Kenneth L. Kearns

Hiking down the Energy Landscape: Progress Toward the Kauzmann Temperature via Vapor Deposition

Physical vapor deposition was employed to prepare amorphous samples of indomethacin and 1,3,5-(tris)naphthylbenzene. By depositing onto substrates held somewhat below the glass transition temperature and varying the deposition rate from 15 to 0.2 nm/s, glasses with low enthalpies and exceptional kinetic stability were prepared. Glasses with fictive temperatures that are as much as 40 K lower than those prepared by cooling the liquid can be made by vapor deposition. As compared to an ordinary glass, the most stable vapor-deposited samples moved about 40% toward the bottom of the potential energy landscape for amorphous materials. These results support the hypothesis that enhanced surface mobility allows stable glass formation by vapor deposition. A comparison of the enthalpy content of vapor-deposited glasses with aged glasses was used to evaluate the difference between bulk and surface dynamics for indomethacin; the dynamics in the top few nanometers of the glass are about 7 orders of magnitude faster than those in the bulk at Tg - 20 K.

Influence of substrate temperature on the stability of glasses prepared by vapor deposition

Physical vapor deposition of indomethacin (IMC) was used to prepare glasses with unusual thermodynamic and kinetic stability. By varying the substrate temperature during the deposition from 190 K to the glass transition temperature (Tg=315 K), it was determined that depositions near 0.85Tg (265 K) resulted in the most stable IMC glasses regardless of substrate. Differential scanning calorimetry of samples deposited at 265 K indicated that the enthalpy was 8 J/g less than the ordinary glass prepared by cooling the liquid, corresponding to a 20 K reduction in the fictive temperature. Deposition at 265 K also resulted in the greatest kinetic stability, as indicated by the highest onset temperature. The most stable vapor-deposited IMC glasses had thermodynamic stabilities equivalent to ordinary glasses aged at 295 K for 7 months. We attribute the creation of stable IMC glasses via vapor deposition to enhanced surface mobility. At substrate temperatures near 0.6Tg, this mobility is diminished or absent, resulting in low stability, vapor-deposited glasses.

Glasses crystallize rapidly at free surfaces by growing crystals upward

The crystallization of glasses and amorphous solids is studied in many fields to understand the stability of amorphous materials, the fabrication of glass ceramics, and the mechanism of biomineralization. Recent studies have found that crystal growth in organic glasses can be orders of magnitude faster at the free surface than in the interior, a phenomenon potentially important for understanding glass crystallization in general. Current explanations differ for surface-enhanced crystal growth, including released tension and enhanced mobility at glass surfaces. We report here a feature of the phenomenon relevant for elucidating its mechanism: Despite their higher densities, surface crystals rise substantially above the glass surface as they grow laterally, without penetrating deep into the bulk. For indomethacin (IMC), an organic glass able to grow surface crystals in two polymorphs (α and γ), the growth front can be hundreds of nanometers above the glass surface. The process of surface crystal growth, meanwhile, is unperturbed by eliminating bulk material deeper than some threshold depth (ca. 300 nm for α IMC and less than 180 nm for γ IMC). As a growth strategy, the upward-lateral growth of surface crystals increases the system’s surface energy, but can effectively take advantage of surface mobility and circumvent slow growth in the bulk.

High-modulus organic glasses prepared by physical vapor deposition.

