crms-free transparent conductive coatings€¦ · biggest global suppliers of raw materials study...
TRANSCRIPT
COST is supported
by the EU Framework Programme Horizon 2020
CRMs-free transparent conductive
coatings
Maria Luisa GRILLI (Italy)
Training School ” CRMs in extreme conditions: focus to
young material scientist, challenges and perspectives”
Budapest,9 July 2018
Solutions for Critical Raw Materials
Under Extreme Conditions
(CRM-EXTREME)
Outline
Transparent condutors Indium Tin Oxide
Fundamentals on transparent conductive oxides
ITO alternative conductive oxides: AZO and metal/AZO
Ultrathin Ni films
Graphene
Transparent conductors (TCs) are widely used in electronic devices, thin film solar cells and touch screens.
Transparent conductors
Indium tin oxide (ITO) dominates the market
Low resistivity ρ~10-4-10-5 Ω cm High transmittance 80-90%
TC in transparent electronics
Transparent electronics is an emerging technology which employs wide band
gap semiconductors for several applications:
displays, smart windows, lighting, solar panels and large area sensors. Main
applications: Transparent Thin Film Transistors (TTFTs).
Obstacles: lack of reliable p-type TCOs.
Source: New Trends in Touch by Jennifer Colegrove
ITO actually dominates the display market, but forecast indicates rapid ITO-alternatives growth.
Transparent conductors in Touch
List of CRMs for EU released in 2017
rock
Study on the review of the list of critical raw materials 2017, https://ec.europa.eu/growth/sectors/raw-materials/specific-interest/critical_en 2017
Biggest global suppliers of raw materials
Study on the review of the list of critical raw materials 2017, https://ec.europa.eu/growth/sectors/raw-materials/specific-interest/critical_en materials
2017
~ 1,000 MT Indium are mined yearly
25-30% of indium mined yearly become
refined indium
25-30% accumulates in residues
40-50% goes to non-indium-capable
refineries and is lost. Furthermore, only 17%
of zinc refiners are producing indium.
There is an opportunity to increase
efficiency and to expand production capacity.
In supply (excluding China)
In is a by-product of lead+zinc production as it is mostly contained in sphalerite, a lead-zinc sulfide mineral. EU32 produce 7,5% of world production of lead, 8.6% of zinc EU32 Indium production approaches to 0 “Supply and demand seem to balance today on EU level but in order to ensure that the EU industry is self sustaining in the future, it will be necessary to retain indium – containing scrap, increase recovery (primary and secondary) and recycling / refining processes”
“Substitutability index (measure of the difficulty in substituting the material) 0,9 Recycling rate (recycled content from old scrap) 0,3% Import Dependence 100% Economic importance 6,7 Supply Risk 2,0 Environmental Country Risk 1,7”
Indium is a «critical raw material» for European Commission
Source: Report of the Ad-hoc Working Group on defining critical raw materials Critical raw materials for the EU, 2010
IIn ranks 61st in abundance of all
chemical elements. 240 ppb in the
earth’s crust
Indium consumers
Key consumers of In (2009)
“Future increases in indium production are expected to
be easily accomplished…
… indium can enjoy virtually infinite growth
in use without supply limitations.” Indium Geology, Mineralogy, and Economics,
Ulrich Schwarz-Schampera & Peter M. Herzig, 2002
Source: The Future of Indium Supply and ITO, William A. Jackson, 2012
•Indium/ITO cost impact on screen area: $4.80/m2
•Includes sputter process cost and yield, at $800/kg indium price (In price $550/kg at 30-3-2012)
•A 42” screen uses (0.52m x 0.9m x $4.8/m2)= $2.24 in ITO direct material
•TV retail price > $430
• 2018 In price $280-600/kg depending on purity and supplier
Source: William Jackson,The Future of Indium Supply and ITO, Indium Corporation, 2012
Cost Impact of ITO Per LCD Panel
C athode
E TL
H TL
A node
S ubstrate
LIGHT
-
+
Voltage
Electron Transport Layer /
Electroluminescent Layer
Hole Transport Layer
C athode
E TL
H TL
A node
S ubstrate
LIGHTLIGHT
-
+
Voltage -
+
-
+
Voltage
Electron Transport Layer /
Electroluminescent Layer
Hole Transport Layer
Source: Wikimedia
LCD display
OLED emission
The technology transition from LCD to OLED can cut ITO consumption by 50% !
