Electrochemical growth of CoNi and Pt-CoNi soft magnetic composites on an alkanethiol monolayer-modified ITO substrate

CoNi and Pt-CoNi magnetic layers on indium-tin oxide (ITO) substrates modified by an alkanethiol self-assembled monolayer (SAM) have been electrochemically obtained as an initial stage to prepare Semiconducting layer-SAM-Magnetic layer hybrid structures. The best conditions to obtain the maximum compactness of adsorbed layers of dodecanethiol (C12-SH) on ITO substrate have been studied using contact angle, AFM, XPS and electrochemical tests. The electrochemical characterization (electrochemical probe or voltammetric response in blank solutions) is fundamental to assure the maximum blocking of the substrate. Although the electrodeposition process on the SAM-modified ITO substrate is very slow if the blocking of the surface is significant, non-cracked metallic layers of CoNi, with or without a previously electrodeposited seed-layer of platinum, have been obtained by optimizing the deposition potentials. Initial nucleation is expected to take place at the pinhole defects of the C12-SH SAM, followed by a mushroom-like growth regime through the SAM interface that allows the formation of a continuous metallic layer electrically connected to the ITO surface. Due to the potentiality of the methodology, the preparation of patterned metallic deposits on ITO substrate using SAMs with different coverage as templates is feasible.

The functionalisation and nanopatterning of substrates by means of self-assembled monolayers (SAMs) has been exploited for low-cost, high aspect ratio hierarchical growth applications in molecular spintronics 1 , selective adsorption of nanostructures 2-6 , biosensing 7 and nanolithography 8 .
Alkanethiol SAMs have been widely utilized for modifying conducting substrates, in particular welldefined Au surfaces [9][10][11][12][13][14][15][16][17] . Among all plausible nanomodifications, the formation of continuous metallic layers over SAM-modified Au electrodes has drawn a burgeoning interest. Several methodologies for obtaining a continuous metallic layer on the top of a SAM have been explored, the most performed being electrochemical methods. Unlike the cost-intensive physical methods such as vacuum layer deposition 18,19 or metal nano-transfer printing [20][21][22][23][24] , electrochemical methods stand out as an excellent choice due to their low cost and environment-friendly nature that allow easy tailoring of the composition, crystalline structure and thickness of metallic layers by an exhaustive control of both deposition conditions and electrolytic bath composition 25 . Kolb and co-workers intensively pursued the selective metallization of SAM surfaces by means of Cu or Ag underpotential deposition (UPD) [26][27][28][29] , but diffusion and nucleation of the metallic ions at the Au-SAM interface impeded the selective formation of a continuous film on the top of the SAM. The presence of nanopores, even in denselypacked alkanethiol SAMs, favoured the electrocrystallisation of the metal at the SAM pinholes due to inferior electrical resistance by means of a mushroom-type growth mechanism 30,31 . This phenomenon, undesired for obtaining metal-spacer-metal thin film hybrid structures, was used by Schilardi and coworkers to obtain thin metal films connected to the substrate by means of narrow wires obtained during the columnar growth 32 . Further studies revealed the possibility of using the SAM defects as a low-adherent support to nanopattern electrodeposited metallic films following the morphology of a templated substrate 33,34 .
