Amorphous-crystalline Transformation Research Articles

Phase-change-driven tuning of linear and nonlinear optical properties of Te/As2Se3 bilayer thin films for optoelectronic applications

The present work reports the amorphous-crystalline phase transformation by interdiffusion of Te into the As2Se3 matrix with annealing at various temperatures (100, 150, and 200 °C). Te/As2Se3 bilayer thin films were prepared by electron beam evaporation under high vacuum conditions. The amorphous nature of As2Se3 thin film was confirmed from the X-ray diffraction study. The presence of small peaks in Te/As2Se3 thin film indicates the As4Te4 (monoclinic) and As2Se3 (monoclinic) phases, confirming the amorphous to crystalline phase transformation upon Te interdiffusion and subsequent annealing. The impact of Te top layer deposition on the vibrational modes were also observed from the Raman spectroscopy. The formation of Te agglomerates is confirmed from the morphological analysis which disappeared with further annealing. Furthermore, the water contact angle measurements have provided insight into surface interaction mechanisms of these materials. The linear and nonlinear optical properties were changed upon interdiffusion of Te as observed from UV–Visible spectroscopy. The increase in Eu indicated an increase in disorder hence the decrease in the bandgap. The 3rd order nonlinear susceptibility (χ3) and nonlinear refractive index (n2) increased with annealing. The tuning of linear and nonlinear optical properties by annealing can be used in various optoelectronic applications. Specifically, the phase transformation behaviour of Te/As2Se3 bilayer thin films enables their utilization in phase-change material-based optoelectronic devices.

Pressure-induced metallization in the absence of a structural transition in the layered ferromagnetic insulator Cr2Ge2Te6

We report the crystallographic and electrical transport properties of single crystals of the ferromagnetic two-dimensional (2D) material ${\mathrm{Cr}}_{2}{\mathrm{Ge}}_{2}{\mathrm{Te}}_{6}$ under high pressure. In contrast to previous studies, our high-pressure single-crystal x-ray diffraction under hydrostatic conditions shows prominent anisotropic compressibility in the layered structure of the crystalline $R\overline{3}$ phase of ${\mathrm{Cr}}_{2}{\mathrm{Ge}}_{2}{\mathrm{Te}}_{6}$ without any structural phase transitions up to 20 GPa. Our data confirm a distinct and irreversible crystalline-amorphous transformation in ${\mathrm{Cr}}_{2}{\mathrm{Ge}}_{2}{\mathrm{Te}}_{6}$. The loss of crystallinity starts at 20 GPa; however, the crystalline phase and amorphous state coexist even at the maximum pressure of 31.2 GPa. High-pressure powder x-ray diffraction data and electrical resistivity measurements of ${\mathrm{Cr}}_{2}{\mathrm{Ge}}_{2}{\mathrm{Te}}_{6}$ using NaCl as the pressure-transmitting medium reveal an insulator-to-metal transition in the absence of a phase transition at \ensuremath{\sim}3.9 GPa; at a considerably lower pressure than the previously reported (7--14 GPa). Density functional theory calculations demonstrate the density of states around the Fermi level are primarily dominated by Cr $3d$ and Te $5p$ states. Hence the large reduction of Cr-Te bond lengths within the ${\mathrm{CrTe}}_{6}$ octahedra under compression is most likely responsible for the band-gap closure. This study clarifies that the phase stability and onset of metallization pressure in the ${\mathrm{Cr}}_{2}{\mathrm{Ge}}_{2}{\mathrm{Te}}_{6}$ sample are sensitive to the hydrostatic environments and demonstrates how pressure can be used to tune the physical properties of 2D ferromagnetic compounds.

Amorphous Phase Formation and Heat Treating Evolution in Mechanically Alloyed Ti–Cu Alloy for Biomedical Applications

The present work aims to produce TixCu100-x (x = 90, 80, 70, and 60) amorphous alloys by using high energy ball mill. The microstructure and possible formation of amorphous phases were characterized employing laser granulometry, scanning electron microscopy (SEM), transmission electron microscopy (TEM), and X-ray diffraction (XRD). The amorphous-crystalline transformation at high temperatures was studied according to differential scanning calorimetry (DSC) and XRD. The Ti80Cu20 alloys were obtained in an amorphous state after 30 h of mechanical alloying, and the amorphous phase is stable up to 340 °C. At higher temperatures, this alloy crystallizes, forming the intermetallic compound Ti2Cu, and a substitutional solid solution Ti(Cu).

