Transition Temperature Of Thin Films Research Articles

Effects of the surface energy and surface stress on the phase stability of spin crossover nano-objects: a thermodynamic approach.

Size-induced phase transformation at the nanoscale is a common phenomenon whose understanding is essential for potential applications. Here we investigate phase equilibria in thin films and nanoparticles of molecular spin crossover (SCO) materials. To calculate the size-temperature phase diagrams we have developed a new nano-thermodynamic core-shell model in which intermolecular interactions are described through the volume misfit between molecules of different spin states, while the contributions of surface energy and surface stress are explicitly included. Based on this model, we rationalize the emergence of previously-reported incomplete spin transitions and the shift of the transition temperature in finite size objects due to their large surface-to-volume ratio. The results reveal a competition between the elastic intermolecular interaction and the internal pressure induced by the surface stress. The predicted transition temperature of thin films of the SCO compound [Fe(pyrazine)][Ni(CN)4] follows a clear reciprocal relationship with respect to the film thickness and the transition behavior matches the available experimental data. Importantly, all input parameters of the present model are experimentally accessible physical quantities, thus providing a simple, yet powerful tool to analyze SCO properties in nano-scale objects.

The glass transition temperature of thin films: A molecular dynamics study for a bead-spring model.

Molecular dynamics simulations were carried out on free-standing liquid films of different thicknesses h using a bead-spring model of 10 beads per chain. The glass transition temperatures, Tg, of the various films were determined from plots of the internal energy versus temperature. We used these simulations to test the validity of our earlier conjecture that the glass transition of a confined liquid could be approximated by pre-averaging over the non-uniform density profile of the film. Using the density profiles from our simulations, we computed the average density of the free-standing films as a function of temperature. In all our film simulations we found, within the error of the simulation, that Tg of the film occurred at the same density (or packing fraction) as the bulk system at the bulk glass transition temperature TgB. By equating these densities at their respective glass transition temperatures, as suggested by the simulations, we deduced that Tg/TgB is proportional to h0/h. This is consistent with previous simulations and experimental data. Moreover, the parameter h0 is determinable in our model from the density profile of the films.

Hysteresis and change of transition temperature in thin films of Fe{[Me2Pyrz]3BH}2, a new sublimable spin-crossover molecule.

Thin films of the spin-crossover (SCO) molecule Fe{[Me2Pyrz]3BH}2 (Fe-pyrz) were sublimed on Si/SiO2 and quartz substrates, and their properties investigated by X-ray absorption and photoemission spectroscopies, optical absorption, atomic force microscopy, and superconducting quantum interference device. Contrary to the previously studied Fe(phen)2(NCS)2, the films are not smooth but granular. The thin films qualitatively retain the typical SCO properties of the powder sample (SCO, thermal hysteresis, soft X-ray induced excited spin-state trapping, and light induced excited spin-state trapping) but present intriguing variations even in micrometer-thick films: the transition temperature decreases when the thickness is decreased, and the hysteresis is affected. We explain this behavior in the light of recent studies focusing on the role of surface energy in the thermodynamics of the spin transition in nano-structures. In the high-spin state at room temperature, the films have a large optical gap (∼5 eV), decreasing at thickness below 50 nm, possibly due to film morphology.

X-ray reflectivity study of the glass transition temperature of thin films

The glass transition takes place in amorphous materials (like polymers) during heating or cooling, and can be described as reversible transition from a hard and brittle state into a rubber-like state. Although physical properties of the material change significantly during the glass transition, this is not a phase transition of the material. The temperature at which the transition between the glassy and rubbery state occurs is called the glass transition temperature, and this temperature is always lower than the melting temperature. Thermodynamically, the glass transition is associated with transfer of heat between the system and its surrounding and with an abrupt volume change. Previously it was shown that the glass transition temperature of nano-films is different from that of bulk materials [1], which signifies the importance of determining this parameter for such systems. In the current work, we use quick X-ray reflectivity (qXRR) measurements to determine the glass transition temperature of polyvinyl acetate (PVAc). PVAc is rubbery synthetic polymer with the formula (C4H6O2), a density of 1.18 g/cm3, and a glass transition temperature for bulk material of 30oC [2]. Regular X-ray reflectivity measurements are based on θ/2θ scans at grazing incidence and typically require 0.5-1.5 h for a single scan. The qXRR technique is based on simultaneous measurement of the whole angular x-ray reflectivity profile and is suitable for in-situ measurement without moving the sample and/or the x-ray optics. Thus, the qXRR technique allows for very fast measurement of the x-ray reflectivity curves (duration of each scan is typically 0.1–20 sec [3]), which permits studying the time evolution of chemical, thermal, and mechanical changes at the surface and interface of different materials. X-ray reflectivity measurements give information about both density and thickness of thin films, and are suitable for studying glass transition phenomena. Nano-thickness PVAc layers on a Si substrate were examined with the qXRR technique, with x-ray reflectivity scans (each 10-seconds in duration) being recorded while temperature was changed from 20 to 50oC (total of 331 scans over 7 hours and 46 minutes). In the current paper, the experimental setup, the data-processing, and the analysis of the results from the qXRR measurements will be presented.

