The morphology of organic nanocolumn arrays: Amorphous versus crystalline solids
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artin Oehzelt Institut fu¨r Experimentalphysik, Johannes Kepler Universita¨t Linz, A-4040 Linz, Austria
Steffen Duhm, Ju¨rgen P. Rabe, and Norbert Kochb) Institut fu¨r Physik, Humboldt-Universita¨t zu Berlin, D-12489 Berlin, Germany (Received 16 June 2008; accepted 15 December 2008)
The morphology of nanocolumns grown by glancing angle deposition is studied for molecular materials forming amorphous and crystalline solids. Amorphous tris(8-hydroxyquinoline)aluminum nanocolumn arrays were obtained at sample rotation speeds varying from 0.3 rpm (revolutions per minute) to 30 rpm. For crystalline pentacene, an array of regular nanocolumns formed at a rotation speed of 3 rpm, while higher and lower rotation speeds led to a wide distribution of column heights and shapes. The incoming molecular flux and the molecular diffusion length on column surfaces, both dependent on rotation speed, were found to govern the resulting morphology of crystalline pentacene nanocolumns. I. INTRODUCTION
Recently, the fabrication of nanocolumn arrays based on organic semiconductor materials was reported, for example, made of tris(8-hydroxyquinoline)aluminum (Alq3),1,2 pentacene (PEN), and C60.3 Similar to carbon nanotubes, these organic nanocolumn arrays have significant potential for realizing functional devices, such as sensors, field emitters, and nanoelectronic devices.1–8 An advantage of organic nanocolumn arrays is that they can be directly deposited onto large-area flexible substrates at room temperature. They are fabricated by glancing angle deposition (GLAD), comprising the combination of an oblique-angle of incidence of the deposited material (with respect to the substrate surface) and simultaneous substrate rotation, which leads to the formation of columnar structures on the nanometer scale in all three dimensions.9,10 GLAD has been used before to fabricate nanoscale structures of inorganic materials, for example, silicon or various metals.11–15 The geometric form of nanostructures grown by GLAD depends on a number of factors including the oblique angle of incidence, the distance between source and substrate, and the substrate rotation speed (do/dt).12–15 Here, o is the azimuthal angle about the substrate surface normal. A variety of thin-film
Address all correspondence to these authors. a) e-mail: [email protected] b) e-mail: [email protected] DOI: 10.1557/JMR.2009.0163 1492
http://journals.cambridge.org
J. Mater. Res., Vol. 24, No. 4, Apr 2009 Downloaded: 07 Apr 2015
morphologies have been fabricated using vastly different rotation speeds, including separated straight columns,13 zig-zag structures,14 and helices.10 In terms of thin film growth processes, organic molecules differ from inorganic atoms as the molecule-molecule and moleculesubstrate interactions are usually much weaker than atom-atom and atom-substrate interactions, which has a distinct impact on thin film growth.16,17 Therefore, the effect of growth parameters on the morphology of organic nanocolumn arrays has to be studied. In the present work
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