Sol-gel chemistry of transition metal oxides

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

Colloidal nanocrystals (NCs, i.e., crystalline nanoparticles) have become an important class of materials with great potential for applications ranging from medicine to electronic and optoelectronic devices. Today’s strong research focus on NCs has been prompted by the tremendous progress in their synthesis. Impressively narrow size distributions of just a few percent, rational shape-engineering, compositional modulation, electronic doping, and tailored surface chemistries are now feasible for a broad range of inorganic compounds. The performance of inorganic NC-based photovoltaic and light-emitting devices has become competitive to other state-of-the-art materials. Semiconductor NCs hold unique promise for near- and mid-infrared technologies, where very few semiconductor materials are available. On a purely fundamental side, new insights into NC growth, chemical transformations, and self-organization can be gained from rapidly progressing in situ characterization and direct imaging techniques. New phenom...

Prospects of Nanoscience with Nanocrystals

The field of nanocrystal quantum dots (QDs) is already more than 30 years old, and yet continuing interest in these structures is driven by both the fascinating physics emerging from strong quantum confinement of electronic excitations, as well as a large number of prospective applications that could benefit from the tunable properties and amenability toward solution-based processing of these materials. The focus of this review is on recent advances in nanocrystal research related to applications of QD materials in lasing, light-emitting diodes (LEDs), and solar energy conversion. A specific underlying theme is innovative concepts for tuning the properties of QDs beyond what is possible via traditional size manipulation, particularly through heterostructuring. Examples of such advanced control of nanocrystal functionalities include the following: interface engineering for suppressing Auger recombination in the context of QD LEDs and lasers; Stokes-shift engineering for applications in large-area luminesce...

Spectroscopic and Device Aspects of Nanocrystal Quantum Dots

Semiconductor crystallites of characteristic dimension \sim20-100 A can be made by precipitation in liquids and dielectrics. These crystallites have bulk-like internal lattices. The optical spectra show partially resolved discrete features that result from carrier spatial confinement in three dimensions. Spectral shifts of more than 1 eV are observed. The electronic wavefunctions of small crystallites are discussed within the effective mass approximation. The pattern of discrete hole states is qualitatively different in small crystallites than in slab superlattices. In small CdS crystallites, luminescence is observed from trapped carriers that may be in localized surface states. The recombination emission is strongly coupled to crystallite phonons.

Zero-dimensional "excitons" in semiconductor clusters

Illumination of semiconductor colloids such as ZnS , CdS and Ti02, generates electrons and positive holes which may react with the aqueous solvent or dissolved substances  In ZnS and CdS , recombination of the charge carriers is often accompanied by emission of light The absorption spectra of very small CdS or ZnS particles (1 3 nm) show changes which indicate a transition from semiconductor to polymolecular material The conduction band energy is shifted, and the exciton transition band becomes more pronounced  The fluorescence spectra of small CdS particles contain a band at the onset of absorption and two bands at longer wavelengths  They are attributed to emission from the short-lived exciton state and from longer-lived states of trapped electrons and holes The chemical reactions described include the photo-anodic and the photocathodic dissolution of colloidal sulfides, the reduction of water and carbon dioxide , and the oxidation of alcohols and sulfite anions  Two-electron (two-hole) and one-electron (one-hole) transfer mechanisms are discussed Flash photolysis studies of colloidal Ti02 gave the absorption spectra of long-lived electrons and positive holes which are trapped in surface states of different energies Chemical reactions of these charge carriers are also described, including the reduction of tetranitromethane, the oxidation of SCN and OHanions, and of various organic compounds The electrochemical nature of all these photo-reactions is emphasized

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A. Henglein

Papers

Catalysis of photochemical reactions by colloidal semiconductors