Drop-casting gold nanoparticles on tungsten diselenide effectively ‘brightens’ the ‘dark excitons’

Materials such as tungsten diselenide (WSe2) and molybdenum diselenide are being studied keenly for their opto-electronic properties – 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. 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.

Two-dimensional material

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. “The most challenging aspect of this study was the controlled photoluminescence measurement of these materials from room temperature to 100 K,” says Pramoda Kumar Nayak, a DST Ramanujan Fellow at the Department of Physics of IIT Madras and an author of the paper.

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.

Excitons

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.

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. But, its application to 2D systems is in nascent stage. So, I thought that if we 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,” says Dr Nayak.