Crystallization Of Amorphous Films Research Articles

Impact of Crystal Nuclei on Dissolution of Amorphous Drugs.

Amorphous drugs are used to improve bioavailability of poorly water-soluble drugs. Crystallization must be managed to take full advantage of this formulation strategy. Crystallization of amorphous drugs proceeds in a sequence of crystal nucleation and growth, with different kinetics. At low temperatures, crystal nucleation is fast, but crystal growth is slow. Therefore, amorphous drugs may generate dense but nanoscale crystal nuclei. Such tiny nuclei cannot be detected using routine powder X-ray diffraction (PXRD) and polarized light microscopy (PLM). However, they may negate the dissolution advantage of amorphous drugs. In this work, for the first time, the impact of crystal nuclei on dissolution of amorphous drugs was studied by monitoring the real-time dissolution from amorphous drug films, with and without crystal nuclei, and the evolving crystallinity in the films. Three model drugs (ritonavir/RTV, posaconazole/POS, and nifedipine/NIF) were chosen to represent different crystallization tendencies in the supercooled liquid state, namely, slow-nucleation-and-slow-growth (SN-SG), fast-nucleation-and-slow-growth (FN-SG), and fast-nucleation-and-fast-growth (FN-FG), respectively. We find that although the amorphous films containing nuclei do not show obvious differences from the nuclei-free films under PLM and PXRD before dissolution, they have inferior dissolution performance relative to the nuclei-free amorphous films. For SN-SG drug RTV, crystal nuclei have negligible impact on the crystallization of amorphous films, dissolution rate, and supersaturation achieved. However, they cause earlier de-supersaturation by inducing crystallization in solution as heterogeneous seeds. For FN-SG drug POS and FN-FG drug NIF, crystal nuclei accelerate crystallization in the amorphous films leading to lower supersaturation achieved with POS, and elimination of any supersaturation with NIF. Dissolution profiles of amorphous films can be further analyzed using a derivative function of the apparent dissolution rate, which yields amorphous solubility, initial intrinsic dissolution rate, and onset of crystallization in the amorphous films. This study highlights that although crystal nuclei are undetectable with routine analytical methods, they can significantly negate, or even eliminate, the dissolution advantage of amorphous drugs. Hence, understanding crystal nucleation process and developing approaches to prevent it are necessary to fully realize the benefits of amorphous solids.

Dielectric functions evolution and electronic bandgap manipulation by silicon doping for Sb2Te3 phase change films: Temperature dependent spectroscopic ellipsometry study

The dielectric function evolution and electronic bandgap manipulation by silicon (Si) doping for Sb2Te3 phase change films have been investigated by temperature dependent spectroscopic ellipsometry measurements. During the phase change from amorphous to rhombohedral structures, the significant contrast of electronic band structure for pure Sb2Te3 and Si-doped Sb2Te3 (SST) films as functions of temperature (210–620 K) and Si concentration (0%–12%) has been systematically studied by analyzing the dielectric functions, Tauc gap energy, and partial spectral weight integral. The distinct differences can be mainly attributed to the increment of structure order degree, originated from the change of local bonding arrangement. Based upon the evolutions of Tauc gap energy and partial spectral weight integral with increasing temperature for all four samples, it can be concluded that Si doping can inhibit the crystallization of amorphous films and accelerate the phase change process by serving as nanoscale heaters, which is helpful in improving the thermal stability of amorphous films. The elevated crystallization temperature and phase change rate by Si doping contribute to the dependability and endurance for SST-based phase change memory. The present data provide an important direction on the physical mechanism investigation of Si doping Sb2Te3 by optical techniques.

Layer polymorphous crystallizations of amorphous films: structural and kinetic aspects

Structural and geometric aspects of layer polymorphic crystallization of thin amorphous films were considered. The review of electron microscopic studies of crystallization “in situ” of amorphous films under the influence of the electron beam was provided. Digital video recording in real time t of structural and phase changes occurring in the film was applied. Kinetic crystallization curves were constructed and the parameters reflecting the growth of crystals of various morphologies in amorphous film were defined. Electron-beam crystallization of amorphous films is accompanied by the formation of Cr2O3 and V2O3 crystals of three basic morphological forms: disk-shaped, sickle-shaped and needle-shaped, the kinetics of growth of which is different. The growth of disk-shaped crystals occurs at a constant rate; the degree of crystallinity x ∼ t 2, and the rate of change in the fraction of the crystalline phase x't ∼ t. Sickle-shaped (at the initial stage) crystal, as it grows, changes its morphology in the sequence: sickle-shaped → ring-shaped → disk-shaped crystal.

