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

In this study, we present an investigation on the growth of thin Mo2C crystals via chemical vapor deposition using CH4. Optical microscopy (OM), scanning electron microscopy (SEM), atomic force microscopy(AFM), and Raman spectroscopy studies show that the morphology and the thickness of Mo2C crystals are strongly affected by the impurities in the system, the thickness of the copper substrate, and the graphene presence on Cu surface prior to Mo2C formation. Our studies show that during the CVD process, orthorhombic Mo2C crystals grow along the [100] direction on two different regions: directly on Cu surface or on graphene covered regions. Mo2C crystals that form on graphene are found to be thinner and less defective compared to the ones formed on the Cu surface. This is attributed to graphene acting as an additional diffusion barrier for Mo atoms diffusing through the copper. In addition to the graphene beneath the Mo2C crystal, Raman studies indicate that graphene may grow also on top of the Mo2C crystal, forming a graphene/Mo2C/graphene sandwich structure which may offer interesting properties for electronic applications.

Mechanical properties of transition metal dichalcogenides (TMDCs) are relevant to their prospective applications in flexible electronics. So far, the focus has been on the semiconducting TMDCs, mostly MoX2 and WX2...

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).

Electric field driven reversible phase transitions in two-dimensional (2D) materials are appealing for their potential in switching applications. Here, we introduce potassium intercalated MnO 2 as an exemplary case. We demonstrate the synthesis of largearea single-crystal layered MnO 2 via chemical vapor deposition as thin as 5 nm. These crystals are spontaneously intercalated by potassium ions during the synthesis. We showed that the charge transport in 2D K-MnO 2 is dominated by motion of hydrated potassium ions in the interlayer space. Under a few volts bias, separation of potassium and the structural water leads to formation of different phases at the opposite terminals, and at larger biases K-MnO 2 crystals exhibit reversible layered-to-spinel phase transition. These phase transitions are accompanied by electrical and optical changes in the material. We used the electric field driven ionic motion in K-MnO 2 based devices to demonstrate the memristive capabilities of two terminal devices.