Angelica M. Stacy

Optical Negative Refraction in Bulk Metamaterials of Nanowires

Negative refraction in metamaterials has generated great excitement in the scientific community. Although negative refraction has been realized in microwave and infrared by using metamaterials and by using two-dimensional waveguide structures, creation of a bulk metamaterial showing negative refraction at visible frequency has not been successful, mainly because of the significant resonance losses and fabrication difficulties. We report bulk metamaterials made of nanowires that show such negative refraction for all incident angles in the visible region. Moreover, the negative refraction occurs far from any resonance, resulting in a low-loss and a broad-band propagation at visible frequencies. These remarkable properties can substantially affect applications such as imaging, three-dimensional light manipulation, and optical communication.

Insights into the Electrodeposition of Bi2Te3

In this paper, the processes associated with the electrodeposition of bismuth telluride (Bi 2 Te 3 ), a thermoelectric material, are reported along with an analysis of the composition and crystallinity of the resulting films. The electrodeposition can be described by the general reaction 3HTeO 2 + + 2Bi 3+ + 18e - + 9H + → Bi 2 Te 3 + 6H 2 O. Cyclic voltammetry studies of Bi, Te, and Bi/Te dissolved in I M HNO 3 reveal two different underlying processes depending on the deposition potential. One process involves the reduction of HTeO + 2 to Te 0 and a subsequent interaction between reduced Te 0 and Bi 3+ to form Bi 2 Te 3 . A second process at more negative reduction potentials involves reduction of HTeO + 2 to H 2 Te followed by the chemical interaction with Bi 3+ . Both processes result in the production of crystalline Bi 2 Te 3 films in the potential range -0.1 < E < -0.52 V vs. Ag/AgCl (3 M NaCI) on Pt substrates as determined by powder X-ray diffraction (XRD). Electron probe microanalyses and XRD reveal that the films are bismuth-rich and less oriented for more negative deposition potentials.

Raman spectra of IVB and VIB transition metal disulfides using laser energies near the absorption edges

In this paper, we present Raman spectra of ZrS2, HfS2, MoS2 and WS2 using laser energies near the energies of the absorption edges. The Raman spectra probe the properties of the first-excited electronic state and the nature of the electron-phonon coupling. The spectra of the IVB disulfides are independent of the laser excitation energy, suggesting weak electron-phonon interaction. In contrast, additional Raman bands appear in the spectra of the VIB disulfides as the laser energy approaches the band gap energy. The new modes in the spectra of MoS2 and WS2 cannot be assigned as first-order processes nor as combination bands of the phonons with zero momentum. The resonance Raman scattering of MoS2 is analyzed in terms of second-order scattering due to the coupling of phonon modes of nonzero momentum with an electronic transition associated with excitonic states.

Carbon and oxygen atom mobility during activation of Mo2C catalysts

Abstract Temperature-programmed desorption (TPD) and reduction (TPR) were used to study the activation of Mo 2 C for ethylene hydrogenation at 298 K. Mo 2 C can incorporate large amounts of oxygen, and since oxygen is a poison for the reaction, it must be removed from the surface to activate the catalyst. As oxygen is removed from the surface by evacuation or reduction, it is replenished by diffusion of oxygen from the bulk. Therefore, the equivalent of several monolayers must be removed to obtain an oxygen-free surface, although it is not necessary to remove all the bulk oxygen. Furthermore, the oxygen cannot be removed by evacuation or reduction without also removing a significant amount of bulk carbon. Although the initial activity of the catalyst for ethylene hydrogenation is independent of the amount of carbon removed, the rate of deactivation of the Mo 2 C strongly depends on the extent of carbon removal. As more carbon is removed, the rate of deactivation increases and the shape of the activity-time curve (deactivation curve) changes from an exponential decay to a sigmoidal shape. TPR spectra of oxygen chemisorbed on the surface after two different activation procedures showed that the surface of the catalyst depends on the extent of carbon removal. As more carbon is removed from the Mo 2 C, the oxygen is bound more tightly to the surface. This suggests that ethylene also might be bound more tightly on the carbon-deficient catalyst, which is consistent with the higher rate of deactivation.

Thermal properties of electrodeposited bismuth telluride nanowires embedded in amorphous alumina

Bismuth telluride nanowires are of interest for thermoelectric applications because of the predicted enhancement in the thermoelectric figure-of-merit in nanowire structures. In this letter, we carried out temperature-dependent thermal diffusivity characterization of a 40nm diameter Bi2Te3 nanowires∕alumina nanocomposite. Measured thermal diffusivity of the composite decreases from 9.2×10−7m2s−1 at 150Kto6.9×10−7m2s−1 at 300K and is lower than thermal diffusivity of unfilled alumina templates. Effective medium calculations indicate that the thermal conductivity along nanowires axis is at least an order of magnitude lower than thermal conductivity of the bulk bismuth telluride.

