrate temperature of 300°C the nanowire size increased, and the density decreased
along with coalescence of the nanowires.
At source temperatures of 450°C, nanowires were observed at all substrate temperatures
with trend the showed a decrease in density and an increase in thickness with increasing
temperature.
Despite the uniform crystal phase, it is evident from the temperature variation, that different
nanostructures are formed. It would appear that control of the mass flux and thermal diffusion of
molecules at the substrate surface are responsible for the formation of high aspect ratio nanowires
as opposed to the more coarse nanopillars and certainly the island clusters. This presents the
kinetics of the aforementioned growths in terms of low flux conditions that exist at low source
temperatures, and high flux conditions that exist at higher source temperatures.
Under high flux conditions, supersaturated ZnPc exists in the vapor phase at the substrate
surface by means of a high sublimation rate from the source diffusing vertically by a temperature
gradient. The rate of material arrival on the surface then dominates the ability of the ZnPc
molecules to diffuse across a free surface. This illustrates the kinetics as being diffusion limited,
and since the fastest growth axis of the ZnPc nanostructures will be in the π−π stacking direction,
anisotropic growth is preferrential.
Under low flux conditions, the kinetics change from a diffusion limited to a mass limited
growth mechanism. The nanostructures formed under these conditions are island clusters where
the diffusion barriers are low enough for the ZnPc molecules to diffuse towards the low density
nucleation sites.
52
The conditions inside the growth window extremes are a marked balance between these two
mechanisms. As shown in the 400°C source temperature conditions from figure 3-9, the substrate
temperature controlling surface diffusion can either be smaller which form nanopillars due to an
initial increase in nucleation density or larger forming jagged platelets due to a decrease in
nucleation density.
53
Nanowire Growth Time
To investigate growth rate of the nanostructures, both temperatures and pressure were held
constant at Tsource 450°C, Tsubstrate 250°C, and 17mTorr respectively, and the growth time was
varied between 30 seconds and 5 mins. These two values represent a reasonable range for the
growth under both a high flux and low flux condition where the structures can be identified. The
images in Figure 10A have a similar high aspect ratio, nanowire morphology, but the scale of the
longer growth time, shows the wires are much longer and have a larger diameter. This cross-
sectional view of these growths quantitatively confirms the size difference where the height of the
wires increases greatly from 500nm to 16 μm, while the width of the wires increases from 125nm
to 750nm on average, throughout the 5-minute time frame.
Figure 3-10. (A) Cross-sectional FESEM images showing the ZnPc nanostructure evolution on
graphene-coated Si<100> as a function of growth times between 30 seconds and 5 minutes, for
the Tsource 450°C, Tsubstrate 250°C, 17mTorr condition. (B) Plot of the nanowire dimensions as a
function of growth time.
From Figure 3-10B, the nanostructure size is seen to increase roughly linear with time. Given
the constant mass flux from the source material to the substrate, preferential adsorption of the ZnPc
molecules from the vapor phase to the nanostructure growth front appears to be supported from
the thermodynamic drive of free energy reduction. Additionally, the increase in size of each single
crystal nanostructure can be described by Ostwald ripening.
54
XRD Characterization of ZnPc Growth
In order to obtain information about ZnPc in powder form, identify the phase of the
material, and analyze the out-of-plane ordering of ZnPc molecules on both graphene and the native
oxide silicon substrate, XRD measurements were conducted.
As a reference, and to verify the information in the XRD database, a measurement of the
bulk ZnPc in powder form was taken and this spectrum is shown in figure 3-11. These diffraction
peak positions were compared with the standard JCPDS-ICDD tables (PDF 00-039-1882) and
found to coincide well for the β-ZnPc structure. It should be noted that the ZnPc XRD peaks will
all fall below a two-theta value of 30°.
Figure 3-11. XRD measurement of the starting ZnPc bulk powder compared against the
PCPDS-ICDD database peak positions for β-ZnPc. The peak position agreement confirms the
starting material to be in the beta phase.
55
As it was shown in the SEM micrograph in figure 3-7, the ZnPc nanowires on the native
oxide silicon region were oriented parallel to the substrate surface. To gain insight into the
molecular stacking on this surface, ZnPc was deposited onto a native oxide silicon substrate with
no transferred graphene, using the same deposition conditions. This XRD spectrum for this sample
is shown in figure 3-12.
Figure 3-12. XRD measurement of ZnPc deposited on a native oxide silicon substrate and
compared against the PCPDS-ICDD database peak positions for β-ZnPc. The peak position
agreement confirms the deposited nanowires to be in the beta phase.
Once again, the acquired spectrum matches well with the β-ZnPc database peak positions.
In addition, the peak position of the most noticeable feature was the Bragg reflection observed at
2θ = 7.1646°. This peak corresponds to an interplanar spacing of 12.32825Å and has been assigned
to the (200) plane of the β-ZnPc monoclinic structure [24]. The intensity of this reflection feature
implies that ZnPc molecules are oriented perpendicularly to the surface of the Si substrate,
confirming an edge-on orientation.
56
For a ZnPc film deposited on graphene, the XRD pattern changes drastically. A diffraction
peak emerged at 2θ = 27.88°, corresponding to an interplanar spacing of 3.20 Å, implying it
corresponds to the (112) plane of β-phase ZnPc. The intensity of the peak at 2θ = 7.07° was
significantly lower than that of ZnPc film deposited on Si substrate. This indicated that the vast
majority of ZnPc molecules adopted a face-on orientation due to the graphene. The presence of the 2θ
= 7.07° peak can also be attributed to incomplete coverage of the graphene, leaving some ZnPc
molecules to be grown on the native oxide silicon with an edge-on orientation.
