In situ observation of fast surface dynamics during the vapor-deposition of a stable organic glass.
basic_science · Level V
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- Record sourced from PubMed, PMID 33242316.
- Also identified by DOI 10.1039/d0sm01916j.
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Abstract
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<sup>-1</sup> onto a substrate at T<sub>dep</sub> = 75 K = 0.82T<sub>g</sub>, 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.