Two-step devitrification of ultrastable glasses.

The discovery of ultrastable glasses raises novel challenges about glassy systems. Recent experiments studied the macroscopic devitrification of ultrastable glasses into liquids upon heating but lacked microscopic resolution. We use molecular dynamics simulations to analyze the kinetics of this transformation. In the most stable systems, devitrification occurs after a very large time, but the liquid emerges in two steps. At short times, we observe the rare nucleation and slow growth of isolated droplets containing a liquid maintained under pressure by the rigidity of the surrounding glass. At large times, pressure is released after the droplets coalesce into large domains, which accelerates devitrification. This two-step process produces pronounced deviations from the classical Avrami kinetics and explains the emergence of a giant lengthscale characterizing the devitrification of bulk ultrastable glasses. Our study elucidates the nonequilibrium kinetics of glasses following a large temperature jump, which differs from both equilibrium relaxation and aging dynamics, and will guide future experimental studies.

Anatomy of the dielectric behavior of methyl-m-toluate glasses during and after vapor deposition.

Glassy films of methyl-m-toluate have been vapor deposited onto a substrate equipped with interdigitated electrodes, facilitating in situ dielectric relaxation measurements during and after deposition. Samples of 200 nm thickness have been deposited at rates of 0.1 nm/s at a variety of deposition temperatures between 40 K and Tg = 170 K. With increasing depth below the surface, the dielectric loss changes gradually from a value reflecting a mobile surface layer to that of the kinetically stable glass. The thickness of this more mobile layer varies from below 1 to beyond 10 nm as the deposition temperature is increased, and its average fictive temperature is near Tg for all deposition temperatures. Judged by the dielectric loss, the liquid-like portion of the surface layer exceeds a thickness of 1 nm only for deposition temperatures above 0.8Tg, where near-equilibrium glassy states are obtained. After deposition, the dielectric loss of the material positioned about 5-30 nm below the surface decreases for thousands of seconds of annealing time, whereas the bulk of the film remains unchanged.

Surface equilibration mechanism controls the molecular packing of glassy molecular semiconductors at organic interfaces.

Glasses prepared by physical vapor deposition (PVD) are anisotropic, and the average molecular orientation can be varied significantly by controlling the deposition conditions. While previous work has characterized the average structure of thick PVD glasses, most experiments are not sensitive to the structure near an underlying substrate or interface. Given the profound influence of the substrate on the growth of crystalline or liquid crystalline materials, an underlying substrate might be expected to substantially alter the structure of a PVD glass, and this near-interface structure is important for the function of organic electronic devices prepared by PVD, such as organic light-emitting diodes. To study molecular packing near buried organic-organic interfaces, we prepare superlattice structures (stacks of 5- or 10-nm layers) of organic semiconductors, Alq3 (Tris-(8-hydroxyquinoline)aluminum) and DSA-Ph (1,4-di-[4-(N,N-diphenyl)amino]styrylbenzene), using PVD. Superlattice structures significantly increase the fraction of the films near buried interfaces, thereby allowing for quantitative characterization of interfacial packing. Remarkably, both X-ray scattering and spectroscopic ellipsometry indicate that the substrate exerts a negligible influence on PVD glass structure. Thus, the surface equilibration mechanism previously advanced for thick films can successfully describe PVD glass structure even within the first monolayer of deposition on an organic substrate.

Using 4D STEM to Probe Mesoscale Order in Molecular Glass Films Prepared by Physical Vapor Deposition.

Physical vapor deposition can be used to prepare highly stable organic glass systems where the molecules show orientational and translational ordering at the nanoscale. We have used low-dose four-dimensional scanning transmission electron microscopy (4D STEM), enabled by a fast direct electron detector, to map columnar order in glassy samples of a discotic mesogen using a 2 nm probe. Both vapor-deposited and liquid-cooled glassy films show domains of similar orientation, but their size varies from tens to hundreds of nanometers, depending on processing. Domain sizes are consistent with surface-diffusion-mediated ordering during film deposition. These results demonstrate the ability of low-dose 4D STEM to characterize a mesoscale structure in a molecular glass system which may be relevant to organic electronics.

Engineering the glass structure of a discotic liquid crystal by multiple kinetic arrests.