The minor peaks at 2θ = 9.37° and 25.14° have a calculated interplanar spacing of 9.43Å and
3.5Å respectively. These positions and interplanar spacing values correspond to the (101) plane
and the (-312) plane, providing further evidence the sample is beta phase. The remaining peak at
2θ = 26.7° is the characteristic (002) peak from multi-layer graphene. This XRD pattern is shown in
figure 3-13.
Figure 3-13. XRD measurement of ZnPc deposited on CVD graphene transferred onto a native
oxide silicon substrate. The peak positions at (1) 7.07° (2) 9.37° (3) 25.14° (4) 26.7° (5) 27.88°
correspond to β-ZnPc with some multi-layer graphene domains present.
57
Chapter 4 – Transport & Growth Kinetics
Overview
Any discussion about physical vapor deposition (PVD) kinetics starts with an overview of the
vacuum system involved. Since PVD has been a widely implemented technique for the fabrication
of thin films it is common to find commercial systems with setups similar to the schematic in
figure 4-1. After evacuating the chamber to a reduced pressure environment, three key steps are
undertaken:
1. Sublimation of a solid source material (typically a powdered material for organics)
2. Diffusion of the vapor phase material
3. Condensation of the vapor phase material onto a target substrate
From this general setup, the newly built Mk I reactor setup was engineered to maintain the essential
equipment while specifically amending the surrounding pieces to optimize organic vapor
deposition for nanostructure fabrication. Starting with the removal of the turbo pump, the system
pressure range is altered from a 10-6 Torr range to a 10-3 Torr range. This is the most critical
alteration and the effect on the kinetics will be outlined in subsequent sections.
Following the discussion on transport kinetics during the PVD process, an analysis of the
nucleation and growth of ZnPc onto the graphene templated substrates is undertaken. This section
seeks to explain the unique role of graphene at the ZnPc-substrate interface.
Figure 4-1. Typical commercial physical vapor deposition equipment setup
58
Sublimation Process
Control of the physical vapor deposition process starts with the evaporation kinetics. Thermal
energy is transferred to the solid source, raising the temperature until the sublimation threshold is
reached. It is at this point that efficient deposition is possible, and to quantitatively define this
temperature-pressure relationship, the Antoine equation was employed as shown below.
𝐿𝑜𝑔 𝑃 = − 𝐴
𝑇+ 𝐵
The Antoine coefficients for ZnPc were found to be A = 10523 and B = 9.52 based on the
experimental work of Semyannikov et al. The plot of ZnPc vapor pressure is therefore extrapolated
as a function of temperature and shown in figure 4-2. From this plot, the vapor pressure of ZnPc
is only significant enough for effective evaporation above 350 ⁰C.
Figure 4-2. Vapor pressure-temperature relationship for ZnPc. Operating source temperature
was held between 350-450°C for these experiments.
59
By definition, the maximum evaporation rate is attained when the number of vapor
molecules emitted will exert the equilibrium vapor pressure without those same molecules
returning to the solid. The Hertz-Knudsen equation was developed to measure this surface rate and
is shown below:
𝛷𝑒 = 𝑎𝑁𝐴(𝑃𝑒 − 𝑃ℎ)
(2𝜋𝑀𝑅𝑇)1 2⁄
To maximize the evaporation rate, the sticking coefficient, αe, should be one, though solids
typically have values much less than that, and the pressure, Ph, should be zero. Under these
circumstances, the evaporation rate can be controlled with the source temperature alone, as the
equilibrium vapor pressure is temperature dependent.
Applying the Hertz-Knudsen equation to the ZnPc system, the following variables are defined:
Coefficient of Evaporation (a) 0.1 < a < 1
Partial pressure (p) Pa 0.0044 < p < 0.9466
Avogadro’s # (Na) 6.02E+23
Molecular mass (M) kg 0.57791
Gas const. (R) J/mol∙K 8.314
Temperature (T) ⁰K 623.15 < T < 723.15
Density (ρ) g/cm3 1.5
Solving the Hertz-Knudsen equation shows that the evaporation flux is on the order of 1017 for
the ideal evaporation conditions (a=1). The plot for several efficiencies is shown in figure 4-3.
60
Figure 4-3. Flux-temperature relationship for ZnPc. Ideal evaporation condition is represented
by the a=1 series, with a very low efficiency evaporation represented by the a=0.1 series.
61
Vapor Transport
Transport in physical vapor deposition systems is usually predicated on the mean free
path being much larger than the characteristic length. The mean-free path is defined as the mean
distance traveled by a molecule between collisions and is highly dependent on the pressure. The
equation employed in the calculation is derived from the kinetic theory of gasses and is shown
below.
𝜆𝑚𝑓𝑝 = 1
√2𝜋𝑑𝑐2𝑛
Part of the assumptions made from the kinetic theory of gases, include a molecule with a
spherical geometry. Additionally, in the vast majority of cases the source material is well
characterized, and the collision diameter can be obtained by literature. In the case of many small
molecule organics, the molecular geometry may be non-spherical, or the kinetic diameter may be
unknown. This is indeed the case for ZnPc, which is a planar molecule.
Using JSmol computer software to model the molecular structure and the electrostatic
surface, the kinetic diameter of the molecule can be estimated. The molecular surface is shown in
figure 4-4 with the diameter measured to be 1.5 nanometers.
Solving the mean-free path equation and plotting the result as a function of temperature, it
is evident there is a significant decrease in mean-free path over the deposition temperature range
of 350⁰C to 450⁰C. This plot is shown in figure 4-5.
