Ferroelectrics have recently attracted attention as a candidate class of materials for use in photovoltaic devices, and for the coupling of light absorption with other functional properties. In these materials, the strong inversion symmetry breaking that is due to spontaneous electric polarization promotes the desirable separation of photo-excited carriers and allows voltages higher than the bandgap, which may enable efficiencies beyond the maximum possible in a conventional p-n junction solar cell. Ferroelectric oxides are also stable in a wide range of mechanical, chemical and thermal conditions and can be fabricated using low-cost methods such as sol-gel thin-film deposition and sputtering. Recent work has shown how a decrease in ferroelectric layer thickness and judicious engineering of domain structures and ferroelectric-electrode interfaces can greatly increase the current harvested from ferroelectric absorber materials, increasing the power conversion efficiency from about 10(-4) to about 0.5 per cent. Further improvements in photovoltaic efficiency have been inhibited by the wide bandgaps (2.7-4 electronvolts) of ferroelectric oxides, which allow the use of only 8-20 per cent of the solar spectrum. Here we describe a family of single-phase solid oxide solutions made from low-cost and non-toxic elements using conventional solid-state methods: [KNbO3]1 - x[BaNi1/2Nb1/2O3 - δ]x (KBNNO). These oxides exhibit both ferroelectricity and a wide variation of direct bandgaps in the range 1.1-3.8 electronvolts. In particular, the x = 0.1 composition is polar at room temperature, has a direct bandgap of 1.39 electronvolts and has a photocurrent density approximately 50 times larger than that of the classic ferroelectric (Pb,La)(Zr,Ti)O3 material. The ability of KBNNO to absorb three to six times more solar energy than the current ferroelectric materials suggests a route to viable ferroelectric semiconductor-based cells for solar energy conversion and other applications.

The combined effects of strain and phonon confinement are seen to explain why the Raman peak near 464 cm Ϫ1 in CeO 2Ϫy nanoparticles shifts to progressively lower energies and the lineshape of this feature gets progressively broader and asymmetric ͑on the low-energy side͒ as the particle size gets smaller. The increasing lattice constant measured for decreasing particle size explains this Raman shift well. The linewidth change is fairly well explained by the inhomogenous strain broadening associated with the small dispersion in particle size and by phonon confinement. The spectra are also likely to be directly affected by the presence of oxygen vacancies. Comparison of the temperature dependence of the Raman lineshape in the nanoparticles and the bulk shows that phonon coupling is no faster in the nanoparticles, so size-dependent phonon coupling does not contribute to the large nanoparticle peak red shifts and broadening at room temperature. Irreversible thermally induced changes are observed in the Raman peak position of the nanoparticles.

Control of nanocrystal surface defects for efficient charge extraction in polymer-ZnO photovoltaic systems J. Appl. Phys. 112, 066103 (2012) Experimental surface-enhanced Raman scattering response of two-dimensional finite arrays of gold nanopatches Appl. Phys. Lett. 101, 111606 (2012) Nano-hillock formation in diamond-like carbon induced by swift heavy projectiles in the electronic stopping regime: Experiments and atomistic simulations Appl. Phys. Lett. 101, 113115 (2012) Mass transport and thermal stability of TiN/Al2O3/InGaAs nanofilms

We report scanned probe characterizations of the ferroelectric phase transition in individual barium titanate (BaTiO3) nanowires. Variable-temperature electrostatic force microscopy is used to manipulate, image, and evaluate the diameter-dependent stability of ferroelectric polarizations. These measurements show that the ferroelectric phase transition temperature (TC) is depressed as the nanowire diameter (dnw) decreases, following a 1/dnw scaling. The diameter at which TC falls below room temperature is determined to be approximately 3 nm, and extrapolation of the data indicates that nanowires with dnw as small as 0.8 nm can support ferroelectricity at lower temperatures. We also present density functional theory (DFT) calculations of bare and molecule-covered BaTiO3 surfaces. These calculations indicate that ferroelectricity in nanowires is stabilized by molecular adsorbates such as OH and carboxylates. These adsorbates are found to passivate polarization charge more effectively than metallic electrodes, explaining the observed stability of ferroelectricity in small-diameter BaTiO3 nanowires.

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The perovskite phase of cesium lead iodide (α-CsPbI or "black" phase) possesses favorable optoelectronic properties for photovoltaic applications. However, the stable phase at room temperature is a nonfunctional "yellow" phase (δ-CsPbI). Black-phase polycrystalline thin films are synthesized above 330 °C and rapidly quenched to room temperature, retaining their phase in a metastable state. Using differential scanning calorimetry, it is shown herein that the metastable state is maintained in the absence of moisture, up to a temperature of 100 °C, and a reversible phase-change enthalpy of 14.2 (±0.5) kJ/mol is observed. The presence of atmospheric moisture hastens the black-to-yellow conversion kinetics without significantly changing the enthalpy of the transition, indicating a catalytic effect, rather than a change in equilibrium due to water adduct formation. These results delineate the conditions for trapping the desired phase and highlight the significant magnitude of the entropic stabilization of this phase.

We report strong enhancement (approximately 10(3)) of the spontaneous Raman scattering from individual silicon nanowires and nanocones as compared with bulk Si. The observed enhancement is diameter (d), excitation wavelength (lambda(laser)), and incident polarization state dependent, and is explained in terms of a resonant behavior involving incident electromagnetic radiation and the structural dielectric cross section. The variation of the Raman enhancement with d, lambda(laser), and polarization is shown to be in good agreement with model calculations of scattering from an infinite dielectric cylinder.

The reflectance of porous silicon carbide ͑PSC͒ thin films on SiC substrates is measured in the infrared reststrahlen region by Fourier transform infrared reflectance spectroscopy and is compared to simulated spectra based on phenomenological and Bergman statistical effective-medium dielectric functions. The phenomenological models evaluated include the Bruggeman, cavity-and sphere-Maxwell-Garnett ͑C-MG and S-MG͒, Landau-Lifshitz/Looyenga ͑LLL͒, and Monecke models. In addition, modifications to the Bruggeman and C-MG models with variable particle shapes and surface layers are examined. Hybrid versions of the C-MG and LLL models are also considered, alternatively by using a phenomenological mixing approach, which gives a direct physical interpretation of the topology, and by directly mixing the statistical spectral density functions of the C-MG and LLL effective dielectric functions. This latter statistical hybrid model gives the best ͑and quite good͒ agreement with experiments. The differences in the hybrid models are understood by comparing their spectral density functions. The dip ͑or splitting͒ in the PSC film reststrahlen band is attributed to surface optical phonon modes.