Despite of the aforementioned, alkanethiol SAM formation on non-metallic surfaces has been scarcely studied, the most known examples being over SnO 2 (110) 35 , manganite (La 2/3 Sr 1/3 MnO 3 ) 36 , magnetite (Fe 3 O 4 ) 37 or fluorine-doped tin oxide 38 . One of the most promising substrates for such purpose is indium-tin oxide (ITO). ITO, a transparent semiconducting oxide, has extensively been used in numerous applications such as liquid-crystal displays (LCD), organic light-emitting diodes (OLEDs) 39 and solar cells 40 due to the combination of its optical transparency along with its electrical conductivity 41 . As a consequence, the formation of a highly-compact alkanethiol SAM above an ITO surface could easily modulate its electronic properties, thus expanding its application range. Several works have attempted to pursue this, with variable results: Grunze and co-workers studied the formation of octadecanethiol (C18-SH) or hexadecanethiol (C16-SH) SAMs over freshly-deposited or UV/ozone cleaned ITO substrates, by immersion in the neat liquid or in an ethanol solution, obtaining high-compactness SAMs 42 . Karsi et al. managed to obtain compact decanethiol (C10-SH), dodecanethiol (C12-SH) and octadecanethiol (C18-SH) SAMs over ITO commercial substrates by immersion in the neat liquid, using a pre-treatment method alternative to UV/ozone 43 . However, the electrochemical characterization of the substrates to evaluate defects in the SAM or a further deposition of a metallic layer above the ITO-SAM structure was not evaluated.
The electrodeposition of metallic alloys on SAM-modified semiconducting substrates has not been accurately studied. These studies present high interest in order to confer magnetic or electronic properties for the development of devices for electronics 44 and spintronics 45 . Along with this, the formation of a strongly bonded SAMs on a semiconducting surface, if allowing the formation of metallic thin films on it, would hamper diffusion of metallic layer constituents in the SiO2 semiconductor substrate 46 . Moreover, thiol SAM-ITO substrates have been proposed to improve compactness and growth rate of metallic layers with respect to bare ITO 47 , as well as to inhibit Ni electroless deposition over Pd seeds 48 . Thus, the electrodeposition of metallic layers, either ferromagnetic or non-magnetic, over alkanethiol-modified ITO substrates, deserves further investigation.
In this report we analyse the electrochemical preparation of metallic deposits of CoNi or Pt-CoNi composite on an alkanethiol SAM-modified ITO substrate. The C12-SH SAM formed by immersion over a commercial ITO substrate is analysed by means of X-ray photoelectron spectroscopy (XPS), atomic force microscopy (AFM), electrochemical probe and contact angle measurements. The optimization of the incubation time, C12-SH solution concentration and ITO pretreatment are demonstrated to be critical for the compactness of the SAM. Electrodeposition of CoNi and Pt/CoNi on the ITO-SAM structure is studied in order to obtain non-magnetic or ferromagnetic hybrid structures. Surface morphology and magnetic response of the ITO-SAM-CoNi structure is evaluated by SEM and SQUID measurements. Although the electrodeposition process of the magnetic alloy is drastically hindered by the presence of the SAM layer, electron exchange can start through SAM surface defects leading to a continuous metallic layer on the SAM surface electrically connected to the ITO substrate by means of nanometric columns, probably formed by the accepted mushroom growth mechanism proposed for the electrodeposition on metallic substrates as gold.  -dodecanethiol (C12-SH) (Merck, ≥ 98%, synth. grade) was used as received. 5 mM and 50 mM thiol solutions were prepared with absolute ethanol (Panreac, UV-IR-HPLC PAI) and preserved at 5°C prior to use.