Local structure and oxide-ion motion in defective perovskites

Abstract Macroscopic thermodynamic and transport properties of disordered materials are determined largely by their local structure, which may differ substantially from long-range crystalline symmetry. In order to better understand local structure and ionic motion in highly disordered perovskite oxides, we have investigated several cubic perovskites using high-temperature oxygen-17 NMR in conjunction with other experimental techniques. Materials we have studied include Ba(In 0.67 Zr 0.33 )O y , Ba(In 0.67 Ce 0.33 ) O y , (La 0.5 Ba 0.5 ) (Co 0.7 Cu 0.3 O y , and (La 0.6 Sr 0.4 ) (Co 0.8 Cu 0.2 ) O y . We show that despite having the long-range cubic symmetry as determined by X-ray and neutron powder diffraction, these materials possess microdomains with layered structures on a short length scale (50–500 A). These microdomains are apparent in HRTEM images of these materials, and manifest themselves as unit cell doublings in the electron diffraction patterns. Neutron powder profile refinements and oxygen- 17 DAS NMR both suggest that oxygen nuclei are displaced from sites of cubic symmetry in a manner reminiscent of layered perovskite-related structures. As is the case with known layered materials, the high temperature oxygen- 17 spectra and relaxation measurements show that few oxygen atoms are mobile below 800°C due to trapping of oxygen-ion vacancies in ordered layers. In the case of (La 0.6 Sr 0.4 ) (Co 0.8 Cu 0.2 ) O y , estimates of the vacancy trapping energy and the vacancy migration energy, extracted from NMR, appear to rationalize macroscopic transport measurements.

Oxygen Isotope Effect in High-Temperature Oxide Superconductors

The effect of oxygen isotope substitution on the superconducting transition temperature, Tc, has been measured for BaBi0.25Pb0.75O3 (Tc, ≈ 11 K) and Lal1.85 Ca0.15CuO4 (Tc ≈ 20 K), and is compared to the shifts observed for La1.85Sr0.15CuO4 (Tc ≈ 37 K) and YBa2Cu3O7 (Tc ≈ 92 K). For all four materials, the transition temperature is shifted to lower temperature upon substitution of oxygen-18 for oxygen-16. The observed shifts demonstrate that phonons are involved in the electron-pairing mechanism in these oxide superconductors.

Pulsed laser deposition of skutterudite thin films

Skutterudite (CoSb3 and IrSb3) thin films of high phase purity and crystallinity have been prepared by pulsed laser deposition. It is found that while the crystallinity of the films increases with increasing temperature, the phase purity decreases due to antimony volatility. The skutterudite phase can be stabilized to higher temperature by addition of excess antimony to the pulsed laser deposition target. Target-to-substrate distance must be optimized to obtain stoichiometric skutterudite films with low stress. A decrease in the flux of energetic particles and changes in particle stoichiometry with increasing distance caused by plume tilting results in lower film stress, but the diantimonide phase is nucleated if the distance is too large. It is possible to further reduce compressive stress by growing films in an inert background gas to decrease the flux of energetic particles. Overall, the highest quality films are grown from targets with excess antimony at a temperature of 270 °C with a target-to-substr...

Specific heat of YBa2Cu3O7

Abstract The specific heats of five YBa 2 Cu 3 O 7 samples were measured in the range 0.4–100K in fields of 0, 3.5 and 7T. For two samples Cr was partially substituted for Cu. This produced only a small decrease in T c (0.35 K/mole % Cr) but a strong suppression of the Meissner effect and †C/T c , which provided a basis for estimating the normal-state specific heat near T c . A third sample had Cr present in an impurity phase. Two undoped samples showed evidence of fluctuation effects near T c . A comparison of these results with other work is given.

Magnetic Field Dependence of the Specific Heat of Some High-Tc Superconductors

Measurements of specific heat, C, were made on samples of La2CuO4−y (La1), La1.85M0.15CuO4−y (Ca1, Sr1, Sr2, Ba1 and Ba2) and YBa2Cu3O9−y (Y1). Meissner effect measurements were made in 12.5G. The high-temperature onset of a change in magnetic susceptibility, χ, was taken as Tc, and the transition width, ΔTc, as the temperature interval of the 10–90% change in χ. Calculation of the fractional Meissner effect, −4πχV, was based on the total sample volume, with χV corrected for demagnetizing effects. Figure 1 is a plot of −4πχV vs T.