Scientists Measure Elusive Particles that Cannot be Observed Using Light

For the first time, researchers have directly observed and quantified mysterious particles, known as dark excitons, that otherwise cannot be visualized by light. This breakthrough ends a decade-long search for a potential new group of exceptionally thin, 2D semiconductors.

The new class of semiconductors, known as transition metal dichalcogenide monolayers (TMDs), have structures similar to graphene. TMDs are made up of just a single layer of atoms that are arranged to form a hexagonal crystal lattice. Image Credit: 3113Ian at English Wikipedia.

The robust method could redefine research works on excitons and 2D semiconductors, and holds immense implications for upcoming technological devices, ranging from LEDs and solar cells to lasers and smartphones. The novel method has been explained in the leading Science journal.

Excitons are essentially excited states of matter present inside semiconductors—a crucial component used in a majority of present-day technologies. Excitons are formed when electrons within the semiconductor material are activated by light to a higher state of energy, and a “hole” is left behind at the energy level, where the electrons had earlier resided.

Holes are the absence of an electron, and so carry the opposite charge to an electron. These opposite charges attract, and electrons and holes bind together to form excitons that can then move throughout the material.

Keshav Dani, Study Senior Author and Professor, Okinawa Institute of Science and Technology Graduate University

Professor Dani also heads the Femtosecond Spectroscopy Unit at the same institute.

In standard semiconductors, excitons are extinguished within a few billionths of a second following their development. They can also be “fragile,” rendering them too challenging to analyze and exploit. However, about 10 years ago, investigators identified 2D semiconductors, in which the excitons were found to be stronger.

Robust excitons give these materials really unique and exciting properties, so there have been a lot of intense studies worldwide aimed at using them to create new optoelectronic devices. But at the moment, there is a major limitation with the standard experimental technique used to measure excitons.

Dr Julien Madéo, Study Co-First Author and Staff Scientist, Femtosecond Spectroscopy Unit, Okinawa Institute of Science and Technology Graduate University

Scientists are presently using optical spectroscopy methods, essentially quantifying the kind of wavelengths of light that are reflected, absorbed, or produced by the semiconductor material—to yield data relating to the energy states of excitons. However, optical spectroscopy techniques can only record a small part of the actual picture.

For a long time, investigators have known that only bright excitons—a kind of exciton—are capable of interacting with light. But there are also other types of excitons, such as momentum-forbidden dark excitons.

The electrons in this kind of dark exciton exhibit an entirely different momentum from the holes to which they are attached, and this inhibits them from absorbing light. This fact also implies that electrons in dark excitons exhibit a different momentum from the electrons found in bright excitons.

We know they exist, but we cannot directly see them, we cannot directly probe them, and therefore we do not know how important they are, or how much they impact the optoelectronic properties of the material,” Dr Madéo added.

Shining Light Dark Excitons

For the first time, the researchers successfully visualized dark excitons by altering a robust method. This method was mostly used for studying single, untethered electrons in the past.

It wasn’t clear how this technique would work for excitons, which are composite particles wherein the electrons are bound. There was a lot of theoretical work in the scientific community discussing the validity of this approach.

Keshav Dani, Study Senior Author and Professor, Okinawa Institute of Science and Technology Graduate University

According to the researchers’ technique, if a ray of light containing photons of sufficient energy is applied to bombard excitons in the semiconductor material, the energy emitted by the photons would break down the excitons and directly knock off the electrons from the material.

By quantifying the direction at which the electrons exit from the material, the team would be able to establish the initial momentum of the electrons, especially when they are part of excitons. Therefore, the researchers would be able to both visualize and distinguish the dark excitons from the bright excitons.

However, to apply this latest method, some vast technical challenges have to be solved. The researchers must create light pulses containing high-energy extreme ultraviolet photons. These photons should be able to split the excitons and knock off the electrons from the material. The instrument should then be able to quantify the angle and energy of these electrons.

Moreover, since excitons have a short lifespan, the instrument should be able to operate on timescales of less than one thousand billionths of a second. Finally, the instrument must also have sufficiently high spatial resolution to quantify the samples of 2D semiconductors, which usually come only in micron-scale sizes.

When we solved all the technical problems, and turned on the instrument, basically there on our screen were the excitons—it was really amazing,” added Dr Michael Man, the co-first author of the study and also from the Femtosecond Spectroscopy Unit at the Okinawa Institute of Science and Technology Graduate University.

The team observed that, as estimated before, both dark and bright excitons were present in the semiconductor material. But to their amazement, they also discovered that the material is governed by dark excitons, which easily outnumbered the bright excitons.

The researchers further noted that under specific conditions, as the excited electrons spread across the material and altered the momentum, the excitons could change between dark or bright states.

The dominance of the dark excitons and the interplay between the dark and bright excitons suggests that dark excitons impact this new class of semiconductors even more greatly than anticipated,” added Dr Madéo.

This technique is a real breakthrough. Not only does it provide the first observation of dark excitons and illuminate their properties, but it ushers in a new era in the study of excitons and other excited particles,” concluded Professor Dani.

Dynamics of dark and bright excitons

By varying the time between the exciton-generating pump pulse and the extreme ultraviolet pulse that kicked out the electrons, the scientists were able to construct a video showing how dark excitons (red) and bright excitons (blue) changed over time. Video Credit: Okinawa Institute of Science and Technology Graduate University.

Journal Reference:

Madéo, J., et al. (2020) Directly visualizing the momentum-forbidden dark excitons and their dynamics in atomically thin semiconductors. Science.

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