Compositional modulation superlattices

Figure 8.21 Schematic representations of normal and modulated crystal structures and diffraction patterns (a) a normal superlattice, formed by the repetition of an anion substitution (b) part of the diffraction pattern of (a) (c) a crystal showing a displacive modulation of the anion positions (d) a crystal showing a compositional modulation of the anion conditions, (the change in the average chemical nature of the anion is represented by differing circle diameters) (e) part of the diffraction pattern from (c) or (d) (f) a modulation wave at an angle to the unmodulated component (g) part of the diffraction pattern from (f). Metal atoms are represented by shaded circles and non-metal atoms by open circles...

Figure 2 shows the band structures of several homopolymers and pyrrole-bithiophene copolymers estimated by electrochemical and optical methods as examples. A combination of these homopolymers and/or copolymers implies various kinds of superlattice structures. The electrochemical preparation of both homopolymer multiheterolayers and/or copolymer multiheterolayers results in a superlattices. The electrochemical copolymerization method as used to prepare heterolayers was easier than in the homopolymer heterolayers. The copolymer multi heterolayers are prepared by simply changing the applied electrode potential. On the contrary, the latter needs exchange of the mother solutions. The present electrocopolymerization method which makes compositionally modulated copolymer heterolayers possible is considered to be one of the most fascinating methods to fabricate organic superlattices. 

Fig. 50. Summary of the X-ray scattering from a GcFY superlattice. Top three models for the magnetic-moment modulation. Bottom the calculated flipping ratio corresponding to no magnetic contribution from the two interfacial layers (dashed curve), a smooth decrease in the moment (solid curve), and a uniform reduction in moment (i.e., a moment modulation that comes from the composition modulation) (dotted curve), compared with the values measured at 8.04 keV Key the satellite peaks are labelled as Q l,m), so that the (002) Bragg peak in this notation is written as Q 2,0), and its first-order superlattice peaks Q 2,1), etc. (From Vettier et al. 1986.)...

We have recently demonstrated that it is possible to electrodeposit nanoscale ceramic superlattices based on the TlPbO system (5). The idea of electrochemically depositing nanomodulated superlattices is not new, but it has not been applied previously to the deposition of nonmetallic materials. Several research groups have shown that compositionally modulated metallic alloys can be electrochemically deposited from a single plating bath by cycling either the potential or current (6-9). The interest in nanomodulated metallic systems stems from their enhanced mechanical and magnetic properties (7,10),... 

A second application of current interest in which widely separated length scales come into play is fabrication of modulated foils or wires with layer thickness of a few nanometers or less [156]. In this application, the aspect ratio of layer thickness, which may be of nearly atomic dimensions, to workpiece size, is enormous, and the current distribution must be uniform on the entire range of scales between the two. Optimal conditions for these structures require control by local mechanisms to suppress instability and produce layer by layer growth. Epitaxially deposited single crystals with modulated composition on these scales can be described as superlattices. Moffat, in a report on Cu-Ni superlattices, briefly reviews the constraints operating on their fabrication by electrodeposition [157]. 

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... 

The rich structure of superlattice harmonics predicted by the rectangular model is mirrored by the superlattice peaks observed in the Dy/Lu sample of fig. 15. At the opposite extreme, ufien only a sinusoidal modulation of composition exists, a single... 

The best evidence for a superlattice is the observation of satellites around the Bragg reflection in x-ray diffraction (XRD)." The satellites are caused by the superperiodicity in the system, because the x-ray pattern is the Fourier transform of the product of the lattice and modulation functions convoluted with the basis. For composition waveforms that vary sinusoidally, only first order satellites are expected, because only one Fourier term is needed to describe a sine wave. As the waveform... 

Superlattices of metals, semiconductors, and ceramics - can be produced by single bath electrodeposition.il In addition to superlattices in which the composition is modulated, it is also possible to alternate layers of metal oxides with varying defect chemistry. The defect chemistry superlattices are produced by simply pulsing the applied potential during deposition. A cross-sectional SEM image of a compositional superlattice based on magnetite is shown in Figure 17.18. ... 

A superlattice is a multilayer structure with coherent stacking of atomic planes (7). An idealized superlattice structure with square-wave modulation of composition and/or structure is shown in Figure 1. The thicknesses of the layers are not necessarily equal, as long as the structure is periodic. When the modulation wavelength is in the nanometer range, each layer is only a few unit cells thick. Quantum confinement of carriers in rianomodulated materials often leads to technologically important optical and electrical properties which are intermediate between those of discreet molecules and extended network solids. 

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