Photophysics and Electronic Structure of Lateral Graphene/MoS and Metal/MoS Junctions.

Integration of semiconducting transition metal dichalcogenides (TMDs) into functional optoelectronic circuitries requires an understanding of the charge transfer across the interface between the TMD and the contacting material. Here, we use spatially resolved photocurrent microscopy to demonstrate electronic uniformity at the epitaxial graphene/molybdenum disulfide (EG/MoS) interface. A 10× larger photocurrent is extracted at the EG/MoS interface when compared to the metal (Ti/Au)/MoS interface.

Flat Bands and Mechanical Deformation Effects in the Moiré Superlattice of MoS-WSe Heterobilayers.

It has recently been shown that quantum-confined states can appear in epitaxially grown van der Waals material heterobilayers without a rotational misalignment (θ = 0°), associated with flat bands in the Brillouin zone of the moiré pattern formed due to the lattice mismatch of the two layers. Peaks in the local density of states and confinement in a MoS/WSe system was qualitatively described only considering local stacking arrangements, which cause band edge energies to vary spatially. In this work, we report the presence of large in-plane strain variation across the moiré unit cell of a θ = 0° MoS/WSe heterobilayer and show that inclusion of strain variation and out-of-plane displacement in density functional theory calculations greatly improves their agreement with the experimental data.

Quantum-Confined Electronic States Arising from the Moiré Pattern of MoS-WSe Heterobilayers.

A two-dimensional (2D) heterobilayer system consisting of MoS on WSe, deposited on epitaxial graphene, is studied by scanning tunneling microscopy and spectroscopy at temperatures of 5 and 80 K. A moiré pattern is observed, arising from lattice mismatch of 3.7% between the MoS and WSe.

Realizing Large-Scale, Electronic-Grade Two-Dimensional Semiconductors.

Atomically thin transition metal dichalcogenides (TMDs) are of interest for next-generation electronics and optoelectronics. Here, we demonstrate device-ready synthetic tungsten diselenide (WSe) via metal-organic chemical vapor deposition and provide key insights into the phenomena that control the properties of large-area, epitaxial TMDs. When epitaxy is achieved, the sapphire surface reconstructs, leading to strong 2D/3D (i.

Magnitude of the current in 2D interlayer tunneling devices.

Using the Bardeen tunneling method with first-principles wave functions, computations are made of the tunneling current in graphene/hexagonal-boron-nitride/graphene (G/h-BN/G) vertical structures. Detailed comparison with prior experimental results is made, focusing on the magnitude of the achievable tunnel current. With inclusion of the effects of translational and rotational misalignment of the graphene and the h-BN, predicted currents are found to be about 15×  larger than experimental values.

Large scale 2D/3D hybrids based on gallium nitride and transition metal dichalcogenides.

Two and three-dimensional (2D/3D) hybrid materials have the potential to advance communication and sensing technologies by enabling new or improved device functionality. To date, most 2D/3D hybrid devices utilize mechanical exfoliation or post-synthesis transfer, which can be fundamentally different from directly synthesized layers that are compatible with large scale industrial needs. Therefore, understanding the process/property relationship of synthetic heterostructures is priority for industrially relevant material architectures.

Epitaxial graphene homogeneity and quantum Hall effect in millimeter-scale devices.

Quantized magnetotransport is observed in 5.6 × 5.6 mm epitaxial graphene devices, grown using highly constrained sublimation on the Si-face of SiC(0001) at high temperature (1900 °C).

Tuning electronic transport in epitaxial graphene-based van der Waals heterostructures.

Two-dimensional tungsten diselenide (WSe2) has been used as a component in atomically thin photovoltaic devices, field effect transistors, and tunneling diodes in tandem with graphene. In some applications it is necessary to achieve efficient charge transport across the interface of layered WSe2-graphene, a semiconductor to semimetal junction with a van der Waals (vdW) gap. In such cases, band alignment engineering is required to ensure a low-resistance, ohmic contact.

Scanning Tunneling Microscopy and Spectroscopy of Air Exposure Effects on Molecular Beam Epitaxy Grown WSe2 Monolayers and Bilayers.

The effect of air exposure on 2H-WSe2/HOPG is determined via scanning tunneling microscopy (STM). WSe2 was grown by molecular beam epitaxy on highly oriented pyrolytic graphite (HOPG), and afterward, a Se adlayer was deposited in situ on WSe2/HOPG to prevent unintentional oxidation during transferring from the growth chamber to the STM chamber. After annealing at 773 K to remove the Se adlayer, STM images show that WSe2 layers nucleate at both step edges and terraces of the HOPG.

Probing Critical Point Energies of Transition Metal Dichalcogenides: Surprising Indirect Gap of Single Layer WSe2.

By using a comprehensive form of scanning tunneling spectroscopy, we have revealed detailed quasi-particle electronic structures in transition metal dichalcogenides, including the quasi-particle gaps, critical point energy locations, and their origins in the Brillouin zones. We show that single layer WSe2 surprisingly has an indirect quasi-particle gap with the conduction band minimum located at the Q-point (instead of K), albeit the two states are nearly degenerate. We have further observed rich quasi-particle electronic structures of transition metal dichalcogenides as a function of atomic structures and spin-orbit couplings.

