Semiconductor nano-particles (quantum dots) exhibit novel optical, electronic and magnetic properties due to quantum size confinement and extremely large surface-to-volume (S/V) ratio relative to bulk. They have potential applications in emerging technologies ranging from nano-electronics to opto-electronics and solar energy conversion based on photocatalysis. Quantum confinement affects both static and dynamic properties of charge carriers while the large S/V ratio is often associated with enhanced activity of surface sites and critically affects the charge carrier behavior and the materials properties. The properties of the nano-particles can also be affected by the shape of the particles.
Recently our group has successfully applied femtosecond laser spectroscopy to study the ultrafast charge carrier dynamics in a series of semiconductor nano-particle systems, including CdS, TiO2, ZnS, AgI, CuS, PbS, and PbI2. Our primary goal was to gain a better understanding of the effects of particle size, shape and surface on the fundamental charge carrier behavior in semiconductor nano-particles with emphasis on carrier-carrier, carrier-surface and carrier-lattice interactions. This understanding is critical to the design and development of new, advanced nano-materials using molecular scale approaches. We have found that in all the systems studied the charge carrier relaxation process is dominated by surface trap states, which act to reduce the carrier lifetime by trapping the carriers and to cause non-radiative decays. In CdS nano-particles, we have found that surface trapping occurs in < 100 fs. The strong surface dependence can be useful or harmful, depending on the applications of interest. For instance, fast trapping and associated non-radiative decays are undesirable for luminescence applications but often useful for photocatalytic applications. We have developed ways to chemically control and modify the particle surface to obtain the desired properties. One of the interesting observations made in these studies was that at high excitation intensities exciton-exciton annihilation becomes important in systems such as CdS and ZnS, due to strong carrier-carrier interaction as a consequence of a high density of excitons or charge carriers created within a small confined volume. The decay due to exciton-exciton annihilation has a lifetime of about 1-3 ps, which is relatively insensitive to the surface or electronic properties of the particles. This phenomenon has potential applications in non-linear optics and optical switches.
Our latest work on PbI2 nano-particles has shown quantum beats with a period of 1.7 to 6 ps as a result of coherent excitation of two close-lying surface trap states. The quantum beats are superimposed on a slow 60 ps decay due to charge carrier relaxation. This study demonstrates that quantum beats contain important information about surface states. We have also observed interesting solvent (surface) and wavelength dependence of the quantum beats and relaxation dynamics. In this case particle size was found to be unimportant in the size range of 3-60 nm. Comparison between the different systems studied, as a function of particle size, surface, shape and excitation intensity, showed both similarities and intriguing differences. These results have significant implications in applications of semiconductor nano-materials in various areas.
Jin Z. Zhang, Ph.D.
Associate Professor of Chemistry, Department of Chemistry and Biochemistry
University of California, Santa Cruz, CA 95064, USA
phone: (831)-459-3776; fax: (831)-459-2935 [Note that the old area code 408 has been changed into 831 for Santa Cruz effective July 11, 1998]