One-dimensional metal oxide nanowires, such as In 2 O 3 , [1] ZnO, [2] SnO 2 , [3] CdO, [4] and CuO [5] nanowires, have attracted a lot of attention because of their unique properties for applications ranging from nanoelectronic devices to gas sensors. Among them, SnO 2 is particularly interesting and has many important applications. For instance, SnO 2 is a very important n-type semiconductor with a large bandgap (E g = 3.6 eV at 300 K [6] ), thus making it ideal to work as transparent conducting electrodes for organic light emitting diodes and solar cells. [7±9] In addition, SnO 2 thin films have been extensively studied and used as chemical sensors for environmental and industrial applications.[7±9] SnO 2 in the nanowire form has enormous potential to work as building blocks for nanoelectronics, and is also expected to offer superior chemical sensing performance due to the enhanced surface to volume ratio. Despite the utmost importance, only a relatively small effort has been directed toward the synthesis of SnO 2 whiskers, nanorods, and more recently nanobelts. [3,10±12] Much is left to be explored, especially for the synthesis of high-quality, single-crystalline SnO 2 nanowires with precisely controlled diameters below 30 nm, as required for high-performance field-effect nanowire transistors. In this paper, we report an efficient and reliable laser-ablation approach for large-scale synthesis of SnO 2 nanowires. Precise control over the nanowire diameters has been achieved by using monodispersed gold clusters as the catalyst. Detailed material analysis, such as transmission electron microscopy (TEM) and X-ray diffraction (XRD), were used to confirm the single-crystalline nature of our nanowires. In addition, field effect transistors (FETs) have been constructed based on individual SnO 2 nanowires with on/off ratios up to 10 3 . These nanowire transistors were further demonstrated to work as sensitive UV and polarized UV detectors.A quartz tube furnace was used for our SnO 2 synthesis, where a pure Sn target was placed at the upper-stream of the tube outside the hot zone of the furnace, and Si±SiO 2 substrates covered with 20 nm gold catalytic clusters were placed in the middle of the quartz tube. The tube was then purged with 0.02 % oxygen diluted in argon, followed by heating of the furnace to 900 C. The Sn target was then ablated with a Nd:YAG laser to supply Sn vapor, which was carried downstream by the oxygen±argon mixture. The chamber was maintained at 400 torr during the laser ablation, and the typical reaction time used was about 10±30 min. Our synthesis follows the well-known vapor±liquid±solid (VLS) growth mechanism, where the Sn vapor first diffuses into the gold catalytic particles, and grows out and reacts with O 2 to form SnO 2 once the Sn±Au alloy reaches supersaturation. Continued addition of Sn into the Sn±Au nanoparticle feeds the SnO 2 growth and eventually the diameter of the SnO 2 nanowire is directly linked to the catalytic particle size. After cooling down, the samples were characteriz...