Quantum confinement structures, synthesis

Quantum dots are the engineered counterparts to inorganic materials such as groups IV, III-V and II-VI semiconductors. These structures are prepared by complex techniques such as molecular beam epitaxy (MBE), lithography or self-assembly, much more complex than the conventional chemical synthesis. Quantum dots are usually termed artificial atoms (OD) with dimensions larger than 20-30 nm, limited by the preparation techniques. Quantum confinement, single electron transport. Coulomb blockade and related quantum effects are revealed with these OD structures (Smith, 1996). 2D arrays of such OD artificial atoms can be achieved leading to artificial periodic structures. 

Microfabrication is increasingly central to modern science and technology. Many opportunities in technology derive from the ability to fabricate new types of microstructures or to reconstitute existing structures in down-sized versions. The most obvious examples are in microelectronics. Microstructures should also provide the opportunity to study basic scientific phenomena that occur at small dimensions one example is quantum confinement observed in nanostructures [1]. Although microfabrication has its basis in microelectronics and most research in microfabrication has been focused on microelectronic devices [2], applications in other areas are rapidly emerging. These include systems for microanalysis [3-6], micro-volume reactors [7,8], combinatorial synthesis [9], micro electromechanical systems (MEMS) [10, 11], and optical components [12-14]. 

The chapters in this volume present detailed insights into the synthesis-structure-properties relationships of nanostructured materials. In particular, the catalytic and photocatalytic properties of nanoclusters and nanostructured materials with ultrahigh surface-to-volume ratio are demonstrated. The gas absorption characteristics and surface reactivity of nanoporous and nanocrystalline materials are shown for various separation and reaction processes. In addition, the structural manipulation, quantum confinement effects, transport properties, and modeling of nanocrystals and nanowires are described. The biological functionality and bioactivity of nanostructured ceramic implants are also discussed. 

The latest development in the subfield of surface-modified semiconductor nanocrystals is the synthesis of three-layered colloidal particles [55-58]. The novel structures consist of a size-quantized semiconductor particle acting as the core spherically covered by several monolayers of another semiconductor material, which by themselves are surrounded by several monolayers of, again, the core material acting as the outermost shell. These particles are called quantum dot quantum wells (QDQWs) or, metaphorically, nano-onions. The more scientific naming is motivated by the analogy to real quantum wells, which are semiconductor structures with alternating layers of two semiconductor materials exhibiting quantum confinement in one dimension in at least one of the materials. 

During the last decade STM has proven to be a unique tool for the synthesis of novel structures. Perhaps the most elegant demonstration of this was the positioning of individual Xe atoms on Ni(l 10) with atomic precision in a low-temperature UHV experiment [516], A variety of structures that exhibit the physics of quantum confinement have been produced in this manner [517], and more recently, the manipulation of individual molecules at room temperature has been demonstrated [518,519], It is now clear that there are several possible mechanisms for atomic and/or molecular manipulation [520], Similarly, two reviews of various related schemes for sub-pm surface modification are also available [521,522], In addition to published... 

The study of confined quantum systems has attracted increasing attention from several research groups in the world due to the unusual physical and chemical properties exhibited by such systems when subject to spatial limitation. Such novel properties, not present in conventional materials, have marked a new era for the synthesis of modern materials - structured at the nanoscale - and leading to what is now called nanotechnology. 

Specifically, this volume focuses on the synthesis, processing, and structural tailoring of nanocrystalline and nanoporous materials. Nanocrystalline materials possess unique hybrid properties characteristic of neither the molecular nor the bulk solid-state limits and may be confined in nanometersized domains in one, two, or three dimensions for unusual size-dependent behavior. Nanoporous materials, characterized by well-defined pores or cavities in the nanometer size regime and controlled pore diameter and structure, give rise to unique molecular sieving capabilities and ultrahigh internal surface areas. Nanoporous structures also act as hosts and templates for the fabrication of quantum dots and quantum wires. 

Charge carriers in semiconductors can be confined in one spatial dimension (ID), two spatial dimensions (2D), or three spatial dimensions (3D). These regimes are termed quantum films, quantum wires, and quantum dots as illustrated in Fig. 9.1. Quantum films are commonly referred to as single quantum wells, multiple quantum wells or superlattices, depending on the specific number, thickness, and configuration of the thin films. These structures are produced by molecular beam epitaxy (MBE) and metalorganic chemical vapor deposition (MOCVD) [2j. The three-dimensional quantum dots are usually produced through the synthesis of small colloidal particles. 

In the past two decades, composite nanoparticles have attracted great attention for their special and multiple properties. The synthesis of composite nanoparticles has not been confined to simply integration of some categories with special effects. The structure has become much more complex, and the scale of composite has been reduced to less than 1-20 nm, which can be referred to as quantum dots. According to the structural features, composite nanoparticles can be mainly grouped into three categories simple hybrid, core/shell structured composite nanoparticles, and multifunctional quantum dots, as shown in Fig. 1. 

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