Eur. Phys. J. Special Topics 189, 181-186 (2010)
https://doi.org/10.1140/epjst/e2010-01321-1

Regular Article

Dynamics in ultrathin liquid films studied by simultaneous dielectric spectroscopy (DRS) and organic molecular beam deposition (OMBD)

¹ Department of Physics and Astronomy, Katholieke Universiteit Leuven, Celestijnenlaan 200D, Leuven 3001, Belgium
² Department of Chemistry and Biochemistry, University of Berne, Freiestr. 3, 3012 Berne, Switzerland

^a e-mail: wubbenhorst@fys.kuleuven.be

Received: 1 September 2010
Revised: 14 September 2010
Published online: 12 November 2010

Abstract

Real-time dielectric relaxation spectroscopy for a molecular beam deposited glass forming liquids is proposed as a versatile approach for the study of the dynamic glass transition in geometric confinement. To achieve the highest sensitivity down to monomolecular organic layers in a wide frequency range (0.1–10⁷  Hz) during simultaneous deposition and desorption, we have used μm spaced interdigitated electrodes under ultrahigh vacuum conditions. Experiments using glycerol deposited on fused silica at − 40  ^∘C revealed a dielectric glass transition process for a layer thickness as low as 0.7 nm. While its peak position hardly changes upon thickness reduction, a clear broadening is observed that implies an increasing heterogeneous mobility scenario for the thinnest films caused by molecules being part of a reduced (at the substrate) or enhanced (free surface) mobility layer. This finding is supported by desorption experiments that reveal a strong retardation of the desorption rate for films below 1 nm.