a thesis submitted in partial fulfillment for the degree
TRANSCRIPT
1
Design and evaluation of piezoelectric response in a
device based on liquid-phase exfoliated π΄ππΊπ
A thesis submitted in partial fulfillment for the degree
of Bachelor of Science
Author: Jhon Vergel
In the
Science faculty
Physics Department
Advisor:
Yenny Hernadez PhD
BogotΓ‘ D.C. Colombia
2019
Design and evaluation of piezoelectric response in a
device based on liquid-phase exfoliated π΄ππΊπ
Jhon Sebastian Vergel Capacho
2
Universidad de los Andes
Abstract Two-dimensional materials have attracted much interest in the last
years mainly because when quantum confinement is in place new
properties appear. That is the case of single layer or odd layers of
πππ2. When confined to 2 dimensions, πππ2 presents new
properties, this change is due in part by the loss of centrosymmetry.
Additionally, second-order responses appear like second-harmonic
generation and piezoelectricity, in the case of πππ2.Then is
essential to characterize this type of material in its 2D forms. Our
study shows a way to deposit MoS2 using spray coating, and with
this process we can study the topology of the MoS2 flakes; we also
see the preferred way in which the MoS2 stacks. Two materials
were studied, graphene and PMMA, as a possible energy
transductor, for piezoelectric measurements, and it was found that
in the current configuration graphene does not work as a
transductor, and there is need to do further investigation on the
PMMA
Table of contents Abstract ....................................................................................................................................... 2
Introduction ................................................................................................................................ 5
Theoretical background and previous work ............................................................................ 8
2.1. Molybdenum Disulfide monolayer properties .............................................................. 8
2.1.1. Piezoelectric effect in transition metal dichalcogenides .......................................... 10
2.1.2. Second Harmonic Generation ................................................................................... 12
2.2. Previous work ................................................................................................................ 13
Materials and methods ............................................................................................................ 14
3 .1) π΄ππΊπ liquid-phase exfoliation ..................................................................................... 14
3 .2) Graphene electroquimical exfoliation ........................................................................ 17
3.4) PMMA and PDMS synthesis ....................................................................................... 18
3 .5) Spin coating to deposit polymers ................................................................................ 20
3 .6) Spray coating to deposit nanolayers .......................................................................... 21
3
3 .7) Silver layer by thermal evaporation .......................................................................... 22
3 .8) Heat treatment ............................................................................................................. 22
3 .9) Experimental assembly for structural properties ..................................................... 23
3 .10) Experimental assembly for absorbance curve and concentration curve .............. 24
Results and analysis ................................................................................................................. 26
4.1. Liquid phase exfoliation results ................................................................................... 26
4.2. π΄ππΊπ layer ..................................................................................................................... 29
4.3. Graphene as an energy transducer .............................................................................. 34
4.4. PMMA as an energy transducer .................................................................................. 37
Conclusions and future work .................................................................................................. 40
References .................................................................................................................................. 42
FIGURE 1 (A) SIDE VIEW OF THE STRUCTURE OF πππ2 IN BLOCK; (B) TOP VIEW OF A
SINGLE LAYER OF πππ 2;(C) SIDE VIEW OF A SINGLE LAYER πππ2. IN COLOR YELLOW THE S-ATOMS AND IN COLOR GREY MO-ATOMS [7] ............................................
9 FIGURE 2 PROPOSED PROTOTYPE TO MEASURE PIEZOELECTRICITY IN LIQUID PHASE
EXFOLIATED πππ2. ....................................................................................................................... 14 FIGURE 3 PROCESS OF LIQUID PHASE EXFOLIATION FOR πππ2. A) PICTURE SHOWING THE
ULTRASONIC BATH. THIS PROCESS IS NEEDED TO MAKE A HOMOGENEOUS MIXTURE.B) PICTURE SHOWING SONIC TIP NEEDED TO INTRODUCE MECHANICAL FORCES TO ASSIST THE EXFOLIATION. C) VACUUM FILTER ARRANGEMENT USED TO KEEP ONLY THE EXFOLIATED MATERIAL D) CENTRIFUGE USED TO SEPARATE THE MORE MASSIVE PARTICLES FROM THE LIGHTER. E)-F) IN E SHOWS THE PRODUCT OF THE CENTRIFUGE. IN F IS SHOWN, ONLY THE MATERIAL THAT FLOATED IS PRESENT, AS THIS IS THE PRODUCT OF THE EXFOLIATION PROCESS. ...........................
15 FIGURE 4 PROCESS OF ELECTROCHEMICAL EXFOLIATION FOR GRAPHENE. A)
ASSEMBLY FOR THE ELECTROCHEMICAL EXFOLIATION, ON THE LEFT THE POWER
SOURCE AND ON THE RIGHT THE BEFORE AND AFTER B) VACUUM FILTER ARRANGEMENT USED TO KEEP ONLY THE EXFOLIATED MATERIAL C) CENTRIFUGE USED TO SEPARATE THE HEAVIER PARTICLES FROM THE LIGHTER D) SHOWS THE PRODUCT OF THE CENTRIFUGE E) SUSPENSION WITH ONLY GRAPHENE IN IT. .............................
17 FIGURE 5 ON THE LEFT PDMS FILLED WITH BUBBLES, ON THE RIGHT AFTER 10 MINUTES
ON THE VACUUM CHAMBER WITHOUT BUBBLES ............................................................... 19 FIGURE 6 SPINCOATER MODEL WS-650MZ-23NPPB0. USEFUL IN THE PROSSES OF PMMA
COATING, IT ALLOWS US TO CONTROL THE THICKNESS WITH THE VARIATION OF THE ANGULAR SPEED. ................................................................................................................. 20
FIGURE 7 SPRAY COATER, CONSISTING OF A MECHANICAL BOX AND A HEATING PLATE. THE SAMPLES SIT IN THE MIDDLE AT A HIGH TEMPERATURE. .......................................
21
4
FIGURE 8 CALIBRATION CURVE USING THE 635NM MAXIMUM FROM THE ABSORPTION
SPECTRA. THE LINEAR REGRESSION FOUND RELATES TO THE KNOWN CONCENTRATION OF πππ2 MONOLAYER WITH THE ABSORBANCE MEASURED AT THIS POINT. ERROR BARS SHOW THE STANDARD ERROR ON THE MEAN. ....................
