Graphene multiplies the power of light

January 19, 2015
Graphene multiplies the power of light
Credit: ThinkStock

Could graphene turn light to electricity? Scientists have shown that graphene can convert a single photon into multiple electrons, showing much promise for future photovoltaic devices.

Graphene is a material that has gathered tremendous popularity in recent years, due to its extraordinary strength and weight. It can be generated by literally peeling it off from graphite, or by growing it on top of various materials, which makes its production cost-effective. Studies have hinted that graphene can also be used as a photovoltaic material, turning light into electricity. Using a cutting-edge spectroscopic method, scientists at EPFL and collaborators have demonstrated that by absorbing a single photon, graphene can generate multiple electrons that have enough energy to drive an electrical current. The work is published in Nano Letters.

Graphene is fascinating in terms of fundamental physics, because it is better at conducting electricity at room temperature than e.g. copper, which makes it ideal for ultra-fast circuits. In addition, graphene has been shown to conduct electricity after absorbing light, which means that it could also be used in photovoltaic devices. But until now, graphene's potential for efficient light-to-electricity conversion was not well understood.

This is a challenging task as this conversion takes place on a femto-second scale (10-15 sec; a quadrillionth of a second), too fast for conventional techniques to detect electron movement. To overcome this obstacle, Jens Christian Johannsen from Marco Grioni's lab at EPFL, with colleagues at Aarhus University and ELETTRA in Italy, employed a sophisticated technique called "ultrafast time- and " (trARPES). The experiments were carried out at the world-renowned Rutherford Appleton Laboratory in Oxford.

With this method, a small sample of graphene is placed in an . The graphene is then hit with an ultrafast 'pump' pulse of laser light. This excites the electrons in graphene, "raising" them to higher energy states where they can actually drive an . While the electrons are in those states, the graphene sample is hit with a time-delayed, 'probe' pulse that literally takes a snapshot of the energy each electron has at that moment. The sequence is repeated rapidly for different time points, like a stop-motion movie, and captures the dynamics of the electrons in a live-action sequence.

One photon, many electrons

The scientists used "doped" samples of graphene, which means that they added or subtracted electrons from it by chemical means. The experiment revealed that, when doped graphene absorbs a single photon, this can excite several electrons and do so proportionally to the degree of doping. The photon excites an electron, which then rapidly "falls" back down to its ground state of energy. As it does so, the "fall" excites two more on average as a knock-on effect. "This indicates that a photovoltaic device using doped graphene could show significant efficiency in converting light to electricity", says Marco Grioni.

The scientists have made the first-ever direct observation of graphene's photon-electron multiplication effect, which makes the material a very promising building block for any device that relies on converting light into electricity. For example, novel using could harvest light energy across the entire solar spectrum with lower energy loss than current systems.

Building on their cutting-edge technology and experimental success, the scientists are now planning to explore similar effects in other two-dimensional materials, such as molybdenum disulphide (MoS2), a material that is already in the limelight for its remarkable electronic and catalytic properties.

Explore further: Scientists show that graphene is highly efficient in converting light to electricity

More information: Johannsen JC, Ulstrup S, Crepaldi A, Cilento F, Zacchigna M, Miwa JA, Cacho C, Chapman RT, Springate E, Fromm F, Raidel C, Seyller T, King PDC, Parmigiani F, Grioni M, Hofmann P. "Tunable Carrier Multiplication and Cooling in Graphene." Nano Letters December 2 2014. DOI: 10.1021/nl503614v

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16 comments

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gkam
2.3 / 5 (3) Jan 19, 2015
Too many new developments. I wish I had some coal stock, so I could sell it.

This could put the last nail in the coffin of coal. And hopefully, nukes.

But I worry about the health effects of such sharp materials. They cut tissue. Graphite fibers are opaque to radio frequencies and lodge in the lung, so I expect some problems with the Fullerines, as well. I helped to build and operate the system that tested them for NASA.
gkam
2.3 / 5 (3) Jan 19, 2015
Sorry about the unclear post above: I tested the graphite fiber materials, not the Fullerines.
marinespill
not rated yet Jan 19, 2015
I should have stopped reading after the first line when the writer claimed they could convert photons into electrons. Kinda killed any hope of real information in the article.
Bill C_
not rated yet Jan 19, 2015
this is a great step forward towards the matter-antimatter reactor with nearly 100% efficiency in converting the annihilation product photons directly into electrical energy for EM fields and propulsion !!!!!!