62
Figure 4.4. JSmol model of the metal free phthalocyanine structure. The electrostatic surface is
shown to aid in measuring the kinetic diameter of 1.5 nm.
Given the higher-pressure regime of the new system compared to more traditional
commercial setups, the much smaller mean-free path of the ZnPc transport prompts revisiting
whether or not the system can maintain free-molecular flow.
Using the Knudsen number calculated below:
𝐾𝑛 = 𝜆
𝐷
λ is the mean-free path
D is the characteristic length
In order to achieve free-molecular flow, the Knudsen number must be greater than 1, else it will
fall into a Knudsen flow or viscous flow range whereby inter-particle collisions occur frequently.
63
Figure 4-5. The calculated mean-free path vs temperature. The log scale on the vertical axis
emphasizes the significant decrease in MFP over the deposition range in this system.
From figure 4-5, the MFP during the relevant deposition temperatures ranges from 10cm
to 100μm. This severely limits the characteristic length of the deposition chamber in order to
maintain free-molecular flow. In the PVD system implemented in this work, this characteristic
length is a 2.5cm diameter quartz tube.
Since this MFP was calculated based on the ZnPc vapor pressure attained at each
temperature, it should follow that at the deposition pressures utilized the ZnPc partial pressure
approaches but never reaches the total composition of the global pressure within the reactor and
therefore the MFP in the experiments will be primarily determined by the global pressure.
64
Inside the reduced atmosphere of the quartz cuvette the transport kinetics, can be modeled
as a 1-dimensional diffusion process. Mathematically, it can be described as a coupled flux
equation:
where JM is the mass flux, LMQ is the kinetic coefficient relating mass to heat, LMM is the kinetic
coefficient relating mass to mass, T is temperature, x is a linear position in one dimension, and μ
is the chemical potential.
This equation assumes that the kinetic process is linear and has no memory contributions.
Therefore, the flux can be expressed as a Maclaurin series expansion of the affinities with coupling
kinetic coefficients responsible for the cross effects from momentum transfer, and the direct kinetic
coefficients representing its conjugate driving force.
Additionally, the mass diffusivity coefficient’s temperature dependence in the vapor phase
can be expressed using Chapman–Enskog theory. This is shown in the equation below:
where T is the temperature, Mi is the molecular weight of the ith component, P is the pressure, σAB
is the Lennard Jones radius, and ΩD,AB (T,εAB, σAB ) is an empirical function ranging from 0.5 to
2.7, where εAB is the Lennard-Jones intermolecular potential field.
Ultimately, the vapor phase material diffuses vertically according to the classical error
function below,
−−=
Tdx
dTL
dx
dT
TLJ MMMQM
1
( )
=
Dt
xCtxC
4erfc, 0
65
This is assuming that the surface concentration above the ZnPc powder is constant, and the
system represents a semi-infinite diffusion problem. It should be a reasonable assumption given
the temperature range and the large pressure suppression of the ZnPc vapor at time zero, followed
by a steady-state evaporation of the powder during the diffusion process.
As the mass diffuses, supersaturation of the material in the vapor phase will condense at a
cooler temperature where the vapor desaturates, resulting in crystal nucleation and growth. This
process is illustrated in figure 4-6.
Figure 4-6. Mass diffusion of material through the temperature gradient followed by crystal
nucleation and growth
66
Deposition
As vapor transport proceeds, and the system moves to a steady state deposition condition,
the impingement flux at the substrate should equal the evaporation rate from the source material.
Mathematically, the deposition rate can be quantified by the equation below:
𝐷𝑒𝑝𝑜𝑠𝑖𝑡𝑖𝑜𝑛 𝑅𝑎𝑡𝑒 = 𝛷𝑒𝑀
𝜌𝑁𝐴
Where Φ is the impingement flux, M is the molecular mass of the source material, ρ is the density,
and NA is Avogadro’s number.
Based on the developed data relating evaporation flux to temperature, the deposition rates
can be calculated and shown in figure 4-7. Provided the maximum evaporation condition of a = 1,
and that the flux is unimpeded vertically during transport, the growth rate at the substrate is
calculated between 12.25 nm/sec at 350°C and 2469.25 nm/sec at 450°C.
However, when looking at the cross-sectional SEM results, the average length of the ZnPc
nanopillars ranged from 500nm at a 450°C growth for 30 seconds, to 16μm at a 450°C growth for
5 minutes. This would equate to a growth rate of 16.67nm/sec and 53.3nm/sec for the respective
conditions. These much lower rates imply that the nanostructure growth rate varies significantly
from the expected values based on mass flux calculations.
To achieve the experimental values, the impingement flux would need to be stifled. One
possibility is that the sticking coefficient of ZnPc during the heteroepitaxy phase of the growth is
on the order of 5E-3, and 2E-2 during the homoepitaxy phase. The other possibility is that given
the viscous flow of the ZnPc at the higher deposition temperatures, the decreased mean-free path
would lead to material being deposited more on the quartz cuvette sidewalls, especially as the
molecules reached the lower end of the temperature gradient, leading to an overall drop in
impingement rate at the substrate surface.
In agreement with the conservation of mass, the preponderance of evidence suggests that
the impingement flux delta between the source and substrate can be explained by the limited mean
free path.
67
Figure 4-7. The calculated deposition rate vs temperature. Comparisons to cross-sectional SEM
measurements of thickness, the sticking coefficient is between 0.1 and 0.5.