ITO
ITO
ITO
Mitigate the risks by alternative products
ITO = 90 wt% In2O3 + 10 wt% SnO2
(In = 78 wt%)
Source: NATURE PHOTONICS, VOL 6 (DEC. 2012), p.809
Mitigate the risk by alternative materials
Cost pressure pushed the transition from ITO to
AZO and FTO in solar cells
Red rectangle: target region for TCE applications.
Source: NATURE PHOTONICS, VOL 6 (DEC. 2012), p.809
Conductivity (r-1) is driven by carrier concentration and mobility. Transparency is driven by plasma wavelength (light reflection by free charge carriers), that should be as far in the NIR as possible
Metals are opaque, polymers and SWNT are not very conductive Graphene can be a good option in the future
Mitigate the risk by alternative materials
ITO deposition is becoming fully compatible with
printing and roll-to-roll processing on large area
Alternative solutions need to create a great value
proposition in their specific application market
LCD manufacturers are extremely reluctant to switch
away from ITO
Switch would need to disrupt established
manufacturing lines
Mitigate the risk by alternative materials
Transparent conductive oxides (TCO)
State of the art of TCO
About 20 different types of doped binary oxides have been proposed as TCO. ITO dominates the market, AZO, GZO and FTO came close to ITO optical and electrical properties. Several ITO alternative materials including amorphous TCO, ultrathin metal films, graphene, carbon based transparent conductors and hybrids are under intensive investigations.
Conductivity: fundamentals
Semiconductors: tipically Egap< 2eV TCO semiconductor with
Egap~3-4 eV
An energy band is a range of allowed electron energies. The energy band in a metal is only partially filled with electrons. Metals have overlapping valence and conduction bands. Semiconductors and insulators have a gap of of not allowed electron energies.
Insulators: tipically Egap> 5eV
TCO conductivity: fundamentals
TCO are degenerate semiconductors: number of carriers is very high (n~1020-1021 cm-3) so that the Fermi level is located inside the conduction band for an n- type semiconductor, and inside the valence band for a p-type semiconductor. Degenerate semiconductors behave electronically like metals, and can be modeled by Drude’s theory.
Degenerate n-type semiconductor
Zachary M Gibbs et al 2013 New J. Phys. 15 075020
ITO conductivity
In a TCO carriers are generated by: Doping (substitution of impurities of metal cations) Lack of stoichiometry (defects): oxygen vacancies, interstitial metal cations, etc.
Doping mechanism in ITO (In2O3:SnO2)
Source: Bright adapted from Granqvist (1989)
A material with a high conductivity σ must have a high carrier concentration n (electrons) or p (holes) and a high carrier mobility μp,n:
Mobility is:
where e is the elementary charge, τ is the mean time between collisions, and m* is the effective electron mass. Maximum carrier concentration is limited by dopand solubility in the host material (generally n~1021 cm-3), while carrier mobility is limited by scattering processes.
ennp,
Conductivity fundamentals
*m
e
Conductivity fundamentals
For any given TCO exists a resistivity
minimum (resistivity well), where contributions from doping and oxygen
vacancies scattering are optimized. .
At low doping levels resistivity is high due to a lack of conduction electrons (holes): At high doping levels resistivity is high due to scattering from impurities: In a TCO oxygen deficiency has got also a fundamental role in the electrical conductivity.
n
1r
r
1
Optical and electrical properties
The number of carrier n determines both the electrical (1) and optical (2) properties by: 1) 2) Generally the maximum allowable carrier density depends on the required spectral window of transparency. The best strategy for optimizing TCO materials is to limit the carrier concentration and increase the carrier mobility.
en
*
2
m
ne
o
p
dn
A
Optical properties of TCOs
Three different wavelength regions: ABSORPTION, TRANSMISSION and REFLECTION can can be
identified in the optical trasmittance of a TCO film.
