# Nanostructured gold-coated AFM tips generated by potentiostatic electrodeposition for tip-enhanced Raman spectroscopy

Yuhan Huang, David Talaga, Patrick Garrigue, Gerardo Salinas, Gary Cooney, Stéphane Reculusa, Alexander Kuhn, Laurent Bouffier, SEBASTIEN BONHOMMEAU

► To cite this version:

Yuhan Huang, David Talaga, Patrick Garrigue, Gerardo Salinas, Gary Cooney, et al.. Nanostruc- tured gold-coated AFM tips generated by potentiostatic electrodeposition for tip-enhanced Raman spectroscopy. Chemical Physics Letters, 2023, 832, pp.140893. 10.1016/j.cplett.2023.140893. hal- 04772742

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Distributed under a Creative Commons Attribution 4.0 International License
Nanostructured gold-coated AFM tips generated by
potentiostatic electrodeposition for tip-enhanced
Raman spectroscopy

Yuhan Huang,ª David Talaga,ª Patrick Garrigue,ª Gerardo Salinas,ª Gary S. Cooney,ª Stéphane Reculusa,ª Alexander Kuhn,ª Laurent Bouffierª,\* and Sébastien Bonhommeauª,\*

ª Univ. Bordeaux, CNRS, Bordeaux INP, ISM, UMR 5255, F-33400 Talence, France.

\* To whom correspondence should be addressed. E-mail: laurent.bouffier@u-bordeaux.fr, sebastien.bonhommeau@u-bordeaux.fr

**KEYWORDS.** Tip-enhanced Raman spectroscopy, Potentiostatic electrodeposition, Graphene oxide, Carbon nanotubes.

**ABSTRACT.**

Potentiostatic electrodeposition was used to manufacture gold-coated atomic force microscopy (AFM) tips for applications in tip-enhanced Raman spectroscopy (TERS). The growth of the gold film was found to be initiated by the instantaneous nucleation mechanism on the doped-silicon AFM tip taken as substrate. This cost-effective method allowed robust AFM-TERS probes with localized surface plasmon resonances in the visible range to be produced with 53% yield. These probes enable TERS mapping of a graphene oxide flake and a carbon nanotube to be performed
with lateral spatial resolution below 20 nm, which was uncorrelated with the curvature radius of
the metallized tip apex.

**1. Introduction.**

Tip-enhanced Raman spectroscopy (TERS) has emerged as a powerful technique for the
molecular characterization of nanomaterials and biomolecules with nanoscale spatial resolution
[1-3]. It takes advantage of the high lateral spatial resolution of scanning probe microscopies, such
as atomic force microscopy (AFM) or scanning tunnelling microscopy (STM), and the chemical
specificity of Raman spectroscopy. In TERS, laser light excites the localized surface plasmon
resonance (LSPR) of a tapered metal nanoparticle probe positioned at nanoscale distance from the
sample. This resonant excitation produces an intense and confined electromagnetic field at the
probe apex, and leads to enhanced Raman signatures of nanometre-sized sample regions. Many
types of fabrication methods have been proposed in the literature to manufacture TERS probes,
but none of them prevail so far [4-6]. STM-TERS tips are generally manufactured by
electrochemical etching [7] or by using micro-/nanofabrication methods, such as focused ion beam
milling [8] or template stripping [9]. In contrast, AFM-TERS tips are often obtained by physical
vapor deposition (PVD) of noble metals (and additional anchoring and/or insulating capping layers
[10,11]) or by micro-/nanofabrication as well as by electrochemical methods. An easy access to
high-performance AFM-TERS probes opens opportunities for many applications owing to the
versatility of AFM for the characterization of materials compared with STM, which always
requires a conductive substrate. Commonly, PVD techniques have been preferred since the
metallization procedure is simple, fast and inexpensive. However, the robustness and lifetime of
such metal coatings is highly variable. Although their stability can be improved by protecting them
with insulating capping layers, additional coatings must not be thicker than 1-2 nm, in order to
preserve a significant electromagnetic enhancement at the metal tip apex. This constraint ideally
involves the use of costly atomic layer deposition of insulating materials (SiO₂ or Al₂O₃ in general)
or tedious wet chemical methods [12]. Similarly, micro-/nanofabrication methods can allow for
the manufacturing of TERS-effective probes with high reproducibility, but their high cost and the
required specific expertise to handle them prevent most TERS users from using them in practice.
Several companies also propose TERS probes, but their performances are very variable for a quite
prohibitive cost. In this context, pulsed metal electrodeposition was described as an alternative to
design gold and silver AFM-TERS tips with high enhancement and long lifetime [13,14]. The
effectiveness of such electrochemically modified AFM-TERS tips was evaluated by probing a
monolayer of a malachite green derivative with high Raman cross-section, but no TERS imaging
was carried out, impeding the full assessment of the robustness of these tips.

