Ultrafast spectroscopy of nanomaterials

Abstract

Ultrafast spectroscopy is a very useful technique to study the electronic and vibrational properties of many solid-state systems. In this dissertation, ultrafast optical-pump optical-probe (OPOP) and terahertz time-domain spectroscopy (THz-TDS) have been used to study low dimensional nanomaterials and bulk oxides which are often used substrates for nanomaterials, thin films and functional materials. The photoexcited electrons in bilayer graphene (BLG) exhibit two relaxation processes — the fast relaxation time is less than 200 fs while the slow one is around a few ps. After the BLG has been intercalated with FeCl3, the sample became hole-doped. Also, there are many horizontal bands in the electronic band structure. The presence of these additional bands has caused three effects in the pump-probe results: (a) pump probe signal changes sign; (b) the signal rise time is much longer than that of BLG; (c) the fast relaxation process disappears leaving only the slow one. The conductivity spectrum of Bernal BLG depends on the impurity concentration, chemical potential, temperature and bias voltage, but is nonetheless almost Drude-like at the lowest frequencies. In twisted BLG, van-Hove singularities develop near the Fermi energy, which results in an enhanced density of states (DOS). Strongly coupled stacking order BLG is investigated by THz-TDS at different temperatures. In the frequency dependence of the real conductivity, superposed on top of a Drude-like response, we observed a strong and narrow peak centered at ~2.7 THz. The overall Drude shape was analyzed using a disorder dependent model, while the conductivity peak at 2.7 THz was attributed to an enhanced DOS at that energy, which is caused by the presence of low-energy van-Hove singularities arising from a commensurate twisting of the top graphene layer relative to the bottom layer. The accurate substrate complex refractive index is a precondition for the THz study for the materials grown on the substrate. Z-cut quartz (CrysTec GmbH) is a very good transparent substrate for THz radiation, as it has a temperature-independent and almost frequency-independent refractive index . The refractive index n(ω) of LaAlO3 (LAO, CrysTec GmbH) is temperature-dependent and increases monotonically with frequency. The values of k(ω) are very small, and no obvious absorption peaks appeared frequency range 0.2—3 THz. After the LAO has been annealed, n(ω) shows sudden changes at certain frequencies where the absorption peaks appear in k(ω). Also, the crystal orientation can affect the complex refractive index. SnO2 nanowires are investigated by THz-TDS as a function of temperature. Results show that SnO2 nanowires are not only transparent in the visible range, but also in the terahertz range 0.5 THz to 2.4 THz. The carrier density increases and scattering time decreases with increasing temperature. The reduction in carrier mobility compared with bulk material indicates the presence of carrier backscattering and localization in the nanowires.