Pb electrodeposition on polycrystalline Cu in the presence and absence of Cl-: A combined oblique incidence reflectivity difference and in situ AFM study

Article abstract of DOI:10.1016/j.susc.2007.02.016

Oblique incidence reflectivity difference (OI-RD) measurements reveal differences in the earliest stages of overpotential-deposited (OPD) growth between Pb electrodeposition on polycrystalline Cu surfaces in the presence and absence of Cl^-. At moderate overpotentials, when only 100 mM ClO₄^- is present, the magnitude of the real part of the OI-RD signal continues to increase after completion of the first underpotential-deposited (UPD) Pb monolayer, but with the addition of 20 mM KCl the magnitude decreases after the UPD monolayer forms. In situ atomic force microscopy (AFM) shows that in the former case the island density is much greater than in the latter. Using OI-RD as a probe, we show additionally that when the substrate potential is returned to a more positive potential in the presence of Cl^-, the UPD Pb monolayer dissolves after the Pb islands disappear.

Full text of DOI:10.1016/j.susc.2007.02.016

Surface Science 601 (2007) 1886–1891
www.elsevier.com/locate/susc
Pb electrodeposition on polycrystalline Cu in the presence
ꢀ
and absence of Cl : A combined oblique incidence reflectivity
difference and in situ AFM study
a
b
b
c
c
Guang Yu Wu , S.-E. Bae , A.A. Gewirth , J. Gray , X.D. Zhu ,
d
a,
*
T.P. Moffat , W. Schwarzacher
a
H.H. Wills Physics Laboratory, University of Bristol, Bristol BS8 1TL, UK
Department of Chemistry and Frederick Seitz Materials Research Laboratory, University of Illinois at Urbana-Champaign, Urbana, IL 61801, USA
b
c
Department of Physics, University of California, Davis, CA 95616, USA
National Institute of Standards and Technology, Materials Science and Engineering Laboratory, Gaithersburg, MD 20899, USA
d
Received 20 November 2006; accepted for publication 19 February 2007
Available online 23 February 2007
Abstract
Oblique incidence reflectivity difference (OI-RD) measurements reveal differences in the earliest stages of overpotential-deposited
ꢀ
(
OPD) growth between Pb electrodeposition on polycrystalline Cu surfaces in the presence and absence of Cl . At moderate overpoten-
ꢀ
tials, when only 100 mM ClO is present, the magnitude of the real part of the OI-RD signal continues to increase after completion of the
4
first underpotential-deposited (UPD) Pb monolayer, but with the addition of 20 mM KCl the magnitude decreases after the UPD mono-
layer forms. In situ atomic force microscopy (AFM) shows that in the former case the island density is much greater than in the latter.
Using OI-RD as a probe, we show additionally that when the substrate potential is returned to a more positive potential in the presence
ꢀ
of Cl , the UPD Pb monolayer dissolves after the Pb islands disappear.
ꢀ
2007 Elsevier B.V. All rights reserved.
Keywords: Atomic force microscopy; Electrochemical methods; Ellipsometry; Electrochemical phenomena; Nucleation; Lead; Copper
1
. Introduction
tial monolayer at a deposition potential positive of the bulk
deposition potential is also distinct from subsequent
growth. The structure and properties of this underpoten-
tial-deposited (UPD) monolayer have been studied exten-
sively. Brisard et al. have shown that Pb UPD on Cu in
Under ultrahigh vacuum (UHV) conditions, previous
studies have shown that Pb grows layer-by-layer on
Cu(111) at low temperatures (e.g., 140 K), stabilized by
the quantum size effect [1,2]. LEED studies showed that
the lattice parameter of the first Pb monolayer is slightly
less than that of bulk Pb. At T P 300 K, the energy penalty
associated with pseudomorphic growth on this compressed
substrate favours the subsequent growth of relaxed 3-D Pb
islands (Stranski–Krastanov growth) [2]. When Pb is elec-
trodeposited on Cu(111) or Cu(100), the growth of the ini-
ꢀ
a chloride-containing electrolyte is accompanied by Cl
desorption [3,4]. Chu et al. used in situ surface X-ray dif-
fraction (XRD) to demonstrate that the UPD Pb mono-
layer on Cu(111) is also compressed compared to bulk
Pb, as for the first monolayer in vacuum deposition [5].
