Electron Transport Through Molecules

Transport of low energy electrons in organic materials has a wide range of possible applications in sensoric or molecular electronic technologies. There are also important implications for the understanding of radiation damage in biological tissue.

We are studying electron transport processes through biological molecules (DNA, peptides) with scanning tunnelling microscopy (STM) under controlled humidity and temperature. Effects of molecular linker groups, polymeric sequence, molecular orientation, structure and conformation and also the concentration of counter ions in an aqueous environment can affect electron transport properties. A study of electron transport through DNA containing deoxyuridine with chemically enhanced donor properties was published in Ref. 1. The strands are bound to an Au surface with a thiol group and organized in a self assembled monolayer as shown schematically in figure 1.

Figure 1. Schematic STM situation with a self assembled monolayer of short double stranded DNA.

The length of the DNA strands makes an obvious difference in such experiments. Compared to the rigid double stranded dsDNA (persistence length ≈ 50 nm) of figure 1, single stranded ssDNA (persistence length ≈ 4 nm) of 50 bases are more bent and entangled to some degree due to a higher flexibility and length. Figure 2 shows results from current / voltage measurements on such ssDNA surrounded by H2O molecules at full surface coverage in comparison with a water film on a clean Au(111) surface. The distributions of slopes of IV curves between ± 15 mV were obtained with more than 1000 scans. Obviously there is lower conductivity through the DNA molecules than through water on clean Au. A strong peak at around slope "0" can be related to the presence of a DNA bandgap, which was not observed with 18 base or shorter strands.

Figure 2. Comparison of slope distributions through 0 V from current voltage measurements.

Figure 3 shows a sample with lithographically produced Au dots on an oxidized Ti surface. Charge transfer properties of the two marked dots were measured with current-voltage scans and averaged with the ± 3σ standard deviation given for the lowest voltages of t he scans. Conductivity through the dots can vary significantly and the oxide layer underneath causes a conductivity gap of about ± 500 mV but a clear reduction of conductivity at higher potentials is observed when the dots are covered with 50 base ssDNA.

With a particular interest in the development of advanced spectroscopic methods, we are using home built instrumentation and controller software, which are adjustable to special experimental requirements and new modes of data acquisition and processing. Small currents of about 0.5 pA up to the milliamp region can be measured with excellent accuracy. We investigate topographic structures with sub nanometer resolution and distance / current or voltage / current characteristics in combination with chemical or geometrical sample parameters. A special feature of our microscope is the optical access to the tunnelling gap from underneath using semitransparent samples, which allows us to carry out accurate positioning of nano structured surfaces under the probe tip and study effects from optical excitation in an evanescent field.

Figure 3. Lithographically produced Au dots on TiOx with current-voltage measurements with and without 50 base ssDNA.