Semiconductors quantum crystallites

Brus L E 1993 NATO ASI School on Nanophase Materials ed G C Had]lpanayls (Dordrecht Kluwer) Allvisatos A P 1996 Semiconductor clusters, nanocrystals and quantum dots Science 271 933 Heath J R and Shlang J J 1998 Covalency In semiconductor quantum dots Chem. See. Rev. 27 65 Brus L 1998 Chemical approaches to semiconductor nanocrystals J. Phys. Chem. Solids 59 459 Brus L 1991 Quantum crystallites and nonlinear optics App/. Phys. A 53 465... 

Kanemoto M, Shiragami T, Pac CJ, Yanagida S Semiconductor photocatalysis — effective photoreduction of carbon-dioxide catalyzed by ZnS quantum crystallites with low-density of surface-defects. J Phys Chem 1992, 96(8) 3521— 3526. 

Yanagida S, Yoshiya M, Shiragami T, Pac CJ, Mori H, Fujita H Semiconductor photocatalysis. 9. Quantitative photoreduction of aliphatic ketones to alcohols using defect-free ZnS quantum crystallites. J Phys Chem 1990, 94(7) 3104— 3111. 

Our goal was to develop hands-on activities and laboratory experiments that could be adapted for delivery to the different constituent groups (2). Semiconductor quantum dots were chosen as the intellectual focus of the new instructional materials. More specifically, colloidal quantum dots, formed from cadmium selenide (CdSe) and cadmium selenide sulfide (CdSexSi.x) alloys were used to demonstrate the colorful trends that are correlated to the physical size of nanosized crystallites as shown in Figure 1. This visual trend vividly illustrates one of the central themes in nanoscience-/ /7y /ca/properties often depend on... 

For materials with a zinc blende cubic lattice, the solution - liquid - solid (SLS) mechanism [132] has been widely used to synthesize wirelike crystallites. The SLS method is analogous to the vapor-liquid-solid (VLS) method [133], which generates single-crystal wires in relatively large quantities. Very recently, quantum rods and wires of InAs [134] and InP [135] with controllable diameters and excellent crystallinity were synthesized by the SLS method. However, the fabrication of III-V semiconductor quantum rods (QRs) remains difficult regarding the control of size, length, and size uniformity. Another difficulty with QRs synthesized by the SLS method is residual metallic catalyst spherules (Au, In) present at the rod tip. The presence of these metal catalyst particles interferes with measurements of the optical and electronic properties of the QRs. 

As the radius of a semiconductor crystallite approaches the exciton-Bohr-radius its electronic properties begin to change, whereupon quantum size effects can be expected. The Bohr radius ub of an exciton is given by... 

The interest in semiconductor QD s as NLO materials has resulted from the recent theoretical predictions of strong optical nonlinearities for materials having three dimensional quantum confinement (QC) of electrons (e) and holes (h) (2,29,20). QC whether in one, two or three dimensions increases the stability of the exciton compared to the bulk semiconductor and as a result, the exciton resonances remain well resolved at room temperature. The physics framework in which the optical nonlinearities of QD s are couched involves the third order term of the electrical susceptibility (called X )) for semiconductor nanocrystallites (these particles will be referred to as nanocrystallites because of the perfect uniformity in size and shape that distinguishes them from other clusters where these characteriestics may vary, but these crystallites are definitely of molecular size and character and a cluster description is the most appropriate) exhibiting QC in all three dimensions. (Second order nonlinearites are not considered here since they are generally small in the systems under consideration.)... 

An interesting development in this field is the recent report by Dameron et al. (88) of the biosynthesis of quantum-sized CdS crystals in the yeast cells Candida glabrata and Schizo saccharomyces pombe. Exposed to Cd ions these cells synthesize certain peptides with an enhanced sulfide production. Small CdS crystals are formed inside the cells. These crystallize in the rock salt structure (and not in the thermodynamically stable hexagonal configuration). The organism controls particle nucleation and growth, so that uniformly sized CdS particles of about 20 A are formed. They show pronounced quantum-size effects. This is the first example of the biosynthesis of quantum-sized semiconductor crystallites. It constitutes a metabolic route for the detoxification of Cd " -infected living cells (see also 89). 

Schmidt H. M. and Weller H. (1986), Photochemistry of colloidal semiconductors. 15. Quantum size effects in semiconductor crystallites—calculation of the energy spectrum for the confined exciton , Chem. Phys. Lett. 129, 615-618. 

Louis E. Brus (NAS) is a professor of chemistry at Columbia University. He has been a pioneer in the synthesis, size control, and spectroscopy of nanometer-scale semiconductor crystallites. His elucidation of quantum-size effects in these materials is central to our understanding of the transition between molecular and bulk behavior. He received a B.S. in chemical physics from Rice University and his Ph.D. in chemical physics from Columbia University. 

Photoluminescence could be due to the radiative annihilation (or recombination) of excitons to produce a free exciton peak or due to recombination of an exciton bound to a donor or acceptor impurity (neutral or charged) in the semiconductor. The free exciton spectrum generally represents the product of the polariton distribution function and the transmission coefficient of polaritons at the sample surface. Bound exciton emission involves interaction between bound charges and phonons, leading to the appearance of phonon side bands. The above-mentioned electronic properties exhibit quantum size effect in the nanometric size regime when the crystallite size becomes comparable to the Bohr radius, qb- The basic physics of this effect is contained in the equation for confinement energy,... 

Quantum confinement is defined as the space where the motions of electrons and holes in a semiconductor are restricted in one or more dimensions. This quantum confinement occurs when the size of semiconductor crystallites is smaller than the bulk exciton Bohr radius. Quantum wells, quantum wires, and quantum dots are confined in one, two, and three dimensions, respectively [1, 2]. The confinement can be created due to electrostatic potentials, the presence of an interface between different semiconductor materials, and the presence of a semiconductor surface. A valence band and a conduction band are separated by an energy range known as the band gap ( g). These amounts of energy will be absorbed in order to promote an electron from the valence band to the conduction band and emitted when the electron relaxes directly fi om the conduction band back to the valence band. By changing the size of the semiconductor nanoparticles, the energy width of the band gap can be altered and the optical and electrical responses of these particles are changed (Fig. 1). 

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