In abstract:

Measuring charge carrier diffusion in an atomically-thin semiconductor

In a major feat of technical skill, a team of interdisciplinary researchers at The University of Manchester have studied the electrochemistry of a single atomic layer of molybdenum disulfide (MoS2) for the first time.

Molybdenum disulfide is an exciting material because it can be exfoliated into two-dimensional (2D) sheets, less than 1nm thick, or 100,000 times thinner than an ordinary sheet of paper. Unlike its more famous cousin, graphene, MOSis a semiconductor, with a band gap similar to silicon, meaning that it can be used to convert light into charge carriers (electrons and holes) and therefore produce electricity. This means that it has huge, but as yet unrealised applications ranging from microelectronics to photocatalytic hydrogen production. In many of these applications, it is important to understand how charge carriers move through and along MoS2 sheets. This is a challenge, not least because it is very difficult to make isolated one-layer-thick sheets of the material.

In a key breakthrough, the researchers have made pristine monolayers and 'few-layers' of MoS2 by mechanical exfoliation, and for the first time have measured two of their key electrochemical-parameters - the electron transfer rate and the electric double-layer capacitance. They found that both increase with the number of layers in the stack. Careful measurements allowed the researchers to differentiate between the charge carrier diffusion between the 2D layers (in a multilayer) and along the 2D layer (in a single monolayer). This adds to our fundamental understanding and opens the way to exploiting MoS2 in applications such as water splitting.

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  • The band gap of a solid refers to the energy gap between the levels filled with electrons and those that are empty.  It is a fundamental property of semiconductors, and determines the frequency of light absorbed (and colour) of solids. The band gap of MoS2, like silicon, matches well with the solar spectrum, making it potentially a great light harvester. 
  • Photoelectrochemical water splitting is being investigated worldwide as a means of obtaining hydrogen fuel cleanly.   Visible light from the Sun does not have enough energy to break the bonds in a water molecule - but when a suitable catalyst is used and a small voltage is supplied, the molecule can be 'split' into oxygen and hydrogen molecules. MoS2 has great potential in this and other energy storage applications.
  • In an electrochemical measurement, the rate of charge carrier movement through or along parts of the device may be measured as a function of changes in the experimental variables (such as the intensity of illumination, as here.)  Measurements of the electron transfer rate, which refers simply to a speed of electron or hole transfer between a molecule in a solution and a solid electrode, allow researchers to understand the mechanism of diffusion of the charge carriers.
  • This detemines how fast a certain reaction, water splitting for example, proceeds.
  • In a liquid solution, the electric double-layer capacitance refers to an electrical phenomenon that appears between an electrode and a liquid electrolyte surrounding it.  At this interface, two layers of ions with opposite polarity, separated by a layer of solvent molecules, form when a voltage is applied.  The layer of solvent behaves like a dielectric in a conventional capacitor - so this is a way of storing electrical energy.
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