In-situ ellipsometry characterization of vapor-deposited stable molecular glasses*

Recent studies [1,2] in physical vapor-deposited (PVD) molecular glasses and liquid-quenched polymers indicate that thin films have higher density than bulk, can access unique phases at low deposition temperatures, and have gradients in their density, as measured through their refractive index. These effects motivate in-situ studies of the evolution of density and stability as a function of film thickness, which were performed using spectroscopic ellipsometry during vapor deposition of N,N’-bis(3-methylphenyl)-N,N’-diphenylbenzidine (TPD) glasses at various deposition rates and substrate temperatures. The in-plane and out-of-plane refractive indices show non-monotonic variations with thickness, starting from high initial values, indicative of high thin film density compared to thick films. The birefringence goes through a maximum at intermediate thicknesses, showing non-homogenous packing along the normal direction. Steady-state properties are achieved at a thickness ≥300 nm, independent of the deposition rate (0.025~1.5 nm/s). These results elucidate the long-range effect of the thin film geometry on the packing and properties of PVD molecular glasses. 

[1] Jin et al., PNAS 118, e2100738118 (2021).

[2] Han et al., JCP 155, 144901 (2021).