68
ZnPc-Graphene Interface
In the previous chapters the effect of graphene on the orientation of ZnPc crystallization
was demonstrated. From both SEM images and the XRD spectra the molecular orientation of ZnPc
was shown to vary according to figure 4-8. In the following section, an explanation for this
behavior is presented from a thermodynamic perspective.
Figure 4-8. Molecular orientation of ZnPc when deposited on silicon vs single-layer
graphene/silicon. (a) Edge-on molecular packing (b) Face-on molecular packing
69
It has been established that the ordering of organic molecules in films is governed by the
interplay between both intermolecular and molecule−substrate interactions [49]. Many of these
initial discoveries were in the context of researching self-assembled monolayers of organic
molecules on metal surfaces. As part of these surface studies, it was shown weak molecule-
substrate interaction would be dominated by the molecule-molecule interaction causing the
molecules to pack in an edge-on orientation. In the opposite case, a strong molecule-substrate
interaction was shown to cause molecules to pack in a face-on orientation.
It was therefore suggested that the orientation of the molecule could be controlled by the
substrate electronic structure. In detail, the interaction strength of a π-conjugated system with the
substrate is controlled by the local density of states (LDOS) at or near the Fermi level at the
substrate surface [50].
T. Wang et al. [51] recently published a study seeking to confirm that theory for the
interaction of a 10nm ZnPc film grown on graphene and SiO2. In this study, they were able to
characterize the ionization potential of each sample using ultraviolet photoelectron spectroscopy.
It was shown that the ionization potential of ZnPc on graphene was 5.28eV and 4.82eV for ZnPc
on SiO2. This potential difference of 0.46eV was compared against previous studies of other
organic materials and was in agreement with the energy difference associated with a face-on vs
edge-on orientation.
In an additional study using first-principles calculations of the ZnPc isomorph, CuPc,
molecules were introduced on graphene in both face-on and edge-on orientation. The CuPc binding
energy to graphene is 3.37 eV and if van der Waal contributions are not considered; the binding
reduces to 0.1 eV on graphene [52]. This is an important result given that the van der Waal
interactions are directly correlated to the π orbitals of both the molecule and the substrate.
From these studies, coupled with the XRD characterization performed, it is clear that the
graphene has a profound effect on the ZnPc molecular orientation at the interface and the cause of
this effect can be traced to the LDOS of the silicon substrate from which the ionization potential
varies.
70
Nucleation & Growth
A discussion on the nucleation and growth of ZnPc on graphene templated substrates
couples closely to the effects of graphene presented in the previous section. As such, the discussion
will seek to explain the growth mode from both a thermodynamic and kinetic perspective.
Starting with the initial epitaxial condition, it follows from the previous section that the
molecule-substrate interaction will be strong, inducing the ZnPc molecules to pack in a face-on
configuration. It is also noticeable that ZnPc molecules will preferentially nucleate at step edges,
wrinkles or at grain boundaries of graphene prior to complete coverage. Evidence of this is shown
in figure 4-9.
Figure 4-9. Preferential nucleation of ZnPc at the grain boundary of single-layer
graphene/silicon.
71
It has been reported that the diffusion barrier for the isomorphic CuPc on graphene is
0.16eV [53]. With the contribution to van der Waals forces primary emanating from the Pc skeletal
structure, this value is likely representative of the range ZnPc would be in. In which case, the
barrier is sufficiently small to enable diffusion at room temperature. Using the combination of
molecular orientation and proficient diffusion, it is likely that the first layer will be a continuous
film in a face-on packing.
After the first layer, molecule–molecule interactions begin to emerge over the molecule-
substrate interactions. At this point, subsequent layers will start experiencing strain causing non-
planar orientations to emerge. This change in planarity is indicative of the Stranski–Krastanov (S–
K) growth mode, which sees a 2D layer-by-layer growth transition into a 3D island growth.
In the work of Wang et al. [50], the S-K growth mode was identified for the growth of
ZnPc thin films on Au substrates. Recalling the similar ionization energy for ZnPc films on Au
and graphene, it is likely that the S-K growth mode is present here as well. The S-K growth mode
was also found in the growth of a ZnPc:C60 co-deposition onto a silicon substrate [54]. In that
work, the lattice mismatch between the film and the substrate were found to relax within 8
molecular levels, resulting in a columnar surface. The SEM image in figure 4-10 of a ZnPc growth
onto a graphene substrate provides some evidence for many of the phenomenon described here.
Figure 4-10. ZnPc nucleation along a step edge, showing an apparent 2D film and 3D island
transition.
72
For completion, it should be noted that following the nucleation according to the S-K
growth mode, the fast growth in the vertical direction can be explained by the growth axis in the
π−π stacking direction being favored. This fact coupled with the large thermal motion of ZnPc
deposited at high temperatures, causes the surface diffusion to favor island growth. Further details
about the resultant morphology were touched upon in the systematic study in chapter 3.
73
Chapter 5 – Substrates, Layers, Polymorphism & Defects
Overview
In the previous chapters the focus has been on understanding the growth effects by
analyzing the system parameters for a common graphene coated substrate. In the systematic study
this sample was silicon <100>. Doing so allowed the parametric study to decouple each effect.
These experiments ultimately provided a comprehensive summary of the extrinsic factors effecting
the ZnPc growth in the reactor.
Moving forward, this chapter will explore several additional effects that can change the
morphology of the ZnPc thin film:
1) The substrate material CVD graphene is transferred on
2) The number of layers of graphene present
3) The polytype of ZnPc
The last section of the chapter will briefly explore the effect of defects in the graphene, and any
potential effects on nucleation.
