Session X12: Focus Session: Graphene: Growth, Mechanical Exfoliation, and Properties - Morphology and Interactions

Atomic Scale Properties of Chemically Doped Graphene

In monolayer graphene, substitutional doping during growth can be used to alter its electronic properties. We used scanning tunneling microscopy, Raman spectroscopy, x-ray spectroscopy, and first principles calculations to characterize individual nitrogen dopants in monolayer graphene grown on a copper substrate. Individual nitrogen atoms were incorporated as graphitic dopants, and a fraction of the extra electron on each nitrogen atom was delocalized into the graphene lattice. The electronic structure of nitrogen-doped graphene was strongly modified only within a few lattice spacings of the site of the nitrogen dopant. These findings show that chemical doping is a promising route to achieving high-quality graphene films with a large carrier concentration. Ref: L. Zhao et al, \textit{Science} \textbf{333},999 (2011).   

Quantum Hall Effect in Graphene Bilayers Grown on Copper by Chemical Vapor Deposition

We report an investigation of quantum Hall effect in graphene bilayers grown on Cu substrates by chemical vapor deposition. Raman spectroscopy of the as grown graphene bilayers reveals a position dependent full width half maximum of the 2D peak, ranging from 22 to 55 cm$^{-1}$, suggesting the bilayer is a mixture of Bernal stacked and decoupled graphene monolayers. Using scanning Raman spectroscopy we identify areas with either wide (45-55 cm$^{-1})$, as well as narrow (22-26 cm$^{-1})$ 2D peaks, in order to fabricate back-gated Hall bars on such grains. Magnetotransport measurements in bilayer regions characterized by a wide 2D peak reveal quantum Hall states (QHS) at filling factors $\nu = \pm$ 4, 8, 12 consistent with a Bernal stacked bilayer, which develop at magnetic fields higher than 15 T. In contrast, magnetotransport measurements in bilayer regions defined by a narrow 2D peak shows QHSs down to 3 T, and with a sequence consisting of the superposition of the QHSs of two independent monolayers. We compare the QHS energy gaps extracted from activation measurements with the theoretical Landau level (LL) separation, and estimate the LL broadening.   

Layer Number and Stacking Order Imaging of Few-layer Graphenes by Transmission Electron Microscopy

A method using transmission electron microscopy (TEM) selected area electron diffraction (SAED) patterns and dark field (DF) images is developed to identify graphene layer number and stacking order by comparing intensity ratios of SAED spots with theory. Graphene samples are synthesized by ambient pressure chemical vapor depostion and then etched by hydrogen in high temperature to produce samples with crystalline stacking but varying layer number on the nanometer scale. Combined DF images from first- and second-order diffraction spots are used to produce images with layer-number and stacking-order contrast with few-nanometer resolution. This method is proved to be accurate enough for quantative stacking-order-identification of graphenes up to at least four layers. This work was partially supported by Science of Precision Multifunctional Nanostructures for Elecrical Energy Storage, an Energy Frontier Research Center funded by the U.S. DOE, Office of Science, Office of Basic Energy Sciences under Award Number DESC0001160.   

Metal surface melting effect on the formation of graphene wrinkles

The synthesis of high quality large area graphene is a significant challenge that needs to be overcome for practical applications of this material. Therefore, understanding the growth mechanism of graphene is crucial in order to explain the origin of defects on it. Different thickness of Copper and Nickel films deposited on silica substrates, and foils with various degrees of purity were used in this study. We investigated the changes in the surface morphology of the thin films and foils with temperature at 860-1100$^{\circ}$C with and without growth of graphene by Chemical Vapor Deposition method. Detailed investigation by Raman spectroscopy, SEM and AFM analysis revealed that thermal treatment of the metal substrates at high temperatures $\sim $1000$^{\circ}$C causes formation of dendritic like structures on the surface. We attributed these structures to the non-equilibirium solidification of the melted surface of the metal during the cooling, which are also present in the case of graphene growth. We concluded that the reconstruction of the metal surface morphology in the case of graphene growth, due to the surface melting, significantly affects on the final topography of the graphene wrinkles and, thereby on the quality of graphene.   

Growth and Characterization of Graphene-Boron Nitride Heterostructures

Graphene has been used to explore the fascinating properties of two-dimensional sp$^{2}$ carbon, and shows great promise for applications. Heterostructures of graphene (G) and hexagonal boron nitride (h-BN) have the potential for extended functionality, e.g., providing high carrier mobilities in graphene devices supported on h-BN and giving rise to emergent electronic behavior near in-plane G/h-BN junctions. While significant progress has been made recently in separate graphene and boron nitride growth on transition metals, the controlled synthesis of high-quality G/h-BN heterostructures poses new challenges. We discuss the fundamental growth mechanisms underlying the synthesis of G/h-BN heterostructures, studied by a combination of in-situ surface microscopy methods. Real-time low-energy electron microscopy (LEEM) provides a mesoscale view of the nucleation and growth of h-BN in the presence of graphene, and vice-versa. LEEM imaging together with diffraction and angle resolved photoemission spectroscopy (micro-ARPES) gives insight into the interaction between graphene and h-BN. Scanning tunneling microscopy has been used to probe intermixing and the atomic-scale structure of interfacial boundaries. Combining real-time and atomic-resolution imaging, we identify successful approaches for achieving atomically sharp G/h-BN junctions.   

