Jan 19 2015
Researchers at Ecole polytechnique fédérale de Lausanne (EPFL), along with their collaborators have demonstrated the ability of graphene to convert a single photon into multiple electrons that possess energy that is sufficient enough for driving an electrical current. The team used an advanced spectroscopic method for this demonstration. This discovery holds great promise for development of future photovoltaic devices.
For this project, EPFL’s Laboratory of Photoelectron Spectroscopy has collaborated with Aarhus University, the Universität Erlangen Nürnberg, the University of St. Andrews, Elettra-SincrotroneTrieste S.C.p.A, the Rutherford Appleton Laboratory, Technische Universität Chemnitz, the University of Trieste and the IOM-CNR Laboratorio TASC.
Graphene is light in weight and demonstrates extraordinary strength, due to which it has gained immense popularity during recent times. It can be produced at low cost as it can be grown on top of different types of materials or by peeling it off from the graphite material. Studies performed until now have hinted at the possibility of using graphene as a photovoltaic material for converting light into electricity. In this study, the researchers have used an advanced spectroscopic method to demonstrate the potential of graphene to turn light in to electricity.
When considered from the point of fundamental physics, graphene arouses great interest. This is because, when compared with other materials like copper, it is a better conductor of electricity at room temperature. This property makes it suitable for applications in ultra-fast circuits. Furthermore, it has been demonstrated that after absorption of light, graphene can conduct electricity, which would allow it to be used for photovoltaic devices. However, until now, the potential for graphene to convert light-to-electricity efficiently has not been well understood.
This light-to-electricity conversion occurs on a femto-second scale, which is a quadrillionth of a second. This is very fast and conventional techniques do not have the ability to detect movement of electrons on such a scale. To address this challenge, “ultrafast time- and angle-resolved photoemission spectroscopy” (trARPES), which is a sophisticated technique, was used by Jens Christian Johannsen from EPFL Marco Grioni’s lab along with collaborators from ELETTRA in Italy and the Aarhus University. This study was performed at Rutherford Appleton Laboratory in Oxford.
In this method, the researchers placed a graphene sample in an ultra-high vacuum chamber. They then used an ultrafast ‘pump’ pulse of laser light to hit this graphene sample. This results in the electrons in graphene becoming excited. They are then raised to higher energy states where they are able to drive an electrical current. When the electrons are in these higher energy states, a time-delayed, ‘probe’ pulse is used to hit the graphene sample.This provides a snapshot of the energy possessed by each electron at that time. Similar to a stop-motion movie, the researchers repeated the sequence rapidly for various time points. This enabled capture of the electron’s dynamics in a live-action sequence.
“Doped” samples of graphene were used for the experiment. This involved using chemical methods to add or remove electrons. The researchers found that when a single photon was absorbed by the doped graphene, it could excite numerous electrons and this occurred in proportion to the amount of doping. An electron is excited by the photon, and this makes it to “fall” back to its ground state of energy rapidly. When this occurs, on an average two more electrons are excited by the “fall” due to a knock-on effect.
This indicates that a photovoltaic device using doped graphene could show significant efficiency in converting light to electricity.
Marco Grioni
This is considered to be the first-ever direct study of the photon-electron multiplication effect of graphene. This makes graphene a potential building block for devices that depend on conversion of light into electricity. When compared with currently used systems, novel photovoltaic devices that utilize graphene would have the ability to harvest light energy with lower energy loss. This harvesting could occur across the entire solar spectrum.
The researchers are now intending to study similar effects in molybdenum disulphide (MoS2), and other such two-dimensional materials based on the advanced technology and success of the present experiment. Molybdenum disulphide is recognized for its amazing catalytic and electronic properties.
This study has been reported in the journal, Nano Letters.
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