Jiuxiang Dai

Deterministic conversion of 2D transition metal dichalcogenides (TMDs) to Janus quantum layers is achieved via a novel synthesis approach. Integrated in situ spectroscopy measurements enable detailed insights into classical TMDs to Janus layer conversion to reach excitonic-grade Janus layer.

Abstract

Named after the two-faced Roman god of transitions, transition metal dichalcogenide (TMD) Janus monolayers have two different chalcogen surfaces, inherently breaking the out-of-plane mirror symmetry. The broken mirror symmetry and the resulting potential gradient lead to the emergence of quantum properties such as the Rashba effect and the formation of dipolar excitons. Experimental access to these quantum properties, however, hinges on the ability to produce high-quality 2D Janus monolayers. Here, these results introduce a holistic 2D Janus synthesis technique that allows real-time monitoring of the growth process. This prototype chamber integrates in situ spectroscopy, offering fundamental insights into the structural evolution and growth kinetics, that allow the evaluation and optimization of the quality of Janus monolayers. The versatility of this method is demonstrated by synthesizing and monitoring the conversion of SWSe, SNbSe, and SMoSe Janus monolayers. Deterministic conversion and real-time data collection further aid in conversion of exfoliated TMDs to Janus monolayers and unparalleled exciton linewidth values are reached, compared to the current best standard. The results offer an insight into the process kinetics and aid in the development of new Janus monolayers with high optical quality, which is much needed to access their exotic properties.

This review briefly introduces preparation methods, oxidation mechanisms, a comprehensive summarization of factors affecting oxidation and a detailed account of strategies to mitigate MXenes degeneration. It furthermore offers an overview of in situ synthesis MXene derivatives with enhanced performance, and presents perspectives for the challenges and opportunities for the protection and application of MXenes and their derivatives.

Abstract

As an emerging star of 2D nanomaterials, 2D transition metal carbides and nitrides, named MXenes, present a large potential in various research areas owing to their intrinsic multilayer structure and intriguing physico-chemical properties. However, the fabrication and application of functional MXene-based devices still remain challenging as they are prone to oxidative degradation under ambient environment. Within this review, the preparation methods of MXenes focusing on the recent investigations on their thermal structure–stability relationships in inert, oxidizing, and aqueous environments are systematically introduced. Moreover, the key factors that affect the oxidation of MXenes, such as, atmosphere, temperature, composition, microstructure, and aqueous environment, are reviewed. Based on different scenarios, strategies for avoiding or delaying the oxidation of MXenes are proposed to encourage the utilization of MXenes in complicated environments, especially at high temperature. Furthermore, the chemistry of MXene-derived oxides is analyzed, which can offer perspectives on the further design and fabrication of novel 2D composites with the unique structures of MXenes being preserved.

Room-temperature 2D ferromagnetism is successfully realized in topological-insulator-induced van der Waals Fe₃GeTe₂, which is evidenced by ultrafast THz emission spectroscopy. Furthermore, the direction of the external magnetic field is rotated and the sample's front surface is reversed to probe the THz radiation symmetry. The results reveal that the spin-to-charge conversion effect is the dominant THz radiation mechanism.

Abstract

Future information technologies for low-dissipation quantum computation, high-speed storage, and on-chip communication applications require the development of atomically thin, ultracompact, and ultrafast spintronic devices in which information is encoded, stored, and processed using electron spin. Exploring low-dimensional magnetic materials, designing novel heterostructures, and generating and controlling ultrafast electron spin in 2D magnetism at room temperature, preferably in the unprecedented terahertz (THz) regime, is in high demand. Using THz emission spectroscopy driven by femtosecond laser pulses, optical THz spin-current bursts at room temperature in the 2D van der Waals ferromagnetic Fe₃GeTe₂ (FGT) integrated with Bi₂Te₃ as a topological insulator are successfully realized. The symmetry of the THz radiation is effectively controlled by the optical pumping incidence and external magnetic field directions, indicating that the THz generation mechanism is the inverse Edelstein effect contributed spin-to-charge conversion. Thickness-, temperature-, and structure-dependent nontrivial THz transients reveal that topology-enhanced interlayer exchange coupling increases the FGT Curie temperature to room temperature, which provides an effective approach for engineering THz spin-current pulses. These results contribute to the goal of all-optical generation, manipulation, and detection of ultrafast THz spin currents in room-temperature 2D magnetism, accelerating the development of atomically thin high-speed spintronic devices.

A van der Waals on epitaxial HgCdTe for uncooled mid-wavelength infrared photodetectors with fast response and high detectivity at spectral blackbody detection is introduced. Meaningfully, the device exhibits a fast response time of 13 ns, peak detectivity of 2 × 10¹⁰ and 10¹¹ cm Hz^1/2 W⁻¹ under blackbody radiation at 300 K and at 77 K.

