huangpanqi

Although the research of 2D photocatalysts has made great progress in the past decades, there are still many challenges in understanding the deep relationship between the surface state and the reaction mechanism. The surface modification strategies and reaction mechanisms of 2D photocatalysts are reviewed, and some useful views are put forward for future research in this field.

Abstract

2D materials show many particular properties, such as high surface-to-volume ratio, high anisotropic degree, and adjustable chemical functionality. These unique properties in 2D materials have sparked immense interest due to their applications in photocatalytic systems, resulting in significantly enhanced light capture, charge-transfer kinetics, and surface reaction. Herein, the research progress in 2D photocatalysts based on varied compositions and functions, followed by specific surface modification strategies, is introduced. Fundamental principles focusing on light harvesting, charge separation, and molecular adsorption/activation in the 2D-material-based photocatalytic system are systemically explored. The examples described here detail the use of 2D materials in various photocatalytic energy-conversion systems, including water splitting, carbon dioxide reduction, nitrogen fixation, hydrogen peroxide production, and organic synthesis. Finally, by elaborating the challenges and possible solutions for developing these 2D materials, the review is expected to provide some inspiration for the future research of 2D materials used on efficient photocatalytic energy conversions.

Nature, Published online: 09 March 2022; doi:10.1038/s41586-021-04360-y

A feasible strategy to construct high-performance and low-power chemical vapor deposition-grown polycrystalline molybdenum ditelluride thin film transistors with solution-processed ternary Hf_0.5Zr_0.5O₂/HfAlO₂ high-k dielectric is demonstrated.

Abstract

Besides the widely investigated potential as alternative to silicon in microelectronics, semiconducting 2D transition metal dichalcogenides (TMDs) also show appealing prospects in thin film transistors (TFTs)-based applications, while still suffer from insufficient demonstration. Herein, the authors systematically report high-performance and low-power chemical vapor deposition-grown polycrystalline molybdenum ditelluride (MoTe₂) TFTs with solution-processed ternary Hf_0.5Zr_0.5O₂/HfAlO₂ high-k dielectric. Benefitting from the optimized high quality HfAlO₂ film synthesis and proper postannealing treatment, the constructed MoTe₂ TFTs exhibit a high mobility ≈27.24 cm² V⁻¹ S⁻¹, a current on/off ratio ≈6.43 × 10⁵, threshold voltage ≈−3.27 V, and subthreshold swing (SS) value ≈152.4 mV dec⁻¹, respectively. Moreover, by supplementing another solution-processed layer of ferroelectric Hf_0.5Zr_0.5O₂ to form double layer of HfAlO₂/Hf_0.5Zr_0.5O₂ dielectric, the device performance can be further improved with an ignorable hysteresis, increased mobility of ≈55.53 cm² V⁻¹ S⁻¹, and significantly reduced SS value of ≈110.16 mV dec⁻¹, respectively. The current investigation offers a feasible strategy to fabricate high-performance and low-power MoTe₂ TFTs for potential TMDs-based TFTs applications.

Twist angle induces various Moiré-related properties in 2D materials, but most studies only focus on bilayer systems. Here, via a folding strategy, multilayer MoS₂ Moiré superlattices are fabricated whose interlayer coupling, indirect bandgap, and degree of circular polarization (DOCP) are tunable by twist angle. The highest DOCP for folded bilayer MoS₂ can reach 86% above liquid nitrogen temperature.

Abstract

Twist angle provides a new degree of freedom for 2D material modifications. In principle, the intrinsic properties of twisted multilayers can be regulated by twist angle between each adjacent layer and thus have greater tunability than widely studied bilayer structures. Considering its complexity, it is important to first investigate the simplest twisted multilayers with only one interface twisted. In this work, multilayer Moiré superlattices with only one twisted interface via paraffin-assisted folding of non-twisted stacked (highly symmetrically stacked) multilayer MoS₂ are successfully fabricated, and their twist-angle dependent optical properties are systematically studied. Compared to non-twisted stacked multilayer MoS₂, the one-interface-twisted multilayers show a 2–3.5 times higher PL intensity, and their interlayer coupling, indirect bandgap, and degree of circular polarization (DOCP) are tunable by twist angle. Notably, the DOCP for the one-interface-twisted four-layer (folded bilayer) can reach 86%, which is the highest value ever reported for transition metal dichalcogenide homostructures above liquid nitrogen temperature. This work provides a solid base for understanding twist-angle dependent properties of twisted multilayer 2D-materials.

A controllable Fe doping strategy is developed in centimeter-sized monolayer MoS₂ films with ultralow contact resistance. Excellent device performance featured with ultrahigh electron mobility and on/off current ratio is achieved, thanks to the ultralow electron effective mass. Unidirectional Fe-MoS₂ domains are prepared on 2 in. commercial c-plane sapphire, suggesting the feasibility of synthesizing wafer-scale single-crystal semiconductors with outstanding device performance.

