Transition‐Metal Dichalcogenide NiTe2: An Ambient‐Stable Material for Catalysis and Nanoelectronics

By means of theory and experiments, the application capability of nickel ditelluride (NiTe₂) transition‐metal dichalcogenide in catalysis and nanoelectronics is assessed. The Te surface termination forms a TeO₂ skin in an oxygen environment. In ambient atmosphere, passivation is achieved in less than 30 min with the TeO₂ skin having a thickness of about 7 Å. NiTe₂ shows outstanding tolerance to CO exposure and stability in water environment, with subsequent good performance in both hydrogen and oxygen evolution reactions. NiTe₂‐based devices consistently demonstrate superb ambient stability over a timescale as long as one month. Specifically, NiTe₂ has been implemented in a device that exhibits both superior performance and environmental stability at frequencies above 40 GHz, with possible applications as a receiver beyond the cutoff frequency of a nanotransistor.     

Recent Developments in One‐Dimensional Inorganic Nanostructures for Photodetectors

With large surface‐to‐volume ratios and Debye length comparable to their small sizes, one‐dimensional inorganic nanostructures have extensively been investigated and widely used to fabricate high‐performance nano­scale electronic and optoelectronic devices. This feature article reviews the state‐of‐the‐art research activities that focus on the one‐dimensional inorganic nanostructures and their photodetector applications. It begins with a survey of one‐dimensional inorganic nanostructures and the fundamentals of photodetectors. Some remarkable photoresponse characteristics are then presented, which are organized into sections covering several kinds of important nanostructures, such as ZnO, V₂O₅, ZnS, In₂Se₃, InSe, CdS, CdSe, ZnSe, Sb₂Se₃, ZrS₂, Ag₂S, and Zn_xCd_1‐xSe. Each section describes the corresponding photodetective properties in detail. Finally, the article concludes with some perspectives and outlook on the future developments in the field.     

Recent Progress on 2D Noble‐Transition‐Metal Dichalcogenides

Emerging classes of 2D noble‐transition‐metal dichalcogenides (NTMDs) stand out for their unique structure and novel physical properties in recent years. With the nearly full occupation of the d orbitals, 2D NTMDs are expected to be more attractive due to the unique interlayer vibrational behaviors and largely tunable electronic structures compared to most transition metal dichalcogenide semiconductors. The novel properties of 2D NTMDs have stimulated various applications in electronics, optoelectronics, catalysis, and sensors. Here, the latest development of 2D NTMDs are reviewed from the perspective of structure characterization, preparation, and application. Based on the recent research, the conclusions and outlook for these rising 2D NTMDs are presented.     

Two‐Dimensional Molybdenum Trioxide and Dichalcogenides

In the quest to discover the properties of planar semiconductors, two‐dimensional molybdenum trioxide and dichalcogenides have recently attracted a large amount of interest. This family, which includes molybdenum trioxide (MoO₃), disulphide (MoS₂), diselenide (MoSe₂) and ditelluride (MoTe₂), possesses many unique properties that make its compounds appealing for a wide range of applications. These properties can be thickness dependent and may be manipulated via a large number of physical and chemical processes. In this Feature Article, a comprehensive review is delivered of the fundamental properties, synthesis techniques and applications of layered and planar MoO₃, MoS₂, MoSe₂, and MoTe₂ along with their future prospects.     

Tailoring the Surface Chemical Reactivity of Transition‐Metal Dichalcogenide PtTe2 Crystals

PtTe₂ is a novel transition‐metal dichalcogenide hosting type‐II Dirac fermions that displays application capabilities in optoelectronics and hydrogen evolution reaction. Here it is shown, by combining surface science experiments and density functional theory, that the pristine surface of PtTe₂ is chemically inert toward the most common ambient gases (oxygen and water) and even in air. It is demonstrated that the creation of Te vacancies leads to the appearance of tellurium‐oxide phases upon exposing defected PtTe₂ surfaces to oxygen or ambient atmosphere, which is detrimental for the ambient stability of uncapped PtTe₂‐based devices. On the contrary, in PtTe₂ surfaces modified by the joint presence of Te vacancies and substitutional carbon atoms, the stable adsorption of hydroxyl groups is observed, an essential step for water splitting and the water–gas shift reaction. These results thus pave the way toward the exploitation of this class of Dirac materials in catalysis.     

