Analyzing Dirac Cone and Phonon Dispersion in Highly Oriented Nanocrystalline Graphene

Chemical vapor deposition (CVD) is one of the most promising growth techniques to scale up the production of monolayer graphene. At present, there are intense efforts to control the orientation of graphene grains during CVD, motivated by the fact that there is a higher probability for oriented grains to achieve seamless merging, forming a large single crystal. However, it is still challenging to produce single-crystal graphene with no grain boundaries over macroscopic length scales, especially when the nucleation density of graphene nuclei is high. Nonetheless, nanocrystalline graphene with highly oriented grains may exhibit single-crystal-like properties. Herein, we investigate the spectroscopic signatures of graphene film containing highly oriented, nanosized grains (20–150 nm) using angle-resolved photoemission spectroscopy (ARPES) and high-resolution electron energy loss spectroscopy (HREELS). The robustness of the Dirac cone, as well as dispersion of its phonons, as a function of graphene’s grain size and before and after film coalescence, was investigated. In view of the sensitivity of atomically thin graphene to atmospheric adsorbates and intercalants, ARPES and HREELS were also used to monitor the changes in spectroscopic signatures of the graphene film following exposure to the ambient atmosphere

Highly Active and Selective Zr/MCF Catalyst for Production of 1,3-Butadiene from Ethanol in a Dual Fixed Bed Reactor System

Copper and zirconium oxide clusters were highly dispersed on mesocellular siliceous foam (MCF), a mesoporous silica support with ultra large, interconnected nanopores. These catalysts (denoted as Cu/MCF and Zr/MCF) were separately loaded into two fixed bed reactors as catalysts for the conversion of ethanol (EtOH) to 1,3-butadiene (BD). Under optimal conditions, high BD selectivity (up to 73%) and ethanol conversion (up to 96%) were achieved at weight hourly space velocities of 1.5 and 3.7 h^–1. This translates to an unprecedented productivity of 1.4 g_BD/g_catalyst h^–1 (208 g_BD/l_catalyst h^–1). The high catalytic performance is attributed to the highly selective and active catalysts. The EtOH dehydrogenation activity of Cu/MCF could be accurately controlled in the first reactor, which delivers a fixed ratio of the acetaldehyde/EtOH mixture to Zr/MCF in the second reactor. The optimal ratio minimizes EtOH dehydration to ethylene by Zr/MCF, while maximizing the selectivity to BD. MCF was found to be superior over commercial porous silica in terms of EtOH conversion, BD selectivity, and tolerance to coking. High BD selectivity was maintained with a slight decrease in EtOH conversion over 42 h, which was readily restored upon regeneration by thermal treatment in air

Gate-Tunable Giant Stark Effect in Few-Layer Black Phosphorus

Two-dimensional black phosphorus (BP) has sparked enormous research interest due to its high carrier mobility, layer-dependent direct bandgap and outstanding in-plane anisotropic properties. BP is one of the few two-dimensional materials where it is possible to tune the bandgap over a wide energy range from the visible up to the infrared. In this article, we report the observation of a giant Stark effect in electrostatically gated few-layer BP. Using low-temperature scanning tunnelling microscopy, we observed that in few-layer BP, when electrons are injected, a monotonic reduction of the bandgap occurs. The injected electrons compensate the existing defect-induced holes and achieve up to 35.5% bandgap modulation in the light-doping regime. When probed by tunnelling spectroscopy, the local density of states in few-layer BP shows characteristic resonance features arising from layer-dependent sub-band structures due to quantum confinement effects. The demonstration of an electrical gate-controlled giant Stark effect in BP paves the way to designing electro-optic modulators and photodetector devices that can be operated in a wide electromagnetic spectral range

Resolving the Spatial Structures of Bound Hole States in Black Phosphorus

Understanding the local electronic properties of individual defects and dopants in black phosphorus (BP) is of great importance for both fundamental research and technological applications. Here, we employ low-temperature scanning tunnelling microscope (LT-STM) to probe the local electronic structures of single acceptors in BP. We demonstrate that the charge state of individual acceptors can be reversibly switched by controlling the tip-induced band bending. In addition, acceptor-related resonance features in the tunnelling spectra can be attributed to the formation of Rydberg-like bound hole states. The spatial mapping of the quantum bound states shows two distinct shapes evolving from an extended ellipse shape for the 1s ground state to a dumbbell shape for the 2p_<i>x</i> excited state. The wave functions of bound hole states can be well-described using the hydrogen-like model with anisotropic effective mass, corroborated by our theoretical calculations. Our findings not only provide new insight into the many-body interactions around single dopants in this anisotropic two-dimensional material but also pave the way to the design of novel quantum devices

Surface Functionalization of Black Phosphorus via Potassium toward High-Performance Complementary Devices

Two-dimensional black phosphorus configured field-effect transistor devices generally show a hole-dominated ambipolar transport characteristic, thereby limiting its applications in complementary electronics. Herein, we demonstrate an effective surface functionalization scheme on few-layer black phosphorus, through in situ surface modification with potassium, with a view toward high performance complementary device applications. Potassium induces a giant electron doping effect on black phosphorus along with a clear bandgap reduction, which is further corroborated by in situ photoelectron spectroscopy characterizations. The electron mobility of black phosphorus is significantly enhanced to 262 (377) cm² V^–1 s^–1 by over 1 order of magnitude after potassium modification for two-terminal (four-terminal) measurements. Using lithography technique, a spatially controlled potassium doping technique is developed to establish high-performance complementary devices on a single black phosphorus nanosheet, for example, the p–n homojunction-based diode achieves a near-unity ideality factor of 1.007 with an on/off ratio of ∼10⁴. Our findings coupled with the tunable nature of in situ modification scheme enable black phosphorus as a promising candidate for further complementary electronics

Chemical Vapor Deposition of Large-Size Monolayer MoSe₂ Crystals on Molten Glass

We report the fast growth of high-quality millimeter-size monolayer MoSe₂ crystals on molten glass using an ambient pressure CVD system. We found that the isotropic surface of molten glass suppresses nucleation events and greatly improves the growth of large crystalline domains. Triangular monolayer MoSe₂ crystals with sizes reaching ∼2.5 mm, and with a room-temperature carrier mobility up to ∼95 cm²/(V·s), can be synthesized in 5 min. The method can also be used to synthesize millimeter-size monolayer MoS₂ crystals. Our results demonstrate that “liquid-state” glass is a highly promising substrate for the low-cost growth of high-quality large-size 2D transition metal dichalcogenides (TMDs)

Mo-Terminated Edge Reconstructions in Nanoporous Molybdenum Disulfide Film

The catalytic and magnetic properties of molybdenum disulfide (MoS₂) are significantly enhanced by the presence of edge sites. One way to obtain a high density of edge sites in a two-dimensional (2D) film is by introducing porosity. However, the large-scale bottom-up synthesis of a porous 2D MoS₂ film remains challenging and the correlation of growth conditions to the atomic structures of the edges is not well understood. Here, using molecular beam epitaxy, we prepare wafer-scale nanoporous MoS₂ films under conditions of high Mo flux and study their catalytic and magnetic properties. Atomic-resolution electron microscopy imaging of the pores reveals two new types of reconstructed Mo-terminated edges, namely, a distorted 1T (DT) edge and the Mo-Klein edge. Nanoporous MoS₂ films are magnetic up to 400 K, which is attributed to the presence of Mo-terminated edges with unpaired electrons, as confirmed by density functional theory calculation. The small hydrogen adsorption free energy at these Mo-terminated edges leads to excellent activity for the hydrogen evolution reaction