Chemical vapor deposition of silica micro- and nanoribbons using step-edge localized water

Article abstract of DOI:10.1021/jp034452k

Amorphous silica (SiO₂) ribbons were prepared by hydrolyzing SiCl₄ at highly oriented pyrolytic graphite (HOPG) surfaces on which water had condensed. Exposure to humid air (>30% relative humidity (RH)) at room temperature caused water to condense selectively at hydrophilic step edges on an otherwise hydrophobic graphite surface. Upon exposure of this surface to SiCl₄ vapor, a reaction with condensed water at step edges caused the formation of SiO₂ according to the reaction SiCl₄ + 2H₂ + SiO₂ + 4HCl. The shape and size of the SiO₂ nanostructures varied with the RH: Below 20% RH, nanoparticles of SiC₂, aligned at step edges, were observed. For RH of 35-50%, continuous nano- and microribbons of SiO₂ were obtained. For higher RH, micron-scale silca particles and ribbons were both observed. At a RH of 35%, silica ribbons as small as 80 nm (width) ?? 20-40 nm (height) up to 500 ??m in length could be prepared. Repetitive dosing of the HOPG surface in the humid environment with SiCl₄ produced nano- and microribbons with a height and width that increased in proportion to square root of the SiCl₄ dose.

Full text of DOI:10.1021/jp034452k

© Copyright 2003 by the American Chemical Society
VOLUME 107, NUMBER 23, JUNE 12, 2003
LETTERS
Chemical Vapor Deposition of Silica Micro- and Nanoribbons Using Step-Edge Localized
Water
Michael P. Zach,† John T. Newberg, Luiza Sierra, John C. Hemminger, and
Reginald M. Penner*
Department of Chemistry, UniVersity of California, IrVine, California 92697-2025
ReceiVed: February 21, 2003
Amorphous silica (SiO2) ribbons were prepared by hydrolyzing SiCl4 at highly oriented pyrolytic graphite
(HOPG) surfaces on which water had condensed. Exposure to humid air (>30% relative humidity (RH)) at
room temperature caused water to condense selectively at hydrophilic step edges on an otherwise hydrophobic
graphite surface. Upon exposure of this surface to SiCl4 vapor, a reaction with condensed water at step edges
caused the formation of SiO2 according to the reaction SiCl4 + 2H2O f SiO2 + 4HCl. The shape and size
of the SiO2 nanostructures varied with the RH: Below 20% RH, nanoparticles of SiO2, aligned at step edges,
were observed. For RH of 35-50%, continuous nano- and microribbons of SiO2 were obtained. For higher
RH, micron-scale silca particles and ribbons were both observed. At a RH of 35%, silica ribbons as small as
80 nm (width) x 20-40 nm (height) up to 500 µm in length could be prepared. Repetitive dosing of the
HOPG surface in the humid environment with SiCl4 produced nano- and microribbons with a height and
width that increased in proportion to square root of the SiCl4 dose.
I. Introduction
employed laser ablation to control the diameters of crystalline
silicon nanowires prepared by VLS at 440 °C.
Micron-scale fibers composed of silicon dioxide and other
dielectrics are technologically important as conduits for light.1,2
New methods for preparing dielectric nanofibers have recently
emerged. Wang, Gole, and co-workers3-6 have described a
technique for preparing “nanobelts” composed of a variety of
metal oxides including SiO2, SnO2, ZnO, In2O3, and others.
These single crystalline structures are prepared by thermal
evaporation and vapor transport of the metal oxide from powders
at ≈1000-1400 °C. Wang et al.7 have also demonstrated that
amorphous silica nanowires can be obtained by vapor-liquid-
solid (VLS) synthesis at 1150 °C using gallium catalyst droplets.
