Elastic modulus of amorphous SiO2 nanowires

Article abstract of DOI:10.1063/1.2165275

Amorphous SiO2 nanowires with diameter ranging from 50 to 100 nm were synthesized using chemical vapor deposition (CVD) under an argon atmosphere at atmospheric pressure. Nanoscale three-point bending tests were performed directly on individual amorphous

Full text of DOI:10.1063/1.2165275

Elastic modulus of amorphous SiO 2 nanowires
Hai Ni, Xiaodong Li, and Hongsheng Gao
Citation: Applied Physics Letters 88, 043108 (2006); doi: 10.1063/1.2165275
View online: http://dx.doi.org/10.1063/1.2165275
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APPLIED PHYSICS LETTERS 88, 043108 ͑2006͒
Elastic modulus of amorphous SiO2 nanowires
Hai Ni, Xiaodong Li,a͒ and Hongsheng Gao
Department of Mechanical Engineering, University of South Carolina,
300 Main Street, Columbia, South Carolina 29208
͑Received 17 October 2005; accepted 7 December 2005; published online 25 January 2006͒
Amorphous SiO2 nanowires with diameter ranging from 50 to 100 nm were synthesized using
chemical vapor deposition ͑CVD͒ under an argon atmosphere at atmospheric pressure. Nanoscale
three-point bending tests were performed directly on individual amorphous SiO2 nanowires using an
atomic force microscope ͑AFM͒. Elastic modulus of the amorphous SiO2 nanowires was measured
to be 76.6 7.2 GPa, which is close to the reported value of the bulk SiO2 and thermally grown SiO2
thin films, but lower than that of plasma-enhanced CVD SiO2 thin films. The amorphous SiO2
nanowires exhibit brittle fracture failure in bending. © 2006 American Institute of Physics.
͓DOI: 10.1063/1.2165275͔
One-dimensional ͑1D͒ nanomaterials have been found to
possess remarkable properties, such as electrical, optical,
catalytic, and mechanical properties.1–6 These intriguing
properties of 1D nanomaterials offer great opportunities to
use them as potential functional and structural nanobuilding
blocks in nanoscale electronic, optical, bioengineering, and
micro/nanoelectromechancial systems applications, and have
triggered great efforts to synthesize various 1D nanomateri-
als, such as tubes and/or wires of C,4,7–10 B,11–13 Si,14–16
SiO2,17–19 and GaN,20–22 to name a few.
As one of the most important photoluminescence mate-
rials, SiO2 has been widely used in the electronic industry.17
Recently, SiO2 nanowires have shown a great promise as
functional and structural nanobuilding blocks in nanoelec-
tronics. Various methods, such as chemical vapor deposition
͑CVD͒,18,23–25 laser ablation,26 sol-gel,27 and thermal evapo-
ration methods,28 have been used to produce SiO2 nanowires
In this study, amorphous SiO2 nanowires were synthe-
sized using CVD via a vapor-liquid-solid method, after char-
acterizing their microstructure and composition using vari-
ous electron microscopy techniques, we measured the elastic
modulus of the amorphous SiO2 nanowires by directly bend-
ing individual suspended wires using an AFM tip. This letter
also reports a reliable nanowire clamping technique for the
manipulation and mechanical testing of 1D nanomaterials.
The amorphous SiO2 nanowires were synthesized using
CVD through a vapor-liquid-solid method. During the syn-
thesis, high-purity argon flow rate varied from 50 to 270
sccm ͑standard cubic centimeters per minutes͒, through a
three-zone tube furnace with temperature control for each
zone. Silicon powder ͑99.995% pure͒ was used as source
material and was placed near the center of the furnace with a
temperature of 1075 °C. The silicon ͑100͒ wafer with a 20
nm gold coating was used as the substrate and placed near
the lower-temperature zone ͑900 °C͒ and downstream of the
argon flow. After holding 30 to 60 min at the controlled
temperature, the nanowires were removed from the tube fur-
nace and immediately examined by scanning electron mi-
croscopy ͑SEM͒ ͑FEI Quanta 200͒, energy dispersive x-ray
͑EDX͒ spectroscopy, and transmission electron microscopy
͑TEM͒ ͑Hitachi H-8000͒.
The nanowire containing solution was dropped onto a
standard AFM reference sample ͑Veeco Metrology Group͒
with uniform channels. In order to avoid nanowire sliding
during the bending tests, both ends of the nanowires, which
bridged the channel, were clamped by electron-beam-
induced deposition ͑EBID͒ of carbonaceous materials
͑paraffin͒.36 A Veeco Dimension 3100 AFM was used to per-
form three-point bending tests by directly indenting the cen-
ter of a suspended nanowire which bridged the channel with
a tapping mode silicon AFM tip.
on
a large scale, from well-aligned to nanoflower
morphology.18,19 In order to integrate these nanowires into
functional nanodevices, a precise measurement of the me-
chanical properties is of significant importance, because me-
chanical failure of these nanobuilding blocks may lead to the
malfunction or even fatal failure of the entire devices.
Recent studies have revealed that material properties are
size dependent. The extremely small dimensions of 1D na-
nomaterials impose a tremendous challenge to many existing
testing and measuring techniques for experimental studies of
their mechanical properties. An atomic force microscope
͑AFM͒, having high spatial resolution and force sensing ca-
pabilities, has been proven to be a robust mechanical prop-
erty characterization tool for 1D nanomaterials. The AFM
combined with a nanoindenter was used to measure the me-
chanical properties of silver nanowires,29 ZnS nanobelts,30
and Cu2O nanocubes.31 The AFM contact mode was used to
extract the elastic modulus from the force-deflection re-
