The materials we use in everyday life have a three-dimensional structure.

In 2004, a two-dimensional solid, graphene, was first realised by isolating a single layer of carbon atoms. This discovery paved the way for the realisation of a large class of two-dimensional crystalline materials, whose physical properties differ drastically from those of three-dimensional materials. More recently, it has been observed how, by joining individual 2D layers of a pair of different materials, it is possible to combine the unique and complementary properties of the basic constituents, enabling the realisation of innovative devices in the field of photonics and electronics.

One of the most impressive application solutions for this family of solids, known as 'heterostructures', has been achieved by combining graphene with certain two-dimensional semiconductors (Transition metal dichalcogenides). Their implementation makes it possible to create light detectors that, thanks to the very high electron mobility of graphene and the excellent absorption capacity of the semiconductor, are significantly faster and more efficient than the technologies currently used.

The physical mechanism underlying the conversion and transfer of the light captured by the semiconductor into an electrical signal in graphene, although crucial for the optimal design of these light detectors, is still hotly debated.

Today, the Femtoscopy research team directed by Tullio Scopigno of the Department of Physics of Sapienza University of Rome, in collaboration with the Strasbourg Institute of Material Physics and Chemistry and the Italian Institute of Technology, which participates thanks to the support of the European “Graphene Flagship” project, has come up with results that are fundamental for understanding the microscopic processes underlying devices that interact with light and convert electrical signals into optical signals and vice versa.

The study, published in the journal PNAS, first established that energy transfer between the two materials occurs very quickly, a time quantified in a few picoseconds (a picosecond is the thousandth part of a billionth of a second).

Furthermore, by monitoring the temperature of the graphene electrons, the researchers described the mechanisms of energy exchange between the two materials during this tiny fraction of a second.

Initially, it was believed that the conversion of light absorbed by the semiconductor into an electric current in graphene required a positive or negative net charge transfer.

The experiment showed that the light energy absorbed by the semiconductor is converted into thermal agitation of the electrical charges in the graphene. Specifically, in the first moments following the absorption of light by the semiconductor, what is transferred to the graphene is a packet of energy associated with a neutral exciton (consisting of a pair of positive and negative charges bound together), not a net charge. The entire exciton is transferred to the graphene (hence electrically neutral) and not a single charged fragment as initially thought.

'These results were possible thanks to an innovative spectroscopic technique,' says Tullio Scopigno, 'which employs pairs of ultra-short light pulses lasting one picosecond. The first pulse deposits energy in the semiconducting material. The second, by measuring the temperature of the electrons in the graphene, makes it possible to probe the transfer of energy and/or charge between the two materials'.

Understanding how energy exchanges take place between graphene and other two-dimensional materials is a key step towards implementing state-of-the-art, rationally designed optoelectronic devices such as solar cells, LEDs, touchscreens and photodetectors.

References:

Picosecond energy transfer in a transition metal dichalcogenide–graphene heterostructure revealed by transient Raman spectroscopy – Carino Ferrante, Giorgio Di Battista, Luis E. Parra López, Giovanni Batignani, Etienne Lorchat, Alessandra Virga, Stéphane Berciaud, Tullio Scopigno - Proceedings of the National Academy of Sciences (2022) https://doi.org/10.1073/pnas.2119726119

Further Information

Tullio Scopigno
Department of Physics
tullio.scopigno@uniroma1.it