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

Abstract

We have examined the growth of bilayers and superlattices of pentacene and perylene derivatives (PTCDI-C n ) using in situ real time X-ray synchrotron radiation techniques and ex situ atomic force microscopy. We find that the growth of PTCDI-C n 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, C n : the smoothest films are formed with PTCDI-C 5 , while the roughest are formed with PTCDI-C 13 . 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-C n . 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-C n ) is not favored, resulting in significant reorganization. Well-ordered superlattices of pentacene and PTCDI-C n 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-C n 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.

Interplay between Charge Carrier Mobility, Exciton Diffusion, Crystal Packing, and Charge Separation in Perylene Diimide-Based Heterojunctions

“…The preferential crystal orientation of PDI-octyl and -hexhep are in accordance with literature. 19,30,45 Comparison of the powder with the thin film diffractograms shows the preferred crystal orientation in the thin film since it only has one reflection, in contrast to the powder spectrum. The extent of crystallization of PDI-octyl does not seem to be larger than in PDI-hexhep.…”

Influence of C60 co-deposition on the growth kinetics of diindenoperylene–From rapid roughening to layer-by-layer growth in blended organic films

“…24,25 Real time monitoring of the scattering intensity at the anti-Bragg 26,27 point during the thin film growth allows to discriminate between the three basic growth modes: 28 the stepflow, the layer-by-layer, and three-dimensional (i.e., growth accompanied by the formation of islands higher than one mono-layer) growth modes, which result in constant scattered intensity, periodic intensity oscillations, and steep intensity decay, respectively. In the anti-Bragg regime, various probes including specular reflected thermal energy He atoms, 28 high energy electron diffraction (RHEED), 29,30 and X-ray reflection in the anti-Bragg point 26,[31][32][33][34][35] have been used to follow the growth mode and detect growth instabilities. Additional information on atomic or molecular island density evolution, which may exhibit oscillations due to island nucleation and coalesce during growth, can be obtained by monitoring diffuse (off-specular) X-ray scattering.…”

A Computational Predictive Approach for Controlling the Morphology of Functional Molecular Aggregates on Substrates

“…The simulation approach described above was applied to the investigation of the growth of PTCDI‐C13 layers at the interface with graphene in different environmental and processing conditions. As shown in previous works, PTCDI‐C13 exhibits a strong tendency to form stable crystalline 2D aggregates at the solid state . In the most thermodynamically stable 2D phase, PTCDI‐C13 molecules are aligned in a co‐facial (CF) configuration, which maximizes π–π stacking interactions in aggregates .…”

“…We apply our protocol to the case of the perylene diimide derivative N , N ′‐ditridecylperylene‐3,4,9,10‐tetracarboxylic diimide (PTCDI‐C13), a state‐of‐the‐art multifunctional n‐type organic semiconductor, at the interface with graphene, a prototypical 2D substrate . The remarkable semiconductor properties of N ‐alkyl‐substituted perylene diimide derivatives are mostly related to the strong π‐stacking interactions occurring between the aromatic cores of adjacent molecules in aggregates, and to the efficient packing assisted by the side alkyl chains . These structural peculiarities lead also to the occurrence of several stable and metastable crystalline and liquid crystalline phases in aggregates, depending on the details of the molecular structure.…”