Glasses denser than the supercooled liquid.

When aged below the glass transition temperature, [Formula: see text], the density of a glass cannot exceed that of the metastable supercooled liquid (SCL) state, unless crystals are nucleated. The only exception is when another polyamorphic SCL state exists, with a density higher than that of the ordinary SCL. Experimentally, such polyamorphic states and their corresponding liquid-liquid phase transitions have only been observed in network-forming systems or those with polymorphic crystalline states. In otherwise simple liquids, such phase transitions have not been observed, either in aged or vapor-deposited stable glasses, even near the Kauzmann temperature. Here, we report that the density of thin vapor-deposited films of N,N'-bis(3-methylphenyl)-N,N'-diphenylbenzidine (TPD) can exceed their corresponding SCL density by as much as 3.5% and can even exceed the crystal density under certain deposition conditions. We identify a previously unidentified high-density supercooled liquid (HD-SCL) phase with a liquid-liquid phase transition temperature ([Formula: see text]) ∼35 K below the nominal glass transition temperature of the ordinary SCL. The HD-SCL state is observed in glasses deposited in the thickness range of 25 to 55 nm, where thin films of the ordinary SCL have exceptionally enhanced surface mobility with large mobility gradients. The enhanced mobility enables vapor-deposited thin films to overcome kinetic barriers for relaxation and access the HD-SCL state. The HD-SCL state is only thermodynamically favored in thin films and transforms rapidly to the ordinary SCL when the vapor deposition is continued to form films with thicknesses more than 60 nm.

Polyamorphism of vapor-deposited amorphous selenium in response to light.

Enhanced surface mobility is critical in producing stable glasses during physical vapor deposition. In amorphous selenium (a-Se) both the structure and dynamics of the surface can be altered when exposed to above-bandgap light. Here we investigate the effect of light on the properties of vapor-deposited a-Se glasses at a range of substrate temperatures and deposition rates. We demonstrate that deposition both under white light illumination and in the dark results in thermally and kinetically stable glasses. Compared to glasses deposited in the dark, stable a-Se glasses formed under white light have reduced thermal stability, as measured by lower density change, but show significantly improved kinetic stability, measured as higher onset temperature for transformation. While light induces enhanced mobility that penetrates deep into the surface, resulting in lower density during vapor deposition, it also acts to form more networked structures at the surface, which results in a state that is kinetically more stable with larger optical birefringence. We demonstrate that the structure formed during deposition with light is a state that is not accessible through liquid quenching, aging, or vapor deposition in the dark, indicating the formation of a unique amorphous solid state.

The role of intramolecular relaxations on the structure and stability of vapor-deposited glasses.

Stable glasses (SGs) are formed through surface-mediated equilibration (SME) during physical vapor deposition (PVD). Unlike intermolecular interactions, the role of intramolecular degrees of freedom in this process remains unexplored. Here, using experiments and coarse-grained molecular dynamics simulations, we demonstrate that varying dihedral rotation barriers of even a single bond, in otherwise isomeric molecules, can strongly influence the structure and stability of PVD glasses. These effects arise from variations in the degree of surface mobility, mobility gradients, and mobility anisotropy, at a given deposition temperature (Tdep). At high Tdep, flexible molecules have access to more configurations, which enhances the rate of SME, forming isotropic SGs. At low Tdep, stability is achieved by out of equilibrium aging of the surface layer. Here, the poor packing of rigid molecules enhances the rate of surface-mediated aging, producing stable glasses with layered structures in a broad range of Tdep. In contrast, the dynamics of flexible molecules couple more efficiently to the glass layers underneath, resulting in reduced mobility and weaker mobility gradients, producing unstable glasses. Independent of stability, the flattened shape of flexible molecules can also promote in-plane orientational order at low Tdep. These results indicate that small changes in intramolecular relaxation barriers can be used as an approach to independently tune the structure and mobility profiles of the surface layer and, thus, the stability and structure of PVD glasses.

Effect of Nanopore Geometry in the Conformation and Vibrational Dynamics of a Highly Confined Molecular Glass.

The effect of nanoporous confinement on the glass transition temperature (Tg) strongly depends on the type of porous media. Here, we study the molecular origins of this effect in a molecular glass, N,N'-bis(3-methylphenyl)-N,N'-diphenylbenzidine (TPD), highly confined in concave and convex geometries. When confined in controlled pore glass (CPG) with convex pores, TPD's vibrational spectra remained unchanged and two Tg's were observed, consistent with previous studies. In contrast, when confined in silica nanoparticle packings with concave pores, the vibrational peaks were shifted due to more planar conformations and Tg increased, as the pore size was decreased. The strong Tg increases in concave pores indicate significantly slower relaxation dynamics compared to CPG. Given TPD's weak interaction with silica, these effects are entropic in nature and are due to conformational changes at molecular level. The results highlight the role of intramolecular degrees of freedom in the glass transition, which have not been extensively explored.

