Structural and Optical Properties of Multilayered Cobalt/M (with M= Platinum or Rhodium) Thin Films Nanowires Electrodeposited into Ion Track-Etched Polycarbonate Membra

Since the oscillating interchange interaction has been found in multilayered magnetics materials, the interlayers coupling in transition metals thin films (Co, Fe, Ni) separated by non magnetics elements (Pt, Cu, Ag, Au etc) is of great interest [1,2]. Multilayers nanomaterials are of considerable industrial interest because of their specific properties and many promising areas of applications in electronics or optics like X-ray and UV mirrors, giant magnetic resistance and magnetic recording, etc. [3]. One-dimensional nanomaterials such as nanorods, Nanowires (NWs) etc… have been attracting much attention due to their important role in almost all areas of nanoscience and nanotechnology [4]. Particularly, metallic nanowires are considered to be promising candidates for future electronic, sensing, and magnetic and optoelectronic applications. The performance of nanowires depends strongly on parameters like composition, shape, size, structure and also spatial distribution which are predetermined by the fabrication method and conditions. Cobalt is an important ferromagnetic material with large coercivity and high Curie temperature (≈1388 K) and is widely used in presently available magnets and magnetic recording media. In the past decade, Co nanowires have attracted intensive research interest stimulated by their diverse application in ultra high density information storage media [5]. Additionally, some novel applications like field emission devices have been explored and reported in recent years [6,7]. To produce Co/M NWs, several fabrication methods have been developed, including template-assisted assembly [8], lithography [4], magnetic field-induced self-assembly [9] and use of diblock copolymers [10,11]. Using polymer membrane filters, ferromagnetic metal nanowires have been synthesized so far. Whitney et al. reported that the arrays of Ni and Co nanowires were electrodeposited in polymer templates with the nanometer-sized pores prepared by nuclear track etching technique [12]. They found that the preferred magnetization direction is perpendicular to the film plane and enhanced coercive force as high as 680 Oe. Piraux et al. reported that the array of Co/Cu multilayered nanowires with GMR response was electrodeposited in nanoporous polymer template [13]. Blondel et al. also reported that Co/Cu and Ni-Fe/Cu multilayered nanowires with GMR response was demonstrated [14]. They synthesized the multilayered nanowires with length of 6 μm, diameter of 80 nm and each layer thickness of 5~10 nm into the nanopores of ion track-etched polycarbonate membrane filters. As demonstrated by numerous works [14,15] the fabrication of metallic nanowires in ion track-etched templates by electrochemical deposition possesses several unique advantages such as the control over size and crystallinity. In this contribution, Co nanowires were electrochemically grown in ion track-etched polycarbonate membranes and their morphology, composition, microstructure and magnetic properties of the wires were studied [16]. Additionally, for the first time, the optical properties in the UV-vis-NIR spectral range of well aligned multilayered Co/M (M=Pt, Rh) thin films nanowires, being embedded in an ion-track template, are investigated. This paper explores the properties of multilayered thin films NWs electrodeposited in ion track-etched polycarbonate membranes in terms of reflection across the solar spectrum.

The electrodeposition has been carried out in a conventional three electrodes Pyrex cell (50 mL) under an AUTOLAB PGSTAT (model 302N) control at room temperature. All potentials of the working electrode were measured with respect to a Saturated Calomel Electrode (SCE) reference (Ag/AgCl) and the Counter Electrode (CE) was platinum as presented in the figure 1 where we could distinguish the autolab (a), and the electrochemical cell (b) (with its three electrodes, WE, CE and RE) used for this work.

 

Polycarbonate (PC) membranes (from Millipore) of thickness δ≈7 mm and pores diameter ranging from 100 to 400 nm were used to produce multilayered Co/M (M=Pt, Rh) thin films nanowires. A gold (Au) seed layer was first evaporated on the backside of the PC porous membrane to cover the pores. And this backing layer served as cathode (working electrode WE) during the subsequent electrochemical deposition of the multilayered Co/M thin films nanowires. During the deposition, a negative constant voltage was applied for Co and M reduction respectively and the electric current was monitored as a function of time. The multilayered Co/M thin films nanowires were prepared alternating elemental target into PC template. The substrates dimension was (25px7x10^-3) mm³. The figure 2 shows the schematic transversal view of the PC membrane where d is the thickness of the alternating deposited element Co or M.

 

The electrolyte used in this work consisted of an aqueous solution as shown in the following table N° 1. To avoid the element M in the cobalt layer during the electrodeposition, we took the concentration of cobalt 100 times the concentration of the element M.