Phase change behavior improvement of Sb2Te3 films by Si doping: Raman scattering evidence at elevated temperatures

The dynamic crystallization process of silicon (Si) doped Sb2Te3 (SST) films as a function of temperature (210–620 K) and Si concentration (0%–12%) has been investigated by temperature dependent Raman scattering. Based upon the evolution of original Raman spectra and surface morphologies with increasing temperature for the samples, it can be concluded that the temperature ranges for intermediate transition states are estimated to be 160, 120, and 80 K, corresponding to SST7%, SST9%, and SST12% films, respectively. This phenomenon can also be summarized by the thermal evolution of the proportion of the rhombohedral phase derived from Raman phonon modes. It illustrates that Si introduction can suppress the crystallization of amorphous films, increase the crystallization temperature, and accelerate the crystallization process of the intermediate transition state. The phase change behavior improvement by Si doping is of great significance for further study on the amorphous thermal stability and endurance of SST-based data storage devices from the optical perspective.

Formation of catalyst nanoparticles for the growth of carbon nanotubes during annealing of amorphous Co-Zr-O films

The formation of nanosized catalyst particles for the growth of carbon nanotubes can be carried out during the crystallization of amorphous films consisting of two metals, one of which has higher free energy of the oxide. During annealing in the presence of oxygen, this metal is oxidized with the reduction of the second metal, which leads to the formation of nanoparticles embedded in the oxide of the first metal. This the process has been experimentally and theoretically studied by the example of the formation of cobalt nanoparticles on the surface of amorphous Co-Zr-O films as a result of the decomposition of a supersaturated solid solution and mechanical stresses arising during the oxidation of zirconium. We have proposed a mechanism for the formation of catalyst nanoparticles and phenomenological model of this process developed on the basis of the phase-field theory. Keywords: carbon nanotubes, nanoparticles of catalyst, phase transitions, decomposition of supersaturated solid solutions.

Формирование наночастиц катализатора роста углеродных нанотрубок при отжиге аморфных пленок Co-Zr-O

The formation of nanosized catalyst particles for the growth of carbon nanotubes can be carried out during the crystallization of amorphous films consisting of two metals, one of which has higher free energy of the oxide. During annealing in the presence of oxygen, this metal is oxidized with the reduction of the second metal, which leads to the formation of nanoparticles embedded in the oxide of the first metal. This the process has been experimentally and theoretically studied by the example of the formation of cobalt nanoparticles on the surface of amorphous Co-Zr-O films as a result of the decomposition of a supersaturated solid solution and mechanical stresses arising during the oxidation of zirconium. We have proposed a mechanism for the formation of catalyst nanoparticles and phenomenological model of this process developed on the basis of the phase-field theory.

Dependence on Film Thickness of Guest-Induced c Perpendicular Orientation in PPO Films.

For poly(2,6-dimethyl-1,4-phenylene)oxide (PPO) films exhibiting nanoporous-crystalline (NC) phases, c⊥ orientation (i.e., crystalline polymer chain axes being preferentially perpendicular to the film plane) is obtained by crystallization of amorphous films, as induced by sorption of suitable low-molecular-mass guest molecules. The occurrence of c⊥ orientation is relevant for applications of NC PPO films because it markedly increases film transparency as well as guest diffusivity. Surprisingly, we show that the known crystallization procedures lead to c⊥ oriented thick (50–300 μm) films and to unoriented thin (≤20 μm) films. This absence of crystalline phase orientation for thin films is rationalized by fast guest sorption kinetics, which avoid co-crystallization in confined spaces and hence inhibit formation of flat-on lamellae. For thick films exhibiting c⊥ orientation, sigmoid kinetics of guest sorption and of thickening of PPO films are observed, with inflection points associated with guest-induced film plasticization. Corresponding crystallization kinetics are linear with time and show that co-crystal growth is poorly affected by film plasticization. An additional relevant result of this study is the linear relationship between WAXD crystallinity index and DSC melting enthalpy, which allows evaluation of melting enthalpy of the NC α form of PPO (ΔHmο = 42 ± 2 J/g).

