The substrate of choice turned out to be vertically arranged, caespitose, densely packed carbon nanotubes (CNT) that guarantee this high density of 'hot spots'. The group developed techniques to grow dense forests of CNTs in a uniform and controlled manner. The availability of this expertise was one of the principal motivations for using nanotubes as the basis for highly sensitive SERS sensors, says Park.
A spaghetti-like surface
The tips of the CNTs are sharply curved, and the researchers coated these tips with gold and hafnium dioxide, a dielectric insulating material. The point of contact between the surface of the sensor and the sample thus resembles a plate of spaghetti topped with sauce. However, between the strands of spaghetti, there are numerous randomly arranged holes that let through scattered light, and the many points of contact -- the 'hot spots' -- amplify the signals.
"One method of making highly sensitive SERS sensors is to take advantage of the contact points of metal nanowires," explains Park. The nano-spaghetti structure with metal-coated CNT tips is perfect for maximising the density of these contact points.
Indeed, Bond explains, the wide distribution of metallic nano-crevices in the nanometre range, well recognised to be responsible for extreme electromagnetic enhancement (or hot spots) and highly pursued by many research groups, has been easily and readily achieved by the team, resulting in the intense and reproducible enhancements.
The sensor differs from other comparable ultra-sensitive SERS sensors not only in terms of its structure, but also because of its relatively inexpen
|Contact: Hyung Gyu Park|