Phase Transition In Thin Films Research Articles

Tetrafluorosubstituted titanyl phthalocyanines: Structure of single crystals and phase transition in thin films

In this work, tetrafluorinated titanyl phthalocyanines with F-substituents in both peripheral (TiOPcF4-p) and non-peripheral (TiOPcF4-np) positions were synthesized and characterized by the methods of elemental analysis, IR and electronic absorption spectroscopies. The influence of the position of F-substituents on the structure of TiOPcF4-p and TiOPcF4-np single crystals and thin films grown by vacuum sublimation was studied. TiOPcF4-p crystallizes in the I4/m space group and its molecules form uniform stacks along the tetragonal axis. Introduction of F-substituents to non-peripheral positions leads to the change of the crystal structure: TiOPcF4-np crystallizes in the P-1 space group and its molecules are packed into 2D layers. Both phthalocyanines form oriented polycrystalline films when deposited by thermal sublimation in vacuum, but their phase composition is different from that of single crystals. The effect of post-deposition annealing of TiOPcF4-p and TiOPcF4-np films on their phase composition, morphology, optical absorption and Raman spectra, as well as conductivity was revealed. It was shown that a phase transition, which led to the appearance of an additional band in the near-IR region in the optical absorption spectrum, was observed in TiOPcF4-p films when heated, whereas no phase transition was observed in TiOPcF4-np films.

Phase behavior of P(NIPAM-g-PLA) films for cell sheet technology assessed with atomic force microscopy

Thermoresponsive graft copolymers of N-isopropylacrylamide with polylactide (P(NIPAM-g-PLA)) are promising materials for the preparation of cell supports in cell sheet technology. Here, we studied the thermal behavior of thin and thick films prepared by spin-coating from P(NIPAM-g-PLA) (fresh and stored for 1 year), using atomic force microscopy. P(NIPAM-g-PLA) copolymers of different molecular weights were synthesized. The films’ swelling, Young’s moduli and roughness were tracked when lowering the temperature to that of the phase transition. The obtained thermal dependencies demonstrated that, in contrast to neat PNIPAM films, the prepared films efficiently dissolved at the temperature corresponding to the LCST of the copolymer in a solution. After a year of storage in ambient conditions, certain signs of polymer deterioration due to water uptake were observed. The obtained data on the phase transition in thin films of P(NIPAM-g-PLA) are of importance for their potential use as cell supports in the cell sheet technology.

Features of the Phase Transition in Thin Films of AgI Superionic Semiconductor

Features of the Phase Transition in Thin Films of AgI Superionic Semiconductor

Non-spontaneous symmetry breaking, chaos, and universality in 2D superconducting phase transition

This research paper explores the intriguing phenomenon of the superconductor-metal–insulator phase transition in thin films, examining it from a theoretical standpoint. Our study revolves around the proposition that the process of U(1) symmetry breaking in the Landau–Ginzburg theory might not be entirely spontaneous. Building on this insight, we derive critical parameters characterizing the superconducting phase transition. Our findings demonstrate that the application of an electric field can effectively control the phase transition, leading to the suppression of the supercurrent at specific electric potential values, which is consistent with recent research. Furthermore, we have developed a robust relationship for the nonlinear resistivity that accurately simulates experimental measurements below the critical temperature. This derived relation adopts the form of logistic functions, providing a systematic framework to describe the system within the realm of chaos theory. Moreover, we establish a link with the Berezinskii–Kosterlitz–Thouless theory, highlighting the universality of the topological transition. However, this universality breaks down under the influence of multiple control parameters. To delve further into the underlying reasons for the collapse of universality, we turn to the study of Markus–Lyapunov fractals, which offers a deep understanding into the system’s behavior in the presence of varying external influences..

Analysis of Vibrational Spectra of Tetrafluoroethane Glasses Deposited by Physical Vapor Deposition.

This paper presents the results obtained in the study of structural phase transitions in thin films of R134A. The samples were condensed on a substrate by physical deposition of R134A molecules from the gas phase. Structural phase transformations in samples were investigated by observing the changes in characteristic frequencies of Freon molecules in the mid-infrared range with the help of Fourier-transform infrared spectroscopy. The experiments were carried out in the temperature range from 12 to 90 K. A number of structural phase states, including glassy forms, were detected. The changes in thermogram curves at fixed frequencies of half-widths of absorption bands of R134A molecules were revealed. These changes indicate a large bathochromic shift of these bands at frequencies of ν = 842 cm-1, ν = 965 cm-1, and ν = 958 cm-1 and a hypsochromic shift of the bands at frequencies of ν = 1055 cm-1, ν = 1170 cm-1, and ν = 1280 cm-1 at temperatures from T = 80 K to T = 84 K. These shifts are related to the structural phase transformations in the samples.

