Pressure-induced transition-temperature reduction in ZnS nanoparticles

The study of the structural transition in nanoscale materials is of particular interest for their potential applications. In the present study, we have observed a lower temperature T = 250?°C for the phase transition from the sphalerite structure to the wurtzite structure in ZnS nanoparticles under a pressure of 1?GPa, as compared to those, T = 400 and 1020?°C, for ZnS nanoparticles and bulk ZnS under normal pressure, respectively. The reduced transition temperature is attributed to the applied pressure leading to tight particle-particle contacts, which change the surface (or interfacial) environment of the nanoparticles and thus their surface (or interfacial) energy.     

Formation of strain-free GaAs-on-Si structures by annealing under ultrahigh pressure

Theoretical and experimental discussions on a novel method to solve the thermal mismatch problem in heteroepitaxial growth have been reviewed. It has been predicted theoretically for structures such as Ge/Si and GaAs/Si that the difference in thermal expansion coefficients can be compensated for by the elastic strain generated by hydrostatic pressure. This theoretical prediction has been verified experimentally using GaAs-on-Si structures, in which the structures are formed by metalorganic chemical vapor deposition and subsequently annealed under ultrahigh pressure. It has been found that for annealing at pressures up to 2.1 GPa, the strain in GaAs films decreases linearly with increasing pressure and becomes zero at a pressure of around 1.9 GPa. It has also been found that the strain depends weakly on the annealing temperature, which ranged from 300 to 500 °C. Concerning the crystalline quality of the annealed GaAs films, a slight increase in the minimum channeling yield in Rutherford backscattering spectrometry has been observed in the samples with broad-area GaAs films. It has been found, however, that degradation in the crystalline quality can be avoided by etching the GaAs films in a pattern of stripes 10 μm wide.     

Effect of annealing on the structure and magnetic property of Zn0.9Fe0.1S nanoparticles

Zn_0.9Fe_0.1S nanoparticles were synthesized in microemulsion and then annealed in nitrogen atmosphere at different temperatures. The XRD results show that the samples annealed below 300 °C are pure cubic ZnS. Higher annealing temperature introduces the Fe₃O₄ phase. The X-ray absorbing fine structure (XAFS) results indicate that annealing induces the Fe ions to change from divalence to trivalence and crystalline quality gradually improves with the increase of annealing temperature. In view of the structural characteristics indicated by XRD and XAFS, the origin of ferromagnetism at low annealing temperature may be attributed to trivalent Fe ions.     

Effect of Nb and Sc doping on the phase transformation of sol-gel processed TiO2 nanoparticles

Nb and Sc doped TiO2 nanoparticles were synthesized via sol-gel technique. Dopant concentration of each element was varied from 0.5 to 1.5 atomic%. The effect of metal ion doping and calcination temperatures on anatase to rutile phase transformation has been investigated. Samples were analyzed by various analytical methods such as X-ray diffraction (XRD), Transmission Electron Microscope (TEM), X-ray Photoelectron Spectroscopy (XPS) and Energy Dispersive X-ray Spectroscopy (EDS). XRD analyses showed that Nb and Sc doped samples calcined at 300 degrees C and 350 degrees C, respectively, were crystalline and had an anatase structure. Results showed that anatase was stable up to 700 degrees C annealing temperature for samples doped with 0.5 atomic% Nb. There was a sharp transition from anatase to rutile phase above 700 degrees C and complete rutile structure was obtained at 750 degrees C. However, the transformation from anatase to rutile was not so sharp in samples doped with 1.0 atomic% and 1.5 atomic% Nb. Results indicated that higher concentration of Nb helps to stabilize the anatase phase. For samples doped with 0.5 atomic% Sc, anatase phase is stable up to 650 degrees C. Transformation from anatase to rutile starts at temperature above 650 degrees C and 100% rutile phase was obtained at 800 degrees C while for samples doped with 1.0 atomic% and 1.5 atomic% Sc, the complete transformation from anatase to rutile takes place at an even higher temperature. Results indicate that increasing the calcination time from 0.5 to 2.0 hours at 500 degrees C does not affect the stability of anatase phase. However, TEM and XRD data showed that the increase in the annealing time leads to an increase in particles size. The rutile to anatase concentration ratio increased with temperature above the phase transformation temperature. The activation energy for the phase transformation from anatase to rutile for doped and undoped samples was also measured. There was a general rise in the activation energy with increasing dopant concentration.     

