Perspective: Highly stable vapor-deposited glasses

Direct Numerical Analysis of Dynamic Facilitation in Glass-Forming Liquids

We propose a computational strategy to quantify the temperature evolution of the timescales and length scales over which dynamic facilitation affects the relaxation dynamics of glass-forming liquids at low temperatures, which requires no assumption about the nature of the dynamics. In two glass models, we find that dynamic facilitation depends strongly on temperature, leading to a subdiffusive spreading of relaxation events which we characterize using a temperature-dependent dynamic exponent. We also establish that this temperature evolution represents a major contribution to the increase of the structural relaxation time.

Dielectric Study of n-Propanol during Physical Vapor Deposition: No Surface Mobility and No Kinetic Stability

Dielectric relaxation experiments have been performed on n-propanol (NPOH) films during physical vapor deposition at temperatures above and below its glass transition, Tg = 97 K. The results for NPOH are compared with those of analogous experiments on methyl-m-toluate (MMT) and 2-methyltetrahydrofuran (MTHF), with all three deposited at the same reduced temperature, 0.82Tg. While MMT and MTHF display clear signs of a highly mobile surface layer, no such feature is observed for NPOH. The existence of this in situ observed mobile surface layer correlates perfectly with the material's ability to form kinetically stable glasses, as NPOH differs from MMT and MTHF by not displaying kinetic stability.

Surface Glass Transition Temperature Region of Diarylethene Films Determined by Nano-Marangoni Effect

For the last two decades, research has addressed whether the glass transition temperature and the molecular motions on the surface of organic films are significantly different from those inside the bulk glasses. It is reported that the surface of the photochromic diarylethene film prepared by vacuum deposition has fluidity and the vacuum deposition of small amount of rubrene molecules induces surface tension fluctuations, generating dents due to the Marangoni flow in nanoscale. The depth of the dents increases in proportion to these radii for the colorless diarylethene film with a bulk glass transition temperature (Tg) close to room temperature. On the other hand, in the colored diarylethene obtained by UV irradiation to the colorless film, the depth becomes constant at a certain level. The Tg distribution in the depth direction is clarified based on an analysis of the dent depth. By approximating the obtained Tg depth distribution with an exponential function, the outermost surface Tg is about 100 K lower than the bulk Tg in the case of photoisomerized diarylethene.

Vapor-to-glass preparation of biaxially aligned organic semiconductors

Physical vapor deposition (PVD) provides a route to prepare highly stable and anisotropic organic glasses that are utilized in multi-layer structures such as organic light-emitting devices. While previous work has demonstrated that anisotropic glasses with uniaxial symmetry can be prepared by PVD, here, we prepare biaxially aligned glasses in which molecular orientation has a preferred in-plane direction. With the collective effect of the surface equilibration mechanism and template growth on an aligned substrate, macroscopic biaxial alignment is achieved in depositions as much as 180 K below the clearing point TLC-iso (and 50 K below the glass transition temperature Tg) with single-component disk-like (phenanthroperylene ester) and rod-like (itraconazole) mesogens. The preparation of biaxially aligned organic semiconductors adds a new dimension of structural control for vapor-deposited glasses and may enable polarized emission and in-plane control of charge mobility.

Highly stable petroleum pitches provide access to the deep glassy state

Differential scanning calorimetry (DSC) was used to study the fast aging behavior of two petroleum pitch materials despite being only three to five years old. We observe that these highly aromatic pitches with broad distributions of both molecular weight and aromaticity exhibit large enthalpic relaxation endotherms in initial DSC heating scans, and 20-32 °C reductions in the fictive temperature and 0.35-0.87 of θK, which are indicative of aged glasses similar to ultrastable glasses and 20 MA aged amber. Quantifying the degree of thermodynamic stability relative to the Kauzmann temperature vs. the aging time demonstrates that these materials age just as quickly as low fragility metallic glasses. Additionally, we observe that pitches age faster than polymers reported in the literature when compared using down-jump experiments. We hypothesize that the fraction of higher aromaticity of pitch molecules plays a crucial role in faster dynamics. The unique aging behavior and the ability to produce pitches in bulk quantities using pilot-scale equipment, while being possible to tailor their molecular composition, make them a useful material for studying complex aging dynamics in the deep glassy state.

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.

