Point contact current–voltage measurements on individual organic semiconductor grains by conducting probe atomic force microscopy

Conducting probe atomic force microscopy (CP-AFM) was used to make point contact current–voltage (I–V) measurements on individual microscopic grains of the organic semiconductor sexithiophene (6T). The 6T grains ranged from 1 to 6 molecules (2–14 nm) in thickness, 1–2 μm in length and width, and were deposited by thermal evaporation onto SiO2 substrates previously patterned with 200 nm wide Au wires. Au-coated AFM probes were used to image the substrates in air to identify individual 6T grains which grew in contact with a wire. The same probes were used to record the I–V characteristics of single grains. Analysis of the differential resistance as a function of probe wire separation yielded typical grain resistivities of 100 Ω cm and contact resistances of ∼100 MΩ. Over the 0–3 V range probed, the shape of the I–V curves can be attributed to a combination of the nonlinear I–V characteristics of the Au-6T junctions and the ohmic response of the grain. In general, we have shown that CP-AFM is a reliable method for correlating electrical transport properties with microscopic morphology in organic semiconductors.