THE JOURNAL OF CHEMICAL PHYSICS 129, 164702 共2008兲
Synthesis, growth mechanism, and light-emission properties of twisted
SiO2 nanobelts and nanosprings
Z. Y. Zhang,1 X. L. Wu,1,a兲 L. L. Xu,1 J. C. Shen,1 G. G. Siu,2 and Paul K. Chu2
1
National Laboratory of Solid State Microstructures and Department of Physics, Nanjing University,
Nanjing 210093, People’s Republic of China
2
Department of Physics and Materials Science, City University of Hong Kong, Kowloon, Hong Kong, China
共Received 17 January 2008; accepted 5 September 2008; published online 22 October 2008兲
Amorphous twisted SiO2 nanobelts have been synthesized on Si wafers using facile thermal
evaporation. These nanobelts are produced together with SiO2 nanowires and a small quantity of
SiO2 nanosprings. Spectral and microstructural analyses suggest that the twisted SiO2 nanobelts and
nanosprings form via a polar surface driven process. Spontaneous polarization on the very thin polar
crystalline SiO2 layers on the amorphous SiO2 nanobelt and nanospring surfaces makes the nuclei
rearrange orderly and causes the nanobelt and nanowire to roll up at a certain twisty angle. The
cathodoluminescence spectrum acquired from these SiO2 nanostructures reveals three emission
bands at 4.4, 3.7, and 2.7 eV originating from oxygen-related defect centers. The polar surface
driven mechanism can adequately explain the growth of these novel twisty nanobelts and
nanosprings which have potential applications in sensors, transducers, resonators, and photonics.
© 2008 American Institute of Physics. 关DOI: 10.1063/1.2993164兴
I. INTRODUCTION
Since the fabrication of the first helical nanotubes of
graphitic carbon,1 considerable efforts have been made to
produce quasi-one-dimensional nanostructures in the forms
of tubes, wires, and belts because of their interesting geometries, novel properties, and potential applications.1–4 Among
the numerous types of quasi-one-dimensional nanostructures,
the helical nanostructures such as helical SiO2 nanosprings5
and amorphous Si carbide nanosprings6 show unique morphologies and properties. Hence, synthesis and characterization of these nanostructures are of interest. Using the difference in the stress on two surfaces of a film, rings and tubes of
strained bilayer thin films such as Si/ SiGe have been made.7
Helical ZnO nanosprings8 and nanobelts9 have also been obtained by a solid-vapor growth process. As important microelectronic materials, helical Si and Si oxide nanostructures
have attracted remarkable attention in the past years due to
their unique properties and promising applications in nonvolatile memory devices, mesoscopic research, nanoengineering, optoelectronics devices, and nanodevices.5,10–12
However, only a few helical Si oxide nanostructures have
hitherto been observed, and these helical nanostructures such
as helical SiO2 nanosprings,5 silica multinanowire
nanospring,13 helical SiOx nanorings and nanosprings,14 and
helical crystalline SiC / SiO2 core-shell nanowires15 possess a
springlike structure.
In the work reported here, we demonstrate the facile
synthesis of novel twisted Si oxide SiO2 nanobelts together
with SiO2 nanosprings using a thermal evaporation technique. The Si oxide nanospring was first observed in 2003
and metal catalysts such as Fe and Au were believed to play
a兲
Author to whom correspondence should be addressed. Electronic mail:
[email protected].
0021-9606/2008/129共16兲/164702/6/$23.00
an important role in the formation process.5,13 The helical
structure of nanosprings is indicative of a highly asymmetric
growth mode. To explain the growth asymmetry, contact
angle anisotropy 共CAA兲 at the catalyst-nanowire interface
has been proposed.5,13–16 In our experiments, the vapor
source is composed of pure silicon monoxide powders but no
metal catalyst is used. This new process thus eliminates
metal impurity contamination and produces pure silicon oxide nanostructures. The helical SiO2 nanostructures were
formed on some single crystalline Si wafers placed in an
alumina tube inside a furnace. The vapor source composed of
silicon oxide powders was placed in an alumina crucible located in front of the alumina boat. Si 共100兲 wafers were
placed on the alumina boat to collect the products. Nearly
white powders 共product A兲 and a light yellow spongelike
product 共product B兲 were formed on the Si wafers at distances of 11 and 13 cm on the alumina crucible, respectively.
