Synthesis of large-scale atomic-layer SnS<Subscript>2</Subscript> through chemical vapor deposition

Two-dimensional layers of metal dichalcogenides have attracted much attention because of their ultrathin thickness and potential applications in electronics and optoelectronics.Monolayer SnS2,with a band gap of ～2.6 eV,has an octahedral lattice made of two atomic layers of sulfur and one atomic layer of tin.Till date,there have been limited reports on the growth of large-scale and high quality SnS2 atomic layers and the investigation of their properties as a semiconductor.Here,we report the chemical vapor deposition (CVD) growth of atomic-layer SnS2 with a large crystal size and uniformity.In addition,the number of layers can be changed from a monolayer to few layers and to bulk by changing the growth time.Scanning transmission electron microscopy was used to analyze the atomic structure and demonstrate the 2H stacking poly-type of different layers.The resultant SnS2 crystals is used as a photodetector with external quantum efficiency as high as 150％,suggesting promise for optoelectronic applications.     

Composition-Controllable Syntheses and Property Modulations from 2D Ferromagnetic Fe5Se8 to Metallic Fe3Se4 Nanosheets

Exploring new-type 2D magnetic materials with high magnetic transition temperature and robust air stability has attracted wide attention for developing innovative spintronic devices. Recently, intercalation of native metal atoms into the van der Waals gaps of 2D layered transition metal dichalcogenides (TMDs) has been developed to form 2D non-layered magnetic TMDs, while only succeeded in limited systems (e.g., Cr₂S₃, Cr₅Te₈). Herein, composition-controllable syntheses of 2D non-layered iron selenide nanosheets (25% Fe-intercalated triclinic Fe₅Se₈ and 50% Fe-intercalated monoclinic Fe₃Se₄) are firstly reported, via a robust chemical vapor deposition strategy. Specifically, the 2D Fe₅Se₈ exhibits intrinsic room-temperature ferromagnetic property, which is explained by the change of electron spin states from layered 1T'-FeSe₂ to non-layered Fe-intercalated Fe₅Se₈ based on density functional theory calculations. In contrast, the ultrathin Fe₃Se₄ presents novel metallic features comparable with that of metallic TMDs. This work hereby sheds light on the composition-controllable synthesis and fundamental property exploration of 2D self-intercalation induced novel TMDs compounds, by propelling their application explorations in nanoelectronics and spintronics-related fields.     

Potential 2D Materials with Phase Transitions: Structure,Synthesis, and Device Applications

Layered materials with phase transitions, such as charge density wave (CDW) and magnetic and dipole ordering, have potential to be exfoliated into monolayers and few‐layers and then become a large and important subfamily of two‐dimensional (2D) materials. Benefitting from enriched physical properties from the collective interactions, long‐range ordering, and related phase transitions, as well as the atomic thickness yet having nondangling bonds on the surface, 2D phase‐transition materials have vast potential for use in new‐concept and functional devices. Here, potential 2D phase‐transition materials with CDWs and magnetic and dipole ordering, including transition metal dichalcogenides, transition metal halides, metal thio/selenophosphates, chromium silicon/germanium tellurides, and more, are introduced. The structures and experimental phase‐transition properties are summarized for the bulk materials and some of the obtained monolayers. In addition, recent experimental progress on the synthesis and measurement of monolayers, such as 1T‐TaS₂, CrI₃, and Cr₂Ge₂Te₆, is reviewed.     

On the oxidation of VS2 2D platelets using tip-enhanced Raman spectroscopy

2D materials are enabling disruptive advancements in electronic and photonic devices yielding to the development of sensing and wearable materials and in the field of energy production and storage as key components of photovoltaic technology and batteries. Nevertheless, little attention has been paid to TMDs and oxides that contain vanadium, as it is the case of vanadium disulfide (VS₂) and vanadium dioxide (VO₂). In this study we review the synthesis and characterization using Raman spectroscopy of VS₂ and its oxidized states. Laser-induced oxidation occurring during the Raman experiments in ambient conditions is described and plateau values of laser power levels to induce oxidation are provided. Furthermore, tip-enhanced Raman spectroscopy (TERS) spectra and maps are conducted to reveal at the single flake level the onset of oxidation mechanisms at the surface of the 2D platelets.     

Thickness and temperature dependent electrical properties of ZrS<Subscript>2</Subscript> thin films directly grown on hexagonal boron nitride

Two-dimensional ZrS₂ materials have potential for applications in nanoelectronics because of their theoretically predicted high mobility and sheet current density. Herein, we report the thickness and temperature dependent transport properties of ZrS₂ multilayers that were directly deposited on hexagonal boron nitride (h-BN) by chemical vapor deposition. Hysteresis-free gate sweeping, metalinsulator transition, and T ^–γ (γ ~ 0.82–1.26) temperature dependent mobility were observed in the ZrS₂ films.

