Synthesis and photoluminescent properties of size-controlled germanium nanocrystals from phenyl trichlorogermane-derived polymers

Article abstract of DOI:10.1021/ja710286a

We report the preparation of luminescent oxide-embedded germanium nanocrystals (Ge-NC/ GeO₂) by the reductive thermal processing of polymers derived from phenyl thchlorogermane (PTG, C₆H ₅-GeCl₃). Sol-gel processing of PTG yields air-stable polymers with a Ge:O ratio of 1:1.5, (C₆H₅GeO _1.5)_n, that thermally decompose to yield a germanium rich oxide (GRO) network. Thermal disproportionation of the GRO results in nucleation and initial growth of oxide-embedded Ge-NC, and subsequent reaction of the GeO₂ matrix with the reducing atmosphere results in additional nanocrystal growth. This synthetic method affords quantitative yields of composite powders in large quantities and allows for Ge-NC size control through variations of the peak thermal processing temperature and reaction time. Freestanding germanium nanocrystals (FS-Ge-NC) are readily liberated from Ge-NC/GeO₂ composite powders by straightfoward dissolution of the oxide matrix in warm water. Composites and FS-Ge-NC were characterized using thermogravimetric analysis (TGA), Fourier transform infrared spectroscopy (FTIR), Raman spectroscopy, X-ray powder diffraction (XRD), X-ray photoelectron spectroscopy (XPS), transmission electron microscopy (TEM), selected area electron diffraction (SAED), energy dispersive X-ray spectroscopy (EDX), and photoluminescence (PL) spectroscopy.

Full text of DOI:10.1021/ja710286a

Published on Web 02/27/2008
Synthesis and Photoluminescent Properties of
Size-Controlled Germanium Nanocrystals from Phenyl
Trichlorogermane-Derived Polymers
Eric J. Henderson, Colin M. Hessel, and Jonathan G. C. Veinot*
Department of Chemistry, UniVersity of Alberta, Edmonton, Alberta, Canada
Received November 13, 2007; E-mail: jveinot@ualberta.ca.
Abstract: We report the preparation of luminescent oxide-embedded germanium nanocrystals (Ge-NC/
GeO2) by the reductive thermal processing of polymers derived from phenyl trichlorogermane (PTG, C6H5-
GeCl3). Sol-gel processing of PTG yields air-stable polymers with a Ge:O ratio of 1:1.5, (C6H5GeO1.5)n,
that thermally decompose to yield a germanium rich oxide (GRO) network. Thermal disproportionation of
the GRO results in nucleation and initial growth of oxide-embedded Ge-NC, and subsequent reaction of
the GeO2 matrix with the reducing atmosphere results in additional nanocrystal growth. This synthetic method
affords quantitative yields of composite powders in large quantities and allows for Ge-NC size control through
variations of the peak thermal processing temperature and reaction time. Freestanding germanium
nanocrystals (FS-Ge-NC) are readily liberated from Ge-NC/GeO2 composite powders by straightfoward
dissolution of the oxide matrix in warm water. Composites and FS-Ge-NC were characterized using
thermogravimetric analysis (TGA), Fourier transform infrared spectroscopy (FTIR), Raman spectroscopy,
X-ray powder diffraction (XRD), X-ray photoelectron spectroscopy (XPS), transmission electron microscopy
(TEM), selected area electron diffraction (SAED), energy dispersive X-ray spectroscopy (EDX), and
photoluminescence (PL) spectroscopy.
mal conversion and serve as biomarkers for cell signaling.8
While the interest in the optical properties of Ge nanoparticles
remains strong, much attention has recently been given to the
electronic properties of germanium nanostructures. Specifically,
germanium nanowires have shown promise as microelectronic
interconnects,9 in large part because of the development of
synthetic procedures that yield large quantities of well-defined
materials.10-13 In addition, oxide-embedded Ge-NC exhibit
promising charge retention properties with potential applications
in nonvolatile memory.14-16 It is thought that the smaller band
gap of Ge may provide improved data retention and write and
erase speeds relative to commercially available Si-based systems.
1. Introduction
The study of Group IV semiconductor nanostructures has
received much attention over the past several decades, due in
large part to their unique optical and electronic properties. The
discovery of visible photoluminescence (PL) from silicon and
germanium nanostructures1,2 has not only challenged bulk
semiconductor band structure models3 but also significantly
expanded the potential applications of these elemental semi-
conductors.4 Optical applications that once seemed impossible
because of the forbidden indirect band gap transition of Si and
Ge may soon become reality as a result of size-induced PL. In
addition, freestanding (FS) Group IV semiconductor nanocrys-
tals have been proposed as nontoxic diagnostic and therapeutic
agents.5 These systems afford significant advances over status
quo toxic compound semiconductor analogues that require
elaborate surface passivation and encapsulation.6 In this regard,
recent reports have shown proof-of-concept application of
freestanding silicon nanocrystals (FS-Si-NC) as luminescent
biological probes.7 In addition, freestanding germanium nanoc-
rystals (FS-Ge-NC) have also been shown to exhibit photother-
While the optical characteristics of Si-NC have been studied
extensively, Ge-NC have not been investigated in as much detail.
