We present large-scale reproducible fabrication of multifunctional ultrasharp metallic structures on planar substrates with capabilities including magnetic field nanofocusing and plasmonic sensing. Objects with sharp tips such as wedges and pyramids made with noble metals have been extensively used for enhancing local electric fields via the lightning-rod effect or plasmonic nanofocusing. However, analogous nanofocusing of magnetic fields using sharp tips made with magnetic materials has not been widely realized. Reproducible fabrication of sharp tips with magnetic as well as noble metal layers on planar substrates can enable straightforward application of their material and shape-derived functionalities. We use a template-stripping method to produce plasmonic-shell-coated nickel wedge and pyramid arrays at the wafer-scale with tip radius of curvature close to 10 nm. We further explore the magnetic nanofocusing capabilities of these ultrasharp substrates, deriving analytical formulas and comparing the results with computer simulations. These structures exhibit nanoscale spatial control over the trapping of magnetic microbeads and nanoparticles in solution. Additionally, enhanced optical sensing of analytes by these plasmonic-shell-coated substrates is demonstrated using surface-enhanced Raman spectroscopy. These methods can guide the design and fabrication of novel devices with applications including nanoparticle manipulation, biosensing, and magnetoplasmonics.
Bibliographical noteFunding Information:
This work was supported by grants from the National Science Foundation (NSF CAREER Award for S.K., N.J.W., L.M.O., S.H.O.; NSF Graduate Research Fellowship for L.M.O.), Seagate Technology through the Center for Micromagnetics and Information Technologies (MINT) at the University of Minnesota (J.S., T.Q., S.H.O., R.H.V.), EPSRC (J.B.E., C.K.W., and N.J.L.), ERC (J.B.E.), and the MnDrive Initiative (N.J.W. and S.H.O). S.H.O. also acknowledges the NSF program: Research Opportunities in Europe for NSF CAREER awardees (DBI 1338654), which supported this collaboration between the University of Minnesota and Imperial College London. S.K. was supported by the Doctoral Dissertation Fellowship from the University of Minnesota. Device fabrication was performed at the Minnesota Nano Center at the University of Minnesota, which receives support from the NSF through the National Nanotechnology Coordinated Infrastructure. Computational modeling using COMSOL Multiphysics was performed through the University of Minnesota Supercomputing Institute.
- magnetic nanofocusing
- magnetic trapping
- template stripping