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

Flexible Black-Phosphorus Nanoflake/Carbon Nanotube Composite Paper for High-Performance All-Solid-State Supercapacitors

We proposed a simple route for fabrication of the flexible BP nanoflake/carbon nanotube (CNT) composite paper as flexible electrodes in all-solid-state supercapacitors. The highly conductive CNTs not only play a role as active materials but also increase conductivity of the hybrid electrode, enhance electrolyte shuttling and prevent the restacking between BP nanoflakes. The fabricated flexible all-solid-state supercapacitor (ASSP) device at the mass proportion of BP/CNTs 1:4 was found to deliver the highest volumetric capacitance of up to 41.1 F/cm³ at 0.005 V/s, superior to the ASSP based on the bare graphene or BP. The BP/CNTs (1:4) device delivers a rapid charging/discharging up to 500 V/s, which exhibits the characteristic of a high power density of 821.62 W/cm³, while having outstanding mechanical flexibility and high cycling stability over 10 000 cycles (91.5% capacitance retained). Moreover the BP/CNTs (1:4) ASSP device still retains large volumetric capacitance (35.7 F/cm³ at the scan rate of 0.005 V/s) even after 11 months. In addition, the ASSP of BP/CNTs (1:4) exhibits high energy density of 5.71 mWh/cm³ and high power density of 821.62 W/cm³. As indicated in our work, the strategy of assembling stacked-layer composites films will open up novel possibility for realizing BP and CNTs in new-concept thin-film energy storage devices

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

Low-Temperature Diffusion of Oxygen through Ordered Carbon Vacancies in Zr₂C_<i>x</i>: The Formation of Ordered Zr₂C_<i>x</i>O_<i>y</i>

Investigations are performed on low-temperature oxygen diffusion in the carbon vacancy ordered ZrC_0.6 and thus induced formation of the oxygen atom ordered ZrC_0.6O_0.4. Theoretically, a superstructure of Zr₂CO can be constructed via the complete substitution of carbon vacancies with O atoms in the Zr₂C model. In the ordered ZrC_0.6, the consecutive arrangement of vacancies forms the vacancy channels along some zone axes in the C sublattice. Through these vacancy channels, the thermally activated oxygen diffusion is significantly facilitated. The oxygen atoms diffuse directly into and occupy the vacancies, producing the ordered ZrC_0.6O_0.4. Relative to the ordered ZrC_0.6, the Zr positions are finely tuned in the ordered ZrC_0.6O_0.4 because of the ionic Zr–O bonds. Because of this fine adjustment of Zr positions and the presence of oxygen atoms, the superstructural reflections are always observable in a selected area electron diffraction (SAED) pattern, despite the invisibility of superstructural reflections in ZrC_0.6 along some special zone axes. Similar to the vacancies in ordered ZrC_0.6, the ordering arrangement of O atoms in the ordered ZrC_0.6O_0.4 is in nanoscale length, thus forming the nano superstructural domains with irregular shapes