Paper EM+AS+EN+TF-ThM1
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

Thursday, October 31, 2013, 8:00 am, Room 101 B

Over the past several years significant advances have been made concerning our understanding of the growth of crystalline small molecule organic thin films consisting of a single component. An important challenge in organic electronics is to develop and improve methods to integrate both p-type and n-type small molecule organic semiconductors into the same device microstructure. Thus, developing an understanding of the molecular scale events that lead to heterojunction formation is essential. In this work we present results concerning nucleation and growth of a series of n-type organic semiconductors: PTCDI-C_n, where the length of the alkyl tail (C_n) attached to the perylene core of these molecules has been varied from n = 5, 8 and 13. In addition to examinations of the growth of these molecules themselves, we have also examined the growth of these molecules on ultrathin films of the p-type semiconductor pentacene, and vice-versa, the growth of pentacene on layers of PTCDI-C_n. In this work we make use of molecular beam techniques to deliver PTCDI-C_n and/or pentacene, and growth of these layers is monitored in situ and in real time using X-ray synchrotron radiation, and ex situ using atomic force microscopy. Concerning the growth of PTCDI-C_n on 1 monolayer (ML) of pre-deposited pentacene we find substantial differences between the C₅ and C₁₃ variants of this molecule: X-ray intensity oscillations at the anti-Bragg scattering condition, signifying layer-by-layer (LbL) growth, are much more extended for the C₅ variant. Interestingly, despite the extended nature of LbL growth for this molecule, the thin film crystallinity formed from C₅ is significantly reduced when compared to the C₁₃ variant, suggesting that the molecular interactions that provide the driving forces for crystallization, may also contribute to higher barriers for step-edge crossing events—highly crystalline but rough thin films. More dramatic differences emerge as we examine the sequence of deposition. While each perylene variant grows approximately layer-by-layer on 1 ML of pentacene for several layers of PTCDI-C_n, when this order is reversed, and pentacene is grown on 1 ML of PTCDI-C_n dramatic changes occur: growth is immediately 3D, and a very rough morphology is formed. Ex situ analysis using AFM reveals that the thin film morphology is rougher than what would be formed from simple random deposition. Examination of multilayer structures, e.g., A/B/A/B… where A is PTCDI-C_n and B is pentacene, shows that the growth of roughness tends to reflect this asymmetry, where the pentacene cycle tends to roughen the vacuum|film interface, while the PTCDI-C_n cycle tends to smoothen the interface.