Kipp van Schooten
Wednesday, December 5, 2012
3:30pm (110 INSCC)
Title: Optically Active Charge Traps & Chemical Defects in Semiconducting Nanocrystals Probed by Pulsed Optically Detected Magnetic Resonance
The forefront of current nanoscience initiatives includes the investigation and development of semiconducting colloidal nanocrystals for optoelectronic device concepts. Being highly facile in their synthesis, a wide range of sizes, morphologies, materials, interactions and effects can easily be engineered. Their solution-processability also offers the prospect of extremely cheap device manufacturing. Additionally, this material class makes available a type of “playground” for generating and observing novel quantum effects within reduced dimensions.
Since the surface-to-volume ratio is very large in these systems, unsatisfied surface states are able to dominate the energetics of these particles. Serving as charge “trap” states, their effect on observables is readily seen, for instance, in single particle photoluminescence blinking. Unfortunately, most methods used to observe their influence are inherently blind to the chemical identity of these sites. In absence of such structural information, systematically engineering a robust passivation system becomes problematic.
The development of pulsed optically detected magnetic resonance (pODMR) as a method for directly addressing the chemical nature of optically active charges while under trapping conditions is the primary tenet of this thesis work. Several trapping channels are observed in CdS nanorods, while two in particular are correlated, demonstrating for the first time that both electrons and holes are able to be trapped within the same nanoparticle at the same time. The intrinsically long spin coherence lifetime for these states allows for the spin multiplicity and degree of isolation to be explored, and opens the possibility of highly precise chemical fingerprinting through electron spin echo envelop modulation (ESEEM). Demonstration of novel effects is also performed for CdS/CdSe heterostructure tetrapods, such as coherent control of the light-harvesting process and remote readout of spin information.