Mechanism of formation and electronic structure of semiconducting ZnSb nanoclusters electrodeposited from an ionic liquid

Article abstract of DOI:10.1016/j.electacta.2007.07.034

Electrocrystallization of Sb and the compound semiconductor ZnSb has been investigated by in situ SPM methods at the electrified ionic liquid/Au(1 1 1) interface at an elevated temperature of 50 °C for the first time employing the ionic liquid ZnCl₂-[C₄mim]⁺Cl^- (45:55). Prior to the underpotential deposition (UPD) process of Sb, ZnCl₃^- anions adsorb on the gold surface at the open-circuit potential (OCP). An ordered region - showing the characteristic of a Moire?-like pattern - coexists with a disordered region indicative of an interfacial phase transition. When the potential is reduced to -0.40 V versus Pt/Pt(II), 2D electrocrystallization of Sb starts showing a typical fenced(sqrt(3) × sqrt(3)) structure of the first monolayer. Further decreasing the potential to -0.5 V a second layer of Sb islands occurs. Stepping the potential from the UPD region to -0.60 V, the OPD of Sb sets in showing randomly dispersed clusters of homogeneous size. Near the ZnSb deposition potential, at ～-0.95 V, a nearly homogeneous distribution of clusters of spherical shape with diameters up to 15 nm is found. Their corresponding STS curves exhibit an obvious semiconducting behaviour with a gap-energy of ～0.6 ± 0.2 eV. Experiments at deposition conditions on the Sb-rich or Zn-rich side relative to the ZnSb deposition potential show an obvious doping effect - in the case of Zn excess - which is revealed by the corresponding normalized conductance (NC) spectra.

Full text of DOI:10.1016/j.electacta.2007.07.034

Electrochimica Acta 53 (2007) 518–524
Mechanism of formation and electronic structure of semiconducting
ZnSb nanoclusters electrodeposited from an ionic liquid
∗
Olivier Mann, Werner Freyland
Institute of Physical Chemistry, University of Karlsruhe, Kaiserstr. 12, 76128 Karlsruhe, Germany
Received 22 March 2007; received in revised form 14 June 2007; accepted 7 July 2007
Available online 22 July 2007
Abstract
Electrocrystallization of Sb and the compound semiconductor ZnSb has been investigated by in situ SPM methods at the electrified ionic
◦
+
−
liquid/Au(1 1 1) interface at an elevated temperature of 50 C for the first time employing the ionic liquid ZnCl
2
–[C
4
mim] Cl (45:55). Prior to the
−
underpotential deposition (UPD) process of Sb, ZnCl
3
anions adsorb on the gold surface at the open-circuit potential (OCP). An ordered region –
showing the characteristic of a Moir e´ -like pattern – coexists with a disordered region indicative of an interfacial phase transition. When the potential
ꢀ
√
√ ꢁ
is reduced to −0.40 V versus Pt/Pt(II), 2D electrocrystallization of Sb starts showing a typical
3 × 3 structure of the first monolayer. Further
decreasing the potential to −0.5 V a second layer of Sb islands occurs. Stepping the potential from the UPD region to −0.60 V, the OPD of Sb sets
in showing randomly dispersed clusters of homogeneous size. Near the ZnSb deposition potential, at ∼−0.95 V, a nearly homogeneous distribution
of clusters of spherical shape with diameters up to 15 nm is found. Their corresponding STS curves exhibit an obvious semiconducting behaviour
with a gap-energy of ∼0.6 ± 0.2 eV. Experiments at deposition conditions on the Sb-rich or Zn-rich side relative to the ZnSb deposition potential
show an obvious doping effect – in the case of Zn excess – which is revealed by the corresponding normalized conductance (NC) spectra.
©
2007 Elsevier Ltd. All rights reserved.
Keywords: Electrocrystallization; Compound semiconductors; In situ scanning probe microscopy; Ionic liquid electrolyte; Elevated temperature
1
. Introduction
of the electrolytes. Here, ionic liquids have the special advantage
of high-electrochemical windows up to ∼6 V [4]. Thus, a wider
range of electrodeposited nanostructured compound semicon-
ductors is accessible.