Tunable Negative Differential Resistance and Resistive Switching Properties of Amorphous WOₓ Devices

The development of multifunctional electric device is of great significance for improving the integration dense of integrated circuits in the future. Herein, amorphous WO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x</sub> -based memristor was fabricated and the mechanism of resistance switching was studied in detail. The device exhibited a stable coexistence of bipolar resistive switching (RS) and negative differential resistance (NDR) behaviors at room temperature, which could modulated by changing the annealing time at 300 °C. The optimal high-resistance state (HRS)/low-resistance state (LRS) resistance ratio of ~30 was obtained by postannealing in 100 min. However, both RS and NDR behaviors disappeared with the crystallization of amorphous films when the annealing time was 300 min. The Ohmic conduction mechanism should be responsible for the charge transport of LRS, while the HRS transmission was corporately dominated by the trap-controlled space charge limited conduction (SCLC) and Poole-Frenkel (P-F) emission. The change of trap levels modulated by postannealing was calculated and considered to be the reason for the change of RS and NDR behaviors. Raman spectroscopy provided evidence for the formation of strong W-O bonds, which contributed to the disappearance of RS and NDR behaviors.

C-Perpendicular Orientation of Poly(ʟ-lactide) Films.

Poly(ʟ-lactide) (PLLA) films, even of high thickness, exhibiting co-crystalline and crystalline α phases with their chain axes preferentially perpendicular to the film plane (c⊥ orientation) have been obtained. This c⊥ orientation, unprecedented for PLLA films, can be achieved by the crystallization of amorphous films as induced by low-temperature sorption of molecules being suitable as guests of PLLA co-crystalline forms, such as N,N-dimethylformamide, cyclopentanone or 1,3-dioxolane. This kind of orientation is shown and quantified by two-dimensional wide-angle X-ray diffraction (2D-WAXD) patterns, as taken with the X-ray beam parallel to the film plane (EDGE patterns), which present all the hk0 arcs centered on the meridian. PLLA α-form films, as obtained by low-temperature guest-induced crystallization, also exhibit high transparency, being not far from those of the starting amorphous films.

Thermoelectric Properties of Amorphous and Nanocrystallic Cr0.33Si0.67 Films at Different Annealing Stages in the Range 100–900 K

The thermoelectric properties of thin Cr0.33Si0.67 films in amorphous and crystalline states, as well as at different annealing stages in the temperature range 100–900 K have been studied. The crystallization of amorphous films at a temperature of ~550 K is accompanied by an increase in the resistivity and thermoelectric power. Amorphous films crystallize with the formation of CrSi2 nanocrystals with an average grain size of 10–20 nm. The increase in resistivity is due to the appearance of a crystalline phase and the formation of interphase boundaries between the crystalline and amorphous phases. The scattering of charge carriers at interphase boundaries is selective, which leads to the appearance of an additional contribution to the thermopower.

Structural and kinetic aspects of electron-beam crystallization of amorphous films of antimony sulfide

Electron-beam crystallization of amorphous films of Sb2S3 of both stoichiometric composition and with the excess of Sb was studied by the methods of transmission electron microscopy “in situ” with video registration of structural changes. It was revealed, that Sb2S3 films of stoichiometric composition crystallize polymorphous in a single stage. Films with the excess of Sb crystallize in two stages. There is predominant crystallization of Sb during the first stage of the process, and subsequent crystallization of matrix of Sb2S3 during the second stage. The kinetic curves, demonstrating the dependence on time of the density and size of micro-crystals of Sb, as well as of the dependence on time of the size of Sb2S3 crystals and of fraction of the crystalline phase were obtained with the help of frame-by-frame analysis of the video. Primary crystallization of Sb2S3 occurs in accordance with the layer polymorphous crystallization mode with the relative length δ0 ≈ 4992. The dependence on time of the fraction crystallized has a quadratic character. Secondary crystallization of Sb2S3 occurs also in accordance with the layer polymorphous crystallization mode with the relative length δ0 ≈ 9725. The dependence on time of the fraction crystallized can be described by power function with the exponent 0.68.

Kinetics of electron beam crystallization of amorphous films of Yb2O2S

Electron-beam crystallization of amorphous films of Yb2O2S was studied by the methods of transmission electron microscopy “in situ” with video registration of structural changes. The kinetic curves, demonstrating the dependence on time of the crystals density and of the fraction of the crystalline phase was obtained with the help of frame-by-frame analysis of the video. The formation of the polycrystalline film occurs at a constant crystal growth rate and at a constant intensity of nucleation of crystallization centers, which corresponds to α-version of the Kolmogorov's model. The phase transformation in amorphous films of Yb2O2S occurs in accordance with the island polymorphous crystallization mode with the relative length δ0≈ 1030. At small layer thicknesses of Yb2O2S (d ≤ 20 nm) the effect of stabilization of the amorphous state was observed.