26 FIGURE 9 ABSORPTION SPECTRA FOR THE πππ2. THE LETTERS REPRESENT THE METHOD
USED, Rβ DESCRIBES A METHOD IN WHICH THE SONIC TIP STEP GOES FOR 4 HOURS
AND Jβ 3 HOURS. ALSO, THE 630NM AND 690NM LINE SHOWS WERE THE EXPECTED MAXIMUMS SHOULD APPEAR IF IS THE CASE OF EXFOLIATED πππ2. ...........................
27 FIGURE 10 FLUORESCENCE SPECTRUMS FOR EXFOLIATED πππ2, THREE SPECTRUMS ARE
SHOWN AT 370NM,300NM,500NM. THE INTENSITY IS ON ARBITRARY UNITS. ..............
29 FIGURE 11 OPTICAL IMAGES OF πππ2 DEPOSITED OVER SIO, THE MASS OF THE
MATERIAL DEPOSITED GROWS FROM A) TO B). ................................................................... 30 FIGURE 12 SEM IMAGES FOR THE πππ2 DEPOSITED OVER SIO, THE MASS OF THE
MATERIAL DEPOSITED GROWS FROM A) TO B). ................................................................... 31 FIGURE 13 SEM IMAGE OF A CRISTAL OF πππ2 DEPOSITED OVER SIO; THE TRIANGULAR
SHAPE IS EXPECTED FROM THE LATTICE DISTRIBUTION. ................................................ 32 FIGURE 14 COMPOSITION ANALYSIS FOR A NANOLAYERED πππ2, THE TOP IMAGE
SHOWS SEM RESULTS, AND THE BOTTOM ONE THE POSSIBLE ELEMENTS IN THE IMAGE. ............................................................................................................................................. 33
FIGURE 15 COMPOSITION ANALYSIS FOR STRANGE STRUCTURES IN THE SAMPLES OF πππ2 OVER SIO, THE TOP IMAGE SHOWS SEM RESULTS, AND THE BOTTOM ONE THE POSSIBLE ELEMENTS IN THE IMAGE. THE STRUCTURES ON THE IMAGE ARE FOUND TO BE CONTAMINATION. ............................................................................................................ 33
FIGURE 16 OPTICAL COMPARISON OF THE BEST RESULTS FOR THE HEAT TREATMENT
PROCEDURE. IN A) NO TREATMENT APPLIED, B) CONVECTION OVEN 2 HOURS, AND C) CONVECTION OVEN 4 HOURS. .............................................................................................. 34
FIGURE 17 GRAPHENE ABSORBANCE CURVE. 4 SAMPLES MADE WITH THE SAME
PROCEDURE ARE COMPARED. 258 NM IS THE EXPECTED MAXIMA FOR GRAPHENE; THE CASE OF GRAPHITE IS AROUND 275 NM. ........................................................................
35 FIGURE 18 SIO-CR-AU-πππ2-GRAPHENE SAMPLES IN ORDER OF LAYERS. THE TOP
SIDE SHOWS THE GRAPHENE FORMING LAYER, WHICH IS REFLECTIVE. ON THE
BOTTOM, THE MASK USED PROTECT THE SIO-CR-AU LAYERS. .......................................................... 36
FIGURE 19 SIO-CR-AU-πππ2. -GRAPHENE-AG LAYERS IN SCALING ORDER, ON TOP OF THE
SAMPLES. SIO-CR-AU-MOS2-GRAPHENE LAYERS ON THE MIDDLE INTERFACE. AND SIO-CR-AU ON THE BOTTOM. ..................................................................................................... 37 FIGURE 20 PMMA THICKNESS TEST SAMPLES. .............................................................................. 38
FIGURE 21 SEM IMAGE OF THE LATERAL VIEW OF THE 9000RPM SAMPLE OF PMMA. THE
BOTTOM PART IS THE SIO-CR-AU-MOS2, THE FLAKES OF MOS2 LOOKING LIKE LITTLE BUMPS, OVER THAT A BIG GAP AND THEN THE PMMA THE SIGNIFICANT LAYER ON THE TOP OF THE IMAGE. ........................................................................................ 39
EQUATION 1 BASIS VECTORS FOR THE CELL UNIT OF πππ2 ....................................................... 8 EQUATION 2 A) DIRECT PIEZOELECTRIC EFFECT, WHERE P IS THE POLARIZATION (ππΆπ2)
π IS THE STRESS(ππ2) AND D IS THE PIEZOELECTRIC COEFFICIENT(ππΆπ);B) SHOWS THE INVERSE PIEZOELECTRIC COEFFICIENT ........................................................................ 11
EQUATION 3 SHOWS THE PIEZOELECTRIC FORMULA IN MATRIX REPRESENTATION, AND THE EXPRESSION TO FIND THE COMPONENTS OF THE POLARIZATION. .......................
11 EQUATION 4 RELATES THICKNESS T AND ANGULAR VELOCITY IN THE PROCESS OF SPIN-
5
COATING [18] .................................................................................................................................. 20 EQUATION 5 BEER-LAMBERT LAW. RELATES ABSORBANCE AND CONCENTRATION. A IS
THE ABSORBANCE; π IS THE MOLAR COEFICCENT; C IS THE MOLAR CONCENTRATION, AND L IS THE OPTICAL LENGTH. .......................................................... 25
TABLE 1 SAMPLES OF THE πππ2 DEPOSIT TEST, WITH DIFFERENT MASSES .........................
30 TABLE 2 INFORMATION ABOUT THE MULTIPLES TEST MADE TO ELIMINATE THE
BUBBLES THAT APPEARED ON THE PMMA ............................................................................ 40
Chapter 1
Introduction
The piezoelectric effect is the direct relationship between mechanical and electrical
systems. This type of effect occurs linearly in materials that have a noncentrosymmetric
crystal structure, which means it is not invariant under a mirror transformation. Then a
medium can be defined as piezoelectric if given an external mechanical stress one gets a
dielectric displacement. In the same way, given an external electric field, one gets a strain
in the material. [1] Since 1880, when the Pierre brothers first described it, this effect had
been studied both in a practical and a theoretical way.
