Nanocrystals Emit Light by Tunneling Electrons

Illustration of a nanosized device made of two joined silver single crystals that generate light by inelastical electron tunneling. (Source: S. Bopp)

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 device emits light by 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. His 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 the polymer 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 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. (Source: UCSD)

Reference: H. Qian et al.: Efficient light generation from enhanced inelastic electron tunnelling, Nat. Phot., online 23 July 2018; DOI: 10.1038/s41566-018-0216-2

Link: Photonics (Z. Liu), Dept. of Electrical and Computer Engineering, University of California San Diego, La Jolla, USA

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