Abstract
Nonlinear plasmonics requires the use of high-intensity laser sources in the visible and near/mid-infrared spectral ranges to characterise the potential enhancement of the vibrational fingerprint of chemically functionalised nanostructured interfaces aimed at improving the molecular detection threshold in nanosensors. We used Two-Colour Sum-Frequency Generation (2C-SFG) nonlinear optical spectroscopy coupled to the European CLIO Free Electron Laser in order to highlight an energy transfer in organic and inorganic interfaces built on a silicon substrate. We evidence that a molecular pollutant, such as thiophenol molecules adsorbed on small gold metal nanospheres grafted on silicon, was detected at the monolayer scale in the 10 µm infrared spectral range, with increasing SFG intensity of three specific phenyl ring vibration modes reaching two magnitude orders from blue to green–yellow excitation wavelengths. This observation is related to a strong plasmonic coupling to the thiophenol molecules vibrations. The high level of gold nanospheres aggregation on the substrate allows us to dramatically increase the presence of hotspots, revealing collective plasmon modes based on strong local electric fields between the gold nanoparticles packed in close contact on the substrate. This configuration favors detection of Raman active vibration modes, for which 2C-SFG spectroscopy is particularly efficient in this unusual infrared spectral range.
Highlights
Nonlinear optical Two-colour Sum-Frequency Generation (2C-SFG) spectroscopy of metal [1,2,3] and semiconducting [4,5,6] nanoparticles constitutes a reliable probe of the surface molecular chemistry of nanostructured samples in catalysis andchemical sensing [7]
We need two high-intensity laser sources in the infrared (IR) and in the visible spectral ranges. While it is possible from almost 20 years [28,29] to take profit from a visible wavelength tunable from violet to red in such spectroscopy, highlighting the effect of electronic properties of any kind of interface, the major drawback was the access of the infrared wavelength in optical parametric oscillators/amplifiers (OPO/OPA)
The CLIO Free Electron Laser has been revealed as a powerful and pioneering tool when coupled to SFG spectroscopy, giving a unique optical setup that led to numerous experimental firsts during 25 years of operation, thanks to its unique delivered power in the infrared spectral range, opening the door to nonlinear optical spectroscopy in the vibrational fingerprint as illustrated in the present work
Summary
Nonlinear optical Two-colour Sum-Frequency Generation (2C-SFG) spectroscopy of metal [1,2,3] and semiconducting [4,5,6] nanoparticles constitutes a reliable probe of the surface molecular chemistry of nanostructured samples in catalysis and (bio)chemical sensing [7]. In such systems, the evolution of various physico-chemical properties impacted by surface plasmons and excitons can be analysed [8,9].
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