Metal nanostructures are known to induce absorption or scattering through localized surface plasmon resonance (LSPR) but their resonance wavelength is mostly limited to the visible spectrum thereby limiting scope of plasmonic applications. Metal nanostructures can be designed to have infrared resonance, but this requires the use of complex shapes such as nanoholes or nanoshells and large particles (>100 nm), which has slowed the development of infrared plasmonics. Applications such as surface-enhanced infrared absorption (SEIRA), a spectroscopy technique used for applications ranging from studies of catalytic processes to molecular sensing, are limited currently by the achievable concentration of infrared light into highly localized near fields. In our work, we have been developing a new class of plasmonic nanomaterials that are doped metal oxide semiconductor nanocrystals (<20nm) with which we can enrich the possibilities for infrared plasmonics and even introduce dynamic tunability. Unlike nanofabricated complex patterns, these NCs are simple to fabricate on a large scale using solution phase synthesis.
High near field enhancement has been a key to many applications of plasmonic nanostructures, yet the potential for plasmonic field enhancement by semiconductor nanocrystals has been uncertain. We use the discrete dipole approximation (DDA) to predict the field enhancement around single nanocrystals and dimers and to suggest optimal nanocrystal morphologies. Indium-doped cadmium oxide is considered as a prototypical material. The computed extinction spectra were sensitive to changes in doping level, suggesting wide-ranging opportunities for on-device tunability of the infrared LSPR. We have shown that with this class material we can achieve high scattering efficiencies and efficient local heat production with 100 nm particles making them suitable for photothermal therapies and simultaneous bio imaging. For applications based on near field enhancement like SEIRA, single particles and dimers of nanocrystals demonstrate strong shape- and wavelength-dependent near field enhancement.