Jonathan E. Spanier

Cerium oxide nanoparticles: Size-selective formation and structure analysis

Nanoparticles of cerium oxide with a narrow size distribution (±15%) are prepared by mixing cerium nitrate solution with an ammonium reagent. High-resolution transmission electron microscopy (TEM) indicates that over 99% of the synthesized particles are single crystals. TEM and photon absorption are used to monitor particle size. The lattice parameter increases up to 0.45% as the particle size decreases to 6 nm, as observed with x-ray diffraction. Raman spectra also suggest the particle-size effect and concomitant lattice expansion. The lattice expansion can be explained by increased concentrations of point defects with decreasing particle size.

Visible thermal emission from sub-band-gap laser excited cerium dioxide particles

Cerium dioxide particles excited in air with sub-band-gap radiation emit very broad radiation in the visible spectrum above a threshold intensity that decreases with increasing ambient temperature. Concomitant with this emission is the near disappearance of the Stokes and anti-Stokes Raman scattering peaks. Both phenomena are reversible in air up to just above threshold, and are seen for nanoparticles and several-micron-diameter particles with particle diameter comparable to or smaller than the laser focus. Temperature estimates using the Stokes/anti-Stokes scattering intensity ratio suggest there is laser heating due to small intragap absorption and possible nonlinear processes, given the very slow thermal conduction. The broad emission in this loose powder may well be due to thermal emission, on the basis of spectral fitting of the high-energy part of the spectrum to a blackbody radiator at ∼1200–1400u200a°C, although luminescence from a new phase is a possibility. The sudden decrease in Raman scattering and increase in emission in air are consistent with a transition to a new, possibly luminescent, phase, as is the continued disappearance of the Raman peaks in forming gas when the laser power is reduced below the upstroke threshold. Oxygen point defects and their complexes may play an important role in many of these processes.

Optical Transmission, Photoluminescence, and Raman Scattering of Porous SiC Prepared from p-Type 6H SiC.

The optical transmission, temperature-dependence of the photoluminescence (PL), and Raman scattering of porous SiC prepared from p-type 6H-SiC are compared with those from bulk p-type 6H-SiC. While the transmission spectrum of bulk SiC at room temperature reveals a relatively sharp edge corresponding to its band gap at 3.03 eV, the transmission edge of porous SiC (PSC) is too wide to determine its band gap. It is believed that this wide edge might be due to surface states in PSC. At room temperature, the PL from PSC is 20 times stronger than that from bulk SiC. The PL PSC spectrum is essentially independent of temperature. The relative intensities of the Raman scattering peaks from PSC are largely independent of the polarization configuration, in contrast to those from bulk SiC, which suggests that the local order is fairly random.

Dynamics of Photogenerated Surface Charge on BiFeO3 Films

We report on the spatial and temporal evolution of photoinduced charge generation and carrier separation in heteroepitaxial BiFeO(3) thin films deposited on Nb:SrTiO(3) as measured in ambient at room temperature with Kelvin probe and piezoresponse force microscopy. Contributions from the self-poled and ferroelectric polarization charge are identified from the time evolution of the correlated surface potential and ferroelectric polarization in films as grown and following poling, and at different stages and intensities of optical illumination. Variations in the surface potential with bias voltage, switching history, and illumination intensity indicate how both bulk ferroelectric photovoltaic and the domain wall offset potential mechanisms contribute to the photogenerated charge.

Single-crystal, mesoscopic films of lead zinc niobate-lead titanate: Formation and micro-Raman analysis

A process to form thin films of lead zinc niobate–lead titanate (PZN–PT) from a bulk crystal for microelectronic and microelectromechanical device applications is presented. The structural phase transitions and ferroelectric ordering in unpoled crystalline bulk and thin-film relaxor PZN–PT are studied from −190u200a°C to 600u200a°C using polarized micro-Raman scattering. The structural phase transitions in this material are observed by distinct changes in the polarization selectivity. The results for the thin film and bulk crystal are in good agreement for a wide range of the temperatures studied, indicating that the thin-film PZN–PT retains much, if not all, of the structural and ferroelectric properties of the original bulk substrate.

Local probing of magnetoelectric coupling and magnetoelastic control of switching in BiFeO3-CoFe2O4 thin-film nanocomposite

We report on the combination of piezoresponse force microscopy (PFM), magnetic force microscopy, and local ferroelectric switching with magnetic field for the study of a thin-film magnetoelectric (ME) nanocomposite. The collection of PFM under an applied variable magnetic field within a polycrystalline perovskite-spinel BiFeO3-CoFe2O4 (BFO-CFO) 0-3 type thin-film nanocomposite enables quantitative and proximal measurement of magnetoelastic strain-driven ME response. Combination of measurement of the as-grown strain state with local measurements of microstructure and macroscopic magnetization permits local mapping of ME coupling.

Vapor-phase epitaxial growth on porous 6H–SiC analyzed by Raman scattering

SiC vapor-phase epitaxy on porous silicon carbide (PSC) substrates formed by electrochemical anodization is reported. Raman scattering indicates that the polytype of the optically smooth SiC grown on PSC formed in both p-type and n-type 6H substrates is 6H. The Raman scattering selection rules in these films are the same as those observed in the bulk substrate and epilayers grown on bulk, indicating high crystalline quality. The formation of epitaxial 6H–SiC on porous 6H–SiC may open up new possibilities for dielectric device isolation, fabrication, and epitaxial lift-off.

Electrochemical Impedance Spectroscopy of 6H-SiC in Aqueous Hydrofluoric Acid

Polarization and electrochemical impedance spectroscopy of p-type 6H-SiC electrodes in an aqueous hydrofluoric acid solution is performed during the formation of porous SiC. The polarization of p-type SiC permits the identification of the flatband voltage, a Tafel-like region, and a limiting current which is attributed to the onset of the electropolishing regime. For some of the potential range studied over which porous layers form, the impedance data are characterized by a single high-frequency semicircle and there is a second low-frequency semicircle at intermediate potentials, The dependence of the fitted capacitance and resistance on the applied voltage suggests that the SiC-electrolyte interface is controlled under different regimes of potential by the space-charge layer, the interaction with surface states, and the formation of a passivating film.

Mesoscopic Free Path of Nonthermalized Photogenerated Carriers in a Ferroelectric Insulator

We show how finite-size scaling of a bulk photovoltaic effect-generated electric field in epitaxial ferroelectric insulating BaTiO_{3}(001) films and a photo-Hall response involving the bulk photovoltaic current reveal a large room-temperature mean free path of photogenerated nonthermalized electrons. Experimental determination of mesoscopic ballistic optically generated carrier transport opens a new paradigm for hot electron-based solar energy conversion, and for facile control of ballistic transport distinct from existing low-dimensional semiconductor interfaces, surfaces, layers, or other structures.

Magnetostrictive stress reconfigurable thin film resonators for near direct current magnetoelectric sensors

The magnetic response of microdevices is significantly enhanced at structural resonance allowing for improved sensitivity and signal-to-noise ratio. Here, free-standing thin film CoFe bridge resonators have been fabricated and investigated. It is shown that the strong magnetic field dependence of the fundamental resonance frequency is a function of magnetic field orientation due to stress-induced anisotropy. These devices may offer a new approach for developing fully integrated resonant magnetic field sensing technology.