Electrodeposition of semiconductor nanostructures has sev-
eral advantages over conventional vapour deposition: the
preparation is performed typically near room temperature, the
amount of deposited material can be precisely controlled by
the charge passed, and, employing modern scanning probe
microscopy (SPM) techniques, the morphology and elec-
tronic structure of the deposit can be determined in situ.
For electrodeposition of semiconducting nanomaterials such
as quantum dots, nanowires or thin films, several routes have
been developed including hybrid electrochemical/chemical syn-
thesis, co-deposition or electrochemical atomic layer epitaxy
In a previous investigation we have reported on a first attempt
of nanoscale electrodeposition of the compound semiconductor
+
−
AlSb from the ionic liquid AlCl3–[C4mim] Cl (1:1) con-
taining SbCl3 [5]. In this melt the deposition potential of the
stoichiometric compound AlSb is well separated from that of
pure Al and Sb, respectively. In this communication, we focus
asasecondexampleonthecompoundsemiconductorZnSbelec-
+
−
trodeposited from the ionic liquid ZnCl2–[C4mim] Cl (45:55)
plus 2 mM SbCl3. The smallest energy gap in ZnSb is an indi-
rect one with a value of 0.53 eV in the bulk [6]. In the following
we describe the electrochemical preparation of ZnSb on the
nanometer scale and present first results of its characterization,
morphology and electronic structure, obtained by in situ elec-
trochemical SPM experiments. For better understanding of the
ZnSb phase formation we also studied the electrocrystalliza-
tion of pure Sb at under- and overpotential (UPD and OPD)
conditions.
(
ECALE), for recent reviews see Refs. [1–3]. However, the
choice of compound semiconductors, which can be prepared
by electrodeposition is limited by the electrochemical windows
∗
Corresponding author. Tel.: +49 721 608 2100; fax: +49 721 608 6662.
E-mail address: werner.freyland@chemie.uni-karlsruhe.de (W. Freyland).
URL: www.ipc.uni-karlsruhe.de (W. Freyland).
0
013-4686/$ – see front matter © 2007 Elsevier Ltd. All rights reserved.
doi:10.1016/j.electacta.2007.07.034

O. Mann, W. Freyland / Electrochimica Acta 53 (2007) 518–524
519
2
. Experimental
been determined following the evaluation method described by
Feenstra [9].
The electrochemical and in situ STM investigations
were performed using the ionic liquid electrolyte 1-butyl-
3. Results and discussion
+
−
3-methylimidazolium chloride ([C4mim] Cl ) which was
prepared from 1-chlorobutane and 1-methylimidazole (both
from Merck) and purified by recrystallization from acetonitrile.
ZnCl2 (Alfa, >99.99%) was used as received. Under inert atmo-
sphere (argon with O2 and H2O concentrations <1 ppm), ZnCl2
3
.1. Cyclic voltammetry
An overview of the redox processes occurring during the
antimony and the respective alloy electrodeposition at ele-
vated temperature is presented by the CVs in Fig. 1. Antimony
was deposited from 2 mM SbCl3 solution at the electrified
{
+
−
and [C4mim] Cl crystals were mixed in a 45:55 molar ratio
◦
and stirred at 60 C for 2 days yielding a slightly viscous color-
+
−
less liquid. A viscous SbCl3–[C4mim] Cl melt was obtained
by mixing SbCl3 (Alfa, > 99.99%) with [C4mim] Cl in a
5:55 molar ratio. The effective Sb(III) concentration was pre-
paredbyadding the appropriate amountof SbCl3–[C4mim] Cl
to the ZnCl2–[C4mim] Cl (45:55) melt and then stirred for
4 h. A 200–300 nm Au(1 1 1) film evaporated on borosilicate
+
−
ZnCl2–[C4mim] Cl } (45:55) ionic liquid/Au(1 1 1) interface
+
−
◦
at 50 C. To get more detailed insight of the processes involved
a CV was recorded also at lower scan rate which is presented
in Fig. 1b. Additional peaks around −0.40 V are clearly visible
which were not observed in the case of the pure ionic liquid.
