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The development of a robust method for integrating high-performance semiconductors on flexible plastics could enable exciting avenues in fundamental research and novel applications. One area of vital relevance is chemical and biological sensing, which if implemented on biocompatible substrates, could yield breakthroughs in implantable or wearable monitoring systems. Semiconducting nanowires (and nanotubes) are particularly sensitive chemical sensors because of their high surface-to-volume ratios. Here, we present a scalable and parallel process for transferring hundreds of pre-aligned silicon nanowires onto plastic to yield highly ordered films for low-power sensor chips. The nanowires are excellent field-effect transistors, and, as sensors, exhibit parts-per-billion sensitivity to NO 2 , a hazardous pollutant. We also use SiO 2 surface chemistries to construct a 'nano-electronic nose' library, which can distinguish acetone and hexane vapours via distributed responses. The excellent sensing performance coupled with bendable plastic could open up opportunities in portable, wearable or even implantable sensors.The fabrication of electronic devices on plastic substrates has attracted considerable recent attention owing to the proliferation of handheld, portable consumer electronics. Plastic substrates possess many attractive properties including biocompatibility, flexibility, light weight, shock resistance, softness and transparency [1][2][3] . However, most plastics deform or melt at temperatures of only 100−200 °C, placing severe limitations on the quality of semiconductors that can be grown directly on plastic. Central to continued advances in highperformance plastic electronics is the development of robust methods for overcoming this temperature restriction. Recently, three categories of approaches have emerged to address this problem.The first approaches are crystallization methods, in which an inferior inorganic semiconductor is vapour deposited at low temperatures onto plastic, and subsequently crystallized. An example is the conversion of amorphous silicon into polycrystalline silicon via laser crystallization 4 . Polysilicon thin-film transistors (TFTs) made in this way have yielded electron mobilities up to 250 cm 2 V −1 s −1 and hole mobilities up to 65 cm 2 V −1 s −1 (refs 5-7). However, this approach suffers from an inherent dichotomy between achieving high performance, which requires larger crystal grain sizes, and achieving homogeneity, which requires smaller grain sizes for uniformity in number of grain boundaries per © 2007 Nature Publishing Group * Correspondence and requests for materials should be addressed to J.R.H. heath@caltech.edu. Author contributions Competing financial interestsThe authors declare no competing financial interests. NIH Public Access TRANSFER OF NANOWIRES ONTO PLASTICThe dry transfer process uses the fact that the SNAP procedure is carried out on silicon-onoxide (SOI) wafers, as the buried silica can be readily etched to free the wires for transfer. Figure 1 summa...

Photoelectrochemical (PEC) water splitting promises a solution to the problem of large-scale solar energy storage. However, its development has been impeded by the poor performance of photoanodes, particularly in their capability for photovoltage generation. Many examples employing photovoltaic modules to correct the deficiency for unassisted solar water splitting have been reported to-date. Here we show that, by using the prototypical photoanode material of haematite as a study tool, structural disorders on or near the surfaces are important causes of the low photovoltages. We develop a facile re-growth strategy to reduce surface disorders and as a consequence, a turn-on voltage of 0.45 V (versus reversible hydrogen electrode) is achieved. This result permits us to construct a photoelectrochemical device with a haematite photoanode and Si photocathode to split water at an overall efficiency of 0.91%, with NiFeOx and TiO2/Pt overlayers, respectively.

Single-walled carbon nanotubes (SWNT) are grown by a plasma enhanced chemical vapor deposition (PECVD) method at 600°C. The nanotubes are of high quality as characterized by microscopy, Raman spectroscopy, and electrical transport measurements. High performance field effect transistors are obtained with the PECVD nanotubes. Interestingly, electrical characterization reveals that nearly 90% of the nanotubes are semiconductors and thus highly preferential growth of semiconducting over metallic tubes in the PECVD process. Control experiments with other nanotube materials find that HiPco nanotubes consist of ∼61% semiconductors, while laser ablation preferentially grows metallic SWNTs (∼70%). The characterization method used here should also be applicable to assessing the degree of chemical separation of metallic and semiconducting nanotubes.Single-walled carbon nanotubes (SWNTs) have been established as ballistic metallic and semiconducting molecular wires potentially useful for future high performance electronics. 1-4 To realize this potential, it is necessary to achieve preferential growth of semiconducting versus metallic nanotubes or enable high degrees of separation 5-8 of the two types of nanotubes. Here, we present synthesis of high quality SWNTs by a plasma enhanced CVD method at 600°C, and an unexpected result that the PECVD method preferentially grows semiconducting nanotubes at a high percentage of ∼90%. The preferential growth has prompted us to investigate the percentages of semiconducting (s-SWNT) and metallic SWNTs (m-SWNT) in materials grown by other methods, both as control experiments and to elucidate these previously unknown parameters for some of the widely used nanotube materials. We conclude that the relative abundances of semiconducting and metallic nanotubes grown by various methods are different and do not necessarily follow the 2:1 ratio expected for random chirality distribution. Highly preferential growth of a certain type of SWNT can occur depending on the growth method. The results and characterization method presented here should also have implications to chemical separation of nanotubes.A home-built radio frequency (RF, 13.56 MHz) 4-in. remote PECVD system 9 was used for nanotube growth (Figure 1). The plasma discharge source consisted of a copper coil wound around the outside of the 4-in. quartz tube near the feed-gas entrance. We operated the plasma in capacitive mode with the interior furnace wall acting as an electrode and the coil acting as the counter electrode. This created a low-density plasma that propagated down the interior of the quartz tube and reached the sample placed at the center of the tube reactor, 40 cm away from the plasma coil. The