Light. It gives us the ability to see and experience our surroundings, to observe and analyse objects and phenomena, and it is crucial to many forms of life. Microscopes can help us to see things too small for the human eye, however, below a certain size, some objects are so small that they are invisible to regular white light and can’t be studied without particular tools. An electron microscope is like a regular microscope, however it uses a beam of electrons, instead of light, to image very small objects.
Light behaves as a wave, which puts a lower limit on the size of object it can be used to study. If the wavelength of light is larger than the object you’re trying to study, it will go right by without interacting with the object, meaning this can’t be seen. Electrons also behave as waves, however as these have a much smaller wavelength, they can be used to image much smaller objects, all the way down to the atomic level.
This technique can therefore be used to study a wide range of nanoscale materials, that is, objects which have at least one dimension at the nanoscale. A nanometre (nm) is one billionth of a meter. A rough comparison is if a marble were 1 nm in diameter, a metre would be roughly equivalent to the diameter of Earth. Materials like metals, plastics and minerals often behave very differently at the nanoscale to their regular, larger, formats, which we tend to be more familiar with.
Gold nanoparticles, small sphere-like objects made of gold, are roughly 100 nm in diameter. These are represented in Figure 1. When two of these are placed close together and white light is shone on them, an effect called a plasmon is produced. A plasmon refers to a collective wave of electrons forming and moving together at the surface of a material, like a wave across the surface of an ocean. When a plasmon is produced in the small gap between two gold nanoparticles, the energy produced by the electron wave can trap the light shone on the nanoparticles. Picture a magnifying glass held so that it catches and concentrates sunlight. This concentration of light is also a concentration of energy, and can be used to encourage reactions, like when this process can be used to start a fire or burn a sheet of paper. Trapping light in this nanogap between the nanoparticles has a similar effect, where the energy in this region is increased and can be used to improve chemical reactions or perform processes like data transfer, storage, and environmental and biological sensing.
In order to optimise and create predictable nanoparticle and light interactions, it is important to understand how the gold nanoparticles are interacting with each other, what the nanogap looks like, and whether the material undergoes any changes when plasmons are produced and studied. Electron microscopy can be used to study this, as the electron beam can be used to visualise the nanoparticle interactions and nanogap geometry and locations, which will inform how these can be better synthesised and implemented, which may have a significant positive environmental and industrial impact.
NanoDTC PhD Student, c2022