The semiconductor ZnO has gained substantial interest in the research community in part because of its large exciton binding energy (60meV) which could lead to lasing action based on exciton recombination even above room temperature. Even though research focusing on ZnO goes back many decades, the renewed interest is fueled by availability of high-quality substrates and reports of p-type conduction and ferromagnetic behavior when doped with transitions metals, both of which remain controversial. It is this renewed interest in ZnO which forms the basis of this review. As mentioned already, ZnO is not new to the semiconductor field, with studies of its lattice parameter dating back to 1935 by Bunn [Proc. Phys. Soc. London 47, 836 (1935)], studies of its vibrational properties with Raman scattering in 1966 by Damen et al. [Phys. Rev. 142, 570 (1966)], detailed optical studies in 1954 by Mollwo [Z. Angew. Phys. 6, 257 (1954)], and its growth by chemical-vapor transport in 1970 by Galli and Coker [Appl. Phys. ...

/pdf/a-comprehensive-review-of-zno-materials-and-devices-21a3zjadem.pdf

A comprehensive review of zno materials and devices

Zinc oxide is a unique material that exhibits semiconducting and piezoelectric dual properties. Using a solid–vapour phase thermal sublimation technique, nanocombs, nanorings, nanohelixes/nanosprings, nanobelts, nanowires and nanocages of ZnO have been synthesized under specific growth conditions. These unique nanostructures unambiguously demonstrate that ZnO probably has the richest family of nanostructures among all materials, both in structures and in properties. The nanostructures could have novel applications in optoelectronics, sensors, transducers and biomedical sciences. This article reviews the various nanostructures of ZnO grown by the solid–vapour phase technique and their corresponding growth mechanisms. The application of ZnO nanobelts as nanosensors, nanocantilevers, field effect transistors and nanoresonators is demonstrated.

/pdf/zinc-oxide-nanostructures-growth-properties-and-applications-8d5hcz3lg8.pdf

Zinc oxide nanostructures: growth, properties and applications

In the past ten years we have witnessed a revival of, and subsequent rapid expansion in, the research on zinc oxide (ZnO) as a semiconductor. Being initially considered as a substrate for GaN and related alloys, the availability of high-quality large bulk single crystals, the strong luminescence demonstrated in optically pumped lasers and the prospects of gaining control over its electrical conductivity have led a large number of groups to turn their research for electronic and photonic devices to ZnO in its own right. The high electron mobility, high thermal conductivity, wide and direct band gap and large exciton binding energy make ZnO suitable for a wide range of devices, including transparent thin-film transistors, photodetectors, light-emitting diodes and laser diodes that operate in the blue and ultraviolet region of the spectrum. In spite of the recent rapid developments, controlling the electrical conductivity of ZnO has remained a major challenge. While a number of research groups have reported achieving p-type ZnO, there are still problems concerning the reproducibility of the results and the stability of the p-type conductivity. Even the cause of the commonly observed unintentional n-type conductivity in as-grown ZnO is still under debate. One approach to address these issues consists of growing high-quality single crystalline bulk and thin films in which the concentrations of impurities and intrinsic defects are controlled. In this review we discuss the status of ZnO as a semiconductor. We first discuss the growth of bulk and epitaxial films, growth conditions and their influence on the incorporation of native defects and impurities. We then present the theory of doping and native defects in ZnO based on density-functional calculations, discussing the stability and electronic structure of native point defects and impurities and their influence on the electrical conductivity and optical properties of ZnO. We pay special attention to the possible causes of the unintentional n-type conductivity, emphasize the role of impurities, critically review the current status of p-type doping and address possible routes to controlling the electrical conductivity in ZnO. Finally, we discuss band-gap engineering using MgZnO and CdZnO alloys.