Use of polystyrene brushes to investigate the role of interface between substrates and thin homogeneous films

ABSTRACTThe viscoelastic properties of thin polystyrene (PS) films depend on confinement, as it can modify the molecular dynamics affecting the glass transition. In the recent past, the authors have investigated the region next to the free interface by means of an atomic force microscope suitably modified to monitor the indentation of a tip into a film during a given lapse of time while applying a constant load. Herein, to explore the interface with the substrate, the authors report on experiments in which PS brushes grafted to native silicon oxide were used. It was found that the film wettability on brushes and H‐terminated silicon can be highly improved when compared with native silicon oxide. In addition, the glass transition temperature of thin films increases up to the bulk value in the case of film/brush combinations with high molecular weight or films with high molecular weight on H‐terminated silicon. Data are discussed according to hypotheses such as residual solvent presence, interface free volume, and molecular mechanical coupling. These observations can be of great interest for nanotechnological applications, especially in those instances where one needs to tailor the temperature dependence of viscoelastic properties of thin films. © 2013 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2013, 51, 1149–1156

Defect-induced shift of the Peierls transition in TTF–TCNQ thin films

In this paper we investigate the influence of the substrate material and film thickness on the Peierls transition temperature in tetrathiafulvalene–tetracyanoquinodimethane (TTF–TCNQ) thin films, grown by physical vapor deposition. Our analysis shows that the substrate material and the growth conditions strongly influence the film morphology. In particular, we demonstrate that the Peierls transition temperature in thin films is lower than in TTF–TCNQ single crystals. We argue that this effect arises due to defects, which emerge in TTF–TCNQ thin films during the growth process.

Vitaly Lazarevich Ginzburg

Vitaly Lazarevich Ginzburg

Dielectric Behavior of Glass Transition and Dynamics in Thin Polymer Films

Glass transition temperature (Tg) and dynamics of thin films of polystyrene (PS) and poly(methyl methacrylate) (PMMA) are investigated using dielectric relaxation spectroscopy and related methods. In case of thin films of polystyrene, the glass transition temperature could successfully be obtained as a function of film thickness and the obtained results are consistent with those observed using other techniques such as ellipsometry. In accordance with the decrease in the glass transition temperature in thin films, the dynamics of the α-process becomes faster with decreasing film thickness and the distribution of the α-relaxation times becomes broader. In case of thin films of poly(methyl methacrylate), the thickness dependence of the α- and β-processes could be observed using the dielectric relaxation spectroscopy. Both the temperatures Tα and Tβ, at which the dielectric losses due to the α- and β-processes show a maximum at a given frequency, decrease with decreasing film thickness. Below a critical thickness, both Tα and Tβ begin to decrease rapidly with decreasing thickness. In order to investigate the positional dependence of the dynamics of the α-processes for polystyrene thin films, we utilize bilayer films, where a thin film of PS labeled with a dye is located on top of the film of unlabeled PS. For such systems, we could extract the contribution only from the upper labeled layer of PS. The glass transition temperature of the upper labeled layer between unlabeled PS layer and aluminum electrode is lower than that of the bulk labeled polystyrene. This result supports that the sample geometry commonly used for dielectric measurements of ultra thin films is the same to that used for other methods as far as the glass transition dynamics are concerned.