The growth of porous ZnO nanowires by thermal oxidation of ZnS nanowires

The growth of porous ZnO nanowires (NWs) via phase transformation of ZnS NWs at 500-850 degrees C in air was studied. The ZnS NWs were first synthesized by thermal evaporation of ZnS powder at 1100 degrees C in Ar. On subsequent annealing at 500 degrees C in air, discrete ZnO epilayers formed on the surface of ZnS NWs. At 600 degrees C, polycrystalline ZnO and the crack along the (0001) interface between the ZnO epilayer and ZnS NW were observed. At 700-750 degrees C ZnS NWs transformed to ZnO NWs, meanwhile nanopores and interfacial cracks were observed in the ZnO NWs. Two factors, the evaporation of SO2 and SO3 and the stress induced by the incompatible structure at the interface of ZnO epilayer and ZnS NW, can be responsible for the formation of porous ZnO NWs from ZnS NW templates on annealing at 700-750 degrees C in air. Rapid growth of ZnO at 850 degrees C could heal the pores and cracks and thus resulted in the well-crystallized ZnO NWs.     

Mechanical behaviour of polycrystalline BeO,Al2O3 and AlN at high pressure

The mechanical behaviour of various types of BeO, Al₂O₃, and AlN have been investigated at confining pressures up to 1.25 GPa, at 25° C, and at strain rates of 3 to 7×10^–5 sec^–1. The stress-strain data taken in uniaxial compressive-stress loading indicate the BeO aggregates undergo a transition from brittle fracture at low pressures to plastic flow at high pressures. Depending on the fabrication process, this transition pressure in BeO occurs at 0.4 to 0.7 GPa. Concurrently, the ultimate compressive strength of BeO increases from 1.0 to 1.9 GPa at 0.1 MPa pressure to over 4.0 GPa at 1.O GPa. Alumina remains brittle at all pressures up to 1.25 GPa; its strength increases from 4.5 GPa at 0.1 MPa pressure to over 6.0 GPa at 1.25 GPa. Aluminium nitride behaves similarly to BeO, having a brittle-ductile transition at 0.55 GPa. Its ultimate strength increases from 3.2 GPa at 0.1 MPa pressure to 4.7 GPa at 0.8 GPa. The distortional strain energy (proportional to the area under the stress-strain curve) absorbed by each material during compression at pressure was calculated and compared to available data from the literature. Alumina shows a degraded energy absorption with pressure, but both BeO and AlN yield a strongly enhanced performance at moderate pressures. Beryllium oxide and AlN thus appear to be promising structural materials for certain applications where high strengths and ductilities are required at moderate pressures.     

Solid-solid transition temperatures in SrCl2 from differential thermal analyses to 0.6 GPa

The reversible phase transition between the high-temperature, cubic C1 and the low-temperature, orthorhombic C23 polymorphs of SrCl₂ has been investigated by differential thermal analysis under hydrostatic pressure to 0.63 GPa. The C23→C1 transition temperature varies linearly with pressure at the rate of 0.42₄ μK(Pa)^?1 from the highest pressures down to ca. 0.34 GPa, and also linearly with slope 1.73 μK(Pa)^?1 at pressures ?0.26 GPa; the reverse, C1→C23 transition is not observed or deduced ?0.21 GPa. The observed curvature for the C1–C23 phase boundary over the range ?0.26–0.34 GPa, 1000–1050 K can be attributed to intersection with the “diffuse” transition in C1; the latter transition, however, could not be observed unambiguously. Linear extrapolation to 0.1 MPa places the C23→C1 transition near 553 K, which implies that C23 - not C1 - is the stable low-temperature polymorph. The recently-investigated transitions in PbF₂ closely parallel these in in SrCl₂.     