This observation is in good agreement with a previous study
where additional peaks were observed after addition of Sb in
a chloroaluminate ionic liquid [5]. Starting from the anodic
potential limit, a broad hump – of low intensity – around 0.0 V
is observed in the inset of Fig. 1a. Since this hump is inde-
pendent of the presence of Sb in the solution, it is reasonable
to interpret it by the chemisorption of zinc-trichloride anions
4
+
−
+
−
2
glasscoveredbychromium(12 mm × 12 mm, BerlinerGlasKG,
Germany) was used as working electrode. Prior to use, the sub-
strate was annealed in a hydrogen flame for 3 min and then
slowly cooled down under nitrogen stream (Messer, Griesheim,
5
N).
Electrochemical measurements were performed in a tradi-
tional three-electrode teflon cell with Pt wires used as quasi
reference and counter electrodes. In this article all potentials
were measured versus the quasi reference electrode Pt/Pt(II).
Cyclic voltammograms (CVs) were recorded using an AutoLab
potentiostat (PGSTAT 30, Eco Chemie, The Netherlands).
In situ scanning tunneling microscopy (STM) measurements
were carried out in both constant current and height modes using
a molecular imaging (MI) controller. For the STM experiments,
tunneling tips were freshly prepared from Pt–Ir wires by an elec-
−
1
trochemical etching process in 3.5 mol l NaCN. Coating of
the tips via electrophoresis with an epoxide electropaint (BASF
ZQ 84-3225 0201, Germany) was performed in order to mini-
mize the faradaic current. To avoid contamination of the ionic
liquid, all the STM experiments were performed with a home
built microscope inside a vacuum-tight stainless steel vessel
filled with argon. The whole setup was installed on a vibration-
damping table. For more details concerning the construction
of microscope plus heater, assembling of the microscope, and
measurements under argon atmosphere, cf. Ref. [7].
In order to carry out experiments at elevated temperature a
special design of the sample holder, including a Mo wire heat-
ing system, was built. A Ni–NiCr thermocouple was used and
connected at a distance of ∼3 mm from the sample to monitor
the temperature stability. A power supply (Hewlett Packard) of
high-stability – to minimize as much as possible the thermal
drift – was connected to the above heating element. The whole
system was set up to the desired temperature and left for thermal
stabilization during half a day.
Special care was taken to record tunneling spectra at dif-
ferent sites of the STM images. For this aim the I–U curves
weretakeninthecurrentimagingtunnelingspectroscopy(CITS)
mode [8] and were averaged over 5–10 measurements under
constant conditions. From the differential I–U curves the nor-
malized differential conductance spectra (dI/dU)/(I/U) have
+
−
Fig. 1. (a) Cyclic voltammogram of 2 mM SbCl –[C mim] Cl in a molar
3
4
+
−
ratio of 45:55 in ZnCl2–[C4mim] Cl (45:55) on Au(1 1 1) recorded at a scan
rate of 20 mV s 1. Inset shows the magnification around 0.0 V at a scan rate
−
−
1
of 100 mV s and (b) magnification of the CV around −0.4 V to get a clear
−
1
resolution of the corresponding peaks at a sweep rate of 10 mV s . The CVs
◦
were recorded at 50 C.

5
20
O. Mann, W. Freyland / Electrochimica Acta 53 (2007) 518–524
◦
Fig. 2. (a) Large-scale STM image of gold at −0.17 V, Etip = 0.03 V, It = 1.3 nA showing the adsorption of an adlayer at 50 C and (b) higher resolved STM image
◦
showing the Moir e´ -like pattern at 50 C.
which is demonstrated by STM investigations presented below.
The absence of a pronounced peak in case of adsorption of
anions is presumably due to slow kinetics involved in the pro-
cess. Reducing the potential towards more negative values, two
new processes C2/A2 and C3/A3 appear with reduction peaks at
corresponding dissolution peak A4 is observed at −0.55 V and
it tends to increase and merge with the anodic peaks A5 and
A6 depending on the reversal deposition potential. At potentials
negative of that of peak C4, Zn and Sb codeposition has to be
considered which finally leads to a broad peak C5 at −1.0 V
where the ZnSb compound deposition is expected (see Fig. 1a).
