The Ag+ induced solution–liquid–solid growth, photoluminescence and photocatalytic activity of twinned ZnSe nanowires

Cubic ZnSe nanowires with periodically alternating twins along the wire growth direction are synthesized in the ZnCl₂–Na₂SeO₃–AgNO₃–ethylenediamine (EN)-ethylene glycol (EG)-polyvinyl–pyrrolidone (PVP) solvothermal system at 180°C for 12 h. The twinned ZnSe nanowires have diameters of 75±10 nm and lengths of >10 micrometers, and grow along 〈111〉 direction. The role of AgNO₃ in the formation of ZnSe nanowires was investigated, and an Ag⁺ induced solution–liquid–solid growth mechanism is also proposed to account for the conversion of microspheres assembled from ZnSe nanocrystallites into ZnSe nanowires. Compared with ZnSe microspheres, the as-prepared twinned ZnSe nanowires exhibit stronger band edge emissions of the wurtzite- and zinc-blende-structured ZnSe and lower deep defect related emission, and their photocatalytic ability is weaker than that of ZnSe microspheres. The results suggest that this simple, mild, one-step solution approach to fabricate ZnSe nanowires may be employed for the synthesis of other selenium compounds with one dimensional nanostructures, and provides opportunities for both fundamental research and technological applications.     

Preparation and rapid thermal annealing of AlN thin films grown by molecular beam epitaxy

AlN films were grown at 785 ^°C on (0001) sapphire substrates by radio-frequency assisted molecular beam epitaxy. Post-growth rapid thermal annealing (RTA) was carried out from 900 to 1200 ^°C for 10 s in flowing N₂. The morphological and structural properties of the AlN epilayers before and after the RTA were studied by atomic force microscopy, x-ray diffraction and transmission electron microscopy. It is found that the threading dislocations can be decreased to an order of magnitude by using an interlayer growth method. The surface roughness (RMS) of the AlN thin films becomes larger with the increase of annealing temperature. The full width at half maximum of AlN (0002) rocking curve reaches its minimum after the RTA at 1000 ^°C.     

Synchrotron-based investigation of iron precipitation in multicrystalline silicon

We report on investigations of the precipitation of iron in block-cast multicrystalline silicon using the techniques of X-ray beam induced current, X-ray fluorescence microscopy and X-ray absorption microspectroscopy. The samples studied were intentionally contaminated with iron and annealed at temperatures between 850 and 1050 ^°C. Annealing at 950 ^°C was found to lead to well detectable iron precipitation inside the grains and at grain boundaries. Small only iron clusters were detected after the 850 ^°C anneal while no iron clusters were found after the 1050 ^°C treatment. X-ray absorption near edge structure analyses of the iron clusters revealed mostly iron silicide and in one case iron oxide. Under the given condition at the beamline, the detection sensitivity for iron was estimated to be 4×10⁷ atoms, corresponding to a spherical FeSi₂ particle of 40 nm radius.     

The conversion of wettability in transparent conducting Al-doped ZnO thin film

We report on the systematic changes of surface wettability in one of the most promising transparent conducting oxide materials, Al-doped ZnO (AZO) thin films. It was revealed that the characteristic surface wettability, which would make a key role in adhesion with other layers of optoelectronic device, can be largely changed by Al concentrations and film growth temperature. Keeping the electrical conductivity constant, the water contact angle (WCA) of a 2 mol% AZO film was changed by about 50 ^°C depending on the surface roughness. In the samples grown at 300 ^°C, the roughness enhancement was large and a hydrophobic surface formed, whereas in the samples grown at 500 ^°C a hydrophilic surface formed. We attributed the variation in surface wettability with growth temperature to changes in surface morphology. This result suggests that 2 mol% Al doping concentration can be considered as a critical concentration in changing of surface morphology of AZO as well as in electrical properties.     

Emission of Si nanoclusters of different phases in amorphous hydrogenated silicon

This paper presents the results of PL spectrum studies for Si nano-clusters in an amorphous silicon matrix. The four amorphous Si layers were prepared by the hot-wire CVD method on glass substrates at a temperature of 250 ^°C and different filament temperatures in the range of 1650–1950 ^°C. The joint analysis of PL and X ray diffraction results dependant on technological conditions has been done. PL bands deal with Si nanocrystals and amorphous Si nanoclusters are discussed as well.     

