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

TL;DR

This paper demonstrates that charge density gradients in non-uniform graphene devices induce a photothermoelectric effect, explaining observed photocurrent behaviors near charge neutrality and challenging previous interpretations of intrinsic photoresponse.

Contribution

It identifies the photothermoelectric effect as the primary source of photocurrent in non-uniform graphene, providing a comprehensive explanation for experimental observations.

Findings

PTE effect explains sign and spatial distribution of photocurrent

Photothermoelectric effect accounts for temperature dependence

Charge density gradients induce significant photocurrent signals

Abstract

Q. 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 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) which is strongly peaked around charge neutrality, i.e. 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 the intrinsic photocurrents in graphene. We propose future experiments which may disentangle the contributions of PTE and intrinsic photocurrent in graphene devices.