Wu and co-workers8 prepared amorphous SiO2 nanowires by a
carbothermal reduction reaction between silicon dioxide and
activated carbons at 1350 °C. Leiber and co-workers9,10 have
In this paper we demonstrate that graphite step edges can be
used to template the formation of SiO2 nanoribbons at room
temperature using the strategy shown schematically in Figure
1. The basal plane surface of a highly oriented pyrolytic graphite
(HOPG) crystal is traversed by a high density (103-104 cm-2
)
of step edge defects that are approximately linear and up to a
millimeter in length. These step edges can be used to template
the growth by electrodeposition of metal11 or conductive metal
oxide (e.g., MoO2)12,13 nanowires, but insulators such as SiO2
cannot be electrodeposited. Upon equilibration of a graphite
surface with humid air, water condenses preferentially at
hydrophilic defects on the surfacesespecially step edges. SiO2
nano- and microribbons were obtained by exposing this “wet”
HOPG surface to the SiCl4 vapor:
* Corresponding author. E-mail: rmpenner@uci.edu.
† Current address: Department of Earth and Planetary Sciences, Uni-
versity of California, Berkeley, California 94720-4767.
SiCl4 + 2H2O f SiO2 + 4HCl
(1)
10.1021/jp034452k CCC: $25.00 © 2003 American Chemical Society
Published on Web 05/17/2003

5394 J. Phys. Chem. B, Vol. 107, No. 23, 2003
Letters
Figure 2. Experimental apparatuses used for creating nanoribbons in
this work. (A) HOPG was suspended on a glass microscope slide within
a plastic centrifuge tube containing a few milliliters of a sulfuric acid
and water mixture. The relative humidity was adjusted using the concen-
tration of acid (see Figure 3A). A syringe needle was inserted through
the cap to deliver the SiCl4 to a glass bead located 10 mm from the
HOPG. The bead prevented liquid SiCl4 from making direct contact
with the HOPG. (B) A stainless steel reaction chamber facilitated precise
dosing of the SiCl4 onto the HOPG surface. The dosing was controlled
by filling a 5 cm length of a stainless steel tube (1/16 in. diameter o.d.,
0.4 mm i.d.) with argon and SiCl4 vapor from a 200 mL glass vessel
pressurized to 2 atm with argon and allowed to reach equilibrium with
liquid SiCl4. Two Clippard solenoid valves were used to deliver 25 µL
volumes of SICl4 vapor to the reaction chamber. These were either
controlled manually or controlled via a computer interface and
LabView. Additional ports to the chamber were used for a roughing
pump, argon purge, thermocouple vacuum gauge, and a second dosing
loop used for other reagents or independent water vapor dosing.
Figure 1. Method for forming SiO2 micro- and nanoribbons on HOPG
step edges at room temperature.
Previously, reaction 1 has been used to prepare silica films using
atomic layer epitaxy (ALE) and atomic layer processing (ALP).
In experiments by George et al.,14-16 silicon surfaces were
exposed alternately to water and SiCl4 under conditions where
both reagents underwent surface-limited reactions. Ultrathin
(<15 nm) silica layers of the desired thickness were thereby
built up in 0.8-1.0 nm steps. Implementation of ALE involving
reaction 1 has required either elevated temperatures (>600 K16)
or the use of catalysts (NH3,17,18 pyridine19). We demonstrate
here that high-purity SiO2 ribbons can be obtained using reaction
1 at room temperature without catalysts.
loop evacuated to the pressure of the main chamber (<6 mTorr),
the first valve to the flask would be held open and the dosing
loop was allowed to fill with argon saturated with SiCl4 vapor.
With the first valve closed the second valve was then opened
and allowed to dump its contents to the chamber. This procedure
was repeated to deliver additional doses of SiCl4. HOPG crystals
were freshly cleaved and suspended by a copper alligator clip
over a 50:50 mixture of H2SO4 and water, resulting in a RH )
37%.
