An ion-beam technique has been employed to fabricate nanoscale needlelike structures in ZnO thin films on silicon and plastic substrates at room temperature. The ZnO nanoneedles showed a single-crystalline wurtzite structure, the stem of which was around 100 nm in diameter. The sharp tips of the nanoneedles exhibited an apex angle of 20°as measured by transmission electron microscopy. Room-temperature ultraviolet random lasing action was observed in the ZnO nanoneedle arrays under 355 nm

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... excitation. One-dimensional ͑1D͒ semiconductor nanostructures are expected to provide functional components for future electronic, optoelectronic, and nanoelectromechanical systems. 1 There has been considerable interest in the growth of 1D semiconductor nanostructures on silicon substrates as well as temperature-sensitive substrates like plastic. The use of Si substrates enables the integration of nanomaterials in Sibased electronic devices. It is also desirable to fabricate semiconductor nanostructures on plastic substrates as it enables the development of high-performance electronic and photonic devices with the potential to impact a broad spectrum of applications. 2 Despite significant progress in deposition techniques such as vapor-liquid solid ͑VLS͒, 3 thermal evaporation condensation, 4 and metalorganic chemical-vapor deposition ͑MOCVD͒, 5 the synthesis of semiconductor nanostructures still requires growth temperatures of above 400°C. Here, we report the growth of aligned ZnO nanoneedles on Si and plastic substrates using a simple ion-beam technique. In principle, this technique is capable of fabricating a desired type and size of nanostructure on any solid substrate. 6,7 As 1D ZnO nanostructures have attracted considerable attention due to their optical properties and potential application in nanoscale photonics, 8 it is of significant interest in fabricating ZnO nanostructures directly on plastic substrates. Although low-temperature ͑Ͻ100°C͒ aqueoussolution-based synthesis methods have been demonstrated to grow aligned ZnO nanostructures, a seed layer is required which may lead to contamination. 9,10 The ion-beam technique should lead to a new approach to fabricate semiconductor nanostructures selectively on any substrate, especially on temperature-sensitive substrates, which may find applications in the emerging plastic electronic and optoelectronic devices. ZnO epilayers were grown using the filtered cathodic vacuum arc ͑FCVA͒ technique. The apparatus of the FCVA has been described elsewhere. 11 High-purity zinc ͑99.9% pu-rity͒ was used as the cathode material and oxygen gas was used as the reactant gas. Typically, the base pressure was kept at approximately 2.7ϫ 10 −4 Pa, and the pressure during deposition was about 1.3ϫ 10 −2 Pa. An arc current of 60 A was used to generate the plasma, and the axial and curvilinear fields were produced by a magnetic field of strength about 40 mT to steer the plasma. The ZnO thin films were deposited at a substrate temperature of 200°C onto Si and plastic substrates. The ZnO nanostructures were fabricated by an ion-beam system comprised of an ultrahigh vacuum scanning electron microscope ͑UHV-SEM͒ ͑JEOL; JAMP-10S͒ and a differentially pumped microbeam ion gun ͑JEOL; MIED͒. The sputtering was done with 3 keV Ar + ions focused into a microbeam 380 m in diameter with mean ion-current densities of 220 A/cm 2 . A thin carbon film was deposited prior to sputtering to enhance the ion-induced formation of nanostrucures. 12 Ion irradiation was carried out at room temperature for 30 and 60 min. The residue of carbon layer was then removed by ethanol before characterization. Optical characteristics of the devices at room temperature were studied under optical excitation by a 355-nm frequency tripled Nd:YAG ͑yttrium aluminum garnet͒ pulse laser ͑10 Hz, 6 ns pulse width͒. Optical pump was achieved by using a cylindrical lens to focus a pump stripe of 5 mm length and 60 m width onto the sample. A polarizer was set before the detector in order to analyze the polarization properties of the lasing light. We shall first illustrate the fabrication of ZnO nanoneedles on silicon substrates. The key steps in the fabrication process include ͑i͒ the deposition of a 420-nm-thick SiO 2 on Si by thermal oxidation; ͑ii͒ the deposition of a 300-nm-thick ZnO film; ͑iii͒ the deposition of a thin carbon layer to enhance the ion-induced formation of nanostructures; and ͑iv͒ the irradiation of the sample by Ar + ion beam at room temperature. The ZnO nanostructures were formed selectively and uniformly on the ZnO surface. The growth mechanism a͒