Cross-sectioning spatio-temporal Co-In electrodeposits: disclosing a magnetically-patterned nanolaminated structure

Micrometer-thick cobalt-indium (Co-In) films consisting of self-assembled layers parallel to the cathode plane, and with a periodicity of 175 ± 25 nm, have been fabricated by electrodeposition at a constant current density. These films, which exhibit spatio-temporal patterns on the surface, grow following a layer-by-layer mode. Films cross-sections were characterized by electron microscopies and electron energy loss spectroscopy. Results indicate the spontaneous formation of nanolayers that span the whole deposit thickness. A columnar structure was revealed inside each individual nanolayer which, in turn, was composed of well-distinguished In- and Co-rich regions. Due to the dissimilar magnetic character of these regions, a periodic magnetic nanopatterning was observed in the cross-sectioned films, as shown by magnetic force microscopy studies. counter . The on-top morphology of the films was characterized with a Zeiss Merlin field emission scanning electron microscope (FE-SEM). The average Co content in the coatings was determined by energy dispersive X-ray spectroscopy (EDX) operated at 20kV. The structure of the films was determined by X-ray diffraction (XRD) using a Philips X'Pert Diffractometer in Bragg–Brentano geometry using Cu Kα radiation (note that both wavelengths  (K α1 ) = 1.5406 Å and  (K α2 ) = 1.5443 Å were used in intensity proportion of I(K α2 ) = I(K α1 ) = 0.5) in the 25–125º 2  range (0.03º step size and 10 s holding time). Films’ cross-section was prepared differently depending on whether transmission electron microscopy (TEM) of FE-SEM was targeted. For TEM purposes, cross-sections were prepared by embedding the film in epoxy resist followed by cutting thin slices with an ultramicrotome (Leica EM UC6, Leica Microsystems Ltd., Milton Keynes, UK) using a 35° diamond knife at room temperature. Analyses were performed on a FEI Tecnai20 high-resolution S/TEM operated at 200 kV, equipped with energy dispersive X-ray detector. For SEM analyses (Zeiss Merlin), the films were embedded in a conductive epoxy resin, grinded to remove the resin, and polished using Struers MD-Largo composite disc onto which 9 µm water based diamond suspension was applied. The room temperature magnetic properties were measured using a vibrating sample magnetometer (VSM) from Oxford Instruments. Hysteresis loops were recorded under a

Besides, the deposition rates are rather slow. Electrodeposition has been widely utilized to produce micrometer-thick structures composed of alternated layers (metals, alloys and even polymers) in different configurations (films, rods and wires). Because it works at room pressure and temperature, electrodeposition possesses many attractive features like low cost and industrial scalability [18]. Layered coatings are typically fabricated from a single electrolyte by conveniently switching the deposition potential or current density [19] or by using different electrolytes [20]. The former approach is preferred when smaller layer thicknesses and a high number or repetitions are desired.
The in-situ formation of spatio-temporal patterns during the electrodeposition of some binary systems such as Ag-Bi, Ag-Cd, Ag-In, Ag-Sb or Co-In was previously demonstrated [21][22][23][24][25][26][27]. The spatio-temporal patterns typically consist of a mixture of waves, targets and spirals whose relative proportion was shown to depend on a number of factors, such as the nature of the system and the applied current density. Interestingly, it was recently found that the Co-In system not only exhibits the spatio-temporal selfpattern on the surface, but also a fancy layer-by-layer growth type [28]. Although the spontaneous formation of layered deposits with a repeat length of tens or hundreds of nanometers has been observed in a quite few instances [29][30][31], this growth mode in immiscible systems exhibiting spatio-temporal patterns has not been studied in detail.
The aim of this work is to characterize the layer-by-layer growth in electrodeposited spatio-temporal Co-In films using electron microscopy techniques. The goal is to explore the cross-section of the electrodeposits in order to determine whether composition is homogeneous across the nanolayers or, instead, it varies within each layer or from one layer to another. This could eventually lead to dissimilar magnetic properties and hence, confer these materials new technological functionalities. In such a case, Co-In nanolaminates would be spontaneously produced from a single electrolyte under nominally constant conditions, which represents a step forward within the field of electrodeposited multilayered materials.

Experimental
Deposition of Co-In coatings was conducted in a three-electrode cell connected to a PGSTAT302N Autolab potentiostat/galvanostat (Ecochemie). A double junction Ag|AgCl (E=+0.210V/SHE) reference electrode (Metrohm AG) was used with 3M KCl inner solution and 1M NaCl outer solution. A platinum sheet served as a counter electrode. Silicon (111) substrates with e-beam evaporated Ti (100 nm)/Au (125 nm) adhesion/seed layers were used as cathodes for Co-In growth. The working area of the Au/Ti/Si substrates was 5 x 5 mm 2 . Co-In deposits of 10 m in thickness were obtained at constant current densities from -10 mA cm -2 to -20 mA cm -2 .
The on-top morphology of the films was characterized with a Zeiss Merlin field emission scanning electron microscope (FE-SEM). The average Co content in the coatings was determined by energy dispersive X-ray spectroscopy (EDX) operated at 20kV. The structure of the films was determined by X-ray diffraction (XRD) using a Philips X'Pert Diffractometer in Bragg-Brentano geometry using Cu Kα radiation (note that both wavelengths (K α1 ) = 1.5406 Å and (K α2 ) = 1.5443 Å were used in intensity proportion of I(K α2 ) = I(K α1 ) = 0.5) in the 25-125º 2 range (0.03º step size and 10 s holding time). Films' cross-section was prepared differently depending on whether transmission electron microscopy (TEM) of FE-SEM was targeted. For TEM purposes, cross-sections were prepared by embedding the film in epoxy resist followed by cutting thin slices with an ultramicrotome (Leica EM UC6, Leica Microsystems Ltd., Milton Keynes, UK) using a 35° diamond knife at room temperature. Analyses were performed on a FEI Tecnai20 high-resolution S/TEM operated at 200 kV, equipped with energy dispersive X-ray detector. For SEM analyses (Zeiss Merlin), the films were embedded in a conductive epoxy resin, grinded to remove the resin, and polished using Struers MD-Largo composite disc onto which 9 µm water based diamond suspension was applied. The room temperature magnetic properties were measured using a vibrating sample magnetometer (VSM) from Oxford Instruments. Hysteresis loops were recorded under a maximum applied field of 700 Oe applied along the parallel and perpendicularto-plane directions. Atomic and magnetic force microscopy (AFM/MFM) images were acquired using Dual Scope C-26 system from Danish Micro Engineering. The MFM maps were taken at a tip lift height of 100 nm.

