Alkyl-terminated crystalline Ge nanoparticles prepared from NaGe: Synthesis, functionalization and optical properties

Article abstract of DOI:10.1016/j.jssc.2008.06.018

High purity NaGe was directly prepared by a low-temperature reaction of NaH and Ge. The product was characterized by powder X-ray diffraction (XRD), scanning electron microscopy (SEM), and energy-dispersive X-ray (EDX) spectroscopy. This material is a useful starting reagent for the preparation of Ge nanoparticles. Hydrogen-terminated germanium (Ge) nanoparticles were prepared by reaction of NaGe with NH₄Br. These Ge nanoparticles could be prepared as amorphous or crystalline nanoparticles in quantitative yields and with a narrow size distribution. The nanoparticles were functionalized via thermally initiated hydrogermylation with 1-eicosyne, CH₃(CH₂)₁₇C≡CH to produce alkyl-terminated Ge nanoparticles. The modified Ge nanoparticles were characterized by powder XRD, transmission electron microscopy (TEM), Fourier transform infrared (FT-IR) and Raman spectroscopy, and photoluminescence (PL) spectroscopy. The alkyl-functionalized Ge nanoparticles can be expected to have promising applications in many technological and biological areas.

Full text of DOI:10.1016/j.jssc.2008.06.018

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Journal of Solid State Chemistry 181 (2008) 1628– 1633
Contents lists available at ScienceDirect
Journal of Solid State Chemistry
journal homepage: www.elsevier.com/locate/jssc
Alkyl-terminated crystalline Ge nanoparticles prepared from NaGe:
Synthesis, functionalization and optical properties
Ã
Xuchu Ma, Fengyi Wu, Susan M. Kauzlarich
Department of Chemistry, University of California at Davis, One Shields Avenue, Davis, CA 95616, USA
a r t i c l e i n f o
a b s t r a c t
Article history:
High purity NaGe was directly prepared by a low-temperature reaction of NaH and Ge. The product was
characterized by powder X-ray diffraction (XRD), scanning electron microscopy (SEM), and energy-
dispersive X-ray (EDX) spectroscopy. This material is a useful starting reagent for the preparation of Ge
nanoparticles. Hydrogen-terminated germanium (Ge) nanoparticles were prepared by reaction of NaGe
Received 10 May 2008
Received in revised form
9
June 2008
Accepted 11 June 2008
Available online 13 June 2008
with NH
quantitative yields and with a narrow size distribution. The nanoparticles were functionalized via
thermally initiated hydrogermylation with 1-eicosyne, CH (CH
4
Br. These Ge nanoparticles could be prepared as amorphous or crystalline nanoparticles in
Keywords:
3
2
)
17CꢀCH to produce alkyl-terminated Ge
Zintl salt NaGe
Ge nanoparticles
Functionalization
Photoluminescence
nanoparticles. The modified Ge nanoparticles were characterized by powder XRD, transmission electron
microscopy (TEM), Fourier transform infrared (FT-IR) and Raman spectroscopy, and photoluminescence
(PL) spectroscopy. The alkyl-functionalized Ge nanoparticles can be expected to have promising
applications in many technological and biological areas.
&
2008 Elsevier Inc. All rights reserved.
1. Introduction
In this paper, we report the synthesis of Ge nanoparticles from
the reaction of Zintl salt, NaGe, with NH Br. We also present a
4
Group 14 semiconductor nanoparticles have useful applica-
tions in many areas of technology and biology. Silicon in
nanoform, for example, has shown enhanced photovoltaic effects
new, simple synthetic route to NaGe from the reaction of NaH
with Ge powder that does not involve the use of niobium or
4
tantalum tubing. The reaction of a metal silicide with NH Br has
[
1], and thermoelectric effects [2,3]. While there has been intense
been shown to be successful in synthesizing hydrogen-terminated
Si nanoparticles [31,32] and we have extended this route to
produce hydrogen-terminated Ge nanoparticles. This is a simple
oxidation–reduction route that provides gram quantities of high
purity Ge nanoparticles with hydrogen on the surface. The
resulting nanoparticles can be easily reacted with alkenes or
alkynes via a hydrogermylation reaction [33] to produce alkyl
bonded, functionalized nanoparticles.
