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Dissertation Information for Jianchang Guo

NAME:
- Jianchang Guo

DEGREE:
- Ph.D.

DISCIPLINE:
- Chemistry

SCHOOL:
- Emory University (USA) (2006)

ADVISORS:
- Tianquan Lian

COMMITTEE MEMBERS:
- Karl S. Hagen
- Michael C. Heaven

MPACT Status: Fully Complete

Title: Controlling photoinduced electron transfer dynamics between molecular adsorbates and semiconductor nanoparticles

Abstract: Photoinduced electron transfer (ET) dynamics from molecular adsorbates to inorganic semiconductor (SC) nanocrystalline thin films have been investigated by ultrafast transient absorption spectroscopy. The dependence of ET rate on various factors, such as the nature of semiconductors, doping level, the thickness of insulating overlayer, the particle size of quantum dots and the adsorbate anchoring bridge length was examined by choosing proper dyes and semiconductor nanoparticles. The dependence of ET dynamics on the nature of semiconductors was investigated by using Ru polypyridyl complex-sensitized Nb 2 O 5 and In 2 O 3 films and comparing with previous studies of Ru dyes on SnO 2 , ZnO, and TiO 2 . The injection kinetics of Ru dyes on Nb 2 O 5 are biphasic, with an ultrafast component (<100 Es, 37%) and a much slower (ps or longer timescale) component, corresponding to injection from unthermalized and thermalized excited states of dye molecules, respectively. After excitation of the adsorbates at 532 nm, the injection kinetics from RuN 3 to In 2 O 3 are similar to those of SnO 2 . These kinetics traces show a negligible <100 fs injection component and are very different from those to TiO 2 . A much faster injection rate from unthermalized excited states is observed for d-type semiconductors (Nb 2 O 5 and TiO 2 ) than that for s-type semiconductors (ZnO, In 2 O 3 , and SnO 2 ), likely due to orders of magnitude higher density of states in the former. The dependence of ET dynamics on the doping level was investigated by using Re polypyridyl complexes and a conjugated polymer (MEH-PPV) sensitized antimony doped SnO 2 (ATO) nanoparticles. In particles with different levels of Sb dopant, the rates of electron injection from adsorbate excited states to ATO nanoparticles are independent of dopant level, and the rates of recombination increase with the doping level. The observed similar forward electron injection rates are attributed to negligible changes of available accepting states in the conduction band at the doping levels studied. The dependence of the recombination rate on conduction band electron density is explained by a continuous-time random walk model. The dependence of ET dynamics on the thickness of an insulating overlayer was investigated by using Ru and Re polypyridyl complex sensitized Al 2 O 3 -coated TiO 2 and SnO 2 nanocrystalline thin films. The injection rates from dye to Al 2 O 3 -coated semiconductor nanoparticles decrease with an increasing number of Al 2 O 3 overlayers. The uniformity of Al 2 O 3 is examined by using catechol as a probe molecule. It is found that catechol could still form charge transfer complexes with TiO 2 even at three Al 2 O 3 overlayers, indicating that the Al 2 O 3 overlayer may be inhomogeneously distributed on the semiconductor nanoparticle surface. The dependence of ET dynamics on the size of quantum dots (qdots) were investigated by monitoring the fluorescence decay of dyes bound to qdots. Electron injection from C343 to 2.4 and 3.4 nm CdS qdots is observed, while no injection is observed from RdB to CdS qdots. This is attributed to the difference of relative energetics between C343/RdB and CdS qdots. The ET rate is faster from C343 to 3.4 nm CdS than to 2.4 nm CdS, corresponding to a lower conduction band edge of large CdS qdots. The dependence of ET dynamics on anchoring bridge length was investigated by using phenylene-bridged PDI and methylene-bridged C343 on SnO 2 or TiO 2 . The ET rates decrease with increasing PDI loading, which was attributed to the decrease of accepting states due to the formation of PDI excimer. No bridge length dependence is observed on both phenylene-bridged PDI and methylene-bridged C343. We suspect the dyes are sitting on the surface through conjugated [pi] systems due to the strong interaction of [pi] rings and semiconductor. Further experiments and simulations may be needed to clarify the results. Thin films with mixed nanoparticles were prepared by using Layer-by-Layer (LBL) assembly based on hydrogen bonding. Time-resolved fluorescence spectroscopy studies of the LBL film with mixed 25 and 45 Å (CdSe)ZnS (core)shell qdots show faster decay than the film with pure 25 Å (CdSe)ZnS qdots, indicating efficient energy transfer from small to large qdots. Possible structure of LBL films is discussed.

MPACT Scores for Jianchang Guo

A = 0
C = 0
A+C = 0
T = 0
G = 0
W = 0
TD = 0
TA = 0
calculated 2008-02-17 23:49:47

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