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Structural Order of the Molecular Adlayer Impacts the Stability of Nanoparticle-on-Mirror Plasmonic Cavities

Immense field enhancement and nanoscale confinement of light are possible within nanoparticle-on-mirror (NPoM) plasmonic resonators, which enable novel optically activated physical and chemical phenomena and render these nanocavities greatly sensitive to minute structural changes, down to the atomic scale. Although a few of these structural parameters, primarily linked to the nanoparticle and the mirror morphology, have been identified, the impact of molecular assembly and organization of the spacer layer between them has often been left uncharacterized. Here, we experimentally investigate how the complex and reconfigurable nature of a thiol-based self-assembled monolayer (SAM) adsorbed on the mirror surface impacts the optical properties of the NPoMs. We fabricate NPoMs with distinct molecular organizations by controlling the incubation time of the mirror in the thiol solution. Afterward, we investigate the structural changes that occur under laser irradiation by tracking the bonding dipole plasmon mode, while also monitoring Stokes and anti-Stokes Raman scattering from the molecules as a probe of their integrity. First, we find an effective decrease in the SAM height as the laser power increases, compatible with an irreversible change of molecule orientation caused by heating. Second, we observe that the nanocavities prepared with a densely packed and more ordered monolayer of molecules are more prone to changes in their resonance compared to samples with sparser and more disordered SAMs. Our measurements indicate that molecular orientation and packing on the mirror surface play a key role in determining the stability of NPoM structures and hence highlight the under-recognized significance of SAM characterization in the development of NPoM-based applications.


