[Paper Review] Single-atom control of the optoelectronic response in sub-nanometric cavities

P. García‐González, Alejandro Varas+2 more|arXiv (Cornell University)|Mar 20, 2019

Gold and Silver Nanoparticles Synthesis and ApplicationsMaterials Science3 citations

TL;DR

This study demonstrates single-atom control over the optoelectronic response in sub-nanometric plasmonic junctions using ab-initio time-dependent density functional theory. By tuning the atomic identity of a single-atom bridge between two Na297 nanoparticles, the researchers show that both plasmonic resonances and photoinduced current can be dramatically tuned due to strong coupling between localized surface plasmons and charge-transfer excitations, enabling dual control over optical and transport properties at the atomic scale.

ABSTRACT

By means of ab-initio time dependent density functional theory calculations carried out on an prototypical hybrid plasmonic device (two metallic nanoparticles bridged by a one-atom junction), we demonstrate the strong interplay between photoinduced excitation of localized surface plasmons and electron transport through the single atom. Such an interplay is remarkably sensitive to the atomic orbitals of the junction. Therefore, we show the possibility of a twofold tuning (plasmonic response and photoinduced current across the juntion) just by changing a single atom in the device.

Motivation & Objective

• To understand the interplay between localized surface plasmons and electron transport in hybrid plasmonic-molecular junctions.
• To investigate how atomic-scale variations in a single-atom bridge affect the optoelectronic response of sub-nanometric plasmonic devices.
• To demonstrate that the optical and transport properties of such systems can be co-tuned by changing only the identity of the bridging atom.
• To provide a first-principles explanation of the coupling between plasmonic modes and charge-transfer excitations in atomic-scale junctions.

Proposed method

• Ab-initio time-dependent density functional theory (TDDFT) calculations were performed on a prototypical system: two Na297 icosahedral clusters separated by 0.72 nm, bridged by a single atom.
• The linear optical response was computed using the quasi-static approximation and a delta-kick electric field perturbation to simulate monochromatic light excitation along the dimer axis.
• The induced current and dipole response were extracted from the time evolution of the electron density following the perturbation.
• The system's electronic structure and density of states (DOS) were analyzed to correlate atomic orbital character with optoelectronic response.
• Phase shifts of the AC current were calculated as φc(ω) = arctan[Im I(ω)/Re I(ω)] to probe the dynamics of charge transfer and plasmonic coupling.
• The calculations used the OCTOPUS code with norm-conserving pseudopotentials and the generalized gradient approximation (GGA) for exchange-correlation.

Experimental results

Research questions

• RQ1 How does the identity of a single-atom bridge influence the plasmonic response of a sub-nanometric plasmonic junction?
• RQ2 To what extent can photoinduced current through the junction be tuned by atomic-level changes in the bridge?
• RQ3 What is the role of charge-transfer (CT) excitations in modifying the phase and amplitude of the AC current in the presence of plasmonic resonances?
• RQ4 How does the coupling between surface plasmon resonances and CT modes affect the optical absorption spectrum and current response?
• RQ5 Can the optoelectronic response be simultaneously tuned in both plasmonic and transport channels via a single-atom change?

Key findings

• The photoinduced current is strongly enhanced at plasmon resonance frequencies, particularly when charge-transfer (CT) modes are present, such as in Fe- and Mg-bridged systems.
• For Fe-bridged junctions, a strongly hybridized CT mode appears at ~3.5 eV, contributing significantly to both the optical absorption spectrum and the current, causing a split in the bonding dipole plasmon (BDP) peak.
• The phase of the AC current, φc(ω), shifts from ~−π/2 in the static limit for CT-dominated systems (e.g., Al- and Mg-bridged) to ~π/2 at the BDP frequency, indicating a transition from tunneling to resonant plasmonic response.
• In the plasmonic frequency range (2–4 eV), the phase at the BDP frequency is closer to π than to π/2 for CT-active systems, a distinctive signature of CT-plasmon coupling.
• The current response for the bare Na297 dimer is in-phase with the external field (φc ≈ 0) at low frequencies, while for Ar-bridged systems, a phase shift appears at lower frequencies due to new excitation channels above the barrier.
• The system's response is highly sensitive to the atomic orbital character of the junction, enabling dual tuning of plasmonic and transport properties through a single atomic substitution.