%\documentstyle[12pt,twoside,epsfig]{article}
\documentstyle[12pt,twoside]{article}
\topmargin-1cm
\oddsidemargin-5mm
\evensidemargin-5mm
\textwidth17cm \textheight23cm
\pagestyle{headings}
\righthyphenmin=20
\sloppy
\renewcommand{\thesection}{\arabic{section}.}
\makeatletter
\renewcommand{\section}{\@startsection{section}{1}{\parindent}%
{3.5ex plus 1ex minus .2ex}{2.3ex plus .2ex}{\large\bf}}
\newcommand{\@cit}[2]{{#1\if@tempswa , #2\fi}}
\makeatother
\newcommand{\gcit}[2]{[\cite{#1}--\cite{#2}]}
%\newcommand{\cit}[1]{[\cite{#1}]}

%\newcommand{\gcite}[2]{[\cite{#1}--\cite{#2}]}


\begin{document}
\makeatletter
\renewcommand{\@oddhead}{Mater.Phys.Mech. {\bf 1} (2000) 1-10\hfil\thepage}
\renewcommand{\@oddfoot}{\copyright 2000 Advanced Study Center Co. Ltd.\hfil}
\makeatother

\vspace*{2cm}
\begin{center}
{\Huge\sf\bf
Nanostructured Ni films by polyol electroless deposition}
\end{center}
\vspace{0.3cm}


\centerline{\bf J. Zhang$^1$, G.-M. Chow$^1$, S. H. Lawrence$^2$ and C.R. Feng$^2$}\noindent
\begin{center}
{\small
$^1$Department of Materials Science, National University of Singapore\\
Kent Ridge, Singapore 117543, Republic of Singapore\\
{\it E-mail}: mascgm@nus.edu.sg\\
$^2$ Naval Research Laboratory, Washington, DC 20375, USA}
\end{center}
\begin{quote}
{\small Received 17 November 1999}
\end{quote}
\begin{quote}
{\small{\bf Abstract.}
Nanostructured Ni films were deposited on Cu by reducing nickel acetate tetrahydrate in 
refluxing ethylene glycol at about 194~$^\circ$C.  The effects of deposition time ($t$) and Pt as 
nucleating aid were investigated in this polyol process.  The chemistry of deposition changed 
with increasing $t$.  At longer $t$, corrosive attack of the solvent on the deposited films led to 
dissolution and refinement of Ni crystals.   Saturation magnetization increased with $t$ and 
decreased with increasing Pt precursor concentration.  The deposited Ni films were 
magnetically saturated.  They showed in-plane magnetization anisotropy and perpendicular 
coercivity anisotropy.  The use of Pt caused rapid powder precipitation but did not enhance 
film deposition.  The Pt-catalyzed Ni films showed a (200) texture and a higher squareness 
ratio.}
\end{quote}

\section{Introduction}

Nanostructured films have attracted much attention because of their unique properties 
that arise from effects of size reduction and large amount of interfaces [1]. These films can be 
prepared by many methods, for example, sputtering, laser ablation, electrodeposition and 
electroless deposition. For uniform metallization of non-planar and/or hidden surfaces, 
solution chemistry is better suited than the physical vapor methods, since it is not limited by 
the line-of-sight deposition. 

An electroless polyol method, which has been mainly used to synthesize micron, 
submicron and nanostructured powders [1-8], has been recently applied for deposition of 
nanostructured metal films, for example, Cu film on AlN substrate [9] and Ni$_x$Co$_{100-x}$ film on 
Cu substrate [10]. The Ni$_x$Co$_{100-x}$ films exhibited good magnetic properties with 
perpendicular coercivity anisotropy [10]. The structure of these films have been recently 
investigated using anomalous x-ray scattering to determine if a solid solution or phase-
separated composite was formed and the element-selected long range order [11]. To date, the 
film deposition chemistry and mechanisms of this polyol process are not yet well understood. 
Unlike physical vapor deposition where the flux of atoms can be maintained constant, the 
solution chemistry (thus deposition chemistry) may vary as a function of deposition (reaction) 
time.
In this paper, we report our investigation of the magnetic properties of polyol-deposited 
Ni films and their dependence on the deposition time ($t$). The effects of increasing $t$ on 
crystallite size, film thickness, microstructures and magnetic properties were studied. The 
influence of Pt as nucleating aid, which has been used in polyol synthesis of powders [12], on 
Ni film deposition was also studied.

