Electron Field Emission of Geometrically Modulated Monolayer Semiconductors

Electron field emission, electrons emitted from solid surfaces under high electric field, offers significant scientific interests in materials sciences and potential optoelectronics applications. 2D atomic layers, such as MoS₂, exhibit fascinating properties for diverse applications in next‐generation nanodevices and rich physical phenomena for fundamental research. However, the study on field emission of semiconducting monolayers is lacking owing to its low efficiency and stability of electron emission. Here, electron field emission of the geometrically modulated monolayer semiconductors suspended with 1D nanoarrays is demonstrated. Chemical vapor deposition synthesis of two prototype monolayers of transition metal dichalcogenides (TMD), MoS₂ and MoSe₂, is presented and their diverse band structures offer an ideal platform to explore the fundamental process of the electron emission in the TMD. Geometrical modulation and charge transfer of the semiconducting monolayers can be significantly tuned with the structural suspension with the 1D ZnO nanoarrays. Possible mechanisms on the enhanced electron emission of the 2D monolayers are discussed. With geometrical control of the monolayers, a highly efficient and stable electron emission of TMD monolayers is achieved in low turn‐on electric fields, enabling applications on electrons sources and opening a new avenue toward geometrically tuned atomic layers.     

Significant Exciton Brightening in Monolayer Tungsten Disulfides via Fluorination: n‐Type Gas Sensing Semiconductors

Monolayer transition‐metal dichalcogenides (TMDCs) have recently emerged as promising candidates for advanced photonic and valleytronic applications due to their unique optoelectronic properties. However, the low luminescence efficiency of monolayer TMDCs has significantly hampered their use in these fields. Here it is reported that the photoluminescence efficiency of monolayer WS₂ can be remarkably enhanced up to fourfold through the fluorination, surpassing the reported performance of molecular and/or electrical doping methods. Its degree is easily controlled by changing the fluorine plasma duration time and can also be reversibly tuned via additional hydrogen plasma treatment, allowing for its versatile tailoring for interfacial band alignment and customized engineering. The striking photoluminescence improvement occurs via a substantial transition of trions to excitons as a result of the strong electron affinity of fluorine dopants, and the fluorination enables unprecedented detection of n‐type NH₃ gas in WS₂ due to changed excitonic dynamics showing excellent sensitivity (at least down to 1.25 ppm). This work provides valuable strategies and insights into exciton physics in monolayer TMDCs, opening up avenues toward highly‐efficient 2D light emitters, photovoltaics, nanosensors, and optical interconnects.     

Dirac Fermions in Blue Phosphorene Monolayer

2D materials beyond graphene and in particular 2D semiconductors have raised interest due to their unprecedented electronic properties, such as high carrier mobility or tunable bandgap. Blue phosphorene is an allotrope of black phosphorene that resembles graphene as it presents a honeycomb structure. However, it is known to have semiconductor character and the crucial point is to determine whether this hexagonal phase of phosphorene presents Dirac fermions as in graphene. Here, the first compelling experimental evidence of Dirac fermions in blue phosphorene layer grown on Cu(111) surface is presented. The results highlight the formation of a highly ordered blue phosphorene sheet with a clear Dirac cone at the high symmetry points of the Brillouin Zone. The charge carriers behave as massless relativistic particles. Therefore, all the expectations held for graphene, such as high-speed electronic devices based on ballistic transport at room temperature, may also be applied to blue phosphorene.     

Photoluminescent Arrays of Nanopatterned Monolayer MoS2

Monolayer 2D MoS₂ grown by chemical vapor deposition is nanopatterned into nanodots, nanorods, and hexagonal nanomesh using block copolymer (BCP) lithography. The detailed atomic structure and nanoscale geometry of the nanopatterned MoS₂ show features down to 4 nm with nonfaceted etching profiles defined by the BCP mask. Atomic resolution annular dark field scanning transmission electron microscopy reveals the nanopatterned MoS₂ has minimal large‐scale crystalline defects and enables the edge density to be measured for each nanoscale pattern geometry. Photoluminescence spectroscopy of nanodots, nanorods, and nanomesh areas shows strain‐dependent spectral shifts up to 15 nm, as well as reduction in the PL efficiency as the edge density increases. Raman spectroscopy shows mode stiffening, confirming the release of strain when it is nanopatterned by BCP lithography. These results show that small nanodots (≈19 nm) of MoS₂ 2D monolayers still exhibit strong direct band gap photoluminescence (PL), but have PL quenching compared to pristine material from the edge states. This information provides important insights into the structure–PL property correlations of sub‐20 nm MoS₂ structures that have potential in future applications of 2D electronics, optoelectronics, and photonics.     

半导体/SiO2纳米复合材料的溶胶-凝胶制备

总结了溶胶-凝胶法制备半导体/SiO2纳米复合材料的研究进展。目前,通过该方法已制备了镶嵌在SiO2玻璃中的II-VI族I、II-V族I、-VII族和IV族等半导体;形态上,除玻璃块体和薄膜外,还出现了核壳结构的纳米颗粒和同轴纳米电缆。     

Electrical Switching in TlSbSe2 Chalcogenide Semiconductors

Electrical measurements were performed on TlSbSe₂ ternary crystals in the temperature range 293–413 K. The obtained I–V characteristics consist of two regions: an Ohmic region at low current densities, and nonlinear regions having negative differential resistance (NDR) at moderate and higher current densities. The nonlinear behavior of the I–V curves was studied at different ambient temperatures. The sample temperature and the threshold voltage of the NDR region were also examined as a function of the ambient temperature. We detected that the investigated samples exhibit threshold-type switching and propose that the switching mechanism has an electronic origin.     

