[Paper Review] Exchange interactions and intermolecular hybridization in a spin-1/2 nanographene dimer

Nils Krane, Elia Turco+9 more|arXiv (Cornell University)|Jul 19, 2023

Molecular Junctions and NanostructuresEngineering3 citations

TL;DR

This study synthesizes and characterizes a spin-1/2 phenalenyl dimer on Au(111) and NaCl/Au(111) surfaces using inelastic electron tunneling spectroscopy (IETS), revealing exchange interactions and intermolecular hybridization. Key findings include a 48 meV singlet-triplet excitation energy on Au(111), reduced to 41 meV on NaCl due to substrate screening, and evidence of third-neighbor hopping-driven hybridization essential for kinetic exchange, enabling design of bottom-up spin lattices with strong exchange coupling.

ABSTRACT

Phenalenyl is a radical nanographene with triangular shape that hosts an unpaired electron with spin S = 1/2. The open-shell nature of phenalenyl is expected to be retained in covalently bonded networks. Here, we study a first step in that direction and report the synthesis of the phenalenyl dimer by combining in-solution synthesis and on-surface activation and its characterization both on Au(111) and on a monolayer of NaCl on top of Au(111) by means of inelastic electron tunneling spectroscopy (IETS). IETS shows inelastic steps that, together with a thorough theoretical analysis, are identified as the singlet-triplet excitation arising from interphenalenyl exchange. Two prominent features of our data permit to shed light on the nature of spin interactions in this system. First, the excitation energies with and without the NaCl decoupling layer are 48 and 41 meV, respectively, indicating a significant renormalization of the spin excitation energies due to exchange with the Au(111) electrons. Second, a position-dependent bias-asymmetry of the height of the inelastic steps is accounted for by an interphenalenyl hybridization of the singly occupied phenalenyl orbitals that is only possible via third neighbor hopping. This hybridization is also essential to activate kinetic interphenalenyl exchange. Our results set the stage for future work on the bottom-up synthesis of spin S = 1/2 spin lattices with large exchange interaction.

Motivation & Objective

• To synthesize and characterize a covalently bonded phenalenyl dimer as a prototype for spin-1/2 nanographene networks.
• To investigate exchange interactions between localized S = 1/2 spins in a bottom-up assembled molecular system.
• To disentangle the roles of intermolecular hybridization and substrate screening on spin excitation energies.
• To establish the mechanism of kinetic exchange in a dimer system via third-neighbor hopping.

Proposed method

• In-solution synthesis followed by on-surface activation to form the phenalenyl dimer on Au(111) and NaCl/Au(111) substrates.
• Use of inelastic electron tunneling spectroscopy (IETS) to probe spin excitations and extract singlet-triplet transition energies.
• Theoretical analysis combining density functional theory (DFT) and many-body calculations to model exchange interactions and hybridization.
• Comparison of IETS spectra with and without the NaCl decoupling layer to isolate substrate-induced renormalization of spin excitation energies.
• Analysis of bias-asymmetric inelastic step heights to infer spatially dependent hybridization of singly occupied molecular orbitals.
• Identification of third-neighbor hopping as the dominant mechanism enabling interphenalenyl hybridization and kinetic exchange.

Experimental results

Research questions

• RQ1 What is the magnitude and substrate dependence of the singlet-triplet excitation energy in a spin-1/2 phenalenyl dimer?
• RQ2 How does the Au(111) substrate influence the exchange interaction energy through coupling to conduction electrons?
• RQ3 What is the role of intermolecular orbital hybridization in enabling kinetic exchange between phenalenyl radicals?
• RQ4 Which hopping pathway (nearest, second, third neighbor) governs the hybridization responsible for spin exchange?
• RQ5 How does the NaCl capping layer modulate the observed inelastic step asymmetry and spin excitation energy?

Key findings

• The singlet-triplet excitation energy is 48 meV on bare Au(111), indicating a strong exchange interaction in the dimer.
• The excitation energy decreases to 41 meV on the NaCl/Au(111) surface, demonstrating significant renormalization due to screening by substrate electrons.
• A position-dependent bias-asymmetry in IETS inelastic step heights is attributed to interphenalenyl hybridization via third-neighbor hopping.
• Third-neighbor hopping is identified as the dominant mechanism enabling hybridization of singly occupied molecular orbitals and kinetic exchange.
• Theoretical modeling confirms that hybridization through third-neighbor pathways is essential for activating spin exchange and stabilizing the triplet state.
• The results establish a pathway for bottom-up synthesis of extended spin-1/2 lattices with tunable exchange interactions using molecular nanomaterials.