Hot carriers in epitaxial graphene sheets with and without hydrogen intercalation: role of substrate coupling.

The development of graphene electronic devices produced by industry relies on efficient control of heat transfer from the graphene sheet to its environment. In nanoscale devices, heat is one of the major obstacles to the operation of such devices at high frequencies. Here we have studied the transport of hot carriers in epitaxial graphene sheets on 6H-SiC (0001) substrates with and without hydrogen intercalation by driving the device into the non-equilibrium regime.

Spatially resolved mapping of electrical conductivity across individual domain (grain) boundaries in graphene.

All large-scale graphene films contain extended topological defects dividing graphene into domains or grains. Here, we spatially map electronic transport near specific domain and grain boundaries in both epitaxial graphene grown on SiC and CVD graphene on Cu subsequently transferred to a SiO2 substrate, with one-to-one correspondence to boundary structures. Boundaries coinciding with the substrate step on SiC exhibit a significant potential barrier for electron transport of epitaxial graphene due to the reduced charge transfer from the substrate near the step edge.

Atomic-scale mapping of thermoelectric power on graphene: role of defects and boundaries.

The spatially resolved thermoelectric power is studied on epitaxial graphene on SiC with direct correspondence to graphene atomic structures by a scanning tunneling microscopy (STM) method. A thermovoltage arises from a temperature gradient between the STM tip and the sample, and variations of thermovoltage are distinguished at defects and boundaries with atomic resolution. The epitaxial graphene has a high thermoelectric power of 42 μV/K with a big change (9.

Low-energy electron reflectivity from graphene: first-principles computations and approximate models.

A computational method is developed whereby the reflectivity of low-energy electrons from a surface can be obtained from a first-principles solution of the electronic structure of the system. The method is applied to multilayer graphene. Two bands of reflectivity minima are found, one at 0-8 eV and the other at 14-22 eV above the vacuum level.

Charge transfer between isomer domains on n+-doped Si(111)-2 × 1: energetic stabilization.

Domains of different surface reconstruction-negatively or positively buckled isomers-have been previously observed on highly n-doped Si(111)-2 × 1 surfaces by angle-resolved ultraviolet photoemission spectroscopy and scanning tunneling microscopy/spectroscopy. At low temperature, separate domains of the two isomer types are apparent in the data. It was argued in the previous work that the negative isomers have a lower energy of their empty surface states than the positive isomers, providing a driving force for the formation of the negative isomers.

Coexistence of negatively and positively buckled isomers on n+-doped Si(111) − 2 × 1.

A long-standing puzzle regarding the Si(111) − 2 × 1 surface has been solved. The surface energy gap previously determined by photoemission on heavily n-doped crystals was not compatible with a strongly bound exciton known from other considerations to exist. New low-temperature angle-resolved photoemission and scanning tunneling microscopy data, together with theory, unambiguously reveal that isomers with opposite bucklings and different energy gaps coexist on such surfaces.

Ultrafast transient absorption microscopy studies of carrier dynamics in epitaxial graphene.

Transient absorption microscopy was employed to image charge carrier dynamics in epitaxial multilayer graphene. The carrier cooling exhibited a biexponential decay that showed a significant dependence on carrier density. The fast and slow relaxation times were assigned to coupling between electrons and optical phonon modes and the hot phonon effect, respectively.

Adatom kinetics on and below the surface: the existence of a new diffusion channel.

Employing density-functional theory in combination with scanning tunneling microscopy, we demonstrate that a thin metallic film on a semiconductor surface may open an efficient and hitherto not expected diffusion channel for lateral adatom transport: adatoms may prefer diffusion within this metallic layer rather than on top of the surface. Based on this concept, we interpret recent experiments: We explain why and when In acts as a surfactant on GaN surfaces, why Ga acts as an autosurfactant, and how this mechanism can be used to optimize group-III nitride growth.

Spontaneous formation of indium-rich nanostructures on InGaN(0001) surfaces.

InGaN(0001) surfaces prepared by molecular beam epitaxy have been studied using scanning tunneling microscopy and first-principles total energy calculations. Nanometer-size surface structures are observed consisting of either vacancy islands or ordered vacancy rows. The spontaneous formation of these structures is shown to be driven by significant strain in the surface layers and by the relative weakness of the In-N bond compared to Ga-N.

High failure rate of cementless threaded acetabular cups: a radiographic and histologic study in the goat.

We studied the fixation of the Mecron cementless titanium screw cup radiographically and histologically in 20 dwarf-goats after periods of 0, 6, 26 and 52 weeks. In only 3 goats did histology show good bone-implant contact, whereas in the other 17 goats a fibrous membrane interface was seen. This high failure rate is caused by the poor primary fixation and should be a warning against the use of this implant.

Failure of the Mecring screw-ring acetabular component in total hip arthroplasty. A three to seven-year follow-up study.

We prospectively studied the results of 411 consecutive total hip arthroplasties with a Mecring screw-ring acetabular component inserted without cement combined with a Stanmore femoral stem inserted with cement. The duration of follow-up ranged from three to seven years (mean, four years and six months). Three hundred and thirty-one patients (378 hips) were available for physical examination and had a complete set of radiographs.