X-ray scattering has been used to characterize the columnar packing and the π stacking in a glass-forming discotic liquid crystal. In the equilibrium liquid state, the intensities of the scattering peaks for π stacking and columnar packing are proportional to each other, indicating concurrent development of the two orders. Upon cooling into the glassy state, the π-π distance shows a kinetic arrest with a change in the thermal expansion coefficient (TEC) from 321 to 109 ppm/K, while the intercolumnar spacing exhibits a constant TEC of 113 ppm/K. By changing the cooling rate, it is possible to prepare glasses with a wide range of columnar and π stacking orders, including zero order. For each glass, the columnar order and the π stacking order correspond to a much hotter liquid than its enthalpy and π-π distance, with the difference between the two internal (fictive) temperatures exceeding 100 K. By comparison with the relaxation map obtained by dielectric spectroscopy, we find that the δ mode (disk tumbling within a column) controls the columnar order and the π stacking order trapped in the glass, while the α mode (disk spinning about its axis) controls the enthalpy and the π-π spacing. Our finding is relevant for controlling the different structural features of a molecular glass to optimize its properties.

Understanding the Fragile-to-Strong Transition in Silica from Microscopic Dynamics.

In this work, we revisit the fragile-to-strong transition (FTS) in the simulated BKS silica from the perspective of microscopic dynamics in an effort to elucidate the dynamical behaviors of fragile and strong glass-forming liquids. Softness, which is a machine-learned feature from local atomic structures, is used to predict the microscopic activation energetics and long-term dynamics. The FTS is found to originate from a change in the temperature dependence of the microscopic activation energetics. Furthermore, results suggest there are two diffusion channels with different energy barriers in BKS silica. The fast dynamics at high temperatures is dominated by the channel with small energy barriers (<∼1 eV), which is controlled by the short-range order. The rapid closing of this diffusion channel when lowering temperature leads to the fragile behavior. On the other hand, the slow dynamics at low temperatures is dominated by the channel with large energy barriers controlled by the medium-range order. This slow diffusion channel changes only subtly with temperature, leading to the strong behavior. The distributions of barriers in the two channels show different temperature dependences, causing a crossover at ∼3100 K. This transition temperature in microscopic dynamics is consistent with the inflection point in the configurational entropy, suggesting there is a fundamental correlation between microscopic dynamics and thermodynamics.

Surface Diffusion Is Controlled by Bulk Fragility across All Glass Types.

Surface diffusion is vastly faster than bulk diffusion in some glasses, but only moderately enhanced in others. We show that this variation is closely linked to bulk fragility, a common measure of how quickly dynamics is excited when a glass is heated to become a liquid. In fragile molecular glasses, surface diffusion can be a factor of 10^{8} faster than bulk diffusion at the glass transition temperature, while in the strong system SiO_{2}, the enhancement is a factor of 10. Between these two extremes lie systems of intermediate fragility, including metallic glasses and amorphous selenium and silicon. This indicates that stronger liquids have greater resistance to dynamic excitation from bulk to surface and enables prediction of surface diffusion, surface crystallization, and formation of stable glasses by vapor deposition.

A liquid with distinct metastable structures: Supercooled butyronitrile.

The dielectric relaxation behavior of the molecular glass former butyronitrile is revisited by measuring both bulk samples cooled from the melt and samples obtained by physical vapor deposition. We find that the dielectric constant in the viscous regime of the bulk liquid is much higher than reported previously, reaching εs = 63 at T = 103 K, i.e., just above the glass transition temperature Tg = 97 K. By contrast, varying the deposition temperature and rate of vapor-deposited samples leads to dielectric constants in a range between 4.5 and 63 at T = 103 K. Values much below εs = 63 persist for thousands of seconds, where the dielectric relaxation time is about 0.1 s. The observations can be interpreted by the formation of clusters in which pair-wise anti-parallel dipole orientation is the preferred state at temperatures well below the glass transition. These non-crystalline clusters are long-lived even above Tg, where the remaining volume fraction is in the state of the equilibrium polar liquid.

Vapor deposition rate modifies anisotropic glassy structure of an anthracene-based organic semiconductor.

