Nav: Home

Nanocrystals emit light by efficiently 'tunneling' electrons

July 23, 2018

Using advanced fabrication techniques, engineers at the University of California San Diego have built a nanosized device out of silver crystals that can generate light by efficiently "tunneling" electrons through a tiny barrier. The work brings plasmonics research a step closer to realizing ultra-compact light sources for high-speed, optical data processing and other on-chip applications.

The work is published July 23 in Nature Photonics.

The device emits light by a quantum mechanical phenomenon known as inelastic electron tunneling. In this process, electrons move through a solid barrier that they cannot classically cross. And while crossing, the electrons lose some of their energy, creating either photons or phonons in the process.

Plasmonics researchers have been interested in using inelastic electron tunneling to create extremely small light sources with large modulation bandwidth. However, because only a tiny fraction of electrons can tunnel inelastically, the efficiency of light emission is typically low--on the order of a few hundredths of a percent, at most.

UC San Diego engineers created a device that bumps that efficiency up to approximately two percent. While this is not yet high enough for practical use, it is the first step to a new type of light source, said Zhaowei Liu, a professor of electrical and computer engineering at the UC San Diego Jacobs School of Engineering.

"We're exploring a new way to generate light," said Liu.

Liu's team designed the new light emitting device using computational methods and numerical simulations. Researchers in the lab of Andrea Tao, a professor of nanoengineering at the UC San Diego Jacobs School of Engineering, then constructed the device using advanced solution-based chemistry techniques.

The device is a tiny bow-tie-shaped plasmonic nanostructure consisting of two cuboid, single crystals of silver joined at one corner. Connecting the corners is a 1.5-nanometer-wide barrier of insulator made of a polymer called polyvinylpyrrolidone (PVP).

This tiny metal-insulator-metal (silver-PVP-silver) junction is where the action occurs. Electrodes connected to the nanocrystals allow voltage to be applied to the device. As electrons tunnel from a corner of a silver nanocrystal through the tiny PVP barrier, they transfer energy to surface plasmon polaritons--electromagnetic waves that travel along the metal-insulator interface--which then convert that energy to photons.

But what makes this particular junction more efficient at tunneling electrons inelastically is its geometry and extremely tiny size. By joining two silver single crystals together at their corners with a tiny barrier of insulator in between, researchers essentially created a high quality optical antenna with a high local density of optical states, resulting in more efficient conversion of electronic energy to light.

Metal-insulator-metal junctions have had such low light emission efficiency in the past because they were constructed by joining metal crystals along an entire face, rather than a corner, explained Liu. Giving electrons a high quality optical antenna with a much smaller gap to tunnel through allows efficient light emission, and this kind of structure has been difficult to fabricate with nanolithography methods used in the past, he said.

"Using chemistry, we can build these precise nanosized junctions that allow more efficient light emission," said Tao. "The fabrication techniques we use give us atomic level control of our materials--we can dictate the size and shape of crystals in solution based on the reagents we use, and we can create structures that have atomically flat faces and extremely sharp corners."

With additional work, the team aims to further boost efficiency another order of magnitude higher. They are exploring different geometries and materials for future studies.
-end-
Paper title: "Efficient light generation from enhanced inelastic electron tunneling." Co-authors include joint co-first authors Haoliang Qian and Su-Wen Hsu; Kargal Gurunatha, Conor T. Riley, Jie Zhao and Dylan Lu.

This work is supported by the Defense Advanced Research Projects Agency (DARPA) Microsystems Technology Office (W911NF-16-2-0156).

University of California - San Diego

Related Electrons Articles:

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.
New method for detecting quantum states of electrons
Researchers in the Quantum Dynamics Unit at the Okinawa Institute of Science and Technology Graduate University (OIST) devised a new method -- called image charge detection -- to detect electrons' transitions to quantum states.
Slow electrons to combat cancer
Slow electons can be used to destroy cancer cells - but how exactly this happens has not been well understood.
How light steers electrons in metals
Researchers in the Department of Physics of ETH Zurich have measured how electrons in so-called transition metals get redistributed within a fraction of an optical oscillation cycle.
Twisting whirlpools of electrons
Using a novel approach, EPFL physicists have been able to create ultrafast electron vortex beams, with significant implications for fundamental physics, quantum computing, future data-storage and even certain medical treatments.
Inner electrons behave differently in aromatic hydrocarbons
In an international research collaboration between Tsinghua University in Beijing and Sorbonne University in Paris, scientists found that four hydrocarbon molecules, known for their internal ring structure, have a lower threshold for the release of excess energy than molecules without a similar ring structure, because one of their electrons decays from a higher to a lower energy level, a phenomenon called the Auger effect.
Exotic spiraling electrons discovered by physicists
Rutgers and other physicists have discovered an exotic form of electrons that spin like planets and could lead to advances in lighting, solar cells, lasers and electronic displays.
Racing electrons under control
The advantage is that electromagnetic light waves oscillate at petaherz frequency.
Electrons go with the flow
You turn on a switch and the light switches on because electricity 'flows'.
Tying down electrons with nanoribbons
Nanoribbons are promising topological materials displaying novel electronic properties. UC Berkeley chemists and physicists have found a way to join two different types of nanoribbon to create a topological insulator that confines single electrons to the junction between them.
More Electrons News and Electrons Current Events

Best Science Podcasts 2019

We have hand picked the best science podcasts for 2019. Sit back and enjoy new science podcasts updated daily from your favorite science news services and scientists.
Now Playing: TED Radio Hour

Rethinking Anger
Anger is universal and complex: it can be quiet, festering, justified, vengeful, and destructive. This hour, TED speakers explore the many sides of anger, why we need it, and who's allowed to feel it. Guests include psychologists Ryan Martin and Russell Kolts, writer Soraya Chemaly, former talk radio host Lisa Fritsch, and business professor Dan Moshavi.
Now Playing: Science for the People

#538 Nobels and Astrophysics
This week we start with this year's physics Nobel Prize awarded to Jim Peebles, Michel Mayor, and Didier Queloz and finish with a discussion of the Nobel Prizes as a way to award and highlight important science. Are they still relevant? When science breakthroughs are built on the backs of hundreds -- and sometimes thousands -- of people's hard work, how do you pick just three to highlight? Join host Rachelle Saunders and astrophysicist, author, and science communicator Ethan Siegel for their chat about astrophysics and Nobel Prizes.