Electron transfer in quantum-dot-sensitized ZnO nanowires: ultrafast time-resolved absorption and terahertz study

Photoinduced electron injection dynamics from CdSe quantum dots to ZnO nanowires is studied by transient absorption and time-resolved terahertz spectroscopy measurements. Ultrafast electron transfer from the CdSe quantum dots to ZnO is proven to be efficient already on a picoseconds time scale (τ = 3-12 ps). The measured kinetics was found to have a two-component character, whose origin is discussed in detail. The obtained results suggest that electrons are injected into ZnO via an intermediate charge transfer state.     

Synthesis and characterization of ZnO nanowires by thermal oxidation of Zn thin films at various temperatures

In this research high-quality zinc oxide (ZnO) nanowires have been synthesized by thermal oxidation of metallic Zn thin films. Metallic Zn films with thicknesses of 250 nm have been deposited on a glass substrate by the PVD technique. The deposited zinc thin films were oxidized in air at various temperatures ranging between 450 °C to 650 °C. Surface morphology, structural and optical properties of the ZnO nanowires were examined by scanning electron microscope (SEM), X-ray diffraction (XRD), energy dispersive X-ray (EDX) and photoluminescence (PL) measurements. XRD analysis demonstrated that the ZnO nanowires has a wurtzite structure with orientation of (002), and the nanowires prepared at 600 °C has a better crystalline quality than samples prepared at other temperatures. SEM results indicate that by increasing the oxidation temperature, the dimensions of the ZnO nanowires increase. The optimum temperature for synthesizing high density, ZnO nanowires was determined to be 600 °C. EDX results revealed that only Zn and O are present in the samples, indicating a pure ZnO composition. The PL spectra of as-synthesized nanowires exhibited a strong UV emission and a relatively weak green emission.     

SERS spectroscopy and SERS imaging of Shewanella oneidensis using silver nanoparticles and nanowires

Facile and reproducible SERS signals from Shewanella oneidensis were obtained utilizing silver nanoparticles (AgNPs) and silver nanowires (AgNWs). Additionally, SERS images identify the distribution of SERS hot-spots. One important observation is the synergistically enhanced SERS signal when AgNPs and AgNWs are used in conjunction, due to constructively enhanced electromagnetic field.     

A wet chemical preparation of transparent conducting thin films of Ga-doped ZnO nanoparticles

Crystalline gallium doped zinc oxide (GZO) nanopowders were synthesized using hydrothermal treatment processing. The doping concentration affected the phase structure as well as the shape of the nanopowder from nano rod-like structure to nanoparticulate one. The specific BET surface area increases with increasing the gallium doping concentration. Transparent conducting films were deposited on borosilicate glass substrate by spin coating using sols containing GZO nanoparticles dispersed in 1-propanol. The films are crystalline with a hexagonal structure. The effect of Ga doping concentration, sintering temperature and thickness of the layers has been investigated. The lowest resistivity achieved was 6.4 × 10^?2 Ω cm for a thickness of 150 nm. The films present a transmittance in the visible range as high as 90 %.     

Conductivity of solvated electrons in hexane investigated with terahertz time-domain spectroscopy

We present investigations of the transient photoconductivity and recombination dynamics of quasifree electrons in liquid n-hexane and cyclohexane performed using terahertz time-domain spectroscopy (THz-TDS). Quasifree electrons are generated by two-photon photoionization of the liquid using a femtosecond ultraviolet pulse, and the resulting changes in the complex conductivity are probed by a THz electromagnetic pulse at a variable delay. The detection of time-domain wave forms of the THz electric field permits the direct determination of both the real and the imaginary part of the conductivity of the electrons over a wide frequency range. The change in conductivity can be described by the Drude model, thus yielding the quasifree electron density and scattering time. The electron density is found to decay on a time scale of a few hundred picoseconds, which becomes shorter with increasing excitation density. The dynamics can be described by a model that assumes nongeminate recombination between electrons and positive ions. In addition, a strong dependence of the quasifree electron density on temperature is observed, in agreement with a two-state model in which the electron may exist in either a quasifree or a bound state.     