High Chern number in strained thin films of dilute magnetic topological insulators

The quantum anomalous Hall effect was first observed experimentally by doping the ${\mathrm{Bi}}_{2}{\mathrm{Se}}_{3}$ materials family with chromium, where 5% doping induces an exchange field of around 0.1 eV. In ultrathin films, a topological phase transition from a normal insulator to a Chern insulator can be induced with an exchange field proportional to the hybridization gap. Subsequent transitions to states with higher Chern numbers require an exchange field larger than the (bulk) band gap, but are prohibited in practice by the detrimental effects of higher doping levels. Here, we show that threshold doping for these phase transitions in thin films is controllable by strain. As a consequence, higher Chern states can be reached with experimentally feasible doping, sufficiently dilute for the topological insulator to remain structurally stable. Such a facilitated realization of higher Chern insulators opens prospects for multichannel quantum computing, higher-capacity circuit interconnects, and energy-efficient electronic devices at elevated temperatures.

Molecular dynamics data-driven study of leidenfrost phenomena in context to liquid thin film phase transformation

In many micro and nanoscale applications of thin film phase transition, identifying the circumstances that allows stable liquid contact with a heated surface is critical. Using molecular dynamics (MD) simulations, our current data driven machine learning-based study attempts to examine the properties of liquid interaction with a solid surface at nanoscale. A few nanometer-thick liquid argon layer over a platinum surface has been used to simulate a liquid-solid contact system. The wall temperature is raised linearly after necessary initial equilibration of the entire system, with different boundary heating rates for various surface wetting conditions, namely hydrophobic, hydrophilic, and superhydrophilic. Our current investigation shows that the heating condition, liquid film thickness, and surface wetting condition all have a significant impact on the type of liquid wall contact that persists during the phase transition phenomena of thin liquid argon film (i.e., normal evaporation or explosive boiling). In the event of normal evaporation, a stable liquid contact with the solid surface continues, however in the case of explosive boiling, the liquid film is splashed away from the solid surface resembling the macroscopic Leidenfrost effect. For various system configurations in regard to liquid initial film thickness, liquid heating rate as well as solid-liquid interaction, a wide variation of the onset time as well as the wall temperature of boiling explosion have been found in the present study. An accurate mapping of the Leidenfrost conditions in context to nanoscale thin film liquid-vapor phase process has been generated using a predictive model based on deep neural networks that has been designed, trained and cross validated against molecular dynamics data of the present study.

Conformational Change of Alkyl Chains at Phase Transitions in Thin Films of an Asymmetric Benzothienothiophene Derivative.

Among many promising organic semiconducting materials, 2-decyl-7-phenyl[1]benzothieno[3,2-b][1]benzothiophene (Ph-BTBT-C10) shows outstanding device performances for organic field-effect transistors. This compound has a highly ordered liquid crystalline state, i.e., the smectic E (SmE) phase. Although the transition from the crystalline state to the SmE phase is believed to accompany melting of the alkyl chains, no spectroscopic evidence has been found so far. In this study, the conformational change of the decyl chains in Ph-BTBT-C10 films across the phase transition is analyzed by temperature-dependent measurements in situ using infrared spectroscopy. The spectral analysis reveals that the polycrystalline film has latent conformational disorder (the gauche conformer), the rate of which becomes more pronounced with the heat treatment. As expected, melting of the decyl chains is observed above the transition temperature to the SmE phase. This study also highlights the discovery of some key bands sensitive to the phase transitions in liquid crystalline organic semiconductors.

