In this paper, silver thin films deposited on SiO2 substrates with a germanium wetting layer fabricated by electron-beam evaporation were studied. The characterization methods of XTEM, FTIR, XRD and XRR were used to study the structural properties of silver thin films with various thicknesses of germanium layers. Silver films deposited with very thin (1-5nm) germanium wetting layers show about one half of improvement in the crystallite sizes comparing silver films without germanium layer. The surface roughness of silver thin films significantly decrease with a thin germanium wetting layer, reaching a roughness minimum around 1-5nm of germanium, but as the germanium layer thickness increases, the silver thin film surface roughness increases. The relatively higher surface energy of germanium and bond dissociation energy of silver-germanium were introduced to explain the effects the germanium layer made to the silver film deposition. However, due to the Stranski-Krastanov growth mode of germanium layer, germanium island formation started with increased thickness (5-15nm), which leads to a rougher surface of silver films. The demonstrated silver thin films are very promising for large-scale applications as molecular anchors, optical metamaterials, plasmonic devices, and several areas of nanophotonics.
Juan J. Díaz León, Matthew Garrett, Junce Zhang, Katherine Han, R. Ernest Demaray, Roger Anderson, Allan Lewandowski, William Bottenberg, Nobuhiko Kobayashi
Concentrated solar energy has proven to be an efficient approach for both solar thermal energy applications and photovoltaics. Here, we propose a passive optical device, the Adiabatic Optical Coupler (AOC), that efficiently couples concentrated solar light from a primary solar concentrator into an optical fiber, enabling light collection and energy conversion/storage to be geographically separated, thus maximizing the overall system efficiency. The AOC offers secondary concentration of concentrated solar light through an adiabatic optical mode conversion process. Solar light, highly focused by this two stage concentrator, is delivered by optical fiber to either be subsequently converted to electricity or thermally stored. The ability to transport high energy light flux eliminates the need for high temperature working fluids in solar-thermal systems. In order to design the AOC and related peripherals, we used various modeling tools to cover different optical regimes at macroscopic and microscopic scales. We demonstrated a set of optical thin films with spatially varied refractive index up to 3 and negligible optical absorption by using proprietary sputtering technique to fabricate the AOC. We further studied the films using experimental measurements and theoretical analysis to optimize their optical properties. Preliminary cost analysis suggests that solar thermal power generation systems that employ our S2F concept could offer the cost and efficiency required to achieve the 2020 SunShot initiative levelized cost of electricity (LCOE) target. Success of this endeavor could change the energy conversion paradigm, and allow massively scalable concentrated solar energy utilization.
Semiconductor-oxide nanostructure devices can be a very intriguing material platform if optoelectronic properties of the original semiconductor nanostructures can be tuned by explicitly controlling properties of the oxide coating. This paper describes our finding that optical properties of semiconductor nanowires can be tuned by depositing a thin layer of metal oxide. In this experiment, indium phosphide nanowires were grown by metal organic chemical vapor deposition on silicon substrates with gold catalyst. The nanowires formed three-dimensional nanowire networks from which collective optical properties were obtained. The nanowire network was coated with an aluminum oxide thin film deposited by plasma-enhanced atomic layer deposition. We studied the dependence of the peak wavelength of photoluminescence spectra on the thickness of the oxide coatings. We observed continuous blue shift in photoluminescence spectra when the thickness of the oxide coating was increased. The observed blue shift is attributed to the Burstein-Moss effect due to increased carrier concentration in the nanowire cores caused by repulsion from an intrinsic negative fixed charge from the oxide surface. Samples were further characterized by scanning electron microscopy, transmission electron microscopy, and selective area diffractometry in an attempt to explain the physical mechanisms for the blue shift.
The growth of silicon core-shell nanowires with a crystalline-core and a polycrystalline-shell on copper substrates pretreated with carbon via Plasma Enhanced Chemical Vapor Deposition (PECVD) was demonstrated. The nanowire diameters range from 120 to 250nm with 10-20nm crystalline cores. The overall large diameter enables easier methods of forming an electrical/thermal contact while the small core maintains the benefits of nanowires. By altering the copper surface with carbon, highly dense silicon nanowire networks can be directly grown on copper substrates, which could allow for efficient and economical incorporation of silicon nanowires into such applications as thermoelectric devices.
