Characterization of evaporated thin films of merocyanine dyes

Vacuum evaporation of a merocyanine dye, 3‐ethyl‐5 [2‐(3‐ethyl‐2‐benzothiazolidene) ethyldene]‐2‐thioxo‐4‐thiazolidione, onto different substrates such as bare and surface‐oxidized silicon wafers and a quartz glass substrate is done. There is a difference in color between a film deposited on the bare silicon substrate and one on either a quartz glass plate or a silicon wafer with a thick (96.5‐nm) oxide layer. The dye film on quartz has an absorption maximum at 560 nm, whereas that on bare silicon has one at 582 nm. The bare crystalline silicon surface may give dye molecules more opportunity to aggregate than amorphous silicon dioxide or quartz surfaces. The dye molecules are preferentially oriented along the [100] direction of the substrate crystal, with the carbonyl group parallel to and the conjugated aromatic plane perpendicular to the substrate surface, although there is no detectable difference of orientation between the films deposited on these different substrates.