We control the anisotropic molecular packing of vapor-deposited glasses of ABH113, a deuterated anthracene derivative with promise for future organic light emitting diode materials, by changing the deposition rate and substrate temperature at which they are prepared. We find that at substrate temperatures from 0.65 Tg to 0.92 Tg, the deposition rate significantly modifies the orientational order in the vapor-deposited glasses as characterized by x-ray scattering and birefringence. Both measures of anisotropic order can be described by a single deposition rate-substrate temperature superposition (RTS). This supports the applicability of the surface equilibration mechanism and generalizes the RTS principle from previous model systems with liquid crystalline order to non-mesogenic organic semiconductors. We find that vapor-deposited glasses of ABH113 have significantly enhanced density and thermal stability compared to their counterparts prepared by liquid-cooling. For organic semiconductors, the results of this study provide an efficient guide for using the deposition rate to prepare stable glasses with controlled molecular packing.

Surface diffusion of a glassy discotic organic semiconductor and the surface mobility gradient of molecular glasses.

Surface diffusion has been measured in the glass of an organic semiconductor, MTDATA, using the method of surface grating decay. The decay rate was measured as a function of temperature and grating wavelength, and the results indicate that the decay mechanism is viscous flow at high temperatures and surface diffusion at low temperatures. Surface diffusion in MTDATA is enhanced by 4 orders of magnitude relative to bulk diffusion when compared at the glass transition temperature Tg. The result on MTDATA has been analyzed along with the results on other molecular glasses without extensive hydrogen bonds. In total, these systems cover a wide range of molecular geometries from rod-like to quasi-spherical to discotic and their surface diffusion coefficients vary by 9 orders of magnitude. We find that the variation is well explained by the existence of a steep surface mobility gradient and the anchoring of surface molecules at different depths. Quantitative analysis of these results supports a recently proposed double-exponential form for the mobility gradient: log D(T, z) = log Dv(T) + [log D0 - log Dv(T)]exp(-z/ξ), where D(T, z) is the depth-dependent diffusion coefficient, Dv(T) is the bulk diffusion coefficient, D0 ≈ 10-8 m2/s, and ξ ≈ 1.5 nm. Assuming representative bulk diffusion coefficients for these fragile glass formers, the model reproduces the presently known surface diffusion rates within 0.6 decade. Our result provides a general way to predict the surface diffusion rates in molecular glasses.

Vapor deposition of a nonmesogen prepares highly structured organic glasses.

We show that glasses with aligned smectic liquid crystal-like order can be produced by physical vapor deposition of a molecule without any equilibrium liquid crystal phases. Smectic-like order in vapor-deposited films was characterized by wide-angle X-ray scattering. A surface equilibration mechanism predicts the highly smectic-like vapor-deposited structure to be a result of significant vertical anchoring at the surface of the equilibrium liquid, and near-edge X-ray absorption fine structure (NEXAFS) spectroscopy orientation analysis confirms this prediction. Understanding of the mechanism enables informed engineering of different levels of smectic order in vapor-deposited glasses to suit various applications. The preparation of a glass with orientational and translational order from a nonliquid crystal opens up an exciting paradigm for accessing extreme anisotropy in glassy solids.

Surface mobility in amorphous selenium and comparison with organic molecular glasses.

Surface diffusion is important for a broad range of chemical and physical processes that take place at the surfaces of amorphous solids, including surface crystallization. In this work, the temporal evolution of nanoholes is monitored with atomic force microscopy to quantify the surface dynamics of amorphous selenium. In molecular glasses, the surface diffusion coefficient has been shown to scale with the surface crystal growth rate (us) according to the power relation us ≈ Ds 0.87. In this study, we observe that the same power law applies to surface crystallization of amorphous selenium, a representative inorganic polymer glass. Our study shows that the surface diffusion coefficient can be used to quantitatively predict surface crystallization rates in a chemically diverse range of materials.

Anisotropic Vapor-Deposited Glasses: Hybrid Organic Solids.

