Crystalline Phase Transition Research Articles

Tellurium Single-Crystal Arrays by Low-Temperature Evaporation and Crystallization.

Thermally evaporated tellurium possesses an intriguing crystallization behavior, where an amorphous to crystalline phase transition happens at near-ambient temperature. However, a comprehensive understanding and delicate control of the crystallization process for the evaporated Te films is lacking. Here, the kinetics and dynamics of the crystallization of thermally evaporated Te films is visualized and modeled. Low-temperature processing of highly crystalline tellurium films with large grain size and preferred out-of-plane orientation ((100) plane parallel to the surface) is demonstrated by controlling the crystallization process. Tellurium single crystals with a lateral dimension of up to 6µm are realized on various substrates including glass and plastic. Field-effect transistors based on 5°C crystallized Te single grains (6-nm-thick) exhibit an average effective hole mobility of ≈100 cm2 V-1 s-1 , and on/off current ratio of ≈3 × 104 .

Systematic Exploration of the Synthetic Parameters for the Production of Dynamic VO2(M1).

Thermochromic dynamic cool materials present a reversible change of their properties wherein by increasing the temperature, the reflectance, conductivity, and transmittance change due to a reversible crystalline phase transition. In particular, vanadium (IV) dioxide shows a reversible phase transition, accompanied by a change in optical properties, from monoclinic VO2(M1) to tetragonal VO2(R). In this paper, we report on a systematic exploration of the parameters for the synthesis of vanadium dioxide VO2(M1) via an easy, sustainable, reproducible, fast, scalable, and low-cost hydrothermal route without hazardous chemicals, followed by an annealing treatment. The metastable phase VO2(B), obtained via a hydrothermal route, was converted into the stable VO2(M1), which shows a metal–insulator transition (MIT) at 68 °C that is useful for different applications, from energy-efficient smart windows to dynamic concrete. Within this scenario, a further functionalization of the oxide nanostructures with tetraethyl orthosilicate (TEOS), characterized by an extreme alkaline environment, was carried out to ensure compatibility with the concrete matrix. Structural properties of the synthesized vanadium dioxides were investigated using temperature-dependent X-ray Diffraction analysis (XRD), while compositional and morphological properties were assessed using Scanning Electron Microscopy, Energy Dispersive X-ray Analysis (SEM-EDX), and Transmission Electron Microscopy (TEM). Differential Scanning Calorimetry (DSC) analysis was used to investigate the thermal behavior.

Pre-Crystallization Criteria and Triple Crystallization Kinetic Parameters of Amorphous–Crystalline Phase Transition of As40S45Se15 Alloy

This framework focuses mainly on a detailed study of the pre-crystallization criteria that characterize the As40S45Se15 glassy alloy in various heating rates ranging from 5 to 40 (K/min) by Differential Scanning Calorimetry (DSC). These criteria aim to clarify the relationship of the tendency of glass-forming by the heating rate for the investigated glassy alloy. The crystallization parameters were calculated using different methods.The activation energy of crystallization Ec(χ) as a function of conversion (χ) was obtained using the iso-conversional models of Flynn–Wall–Ozawa (FWO), Starink and Kissinger–Akahira–Sunose (KAS). The results show a slight increase of Ec(χ) with conversion (χ) which accounts for a single-step mechanism controlling the crystallization process. Moreever, the conversion dependence of the Avrami exponent n(χ) show an increase with conversion (χ), average values of n(χ) can be accounted for two and three-dimensional crystal growth with heterogeneous nucleation. On the other hand, the fitting of the experimental DSC data to the calculated DSC curves indicated that the crystallization process of the studied glasses cannot be satisfactorily described by the Johnson–Mehl–Avrami (JMA) model. On the contrary (SB) model is more suitable to describe the crystallization process for the studied of As40S45Se15 Alloy. Finally, the crystalline structure of the study sample was recognized by X-ray diffraction (XRD) and electron scanning microscope (SEM).