Based on the bast amount of studies, nowadays, piezoelectric devices are widely used in
all kinds of electronic devices like smartphones, alarm clocks, peacemakers, among
others. Traditionally, piezoelectricity is found and highly studied in bulk materials, also
seen as three-dimensional systems [2].
However, some materials show different properties in bulk and as they do in 2 or 1
dimensions. Given this, in recent times it became more attractive to take well-known bulk
materials and lower their dimensionality and observe the change in their properties. In
particular, taking an interest in thin films that show piezoelectricity, modeled as a
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bidimensional film. Therefore, if one now confines the system to a bidimensional film, it
is granted the development of new and more precise devices. This reduction in size is a
direct cause of the symmetry of the material. For example, hexagonal structures, when
being confined to 2-dimensional films, present a break in the symmetry in such a way that
piezoelectricity arises. This appearance has allowed prediction of this type of behavior in
graphene oxides, hexagonal BN monolayers also, transition metals dichalcogenides. [1]
As a consequence of the above, πππ2 monolayers generate high interest given their
possible applications on devices that take advantage in a novel way of their
piezoelectricity, which has shown that it can be more efficient in the production of
mechanical energy to electricity than its counterparts in bulk. Also, thin layers have more
versatility because of the opportunities of fabricating nanodevices, resulting in a broad
interest in the area of biomedicine. [3] Then, this interest in the area has spawned
numerous studies around monolayer πππ2, and few-layered MoS2, from its synthesis,
properties, both in a theoretical and experimental way. [4]
Furthermore, in πππ2, the odd-numbered layers had been first theoretically proposed to
have piezoelectricity, because they can maintain atomic structures in monolayers and this
state occurs a break in the inversion symmetry. The break means a non-centrosymmetric
making a system suitable for piezoelectricity.
Additionally, it exists experimental evidence of the piezoelectricity in 2D monolayers and
even odd layers. Nevertheless, the low dimensionality comes with problems, like been
thermodynamically unstable. [5]
The main objective of this study is the elaboration of processes to deposit, in a Si
substrate, the liquid phase exfoliated πππ2, and study some of its properties like a second-
order phenomenon and the extinction coefficient; also, study the πππ2 crystals optically
and morphologically. Furthermore, find a suitable material for energy transduction to
implement a device based on the liquid phase exfoliated πππ2 over a conductive substrate
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to evaluate the piezoelectric response macroscopically. With those objectives in mind,
standardized way of deposit crystals of πππ2 over a thin conductive layer, of gold or
silver, thermally evaporated over a silicon substrate was developed. Also, this study
focuses on evaluating the voltage output of the πππ2 using graphene and
PMMA as a transductor.
8
Chapter 2
Theoretical background and previous work
2.1. Molybdenum Disulfide monolayer properties
Molybdenum disulfide (πππ2), is part of the metals transition dichalcogenides, it has a
hexagonal crystal structure of 2 S atoms, and 1 Mo atom, shown Figure 1 is a pictorial
representation of the crystal structure of the monolayer, lack of a centrosymmetric
arrangement is one of the critical properties that differentiate the bulk and the monolayer
forms of this material. Then the unit cell can be characterized by the lattice parameters Ξ±,
the in-plane lattice constant, and c, the out-of-plane lattice constant.
[6]
Equation 1 Basis vectors for the cell unit of πππ2
9
Figure 1 (a) Side view of the structure of πππ2 in block; (b) Top view of a single layer of
πππ 2;(c) Side view of a single layer πππ2. In color yellow the S-atoms and in color grey
Mo-atoms [7]
πππ2 is diamagnetic, with an indirect bandgap semiconductor; it has optical properties,
such as photoluminescence, showing two excitonic peaks at 1.92eV (A), and 2.08eV (B).
Some properties differ when confined to 2 dimensions. First of all, the
piezoelectric effect emerges. Another different property is that the bulk bandgap is
indirect, and the monolayer band gap is direct. In monolayer, πππ2 shows an optical gap
of 1.8-1.9 eV [6]. Also, the photoluminescence intensity increase in a 1000-fold and other
optical properties as the second harmonic generation (SHG) can be measured. [8]
In this section, we will discuss how piezoelectricity works, and how the process
of SHG, since both are second-order nonlinear phenomena present in nanolayers of πππ2
important in the characterization of the material in its 2D configuration.
10
2.1.1. Piezoelectric effect in transition metal
dichalcogenides
Piezoelectricity is one of the properties intimately related to the crystal structure
of the material. Given a molecular structure where negative and positive charges coincide
in the center of symmetry is clear that the molecule results in an electrically neutral
arrangement. Now, if one applies external mechanical stress to the molecule in a given
axis, it is possible to displace the charges resulting in a dipole moment. In other words,
polarizing the material. [9]. This process of converting mechanical stress into a
polarization is what is known as the piezoelectric effect, and the process in which an
external magnetic field causes a strain in the material is known as the inverse piezoelectric
effect. Furthermore, for the molecule to stop being neutral after external stress is applied
is necessary for it to be non-centrosymmetric. If a stress is applied in any direction in a
centrosymmetric crystal, the crystal will remain neutral, granting no polarization.
Transition metal dichalcogenides, in bulk, are centrosymmetric due to being
experimentally observed to have a 2H structure with alternating species [6]. The force
between the layers of the 2H-TMDC is of the van der walls type, making possible the
separation in monolayers given this, it is clear that there is a break in the inversion center,
ergo piezoelectricity arises. Additionally, it can be proven mathematically that the lack of
centrosymmetry has the consequence that all the odd rank tensors properties may be
nonzero. In general, the piezoelectric tensor is of the third-rank. [10].
The mathematical approximation can be done with tensors and can be better
understood using matrixes as the preferred representation. The formula for the direct
effect and the converse effect can be seen in Equation 2, and this is the general form;
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nevertheless, the piezoelectric effect is orientation-dependent so that it can be express in
matrix form.
π)π = ππ
π) π = ππΈ
Equation 2 a) Direct piezoelectric effect, where P is the polarization, π is the
Stress and d is the piezoelectric coefficient b) Shows the inverse piezoelectric
coefficient
Equation 3 shows the piezoelectric formula in matrix representation, and the expression
to find the components of the polarization.