Adv. Mater. 2010, 22, 39–42 2010 WILEY-VCH Verlag Gmb T IO N A wide range of packing structures are available to glasses, with more efficient packing leading to materials exhibiting higher moduli. Aging a glass allows for better packing and a higher modulus, but even long aging times increase themodulus by only a few percent; preparing high-modulus materials in this manner is impractical. Using Brillouin light scattering spectroscopy, we show that physical vapor deposition can be used to circumvent this kinetic limitation and produce glasses whose moduli exceed those of the ordinary glass by up to 19%. These high-modulus glasses resist thermal treatment and take at least 10 times longer than the structural relaxation time to transform to the supercooled liquid. The facile production of high-modulus glasses will likely prove useful for fundamental investigations and coating technologies. Unlike their crystalline counterparts, glasses have a nearly limitless array of packing arrangements. As a supercooled liquid is cooled, molecular motions eventually slow to such an extent that equilibrium cannot be maintained. Below this transition temperature Tg, a mechanically stable, non-equilibrium glass is formed. Glasses slowly evolve towards equilibrium (i.e., aging) in a process that optimizes packing and creates higher moduli materials. The structural relaxation time ta dictates the rate at which this process takes place, and due to the steep temperature dependence near Tg, ta is on the order of days only a few degrees below Tg. The aging process thus increases the moduli so slowly that in practice changes of only a few percent are possible. If high-modulus amorphous materials are to be utilized for science and technology, new preparation techniques are needed which circumvent these kinetic restrictions and allow for more optimized amorphous packing. In this work, we show that high-modulus organic glasses can be made efficiently with physical vapor deposition. Using this preparation technique, we can avoid the kinetic limitations of aging and prepare high-modulus glasses in a matter of hours. Enhanced dynamics at the surface of amorphous materials allows for rapid configurational sampling in the top few nanometers. Vapor deposition can build an efficiently packed amorphous material in a layer-by-layer fashion by taking advantage of enhanced surface dynamics and thus is not limited by the slow relaxation kinetics of the bulk. We determine the mechanical properties of such vapor-deposited films using Brillouin light scattering spectroscopy (BLS). Because of the non-destructive nature of BLS, the moduli of the as-deposited glass, the supercooled liquid, and ordinary glass (created by cooling the liquid) can be determined from a single sample. Physical vapor depositions were performed separately on two organic-glass-formingmaterials: indomethacin (IMC, Tg1⁄4 315K) and trisnaphthylbenzene (TNB, Tg1⁄4 348K). These twomolecules are well-known glass-formers, and their glasses have been previously prepared with physical vapor deposition. During deposition, the temperature of the substrate Tsubstrate and the deposition rate are the important control parameters. For this work, Tsubstrate was near 0.85 Tg, i.e., 265K for IMC and room temperature ( 295K) for TNB. The rate of the deposition in all cases was held at 0.2 0.03 nm s 1 until a thickness of 10–15mm was reached. It has previously been shown that these deposition conditions produce glasses with low enthalpy, high density, and high kinetic stability. Tsubstrate was controlled by attaching the SiO2 substrates to a copper temperature stage (Fig. 1a). The deposition rate was controlled by adjusting the temperature of a crucible containing either IMC or TNB. Further details are given in the Experimental section. BLS detects photons that are inelastically scattered by propagating density fluctuations (phonons). The frequency shift between the incident laser and the scattered light is analyzed by a high-resolution tandem Fabry–Perot interferometer (details in Experimental section). The incident laser polarization was chosen to be perpendicular to the scattering plane. Scattered light polarized perpendicular and parallel to the incident polarization was measured in separate experiments, providing access to scattering from longitudinal and transverse phonons, respectively. The scattering from these two polarizations is shown in Figure 1d for vapor-deposited IMC glass at 298 K (lower panel) and supercooled liquid IMC at 336K (upper panel). Stokes and anti-Stokes shifts by the longitudinal (L) and transverse (T) phonons are observed to the left and right of the Rayleigh line region (shaded area), respectively. The vertical lines drawn on the Stokes side of the spectrum illustrate the temperatureindependent scattering of the SiO2 substrate and the temperature-dependent scattering of IMC. The absence of longitudinal phonons in the transverse spectrum indicates no birefringence in the vapor-deposited sample within the sensitivity of BLS. This is fully consistent with the amorphous nature of the stable glass as documented by wide-angle X-ray scattering and observations of crystal growth on very long timescales. The peaks in the BLS spectra provide access to the phase velocities cl,t for the L and T phonons and through this route the moduli can be obtained. Spectra similar to those found in

Physical vapor deposition as a route to hidden amorphous states

Stable glasses of indomethacin (IMC) were prepared by using physical vapor deposition. Wide-angle X-ray scattering measurements were performed to characterize the average local structure. IMC glasses prepared at a substrate temperature of 0.84 Tg (where Tg is the glass transition temperature) and a deposition rate of 0.2 nm/s show a broad, high-intensity peak at low q values that is not present in the supercooled liquid or melt-quenched glasses. When annealed slightly above Tg, the new WAXS pattern transforms into the melt-quenched glass pattern, but only after very long annealing times. For a series of samples prepared at the lowest deposition rate, the new local packing arrangement is present only for deposition temperatures below Tg −20 K, suggesting an underlying first-order liquid-to-liquid phase transition.

