Ultrafast spectroscopic investigations of cadmium chalcogenides: Nanoscale electronic relaxation and transfer

Bryan Thomas Spann, Purdue University

Abstract

Harnessing solar energy more effectively remains one of the most important scientific challenges in recent history. Various strategies have been developed to capture the sun's energy to generate usable electricity. Recently, advances in chemistry have allowed researchers to synthesize semiconducting nanocrystals which show great promise in capturing and converting solar energy in a cheap and efficient way. In this dissertation, aspects of energy conversion processes in semiconducting nanocrystals are explored to elucidate their potential for photovoltaic applications. Various forms of linear and non-linear optical spectroscopy techniques were employed to explore electronic relaxation and transfer phenomena in nanoscale cadmium chalcogenide materials and heterojunctions. Ultrafast transient absorption studies were performed on various sizes of CdSe quantum dots (QDs) and quantum rods (QRs) with similar bandedge energies. These studies reviled that QRs have increased intraband relaxation times when compared with QDs as a result of an ultrafast formation of a 1D exciton along the elongated axis of the QR. The formation of the 1D exciton reduces the electron-to-hole scattering potential, consequently reducing the Auger thermalization mechanism. Furthermore, QD samples made in film form showed increased intraband relaxation times as a result of a hydrazine treatment which removed (in part) the organic ligands attached to the surface. As a result of removing the ligands, the ligand based relaxation pathway for the holes was also reduced, causing longer intraband relaxation. In addition to the studies on CdSe nanocrystals (NCs), ultrafast spectroscopy was used to study aspects of charge transfer in CdS – TiO2 NC heterojunctions. This study revealed a means of increasing photo-induced ultrafast charge transfer in successive ionic layer adsorption and reaction (SILAR) CdS–TiO2 NC heterojunctions using pulsed laser sintering of TiO2 nanocrystals. The enhanced charge transfer was attributed to both morphological and phase transformations. Laser sintering resulted in varying degrees of anatase to rutile phase transformation of the TiO 2, producing thermodynamically more favorable conditions for charge transfer by increasing the change in free energy between the CdS donor and TiO2 acceptor states. Finally, aspects of apparent hot electron transfer as a result of the SILAR process which allows CdS to be directly adsorbed to the TiO2 surface. Finally, ultrafast transient absorption spectroscopy with temporal pulse shaping was employed to manipulate coherent phonon excitation and quantify the strength of electron-phonon coupling in CdTe1-xSex NCs. Raman active CdSe and CdTe longitudinal optical phonon (LO) modes are excited and probed in the time domain. Furthermore, by temporally controlling pump pulse pairs to coherently excite and cancel coherent phonons in the CdTe 1-xSex NCs, estimates of the relative amount of optical energy that is coupled to the coherent CdSe LO mode. In summary, ultrafast spectroscopy was used to explore charge and energy carrier relaxation and transfer mechanisms in nanoscale cadmium chalcogenides both in colloidal and film forms. While the results presented here cover a broad range of understanding photo-physical phenomena in these materials, it is evident that more work is required to further understand the many-body interactions that are present in the semiconducting NC films.

Degree

Ph.D.

Advisors

Xu, Purdue University.

Subject Area

Physical chemistry|Mechanical engineering|Nanoscience

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