An important difference between Si and Ge arises because of
the smaller carrier effective masses and larger dielectric constant
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J. AM. CHEM. SOC. 2008, 130, 3624-3632
10.1021/ja710286a CCC: $40.75 © 2008 American Chemical Society

Size-Controlled Germanium Nanocrystals
A R T I C L E S
associated with Ge.17 As a result, interest in Ge-NC is increasing
in part because it is reasonable that quantum confinement effects
will emerge for larger particle sizes. It is well-known that
quantum confinement-induced photoluminescence from semi-
conductor nanocrystals is size-dependent. Charge retention
properties of oxide-embedded nanocrystals have also been
shown to be strongly dependent on particle size distributions.18
Still, the effects of nanocrystal composition and shape on these
properties are poorly understood and remain the subject of active
research. Hence, it is necessary to develop versatile synthetic
techniques that yield materials of well-defined size, shape, and
composition before the full potential of Ge-NCs can be realized
and practically implemented in the aforementioned applications.
Establishing such methods will also facilitate a fundamental and
comprehensive understanding of size-dependent properties of
Ge-NC.
and electrochemical46 etching of Ge wafers, and laser-induced
air breakdown processing.47 Although these synthetic techniques
have proven successful in the preparation of size-selected Ge-
NC, the use of elaborate experimental infrastructure and
specialized precursors limit their widespread use. In addition,
challenges including limited sample sizes, broad size distribu-
tions, and contamination from residual reaction byproducts could
limit the practicality of these methods. Of particular significance,
it has recently been proposed that residual organic byproducts
from solution-based syntheses could hinder accurate optical
characterization.48
Reports employing substoichiometric oxides of germanium
(GeOx, 0 e x e 2), germanium rich oxides (GRO), as Ge-NC
precursors are surprisingly sparse throughout the literature.
Analogous oxides have been studied extensively for Si (SROs)
and are versatile precursors for the generation of oxide embed-
ded Si-NC. We recently demonstrated that near-monodisperse
oxide-embedded Si-NC could be prepared by reductive thermal
processing of hydrogen silsesquioxane (HSQ, Si8O12H8).49,50 As
with other SiOx-based systems (0 e x e 2), these substoichio-
metric silicon oxides disproportionate into elemental Si and the
stoichiometric oxide, SiO2. There are only limited examples of
GeOx films that have been prepared by magnetron cosputtering51
and electron beam15,52 evaporation. As is the case for their
silicon-based equivalents, GROs disproportionate with appropri-
ate thermal processing and under ideal conditions yield Ge-NC
embedded in a GeO2-like matrix. Although successful, these
methods produced small quantities of Ge-NC and provided
limited control over particle size and size distribution.
To date, there has been much effort devoted to establishing
robust synthetic techniques that meet these requirements.
Existing methods for preparing FS-Ge-NC include solution-
phase reduction of Ge(II)19 and Ge(IV)17,20-23 precursors,
metathesis of Ge Zintl salts with GeCl4,24-26 supercritical
thermolysis,27,28 and solution-phase thermal decomposition.29-31
Oxide-embedded Ge-NC have been prepared by chemical vapor
deposition of GeO,16,32 ion implantation33,34 and cosputtering2,35-40
followed by thermal annealing, sol-gel processing,41-44 stain45
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Here we report the preparation of size-controlled germanium
nanocrystals embedded in germanium oxide (Ge-NC/GeO2) via
thermal processing of an organic functionalized germanium rich
oxide obtained from the hydrolysis and subsequent condensation
of phenyl trichlorogermane (PTG, C6H5GeCl3). The reaction
conditions of the present synthetic strategy were chosen to yield
a GRO with a Ge:O ratio of 1:1.5, the same “metal”:oxide ratio
exhibited by HSQ (Vide supra). The resulting sol-gel polymers,
(C6H5GeO1.5)n, obtained from the condensation of PTG, were
prepared using standard laboratory techniques, and subsequent
reductive thermal processing in a conventional tube furnace
yielded oxide embedded Ge-NC. The presented preparative
method yields near-monodisperse, oxide-embedded, luminescent
nanocrystals whose size may be tailored by varying thermal
processing conditions. The present approach also enables
straightforward liberation of FS-Ge-NC from the surrounding
oxide matrix upon exposure to warm (60 °C) water. The aqueous
liberation procedure described herein yields size-controlled FS-
Ge-NC bearing oxide/hydroxide surface termination.
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9
J. AM. CHEM. SOC. VOL. 130, NO. 11, 2008 3625

A R T I C L E S
Henderson et al.
Table 1. Summary of Samples Investigated. All Samples Were
Processed in Slightly Reducing Atmosphere (5% H2/95% Ar)
7.0 software. (C6H5GeO1.5)n polymer samples were placed in a platinum
pan and heated in 5% H2/95% Ar from room temperature to 900 °C at
20 °C/min.
Fourier Transform Infrared Spectroscopy (FTIR). FTIR spec-
troscopy of free-flowing powders of 1-9 was performed using a Nicolet
Magna 750 IR spectrophotometer.
Raman Spectroscopy. Raman spectroscopy was performed using
a Renishaw inVia Raman microscope equipped with a 785 nm diode
laser and a power of 3.98 mW on the sample.