Introduction
Metallic nanostructures are known to support localized surface plasmon resonances (LSPRs) in the visible and near-infrared range. When placed in close proximity, the LSPRs supported by individual nanoparticles hybridize to form new modes, 1 some of them providing a large enhancement of the incident electromagnetic (EM) field inside the gap or dielectric layer between the nanostructures. 2 These new LSPRs and the corresponding magnitude of electromagnetic field enhancement depend on a number of factors including the material composition, morphology, and dimensions of the nanostructures and the spacer between them. 3 Several hundred-fold field enhancement along with sub-nanometric spatial sensitivity of such structures are utilized in numerous applications: sensing and chemical fingerprinting with surface enhanced Raman scattering (SERS), 4,5 nonlinear generation of light, 6,7 strong light matter interaction, 8,9 nanoscale chemistry, 10,11 photodetection, 12 and integrated electricalto-optical conversion, 13,14 to cite a few examples. Plasmonic nanocavities can also function as nanoscale sources of heat due to the Joule effect 15 and can be employed for several biological and medical applications. 16,17 Owing to the above mentioned and many more potential applications, numerous plasmonic nanostructures have been developed over the years. 18,19 While state-of-the-art photolithographic techniques can be used for structures with critical dimensions between 10 nm -100 nm, reproducible fabrication of sub-10 nm devices is best achieved using molecular self-assembly, which enables the realization of nanocavities with sub-nanometer gap sizes. 20,21 Among these nanostructures, the nanoparticle-on-mirror (NPoM) geometry offers unpar-alleled ease of fabrication with substantial reproducibility. The NPoM consists of a nanoparticle positioned above a flat metal film (the mirror) with a well-defined gap between them.
When the NPoM is illuminated, the charge oscillations initiated in the nanoparticle form an inverted image within the mirror surface; their mutual coupling results in the overall plasmon resonances. The fundamental properties of these plasmonic modes have been investigated in a number of pioneering works. [22][23][24][25] Commonly found plasmon modes in the NPoM are the transverse mode of the nanoparticle and the bonding dipole mode (labeled here l1) between the nanoparticle and the mirror. Moreover, higher order dipole modes and cavity modes may also exist in the gap due to facets present on the surface of the nanoparticle. 26 To the best of our knowledge, the first demonstration of NPoM can be traced back to the enhancement of light emission from metal-insulator-metal tunnel junctions that was first observed by McCarthy et al. 27 in 1977. A few groundbreaking experimental studies include the works of Holland et al.; 28 with silver flakes on top of silver films with lithium fluoride spacer, and Kume et al.; 29 with silver nanoparticles embedded in silica matrix on top of silver film. NPoM structures with SAM as spacer layer were introduced at the beginning of the 21 st century by Hutter et al. 30 and Okamoto et al. 31 The ease of fabrication and the reproducibility of SAM-based NPoM bypasses the limits imposed by conventional fabrication processes, and this approach has been shown to be compatible with deterministic positioning of the nanoparticles. 32 As the gap size is decreased below 1 or 2 nm, quantum mechanical effects can be observed at room temperature. 33 The immense EM field enhancement and extremely small mode volume make the LSPRs of these nanocavities susceptible to minute changes in the NPoM structure, 3 especially to the structure and configuration of SAM layers. 34 Conformational changes of molecules within a break junction are known to affect the conductivity of the junction. 35,36 While the adsorption of thiolated molecules on noble metals has been extensively studied, 37,38 the dynamics of molecular species within the SAM and its impact on the optical response of NPoM remain unclear.
Decades of fundamental surface science studies combining local spectroscopies, electro-chemistry, and theoretical modelling 39,40 have shed light into the two-step process of the SAM formation. 41,42 The first step is characterized by fast adsorption and desorption processes, where thiol molecules adsorb to the Au surface in lying-down conformation (i.e. with the principal molecular axis being near-parallel to Au surface) and remain highly mobile. 41 The second step leads to slow re-organization of the initial phase into a densely packed phase in which the molecules stand in the upright configuration (with the principle molecular axis being close to the surface normal). 43 SAM re-organization partially proceeds through the transport of the thiol-Au ad-atom moieties that remain mobile once the strong thiol-Au bond is formed. 44 This process leads to the formation of Au islands 45 -a process known as surface reconstruction.
The two-step process generally describes the formation of thiol-based SAMs on Au. However, the adsorption kinetics and its final SAM structure strongly depend on the surface conditions of the Au substrate and on the preparation protocol. The critical surface con- on the Au surface has been observed even in densely packed SAMs prepared on monocrystalline surfaces. Local studies using scanning tunneling microscopy (STM) reveal significant heterogeneities in SAM ordering, even on spatial scales below 50 nm. Having in mind the parameters affecting the SAM preparation, it becomes clear that perfect single domain SAM layers are a rare exception rather than reality, especially on polycrystalline Au substrates.
In this article, we study how the organization and orientation of biphenyl-1,4-thiol (BPhT) molecules within NPoM plasmonic nanocavities affect their optical properties, and how these properties are modified under focused near-infrared laser excitation with power levels ranging from few µW/µm 2 to mW/µm 2 . The molecular orientation and packing den-sity is tuned using variable incubation time of the gold film in the BPhT solution. For clarity we limit ourselves to two incubation times, 2 hours (labeled S2) and 24 hours (labeled S24), ensuring significantly different SAM morphologies. The differences between S2 and S24 were assessed using reductive desorption and STM, confirming that the longer incubation of S24 leads to denser packing of the molecules and more pronounced surface reconstruction of the gold film compared to the shorter incubation of S2, which yields a sparser monolayer of lying-down molecules. Many individual NPoMs on each type of sample were optically characterised using darkfield scattering and Raman spectroscopy under increasing laser illumination power. We find that changing the SAM incubation time has a significant impact on the plasmon resonance wavelength and on the stability of the NPoM cavities, with the observation that a more ordered and densely packed monolayer results in a less stable plasmonic response, rapidly red-shifting and degrading under laser illumination. We attribute this observation to heat-induced reorientation of the molecules toward a flatter configuration, together with the increased mobility of gold adatoms after surface restructuring, which facilitate the formation of conductive bridges between the gold film and the nanoparticle in samples with longer incubation times. Our study reveals the overlooked critical role of SAM preparation conditions in fabricating robust and reproducible NPoM plasmonic resonators.
It contributes to a better understanding of plasmonic nanojunctions and should stimulate further research at the interface between plasmonics and surface science in order to push nano-plasmonics closer to applications.

Electrochemical (EC) Characterization
We characterize the molecular ordering of BPhT SAMs using reductive desorption (AuSC6H4C6H5+ e -→ Au 0 + SC 6 H 4 C 6 H 5 ) with the specific signature being cathodic waves. The peak positions, i.e. potential of the reductive desorption, strongly depend on the lateral inter-actions between BPhT molecules 46,49 and are indirectly related to the density of the SAM packing. The desorption peak of densely packed SAMs is thus shifted to the more negative potentials (Table S1). The peak width and the area under the peak depend on the number of the thiol molecules desorbing from the Au surface at a given potential. Broad desorption peaks are thus related to the ill-defined SAMs of low-packing density. We note that EC characterization is a global characterization technique where SAM desorbs over the whole sample surface. It provides information on the molecular ordering over the macroscopic scale. Figure 1a shows the cyclic voltammograms of S24 and S2 prepared on glass/Cr/Au substrate. Cathodic peaks at -0.88 V and -0.85 V versus Ag/AgCl reference electrode correspond to reductive SAM desorption on S24 and S2, respectively. Since desorption of highly-ordered SAM proceeds at more negative potentials, the shift of ≈ -30 mV for S24 suggests a high degree of SAM ordering. Moreover, broad desorption peak on S2, with the center slightly shifted towards positive potentials, implies that BPhT SAM on S2 is globally in ill-defined phase. We also note the fine structure in the S2 voltammetric wave (Figure 1a inset). The shoulder shifted to the negative potentials with respect to the main peak might arise from small SAM domains (< 50 nm 2 ) being in the phase of denser packing than ill-defined phase characterized by the main S2 desorption peak (Fig. S5).

