Controllable synthesis,characterization and optical properties of Ag@AgCl coaxial core‐shell nanocables

Core‐shell structures often exhibit improved physical and chemical properties. Developing a relatively general, facile, and low temperature synthetic approach for core‐shell structures with complex compositions is still a particularly challenging work. Here we report a general chemical conversion route to prepare high quality Ag@AgCl coaxial core‐shell nanocables via the redox reaction between Ag nanowires and FeCl₃ in solution. The powder X‐ray diffraction of the Ag@AgCl coaxial core‐shell nanocables shows additional diffraction peaks corresponding to AgCl crystals apart from the signals from the Ag nanowire cores. Scanning electron microscopy and transmission electron microscopy images of the Ag@AgCl coaxial core‐shell nanocables reveal that the Ag nanowires are coated with AgCl nanoparticles. The effect of the molar ratio of Fe:Ag on the morphology and optical absorption of the Ag@AgCl coaxial core‐shell nanocables is systematically investigated. The result shows that the optical absorption of Ag nanowires decreases gradually and that of AgCl nanoparticles improves gradually with the increase of the molar ratio of Fe:Ag. The formation process of the Ag@AgCl coaxial core‐shell nanocables has been discussed in detail. The present chemical conversion approach is expected to be employed in a broad range of applications to fabricate innovative core‐shell structures with different compositions and shapes for unique properties. (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Photoacoustic studies on the thermal properties of the NLO compound: L‐alaninium maleate

Studies on thermal and optical properties are very essential for a nonlinear optical material. Photoacoustics (PA) is a recently developed non‐destructive testing (NDT) tool for studying such properties. L‐Alaninium maleate (C₃H₈NO₂⁺·C₄H₃O₄^‐), an organic non‐linear optical (NLO) material was synthesized using submerged seed solution method and was characterized by single crystal X‐ray diffraction (XRD) and density. PA technique was used to determine the thermal parameters of LAM crystals. Moreover, possible phase transition analysis was conducted to ascertain the thermal stability. Scanning electron microscopic (SEM) studies were also carried out and crystal‐packing features discussed. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Influence of the preparation route on the optical properties of dosimetric phosphors based on rare‐earth doped polycrystalline strontium borates

Most of commercial dosimetric phosphors consist of suitably doped polycrystalline compounds. Whereas the activation issue has been usually addressed, less attention has been paid to the influence of host preparation in the final perfomance of the products. In this paper different routes have been followed to synthesise phosphors based on rare‐earth doped strontium borates. The structural and optical properties of the materials have been given special consideration. Finally, thermoluminescence glow curves have been recorded in order to assess which of the preparation routes yields the most efficient compound. (© 2004 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Optical and mechanical properties of a new second order nonlinear optical crystal urea l‐threonine

A new second order non‐linear optical organic crystal Urea L‐threonine has been grown by aqueous solution slow cooling technique. The grown crystal is non hygroscopic and transparent in the visible region. The solubility of the grown crystal was found. The crystal structure and perfection were determined using powder XRD and High resolution XRD. The molecular structure was confirmed by ¹H‐NMR spectrum. Thermal properties of the crystal have been studied using thermogravimetric (TG) and differential thermal analysis (DTA). The dielectric response of the crystal with varying frequencies at different temperatures was studied. UV‐Visible spectrum was recorded to study the optical transparency of the grown crystals. The second order non linear optical property was examined by Kurtz powder technique and found that the second order susceptibilities is 1.2 KDP at wavelength 1064 nm. The mechanical behaviour was studied by Vickers Microhardness test. The results are discussed in detail. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Evolution of real structure in Ge‐Si mosaic crystals

The structure and properties of Czochralski (Cz)‐grown Ge_1‐xSi_x mosaic crystals were investigated using optical microscopy, atomic force microscopy, X‐ray diffraction analysis, microprobe analysis, FTIR and transmission electron microscopy. The role of segregation, form of solid‐liquid interface and dislocation generation in the development of mosaic structure were analyzed and used for optimization of growth parameters such as Si concentration and growth rate. The dislocation density estimated experimentally was compared with the calculated data. Composition fluctuations caused by formation of cellular structure at the interface lead to a local lattice misorientation that is one of the reasons for crystal mosaicity. Model of mosaic structure generation in terms of dislocation density and composition variations is presented.     

