Campione Marcello, Sassella Adele, Moret Massimo, Marcon Valentina, Raos Guido
INFM and Department of Materials Science, Università degli Studi di Milano Bicocca, V. R. Cozzi 53, I-20125 Milano, Italy.
J Phys Chem B. 2005 Apr 28;109(16):7859-64. doi: 10.1021/jp0453616.
Growth studies of ultrahigh vacuum deposited thin films are often carried out ex situ, assuming the total film mass reached at the end of the deposition is preserved in the subsequent stages of film preparation. Many kinetic models commonly adopted to analyze quantitatively the mechanism of growth take into account the role of the deposition rate of molecules on the substrate surface, their diffusion, and their possible desorption. Within this framework, a strong simplification (and approximation) of the model is achieved when considering a regime of complete condensation (i.e., neglecting the possibility of re-evaporation of the deposited molecules, both during the deposition and the postdeposition stages of growth). Here, we demonstrate that, for molecular materials of relatively small organic molecules physisorbed on inert surfaces, this phenomenon may strongly affect not only the surface dynamics during deposition but also the postdeposition stage of thin film preparation. Some examples showing clearly its effects on the surface of single crystals and the thin film phase are reported and discussed. Finally, a quantitative description of desorption is provided by comparing the prediction of thermodynamics for the quaterthiophene/silica system with the experimental observation of the growth dynamics of the film and the results of approximate kinetic models. The thermodynamic model employs the surface free energies of a quaterthiophene crystal, which are evaluated by molecular simulation using a newly developed force field.
超高真空沉积薄膜的生长研究通常是在非原位条件下进行的，假设在沉积结束时达到的总膜质量在随后的膜制备阶段得以保留。许多常用于定量分析生长机制的动力学模型都考虑了分子在基底表面的沉积速率、扩散以及可能的脱附作用。在此框架内，当考虑完全凝聚状态（即忽略沉积分子在沉积过程和生长的沉积后阶段重新蒸发的可能性）时，模型会得到极大的简化（和近似）。在此，我们证明，对于物理吸附在惰性表面上的相对较小有机分子的分子材料，这种现象不仅会强烈影响沉积过程中的表面动力学，还会影响薄膜制备的沉积后阶段。本文报道并讨论了一些清楚显示其对单晶表面和薄膜相影响的例子。最后，通过将四噻吩/二氧化硅体系的热力学预测与薄膜生长动力学的实验观察结果以及近似动力学模型的结果进行比较，给出了脱附的定量描述。热力学模型采用了四噻吩晶体的表面自由能，该自由能通过使用新开发的力场进行分子模拟来评估。
