Hot electron physics and applications

Generation, lifetime, and extraction of hot electrons at the surfaces of metallic materials and plasmonic nanostructures have been extensively studied and discussed in the last 10 years. This small but highly energetic population of non-thermal carriers—with energies well above the Fermi level—has expanded our perspective dramatically on controlling light-matter interactions and presents a variety of new challenges and opportunities. Our current understanding of their properties shows the way toward applications across many disciplines, including energy conversion, photocatalysis, photodetection, quantum chemistry, or optically-active materials, among others. Hot-electron behavior determines the properties of optically-excited materials at very short timescales, between the femtosecond and the picosecond scale. Although ultrafast probing techniques are required for their study, their short lifetimes also open the possibility of using them in devices with short response times.¹ In metals, hot carriers occur as a consequence of damping processes, which excite carriers into energetic states. This characteristic excess energy is particularly promising for applications such as solar energy harvesting, where strategies for reliably extracting hot carriers are highly desirable. Thus, fully understanding the dynamics of hot carriers and how to best exploit them can allow the development of materials and techniques to capture what would otherwise be dissipative losses in, e.g., semiconductor photovoltaic cells^2,3 or plasmonic photocatalytic systems.^4–6 Besides their interest in driving quantitative gains in the efficiency of energy conversion scenarios, they have been shown to drive physical mechanisms underscoring qualitatively new tools in light-driven processes. Examples of this include controlled nanocrystal growth and etching^7,8 and facilitating phase changes in neighboring materials.⁹ Furthermore, the physics of hot carriers can be exploited in processes of light emission through inelastic electron tunneling^10,11 or luminescence,¹² and for lasing in hybrid devices.¹³ Such exciting advances notwithstanding, this field still holds open questions and stands to gain from fundamental research, shedding light over topics such as how to characterize the internal hot carrier energy distribution in plasmonic structures.¹⁴ Consequently, the study of hot carrier physics is at a fertile stage where fundamental and applied work closely and fruitfully interact to expand the frontiers of the field. In the next section we describe the contributions of the “Hot Electron Physics and Applications” Special Topic in Journal of Applied Physics toward moving this exciting field forward.

The “Hot Electron Physics and Applications” Special Topic Collection presents work that cuts across fundamental and applied divisions in a range of lines of inquiry open in the field of study of hot electrons, addressing fundamental questions related to hot electron dynamics, and probing different approaches for increasing the efficiency and versatility of these energetic carriers in applied settings, such as photovoltaics and photocatalysis. Likewise, this selection of articles explores the behavior of a variety of materials, covering the study of plasmonic and semiconductor systems. In this latter category we can highlight studies contextualized within the improvement of solar cell efficiency, such as the characterization of hot carrier spatial distribution and thermalization using hyperspectral luminescence imaging in multi-quantum well structures,¹⁵ or a detailed study of hot carrier thermalization in GaAs absorber layers and the changes over volume and surface thermalization mechanisms in thin layers with varying thickness.¹⁶ Additionally, Ferry et al. present a general discussion on the physics of hot carrier solar cells and outline a proposal for using semiconductors with band structures offering metastable valleys where hot carriers suffer lower phonon-mediated losses.¹⁷ Outside of the context of energy harvesting, the Special Topic also features studies on the relevance of hot carrier physics in the application of semiconductor as light sources, such as the demonstration of enhanced emission intensity on a polycrystalline silicon light-emitting device by designing a cascading multijunction system supporting an avalanche mode of carrier injection.¹⁸ In the context of THz sources, Ramonas et al. study the correlation between excitations in the GHz range and THz oscillations in GaN quantum wells, mediated by the coupling of longitudinal optical phonons with hot electrons.¹⁹ 

Among the featured articles studying plasmonic materials, most have focused on aspects of hot carrier physics for light harvesting and conversion, but this field can also open windows to new physics and applications. A good example of such opportunities can be found in the work by Zhu et al., which explores a model of hot carrier excitation and recombination to explain the origin of above-threshold upconverted photons in electrically-driven plasmonic tunnel junctions, critically contrasting it with alternative theoretical descriptions.²⁰ At the same time, much of the interest of plasmonic hot carriers relates to their promise in solar harvesting, and underscore the importance of fully characterizing the capabilities of different materials in applications such as photocatalysis. The work by Fujita et al. creates spectrally comparable nanoarrays of Au and Ag, two relevant materials with ample presence in the literature, to set up a relevant comparison between these materials in terms of their capabilities to excite intraband hot carriers and their consequent effect driving a chemical transformation.²¹ Feng et al. use SERS spectroscopy as a technique to monitor the evolution of an optically-driven Suzuki-Miyaura reaction catalyzed with Au@Pd nanoparticles, and show that the reaction was driven by hot electrons rather than being a thermal effect.²² In contrast, Feng et al. use the photothermal properties of an array of Au half-shells to drive the synthesis of ZnO microspheres, structures that are then used to excite electron-hole pairs and act as photocatalytic agents in the reduction of AgNO3.²³ 

