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

   https://doi.org/10.1073/pnas.2009852117  

   Zhang, Aixi ; Jin, Yi ; Liu, Tianyi ; Stephens, Richard B. ; Fakhraai, Zahra ( September 2020 , Proceedings of the National Academy of Sciences)

   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.
2. The Effect of Thin Interfacial Layer on the Mechanical Properties of Metal/Zerodur Heterogeneous Bonding

   https://doi.org/10.1149/MA2021-01512000mtgabs  

   Klokkevold, Katherine ; Keeven, Weston ; Clevenger, Michael ; Liu, Mingyuan ; Song, Han Wook ; Lee, Sunghwan ( May 2021 , ECS Meeting Abstracts)

   Heterogeneous bonding between metals and ceramics is of significant relevance to a wide range of applications in the fields of industry, defense, and aerospace. Metal/ceramic bonding can be used in various specific part applications such as vacuum tubes, automotive use of ceramic rotors, and rocket igniter bodies. However, the bonding of ceramic to metal has been challenging mainly due to (1) the low wettability of ceramics, on which the adhesion of molten adhesive bonders is limited and (2) the large difference between the coefficients of thermal expansion (CTE) of the two dissimilar bonded materials, which develops significant mechanical stresses at the interface and potentially leads to mechanical failures. Vapor-phase deposition is a widely used thin film processing technique in both academic research laboratories and manufacturing industries. Since vapor phase coatings do not require wettability or hydrophobicity, a uniform and strongly adherent layer is deposited over virtually any substrate, including ceramics. In this presentation, we report on the effect of vapor phase-deposited interfacial metal layers on the mechanical properties of bonding between stainless steel and Zerodur (lithium aluminosilicate-based glass ceramic). Direct-current magnetron sputtering was utilized to deposit various thin interfacial layers containing Ti, Cu, or Sn. In addition, to minimize themore »unfavorable stress at the bonded interface due to the large CTE difference, a low temperature allow solder, that can be chemically and mechanically activated at temperatures of approximately 200 °C, was used. The solder is made from a composite of Ti-Sn-Ce-In. A custom-built fixture and universal testing machine were used to evaluate the bonding strength in shear, which was monitored in-situ with LabView throughout the measurement. The shear strength of the bonding between stainless steel and Zerodur was systematically characterized as a function of interfacial metal and metal processing temperature during sputter depositions. Maximum shear strength of the bonding of 4.36 MPa was obtained with Cu interfacial layers, compared to 3.53 MPa from Sn and 3.42 MPa from Ti adhesion promoting layers. These bonding strengths are significantly higher than those (~0.05 MPa) of contacts without interfacial reactive thin metals. The fracture surface microstructures are presented as well. It was found that the point of failure, when Cu interfacial layers were used, was between the coated Cu film and alloy bonder. This varied from the Sn and Ti interfacial layers where the main point of failure was between the interfacial film and Zerodur interface. The findings of the effect of thin adhesion promoting metal layers and failure behaviors may be of importance to some metal/ceramic heterogeneous bonding studies that require high bonding strength and low residual stresses at the bonding interface. The authors gratefully acknowledge the financial support of the Improvement of Measurement Standards and Technology for Mechanical Metrology (Grant No. 20011028) by KRISS.« less
3. Generic packing motifs in vapor-deposited glasses of organic semiconductors

   https://doi.org/10.1039/c9sm01155b  

   Bagchi, Kushal ; Gujral, Ankit ; Toney, M. F. ; Ediger, M. D. ( October 2019 , Soft Matter)

   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 ∼0.8 T g (where T g 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.
4. In situ observation of fast surface dynamics during the vapor-deposition of a stable organic glass

   https://doi.org/10.1039/d0sm01916j  

   Thoms, E. ; Gabriel, J. P. ; Guiseppi-Elie, A. ; Ediger, M. D. ; Richert, R. ( December 2020 , Soft Matter)

   By measuring the increments of dielectric capacitance (Δ C ) 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 = 0.82 T 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.
5. Glasses denser than the supercooled liquid

   https://doi.org/10.1073/pnas.2100738118  

   Jin, Yi ; Zhang, Aixi ; Wolf, Sarah E. ; Govind, Shivajee ; Moore, Alex R. ; Zhernenkov, Mikhail ; Freychet, Guillaume ; Arabi Shamsabadi, Ahmad ; Fakhraai, Zahra ( July 2021 , Proceedings of the National Academy of Sciences)

   When aged below the glass transition temperature,${\mathit{T}}_{\mathit{g}}$, 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 ofN,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 (${\mathit{T}}_{\mathit{L}\mathit{L}}$) ∼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-depositedmore »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.

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