Exciton luminescence and many-body effect of monolayer WS2 at room temperature

Monolayer transition metal dichalcogenides favor the formation of a variety of excitonic quasiparticles,and can serve as an ideal material for exploring room-temperature many-body effects in two-dimensional systems.Here,using mechanically exfoliated monolayer WS;and photoluminescence(PL)spectroscopy,exciton emission peaks are confirmed through temperature-dependent and electric-field-tuned PL spectroscopy.The dependence of exciton concentration on the excitation power density at room temperature is quantitatively analyzed.Exciton concentrations covering four orders of magnitude are divided into three stages.Within the low carrier concentration stage,the system is dominated by excitons,with a small fraction of trions and localized excitons.At the high carrier concentration stage,the localized exciton emission from defects coincides with the emission peak position of trions,resulting in broad spectral characteristics at room temperature.

二维铁电半导体层级处理模块设计及低功耗高性能人工视觉系统应用

The growth of data and Internet of Things challenges traditional hardware,which encounters efficiency and power issues owing to separate functional units for sensors,memory,and computation.In this study,we designed an a-phase indium selenide(a-In_(2)Se_(3))transistor,which is a two-dimensional ferroelectric semiconductor as the channel material,to create artificial optic-neural and electro-neural synapses,enabling cutting-edge processing-in-sensor(PIS)and computing-in-memory(CIM)functionalities.As an optic-neural synapse for low-level sensory processing,the a-In_(2)Se_(3)transistor exhibits a high photoresponsivity(2855 A/W)and detectivity(2.91×10^(14)Jones),facilitating efficient feature extraction.For high-level processing tasks as an electro-neural synapse,it offers a fast program/erase speed of 40 ns/50μs and ultralow energy consumption of 0.37 aJ/spike.An AI vision system using a-In_(2)Se_(3)transistors has been demonstrated.It achieved an impressive recognition accuracy of 92.63%within 12 epochs owing to the synergistic combination of the PIS and CIM functionalities.This study demonstrates the potential of the a-In_(2)Se_(3)transistor in future vision hardware,enhancing processing,power efficiency,and AI applications.

超薄金属性MoO_(2)纳米片可控合成及其范德华接触应用研究

在二维半导体与金属材料间引入范德华接触构建器件被认为是解决二维材料电接触问题的有效途径之一.然而,迄今为止,研究主要集中在半导体材料合成与改性上,而对金属材料的制备和性能的研究较少.在这项工作中,我们报道了利用化学气相沉积法可控合成厚度从3.5到10^(6)nm的层状MoO_(2)金属二维纳米片.利用X射线衍射、扫描隧道显微镜和透射电子显微镜对制备的MoO_(2)纳米片进行了系统表征,结果表明,制备的MoO_(2)为单斜晶型、晶质质量高、稳定性好.电学表征表明,MoO_(2)具有优良的导电性能,其导电率超过10^(6)S m^(-1),可与石墨烯和某些金属相媲美.此外,我们还通过引入MoO_(2)薄片作为范德华接触材料,探索了其在MoS_(2)场效应晶体管中的接触应用.所获得的MoS_(2)场效应晶体管表现出低肖特基势垒(36 m e V)和高载流子迁移率(210 cm^(2)V^(-1)s^(-1),10 K).这项工作为金属二维材料的可控制备和应用提供了新思路,并有望促进二维材料电子器件的发展.

Light-triggered two-dimensional lateral homogeneous p-n diodes for opto-electrical interconnection circuits

The realization of light-triggered devices where light is used as external stimulus to control the device performances is a long-standing goal in modern opto-electrical interconnection circuits.In this work,it reveals that light illumination can induce the formation of p-n junctions along two-dimensional conduction channels.The results indicate that the dominant charge carrier type and density in black phosphorus(BP)conduction channel can be effectively modulated by the underlying cadmium sulfide(CdS)photogate layer under light illumination.This enables flexible switching of the working state between BP resistor and BP p-n diode in the designed semi-photo-gate transistor(SPGT)devices when switching the light on and off(ultra-low threshold light power).Simultaneously,the achieved BP p-n junctions also exhibit ultra-high photoresponsivity and evident photovoltaic properties.That is to say,light can be employed as external stimulus to define the BP p-n junctions,and in turn the p-n junctions will further convert the light into electrical power,showing all-in-one opto-electrical interconnection properties.Moreover,the SPGT device architecture is also applicable for construction of other ambipolar semiconductor-based(WSe2-and MoTe2-based)p-n diodes.Such universal all-in-one light-triggered lateral homogeneous pn junctions with ultra-low energy consumption should open a new pathway toward novel optoelectronic devices and deliver various new applications.