Abstract

2D semiconductors are emerging as plausible candidates for next-generation “More-than-Moore” nanoelectronics to tackle the scaling challenge of transistors. Wafer-scale 2D semiconductors, such as MoS₂ and WS₂, have been successfully synthesized recently; nevertheless, the absence of effective doping technology fundamentally results in energy barriers and high contact resistances at the metal–semiconductor interfaces, and thus restrict their practical applications. Herein, a controllable doping strategy in centimeter-sized monolayer MoS₂ films is developed to address this critical issue and boost the device performance. The ultralow contact resistance and perfect Ohmic contact with metal electrodes are uncovered in monolayer Fe-doped MoS₂, which deliver excellent device performance featured with ultrahigh electron mobility and outstanding on/off current ratio. Impurity scattering is suppressed significantly thanks to the ultralow electron effective mass and appropriate doping site. Particularly, unidirectionally aligned monolayer Fe-doped MoS₂ domains are prepared on 2 in. commercial c-plane sapphire, suggesting the feasibility of synthesizing wafer-scale 2D single-crystal semiconductors with outstanding device performance. This work presents the potential of high-performance monolayer transistors and enables further device downscaling and extension of Moore's law.

The transfer of graphene from growth substrates onto target substrates is highly needed for further applications of graphene. However, the as-transferred graphene films still suffer from the issues of cracks, surface contaminants, wrinkles, and unintentional doping. In the review, the focus is on these issues and corresponding solutions.

Abstract

Owing to the fascinating properties of graphene, fulfilling the promising characteristics of graphene in applications has ignited enormous scientific and industrial interest. Chemical vapor deposition (CVD) growth of graphene on metal substrates provides tantalizing opportunities for the large-area synthesis of graphene in a controllable manner. However, the tedious transfer of graphene from metal substrates onto desired substrates remains inevitable, and cracks of graphene membrane, transfer-induced doping, wrinkles as well as surface contamination can be incurred during the transfer, which highly degrade the performance of graphene. Furthermore, new issues can arise when moving to large-scale transfer at an industrial scale, thus cost-efficient and environment-friendly transfer techniques also become imperative. The aim of this review is to provide a comprehensive understanding of transfer-related issues and the corresponding experimental solutions and to provide an outlook for future transfer techniques of CVD graphene films on an industrial scale.

Orientated growth of Te with a thickness down to 5 nm is realized on the three-fold symmetric substrates (WSe₂, WS₂, MoSe₂, and MoS₂). This method is extended to the growth of SeTe alloys, providing flexibility for band engineering. Finally, growth of single-crystal-textured Te film is demonstrated on the low-symmetric surface of WTe₂.

Abstract

Tellurium, as an elemental van der Waals semiconductor, has intriguing anisotropic physical properties owing to its inherent 1D crystal structure. To exploit the anisotropic and thickness-dependent behavior, it is important to realize orientated growth of ultrathin tellurium. Here, van der Waals epitaxial growth of Te on the surface of 2D transition metal dichalcogenides is systematically investigated. Orientated growth of Te with a thickness down to 5 nm is realized on three-fold symmetric substrates (WSe₂, WS₂, MoSe₂, and MoS₂), where the atomic chains of Te are aligned with the armchair directions of substrates. 1D/2D moiré superlattices are observed for the Te/WSe₂ heterostructure. This method is extended to the growth of SeTe alloys, providing flexibility for band engineering. Finally, growth of textured Te film is demonstrated on the lower-symmetry surface of WTe₂.

The latest progress of additives-assisted chemical vapor deposition growth of 2D materials is reviewed, mainly summarized from the perspectives of promote effects, applications, and growth mechanisms, providing a guidance for 2D materials to move from labs toward industries.

Abstract

2D materials are increasingly becoming key components in modern electronics because of their prominent electronic and optoelectronic properties. The central and premise to the entire discipline of 2D materials lie in the high-quality and scaled preparations. The chemical vapor deposition (CVD) method offers compelling benefits in terms of scalability and controllability in shaping large-area and high-quality 2D materials. The past few years have witnessed development of numerous CVD growth strategies, with the use of additives attracting substantial attention in the production of scaled 2D crystals. This review provides an overview of different additives used in CVD growth of 2D materials, as well as a methodical demonstration of their vital roles. In addition, the intrinsic mechanisms of the production of scaled 2D crystals with additives are also discussed. Lastly, reliable guidance on the future design of optimal CVD synthesis routes is provided by analyzing the accessibility, pricing, by-products, controllability, universality, and commercialization of various additives.