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Solvothermal synthesis of hexagonal ZnO nanorods and their photoluminescence properties

Pure hexagonal ZnO nanorods were synthesized by low-temperature (90 °C) solvothermal treatment of zinc acetate in 40-80 wt.% hydrazine hydrate aqueous solutions. The products were characterized by means of powder X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electronic microscopy (TEM), selected area electron diffraction (SAED), and room temperature photoluminescence (RTPL) spectra. They show a strong UV emission at around 380 nm upon excitation at 360 nm using a Xe lamp at room temperature. The influence on the quality of the nanorods was investigated while the content of the solvent changed. The as-synthesized ZnO nanorods are promising materials for nanoscale optoelectronic devices due to their excellent UV emission properties.     

Recent Progress in CVD Growth of 2D Transition Metal Dichalcogenides and Related Heterostructures

In recent years, 2D layered materials have received considerable research interest on account of their substantial material systems and unique physicochemical properties. Among them, 2D layered transition metal dichalcogenides (TMDs), a star family member, have already been explored over the last few years and have exhibited excellent performance in electronics, catalysis, and other related fields. However, to fulfill the requirement for practical application, the batch production of 2D TMDs is essential. Recently, the chemical vapor deposition (CVD) technique was considered as an elegant alternative for successfully growing 2D TMDs and their heterostructures. The latest research advances in the controllable synthesis of 2D TMDs and related heterostructures/superlattices via the CVD approach are illustrated here. The controlled growth behavior, preparation strategies, and breakthroughs on the synthesis of new 2D TMDs and their heterostructures, as well as their unique physical phenomena, are also discussed. Recent progress on the application of CVD‐grown 2D materials is revealed with particular attention to electronics/optoelectronic devices and catalysts. Finally, the challenges and future prospects are considered regarding the current development of 2D TMDs and related heterostructures.     

原子氧环境中聚酰亚胺的质量变化和侵蚀机制

用石英晶体微天平(QCM)原位监测并研究了聚酰亚胺薄膜在地面原子氧模拟装置中暴露时的质量变化.结果表明,聚酰亚胺薄膜在较低的原子氧束流通量暴露的初期,试样的质量先增加后降低,质量的降低与暴露的时间成正比.在高原子氧束流通量暴露的初期,试样质量的增加不明显,甚至一开始就发生稳态氧化失重.实验数据拟合的结果表明,原子氧对聚合物造成的侵蚀主要发生在有氧原子吸附的表面.质量的增加是由于较低的原子氧通量没有能完全氧化聚合物的表面.原子氧对聚合物材料的侵蚀机制服从Langmuir吸附理论.     

The Role of Graphene and Other 2D Materials in Solar Photovoltaics

2D materials have attracted considerable attention due to their exciting optical and electronic properties, and demonstrate immense potential for next‐generation solar cells and other optoelectronic devices. With the scaling trends in photovoltaics moving toward thinner active materials, the atomically thin bodies and high flexibility of 2D materials make them the obvious choice for integration with future‐generation photovoltaic technology. Not only can graphene, with its high transparency and conductivity, be used as the electrodes in solar cells, but also its ambipolar electrical transport enables it to serve as both the anode and the cathode. 2D materials beyond graphene, such as transition‐metal dichalcogenides, are direct‐bandgap semiconductors at the monolayer level, and they can be used as the active layer in ultrathin flexible solar cells. However, since no 2D material has been featured in the roadmap of standard photovoltaic technologies, a proper synergy is still lacking between the recently growing 2D community and the conventional solar community. A comprehensive review on the current state‐of‐the‐art of 2D‐materials‐based solar photovoltaics is presented here so that the recent advances of 2D materials for solar cells can be employed for formulating the future roadmap of various photovoltaic technologies.     

Structure and photoconductive properties of dip-deposited SnS and SnS2 thin films and their conversion to tin dioxide by annealing in air

SnS and SnS₂ thin films have been prepared by the dip technique. In this technique, a substrate was dipped into an alcoholic solution of the corresponding chloride and thiourea and then withdrawn vertically at a controlled speed, and finally baked in a high temperature furnace at atmospheric condition. XRD and SEM data suggest that good quality SnS and SnS₂ films are obtained at a baking temperature of 300 and 360°C, respectively. Values of band gap for SnS and SnS₂ obtained from spectral response of photoconductivity are 1.4 and 2.4 eV, respectively. The indirect allowed band gap values for SnS₂ film obtained from optical absorption measurements are 1.95 and 2.05 eV. Open-air annealing of both SnS and SnS₂ films at 400°C converts them to transparent conducting SnO₂.     

