Free surfaces cause reductions in the glass transition temperature of thin polystyrene films

The effect of free surfaces on the glass transition temperature (T(g)) of thin polystyrene films was studied. Measurements were performed on films (8 nm相似文献   

Probing slow dynamics in supported thin polymer films

We have used variable cooling rate ellipsometric measurements to probe the slow dynamics in thin supported polystyrene films. For the slowest cooling rates (approximately 1 K/min) the measured Tg values are reduced below the bulk value with the measured Tg of 341 K for a 6 nm film. As the cooling rate is increased the Tg reductions become smaller until at cooling rates >90 K/min there is only slight evidence for a film-thickness-dependent Tg value. By relating the cooling rate to a relaxation time, we show that the relaxation dynamics of the thin films appears to become Arrhenius with an activation energy that decreases with decreasing film thickness. We discuss this in terms of a possible connection to a length scale for cooperative motion. Finally, the results can be used to resolve a number of outstanding contradictory reports in the literature.     

Dramatic reduction of the effect of nanoconfinement on the glass transition of polymer films via addition of small-molecule diluent

The effect of nanoconfinement on the glass transition temperature T(g) in thin polymer films is studied as a function of added small-molecule diluent or plasticizer. The decrease [increase] in T(g) found in nanoconfined, neat polystyrene [poly(2-vinyl pyridine)] is suppressed by added diluent, with 13-20 nm thick polystyrene films exhibiting bulk T(g) upon addition of 9 wt % pyrene or 4 wt % dioctylphthalate [corrected]. This is explained by a connection between the size scale of the cooperative dynamics associated with T(g), which decreases with added diluent, and the size scale of the nanoconfinement effect.     

Substrate and chain size dependence of near surface dynamics of glassy polymers

We use nanohole relaxation to study the surface relaxation of films of glassy isotactic poly (methyl methacrylate) (i-PMMA) films. These measurements allow us to obtain the time dependent relaxation function at a number of different sample temperatures for the first 2-3 nm of the free surface in a system often used as a model system for the effect of the substrate on thin film dynamics. The surface is observed to relax at temperatures up to 42 K below the bulk Tg value, even on systems where the thin film Tg is known to be greater than the bulk value. We are able to determine the range over which the substrate directly affects the free surface relaxation, and determine a surprisingly large (Mw independent) limiting thickness of approximately 180 nm where the free surface relaxation is not affected by the substrate. For thick films (h>200 nm) we find an unexpected linear Mw dependence of the near surface relaxation time.     

Molecular mobility in polymer thin films

Fluorescence recovery after photobleaching was used to measure in-plane dye-probe diffusion coefficients, D, in thin films of monodisperse polystyrene supported on fused quartz substrates. The substrates were prepared with a high density of surface hydroxyl groups which interact favorably with repeat units of the polymer. The effects of temperature and film thickness were investigated, at temperatures above the bulk glass transition of the polymer, T(g), and in the range of film thicknesses from 1-10(2) times the radius of gyration (R(g)) of individual polymer molecules. As the film thickness decreases towards R(g) the value of D increases above the bulk values, with significant effects first appearing in films approximately 20R(g). In the thinnest films studied, about 4R(g), the values of D lie as much as two orders over bulk values. At the same time, the temperature dependence of D becomes much weaker than in bulk. Analysis by free volume theory indicates that apparent values of both T(g) and the thermal expansion coefficient for liquid state, alpha(L), decrease as the film thickness decreases. The possible effects of surface segregation of the dye probe are discussed.     

Direct imaging of nanoparticle embedding to probe viscoelasticity of polymer surfaces

Atomic force microscopy was used to study the embedding of gold nanoparticles into the surface of polystyrene films. The rate of embedding was determined at temperatures near the bulk glass transition temperature T(g) by measuring the apparent nanosphere height as a function of annealing time. In particular, relative height measurements of the adhered particles were made at temperatures below the bulk T(g) value. In the absence of enhanced surface dynamics or yield processes no embedding is expected to occur for T相似文献   

