Conductivity in CdSe quantum dots and TiO2 nanoparticles: what can THz spectroscopy tell us?

Terahertz spectroscopy emerged about 14 years ago with the demonstration that nearly single cycle pulses of far-infrared (FIR) radiation could be generated, sent through free space, and subsequently detected in the time-domain. Since then, THz spectroscopy has become an active area with studies ranging from condensed matter physics to gas-phase spectroscopy to biomedical imaging. One of the most unique aspects of THz spectroscopy is that the pulses are of sub-picosecond duration, and it is possible to characterize the time-dependent ac conductivity in the FIR on ultrafast timescales. That is, it is a non-contact electrical probe with sub-picosecond temporal resolution. We have applied time-resolved THz spectroscopy to novel systems such as low-temperature grown GaAs, semiconductor quantum dots, and sintered colloidal TiO₂. This paper will describe the transient photoconductivity in CdSe quantum dots and nanocrystalline TiO₂ as a function of size and morphology.