Three‐step process improves crystal quality of Cu2ZnSnS4 absorber layer and efficiency of solar cell

Cu₂ZnSnS₄ (CZTS) films are fabricated using the three‐step process, including deposition, preheating and sulfurization of Cu–Zn–Sn (CZT) precursors. The effect of preheating temperature on structures, morphologies, and optical properties of CZTS films is investigated detailedly by X‐ray diffraction, Raman spectra, scanning electron microscopy, and UV spectrophotometer. It is found that the proper preheating temperature can improve the crystal quality of CZTS films. The prepared CZTS film by sulfurizing the preheated precursor at 300 °C presents high crystallinity, uniform surface morphology, and suitable optical properties. Compared with two‐step process, three‐step method can not only improve crystal quality of CZTS films but also decline sulfurization temperature. We also discuss the mechanism of improving the properties of CZTS absorber layer by the preheating route in detail. In addition, the experimental results also indicate that solar cell prepared by three‐step method displays higher conversion efficiency.     

Synthesis of radial‐like ZnO structure by hydrothermal method with ZnSO4·7H2O and Zn(CH3COO)2·2H2O as zinc sources

Radial‐like ZnO structures were prepared using zinc sulfate (ZnSO₄·7H₂O) and zinc acetate [Zn(CH₃COO)₂·2H₂O] as zinc sources by a facile template‐free hydrothermal method in this paper. Structural and optical properties of radial‐like ZnO structures are characterized by X‐ray diffraction (XRD), field emission scanning electron microscopy (FESEM), UV‐vis spectrophotometer and photoluminescence measurement (PL). It has been found that the distinct surface morphologies of radial‐like ZnO structures grown by different zinc sources. Slim radial‐like ZnO with a hexagonal wurtzite structure is grown by using ZnSO₄·7H₂O as zinc sources, whereas coarse radial‐like ZnO with zincite structure is achieved by zinc acetate. The UV‐vis absorption spectra of them both display an obvious and significant absorption in the ultraviolet region. The room temperature PL spectra of ZnO structures grown by two different zinc sources possess a common feature that consists of a strong ultraviolet (UV) peak and visible emission band.     

Influence of Non‐Stoichiometric Defects on Optical Properties in LiNbO3

We present a band structure approach with a molecular dynamics cluster optimization which accounts for the various structural modifications related to the non‐stoichiometry of LiNbO₃ crystals. The variation of the optical properties with the deviation from the stoichiometric composition can be understood within this approach. Particular role of the electron‐phonon contributions to the electrooptics coefficient is shown. Model calculations yield a large dependence of the electrooptis coefficient r₂₂ on the crystal composition, in agreement with the experimental data. The observed minimum of the r₂₂ coefficient versus the non‐stoichiometry is interpreted as originated from the non‐centrosymmetry in the electrostatic potential distribution around Nb‐O₆ clusters.     

Nd(III) and Yb(III) ions incorporated in Y4Al2O9 obtained by sol‐gel method: synthesis,structure, crystals and luminescence

The nanocrystalline powders of Y₄Al₂O₉ (YAM) pure and doped by Nd, Yb and codoped by Nd and Yb were obtained via modified sol‐gel method. These powders were characterized by X‐ray diffraction method, scanning electron microscopy and high resolution scanning electron microscopy, luminescence spectroscopy and differential thermal analysis (DTA). We obtained single phase powders of crystalline structure with average size 70 nm exhibiting interesting luminescent properties. Efficient non‐radiative energy transfer between Nd and Yb was found. DTA confirmed the phase transition at about 1400 °C. From these nanocrystalline powders, the crystals YAM:Yb, YAM:Yb,Nd were grown by micro‐pulling down technique. They were cracking during cooling owing to the phase transition. Luminescent properties of YAM:Nd,Yb crystals were identical with properties of corresponding nanopowders within experimental incertitude. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Effect of different metal ions on structural,thermal, spectroscopic and optical properties of ATCC and ATMC single crystals