Ultra-high temperature flexure and strain driven amorphization in polycrystalline boron carbide bulks

The temperature dependence of the flexural strength of sintered boron carbide was studied. At temperatures above 2000 °C, B 4 C showed an ultrahigh flexural strength exceeding 1 GPa which was accompanied by a change in the deformation mechanism from brittle fracture to plastic deformation . Scanning transmission electron microscopy (STEM) observations revealed that the amorphization can be of microstructural origin for the observed plasticity in B 4 C at temperatures above 2000 °C and a strain rate of 3•10 −3 s −1 . The amorphization occurs inside of the severely deformed grains. Flexural tests below 2000 °C provided evidence for the formation of stacking faults or dislocations, which are ordinary defects after the flexural tests. The results at 2000 °C suggest that the magnitude of the tensile stresses imposed on the B 4 C grains during deformation in flexure and the total strain transferred to a ceramic during the deformation process play the dominant role in the crystalline-amorphous transformation.

A survey on crystallization kinetic behavior of direct current magnetron sputter deposited NiTi thin films

The purpose of this work is to assess the crystallization kinetic properties of the direct current (DC) magnetron sputtered NiTi thin films. NiTi thin films were deposited on NaCl substrate using a magnetron sputtering method. Differential scanning calorimetry (DSC) was employed at different heating rates of 5, 20, and 30 K min−1 to investigate the crystallization kinetic behavior. Only one dominant peak emerged during amorphous-crystalline transformation at different heating rates. The activation energy values during crystallization were 321.81, 288.86, and 234.56 kJ mol−1 at heating rates of 5, 20, and 30 K min−1, respectively. The average value of the Avrami exponent for the studied films was 2.2. The predominant mechanisms governing the nucleation and growth processes were continuous nucleation and diffusion-controlled two-dimensional growth. We envisage that the results of this systematic work will create new paradigms and opportunities in industrial-scale applications of NiTi films in microelectrochemical systems.

Investigation of amorphous-crystalline transformation induced optical and electronic properties change in annealed As50Se50 thin films

The present work reports the amorphous-crystalline phase transformation in thermally evaporated As50Se50 thin films upon annealing at below Tg (423 K) and above Tg (523 K). The structural transition was probed by XRD, Raman and X-ray photoelectron spectroscopy. The composition and surface morphology were probed by EDS and FESEM techniques. The transmittance and reflectance spectra over the wavelength range 500 nm–1200 nm were used to deduce the optical parameters. The various optical parameters of the as-prepared and annealed As50Se50 thin films were estimated and discussed in terms of density of defect states and disorders. The indirect optical energy gap decreased for the 423 K annealed film and abruptly increased for 523 K annealed film as compared to the as-prepared film. The Swanepoel envelope method, WDD model, and Sellemeire postulates were employed for the analysis of refractive index, static refractive index, oscillator energy, dispersion energy, oscillator wavelength and dielectric constant. The changes in the linear and nonlinear properties showed opposite behavior for the two annealed films. The non-linear refractive index and 3rd order susceptibility were found to be increased for 423 K annealed film and decreased for the 523 K annealed film. The optical as well as the electrical conductivity changed with annealing and the electrical susceptibility increased for 523 K annealed film. The tunable optical properties can be applied for several optoelectronic applications. The amorphous to crystalline structural transformation occurred at annealing above Tg. The 3rd order susceptibility was found to be more at 423 K annealing and less at 523 K annealing.

Theoretical and experimental study of reversible intercalation of Li ions in the Jarosite NaFe3(SO4)2(OH)6 structure

