Syntheses of WS2 Monolayer and vdW PdSe2/WS2 Heterostructure for Electronic and Optoelectronic Devices


Student thesis: Doctoral Thesis

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Awarding Institution
Award date20 May 2022


Semiconducting 2D nanomaterials are demonstrated to have great potential for applications in next-generation electronics and optoelectronics. For instance, WS2 is an exciting kind of semiconducting 2D nanomaterial, which exhibits many novel phenomena, such as the layer-dependent band gap and band structure, high photoluminescence (PL) quantum yield, large exciton binding energy, valley-dependent transition selection rule, and so forth. These unique characteristics would make WS2 an ideal active device material for high-performance transistors, photodetectors, light-emitting devices, and many others. Because of their fascinating properties, two dimensional (2D) transitional metal dichalcogenides (TMDs) have attracted a lot of attention for developing next-generation electronics and optoelectronics. In this case, synthesizing large single-domain 2D TMDs becomes urgently desired for technological utilizations. Until now, various modified CVD methods have then been explored to achieve high-quality large single-domain 2D TMDs during the past few years. It has been exemplified that it is feasible to enlarge the single-domain size by accelerating the lateral growth. Also, controlling the nucleation density is another versatile method for the synthesis of large single-domain TMDs.

On the other hand, in order to widen the spectral response range and to enhance the responsivity of TMDs devices, van der Waals (vdW) heterostructures integrated of two different TMDs materials have been extensively explored to take advantages of not only the improved light absorption but also the interlayer transition of the vdW TMDs heterostructure under illumination. It has been shown that 2D vdW TMDs heterostructures tend to exhibit extraordinary light–matter interaction due to their appropriate band alignment, extending the spectral response. More importantly, these heterointerfaces can also separate the photo-excited charge carriers more efficiently for the existence of built-in electric field, leading to improved photodetection performance such as large photogain, and high photoresponsivity.

Here, utilizing a NaOH promoter and W foils as the W source, we have successfully achieved the fabrication of ultralarge single-domain monolayer WS2 films via a modified chemical vapor deposition method. With the proper introduction of a NaOH promoter, the single-domain size of monolayer WS2 can be increased to 550 μm, while the WS2 flakes can be well controlled by simply varying the growth duration and oxygen concentration in the carrier gas. Importantly, when they are fabricated into global backgated transistors, WS2 devices exhibit respectable peak electron mobility up to 1.21 cm2 V−1 s −1, which is comparable to those of many state-of-the-art WS2 transistors. Photodetectors based on these single-domain WS2 monolayers give an impressive photodetection performance with a maximum responsivity of 3.2 mA W−1 . All these findings do not only provide a cost-effective platform for the synthesis of high-quality large single-domain 2D nanomaterials, but also facilitate their excellent intrinsic material properties for the next-generation electronic and optoelectronic devices.

Furthermore, vdW PdSe2/WS2 heterostructure is developed for robust high-performance broadband photodetection from visible to infrared optical communication band. These heterostructure devices are simply formed by direct selenization of Pd films pre-deposited on the chemical vapor deposited monolayer WS2, followed by wet-transfer onto device substrates with pre-patterned electrodes. Importantly, the obtained heterostructure device exhibits an impressive broadband spectral photoresponse with response times less than 100 ms for different wavelength regions (532 to 1550 nm), where this performance is significantly better than that of pristine monolayer WS2 devices. This performance enhancement is attributed to the type I band alignment of the heterostructure. Under illumination, both intralayer and interlayer excitations are involved to generate carriers in the relevant layer, enabling the broadband photoresponse. Photocarriers would then undergo charge separation in the depletion region with electrons transferred into the charge transport layer of WS2 through the built-in electric field, followed by the relaxation to valance band via interlayer or intralayer transition. All these findings can indicate the promising potential of vdW PdSe2/WS2 heterostructures for next-generation high-performance optoelectronics.

    Research areas

  • monolayer WS2, CVD, charge trapping, vdW TMDs heterostructure, interlayer transition, broadband photodetection