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.

Major applications

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.