The world we live in today is largely driven by advancements made possible by research into electronic materials. The computer that you’re reading this from is a complex machine constructed with a legion of transistors carefully moving information throughout the device. Those transistors are made of a material called silicon, a valuable semiconductor.
From an electronic perspective, there are three types of materials: insulators, conductors, and semiconductors. Insulators are usually made up of strongly polarized ions. These ions don’t share their electrons with each other, so charge stays bound in one place and can’t carry a current. Conductors are the exact opposite: the atoms inside bind with each other in such a way that their electrons can travel from one atom to another, and current passes through with hardly any trouble.
As the name suggests, semiconductors are somewhere between insulators and conductors. They can carry current, but their ability to do so is strongly influenced by their chemical makeup. In silicon and similar substances, even minuscule impurities can alter the conductive behavior drastically by altering the material’s electronic states, which control how many charges can move around. This might sound troublesome for the people who process silicon for energy and electronic devices (and it certainly is!), but it also enables precise control of electronic properties within a single material. Through a process called doping, in which impurities are introduced in minute yet controlled amounts, engineers can use silicon alone as a supremely versatile substance, which is why it is the critical component of solar cells, transistors, thermoelectric devices, and a host of other sensitive technologies that have become critical to society in the 21st century. Unfortunately, doping can only be performed in the pre-processing stage of material creation. Once silicon is in a chip, its chemical makeup cannot be changed, so the properties endowed by doping—electrical conductivity , for instance—are static.
Finding materials that can switch their properties by a simple electronic input is of great importance, and researchers within the Catalyst Design for Decarbonization Center have observed this behavior in a most unexpected material. This research center, known also as CD4DC, is an EFRC bringing together scientists from universities and laboratories across the country to advance the use of modern materials in energy and catalysis. The Anderson lab focuses part of its research efforts at the University of Chicago on metal–organic frameworks (MOFs for short), which are prized for their molecular-scale porosity and chemical versatility. MOFs are also generally easy to make because of their similarity to traditional salts, which can be crystallized with little more than their constituent ions and some patience. A trade-off for easy synthesis is the characteristic electrical insulation that salts possess, so MOFs are rarely conductive. To tackle this issue, the Anderson lab attempts to build MOFs with metal–sulfur bonds, which are more similar to the bonds seen in semiconductors than the metal–oxygen or metal–nitrogen bonds present in most MOFs. With the right combination of molecules and synthetic techniques, these materials might demonstrate long-sought conductivity.
A recent study published by the Anderson lab reports a MOF made up of iron and a sulfur-containing organic bridge. Not only is this MOF a semiconductor; by exposing it to straightforward chemical treatment, its conductivity can be improved and switched between p-type and n-type (the two types of semiconductors). At face value, this doesn’t sound too different from how silicon is processed, but the difference lies in what exactly about the material is being changed. In silicon, different elements are introduced into the bulk material. In this case, the chemical treatment simply removes electrons from the organic bridges. Like removing cars from a crowded highway, electrons can move more liberally through the material after treatment.
Because only the electrons and a few exchangeable ions are affected by this treatment, it can theoretically be done within a device, and the range of electronic states accessible make it an impressive candidate for several applications. “The material’s…flexibility makes us hopeful that it can perform well in situations where multiple [electronic] equivalents are required,” says John Anderson, the principal investigator overseeing the research. During our discussion of his interest in the material, he explained that such applications could include lithium-ion batteries, where a wide range of electronic states accompanying ion storage are valued for optimal performance. The sequential and discrete electronic states demonstrated by the MOF also make it a unique contender for high-demand catalytic reactions—such as those used to synthesize sustainable liquid fuels—during which many electrons must be transferred in rapid sequence. Lei Wang, a postdoctoral scholar in the Anderson lab and project spearhead, is currently investigating the material’s performance in simple hydrogenation reactions to gauge its further potential.
Unfortunately, the chemical makeup that endows the MOF with its interesting properties also makes it challenging to synthesize and work with. “The challenge is the stability,” says Wang. “This material does not have the stability to moisture and oxygen compared to [traditional] MOFs.” Because of the tendency for this material and other sulfur-based MOFs to degrade in ambient air, the researchers in the Anderson lab rely on a procedural groundwork crucial for developing these sensitive materials for effective characterization. So while they are excited to see what else is in store for this interesting material, Anderson, Wang, and their colleagues will continue their efforts to create similar materials that can perform well in air and possibly be implemented into other electronic devices.