Product name	Concentration [mM]
CoSO4·7H2O (99,9%)	1	100
M=K2PtCl6 or RhCl3 (99,8 %)	0.01	1
H3BO4 (99,5 %)	40

The morphology of the thin films wires were studied by means of Scanning Electron Microscopy (SEM). For this last purpose, the PC membrane template was dissolved in ones drops of dichloromethane (CH₂Cl₂).

In addition, the microstructure and optical properties of the wire arrays were investigated by X-Ray Diffraction (XRD), the optical reflectance of the as-prepared multilayered Co/M thin films nanowires has been characterized using UV-Visible-Near Infrared Spectrophotometer (Agilent Technologies Cary Series 5000 UV-Vis-NIR Spectrophotometer) equipped with an integrating device. Reflectance spectra of the samples were measured at Room Temperature (RT) between 800nm and 2500nm. All measurements were performed in double beam mode, using reduced slit height and zero/baseline correction. Prior to the measurements, each sample was positioned using the sample clip while baseline correction was performed before the acquisition of sample spectra in order to set 0 and 100% Transmission (T) values. This is particularly important when measuring samples with low reflectance.

In order to choose the appropriate deposition potentials, the electrolyte was characterized by the Cyclic Voltammogram (CV). Figure 3 shows the stabilized cyclic voltammograms obtained for the electrolyte used to deposit cobalt/M thin films nanowires.

The curve show the cathodic and anodic peaks associated to the deposition and dissolution of cobalt, respectively. There is no considerable current flowing in the potential region between 0.0 V and -1.4 V. Cobalt depositions begin at around -0.9 V. As the absolute value of cathode potential increases from -0.80 V to -1.2 V, the current also increases. Between -1.2 V and -1.4 V, the increase of current slows down. After -1.4 V, it begins to increase with the increase of the potential. When the scan is reversed, the nucleation loop is observed. At around -0.05 V an oxidation peak of cobalt is detected. According to these results and the appearance of the films, the optimum electrodeposition potential range was decided to be from -0.80 V to -1.20V. The selected potential value is E_d (Co)≈-0.90V. The same was done for M deposition potential which was decided to be E_d (M)≈- 0.65V. Multilayered Co/M thin films NWs were synthesized alternatingly switching cathode potential from -0.65 V (for M layer) to -0.90 V (for Co layer) as shown in figure 4. The current-time transients were also recorded to understand the nucleation and growth mechanism of Co/M thin films nanowires. Figure 4 shows the (a) potential-time curve and (b), (c) and (d) current-time transients of the thin films nanowires.

 

According to this figure, when the potential is switched from -0.90 V to -0.65 V, anodic current is observed for Co/Pt. This is resulting from the dissolution of electrodeposited Co, because -0.65V is nobler than the equilibrium potential of Co. At this potential, it is estimated that the Pt deposition and Co dissolution will proceed simultaneously. According to the time - dependence of cathodic current during electrodeposition of Co/Pt multilayered nanowires as shown in figure 4(d), filling time was around 10000 s and the deposition rate was estimated to be about ≈0.7 nm/s.

The phase structure and crystalline orientation of the as-prepared multilayered Co/M thin films NWs were investigated by XRD. Figures 5(a and b) represents the XRD patterns of as prepared multilayered Co/M thin films NWs. From this analysis, it is found that the prepared particles show Hexagonal Closed Packed (HCP) crystal structure with lattice reflections of (10.0), (00.2), (10.1) and (11.0). However the relative intensity of the signal at 2q=47.942° for Co/Pt, corresponding to the (10.1) plane, is significantly larger than for Co powder, evidencing a preferred crystallographic orientation along the (10.1) direction. [17] Predicting that nanowires of fcc materials (Co, Pt, Rh,) which are deposited potentiostatically grow preferentially along the (10.1) direction because of surface energy minimization. However, cobalt exists in two different phases-Hexagonal Closed Packed (HCP) and face centered cubic-where the fcc phase is a high temperature phase and therefore is metastable at RT. At RT this phase can also be obtained in some specific ways [7,18-20]. For electrochemically deposited Co, it has been reported that fcc structured films and nanowires were successfully fabricated with electrolytes having low pH values [7,16]. A. Kazadi et al. [3,20,21] proposed that the fcc formation is promoted by co-deposition of atomic hydrogen. In the present work, the Cobalt electrolyte consisted of 40 g/l H₃BO₃ providing a certain number of hydrogen ions which may be co-deposited during the electrochemical wire growth. According to the aforementioned mechanism, this co-deposition may promote the formation of fcc wires. Beside, Hexagonal Close Packed (HCP) (10.0), (00.2), (10.1) and (11. 0) peaks were observed at all patterns. However, in a study [20] hcp at high pH (∼5.7) and fcc at low pH (∼1.6) structures were observed in electrodeposited cobalt.

