Nanopatterning electrically insulating oxide lines on organic electronic surfaces can play a role in fabricating future nanoscale devices. Here we write oxide features on rubrene single crystal surfaces by performing local anodic oxidation using the tip of an atomic force microscope. Oxide feature height increases with voltage bias and decreases with tip writing speed, and gaps as small as 22 nm at the surface between two parallel oxide lines were realised. Conductance tomography is employed in a unique way to determine the depths of oxide features, by exposing subsurface layers of the patterned material without using chemical etching while simultaneously mapping material conductance. The oxide line depth exceeds its height, with the depthto-height ratio frequently being more than 1.6. A critical electric field of ~3×10 6 V/cm is identified, below which the oxide growth ceases, resulting in a maximum oxide vertical extent of about ~60 nm at a voltage bias of ~20 V.
Electronic devices made from organic materials have the potential to support a more ecologically friendly and affordable future. However, the ability to fabricate devices with well-defined and reproducible electrical and optical properties is hindered by the sensitivity to the presence of chemical impurities. Oxygen in particular is an impurity that can trap electrons and modify conductive properties of some organic materials. Until now the 3-dimensional profiling of oxygen species in organic semiconductors has been elusive and the effect of oxygen remains disputed. In this study we map out high-spatial resolution 3-dimensional distributions of oxygen inclusions near the surface of single crystal rubrene, using Time of Flight Secondary Ion Mass Spectroscopy (ToF-SIMS). Channels of diffused oxygen are found extending from uniform oxygen inclusion layers at the surface. These channels extend to depths in excess of 1.8 μm and act as an entry point for oxygen to diffuse along the ab-plane of the crystal with at least some of the diffused oxygen molecularly binding to rubrene. Our investigation of surfaces at different stages of evolution reveals the extent of oxygen inclusion, which affects rubrene's optical and transport properties, and is consequently of importance for the reliability and longevity of devices.
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