There is, I think, something ethereal about i —the square root of minus one. I remember first hearing about it at school. It seemed an odd beast at that time—an intruder hovering on the edge of reality.

Usually familiarity dulls this sense of the bizarre, but in the case of i it was the reverse: over the years the sense of its surreal nature intensified. It seemed that it was impossible to write mathematics that described the real world in …

I and i

This article provides a comprehensive review of current research activities that concentrate on one-dimensional (1D) nanostructures—wires, rods, belts, and tubes—whose lateral dimensions fall anywhere in the range of 1 to 100 nm. We devote the most attention to 1D nanostructures that have been synthesized in relatively copious quantities using chemical methods. We begin this article with an overview of synthetic strategies that have been exploited to achieve 1D growth. We then elaborate on these approaches in the following four sections: i) anisotropic growth dictated by the crystallographic structure of a solid material; ii) anisotropic growth confined and directed by various templates; iii) anisotropic growth kinetically controlled by supersaturation or through the use of an appropriate capping reagent; and iv) new concepts not yet fully demonstrated, but with long-term potential in generating 1D nanostructures. Following is a discussion of techniques for generating various types of important heterostructured nanowires. By the end of this article, we highlight a range of unique properties (e.g., thermal, mechanical, electronic, optoelectronic, optical, nonlinear optical, and field emission) associated with different types of 1D nanostructures. We also briefly discuss a number of methods potentially useful for assembling 1D nanostructures into functional devices based on crossbar junctions, and complex architectures such as 2D and 3D periodic lattices. We conclude this review with personal perspectives on the directions towards which future research on this new class of nanostructured materials might be directed.

One‐Dimensional Nanostructures: Synthesis, Characterization, and Applications

The invention concerns a quantum cascade laser comprising in particular a gain region (14) consisting of several layers (20) each including: alternating strata of a first type (28) defining each an AllnAs quantum barrier and strata of a second type (28) defining each an InGaAs quantum barrier, and injection barriers (22), interposed between two of the layers (20). The layers of the gain region (14) form each an active zone extending from one to the other of the injection barriers (22) adjacent thereto. The strata (26, 28) are dimensioned such that: each of the wells comprises, in the presence of an electric field, at least a first upper subband, a second median subband, and a third lower subband, and the probability of an electron being present in the first subband is highest in the proximity of one of the adjacent injection barriers, in the second subband in the median part of the zone and in the third subband in the proximity of the other adjacent barriers. The laser is formed by a succession of active zones and injection barriers, without interposition of a relaxation zone.

https://science.sciencemag.org/content/sci/264/5158/553.full.pdf

Quantum cascade laser

基礎講座 電気泳動(Electrophoresis)

An obstacle to the use of graphene as an alternative to silicon electronics has been the absence of an energy gap between its conduction and valence bands, which makes it difficult to achieve low power dissipation in the OFF state We report a bipolar field-effect transistor that exploits the low density of states in graphene and its one-atomic-layer thickness Our prototype devices are graphene heterostructures with atomically thin boron nitride or molybdenum disulfide acting as a vertical transport barrier They exhibit room-temperature switching ratios of ≈50 and ≈10,000, respectively Such devices have potential for high-frequency operation and large-scale integration

http://repositorium.sdum.uminho.pt/bitstream/1822/21512/1/1112.4999_Science.pdf

Field-effect tunneling transistor based on vertical graphene heterostructures.

Two‐dimensional electron gas (2DEG) in a quantum well or inversion layer, unlike an ordinary grounded metallic plane, does not completely screen an applied transverse electric field. Owing to its Fermi degeneracy energy, a 2DEG manifests itself as a capacitor in series, whose capacitance per unit area equals CQ=me2/πℏ2, where m is the effective electron mass in the direction transverse to the quantum well. Partial penetration of an external field through a highly conducting 2DEG allows the implementation of several novel high‐speed devices, including a three‐terminal resonant‐tunneling transistor and a gate‐controlled thermionic emission transistor.

Quantum capacitance devices

The speed of operation of negative differential resistance (NDR) devices based on resonant tunneling in a double‐barrier quantum‐well structure is considered. It is shown that the intrinsic RC delay of a single barrier limits the frequency of active oscillations to fmax =1/(2πτ), where τ=eα−1(λ/c)exp(4πd/λ) with λ being the de Broglie wavelength of the tunneling electron, d the barrier thickness, e the dielectric permittivity, c the speed of light, and α≊1/137 the fine‐structure constant. The relevance of this estimate to recent experimental results is discussed. An alternative mechanism for the NDR is proposed—not involving resonant tunneling. It should be observable in various single‐barrier structures in which tunneling occurs into a two‐dimensional system of states. In a double‐barrier structure, specially designed experiments are required to distinguish this effect from resonant tunneling.

Frequency limit of double‐barrier resonant‐tunneling oscillators

We have reconsidered the problem of the critical layer thickness hc for growth of strained heterolayers on lattice‐mismatched substrates, using a new approach which allows us to determine the spatial distribution of stresses in a bi‐material assembly and include the effects of a finite size of the sample. The possibility of dislocation‐free growth of lattice‐mismatched materials on porous silicon substrates is discussed as an example of a more general problem of heteroepitaxial growth on small seed pads of lateral dimension l, having a uniform crystal orientation over the entire substrate wafer. It turns out that for a given mismatch f, the critical film thickness hlc strongly depends on l, rising sharply when the latter is sufficiently small, l≲lmin. The characteristic size lmin( f ) below which, effectively, hlc( f )→∞, is determined in terms of the experimentally known (or calculated for growth on a monolithic substrate) function h∞c( f )≡hc( f ). When l≲lmin, then the entire elastic stress in the epit...

New approach to the high quality epitaxial growth of lattice‐mismatched materials

The transport properties of solids, as well as the many optical phenomena in them are determined by the scattering of current carriers. This book elucidates the state of the art in the research on the scattering mechanisms for current carriers in metals and semiconductors, and describes experiments in which these mechanisms are most dramatically manifested. With the knowledge of the probability of elementary scattering events and the application of the idea of a test particle the following subjects are dealt with: electronic transport theory based on the test-particle and correlation-function concepts; scattering by phonons, impurities, surfaces, magnons and dislocations; electron-electron scattering and the electron temperature; two-phonon scattering, spin-flip scattering, scattering in degenerate and many band models. 291 refs.; 104 figs.; 11 tabs.

Carrier Scattering in Metals and Semiconductors

Properties of GexSi1−x strained‐layer p‐i‐n detectors, in which the strained‐layer superlattice itself was used as an absorption region, have been studied for the first time. These devices were grown on (100)Si by molecular beam epitaxy. Using waveguide geometry we have obtained internal quantum efficiencies on the order of 40% at 1.3 μm in superlattices with the Ge fraction x=0.6. The superlattice detectors show the frequency response bandwidth of over 1 GHz and uniformly excellent electrical characteristics for values of x as large as 0.8.

Serge Luryi

Papers

Quantum capacitance devices

Frequency limit of double‐barrier resonant‐tunneling oscillators

New approach to the high quality epitaxial growth of lattice‐mismatched materials

Carrier Scattering in Metals and Semiconductors

GexSi1−x strained‐layer superlattice waveguide photodetectors operating near 1.3 μm