Great Job !!!!!!!!! Great discovery !!!!!!!

Bill C.
gkam
1 / 5 (1) Jan 19, 2015
Perhaps marinespill could look up Einstein's first Nobel Prize.
dan42day
not rated yet Jan 19, 2015
With all these potential improvements to the efficiency of photovoltaics I keep reading about, it's only a matter of time before we will be able to power an entire city with the light from a single handheld flashlight!
Cg_A
not rated yet Jan 19, 2015
The graphene is then hit with an ultrafast 'pump' pulse of laser light. This excites the electrons in graphene, "raising" them to higher energy states where they can actually drive an electrical current.

This reminds me of throwing a rock in a swimming pool and calling the ripples a 'current' albeit in all directions from the point of inpact. Is that what you're thinking? Let's look at this a little closer.
a laser pulse of the right frequency can excite a large group of electrons, not just one, (the rock in the pool) and yes, these electrons can then give up the excitation with radiation that could potentially excite other nearby electrons in the graphene 'ring'. This could spread as a wave of electrons being excited and then 'dropping out' further propagating the wave. (toward the edge of the pool). I would not call this an electric current unless the effect propagates to the ends of the graphene film and is measured as an increase in free electron presence there.
Cg_A
not rated yet Jan 19, 2015
Further, (more like shock therapy back in the day) pulsing the surface with a hot laser may have raised the energy level of the electrons more than one level, ie, conduction band plus one or more levels.. something sunshine is very very unlikely to do.. sort of like ringing a bell with a bazooka shot when the average bell is rung with a tap of a spoon. Bottom line, I don't think there is anything to be learned from this report.
gkam
1 / 5 (1) Jan 19, 2015
CgA, give the researchers credit for hearing all that already.
swordsman
not rated yet Jan 20, 2015
A radio antenna does this, too, except that the "photon" is of much longer wavelength.
TheGhostofOtto1923
1 / 5 (3) Jan 20, 2015
Too many new developments. I wish I had some coal stock, so I could sell it.

This could put the last nail in the coffin of coal. And hopefully, nukes.

But I worry about the health effects of such sharp materials. They cut tissue. Graphite fibers are opaque to radio frequencies and lodge in the lung, so I expect some problems with the Fullerines, as well. I helped to build and operate the system that tested them for NASA.
Must have been one of the many 6 month jobs you had.
gkam
3 / 5 (2) Jan 20, 2015
The report abstract is in a NASA Technical Memorandum.

What did you do? Anything at all?
TheGhostofOtto1923
1 / 5 (3) Jan 21, 2015
The report abstract is in a NASA Technical Memorandum.

What did you do? Anything at all?
What we've done is immaterial to the facts we post. Conversely, nobody trusts a lying braggart whose facts are easily disproven, no matter what he's done or pretended to do.
gkam
3 / 5 (2) Jan 21, 2015
Sorry about your personal vendetta.

Do you want to see the report?
TheGhostofOtto1923
1 / 5 (3) Jan 21, 2015
Sorry about your personal vendetta
Dude, Ive been treating people like you exactly the same way here at physorg for some years now. Like I say I cant stand bullshit artists, which is what you have proven yourself to be.
Do you want to see the report?
Why? It is irrelevant to the thread. It lends absolutely no credence to what you post, which is typically crap.

If your crap cant stand by itself, what makes you think some 40yo report is going to help it do so? Bragging about it only makes you look pathetic.
Bartdude
5 / 5 (2) Jan 26, 2015
One thing that holds stronger than any graphene bond: life facts. We're no better than the way we treat our peers. In the case of TheGhostofOtto1923, this rule posits that all criticism you heap on another, merely rests in your personal karmic radiation field, it can't travel as far as you must believe it does. Mental telepathy is for gypsies and thieves, and the religious.

Now, if you don't mind, I'm here to learn science, not teach psychology.

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