A Molecular Shape Recognitive HPLC Stationary Phase Based on a Highly Ordered Amphiphilic Glutamide Molecular Gel.

Chiral glutamide-derived lipids form self-assembled fibrous molecular gels that can be used as HPLC organic phases. In this study, HPLC separation efficiency was improved through the addition of branched amphiphilic glutamide lipids to the side chains of a terminally immobilized flexible polymer backbone. Poly(4-vinylpyridine) with a trimethoxysilyl group at one end was grafted onto the surface of porous silica particles (Sil−VP15, polymerization degree = 15), and the pyridyl side chains were quaternized with a glutamide lipid having a bromide group (BrG). Elemental analysis indicated that the total amount of the organic phase of the prepared stationary phase (Sil−VPG15) was 38.0 wt%, and the quaternization degree of the pyridyl groups was determined to be 32.5%. Differential scanning calorimetric analysis of a methanol suspension of Sil−VPG15 indicated that the G moieties formed a highly ordered structure below the phase transition temperature even on the silica surface, and the ordered G moieties exhibited a gel-to-liquid crystalline phase transition. Compared with a commercially available octadecylated silica column, the Sil−VPG15 stationary phase showed high selectivity toward polycyclic aromatic hydrocarbons, and particularly excellent separations were obtained for geometrical and positional isomers. Sil−VPG15 also showed highly selective separation for phenol derivatives, and bio-related molecules containing phenolic groups such as steroids were successfully separated. These separation abilities are probably due to multiple interactions between the elutes and the highly ordered functional groups, such as the pyridinium and amide groups, on the highly ordered molecular gel having self-assembling G moieties.

Octylammonium Sulfate Decoration Enhancing the Moisture Durability of Quasi‐2D Perovskite Film for Light‐Emitting Diodes

AbstractQuasi‐2D perovskite exhibits excellent luminescence properties for highly efficient perovskite light‐emitting diodes (PeLEDs). However, the lattice degradation and crystalline phase transition induced by water limit the PeLED's development for application and commercialization. It is demonstrated that the organic additives effectively reduce the defect density, while the bonding strength of these organics and perovskite becomes weak under the high electric field and the Joule heat in an operating PeLED. Thus, an alternative additive for forming strong bonding with perovskite is promising to improve the stability of perovskite film and PeLEDs simultaneously. Here, it is shown that octylammonium sulfate decoration effectively enhances the moisture durability of PEA2(CsPbBr3)4PbBr4 quasi‐2D perovskite film by generating PbSO4 on the surface. The strong bonding interaction of SO42− and Pb2+ ions leads to the reduced defect density, enhanced lattice stability, and robust photoluminescence quantum yield of perovskite film, which is confirmed by the density functional theory calculation. Moreover, the generated PbSO4 reduces the internal resistance and adjusts the band structure of perovskite film to enhance the carrier transport and injection balance. The PeLEDs based on PbSO4 decorated perovskite film exhibit enhanced operational lifetime and the maximum external quantum efficiency of 10.84%.

Charge density waves and their transitions in anisotropic quantum Hall systems

In recent experiments, external anisotropy has been a useful tool to tune different phases and study their competitions. In this paper, we look at the quantum Hall charge density wave states in the N = 2 Landau level. Without anisotropy, there are two first-order phase transitions between the Wigner crystal, the 2-electron bubble phase, and the stripe phase. By adding mass anisotropy, our analytical and numerical studies show that the 2-electron bubble phase disappears and the stripe phase significantly enlarges its domain in the phase diagram. Meanwhile, a regime of stripe crystals that may be observed experimentally is unveiled after the bubble phase gets out. Upon increase of the anisotropy, the energy of the phases at the transitions becomes progressively smooth as a function of the filling. We conclude that all first-order phase transitions are replaced by continuous phase transitions, providing a possible realisation of continuous quantum crystalline phase transitions.