On the other hand, Equation 3 shows the matrix representation of the direct
piezoelectric effect. Where ππ relates to the direction of the stress applied 1,2,3 are the
axes x,y,z, and 4,5,6 to shear stress.
Hence, using the mathematical approach, it was predicted piezoelectric in
TMDCS, in this project is of interest just one of them, πππ2. Between the theoretical
prediction of piezoelectricity in monolayers of πππ2, and now, multiple studies have
shown that given the symmetry of the crystal and the two dimensions approach; there are
only a couple of d coefficients that will be independent and different to 0. The principal
being π11, calculated to be 2.91πππ The first subscript is the strain direction, and the second
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one is the polarization direction. Meaning the most significant piezoelectric response is in
the X-axis, or by and the response in the same direction. [11]On the other hand, in this
project, the coefficient at interest is the π13 , several times lower than the π11 also a
nonzero coefficient, since it is related to the π3 ,s tress on the z direction , and it is more
accessible in this direction to design a force actuator. Another exciting direction is the
sheer force which is also dependent on the π16 coefficient but this coefficient is at the
same time depend on the π11, wich means that applying a sheer force give a response on
the x-direction and with the maximum magnitude expected for
the πππ2.
2.1.2. Second order optical response
Other second-order phenomenon of interest in this project ia the second order
nonlinear optical response of the material which could be observed as second harmonic
generation (SHG) or as spontaneous parametric down conversion (SPDC). Moreover,
this kind of phenomenon arises in noncentrosymmetric crystals, where the optical
susceptibility Ο is that of the second order, Ο (2)
In πππ2, it has been measured SHG from atomically thin samples, and it has been
shown a strong SHG in odd layers; this expected since the phenomenon of SHG is stronger
for a non-centrosymmetric crystal. SHG is described by a third-rank tensor in the electric
dipole approximation, which is also the one responsible for the information on the
symmetry of surface layers. Using the mentioned tensor, the inversion symmetry break
can be affirmed if the SHG is largely enhanced. However, SPDC has not been measured
in πππ2 yet, despite having been theoretically predicted [8].
13
2.2. Previous work
πππ2 is a material broadly studied in bulk and the recent decade in nanolayers. Also,
some devices exist that make use of the nonlinear responses of this material, especially
the optical ones as SHG (second harmonic generation), PL(Photoluminicense), and some
light-induced electric properties to develop phototransistors. [12]
However, in the advancement of piezoelectric devices, πππ2 has not been broadly
used. Other materials 2D as ZnO shows how can nanoscale mechanical energy be
converted into macroscopically electrical energy employing the piezoelectric modes, with
power generating efficiency that varies from 17 to 30 percent. [2], [13]. ZnO larger
piezoelectric coefficient is measured to be 44.33 pm/v, but this could vary taking in
account
the fabrication and deposition methods, this coefficient is several times larger than the
one discussed for πππ2 hover using the appropriated energy transducer this is not a
problem, πππ2 presents other properties like thermal resistance. In the development of a
ZnO nanogenerators, a substrate of PTFE is gold-coated to work as a bottom electrode,
then ZnO is deposited in a nanowire arrangement, and PMMA surrounding the nanowires,
at the top is a silver-coated layer as the other electrode [14]
14
Chapter 3
Materials and methods
Now its time to describe the method used to synthesize the needed materials, and
also the methods used to develop the multiple devices. The prototype proposed at first for
the device, consisting of a silicon oxide wafer of 1.5 cm X 1.5 cm with a chrome and thin
gold layer as a conductive bottom electrode over the complete area. Moreover, a πππ2
layer comes over approximately ΒΎ of the area, on top of this comes a transducer layer, it
could be either PMMA or graphene; on top of the transducer is situated a silver-coated
layer as the top electrode and a cable is connected to both of the electrodes to measure
electrical properties.
Figure 2 Proposed prototype to measure piezoelectricity in liquid phase exfoliated πππ2.
3 .1) π΄ππΊπ liquid-phase exfoliation
15
For the πππ2, liquid-phase exfoliation was used. NMP(N-Methyl-2-Pyrrolidone) as
stabilizing solvent was used, assisting in the liquid exfoliation. The mixture for the
exfoliation consists of 7.5ml of π»2π millipore, 12.5 ml NMP and 100mg of powdered
πππ2. It is essential to know that this mixture will produce heat, and for the process to be
successful, it is done with constant contact with ice, of course not directly. Once the
mixture is complete, the next step as is shown in Figure 3, is to sonicate the πππ2 to have
a homogenous compound. The time for this process is 1 hour. Additionally, in the
ultrasonic bath, the temperature is maintained under 15 CΒ°.
Figure 3 Process of liquid phase exfoliation for πππ2. A) picture showing the ultrasonic
bath. This process is needed to make a homogeneous mixture.B) picture showing sonic
tip needed to introduce mechanical forces to assist the exfoliation. C) vacuum filter
arrangement used to keep only the exfoliated material D) centrifuge used to separate the
more massive particles from the lighter. E)-F) in E shows the product of the centrifuge.
In F is shown, only the material that floated is present, as this is the product of the
exfoliation process.
As mentioned before, if the temperature rises, the πππ2 could change its
composition, and the process will not be successful. The next step is B in Figure 3; this is
16
the primary process of the exfoliation. This step works by introducing a sonic force
through the tip. πππ2 has various layers composed with hexagonal planes of S atoms on
either side of a hexagonal plane of Mo atoms, these planes stack on top of each other with
weak van der Waals forces. [15] Exploiting the weak forces between planes, using the
sheer force produced by the sonic tip is possible to separate the planes and get monolayers.
In this process too, is needed the ice because the sonic tip by its movement will introduce
heat. The time for this step depends on the protocol used, as it will be discussed in the
results.
The next step, C in Figure 3, is to use the vacuum filter arrangement to drain all the
NMP, and left behind is just the exfoliated πππ2. The filter used in this process is a 0.1
Β΅m filter, that lets the π»2π and the NMP pass but not the πππ2 flakes. To make sure all
the NMP possible is no longer in the material. π»2π millipore is poured at least five times
to the arrangement. After the process is completed, the only thing left behind is a grey
mass on top of the filter. This mass goes to a redispersion process in H20 millipore. The
volume of H2O is based on the protocol that will is shown in the results; this step is done
in a falcon flask.