In this work, we describe a simple and cost-effective potentiostatic method to manufacture AFM-TERS probes based on gold electrodeposition on n-doped silicon AFM tips. We propose a mechanism to explain the electrodeposition process and indicate ideal conditions to obtain TERS active probes using this method. Depending on the experimental conditions, the resulting AFM-TERS probes have varying morphologies, but at least 50% of them allow TERS imaging of graphene oxide (GO) and/or carbon nanotubes (CNTs) to be realized with lateral spatial resolution (LR) below 20 nm, irrespective of the size of the tip apex.

## 2. Materials and Methods

### 2.1 *Electrodeposition setup*

Gold-coated TERS tips were obtained by a potentiostatic electrodeposition method, using a
conventional three-electrode electrochemical cell (Fig. 1a).
The working electrode (WE) was a commercial n-doped silicon AFM tip (AppNano ACCESS-FM, 2.7 N/m spring constant, 60 kHz resonance frequency), the reference electrode (RE) was a Ag/AgCl electrode and the helical counter electrode (CE) was a gold wire (Thermo scientific, gold purity 99.9%, 0.25 mm diameter). A cyanide-free Metalor ECF60 solution was chosen as gold plating bath. Note that the AFM tip was mounted on a modified cantilever holder (Bioscope II, Bruker), allowing for the immersion of the AFM tip in the plating solution as well as the electrical wiring (Fig. S1 in supplementary materials). Such a setup enables a precise control of the immersion depth and prevents any deleterious contact between metallic parts of the holder and the plating solution. A chemical pre-treatment was necessary prior to electrodeposition in order to remove the nanometre-thick insulating silicon oxide layer, always covering silicon AFM tips. It consisted in chemical etching of as-received AFM tips for 2 min in 5% HF (Caution! HF is highly corrosive, can cause deep tissue damage and systemic toxicity) aqueous solution (prepared after dilution of a 48% v/v HF solution in water from Sigma-Aldrich, 99.99%), followed by thorough rinsing with Milli-Q water and drying with nitrogen gas. Freshly etched AFM tips then behaved as an efficient WE on top of which gold was electrodeposited when the applied potential was negative enough to trigger the electrochemical reduction reaction. To determine the onset potential of this reaction, linear sweep voltammetry (LSV) was performed with a potentiostat (μ-Autolab type III Metrohm) controlled by the NOVA 2.1.5 software. The onset potential indicates the threshold value at which the gold reduction takes place (around -0.5 V vs Ag/AgCl) (Fig. 1b). It is noteworthy that many different methods have been reported to estimate the onset potential [15].

Here, the onset potential is defined qualitatively as the value when the current deviates from the background by 10%. All the potentiostatic electrodeposition experiments were performed by using a PalmSens 4 potentiostat triggered by the PSTrace 5.9 software. The AFM tip was immersed in
the plating solution and positioned in the middle of the area surrounded by the CE as shown in Fig. 1a. Potentials of -1.7 V or -1.8 V vs Ag/AgCl, as well as electrodeposition times of 1 or 2 s, were used to manufacture gold modified TERS tips.

## 2.2 Scanning electron microscopy

Secondary electron images of metallized tips were recorded after TERS measurements using a Tescan Vega 3 scanning electron microscope (SEM), equipped with a XFlash 630M energy dispersive spectroscopy (EDS) detector (Bruker). A 10 mm working distance and a 15 kV acceleration voltage were employed for SEM imaging.

## 2.3 Dark-field scattering microscopy

Dark-field scattering microscopy (DFSM) was performed in total-internal reflection (TIR) geometry using an instrument described in detail elsewhere [16,17]. DFSM in TIR is particularly relevant to specifically characterize LSPR of the apex of AFM-TERS probes as a function of their shape, their metal composition, and the incident light polarization [18-21]. Briefly, DFSM measurements were carried out using intense white light irradiation with a polarisation state parallel (p-polarisation) or perpendicular (s-polarisation) to the incidence plane. The angle of incidence $\alpha_i$ was set above the critical angle $\alpha_c$ ($\alpha_i = 45^\circ > \alpha_c \sim 42^\circ$) by means of a 60x oil immersion TIR objective (OLYMPUS, 1.49 NA). The diameter of the analysed sample area was 2.6 µm.

