Monolayer WS2 (Tungsten Disulfide) with a direct-energy gap and excellent photoluminescence quantum yield at room temperature shows potential applications in optoelectronics. However, controllable synthesis of large-area monolayer WS2 is still challenging because of the difficulty in controlling the interrelated growth parameters. Herein, we report a facile and controllable method for synthesis of large-area monolayer WS2 flakes by direct sulfurization of powdered WO3 (Tungsten Trioxide) drop-casted on SiO2/Si substrates in a one-end sealed quartz tube. The samples were thoroughly characterized by an optical microscope, atomic force microscope, transmission electron microscope, fluorescence microscope, photoluminescence spectrometer, and Raman spectrometer. The obtained results indicate that large triangular monolayer WS2 flakes with an edge length up to 250 to 370 μm and homogeneous crystallinity were readily synthesized within 5 min of growth. We demonstrate that the as-grown monolayer WS2 flakes show distinctly size-dependent fluorescence emission, which is mainly attributed to the heterogeneous release of intrinsic tensile strain after growth.

Transition-metal dichalcogenides (TMDs), including molybdenum disulfide (MoS2) and tungsten disulfide (WS2), with appealing properties have recently become promising alternatives to graphene with semimetal and low on/off current ratio properties as the sensing channel in field-effect transistor (FET) biosensors. However, the efficiency of DNA-based FET devices strongly depends on how DNA probes are tethered to the nanomaterial channels. As against covalent attachment, simple DNA physisorption has become increasingly popular, and a DNA sequence with strong affinity for nanomaterials is still highly sought after. Recently, poly-cytosine (poly-C) DNA was found to be strongly adsorbed to many common nanomaterials, including WS2. Herein, a diblock DNA probe containing a (poly-C) (C15) was used to attach to a chemical vapor deposition (CVD)-grown monolayer WS2 surface; meanwhile, the target complementary DNA (cDNA) was hybridized to the other block of the DNA probe. The biosensor developed following this strategy led to a limit of detection down to 3 aM within a concentration range spanning over approximately 7 orders of magnitude (10–16 to 10–9 M), which was lower than those of the previously reported TMDs and a good competitor to graphene FET DNA biosensors. Moreover, the proposed WS2 FET DNA biosensor showed high specificity capable of distinguishing the cDNA from non-cDNA, one-base mismatched DNA, two-base mismatched DNA, and three-base mismatched DNA, making our strategy an exciting avenue for disease diagnosis. The authors are convinced that this work extends the CVD synthesis of WS2 and its promise in biosensing application-based FETs.

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