The next step is D in Figure 3, using the centrifuge for 1.5 hours at 1.2k RPM. The
dispersion of πππ2 suffers a mass separation on the principle that more massive crystals
move faster and farther from the central point of the system. In the end, in the recipient is
left the heavier masses on the bottom and walls, and the lighter masses floating in the
dispersion, this part that floats is the one at interest, as mentioned before, this project has
a particular interest in monolayers. The monolayers are the lighter in the bunch, so from
E to F, only the upper part is taken to ensure taking mostly monolayers.
17
3 .2) Graphene electroquimical exfoliation
Figure 4 process of electrochemical exfoliation for graphene. A) assembly for the
electrochemical exfoliation, on the left the power source and on the right the before and
after B) vacuum filter arrangement used to keep only the exfoliated material C)
centrifuge used to separate the heavier particles from the lighter D) shows the product
of the centrifuge E) suspension with only graphene in it.
The graphene is obtained by taking a piece of graphite and exfoliating it. In Figure
4 the overall process to produce graphene by electrochemical exfoliation is shown, in A
the voltage source and the main assembly for the exfoliation composed of a piece of
graphite paper, with dimensions 1.5cm X 1.5cm X 0.03mm, in the working electrode, and
a platinum rod as the auxiliary electrode are displayed. This setup is submerged partially
in 100ml of π»2ππ4 at 0.1M. A constant voltage of 8.5 V passes through the system,
causing a dissociation on the ions of the acid. These ions are driven by the electric
potential to move to de electrodes, the π»+ are attracted to the platinum rod and the ππ4β
is attracted to the graphite. The ππ4β is the exfoliating factor, interfering between the Van
der Waals forces that exist between the layers of graphite, leaving the lightest layers of
graphene floating.
18
Afterwards, on the flask is left mostly graphite layers suspended in the acid, the
next step is to put the suspension in a vacuum filter system as seen in B, using a 0.1 Β΅m
filter. After at least five uses of π»2π millipore to drain al the acid, the dry material is put
in a falcon recipient with 50 ml of π»2π and then was put in the centrifuge, as seen in c,
for 90 minutes at 3500 rpm for 1 hour. The product of the centrifuge can be seen in D, the
massive particles attach to the sides and the bottom, and the useful part for this project is
the graphene floating in the upper part, this material was extracted and can be seen in E.
3.4) PMMA and PDMS synthesis
Polymethylmethacrylate (PMMA), is a synthetic polymer that comes from the monomer
methylmethacrylate. PMMA is most notable for its use as a replacement for glass because
it is a transparent thermoplastic, has high resistance, and is a dielectric. This last property
is of especial interest in this research. [16]
There are multiple ways to synthesize PMMA; however, one of the cheapest ways is
solution polymerization of PMMA. This method consists of the use of an MMA resin and
a catalyst. In this process, the mixture is used in a 1:1 proportion and stir using a magnetic
stirrer hot plate for 20 minutes. Additionally, acetone is used to control the density of the
mixture to facilitate the process of homogenization and deposition. The acetone
evaporates a room temperature leaving only the PMMA in the end.
On the other hand, to deposit in a flexible substrate is often used
PDMS(Polydimethylsiloxane), which is a type of organic silicon, useful again for its easy
use and easy fabrication and dielectric constant of 2.4. [17] PDMS consist too in a resing
of
19
DMS and a catalyst that polymerizes the mixture, the ratio used is 20:1. To stir is used a
plastic fork; this leads to bubbles, so 10 minutes in a vacuum chamber is used to get rid
of them.
Figure 5 On the left PDMS filled with bubbles, on the right after 10 minutes on the
vacuum chamber without bubbles
Moreover, the application of these materials, PMMA and PDMS, is over the layer of
πππ2 to conduct the energy created in the process of piezoelectric conversion, as shown
before the mechanical energy gets converted to a polarization, is then used the dielectric
properties of those polymers to transport the energy to a metallic contact.
20
3 .5) Spin coating to deposit polymers.
Figure 6 Spincoater model WS-650MZ-23NPPB0. Useful in the prosses of PMMA
coating, it allows us to control the thickness with the variation of the angular speed.
The next step is to deposit the PMMA and PDMS over a Si substrate. It is used then a
spin coater, the one in Figure 6 since this method relates the thickness and angular
velocity:
To deposit the PMMA and PDMS, a spin coater is used. The thickness and the angular
velocity are related in this method as Equation 4 shows.
π‘ β1
βπ€
Equation 4 Relates thickness t and angular velocity in the process of spin-coating [18]
Then the variable to adjust is only the angular velocity, given that the method does not
relate the volume, time, nor another variable; the constant depends only on the material.
Later, in the results chapter, it will be shown that the angular velocity is not the only
variable overall that needs to be taken into account. Also, it is essential to control the
21
amount of acetone in the case of the PMMA and the method of pouring the material; there
are two options, static and dynamic. In the static case, a 500ml drop of the material is
poured before the process starts, and in the dynamic case, the pouring process occurs
during a slow angular velocity step.
3 .6) Spray coating to deposit nanolayers
Figure 7 Spray coater, consisting of a mechanical box and a heating plate. The samples
sit in the middle at a high temperature.
22
The spray coater in Figure 7, shows the method used to deposit the nanomaterials on the
silicon substrate. For the πππ2 the mechanical part moves at the medium setting to let the
material being deposited evaporate, the heating plate is set to 200 CΒ°., that is the boiling
temperature for NMP, and it is over the water boiling temperature. The idea is to spray
the substrate with the πππ2 suspension, and at this temperature, all but the πππ2 should
evaporate. On the other hand, the settings for the graphene are the same, with the
exception that the temperature is not enough to evaporate any left sulfuric acid, so the
temperature is only used to evaporate the water quickly and keep the layer homogenous.
The quantity of material is discussed later in the results section.
3 .7) Silver layer by thermal evaporation
Thermal evaporation is a process in which a material, in this case, silver, is heated
to its boiling point. Using a vacuum chamber, this material undergoes condensation over
the substrate. This process is used to create a 100 nm layer over the part of the device
needed, in this project it is used to create a contact over the layer the possible transducers,
and it was also studied as a possible conducting layer to replace the gold,and this way
have a process that can be done entirely at the University of Los Andes while also
diminishing the costs. In the case of silver as a contact, the process is complemented with
silver paint to contact a cable and make current and voltage measurements.
