Anhydrous solution synthesis of germanium nanocrystals from the germanium(II) precursor Ge[N(SiMe3)2]2

Article abstract of DOI:10.1039/b416066e

A convenient, simple, single-source solution synthesis of Ge nanocrystals via thermal reduction of Ge(II) precursor Ge[N(SiMe₃) ₂]₂ in a non-coordinating solvent at 300°C and 1 atm Ar is described. The Royal Society of Chemistry 2005.

Full text of DOI:10.1039/b416066e

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Anhydrous solution synthesis of germanium nanocrystals from the
germanium(II) precursor Ge[N(SiMe3)2]2{
Henry Gerung,a Scott D. Bunge,b Timothy J. Boyle,*b C. Jeffrey Brinkerab and Sang M. Hana
Received (in Cambridge, UK) 18th October 2004, Accepted 1st February 2005
First published as an Advance Article on the web 16th February 2005
DOI: 10.1039/b416066e
decompose under milder conditions without the need for
additional reducing agents. The reduction potential of Ge(II) to
A convenient, simple, single-source solution synthesis of Ge
nanocrystals via thermal reduction of Ge(II) precursor
Ge[N(SiMe3)2]2 in a non-coordinating solvent at 300 uC and
1 atm Ar is described.
Ge(0) is +0.247 V, whereas that of Ge(IV) to Ge(0) is +0.124 V.22
similar approach has been developed successfully for the solution-
A
based synthesis of Cu, Ag, and Au NCs.23
Crystalline semiconductor nanocrystals (NCs) have been
extensively researched for their potential applications in optoelec-
tronics,1 photovoltaics,2 and biological imaging.3 These applica-
tions are dependant upon the quantum confinement effect, which
gives rise to the size-dependent electronic and optical properties
found in these NCs.4 To date, much attention has been given to
the development of convenient, large-scale solution synthesis of
crystalline semiconductor NCs such as CdSe,5 InP,6 and AgBr.7
These semiconductor NCs have been produced with a narrow size
distribution and definitive morphological control in order to
‘‘tune’’ their properties.8 In contrast, research in Group IV NCs
(e.g., Si and Ge) has been less vigorous since these materials
possess an indirect bandgap; however, interest has increased due to
the discovery of luminescence in porous Si9 and the development
of synthetic routes to Si NCs.10 A recent report demonstrates
stimulated emission and light amplification from Si NCs
embedded in a layer of SiO2 upon single-pass transmission of
390 nm excitation with a net material gain comparable to III–V
quantum dots.11
For the Ge(II) precursor, we chose the amido based
Ge[N(SiMe3)2]2 which can be prepared at room temperature and
pressure following previously reported synthetic methods.24 The
selection of Ge[N(SiMe3)2]2 is also based on the ease of synthesis,
the absence of potential halide contamination, and the labile
amido ligand sets. Full details of the Ge NC syntheses are supplied
in the experimental section.{25 As shown by Fourier transform
infrared (FTIR) data (vide infra), oleylamine encapsulates the
surface of Ge NCs to prevent agglomeration in an y89% yield.
