The nature of encapsulated silicon nanoclusters in zeolite Y

Article abstract of DOI:10.1063/1.123382

Luminescent silicon clusters have been synthesized by chemical vapor deposition of Si₂H₆ into the supercages of H₃₂Na₂₄Y zeolite. The encapsulated Si clusters are air stable and exhibit a room-temperature photoluminescence in the green-yellow region. The electronic band gap of the cluster is determined to be about 2.4 eV from photoelectron and photoabsorption spectroscopies. A saturation loading of seven disilane molecules per supercage was determined directly from multiple quantum ¹H nuclear magnetic resonance spin counting of the precursor material. Intrazeolitic Si_n clusters are formed after heating with loss of SiH₄ and H₂. The new quantitative spin counting result is consistent with an average value of 12±2 Si/cluster obtained from previous indirect measurements using x-ray photoelectron spectroscopy and Si K-edge x-ray absorption near-edge spectroscopy [J. He, D. D. Klug, J. S. Tse, C. I. Ratcliffe, and K. F. Preston, Appl. Phys. Lett. 71, 3194 (1997)]. This observation reveals a mechanism for the conversion of gaseous Si₂H₆ into silicon nanoclusters.

Full text of DOI:10.1063/1.123382

The nature of encapsulated silicon nanoclusters in zeolite Y
J. He, Y. Ba, C. I. Ratcliffe, J. A. Ripmeester, D. D. Klug, and J. S. Tse
Citation: Applied Physics Letters 74, 830 (1999); doi: 10.1063/1.123382
View online: http://dx.doi.org/10.1063/1.123382
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APPLIED PHYSICS LETTERS
VOLUME 74, NUMBER 6
8 FEBRUARY 1999
The nature of encapsulated silicon nanoclusters in zeolite Y
J. He, Y. Ba, C. I. Ratcliffe, J. A. Ripmeester, D. D. Klug, and J. S. Tsea)
Steacie Institute for Molecular Sciences, National Research Council of Canada, Ottawa, Canada K1A 0R6
͑Received 21 September 1998; accepted for publication 7 December 1998͒
Luminescent silicon clusters have been synthesized by chemical vapor deposition of Si2H6 into the
supercages of H32Na24Y zeolite. The encapsulated Si clusters are air stable and exhibit a
room-temperature photoluminescence in the green-yellow region. The electronic band gap of the
cluster is determined to be about 2.4 eV from photoelectron and photoabsorption spectroscopies. A
saturation loading of seven disilane molecules per supercage was determined directly from multiple
1
quantum H nuclear magnetic resonance spin counting of the precursor material. Intrazeolitic Sin
clusters are formed after heating with loss of SiH4 and H2. The new quantitative spin counting result
is consistent with an average value of 12Ϯ2 Si/cluster obtained from previous indirect
measurements using x-ray photoelectron spectroscopy and Si K-edge x-ray absorption near-edge
spectroscopy ͓J. He, D. D. Klug, J. S. Tse, C. I. Ratcliffe, and K. F. Preston, Appl. Phys. Lett. 71,
3194 ͑1997͔͒. This observation reveals a mechanism for the conversion of gaseous Si2H6 into silicon
nanoclusters. © 1999 American Institute of Physics. ͓S0003-6951͑99͒02506-1͔
The encapsulation of semiconductor nanoclusters in the
pores of zeolite molecular sieves has been actively explored
as a means of producing tunable light-emitting materials.1–5
The well-defined and well-organized cavities provided by a
zeolite host serve as an ideal environment to control the en-
capsulated cluster’s size and shape and to assemble the clus-
ters in an ordered array. There is also the advantage of sta-
bilizing the normally very unstable nanosized clusters
through encapsulation and coordination to the host frame-
work. While encapsulated II–IV semiconductor clusters in
zeolites have been extensively studied,6–10 relatively little
work has been systematically carried out on III–V ͑Ref. 11͒
and IV ͑Refs. 12 and 13͒ clusters, particularly Si clusters, in
such systems.