Scanning Tunneling Microscopy (STM)
SAMs formed upon two different incubation times, i.e. S2 and S24, were also studied using STM. We highlight that STM characterization was performed on the mirror substrate (i.e. template stripped Au) following the same procedure as for NPoM preparation. As discussed earlier, SAM characterization on the mirror substrate is of great importance since molecular ordering depends on surface crystallinity, the size of the terraces, and the presence of surface defects.

Optical Characterization
The spectral position of the dipole mode is known to be extremely sensitive to structural parameters of the NPoM such as nanoparticle size and shape along with the height, refractive index, and conductivity of the gap layer. [53][54][55] We study about 10 NPoMs on each sample.  while keeping the total laser exposure to 5 minutes for each power. After laser exposure the DF spectrum is measured, and the cycle is repeated after an increase of the laser power.
Examples of typical changes to the DF spectra upon laser exposure for the two different samples are shown in Figure 2(a) and 2(b). For S24 the dipole mode appears to redshift already after first laser exposure with ~ 8 µW, without significant change in the peak scattering intensity. The DF spectra measured after each exposure show that the dipole mode continues to red shift along with declining peak intensity as the laser is ramped to higher powers. After the fifth exposure with about 170 µW, the dipole mode has shifted close to 800 nm and the intensity has dropped noticeably compared to the initial value ( Figure 2a, green line). The higher order dipole mode has also red shifted to 700 nm with an increase in peak intensity. Upon further increase of laser power the spectrum suddenly blue-shifts, indicating the formation of a conductive bridge between the AuNS and the mirror underneath and finally the AuNS is completely fused with the mirror (Fig. S8). 57 The evolution is quite different for the NPoMs on S2, which is the sample with disordered SAM with lying-down molecules. In the example shown in Figure 2(b), the dipole mode is initially located around 800 nm (blue line in lower panel) and does not appear to red shift as readily as observed for S24 when the laser power is increased. The peak intensity also remains unchanged as measured after each exposure. It is only after laser powers greater than 400 µW that a significant red-shift and a decrease in intensity of the dipole mode are observed. Bridge formation followed by fusion of AuNS to the mirror similar to that observed for S24 occurs at even higher powers (Fig. S9).
Multiple NPoMs across S24 and S2 studied with the same procedure disclose the distinct and layer dependent evolution of the dipole mode with increasing laser power, as presented in Figure 2 Figure 2c. Importantly, though, measurements with laser wavelength at 632, 710 and 760 nm indicate that laser detuning from the the dipole mode does not have a major impact on the behavior of the NPoMs (Fig. S7), so that the difference in stability observed between S2 and S24 cannot be simply attributed to their different initial resonance position.
The changes in peak intensity of the dipole mode with laser power are slightly different for the two types of NPoMs (Figure 2d). While the continuous decrease in the peak intensity for NPoMs on S24 is correlated with the red-shift of the peak wavelength, the decrease in gap height alone cannot explain the decrease in peak intensity as shown by BEM simulations in Figure 3(d). This discrepancy is attributed to phenomena such as electron tunneling and charge screening that begin to play a significant role in sub-nanometer gap sizes.

