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Search Results: 1 - 10 of 303835 matches for " Brian J. LeRoy "
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Graphene on Hexagonal Boron Nitride
Matthew Yankowitz,Jiamin Xue,Brian J. LeRoy
Physics , 2014, DOI: 10.1088/0953-8984/26/30/303201
Abstract: The field of graphene research has developed rapidly since its first isolation by mechanical exfoliation in 2004. Due to the relativistic Dirac nature of its charge carriers, graphene is both a promising material for next-generation electronic devices and a convenient low-energy testbed for intrinsically high-energy physical phenomena. Both of these research branches require the facile fabrication of clean graphene devices so as not to obscure its intrinsic physical properties. Hexagonal boron nitride has emerged as a promising substrate for graphene devices, as it is insulating, atomically flat and provides a clean charge environment for the graphene. Additionally, the interaction between graphene and boron nitride provides a path for the study of new physical phenomena not present in bare graphene devices. This review focuses on recent advancements in the study of graphene on hexagonal boron nitride devices from the perspective of scanning tunneling microscopy with highlights of some important results from electrical transport measurements.
Local Spectroscopic Characterization of Spin and Layer Polarization in WSe$_2$
Matthew Yankowitz,Devin McKenzie,Brian J. LeRoy
Physics , 2015, DOI: 10.1103/PhysRevLett.115.136803
Abstract: We report scanning tunneling microscopy (STM) and spectroscopy (STS) measurements of monolayer and bilayer WSe$_2$. We measure a band gap of 2.21 $\pm$ 0.08 eV in monolayer WSe$_2$, which is much larger than the energy of the photoluminescence peak indicating a large excitonic binding energy. We additionally observe significant electronic scattering arising from atomic-scale defects. Using Fourier transform STS (FT-STS), we map the energy versus momentum dispersion relations for monolayer and bilayer WSe$_2$. Further, by tracking allowed and forbidden scattering channels as a function of energy we infer the spin texture of both the conduction and valence bands. We observe a large spin-splitting of the valence band due to strong spin-orbit coupling, and additionally observe spin-valley-layer coupling in the conduction band of bilayer WSe$_2$.
Local Spectroscopy of the Electrically Tunable Band Gap in Trilayer Graphene
Matthew Yankowitz,Fenglin Wang,Chun Ning Lau,Brian J. LeRoy
Physics , 2013, DOI: 10.1103/PhysRevB.87.165102
Abstract: The stacking order degree of freedom in trilayer graphene plays a critical role in determining the existence of an electric field tunable band gap. We present spatially-resolved tunneling spectroscopy measurements of dual gated Bernal (ABA) and rhombohedral (ABC) stacked trilayer graphene devices. We demonstrate that while ABA trilayer graphene remains metallic, ABC trilayer graphene exhibits a widely tunable band gap as a function of electric field. However, we find that charged impurities in the underlying substrate cause substantial spatial fluctuation of the gap size. Our work elucidates the microscopic behavior of trilayer graphene and its consequences for macroscopic devices.
Scanning gate microscopy of ultra clean carbon nanotube quantum dots
Jiamin Xue,Rohan Dhall,Stephen B. Cronin,Brian J. LeRoy
Physics , 2015,
Abstract: We perform scanning gate microscopy on individual suspended carbon nanotube quantum dots. The size and position of the quantum dots can be visually identified from the concentric high conductance rings. For the ultra clean devices used in this study, two new effects are clearly identified. Electrostatic screening creates non-overlapping multiple sets of Coulomb rings from a single quantum dot. In double quantum dots, by changing the tip voltage, the interactions between the quantum dots can be tuned from the weak to strong coupling regime.
Evolution of the electronic band structure of twisted bilayer graphene upon doping
Shengqiang Huang,Matthew Yankowitz,Kanokporn Chattrakun,Arvinder Sandhu,Brian J. LeRoy
Physics , 2015,
Abstract: The electronic band structure of twisted bilayer graphene depends on the twist angle between the two layers. In particular, van Hove singularities occur in the density of states when the two sets of monolayer bands cross. An enhanced Raman G peak is observed when the excitation laser is resonant with the energy separation of these singularities. Using Raman spectroscopy, we monitor the evolution of the electronic band structure for various twist angles upon charge doping. The variation of the Raman G peak area with charge density reveals changes in the electronic band structure, and a decrease in the energy of the resonant scattering pathway is inferred from the twist angle dependence. This decrease is due to a charge density asymmetry between the two layers. Our results demonstrate that the electronic and optical properties of twisted bilayer graphene can be controlled by doping.