Anisotropic behaviour of the glass transition temperature in thin films of Langmuir–Blodgett deposited side chain polymers

Langmuir–Blodgett (LB) films of side chain polyamic acids were formed, deposited onto optical waveguides and imidized via heating to the corresponding polyimides on the optical waveguides. The LB film formation of the side chain polyamic acids, the imidization reaction and the glass transition behaviour of the polyimide thin films were investigated. Polarisation dependent waveguide mode spectroscopy was applied therefore. It was found that the glass transition temperature (Tg) of bulk and LB thin film samples of these polymers are identical and only depend on the spacer length between the side chain and backbone, not on film thickness. Our investigation shows that in the case of the ultrathin film geometry, the glass transition temperature exhibits an anisotropic behaviour. We find the classical kink in the temperature scan (a change in slope of the free volume vs. temperature plot) due to the thermal expansion change at Tg, in p-polarisation only, which is perpendicular to the main chain orientation. In the s-polarisation measurements (parallel to the main chains) no kink was found, only an offset in the vicinity of Tg. We attribute the anisotropic thermal behaviour to a transition, the freezing of segmental motion, in the side chains only.

Novel micro-thermal characterisation of thin film NiTi shape memory alloys

Abstract A novel approach in determining the transition temperatures of thin films of NiTi shape memory alloys was investigated and compared with conventional techniques. The technique is based on micro-thermal analysis utilising a scanning thermal microscope (SThM). This method has the potential to allow the transformation temperatures of thin films to be investigated in situ. In this study, thermal micro-analysis using an SThM was investigated on thin film samples of 47.5 at.% Ni and 49.6 at.% Ni in Ti. Thin amorphous NiTi films were deposited on silicon substrates using DC plasma assisted sputtering. The thin films were recrystallised and subsequently characterised by SThM. Results were then compared with DSC, XRD and resistivity measurements. The SThM sensor variation with temperature showed clear onset and completion of transformation from martensite to austenite during a heating cycle. The small probe size of 1 μm2 allowed variations in transformation temperatures across the NiTi thin film to be investigated. A range of transition temperatures was observed, which was consistent with the difference between AS and AF as determined by resistivity and bulk DSC measurements. The linear contraction between martensite and austenite was also measured by this technique and was found to be similar to the theoretical transformation of 0.45%. This novel method will provide a route to investigate both the shape memory transformation temperatures of NiTi thin films and their spatial variation across devices such as micro-actuators.

Thickness Dependence of the Glass Transition Temperature in Thin Films of Partially Miscible Polymer Blends

The thickness dependence of the glass transition temperature, Tg(h), has been investigated using ellipsometry at a variable temperature for thin films of partially miscible stereoregular poly(methyl methacrylate) (PMMA)−low-molecular-weight poly(ethylene oxide) (PEO) blends. These values were compared to Tg(h) values of pure stereoregular PMMA thin films, and the miscibility of PMMA/PEO systems is assessed by the so-called plasticizing effect of PEO, that is, ΔTg*. In thin-film geometry, the miscibility of i-PMMA is enhanced compared with that in the bulk and becomes higher than that for s-PMMA in contrary to the bulk behavior of the blends. The conformation energy ΔE of the PMMA chains in the thin film increase much more for s-PMMA than for i-PMMA, indicating larger gauche to trans conformational rearrangement for the former isomer in the confined geometry. These large local modifications of the s-PMMA conformation in the thin film could result in modification of the mixing entropy and, therefore, be the...

Ideal glass transitions in thin films: An energy landscape perspective

We introduce a mean-field model for the potential energy landscape of a thin fluid film confined between parallel substrates. The model predicts how the number of accessible basins on the energy landscape and, consequently, the film’s ideal glass transition temperature depend on bulk pressure, film thickness, and the strength of the fluid–fluid and fluid–substrate interactions. The predictions are in qualitative agreement with the experimental trends for the kinetic glass transition temperature of thin films, suggesting the utility of landscape-based approaches for studying the behavior of confined fluids.

Evidence for size-dependent mechanical properties from simulations of nanoscopic polymeric structures

Discontinuous molecular dynamics simulations of a model polymer have been conducted to investigate the glass transition of ultrathin films and the mechanical properties of nanoscopic structures. Continuum mechanics models have been applied to interpret simulation data and extract apparent Young’s Moduli. Consistent with experiments, the results of simulations indicate that the glass transition temperature of thin films can be higher or lower than that of the bulk, depending on the nature of polymer–substrate interactions. Simulations also indicate that the mechanical properties of nanoscopic structures can be considerably different from those of the bulk. An analysis of molecular strain distributions in nanostructures undergoing a deformation indicate that significant stress relaxation occurs at air–polymer interfaces. A comparison of these distributions to the results of continuum, finite-element calculations reveal pronounced differences between the continuum and molecular approaches.