n>>k k>>n k>>n
k extinction coefficient, n refractive index
Source: Bright
Optical and electrical properties
The transmission in the near UV is limited by Eg, because photons with energy larger than Eg are absorbed. By increasing doping (n) the absorbing edge shifts towards higher energy (lower wavelengths) due to Burstein Moss shift.
k extinction coefficient,
n refractive index
300 350 400 450 5000
20
40
60
80
100
Burstein Moss shift
Tra
nsm
itta
nce %
Wavelength (nm)
l<lgap
high
absorptionn
k>> n
I region
II region
Increasing doping
In the NIR region, reflection at the plasma frequency is due to free carriers. By increasing doping (n density of electrons), moves towards lower wavelengths.
pl
Optical and electrical properties
500 1000 1500 2000 25000
20
40
60
80
100
Increasing doping
Tra
nsm
itta
nce %
Wavelength (nm)
K>>n
k extinction coefficient,
n refractive index
dn
A
p
*
2
m
ne
o
p
In the NIR, absorption of photons by free carriers is:
II region III region
p
p
l
2
Figure of merit
In 1976, Haacke provided a simple parameter measuring the goodness of a TCO which combines the transmittance T and the sheet resistance Rs:
Where
Where r is the resistivity and t the thickness of the layer. The sheet resistance is normally expressed in ohms/square or
Ω/. Usually T is measured @550nm, but the average value over
the application spectral range (usually 400-1000 nm) should be
better considered.
tRs
r
sR
T 10
Additional important parameters, apart resistivity and transmittance: work function: compatibility with the active
electronic material. Important in solar cells and OLED.
Chemical stability Mechanical resistance: to abrasion, flexion,
etc. Reliability: average replacement cycle time
Other TCO requirements
TCO deposition methods
Transparent conductive materials can be prepared using a wide variety of thin-film deposition methods. PVD methods: evaporation, magnetron sputtering, molecular beam epitaxy, pulsed laser ablation, etc. (CVD) methods: high-temperature CVD, metal–organic CVD (MOCVD), atomic layer deposition, etc. Chemical methods: spin coating, spray pyrolysis, etc. The choice of the best film growth process must take into account:
Deposition temperature
Temperature tolerance of substrate
Costs
TCO deposition methods
n- and p-type TCOs developed at ENEA
ITO films
Al doped ZnO (AZO) films
AZO/Ag/AZO multilayers
TiOx films
TiOx /Ag (Ti)/ TiOx multilayers
NiOx films
TiO2 buffer layers for perovskites solar cells
NiO buffer layers for CZTS solar cells
n-type
p-type
Others (than TCOs) TC developed at ENEA
Graphene
CNTs
Conductive polymers (PEDOT:PSS) and
hybrids
Ultrathin metal films (UTMFs) and metal
grids: Ag, Ni
Optical Coatings Laboratory facilities
Deposition systems: radiofrequency sputtering
dual ion beam sputtering
e-beam evaporation (ion assistance)
RF sputtering system used for TCOs fabrication
Influence of sputtering parameters
In a TCO film, oxygen partial pressure determines the amount of doping by oxygen vacancies control of oxygen during the deposition is CRUCIAL! Substrate temperature is very important to promote doping and crystallization of growing films. Typical process parameters: pressure, oxygen partial pressure, temperature, plasma condition, power, deposition geometry, etc. play an indirect role on the optoelectrical performance influencing the CRISTALLINITY and MORPHOLOGY of films.
Sample Pressure
Pa
O2,
sccm
Ar,
sccm
O2,
%
T
(°C)
Film
thickness
nm
Sheet
resistance
Ω/
ITO_2
0.16 0.2 2.5 8.9 RT 748 150
ITO_11 0.17 0.24 3 8.9 RT 252 2.5x103
ITO_16 0.33 - 10 0 290 914 3
Electrical characteristics are strongly thickness dependent: films with lower
thickness are less conductive
Films grown on pre-heated substrates show superior performances.