Substrate pretreatment and SAM preparation
ITO-coated (25nm) glass (2 mm) substrates were pre-treated by means of ultrasonication in ultrapure water and a subsequent immersion in freshly prepared saturated NaOH solution (Merck, pellets, analytical grade) and pure H 2 SO 4 (Acros Organics, 96%). The freshly pre-treated ITO substrates were immersed in 5mM C12-SH, 50mM C12-SH or in pure C12-SH for different time at room temperature in a hermetically closed opaque glass vial. After incubation, samples were carefully rinsed in copious amounts of absolute ethanol and ultrapure water to remove physisorbed thiols, and blown dry with N 2 .

SAM characterization
AFM measurements were performed in a AFM Nanotech Microscope (Nanotech Electrónica S.L., Cervantes AFM system) in tapping mode using a 4.6 µm, 45 µm, 160 µm (thickness, width, length) Arrow-NCR-20 monolithic, highly doped silicon tip (Nanoworld). Analysis of the topographical images (mean surface roughness) was performed by means of the WSxM 5.0 software 49 . XPS measurements were carried out in a PHI ESCA-5500 Multitechnique System (Physical Electronics), with a monochromatic X-ray source (Al Kα line of 1486.6 eV energy and 350 W), placed perpendicular to the analyzer axis and calibrated using the Ag 3d 5/2 line with a full width at half maximum (FWHM) of 0.8 eV. The analysed area was a circle of 0.8 mm diameter, and the selected resolution for the spectra was 187.85eV of pass energy and 0.8eV/step for the general spectra. For high-resolution spectra of the elements, 58.7 eV of pass energy and 0.25 eV/step were used as XPS acquisition parameters. All measurements were made in an ultra-high vacuum (UHV) chamber, with a pressure between 5·10 -9 and 2·10 -8 torr. Angle-resolved XPS (AR-XPS) measurements were performed with the experimental conditions stated previously, but varying the angle between the surface and analyser from 15° to 90°. CoNi deposits were etched by means of argon ions sputtering up to few nm prior to AR-XPS acquisition. Multipak 8.2 software was used for digital acquisition and peak deconvolution (single Gaussian curves fit), and all spectra were calibrated by setting the C1s C-  followed by a basic-acid media treatment 43 . The results of using these procedures in our ITO commercial samples, monitored by contact-angle measurements, are summarized in Table 1. It is noticeable the irreproducibility of the solvent-based treatments proposed by Clark et al., as they yield contact angles much higher than the value expected (θ th~ 30°) 52 . On the other hand, the basic-acid media treatment manages to obtain a highly-hydrophilic surface (θ f <5°), in agreement with the theoretical value reported for this treatment (θ th <5°) 43 , that guarantees successful and reproducible removal of both physisorbed inorganic and organic impurities. As a result, all ITO samples will be pretreated by this procedure prior to SAM formation.  inform that C12-SH molecules chemisorption kinetics has not reached its saturation rate. As a consequence, the resulting SAM is expected to exhibit an inhomogeneous, short-range orientational order due to its low compactness. At longer incubation times, effective SAM reorganization along the surface and further C12-SH molecules chemisorption occurs, allowing the formation of a more densely packed structure. This is supported by the fact that the maximum contact angle stabilizes at θ~100°, independently of the C12-SH solution concentration and incubation time. This value is similar to those attributed to a densely packed C12-SH SAM on ITO 43 , initially confirming the capability of forming

XPS analysis
The interaction involved in the formation of a C12-SH SAM over ITO was elucidated by means of XPS measurements. The high-resolution XPS spectrum analysis of the sulphur S2p region, shown in C12-SH sample, more than one oxidized thiol species seems to contribute to the observed XPS signal.
The XPS analysis allows detecting chemisorption of C12-SH molecules to ITO by means of a covalent bonding between the thiolate end group and In or Sn. However, other highly oxidized sulphur species originate the second component, as the manner that a densely-packed SAM layer cannot be formed.

AFM measurements
ITO topography imaging before and after the adsorption of a C12-SH layer was performed by means of tapping-mode AFM. Prior to this, the analysis of the topography of the ITO before (Fig. 1E) and after (Fig. 1F) undergoing the basic-acid pretreatment was performed, by evaluating the influence of the pretreatment in the surface root mean square roughness (RMS). Only a slight modification in RMS was observed (1 nm prior to pretreatment and 3 nm after pretreatment); in both cases usual characteristic holes at the ITO surface were observed. As a consequence, despite of its aggressive The presence of physisorbed molecules at the holes might explain the XPS S2p signals observed at higher binding energies, ascribed in this report to the in-situ oxidation of unbounded thiol molecules or to the presence of oxidized thiol impurities. The removal of these molecules is expected to be hampered by the difficulty of the solvent to penetrate the nanometric holes.

