Elasticity of two-photon-fabricated nano-wires

Abstract

We have found that a polymer nano-wire with a radius of around 100 nm has a shear modulus three order of magnitude smaller than that measured from the bulk of the same material. This large difference in shear modulus indicates that elasticity of nano-scale structures is not scalable from bulk material. To understand the elasticity of polymer nano-scale structures, we evaluated influence of fabrication conditions on elasticity of photopolymer nano-wires. We fabricated a nano-wire by two-photon polymerization into a shape of a coil spring: geometry capable to magnify mechanical deformations. We stretched the spring by laser trapping, and from force-strain relation we calculated the shear modulus of the polymer wire. When the laser power for fabrication increased by 20 % from polymerization threshold, the shear modulus increased from 0.39 MPa and saturated at 0.77 MPa. We used Raman spectroscopy to investigate the progress of polymerization reaction for each laser power. From the Raman spectra, the cause of lower shear modulus in the nano-wire fabricated by lower laser power was remarked as inadequate polymerization process. Additional UV irradiation hardened the nano-wire fabricated by lower laser power¹. The result demonstrates that the combinative use of two-photon fabrication and subsequent UV irradiation makes it possible to control both the spatial resolution in two-photon fabrication and elasticity of polymer structures.