3D periodic composite nanopatterns with superior mechanical properties: the effect of nanoparticles on pattern contrast and mechanical properties†
Three-dimensional (3D) periodic nanopatterns have gained much interest due to their potential applications, ranging from photonics to biological tissue engineering. Here, 3D silica nanoparticle/SU8 composite patterns were fabricated by holographic lithography. Although a uniform composite photoresist film was obtained by mixing epoxy-functionalized silica nanoparticles, the unavoidable scattering by the silica nanoparticles sufficiently changed the photosensitivity of the photoresists, resulting in the decrease of pattern contrast. With careful optimization of the light exposure condition, as well as increased polymer rigidity due to uniform dispersion of the nanoparticles in the polymer matrix, we were able to obtain high contrast 3D nanopatterns with up to 5 wt% silica content. We characterized the mechanical properties by the nanoindentation technique. The incorporation of silica nanoparticles remarkably improved the mechanical stability of the resulting 3D patterns. The mechanical properties of the composite nanopatterns displayed a Young's modulus of 3.8 GPa and a hardness of 0.05 GPa. Although the 3D patterns have a pore volume of around 50%, these mechanical properties are similar to those previously reported for the bulk SU8 films.