In recent years, nanocrystals of metal sulfide materials have attracted scientific research interest for renewable energy applications due to the abundant choice of materials with easily tunable electronic, optical, physical and chemical properties. Metal sulfides are semiconducting compounds where sulfur is an anion associated with a metal cation; and the metal ions may be in mono-, bi- or multi-form. The diverse range of available metal sulfide materials offers a unique platform to construct a large number of potential materials that demonstrate exotic chemical, physical and electronic phenomena and novel functional properties and applications. To fully exploit the potential of these fascinating materials, scalable methods for the preparation of low-cost metal sulfides, heterostructures, and hybrids of high quality must be developed. This comprehensive review indicates approaches for the controlled fabrication of metal sulfides and subsequently delivers an overview of recent progress in tuning the chemical, physical, optical and nano- and micro-structural properties of metal sulfide nanocrystals using a range of material fabrication methods. For hydrogen energy production, three major approaches are discussed in detail: electrocatalytic hydrogen generation, powder photocatalytic hydrogen generation and photoelectrochemical water splitting. A variety of strategies such as structural tuning, composition control, doping, hybrid structures, heterostructures, defect control, temperature effects and porosity effects on metal sulfide nanocrystals are discussed and how they are exploited to enhance performance and develop future energy materials. From this literature survey, energy conversion currently relies on a limited range of metal sulfides and their composites, and several metal sulfides are immature in terms of their dissolution, photocorrosion and long-term durability in electrolytes during water splitting. Future research directions for innovative metal sulfides should be closely allied to energy and environmental issues, along with their advanced characterization, and developing new classes of metal sulfide materials with well-defined fabrication methods.

Recent advances in metal sulfides: from controlled fabrication to electrocatalytic, photocatalytic and photoelectrochemical water splitting and beyond

Electrical metal contacts to two-dimensional (2D) semiconducting transition metal dichalcogenides (TMDCs) are found to be the key bottleneck to the realization of high device performance due to strong Fermi level pinning and high contact resistances (Rc). Until now, Fermi level pinning of monolayer TMDCs has been reported only theoretically, although that of bulk TMDCs has been reported experimentally. Here, we report the experimental study on Fermi level pinning of monolayer MoS2 and MoTe2 by interpreting the thermionic emission results. We also quantitatively compared our results with the theoretical simulation results of the monolayer structure as well as the experimental results of the bulk structure. We measured the pinning factor S to be 0.11 and −0.07 for monolayer MoS2 and MoTe2, respectively, suggesting a much stronger Fermi level pinning effect, a Schottky barrier height (SBH) lower than that by theoretical prediction, and interestingly similar pinning energy levels between monolayer and bulk Mo...

Fermi Level Pinning at Electrical Metal Contacts of Monolayer Molybdenum Dichalcogenides.

Two-dimensional (2D) materials have attracted tremendous research interest since the breakthrough of graphene. Their unique optical, electronic, and mechanical properties hold great potential for harnessing them as key components in novel applications for electronics and optoelectronics. Their atomic thickness and exposed huge surface even make them highly designable and manipulable, leading to the extensive application potentials. What's more, after acquiring the qualification for being the candidate for next-generation devices, the assembly of 2D materials monomers into mass or ordered structure is also of great importance, which will determine their ultimate industrialization. By designing the monomers and regulating their assembling behavior, the exploration of 2D materials toward the next-generation circuits can be spectacularly achieved. In this review, we will first overview the emerging 2D materials and then offer a clear guideline of varied physical and chemical strategies for tuning their properties. Furthermore, assembly strategies of 2D materials will also be included. Finally, challenges and outlooks in this promising field are featured on the basis of its current progress.

