Proven: Graphene Makes Multiple Electrons From Light

http://spectrum.ieee.org/nanoclast/green-tech/solar/graphene-gets-another-boost-in-high-conversion-efficiency-photovoltaics

By Dexter Johnson

Photo: Getty Images 

Researchers at École Polytechnique Fédérale de Lausanne (EPFL) in Switzerland have for the first time observed and measured graphene converting a single photon into multiple electrons in a photovoltaic device.  This work should buoy hopes that graphene can serve as a material for photovoltaics with very high energy-conversion efficiencies.

The discovery builds on work conducted last year by the Barcelona-based Institute of Photonic Science (ICFO). ICFO scientists were able to indirectly show that graphene is capable of converting one photon into multiple electrons. In that research, the team excited the graphene by exposing it to photons of different energies (colors). They then used a pulse of terahertz radiation to measure the resulting hot-electron distribution. They determined that a higher photon energy (violet) resulted in higher numbers of hot electrons than a lower photon energy (infrared).

In this most recent EPFL work, the researchers had to devise a way to measure the conversion process, which occurs on a femto-second scale (10-15seconds). That’s far faster than any conventional method for detecting electron movement.

The team turned to a new technique called ultrafast time- and angle-resolved photoemission spectroscopy” (trARPES). The measurements themselves, the results of which were published in the journal Nano Letters, took place at the Rutherford Appleton Laboratory at Oxford University.

The graphene was place in an ultra-high vacuum chamber where the material was then hit with an ultrafast “pump” pulse of laser light. The laser light excites the electrons in the graphene bringing them to a higher energy state . In this heightened state, the graphene is then hit with a time-delayed, “probe” pulse that serves to take a snapshot of the energy each electron has at that moment. By doing this numerous times, the researchers create a kind of stop-motion movie of the conversion process.

“This indicates that a photovoltaic device using doped graphene could show significant efficiency in converting light to electricity,” said Marco Grioni of EPFL in a press release.

While nanomaterials in photovoltaics have held out the promise of converting a single photon into multiple electrons in research dating back to 2004, there have been skeptics as to whether this ability will actually lead to higher conversion efficiencies.

Eran Rabani, a researcher at Tel Aviv University, back in 2011 declared that he was not so convinced by the research on electron multiplication.

"Our theory shows that current predictions to increase efficiencies won't work,” Rabani said in a press release at the time. “The increase in efficiencies cannot be achieved yet through Multiexciton Generation, a process by which several charge carriers (electrons and holes) are generated from one photon."

This skepticism may account for why so much energy has been devoted to measuring and characterizing the generation of multiple electrons from a single photon.

But if multiple electron generation can—as some hope—boosted conversion efficiency to 60 percent from what was thought to be a 32 percent limit, then proving that the event indeed occurs is well worth it.

Semi-OT Fantastic Flash Memory Combines Graphene and Molybdenite

My Note: graphene transistors are believed to be  capable of switching at frequencies in the  terahertz range ,  it remains to be determined if  molybdenite has similar capability.

http://www.sciencedaily.com/releases/2013/03/130319144535.htm

EPFL scientists have combined two materials with advantageous electronic properties -- graphene and molybdenite -- into a flash memory prototype that is very promising in terms of performance, size, flexibility and energy consumption.

After the molybdenite chip, we now have molybdenite flash memory, a significant step forward in the use of this new material in electronics applications. The news is even more impressive because scientists from EPFL's Laboratory of Nanometer Electronics and Structures (LANES) came up with a truly original idea: they combined the advantages of this semiconducting material with those of another amazing material -- graphene. The results of their research have recently been published in the journal ACS Nano.
Two years ago, the LANES team revealed the promising electronic properties of molybdenite (MoS2), a mineral that is very abundant in nature. Several months later, they demonstrated the possibility of building an efficient molybdenite chip. Today, they've gone further still by using it to develop a flash memory prototype -- that is, a cell that can not only store data but also maintain it in the absence of electricity. This is the kind of memory used in digital devices such as cameras, phones, laptop computers, printers, and USB keys.
An ideal "energy band"
"For our memory model, we combined the unique electronic properties of MoS2 with graphene's amazing conductivity," explains Andras Kis, author of the study and director of LANES.
Molybdenite and graphene have many things in common. Both are expected to surpass the physical limitations of our current silicon chips and electronic transistors. Their two-dimensional chemical structure -- the fact that they're made up of a layer only a single atom thick -- gives them huge potential for miniaturization and mechanical flexibility.

Although graphene is a better conductor, molybdenite has advantageous semi-conducting properties. MoS2 has an ideal "energy band" in its electronic structure that graphene does not. This allows it to switch very easily from an "on" to an "off" state, and thus to use less electricity. Used together, the two materials can thus combine their unique advantages.
Like a sandwich
The transistor prototype developed by LANES was designed using "field effect" geometry, a bit like a sandwich. In the middle, instead of silicon, a thin layer of MoS2 channels electrons. Underneath, the electrodes transmitting electricity to the MoS2 layer are made out of graphene. And on top, the scientists also included an element made up of several layers of graphene; this captures electric charge and thus stores memory.
"Combining these two materials enabled us to make great progress in miniaturization, and also using these transistors we can make flexible nanoelectronic devices," explains Kis. The prototype stores a bit of memory, just a like a traditional cell. But according to the scientist, because molybdenite is thinner than silicon and thus more sensitive to charge, it offers great potential for more efficient data storage