\section{Experimental}

Polycrystalline, coarse-grained copper with (200) texture was used as substrates 
without any pre-deposition surface treatment or polishing. 0.1 M nickel acetate tetrahydrate 
was suspended in 200 ml ethylene glycol. Hexachloroplatinic acid in small quantity was 
added in reactions where the effects of nucleating aid were investigated. The ratio of 
concentration of Pt to Ni ([Pt]/[Ni]) was estimated from the starting precursors. The substrate 
was vertically suspended in the mixture, which was brought to reflux at about 194 $^\circ$C. 
Deposition time ($t= 0.5$, 1, 2 and 3~$h$) was taken on the base of the time required to reach the 
the refluxing temperature. After each deposition, the coated sample was immediately 
removed from the mixture, rinsed with distilled water and acetone or 2-propanol, and 
subsequently air-dried for characterization.

\makeatletter
\renewcommand{\@evenhead}{\thepage\hfil {\it J.Zhang et al}}
\renewcommand{\@oddhead}{\thepage\hfil {\it Nanostructured Ni films}}
\renewcommand{\@oddfoot}{}
\makeatother


The structure of the films was studied using x-ray diffraction (XRD) in the
$\theta-2\theta$ mode.
The diffraction peaks were analyzed using a profile fit routine and the FWHM of the specular 
peaks were used for estimation of average crystallite size and strain of these films. It was
assumed that the total line broadening (B$_{total}$) was the linear sum of that due to size and strain. 
A plot of (B$_{total}\cos\theta$) vs $\sin\theta$ yielded the strain and the size from the slope and intercept, 
respectively. The microstructure was investigated using scanning electron microscopy (SEM) 
and transmission electron microscopy (TEM). Cross-sectioned samples were checked with 
SEM for film thickness determination. The hysteresis loops were measured using vibrating 
sample magnetometry (VSM) in the parallel ($\parallel$) and perpendicular ($perp$) directions to the film 
plane using a maximum magnetic field of $\pm$50kOe.

\section{Results and discussions}

The dependence of film thickness on $t$ and the [Pt]/[Ni] is shown in Fig.1.
For $t=3h$,
the decrease in film thickness may be related to the corrosive attack of ethylene glycol on 
deposited Ni films. It has been reported for other systems involving chemical oxidation of 
ethylene glycol at elevated temperatures, oxalic acid was identified as a product. Both it and 
various organic acids that were formed as intermediates are corrosive to metals [13]. Nickel 
and Pt are miscible below 200 $^\circ$C. When Pt was used as nucleating aid, the reaction 
completed at about 0.5$h$ for [Pt]/[Ni] of $1.5\cdot10^{-2}$.
When [Pt]/[Ni]$\le2\cdot10^{-5}$, the reaction took
about 1~$h$ to finish. The use of Pt precursors for aiding nucleation of Ni nanocrystals has been 
reported [12]. The similar results were reported for Pd (II) and Cu (II) acetates in 2-
ethoxyethanol at 135 $^\circ$C, in which initial Pd reduction assisted the reduction of copper [14]. 
In this work, Pt precursor only promoted rapid precipitation of Ni powder but it did not 
enhance film deposition, since thinner films were obtained compared to those deposited 
without Pt (Fig. 1). The undesirable competition between powder precipitation and film 
formation can be minimized by application of electric field during deposition and lower 
deposition temperature [15].

%\begin{figure}[p]
%\centerline{\epsfig{file=fig1.eps, height=3.5in,width=3.5in}}
%\vspace*{20cm}


%{\small{\it Figure 1:}
%Dependence of Ni film thickness on deposition time.
%}
%\end{figure}


The XRD results showed that Ni was polycrystalline fcc. The average crystallite size and 
strain of Ni films are shown in Fig. 2. The crystallite size reached a maximum of about 138 
nm at $t=2h$, but it significantly decreased to 40 nm at $t = 3h$. The decrease in crystallite size 
and film thickness at longer $t$ was the result of corrosive attack of the solvent on the deposited 
films, leading to dissolution and re-deposition of finer particles. The micro-strain of these 
films changed from tensile to compressive with increasing $t$. Fig. 3 shows the SEM 
micrographs of Ni films deposited at $t=2$ and $3 h$, respectively. It can be seen that sintering 
of films occurred (thus largest crystallite size as shown in Fig. 2) at $t=2h$, whereas finer 
particles were observed in film deposited at $t=3h$. Note that the particles seen in SEM 
consisted of nanostructured crystallites (measured by x-ray line broadening). Fig. 4 shows a 
TEM micrograph of Ni crystallites on Cu substrate deposited at $t=2h$.

