INVESTIGATION OF CHARGE-TRANSPORT IN THIN, DOPED SEXITHIOPHENE CRYSTALS BY CONDUCTING PROBE ATOMIC-FORCE MICROSCOPY

Conducting probe atomic force microscopy (CPAFM) was used to measure t he electrical transport characteristics of 2-14 nm thick doped crystal lites of the organic semiconductor sexithiophene (6T) grown on Au and SiO2 substrates by vacuum sublimation. To make the measurements, an AF M was modified to allow in situ switching from tapping mode imaging to point contact electrical characterization with an Au-coated tip. The crystals were characterized structurally by molecular contrast AFM ima ging and consist of layers of 6T molecules oriented with their long ax es nearly perpendicular to the substrate. For crystals grown on Au sub strates, transport is probed through the thickness of the crystals (i. e., the vertical direction) using a CPAFM tip and the substrate as ele ctrical contacts. On SiO2 substrates, transport is measured parallel t o the substrate between the CPAFM tip and a nanofabricated Au electrod e in contact with the crystallite. The measurements on Au reveal an un expected dependence of the conductance on crystallite thickness, namel y that conductance is greatest for crystals that are three 6T layers t hick, not one layer. Both the vertical and horizontal conductance meas urements show nonohmic behavior which may arise from an energy barrier to charge injection at the Au-6T interface. The reproducibility of th e CPAFM methodology for probing transport in these extremely thin orga nic crystals and the observation of nonohmic behavior underscore the i mportance of nanoscale transport measurements afforded by CPAFM.