When the Sequence of Thin Film Deposition Matters: Examination of Organic-on-Organic Heterostructure Formation Using Molecular Beam Techniques and in Situ Real Time X‑ray Synchrotron Radiation

• Published in: Journal of physical chemistry. C 2016-03, Vol.120 (11), p.6165-6179
• Main Authors: Kish, E. R, Nahm, R. K, Woll, A. R, Engstrom, J. R
• Format: Article
• Language: English
• ISSN: 1932-7447; 1932-7455
• DOI: 10.1021/acs.jpcc.6b01717

We have examined the growth of bilayers and superlattices of pentacene and perylene derivatives (PTCDI-Cn) using in situ real time X-ray synchrotron radiation techniques and ex situ atomic force microscopy. We find that the growth of PTCDI-Cn layers on 1 monolayer (ML) of pentacene is initially 2D layer-by-layer (LbL), eventually transitioning to a mode of growth that is more 3D after several monolayers have been deposited. We find that the extent of 2D LbL growth depends on the length of the alkyl end chains, Cn: the smoothest films are formed with PTCDI-C5, while the roughest are formed with PTCDI-C13. These observations reflect a difference in the Ehrlich–Schwoebel barrier for step-edge crossing with alkyl end-chain length. When the sequence of deposition is reversed, we observe spectacular changes in the evolution of surface roughness for the growth of pentacene thin films on 1 ML of PTCDI-Cn. The growth is immediately 3D, while still remaining crystalline. The morphology of these thin films indicates significant reorganization of the deposited pentacene during and/or subsequent to growth, producing 3D islands that can only form if “uphill” transport is operative. Surface energy is driving this process, where the growth of high surface energy layers (pentacene) on low surface energy materials (PTCDI-Cn) is not favored, resulting in significant reorganization. Well-ordered superlattices of pentacene and PTCDI-Cn cannot be grown, consistent with our results on the bilayers. We find that thin film roughness increases abruptly with the deposition of the second ML of pentacene or the formation of the first pentacene-on-PTCDI-Cn interface. Our results indicate that the successful growth of superlattices of small molecular organic thin films will require matching of surface energies to minimize the driving forces for reorganization.