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 1Synthesis and photoluminescence property of silicon carbon nanowires
synthesized by the thermal evaporation method
Enlei Zhanga, Yuanhong Tanga,b, , Yong Zhanga, Chi Guoa
a
College of Materials Science and Engineering, Hunan University, Changsha 410082, People’s Republic of China
b Powder Metallurgy Research Institute, Central South University, Changsha 410083, People’s Republic of China
a r t i c l e i n f o
Article history:
Received 13 November 2008
Received in revised form
18 November 2008
Accepted 18 November 2008
Available online 27 November 2008
PACS:
81.07.Bc
81.40.Tv
Keywords:
Nanostructures
Crystal growth
Electron microscopy
Optical properties
a b s t r a c t
The purity ofb-SiC nanowires is raised obviously by using an ordered nanoporous anodic aluminum oxide template by the thermal evaporation method without any metal catalyst The microstructures were characterized by scanning electron microscopy, energy-dispersive X-ray spectroscopy, X-ray diffraction and high-resolution transmission electron microscopy The results show that the synthesized products mainly consist of nanowires, which are single-crystallineb-SiC with diameters of about 50 nm and tens of micrometers long The nanowires axes lie along the /111S direction and possess a high density of planar defects The b-SiC nanowires exhibit the strong photoluminescence peaks at wavelength 400 nm, which is significantly shifted to the blue compared with the reported luminescence results of SiC materials The blueshift may be ascribed to morphology, quantum size confinement effects
of the nanomaterials and abundant structure defects that existed in the nanowires Finally, the growth mechanism of SiC nanowires and the effect of anodic aluminum oxide template are also analyzed and discussed
&2008 Elsevier B.V All rights reserved
1 Introduction
Recently, one-dimensional structures such as wires, rods, belts
and tubes have become the focus of intensive research because of
their unique applications in functional materials and the
fabrica-tions of the nanoscale devices[1–3] As an important wide
band-gap semiconductor with high electron mobility, SiC nanowires
would be favorable for applications in temperature,
high-power and high-frequency nanoscale devices[4] Recent results
[5] show that the elasticity and strength of SiC nanowires are
considerably greater than those of SiC whiskers and bulk SiC
A variety of methods on the synthesis of SiC nanowires have
been developed, including laser ablation [6,7], chemical vapor
deposition via silicon precursor [8–11], physical evaporation,
hydrothermal method[12,13]and catalyst-assisted vapor liquid
solid mechanism[14] However, these products are available at
the cost of either high pure and expensive carbon nanotube or the
hazardous and easily explosive silicon (carbon) precursor of SiH4
or SiCl4(CH4) In addition, the synthesized products were of low
yield and with much SiC bulk Thus, large-scale synthesis of pure
b-SiC nanowires still remains a challenge to be considered for the above-mentioned disadvantage
In this work, we have developed a simple method for synthesizing large-scale pureb-SiC nanowires by heat-activated carbon with SiO powders using anodic aluminum oxide (AAO) template without any metal catalyst SiO powders cannot react with activated carbon directly because of AAO template The synthesized SiC nanowires were of high yield without much bulk The synthesized nanowires consist about 50 nm diameter core wrapped with an amorphous SiO2 sheath The crystal growth direction /111S is clearly observed Photoluminescence spec-trum centered at 400 nm is referred to the SiC nanowires Based
on an analysis of experimental conditions, a growth mechanism and the effect of AAO template are proposed to explain the formation of pure SiC nanowires
2 Experimental 2.1 Fabrication process of the AAO template The AAO template was prepared by a two-step aluminum anodic oxidation process similar to that previously described, to obtain a uniform pore structure[15,16] Prior to anodization, the high-purity aluminum thin sheets (99.99%) were annealed at
600 1C for 2 h, rinsed in distilled water and then electropolished to
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Physica E
1386-9477/$ - see front matter & 2008 Elsevier B.V All rights reserved.
Corresponding author at: College of Materials Science and Engineering, Hunan
University, Changsha 410082, People’s Republic of China Tel./fax: +86 7318821778.
E-mail address: yhtang2000@163.com (Y.H Tang).
