Separation-dependent electronic transparency of monolayer graphene membranes on III-V semiconductor substrates

Exfoliation of Graphene onto Si(111)-7 × 7 under Ultrahigh Vacuum Provides Some Protection against Exposure to Air

The investigation of graphene as a protective coating for different materials has been an area of active research for well over a decade. However, graphene's ability to protect clean, reconstructed silicon from ambient gases has remained uninvestigated. Here, we describe the use of a clean ultrahigh vacuum transfer method to deposit graphene onto the Si(111)-7 × 7 reconstruction. Using a combination of X-ray photoelectron spectroscopy (XPS) and scanning tunneling microscopy (STM), we confirmed the successful transfer of graphene onto the surface. We then exposed the graphene-protected surface to 120 L of ambient air in the vacuum chamber and found that although the reconstruction is destroyed on unprotected terraces, clean and ordered silicon can be imaged through the transferred graphene. Exposing the sample to ambient air at atmospheric pressure for 2 days produces a strong signature of oxidation in XPS, and STM images revealed that although graphene can still be identified, the regions beneath the graphene appear amorphous. This work demonstrates that graphene provides some protection against the oxidation of Si(111)-7 × 7 by air, but that this protective ability is not sufficient to forestall oxidation during prolonged exposure to atmospheric conditions.

Pinhole-seeded lateral epitaxy and exfoliation of GaSb films on graphene-terminated surfaces

Remote epitaxy is a promising approach for synthesizing exfoliatable crystalline membranes and enabling epitaxy of materials with large lattice mismatch. However, the atomic scale mechanisms for remote epitaxy remain unclear. Here we experimentally demonstrate that GaSb films grow on graphene-terminated GaSb (001) via a seeded lateral epitaxy mechanism, in which pinhole defects in the graphene serve as selective nucleation sites, followed by lateral epitaxy and coalescence into a continuous film. Remote interactions are not necessary in order to explain the growth. Importantly, the small size of the pinholes permits exfoliation of continuous, free-standing GaSb membranes. Due to the chemical similarity between GaSb and other III-V materials, we anticipate this mechanism to apply more generally to other materials. By combining molecular beam epitaxy with in-situ electron diffraction and photoemission, plus ex-situ atomic force microscopy and Raman spectroscopy, we track the graphene defect generation and GaSb growth evolution a few monolayers at a time. Our results show that the controlled introduction of nanoscale openings in graphene provides an alternative route towards tuning the growth and properties of 3D epitaxial films and membranes on 2D material masks.

Remote epitaxy represents a promising method for the synthesis of thin films on lattice-mismatched substrates, but its atomic-scale mechanisms are still unclear. Here, the authors demonstrate the growth of exfoliatable GaSb films on graphene-terminated GaSb (001) via seeded lateral epitaxy, showing that pinhole defects in graphene serve as selective nucleation sites.

Atomically flat and thermally stable graphene on Si(111) with preserved intrinsic electronic properties

Silicon and graphene are two wonder materials, and their hybrid heterostructures are expected to be very interesting fundamentally and practically. In the present study, by adopting fast dry transfer and ultra-high vacuum annealing, atomically flat monolayer graphene is successfully prepared on the chemically active Si(111) substrate. More importantly, the graphene overlayer largely maintains its intrinsic electronic properties, as validated by the results of the energy-dependent electronic transparency, Dirac point observation and quantum coherence characteristics, and further confirmed by first-principles calculations. The intrinsic properties of graphene are retained up to 1030 K. The system of atomically flat and thermally stable graphene on a chemically active silicon surface with preserved inherent characteristics renders the graphene/silicon hybrid a promising system in the design of high-performance devices and the exploitation of interfacial topological quantum effects.

Solution-Synthesized Chevron Graphene Nanoribbons Exfoliated onto H:Si(100)

There has been tremendous progress in designing and synthesizing graphene nanoribbons (GNRs). The ability to control the width, edge structure, and dopant level with atomic precision has created a large class of accessible electronic landscapes for use in logic applications. One of the major limitations preventing the realization of GNR devices is the difficulty of transferring GNRs onto nonmetallic substrates. In this work, we developed a new approach for clean deposition of solution-synthesized atomically precise chevron GNRs onto H:Si(100) under ultrahigh vacuum. A clean transfer allowed ultrahigh-vacuum scanning tunneling microscopy (STM) to provide high-resolution imaging and spectroscopy and reveal details of the electronic structure of chevron nanoribbons that have not been previously reported. We also demonstrate STM nanomanipulation of GNRs, characterization of multilayer GNR cross-junctions, and STM nanolithography for local depassivation of H:Si(100), which allowed us to probe GNR-Si interactions and revealed a semiconducting-to-metallic transition. The results of STM measurements were shown to be in good agreement with first-principles computational modeling.

