ZnO nanowires and nanobelts are two representatives of one-dimensional semiconductor nanomaterials possessing potential applications as optoelectronic and sensor devices. In this study, we applied a vapour-transport-deposition method to synthesize both types of nanostructures using relatively low temperatures (860 • C) by controlling the source materials. We found that the resulting product under similar growth conditions can be switched between [0001]-axial nanowires and 1120-axial nanobelts simply by adding indium to the source. The former appear as ordered vertical arrays of pure ZnO while the latter are belts without spatial ordering. Both represent defect-free single crystals grown via the vapour-liquid-solid mechanism using nanosphere lithography-fabricated catalyst Au templates. Examination of the early growth stage suggests that the dissolution of In into Au influences the nucleation of ZnO at the solid-liquid interface, and subsequently defines the structure and crystallographic orientation of the nanobelts. The optical properties of both nanostructures are studied by photoluminescence and resonant Raman scattering, which indicate consistently that the doped nanobelts have a higher carrier concentration than the nanowires.

Single crystals of the charge-transfer salt picene/2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane have been grown using physical vapor transport. The crystal structure was determined using single-crystal X-ray diffraction. It was found that the crystals grow in a 1:1 molecular ratio and adopt a monoclinic structure with alternate stacking. Both Xray data and Raman measurements show that the grown crystals are of good quality. From structure and infrared data, the charge transfer between acceptor and donor molecules was estimated to be approximately 0.14−0.19 electron. Transport measurements indicate a nonmetallic ground state with an activation energy of 0.6 eV. The supporting density functional theory calculations on molecular model systems as well as on crystalline structures confirm the amount of charge transfer and provide first insights into the electronic structure of the new material.

Resonant Raman study reveals the noticeable effect of the ligand exchange on the nanocrystal (NC) surface onto the phonon spectra of colloidal CdTe NC of different size and composition. The oleic acid ligand exchange for pyridine ones was found to change noticeably the position and width of the longitudinal optical (LO) phonon mode, as well as its intensity ratio to overtones. The broad shoulder above the LO peak frequency was enhanced and sharpened after pyridine treatment, as well as with decreasing NC size. The low-frequency mode around 100 cm-1 which is commonly related with the disorder-activated acoustical phonons appears in smaller NCs but is not enhanced after pyridine treatment. Surprisingly, the feature at low-frequency shoulder of the LO peak, commonly assigned to the surface optical phonon mode, was not sensitive to ligand exchange and concomitant close packing of the NCs. An increased structural disorder on the NC surface, strain and modified electron-phonon coupling is discussed as the possible reason of the observed changes in the phonon spectrum of ligand-exchanged CdTe NCs.PACS: 63.20.-e, 78.30.-j, 78.67.-n, 78.67.Bf

Raman and infrared phonon spectra of ultrasmall (1.8 nm) colloidal CdS nanoparticles (usNPs) are presented. Multiphonon scattering by optical phonons up to the third order is observed in the Raman spectra at low temperature and resonant (325 nm) excitation. The first-order optical phonon peak is a superposition of several components, two of which can be assigned to surface optical (SO) and longitudinal optical (LO) modes, respectively. The LO mode, being markedly broadened compared to that of spectra of regular (>2 nm) NPs, is related to phonon confinement and bond distortion induced by a significant structural relaxation in usNPs. A shoulder observed above the LO frequency is either due to the density of phonon states induced by distorted surface bonds or due to higher-order scattering processes involving optical and acoustic phonons. The Raman peaks of usNPs do not reveal the upward shift and narrowing upon decreasing temperature from 300 K down to 85 K typical for crystalline semiconductors, even though their intensity increases as expected. The abnormal thermal behavior of phonon peaks is likely related to the significant structural reorganization of the usNP lattice. A broad feature in the range of 200−300 cm −1 observed in the infrared phonon spectrum of usNPs correlates with the Raman data and is distinct from the SO mode previously reported for NPs of larger size. ■ INTRODUCTIONUltrasmall semiconductor nanoparticles (usNPs), or "magicsize" clusters, have recently attracted much attention due to their unique physical properties. 1−6 The growth rate of colloidal NPs deviates from the prediction of the classic thermodynamic growth theory as the NPs attain the magic closed-shell structure resulting from the existence of a chemical potential well. 6 For CdSe, CdS, and several other II−VI compounds, the most frequently observed magic structure comprises 32−34 molecular units corresponding to a diameter of around 1.7−1.9 nm. 1,6,7 The usNPs reveal either narrow blue 8 or broad-band quasi-white photoluminescence (PL), making this kind of NP promising for light-emission applications. [2][3][4]9,10 Studying NPs of such a small size is also of fundamental importance, as it allows the existing models to be verified in the regime of strong electron and phonon confinement. 11−15 Understanding and controlling the electronic and optical properties of usNPs is, however, complicated due to a considerable contribution from surface atoms, the number of which is comparable to the number of fully coordinated inner atoms, as well as due to the coupling between the electronic states in the NP and ligands. 16 The lattice vibration (phonon) spectra can provide valuable information on the structure and physical properties of semiconductor NPs and their interaction with the environment. 15−22 Here we report on resonant Raman scattering and infrared (IR) studies of phonons in CdS usNPs prepared by "wet" colloidal chemistry under mild conditions. We observe for the first time multiple Raman scattering by optical phonons up to the th...

