Transformation Of Thin Films Research Articles

Effect of uni- and biaxial strain on phase transformations in Fe thin films

Using molecular-dynamics simulation, we study the phase transformations in Fe thin films induced by uni- and biaxial strain. Both the austenitic transformation of a body-centered cubic (bcc) film at the equilibrium temperature of the face-centered cubic (fcc)–bcc transformation and the martensitic transformation of an undercooled fcc film are studied. We demonstrate that different strain states (uni- or biaxial) induce different nucleation kinetics of the new phase and hence different microstructures evolve. For the case of the austenitic transformation, the direction of the applied strain selects the orientation of the nucleated grains of the new phase; the application of biaxial strain leads to a symmetric twinned structure. For the martensitic transformation, the influence of the strain state is even more pronounced, in that it can either inhibit the transformation, induce the homogeneous nucleation of a fine-dispersed array of the new phase resulting in a single-crystalline final state, or lead to the more conventional mechanism of heterogeneous nucleation of grains at the free surfaces, which grow and result in a poly-crystalline microstructure of the transformed material.

A study on phase transformation of SnOx thin films prepared by reactive magnetron sputtering

In the paper, SnOx thin films were deposited by reactive magnetron sputtering from a tin target in O2 containing working gas. The evolution from Sn-containing SnO to tetravalent SnO2 films was investigated. The films could be classified into three groups according to their optical band gaps, which are Eg<2.5eV, Eg=3.0–3.3eV and Eg>3.7eV. The electric measurements show that high conductivity can be obtained much easier in SnO2 than in SnO films. A high electron mobility of 15.7cm2V−1s−1, a carrier concentration of 1.43×1020cm−3 and a resistivity of 2.8×10−3Ωcm have been achieved in amorphous SnO2 films. Films with the optical band gap of 3.0–3.3eV remain amorphous though the substrate temperature is as high as 300°C, which implies that °btaining high mobility in p-type SnO is more challenging in contrast to n-type SnO2 films.

Driving forces for texture transformation in thin Ag films

The well-known thickness-dependent (111)-to-(100) texture transformation in thin FCC films is usually attributed to a competition between interface and strain energies. In this model, thin films retain their (111) texture due to the lower energy of the (111) interface, while thick films transform to (100) due to the lower stiffness and thus strain energy of a (100) film. However, recent work has called this model into question, suggesting that neither the stress nor the interface energy play a dominant role in texture transformation. We investigated the driving forces involved in this transformation by using a bulge test apparatus to induce different stresses in thin Ag films under identical annealing conditions. In situ synchrotron XRD measurements show the change in texture during annealing, and reveal that applied stresses have no effect on the transformation. Stress analysis shows that differences in driving forces for texture transformation due to applied bulge pressure were significant (≈200 kJ/m3), suggesting that a different, much larger driving force must be responsible. Reduction in defect energy has been proposed as an alternative. However, vacancy and dislocation densities must be exceptionally high to significantly exceed the strain energy and do not provide obvious orientation selection mechanisms. Nanotwins in reported densities are shown to provide greater driving force (≈1000 kJ/m3) and may account for orientation selection. The large difference between the calculated strain and defect energies and the driving force for grain growth (21,100 kJ/m3) casts doubt on the applicability of a simple thermodynamic model of texture transformation.

Kinetics of phase transformations in thin CuIn5Se8 films

The processes of the phase formation and phase transitions in thin CuIn5Se8 layers are studied by the kinematic electron-diffraction method. It is shown that Cu2Se and In2Se3 films obtained in the ratio of 1: 5 are formed in the amorphous state. The parameters of kinematic crystallization for amorphous CuIn5Se8 films are obtained. The dimension of crystal-nucleus growth, and the activation energies of nucleation and crystal growth are determined.

Supercapacitive Properties of Cobalt Oxide Thin Films Prepared by Electrostatic Spray Deposition Technique at Different Substrate Temperature

In this work, cobalt oxide thin films were prepared by electrostatic spray deposition (ESD) technique. The influence of the substrate temperatures on properties of film was investigated. Phase transformation of cobalt oxide thin films due to the effect of different substrate temperature was also observed. Cyclic voltammetry was used to measure the performance of cobalt oxide supercapacitor. At higher substrate temperature, the cobalt oxide thin films exhibit the high specific capacitance due to the effect of phase transformation in cobalt oxide films.