X-ray Photoelectron Spectroscopy. X-ray photoelectron
spectroscopy (XPS) was carried out using an ESCALAB MKII
photoelectron spectrometer (VG Scientific). The ESCALAB
MKII is a multitechnique surface analysis instrument based on
an ultrahigh vacuum (UHV) system consisting of three sepa-
rately pumped, interconnected chambers (sample preparation,
fast sample entry, and spectroscopy). The fast entry chamber
allowed rapid sample transfer from air to UHV pressures (base
pressures during XPS analysis were in the low-to-mid 10-10
Torr range). The XPS experiments were performed in the
spectroscopy chamber using a standard Mg anode X-ray source
(and the Mg KR X-rays at 1253.6 eV) and a 150 mm
hemispherical electron energy analyzer. The spectra presented
here were acquired using an analyzer pass energy of 20 eV.
Under these conditions the spectrometer energy resolution was
∼0.8 eV. Samples were prepared by supporting the graphite
surface, upon which the nanowires were prepared, on a copper
sample holder using conductive colloidal silver paste (Ted Pella).
Binding energies were referenced to the C(1s) peak at 284.5 eV.
Microscopy. Scanning electron microscopy (SEM) was
carried out on uncoated samples using a Philips FEG-30XL
II. Experimental Methods
Silicon Dioxide Deposition. Two types of controlled humid-
ity chambers were employed for the chemical vapor deposition
of silicon dioxide step edge coatings in this work. The first
consisted of a capped 50 mL polypropylene disposable centri-
fuge tube (Figure 2A). The humidity in these tubes was fixed
by equilibrating the interior of the tube with several milliliters
of a sulfuric acid/water mixture. The HOPG crystal was placed
on the glass microscope slide that spanned the width of the
centifuge tube. After an equilibration time of at least 15 min,
∼0.05 mL of SiCl4 (99.998%, Aldrich) was delivered onto a 4
mm glass bead affixed to the glass slide ∼10 mm away from
the graphite crystal on the glass slide. The evaporation and
reaction of SiCl4 with adsorbed water occurred within seconds
but the graphite was maintained in the tube for 5 min to ensure
the reaction was complete. HOPG samples were then removed
into ambient laboratory air and analyzed by AFM, SEM, TEM,
SAED, EDAX, and XPS (see below).
A stainless steel chamber with a volume of 150 mL was
specially built to carry out experiments in which HOPG surfaces
were exposed to pulses of SiCl4 vapor while in a controlled
humidity environment. The dosing system attached to this
chamber consisted of a glass flask containing a few milliliters
of SiCl4 pressurized to 1 atm with argon. Two solenoid valves
separated the flask from the main chamber with a stainless steel
loop (volume ) 25 µL) connecting the two valves. With the

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J. Phys. Chem. B, Vol. 107, No. 23, 2003 5395
Figure 3. SEM images showing the effect of the humidity on the selectivity of SiO2 deposition and showing the morphology of the resulting
deposit. These samples were prepared using the apparatus shown in Figure 2A. (A) The sulfuric acid-water solution in the centifuge tube produced
a constant humidity in accordance with the relationship plotted here. The SEM images shown in (B)-(F) were acquired at the humidity values
indicated in this diagram. (B) Virtually no deposition was observed for HOPG surfaces equilibrated near 0% RH (over pure H2SO4). (C) At ∼6%
RH, deposition of SiO2 was confined to step edges, but discontinuous structures were obtained. (D) At ∼37% RH, SiO2 was again confined to step
edges, and many of these “nanoribbons” were continuous for hundreds of microns. (E) At ∼84% RH, SiO2 deposited both at terraces and on step
edges. These deposits had a beaded appearance. (F) At 100% RH (over pure water), micron scale hemispheres of SiO2 were observed on top of
decorated step edges.
microscope equipped with EDAX elemental analysis capabili-
ties. TEM was performed on a Phillips CM-20. Noncontact
AFM was accomplished using a Park Scientific Instruments
Autoprobe CP with a noncontact head, a 10 µm scanner, and
noncontact Ultralevers (A and B tips with resonant frequencies
of 70-90 kHz).
