Time-dependent changes in the growth of ultrathin ionic liquid films on Ag(111)

Various amounts of the ionic liquids (ILs) [C₁C₁Im][Tf₂N] and [C₈C₁Im][Tf₂N] were deposited in vacuo by
physical vapour deposition (PVD) on single crystalline Ag(111) at room temperature and subsequently
monitored by angle-resolved X-ray photoelectron spectroscopy (ARXPS) as a function of time. For
very low coverages of up to one closed molecular layer, an initial wetting layer was rapidly formed for
both ILs. Deposition of higher amounts of [C₁C₁Im][Tf₂N] revealed an initial three-dimensional
film morphology. On the time scale of hours, characteristic changes of the XPS signals were observed.
These are interpreted as island spreading and a transformation towards a nearly two dimensional
[C₁C₁Im][Tf₂N] film as the final state. In contrast, a film morphology close to 2D was found from the very
beginning for [C₈C₁Im][Tf₂N] deposited on Ag(111) demonstrating the influence of the alkyl chain length
on the growth kinetics. These studies also highlight the suitability of time-resolved ARXPS for the
investigation of IL/solid interfaces, which play a crucial role in IL thin film applications such as in
catalysis, sensor, lubrication, and coating technologies.