Rapid and topotactic transformation from octacalcium phosphate to hydroxyapatite (HAP): a new approach to self-organization of free-standing thin-film HAP-based nanohybrids

Biomineralization-inspired processing is attractive for the preparation of functionalized inorganic/organic polymer hybrid materials because the materials are obtained under mild conditions and by using organic templates. As for the formation processes of ordered nanocrystalline hydroxyapatite (HAP), the preparation of self-standing hybrid films based on HAP has not yet been established. In the present study, self-standing thin-film hybrids composed of HAP and poly(vinyl alcohol) (PVA) are obtained by rapid and topotactic transformation of thin films based on octacalcium phosphate (OCP) as a precursor in the organic polymer matrix. Bioinspired crystallization of calcium phosphate on the PVA matrix in the presence of poly(acrylic acid) leads to the formation of nanocomposite structures with oriented OCP nanorod crystals 2–4 nm in width and 10–30 nm in length. The nanostructures allow the composites to transform rapidly into a HAP/PVA hybrid thin film in water. The transformation proceeds without a change in the original OCP/PVA nanostructures, resulting in the formation of a HAP/PVA hybrid thin film with oriented HAP nanorod crystals 5–6 nm in width and 20–50 nm in length. The HAP/PVA hybrids have been obtained as self-standing films with submicrometer scale thickness. The ratio of organic to inorganic components in the self-standing hybrid thin films is similar to that in bones of vertebrates.

Phase transformations in thin iron oxide films: Spectromicroscopic study of velocity and shape of the reaction fronts

Combining low energy electron microscopy (LEEM) with low energy electron diffraction (LEED) and x-ray photoemission electron microscopy (XPEEM), we studied the phase transformations between Fe3O4, γ-Fe2O3, and α-Fe2O3, grown as 10nm thin oxide films on Pt(111) and Ag(111) single crystals. These transformations occur as moving reaction fronts in most cases, the shapes and velocities of which show strong dependences on temperature and oxygen pressure, but also on defects like step bunches of the supporting substrate and domain boundaries in the initial oxide film. While the non-uniform moving fronts make quantitative analysis difficult, we have extracted approximate values for the average front velocities. We discuss these as well as the qualitative information on the non-uniform fronts in terms of the known geometric situations and the likely motional steps.

Martensitic transformation in as-grown and annealed near-stoichiometric epitaxial Ni2MnGa thin films

Magnetic shape memory nanostructures have a great potential in the field of the nanoactuators. The relationship between dimensionality, microstructure and magnetism characterizes the materials performance. Here, we study the martensitic transformation in supported and free-standing epitaxial Ni47Mn24Ga29 films grown by sputtering on (0 0 1) MgO using a stoichiometric Ni2MnGa target. The films have a Curie temperature of ~390 K and a martensitic transition temperature of ~120 K. Similar transition temperatures have been observed in films with thicknesses of 1, 3 and 4 μm. Thicker films (with longer deposition time) present a wider martensitic transformation range that can be associated with small gradients in their chemical concentration due to the high vapour pressure of Mn and Ga. The magnetic anisotropy of the films shows a strong change below the martensitic transformation temperature. No features associated with variant reorientation induced by magnetic field have been observed. Annealed films in the presence of a Ni2MnGa bulk reference change their chemical composition to Ni49Mn26Ga25. The change in the chemical composition increases the martensitic transformation temperature, being closer to the stoichiometric compound, and reduces the transformation hysteresis. In addition, sharper transformations are obtained, which indicate that chemical inhomogeneities and defects are removed. Our results indicate that the properties of Ni–Mn–Ga thin films grown by sputtering can be optimized (fixing the chemical concentration and removing crystalline defects) by the annealing process, which is promising for the development of micromagnetic shape memory devices.

Phase Transformations in the Pulsed Laser Deposition Grown TiO2 Thin Films as a Consequence of O2 Partial Pressure and Nd Doping

In this work, we present the pronounced phase transformation in the TiO2 thin films grown in simultaneously varied oxygen (O2) partial pressure and doping concentration of neodymium (Nd). Pulsed laser deposition (PLD) was employed for the synthesis of TiO2 films in varied O2 partial pressures from 100 mTorr (O2 rich) to 0 mTorr (O2 absent) for each Nd doping concentration (1.0, 1.5, and 2.0 at. %). The phase and structural studies confirmed the systematic phase transformation from the anatase to the rutile to the metastable and finally to the amorphous phase as the O2 content was reduced and Nd doping concentration was increased. A drastic reduction in the crystallite size was also observed. XPS confirmed the reduction of Ti (Ti4+ to Ti3+), creation of O vacancies, and consequently increased nonstoichiometry and short-range ordered TiO2 lattice. The experimental findings were complemented with the density functional theory calculations of surface energies and crystal facet areas of the anatase and rutile ...