The term "organic solids" encompasses both crystals and glasses. Organic crystals are commonly grown for purification and structure determination and are being extensively explored for applications in organic electronics including field effect transistors. The ability to control the packing of one molecule relative to its neighbors is of critical importance for most uses of organic crystals. Often, anisotropic packing is also highly desirable as it enhances charge transport and optimizes light absorption/emission. When compared to crystals, the local packing in organic glasses is highly disordered and often isotropic. Glasses, however, offer two key advantages with respect to crystals. First, glasses typically lack grain boundaries and thus exhibit better macroscopic homogeneity. Second, glass composition can often be varied over a wide range while maintaining homogeneity. Besides electronic materials, many modern plastics used in a wide range of technologies are organic glasses, and the glassy state is being increasingly utilized to deliver pharmaceuticals because of higher bioavailability. In this article, we introduce vapor-deposited organic glasses as hybrid materials that combine some of the useful features of crystals and traditional liquid-cooled glasses. Physical vapor deposition produces glasses by directly condensing molecules from the gas phase onto a temperature-controlled substrate and allows film thickness to be controlled with nanometer precision. Just as liquid-cooled glasses, vapor-deposited glasses have smooth surfaces and lack grain boundaries. These attributes are critical for applications such as organic light emitting diodes (OLEDs), in which vapor-deposited glasses of organic semiconductors form the active layers. In common with crystals, vapor-deposited glasses can exhibit anisotropic packing, and the extent of anisotropy can be comparable to that of the typical organic crystal. For vapor-deposited glasses, in contrast to crystals, anisotropic packing can generally be controlled as a continuous variable. Deposition conditions can be chosen to produce glasses with significant molecular orientation (molecules "standing up" or "lying down" relative to the substrate), and π-stacking can be directed along different directions relative to the substrate. Over the last five years, we have gained a fundamental understanding of the mechanism that controls the anisotropy of vapor-deposited glasses and learned how to control many aspects of anisotropic packing. Two key elements that enable such control are the high mobility present at the surface of an organic glass and the tendency of the surface to promote anisotropic packing of molecules. In contrast to traditional epitaxial growth, for vapor-deposited glasses, the free surface (not the substrate) acts as a template that controls the structure of a growing film. The structure of any given layer is decoupled from those beneath it, thereby providing considerable freedom in producing layered glassy structures.

Activation Entropy as a Key Factor Controlling the Memory Effect in Glasses.

As opposed to the common monotonic relaxation process of glasses, the Kovacs memory effect describes an isothermal annealing experiment, in which the enthalpy and volume of a preannealed glass first increases before finally decreasing toward equilibrium. This interesting behavior has been observed for many materials and is generally explained in terms of heterogeneous dynamics. In this Letter, the memory effect in a model Au-based metallic glass is studied using a high-precision high-rate calorimeter. The activation entropy (S^{*}) during isothermal annealing is determined according to the absolute reaction rate theory. We observe that the memory effect appears only when the second-annealing process has a large S^{*}. These results indicate that a large value of S^{*} is a key requirement for observation of the memory effect and this may provide a useful perspective for understanding the memory effect in both thermal and athermal systems.

In situ observation of fast surface dynamics during the vapor-deposition of a stable organic glass.

By measuring the increments of dielectric capacitance (ΔC) and dissipation (Δtan δ) during physical vapor deposition of a 110 nm film of a molecular glass former, we provide direct evidence of the mobile surface layer that is made responsible for the extraordinary properties of vapor deposited glasses. Depositing at a rate of 0.1 nm s-1 onto a substrate at Tdep = 75 K = 0.82Tg, we observe a 2.5 nm thick surface layer with an average relaxation time of 0.1 s, while the glass growing underneath has a high kinetic stability. The level of Δtan δ continues to decrease for thousands of seconds after terminating the deposition process, indicating a slow aging-like increase in packing density near the surface. At very low deposition temperatures, 32 and 42 K, the surface layer thicknesses and mobilities are reduced, as are the kinetic stabilities.

Surface diffusion in glasses of rod-like molecules posaconazole and itraconazole: effect of interfacial molecular alignment and bulk penetration.

The method of surface grating decay has been used to measure surface diffusion in the glasses of two rod-like molecules posaconazole (POS) and itraconazole (ITZ). Although structurally similar antifungal medicines, ITZ forms liquid-crystalline phases while POS does not. Surface diffusion in these systems is significantly slower than in the glasses of quasi-spherical molecules of similar volume when compared at the glass transition temperature Tg. Between the two systems, ITZ has slower surface diffusion. These results are explained on the basis of the near-vertical orientation of the rod-like molecules at the surface and their deep penetration into the bulk where mobility is low. For molecular glasses without extensive hydrogen bonds, we find that the surface diffusion coefficient at Tg decreases smoothly with the penetration depth of surface molecules and the trend has the double-exponential form for the surface mobility gradient observed in simulations. This supports the view that these molecular glasses have a similar mobility vs. depth profile and their different surface diffusion rates arise simply from the different depths at which molecules are anchored. Our results also provide support for a previously observed correlation between the rate of surface diffusion and the fragility of the bulk liquid.

