Synthesis and characterization of semiconductor nanomaterials: lead sulfide, cadmium sulfide, and cadmium telluride

• Main Author: Wu, Fanxin
• Format: Dissertation
• Language: English
• Published: ProQuest Dissertations & Theses 01-01-2004
• Subjects: Chemistry; Materials science; Physical chemistry
• ISBN: 0496746685; 9780496746682

Charge carrier dynamics in semiconductor nanoparticles (NPs) such as CdS, CdTe, and PbS have been studied using femtosecond transient absorption spectroscopy, static and time-resolved fluorescence measurements. CdS NPs with different surface capping enable us to determine lifetimes of linear relaxation processes from bandedge states and deep trap states in CdS NPs. Under high excitation intensities, surface-passivated (SP) CdS NPs show a fast nonlinear decay in addition to the slow linear decay, while surface-unpassivated (UP) samples only show a single exponential decay. The fact that SP samples with lower density of states (DOS) are easier to reach trap state saturation than UP samples with higher DOS provide new and stronger evidence for the model of exciton-exciton annihilation. Charge carrier dynamics of thiol-capped CdTe have been investigated using both femtosecond transient absorption spectroscopy and nanosecond time-resolved fluorescence measurements. An unusual decrease of fluorescence with excitation intensity has been observed under relatively high excitations for CdTe NPs, which is in contrast to that of CdS where fluorescence only increases monotonically with increasing excitation under the same range. CdTe NPs exhibits stronger nonlinear properties than CdS NPs possibly due to the stronger quantum confinement effect and appears to be potentially useful for nonlinear optical applications. PbS NPs with various surface capping and sizes all show similar dynamic properties. It may be possible to observe size and or surface dependence in the near infrared region, which is out of our detection limit. Compared to PbS NPs, US NPs show different size-dependent dynamic profiles. CdS NPs and NRs (nanorods) with different sizes all show similar time constants for the slow linear decay. However, larger CdS NPs show a more pronounced fast nonlinear decay component, reflected in smaller time constant and larger amplitude. The size dependence of the nonlinear component in CdS NPs has been explained by considering a number of factors including quantum confinement effect, surface states, and number of excitons per NP. The nonlinear decay processes appear to be dominated by the volume factor (number of excitons per particle). High pressure fluorescence measurements have been used to monitor the phase transitions in CdTe NPs. The transition pressure from zinc-blende/cinnarbar to rocksalt phase in CdTe solution has been elevated from that of bulk CdTe and that of CdTe in solid form. The elevation of transition pressure and enhanced stability have been explained by a larger surface tension and inhomogeneous strain in CdTe NP solution.