Đây là một bài báo khoa học về dây nano silic trong lĩnh vực nghiên cứu công nghệ nano dành cho những người nghiên cứu sâu về vật lý và khoa học vật liệu.Tài liệu có thể dùng tham khảo cho sinh viên các nghành vật lý và công nghệ có đam mê về khoa học
Trang 1A structural model of one-dimensional thin silica nanowires
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 nanodevices Silica (SiO2),
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 nanowires 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
of their geometric and electronic structures
When forming bulk crystal, silica is a
three-dimen-sional (3D) network of corner-sharing SiO4tetrahedra,
frequently six-membered rings (refer to an Si–O–Si–
O ring containing six Si atoms) However, the smaller
four-, three-, and two-membered rings have also been
found to exist in the surface or interior of amorphous
and crystalline silica, as well as vitreous silica [9–22]
In particular, two-membered rings (2MRs) exists not only in silica-w at high temperature [15], but also in Si–O-plasma reactions [16] as well as in the condensa-tion of vicinal hydroxyls or the thermodynamic rear-rangement of the pure silica structure at the surfaces
of amorphous and crystalline silica at high temperature
[17–22] The structural diversity creates opportunities for ma-terials with designed structures and properties Recently, Bromley et al.[23]proposed a structural model of silica molecular chains based on the edge-sharing 2MR (ES-2MR) units They found that the chains are energetically less stable than the corresponding molecular rings for
n > 11 (n is the number of SiO2units)[23] Here, we pro-pose a new model of thin silica molecular chains based
on spiro union 2MR (SU-2MR) units, aiming at provid-ing insight into the growth of 1D silica nanowires by searching for the preponderant structures via quantum 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: aprqz@cityu.edu.hk (R.Q Zhang).
www.elsevier.com/locate/cplett Chemical Physics Letters 394 (2004) 437–440
Trang 2SU-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 SiO2units, unlike the ES-2MR chains which
con-tain either even-n SiO2 units or odd-n SiO2 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 (SiO2)nwith size varying from n = 2 to 26, using
den-sity functional theories available in both SIESTA 1.3
[24–26]and GAUSSIAN 98[27]codes The SIESTA
geo-metric optimizations were first performed using the
Per-dew–Burke–ErnzerhofÕs [28] Generalized Gradient
Approximation (GGA) functional with the double-f
plus polarization orbital (DZP) And further Gaussian
calculations were carried out at the B3LYP/6-31G(d)
level of theory, which has been confirmed to be enough
accurate for describing silica systems[29]
The geometries of the two kinds of the silica chains
show their respective unique characteristics Firstly,
the shapes and mutual positions of basic 2MR rings
are different In ES-2MR chains, all of the Si atoms form
a line with the planar 2MRs perpendicular to each
other Whereas, in SU-2MR chains, the Si atoms divide
into two parallel arrays linked by the bridging oxygen
(BO) atoms Secondly, the SU-2MR chain is more
com-pact than the ES-2MR chain The former possesses
much larger radial size (0.4 nm) and thus is relatively
shorter than the latter at the same size Thirdly, we
found that the SU-2MR chains tend to be slightly
curved as n > 20 due to the poorer symmetry, while long
ES-2MR chains still keep their straight line
To evaluate the energetic stability of these chains, we
calculated their binding energies (BEs) per SiO unit as
shown inFig 1, which is defined as the energy necessary
to dissociate the cluster into SiO2monomers 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-2MR 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/SiO2 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 (SiO2)n chains SU-2MR chains are more rapidly stabilized with in-creasing n, due to both its relatively 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-ble in the initial stage of the silica nanoparticle embryo However, longer silica nanowires prefer SU-2MR struc-tures A growth mode change from the ES-2MR chain could be expected at n 9 with growth direction change and an open up of the end 2MR to facilitate the fol-lowed SU-2MR growth
To further test the structural stability of these molec-ular chains, we performed density functional molecmolec-ular dynamics simulations using SIESTA for chains with dif-ferent sizes at several temperatures, i.e., 500, 1000, 1500,
2000 and 3000 K The simulation time step was chosen
to be 1 fs, and the relaxed steps was set to 1000 Atomic forces are calculated using the Hellmann–Feymann
the-Fig 2 HOMO–LUMO gaps of the silica molecular chains as a function of n for (a) SU-2MR chains, and (b) ES-2MR chains The insets show the isodensity surfaces of the HOMO and LUMO of the SU-2MR chain at n = 12.
Fig 1 Binding energy per SiO 2 unit as a function of the silica cluster
size for (a) the SU-2MR chains, (b) the ES-2MR chains, and (c) the
ES-2MR rings The insets show the geometries of the representative
clusters.
438 D.J Zhang, R.Q Zhang / Chemical Physics Letters 394 (2004) 437–440
Trang 3orem; NewtonÕs equations are integrated by means of
VerletÕs algorithm We found that all these chains retain
their initial connectivity throughout the whole
simula-tions even at 3000 K which is far higher than the
synthe-sis conditions envisaged (1000 K) It is indicated that
both ES-2MR and SU-2MR chains are extremely
ther-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 2show 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 model of 1D silica
nano-wires.Fig 3schematically 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 out by their ionization potentials (IP) We calculated their vertical IPs for several representative structures For example, the vertical IP of the ES-2MR chain at
n = 12 is 14.82 eV, and that for the SU-2MR chain at
n = 18 is 9.79 eV The high ionization potential facili-tates their separation as neutral species from other silica clusters in an ionizing environment[31]
In conclusion, we presented a new SU-2MR model of silica molecular chains It is proposed to be a more ap-propriate growth model of 1D silica nanowires because
of their higher energetic and thermal stabilities and chemical reactivity than those of ES-2MR chains pro-posed early by Bromley et al., as n > 8
Acknowledgements The work described in this Letter was supported by two grants from the Research Grants Council of the Hong Kong Special Administrative Region, China [pro-ject No CityU 1011/01P; and pro[pro-ject No CityU 1033/ 00P]
Fig 3 Schematic illustrations of the silica nanowire growth in (a) a monomer growth mode, and (b) a dimer growth mode.
D.J Zhang, R.Q Zhang / Chemical Physics Letters 394 (2004) 437–440 439
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