3 .8) Heat treatment
Another process that was necessary to add was a heat treatment step. This step comes
right after the deposition of πππ2; the wafer goes to either a vacuum oven or a
conventional convection oven, different times and temperatures were used as shown in
23
the next section. This process is to eliminate the chemical residues that can be still in the
wafer since the melting point of πππ2 is 2.375 Β° C; There is no risk to eliminate the
monolayers, and the boiling point of water is 100 Β°C; of NMP is 200Β°C but under vacuum
goes down to 105 Β°C
3 .9) Experimental assembly for structural
properties
Multiple assemblies were present in the progress of this project. First of all, an optical
microscope was used as a way to observe the differences in the samples when variables
like concentration and temperature in the heat process, were changed.
The process of taking images is simple. First, the sample goes to a sample holder, using
a computer, and the program from Olympus pictures are taken, depending on the
magnification a reference is taken too at the same height and focus. To process the image
first the reference is used to create a relation between pixels and meters; a bar with its size
in meters is inserted in the image, and a black and white filter is overlaid over the image
to have better contrast.
Second of all, a scanning electron microscope (SEM)enables us to see the sample almost
on a nanometric scale. The SEM works by scanning the sample with a focused electron
beam; the beams then undergo processes of scattering and interactions with the sample,
and later a detector obtains the angle and intensity of the electrons after the interactions.
The preparation of the samples is as follows; the exfoliated πππ2 is on top of a SiO wafer
without a gold-coated layer; this is crucial because a conducting surface can mess with
the electron beam; thus, the signal obtained. The sample then goes in the SEM, and the
24
rest of the analysis is internal in the program, in the end, an image is obtained like Figure
13
Finally, an AFM, Asylum Research MFP-3D-BIO, is used too to understand the
topography of the sample. AFM stands for an atomic force microscope; this method works
by using a cantilever with a sharp tip, sometimes in the scale of atoms, to scan a surface
either in contact or non-contact modes. A laser is pointed to the back of the cantilever,
and when the cantilever gets deflected by repulsive forces the variation on the cantilever
gets detected by the laser, and this information is the one that shows the topography. Not
only shows the area but also the depth in the sample.
3 .10) Experimental assembly for absorbance curve
and concentration curve
It is critical to ensure the correct exfoliation of the πππ2; with this end in mind, a Uv-vis
was used. A spectrophotometer is an instrument that allows measuring, to a given
wavelength, beam how much signal is absorbed by the sample. Every crystal absorbs in
different wavelengths, and so the curve generated differs one from the other, in the case
of πππ2, a water suspension of bulk material have an absorbance curve different from the
absorbance curve of liquid-phase exfoliated πππ2. To prepare the sample is needed, the
πππ2 after the exfoliation still suspended in water, first a reference is taken, from 200nm
to 1100nm in wavelength, this reference is from the same water used in the process of
exfoliation without the πππ2. Next, a measure of the sample is taken in the same range
as before; the sample goes in a quartz recipient of known volume. In the end, an
absorbance curve is obtained.
25
On the other hand, the maximums of the mentioned absorbance curve can be
related to the concentration of the sample by doing a calibration curve. This relationship
follows the beer lambert law, Equation 5. This law represents a linear relation between
absorbance and the molar concentration. After exfoliation, the mass is an unknown, so in
order to do a calibration curve, it is necessary to filtrate the exfoliated πππ2, weight its
mass, and disperse in a known volume. Knowing the volume and the mass we have the
concentration, then in order to get the calibration curve, we take the points of absorbance
in the maximums; with each point, it is possible to make a calibration. To make a curve,
it is needed multiple points, so this process is done the times that are needed.
π΄ = πππ
Equation 5 Beer-Lambert law. Relates absorbance and concentration. A is the
absorbance; π is the molar coeficcent; c is the molar concentration, and l is the optical
length.
26
Chapter 4
Results and analysis
4.1. Liquid phase exfoliation results
In this subsection, we will discuss the results obtained for the properties of liquid
phase exfoliated πππ2. First, we want to know the concentration of the liquid samples,
because we are using it as a variable, making sure the other parameters are constant. Thus,
a calibration curve is made; in Figure 9, the absorption spectra show maximums at 635
nm and 690 nm. So Figure 8 shows a calibration curve, using the maximum on the
absorption spectra located at 690 nm.
Figure 8 Calibration curve using the 635nm maximum from the absorption spectra. The
linear regression found relates to the known concentration of πππ2 monolayer with the
absorbance measured at this point. Error bars show the standard error on the mean.
27
From the regressions is possible from now on establish the concentration in the rest of
the exfoliations, hence is possible to establish the mass too. Moreover, the regression is
useful to calculate the coefficient of the Beer-Lambert law(extinction coefficient), Using
Equation 5.
When compared with the study of [19], where the mean coefficient found was 690
ml/ππβπ it is a difference of 23 percent when compared with our coefficient.
The procedure described in the following section to produce πππ2 monolayers
were developed in the nanomaterials laboratory. The exact times depended on the
protocol; various protocols were developed, in another unpublished study of the
Nanomaterials laboratory, to optimize the production of nanolayers. The two protocols
dimed the best for monolayer production are named Rβ and J.β They only differ in the
time of the sonic tip step, in Rβ the time is 4 hours and in Jβ is 3 hours.
Figure 9 Absorption spectra for the πππ2. The letters represent the method used, Rβ
describes a method in which the sonic tip step goes for 4 hours and Jβ 3 hours. Also, the
630nm and 690nm line shows were the expected Maximums should appear if is the case
of exfoliated πππ2.
28
Additionally, Figure 9 shows the results obtained with the spectrophotometer for two
samples of Rβ and two of J.β Then the protocol selected for the exfoliation process from
now on is Jβ since systematically can produce a more significant concentration, this is
important since the mass is going to be the only variable on the process of depositing the
πππ2 over the SiO wafers. Also, a challenge shown in the same figure is that even when
doing the same protocol, the concentration varies, this can be explained in the filtration
process, since not all the filters are equal, sometimes more material is left over the filter
and not dispersed in the water.