/pdf/fundamentals-of-zinc-oxide-as-a-semiconductor-1s4l8bh009.pdf

Fundamentals of zinc oxide as a semiconductor

We have used the pH-induced self-assembly of a peptide-amphiphile to make a nanostructured fibrous scaffold reminiscent of extracellular matrix. The design of this peptide-amphiphile allows the nanofibers to be reversibly cross-linked to enhance or decrease their structural integrity. After cross-linking, the fibers are able to direct mineralization of hydroxyapatite to form a composite material in which the crystallographic c axes of hydroxyapatite are aligned with the long axes of the fibers. This alignment is the same as that observed between collagen fibrils and hydroxyapatite crystals in bone.

http://science.sciencemag.org/content/sci/294/5547/1684.full.pdf

Self-assembly and mineralization of peptide-amphiphile nanofibers

The assembly of semiconductor nanowires and carbon nanotubes into nanoscale devices and circuits could enable diverse applications in nanoelectronics and photonics1. Individual semiconducting nanowires have already been configured as field-effect transistors2, photodetectors3 and bio/chemical sensors4. More sophisticated light-emitting diodes5 (LEDs) and complementary and diode logic6,7,8 devices have been realized using both n- and p-type semiconducting nanowires or nanotubes. The n- and p-type materials have been incorporated in these latter devices either by crossing p- and n-type nanowires2,5,6,9 or by lithographically defining distinct p- and n-type regions in nanotubes8,10, although both strategies limit device complexity. In the planar semiconductor industry, intricate n- and p-type and more generally compositionally modulated (that is, superlattice) structures are used to enable versatile electronic and photonic functions. Here we demonstrate the synthesis of semiconductor nanowire superlattices from group III–V and group IV materials. (The superlattices are created within the nanowires by repeated modulation of the vapour-phase semiconductor reactants during growth of the wires.) Compositionally modulated superlattices consisting of 2 to 21 layers of GaAs and GaP have been prepared. Furthermore, n-Si/p-Si and n-InP/p-InP modulation doped nanowires have been synthesized. Single-nanowire photoluminescence, electrical transport and electroluminescence measurements show the unique photonic and electronic properties of these nanowire superlattices, and suggest potential applications ranging from nano-barcodes to polarized nanoscale LEDs.

Growth of nanowire superlattice structures for nanoscale photonics and electronics.

Room-temperature ultraviolet lasing in semiconductor nanowire arrays has been demonstrated The self-organized, oriented zinc oxide nanowires grown on sapphire substrates were synthesized with a simple vapor transport and condensation process These wide band-gap semiconductor nanowires form natural laser cavities with diameters varying from 20 to 150 nanometers and lengths up to 10 micrometers Under optical excitation, surface-emitting lasing action was observed at 385 nanometers, with an emission linewidth less than 03 nanometer The chemical flexibility and the one-dimensionality of the nanowires make them ideal miniaturized laser light sources These short-wavelength nanolasers could have myriad applications, including optical computing, information storage, and microanalysis

Room-temperature ultraviolet nanowire nanolasers

no attention has been given to the photoconducting properties of nanowires despite the exciting possibilities for use in optoelectronic circuits. Here, we show the possibility of creating highly sensitive nanowire switches by exploring the photoconducting properties of individual semiconductor nanowires. The conductivity of the ZnO nanowires is extremely sensitive to ultraviolet light exposure. The light-induced conductivity increase allows us to reversibly switch the nanowires between “OFF” and “ON” states, an optical gating phenomenon analogous to the commonly used electrical gating. [2,3,10]

/pdf/nanowire-ultraviolet-photodetectors-and-optical-switches-256c69tu0y.pdf

Nanowire ultraviolet photodetectors and optical switches

Room-temperature ultraviolet lasing in semiconductor nanowire arrays has been demonstrated. The self-organized, oriented zinc oxide nanowires grown on sapphire substrates were synthesized with a simple vapor transport and condensation process. These wide band-gap semiconductor nanowires form natural laser cavities with diameters varying from 20 to 150 nanometers and lengths up to 10 micrometers. Under optical excitation, surface-emitting lasing action was observed at 385 nanometers, with an emission linewidth less than 0.3 nanometer. The chemical flexibility and the one-dimensionality of the nanowires make them ideal miniaturized laser light sources. These short-wavelength nanolasers could have myriad applications, including optical computing, information storage, and microanalysis.