Hot carrier photodetection is another relevant application for plasmonic nanostructures, and it has attracted significant attention. McPolin et al. demonstrate the promise of using cavity-based plasmonic hot carrier photodetectors, affording both large performance gains and the possibility of creating dynamically tunable devices.²⁴ When using static systems, it is important to have a precise understanding of their tuning capabilities at the fabrication stage. Rahaman et al. explored the effect of annealing in reshaping the nanoantennas in plasmonic patterned metasurfaces and its subsequent impact on their resonant spectra, contrasting the spectral robustness of metamaterials made with different metals against high temperatures.²⁵ Regardless of the specific application of light conversion, understanding the mechanisms allowing the transfer of energy from the electronic degrees of freedom in the nanostructures is key to improve device efficiency. Du et al. studied the charge recombination in TiO₂ loaded with Au nanoparticles using transient absorption spectroscopy, exploring its dependence on the size of the plasmonic nanoparticles and proposing a mechanistic description of the process.²⁶ Naldo et al. present an analysis of the two-temperature model of electron thermalization and its usage in characterizing electron-phonon coupling in metals, including a detailed discussion on the sensitivity of the model in determining the physical parameters of the material.²⁷ 

Importantly, when exploring the possibilities of nanophotonic systems using plasmonic materials we are not limited to tailoring their responses through their geometry, and there is vibrant research in the field exploring the usage of materials beyond coinage metals. Wen et al. provide a thorough theoretical overview of the potential of using low-loss Na plasmonic metasurfaces for the efficient generation of hot carriers and their collection into a Si layer, and contrasting its properties with those of commonly used plasmonic metals.²⁸ Shinde et al. demonstrate the capabilities of TiN as a plasmonic material driving photoelectrochemical conversion through a wide spectral excitation of hot carriers, and explore different injection scenarios selecting both spectral sensitivity and type of injected carrier when combining TiN nanodisks with two distinct titanate-based oxide catalysts.²⁹ The work by Tofanello et al., which characterizes Au-enhanced hematite photoanodes in distinct configurations with and without Al oxides as interfacing layers, provides insights on the relevance of engineering the metal/semiconductor interfaces in plasmon-enhanced photoelectrocatalytic setups.³⁰ 

Altogether, the research curated for the “Hot Electron Physics and Applications” Special Topic Collection showcases and advances several aspects of the study of hot carriers, from characterization to exploitation, including discussion on new ways of approaching and evaluating known models and phenomena. The field of hot carrier physics lies at the core of the study of light-matter interaction for a wide range of materials, and these are important and timely contributions to a topic that not only probes fundamental questions in material science, but also holds the potential to significantly extend our capabilities in key areas of technological development, such as optoelectronics, telecommunications and solar energy harvesting. We hope that these contributions are found to be useful by the research community at large, and serve to spark new ideas to drive the field forward.

We want to thank the Associate Editor Professor Paolo Vavassori for leading the creation of this Special Topic in Hot Electron Physics, the staff and editors of the Journal of Applied Physics for their invaluable assistance at all stages of its elaboration, and the authors and reviewers for contributing with their work to the success of this Special Issue. L.V.B. acknowledges support from the Xunta de Galicia (Centro singular de investigación de Galicia accreditation 2019-2022), the European Union (European Regional Development Fund - ERDF) and the National Natural Science Foundation of China (Project No. 12050410252). E.C. acknowledges funding and support from the Deutsche Forschungsgemeinschaft (DFG, German Research Foundation) under Germanýs Excellence Strategy – EXC 2089/1–390776260, the Bavarian program Solar Energies Go Hybrid (SolTech), the Center for NanoScience (CeNS) and the European Commission through the ERC Starting Grant CATALIGHT (802989). S.I. acknowledges the financial support from the JSPS KAKENHI (17H04801, 19H04331), the TEPCO Memorial Foundation, and JST PRESTO (JPMJPR19I2). R.F.O. acknowledges support from the UK Engineering and Physical Science Research Council EP/M013812/1.