Oxygen partial pressure has got a detrimental effect on the electrical
conductivity
Indium Tin Oxide: Influence of sputtering parameters
Deposition conditions and sheet resistance of ITO films
FT-IR Spectrophotometer
Cryogenic system for T
measurement
W-VASE ellipsometer
UV-Vis Spectrophotometer
Optical and electrical characterization systems
Hall-effect apparatus
Four point probe apparatus
500 1000 1500 2000 25000
20
40
60
80
100
RT
T,R
(%)
Wavelength (nm)
Thin Solid Films 515, 8469–8473, 2007
Indium Tin Oxide: Optical and electrical properties
2
Heated substrate Non heated substrate
ρsheet= 3 Ω/
ρsheet= 2.5 KΩ/
ρsheet= 150 Ω/
Sample T
(°C)
d
(nm)
r (Ω/)
ITO_2
RT 748 150
ITO_11 RT 252 2.5x103
ITO_16 290 914 3
Indium Tin Oxide: optical constants
Applied Physics A 89, 1, 63-72, 2007.
Highly conductive
Moderate conductivity
300 400 500 600 700 800 900 1000
0
10
20
30
40
50
60
70
80
90
100
Rsheet
(ITO CCR 13)
=203 /sq
Rsheet
(ITO CCR 14)
=35 /sq
Tra
nsm
itta
nce
(%
)
Wavelength (nm)
Indium Tin Oxide: effect of substrate temperature
T=200°C
Non heated
t=140 nmm
Improved characteristics also at relatively low substrate heating (Tsub=200 °C)
Indium Tin Oxide: morphology
Sputtering parameters,
including oxygen partial
pressure have got a strong
influence on films’
morphology.
black metallic-like film
As deposited Annealed
Comparison of films of
different thickenss (q-2q) Comparison of films of different thickenss
(grazing incidence)
Indium Tin Oxide: structure
The deposited ITO films exhibit the cubic-bixbyite structure of indium
oxide with more or less pronounced preferential orientation.
1100 nm
172 nm
8-layer coating (ITO/TiO2/SiO2) on glass CIE color diagram (D65) for T and R
400 500 600 700 800 Wavelength (nm)
0
20
40
60
80
100
300 400 500 600 700 800 900
Wavelength (nm)
Rs
T
R
Ts
T
R
ITO-SIO2-TIO2-8layers
T
R
0° incidence angle
Proc. SPIE, 2007.
ITO against UV and IR
Objectives
Null transmission out of the visible (UV e NIR) to avoid radiation induced damage.
Low reflection in the visible (410-680 nm) and optimization of color rendering to improve the observer vision.
Applied Physics A 89, 1, 63-72, 2007.
Physica Status solidi A 210 (4), 748-754,2013.
ITO alternatives: Al doped ZnO (AZO)
Sample r.f. power
(W)
pAr
(mbar)
T
(°C)
magnetic
field (G)
AZO34 300 7x10-3 240 32
AZO58 220 1x10-2 200 48
Growth conditions
Sample d
(nm)
ρsheet
(Ω/)
ρ(Ωcm) N
(cm-3)
m
(cm2/V
s)
Tvis
(%)
FTC=T10/Rs
(mΩ−1)
AZO34 776 43.5 3.4 10-3 1.85 1020 10.1 91 8
AZO58 864 32.9 2.8 x 10-3 1.86 1020 11.8 91 10
Films properties
500 1000 1500 2000 2500
0,0
0,2
0,4
0,6
0,8 AZO34
AZO58
Extinctio
n c
oe
ffic
ient
Wavelength (nm)
AZO: optical properties
Optical constants
Optical constants were calculated from the transmittance and reflectance
spectra in the range 300–2500 nm, basing on the model of homogeneous film.
AZO film’s morphology
Evolution of morphology vs resistivity
decrease:
Morphology changes gradually from a
regular distribution of small round grain
size to a distribution of irregularly
shaped grain of increasing dimension
AZO film’s structure
Lower range of transmittance due to the metal.
Higher conductivity due to the metal.
500 1000 1500 2000 2500
0
10
20
30
40
50
60
70
80
90
100
AZO/Ag/AZO
B270 glass
AZO
Tra
nsm
itta
nce
, %
Wavelength,nm
Tvis average 80%
rsheet 16 Ω/ RT deposition
Multilayers AZO/Ag/AZO
J . Appl. Phys. 114, 094509 (2013).
Phys. Status Solidi C, 1–4 (2016).
