Arrays of well‐aligned single‐crystal zinc oxide (ZnO) nanowires of uniform diameter and length have been synthesized on a (100) silicon substrate via a simple horizontal double‐tube system using chemical vapor transport and condensation method. X‐ray diffraction and transmission electron microscopy (TEM) characterizations showed that the as‐grown nanowires had the single‐crystal hexagonal wurtzite structure with detectable defects and a <0002> growth direction. Raman spectra revealed phonon confinement effect when compared with those of ZnO bulk powder, nanoribbons, and nanoparticles. Photoluminescence exhibited strong ultraviolet emission at 3.29 eV under 355 nm excitation and green emission at 2.21 eV under 514.5 nm excitation. No catalyst particles were found at the tip of the nanowires, suggesting that the growth mechanism followed a self‐catalyzed and saturated vapor–liquid–solid (VLS) model. Self‐alignment of nanowires was attributed to the local balance and steady state of vapor flow at the substrate. The growth technique would be of particular interest for direct integration in the current silicon‐technology‐based optoelectronic devices.

Semiconductor nanowires and nanoribbons are under intensive study as building blocks for next-generation nanodevices.[1±7] Much effort has focused on developing nanowires and nanoribbons for optoelectronic applications such as lasers, [2,3,7±10] waveguides, [11] and optical switches. [12] In 2003, lasing in a single ZnS nanoribbon [2] and in ZnO nanowires and nanoribbons [7] were reported by Lee's and Yang's groups, respectively. Recently, we have also observed lasing in CdS nanoribbons.[9] So far, lasing activity in nanoribbons and nanowires has been obtained only for fixed wavelengths corresponding to their near-bandgap emission. Yet, for practical applications the ability to fabricate lasers of a predetermined wavelength is often critical. In this work, we show that nanoribbons of the ternary alloy Zn x Cd 1±x S are able to sustain lasing action for a broad range of compositions of x. Specifically, we can continuously change the lasing emission of Zn x Cd 1±x S nanoribbons in two spectral regions, 485 to 515 nm and 340 to 390 nm, by controlling the composition x close to CdS (0.25 ³ x ³ 0) and ZnS (0.75 £ x £ 1), respectively. These results suggest the exciting possibility that semiconductor nanolasers of preselected ªtunableº wavelengths covering the full visible spectrum may be achieved using nanoribbons or nanowires via control of their composition, size, and dimensionality. Bandgap engineering has already been utilized in group II± VI materials for preparing superlattices, [13] heterostructures, [14] and quantum wells, [15,16] which can produce bluegreen lasers. To date, group II±VI materials are considered potential contenders for optoelectronic applications, [17] although major problems [18±21] continue to hamper their development. Notably, these problems include 1) lack of high-quality bulk single crystals of group II±VI semiconductors suitable for use as substrates; 2) presence of defects derived from the polytypism of group II±VI compounds; and 3) lack of reproducible doping to obtain the desired electrical conductivity. COMMUNICATIONS

High-density and high-quality CdS nanoribbons were synthesized by a thermal evaporation process. The nanoribbons were characterized by scanning electron microscopy, transmission electron microscopy, x-ray diffraction, and photoluminescence spectroscopy. CdS nanoribbons were found to be single crystals of high phase purity and low defect density. Lasing was observed in the CdS nanoribbons upon optical pumping. The growth of CdS nanoribbons was explained by the vapor–liquid–solid mechanism.