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\begin{document}

\begin{center}
 
{\Large Supplementary material - Highly ordered vertical nickel nanotubes and nanowires on thin substrate for high power lasers experiments}

Stefania Ionescu$^{1,2}$, Cristina Gheorghiu$^{1}$, Valentin Lupu$^{2}$, Daniel Popa$^{1}$, Kazuo Tanaka$^{1,3}$, Victor Leca$^{1}$

$^{1}$Extreme Light Infrastructure - Nuclear Physics (ELI-NP), “Horia Hulubei” National Institute for Physics and Nuclear Engineering (IFIN-HH), 30 Reactorului Street, RO-077125 Bucharest-Magurele, Romania;
$^{2}$National University of Science and Technology POLITEHNICA Bucharest, Splaiul Independentei no. 313, RO-060042 Bucharest, Romania;
$^{3}$Institute of Laser Engineering, Osaka University, Yamada-oka 2-6, Suita, Osaka 565-0871, Japan

\end{center}

\section{Electropolishing - roughness measurements}
An electropolishing study was performed to find the required time to obtain a mirror-like polished surface of the aluminum foil. The electrolyte was 70$\%$  perchloric acid solution, combined with ethanol in a ratio acid to solvent of 1:4, cooled at 0$^{\circ}$C. 10 V were applied to the aluminum foil for 1 to 6 minutes, without stirring. After 6 minutes, the electropolishing showed good results, presented in Figure~\ref{fig:electropolish.png} (left), where it can be observed that the lines that resulted from the cold rolling step were removed. 
From the roughness analysis performed with the optical profilometer (Sensofar, S Neox), presented in Table~\ref{table:Roughness analysis}, we can conclude that the 20 ${\mu}$m electropolished foil and the 30${\mu}$m cold rolled foil have similar surface topography, although the electropolished samples have slightly lower roughness. 
\begin{table*}[h]
  \centering
  \begin{threeparttable}
    \caption{Roughness analysis}
    \label{table:Roughness analysis}
    \begin{tabular}{p{0.07\linewidth} p{0.3\linewidth} p{0.12\linewidth} p{0.1\linewidth} p{0.1\linewidth} p{0.15\linewidth}}
      \toprule 
      \multicolumn{1}{c}{Sample thickness}
      & \multicolumn{1}{c}{Electropolishing duration, min}
      & Sa, ${\mu}$m
      & Sq, ${\mu}$m
      & $\Delta z$, ${\mu}$m
      & Magnification\\
      \midrule
     20 & 1 & 0.8 & 1 & 2.2 & 100\\
      \midrule
       20 & 2 & 0.6 & 0.8 & 2.5 & 100\\
      \midrule
      20 & 3 & 0.45 & 0.76 & 1.12 & 100\\
      \midrule
       20 & 4 & 0.3 & 0.4 & 1.25 & 100\\
      \midrule
       20 & 5 & 0.23 & 0.29 & 1 & 100\\
      \midrule
      20 & 6 & 0.15 & 0.2 & 0.5 & 150\\
      \midrule
       30 & 0 & 0.26 & 0.33 & 0.8 & 100\\
      \midrule
      50 & 2 & 0.49 & 0.61 & 1.5 & 100\\
      \bottomrule
      \end{tabular}
      \end{threeparttable}
\end{table*}

The surface roughness analysis was carried out using interferometric mode, with 100 x objective, and confocal mode, with 150 x objective, respectively. The results obtained for the 30 ${\mu}$m cold rolled foil and 20 ${\mu}$m electropolished foils are presented in Figure~\ref{fig:polish.png}. For comparison, the data for the 2 min electropolished 50 ${\mu}$m foil is also given in the Table. A peak-to-valley measurement was performed for each sample and was included in the Table\ref{table:Roughness analysis}, as the $\Delta z$ value.
\begin{figure*}[h]
\centering
\includegraphics[width=1\textwidth]{figures/polish.png}
\caption{\label{fig:wide} Profilometry analysis of the 30 ${\mu}$m cold rolled foil (left) and of the 20 ${\mu}$m electropolished for 6 minutes foil (right).}
\label{fig:polish.png}
\end{figure*}

\begin{figure*}[h]
\centering
\includegraphics[width=0.8\textwidth]{figures/electropolish.png}
\caption{\label{fig:wide}SEM images of  electropolished sample (top), and cold rolled sample (bottom); showing the aluminum plate unanodized (a and e), top pores view (b and f), bottom pores view (c and g) , and the resulted deposited nickel nanostructures (d and h).}
\label{fig:electropolish.png}
\end{figure*}
There is a difference in the arrangement of the upper open pores after anodization, for the electropolished and unpolished aluminum foil (see Figure~\ref{fig:electropolish.png} a and e- unanodized aluminum and b and f- anodized aluminum template). The pores formed in the polished sample were much more ordered, as shown in Figure~\ref{fig:electropolish.png} (b). On the bottom side, for the barrier layer there isn't a major change visible (Figure~\ref{fig:electropolish.png} c and g). However, after the electrodeposition of the nanostructures in both templates, which were treated under the same conditions, no difference was noticed in the resulted structures, see Figure~\ref{fig:electropolish.png} (d,h). Therefore, we may conclude that the electropolishing step will improve the upper pores arrangement, but won't influence the final nanostructures, if the deposition is performed at the bottom of the open pores.  

