Understanding the mechanisms involved in chemical vapour deposition (CVD) synthesis of atomically thin transition metal dichalcogenides (TMDCs) requires the precise control of numerous growth parameters. All the proposed mechanisms and their relation to the growth conditions are inferred from characterising intermediate formations obtained by stopping the growth blindly. To fully understand the reaction routes that lead to the monolayer formation, real time observation and control of the growth are needed. Here, we demonstrate how a custom-made CVD chamber that allows real time optical monitoring can be employed to study the reaction routes that are critical to the production of the desired layered thin crystals in salt assisted TMDC synthesis. Our real time observations reveal the reaction between the salt and the metallic precursor to form intermediate compounds which lead to the layered crystal formation. We identified that both the vapour-solid-solid and vapour-liquid-solid growth routes are in an interplay. Furthermore, we demonstrate the role H2 plays in the saltassisted WSe2 synthesis. Finally, we guided the crystal formation by directing the liquid intermediate compound through pre-patterned channels. The methods presented in this article can be extended to other materials that can be synthesized via CVD. Main Text:Chemical vapour deposition (CVD) synthesis of two-dimensional (2D) transition metal dichalcogenides (TMDCs) involves deposition of gaseous precursors on to a substrate to facilitate the crystallization in the desired crystal structure 1,2,3,4,5 . In a typical CVD synthesis, a transition metal containing precursor is placed in a tube furnace with a chalcogen precursor and a target substrate. Ar/H2 mixture carries the vaporised chalcogen precursor and the metal compounds to form atomically thin layers on the target substrate. Salts are also added to the conventionally used metal oxide precursors to form more volatile intermediate compounds 6,7,8 . This increases the monolayer formation rate and allows the synthesis of otherwise difficult to synthesize 2D TMDCs 9 . The setup described above has been used to produce atomically thin TMDC crystals in various morphologies. However, optimization of the growth parameters requires blind trial and errors, and even the optimized recipes offer limited control in terms of number of layers, crystal phase and morphology.There are two growth modes in CVD synthesis of TMDCs. (1) Vapour-Solid-Solid (VSS): Vaporized precursors are adsorbed on the substrate and form crystals via surface diffusion and bond formation at an elevated temperature 10 , and (2) Vapour-Liquid-Solid (VLS): Supersaturated liquid droplets containing the constituent elements form the crystals 11 . Figure 1a depicts these growth modes. Despite many studies on the CVD growth mechanisms of few layer TMDCs, it is unclear which growth mode prevails under different growth conditions. The

Light induced current in two-dimensional (2D) layered materials emerges from mechanisms such as photothermoelectric effect, photovoltaic effect or nonlocal hot carrier transport. Semiconducting layered transition metal dichalcogenides have been studied extensively in recent years as the generation of current by light is a crucial process in optoelectronic and photovoltaic devices. However, photocurrent generation is unexpected in metallic 2D layered materials unless a photothermal mechanism is prevalent. Typically, high thermal conductivity and low absorption of the visible spectrum prevent photothermal current generation in metals. Here, we report photoresponse from two-terminal devices of mechanically exfoliated metallic 3R-NbS2 thin crystals using scanning photocurrent microscopy (SPCM) both at zero and finite bias. SPCM measurements reveal that the photocurrent predominantly emerges from metal/NbS2 junctions of the two-terminal device at zero bias. At finite biases, along with the photocurrent generated at metal/NbS2 junctions, now a negative photoresponse from all over the NbS2 crystal is evident. Among our results, we realized that the observed photocurrent can be explained by the local heating caused by the laser excitation. These findings show that NbS2 is among a few metallic materials in which photocurrent generation is possible. TextPhotocurrent generation in semiconducting 2D layered materials is dominantly due to photothermal [1], photovoltaic effects [2,3] as well as excitation of nonlocal hot carriers [4][5][6]. In metals, these mechanisms typically do not result in photocurrent generation except in a few cases. Photothermal effects are generally not significant in metals because of typical high thermal conductivity and low absorption of the optical excitation. Moreover, optically excited electrons in metals would not have a measurable contribution to the large number of intrinsic electrons near the Fermi level when a bias is applied. In a few cases such as metallic carbon nanotubes [7], graphene [4,8,9] and gold nanoparticle networks [10], photocurrent generation has been reported. Light induced current generation in metallic 2D layered transition metal dichalcogenide (TMDC) is unprecedented, thus, it is not clear what mechanism will be prevalent. Here, we investigate photoresponse of mechanically exfoliated thin Niobium Disulfide (NbS2) crystals using scanning photocurrent microscopy (SPCM) as an exemplary metallic TMDC.NbS2 can be found in layered form both in hexagonal (2H) and rhombohedral (3R) polytypes. While both polytypes are metallic [11], only 2H-NbS2 is superconducting. It is reported in the literature [12][13][14][15]] that 3R-NbS2 shows a conductivity maximum, ranging from 20K to 50K, whose origin is still under debate [16,17]. The conductivity maximum may indicate charge density wave correlations [18] as well as weak localization [19], strong Coulomb correlations [20] or the Kondo effect [21]. Sketches of the 3R structure are depicted in Fig. 1(a) and 1(b).

This work presents an automated three-point bending apparatus that can be used to study strain engineering and straintronics in two-dimensional materials. We benchmark the system by reporting reproducible strain tuned micro-reflectance, Raman, and photoluminescence spectra for monolayer molybdenum disulfide (MoS2). These results are in good agreement with reported literature using conventional bending apparatus. We further utilize the system to automate strain investigations of straintronic devices by measuring the piezoresistive effect and the strain effect on photoresponse in an MoS2 electrical device. The details of the construction of the straightforward system are given and we anticipate it can be easily implemented for study of strain engineering and straintronics in a wide variety of 2D material systems.

Strain Engineering In article number 2201091, Andres Castellanos‐Gomez and co‐workers present an automated experimental setup to perform strain engineering experiments with two‐dimensional materials with unprecedented accuracy and reproducibility has been developed. The setup employs a motorized stage and three‐point bending apparatus geometry to control the applied strain on the materials, enabling changes of strain in steps as small as 10−6%.

Thermal conductivity measurement techniques for materials with nanoscale dimensions require fabrication of very complicated devices or their applicability is limited to a class of materials. Discovery of new methods with high thermal sensitivity are required for the widespread use of thermal conductivity measurements in characterizing materials' properties. We propose and demonstrate a simple non-destructive method with superior thermal sensitivity to measure the in-plane thermal conductivity of nanosheets and nanowires using the bolometric effect. The method utilizes laser beam heating to create a temperature gradient, as small as a fraction of a Kelvin, over the suspended section of the nanomaterial with electrical contacts. Local temperature rise due to the laser irradiation alters the electrical resistance of the device, which can be measured precisely. This resistance change is then used to extract the temperature profile along the nanomaterial using thermal conductivity as a fitting parameter. We measured the thermal conductivity of V 2 O 3 nanosheets to validate the applicability of the method and found an excellent agreement with the literature. Further, we measured the thermal conductivity of metallic 2H-TaS 2 for the first time and performed ab initio calculations to support our measurements. Finally, we discussed the applicability of the method on semiconducting nanosheets and performed measurements on WS 2 and MoS 2 thin flakes.