Dispersion spectroscopy of liquid thin films

Summary A new analytical method is presented based on measurement of dispersion spectra (refractive index versus wavelength) of liquid samples with a fast scanning diode array spectrometer. It proves to be suitable for the analysis of nanoliter volumes where classical absorption spectroscopy fails. Future applications include sample identification and discrimination (micro-analysis) and HPLC detection.

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Dispersionsspektroskopie von Flüssigkeits-Dünnschichten

Zusammenfassung Die hier vorgestellte analytische Meßmethode beruht auf der Messung der spektralen Dispersion der Probenflüssigkeit mit Hilfe eines Diodenzeilenspektrometers, das sich vor allem durch seine hohe Meßgeschwindigkeit auszeichnet. Es wird gezeigt, daß sich mit dieser Methode auch Volumina im Nanoliterbereich analysieren lassen, die der klassischen Absorptionsspektroskopie nicht mehr zugänglich sind. Die zukünftigen Anwendungen sind in der Probenidentifikation und -unterscheidung zu sehen, wie sie z. B. in der HPLC gefordert ist.

     

Ordered porous ZnO thin films formed by dip-coating method using PS templates

Monolayer polystyrene spheres (∼400 nm) array templates were assembled orderly on clean glass substrates by dip-coating method from emulsion of polystyrene (PS). Porous ZnO thin films were also prepared by dip-coating method to fill the interstices among the close-packed PS templates with ZnO and annealing to remove the PS templates. Results showed that ZnO sol concentration and dipping time of PS templates in sol had great influences on the morphology of ordered porous ZnO thin films. There was a shrinkage ratio of about 30% from pore to PS. SEM observation showed that the PS array templates had face-centered cubic close-packing. X-ray diffraction (XRD) spectra showed the porous ZnO thin film was wurtzite structure. The optical transmittance decreased with decreasing wavelength of lights, but was kept above 80% beyond the wavelength of 550 nm. Optical band-gap of the porous ZnO thin film annealed at 500°C was 3.22 eV.     

Formation of gold-silver nanowires in thin surfactant solution films

Thin, long gold/silver nanowires were grown on substrates in thin surfactant solution films. This growth process occurred exclusively in thinning aqueous films as the water evaporated, and elongated surfactant template structures were formed. The nanowire growth depended on the presence of a relatively high concentration of silver ions (typical Ag:Au mole ratio of 1:1). Tuning the pH value to about 5 in the growth solution was crucial for the nanowire growth. Further development of this process may lead to a simple wet chemical technique for the fabrication of relatively uniform arrays of metal nanowires on surfaces.     

Calorimetry of nanoparticles,surfaces, interfaces,thin films,and multilayers

Calorimetry gives insight into the stability of nanophase materials. Using TiO₂ as an example, the interplay of energetics of polymorphism, surface energy, and surface hydration is discussed. Oxide melt solution calorimetry, water adsorption calorimetry, and adiabatic heat capacity studies together show the following. The metastability of bulk polymorphs increases in the order rutile, brookite, anatase, while the surface energy increases in the opposite order. This leads to crossovers in phase stability at the nanoscale, which appears to be a general phenomenon. Hydration plays a major role in stabilizing nanoparticles and the first layers of water are tightly bound. There is little excess heat capacity and no significant excess vibrational entropy in nanophase rutile or anatase. Further applications of calorimetry to thin films, interfaces, multilayers, and sub-milligram samples are presented.     

Far-infrared characteristics of ZnS nanoparticles measured by terahertz time-domain spectroscopy

The optical and dielectric properties of ZnS nanoparticles are studied by use of terahertz time-domain spectroscopy (THz-TDS) over the frequency range from 0.3 to 3.0 THz. The effective medium approach combined with the pseudo-harmonic model of the dielectric response, where nanoparticles are embedded in the host medium, provides a good fit on the experimental results. The extrapolation of the measured data indicates that the absorption is dominated by the transverse optical mode localized at 11.6+/-0.2 THz. Meanwhile, the low-frequency phonon resonance of ZnS nanoparticles is compared with the single-crystal ZnS. The THz-TDS clearly reveals the remarkable distinction in the low-frequency phonon resonances between ZnS nanoparticles and single-crystal ZnS. The results demonstrate that the acoustic phonons become confined in small-size nanoparticles.     