The influence of ligand exchange for pyridine onto the structure and phonon spectra of oleic acid-stabilized CdSe nanocrystals (NCs) is studied by resonant Raman and optical absorption spectroscopy, nuclear magnetic resonance and transmission electron microscopy. The removal of oleic acid ligand by pyridine treatment results in change of intensity ratio of the longitudinal optical (LO) phonon peak to its overtones. The latter effect is attributed to a changed electron-phonon coupling in NCs upon introduction of the hole-capturing ligand (pyridine). The upward shift and broadening of the LO phonon peak are also observed and supposed to be the result of interplay between partial oxidation of the NC and strain induced by surface reconstruction. The relative contribution of these two effects is found to be dependent on the NC size. The activation of two additional Raman features, in the low-frequency range and above the LO band, for pyridine-treated NCs is supposed to be related with induced disorder or reconstruction on the NC surface. No noticeable effect of the surface treatment and concomitant NC aggregation onto the surface optical phonon mode was observed.

Epitaxial films and ordered arrays of submicron structures of nickel and cobalt ferrites were deposited on Nb doped SrTiO 3 by pulsed laser deposition. X-Ray diffraction and atomic force microscopy showed that the films have a good crystalline quality and smooth surfaces. A larger number of phonon bands was observed in the polarization dependent Raman spectra of the ferrite films than expected for the cubic spinel structures. This is explained by short range ordering of the Ni 2þ (or Co 2þ) and Fe 3þ cations at the octahedral sites inducing a lowering of the symmetry. The same behavior was also observed in the Raman spectra measured for the submicron structures, suggesting the same cation distribution as in the films. The diagonal components of the dielectric function for nickel and cobalt ferrites are determined from ellipsometry in the 0.73-5 eV photon energy range. The absorption edge was analyzed using a bandgap model and the energies for the indirect and direct optical transitions were calculated. It was found that both nickel and cobalt ferrites are indirect bandgap materials with bandgaps of 1.65 eV and 1.42 eV, respectively, while the first direct transitions lie at 2.69 eV and 1.95 eV, respectively. Magneto-optical Kerr effect spectroscopy in combination with spectroscopic ellipsometry allowed the off-diagonal elements of the dielectric tensor to be determined in the energy range from 1.7 eV to 5 eV. V

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Epitaxial BiFeO3 films pulsed laser deposited on SrTiO3, Nb:doped SrTiO3, and DyScO3 were studied using variable angle spectroscopic ellipsometry, vacuum ultraviolet ellipsometry, micro-Raman spectroscopy, and x-ray diffraction. The energy band gap of the film deposited on DyScO3 is 2.75 eV, while the one for the film deposited on Nb:doped SrTiO3 is larger by 50 meV. The blueshift in the dielectric function of the BiFeO3 films deposited on Nb:doped SrTiO3 compared to the films deposited on DyScO3, indicates a larger compressive strain in the films deposited on Nb:doped SrTiO3. This is confirmed by Raman spectroscopy and by high resolution x-ray diffraction investigations.