Constant M. Guédon, Hennie Valkenier,Troels Markussen, Kristian S. Thygesen, Jan C. Hummelen, and Sense Jan van der Molen
Nature Nanotechnology 2012, doi:10.1038/nnano.2012.37
Link to Letter: http://www.nature.com/nnano/journal/vaop/ncurrent/full/nnano.2012.37.html
Link to News and Views:
http://www.nature.com/nnano/journal/vaop/ncurrent/full/nnano.2012.46.html Quantum interference (QI) effects in molecular junctions have been predicted theoretically and several potential applications within molecular electronics, switches, or thermoelectric devices have been suggested. However experimental evidence has so far only been indirect. In this paper a conducting atomic force microscope (C-AFM) is used to measure the conductance of a monolayer film of molecules on a gold substrate. The five molecules studied are shown in Fig. 1 together with a schematic picture of the C-AFM setup. One of these molecules (AQ-MT) show clear and direct evidence of destructive interference seen as a pronounced dip in the conductance.
 |
|
Figur 1: Panel a shows the chemical structure of the considered molecules. AQ-DT and AQ-MT are both cross-conjugated and are theoretically expected to exhibit QI in the HOMO-LUMO gap. The linear conjugated molecules are theoretically expected to have a high conductance. |
The measured conductance values are in good agreement with electron transport calculations based on density functional theory. Figure 2 shows some of the calculated transmission functions. The cross-conjugated molecules AQ-DT and AQ-MT have a clear dip in the transmission functions – a clear sign of destructive QI.
 |
Figur 2: Calculated electronic transmission functions. The cross-conjugated molecules AQ-DT and AQ-MT display a destructive QI feature seen as a sharp dip in the transmission. The linear conjugated molecule AC-DT has a transmission around the Fermi energy which is several orders of magnitude larger than AQ-DT and AQ-MT. |
Allowing for a small shift in the Fermi energy (which is difficult to calculate accurately) we obtain an excellent agreement between the measurements and the calculated conductance as shown in Figure 3.
 |
Figur 3: Measured conductance histogram. Light colors indicate the most frequently observed conductance. The purple line show the calculated conductance, when the Fermi energy is located at the transmission minimum. |