Exciton Formation and Dissociation One-dimensional Graphene Nanoribbons

Dr. Hai Wang1,2, Dr. Alex Tries2, Mr. Guanzhao Wen2
1Utrecht University, Utrecht, Netherlands. 2MPI for Polymer Research, Mainz, Germany


Carbon-based nanostructures, such as zero-dimensional (0D) graphene quantum dots (GQDs), and one-dimensional (1D) graphene nanoribbons (GNRs) have emerged as promising building blocks for organic optoelectronics due to their tunable optical and electronic properties[1]. Understanding the dynamics and transport properties of photogenerated charge carriers is of great fundamental research interest and essential for device applications. 

I would like to present our recent efforts in understanding charge carrier dynamics in one-dimensional GNRs using ultrafast terahertz (THz) spectroscopy. First, we unveil the gigantic excitonic effects of solution-dispersed GNRs, exhibiting a large exciton binding energy of up to ~700 meV (see Figure 1) due to strong Coulomb interactions between photogenerated electron-hole pairs. Following above-electronic-bandgap excitations, photogenerated charge carriers appear transiently free (within 1 ps) and rapidly condense into exciton states in ~ps time scales owing to strong electron-phonon interactions in GNRs. While the strong exciton effects in GNRs can be beneficial for light emission applications, they pose substantial challenges for solar energy production e.g. solar cells where excitons are required to be separated. In the second part of my talk, I will discuss an intriguing hot carrier dissociation as one critical exciton-splitting mechanism in GNRs, and unveil how the length of GNRs plays a critical role in dictating the free carrier lifetime. These studies are not only relevant for fundamental photophysics but also demonstrate the bright future of carbon nanomaterials of organic optoelectronics.