X-ray Powder Diffraction (XRD). XRD was performed using an
INEL XRG 3000 X-ray diffractometer equipped with a Cu KR radiation
source (λ ) 1.54 Å). Bulk crystallinity for samples 1-9 was evaluated
on finely ground samples mounted on a low-intensity background
silicon (100) sample holder.
processing
processing
time (h)
mean crystal diameter
(XRD) (nm)a
sample
temp (°C)
1
2
3
4
5
6
7
8
9
n/a
n/a
1
1
1
1
1
2
5
24
n/a
n/a
n/a
6.2
10.3
16.3
7.9
11.5
12.7
400
500
525
550
600
525
525
525
a Values were calculated from Debye-Scherrer analysis of XRD peak
broadening.
X-ray Photoelectron Spectroscopy (XPS). XPS was performed
using a Kratos Axis Ultra instrument operating in energy spectrum mode
at 210 W. The base pressure and operating chamber pressure were
maintained at e10-7 Pa. A monochromatic Al KR source (λ ) 8.34
Å) was used to irradiate the samples, and the spectra were obtained
with an electron takeoff angle of 90°. To control sample charging, the
charge neutralizer filament was used during all experiments. Wide
survey spectra were collected using an elliptical spot with 2 and 1 mm
major and minor axis lengths, respectively, and a 160 eV pass energy
with a step of 0.33 eV. CasaXPS (Vamas) software was used to process
high-resolution spectra. All spectra were calibrated to the C1s emission
(284.8 eV). After calibration, the background from each spectrum was
subtracted using a Shirley-type background to remove most of the
extrinsic loss structure. Sample compositions were determined from
the emission intensities of the survey spectra using appropriate
sensitivity factors.
Transmission Electron Microscopy (TEM). TEM, energy disper-
sive X-ray spectroscopy (EDX), and selected-area electron diffraction
(SAED) were performed using a JEOL-2010 (LaB6 thermionic emission
source) electron microscope with an accelerating voltage of 200 keV.
TEM samples of FS-Ge-NC were drop-coated from an ethanol
suspension onto a carbon-coated copper grid.
Photoluminescence Spectroscopy (PL). PL spectra were evaluated
at room temperature using the 325 nm line of a He-Cd laser excitation
source, and emission was detected with a fiber-optic digital charge-
coupled device (CCD) spectrometer whose spectral response was
normalized using a standard blackbody radiator. Thin films of finely
ground powders of 1-9 were drop-coated onto silicon wafers from an
ethanol suspension. PL spectra of cloudy ethanol solutions of FS-Ge-
NC were acquired in an identical manner.
The formation, growth, and optical properties of oxide
embedded Ge-NC described herein were evaluated as a function
of processing time and temperature. These materials were
characterized using thermogravimetric analysis (TGA), Fourier
transform infrared spectroscopy (FTIR), Raman spectroscopy,
X-ray diffraction (XRD), X-ray photoelectron spectroscopy
(XPS), transmission electron microscopy (TEM), energy dis-
persive X-ray spectroscopy (EDX), selected-area electron dif-
fraction (SAED), and photoluminescence (PL) spectroscopy.
2. Experimental Details
Reagents and Materials. Phenyl trichlorogermane (C6H5GeCl3,
98%) was purchased from Sigma Aldrich, used as received, and stored
in a nitrogen glovebox in subdued light. High-purity DI water (18.2
MΩ/cm) was obtained from a Barnstead Nanopure Diamond purifica-
tion system. Isopropyl alcohol (IPA, g99.5%) was purchased from
Fisher Scientific and used as received.
Bulk (C6H5GeO1.5
) Polymer Preparation (1). In a typical syn-
n
thesis, 16 mL of a 65% (v/v) solution of IPA in DI water was added
dropwise to 4.5 mL of C6H5GeCl3 (7.13 g, 27.8 mmol) under inert
atmosphere with vigorous stirring, using standard Schlenk techniques.
The clear and colorless C6H5GeCl3 immediately turned cloudy white
as the IPA solution was added, and hydrolysis of C6H5GeCl3 was
confirmed by monitoring the pH of the reaction mixture (pH ) 1).
The cloudy white mixture was gently heated at 60 °C and stirred for
24 h to ensure complete condensation. The resulting white solid
precipitate was isolated by vacuum filtration, washed several times with
DI water, and dried in Vacuo. The white solid product 1 (3.7 g) was
obtained in yields greater than 90%, and is stable under ambient
conditions.
3. Results and Discussion
We report the preparation of oxide-embedded germanium
nanocrystals (Ge-NC/GeO2) from the reductive thermal process-
ing of a polymer prepared via the controlled hydrolysis and
condensation of phenyl trichlorogermane (PTG, C6H5GeCl3).
Phenyl-substituted germanes are known to decompose at tem-
peratures as low as 300 °C and have been shown to be effective
Bulk Ge-NC/GeO2 Composite Preparation (2-9). (C6H5GeO1.5
)
n
polymer, 1, was placed in a quartz reaction boat and transferred to a
high-temperature tube furnace. Samples were heated to defined peak
processing temperatures at 18 °C/min for predetermined times under
slightly reducing atmosphere (5% H2/95% Ar). The resulting solid Ge-
NC/GeO2 composites were obtained in yields of ca. 90% with respect
to germanium content. After being cooled to room temperature, the
solid composites were mechanically ground in an agate mortar and
pestle. Reaction parameters and sample identification numbers are
summarized in Table 1.
precursors for germanium nanostructures.10 The (C6H5GeO1.5 n
)
polymer, 1, was prepared using modified sol-gel procedures
in which water and isopropyl alcohol (IPA) were added to PTG.
