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The relaxor ferroelectric Pb(Zn1/3Nb2/3)O3–0.08PbTiO3 (PZN–0.08PT) single crystal has been investigated using polarized micro-Raman scattering technique in a wide temperature range from -150 to 350 °C. Raman spectra of the crystal at temperatures lower than 90 °C show 11 modes over a frequency range of 100–1000 cm-1. There are 12 Raman active vibration modes in the monoclinic structure of relaxor ferroelectrics according to the group theory analysis. Thus the structure of PZN-0.08PT single crystal at low temperatures is confirmed as monoclinic. Two structural phase transitions in the crystal were observed when the temperature was increasing. First, the monoclinic-tetragonal (M-T) phase transition happens at 90 °C. Second, the tetragonal-cubic (T-C) phase transition takes place at 170 °C. [ABSTRACT FROM AUTHOR]

Amorphous twisted SiO2 nanobelts have been synthesized on Si wafers using facile thermal evaporation. These nanobelts are produced together with SiO2 nanowires and a small quantity of SiO2 nanosprings. Spectral and microstructural analyses suggest that the twisted SiO2 nanobelts and nanosprings form via a polar surface driven process. Spontaneous polarization on the very thin polar crystalline SiO2 layers on the amorphous SiO2 nanobelt and nanospring surfaces makes the nuclei rearrange orderly and causes the nanobelt and nanowire to roll up at a certain twisty angle. The cathodoluminescence spectrum acquired from these SiO2 nanostructures reveals three emission bands at 4.4, 3.7, and 2.7 eV originating from oxygen-related defect centers. The polar surface driven mechanism can adequately explain the growth of these novel twisty nanobelts and nanosprings which have potential applications in sensors, transducers, resonators, and photonics. [ABSTRACT FROM AUTHOR]

Efficient resonant electron transfer from the surface bonding structure to the conduction band of quantum confined Si nanocrystals is observed by Si nanocrystals in a toluene suspension. Based on the electron transfer mechanism, the enhancement of photoluminescence originates from the band-to-band recombination in the p-type Si nanocrystals suspended in a toluene solution. The energy levels of the electrons in the Si nanocrystals chemisorbed with toluene molecules are calculated using the method of linear combination of atomic orbitals, and the characteristics of the obtained density of states is in good agreement with the observed photoluminescence properties. [ABSTRACT FROM AUTHOR]

<111>-oriented p-type Si wafer with a resistivity of 1–5 Ω cm was implanted with Fe+ and then annealed at 1100 °C in N2 for 60 min, followed by anodization in a solution of HF to form porous structure with β-FeSi2 nanocrystallites. Photoluminescence (PL) spectral measurements show that a strong PL peak appears in the range of 610–670 nm. The position of the PL peak remains unchanged, but its intensity increases with the storage time in air until about three months and then saturates. C60 molecules were chemically coupled on the porous structure through a kind of silane coupling agent to form a nanocomposite. It is revealed that the stable PL peak monotonically shifts to a pinning wavelength at 570 nm. Experimental results from PL, PL excitation, Raman scattering, and x-ray diffraction measurements clearly show that the pinned PL originates from optical transition in C60-related defect states, whereas the photoexcited carriers occur in the β-FeSi2 nanocrystallites formed during anodization. This work opens a new way to tailor nanometer environment for seeking optimal luminescent properties. [ABSTRACT FROM AUTHOR]

Photoluminescence (PL) measurements have been performed on the nanocomposites of higher fullerene-coupled porous silicon (PS) nanocrystals. For the C70/PS and C76(78)/PS nanocomposites, the PL spectra show a pinning wavelength at ∼565 nm and for the C84/PS and C94/PS nanosystems the pinning wavelength is at ∼590 nm. The PL pinning property is closely related to the sorts of the coupled fullerenes. A band mixing model of direct and indirect gaps in a nanometer environment consisting of nc-Si core, SiO2 surface layer, and coupled fullerene has been proposed for calculation of electronic states. Good agreement is achieved between the experiments and theory. [ABSTRACT FROM AUTHOR]

