Synthesis and Raman Detection of 5-Amino-2-mercaptobenzimidazole Self-Assembled Monolayers in Nanoparticle-on-a-Mirror Plasmonic Cavity Driven by Dielectric Waveguides

Functionalization of metallic surfaces by molecular monolayers is a key process in fields such as nanophotonics or biotechnology. To strongly enhance light–matter interaction in such monolayers, nanoparticle-on-a-mirror (NPoM) cavities can be formed by placing metal nanoparticles on such chemically functionalized metallic monolayers. In this work, we present a novel functionalization process of gold surfaces using 5-amino-2-mercaptobenzimidazole (5-A-2MBI) molecules, which can be used for upconversion from THz to visible frequencies. The synthesized surfaces and NPoM cavities are characterized by Raman spectroscopy, atomic force microscopy (AFM), and advancing–receding contact angle measurements. Moreover, we show that NPoM cavities can be efficiently integrated on a silicon-based photonic chip performing pump injection and Raman-signal extraction via silicon nitride waveguides. Our results open the way for the use of 5-A-2MBI monolayers in different applications, showing that NPoM cavities can be effectively integrated with photonic waveguides, enabling on-chip enhanced Raman spectroscopy or detection of infrared and THz radiation.

the minimum angle was observed.This advancing-receding cycle was repeated three times.The mean values of the maximum (minimum) angles from the remaining four cycles of the advancing (receding) contact angles were calculated.
Three samples were used to measure the differences between a 5-A-2MBI functionalized and a non-functionalized gold surface.Figure S.1a shows the results of the WCA measurements, which correspond to the WCA of a non-functionalized Au sample, bare Au substrate immersed for 12h in Absolute Ethanol and finally a Au-5-A-2MBI functionalized sample.Different WCA values were obtained for each case, probing that the CA method is useful for probing the surface functionalization: 76˚ for nonfunctionalized gold, 62˚ for non-functionalized gold + Ethanol, and 54˚ for functionalized gold.
As expected, 5-A-2MBI SAM provides more hydrophilic surfaces when compared with bare Au surface, attributed to the presence of amines and other N groups.

SI.2: Atomic Force Microscopy (AFM) imaging
An Alpha300 RA (Raman-AFM) instrument from WITec was employed for the AFM sample characterization.All measurements were performed in AC mode.Sharp silicon probes without coating (K ~ 42 N/m, f 0 ~ 320 kHz) were purchased from PPP-NCH (Nanosensors).All AFM images were processed with WSxM software from Nanotec Electrónica S.L. 1 Figure S1b

SI.3: Drop casting Au-NP deposition
The drop casting was performed by delivering an 8 μL drop of 60 nm Au-NP solution onto the 5-A-2MBI functionalized Au sample, left for 5 minutes and then rinsed with Mili-Q water.Finally, the substrate was dried under the N 2 stream.Water suspension of spheric Citrate-capped 60 nm Au-NPs were purchased from Nanopartz™.The substrate was protonated to improve the NP's delivery.Protonation is a technique that harnesses the property of certain molecules to become positively charged.In this case, the 5-A-2MBI molecule has an amine group -NH 2 , which can eventually become -NH + dark-field optical images taken before and after sample protonation, followed by drop casting of 60 nm and 150 nm diameter Au-NPs, respectively.We can see an increase in the density of Au-NPs for both NP sizes.Also, a better distribution of single NPs on the gold surface can be seen, especially for NPs of 60 nm in a functionalized Au surface.In percentages, considering the bare gold, the density increases by approximately 600 % after protonation and 800% after functionalized surface protonation for 60nm Au-NPs.
For 150 nm Au-NPs, the increment of the NP numbers for a bare gold surface is around 42 %, and there is an increase of approximately 300 % in the density of 150 nm Au-NPs for protonated functionalized surfaces.

SI.5: NP positioning
Spherical citrate-capped 150 nm Au-NPs were deterministically positioned on top of plasmonic nanocavities consisting of a 650 x 650 nm squared Au patch lithographed at the intersection of two orthogonal 600 x 220 nm Si 3 N 4 waveguides.After functionalization of the Au patch, the transfer of 150 nm Au-NPs was performed using a micro-contact printing technique developed by our group, allowing the controlled positioning of individual Au NPs with sub-micron accuracy. 2Water suspensions of spheric Citrate-capped 150 nm Au-NPs were purchased from Nanopartz™.