WHAT CAN WE LEARN FROM COMPARISON BETWEEN CUPRATES AND HE FILMS?: PHASE SEPARATION AND FLUCTUATING SUPERFLUIDITY

In the underdoped, overdoped, Zn-doped or stripe-forming regions of high-Tc cuprate superconductors (HTSC), the superfluid density ns/m* at T→ 0 shows universal correlations with Tc. Similar strong correlations exist between 2-dimensional superfluid density and superfluid transition temperature in thin films of 4 He in non-porous or porous media, and 4 He /3 He film adsorbed on porous media. Based on analogy between HTSC and He film systems, we propose a model for cuprates where: (1) the overdoped region is characterized by a phase separation similar to 4 He /3 He ; and (2) pair (boson) formation and fluctuating superconductivity occur at separate temperatures above Tc in the underdoped region.

Superconducting NdBa/sub 2/Cu/sub 3/O/sub 7-δ/ thin films prepared by the laser ablation method

NdBa/sub 2/Cu/sub 3/O/sub 7-/spl delta//(NBCO) thin films were deposited on (100) SrTiO/sub 3/(STO) substrates by the laser ablation method. The transition temperature of thin films was very sensitive to the deposition parameters such as substrate temperature, chamber pressure, and the distance between target and substrate. Thin films of T/sub c//spl sim/93 K were obtained when we deposited at 770/spl deg/C and 800 mTorr of oxygen pressure. The NBCO thin films were grown epitaxially with c-axis preferred orientation. The critical current density of the thin films was /spl sim/1/spl times/10/sup 6/ A/cm/sup 2/ at 77 K. We have also studied the surface morphology with AFM, and the typical smoothness is /spl sim/3 nm.

Transmission electron microscopy investigations of defects in molecular beam epitaxy-grown oxide films

With a view to increasing the superconductive transition temperature of thin films of La 2CuO 4 grown by molecular beam epitaxy, films were grown on substrates of lattice parameters such that the film-substrate misfit became small and tensile, or compressive. The microstructure of these films was investigated using transmission electron microscopy and qualitative correlations with physical properties suggested.

Diffusive Boundary Effect on the Superconducting Transition Temperature of Thin Films in Magnetic Field

The diffusive boundary effect on the superconducting transition temperature of a thin film is investigated in the case where the magnetic field is parallel to the film surfaces. In our model, the rough surfaces (the diffusive boundaries) are described by dirty layers, and the scattering effects on the film surfaces are replaced by the impurity scattering in imaginarily contacted dirty layers. The linearized gap equation is analytically derived by a quasi-classical Green's function method, which correctly includes the diffusive boundary effects. It can be shown numerically that the transition temperature is not enhanced by the magnetic field, even in the diffusive boundary condition.

Superconducting Transition Temperature of Thin Films in Magnetic Field

Superconducting transition temperature of thin films in magnetic field is investigated. The linearized gap equation is derived by a quasiclassical Green's function method, which correctly includes the boundary effects and the impurity scattering effects. From the properties of the gap equation, it can be rigorously shown that the transition temperature is not enhanced by magnetic field. The temperature dependence of the critical field is numerically calculated in the clean limit.

Increase in the superconducting transition temperature of thin films as a result of a normal metal deposition on their surface

The superconducting transition temperature T c of quench condensed thin films of bismuth and gallium have been observed to rise as a result of the deposition of silver on the films surface. Increase in the critical temperature δT c was depended on the thickness of the superconducting film and δT c≈1K for very rhin films. The effect stems from a suppression of the localization of electrons due to a decrease of the films resistance.

Superconducting transition temperatures of InGa dilute alloys (FCT)

Abstract The superconducting transition temperature of thin film In and In1−xGax (x=.04, .08, .12) samples was measured resistively. These results, which complement those on InTl previosly obtained, permit a wider study of the superconductivity of this isoelectronic series (GaInTl).