Jiuxiang Dai

Herein, for the first time, large-scale and patterned v-MoS₂ nanosheets (NS) using chemical vapor deposition (CVD) method and a TiO₂-induced layer are synthesized. This method can be used to fabricate various functional devices without etching. The photodetector is fabricated directly using square-patterned v-MoS₂ NS arrays. The method proposed could be extended to form other 2D materials.

Molybdenum disulfide (MoS₂) has recently gained attention due to the exciting and various nanostructures created by different synthesis processes and their superior physical and chemical properties. Although large-scale production with various nanostructures is achieved, only patterned film growth is demonstrated in specific locations. Traditional photolithography and etching processes are not suitable for patterning of 3D structures due to the huge height difference. The large-scale patterned growth of MoS₂ with different nanostructures is critical for sophisticated, customized electronic and optoelectronic devices. Herein, for the first time, the synthesis of large-scale and patterned MoS₂ nanosheets with vertically standing morphology (v-MoS₂ NS) using a chemical vapor deposition (CVD) method and a TiO₂-induced layer is reported. This method enables the preparation of patterned v-MoS₂ NS arrays with clean boundaries, uniform distributions, controllable locations, and various shapes, which can be used to fabricate various functional devices without etching. Next, it is demonstrated that the photodetectors can be fabricated directly using square-patterned v-MoS₂ NS arrays. The method proposed could be extended to form other 2D materials with vertical NS structures and provides a lot of new chances to legitimately design functional devices and systems in the future.

Nature Electronics, Published online: 14 October 2021; doi:10.1038/s41928-021-00653-2

This paper introduces Ta-doped MoSe₂ via substitutional doping. The transfer characteristic of the synthesized MoSe₂ can be controllably switched from n-type to ambipolar, and to p-type. Based on the Ta-doped MoSe₂, outstanding homojunction photodetectors (high external quantum efficiency ≈42% and fast response speed ≈20 µs) and inverters (high voltage gain ≈34) are demonstrated.

Abstract

2D materials, of which the carrier type and concentration are easily tuned, show tremendous superiority in electronic and optoelectronic applications. However, the achievements are still quite far away from practical applications. Much more effort should be made to further improve their performance. Here, p-type MoSe₂ is successfully achieved via substitutional doping of Ta atoms, which is confirmed experimentally and theoretically, and outstanding homojunction photodetectors and inverters are fabricated. MoSe₂ p–n homojunction device with a low reverse current (300 pA) exhibits a high rectification ratio (10⁴). The analysis of dark current reveals the domination of the Shockley–Read–Hall (SRH) and band-to-band tunneling (BTB) current. The homojunction photodetector exhibits a large open-circuit voltage (0.68 V) and short-circuit currents (1 µA), which is suitable for micro-solar cells. Furthermore, it possesses outstanding responsivity (0.28 A W⁻¹), large external quantum efficiency (42%), and a high signal-to-noise ratio (≈10⁷). Benefiting from the continuous energy band of homojunction, the response speed reaches up to 20 µs. Besides, the Ta-doped MoSe₂ inverter exhibits a high voltage gain (34) and low power consumption (127 nW). This work lays a foundation for the practical application of 2D material devices.

Memtransistors

In article number 2103175, Ye Zhou and co-workers utilize two-dimensional WSe₂ as channel material to fabricate a memtransistor for synaptic plasticity simulation. Benefitting from the multi-terminal input and high adjustability, the memtransistor can mimic both homosynaptic and heterosynaptic plasticity without introducing an extra terminal and can simultaneously offer versatile reconfigurability of excitatory and inhibitory plasticity.

A large tunneling magnetoresistance is observed in all van der Waals spin valves based on a Fe₃GeTe₂/InSe/Fe₃GeTe₂ (ferromagnet/semiconductor/ferromagnet) heterojunction. Devices with the same thickness of the InSe layer reveal either a tunneling or a metallic transport behavior due to pinholes. This finding suggests opportunities for further development of this spin valve as a platform for nonvolatile memories and spin-logic applications.

