The use of surface-initiated polymerization to reduce template feature size and facilitate fabrication of 3D freestanding nanostructures

We report the use of surface-initiated polymerization (SIP) to facilitate nanolithography-based template-directed synthesis of various 3D freestanding nanostructures (sub-10 nm) with arbitrary 3D anisotropic shapes and different compositions on the nano/ molecular scale in a designated way. In particular, SIP is combined with "top-down" nanolithographic techniques, such as electron-beam lithography, to fabricate nanotemplates with 3D recessed nanosites that possess both reduced feature size and anisotropic surface functionalities. Implementation of SIP can address the technical challenge for nanolithography to directly produce molecular-scale feature sizes, since the scale gap between what nanolithography can achieve and molecular feature sizes can be bridged, both in terms of size as well as chemical functionality, by the SIP-induced polymer layer. More significantly it enables a nanotemplate with tunable surface chemistry, leading to appropriate registration of nano-objects onto the template for molding as well as effective de-molding, resulting in 3D conformal freestanding synthetic nanostructures. In this work both 0-D polystyrene colloidal nanoparticles (NPs) and polystyrene-iron(III) oxide composites NPs are properly assembled and molded by the templates resulting in different nanostructures with desirable shapes and functionalities. The demonstrated 3D freestanding nanostructures (sub-10 nm) include "nanomushrooms," "nanobuttons," "nanospikes," "nanospaghetti," and "nanoflowers". In addition, the resultant PS-Fe3O4 composite nanostructures and their assemblies show ferromagnetic properties. Although the template-based surface synthesis produces low quantities compared to a traditional "bottom- up" chemical synthesis method, the templates allow for specific design of shapes and chemical functionalities of NPs to be directly designed, which may be advantageous in many applications such as drug delivery or imaging.