Modified Mirror Reaction Metals Apertureless Probes

Daniel S. Burgess

A research team from the University of Leeds in the UK, Riken research institute in Wako, Japan, and Peking University has demonstrated that a variant of the familiar mirror reaction can deposit silver on the tip of atomic force microscope (AFM) probes, making them suitable for use in apertureless scanning near-field optical microscopy. Experiments confirm that such modified probes can collect near-field Raman images with spatial resolutions of 24 nm.

In the technique, a silicon AFM probe is dipped into an [Ag(NH₃)₂]⁺ solution, forming a silver particle at the apex of the tip. The size of the particle is dependent on the length of time that the probe is immersed in the solution — on the order of minutes. The mechanisms responsible for the preferential deposition of the silver at the tip are unclear.

Tests with nearly uncoated tips and those with 80-nm-diameter or 1.2-μm-diameter particles confirmed Raman enhancement at an excitation wavelength of 488 nm. A tip that features an 80-nm particle, which required an immersion time of six minutes, produced high-resolution tip-enhanced Raman images of single-walled carbon nanotubes.

The researchers note that other techniques can produce such probes, but they deposit silver particles that are too small for Raman enhancement. Annealing increases the size of the particles, but it can deform the tips. The chemical process does not suffer these limitations.

Applied Physics Letters, June 27, 2005, 263111.