IPA was added to the reaction solution to control the condensa-
tion of PTG by simultaneously lowering the water concentration
and slowing hydrolysis rates by forming alkoxy-substituted
intermediates. It was only with complete condensation that cross-
linked (C6H5GeO1.5)n polymer networks possessed the targeted
Ge:O ratio of 1:1.5. It is reasonable that the hydrolysis and
condensation of partially cross-linked isopropoxy-substituted
(OPri) phenyl germanes were catalyzed by liberated HCl and
driven to completion by preferential evaporation of IPA with
Liberation of FS-Ge-NC. FS-Ge-NC were liberated from composite
8 by dissolution of the GeO2 matrix in warm water. In a typical
liberation procedure, 0.04 g of 8 was stirred in 30 mL of DI water at
ca. 60 °C for predetermined times ranging from 30 min to 20 h. The
initial black suspension turned light brown as the reaction progressed.
The FS-Ge-NC were isolated by centrifugation and washed with DI
water and ethanol.
Thermogravimetric Analysis (TGA). TGA was performed using
a Perkin-Elmer Pyris 1 TGA equipped with Pyris Thermal Analysis
9
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Size-Controlled Germanium Nanocrystals
A R T I C L E S
Figure 1. Photographs of Ge-NC/GeO2 composites thermally processed,
Top: at varied peak temperatures, 1-6, for 1 h, Bottom: for varied times
at 525 °C, 4, 7-9, noted in Table 1.
Scheme 1. Schematic of (A) Hydrolysis and (B) Condensation
Reactions for the Formation of 1 via Sol-Gel Processing of
Phenyl Trichlorogermane (PTG) (Ph ) C6H5)
Figure 2. FTIR spectra of samples 1-6 illustrating the influence of peak
processing temperatures in 5% H2/95% Ar atmosphere.
phenyl group vibrational modes, including aromatic C-H
stretching at 3100-3000 cm-1, aromatic CdC stretching at ca.
1435 cm-1, in-plane C-H bending at ca. 1000 cm-1, and out-
of-plane C-H bending at ca. 695 cm-1 and 735 cm-1. The FTIR
spectrum of 1 also displays intense Ge-O-Ge stretching modes
at ca. 850 cm-1, consistent with a hydrolyzed/condensed
polymer.52,54,55 The absence of hydroxyl O-H stretching at ca.
3200 cm-1 and Ge-OH stretching at ca. 750 cm-1 55 confirms
complete condensation of hydrolyzed PTG. Consistent with our
TGA observations, increased processing temperature, 2-6, was
accompanied by a decrease and eventual loss of IR absorptions
associated with phenyl functionalities. The evolution of the Ge-
O-Ge stretching region from well-defined structured peaks to
a broad featureless band is consistent with the transformation
of a well-defined Ge-O structure in 1 to an extended germa-
nium oxide network in Ge-NC/GeO2 composites.50 In addition,
broadening and shifting of the Ge-O-Ge stretching mode to
higher energy is consistent with the formation of germanium
oxides with higher oxygen content52 and supports the thermal
transformation of “GeO1.5” to “GeO2”.
FTIR spectra were monitored as a function of processing time
for samples treated at 525 °C under 5% H2/95% Ar, 4, 7-9.
With increased processing time, the Ge-O-Ge stretching band
intensity decreased until only trace absorption was observed for
samples processed for 24 h, 9. We propose this trend results
from GeO2 reduction by H2 in the processing atmosphere, and
is supported by XPS observations (Vide infra). A gradual
narrowing of the Ge-O-Ge stretching band was also observed
with increased processing time, and is caused by a decreased
intensity of the high-energy component of the asymmetric band
(Supporting Information, S1). It is reasonable that this could
result from the preferential reduction of higher oxidation state
species (i.e., GeO2). The role of H2 in the oxide reduction and
nanocrystal growth mechanism is currently being investigated
in our laboratory.
gentle heating (60 °C) and stirring for 24 h. The hydrolysis and
condensation reactions for the formation of 1 from PTG are
summarized in Scheme 1.
Thermal processing of 1 in 5% H2/95% Ar at g300 °C
decomposed the polymer to yield a “true” GRO with a Ge:O
ratio of 1:1.5. The white solid (C6H5GeO1.5)n, 1, darkened upon
thermal processing, and the color and crystallinity of the
resulting composites, 2-9, depended on the peak processing
temperature and processing time (See Figure 1).
Thermogravimetric Analysis (TGA). Thermogravimetric
analysis provides valuable insight into the decomposition
products and a possible thermal decomposition pathway of 1.
Following the loss of residual solvent below 200 °C, thermal
treatment of 1 in a slightly reducing atmosphere (5% H2/95%
Ar) resulted in a weight loss (ca. 38%), consistent with partial
liberation of the phenyl substituents. The onset of thermal
decomposition occurs at 300 °C, in agreement with previous
reports of phenyl germane decomposition.10 Coupling the TGA
to a FTIR spectrometer allowed qualitative probing of the
reaction byproducts and confirmed the liberation of aromatic
fragments. Upon heating to higher temperatures (i.e., >700 °C),
additional weight loss (ca. 5%) was observed, that we attribute
to evaporation of Ge. While investigations of the reaction
between GeO2 and Ge at temperatures as low as 375 °C have
reported the loss of volatile GeO in UHV,53 this is unlikely for
the present system at atmospheric pressure. Upon extended
thermal processing (g1 h) of bulk samples, an experimental
weight loss of ca. 50% was observed, consistent with near-
complete removal of the phenyl moieties and partial evaporation
of Ge.
