Controlling Interface Morphology and Layer Crystallinity in Organic Heterostructures: Microscopic View on C₆₀ Island...

Controlling the crystallinity of organic thin films is an important aspect in the improvement of organic electronic devices. However, because of high molecular mass, structural anisotropy, and weak intermolecular van der Waals bonding, crystalline ordering is not easily accomplished. While film preparation at elevated substrate temperature often improves the crystalline quality, this approach cannot be applied to temperature-sensitive materials such as plastic foils used as substrates for flexible electronics. Here, we examine in detail a low-temperature approach to improve film crystallinity by using ultrathin pentacene (PEN) buffer layers that allow crystalline growth of buckminsterfullerene (C₆₀) thin films while without such buffer layers, only amorphous fullerene films are formed upon room-temperature deposition on various support substrates. Remarkably, this effect depends critically on the thickness of the PEN buffer and requires a thickness of at least two monolayers to induce crystalline growth, whereas a buffer layer consisting of a monolayer of PEN again yields amorphous C₆₀ films. Combining crystallographic investigations by X-ray diffraction and atomic force microscopy measurements, we determine distinct nucleation sites on buffer layers of different thickness, which are correlated to the amorphous, respectively crystalline C₆₀ islands. Our microscopic analysis reveals distinct differences for the nucleation and diffusivity of fullerenes on the PEN monolayer and on thicker buffer layers, which are attributed to the molecular arrangement in the PEN monolayer. Finally, we show that the crystalline C₆₀ films are exclusively (111)-oriented and the fullerene islands are even heteroepitaxially aligned on the PEN buffer.