AZO/metal trilayers
Ag morphology
Goal: Ag percolation at the lower thickness is desirable
Non percolating Ag: insulator Percolating Ag: conductive
Film thicknesses reported in the table
were estimated on the basis of
deposition rates, obtained from the
thickness of thicker reference samples
measured by a surface profilometer
KLA-Tencor P-10.
Ni films were fabricated on fused silica substrates by
radio frequency sputtering in pure Ar atmosphere
(15 sccm, pAr = 1x10-2 mbar) at room temperature and
several radio frequency powers in the range 50-300 W.
Compact and stable ultrathin films were obtained at the
high r. f. powers of 200 and 300 W.
RF sputtering plant Name Power
(W)
Time
(s)
Thickness
(nm)
RS
(Ω/sq)
ρ
(Ω·cm x10-5)
Sample 1 200 30 3.3 170 5.6
Sample 2 200 20 2.2 390 8.3
Sample 3 200 30 3.3 164 5.4
Sample 4 300 11 2.1 600 12.0
Sample 5 300 20 3.7 160 5.9
Sample 6 300 15 2.8 245 6.7
Table 1. Ni films characteristics
Ni films, despite their inferior electrical properties with respect to silver films, show
higher stability, wider transparency spectral range, which allows integration in UV
light emitting diodes (LED) or infrared (IR) detectors, and high compatibility with
nearly all organic and semiconductor materials and related processes
Ultrathin Ni films
Transmittance and electrical properties
Transmittance of Ni films vs. sheet resistance
Spectral transmittance of Ni films
thinnest sample
• Sheet resistance of the films was
measured by four point probe method.
Resistivity in the range of 10-5 Ω·cm
was achieved.
• Aging was not affecting the electrical
properties of the films, even after 2-
year exposure to the ambient
atmosphere.
• Spectral transmittance T and
reflectance R of the films were
measured at normal incidence by the
Perkin-Elmer Lambda 950
spectrophotometer, showing an almost
flat behavior over a wide spectrum and
with UV and NIR transparency much
higher compared to ITO films.
The roughness of the films measured by AFM on two different scanning areas was less than 0.5 nm.
FE-SEM micrographs showed compact and dense films even at low thickness values.
Ni films morphology
AFM micrographs of Sample 3 on 4 µm2 and 100 µm2 areas. SEM micrograph of Sample 3
RMS = 0.21 nm RMS = 0.46 nm
Optical constants determination
Multi-angle R/T measurements were performed using the
Agilent Cary 7000 spectrophotometer equipped with the
Universal Measurement Accessory (UMA).
A new spectrophotometric method suitable for ultrathin
films (see Th.B4) has been applied to Sample 1.
The continuous increase of the extinction coefficient (k)
at longer wavelengths denotes the Drude behavior of
the material and confirms the conducting property,
even for such ultrathin layer.
By multi-angle analysis of R and T data, the estimated
thickness was 5.2 nm, a value higher than the one
predicted by the deposition rate (3.3 nm). A thickness
of ~ 5 nm was inferred by ellipsometric measurements.
The comparison of these optical constants to those
of a reference thicker film (Palik), shows that the
trends are basically the same, while the absolute
values of the extinction coefficient are lower.
Resistivity of films is quite stable.
Slight increase in the resistance is
accompanied by a slight increase in
the transmittance value.
Resistance of Ni films vs time
Ultrathin Ni film: stability
XPS surface analysis
Films are protected by an ultrathin NiO layer: XPS measurements
showed that within 1 nm depth both Ni and NiO are present.
MgKa 1253,6 eV
Fabrication of a silver grid on Ni films to decrease film resistivity
f
Grid Filling Factor
F=W/(Gs+ W) is 0.048. (W= 50 m), Gs=1mm
Thin Solid Films 594 (2015) 261-265.
Optical Interference Coatings, Tucson, 2016.
Trasmittance of a Ni film with and
without the metal grid
W=50 m
ρsheet= 20Ω/
The non-existence of p-type transparent conducting oxides is thought to originate from a general characteristic in the electronic structure of oxides: the localized oxygen p nature of the valence band in most oxides makes those bands very flat and leads to large hole effective masses (low mobility). Therefore, any finding of a p-type conducting oxide must include modification of the energy band structure to reduce the localization behaviour, which in turn requires new insight into the relation between electronic structure and properties of oxide materials.
