The colour of a material is often determined by the types of bonds and geometry in which the atoms in the material are arranged. As a result, measuring the colour of a material means gaining insight into how the material is put together. This information in turn determines how the material can be used. Even in glasses, a broad class of materials that are characteristically ‘jumbled up’ and lacking long-range order, the colour of the glass reflects how atoms near each other are arranged and bonded to one another.

Recent progress in the development and processing of glasses made from metal-organic frameworks (MOFs) has led to new glasses that are mixed on the nanoscale, far smaller than can be seen or measured with conventional techniques. In this work, a cobalt-based MOF that is typically purple in colour was blended together with a zinc-based MOF which gives a clear glass when prepared in isolation. Because of the small size of ‘islands’ of the either cobalt- or zinc-based MOF in the combined glass, Sean Collins, Louis Longley, Tom Bennett, and Paul Midgley worked with collaborators at the SuperSTEM and Rutherford Appleton laboratories to use electron beams to ‘see’ the spectroscopic signatures—that is, the colour—of the domains of cobalt- and zinc-based MOFs within the glass. 

This advance in seeing colour at the nanoscale in MOF glasses demonstrates that electron beams (in the form of electron energy loss spectroscopy or EELS) can be used to do the kind of measurements typically done in a materials chemistry lab—but now in just about 100 nanometers of material. This progress will allow researchers to better understand the details of how MOF glasses interact with light but also how the atoms are organized within the glass or when and how it can be expected to break.

Figure: Nanoscale spectroscopy in the electron microscope reveals the local colour and bonding geometry in a mixed cobalt- and zinc-based metal-organic framework glass.