## 2.4 Tip-enhanced Raman spectroscopy

TERS measurements were carried out on a side-illumination NanoRaman system (HR Evo Nano, HORIBA Scientific), consisting of an Omegascope atomic force microscope combined with a LabRAM HR Evolution Raman spectrometer. A linearly polarized laser light (633 nm, 170 µW) was focused on the sample using a 100x long-working distance objective (MITUTOYO, 0.7 NA).
TERS spectra were measured using a 600 grooves/mm diffraction grating. TERS mapping was performed in the hybrid contact mode developed by HORIBA, for which the TERS tip was in contact with the sample surface only during the measurement of the TERS signal [22]. The integration time per spectrum was 3 s. All TERS measurements were made by exciting gold-coated TERS probes manufactured by potentiostatic electrodeposition. The laser fluence was also optimized to avoid any damage through laser-induced sample heating. The 521 cm⁻¹ Raman band of a benchmark silicon chip was used as reference to verify regularly the spectrometer calibration. The investigated sample was purchased from HORIBA as test sample containing GO sheets and CNTs deposited on a gold substrate. TERS spectra presented here were obtained after a slight smoothing using the Savitzky-Golay smoothing algorithm (LabSpec 6 software).

# 3. Results and Discussion

## 3.1 Electrodeposition process

The potentiostatic electrodeposition of gold on n-doped silicon AFM tips is based on the nucleation and growth of a noble metal layer. As nucleation points are randomly distributed on silicon, a homogeneous metallic film should fully cover the tip apex. However, the electrodeposition of gold on gold is more favourable than that of gold on silicon, due to the lower conductivity of the semiconductor. The electrodeposition onto a silicon AFM probe follows the Volmer-Weber mechanism [23]. Following the initial 3-D nucleation, metallic islands keep growing, and finally merge into a continuous film. The coating homogeneity and thickness are governed by the overpotential applied to the WE and the electrodeposition time. According to theory, the energy barrier to trigger gold reduction and the corresponding radius of the gold nuclei both decrease with increasing overpotential [24]. When the applied potential is comprised between the onset potential (-0.5 V vs Ag/AgCl) and -1.7 V vs Ag/AgCl, gold is deposited on the cantilever
and the chip, but not on the tip apex. This behaviour suggests that a different nucleation energy is required at the tip apex compared to other parts of the WE. This phenomenon might be associated to the non-uniform doping level, along the semiconductor substrate, and/or the fact that the driving force is too weak to cross the energy barrier at low overpotentials. For long electrodeposition times, the metal layer might be quite heterogeneous albeit thick. On the other hand, when the applied potential is more negative than -1.8 V vs Ag/AgCl, the control of the electrodeposition kinetics becomes rather difficult. The latter experimental conditions are thus inadequate to produce AFM-TERS tips. Therefore, potentials of -1.7 V / -1.8 V vs Ag/AgCl and short electrodeposition times of 1 s / 2 s turned out to be the optimal working parameters to obtain a gold coating, which homogeneously covers the whole AFM tip apex (Fig. 2a-d). For these experimental parameters, the gold roughness is quite different from one tip to another, but this is not a critical problem for TERS efficiency, as discussed later. It is worth emphasizing that the optimal potential depends on the conductivity of the WE, i.e. the doping level of the AFM tip. From one batch of AFM cantilevers to another, the doping can be slightly different and therefore the required potential might vary relative to the -1.7 V / -1.8 V vs Ag/AgCl potential considered here. However, the proposed methodology can be easily transposed to other materials in a straightforward manner. For example, for AFM tips made out of a more conductive material such as tungsten, the potential required for gold electrodeposition should be less negative than the one required to metallize n-doped silicon.

Instantaneous and progressive nucleation are two major models used to describe the nucleation and growth mechanisms. For instantaneous nucleation, all the nuclei are formed at the same time and grow with the same rate during the whole electrodeposition process. On the contrary, the number of nuclei does increase during the electrodeposition in the case of a progressive nucleation.
A theoretical framework to calculate the potentiostatic maximum current was suggested to better distinguish instantaneous and progressive nucleation pathways [25]. Based on this theory, Scharifker and Hills further proposed a non-dimensional model of instantaneous and progressive nucleation. The two dimensionless parameters are the current (I) divided by the maximum current reached during electrodeposition (I<sub>m</sub>) and the corresponding time ratios (t/t<sub>m</sub>) [26].

