Origin of Translational and Dipolar Order in Vapor-Deposited Alq3 glasses*

Structural anisotropy in vapor-deposited molecular glasses has attracted interest both from a fundamental physics and organic electronics perspective. It has been shown previously that the bulk orientational anisotropy in vapor-deposited glasses is inherited from the free surface of the equilibrium supercooled liquid (surface equilibration mechanism). However, it is unclear if this mechanism can explain other common structural features of vapor-deposited glasses such as a tendency for center of mass ordering and dipolar alignment. To test if surface equilibration can explain these structural features, we perform X-ray scattering studies and molecular dynamics simulations of thin films of common OLED (organic light emitting diode) molecule Alq3. From x-ray scattering we find a substrate temperature dependent tendency for center of mass ordering in vapor-deposited Alq3 glasses. Simulations reveal that the free surface of the equilibrium liquid exhibits a tendency for center of mass ordering and dipolar order. The surface equilibration mechanism can explain our observation of center of mass ordering in vapor-deposited Alq3 glasses and the existence of dipolar alignment reported by other groups.