In situ spectroscopic ellipsometry monitoring of multilayer growth dynamics via molecular layer epitaxy

Real time monitoring of organic monolayer self-assembly by molecular layer epitaxy (MLE) processes was studied by in situ spectroscopic ellipsometry techniques. For the MLE of imide-based organic heterostructures using chemisorption of 3,4,7,8-naphthalenetetracarboxylic dianhydride (NTCDA) and 1,6-diamino-n-hexane (DAH) on prefunctionalized surfaces, in situ ellipsometry reveals that the reaction kinetics can be best fitted to an S-shaped deposition curve with saturated coverage of about 20 min by the Langmuir-Hinshelwood model, with a slow initial phase, followed by a faster second phase. The rate of deposition at each moment is proportional to the number of empty sites multiplied by the number of occupied sites. Calculated deposition rate constants for every pulse are k_T = 5.6 × 10^-5 s^-1 for first deposition of NTCDA on a template (T-layer), decreasing to k_A = 1.5 × 10^-5 s^-1 for NTCDA and to k_B = 7.2 × 10^-6 s^-1 for DAH assembly pulses correspondingly. A modified Rudzinski-Aharoni kinetic model for adsorption that correlates adsorption energy with valid numbers of reactive sites was used to estimate an equilibrium surface absorption energy of 16 kcal for a NTCDA layer and 29 kcal for a DAH layer.