Nanoscale electrocrystallisation of Sb and the compound semiconductor AlSb from an ionic liquid

Article abstract of DOI:10.1039/b517243h

Aluminium antimonide nanoclusters with an apparent band gap energy of 0.92 ± 0.2 eV have been electrodeposited from the neutral ionic melt AlCl ₃-1-butyl-3-methylimidazolium chloride {AlCl₃-[C ₄mim]⁺Cl^-

Full text of DOI:10.1039/b517243h

View Article Online / Journal Homepage / Table of Contents for this issue
COMMUNICATION
www.rsc.org/chemcomm | ChemComm
Nanoscale electrocrystallisation of Sb and the compound semiconductor
AlSb from an ionic liquid
C. L. Aravinda and W. Freyland*
Received (in Cambridge, UK) 7th December 2005, Accepted 27th February 2006
First published as an Advance Article on the web 10th March 2006
DOI: 10.1039/b517243h
+
2
9
Aluminium antimonide nanoclusters with an apparent band
with [C
mim] Cl in a molar ratio of 45 : 55. A melt containing
4
gap energy of 0.92 ¡ 0.2 eV have been electrodeposited from
Sb(III) with an effective concentration of y1 mM was prepared by
+
2
the neutral ionic melt AlCl –1-butyl-3-methylimidazolium
adding an appropriate amount of the SbCl –[C
3
mim] Cl melt to
4
3
+
2
the {AlCl –[C mim] Cl melt and stirring for 24 h. All the above
+
2
chloride {AlCl –[C mim] Cl } at room temperature and have
3 4
3
4
processes were carried out under high purity argon atmosphere at
a temperature of 294 K.
been characterized in-situ by electrochemical scanning tunneling
microscopy (STM) and spectroscopy (STS).
An overview of the redox processes during Sb electrodeposition
+
2
3 4
from an {AlCl –[C mim] Cl } ionic liquid is obtained by cyclic
The enormous interest in nanoscale materials stems from their
fascinating size dependent properties, in particular, specific
variation of their catalytic, electronic, or mechanical character-
istics. The size, structure and growth patterns and hence their
properties can depend on the preparation techniques. Focusing
here on electrochemical techniques, room temperature molten salts
or ionic liquids (ILs) have proven to be particularly attractive
electrolytes for the deposition of various materials. They are
characterized by low melting points, very low vapour pressures,
high electrolytic conductivities and especially large electrochemical
windows of up to 6 V, see e.g. ref. 1. It is this last characteristic that
enables the electrodeposition of new materials which due to their
large deposition potential cannot be deposited from aqueous
electrolytes. Recently, ILs have been used successfully to deposit a
variety of nanostructured materials such as functionalized
voltammetric studies. Fig. 1 shows the voltammograms on
Au(111) recorded at different sweep rates in the melt containing
y1 mM Sb(III). The redox couple C1/A1 around 730 mV
corresponding to a two electron oxidation process of Sb(III) to
Sb(V) is observed in agreement with earlier studies in a similar
1
0
2
melt. In the present neutral melt Al(III) mainly exists as AlCl4
and is not reducible in the potential range of Sb deposition and
hence excludes the deposition of Al. SbCl due to the equilibrium
3
2
reaction with AlCl4 forms SbCl4 and SbCl2 complex ions.
2
+
11
The reduction waves indicated in the range from C2 to C3 we
attribute to the underpotential deposition (UPD) of Sb, see below.
The reduction peak C4 exhibits a clear asymmetric wing towards
cathodic potential which can be explained by Al codeposition, see
the STM results below. The broad hump C5 is due to AlSb
deposition. The strong anodic peak around 290 mV (A2)
encompasses several oxidation processes including Sb and
Al Sb stripping. With increasing Sb(III) concentration in the
2
3
nanoporous gold, C60 fullerene thin films, elemental semicon-
4
ductors, nanocrystalline metals and alloys by electrochemical
5
means. The research in this direction is motivated by the prospect
x
y
to control the various deposition parameters precisely and to
design the strategy for deposition of metal, alloy and semicon-
ductor nanostructures by employing in-situ nanoscale probe
techniques. The group III–V compound semiconductors in general
and Sb based semiconductors in particular are finding increasing
importance for use as barrier materials in high speed electronics
melt, this peak splits into two anodic peaks A2 and A3 at 10 mV
and 440 mV, respectively, as shown in the inset of the Fig. 1.
6,7
and long-wavelength optoelectronic devices. Here we present the
first report on nanoscale electrodeposition of the compound
2
+
x y 3 4
semiconductor Al Sb from an {AlCl –[C mim] Cl } melt by
in-situ characterization with electrochemical STM and STS. The
bulk Al–Sb alloy phase diagram is characterized by a stoichio-
metric compound AlSb which melts congruently at 1058 uC.
+
2
8
4
[C mim] Cl was synthesized as described earlier. Anhydrous
AlCl (Fluka, >99%) was sublimed twice under vacuum and the
3
resulting white crystals were used for the melt preparation. A
+
2
3 4
neutral {AlCl –[C mim] Cl } melt was obtained by mixing equal
+
2
moles of AlCl crystals and [C mim] Cl . A highly viscous SbCl –
3
4
3
+
2
mim] Cl melt was formed by mixing SbCl
[
C
4
3
(Alfa, >99.99%)
+
2
Fig. 1 Cyclic voltammograms of {AlCl
3 4
–[C mim] Cl } (1 : 1) ionic melt
Institute of Physical Chemistry, University of Karlsruhe (TH),
Kaiserstrasse 12, D-76128, Karlsruhe, Germany.
E-mail: Werner.Freyland@chem-bio.uni-karlsruhe.de;
Fax: +49-721-608-6662; Tel: +49-721-608-2100
containing y1 mM Sb(III) on Au(111) at sweep rates of 20, 40, 60 and
21
8
0 mV s . Inset shows the cyclic voltammogram recorded in melt con-
21
taining y5 mM Sb(III) at a sweep rate of 100 mV s . Temperature 294 K.
This journal is ß The Royal Society of Chemistry 2006
Chem. Commun., 2006, 1703–1705 | 1703