Structure-property correlations in molybdenum trioxide thin films and nanoparticles

Molybdenum trioxide films were grown by thermal evaporation of MoO3 powder on silica, silicon, and alumina substrates and their crystal structure; short−range order, stoichiometry, surface morphology, optical, electrical, and gas-sensing properties were studied. Optical band gap decreases from 2.88 eV to 1.88 eV with post−deposition annealing and crystallization of amorphous film into the orthorhombic phase. Refractive indices and thicknesses of amorphous and crystalline films were determined from the analysis of interference fringes in the UV-visible transmittance and reflectance spectra. Nanoparticles of MoO3 were produced by solvo-thermal method and their structural, surface morphology and gas-sensing properties were compared with those of thin films. High−resolution Raman studies were performed on amorphous and crystalline film, bulk and nanopowder samples, these studies revealed that the short-range structure of amorphous MoO3 is similar to that of the orthorhombic phase. The relative intensities of low frequency Raman peaks at 285 cm−1 and 291 cm−1 show correlation with the oxygen concentration in the samples and it is found that Raman spectroscopy can be used to determine the Mo−O stoichiometry. The gas sensing properties of crystalline film and nanoparticles were studied towards H2S and C2H5OH. Thin films show H2S response of 2.8 towards 150 ppm of H2S, and a response of 2.6 for 2000 ppm of C2H5OH. Nanoparticles exhibit higher H2S and C2H5OH response of ∼6 and faster response times. Maximum response towards the two gases is in the operating temperature range of 453 K to 500 K and is due to the reaction of H2S and C2H5OH molecules with surface adsorbed O− ions, and there is little or no role of surface redox reactions.

Kinetics of autoepitaxial crystal growth in amorphous films of Cr2O3

Kinetics of autoepitaxial crystal growth in amorphous films of Cr2O3 were studied by the methods of transmission electron microscopy “in situ” with video registration of structural changes. With frame-by-frame analysis of the video, the kinetic curves of the time dependence of the radius, length and width of growing crystals with different morphological types (needle-shaped and disc-shaped crystals) were obtained. The time dependence of fraction of the crystallized material was determined. Crystallization front may be splitting into slices that can move at different speed. The conclusion was made that the autoepitaxial crystal growth is the type of layer polymorphous crystallization of amorphous films.

Crystallization of submicron amorphous hydrogenated silicon films with different hydrogen concentration by nanosecond ruby laser irradiation

Pulse laser annealing remains an actual problem aimed to crystallization of amorphous films on nonrefractive substrates. But, the most previous works studied laser crystallization of relatively thin (<300 nm) a-Si:H films and mainly used excimer lasers. But, excimer lasers are not suitable for crystallization of submicron a-Si:H films due to low penetration depth in a-Si:H at such wavelengths. The problem can be resolved by using lasers with longer wavelengths. The desirable result of crystallization also depends on the choice of proper laser fluence, which is different for films with different hydrogen concentrations. In this work, the processes of a pulsed ruby laser induced crystallization of submicron (0.7 μm) amorphous hydrogenated silicon films with different hydrogen concentrations (2, 12, and 39 at. %) by different laser fluences were investigated. The films were prepared on glass substrates by plasma enhanced chemical vapor deposition technique followed by isothermal annealing in nitrogen atmosphere. The laser annealing (λ = 694 nm) was carried out at a pulse duration of 80 ns (full width at half-maximum) in the fluence range from 0.6 to 2.1 J/cm2. The laser fluence thresholds for surface area crystallization were found for different hydrogen concentrations in the films. The increase of hydrogen concentration leads to an increase of the threshold energy density (laser fluence) for surface area crystallization due to a decrease of light absorption in the films with a higher hydrogen concentration. Also, it was shown that ruby laser radiation can penetrate and partially crystallize the full depth of the submicron a-Si:H film, but the problem of homogeneity remains.Pulse laser annealing remains an actual problem aimed to crystallization of amorphous films on nonrefractive substrates. But, the most previous works studied laser crystallization of relatively thin (<300 nm) a-Si:H films and mainly used excimer lasers. But, excimer lasers are not suitable for crystallization of submicron a-Si:H films due to low penetration depth in a-Si:H at such wavelengths. The problem can be resolved by using lasers with longer wavelengths. The desirable result of crystallization also depends on the choice of proper laser fluence, which is different for films with different hydrogen concentrations. In this work, the processes of a pulsed ruby laser induced crystallization of submicron (0.7 μm) amorphous hydrogenated silicon films with different hydrogen concentrations (2, 12, and 39 at. %) by different laser fluences were investigated. The films were prepared on glass substrates by plasma enhanced chemical vapor deposition technique followed by isothermal annealing in nitrogen atmosph...