Additional Dielectric Materials Deposited. All other depo-
sitions were done using the centrifuge tubes with the equilibra-
tion of the HOPG done over the 50:50 H2SO4/H2O mixture
while the same air-sensitive syringe techniques were observed.
Titanium dioxide was deposited from the TiCl4 precursor
(99.9%, Aldrich). Germanium oxide was deposited from the
GeCl4 precursor (99.99%, Aldrich).
RH from 10 to 20%, exposure of the graphite to SiCl4 caused
the formation of hemispherical nanometer and micron-scale
particles composed of SiO2 , as seen in Figure 3C. These
particles formed preferentially at defect sites of the HOPG
surface (especially step edges) because these defects are
hydrophilic whereas the surrounding defect-free basal plane is
hydrophobic. As the RH was increased to between 30 and 45%,
the quantity of condensed water increased and the condensate
at step edges evolved into continuous hemicylindrical ribbons
or “wires”. A RH value of 37% was close to optimum in terms
of favoring this morphology for the silica deposit. Exposure of
this surface to SiCl4 resulted in the formation of SiO2 ribbons
at steps as seen in Figure 3D. If the graphite surface was
equilibrated with RH >60%, droplets of water formed at both
defects, and also on defect-free regions of the basal plane. The
corresponding morphology of the SiO2 obtained at these high
humidities is shown in Figure 3E,F.
III. Results and Discussion
The strategy outlined in Figure 1 was first implemented using
closed, polypropylene centrifuge tubes (volume ) 50 mL, Figure
2A). A few milliliters of a sulfuric acid/water mixture at the
bottom of these tubes fixed the relative humidity (RH) according
to the relationship plotted in Figure 3A. A graphite crystal was
placed on a glass slide suspended over this sulfuric acid solution.
After an equilibration period of at least 15 min, a ≈50 µL droplet
of SiCl4 was syringed onto a 4 mm glass bead affixed to the
glass slide, located 10 mm away from the graphite crystal. This
glass bead prevented liquid SiCl4 from contacting the HOPG.
SiCl4 vapor was then permitted to diffuse and react with
adsorbed water on the graphite surface. This process was
complete in an instant, but 5 min was allowed to ensure
complete reaction of all precursor.
In Figure 4 two SEM images show the silica nano- and
microribbons formed using this procedure at RH ≈37%. The
parallel nature of step edges on cleaved graphite results in the
formation of large arrays of parallel silica ribbons, as seen clearly
in the low magnification SEM image of Figure 4A. It is also
apparent from this image that many thousands of silica ribbons
with lengths of 200-500 µm were produced in a typical
experiment. The length of the individual ribbons is a reflection
of the fact that step edges can traverse the entire diameter of
the columnar grains within the HOPG. For the ZYB grade
HOPG utilized for these experiments, many of these grains had
diameters in excess of 500 µm. Thus, in a typical CVD
experiment, many thousands of silica ribbons with lengths of
200-500 µm were produced. Typical SiO2 ribbons can be seen
in the cross-section of the SEM image shown in Figure 4B.
Bending the graphite surface caused these SiO2 fibers to release
Scanning electron microscopy (SEM) images of graphite
surfaces prepared using this procedure are shown in Figure 3.
In dry air (equilibrated with 100% H2SO4), almost no SiO2 was
observed on the graphite surface, as shown in Figure 2B. For

5396 J. Phys. Chem. B, Vol. 107, No. 23, 2003
Letters
Figure 4. Two SEM images of silica nanoribbons prepared using 37%
RH and the apparatus of Figure 2A. (A) This low magnification image
shows the organization of nanoribbons on the HOPG surface. This
organization is a reflection of the organization of step edges on the
HOPG surface. Many of the silica nanoribbons on this surface exceeded
0.5 mm in length. (B) Silica ribbons are flexible and can be removed
from the substrate by flexing the HOPG. This perspective image shows
a closeup of an area that has been folded. The mechanical properties
of these ribbons, coupled with their weak adhesion to the HOPG surface,
allows one to separate the ribbons from the HOPG surface, giving free-
standing ribbons.
from the surface. These SiO2 fibers were ribbon-shaped with
an aspect ratio (A ) width/height) of 2-5.