Room-Temperature Ultraviolet Nanowire Nanolasers.

One-dimensional nanostructures having uniform diameters of less than approximately 200 nm. These inventive nanostructures, which we refer to as “nanowires”, include single-crystalline homostructures as well as heterostructures of at least two single-crystalline materials having different chemical compositions. Because single-crystalline materials are used to form the heterostructure, the resultant heterostructure will be single-crystalline as well. The nanowire heterostructures are generally based on a semiconducting wire wherein the doping and composition are controlled in either the longitudinal or radial directions, or in both directions, to yield a wire that comprises different materials. Examples of resulting nanowire heterostructures include a longitudinal heterostructure nanowire (LOHN) and a coaxial heterostructure nanowire (COHN).

Nanowires, nanostructures and devices fabricated therefrom

Recent research in the field of nanometer-scale electronics has focused on two fundamental issues: the operating principles of small-scale devices1 and schemes that lead to their realization and eventual integration into useful circuits.2 The availability of a nanoscale toolbox is the key for this field of research. Among the many potential building blocks within this nanoscale toolbox, nanowires are considered one of the key components because they can be used as interconnects and other functional devices in nanoelectronics.3 Unfortunately, although several processes have been developed for the syntheses of semiconductor nanowires,4 few methods have been developed for preparing free-standing, uniform metal nanowires. Among them, template synthesis (in porous matrixes such as porous Al2O3 films5 and mesoporous silica6) and step-edge decoration7 are considered as effective approaches. The step-edge decoration method was recently employed to produce Mo nanowires of 100 nm thickness.7 Metal nanowires have also been prepared by using DNA8 and carbon nanotubes as templates.9 Herein we report a simple chemical process for synthesizing long, free-standing metal nanowires. LiMo3Se3 nanowires are used as both reducing agents and sacrificial templates in this study to yield continuous metal nanowires. The metal nanowires generally have diameters of 10100 nm and lengths of several micrometers. These metal nanowires display small ohmic resistances at room temperature, indicating that these wires could prove useful as interconnects in nanoelectronic circuits. Previously, DNA has been used as a biotemplate for making metallic nanowires,8 although the continuity of the resulting metal nanowires has been problematic. Since DNA itself does not possess any reducing/oxidizing capability, the synthesis generally is a two-step process, which involves metal activation followed by chemical reduction. Nevertheless, the construction of electronic circuits based only on native DNA remains problematic, mainly due to the high resistance of DNA that limits its potential applications in this regard. Recently, Braun and co-workers presented a new approach by fixing DNA between two contacts and utilizing it as a template for the construction of a silver nanowire.8 This technique uses the molecular recognition properties of the molecule for the defined buildup of a nanostructure and installs its electrical functionality by the directed construction of a metallic wire on the biotemplate. However, the reported 100 nm thick silver wires displayed a nonconducting gap for small bias voltages. In this study, we choose [Mo3Se3]∞ molecular chains as our experimental system for two important reasons. First, these molecular chains are obtained by dissolution of crystals of quasi1D materials LiMo3Se3 in polar solvent.10 Each chain is made of staggered stacks of triangular Mo3Se3 with a diameter of 