The process of solid-state dewetting of Au thin films studied by in situ scanning transmission electron microscopy

Solid-state dewetting describes the transformation of thin films into a set of particles or droplets at temperatures well below the melting temperature of the bulk. In this work in situ scanning transmission electron microscopy has been used to study the dewetting behavior of discontinuous Au thin films (15nm and 22nm thick) on amorphous silicon nitride membranes at temperatures ranging from 300°C to 600°C. The combination of microscopic and statistical information enabled not only the qualitative discussion of the observed processes but also the quantification of the kinetics as well as the development of a model of the underlying morphological mechanism. A model-free master curve approach to the temporal evolution of the covered area at different temperatures is used to determine the activation energy of dewetting (1.04±0.14eV for the 15nm thick film). A closer inspection reveals a multiple power law behavior, which is discussed in the frame of depercolation. Retraction of finger-like structures is found to be the dominant morphological mechanism based on the observed linear relationship between covered area and boundary length.

Optical diagnostics of the laser-induced phase transformations in thin germanium films on silicon, sapphire, and fused silica

The in-situ procedure is used to study the modification of thin (200–600 nm) germanium films induced by nanosecond pulses of a ruby laser. The films are produced using the ion-beam or magnetron sputtering on single-crystalline silicon (Si), sapphire (Al2O3), and fused silica (α-SiO2) substrates. The results on the dynamics of the laser-induced processes are obtained using the optical probing of the irradiated region at wavelengths of λ = 0.53 and 1.06 μm. The results of probing make it possible to determine the threshold laser energy densities that correspond to the Ge and Si melting and the generation of the Ge ablation plasma versus the amount of deposited Ge and thermophysical parameters of the substrate. The reflection oscillograms are used to obtain the dependences of the melt lifetime on the laser-pulse energy density.

Effect of the polymer concentration on the Rayleigh-instability-type transformation in polymer thin films coated in the nanopores of anodic aluminum oxide templates.

We study the Rayleigh-instability-type transformation of polystyrene (PS) thin films coated in the nanopores of anodic aluminum oxide (AAO) templates. The PS thin films are fabricated using a solution-wetting method, in which the wall thicknesses are controlled by the concentrations of the polymer solutions and the diameters of the nanopores. By thermal annealing, the surfaces of the thin films undulate and the morphologies transform from nanotubes to Rayleigh-instability-induced nanostructures (short nanorods) and long nanorods. To understand the mechanism of the morphology transformation further, we construct the morphology diagrams by annealing the PS thin films at different temperatures and times. We observe that the morphology diagrams of the PS thin films prepared by different concentrations are similar, indicating that the transformation kinetics are not affected by the film thicknesses. The values of the undulation wavelengths, however, are controlled by the film thicknesses and the diameters of the nanopores.

High-Temperature Ammonolysis of Thin Film Ta2O5 Photoanodes: Evolution of Structural, Optical, and Photoelectrochemical Properties

Tantalum nitride (Ta3N5) and oxynitride (TaON) are promising materials for photoelectrochemical (PEC) water splitting due to near-ideal band gaps and band edge positions. However, the high-temperature ammonolysis process that is usually used to make these materials depends sensitively on the process parameters and specific design of the annealing system, and reproducing highly efficient (oxy)nitride photoanodes from recipes reported by other laboratories has proven to be challenging. To understand and monitor the nitridation process in more detail, we employ an optical absorption spectroscopy technique that allows us to follow the transformation of thin Ta2O5 films in situ at temperatures up to 800 °C. Our results show that the incorporation of nitrogen in a dry ammonia atmosphere starts at 575 °C and is accompanied by a gradual red-shift of the Ta2O5 absorption edge and an expansion of the lattice due to the larger ionic radius of N3– relative to O2–. Although coloration of the material due to an N-2p → Ta-3d transition occurs readily, the films do not show any visible-light PEC activity until the nitrogen concentration is high enough to form a continuous N-2p impurity band. Ta3N5 is found to be the only thermodynamically stable phase between 575 and 800 °C, with no traces of TaON. Longer nitridation times result in lower defect concentrations, larger grain sizes, and improved PEC performance. The photocurrent of well-crystallized films is limited by slow water oxidation kinetics. This can be effectively remedied by depositing IrO2 nanoparticles as a water oxidation cocatalyst, which results in external quantum efficiencies of up to 45%. The smaller enhancement of the PEC performance at longer wavelengths reveals that hole transport in Ta3N5 limits the water splitting performance of IrO2-catalyzed Ta3N5 photoanodes.

Reversible J- to H-aggregate transformation in thin films of a perylenebisimide organogelator

A perylene bisimide organogelator is shown to behave as a reversible stimuli responsive material: thermal annealing and contact with organic non solvents allow to switch back and forth between a green J-type (Form I) and a red H-type (Form II) aggregate in thin films and powders of a N,N′-substituted H-bonding perylenebisimide (PBI-C10).

