Superlattices absorption

When the silver nanocrystals are organized in a 2D superlattice, the plasmon peak is shifted toward an energy lower than that obtained in solution (Fig. 6). The covered support is washed with hexane, and the nanoparticles are dispersed again in the solvent. The absorption spectrum of the latter solution is similar to that used to cover the support (free particles in hexane). This clearly indicates that the shift in the absorption spectrum of nanosized silver particles is due to their self-organization on the support. The bandwidth of the plasmon peak (1.3 eV) obtained after deposition is larger than that in solution (0.9 eV). This can be attributed to a change in the dielectric constant of the composite medium. Similar behavior is observed for various nanocrystal sizes (from 3 to 8 nm). 

Oonishi, T., Sato, S., Yao, H. and Kimura, K. (2007) Three-dimensional gold nanopartide superlattices Structures and optical absorption characteristics. Journal of Applied Physics, 101, 114314. 

Fig. 9.3.5 (A) Scanning electron microscopy on concentrated solution of 4.5-nm silver particles ([(Ag) ] = 4 X I0 3 M). Large aggregates on silver multilayers are present. (B) Absorption spectra of free 4.5-nm silver nanoparticles dispersed in hexane before (solid line) leaving a drop on the support, after washing the support with hexane (dashed line), and deposition on the support forming a 3D superlattice (a). ([(Ag) ] = 4 X I0 3 M).

For the absorption spectrum, since the translational invariance of the initial lattice is lost, the wave vector is defined modulo a vector of the reciprocal superlattice thus, we must bring the absorption wave vector k (k = 0 and k = (2n/b)h) to a vector of the first zone of the superlattice ... 

Crystalline and amorphous silicons, which are currently investigated in the field of solid-state physics, are still considered as unrelated to polysilanes and related macromolecules, which are studied in the field of organosilicon chemistry. A new idea proposed in this chapter is that these materials are related and can be understood in terms of the dimensional hierarchy of silicon-backbone materials. The electronic structures of one-dimensional polymers (polysilanes) are discussed. The effects of side groups and conformations were calculated theoretically and are discussed in the light of such experimental data as UV absorption, photoluminescence, and UV photospectroscopy (UPS) measurements. Finally, future directions in the development of silicon-based polymers are indicated on the basis of some novel efforts to extend silicon-based polymers to high-dimensional polymers, one-dimensional superlattices, and metallic polymers with alternating double bonds. 

Superlattice and low-dimensional physics are some of the most interesting subjects in solid-state physics. A challenging problem in this field is the formation of quantum wire and quantum box structures by using ultra-high technology such as MBE, MOCVD (metallorganic chemical vapor deposition), and related frontier microprocessing. However, this problem has not yet been solved. Poly silane is probably a perfect quantum wire in itself The absorption spectrum of polysilane clearly shows the characteristics of a one-dimensional quantum wire. Even a quantum box or a one-dimensional superlattice can be formed by chemical polymerization, which may be the simplest way. 

An example of a one-dimensional superlattice structure is structure 1, which is an ordered copolymer. The skeleton is formed by silicon and germanium atoms. A unit cell is three times larger than that of a homopolymer. The band structure of this ordered copolymer changes to the zone-folded profile, which may result in a characteristic absorption spectrum. 

Nanocomposites in the form of superlattice structures have been fabricated with metallic, " semiconductor,and ceramic materials " " for semiconductor-based devices. " The material is abruptly modulated with respect to composition and/or structure. Semiconductor superlattice devices are usually multiple quantum structures, in which nanometer-scale layers of a lower band gap material such as GaAs are sandwiched between layers of a larger band gap material such as GaAlAs. " Quantum effects such as enhanced carrier mobility (two-dimensional electron gas) and bound states in the optical absorption spectrum, and nonlinear optical effects, such as intensity-dependent refractive indices, have been observed in nanomodulated semiconductor multiple quantum wells. " Examples of devices based on these structures include fast optical switches, high electron mobility transistors, and quantum well lasers. " Room-temperature electrochemical... 

Coimolly S, Fullam S, Korgel B, Fitzmaurice D (1998) Time-resolved small-angle X-ray scattering studies of nanocrystal superlattice self-assembly. J Am Chem Soc 120 2969-2970 deGroot F (2001) High-resolution X-ray emission and X-ray absorption spectroscopy. Chem Rev 101 1779-1808... 

This chapter concentrates on the design of efficient dipolar NLO chromophores and the different approaches for their incorporation in non-centrosymmetric materials, including guest-host polymer systems, chromophore-functionalized polymers (side-chain and main-chain), cross-linked chromophore-macromolecule matrices, dendrimers, and intrinsically acentric self-assembled chromophoric superlattices. The different architectures will be compared together with the requirements (e.g., large EO coefficient, low optical absorption, high stability, and processability) for their incorporation into practical EO devices. First, a brief introduction to nonlinear optics is presented. 

In this chapter we review the work done in our laboratory on the structure (Part II), optical absorption (Part III), photoluminescence (Part IV), and electrical transport (Part V) of a-Si H/a-SiNjci H superlattices. Results with single quantum well structures are discussed by Kukimoto in Chapter 12 of Volume 2ID. 

Optical absorption has been studied in superlattices made of a-Si H layers alternating with a-SiN iH, a-Si, tQ H, or a-Ge H (Abeles and Tiedje, 1983 Tiedje et al., 1984). Figure 4 shows the optical absorption coefficienta versus photon energy of a series of a-Si H/a-SiN H superlattices about 1 fiia thick, in which the thickness of the a-Si H layer Ls is varied and the a-SiN , H layer thickness 35 A is held fixed. The large blue shift in the optical absorption edge with decreasing has been attributed to an increase... 

Fig. 6. Optical absorption coefficient a for the a-Si H/a-SiN H superlattice for values of wbll width Lg = 400,20, and 8 A plotted as (otE) versus photon energy E. The experimental data aregivenby the full circles. The solid curves were calculated for the density-of-states model shown in Fig. Sb. The extrapolation of the gap E, is shown for the case of 1. — 20 A by the dashed line. (From Tiedje et al. (1984).]...

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