Enhanced Photoresponse of SnSe-Nanocrystals-Decorated WS₂ Monolayer Phototransistor

Single-layer WS₂ has shown excellent photoresponse properties, but its promising applications in high-sensitivity photodetection suffer from the atomic-thickness-limited adsorption and band-gap-limited spectral selectivity. Here we have carried out investigations on WS₂ monolayer based phototransistors with and without decoration of SnSe nanocrystals (NCs) for comparison. Compared to the solely WS₂ monolayer, SnSe NCs decoration leads to not only huge enhancement of photoresponse in visible spectrum but also extension to near-infrared. Under excitation of visible light in a vacuum, the responsivity at zero gate bias can be enhanced by more than 45 times to ∼99 mA/W, and the response time is retained in millisecond level. Particularly, with extension of photoresponse to near-infrared (1064 nm), a responsivity of 6.6 mA/W can be still achieved. The excellent photoresponse from visible to near-infrared is considered to benefit from synergism of p-type SnSe NCs and n-type WS₂ monolayer, or in other words, the formed p-n heterojunctions between p-type SnSe NCs and n-type WS₂ monolayer

Ultrahigh-Gain and Fast Photodetectors Built on Atomically Thin Bilayer Tungsten Disulfide Grown by Chemical Vapor Deposition

The low responsivity observed in photodetectors based on monolayer transition-metal dichalcogenides has encouraged the pursuit of approaches that can efficiently enhance the external quantum efficiency, which relies predominantly on the light absorption, the lifetime of the excess carriers, and the charge collection efficiency. Here, we demonstrate that phototransistors fabricated on large-area bilayer tungsten disulfide (WS₂) grown by chemical vapor deposition exhibit remarkable performance with photoresponsivity, photogain, and detectivity of up to ∼3 × 10³ A/W, 1.4 × 10⁴, and ∼5 × 10¹² Jones, respectively. These figures of merit of bilayer WS₂ provide a significant advantage over monolayer WS₂ due to the greatly improved carrier mobility and significantly reduced contact resistance. The photoresponsivity of bilayer WS₂ phototransistor can be further improved to up to 1 × 10⁴ A/W upon biasing a gate voltage of 60 V, without evident reduction in detectivity. Moreover, the bilayer WS₂ phototransistor exhibits a high response speed of less than 100 μs, large bandwidth of 4 kHz, high cycling reliability of over 10⁵ cycles, and spatially homogeneous photoresponse. These outstanding figures of merit make WS₂ bilayer a highly promising candidate for the design of high-performance optoelectronics in the visible regime

Study of the Decomposition and Phase Transition of Uranium Nitride under UHV Conditions via TDS, XRD, SEM, and XPS

Uranium nitrides are among the most promising fuels for Generation IV nuclear reactors, but until now, very little has been known about their thermal stability properties under nonequilibrium conditions. In this work, thermal decomposition of nitrogen-rich uranium nitride (denoted as UN_2–<i>x</i>) under ultrahigh-vacuum (UHV) conditions was investigated by thermal desorption spectroscopy (TDS). It has been shown that the nitrogen TDS spectrum consists of two peaks at about 723 and 1038 K. The X-ray diffraction, scanning electron microscopy, and X-ray photoelectron microscopy results indicate that UN_2–<i>x</i> (UN₂ phase) decomposed into the α-U₂N₃ phase in the first step and the α-U₂N₃ phase decomposed into the UN phase in the second step

Sulfur-Doped Black Phosphorus Field-Effect Transistors with Enhanced Stability

Black phosphorus (BP) has drawn great attention owing to its tunable band gap depending on thickness, high mobility, and large <i>I</i>_on/<i>I</i>_off ratio, which makes BP attractive for using in future two-dimensional electronic and optoelectronic devices. However, its instability under ambient conditions poses challenge to the research and limits its practical applications. In this work, we present a feasible approach to suppress the degradation of BP by sulfur (S) doping. The fabricated S-doped BP few-layer field-effect transistors (FETs) show more stable transistor performance under ambient conditions. After exposing to air for 21 days, the charge-carrier mobility of a representative S-doped BP FETs device decreases from 607 to 470 cm² V^–1 s^–1 (remained as high as 77.4%) under ambient conditions and a large <i>I</i>_on/<i>I</i>_off ratio of ∼10³ is still retained. The atomic force microscopy analysis, including surface morphology, thickness, and roughness, also indicates the lower degradation rate of S-doped BP compared to BP. First-principles calculations show that the dopant S atom energetically prefers to chemisorb on the BP surface in a dangling form and the enhanced stability of S-doped BP can be ascribed to the downshift of the conduction band minimum of BP below the redox potential of O₂/O₂^–. Our work suggests that S doping is an effective way to enhance the stability of black phosphorus

Strain Release Induced Novel Fluorescence Variation in CVD-Grown Monolayer WS₂ Crystals

Tensile strain is intrinsic to monolayer crystals of transition metal disulfides such as Mo­(W)­S₂ grown on oxidized silicon substrates by chemical vapor deposition (CVD) owing to the much larger thermal expansion coefficient of Mo­(W)­S₂ than that of silica. Here we report fascinating fluorescent variation in intensity with aging time in CVD-grown triangular monolayer WS₂ crystals on SiO₂ (300 nm)/Si substrates and formation of interesting concentric triangular fluorescence patterns in monolayer crystals of large size. The novel fluorescence aging behavior is recognized to be induced by the partial release of intrinsic tensile strain after CVD growth and the induced localized variations or gradients of strain in the monolayer crystals. The results demonstrate that strain has a dramatic impact on the fluorescence and photoluminescence of monolayer WS₂ crystals and thus could potentially be utilized to tune electronic and optoelectronic properties of monolayer transition metal disulfides