分子动力学方法模拟压强对Bi4Ti3O12铁电相变行为的影响

在壳模型的基础上, 通过分子动力学方法模拟了压强对Bi₄Ti₃O₁₂(BIT)铁电相变行为的影响. 为了提高模拟的准确性, 在原有势参数的基础上增加了Ti-Ti短程相互作用势. 计算得出了温度为300K时BIT单晶的铁电正交B2cb相在x方向和z方向的自发极化强度分别为39.4μC/cm²和0, 与实验结果较好的吻合. 然后模拟了压强对BIT相变行为的影响. 模拟结果表明: BIT单晶在压强从-2 GPa到24 GPa范围内, 经历了两次结构相变, 分别发生在 6 GPa和20 GPa处. 这种对称性的改变类似于在环境压力条件下温度导致BIT单晶对称性的改变. 因而模拟结果为研究压强引起BIT的相变行为提供了理论依据.     

Local structure at Mn2+ ions in vacuum annealed small cubic ZnS nanocrystals self-assembled into a mesoporous structure

A mesoporous structure of self-assembled nanocrystals of cubic ZnS doped with Mn2+ ions with a homogeneous distribution of pores of similar size was synthesized at room temperature by a surfactant-assisted liquid-liquid reaction. The component nanocrystals exhibit a high crystallinity and a tight size distribution centered at 2 nm, as well as the narrowest Electron Paramagnetic Resonance (EPR) spectra linewidth and the best resolution reported so-far, effects attributed to self-assembling. The observed EPR spectra consist of lines from the substitutional Mn2+(I) and surface Mn2+(II) and Mn2+(III) centers. Here we show that, in contrast with previous reports, our EPR spectra are highly sensitive to structural changes during pulse annealing in vacuum up to 500 degrees C. The changes are related to the transformation of the surface Mn2+ centers in new Mn2+ centers, attributed to an oxidation process in which the thermal decomposition of the Tween 20 additive, also observed by EPR, seems to be involved. We have also been able to observe, for the first time by EPR spectroscopy, the formation of the ZnO phase and the nanocrystals size increase, which occur during annealing up to 500 degrees C, structural changes confirmed by XRD and TEM observations on the samples previously investigated by EPR.     

Pressure-Induced Structural Transitions of the Zinc Sulfide Nano-particles with Different Sizes

ZnS nano-particles with average sizes of 10 nm and 5 nm were fabricated by sol-gel method, and their pressure-induced phase transformations were in-situ examined in a diamond anvil cell by energy dispersive X-ray diffraction （EDXD） from ambient pressure to 35.0 GPa. From the obtained interplanar spacing data,the volume compression ratios were derived at different pressures, and then the bulk modulus and its pressure derivative were obtained by fitting to the Murnaghan equation. It is found that both ZnS nano-particles initially in the zinc-blende phase transformed to cubic NaCl structure in the presence of pressure and the transition was reversible when the pressure was released. Moreover, it is suggested that a smaller particle size will induce a larger transition pressure.     

Pressure and Temperature Effects on Point-Defect Equilibria and Band Gap of ZnS

The pressure (100–200 MPa) and temperature (900–1100°C) effects on the equilibria of native point defects and background impurities in Zn-enriched ZnS are studied using cathodoluminescence and transmission spectra. The optimal conditions are found under which high pressures and temperatures accelerate migration of defects and impurities. The associated structural and compositional changes are studied by scanning electron microscopy. The increase in the concentration of dissolved oxygen at high pressures and temperatures is accompanied by a reduction in the band gap of ZnS, growth and blue shift (to 395–400 nm) of the shorter wavelength component of the SA blue emission in ZnS, and quenching of the longer wavelength component (445 nm). In addition, at 300 K a free-exciton bandI ₁ emerges at 342 nm. It is shown that the data available in the literature can be used to evaluate the concentration of dissolved oxygen in ZnS · O from its band gap. The effects of different oxygen species on the transmission of ZnS are studied in the range 3.5–15 m.     