Structural Signatures of Ultrastability in a Deposited Glassformer

Glasses obtained from vapor deposition on a cold substrate have superior thermodynamic and kinetic stability with respect to ordinary glasses. Here we perform molecular dynamics simulations of vapor deposition of a model glassformer and investigate the origin of its high stability compared to that of ordinary glasses. We find that the vapor deposited glass is characterized by locally favored structures (LFSs) whose occurrence correlates with its stability, reaching a maximum at the optimal deposition temperature. The formation of LFSs is enhanced near the free surface, hence supporting the idea that the stability of vapor deposited glasses is connected to the relaxation dynamics at the surface.

Film Thickness Dependent Stability and Glass Transition Temperature of Polymer Films Produced by Physical Vapor Deposition

We report measurements of the onset temperature of rejuvenation, T_{onset}, and the fictive temperature, T_{f}, for ultrathin stable polystyrene with thicknesses from 10 to 50 nm prepared by physical vapor deposition. We also measure the T_{g} of these glasses on the first cooling after rejuvenation as well as the density anomaly of the as-deposited material. Both the T_{g} in rejuvenated films and the T_{onset} in stable films decrease with decreasing film thickness. The T_{f} value increases for decreasing film thickness. The density increase typical of stable glasses also decreases with decreasing film thickness. Collectively, the results are consistent with a decrease in apparent T_{g} due to the existence of a mobile surface layer, as well as a decrease in the film stability as the thickness is decreased. The results provide the first self-consistent set of measurements of stability in ultrathin films of stable glass.

Hiking down the Free Energy Landscape Using Sequential Solvent and Thermal Processing for Versatile Ordering of Block Copolymer Films

The kinetics and morphology of the ordering of block copolymer (BCP) films are highly dependent on the processing pathway, as the enthalpic and entropic forces driving the ordering processes can be quite different depending on process history. We may gain some understanding and control of this variability of BCP morphology with processing history through a consideration of the free energy landscape of the BCP material and a consideration of how the processing procedure moves the system through this energy landscape in a way that avoids having the system becoming trapped into well-defined metastable minima having a higher free energy than the target low free energy ordered structure. It is well known that standard thermal annealing (TA) of BCPs leads to structures corresponding to a well-defined stable free energy minimum; however, the BCP must be annealed for a very long time before the target low free energy structures can be achieved. Herein, we show that the same target low-energy structure can be achieved relatively quickly by subjecting as-cast films to an initial solvent annealing [direct immersion annealing (DIA) or solvent vapor annealing (SVA)] procedure, followed by a short period of TA. This process relies on lowering the activation energy barrier by reducing the glass-transition temperature through DIA (or SVA), followed by a multi-interface chain rearrangement through sequential TA. This energy landscape approach to ordering should be applicable to the process design for ordering many other complex materials.

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.

Creating bulk ultrastable glasses by random particle bonding

A recent breakthrough in glass science has been the synthesis of ultrastable glasses via physical vapor deposition techniques. These samples display enhanced thermodynamic, kinetic and mechanical stability, with important implications for fundamental science and technological applications. However, the vapor deposition technique is limited to atomic, polymer and organic glass-formers and is only able to produce thin film samples. Here, we propose a novel approach to generate ultrastable glassy configurations in the bulk, via random particle bonding, and using computer simulations we show that this method does indeed allow for the production of ultrastable glasses. Our technique is in principle applicable to any molecular or soft matter system, such as colloidal particles with tunable bonding interactions, thus opening the way to the design of a large class of ultrastable glasses.

Designing Glass and Crystalline Phases of Metal-Bis(acetamide) Networks to Promote High Optical Contrast

Owing to their high tunability and predictable structures, metal-organic materials offer a powerful platform to study glass formation and crystallization processes and to design glasses with unique properties. Here, we report a novel series of glass-forming metal-ethylenebis(acetamide) networks that undergo reversible glass and crystallization transitions below 200 °C. The glass-transition temperatures, crystallization kinetics, and glass stability of these materials are readily tunable, either by synthetic modification or by liquid-phase blending, to form binary glasses. Pair distribution function (PDF) analysis reveals extended structural correlations in both single and binary metal-bis(acetamide) glasses and highlights the important role of metal-metal correlations during structural evolution across glass-crystal transitions. Notably, the glass and crystalline phases of a Co-ethylenebis(acetamide) binary network feature a large reflectivity contrast ratio of 4.8 that results from changes in the local coordination environment around Co centers. These results provide new insights into glass-crystal transitions in metal-organic materials and have exciting implications for optical switching, rewritable data storage, and functional glass ceramics.

