# Mesoscale Compositionally Modulated Nanocrystalline Ni-Fe Electrodeposits for Nanopatterning Applications

Authors: P. Egberts; G. D. Hibbard
Journal: Journal of Nanomaterials
 (2008)

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## Abstract

A considerable range of surface nanostructures can be fabricated by the selective dissolution of elements or phases from metallic alloys. Selectively etched electrodeposited multilayers may find useful application in optoelectronic and MEMS devices. One issue with electrodeposited multilayers is that the fine-scale multilayer structure can often exhibit significant waviness if the band layer spacing is on the same order of magnitude as the grain size. In the present study, the mean grain size was reduced to below 10 nm in a compositionally modulated Ni-Fe alloy. Preferential etching on the electroform cross-section resulted in highly uniform and directional surface channels. The evolution of this nanopatterned surface morphology was characterized by atomic force microscopy and directional roughness parameters were obtained.


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## Body

## 1. Introduction

Selective
dissolution  in which one element or phase is preferentially removed from an alloy
can be used to create a considerable range of functional surface nanostructures
[1–5]. For example, electrochemical dealloying of single-phase Au-Ag solid
solutions can be used to create nanoporous surface morphologies [e.g., [1, 2]]. In
the case of multiphase alloys, Re nanowire assemblies have been produced by the
selective etching of the NiAl-Re eutectic [3] and gas permeable nanoporous
superalloy membranes have been fabricated by preferential dissolution of the γ′ phase in a Ni-based superalloy forming
several hundred nm wide interconnected channels [4, 5].The
selective etching of multilayers can also be used to generateμm- and nm-scale surface morphologies. Relatively
large scale dissolution of multilayered electrodeposits can be used to produce
three-dimensional components in microelectromechanical systems (MEMSs), such as
microgears [6] and microradiators [7]. Smaller scale dissolution can be used to
generate nanopatterned surface morphologies for nanoimprint lithography
applications, for example, optoelectronic devices [8, 9]. NiCu/Cu [8], NiFeCu/Cu
[8], and CoNiFeCu/Cu [9] multilayers have been electrodeposited having layer
thicknesses on the order of ~100 to 1000 nm and selectively etched to produce
periodic nanopatterned surfaces. In one study, a nanoimprint master stamp was
created which was used to transfer the surface pattern to a polymeric resin, successfully
replicating the structure of the nanomould [8]. However, in certain cases, the
uniformity of the structure can be significantly disturbed by multilayer
waviness, a phenomenon that was attributed to the grain structure being on a
comparable scale to the individual layer thicknesses [8, 9].One
way to circumvent this issue may be to electrodeposit a compositionally
modulated material in which the grain size is much finer than the individual
layer thicknesses.  Electrodeposited
nanocrystalline materials, in which the grain size is less than 100 nm, have
the additional advantage of enhanced mechanical properties, such as
significantly increased hardness and yield strength compared to conventional
polycrystalline materials [e.g., [10, 11]].
Furthermore, nanocrystalline electrodeposits are particularly attractive
for MEMS applications because the ultrafine and equiaxed grain structure
results in a large number of grain boundaries contained in even the smallest
feature of a microscale device [12]. Ultrafine periodic compositional
modulations have recently been identified in nanocrystalline Ni-Fe alloys [13].  This study uses selective dissolution of Ni-Fe
electrodeposits having a mean grain size of less than 10 nm to create periodic
and regular nanopatterned surfaces.


