Chemical Vapor Deposition Syntheses of Wafer-Scale 2D Transition Metal Dichalcogenide Films toward Next-Generation Integrated Circuits Related Applications

2D semiconducting transition metal dichalcogenides (TMDCs), most with a formula of MX₂ (M=Mo, W; X=S, Se, etc.), have emerged as promising channel materials for next-generation integrated circuits, considering their dangling-bond-free surfaces, moderate bandgaps, relatively high carrier mobilities, etc. Wafer-scale preparation of 2D MX₂ films holds fundamental significance for realizing their applications. Chemical vapor deposition (CVD) is recognized as the most promising method for preparing electronic-grade 2D MX₂ films. This review hereby summarizes the recent progress in CVD syntheses of wafer-scale 2D MX₂ films and their applications in logic operations, data storage, and image capturing/processing related fields. The first part focuses on the wafer-scale syntheses of 2D MX₂ films through designing homogeneous metal precursor supply routes (e.g., precoating soluble precursor, feeding gaseous precursor, designing independent multisource supply or face-to-face metal precursor supply routes). The second part highlights the epitaxial growth of monolayer MX₂ single crystals on single-crystal Au substrates and well-designed sapphire substrates. The third part introduces various functional device/circuit related applications of CVD-derived 2D MX₂ wafers. Finally, challenges and prospects are discussed from the viewpoints of the controlled synthesis, reliable characterization, and damage-free transfer of 2D MX₂, as well as the fabrication and integration of high-performance devices.     

Lattice vibration and Raman scattering of two-dimensional van der Waals heterostructure

Research on two-dimensional(2D) materials and related van der Waals heterostructures(vdWHs) is intense and remains one of the leading topics in condensed matter physics.Lattice vibrations or phonons of a vdWH provide rich information,such as lattice structure,phonon dispersion,electronic band structure and electron–phonon coupling.Here,we provide a mini review on the lattice vibrations in vdWHs probed by Raman spectroscopy.First,we introduced different kinds of vdWHs,including their structures,properties and potential applications.Second,we discussed interlayer and intralayer phonon in twist multilayer graphene and MoS2.The frequencies of interlayer and intralayer modes can be reproduced by linear chain model(LCM)and phonon folding induced by periodical moiré potentials,respectively.Then,we extended LCM to vdWHs formed by distinct 2D materials,such as MoS2/graphene and hBN/WS2 heterostructures.We further demonstrated how to calculate Raman intensity of interlayer modes in vdWHs by interlayer polarizability model.     

制样溶剂及基底对原子力显微镜观测单壁碳纳米管的影响

本文选用不同基底和溶剂进行单壁碳纳米管(SWNTs)样品制备,采用原子力显微镜(atomic force microscope,AFM)观察进行表征和观测。结果表明,溶剂和基底对样品制备十分重要,会直接影响AFM观测效果。以去离子水为溶剂进行样品分散时,SWNTs在石英、硅片和云母三种基底上都出现了不同程度的聚积和重叠,影响观测效果。而采用乙醇为溶剂,在石英或云母为基底进行成像时,均可获得质量良好的图像。选择云母作为基底,可以大大减少基底处理时间,制样方便快速,并且能够获得理想的观测效果。     

非制冷型中长波铟砷锑探测器

工作在中、长波红外波段(波长5~12μm)的红外探测器在红外制导、红外成像、环境监测及资源探测等方面有着重要而广阔的应用前景。目前中国军用和民用对这一波段的非制冷型、快速响应的光子型红外探测器有迫切需求。文中用熔体外延(ME)法在InAs(砷化铟)衬底上生长的InAs0.05Sb0.95(铟砷锑)厚膜单晶,制作了高灵敏度、非制冷型、中长波光导型探测器,探测器上安装了Ge(锗)浸没透镜。傅里叶变换红外(FTIR)吸收光谱显示InAsSb材料的本征吸收边出现在波长8μm以后。InAs0.05Sb0.95探测器的光谱响应波长范围为2~9μm。室温下,在波长6.5μm处的峰值探测率Dλp*达到5.4×109 cm·Hz1/2·W-1,在波长8.0μm和9.0μm处的探测率D*分别为9.3×108和1.3×108 cm·Hz1/2·W-1,显示了InAsSb探测器的优越性能及对红外探测和成像的应用前景。     

