Amorphous-crystalline Phase 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.

Phase formation behavior of alkali-activated calcined clays: effects of the reactive phase and NaOH concentration

ABSTRACT The relevance of calcined clays as sources with a great potential for non-clinker alkali-activated materials has significantly increased in recent decades. This article reports on a study to investigate how NaOH with concentrations in the 5–14 M range affects the mechanical properties, amorphous-crystalline phase transformations, and reaction product composition of alkali-activated calcined kaolinite/montmorillonite cements with reactive phases of 60% and 33%, and with the SiO2/Al2O3 ratios of 1.54 and 1.74. The hardened pastes based on both calcined clays exhibited the highest compressive strength up to 9.1 MPa following their activation with 8 M NaOH. The higher amorphous fraction in the calcined clay provided gradual zeolitisation/crystallisation at Na2O/SiO2 and Na2O/Al2O3 molar ratios in the 0.36–0.63 and 0.56–0.98 ranges, respectively, accompanied by the deterioration of the mechanical characteristics of the samples. The zeolite content in the alkali-activated calcined clay cements with an amorphous fraction of 33% peaked for NaOH at 11 M, and the corresponding molar ratios of Na2O/SiO2 and Na2O/Al2O3 were 0.85 and 1.46, respectively. The main reaction products in the studied systems determined by XRD, TG/DSC, and FTIR spectroscopy analyses were N-A-S-H and zeolite A. The properties and reaction products obtained favor the use of proposed alkali-activated cements for controlled low-strength materials.

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.

Influence of phase transformation on structure–property relationship in quaternary In10Sb10Ag10Se70 chalcogenide films

This work reports the effect of thermal annealing (353–413 K) on structure and physical properties of thermally evaporated In10Ag10Sb10Se70 chalcogenide films. The samples were characterized by X-ray diffraction, transmission electron microscopy, TEM, Raman spectroscopy, optical absorption and photoconductivity measurements. Thermal annealing favours amorphous to polycrystalline structure transition with the growth of Se, AgInSe2 and AgSbSe2 phases. A significant change in position/shape of Raman bands upon annealing has been observed. The indirect optical gap decreases from 1.44 to 0.68 eV while tailing parameter increases with annealing. Photosensitivity decreases from 29.3 to 14.2 with annealing temperature. The decay of photocurrents is well fitted to stretched exponential function; the value of decay time constant increases while local minimum value of dispersion parameter at 353 K has been observed. These results are explained on the basis of amorphous crystalline phase transformation in chalcogenides and are important for developing new materials for emerging technologies.

Reversible fluorescent switching properties of pyrene-substituted acylhydrazone derivatives toward mechanical force and acid vapor with aggregation-induced emission

Four pyrene-substituted acylhydrazone derivatives named PMBH ((E)-N'-(pyren-1-ylmethylene)benzohydrazide), PMPH ((E)-N'-(pyren-1-ylmethylene)picolinohydrazide), PMNH ((E)-N'-(pyren-1-ylmethylene)nicotinohydrazide) and PMIH ((E)-N'-(pyren-1-ylmethylene)isonicotinohydrazide) by functionalizing the phenyl ring and pyridyl ring with pyrene-substituted acylhydrazone moiety have been designed and synthesized, whose properties were investigated. All the four synthesized compounds exhibited favorable aggregation induced emission (AIE) features and reversible mechanofluorochromism (MFC) behaviour. Especially, the intense cyan emission of PMIH changed into orange-yellow emission with a 101 nm red-shift upon grinding. The MFC behavior was associated with the crystalline-amorphous phase transformation between the pristine and ground states, which was confirmed by the investigations of the power X-ray diffraction (PXRD), the differential scanning calorimetry (DSC) and field emission scanning electron microscopy (FESEM). Specifically, PMPH, PMNH and PMIH demonstrated remarkable and reversible acidofluorochromic response due to the presence of free nitrogen atom of pyridyl groups at the terminal position. The chosen approaches showed a new possibility for generating multi-stimuli responsive materials with modulated solid state properties.