Kinetics of crystals growth under electron-beam crystallization of amorphous films of hafnium dioxide

Kinetics of crystals growth under electron-beam crystallization of amorphous films of hafnium dioxide

Effect of Sn Doping on the Crystallization Kinetics of Amorphous TlInS2 Films

The crystallization of amorphous Sn-doped TlInS2 films into three polymorphs has been studied by kinematic electron diffraction. The results demonstrate that the crystallization of 30-nm-thick amorphous films produced by thermal evaporation in high vacuum can be described by the Avrami–Kolmogorov equation: Vτ = V0[1–exp(–kτm)]. Kinematic electron diffraction patterns of the TlIn1–хSnxS2 films have been used to assess the effect of doping with Sn on the growth dimensionality and the activation energy for the crystallization of the amorphous films and the unit-cell parameters of the resultant crystalline materials. Doping extends the temperature range and effective activation energy for the crystallization of the amorphous films.

Optical Properties and Stability of Bilayer Rubrene-Alq3 Films Fabricated by Vacuum Deposition

We report on the optical and structural characterization of the two-component vacuum deposited (VD) rubrene (Rub)-Alq3 films. As is known, Rub-doped OLED active materials demonstrate both promising electroluminescence and transistor characteristics. However, in terms of operational lifetime, the Rub practical application in basic devices has a few draw-backs related to its chemical instability. Our main attention was focused on the role of the Alq3 coverage and the isomeric transformation of a Rub molecule on its chemical stability in these structures. By monitoring the evolution of PL emission in time, we found that the Rub degradation in Rub-Alq3 films is slower than that in vacuum-deposited Rub layers. These results demonstrate that the deposition of an Alq3 layer can be a way to enhance the stability of Rub to the photo-oxidation in optoelectronic devices. The Rub amorphous film crystallization at elevated temperatures in open air was observed for the first time.

Electron microscopic investigation of the kinetics of the layer and island crystallization of amorphous V2O3 films deposited by pulsed laser evaporation

An electron microscopic investigation was performed on the kinetics of the layer and island crystallization of amorphous V2O3 films deposited by pulsed laser evaporation of vanadium in an oxygen atmosphere. The crystallization was initiated by the action of an electron beam on an amorphous film in the column of a transmission electron microscope. The kinetic curves were plotted on the basis of a frame-by-frame analysis of the video recorded during the crystallization of the film. It was found that the layer crystallization of amorphous films is characterized by a quadratic dependence of the fraction of the crystalline phase x on the time t, whereas the island crystallization is described by an exponential dependence of x on t. The kinetic curves of island crystallization of amorphous films were analyzed on the basis of the α-version of the Kolmogorov model. For each type of crystallization, there are specific values of the dimensionless relative length unit δ0, which is equal to the ratio of the characteristic length unit to the parameter characterizing the unit cell of the crystal. It was established that, for the layer crystallization, the relative length unit lies in the range δ0 ~ 4300–4700, whereas for the fine-grained island crystallization, it amounts to δ0 ~ 110.

Kinetics of the electron beam induced crystallization of amorphous ZrO2 films obtained via ion-plasma and laser sputtering

The structure and electron beam induced crystallization kinetics of amorphous ZrO2 films obtained via ion-plasma and laser sputtering were compared. The studies were performed by electron diffraction and transmission electron microscopy with recording video films in situ. The effect of an electron beam on an amorphous film in a vacuum was accompanied by the formation of zirconia microcrystals with an FCC lattice. For laser evaporation, the density of crystallization nuclei was β ~ 109 cm–2, and the characteristic length unit was D0 ~ 0.48 μm. For ion-plasma evaporation, β ~ 1010 cm–2, and D0 ~ 0.06 μm. The kinetic curves of the crystallization of amorphous films were analyzed using the β-variant of the Kolmogorov model as a basis.