The synthesis of Si nanoclusters in dehydrated HY zeo-
lite through the chemical vapor deposition of disilane
(Si2H6) gas was reported recently.10 This encapsulated Si
cluster exhibits strong room-temperature photoluminescence
and the optical absorption edge was found to shift to higher
energy with respect to bulk silicon.15 The exact nature of the
electronic structure, the origin of the luminescence, the clus-
ter formation mechanism and, perhaps more importantly, the
size of the clusters remain uncertain. These key issues are
investigated in the present work. For the framework struc-
ture, we chose to prepare the Si clusters in a partially ex-
changed acidic form of the Y zeolite (H32Na24Y–NaHY),
because this starting material has a much better thermal sta-
bility than the HY zeolite used previously.
pumping at 100 °C for 1 h, the precursor was further heated
to 550 °C at 1 °C/min and held at this temperature for 2 h to
eliminate more H2 and some SiH4. The resulting solid is
stable in air and is pale yellow in color. Analysis of all these
zeolitic materials by x-ray diffraction ͑XRD͒ reveals that the
crystallinity and integrity of the zeolite host is maintained
with no sign of diffraction peaks characteristic of crystalline
silicon (c-Si).
The encapsulation process of Si clusters within the pores
of zeolite includes the anchoring of Si2H6 followed by the
clustering of the entrapped disilane precursor via the elimi-
nation of the H atoms as hydrogen. These chemical pro-
cesses were monitored by in situ infrared ͑IR͒, thermogravi-
metric analysis-mass spectrometer ͑TGA-MS͒, and solid-
state 29Si magic angle spinning ͑MAS͒ NMR spectroscopy.
The appearance of new vibrational bands characteristic of
O–SiHm –SiHn stretching after the initial reaction of disilane
with the zeolite indicate the anchoring of Si2H6. Upon py-
rolysis, the clustering of the anchored disilane is indicated by
the elimination of gaseous silane (SiH4) and hydrogen as
monitored by mass spectrometry, and the gradual decrease of
the Si–H stretching IR intensities. The formation of new
Si–Si bonds is clearly shown from the observation of 29Si
NMR resonances at ϳϪ79 ppm relative to tetramethylsilane
͑TMS͒, which are characteristic of Si nuclei directly bonded
to 3–4 Si atoms. All the spectroscopic evidence unequivo-
cally substantiates the formation of Si clusters inside the
pores of the zeolite.
Silicon clusters were prepared by chemical vapor depo-
sition ͑CVD͒ of disilane gas ͑98%, Matheson͒ within the
pores of NaHY zeolite, to a saturation loading determined
previously. The disilane gas was transferred into a closed
vessel containing the dehydrated–deamminated zeolite pow-
der and the mixture was then heated at 100 °C for 40 h. This
treatment produced intrazeolitic Si cluster precursors. The
loading of Si2H6 was roughly estimated according to the
quantity of zeolite used and the amount of Si2H6 reacted.
After removal of the generated H2 and unreacted Si2H6 by
Prior to the measurement of the photoluminescence ͑PL͒
of intrazeolitic Sin clusters, the luminescent behavior of the
pristine NaHY host was evaluated. Only a weak emission
that may be associated with the hydroxyl groups in the zeo-
lite host was observed around 500 nm at room temperature.
Figure 1 shows the room-temperature PL spectrum of in-
trazeolitic Sin clusters prepared from a saturation loading of
disilane. The emission is found to peak at 560 nm—in the
green-yellow region of the visible spectrum. Samples pre-
pared with smaller disilane loadings, and hence, smaller Si
clusters, also display strong room-temperature photolumines-
cence in the same region. The PL intensity increases with the
a͒
Corresponding author. Electronic mail: tse@jtsg.sims.nrc.ca
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0003-6951/99/74(6)/830/3/$15.00 830 © 1999 American Institute of Physics
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Appl. Phys. Lett., Vol. 74, No. 6, 8 February 1999
He et al.