%\begin{figure}[p]
%\centerline{\epsfig{file=fig2.eps, height=3.5in,width=3.5in}}
%\vspace*{20cm}


%{\small{\it Figure 2:}
%Dependence of crystallite size and strain on deposition time.
%}\end{figure}

%\begin{figure}
%\centerline{\epsfig{file=fig3.eps, height=3.5in,width=3.5in}}
%\vspace*{5.5cm}


%{\small{\it Figure 3:}
%SEM micrographs of Ni films deposited at : a) 2 $h$, b) 3 $h$.
%}\end{figure}

%\begin{figure}
%\centerline{\epsfig{file=fig4.eps, height=3.5in,width=3.5in}}
%\vspace*{6cm}


%{\small{\it Figure 4:}
%TEM bright field micrograph of Ni film on Cu substrate deposited at 2 $h$.  Dark
%particles are Ni.
%}\end{figure}


The XRD patterns of Ni films prepared using Pt catalysts only showed only Ni, without
any evidence of Pt as a separate phase. The average crystallite size (estimated from (111) line 
broadening) increased with decreasing [Pt]/[Ni] precursor molar ratio from 33 to about 70 
nm. The similar results of dependence of Ni particle size on Pt nucleating aid was reported in 
the polyol synthesis of Ni powders [12].

The Ni films were magnetically saturated. Both Ni and Pt-catalyzed Ni films exhibited 
in-plane magnetization anisotropy. Fig. 5 shows the $M_{s\parallel}$ and $M_{s\perp}$? for Ni films deposited at 0.5 
and 3 h. Fig. 6 shows the dependence of $M_{s\parallel}$, $H_{c\parallel}$ and $H_{c\perp}$? of Ni films on t. The $ M_s$ of bulk fcc 
Ni is about $M_s$ is 484 emu/cm$^3$. The values of $M_{s\parallel}$ increased with $t$ and reached 405 emu/cm$^3$ 
for the Ni film deposited $3 h$. This may be due to the increased film density at longer $t$. For 
films consisting of randomly oriented crystallites, the squareness ratio (ratio of remanent 
magnetization, $M_r$, to saturation magnetization, Ms) is typically about 0.5. This ratio was 
between 0.4 and 0.5 for Ni films, except it was higher for that deposited at $1 h$ (about 0.6). 
Fig. 7 shows $M_{s\parallel}$ and the squareness ratio of Ni films deposited with Pt catalysts. $M_{s\parallel}$ 
increased whereas squareness ratio decreased with decreasing [Pt]/[Ni]. The deposition time 
for Ni films using [Pt]/[Ni] of $2\cdot10^{-5}$ and $2\cdot10^{-6}$ was about $1 h$, and was $0.5 h$ for [Pt]/[Ni] of 
$1.5\cdot10^{-2}$. Pt had a predominant effect on $M_{s\parallel}$ and a lower [Pt]/[Ni] ratio led to a higher $M_s$ as
expected. Lower $M_s$ may be related to alloying of Pt with Ni. Examination of XRD data of 
these films showed a (200) texture. This is consistent with the higher squareness ratio 
(between 0.56 to 0.65) of these films. Fig. 8 shows that the films deposited with Pt catalysts 
also had higher perpendicular coercivity. The coercivity anisotropy may be related to the 
microstructures of deposited film [15]. The work is currently underway to investigate the 
structure and microstructure of these films by transmission electron microscopy.

%\begin{figure}[p]
%\centerline{\epsfig{file=fig5.eps, height=3.5in,width=3.5in}}
%\vspace*{20cm}


%{\small{\it Figure 5:}
%Hysteresis loops of Ni films deposited at 0.5 $h$ and 3 $h$, showing magnetic saturation
%and in-plane anisotropy.
%}\end{figure}

%\begin{figure}[p]
%\centerline{\epsfig{file=fig6.eps, height=3.5in,width=3.5in}}
%\vspace*{20cm}


%{\small{\it Figure 6:}
%Dependence of saturation magnetization and coercivity of Ni films on deposition
%time.
%}\end{figure}

%\begin{figure}[p]
%\centerline{\epsfig{file=fig7.eps, height=3.5in,width=3.5in}}
%\vspace*{20cm}


%{\small{\it Figure 7:}
%Dependence of saturation magnetization and square ratio of Ni films on [Pt]/[Ni]
%precursor molar ratio.
%}\end{figure}