Results and discussion
The structural characterization was studied by XRD ( Figure S1  A closer look to the cross-section of the Co-In films was taken by TEM working on STEM mode (Figure 2). The formation of partially stacked individual layers, undulated and rather uniform in thickness was observed, in agreement with previous SEM analyses (Figure 2a and 2b). Interestingly, a kind of columnar structure is observed in each nanolayer (Figure 2c). Such a columnar grain structure is often observed in electrodeposited metals and alloys [33]. The repetition length as measured by TEM was 175 nm ± 25 nm across the entire deposit thickness, thus furnishing a highly, almost perfect, repeatable 2D pattern expanding several micrometers. Three regions were distinguished within each layer. Namely, a wider, dense, bright region followed by a more irregular, fluffy, grey region, and finally a dark area (Figure 2c). EDX line scan analyses across two adjacent nanolayers were conducted in order to shed light on the chemical composition profile (Figure 3a). The above-mentioned heterogeneity was evident as the beam was scanning from the center of one nanolayer toward the center of the adjacent nanolayer (Figure 3b). The relatively dense bright area was enriched in In whereas the spongy-like grey area contained a larger amount of Co on average, although the In and Co signals strongly fluctuate. Remarkably, the dark regions correspond to areas mainly free from material. STEM-EDX line scan analyses across these dark regions confirmed the occurrence of holes in the material ( Figure S1 and S2, Supporting information). Eventual delamination during sample preparation (cutting by ultramicrotome) for STEM analyses that could explain the existence of these gaps is unlikely. Therefore, such apparently 'empty' regions already develop during film deposition. Interestingly, lateral growth is favored over vertical growth in spite of these discontinuities. The occurrence of In-and Co-rich regions was observed in different areas of the cross-sectioned films and therefore was proven neither to be an artifact of the measurement nor an anomaly of the deposit in the region of interest. It was also observed that the nanolayers were joined together mostly through In-rich anchor points (see Figures 2b and 2d).  Previous EDX mappings conducted on the cross-section of a Co-In deposit seemingly indicated that the distribution of Co and In elements was homogeneous [28]. Current EELS analyses do demonstrate that this is not the case and that composition heterogeneities exist at the nanoscale. TEM images of the same region imaged by STEM are displayed in Figure 4a and 4b. Notice that the contrast is opposed to that observed in STEM mode. Bright regions correspond to hollow spaces in the deposit.
The HRTEM image (Figure 4c) demonstrates that the material is polycrystalline, in agreement with previous works [28]. It has been claimed that the spatio-temporal structures do move in the vertical direction. This is possibly connected with the effect of natural convection during electrodeposition and results in the formation of layered coatings [32]. Indeed, a zoomed detail of the very first layers deposited on the substrate reveals that when an undulation or a "defect" appears, this is conformally wrapped by the subsequent layers until the growing front becomes flattened ( Figure 5).  Co-In nanolayered films show a ferromagnetic response at room temperature as has been previously reported [28] and shown here in Figure 6. MFM characterization of the cross-section of the deposits was carried in order to assess any kind of magnetic pattering arising from the layer-by-layer architecture of the Co-In films. Figure 7a shows the AFM image obtained upon scanning an area of 1 x 1 m 2 , in which the periodical layered structure, already observed by SEM and (S)TEM, was captured. In this topological image, alternating dark and bright fringes are observed, corresponding to the In-rich and Co-In lamellae. The corresponding MFM image (Figure 7b) was acquired in remanent state after applying a strong magnetic field perpendicular to the film cross-section. In the MFM image, a magnetic nanopatterning is observed, which correlates well with the layered structured seen in Figure 2a

Conclusions
Co-In nanolayered films were obtained by direct current electrodeposition on Au surfaces. TEM investigations of the films' cross-section showed the occurrence of selfassembled nanolayers along the entire deposit thickness. The layers were not compositionally homogeneous but Co-and In-rich regions coexisted in it. Moreover, each layer showed a columnar structure. A stripe-like magnetic patterning developed as a consequence of the nanolaminated structure. The possibility to electrodeposit, under direct current conditions, a system that spontaneously features a spatio-temporal pattern with micrometer lateral sizes over its surface coupled to a magnetically-patterned nanolayered vertical structure, represents a breakthrough in the field of nanocomposites science.