Chemical sensing applications utilizing surface enhanced Raman spectroscopy (SERS) have drawn significant
attention recently. However, developing a reliable, high performance SERS platform remains a challenge. A novel
SERS substrate based on nanofingers was successfully demonstrated to provide large enhancement reliably and
showed great promise for practical applications. Capillary forces bring the gold caps on the nanofingers into close
proximity upon exposure to a solution containing molecules of interest, trapping molecules within the gaps and
producing greatly enhanced Raman signals. Transmission electron microscopy (TEM) was used to characterize the
structure of the nanofingers, in particular the gaps between finger tips to improve the fundamental understanding of
the structural-performance relationship.
Nanosphere lithography is an effective technique for high throughput fabrication of well-ordered patterns, but
expanding the method to large area coverage of nanoparticles less than 300 nm in diameter while maintaining good order
has proven challenging. Here we demonstrate a nanosphere lithography based technique for fabricating large area, wellordered
arrays of hemispherical metal particles which pushes the limits of these size constraints. First, large area
monolayers of polystyrene (PS) nanospheres are assembled at an air-water interface and then transferred to a submerged
substrate. The submerged substrate is supported at a 10° angle so that the water draining speed can be used to control the
transfer rate, which is essential for hydrophobic substrates such as the polymer-coated glass used in our work.
A double liftoff procedure was used to transfer the PS pattern to a silver particle array on an arbitrary substrate,
achieving tunable control over the final metal particle diameter and spacing in the range of 50-150 nm and 100-200 nm,
respectively. Additional control over particle shape and diameter can be obtained by modifying the substrate surface
energy. For example, depositing silver on ITO-coated glass rather than a more hydrophilic clean glass substrate leads to
a more hemispherical particle shape and a diameter reduction of 20%. Peak wavelength-selective reflection greater than
70% and total extinction greater than 90% were measured. The intensity, position and bandwidth of the main plasmon
resonance of the arrays were shown to have minimal angle dependence up to at least 30° off normal.