The surface topography of bare- and etched- thermally treated copper foils have been examined by optical microscopy, SEM, and STM. Cu foils were subjected to chemical pretreatment and pre- annealing procedures with the intention of improving the quality of Cu film and providing better condi- tions for graphene growth. The results showed that etching copper foils scars the surface and leaves be- hind etch quarries and residue. The protocol that included pre-annealing of foils followed by the growth process was expected to improve the quality of the foil surface, sequentially resulting in a better quality of graphene. However, this procedure produced a crust layer on the foil that sometimes peeled off along with graphene. The STM study indicated that the heat treated Cu surface includes large height variations on the micrometer length scale. Yet, a layer of graphene on top of the Cu surface reduced its apparent surface roughness. Optical micrograph of CVD graphene transferred onto a SiO2 wafer displays
the imprinted surface texture of the Cu grain boundaries on the subsequently grown graphene layer. This observation supports the notion that carbon atoms bind more readily at the Cu grain boundary dur- ing the CVD growth.
The results of Chapter 6 demonstrated the difference in PMMA, PC, and PS thermoplastic poly- mers during the graphene transfer and the importance of the ethanol treatment on the surface charac- terization and electrical properties of graphene. Visible improvement to surface morphology of gra- phene on SiO2 is observed after the ethanol treatment. Besides the change in cleanness of the graphene
surface, all devices that had been treated with ethanol, regardless of choice of polymer aid, showed no- ticeable improvement in carrier mobility. Raman characterization of the graphene samples showed evi- dent blue-shifts in the G and 2D bands which suggest that the graphene samples are more p-doped after the ethanol treatment. Even though, there is a distinct improvement to the electrical properties of gra- phene after the film has been subjected to the ethanol treatment. Graphene transferred by both PC and
PS showed visible tears along with partial laceration of the graphene membrane, unlike graphene trans- ferred by PMMA, which results in less tears of the carbon film.
The future work will consist of the extension of graphene study. Magnetoresistance (MR) measurements on CVD graphene will be performed at room temperature down to liquid helium tem- peratures. Measurement results will be analyzed as a function of temperature variations, and the ex- pected weak localization (WL) features will provide the information about the quality of the sample, as WL is caused by the inter-valley scattering due to the presence of atomically sharp disorders which in turn favors the appearance of WL at zero magnetic field. Given the existence of such structure, the de- pendence of WL on magnetic field will be fitted by the McCann theory on WL [70], and extracted charac- teristics length scales: phase coherence length, , inter-valley scattering length, , and intra-valley scattering length, , will be analyzed as a function of temperature and carrier density. We will also study a dependence of various carrier temperatures as a function of a given current for devices of vari- ous carrier density and mobility.
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APPENDICES
Appendix A: List of Abbreviations
AFM – Atomic Force Microscope BF – Bare foil
CVD – Chemical Vapor Deposition EF – Etched foil
EG – Ethanol treated graphene
G – Graphene transferred by PMMA, PC, and PS – Phase coherence length
– Inter-valley scattering length – Intra-valley scattering length MR – Magnetoresistance
OM – Optical Microscope PC – Polycarbonate PDMS – Polydimethylsiloxane PMMA – Poly(methyl methacrylate) PS – Polystyrene
– Average Roughness
SEM – Scanning Electron Microscope STM – Scanning Tunneling Microscope UHV – Ultra High Vacuum
Appendix B: Methods
Appendix B.1: Graphene Growth in Chapter 4
CVD synthesis of graphene samples were performed on 25 μm thick copper foil using a horizon- tal 1 inch quartz tube furnace system. All foil specimens came from the same roll. The method involved the flow of methane and hydrogen/nitrogen gases. Under vacuum conditions with the base pressure of 36 mTorr, the furnace is heated to 1025 °C with 10 sccm flow of H2N2 reaching
200 mTorr H2N2 pressure. Once the temperature reached 1025 °C, the flow of CH4 is introduced
to the system. Both CH4 with 40 sccm and H2N2 with 10 sccm are flowing during the growth pro-
cess at 1025 °C for 30 min (P 500 mTorr). During the CVD growth, the furnace with the Cu foil required a period of 30 min to reach the operating temperature of 1025 °C. During this period, there was a steady flow of H2N2 in the growth chamber. After heat treatment, the specimens
were cooled to room temperature over a period of two hours.
