TYC Highlight Seminar: A Tale of Two Particles: Hot-carrier transfer and Designing of Alloy Nanostructures for Optical Sensing

SAFB G34 plus the mezzanine on Level 1, Alexander Fleming, Imperial College London

Prof. Paul Erhart, Chalmers University of Technology

In the first part of this talk, I will discuss atomic scale simulations of plasmon-induced hot carrier generation and transfer. Metal nanoparticles are attractive for plasmon-enhanced generation of hot carriers, which may be harnessed in photochemical reactions.

While direct hot-carrier transfer can in principle be particular efficient for increasing photo-catalytic activity, it is difficult to discern experimentally and competes with several other mechanisms. In our work, we analyze the coherent femtosecond dynamics of photon absorption, plasmon formation, and subsequent hot-carrier generation through plasmon dephasing using first-principles simulations [1]. I will show how we can predict the energetic and spatial hot-carrier distributions in small metal nanoparticles and how they vary with particle size and shape. The distribution of hot carriers on a surface is, however, only one part in the transfer process, the other part being the receiving molecule (or semiconductor) [2]. In this context, I will discus how the dependence of the hot-carrier transfer probability on the nanoparticle-molecule distance and configuration. Our simulations show that hot-electron transfer can even be effective at long distances, well outside the region of chemisorption; hot-hole transfer on the other hand is limited to shorter distances. These observations can be explained by the energetic alignment between molecular and nanoparticle states as well as the excitation frequency. The hybridization of the molecular orbitals is the key predictor for hot-carrier transfer in these systems, emphasizing the need to include the effects of ground state hybridization for accurate predictions. Finally, I will show a non-trivial dependence of the hot-carrier distribution on the excitation energy, which could be exploited when optimizing photo-catalytic systems.

In the second part I will present recent results pertaining to the computational design of Pd nanoalloy structures for hydrogen sensing. Pd nanoalloys show great potential as hysteresis-free, reliable hydrogen sensors. Changes in hydrogen pressure translate to changes in hydrogen content and eventually the optical spectrum. Recently, we employed a multi-scale modeling approach to determine optimal conditions for optical hydrogen sensing using Pd-Au alloys. To this end, we combined electronic structure calculations of the dielectric response [3] with atomic scale simulations of the alloy thermodynamics [4] and electrodynamic simulations [5]. We carefully compare the simulation results with experimental data and assess potential sources for discrepancies. Invariably, the results suggest that there is an upper bound to the “optical” sensitivity that cannot be overcome by engineering composition and/or geometry. While the alloy composition has a limited impact on optical sensitivity, it can, however, strongly affect H uptake and consequently the “thermodynamic” sensitivity. Specifically, I will show how the latter can be improved by compositional engineering and even substantially enhanced via the formation of an ordered phase that can be synthesized at higher hydrogen partial pressures. 

[1] T. P. Rossi, P. Erhart, and M. Kuisma, “Hot-Carrier Generation in Plasmonic Nanoparticles: The Importance of Atomic Structure”, ACS Nano 14, 9963 (2020) 

[2] J. Fojt, T. P. Rossi, and P. Erhart, “Hot-carrier transfer across a nanoparticle-molecule junction: The importance of orbital hybridization and level alignment”, Nano Letters, accepted (2022)

[3] J. M. Rahm et al., “A Library of Late Transition Metal Alloy Dielectric Functions for Nanophotonic Applications”, Advanced Functional Materials 30, 2002122 (2020)

[4] J. M. Rahm et al., “A tale of two phase diagrams: Interplay of ordering and hydrogen uptake in Pd-Au-H “, Acta Materialia 211, 116893 (2021) 

[5] P. Ekborg-Tanner et al., “Computational Design of Alloy Nanostructures for Optical Sensing of Hydrogen”, ACS Appl. Nano Mater. 2022, 5, 8, 10225–10236

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Organiser: Johannes Lischner