Confined nanoclusters

Results similar to those discussed for dendrimer-encapsulated Cu and Pt are also obtained for Pd, Ru, and Ni nanoclusters. An example of 40-atom Pd nanoclusters confined within G4-OH is shown in the bottom micrograph of Fig. 14. 

Some nanoclusters, particularly porous nanoclusters, confine reactants into close proximity and accelerate reaction rates via mass action. 

Guo Z, Xiao C, Maligal-Ganesh RV, Zhou L, Goh TW, Li X, et al. Pt nanoclusters confined within metal-organic framework cavities fa chemoselective cinnamaldehyde hydrogenation. ACS Catal 2014 4 1340-8. 

To the best of our knowledge, such a control of nanocluster size through the nanomorphology of the stabilizer cannot be achieved with any other system, not even with those in which the metal nanocluster precursors are physically confined within nanometer-sized cavities [23]. 

Well-defined nanoclusters (w 10-100 A diameter) of several metals have been prepared via the polymerization of metal-containing monomers. The synthetic approach involves the block copolymerization of a metallated norbornene with a hydrocarbon co-monomer which is used to form an inert matrix. Subsequent decomposition of the confined metal complex affords small clusters of metal atoms. For example, palladium and platinum nanoclusters may be generated from the block copolymerization of methyl tetracyclododecane (223) with monomers (224) and (225) respectively. 10,611 Clusters of PbS have also been prepared by treating the block copolymer of (223) and (226) with H2S.612 A similar approach was adopted to synthesize embedded clusters of Zn and ZnS 613,614... 

Polarization, Surface Enhancement and Quantum Confinement in Nanocluster Magnetism. 

H. Noglik and W. J. Pietro, Chem. Mater., 7, 1333 (1995). Surface Functionalization of Cadmium Sulfide Quantum Confined Semiconductor Nanoclusters. 2. Formation of a Quantum Dot Condensation Polymer. 

Surface enhanced Raman spectroscopy (SERS) experiments on silver and gold nanoclusters have demonstrated large enhancement levels and field confinement of 5 nm or less for various samples such as single-walled carbon nanotubes.1 However, the locations of these conditions cannot be controlled but are instead determined by the specific nanostructures used. That is, the target molecules have... 

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. 

It has also been demonstrated that mesoporous materials are viable candidates for optical devices [90]. Silicon nanoclusters were formed inside optically transparent, free-standing, oriented mesoporous silica film by chemical vapor deposition (CVD) of disilane within the spatial confines of the channels. The resulting silicon-silica nanocomposite displayed bright visible photoluminescence and nanosecond lifetimes (Fig. 2.12). The presence of partially polymerized silica channel walls and the retention of the surfactant template within the channels afforded very mild 100-140°C CVD conditions for the formation of... 

For more information/precedents on quantum confinement effects for metalUc nanoclusters, see ... 

During the past decade, a new focus has developed. It was found that semiconductor particles can be made so small, typically into the nanometer size regime, that a quantum confinement effect occurs [6-15]. Particles of this size are often referred to as nanoclusters, nanoparticles, quantum dots, or Q-particles. The structures of these nanometer-sized semiconductor clusters are usually similar to those of the bulk crystals, yet their properties are remarkably different. With the proper surface-capping agents, clusters of varying sizes can be isolated as powders and redissolved into various organic solvents just like molecules. The availability of this new class of materials allows us to study the transition of a material from molecule to bulk solid, as well as its various properties and applications. 

In a bulk semiconductor, photoexcitation generates electron-hole pairs which are weakly bounded by Coulomb interaction (called an exciton). Usually one can observe the absorption band of an exciton only at low temperature since the thermal energy at room temperature is large enough to break up the exciton. When the exciton is confined in an energy potential, the dissociation probability reduces and the overlap of the electron and hole wavefunction increases, which is manifested by a sharper absorption band observable at room temperature. This potential can be due to either a deformation in the lattice caused by an impurity atom or, in the present case, the surface boundary of a nanocluster. The confinement of an exciton by an impurity potential (called bound exciton) is well known in the semiconductor literature [16]. There is considerable similarity in the basic physics between confinement by an impurity potential and confinement by physical dimension. The confinement effects on the absorption cross section of a nanocluster are discussed in Section II. 

Surface plays an important role in excited state relaxation processes. In the ideal case of a three-dimensionally confined exciton, one expects to see strong exciton luminescence due to enhanced overlap of the electron and hole wavefunction. The radiative rate of the exciton should increase with increasing cluster size. In reality, this is generally not observed. Most of the luminescence spectra of semiconductor nanoclusters consist of a stokes-shifted broad luminescence band, usually attributed to emission from surface defects. Sometimes near the band edge, an exciton-like luminescence band can be observed. Various passivation procedures have been used to enhance the exciton luminescence. These are discussed in Section III. 

One subject that attracted much attention is the nonlinear optical properties of these semiconductor nanoclusters [17], The primary objective is to find materials with exceptional nonlinear optical response for possible applications such as optical switching and frequency conversion elements. When semiconductors such as GaAs are confined in two dimensions as ultrathin films (commonly referred to as multiple quantum well structures), their optical nonlinearities are enhanced and novel prototype devices can be built [18], The enhancement is attributed mostly to the presence of a sharp exciton absorption band at room temperature due to the quantum confinement effect. Naturally, this raises the expectation on three-dimensionally confined semiconductor nanoclusters. The nonlinearity of interest here is the resonant nonlinearity, which means that light is absorbed by the sample and the magnitude of the nonlinearity is determined by the excited state... 

Because of the confinement effect on the translational center-of-mass motion of the exciton, the radiative rate of an exciton in a semiconductor nanocluster shows interesting size and temperature dependence it increases with increasing cluster size and decreasing temperature. 

For bulk semiconductors and multiple quantum well structures (one-dimensional confined semiconductors), transport properties are of great interest and are important for practical applications such as transistors and detectors. Semiconductor nanoclusters are usually confined in three-dimensions by insulating matrices such as polymers and glasses, where the transport of carriers is not feasible. To explore transport-related applications of semiconductor clusters, a matrix that is capable of transporting carriers is needed, In addition, the redox properties of the matrix have to allow injection of carriers from semiconductor nanoclusters to the matrix. 

Figure 12.29 shows the I-V characteristics of thin specimen at room temperature, and 253 K were observed by high current source measure unit and plotted by using Metrics Interactive Characterization Software. The overall nature of I-V characteristics is an apparent indicator of the formation of localized energy levels in the host background. The formation of localized energy level may be attributed due to 3-D confinement of quantum dot-like ionic nanoclusters within the dielectric substrate. 

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