Raman Spectroscopy. Raman spectroscopy was used to
evaluate bonding in the Ge-NC core by monitoring the Ge-Ge
optical phonon (OP) vibration that is centered at 302 cm-1 for
bulk crystalline Ge.42 Figure 3A shows the evolution of baseline-
corrected Raman spectra with processing temperature, 2-6.
Composites 2 and 3 contain no elemental germanium at the
sensitivity of the Raman experiment, given the absence of the
characteristic Ge-Ge OP emission. With increased processing
temperature, 4, a broad asymmetric peak emerges at ca. 294
cm-1 which is slightly shifted to lower energy relative to bulk
Fourier Transform Infrared (FTIR) Spectroscopy. Figure
2 shows the evolution of FTIR spectra with increasing peak
processing temperature for samples treated for 1 h in 5% H2/
95% Ar, 1-6. The FTIR spectrum of 1 shows characteristic
(54) Zacharias, M.; Fauchet, P. M. J. Non-Cryst. Solids 1998, 227-230, 1058-
1062.
(55) Jang, J. H.; Koo, J.; Bae, B. S. J. Am. Ceram. Soc. 2000, 83, 1356-1360.
(53) Prabhakaran, K.; Maeda, F.; Watanabe, Y.; Ogino, T. Thin Solid Films
2000, 369, 289-292.
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Figure 3. (A) Baseline-corrected Raman spectra of samples 2-6 processed
at indicated peak temperatures for 1 h in 5% H2/95% Ar. (B) Baseline-
corrected Raman spectra of samples processed for indicated times at 525
°C under 5% H2/95% Ar, 4, 7-9.
Ge. The appearance of this OP vibration in the spectrum of 4
indicates Ge-Ge bonding and suggests formation of elemental
germanium occurs above 500 °C. This conclusion is further
supported by XRD and XPS observations (Vide infra), and is
consistent with reports on the formation of SiO2-embedded Ge-
NC.44 As the processing temperature increases, 4-6, the OP
peak increases in intensity, narrows, and shifts to lower energy;
similar trends have been attributed to Ge-NC growth.38 The
slight decrease in Ge-Ge OP intensity for samples processed
at 600 °C, 6, may be attributed to the loss of Ge through
evaporation, in agreement with our TGA observations (Vide
supra).
It is generally accepted that the confinement of optical
phonons in semiconductor nanocrystals results in asymmetric
broadening of the OP Raman peak and a shift to lower energy
relative to bulk samples.17,34,38 As a result, the Ge-Ge OP in
nanoscale systems shifts to lower energy with decreasing crystal
size. This relationship has previously been used to estimate mean
particle dimensions.15,17,44 For the present study, a slight shift
to lower energy is observed with increased processing temper-
ature (Figure 3A), coincident with an increase in Ge-NC size
as established by XRD (Vide infra). While these findings may
appear contradictory, similar observations have been attributed
to matrix-induced compressive stresses resulting from crystal/
matrix lattice mismatches.32,34,43 As a result of the complex
interplay of phonon confinement and matrix effects on the
energy of the Ge-Ge OP, mean particle diameters were not
determined from the present Raman data.
Figure 3B shows the evolution of baseline-corrected Raman
spectra as a function of thermal processing time at 525 °C, 4,
7-9. All samples exhibit asymmetric OP emissions at lower
energies than expected for bulk Ge, consistent with the presence
of Ge-NC. With increased thermal processing time, the Ge-
Ge OP peaks increase in intensity and narrow, consistent with
nanocrystal growth.
X-ray Diffraction (XRD). Figure 4A shows the dependence
of the X-ray diffraction pattern on peak processing temperature,
1-6. The powder diffraction pattern of 1 is characterized by
several broad reflections, attributed to long-range ordering in
the 3D cross-linked polymer network. It is clear that processing
at e500 °C, 2 and 3, reduces the long-range order in 1, yielding
a noncrystalline solid. The broad feature centered at ca. 2θ )
20° is characteristic of noncrystalline GeO2.56 Processing at 525
°C, 4, generates crystalline Ge nanodomains, as evidenced by
the emergence of diffraction peaks at ca. 27°, 47°, and 54° that
Figure 4. (A) X-ray diffraction patterns of samples processed at indicated
peak processing temperatures, 1-6. (B) X-ray diffraction patterns of samples
processed at 525 °C for various times, 4, 7-9.
are readily indexed to the (111), (220), and (311) reflections of
diamond structure Ge. Following processing at higher temper-
atures, 5 and 6, Ge reflections intensify and narrow, and higher
order reflections corresponding to the (400), (331), (422), (333),
(440), and (531) planes appear. These observations are indicative
of nanocrystal growth. The Debye-Scherrer relationship was
used to estimate nanocrystal size of 4-6 (See Table 1).
Figure 4B shows the evolution of the X-ray diffraction pattern
with varied processing time at 525 °C, 4, 7-9. Increased
processing time results in Ge reflections intensifying and
narrowing, and the appearance of higher order reflections. As
with increasing temperature, this observation is indicative of
nanocrystal growth. The Debye-Scherrer relationship was used
to estimate nanocrystal size as a function of processing time
(see Table 1).