Preparation and optical properties of silver nanowires and silver-nanowire thin films

Silver nanowires and silver-nanowire thin films have attracted much attention due to their extensive applications in Surface-Enhanced Raman Scattering (SERS) and Surface-Enhanced Fluorescence (SEF). Thin films of silver nanowires within polyelectrolyte layers of poly(allylamine hydrochloride) (PAH) and poly(sodium 4-styrenesulfonate) (PSS) were fabricated by the Spin-Assisted Layer-by-Layer (SA-LbL) method. The surface coverage, thickness, and absorbance properties of the silver-nanowire films were controlled by the number of layers deposited. Both transverse and longitudinal surface plasmon (SP) modes of the silver-nanowires were observed in the absorbance spectra, as was evidence for nanowire interaction. Two-dimensional finite difference time-domain (2D FDTD) simulations predict that the maximum field enhancement occurs at the ends and cross-sectional edges of the wires for the longitudinal and transverse modes, respectively. Silver nanowires were synthesized by a facile, high-yield solvothermal approach, which can be easily manipulated to control the aspect ratio of the nanowires. The effects of polyvinylpyrrolidone (PVP) concentration and molecular weight on the growth of the silver nanowires, which are not documented in the original procedure, are discussed. It is shown that the growth mechanism for silver nanowires in the solvothermal synthesis is similar to that reported for the polyol synthesis.     

Photoluminescence spectroscopy of pure pentacene, perfluoropentacene, and mixed thin films

We report detailed temperature dependent photoluminescence (PL) spectra of pentacene (PEN), perfluoropentacene (PFP), and PEN:PFP mixed thin films grown on SiO(2). PEN and PFP are particularly suitable for this study, since they are structurally compatible for good intermixing and form a model donor/acceptor system. The PL spectra of PEN are discussed in the context of existing literature and compared to the new findings for PFP. We analyze the optical transitions observed in the spectra of PEN and PFP using time-dependent density functional theory calculations. Importantly, for the mixed PEN:PFP film we observe an optical transition in PL at 1.4 eV providing evidence for coupling effects in the blend. We discuss a possible charge-transfer (CT) and provide a tentative scheme of the optical transitions in the blended films.     

High-resolution waveguide terahertz spectroscopy of partially oriented organic polycrystalline films

We have characterized the terahertz (THz) vibrational spectroscopy of organic polycrystalline thin films using the new experimental technique of waveguide terahertz time domain spectroscopy (waveguide THz-TDS). The organic materials used in this study are tetracyanoquinodimethane (TCNQ) and 1,3-dicyanobenzene (13DCB). For each material, a thin film is cast onto one of the inner surfaces of a metal parallel plate waveguide (PPWG), followed by measurement of the low-frequency vibrational spectrum using waveguide THz-TDS. The vibrational spectra of the waveguide films are compared to corresponding vibrational spectra of standard pellet samples made by dispersing the organic solid in transparent polyethylene. We show how the waveguide films produce significantly narrower THz vibrational line shapes and reveal additional spectral lines that are obscured by inhomogeneous broadening effects in the pellet samples. When TCNQ waveguide films are cooled to 77 K, vibrational line widths as sharp as 25-30 gigahertz (0.83-1.0 cm(-1)) at the full width at half-maximum are observed, which are among the narrowest far-infrared line widths measured for this material. The origin of the line-narrowing effect for the waveguide films is the suppression of inhomogeneous broadening due to the planar ordering of the film on the waveguide surface. The TCNQ waveguide films are further characterized using optical microscopic evaluation to understand how film morphology affects the THz vibrational spectrum. X-ray diffraction is used to determine the orientation of the polycrystalline TCNQ films on the PPWG surface and to qualitatively explain the different vibrational line strengths observed for the ordered waveguide film relative to the random pellet.     

