Terahertz time-domain spectroscopy studies of nanocomposites
Date of Issue2016-03-29
School of Physical and Mathematical Sciences
Terahertz time-domain spectroscopy (THz-TDS) is a powerful tool which has been used to probe the low-energy excitations in metals and semiconductors. The THz transmission signal is sensitive to the presence of free carriers, phonons, excitons, and the interplay among these degrees of freedom. The most remarkable advantage of THz-TDS is the simultaneous acquisition of the amplitude and phase of the THz pulses, which allows us to obtain the complex permittivity (or equivalently, complex optical conductivity) without using the Kramers-Kronig relationship. The ability to obtain the real and imaginary parts of the optical conductivity severely restricts the physical models that can explain the data. In this thesis, after introducing the experimental setup and data analysis procedure, the main experimental results are presented. Topological insulators (TIs) are fascinating materials with an insulating bulk and conducting surface. The surface states of topological insulators display Dirac-cone dispersion in which the electrons obey a linear energy-momentum ‘relativistic’ relationship. Due to the very small photon energy of THz radiation, the photons can be absorbed, causing intra band transitions in the Dirac cone. By performing THz measurement on a composite of Bi2Te3 TI nanoparticles deposited on single-layer graphene (SLG), we observed an enhanced THz modulation compared to pure SLG, which suggest that there is electron injection from the TI nanoparticles into the graphene. Our results also reveal the presence of multiple phonon modes in Bi2Te3 thin film in our frequency window of 0.3 – 3 THz. Doped ZnO is a type of transparent conductive oxide which can be used in organic photovoltaics. The properties of transparency and conductivity are both desirable, but are often mutually exclusive. To improve the conductivity while retaining the transparency, a good way is to improve the carrier mobility. Our THz-TDS results show that the Ag nanoparticles can improve the electron mobility of Al-doped ZnO. The recently-discovered organometallic halide perovskite CH3NH3PbI3 has been shown to achieve a photovoltaic efficiency of up to 19%. By measuring the THz con-ductivity via THz-TDS, we observed that two phononmodes in the high-temperature tetragonal phase split into four phonon modes in the low-temperature orthorhombic phase. The presence of new phonon modes in the low-temperature orthorhombic phase might have implications on the photovoltaic efficiency at low temperatures.