Efficient control of emission and carrier polarity in WS_(2)monolayer by indium doping

Substitutional doping of two-dimensional(2D)transition metal dichalcogenides(TMDs)has been recognized as a promising strategy to tune their optoelectronic properties for a wide array of applications.However,controllable doping of TMDs remains a challenging issue due to the natural doping of these materials.Here,we demonstrate the controllable growth of indium-doped p-type WS_(2) monolayers with various doping concentrations via chemical vapor deposition(CVD)of a host tungsten(W)source and indium(In)dopant.Scanning transmission electron microscopy confirmed that In atoms successfully substitute the W atoms in the WS_(2) lattice.Intriguingly,the photoluminescence of the doped sample experiences strong intensity modulation by the doping concentration,which first increases remarkably with an enhancement factor up to~35 and then decreases gradually when further increasing the doping concentration.Such a phenomenon is attributed to the progressive change of the exciton to trion ratio as well as the defect concentration in the doped samples.The assignment was further verified by the electric behavior of the fabricated In-doped WS_(2) field effect transistors,which changes regularly from n-type to bipolar and finally to p-type behavior with increasing doping concentration.The successful growth of p-type monolayer WS_(2) and the dual control of its optical and electrical properties by In doping may provide a promising method to engineer the opto-electronic properties of 2D materials.

Record high photoresponse observed in CdS-black phosphorous van der Waals heterojunction photodiode

Two-dimensional(2D)materials have recently received great attention for their atomic thin thickness and thus derived outstanding electrical,optical and optoelectronic properties.Moreover,the dangling-bond-free surfaces of 2D materials enable the direct integration of different materials with various properties through van der Waals(vdW)forces into vdW heterostructures,providing new opportunities for constructing new type devices with superior performances.In this study,we report the vertical assembly of n-type CdS and p-type BP into p-n junctions.The electrically tunable heterojunction device shows a high current rectifying ratio up to8×103at a low bias voltage range of±1 V and an ideality factor of 1.5.More interestingly,the CdS/BP vdW heterojunction exhibits an ultra-high photoresponsivity up to 9.2×105A W-1and an ultra-high specific detectivity of 3.2×1013Jones with a low bias voltage of 1.0 V,which is among the highest in the reported results of 2D heterostructures.While operated at a self-powered mode,the device also exhibits excellent photodetection performances with a high photoresponsivity of0.27 A W-1and a high external quantum efficiency of 76%.Time-resolved photoresponse characterizations indicate that the device possesses a fast response time of about 10 ms.The developed CdS/BP vdW heterojunctions will find potential applications in the next-generation nanoscale electronics and optoelectronics applications.

Defect-induced distinct exciton-exciton interactions in WS2 monolayers

The optoelectronic properties of atomically thin transition metal dichalcogenides(TMDs)are largely influenced by defect populations(DPs).In this work,we fabricate WSmonolayers with different DPs by varying the fabrication methods and further reveal their distinct exciton-exciton interactions.Steady-state photoluminescence(PL)experiments show that the monolayer with the lowest DP shows optimal PL intensity at low excitation power;however,it is overtaken and significantly surpassed by monolayers with higher DPs at high excitation powers.Excitation-power-dependent experiments demonstrate that these monolayers exhibit distinct PL saturation behaviors with the threshold power differing by four orders of magnitude.Combined with in situ PL imaging and time-resolved PL experiments,we attribute such PL evolution discrepancies to the different DPs within these monolayers,which largely influence the exciton diffusion behavior and subsequently bring about distinct nonradiative exciton-exciton annihilations(EEAs).Valley polarization experiments are further employed to re-examine the DPs of these monolayers.This work reveals the distinct PL behaviors and underlying exciton dynamics in TMD monolayers with different DPs,which can largely facilitate the engineering of relevant high-performance devices for practical applications.

Contact and injection engineering for low SS reconfigurable FETs and high gain complementary inverters

The newly emerged two-dimensional(2D) semiconducting materials, owning to the atomic thick nature and excellent optical and electrical properties, are considered as potential candidates to solve the bottlenecks of traditional semiconductors. However, the realization of high performance 2D semiconductorbased field-effect transistors(FETs) has been a longstanding challenge in 2D electronics, which is mainly ascribing to the presence of significant Schottky barrier(SB) at metal-semiconductor interfaces. Here, an additional contact gate is induced in 2D ambipolar FET to realize near ideal reconfigurable FET(RFET)devices without restrictions of SB. Benefitting from the consistently high doping of contact region, the effective SB height can be maintained at ultra-small value during all operation conditions, resulting in the near ideal subthreshold swing(SS) values(132 mV/decade for MoTe2 RFET and 67 mV/decade for WSe2 RFET) and the relatively high mobility(28.6 cm2/(Vs) for MoTe2 RFET and 89.8 cm2/(V s) for WSe2 RFET). Moreover, the flexible control on the doping polarity of contact region enables the remodeling and switching of the achieved unipolar FETs between p-type mode and n-type mode. Based on such reconfigurable behaviors, high gain complementary MoTe2 inverters are further realized. The findings in this work push forward the development of high-performance 2D semiconductor integrated devices and circuits.