Figure 10 Fluorescence spectrums for exfoliated πππ2, three spectrums are shown at
370nm,300nm,500nm. The intensity is on arbitrary units.
On the other hand, fluorescence curves were taken to evaluate second order nonlinear
optical responses of the nanolayered πππ2; the sample preparation is the same as in the
case of the spectrophotometer; only in this case, the sample is shot with a photon beam
29
of a specific wavelength, since the crystal are in suspension the beam hit in different
angles in various crystals, and then is measured the response in a range of wavelengths
bigger than the initial one. On figure Figure 10, it can be seen three fluorescence curves.
The maximums are centered around exactly double the wavelength of the source, or what
is the same half of the frequency. However, from the literature, it is known that the second
harmonic generation is angle dependent, so this does not give useful information about
SHG. To determine second order nonlinear optical responses other kind of setup is
needed.
4.2. π΄ππΊπ layer
After setting the parameters for the πππ2 fabrication, the next step is to deposit the
πππ2 nanolayers. The first test was to change the mass of nanolayered πππ2 to observe
the crystal distributions and topological properties. The idea is to form a complete layer
over the substrate, in this test, conformed only of SiO.
Sample # Mass
1 3.08
2 6.12
3 12.43
Table 1 samples of the πππ2 deposit test, with different masses
30
Figure 11 Optical images of πππ2 deposited over SiO, the mass of the material
deposited grows from a) to b).
In figure 15, it is clear that when incrementing the mass the area density is bigger.
Another other remarks noted with the optical imaging, is that even doubling the mass
from sample 2 to 3, the difference is not that much in the layer formation, this means that
even though more crystal appears when augmenting the mass, those crystals agglomerate
and some spaces are always present. Also, some contamination was present when seen
under the microscope, the color of the crystals was white, and the contamination) n had a
reddish color. To further investigate this contamination and isolate the image of a cluster
of crystals, SEM was used.
a) b )
c)
31
Figure 12 Sem images for the πππ2 deposited over SiO, the mass of the material
deposited grows from a) to b).
Figure 13 Sem image of a cristal of πππ2 deposited over Sio; the triangular shape is
expected from the lattice distribution.
a) b )
C)
32
At this scale, it is abundantly clear that there are spaces between the crystals in all the
samples; even in sample 3, the SEM showed that is more the space empty than the
populated with the clusters of πππ2. The preferred form of the nanolayers of πππ2 is a
triangle; this arises from the hexagonal crystal structure, Figure 13 shows one of this
crystals when is not in a cluster, the triangular shape coincides with the expected result;the
crystal has an approximate area of 0.6 Β΅m and is not a monolayer. However, the substrate
can be seen through the scattered signal, which means the thickness is in the nanometric
scale, although there is no way to know if is in an odd or even state. Figure 14 shows the
composition of the same crystal, Mo, Si, S, are present in a significant fraction confirming
that is a πππ2 crystal; however some c appears, this contamination is inevitable since the
principal produce of the laboratory is graphene and there is not a clean room to deposit
the nanolayers, other kinds of contamination are shown on Figure 15, the composition of
the contamination on that case is Cu,Zn and C. upon investigation it was found that the
source of Cu and Zn was the spray from the spray coater; this model had metallic paint
covering the interior where the suspended πππ2 goes, this covering was eliminated with
a strong acid to avoid this contamination.
33
Figure 14 Composition analysis for a nanolayered πππ2, the top image shows SEM
results, and the bottom one the possible elements in the image.
Figure 15 Composition analysis for strange structures in the samples of πππ2 over SiO,
the top image shows SEM results, and the bottom one the possible elements in the image.
The structures on the image are found to be contamination.
Also, in Figure 19, some drops can be seen in the image, those cannot be water
since the spray coating process occurs at 180 CΒ°, so the other option is NMP. To eliminate
the residual NMP, a sample is prepared with 15.8 mg of πππ2 nanolayers. Then, two
different methods of heat treatment are proposed, first convection oven for 2,3,4 hours at
200 CΒ° and second vacuum oven for 2 hours at 150 CΒ°. Best results for samples with 15.8
mg, in a no-treatment, b convection oven 2 hours, and c 4 hours. The best result is obtained
by using the convection oven for 4 hours, the white covering around the white dots starts
to disappear, and the points stay, in SEM was shown that those points where precisely
the agglomerated πππ2.
34
Figure 16 Optical comparison of the best results for the heat treatment procedure. In a)
no treatment applied, b) convection oven 2 hours, and c) convection oven 4 hours.
From now on, all the samples are prepared with no less than 12.0 mg of exfoliated
πππ2 and no more than 20 mg since the layers tend to agglomerate instead of covering
all the areas available. Also, heat treatment is added after the spray coating. Consisting of
4 hours of 200 CΒ°in a convection oven after.
4.3. Graphene as an energy transducer
a) b)
c)
35
Figure 17 Graphene absorbance curve. 4 samples made with the same procedure are
compared. 258 nm is the expected maxima for graphene; the case of graphite is around
275 nm.
Graphene is a 2D material just as πππ2, so the absorption spectra is useful to
ensure the proper exfoliation of the material. The expected curve for the graphite
synthesized in the nanomaterial laboratory of the University of Los Andes has a maximum
at 258nm, while the graphite has a maximum at around 275nm. Figure 17 shows the
absorption curves for multiple water suspensions of graphene, is accessible to appreciate
that the method is consistent in converting graphite into graphene; however, the 4 curves
shown have different concentration, although they all came from the same roll of graphite
and underwent the same process, with the same instruments and acid. This phenomenon
could be because the electrochemical reaction might be susceptible to humidity, pressure,
and temperature, those variables can not be controlled in the laboratory as of now.
Additionally, there is no need to create a calibration curve for the graphene since
it is known from another unpublished project form the laboratory that once the graphene
forms a layer, it begins to be reflective as it can be seen on Figure 18. The graphene
successfully formed a layer on top of the πππ2. The next step is to deposit a silver contact
on top of the graphene, this process was successful, too, and a complete device is shown
in Figure 19.
