Morphological Transitions in Organic Ultrathin Film Growth Imaged by In Situ Step-by-Step Atomic Force Microscopy

Abstract

In situ atomic force microscopy (AFM) allowed us to investigate the evolution at the early stages of the growth of organic thin films. An ultrahigh-vacuum atomic force microscope, integrated with a Knudsen effusion cell for the sublimation of α-sexithiophene (6T), continuously scans the same region during the deposition of sublimed molecules on native silicon oxide as a function of the substrate temperature. Noncontact AFM images acquired sequentially provide snapshots of the time evolution of the film morphology that is monitored up to the deposition of five monolayers. At all substrate temperatures, a Stranski−Krastanov growth mode of organic films is observed: the first two monolayers grow layer-by-layer (two-dimensional2D), then films evolve into islands (three-dimensional3D). Despite the apparent similarity, we find an anomalous dynamic scaling characterized by the abrupt change of the growth exponent β vs substrate temperature. This novel transition, induced by the substrate temperature, is ascribed to the morphological transition from ziggurat islands to large terraces. The analysis of the evolution of the root-mean-square (RMS) roughness based on the distributed growth model underlines the role of down-hill mass transport for the growth of the first two monolayers, transport that is progressively hindered for the next monolayers.

Polymorphism and structure formation in copper phthalocyanine thin films

“…Whether organic thin films become rougher or smoother on heated substrates is dependent on many parameters, such as the deposition rate (Storzer et al, 2017;Farahzadi et al, 2010) and the chemical structure (Belova et al, 2018;Reisz et al, 2017). Furthermore, the roughness depends on the film thickness and the choice of substrate (Dü rr et al, 2003;Kowarik et al, 2006;Yim & Jones, 2006;Hong et al, 2008;Zhang et al, 2009Zhang et al, , 2011Kim et al, 2010;Hinderhofer et al, 2011;Obaidulla & Giri, 2015;Nahm & Engstrom, 2016;Brillante et al, 2017;Chiodini, Straub et al, 2020). Some molecules such as pentacene form smoother thin films at elevated substrate temperatures (Yagi et al, 2004).…”

Synthetic Data in Quantitative Scanning Probe Microscopy

“…The spectrum of phenomena observed in sub-monolayer and few-layer film growth is surprisingly rich and a variety models have been used to study specific processes, such as island ripening processes and roughening transitions. [ 65 , 83 , 84 , 85 , 86 ].…”

Differential Entropy: An Appropriate Analysis to Interpret the Shape Complexity of Self-Similar Organic Islands

“…In general, OTFs on SiO 2 follow the Stranski-Krastanov (SK) growth mode [ 19 , 20 ], a mixed scenario involving both Frank–Van der Merwe (FM, two-dimensional films) and Volmer–Weber growth modes (VW, three-dimensional films) [ 21 ]. At the SiO 2 interface, the OTF is characterized by two or three complete FM layers [ 21 ].…”

“…These layers, also known as monolayers (MLs), are complete and composed of quasi upright standing molecules [ 22 , 23 , 24 , 25 , 26 , 27 ]. After that, the growth switches to VW and three-dimensional structures form [ 19 ].…”

“…In the past, scaling laws were employed to study, statically [ 1 , 28 , 29 ] and dynamically [ 19 , 26 , 30 , 31 ], the morphological evolution of OTFs for increasing thickness, proving the self-affinity of the OTFs surface profile [ 10 , 32 , 33 , 34 , 35 , 36 ].…”