The evident aggregation-induced emission and the reversible mechano-responsive behavior of carbazole-containing cruciform luminophore

New D-A type cruciform luminophore TFMB-CBCEB bearing electron-donor carbazole unit and electron-acceptor dicyanovinylbenzene segment was designed and prepared. The results show that the emission wavelengths of TFMB-CBCEB are strongly affected by solvent polarity, indicating intramolecular charge-transfer (ICT) transitions. Interestingly, TFMB-CBCEB exhibits AIE behavior with high αAIE of 405 and high solid-state luminescence efficiency of up to 0.810. Moreover, TFMB-CBCEB shows remarkable mechanofluorochromic (MFC) properties. When the as-prepared samples are subjected to grinding, the emission of TFMB-CBCEB red-shifts by 32 nm from the blue regions (λmax = 566 nm) to the orange regions (λmax = 598 nm). The changes of fluorescence emission induced by grinding can be restored by solvent-fuming and regenerated by grinding, suggesting excellent reversibility in the switching processes. PXRD analysis revealed that the MFC mechanism of TFMB-CBCEB attributed to the crystalline-amorphous phase transformation between the original and ground states. Meanwhile, the red-shift in the PL spectra upon grinding comes from the extension in molecular conjugation caused by planarization of molecular conformation and subsequent planar intramolecular charge transfer (PICT) process.

Mechanofluorochromic behavior of anthracene-based acylhydrazone derivatives

Two anthracene-based derivatives, having one or two acylhydrazone branches attached on the anthracene, namely, asymmetrical AHP-1 and symmetrical AHBP-1 were designed. These compounds exhibited mechanofluorochromic behavior with the emission colors reversibly changing upon grinding and recrystallizing. The mechanofluorochromic mechanism was explored and ascribed to the crystalline-amorphous phase transformation. In addition, the red-shifted extent of fluorescent emission of the ground AHBP-1 samples was more than that of AHP-1.

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.

Piezofluorochromism of triphenylamine-based triphenylacrylonitrile derivative with intramolecular charge transfer and aggregation-induced emission characteristics

New triphenylacrylonitrile derivative (DPPA) with triphenylamine moiety as electron donor group has been synthesized. Its emission wavelengths were strongly affected by solvent polarity, indicating intramolecular charge transfer (ICT) transitions. It was found that DPPA was almost non-emissive in tetrahydrofuran (THF), and the emission could be intensified obviously when a great amount of H2O was added, illustrating aggregation induced emission property. It is interesting that the pristine crystal of DPPA emitted green light, and the ground made its emitting color to change into orange, which could be recovered under fumed with dichloromethane (DCM) vapor. The piezofluorochromic behavior might be switched reversibly under the stimulus of external force because of crystalline-amorphous phase transformation, which could be confirmed from the results of PXRD patterns and DSC curves in different solid states. This work might open a new strategy to provide a broad perspective for the development of AIE PFC materials.

Evolving affinity between Coulombic reversibility and hysteretic phase transformations in nano-structured silicon-based lithium-ion batteries

Nano-structured silicon is an attractive alternative anode material to conventional graphite in lithium-ion batteries. However, the anode designs with higher silicon concentrations remain to be commercialized despite recent remarkable progress. One of the most critical issues is the fundamental understanding of the lithium–silicon Coulombic efficiency. Particularly, this is the key to resolve subtle yet accumulatively significant alterations of Coulombic efficiency by various paths of lithium–silicon processes over cycles. Here, we provide quantitative and qualitative insight into how the irreversible behaviors are altered by the processes under amorphous volume changes and hysteretic amorphous–crystalline phase transformations. Repeated latter transformations over cycles, typically featured as a degradation factor, can govern the reversibility behaviors, improving the irreversibility and eventually minimizing cumulative irreversible lithium consumption. This is clearly different from repeated amorphous volume changes with different lithiation depths. The mechanism behind the correlations is elucidated by electrochemical and structural probing.