Jarosite NaFe3(SO4)2(OH)6 (NFSOH) with a trigonal structure (R-3 m space group) was prepared by the hydrothermal synthesis at 150 °C and then ball milled with carbon black. Crystal and local structure, morphology, thermal stability, conductivity and electrochemical properties of the as-prepared composite NaFe3(SO4)2(OH)6/C (NFSOH/C) were investigated. For the first time, the Na/Li electrochemical ion-exchange in NFSOH/C in a hybrid Na/Li cell was studied in detail by theoretical and experimental methods. According to DFT calculations, mixed structurally-stable Na1-xLixFe3(SO4)2(OH)6 Jarosite phases can exist up to complete Na/Li substitution, however, it was not achieved in full experimentally. Upon cycling in a Li cell, the crystalline-amorphous transformation occurs similar to that observed in a Na cell, followed by subsequent decomposition of the mixed Na1-xLixFe3(SO4)2(OH)6. The diffusion paths for both Na+ and Li+ ions in NFSOH and LiFe3(SO4)2(OH)6 structures were determined by the Voronoi-Dirichlet partition (VDP) of crystal space. It was found that the value of the average diffusion coefficient in NFSOH increases by two orders of magnitude after the first cycle both in Na and Li cells.

Crystalline to amorphous transformation in solid-solution alloy nanoparticles induced by boron doping.

We synthesized a palladium-ruthenium-boron (Pd-Ru-B) solid-solution ternary alloy. Elemental mappings confirmed successful alloying of B with Pd-Ru body without changing the particle sizes, demonstrating the first discovery of this ternary alloy. Pair distribution function analysis revealed a drastic decrease in atomic correlation in Pd-Ru nanoparticles by B doping. This result gives the first example of structural transformation from crystalline to amorphous in solid-solution alloy nanoparticles induced by the doping of light elements.

A Strongly Fluorescent Molecular Material Responsive to Physical/Chemical Stimuli and their Coupled Impact.

Molecular materials with weak but extended and pliable supramolecular interactions are versatile candidates for eliciting stimuli-sensitive optical responses. A novel diaminodicyanoquinodimethane (DADQ) molecule, 7,7-bis(2-(2-pyridyl)ethylamino)-8,8-dicyanoquinodimethane (BPEDQ), has been synthesized and structurally characterized; it exhibits enhanced fluorescence emission in the aggregated and solid states, characteristic of DADQs. The pyridine moieties in the molecule, in addition to the amino and cyano groups of the strongly dipolar fluorophore moiety, induce extensive H-bonding interactions which can impart structural integrity to the solid material; this enables reversible crystalline-amorphous transformations triggered by mechanical grinding and solvent fuming. The concomitant fluorescence color and intensity switching are prominent and reversible. Protonation-deprotonation events induced by acidic and basic vapors also produce stark fluorescence response variations; the chemical stimuli also lead to amorphization of the solid. The full cycle of chemical and physical stimuli, and the consequence of their individual and coupled impact on the fluorescence emission, are illustrated using a BPEDQ-doped polymer thin film.

Aminostyrylquinoxalines derived organic materials with remarkable solvatochromic and mechanofluorochromic properties

Four multi-responsive fluorophores (DMQA1, DMQA2, DMQB1, DMQB2) based on 2,3-dimethylquinoxaline were successfully designed and synthesized. It was found that all these fluorophores presented remarkable red-shifted solvatochromic behavior with increasing solvent polarity, in which, DMQA2 displayed 117 nm red-shift from cyclohexane to DCM. Interestingly, these four compounds all exhibited high contrast mechanofluorochromism and DMQA2 received the largest MFC spectral shift (ΔλMFC = 84 nm). Moreover, Power wide-angle X-ray diffraction (PXRD) and Differential scanning calorimetry (DSC) were carried out to clarify the crystalline-amorphous transformation upon grinding. In this work, the quinoxaline template was introduced to develop high-performance MFC materials.

Structural changes in hexagonal WO3 under high pressure

The high pressure of hexagonal-tungsten oxide (h-WO3) has been studied in diamond anvil cell up to 50 GPa at room temperature by angle-dispersive X-ray powder diffraction (AD-XRD). Cell refinement, FWHM and area of peaks and microstructure were analyzed. Results show that below 10 GPa, lattice strains follow linear trend, with bulk modulus B0 = 42(2) GPa, and its pressure derivative B0' = 13(2), and the crystalline grain sizes rapidly decrease in this range. In the range of 10–36 GPa, the lattice strains deviate linear trend, with decreasing amount of crystalline regions, and r.m.s. microstrain dominates this range. After 36 GPa, the lattice strain show a little increase, implying the crystalline-amorphous transformation occurs.