Abstract

Uncooled infrared photodetectors have evoked widespread interest in basic research and military manufacturing because of their low-cost, compact detection systems. However, existing uncooled infrared photodetectors utilize the photothermoelectric effect of infrared radiation operating at 8–12 µm, with a slow response time in the millisecond range. Hence, the exploration of new uncooled mid-wavelength infrared (MWIR) heterostructures is conducive to the development of ultrafast and high-performance nano-optoelectronics. This study explores a van der Waals heterojunction on epitaxial HgCdTe (vdWs-on-MCT) as an uncooled MWIR photodetector, which achieves fast response as well as high detectivity for spectral blackbody detection. Specifically, the vdWs-on-MCT photodetector has a fast response time of 13 ns (77 MHz), which is approximately an order of magnitude faster than commercial uncooled MCT photovoltaic photodetectors. Importantly, the device exhibits a photoresponsivity of 2.5 A W^-1, quantum efficiency as high as 85%, peak detectivity of 2 × 10¹⁰ cm Hz^1/2 W^-1 under blackbody radiation at room temperature, and peak detectivity of up to 10¹¹ cm Hz^1/2 W^-1 at 77 K. Thereby, this work facilitates the effective design of high-speed and high-performance heterojunction uncooled MWIR photodetectors.

Recent progress and ongoing challenges for memtransistors are reviewed in the context of neuromorphic computing in solid-state circuits and systems. Gate-tunable learning and bio-realistic functions in multi-terminal synaptic devices are compared for memtransistors and related floating gate and ferroelectric memories, suggesting opportunities for new architectures that are suitable as hardware accelerators for artificial intelligence algorithms.

Abstract

Due to the increasing importance of artificial intelligence (AI), significant recent effort has been devoted to the development of neuromorphic circuits that seek to emulate the energy-efficient information processing of the brain. While non-volatile memory (NVM) based on resistive switches, phase-change memory, and magnetic tunnel junctions has shown potential for implementing neural networks, additional multi-terminal device concepts are required for more sophisticated bio-realistic functions. Of particular interest are memtransistors based on low-dimensional nanomaterials, which are capable of electrostatically tuning memory and learning behavior at the device level. Herein, a conceptual overview of the memtransistor is provided in the context of neuromorphic circuits. Recent progress is surveyed for memtransistors and related multi-terminal NVM devices including dual-gated floating-gate memories, dual-gated ferroelectric transistors, and dual-gated van der Waals heterojunctions. The different materials systems and device architectures are classified based on the degree of control and relative tunability of synaptic behavior, with an emphasis on device concepts that harness the reduced dimensionality, weak electrostatic screening, and phase-changes properties of nanomaterials. Finally, strategies for achieving wafer-scale integration of memtransistors and multi-terminal NVM devices are delineated, with specific attention given to the materials challenges for practical neuromorphic circuits.

Multilayer heterostructures with localized and sharp lateral interfaces across hundreds of individual van der Waals layers are realized from the anisotropic layered semiconductors SnS and GeS. Cathodoluminescence spectroscopy shows the transfer of electron–hole pairs across the lateral interfaces, demonstrating excellent connectivity without interfacial recombination. The results are promising for applications benefiting from thick layered crystals combined with in-plane carrier manipulation.

Abstract

Research on engineered materials that integrate different 2D crystals has largely focused on two prototypical heterostructures: Vertical van der Waals stacks and lateral heterostructures of covalently stitched monolayers. Extending lateral integration to few layer or even multilayer van der Waals crystals could enable architectures that combine the superior light absorption and photonic properties of thicker crystals with close proximity to interfaces and efficient carrier separation within the layers, potentially benefiting applications such as photovoltaics. Here, the realization of multilayer heterstructures of the van der Waals semiconductors SnS and GeS with lateral interfaces spanning up to several hundred individual layers is demonstrated. Structural and chemical imaging identifies {110} interfaces that are perpendicular to the (001) layer plane and are laterally localized and sharp on a 10 nm scale across the entire thickness. Cathodoluminescence spectroscopy provides evidence for a facile transfer of electron-hole pairs across the lateral interfaces, indicating covalent stitching with high electronic quality and a low density of recombination centers.

Advanced Materials Interfaces, Volume 8, Issue 22, November 23, 2021.

Peter Müller-Buschbaum and co-workers show in their article number 2100191 that ternary nanoswitches can be realized when combining multi-responsive block copolymer films with mixed vapor atmospheres. The combination of amphiphilicity and the co-nonsolvency effect is used to prepare three distinctly different film thicknesses as analyzed with neutron reflectivity and spectroscopy.

Nature Communications, Published online: 23 November 2021; doi:10.1038/s41467-021-26656-3

Here, the authors perform lightwave-driven terahertz scanning tunnelling microscopy and spectroscopy of graphene nanoribbons with atomic resolution in three dimensions, revealing localized wavefunctions that are inaccessible by conventional scanning tunnelling microscopy.

