Quantum confinement of electrons

The unique electronic properties of CNTs are due to the quantum confinement of electrons normal to the CNT axis. In the radial direction, electrons are... 

The emission spectra of nanoparticles of semiconductors such as CdSe and CdTe, known as quantum dots, depend on size. Quantum confinement of electrons within a particle increases the electronic bandgap with decreasing particle size, so that a series of different sized CdSe particles emits a rainbow of colors. 

Gallium phosphide, GaP, has an electron effective mass of 0.82me and a hole effective mass of 0.60 me. Calculate the dimension at which quantum confinement becomes significant for (a) n-type and (b) p-type gallium phosphide, (c) The crystal structure of gallium phosphide consists of layers of GaP each 0.315 nm in thickness how many layers are needed for quantum confinement of electrons or holes ... 

Synthesis of CdS nanoparticles can be performed easily and safely by freshmen students. Based originally on research by Agostiano (P), the procedure has been adapted to use reagents commonly available to a general chemistry laboratory (JO, 11). This experiment illustrates how intermolecular forces affect the formation of micelles and how surfactants behave in oil-water mixtures. The difference in color between the bulk and nanosized CdS is visibly obvious but students can also calculate the nanoparticle size with the aid of a UV/visible spectrophotometer. The explanation for the color difference is based on quantum confinement of electrons and holes in the particle s semiconductor lattice. 

Quantum dots are defined as 0-dimensional semiconductor material as result of quantum confinement of electrons in three dimensions [112, 113]. QDs are generally synthesized from elements in the periodic table belonging to groups 12-14 or 13-15 to form clusters which have dimensions smaller than the exciton Bohr radius [112-115]. QDs have been used to modify electrode surfaces for electrochemical detection of analytes, such as magnesium ions, glutathione and glucose [116-118]. 

Crommie M F, Lutz C P and Eigler D M 1993 Confinement of electrons to quantum corrals on a metal surface Science 262 218... 

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. 

The absence of an enormous enhancement in radiative decay rates in the nanocrystals can also be verified by electronic absorption spectroscopy. The original claim stated that the Mn2+ 47) —> 6A1 radiative decay lifetime dropped from xrad = 1.8 ms in bulk Mn2+ ZnS to xrad = 3.7 ns in 0.3% Mn2+ ZnS QDs ( 3.0 nm diameter) (33). This enhancement was attributed to relaxation of Mn2+ spin selection rules due to large sp-d exchange interactions between the dopant ion and the quantum-confined semiconductor electronic levels (33, 124— 127). Since the Mn2+ 47 > 6Ai radiative transition probability is determined... 

As you may recall from Chapter 4, when an electron is promoted from the valence to conduction bands, an electron-hole pair known as an exciton is created in the bulk lattice. The physical separation between the electron and hole is referred to as the exciton Bohr radius (re) that varies depending on the semiconductor composition. In a bulk semiconductor crystal, re is significantly smaller than the overall size of the crystal hence, the exciton is free to migrate throughout the lattice. However, in a quantum dot, re is of the same order of magnitude as the diameter (D) of the nanocrystal, giving rise to quantum confinement of the exciton. Empirically, this translates to the strongest exciton confinement when D < 2r. ... 

Nanocrystals of metal and semiconductors with diameters in the range 1 to 50 nm form a class of materials with unusual properties which are size-dependent. Excellent electrical conductivity that primarily characterizes a metallic state, becomes a rare entity in small nanocrystals (< 2 nm) due to quantum confinement of the electronic states. Similarly, magnetic metals lose much of the coercivity with diminishing size. On the other hand, chemical properties such as reactivity may show up better at smaller sizes due to a greater number of surface bonding sites and other electronic effects. Considering the importance of nanocrystals in tech-... 

Kolb s group presented a jump-to-contact technique by which the surface can be decorated with small metal clusters provided there is sufficient interaction between a sample and a tip, the effect discussed earlier. By means of this technique, they showed the way to generate different patterns on a different scale. On the nanometer scale they presented the structure made of 12-Cu clusters of 0.8-mn height (see Fig. 29). They reported an interesting observation on the surprising stability against anodic dissolution of the fabricated clusters, and they proposed to explain this effect by quantum confinement of the electrons in the clusters that leads to an energy gap between the electronic states of the clusters. ... 

However, even in perfect ID structures, the mixing of Frenkel and CT excitons destroys this simple picture. Below, following (43), we show that this mixing is responsible for the appearance of new excitonic states, which are localized at the ends of a one-dimensional crystal chain and which are analogous to Tamm surface states of electrons. Their energy can be blue- or red-shifted in comparison with the bulk states. In the case of red-shift, these states can determine the fluorescence spectrum of a molecular chain. They can also play an important role in quantum confinement of the states in the molecular chain. For the description... 

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