Graphitization of activated carbon under high pressures and high temperatures

Activated carbon was treated at 5.0 GPa up to 1600 ^°C and the structural evolution in the graphitization process was investigated. The graphitization temperature is reduced to 1200 ^°C at 5 GPa, as reflected by x-ray diffraction patterns. Honeycomb-like structures come into being in the high-pressure sintering temperature range of 1000–1100 ^°C and slice-like structures appear after graphitization. Raman frequency and half width drop drastically near the graphitization temperature, and the appearance of D and D′ lines indicates there are still some disorder structures in the graphitized activated carbon.     

ZnO nanoparticles with controlled shapes and sizes prepared using a simple polyol synthesis

Nanocrystalline zinc oxide (ZnO) particles with controlled shapes and sizes were prepared at 180 ^°C by a simple polyol method. The amount of water and the method of addition played an important role in determining the characteristics of the synthesized particles. Rod-shaped ZnO particles with major axis lengths of ∼114 nm were obtained by heating the precursor solution, while equiaxial particles with average diameters of ∼24 nm were prepared by injecting water into hot precursor solution. Increasing the amount of water added to the precursor solution enlarged the aspect ratio of the rod-shaped particles and increased the particle size of the equiaxial particles due to enhanced hydrolysis and condensation of the Zn ion complex.     

Low-temperature direct wafer bonding of GaAs/InP

An approach for low-temperature direct wafer bonding of GaAs/InP was presented. The bonding procedure was carried out at temperatures from 350 to 500 ^°C, and the bonded n-GaAs/n-InP specimens were obtained even at a temperature as low as 350 ^°C. The compositional profile on the GaAs/InP heterointerface was studied by X-ray photoelectron spectroscopy. The bonded interfacial properties were also characterized by current–voltage (I–V$I\text{–}V$) and bonding strength measurement. The experimental results revealed an InGaAsP (or/and InGaAs) interlayer formed at the bonded interface, which influences the electrical property as well as the bonding strength. For the specimen bonded at 350 ^°C, the transport of major carriers could be explained by a tunneling effect. But the carrier transport was described by the thermionic emission theory for the specimen bonded at 450 ^°C. Finally, the mechanism of GaAs/InP bonding was discussed.     

Nanoindentation of vertical ZnO nanowires

We report the experimental observations of buckling instabilities of vertical well-aligned single-crystal ZnO nanowires prepared on ZnO:Ga/glass templates. It was found that critical buckling load and buckling energy of the ZnO nanowires were 215 μN and 3.69×10⁻¹¹ J, respectively. It was also found that Young's modulus of the ZnO nanowires were 232 and 454 GPa while critical buckling strains were 0.35% and 0.18% when fixed–fixed column mode and fixed–pinned column mode were used, respectively.     

The influence of annealing temperature on the structure,morphologies and optical properties of ZnO nanoparticles

ZnO nanoparticles (NPs) have been successfully synthesized by the simple solution method at low temperature. The effects of annealing temperature on the structure and optical properties of ZnO NPs were investigated in detail by X-ray diffraction, transmission electron microscopy (TEM), ultraviolet–visible (UV–vis) spectroscopy and photoluminescence (PL) measurements. As the annealing temperature was increased above 180 ^°C the particles morphology evolved from spherical to hexagonal shape, indicating that the average particle size increased from 11 nm to 87 nm. The UV-vis and PL spectra showed a red-shift from 3.62 to 3.33 eV when the annealing temperature was increased.     

The photoluminescence properties of TiO2-sheathed ZnSe nanowires

ZnSe nanowires have been synthesized by thermal evaporation of ZnSe powders on gold-coated Al₂O₃(0 0 0 1) substrates and then sheathed with TiO₂ by sputtering. Our results show that sheathing Zn nanowires with thin TiO₂ layers can significantly enhance the photoluminescence (PL) emission intensity. XPS analysis results suggest that the PL enhancement is attributed to increases in the concentrations of deep levels such as oxygen and titanium interstitials as well as the density of interface states. The PL emission of ZnSe nanowires is also enhanced by thermal annealing. Annealing in an argon atmosphere is more efficient in enhancing the PL emission than annealing in an oxygen atmosphere.     

