Low-temperature synthesis of Ge nanocrystals in zeolite Y

Article abstract of DOI:10.1063/1.117520

A method for obtaining Ge nanocrystals has been developed using the porous structure of zeolite Y as a matrix. Samples were obtained by reduction of GeO₂ nanocrystals, previously grown in zeolite α cages, in H₂ at 470°C. This is apparently the lowest GeO₂ reduction temperature reported and is probably due to the influence of the host. Ge crystallites so formed present an absorption edge at 2.2 eV, which is shifted ～1.4 eV with respect to the bulk value. A softening and an asymmetric broadening of the Raman peak were also observed. We can estimate an average diameter of 3 nm for the Ge nanocrystals from both results.

Full text of DOI:10.1063/1.117520

Lowtemperature synthesis of Ge nanocrystals in zeolite Y
H. Miguez, V. Fornés, F. Meseguer, F. Marquez, and C. Lopez
Citation: Applied Physics Letters 69, 2347 (1996); doi: 10.1063/1.117520
View online: http://dx.doi.org/10.1063/1.117520
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Low-temperature synthesis of Ge nanocrystals in zeolite Y
H. Migueza) and V. Fornes
´
´
´
Instituto de Tecnologıa Quımica (UPV-CSIC), Avenida de los Naranjos s/n 46022, Valencia, Spain
F. Meseguer
Instituto de Ciencia de Materiales (CSIC), Cantoblanco 28049, Madrid, Spain
F. Marquez
´
Instituto de Tecnologia Quımica (UPV-CSIC), Avenida de los Naranjos s/n 46022, Valencia, Spain
C. Lopez
Instituto de Ciencia de Materiales (CSIC), Cantoblanco 28049, Madrid, Spain
͑Received 3 April 1996; accepted for publication 1 August 1996͒
A method for obtaining Ge nanocrystals has been developed using the porous structure of zeolite Y
as a matrix. Samples were obtained by reduction of GeO2 nanocrystals, previously grown in zeolite
␣ cages, in H2 at 470 °C. This is apparently the lowest GeO2 reduction temperature reported and is
probably due to the influence of the host. Ge crystallites so formed present an absorption edge at 2.2
eV, which is shifted ϳ1.4 eV with respect to the bulk value. A softening and an asymmetric
broadening of the Raman peak were also observed. We can estimate an average diameter of 3 nm
for the Ge nanocrystals from both results. © 1996 American Institute of Physics.
͓S0003-6951͑96͒01142-4͔
Semiconductor nanocrystals have gathered great impor-
tance due to the possibility of developing novel electronic
devices and studying quantum confinement effects. Strong
exciton confinement effects are expected when crystal size
(L) value is lower than the exciton effective Bohr radius1,2
(RB). Phonon confinement occurs when L is small enough to
consider the phonon described by a wave packet instead of a
plane wave.3 Several methods using different matrices for
obtaining germanium particles with nanometric size have
been reported.4–8 In most cases high temperatures ͑700–
800 °C͒ are needed. As far as we know, only Craciun et al.5
have previously reported a low temperature (550 °C͒ synthe-
sis of germanium nanocrystals using ultraviolet oxidation of
Si0.8Ge0.2 strained layers.
Zeolites are crystalline, porous, aluminosilicate materials
which are well known as catalysts for organic reactions.9
Zeolite cages act as nanoreactors where chemical processes
take place in milder conditions than usual. Also the uniform
pore size, varying from 0.4 to 10 nm depending on the type
of zeolite, make them suitable to be used as hosts for semi-
conductor nanoparticles. Although many different zeolites
have already been used as matrices for obtaining semicon-
ductor nanocrystals with uniform size,10,11 the improvement
of their synthesis reactions has apparently not yet been re-
ported. Many works are reported on type II–VI and III–V
nanoparticles synthesized in zeolites, but few on type IV.12
We present here a novel and simple chemical method for
obtaining germanium nanocrystals using the ␣ cages of zeo-
lite Y as hosts. Zeolite Y ͑faujasite-type͒ possesses two dif-
ferent quasispherical cages, the sodalite cage or ␤-cage ͑di-
ameter 0.8 nm͒ and the supercage or ␣-cage ͑diameter 1.3
nm͒. Initially, GeO2 crystallites were formed by the hydroly-
sis of gaseous GeCl4 at room temperature by H2O trapped in
the zeolite ␣ cages, since the GeCl4 molecule is too large to
enter the ␤ cage. The remaining GeCl4 and HCl were re-
moved by heating and vacuum cleaning. Following this,
Ge0 nanocrystals were obtained by reduction in H2. This
process was monitored in situ using temperature pro-
grammed reduction, where a flux of H2 ͑15%͒ in Ar was
passed over the oxidized sample as the temperature increased
with a rate of dT/dtϭ1°C/min. Two different H2 uptake
peaks at 410 and 470 °C were observed, which corresponded
to GeO and Ge0, respectively, as was confirmed by x-ray
photoelectron spectroscopy in situ analysis. Ge nanocrystals
formation takes place at 470 °C, ϳ200 °C lower than the
reduction temperature of bulk GeO2.8 The low reduction
temperature of GeO2observed in zeolite Y could be related to
the effect of the zeolite cage walls on the GeO2 molecular
orbitals. Molecular orbital calculations performed by semi-
empirical and ab initio methods could support this assump-
tion. These kinds of calculations have already shown that the
lowering of the activation energy of several processes in-
