Dr. Alexander Efros Wins Sigma Xi Pure Science Award

12/18/2006 - 65-06r
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Dr. Alexander Efros, a senior researcher at the Naval Research Laboratory (NRL), has received the 2006 Sigma Xi Pure Science Award. Dr. Efros, a theoretical and computational physicist, is recognized for "pioneering and fundamental contributions to the theory of nano-scale semiconductors and the model describing their electronic and optical properties."

Dr. Efros is recognized as a pioneer in the development of the theory of the optical and electronic properties of semiconductor nanocrystals. The seminal theories constructed by Dr. Efros provide a basic foundation for understanding nanocrystal optical properties, their electronic structure, and their linear and nonlinear optical responses.

Nano-scale semiconductors are a new class of optical materials that essentially constitute a new form of matter that can be considered as "artificial atoms." Like atoms, they have discrete optical energy spectra that are tunable over a wide range by varying the nanocrystals' size and shape. Scientists can manipulate them with nanometer precision to form nanocrystal molecules and three-dimensional ordered arrays of "supercrystals," which can be inserted into other materials as dopants or joined to a larger molecule to form a super molecule. This provides scientists with an almost unlimited number of new "atomic elements" to form new materials.

The unusual properties of nano-scale semiconductors, which include size- and shape-controlled tunability of their electronic, optical, and transport properties, combined with the ability to chemically and physically manipulate these "free-standing" nanostructures with nanometer precision, open exciting new opportunities for the development of novel materials with a wide range of applications. Two such applications are of particular interest to the Navy. The first application involves the labeling of a biological molecule by luminescent and photo-stable nanocrystals for applications in biology and medicine. Success in this area suggests it will be possible to create super-sensitive, nanocrystal-based detectors for various biological agents and dangerous chemical compounds. The second application is based on the observation of tunable gain and stimulated emission in CdSe nanocrystals at room temperature. These effects indicate a potential for constructing a PbSe or HgTe nanocrystal-based laser working at 1.6 microns. This is the only long wave length that propagates through the atmosphere without absorption and scattering and is not harmful to human vision. Lasers of this type would be useful in special communications applications.

Spherical semiconductor nanocrystals are the most heavily studied of the nano-scale semiconductors. The strong size dependence of nanocrystal optical properties was discovered independently more than 20 years ago in two different materials: in semiconductor-doped glasses by A. I. Ekimov (1981) and in colloidal solutions by L. Brus (1983). There were many similarities in these discoveries. In both cases, nanocrystal growth resulted from the natural thermodynamic process of solid phase separation of a solution supersaturated with a semiconductor material, a process similar to that which forms water droplets in very humid air. The nanocrystals droplets are the nuclei of a new solid phase. Nanocrystal size is controlled by diffusion of the atoms or ions to the growing nuclei and by the degree of supersaturation. The final size of the nanocrystal is controlled by the duration of the phase-separation process and is fixed at its interruption, a process termed "arrested precipitation."

Semiconductor nanocrystals are a proving ground for ideas about three-dimensional confinement, which occurs when electron and hole free motion is spatially confined in all three dimensions. The quasi-spherical shape of the nanocrystal, combined with very strong confinement of the carriers, provides a basis for a realistic theoretical description of nanocrystals' electronic and optical properties. At the same time, various chemical means enable researchers to effectively control nanocrystal size. Together, these characteristics have allowed researchers to make unambiguous comparisons of theory with experiment, which leads to several important concepts.

In the early 1980s, when the study of nanocrystal quantum dots had just begun, Dr. Efros showed that their optical properties are controlled by the strength of the electron-hole Coulomb interaction. This interaction must always be taken into account because both electrons and holes are confined in the same crystal volume. Theoretical analyses show that the optical properties of spherical nanocrystals depend strongly on the ratio of the exciton Bohr radius to nanocrystal size. In a "strong- confinement" regime, when the nanocrystal radius is smaller than the exciton Bohr radius, optical spectra can be considered as the spectra of transition between electron and hole quantum size levels. In the opposite "weak-confinement" regime, when the nanocrystal radius is larger than the exciton Bohr radius, optical spectra are determined by confinement of the excitons. The study of different nanocrystals confirmed this approach and today researchers commonly use this concept to describe all types of quantum dots.