Influence of Surface Roughness on the Dynamics and Crystallization of Vapor-Deposited Thin Films

The substrate roughness is a very important parameter that can influence the properties of supported thin films. In this work, we investigate the effect of surface roughness on the properties of a vapor-deposited glass (celecoxib, CXB) both in its bulk and in confined states. Using dielectric spectroscopy, we provide experimental evidence depicting a profound influence of surface roughness on the α-relaxation dynamics and the isothermal crystallization of this vapor-deposited glass. Besides, we have verified the influence of film confinement on varying values of surface roughnesses as well. At a fixed surface roughness value, the confinement could alter both the dynamics and crystallization of vapor-deposited CXB.

Thermally Activated Delayed Fluorescence: Polarity, Rigidity, and Disorder in Condensed Phases

We present a detailed and comprehensive picture of the photophysics of thermally activated delayed fluorescence (TADF). The approach relies on a few-state model, parametrized ab initio on a prototypical TADF dye, that explicitly accounts for the nonadiabatic coupling between electrons and vibrational and conformational motion, crucial to properly address (reverse) intersystem crossing rates. The Onsager model is exploited to account for the medium polarity and polarizability, with careful consideration of the different time scales of relevant degrees of freedom. TADF photophysics is then quantitatively addressed in a coherent and exhaustive approach that accurately reproduces the complex temporal evolution of emission spectra in liquid solvents as well as in solid organic matrices. The different rigidity of the two environments is responsible for the appearance in matrices of important inhomogeneous broadening phenomena that are ascribed to the intertwined contribution from (quasi)static conformational and dielectric disorder.

A density scaling conjecture for aging glasses

The aging rate of glasses has traditionally been modeled as a function of temperature, T , andfictive temperature, while density, ρ, is not explicitly included as a parameter. However, this de-scription does not naturally connect to the modern understanding of what governs the relaxationrate in equilibrium. In equilibrium it is well known that the relaxation rate, γeq , depends on tem-perature and density. In addition a large class of systems obey density scaling which means therate specifically depends on the scaling parameter, Γ = e(ρ)/T , where e(ρ) is a system specificfunction. This paper present a generalization of the fictive temperature concept in terms of a fic-tive scaling paramter, Γfic , and a density scaling conjecture for aging glasses in which the agingrate depends on Γ and Γfic .

Vapor-Deposited Thin Films: Studying Crystallization and α-relaxation Dynamics of the Molecular Drug Celecoxib

Crystallization is one of the major challenges in using glassy solids for technological applications. Considering pharmaceutical drugs, maintaining a stable amorphous form is highly desirable for improved solubility. Glasses prepared by the physical vapor deposition technique got attention because they possess very high stability, taking thousands of years for an ordinary glass to achieve. In this work, we have investigated the effect of reducing film thickness on the α-relaxation dynamics and crystallization tendency of vapor-deposited films of celecoxib (CXB), a pharmaceutical substance. We have scrutinized its crystallization behavior above and below the glass-transition temperature (T_g). Even though vapor deposition of CXB cannot inhibit crystallization completely, we found a significant decrease in the crystallization rate with decreasing film thickness. Finally, we have observed striking differences in relaxation dynamics of vapor-deposited thin films above the T_g compared to spin-coated counterparts of the same thickness.

Transient learning degrees of freedom for introducing function in materials

SignificanceMany protocols used in material design and training have a common theme: they introduce new degrees of freedom, often by relaxing away existing constraints, and then evolve these degrees of freedom based on a rule that leads the material to a desired state at which point these new degrees of freedom are frozen out. By creating a unifying framework for these protocols, we can now understand that some protocols work better than others because the choice of new degrees of freedom matters. For instance, introducing particle sizes as degrees of freedom to the minimization of a jammed particle packing can lead to a highly stable state, whereas particle stiffnesses do not have nearly the same impact.