This latter interpretation is confirmed by the STM and STS
investigations presented below. Finally, a last peak C6 is indi-
cated below −1.2 V which is assigned to the Zn bulk deposi-
tion.
−
0.38 V and near −0.5 V, respectively (see Fig. 1b). On the basis
of STM experiments performed in this region these processes
can be attributed to the UPD of Sb which is in good agree-
ment with previous results [10]. Near −0.60 V, a new reduction
wave C4 corresponding to the OPD process of Sb starts. The
◦
Fig. 3. Large-scale STM image showing the formation of the first Sb monolayer during the UPD process at −0.40 V, Etip = −0.35 V, It = 1 nA at 50 C at different
time intervals: (a) 2 min and (b) 8 min. (c) High-resolution STM image of an almost complete Sb monolayer and (d) inverse FFT of the region delimited by lines in
(
c) showing the structure of the adsorbate. Inset shows the 2D-FFT of the latter region.

O. Mann, W. Freyland / Electrochimica Acta 53 (2007) 518–524
521
Fig. 4. Large-scale STM images showing the layer-by-layer growth of Sb at different potentials: (a) −0.45 V, Etip = 0.05 V, It = 1 nA and (b) −0.55 V, Etip = 0.09 V,
It = 1 nA.
−
−
3
.2. Adsorption of ZnCl3
bromide on Au(1 1 1) [15,16] and PF6 [14]. Thus, the protru-
sions and the Moir e´ -like pattern coexist on the surface which is
indicative of a transition between two phases. It is worth notic-
ing that the adlayer can also be formed under potential control.
Further experiments are under investigation in order to elucidate
Prior to the Sb deposition an interesting observation is
made at open-circuit potential (OCP). Fig. 2a shows a typical
large-scale STM image of Au(1 1 1) at an OCP of −0.17 V. A
well-ordered pattern is observed at this potential which forms
a quasi-hexagonal structure all over the Au(1 1 1) surface. It is
−
the ZnCl3 phase transition and will be published separately.
−
reasonable to assign this pattern to the adsorption of ZnCl3
3.3. Underpotential deposition of Sb
since the trichloride anion is the main Zn(II) species present
in a neutral and slightly basic IL according to literature [11].
It is interesting to point out that the gold steps in Fig. 2a are
obvioulsy not parallel and well defined as expected which may
be due to an enhanced mobility of the Au step atoms caused
Switching the potential towards more negative values with
respect to the OCP, the UPD of Sb is initiated. Fig. 3a and b
present typical large-scale STM images during the first antimony
+
−
layer formation from {ZnCl –[C mim] Cl } (45:55) ionic liq-
2
4
−
by the ZnCl3 adsorption. Similar features were observed in
uid containing 2 mM SbCl at different time intervals. Directly
3
the case of sodium and iodine adsorption on Au(1 1 1) [12,13].
A higher resolution STM image of the adlayer is presented in
Fig. 2b. It is clearly composed of two different areas: region I
presenting a nearly close packed structure and region II com-
posed of structural defects. The distances between bright spots
after switching the potential to −0.40 V Sb islands start to
nucleate randomly along the monoatomic gold step edges (indi-
cated by their undefined shape) as well as on the terraces of
Au(1 1 1). After several minutes the Sb islands, confined to a
monoatomical height, obviously start to merge forming a more
homogeneous adlayer as indicated by the arrows in Fig. 3a and
b. After more than 15 min deposition at constant potential, a 2D
structure appears on the surface representing an almost com-
plete Sb monolayer (Fig. 3c). Small thermal fluctuations due
˚
of region I are found to be 19.5 ± 2 A. This larger spacing can
be associated with a Moir e´ pattern which is known as extremely
sensitive to the misfit between the adlayer and the substrate [14].