Geometric features in lyotropic liquid crystalline phase transitions observed in aqueous surfactant systems

Lyotropic liquid crystalline phases are widely known to be formed in aqueous surfactant mixtures. Although the phase behavior of several surfactants has been elucidated in the past years, a striking absence of detailed discussion on how molecular geometric parameters vary as different liquid crystals are formed is consistently seen in literature. The current work aimed to partially fill this gap, by determining how several structural parameters changed in a variety of phase transitions previously observed in aqueous surfactant binary and ternary mixtures. The main findings reveal that the cell parameter dimensions are strongly correlated with the packing fraction of small aggregates in each phase; the folding and interpenetration of surfactant hydrocarbon chains are most likely to happen in phases with lower curvature, and can greatly affect other structural parameters; added components can alter the aqueous surfactant phase behavior at different extents; and highly ordered phases are favored by a substantial decrease in the cross-sectional areas per surfactant; among others. The obtained results enable important knowledge on lyotropic liquid crystal formation, in addition to providing detailed information on how to control and obtain such phases, thus prospecting new insights in areas where they are highly desired.

Electrical percolation characteristics and other electrical transport properties of amorphous Se85-xTe15Sbx thin films

The effect of Sb at% on different temperature ranges dc-electrical conductivity are reported for thermally evaporated Se85−xTe15Sbx(x = 2.5, 5 and 7.5 at.%) glassy thin films. The structure and surface morphology of the considered samples were studied by the X-Ray Diffraction (XRD), Differential Thermal Analysis (DTA), Fourier Transform Infrared (FTIR) and Scanning Electron Microscope (SEM) measurements. The electrical conduction in the Se85−xTe15Sbx chalcogenide glass system was found to be determined by different conduction mechanisms with different activation energy values at various temperature ranges. The relation between various structural parameters and the electrical activation energies was investigated. The analysis of the high-temperature conductivity data identified that the conduction mechanism on the basis of thermionic emission occurs over the grain boundaries according to Seto's model. In the amorphous to crystalline phase transition region, a model has been modified to describe the conduction mechanism. The modification is considering the percolation effects alongside with the effect of thermally activated electrical conductivity of the formed crystalline grains embedded in the sample amorphous matrix. The calculated electrical conductivity was found to be effectively fitted to the temperature-dependent electrical conductivity measurements for thermally evaporated Se85−xTe15Sbx films and showed promising candidates to be examined in different chalcogenide systems .

Chlorosulfolipid (Danicalipin A) Membrane Structure: Hybrid Molecular Dynamics Simulation Studies.

Chlorosulfolipids (CSLs) are major components of flagellar membranes in sea algae. Unlike typical biological lipids, CSLs contain hydrophilic sulfate and chloride groups in the hydrocarbon tail; this has deterred the prediction of the CSL membrane structure since 1960. In this study, we combine coarse-grained (CG) and atomistic molecular dynamics (MD) simulations to gain significant insights into the membrane structure of Danicalipin A, which is one of the typical CSLs. It is observed from the CG MD that Danicalipin A lipids form a stable monolayer membrane structure wherein the hydrocarbon moieties are sandwiched by hydrophilic sulfate and chloride groups in both the head and tail regions. On the basis of the mesoscopic structure, we built the corresponding atomistic model to investigate the integrity of the CSL monolayer membrane structure. The monolayer membrane comprising bent lipids shows high thermal stability up to 313 K. The gel-liquid crystalline phase transition is observed around 300 K.

Thermally Driven Amorphous‐Crystalline Phase Transition of Carbonized Polymer Dots for Multicolor Room‐Temperature Phosphorescence

AbstractMulticolor carbon dot (CD)‐based nanomaterials offer a variety of opportunities for potential applications in bioimaging, optoelectronic devices, and information security. However, it still remains a challenge to modulate the conjugated π‐structure of CDs to achieve multicolor room‐temperature phosphorescence (RTP). Herein, the authors present a strategy based on thermally driven amorphous−crystalline phase transition to achieve multicolor carbonized polymer dots (CPDs) with the emission color tunable from green to orange‐red. This is the first report on multicolor RTP emission from CDs by means of thermal stimulus. Further investigations reveal that the formation of self‐protective covalently crosslinked frameworks and codoping of multiple heteroatoms play a crucial role in the production of RTP. RTP color tunability can be attributed to different crystalline contents of the conjugated π‐domain within CPDs. Potential application of the developed CPDs as printable and writable security inks for advanced multilevel anti‐counterfeiting and encryption is demonstrated. This work paves a path for the development of multicolor RTP materials and suggests great potential of CDs in exploiting novel optical materials toward intriguing applications.