Microscopic analysis of sound attenuation in low-temperature amorphous solids reveals quantitative importance of non-affine effects

Sound attenuation in low-temperature amorphous solids originates from their disordered structure. However, its detailed mechanism is still being debated. Here, we analyze sound attenuation starting directly from the microscopic equations of motion. We derive an exact expression for the zero-temperature sound damping coefficient. We verify that the sound damping coefficients calculated from our expression agree very well with results from independent simulations of sound attenuation. Small wavevector analysis of our expression shows that sound attenuation is primarily determined by the non-affine displacements’ contribution to the sound wave propagation coefficient coming from the frequency shell of the sound wave. Our expression involves only quantities that pertain to solids’ static configurations. It can be used to evaluate the low-temperature sound damping coefficients without directly simulating sound attenuation.

How do interfaces alter the dynamics of supercooled water?

The structure of liquid water in the proximity of an interface can deviate significantly from that of bulk water, with surface-induced structural perturbations typically converging to bulk values at about ∼1 nm from the interface. While these structural changes are well established it is, in contrast, less clear how an interface perturbs the dynamics of water molecules within the liquid. Here, through an extensive set of molecular dynamics simulations of supercooled bulk and interfacial water films and nano-droplets, we observe the formation of persistent, spatially extended dynamical domains in which the average mobility varies as a function of the distance from the interface. This is in stark contrast with the dynamical heterogeneity observed in bulk water, where these domains average out spatially over time. We also find that the dynamical response of water to an interface depends critically on the nature of the interface and on the choice of interface definition. Overall these results reveal a richness in the dynamics of interfacial water that opens up the prospect of tuning the dynamical response of water through specific modifications of the interface structure or confining material.

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 T_g. 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 D_v(T) + [log D₀ - log D_v(T)]exp(-z/ξ), where D(T, z) is the depth-dependent diffusion coefficient, D_v(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.

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.

Characterization of the Interfacial Orientation and Molecular Conformation in a Glass-Forming Organic Semiconductor

The ability to control structure in molecular glasses has enabled them to play a key role in modern technology; in particular, they are ubiquitous in organic light-emitting diodes. While the interplay between bulk structure and optoelectronic properties has been extensively investigated, few studies have examined molecular orientation near buried interfaces despite its critical role in emergent functionality. Direct, quantitative measurements of buried molecular orientation are inherently challenging, and many methods are insensitive to orientation in amorphous soft matter or lack the necessary spatial resolution. To overcome these challenges, we use polarized resonant soft X-ray reflectivity (p-RSoXR) to measure nanometer-resolved, molecular orientation depth profiles of vapor-deposited thin films of an organic semiconductor Tris(4-carbazoyl-9-ylphenyl)amine (TCTA). Our depth profiling approach characterizes the vertical distribution of molecular orientation and reveals that molecules near the inorganic substrate and free surface have a different, nearly isotropic orientation compared to those of the anisotropic bulk. Comparison of p-RSoXR results with near-edge X-ray absorption fine structure spectroscopy and optical spectroscopies reveals that TCTA molecules away from the interfaces are predominantly planar, which may contribute to their attractive charge transport qualities. Buried interfaces are further investigated in a TCTA bilayer (each layer deposited under separate conditions resulting in different orientations) in which we find a narrow interface between orientationally distinct layers extending across ≈1 nm. Coupling this result with molecular dynamics simulations provides additional insight into the formation of interfacial structure. This study characterizes the local molecular orientation at various types of buried interfaces in vapor-deposited glasses and provides a foundation for future studies to develop critical structure-function relationships.

Connecting Packing Efficiency of Binary Hard Sphere Systems to Their Intermediate Range Structure

Using computed x-ray tomography we determine the three dimensional (3D) structure of binary hard sphere mixtures as a function of composition and size ratio of the particles q. Using a recently introduced four-point correlation function we reveal that this 3D structure has on intermediate and large length scales a surprisingly regular order, the symmetry of which depends on q. The related structural correlation length has a minimum at the composition at which the packing fraction is highest. At this composition also the number of different local particle arrangements has a maximum, indicating that efficient packing of particles is associated with a structure that is locally maximally disordered.

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.