Trang 2achieve a smooth surface Subsequently, the aluminum samples
were anodized in 0.3 M oxalic acid (40 V, 17 1C, 6 h and Al sheet as
an anode) In the first step, anodized layer was removed by
etching in a mixture of phosphoric acid and chromic acid at 60 1C
for 12 h During the second step, the samples rinsed in distilled
water and oxalic acid were anodized again in 0.3 M oxalic acid
(40 V, 16 1C, 10 h and Al sheet as an anode) After the second-step
anodization, the unwanted aluminum matrix was dissolved in
saturated CuCl2 solution at room temperature Finally, the
template was rinsed with distilled water and immersed in 5%
phosphoric acid for about 30 min at room temperature to adjust
the pore diameter and remove the barrier layer at the bottom of
nanoholes
2.2 Synthesis of SiC nanowires
The preparation apparatus for synthesis of SiC nanowires is a
conventional furnace with horizontal alumina tube Solid SiO
powders (1 g, purity 99.9%) were placed in a graphite crucible and
covered with an AAO template The activated carbon (2 g) was
placed on the AAO template Then, the crucible was covered with
a graphite lid, placed in the hot zone inside the alumina tube, as
shown inFig 1 The chamber was flushed with high purity of Ar
(40 sccm) to eliminate O2by means of rotary vacuum pump for
many times Afterwards, the furnace was rapidly heated from
room temperature to 1400 1C at a heating rate of 10 1C/min and
maintained for reaction for 2 h in atmosphere pressure The
sample was taken out when it was cooled down to room
temperature, and the AAO template surface was deposited with
thick layer of light-blue fluffylike products
Morphology and crystal lattice of the samples were observed
by field-emission transmission electron microscopy (TEM, JEOJ
JSM-5600LV) and high-resolution transmission electron
micro-scopy (HR-TEM, JEOL JEM-3010) The crystalline structure was
analyzed by X-ray diffraction (XRD, Semens D5000) The possible
chemical composition of as-grown products was investigated by
energy-dispersive X-ray spectroscopy (EDS) attached to the TEM
The IR measurement was completed on a WQF-410 spectrometer
with a resolution of 0.65 cm1 Phtotoluminescence (PL, Hitatch
F-4500) spectroscopy measurement was performed with xenon
lamp under 354 nm as the excitation source at room temperature
3 Results and discussion
A typical SEM image of the nanoporous AAO template
surface is shown inFig 2 The image shows the nanopores on
the surface with an average pore diameter of about 50 nm, which
are connected to each other to form the nanonetwork Fig 3a
Fig 2 SEM image of the nanoporous AAO template surface.
Fig 3 SEM image ofb-SiC nanowires synthesized by thermal evaporation: (a)
Trang 3displays SEM image of SiC nanowires synthesized using the AAO
template by thermal evaporation It reveals that large quantities of
randomly distributed wire-like products have been obtained The
nanowires are of uniform diameter of about 50 nm and lengths up
to tens of micrometers In addition, it is very important to put the
AAO template over SiO powders To test this, we removed the AAO
template to repeat the above process, and much more bulk was
found, as shown inFig 3b We think that silica source could not
react with activated carbon directly because of using AAO
template, and the template made the concentration of the SiO
vapor increase Thus, overgrowth of the nanowires became
possible by using AAO template
The X-ray diffraction pattern for the obtained sample is shown
inFig 4 As can be seen from the pattern, the major diffraction
peak can be indexed as the (111), (2 0 0), (2 2 0), (3 11) and (2 2 2)
reflections of cubic b-SiC (unit cell parameter a¼0.4389 nm)
These values are almost identical to the known values forb-SiC
(JCPDS Card no 73-1665)
The internal structure of SiC nanowires was characterized by
TEM.Fig 5a displays a typical TEM image of the SiC nanowires,
revealing that the periphery of SiC nanowires is very clean and
straight It also shows that the SiC nanowires possess a high
density of planar defects, stacking faults which are perpendicular
to the wires axes, similar to the already reported results[17–19]
With regard to energetic consideration, the formation of stacking
faults during the growth of SiC nanowires is favorable due to the
contribution of stacking faults themselves with lower energy By
HR-TEM image (Fig 6) observation, we have found that nanowires
have a crystalcore and an amorphous sheath with thickness about
2 nm The SiO2 sheath could be easily removed by etching in
hydrofluoric acid (HF) The thickness of the SiO2sheath could be
controlled by changing the etching time.Fig 6also shows that the
spacing of lattice fringes is 0.25 nm, corresponding to the {111}
plane spacing, and also indicates that nanowire grows along
/111S direction The fast Fourier transform (FFT, inset ofFig 6)
indicates that the nanowires only possess /111S crystal
orienta-tion The possible chemical composition of the sample was
