23 research outputs found
Membrane amplitude and triaxial stress in twisted bilayer graphene deciphered using first-principles directed elasticity theory and scanning tunneling microscopy
Twisted graphene layers produce a moir\'e pattern (MP) structure with a
predetermined wavelength for given twist angle. However, predicting the
membrane corrugation amplitude for any angle other than pure AB-stacked or
AA-stacked graphene is impossible using first-principles density functional
theory (DFT) due to the large supercell. Here, within elasticity theory we
define the MP structure as the minimum energy configuration, thereby leaving
the height amplitude as the only unknown parameter. The latter is determined
from DFT calculations for AB and AA stacked bilayer graphene in order to
eliminate all fitting parameters. Excellent agreement with scanning tunneling
microscopy (STM) results across multiple substrates is reported as function of
twist angle.Comment: to appear in Phys. Rev.
Vertical conduction properties of few-layer epitaxial graphene / n-type 4H-SiC heterojunctions at cryogenic temperatures
Vertical diodes of epitaxial graphene on n 4H-SiC were investigated. The graphene Raman spectraexhibited a higher intensity in the G-line than the 2D-line, indicative of a few-layer graphene film.Rectifying properties improved at low temperatures as the reverse leakage decreased over six ordersof magnitude without freeze-out in either material. Carrier concentration of 10 16 cm 3in the SiCremained stable down to 15 K, while accumulation charge decreased and depletion width increasedin forward bias. The low barrier height of 0.08 eV and absence of recombination-induced emissionindicated majority carrier field emission as the dominant conduction mechanism
Chalcogenide Glass-on-Graphene Photonics
Two-dimensional (2-D) materials are of tremendous interest to integrated
photonics given their singular optical characteristics spanning light emission,
modulation, saturable absorption, and nonlinear optics. To harness their
optical properties, these atomically thin materials are usually attached onto
prefabricated devices via a transfer process. In this paper, we present a new
route for 2-D material integration with planar photonics. Central to this
approach is the use of chalcogenide glass, a multifunctional material which can
be directly deposited and patterned on a wide variety of 2-D materials and can
simultaneously function as the light guiding medium, a gate dielectric, and a
passivation layer for 2-D materials. Besides claiming improved fabrication
yield and throughput compared to the traditional transfer process, our
technique also enables unconventional multilayer device geometries optimally
designed for enhancing light-matter interactions in the 2-D layers.
Capitalizing on this facile integration method, we demonstrate a series of
high-performance glass-on-graphene devices including ultra-broadband on-chip
polarizers, energy-efficient thermo-optic switches, as well as graphene-based
mid-infrared (mid-IR) waveguide-integrated photodetectors and modulators
Investigation of the Epitaxial Graphene/p-SiC Heterojunction
There has been significant research in the study of in-plane charge-carrier transport in graphene in order to understand and exploit its unique electrical properties; however, the vertical graphene–semiconductor system also presents opportunities for unique devices. In this letter, we investigate the epitaxial graphene/p-type 4H-SiC system to better understand this vertical heterojunction. The I–V behavior does not demonstrate thermionic emission properties that are indicative of a Schottky barrier but rather demonstrates characteristics of a semiconductor heterojunction. This is confirmed by the fitting of the temperature-dependent I–V curves to classical heterojunction equations and the observation of band-edge electroluminescence in SiC
The Hide-and-Seek of Grain Boundaries from Moire Pattern Fringe of Two-Dimensional Graphene
Grain boundaries (GBs) commonly exist in crystalline materials and affect various properties of materials. The facile identification of GBs is one of the significant requirements for systematical study of polycrystalline materials including recently emerging two-dimensional materials. Previous observations of GBs have been performed by various tools including high resolution transmission electron microscopy. However, a method to easily identify GBs, especially in the case of low-angle GBs, has not yet been well established. In this paper, we choose graphene bilayers with a GB as a model system and investigate the effects of interlayer rotations to the identification of GBs. We provide a critical condition between adjacent moire fringe spacings, which determines the possibility of GB recognition. In addition, for monolayer graphene with a grain boundary, we demonstrate that low-angle GBs can be distinguished easily by inducing moire patterns deliberately with an artificial reference overlayopen0
Graphene nanoribbon field-effect transistors on wafer-scale epitaxial graphene on SiC substrates
We report the realization of top-gated graphene nanoribbon field effect transistors (GNRFETs) of ∼10 nm width on large-area epitaxial graphene exhibiting the opening of a band gap of ∼0.14 eV. Contrary to prior observations of disordered transport and severe edge-roughness effects of graphene nanoribbons (GNRs), the experimental results presented here clearly show that the transport mechanism in carefully fabricated GNRFETs is conventional band-transport at room temperature and inter-band tunneling at low temperature. The entire space of temperature, size, and geometry dependent transport properties and electrostatics of the GNRFETs are explained by a conventional thermionic emission and tunneling current model. Our combined experimental and modeling work proves that carefully fabricated narrow GNRs behave as conventional semiconductors and remain potential candidates for electronic switching devices