One-dimensional (1D) ZnO nanostructures have been studied intensively and extensively over the last decade not only for their remarkable chemical and physical properties, but also for their current and future diverse technological applications. This article gives a comprehensive overview of the progress that has been made within the context of 1D ZnO nanostructures synthesized via wet chemical methods. We will cover the synthetic methodologies and corresponding growth mechanisms, different structures, doping and alloying, position-controlled growth on substrates, and finally, their functional properties as catalysts, hydrophobic surfaces, sensors, and in nanoelectronic, optical, optoelectronic, and energy harvesting devices.

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One-dimensional ZnO nanostructures: Solution growth and functional properties

ZnO nanostructures for optoelectronics: Material properties and device applications

ZnO has attracted considerable attention for optical device applications because of several potential advantages over GaN, such as commercial availability of bulk single crystals and a larger exciton binding energy (~60 meV compared with ~25 meV for GaN). Recent improvements in the control of background conductivity of ZnO and demonstrations of p-type doping have intensified interest in this material for applications in light-emitting diodes (LEDs). In this paper, we summarize recent progress in ZnO-based LEDs. Physical and electrical properties, bandgap engineering, and growth of n- and p-type ZnO thin films are also reviewed.

Recent Advances in ZnO-Based Light-Emitting Diodes

The pathway towards the realization of optical solid-state lasers was gradual and slow. After Einstein's paper on absorption and stimulated emission of light in 1917 it took until 1960 for the first solid state laser device to see the light. Not much later, the first semiconductor laser was demonstrated and lasing in the near UV spectral range from ZnO was reported as early as 1966. The research on the optical properties of ZnO showed a remarkable revival since 1995 with the demonstration of room temperature lasing, which was further enhanced by the first report of lasing by a single nanowire in 2001. Since then, the research focussed increasingly on one-dimensional nanowires of ZnO. We start this review with a brief description of the opto-electronic properties of ZnO that are related to the wurtzite crystal structure. How these properties are modified by the nanowire geometry is discussed in the subsequent sections, in which we present the confined photon and/or polariton modes and how these can be investigated experimentally. Next, we review experimental studies of laser emission from single ZnO nanowires under different experimental conditions. We emphasize the special features resulting from the sub-wavelength dimensions by presenting our results on single ZnO nanowires lying on a substrate. At present, the mechanism of lasing in ZnO (nanowires) is the subject of a strong debate that is considered at the end of this review.

ZnO nanowire lasers

[∗] S. Xu , C. Xu , Y. Liu , Y. F. Hu , R. S. Yang , Q. Yang , Prof. Z. L. Wang School of Materials Science and Engineering Georgia Institute of Technology Atlanta, Georgia, 30332–0245 (USA) E-mail: zlwang@gatech.edu J. H. Ryou , H. J. Kim , Z. Lochner , S. Choi , Prof. R. Dupuis School of Electrical and Computer Engineering Georgia Institute of Technology Atlanta, Georgia, 30332–0245 (USA) ZnO-based light emitting diodes (LEDs) have been considered as a potential candidate for the next generation of blue/ near-UV light sources, [ 1 ] due to a direct wide bandgap energy of 3.37 eV, a large exciton binding energy of 60 meV at room temperature, and several other manufacturing advantages of ZnO. [ 2 ] While the pursuit of stable and reproducible p-ZnO is still undergoing, [ 3,4 ] heterojunctions of n-ZnO and p-GaN are employed as an alternative approach in this regard by considering the similar crystallographic and electronic properties of ZnO and GaN. [ 5–7 ] Compared with the thin fi lm/thin fi lm LEDs, [ 5,6 , 8 ] which may suffer from the total internal refl ection, n-ZnO nanowire/p-GaN thin fi lm heterostructures are utilized in order to increase the extraction effi ciency of the LEDs by virtue of the waveguiding properties of the nanowires. [ 9–11 ] But in all of these cases, the n-ZnO nanowires are randomly distributed on the substrate, which largely limits their applications in high performance optoelectronic devices. Here in this work, we demonstrate the capability of controlling the spatial distribution of the blue/near-UV LEDs composed of position controlled arrays of n-ZnO nanowires on a p-GaN thin fi lm substrate. The device was fabricated by a conjunction of low temperature wet chemical methods and electron beam lithography (EBL). The EBL could be replaced by other more convenient patterning techniques, such as photolithography and nanosphere lithography, rendering our technique low cost and capable of scaling up easily. Under forward bias, each single nanowire is a light emitter. By Gaussian deconvolution of the emission spectrum, the origins of the blue/nearUV emission are assigned particularly to three distinct electronhole recombination processes. By virtue of the nanowire/thin fi lm heterostructures, these LEDs give an external quantum effi ciency of 2.5%. This approach has great potential applications in high resolution electronic display, optical interconnect, and high density data storage. The design of the LED is shown in Figure 1a . Ordered ZnO nanowire arrays were grown on p-GaN (Figure 1 b–d), [ 12–14 ]

http://www.nanoscience.gatech.edu/paper/2010/10_AM_06.pdf

Ordered Nanowire Array Blue/Near-UV Light Emitting Diodes

Vertically aligned ZnO nanowires were successfully grown on the sapphire substrate by nanoparticle-assisted pulsed laser deposition (NAPLD), which were employed in fabricating the ZnO nanowire-based heterojunction structures. p-GaN/n-ZnO heterojunction light-emitting diodes (LEDs) with embedded ZnO nanowires were obtained by fabricating p-GaN:Mg film/ZnO nanowire/n-ZnO film structures. The current–voltage measurements showed a typical diode characteristic with a threshold voltage of about 2.5 V. Electroluminescence (EL) emission having the wavelength of about 380 nm was observed under forward bias in the heterojunction diodes and was intensified by increasing the applied voltage up to 30 V.

Electroluminescence from ZnO nanowire-based p-GaN/n-ZnO heterojunction light-emitting diodes

Component regulation and crystallization mechanism of CsPbBr3/Cs4PbBr6 perovskite composite quantum dots-embedded borosilicate glass for light emitting application

Amphipathic carbon dots with solvent-dependent optical properties and sensing application

Vertically aligned ZnO nanowires have been successfully synthesized on c-cut sapphire substrates by a catalyst-free nanoparticle-assisted pulsed-laser ablation deposition (NAPLD) in Ar and N2 background gases. In NAPLD, the nanoparticles formed in the background gas by laser ablation are used for the growth of the nanowires. The surface density of the nanowires can be controlled by varying the density of nanoparticles, which is in turn achieved by varying ablation laser parameters such as the energy and the repetition rate. When single ZnO nanowire synthesized in a N2 background gas was excited by 355 nm laser-pulse with a pulse-width of 8 ns, stimulated emission was clearly observed, indicating high quality of the nanowire.

Ruiqian Guo

Papers

Electroluminescence from ZnO nanowire-based p-GaN/n-ZnO heterojunction light-emitting diodes

Component regulation and crystallization mechanism of CsPbBr3/Cs4PbBr6 perovskite composite quantum dots-embedded borosilicate glass for light emitting application

Amphipathic carbon dots with solvent-dependent optical properties and sensing application

Catalyst-free synthesis of vertically-aligned ZnO nanowires by nanoparticle-assisted pulsed laser deposition

Microwave-assisted aqueous synthesis of transition metal ions doped ZnSe/ZnS core/shell quantum dots with tunable white-light emission