Abstract

2D layered chalcogenide semiconductors have been proposed as a promising class of materials for low-dimensional electronic, optoelectronic, and spintronic devices. Here, all-2D van der Waals vertical spin-valve devices, that combine the 2D layered semiconductor InSe as a spacer with the 2D layered ferromagnetic metal Fe₃GeTe₂ as spin injection and detection electrodes, are reported. Two distinct transport behaviors are observed: tunneling and metallic, which are assigned to the formation of a pinhole-free tunnel barrier at the Fe₃GeTe₂/InSe interface and pinholes in the InSe spacer layer, respectively. For the tunneling device, a large magnetoresistance (MR) of 41% is obtained under an applied bias current of 0.1 µA at 10 K, which is about three times larger than that of the metallic device. Moreover, the tunneling device exhibits a lower operating bias current but a more sensitive bias current dependence than the metallic device. The MR and spin polarization of both the metallic and tunneling devices decrease with increasing temperature, which can be fitted well by Bloch's law. These findings reveal the critical role of pinholes in the MR of all-2D van der Waals ferromagnet/semiconductor heterojunction devices.

A joint experiment-theory investigation is conducted to unveil the fracture mechanics of grain boundaries (GBs) in low-symmetry monolayer rhenium disulfide at the single-atom level. GBs having different Re chain alignment with respect to the GB orientation display disparate crack behaviors, indicating the GB type-dependent mechanical failure in anisotropic 2D polycrystals, which gives fundamental insights into GB engineering.

Abstract

Grain boundaries (GBs) play a central role in the fracture of polycrystals. However, the complexity of GBs and the difficulty in monitoring the atomic structure evolution during fracture greatly limit the understanding of the GB mechanics. Here, in situ aberration-corrected scanning transmission electron microscopy and density functional theory calculations are combined to investigate the fracture mechanics in low-symmetry, polycrystalline, 2D rhenium disulfide (ReS₂), unveiling the distinctive crack behaviors at different GBs with atomic resolution. Brittle intergranular fracture prefers to rip through the GBs that are parallel to the Re chains of at least one side of the GBs. In contrast, those GBs, which do not align with Re chains on either side of the GBs, are highly resistant to fracture, impeding or deflecting the crack propagation. These results disclose the GB type-dependent mechanical failure of anisotropic 2D polycrystals, providing new ideas for material reinforcement and controllable cutting via GB engineering.

Quasi-isotropically thermal conductive cellulose reinforced polyvinyl alcohol/nylon12 modified hexagonal boron nitride nanosheet films with high transparency, good electrical insulation, and super-flexibility are successfully fabricated through a vacuum filtration and subsequent self-assembly process. These films are very promising as thermal interface management materials for highly efficient heat dissipation of diverse electronic devices.

Abstract

Polymer-based thermal management materials (TIMs) show great potentials as TIMs due to their excellent properties, such as high insulation, easy processing, and good flexibility. However, the limited thermal conductivity seriously hinders their practical applications in high heat generation devices. Herein, highly transparent, insulating, and super-flexible cellulose reinforced polyvinyl alcohol/nylon12 modified hexagonal boron nitride nanosheet (PVA/(CNC/PA-BNNS)) films with quasi-isotropic thermal conductivity are successfully fabricated through a vacuum filtration and subsequent self-assembly process. A special structure composed of horizontal stacked hexagonal boron nitride nanosheets (h-BNNSs) connected by their warping edges in longitudinal direction, which is strengthened by cellulose nanocrystals, is formed in PVA matrix during self-assembly process. This special structure makes the PVA/(CNC/PA-BNNS) films show excellent thermal conductivity with an in-plane thermal conductivity of 14.21 W m⁻¹ K⁻¹ and a through-plane thermal conductivity of 7.29 W m⁻¹ K⁻¹. Additionally, the thermal conductive anisotropic constants of the as-obtained PVA/(CNC/PA-BNNS) films are in the range of 1 to 4 when the h-BNNS contents change from 0 to 60 wt%, exhibiting quasi-isotropic thermal conductivity. More importantly, the PVA/(CNC/PA-BNNS) films exhibit excellent transparency, super flexibility, outstanding mechanical strength, and electric insulation, making them very promising as TIMs for highly efficient heat dissipation of diverse electronic devices.

The direct atomic layer deposition of Al₂O₃ on monolayer (1L) MoS₂ supported by gold and by a common insulating substrate (Al₂O₃/Si) is investigated. The growth of compact and ultrathin (≈3.6 nm) Al₂O₃ film is obtained on 1L MoS₂/Au, and it is discussed in terms of the peculiar interaction with the metal substrate.