For instantaneous nucleation:

$$ \frac{I^2}{I_m^2} = \frac{1.9542}{t/t_m} \{1 - \exp[-1.2564 (t/t_m)]\}^2 \quad (1) $$

Whereas for progressive nucleation:

$$ \frac{I^2}{I_m^2} = \frac{1.2254}{t/t_m} \{1 - \exp[-2.3367 (t/t_m)]\}^2 \quad (2) $$

To estimate the nucleation behaviour during the electrodeposition process, I<sup>2</sup>/I<sub>m</sub><sup>2</sup> vs t/t<sub>m</sub> was plotted for one of the metal-coated AFM tips (Tip4 in Fig. 2d) and compared with the theoretical curves deduced from relations (1) and (2) (Fig. 2f and Fig. S2 for the corresponding potentiostatic measurement). Before reaching the maximum current at t/t<sub>m</sub> = 1, the observed behaviour deviates from the theoretical curves, partly due to the influence of the capacitive current. However, above this maximum current, the experimental curve (dark red line) overlaps well with the instantaneous nucleation model (grey line), which indicates that the formation and growth of instantaneous nucleation domains is the more likely mechanism occurring during the electrodeposition process. The rather homogeneous gold deposition revealed by SEM in Fig. 2d corroborates this conclusion.

## 3.2 Morphological and plasmonic properties of TERS tips

SEM images of four AFM tips coated with gold by potentiostatic electrodeposition are shown in Fig. 2a-d. EDS measurements confirm the convenient gold coating at the tip end (Fig. S3). Curvature radii of Tip1, Tip2, Tip3, and Tip4 apexes are equal to 40 nm, 37 nm, 170 nm, and 235
nm, respectively (Fig. 2a-d). The increasing size of the tip apexes reflects the increasing thickness of gold deposited on the etched n-doped silicon. By means of DFSM measurements in TIR, the LSPR of a ~120 nm-long end portion of these gold-coated tips can be estimated [11]. The LSPR of a nanostructured gold film is directly related to its optical extinction, which is the sum of its absorption and scattering. For our thick gold layers, the absorption is negligible with respect to the scattering signal measured by DFSM. Variations of the scattered intensity thus reproduce correctly extinction properties of gold-coated AFM tip apexes. From Tip1 to Tip4, the LSPR broadband is maximum at λ<sub>LSPR</sub> = 537 nm, 542 nm, 664 nm and 687 nm, respectively (Fig. 2e). It is seemingly red-shifted when the curvature radius of the tip apex increases. Tip1 and Tip2 exhibit very similar apex dimensions, and they have comparable LSPR hence. This size-dependent red shift of the LSPR can be interpreted by considering the excitation of a dipolar plasmon at the short-end portion of the tip apex [20]. When the apex size increases, the charge separation (i.e. the surface polarization) increases and leads to a reduced restoring force for the plasmon dipole oscillation, a lower frequency for the collective oscillation of electrons and a red shift of the LSPR wavelength [27]. This behaviour is consistent with those reported for individual noble metal nanoparticles [28-30]. However, it is worth mentioning that the dependence on the curvature radius of the tip apex could be qualitatively different if a micrometre-long tip area had been excited [31]. In this case, the increased size of the tip apex would be associated with a decreased aspect ratio of the irradiated metal area. Like for noble metal nanorods [27,32], this would lead to a decrease of the charge separation, an increase of the restoring force for the plasmon dipole oscillation and of the corresponding frequency for the plasmon collective oscillation, thus resulting in a blue shift of the LSPR wavelength [31]. The specific excitation and probing of a short-end portion of the TERS tip by DFSM in TIR appears as the key element to explain the red shift of the LSPR with increasing
apex sizes. The shape of LSPR bands with full widths at half maxima in the 110-190 nm range however suggests the presence of multiple contributions that are difficult to assign precisely, but are visibly related to the size of the tip apex as well as the roughness of the gold film. The LSPR is indeed strongly affected by the shape of noble metal nanostructures at the tip end [33-35], and thus by the roughness of the gold layer. Multipolar plasmon modes may also contribute to DFSM spectra, with potential quadrupolar plasmon modes emerging as a small band and a shoulder at 540 nm for Tip4 and Tip3, respectively [36].

### 3.3 TERS characterization and imaging

As suggested in Fig. 2a and b, the amount of gold deposited on top of AFM tips is not always proportional to the short electrodeposition time. However, it is noteworthy that, although the LSPR is affected by the thickness of the gold layer, the TERS efficiency is not strongly modified by changes in thickness provided that the potential and the electrodeposition time remain equal to -1.7 V / -1.8 V and 1 s / 2 s, respectively. Under these experimental conditions, 53 % of metallized AFM tips (8 out of 15) exhibited an exploitable TERS activity. This value is globally similar to the typical fabrication yield obtained with PVD techniques, such as sputtering [37].

