Fan-Hung Liu

Nitrogen-Doped Graphene Sheets Grown by Chemical Vapor Deposition: Synthesis and Influence of Nitrogen Impurities on Carrier Transport

A significant advance toward achieving practical applications of graphene as a two-dimensional material in nanoelectronics would be provided by successful synthesis of both n-type and p-type doped graphene. However, reliable doping and a thorough understanding of carrier transport in the presence of charged impurities governed by ionized donors or acceptors in the graphene lattice are still lacking. Here we report experimental realization of few-layer nitrogen-doped (N-doped) graphene sheets by chemical vapor deposition of organic molecule 1,3,5-triazine on Cu metal catalyst. When reducing the growth temperature, the atomic percentage of nitrogen doping is raised from 2.1% to 5.6%. With increasing doping concentration, N-doped graphene sheet exhibits a crossover from p-type to n-type behavior accompanied by a strong enhancement of electron-hole transport asymmetry, manifesting the influence of incorporated nitrogen impurities. In addition, by analyzing the data of X-ray photoelectron spectroscopy, Raman spectroscopy, and electrical measurements, we show that pyridinic and pyrrolic N impurities play an important role in determining the transport behavior of carriers in our N-doped graphene sheets.

Low Carrier Density Epitaxial Graphene Devices On SiC

The transport characteristics of graphene devices with low n- or p-type carrier density (∼10(10) -10(11) cm(-2) ), fabricated using a new process that results in minimal organic surface residues, are reported. The p-type molecular doping responsible for the low carrier densities is initiated by aqua regia. The resulting devices exhibit highly developed ν = 2 quantized Hall resistance plateaus at magnetic field strengths of less than 4 T.

Insulator-quantum Hall transition in monolayer epitaxial graphene

We report on magneto-transport measurements on low-density, large-area monolayer epitaxial graphene devices grown on SiC. We observe temperature (T)-independent crossing points in the longitudinal resistivity ρxx, which are signatures of the insulator-quantum Hall (I-QH) transition, in all three devices. Upon converting the raw data into longitudinal and Hall conductivities σxx and σxy, in the most disordered device, we observed T-driven flow diagram approximated by the semi-circle law as well as the T-independent point in σxy near e2/h. We discuss our experimental results in the context of the evolution of the zero-energy Landau level at low magnetic fields B. We also compare the observed strongly insulating behaviour with metallic behaviour and the absence of the I-QH transition in graphene on SiO2 prepared by mechanical exfoliation.

Controlling the Fermi Level in a Single-Layer Graphene QHE Device for Resistance Standard

NMIJ/AIST and NIST are collaborating on the development of graphene-based quantized Hall resistance (QHR) devices. We formed graphene films on SiC(0001) substrates and processed the samples into Hall bar devices using the NIST clean room facility. The electronic transport properties have been observed at the NIST and NMIJ/AIST. We used two methods to control the Fermi level in the samples. One is hydrogen intercalation and the other is photochemical gating. Using the former technique, the Fermi level moved across the Dirac point. For the latter technique, it moved closer to the Dirac point.

Controlling Fermi level in single layer graphene QHE device for resistance standard

The National Metrology Institute of Japan/ National Institute of Advanced Industrial Science and Technology (NMIJ/AIST) and the National Institute of Standards and Technology (NIST) are collaborating on the development of graphene-based quantized Hall resistance devices. We formed graphene films on silicon carbide (0001) substrates and processed the samples into Hall bar devices using the NIST clean room facility. The electronic transport properties have been observed at the NIST and NMIJ/AIST. Hydrogen intercalation and photochemical gating were employed to control the Fermi level in the samples. For the first method, the Fermi level was observed to move across the Dirac point. For the latter technique, it moved closer to the Dirac point.

Probing weak localization in chemical vapor deposition graphene wide constriction using scanning gate microscopy.