0.5 nm. The molecular chain itself is a reducing agent and can be readily oxidized. Second, these individual molecular wires form uniform nanowire bundles with diameters of 2-100 nm in certain polar solvents such as methanol and DMSO. Hence, these nanowires possess dual functionalities, being reductive one-dimensional templates. In fact, the redox chemistry of LiMo3Se3 was previously studied by Tarascon, and it was demonstrated that insertion and extraction of Li/Mo6Se6 is reversible.11 To form uniform metal nanowires, a redox reaction is carried out using LiMo3Se3 nanowires as the reducing agents to reduce aqueous metal ions (e.g., AuCl4, Ag+, PdCl4, PtCl4). Metal ions are reduced and deposited directly on the nanowire templates, while the nanowire templates are oxidized into Mo3Se3 and eventually dissolve in water (Figure 1). Consequently, metal nanowires with diameters of 2-100 nm can be obtained through this cooperative chemical templating process. Metal nanowires of Au, Ag, Pt, and Pd can be readily synthesized. In a typical experiment, a LiMo3Se3 solution was prepared by dissolving 5 mg of LiMo3Se3 in 20 mL of DMSO. A 2 mL drop of metal ion solution (0.005 wt %) was added to this DMSO solution. Using the Au nanowire system as an example, evidence of the reaction is apparent immediately after the solution mixing. The overall solution color develops a pink tint, and a peak at 526 nm appears in the UV-vis absorption spectra, indicating that Au nanocluster nucleation and growth has occurred. Figure 2 shows transmission electron microscopy images taken before (a) and after (b-d) the templating reaction for the Au nanowire system. Before the reaction, the LiMo3Se3 molecular chains self-assemble into nanowire bundles of 10-100 nm in diameter. After the redox reaction, Au nanowires of similar diameters and morphology are obtained. These Au nanowires are continuous and polycrystalline. Figure 2d shows a high-resolution TEM image of an individual Au nanowire with diameter of 15 nm. It can be seen that the Au nanocrystalline domains fuse together at the interface and form a continuous and robust (1) Dekker: C. Phys. Today 1999, 52, 22. (2) (a) Hu, T.; Odom, W.; Lieber, C. M. Acc. Chem. Res. 1999, 32, 435. (b) Collier, C. P.; Wong, E. W.; Behloradsky, M.; Raymo, F. M.; Stoddart, J. F.; Kuekes, P. J.; Williams, R. S.; Heath, J. R. Science 1999, 285, 391. (c) Rueches, T.; Kim, K.; Joselevich, E.; Tseng, G. Y.; Cheung, C.; Lieber, C. M. Science 2000, 289, 94. (3) Huang, M. H.; Mao, S.; Feick, H.; Yan, H.; Wu, Y.; Kind, H.; Weber, E.; Russo, R.; Yang, P. Science 2001, 292, 1897. (4) Wu, Y.; Yang, P. J. Am. Chem. Soc. 2001, 123, 3165. (5) (a) Foss, C. A.; Tierney, M. J.; Martin, C. R. J. Phys. Chem. 1992, 96, 9001. (b) Preston, C. K.; Moskovits, M. J. Phys. Chem. 1993, 97, 8495. (c) Whitney, T. M.; Jiang, J. S.; Searson, P. C.; Chien, C. L. Science 1993, 261, 1316. (6) Huang, M.; Choudrey, A.; Yang, P. Chem. Commun. 2000, 12, 1603. (7) Zach, M. P.; Ng, K. H.; Penner, R. M. Science 2000, 290, 2120 and references therein. (8) Braun, E.; Eichen, Y.; Sivan, U.; Ben-Yoseph, G. Nature 1998, 391, 775. (9) Zhang, Y.; Dai, H. Appl. Phys. Lett. 2000, 77, 3015. (10) Tarascon, J. M.; DiSalvo, F. J.; Carrol, C. H. J.; Walsh, M.; Rupp, L. J. Solid State Chem. 1985, 58, 290. (11) Tarascon, J. M.; Hull, G. W.; DiSalvo, F. J. Mater. Res. Bull. 1984, 19, 915. Figure 1. Schematic illustration of metal nanowire templating reaction between LiMo3Se3 nanowires and metal ions. 10397 J. Am. Chem. Soc. 2001, 123, 10397-10398

Hannes Kind

Papers

Room-temperature ultraviolet nanowire nanolasers

Nanowire ultraviolet photodetectors and optical switches

Room-Temperature Ultraviolet Nanowire Nanolasers.

Nanowires, nanostructures and devices fabricated therefrom

Metal Nanowire Formation Using Mo3Se3- as Reducing and Sacrificing Templates