In accordance with microstructural and spectroscopic analyses, product A comprises primarily of amorphous silicon oxide nanowires with small amounts of twisty SiO2 nanobelts
and SiO2 nanosprings, whereas product B consists of more
conventional Si nanowires. The relative ratios of the amounts
of SiO2 nanowires, twisty SiO2 nanobelts, and SiO2 nanosprings in product A are 60:35:5. Cathodoluminescence 共CL兲
examination reveals three emission bands at 4.4, 3.7, and
2.7 eV and indicates a large number of oxygen-deficient defects in the amorphous SiO2.
II. SAMPLES AND EXPERIMENTS
The twisted SiO2 nanobelts and nanosprings were synthesized utilizing a thermal evaporation technique. The products were formed on single crystalline silicon wafers placed
in an alumina tube inside a furnace. The vapor source
129, 164702-1
© 2008 American Institute of Physics
164702-2
Zhang et al.
consisting of silicon monoxide 共99.99% purity, 3 g兲 powders
was placed in an alumina crucible which was placed in front
of an alumina boat. Clean Si 共100兲 wafers were placed on the
alumina boat at distances of 9 – 15 cm on the alumina crucible to collect the products. The alumina crucible was positioned at the center of the alumina tube in the furnace. The
vaporization system was pumped to 1 ⫻ 10−3 Pa with a diffusion pump. Then the alumina tube was heated to 1300 ° C
under a carrier gas 共99.99% Ar兲 at a typical total pressure of
70 Pa and a flow rate of 150 SCCM 共SCCM denotes cubic
centimeter per minute at STP兲. After the temperature of the
alumina crucible has reached 1300 ° C, the temperature was
held for 3 h. Afterward, the alumina tube was cooled down
to room temperature under Ar gas. Nearly white powder and
a light yellow spongelike products were formed on the Si
共100兲 wafer at distances of 11 and 13 cm in the alumina
crucible, respectively. The schematic of the experimental
setup is the same as that reported in Ref. 17. The morphologies of the two products were characterized by field emission
scanning electron microscope 共SEM兲 共LEO VP1530兲 and
field emission transmission electron microscope 共TEM兲 共FEI
Tecnai G2 20 S-TWIN TEM兲. Energy dispersive spectroscopy 共EDS兲 and x-ray diffraction 共XRD兲 共Rigaku-3015兲
were employed to further characterize the white powders.
The CL spectrum was obtained on a thermal field SEM
共JEOL JSM-7000F兲 equipped with a CL system 共Gatan MonoCL3兲. All the measurements were performed at room
temperature.
FIG. 1. SEM images of the helical SiO2 nanobelts. 共a兲 There are abundant
nanowires and helical nanobelts and the lengths of the helical nanobelts are
about several tens of micrometers. 共b兲 Arrows 1–4 indicate four helical
nanobelts with different twisty degrees. Images 共c兲–共f兲 are the high magnification SEM images of nanobelts 1–4 with different twisty degrees.
J. Chem. Phys. 129, 164702 共2008兲
FIG. 2. SEM images of the helical SiO2 nanosprings. Images 共a兲–共c兲 indicate three helical SiO2 nanosprings with different diameters and helical
pitches. 共d兲 A type of the special nanospring structure. Two nanosprings
grow together side by side.
III. RESULTS AND DISCUSSIONS
The general morphology of product A on Si 共100兲 is
depicted in Fig. 1共a兲. Large quantities of nanowires and
nanobelts can be observed from the SEM image. Their
lengths are of the order of several tens of micrometers. These
nanobelts have different helical pitches h 共a whole repeat
unit of helical belt兲. Four twisty nanobelts are indicated by
arrows 1–4 in the SEM image in Fig. 1共b兲. Figures 1共c兲–1共f兲
show the high magnification SEM images of the four twisty
nanobelts. If we define the twisty degree of a helical nanobelt
as d / h, where d is the belt width, it can be readily observed
that the four nanobelts have fixed twisty degrees and helical
pitches. Figure 1共c兲 shows a schematic for calculating the
twisty degree of a nanobelt. The broken line is the central
axis of the nanobelt. Arrow A is placed in the middle of the
plane of a periodic unit in the nanobelt vertical to the central
axis. Arrow A⬘ is placed in the adjacent periodic unit. The
distance between arrows A and A⬘ is the length of half a
periodic unit of the helical nanobelt h / 2. From Figs.