Nature Electronics, Published online: 22 November 2021; doi:10.1038/s41928-021-00671-0

By understanding the origins of instability in high-mobility amorphous oxide transistors, ultrastable thin-film transistors with mobilities of 70 cm2 (V s)–1 can be fabricated.

Nature Electronics, Published online: 22 November 2021; doi:10.1038/s41928-021-00672-z

This Perspective examines the development of integrated circuits based on layered two-dimensional materials, exploring where they are likely to first find commercial use and considers the challenges than need to be addressed to create highly scaled circuits.

Quasistatic equilibrium growth in confined reaction configurations (enclosure systems, inner-tube setups, sandwiched substrates, etc.) in a chemical vapor deposition system for high quality and large area graphene synthesis.

Abstract

This study reviews the majorly used chemical vapor deposition (CVD) with a focus on confined reaction configurations in which quasistatic equilibrium conditions are obtained for the fabrication of graphene with large size and high quality through controlled nucleation density, feedstock flux, and growth rates. The confinement configurations can also be used to tune the thickness, domain size and shape, and stacking order of the synthetic graphene. The confined CVD reaction configurations discussed include enclosure systems, inner-tube setups, sandwiched substrates, as well as other types of configurations. The advantages and limitations of the different confinement configurations are presented, along ways to optimize the operational parameters for them.

The interstitially tetrahedral O–V–S site in the vdW gap of V-doped 2D SnS₂ establishes the origin of the charge transfer mechanism between metal ion V⁴⁺ 3d and ligand O^2- 2p/S^2- 3p states and the decrease in the band gap by studying synchrotron-based techniques and first-principles density functional theory.

Abstract

Effects of electronic and atomic structures of V-doped 2D layered SnS₂ are studied using X-ray spectroscopy for the development of photocatalytic/photovoltaic applications. Extended X-ray absorption fine structure measurements at V K-edge reveal the presence of VO and VS bonds which form the intercalation of tetrahedral OVS sites in the van der Waals (vdW) gap of SnS₂ layers. X-ray absorption near-edge structure (XANES) reveals not only valence state of V dopant in SnS₂ is ≈4⁺ but also the charge transfer (CT) from V to ligands, supported by V L _α,β resonant inelastic X-ray scattering. These results suggest V doping produces extra interlayer covalent interactions and additional conducting channels, which increase the electronic conductivity and CT. This gives rapid transport of photo-excited electrons and effective carrier separation in layered SnS₂. Additionally, valence-band photoemission spectra and S K-edge XANES indicate that the density of states near/at valence-band maximum is shifted to lower binding energy in V-doped SnS₂ compare to pristine SnS₂ and exhibits band gap shrinkage. These findings support first-principles density functional theory calculations of the interstitially tetrahedral OVS site intercalated in the vdW gap, highlighting the CT from V to ligands in V-doped SnS₂.

Parallel 1D semiconductor channels connected by a superconducting strip are the core materials platform in recent theoretical proposals for exotic quantum states. In order to realize such device geometries, pairs of nanowires are grown in proximity and merged in situ by a common superconducting film. The applicability is demonstrated by implementing the double nanowires in quantum transport devices.

Abstract

Parallel 1D semiconductor channels connected by a superconducting strip constitute the core platform in several recent quantum device proposals that rely, for example, on Andreev processes or topological effects. In order to realize these proposals, the actual material systems must have high crystalline purity, and the coupling between the different elements should be controllable in terms of their interfaces and geometry. A strategy for synthesizing double InAs nanowires by the vapor-liquid-solid mechanism using III-V molecular beam epitaxy is presented. A superconducting layer is deposited onto nanowires without breaking the vacuum, ensuring pristine interfaces between the superconductor and the two semiconductor nanowires. The method allows for a high yield of merged as well as separate parallel nanowires with full or half-shell superconductor coatings. Their utility in complex quantum devices by electron transport measurements is demonstrated.

The fluorescence recovery after photobleaching (FRAP) microscopy, a powerful technology in cell biology, is first utilized to spatiotemporally manipulate and visually monitor the dynamics of photoexcited carrier in attractive 2D transition metal dichalcogenide monolayer (1L-TMD). This allows on-demand access to the photoluminescence dark state of 1L-TMD and deciphering of an interesting photoinduced dedoping effect in 1L-TMD after photodoping.

