Graphene helps protect photocathodes for physics experiments

September 17, 2018 by Jared Sagoff, Argonne National Laboratory
This visualisation shows layers of graphene used for membranes. Credit: University of Manchester

Transforming light into electricity is no mean feat. Some devices, like solar cells, use a closed circuit to generate an electric current from incoming light. But another class of materials, called photocathodes, generate large quantities of free electrons that can be used for state-of-the-art science.

Photocathodes have one significant limitation, which is that they degrade when exposed to air. To prevent this, scientists at the U.S. Department of Energy's (DOE) Argonne, Brookhaven, and Los Alamos national laboratories have developed a way to wrap photocathodes up in a protective coat of atomically thin graphene, extending their lifetimes.

"The thin layer [of graphene] we use provides insulation from air without hampering charge mobility or ."—Junqi Xie, Argonne physicist

Photocathodes work by converting photons of light into electrons through a process known as the photoelectric effect—which essentially involves the ejection of electrons from the surface of a material hit with light of a sufficient frequency. The large quantities of electrons generated by photocathodes can be used in accelerator systems that produce intense electron beams, or in photodetector systems for high-energy physics experiments that operate in low-light environments in which every photon counts.

The relative success of a photocathode material hinges on two distinct qualities: its quantum efficiency and its longevity. "Quantum efficiency refers to the ratio of emitted electrons to incoming photons," said Argonne physicist Junqi Xie.

The higher the quantum efficiency of a given material, the more electrons it can generate.

In the study, Xie and his colleagues looked at a material called potassium cesium antimonide, which has one of the highest quantum efficiencies of any known photocathode in the visible range of the spectrum. But even though the quantum efficiency of the material is high, potassium cesium antimonide photocathodes are susceptible to breaking down when exposed to even very small amounts of air.

According to Xie, there are two ways of making sure the photocathode doesn't interact with air. The first is to operate it in a vacuum, which isn't always feasible. The second is to encapsulate the photocathode with a thin film of material.

To successfully insulate a photocathode, the researchers needed to identify a material that could form layers only a few atoms thick and that was electrically conductive. Graphene, a two-dimensional material made of carbon, satisfied both of these requirements.

"For graphene, you can just use two or three atomic layers; plus, it's optically transparent and has high charge mobility," Xie said. "The thin layer we use provides insulation from air without hampering charge mobility or quantum efficiency."

Proving that a photocathode material can last longer without suffering from quantum efficiency losses represents the key challenge in developing the next generation of these materials, Xie said. "The photocathode itself is pretty good—it's a state-of-the-art photocathode with high . Using graphene helps alleviate concern about the lifetime," he explained.

The graphene-wrapping technique used in this study could in principle be employed in any whose performance suffers when exposed to air. It is especially important for a proposed new generation of photocathodes based on a class of materials called halide perovskites. These could offer even higher quantum efficiencies than potassium cesium antimonide, but face similar challenges when it comes to lifetime.

An article based on the study, "Free-standing bialkali photocathodes using atomically thin substrates," appeared in the July 6 online edition of Advanced Materials Interfaces.

Explore further: New 3-D models illustrate the effect of material roughness on electrons emitted from the surface of a photocathode

More information: Hisato Yamaguchi et al, Photocathode: Free-Standing Bialkali Photocathodes Using Atomically Thin Substrates (Adv. Mater. Interfaces 13/2018), Advanced Materials Interfaces (2018). DOI: 10.1002/admi.201870065

Related Stories

Novel nano material for quantum electronics

September 10, 2018

An international team led by Assistant Professor Kasper Steen Pedersen, DTU Chemistry, has synthesized a novel nano material with electrical and magnetic properties making it suitable for future quantum computers and other ...

A novel graphene quantum dot structure takes the cake

August 23, 2018

In a marriage of quantum science and solid-state physics, researchers at the National Institute of Standards and Technology (NIST) have used magnetic fields to confine groups of electrons to a series of concentric rings within ...

Black gold: Enabling bright, high rep-rate electron beams

February 15, 2013

Free electron lasers (FELs) have proven their worth, but next-generation light sources will have to do better than produce ultrabright x-ray pulses 100 or so times a second. What's needed is megahertz rep rate, a million ...

Recommended for you

Sculpting stable structures in pure liquids

February 21, 2019

Oscillating flow and light pulses can be used to create reconfigurable architecture in liquid crystals. Materials scientists can carefully engineer concerted microfluidic flows and localized optothermal fields to achieve ...

Researchers make coldest quantum gas of molecules

February 21, 2019

JILA researchers have made a long-lived, record-cold gas of molecules that follow the wave patterns of quantum mechanics instead of the strictly particle nature of ordinary classical physics. The creation of this gas boosts ...


Please sign in to add a comment. Registration is free, and takes less than a minute. Read more

Click here to reset your password.
Sign in to get notified via email when new comments are made.