Highly Periodic Metal Dichalcogenide Nanostructures with Complex Shapes,High Resolution,and High Aspect Ratios

The development of high resolution, high aspect ratio metal dichalcogenide nanostructures is one of the most important issues in 2D material researchers due to the potential to exploit their properties into high performance devices. In this study, for the first time a way of fabricating metal dichalcogenide nanostructures with high resolution (<50 nm scale) and high aspect ratios (>120) by chemical vapor deposition assisted secondary sputtering phenomenon is reported. This approach can universally synthesize various types of metal dichalcogenides including MoS₂, WS₂, and SnS₂, implying the possibility for further utilization with selenides and tellurides. Also, this method can produce highly periodic complex patterns such as hole–cylinder, concentric rings, and line patterns, which are unprecedented in previous reports. The feature size and aspect ratio of the metal dichalcogenide structures can be manipulated by controlling the dimensions of the photoresist prepatterns, while the pattern resolution and layer orientation can be manipulated by controlling the thickness of the deposited metal film. It is demonstrated that nanostructures with high resolution and high aspect ratio significantly improve gas‐sensing properties compared with previous metal dichalcogenide films. It is believed that the method can be a foundation for synthesizing various materials with complex patterns for future applications.     

PdTe2 Transition‐Metal Dichalcogenide: Chemical Reactivity,Thermal Stability,and Device Implementation

Palladium ditelluride (PdTe₂) is a novel transition‐metal dichalcogenide exhibiting type‐II Dirac fermions and topological superconductivity. To assess its potential in technology, its chemical and thermal stability is investigated by means of surface‐science techniques, complemented by density functional theory, with successive implementation in electronics, specifically in a millimeter‐wave receiver. While water adsorption is energetically unfavorable at room temperature, due to a differential Gibbs free energy of ≈+12 kJ mol^?1, the presence of Te vacancies makes PdTe₂ surfaces unstable toward surface oxidation with the emergence of a TeO₂ skin, whose thickness remains sub‐nanometric even after one year in air. Correspondingly, the measured photocurrent of PdTe₂‐based optoelectronic devices shows negligible changes (below 4%) in a timescale of one month, thus excluding the need of encapsulation in the nanofabrication process. Remarkably, the responsivity of a PdTe₂‐based millimeter‐wave receiver is 13 and 21 times higher than similar devices based on black phosphorus and graphene in the same operational conditions, respectively. It is also discovered that pristine PdTe₂ is thermally stable in a temperature range extending even above 500 K, thus paving the way toward PdTe₂‐based high‐temperature electronics. Finally, it is shown that the TeO₂ skin, formed upon air exposure, can be removed by thermal reduction via heating in vacuum.     

Periodical Ripening for MOCVD Growth of Large 2D Transition Metal Dichalcogenide Domains

2D semiconductors, especially 2D transition metal dichalcogenides (TMDCs), have attracted ever-growing attention toward extending Moore's law beyond silicon. Metal–organic chemical vapor deposition (MOCVD) has been widely considered as a scalable technique to achieve wafer-scale TMDC films for applications. However, current MOCVD process usually suffers from small domain size with only hundreds of nanometers, in which dense grain boundary defects degrade the crystalline quality of the films. Here, a periodical varying-temperature ripening (PVTR) process is demonstrated to grow wafer-scale high crystalline TMDC films by MOCVD. It is found that the high-temperature ripening significantly reduces the nucleation density and therefore enables single-crystal domain size over 20 µm. In this process, no additives or etchants are involved, which facilitates low impurity concentration in the grown films. Atom-resolved electron microscopy imaging, variable temperature photoluminescence (PL) spectroscopy, and electrical transport results further confirm comparable crystalline quality to those observed in mechanically exfoliated TMDC films. The research provides a scalable route to produce high-quality 2D semiconducting films for applications in electronics and optoelectronics.     

