Abstract

This talk will discuss the photoexcited carrier dynamics of one and two-dimensional nanomaterials studied by using time resolved optical pump-terahertz spectroscopy. The studies include the carrier dynamics of single layer and bilayer graphene, single walled and double walled carbon nanotubes and transition metal di-chalcogenides. In single layer graphene, the real part of terahertz photoconductivity, was found to be negative at high doping and positive at low doping throughout the spectral range 0.5-2.5 THz. In contrast, for moderately doped bilayer graphene, real part of photoconductivity was negative at low frequency and positive on high frequency part of the spectra. In addition, opening a gap across the Dirac point through hydrogen doping results in the sign change of photoconductivity from negative to positive. The role of short-range scattering arising from charge neutral defects such as ripples, grain boundaries etc, long-range Coulomb scattering due to charged defects from underlying substrate, acoustic phonon, and surface-optical phonon scattering all are evaluated quantitatively to explain the photoinduced terahertz conductivity spectra for all these cases. The measured photo-conductivities for single wall and double wall carbon nanotubes were found to be different by their sign and spectral responses which were also described quantitatively by Boltzmann transport model of electrical conductivity. In transition metal di-chalcogenides, the carrier relaxation was faster at higher pump fluence which was understood by rate equation model incorporating defect-mediated electrons, holes, and excitons capture and recombination rates.