Similar behaviour was observed for the adsorption of iodine and
Fig. 5. (a) Large-scale STM image showing the 3D Sb growth at −0.6 V, Etip = 0.4 V, It = 1 nA and (b) height profile of the associated area marked with an arrow on
the STM image.

5
22
O. Mann, W. Freyland / Electrochimica Acta 53 (2007) 518–524
◦
to measurements at an elevated temperature of 50 C made it
difficult to get perfect atomically resolved images of the Sb
adlayer structure as seen by the slightly distorted structure of the
adlayer. Nevertheless, after the 2D-FFT filtering of the region
delimited in Fig. 3c the structure of the Sb adlayer could be
determined. A hexagonal structure (inset of Fig. 3d) is observed
and the inverse 2D-FFT (see Fig. 3d) allows evaluating the lat-
tice parameter more precisely. Thus, the distances between two
˚
nearest neighbours are found to be equal to 5.3 ± 0.2 A typical
ꢂ
ꢃ
√
√
for a
3 ×
3
structure. This result is in good agreement
with the structure described previously for the adsorption of Sb
on Au(1 1 1) [5,17].
To monitor the growth mechanism of the Sb UPD, the
potential was decreased to −0.45 V and images were taken as
presented in Fig. 4a. Obvioulsy, the growth of the Sb layer
becomes faster and after ∼10 min polarization time roughly 45%
of the Au layer is covered by Sb. However, before a complete
adlayer has formed, a second layer of Sb starts to grow on the
monoatomically high-Sb islands – forming the first layer – as
confirmed by the height profile presented in the inset of Fig. 4a.
Decreasing the potential to −0.55 V, (corresponding to process
C3 in Fig. 1) leads to the morphology presented in Fig. 4b. The
growth of a second layer of Sb islands is indicated.
3
.4. Overpotential deposition of Sb
Near the potential of −0.6 V (process C4) a characteristic
morphology of 3D deposits is shown in Fig. 5. Prior to 3D depo-
sition the potential was kept constant at −0.4 V in the UPD range
until one complete monolayer of Sb had formed followed by a
direct jump to −0.60 V. The resulting STM picture, shown in
Fig. 5a, presents a homogeneous distribution of clusters of uni-
form size all over the surface. The clusters have a typical width
of 15 ± 1 nm as shown in the profile in Fig. 5b. When the poten-
tial is decreased towards more negative values the morphology
of the deposit changes yielding bigger clusters. A similar mor-
phology of Sb clusters has been observed in the study of Sb
deposition on Au(1 1 1) from a chloroaluminate melt [5].
3
.5. Electrocrystallization of the compound semiconductor
ZnSb
Fig. 6. (a) Large-scale STM image after switching the potential to −0.95 V,
Etip = −0.75 V, It = 1.6 nA showing the growth of the ZnxSb1−x alloy and (b)
STS spectrum recorded for the cluster indicated with an arrow on the STM image
and (c) normalized differential conductivity spectrum of the cluster marked with
an arrow on the STM image.
According to the CV measurements (Fig. 1) the formation of
the compound ZnSb is expected at a potential around −1.0 V.
Results of STM and STS measurements at a potential of −0.95 V
are presented in Fig. 6. At a first sight, the distribution of small
clusters of nearly spherical shape on the surface is random. Their
size distribution is rather homogeneous, as presented in the STM
image, except for several bigger clusters located in the middle
of the picture. The size of the clusters ranges from 5 to 10 nm
with exception of the bigger clusters having diameters up to
ones exhibit no clear semiconducting behaviour and are more
consistent with the semimetallic characteristics of bulk Sb, see
also Ref. [5]. Therefore the formation of semiconducting ZnSb
clusters seems to be not homogeneous all over the surface which
may indicate a slow formation process. In order to get a first
estimate of the band structure of the bigger semiconducting
clusters we have analyzed their I–U-curves by the method of
Feenstra [18] which yields the normalized differential conduc-
tance spectra, (dI/dU)/(I/U). An example is shown in Fig. 6c.