Magnetic Zn1-xCoxO nanorods were fabricated via direct hydrothermal synthesis. The measurements of x-ray diffraction, x-ray photoemission spectroscopy, and optical absorption spectra demonstrate the presence of cobalt in the +2 state in a tetrahedral crystal field, which indicates that Co ions have been doped into the nanorods. The observations of morphology and microstructure reveal that the Zn1-xCoxO nanorods grow along the [0002] direction through Ostwald ripening not only competing with but being assisted by oriented attachment. The field dependence of magnetization (M-H curves) of the Zn1-xCoxO nanorods measured at 300 K shows their ferromagnetic characteristics. The coercive fields (Hc) were obtained to be 98 and 36 Oe for nominal x=0.029 and 0.056, respectively. Our experimental results provide one effective method for fabrication of transition metal doped ZnO nanostructures with room-temperature ferromagnetism by direct chemical synthesis. [ABSTRACT FROM AUTHOR]

A low-frequency Raman vibration mode, whose peak position and linewidth are independent of the sizes of Ge nanocrystals and the polarization configuration of incident excitation light, was observed in silicon oxide films embedded with Ge nanocrystals which were prepared using magnetron cosputtering of SiO2–Ge–Si targets. The peak position of the Raman mode is sensitive to the content of Si in the matrix. After the sample is annealed above a special temperature that increases with the content of Si, the Raman mode disappears. Microstructural observations and spectral analyses disclose that this low-frequency Raman mode arises from a local structure which is positioned at the surfaces of Ge nanocrystals and consists of Ge, Si, and O atoms. High-temperature annealing leads to the removal of Ge atoms from the local structure. As a result, the local vibration mode vanishes. [ABSTRACT FROM AUTHOR]

Using an anodic alumina membrane with an ordered nanopore array as a template, we have fabricated a two-dimensional light-emitting nanopolymer array by embedding the luminescence polymer poly[2-methoxy-5-(2′-ethyl-hexyloxy)-p-phenylene vinylene] (MEH-PPV) into the nanopores. It is revealed experimentally that the conformation of the MEH-PPV chains in the nanopores is in the form of a bunch of polymer chains. The number of the polymer chains in a bunch depends on the concentration of the polymer solution and the diameters of nanopores in the template. Investigation on the photoluminescence spectra of the nano-MEH-PPV array annealed under different temperatures shows that the nanopores can effectively hinder the relaxation of MEH-PPV, which leads to its high thermal stability. [ABSTRACT FROM AUTHOR]

Ge and Si nanocrystals (nc-Ge and nc-Si) with average sizes in the range of 2–7 nm, embedded in silica matrix, were fabricated for investigating their acoustic-phonon vibrational properties. The freely elastic sphere theory was found to be unsuitable for explaining the low-frequency phonon vibration character of both nc-Ge and nc-Si in our current experiments. We have assigned the observed low-frequency Raman peaks to “LA-like mode” and “TA-like mode” in terms of their polarization and depolarization behaviors. In addition, it is revealed that the lattice contraction phenomenon exists in nc-Ge and nc-Si with sizes smaller than 4 nm, which leads to a contrary effect against matrix traction on the phonon vibrational frequencies. [ABSTRACT FROM AUTHOR]

Alumina nanotube (ANT) and nanowire (ANW) arrays were produced from highly ordered porous anodic alumina (PAA) by etching in phosphoric acid and subsequent thermal treatment. The domain structures that have small sizes provide more boundaries and break the hexagonal symmetry of the PAA template, thereby inducing star-like splitting modes. Using very thin PAA films, ANTs and their arrays are fabricated, whereas ANWs and their arrays are produced when using thick films. This is due to the thin wall between nanopores. Our results provide some mechanistic and fundamental information pertaining to the fabrication of nanostructures via a PAA template. [ABSTRACT FROM AUTHOR]

