[Paper Review] Hot-carrier transfer across a nanoparticle-molecule junction: The importance of orbital hybridization and level alignment

Jakub Fojt, Tuomas Rossi+1 more|arXiv (Cornell University)|Jun 10, 2022

Gold and Silver Nanoparticles Synthesis and Applications74 references49 citations

TL;DR

This study uses first-principles real-time time-dependent density functional theory (RT-TDDFT) to investigate hot-carrier transfer across Ag, Au, and Cu nanoparticle–CO molecule junctions, revealing that electron transfer depends non-monotonically on distance and is strongly influenced by orbital hybridization and energy level alignment. The key finding is that hot-electron transfer remains effective up to 5 Å from the nanoparticle, even beyond chemisorption distances, while hot-hole transfer is limited to shorter ranges, highlighting the critical role of ground-state hybridization for accurate prediction of hot-carrier efficiency in plasmonic photocatalysis.

ABSTRACT

While direct hot-carrier transfer can increase photo-catalytic activity, it is difficult to discern experimentally and competes with several other mechanisms. To shed light on these aspects, here, we model from first principles hot-carrier generation across the interface between plasmonic nanoparticles and a CO molecule. The hot-electron transfer probability depends non-monotonically on the nanoparticle-molecule distance and can 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, we show a non-trivial dependence of the hot-carrier distribution on the excitation energy, which could be exploited when optimizing photo-catalytic systems.

Motivation & Objective

• To understand the mechanisms governing hot-carrier transfer across plasmonic nanoparticle–molecule junctions, particularly in direct hot-carrier transfer processes.
• To investigate how geometric configuration, distance, and material (Ag, Au, Cu) influence hot-carrier generation efficiency.
• To determine the role of orbital hybridization and energetic level alignment in mediating hot-carrier transfer beyond chemisorption distances.
• To quantify the dependence of hot-carrier distribution on excitation frequency and system geometry for optimizing photocatalytic efficiency.

Proposed method

• First-principles real-time time-dependent density functional theory (RT-TDDFT) is used to simulate electron dynamics after ultrafast laser excitation of plasmonic nanoparticles.
• A Gaussian laser pulse tuned to the localized surface plasmon resonance (3.8 eV) drives the system, and carrier generation is analyzed post-decay via Kohn-Sham state populations.
• Hot-carrier fractions are computed in the molecule and nanoparticle using spatial probability densities and energetic distributions, with contributions separated by spin and orbital character.
• The system is modeled across multiple adsorption sites: (111) on-top, (111) fcc, (100) hollow, and corner, with rigid translations to probe distance dependence.
• Ground-state hybridization is quantified via projected density of states (PDOS) and wave function overlap, with Kohn-Sham eigenvalues used to assess level alignment.
• The analysis includes spatio-energetic probability densities and carrier fractions in Voronoi regions to isolate molecular contributions.

Experimental results

Research questions

• RQ1 How does the distance between a plasmonic nanoparticle and a CO molecule affect hot-electron transfer efficiency?
• RQ2 What is the role of orbital hybridization and level alignment in determining hot-carrier transfer at varying interfacial distances?
• RQ3 Why does hot-electron transfer remain effective at long distances (up to 5 Å), while hot-hole transfer is limited to shorter ranges?
• RQ4 How does the excitation frequency influence the distribution and localization of hot carriers in the system?
• RQ5 Can non-monotonic carrier transfer behavior be explained by hybridization and energetic alignment rather than just wave function overlap?

Key findings

• Hot-electron transfer probability remains significant (0.5–2%) at distances up to 5 Å from the nanoparticle, indicating effective transfer well beyond chemisorption distances.
• Hot-hole transfer is limited to shorter distances, with maximum transfer efficiency of 0.2–0.8%, and decays smoothly with increasing separation.
• The non-monotonic distance dependence of electron transfer is explained by resonant coupling via hybridized molecular and nanoparticle states, not just wave function overlap.
• Orbital hybridization is the dominant predictor of hot-carrier transfer efficiency, emphasizing the need to include ground-state hybridization in predictive models.
• The hot-carrier distribution exhibits a non-trivial dependence on excitation energy, with distinct features at 2.1, 2.7, 3.3, and 3.9 Å, indicating tunable response across different geometries.
• The (111) fcc site shows the most featureless behavior, while other sites display multiple peaks in transfer efficiency, indicating site-specific control potential.