1共c兲–1共e兲, the twisty degrees can roughly be calculated as
0.16, 0.12, and 0.28 for the three shown nanobelts. The
larger the twisty degree is, the more drastic the nanobelt is
twisted. Therefore, the twisty degree can roughly reflect the
twisty level 关for nanobelt 4 in Fig. 1共f兲, the twisty degree is
difficult to be calculated due to the complicated belt morphology兴. The thicknesses and widths of the four helical
nanobelts are in the ranges of 40–80 and 400– 800 nm, respectively, and the thickness to width ratios are all about
1:10. The general morphology of product B is exhibited in
the inset of Fig. 3. Large quantities of nanowires can again
be observed in the SEM image and their lengths are also of
the order of several tens of micrometers.
Upon careful examination of product A by SEM, a small
amount of helical nanosprings can be identified. The SEM
images of the nanosprings are shown in Fig. 2. The diameters
and helical pitches of the three nanosprings in Figs. 2共a兲–2共c兲
are 350 and 140 nm, 580 and 880 nm, 210 and 200 nm, re-
164702-3
Light-emission properties of twisted SiO2 nanobelts and nanosprings
FIG. 3. XRD spectra of products A 共a兲 and B 共b兲 on Si wafers. The inset
shows the SEM image of product B.
spectively. The diameters of the nanowires, which form the
three nanosprings, are 140, 170, and 90 nm, respectively.
The three nanosprings do not have the same diameters and
helical pitches, suggesting that their growth environments
are different. With the exception of the single nanospring,
two nanosprings are observed to grow side by side and oriented attachment is indicated in Fig. 2共d兲.
In order to identify the structure of the nanowires, helical
nanobelts, and nanosprings, we performed XRD measurements on the products and the results are shown in Fig. 3.
The XRD spectrum acquired from product B, which mainly
contains Si nanowires, reveals three strong diffraction peaks
from crystalline Si 共111兲, 共220兲, and 共311兲 and one broad
peak from amorphous Si oxide 关Fig. 3共b兲兴.18–20 This indicates
that the product is mainly composed of crystalline Si nanostructure in addition to a small amount of the amorphous
component. In comparison, the XRD spectrum obtained from
product A exhibits two broad diffraction peaks originating
from amorphous Si oxide nanostructures as well as three
weak crystalline Si peaks 关Fig. 3共a兲兴.20 Note that the 共100兲
peak originates from the Si substrate. The XRD results indi-
J. Chem. Phys. 129, 164702 共2008兲
cate that under our experimental conditions, the distance between the vapor source and Si substrate is a very important
parameter in the formation of the different nanostructures.
To accurately determine the structures of the nanowires,
helical nanobelts, and helical nanosprings, we conducted
high-resolution TEM 共HRTEM兲, selected-area electron diffraction 共SAED兲, and EDS. Figures 4共a兲–4共c兲 display the
HRTEM images showing that these nanostructures are primarily amorphous. The SAED pattern shown in the inset in
Fig. 4共b兲 corroborates it. Figure 4共d兲 exhibits the typical EDS
spectrum of these nanostructures disclosing the presence of
C, O, Cu, and Si. Since the Cu and C signals originate from
the copper grids and carbon film covering the grids, the
nanostructures contain only Si and O, hence SiO2.
Many helical nanostructures have been fabricated
previously,5–9,13–16 and several growth models such as
CAA,5,6,16,21 screw-dislocation-driven 共SCD兲 growth,1,22,23
and polar-surface-driven 共PSD兲 growth have been
proposed.8,9,24 The CAA model, which has been used to explain the growth of amorphous helical nanowires, postulates
that the helical growth is due to the CAA at the catalystnanowire interface. In our experiments, there exist Si atoms
and SiO2 molecules in the mixed vapor. Mostly SiO2 molecules condense and form nanowires on the Si substrate at a
distance of 11 cm but a small quantity of Si atoms also condense and agglomerate on the Si crystallites on the substrate.