36
Figure 18 SiO-Cr-Au-πππ2-Graphene samples in order of layers. The top side shows the
graphene forming layer, which is reflective. On the bottom, the mask used protect the SiO-
Cr-Au layers.
37
Figure 19 SiO-Cr-Au-πππ2. -Graphene-Ag layers in scaling order, on top of the
samples. SiO-Cr-Au-MoS2-Graphene layers on the middle interface. And SiO-Cr-Au on
the bottom.
Finally, the graphene is studied as a transducer. The idea is to use the graphene to pick
up the signal from al the crystals that are in a monolayer or odd arrangement. However,
when measuring continuity, it was shown that the graphene was directly connected with
the gold, meaning that the circuit was shorted. This configuration does not represent the
ideal case, with this design then is not possible to use the graphene as a transducer, it was
observed that the πππ2 does not form a complete layer so when the graphene was
deposited not only over the πππ2 but in-between too, and, the graphene is a good electric
conductor, the signal that the πππ2 could output cannot be registered since there is a short
circuit.
4.4. PMMA as an energy transducer
First of all, it is necessary to determine the preferred thickness of the layer, using Equation
4 we know that the variable to change is the angular velocity of the spin coater. The first
test was to variate the angular velocity; the PMMA was then deposited on glass wafers of
the same area of the real ones. Since the πππ2 is expected to have a nanometric height
the PMMA wanted height is higher than 100nm but no so high that the signal dissipates.
Three of the samples from this test are displayed in Figure 20, while it may be to see in a
photo, to the naked eye is clear qualitatively wich one is thicker. Samples for 1000rpm,
to 12000rpm in 2000 steps were made, the best result was 9000rpm; at this angular
velocity, the PMMA layer looks thick enough to cover the nanolayer of πππ2. Fewer rpm
the layers look so tick that the caliper still could measure the layer. More rpm, the PMMA
will not form a complete layer; in the corners and borders, the PMMA was missing.
38
Additionally, a process of 1000 rpm was added before the selected 9000 rpm. This
permitted a more homogeneous layer with no bumps or valleys.
Figure 20 PMMA Thickness test samples.
On the other hand, a complete device was made just like in the graphene case but
changing that material to PMMA. This time the circuit was not shorted. However, when
measured in the oscilloscope, it was clear that this time, it simulated an open circuit.SEM
was used to observe a lateral view of the PMMA. From this process was found that the
thickness of the PMMA layer at 9000rpm is, on average 5Β΅m.
Furthermore, in the image appear to be a space between the PMMA and the πππ2, this
could be the reason that the device appeared as an open circuit. The next step was to
eliminate the air bubbles before the PMMA dries. To this end, a vacuum chamber and
convection over were used right after the process of spin coating.
39
Figure 21 SEM image of the Lateral view of the 9000rpm sample of PMMA. The bottom
part is the SiO-Cr-Au-MoS2, the flakes of MoS2 looking like little bumps, over that a big
gap and then the PMMA the significant layer on the top of the image.
Procedure used Qualitative description
The vacuum chamber 10 minutes The bubbles rose to the surface, the same
amount
Convection oven 30 minutes 80 CΒ° Fewer bubbles but some still inside the
PMMA
Convection oven 30 minutes 100 CΒ° Fewer bubbles but some still inside the
PMMA
Convection 15 minutes 120 CΒ° Fewer bubbles but some still inside the
PMMA
The vacuum chamber 10 minutes then
Convection oven 30 minutes 100 CΒ°
Almost all the bubbles disappeared, but
the PMMA disappeared entirely from the
corners and borders
Table 2 Information about the multiples test made to eliminate the bubbles that appeared
on the PMMA
Following the results from Table 2, the method chosen was a vacuum chamber for 10
minutes and then convection oven 30 minutes at 100 CΒ°. A new device was done
following all the same parameters plus the process to get rid of the bubbles. The devices
had no apparent bubbles; nevertheless, due to the time scope of this project, it was no
possibility to add the silver contact and measure the ability of PMMA as a transducer nor
the macroscopic piezoelectric response of the πππ2 layer.
40
Chapter 5
Conclusions and future work
β’ The process of liquid-phase exfoliation of πππ2 was successfully replicated,
obtaining a suspension of nanolayers, corroborated with the absorption curve.
β’ The process of electrochemical exfoliation of graphene was successfully
replicated, obtaining a suspension of nanolayers corroborated with the absorption
curve.
β’ The Beer-Lambert law was corroborated, finding a linear relationship between
concentration and absorbance. Hence, a calibration curve was successfully made.
β’ The extinction coefficient for exfoliated πππ2was found to 545.2 ππ
ππβπFor the
690 maxima. Also, it is known to be an acceptable result since when comparing
to the literature is only 23 percent different, and the difference can arise from the
use of different instruments.
β’ The second-order nonlinear phenomenon of second-harmonic generation was not
measured successfully in the water suspended exfoliated πππ2. It is needed an
optical configuration, where the orientation of the crystal is known, and the
detector is only in the expected angle to measure the outgoing photon.
β’ The πππ2was deposited on a silicon wafer with success, via spray coating. It was
not found in the literature anyone that has done this before. There is still work to
improve this method but overall is cheaper than other methods like CVD.
β’ Graphene is not useful as a transducer in the current configuration; the high
conductivity of the graphene causes the device to short circuit.
41
β’ Exfoliated πππ2 does not form a complete layer when deposited with spray
coating. The πππ2 flakes tend to agglomerate.
β’ Evaporated silver does not adhere to SiO wafers due to small adhesives forces,
perturbation as temperature, mechanical forces or chemical reactions, can
compromise the adhesiveness of the silver.
β’ It was developed a method to deposit PMMA with a thickness of 5Β΅m and a
method to extract the bubbles from the deposited PMMA.
Due to time constraints, some tests could not be reported. It was planned to finish a device
to prove PMMA transducing properties. Also, another substrate was going to be tested,
this is the case of flexible ITO, and PDMS was going to be tested as the transducer. It is
imperative to diminish the contamination, using a cleanroom. Future work towards
measuring the piezoelectric effect on the πππ2 and finding the conversion constant could
be made by completing the device proposed with the appropriated transducer. Also, Pl
measurements are needed to corroborate the existence of second order optical response in
the MoS2 once is deposited in a substrate and not suspended in water.
42
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