In the technological field, finding such a material may open the way to new
applications such as ultraviolet-emitting diodes.
Search of p-type TCOs
*m
e
Search of p-type TCOs
Nature Communications 4, Article number: 2292
Milestones in Transparent Electronics: discovery of semi-transparent p-type TCOs
The first report of a semitransparent p-type conducting thin film
of nickel oxide (NiO) was published in 1993 by Sato et. al from
Kanazawa Institute of Technology, Japan. They observed about
40% transmittance of the NiO films in the visible region
The field of p-type TCOs received most of its impulse in 1997
when Kawazoe et al. demonstrated that CuAlO2 delafossite
could show encouraging p-type conductivity (1 S/cm) and
optical transparency in the visible range.
State of art p-type TCOs
Most p-type TCOs have a delafossite structure with composition CuMO2, where M is a trivalent cation such as: Al, Ga, In, Cr, Y, Sc, La, B, developed by “chemical
modulation of valence band (CMVB)”. Trivalent M can be substituted by divalent cations, such as Mg, Fe and Ca.
Only a few examples of high conductivity p-type films exist: CuCr1-xMgxO2 ϭ=220 S/cm
NixCo3−xO4 ϭ=375 S/cm
LaCuOSe ϭ=910 S/cm
BaCuSF ϭ=260 S/cm
Bi2Sr2CoO2Oy ϭ=182 S/cm
NiO ϭ=100 S/cm Tvis=40%
Transmittance @550nm is low as compared to n type films, up to 70 %. Such high transmittance are achieved mainly for low conductive films (≤1 S/cm).
400 600 800 10000
20
40
60
O2=100 %
thickness = 35 nm
O2=66 %
O2=50 %
O2=30 %
PRF
=250 W
Tra
nsm
itta
nce (
%)
Wavelength (nm)0 25 50 75 100
0.0
0.2
0.4
0.6
PRF
=250 W
Resis
tivity
( c
m)
Oxygen partial pressure (%)
XPS spectra of Ni 2p clearly show the mixed NiO (Ni+2) at 854.5 eV and
Ni2O3 (Ni+3) phases at 855.8 eV.
Minimum resistivity 1.6x10-2 Ωcm.
Semiconductor Science & Technology, vol. 31, 5, 055016, 2016.
NiO p-type films developed at ENEA
900 890 880 870 860 850 840
Ni3+ Ni
2+
Ni 2p3/2
Ni 2p1/2
a)Ni 2p
Inte
nsity (
arb
. units)
Binding Energy (eV)
536 534 532 530 528 526
b)NiO
Ni2O
3
O 1s
Inte
nsity (
arb
. units)
Binding Energy (eV)
20 40 60 80 100
0.6
0.7
0.8
0.9
Ni 2O
3 / N
iO
Oxygen partial pressure (%)
Effect of oxygen partial pressure on the ratio Ni3+/Ni2+
Conductivity increases by increasing the Ni3+ ions,
due to nickel vacancies and interstitial oxygen
associated with Ni+3 ions.
NiO p-type films developed at ENEA
XPS spectra of Ni 2p clearly show the mixed NiO (Ni+2) at
854.5 eV and Ni2O3 (Ni+3) phases at 855.8 eV.
Graphene
Grafene is the single sp2 hybridized planar unit of graphite.
Optical transparency derives only from the low thickness, since one atomic layer of carbon absorbs about 2% of the visible radiation.
High carrier mobility is also related to the large crystal size along the sp2 plane.
Oxford dictionary:
Graphene: a form of carbon consisting of planar sheets
which are one atom thick, with the atoms arranged in a
honeycomb-shaped lattice.
Only single crystal, one layer qualifies as graphene
The definition has been often extended to few
graphene layers, thin graphite
Graphene facts
• Graphene is a single, defect-less, sp2 bonded basal plane of Graphite
• Graphene is not a semiconductor
• It is both a surface and one single organic molecule
• It is interfacial when not in graphite
1. It can be modified by adding different groups (Graphane, Graphone, Graphene
Oxide,...,∞). For instance, by adding one H for each C, Graphane is a fully sp3
hybridized electrical insulator, like diamond.
