Scientists at the Research Institute for Electronic Science at Hokkaido University have demonstrated a way to enhance the photoluminescence (PL) emission of gold by fabricating well defined nanostructure arrays of the metal.1

Metals are not usually strong light emitters but emission intensity can be enhanced by interaction with localised collective electronic excitations, known as surface plasmons. Plasmonic modes are strongly localised in nanoparticles and promote interband excitation, which has been reported to yield strong luminescence from nanoparticles of gold and silver.

Hiroaki Misawa and colleagues used electron beam lithography to fabricate square blocks about 100 nm square and 40 nm high, and varied the gaps between adjacent blocks. “Use of a top-down technique instead of self assembly enabled us to prepare large arrays of blocks having nearly identical size and orientation, and yielded high plasmonic field enhancement, which determines the excitation efficiency,” says Misawa. The scientists varied the gap size in order to control the localisation of the surface plasmon modes and consequently the intensity of luminescence from the gold nanostructures.

The researchers investigated pairs of blocks aligned along one of the diagonals and found that for large gaps, the intensity of PL in the visible range was as weak as that from smooth gold surfaces. However, the emission was enhanced by several orders of magnitude when the size of gap was decreased so as to approach zero.

Fig. 1: Gold nanoblocks in displayed in a checkerboard configuration.

To further validate the effect observed in the pairs of blocks, the researchers fabricated structures in which the nanoblocks were arranged in checkerboard configurations (Fig. 1). Again, the PL emission was strongly enhanced when the gap between corners of adjacent blocks was below 14 nm but emission was almost undetectable for larger gaps.

“These results demonstrate a nano-engineered metallic light-emitting material whose emission efficiency is controllable via structural tailoring, in this case by varying the width of the nanogap,” says Misawa. “Such structures can be used in applications related to high-resolution fluorescence imaging and single-molecule fluorescence quenching spectroscopy, which so far have relied on chemicallytailored organic molecules as light emitters.”