Fig. 3 shows TERS spectra of a sample containing GO flakes and CNTs under $λ_{exc}$ = 633 nm laser excitation. Three different AFM-TERS probes were used, namely Tip1, Tip2, and Tip3, which were described in Fig. 2 (the TERS spectrum associated with an additional tip called Tip5 is provided in Fig. S4). Their LSPR could be excited at 633 nm (Fig. 2e and Fig. S5). When Tip1 and Tip2 are engaged on the sample surface, similar TERS intensities are observed, while the TERS intensities appear nearly three times higher with Tip3, for which the LSPR is more efficiently excited under 633 nm irradiation ($λ_{exc} < λ_{LSPR}$) [38]. When these AFM-TERS probes are withdrawn, no TERS band can be identified. For Tip1 and Tip3, the TERS signature of a GO
flake is observed, with a characteristic D band centred at 1350 cm⁻¹ and a G band at 1580 cm⁻¹ (Fig. 3a and c). For Tip2 (Fig. 3b), the TERS signatures suggest the presence of an oxidized and hydrogenated multi-walled carbon nanotube (MWCNT) of nearly 10 nm diameter according to the AFM images displayed in Fig. 3e. TERS bands emerge at 1576, 1485, and 1200 cm⁻¹ (Fig. 3b). The band centred nearby 1570 and 1485 cm⁻¹ can be assigned to the G band of the MWCNT, and CH₂ deformations or scissoring vibrations, respectively [1,22,39,40]. Bands at 1200 and 1120 cm⁻¹ should correspond to C-O-C stretching vibrations [22,40]. Remarkably, as the excited sample areas are 5-10 nm high (Fig. 2d-f), the gap mode effect between the gold tip apex and the gold substrate is expected not to be very effective. This effect is known to increase significantly the TERS enhancement factor when the tip-substrate distance is shorter than 1-2 nm [41,42]. The possibility to detect TERS signals in the absence of significant gap mode effect is promising for future applications in materials science and/or biology. In our experimental conditions, no far-field signal from the sample could be detected. Thus, the optical contrast, defined as the ratio of near-field and far-field signals, cannot be precisely estimated although it is clearly high enough to allow for TERS imaging. Finally, we were able to collect TERS spectra with AFM-TERS probes, which had already been prepared 12 days before, provided that they were stored in inert atmosphere in the meantime. This behaviour suggests a good chemical stability of metallized tips. Moreover, no gold peeling-off was observed after TERS measurements, which proves a certain robustness of the gold coatings.

Not only does the TERS technique aims at amplifying the Raman signal, but it also pursues the goal of improving the LR with respect to far-field confocal Raman microscopy. Fig. 3 demonstrates the TERS efficiency of AFM-TERS probes manufactured by potentiostatic electrodeposition of gold. However, it does not give any information about the LR. The LR can be
assessed by analysing TERS intensity variations across a MWCNT or the edge of a GO flake. Fig. 4b shows TERS intensity profiles plotted by integrating intensities of the 1590 cm⁻¹ TERS band of a MWCNT (Fig. 4c). These spectra were collected at eleven points along the white arrow indicated in Fig. 4a. The full width at half maximum extracted from the resulting TERS intensity profiles (FWHM=18 nm) suggests that the LR is better than 20 nm. Note that this LR is close to the FWHM extracted from the AFM height profile (FWHM=13 nm) determined along the white arrow drawn in the AFM image of Fig 4a, which has been performed using an uncoated tip to reproduce as faithfully as possible the genuine dimensions of the investigated MWCNT, and avoid AFM image broadening effects by convolution of the real size of the MWCNT with the apex size of Tip3 used for TERS (Fig. 2c).

The LR below 20 nm is in line with those usually reported for noble metal tips obtained by PVD techniques [11,22,37,43], even though nanometre LR have also been reached with these latter methods [44]. It is apparently uncorrelated to the dimension of the tip apex, which is probably linked to the presence of nanoscale metal protrusions at the very end of the rough AFM-TERS probe. At such metal geometrical singularities, the local charge density can be increased and so is the lightning-rod effect. Under laser light excitation, the intense electromagnetic enhancement can then be highly confined in an area of dimensions down to the sub-nanometre scale [45].

Finally, the TERS efficiency and robustness of AFM-TERS probes is demonstrated by carrying out TERS imaging. Fig. 5b and 5c show TERS maps of a portion of MWCNT obtained by intensity integration of the D band (red) and the G band regions (green), centred at 1320 and 1590 cm⁻¹ respectively. Interestingly, this MWCNT is neither oxidized nor hydrogenated in contrast to the MWCNT examined in Fig. 3b. In Fig. 5a, the height of the MWCNT seems to be nearly 10 nm, while its width is about 50 nm. In this case, the AFM height image has been obtained using Tip4,
which has also been kept for TERS measurements. The 50 nm width is thus related to the
convolution of the genuine size of the MWCNT and the apex of Tip4 with 235 nm-large curvature
radius (Fig. 2d). In contrast, in TERS maps (Fig. 5b and c), it clearly appears that the TERS signal
emerges at a single 10 nm-large pixel along the MWCNT. This observation is fully consistent with
the expected tubular structure of a MWCNT with 10 nm height, and therefore 10 nm width.