Low-temperature scanning gate microscopy (LT-SGM) studies of graphene allow one to obtain important spatial information regarding coherent transport such as weak localization (WL) and universal conductance fluctuations. Although fascinating LT-SGM results on pristine graphene prepared by mechanical exfoliation have been reported in the literature, there appears to be a dearth of LT-SGM results on chemical vapor deposition (CVD)-grown graphene whose large scale and flexible substrate transferability make it an ideal candidate for coherent electronic applications. To this end, we have performed LT-SGM studies on CVD-grown graphene wide constriction (0.8 μm), which can be readily prepared by cost-effective optical lithography fully compatible with those in wafer foundry, in the WL regime. We find that the movable local gate can sensitively modulate the total conductance of the CVD graphene constriction possibly due to the intrinsic grain boundaries and merged domains, a great advantage for applications in coherent electronics. Moreover, such a conductance modulation by LT-SGM provides an additional, approximately magnetic-field-independent probe for studying coherent transport such as WL in graphene and spatial conductance variation.

Localization and electron-electron interactions in few-layer epitaxial graphene

This paper presents a study of the quantum corrections caused by electron-electron interactions and localization to the conductivity in few-layer epitaxial graphene, in which the carriers responsible for transport are massive. The results demonstrate that the diffusive model, which can generally provide good insights into the magnetotransport of two-dimensional systems in conventional semiconductor structures, is applicable to few-layer epitaxial graphene when the unique properties of graphene on the substrate, such as intervalley scattering, are taken into account. It is suggested that magnetic-field-dependent electron-electron interactions and Kondo physics are required for obtaining a thorough understanding of magnetotransport in few-layer epitaxial graphene.

Dirac fermion heating, current scaling, and direct insulator-quantum Hall transition in multilayer epitaxial graphene

We have performed magnetotransport measurements on multilayer epitaxial graphene. By increasing the driving current I through our graphene devices while keeping the bath temperature fixed, we are able to study Dirac fermion heating and current scaling in such devices. Using zero-field resistivity as a self thermometer, we are able to determine the effective Dirac fermion temperature (TDF) at various driving currents. At zero field, it is found that TDF ∝ I≈1/2. Such results are consistent with electron heating in conventional two-dimensional systems in the plateau-plateau transition regime. With increasing magnetic field B, we observe an I-independent point in the measured longitudinal resistivity ρxx which is equivalent to the direct insulator-quantum Hall (I-QH) transition characterized by a temperature-independent point in ρxx. Together with recent experimental evidence for direct I-QH transition, our new data suggest that such a transition is a universal effect in graphene, albeit further studies are required to obtain a thorough understanding of such an effect.

High Current-Induced Electron Redistribution in a CVD-Grown Graphene Wide Constriction

Investigating the charge transport behavior in one-dimensional quantum confined system such as the localized states and interference effects due to the nanoscale grain boundaries and merged domains in wide chemical vapor deposition graphene constriction is highly desirable since it would help to realize industrial graphene-based electronic device applications. Our data suggests a crossover from interference coherent transport to carriers flushing into grain boundaries and merged domains when increasing the current. Moreover, many-body fermionic carriers with disordered system in our case can be statistically described by mean-field Gross-Pitaevskii equation via a single wave function by means of the quantum hydrodynamic approximation. The novel numerical simulation method supports the experimental results and suggests that the extreme high barrier potential regions on graphene from the grain boundaries and merged domains can be strongly affected by additional hot charges. Such interesting results could pave the way for quantum transport device by supplying additional hot current to flood into the grain boundaries and merged domains in one-dimensional quantum confined CVD graphene, a great advantage for developing graphene-based coherent electronic devices.

Imaging coherent transport in chemical vapor deposition graphene wide constriction by scanning gate microscopy

We use a scanning gate microscopy to perturb coherent transport in chemical vapor deposition (CVD) graphene wide constriction. Particularly, we observe conductance oscillations in the wide constriction region (W ∼ 800 nm) characterized by spatial conductance variations, which imply formation of the nanometer-scale ring structure due to the merged domains and intrinsic grain boundaries. Moreover, additional hot charges from high current can suppress the coherent transport, suggesting that the hot carriers with a wide spreading kinetic energy could easily tunnel merged domains and intrinsic grain boundaries in CVD-grown graphene due to the heating effect, a great advantage for applications in graphene-based interference-type nano-electronics.