Graphene is an attractive candidate for many optoelectronic applications because of its vanishing bandgap and high carrier mobility. An essential process for such applications is the dissipation of the energy of photo-excited charge carriers in graphene. Two competing energy relaxation mechanisms for optically excited carriers exist [1]: They can (i) thermalize with intrinsic carriers near the Fermi level, heating them to higher energy states through the process of ‘hot carrier multiplication’; or (ii) the excess energy of the optically excited carrier can be lost via emission of phonons. With optical excitation-THz probe spectroscopy, an optical method for probing photoconductivities on ultrafast timescales, we reveal highly efficient energy transfer from an optically excited carrier to multiple heated charge carriers (relaxation path (i)) . Carrier heating is also highly efficient when intrinsic carriers are accelerated by a strong electric field [2]. We further show how the branching ratio between (i) and (ii) can be tuned externally [3].While presenting an advantage for some applications, the vanishing bandgap of graphene can also be a disadvantage in applications such as photovoltaics. A chemical synthesis approach was recently shown for making well defined, narrow graphene nanoribbons (GNRs) with widths as small as ~1 nm.[4] In these structures, carrier confinement in he lateral dimension induces a bandgap corresponding to visible wavelengths. Carbon nanotubes (CNTs) are similar one-dimensional graphene nanostructures with potential bandgap. The complex photoconductivity of semiconducting GNRs and CNTs reveals that, while the mechanism of photoconductivity is very similar in the two nanostructures, the charge mobility is quite different [5].  

[1] K. J. Tielrooij, et al. Photoexcitation cascade and multiple hot-carrier generation in graphene, Nature Phys. 2013, 9, 248.

[2] Z. Mics, et al., Thermodynamic picture of ultrafast charge transport in graphene, Nature Comm. 2015, 6, 7655.

[3] S. A. Jensen , et al. Competing Ultrafast Energy Relaxation Pathways in Photoexcited Graphene, Nano Lett. 2014, 14, 5839.

[4] A. Narita, et al. , Synthesis of structurally well-defined and liquid-phase-processable graphene nanoribbons, Nature Chem. 2014, 6, 126.

[5] S. A. Jensen, et al. , Ultrafast photoconductivity of graphene nanoribbons and carbon nanotubes, Nano Lett. 2013, 13, 5925.