interest in solution synthesis of Si nanoparticles [4], Ge nano-
particles have received significantly less attention, perhaps due to
controversy over the origin of light emission from small, surface
passivated nanoparticles [5]. However, Ge nanoparticles have
many applications also, such as precursors for thin film formation
[
6–8], for porous germanium (Ge) [9], and, regardless of the origin
of light emission, for bioprobes [10,11].
Recently, Ge nanoparticles have been prepared via solution
routes by a number of approaches, such as metathesis of Ge Zintl
salts with GeCl
4
[12–14], supercritical thermolysis [15,16], reduc-
2. Experimental section
tion of Ge(II) [17] and Ge(IV) precursors [18–22], and thermal
decomposition of organogermanes [22–25]. Though great pro-
gress has been made in recent years, the synthesis of Ge
nanoparticles via solution routes remains an intense area of
research [26]. In recent years, the focus has been on the synthesis
of gram quantities of Ge nanoparticles [27], control of size [21,28],
and the functionalization of their surface [29,30].
2.1. Materials
Sodium hydride powder (NaH, 95%, Sigma-Aldrich), Ge powder
(Ge, X99%, Sigma-Aldrich), and 1-eicosyne (94%, GFS) were used
as purchased without further treatment. Ammonium bromide
(NH Br, X99.99%, Sigma-Aldrich) was dried under dynamic
4
vacuum at 100 1C for 12 h prior to usage. Diethylene glycol dibutyl
ether (X98%, Sigma-Aldrich) was degassed before using. Glass-
ware was dried overnight at 120 1C and transferred hot into an N -
2
Ã
filled glove box. Unless otherwise stated, all samples are oxygen-
sensitive and precautions must be taken to avoid exposure to
Corresponding author. Fax: +1530 752 8995.
E-mail address: smkauzlarich@ucdavis.edu (S.M. Kauzlarich).
0
022-4596/$ - see front matter & 2008 Elsevier Inc. All rights reserved.
doi:10.1016/j.jssc.2008.06.018

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X. Ma et al. / Journal of Solid State Chemistry 181 (2008) 1628–1633
1629
oxygen. All manipulations were carried out under dry N
2
or Ar gas
sided carbon tape on specimen holders. The specimen holders
were kept in a N -filled jar in a glove box and quickly transferred
(
X99% purity), in either a glove box filled with N or on a standard
2
2
Schlenk line, using standard anaerobic and anhydrous techniques.
NaGe was prepared from NaH and Ge powder. The pre-milled
mixture of NaH and Ge (molar ratio of 1.2:1) was placed into an
alumina crucible with an alumina cover in a quartz tube, and then
heated at 270 1C for 3 h under flowing argon.
to the SEM instrument. The time for exposing to air was less than
3 s. TEM samples were prepared by dipping holey-carbon-coated,
400-mesh electron microscope grids into the hexane colloid
solution and drying them at 120 1C overnight. Fourier transform
infrared (FT-IR) spectroscopic data of the oily phase were obtained
using a Shimadzu IR Prestige 21 equipped with a diffuse
reflectance accessory. Photoluminescence (PL) spectra of the oily
phase containing the Ge nanoparticles were measured on a
FluoroMax-3P fluorometer, where the Ge nanoparticles were
dispersed in hexane.
2
.2. Preparation of crystalline ð1Þ or amorphous ð2Þ hydrogen-
terminated germanium nanoparticles
Hydrogen-terminated crystalline Ge nanoparticles were
synthesized by a solid-state process (1) or a solution process (2).
(
1) Solid-state process: NaGe (0.1912 g, 2 mmol) and NH
4
Br
3. Results and discussion
(0.3916 g, 4 mmol) were mixed in a glove box and placed into an
alumina crucible, which was then placed in a one end sealed
quartz tube with stopcock at the other end. The quartz tube was
seated in a horizontal tube furnace under flowing of argon and
heated at 300 1C for 12 h. This reaction produced hydrogen-
terminated crystalline Ge nanoparticles and NaBr.