Electrochemical response on ITO-SAM substrates
In order to test the compactness of the SAM, the voltammetric response of a test electrochemical process (electrochemical probe) on the ITO-SAM substrate was performed. The charge transfer is intrinsically dependent on both the nature of the electrode surface as well as the structure of the electrical double-layer 54 . The presence of a SAM (dielectric nature) on an electrode hinders the electron transfer either by an effective reduction of the electrode electroactive area or by impeding the approach of the redox species to its surface. The Fe(CN 6 ) 3-/ Fe(CN 6 ) 4redox pair is a standard probe because it implies a reversible, one-electron, outer-sphere redox reaction (E=0.153 V, k= 0.03cm·s -1 ) 55 .
If the compactness of the SAM is high, the redox pair Fe(CN 6 ) 3-/ Fe(CN 6 ) 4will not manage to get in direct contact with the ITO surface, leading to a substantial decay in the electrochemical current density of the redox processes involved. If the compactness of the SAM is low, the electroactive species will manage to perform electron transfer processes through SAM surface defects, observing a non-negligible current density. Figure 2 shows the cyclic voltammograms obtained in a 2mM Fe(CN 6 ) 3-/ Fe(CN 6 ) 4solution for the C12-SH SAM modified ITO electrodes at different times of incubation in 5mM or pure C12-SH. For the 5mM C12-SH incubated ITO samples, a significant decay of the current density was observed at 2 h of incubation, but a similar electrochemical response was obtained for 5 h of incubation, which reveals that a limit in the coverage of the ITO was attained and that, accordingly, a densely-packed SAM was not formed in these conditions. The presence of a low compact C12-SH layer allows the approaching of the electroactive species (through the SAM defects) to the ITO electrode and the

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Physical Chemistry Chemical Physics subsequent electron transfer. When longer inmersion times were tested, the electrochemical response of the redox probe increased, indicating the presence of an even less compact SAM. This could be attributed to the fact that, in an ethanol media, a dynamic competitive adsorption process could occur.
As the interaction of alkanethiols with ITO is expected to be weaker than to a gold surface, ethanol molecules may replace C12-SH adsorbed molecules at the ITO surface upon long modification times, leading to a decrease in the SAM compactness. A similar phenomenon was reported for silane-based Both ITO and ITO-SAM exhibit in the -0.5/+1.5 V potential range only redox processes concerning the solvent (massive hydrogen evolution or oxygen formation) (Fig. 2C). This issue shows the wide electrochemical window exhibited by both samples, confirming the stability of the ITO-SAM system Page 14 of 39 Physical Chemistry Chemical Physics in acidic media that allows further metal electrodeposition. Moreover, the ITO-SAM sample exhibits a significant displacement of redox processes to higher absolute potentials values as well as a lower interfacial capacitance (2.5 µF·cm -2 ) with respect to bare ITO (6.25 µF·cm -2 ) in the 0.9-1.1V voltage range, as a consequence of the organic layer adsorption. Theoretical interfacial capacitance (C th ) value for a C12-SH monolayer (assuming dielectric constant ε=2.5, molecular length d=17.6 Å, and tilt angle with respect to normal surface axis α= 0°) 59,60 is 1.3 µF·cm -2 . Differences observed between both theoretical and experimental values can be ascribed to deviations with respect to the parallel plate model assumed for calculating C th and to the presence of defects in the SAM. A more tilted conformation of C12-SH molecules present in a SAM with surface defects with respect to a denselypacked C12-SH SAM leads to an overestimation of the distance between the parallel plates, i. e. the electrical double layer thickness, consequently leading to higher interfacial capacitance values.

Electrodeposition of a ferromagnetic CoNi alloy over ITO-SAM: electrochemical study
The cyclic voltammograms of ITO and ITO-SAM substrates in an electrolytic bath containing Co(II) and Ni(II) will allow us to determine whether electrodeposition is possible over these substrates and if this deposition leads to the formation of a metallic alloy. The voltammetric profile obtained over ITO is similar to that obtained in other substrates such as vitreous carbon, silicon, Si/Ti/Ni or Si/Cr/Cu 61 , and the deposition process starts at approx. -750 mV (Fig. 3, curve a). Over the ITO-SAM structure, deposition current is observed only at more negative potentials (E= -850 mV) and the process is drastically slower (Fig. 3,  The potentiostatic electrodeposition of the CoNi alloy was tested, at two deposition rates, on both ITO and ITO-SAM substrates. In each case the deposition potentials were adjusted to attain similar j-t profiles on ITO and ITO-SAM, in order to compare the deposits obtained over the two substrates at similar deposition rate. Over the ITO-SAM substrate, more negative potentials were applied to obtain the same j-t profile than over bare ITO, due to the blocking effect of the SAM in the electron transfer process. The deposition at the more negative potentials (-0.94 V for ITO, -1.1 V for ITO-SAM) (curves g and f in Figure 4), selected to achieve growth rates similar to those obtained for CoNi deposits on Si/Ti/Au substrates 61 leads to incoherent, low adherence deposits with micrometric cracks. However, when the deposition process is slower (0.78 V for ITO, -0.92 V for ITO-SAM) (curves b and d in Figure 4) uniform, non-cracked deposits were obtained. The FE-SEM micrographs (Figs. 4B and 4C) show that both deposits present a thin-grain, nodular morphology, whose grain size cannot be estimated quantitatively due to the technique resolution limits, this commonly observed with some CoNi Upon the application of a X-ray incident beam, the scape depth of the photoelectrons emitted from the sample is highly dependent on α. Nearly grazing X-ray incidence (low α values) measurements are sensitive to the more superficial atomic/molecular layers, in this case the surface of the SAM, due to the lower scape depth (kinetic energy) of the photoemitted electrons detected. Perpendicular X-ray incidence (high α values) measurements are more sensitive to the less superficial atomic/molecular layers, in this case to the ITO surface, due to higher scape depth of the photoemitted electrons detected. Thus, CoNi deposit is mainly accumulated at the top of the SAM should lead to higher photoemission intensities of CoNi at low α than at high α. When going to X-ray normal incidence (α=90°) to nearly grazing incidence (α=15°), a steep decay in the In3d 5/2 /Co2p 3/2 and In3d 5/2 /Ni2p 3/2