Moffat has shown that the early stages of Pb UPD on
Cu(001) were accompanied by surface alloy formation
[
6], and the same might be expected for Pb UPD on
Cu(111) as UHV experiments have already demonstrated
surface alloy formation for Pb deposition on both the
(111) and (001) faces of Cu [7,8].
*
Corresponding author. Tel.: +44 117 9288709; fax: +44 117 9255624.
E-mail address: w.schwarzacher@bristol.ac.uk (W. Schwarzacher).
0
039-6028/$ - see front matter ꢀ 2007 Elsevier B.V. All rights reserved.
doi:10.1016/j.susc.2007.02.016

G.Y. Wu et al. / Surface Science 601 (2007) 1886–1891
1887
In contrast to Pb UPD on Cu, very little attention has
been paid to the subsequent growth of Pb at potentials neg-
ative of the bulk deposition potential, i.e., overpotential
deposition (OPD). Here we present the results of such a
study for Pb OPD on polycrystalline Cu in the presence
and absence of chloride, using a combination of oblique
incidence reflectivity difference (OI-RD) and in situ
AFM. OI-RD is a form of polarization-modulation ellips-
ometry (PME), optimized to probe changes that take place
at the surface of a substrate [9,10]. The essential difference
between OI-RD and conventional PME is that, prior to an
experiment, the Pockels cell and polarizer are used to nul-
lify the first and second harmonics of the phase-modulated
light. Its sub-monolayer sensitivity and speed means that it
can be used to follow the early stages of electrodeposition
processes even involving fast kinetics. OI-RD has been
used to study the electrodeposition of Co on Au [11] and
to characterize gas adsorption and film growth under
UHV conditions [9,10,12]. However, like most optical mea-
surements, the interpretation of OI-RD results relies on
appropriate knowledge of the surface morphology of the
thin-film system under investigation [13,14]. We have,
therefore, used in situ AFM to interpret the data from
the OI-RD measurements.
before passing through a Pockels cell. After reflection from
the sample, the light passes through an analyzer before
being detected by the photodiode. The measured intensity
has terms in various harmonics of the modulation fre-
quency X, and during an OI-RD experiment we monitor
I(2X) using a lock-in amplifier. We used a computer-aided
data acquisition system to collect the optical and electro-
chemical data simultaneously. Prior to electro-deposition,
the transmission axis of the analyser and the Pockels cell
are adjusted so that I(2X) is close to zero. I(2X) is propor-
tional to the real part of D ꢀ D , i.e., Re{D ꢀ D }, where
p
s
p
s
we define D = (r ꢀ r )/r and D = (r ꢀ r )/r , with r
p
p
p0 p0
s
s
s0 s0
p
and r being the sample reflectivity for p- and s-polarized
s
light during electrodeposition and rp0 and rs0 the corre-
sponding values for the substrate [10]. For a simple three-
layer system (substrate–film–electrolyte), Re{D ꢀ D } is
p
s
proportional to the film thickness for an optically smooth
film, and depends on the optical dielectric constants of the
substrate, the film, and the electrolyte [9].
For in situ AFM measurements, a Molecular Imaging
PicoSPM 300 was used working in contact mode with can-
ꢀ1
tilevers having a force constant of 0.58 N m . Images
were acquired over areas of either 15 lm · 15 lm or
30 lm · 30 lm with a resolution of 256 · 256 pixels.
