Apr 5, 2011
Molecular strain sensing based on quantum charge transport
Scientists based in the Netherlands have developed a technique to study the resistance of single molecules while straining them. The tunable platform can be thought of as an extremely sensitive strain sensor. Its electrical resistance is basically governed by quantum tunneling and tiny variations in distance (picometres) lead to large changes in resistance.
The group's devices consist of a flexible substrate on top of which gold nanoparticles are self-assembled into a two-dimensional triangular lattice. The nanoparticles are protected by a shell of alkanethiols, which fixes the interparticle distance. In this way, stable room-temperature devices are created. Next, the researchers place a sample in a three-point bending set-up and measure its resistance (see image). When the substrate is bent, the nanoparticle network is strained, leading to a tiny increase in interparticle distance (&symp;30 picometers for 1 mm pushing rod displacement). The resulting change in resistance, however, is easily detectable (see thumbnail - plot [a]).
As a next step, the team inserted conjugated OPE3-dithiol molecules into the network. These form molecular bridges between neighbouring nanoparticles, leading to a large decrease in device resistance. Now, when the same straining experiment is performed, the normalized response, or equivalently the sensor's sensitivity, was observed to decrease by a factor of six (see thumbnail - plot [b]). This interesting modification results from the profoundly different conductance properties of conjugated molecules and alkanes.
First, the platform may be applied as an extremely sensitive strain sensor. For example, replacing the laser deflection system on an AFM in order to perform experiments that are sensitive to light. By using different molecular species, the sensitivity of the strain sensor can be tuned at will. For the team, however, the projected application is more fundamental. Here, the new tool paves the way for charge transport studies of strain-sensitive, switchable molecules.
The researchers presented their work in the journal Nanotechnology.
About the author
Constant Guédon is a PhD student in the group of Sense Jan van der Molen at the Kamerlingh Onnes Laboratory, Leiden University, the Netherlands. The group's key interest is in charge transport through the smallest systems imaginable: nanoparticles, molecules and even atoms. We thank our co-authors who synthesized the OPE3-molecules (E H Valkenier, J C Hummelen) and helped with the experiments (J Zonneveld).