Elastic Strain Engineering of Low-dimensional Nanostructures: Tuning Functional Properties by Mechanical Stretching

Description

“Strain engineering” originally refers to a general strategy employed in semiconductormanufacturing to enhance device performance (or saying, electron/hole mobility) bymodulating strain in the transistor channel. Recently a new concept of “elastic strainengineering (ESE)” has been proposed to tailor the physical and chemical properties bymechanically changing the lattice strain (elastic strain). Unlike those strained byepitaxial growth in conventional strain engineering, the functional properties tuned byelastic strain engineering can be flexible, continuous and reversible. As a theoreticalconcept, tuning lattice parameter for desired properties wasn’t new, but one of the keyfactors to experimentally achieve ESE was to have materials that can sustain very largeelastic strain (e>1%) without being relaxed by plasticity or fracture (most conventionalmaterials can only sustain 0.2-0.3% elastic strain). Until recently, as the advances insynthesis and the explosive proliferation of nanomaterials with “ultra-strength”, ESEhas become an experimental reality.Despite that elastic strain engineering has been theoretically proposed as a powerful wayof fine-tuning nanomaterials’ properties to create functional nanodevices with novelapplications, experimental studies are still very scarce. So in this project, we propose tocarry out a range of in situ nanomechanical, electro-mechanical and spectroscopycharacterizations for a few representative low-dimensional nanomaterials, to quantifytheir tensile elastic strain limits and “ultra-strength”---the foundation of ESE.Through the in situ TEM, we try to further understand the origin of “ultra-strength”,and investigate the corresponding size effect in crystal elasticity at the atomic level.Through in situ tensile straining, we will quantitatively characterize the piezoresistiveresponses of semiconductor (Si and Ge) nanowires as well as 2-D atomic sheets (e.g.MoS2) with large elastic strain, and investigate conductance characteristics of strainedultrathin Au nanowires. In addition, we will try to tailor the bandgap structures ofextremely strained semiconductor nanowires (e.g. Ge nanowire was predicted to have anindirect-to-direct bandgap transition ate>5%) and possibly 2-D atomic sheets.The successful implementation of this project will strengthen our understanding on theconcept of ESE and allow us to identify a few “ultra-strength” materials with highlytunable functional properties for device applications, for which the potential commercialvalue has been already demonstrated by the billion-dollar strained semiconductorindustry. Scientifically, our in situ experiment results will greatly complement thetheoretical and modeling efforts, and together they shall provide unprecedented detailsand quantitative insights into how nanomaterials response to applied strain with desiredfunctional properties.