“Reconstructable mask lithography (RML) is the first method able to create plasmonic heterogeneous nanoparticle oligomers within a volume the size of a virus particle as well as produce these nanostructures over large areas in ordered arrays,” explains team leader Teri Odom. “Being able to tune each metal nanoparticle in an oligomer will allow us to optimize nanostructure architectures for a wide range of plasmon-enhanced applications.”

Metallic nanostructures can be used to make sophisticated nanoscale devices, especially high-density information storage and optical interconnects. The optical properties of these nanomaterials are directly related to so-called localized surface plasmon resonances that depend on collective excitations of conduction electrons at the metal nanoparticle surface.

The electromagnetic field surrounding a metallic nanoparticle that has been excited by light at frequencies near the plasmon resonance extends only over short distances. Thus, similar to how atoms need to be close to form molecules, metal nanoparticles need to be situated within tens of nanometres from each other to form new plasmonic nanoclusters, or oligomers (such as dimers, trimers, tetramers and so on). Each type of cluster has its own unique set of localized surface plasmon modes.

Model predicts best nanoclusters

Odom’s team has developed a model to predict the best such nanoclusters for specific applications – in this case hydrogen gas sensing. The researchers then use RML to actually make these structures.

“RML combines two techniques to create plasmonic nanoparticle oligomers: photolithography and tilted metal evaporation,” Odom tells nanotechweb.org. “Key to this lithography process is the use of removable and additive metal deposition masks.”

The researchers began by using high-angle Cr deposition to construct a temporary mask containing slit-like features atop a Ti hole array on an etched Si template. This Cr mask determines how big the final plasmonic structures will be and where they will be positioned in the assembly. The Cr slit mask can be removed and rebuilt between each series of plasmonic nanoparticle deposition steps.

Hydrogen sensing – but not only

“Since we can design each nanoparticle within an assembly independently, we are able to construct a wide range of different hetero-oligomer arrays,” says Odom.

The team used its large-area (cm2) oligomer arrays, containing gold and palladium nanoparticles, to detect hydrogen gas. When the Pd nanoparticles incorporate hydrogen, their dielectric properties as well as their lattice constant change. Such changes can be measured by analysing the light transmitted through the structures. The structures were able to sense both low (2%) and high (100%) concentrations of hydrogen – which is promising for applications in safety and fuel-cell industries, adds Odom.

“Hydrogen sensing is only one application that we used to highlight the potential of our fabrication strategy,” she says. “In future work, we will be looking at using plasmonic hetero-oligomers to observe and monitor chemical reactions at the nanoscale.”

The current work is detailed in ACS Nano.