超细铁酸镉材料的制备及其气敏性能研究

以FeSO4·7H2O和CdCl2·2.5H2O为原料,采用化学共沉淀法制备了纳米尺寸CdFe2O4粉体。利用X射线衍射仪、透射电镜对粉体的形貌、结构进行了表征。研究表明,用化学共沉淀法制备的CdFe2O4结晶情况良好,分布均匀,平均粒径约为80 nm。以CdFe2O4纳米粉体为原料制备了厚膜气敏元件,在工作温度为400左右时,该元件对乙醇具有较高的灵敏度、好的稳定性和选择性。并对气敏机理给予了解释。     

Gate-Controlled Polarity-Reversible Photodiodes with Ambipolar 2D Semiconductors

A photosensor with an amplitude-tunable and polarity-reversible response under gate modulation has potential as a computational photosensor, which can provide more recognition degree of data to enhance signal processing efficiency. Although, the ambipolar 2D semiconductors possess unique gate-tunable properties, the question of how to utilize this property to design polarity-reversible photodiodes for intelligent applications remains unanswered. Here, gate-controllable polarity-reversible photodiodes based on ambipolar 2D semiconductors with an asymmetrically metal-contacted architecture are proposed. By controlling the gate-field, the local carrier type and density profile can be manipulated in the channel due to the partial shielding feature of the asymmetrically metal-contacted architecture, resulting in a polarity-reversible photodiode. The reported WSe₂-based photodiode possesses excellent rectifying behavior with a rectification ratio over 10⁵, photovoltaic performance with 90% external quantum efficiency, and 2.3% power conversion efficiency under gate regulation. Meanwhile, the device exhibits reversible polarity of photovoltage from a negative to positive state under gate control. By utilizing the reversible photovoltage of the WSe₂ photodiode, an optoelectronic switch with a photovoltage polarity signal is demonstrated without a bias voltage. This photovoltage-reversible homodiode paves the way to develop 2D devices with multiple operation modes for potential applications in high-efficiency photovoltaics, intelligent vision sensors, and logic optoelectronics.     

Laser‐Assisted Chemical Modification of Monolayer Transition Metal Dichalcogenides

Laser‐assisted chemical modification is demonstrated on ultrathin transition‐metal dichalcogenides (TMDs), locally replacing selenium by sulfur atoms. The photoconversion process takes place in a controlled reactive gas environment and the heterogeneous reaction rates are monitored via in situ real‐time Raman and photoluminescence spectroscopies. The spatially localized photoconversion results in a heterogeneous TMD structure, with chemically distinct domains, where the initial high crystalline quality of the film is not affected during the process. This has been further confirmed via transmission electron microscopy as well as Raman and photoluminescence spatial maps. This study demonstrates the potential of laser‐assisted chemical conversion for on‐demand synthesis of heterogeneous 2D materials with applications in nanodevices.     

Large‐Area Synthesis of Continuous and Uniform MoS2 Monolayer Films on Graphene

Heterostructures composed of multiple layers of different atomically thin materials are of interest due to their unique properties and potential for new device functionality. MoS₂‐graphene heterostructures have shown promise as photodetectors and vertical tunnel transistors. However, progress is limited by the typically micrometer‐scale devices and by the multiple alignments required for fabrication when utilizing mechanically exfoliated material. Here, the synthesis of large‐area, continuous, and uniform MoS₂ monolayers directly on graphene by chemical vapor deposition is reported, resulting in heterostructure samples on the centimeter scale with the possibility for even larger lateral dimensions. Atomic force microscopy, photoluminescence, X‐ray photoelectron, and Raman spectroscopies demonstrate uniform single‐layer growth of stoichiometric MoS₂. The ability to reproducibly generate large‐area heterostructures is highly advantageous for both fundamental investigations and technological applications.     

Modulating Electronic Structure of Monolayer Transition Metal Dichalcogenides by Substitutional Nb-Doping

Modulating electronic structure of monolayer transition metal dichalcogenides (TMDCs) is important for many applications, and doping is an effective way toward this goal, yet is challenging to control. Here, the in situ substitutional doping of niobium (Nb) into TMDCs with tunable concentrations during chemical vapor deposition is reported. Taking monolayer WS₂ as an example, doping Nb into its lattice leads to bandgap changes in the range of 1.98–1.65 eV. Noteworthy, electrical transport measurements and density functional theory calculations show that the 4d electron orbitals of the Nb dopants contribute to the density of states of Nb-doped WS₂ around the Fermi level, resulting in an n- to p-type conversion. Nb-doping also reduces the energy barrier of hydrogen absorption in WS₂, leading to an improved electrocatalytic hydrogen evolution performance. These results highlight the effectiveness of controlled doping in modulating the electronic structure of TMDCs and their use in electronic related applications.     

集成电路与半导体

安森美公司PFC控制器 安森美(ON Semiconductor)公司推出两款功率因数修正(PFC)控制器,针对输入功率在75W到3kW的应用而设计.NCP1653和NCP1601控制器非常适于如下设备电源的功率因数修正:电视、平板监视器、台式PC和笔记本适配器SMPS、离线充电器以及白色家电(如冰箱、洗衣机和干燥机).NCP1653是一种频率固定、电流模式PFC控制器,其设计可以有效地驱动中、高功率(100W到3kW)连续导通模式(CCM)升压变换器.