View Article Online
In-situ STM experiments were performed in feedback control
1
mode with a Molecular Imaging Picoscan STM controller under
potentiostatic conditions with a specially designed in house built
electrochemical STM setup as described in ref. 12. A fresh 200–
3
00 nm thick gold film evaporated onto a chromium covered
2
quartz plate (12 6 12 mm , Berliner Glas KG, Germany) was
annealed in a H flame and cooled slowly in a N stream prior to
2
2
use as a working electrode. A pre-cleaned Teflon electrochemical
2
cell (effective area of 0.36 cm ) was sealed on to the gold substrate
using a Teflon coated O-ring. An Al ring and Al wire (>99.99%,
Alfa Acer, Germany) dipped into the melt served as the counter
and reference electrodes, respectively. The STM tip was freshly
prepared by etching tungsten wire (0.25 mm diameter, >99.98%,
2
1
Alfa) in a NaOH solution (2 mol l ). To avoid the effect of
faradaic currents, the tips were coated via electrophoresis with an
epoxide electropaint (BASF ZQ 84-3225 0201, Germany) and
cured at 150 uC for 2 h and subsequently at 200 uC for 10 min. The
whole setup for the EC-STM studies was assembled in an argon
filled glovebox (O and H O , 1 ppm) and mounted in a clean
2
2
and Ar-filled airtight stainless steel container to prevent contam-
ination and to ensure relatively long measurement times.
To get an insight into the electrodeposition process here we
focus on the results of the STM experiments performed in two
different potential regions: first, the UPD and overpotential
deposition (OPD) region of Sb and, secondly, the deposition of
x y
Al Sb nanoclusters. In the potential range positive of the redox
couple C1/A1, terrace structures of flame annealed Au(111) were
seen. Upon changing the potential into the UPD range, 6–16 nm
wide and up to y45 nm long Sb 2D nanostripes appear all over
the substrate surface (Fig. 2a). The height of the nanostripes is
˚
2
.7 ¡ 0.2 A (see height profile). This value is expected for a
monolayer of Sb. An atomically resolved STM image of the 2D
stripes reveals a highly ordered structure of the Sb !7 6 !7
superlattice having quasi-hexagonal symmetry with interatomic
˚
˚
distances of 7.6 ¡ 0.2 A and 7.8 ¡ 0.2 A (Fig. 2b). A similar
superlattice structure has been observed by Ward and Stickney by
1
3
LEED studies of the deposition of Sb on Cu(111). After about
0 min the 2D nanostripes merge with one another and a two
3
dimensional worm like network structure evolves (STM picture
not shown) and subsequently develops into an almost complete
monolayer. Reducing the potential further below 0 V a large
number of small monoatomically high islands occurs (Fig. 2c)
which exhibit similar facets and identical symmetry (Fig 2d) as that
of the 2D nanostripes. The islands coalesce to form deposit
domains and begin to grow three dimensionally by the Stransky–
Krastanov growth mode when further deposition was promoted at
decreasing potentials. When the substrate potential was extended
well below 2500 mV, drastic changes in the STM pictures were
x y
seen. Three dimensional clusters of Al Sb appear. At a substrate
potential of 21000 mV the clusters continue to grow in 3D with
time to form narrowly size dispersed clusters exhibiting a height of
y6 nm and a width of y50 nm (see height profile in Fig. 3).
2
Fig. 2 (a) STM image (100 6 100 nm ) showing the UPD of Sb 2D
nanostripes at 100 mV on Au(111) and the height profile (see double
headed arrow), (b) 13 nm 6 20 nm high resolution STM image of a Sb
2
Al Sb
x
y
clusters were characterized by in-situ I–U spectroscopy
nanostripe with !7 6 !7 super lattice, (c) STM image (100 6 100 nm )
showing the OPD of Sb at 2250 mV and (d) 11 6 11 nm high resolution
2
positioning the tip just above the cluster of interest (see e.g. the tip
position in Fig. 3a). The STS curves were measured at different
sites and clusters and were recorded rapidly (y200 ms) at a set
point current of 20 nA. Since the tip bias voltage was scanned in a
wide range from 21.2 to 1.2 V during the acquisition of I–U
curves, the quality of the tip coating was carefully monitored
STM image of a Sb island deposited on UPD Sb monolayer. Etip
=
250 mV, Itunn = 1 nA.
before and after the spectra acquisition which showed Faraday
currents of ,20 pA. For comparison, spectra of the bare gold
1
704 | Chem. Commun., 2006, 1703–1705
This journal is ß The Royal Society of Chemistry 2006

