Paper of the Week for May 30th, 2018

May 30, 2018

The paper can be found here: Extraordinary plasticity of an inorganic semiconductor in darkness

*Disclaimer: I am far from a materials scientist. In fact, I’m probably closer to a layperson when it comes to inorganic materials and semiconductors. Comments and corrections are welcome. I’m always looking to learn!

Welcome to Paper of the Week!

Every week, I’m going to pick a paper that caught my eye (usually from Twitter if I’m being honest) and tell you about it!

This week’s paper is titled “Extraordinary plasticity of an inorganic semiconductor in darkness,” and it comes from a lab in the Department of Materials Physics, Nagoya University in Japan. This is definitely the first time I’ve ever seen the term “extraordinary” used in the title of a journal article, so this paper would have been a serious contender for that alone.

Let’s start from the basics. As you may have already noticed, some materials conduct electricity and some do not. Whether or not a particular material conducts electricity depends on the quantity of free electrons and holes (locations in the material with no electrons) in the material. Glass, which is an insulator, does not have free electrons, and therefore does not conduct electricity.

Thermal energy, or heat, can change the number of free electrons and holes present in a material, as can light irradiation. Increasing the number of free electrons and holes decreases the resistance of the material. If an electrical field is applied to this material (for example, by creating a voltage difference across the material), the holes and free electrons will become excited, and the material will become electrically conductive relative to the number of free electrons and holes. Semiconductors are these such materials, where some of the electrons are bound in covalent bonds, and some move freely within the material.

This is where darkness and plasticity come in. The number of free electrons and holes present in a material depends somewhat on the material’s crystalline structure, and that structure changes when stress or strain are induced into that material. Therefore, the properties of the crystalline structure of the material, including plasticity, can be altered with applied energy, in the form of thermal energy or light irradiation. In theory, you could have a material that becomes less conductive when bent, only for that reduction to be counteracted by light irradiation of the material.

But what about the absence of light? This paper found that a zinc sulfide crystal, when placed in total darkness, became significantly more plastic, or deformable. Crystals that were deformed under either LED or UV light reached roughly 2% strain before yielding, whereas crystals deformed in total darkness reached 45% strain. That’s a crazy amount of deformation for a crystal that could barely withstand stress without breaking in normal light!

We talked earlier about how light irradiation of a material could increase the number of free electrons and holes, so I would expect that while the total darkness might make the crystals more deformable, it would also reduce the energy of the free electrons and holes. Interestingly (to me, a person who does not work in this field), that’s not quite what happens. In the next section of the paper, which looks at the light-absorption characteristics of the material, the authors note that the optical band gap (or the amount of energy required for an electron to leave the covalent bond of the crystal structure and become free) decreases at 35% strain. While they did not show any data on conductivity tests between light-deformed and darkness-deformed crystal samples, I would think that this means that the conductance is similar between the two crystals, because of the reduction in the bandgap would allow previously free electrons to remain free. Alternatively, the conductance could decrease, but not as much as one would expect.

The paper goes on to discuss why this increased plasticity in darkness might occur, but in the interest of not misleading anyone, I won’t cover that since I didn’t fully understand it myself.

Why should you care? Well, semiconductors like zinc sulfide power many of the electronics we use today, and in recent years, we’ve seen technology begin to shift towards flexible electronics that could more easily interface with the average person’s body. If darkness allows for semiconductors to become more flexible while maintaining their conductive properties (or if that flexibility allows for the acquisition of new measurements due to changes in resistance based on deformation), then we could see some interesting new technology come of discoveries like this.

Discussion, links, and tweets

I'm a graduate student at Harvard-MIT HST. Follow me on Twitter; to see what I'm up to today, and tweet at me if you want to chat!