Flexible electronics and optoelectronics of 2D van der Waals materials

Flexible electronics and optoelectronics exhibit inevitable trends in next-generation intelligent industries,including healthcare and wellness,electronic skins,the automotive industry,and foldable or rollable displays.Traditional bulk-material-based flexible devices considerably rely on lattice-matched crystal structures and are usually plagued by unavoidable chemical disorders at the interface.Two-dimensional van der Waals materials(2D VdWMs)have exceptional multifunctional properties,including large specific area,dangling-bond-free interface,plane-to-plane van der Waals interactions,and excellent mechanical,electrical,and optical properties.Thus,2D VdWMs have considerable application potential in functional intelligent flexible devices.To utilize the unique properties of 2D VdWMs and their van der Waals heterostructures,new designs and configurations of electronics and optoelectronics have emerged.However,these new designs and configurations do not consider lattice mismatch and process incompatibility issues.In this review,we summarized the recently reported 2D VdWM-based flexible electronic and optoelectronic devices with various functions thoroughly.Moreover,we identified the challenges and opportunities for further applications of 2D VdWM-based flexible electronics and optoelectronics.

与硅基技术兼容的二维过渡金属硫族化合物电子器件

作为现代信息社会的物理基石,以硅基材料为核心的集成电路极大推动了人类现代文明的进程.但是,随着晶体管特征尺寸微缩逐渐接近物理极限,传统硅基材料出现了电学性能衰退、异质界面失稳等挑战,导致集成电路数据处理能力提升难、功耗急剧增加等问题产生.超薄二维过渡金属硫族化合物(transition metal dichalcogenides,TMDCs)具有表面平整无悬挂键、电输运性能优异、静电控制力强、化学性质稳定等优势,可有效解决上述问题,被认为是后摩尔时代集成电路的最具潜力候选材料之一.目前,二维TMDCs集成电路研究在多个关键领域均取得了突破性成果,但距离产业化应用仍需要克服一些挑战.本文着重介绍了二维TMDCs材料与电子器件在集成电路应用的各方面优势,系统阐明了二维TMDCs集成电路在材料控制生长、范德华界面优化以及器件设计构筑等方面的关键科学问题,提出了相应解决办法和应对措施,分析了二维TMDCs集成电路产业化进程中的综合性挑战,明确了“与硅基技术兼容”二维TMDCs集成电路发展路线的优势、可行性与突破方向.

Universal transfer of full-class metal electrodes for barrier-free two-dimensional semiconductor contacts

Metal–semiconductor contacts are crucial components in semiconductor devices.Ultrathin two-dimensional transition-metal dichalcogenide semiconductors can sustain transistor scaling for next-generation integrated circuits.However,their performance is often degraded by conventional metal deposition,which results in a high barrier due to chemical disorder and Fermi-level pinning(FLP).Although,transferring electrodes can address these issues,they are limited in achieving universal transfer of full-class metals due to strong adhesion between pre-deposited metals and substrates.Here,we propose a nanobelt-assisted transfer strategy that can avoid the adhesion limitation and enables the universal transfer of over 20 different types of electrodes.Our contacts obey the Schottky–Mott rule and exhibit a FLP of S=0.99.Both the electron and hole contacts show record-low Schottky barriers of 4.2 and 11.2 meV,respectively.As a demonstration,we construct a doping-free WSe_(2) inverter with these high-performance contacts,which exhibits a static power consumption of only 58 pW.This strategy provides a universal method of electrode preparation for building high-performance post-Moore electronic devices.

Silicon-processes-compatible contact engineering for twodimensional materials integrated circuits

Two-dimensional(2D)semiconductors,especially transition metal dichalcogenides(TMDCs),have been proven to be excellent channel materials for the next-generation integrated circuit(IC).However,the contact problem between 2D TMDCs and metal electrodes has always been one of the main factors restricting their development.In this review,we summarized recent work on 2D TMDCs contact from the perspective of compatible integration with silicon processes and practical application requirements,including the contact performance evaluation indicators,special challenges encountered in 2D TMDCs,and recent optimization methods.Specifically,we sorted out and highlighted the performance indicators of 2D TMDCs contacts,including contact resistance(RC),contact scaling,contact stability,and contact electrical/thermal conductivity.Special challenges of 2D TMDCs and metal contact,such as severe Fermi level pinning,large RC,and difficult doping,are systematically discussed.Furthermore,typical methods for optimizing 2D TMDCs RC,edge contact strategies for scaling contact lengths,and solutions for improving contact stability are reviewed.Based on the current research and problems,the development direction of 2D TMDCs contacts that meet the silicon-based compatible process and application performance requirements is proposed.