74
Substrate Effects
In chapter 4, a thermodynamic and kinetic analysis of the growth mode was presented
which established that the orientation of the molecules could be affected by the density of states
present in the underlying substrate. This concept helped explain the growth mode of ZnPc on
graphene transferred onto a silicon substrate. However, a greater array of substrates would need
to be tested to confirm that the theory presented is applicable to the work being performed in this
project.
It is therefore prudent to explore the effect on a range of commonly utilized substrates.
These would include sapphire, copper, and SiO2 with both a 90nm and a 280nm oxide layer. As a
reminder, when ZnPc was grown on top of the graphene coated silicon <100> substrates under
high flux conditions, the result was high aspect ratio nanowires that grew preferentially onto the
graphene domains in a vertical orientation.
Noting the single crystalline nature of the underlying silicon substrates, the first alternative
substrate investigated was c-plane sapphire. Investigating sapphire compares the common high
crystallinity trait while also noting the semiconducting versus insulating nature of silicon and
sapphire respectively.
Growing the ZnPc under high flux conditions once again produced high aspect ratio
nanowires that grew preferentially onto the graphene domains in a vertical orientation. This result
is shown in figure 5-1.
The vertical orientation of the nanowires gives evidence that a single crystalline underlying
substrate, similar to silicon, will promote the growth in a face-on packing. Additionally, the
insulating nature of the underlying substrate had no effect on the growth morphology.
To fully compare sapphire to silicon within the context of ionization potential, ultraviolet
photoelectron spectroscopy could be performed. Unfortunately, this work falls outside the scope
of this project. Based on this empirical evidence, it would appear that the ionization potential is in
the range of 5.28eV.
It should also be noted that the ZnPc growth on bare sapphire has an edge-on orientation
noted by the long aspect ratio in the (100) plane direction.
75
Figure 5-1. ZnPc on single layer graphene/C-plane sapphire shows vertical orientation while on
bare sapphire the crystals are parallel to the surface.
Evidence of the single crystalline influence is further explored with the growth on graphene
directly synthesized on copper. Growth on this substrate will also help answer the question of
whether the conducting nature of the substrate influences the morphology. This substrate has the
additional element of answering whether the CVD transfer process has an effect. The previous
samples have each gone through a FeCl etch and transfer, which may have variability in graphene
surface contact. In the end, the ZnPc grown on a graphene/copper substrate showed the nanowire
morphology under the same growth conditions. This helps verify the common single crystal
element, as well as the negligible effect of the transfer process or material conductivity. The
growth is shown in figure 5-2.
76
Figure 5-2. ZnPc on single layer graphene/copper shows vertical orientation while on bare
copper the crystals are parallel to the surface.
This result could be expected given the high molecule-substrate interaction strength of
copper and ZnPc. Thus, both the copper and graphene both serve to influence a face-on packing
motif. The thickness of the growth here partially conceals information about the growth on the
bare copper.
A second deposition on bare copper under low flux conditions appeared to show the ZnPc
growth in a face-on packing when viewed with a SEM. This result would be consistent with the
ionization potential theory in explaining the face-on packing and can be seen in figure 5-3.
However, the orientation of these nanostructures would need to be confirmed using XRD.
Unfortunately, the XRD instrumentation used was unable to resolve a spectrum for the
small sized nanostructures grown, prompting the need for future work to resolve this question.
77
Figure 5-3. ZnPc on bare copper shows vertical orientation due to a strong molecule-substrate
interaction.
Following the study of single crystalline substrates, an amorphous substrate was
investigated. In this case, readily available SiO2 with a 280nm oxide surface. Single layer graphene
was transferred onto the SiO2 substrate using the same process as with the aforementioned
substrates. Given a common set of growth conditions, the ZnPc grew as a combination of
nanopillars and islands with no nanowires observed. On the bare SiO2 the ZnPc was once again
oriented parallel to the substrate. This would indicate that the amorphous oxide layer is playing a
role in the structure morphology even through a single layer graphene. This growth is shown in
figure 5-4.
In the work presented by T. Wang et al [51], the ionization potential derived from UPS
characterization for graphene transferred onto SiO2 was 5.28eV. This would indicate that the
molecules would pack in a face-on orientation, and this is confirmed visually from the SEM image
that the ZnPc growth on SLG is indeed vertical.
78
Despite its orientation though, it is clear that the morphology observed is different than the
high aspect ratio nanowires previously seen on other substrates. One potential explanation for this
behavior is that the weaker substrate-molecule interaction strength of the SiO2 could be allowing
the relaxation to occur more quickly in the molecular layer stacking. It is also possible that the
weaker substrate-molecule interaction strength is allowing for slightly greater diffusion after the
1ML coalescence. That would lead to slightly larger domains parallel to the surface.
Further investigation of this growth morphology would be needed to confirm any theory
presented here.
Figure 5-4. ZnPc on single layer graphene/SiO2 shows vertical orientation while on bare SiO2
the crystals are parallel to the surface.
79
It is worth noting, that one of the previous growths was performed on unetched Silicon,
where there would be a thin native oxide layer. This native oxide layer did not prevent the growth
of nanowires from forming, which leads to questioning the role of thickness in the oxide layer.
Therefore, analyzing the growth on a thinner oxide layer was undertaken with a SiO2 substrate of
90nm oxide layer, shown in figure 5-5. The effect again was that ZnPc grew with a combination
of nanopillars and islands. This would indicate that the ability of the ZnPc to maintain nanowire
morphologies would be limited to only the thinnest of oxide thicknesses. Overall, we see a clear
difference between the single crystal and amorphous underlying substrate.