Abstract

In this paper, the authors demonstrate the atomic layer deposition (ALD) of highly homogeneous and ultrathin (≈3.6 nm) Al₂O₃ films with very good insulating properties (breakdown field of ≈10–12 MV cm⁻¹) directly onto monolayer (1L) MoS₂ exfoliated on gold. Differently than in the case of 1L MoS₂ supported by a common insulating substrate (Al₂O₃/Si), a better nucleation process of the high-k film is observed on the 1L MoS₂/Au system since the ALD early stages. Atomic force microscopy analyses show a ≈50% Al₂O₃ surface coverage just after 10 ALD cycles, its increase to >90% (after 40 cycles), and a uniform ≈3.6 nm film (after 80 cycles). The Al₂O₃ density on bilayer MoS₂ is found to be significantly reduced with respect to 1L MoS₂/Au, suggesting a role of screened interface charges with the metal substrate on the adsorption of ALD precursors. Finally, Raman and photoluminescence spectroscopy show a p-type doping and tensile strain of 1L MoS₂ induced by the Au substrate, providing an insight on the evolution of vibrational and optical properties after the Al₂O₃ deposition. The direct ALD growth of Al₂O₃ on large-area 1L MoS₂ induced by the Au underlayer can be of wide interest for electronic applications.

Bottom-Up Fabrication

In article number 2103708, Joon-Ho Oh, Ka-Hyun Kim, and co-workers present the unprecedented fabrication of crystalline silicon wafers and solar cells by a fully bottom-up growth method without wasting substrate, in contrast to conventional technologies that sacrifice large amounts of raw material. A plasma-assisted epitaxially grown silicon seed layer with a self-organized nanogap is key for the realization of the fully bottom-up process. The results represent a technological breakthrough in advanced silicon microelectronics and photovoltaics.

Self-Healing Devices

An innovative concept of a self-healed electrode-channel system employing ultrathin metallic copper monosulfide (CuS) and monolayered molybdenum disulfide (MoS₂) is demonstrated by John Hong, SeungNam Cha, and co-workers in article number 2102091 for the design of defect-curable transistors and phototransistors. Excess sulfur adatoms from the CuS electrode spontaneously heal the defect sites in the MoS₂ channel for record-high transistor and phototransistor performance.

Ripples are strongly coupled with ferroic orders in 2D ferroics. Taking monolayer ferroelectric–ferroelastic GeSe as an example, it is shown that rippling ferroic phase transition and domain switching can render ultrahigh piezoelectric performance over a broad temperature range in 2D materials. This provides valuable guidance for ripple engineering in 2D materials with potential applications in 2D flexible electronics.

Abstract

Ripples are a class of native structural defects widely existing in 2D materials. They originate from the out-of-plane flexibility of 2D materials introducing spatially evolving electronic structure and friction behavior. However, the effect of ripples on 2D ferroics has not been reported. Here a molecular dynamics study of the effect of ripples on the temperature-induced ferroic phase transition and stress-induced ferroic domain switching in ferroelastic-ferroelectric monolayer GeSe is presented. Ripples stabilize the short-range ferroic orders in the high-temperature phase with stronger ferroicity and longer lifetime, thereby increasing the transition temperature upon cooling. In addition, ripples significantly affect the domain switching upon loading, changing it from a highly correlated process into a ripple-driven localized one where ripples act as source of dynamical random stress. These results reveal the fundamental role of ripples on 2D ferroicity and provide theoretical guidance for ripple engineering of controlled phase transition and domain switching with potential applications in flexible 2D electronics.

This is the first study to use a molten-salt-assisted electrochemical etching (MS-E-etching) method to synthesize Ti₃C₂Cl₂ without metallic impurities. The T _x surface terminations could be in situ modified from −Cl to −O and/or −S in this one-pot process. The obtained −O-terminated Ti₃C₂T _x electrode exhibited a significant increase in capacity.

