Dirac-point photocurrents due to the photothermoelectric effect in non-uniform graphene devices

Ma et al.1 recently reported a strong photocurrent associated with charge neutrality in graphene devices with non-uniform geometries, which they interpreted as an intrinsic Dirac-point (IDP) photoresponse enhanced by the momentum non-relaxing nature of electron–electron collisions at charge neutrality. Here we argue that gradients in charge carrier density give rise to a photothermoelectric effect (PTE) that is strongly peaked around charge neutrality, that is at p–n junctions, and such p–n junctions naturally arise at the edges of graphene devices due to fringing capacitance. Using known parameters, the PTE effect in the presence of charge density gradients predicts the sign, spatial distribution, gate voltage dependence and temperature dependence of the photoresponse in non-uniform graphene devices, including predicting the observed sign change of the signal away from charge neutrality and the non-monotonic temperature dependence, neither of which is explained by IDP photocurrents in graphene. We propose future experiments that may disentangle the contributions of PTE and intrinsic photocurrent in graphene devices.