\section{Thermal treatment}
\begin{figure*}[h]
\centering
\includegraphics[width=0.6\textwidth]{figures/ebsd2.png}
\caption{\label{fig:wide}EBSD analysis of untreated (left) and thermal treated (right) aluminum foils.} 
\label{fig:ebsd2.png}
\end{figure*}
The thermal treatment step was performed in a calcination oven, at 500$^{\circ}$C, for 3 h, with 50 minutes ramp-up, followed by overnight cooling. This step was necessary to increase the grain size in the aluminum foil, which resulted in larger areas of ordered domains of pores during anodization. To verify this, the electron backscatter diffraction (EBSD, Bruker), embedded in the SEM (Maia 3, Tescan), was used. The sample was placed at an angle of 70$^{\circ}$ with respect to the electron beam, facing the EBSD detector. The results, presented in Figure~\ref{fig:ebsd2.png}, indicate that the average grain size increased from 5.18 ${\mu}$m to over 393 ${\mu}$m, while tens of grains can be observed in the untreated one, only three grains are partially visible in the EBSD map of the annealed sample.

\section{Anodization at high voltages}
For both COPE and OPE electrolytes, anodizations at different voltages were carried out in order to determine the most stable conditions for a reproducible process, but a thorough determination of the highly ordered templates is still to be conducted. The highest voltages at which the process was still controllable were found to be 290 V for COPE and 130 V for OPE. The template obtained with COPE is presented in Figure~\ref{fig:highVoltage.png} (a,b), showing pore diameters of 300 nm and distances between pores of 300 nm. The template was obtained by increasing gradually the voltage from 240 V to 290 V, for 30 minutes, and anodized for 21 h in 1 single step, at 10-13 $^{\circ}$C, followed by dissolving in phosphoric acid for 2 h and for 10 min in NaOH. For the nanowires deposition, the template was coated with 200 nm platinum film and electrodeposition was performed for 6 minutes. In Figure~\ref{fig:highVoltage.png} (d), nickel nanowires with diameters bigger than 300 nm and distances between them of 300 nm are showed, although area covered with nanowires were not obtained homogeneously, as depicted in in Figure~\ref{fig:highVoltage.png} (c). Optimization of the surface quality can be performed to obtain a more homogeneous template. The template obtained in OPE electrolyte, presented in Figure~\ref{fig:highVoltage.png} (e,f), was fabricated by 2 steps of anodization, the first step for 3 h and the second step for 24 h, at 0 $^{\circ}$C, and shows pores of 150-200 nm and similar interpore distances, while the obtained nanotubes, presented in Figure~\ref{fig:highVoltage.png} (g,h), are covering homogeneously the whole area.

\begin{figure*}[h]
\centering
\includegraphics[width=0.8\textwidth]{figures/highVoltage.png}
\caption{\label{fig:wide}SEM images of porous alumina fabricated in COPE electrolyte at 290 V (a) top pores, (b) bottom open pores, and the deposited nickel nanowires (c,d) and porous alumina fabricated in OPE electrolyte at 130 V (e) top pores, (f) bottom open pores, and the deposited nickel nanowires (g,h)}
\label{fig:highVoltage.png}
\end{figure*}


\section{Nanostructures size distribution} 
\begin{figure*}[h]
\centering
\includegraphics[width=0.5\textwidth]{figures/histo.png}
\caption{\label{fig:wide}SEM images of Nickel nanotubes (upper images) and nanowires (lower images) and the corresponding histograms for the diameter distribution}
\label{fig:histo.png}
\end{figure*}
The inner diameters of the nanotubes deposited in a template anodized in OPE electrolyte were obtained by fitting the central pore shape to an ellipsoid and averaging the Feret diameters using Fiji software \cite{Schindelin2012}. The resulting histogram from Figure~\ref{fig:histo.png} follows a normal distribution with a mean of 163.65 nm, and standard deviation of 19.88 nm. This significant variation of diameters is in accordance with the image, as the pores appear of various shapes and significantly different sizes.
The diameters of the nanowires deposited in the template anodized at 240 V with COPE electrolyte were found similarly to the nanotubes inner diameters. These diameters also fit a normal distribution, but with a mean of 0.44${\mu}$m and a very low standard deviation of 0.045${\mu}$m, suggesting a well controlled growth process. The characteristics of the samples presented in Figure~\ref{fig:histo.png} are dependent on the etching time of the template, as well as on the  electrodeposition parameters, therefore, while preparing nanostructures in a template obtained by the same anodization process, the final result can be different.

\section{Samples surface area}
The area covered by the nanostructures depends on the size of the template, which is given by the initial dimensions of the aluminum foil used for anodization. In this case usually a foil of 3-4x1 $cm^2$ was prepared, although the AAO template is fragile and frail, is often reduced to smaller size, resulting in this case in nanostructured targets of 1-2 $cm^2$, which are homogeneously distributed, as presented in Figure~\ref{fig:area.png} for Ni nanowires and nanotubes.
\begin{figure*}[h]
\centering
\includegraphics[width=0.8\textwidth]{figures/area.png}
\caption{\label{fig:wide}SEM images of Nickel nanowires and nanotubes from higher to lower magnification. (d) showing pieces of targets cut and mounted on a holder.}
\label{fig:area.png}
\end{figure*}

\end{document}