In this presentation, I will give a brief overview of our research activities in the areas of synthesis, characterization, and applications of nanoscale structures built from semiconductor and metal nanocrystals. Using colloidal nanochemistry, we can fabricate nanocrystals of II-VI, III-V and IV-VI semiconductors with nearly atomic precision in a wide range of sizes and shapes. We can combine different semiconductors to produce complex multishell structures or fabricate hybrid, bi-functional nanocrystals comprising magnetic metals and semiconductors. To characterize electronic and optical properties of synthesized nanomaterials, we apply novel spectroscopic tools that provide single-nanoparticle sensitivity. We use a single-nanocrystal, photoluminescence-excitation technique to study the structure of optical transitions not affected by ensemble averaging effects [1]. By using broad-band, near-field extinction spectroscopy, we study amplitude and phase spectra of plasmon oscillations in individual metal nanocrystals [2]. Our spectroscopic studies reveal a number of unusual physical behaviors arising from quantum-mechanical confinement of electronic excitations in individual nanoparticles and/or strong interfacial interaction between nanoparticles in assembled structures. We use new functionalities produced by the nanosize regime as "enablers" of such novel technologies as color-selectable nanocrystal lasing [3], energy-transfer-powered light emitting diodes [4], and carrier-multiplication enhanced solar cells [5].

[1] H. Htoon, P. J. Cox, and V. I. Klimov, Phys. Rev. Lett. 93, 187402 (2004).
[2] A. A. Mikhailovsky, M. A. Petruska, M. I. Stockman, and V. I. Klimov, Opt. Lett. 28, 1686 (2003).
[3] V. I. Klimov, S. A. Ivanov, J. Nanda, M. Achermann, J. McGuire, and A. Piryatinski, Nature (2007).
[4] M. Achermann, M. A. Petruska, S. Kos, D. Smith, D. D. Koleske, and V. I. Klimov, Nature 429, 642 (2004).
[5] R. D. Schaller and V. I. Klimov, Phys. Rev. Lett. 92, 186601 (2004).