Current research into semiconductor clusters is focused on the properties of quantum dots-fragments of semiconductor consisting of hundreds to many thousands of atoms-with the bulk bonding geometry and with surface states eliminated by enclosure in a material that has a larger band gap. Quantum dots exhibit strongly size-dependent optical and electrical properties. The ability to join the dots into complex assemblies creates many opportunities for scientific discovery.

Semiconductor Clusters, Nanocrystals, and Quantum Dots

The interest in nanoscale materials stems from the fact that new properties are acquired at this length scale and, equally important, that these properties * To whom correspondence should be addressed. Phone, 404-8940292; fax, 404-894-0294; e-mail, mostafa.el-sayed@ chemistry.gatech.edu. † Case Western Reserve UniversitysMillis 2258. ‡ Phone, 216-368-5918; fax, 216-368-3006; e-mail, burda@case.edu. § Georgia Institute of Technology. 1025 Chem. Rev. 2005, 105, 1025−1102

Chemistry and properties of nanocrystals of different shapes.

We report a synthesis of highly luminescent (CdSe)ZnS composite quantum dots with CdSe cores ranging in diameter from 23 to 55 A. The narrow photoluminescence (fwhm ≤ 40 nm) from these composite dots spans most of the visible spectrum from blue through red with quantum yields of 30−50% at room temperature. We characterize these materials using a range of optical and structural techniques. Optical absorption and photoluminescence spectroscopies probe the effect of ZnS passivation on the electronic structure of the dots. We use a combination of wavelength dispersive X-ray spectroscopy, X-ray photoelectron spectroscopy, small and wide angle X-ray scattering, and transmission electron microscopy to analyze the composite dots and determine their chemical composition, average size, size distribution, shape, and internal structure. Using a simple effective mass theory, we model the energy shift for the first excited state for (CdSe)ZnS and (CdSe)CdS dots with varying shell thickness. Finally, we characterize the...

(CdSe)ZnS Core-Shell Quantum Dots - Synthesis and Characterization of a Size Series of Highly Luminescent Nanocrystallites

▪ Abstract Solution phase syntheses and size-selective separation methods to prepare semiconductor and metal nanocrystals, tunable in size from ∼1 to 20 nm and monodisperse to ≤5%, are presented. Preparation of monodisperse samples enables systematic characterization of the structural, electronic, and optical properties of materials as they evolve from molecular to bulk in the nanometer size range. Sample uniformity makes it possible to manipulate nanocrystals into close-packed, glassy, and ordered nanocrystal assemblies (superlattices, colloidal crystals, supercrystals). Rigorous structural characterization is critical to understanding the electronic and optical properties of both nanocrystals and their assemblies. At inter-particle separations 5–100 A, dipole-dipole interactions lead to energy transfer between neighboring nanocrystals, and electronic tunneling between proximal nanocrystals gives rise to dark and photoconductivity. At separations <5 A, exchange interactions cause otherwise insulating ass...

Synthesis and Characterization of Monodisperse Nanocrystals and Close-Packed Nanocrystal Assemblies

Semiconductor nanocrystals exhibit a wide range of size-dependent properties. Variations in fundamental characteristics ranging from phase transitions to electrical conductivity can be induced by controlling the size of the crystals. The present status and new opportunities for research in this area of materials physical chemistry are reviewed.

Perspectives on the Physical Chemistry of Semiconductor Nanocrystals

The synthetic procedure, the characterization, and some photophysical properties of a quantum dot quantum well (QDQW) system are described in detail. The novel structures prepared via wet chemical methods consist of a core of size-quantized CdS and a well of 1-3 monolayers of HgS capped by 1-5 monolayers of CdS acting as the outermost shell. Additionally, theoretical calculations based on the effective mass approximation appropriate to describe the 1s-1s electronic transition of the composite particles are presented

Preparation, characterization, and photophysics of the quantum dot quantum well system cadmium sulfide/mercury sulfide/cadmium sulfide

An extended theoretical approach for calculating the 1s-1s electronic transition in spherically layered semiconductor quantum dots is presented. The extension over the common effective-mass approximation includes the implementation of the Coulomb interaction and finite potential wells at the particle boundaries. The calculations are carried out for the quantum-dot quantum well CdS/HgS/CdS and compared to recently available experimental results. The wave functions of electrons and holes spreading over the entire structure and the probabilities of presence in the different layers, as well as outside the structure in the surrounding dielectric water, are presented.

Quantum-dot quantum well CdS/HgS/CdS: Theory and experiment.

Small gold cluster anions ${{\mathrm{Au}}_{n}}^{\ensuremath{-}}$ are known for their unusual two-dimensional (2D) structures, giving rise to properties very different from those of bulk gold. Previous experiments and calculations disagree about the number of gold atoms ${n}_{c}$ where the transition to 3D structures occurs. We combine trapped ion electron diffraction and state of the art electronic structure calculations to resolve this puzzle and establish ${n}_{c}=12$. It is shown that theoretical studies using traditional generalized gradient functionals are heavily biased towards 2D structures. For a correct prediction of the 2D-3D crossover point it is crucial to use density functionals yielding accurate jellium surface energies, such as the Tao-Perdew-Staroverov-Scuseria (TPSS) functional or the Perdew-Burke-Ernzerhof functional modified for solids (PBEsol). Further, spin-orbit effects have to be included, and large, flexible basis sets employed. This combined theoretical-experimental approach is promising for larger gold and other metal clusters.

2D-3D transition of gold cluster anions resolved

Size-selected, ligand-free gold clusters with diameters lessthan 2nm can be routinely generated in the gas phase. Thepronounced size dependence of their physical and chemicalproperties is one of their most important features. Surface-deposited gold clusters are particularly interesting for appli-cations in nanotechnology and heterogeneous catalysis.

Au34−: A Chiral Gold Cluster?

We report the experimental structure determination of cold, mass selected Ag55+/- cluster ions using the recently developed technique of trapped ion electron diffraction. By comparison of experimental and theoretical molecular scattering functions and consideration of computed total energies, we show that Ag55+ constitutes an ideal Mackay icosahedron, whereas Ag55- is a weakly Jahn−Teller distorted icosahedron. Isomers of other structural types, for example, decahedral or close-packed, could be ruled out. The candidate structures were obtained by density functional theory calculations.

Detlef Schooss

Papers

Preparation, characterization, and photophysics of the quantum dot quantum well system cadmium sulfide/mercury sulfide/cadmium sulfide

Quantum-dot quantum well CdS/HgS/CdS: Theory and experiment.

2D-3D transition of gold cluster anions resolved

Au34−: A Chiral Gold Cluster?

The structures of Ag55+ and Ag55- : trapped ion electron diffraction and density functional theory.