A novel metal‐organic coordination complex nonlinear optical crystals, tri‐allylthiourea cadmium chloride [(CdCl₂(AT)₃] and tri‐allylthiourea mercury chloride [(HgCl₂(AT)₃] abbreviated as ATCC, ATMC (AT is Allylthiourea i.e.,CH₂=CHCH₂NHCSNH₂) has been synthesized and grown as single crystals. It was synthesized in deionised water and further recrystallized to improve its purity. Single crystals of the allylthiourea co‐ordination complex nonlinear optical crystals tri allylthiourea cadmium chloride (ATCC) with dimensions of 14x14x10 mm³ and tri allylthiourea mercury chloride (ATMC) with dimensions of 15x15x12 mm³ were grown successfully from aqueous solution by solvent evaporation as well as by temperature lowering method. It exhibits powder SHG efficiencies higher than that of a well known organic NLO crystal Urea. The solubility of the as grown crystals was estimated from the aqueous solution and the effect of different metal ions on the grown crystals, structural, thermal, spectral and optical properties were analyzed. XRD studies the reveals the same structure of both materials. Influence of the different central metal (Cd and Hg) atoms, changing the thermal properties of the materials when NLO complexes formed with the common ligand allylthiourea. The metal co‐ordination was confirmed form the spectroscopic analysis. From the UV transmittance studies, red shift was from the transparency cut‐off wavelengths. The value is 285nm for ATCC is and is 335nm ATMC, Non‐linear an optical study confirms the suitabilities of the as grown crystals for the non linear optical applications. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

X‐Ray Diffuse Scattering on Self‐Organized Mesoscopic Structures

We highlight the great potential of x‐ray diffuse scattering for the characterization of mesoscopic structures. The mesoscopic length scale plays a fundamental role at semiconductors, since structures with spatial dimensions in the nanometer regime exhibit quantum size effects. Also the driving forces during epitaxial growth of these structures are most relevant at mesoscopic length scales and they may lead to self‐organization processes in that structures can form spontaneously during epitaxial growth. One of the most interesting features are quantum dots (QDs) since they exhibit zero‐dimensional electronic properties and thus show an impressive potential for applications in optoelectronic devices. X‐ray diffuse scattering is an excellent tool to investigate shape, size, and positional correlation and to characterize the strain field inside and in the vicinity of such self‐organized mesoscopic structures. It provides non‐destructive structural information over the entire mesoscopic length scale. X‐ray scattering is complementary to direct imaging techniques in that it gives a statistical average over large ensembles with high strain sensitivity. The application of different scattering techniques for the structural characterization of mesoscopic structures will be demonstrated. For the example of SiGe islands on Si (001) substrate it will be shown that x‐ray diffuse scattering is able to reveal details of germanium distribution in such islands. In a second example the investigation of the asymmetric shape and size of InP QDs that are grown on (001) In_0.48Ga_0.52 P/GaAs is described. The influence of positional correlation on the diffuse scattering will be discussed.     