Fig. 5e shows two TERS images superimposed to the tapping-mode topographical image of a GO flake, which were collected with 20 nm scanning steps using another tip denoted Tip6. Tip6 was fabricated by applying -1.7 V potential on the WE for 1 s. Marker bands of GO, namely the D band at 1330 cm⁻¹ and the G band at 1595 cm⁻¹, were essentially observed at the edge of the flake, as already reported for carboxyl-modified GO sheets [22]. The most likely cause of the higher Raman signal enhancement at this location is the disruption of the GO lattice leading to a better alignment of the axial polarization of the tip-enhanced near-field with the GO Raman polarizability tensor. On the TERS images in Fig. 5e, the edge of the GO flake is detectable by a single line of pixels, which is in line with our preceding observations on a MWCNT (Fig 5a-c) and confirms the nanoscale LR below 20 nm.

**Conclusion**

A cost-effective potentiostatic electrodeposition method has been developed to manufacture
robust gold-coated AFM-TERS probes with good chemical stability and reasonable 53% yield.
The growth of the gold film has been shown to be triggered by initial instantaneous nucleation of
gold. Even though our study focused on the gold coating of n-doped silicon AFM cantilevers, the
described methodology could be easily transposed to other types of materials. We demonstrated
the possibility to perform TERS imaging using these metallized AFM tips, with a good optical
contrast and convenient LR below 20 nm. Finally, the LSPR of these AFM-TERS probes appears
relatively broad and may allow for TERS experiments at various visible wavelengths. Further
works will be however necessary to thoroughly evaluate the potential of these probes for TERS
characterization and imaging using different experimental configurations and surrounding media.

ACKNOWLEDGMENT

The PhD scholarship of YH and the experimental work have been funded by the Agence Nationale
de la Recherche under contract No. ANR-20-CE29-0004. GSC and SB also thank the University
of Bordeaux for the MESRI PhD grant of GSC. DT and SB acknowledge the Région Nouvelle
Aquitaine for financial support under contract no. 2019-1R1M03. TERS measurements were
performed at the platform SIV (Spectroscopie et Imagerie Vibrationnelle) funded by the European
Union (FEDER) and the Région Nouvelle Aquitaine. GS and AK acknowledge financial support
by the European Research Council (ERC) under the European Union's Horizon 2020 research and
innovation program (grant agreement no 741251, ERC Advanced grant ELECTRA). YH and LB
acknowledge Tatjana Safarik for scientific discussions.

REFERENCES

[1] P. Verma, Chem. Rev. 117 (2017) 6447-6466.

[2] D. Kurouski, A. Dazzi, R. Zenobi and A. Centrone, Chem. Soc. Rev. 49 (2020) 3315-3347.

[3] S. Bonhommeau, G. S. Cooney and Y. Huang, Chem. Soc. Rev. 51 (2022) 2416-2430.

[4] X. Shi, N. Coca-López, J. Janik and A. Hartschuh, Chem. Rev. 117 (2017) 4945-4960.

[5] S. Bonhommeau and S. Lecomte, ChemPhysChem 19 (2018) 8-18.

[6] P. Pienpinijtham, Y. Kitahama and Y. Ozaki, Nanoscale 14 (2022) 5265-5288.
[7] T. Touzalin, S. Joiret, I. T. Lucas, E. Maisonhaute, Electrochem. Commun. 108 (2019) 106557.

[8] T. L. Vasconcelos, B. S. Archanjo, B. Fragneaud, B.S. Oliveira, J. Riikonen, C. Li, D. S. Ribeiro, C. Rabelo, W. N. Rodrigues, A. Jorio, C. A. Achete and L. G. Cançado, ACS Nano 9 (2015) 6297-6304.

[9] T. W. Johnson, Z. J. Lapin, R. Beams, N. C. Lindquist, S. G. Rodrigo, L. Novotny, and S.-H. Oh, ACS Nano 6 (2012) 9168-9174.

[10] A. Taguchi, J. Yu, P. Verma and S. Kawata, Nanoscale 7 (2015) 17424-17433.

[11] D. Talaga, A. Bremner, T. Buffeteau, R.A.L. Vallée, S. Lecomte, S. Bonhommeau, J. Phys. Chem. Lett. 11 (2020) 3835-3840.

[12] Y.-P. Huang, S.-C. Huang, X.-J. Wang, N. Bodappa, C.-Y. Li, H. Yin, H.-S. Su, M. Meng, H. Zhang, B. Ren, Z.-L. Yang, R. Zenobi, Z.-Q. Tian and J.-F. Li, Angew. Chem. Int. Ed. 57 (2018) 7523-7527.

