Jiuxiang Dai

Although various van der Waals stacking systems for preparing multifunctional devices are reported, their complexity in fabrication and/or inflexibility in reversible operation limit the possibility of multifunctional integrations and the requirement of the fast-growing internet of things paradigm. Here, a simple and effective rapid temperature annealing strategy for reversibly controlling and optimizing the electronic properties of MoTe₂-based heterostructure electronics is proposed/realized.

Abstract

Van der Waals (vdW) heterostructures—in which layered materials are purposely selected to assemble with each other—allow unusual properties and different phenomena to be combined and multifunctional electronics to be created, opening a new chapter for the spread of internet-of-things applications. Here, an O₂-ultrasensitive MoTe₂ material and an O₂-insensitive SnS₂ material are integrated to form a vdW heterostructure, allowing the realization of charge-polarity control for multioperation-mode transistors through a simple and effective rapid thermal annealing strategy under dry-air and vacuum conditions. The charge-polarity control (i.e., doping and de-doping processes), which arises owing to the interaction between O₂ adsorption/desorption and tellurium defects at the MoTe₂ surface, means that the MoTe₂/SnS₂ heterostructure transistors can reversibly change between unipolar, ambipolar, and anti-ambipolar transfer characteristics. Based on the dynamic control of the charge-polarity properties, an inverter, output polarity controllable amplifier, p-n diode, and ternary-state logics (NMIN and NMAX gates) are demonstrated, which inspire the development of reversibly multifunctional devices and indicates the potential of 2D materials.

A study of a newly discovered graphene-water membrane is accomplished. It provides a promising technique for high-quality graphene process with ultra-high flatness, and free of defects, leveraging the high surface tension of pure water. These advantages enable high-performance device fabrication for electronic and biological research.

Abstract

It is widely accepted that solid-state membranes are indispensable media for the graphene process, particularly transfer procedures. But these membranes inevitably bring contaminations and residues to the transferred graphene and consequently compromise the material quality. This study reports a newly observed free-standing graphene-water membrane structure, which replaces the conventional solid-state supporting media with liquid film to sustain the graphene integrity and continuity. Experimental observation, theoretical model, and molecular dynamics simulations consistently indicate that the high surface tension of pure water and its large contact angle with graphene are essential factors for forming such a membrane structure. More interestingly, water surface tension ensures the flatness of graphene layers and renders high transfer quality on many types of target substrates. This report enriches the understanding of the interactions on reduced dimensional material while rendering an alternative approach for scalable layered material processing with ensured quality for advanced manufacturing.

Chemical vapor deposition can produce large-area graphene films with fine scalability and controllability, and graphene doping films have become an important pathway for realizing their desired applications. In this review, the aim is to summarize the strategies for graphene doping and to compare doping performance including doping-induced defects and scattering, as well as doping stability and uniformity.

Abstract

Graphene films have been regarded as potential materials for future applications in electronics and optoelectronics. Among various synthetic approaches, chemical vapor deposition (CVD) approach can produce large-area graphene films with excellent scalability, controllability, and quality. Graphene doping, that is capable of improving carrier concentration and shifting the Fermi level position of graphene, has become an important pathway for realizing its desired applications. Physical adsorption of dopants on graphene surface can dope the graphene system through surface charge transfer and preserve the graphene lattice; however, the low doping stability strongly hinders its applications. The substitutional doping can achieve fine doping stability by incorporating heteroatoms, such as nitrogen and boron, into graphene lattice, but suffers from low carrier mobility owing to the presence of defects and disorders. In this review, the aim is to provide a comprehensive understanding of application-related doping performance including doping stability, doping uniformity, carrier concentration, carrier mobility, electrical conductivity, and optical transparency. The aim of the review is also to provide an outlook for future doping techniques of CVD graphene films toward various applications.

Atomic layer precision thinning of MoTe₂ at nanoscale lateral resolution is demonstrated by introducing laser irradiated hot tip. The contact of the hot tip on MoTe₂ surface promotes oxidation at nanoscale resolution which is simultaneously removed by thermomechanical scribing. This process completes the etching of the topmost layer of MoTe₂ while maintaining the high crystallinity of thinned MoTe₂ flake.

Abstract

A key feature of 2D transition metal dichalcogenides (2D-TMDCs) is that their properties are strongly dependent on their thickness, typically appearing in ultrathin mono- or few- layers. Thus, precise control of functional nanostructure is critical for fundamental research and applications in 2D-TMDCs. Here, atomic layer precision thinning of molybdenum ditelluride (MoTe₂) at nanoscale lateral resolution by introducing laser irradiated hot tip is demonstrated. The contact of the hot tip on MoTe₂ surface promotes oxidation at nanoscale resolution which is simultaneously removed by thermomechanical scribing. This process completes the atomic layer precision thinning of MoTe₂ while maintaining the high crystallinity of thinned MoTe₂ flake. Further, the electrical properties of the MoTe₂ flake are intact after thinning, which proves that the thinned MoTe₂ flake obtained by these methods can potentially be utilized for device fabrication. It is believed that the work will enable applications of 2D-TMDCs that require nanoscale resolution with controlled thickness.

