Surface glass transition temperature characterized by metal-atom deposition/desorption on organic films

Surfaces and interfaces play an important role in obtaining high-performance organic devices. An essential property of organic films is the surface glass transition temperature (surface-Tg) and many methods for characterizing surface-Tg have been studied. We propose a novel method for characterizing surface-Tg based on metal-vapor atom deposition and desorption. We monitored metal-vapor deposition and desorption on organic surfaces using double quartz crystal microbalances. Mg vapor is not deposited on organic surfaces with a low bulk-Tg such as a colorless photochromic diarylethene (DAE) film. This deposition phenomenon is caused by Mg-atom desorption from the surface based on active surface molecular motion. However, Mg deposition began after a certain time of continuous evaporation (deposition-threshold time). The threshold time elongated with increased substrate temperature and elongated dramatically at a substrate temperature several degrees below the bulk-Tg for DAE. The surface molecular motion becomes active and the metal-atom desorption accelerates as the temperature neared the surface-Tg. Thus a temperature with a dramatic elongation of the threshold time indicates the surface-Tg. This method can be applied to a variety of organic films.