volving organic molecules is due to the effect of the cage on
the molecular orbitals of the inclusions.13,14
In order to study separately the optical properties of the
sample at the different stages of the reduction process, three
different reduction experiments were performed. Figure 1
shows the H2 uptake curves obtained at different final tem-
peratures during the zeolite/GeO2 sample reduction. The fi-
nal temperatures employed were 410 °C ͓see Fig. 1͑a͒, GeO
formation͔, 470 °C ͓see Fig. 1͑b͒, Ge0 nanocrystals forma-
tion͔, and 900 °C ͓see Fig. 1͑c͔͒. The third experiment was
performed to prove that the stability of the Ge0 nanocrystals
depends on the zeolite structure, which ruptures at 900 °C.
This was proven by x-ray diffraction.
The optical absorption of the samples was measured
through diffuse reflectance ͑DR͒ spectroscopy ͑see Fig. 2͒.
The first sample ͑GeO in zeolite͒ did not absorb in the region
of interest ͓see Fig. 2͑a͔͒. The second sample ͑Ge0 nanocrys-
tals͒ presents an absorption edge at ϳ2.2 eV ͓560 nm, see
Fig. 2͑b͔͒, which is blue shifted ϳ1.4 eV with respect to the
bulk value, 0.79 eV ͑1570 nm͒. The sharpness of the absorp-
a͒
Electronic mail: becitq@itq.upv.es
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FIG. 3. Raman spectra of the samples reduced at ͑a͒ 410 °C, ͑b͒ 470 °C,
and ͑c͒ 900 °C.
confinement regime (RBӷL). The formula derived by Brus2
is usually employed to relate the shift of the optical absorp-
tion edge to the nanocrystal radius in this regime. We have
used this formula to estimate the crystal size, obtaining a 3
nm diameter for a band-gap shift of 1.4 eV. On the other
side, the shape of the Raman spectrum corresponds to a
nanocrystal particle (LϾ2 nm͒ rather than a cluster (LϽ2
nm͒,4 as would be expected if the particle was strictly in-
cluded in the ␣ cage. This can be justified if we consider that
the ␣ cages are interconnected through windows of 0.7 nm
and, therefore, the Ge0 nanoparticles may be formed without
constraints through the windows. Both the softening and the
asymmetric broadening at low energy of Raman peak ͓see
Fig. 3͑b͔͒ are in good agreement with the effect of relaxation
of the q-vector selection rule due to quantum size confine-
ment. Considering these estimates, a fit of the experimental
Raman line shape was attempted using the formulas derived
by Campbell and Fauchet15 to describe the Raman spectrum
of nanocrystalline spherical semiconductors. Calculations
were performed employing the phonon dispersion relation of
Ge at 300 °K.16,17 A fairly accurate fit was obtained consid-
ering the crystal size of 3 nm ͑see Fig. 4, solid line͒, agreeing
with the result obtained from DR measurements. However,
the effect of the surface on the optical properties of nanopar-
ticles are not dismissible and some contribution to the broad
tail may arise from it.
FIG. 1. H2 uptake curves obtained with temperature programmed reduction
corresponding to the reduction processes at ͑a͒ 410 °C, ͑b͒ 470 °C, and ͑c͒
900 °C.
tion edge shows a fairly uniform nanocrystal size distribu-
tion. When nanocrystals aggregate due to the collapse of the
zeolite structure at 900 °C, the absorption edge of the sample
is observed to be close to that of bulk Ge ͓see Fig. 2͑c͔͒.
Raman spectra were obtained for the three cases using
a Dilor XY system ͑see Fig. 3͒ and the 514.5 nm line of an
Arϩ laser. A power of 5 mW was used in order to avoid
spurious effects caused by the laser induced heating of the
sample. Th sample obtained after reduction up to 410 °C did
not show a Raman peak in the region of interest, indicating
the absence of Ge0 in the zeolite ͓see Fig. 3͑a͔͒. The sample
reduced up to 470 °C presents a clear peak at 296 cmϪ1, red
shifted 4.5 cmϪ1 with respect to the Ge bulk ⌫25 phonon
Ј
value. A large and broad tail is also observed on the low
frequency side of the peak ͓see Fig. 3͑b͔͒. When the sample
is heated to 900 °C, breakdown of the zeolite structure oc-
curs. The germanium nanocrystals are no longer confined
and aggregation takes place. Therefore, the Raman peak nar-
rows and recovers the bulk frequency value ͓see Fig. 3͑c͔͒.
As Ge has a large exciton Bohr effective radius (ϳ25
nm͒, it is a suitable material for studying the exciton strong
In conclusion, we have developed a new chemical
method for obtaining uniform size germanium nanocrystals
at low temperature (470 °C͒ using the zeolite Y ␣ cages as
FIG. 2. DR spectra, in Kubelka–Munk units, of the samples reduced at ͑a͒
410 °C, ͑b͒ 470 °C, and ͑c͒ 900 °C.
FIG. 4. Raman spectrum of nanocrystalline germanium in sample ͑b͒ and
the fit performed considering the crystal size of 3 nm.
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2348 Appl. Phys. Lett., Vol. 69, No. 16, 14 October 1996 Miguez et al.
On: Wed, 24 Dec 2014 22:43:52

hosts. Not only are the nanocrystalline properties not appar-
ently modified by the host, but, in fact, the cage also favors
the reduction process. Raman and DR spectroscopy show
that the stability of the nanocrystals is imposed by the host
features. The crystal size estimations based on both spec-
troscopies are in good agreement.
We would like to acknowledge A. Corma and R. May-
oral for the useful discussions. This research has been sup-
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