Synthesis of multi-wall carbon nanotubes by the pyrolysis of ethanol on Fe/MCM-41 mesoporous molecular sieves

Ordered hexagonal arrangement MCM-41 mesoporous molecular sieves were synthesized by the traditional hydrothermal method, and Fe-loaded MCM-41 mesoporous molecular sieves (Fe/MCM-41) were prepared by the wet impregnation method. Their mesoporous structures were testified by X-ray diffraction (XRD) and the N₂ physical adsorption technique. Carbon nanotubes (CNTs) were synthesized by the chemical vapor deposition (CVD) method via the pyrolysis of ethanol at atmospheric pressure using Fe/MCM-41 as a catalytic template. The effect of different reaction temperatures ranging from 600 to 800 ^°C on the formation of CNTs was investigated. The resulting carbon materials were characterized by various physicochemical techniques such as transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HRTEM), and Raman spectroscopy. The results show that multi-wall carbon nanotubes (MWCNTs) with an internal diameter of ca. 7.7 nm and an external diameter of ca. 16.9 nm were successfully obtained by the pyrolysis of ethanol at 800 ^°C utilizing Fe/MCM-41 as a catalytic template.     

Growth and high-temperature electromechanical properties of Ca3NbX3Si2O14 (X=Ga and Al) piezoelectric crystals

Piezoelectric single crystals of Ca₃NbX₃Si₂O₁₄ (CNXS, X=Ga and Al) with ordered langasite structure were successfully grown using the Czochralski technique. The structure was analyzed by X-ray powder diffraction, and the lattice parameters for CNAS were found to decrease slightly when compared to CNGS, due to the smaller ion radius of Al. The dielectric, piezoelectric and electromechanical properties were studied as function of temperature from 30 ^°C to 900 ^°C, showing a stable temperature-dependent behavior. Of particular significant is their high mechanical quality factor and electrical resistivity at elevated temperature, demonstrating CNXS crystals to be promising candidates for high-temperature applications.     

AlGaN/GaN heterostructure grown on 1 -tilt sapphire substrate by MOCVD

AlGaN/GaN epitaxial layers were grown on 0^°-tilt and 1^°-tilt sapphire substrates by metalorganic chemical vapor deposition (MOCVD). With exactly the same growth conditions, it was found that dislocation density was smaller and crystal quality was better for the AlGaN/GaN epitaxial layers prepared on 1^°-tilt sapphire substrate. We also found that AlGaN/GaN epitaxial layers on 1^°-tilt sapphire substrate were grown with step growth mode while those on 0^°-tilt substrate were grown with two-dimensional island growth. From the temperature-dependent mobility, it was found that crystal quality of the AlGaN/GaN epitaxial layer prepared on 1^°-tilt sapphire substrate is better.     

The roles of zinc selenide sandwiched between organic layers and its applications in white light-emitting diodes

In this paper, the roles of zinc selenide (ZnSe) sandwiched between organic layers, i.e. organic/ZnSe/aluminum quinoline (Alq₃), have been studied by varying device structure. A broad band emission was observed from ITO/poly(N-vinylcarbazole)(PVK)(80 nm)/ZnSe(120 nm)/ Alq₃(15 nm)/Al under electric fields and it combined the emissions from the bulk of PVK, ZnSe and Alq₃, however, emission from only Alq₃ was observed from trilayer device ITO/N,N^′-bis-(1-naphthyl)-N,N^′-diphenyl-1, 1^′-biphenyl-4, 4^′-diamine (NPB) (40 nm)/ZnSe(120 nm)/ Alq₃(15 nm)/Al. Consequently the luminescence mechanism in the ZnSe layer is suggested to be charge carrier injection and recombination. By thermal co-evaporating Alq₃ and 4-(dicyanomethylene)-2-t-butyl-6-(1,1,7,7-tetramethyljulolidyl-9-enyl)-4H-pyran (DCJTB), we get white light emission with a Commission Internationale de l’E clairage (C.I.E) co-ordinates of (0.32, 0.38) from device ITO/PVK(80 nm)/ZnSe(120 nm)/ Alq₃:DCJTB(0.5 wt% DCJTB)(15 nm)/Al at 15 V and the device performs stably with increasing applied voltages.     

