Detection of electron tunneling across plasmonic nanoparticle–film junctions using nitrile vibrations

Abstract

The significant electric field enhancements that occur in plasmonic nanogap junctions are instrumental in boosting the performance of spectroscopy, optoelectronics and catalysis. Electron tunneling, associated with quantum effects in small junctions, is reported to limit the electric field enhancement. However, observing and quantitatively determining how tunneling alters the electric fields within small gaps is challenging due to the nanoscale dimensions and heterogeneity present experimentally. Here we report the use of a nitrile probe placed in the nanoparticle-film gap junctions to demonstrate that the change in the nitrile stretching band associated with the vibrational Stark effect can be directly correlated with the local electric field environment modulated by gap size variations. The Stark shift arises correlates with plasmon resonance shifts associated with electron tunneling across the gap junction. Time dependent changes in the nitrile band with extended illumination further support a build up of charge associated with optical rectification in the coupled plasmon system. Computational models agree with our experimental observations that the frequency shifts arise from a vibrational Stark effect. Large local electric fields associated with the smallest gap junctions give rise to significant Stark shifts. These results indicate that nitrile Stark probes can measure the local field strengths in plasmonic junctions and monitor the subtle changes in the local electric fields resulting from electron tunneling.

Photoinduced Plasmon-Driven Chemistry in trans-1,2-Bis(4-pyridyl)ethylene Gold Nanosphere Oligomers

“…Similarly, the 1588 cm –1 assigned to molecule–surface binding, shifts 7 cm –1 after eight successive intervals of pumping time. Previous studies have demonstrated that molecules are capable of directly measuring the modulation of local electromagnetic fields due to electron tunneling in plasmonic nanogaps. , The resulting time-dependent vibrational shifts are due to a buildup of charge due to optical rectification in strongly coupled nanoparticle systems. A recent study on charge buildup of Au nanoparticles concluded that steady-state light excitation can lead to photoelectrochemical potentials as high as 240 mV .…”

Light-Induced Voltages in Catalysis by Plasmonic Nanostructures

“…30 mV across the nanogap. Schultz and co-workers found optical rectification by aggregates of Au nanorods with nanogaps. − Under visible-light excitation, the nanostructures were found to sustain a DC potential, as evidenced by the Stark shift of the vibrational mode of a reporter molecule.…”

Photothermal Microscopy of Coupled Nanostructures and the Impact of Nanoscale Heating in Surface-Enhanced Raman Spectroscopy

“…5–6 In a chemically inert environment, the hot carriers transfer their energy to the metal lattice, converting the absorbed light into heat. 5, 7–8 These plasmonic properties of gold NPs have been used for various applications, such as: SERS, 9–13 TERS, 14–15 solar energy conversion, 16–18 photocatalysis, 19–20 and photothermal therapy. 21–23 …”