Journal
ACS APPLIED MATERIALS & INTERFACES
Volume 10, Issue 16, Pages 14175-14182Publisher
AMER CHEMICAL SOC
DOI: 10.1021/acsami.8b02177
Keywords
indium oxide; sexiphenyl; organic molecules; atomic force microscopy; scanning tunneling microscopy; adsorption site; monolayer
Funding
- FWF project [T759-N27, F4505, Wittgensteinpreis Z250]
- U.S. Department of Energy, Basic Energy Sciences, Materials Sciences and Engineering Division [DE-AC05-00OR22725]
- European Research Council (Advanced Grant OxideSurfaces) [ERC-2011-ADG_20110209]
- Austrian Science Fund (FWF) [T759] Funding Source: Austrian Science Fund (FWF)
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The performance of an organic semiconductor device is critically determined by the geometric alignment, orientation, and ordering of the organic molecules. Although an organic multilayer eventually adopts the crystal structure of the organic material, the alignment and configuration at the interface with the substrate/electrode material are essential for charge injection into the organic layer. This work focuses on the prototypical organic semiconductor para-sexiphenyl (6P) adsorbed on In2O3(111), the thermodynamically most stable surface of the material that the most common transparent conducting oxide, indium tin oxide, is based on. The onset of nucleation and formation of the first monolayer are followed with atomically resolved scanning tunneling microscopy and noncontact atomic force microscopy (nc-AFM). Annealing to 200 degrees C provides sufficient thermal energy for the molecules to orient themselves along the high-symmetry directions of the surface, leading to a single adsorption site. The AFM data suggests an essentially planar adsorption geometry. With increasing coverage, the 6P molecules first form a loose network with a poor long-range order. Eventually, the molecules reorient into an ordered monolayer. This first monolayer has a densely packed, well ordered (2 x 1) structure with one 6P per In2O3(111) substrate unit cell, that is, a molecular density of 5.64 x 10(-3) cm(-2).
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