Microstructure and nanohardness of the diluted magnetic semiconducting Cd1−xMnxS nano-crystalline films

Cd_1−xMn_xS nano-crystalline films (0 ≤ x ≤ 0.5) were formed on glass substrates by thermal evaporation technique at room temperature (300 K). AFM studies showed that all the films were in nano-crystalline form with the grain size varying in the range between 36 and 58 nm and exhibited hexagonal structure of the host material. The lattice parameters varied linearly with composition, following Vegard's law in the entire composition range. The nanohardness and Young's modulus decreased sharply with ‘Mn’ content upto x = 0.3 and increased with high Mn content.     

Effect of incorporating nonlanthanoidal indium on optical properties of ferroelectric Bi4Ti3O12 thin films

Bi₄Ti₃O₁₂ (BTO) and Bi_3.25In_0.75Ti₃O₁₂ (BTO:In) thin films were prepared on fused quartz and LaNiO₃/Si (LNO) substrates by chemical solution deposition (CSD). Their microstructures, ferroelectric and optical properties were investigated by X-ray diffraction, scanning electron microscope, ferroelectric tester and UV-visible-NIR spectrophotometer, respectively. The optical band-gaps of the films were found to be 3.64 and 3.45 eV for the BTO and BTO:In films, respectively. Optical constants (refractive indexes and extinction coefficients) were determined from the optical transmittance spectra using the envelope method. Following the single electronic oscillator model, the single oscillator energy E₀, the dispersion energy E_d, the average interband oscillator wavelength λ₀, the average oscillator strength S₀, the refractive index dispersion parameter (E₀/S₀), the chemical bonding quantity β, and the long wavelength refractive index n_∞ were obtained and analyzed. Both the refractive index and extinction coefficient of the BTO:In films are smaller than those of the BTO films. Furthermore, the refractive index dispersion parameter (E₀/S₀) increases and the chemical bonding quantity β decreases in the BTO and BTO:In films compared with those of bulk.     

Ultra‐high long‐term stability of oxide‐TTFTs under current stress

In this letter the stability of transparent thin‐film transistors (TTFTs) based on the ZnO–SnO₂ (ZTO) material system is investigated. Bottom‐gate devices have been subject to electrical stress via a gate–source bias of 10 V and a drain‐source bias of 10 V leading to a drain–source current of 188 µA. In optimized TTFTs with a composition of [Zn]:[Sn] = 36:64 the relative change of the saturated field effect mobility was less than 1% and the threshold voltage shift was about 320 mV after 1000 hours of operation. This extraordinary stability of ZTO TTFTs underlines their suitability as drivers in active matrix OLED displays. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Towards ideal hexagonal self‐ordering of TiO2 nanotubes

The present work reports on key factors that influence the degree of order in anodic TiO₂ nanotube layers. We show that the anodization voltage and the Ti purity are of crucial importance for the ideality of self‐organization within the nanotube layers and that repeated anodization can significantly improve hexagonal ordering. Optimizing each factor significantly reduces the variation in the average pore diameter and strongly reduces the areal density of polygon ordering/packing errors. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Variable-range hopping in Fe70Pt30 catalyzed multi-walled carbon nanotubes film

Using low-pressure chemical vapour deposition (LPCVD), multi-walled carbon nanotubes (MWNTs) are grown on nanocrystalline Fe₇₀Pt₃₀ film. The Fe₇₀Pt₃₀ nanocrystalline film is deposited by vapour condensation technique. The size of the nanoparticles varies from 5–10 nm, as inferred from SEM micrographs of Fe₇₀Pt₃₀ film. SEM and TEM observations of as-grown CNTs film reveal that these are multi-walled and their diameter varies from 30–80 nm and length is of the order of several micrometers respectively. There is a structural change from ordinary geometry of CNTs to bamboo shaped as suggested by TEM image. Raman spectra shows sharp G and D bands with a higher intensity of G band showing the presence of graphitic nature of the nanotubes. An experimental study of the temperature dependence of electrical conductivity of MWNTs film is done over a wide temperature range from (293–4 K). The measured data gives a good fit to variable-range hopping (VRH) and the results are interpreted using Mott's (VRH) model. The conduction mechanism of the MWNTs film shows a crossover from the exp[ -(T_o/T)^1/4] law in the temperature range (293–110 K) to exp[ -(T_m/T)^1/3] in the low temperature range (110–4 K). This behaviour is attributed to temperature-induced transition from three-dimension (3D) to two-dimension (2D) VRH. Various Mott's parameters like characteristic temperature (T_m), density of states at Fermi level N(E_F), localization length (ξ), hopping distance (R), hopping energy (W) have also been calculated using above-mentioned model.     