Various strategies for production of boron nanosheets (BNS) along with their merits and demerits are described in this article. High-quality and uniform BNS are produced from low-cost boron chunks by using an ultrasonication-assisted liquid-phase exfoliation. The catalytic potential is investigated for the degradation of organic pollutants in the presence of peroxymonosulphate as an oxidizing agent.

Abstract

High-end technological applications of thin boron nanosheets (BNS), including single layer borophene, have inspired the scientific community to develop fabrication routes to achieve scalable preparation. Among the different strategies, liquid-phase exfoliation is one of the facile methods. However, this method has certain drawbacks, such as the use of high-cost boron precursors (boron powder having particle size in micrometer) and possesses lower quality and size nonuniformity in the as-prepared BNS. To address these issues, the production of high-quality and uniform BNS is reported from low-cost boron chunks (bulk boron having > 1 cm particle size) by using an ultrasonication-assisted liquid-phase exfoliation method. The as-prepared BNS shows strong optical fluorescence characteristics and exhibits good electrical and photoconductivity values indicating its suitability for various applications. As an application study, the catalytic potential of the as-prepared BNS is explored for the degradation of diverse organic pollutants using peroxymonosulfate as an oxidizing agent. The density functional theory is used to calculate the energy minima of different boron crystallographic phases along with the interactions of peroxymonosulfate with BNS. The facile strategy reported here is expected to pave the way for scalable production of ultrathin BNS from a low-cost precursor.

Here, in situ growth of 2D MoO₂/MoS₂ heterostructures on the centimeter-scale substrate is shown. Noteworthy, the creation of a clean interface between atomic thin MoO₂ (metallic) and MoS₂ (semiconducting) results in a different electronic structure, which shows an improved HER performance with an overpotential of 60 mV at 10 mA cm⁻² and a Tafel slope of 47 mV dec⁻¹.

Abstract

2D material-based heterostructures are constructed by stacking or spicing individual 2D layers to create an interface between them, which have exotic properties. Here, a new strategy for the in situ growth of large numbers of 2D heterostructures on the centimeter-scale substrate is developed. In the method, large numbers of 2D MoS₂, MoO₂, or their heterostructures of MoO₂/MoS₂ are controllably grown in the same setup by simply tuning the gap distance between metal precursor and growth substrate, which changes the concentration of metal precursors feed. A lateral force microscope is used first to identify the locations of each material in the heterostructures, which have MoO₂ on the top of MoS₂. Noteworthy, the creation of a clean interface between atomic thin MoO₂ (metallic) and MoS₂ (semiconducting) results in a different electronic structure compared with pure MoO₂ and MoS₂. Theoretical calculations show that the charge redistribution at such an interface results in an improved HER performance on the MoO₂/MoS₂ heterostructures, showing an overpotential of 60 mV at 10 mA cm⁻² and a Tafel slope of 47 mV dec⁻¹. This work reports a new strategy for the in situ growth of heterostructures on large-scale substrates and provides platforms to exploit their applications.

In this work, large area synthesized n-type MoS₂ and p-type vanadium doped WSe₂-based field effect transistors are integrated, with non-volatile and analog storage capabilities to demonstrate digital logic and neuromorphic computing primitives highlighting the applicability of the fabrication process flow to realize 2D heterogeneous integrated circuits.

Abstract

Atomically thin, 2D, and semiconducting transition metal dichalcogenides (TMDs) are seen as potential candidates for complementary metal oxide semiconductor (CMOS) technology in future nodes. While high-performance field effect transistors (FETs), logic gates, and integrated circuits (ICs) made from n-type TMDs such as MoS₂ and WS₂ grown at wafer scale have been demonstrated, realizing CMOS electronics necessitates integration of large area p-type semiconductors. Furthermore, the physical separation of memory and logic is a bottleneck of the existing CMOS technology and must be overcome to reduce the energy burden for computation. In this article, the existing limitations are overcome and for the first time, a heterogeneous integration of large area grown n-type MoS₂ and p-type vanadium doped WSe₂ FETs with non-volatile and analog memory storage capabilities to achieve a non–von Neumann 2D CMOS platform is introduced. This manufacturing process flow allows for precise positioning of n-type and p-type FETs, which is critical for any IC development. Inverters and a simplified 2-input-1-output multiplexers and neuromorphic computing primitives such as Gaussian, sigmoid, and tanh activation functions using this non–von Neumann 2D CMOS platform are also demonstrated. This demonstration shows the feasibility of heterogeneous integration of wafer scale 2D materials.

A framework of controllable synthesis routes for ultrathin 2D MOFs are discussed, followed by illustrations of promising applications relating to various electrical devices and integrated circuits. It is concluded by outlining how 2D MOFs can be synthesized in a simpler, highly efficient, low cost, and environmentally friendly way which opens up their opportunities in advanced electrical devices and integrated circuits.