## 2. Experimental

A
free-standing nanocrystalline Ni-53%Fe electrodeposit was obtained from
Integran Technologies Inc. of Toronto, Canada. Grain size reduction to the
nm-scale can be achieved by controlling the electrodeposition parameters and
bath chemistry such that the nucleation of new grains is favored over the
growth of existing grains [14, 15]. The sample used in the present study was
produced from an aqueous solution using an electrodeposition process similar to
the one reported by Cheung et al. [16]. It was plated on a 150 mm × 150 mm Ti
cathode to a thickness of ~0.5 mm and mechanically stripped from the substrate.
The alloy (Ni and Fe) and impurity (O and H) compositional modulations self-assembled
during deposition and have been attributed to periodic fluctuations in pH at
the cathode surface [13].The overall Ni-Fe
alloy composition was measured by energy dispersive X-ray spectroscopy (EDS) in
a scanning electron microscope (SEM). The nanostructure of the electrodeposit
was characterized by transmission electron microscopy (TEM). Samples were
thinned to electron transparency by jet-electropolishing in a 10% perchloric
acid, 15% acetic acid, and 75% methanol solution at −30°C with an applied voltage
of 10 V. The nanopatterned surface morphologies were characterized by SEM and
atomic force microscopy (AFM).  Coupons
were mounted in cross-section and prepared by mechanical grinding and polishing
to obtain a mirror finish. Samples were etched in a 50% acetic acid and 50%
nitric acid solution for times ranging from 10 seconds to 160 seconds. Constant
deflection mode was used in the AFM and five scans (15 μm × 15 μm scan area) were obtained for each surface
condition. Profile plots across the banded multilayers were obtained by
averaging the AFM data files and were used to measure band spacing and height.


