Improving organic materials structure and performance to use them in competitive devices and sensors

Growing ordered crystals using organic molecules instead of atoms is intrinsically harder. The asymmetry of molecules gives rise to defects and usually ends in polycrystalline materials in the best case. Their disorder always hindered the use of organic materials in many applications. Nowadays several new growth techniques are being developed, some of which can largely improve the control on organic materials growth. This leads to better structure, better chemical-physical properties and thus better performance when such materials are used in applicative devices. Conjugated small molecules are very interesting both as a building block to study the growth model of crystalline organic films and as a very good performance organic material. Vacuum deposition has shown to be the most suitable technique to get high purity ordered films. Nevertheless some problems remain due to the high anisotropy of organics that make easy the formation of different polymorphs or/and orientations that strongly hinder high quality films. The supersonic molecular beam deposition (SuMBD) technique, developed at the IMEM Nanoscience Lab, allows a better control on the growth. The kinetic energy of the impinging molecules is the key factor that affects the growth improving the assembling processes of molecules and their surface mobility. Here we report on the growth of different small conjugated molecules investigating the influence of deposition parameters. We will show how, tuning properly the supersonic beam, it is possible to obtain very ordered thin films with large and smooth monocrystalline islands. A higher kinetic energy always improves the quality of the film in terms of structure, size of the grains and thus also of the device performance. The deposition regime obtainable by SuMBD, permits to achieve field effect mobility values that are the state of the art even in absence of any self-assembled monolayer and ex-situ. Such improved device performance can make organic crystalline materials a good competitor to other materials in a wide range of applications including the sensing field.