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Thermometry of plasmonic heating by Inelastic Electron Tunneling Spectroscopy (IETS) Nirit Nachman, and Yoram Selzer Nano Lett., Just Accepted Manuscript • DOI: 10.1021/acs.nanolett.7b03153 • Publication Date (Web): 23 Aug 2017 Downloaded from http://pubs.acs.org on August 23, 2017
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Nano Letters
Thermometry of plasmonic heating by Inelastic Electron Tunneling Spectroscopy (IETS) Nirit Nachman & Yoram Selzer* School of Chemistry, Tel Aviv University, Tel Aviv 69978, Israel
[email protected] The electronic and lattice heating accompanying plasmonic structures under illumination is suggested to be utilized in a broad range of thermoplasmonic applications. Specifically, in molecular electronics, precise determination of the temperature of illuminated junctions is crucial, since the temperature dependent energy distribution of charge carriers in the leads affects the possibility to steer various light-controlled conductance processes. Existing optical methods to characterize the local temperature in all these applications lack the spatial resolution to probe the few nanometers in size hot spots and therefore typically report average values over a diffraction limited length scale. Here we demonstrate that Inelastic Electron Tunneling Spectroscopy (IETS) of molecular junctions based on thiolalkyl chains can be used to precisely measure the temperature of metal nano-scale gaps under illumination. The nature of this measurement guarantees that the reported temperature indeed characterizes the confined volume in which heat is produced by the relaxation of hot carriers. Using a simple model we suggest that the accuracy of the method enables also to semi-quantify the energy distribution of the hot carriers. Keywords: plasmons, inelastic tunneling, hot carriers, molecular junctions, thermoplasmonics.
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The heating that is accompanying plasmonic structures under illumination is suggested to be used in a large variety of applications such as photo-thermal therapeutics, heat-assisted magnetic recording, drug release, catalysis and solar thermal energy harvesting1,2. Measurement of the temperature gradients formed in these applications has been demonstrated by several noninvasive far field optical methods such as refractive index variation3, fluorescence4,5 and Raman6,7. In addition to the fact that their spatial resolution is diffraction limited, they are also based on the calibrated thermal response of neighboring materials and therefore do not directly probe the temperature of the metallic nanostructures themselves. To overcome this limitation methods such as anti-Stokes electronic Raman scattering
8,9
and measurement of the optically
induced bolometric change in the resistivity of illuminated metal nanowires10,11 have been introduced. Still, the spatial resolution of these approaches is coarser than the expected size of what we will refer to as the plasmonic hot spots. This size is a direct outcome of the heating mechanism. Heating commences once a quantum of plasmon, with an energy of ħω is damped non-radiatively via Landau damping to create electron-hole pairs1,12-18. The formed hot carriers, are distributed over a certain energy range above and below the Fermi energy, depending on the plasmon energy and on momentum matching considerations. The relaxation of the hot carriers is taking place by electron-electron and electron-phonon interactions to result in a Fermi-like distribution with a high electronic temperature Te. The time scale of this process is few tens of femtoseconds and considering the mean free path of the carriers within a length scale of few tens of nanometers. The formed hot spot continues to relax by subsequent interaction with the lattice over a time scale of ~1-2 picoseconds to result in a higher local lattice temperature, Tl.. Final relaxation to the temperature of the surroundings is taking place also by heat diffusion within a relatively long time scale of 100ps-10ns.
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Thermal probing of a hot spot, with a characteristic size of few nanometers, becomes experimentally highly challenging when measurement of the temperatures within nano-scale metallic gaps is desired. Such measurements are of great significance in molecular junctions and junctions embedded with quantum dots, where many methods have been suggested to plasmonically control and steer conduction processes within such junctions19-24, yet their effectivity and the stability of the junctions critically depends on the local values of Te and Tl under illumination. Here we show that Inelastic Electron Tunneling Spectroscopy (IETS) of thiol-alkyl chains within nano-scale Au gaps can be used as a precise thermometer of the steady state in-gap values of both Te and Tl under illumination (figure 1). We also show that the inherent locality of the IETS measurement enables to use the determined values of Te in a self-consistent calculation that supports theoretical predictions regarding the role of momentum matching on the energy distribution of hot carriers formed by intraband transitions in Au.
Figure 1. Hot carriers generation and heating in a metal-insulator-metal (MIM) structure. A surface plasmon polariton (SPP) with an energy of ħω is created within an MIM structure, in which the insulator is a monolayer of thiol-alkyl chain molecules. The purple line is a schematic profile of the electric field in the gap and the metal leads. Relaxation of the hot electrons (e-) and holes (h+) formed by Landau damping of the SPP results in higher local electronic (Te) and lattice (Tl) temperatures.
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IETS of molecular junctions has been studied extensively both experimentally and theoretically25-31. It is based on the effect that once the available energy for a tunneling offresonance electron under a certain applied bias, eV, is larger than the energy associated with a
certain vibrational mode of the junction, , excitation of this mode by the tunneling electrons
becomes possible and as a result an inelastic tunneling channel through the junction is opened (figure 2). This additional channel results in a minute increase in the overall conductance of the junction which can be resolved as a peak in the second derivative d2I/dV2 of its I-V curve. The full width at half maximum (FWHM) of this peak depends on the smearing of the electronic distribution within the leads and is analytically shown in the case of a simple tunneling barrier32 to be equal to 5.4kBTe, where kB is the Boltzmann coefficient.
Figure 2. Inelastic Electron Tunneling Spectroscopy as a Te thermometer. (a) A schematic of elastic and inelastic tunneling processes under an arbitrary potential barrier between two metal leads. The inelastic process commences once the applied bias potential enables excitation of a certain vibrational mode of the barrier with an energy ħω0. (b) The small increment in the measured current as a result of the additional inelastic channel is resolved as a peak in the second derivative of the I-V curve, located at a bias potential of ħω0/e. The width of the peak is Te dependent, here Te(red curve)>Te(blue curve).
Thus, high resolution vibrational spectra with this method necessitates measurements at low temperatures. However, if the energy values of the vibrational modes of a measured junction are well separated, their corresponding IETS peaks can serve as an effective thermometer for Te within a relatively broad range of temperatures. As we show below, junctions based on alkyl
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chains comply with this demand. In addition, by the nature of the conductance process the probed temperature characterizes the electronic temperature within a distance of few inelastic mean free paths from the interface, a zone that is comparable in size to the plasmonic hot spot. In molecular junctions the shape and width of the IETS peaks depend in addition to the smearing within the leads also on molecular attributes such as the energy position of the electronic levels and their coupling to the leads. However, these contributions become negligible when the levels are located more than 1eV above or below the Fermi level33. Junctions with thiol-alkyl chains comply with this demand as well34 and can be treated to a first approximation as a simple tunneling barrier with a 5.4kBTe width dependency of the peaks. An additional reason for using thiol-alkyl chains in the experiments originates from their low conductance at low bias values (