Cu/Au/glass substrates were prepared by vacuum evap-
oration of an Au film onto a glass microscope slide. A
100 nm thick Cu film was then deposited on the Au from
a 0.3 M CuSO /1.2 M H SO electrolyte containing trace
We find that for the Pb on Cu system studied, the OI-
RD signal is dependent on the deposition potential, and
ꢀ
whether Cl is present in the electrolyte. With the help of
in situ AFM, we show that the observed differences in
the OI-RD signal are due to dramatic differences in the
density and the average size of 3-D islands formed during
OPD. The combination of AFM and OI-RD makes an ex-
tremely powerful tool for electrodeposition studies.
4
2
4
ꢀ
ꢀ2
Cl at a current density of 20 mA cm . The Cu substrate
was washed with pure water and dried with N before being
2
immersed into the Pb electrolyte and held at a potential of
ꢀ0.2 V prior to Pb deposition. For the Pb electrodeposit-
ion experiments we prepared perchlorate-based electrolytes
2
+
2
. Experimental
containing 100 mM HClO
from either lead perchlorate Pb(ClO4)2 or lead oxide
PbO). Electrolytes were prepared using ultrapure water
and 99.999% HClO to minimize chloride contamination.
4
(perchloric acid) + 1 mM Pb
(
The experimental arrangement for OI-RD is shown in
Fig. 1. Light from a 15 mW He–Ne laser passes through a
polarizer and is then modulated by the photoelastic modu-
lator (PEM90, Hinds Instruments) between p- (electric field
parallel to the reflection plane) and s- (electric field perpen-
dicular to the reflection plane) at a frequency of X = 50 kHz
4
For some experiments, we deliberately added chloride in
the form of 20 mM KCl. Electrodeposition experiments
were carried out either in the optical cell shown in Fig. 1,
or in the AFM cell. In the optical cell we used a saturated
calomel (SCE) or mercury sulphate (MSE) reference elec-
trode while in the AFM cell we used a Pb wire as a refer-
1
ence. All potentials are quoted with respect to SCE.
3. Results and discussion
Fig. 2a and b show cyclic voltammograms (CV) ob-
tained at a rate of 10 mV/s, in a solution containing
100 mM HClO and 1 mM Pb(ClO ) with and without
4 4 2
20 mM KCl, respectively. Although there are differences
in the precise peak positions, qualitatively, our results are
in good agreement with earlier studies on Cu single crystals
Fig. 1. Sketch of the electrochemical cell and optical set up for the oblique
incidence reflectivity difference experiment. Laser: 15 mW He–Ne laser;
PL: Glan-Thompson polarizers; PEM: photoelastic modulator (Hinds
Instruments); PC: Pockels cell; CE: counter electrode; RE: reference
electrode; WE: working electrode (Cu/Au/glass); A: analyzer; PD: biased
silicon photodiode.
1
Although the SCE could introduce trace chloride to our nominally
chloride-free electrolyte, it did not make any noticeable difference to our
results whether this or the MSE was used.

1888
G.Y. Wu et al. / Surface Science 601 (2007) 1886–1891
2
1
0
1
AII
0.020
UPD monolayer forms
AI
-
0.5 V
0.015
0.010
UPD monolayer
dissolves
CIb
CIa
-
CII
-0.2 V
0
-0.2 V
80
-
0.6
-0.5
-0.4
ESCE (V)
-0.3
-0.2
20
40
60
100
time (s)
3
2
1
0
AII
Bulk Pb deposition
0
0
.025
.020
AI
-
0.54 V
Dissolution
of Pb
-
-
1
2
CIb
0.015
CIa
CII
-
0.2 V
0.010
-0.2 V
80
-
0.6
-0.5
-0.4
-0.3
-0.2
ESCE (V)
0
20
40
time (s)
60
100
Fig. 2. Cyclic voltammograms for a (111)-textured polycrystalline Cu
substrate measured in: (a) 100 mM HClO + 1 mM Pb(ClO ; (b)
+ 20 mM KCl. The scan rate was
0 mV s and all potentials are quoted with respect to SCE.