X-ray Photoelectron Spectroscopy (XPS). Figure 5A shows
the evolution of the Ge 3d spectral region for 1-6. As expected,
the spectrum for 1 shows a single broad emission with a peak
binding energy at ca. 32 eV, suggesting an intermediate
germanium oxide species.57 On the basis of the emission
intensities of the survey spectrum of 1 and using appropriate
sensitivity factors, Ge and O have relative concentrations of 1
to 1.5. This further confirms complete hydrolysis of the PTG
and is consistent with our FTIR observations. In addition, there
is no detectable residual Cl in the survey spectrum of 1. XP
analysis shows the appearance of a shoulder at ca. 29.5 eV for
3-6, that we attribute to the presence of Ge(0). We also observe
a shift of the strongest germanium oxide feature to higher
binding energy (33 eV), consistent with the formation of a GeO2
matrix. Similar binding energies have been obtained in XP
(56) Viswanathamurthi, P.; Bhattarai, N.; Kim, H. Y.; Khil, M. S.; Lee, D. R.;
Suh, E. K. J. Chem. Phys. 2004, 121, 441-445.
(57) Molle, A.; Bhuiyan, M. N. K.; Tallarida, G.; Fanciulli, M. Mater. Sci. Semin.
Proc. 2006, 9, 673-678.
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Figure 7. Suspensions of Ge-NC/GeO2 composite in warm DI water after
(A) 30 min, and (B) 20 h.
growth may be controlled by regulating the processing atmo-
sphere composition.
Reports describing the formation and growth of Ge-NC
embedded in silica have proposed that initial reduction of GeO2
(or GeOx) leads to nucleation and subsequent crystal growth.40
In contrast, for the present system we propose that following
thermal decomposition of 1, thermally induced disproportion-
ation generates Ge(0) and Ge(IV) (i.e., GeO2) from the GRO
network, Scheme 2A. This process causes particle nucleation
and crystal growth. If reduction of GeOx was the primary event
leading to nucleation and crystal growth, as previously proposed
in the literature,40 one would expect the emission at ca. 32 eV
to decrease and be replaced by a Ge(0) emission; no GeO2-like
species would be observed. This is clearly not the case for the
present system. As a result, we propose that with prolonged
heating, the GeO2 matrix is reduced to Ge(0) by atmospheric
H2. This second source of elemental Ge provides an additional
growth pathway for Ge-NC, Scheme 2B.
Figure 5. (A) X-ray photoelectron spectra of 1-6 showing the thermal
disproportionation of 1 into Ge and GeO2. (B) X-ray photoelectron spectra
of 4, 7-9, showing the thermal reduction of GeO2 to Ge, and the resulting
growth of Ge-NC as a function of processing time at 525 °C.
Photoluminescence (PL) Spectroscopy. As previously men-
tioned, the PL properties of Ge-NC are of increasing interest
because quantum confinement effects are expected to appear
for relatively large Ge-NC.17 In contrast to silicon nanomaterials,
the PL response of Ge-NC has not been thoroughly investigated.
The size-dependent PL of Si-NC is generally accepted as a
manifestation of quantum confinement.59 It has been proposed
that for nanoscale semiconductor structures with dimensions
smaller than the Bohr exciton radius, confinement of charge
carriers results in an enhancement of radiative recombination
processes and increase in photon energy with decreasing crystal
size. For Ge-NC, reports are often contradictory about the critical
crystal size required for quantum confinement effects to emerge.
The Bohr exciton radius for bulk Ge has been reported to be
11.5 nm,20 17.7 nm,59 and 25.3 nm.15 Further complicating a
detailed understanding of the optical properties of Ge-NC, there
are also numerous direct and indirect transitions that characterize
bulk Ge.17 Nevertheless, there have been numerous reports for
quantum confined emission in Ge-NC, with the vast majority
exhibiting size-dependent PL in the visible region of the
electromagnetic spectrum.20,25,31,32 Still, similar PL character-
istics have been attributed to ultrasmall molecular-like Ge
clusters, interfacial defects, surface oxides, and matrix ef-
fects.29,36,37,43,45,52,54 Furthermore, quantum confined emission
has also been reported in the NIR for Ge nanocrystals of similar
Figure 6. Dependence of the photoluminescence spectra of Ge-NC/GeO2
composite powders on (A) processing temperature, 1-6, and (B) processing
time at 525 °C, 4, 7-9. All spectra were collected with λex ) 325 nm.
analysis of other oxidized germanium nanoscale systems.58 The
evolution of the Ge(0) and Ge(IV) emissions suggests a
thermally induced disproportionation reaction (i.e., 4GeO1.5 f
Ge + 3GeO2) is occurring. For the present system, the relative
amount of Ge(0) increases with processing temperature, con-
sistent with nanocrystal growth. The slight decrease in the
intensity of the Ge(0) emission at 600 °C, 6, may result from
evaporation loss of elemental Ge, and agrees with present
Raman and TGA observations (Vide supra).
Figure 5B shows the evolution of the XP spectra with
processing time at 525 °C, 4, 7-9. Prolonged reductive thermal
processing of 1 increases the concentration of Ge(0) at the
expense of Ge(IV), suggesting GeO2 is being reduced by H2 in
the processing atmosphere. The dramatic change in the amount
of Ge(0) highlights that processing time may afford control of
Ge-NC size. It also underscores the possibility that Ge-NC
(59) Cullis, A. G.; Canham, L. T.; Calcott, P. D. J. J. Appl. Phys. 1997, 82,
909-965.