Computer Simulation of Phase Transitions in Thin Films with an Antidote Lattice

We investigate the magnetic phase transition in a thin film with an antidote lattice by computer simulation. A lattice of non-magnetic antidotes is present in a thin film of several atomic layers. The antidotes form a rectangular lattice. We are looking at two forms of antidotes. The Ising model and Wolf’ cluster algorithm simulate the system’s magnetic behavior. Antidotes act on additional surfaces of the system. This results in a change in the Curie temperature of the system. Dependence of phase transition temperature on holes size and shape is obtained. The phase transition temperature depends on the size of the hole by logarithmic law. The Curie temperature for triangular holes is lower than for square holes. We investigated the magnetization of a thin film with an antidote lattice and constructed a hysteresis loop. The hysteresis loop expands as the hole size decreases. Coercive force depends on the size and shape of the holes. Coercive force varies by nonlinear law.

Exploiting Spin Fluctuations for Enhanced Pure Spin Current.

We demonstrate the interplay of pure spin current, spin-polarized current, and spin fluctuation in 3d Ni_{x}Cu_{1-x}. By tuning the compositions of the Ni_{x}Cu_{1-x} alloys, we separate the effects due to the pure spin current and spin-polarized current. By exploiting the interaction of spin current with spin fluctuation in suitable Ni-Cu alloys, we obtain an unprecedentedly high spin Hall angle of 46%, about 5 times larger than that in Pt, at room temperature. Furthermore, we show that spin-dependent thermal transport via anomalous Nernst effect can serve as a sensitive magnetometer to electrically probe the magnetic phase transitions in thin films with in-plane anisotropy. The enhancement of spin Hall angle by exploiting spin current fluctuation via composition control makes 3d magnets functional materials in charge-to-spin conversion for spintronic application.

Secondary phase limited metal-insulator phase transition in chromium nitride thin films

Chromium nitride (CrN) is a well-known hard coating material that has found applications in abrasion and wear-resistant cutting tools, bearings, and tribology applications due to its high hardness, high-temperature stability, and corrosion-resistant properties. In recent years, CrN has also attracted significant interest due to its high thermoelectric power factor, and for its unique and intriguing metal-insulator phase transition. While CrN bulk single-crystals exhibit the characteristic metal-insulator transition accompanied with structural (orthorhombic-to-rocksalt) and magnetic (antiferromagnetic-to-paramagnetic) transition at ∼260–280 K, observation of such phase transition in thin-film CrN has been scarce, and the exact cause of the absence of such transitions in several studies is not well-understood. In this work, the formation of the secondary metallic Cr2N phase during the growth is demonstrated to inhibit the observation of metal-insulator phase transition in CrN thin films. When the Cr-flux during deposition is reduced below a critical limit, epitaxial and stoichiometric CrN thin film is obtained that reproducibly exhibits the phase transition. Annealing of the mixed-phase film inside reducing NH3 environment converts the Cr2N into CrN, and a discontinuity in the electrical resistivity at ∼277 K appears, which supports the underlying hypothesis. Demonstration of the inhibited metal-semiconductor phase transition in CrN due to the presence of secondary Cr2N phase is similar to the previous finding of the substantial change in its mechanical hardness and reduction in thermoelectric properties. A clear demonstration of the origin behind the controversy of the metal-insulator transition in CrN thin films marks significant progress and would enable its nanoscale device realization.

Metal-Insulator phase transition in thin films of vanadium dioxide doped with indium

The electrical conductivity of thin polycrystalline films V(1-x)InxO2 has been studied in a wide temperature range covering the regions of existence of both the metallic and insulator phases. It is shown that with increasing indium concentration, the temperature of the metal-insulator phase transition decreases, and the width of the temperature region of phase coexistence monotonically increases. To explain the temperature dependence of the electrical conductivity of the insulator phase V(1-x)InxO2, a model of hopping conductivity is applied, taking into account the effect of thermal vibrations of atoms on the resonance integral. Calculated the values of the parameter ε depend on the degree of doping of VO2. Keywords: phase transition, vanadium oxides, thin films, polaron, electrical conductivity.

Thin Thermoresponsive Polymer Films for Cell Culture: Elucidating an Unexpected Thermal Phase Behavior by Atomic Force Microscopy.

