Simple Experimental Procedures to Distinguish Photothermal from Hot

Baffou et al. Light: Science & Applications (2020) 9:108 Official journal of the CIOMP 2047-7538 https://doi.org/10.1038/s41377-020-00345-0 www.nature.com/lsa REVIEW ARTICLE Open Access Simple experimental procedures to distinguish photothermal from hot-carrier processes in plasmonics Guillaume Baffou1, Ivan Bordacchini2,AndreaBaldi 3,4 and Romain Quidant2,5,6 Abstract Light absorption and scattering of plasmonic metal nanoparticles can lead to non-equilibrium charge carriers, intense electromagnetic near-fields, and heat generation, with promising applications in a vast range of fields, from chemical and physical sensing to nanomedicine and photocatalysis for the sustainable production of fuels and chemicals. Disentangling the relative contribution of thermal and non-thermal contributions in plasmon-driven processes is, however, difficult. Nanoscale temperature measurements are technically challenging, and macroscale experiments are often characterized by collective heating effects, which tend to make the actual temperature increase unpredictable. This work is intended to help the reader experimentally detect and quantify photothermal effects in plasmon-driven chemical reactions, to discriminate their contribution from that due to photochemical processes and to cast a critical eye on the current literature. To this aim, we review, and in some cases propose, seven simple experimental procedures that do not require the use of complex or expensive thermal microscopy techniques. These proposed procedures are adaptable to a wide range of experiments and fields of research where photothermal effects need to be assessed, such as plasmonic-assisted chemistry, heterogeneous catalysis, photovoltaics, biosensing, and enhanced molecular spectroscopy. 1234567890():,; 1234567890():,; 1234567890():,; 1234567890():,; Introduction pairs with an energy as high as a few eV. Such non- Driving chemical reactions with plasmonic nano- thermal, highly energetic charge carriers are termed hot particles is a rapidly growing field of research, with carriers in solid-state physics because they markedly potential applications of high economic and industrial deviate from the thermalized Fermi–Dirac energy dis- impact. Localized surface plasmon resonances in metal tribution of the free electrons in the metal. The transfer of nanoparticles can catalyze chemical reactions via optical these hot charge carriers from the nanoparticle to the near-field enhancement, heat generation, and hot-charge surrounding molecular adsorbates or photocatalytic 1 carrier injection . The latter mechanism, based on the use materials (such as TiO2) is capable of driving electronic of non-equilibrium electrons and holes to activate redox and chemical processes in the nanoparticle vicinity. Since reactions, was initially proposed in 2004, paving the way 2010, there has been a sudden rise in the number of to a very active branch of research in plasmonics2,3. publications related to hot-carrier plasmonics, driven by Photon absorption in plasmonic metal nanoparticles seminal work from the groups of Moskovits4,5, Halas6 and results in the excitation of non-equilibrium electron–hole Linic7,8, among others, and forecasting applications in nanochemistry8,9, water-splitting5,7,10, optoelectronics6,11, – and photovoltaics4,12 14. Correspondence: Guillaume Baffou ([email protected]) Depending on the application, different definitions of 1Institut Fresnel, CNRS, Aix Marseille University, Centrale Marseille, Marseille, hot electrons in plasmonics have been used, and some France clarification has to be made before going further to avoid 2ICFO-Institut de Ciències Fotòniques, The Barcelona Institute of Science and Technology, 08860Castelldefels, Barcelona, Spain confusion and ambiguity. After a photon is absorbed by a Full list of author information is available at the end of the article © The Author(s) 2020 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 linktotheCreativeCommons 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/. Baffou et al. Light: Science & Applications (2020) 9:108 Page 2 of 16 metal nanoparticle, a very energetic electron–hole pair is light, τe-e ~ 50 fs and hν are the photon energy. For a gold 4 2 created, with an energy equal to the photon energy hν. nanosphere, 50 nm in radius (σabs = 2×10 nm ) illumi- This energy is shared between these two carriers with a nated at 530 nm with I = 5×104 W/m2 (a typical value – – ratio that depends on where the excited electron origi- from the literature7 9,26 28), the time-average number of nates from within the conduction band15. These primary hot electrons in the nanoparticle under steady-state 15–17 −4 hot carriers are usually coined quasi-ballistic carriers . illumination hiNhot eÀ is ~10 . This low number means Within a few tens of fs18, the primary hot carriers ther- that for irradiances typically used in plasmon-assisted malize with the other electrons of the metal through photochemical experiments, primary hot-charge carriers electron–electron inelastic scattering events. These sub- are available for only ~0.01% of the time, i.e., on very brief sequent thermalized charge carriers have a strongly occasions. Under continuous-wave (CW) illumination, a reduced energy compared with the primary hot electrons, hot carrier always thermalizes before the absorption of the less than a few tenths of eV. However, they have also been next photon, such that no primary hot-carrier population called “hot” by a large part of the community, especially exists. A large population of primary hot carriers can exist those working with pulsed lasers, as such low energies still only under pulsed illumination where thousands of correspond to electronic temperatures on the order of a photons can be absorbed during τe-e. However, under few thousands of K. These thermalized, “warm” elec- the conditions mentioned above, the nanoparticle still trons16 should be distinguished from the primary, quasi- absorbs ~3 billion photons per second, generating 3 ballistic hot electrons because of their lower energy and billion primary hot electron–hole pairs. Thus, the very their longer lifetimes, of the order of picoseconds, dictated small lifetime of the primary hot carriers is unfavorable by multiple, sequential electron–phonon scattering pro- but does not necessarily preclude detectable hot-carrier- cesses. In hot-carrier-assisted plasmonic chemistry, only assisted processes, a priori. The community is aware of primary hot electrons have enough energy to contribute this issue, and several recent studies directly analyze the to chemical reactions. respective contributions of hot electrons and photother- The actual involvement of hot carriers in several mal effects, as shown in Fig. 1. Nevertheless, although chemistry experiments has been recently questioned, with plasmonic nanoparticles are excellent light-to-heat con- the proposition of alternative mechanisms, such as direct verters, the associated temperature increase is often photoexcitation of hybrid particle–adsorbate com- difficult to predict and measure. – – plexes19 21 or simple heat generation22 24. Indeed, the This article is intended to help experimentalists dis- further thermalization of these excited carriers via criminate between thermal and non-thermal effects in electron–phonon scattering leads to heating of the entire plasmon-driven chemical processes. To this aim, we nanoparticle and further heat diffusion to the surrounding propose seven simple experimental procedures that avoid reaction medium, suggesting that photothermal effects the use of complex or expensive thermal microscopy may also contribute to the observed reactivity techniques, which may sometimes be inaccurate29,30. enhancement25. These procedures are described hereinafter and critically The main concern with primary hot carriers resides in illustrated with some practical examples from the litera- their very short lifetime. They thermalize via ture. In the last section, we also provide some practical electron–electron scattering within a time scale τe-e of a guidance on how to avoid common pitfalls when using few tens of fs for gold18, making any interaction with the numerical simulations to estimate photothermal effects in surrounding environment a low-probability event. The plasmonic systems, highlighting the importance of col- time-average number of primary hot electrons generated lective photothermal effects in plasmonics. in a single nanoparticle under illumination can be quan- tified using this simple expression: Procedure # 1: varying the illumination power In the case of a photochemical process in plasmonics, σ τ absI eÀe such as near-field enhancement of photochemical reac- hiN À ¼ ð1Þ hot e hν tions or hot-charge-carrier-assisted redox reactions at the where σabs is the absorption cross section of the nanoparticle

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