## 3. Results and Discussion

TEM
characterization showed a uniform and equiaxed nanostructure throughout the
electron transparent region of the foil. Figure1 presents typical bright field
and dark field TEM images and selected area electron diffraction (SAED)
pattern. The SAED pattern exhibited only the γ phase
(FCC), which is consistent with previous studies of electrodeposited
nanocrystalline Ni-Fe of similar concentration [e.g., [11, 16–18]]. The mean
grain size was measured from dark field TEM micrographs. The grain size
distribution of the Ni-53%Fe electrodeposit is presented in Figure 2. Over the approximately
550 grains measured, the sizes ranged from <5 nm to ~20 nm, with a mean of 9 nm and a standard deviation of 4 nm; the grain size distribution followed a
lognormal distribution (with μ = 8.3 nm
and σ = 0.46 nm), which is typical of nanocrystalline Ni-based electrodeposits [e.g., [11, 16–18]].TEM characterization of nanocrystalline Ni-Fe electrodeposit showing bright field (a) and dark field (b) images and selected area electron diffraction pattern (c).
(a)(b)(c)Figure 2
Grain size distribution of the nanocrystalline Ni-Fe electrodeposit.At lower
magnifications, the effect of compositional modulations in the electrodeposit
could be seen through periodic changes in the TEM foil thickness, Figure3. EDS
line-scan characterization in a scanning transmission electron microscope
(STEM) showed significant variation in the iron and nickel X-ray intensity,
where dark (thicker) bands had a higher iron concentration than the lighter
(thinner) bands [13]. Time-of-flight secondary ion mass spectrometry (TOF-SIMS)
characterization on a polished cross-section from the same electrodeposit
showed periodic contrast in the oxygen and hydrogen signals; this was
interpreted in terms of periodic codeposition of nickel hydroxides [13]. The
effect of these alloy and impurity compositional modulations could be seen in
polished and etched cross-sections of the Ni-Fe electrodeposit, Figure 4. A pattern
of straight recessed channels (average spacing of 650 nm) that were continuous
along the cross-section and aligned perpendicular to the growth direction could
be seen throughout the deposit. These bands were remarkably straight and did
not exhibit the waviness that has been seen in previous studies [8, 9]. In the
present case, the band morphology is decoupled from the grain size; at 9 nm
(Figure 2) the mean grain size is nearly two orders of magnitude smaller than
the average band spacing of ~650 nm (Figure 4). This scaling difference should
have the effect of largely eliminating any potential crystallographic
orientation dependence on the dissolution kinetics, promoting ultrafine
etchability.Figure 3
Low magnification plan view TEM bright field image showing thickness contrast across the compositionally modulated bands.Figure 4
Typical SEM image of a polished and etched cross-section illustrating the channel morphology.Development of the
selectively etched channels was investigated by tracking the surface morphology
for etching times ranging from 5 to 160 seconds. Figure5 presents AFM profile
plots recorded after 5 seconds, 10 seconds, 20 seconds, 40 seconds, 80 seconds,
and 160 seconds of immersion in the 50% H3NO3-50% CH3COOH
etching solution. There is little apparent order to the AFM surface plot for the
shortest etching time of 5 seconds and the maximum band depth (i.e., the
largest difference between adjacent peaks and valley) is only ~50 nm.  By 10 seconds of etching, certain bands have
begun to etch deeply, some down to more than ~200 nm, but most etched channels
are quite shallow. The most regular structure is observed after 40 seconds of
etching, where the average channel depth is ~120 nm. The deepest channels (~500 nm) were observed in the 80-second-etched sample. Beyond this time, the maximum
relative channel depth decreased and most channels were relatively shallow, on
the order of  ~150 nm.AFM surface plots of the banded surface morphology after (a) 5 s, (b) 10 s, (c) 20 s, (d) 40 s, (e) 80 s, and (f) 160 s immersion in 50% acetic acid 50% nitric acid etching solution.
(a)(b)(c)(d)(e)(f)The
one-dimensional periodic surface morphology was quantified from profile plots
in which the average height along the bands was plotted as a function of
position across the bands, typical examples are shown for each etching time in
Figure6. Band spacing and band depth were measured by quantifying the
distribution of local maxima and minima on the surface profile plots.
Individual bands were identified by a change in curvature at a local maxima or
minima, corresponding to a height change of approximately 5 nm. Band spacing
was defined as the distance between adjacent maxima and minima and band height
was defined as the change in height between adjacent maxima/minima pairs.AFM profile plots after (a) 5 s, (b) 10 s, (c) 20 s, (d) 40 s, (e) 80 s, and (f) 160 s immersion in 50% acetic acid 50% nitric acid etching solution.
(a)(b)(c)(d)(e)(f)Band height and
band spacing are plotted as a function of etching time in Figure7. The
increase in band height is approximately linear for etching times between 5 seconds
and 80 seconds, from which a differential etching rate of 3.2±0.1 nm/s was determined, that is, the difference in rate of material loss for most
active versus least active regions of the bands (Figure 7(a)). The plot of band
spacing as a function of etching time (Figure 7(b)) is somewhat more complex.
There is initially a decrease in effective band spacing from 5 seconds to 10 seconds,
which occurs during the period when preferential band etching is becoming
activated. From 10 seconds to 40 seconds the band spacing is constant at ~650 nm. The effective band spacing increases in the 80-second- and 160-second-etched
samples, once the surface becomes over-etched.Mean band height (a) and band spacing (b) as a function of etching time in a solution of 50% acetic acid and 50% nitric acid.  Error bars indicate the standard error in the mean.
(a)(b)Atoms in the
intercrystalline region are in a nonequilibrium state and are generally
expected to have lower corrosion resistance [19]. In conventional
polycrystalline materials this can be seen as localized grain boundary attack.
Grain size reduction to the nm-scale is expected to have a significant effect
on the overall dissolution rate, largely because of the increased grain
boundary and triple-line density intersecting the free surface [20, 21]. A study
of the corrosion behavior of nanocrystalline and conventional polycrystalline
Ni found a larger passivation current density for nanocrystalline compared to
polycrystalline Ni [20]. X-ray photoelectron spectroscopy of the same samples
