UC Berkeley

Chalcogen-based materials, specially those containing S, Se or Te, can can be successfully manufactured into thin films with semiconducting properties. Transition metal dichalcogenides (TMDCs) are a class of layered materials whose electronic properties are known to depend on the elastic state. Mono- and bi-layer WSe2, for example, undergoes a strain-induced direct to indirect band gap transition. This feature, in principle, could be of great use to design single-material electronic devices were circuits are “drawn” in different elastic states. While it is possible to grow TMDCs in a variety of elastic states, precise control of local strain is yet to be achieved. The first part of this dissertation develops a theoretical model for an empirically-tested method of strain release for TMDCs with built-in strain: the solvent evaporation-mediated decoupling (SEMD) method. The purpose of building such model is to generate knowledge about the underlying mechanism that allows the strain release, and once this process is better understood, to propose further generalizations and experiments that could lead to a better engineering of the elastic states.

The SEMD process consists simply in letting a droplet of liquid solvent evaporate on top of a strained layer. The process was first reported in WSe2 monolayers grown by physical vapor deposition (PVD) on top of amorphous silica; in this case, as the system cooled down from growth to room temperature, strain build up in the film due to the mismatch between its thermal expansion coefficient and that of the substrate. By finding the elastic equilibrium state of a Hamiltonian that takes into account the elastic, interfacial, and substrate-film interaction energies, the proposed model finds an in-plane deformation beyond the film’s initial strain as the main effect of the solvent droplet, and identifies it as the strain release mechanism. Furthermore, the proposes a selectivity criteria based on the contact angle that determines whether the SEMD process would be successful for a given initial elastic state.

The second part of the thesis is about thin films made of Te. Te-based materials were heavily studied in the 1980’s due to their applications in data storage, and are now experiencing a revival mainly because its potential application in electronic and optic devices. Due to the chain-like nature of the crystalline Te structure, this material's properties are highly anisotropic, with the conductivity, for example, being greater in the direction parallel to the chains. Because of this anisotropy, high-quality devices require large areas of single-crystalline Te.

Here, we explore the crystallization process of vapor-grown amorphous Te under different temperatures and, using tools from classical nucleation theory, investigate the nucleation and growth rate of crystals in an amorphous matrix. As a low nucleation rate will necessarily result in slow growth, another mechanism to grow Te single crystals is needed. An alternative route is offered by growing Te upon WTe2, a substrate with two-fold symmetry that favors chain alignment along its high-symmetry direction. Crystals nucleating on top of WTe2 are found to have single crystal-like texture, and thus be suitable for device applications.