Nanotechnology offers the promise of enabling revolutionary advances in diverse areas ranging from electronics, optoelectronics, and energy to healthcare. Underpinning the realization of such advances are the nanoscale ma te rials and corresponding nanodevices central to these application areas. Semiconductor nanowires and nanobelts are emerging as one of the most powerful and diverse classes of functional nanoma terials that are having an impact on science and technology. In this issue of MRS Bulletin, several leaders in this vibrant field of research present brief reviews that highlight key aspects of the underlying materials science of nanowires, basic device functions achievable with these materials, and developing applications in electronics and at the interface with biology. This article introduces the controlled synthesis, patterned and designed self-assembly, and unique applications of nanowires in nanoelectronics, nano-optoelectronics, nanosensors, nanobiotechnology, and energy harvesting.

Zinc oxide is a unique material that exhibits semiconducting, piezoelectric, and pyroelectric multifunctionalities. By controlling the size and orientation of the polar surfaces of ZnO nanobelts, single - crystal nanocombs, nanorings, nanohelices, nanosprings, and nanobows of ZnO have been synthesized. This article centers on the fundamental growth mechanism and fabrication of electromechanical devices based on piezoelectric ZnO nanostructures, including a nanogenerator using aligned ZnO nanowires for converting nanoscale mechanical energy into electric energy. The mechanism of the electric nanogenerator relies on the unique coupling of the piezoelectric and semiconducting properties of ZnO, which is the fundamental principle of nano - piezotronics, a new field using the piezoelectric effect for fabricating electronic devices and components. The approach has the potential of converting biological mechanical energy, acoustic/ultrasonic vibration energy, and biofluid hydraulic energy into electricity, demonstrating a new pathway for self - powering of wireless nanodevices and nanosystems.

Semiconducting nanowires are emerging as a route to combine heavily mismatched materials. The high level of control on wire dimensions and chemical composition makes them promising materials to be integrated in future silicon technologies as well as to be the active element in optoelectronic devices.

This ar ticle reviews the recent progress in epitaxial growth of nanowires on non-corresponding substrates. We highlight the advantage of using small dimensions to facilitate accommodation of the lattice strain at the surface of the structures. More specifically, we will focus on the growth of III-V nanowires on Group IV substrates. This approach enables the integration of high-perform ance III-V semiconductors monolithically into mature silicon technology, since fundamental issues of III-V integration on Si such as lattice and thermal expansion mismatch can be overcome. Moreover, as there will only be one nucleation site per crystallite, the system will not suffer from antiphase boundaries.

Issues that affect the electronic properties of the heterojunction, such as the crystallographic quality and diffusion of elements across the heterointerface, will be discussed. Finally, we address potential applications of vertical III-V nanowires grown on silicon.

One-dimensional (1D) semiconducting oxide nanostructures such as ZnO, SnO2, and In2O3 have been extensively studied due to their excellent optical and electrical properties. Growth of 1D nanostructures with precisely controlled size, phase purity, crystallinity, and chemical composition still presents numerous challenges. In this review, we report the recent progress on the synthesis of binary oxide nanostructures consisting of different oxides through a simple and effective vapor transport approach in our research. By controlling the experimental conditions, this approach enables the synthesis of various multicomponent binary oxide nanowires.

We describe the production of hierarchical branched nanowire structures by the sequential seeding of multiple wire generations with metal nanoparticles. Such complex structures represent the next step in the study of functional nanowires, as they increase the potential functionality of nanostructures produced in a self-assembled way. It is possible, for example, to fabricate a variety of active heterostructure segments with different compositions and diameters within a single connected structure. The focus of this work is on epitaxial III-V semiconductor branched nanowire structures, with the two materials GaP and In As used as typical examples of branched structures with cubic (zinc blende) and hexagonal (wurtzite) crystal structures. The general morphology of these structures will be described, as well as the relationship between morphology and crystal structure.

A new concept of macroelectronics using assembled semiconductor nanowire thin films holds the promise of significant performance improvement. In this new concept, a thin film of oriented semiconductor nanowires is used to produce thin-film transistors (TFTs) with conducting channels formed by multiple parallel single-crystal nanowire paths. There fore, charges travel from source to drain within single crystals, ensuring high carrier mobility. Recent studies have shown that high-performance silicon nanowire TFTs and high-frequency circuits can be readily produced on a variety of substrates including glass and plastics using a solution assembly process. The device performance of these nanowire TFTs not only greatly surpasses that of solution-processed organic TFTs, but is also significantly better than that of conventional amorphous or polycrystalline silicon TFTs, approaching single-crystal silicon-based devices. Furthermore, with a similar frame-work, Group III-V or II-VI nanowire or nanoribbon materials of high intrinsic carrier mobility or optical functionality can be assembled into thin films on flexible substrates to enable new multifunctional electronics/optoelectronics that are not possible with traditional macroelectronics. This can have an impact on a broad range of existing applications, from flat-panel displays to image sensor arrays, and enable a new generation of flexible, wearable, or disposable electronics for computing, storage, and wireless communication.

The interface between nanosystems and biosystems is emerging as one of the broadest and most dynamic areas of science and technology, bringing together biology, chemistry, physics, biotechnology, medicine, and many areas of engineering. The combination of these diverse areas of research promises to yield revolutionary advances in healthcare, medicine, and the life sciences through the creation of new and powerful tools that enable direct, sensitive, and rapid analysis of biological and chemical species. Devices based on nanowires have emerged as one of the most powerful and general platforms for ultrasensitive, direct electrical detection of biological and chemical species and for building functional interfaces to biological systems, including neurons. Here, we discuss representative ex amples of nanowire nanosensors for ultrasensitive detection of proteins and individual virus particles as well as recording, stimulation, and inhibition of neuronal signals in nanowire-neuron hybrid structures.

The following article is based on the Outstanding Young Investigator Award presentation given by Ju Li on April 19, 2006, at the Materials Research Society Spring Meeting in San Francisco. Li received the award “for his innovative work on the atomistic and first-principles modeling of nanoindentation and ideal strength in revealing the genesis of materials deformation and fracture.”

Defect nucleation plays a critical role in the mechanical behavior of materials, especially if the system size is reduced to the submicron scale. At the most fundamental level, defect nucleation is controlled by bond breaking and reformation events, driven typically by mechanical strain and electronegativity differences. For these processes, atomistic and first-principles calculations are uniquely suited to provide an unprecedented level of mechanistic detail. Several connecting threads incorporating notions in continuum mechanics and explicit knowledge of the interatomic energy landscape can be identified, such as homogeneous versus heterogeneous nucleation, cleavage versus shear-faulting tendencies, chemomechanical coupling, and the fact that defects are singularities at the continuum level but regularized at the atomic scale. Examples are chosen from nano-indentation, crack-tip processes, and grain-boundary processes. In addition to the capacity of simulations to identify candidate mechanisms, the computed athermal strength, activation energy, and activation volume can be compared quantitatively with experiments to define the fundamental properties of defects in solids.