Temperature dependent elastic, lattice vibronic and thermal properties of nanomaterials
Date of Issue2008
School of Electrical and Electronic Engineering
It is well known that the physical properties of a macroscopic system can be well described using classical approaches such as the Gibbs free energy or the continuum medium mechanics, which relates the measurable quantities directly to the external applied stimulus such as temperature, pressure, chemical composition, electric and magnetic field, etc, without considering the atomistic origin. At atomic scale, quantum effect becomes dominant and the physical properties of a small object can be reliably obtained from computations by solving the Schrödinger equations or Newton motion equations with a sum of averaged interatomic potentials as a key element. However, for a small system in nanometer scale, both classical and quantum approaches have their limitations. Therefore, an effective approach solving the difficulties encountered by both classical and quantum approximations has been a great challenge. The recently advanced bond-order-length-strength (BOLS) correlation suggests that the size dependent material property is mainly attributed to the interaction between under-coordinated atoms in the surface skins. The coordination number imperfection in the surface skin leads spontaneous bond length contraction and bond energy strengthening. This causes densification and localization of charge, energy, mass at surface region, and hence modifies atomic coherency, Hamiltonian, etc. By extending the BOLS correlation mechanism to temperature domain, an approach of local bond average (LBA) has been developed in this thesis, which states that: (i) the entire specimen or a specific location of a specimen can be represented by a representative bond; (ii) the detectable quantity of a specimen can be obtained once the relationship between this detectable quantity and the bond identities (bond order, nature, length, and strength) of the representative bond and the response of these bond identities to the stimulus is established. This thesis discusses the size and temperature dependent elastic, lattice vibronic,and thermal properties of various materials. Deeper insight into the mechanism behind the size and temperature dependence together with analytical solutions is presented. Theoretical reproductions of experimental observations reveal that the surface coordination number imperfection induced spontaneous bond contraction and bond energy strengthening dictate the size-induced mechanical strength enhancement, optical Raman frequency redshift, and thermal conductivity variation; whereas the thermally-driven bond length expansion and bond energy weakening lead to mechanical strength depression, Raman frequency redshift, as well as surface energy reduction. Reproductions of size and temperature dependent Young’s modulus, Raman frequency shift, thermal expansion coefficient, thermal conductivity and surface energy for various materials, such as metals, group IV, group III-nitrides, and carbon based materials give quantitative information about the atomic cohesive energy and the dimer frequency, which is beyond the scope of currently available approaches in literature. Current progress in BOLS correlation and LBA approximation could pave a path to bridge the gap between the classical approach in macroscopic system and the quantum confinement approach in atomic level by considering the interatomic bond formation, dissociation, relaxation, vibration and associated energetic response of corresponding atoms and electrons and the consequences on the measurable quantities.