Semiconductor dots, confined atoms

Quantum dots are semiconductors composed of atoms from groups II-VI or III-V elements of the periodic table, for example, CdSe, CdTe, and InP (39). Their brightness is attributed to the quantization of energy levels due to confinement of an eleetron in a three-dimensional box. The optical properties of quantum dots can be manipulated by synthesizing a (usually stabilizing) shell. Such Q-dots are known as core-shell quantum dots, for example, CdSe-ZnS, InP-ZnS, and InP-CdSe. In this section, we will discuss the different properties of quantum dots based on their size and composition. 

The STL technique has by now been extensively applied to generate light emission from individual semiconductor and metal quantum dots. These nanoparticles are generally smaller than 100 nm and larger than a few atoms. In the case of semiconductors, quantum confinement causes the energy level structure... 

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. 

Ideal semiconductor quantum dot structures should exhibit a delta-function-like (atomic-like) density-of-states for both electrons mid holes [4], Optical excitations in such structures are excitonic in nature, since an electron mid a hole confined in a quantum dot necessarily interact via their Coulomb interaction mid, therefore, form mi exciton. Consequently, a single electron-hole pair in a quantum dot corresponds to mi exciton, whereas doubly occupied electron and hole states (both spin states) correspond to a biexciton. Since the Coulomb interaction of the particles is inevitable, it makes no sense to distinguish between excitons mid free electrons mid holes within a quantum dot. Optical gain... 

The confinement model is also useful to systematically study effects on an atom or molecule trapped in a microscopic cavity or in fullerenes [72-78]. As mentioned above, some of the system observables undergo changes as a result of spatial confinement. The same situation is found at a nanoscopic scale in artificial systems constructed within semiconductors [79-87,172-188], such as two-dimensional quantum wells, quantum wires and quantum dots. Properties of a hydrogen-like impurity in a 2D quantum well have been investigated by several authors [172,173,185-188], who have concluded that particular features associated with the states, as well as the properties of an impurity, are determined, among other factors, by the size of the confining structure. Other applications of confined systems refer to Metal properties [147,148], astrophysical spectroscopic data [40,146], phase transitions [155], matter embedded in electrical fields [68], nuclear models [164], etc. For a detailed list of references, several review articles [25, 48,54,95,125,127] are available. 

In a quantum dot, which is also often called an artificial atom, the excitons are confined in all three spatial dimensions. In a bulk semiconductor, an electron-hole pair is bound within the Bohr exciton radius, which is characteristic for each type of semiconductor. A quantum dot is smaller than the Bohr exciton radius, which causes the appearance of discrete energy levels. The bandgap, AE, between the valance and conduction band of the semiconductor is a function of the nanocrystaTs size and shape. Q-dots feature slightly lower luminescence quantum yields than traditional organic fluorophors but... 

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