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Time to put on your 3D glasses

Sean Collins on how electron microscopy reveals optical physics of “nano” gold and silver in 3D.

A diverse range of technologies today rely on tiny light-driven “nano-optical” devices for everything from the detection of trace chemical agents, to sensors for biomedical devices, light-trapping structures for solar cells and novel photo-catalysts for the chemical industries. Many of these applications rely on physics of metals like gold and silver. These metals are unique in that when they are structured at the “nano”-scale (about 1 million times smaller than a dot made by a pencil) they concentrate and scatter visible light in special ways.

The exact colour of the light particles that such “nano” gold and silver particles absorb and scatter depends on special resonance conditions determined by the size, shape and chemical environment around the particles. But because these particles are so small, typically many times smaller than the wavelength of the light they manipulate, conventional light microscopy cannot “see” the resonance modes. Moreover, because of the physics of how light scatters, conventional light microscopes cannot resolve objects less than about half the wavelength of visible light.

Instead, using beams of electrons, in the past 10 years researchers have been able to image these particles using a technique called electron energy loss spectroscopy (EELS). This technique, however, has been limited because it has been possible only to form two-dimensional images of the resonance modes when in fact they are physically present in three dimensions (3D). The interpretation of EELS images has also been complicated by difficulties in quantitatively measuring optical properties from signals generated by electrons rather than photons.

In work published in ACS Nano this month, Steve Barrow and Paul Midgley at Cambridge, Paul Mulvaney at the University of Melbourne (Australia), Alison Funston at Monash University (Australia), David Rossouw and Gianluigi Botton at McMaster University (Canada) and I have used EELS to precisely determine the symmetry of the resonance modes of different 2D and 3D arrangements of three- and four-particle aggregates of “nano” gold particles. These particle ensembles are of particular interest for special optical effects that are possible when several particles are coupled together. In this case, the particles were self-assembled using DNA to bind them to each other.

The modes can be thought of as analogous to the resonances a violinist achieves by defining the length of a string with the position of a finger. For a particular geometry of the gold particles, the analogous “finger” is placed in a different position and each particle assembly has a different “melody” – or in the case of light, a different set of colours that it absorbs and scatters in a specifically defined way in 3D space. For these gold and silver particles, the resonating modes can be described in terms of charges on the surface of the particles, so called “surface plasmons” or standing waves of surface charge (rather than the standing waves of a violin string). By taking EELS images and comparing these experimental measurements with detailed simulations of the underlying surface charges, we effectively noted down the unique melodies of different possible combinations of three or four particles of gold. By repeating the measurements for multiple different arrangements of the particles, we were then able to work out the systematic variations present in the recorded patterns.

This work builds closely on previous work published last year in ACS Photonics, showing the first direct experimental 3D measurement of these critical mode-specific surface charges for a single silver particle.

The measurement of 3D resonance modes in gold and silver particles is critical for understanding the physics that supports the optical technologies based on such structures and for opening up new areas for designing novel optical effects in the smallest nanoscale volumes possible.

*Dr Sean Collins completed his PhD as a Gates Cambridge Scholar (2012) in March 2016. Picture credit: Reprinted with permission from S.J. Barrow, S.M. Collins, D. Rossouw, et al. ACS Nano, 2016, ASAP. DOI: 10.1021/acsnano.6b03796. Copyright 2016 American Chemical Society.