(58) Hanrath, T.; Korgel, B. A. J. Am. Chem. Soc. 2004, 126, 15466-15472.
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Figure 8. Transmission electron micrograph of FS-Ge-NC after (A) 30 min, (B) 30 min, and (C) 2.5 h in warm DI H2O. All scale bars ) 100 nm. Insets:
SAED patterns with indicated reflections characteristic of crystalline Ge.
Scheme 2. A Schematic Representation of the Generation of Ge-NC/GeO2 Composites (Ph ) C6H5) (A) from the Thermal
Disproportionation of Decomposed 1, and (B) Subsequent Growth of Ge-NC from the Reduction of the GeO2 Matrix by Atmospheric H2
sizes.35 From this array of examples, it is clear that the source-
(s) of PL emanating from Ge-NC remain the subject of some
scientific controversy.
Scheme 3. Schematic Representation of the Liberation of
FS-Ge-NC through the Dissolution of the GeO2 Matrix in Warm DI
Water
Figure 6A shows the normalized PL spectra for 1-6. The
sharp peak at ca. 441 nm is a spectral artifact arising from the
He-Cd laser excitation source. Unprocessed 1 exhibits a sharp
PL band with an emission maximum centered at ca. 390 nm.
With increased processing temperature, 2-6, this band broadens
asymmetrically while also shifting to lower energy. The PL
response of 6 shows a maximum centered at ca. 440 nm and
resembles the PL behavior of amorphous GeO2 (Figure 6A).
We attribute this spectral feature to an oxide-mediated radiative
transition, that arises either from Ge-NC surface oxide states
or noncrystalline GeO2 matrix. Figure 6A also shows that
increased processing temperature results in the emergence of a
low energy shoulder that monotonically red-shifts with increased
temperature (2-6). This red-shift is consistent with quantum
confined excitons in nanocrystals of increasing size, as indicated
by Debeye-Sherrer analysis of XRD peak broadening, Table
1. However, in order to conclusively identify the source of this
emission, detailed spectroscopic investigations, including X-ray
excited optical luminescence (XEOL) spectroscopy, are actively
being pursued in our group. Interestingly, upon prolonged
exposure to the 325 nm laser excitation source, the “oxide”
spectral feature was photobleached and the “quantum confined”
component did not vary in intensity. This is consistent with
reports of PL stability in semiconductor quantum dots.
°C, 4, 7-9. As processing time increased, the spectral feature
attributed to quantum confinement in Ge-NC red-shifted, until
it was completely resolved from the oxide-associated feature.
Again, this is consistent with quantum-confined excitons in Ge-
NC of increasing size. It can also be seen in Figure 6B that
band at ca. 400 nm is of much lower intensity in the PL spectrum
for 9. This is qualitatively consistent with the loss of GeO2 as
established by XPS analysis (Vide supra) and further supports
our assignment of this band to an oxide-based transition.
Liberation of Freestanding Germanium Nanocrystals (FS-
Ge-NC). FS-Ge-NC were readily liberated from their oxide
matrix via dissolution of GeO2 in warm deionized (DI) water
(ca. 60 °C). In comparison, many FS-Ge-NC synthetic methods
employ toxic/corrosive etchants and elaborate purification
procedures for nanocrystal liberation. For example, silica-
embedded Ge-NC require hydrofluoric acid (HF),34 and solution-
based syntheses require numerous washing steps with organic
solvents to free the nanocrystals from residual surfactant and
other reaction byproducts.20,25 A schematic representation of the
liberation process is presented in Scheme 3.
Figure 6B shows the dependence of normalized PL spectra
of Ge-NC/GeO2 composites on thermal processing time at 525
In addition to particle liberation, we also found that prolonged
exposure to DI water resulted in a gradual, controlled decrease
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Size-Controlled Germanium Nanocrystals
A R T I C L E S
Figure 9. Measured size distribution of (A) FS-Ge-NC isolated after 30 min in warm H2O (mean diameter ) 5.5 nm, n ) 316, σ ) 0.6 nm), and (B)
FS-Ge-NC isolated after 2.5 h in warm H2O (mean diameter ) 4.5 nm, n ) 534, σ ) 0.6 nm).
Figure 10. Energy dispersive X-ray spectra of FS-Ge-NC isolated from 8 after (A) 30 min, and (B) 2.5 h, in warm H2O. Peaks labeled with * are due to
the TEM sample holder.
in Ge-NC size. Qualitatively, this was manifested in a change
in the color of the reaction mixture from black after 30 min to
light brown after 20 h (See Figure 7). We propose this results
from sequential oxidation/dissolution steps, a phenomenon that
has previously been reported for Ge wafers submerged in
water.60 A similar strategy that employed solutions of HNO3/
HF has been employed for the size tuning of Si-NC.61 The
long reaction times required for modifying the Ge-NC size with
warm H2O is very beneficial, as it allows for precise control
over nanocrystal dimension. A study of the mechanism of oxide
dissolution and crystal size tailoring are the subject of ongoing
investigation in our laboratory. The FS-Ge-NC were character-
ized with TEM, EDX, SAED, and PL spectroscopy.