%\begin{figure}[p]
%\centerline{\epsfig{file=fig8.eps, height=3.5in,width=3.5in}}
%\vspace*{20cm}


%{\small{\it Figure 8:}
%Dependence of coercivity of Ni films on [Pt]/[Ni] precursor molar ratio.
%}\end{figure}

\section{Summary}


Nanostructured Ni films were deposited by the polyol method on Cu substrates with or 
without Pt as nucleating aid. The solution chemistry varied with $t$. At longer $t$, corrosive 
attack of the solvent on deposited films led to dissolution and re-deposition of Ni. Saturation 
magnetization increased with $t$ and decreased with increasing Pt precursor concentration. The 
magnetically saturated films showed in-plane magnetization anisotropy and perpendicular 
coercivity anisotropy. The use of Pt favored rapid powder precipitation but did not enhance 
film deposition. The Pt-catalyzed Ni films showed a (200) texture and a higher squareness 
ratio. Further work is needed to understand the solution chemistry and film deposition 
chemistry in the polyol process.
\vspace*{2ex}

{\bf Acknowledgement.}
GMC thank the support of this work by the Academic Research Grant of the National 
University of Singapore and the US Office of Naval Research. J. Zhang was supported by the 
NUS postgraduate research scholarship.
\vspace*{4ex}

{\large\bf References}\\

\noindent
[1] G.M. Chow, I. Ovid'ko and T. Tsakalakos (eds.), Nanostructured Films and Coatings, 
NATO Advanced Research Workshop, Santorini, Greece (Kluwer Publications, the 
Netherlands, 2000, in press).

\noindent
[2] F. Fi$\acute e$vet, J.P. Lagier and M. Figlarz // Mater. Res. Soc. Bull.  (1989) 29.

\noindent
[3]   F. Fi$\acute e$vet, J.P. Lagier, B. Blin, B. Beaudoin and M. Figlarz // Solid State Ionics {\bf 32/33}
(1989) 198.

\noindent
[4]   G. Viau, F. Ravel, O. Acher, F. Fievet -Vincent and F. Fievet // J. Appl. Phys. {\bf 76}
(1994) 6570.

\noindent
[5]             L.K. Kurihara, G.M. Chow and P.E. Schoen // Nanostructur. Mater. {\bf 5} (1995) 607.

\noindent
[6]		G.M. Chow, L.K. Kurihara, K.M. Kemner, P.E. Schoen, W.T. Elam, A. Ervin, S.
Keller, Y.D. Zhang, J. Budnick and T. Ambrose // J. Mater. Res. {\bf 10} (1995) 1546.

\noindent
[7]             G. Viau, F. Fievet -Vincent, and F. Fievet // Solid State Ionics {\bf 84} (1996) 259.


\noindent
[8]             G. Viau, F. Fievet -Vincent and F. Fievet // J. Mater. Chem. {\bf 6} (1996) 1047.

\noindent
[9]		G.M. Chow, L. K. Kurihara, D. Ma, C. R. Feng, P.E. Schoen and L.J. Martinez-
Miranda // Appl. Phys. Lett. {\bf 70} (1997) 2315.

\noindent
[10]	G.M. Chow, J. Ding, J. Zhang, K.Y. Lee, D. Surani and S.H. Lawrence // Appl. Phys.
Lett. {\bf 74} (1999) 1889.

\noindent
[11]	G.M. Chow, W.C. Goh, Y.K. Hwu, T.S. Cho, J.H. Je, H.H. Lee, H.C. Kang, D.Y. Noh,
C. K. Lin and W. D. Chang // Appl. Phys. Lett. {\bf 75} (1999) 2503.

\noindent
[12]	M.S. Hedge, D. Larcher, L. Dupont, B. Beaudion, K. Tekaia-Elhsissen and J.-M.
Tarascon // Solid State Ionics {\bf 93} (1997) 33.

\noindent
[13]	C.B. Jordan, Engine Coolant Testing, Second Symposium, (American Society for
Testing of Metals, 1986), p. 249.

\noindent
[14]    J.S. Bradley, E.W. Hill, C. Klein, B. Chaudret, and J. Duteil // Chemistry of Materials {\bf 5}
(1993) 254.

\noindent
[15]	G.M. Chow, J. Zhang, Y.Y. Li, J. Ding and W.C. Goh // Mat. Sci. Eng. A, in press.


\end{document}