Photoconduction spectroscopy of p-type GaSb films

Excess carrier lifetimes (77 K) have been measured as function of the absorbed flux density in undoped p-type gallium antimonide films (GaSb/GaAs) using steady state photoconductivity measurements with the illumination wavelength of 1.1 μm. Using the results from Hall effect measurements along with the relations describing the lifetimes of the excess minority carriers in the bulk of the films and at the surface, the theoretical values of the effective excess carrier lifetime in the materials were also calculated. Discrepancies between the experimental and theoretical results were described using a two-layer model, by considering the variation in the charge distribution within the layer due to the presence of surface states, as well as the band offset between the layer and the substrate. Theoretical modeling of the experimental result yields values of different parameters such as band bending at the surface, minimum value of Shockley–Read–Hall lifetime and maximum value of the surface recombination velocity.     

Experimental temperature dependence of the magnetization of surface layers of Fe at interfaces with nonmagnetic materials

Theoretical studies have investigated the temperature dependence of the magnetization at the surface of ferromagnets. For isotropic exchange uniform throughout the sample, the temperature dependence of the surface magnetization varies asM(T)/M(0)≈(1−BT ^3/2) with aB factor at least twice as great as the bulk value. For cases in which the exchange interaction is different in the surface layer, or in which there is a significant surface exchange anisotropy, a different temperature dependence may result. These theoretical results have seen little experimental test. We report here measurements made on epitaxial (110) Fe films in which different temperature dependences are found depending upon the interfacial material chosen as a covering layer.     

Thickness dependence of the dynamics in thin films of isotactic poly (methylmethacrylate)

The film thickness dependence of both the glass transition temperature (T(g)) and the 1 kHz alpha relaxation were studied for thin films of isotactic Poly (methylmethacrylate) (i-PMMA) supported on aluminium substrates. Films in the thickness range 7-200 nm were studied. The ellipsometrically determined T(g) was found to show reductions for films thinner than 60 nm, with the largest observed reduction being 12 K for a 7 nm thick film. Measurements of the T(g) were also performed on i-PMMA films supported on silicon substrates. Dielectric studies of the temperature dependent 1 kHz alpha relaxation peak, showed that the position (T(alpha)) and shape of the peak have no film thickness dependence. This was shown to hold for films with one free surface and films with a 30 nm thermally evaporated capping layer. Capping the films was shown to have no effect on the thickness dependence of either T(g) or T(alpha). The implications of these results are discussed further and the different film thickness dependencies of T(g) and T(alpha) are discussed. This is done within the framework of the Vogel-Fulcher-Tamann (VFT) theory of glass forming materials and also in the context of the existence of a dynamic correlation length xi.     

Hydrophobic switching nature of methylcellulose ultra-thin films: thickness and annealing effects

We have studied the thermosensitive property of methylcellulose (MC) thin films supported on Si substrate by static sessile drop contact angle measurements, and their surface properties and thin film structure by x-ray reflectivity (XRR) and atomic force microscopy (AFM) techniques. From the static sessile drop contact angle measurements, the MC thin films showed the characteristic hydrophilic-to-hydrophobic transition at ～70?°C, which is the lower critical solution temperature of the bulk solution volume phase separation transition. For films with thickness d ≤ R(g), the onset of such a transition is affected by the film thickness while very thick films, d ? R(g), yielded higher contact angles. Annealing the MC thin films with thicknesses ～200 ? (near the radius of gyration, R(g), of the polymer) below the bulk glass transition temperature (T(g) ～ 195?° C) would not change the hydrophobic switch nature of the film but annealing 'at' and above the bulk T(g) would change its surface property. From surface topography images by AFM, there were no significant changes in either the roughness or the film texture before and after annealing. With XRR data, we were able to determine that such changes in the surface properties are highly correlated to the film thickness changes after the annealing process. This study, we believe, is the first to examine the thermal annealing affects on the thermal response function of a thermoresponsive polymer and is important for researching how to tailor the hydrophobic switching property of MC thin films for future sensing applications.     

Evidence for p-type doping of InN

The first evidence of successful p-type doping of InN is presented. It is shown that InN:Mg films consist of a p-type bulk region with a thin n-type inversion layer at the surface that prevents electrical contact to the bulk. Capacitance-voltage measurements indicate a net concentration of ionized acceptors below the -type surface. Irradiation with 2 MeV He+ ions is used to convert the bulk of InN:Mg from p to n-type, at which point photoluminescence is recovered. The conversion is well explained by a model assuming two parallel conducting layers (the surface and the bulk) in the films.     

