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New Possibilities for Nano-sized Optical Filters

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21 Oct 2019
New Possibilities for Nano-sized Optical Filters

Tailoring two-dimensional nano-materials into optical filters and thermoelectric devices possible, suggest theoretical studies.

Graphene is a form of carbon with a single layer of atoms in a honeycomb like hexagonal structure in a single plane. The winners of the 2010 Physics Nobel Prize, Andre Geim and Kostya Novoselov, isolated graphene from graphite for the first time in 2004, using sticky tape. It kicked off a massive amount of research on this wonder material, and also got researchers to try and make other two-dimensional (2D) materials.

Scientists predict at least 700 stable two-dimensional materials. Though many of them are yet to be synthesized, scientists are carrying out theoretical studies to investigate their properties. These materials are likely to alter the electronics industry radically as they could be used in photovoltaics, electronics and cancer theranostics.

Prof Bhaskaran Muralidharan and Dr Alestin Mawrie of the Indian Institute of Technology Bombay have researched a specific category of two-dimensional nanomaterials, called semi-Dirac materials. Their theoretical studies show that it is possible to engineer semi-Dirac materials to make optical filters and efficient thermoelectric nanodevices. This study is published in the journal Physical Review B.

Semi-Dirac materials have special properties

Many two-dimensional materials are excellent semiconductors. Dirac materials are a particular type of 2D materials that could be used to make electronics, and could also be useful in desalination and DNA sequencing. One needs to modify fewer parameters to modify the properties of these materials as compared to conventional metals. 

In Dirac materials, the charge carriers move within the material at a velocity close to that of light. Whereas in semi-Dirac materials, the charge carrier movement is not the same in all directions within the 2D plane. The charge carrier velocity is close to that of light in just one direction but much smaller in a direction perpendicular to the former. This unique nature of semi-Dirac materials leads to some peculiarity in their electronic properties such as optical conductivity and thermoelectricity.

The optical conductivity of a material decides its response to electromagnetic waves, including light. Semi-Dirac materials can have a very high optical conductivity for electromagnetic waves of a specific frequency and specific polarisation. The polarisation of an electromagnetic wave indicates the direction in which its electric field component oscillates, in reference to the direction of propagation. When the electrical field oscillates in a single direction in the plane (say x-y plane) perpendicular to the direction of propagation (the z-axis), it is called a linearly polarised wave. If the direction of oscillation is along the x-direction, we may call it x polarised wave. A wave in which the electric field oscillates along the y-axis is called y polarised wave.

A voltage difference across a piece of thermoelectric material causes a temperature difference across it and vice versa. Such materials can be used as heat pumps to remove heat from nano-devices, thus increasing their longevity and efficiency.

The researchers at IIT Bombay wanted to study how the physical properties of semi-Dirac materials change when they alter various parameters associated with them. They focused on how the optical conductivity and thermoelectric properties vary with a tunable parameter called the gap parameter.

Tuning optical conductivity to make optical filters

Optical conductivity of a material can indicate if the material is transparent or opaque. For instance, an increased optical conductivity would mean higher absorption of electromagnetic waves.

The researchers used computations to understand the response of semi-Dirac materials to x and y polarised light. They showed that the gap parameter could be tuned to adjust the optical conductivity of the semi-Dirac materials. It could then help to engineer the material to have high optical conductivity for a light of a specific frequency and polarisation. The material could also be made to have high conductivity for, say, y polarised light, thus allowing only the x polarised light to pass through. Further, for different materials, the researchers have identified the frequency of light for which this high direction-dependence of the optical conductivity happens.

“The high degree of direction dependence of optical conductivity suggests that the Semi-Dirac materials can be used as a unique transparent conductor with given transparency along one direction while showing very high absorption (opaqueness) along the perpendicular direction. This property can be used to probe some of the physics of 2D semi-Dirac materials,” says Prof. Muralidharan regarding the potential implications of his study.

Optimising power and efficiency of thermoelectric devices

Thermoelectric nanodevices may not be very efficient in the operating condition that gives the highest output power. In the operating condition that has the best possible efficiency, the power may not be enough. One of the challenges in designing these devices is obtaining optimal conditions that ensure enough power output at the best possible efficiency. 

Prof Muralidharan and Dr Mawrie have theoretically calculated the efficiency and power for various values of the gap parameter of 2D semi-Dirac-based nano thermoelectric devices. They found that for specific values of the gap parameter, these materials can operate at high power with very high efficiency. Thus, it may be possible to tune the gap parameter to get optimal power and efficiency in these devices. This can open up new avenues in design of thermoelectric nanodevices, especially thermoelectric heat pumps.

The findings show the potential of 2D semi-Dirac materials, but we must note that these are theoretical results and proposals only.

“We hope that some experimental groups will look up our results and conduct necessary experiments to ascertain our results,” says Prof. Muralidharan.

This article has been run past the researchers, whose work is covered, to ensure accuracy.

Editor's Note: The article was updated with an additional reference to the article as per the request of the researchers.