Rational Design on Wrinkle-Less Transfer of Transition Metal Dichalcogenide Monolayer by Adjustable Wettability-Assisted Transfer Method

Transfer-induced wrinkles are universal issues when transferring transition metal dichalcogenide (TMDC) monolayer from an as-grown substrate to a target substrate. The undesired transfer-induced wrinkles can mainly be attributed to wettability, which refers to the ability of a liquid to come in contact with a solid surface. Herein, an adjustable wettability-assisted transfer (AWAT) method with different mixtures of transfer media to reduce the density of wrinkles is developed. By manipulating the wettability of the transfer medium with different ratios of alcohol and de-ionized (DI) water, the TMDC monolayer is smoothly attached to the target substrate, thus achieving a wrinkle-less transferred TMDC monolayer. With this method, the density of wrinkles can be decreased by ≈ 15–20% compared with the conventional transfer method by pure DI water. The transferred MoS₂ monolayer with the AWAT method can achieve enhanced carrier mobility from ≈ 20 to ≈ 35 cm² V⁻¹ s⁻¹ in average, which is 30 times larger than that transferred by pure DI water. The AWAT method applied to a WS₂ monolayer onto a SiO₂/p⁺-Si substrate and a MoS₂ monolayer onto a HfO₂/p⁺-Si substrate are demonstrated, which is beneficial in research and applications involving the transfer of TMDC monolayer.     

One‐Dimensional Metal‐Oxide Nanostructures: Recent Developments in Synthesis,Characterization, and Applications

1D metal‐oxide nanostructures have attracted much attention because metal oxides are the most fascinating functional materials. The 1D morphologies can easily enhance the unique properties of the metal‐oxide nanostructures, which make them suitable for a wide variety of applications, including gas sensors, electrochromic devices, light‐emitting diodes, field emitters, supercapacitors, nanoelectronics, and nanogenerators. Therefore, much effort has been made to synthesize and characterize 1D metal‐oxide nanostructures in the forms of nanorods, nanowires, nanotubes, nanobelts, etc. Various physical and chemical deposition techniques and growth mechanisms are exploited and developed to control the morphology, identical shape, uniform size, perfect crystalline structure, defects, and homogenous stoichiometry of the 1D metal‐oxide nanostructures. Here a comprehensive review of recent developments in novel synthesis, exceptional characteristics, and prominent applications of one‐dimensional nanostructures of tungsten oxides, molybdenum oxides, tantalum oxides, vanadium oxides, niobium oxides, titanium oxides, nickel oxides, zinc oxides, bismuth oxides, and tin oxides is provided.     

Two‐Dimensional Nanostructured Materials for Gas Sensing

Two‐dimensional (2D) nanostructures are highly attractive for fabricating nanodevices due to their high surface‐to‐volume ratio and good compatibility with device design. In recent years 2D nanostructures of various materials including metal oxides, graphene, metal dichalcogenides, phosphorene, BN and MXenes, have demonstrated significant potential for gas sensors. This review aims to provide the most recent advancements in utilization of various 2D nanomaterials for gas sensing. The common methods for the preparation of 2D nanostructures are briefly summarized first. The focus is then placed on the sensing performances provided by devices integrating 2D nanostructures. Strategies for optimizing the sensing features are also discussed. By combining both the experimental results and the theoretical studies available, structure‐properties correlations are discussed. The conclusion gives some perspectives on the open challenges and future prospects for engineering advanced 2D nanostructures for high‐performance gas sensors devices.     

Performances of Liquid‐Exfoliated Transition Metal Dichalcogenides as Hole Injection Layers in Organic Light‐Emitting Diodes

2D transition metal dichalcogenide (TMD) nanosheets, including MoS₂, WS₂, and TaS₂, are used as hole injection layers (HILs) in organic light‐emitting diodes (OLEDs). MoS₂, WS₂, and TaS₂ nanosheets are prepared using an exfoliation by ultrasonication method. The thicknesses and sizes of the TMD nanosheets are measured to be 3.1–4.3 nm and more than 100 nm, respectively. The work functions of the TMD nanosheets increase from 4.4–4.9 to 4.9–5.1 eV following ultraviolet/ozone (UVO) treatment. The turn‐on voltages at 10 cd m^?2 for UVO‐treated TMD‐based devices decrease from 7.3–12.8 to 4.3–4.4 V and maximum luminance efficiencies increase from 5.74–9.04 to 12.01–12.66 cd A^?1. In addition, this study confirms that the stabilities of the devices in air can be prolonged by using UVO‐treated TMDs as HILs in OLEDs. These results demonstrate the great potential of liquid‐exfoliated TMD nanosheets for use as HILs in OLEDs.     

