Fabrication Of Low-dimensional Nanostructures Research Articles

Recent Progress in the Fabrication of Low Dimensional Nanostructures via Surface-Assisted Transforming and Coupling

Polymerization of functional organics into covalently cross-linked nanostructures via bottom-up approach on solid surfaces has attracted tremendous interest recently, due to its appealing potentials in fabricating novel and artificial low dimensional nanomaterials. While there are various synthetic approaches being proposed and explored, this paper reviews the recent progress of on-surface coupling strategies towards the synthesis of low dimensional nanostructures ranging from 1D nanowire to 2D network and describes their advantages and drawbacks during on-surface process and phase transformations, for example, from molecular self-assembly to on-surface polymerization. Specifically, Ullmann reaction is discussed in detail and the mechanism governing nanostructures’ transforming effect by surface treatment is exploited. In the end, it is summarized that the hierarchical polymerization combined with Ullmann coupling makes it possible to realize the selection of different synthetic pathways and phase transformations and obtain novel organometallic nanowire with metalorganic bonding.

A simple solution for an intense terahertz emitter

Terahertz (THz, in the far IR region) imaging and spectroscopy offers the ability to discover chemical composition noninvasively. Applications include medical imaging, security screening, and pharmaceutical quality control. Semiconductors can produce pulsed THz radiation when their surface is illuminated by a sub-picosecond laser pulse, rather than continuous wave sources, such as a quantum cascade laser. Yet despite several technical advances, the output power is low. Thus the development of higher power semiconductor THz emitters is of considerable practical significance.1 THz generation occurs when a semiconductor is illuminated by an ultrafast laser pulse with photon energy greater than the semiconductor bandgap. A large number of electron and hole pairs are created and accelerated in opposite directions by the electric field. The resulting charge separation forms a dipole that emits a coherent THz pulse. THz radiation generated within a material of high refractive index n has an extraction problem: at the semiconductor surface, all rays outside an ‘emission cone’ of half-angle α = sin−1 1 n to the surface normal (shown as a grey cone in Figure 1) suffer total internal reflection and do not escape from the device. The fraction of power extracted depends on the orientation of the dipole axis relative to the surface. Several methods exist to increase the extraction efficiency, such as applying an external magnetic field,2 using a coupling prism,3 or fabrication of low-dimensional nanostructures.4 For example, under a magnetic field the Lorentz force—the force on a point charge due to electromagnetic fields—can rotate the orientation of the dipole to coincide with the emission cone as shown in Figure 1(a). For the extreme case of a dipole parallel to the surface, the emitted power would increase by more than a factor of 20. But these methods require cumbersome equipment, such as a strong magnet, or special sample growth techniques. We simply changed the growth orientation of the Figure 1. (a) By applying an external magnetic field (B), the THz dipole can be rotated and the radiation pattern can be overlapped with the emission cone to enhance the free-space radiation. (b) The wurtzite (hexagonal crystal) structure of indium nitride (InN). The InN film is grown along the a-axis and the in-plane electric field is formed on the a-plane.

Recent advances in hydrothermal syntheses of low dimensional nanoarchitectures

Recently, considerable attention has been focused on low-dimensional nanomaterials owing to their unique physical properties and potential applications in sensors, magnetics, electric transportation, optics, and even as building blocks for nanoscale devices. The past couple of decades have witnessed the development on new mild soft solution based strategies for fabrication of low dimensional nanostructures. In particular, hydrothermal synthesis, as a traditional method but a good choice, has provided various general routes for the rational synthesis of a variety of low dimensional nanostructured materials such as nanorods, nanowires, nanosheets, nanotubes, and their assemblies with controllable size, shape, phase, length scale, and structural complexity. This review gives an overview of recent advances in hydrothermal syntheses of low dimensional nanoarchitectures. Recent progresses undoubtedly emphasise that a rich family of inorganic nanostructures can be accessible by this facile approach and their shape, size, and structures can be controlled. However, hydrothermal synthesis for rational design of nanomaterials is still at its infancy and far beyond its maturity even though this traditional approach has been studied for many years. The further understanding of the reaction kinetics and formation mechanics in this reaction system by taking advantage of various characterisation techniques will make this route become a mature and reliable technique in future for the rational design of diverse nanomaterials with expected well-defined shapes, sizes, phases, and compositions, and even with unique structural specialty and complexity. These new nanobuilding blocks of different functionalities will find potential applications in nanoscience and nanotechnology.

X-ray methods for strain and composition analysis in self-organized semiconductor nanostructures

The fabrication of low-dimensional nanostructures (e.g. quantum wires or quantum dots) is presently among the most exciting challenges in semiconductor technology. The electronic and optical properties of these systems depend decisively on structural parameters, such as size, shape, elastic strain, chemical composition and positional correlation among the nanostructures. X-ray scattering methods have proven to be an excellent tool to get access to these parameters. Beyond its sensitivity to deformations of the crystal lattice, it is sensitive to fluctuations of the surface and interface morphology on length scales ranging from 0.1 nm up to several μm. The small dimensions and the corresponding weak scattering signal require the use of highly brilliant synchrotron radiation. Recent methodological developments and their application to the material system Ge on Si are discussed. To cite this article: T.H. Metzger et al., C. R. Physique 6 (2005).