 

In figure 6, we could observe that the wires thickness has linear relationship with respect to the synthesis time. This indicates that there is indeed a linear relationship between the length of the nanowires and the above-mentioned time for each deposition potential.

As shown in the table N° 2, we could see the PC membrane parameters used in this work.

 

Parameter	Value
Pore diameter [nm]	≈ 400
Membrane thickness [μm]	≈ 7
Pore density [pores/cm2]	≈ 2x106

The SEM micrograph shown in figure 7(a) displays the morphology and size of PC template before deposition of the Co/M nanowires. And the figure 7(b) shows the top surface of PC template with hemispherical caps formed after deposition. The wires are aggregated and randomly distributed on the substrate. The wire aggregation is ascribed to surface tension effects of solvent droplets when drying the sample after template dissolution. The detailed description of this phenomenon can be found elsewhere [22]. The SEM image presented in figure 9 and the AFM image in figure 8(a) reveal that the wires have excellent cylindrical smooth shape (highly polished surfaces), uniform deposit and homogeneous contours along their long axis. From figure 8(b), the MFM image shows the magnetic configuration of as-grown thin films, observed by magnetic force microscopy. One could notes that the magnetization is perpendicular to the wires axis in conformity to the result found for the Co wires. The polarity alternation suggests the reversal of magnetization in the perpendicular plane in function of wires’ length.

The total reflectance spectra of all the samples (Figure 9) clearly indicate the influence of layers number and electrolyte type on the interference pattern. The occurrence of interference fringes obtained for each sample is due to partial reflection on the top surface (PC template-air interface) and on the bottom surface (PC template - Au interface). For a determined wavelength, if the reflected rays on the both surfaces are in phase, they interfere constructively and originate a maximum of the reflectance spectrum. If they have opposite phases, a destructive interference occurs. And this causes a minimum on the reflectance spectrum. The number of interference maxima and minima is directly related to the multilayered Co/M thin films NWs layer thickness. Figure 9 shows the specular reflectance spectra of (D_p=400nm) the as-prepared multilayered Co/M thin films NWs being embedded in the PC template were measured with UV light propagating obliquely with 30° along the wires’ long axis in a standard transmission mode. From the optical spectra, in the optical response, the multilayered Co/M thin films NWs show a broad reflectance band from ≈900nm to ≈2500nm which is ascribed to the so-called ‘‘surface plasmon resonance” (SPR) [23], the collective oscillation of conduction electrons within the wires driven by the electric field of the incident light.

As far as we know, this reflectance oscillation could be also due to the oscillatory exchange coupling across the nonmagnetic layers M in Cobalt layers as function of thickness of M layers. It follows that the oscillations in these systems are approximately periodic, and the period change with the change of thickness. This phenomena has been discovered by Parkin et al., [1]. Therefore, it is expected that the cylindrical shape of the nanowires has a significant influence on their optical properties.

Multilayered Co/M thin films nanowires having a face centered cubic and a hexagonal compact mixed lattices were successfully fabricated by electrochemical deposition in ion track-etched polycarbonate templates. From X-ray diffraction characterization the wires exhibit a strong (10.1) texture. The SEM and AFM images displayed the morphology and revealed that the wires have excellent cylindrical shape, smooth and homogeneous contours along their long axis. MFM image shows that the magnetization is perpendicular to the wires axis in conformity to the result found for the Co wires. The polarity alternation suggests the reversal of magnetization in the perpendicular plane in function of wires’ length. From the optical spectra, the multilayered thin films NWs show a broad reflectance band from ≈900nm to ≈2500nm which is ascribed to a surface plasmon-resonance and the thin films behavior. The magnetization reversal strongly depends on the variation of the aspect ratio (thickness/diameter) between cobalt and M (M=Pt, or Rh) layers. Therefore, the change of the thickness (wires length) or the pores diameter (wide) was seen to control the properties of the multilayered thin films NWs and hence their properties could be modified for desired purpose. Hence one can say that the multilayered Co/M thin films NWs stack with many layers can be a good candidate for electro-optic, telecommunications, solar concentrator and architectural applications.