Fig. 3. OI-RD signal jRe{D
p
ꢀ D }j (not normalized) recorded when
s
4
4 2
)
stepping the Cu substrate potential (a) from E = ꢀ0.2 V to E = ꢀ0.5 V for
t = 50 s (underpotential deposition – UPD) and (b) from E = ꢀ0.2 V to
E = ꢀ0.54 V for t = 50 s (overpotential deposition – OPD). The electro-
1
1
4 4 2
00 mM HClO + 1 mM Pb(ClO )
ꢀ1
4 4 2
lyte was 100 mM HClO + 1 mM Pb(ClO ) .
[
3,4]. In both CV’s a double-peak structure labelled ‘CIa’
and ‘C ’ is seen during the cathodic scan around ꢀ0.4 V,
applying a potential E = ꢀ0.5 V (just positive of the bulk
deposition potential) to the Cu substrate for t = 50 s in
Ib
corresponding to Pb UPD, while bulk deposition com-
mences at ꢁꢀ0.54 V. Peaks corresponding to bulk and
UPD Pb dissolution, labelled ‘A ’ and ‘A ’ respectively
ꢀ
the absence of Cl , jRe{D ꢀ D }j increases rapidly due
p
s
to UPD Pb and then remains constant at the value corre-
sponding to the UPD monolayer, indicating that there is
no subsequent bulk Pb deposition. On returning the sub-
strate potential to E = ꢀ0.2 V, the UPD monolayer dis-
solves and jRe{D ꢀ D }j returns to its initial value. If a
II
I
are seen during the anodic scan. In agreement with studies
on single crystal substrates, the splitting of the cathodic
and anodic UPD peaks C and A is less in the presence
I
I
ꢀ
ꢀ
of Cl than in its absence, signifying that Cl enhances
the kinetics of Pb UPD deposition. Furthermore, the aver-
age position of the UPD peaks is closer to the bulk depo-
p
s
potential negative of the bulk deposition potential,
E = ꢀ0.54 V is applied, Fig. 3b shows that OPD Pb (bulk
deposition) causes jRe{D ꢀ D }j to increase beyond the
ꢀ
sition potential in the presence of Cl , signifying that the
p
s
binding energy of the UPD monolayer to the Cu substrate
value corresponding to the UPD monolayer, after a few
seconds delay (nucleation time). In situ AFM (Fig. 4)
shows that numerous sub-micron Pb islands form at
E = ꢀ0.54 V. The wide range of sizes observed suggests
that nucleation is progressive. The sub-micron islands are
seen to dissolve when the substrate potential is returned
to E = ꢀ0.2 V.
is less in this case [3]. The two cathodic UPD peaks C and
Ia
CIb observed in CVs on polycrystalline Cu substrates may
be associated with Cu sites having different geometries, as
their separation in Fig. 2b is about 30 mV, which is close
to the difference between the peak potentials for Pb UPD
deposition on Cu(111) and Cu(100) in the presence of
ꢀ
ꢀ
Cl (ꢀ0.37 V and ꢀ0.40 V, respectively [3,6]).
In the presence of Cl , the OI-RD response during UPD
Fig. 3a shows that the formation of the UPD Pb mono-
layer is clearly seen in the OI-RD signal. As a result of
has the same sign and order of magnitude as in the absence
ꢀ
of Cl (Fig. 5a). During OPD at E = ꢀ0.54 V, however,

G.Y. Wu et al. / Surface Science 601 (2007) 1886–1891
1889
there is a significant change. Fig. 5b shows that after com-
pletion of the UPD monolayer, jRe{D ꢀ D }j starts to fall
p
s
instead of continuing to rise. The opposing effects of the
UPD and OPD layers on the OI-RD signal imply that
the optical response of the material deposited at
ꢀ
E = ꢀ0.54 V in the presence of Cl is very different from
that of the dense array of sub-micron islands deposited at
ꢀ
this potential in the absence of Cl (Fig. 4).
