Electrodeposition of stable and narrowly dispersed germanium
nanoclusters from an ionic liquid
Frank Endres* and Sherif Zein El Abedin
University of Karlsruhe, Institute for Physical Chemistry, D-76128 Karlsruhe, Germany.
E-mail: frank.endres@chemie.uni-karlsruhe.de
Received (in Cambridge, UK) 22nd November 2001, Accepted 12th March 2002
First published as an Advance Article on the web 22nd March 2002
Germanium nanoclusters with a narrow height distribution
have been electrodeposited from a dilute solution of GeCl in
the ionic liquid 1-butyl-3-methylimidazolium hexafluoro-
phosphate [BMIm]PF : under the reported conditions the
lateral sizes of most of the clusters range between 20 and 30
nm while their heights vary from 1 to 10 nm with most of
them between 1 and 5 nm.
metallic Ge layer of 300 pm in maximum thickness forms on
gold before bulk growth sets in at E < 0 V. The second peak is
correlated with the electrodeposition of germanium from Ge(II
4
)
6
species, at 21 V reduction of the organic cation begins. On the
basis of the present data we cannot comment on further
intermediate Ge redox states. Platinum or tungsten electrodes
can be used as the counter electrode, and we did not find any
disturbance by products that are formed at the counter electrode.
Pt was used as a ‘quasi’ reference electrode, and it gives a
sufficiently stable electrode potential as soon as a certain
amount of Ge(II) is formed, provided the solution does not
contain water. At E > 1 V gold oxidation sets in, starting at the
steps between different Au(111) terraces. In part the oxidation
peaks at E > 1 V are also due to Ge oxidation. From the cyclic
voltammogram the deposition of Ge seems to be irreversible as
there is no clear stripping peak. Such irreversibility has also
In the last few years the interest in ionic liquids as reaction
media for chemical processes has considerably increased. Many
combinations of cations and anions exist1–3 and they have,
depending on the ion-combination, wide electrochemical win-
dows and negligible vapour pressures over wide temperature
6
ranges. [BMIm]PF , for example, has an electrochemical
4
window of a little more than 4 V on Au(111). It is limited in the
cathodic regime by the reduction of the organic cation while at
the anodic limit gold oxidation sets in. Due to their wide
electrochemical windows ionic liquids give in general access to
elements, that can otherwise not be electrodeposited from
aqueous solutions like e.g. Al, Ti, Si and Ge. Germanium
nanoclusters and quantum dots with dimensions of only a few
nanometers have been intensively investigated in the past in
basic research. Such small Ge clusters show, for example, a
been observed for Si, which has been electrodeposited from
8
SiCl
4
in organic solutions. Nevertheless Ge can be removed
completely from the surface. It is known that Ge(IV) halides
attack elemental Ge in a chemical reaction (Ge + Ge(IV) ? ꢀ
9
Ge(II) ) and with respect to electrooxidation this process seems
to be kinetically favorable. More details on this dissolution/
electrodissolution can be found in refs. 4 and 7. The two Ge
reduction peaks are mainly controlled by diffusion as the peak
currents rise linearly with the square root of scan rate (see also
ref. 10).
5
photoluminescence which is shifted to higher energies with
6
decreasing particle size, thus quantum size effects are present.
Most of such studies were performed under ultrahigh vacuum
conditions which would complicate a possible future nano-
technological process. Therefore we were seeking a method to
prepare germanium by electrochemical means. In a recent
study7 we have reported in detail about in situ scanning
tunnelling microscope (STM) results on germanium electro-
The in situ STM experiments were performed with a
Molecular Imaging PicoScan STM controller in feedback mode
under potentiostatic conditions with in house built STM heads
ꢀ ꢀ
that allow measurements under inert gas (H O and O below ꢀ
ppm).11 Typical setpoints for the STM measurement are in the
range 1–ꢀ nA while for the semiconductor nanoparticles probed
here the tunnelling voltage should be at least +500 mV. Fig.
ꢀ(a)–(c) show germanium nanoclusters on Au(111) as probed in
situ by the STM. They were made in the following way: with
retracted tip (in order to exclude any influence of the tip on the
deposition process) the electrode potential was set from the
open circuit value (about 1 V vs. Ge) for 1 h to -300 mV vs. Ge,
and held constant Then the tip was approached with a bias of
deposition on Au(111). From [BMIm]PF
6
, which was saturated
either with GeCl or GeBr , a thin Ge layer with a rather
4
4
metallic behaviour and a maximum thickness of 300 pm forms
before bulk growth sets in. Bulk deposition starts with
nanoclusters and nanosized micrometer thick layers with a
typical band gap of 0.7 ± 0.1 eV (shown by in situ tunnelling
spectroscopy) can easily be obtained. However, from saturated
solutions the growth of these nanoclusters is either too fast for
size dependent studies, or the growth has to be surveyed for
several hours in situ (by selecting the proper electrode potential)
in order to perform the spectroscopic measurements ‘at the right
time’.
In the present communication we report that narrowly
dispersed Ge nanoclusters can be made by electrochemical
means on a reasonable time scale and that these clusters are
stable even during permanent probing in situ with the STM.
Fig. 1 shows the cyclic voltammogram of [BMIm]PF
6
on
Au(111) with GeCl in an approximate concentration of 5 ± ꢀ 3
4
2
3
21
21
ꢀ
1
0
mol l
(v = 10 mV s , electrode area: 0.5 cm ),
calibrated vs. the bulk deposition of germanium. The CV was
acquired with the potentiostat that was delivered together with
the employed STM controller (see below). Au(111) samples
(
gold on mica) were purchased from the Molecular Imaging
Corporation and annealed at 900 °C under vacuum prior to use.
Starting at 1 V, towards the cathodic regime the first peak at
+
ꢀ50 mV is mainly correlated with the reduction of Ge(IV) to
Fig. 1 Cyclic voltammogram of [BMIm]PF on Au(111), c(GeCl ) = (5 ±
6
4
7
23
21
21
Ge(II). Furthermore, as with saturated solutions, a thin rather
ꢀ) 3 10 mol l , v = 10 mV s
.
8
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CHEM. COMMUN., 2002, 892–893
This journal is © The Royal Society of Chemistry 2002