Observation and analysis of flat-lying molecular layer via scanning probe microscopy

The analysis of the formation of ultra-thin organic films is a very important issue. In fact, it is known that the properties of organic field effect transistors are strongly affected by the early growth stages. For instance, in the case of sexithiophene deposited on thermal SiOx, the presence of domains made of molecules with the backbone parallel to the substrate surface has been indirectly evidenced by photoluminescence spectroscopy and directly imaged by Ultrasonic Force Microscopy. On the contrary, photoluminescence spectroscopy have failed to observe sexithiophene deposited on native SiOx. In this paper, we show how Scanning Probe Microscopy techniques combined with post-annealing process are able to identify a flat-lying molecular layer on the native SiOx. By using Lateral Force Microscopy and Atomic Force Microscopy in Intermittent Contact we investigate its mechanical properties and, by using Ultrasonic Force Microscopy, we directly identify the structure of flat-lying molecular layer. Finally, we show how the post annealing process can address the growth of organic materials towards a “driven layer-by-layer growth” to both improve the surface coverage and reduce the domain boundaries effects in organic field effect transistor.