image: The transparency of a device at the 2-D limit can be tuned using an electric bias, where different type of optical processes that simply define the way we see objects can be modify externally. This gives the control on properties that until very recently were not thought to be manipulated arbitrarily. view more
Credit: Queen's University Belfast
Researchers at Queen's University Belfast and ETH Zurich, Switzerland, have created a new theoretical framework which could help physicists and device engineers design better optoelectronics, leading to less heat generation and power consumption in electronic devices which source, detect, and control light.
Speaking about the research, which enables scientists and engineers to quantify how transparent a 2D material is to an electrostatic field, Dr Elton Santos from the Atomistic Simulation Research Centre at Queen's, said: "In our paper we have developed a theoretical framework that predicts and quantifies the degree of 'transparency' up to the limit of one-atom-thick, 2D materials, to an electrostatic field.
"Imagine we can change the transparency of a material just using an electric bias, e.g. get darker or brighter at will. What kind of implications would this have, for instance, in mobile phone technologies? This was the first question we asked ourselves. We realised that this would allow the microscopic control over the distribution of charged carriers in a bulk semiconductor (e.g. traditional Si microchips) in a nonlinear manner. This will help physicists and device engineers to design better quantum capacitors, an array of subatomic power storage components capable to keep high energy densities, for instance, in batteries, and vertical transistors, leading to next-generation optoelectronics with lower power consumption and dissipation of heat (cold devices), and better performance. In other words, smarter smart phones."
Explaining how the theory could have important implications for future work in the area, Dr Santos added: "Our current model simply considers an interface formed between a layer of 2D material and a bulk semiconductor. In principle, our approach can be readily extended to a stack of multiple 2D materials, or namely, van der Waals heterostructures recently fabricated. This will allow us to design and predict the behaviour of these cutting-edge devices in prior to actual fabrication, which will significantly facilitate developments for a variety of applications. We will have an in silico search for the right combination of different 2D crystals while reducing the need for expensive lab work and test trials."
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Further information on the Atomistic Simulation Research Centre at Queen's is available online at http://titus.phy.qub.ac.uk/
Media inquiries to Michelle Cassidy/Anne-Marie Clarke, Queen's Communications Office. Tel: +44 (0)28 90 97 5310 or email comms.officer@qub.ac.uk
Notes to Editors
1) Dr Elton Santos is available for interview. Please telephone +44 (0)28 90 97 5310 to arrange.
2) The present multiscale approach considers the penetration of the field effect through a monolayer 2D material in a metal-oxide-2D material semiconductor (MOGS) quantum capacitor (QC). By using graphene as the model system, the team at Queen's and Zurich ETH developed a macroscopic model to describe the charge distribution in graphene and the semiconductor. These results are corroborated by atomistic ab initio calculations at the level of density functional theory, including van der Waals interactions, which are important due to the nanoscale of the problem (e.g. 10-9 m), and Poisson-Boltzmann equation methods, importantly at micro-scale (e.g. 10-6 m).
The team next defined and formulated an index or parameter that quantified the transparency of a monolayer 2D material to an electric displacement field and showed that the transparency is determined by the combined effect of 2D material quantum capacitance and the semiconductor capacitance (classical one studied in high-school). By calculating the quantum capacitance for a variety of 2D materials using accurate hybrid functionals, they predicted the ranking for a variety of 2D compounds according to their transparency to an electric displacement field as follows: graphene > silicene > germanene > WS2 > WTe2 > WSe2 > MoS2 > MoSe2 > MoTe2, when the majority carrier is electron.
3) An image to accompany the story is available by clicking on the link below: https://www.dropbox.com/s/tocvjdpp9dydya6/GRAPH.tif?dl=0
CAPTION: The transparency of a device at the 2D limit can be tuned using an electric bias, where different type of optical processes that simply define the way we see objects can be modify externally. This gives the control on properties that until very recently were not thought to be manipulated arbitrarily.
4) Funding acknowledgements: E.J.G.S. acknowledges the use of computational resources from the UK national high performance computing service, ARCHER, for which access was obtained via the UKCP consortium and funded by EPSRC grant ref EP/K013564/1; and the Extreme Science and Engineering Discovery Environment (XSEDE), supported by NSF grants number TG-DMR120049 and TG-DMR150017. The Queen's Fellow Award through the startup grant number M8407MPH is also acknowledged and the PhD studentship from the Energy PRP funded by NI-DEL and Queen's University Belfast