Dielectric relaxations in ultrathin isotactic PMMA films and PS-PMMA-PS trilayer films

The local and cooperative dynamics of supported ultrathin films ( L = 6.4 - 120 nm) of isotactic poly(methyl methacrylate) (i-PMMA, Mn = 118 x 10(3) g/mol) was studied using dielectric relaxation spectroscopy for a wide range of frequencies (0.1 Hz to 10(6) Hz) and temperatures (250 - 423 K). To assess the influence of the PMMA film surfaces on the glass transition dynamics, two different sample geometries were employed: a single layer PMMA film with the film surfaces in direct contact with aluminum films which act as attractive, hard boundaries; and a stacked polystyrene-PMMA-polystyrene trilayer film which contains diffuse PMMA-PS interfaces. For single layer films of i-PMMA, a decrease of the glass transition temperature T(g) by up to 10 K was observed for a film thickness L < 25 nm (comparable to R(EE)), indicated by a decrease of the peak temperature T(alpha) in the loss epsilon(")(T) at low and high frequencies and by a decrease in the temperature corresponding to the maximum in the apparent activation energy E(a)(T) of the alpha-process. In contrast, measurements of i-PMMA sandwiched between PS-layers revealed a slight (up to 5 K) increase in T(g) for PMMA film thickness values less than 30 nm. The slowing down of the glass transition dynamics for the thinnest PMMA films is consistent with an increased contribution from the less mobile PMMA-PS interdiffusion regions.     

Mobility in thin polymer films ranging from local segmental motion, Rouse modes to whole chain motion: A coupling model consideration

Large increases of mobility of local segmental relaxation observed in polymer films as the film thickness is decreased, as evidenced by decreases of the glass temperature, are not found for relaxation mechanisms that have longer length scales including the Rouse relaxation modes and the diffusion of entire polymer chains. We show that the coupling model predictions, when extended to consider polymer thin films, are consistent with a large increase of the mobility of the local segmental motions and the lack of such a change for the Rouse modes and the diffusion of entire polymer chains. There are two effects that can reduce the coupling parameter of the local segmental relaxation in thin films. One is the chain orientation that is induced parallel to the surface when the film thickness h becomes smaller than the end-to-end distance of the chains and the other is a finite-size effect when h is no longer large compared to the cooperative length scale. Extremely thin ( ≈ 1.5 nm) films obtained by intercalating a polymer into layered silicates have thickness significantly less than the cooperative length scale near the bulk polymer glass transition temperature. As a result, the coupling parameter of the local segmental relaxation in such thin films is reduced almost to zero. With this plausible assumption, we show the coupling model can explain quantitatively the large decrease of the local segmental relaxation time found experimentally. Received 1 August 2001 and Received in final form 1 December 2001     

Enhancement of the superconducting transition temperature of MgB2 by a strain-induced bond-stretching mode softening

We report a systematic increase of the superconducting transition temperature T(c) with a biaxial tensile strain in MgB2 films to well beyond the bulk value. The tensile strain increases with the MgB2 film thickness, caused primarily by the coalescence of initially nucleated discrete islands (the Volmer-Weber growth mode.) The T(c) increase was observed in epitaxial films on SiC and sapphire substrates, although the T(c) values were different for the two substrates due to different lattice parameters and thermal expansion coefficients. We identified, by first-principles calculations, the underlying mechanism for the T(c) increase to be the softening of the bond-stretching E(2g) phonon mode, and we confirmed this conclusion by Raman scattering measurements. The result suggests that the E(2g) phonon softening is a possible avenue to achieve even higher T(c) in MgB2-related material systems.     

Evidence of glass transition in thin films of maleic anhydride derivatives: Effect of the surfactant coadsorption

The glass transition temperature of poly (maleic anhydride-alt-1-octadecen) and poly (styreneco-maleic anhydride) cumene-terminated thin films has been measured by mechanical relaxation of Langmuir films of these polymers. The dynamical properties show glass-like features (non-Arrhenius relaxation times and non-Debye mechanical response) interpreted by the coupling model. The glass transition temperature values determined by a mechanical relaxation experiment (step-compression) agree very well with those obtained by surface potential measurements. It is found that the glass transition temperature values in thin films decrease by about 100K as compared with those corresponding to the bulk polymers. The coadsorption of the water-insoluble surfactant DODAB decreases the glass transition temperature.     