Strong Interlayer Transition in Few-Layer InSe/PdSe2 van der Waals Heterostructure for Near-Infrared Photodetection

Near infrared (NIR) photodetectors based on 2D materials are widely studied for their potential application in next generation sensing, thermal imaging, and optical communication. Construction of van der Waals (vdWs) heterostructure provides a tremendous degree of freedom to combine and extend the features of 2D materials, opening up new functionalities on photonic and optoelectronic devices. Herein, a type-II InSe/PdSe₂ vdWs heterostructure with strong interlayer transition for NIR photodetection is demonstrated. Strong interlayer transition between InSe and PdSe₂ is predicted via density functional theory calculation and confirmed by photoluminance spectroscopy and Kelvin probe force microscopy. The heterostructure exhibits highly sensitive photodetection in NIR region up to 1650 nm. The photoresponsivity, detectivity, and external quantum efficiency at this wavelength respectively reaches up to 58.8 A W⁻¹, 1 × 10¹⁰ Jones, and 4660%. The results suggest that the construction of vdWs heterostructure with strong interlayer transition is a promising strategy for infrared photodetection, and this work paves the way to developing high-performance optoelectronic devices based on 2D vdWs heterostructures.     

Synthesis,properties, and applications of large-scale two-dimensional materials by polymer-assisted deposition

Two-dimensional(2D) materials have attracted considerable attention because of their novel and tunable electronic,optical, ferromagnetic, and chemical properties. Compared to mechanical exfoliation and chemical vapor deposition, polymer-assisted deposition(PAD) is more suitable for mass production of 2D materials owing to its good reproducibility and reliability. In this review, we summarize the recent development of PAD on syntheses of 2D materials. First, we introduce principles and processing steps of PAD. Second, 2D materials, including graphene, MoS2, and MoS2/glassy-graphene heterostructures, are presented to illustrate the power of PAD and provide readers with the opportunity to assess the method. Last, we discuss the future prospects and challenges in this research field. This review provides a novel technique for preparing 2D layered materials and may inspire new applications of 2D layered materials.     

Low-loss three-dimensional waveguide crossings using adiabatic interlayer coupling

The silica-based planar lightwave circuit (PLC) is a promising technology for application to a reconfigurable optical add/drop multiplexer (ROADM), which contains arrayed waveguide gratings (DEMUX/MUX), variable optical attenuators (VOAs), switches (SWs), and tap monitors (TAPs) as shown in Fig. 1. Specifically, single-chip integration with higher delta (D) contrast waveguides, which allow a small bending radius, enables us to realise compact and low-cost ROADM modules [1]. The critical problem with regards to single-chip integration is the large excess loss caused by conventional two-dimensional (2D) waveguide crossings as shown by the broken line in Fig. 1, because a large channel number of more than 40 ch is strongly required in ROADMsystems, and the crossing loss will accumulate in proportion to the channel number. On the other hand, three-dimensional (3D) waveguide crossings are attractive because they are fundamentally lossless [2, 3]. The most important technical aspect with regards to 3D waveguide crossings is to find a way to interconnect between vertically stacked waveguides. Various approaches have been proposed for realising interlayer coupling. One approach is direct writing using a femtosecond laser [2]. The problem with this approach is that the writing process time increases in proportion to the number of channels. Another approach involves using vertical directional couplers to realise interlayer coupling of the double waveguide structures [3, 4]. In this case, a severe fabrication tolerance is required if we are to form a directional coupler with 100% coupling. To overcome these problems, we propose and demonstrate 3D waveguide crossings based on a triple waveguide layer structure and adiabatic double/stacked-core modefield (MF) converters. By adopting simple mode conversion with the middle waveguide layer, we can provide stable low-loss interlayer coupling between vertically stacked upper and lower waveguides.     

Seed‐Induced Vertical Growth of 2D Bi2O2Se Nanoplates by Chemical Vapor Transport

As two‐dimensional (2D) layered materials attract more attention owing to their unique optical, electrical, and thermal properties, there are persistent efforts to grow high‐quality 2D layered materials for fundamental research and device applications. While large‐area 2D layered materials with high crystal quality can be obtained through chemical vapor transport, the strong binding between 2D layered materials and substrates poses a significant challenge for attempts to reveal their intrinsic properties and to use these 2D building blocks for constructing advanced heterostructured devices. Therefore, it would be ideal to grow high‐quality 2D materials with minimized contact and binding with substrate. Through both calculation and experiment, it is demonstrated that by introducing a seed layer at the nucleation stage, the crystallographic disregistry and the corresponding adhesion energy between 2D materials and substrate can be altered, resulting in a change of crystal surface in contact with the substrate, and therefore vertical growth of 2D materials on substrates. As an example, it is demonstrated that with Bi₂O₃ serving as a seed layer, vertical growth of 2D plates of Bi₂O₂Se on mica substrates can be realized. These vertically grown 2D nanoplates of Bi₂O₂Se can be conveniently transferred with their thermal properties investigated for the first time.     

