We have observed unusual ferromagnetic properties in single-crystalline CoSi nanowire ensemble, in marked contrast to the diamagnetic CoSi in bulk. High-density freestanding CoSi nanowires with B20 crystal structure are synthesized by a vapor-transport-based method. The reaction of cobalt chloride precursor with a Si substrate produces high-aspect-ratio CoSi nanowires. The high-resolution transmission electron microscopy and electron diffraction studies reveal superlattice structure in CoSi nanowires with twice the lattice parameter of simple cubic CoSi lattice. The zero-field-cooled and field-cooled (ZFC-FC) measurements from the nanowire ensemble show freezing of the disordered surface spins at low temperatures. The magnetoresistance (MR) measurements of single nanowire devices show a negative MR whose magnitude gets larger at lower temperatures.

We report vapor-phase synthesis of single-crystalline freestanding Ag nanowires (NWs) and polarized surface-enhanced Raman scattering (SERS) of a single NW. To the best of our knowledge, vapor-phase synthesis of single-crystalline Ag NWs and polarized SERS on an individual NW have not yet been reported.Silver has the highest electrical and thermal conductivity among all metals. In addition, because of the nature of its optical constants, silver shows the most effective SERS among all metals in the visible region. 1 Since the intensity of the SERS signal depends strongly on the detailed morphology of the Ag nanostructures, it is most important to have a well-defined and well-characterized system with a clean surface for the preparation of reliable sensors based on SERS detection. Current fabrication processes for the production of SERSactive substrates, however, are often irreproducible to a certain degree, thereby leading to inconsistent optical properties and, thus, significantly fluctuating enhancement factors for substrates prepared by apparently the same procedure. Irreproducible SERS enhancement would be detrimental to quantitative and reliable sensing, which would be most critical for medical diagnosis.As a first step toward fabrication of well-controlled nanobiosensors employing SERS technique, we searched for a method to synthesize well-characterized free-standing single-crystalline Ag NWs with a clean surface. This led us to the adoption of vaporphase synthesis rather than solution phase. While vapor-phase synthesis is one of the most widely used methods for the synthesis of 1D nanostructures, it has been used mainly for the synthesis of semiconductor NWs and nanotubes. Only a few metallic NWs have been synthesized through vapor phase, 2 and most of the reported methods for the synthesis of Ag NWs are wet chemical methods involving templates, surfactants, or capping agents. 3 Our synthetic method is unique in that it uses only a single reactant, Ag 2 O, without using any templates or catalysts. In a typical synthesis, 0.2 g of Ag 2 O powder was placed in an alumina boat in the middle of a 1 in. diameter horizontal quartz tube furnace. The NWs were grown at a few centimeters downstream from the precursor on a Si substrate. At high temperature (T 1 ) 900-1000°C ), the precursor vapor was carried downstream by the flow of 500 sccm of Ar at 5-10 Torr to a lower temperature zone (T 2 ) 500°C), where Ag NWs were grown. The noncatalytic growth of the NWs and the preferred formation of large particles over NWs at higher precursor temperatures demonstrated that the Ag NWs were formed by a vapor-solid mechanism. 2a A representative SEM image in Figure 1a shows good density of straight NWs tens of micrometers long on a Si substrate. The insets are SEM and TEM images of Ag NWs, showing that the tips of NWs are round, and NWs have clean surfaces and diameters of 80-150 nm.The XRD patterns of as-grown NW ensembles (Supporting Information Figure S1) are indexed perfectly to the face-centered cubic (fcc) crystal str...