Revealing surface oxidation on the organic semi-conducting single crystal rubrene with time of flight secondary ion mass spectroscopy

Abstract

To address the question of surface oxidation in organic electronics the chemical composition at the surface of single crystalline rubrene is spatially profiled and analyzed using Time of Flight - Secondary Ion Mass Spectroscopy (ToF-SIMS). It is seen that a uniform oxide (C42H28O) covers the surface while there is an increased concentration of peroxide (C42H28O2) located at crystallographic defects. By analyzing the effects of different primary ions, temperature and sputtering agents the technique of ToF-SIMS is developed as a valuable tool for the study of chemical composition variance both at and below the surface of organic single crystals. The primary ion beams C60(3+) and Bi3(+) are found to be most appropriate for mass spectroscopy and spatial profiling respectively. Depth profiling of the material is successfully undertaken maintaining the molecular integrity to a depth of ~5 μm using an Ar cluster ion source as the sputtering agent.

Photovoltaic Effect at the Schottky Interface with Organic Single Crystal Rubrene

“…The origin of doping which shifts the Fermi level has not been addressed in this work. Previously, electronic states at 0.25–0.35 eV above the HOMO were identified and assigned to oxygen doping, while the structural defects were found to facilitate the oxidation . However, the structural defects were discussed as the sole origin of the same features in the photoluminescence spectra too .…”

Channels of oxygen diffusion in single crystal rubrene revealed

“…Previous studies have shown that these oxide species of rubrene bind to different sites on the surface with rubrene peroxide preferentially being found at the sites of defects and rubrene oxide forming a more uniform covering of the surface. 16 As can be seen in Figure 2 there is a clear correlation between each of the ionic species tracked and the locations of the pillars. Several of the pillars can be identified even in the low count rubrene peroxide map.…”

“…21 A typical crystal can be seen in previous reports from our group along with a diagrammatic representation of the crystalline packing. 16,17 This study looks at the ab-plane surface. Crystals were mounted on an aluminium substrate using carbon paint.…”

“…In this study we will investigate oxygen in a single crystal organic semiconductor, rubrene, which exhibits excellent optical and electronic properties, but in which oxygen incorporation is ill-understood and debated within the literature. [10][11][12][13][14][15][16][17][18][19] In this paper we map out 3-dimensional distributions of oxygen inclusions near the surface of rubrene in order to determine the uniformity of oxide distribution and how this should inform the design of electronic devices and analysis of surface measurements. Oxide mapping is undertaken using highspatial resolution Time of Flight Secondary Ion Mass Spectroscopy (ToF-SIMS), for the first time.…”

Local anodic oxidation lithography on organic semiconductor crystals: Oxide depth revealed by conductance tomography

“…Rubrene is considered the prototype single crystal organic semiconductor because of its relatively large charge carrier mobility (10-40 cm 2 /(V•s)) [10][11][12] and long exciton diffusion lengths (~4 microns) [13,14]. Oxidation properties of the single crystal have been studied in detail; in addition to evidence of an epitaxial native oxide layer [15], oxygen-related species have been observed uniformly on the surface and preferentially at structural defects [16], as well as under the surface [17], having accessed preferential channels into the bulk before spreading laterally [18]. Meanwhile, multiple oxygen incorporation mechanisms have been proposed, each with unique oxygen exposure methods, suggesting varying effects on rubrene device performance.…”