The reason behind the OI-RD response in the presence
-
of Cl is revealed by in situ AFM images (Fig. 6). There
are tip artefacts in this image, but the differences in island
size and density between this figure and Fig. 4 are obvious.
ꢀ
In the presence of Cl , micron-sized Pb islands form with
an average island-island separation of the order of 10 lm
so that the surface is optically rough. From such a surface,
the OI-RD signal is dominated by the disproportional scat-
tering of s- and p-polarized light by the wavelength-sized
and multi-wavelength-spaced Pb islands. This is because
the efficiency for the s-polarized component to be scattered
into the specular direction is markedly different from that
for the p-polarized component. Our results so far suggest
4
Fig. 4. 15 · 15 lm in situ AFM image obtained in 100 mM HClO + 1 mM
2
+
Pb electrolyte during growth at ꢀ0.54 V. The island size increases from
the top of the figure (start of scan) to the bottom (end of scan) as growth
proceeds.
ꢀ
that nucleation in the presence of Cl is instantaneous.
Furthermore, when the substrate potential is cycled to
deposit and re-dissolve OPD Pb, there is some evidence
that islands form at the same locations in successive cycles,
suggesting that they nucleate at defects.
0
0
0
0
0
.05
.04
.03
.02
.01
UPD monolayer forms
ꢀ
Precisely why the presence of Cl has such a drastic
-
0.52 V
effect on the nucleation density of Pb islands during OPD
remains to be established, but a possible explanation could
be differences in the structure of the UPD layer in the pres-
ꢀ
ꢀ
ence and absence of Cl . Even though Cl desorption has
been reported in association with Pb UPD monolayer for-
mation on Cu [3], our results make it seem likely that there
UPD monolayer
dissolves
ꢀ
is still Cl adsorbed on the Pb prior to OPD deposition.
XRD studies showed that the presence of Cl had a non-
-
0.2 V
-0.2 V
ꢀ
negligible effect on the structure of the UPD monolayer,
affecting both the degree of order and the Pb–Pb lattice
0
20
40
time (s)
60
80
100
Bulk Pb deposition
0
.04
.03
.02
.01
UPD
monolayer
remains
after bulk Pb
dissolves ?
0
-
0.54 V
0
0
-
0.2 V
80
-
0.2 V
0
20
40
60
100
time (s)
Fig. 5. OI-RD signal jRe{D
p
ꢀ D }j (not normalized) recorded when
s
stepping the Cu substrate potential (a) from E = ꢀ0.2 V to E = ꢀ0.52 V
for t = 50 s (UPD) and (b) from E = ꢀ0.2 V to E = ꢀ0.54 V for t = 50 s
(OPD). The electrolyte was 100 mM HClO
4
+ 1 mM Pb(ClO
4
)
2
+ 20 mM
4
Fig. 6. 30 · 30 lm in situ AFM image obtained in 100 mM HClO +
2
+
KCl.
1 mM Pb + 20 mM KCl electrolyte during growth at ꢀ0.54 V.

1890
G.Y. Wu et al. / Surface Science 601 (2007) 1886–1891
spacing as a function of potential [5]. Enhanced surface dif-
ꢀ
fusion in the presence of Cl could also favour the forma-
tion of fewer larger islands. In any case, more work is
required to establish any parallels with other systems.
Fig. 5b reveals a further interesting phenomenon. When
the substrate potential is returned to E = ꢀ0.2 V, there is a
spike in jRe{D ꢀ D }j before the signal returns to its value
p
s
prior to the start of Pb electrodeposition. This suggests that
a compact Pb layer (for which the sign of Re{D ꢀ D } is
p
s
opposite to that of the sparse large islands) remains for a
short time after the large islands dissolve. Our observation
of this transitory effect is a nice example of how OI-RD
and in situ AFM measurements complement each other,
as it would be extremely difficult to interpret the OI-RD
data without the AFM images, but our in situ AFM by it-
self has neither the spatial nor time resolution to reveal the
persistence of the UPD monolayer after dissolution of the
large islands.