Design of a homologous series of molecular glassformers.

We design and synthesize a set of homologous organic molecules by taking advantage of facile and tailorable Suzuki cross coupling reactions to produce triarylbenzene derivatives. By adjusting the number and the arrangement of conjugated rings, the identity of heteroatoms, lengths of fluorinated alkyl chains, and other interaction parameters, we create a library of glassformers with a wide range of properties. Measurements of the glass transition temperature (Tg) show a power-law relationship between Tg and molecular weight (MW), with of the molecules, with an exponent of 0.3 ± 0.1, for Tg values spanning a range of 300-450 K. The trends in indices of refraction and expansion coefficients indicate a general increase in the glass density with MW, consistent with the trends observed in Tg variations. A notable exception to these trends was observed with the addition of alkyl and fluorinated alkyl groups, which significantly reduced Tg and increased the dynamical fragility (which is otherwise insensitive to MW). This is an indication of reduced density and increased packing frustrations in these systems, which is also corroborated by the observations of the decreasing index of refraction with increasing length of these groups. These data were used to launch a new database for glassforming materials, glass.apps.sas.upenn.edu.

Synthesis, Electronic Structure, and Reactivity Studies of a 4-Coordinate Square Planar Germanium(IV) Cation.

A tetra-coordinate, square planar germanium(IV) cation [(TPFC)Ge](+) (TPFC = tris(pentafluorophenyl)corrole) was synthesized quantitatively by the reaction of (TPFC)Ge-H with [Ph3C](+)[B(C6F5)4](¯). The highly reactive [(TPFC)Ge](+) cation reacted with benzene to form phenyl complex (TPFC)Ge-C6H5 through an electrophilic pathway. The key intermediate, a σ-type germylium-benzene adduct, [(TPFC)Ge(η(1)-C6H6)](+), was isolated and characterized by single-crystal X-ray diffraction. Deprotonation of [(TPFC)Ge(η(1)-C6H6)](+) cation led to the formation of (TPFC)Ge-C6H5. [(TPFC)Ge](+) also reacted with ethylene and cyclopropane in benzene at room temperature to form (TPFC)Ge-CH2CH2C6H5 and (TPFC)Ge-CH2CH2CH2C6H5, respectively. The observed electrophilic reactivity is ascribed to the highly exposed cationic germanium center with novel frontier orbitals comprising two vacant sp-hybridized orbitals that are not conjugated to π-system. The three electron-withdrawing pentafluorophenyl groups on the corrole ligand also enhance the electrophilicity of the cationic germanium corrole.

PPDM++: Parallel Point Detection and Matching for Fast and Accurate HOI Detection.

Human-Object Interaction (HOI) detection aims to understand human activities by detecting interaction triplets. Previous HOI detection methods adopt a two-stage instance-driven paradigm. Unfortunately, many non-interactive human-object pairs generated by the first stage are the main obstacle impeding HOI detectors from high efficiency and promising performance. To remedy this, we propose a novel top-down interaction-driven paradigm, detecting interactions first and bridging interactive human-object pairs through interactions. We formulate HOI as a point triplet human point, interaction point, object point and design a Parallel Point Detection and Matching (PPDM) framework. We further take advantage of two-stage methods and propose a novel framework, PPDM++, that detects the interactive human-object pairs by PPDM, then extracts region features for each pair to predict actions. The core of PPDM/PPDM++ is to convert the instance-driven bottom-up paradigm to an interaction-driven top-down paradigm, thus avoiding additional computation costs from traversing a tremendous number of non-interactive pairs. Benefiting from the advanced paradigm, PPDM/PPDM++ has achieved significant performance gains with high efficiency. PPDM-DLA-34 has achieved 19.94 mAP with 42 FPS as the first real-time HOI detector, and PPDM++-SwinB achieves 30.1 mAP with 17 FPS on HICO-DET dataset. We also built an application-oriented database named HOI-A, a supplement to the existing datasets.

The Evolution of MXenes Conductivity and Optical Properties Upon Heating in Air.