Transmission electron microscopy (TEM) is used to characterize
the shape, size, and crystallinity of the synthesized Ge NCs. A
TEM image of typical Ge NCs is shown in Fig. 1. The crystallinity
of the particles is shown by the selected area electron diffraction
(SAED) pattern in Fig. 1a. The diffraction pattern shows
˚
d-spacings of 3.26, 2.00, 1.70, and 1.41 A, which matches the
d-spacings of bulk Ge (111), (220), (311), and (400) cubic phase
reflections. A high-resolution TEM image of one of the particles in
Fig. 1b shows the cubic lattice structure of the Ge NCs. The FTIR
and TEM characterizations do not reveal the presence of GeO2 on
In comparison to Si NCs, Ge NCs have a larger exciton Bohr
radius, which imparts a strong quantum confinement independent
of the relatively large NC radius.12 Even though bulk Ge is an
indirect bandgap material, Ge NCs have been found to behave as
a direct bandgap material.13 This transformation opens up the
possibility of using Ge NCs as light-emitting or power-generating
elements. In addition, modification of the Ge surface with organic
layers could bring Ge NCs into further use as chemical sensors.14
Previously reported Ge NC syntheses rely on direct reduction of
Ge(IV) precursors to Ge(0) by utilizing Na/K as a reducing agent,15
17
18
Zintl salt as a precursor,16 reduction of GeCl4 or GeI4 using
LiAlH4 as reducing agent, high-pressure reduction,19 high
temperature supercritical fluid,20 and laser illumination.21
However, the presence of reducing agents or salt by-products in
the reaction makes the separation and purification processes, as
well as control over Ge NC surface, more difficult. Herein, we
demonstrate a simple, single source solution synthesis route of Ge
NCs via thermal reduction of a Ge(II) precursor to Ge(0). The
reasoning is based on the reduction potentials of Ge(II) and Ge(IV),
where a Ge(II) precursor, compared to a Ge(IV) precursor, will
Fig. 1 TEM images of Ge NCs. (a) Selected area electron diffraction
˚
(SAED) of Ge NCs with d-spacings of 3.26, 2.00, 1.70, and 1.41 A
{ Electronic supplementary information (ESI) available: X-Ray diffraction
of Ge NC powder. See http://www.rsc.org/suppdata/cc/b4/b416066e/
*tjboyle@sandia.gov
corresponding to Ge (111), (220), (311), and (400), respectively. (b) High-
˚
resolution image of the Ge NCs with a d-spacing of 3.26 A for Ge (111).
1914 | Chem. Commun., 2005, 1914–1916
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the particles. The average particle size, shown in Fig. 2, is 7 ¡ 4 nm
over 119 particles and measured from the longest dimension of the
particle. Cumulative data show that y95% of the particles are less
than 8 nm.
C–H bending mode and the three peaks at 2850, 2922, and
2955 cm21 represent the C–H stretching modes of the oleylamine
carbon chain. The large peak at 3500 cm21 has been assigned to
the MeOH used in the separation step. The presence of various
N–H peaks suggests that amines are bound to the surface of the
Ge NCs. Upon exposure to air for an extended period of 5 months,
no substantial change is observed in the FTIR spectrum (Fig. 3c).
The Ge–O stretch (800–1000 cm21) cannot be resolved from the
spectra due to its overlap with the N–H wagging mode.27
However, no appreciable increase in the Ge–O stretching mode
at 850 cm21 is observed, even after 5 months of exposure in air.
This result suggests that formation of GeO2 is minimal,
presumably due to the encapsulation of the Ge NC surface by
the oleylamine ligands.
As an indirect measure of amine surface passivation, we
compare the FTIR spectrum of pure oleylamine (Fig. 3a) with
that of Ge NCs, taken with respect to the Ar ambient background
(Fig. 3b). Prior to sampling, the Ge NCs are washed{ to remove
excess surfactants and by-products. The presence of oleylamine
group on Ge NCs (Fig. 3b) is indicated by the N–H wagging mode
from 650–900 cm21; NH2 bending modes at 909, 964, and
993 cm21; and NH2 scissor mode at 1568 cm21 26
The spectrum
.
shown in Fig. 3b also reveals the characteristic peak of the C–N
stretch at 1042 cm21 which suggests that the C–N bonds in amine
groups, and therefore oleylamine ligands, remain intact, encapsu-
lating the Ge NCs. The peak at 1468 cm21 is associated with the
Based on the FTIR data, we deduce the following reaction
mechanism. Upon injection of Ge[N(SiMe3)2]2 and oleylamine
into hot octadecene solution, the Ge–N bond is cleaved by the
oleylamine group and Ge is subsequently reduced. The reported
Ge–N and Si–N bond strengths are 55 and 78 kcal mol21
,
respectively, which indicate that Ge–N is more susceptible than
Si–N to cleavage.28 Furthermore, primary amines do not dissociate
in the presence of Ge, but upon absorption on Si, primary
amines will undergo N–H dissociation, resulting in the formation
of a Si–H bond with a characteristic IR stretching mode at
2066 cm21 26
. The absence of a Si–H peak in these spectra indicates
that no detectable Si is present on the sample. The absence of Si in
the Ge NCs is confirmed through energy dispersive X-ray
spectroscopy (EDS) investigations (shown in Fig. 4).
