PEI-SWCNTs in a concentration of 1 µg ml -1 were deposited on a Cr/Au (10 nm/200 nm) Si substate, to provide a conductive uniform substrate for the Kelvin probe measurements. Kelvin probe was performed by means of an Oxford Instruments Asylum research MFP-3D equipped with Asylum electrilever Ti/Ir tips (nominal resonant frequency =75 kHz, Spring Constant = 2 N m -1 and a nominal tip curvature radius of 30 nm) in a dual pass mode (NAP mode) at a second pass elevation of 50 nm. To compute the sample workfunction, the tip was carefully calibrated on a freshly cleaved Barnal-stacked HOPG sample (ZYA, NT-MDT) with a nominal worfunction of 4.6 eV and a uniformity of 1.8 mV rms , taken as measurement accuracy, using the same parameters.