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Research Topics

1. Nitrogen-doped graphene sheets grown by chemical vapor deposition

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. [Please see Y.-F. Lu et al. ACS Nano 7, 6522 (2013) for details.]

2. Controllable disorder in a hybrid nanoelectronic system: realization of a superconducting diode

We have studied a hybrid nanoelectronic system which consists of an AlGaAs/GaAs two-dimensional electron gas (2DEG) in close proximity (~70 nm) to an Al superconducting nanofilm. By tuning the current through the Al film, we can change the conductance of the 2DEG and furthermore vary the effective disorder in the Al superconducting film in a controllable way. When a high current is injected into the film, screening which couples the Al film and the 2DEG results in a collapse of anti-symmetric behavior in the current-voltage characteristics, V(I)-V(-I), which holds true in a conventional superconductor. Our results may open a new avenue of experimentally realizing a superconducting diode. [Please see S.-T. Lo et al. Sci. Rep. 3, 2274 (2013) for details.]

3. Fractional quantum Hall effect in a high Landau level in bilayer graphene

The fractional quantum Hall effect is a canonical example of electron-electron interactions producing new ground states in many-body systems. Most fractional quantum Hall studies have focussed on the lowest Landau level, whose fractional states are successfully explained by the composite fermion model. In the widely studied GaAs-based system, the composite fermion picture is thought to become unstable for the N≥2 Landau level, where competing many-body phases have been observed. Here we report magneto-resistance measurements of fractional quantum Hall states in the N=2 Landau level (filling factors 4<|v|<8) in bilayer graphene. In contrast with recent observations of particle-hole asymmetry in the N=0/N=1 Landau levels of bilayer graphene, the fractional quantum Hall states we observe in the N=2 Landau level obey particle-hole symmetry within the fully symmetry-broken Landau level. Possible alternative ground states other than the composite fermions are discussed. [Please see G. Diankov et al., Nat. Commun. 7, 13908 (2016) for details.]

4. Temperature dependence of electron density and electron–electron interactions in graphene

We report carrier density measurements and electron-electron (e-e) interactions in monolayer epitaxial graphene grown on SiC. The temperature (T)-independent carrier density determined from the Shubnikov-de Haas (SdH) oscillations clearly demonstrates that the observed logarithmic temperature dependence of the Hall slope in our system must be due to e-e interactions. Since the electron density determined from conventional SdH measurements does not depend on e-e interactions based on Kohn's theorem, SdH experiments appear to be more reliable compared with the classical Hall effect when one studies the T dependence of the carrier density in the low T regime. On the other hand, the logarithmic T dependence of the Hall slope δRxy/δB can be used to probe e-e interactions even when the conventional conductivity method is not applicable due to strong electron-phonon scattering. [Please see C. W. Liu et al., 2D Mater. 4, 025007 (2017) for details.]

5. Non-saturating magnetoresistance in graphene

We report large, non-saturating magnetoresistance (MR) of ~140% in single layer chemical vapor deposition (CVD) graphene with an h-BN capping layer at room temperature at B = 9 T. Based on the classical model developed by Parish and Littlewood, our results show that the MR is proportional to the average mobility <μ> and decreases with increasing temperature. In contrast, in a large-area, extremely homogenous single layer epitaxial graphene (EG) device, the MR is saturating and is inversely proportional to <μ>, which is consistent with the finite resistance network picture. By comparing the results obtained from CVD graphene with an h-BN capping layer with those from the EG device, we show that the non-saturating linear characteristics come from multi-channel current paths in a two-dimensional plane due to the intrinsic grain boundaries and domains of CVD graphene by capping an h-BN layer that increase the <μ> of CVD graphene. Our results on CVD graphene with an h-BN capping layer pave the way for industrial schemes of graphene-based and air-stable magnetic field sensors with a linear, large response at room temperature. [Please see C. Chuang et al., Carbon 136, 211 (2018) for details.]

 

 

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Equipment

IMAG4042.jpg     IMAG4043.jpg

Oxford He3/He4 Dilution Cryostat System

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                                    Top-loading  He3 System

 

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                 Probe & TO-five                               MOTIC Microscope

 

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              Turbo Pumping Station                       15 T superconductor magnet

 

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