High‐Performance Transition Metal Dichalcogenide Photodetectors Enhanced by Self‐Assembled Monolayer Doping

Most doping research into transition metal dichalcogenides (TMDs) has been mainly focused on the improvement of electronic device performance. Here, the effect of self‐assembled monolayer (SAM)‐based doping on the performance of WSe₂‐ and MoS₂‐based transistors and photodetectors is investigated. The achieved doping concentrations are ≈1.4 × 10¹¹ for octadecyltrichlorosilane (OTS) p‐doping and ≈10¹¹ for aminopropyltriethoxysilane (APTES) n‐doping (nondegenerate). Using this SAM doping technique, the field‐effect mobility is increased from 32.58 to 168.9 cm² V^?1 s in OTS/WSe₂ transistors and from 28.75 to 142.2 cm² V^?1 s in APTES/MoS₂ transistors. For the photodetectors, the responsivity is improved by a factor of ≈28.2 (from 517.2 to 1.45 × 10⁴ A W^?1) in the OTS/WSe₂ devices and by a factor of ≈26.4 (from 219 to 5.75 × 10³ A W^?1) in the APTES/MoS₂ devices. The enhanced photoresponsivity values are much higher than that of the previously reported TMD photodetectors. The detectivity enhancement is ≈26.6‐fold in the OTS/WSe₂ devices and ≈24.5‐fold in the APTES/MoS₂ devices and is caused by the increased photocurrent and maintained dark current after doping. The optoelectronic performance is also investigated with different optical powers and the air‐exposure times. This doping study performed on TMD devices will play a significant role for optimizing the performance of future TMD‐based electronic/optoelectronic applications.     

Potential-Driven Semiconductor-to-Metal Transition in Monolayer Transition Metal Dichalcogenides

The potential-driven semiconductor-to-metal transition is investigated in monolayer transition metal dichalcogenides by employing a new proposed method, i.e., the fixed-potential method (FPM). Under the same voltage, the semiconducting and metallic phases will be charged differently due to their different electronic properties. The potential-driven phase transition process is simulated by the injection of unequal electrons in the semiconducting and metallic phases. The unequal electron injection is more consistent with the actual experimental process, although equal electron injection also can theoretically induce a phase transition. MoTe₂ is chosen as a prototypical example to examine the physical mechanism. When the fixed electrode potential is above the potential of zero-charge, excess electrons are injected into the metallic 1T’ phase instead of the semiconducting 2H phase, stabilizing the 1T’ phase. In addition, the potential-dependent kinetics, in which the charge transfer is fluctuating, suggests that increasing the electrode potential will decrease the kinetic barrier of the 2H→1T’ transition process. The calculated relative transition voltage of 2.5 V agrees well with the experimental results, demonstrating the validity of the FPM. This study provides new insight into potential-driven semiconductor-to-metal phase transitions and suggests a new theoretical approach for studies under constant voltage conditions.     

Chemical Vapor Deposition Syntheses of Wafer-Scale 2D Transition Metal Dichalcogenide Films toward Next-Generation Integrated Circuits Related Applications

2D semiconducting transition metal dichalcogenides (TMDCs), most with a formula of MX₂ (M=Mo, W; X=S, Se, etc.), have emerged as promising channel materials for next-generation integrated circuits, considering their dangling-bond-free surfaces, moderate bandgaps, relatively high carrier mobilities, etc. Wafer-scale preparation of 2D MX₂ films holds fundamental significance for realizing their applications. Chemical vapor deposition (CVD) is recognized as the most promising method for preparing electronic-grade 2D MX₂ films. This review hereby summarizes the recent progress in CVD syntheses of wafer-scale 2D MX₂ films and their applications in logic operations, data storage, and image capturing/processing related fields. The first part focuses on the wafer-scale syntheses of 2D MX₂ films through designing homogeneous metal precursor supply routes (e.g., precoating soluble precursor, feeding gaseous precursor, designing independent multisource supply or face-to-face metal precursor supply routes). The second part highlights the epitaxial growth of monolayer MX₂ single crystals on single-crystal Au substrates and well-designed sapphire substrates. The third part introduces various functional device/circuit related applications of CVD-derived 2D MX₂ wafers. Finally, challenges and prospects are discussed from the viewpoints of the controlled synthesis, reliable characterization, and damage-free transfer of 2D MX₂, as well as the fabrication and integration of high-performance devices.     

