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
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journal homepage: www.elsevier.com/locate/matlet
Preparation and photoluminescence of high density SiOx nanowires with FezOa nanoparticles catalyst
XJ Wang **, B Dong”, Z Zhou?
* School of Electronics Engineering and Computer Science, Peking University, Beijing 100871, China
> School of Science, Dalian Nationalities University, Dalian 116600, China
Article history:
Received 6 January 2009
Accepted 29 January 2009
Available online 12 February 2009
Large scale, high density SiOx nanowires have been synthesized using a novel Fe;04 nanoparticles catalyst The lengths of SiOx nanowires are in the range of several tens to hundreds of micrometers, and the diameters
of nanowires are 20-80 nm Transmission electron microscopy and high-resolution transmission electron microscopy show that the SiOx nanowires are amorphous, and energy dispersive X-ray spectrometry analysis reveals that SiOx nanowires consist of Si and O elements in an atomic ratio of approximately x = 1.4-1.7 The mony vapor-liquid-solid (VLS) mechanism is the main formation mechanism of SiOx nanowires The SiOx
ultraviolet fluorescent light excitation is applied at room temperature The SiOx nanowires with good Keywords: photoluminescence properties are promising candidates for ultraviolet-blue optical emitting devices
SiOx nanowires
Photoluminescence
© 2009 Elsevier B.V All rights reserved
1 Introduction
Since the discovery of carbon nanotubes by lijima [1], one-
dimensional nanomaterials have stimulated great interest because
of their potential fundamental and practical applications in many
areas such as material research, chemistry, physics and engineering,
etc Among the one-dimensional nanomaterials, silicon oxide nano-
wires are of great significance in the fields of photoluminescence (PL),
localization of light, near-field optical microscopy, low-dimensional
waveguide, and nano-inter-connection integrated optical devices
[2,3] Several methods, including chemical vapor deposition [4],
carbon-assisted methods [5], laser ablation [2], and thermal evapora-
tion [6] have been developed to prepare silicon oxide nanowires Since
the majority of silicon oxide nanowires fabrication methods are
catalyst-based methods, different kinds of catalysts have been used,
such as Au, Pd—Au, Fe, Ga, Ga-In, Ni, In-Ni, Sn, and Co [2,3,7]
Magnetite (Fe30,) is a common magnetic iron oxide that has a cubic
inverse spinel structure with oxygen forming a fcc closed packing and
iron cations occupying interstitial tetrahedral sites and octahedral
sites In the past decades, Fe30, is becoming more and more attractive
because of its intrinsic half-metallic ferromagnetic nature, which can
be widely used in catalysts, biological assays, chemical sensors, and
superparamagnets [8,9] Recently, efforts have been devoted to
preparation of Fe30, nanoparticles, nanowires and nanotubes In
2002, Sun and Zeng [10] prepared the size-controlled monodisperse
* Corresponding author Tel.: +86 1062882397
E-mail address: xjwangdlut@hotmail.com (XJ Wang)
0167-577X/$ - see front matter © 2009 Elsevier B.V All rights reserved
doi:10.1016/j.matlet.2009.01.084
Fe30,4 nanoparticles with 4-16 nm by high-temperature solution phase reaction of iron (III) acetylacetonate (Fe(acac)3) in the presence
of alcohol, oleic acid, and oleylamine The size-controlled mono-
disperse Fe30, nanoparticles can be used for a wide range of disciplines, including magnetic fluids, catalysis, biotechnology/bio- medicine, magnetic resonance imaging, data storage, and environ- mental remediation In the paper, we for the first time prepared the large scale, high density SiOx nanowires using the novel Fe304 nanoparticles as catalyst Photoluminescence of SiOx nanowires fabricated by the method was measured, in order to explore the possibility of application in light emitting devices
2 Experimental Fe304 nanoparticles with diameter less than 10 nm were synthesized by high temperature solution phase reaction of iron (III) acetylacetonate with 1,2-dodecandiol in the presence of oleic acid and oleylamine The growth of SiOx nanowires was conducted in a horizontal tube furnace with a quartz tube The mixture of Fe30, liquid-drops and silicon powders was deposited on Si (111) wafer The SiOx nanowires grew by a two-step raising temperature method In the first procedure, when the temperature of reaction region reached
400 °C, the sample was transferred to reaction region rapidly and kept there for 30 min to eliminate the remained oleic acid and oleylamine Then, the sample was returned to out of reaction region Ar gas flow rate kept up 300 sccm through this procedure In the second procedure, when the temperature of reaction region was increased
to 1000-1200 °C, the sample was transferred to reaction region
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30
20 (degree)
Fig 1 (a) TEM image of Fe30, nanoparticles, (b) XRD pattern of FeO, nanoparticles,
rapidly again The growth of SiOx nanowires started The reaction
lasted for 2 h in this procedure under a constant 50 sccm flow rate of
Ar gas After the furnace was cooled to room temperature, the white
product was found on the surface of Si(111) substrate Field-emission
scanning electron microscope (FESEM) (XL-SFEG, FEI Corp.) with
energy-dispersive X-ray spectroscopy (EDS) was used for morpholo-
gical observation and element composition analysis of SiOx nano-
wires X-ray diffraction (XRD) (D/Max-2400, Rigaku Corp.) with
CuKa radiation was assumed for phase structure of Fe;0, nanopar-
ticles Transmission electron microscopy (TEM) (Tecnai-20, PHILIPS
Corp.) and high-resolution transmission electron microscopy
(HRTEM) (Tecnai F20, FEI Corp.) with electron energy loss spectro-
meter (EELS) were employed to perform the microanalysis of SiOx
(a)
nanowires Photoluminescence measurement was performed at a fluorescence spectrometer (F4500, HITACHI corp.) with a resolution of 1.0 nm
3 Results and discussion
Fig 1(a) shows TEM image of Fe30, nanoparticles It can be seen that the monodisperse Fe30, nanoparticles with diameter about 8 nm were observed from the TEM image X-ray diffraction is used to obtain structure information of Fe30, nanoparticles, as shown in Fig 1(b) It indicated the highly crystalline cubic spinel structure was detected from the XRD pattern The reflection peak positions and relative
Spot Magn Det WD -———— es
3.0 10000x SE 50
mT
Fig 2 SEM images and EDS of SiOx nanowires deposited on the Si(111) substrate at the reaction temperature of 1000 °C for 2 h (a) and (b), The lower magnification SEM images of the nanowires, (c) The higher magnification SEM image of nanowire tops, (d) EDS of the nanowires.