Although no metal catalyst is used, the Si nanocrystallites
can substitute for the metal catalyst. They are buried in the
SiO2 nanowire and form the “catalyst”/nanowire interface.
Our XRD results show the presence of weak crystalline diffraction peaks 关Fig. 3共a兲兴. As a result, the CAA model seems
possible to be used for explaining the growth of the helical
nanobelts. However, we should notice that the amount of the
Si nanocrystallites is very small. They may mainly be localized in the nanowire sample. Therefore, this CAA model
should not be a good candidate to interpret the growth of the
currently helical nanobelts. The SCD model, which has been
FIG. 4. TEM images of 共a兲 nanowires, 共b兲 helical nanobelts, and 共c兲 nanosprings. The inset in 共b兲 is a typical
SAED pattern of these SiO2 nanostructures, indicating
that they are amorphous. 共d兲 A typical EDS spectrum
acquired from these SiO2 nanostructures.
164702-4
J. Chem. Phys. 129, 164702 共2008兲
Zhang et al.
FIG. 5. HRTEM image of the side edge of an amorphous Si oxide nanobelt.
Its surface shows a crystalline SiO2 layer with thickness of about 5.5 nm.
used to explain the growth of helical nanostructures with a
nanocrystalline core, stipulates that the helical growth results
from screw dislocations in the crystal lattice. This model
cannot also be used to explain the formation of amorphous
helical nanostructures. The PSD model appears to be the
only plausible one. A key point for the PSD model is the
presence of a polar crystalline SiO2 surface on the amorphous SiO2. This thin crystalline SiO2 layer can be deduced
from XRD spectrum25,26 and directly observed by HRTEM.
In previous XRD spectra, the first-order Bragg reflection
peaks with a lattice spacing of 4.1⫾ 0.15 Å and the broad
peaks at 2.03⫾ 0.15 Å are associated with the crystalline
SiO2 phase.26 Based on our XRD results in Fig. 3, it can be
inferred that a thin crystalline SiO2 layer should exist on the
surface of the amorphous SiO2 nanobelts. This crystalline
SiO2 layer is thin and thus difficult to be observed. Fortunately, we obtained some acceptable HRTEM images from
the side edges. A typical result is presented in Fig. 5. We can
see that the side edge of the amorphous Si oxide nanoblet
shows a layer of thin crystalline SiO2 with thickness of about
5.5 nm. The lattice spacing of the crystalline SiO2 layer is
about 0.51 nm, corresponding to the spacing of the 共010兲
plane of ␤-SiO2 phase 共cristobalite兲. The structure of ␤-SiO2
phase and a sectional view vertical to the 共010兲 plane are
plotted in Fig. 6. From Fig. 6共b兲, we can see that there exist
many polar planes such as 兵100其, 兵101其, 兵201其, and 兵302其
which are vertical to the 共010兲 plane. These polar planes
have different charge sorts and densities. For example, for
the two neighboring planes 共marked as 1 and 2兲 along the
(a)
(b)
2
1
3
4
FIG. 6. 共Color online兲 The structure 共a兲 of ␤-SiO2 phase 共cristobalite兲 and a
sectional view vertical to the 共010兲 plane 共b兲.
FIG. 7. SEM images of the helical SiO2 nanobelts prepared with shorter
growth times. These SiO2 nanobelts have a highly periodic structure on the
surface. The fringe spacing intervals are equidistant.
具100典 direction, the effective charge for plane 1 is negative
and that for plane 2 is positive. Similar situation can be seen
for the two neighboring planes 共marked as 3 and 4兲 along the
具101典 direction.
