The effect of substrate temperature on the structure and properties of vacuum deposited films

Abstract

A Study of vacuum deposited films of the charge transfer complex (TTF)(TCNQ) and several polycyclic aromatic hydrocarbon sis described. Fluorescence techniques have been used to investigate the annealing behaviour of low temperature deposits of anthracene, pyrene, perylene and tetracene, in order to establish the cause of the structural optimum, or singularity which is known to occur when films of some of these materials are deposited onto substrates maintained at one third their normal boiling points (0.33 T(_B)). Evidence is presented for the formation of a metastable state in the vicinity of 0.33 T(_B) in the case of anthracene, pyrene and perylene. A detailed study of the topography of (TTF)(TCNQ) films has revealed a pronounced structural singularity at a deposition temperature of 213K which was accompanied by an apparent reduction in the activation energy for conduction. However, it was shown that this apparent optimisation of electrical properties was associated with crystallite orientation effects rather than a reduction in the influence of grain boundaries. In-situ infrared absorption spectroscopy has confirmed that films of (TTF)(TCNQ), either deposited at, or annealed to 213K adopt a neutral lattice structure with no evidence of charge transfer between donors and acceptors. Annealing experiments have established that this neutral lattice possesses short range order and has a metastable, glass-like, structure. Furthermore, conclusive evidence is presented for the formation of a disordered phase of (TTF)(TCNQ) below 0.33 T(_B) which exhibits anomalous infrared activity of the totally symmetric vibrational modes of both (TTF) and (TCNQ). This observation is considered to be analogous to the formation of excimers in disordered films of the aromatic hydrocarbons, prepared below 0.33 T(_B). On the basis of this evidence a model is discussed in which the observation of an optimum substrate temperature effect in Evaporated films of molecular solids is attributed to the formation of a dense metastable state which has a thermodynamic stability intermediate between the crystalline and disordered state of materials.