Gradient in refractive index reveals denser near free surface region in thin polymer films

A gradient in refractive index that is linear in magnitude with depth into the film is used to fit ellipsometric data for thin polymer films of poly(methyl methacrylate) (PMMA), polystyrene (PS), and poly(2-vinyl pyridine) (P2VP). We find that the linear gradient model fits provide more physically realistic refractive index values for thin films compared with the commonly used homogeneous Cauchy layer model, addressing recent reports of physically unrealistic density increases. Counter to common expectations of a simple free volume correlation between density and dynamics, we find that the direction of refractive index (density) gradient indicates a higher density near the free surface, which we rationalize based on the observed faster free surface dynamics needed to create vapor deposited stable glasses with optimized denser molecular packings. The magnitude of refractive index gradient is observed to be three times larger for PMMA than for PS films, while P2VP films exhibit a more muted response possibly reflective of a decoupling in free surface and substrate dynamics in systems with strong interfacial interactions.

Deposition control of model glasses with surface-mediated orientational order

We introduce a minimal model of solid-forming anisotropic molecules that displays, in thermal equilibrium, surface orientational order without bulk orientational order. The model reproduces the nonequilibrium behavior of recent experiments in which a bulk nonequilibrium structure grown by deposition contains regions of orientational order characteristic of the surface equilibrium. This order is deposited, in general, in a nonuniform way because of the emergence of a growth-poisoning mechanism that causes equilibrated surfaces to grow slower than non-equilibrated surfaces. We use evolutionary methods to design oscillatory protocols able to grow nonequilibrium structures with uniform order, demonstrating the potential of protocol design for the fabrication of this class of materials.

Relaxation moduli of glass-forming systems: temperature effects and fluctuations

Equilibrium and dynamical properties of a two-dimensional polydisperse colloidal model system are characterized by means of molecular dynamics (MD) and Monte Carlo (MC) simulations. We employed several methods to prepare quasi-equilibrated systems: in particular, by slow cooling and tempering with MD (method SC-MD), and by tempering with MC dynamics involving swaps of particle diameters (methods Sw-MD, Sw-MC). It is revealed that the Sw-methods are much more efficient for equilibration below the glass transition temperature T_g leading to denser and more rigid systems which show much slower self-diffusion and shear-stress relaxation than their counterparts prepared with the SC-MD method. The shear-stress relaxation modulus G(t) is obtained based on the classical stress-fluctuation relation. We demonstrate that the α-relaxation time τ_α obtained using a time-temperature superposition of G(t) shows a super-Arrhenius behavior with the VFT temperature T₀ well below T_g. We also derive novel rigorous fluctuation relations providing isothermic and adiabatic compression relaxation moduli in the whole time range (including the short-time inertial regime) based on correlation data for thermostatted systems. It is also shown that: (i) the assumption of Gaussian statistics for stress fluctuations leads to accurate predictions of the variances of the fluctuation moduli for both shear (μ_F) and compression (η_F) at T⪆T_g. (ii) The long-time (quasi-static) isothermic and adiabatic moduli increase on cooling faster than the affine compression modulus η_A, and this leads to a monotonic temperature dependence of η_F which is qualitatively different from μ_F(T) showing a maximum near T_g.

Enhanced medium-range order in vapor-deposited germania glasses at elevated temperatures

Glasses are nonequilibrium solids with properties highly dependent on their method of preparation. In vapor-deposited molecular glasses, structural organization could be readily tuned with deposition rate and substrate temperature. Here, we show that the atomic arrangement of strong network-forming GeO₂ glass is modified at medium range (<2 nm) through vapor deposition at elevated temperatures. Raman spectral signatures distinctively show that the population of six-membered GeO₄ rings increases at elevated substrate temperatures. Deposition near the glass transition temperature is more efficient than postgrowth annealing in modifying atomic structure at medium range. The enhanced medium-range organization correlates with reduction of the room temperature internal friction. Identifying the microscopic origin of room temperature internal friction in amorphous oxides is paramount to design the next-generation interference coatings for mirrors of the end test masses of gravitational wave interferometers, in which the room temperature internal friction is a main source of noise limiting their sensitivity.