Thin transparent W-doped indium-zinc oxide (WIZO) layer on glass

Annealing effect on structural and electrical properties of W-doped IZO (WIZO) films for thin film transistors (TFT) was studied under different process conditions. Thin WIZO films were deposited on glass substrates by RF magnetron co-sputtering technique using indium zinc oxide (10 wt.% ZnO-doped In2O3) and WO3 targets in room temperature. The post annealing temperature was executed from 200 degrees C to 500 degrees C under various O2/Ar ratios. We could not find any big difference from the surface observation of as grown films while it was found that the carrier density and sheet resistance of WIZO films were controlled by O2/Ar ratio and post annealing temperature. Furthermore, the crystallinity of WIZO film was changed as annealing temperature increased, resulting in amorphous structure at the annealing temperature of 200 degrees C, while clear In2O3 peak was observed for the annealed over 300 degrees C. The transmittance of as-grown films over 89% in visible range was obtained. As an active channel layer for TFT, it was found that the variation of resistivity, carrier density and mobility concentration of WIZO film decreased by annealing process.     

Structural evolution of La-Cr-O thin film: Part II. Elasto-plastic properties by nanoindentation

The goal of this research is to study the elasto-plastic properties of La-Cr-O thin films deposited by RF-magnetron sputtering on stainless steel interconnect materials after annealing at high temperatures in air. Elastic modulus, hardness and yield pressure derived from nanoindentation data are reported for thin films in different structural states. The amorphous film has an estimated elastic modulus of 174 GPa. The moduli of annealed films are calculated to be 150, 185 and 120 GPa after annealing at 500 °C, 600 °C and 800 °C, respectively. The film annealed at 800 °C has the lowest hardness and is dramatically different from the other structural states due to formation of the nanoporosity. The amorphous film and the films annealed at 500 °C and 600 °C both have hardness of 14 GPa, which is close to the value estimated by modeling.     

Spherical hexagonal tellurium nanocrystals: fabrication and size-dependent structural phase transition at high pressure

Single-crystalline spherical nearly monodisperse tellurium (Te) nanocrystals (NCs) with average diameters of 20 and 90?nm, respectively, have been fabricated for the first time by a facile solution sonochemistry process. The structural characterizations show that the as-synthesized Te NCs have pure hexagonal structure, as revealed by x-ray diffraction (XRD), selected-area electron diffraction (SAED), energy-dispersive x-ray (EDX) spectroscopy, and high-resolution transmission electron microscopy (HRTEM) methods. The size-dependent structural phase transition of Te NCs up to the high pressure of 20?GPa has been investigated in a diamond anvil cell using resistance measurement at room temperature, and compared with the behavior of bulk Te under identical conditions. The experimental results indicate that 20?nm Te NCs, 90?nm NCs, and bulk Te all undergo two phase transitions up to 20?GPa, their respective transition pressures being about 7.2 and 10.3?GPa, 5.9 and 8.8?GPa, and 4.0 and 6.8?GPa. This indicates that the phase transition pressures are higher for the smaller NCs. In this paper we discuss the size-dependent structural phase transitions, the sluggishness of the phase transition process, and the fluctuating properties of the phase transition products at high pressure. The present work might open an avenue to real-time detection of the dynamics of the phase transition in bulk and nanoscale materials at high pressure, and also could serve as a guide to tailoring the microscopic properties of materials.     

Rietveld analysis of the effect of annealing atmosphere on phase evolution of nanocrystalline TiO2 powders

The structural evolution of nanocrystalline TiO2 was studied by X-ray diffraction (XRD) and the Rietveld refinement method (RRM). TiO2 powders were prepared by the sol-gel technique. Post annealing of as-synthesized powders in the temperature range from 500 degrees C to 800 degrees C under air and argon atmospheres led to the formation of TiO2 nanoparticles with mean crystallite size in the range of 37-165 nm, based on the Rietveld refinement results. It was found that the phase structure, composition, and crystallite size of the resulting particles were dependent on not only the annealing temperature, but also the annealing atmosphere. Rietveld refinement of the XRD data showed that annealing the powders under argon atmosphere promoted the polymorphic phase transformation from anatase to rutile. Field emission scanning electron microscopy (FESEM) was employed to investigate the morphology and size of the annealed powders.     

A surface phase transition of supported gold nanoparticles

A thermal phase transition has been resolved in gold nanoparticles supported on a surface. By use of asynchronous optical sampling with coupled femtosecond oscillators, the Lamb vibrational modes could be resolved as a function of annealing temperature. At a temperature of 104 degrees C the damping rate and phase changes abruptly, indicating a structural transition in the particle, which is explained as the onset of surface melting.     