SI.6: Numerical simulations
Simulations were conducted based on the geometries described in the main text.The simulations were performed according to the main text.Figure S3a shows the structure with the port positions, which are essential for the whole study.The results were obtained by considering two separate modes: excitation and collection.On the one hand, for the excitation, a TM coupling mode (Figure S3b) had to be considered so that the electric field was vertically oriented, allowing interaction with the 5-A-2MBI SAM.Additionally, we assumed that Port 1 is the input port where the electromagnetic field is coupled to the waveguide.The enhanced electric field was measured by placing probes in the middle of the gap between the nanoparticle and the gold surface (Figure S3c).
On the other hand, to assess the theoretical collection of the emitted signal, we considered a point dipole exactly in the same location where the probes were positioned, in the middle of the SAM below the NP (Figure S3d).The signal is emitted by the dipole, coupled to the waveguide, and directed towards Port 1, whose S-parameters indicate the intensity.Simulations were conducted with and without a metallic structure to study the SERS effect of the structure.
The mesh played an important role in optimizing the calculation time.For this purpose, a hexahedral mesh was considered, paying particular attention to critical areas such as the gap, where a more precise mesh was considered.Additionally, the waveguide is made of silicon nitride (n=2), and a refractive index of n=1.8 was considered for the SAM, 3 n=1.45for the SiO 2 structure on which the gold patch is positioned.For all gold structures, the specifications of Johnson and Christy from 1972 were selected. 4

SI.7: Fabrication of the photonic chip
The waveguide structures were fabricated on standard silicon nitride samples with a thickness of 300 nm and a buried oxide layer thickness of 3.27 µm.The fabrication is based on an electron-beam (e-beam) direct-writing process performed on a coated 300 nm negative (Man-2403) resist film.The mentioned e-beam exposure, performed with a Raith150 tool, was optimized to reach the required dimensions, employing an acceleration voltage of 20 KeV and an aperture size of 30 um.After developing, the resist patterns were transferred into the Silicon nitride employing an optimized Inductively Coupled Plasma-Reactive Ion Etching process with fluoride gases.A second lithography e-beam process, in this case using positive resist PMMA, was carried out to fabricate the metal patch prior to an evaporation process of 40 nm of gold and a lift-off process employing MNP as solvent.

SI.8: Waveguide-driven Raman spectroscopy
Spectrometer alpha300 RA (Raman-AFM) from WITec provides information about the AntiStokes Raman scattering in the 80 -3000 cm−1 range, with monochromatic 532 nm (green) and 633 nm (red) laser illumination and 100x objective.Raman measurements were performed at 532%nm (green) excitation, a power of P = 7 mW, a grating G = 600 l/mm and a 100x objective.The Raman images were scanned at 300%×%300 points, with 0.035%s integration time at each point.
A 785 nm diode laser was coupled using an 40x objective into the mounted photonic chip, with an imaging camera collecting scattered light from above to a certain when coupling into the waveguide was achieved.Before reaching the waveguide, the laser passed through a 50% beam splitter, so returning light from the waveguide could be collected and focused into a monochromator and CCD (Andor) for measuring Raman spectra.These were calibrated using the signal from the Si surface.
Figure S1b displays an AFM topography image of an Au-5-A-2MBI functionalized

Figure S1 .
Figure S1.a) The blue curve corresponds to the non-functionalized Au sample, while the

3 (
Figure S.2a).This method consists of immersing the sample in a strong acid solution as hydrochloric acid (HCl).Consequently, the surface will acquire a positive charge, making it more efficient in attracting negatively charged Au-NPs.To quantify the improvement in sample protonation for Au-NPs deposition, we conducted large-area optical dark-field imaging at 50x magnification (From Figure S.2b to i). Figure S.2b and Figure S.2c display

Figure S 2 .
Figure S 2. a) Schematic representation of the surface protonation method using HCl

Figure S 3 .
Figure S 3. a) Scheme of the complete simulated structure with the specifications of the