Fundamental processes in semiconductor nanocrystals.

Abstract

Colloidal semiconductor nanocrystals attract a great deal of attention due to the possibility of tuning their optical and electronic properties by variation of shape, size and composition. Colloidal nanocrystals have been synthesized in the form of spherical quantum dots, one-dimensional nanorods and nanowires, two-dimensional nanoplates and nanosheets, as well as three-dimensional multipods. Besides nanocrystals composed of a single semiconductor material, heterostructures of different materials have also been realized. The surface of colloidal nanocrystals is usually passivated by molecular or ionic ligands. Wet-chemical synthesis procedures offer prospects for cheap production of materials in large quantities. Electronic excited states (excitons) of nanocrystals can decay by fluorescence with kinetics and quantum yields that depend on composition, shape and size. Mutual electronic coupling of nanocrystals can be realized in thin films. Photoexcitation of such a nanocrystal film can yield free mobile charge carriers. The luminescent properties of nanocrystals are of interest for their application as fluorescent tags in biological systems, or spectral up/down conversion. Thin films of nanocrystals can be applied as the active material in light-emitting diodes, photovoltaic cells, transistors or logic gates. In view of the above-mentioned applications, as well as from a fundamental scientific perspective, it is of great importance to study how the composition, shape and size of nanocrystals influence the properties of electronic excited states (excitons) and charge carriers (excess electrons or holes). This special issue provides a series of studies on the factors that govern the optical and electronic properties of colloidal semiconductor nanocrystals. Numerous studies have shown that single electronically excited nanocrystals exhibit fluorescence intermittency (blinking). The duration of ‘‘blinking-on’’ and ‘‘blinkingoff’’ periods can last from seconds to minutes. These periods obey power law statistics with exponential tails. Zhu and Marcus have extended the equations to describe blinking dynamics for the situation for higher excitation densities, at which biexcitons are present. Smyder et al. report a related experimental study of the effects of pulsed versus continuous wave laser excitation on the blinking of quantum dots. For pulsed excitation the probability of a long ‘blinking-on’ period is lower and the fluorescence lifetime is shorter. Several studies have been carried out to avoid blinking by fabrication of photo-stable nanocrystals with a core–shell hetero-structure with a giant shell or an alloy graded shell. As shown by Kershaw et al. the composition of CdxHg1 xTe alloyed colloidal quantum dots strongly affects the competition of hot exciton relaxation by carrier multiplication and phonon emission. The interplay between these two processes is strongly dependent on the electron effective mass, which can be tuned by variation of the composition and may be considerably lower than the hole effective mass. A theoretical study by Delerue demonstrates that a honeycomb super-lattice of covalently connected CdSe quantum dots has conduction bands with Dirac points analogous to graphene. In addition quantum interference effects give rise to the presence of a non-trivial flat band. Electrical and thermal transport in thin films of colloidal nanocrystals is to a large extent determined by the coupling between adjacent nanocrystals, which depends on the nature of the surface ligands. Park et al. report the effects of the surface ligands on the trap density in a bilayer field-effect transistor (FET) consisting of a thin PbS nanocrystal film interfaced with vacuum-deposited pentacene. Studies by Li et al. show that the efficiency of a photovoltaic cell based on a polymer and tetrapod nanocrystal blend is much higher than that of one using a blend with quantum dots of the same material. The work of Gu et al. shows that the open-circuit voltage of dye-sensitized solar cells can be improved by doping TiO2 nanocrystals with tantalum. Claudio et al. discuss enhancement of charge mobility and reduction of thermal a Department of Chemistry and Solid State Institute, Technion, Haifa 3200, Israel b Optoelectronic Materials Section, Department of Chemical Engineering, Delft University of Technology, Julianalaan 136, 2628 BL Delft, The Netherlands DOI: 10.1039/c4cp90174f