View Article Online
Fig. 4 A typical I–U tunneling spectrum for Al
position see Fig. 3a). Inset shows I–U tunneling spectrum for the bare Au
111) substrate. Itunn = 20 nA.
x y
Sb cluster (for tip
(
nanometer scale from an ionic liquid. The method also looks very
promising to deposit various Sb based compound semiconductor
nanostructures containing two or more elements such as InSb,
GaSb, AlInSb and GaInAlSb. The 2D layer of Sb deposited in this
study was found to be quite stable even at open circuit potential.
Since the UPD of Al can also be achieved just by changing the
5
acidity of the melt, there is a possibility to deposit epitaxial Al–Sb
compound semiconductor layers by electrochemical atomic layer
15
epitaxy.
2
Financial support of this work by the DFG Center of Func-
tional Nanostructures, University of Karlsruhe, is acknowledged.
Fig. 3 Representative in-situ STM pictures (200 6 100 nm ) showing the
D growth of Al Sb nanoclusters with time: (a) after 3 min, (b) after 8 min
3
x
y
and (c) after 20 min. E = 21 V, Etip = 20.7 V, Itunn = 1 nA; and the height
profiles for the nanocluster.
Notes and references
1
Ionic Liquids in Synthesis, ed. P. Wasserscheid and T Welton, Wiley-
VCH, Weinheim, 2003.
J. F. Huang and I. W. Sun, Adv. Funct. Mater., 2005, 15, 989.
surface were also recorded prior to the deposition which show
typical metallic behavior (see inset Fig. 4). A tunneling spectrum
obtained for an Al Sb cluster of y20 nm in size is shown in Fig. 4.
2
3 D. Deutsch, J. Tar a´ bek, M. Krause, P. Janda and L. Dunsch, Carbon,
004, 42, 1137.
4
5
x
y
2
From the spectrum it is apparent that the cluster exhibits
I. Mukhopadhyay and W. Freyland, Chem. Phys. Lett., 2003, 377, 223.
C. L. Aravinda and W. Freyland, Chem. Commun., 2004, 2754; C.
L. Aravinda, I. Mukhopadhyay and W. Freyland, Phys. Chem. Chem.
Phys., 2004, 6, 5225.
semiconductor behavior. The band gap is found to be 0.92 ¡
0
.2 eV which is clearly smaller than the bulk value for the indirect-
14
gap semiconductor AlSb, i.e. y1.6 eV. The difference between
these values may be due to a size effect of the cluster studied and in
addition, deviations from stoichiometry which result in a relatively
high doping of the cluster. Interestingly, with increasing size of the
cluster the energy gap is also found to increase. A detailed study of
the dependence of the energy gap on the cluster size and the melt
composition is under way and will be published in the future.
Hitherto most of the methods reported to fabricate compound
semiconductor nanostructures are based on molecular beam or
sputtering techniques under UHV conditions. The present EC-
STM study shows the viability of electrochemical methods to
deposit less noble compound semiconductors such as AlSb on the
6
7
M. Razeghi, Eur. Phys. J.: Appl. Phys., 2003, 23, 149.
H. Baaziz, Z. Charifi and N. Bouarissa, Mater. Chem. Phys., 2001, 68,
1
97.
8 F. Endres and W. Freyland, J. Phys. Chem. B, 1998, 102, 10229.
K. Carpenter and M. W. Verbrugge, J. Mater. Res., 1994, 9, 2584.
0 M. H. Yang and I. W. Sun, J. Appl. Electrochem., 2003, 33, 1077.
1 M. Lipsztajn and R. A. Osteryoung, Inorg. Chem., 1985, 24, 3492.
2 A. Shkurankov, F. Endres and W. Freyland, Rev. Sci. Instrum., 2002,
73, 102.
3 C. Ward and J. L. Stickney, Phys. Chem. Chem. Phys., 2001, 3, 3364.
9
1
1
1
1
14 S. M. Sze, Physics of Semiconductor Device, 2nd edn, Wiley Interscience,
New York, 1981, pp. 848–849.
5 J. L. Stickney, in Electroanalytical chemistry A Series of Advances, ed.
A. J. Bard and I. Rubinstein, Marcel Dekker, New York, 1999, p. 75.
1
This journal is ß The Royal Society of Chemistry 2006
Chem. Commun., 2006, 1703–1705 | 1705