polarized in the passive region showed a more defective passive layer for the
nanocrystalline material [21]. The increased dissolution rate of nanocrystalline
materials can also be seen from chemical machining studies of polycrystalline
[22] and nanocrystalline Ni [23]; machining rates in nanocrystalline Ni were
approximately an order of magnitude higher than conventional polycrystalline Ni
for the same FeCl3 aqueous solution concentrations [23].In the present
case, the characteristic grain boundary spacing is more than an order of
magnitude finer than the sub-μm channel spacing. It should be noted that no
significant change in grain size or grain-size distribution was seen across the
compositional bands. Therefore, while grain-size reduction likely has a
significant effect on the overall dissolution rate, it likely has comparatively
little effect on the differential etching rate that forms the surface nanopattern.  More significant are the Fe and Ni
compositional variations in the through thickness direction; STEM/EDS line
profiles across the compositional variations found that the relative intensity
of the Fe signal ranged from ~30% to ~50% [13]. While it is known that the
primary passivation potential of binary Ni-Fe alloys generally increase with
increasing Ni concentration [24], comparatively little study has been conducted
on the corrosion behavior of these alloys in nanocrystalline form. A study on the
pitting behavior of nanocrystalline Ni-18%Fe found that it was more susceptible
to pitting corrosion after significant grain growth had occurred during
annealing [25]. Another study comparing the corrosion resistance of
electrodeposited nanocrystalline Ni-W and Ni-Fe-W alloys reported poor
corrosion resistance for the ternary alloy because of preferential dissolution
of Fe [26]. While alloy concentration effects on the corrosion rate of
electrodeposited nanocrystalline Ni-Fe alloys remain to be clearly established,
the differential etching seen in the present study may be related to the
preferential corrosion of compositional bands having higher Fe concentrations.The  evolving
surface morphology can also be quantified in terms of surface roughness parameters.
Figure8 presents the average roughness, Ra ,
and the root-mean-square roughness, RRMS ,
measured perpendicular to the band orientation as a function of etching time. Both
roughness parameters (Figure 8) followed a similar trend to that of the band
height (Figure 7(a)) as a function of etching time. One measure of the band
straightness is given by the angular distribution in surface roughness. Line profile
average roughness values Ra and RRMS were obtained in
10°
increments from 0°
(parallel to the bands) to 90° (perpendicular to the bands) and are presented as a
series of polar plots in Figure 9. In each case, the surface roughness
perpendicular to the bands is ~6 times greater than the surface roughness
parallel to the bands.  For example, after
40 seconds of etching time the Ra value is 62±7.6 nm perpendicular to the bands and 11±4.4 nm parallel to
the bands. In fact the average roughness is nearly constant at ~60 nm over the
angular range of 10°
to 90°;
it is only when the surface roughness is measured nearly parallel to the bands
that it decreases significantly.Evolution of the average roughness (a) and RMS roughness (b) with increasing etching time.  Error bars give the standard error in the mean.
(a)(b)Polar plots giving the average roughness (a) and RMS roughness (b) as a function of angle measured with respect to the band orientation (0° is parallel to bands).  Error bars give the standard error in the mean.
(a)(b)In
addition to surface modification by varying the etching conditions, it is
possible to manipulate the mesoscale compositional modulations themselves
during electrodeposition. Self-assembled compositional modulations in
conventional electrodeposits can be manipulated through synthesis parameters
such as temperature, duty cycle, and plating current [27]. Periodic adjustments
to these parameters during deposition of nanocrystalline electrodeposits could
be used to modify the compositional modulations and provide a mechanism for
controlling the resultant band structure. In addition, the overall compositional
modulations are conformal to the cathode surface. This means that curved
channels can be engineered by designing appropriate cathode geometries.The
ability to engineer the surface structure of nanocrystalline Ni-Fe alloys may
add new functionalities to nanocrystalline electrodeposits in micro and
nanoscale applications.  Conventional Ni alloy electrodeposits
have become increasingly important components in MEMS devices [e.g., [28]], and reducing
the grain size of these deposits to the nanocrystalline regime may
significantly improve their performance [12]. Nanocrystalline electrodeposits
have substantially increased strength, hardness, and wear resistance compared
to conventional polycrystalline electrodeposits [e.g., [29]]. For example, more
than a factor of three increase in yield strength is often observed, which can
correspond to an approximately ten fold increase in the elastic energy storage
capacity [29]. Furthermore, nanostructured electrodeposits offer increased
uniformity in elastic properties when device components are scaled to the
microscale regime and below [12]. Finally, the surface roughness
directionality illustrated in the present study may provide new opportunities
for controlling the relative movement of micro and nanoscale components by
frictional forces.


## 4. Conclusions

Selective
dissolution of nanocrystalline Ni-Fe electrodeposits was used to create
periodic and regular nanopatterned surfaces. TEM characterization showed an
ultrafine and uniform nanocrystalline grain size. Self-assembled alloy and
impurity compositional modulations in the electrodeposit resulted in preferential
etching of recessed channels that were aligned perpendicular to the growth
direction and were continuous along the cross-section. AFM was used to
characterize the nanopatterned surfaces; before over-etching, there was an
initial linear differential etching rate between peak height and valley depth.
The surface morphology was also quantified in terms of roughness parameters and
their angular distribution. The surface roughness was approximately constant over
an angular range from 10° to 90° inclination to the channels and nearly six times greater
than parallel to the channels. Engineering the surface structure of Ni-Fe alloy
electrodeposits may add new functionalities to nanocrystalline materials in
micro and nanoscale applications.



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