Structural and Optical Properties of Vacuum Coated and Annealed Copper Phthalocyanine (CUPC) Thin Films

Heat treatment is able to cause the structural transformation of phthalocyanine thin film during the post-deposition heat treatment. The annealing effects of the process on the film characteristics of vacuum deposited copper phthalocyanine thin films were studied. However, the pre-annealing leads to a larger-grain and a more compact structure. Structure transformation was inhibited by post-deposition annealing at 300oC. Thin films of CuPc were deposited using vacuum evaporation technique method onto glass substrates. The films were annealed at 4 different temperatures (323, 333, 343 and 353K) in air at different intervals of time. Optical studies were done and the results are discussed. The optical band gap for the film as obtained from the graph for all the thin films are tabulated and analyzed. X ray diffraction studies on the deposited film showed crystallinity with a major peak for CuPc.

Spontaneous shape transition of thin films into ZnO nanowires with high structural and optical quality.

ZnO nanowires are usually formed by physical and chemical deposition techniques following the bottom-up approach consisting in supplying the reactants on a nucleation surface heated at a given temperature. We demonstrate an original alternative approach for the formation of ZnO nanowire arrays with high structural and optical quality, which is based on the spontaneous transformation of a ZnO thin film deposited by sol-gel process following a simple annealing. The development of these ZnO nanowires occurs through successive shape transitions, including the intermediate formation of pyramid-shaped islands. Their nucleation under near-equilibrium conditions is expected to be governed by thermodynamic considerations via the total free energy minimization related to the nanowire shape. It is further strongly assisted by the drastic reordering of the matter and by recrystallization phenomena through the massive transport of zinc and oxygen atoms towards the localized growth areas. The spontaneous shape transition process thus combines the easiness and low-cost of sol-gel process and simple annealing with the assets of the vapor phase deposition techniques. These findings cast a light on the fundamental mechanisms driving the spontaneous formation of ZnO nanowires and, importantly, reveal the great technological potential of the spontaneous shape transition process as a promising alternative approach to the more usual bottom-up approach.

In situ observation of the spatial distribution of crystalline phases during pressure-induced transformations of indented silicon thin films

Abstract

Peculiarities of phase transformations in SiC crystals and thin films with in-grown original defects

Phase transformations of SiC crystals and thin films with in-grown original defects have been studied. The analysis of absorption, excitation and low-temperature photoluminescence spectra testifies to formation of new micro-phases during the growth. The complex spectra can be decomposed into similar structure-constituting spectra shifted against each other on the energy scale. These spectra are indicative of formation of new nanophases. Taking into account the position of the short-wave edge in the zerophonon part of the SF-i spectra as well as the position of corresponding excitation spectra and placing them on the well-known linear dependence of the exciton gap (Egx) on the percentage of hexagonally in different polytypic structures, one can obtain a hint to the percentage of hexagonally in the new metastable structures appearing in the 6H (33) matrix or in the growth process. The SF spectra are indicative of the appearance of these metastable structures.

The Mechanism of Collapse of Heterogeneous Lipid Monolayers

Collapse of homogeneous lipid monolayers is known to proceed via wrinkling/buckling, followed by folding into bilayers in water. For heterogeneous monolayers with phase coexistence, the mechanism of collapse remains unclear. Here, we investigated collapse of lipid monolayers with coexisting liquid-liquid and liquid-solid domains using molecular dynamics simulations. The MARTINI coarse-grained model was employed to simulate monolayers of ∼80 nm in lateral dimension for 10–25 μs. The monolayer minimum surface tension decreased in the presence of solid domains, especially if they percolated. Liquid-ordered domains facilitated monolayer collapse due to the spontaneous curvature induced at a high cholesterol concentration. Upon collapse, bilayer folds formed in the liquid (disordered) phase; curved domains shifted the nucleation sites toward the phase boundary. The liquid (disordered) phase was preferentially transferred into bilayers, in agreement with the squeeze-out hypothesis. As a result, the composition and phase distribution were altered in the monolayer in equilibrium with bilayers compared to a flat monolayer at the same surface tension. The composition and phase behavior of the bilayers depended on the degree of monolayer compression. The monolayer-bilayer connection region was enriched in unsaturated lipids. Percolation of solid domains slowed down monolayer collapse by several orders of magnitude. These results are important for understanding the mechanism of two-to-three-dimensional transformations in heterogeneous thin films and the role of lateral organization in biological membranes. The study is directly relevant for the function of lung surfactant, and can explain the role of nanodomains in its surface activity and inhibition by an increased cholesterol concentration.

Phase Transformation in Ultra-Thin Films

Thickness dependent structural phase transformation in thin polycrystalline metal films has been reviewed. Various effects of film thickness reduction on film microstructure have been identified. Film thickness dependent structural phase transformation has been treated thermodynamically taking polycrystalline titanium (Ti) thin film as model example.