A high-quality saturable absorber (SA) based on GeAs₂ nanosheets is introduced. The saturation intensity and the modulation depth are measured to be 1.23 GW cm⁻² and 5.2%, respectively. By incorporating this GeAs₂ SA into fiber lasers, a high stable ultrafast fiber laser with a pulse duration of 371 fs and a single-frequency fiber laser with a linewidth of ≈678 Hz are demonstrated, respectively.

Abstract

As a new IV–V group semiconductor, germanium-diarsenide (GeAs₂) compounds have attracted considerable attention due to their outstanding optical and electrical properties, thickness-dependent bandgap, in-plane anisotropy, and excellent optical absorption. However, the potential of GeAs₂ in the field of ultrafast and ultranarrow fiber laser has not been studied. In this article, a high-quality GeAs₂ nanosheets saturable absorber (SA) is successfully prepared by liquid-phase exfoliation. The nonlinear optical characteristics of GeAs₂ nanosheets have been investigated based on a balanced twin-detector measurement system. The modulation depth, nonsaturable loss, and saturation intensity are measured to be 5.2%, 24%, and 1.23 GW cm⁻², respectively. GeAs₂ has been successfully applied as an SA in an ultrafast and single-frequency fiber laser. A stable mode-locked laser pulses operation with a duration as short as 371 fs and a repetition rate of 8.19 MHz at a wavelength of 1560 nm is achieved. Moreover, ultranarrow fiber lasers with a high signal-to-noise ratio of 80 dB and a linewidth of ≈678 Hz are obtained. The findings validate that 2D GeAs₂ can be used as an SA and has promising applications in ultrafast and ultranarrow photonics.

MXenes are promising for future electronics and optoelectronics; however, previously reported patterning methods lack efficiency, resolution, and compatibility with mainstream semiconductor processing. Here, a wafer-scale combination patterning method with a resolution up to the micrometer scale is developed, resulting in an integrated array of 1024-pixel Ti₃C₂T _x /Si photodetectors with a record-high detectivity of 7.73 × 10¹⁴ Jones.

Abstract

As a rapidly growing family of 2D transition metal carbides and nitrides, MXenes are recognized as promising materials for the development of future electronics and optoelectronics. So far, the reported patterning methods for MXene films lack efficiency, resolution, and compatibility, resulting in limited device integration and performance. Here, a high-performance MXene image sensor array fabricated by a wafer-scale combination patterning method of an MXene film is reported. This method combines MXene centrifugation, spin-coating, photolithography, and dry-etching and is highly compatible with mainstream semiconductor processing, with a resolution up to 2 µm, which is at least 100 times higher than other large-area patterning methods reported previously. As a result, a high-density integrated array of 1024-pixel Ti₃C₂T _x /Si photodetectors with a detectivity of 7.73 × 10¹⁴ Jones and a light–dark current ratio (I _light/I _dark) of 6.22 × 10⁶, which is the ultrahigh value among all reported MXene-based photodetectors, is fabricated. This patterning technique paves a way for large-scale high-performance MXetronics compatible with mainstream semiconductor processes.

Dual interfacial layers, composed of a top covalent anchored carbon dots layer and a Mo:BiVO₄ shallow layer with enriched oxygen vacancies, are constructed to synchronously boost charge-carrier kinetics and inhibit photocorrosion, which results in a BiVO₄ photoanode with a photocurrent density of 6.08 mA cm⁻², and operational stability up to 120 h at 1.23 V_RHE.

Abstract

The semiconductor–liquid junction (SCLJ), the dominant place in photoelectrochemical (PEC) catalysis, determines the interfacial activity and stability of photoelectrodes, whcih directly affects the viability of PEC hydrogen generation. Though efforts dedicated in past decades, a challenge remains regarding creating a synchronously active and stable SCLJ, owing to the technical hurdles of simultaneously overlaying the two advantages. The present work demonstrates that creating an SCLJ with a unique configuration of the dual interfacial layers can yield BiVO₄ photoanodes with synchronously boosted photoelectrochemical activity and operational stability, with values located at the top in the records of such photoelectrodes. The bespoke dual interfacial layers, accessed via grafting laser-generated carbon dots with phenolic hydroxyl groups (LGCDs-PHGs), are experimentally verified effective, not only in generating the uniform layer of LGCDs with covalent anchoring for inhibited photocorrosion, but also in activating, respectively, the charge separation and transfer in each layer for boosted charge-carrier kinetics, resulting in FeNiOOH–LGCDs-PHGs–MBVO photoanodes with a dual configuration with the photocurrent density of 6.08 mA cm⁻² @ 1.23 V_RHE, and operational stability up to 120 h @ 1.23 V_RHE. Further work exploring LGCDs-PHGs from catecholic molecules warrants the proposed strategy as being a universal alternative for addressing the interfacial charge-carrier kinetics and operational stability of semiconductor photoelectrodes.