Appendix B.2: Graphene Growth in Chapter 6
Commercially available 25 μm Cu foils were used. All foil specimens came from the same roll. The heat treatment of the foils was carried out in a mechanically pumped CVD system consisting of 1" furnace with 1" o.d. quartz tube. The CVD system was supplied with high purity H2:N2 [10%:90%] and CH4. The growth process took 30 minutes at 1000 °C under H2N2:CH4 [10 sccm :
65 sccm] with P=375 mTorr. Before CVD growth, the furnace with the Cu foil required a period of 30 minutes to reach the operating temperature of 1000 °C. During this period, there was a steady flow of H2N2 in the growth chamber with P=216 mTorr. After the growth has been com-
Appendix B.3: Graphene Transfer in Chapter 4
PMMA transfer method was utilized as a supporting mechanical substrate. PMMA solution was spin-coated onto the graphene/Cu substrate with 500 rpm for 3 seconds following by 4500 rpm for 60 seconds and then back to 500 rpm for 5 seconds. The copper film was chemical etched with Fe(NO3)3(1M) solution, followed by thorough rinsing with deionized water. Silicon dioxide
served as a substrate of choice for the graphene/polymer system. The graphene/polymer sup- port membrane was allowed to air-dry for the period of 24 hours on the SiO2 substrate. Then,
the PMMA was soaked in acetone overnight and dried with nitrogen gas. Appendix B.4: Graphene Transfer in Chapter 6
Poly(methyl methacrylate) (PMMA), polycarbonate (PC), and polystyrene (PS) transfer methods were utilized as a supporting mechanical substrates. These polymer glass solutions were spin- coated onto the graphene/Cu substrate with 500 rpm for 3 seconds following by 4500 rpm for 60 seconds and then back to 500 rpm for 5 seconds. The copper film was chemically etched with FeCl3 (0.34 M) solution, followed by thorough rinsing with deionized water. The SiO2 served as a
substrate of choice for graphene/polymer membrane. The membrane was allowed to air-dry for the period of 24 hours on the SiO2 substrate. Then, the PMMA was soaked in acetone, PC - in di-
chloromethane, and PS - in toluene baths for 20 hours and dried with nitrogen gas. Chemical treatment with ethanol for each graphene sample on SiO2 substrate was performed for 20
hours.
Appendix B.5: Sample Fabrication Technique in Chapter 6
Graphene sheet deposition onto SiO2 substrate was performed using the support of the pro-
posed thermoplastic polymers after the etching process of Cu foil in FeCl3 solution has been
treatment for graphene Hall bar development. Ethanol treated devices were submerged in eth- anol bath for 20 hours prior to the measurement performance.
Appendix B.6: Optical Technology Classifications in Chapter 4
Optical images were taken with Metallographic microscope, XJP-H100 series. SEM characteriza- tion was performed using Cambridge Instruments, Stereoscan 360 with a substrate at room temperature with chamber vacuum at 8.510-6 Torr and column vacuum at 1.110-7 Torr. STM classification was done using Ultra High Vacuum (UHV) RHK chamber with a substrate at room temperature with chamber vacuum at 110-7 Torr.
Appendix B.7: Optical Technology Classifications in Chapter 6
SEM characterization was performed using Cambridge Instruments, Stereoscan 360 with a sub- strate at room temperature with chamber vacuum at 8.510-6 Torr and column vacuum at 1.110-7 Torr. Raman spectra were attained on CVD graphene on SiO2 substrate transferred by
various polymer glass support methods. The measurements are performed at room tempera- ture with a LabRAM HR spectrometer at 532 nm wavelength laser excitation, with scan area of 1 μm in diameter. A 50 objective is used.