MXenes, a family of 2D transition-metal carbides and nitrides, have excellent electrical conductivity and unique optical properties. However, MXenes oxidize in ambient conditions, which is accelerated upon heating. Intercalation of water also causes hydrolysis accelerating oxidation. Developing new tools to readily characterize MXenes' thermal stability can enable deeper insights into their structure-property relationships. Here, in situ spectroscopic ellipsometry (SE) is employed to characterize the optical properties of three types of MXenes (Ti3 C2 Tx , Mo2 TiC2 Tx , and Ti2 CTx ) with varied composition and atomistic structures to investigate their thermal degradation upon heating under ambient environment. It is demonstrated that changes in MXene extinction and optical conductivity in the visible and near-IR regions correlate well with the amount of intercalated water and hydroxyl termination groups and the degree of oxidation, measured using thermogravimetric analysis. Among the three MXenes, Ti3 C2 Tx and Ti2 CTx , respectively, have the highest and lowest thermal stability, indicating the role of transition-metal type, synthesis route, and the number of atomic layers in MXene flakes. These findings demonstrate the utility of SE as a powerful in situ technique for rapid structure-property relationship studies paving the way for the further design, fabrication, and property optimization of novel MXene materials.

Progressive Language-Customized Visual Feature Learning for One-Stage Visual Grounding.

Visual grounding is a task to localize an object described by a sentence in an image. Conventional visual grounding methods extract visual and linguistic features isolatedly and then perform cross-modal interaction in a post-fusion manner. We argue that this post-fusion mechanism does not fully utilize the information in two modalities. Instead, it is more desired to perform cross-modal interaction during the extraction process of the visual and linguistic feature. In this paper, we propose a language-customized visual feature learning mechanism where linguistic information guides the extraction of visual feature from the very beginning. We instantiate the mechanism as a one-stage framework named Progressive Language-customized Visual feature learning (PLV). Our proposed PLV consists of a Progressive Language-customized Visual Encoder (PLVE) and a grounding module. We customize the visual feature with linguistic guidance at each stage of the PLVE by Channel-wise Language-guided Interaction Modules (CLIM). Our proposed PLV outperforms conventional state-of-the-art methods with large margins across five visual grounding datasets without pre-training on object detection datasets, while achieving real-time speed. The source code is available in the supplementary material.

Role of Molecular Layering in the Enhanced Mechanical Properties of Stable Glasses.

The density, degree of molecular orientation, and molecular layering of vapor-deposited stable glasses (SGs) vary with substrate temperature (Tdep) below the glass-transition temperature (Tg). Density and orientation have been suggested to be factors influencing the mechanical properties of SGs. We perform nanoindentation on two molecules which differ by only a single substituent, allowing one molecule to adopt an in-plane orientation at low Tdep. The reduced elastic modulus and hardness of both molecules show similar Tdep dependences, with enhancements of 15-20% in reduced modulus and 30-45% in hardness at Tdep ≈ 0.8Tg, where the density of vapor-deposited films is enhanced by ∼1.4% compared to that of the liquid-quenched glass. At Tdep < 0.8Tg, one of the molecules produces highly unstable glasses with in-plane orientation. However, both systems show enhanced mechanics. Both the modulus and hardness correlate with the degree of layering, which is similar in both systems despite their variable stability. We suggest that nanoindentation performed normal to the film's surface is influenced by the tighter packing of the molecules in this direction.

Exploring the Importance of Surface Diffusion in Stability of Vapor-Deposited Organic Glasses.

Stable glasses are formed during physical vapor deposition (PVD), through the surface-mediated equilibration process. Understanding surface relaxation dynamics is important in understanding the details of this process. Direct measurements of the surface relaxation times in molecular glass systems are challenging. As such, surface diffusion measurements have been used in the past as a proxy for the surface relaxation process. In this study, we show that the absence of enhanced surface diffusion is not a reliable predictor of reduced ability to produce stable glasses. To demonstrate, we have prepared stable glasses (SGs) from two structurally similar organic molecules, 1,3-bis(1-naphthyl)-5-(2-naphthyl)benzene (TNB) and 9-(3,5-di(naphthalen-1-yl)phenyl)anthracene (α,α-A), with similar density increase and improved kinetic stability as compared to their liquid-quenched (LQ) counterparts. The surface diffusion values of these glasses were measured both in the LQ and SG states below their glass transition temperatures ( Tgs) using gold nanorod probes. While TNB shows enhanced surface diffusion in both SG and LQ states, no significant surface Tg diffusion is observed on the surface of α,α-A within our experimental time scales. However, isothermal dewetting experiments on ultrathin films of both molecules below Tg indicate the existence of enhanced dynamics in ultrathin films for both molecules, indirectly showing the existence of an enhanced mobile surface layer. Both films produce stable glasses, which is another indication for the existence of the mobile surface layer. Our results suggest that lateral surface diffusion may not be a good proxy for enhanced surface relaxation dynamics required to produce stable glasses, and thus, other types of measurements to directly probe the surface relaxation times may be necessary.