[13] L.-K. Yang, T.-X. Huang, Z.-C. Zeng, M.-H. Li, X. Wang, F.-Z. Yang and B. Ren, Nanoscale 7 (2015) 18225-18231.

[14] T.-X. Huang, C.-W. Li, L.-K. Yang, J.-F. Zhu, X. Yao, C. Liu, K.-Q. Lin, Z.-C. Zeng, S.-S. Wu, X. Wang, F.-Z. Yang and B. Ren, Nanoscale 10 (2018) 4398-4405.

[15] C. Batchelor-McAuley, Curr. Opin. Electrochem. 37 (2023) 101176.

[16] D. Talaga, M. Comesaña-Hermo, S. Ravaine, R. A. L. Vallée and S. Bonhommeau, J. Opt. 17 (2015) 114006.

[17] M. Plissonneau, A. Madeira, D. Talaga, S. Bonhommeau, L. Servant, R. A. L. Vallée, C. Labrugère, I. A. Goldthorpe, L. Pautrot-D'Alencon, T. Le Mercier and M. Treguer-Delapierre, ChemNanoMat 5 (2019) 1044-1049.
[18] C. C. Neacsu, G. A. Steudle and M. B. Raschke, Appl. Phys. B 80 (2005) 295-300.

[19] R. Wang, J. Wang, F. Hao and Q. Tian, Appl. Opt. 49 (2010) 3905-3908.

[20] T. Mino, Y. Saito and P. Verma, ACS Nano 8 (2014) 10187-10195.

[21] I. Maouli, A. Taguchi, Y. Saito, S. Kawata and P. Verma, Appl. Phys. Exp. 8 (2015) 032401.

[22] W. Su, N. Kumar, A. Krayev and M. Chaigneau, Nature Commun. 9 (2018) 2891.

[23] Y. D. Gamburg, and G. Zangari, Theory and practice of metal electrodeposition, Springer Science & Business Media, New York, NY, 2011.

[24] F. Nasirpouri, Electrodeposition of nanostructured materials, Vol. 62, Springer, Princeton, NJ, 2017.

[25] G. Gunawardena, G. Hills, I. Montenegro, and B. Scharifker, J. Electroanal. Chem. 138 (1982) 225-239.

[26] B. Scharifker and G. Hills, Electrochim. Acta 28 (1983) 879-889.

[27] M. Rycenga, C. M. Cobley, J. Zeng, W. Li, C. H. Moran, Q. Zhang, D. Qin and Y. Xia, Chem. Rev. 111 (2011) 3669-3712.

[28] U. Kreibig and M. Vollmer, Optical Properties of Metal Clusters, Vol. 25, Springer, Berlin, 1995.

[29] C. Bohren and D. R. Huffman, Absorption and Scattering of Light by Small Particles, Wiley, Hoboken, NJ, 1998.

[30] Q. Zhang, W. Li, C. Moran, J. Zeng, J. Chen, L.-P. Wen and Y. Xia, J. Am. Chem. Soc. 132 (2010) 11372-11378.

[31] T. Itoh, Y. S. Yamamoto, T. Suzuki, Y. Kitahama and Y. Ozaki, Appl. Phys. Lett. 108 (2016) 021604.

[32] P. Pienpinijtham, Y. Kitahama and Y. Ozaki, J. Raman Spectrosc. 52 (2021) 1997-2017.
[33] V. Myroshnychenko, J. Rodríguez-Fernández, I. Pastoriza-Santos, A. N. Funston, C. Novo, P. Mulvaney, L. M. Liz-Marzán and F. J. García de Abajo, Chem. Soc. Rev. 37 (2008) 1792-1805.

[34] H. Guo, F. Ruan, L. Lu, J. Hu, J. Pan, Z. Yang and B. Ren, J. Phys. Chem. C 113 (2009) 10459-10464.

[35] D. Talaga and S. Bonhommeau, Chem. Phys. Lett. 615 (2014) 89-93.

[36] H. Wei, A. Reyes-Coronado, P. Nordlander, J. Aizpurua and H. Xu, ACS Nano 4 (2010) 2649-2654.

[37] S. Bonhommeau, D. Talaga, J. Hunel, C. Cullin, S. Lecomte, Angew. Chem. Int. Ed. 56 (2017) 1771-1774.

[38] N. Félidj, J. Aubard, G. Lévi, J.R. Krenn, A. Hohenau, G. Schider, A. Leitner, F. R. Aussenegg, Appl. Phys. Lett. 82 (2003) 3095-3097.

[39] D. J. Sánchez-Trujillo, L. V. Osorio-Maldonado, J. J. Prías-Barragán, Sci. Rep. 13 (2023) 13, 4810.