Identification of the Key Parameters for Horizontal Transition Dipole Orientation in Fluorescent and TADF Organic Light-Emitting Diodes

In organic light-emitting diodes (OLEDs), horizontal orientation of the emissive transition dipole moment (TDM) can improve light outcoupling efficiency by up to 50% relative to random orientation. Therefore, there have been extensive efforts to identify drivers of horizontal orientation. The aspect ratio of the emitter molecule and the glass-transition temperature (T_g ) of the films are currently regarded as particularly important. However, there remains a paucity of systematic studies that establish the extent to which these and other parameters control orientation in the wide range of emitter systems relevant for state-of-the-art OLEDs. Here, recent work on molecular orientation of fluorescent and thermally activated delayed fluorescent emitters in vacuum-processed OLEDs is reviewed. Additionally, to identify parameters linked to TDM orientation, a meta-analysis of 203 published emitter systems is conducted and combined with density-functional theory calculations. Molecular weight (MW) and linearity are identified as key parameters in neat systems. In host-guest systems with low-MW emitters, orientation is mostly influenced by the host T_g , whereas the length and MW of the emitter become more relevant for systems involving higher-MW emitters. To close, a perspective of where the field must advance to establish a comprehensive model of molecular orientation is given.

Nature of dynamic gradients, glass formation, and collective effects in ultrathin freestanding films

Molecular, polymeric, colloidal, and other classes of liquids can exhibit very large, spatially heterogeneous alterations of their dynamics and glass transition temperature when confined to nanoscale domains. Considerable progress has been made in understanding the related problem of near-interface relaxation and diffusion in thick films. However, the origin of "nanoconfinement effects" on the glassy dynamics of thin films, where gradients from different interfaces interact and genuine collective finite size effects may emerge, remains a longstanding open question. Here, we combine molecular dynamics simulations, probing 5 decades of relaxation, and the Elastically Cooperative Nonlinear Langevin Equation (ECNLE) theory, addressing 14 decades in timescale, to establish a microscopic and mechanistic understanding of the key features of altered dynamics in freestanding films spanning the full range from ultrathin to thick films. Simulations and theory are in qualitative and near-quantitative agreement without use of any adjustable parameters. For films of intermediate thickness, the dynamical behavior is well predicted to leading order using a simple linear superposition of thick-film exponential barrier gradients, including a remarkable suppression and flattening of various dynamical gradients in thin films. However, in sufficiently thin films the superposition approximation breaks down due to the emergence of genuine finite size confinement effects. ECNLE theory extended to treat thin films captures the phenomenology found in simulation, without invocation of any critical-like phenomena, on the basis of interface-nucleated gradients of local caging constraints, combined with interfacial and finite size-induced alterations of the collective elastic component of the structural relaxation process.

Mobility gradients yield rubbery surfaces on top of polymer glasses

Many emerging materials, such as ultrastable glasses^1,2 of interest for phone displays and OLED television screens, owe their properties to a gradient of enhanced mobility at the surface of glass-forming liquids. The discovery of this surface mobility enhancement^3-5 has reshaped our understanding of the behaviour of glass formers and of how to fashion them into improved materials. In polymeric glasses, these interfacial modifications are complicated by the existence of a second length scale-the size of the polymer chain-as well as the length scale of the interfacial mobility gradient^6-9. Here we present simulations, theory and time-resolved surface nano-creep experiments to reveal that this two-scale nature of glassy polymer surfaces drives the emergence of a transient rubbery, entangled-like surface behaviour even in polymers comprised of short, subentangled chains. We find that this effect emerges from superposed gradients in segmental dynamics and chain conformational statistics. The lifetime of this rubbery behaviour, which will have broad implications in constraining surface relaxations central to applications including tribology, adhesion, and surface healing of polymeric glasses, extends as the material is cooled. The surface layers suffer a general breakdown in time-temperature superposition (TTS), a fundamental tenet of polymer physics and rheology. This finding may require a reevaluation of strategies for the prediction of long-time properties in polymeric glasses with high interfacial areas. We expect that this interfacial transient elastomer effect and TTS breakdown should normally occur in macromolecular systems ranging from nanocomposites to thin films, where interfaces dominate material properties^5,10.