Structural and optical properties of vapour‐etching based porous silicon

The structural and optical properties of porous silicon (PS) layers prepared by Vapour‐Etching (VE) of moderately and heavily boron‐doped Si substrates are investigated. The VE technique produces rough PS layers that are essentially formed of interconnected cluster‐like structures. Optical investigations indicate that this surface roughness enables the PS layers to be used as antireflection coatings in silicon based devices. These optical characteristics are investigated by optical reflectivity and light scattering. The local chemical state and the microstructure of the PS layers are studied by electron energy loss spectroscopy (EELS) in transmission electron microscopy (TEM), and are correlated to the red photoluminescence (PL). TEM studies point out that the cluster‐like interconnected structures are composed of luminescent nanocrystallites. PL measurements display that both quantum confinement and surface passivation determine the electronic states of the silicon nanocrystallites. The complex dielectric function is calculated from the experimental single‐scattering distribution spectrum using a Kramers Kronig analysis. The first resonance peak in the imaginary part is observed at 2.3 eV; two other broadened features appear at 4.7 and 8.8 eV. The latter is generally related to an interface plasmon resulting from the silicon‐silicon oxide interface. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Photoacoustic study of the thermal and optical properties of hippuric acid

Photoacoustic spectroscopy (PAS) is a recently developed, non‐destructive testing (NDT) tool, used for analyzing the physical properties of materials. This method is used here to determine the thermal diffusivity and conductivity of the gel‐grown single crystals of hippuric acid, a minor constituent of urinary calculi and also a non‐linear optical (NLO) material. Optical absorption of the specimen was studied using its PA spectrum and compared with UV‐visible absorption spectra. Second harmonic generation (SHG) conversion efficiency was also carried out. Characterization of the crystals was made using X‐ray powder diffraction and density determination. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Nonlinear optical properties of ZnS nanocrystals incorporated within the polyvinyl alcohole photopolymer matrices

Photoinduced nonlinear optical effects in ZnS nanocrystallites incorporated within the photopolymer polyvinyl alcohol matrices were studied. As a basic method we have used photoinduced optical second harmonic generation (SHG). The Q‐switched HF‐laser (λ = 2.64.μm; pulse duration 30‐40 ps) was used as a source of probing (fundamental) light beam and the pulsed nitrogen (λ = 337.7 nm) was applied as a source of the photoinducing pump light. With increasing photoinducing beam power, the output SHG signal (at λ = 1.32 μm) increases. The maximal second‐order non‐linear optical susceptibility corresponds to tensor component d₁₂₃ = d₁₄ = 3.8 ± 0.2 pm/V at a photoinducing power density equal to about 1.25 GW/cm². With decreasing temperature, the SHG signal increases within the temperature range 25‐30 K. Time‐dependent pump‐probe measurements of the SHG indicate an existence of the SHG maximum for a pump‐probe delaying time 20 ps. The ZnS hexagonal structural components play a crucial role in the observed photoinduced second‐order non‐linear optical effects. Large values of the non‐linear optical constants as well the good technological parameters open a possibility to enhance the non‐linear optical susceptibilities of the investigated ZnS nanocrystallites.     

Solution Growth and Characterisation of L‐alanine Single Crystals

Single crystals of L‐alanine have been grown from buffered aqueous solutions and characterised as to their optical quality via wavefront distortion analysis, electrooptical response and harmonic generation efficiency. Refraction indices as well as phase matching loci were found in satisfactory agreement with previously published data for crystals grown in non buffered solution. Estimates for the electro‐optical response are reported for the first time. The experimentally observed crystal habit is discussed in terms of morphological importance (M.I.), which was found to be in disagreement with the predictions of crystallographic criteria based on the interplanar distance d_hkl, on the periodic bond chains (PBC) and the attachment energy E_att. This disagreement is tentatively attributed to the presence of impurities and non‐appropriate supersaturation conditions.     

Structural,microchemical and superconducting properties of ultrathin NbN films on silicon

Structural, chemical and superconducting properties of thin NbN films used for development of fast and sensitive hot‐electron bolometer (HEB) detectors for wide spectra range are reported. The thin NbN films with a thickness between 4 and 10 nm were deposited on the (001)Si substrates by magnetron sputtering. In order to investigate the film morphology and microchemistry, diffraction‐contrast and high‐resolution transmission electron microscopy (TEM) in combination with scanning TEM and electron energy loss spectroscopy (EELS) were performed. In addition, the zero‐resistance critical temperature of the NbN films was measured and correlated to their thickness. The interrelations between fabrication conditions, crystalline and superconducting properties of the differently thick NbN films are discussed. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Synthesis,growth and characterization of l‐alaninium oxalate ‐ a novel organic NLO crystal