Electronic structure and magnetism of metastable bcc Co(001)

The electronic structure and magnetism of thin films of metastable bcc phase Co(001) consisting of one-, five- and nine-layers is determined by means of all-electron local spin density full potential linearized augmented plane wave (FLAPW) calculations at the lattice constant found for bcc Co stabilized on a GaAs substrate by Prinz. Band structure, surface states, density of states, charge and spin densities and contact hyperfine fields are presented. The center layer of the many-layer films yields a spin magnetic moment of 1.76μ_B which agrees well with results of bulk calculations for bcc Co. The surface layer has a moment (1.94μ_B) which is only 10% greater than the bulk value - in sharp contrast to the enhancement found for other transition metal surfaces [35% for Fe(001), 20% for Ni(001) and over 300% for Cr(001)]. Unlike these other transition metal surfaces, even the first layer below the surface layer in Co has the bulk magnetic moment - which indicates that the spin density, like the charge density, shows very short range screening of the surface-vacuum interface.     

Surface morphology characterization of pentacene thin film and its substrate with under-layers by power spectral density using fast Fourier transform algorithms

Surface morphology of pentacene thin films and their substrates with under-layers is characterized by using atomic force microscopy (AFM). The power values of power spectral density (PSD) for the AFM digital data were determined by the fast Fourier transform (FFT) algorithms instead of the root-mean-square (rms) and peak-to-valley value. The PSD plots of pentacene films on glass substrate are successfully approximated by the k-correlation model. The pentacene film growth is interpreted the intermediation of the bulk and surface diffusion by parameter C of k-correlation model. The PSD plots of pentacene film on Au under-layer is approximated by using the linear continuum model (LCM) instead of the combination model of the k-correlation model and Gaussian function. The PSD plots of SiO₂ layer on Au under-layer as a gate insulator on a gate electrode of organic thin film transistors (OTFTs) have three power values of PSD. It is interpreted that the specific three PSD power values are caused by the planarization of the smooth SiO₂ layer to rough Au under-layer.     

Unveiling the nature of three-dimensional orbital ordering transitions: the case of e(g) and t(2g) models on the cubic lattice

We perform large scale finite-temperature Monte Carlo simulations of the classical e(g) and t(2g) orbital models on the simple cubic lattice in three dimensions. The e(g) model displays a continuous phase transition to an orbitally ordered phase. While the correlation length exponent ν ≈ 0.66(1) is close to the 3D XY value, the exponent η ≈ 0.15(1) differs substantially from O(N) values. At T(c) a U(1) symmetry emerges, which persists for T < T(c) below a crossover length scaling as Λ ～ ξ(a), with an unusually small a ≈ 1.3. Finally, for the t(2g) model we find a first order transition into a low-temperature lattice-nematic phase without orbital order.     

金刚石薄膜红外椭圆偏振参量的计算与拟合

用红外椭圆偏振仪对热丝化学气相沉积法制备的金刚石薄膜的光学参量进行了测量。由于表面状态和界面特性的差异，分别对镜面抛光硅片和粗糙氧化铝基片上的金刚石薄膜建立了不同的模型，并在此基础上进行了测试结果的计算拟合。为了综合反应诸如表面粗糙度等表面界面因素对测试结果的影响，根据衬底特性将表面层和界面层分离出来，并采用Bruggeman有效介质方法对它们的影响进行了近似处理。结果表明，硅衬底上金刚石薄膜的椭偏数据在模型引入了厚度为879nm的表面粗糙层之后能得到很好的拟合。而对于氧化铝衬底上的金刚石薄膜而言，除了在薄膜表面引入了粗糙层之外，还必须在衬底和金刚石界面处加入一层由体积分数为0．641的氧化铝、体积分数为0．2334的金刚石和体积分数为0．1253的空隙组成的复合过渡层(厚度995nm)，才能使计算值与实验参量很好地吻合。