Figure 5-5. ZnPc on single layer graphene/90 nm SiO2 shows vertical orientation while on bare
SiO2 the crystals are parallel to the surface.
80
Multilayer Graphene
One of the interesting results Xiao et al [52] observed when depositing CuPc on graphene,
was that at high temperatures, CuPc had preferential nucleation on SLG rather than FLG.
Additionally, at room temperature they found no selective growth for the CuPc molecules to form
continuous films on both SLG and FLG at high coverage, indicating that the film morphology and
CuPc grain size are not influenced by the number of graphene layers.
Similarly, Wang et al. [55] observed that at room temperatures, F16CuPc molecules form a
fully covered film on both SLG and FLG at high coverage, but at low coverage the CuPc molecules
selectively adsorb on SLG, resulting from a subtle difference in their electronic structures.
To confirm this result for ZnPc, the experiment was repeated using graphene transferred
onto a silicon substrate and a high flux (Tsrc=450⁰C) low substrate temperature (Tsub=40⁰C) growth
condition. The SEM image in figure 5-6 shows the result of this growth.
Figure 5-6. ZnPc shows preferential nucleation on SLG at low substrate temperatures.
81
Empirically, the results obtained using similar growth conditions with ZnPc diverge from
the results mentioned by Xiao et al and Wang et al. It is evident that the ZnPc has grown
preferentially on the single layer region even at low substrate temperatures. This difference in
result, motivated further investigation into the effect of the number of layers of graphene on the
ZnPc morphology.
The development of growing concentric AB stacked isolated domains provided an elegant
setup to test on. As shown in figure 5-7, the isolated domains of concentric AB stacked graphene
are an ideal setup to observe the morphology change at the layered interface. Logistically, the
prevalence of these AB stacked domains avoids having to spend excessive time isolating a smaller
feature from a large sample.
Figure 5-7. SEM image of an isolated concentric domain of AB stacked graphene highlighting
the linear progression of layers.
82
Though SEM micrographs can visually show the regions where each layer exists,
confirmation can be had through Raman spectroscopy. However, instead of taking a single point
measurement, the approach is to perform a 2D Raman mapping of the region. From there, the
full width half max of the 2D-band, and the G:2D ratio can be measured with high spatial
resolution over the entire domain. The Raman map, corresponding to figure 5-7, is shown in
figure 5-8 and figure 5-9 for the FWHM of the 2D band and the 2D:G ratio.
First using the Raman map in figure 5-8, the blue region representing a 25 cm-1 FWHM
is representative of a single graphene layer. Tracing a line from this region towards the center of
the AB stacked domain, a stepwise increase in the FWHM occurs until the center region at 55cm-
1 which is representative of multilayer graphene.
Figure 5-8. 2D Raman mapping of an isolated concentric domain of AB stacked graphene
highlighting the FWHM of the 2D Band.
83
Figure 5-9. 2D Raman mapping of an isolated concentric domain of AB stacked graphene
highlighting the 2D/G peak intensity ratio.
If the same exercise is performed using figure 5-9, the light blue colored region showcasing
a 2:1 ratio further indicates this region to be single layer. As the trace is followed towards the
domain center, the ratio decreases stepwise in accordance with each additional layer of graphene.
After confiming the nature of these AB stacked domains, a sample was transferred onto
SiO2 for growth. Using a medium flux (Tsrc = 375⁰C) and a high substrate temperature (Tsub =
250⁰C), the growth shown in figure 5-10 was obtained.
Similar to the previous result of selective growth on SLG, it is evident from this figure
that each graphene layer has a different microstructure. The most drastic change in morphology
is between the single layer region and the bi-layer region. The SLG region demonstrates the
same morphology seen in the previous section. However, directly across the grain boundary to
the bi-layer region the nanopillar structure disappears and the region adopts a clustered island
morphology. On subsequent graphene layers, the same morphology as the bi-layer region
appears but at a slightly twisted angle.
84
Figure 5-10. Growth on bi-layered and multilayered graphene shows the clustered island
morphology, while on single layer the nanopillar morphology is seen.
To further investigate this layer-morphology connection, a similar experiment was carried
out on a multilayer domain grown on a copper substrate. Using a high flux (Tsrc = 425⁰C) and a
high substrate temperature (Tsub = 250⁰C), the growth shown in figure 5-11 was obtained.
Again, the morphology transition as a function of the number of layers is shown. The minor
change in this sample is that the growth on SLG has a nanowire morphology, the growth on bilayer
has nanopillar morphology, and the multilayer regions have the island cluster growth.
Overall, the multilayer domains demonstrate a discrete morphology between layers, though
it is interesting that for a common layer number, the structures on copper and SiO2 are different.
Even more interesting is the detail that the step sequence is the same, but the morphology sequence
on copper is “phase-shifted” relative to SiO2.
With such a pattern, it is possible that the same mechanism driving the morphology
difference between substrates, is also responsible for the shift between SLG and bilayer graphene.
85
Figure 5-11. Growth on layered graphene copper. SLG shows expected nanowire morphology,
bi-layered shows nanopillar morphology and multilayered graphene shows the clustered island
morphology.
To address the difference in morphology from an energy viewpoint, information about the
band structure of multilayer graphene on SiO2 was needed. The most accessible characterization
method to gain this insight was kelvin probe microscopy. This characterization tool has the ability
to provide several pieces of useful information in addition to the difference in work function
between SLG and MLG.