Application of poly-N-isopropylacrylamide (PNIPAM) and its more hydrophobic copolymers with N-tert-butylacrylamide (NtBA) as supports for cell sheets has been validated in numerous studies. The binary systems of these polymers with water are characterized by a lower critical solution temperature (LCST) in a physiologically favorable region. Upon lowering the temperature below the LCST, PNIPAM chains undergo a globule-to-coil transition, causing the film dissolution and cell sheet detachment. The character of the PNIPAM-water miscibility behavior is rather complex and not completely understood. Here, we applied atomic force microscopy to track the phase transition in thin films of linear thermoresponsive (co)polymers (PNIPAM and PNIPAM-co-NtBA) prepared by spin-coating. We studied the films' Young's modulus, roughness, and thickness in air and in distilled water in a full thermal cycle. In dry films, in the absence of water, all the measured parameters remained invariant. The swollen films in water above the LCST were softer by 2-3 orders of magnitude and about 10 times rougher than the corresponding dry films. Upon lowering the temperature to the LCST, the films passed through the phase transition observed as a drastic drop of Young's modulus (about an order of magnitude) and decrease in roughness in both polymers in a narrow temperature range. However, the films did not lose their integrity and demonstrated almost fully reversible changes in the mechanical properties and roughness. The thermal dependence of the films' thickness confirmed that they dissolved only partially and required an external force to induce the complete destruction. The reversible thermal behavior which is generally not expected from non-cross-linked polymers is a key finding, especially with respect to their practical application in cell culture. Both the thermodynamic and kinetic factors, as well as the confinement effect, may be responsible for this peculiar film robustness, which requires overcooling and the aid of an external force to destroy the film.

Strain-Induced Magnetic Transitions in SrMO2.5 (M = Mn, Fe) Thin Films with Ordered Oxygen Vacancies.

We examine the epitaxial-strain-induced phase transitions in thin films of perovskite-derived SrMnO2.5 and SrFeO2.5 exhibiting ordered oxygen vacancies (OOVs). We find that SrMnO2.5 hosts multiple magnetic transitions to other ordered states, including antiferromagnetic (AFM) and ferromagnetic (FM) orders of E*-AFM, C-AFM, and FM types depending on the compressive or tensile strain state. In contrast, no magnetic transitions occur in thin-film SrFeO2.5 (G-AFM to FM), whereas its bulk phase exhibits a hydrostatic pressure-induced AFM-to-FM transition. We explain the origin of these dependencies on the transition-metal configuration, that is, d4 Mn versus d5 Fe, and the relative orientation of the OOVs in the Ca2Mn2O5-type structure with respect to the epitaxial interface. We find that the magnetic phase stability can be predicted by using exchange striction arguments, with FM (AFM) spin interactions preferring longer (shorter) Mn-O bonds in the square pyramidal MnO5 unit comprising SrMnO2.5. Because the Mn-O bond lengths directly shrink or elongate to accommodate the applied stress without considerable polyhedral rotations, we show that compressive and tensile strain tune the unit cell structure to favor different combinations of exchange interactions that stabilize the various magnetic spin orders. Our study shows that the strong coupling between the OOV structure and spin orders with epitaxial strain is a promising route to achieve picoscale control of functional electronic and magnetic responses in complex oxide thin films.

Impact of anharmonic effects on two phases of VO2 thin-film phase transition materials

Vanadium dioxide (VO2) is a strong correlation material, which can undergo transition from the monoclinic phase to the rutile phase near 340 K at ambient pressure. This phase transition can be controlled by extreme changes of optical, electrical and thermodynamic properties, and thus VO2 film possesses various potential device applications. Physically, lattice dynamics play a critical role in the investigation of the phase mechanism and properties of VO2. Given that widely adopted methods such as the density-functional perturbation theory and the quasiharmonic approximation (QHA) are typically only suitable to calculate the VO2 monoclinic phase, in this work we employ the temperature-dependent effective potential (TDEP) method to calculate the phonon properties of the two phases of VO2 by considering the anharmonicity of the lattice vibrations. Assisted by the experimental data, a resistance–temperature phase diagram has been constructed. The results show that the monoclinic phase is unstable because of the existence of the imaginary frequency. Compared with the thermal expansion, calculated by the QHA and TDEP methods, it is demonstrated that the TDEP method is reliable for the stable rutile phase, in which the imaginary frequency disappears. Simultaneously, it is shown that the calculated Gibbs free energy of VO2 agrees with the experimental results. Our work provides robust evidence tthat the anharmonicity in the lattice dynamics of VO2 cannot be ignored.