Physical and electrical properties of graphene folds grown by Chemical Vapor Deposition

We found that there is a large density of wrinkles on CVD graphene transferred to a SiO$_{2}$/Si substrate. At these graphene wrinkles, the SEM signal intensity is lower, the AFM height is higher, and the Raman G-band intensity is stronger as compared to the surrounding single-layer graphene, due to extra layers of graphene at the wrinkles. TEM images confirmed that wide wrinkles are folds instead of ridges. The channel resistance near the Dirac point along graphene folds is significantly lower than the resistance without folds. However, as the gate field or the carrier density is increased, the difference between graphene channel along the fold and without the fold is reduced, due to carrier screening.   

Superior Mobility in Chemical Vapor Deposition Synthesized Graphene by Grain Size Engineering

Chemical vapor deposition (CVD) offers a promising method to produce large-area films of graphene, crucial for commercial realization of graphene-based applications. However, electron transport in CVD grown graphene has continued to fall short of the performance demonstrated by graphene derived from mechanical exfoliation. Lattice defects and grain boundaries developed during growth, structural defects and chemical contamination introduced during transfer, and charged scatterers present in sub-optimal dielectric substrates have all been identified as sources of disorder in CVD grown graphene devices. We grow CVD graphene and fabricate field-effect transistors, attempting to minimize potential sources of disorder. We reduce density of grain boundaries in CVD graphene by controlling domain sizes up to 250 microns. By transferring CVD graphene onto h-BN utilizing a dry-transfer method, we minimize trapped charges at the interface between graphene and in the underlying substrate. We report field-effect mobilities up to 110,000 cm2V-1s-1 and oscillations in magnetotransport measurements below 1 T, confirming the high quality and low disorder in our CVD graphene devices.   

Transfer-Free, Wafer-Scale, and Patterned Synthesis of Graphene on Dielectric Substrates

We report a method for the direct, wafer-scale synthesis of graphene on dielectric substrates using a solid carbon source. Graphene films were synthesized through the thermal decomposition of poly(methyl methacrylate) on copper coated quartz and Si/SiO2 substrates in a low pressure H2/Ar environment. The Cu film partially evaporated during growth, leaving a graphene layer directly on the dielectric substrate. Polyacrylamide was used for synthesis of N-doped graphene due to its nitrogen content. A similar method with process optimization was utilized to grow graphene without any external carbon source. Trace amounts of carbon in metal films result in direct formation of graphene on insulators through a segregation process. A wet etch step allowed complete removal of the metal film, leading to continuous graphene coverage of the surface. This technique utilized for patterned synthesis of graphene and can be used in Si-wafer compatible device fabrication.   

Transport studies of dual-gated ABA- and ABC-stacked trilayer graphene

We present electrical transport studies of dual-gated ABA- and ABC-stacked trilayer graphene field effect transistors. Employing high-quality thin HfO$_{2}$ layers as the top and back gate dielectrics, we independently tune the carrier density and control the band structure of trilayer graphene via the application of a perpendicular electric field E$_{perp}$. The large gating efficiency of the two gates (5.53 x 10$^{12} $/cm$^{2}$ per Volt) and their high breakdown voltage ($>$ 6 V) enable us to reach exceedingly large carrier densities and E$_{perp}$ values, which results in wide tuning of the conductivity of the trilayer devices. Results on ABA-stacked trilayer graphene confirm its semi-metallic nature and reveal evidence of the band structure changes induced by E$_{perp}$. The resistance at the charge neutrality point of ABC-stacked trilayer graphene increases by many orders of magnitude with increasing E$_{perp}$ due to the gradual opening of a band gap. Our results suggest a saturation of the gap size at perpendicular displacement fields greater than 3.5 V/nm, in agreement with theoretical calculations.   

Graphene growth by CVD using liquid precursors

Chemical vapor deposition (CVD) represents an attractive route to synthesize large-area graphene. Its catalytic growth using metals has been studied in recent years, the most popular being nickel and copper. We have successfully grown graphene on copper by CVD at ambient pressure using alcohols as the carbon feedstock. The produced materials are analyzed by SEM, TEM, Raman spectroscopy and transferred onto Si/SiO$_{2}$ substrates using standard methods. Raman fingerprint of monolayer graphene is present in our samples. This technique represents a safer and more versatile alternative to the production of graphene compared to the synthesis of graphene using methane at low pressure.   

Vibrational properties in graphene-on-metal systems: A scanning tunneling microscopy study and density functional theory calculations

Combing inelastic electron tunneling microscopy (IETS) measured by scanning tunneling microscopy and quantum mechanical calculations based on density functional theory, we investigate and compare the vibrational properties of graphene on various transition metal substrates. The observed d2I/dV2 spectra and mappings indicate there is a mode dependant spatial localization for graphene on Ru(0001) surface In contrast, vibrations of graphene on Pt(111) surface and Ni(111) surface is homogeneous. Out-of-plane vibration modes of graphene are tuned by the different interactions on these three substrates. Vibrational density of states of graphene and graphene/Ni(111) system are calculated and make comparison to the IETS experiments. We also calculate the inelastic and elastic tunneling coefficients in Graphene/Ni(111) system to understand the missing peak in IETS experiments. Our results point to the importance of interfacial bonding on phonon properties and, consequently, electronic and thermal transport properties of graphene based devices.