Fabrication of cosputtered Zn–In–Sn–O films and their applications to organic light-emitting diodes

Zn–In–Sn–O (ZITO) films have been grown by rf magnetron cosputtering system from ceramic oxide targets of ZnO and ITO onto glass substrate. X-ray diffraction analysis shows that the microstructure is amorphous below the substrate temperature of 250 ^°C. The films exhibit sheet resistance as low as 16.7 Ω/□ and optical transparency comparable to grater than that of Sn-doped indium oxide (ITO) films. The work function ranged 5.05–5.19 eV, which is a higher work function compared to ITO (4.7 eV). The fabricated ZITO films are used in fabrication of organic light-emitting diodes (OLEDs). The ZITO anode with the zinc content of 12.5 at.% [Zn/(Zn+In+Sn)] fabricated at 250 ^°C-based OLED shows lower turn-on voltage and higher current density compared to that of ITO-based control device.     

Synthesis,phase composition,and magnetic properties of iron nanowires prepared in the pores of polymer track-etched membranes

Arrays of iron nanowires prepared by the method of galvanic filling of polymer track-etched membrane pores (matrix synthesis) under different electrolysis modes and electrolyte temperatures have been studied. The conditions of the synthesis have been analyzed. The optimal composition and electrolyte temperature have been found. The phase composition and magnetic properties of nanowires have been studied using the methods of electron microscopy, X-ray diffraction, elemental energy-dispersive microanalysis, and Mössbauer spectroscopy. The average nanowire diameter is 100–200 nm. The length varies from 6 to 10 μm. The surface density is ～10⁸ cm^?2 at the average distance to each other of about 1 μm. It has been established that the basis of nanowires is formed by the metal iron nanocomposite that manifests the magnetic properties of bulk α-Fe. It has been found that the preferred orientation of the magnetization inside the iron nanowires arises for an array prepared at a potential of ?750 mV.     

Fabrication and magnetic properties of ordered Fe60Pb40 nanowire arrays electrodeposited in AAO templates

Ordered ferromagnetic-nonmagnetic heterogeneous Fe₆₀Pb₄₀ nanowire arrays were successfully fabricated by alternating current (AC) electrodeposition into nanoporous alumina templates. Transmission electron microscopy (TEM) image and selected-area diffraction (SAED) pattern analysis showed that the Fe₆₀Pb₄₀ nanowires are polycrystalline with an average diameter of 22 nm and lengths up to several micrometers. X-ray diffraction (XRD) observations indicated that α-Fe and fcc Pb phase coexist and do not form metastable alloy phase. The as-deposited samples were annealed at 200, 300, 400 and 500 ^°C, respectively. Magnetic measurements showed that nanowires have high magnetic anisotropy with their easy axis parallel to the nanowire arrays, and the coercivity of the samples increased with the annealing temperature up to 400 ^°C and reached a maximum (2650 Oe). The change of magnetic properties associated with the microstructure was discussed.     

Simulation of nonlinear optical absorption in ZnSe:Co crystals

The paper presents theoretical approach to simulation of nonlinear optical absorption in zinc selenide crystals doped with cobalt (II) ions (ZnSe:Co²⁺), which was reported by us earlier (Opt. Laser Technology, V. 35, (2003), 169). We used ZnSe:Co²⁺ crystals as saturable absorbers for generation of giant-pulse eye-safe laser radiation. It was found that minimal optical losses (maximal final transmission) occurred for ZnSe samples containing 1.6×10¹⁹ cm⁻³ of Co²⁺ ions. Band structure and photoinduced molecular dynamics simulations were performed to explain the parabolic dependence of optical losses versus Co²⁺ concentration. The minimum was shown to be the result of photoinduced anharmonic electron–phonon interaction.     

Electrical activities of stacking faults and partial dislocations in 4H-SiC homoepitaxial films

The electrical activities of stacking faults (SFs) and partial dislocations in 4H-SiC homoepitaxial films were investigated by using the electron-beam-induced current (EBIC) technique. The basal plane dislocation was dissociated into Si_(g) 30^° and C_(g) 30^° partials under electron-beam irradiation, with a SF formed in between. The SF shows bright contrast at RT and dark contrast at 50 K in EBIC images. The reasons were discussed according to the quantum-well state of SF. C_(g) 30^° partial is always more electrically active than Si_(g) 30^° partial at each specific accelerating voltage. The EBIC contrasts of those two partials were discussed with the number of recombination centers.