Abstract

Surface terminations of two-dimensional MXene (Ti₃C₂T _x ) considerably impact its physicochemical properties. Commonly used etching methods usually introduce -F surface terminations or metallic impurities in MXene. We present a new molten-salt-assisted electrochemical etching method to synthesize fluorine-free Ti₃C₂Cl₂. Using electrons as reaction agents, cathode reduction and anode etching can be spatially isolated; thus, no metallics are present in the Ti₃C₂Cl₂ product. The surface terminations can be in situ modified from −Cl to −O and/or −S, which considerably shortens the modification steps and enriches the variety of surface terminations. The obtained −O-terminated Ti₃C₂T _x are excellent electrode materials for supercapacitors, exhibiting capacitances of 225 F g⁻¹ at 1.0 Ag⁻¹, good rate performance (91.1 % at 10 Ag⁻¹), and excellent capacitance retention (100 % after 10000 charge/discharge cycles at 10 Ag⁻¹), which is superior to multi-layered Ti₃C₂T _x prepared by other etching methods.

Highly crystalline macroassembled graphene nanofilms prepared by a camphor-assisted cooling–contraction strategy show excellent physical properties and advantages in high-performance electronic and optoelectronic devices.

Abstract

A “cooling–contraction” method to separate large-area (up to 4.2 cm in lateral size) graphene oxide (GO)-assembled films (of nanoscale thickness) from substrates is reported. Heat treatment at 3000 °C of such free-standing macroscale films yields highly crystalline “macroassembled graphene nanofilms” (nMAGs) with 16–48 nm thickness. These nMAGs present tensile strength of 5.5–11.3 GPa (with ≈3 µm gauge length), electrical conductivity of 1.8–2.1 MS m⁻¹, thermal conductivity of 2027–2820 W m⁻¹ K⁻¹, and carrier relaxation time up to ≈23 ps. As a demonstration application, an nMAG-based sound-generator shows a 30 µs response and sound pressure level of 89 dB at 1 W cm⁻². A THz metasurface fabricated from nMAG has a light response of 8.2% for 0.159 W mm⁻² and can detect down to 0.01 ppm of glucose. The approach provides a straightforward way to form highly crystallized graphene nanofilms from low-cost GO sheets.

Research progress for elemental 2D materials in the areas of synthesis, properties, and applications in catalytic nitrogen reduction for electrochemical ammonia synthesis is examined systematically. The major challenges in the solution-processed synthesis of elemental 2D materials and their use for electrochemical ammonia synthesis are discussed.

Abstract

Graphene and related elemental 2D materials have become core materials in nanotechnology and shown great promise for industrially important electrocatalysis reactions. Although excellent progress has been made over the past few years, research into the field of elemental 2D materials beyond graphene is still at an early stage. Importantly, recent research has revealed the promising efficacy of elemental 2D materials as effective nitrogen reduction reaction (NRR) electrocatalysts due to their many excellent properties including high surface activities, acting as active sites for effective functionalization and defect engineering. This review provides a comprehensive account of recent advances in elemental 2D materials with a major focus on the solution-based synthesis routes and their applications in electrocatalytic NRR for ammonia (NH₃) production. After a concise overview of elemental 2D materials, the advantages and challenges of currently available methods for the synthesis of these 2D materials are discussed. Then, the review focuses on the use of these emerging 2D materials in the electrocatalytic reduction of N₂ for sustainable (NH₃) synthesis. Finally, the challenges still to be addressed, and important perspectives in this attractive field are emphasized.

Publication date: December 2021

Source: Materials Today, Volume 51

Author(s): Han-xin Wei, Lin-bo Tang, Ying-de Huang, Zhen-yu Wang, Yu-hong Luo, Zhen-jiang He, Cheng Yan, Jing Mao, Ke-hua Dai, Jun-chao Zheng

Publication date: October 2021

Source: Materials Today, Volume 49

Author(s): Cordelia Sealy

Publication date: October 2021

Source: Materials Today, Volume 49

Author(s): Laurie Winkless

Publication date: October 2021

Source: Materials Today, Volume 49

Author(s): Laurie Donaldson

Author(s): Maxim Trushin and A. H. Castro Neto

Two-dimensional (2D) materials can roll up, forming stable scrolls under suitable conditions. However, the great diversity of materials and fabrication techniques has resulted in a huge parameter space significantly complicating the theoretical description of scrolls. In this Letter, we describe a u...

[Phys. Rev. Lett. 127, 156101] Published Fri Oct 08, 2021