Gold nanodots are stable, both physically and chemically, and are biocompatible. They also very efficiently absorb light thanks to plasmon resonance (the collective motion of conduction electrons). Light fields are enhanced when they are resonant with these plasmons, and as such, the dots are now being extensively used in optical biosensing that is based on strong surface plasmon resonance. They are also good for making electrical contacts in nanoscale devices and as catalysts for assembling quasi-one dimensional nanostructures.

Researchers generally synthesize gold nanoparticles in solution, but positioning these particles on a chip remains challenging. Conventional nanopatterning like electron beam lithography and extreme ultraviolet interference are good techniques but they are expensive. Although low-cost alternatives such as nanoimprint and nanosphere lithography exist, they are severely limited because they rely on complicated resists and lift-off processing.

The new technique developed by Ion Tiginyanu and Eduard Monaico from the Academy of Sciences and Technical University of Moldova, and Kornelius Nielsch of Hamburg University is much simpler and involves electroplating the gold nanodots on the porous semiconductors indium phosphide and gallium phosphide using a pulsed voltage. The dots initially nucleate on the semiconductor surfaces and then grow until they reach around 20 nm in size. The pulsed electrodeposition continues to create new nanodots, and the density of the dots produced depends on the number and width of the applied cathodic voltage pulses.

The researchers observed the gold nanodots uniformly deposit as a monolayer on the entire surface of the semiconductors. According to a model they have developed to explain this effect, a Schottky barrier emerges at the metal-semiconductor interface as soon as the dots reach a threshold size. “Since the barrier potential is oriented in the opposite direction to that of the applied cathodic voltage, we can assume that the modified local potential stops electrodeposition within the area of a dot that has reached the threshold size,” explains Tiginyanu. “At the same time, to keep the process running, new dots continue to nucleate and grow.”

"We can imagine the electrodeposition as a ‘hopping’ process, that is, electroplating ‘jumps’ to other areas on the semiconductor as soon as one or more dots reach a certain threshold diameter,” he tells nanotechweb.org. “The hopping process continues until the entire surface of the semiconductor is covered by a monolayer of self-assembled gold nanodots.”

Even though the semiconductor surfaces are highly porous, the gold nanodots distribute themselves rather uniformly over them, he says. “Despite the changing morphology, that is, alternating buried pores and semiconducting walls, the dots nucleate and grow along the entire top surface.”

According to the team, electroplating the semiconductor surfaces for 10 seconds with pulses lasting just 100 microseconds is enough to cover practically the whole surface with a monolayer of gold nanodots.

“Our approach could widen the potential applications for metal nanodots,” says Tiginyanu. “For example, they might be used to make plasmonic photonic crystals, optoelectronic on-chip interconnects, and chemical and biological sensors based on surface enhanced Raman scattering – a technique that exploits the interactions between light and matter by taking advantage of surface plasmons.”

The research is detailed in ECS Electrochemistry Letters doi: 10.1149/2.0041504eel.