- Scientists observe high intensity light emitting from individual graphene nanoribbons.
- It’s 100 times more intense than the previous single-molecular optoelectronic device.
- The color of light, emitted by 7-atom-wide nanoribbons, can be altered by adjusting the voltage.
Graphene nanoribbons (GNRs), strips of graphene with width less than 50 nanometers, are known for their highly tunable band gaps. For the first time, scientists have observed light emitting from individual graphene nanoribbons.
A team of European researchers from Université de Strasbourg, France, and the CNR-Nanoscience Institute, Italy, has demonstrated the feasibility of controllable graphene-based light emitting devices.
Unlike full graphene sheets, GNRs (when cut into very thin pieces) obtain a sizable optical band gap. The 7-atom-wide nanoribbons emit light at a high intensity and the color of light can be altered by adjusting the voltage. One day, this might lead to a development of bright graphene based light sources.
While conducting experiments, scientists found that the individual graphene nanoribbons were flashing a bright and narrow band of red light at higher intensity than bright light emitting devices made of carbon nanotubes. It was emitting almost 10 million photos per second, almost 100 times more intense than the previous optoelectronic device (single-molecular).
The color of the light can be tweaked by shifting the energy of the main peak with voltage. To form an optoelectronic circuit, scientists suspended individual nanoribbons between a gold substrate Au(111) and a tip of a scanning tunnelling microscope. This decreases the coupling between the GNR and the electrodes that otherwise quenches the emission.
Reference: Nano Letters | doi: 10.1021/acs.nanolett.7b03797
The 7-atom-wide armchair GNRs present an optical gap in the visible spectrum (2 eV) and can be obtained by on-surface synthesis. The electroluminescence spectrum of such a junction highly depends on the nature of the GNR tip connection.
GNR is partially suspended by a microscope tip | Credit: American Chemical Society
In the case of a covalent bond, a sharp and intense luminescence peak emerges, whose energy changes with tip-sample distance and voltage. A transition energy of 1.16 eV is obtained when extrapolating to an unbiased junction, which is quite lower than the optical gab (2 eV) reported for 7-atom-wide armchair GNRs on Au.
They carried out ab initio calculations at GW BetheSalpeter equation level to simulate the optical spectra of finite size of GNRs. The calculations show that bright emission involves electronic states localized at the graphene nanoribbon termini. The emission band can be assigned to a transition between a state localized at a GNR extremity and a state delocalized along the ribbon.
The next step is to study the aspect ratio (width) of GNR on emission and impacts of defects by integrating GNR devices into bigger circuitry to generate robust, bright and controllable light emitting device based on graphene.