Observation of low heat capacities for vapor-deposited glasses of indomethacin as determined by AC nanocalorimetry

Highly stable glass films of indomethacin (IMC) with thicknesses ranging from 75 to 2900 nm were prepared by physical vapor deposition. Alternating current (AC) nanocalorimetry was used to evaluate the heat capacity and kinetic stability of the glasses as a function of thickness. Glasses deposited at a substrate temperature of 0.84T(g) displayed heat capacities that were approximately 19 J/(mol K) (4.5%) lower than glasses deposited at T(g) (315 K) or the ordinary glass prepared by cooling the liquid. This difference in heat capacity was observed over the entire thickness range and is significantly larger than the approximately 2 J/(mol K) (0.3%) difference previously observed between aged and ordinary glasses. The vapor-deposited glasses were isothermally transformed into the supercooled liquid above T(g). Glasses with low heat capacities exhibited high kinetic stability. The transformation time increased by an order of magnitude as the film thickness increased from 75 to 600 nm and was independent of film thickness for the thickest films. We interpret these results to indicate that the transformation of stable glass into supercooled liquid can occur by either a surface-initiated or bulk mechanism. In these experiments, the structural relaxation time of the IMC supercooled liquid was observed to be nearly independent of sample thickness.

Highly Stable Indomethacin Glasses Resist Uptake of Water Vapor

Mass uptake of water vapor was measured as a function of relative humidity for indomethacin glasses prepared using physical vapor deposition at different substrate temperatures. Highly stable glasses were produced on substrates at 265 K (0.84Tg) by depositing at 0.2 nm/s while samples similar to melt-cooled glasses were produced at 315 K and 5 nm/s. Samples deposited at 315 K absorb approximately the same amount of water as glasses prepared by supercooling the melt while stable glasses absorb a factor of 5 less water. Unexpectedly, the diffusion of water in the stable glass samples is 5-10 times faster than in the glass prepared by cooling the liquid.

Calorimetric Evidence for Two Distinct Molecular Packing Arrangements in Stable Glasses of Indomethacin

Indomethacin glasses of varying stabilities were prepared by physical vapor deposition onto substrates at 265 K. Enthalpy relaxation and the mobility onset temperature were assessed with differential scanning calorimetry (DSC). Quasi-isothermal temperature-modulated DSC was used to measure the reversing heat capacity during annealing above the glass transition temperature Tg. At deposition rates near 8 A/s, scanning DSC shows two enthalpy relaxation peaks and quasi-isothermal DSC shows a two-step change in the reversing heat capacity. We attribute these features to two distinct local packing structures in the vapor-deposited glass, and this interpretation is supported by the strong correlation between the two calorimetric signatures of the glass to liquid transformation. At lower deposition rates, a larger fraction of the sample is prepared in the more stable local packing. The transformation of the vapor-deposited glasses into the supercooled liquid above Tg is exceedingly slow, as much as 4500 times slower than the structural relaxation time of the liquid.

Isothermal desorption measurements of self-diffusion in supercooled o-terphenyl

Isothermal desorption of o-terphenyl thin-film bilayers was used to measure self-diffusion coefficients of supercooled o-terphenyl near the glass transition temperature (Tg=243 K). Diffusion coefficients from 10(-15.5) to 10(-12) cm2 s(-1) were obtained between 246 and 265 K. Protio and deuterio o-terphenyl were sequentially vapor deposited, then annealed to simultaneously diffuse and desorb the sample in a vacuum chamber. During the desorption of the bilayer, the concentration of each isotope was detected by a mass spectrometer, which revealed the extent of interfacial broadening. In these experiments, isotopic interdiffusion is indistinguishable from self-diffusion and the measured interfacial broadening is consistent with Fickian diffusion. The samples prepared under several different deposition conditions yielded the same self-diffusion coefficients, indicating that the experiments were conducted in the equilibrium supercooled liquid state.

Molecular orientation, thermal behavior and density of electron and hole transport layers and the implication on device performance for OLEDs

Recent progress has shown that molecular orientation in vapor-deposited glasses can affect device performance. The deposition process can result in films where the molecular axis of the glass material is preferentially ordered to lie parallel to the plane of the substrate. Here, materials made within Dow’s Electronic Materials business showed enhanced performance when the orientation of the molecules, as measured by variable angle spectroscopic ellipsometry, was oriented in a more parallel fashion as compared to other materials. For one material, the anisotropic packing was observed in the as-deposited glass and was isotropic for solution-cast and annealed films. In addition, the density of an as-deposited N,N′-bis(naphthalene-1-yl)-N,N′-bis(phenyl)-2,2′-dimethylbenzidine (NPD) film was 0.8% greater than what was realized from slowly cooling the supercooled liquid. This enhanced density indicated that vapor-deposited molecules were packing more closely in addition to being anisotropic. Finally, upon heating the NPD film into the supercooled liquid state, both the density and anisotropic packing of the as-deposited glass was lost.