The properties of a material depend on the type of motion its electrons can execute, which depends on the space available for them (i.e., on the degree of their spatial confinement). Thus, the properties of each material are characterized by a specific length scale, usually on the nanometer dimension. If the physical size of the material is reduced below this length scale, its properties change and become sensitive to its size and shape. In this Account we describe some of the observed new chemical, optical, and thermal properties of metallic nanocrystals when their size is confined to the nanometer length scale and their dynamical processes are observed on the femto- to picosecond time scale.

Some interesting properties of metals confined in time and nanometer space of different shapes.

The synthesis and study of so-called “nanoparticles”, particles with diameters in the range of 1−20 nm, has become a major interdisciplinary area of research over the past 10 years. Semiconductor nanoparticles promise to play a major role in several new technologies. The intense interest in this area derives from their unique chemical and electronic properties, which gives rise to their potential use in the fields of nonlinear optics, luminescence, electronics, catalysis, solar energy conversion, and optoelectronics, as well as other areas. The small dimensions of these particles result in different physical properties from those observed in the corresponding macrocrystalline, “bulk”, material. As particle sizes become smaller, the ratio of surface atoms to those in the interior increase, leading to the surface properties playing an important role in the properties of the material. Semiconductor nanoparticles also exhibit a change in their electronic properties relative to that of the bulk material; as th...

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Nanocrystalline semiconductors: Synthesis, properties, and perspectives

Inorganic nanocrystals with tailored geometries exhibit unique shape-dependent phenomena and subsequent utilization of them as building blocks for the fabrication of nanodevices is of significant interest. Herein, we review the recent developments in the shape control of colloidal nanocrystals with a focus on the scientifically and technologically important semiconductor and metal oxide nanocrystals obtained by nonhydrolytic synthetic methods. Many structurally unprecedented motifs have been discovered including polyhedrons, rods and wires, plates and prisms, and other advanced shapes such as branched rods, stars, inorganic dendrites, and dumbbells. The currently proposed shape-guiding mechanisms are presented and the important pioneering studies on the assembly of shape-controlled nanocrystals into ordered superlattices and the fabrication of prototype advanced nanodevices are discussed.

Shape Control of Semiconductor and Metal Oxide Nanocrystals through Nonhydrolytic Colloidal Routes

This review is concerned with quantum confinement effects in low-dimensional semiconductor systems. The emphasis is on the optical properties, including luminescence, of nanometre-sized microcrysta...

Low-dimensional systems: quantum size effects and electronic properties of semiconductor microcrystallites (zero-dimensional systems) and some quasi-two-dimensional systems

This is a review and analysis of the optical properties of quantum crystallites, with principal emphasis on the electro-optic Stark effect and all optical third order nonlinearity. There are also introductory discussions on physical size regimes, crystallite synthesis, quantum confinement theory, and linear optical properties. The experiments describe CdSe crystallites, exhibiting strong confinement of electrons and holes, and CuCl crystallites, exhibiting weak confinement of the exciton center of mass. In the CdSe system, neither the Stark effect nor the third order nonlinearity is well understood. The Stark shifts appear to be smaller than calculated, and field inducted broadening also occurs. The third order nonlinearity is only modestly stronger than in bulk material, despite theoretical prediction. Unexpectedly large homogeneous widths, due to surface carrier trapping, in the nominally discrete crystallite excited states appear to be involved. The CuCl system shows far narrower spectroscopic homogeneous widths, and corresponds more closely to an ideal quantum dot in the weak confinement limit. CuCl also exhibits exciton superradiance at low temperature. Surface chemistry and crystallite encapsulation are critical in achieving the predicted large Stark and third order optical effects in II-VI and III-V crystallites.

Quantum crystallites and nonlinear optics

Luminescence and photophysics of cadmium sulfide semiconductor clusters: the nature of the emitting electronic state

Metal selenide clusters have been made and characterized, using the arrested precipitation colloidal technique. A comparison of sulfide and selenide spectra enables observation of the effect of changes in the highest occupied molecular orbitals upon cluster electronic properties. The first and second excited electronic states are both observed as a function of size in ZnSe clusters. The systematic dependence of the spectra lead to assignment of the higher state to a 1S‐type hole based upon the split‐off valence band. It is shown that the energy spectrum of discrete hole states is controlled by the spin‐orbit energy and the isotropic hole mass in small, highly symmetrical clusters. This result contrasts with the heavy hole and light hole states observed for planar confinement. In ≂ 20 A diameter ZnS clusters, there is a strong vibronic temperature dependence in the excited state spectra, while in clusters of smaller gap materials such vibronic effects are very minor. We conjecture that lifetime broadening ...

N. Chestnoy

Papers

Luminescence and photophysics of cadmium sulfide semiconductor clusters: the nature of the emitting electronic state

Higher excited electronic states in clusters of ZnSe, CdSe, and ZnS: Spin‐orbit, vibronic, and relaxation phenomena

Luminescence and Photophysics of CdS Semiconductor Clusters: The Nature of the Emitting Electronic State.