Novel Electronic and Magnetic Properties of Two‐Dimensional Transition Metal Carbides and Nitrides

Layered MAX phases are exfoliated into 2D single layers and multilayers, so‐called MXenes. Using first‐principles calculations, the formation and electronic properties of various MXene systems, M₂C (M = Sc, Ti, V, Cr, Zr, Nb, Ta) and M₂N (M = Ti, Cr, Zr) with surfaces chemically functionalized by F, OH, and O groups, are examined. Upon appropriate surface functionalization, Sc₂C, Ti₂C, Zr₂C, and Hf₂C MXenes are expected to become semiconductors. It is also derived theoretically that functionalized Cr₂C and Cr₂N MXenes are magnetic. Thermoelectric calculations based on the Boltzmann theory imply that semiconducting MXenes attain very large Seebeck coefficients at low temperatures.     

Controlled Fabrication of Multitiered Three‐Dimensional Nanostructures in Porous Alumina

We present the fabrication of multitiered branched porous anodic alumina (PAA) substrates consisting of an array of pores branching into smaller pores in succeeding tiers. The tiered three‐dimensional structure is realized by sequentially stepping down the anodization potential while etching of the barrier layer is performed after each step. We establish the key processing parameters that define the tiered porous structure through systematically designed experiments. The characterization of the branched PAA structures reveals that, owing to constriction, the ratio of interpore distance to the anodization potential is smaller than that for pristine films. This ratio varies from 1.8 to 1.3 nm V^?1 depending on the size of the preceding pores and the succeeding tier anodization potential. Contact angle measurements show that the multitiered branched PAA structures exhibit a marked increased in hydrophilicity over two‐dimensional PAA films.     

Laser‐Assisted Chemical Modification of Monolayer Transition Metal Dichalcogenides

Laser‐assisted chemical modification is demonstrated on ultrathin transition‐metal dichalcogenides (TMDs), locally replacing selenium by sulfur atoms. The photoconversion process takes place in a controlled reactive gas environment and the heterogeneous reaction rates are monitored via in situ real‐time Raman and photoluminescence spectroscopies. The spatially localized photoconversion results in a heterogeneous TMD structure, with chemically distinct domains, where the initial high crystalline quality of the film is not affected during the process. This has been further confirmed via transmission electron microscopy as well as Raman and photoluminescence spatial maps. This study demonstrates the potential of laser‐assisted chemical conversion for on‐demand synthesis of heterogeneous 2D materials with applications in nanodevices.     

Few‐Layered WS2 Nanoplates Confined in Co,N‐Doped Hollow Carbon Nanocages: Abundant WS2 Edges for Highly Sensitive Gas Sensors

Edges of 2D transition metal dichalcogenides (TMDs) are well known as highly reactive sites, thus researchers have attempted to maximize the edge site density of 2D TMDs. In this work, metal‐organic framework (MOF) templates are introduced to synthesize few‐layered WS₂ nanoplates (a lateral dimension of ≈10 nm) confined in Co, N‐doped hollow carbon nanocages (WS₂_Co‐N‐HCNCs), for highly sensitive NO₂ gas sensors. WS₂ precursors are assembled in the surface cavity of Co‐based zeolite imidazole framework (ZIF‐67) and subsequent pyrolysis produced WS₂_Co‐N‐HCNCs. During the pyrolysis, the carbonized ZIF‐67 are doped by Co and N elements, and the growth of WS₂ is effectively suppressed, creating few‐layered WS₂ nanoplates functionalized Co‐N‐HCNCs. The WS₂_Co‐N‐HCNCs exhibit outstanding NO₂ sensing characteristics at room temperature, in terms of response (48.2% to 5 ppm), selectivity, response and recovery speed, and detection limit (100 ppb). These results are attributed to the enhanced adsorption and desorption kinetics of NO₂ on abundant WS₂ edges, confined in the gas permeable HCNCs. This work opens up an efficient way for the facile synthesis of edge abundant few‐layered TMDs combined with porous carbon matrix via MOF templating route, for applications relying on highly active sites.