Wide range optical and electrical contrast modulation by laser-induced phase transitions in GeTe thin films

The paper presents the results of a computational thermo-kinetic model of laser heating of GeTe thin films and their subsequent thermoconductive and radiative cooling. A comparison of this model and experimental results of observing of reversible phase transitions in thin films under impact of nanosecond pulsed laser radiation with «top hat» beam intensity distribution was performed. It was shown that the subsurface crystallization of amorphous GeTe thin film obtained on SiO2 substrates is started at threshold laser radiation energy density E = 7.5 mJ/cm2. Then crystallization process propagates up to substrate primarily in the rhombohedral α-GeTe phase in the range of E = 18.6–32 mJ/cm2 with the subsequent transition to the cubic β-GeTe phase in the range of E = 32–47.6 mJ/cm2 reaching the necessary temperatures for corresponding transitions. The reverse crystalline-amorphous transition is started at the laser energy density 62 mJ/cm2, when the film temperature reaches the melting temperature and is observed up to energy density 93 mJ/cm2 without ablation damage. It was shown that the values of energy densities, required for α-/β-phase transitions and reamorphization for the films obtained on native oxidized p-Si (100) substrates in comparison with ones required for the same transitions in films on SiO2 substrates are more on about 20–30%, because of significant higher thermal conductivity of p-Si (100) substrate. The high optical contrast in the spectral range from 200 nm to 22000 nm and high difference in electrical conductivity from σam = 0.66 Ω−1 cm−1 to σcr = 26.8 Ω−1 cm−1 between amorphous and cubic crystalline states was demonstrated. The observed experimental results are in good agreement with computational thermo-kinetic model.

Surface-induced phase transitions in thin films of dendrimer block copolymers

Abstract The phase morphologies and phase transitions of dendrimer block copolymer thin films confined between two homogeneous, planar hard substrates had been investigated by a three-dimensional real space self-consistent field theory (SCFT). From the perspectives of property and strength of the preferential substrate, when the film system confined within neutral substrates, the thinner film was easier to take the undulated and perpendicular cylinder phases. For the attractive preference of the substrate on block segment A, the polymer films tended to take the surface-wetting structures that was composed by block segment A. On the contrary, for the repulsive preference of the substrate on block segment A, a phase transition of cylinder-lamellae could be observed increasing with the relative surface strength of the preferential substrate.

Metal–Insulator Phase Transition in Thin Films of a Nickel-Doped Vanadium Dioxide

The electrical conductivity of thin polycrystalline VNixO2 films has been studied in a wide range of temperatures, which covers the regions of both metallic and insulator phase. It is shown that the metal–insulator phase transition temperature decreases as the nickel concentration increases, and the temperature range of coexistence of the phases increases monotonically. The temperature dependence of the conductivity of the insulating VNixO2 phase is explained using the hopping conduction model that takes into account the influence of thermal atomic vibrations on the resonance integral. Parameter e has been calculated as a function of the doping level of VO2.

Simulation of Wetting Phase Transitions in Thin Films

Based on the hydrodynamic model, the kinetics of wetting phase transitions in nanoscale liquid films on the substrate surface is analyzed. In the range of metastable states, the features of the formation of equilibrium clusters are studied and corresponding film thickness distributions are calculated. In the range of unstable states, the kinetics of the phase transition resulting in cluster formation is analyzed. In the early stage, regions shaped as holes with a film thickness close to the equilibrium one are formed. Hole coalescence leads to a film material redistribution followed by clustering. For this process, the kinetics of the average hole size and concentration is calculated. For formed clusters, the kinetics of the average radius, average height, concentration, and their radius and height distribution function are studied.

A tunable thermal switching device based on Joule heating-induced metal–insulator transition in VO2 thin films via an external electric field

A solid state thermal switching device can regulate carrier transport by triggering of its critical transition temperature (Tc) by applied external thermal energy. Continuous control of the Tc of the thermal switch by the metal–insulator transition (MIT) phenomenon makes such devices widely usable. In this research, tunable thermal switching devices were fabricated, and characterization of the MIT in VO2 thin film phase transition material was studied as a function of temperature and the external applied electric field. We observed reversible abrupt changes of the electrical resistivity by approximately three orders of magnitude at Tc = 62.3 °C for VO2 thin film on a SiO2/Si substrate. The MIT induced by the external electric field successfully controlled the Tc of the thermal switch between 60 °C and 47 °C (as a linear relationship). We found that the Joule heating effect, rather than electric field breakdown, was a dominant mechanism due to the configuration of the device.