Influence of the charge concentration on the density of states in a two-dimensional superconducting system

The gap and the density of states of high-T_c superconductors have been a subject of paramount interest. In order to explain the observed experimental behavior several pairing mechanisms in high-temperature superconductivity have been considered, by theoretical calculations. In this work, within the BCS scheme, a two-band model with energy band overlapping is introduced. The gap parameter and the density of states in a two-dimensional superconducting system are studied as functions of the charge concentration. This model is applied to Bi2212 in order to obtain numerical results.     

Growth behaviour of ZnO thin films and nanowires on SrTiO3 substrates

We report on comparative investigations of ZnO thin films and nanowires grown on SrTiO₃ (STO) single crystal substrates. Using pulsed laser deposition technique, we could grow ZnO thin films with ()- and (0001)-orientations on (100)- and (110)-orientated STO substrates, respectively. ZnO nanowires, grown by vapour condensation method with Au catalyst layers, did not show preferential alignment on either of the STO substrates. When the ZnO(0001)/STO(110) film was used as seed layer, we obtained dense and vertically aligned nanowires. Whereas, few and inclined nanowires were grown on the ZnO()/STO(100) film. We discuss possible origins to cause all the observations.     

Improved beamforming using curved sparse 2D arrays in ultrasound

In this work we have investigated the effect of curving phase-steered sparse periodic two-dimensional arrays in one direction, and relate this effect to the geometry of the arrays. We have shown that curving is equivalent to removing some of the element periodicity, thus adding some “randomness” to the layout. Compared to flat phase-steered periodically sparse two-dimensional arrays, curving offers an even greater suppression of grating lobes located at directions along the curvature. The class of arrays yielding improved performance due to this suppression of grating lobes has been characterized.The point spread functions of some previously proposed array layouts, shown to be promising for ultrasonic imaging, have been simulated. The arrays have been simulated with various number of elements as well as various focal points, with array and field parameters typical to those in volumetric cardiac imaging. On a 48 × 48 element grid with a transducer center frequency of 3 MHz and the target at 40 mm, reductions in the peak sidelobe level of up to 12 dB were recorded for some critical steering directions, without significant differences in the beamwidth. The integrated sidelobe ratio was also examined, showing an almost equivalent performance as the flat array. This study shows that, without adding any complexity to the system, the overall image quality of a volumetric imaging system can be improved significantly by curving the array in one direction.     

Implications of transient changes of optical and surface properties of solids during femtosecond laser pulse irradiation to the formation of laser-induced periodic surface structures

The formation of laser-induced periodic surface structures (LIPSS) upon irradiation of silicon wafer surfaces by linearly polarized Ti:sapphire femtosecond laser pulses (pulse duration 130 fs, central wavelength 800 nm) is studied experimentally and theoretically. In the experiments, so-called low-spatial frequency LIPSS (LSFL) were found with periods smaller than the laser wavelength and an orientation perpendicular to the polarization. The experimental results are analyzed by means of a new theoretical approach, which combines the widely accepted LIPSS theory of Sipe et al. with a Drude model, in order to account for transient (intra-pulse) changes of the optical properties of the irradiated materials. It is found that the LSFL formation is caused by the excitation of surface plasmon polaritons, SPPs, once the initially semiconducting material turns to a metallic state upon formation of a dense free-electron-plasma in the material and the subsequent interference between its electrical field with that of the incident laser beam resulting in a spatially modulated energy deposition at the surface. Moreover, the influence of the laser-excited carrier density and the role of the feedback upon the multi-pulse irradiation and its relation to the excitation of SPP in a grating-like surface structure is discussed.     

Microstructural characterization of Ti-C-N thin films prepared by reactive crossed beam pulsed laser deposition

In this work, Raman spectroscopy has been used to characterize Ti-C-N thin films in order to obtain information about the microstructure of the deposited materials, and in particular to study the effects due to the carbon incorporation into the TiN lattice. Ti-C-N thin films were prepared using a crossed plasma configuration in which the ablation of two different targets, titanium and carbon, in a reactive atmosphere was performed. With this configuration, the carbon content in the films was varied in an easy way from 5.0 at% to 40.0 at%. Thin film composition was determined from Non-Rutherford Backscattering Spectroscopy (NRBS) measurements. X-ray photoelectron spectroscopy and X-Ray diffraction measurements were also carried out in order to characterize the films in more detail, with this being used to give support to the interpretation of the Raman spectra. The Raman results revealed that at lower carbon concentrations a solid solution Ti(C, N) is formed, whilst at higher carbon concentrations a nanocomposite, consisting of nanocrystalline TiCN and TiC immersed in an amorphous carbon matrix is obtained.