Controlled growth of two-dimensional layered semiconductors from laser-synthesized nanoparticles

Full text: Two-dimensional layered semiconducting materials, notably the metal chalcogenides and dichalcogenides (GaS, GaSe, GaTe, InSe, etc. and MoS₂, WS₂, MoSe₂, NbSe₂, etc.) have recently attracted significant renewed attention due to the novel physical, chemical, electrical and optical properties that can emerge when they are isolated in nanosheets of single- or few-layer thickness. Developing new methods for the facile synthesis of layered materials is crucial for emerging applications in functional devices. Here we demonstrate the use of pulsed laser vaporization (PLV) to form stoichiometric nanoparticles that serve as building blocks for the synthesis of 2D GaSe and MoSe₂ layered semiconductors. Our PLV approach offers a new synthesis solution to address the challenges of conventional vapor phase growth methods (e.g., CVD), by taking advantage of the spatial confinement of the ablation plume in relatively high background gas pressures to preserve the stoichiometric transfer of material from a bulk target to predefined locations on the substrate while providing sufficient kinetic energy for surface diffusion. After ablation of crystalline targets, the propagation of the laser ablation plume was characterized by in situ ICCD-imaging and ion probe analysis. As shown in Fig. 1, we demonstrate the formation of 2D crystals in either nanosheet networks (200 nm domain sizes) or large single crystalline domains (100 μm lateral sizes) with controlled stoichiometry, number of layers, crystallite size, and growth location. Small-domain nanosheet networks are formed by direct deposition of stoichiometric nanoparticles onto heated substrates in an argon background pressure of ~1 Torr. To grow large 2D crystals from nanoparticles grown by laser vaporization, room temperature-deposited nanoparticles were sandwiched between two substrates to form a confined vapor transport growth system. Heating from the source substrate side in an inert background gas develops a natural temperature gradient that evaporates the confined nanoparticles from the source substrate to the cooler receiver substrate where crystalline 2D nanosheets are observed to grow in pre-patterned locations. This novel PLV-based synthesis and processing method offers a unique approach for the controlled growth of large-area, metal chalcogenides with a controlled number of layers and sizes in patterned growth locations for optoelectronics and energy related applications. (author)