Electronic and Mechanical Properties of Graphene-Germanium Interfaces Grown by Chemical Vapor Deposition

Epitaxially oriented wafer-scale graphene grown directly on semiconducting Ge substrates is of high interest for both fundamental science and electronic device applications. To date, however, this material system remains relatively unexplored structurally and electronically, particularly at the atomic scale. To further understand the nature of the interface between graphene and Ge, we utilize ultrahigh vacuum scanning tunneling microscopy (STM) and scanning tunneling spectroscopy (STS) along with Raman and X-ray photoelectron spectroscopy to probe interfacial atomic structure and chemistry. STS reveals significant differences in electronic interactions between graphene and Ge(110)/Ge(111), which is consistent with a model of stronger interaction on Ge(110) leading to epitaxial growth. Raman spectra indicate that the graphene is considerably strained after growth, with more point-to-point variation on Ge(111). Furthermore, this native strain influences the atomic structure of the interface by inducing metastable and previously unobserved Ge surface reconstructions following annealing. These nonequilibrium reconstructions cover >90% of the surface and, in turn, modify both the electronic and mechanical properties of the graphene overlayer. Finally, graphene on Ge(001) represents the extreme strain case, where graphene drives the reorganization of the Ge surface into [107] facets. From this work, it is clear that the interaction between graphene and the underlying Ge is not only dependent on the substrate crystallographic orientation, but is also tunable and strongly related to the atomic reconfiguration of the graphene-Ge interface.

Intrinsic Defects, Fluctuations of the Local Shape, and the Photo-Oxidation of Black Phosphorus

Black phosphorus is a monatomic semiconducting layered material that degrades exothermically in the presence of light and ambient contaminants. Its degradation dynamics remain largely unknown. Even before degradation, local-probe studies indicate non-negligible local curvature-through a nonconstant height distribution-due to the unavoidable presence of intrinsic defects. We establish that these intrinsic defects are photo-oxidation sites because they lower the chemisorption barrier of ideal black phosphorus (>10 eV and out of visible-range light excitations) right into the visible and ultraviolet range (1.6 to 6.8 eV), thus enabling photoinduced oxidation and dissociation of oxygen dimers. A full characterization of the material's shape and of its electronic properties at the early stages of the oxidation process is presented as well. This study thus provides fundamental insights into the degradation dynamics of this novel layered material.

Ab initio investigation of the electronic properties of graphene on InAs(111)A

The equilibrium geometry and electronic structure of graphene on the most stable In-vacancy InAs(111)A surface has been investigated using the density functional and pseudopotential theories. The equilibrium distance between graphene and InAs(111) is found to be 3.05 Å with adsorption energy approximately 38 meV/C atom. According to our electronic band calculation, there is a re-distribution of the charge density around the graphene sheet, which leads to the development of a dipole moment along the surface normal. Scanning tunnelling microscopy image simulations suggest that the InAs(111) substrate is visible through the graphene layer.

Scanning tunneling microscopy study and nanomanipulation of graphene-coated water on mica

We study interfacial water trapped between a sheet of graphene and a muscovite (mica) surface using Raman spectroscopy and ultrahigh vacuum scanning tunneling microscopy (UHV-STM) at room temperature. We are able to image the graphene-water interface with atomic resolution, revealing a layered network of water trapped underneath the graphene. We identify water layer numbers with a carbon nanotube height reference. Under normal scanning conditions, the water structures remain stable. However, at greater electron energies, we are able to locally manipulate the water using the STM tip.

Inducing electronic changes in graphene through silicon (100) substrate modification

We have performed scanning tunneling microscopy and spectroscopy (STM/STS) measurements as well as ab initio calculations for graphene monolayers on clean and hydrogen(H)-passivated silicon (100) (Si(100)/H) surfaces. In order to experimentally study the same graphene piece on both substrates, we develop a method to depassivate hydrogen from under graphene monolayers on the Si(100)/H surface. Our work represents the first demonstration of successful and reproducible depassivation of hydrogen from beneath monolayer graphene flakes on Si(100)/H by electron-stimulated desorption. Ab initio simulations combined with STS taken before and after hydrogen desorption demonstrate that graphene interacts differently with the clean and H-passivated Si(100) surfaces. The Si(100)/H surface does not perturb the electronic properties of graphene, whereas the interaction between the clean Si(100) surface and graphene changes the electronic states of graphene significantly. This effect results from the covalent bonding between C and surface Si atoms, modifying the π-orbital network of the graphene layer. The local density of states shows that the bonded C and Si surface states are highly disturbed near the Fermi energy.