Stimuli responsive and reversible crystalline–amorphous transformation in a molecular solid: fluorescence switching and enhanced phosphorescence in the amorphous state

A diaminodicyanoquinodimethane molecule based material undergoes reversible crystalline–amorphous transformation upon external stimuli. Significantly, delayed fluorescence and phosphorescence are observed in the material, with an unusual enhancement of the latter in the amorphous state, in sharp contrast to the decrease in fluorescence.

Impact of Heat Treatment on The Structural and Optical Properties of Ge4Se60Te30In6 Phase Change Thin Film

Abstract Bulk Ge4Se60Te30In6 chalcogenide glass was synthesized by melt quenching process. Thin films of prepared glassy alloy were deposited onto highly cleaned glass substrate using thermal evaporation technique. In the present work the effect of annealing temperature on the structural and optical behavior of Ge4Se60Te30In6 thin film is investigated by DSC, XRD and UV-Vis spectroscopy. Optical transmittance with wavelength was recorded, for as-prepared and annealed films by a double beam spectrophotometer in the wavelength span 300 to 1000 nm. The optical constants like α, k and Eg were calculated as a function of annealing temperature. The optical transition was found to be allowed direct transition. The decrease of optical band gap with increasing the annealing temperature from 373K to 413 K was explained by Mott and Davis model. The decrease of optical bandgap with increase of annealing temperature were explained by structural relaxation as well as change in defect states and density of localized states due to amorphous-crystalline transformation.

Optical and electrical properties of thermally evaporated Se90Sb10 thin film

Chalcogenide Se90Sb10 thin films are deposited by thermal evaporation from the bulk alloy. X-ray diffraction examination for the annealed films shows the amorphous-crystalline transformation. This is beneficial for optical disk data storage technology. The crystallinity is improved by increasing the annealing temperature. The films annealed at relatively low temperatures exhibit highly transparence reaching to about 90% at incident light of wavelength of 900 nm. The as-prepared and annealed Se90Sb10 films reveal an indirect allowed optical transition. The annealed film at 473 K has an optical band gap of 1.676 eV which is suitable value for solar cell as photovoltaic application. Both the indirect optical energy band gap (Eg) and the oscillator energy (Eo) decrease whereas the oscillator strength (Ed) increases with increasing the annealing temperature. The annealing increases the conductivity and decreases the activation energy for conduction resulting in enhancement of film properties for adapting to solar cells.

Preparation, characterizations and anti-pollutant activity of 7,3',4'-trihydroxyisoflavone nanoparticles in particulate matter-induced HaCaT keratinocytes.

Background7,3′,4′-Trihydroxyisoflavone (734THI), a secondary metabolite derived from daidzein in soybean, possesses several biological activities, including antioxidant, skin whitening and anti-atopic dermatitis properties, but the poor aqueous solubility of 734THI has limited its application in medicine and cosmetic industry.MethodsThe aim of the present study was to improve the physicochemical properties of 734THI using planetary ball mill preparation under a solvent-free process to improve its solubility and anti-pollutant activity.Results734THI nanoparticle powder (734THIN) was successfully prepared by the planetary ball mill technique using polyvinylpyrrolidone K30 as the excipient. 734THIN effectively increased the aqueous solubility and cellular uptake of 734THI by improving its physicochemical properties, including particle size reduction, crystalline–amorphous transformation and intermolecular hydrogen bonding with polyvinylpyrrolidone K30. In addition, 734THIN inhibited the overexpression of COX-2 and MMP-9 by downregulating MAPK pathway signaling in particulate matter-exposed HaCaT keratinocytes, while raw 734THI in PBS with low aqueous solubility did not show any anti-inflammatory or antiaging activity.Conclusion734THIN may be used as an additive in anti-pollutant skin care products for preventing particulate matter-induced inflammation and aging in skin.

Water vapor corrosion behavior and failure mechanism of plasma sprayed mullite/Lu2Si2O7-Lu2SiO5 coatings

A new coating with mullite as the inner layer and Lu2Si2O7-Lu2SiO5 (lutetium disilicate-lutetium monosilicate, LuDS-LuMS) composite as the top layer was designed and fabricated on the surface of porous SiC substrate by atmospheric plasma spraying (APS). The microstructure evolution, phase transformation, inter-diffusion and failure mechanism of the coated samples during steam cycling at 1450 °C were systemically investigated. The results indicated that the coated samples maintain weight gain and the average weight gain rate is 1.12 × 10−1 mg/cm2 h. The thermal expansion coefficient (CTE) mismatches between the coating and the substrate as well as between the two ceramic layers have produced a thermal mismatch stress. In addition, the chemical reaction induced the sintering of LuDS-LuMS layer, the amorphous-crystalline phase transformation and the transformation between LuDS and LuMS phases could result in aging stress. With the accumulation of thermal mismatch stress and aging stress, the through and horizontal cracks have appeared in the coating, leading to the coating failure.