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Fig 3 TEM and HRTEM images of SiOx nanowires
intensities of Fe;0, nanoparticles agree well with standard Fe304 XRD
card (JCPDS No 85-1436)
Fig 2(a)-(c) shows the SEM images of SiOx nanowires deposited
on the Si(111) substrate at the reaction temperature of 1000 °C for 2 h
Large scale, high density nanowires are uniformly covered on the
surface of Si(111) substrate, as illustrated in Fig 2(a) and (b) The
lengths of these nanowires are in the range of several tens to hundreds
of micrometers Fig 2(c) shows the higher magnification SEM image
of nanowire tops It can be clearly seen that, at every one top of these
nanowires, there is a droplet whose diameter is much larger than that
of related nanowire Similar products were also observed at reaction
temperatures of 1100 and 1200 °C The composition analysis of the
products was achieved using EDS Fig 2(d) shows EDS spectrum of the
nanowires, indicating that the products only consist of silicon and
oxygen elements Quantitative analysis shows that the atomic ratio of
SI:O Is 1:(1.4-1.7), suggesting that the SiO; 4-17 nanowires have been
synthesized by the method The EDS measurement achieved on the
nanowire top displays the existence of a small amount of elemental
iron Further sample characterization was carried out using transmis-
sion electron microscopy and high-resolution transmission electron
microscopy Fig 3(a) shows the low magnification TEM images of SiOx
nanowires The diameters of SiOx nanowires are in the range of 20-
80 nm The global droplets on nanowire tops were not observed
because the global droplets have no strong connection with
nanowires, and separated from related nanowires when these
nanowires were ultrasonically dispersed in ethanol Fig 3(b) shows
the HRTEM images of the nanowire No crystal structure was
observed Electron diffraction was done to determine its structure,
as shown in the inset of Fig 3(b), suggested that the nanowire is
amorphous Chemical composition of the nanowire was analyzed by
L 405 nm 465 nm
L 4 L 1 1
Wavelength (nm)
Fig 4 The room-temperature photoluminescence spectrum of SiOx nanowires under
the 250 nm ultraviolet fluorescent light excitation
electron energy loss spectroscopy Si and O peaks were observed, and
no iron, carbon and other impurities could be detected, revealing that the nanowire is pure SiOx nanowire, which is consistent with result of
EDS spectrum
The vapor-liquid-solid (VLS), solid-liquid-solid (SLS), and oxide- assisted (OA) growth mechanisms have been used to explain the growth of silicon nanowires and silicon oxide nanowires In our experiments, Fe30, nanoparticles are employed as catalysts to fabricate SiOx nanowires We have also tried to synthesize SiOx nanowires without Fe30, catalysts, but no one-dimensional SiOx nanomaterials were found on the surface of Si(111) substrate Moreover, EDS also detected the existence of iron only at the top of SiOx nanowires The fact suggested that the Fe30, catalysts play a key
role in the formation of SiOx nanowires, and a VLS mechanism is the
most probable growth mechanism Here, we give a description for the possible formation of SiOx nanowires Firstly, with the temperature increased from 400 °C to 1000 °C, Si vapor is generated at high temperature by the vaporization of silicon powder Meanwhile, Fe-Si-
O nanoclusters form on the surface of Si(111) substrate, which act as nuclei for the formation of SiOx nanowires As the droplets become supersaturated, amorphous silicon nanomires are formed by the reaction between Si and O Oxygen source may come from two factors, one is the oxygen in Fe30, nanoparticles, the other is the remainder oxygen in the reaction chamber The presence of a small amount of O
is not expected to change the Fe-Si phase diagram significantly, but in the meanwhile it acts as the oxygen source during the silicon oxide growth
Fig 4 shows the room-temperature PL spectrum of SiOx nanowires under the 250 nm ultraviolet fluorescent light excitation Two broad
PL emission peaks are clearly observed at the center wavelengths of about 405 nm (3.06 eV) and 465 nm (2.67 eV) The PL properties of various silica nanowires have been studied extensively [2] It has been suggested that the 2.7 eV band is attributed to the neutral oxygen vacancy (=Si-Si=), while the 3.0 eV band corresponds to some
intrinsic diamagnetic defect center, such as twofold coordinated
silicon lone-pair centers (O-Si-O) Therefore, we believe that the PL emissions from the SiOx nanowires should be attributed to the above- mentioned defect centers, which arise from the oxygen deficiency The ultraviolet-blue emission properties of SiOx nanowires are of significant interest for their potential ultraviolet-blue emitting device applications
4 Conclusions
Large scale, high density SiOx nanowires have been synthesized by using a novel 8 nm Fe30, nanoparticles catalyst The SiOx nanowires with lengths of several tens to hundreds of micrometers and diameters of 20-80 nm grew by a two-step raising temperature process based on the VLS mechanism TEM and HRTEM show that the
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