Coal properties find their way into semiconductor devices

A team of researchers at Massachusetts Institute of Technology (MIT) has begun experimentation with using un-purified materials for semiconductors such as coal. Most semiconductors nowadays are made from refined materials, such as silicon, which require several expensive purification steps to become suitable for fabricating devices.

Coal contains mixtures of carbon, hydrogen, sulfur, oxygen and nitrogen in various proportions depending on the type of coal. Researchers from MIT's department of materials science and engineering are looking for applications suitable for thin films that they've made from slurries of types of coal found in nature, rather than adapting the coal's formulae to suit the application. So far the MIT researchers have made a solid-state heater, suitable for car windshields, defrosting airplane wings, or for use in biomedical implants.

Figure 1: The chemical components of coal make it a good candidate for many types of electronic components. (Source: MIT)

"With this work [on heaters] we are barely tapping the potential of coal as an electronic material," said research scientist Nicola Ferralis. He performed the work with professor Jeffrey Grossman and Brent Keller, a doctoral student. The researchers are determining the relation between coal chemistry and band gap. "The goal is to enable the use of different types of coal for the creation of different types of junction devices," said Ferralis.

The ultimate goal, said Ferralis, is the design and implementation of transistors as well as solar cells.

"This is not as far-fetched as it may seem. We have created in the past all-carbon solar cells made with synthetic carbon nanomaterials: the move to coal is a 'natural' step forward, as coal shares many of the same nanostructural and chemical properties of pure carbon structures," Ferralis noted.

Don't underestimate stability

One of the biggest advantages of coal over organic semiconductors, is that they are stable under normal atmospheric conditions. Organic semiconductors, on the other hand must be protected from the humidity and oxygen in the atmosphere with special films or glass encasement.

Figure 2: A simple heating device made by the researchers from unrefined pulverised coal, shown at left under visible light and at right in infrared light. (Source: MIT)

"This is a great advantage of highly aromatic/graphitic materials. Our devices can run unaltered at very high temperature in air for extended periods of times. We do have experience in similar and more complex devices (like solar cells) made out of nanomaterials with similar composition of aromatic carbon, and their stability in air (without any encapsulation) is excellent. The simple fabrication and the lack of encapsulation makes device fabrication simple and potentially extremely cheap," Ferralis stated.

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