Size dependency of the optical, thermal and vibrational properties of ZNO and BATIO3 at the nanoscale

• Main Author: Ma, Shouzhi
• Other Authors: Sun Changqing
• Format: Theses and Dissertations
• Language: English
• Published: 2013
• Subjects: DRNTU::Engineering
• Online Access: http://hdl.handle.net/10356/51235
• Institution: Nanyang Technological University

With device miniaturization, the size effect on band gap, phonon vibration, thermal stability, ferroelectricity and wettability are attracting increasing interests with unfortunately unclear physical origins. It is the intent of this work to find the reasons for size-induced property variation, and the quantitative correlations between size and various properties. On the basis of bond order-length-strength (BOLS) correlation and nonbonding electron polarization (NEP), detectable quantities are functionally correlated to the order, nature, length, and energy of the representative bond of the specimen. By understanding materials from the perspective of bonding science, many physical properties can be interrelated, and a general guideline can be formed to control crystal growth and performance. A core-shell model has been developed to study the increase of the bond energy and binding energy density, and the reduction of atomic cohesive energy, due to nanostructures size and shape. The core retains its bulk nature, while the shell behaves differently as a result of bond contraction due to surface atomic deficiency. Consequently, there is a localized densification of charge, energy, and mass. The shell atoms suffer from enhanced Hamiltonian and suppressed atomic cohesive energy, which dominate properties such as band gap, phonon vibration, thermal stability, and wettability. In order to validate the proposed core-shell model, conductive zinc oxide thin films with different grain sizes were prepared and investigated. Grazing incident X-ray diffraction confirmed that there is spontaneous contraction of bonds between undercoordinated atoms in the surface shell. Wettability IV measurements suggested that the densely and tightly-trapped charges polarize existing surface nonbonding electrons. The Coulomb repulsion between the sample surface and the electric dipoles locked in the liquid skin is the reason for hydrophobicity. Hall measurement supported that the localized polarization turns the surface nonbonding electrons to be tightly-locked dipoles and less conductive. Raman spectroscopy showed that size-induced softening of the A 1 (LO) optical phonons arises from collective interatomic interaction while the stiffening of E 2 (high) phonons from dimer interaction. Phonon vibrational frequency shift is quantitatively correlated to the grain sizes. Post-annealing determines the critical size by equating the thermal and the cohesive energy of the undercoordinated atoms in the surface skin. The perturbed surface energy in turn induces local strain and quantum entrapment, perturbing the Hamiltonian and hence the band gap. Besides properties that directly related to the near-surface binding energy and cohesive energy, there are properties that come from both short-range and long-range interactions, such as ferroelectricity. Therefore, a dual-shell model is also established. One surface shell of three atomic layers represents the short- range interaction due to the shorter and stronger bonds between the under- coordinated atoms. The other shell of K c surface layers characterizes the long- range dipole-dipole interaction, where K c is the critical number of atomic layers. Barium titanate ferroelectrics with different sizes were prepared and demonstrated good agreement with the proposed model. The modeling predictions have been verified by the presented experimental observations and results documented by others. The formulation provides a better understanding and guideline for crystal growth and performance, and makes predictive design and fabrication of functional nanomaterials into reality.