Anatomy of a vapor deposited glass*

Glasses have been prepared by vapor deposition using methyl-m-toluate (MMT), a polar glass forming molecular material with a glass transition temperature T_g = 170 K. Films of about 200 nm thickness have been deposited at rates of 0.1 nm/s onto a borosilicate substrate with deposition temperatures between 40 and 170 K. MMT is known to form kinetically highly stable glasses at or near 0.85T_g = 145 K, and it is simple in the sense that vapor deposited glasses recover the properties of the melt cooled liquid if warmed above T_g.

During and after deposition, the dielectric permittivity of MMT films has been measured with a resolution of tanδ = 10^-7 via an interdigitated electrode structure with a geometric capacitance of 2.2 pF. In all cases, the capacitance of the cell rose linearly with time, indicative of a constant deposition rate. During deposition, the dielectric loss of the film rises quickly at first, and then near linearly with time. The analysis of these deposition curves reveals a slightly different structure near the substrate, a bulk of the film that does not change the loss behavior for 50,000 s after deposition, and a surface layer of more mobile material. The thickness of this layer is at or below 1 nm for deposition at low temperature and sharply rises to above 10 nm average thickness at T_g. The dielectric loss, ε'', understood as indicator of the fictive temperature T_fic, is observed to change gradually from the mobile surface with T_fic > T_g to the much lower value of the stable glass, where T_fic is assumed to be close to the deposition temperature. After the deposition had stopped, this mobile surface layer undergoes considerable reduction of its dielectric loss, and this annealing effect is believed to arise from material positioned 5 to 30 nm below the surface. The picture of a fictive temperature gradient near the surface as derived from these observations differs from that of a distinct mobile layer that quickly and directly converts to the kinetically stable glass.