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. 2021 Jun 9;10:123. doi: 10.1038/s41377-021-00551-4

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© The Author(s) 2021

Open Access This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made. The images or other third party material in this article are included in the article’s Creative Commons license, unless indicated otherwise in a credit line to the material. If material is not included in the article’s Creative Commons license and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this license, visit http://creativecommons.org/licenses/by/4.0/.

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Fig. 1 — a–f Photon-type mechanisms: a The photovoltaic (PV) effect. The photo-excited e-h pairs driven by the built-in electric fields contribute to the photocurrents. b The internal photon emission (IPE) effect. For example, in a G-Si Schottky junction, the photo-excited hot carriers in graphene emitted over the Schottky barrier contribute to the photocurrent. c The direct tunneling (DT). d The Fowler-Nordheim (F-N) tunneling. e The photoconductive (PC) effect. The carrier density increment leads to the change of channel conductivity in a phototransistor. f The photo-gating (PG) effect. The photo-induced gating leads to the change of the channel conductivity in a phototransistor. g The thermal relaxation process in 2DMs. The carrier–carrier scattering results in the increment of the electron temperature T_e, and then the optical/acoustic phonon emissions lead to the increment of lattice temperature T_L. h, i Thermal-type mechanisms: h The bolometric (BOL) effect. The increment of T_e or T_L can be extracted by different read-outs. i The photo-thermoelectric (PTE) effect. Here Seebeck effect plays a major role. In the present example, S₁ and S₂ are respectively the positive and negative Seebeck coefficients.