Metallic Nanoglasses with Promoted β-Relaxation and Tensile Plasticity

The secondary (β) relaxation is an intrinsic feature of glassy systems and is crucial for the mechanical properties of metallic glasses. However, it remains puzzling what structural features control the β-relaxation fundamentally. Here, we use the recently developed nanoglasses exhibiting well-defined structural features at the nanometer scale to interrogate such structure-dynamics relations. We show that an electrodeposited Ni_77.5P_22.5 nanoglass exhibits promoted β-relaxation and enhanced microscale tensile plasticity over the most rapidly melt-quenched metallic glass with the same composition. Structurally, the β-relaxation is sensitive to the interfacial regions among grains in the nanoglasses. Our results reveal a clear correlation between the amorphous nanostructures and the β-relaxation. It seems that the nanostructuring represents a novel route to obtain high-energy glassy states, that is, the inverse problem of the ultrastable glass.

Polymers under nanoconfinement: where are we now in understanding local property changes?

Polymers are increasingly being used in applications with nanostructured morphologies where almost all polymer molecules are within a few tens to hundreds of nanometers from some interface. From nearly three decades of study on polymers in simplified nanoconfined systems such as thin films, we have come to understand property changes in these systems as arising from interfacial effects where local dynamical perturbations are propagated deeper into the material. This review provides a summary of local glass transition temperature T_g changes near interfaces, comparing across different types of interfaces: free surface, substrate, liquid, and polymer-polymer. Local versus film-average properties in thin films are discussed, making connections to other related property changes, while highlighting several historically important studies. By experimental necessity, most studies are on high enough molecule weight chains to be well entangled, although aspects that connect to lower molecule weight materials are described. Emphasis is made to identify observations and open questions that have yet to be fully understood such as the evidence of long-ranged interfacial effects, finite domain size, interfacial breadth, and chain connectivity.

Internal molecular conformation of organic glasses: A NEXAFS study

The origin of the exceptional stability of molecular glasses grown by physical vapor deposition (PVD) is not well understood. Differences in glass density have been correlated with thermodynamic stability for thin films of N,N'-Bis(3-methylphenyl)-N,N'-diphenylbenzidine (TPD) grown by PVD at specific substrate temperatures below the glass transition temperature. However, the relationship between the internal conformation of glass molecules and the thermodynamic properties of molecular glasses is not well studied. We use carbon 1s near edge x-ray absorption fine structure (NEXAFS) spectroscopy to examine different TPD sample preparations in which differences in the thermodynamic stability of the glass are known. Density functional theory simulations of the NEXAFS spectra of TPD allow us to attribute spectroscopic differences to changes in the internal conformation of the TPD molecule and relate this conformation to the stability of the TPD glass. This provides a direct experimental measurement of the internal conformation of molecules forming an organic glass.

Vitrification of octonary perylene mixtures with ultralow fragility

Strong glass formers with a low fragility are highly sought-after because of the technological importance of vitrification. In the case of organic molecules and polymers, the lowest fragility values have been reported for single-component materials. Here, we establish that mixing of organic molecules can result in a marked reduction in fragility. Individual bay-substituted perylene derivatives display a high fragility of more than 70. Instead, slowly cooled perylene mixtures with more than three components undergo a liquid-liquid transition and turn into a strong glass former. Octonary perylene mixtures display a fragility of 13 ± 2, which not only is a record low value for organic molecules but also lies below values reported for the strongest known inorganic glass formers. Our work opens an avenue for the design of ultrastrong organic glass formers, which can be anticipated to find use in pharmaceutical science and organic electronics.

Ultrastable Glassy Polymer Films with an Ultradense Brush Morphology

Glassy polymer films with extreme stability could enable major advancements in a range of fields that require the use of polymers in confined environments. Yet, from a materials design perspective, we now know that the glass transition temperature (T_g) and thermal expansion of polymer thin films can be dramatically different from those characteristics of the bulk, i.e., exhibiting confinement-induced diminished thermal stability. Here, we demonstrate that polymer brushes with an ultrahigh grafting density, i.e., an ultradense brush morphology, exhibit a significant enhancement in thermal stability, as manifested by an exceptionally high T_g and low expansivity. For instance, a 5 nm thick polystyrene brush film exhibits an ∼75 K increase in T_g and ∼90% reduction in expansivity compared to a spin-cast film of similar thickness. Our results establish how morphology can overcome confinement and interfacial effects in controlling thin-film material properties and how this can be achieved by the dense packing and molecular ordering in the amorphous state of ultradense brushes prepared by surface-initiated atom transfer radical polymerization in combination with a self-assembled monolayer of initiators.