Sample preparation
The NPoM nanocavities are fabricated starting with the mirror: an ultraflat Au film obtained by template stripping. 66 In order to facilitate the identification of individual NPoMs distinct markers are produced on the mirror surface by patterning the template. 67 The tem-

Optical setup
The experimental setup consists of a confocal microscope designed to perform Raman and DF scattering spectroscopy. The laser exposure is carried out by a continuous wave (CW) Titanium Sapphire (Ti:Sapph) laser from Spectra Physics (Model 3900, pumped by Millenia 5W) tuned at 710 nm. The laser line is filtered using a bandpass filter (BP) (Fig. S6). white light source focused on the sample by a secondary objective (Olympus Plan N 4x 0.1NA) at angle of 81 degree from the normal to the sample plan. In order to use the supercontinuum as an interference free white light source, its transverse spatial coherence length must be reduced. This is achieved by a procedure described elsewhere. 69 For all the measurements, signal from the sample is collected by the high NA objective. The acquired signal is split and a small portion is used to image the sample plane while the rest is passed through spectral filters and finally recorded using a spectrograph (Andor Kymera 193i) with CCD (Andor iDus). In case of Raman measurements, to remove laser reflection and Rayleigh scattering, the acquired signal is split by 90T/10R beam-splitter (BS); where transmission and reflection are used to filter out anti-Stokes (short pass: SP) and Stokes (long pass: LP) signals, respectively. After the filters, the anti-Stokes and Stokes are first aligned to be vertically parallel using a D-shape mirror and afterwards focused onto the spectrometer slit.
For the collection of scattering spectra, the SP filter is moved out of the beam path and the reflection from the BS is blocked by a shutter. The binning region on the CCD is also reduced to act as a virtual spatial filter and minimize the collection area on the sample.
In order to compensate for the spectral response of the system in DF measurement the sample was replaced by a silver mirror tilted at 40 degrees on the horizontal plane with respect to the optical axis of the high NA objective. The specular reflection of the white light source is then recorded with the spectrometer. The acquired spectra are averaged, smoothed and normalized to obtain the DF spectral response. During the DF measurement of NPoMs, after baseline subtraction the scattering spectra are divided by the DF spectral response. To estimate the spectral response for the Raman measurements, a calibrated white light source (SLS201L ThorLabs) was used to illuminate the system via the high NA objective. The LP and SP filters are removed and spectra are collected separately for Stokes and antiStokes paths, hence the spectral response is computed separately for each path. The acquired spectra are then averaged, smoothed and divided by the calibration spectrum. These are then normalized by the maximum of antiStokes side In order to obtain the normalized spectral response of Stoke and antiStokes path. In order to correct a Raman spectrum both response spectra are combined at the laser wavelength and then used to divide the Raman spectrum.

Supporting Information Available
BPhT trusions; yet, no periodic arrangement is observed. The lower facet appears nanostructured due to the curved terraces (lateral size > 10 nm) and dark, round features (i.e., pits formed upon reconstruction). These differences in the appearance of the crystalline facets imply that different SAM phases co-exist along S24. Heterogeneities in BPhT SAM ordering on the samples prepared upon prolonged incubation in ethanoic solutions is in agreement with earlier reports. 50,51 We speculate that BPhT SAMs on adjacent Au facets are in denselypacked phases with the small tilt angles between the principal molecular axis and the surface normal, as expected for SAMs prepared upon long immersion. 51 Also, we take this opportunity to remind the reader that the template stripped Au (i.e., the mirror in NPoM) is a polycrystalline surface with mostly (111) crystalline facets -it contains (100)    The parameters for the reductive desorption peaks shown in Figure 1  The distances between protrusions on S24, as-measured from the height profiles superimposed on the molecular rows in Figure 1 in the main text, are 8.9±0.9 Å along the single row and 4.2 ± 1.0 Å across two rows implying on the square lattice close to the (2 √ 3x √ 3) structure ( Figure S4). This finding is in excellent agreement with the paper by Azzam et al. 51 and Leung et al. 50 Figure S2).
Highly ordered BPhT SAMs domains on S2, having lateral size < 10 nm, co-exists with disordered phase shown in Figure 1 Figure S10: Electronic Raman temperature calculated by fitting the anti-Stokes background of multiple NPoMs with two exponentials. The large variations in temperature at power below 10 µW can be ignored due to poor signal to noise ratio. The initial temperature for NPoMs with low incubation time SAM was found to lower than those prepared with higher incubation time. Hence, the heat present within the nanogap could explain the molecular reorientation and decrease in gap height.

SI 11: Raman spectra S2 and S24
Figure S11  Figure S12: (a) The evolution of DF spectra on S24 as the laser power is increased. The red dot indicate the evolution of the bonding dipolar mode l1 however after 99µW is it not possible to identify l1. (b) Comparing the first (red) and the last (green) DF spectra show a redshift and decrease in the peak intensity of l1. The higher order dipole mode l2 also redshifts but shows an increase in its intensity marked by blue dashed arrow.