2. Graphene and its related «surface-molecules» can be produced by CVD and then
transferred at low temperature to top almost any surface
3. For applications one must consider both in-plane and perpendicular properties:
“physical and chemical”
• Graphene and derivatives are grown by CVD
on copper
• Chemical Vapour Deposition (CVD) is a
cheap and effective technology
• Low temperature, wet transfer processing
• Interfacing with well established technologies
(i.e. solar devices)
Graphene growth and applications at ENEA
The ENEA CVDs: two home made thermal reactors
Coaxial furnace
Inductive heating Hot filament plasma CVD
Efficient CVD growth from ethanol
Using ethanol the full growth process can be as fast as few seconds: 20s in the figures
Low disorder «D» band, thin and well formed
graphene films can be grown at high temperature
using Ethanol, same as methane
Rapid and highly efficient growth of graphene on copper by chemical vapor deposition of ethanol,
Thin solid films, 571 (2014) 139-144
High-Temperature Growth of Graphene Films on Copper Foils by Ethanol Chemical Vapor Deposition,
J. Phys. Chem. C 2013, 117, 21569−21576
Graphene Transfer
The etching of copper is most frequently performed using protective layers for graphene, such as PMMA based resists or “thermal release tape”
Extraction can also be performed without using a protective layer. Copper foils are left floating on the top of an etching bath, graphene films are finally scooped and transferred into clean water. The free floating method is clean but leads to discontinuous films, unsuitable for devices
An innovative way for graphene transfer was developed in our laboratories, using an intrinsically clean type of resist which sublimates at ambient temperature
Graphene Transfer: an intrinsically clean method based on Cyclododecane
The «temporary resist» is spin coated from a cyclododecane cyclohexane solution. The resist sublimates at STP and no solvents are required, nor post transfer high temperature processes are required for removing residues (such as with PMMA).
The XPS C1s (above) and the Raman spectra are unaltered by the transfer.
In figure a) the optical micrograph of a sample transferred by the free floating method, in figure b) a sample transferred with CD
Cyclododecane C12H24 is a volatile binding medium, and can be used as
temporary binder for sealing and conservation of friable and structurally
weak materials: transport of archaeological objects and art conservation
Cyclododecane as support material for clean and facile transfer of large-area
few-layer graphene. APPLIED PHYSICS LETTERS 105, 113101 (2014)
Scheme of the Cyclododecane assisted transfer
65ºc
90ºc
• No solvents are required: transfer on all polymers
• Low temperature processing: transfer on heat sensitive
substrates
• Work in progress: dry transfer variant for on moisture
sensitive substrates
x N
Ammonium Persulfate
ITO is a unique material with unrivalled performance and tech’s, suitable for the very large areas set by modern display technologies.
Indium, the base element in ITO, is a CRM for EU.
Alternative solutions need to create a great value proposition in their specific application market. No winner yet. Recycling is still unsatisfactory.
At ENEA:
AZO films and AZO/Ag/AZO multilayers deposited respectively at T<250oC and RT, show optical and electrical properties comparable to ITO films.
Ultrathin Ni films have lower conductivity and lower visible transmittance than ITO films. Thin metal grids were used to decrease the resistivity of such films. However, the high transmittance values in the whole UV-Vis-NIR spectral range make ultrathin Ni films the best choice for NIR and UV applications.
High quality quality CVD graphene has been produced and an innovative cyclodecane assisted transfer was develop to transfer single layer and multilayer graphene on different substrates.
Conclusions
Acknowledgments
Anna Sytchkova, ENEA Nicola Lisi, ENEA
Angela Piegari, ENEA
Francesca Menchini, ENEA
Sylvia Boycheva,
University of Sofia Dario della Sala, ENEA
CRM-EXTREME
www.crm-extreme.eu
http://www.cost.eu/COST_Actions/ca/CA15102
Solutions for Critical Raw Materials
Under Extreme Conditions
(CRM-EXTREME)
Maria Luisa GRILLI (Italy) ENEA-Italian National Agency for New Technologies,
Energy and Sustainable Economic Development