Indium phosphide (InP) nanowire networks coated with gold were characterized by Raman spectroscopy. First, InP nanowire networks were grown via metal organic chemical vapor deposition (MOCVD) on silicon substrates with gold catalyst. Subsequently, gold was deposited by thermal evaporation on the grown InP nanowire networks. Different nominal thicknesses of gold were deposited, and then the goal coated InP nanowire networks were annealed in vacuum. Raman spectroscopy was used to study the dependence of InP phonon modes on the thickness of the gold coating. The study shows the gold coating decreases the longitudinal optical phonon mode signal of InP as the thickness increases.
Publisher’s Note: This paper, originally published on 19 September 2013, was replaced with a corrected/revised version on 18 October 2013. If you downloaded the original PDF but are unable to access the revision, please contact SPIE Digital Library Customer Service for assistance.
KEYWORDS: Indium, Semiconductors, Thermoelectric materials, Transmission electron microscopy, Scanning electron microscopy, Metalorganic chemical vapor deposition, Gold, Diffraction, Chemical reactions, Nanowires
The ability to make a good electrical/thermal contact to a large area filled with semiconductor nanowires has been a major engineering challenge in developing this type of thermoelectric devices. A practical fabrication process of a top electrical/thermal contact onto a network of randomly oriented intersecting semiconductor nanowires was designed by implementing a sequence of two separated metal organic chemical vapor deposition processes for indium phosphide. In the first step, a nanowire network was grown on a substrate with indium phosphide nanowires grown axially. Subsequently, growth temperature and pressure were altered to change the axial growth to lateral growth that promoted the formation of indium phosphide extending over multiple nanowires. Possible growth mechanisms during the lateral growth and structural properties of the laterally grown segment will be discussed.
Chemical sensing applications utilizing surface enhanced Raman spectroscopy (SERS) have drawn significant
attention recently. However, developing a reliable, high performance SERS platform remains a challenge. A novel
SERS substrate based on nanofingers was successfully demonstrated to provide large enhancement reliably and
showed great promise for practical applications. Capillary forces bring the gold caps on the nanofingers into close
proximity upon exposure to a solution containing molecules of interest, trapping molecules within the gaps and
producing greatly enhanced Raman signals. Transmission electron microscopy (TEM) was used to characterize the
structure of the nanofingers, in particular the gaps between finger tips to improve the fundamental understanding of
the structural-performance relationship.
Indium phosphide (InP) nanowires were grown by metal organic chemical vapor deposition (MOCVD). InP
nanowires grew in the structure of three-dimensional networks in which electrical charges and heat can travel over
distances much longer than the mean length of the constituent nanowires. We studied the dependence of thermoelectric properties on geometrical factors within the InP nanowire networks. The InP nanowire networks show Seebeck coefficients comparable with that of bulk InP. Rather than studying single nanowires, we chose networks of nanowires formed densely across large areas required for large scale production. We also studied the role played by intersections where multiple nanowires were fused to form the nanowire networks. Modeling based on finite-element analysis, structural analysis, and transport measurements were carried out to obtain insights of physical properties at the intersections. Understanding these physical properties of three-dimensional nanowire networks will advance the development of thermoelectric devices.
Semiconducting nanowires are promising materials for a variety of applications, many of which are in optoelectronics where their ability to use quantum confinement to tune transition energy levels and their ability to epitaxially grow on substrates with large lattice-mismatches enable unique opportunities. In addition, the large relative
surface area of nanowire-based devices can be either a benefit as in, for example, sensor applications or challenges such as charge trapping, non-radiative recombination or Fermi-level pinning. In this study, indium phosphide nanowires grown by metal organic chemical vapor deposition were conformally coated with aluminum oxide by atomic layer deposition as a means of controlling the surface states within the indium phosphide region. Photoluminescence spectra from the coated nanowires show a strong blueshift and slight peak broadening as compared to uncoated nanowires despite their increased lateral dimension. This degree of blueshift is unlikely to have been caused by strain associated with the coating because of its relatively thin thickness (~10 nm), and the x-ray diffraction profiles collected from the coated nanowires do not indicate the presence of substantial lattice deformation. It is more likely to be a result of altered chemical state at the surface; the uncoated nanowires have oxygen bound to both indium and phosphorus near the surface as confirmed in our X-ray photoelectron spectroscopy studies. Subsequent aluminum oxide deposition alters
surface atomic bonding thereby modifying the states electronic responsible for optical properties in the nanowires.
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