[40] G. Socrates, Infrared and Raman Characteristic Group Frequencies: Tables and Charts, John Wiley & Sons Ltd.: Chichester, New York, Weinheim, Toronto, Brisbane, Singapore, 2001.

[41] J. Stadler, T. Schmid and R. Zenobi, Nanoscale 4 (2012) 1856-1870.

[42] J. Stadler, B. Oswald, T. Schmid, and R. Zenobi, J. Raman Spectrosc. 44 (2013) 227-233.

[43] D. Talaga, G. S. Cooney, V. Ury-Thiery, Y. Fichou, Y. Huang, S. Lecomte and S. Bonhommeau, J. Phys. Chem. B 126 (2022) 5024-5032.

[44] Z. He, W. Qiu, M. E. Kizer, J. Wang, W. Chen, A. V. Sokolov, X. Wang, J. Hu and M. O. Scully, ACS Photon. 8 (2021) 424-430.

[45] S. Trautmann, J. Aizpurua, I. Götz, A. Undisz, J. Dellith, H. Schneidewind, M. Rettenmayr and V. Deckert, Nanoscale 9 (2017) 391-401.
Figures and Tables

<figure><img src="image_2.png" /><figcaption>**Fig. 1.** (a) Scheme of the three-electrode electrochemical cell, with a working (WE), counter (CE) and reference (RE) electrode. (b) Linear sweep voltammogram of an AFM tip immersed in the commercial gold plating bath. Scan rate: 50 mV/s. The red cross indicates the onset potential.</figcaption></figure>
<figure><img src="image_3.png" /></figure>

<figure><img src="image_4.png" /></figure>

<figure><img src="image_5.png" /></figure>

**Fig. 2.** (a-d) SEM images of four gold-coated AFM tips prepared by potentiostatic electrodeposition with different potentials and electrodeposition times: (a) -1.7 V vs Ag/AgCl and 1 s for Tip1 (green), (b) -1.7 V vs Ag/AgCl and 2 s for Tip2 (blue), (c) -1.8 V vs Ag/AgCl and 1 s for Tip3 (dark yellow), and (d) -1.8 V vs Ag/AgCl and 2 s for Tip4 (dark red). Scale bars: 200 nm. (e) Normalized DFSM spectra of the corresponding tips excited by *p*-polarised white light. The red dashed line indicates the excitation wavelength used for TERS experiments. (f) Non-dimensional $(I/I_m)^2$ vs $t/t_m$ plots for instantaneous nucleation (grey), progressive nucleation (dashed black) and experimental curve for the electrodeposition process of Tip4 (dark red).
<figure><img src="image_6.png" /><figcaption>**Fig. 3.** (a-c) TERS spectra under 633 nm irradiation when the tip is retracted (Tip out) and engaged on the sample (Tip in) using Tip1 (a), Tip2 (b) and Tip3 (c). Integration time: 3 s. (d-f) Tapping-mode AFM topographical image collected with Tip1 (d), Tip2 (e) and Tip3 (f). The red crosses indicate the position corresponding to the TERS spectra plotted in Fig. 3a, 3b and 3c, respectively.</figcaption></figure>
<figure><img src="image_7.png" /><figcaption>**Fig. 4.** (a) Tapping-mode topographical AFM image of a MWCNT scanned with an uncoated tip. (b) AFM height profile (black) along the white arrow in Fig. 4a, and corresponding TERS profile (red) obtained by intensity integration of the TERS band at 1590 cm⁻¹ in Fig. 4c. (c) TERS spectra recorded using Tip3 along the white arrow drawn in Fig. 4a (scanning step: 10 nm, integration time: 3 s). TERS spectra from bottom to top correspond to sample points from left to right along the white arrow in Fig. 4a.</figcaption></figure>
<figure><img src="image_8.png" /><figcaption>**Fig. 5.** (a) Tapping-mode topographical AFM image of a MWCNT scanned with Tip4. (b-c) Overlay of TERS maps obtained by intensity integration of the D band (red) or the G band (green) and the AFM image bounded by the white square in Fig. 5a. Scanning step: 10 nm. (d) Tapping-mode topographical AFM image of a portion of GO flake scanned with Tip6. (e) Overlay of TERS maps obtained by intensity integration of the D and G bands (blue) and the AFM image bounded by the white square in Fig. 5d. Scanning step: 20 nm. (f) TERS spectrum measured at the position</figcaption></figure>
marked by a white cross in Fig. 5b and 5c. The red and green strips indicate the integrated regions
of the spectrum to generate red and green TERS images in Fig. 5b and 5c, respectively. (g) TERS
spectrum measured at the position marked by a white cross in Fig. 5e. The blue strip indicates the
integrated region of the spectrum to generate the blue TERS images in Fig. 5e. Integration time: 3
s.