Quantum dots and wires

Kolesnikova, A.L. Romanov, A.E. (2004). Misfit dislocation loops and critical parameters of quantum dots and wires. Philosophical Magazine Letters, Vol. 84, No. 3, pp. 501-506, ISSN 0950-0839. 

J.B. Li, L.W. Wang, Band-structure-corrected local density approximation study of semiconductor quantum dots and wires. Phys. Rev. B 72(12), 125325 (2005)... 

Figure 17.1. (a) Quantum wells, (b) quantum wires, (c) ordered arrays of quantum boxes, (d) random quantum dots, and (e) an aggregate of nanometer-size grains. 

Specifically, this volume focuses on the synthesis, processing, and structural tailoring of nanocrystalline and nanoporous materials. Nanocrystalline materials possess unique hybrid properties characteristic of neither the molecular nor the bulk solid-state limits and may be confined in nanometersized domains in one, two, or three dimensions for unusual size-dependent behavior. Nanoporous materials, characterized by well-defined pores or cavities in the nanometer size regime and controlled pore diameter and structure, give rise to unique molecular sieving capabilities and ultrahigh internal surface areas. Nanoporous structures also act as hosts and templates for the fabrication of quantum dots and quantum wires. 

Nanometer sized semiconductor dusters, expected to have properties different from those of molecular and bulk semiconductors of the same composition, represent a new class of materials. Interest in their preparation and potential applications as photocatalysts and device components used in quantum electronics and nonlinear optics is growing rapidly. Isolated, so-called nanophase semiconductors can be considered as zero and one dimensional quantum dots and quantum wires. Their electronic, optical, and photochemical properties change with cluster size. Wider electronic band gaps and new absorption maxima in the electronic spectra have been observed as the size of these materials decreases and have been interpreted as quantum size effects. For example, as the dimensions of the semiconductor particle are reduced, a shift to higher energy in the absorption spectrum relative to that of the bulk is generally observed. 

R.J. Tonucci, B.L. Justus, A.J. Campillo, in Nanometer Array Glass Technology for Quantum, Dot and Quantum Wire Applications. Technical Digest, International Quantum Electronics Conference, Wieen, 1992, p. 60... 

Band gap engineetring confined hetetrostruciutres. When the thickness of a crystalline film is comparable with the de Broglie wavelength, the conduction and valence bands will break into subbands and as the thickness increases, the Fermi energy of the electrons oscillates. This leads to the so-called quantum size effects, which had been precociously predicted in Russia by Lifshitz and Kosevich (1953). A piece of semiconductor which is very small in one, two or three dimensions - a confined structure - is called a quantum well, quantum wire or quantum dot, respectively, and much fundamental physics research has been devoted to these in the last two decades. However, the world of MSE only became involved when several quantum wells were combined into what is now termed a heterostructure. 

The birth of the field of carbon nanotubes is marked by the publication by lijima of the observation of multi-walled nanotubes with outer diameters as small as 55 A, and inner diameters as small as 23 A, and a nanotube consisting of only two coaxial cylinders [2]. This paper was important in making the connection between carbon fullerenes, which are quantum dots, with carbon nanotubes, which are quantum wires. FurtheiTnore this seminal paper [2] has stimulated extensive theoretical and experimental research for the past five years and has led to the creation of a rapidly developing research field. 

In conclusion, wc have shown the interesting information which one can get from electrical resistivity measurements on SWCNT and MWCNT and the exciting applications which can be derived. MWCNTs behave as an ultimate carbon fibre revealing specific 2D quantum transport features at low temperatures weak localisation and universal conductance fluctuations. SWCNTs behave as pure quantum wires which, if limited in length, reduce to quantum dots. Thus, each type of CNT has its own features which are strongly dependent on the dimensionality of the electronic gas. We have also briefly discussed the very recent experimental results obtained on the thermopower of SWCNT bundles and the effect of intercalation on the electrical resistivity of these systems. 

Chemical and electrochemical techniques have been applied for the dimensionally controlled fabrication of a wide variety of materials, such as metals, semiconductors, and conductive polymers, within glass, oxide, and polymer matrices (e.g., [135-137]). Topologically complex structures like zeolites have been used also as 3D matrices [138, 139]. Quantum dots/wires of metals and semiconductors can be grown electrochemically in matrices bound on an electrode surface or being modified electrodes themselves. In these processes, the chemical stability of the template in the working environment, its electronic properties, the uniformity and minimal diameter of the pores, and the pore density are critical factors. Typical templates used in electrochemical synthesis are as follows ... 

Nano-structures comments on an example of extreme microstructure In a chapter entitled Materials in Extreme States , Cahn (2001) dedicated several comments to the extreme microstructures and summed up principles and technology of nano-structured materials. Historical remarks were cited starting from the early recognition that working at the nano-scale is truly different from traditional material science. The chemical behaviour and electronic structure change when dimensions are comparable to the length scale of electronic wave functions. Quantum effects do become important at this scale, as predicted by Lifshitz and Kosevich (1953). As for their nomenclature, notice that a piece of semiconductor which is very small in one, two- or three-dimensions, that is a confined structure, is called a quantum well, a quantum wire or a quantum dot, respectively. 

E. Hanamura, Optical Responses of Quantum Wires/Dots and Microcavities... 

Quantum dots are nanometre scale in three dimensions, but structures that are only nanometre scale in two dimensions (quantum wires) or one dimension (quantum wells or films) also display interesting properties. The quantised nature of the bands in nanostructures can lae seen in the density of states. Schematic, theoretical density of states diagrams for bulk material, quantum wells, quantum wires, and quantum dots are pictured in Figure 11.3. 

---