The Dark/Bright exciton fine structure of the ground exciton state in quantum dots is another concept, which in 1994-95 passed the nanocrystal proving ground. At that time the absorption spectra of CdSe nanocrystals were well understood; however, the standard theory could not explain the photoluminescence data seen in new experiments on CdSe nanocrystals. This photoluminescence had a lifetime three orders of magnitude longer than the theory predicted, as well as a surprising red shift (Stokes shift) of photoluminescence from the energy of the exciting resonant light. To solve this puzzle, Dr. Efros and Dr. Mervine Rosen suggested that the electron-hole exchange interaction, which is greatly enhanced by spatial confinement, splits the ground exciton state in nanocrystal quantum dots into ground optically passive (Dark) and excited optically active (Bright) states. This explains that the long photoluminescence radiative lifetime coming from the Dark exciton state and its Stokes shift occur because the nanocrystals were excited via the optically active Bright exciton state. The Dark/Bright exciton theory developed by Dr. Efros and Dr. Rosen was extraordinarily effective in quantitatively describing in detail photoluminescence in nanocrystals and is now commonly used in research on all type of quantum dots.

Today researchers can control nanocrystal shape using various chemical means. These means allow researchers to transform spherical nanocrystal growth into nanorod growth-crystal particles that have a strongly elongated shape, with an aspect ratio on the order of tens. Recently grown nanorods demonstrate highly efficient photoluminescence, a property that is very important for various optical applications. Dr. Efros, in collaboration with his postdoc Dr. Andrew Shabaev, has shown that these important properties are connected with one-dimensional excitons created in nanorods due to their shape anisotropy. This shape anisotropy leads to anisotropy of spatial confinement of the electron and hole motion and to the enhancement of the Coulomb interaction between them. This effect is connected with penetration of the electric field created by the electron- and hole-charged distribution confined in the nanorod into the surrounding matrix, which has a small dielectric constant. The theory developed to explain this effect showed that the radiative decay time of the one-dimensional excitons in CdSe nanorods can be as short as 500 picoseconds, which is 40 times shorter than in spherical nanocrystals. This prediction explains the efficient photoluminescent properties of nanorods.

Despite more than 20 years of research into their nature, however, nanocrystals continue to surprise scientists with unexpected properties. Researchers just recently discovered that one photon of sufficient energy can generate more than one electron-hole pair in PbSe nanocrystals, an effect never observed in bulk semiconductors. Dr. Shabaev and Dr. Efros suggested a theoretical model that describes the unusual effect of multi-exciton generation by a single photon in PbSe nanocrystals and predicted that this effect should exist in all types of nanocrystals. This paper attracted significant attention among researchers because the effect can lead to a new generation of super-efficient solar cells, whose solar energy conversion efficiency can be 20% greater than in existing cells, where one photon generates a single electron-hole pair.

Dr. Efros continues to delve into the electronic and optical properties of nano-scale semiconductor structures and remains a leading theoretical physicist in this field. He has published more than 100 papers on nano-scale semiconductors and has given more than 30 invited presentations on this topic at various international meetings, in addition to more than 150 invited talks at various universities and laboratories. He is co-editor of two books on nano-scale semiconductors and co-organizer of many conferences on this topic. In 2001 Dr. Efros become a Fellow of the American Physical Society. He also is the recipient of the Optical Society of America's 2006 R.W. Wood Prize, along with Louis E. Brus and Alexei Ekimov, for "discovery of nanocrystal quantum dots and pioneering studies of their electronic and optical properties."

Dr. Efros received his Master of Science degree in physical engineering in 1973 and his Ph.D. in physics in 1978, both from the Technical University in Leningrad, USSR. From 1981 to 1990, he worked as a senior researcher at the Ioffe Institute in Leningrad, and from 1990 to 1992 was a senior researcher in the Physics Department of the Technical University of Munich. From 1992 to 1993, Dr. Efros was a visiting scientist at the Massachusetts Institute of Technology and in 1993 came to NRL as a consultant. In 1999 he became a senior researcher in NRL's Materials Science and Technology Division.

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