Quantized semiconductors

Torimoto T, Naohiro T, Nakamura H, Kuwabata S, Sakata T, Mori H, Yoneyama H (2000) Photoelectrochemical properties of size-quantized semiconductor photoelectrodes prepared by two-dimensional cross-linking of monodisperse CdS nanoparticles Electrochim Acta 45 3269-3276... 

Measurement of non-linear optical properties [580] also provides a means for characterizing size-quantized semiconductor particles. Third-order optical non-linearity of size-quantized semiconductor particles has been discussed in terms of resonant and non-resonant contributions [11]. Resonant non-linearity is expected to increase with decreasing particle size and increasing absorption coefficients. 

Highly monodispersed, size-quantized semiconductor particles have been prepared by size fractionation of the colloidal samples by gel electrophoresis [591 ]. The method is illustrated by quoting the published recipe for CdS particle formation [591] ... 

Related to the quantum well structures, "quantum dot" materials, or size-quantized semiconductor particles, have also been recognized to have nonresonant properties that are attractive. A series of small (< 30 A) capped (thiophenolate) CdS clusters has recently been shown to... 

By decreasing the particle size, it is possible to shift the conduction band to more negative potentials and the valence band to more positive potentials as a result of quantization effects [106]. Because of the increase in the effective bandgap, the band edges of the quantized semiconductor particle attain new positions relative to the band edges of the bulk material. Hence, redox processes that cannot occur in bulk materials can be energetically favored in quantized small particles as the conduction and the valence bands become stronger reductant and oxidants, respectively. 

Weller H. (1993), Quantized semiconductor particles a novel state of matter for materials science , Adv. Mat. 5, 88-95. 

Photo-induced electron transfer reactions from quantum well electrodes into a redox system in solution represent an intriguing research area of photoelectrochemistry. Several aspects of quantized semiconductor electrodes are of interest, including the question of hot carrier transfer from quantum well electrodes into solution. The most interesting question here is whether an electron transfer from higher quantized levels to the oxidized species of the redox system can occur, as illustrated in Fig. 9.31. In order to accomplish such a hot electron transfer, the rate of electron transfer must be competitive with the rate of electron relaxation. It has been shown that quantization can slow down the carrier cooling dynamics and make hot carrier transfer competitive with carrier cooling. 

Soon after the discovery that fluorescence occurs in size-quantized semiconductor particles, the question of the underlying mechanism has been addressed. Without the claim of completeness some articles dealing with this topic are listed in Refs. 28 to 35. On the basis of this work, which, in part, has already been reviewed, three studies appeared in the early 1990s whose results are outlined here in some detail. 

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. 

Another example of photoetching application is the preparation of monodisperse CdS nanoparticles by the use of size-selective photoetching technique, which has been recently reported by Yoneyama and his coworkers [45-49]. Meijerink and his coworkers applied this technique to other compound semiconductor particles, such as ZnS, PbS, and ZnO [50, 51]. It is based on the fact that as the band gap of size-quantized semiconductor nanoparticles increases with a decrease in their size, larger CdS nanoparticles can be selectively photoexcited and photocorroded under irradiation by light. 

Yoneyama H Photoreduction of carbon dioxide on quantized semiconductor nanoparticles in solution. Catalysis Today 1997, 39(3) 169-175. 

Metal sulfides have been incorporated into zeolites X, Y, and A. [222, 223] It was concluded that superclusters made up of quantized semiconductors can be formed by comjdetely filling the internal zeolite cavities to the percolation threshold. The supercluster is created by the three dimensional interconnection of... 

Figure 9.6 (a) Schematic plot of the energy E vs. wave vector k for the electron confined in a size-quantized semiconductor crystal. The dark dots over the parabola indicate the discrete and allowed energy values for the transition, (b) The band structure in the form of discrete energy states that suggest molecular-Uke states for the size-quantized particles. indicates the band gap for the bulk semiconductor, whereas g(r) indicates energy separation between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) in the case of size-quantized particles, where E r) > E. ... 

Nanocrystalline particulate films, which exhibit pronounced quantum size effects in three dimensions, are of great interest due to applications in solar cell (108-112) and sensor (57, 113-115) applications. They exhibit novel properties due to not only the SQE manifested by individual nanoparticles but also the total surface area. Unlike MBE and MOCVD methods used to prepare quantum well electrodes, these electrodes can be prepared by conventional chemical routes described in Section 9.5.2.2. For example, II-VI semiconductor particulate films were prepared by using low concentrations of precursors and by controlling the temperature of the deposition bath. Nodes demonstrated the SQE for CdSe thin films deposited by an electroless method (98). The blue shift in the spectra of CdSe films has been demonstrated to be a function of bath temperature. As described in Section 9.5.2.1, electrodeposition of semiconductors in non-aqueous solvents leads to the formation of size-quantized semiconductor particles. On a single-crystal substrate, electrodeposition methods result in epitaxial growth (116, 117), and danonstrate quantum well properties. 

The fabrication of size-quantized semiconductor and metal nanoparticles has attracted a lot of attention (21-23) because of their novel optical and electrical properties and their appealing features as models for basic science. The LB deposition process originally developed for amphiphiles can also be extended... 

Photo-induced electron transfer reactions from quantum well electrodes into a redox system in solution represent an intriguing research area of photoelectrochemistry. Several aspects of quantized semiconductor electrodes are of interest, including the question of hot carrier transfer from quantum well electrodes into... 

As mentioned above, in quantized semiconductors, it was predicted [73, 74], and experimentally verified by analyzing time-resolved photoluminescence (TRPL) spectra [2, 75-78], that the electron cooHng rate can be substantially reduced in quantum wells. For instance, in the case of MQWs, consisting of... 

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