Short introduction to my research

Tue, Jun 2, 2015

This is a short and personal introduction to my research interests. I have tried to keep it non-technical and as brief as possible. I will be also posting some additional short articles explaining the key concepts in more detail as I go. Hopefully even with pictures!

I am a materials chemist. Materials chemistry, like all fields of research, does not have a firm dictionary definition. What I mean by a materials chemist, then, is a chemist who tries to make and understand chemicals that do things: that is, materials with interesting and unusual properties.

My PhD research focussed on two particular areas of materials chemistry and their intersection: structural disorder (and defects) and metal-organic frameworks (MOFs).

Disorder

A slice through a model of amorphous silicon - one of the iconic disordered materials.

Before talking about disordered materials it is necessary to clarify what I mean by order. The prototypical ordered material is a perfect crystal: a material built from a single repeat unit of atoms or molecules, stacked over and over in the same orientation. A crystal therefore is periodic, so just like with wallpaper shifting your position within a crystal a whole number of periods leaves you in an identical environment. Disordered materials are those that do not repeat perfectly. They range from solids with no periodicity at all (amorphous materials, like glass) to crystals which some element that is not fully ordered (e.g. ice, where the oxygen atoms are well-ordered but hydrogen atoms are disordered).

Disorder might sound like it should be detrimental for a material (and in some cases, it is!). But disordered materials are nevertheless very important:

Firstly, disorder is everywhere and unavoidable. Useful disordered materials include both old, such as concrete or glass, and new, such as those in lithium ion batteries or the “phase change chalcogenides” in non-volatile RAM.

Secondly, disorder can in fact be useful: that many technologically important materials are disordered is not a coincidence. The presence of disorder in a material implies that it can form a number of structures that are about as stable as each other. This means it often doesn’t require much energy input to switch between these structures. Battery electrode materials would be one example where the presence of many disordered, low-energy states is essential to the materials’ function.

Finally, by breaking the rules of conventional crystallography, disordered materials can show very strange properties. For example, magnetically disordered systems can contain magnetic monopoles - impossible outside of a crystal! ¹

Dealing with disorder is still tricky for materials chemists. This is partly because many of the tools we have to determine the positions of the atoms within a material, i.e. solve the structure, require ordered crystals. Part of my research has been to try to investigate whether it is possible to solve the structures of the most disordered materials, such as glasses, from experimental data.

The particular idea I am my coworkers introduced was ‘structural simplicity’. We were interested in seeing whether by trying to make models of disordered materials as simple as possible while still fitting the data can improve models of amorphous materials. We showed that simplicity can in fact drastically improve the quality of these models. These methods will therefore hopefully allow us to squeeze more information about the structures of these very important materials from experimental data, improving understanding of both their structures and properties.

Metal-organic frameworks

As well as investigating the fundamentals of structure in disordered materials I am interested in how disorder manifests itself in crystalline materials. In particular, I have been investigating metal-organic frameworks (MOFs). MOFs have only really been seriously studied for about twenty years ² but have become one of the most actively researched classes of materials.

MOFs are solid networks assembled from metal ions or cluster ’nodes’ linked together by organic molecular ’linkers’. [//]: # (-> show UiO-66 diagram) As the organic molecular linkers are much longer than the atoms that make up the links in conventional frameworks, MOFs can be very porous. This openness has been investigated for all kinds of applications, from carbon capture; to gas and liquid separation and catalysts and sensors. By swapping the organic linkers it is also possible to tune the properties of MOFs, in a way very difficult to do for other kinds of materials. MOFs are therefore a bit like molecular lego (or perhaps more accurately, K’nex) kits.

Left: structure of sodalite (SiO₂), a porous zeolite framework material. Right: structure of ZIF-8, a MOF (Zn(2-methylimidazolate)₂) with the same framework topology.

In conventional framework materials the importance disorder (and defects) has been long recognised. The role of disorder and defects in MOFs is comparatively poorly understood. In my PhD work I showed that the modularity of MOFs can be used to controllably introduce disordered defects into MOFs. I also showed, for the first time, that in some cases this disorder is ‘correlated’. These correlations are examples of ’local’ order within the disorder. [//]: # (->order within disorder diagram)

These disordered defects can also used to control the the physical properties of MOFs. By understanding disorder in MOFs better we will hopefully be able to design better materials - no doubt with a few surprises along the way.

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