At more negative deposition potentials, the shape of the
jRe{D ꢀ D }j transient is qualitatively similar in the pres-
p
s
ꢀ
ence and absence of Cl , as seen from Fig. 7. Fig. 8b shows
that the Pb islands that form in the presence of Cl at
ꢀ
0
0
0
0
0
.08
.06
.04
.02
.00
Bulk Pb deposition
-
without Cl
-0.62 V
Dissolution
of Pb
Fig. 8. (a) 15 · 15 lm in situ AFM image of islands formed by Pb
2+
deposition at E = ꢀ0.62 V in 100 mM HClO
b) 30 · 30 lm in situ AFM image of islands formed by Pb deposition at
E = ꢀ0.62 V in the same electrolyte + 20 mM KCl.
4
+ 1 mM Pb electrolyte;
(
-
0.2 V
0
-0.2 V
20
40
time (s)
60
80
100
ꢀ
0.62 V are of a similar size to those that form at
E = ꢀ0.54 V and much larger than those that form in the
0
0
0
0
0
0
.25
.20
.15
.10
.05
.00
ꢀ
absence of Cl at E = ꢀ0.62 V (Fig. 8a). However, the den-
sity of islands is greater than at E = ꢀ0.54 V. It appears to
be the increased density of Pb islands that changes the
effective dielectric constant to a value closer to that of a
compact Pb film, which gives a monotonically increasing
jRe{D ꢀ D }j signal.
Bulk Pb deposition
-
with Cl
p
s
-
0.62 V
4. Conclusion
Dissolution
of Pb
We have shown that the growth mode of bulk-phase Pb
on polycrystalline Cu is heavily modified by the presence of
-0.2 V
-0.2 V
80
ꢀ
ꢀ
Cl in the electrolyte. In the absence of Cl , a high density
of sub-micron sized islands forms and the OI-RD signal
jRe{D ꢀ D }j increases monotonically with increasing Pb
0
20
40
time (s)
60
100
p
s
coverage, as expected for a compact Pb film. In the pres-
ence of Cl , large islands form, with lateral dimensions
Fig. 7. OI-RD signal jRe{D
p
ꢀ D
s
}j (not normalized) recorded when
ꢀ
stepping the Cu substrate potential from E = ꢀ0.2 V to E = ꢀ0.62 V for
up to several lm. At low overpotentials, the island density
is sufficiently low that jRe{D ꢀ D }j decreases with
t= 50 s (a) in 100 mM HClO
4 4 2
+ 1 mM Pb(ClO ) and (b) in 100 mM
HClO + 1 mM Pb(ClO + 20 mM KCl.
4
4 2
)
p
s

G.Y. Wu et al. / Surface Science 601 (2007) 1886–1891
1891
[3] G.M. Brisard, E. Zenati, H.A. Gasteiger, N.M. Markovic
´
, P.N. Ross
increasing Pb coverage after formation of the UPD mono-
layer, while at higher overpotentials it increases monoton-
ically, as in the absence of Cl . The origin of this dramatic
Jr., Langmuir 11 (1995) 2221.
ꢀ
[4] G.M. Brisard, E. Zenati, H.A. Gasteiger, N.M. Markovi c´ , P.N. Ross
Jr., Langmuir 13 (1997) 2390.
change in growth mode is still under investigation, but
could be associated with a change in the structure of the
initial UPD monolayer. OI-RD data also suggest that
[
5] Y.S. Chu, I.K. Robinson, A.A. Gewirth, Phys. Rev. B 55 (1997) 7945.
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observed because of the high speed of OI-RD relative to
our in situ AFM.
3
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