831
FIG. 2. Number of correlated protons vs MQ excitation time for the en-
trapped Si2H6 precursor in NaHY.
Gaussian fit is estimated to be less than 0.8 proton. Since the
NaHY host can only provide four acidic sites for anchoring
Z͑eolite͒O–Si2H5, i.e., 20 H/supercage, the remaining 18
protons come from three chemisorbed Si2H6¯NaHY.
Within the error limit, 4 ZO–Si2H5 and three chemisorbed
Si2H6 giving a total of 38 H is a realistic possibility. This
structural model accounts for a total of seven disilane mol-
ecules entrapped in each supercage, and implies that the
maximum number of Si available to form a cluster is 14
Si/supercage. The MQ NMR spin counting technique was
also used to determine that a maximum of five protons re-
main in the final Si clusters after thermal treatment at
550 °C. Taking into consideration the elimination of H2 and
some SiH4 during the pyrolysis step, the average cluster size
of the disilane precursor is quite consistent with the esti-
mated average size of 12Ϯ2 atoms in the final cluster esti-
mated reported earlier.15 The mechanism of the anchored
disilanes can be envisioned as follows:
FIG. 1. Room-temperature photoluminescence spectrum for the Si clusters/
NaHY sample ͑with 488 nm Arϩ laser excitation͒.
amount of Si but the peak energy does not appear to shift
appreciably. This observation indicates that discrete Si clus-
ters are formed only inside the cages, and that they become
more homogeneously dispersed in the zeolite host with in-
crease of Si loading.
For the intrazeolitic semiconductor nanocluster systems,
where the atomic compositions of the constituent semicon-
ductor cluster are different from those of zeolite ͑Al, Si, and
O͒, chemical ͑or elemental͒ analysis is generally applied to
quantify the size of the cluster per unit cell of zeolite. Fur-
thermore, in the case of II–VI nanoclusters,6,10,11 it is also
possible to measure the coordination shell of the cluster by
using extended x-ray ray absorption fine structures ͑EXAFS͒
or anomalous x-ray scattering ͑AXS͒.25 This, in combination
with chemical analysis, can be employed to study the size
and shape of the cluster. However, neither chemical analysis
nor EXAFS is applicable to the intrazeolitic Si cluster sys-
tem, owing to the overlap of the Si peak from both cluster
and zeolite host. Furthermore, AXS is impracticable for Si
due to the comparatively low-energy K-shell absorption
chemisorbed
↓
ZO–͑Si2H5͒4͑Si2H6͒3→ZO–͑Si12H5͒ϩ12.5 H2↑ϩ2SiH4↑.
↑
↑
anchored
intrazeolitic cluster
X-ray absorption and photoemission spectroscopies can
be used to measure the energies of the conduction- ͑CB͒ and
valence- ͑VB͒ band edges of the Si cluster. From the energy
shifts of the band edges relative to the values of c-Si, one
should be able to characterize the band gap of the cluster. It
was shown earlier that the onset of the K- and L-edge ab-
sorption edge for the Si cluster is blueshifted about 0.4 eV
relative to that of c-Si.15 Similar observations were also re-
ported on other nanostructured Si systems, e.g., por-Si ͑Ref.
16͒ and layered polysilane.17 Figure 3 shows the valence-
band spectra for empty and Si cluster enclathrated NaHY
zeolites. The two spectra are normalized by matching the
maximum of the predominantly framework Si–O ionization
at 14 eV. The difference of these two spectra yields a rea-
sonable valence-band spectrum of the Sin clusters. In order
to compare the valence-band edge of the clusters to that of
c-Si, we calibrated the respective spectra against their Si 2p
core level using a procedure described earlier.18 The cor-
rected valence band spectrum for the Sin cluster is compared
with that of c-Si, in Fig. 3. The energy of the valence- and
1
edge. The two-dimensional ͑2D͒ multiple quantum H NMR
spin-counting technique is employed here to determine the
number of entrapped disilane precursors in the supercage of
zeolite.
