Abstract
Thermoelectric transport and photothermal conversion in nanomaterials have attracted considerable attention, not only due to their fundamental scientific interests, but also their potential applications in thermoelectricity and cancer therapies. In the first part of this thesis, we study low-dimensional effects on thermoelectric materials. In the study, films and nanowires of Bi2Te3 and Sb2Te3 were fabricated by potentiostatical electrodeposition. In the case of 120 nm- Bi2Te3 nanowires, a smaller thermopower is revealed, while observing a better electric conductivity compared to bulk material. By coupling thermal diffusivity and heat capacity data, and then applying a modified effective medium theory, a thermal conductivity of 0.75 W/m-K at 300 K was estimated. From this, the ZT of this nanowire was calculated to be 0.45 at 300 K, and is expected to exceed 1 at T > 350 K, which is larger than bulk value. For Sb2Te3, the as-prepared films and nanowires are amorphous. To obtain a crystalline phase, a further annealing treatment is required. As the diameter of wires reduce to nanoscale, the temperature dependence of resistance for nanowires reveals a phase transition from semimetal-like to semiconducting behavior. The Seebeck coefficients of the crystalline films and nanowire are +100 uV/K at 300 K, equal to the bulk value. The considerable reduction in estimated thermal conductivity of Sb2Te3 nanowires is similar to that of Bi2Te3 nanowires. In the second part, we present the study on cancer photothermolysis mediated by gold nanorods. The surface plasmons of nanorods were excited by laser pulses to investigate the destruction process of cells caused by photothermal effect. The nontoxic AuNRs with an aspect ratio of 3.92 were precisely controlled and synthesized by a chemical method. This yields a resonant peak of AuNRs at 800 nm, which has the maximum optical transmission through tissues. The AuNRs displayed excellent two-photon photoluminescence imaging; this makes them ideal probes for in situ real-time observations. The results revealed internal cavities in cells, created from thermal explosions triggered by AuNRs localized photothermal effect. The energy threshold for cell therapy depended significantly on the number of nanorods taken up per cell. For an ingested AuNR cluster quantity N~10-30 per cell, it is found that the energy fluences E of 93 mJ/cm2 from a pulse laser with a duration of 505 μs can lead to effective cell destruction in a crumbled form. With N~60-100 AuNR clusters, a non-instant, but progressive cell deterioration can be achieved with an energy as low as E=18 mJ/cm2; this result is much lower than the established safety standard for medical lasers (100 mJ/cm2).