We demonstrate that vertical silicon nanowires take on a surprising variety of colors covering the entire visible spectrum, in marked contrast to the gray color of bulk silicon. This effect is readily observable by bright-field microscopy, or even to the naked eye. The reflection spectra of the nanowires each show a dip whose position depends on the nanowire radii. We compare the experimental data to the results of finite difference time domain simulations to elucidate the physical mechanisms behind the phenomena we observe. The nanowires are fabricated as arrays, but the vivid colors arise not from scattering or diffractive effects of the array, but from the guided mode properties of the individual nanowires. Each nanowire can thus define its own color, allowing for complex spatial patterning. We anticipate that the color filter effect we demonstrate could be employed in nanoscale image sensor devices.

Color filters are widely used in color displays, optical measurement devices, and imaging devices. Conventional color filters have usually only one fixed output color. However developing active color filters with controllable color output can lead to more compact and sophisticated color filter-based devices and applications. Recent progress in nanotechnology and new knowledge of the interaction of light with metal nanostructures allow us to capture and control light better than ever. Here we use it to fabricate active color filters, based on arrays of metallic optical nanoantennas that are tailored to interact with light at visible frequencies via excitation of localized surface plasmons. This interaction maps the polarization state of incident white light to visible color. Similarly, it converts unpolarized white light to chromatically polarized light. We experimentally demonstrate a wide range of applications including active color pixels, chromatically switchable and invisible tags, and polarization imaging based on these engineered colored metasurfaces.

Nanowires have unique optical properties and are considered as important building blocks for energy harvesting applications such as solar cells. However, due to their large surface-to-volume ratios, the recombination of charge carriers through surface states reduces the carrier diffusion lengths in nanowires a few orders of magnitude, often resulting in the low efficiency (a few percent or less) of nanowire-based solar cells. Reducing the recombination by surface passivation is crucial for the realization of high-performance nanosized optoelectronic devices but remains largely unexplored. Here we show that a thin layer of amorphous silicon (a-Si) coated on a single-crystalline silicon nanowire, forming a core-shell structure in situ in the vapor-liquid-solid process, reduces the surface recombination nearly 2 orders of magnitude. Under illumination of modulated light, we measure a greater than 90-fold improvement in the photosensitivity of individual core-shell nanowires, compared to regular nanowires without shell. Simulations of the optical absorption of the nanowires indicate that the strong absorption of the a-Si shell contributes to this effect, but we conclude that the effect is mainly due to the enhanced carrier lifetime by surface passivation.

The organic dye filters of conventional color image sensors achieve the red/green/blue response needed for color imaging, but have disadvantages related to durability, low absorption coefficient, and fabrication complexity. Here, we report a new paradigm for color imaging based on all-silicon nanowire devices and no filters. We fabricate pixels consisting of vertical silicon nanowires with integrated photodetectors, demonstrate that their spectral sensitivities are governed by nanowire radius, and perform color imaging. Our approach is conceptually different from filter-based methods, as absorbed light is converted to photocurrent, ultimately presenting the opportunity for very high photon efficiency.

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...

We have synthesized epitaxial Au, Pd, and AuPd nanowire arrays in vertical or horizontal alignment on a c-cut sapphire substrate. We show that the vertical and horizontal nanowire arrays grow from half-octahedral seeds by the correlations of the geometry and orientation of seed crystals with those of as-grown nanowires. The alignment of nanowires can be steered by changing the atom flux. At low atom deposition flux vertical nanowires grow, while at high atom flux horizontal nanowires grow. Similar vertical/horizontal epitaxial growth is also demonstrated on SrTiO(3) substrates. This orientation-steering mechanism is visualized by molecular dynamics simulations.

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