Front-Mediated Melting of Isotropic Ultrastable Glasses

High Density Two-Component Glasses of Organic Semiconductors Prepared by Physical Vapor Deposition

Physical vapor deposition (PVD) is widely utilized for the production of organic semiconductor devices due to its ability to form thin layers with exceptional properties. Although the layers in the device usually consist of two or more components, there is limited understanding about the fundamental characteristics of such multicomponent vapor-deposited glasses. Here, spectroscopic ellipsometry was employed to characterize the densities, thermal stabilities, and optical properties of covapor deposited NPD and TPD glasses across the entire range of composition. We find that codeposited NPD and TPD form high density glasses with enhanced thermal stability. The dependences of density and stability upon substrate temperature are correlated, and the birefringence of the codeposited glasses is determined by the reduced substrate temperature of mixtures. Additionally, we observe that the transformation of a highly stable and dense two-component glass into its supercooled liquid initiates from the free surface and propagates into the bulk at a constant velocity, like single component PVD glasses. All of these features are consistent with the surface equilibration mechanism.

Front propagation in ultrastable glasses is dynamically heterogeneous

Upon heating, ultrastable glassy films transform into liquids via a propagating equilibration front, resembling the heterogeneous melting of crystals. A microscopic understanding of this robust phenomenology is, however, lacking because experimental resolution is limited. We simulate the heterogeneous transformation kinetics of ultrastable configurations prepared using the swap Monte Carlo algorithm, thus allowing a direct comparison with experiments. We resolve the liquid-glass interface both in space and in time as well as the underlying particle motion responsible for its propagation. We perform a detailed statistical analysis of the interface geometry and kinetics over a broad range of temperatures. We show that the dynamic heterogeneity of the bulk liquid is passed on to the front that propagates heterogeneously in space and intermittently in time. This observation allows us to relate the averaged front velocity to the equilibrium diffusion coefficient of the liquid. We suggest that an experimental characterization of the interface geometry during the heterogeneous devitrification of ultrastable glassy films could provide direct experimental access to the long-sought characteristic length scale of dynamic heterogeneity in bulk supercooled liquids.

Composition Dictates Molecular Orientation at the Heterointerfaces of Vapor-Deposited Glasses

Physical vapor deposition (PVD) can prepare organic glasses with a preferred molecular orientation. The relationships between deposition conditions and orientation have been extensively investigated in the film bulk. The role of interfaces on the structure is less well understood and remains a key knowledge gap, as the interfacial region can govern glass stability and optoelectronic properties. Robust experimental characterization has remained elusive due to complexities in interrogating molecular organization in amorphous, organic materials. Polarized soft X-rays are sensitive to both the composition and the orientation of transition dipole moments in the film, making them uniquely suited to probe molecular orientation in amorphous soft matter. Here, we utilize polarized resonant soft X-ray reflectivity (P-RSoXR) to simultaneously depth profile the composition and molecular orientation of a bilayer prepared through the physical vapor deposition of 1,4-di-[4-(N,N-diphenyl)amino]styryl-benzene (DSA-Ph) on a film of aluminum-tris(8-hydroxyquinoline) (Alq3). The bulk orientation of the DSA-Ph layer is controlled by varying deposition conditions. Utilizing P-RSoXR to depth profile the films enables determination of both the bulk orientation of DSA-Ph and the orientation near the Alq3 interface. At the Alq3 surface, DSA-Ph always lies with its long axis parallel to the interface, before transitioning into the bulk orientation. This is likely due to the lower mobility and higher glass transition of Alq3, as the first several monolayers of DSA-Ph deposited on Alq3 appear to behave as a blend. We further show how orientation at the interface correlates with the bulk behavior of a codeposited glass of similar blend composition, demonstrating a straightforward approach to predicting molecular orientation at heterointerfaces. This work provides key insights into how molecules orient during vapor deposition and offers methods to predict this property, a critical step toward controlling interfacial behavior in soft matter.

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.

Surface premelting and melting of colloidal glasses

The nature of liquid-to-glass transition is a major puzzle in science. A similar challenge exists in glass-to-liquid transition, i.e., glass melting, especially for the poorly investigated surface effects. Here, we assemble colloidal glasses by vapor deposition and melt them by tuning particle attractions. The structural and dynamic parameters saturate at different depths, which define a surface liquid layer and an intermediate glassy layer. The power-law growth of both layers and melting front behaviors at different heating rates are similar to crystal premelting and melting, suggesting that premelting and melting can be generalized to amorphous solids. The measured single-particle kinetics reveal various features and confirm theoretical predictions for glass surface layer.

Surface melting of a colloidal glass

Despite their technological relevance, a full microscopic understanding of glasses is still lacking. This applies even more to their surfaces whose properties largely differ from that of the bulk material. Here, we experimentally investigate the surface of a two-dimensional glass as a function of the effective temperature. To yield a free surface, we use an attractive colloidal suspension of micron-sized particles interacting via tunable critical Casimir forces. Similar to crystals, we observe surface melting of the glass, i.e., the formation of a liquid film at the surface well below the glass temperature. Underneath, however, we find an unexpected region with bulk density but much faster particle dynamics. It results from connected clusters of highly mobile particles which are formed near the surface and deeply percolate into the underlying material. Because its thickness can reach several tens of particle diameters, this layer may elucidate the poorly understood properties of thin glassy films which find use in many technical applications.

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 T_g to 0.92 T_g, 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.

Glass Dynamics Deep in the Energy Landscape

When a liquid is cooled, progress down the energy landscape is arrested near the glass transition temperature T_g. In principle, lower energy states can be accessed by waiting for further equilibration, but the rough energy landscape of glasses quickly leads to kinetics on geologically slow time scales below T_g. Over the past decade, progress has been made probing deeper into the energy landscape via several techniques. By looking at bulk and surface diffusion, using layered deposition that promotes equilibration, imaging glass surfaces with faster dynamics below T_g, and optically exciting glasses, experiments have moved into a regime of ultrastable, low energy glasses that was difficult to access in the past. At the same time, both simulations and energy landscape theory based on a random first order transition (RFOT) have tackled systems that include surfaces, optical excitation, and interfacial dynamics. Here we review some of the recent experimental work, and how energy landscape theory illuminates glassy dynamics well below the glass transition temperature by making direct connections between configurational entropy, energy landscape barriers, and the resulting dynamics.

Nucleation and Growth of the Supercooled Liquid Phase Control Glass Transition in Bulk Ultrastable Glasses

We report the anomalous bulk transformation of vapor deposited stable glasses into the liquid state. The transformation proceeds through two competing parallel processes: partial rejuvenation of the stable glass and nucleation and growth of liquid patches within the glass. The kinetics of the transformation extracted from heat capacity curves after isothermal runs is dominated by the heterogeneous nucleation and growth process that initiates at preexisting seeds and propagates radially at a velocity proportional to the alpha relaxation time. Remarkably, the distance between the activation seeds is independent of temperature within experimental uncertainty and amounts to several micrometers, a value in close agreement with the crossover length for TPD glasses. We speculate the initiation sites for the transformation of the glass into the supercooled liquid are localized regions of lower stability (or density).