Self-assembly crystal, manipulated polymorphic crystalline structure and elevated thermal degradation temperature of poly(1,4-butylene adipate): Effects of an aryl bisamide-based compound

A self-organized bisamide-based derivative (TMB-5) as a nucleating agent (NA) induced PBA to develop shish-kebab structure. TMB-5 increased crystallization rate and lowered crystallization time, and displayed a good nucleation effect on PBA. TMB-5 modulated polymorphism and shortened PBA phase transition process. Thermal stability of PBA increased after blending of TMB-5. Mechanisms on crystal growth, controlled polymorphic crystalline structure and phase transition, and elevated thermal stability of PBA were clarified.

Surface structure evolution and Raman response for multipulse, few-cycle, laser damaged ZnSe.

Multiple 11-fs infrared, few-cycle laser pulses were applied to a polycrystal ZnSe surface to study the evolution of surface damage morphologies. The polycrystalline grain boundaries seem to be the initiation site of surface damage and formation of ripples, which evolve as the result of many laser pulses at the same site. Scanning electron microscopy and atomic force microscopy (AFM) were applied to characterize the surface. The crystalline change and material phase transition were examined by confocal Raman spectroscopy. The thermal expansion coefficient increased slightly in the ablated zone compared to the non-ablated zone according to an AFM thermal tip test. The results show the growth and organization of surface ripples and the change of thermal properties as the number of irradiations at each site increases.

Improving dispersive mixing in compatibilized polystyrene/polyamide-6 blends via extension-dominated reactive single-screw extrusion

Abstract Extensional mixing elements (EMEs) that impose extension-dominated flow via stationary single-plane or double-plane hyperbolic converging-diverging channels were previously designed for twin-screw and single-screw extruders (TSE and SSE, respectively). In a recently published work by the authors, reactive extrusion was performed on PS/PA6 polymer blends TSE using EMEs and a crystalline phase transition of the minor phase in these droplets was observed as the size of droplet decreases from micron to submicron. Herein, we expand upon this work to SSE and study: a) The ability of the EMEs to improve dispersive mixing in the same blends; b) Assess the possibility of achieving the same crystalline phase transition in SSEs. The final blends were characterized by DSC, rheologically and morphologically via SEM, and the results show that while EME-based SSE leads to much improved mixing, better than non-EME TSE, the reduction in size of the PA6 disperse phase is not enough to induce the phase transition observed in EME-based TSE.

Evolution of the local structure surrounding nitrogen atoms upon the amorphous to crystalline phase transition in nitrogen-doped Cr2Ge2Te6 phase-change material

Phase-change memory based upon resistive switching due to phase transitions between the amorphous and crystalline phases of chalcogenide-based phase-change materials (PCMs) has been widely studied for the next generation non-volatile memory (NVM). Unlike traditional PCMs such as Ge2Sb2Te5, nitrogen doped Cr2Ge2Te6 (NCrGT) was reported to not exhibit a bulk resistivity change upon the amorphous to crystalline phase transition, instead the contact resistivity contrast in the presence of metal electrodes was found to play an essential role. In the current study, a distinct difference in the local structure around N atoms in amorphous and crystalline NCrGT films was found using X-ray absorption near-edge spectroscopy (XANES) and hard X-ray photoelectron spectroscopy (HAXPES). CrN and GeN bonds and N2 molecular vibrational modes were found to be present in an amorphous NCrGT film, while N atoms were found to be present at Te substitutional positions in the crystalline phase. A previous study revealed that Cr nanoclusters are present in the undoped amorphous phase, whose concentration was found to decrease as crystallization progressed leading to an increase in the resistivity. In the present study, it was determined that N doping inhibits Cr nanocluster formation in the amorphous NCrGT phase and leads to a constant Cr nanocluster volume fraction even after crystallization, ensuring negligible resistance contrast between the amorphous and crystalline phases. These results will prove useful in understanding the role of dopant elements in the promising PCM CrGT.