Co‐Interlayer Engineering toward Efficient Green Quasi‐Two‐Dimensional Perovskite Light‐Emitting Diodes

With respect to three‐dimensional (3D) perovskites, quasi‐two‐dimensional (quasi‐2D) perovskites have unique advantages in light‐emitting devices (LEDs), such as strong exciton binding energy and good phase stability. Interlayer ligand engineering is a key issue to endow them with these properties. Rational design principles for interlayer materials and their processing techniques remain open to investigation. A co‐interlayer engineering strategy is developed to give efficient quasi‐2D perovskites by employing phenylbutylammonium bromide (PBABr) and propylammonium bromide (PABr) as the ligand materials. Preparation of these co‐interlayer quasi‐2D perovskite films is simple and highly controllable without using antisolvent treatment. Crystallization and morphology are readily manipulated by tuning the ratio of co‐interlayer components. Various optical techniques, including steady and ultrafast transient absorption and photoluminescence spectroscopies, are used to investigate their excitonic properties. Photoluminescence quantum yield (PLQY) of the perovskite film is dramatically improved to 89% due to the combined optimization of exciton binding energy and suppression of trap state formation. Accordingly, a high current efficiency of 66.1 cd A^?1 and an external quantum efficiency of 15.1% are achieved for green co‐interlayer quasi‐2D perovskite LEDs without using any light out‐coupling techniques, indicating that co‐interlayer engineering is a simple and effective approach to develop high‐performance perovskite electroluminescence devices.     

半导体激光二极管的光纤耦合技术

将半导体激光二级管（LD）发出的光更高效地注入到光纤中是光纤激光器与光纤放大器研究的先决条件.半导体激光二级管包括二极管单管、条形巴、二维堆栈和二极管阵列等,其各自的耦合技术之间有联系也有区别.分析介绍了有代表性的柱状楔形法、V型槽法、微透镜法等二极管单管与光纤的耦合技术;光纤束耦合法、光束整形法等二极管条形巴与光纤的耦合技术;以及二极管二维堆栈和二极管阵列与光纤的耦合技术等各种光纤耦合技术,比较了这些方法之间的共通点,供今后的研究人员选择和参考.     

Highly Luminescent and Stable Green Quasi‐2D Perovskite‐Embedded Polymer Sheets by Inkjet Printing

Perovskite materials serve as promising candidates for display and lighting due to their excellent optical properties, including tunable bandgaps and efficient luminescence. However, their efficiency and stability must be improved for further application. In this work, quasi‐two‐dimensional (quasi‐2D) perovskites embedded in different polymers are prepared by inkjet printing to construct any luminescent patterns/pictures on the polymer substrates. The optimized quantum yield reaches over 65% by polyvinyl‐chloride‐based quasi‐2D perovskite composites. In addition, as‐fabricated perovskite?polymer composites with patterns show excellent resistance to abrasion, moisture, light irradiation, and chemical erosion by various solvents. Both quantum yield and lifetime are superior to those reported to date. These achievements are attributed to the introduction of the PEA⁺ cations to improve the luminance and stability of perovskite. This patterned composite can be useful for color‐conversion films with low cost and large‐scale fabrication.     

1D Van der Waals Polymers with Nonlinear Optical Performance Approaching Theoretical Upper Limit

Nonlinear optical (NLO) materials are of great importance for applications in lasers, atomic clocks, free-space communication, etc. Herein, inspired by the recent prediction of excellent second harmonic generation (SHG) performance in van der Waals (vdW) materials with 1D building blocks, 14 new NLO materials are found from 244 bulk crystals constructed with 1D polymers using high-throughput first-principles calculations. Nearly half of the new NLO materials exhibit superior NLO performance with SHG susceptibilities approaching the theoretical upper limit. The 2D form of 11 candidates inherits the NLO property covering UV, visible, and infrared regions. Bader charge analysis reveals that the SHG susceptibility is determined by the charge difference of ions on the chains. Finally, it is proposed that superior NLO materials can be found in materials with proper bandgaps and large charge differences on the chains. This work not only screens out candidates with outstanding NLO performance in vdW materials with 1D building blocks but also provides a guideline for the search and design of NLO vdW 1D polymer patterns with excellent NLO properties.     