The composition and morphology of NaGe was determined by
XRD analysis, SEM, and EDX, as shown in Fig. 1. Fig. 1a shows a
XRD pattern of the obtained NaGe (upper curve), which matches
well with that of theoretical NaGe powder (lower curve)
(
2) Solution process: NaGe (0.1912 g, 2 mmol) and NH
4
Br
(0.3916 g, 4 mmol) were added to a Schlenk flask in the glove
box. Degassed diethylene glycol dibutyl ether (80 mL) was added
via a cannula to the flask and heated at 250 1C for 12 h under
vacuum.
2.3. Synthesis of alkyl-terminated germanium nanoparticles
The product from process (1) was mixed with degassed
diethylene glycol dibutyl ether (80 mL) in a Schlenk flask. 1.5 g
of 1-eicosyne was added and the reaction mixture was heated at
2
50 1C for 12 h. The mixture was allowed to cool to room
temperature resulting in a black solid with a yellow solution.
The yellow solution was transferred via cannula to another
Schlenk flask and vacuum-dried; a viscous orange oil was
obtained. The black solid was collected and washed with excessive
amounts of hexane, alcohol and distilled water successively to
remove unreacted 1-eicosyne and NaBr, and then dried en vacuo.
After the reaction mixture of process (2) was cooled down to
room temperature, 1.5 g of 1-eicosyne was added and heated at
2
50 1C for 12 h while stirring. This mixture was allowed to cool to
room temperature and a black solid remained along with a yellow
solution. The yellow solution was transferred via cannula to
another Schlenk flask and vacuum-dried; a viscous orange oil was
obtained. The black solid was collected and washed with excessive
amounts of hexane, alcohol and distilled water to remove
unreacted 1-eicosyne and NaBr and then dried en vacuo.
2.4. Characterization
Powder X-ray diffraction (XRD) data were collected on an INEL
˚
CPS 120 diffractometer with Co radiation (l ¼ 1.78897 A) for NaGe
(using an air sensitive holder) and Bruker D8 Advance X-ray
˚
diffractometer with CuKa radiation (l ¼ 1.54178 A) for Ge nano-
particles that are the solid phase obtained from these reactions.
Raman spectra were obtained with a Renishaw RM1000 Research
Laser Raman Microscope and an Argon laser operating at an
excitation line of 514 nm for the Ge nanoparticles that are the
solid phase obtained from these reactions. Transmission electron
microscopy (TEM) was performed on a Philips CM-12, operating at
8
0 kV. Hitachi S-800T scanning electron microscopes (SEM) and
an Oxford INCA energy-dispersive X-ray (EDX) spectrometer were
employed to analyze chemical composition, with the accelerating
voltage of 20 kV. SEM and EDX sample was prepared by standard
techniques where grain dispersions were supported on double-
Fig. 1. (a) XRD patterns of NaGe: prepared from optimized reactions of NaH with
Ge (upper curve) and calculated from single-crystal X-ray diffraction data (lower
curve), and (b) SEM image of NaGe and the inset is an EDX spectrum.

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X. Ma et al. / Journal of Solid State Chemistry 181 (2008) 1628–1633
calculated from single-crystal XRD data [34]. Lattice parameters
obtained from the powder XRD data of the as-prepared product
originates from oxidization of the sample surface that may occur
during the sample transfer from the glove box to SEM instrument.
No N or Br peaks can be found in the EDX spectrum, indicating
that the as-prepared product is composed of pure NaGe.