Amorphous alumina coatings: processing, structure and remarkable barrier properties

Amorphous aluminium oxide coatings were processed by metalorganic chemical vapour deposition (MOCVD); their structural characteristics were determined as a function of the processing conditions, the process was modelled considering appropriate chemical kinetic schemes, and the properties of the obtained material were investigated and were correlated with the nanostructure of the coatings. With increasing processing temperature in the range 350 degrees C-700 degrees C, subatmospheric MOCVD of alumina from aluminium tri-isopropoxide (ATI) sequentially yields partially hydroxylated amorphous aluminium oxides, amorphous Al2O3 (415 degrees C-650 degrees C) and nanostructured gamma-Al2O3 films. A numerical model for the process allowed reproducing the non uniformity of deposition rate along the substrate zone due to the depletion of ATI. The hardness of the coatings prepared at 350 degrees C, 480 degrees C and 700 degrees C is 6 GPa, 11 GPa and 1 GPa, respectively. Scratch tests on films grown on TA6V titanium alloy reveal adhesive and cohesive failures for the amorphous and nanocrystalline ones, respectively. Alumina coating processed at 480 degrees C on TA6V yielded zero weight gain after oxidation at 600 degrees C in lab air. The surface of such low temperature processed amorphous films is hydrophobic (water contact angle 106 degrees), while the high temperature processed nanocrystalline films are hydrophilic (48 degrees at a deposition temperature of 700 degrees C). It is concluded that amorphous Al2O3 coatings can be used as oxidation and corrosion barriers at ambient or moderate temperature. Nanostructured with Pt or Ag nanoparticles, they can also provide anti-fouling or catalytic surfaces.     

Synthesis of aligned zinc sulfide nanostructures and the influence of experimental conditions on their morphologies and phase

A novel aligned flower-like array and single-crystal nanosheets composed of ZnS quantum wires were fabricated via a simple solution route. In the method, ZnCl2 chemicals reacted with Na2S in 50 ml of ethylenediamine (en) solution containing different amounts of hydrochloric acid (HCl) at 70-80 degrees C. After annealing at 500 degrees C for 1.5 h, wurtzite phase ZnS nanoscaled materials were synthesized. When the amount of HCl was 2 ml and 3 ml, respectively, flower-like structure and nanosheets were obtained. The products were characterized by X-ray diffraction, transmission electron microscopy, high-resolution electron microscopy (HREM), and electron diffraction (ED). The influences of concentration, temperature, and reaction time on the morphologies and phase of ZnS nanostructures were also studied. The photoluminescence peaks are located at approximately 308 nm and approximately 410 nm. The formation mechanism is also discussed here.     

B3–B1 phase transition and pressure dependence of elastic properties of ZnS

We have performed the ab initio calculations based on density functional theory to investigate the B3–B1 phase transition and mechanical properties of ZnS. The elastic stiffness coefficients, C₁₁, C₁₂, C₄₄, bulk modulus, Kleinman parameter, Shear modulus, Reuss modulus, Voigt modulus and anisotropy factor are calculated for two polymorphs of ZnS: zincblende (B3) and rocksalt (B1). Our results for the structural parameters and elastic constants at equilibrium phase are in good agreement with the available theoretical and experimental values. Using the enthalpy–pressure data, we have observed the B3 to B1 structural phase transition at 18.5 GPa pressure. In addition to the elastic coefficients under normal conditions, we investigate the pressure dependence of mechanical properties of both phases: up to 65 GPa for B1-phase and 20 GPa for B3-phase.     

A new approach to the fabrication of ZnO ultrathin films from annealing ZnS nanoparticulate films

The ZnO ultrathin films were fabricated from annealing the ZnS nanoparticulate films. By the layer by layer self-assembly technique, we constructed the ZnS nanoparticulate films from alternating layers of ZnS nanoparticles and polydiallyldimethylammonium chloride (PDDA). The result of the emission spectra indicated that PDDA played a role of passivator. Annealing the ZnS/PDDA films at different temperatures led to the changes in absorption. The SEM images showed that the preannealed film was composed of the uniformly distributing domains, while on the annealed film, there exist some holes formed by the burning of the organic components. The EDS confirmed that ZnS could be converted to ZnO at 500 °C.