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Physical Chemistry Chemical Physics normalized relative intensity ratios is observed, indicating that the CoNi metallic film is mainly located on top of the C12-SH SAM, in agreement with the mushroom-type growth mechanism.
The show the AR-XPS relative intensity ratios of the E) In3d 5/2 and F) S2p 3/2 deconvoluted peaks with respect to the Pt4f 7/2 peak, for the platinum on ITO-SAM structure, as a function of α. The intensity ratios of the analysed elements are again normalized with respect to the core-level emission values obtained at α=90°. As occurred with the CoNi alloy, a steep decay in the In3d 5/2 / Pt4f 7/2 and S2p 3/2 / Pt4f 7/2 normalized intensity ratios is observed upon going from X-ray normal incidence (α=90°) to nearly grazing incidence (α=15°). Uosaki demonstrated that, if both Pt deposit/dithiol SAM and Pt deposit/Au substrate normalized XPS intensity ratios decayed with higher α values, the Pt deposit was preferentially obtained above the SAM 69 .
The presence of a highly-conductive Pt layer grown from the SAM defects but distributed preferentially on the top of the SAM is not only expected to ensure CoNi selective deposition above the SAM, but also to improve both growth rate and applied potential values. The potentionstatic electrodeposition of CoNi was initially performed over ITO-SAM-Pt at the electrodeposition conditions selected for the CoNi deposition above a SAM (-0.92 V, -1C·cm -2 ). A higher stationary current density (approx. -3.2 mA·cm -2 ) (Fig. 4, curve e) is achieved in comparison to the growth rate obtained without Pt, confirming the SAM surface conductivity enhancement produced by Pt presence, but in these conditions the adherence of the CoNi deposit is very low. The application of less negative potentials (-0.88V) (Fig. 4, curve c) to attain similar stationary currents than for CoNi electrodeposition over ITO-SAM, allowed the preparation of a high-adherent CoNi deposit over ITO-SAM-Pt. The FE-SEM micrograph of the resultant deposit (Fig. 7) shows an evident change in the CoNi deposit morphology. Thus, the presence of an underlying Pt layer not only forces additional electrodeposition optimisation but also alters the CoNi characteristic surface morphology. To sum up, the preparation of a Pt-CoNi hybrid composite structure mainly on the top of an ITO-SAM is also possible. Hence, this report widens the experimental techniques available for hybrid composite production over SAM-modified substrates, providing a low cost and environment-friendly method susceptible to be used for different hybrid structures, such as nanopatterning of soft-magnetic thin films over semiconducting substrates (ITO) using SAMs as templates.

Conclusions
Electrodeposition methods allow preparing magnetic layers over ITO-alkanethiol SAM substrates.
XPS studies allow us to propose the chemisorption of the molecules interacting with the ITO surface.
However, in order to assess quantitatively the formation of a partial or compact C12-SH layer covering