Finding could allow ultrafast switching of conduction, and possibly lead to new broadband light sensors
Graphene, an ultrathin form of carbon with exceptional electrical, optical, and mechanical properties, has become a focus of research on a variety of potential uses. Now researchers at MIT have found a way to control how the material conducts electricity by using extremely short light pulses, which could enable its use as a broadband light detector.
The new findings are published in the journal Physical Review Letters, in a paper by graduate student Alex Frenzel, Nuh Gedik, and three others.
The researchers found that by controlling the concentration of electrons in a graphene sheet, they could change the way the material responds to a short but intense light pulse. If the graphene sheet starts out with low electron concentration, the pulse increases the material's electrical conductivity. This behavior is similar to that of traditional semiconductors, such as silicon and germanium.
But if the graphene starts out with high electron concentration, the pulse decreases its conductivity — the same way that a metal usually behaves. Therefore, by modulating graphene's electron concentration, the researchers found that they could effectively alter graphene's photoconductive properties from semiconductorlike to metallike.
The finding also explains the photoresponse of graphene reported previously by different research groups, which studied graphene samples with differing concentration of electrons. "We were able to tune the number of electrons in graphene, and get either response," Frenzel says.
To perform this study, the team deposited graphene on top of an insulating layer with a thin metallic film beneath it; by applying a voltage between graphene and the bottom electrode, the electron concentration of graphene could be tuned. The researchers then illuminated graphene with a strong light pulse and measured the change of electrical conduction by assessing the transmission of a second, low-frequency light pulse.
In this case, the laser performs dual functions. "We use two different light pulses: one to modify the material, and one to measure the electrical conduction," Gedik says, adding that the pulses used to measure the conduction are much lower frequency than the pulses used to modify the material behavior. To accomplish this, the researchers developed a device that was transparent, Frenzel explains, to allow laser pulses to pass through it.
This all-optical method avoids the need for adding extra electrical contacts to the graphene. Gedik, the Lawrence C. and Sarah W. Biedenharn Associate Professor of Physics, says the measurement method that Frenzel implemented is a "cool technique. Normally, to measure conductivity you have to put leads on it," he says. This approach, by contrast, "has no contact at all."
Additionally, the short light pulses allow the researchers to change and reveal graphene's electrical response in only a trillionth of a second.
In a surprising finding, the team discovered that part of the conductivity reduction at high electron concentration stems from a unique characteristic of graphene: Its electrons travel at a constant speed, similar to photons, which causes the conductivity to decrease when the electron temperature increases under the illumination of the laser pulse. "Our experiment reveals that the cause of photoconductivity in graphene is very different from that in a normal metal or semiconductor," Frenzel says.
The researchers say the work could aid the development of new light detectors with ultrafast response times and high sensitivity across a wide range of light frequencies, from the infrared to ultraviolet. While the material is sensitive to a broad range of frequencies, the actual percentage of light absorbed is small. Practical application of such a detector would therefore require increasing absorption efficiency, such as by using multiple layers of graphene, Gedik says.
The research team also included Jing Kong, the ITT Career Development Associate Professor of Electrical Engineering at MIT, who provided the graphene samples used for the experiments; physics postdoc Chun Hung Lui; and Yong Cheol Shin, a graduate student in materials science and engineering. The work received support from the U.S. Department of Energy and the National Science Foundation.
Andrew Carleen | Eurek Alert!
Getting closer to porous, light-responsive materials
26.07.2017 | Kyoto University
25.07.2017 | Vanderbilt University
Strong light-matter coupling in these semiconducting tubes may hold the key to electrically pumped lasers
Light-matter quasi-particles can be generated electrically in semiconducting carbon nanotubes. Material scientists and physicists from Heidelberg University...
Fraunhofer IPA has developed a proximity sensor made from silicone and carbon nanotubes (CNT) which detects objects and determines their position. The materials and printing process used mean that the sensor is extremely flexible, economical and can be used for large surfaces. Industry and research partners can use and further develop this innovation straight away.
At first glance, the proximity sensor appears to be nothing special: a thin, elastic layer of silicone onto which black square surfaces are printed, but these...
3-D shape acquisition using water displacement as the shape sensor for the reconstruction of complex objects
A global team of computer scientists and engineers have developed an innovative technique that more completely reconstructs challenging 3D objects. An ancient...
Physicists have developed a new technique that uses electrical voltages to control the electron spin on a chip. The newly-developed method provides protection from spin decay, meaning that the contained information can be maintained and transmitted over comparatively large distances, as has been demonstrated by a team from the University of Basel’s Department of Physics and the Swiss Nanoscience Institute. The results have been published in Physical Review X.
For several years, researchers have been trying to use the spin of an electron to store and transmit information. The spin of each electron is always coupled...
What is the mass of a proton? Scientists from Germany and Japan successfully did an important step towards the most exact knowledge of this fundamental constant. By means of precision measurements on a single proton, they could improve the precision by a factor of three and also correct the existing value.
To determine the mass of a single proton still more accurate – a group of physicists led by Klaus Blaum and Sven Sturm of the Max Planck Institute for Nuclear...
26.07.2017 | Event News
21.07.2017 | Event News
19.07.2017 | Event News
26.07.2017 | Physics and Astronomy
26.07.2017 | Life Sciences
26.07.2017 | Earth Sciences