The crystalline SiO2 layer constitutes a polar surface and
so it is reasonable to consider that the helical growth of the
nanobelt arises from the polar crystalline SiO2. A polar nanobelt tends to roll into an enclosed ring to reduce its electrostatic energy, and a spiral shape also has the same effects.27,28
Thus, the formation of the helical nanobelts can be understood from the nature of the SiO2 polar surface.8,9,24,27 If
surface charges are uncompensated during growth, spontaneous polarization will induce electrostatic energy and cause
the nanobelt to roll into a circular ring due to minimization
of the electrostatic energy. At the same time, bending of the
nanobelts produces elastic energy.27 As a result, the nanobelts attain their shapes by minimization of the total energy
constituted by polarization and elasticity. The widths of the
four helical nanobelts marked by arrows 1–4 in Fig. 1共b兲 are
estimated to be about 750, 760, 600, and 880 nm and the
lengths of the half-pitches 共periodic units兲 are about 6.5, 5.4,
2.1, and 1.2 ␮m, respectively. The length-to-width ratios of
the four helical nanobelts are calculated to be about 8.7, 7.1,
3.50, and 1.36, respectively. The ratios decrease and twisty
degrees increase monotonically from nanobelts 1–4. This
phenomenon can be adequately explained by growth driven
by the polar crystalline SiO2 surface. The rolling angle per
unit nanobelt length at the same width increases with surface
polarization intensity. When the surface polarization intensity increases, 360° rolling requires a shorter unit length.
Therefore, a gradual decrease in the length-to-width ratio of
the periodic unit 共helical pitch兲 in the helical nanobelts is a
result of the gradual increase in the surface polarization intensity.
To further confirm the growth mechanism of the helical
SiO2 nanobelts, we performed SEM observations on the
nanobelts prepared using a shorter time 共160 min兲 and the
results are presented in Fig. 7. It is clearly shown that when
the growth time is shorter than that needed to form the stable
nanobelts, the surface of the helical nanobelts exhibits alter-
164702-5
Light-emission properties of twisted SiO2 nanobelts and nanosprings
nating dark and bright stripes. These stripes are distributed
on two sides of the central axis and have a very orderly
periodic structure. The spacing interval is the same in each
nanobelt but different in different nanobelts. With decreasing
growth time to 140 min, the amount of the stripelike nanobelts obviously decreases but similar structures can still be
observed except that the thicknesses and widths of the nanobelts decrease. If we further shorten the growth time, the
stripelike nanobelts disappear and the sample mainly shows
the nanowire structures. This phenomenon is indicative of an
orderly growth process of the helical nanobelt. Accordingly,
the growth mechanism of the helical nanobelt is postulated
as follows. The silicon oxide in the vapor source is vaporized
into molecular species at high temperature composed of Si
atoms and stoichiometric SiO2 molecules 关2SiO → Si
+ SiO2兴. They are then transported by the carrier gas to a
lower temperature region and finally condense on the Si substrate. The reason why the compositions of products A and B
produced at different positions are different is that the mass
of SiO2 molecule is bigger than that of Si atom. SiO2 molecules thus condense first on the Si substrate. The condensed
SiO2 molecules rearrange on the Si substrate in a proper
order to attain energy minimization forming small nuclei.
Many nuclei rearrange due to spontaneous polarization on
the polar crystalline SiO2 surface. Strict periodic arrangement of the stripes on the helical nanobelts is a good confirmation of this process. With increasing growth time, SiO2
molecules arriving later are deposited onto the formed nanobelt with the stripes. Since the mobility of SiO2 molecules at
high temperature is high, the low-energy surfaces tend to be
flat and meanwhile, the valleys on the surface continue to be
filled by SiO2 molecules leading to a smooth surface on the
nanobelts. Our analysis based on the experimental results
indicates that the growth of helical SiO2 nanobelts is a PSD
process. Spontaneous polarization due to the thin polar crystalline SiO2 surface on the amorphous SiO2 nanobelts has
two effects on the growth process: 共1兲 making the nuclei
rearrange orderly and 共2兲 causing the nanobelt to roll up at a
certain twisty angle.