A CVD grown multilayer isolated domain sample was prepared on SiO2 and several
regions were scanned. The most relevant region containing three graphene layers in sequential
order is shown in figure 5-12.
86
Figure 5-12. KPFM imaging of a SLG region in between two bilayer regions. (a) surface profile
showing minimal height delta (b) adhesion showing the entire region in uniform contact with the
substrate (c) work function measurement showing small variation between layers.
87
In the KPFM scans shown, the surface profile in figure 5-12a shows minor height
differences between the different graphene layers as expected. Within each domain the only
relevant profiles are from step-bunching effects via graphene strain relaxation. In figure 5-12b the
adhesion is shown to be uniform. Most importantly, in figure 5-12c the contact potential difference
shows the single layer graphene and bilayer graphene at approximately 100meV apart. Between
the bilayer and tri-layer however that potential difference is nearly indistinguishable. These values
agree with the values found by Panchal et al. [56]
Using this information, the work function of the PFQNE-AL probe tip (Φprobe = 4.09eV), and
literature values for SiO2 of 5.28eV, we can conclude that the ionization potential for the bilayer
graphene is approximately 5.16eV. While this may not be a significant difference, a drop of 500meV
between SLG on SiO2 and SiO2 has been shown to differentiate between an edge-on packing and a
face-on packing.[57]
In this regard, it would seem that the reduction in ionization potential is causing a slight tilt
from the surface normal. While this alone might not account for all the factors involved, it is likely one
of the contributors to the difference in morphology.
88
Polymorphism
Throughout this work, the deposition conditions have focused on variable flux rates while
controlling the substrate temperature to be between 200⁰C-300⁰C. Under these conditions, it was
expected that the stable β-phase ZnPc structure would be produced. However, it is far more
common that depositions performed using commercial thermal evaporators produce the metastable
α-polymorph. This can be attributed to performing thin film depositions with the substrate at room
temperature. Accordingly, an investigation into the growth of α-ZnPc was carried out.
Using a silicon <100> substrate, ZnPc was deposited at substrate temperatures below the
200⁰C phase transition point. From the acquired XRD pattern shown in figure 5-13, the
characteristic peak at 2θ = 7.0574° is seen. The prominence of this peak coupled with the absence
of any other significant ZnPc peaks, suggests a different structure when compared to the
characteristic spectrum from the β-phase ZnPc (figure 3-12). Additionally, the experimental
spectrum shown here compares favorably to the XRD data acquired by [26] and [25] for ZnPc, as
well as [52] for isomorphic CuPc. From this XRD comparison it is very likely that this sample is
α-phase ZnPc.
Figure 5-13. XRD spectrum of α-ZnPc grown on silicon using a 460⁰C Tsrc and a 75⁰C Tsub.
89
Further investigation into α-ZnPc, with the inclusion of a graphene template to orient the
phthalocyanine molecules in a face-on configuration, shows a drastic change from the XRD
spectrum on silicon. For a ZnPc film deposited on graphene with a 325⁰C source temperature and
a 160⁰C substrate temperature, the XRD pattern is shown in figure 5-14. A diffraction peak appears
at 2θ = 27.9144⁰, corresponding to an interplanar spacing of 3.2 Å, which has been indexed as the
(112) plane of α-ZnPc. The diffraction peak at 2θ = 7.061488⁰ is significantly less intense
compared to the deposition on silicon (figure 5-13) indicating the majority of the molecules are in
a face-on orientation due to the graphene.
Figure 5-14. XRD spectrum of α-ZnPc grown on graphene/silicon using a 325⁰C Tsrc and a
160⁰C Tsub. The peak at 2θ = 27.9144⁰ suggests a face-on orientation on graphene.
Complimenting the XRD data collected, additional evidence suggesting the presence of α-
ZnPc can be found from SEM images of the growth on graphene. Figure 5-15 shows the
morphology from the XRD sample grown at 325⁰C Tsrc and a 160⁰C Tsub. As a comparison, an
SEM image of α-CuPc grown on graphene transferred onto a silicon substrate is shown in figure
5-16, demonstrating the isomorphism to α-ZnPc.
90
Figure 5-15. SEM image of α-ZnPc grown on graphene/silicon using a 325⁰C Tsrc and a 160⁰C
Tsub.
Figure 5-16. SEM image of α-ZnPc isomorph α-CuPc grown on graphene/silicon from Xiao et
al.[52]
91
Further building upon the α-ZnPc characterization data, ZnPc can undergo a phase
transition with postdeposition thermal treatment, changing from α-ZnPc to β-ZnPc crystallites.
[58]. Applying this logic to our samples, a growth was performed using a source temperature of
325⁰C and a substrate temperature of 160⁰C. SEM images of this growth are shown in figure 5-17
and match the expected morphology for α-ZnPc. Subsequently, a postdeposition thermal treatment
was performed at 300⁰C for 1 hr in an inert argon environment. An SEM image of the ZnPc
morphology post thermal treatment is shown in figure 5-18.
Based on the pre and post SEM images, it would appear that the α-ZnPc domains
changes polytype to β-ZnPc crystallites. The morphologies seen in each case are comparable to
previously ascertained structures during this study.
Figure 5-17. SEM image of α-ZnPc grown on graphene/SiO2 using a 325⁰C Tsrc and a 160⁰C
Tsub.
92
Figure 5-18. SEM image of a ZnPc domain on graphene/SiO2 post thermal treatment. The
appearance of nanowire structures suggests β-ZnPc.
