Grazing Incidence X-ray Scattering Techniques to Study Growth Mechanism of Organic Films

We have summarized the application of in situ and real-time X-ray scattering to investigate the growth of organic thin films, particularly the grazing incidence X-ray diffraction technique with some exposure to the growth oscillations and GISAXS, which are helpful in determining the evolution of the crystal structure and in- and out-of-plane morphology. The importance of the growth mechanism of organic thin films in nature and technology has been addressed along with the different aspects of non-equilibrium growth and the various control parameters involved in it. An X-ray scattering measurement benefits from the fact that it can be used under very different environments like vacuum, gas, liquid and high pressure. The major drawback of these techniques is the possibility of radiation damage to organic samples, in particular for synchrotron experiments, and this has also been discussed in some detail. Several cases of pure and mixed organic thin films have been discussed in the light of the GIXD and the GISAXS technique and the primary inferences from such studies have been elucidated. A lot of important information regarding the growth and structure of pure and mixed films, like influence of the growth parameters (substrate temperature, rate of growth, interaction with the substrate), the kinetics of phase-separation, the mixing behavior of dimensionally similar and dissimilar molecules, the effect of templating and other issues have been addressed. Important future challenges in this field include clear demarcation between kinetic and energetic effects on the morphological evolution. Transitions from a transient thin film phase to a bulk phase have been demonstrated by extending the existing theoretical formalism of growth. Anisotropic interaction parameters, together with tilt angle have been introduced to explain the sudden disappearance of the in-plane order of the equimolar mixed films, although the order is retained in the out-of-plane direction. Kinetically limited phase separation has been followed in real-time and depth-resolved GIXD has been utilized to propose the asymmetry in the domains of the phase-separating mixtures. Additional control parameters have been identified that can influence the film structure such as exposure to light, controlled interruption of growth, and suitable functionalization of the layers by employing templates. Still, to address internal degrees of freedom in molecular materials, both methods and materials are being developed, and together with in situ real-time observation of thin film growth, unprecedented control over structure of novel thin film materials is expected.