空间材料的原子氧侵蚀理论和预测模型

低地轨道环境中的原子氧对航天器材料的侵蚀导致材料的性能变坏甚至失效，原子氧的侵蚀机理和防护技术是当前空间环境效应研究的热点．在对原子氧效应机理已有理解的基础上，准确预测空间材料在低地轨道环境中由原子氧引起的侵蚀效应，可对设计者在工程选材和飞行器设计提供帮助。本文综述了近年来发展的原子氧与空间材料相互作用的理论模型和侵蚀速率预测模型，并对各种模型进行了分析，也指出了关于原子氧效应的研究重点．     

Temperature dependence of SiO2 film growth with plasma-enhanced atomic layer deposition

The growth per cycle (GPC) temperature dependence was investigated for SiO₂ films prepared by plasma-enhanced atomic layer deposition (PEALD). During preparation of PEALD-SiO₂ using bis-diethyl-amino-silane, the GPC was saturated via increasing the precursor dose time and flow rate. The saturated GPC decreased with increasing deposition temperature. GPC saturation curves as a function of precursor dose time were analyzed by a two-step adsorption model, where the amino-silane reversibly adsorbed (physisorption) during the first step, and then irreversibly adsorbed (chemisorption) on the SiO₂ surface upon reaction with surface OH absorbents. This model is in good quantitative agreement with the saturation curve. The GPC value was determined by the surface reaction of amino-silane with OH sites, whose surface density was decreased by increasing the deposition temperature. The GPC saturation became slower with increasing deposition temperature, because the desorption rate of the physisorbed precursor increased with increasing temperature.     

Electrical and photo-electrical properties of MoS2 nanosheets with and without an Al2O3 capping layer under various environmental conditions

The electrical and photo-electrical properties of exfoliated MoS₂ were investigated in the dark and in the presence of deep ultraviolet (DUV) light under various environmental conditions (vacuum, N₂ gas, air, and O₂ gas). We examined the effects of environmental gases on MoS₂ flakes in the dark and after DUV illumination through Raman spectroscopy and found that DUV light induced red and blue shifts of peaks (E¹_2 g and A_1 g) position in the presence of N₂ and O₂ gases, respectively. In the dark, the threshold voltage in the transfer characteristics of few-layer (FL) MoS₂ field-effect transistors (FETs) remained almost the same in vacuum and N₂ gas but shifted toward positive gate voltages in air or O₂ gas because of the adsorption of oxygen atoms/molecules on the MoS₂ surface. We analyzed light detection parameters such as responsivity, detectivity, external quantum efficiency, linear dynamic range, and relaxation time to characterize the photoresponse behavior of FL-MoS₂ FETs under various environmental conditions. All parameters were improved in their performances in N₂ gas, but deteriorated in O₂ gas environment. The photocurrent decayed with a large time constant in N₂ gas, but decayed with a small time constant in O₂ gas. We also investigated the characteristics of the devices after passivating by Al₂O₃ film on the MoS₂ surface. The devices became almost hysteresis-free in the transfer characteristics and stable with improved mobility. Given its outstanding performance under DUV light, the passivated device may be potentially used for applications in MoS₂-based integrated optoelectronic circuits, light sensing devices, and solar cells.     

Large-area vapor-phase growth and characterization of MoS(2) atomic layers on a SiO(2) substrate

Atomic-layered MoS(2) is synthesized directly on SiO(2) substrates by a scalable chemical vapor deposition method. The large-scale synthesis of an atomic-layered semiconductor directly on a dielectric layer paves the way for many facile device fabrication possibilities, expanding the important family of useful mono- or few-layer materials that possess exceptional properties, such as graphene and hexagonal boron nitride (h-BN).     

Direct vapor phase growth process and robust photoluminescence properties of large area MoS2 layers

There has been growing research interest in the use of molybdenum disulfide in the fields of optoelectronics and energy harvesting devices, by virtue of its indirect-to-direct band gap tunability. However, obtaining large area thin films of MoS2 for future device applications still remains a challenge. In the present study, the amounts of the precursors （S and MOO3） were varied systematically in order to optimize the growth of highly crystalline and large area MoS2 layers by the chemical vapor deposition method. Careful control of the amounts of precursors was found to the key factor in the synthesis of large area highly crystalline flakes. The thickness of the layers was confirmed by Raman spectroscopy and atomic force microscopy. The optical properties and chemical composition were studied by photoluminescence （PL） and X-ray photoelectron spectroscopy. The emergence of strong direct excitonic emissions at 1.82 eV （A-exciton, with a normalized PL intensity of -55 × 10^3） and 1.98 eV （B-exciton, with a normalized PL intensity of -5 × 10^3） of the sample at room temperature clearly indicates the high luminescence quantum efficiency. The mobility of the films was found to be 0.09 cm^2/（V.s） at room temperature. This study provides a method for the controlled synthesis of high-quality two-dimensional （2D） transition metal dichalcogenide materials, useful for applications in nanodevices, optoelectronics and solar energv conversion.