Photoinduced hydrophilicity of heteroepitaxially grown ZnO thin films

Single crystalline ZnO thin films were heteroepitaxially grown on sapphire substrates by rf-magnetron sputtering. The ZnO films on sapphire A and C face were oriented along the (0001) direction, whereas the ZnO film on sapphire R face was oriented along the (11-20) direction. The rate of photoinduced hydrophilic conversion strongly depended on the surface crystal structure. The ZnO film oriented along the (11-20) direction exhibited a higher hydrophilicizing rate than those oriented along the (0001) direction. The high hydrophilicizing rate of the ZnO oriented along the (11-20) direction is due to its surface atomic arrangement. The outermost layer of the ZnO surface of the (11-20) face contains oxygen ions, which are considered to be energetically reactive sites and responsible for the hydrophilic conversion.     

Development and characterization of ZnO, Au/ZnO and Pd/ZnO thin films through their adsorptive and catalytic properties

In this paper, we report (a) the development of ZnO thin films prepared by pulsed laser deposition and partially covered with nano-particles Pd or Au and (b) their physicochemical study, in order to investigate their catalytic and/or adsorptive properties. It is the first time where two different and popular methods, namely pulsed laser deposition and reversed flow-inverse gas chromatography, are combined. The inverse gas chromatographic technique with the corresponding time-resolved analysis is used for the first time in order to characterise compounds in the nano-scale domain. We focus on the determination of physicochemical quantities mainly concerning the adsorption in thin films, with (Pd/ZnO) or without (Au/ZnO) catalytic behaviour. Thus, entropy and other important physicochemical quantities are calculated which reveal the mechanism of adsorption as well as of isomerization-hydrogenation of 1-butene and contribute to the study of heterogeneity of thin film surfaces. The programs used have been written in Fortran. An important achievement is also the determination of the standard deviations of the kinetic constants.     

A comment on emission,reflection and transmission spectroscopy of thin films

General formulaes and working equations in terms of surface reflectivities and internal transmittances are provided for spectroscopy of layered systems. It is demonstrated that band distortions or splitting of intense bands can occur in emission or reflection spectroscopy of concentrated films, but not in transmission. Distortions due to variations in reflectivities near absorption bands can be eliminated employing an opaque sample as the reference. In extreme cases, a colder outer layer may result in split emission bands. This effect also is corrected for by an opaque reference. The theory is substantiated by experimental spectra.     

Highly active and sintering-resistant heteroepitaxy of Au nanoparticles on ZnO nanowires for CO oxidation

Gold was supported on commercial ZnO powders(P) and homemade ZnO nanowires(NWs) by a modified deposition–precipitation method. X-ray diffraction and transmission electron microscopy investigation indicated that the size of the Au nanoparticles(NPs) depended strongly on the calcination temperature.The Au NPs were highly dispersed( 5 nm) on both supports with calcination temperatures 400 °C.However, after calcination at 600 °C the Au NPs aggregated much more severely on ZnO P than on ZnO NWs. Gold NPs epitaxially grew into the {10–10} facets of the ZnO NWs after calcination at temperatures 400 °C. Such unique anchoring mechanism accounts for the much better experimentally observed sintering resistance. X-ray photoelectron spectra showed that Au existed as both metallic Au0 and Auδ+species in all the synthesized catalysts with or without calcination treatment; the ratios of Auδ+/Au0,however, varied, depending on the treatment conditions. Catalytic tests showed that the activity for CO oxidation strongly depended on the size of the Au NPs. After calcination at 600 °C, the specific rate for CO oxidation at room temperature decreased about 30 times on Au/ZnO P but only about 4 times on Au/ZnO NW. Stability tests demonstrated that the Au/ZnO NW catalysts had better stability for CO oxidation.