Bismuth Doping Alters Structural Phase Transitions in Methylammonium Lead Tribromide Single Crystals.

We study the effects of bismuth doping on the crystal structure and phase transitions in single crystals of the perovskite semiconductor methylammonium lead tribromide, MAPbBr3. By measuring the temperature-dependent specific heat capacity (Cp), we find that as the Bi doping increases, the phase transition assigned to the cubic to tetragonal phase boundary decreases in temperature. Furthermore, after doping we observe one phase transition between 135 and 155 K, in contrast to two transitions observed in the undoped single crystal. These results appear strikingly similar to previously reported effects of mechanical pressure on perovskite crystal structure. Using X-ray diffraction, we show that the lattice constant decreases as Bi is incorporated into the crystal, as predicted by density functional theory. We propose that bismuth substitutional doping on the lead site is dominant, resulting in BiPb+ centers that induce compressive chemical strain that alters the crystalline phase transitions.

Filamentary superconductivity in wrinkled PtSe2

Platinum diselenide (PtSe2) is a recently discovered Dirac semi-metal, which is theoretically predicted to possess a superconducting transition at an extremely low temperature T c of 2 mK. However, it has not yet to be experimentally reported. We observe a filamentary superconducting transition at 2.3 K in the wrinkled PtSe2 induced by rapid thermal treatment. Certain crystalline deformation or phase transition occurs locally in PtSe2 due to the inhomogeneous strain induced during the temperature cycle, where T c could be significantly enhanced. The possibility of forming degraded PtSe2 or any superconducting compounds at the electric contacts during the thermal treatment is carefully ruled out. We investigate the magnetic field dependence of the superconductivity transition, i.e. the upper critical fields, and find the superconductivity in accordance with the Bardeen–Cooper–Schrieffer theory.

Roles of Phase Purity and Crystallinity on Chloramphenicol Sensing Performance of CuCo2O4/CuFe2O4-based Electrochemical Nanosensors

For the first time, the influences of phase purity and crystallinity on the electrochemical and electrocatalytic properties of CuCo2O4 (CCO) and CuFe2O4 (CFO)-based electrochemical sensors for the detection of chloramphenicol (CAP) are reported. A series of CCO and CFO nanoparticles were prepared by a modified coprecipitation method and then annealed at different temperatures under air (400 °C, 600 °C, 800 °C, and 1000 °C). Surface morphology, the evolution of the crystallite size, and crystalline phase transition, as well as phase purity of CCO and CFO at each annealing temperature, were characterized via different techniques. Their electrochemical properties were analyzed using cyclic voltammetry and differential pulse voltammetry measurements conducted with a PalmSens3 workstation. Results obtained show that the phase purity and crystallinity have decisive effects on their electrocatalytic activity, conductivity, and adsorption efficiency. Under an optimized condition (more namely, annealed 600 °C), both CCO and CFO samples offer high phase purity with low percent of CuO side phase (below 38%), small enough size with a large number of defects and available active sites; particularly, the cubic CFO nanoparticles are present due to its tetragonal phase transition. The modified electrodes with CCO-600 and CFO-600 exhibit a better voltammetric response, a higher synergistic electrocatalytic activity, and a greater electrochemical performance of comparing to other modified electrodes. They respond linearly to chloramphenicol (CAP) in the range from 2.5 to 50 μM. Furthermore, they display am excellent long-term stability, reproducibility, and good selectivity, as well as their capacity of detecting CAP in the real milk sample.