The photoluminescence (PL) of Ge NCs in a toluene solution is
conducted at 1 atm of Ar to confirm the quantum confinement
effect and shown in Fig. 5. UV-visible (UV-vis) spectroscopy
shows continuous absorption across the spectrum with increasing
absorption near the UV region. The NCs are excited with 325 and
442 nm HeCd laser lines to excite NCs of different sizes. The full
width at half maximum (FWHM) of the excitation from the two
laser sources is 2 nm. Since the particles are polydispersed, different
emission wavelengths are observed. The emission spectra exhibit
strong luminescence at 375 and 500 nm, corresponding to different
Fig. 2 Size distribution of Ge NCs showing an average of 7 ¡ 4 nm
from 119 particles, with y95% less than 8 nm.
Fig. 3 FTIR spectrum of (a) oleylamine, (b) Ge NCs after being washed
(time 5 0), and (c) air exposed Ge NCs (time 5 5 months). The
characteristic vibrational modes of NH2 indicate that oleylamine
encapsulates Ge NCs.
Fig. 4 EDS spectrum of Ge NCs dispersed on a carbon coated TEM
copper grid.
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of 32 mmol octadecene, which is heated to 300 uC, an orange solution of
1.0 mmol Ge[N(SiMe3)2]2 in 12 mmol oleylamine is rapidly injected,
whereupon the mixture turns dark brownish-red. The solution is held at
285 uC for 5 min and then allowed to cool to room temperature. After
completion, toluene or chloroform is added, and the solution is extracted
twice with methanol. Excess acetone is used to precipitate the Ge NCs. The
precipitate is separated by centrifugation (33000 rpm for 25 min) and then
re-dispersed in toluene, hexane, or chloroform. The resulting colloid is
stable at room temperature under inert conditions. TEM samples are
prepared in an Ar-filled glovebox by placing a drop of Ge NCs dispersed in
toluene onto a 30-mesh carbon coated copper TEM grid and transported
to the TEM instrument (JEOL 2010) under an atmosphere of Ar. An EDS
spectrum is also collected on the same TEM sample. IR spectra of
oleylamine or Ge NC particles in KBr pellet are obtained with 2 cm21
resolution and 32 scans. Samples for UV-vis and PL (He–Cd laser, CVI
DK 480 monochromator, cooled GaAs photomultiplier) are prepared by
diluting Ge NCs in toluene, hexane or chloroform solution under Ar.
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particle sizes. The corresponding particle sizes based on a
theoretical calculation of the emission wavelengths29 are 5 and
7 nm respectively, in agreement with our observation. Consistent
with published results, the size-dependence manifests the quantum
confinement effect.30
In summary, we have demonstrated a simple, convenient, single-
source Ge NC synthesis via thermolysis of the Ge(II) precursor
Ge[N(SiMe3)2]2. This simple reaction is performed at 1 atm
without the use of reducing agents and with no salt by-products.
Detailed studies are underway to control and improve the Ge NC
size and distribution, and morphology through variations
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This work is supported by the US Office of Basic Energy
Science and the US Department of Energy under Contract DE-
AC04-94AL85000. S.M.H., H.G., and C.J.B. acknowledge the
National Science Foundation (DMR-0094145) for support of this
work. H.G. acknowledges W. Chen, J. Lee, and M. Osinski at the
Center for High Technology Materials for assistance with the PL
measurements. Sandia is a multiprogram laboratory operated by
Sandia Corporation, a Lockheed Martin Company, for the United
States Department of Energy.
Henry Gerung,a Scott D. Bunge,b Timothy J. Boyle,*b
C. Jeffrey Brinkerab and Sang M. Hana
aDepartment of Chemical and Nuclear Engineering, University of New
Mexico, 209 Farris Engineering Center, Albuquerque, NM 87131, USA.
E-mail: meister@unm.edu; Fax: +1 505-277-5433; Tel: +1 505-277-3118
bAdvanced Materials Laboratory, Sandia National Laboratories, 1001
University Boulevard SE, Albuquerque, NM 87106, USA.
E-mail: tjboyle@sandia.gov; Fax: +1 505-272-7336;
Tel: +1 505-272-7625
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Notes and references
{ Experimental: All reactions are performed under standard air sensitive
synthetic methods employing a dual manifold Schlenk-line. To a solution
1916 | Chem. Commun., 2005, 1914–1916
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