Abstract
Novel, optically responsive devices with a host of potential applications have been demonstrated by coupling carbon nanomaterials with photochromic molecules. For light-induced conductance switching in particular, we have recently shown that carbon nanotube-polymer nanocomposites containing azobenzene are very attractive and provide stable and non-degradable changes in conductivity over time at standard laboratory conditions. In these composites, the photoswitching mechanisms are based on light-induced changes in electronic properties and related to the Pool-Frenkel conduction mechanism. However, no link between conductivity switching and the molecular motion of azobenzene chromophores could be found due to application of high elastic modulus polymer matrices. Here we report on single wall carbon nanotube-polymer nanocomposites with a soft polycaprolactone polymer host. Such a system clearly shows the transfer of light-induced, nano-sized molecular motion to macroscopic thickness changes of the composite matrix. We demonstrate that these photomechanical effects can indeed overshadow the electronic effects in conductivity switching behavior and lead to a reversion of the conductivity switching direction near the percolation threshold.
Highlights
Combining photochromic molecules with carbon nanomaterials appears more promising[13]
We investigated multi-wall carbon nanotube-polymer composites, using the same azobenzene derivative, which showed the opposite switching direction above the percolation threshold and weaker overall performance as their single-wall carbon nanotubes (SWCNT) counterparts[25]
The abovementioned investigated systems of polymer composites with carbon nanotubes (CNTs) showed that the electronic effects dominate for photoswitching of conductivity and there was no evidence for participation of a geometrical effect
Summary
Combining photochromic molecules with carbon nanomaterials appears more promising[13]. The results of Mativetsky et al.[9] obtained for metal-molecule-metal junctions with azobenzenes suggest that large switching ratios could be achieved if one were able to transfer the geometrical effect to CNT polymer composite systems, i.e. if one could achieve significant changes in the tunneling gap separation in composites close to the percolation threshold. Whether this approach will be successful is a very interesting question, since the geometrical effect must compete with the other abovementioned mechanisms of light-induced conductance switching found in CNT-based systems so far. Polycaprolactone (PCL) was chosen as a host matrix because it has a low bulk elastic modulus of 300 to 500 MPa26, 27, it is soluble in common organic CNT solvents, and it is miscible with the present azobenzene derivative and does not show phase separation
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