Nonlinear optical material of L‐alaninium oxalate (LAO) has been synthesized and single crystals are grown by slow evaporation technique. The solubility data of LAO is determined in water. Single crystal X‐ray diffraction study showed that LAO belongs to orthorhombic system with a non‐centrosymmetric space‐group P2₁2₁2₁. The functional groups have been identified from FT‐IR spectrum. The UV‐Vis‐NIR spectrum of LAO shows less optical absorption in the entire visible region. The second harmonic generation (SHG) in the material was estimated using Nd:YAG laser. The mechanical properties of the grown crystals are studied using Vickers microhardness tester. The AC and DC conductivity, and dielectric studies are also carried out and reported for the first time. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Channeling‐enhanced EDX for polarity resolved crystal orientation determination

Using channeling‐enhanced energy‐dispersive X‐ray spectroscopy (EDX), we demonstrate polarity sensitive orientation determination of a non‐centrosymmetric crystal in the scanning electron microscope. The authors observe a characteristic asymmetry in the channeling‐enhanced, angle‐dependent EDX data of a GaAs sample, which is in good agreement to simulations using the dynamical theory of diffraction for the incident electron beam. This allows us to assign the orientation of the GaAs crystal according to the non‐centrosymmetric point group. The method shown here overcomes the limitation of a reduced point‐group sensitivity of electron backscatter diffraction patterns and electron channeling patterns for crystalline phases that contain atoms of approximately equal electron scattering cross sections.     

Cobalt phthalocyanine nanowires: Growth,crystal structure,and optical properties

Cobalt phthalocyanine nanowires with new crystal structure and broad optical absorption spectra were fabricated by using organic vapor phase deposition method. The morphology, crystal structure and optical properties of CoPc nanowires were characterized by SEM, X‐ray diffraction, Fourier transform infrared and UV‐visible spectroscopies. Analyses of X‐ray diffraction patterns and Fourier transform infrared spectra indicate that the crystal structure of CoPc nanowires represents a new polymorph, which is designated J‐CoPc. These J‐CoPc nanowires with high directionality (average diameter ∼50 nm) surprisingly possess much broader optical absorption spectra in the visible spectral region than those of the α‐ and β‐phase CoPc, enabling high potential for practical applications in novel molecular electronic/optical devices.     

Crystal structure of the lead bromo-borate Pb<Subscript>2</Subscript>[B<Subscript>5</Subscript>O<Subscript>9</Subscript>]Br from precision single-crystal X-ray diffraction data and the problem of optical nonlinearity of hilgardites

The structural features responsible for the high nonlinear optical activity of crystals of the hilgardite-like anhydrous bromo-borate Pb₂[B₅O₉]Br (space group Pnn2) are analyzed with the use of data obtained from two (precision and conventional) single-crystal X-ray diffraction experiments. The electron-density peaks associated with the stereochemically active lone electron pair are located in free space at two equally probable positions in the vicinity of the Pb(2) atom on the side of the two most distant atoms, Br(1) and Br(2). The stereochemical activity of two nonequivalent lead atoms in the crystal structure increases upon changing over from the Pb(1) atom to the Pb(2) atom. This is in agreement with the behavior of the lone electron pairs in the hilgardite-like hydrated borate Na_0.5Pb₂[B₅O₉](OH)_1.5 · 0.5H₂O and another nonlinear optical borate, namely, Pb₂[B₄O₅(OH)₄](OH)₂ · H₂O, which is related to the compound BiB₃O₆. The most pronounced nonlinear optical properties of the lead bromo-borate Pb₂[B₅O₉]Br as compared to other orthorhombic hilgardites and lead borates are associated with the presence of highly polarized Pb-Br bonds. In this case, the electron density of the lone electron pair of the Pb(2) atom enhances the polarization effect.