IIT MADRAS DEVELOPS MATERIAL WITH PROPERTIES SUITABLE FOR QUANTUM OPTOELECTRONICS
What’s in the news?
Materials such as tungsten diselenide (WSe2) and molybdenum diselenide are being studied keenly for their opto-electronic properties which is a combination of optics and electronics.
- A key property of these materials is photoluminescence, in which the material absorbs light and re-emits it as a spectrum.
- As a matter of fact, researchers from IIT Madras have found a way of enhancing this property about 30 times in tungsten diselenide, by drop-casting gold nanoparticles on to a two-dimensional film.
- The work is published in Applied Physics Letters.
- Consisting of practically one layer of atoms, these materials are two-dimensional in structure.
- Photoluminescence properties can be used in various devices such as quantum LEDs which can be used in communication and computation.
- Experts have opined that the most challenging aspect of this study was the controlled photoluminescence measurement of these materials from room temperature to 100 K.
- As is well known, electrons in semiconductors occupy bands of energy known as valence bands.
- As long as they live in these bands, they do not move and contribute to conduction.
- If excited by a small energy input, they get kicked into what is called the conduction band where they can actually be delocalised and contribute to the conduction by moving around.
- When an electron jumps from the valence to the conduction band, it leaves behind a shadow called a “hole.”
- The electron in the conduction band and the hole in the valence band can bind together and form a composite object (or pseudoparticle) known as an exciton.
- Photoluminescence in tungsten selenide is a result of such excitons.
- There can be two ways in which an exciton can form – when the spins of the component electron and hole are opposite to each other and when they are aligned in teh same direction.
- The former is called a bright exciton and the latter, a dark exciton.
- Because their spins are opposite, the electron and hole forming the bright exciton can recombine, giving out a quantum of light in the process.
- Such a simple way of recombining does not exist for the dark excitons.
- Since there, the spin of the electron and the hole are parallel, their recombination is discouraged by the rule of conservation of angular momentum.
- Hence the dark excitons are longer lived than the bright excitons. The dark excitons need an external influence to help them recombine.
- In their work, the IIT Madras researchers find exactly such an external influence.
The power of gold:
- When they drop-cast gold nanoparticles on the surface of the monolayer tungsten diselenide, they find that the dark excitons couple to the surface fields generated and recombine to give off light quanta.
- Thus, the dark excitons are “brightened” with the help of the gold nanoparticles.
- That plasmonic effect arises due to gold nanoparticles is a well-known concept. However, its application to 2D systems is in nascent stage.
- The scientists thought that if they drop-cast gold nanoparticles onto monolayer WSe2, then it will generate out-of-plane electric field due to plasmonic effect, which can help for spin-flip of conduction band electrons, thereby making dark excitons bright.