Spectral shifts in tip-induced light from plasmonic nanoparticles in air

Mario Zapata-Herrera, Benoît Rogez, Sylvie Marguet, Gérald Dujardin, Elizabeth Boer-Duchemin, and Eric Le Moal

Phys. Rev. B 109, 155433 – Published 26 April 2024

Abstract

In this article, we carry out an in-depth study of the scanning tunneling microscopy-induced luminescence spectra (STML) of individual plasmonic nanoparticles measured in air. When compared to the results of far-field light scattering measured under the same ambient conditions, the STML measurements show spectral shifts and peak broadening of hundreds of meV, even when a nonplasmonic tip is used for STML. We simulate the near-field excitation and the effect of the tip using the finite-element method and show that these effects alone cannot explain the spectral shifts and peak broadening observed for STML experiments in air. However, the experimental results are well reproduced in the numerical simulations if the screening effect of a water meniscus bridge present in the tip-nanoparticle gap is considered. Our results pave the way for finer interpretations of STML experiments in air, where ignoring this additional screening effect can lead to an incorrect mode assignment of the observed resonances.

Received 1 February 2024
Revised 5 April 2024
Accepted 8 April 2024

DOI:https://doi.org/10.1103/PhysRevB.109.155433

Physics Subject Headings (PhySH)

Liquid bridges Surface plasmons

Nanoparticles Tunnel junctions

Finite-element method Light scattering Optical microscopy Scanning probe microscopy Scanning tunneling microscopy

Condensed Matter, Materials & Applied PhysicsAtomic, Molecular & Optical

Authors & Affiliations

Mario Zapata-Herrera ¹, Benoît Rogez ^2,*, Sylvie Marguet³, Gérald Dujardin², Elizabeth Boer-Duchemin ², and Eric Le Moal ^2,†

¹Donostia International Physics Center (DIPC), Donostia-San Sebastián 20018, Spain
²Université Paris-Saclay, CNRS, Institut des Sciences Moléculaires d'Orsay, 91405 Orsay, France
³Université Paris-Saclay, CEA, CNRS, NIMBE, 91191 Gif-sur-Yvette, France

^*Present address: Laboratory Light, nanomaterials and nanotechnologies - L2n, University of Technology of Troyes - CNRS EMR 7004, 12 rue Marie Curie, 10000 Troyes, France.
^†eric.le-moal@universite-paris-saclay.fr

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Issue

Vol. 109, Iss. 15 — 15 April 2024

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Images

Figure 1
(a) Scanning electron micrograph of two nanocubes, 75 nm in edge length, showing their regular shape and rounded corners. (b) Dark-field optical microscopy image of individual nanocubes, in gray scale. (c) Experimental setup used for dark-field optical spectroscopy of single NPs upon illumination with linearly polarized white light at grazing incidence. Light is collected through the substrate. (d) Light scattering spectra of the single NP selected in image (b), measured using the setup described in (c). $s$ and $p$ refer to the orientation of the polarization of the excitation field, either perpendicular ( $s$ ) or parallel ( $p$ ) to the plane of incidence $(y z)$ defined in (c). Thus the spectra labeled $s$ and $p$ correspond to the in-plane (along $x$ ) and out-of-plane (along $z$ ) dipolar resonances of the NP, respectively. The experimental data are background subtracted, corrected for both the power spectrum of the source and for the detection efficiency of the setup and are fit with Lorentzian profiles.
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Figure 2
(a) Experimental setup used for optical microscopy of the STML of NPs on a transparent conductive substrate. (b) Bright-field optical microscopy image of individual nanocubes, in gray scale. (c) Real-space optical microscopy image of the STM-induced luminescence (STML) of the same area upon electrical excitation of the single nanocube in the center of the image shown in (b) (false-color scale). STML spectra of (d) the nanocube shown in image (c) (“NP 1”); (e) STML spectra of another nanocube of the same sample (“NP 2”). Sample bias in both cases is 2.75 V, current set point is 100 pA, and acquisition time is 300 s. The STM tip is located in the center and on the corner of the cube's top face, as indicated on the STM topography image in the inset. The experimental data are corrected for both the theoretical power spectrum of the inelastic tunneling current and for the detection efficiency of the setup. The STML spectra measured at the “center” and “corner” tip positions are expressed in arbitrary units, i.e., they are not normalized with respect to the tunneling current. We have normalized these spectra with respect to their intensity at a photon energy of 1.81 eV, in order to highlight the difference between the two spectra. (f),(g) Fourier-space optical microscopy images of the STML of the same nanocube as in (c) and (d), for “center” and “corner” tip positions (2.75 V, 100 pA, and 120 s). All the STML data shown in this figure have been obtained using the same tungsten tip.
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Figure 3
Numerical simulations of the energy shifts and peak broadening due to tip and near-field excitation effects, which are present in experiments carried out in both air and vacuum. The surrounding medium (including the medium in the gap between the tip and the NP) has an index of refraction of 1. Dashed lines: scattering cross sections of a nanocube for $p$ -polarized illumination [in (a) and (b)]. Dotted line: scattering cross section of a nanocube for $s$ -polarized illumination [in (b)]. Solid lines: emission spectra for a $z$ -oriented oscillating electric dipole located in the center [in (a)] and on the corner [in (b)] of the cube's top face in the absence (red line) and in the presence (blue line) of the tungsten (W) tip. All curves are normalized with respect to their maximum value.
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Figure 4
Effect of the tip-sample water bridge. Similar calculations as in Fig. 3, but this time including the presence of a transparent medium of higher refractive index $n > 1$ (i.e., a water bridge) in the tip-sample gap. From right to left, the curves (blue lines) correspond to the results obtained for $n = 1.30$ , 1.34, 1.38, and 1.42. A vertically ( $z$ )-oriented oscillating electric dipole located in the middle of a 2-nm-sized tip-sample gap, i.e., 1 nm above the center of the cube's top face, is considered. The right hand side of the figure shows the geometry of the water bridge considered in the calculations.
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Figure 5
Comparison: STML vs dark-field optical spectroscopy; experiment vs theory. (a) Light scattering spectrum of a single nanocube measured using dark-field optical spectroscopy under $p$ -polarized illumination [same data as shown in Fig. 1] and STML spectrum of a single nanocube upon local excitation with the tip located in the center of the cube's top face [same data as shown in Fig. 2]. (b) Numerically calculated emission spectra for a vertically ( $z$ )-oriented oscillating electric dipole located 1 nm above the center of the cube's top face, in the absence and in the presence of the tungsten tip and water bridge [same data as shown in Figs. 1 and 4, respectively]. Experimentally, the STML peak shifts by $- 0.35 eV$ and is 0.14 eV wider than the DFM peak. Theoretically, the corresponding values are $- 0.23 eV$ and 0.21 eV.
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