Effects of alkyl chain length on aggregation-induced emission, self-assembly and mechanofluorochromism of tetraphenylethene modified multifunctional β-diketonate boron complexes

Two tetraphenylethene functionalized β-diketonate boron complexes C2TPE-CAR and C16TPE-CAR that were bridged by carbazole bearing alkyl chains of different lengths, have been designed and successfully synthesized. The two donor–acceptor luminogens showed typical twisted intramolecular charge-transfer (TICT) emission. In particular, they exhibited alkyl-dependent aggregation-induced emission (AIE), self-assembly and mechanofluorochromic (MFC) properties. Results showed that C16TPE-CAR with a long hexadecyl chain formed gels in certain solvents and also exhibited gelation-induced emission enhancement. H-aggregates were formed driven by the synergistic effect of π-stacking interaction and van der Waals force in the gel phase. However, C2TPE-CAR, which possessed the short ethyl group, exhibited no self-assembly behavior. Simple mechanical force could change the emission color of C2TPE-CAR and C16TPE-CAR powders from initial bright yellow (571 and 558 nm) to final red (617 and 610 nm), thus high contrast mechanofluorochromism with large spectral shifts of up to 46 and 52 nm, respectively, was observed. The powder XRD studies showed that the MFC behavior might be switched reversibly under the stimulus of external force because of crystalline-amorphous phase transformation.

Reversible crystalline-amorphous phase transformation in Si nanosheets with lithi-/delithiation

Silicon (Si) has a large theoretical capacity of 4200 mAhg−1 and has great potential as a high-performance anode material for Li ion batteries (LIBs). Meanwhile, nanostructures can exploit the potential of Si and, accordingly, many zero-dimensional (0D) and one-dimensional (1D) Si nanostructures have been studied. Herein, we report on two-dimensional (2D) Si nanostructures, Si nanosheets (SiNSs), as anodes for LIBs. These 2D Si nanostructures, with a thickness as low 5 nm and widths of several micrometers, show reversible crystalline–amorphous phase transformations with the lithi-/delithiation by the dimensionality of morphology and large surface area. The reversible crystalline–amorphous phase transformation provides a structural stability of Li+ insertions and makes SiNSs promising candidates for reliable high-performance LIBs anode materials.

Triphenylamine functionalized β-Ketoiminate boron complex exhibiting aggregation-induced emission and mechanofluorochromism

A new triphenylamine functionalized β-Ketoiminate boron complex BF2-TPA that bears the electron donor triphenylamine and the electron acceptor β-iminoenolate boron moiety, has been designed and successfully synthesized. The D–π–A type conjugated compound showed obvious intramolecular charge transfer (ICT) properties, which were evidenced by theoretical calculations and spectral analysis. The compound exhibited evident AIE characteristic with high fluorescence quantum yield in the solid state. BF2-TPA was quite weakly fluorescent in THF, while a significant AIE effect was observed in water/THF (fw = 70%) mixtures with a large AIE factor of 188. Interestingly, BF2-TPA possesses the distinct mechanofluorochromic (MFC) behavior. The as-prepared powder of BF2-TPA could emit strong yellow light centered at 549 nm under UV irradiation, and the fluorescence color changed into orange (590 nm) after grinding, a red shift of 41 nm was observed. Such mechanochromism was reversible upon the treatment of grinding and fuming with CH2Cl2. The powder XRD studies showed that the MFC nature is generated through crystalline–amorphous phase transformation under the stimulus of external force. The relationship between the structure and the properties was studied in detail. The results obtained would be of great help in understanding the MFC mechanism and designing new MFC materials.

Synthesis, crystal structure and reversible mechanofluorochromic properties of a novel phenothiazine derivative

A dibenzoyl phenothiazine derivative containing two benzoyl groups has been synthesized via a Friedel-Crafts reaction. The new phenothiazine gave strong emission in solution and in solid state, and the solid fluorescence quantum yield was as high as 0.63. The single crystal structure of the phenothiazine revealed that the hydrogen bonds CH⋯C (2.864 Å), CH⋯C (2.850 Å), CH⋯O (2.652 Å) would lock the molecular conformation, and the twisted conformation of the molecule lead to poor solid molecular packing, yielding highly emissive behavior in its crystal. Moreover, reversible mechanofluorochromism of the phenothiazine was realized upon grinding/fuming or annealing. The emission color of the phenothiazine changed from green to yellow after grinding and could be restored via fuming with CH2Cl2 or heating. The XRD results show that the mechanofluorochromic nature is generated through crystalline-amorphous phase transformation under external pressure.