sponse of carbon nanotubes32,33 and silicon nanowires.34 Re-
cently, the AFM lateral force mode was used to bend gold
nanowires in order to determine the elastic modulus and frac-
ture strength.35 To date, the mechanical properties of SiO2
nanowires are still lacking. This has limited their applica-
tions in constructing reliable nanodevices.
Figure 1͑a͒ shows the SEM image of the as-grown SiO2
nanowires. EDX analysis, seen in Fig. 1͑b͒, clearly shows
that only oxygen and silicon exist except a small peak from
gold catalyst. Figures 1͑c͒ and 1͑d͒ show the low and high
magnification TEM images of the assynthesized nanowires.
One can clearly see the general morphology of the nano-
wires: The surface is smooth and free of defects, the micro-
structure is homogeneous, and the nanowires have a high
purity. The individual nanowires are uniform with diameter
ranging from 50 nm to100 nm. The TEM selected area elec-
a͒
Author to whom correspondence should be addressed; electronic mail:
lixiao@engr.sc.edu
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0003-6951/2006/88͑4͒/043108/3/$23.00 88, 043108-1 © 2006 American Institute of Physics
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043108-2
Ni, Li, and Gao
Appl. Phys. Lett. 88, 043108 ͑2006͒
FIG. 1. ͑a͒ SEM image showing the morphology of the as-synthesized SiO2
nanowires and ͑b͒ the corresponding EDX result, ͑c͒ low and ͑d͒ high mag-
nification TEM images showing the morphology of the as-synthesized SiO2
nanowires. The inset shows a highly diffuse electron diffraction ring pattern,
indicating amorphous structure of the as-synthesized SiO2 nanowires.
tron diffraction pattern ͓inset in Fig. 1͑c͔͒ shows that no
spot pattern was observed except highly diffuse rings. This
clearly indicates that the as-grown nanowires are amorphous
in nature.
The SiO2 nanowires containing solution was dropped
onto the surface of the Veeco 3D reference standard sample,
and the reference sample was then examined in the SEM for
locating nanowires, which bridged well over the trenches.
Both ends of the suspended nanowires were further clamped
by the EBID process. Figure 2͑a͒ shows such a well aligned,
suspended SiO2 nanowire over a trench. By examining the
carbon peaks in Figs. 2͑b͒ and 2͑c͒, one can clearly see that
after a 2 min EBID, a significant amount of carbonaceous
FIG. 3. ͑Color online͒ ͑a͒ Cantilever and ͑b͒ nanowire deflections obtained
by directly indenting a SiO2 nanowire sitting on a solid Si substrate. ͑c͒
Cantilever and ͑d͒ nanowire deflections obtained on a suspended SiO2 nano-
wire. ͑e͒ and ͑f͒ 3D AFM images showing the suspended SiO2 nanowire
morphology before and after three-point bending testing corresponding to
͑c͒ and ͑d͒, respectively. The nanowire fractured in a brittle manner at a
bending force of 3.3 ␮N. Note: Area “A” was dirt.
material has been deposited onto the surface within the
frames shown in Fig. 2͑a͒. As can be seen later, this thin
layer carbonaceous material and the adhesion force between
the nanowire and the substrate are strong enough to anchor
both ends of the suspended SiO2 nanowire during indentation
bending. Thus, the suspended SiO2 nanowire can be treated
as a double clamped beam.
Before performing each three-point bending test, an
AFM image—together with its height profiles both along and
across the nanowire—was obtained, as shown in Figs.
2͑d͒–2͑f͒, and used to determine the suspended length and
diameter of each nanowire to be tested.
The measured total deflection ͑dt͒ is composed of two
components: The deflection from the cantilever ͑dc͒ and the
deflection from the nanowire ͑dn͒. In order to avoid penetra-
tion of the AFM tip into the nanowire during three-point
bending testing, the maximum applied load, below which no
or negligible amount of penetration occurs, should be iden-
tified. We performed an AFM indentation test directly on a
SiO2 nanowire sitting on a silicon substrate, as shown in
Figs. 3͑a͒ and 3͑b͒. At a 4 ␮N indentation force, the penetra-
tion of the AFM tip into the nanowire, i.e., the nanowire
deflection shown in Fig. 3͑b͒, was within range of 1 nm
after subtracting the cantilever deflection. This force was the
maximum applied load used in all three-point bending tests
in this study.
FIG. 2. ͑Color online͒ ͑a͒ SEM image showing a three-point bending SiO2
nanowire sample bridging a 1 ␮m wide trench, and both ends of the wire
were clamped to the substrate by EBID of carbonaceous materials, ͑b͒ EDX
result from Frames A and B showing the almost absence carbon peak before
EBID, and ͑c͒ EDX result from Frames A and B showing an obvious carbon
peak after EBID, which indicates a layer of carbonaceous material was
deposited within the Frame A and B after 2 min EBID. ͑d͒ AFM image of a
suspended nanowire and its corresponding height profile along ͑e͒ and
Figure 3͑c͒ shows the AFM cantilever-piezo position
along the z-direction curve for a suspended SiO2 nanowire
under an applied bending force of 3.3 ␮N until fracture fail-
ure, and the corresponding applied force-nanowire deflection
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across ͑f͒ the wire for the nanowire length and diameter determination.
relationship is shown in Fig. 3͑d͒. As one can see, the ap-
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043108-3
Ni, Li, and Gao
Appl. Phys. Lett. 88, 043108 ͑2006͒
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Financial support for this study from the National
Science Foundation ͑Grant No. EPS-0296165͒, the ACS
Petroleum Research Fund ͑ACS PRF No. 40450-AC10͒, and
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