Using magnetron cosputtering of SiO2, Ge, and Si targets, Si-based SiO2:Ge:Si films were fabricated for exploring the influence of Si target proportion (PSi) and annealing temperature (Ta) on formation, local structure, and phonon properties of nanocrystalline Si1-xGex (nc-Si1-xGex). At low PSi and Ta higher than 800 °C, no nc-Si1-xGex but a kind of composite nanocrystal consisting of a Ge core, GeSi shell, and amorphous Si outer shell is formed in the SiO2 matrix. At moderate PSi, nc-Si1-xGex begins to be formed at Ta=800 °C and coexists with nc-Ge at Ta=1100 °C. At high PSi, it was disclosed that both optical phonon frequency and lattice spacing of nc-Si1-xGex increase with raising Ta. The possible origin of this phenomenon is discussed by considering three factors, the phonon confinement, strain effect, and composition variation of nc-Si1-xGex. This work will be helpful in understanding the growth process of ternary GeSiO films and beneficial to further investigations on optical properties of nc-Ge1-xSix in the ternary matrix. [ABSTRACT FROM AUTHOR]

Using C60 molecule as a kind of surface-passivated agent to modify the electronic structure of Si nanocrystallites in porous silicon, we disclose that this kind of C60/nanocrystalline Si coupling system can show a strong blue emission at ∼460 nm when stored in air for more than one year. After a full characterization of the photoluminescence properties, we propose a luminescent center in the SiOx layer at the surface of a Si nanocrystallite. It is a pair consisting of an oxygen vacancy and an interstitial oxygen. The interstitial oxygen also forms a peroxy linkage with a neighboring lattice oxygen. Radiative recombination of carriers photogenerated from Si nanocrystallite cores in the luminescent centers results in the observed blue photoluminescence. Neutron irradiation experiments support our assignment of the blue emission mechanism. This work improves the understanding of the origin of blue emission from silicon/oxygen-related nanostructured materials. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

We have investigated the photoluminescence (PL) from anodic alumina membranes with an ordered nanopore array formed on bulk Al foils. Most of the membranes fabricated by anodization in oxalic acid showed a strong PL peak in the blue. Due to an obvious asymmetry, the PL peak can be Gaussian divided into two bands around 405 and 455 nm, having a slight shift with the sample formed in different acid concentrations. The PL excitation (PLE) spectral examinations and analyses revealed that the two blue PL bands originate from optical transitions in two kinds of different oxygen-deficient defect centers, F (oxygen vacancy with two electrons) and F[sup +] (oxygen vacancy with only one electron) centers. Their distributions were discussed on the basis of the observed PL and PLE behaviors. Our experimental results improve the understanding of the blue-emitting property of anodic alumina membranes. [ABSTRACT FROM AUTHOR]

The linearity of contributions to the S-ground-state splitting (zero-field splitting, ZFS) of Gd3+ is confirmed by studies on the quadratic effects of the ZFS mechanisms, which are proved to be negligible in most cases. However, the quadratic crystal field (QCF), though small, may have marginal influence on the superposition model analyses of the experimental spin-Hamiltonian parameters. General expressions are thus obtained for QCF contributions to ZFS in ions with the open-shell f 7 configuration. These are used to calculate the quadratic contributions for Gd3+ in garnet host crystals. They show that these are small and yet should be allowed for when making superpositon model analyses of phenomenological parameters. [ABSTRACT FROM AUTHOR]

We report the determination of the complete set of elastic, piezoelectric, and dielectric constants of a 0.67Pb(Mg1/3Nb2/3)O3-0.33PbTiO3 (0.67PMN-0.33PT) single crystal by using Brillouin scattering in which all experimental data were collected from one [001]cub-poled crystal sample. The values of determined constants are similar to those previously determined from four crystal samples by using a hybrid method combining both ultrasonic and resonant techniques. The directional dependence of the compressional and shear modulus of the crystal in the (001) plane was investigated. A new weak broad peak appears at the low frequency side (∼18 GHz) between 45° and 65° scattering angles and the possible origins of the peak are discussed. [ABSTRACT FROM AUTHOR]

Examines the photoluminescence spectra of partially oxidized porous silicon (Si) coated with Si thin films. Stability of the photoluminescence; Role of the optical transitions in the enhancement and stability of the photoluminescence.