Two-dimensional (2D) amorphous antimony (III) trisulfide nanosheets: Synthesis, photoelectronic property and their transformation to crystalline 1D micro/nanorods

Two-dimensional (2D) crystal materials have attracted intensive research interest because of their multifunctionality. However, the 2D nanosheets (NSs) of amorphous semiconductors are far less explored. We demonstrate here the colloidal synthesis of 2D amorphous antimony (III) trisulfide (Sb2S3) NSs in the mixture solution of different long-chain primary amines. These organic amines act as soft template to define the 2D growth through the coordination of metal cations with amine group. The resultant NSs exhibit distinct 2D characteristic with the thickness of 2–4 nm and the length × width dimensions ranging from several hundreds of nanometer to one to several micrometers. Their photoelectronic property has been evaluated in the indium tin oxide (ITO)/Sb2S3/ITO model device, showing fast temporal photoresponse and photoswitch ability. Through a solid-state thermal-annealing process, the amorphous NSs can be converted to 1D crystalline micro/nanorods, which is due to the high structural anisotropy of crystal phase (stibnite). TG-DSC thermal analysis reveals that the amorphous-crystalline phase transformation is companied by the desorption and/or decomposition of organic surfactant molecules and the S loss.

Self-propagating waves of crystallization in metallic glasses

Self-propagating thermal waves of the amorphous-crystalline transformation in Fe-based metallic glasses, obtained by melt spinning, were observed using a high-speed infrared camera and reported here. Some experimental results are also reported concerning oscillating waves in the CuTi glassy foils. The thermal characteristics and wave propagating velocities, as well as the microstructure and atomic structure transformations, were studied. A comparison of the results with exothermic reaction waves and explosive crystallization shows that the self-propagating waves in metallic glasses are slower and less violent than classical explosive crystallization in deposited films; thus, we suggest naming this phenomenon “soft explosive crystallization.” The experimental data were confirmed by molecular dynamics simulation of the crystallization phenomenon.

Kinetics of a first-order crystalline-amorphous transformation in zirconium tungstate

A combination of in situ volumetric measurements, Raman spectroscopy, x-ray diffraction, differential thermal analysis, and transmission electron microscopy was used to investigate the crystalline-amorphous transformation of zirconium tungstate at high pressure and room temperature. Zirconium tungstate transformed at about 1.2 GPa to an amorphous phase which was recoverable to ambient conditions. The volume contraction and temperature increase in the sample during pressure-induced amorphization indicate that it was a first-order phase transition. The kinetics data are consistent with a model where an amorphous phase occurs by the formation of nuclei without noticeable growth. However, our results conflict with the three-dimensional interface-controlled growth of a new phase in conventional solid-state polymorphic changes.

Effect of annealing on the optical properties of amorphous Se79Te10Sb4Bi7 thin films

Thin films of Se79Te10Sb4Bi7, were prepared by Electron beam deposition technique. The structure of the as-prepared and annealed films has been studied by X-ray diffraction and the surface morphology by the scanning electron microscope (SEM). These studies show that there is a gradual change in structure and the formation of some polycrystalline structures in the amorphous phases is observed when the Se79Te10Sb4Bi7 film is annealed in the temperature range of 333–393K. The optical transmission of these films has been studied as a function of photon wavelength in the range 300–2500nm. It has been found that the optical band gap Egopt decreased with increasing annealing temperature in the range 333–393K. The Urbach energy (Eu), optical conductivity (σopt), imaginary (εi), and real (εr) parts of the complex dielectric constant (ε) and lattice dielectric constant (εL) were also determined. The changes noticed in optical parameters with increasing annealing temperature were explained on the basis of structural relaxation as well as change in defect states and density of localized states due to amorphous-crystalline transformation.