Sign flipping of spontaneous polarization in vapour-deposited films of small polar organic molecules

Films of polar molecules vapour-deposited on sufficiently cold substrates are not only amorphous, but also exhibit charge polarization across their thickness. This is an effect known for 50 years, but it is very poorly understood and no mechanism exists in the literature that can explain and predict it. We investigated this bulk effect for 18 small organic molecules as a function of substrate temperature (30-130 K). We found that, as a rule, alcohol films have the negative end on the vacuum side at all temperatures. Alkyl acetates and toluene showed positive voltages which reached a maximum around the middle of the temperature range investigated. Tetrahydrofuran showed positive voltages which dropped with increasing deposition temperature. Diethyl ether, acetone, propanal, and butanal showed positive film voltages at low temperatures, negative at intermediate temperatures and again positive voltages at higher temperatures. In all cases, film voltages were monitored during heating leading to film evaporation. Film voltages were irreversibly eliminated before film elimination, but voltage profiles during temperature ramps differed vastly depending on compound and deposition temperature. In general, there was a gradual voltage reduction, but propanal, butanal, and diethyl ether showed a change in voltage sign during temperature ramp in films deposited at low temperatures. All these data expand substantially the experimental information regarding spontaneous polarization in vapour-deposited films, but still require complementary measurements as well as numerical simulations for a detailed explanation of the phenomenon.