Date on Master's Thesis/Doctoral Dissertation

8-2017

Document Type

Doctoral Dissertation

Degree Name

Ph. D.

Department

Chemistry

Degree Program

Chemistry, PhD

Committee Chair

Liu, Jinjun

Committee Member

Handa, Sachin

Committee Member

Freelon, Byron

Committee Member

Li, Ying

Author's Keywords

CdSe; nanocluster; quantum dots; PDTC; transient absorption

Abstract

Semiconductor nanocluster (SCNC) research is a rapidly growing field driven by the promising fact that properties of such materials can be tailored by modifying their size, shape, and structure. Combination of nanocluster and organic ligands provides even wider possibilities of design and development of effective and task-specific nanostructures. Understanding of and, eventually, control of energetics, interfacial interaction, and photoinduced processes in such highly heterogeneous structures is critical to invention of novel materials including those in photovoltaic devices. In the newly established Ultrafast Laser Facility (ULF), transient absorption pump-probe spectroscopy (TAPPS) has been employed to investigate the electron transfer (ET) and hole transfer (HT) dynamics in nanostructures with solar cell applications. After a brief introduction of ultrafast laser systems and TAPPS, and a review of previous works from ULF, our recent work on some novel nanocluster-ligand systems will be presented. In this work, ultrasmall (1.6 nm in diameter) cadmium selenide nanoclusters with precise size and mass (Cd34Se34) were passivated by phenyldithiocarbamate (PDTC) ligand monolayers. Because of the quantum confinement effect, the ultrasmall and well-controlled size of the nanoclusters results in discrete and well-resolved electron and hole states in their valence and conduction bands, respectively, which allows a quantitative spectroscopic study of energetics and dynamics of these conjugates. Sub-picosecond ET and HT processes from the SCNC cores to their organic passivating monolayer were observed in TAPPS when excited at higher photon energy than their optical bandgap. Based on results from various control experiments and computational works, photoinduced processes in the SCNC-ligand conjugates have been well understood: Strong coupling between hole states and the ground electronic state of the passivating ligands delocalizes the hole orbitals and facilitates the HT process. In addition, ET from the conduction band of the nanoclusters to the excited states of the ligands creates interfacial charge transfer states with sub-picosecond lifetime. Charge transfer dynamics of CdSe SCNCs with varies of para-substituted derivatives of the PDTC ligand were also studied. The strong quantum confinement and “magical size distribution” of the ultrasmall SCNC core and the versatility of the passivating ligands give these heterogeneous nanostructures very different material functions from the bulk, which leads to unique applications: from quantum dots solar cells, photocatalysis, to biofluorescence sensors and imaging. The understanding of the charge transfer dynamics will provide useful information and guide a better design for the device in different scientific and industry fields.

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