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

With the rise of 2D materials, new physics and new processing techniques have emerged, triggering possibilities for the innovation of electronic and optoelectronic devices. Among them, ambipolar 2D semiconductors are of excellent gate-controlled capability and distinctive physical characteristic that the major charge carriers can be dynamically, reversibly and rapidly tuned between holes and electrons by electrostatic field. Based on such properties, novel devices, like ambipolar field-effect transistors, light-emitting transistors, electrostatic-field-charging PN diodes, are developed and show great advantages in logic and reconfigurable circuits, integrated optoelectronic circuits, and artificial neural network image sensors, enriching the functions of conventional devices and bringing breakthroughs to build new architectures. This review first focuses on the basic knowledge including fundamental principle of ambipolar semiconductors, basic material preparation techniques, and how to obtain the ambipolar behavior through electrical contact engineering. Then, the current ambipolar 2D semiconductors and their preparation approaches and main properties are summarized. Finally, the emerging new device structures are overviewed in detail, along with their novel electronic and optoelectronic applications. It is expected to shed light on the future development of ambipolar 2D semiconductors, exploring more new devices with novel functions and promoting the applications of 2D materials.

Ambipolar 2D Semiconductors and Emerging Device Applications

Two-dimensional materials have garnered interest from the perspectives of physics, materials, and applied electronics owing to their outstanding physical and chemical properties. Advances in exfoliation and synthesis technologies have enabled preparation and electrical characterization of various atomically thin films of semiconductor transition metal dichalcogenides (TMDs). Their two-dimensional structures and electromagnetic spectra coupled to bandgaps in the visible region indicate their suitability for digital electronics and optoelectronics. To further expand the potential applications of these two-dimensional semiconductor materials, technologies capable of precisely controlling the electrical properties of the material are essential. Doping has been traditionally used to effectively change the electrical and electronic properties of materials through relatively simple processes. To change the electrical properties, substances that can donate or remove electrons are added. Doping of atomically thin two-dimensional semiconductor materials is similar to that used for silicon but has a slightly different mechanism. Three main methods with different characteristics and slightly different principles are generally used. This review presents an overview of various advanced doping techniques based on the substitutional, chemical, and charge transfer molecular doping strategies of graphene and TMDs, which are the representative 2D semiconductor materials.

Recent Advances in Electrical Doping of 2D Semiconductor Materials: Methods, Analyses, and Applications.

A correct understanding of the effects of dopants and the electric field on the metal–insulator transition of VO2 remains a challenge. Herein, theoretical and experimental studies are performed to elucidate the role of W dopants and the electric field on the transition. W dopants are found to introduce additional localized electrons in d bands, which induce the splitting of d// orbitals and the V–V dimerization of local V ions in W-doped R-VO2. The experiments on electric-field-driven MIT of VO2 nanofilms indicate that the conductivity of W-doped R-VO2 increases with increasing applied voltage; however, for the pure R-VO2, the conductivity is independent of applied voltages. The phenomena evidence that the electric field would excite the localized d electrons to the conduction band and result in the closure of the d// orbital splitting. This work gives an insight into electric-field-driven MIT and doping engineering of VO2, which would offer opportunities for improving and promoting the applications of VO...

W Doping and Voltage Driven Metal–Insulator Transition in VO2 Nano-Films for Smart Switching Devices

Van der Waals (vdW) heterostructures, integrated two-dimensional (2D) materials with various functional materials, provide a distinctive platform for next-generation optoelectronics with unique flexibility and high performance. However, exploring the vdW heterostructures combined with strongly correlated electronic materials is hitherto rare. Herein, a novel temperature-sensitive photodetector based on the GaSe/VO2 mixed-dimensional vdW heterostructure is discovered. Compared with previous devices, our photodetector exhibits excellent enhanced performance, with an external quantum efficiency of up to 109.6% and the highest responsivity (358.1 mA·W-1) under a 405 nm laser. Interestingly, we show that the heterostructure overcomes the limitation of a single material under the interaction between VO2 and GaSe, where the photoresponse is highly sensitive to temperature and can be further vanished at the critical value. The metal-insulator transition of VO2, which controls the peculiar band-structure evolution across the heterointerface, is demonstrated to manipulate the photoresponse variation. This study enables us to elucidate the method of manipulating 2D materials by strongly correlated electronic materials, paving the way for developing high-performance and special optoelectronic applications.

Mixed-Dimensional Van der Waals Heterostructure Photodetector.

The physical and chemical properties of transition metal dichalcogenides can be effectively tuned by doping or alloying, which is essential for their practical applications. However, the microstructure evolutions and their effects on the physical properties induced by alloying from hetero-atoms with different outermost electronic structures are still unclear. Here, we synthesized Nb-substituted WS2 with various Nb concentrations showing unusual changes of optical behaviors and continuous electrical polarity reversal. The fully softened Raman mode, rapidly quenched photoluminescence, and severe electron scattering can be attributed to the combined effects of charge doping and lattice strain caused by atomic Nb doping. Three types of substitution modes of Nb atoms in the WS2 lattice were observed directly from atomic-resolution scanning transmission electron microscopy. Density functional theory calculations further confirm the role of lattice strain in the evolutions of optical and electrical characteristics. With increasing Nb concentration, n-type, ambipolar, and p-type field-effect transistors can be achieved, indicating the capacity of this doping method to engineer the properties of two-dimensional materials for future electronic applications.

Transition-Metal Substitution-Induced Lattice Strain and Electrical Polarity Reversal in Monolayer WS2.

Vanadium dioxide (VO2) with a metal–insulator transition (MIT) has been supposed as a candidate for optoelectronic devices. However, the MIT temperature (TMIT) above room temperature limits its application scope. Here, high-quality V1–xWxO2 films have been prepared by pulsed laser deposition. On the basis of temperature-dependent transmittance and Raman spectra, it was found that TMIT increases from 241 to 279 K, when increasing the doping concentration in the range of 0.16 ≤ x ≤ 0.20. The interband electronic transitions and orbital structures of V1–xWxO2 films have been investigated via fitting transmittance spectra. Moreover, with the aid of first-principles calculations, an effective orbital theory has been proposed to explain the unique phenomenon. When the W doping concentration increases, the π* and dII orbitals shift toward the π orbital. Meanwhile, the energy gap between the π* and dII orbitals decreases at the insulator state. It indicates that the bandwidth is narrowed, which impedes MIT. In ad...

Manipulating Behaviors from Heavy Tungsten Doping on Interband Electronic Transition and Orbital Structure Variation of Vanadium Dioxide Films.

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Transition-Metal Substitution-Induced Lattice Strain and Electrical Polarity Reversal in Monolayer WS2.

Manipulating Behaviors from Heavy Tungsten Doping on Interband Electronic Transition and Orbital Structure Variation of Vanadium Dioxide Films.