Nav: Home

Shifting dimensions: Exciting excitons in phosphorene

March 25, 2020

Since its discovery in 2014, phosphorene - a sheet of phosphorus atoms only a single atom thick - has intrigued scientists, due to its unique optoelectronic anisotropy. In other words, electrons interact with light and move in one direction only. This anisotropy means that despite being two dimensional (2D), phosphorene shows a mix of properties found in both one-dimensional (1D) and 2D materials. Scientists believe that the distinct quasi-1D nature of phosphorene could be exploited to develop new, innovative optoelectronic devices, from LEDs to solar cells.

Now, scientists from the Femtosecond Spectroscopy Unit at the Okinawa Institute of Science and Technology Graduate University (OIST) have shed light on how excitons - an excited state of matter at the core of optoelectronics - move and interact within phosphorene.

"Because of the anisotropy, excitons behave in a really unique way in phosphorene compared to other 2D materials, which we are only just beginning to understand," said Vivek Pareek, PhD student and first author of the study, published in Physical Review Letters.

Excitons are formed when a material absorbs a photon of light, causing an electron to be excited to a higher energy state. This leaves a positively charged "hole" where the electron used to reside in its initial energy state, which is attracted to the negatively charged excited electron. The resultant bound electron-hole pair - the exciton - can then move through the material and interact with other excitons.

But excitons are short-lived and in time, the excited electrons "fall" back into the holes. To do so, excitons can either emit a photon - a process called radiative recombination - or they can collide with each other, transferring heat to the material - a non-radiative recombination called exciton-exciton annihilation.

"Exciton-exciton interaction, or annihilation, is very different in 1D and 2D systems," explained Pareek. "We can therefore use exciton-exciton annihilation as a tool to probe the nature of interactions in quasi-1D phosphorene."

Probing phosphorene

The scientists used a laser to send two pulses of light at phosphorene - a pump pulse to excite the electrons to form excitons, and a probe pulse to capture how exciton-exciton annihilation occurred during the first hundred picoseconds, which are trillionths of a second. By changing the power of the pump pulse, the researchers altered the initial density of excitons formed.

The team found that as exciton density increased, exciton-exciton annihilation changed in dimension, shifting from 1D to 2D. The researchers show that this dimensional shift occurred due to phosphorene's anisotropic properties, which arise due to the unusual structure of the material. This anisotropy causes excitons to move more rapidly in one specific direction along the lattice and move more slowly in the other direction. Therefore, at low exciton densities, interactions between excitons predominantly occurred only in one dimension - along the more favorable direction. But when the exciton density was increased, resulting in smaller distances between excitons, interactions started to occur in both dimensions.

The scientists also explored the effect of temperature on exciton-exciton annihilation. When the team cooled down the phosphorene flakes, exciton-exciton annihilation reverted from 2D to 1D, even at high exciton densities.

"This study shows that we can control whether exciton-exciton annihilation occurs in one or two dimensions, depending on conditions we set," said Dr. Julien Madéo, OIST staff scientist and co-author of the study. "This reveals a new interesting property of phosphorene, enhancing its prospects as a new material in optoelectronic devices."

Okinawa Institute of Science and Technology (OIST) Graduate University

Related Electrons Articles:

Plasma electrons can be used to produce metallic films
Computers, mobile phones and all other electronic devices contain thousands of transistors, linked together by thin films of metal.
Flatter graphene, faster electrons
Scientists from the Swiss Nanoscience Institute and the Department of Physics at the University of Basel developed a technique to flatten corrugations in graphene layers.
Researchers develop one-way street for electrons
The work has shown that these electron ratchets create geometric diodes that operate at room temperature and may unlock unprecedented abilities in the illusive terahertz regime.
Photons and electrons one on one
The dynamics of electrons changes ever so slightly on each interaction with a photon.
Using light to put a twist on electrons
Method with polarized light can create and measure nonsymmetrical states in a layered material.
What if we could teach photons to behave like electrons?
The researchers tricked photons - which are intrinsically non-magnetic - into behaving like charged electrons.
Electrons in rapid motion
Researchers observe quantum interferences in real-time using a new extreme ultra-violet light spectroscopy technique.
Taming electrons with bacteria parts
In a new study, scientists at the MSU-DOE Plant Research Laboratory report a new synthetic system that could guide electron transfer over long distances.
Hot electrons harvested without tricks
Semiconductors convert energy from photons into an electron current. However, some photons carry too much energy for the material to absorb.
Cooling nanotube resonators with electrons
In a study in Nature Physics, ICFO researchers report on a technique that uses electron transport to cool a nanomechanical resonator near the quantum regime.
More Electrons News and Electrons Current Events

Trending Science News

Current Coronavirus (COVID-19) News

Top Science Podcasts

We have hand picked the top science podcasts of 2020.
Now Playing: TED Radio Hour

Listen Again: Meditations on Loneliness
Original broadcast date: April 24, 2020. We're a social species now living in isolation. But loneliness was a problem well before this era of social distancing. This hour, TED speakers explore how we can live and make peace with loneliness. Guests on the show include author and illustrator Jonny Sun, psychologist Susan Pinker, architect Grace Kim, and writer Suleika Jaouad.
Now Playing: Science for the People

#565 The Great Wide Indoors
We're all spending a bit more time indoors this summer than we probably figured. But did you ever stop to think about why the places we live and work as designed the way they are? And how they could be designed better? We're talking with Emily Anthes about her new book "The Great Indoors: The Surprising Science of how Buildings Shape our Behavior, Health and Happiness".
Now Playing: Radiolab

The Third. A TED Talk.
Jad gives a TED talk about his life as a journalist and how Radiolab has evolved over the years. Here's how TED described it:How do you end a story? Host of Radiolab Jad Abumrad tells how his search for an answer led him home to the mountains of Tennessee, where he met an unexpected teacher: Dolly Parton.Jad Nicholas Abumrad is a Lebanese-American radio host, composer and producer. He is the founder of the syndicated public radio program Radiolab, which is broadcast on over 600 radio stations nationwide and is downloaded more than 120 million times a year as a podcast. He also created More Perfect, a podcast that tells the stories behind the Supreme Court's most famous decisions. And most recently, Dolly Parton's America, a nine-episode podcast exploring the life and times of the iconic country music star. Abumrad has received three Peabody Awards and was named a MacArthur Fellow in 2011.