Exploring Two-Dimensional Materials toward the Next-Generation Circuits: From Monomer Design to Assembly Control

Group 6 transition metal dichalcogenides (G6-TMDs), most notably MoS2, MoSe2, MoTe2, WS2 and WSe2, constitute an important class of materials with a layered crystal structure. Various types of G6-TMD nanomaterials, such as nanosheets, nanotubes and quantum dot nano-objects and flower-like nanostructures, have been synthesized. High thermodynamic stability under ambient conditions, even in atomically thin form, made nanosheets of these inorganic semiconductors a valuable asset in the existing library of two-dimensional (2D) materials, along with the well-known semimetallic graphene and insulating hexagonal boron nitride. G6-TMDs generally possess an appropriate bandgap (1–2 eV) which is tunable by size and dimensionality and changes from indirect to direct in monolayer nanosheets, intriguing for (opto)electronic, sensing, and solar energy harvesting applications. Moreover, rich intercalation chemistry and abundance of catalytically active edge sites make them promising for fabrication of novel energy storage devices and advanced catalysts. In this review, we provide an overview on all aspects of the basic science, physicochemical properties and characterization techniques as well as all existing production methods and applications of G6-TMD nanomaterials in a comprehensive yet concise treatment. Particular emphasis is placed on establishing a linkage between the features of production methods and the specific needs of rapidly growing applications of G6-TMDs to develop a production-application selection guide. Based on this selection guide, a framework is suggested for future research on how to bridge existing knowledge gaps and improve current production methods towards technological application of G6-TMD nanomaterials.

Group 6 transition metal dichalcogenide nanomaterials: synthesis, applications and future perspectives

As is well-known, it is a great challenge that the smooth TiO 2 nanospheres are coated by MoS 2 nanosheets to form the core-shell nanostructure owing to their poor interaction. Herein, we report 3D black N-TiO 2-x @MoS 2 core-shell nanostructures synthesized by a mild and effective strategy combined with a typical hydrothermal reaction and an in situ solid-state chemical reduction method followed by 350 °C calcination under an argon atmosphere. The prepared samples are characterized in detail by X-ray diffraction, Raman, scanning electron microscopy, transmission electron microscopy, and X-ray photoelectron spectroscopy. The results suggest that the 3D N-TiO 2-x @MoS 2 photocatalyst is successfully doped with N and Ti 3+ , and simultaneously coupling with MoS 2 to form the core-shell heterojunction nanostructure. The N and Ti 3+ co-doped and hybrid heterostructures can effectively utilize visible-light and solar energy to degrade methyl orange and produce hydrogen. The degradation rate of methyl orange and the hydrogen production rate are as high as 91.8% and 1.882 mmol h −1  g −1 . To the best of our knowledge, this work is the first instance of combining MoS 2 with N and Ti 3+ co-doped TiO 2 , and the proposed photocatalytic mechanism will provide a new perspective for high activity photocatalyst in future.

Fabrication of 3D flower-like black N-TiO2-x@MoS2 for unprecedented-high visible-light-driven photocatalytic performance

By plasma-enhanced chemical vapor deposition, a molybdenum disulfide (MoS2 ) thin film is synthesized directly on a wafer-scale plastic substrate at below 300 °C. The carrier mobility of the films is 3.74 cm(2) V(-1) s(-1) . Also, humidity is successfully detected with MoS2 -based sensors fabricated on the flexible substrate, which reveals its potential for flexible sensing devices.

Low‐Temperature Synthesis of Large‐Scale Molybdenum Disulfide Thin Films Directly on a Plastic Substrate Using Plasma‐Enhanced Chemical Vapor Deposition

Two-dimensional (2D) metal dichalcogenides like molybdenum disulfide (MoS2) may provide a pathway to high-mobility channel materials that are needed for beyond-complementary metal-oxide-semiconductor (CMOS) devices. Controlling the thickness of these materials at the atomic level will be a key factor in the future development of MoS2 devices. In this study, we propose a layer-by-layer removal of MoS2 using the atomic layer etching (ALET) that is composed of the cyclic processing of chlorine (Cl)-radical adsorption and argon (Ar)(+) ion-beam desorption. MoS2 etching was not observed with only the Cl-radical adsorption or low-energy (<20 eV) Ar(+) ion-beam desorption steps; however, the use of sequential etching that is composed of the Cl-radical adsorption step and a subsequent Ar(+) ion-beam desorption step resulted in the complete etching of one monolayer of MoS2. Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), and atomic force microscopy (AFM) indicated the removal of one monolayer of MoS2 with each ALET cycle; therefore, the number of MoS2 layers could be precisely controlled by using this cyclical etch method. In addition, no noticeable damage or etch residue was observed on the exposed MoS2.