Response of graphene to femtosecond high-intensity laser irradiation
Adam Roberts,Daniel Cormode,Collin Reynolds,Ty Newhouse-Illige,Brian J. LeRoy,Arvinder S. Sandhu
Physics , 2011, DOI: 10.1063/1.3623760
Abstract: We study the response of graphene to high-intensity 10^11-10^12 Wcm^-2, 50-femtosecond laser pulse excitation. We establish that graphene has a fairly high (~3\times10^12Wcm^-2) single-shot damage threshold. Above this threshold, a single laser pulse cleanly ablates graphene, leaving microscopically defined edges. Below this threshold, we observe laser-induced defect formation that leads to degradation of the lattice over multiple exposures. We identify the lattice modification processes through in-situ Raman microscopy. The effective lifetime of CVD graphene under femtosecond near-IR irradiation and its dependence on laser intensity is determined. These results also define the limits of non-linear applications of graphene in femtosecond high-intensity regime.
Optical thickness determination of hexagonal Boron Nitride flakes
Dheeraj Golla,Kanokporn Chattrakun,Kenji Watanabe,Takashi Taniguchi,Brian J. LeRoy,Arvinder Sandhu
Physics , 2013, DOI: 10.1063/1.4803041
Abstract: Optical reflectivity contrast provides a simple, fast and noninvasive method for characterization of few monolayer samples of two-dimensional materials. Here we apply this technique to measure the thickness of thin flakes of hexagonal Boron Nitride (hBN), which is a material of increasing interest in nanodevice fabrication. The optical contrast shows a strong negative peak at short wavelengths and zero contrast at a thickness dependent wavelength. The optical contrast varies linearly for 1-80 layers of hBN, which permits easy calibration of thickness. We demonstrate the applicability of this quick characterization method by comparing atomic force microscopy and optical contrast results.
Optical characterization of electron-phonon interactions at the saddle point in graphene
Adam T. Roberts,Rolf Binder,Nai H. Kwong,Dheeraj Golla,Daniel Cormode,Brian J. LeRoy,Henry O. Everitt,Arvinder Sandhu
Physics , 2013, DOI: 10.1103/PhysRevLett.112.187401
Abstract: The role of electron-phonon interactions is experimentally and theoretically investigated near the saddle point absorption peak of graphene. The differential optical transmission spectra of multiple, non-interacting layers of graphene reveals the dominant role played by electron-acoustic phonon coupling in bandstructure renormalization. Using a Born approximation for electron-phonon coupling and experimental estimates of the dynamic phonon lattice temperature, we deduce the effective acoustic deformation potential to be $D^{\rm ac}_{\rm eff} \simeq 5$eV. This value is in accord with recent theoretical predictions but differs substantially from those obtained using electrical transport measurements.
Long wavelength local density of states oscillations near graphene step edges
Jiamin Xue,Javier Sanchez-Yamagishi,K. Watanabe,T. Taniguchi,Pablo Jarillo-Herrero,Brian J. LeRoy
Physics , 2011, DOI: 10.1103/PhysRevLett.108.016801
Abstract: Using scanning tunneling microscopy and spectroscopy, we have studied the local density of states (LDOS) of graphene over step edges in boron nitride. Long wavelength oscillations in the LDOS are observed with maxima parallel to the step edge. Their wavelength and amplitude are controlled by the energy of the quasiparticles allowing a direct probe of the graphene dispersion relation. We also observe a faster decay of the LDOS oscillations away from the step edge than in conventional metals. This is due to the chiral nature of the Dirac fermions in graphene.
Intrinsic disorder in graphene on transition metal dichalcogenide heterostructures
Matthew Yankowitz,Stefano Larentis,Kyounghwan Kim,Jiamin Xue,Devin McKenzie,Shengqiang Huang,Marina Paggen,Mazhar N. Ali,Robert J. Cava,Emanuel Tutuc,Brian J. LeRoy
Physics , 2014, DOI: 10.1021/nl5047736
Abstract: The electronic properties of two-dimensional materials such as graphene are extremely sensitive to their environment, especially the underlying substrate. Planar van der Waals bonded substrates such as hexagonal boron nitride (hBN) have been shown to greatly improve the electrical performance of graphene devices by reducing topographic variations and charge fluctuations compared to amorphous insulating substrates}. Semiconducting transition metal dichalchogenides (TMDs) are another family of van der Waals bonded materials that have recently received interest as alternative substrates to hBN for graphene as well as for components in novel graphene-based device heterostructures. Additionally, their semiconducting nature permits dynamic gate voltage control over the interaction strength with graphene. Through local scanning probe measurements we find that crystalline defects intrinsic to TMDs induce scattering in graphene which results in significant degradation of the heterostructure quality, particularly compared to similar graphene on hBN devices.
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