Figure 11. Photoluminescence spectra of FS-Ge-NC liberated from 8 after
Transmission Electron Microscopy (TEM). Figure 8 shows
transmission electron micrographs of liberated Ge-NC isolated
after 30 min and 2.5 h in warm DI water. These freestanding
nanocrystals were isolated from sample 8 (i.e., 1 processed at
525 °C for 5 h in 5% H2/95% Ar). Figure 8A shows the Ge-
NC adopt a pseudospherical morphology and are nearly mono-
disperse with a mean diameter of 5.5 nm (n ) 316, σ ) 0.6
nm, Figure 9A) after 30 min in warm DI water. TEM determined
nanocrystal dimensions differ significantly from those obtained
by Scherrer analysis of the XRD pattern of the composite
powder (11.5 nm). This difference is easily understood in the
context of surface oxidation and dissolution by the aqueous
environment, resulting in an overall decrease in Ge-NC size.
This is supported by the observation of a small number of
particles with a TEM measured diameter of ca. 12 nm, Figure
8B, consistent with XRD data. The selected-area electron
diffraction (SAED) pattern of the FS-Ge-NC (Figure 8A, inset)
is characteristic of diamond structure Ge and is consistent with
the XRD patterns obtained for 8. Figure 8C shows FS-Ge-NC
isolated after 2.5 h in warm DI water. The nanocrystal size
30 min and 2.5 h in warm H2O (λex ) 325 nm). The bottom trace is the
bulk composite, 8.
decreased as a result of longer exposure to water, having a TEM
measured mean diameter of 4.5 nm (n ) 534, σ ) 0.6 nm).
From the measured size distribution (Figure 9B), it is evident
the particles remained nearly monodisperse. The SAED pattern
(Figure 8C, inset) is consistent with diamond structure Ge and
shows the nanocrystals maintain crystallinity after prolonged
exposure to water. FTIR spectra of the liberated FS-Ge-NC
showed oxide and hydroxide surface termination (Supporting
Information, S2).
Energy Dispersive X-ray Spectroscopy (EDX). EDX
spectra of FS-Ge-NC isolated from 8 after 30 min (Figure 10A)
and 2.5 h (Figure 10B) in warm DI water indicate the
nanocrystals are composed only of Ge. EDX also shows the
presence of oxygen, which could be attributed to surface
oxidation during sample preparation.
Photoluminescence (PL) Spectroscopy. Figure 11 shows the
normalized PL spectra of liberated Ge-NC after 30 min and
2.5 h in warm DI water. When compared to the bulk composite
powder, 8 (Figure 11, bottom trace), it is clear that the spectral
feature attributed to quantum confinement (ca. 500 nm) has a
(60) Rivillon, S.; Chabal, Y. J.; Amy, F.; Kahn, A. Appl. Phys. Lett. 2005, 87,
1-3.
(61) Sato, S.; Swihart, M. T. Chem. Mater. 2006, 18, 4083-4088.
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greater relative intensity in the liberated Ge-NC. This is
consistent with the dissolution of the GeO2 matrix by the warm
water and a resulting decrease in the intensity of the oxide
feature at ca. 420 nm. Figure 11 shows the PL maximum of
Ge-NCs liberated after 2.5 h is slightly blue-shifted from the
PL peak for the particles liberated after 30 min. This is consistent
with a decrease in crystal size and further supports a quantum
confined emission process.
NC were readily liberated from the oxide matrix by dissolution
of GeO2 in warm H2O, and it was found that they maintained
crystallinity and PL properties. This liberation procedure also
afforded crystal size control based on reaction time.
Acknowledgment. The authors acknowledge funding from
the Natural Sciences and Engineering Research Council of
Canada (NSERC), Canada Foundation for Innovation (CFI),
Alberta Science and Research Investment Program (ASRIP),
and University of Alberta Department of Chemistry. C. W.
Moffat and R. Lister are thanked for assistance with FTIR and
TGA. Prof. M. McDermott is thanked for assistance with Raman
spectroscopy. Prof. A. Meldrum is thanked for access to laser
systems to evaluate photoluminescence spectroscopy. The staff
of the Alberta Centre for Surface Engineering and Sciences
(ACSES) are thanked for XPS analysis. J. Kelly, S. McFarlane,
J. Rodriguez, J. MacDonald, and D. Rollings are also thanked
for useful discussions.
4. Conclusion
We have demonstrated the preparation of oxide-embedded
germanium nanocrystals (Ge-NC/GeO2) from the reductive
thermal processing of a polymer derived from the hydrolysis
and condensation of phenyl trichlorogermane, (C6H5GeO1.5)n.
Following thermal decomposition of the (C6H5GeO1.5)n polymer,
disproportionation of the GRO network resulted in initial Ge-
NC growth, and subsequent reduction of the GeO2 matrix by
H2 in the processing atmosphere resulted in additional crystal
growth. Nanocrystal size was controlled by variations in peak
processing temperature and processing time. Photoluminescence
spectroscopy revealed that Ge-NC/GeO2 composite powders
exhibited visible emission that red-shifted with increased crystal
size, suggesting quantum confined systems. Freestanding Ge-
Supporting Information Available: FTIR spectra of samples
4, 7, 8 and FS-Ge-NC. This material is available free of charge
via the Internet at http://pubs.acs.org.
JA710286A
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3632 J. AM. CHEM. SOC. VOL. 130, NO. 11, 2008