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Nominated by the University of Utah, USA, as an outstanding Ph.D. thesis
Lays the groundwork for further use of Electron Spin Echo Envelop Modulation (ESEEM) and opens the possibility of highly precise chemical fingerprinting
Reveals an astonishingly long memory of spin coherence in semiconductor particles
Colloidal nanocrystals show much promise as an optoelectronics architecture due to facile control over electronic properties afforded by chemical control of size, shape, and heterostructure. Unfortunately, realizing practical devices has been forestalled by the ubiquitous presence of charge "trap" states which compete with band-edge excitons and result in limited device efficiencies. Little is known about the defining characteristics of these traps, making engineered strategies for their removal difficult.
This thesis outlines pulsed optically detected magnetic resonance as a powerful spectroscopy of the chemical and electronic nature of these deleterious states. Counterintuitive for such heavy atom materials, some trap species possess very long spin coherence lifetimes (up to 1.6 µs). This quality allows use of the trapped charge's magnetic moment as a local probe of the trap state itself and its local environment. Beyond state characterization, this spectroscopy can demonstrate novel effects in heterostructured nanocrystals, such as spatially-remote readout of spin information and the coherent control of light harvesting yield.
Content Level »Research
Keywords »CdSe/CdS Nanocrystals - Colloidal Nanocrystals - Electron Spin Resonance - Light Harvesting - Optically Active Charge Traps - Optically Active Charges - Optically Detected Spin Coherence - Pulsed Optically Detected Magnetic Resonance - Seminconducting Nanocrystals - Spin Coherence - Spin Echo Envelop Modulation - Trap States
Introduction.- Colloidal Nanocrystals.- Pulsed Optically Detected Magnetic Resonance (PODMR).- Experimental Methods.- Experimental Considerations of PODMR.- Time-Resolved Optical Spectroscopy.- Nanocrystal Materials.- Sample Preparation.- Spin-Dependent Exciton Quenching and Intrinsic Spin Coherence In CDSE/CDS Nanocrystals.- Chapter Synopsis.- Introduction.- Spectrally Selected, Optically Detected Magnetic Resonance.- Coherence Measurements and Novel Effects.- Conclusion.- Supporting Information.- Toward Chemical Fingerprinting of Deep-Level Defects Sites in CDs Nanocrystals by Optically Detected Spin Coherence.- Chapter Synopsis.- Introduction.- Photoluminescence Decay Dynamics Indicating Long Trapping Lifetimes.- Experimental Methods.- Optically Detected Magnetic Resonance vs. Emission Channel.- Increased Dipolar Coupling of Shallow Trap States Associated with the Defect.- Probing Coherence and ESEEM with Optically Detected Hahn Echoes.- Conclusion.- Summary of Work.- Work in Context.- Publications to Date.