A Simple Method for the Growth of Very Smooth and Ultra-Thin GaSb Films on GaAs (111) Substrate by MOCVD

We present a simple thermal treatment with the antimony source for the metal–organic chemical vapor deposition of thin GaSb films on GaAs (111) substrates for the first time. The properties of the as-grown GaSb films are systematically analyzed by scanning electron microscopy, atomic force microscopy, x-ray diffraction, photo-luminescence (PL) and Hall measurement. It is found that the as-grown GaSb films by the proposed method can be as thin as 35 nm and have a very smooth surface with the root mean square roughness as small as 0.777 nm. Meanwhile, the grown GaSb films also have high crystalline quality, of which the full width at half maximum of the rocking-curve is as small as 218 arcsec. Moreover, the good optical quality of the GaSb films has been demonstrated by the low-temperature PL. This work provides a simple and feasible buffer-free strategy for the growth of high-quality GaSb films directly on GaAs substrates and the strategy may also be applicable to the growth on other substrates and the hetero-growth of other materials.     

Improved ambipolar charge injection in organic field-effect transistors with low cost metal electrode using polymer sorted semiconducting carbon nanotubes

Solution-processed thin film transistors can be implemented using simple and low cost fabrication, and are the best candidates for commercialization due to their application to a range of wearable electronics. We report an ambipolar charge injection interlayer that can improve both hole and electron injection in organic field-effect transistors (OFETs) with inexpensive source-drain electrodes. The solution processed ambipolar injection layer is fabricated by selective dispersion of semiconducting single walled carbon nanotubes using poly(9,9-dioctylfluorene). OFETs with molybdenum (Mo) contacts and interlayer (Mo/interlayer OFETs) exhibit superior performance, including higher hole and electron mobilities, device yield, lower threshold voltages, and lower trap densities than those of bare transistors. While OFETs with Mo contacts show unipolar p-type behaviour, Mo/interlayer OFETs display ambipolar transport due to significant enhancement of electron injection. In the p-type region, transistor performance is comparable to devices with gold (Au). Hole mobility is increased approximately ten-fold over devices with only Mo contacts. The electron mobility of Mo/interlayer OFETs is 0.05 cm²V⁻¹s⁻¹, which is higher than devices with Au electrodes. The p-type contact resistances of Mo/interlayer OFETs are half those of OFETs with Mo contacts. Trap density in Mo/interlayer OFETs is one order magnitude lower than that of pristine devices. We also demonstrate that this approach is extendible to other metals (nickel) and n-type semiconductors with different energy levels. Injection by tunnelling is suggested as the mechanism of ambipolar injection.     

军用纳米金刚石膜的研究与应用综述

对金刚石膜(及类金刚石膜)与传统光学材料的特性作了比较,分析了美军对于金刚石膜军用光学应用的需求以及金刚石膜在现有高科技武器装备中的应用前景,介绍了研制纳米金刚石膜的关键技术问题(主要是CVD和PLD方法)以及国内外研究和应用现状。     

Large‐Scale Synthesis of Bi‐layer Graphene in Strongly Coupled Stacking Order

Large‐scale synthesis of single‐layer graphene (SLG) by chemical vapor deposition (CVD) has received a lot of attention recently. However, CVD synthesis of AB stacked bi‐layer graphene (BLG) is still challenging. Here, we report synthesis of BLG homogeneously at large scale by thermal CVD. The 2D Raman band of CVD BLG splits into four components, suggesting splitting of electronic bands due to strong interlayer coupling. The splitting of electronic bands in CVD BLG is further evidenced by the study of near infrared absorption and carrier dynamics are probed by transient absorption spectroscopy. UV photoelectron spectroscopy invesigation also indiates CVD BLG possesses different electronic structures to those of CVD SLG. The growth mechanism of BLG is found to be related to catalytic activity of the copper (Cu) surface, which is determined by the purity of Cu foils employed in the CVD process. Our work shows that strongly coupled or even AB stacked BLG can be grown on Cu foils at large scale, which is of particular importance for device applications based on their split electronic bands.     