Amorphous helical SiO2 nanosprings have been observed previously using the thermal evaporation
method.5,13,16 The helical structure of nanosprings is indicative of a highly asymmetric growth mode.16 In light of the
evidence that the nanosprings only form from amorphous
nanowires,6,15,16,21 we can rule out these models that are
based on lattice defects such as screw dislocations. With regard to amorphous helical nanosprings such as boron carbide
nanosprings,16,21 Si carbide nanosprings,6 and Si oxide
nanosprings,5,13,16 the growth has been suggested to arise
from growth asymmetry induced by CAA at the catalystnanowire interface. Since no catalyst is utilized in our experiments and the observed Si nanocryslallites are very
small, the nanosprings reported here should not be due to the
CAA growth. If we notice the existence of some polar SiO2
surfaces, we may understand that the polar surface on the
amorphous SiO2 layer similarly plays an important role in
forming the currently observed nanosprings. This is the
asymmetrical charge distribution on nanowire surface that
leads to the formation of the amorphous SiO2 nanosprings.
J. Chem. Phys. 129, 164702 共2008兲
FIG. 8. CL spectrum of the SiO2 nanostructures. The broken lines are the
Gaussian fits.
To study the light-emitting properties of the helical SiO2
nanobelts and nanosprings, we perform CL measurement and
the corresponding results are displayed in Fig. 8. The CL
spectrum consists of three peaks at 285 共4.4 eV兲, 335
共3.7 eV兲, and 460 nm 共2.7 eV兲. In the previous studies on
SiO2 defects, the 2.7 and 4.4 eV lines were generally attributed to optical transitions in oxygen-deficient defect centers
共ODCs兲.29–32 The proposed ODCs include ODCs 共I兲 and
ODCs 共II兲. In ODCs 共I兲 共or relaxed neutral oxygen vacancy,
O3 w Siu Siw O3兲, the adjacent Si atoms relax toward the
vacancy site and form a covalent bond.33 The ODCs 共II兲
include two kinds of defects, the unrelaxed neutral oxygen
vacancy 共O3 w Si¯ Siw O3兲 where the original Si atom positions are maintained34 as well as the twofold coordinated Si
defect 共O2 v Si: 兲.35 According to the silicon-related oxygendeficient centers 共SiODCs兲,31 the singlet 共S1 → S0兲 and triplet
共T1 → T0兲 emissions at 4.4 and 2.7 eV, respectively, can be
observed. Hence, we can correlate the 4.4 and 2.7 eV CL
emission lines with the SiODCs observed from our amorphous SiO2 nanostructures. The 3.7 eV line is generally associated with oxygen vacancy defects in quartz.36 There are
definitely many oxygen vacancy defects in our products and
so the 3.7 eV CL line can be considered to stem from oxygen
vacancy defects in amorphous SiO2, as we have assigned
previously.37
IV. CONCLUSIONS
Novel amorphous helical SiO2 nanobelts have been synthesized by thermal evaporation. They exist together with a
small quantity of amorphous SiO2 nanosprings in the presence of a large amount of amorphous SiO2 nanowires. Spectral and microstructural determination suggests that the
growth of the helical SiO2 nanobelts and nanosprings is
driven by a PSD process. Spontaneous polarization from the
very thin polar crystalline SiO2 layer on the amorphous SiO2
nanobelt and nanospring surface has two effects on the
growth process. First of all, the nuclei rearrange orderly and
secondly, the nanobelts roll up at a certain twisty angle. The
CL spectrum acquired from these SiO2 nanostructures exhibits three emission bands at 4.4, 3.7, and 2.7 eV arising from
the ODCs. These novel nanostructures are of great interest
not only to the understanding of the growth of helical nanostructures via the polar surface driven growth process but
164702-6
also to fundamental physics and optical phenomena. The
SiO2 helical nanobelts and nanosprings have potential applications in sensors, transducers, resonators, and photonics.
ACKNOWLEDGMENTS
This work was jointly supported by the Grants 共Nos.
60576061, SBK20081022, and BK2006715兲 from the National and Jiangsu Natural Science Foundations. Partial support was also from the State Key Program for Basic Research
of China under Grant Nos. 2007CB936301 and
2006CB921803, as well as Hong Kong Research Grants
Council 共RGC兲 Competitive Earmarked Research Grants
共CERG兲 Nos. CityU 112306 and CityU 112307.
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