Effect of Surface Environment on Energy Relaxation Dynamics in Photo-Excited Nanocrystals

The optical properties of semiconducting nanocrystals have considerable application in various fields such as biological imaging, light emitting devices and solar cells. Due to high surface to volume ratio surface structure has a profound effect on the exciton quantum yield and charge carrier dynamics of these nanocrystals. Surface imperfections or surface defects often decrease the exciton quantum yield by trapping the charge carriers and thus affect relaxation dynamics. Surface binding surfactants play an important role in determining optical properties and exciton dynamics as they can remove surface defects through passivation and they can also introduce new trap sites.

Transition metal doped semiconducting nanocrystals especially Mn-doped in II-VI semiconducting host show stoke shifted Mn-emission with high Mn-emission quantum yield applicable in light emitting devices, biological imaging and sensors. Although Mn-emission was found to depend on surface effects, the underlying mechanism and dynamics was not explored in great detail. Thiols are important class of surfactants used to passivate nanocrystals especially to make water soluble nanocrystals. Thiols are hole trapping surfactants and known to quench the emission in CdSe or CdS nanocrytals. In our study, we examined the effect of hole trapping ligand octanethiol on the Mn-luminescence quantum yield and exciton dynamics in Mn-doped CdS/ZnS nanocrystals. Surprisingly Mn-luminescence quantum yield was found to increase in presence of octanethiol in contrary to undoped nanocrystals where octanethiol almost quenched the exciton emission. Combining transient absorption measurements with steady state Mn-photoluminescence and Mn-lifetime measurements, we confirmed the existence of an energy transfer process from octanethiol created hole traps to Mn that enhances the Mn-photoluminescence.

Apart from studying the effect of surface environment in exciton relaxation dynamics in Mn-doped nanocrystals, we have studied the effect of surfactant and solvent on the spin relaxation dynamics in magnetic nanocrystals through transient Faraday rotation measurements. The spin-lattice relaxation rate in spherical Fe_(3)O_(4) nanocrystals depends on the functional group and binding nature of the surfactant. Also, the solvent affects the spin-lattice relaxation rate only when they can access the surface of the nanocrystals. Therefore, thick surfactant passivation prevents the approach of the solvent molecules resulting spin-lattice relaxation rate independent of solvent environment.