Experimental Observation of Strong Exciton Effects in Graphene Nanoribbons.

Alexander Tries,Hai I. Wang,Yiyong Mai,David Beljonne,Fugui Xu,Mathias Kläui,Charusheela Ramanan,Silvio Osella,Pengfei Zhang

doi:10.1021/acs.nanolett.9b04816

Abstract

Graphene nanoribbons (GNRs) with atomically precise width and edge structures are a promising class of nanomaterials for optoelectronics, thanks to their semiconducting nature and high mobility of charge carriers. Understanding the fundamental static optical properties and ultrafast dynamics of charge carrier generation in GNRs is essential for optoelectronic applications. Combining THz spectroscopy and theoretical calculations, we report a strong exciton effect with binding energy up to ∼700 meV in liquid-phase-dispersed GNRs with a width of 1.7 nm and an optical band gap of ∼1.6 eV, illustrating the intrinsically strong Coulomb interactions between photogenerated electrons and holes. By tracking the exciton dynamics, we reveal an ultrafast formation of excitons in GNRs with a long lifetime over 100 ps. Our results not only reveal fundamental aspects of excitons in GNRs (strong binding energy and ultrafast exciton formation etc.) but also highlight promising properties of GNRs for optoelectronic devices.

Highlights

Owing to their massless nature, charge carriers in graphene can possess an extremely high mobility,[1,2] which makes graphene a promising platform for microelectronic[3] and spintronic[4] devices
We find that excitons in graphene nanoribbons (GNRs) can be formed within 0.8 ps from the initial free charges following a direct photoexcitation of charges into the conduction band, which may be due to a strong electron−phonon coupling effect, facilitating fast energy dissipation of hot carriers
The generated excitons are found to be long-lived over 100 ps, rendering GNRs promising for optoelectronic applications

Summary

■ INTRODUCTION

Owing to their massless nature, charge carriers in graphene can possess an extremely high mobility,[1,2] which makes graphene a promising platform for microelectronic[3] and spintronic[4] devices. Huang et al.[31] provided evidence that the lowest exciton transition in GNR is vibronic in nature While all these static and ultrafast studies provide strong evidence for a large exciton effect, a direct access and experimental quantification of the exciton binding energy in GNRs remains challenging. A way to circumvent this problem is to combine more sophisticated one- and two-photon spectroscopy techniques that quantify the 1S−2P splitting of the exciton states.[34] This approach has been successfully applied for quantifying EB in different nanostructures, including carbon nanotubes[35,36] or conjugated polymers,[37] by further modeling the energy difference between 2P states and the electronic band continuum This method requires a relatively high photoluminescence quantum yield of the materials, which may limit its general applicability. Our results demonstrate fundamental aspects of excitons in GNRs and highlight the great promise of GNRs for optoelectronic devices

■ RESULTS AND DISCUSSION

■ CONCLUSION

■ ACKNOWLEDGMENTS

■ REFERENCES