A novel crystal configuration of sandwiched S-Mo-Se structure (Janus SMoSe) at the monolayer limit has been synthesized and carefully characterized in this work. By controlled sulfurization of monolayer MoSe2 the top layer of selenium atoms are substituted by sulfur atoms while the bottom selenium layer remains intact. The peculiar structure of this new material is systematically investigated by Raman, photoluminescence and X-ray photoelectron spectroscopy and confirmed by transmission-electron microscopy and time-of-flight secondary ion mass spectrometry. Density-functional theory calculations are performed to better understand the Raman vibration modes and electronic structures of the Janus SMoSe monolayer, which are found to correlate well with corresponding experimental results. Finally, high basal plane hydrogen evolution reaction (HER) activity is discovered for the Janus monolayer and DFT calculation implies that the activity originates from the synergistic effect of the intrinsic defects and structural strain inherent in the Janus structure.

Janus Monolayer Transition Metal Dichalcogenides

Inorganic semiconductor nanostructures are ideal systems for exploring a large number of novel phenomena at the nanoscale and investigating the size and dimensionality dependence of their properties for potential applications. The use of such nanostructures with tailored geometries as building blocks is also expected to play crucial roles in future nanodevices. Since the discovery of carbon nanotubes much attention has been paid to exploring the usage of inorganic semiconductor nanostructures as field-emitters due to their low work functions, high aspect ratios and mechanical stabilities, and high electrical and thermal conductivities. This article provides a comprehensive review of the state-of-the-art research activities in the field. It mainly focuses on the most widely studied inorganic nanostructures, such as ZnO, ZnS, Si, WO3, AlN, SiC, and their field-emission properties. We begin with a survey of inorganic semiconductor nanostructures and the field-emission principle, and then discuss the recent progresses on several kinds of important nanostructures and their field-emission characteristics in detail and overview some additional inorganic semiconducting nanomaterials in short. Finally, we conclude this review with some perspectives and outlook on the future developments in this area.

Inorganic semiconductor nanostructures and their field-emission applications

Breaking space-time symmetries in two-dimensional crystals (2D) can dramatically influence their macroscopic electronic properties. Monolayer transition metal dichalcogenides (TMDs) are prime examples where the intrinsically broken crystal inversion symmetry permits the generation of valley-selective electron populations [1–4], even though the two valleys are energetically degenerate, locked by time-reversal symmetry. Lifting the valley degeneracy in these materials is of great interest because it would allow for valley-specific band engineering and offer additional control in valleytronic applications. While applying a magnetic field should in principle accomplish this task, experiments to date have observed no valley-selective energy level shifts in fields accessible in the laboratory. Here we show the first direct evidence of lifted valley degeneracy in the monolayer TMD WS2 [5]. By applying intense circularly polarized light, which breaks time-reversal symmetry, we demonstrate that the exciton level in each valley can be selectively tuned by as much as 18 meV via the optical Stark effect. These results offer a novel way to control valley degree of freedom and may provide a means to realize new valley-selective Floquet topological phases [6–8] in 2D TMDs.

http://absimage.aps.org/image/MAR15/MWS_MAR15-2014-002163.pdf

Valley selective optical Stark effect in monolayer WS2

Applications of the non-line-of-sight vapor deposition techniques, such as chemical vapor deposition (CVD) and atomic layer deposition (ALD), offer unique opportunities to produce well-defined high surface area current collectors, thin films or various nanostructures of active (ion-storage) materials, protective coatings, solid electrolytes and improved separators. These features hold significant promise for solving emerging issues in advanced LiBs and supercapacitors. This paper reviews recent developments and applications of CVD and ALD for these energy storage devices, providing selected examples and outlining critical challenges for further exploration.

Chemical vapor deposition and atomic layer deposition for advanced lithium ion batteries and supercapacitors

Semiconductor nanowires have attracted extensive interest as one of the best-defined classes of nanoscale building blocks for the bottom-up assembly of functional electronic and optoelectronic devices over the past two decades. The article provides a comprehensive review of the continuing efforts in exploring semiconductor nanowires for the assembly of functional nanoscale electronics and macroelectronics. Specifically, we start with a brief overview of the synthetic control of various semiconductor nanowires and nanowire heterostructures with precisely controlled physical dimension, chemical composition, heterostructure interface, and electronic properties to define the material foundation for nanowire electronics. We then summarize a series of assembly strategies developed for creating well-ordered nanowire arrays with controlled spatial position, orientation, and density, which are essential for constructing increasingly complex electronic devices and circuits from synthetic semiconductor nanowires. Next, we review the fundamental electronic properties and various single nanowire transistor concepts. Combining the designable electronic properties and controllable assembly approaches, we then discuss a series of nanoscale devices and integrated circuits assembled from nanowire building blocks, as well as a unique design of solution-processable nanowire thin-film transistors for high-performance large-area flexible electronics. Last, we conclude with a brief perspective on the standing challenges and future opportunities.

Nanowire Electronics: From Nanoscale to Macroscale

Electronic and optical properties of van der Waals heterostructures of g-GaN and transition metal dichalcogenides

Electronic and optical properties of titanium-doped GaN nanowires

Optoelectronic properties of pristine and vacancy defect monolayer tungsten diselenide (WSe2) have been investigated by the first principles calculations. The results predicate that Se defect monolayer WSe2 is direct semiconductor whereas the W defect monolayer WSe2 is metallic. The Se defect can decrease the work function for monolayer WSe2, however, the W defect can increase the work function for monolayer WSe2. The absorption edge for defect monolayer WSe2 occurs obviously red-shift, and the energy loss of electron transmitting in defect monolayer WSe2 is faster than pristine monolayer WSe2. The work gives a theoretical guidance for the fabrication of monolayer WSe2 optoelectronic nanodevices.

Effect of vacancy defect on optoelectronic properties of monolayer tungsten diselenide

The stability, electronic, and optical behaviours of XSSe/SiC (X = Mo, W) vdWs heterostructures have been investigated by first-principles calculations. The observed XSSe/SiC vdWs heterostructure is semiconductor with indirect bandgap, and it possesses type-II band alignment, which will promote the segregation of photogenerated carriers. In particular, the charge reallocation cause a large potential drop across XSSe/SiC vdWs heterostructure is 5.748 eV (MoSSe/SiC) and 5.849 eV (WSSe/SiC). Charge transfer demonstrates that the XSSe acts as an acceptor, while the SiC acts as a donor. Moreover, the tensile and compressive strain can regulate the band structures of the XSSe/SiC heterostructures from direct band gap to indirect band gap. Meanwhile, the vertical electric field can not change the band type of the XSSe/SiC heterostructures, but can regulate the band gap of the XSSe/SiC heterostructures. Importantly, the XSSe/SiC vdWs heterostructures have many strong peaks in the visible light region, resulting to efficient use of the solar energy. These attractive properties illustrate that the XSSe/SiC vdWs heterostructures are useful for attaining high-efficiency photocatalyst.

Zhen Cui

Papers

Electronic and optical properties of van der Waals heterostructures of g-GaN and transition metal dichalcogenides

Electronic and optical properties of titanium-doped GaN nanowires

Effect of vacancy defect on optoelectronic properties of monolayer tungsten diselenide

Janus XSSe/SiC (X = Mo, W) van der Waals heterostructures as promising water-splitting photocatalysts

Adsorption of CO, NH3, NO, and NO2 on pristine and defective g-GaN: Improved gas sensing and functionalization