How to "measure" a structural relaxation time that is too long to be measured?

It has recently become possible to prepare ultrastable glassy materials characterized by structural relaxation times, which vastly exceed the duration of any feasible experiment. Similarly, new algorithms have led to the production of ultrastable computer glasses. Is it possible to obtain a reliable estimate of a structural relaxation time that is too long to be measured? We review, organize, and critically discuss various methods to estimate very long relaxation times. We also perform computer simulations of three dimensional ultrastable hard spheres glasses to test and quantitatively compare some of these methods for a single model system. The various estimation methods disagree significantly, and non-linear and non-equilibrium methods lead to a strong underestimate of the actual relaxation time. It is not yet clear how to accurately estimate extremely long relaxation times.

Physical vapor deposition of a polyamorphic system: Triphenyl phosphite.

In situ AC nanocalorimetry and dielectric spectroscopy were used to analyze films of vapor-deposited triphenyl phosphite. The goal of this work was to investigate the properties of vapor-deposited glasses of this known polyamorphic system and to determine which liquid is formed when the glass is heated. We find that triphenyl phosphite forms a kinetically stable glass when prepared at substrate temperatures of 0.75-0.95Tg, where Tg is the glass transition temperature. Regardless of the substrate temperature utilized during deposition of triphenyl phosphite, heating a vapor-deposited glass always forms the ordinary supercooled liquid (liquid 1). The identity of liquid 1 was confirmed by both the calorimetric signal and the shape and position of the dielectric spectra. For the purposes of comparison, the glacial phase of triphenyl phosphite (liquid 2) was prepared by the conventional method of annealing liquid 1. We speculate that these new results and previous work on vapor deposition of other polyamorphic systems can be explained by the free surface structure being similar to one polyamorph even in a temperature regime where the other polyamorph is more thermodynamically stable in the bulk.

Generic packing motifs in vapor-deposited glasses of organic semiconductors.

We study the structure of vapor-deposited glasses of five common organic semiconductors as a function of substrate temperature during deposition, using synchrotron X-ray scattering. For deposition at a substrate temperature of ∼0.8Tg (where Tg is the glass transition temperature), we find a generic tendency towards "face-on" packing in glasses of anisotropic molecules. At higher substrate temperature however this generic behavior breaks down; glasses of rod-shaped molecules exhibit a more pronounced tendency for end-on packing. Our study provides guidelines to create face-on and end-on packing motifs in organic glasses, which can promote efficient charge transport in OLED and OFET devices respectively.

Dielectric properties of vapor-deposited propylbenzenes.

Dielectric susceptibility data of vapor-deposited films of iso-propylbenzene (IPB) and n-propylbenzene (NPB) have been recorded across a wide range of deposition temperatures, Tdep, mostly below the glass transition temperature, Tg. The results for the real and imaginary components of dielectric susceptibility are compared with recently published results for 2-methyltetrahydrofuran (MTHF). Common to all three systems are the following: (i) increased kinetic stability seen as higher onset temperature for the transformation to the liquid state for Tdep ≈ 0.85Tg; (ii) the reduction of the dielectric loss (χ″) for as-deposited glasses, a signature of increased packing density that is maximal for Tdep ≈ 0.85Tg; and (iii) a reduced level of the storage component (χ') for as-deposited glasses, an effect that is almost deposition temperature invariant for Tdep < Tg. Material specific behavior is observed when heating the as-deposited films to 1.2Tg: IPB and NPB transform directly into the ordinary liquid state if judged on the basis of dielectric susceptibility, whereas MTHF has been reported to enter an unusual liquid state prior to a liquid-liquid transition at higher temperatures. These results are discussed in the context of the curious scattering results reported by Ishii et al. for some benzene derivatives, which hint at a liquid-liquid transformation.