Abstract

The development of advanced electronic devices is boosting many aspects of modern technology and industry. The ever-increasing demand for advanced electrical devices and integrated circuits calls for the design of novel materials, with superior properties for the improvement of working performance. In this review, a detailed overview of the synthesis strategies of 2D metal organic frameworks (MOFs) acquiring growing attention is presented, as a basis for expansion of novel key materials in electrical devices and integrated circuits. A framework of controllable synthesis routes to be implanted in the synthesis strategies of 2D materials and MOFs is described. In short, the synthesis methods of 2D MOFs are summarized and discussed in depth followed by the illustrations of promising applications relating to various electrical devices and integrated circuits. It is concluded by outlining how 2D MOFs can be synthesized in a simpler, highly efficient, low-cost, and more environmentally friendly way which can open up their applicable opportunities as key materials in advanced electrical devices and integrated circuits, enabling their use in broad aspects of the society.

Low temperature (<100 °C) processed, fully-printed, narrow-channel In₂O₃ thin film transistors and depletion load-type inverters are fabricated onto polyethylene terephthalate substrates. The devices exhibit signal gain >200 and operation frequency >300 kHz at a low supply voltage of V _DD = 2 V, and an excellent tensile strain tolerance up to 5%.

Abstract

The major limitations of solution-processed oxide electronics include high process temperatures and the absence of necessary strain tolerance that would be essential for flexible electronic applications. Here, a combination of low temperature (<100 °C) curable indium oxide nanoparticle ink and a conductive silver nanoink, which are used to fabricate fully-printed narrow-channel thin film transistors (TFTs) on polyethylene terephthalate (PET) substrates, is proposed. The metal ink is printed onto the In₂O₃ nanoparticulate channel to narrow the effective channel lengths down to the thickness of the In₂O₃ layer and thereby obtain near-vertical transport across the semiconductor layer. The TFTs thus prepared show On/Off ratio ≈10⁶ and simultaneous maximum current density of 172 µA µm⁻¹. Next, the depletion-load inverters fabricated on PET substrates demonstrate signal gain >200 and operation frequency >300 kHz at low operation voltage of V _DD = 2 V. In addition, the near-vertical transport across the semiconductor layer is found to be largely strain tolerant with insignificant change in the TFT and inverter performance observed under bending fatigue tests performed down to a bending radius of 1.5 mm, which translates to a strain value of 5%. The devices are also found to be robust against atmospheric exposure when remeasured after a month.

Nature Materials, Published online: 18 July 2022; doi:10.1038/s41563-022-01308-z

Knowledge of band structure aids in understanding charge transport behaviour, yet it has proved impossible to measure the conduction (LUMO) band of organic semiconductors, in particular due to sample degradation by the electron beam. To address this, the authors developed and used AR-LEIPS to reveal the LUMO band dispersion of pentacene.

npj 2D Materials and Applications, Published online: 18 July 2022; doi:10.1038/s41699-022-00319-3

Exploration of two-dimensional molybdenum-borides and potential applications

Nature Communications, Published online: 18 July 2022; doi:10.1038/s41467-022-31886-0

Chemical vapor deposition enables the scalable production of 2D semiconductors, but the grown materials are usually affected by high defect densities. Here, the authors report a hydroxide vapour phase deposition method to synthesize wafer-scale monolayer WS2 with reduced defect density and electrical properties comparable to those of exfoliated flakes.

Nature Communications, Published online: 18 July 2022; doi:10.1038/s41467-022-31800-8

Dislocation climb is crucial to plasticity and creep of materials. Here, the authors report real-time atomic-scale observations of grain boundary dislocation climb in nanostructured Au at room temperature. The dislocation climb occurs by reconstruction of two atomic columns in the dislocation core.

Nature Communications, Published online: 15 July 2022; doi:10.1038/s41467-022-31598-5

Lockdowns due to the pandemic in the last two years forced a critical number of chip-making facilities across the world to shut down, giving rise to the chip shortage issues. Prof. Meng-Fan (Marvin) Chang (National Tsing Hua University, TSMC—Taiwan), Prof. Huaqiang Wu (Tsinghua University—China), Dr. Elisa Vianello (CEA-Leti—France), Dr. Sang Joon Kim (Samsung Electronics—South Korea) and Dr. Mirko Prezioso (Mentium Techn.—US) talked to Nature Communications to better understand whether and to what extent this crisis has impacted the development of in-memory/neuromorphic chips, an emerging technology for future computing.

Publication date: September 2022

Source: Materials Today, Volume 58

Author(s): Weifei Fu, Antonio Gaetano Ricciardulli, Quinten A. Akkerman, Rohit Abraham John, Mohammad Mahdi Tavakoli, Stephanie Essig, Maksym V. Kovalenko, Michael Saliba

Nature Nanotechnology, Published online: 14 July 2022; doi:10.1038/s41565-022-01165-6

This Review elaborates on the recent developments and the future opportunities and challenges of fundamental research on semiconductor moiré materials, with a particular focus on transition metal dichalcogenides.