Abstract

Tailoring the photoluminescence (PL) of semiconductors through spatiotemporal manipulation of dynamics of photoexcited carriers is of paramount importance for understanding the emitting mechanism and developing high-performance devices. Herein, fluorescence recovery after photobleaching (FRAP) microscopy, a powerful tool in biology, is first utilized to simultaneously manipulate and monitor the dynamics of photoexcited carrier in attractive 2D transition metal dichalcogenide monolayers (1L-TMDs). This allows on-demand access to the PL dark state of 1L-TMDs, based on the triggered exciton–exciton annihilation by pump beam-initiated photodoping. Using a 0.7-µm-diametered pump laser, the PL dark region can be facilely tailored from ≈0.5 to ≈5 µm over a 10-µm-1L-WS₂ flake. An interesting photoinduced dedoping effect in 1L-TMDs after photodoping is discovered by FRAP, which has not been observed before and might account for the non-blinking emission of 1L-TMDs. The revealed mono-exponential photo-dedoping can also be kinetically tailored by ≈170-fold (k: 0.11–19.00 s^-1) by humidity, power of incident laser and type of 1L-TMDs. This study demonstrates the power of FRAP microscopy in exploring the effect of photoexcited carrier dynamics on the PL property of semiconductors, holding promises for understanding light-emitting mechanism and optimizing operational parameters for optoelectronic devices.

Electrochemical random-access memories using multilayered 2D titanium carbide MXene that combine the speed, linearity, write noise, switching energy, and endurance metrics essential for parallel acceleration of artificial neural networks with near ideal numerical accuracy in image recognition simulations are reported. The multilayered 2D MXene films are also stable after heat treatment needed for back-end-of-line integration with Si electronics.

Abstract

Synaptic devices with linear high-speed switching can accelerate learning in artificial neural networks (ANNs) embodied in hardware. Conventional resistive memories however suffer from high write noise and asymmetric conductance tuning, preventing parallel programming of ANN arrays. Electrochemical random-access memories (ECRAMs), where resistive switching occurs by ion insertion into a redox-active channel, aim to address these challenges due to their linear switching and low noise. ECRAMs using 2D materials and metal oxides however suffer from slow ion kinetics, whereas organic ECRAMs enable high-speed operation but face challenges toward on-chip integration due to poor temperature stability of polymers. Here, ECRAMs using 2D titanium carbide (Ti₃C₂T _x ) MXene that combine the high speed of organics and the integration compatibility of inorganic materials in a single high-performance device are demonstrated. These ECRAMs combine the speed, linearity, write noise, switching energy, and endurance metrics essential for parallel acceleration of ANNs, and importantly, they are stable after heat treatment needed for back-end-of-line integration with Si electronics. The high speed and performance of these ECRAMs introduces MXenes, a large family of 2D carbides and nitrides with more than 30 stoichiometric compositions synthesized to date, as promising candidates for devices operating at the nexus of electrochemistry and electronics.

Phototransistors

Metal–oxide-based phototransistors conventionally have a limit to their detection range and display a persistent photocurrent (PPC) phenomenon, which may hinder their potential applications in devices. In article number 2006091, Hyun Jae Kim and co-workers review the latest approaches for the absorption layer, according to surface treatment, structural engineering, and the absorption materials. In addition, they also discuss emerging applications and a general outlook. A metal–oxide–semiconductor-based phototransistor structure for light detection is depicted.

With the assistance of environmental scanning electron microscope, the high hydrophilic nature of pristine graphene with an extremely low water contact angle of ≈30° is confirmed, using the suspended, superclean graphene membrane to exclude the interference by substrates and contamination. This high hydrophilicity is revealed to originate from the charge transfer between graphene and water molecules through H–π interaction.

Abstract

The wettability of graphene remains controversial owing to its high sensitivity to the surroundings, which is reflected by the wide range of reported water contact angle (WCA). Specifically, the surface contamination and underlying substrate would strongly alter the intrinsic wettability of graphene. Here, the intrinsic wettability of graphene is investigated by measuring WCA on suspended, superclean graphene membrane using environmental scanning electron microscope. An extremely low WCA with an average value ≈30° is observed, confirming the hydrophilic nature of pristine graphene. This high hydrophilicity originates from the charge transfer between graphene and water molecules through H–π interaction. The work provides a deep understanding of the water–graphene interaction and opens up a new way for measuring the surface properties of 2D materials.

Publication date: 15 December 2021

Source: Joule, Volume 5, Issue 12

Author(s): Ian Marius Peters, Jens Hauch, Christoph Brabec, Parikhit Sinha

Author(s): Yucheng Jiang, Anpeng He, Run Zhao, Yu Chen, Guozhen Liu, Hao Lu, Jinlei Zhang, Qing Zhang, Zhuo Wang, Chen Zhao, Mingshen Long, Weida Hu, Lin Wang, Yaping Qi, Ju Gao, Quanying Wu, Xiaotian Ge, Jiqiang Ning, Andrew T. S. Wee, and Cheng-Wei Qiu

A new photoelectric device can convert light into charge that it can then store indefinitely.

[Phys. Rev. Lett. 127, 217401] Published Fri Nov 19, 2021