A higher degree of control over the silica growth process
was obtained using a stainless steel reaction chamber in
conjunction with a system for delivering pulses of SiCl4 vapor.
The apparatus we employed, shown schematically in Figure 2B,
allowed SiCl4 vapor to be delivered in identical 25 µL pulses
into a 150 mL stainless steel reaction chamber that was
evacuated to <6 mTorr and maintained at an RH close to 37%
using 1:1 H2SO4:H2O. The pulse duration and the equilibration
time between pulses were both 1 s. We measured the mean
width of silica ribbons using SEM (e.g., Figure 5A,B) and the
mean height using atomic force microscopy (AFM, Figure 5C,D)
as a function of the number of SiCl4 pulses, N. Both dimensions
Figure 5. Plots of average width (or diameter) and average height as
a function of the number of SiCl4 doses for silica ribbons prepared
using the apparatus shown in Figure 2B. In these experiments, the
HOPG surface was equilibrated with argon at 37% RH. SEM images
were used to extract the widths; AFM images were analyzed to
determine the ribbon heights. The error bars shown in this Figure
represent (1σ for the mean value for each data point. (A), (B) SEM
images of the surface after 32 and 128 doses. (C), (D) AFM images
and topography traces of the surface after 8 and 128 doses.
increased smoothly with N, and in direct proportion to N1/2
.
Assuming the reaction probability at nascent silica ribbons
remains constant over the duration of the experiment, the
increase in both the width and height is predicted to be linear
with the square root of the number of pulses applied, as shown
by eq 2:
5 also shows the threshold at which ensembles of SiO2 particles
arrayed along step edges coalesced into continuous ribbons. We
found that the smallest continuous silica ribbons obtainable using
this approach were approximately 75 nm in diameter and 45
nm in height. These dimensions were achieved using four SiCl4
pulses under the conditions of our experiment. A smaller number
of pulses produced hemispherical silica particles along step
edges, as shown in the AFM image of Figure 5C.
2RNPSiCl VSiCl Vm
4
4
r(N) )
(2)
x
AπRTl
In this equation, N is the number of SiCl4 pulses, R is the fraction
of the injected SiCl4 that reacts with nascent silica ribbons, PSiCl
is the vapor pressure of SiCl4 at 23 °C (∼0.255 atm), VSiCl is
4
4
volume of SiCl4 injected with each pulse, Vm is the molar
volume of the SiO2, A is the aspect ratio of the semi-elliptical
SiO2 wires, and l is the total length of active step edges. Figure
Transmission electron microscopy (TEM) and selected area
electron diffraction (SAED) of these silica nanowires revealed
them to be amorphous. Energy-dispersive X-ray elemental

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J. Phys. Chem. B, Vol. 107, No. 23, 2003 5397
Figure 6. X-ray photoelectron spectra of an HOPG surface prepared in the apparatus of Figure 2A using RH ) 37%. Si(2p) and O(1s) binding
energies are both consistent with SiO2. The Cl(2p) region showed no detectible chloride. The binding energy ranges indicated in this figure were
obtained from ref 20.
analysis (EDAX) showed the presence of silicon and oxygen.
Contaminating elements were not detected.
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produced using this approach was possible using X-ray photo-
electron spectroscopy. Shown in Figure 6 are the Si(2p), O(2s),
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In preliminary experiments, we have demonstrated that nano-
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Acknowledgment. This work was funded by the National
Science Foundation (CHE-0111557). M.P.Z. acknowledges
support from the American Chemical Society, Division of
Analytical Chemistry Fellowship sponsored by Merck and
Company. J.C.H. acknowledges funding support from the
Department of Energy through contract DE-FG03-96ER45576.
Finally, we thank Dr. Art Moore of Advanced Ceramics Inc.
for donations of graphite.
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