Two-dimensional transition metal dichalcogenides for post-silicon electronics

Rapid advancements in information technology push the explosive growth in data volume,requiring greater computing-capability logic circuits.However,conventional computing-capability improving technology,which mainly relies on increasing transistor number,encounters a significant challenge due to the weak field-effect characteristics of bulk siliconbased semiconductors.Still,the ultra-thin layered bodies of two-dimensional transition metal dichalcogenides(2D-TMDCs)materials enable excellent field-effect characteristics and multiple gate control ports,facilitating the integration of the functions of multiple transistors into one.Generally,the computing-capability improvement of the transistor cell in logic circuits will greatly alleviate the challenge in transistor numbers.In other words,one can only use a small number,or even just one,2DTMDCs-based transistors to conduct the sophisticated logic operations that have to be realized by using many traditional transistors.In this review,from material selection,device structure optimization,and circuit architecture design,we discuss the developments,challenges,and prospects for 2D-TMDCs-based logic circuits.

Synergistic-engineered van der Waals photodiodes with high efficiency

Van der Waals(vdW)heterostructures based on two-dimensional transitionmetal dichalcogenides have provided unprecedented opportunities for photovoltaic detectors owing to their strong light-matter interaction and ultrafast interfacial charge transfer.Despite continued advancement,insufficient control of photocarrier behaviors still limits the external quantum efficiency(EQE)and operation speed of such detectors.Here,we propose a synergistic strategy of contact-configuration design and thickness-modulation to construct high-performance vdW photodiodes based on the typical type II heterostructure(MoS2/WSe2).Through integrating three contact architectures into one device to exclude other factors,we solid the superiority of designed 1L-MoS2/WSe2/graphene heterostructures incorporating efficient photocarrier collection and gate modulation.Together with leveraging the layer-numberdependent properties of WSe2,we observe the critical thickness of WSe2(11 layers)for the highest EQE,which verifies the thickness-dependent competition between photocarrier generation,dissociation,and collection.Finally,we demonstrate the synergistic-engineered vdW heterostructure can trigger record-high EQE(61%)and manifest ultrafast photoresponse(4.1μs)at the atomically thin limit(8 nm).The proposed strategy enables architecture-design and thickness-engineering to unlock the potential to realize high-performance optoelectronic devices.

Strain modulation on graphene/ZnO nanowire mixed- dimensional van der Waals heterostructure for high- performance photosensor

The mixed-dimensional van der Waals （vdW） heterostructure is a promising building block for strained electronics and optoelectronics because it avoids the bond fracture and atomic reconstruction under strain. We propose a novel mixed-dimensional vdW heterostructure between two-dimensional graphene and a one-dimensional ZnO nanowire for high-performance photosensing. By utilizing the piezoelectric properties of ZnO, strain modulation was accomplished in the mixed-dimensional vdW heterostructure to optimize the device performance. By combining the ultrahigh electrons transfer speed in graphene and the extremely long life time of holes in ZnO, an outstanding responsivity of 1.87 ×10^5 A/W was achieved. Under a tensile strain of only 0.44% on the ZnO nanowire, the responsivity was enhanced by 26%. A competitive model was proposed, in which the performance enhancement is due to the efficient promotion of the injection of photogenerated electrons from the ZnO into the graphene caused by the strain-induced positive piezopotential. Our study provides a strain-engineering strategy for controlling the behavior of the photocarriers in the mixed-dimensional vdW heterostructure, which can be also applied to other similar systems in the future.

Record-high saturation current in end-bond contacted monolayer MoS_(2) transistors

Monolayer two-dimensional(2D)semiconductors are emerging as top candidates for the channels of the future chip industry due to their atomically thin body and superior immunity to short channel effect.However,the low saturation current caused by the high contact resistance(R_(c))in monolayer MoS2 field-effect transistors(FETs)limits ultimate electrical performance at scaled contact lengths,which seriously hinders application of monolayer MoS_(2 )transistors.Here we present a scalable strategy with a clean end-bond contact scheme that leads to size-independent electrodes and ultralow contact resistance of 2.5 kΩ·μm to achieve record high performances of saturation current density of 730μA·μm^(-1)at 300 K and 960μA·μm^(-1)at 6 K.Our end-bond contact strategy in monolayer MoS2 FETs enables the great potential for atomically thin integrated circuitry.

