High Deviatoric Strain Engineering/

Abstract

The structure of a material can be tuned reversibly or irreversibly by imposing elastic or inelastic strain, leading to change of properties. This defines the concept of strain engineering, which includes both elastic strain engineering (ESE) and inelastic strain engineering (ISE). In this thesis, we study ESE and ISE by deviatoric (nonhydrostatic) strain. For ESE, we model how imposition of slowly-varying inhomogeneous elastic strain can induce the electronic structure changes of semiconductor crystals. The strain-dependent shift of valence and conduction band energy levels leads to the formation of electronic and hole bound states in in-homogeneously strained crystals, whose energy levels can be dynamically tuned by the strain field. We developed a new envelope function method with strain-parametrized basis set that can solve the electronic structure of such inhomogeneously strained crystals by incorporating the local electronic structure information obtained from unit-cell level first-principles calculation of homogeneously strained crystals. For ISE, we study the deviatoric strain induced phase transformation and internal structure evolution in soft matter systems. Using largescale molecular dynamics simulation, we demonstrate that controlled sintering of the nanocrystals in self-assembled superlattices of alkanethiol-passivated gold nanoparticles can happen at room temperature through deviatoric stress-induced displacement of the organic ligands. We find that combining a hydrostatic pressure of order several hundred megapascal and a critical deviatoric stress along the nearest-neighbor direction of gold nanoparticle superlattices leads to ordered sintering of gold nanocrystals and the formation of gold nanowire arrays. Similar phenomena can happen in binary superlattices of gold and silver nanoparticles, and we predict the formation of gold-silver multijunction nanowire arrays through deviatoric-stress driven sintering of nanoparticles. We also simulate the plastic flow of two dimensional amorphous granular pillars subjected to athermal, uniaxial and quasistatic deformation. We find that for the athermal granular pillars under inhomogeneous load, the cumulative local deviatoric strains of particles with respect to their neighbors play the role of time in thermal systems, and drive the crossover of non-affine particle displacements from ballistic motion to diffusion. The result suggests that in disordered solids, deviatoric strain alone can drive particle diffusion even at zero vibrational temperature.