Factors correlating to enhanced surface diffusion in metallic glasses

The enhancement of surface diffusion (D_S) over the bulk (D_V) in metallic glasses (MGs) is well documented and likely to strongly influence the properties of glasses grown by vapor deposition. Here, we use classical molecular dynamics (MD) simulations to identify different factors influencing the enhancement of surface diffusion in MGs. MGs have a simple atomic structure and belong to the category of moderately fragile glasses that undergo pronounced slowdown of bulk dynamics with cooling close to the glass transition temperature (T_g). We observe that D_S exhibits a much more moderate slowdown compared to D_V when approaching T_g, and D_S/D_V at T_g varies by two orders of magnitude among the MGs investigated. We demonstrate that both the surface energy and the fraction of missing bonds for surface atoms show good correlation to D_S/D_V, implying that the loss of nearest neighbors at the surface directly translates into higher mobility, unlike the behavior of network-bonded and hydrogen-bonded organic glasses. Fragility, a measure of the slowdown of bulk dynamics close to T_g, also correlates to D_S/D_V, with more fragile systems having larger surface enhancement of mobility. The deviations observed in the fragility-D_S/D_V relationship are shown to be correlated to the extent of segregation or depletion of the mobile element at the surface. Finally, we explore the relationship between the diffusion pre-exponential factor (D₀) and the activation energy (Q) and compare it to a ln(D₀)-Q correlation previously established for bulk glasses, demonstrating similar correlations from MD as in the experiments and that the surface and bulk have very similar ln(D₀)-Q correlations.

Sign flipping of spontaneous polarization in vapour-deposited films of small polar organic molecules

Films of polar molecules vapour-deposited on sufficiently cold substrates are not only amorphous, but also exhibit charge polarization across their thickness. This is an effect known for 50 years, but it is very poorly understood and no mechanism exists in the literature that can explain and predict it. We investigated this bulk effect for 18 small organic molecules as a function of substrate temperature (30-130 K). We found that, as a rule, alcohol films have the negative end on the vacuum side at all temperatures. Alkyl acetates and toluene showed positive voltages which reached a maximum around the middle of the temperature range investigated. Tetrahydrofuran showed positive voltages which dropped with increasing deposition temperature. Diethyl ether, acetone, propanal, and butanal showed positive film voltages at low temperatures, negative at intermediate temperatures and again positive voltages at higher temperatures. In all cases, film voltages were monitored during heating leading to film evaporation. Film voltages were irreversibly eliminated before film elimination, but voltage profiles during temperature ramps differed vastly depending on compound and deposition temperature. In general, there was a gradual voltage reduction, but propanal, butanal, and diethyl ether showed a change in voltage sign during temperature ramp in films deposited at low temperatures. All these data expand substantially the experimental information regarding spontaneous polarization in vapour-deposited films, but still require complementary measurements as well as numerical simulations for a detailed explanation of the phenomenon.

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 T_dep = 75 K = 0.82T_g, 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.

Anisotropy and anharmonicity in polystyrene stable glass

We have used ellipsometry to characterize the anisotropy in stable polymer glasses prepared by physical vapor deposition. These measurements reveal birefringence values (as measured by the magnitude of in-plane vs out-of-plane refractive index) less than 0.002 in vapor-deposited polystyrenes with N from 6 to 12 and with fictive temperatures between 10 K and 35 K below the T_g values. We have measured the thermal expansivity of these stable glasses and compared to ordinary rejuvenated glass. The thermal expansivity of the stable glasses is less than that of ordinary glass with a difference that increases as the fictive temperature T_f decreases.

Ultrastable monodisperse polymer glass formed by physical vapour deposition

Stable glasses prepared by vapour deposition are an analogue of glassy materials aged for geological timescales. The ability to prepare such materials allows the study of near-ideal glassy systems. We report the preparation and characterization of stable glasses of polymers prepared by physical vapour deposition. By controlling the substrate temperature, deposition rate and polydispersity, we prepared and characterized a variety of stable polymer glasses. These materials display the kinetic stability, low fictive temperatures and high-density characteristic of stable glasses. Extrapolation of the measured transformation times between the stable and normal glass provides estimates of the relaxation times of the equilibrium supercooled liquid at temperatures as much as 30 K below the glass transition temperature. These results demonstrate that polymer stable glasses are an exciting and powerful tool in the study of ultrastable glass and disordered materials in general.