Microscopic pump-probe optical technique to characterize the defect of monolayer transition metal dichalcogenides

Monolayer transition metal dichalcogenides(TMDs)are ideal materials for atomically thin,flexible optoelec.tronic and catalytic devices.However,their optoelectrical performance such as quantum yield and carrier mobility often shows below theoretical expectations due to the existence of defects.For monolayer TMD-based devices,finding a low-cost,time-efficient, and nondestructive technique to visualize the change of defect distribution in the space domain and the defect-induced change of the carrier's lifetime is vital for optimizing their optoelectronic properties.Here, we propose a microscopic pump-probe technique to map the defect distribution of monolayer TMDs.It is found that there is a linear relationship between transient differential reflection intensity and defect density,suggesting that this technique not only realizes the visualization of the defect distribution but also achieves the quantitative estimation of defect density.Moreover,the carrier lifetime at each point can also be obtained by the technique. The technique used here provides a new route to characterize the defect of monolayer TMDs on the micro-zone, which will hopefiilly guide the fabrication of high-quality two-dimensional (2D) materials and the promotion of optoelectrical performance.

Single-Atom Vacancy Doping in Two-Dimensional Transition Metal Dichalcogenides

CONSPECTUS:Faced with the growing quests of higher-performance chips,developing new channel semiconductors immune to short channel effects has become a realistic option for continuing Moore’s Law.With outstanding gate electrostatic capacitance,stable chemical properties,and suitable bandgap,two-dimensional(2D)transition metal dichalcogenides(TMDCs)are considered as potential candidates for next-generation channel materials.However,the practical applications of 2D TMDCs are severely limited by stable,precise,and controllable doping technologies,due to their ultrathin body and dangling bond-free surface.Compared to three-dimensional semiconductors,donors in 2D semiconductors need larger ionization energy which can be attributed to the reduced screening of Coulomb interaction and the larger bandgap induced by quantum confinement.Limited by the ultrathin body of 2D TMDCs and the strong film−substrate charge transfer,typical silicon-based substitutional doping technology encounters some headache difficulties in 2D TMDCs and hardly achieves high-concentration doping.The other two doping technologies also cannot take on this task either;local gate electrostatic doping cannot leave the aid of the external electric field.And surface charge transfer doping of molecule adsorbents behaves unstably(e.g.,thermal desorption)or ineffectively modifies the original electronic structure.Fortunately,single-atom vacancies can effectively and precisely adjust the carrier concentration of 2D TMDCs and significantly enhance their conductivity.Therefore,clarifying the work rules and function mechanism of single-atom vacancy doping in 2D TMDCs is beneficial in creating a brand-new optimization strategy of electrical properties and overcoming the technical obstacles of the“lab-to-fab”transition for their practical applications in high-performance electronics and optoelectronics.In this Account,we summarize the state-of-the-art progress in single-atom vacancy doping in 2D TMDCs and highlight the applications in optoelectronic and electronic devices.First,the common defects and the density-largest-defect type in 2D TMDCs are demonstrated through experimental characterizations.Second,the healing and manufacturing strategies of chalcogen vacancies in 2D TMDCs are systematically summarized.Third,we clarify the doping mechanism of single-atom vacancies in 2D TMDCs and its regulation of the electrical properties including carrier concentration and carrier mobility.Fourth,the correlations between chalcogen vacancies in 2D TMDCs and the optical signals from Raman and photoluminescence spectroscopies are established,which will help to quickly and nondestructively evaluate the chalcogen vacancy concentration.Fifth,the current applications of single-atom vacancy doping of 2D TMDCs materials are reviewed,including complementary metal−oxide semiconductor(CMOS)logic inverters,homojunctions,Schottky diodes,and photovoltaic devices.Finally,the potential challenges and future development trends of single-atom vacancy doping for next-generation electronic and optoelectronic devices are pointed out.Overall,this Account guides on controllable and precise doping technologies for researchers in these fields from materials,electronics,and optoelectronics to promote the practical applications of 2D TMDCs.