Tunable Optical Mode Ferromagnetic Resonance in FeCoB/Ru/FeCoB Synthetic Antiferromagnetic Trilayers under Uniaxial Magnetic Anisotropy

Ferromagnetic resonance (FMR) is one of the most important characteristics of soft magnetic materials, which practically sets the maximum operation speed of these materials. There are two FMR modes in exchange coupled ferromagnet/nonmagnet/ferromagnet sandwich films. The acoustic mode has relatively lower frequency and is widely used in radio‐frequency/microwave devices, while the optical mode is largely neglected due to its tiny permeability even though it supports much higher frequency. Here, a realistic method is reported to enhance the permeability in the optical mode to an applicable level. FeCoB/Ru/FeCoB trilayers are carefully engineered with both uniaxial magnetic anisotropy and antiferromagnetic interlayer exchange coupling. This special magnetic structure exhibits a high optical mode frequency up to 11.28 GHz and a maximum permeability of 200 at resonance. An abnormally low inverse switch field (<200 Oe, less than 1/5 of the single layer) is observed which can effectively switch the system from optical mode with higher frequency into acoustic mode with lower frequency. The optical mode frequency and inverse switch field can be controlled by tailoring the interlayer coupling strengths and the uniaxial anisotropy fields, respectively. The tunable optical mode resonance thus can increase operation frequency while reduce operation field overhead in FMR based devices.     

Optoelectronics based on 2D TMDs and heterostructures

2D materials including graphene and TMDs have proven interesting physical properties and promising optoelectronic applications. We reviewed the growth, characterization and optoelectronics based on 2D TMDs and their heterostructures, and demonstrated their unique and high quality of performances. For example, we observed the large mobility, fast response and high photo-responsivity in MoS₂, WS₂ and WSe₂ phototransistors, as well as the novel performances in vdW heterostructures such as the strong interlayer coupling, am-bipolar and rectifying behaviour, and the obvious photovoltaic effect. It is being possible that 2D family materials could play an increasingly important role in the future nano- and opto-electronics, more even than traditional semiconductors such as silicon.     

Ethanol Assisted Transfer for Clean Assembly of 2D Building Blocks and Suspended Structures

As two dimensional materials (2D materials) demonstrate unique and diverse properties, clean transfer methods can serve a cornerstone for creative assembly of these 2D building blocks for both fundamental explorations and versatile applications. One of the major challenges for preserving the pristine properties of 2D materials during transfer and construction is to debond 2D materials from original substrates without inducing structural damage and external contamination. In this work, through both molecular dynamic studies and experimental demonstration, it is found that droplets of ethanol, a common and environmental friendly solvent, can be used to effectively reduce the adhesion energy between 2D materials and substrates, and therefore enable a clean transfer method for 2D mterials. Various assembled structures based on 2D building blocks, such as van der Waals heterostructures, predesigned artificial patterns, 2D materials on suspended devices are all demonstrated. Thermal conductivity measurements of MoS₂ nanosheets on a suspended microbridge device also confirm the successful application of suspended 2D transfer. It is expected that this ethanol assisted transfer method can enable clean assembly of 2D building blocks for construction of novel structures and suspended devices.     

Progress and Insight of Van der Waals Heterostructures Containing Interlayer Transition for Near Infrared Photodetectors

Van der Waals (vdWs) heterostructures enable bandgap engineering of different 2D materials to realize the interlayer transition via type-II band alignment leading to broaden spectrum that is beyond the cut-off wavelength of individual 2D materials. Interlayer transition has a significant effect on the optoelectronic performance of vdWs heterostructure devices, and strong interlayer transition in 2D vdWs heterojunction is always demandable for sufficient charge transfer and rapid speed response. Herein, a state-of-the-art review is presented on recent progress on interlayer transition in vdWs heterostructures for near-infrared (NIR) photodetectors. First, the general synthesis techniques for vdWs heterostructures, band alignments in the vdWs heterostructures are provided. Then, the mechanism of interlayer transition in vdWs heterostructure and recent progress on interlayer transition in vdWs heterostructures for NIR photodetectors are summarized. Afterward, some worthy applications of NIR photodetectors are reviewed in related areas of this topic. At the last, an outlook, challenges, and future research directions of vdWs heterostructures for photodetectors at broaden response spectrum are presented.