In multiple-quantum ͑MQ͒ 1H NMR, individual spins
͑protons͒ become correlated with each other through their
dipolar couplings as a function of time.14 Such MQ coher-
ences are generated by applying an appropriate radio-
frequency pulse sequence to the sample for a time ␶, thus
inducing a Hamiltonian to force the spins to act collectively.
For a finite coupled proton system ͑e.g., in the form of a
cluster͒, the growth of coherences levels off at a value cor-
responding to the number of protons in the cluster. A plot of
the number of correlated spins as a function of time for the
sample of disilane entrapped in NaHY is shown in Fig. 2.
The correlated spin numbers were obtained by fitting the
experimental intensities of the MQ coherences to a Gaussian
distribution model. As expected, the correlated spin number
increases with MQ excitation time and finally levels off at
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N(␶)Ϸ38Ϯ1 after 1500 ␮s. The error associated with the
L-band edge is determined as the intercept of the extrapo-
128.248.55.97 On: Tue, 09 Dec 2014 05:30:42

832
Appl. Phys. Lett., Vol. 74, No. 6, 8 February 1999
He et al.
structure can be accommodated quite comfortably in the zeo-
lite cage.
In conclusion, a topotactic synthetic pathway has been
used to assemble periodic three-dimensional arrays of Sin
clusters through the chemical vapor deposition of Si2H6
within the supercages of NaHY zeolite. 2D multiple quantum
1H NMR spin counting indicates that there are four anchored
ZO-Si2H6 and three chemisorbed Si2H6 units making a total
of 14 Si in each supercage at saturation loading. Upon ther-
mal treatment, a clustering reaction takes place. Taking into
account the probable error in the measurement of the inten-
sity in Si K-XANES, a cluster size of 12Ϯ2 Si atoms is
obtained. These Sin clusters have a band gap of 2.4 eV and
exhibit a room-temperature photoluminescence in the green-
yellow region.
1 G. D. Stucky and J. E. MacDougall, Science 247, 669 ͑1990͒; V. I. Sr-
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͑b͒ c-Si and Si-clusters ͑obtained from the difference of the Si clusters/
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with the base line.18,19 The valence-band edge of the Sin
clusters shifts by 0.9 eV to higher binding energy when com-
pared with c-Si. This observation suggests that the VB in Sin
clusters is 0.9 eV lower than the valence-band maximum in
c-Si. It is noteworthy that the shift of the VB energy level
relative to c-Si is significantly larger than that of the CB
energy level. This trend is consistent with recent experimen-
tal studies on porous Si ͑Ref. 18͒ and the calculations on
hydrogenated Si clusters.20 Taking the sum of the VB and
CB band edge shifts of 1.3 (ϭ0.9ϩ0.4) eV and the indirect
band gap of c-Si ͑1.1 eV͒, the band gap of the Si clusters is
estimated to be 2.4 eV. This result is consistent with the PL
peak energy ͑2.2 eV, see below͒ and the optical gap ͑2.22
eV͒ on similar Sin clusters in dehydrated HY zeolite.13͑a͒
This value is much smaller than the 3–4 eV band gap for
‘‘free’’ Si clusters of similar size deduced from theoretical
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suggested that the relationship between the band gap and size
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The maximum of the luminescence peak of the intrazeo-
lite Sin cluster is found to be about 560 nm ͑2.2 eV͒. This
value is very close to the estimated band gap. The consis-
tency between the luminescence peak and the band gap sug-
gests that the luminescence arises predominantly from the
radiative electron–hole recombination between VB and CB
levels, and no midgap surface states or defects are involved
in the process. Incidentally, the position of the PL peak is
very close to the luminescence at 2.25 eV observed in the
syn-tricyclooctasilane molecule.23 It is plausible that the in-
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trazeolitic Si clusters may be
a derivative of syn-
tricyclooctasilane and have a similar basic ladder-like
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structure.24 Molecular modeling calculations show that this
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