A structural model of one-dimensional thin silica nanowires
D.J. Zhang, R.Q. Zhang
*
Department of Physics and Materials Science, City University of Hong Kong, 83 Tat Chee Avenue, Koloon, Hong Kong SAR, China
Received 27 June 2004; in final form 28 June 2004
Available online 31 July 2004
Abstract
We report a new structural model of silica molecular wire based on spiro union two-membered ring (SU-2MR) units. As revealed
by density functional calculations, the SU-2MR wire is formed by parallel 2MRs bridged by oxygen atoms and is energetically more
favorable, thermally more stable and chemically more reactive at the tip than the edge-sharing two-membered ring molecular chain
proposed early. The SU-2MR molecular chain would be considered as an appropriate structural model of one-dimensional thin
(0.4 nm) silica nanowires.
Ó 2004 Elsevier B.V. All rights reserved.
One-dimensional (1D) nanomaterials are being inten-
sively researched because of their great potentials in
mesoscopic physics and in nanod evices. Silica (SiO
2
),
which is the important component in glass, catalyst,
Si-based microelectronic derives and optical fibers, is
an increasingly important candidate to form 1D nano-
materials. Significant progresses have been made in
synthesizing silica nanowires with a variety of methods
[1–6]. Recently, very long aligned silica nan owires with
thin diameters of 5–10 nm has been synthesized by Hu
et al. [6] through thermal oxidation of silicon wafers.
Theoretical investigation of atomic structures of 1D
quantum wires is fundamentally important for under-
standing their overall properties and growth mechanism.
In contrast to the intensive study on silicon nanowires
[7,8], little has been done about silica nanowires in terms

mechanical calculations.
The insets of Fig. 1 show representative configura-
tions of the SU-2MR molecular chains, and the ES-
2MR molecular chains and rings, respectively. To retain
the stoichiometry, the chains are terminated at either
end by non-bridging oxygen (NBO) atoms. Three other
termination modes have also been considered for the
0009-2614/$ - see front matter Ó 2004 Elsevier B.V. All rights reserved.
doi:10.1016/j.cplett.2004.07.041
*
Corresponding author. Fax: +852 2788 7830.
E-mail address: (R.Q. Zhang).
www.elsevier.com/locate/cplett
Chemical Physics Letters 394 (2004) 437–440
SU-2MR chains. However, they were found to be ener-
getically less favorable than the mode described in
Fig. 1. Note that the SU-2MR chains consist of only
even-n SiO
2
units, unlike the ES-2MR chains which con-
tain either even-n SiO
2
units or odd-n SiO
2
units. We
have performed geometric optimizations and molecular
dynamics simulations for SU-2MR molecular chains in
comparison with those of ES-2MR chains and rings,
for (SiO
2

2
unit as
shown in Fig. 1, which is defined as the energy necessa ry
to dissociate the cluster into SiO
2
monomers. Note that
this energy index is equivalent to the strain energy rela-
tive to a-silica used in our previous work [30]. Compar-
ison with the ES-2MR chains [curve (b)], our SU-2M R
chains are energetically less favorable for smaller sizes
(n < 9). For example, the BE of the SU-2MR chain at
n = 6 is smaller than the corresponding ES-2MR chain
by 0.20 eV/SiO
2
. The larger stability for these small
ES-2MR chains is related to their smaller fraction of
NBOs. However, this factor would become less impor-
tant with the increase of chain length. Instead, the in-
trinsic internal strain on 2MRs for larger chains
remain a crucial factor in stabilizing (SiO
2
)
n
chains.
SU-2MR chains are more rapidly stabilized with in-
creasing n, due to both its relative ly small number of
2MRs and the attendance of BO atoms. As shown in
Fig. 1, the energetic stability of the SU-2MR chains ex-
ceeds those of ES-2MR chains and also rings as n >8.It
is indicated that ES-2MR chains would be more favora-

mally stable, and very resistant to collapse or rupture.
Moreover, the thermal stability of these chains is not
sensitive to the cluster size, confirming their intrinsically
structural rationality.
To examine their electronic properties and the reac-
tivity, we calculated the energy gaps between the highest
occupied molecular orbitals (HOMOs) and the lowest
unoccupied molecular orbitals (LUMOs) of these mo-
lecular chains. As shown in Fig. 2, the gaps for the
two kinds of molecular chains rapidly level off to a con-
stant, 6.45 eV for ES-2MR chains as n > 14, and 5.91 eV
for SU-2MR chains as n > 18. Both the HOMOs and
LUMOs of these chains highly localize at the ends of
the chains, making mainly these regions responsible
for their energy gaps. As an example, the insets of
Fig. 2 show the isodensity surfaces of the HOMO and
LUMO states of the SU-2MR chain at n = 12, respec-
tively. The energy gap is a signature of the chemical re-
activity of a system. Compared to the ES-2MR chains,
the relatively smaller gaps of SU-2MR chains indicate
higher chemical reactivities, facilitating the continuous
growth of the chains. Hence, the SU-2MR chain may
be a more reasonable growth mod el of 1D silica nano-
wires. Fig. 3 schematically illustrates the growth mecha-
nisms [monomer mode (a) and dimer mode (b)] of the
silica nanowires according to the present SU-2MR
model, in which the most stable linear monomer and
rhomb dimer are regarded as preferential growth precur-
sors, respectively.
The relative reactivity of these chains is also borne

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440 D.J. Zhang, R.Q. Zhang / Chemical Physics Letters 394 (2004) 437–440