Controlled Layer-by-Layer Etching of MoS₂.

Bulk layers of MoS2 were synthesized in situ on a polymer substrate at low temperature. The negative charges carried by the layered MoS2 are used to immobilize horseradish peroxidase conjugated IgG via the electrostatic attraction, forming an Au–MoS2/HRP hybrid. Trace H2O2 released from IgG-horseradish peroxidase was successfully evaluated in the linear range of 0–20 ng mL−1.

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In situ synthesis of MoS2 on a polymer based gold electrode platform and its application in electrochemical biosensing

A CoFeB thin film composing a magnetic tunneling junction of CoFeB/MgO/CoFeB was etched in an inductively coupled plasma (ICP) etching system using CO/NH 3 gas mixtures, and its etch characteristics were compared with those of the CoFeB thin film etched using Cl 2 /Ar. When Cl 2 /Ar was used to etch the CoFeB thin film, even though its etch rate was faster than that of the CoFeB thin film etched using CO/NH 3 , a rough CoFeB surface could be observed due to the corrosion of the CoFeB surface during exposure to the air in addition to the significant change of surface composition. On the other hand, no corrosion of the CoFeB thin film was observed after the etching using CO/NH 3 . When the ratio of CO/NH 3 was varied, the highest etch rate of 12 nm/min could be observed at the ratio of 1:3 compared to about 4 nm/min for CO or NH 3 at the ICP source power of 700 W, bias power of 300 W, and 5 mTorr of operating pressure. The highest etch rate was related to the formation of volatile metal carbonyls between metal and CO, where NH 3 appeared to assist the easier formation of metal carbonyl by preventing the dissociation of CO into C and CO 2 .

Etching of CoFeB Using CO ∕ NH3 in an Inductively Coupled Plasma Etching System

Etch damage of Ge2Sb2Te5 (GST) has been investigated after etching in halogen inductively coupled plasmas (ICPs) such as CF4, Cl2, and HBr. X-ray photoelectron spectra of Ge, Sb, and Te on the etched surfaces showed different depths of etch damage depending on the halogen-based plasmas. The blank GST etched by CF4 showed a lower total-halogen-percentage remaining on the etched surface than that etched by Cl2 even though the depth of halogenations was the deepest for the GST etched by CF4 owing to the highest reactivity. However, when a GST feature masked by SiO2/Ti/TiN was etched, owing to the reaction of O from the SiO2 mask and C from CF4, a thinner/or no C–F polymer appeared to be formed on the etched sidewall; therefore, the highest halogenation was observed on the GST etched by CF4. Among the halogen-gases investigated, HBr showed the lowest damage due to the lowest reactivity.

https://iopscience.iop.org/article/10.1143/JJAP.50.086501/pdf

Min-Hwan Jeon

Papers

Low‐Temperature Synthesis of Large‐Scale Molybdenum Disulfide Thin Films Directly on a Plastic Substrate Using Plasma‐Enhanced Chemical Vapor Deposition

Controlled Layer-by-Layer Etching of MoS₂.

In situ synthesis of MoS2 on a polymer based gold electrode platform and its application in electrochemical biosensing

Etching of CoFeB Using CO ∕ NH3 in an Inductively Coupled Plasma Etching System

Etch Damage of Ge$_{2}$Sb$_{2}$Te$_{5}$ for Different Halogen Gases