Surface Potential Mapping of SAM‐Functionalized Organic Semiconductors by Kelvin Probe Force Microscopy

Abstract

Previously, it has been shown that formation of self assembled monolayers (SAMs) of alkyl-silanes on the surfaces of π -conjugated organic molecular crystals results in an orders-ofmagnitude increase in the surface conductivity of these semiconducting materials. [ 1 ] The magnitude of the effect depends on the chemical structure of the alkyl tail of the SAM: a fl uorinated SAM (tridecafl uoro-1,1,2,2-tetrahydrooctyl)trichlorosilane, or FTS) results in a 10 4 -fold increase in surface conductivity of rubrene crystals, for example, while a non-fl uorinated SAM (n-octyltrichlorosilane, or OTS) raises the conductivity only by a factor of 10. These numbers are quoted relative to the surface conductivity of the built-in conduction channel for pristine rubrene crystals, σ 0 ∼ 10 − 9 –10 − 8 S per square, with carrier densities n 0 ∼ (0.6–6) × 10 9 cm − 2 . [ 2 , 3 ] Generally alkyl-silanes functionalized with stronger electron withdrawing groups induce a greater surface conductivity change in p-type organic semiconductors. This effect is a result of interfacial charge-transfer doping where the silane derivative withdraws electrons from the organic semiconductor surface, thereby inducing mobile charge carriers (holes in the case of rubrene) for transport. The interfacial charge-transfer doping that occurs when silanes bind to rubrene will result in surface dipoles and work function shifts for the rubrene surface. Indeed, it is well known that SAMs modify the work functions of both metals and semiconductors by surface dipoles. [ 4 , 5 ] Measuring the resulting work function shifts provides a way to quantify the effects of silane treatment in terms of the band edge offsets from the Fermi level, which can then be related to doping levels in the organic semiconductor. Accordingly, in this work we have quantifi ed the degree of charge-transfer doping and the magnitude of the rubrene band energy modulation induced by OTS and FTS SAMs by performing surface potential measurements with Kelvin probe force microscopy (KFM). [ 6–10 ] In addition we have utilized the sub-50 nm spatial resolution of KFM to visualize important aspects of SAM nucleation and growth on rubrene crystals. Our results are the fi rst quantitative assessment of charge-transfer doping of organic semiconductors by silanes.