Vacuum ultraviolet photoabsorption spectroscopy of space-related ices: formation and destruction of solid carbonic acid upon 1 keV electron irradiation

Context. Carbonic acid (H2CO3) is a weak acid relevant to astrobiology which, to date, remains undetected in space. Experimental work has shown that the β-polymorph of H2CO3 forms under space relevant conditions through energetic (UV photon, electron, and cosmic ray) processing of CO2- and H2O-rich ices. Although its α-polymorph ice has been recently reassigned to the monomethyl ester of carbonic acid, a different form of H2CO3 ice may exist and is synthesized without irradiation through surface reactions involving CO molecules and OH radicals, that is to say γ-H2CO3. Aims. We aim to provide a systematic set of vacuum ultraviolet (VUV) photoabsorption spectroscopic data of pure carbonic acid that formed and was destroyed under conditions relevant to space in support of its future identification on the surface of icy objects in the Solar System by the upcoming Jupiter ICy moons Explorer mission and on interstellar dust by the James Webb Space Telescope spacecraft. Methods. We present VUV photoabsorption spectra of pure and mixed CO2 and H2O ices exposed to 1 keV electrons at 20 and 80 K to simulate different interstellar and Solar System environments. Ices were then annealed to obtain a layer of pure H2CO3 which was further exposed to 1 keV electrons at 20 and 80 K to monitor its destruction pathway. Fourier-transform infrared (FT-IR) spectroscopy was used as a secondary probe providing complementary information on the physicochemical changes within an ice. Results. Our laboratory work shows that the formation of solid H2CO3, CO, and O3 upon the energetic processing of CO2:H2O ice mixtures is temperature-dependent in the range between 20 and 80 K. The amorphous to crystalline phase transition of H2CO3 ice is investigated for the first time in the VUV spectral range by annealing the ice at 200 and 225 K. We have detected two photoabsorption bands at 139 and 200 nm, and we assigned them to β-H2CO3 and γ-H2CO3, respectively. We present VUV spectra of the electron irradiation of annealed H2CO3 ice at different temperatures leading to its decomposition into CO2, H2O, and CO ice. Laboratory results are compared to Cassini UltraViolet Imaging Spectrograph observations of the 70−90 K ice surface of Saturn’s satellites Enceladus, Dione, and Rhea.

Influence of BaO on the catalytic properties and durability of Pd-CZLA for automobile exhaust

Co-precipitation was used to prepare CeO2-ZrO2-La2O3 (CZL) support, and impregnation was employed to load nano-aluminum sol and Pd to prepare a Pd-CZLA catalyst that can be directly used in car exhaust pipe for the secondary purification of exhaust gas. The specific surface area of the catalyst can reach 143.2 m2/g. At 400 0 C, the catalytic efficiency of the catalyst was 76.47%, 70.59%, and 75.41% for C3H8, CO, and NOx, respectively. After modification through doping with 8% BaO in nano-alumina, the catalyst has enhanced high-temperature stability and maintained its high catalytic activity after aging at 1100 0 C. Scanning electron micrograph showed that the high-temperature sintering of the catalyst is slowed down when modified by BaO addition. Combined barium oxide and nano-alumina acts as a stabilizer that prevents the crystalline phase transition of alumina at high temperature and further stabilizes its structure. This material also stabilizes Pd dispersion and improves CZA supportive protection at high temperature.

Proposal for a universal nonvolatile logic device based on the phase change magnetic material

Memory devices that are capable of logic functions have shown great potential in constructing non-von Neumann parallel computing architecture. Here, we proposed a universal nonvolatile logic device, utilizing the amorphous–crystalline phase transition behavior and phase change control of ferromagnetism property of Mn-doped GeTe phase change magnetic material (PCMM). The materials implication (IMP) logic in one step and sequential NAND logic were presented theoretically, based on which the functional completeness of Boolean logic could be realized. The logic operating results can be stored directly in the residual magnetization of the PCMM. This proposal may promote the design and future experimental implementation of next generation electromagnetic devices for data storage and computing.