Fine-tuning the mechanofluorochromic properties of benzothiadiazole-cored cyano-substituted diphenylethene derivatives through D–A effect

We synthesized three new benzothiadiazole-cored cyano-substituted diphenylethene derivatives (PT-OMe, PT-H, and PT-CF3) with different methoxy, hydrogen, and trifluoromethyl end groups, and the synthesis confirmed by standard spectroscopic methods. These end groups endowed them with different donor–acceptor (D–A) effects, and they provide them with a peculiar and completely opposite mechanofluorochromic property. Red-shifted mechanofluorochromic features were found in the PT-OMe and PT-H compounds, while on the contrary, PT-CF3 showed blue-shifted mechanofluorochromic behavior. The mechanofluorochromic mechanism was explored and attributed to the metastable state of the ground compounds and the crystalline-amorphous phase transformation between the original and ground states. Moreover, these derivatives showed reversible significant mechanofluorochromic properties and reproducibility between ground and annealed states, making them promising stimuli-responsive and smart luminescent materials for mechanosensors, fluorescence switches and light-emitting device applications. The introduction of the D–A effect strategy demonstrated in this work would provide a new path to fine tune the optical features of mechanofluorochromic materials with unique and diverse fluorescent properties.

Phase transformation kinetics and optical properties of Ga–Se–Sb phase-change thin films

Phase transformation kinetics in Ga25Se75−xSbx glasses have been determined by non-isothermal differential scanning calorimetric measurements at heating rates of 5, 10, 15, 20 and 25K/min. The values of glass transition (Tg) and crystallization temperature (Tc) are found to be composition and heating rate dependent. The activation energy of crystallization and glass transition have been determined from the dependence of Tc and Tg on the heating rate. Thin films of Ga25Se75−xSbx glasses have been prepared by vacuum evaporation technique with thickness 400nm. These thin films were crystallized by thermal annealing and laser-irradiation. The phase change phenomena have been studied by measuring optical absorption of as-prepared and crystallized thin films in the wave length region 400–900nm. The optical absorption data indicate that the absorption mechanism is non-direct transition. Optical band gap values decrease with increase in Sb contents in Ga–Se as well as with increase in annealing temperature and laser-irradiation time. The optical band gap is shifted due to crystallization by annealing/laser-irradiation. As the phase of the films changes from amorphous to crystalline, a non sharp change of the optical band gap is observed. This gradual decrease in optical band gap was explained to be a result of an amorphous–crystalline phase transformation.

Study of Amorphous-Crystalline Phase Transformations by DSC and Dilatometer in the Case of Al-Based Amorphous Alloys

The amorphous-crystalline transformation is studied by DSC and dilatometer. From the DSC signal the characteristic temperatures and the heat evolved during each transformation step can be determined. The dilatometer depicts the thermal contractions accompanying the changes in topological short range order. Although the characteristics temperatures determined by DSC and dilatometer, two methods are slightly different their dependences on the heating rate are very similar, where from the activation energies of each transformation steps can be determined using the Kissinger plots. As an example, two aluminum-based amorphous ribbon samples (Al88Mm5Ni5Co2 and Al85Y8Ni5Co2) will be measured and compared.

Polarity-dependent reversible resistance switching in as-deposited AgInSbTe phase change film

Abstract The electrical properties of a nanoscale prototype capacitor-like cell structure using an amorphous Ag8In14Sb55Te23 (AIST) as the active material were investigated. A polarity-dependent reversible switching effect which is different from the amorphous–crystalline phase transformation has been observed. The reversible resistance switching between a high resistance state (HRS) and low resistance state (LRS) was induced by bias amplitude and polarity. The electrical resistance ratio of HRS/LRS was ∼10:1. The AIST phase change material is shown to be very promising for multi-state data storage applications.

Piezofluorochromism and morphology of a new aggregation-induced emission compound derived from tetraphenylethylene and carbazole

A new piezofluorochromic aggregation-induced emission compound was synthesized. The compound had morphology-alterable emission property and its amorphous and crystalline aggregates with various emissions were obtained through evaporation from the solutions in different solvent systems. The spectroscopic properties and morphological structures of the compound were reversed upon pressing and annealing (or fuming). The results show that the piezofluorochromic nature is generated through crystalline–amorphous phase transformation under external pressure. The reason for the phase transformation is ascribed to the twisted conformation of the molecule which leads to poor solid molecular packing and formation of some cavities in the interfaces of lamellar layers, as confirmed by single crystal X-ray diffraction analysis.