93
Graphene Defects
It has been shown both in this work and in literature [59] that defects in graphene
promote higher binding energies with metal nanoparticles. It is therefore likely that during the
initial monolayer construction, ZnPc would preferentially nucleate in these locations. It is not
known however, how this preferential nucleation will affect long range ordering in thicker films,
nor if the manifestation of particular morphologies can be attributed to ZnPc causing defects in
the graphene during nucleation or growth.
To investigate these ideas, two studies were carried out. The first sought to address
whether purposely introducing defects into the graphene would modify the microstructure of
ZnPc grown upon it. The second involved characterizing a specific region of the graphene
surface both pre and post ZnPc deposition and utilizing Raman spectroscopy to check for an
increase in defects.
Starting with the first study, CVD graphene was transferred onto SiO2 substrates and then
characterized using Raman spectroscopy paying specific attention to the intensity of the D-band
at 1350cm-1. Each of these samples would then undergo an O2 plasma etch at 50W for between 1
and 5 seconds. These samples would then be characterized again with Raman to quantify the
increase in defects. These samples would finally be grown upon using a high flux (Tsrc=450⁰C)
high substrate temperature (Tsrc=250⁰C) growth condition. The results of this study are shown in
figure 5-19.
The comparison of defect density to growth morphology demonstrated no significant
change to the morphology either between increasing defect densities, or as a comparison to the
reference sample. The growth conditions chosen resulted in a dense array of nanopillars which
may be concealing further information that a suppressed growth might reveal. However, based
on the available information, it would suggest that an increase in defects present in the graphene
have no affect on the long-range order in ZnPc crystallization.
94
Figure 5-19. Comparison of defect density and growth morphology at different plasma etching
times. (a,b) Reference (c,d) 1 second (e,f) 3 seconds (g,h) 5 seconds
95
The second study, investigating if the growth induced defects in the underlying graphene,
was carried out by transferring CVD graphene onto a SiO2 substrate and then characterizing using
Raman spectroscopy. Performing this study on a multilayer graphene domain would provide even
more information. The investigated domain and associated Raman spectra prior to growth is shown
in figure 5-20.
Figure 5-20. Pre-growth characterization of a multilayer graphene domain. (a) optical
micrograph (b) 2D:G ratio demonstrating a layered structure (c) Raman spectra showing no
defects.
96
Figure 5-21. Post-growth characterization of a multilayer graphene domain. (a) SEM
micrograph (b) SLG spectrum (c)Bilayer spectrum (d) tri-layer spectrum (e) quad-layer
spectrum. The band at 1340cm-1 is characteristic of ZnPc and no significant D band appears at
1350cm-1
97
From the post growth analysis shown in figure 5-21, it can be concluded that the presence
of a band at 1340cm-1 can be attributed to the characteristic peak of ZnPc. Since no significant D
band appears at 1350cm-1 it can be concluded that the growth of ZnPc on a graphene surface
doesn’t coincide with the development of any defects. Overall, it would appear that defects may
not have as dramatic effect on the nucleation and growth of ZnPc nor will the proposed growth
mode cause defects.
98
Chapter 6 – Conclusions & Future Work
Zinc Phthalocyanine is a material of great interest for a wide range of electronic
applications. In particular, ZnPc possesses some exceptional properties (described in chapter 1)
ideal for several key electron devices. Two applications of interest are solar cells and gas sensors
for chemicals such as ammonia. These two devices rely on a preferential orientation to maximize
charge transport. However, establishing an interface capable of face-on packing while also being
transparent has reduced the material choices considerably. Complete control over the surface
morphology by integrating an electrically active buffer layer can overcome these limitations to
produce novel device architectures.
This thesis has attempted to expand the knowledge surrounding ZnPc and its related
phthalocyanine family and their interaction with graphene towards overcoming material-related
limitations in organic electronics. This was accomplished by constructing a new physical vapor
deposition system with an effective growth methodology capable of establishing both standard and
unconventional growth regimes. Next, a systematic approach to understanding the growth
morphology based on extrinsic growth parameters was studied leading to distinct microstructure
control. Finally, a thermodynamic and kinetic treatment was applied to the growth mode that could
explain aberrations in the surface morphology when growing under a variety of conditions,
qualities, and interfacial stacks.
Epitaxial graphene as grown from SiC sublimation and post hydrogenation was initially
explored as an additional investigation to address possible effects from graphene character and
substrate contact. However, limitations in achieving a true single layer growth meant direct
comparisons to CVD transferred graphene would be untenable.
99
Future Work
The present research for this thesis has successfully demonstrated an inexpensive and
highly controlled process for physical vapor deposition of the organic semiconductor ZnPc to
achieve a preferential surface morphology. Several areas of additional research can be addressed
however, to further confirm some of the growth theories presented here, which will lead to a more
efficient application in devices.
First, the predominant evidence pointing towards the Stanski-Krastanov growth mode
involving a compressive stress relaxation driving a 2D to 3D transition can be studied. It has been
established that Raman spectroscopy measurements on the graphene 2D peak position can be
utilized to quantify stress. Additionally, UPS measurements of ZnPc grown on a wider range of
substrates can establish a more comprehensive list of induced orientations.
Second, with the advancement of 2D materials for electronic applications the exploration
of this interface with π-conjugated molecules can lead to a greater understanding of surface
thermodynamics and kinetics in thin film growth.
Finally, the extension of ZnPc into the application space with its growth onto similarly
planar molecules of low lattice mismatch such as Perylenetetracarboxylic dianhydride (PTCDA)
to form vertically π-stacking PN-junctions can be explored. This type of orientation optimized
organic device could take advantage of large surface areas and high absorption coefficients to
produce higher efficiency solar cells.
100
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