Effect of layer and stacking sequence in simultaneously grown 2H and 3R WS2 atomic layers.

In two-dimensional layered materials, layer number and stacking order have strong effects on the optical and electronic properties. Tungsten disulfide (WS2) crystal, as one important member among transition metal dichalcogenides, has been usually prepared in a layered 2H prototype structure with space group P63/mmc ([Formula: see text]) in spite of many other expected ones such as 3R. Here, we report simultaneous growth of 2H and 3R stacked multilayer (ML) WS2 crystals in large scale by chemical vapor deposition and effects of layer number and stacking order on optical and electronic properties. As revealed in Raman and photoluminescence (PL) measurements, with an increase in layer number, 2H and 3R stacked ML WS2 crystals show similar variation of PL and Raman peaks in position and intensity. Compared to 2H stacked ML WS2, however, 3R stacked one always exhibits the larger red (blue) shift of Raman [Formula: see text] (A1g) peak and the appearance of PL A, B and I peaks at lower energies. Thereby, PL and Raman features depend on not only layer number but also stacking order. In addition, circularly polarized luminescence from two prototype WS2 crystals under circularly polarized excitation has also been investigated, showing obvious spin or valley polarization of these CVD-grown multilayer WS2 crystals.

One-Step Growth of Spatially Graded Mo1- xW xS2 Monolayers with a Wide Span in Composition (from x = 0 to 1) at a Large Scale.

Alloying is an effective way to modulate material's properties. In particular, graded alloying within a single domain of two-dimensional transition-metal chalcogenide (2D-TMC) is of great technological importance, for example, for achieving band gap modulations. Here, we report a facile method to grow gradient alloying of Mo1- xW xS2 monolayers with large domain sizes and high crystal qualities via the chemical vapor deposition technique. The as-grown Mo1- xW xS2 monolayers have a gradient composition of W from x = ∼0 to ∼1 in a single domain with a lateral dimension up to 300 µm, and the span in band gap can be readily tuned. Owing to the grading in band offsets, the compositionally graded Mo1- xW xS2 alloy monolayer demonstrates an excellent rectifying effect with the ratio of forward to reverse current up to ∼104. Moreover, phototransistors based on the compositionally graded Mo1- xW xS2 monolayers exhibit a high responsivity up to 298.4 A/W in the visible light regime, and particularly a decent responsivity of 28.7 A/W in the near-infrared regime. The control of band gap offset gradient and span in alloyed 2D-TMC semiconductors provides an additional degree of freedom in designing fascinating applications in achieving multifunctional optoelectronic devices on individual substrates.

Highly sensitive and fast monolayer WS2 phototransistors realized by SnS nanosheet decoration.

Two-dimensional chalcogenide monolayers are strong candidates for next-generation flexible and transparent optoelectronics. Due to the intrinsic ultrathin thickness and limited optical absorption, however, their responsivity is normally low. Here we develop a simple and low-cost method to fabricate high-performance hybrid phototransistors of monolayer WS2 with significantly enhanced responsivity and an extended spectral response range, by virtue of surface decoration with liquid-phase exfoliated SnS nanosheets (NSs). The hybrid phototransistors show a much enhanced responsivity of ∼2 A W-1 and an ultrahigh light/dark signal-to-noise ratio of 106 under 457 nm excitation, exhibiting a significant increase of 3 orders of magnitude in responsivity and a 100 fold increase in signal-to-noise ratio, compared with pure WS2 devices. Our hybrid photodetectors also exhibit a respectable response speed, with a rise and decay time of 51 µs and 98 µs, respectively. After optimal surface decoration with narrow bandgap SnS NSs atop a monolayer WS2 channel, an emergent optical responsivity in the near infrared region (1064 nm) is also observed.

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.

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.