∼
15 nm. An example of an I–U curve recorded at a spheri-
cal cluster of ∼15 nm diameter marked in Fig. 6a is given in
Fig. 6b. This curve exhibits clearly semiconducting character-
istics and indicates a voltage gap of ∼0.6 eV. However, further
spectra taken for smaller clusters in the vicinity of the bigger

O. Mann, W. Freyland / Electrochimica Acta 53 (2007) 518–524
523
Fig. 7. (a) Large-scale STM image showing the beginning of the ZnxSb1−x alloy formation at −0.8 V, Etip = −0.60 V, It = 1.6 nA, (b) large-scale STM image after
switching the potential toward −1.05 V, Etip = −0.80 V, It = 1.6 nA and (c and d) are the respective normalized differential conductivity spectra of the clusters marked
by arrows in the STM images.
Taking into account the noise level of the original I–U spec-
tra it is difficult to discuss the weak structures on both sides of
the Fermi level, EF, at zero bias voltage. They might indicate
impurity states, might be due to band bending as observed by
Feenstra [9] and Matino [19] or might be a result of an inho-
mogeneous deposition of the compound, although this is not
clear given the experimental uncertainties. However, the onset
of the valence-band edge is well-defined yielding an extrap-
olated value of Ev ∼ −0.35 V. This extrapolation procedure is
common practice in the interpretation of conductance-spectra,
see, e.g., [18]. For the conductance-band edge the estimation is
less straightforward in view of the band onset. However, a value
of Ec ∼ 0.2 ± 0.1 eV is extrapolated – with an adequate uncer-
tainty – so that an approximate value of the band-gap energy of
Eg ∼ 0.6 ± 0.2 eV is obtained. This is in reasonable agreement
with the indirect gap energy of bulk ZnSb ranging from 0.53 eV
conductor formation presented in Fig. 6a, respectively. In both
cases the morphology of clusters is very similar to that of the
ZnSb compound clusters in Fig. 6. In comparison with the band
structure of the ZnSb compound (Fig. 6c), no significant vari-
ation of the band-gap is observed for excess Sb (n-doping) in
view of the experimental uncertainties given above whereas a
pronounced reduction of the band gap to ∼0.35 eV is observed
for excess Zn (p-doping), see Fig. 7c and d, respectively. In
addition, an enhancement of the conduction band of the Sb-rich
compound is observed in comparison with the Zn-rich one which
ischaracteristicofann-doping. Asimilartrendwasobservedina
previous study of the AlSb compound [5]. However, before more
quantitative conclusions on the change of the electronic struc-
ture due to doping can be drawn systematic studies at various
depositionpotentialsarenecessary. Inaddition, thehomogeneity
of the semiconducting clusters and the effect of possible surface
states on the conductance spectra have to be investigated in more
detail.
[
20] to 0.60 eV [21].
3
.6. Electrodeposition off of the compound ZnSb
For a first test on the effect of doping of the semiconducting
4. Summary and conclusions
clusters we have studied the variation of the conductance spec-
tra at deposition potentials off of that of the ZnSb compound.
The results of the respective STM images and conductance spec-
tra are compared in Fig. 7. The sequence of acquisition of the
pictures was done as follows; Fig. 7a obtained at a deposition
potential of −0.8 V and Fig. 7b at a deposition potential of
The electrocrystallization of Sb and semiconducting ZnSb
have been investigated by in situ SPM methods employing an
+
−
ionic liquid, the {ZnCl2–[C4mim] Cl } (45:55) melt contain-
ing SbCl3. For the first time, electrochemical SPM experiments
with an ionic liquid have been carried out at elevated temperature
◦
−
1.05 V were recorded prior to and after the compound semi-
of 50 C achieving high-resolution STM imaging.

5
24
O. Mann, W. Freyland / Electrochimica Acta 53 (2007) 518–524
PriortotheUPDreaction, anorderedlayerpresentingaquasi-
hexagonal structure of adsorbed ZnCl3 anions is found. At
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−
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ꢂ
ꢃ
√
√
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[
2] R.M. Penner, in [1], p.1.
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Acknowledgements
We acknowledge financial support of this project by the DFG
Center of Functional Nanostructures (CFN) at the University of
Karlsruhe and the Fonds Chemische Industrie.
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