˚
˚
˚
are a ¼ 12.32770.005 A, b ¼ 6.70570.002 A, c ¼ 11.41770.004 A,
and b ¼ 120.0070.041, consistent with the published parameters
˚
˚
˚
of NaGe (a ¼ 12.33 A, b ¼ 6.70 A, c ¼ 11.42 A, and b ¼ 120.01). Two
small peaks at 21.81 and 53.21 in Fig. 1a are belonging to the air
sensitive holder used to obtain the data. Fig. 1b is a SEM image of
the resulting NaGe, indicating that the diameter of NaGe particles
is in the range of 1.5–3 mm. An EDX spectrum of the products
obtained from optimized reactions of NaH with Ge is shown in
inset of Fig. 1b. From inset of Fig. 1b, Na and Ge elements are
detected and atomic ratio of Na:Ge is 1:1.05, consistent with the
stoichiometric formula of NaGe. The carbon (C) signal comes from
the carbon tape (support), while the oxygen (O) signal most likely
NaGe is reacted with NH
4
Br to produce hydrogen-terminated
Ge nanoparticles according to the reaction:
NaGe þ NH Br ! GeÞ ꢁ H þ NH þ NaBr
4
3
where Ge)ꢁH represents hydrogen-terminated Ge nanoparticles.
The hydrogen-terminated Ge material is stable in air for less than
1 h.
This reaction was run either under flowing argon at 300 1C
(process (1)) or in diethylene glycol dibutyl ether at 250 1C
(process (2)). Fig. 2a and b show powder XRD patterns of the solid
phase products obtained from processes (1) and (2), respectively,
after capping with 1-eicosyne. For products prepared from (1),
five diffraction peaks at 27.31, 45.31, 53.81, 66.11 and 73.01
correspond to the (111), (220), (311), (400) and (331) reflections
of cubic-structure crystalline Ge (JCPDS files no. 04-0545,
˚
a ¼ 5.657 A). In Fig. 2b, the observation of two broad diffraction
peaks at approximately 27.31 and 50.01 together with additional
crystalline peaks suggests a mixture of amorphous and crystalline
Ge [20,21]. It is noteworthy that no signals from Ge dioxide are
observed in the XRD patterns, though the powders are washed
with distilled water. This is assumed to be the case since the
2
surfaces have been terminated with alkane and GeO is water
soluble. The yellow oily phase did not give rise to an observable
powder diffraction, presumably because the nanoparticles were
embedded in an organic matrix similar to what we have observed
in reduction reactions to form Ge nanoparticles [21].
Morphology and size distribution of the alkyl-terminated Ge
nanoparticles obtained from processes (1) and (2) were observed
by TEM. The nanoparticles from the oily phase, as shown in Fig. 3,
are isolated from each other and well-dispersed on the grid. Fig.
3
a and b exhibit typical micrographs of alkyl-terminated crystal-
Fig. 2. XRD patterns of Ge nanoparticles obtained by: (a) process (1) (upper curve)
and (b) process (2) (middle curve). Calculated peaks with indices for crystalline Ge
are provided.
line and amorphous Ge nanoparticles, respectively. The size
distribution for crystalline Ge nanoparticles is 4.071.0 nm and
Fig. 3. TEM images and size distribution histograms of alkyl-terminated Ge nanoparticles: (a) crystalline and (b) amorphous.

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1631
Fig. 5. Raman spectra of alkyl-terminated (a) crystalline and (b) amorphous Ge
nanoparticles.
asymmetric bends of the methyl group, respectively. Although FT-
IR spectroscopy does not provide definitive proof that organic
ligands are bonded to the Ge nanoparticle surface, results from FT-
IR spectra are consistent with the presence of a hydrocarbon layer
on the particle surface [17].
Raman spectroscopy could be used to evaluate bonding in the
Ge core by monitoring Ge–Ge optical phonon (OP) vibration [26].
Fig. 5 shows Raman spectra of the Ge nanoparticles that are the
solid phase obtained from these reactions. Fig. 5a is a Raman
spectrum of Ge nanoparticles obtained from process (1), where a
ꢁ
1
peak positioned at ꢂ297 cm can be observed. According to the
literature, this peak can be attributed to crystalline Ge where the
ꢁ1
Ge–Ge stretch is 302 cm [26]. Fig. 5b is a Raman spectrum of Ge
nanoparticles from process (2), showing two Raman shifts at
ꢁ
1
about 154 and 273 cm . These two Raman peaks are character-
istic of amorphous Ge [35].
PL spectroscopy was used to investigate the optical properties
of the Ge nanoparticles in the oily phase. Fig. 6a and b show the
normalized PL spectrum of alkyl-terminated crystalline and
amorphous Ge nanoparticles in hexane prepared from processes
Fig. 4. FT-IR spectra of (a) hydrogen-terminated crystalline Ge and (b) alkyl-
terminated Ge nanoparticles obtained from processes (1) and (2) and terminated
via hydrogermylation, respectively.
(1) and (2), respectively. Fig. 6a shows a series of intense
luminescence peaks with a relatively narrow region from 368 to
415 nm, with excitation from 320 to 350 nm. The maximum
intensity emission is centered at 368 nm with an excitation
wavelength of 320 nm. A theoretical calculation shows that
that for amorphous Ge nanoparticles is 1.970.5 nm. Size distribu-
tion histograms of Ge nanoparticles are shown in Fig. 3a and b,
and these are based on surveys of 500 particles from different
regions of the TEM grids. No aggregation is observed.
indirect band-gap energy (Egap) of Ge nanocrystal is size-
Fig. 4a shows FT-IR spectrum of hydrogen-terminated crystal-
line Ge, and Fig. 4b presents those of alkyl-terminated crystalline
and amorphous Ge nanoparticles from the oily phase obtained
from process (1) and (2), respectively. The spectra of both samples
correspond well with stretches expected for the organic groups.
The IR absorption spectrum of the hydrogen-terminated crystal-
line Ge, shown in Fig. 4a, exhibits the characteristic stretching
dependent and the calculated Egap for ꢂ5 nm Ge nanocrystals is
about 2.1 eV [36,37]. This is not consistent with the observed PL
excitation spectra in Fig. 6a. Therefore, as a possible explanation,
we suggest that PL emission from Ge nanocrystallite positioned
at ꢂ400 nm might be due to two reasons: (1) vibrations of
hydrocarbon layer absorbed on the surface of these synthesized
Ge nanocrystals, (2) defects on surface of Ge nanocrystals [37]. Fig.
6b shows a series of intense luminescence peaks with a relatively
narrow region from 411 to 438 nm, with excitation from 350 to
380 nm. The maximum intensity emission is centered at 423 nm
with an excitation wavelength of 360 nm. Changes in the
excitation wavelength excite different size populations of nano-
particles, resulting in the variation of emission wavelengths.
Though some questions about the PL spectra remain, the larger
increase in energy of the PL relative to band-gap of bulk Ge and
monotonic shift in PL wavelength with excitation wavelength
ꢁ
1
vibration mode band n(Ge–H) at 2100 cm [33], and the other
ꢁ
1
two peaks at 3500, and 1640 cm correspond to N–H stretching
and bending modes. The N–H modes may be due to the presence
4
of NH Br, as this sample was not washed with water, but the
solution of the mixture was drop dried on a KBr salt plate. The FT-
IR spectra in Fig. 4b reveal three peaks at 2961, 2925, and
ꢁ
1
2
872 cm that correspond to C–H stretching modes for CH
CH groups, indicating the presence of hydrocarbon species. The
peaks at 1464 and 1380 cm are attributed to the symmetric and
2
and
3
ꢁ1

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Fig. 7. XRD patterns of Ge nanoparticles from process (1) obtained at different
times: (a) 12 h, (b) 8 h and (c) 2 h.
by a two-step refluxing process (2) and thermal hydrogermyla-
tion. The average size of Ge nanoparticles ranges from 1 to 5 nm
with
a narrow size distribution. Such alkyl-terminated Ge
nanoparticles displayed strong PL in the blue region of the visible
spectrum. The growth conditions of this new method can be
further optimized for synthesis of crystalline nanoparticles.
Acknowledgment
We thank NSF (DMR-0600742) and Nanohmics, Inc. for
funding.
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