analyzed through the EDS data recorded from several pure
nanowires (Fig 5b) The presence of peaks demonstrates that
the nanowires are composed of Si, C and small amount of O It is
found that the molecular ratio of Si/C/O of the nanowires is about
3:2:2, which corresponds well to the standard SiC and SiO2
The small quantity of oxygen may come from the resident oxide
layer The IR spectrum of the as-synthesized SiC nanowire samples
(Fig 7) also shows b-SiC characteristic absorption band at
791 cm1 and SiO2 characteristic absorption bands at 470 and
1000 cm1[13,20] The SiO2 characteristic absorption peaks are quite intense possibly due to the SiO2 outer layers of the SiC nanowires; this confirms the composition of nanowires
To investigate PL properties of the synthesized b-SiC nano-wires, the corresponding measurement was carried out at room temperature and a PL spectrum (Fig 8) was obtained When excited with light from a xenon source (excitation wavelength
354 nm), the nanowires have an emission band between 330 and
600 nm It is clear that a strong peak centered at 400 nm is observed Compared with previously reported luminescence from the bulk [21], film [22]and nanowire[23]of SiC, the emission peak for b-SiC nanowires is obviously shifted to the blue The emergence of the peak with a blueshift is due to the existence of oxygen defects in the amorphous layer, the special rough core– shell interface and the morphology effects such as stacking faults
in the nanowires’ core [24] It also may be attributed to the quantum confinement effect because of the small size[23,25] Clearly, no metal catalyst was employed during the whole procedure Thus, the growth mechanism may not follow the previously reported vapor–liquid–solid (VLS) model On the basis
of experiments, we suggest a possible growth model for b-SiC nanowires The chemical reaction equations during the process can be described as in the following
700
600
500
400
300
200
100
0
2θ (°)
β-Sic (200)
β-Sic (220)
β-Sic
(111)
β-Sic (311)
β-Sic (222)
Fig 5 (a) TEM image ofb-SiC nanowires (b) The EDS spectrum ofb-SiC nanowire.
Trang 4When the furnace is heated to a high temperature, SiO vapor is
generated, and SiO gas pressure can be maintained much higher in
the graphite crucible Hence, under very high SiO partial pressure
the disproportionation reaction of gaseous SiO into Si and SiO2can
take place according to the reaction (1) It was found that if the
concentration of the vapor was high, overgrowth of the nanowires
became possible [26] According to the oxide-assisted growth
mechanism, silica decomposed from SiO is believed to play an
important role, significantly enhancing the nucleation and
one-dimensional growth of Si nanowires, which are clothed with a SiO2sheath
where s and g in the brackets refer to solid and gas state, respectively The reaction temperature being 1400 1C, the Si nanowires with a SiO2 sheath as templates would react with activated carbon to form SiC nanowires according to the following reactions (2) and (3):
In fact, reaction (3) proceeds through two stages in which a gaseous intermediate SiO gas is generated according to the following reaction (4) Once CO is formed, SiO maybe produced according to reaction (5):
The SiO vapor formed in above steps subsequently reacts further with carbon and CO according to the following reaction:
According to thermodynamics calculation for reactions (6) and (7), the standard free energy changes are approximately 77.4 and
39.2 kJ/mol at 1400 1C, respectively Therefore, both reactions should proceed The generated CO2 vapor can be taken into reaction (8) leading to the formation of CO vapor
During the cooling stage, reaction (9) can occur,
Since SiC has much higher melting point than SiO2, the solidification of SiC occurs faster than that of SiO2 and the amorphous, viscous SiO2 may enclose the crystalline SiC nano-wires[27,28] This reaction leads to the decrease in enthalpy and Gibbs energy at temperature below 900 1C As compared with reactions (2) and (3), this reaction is thermodynamically favor-able, and produces large mounts of SiC/SiO composite nanowires
Fig 6 HR-TEM image of b-SiC nanowire The inset is the corresponding fast
Fourier transform (FFT).
Fig 7 IR spectrum of the as-synthesized SiC nanowires sample.
Fig 8 Room-temperature PL spectrum ofb-SiC nanowires.
Trang 54 Conclusions
In summary, scales of pure crystallineb-SiC nanowires with
diameters about 50 nm were synthesized using AAO template by
direct thermal evaporation without any metal catalyst at high
temperature The as-synthesized products mainly consist ofb-SiC
nanowires By means of XRD, SEM, EDS, IR and TEM (HR-TEM),
b-SiC nanowires have been characterized and discussed in detail
The growth direction of nanowires lies along the /111S
direction The tentative growth model according to the SiC
nanowires growth process was suggested Finally, optical property
is found in the photoluminescence emission from b-SiC
nano-wires, which is different from previous observations of SiC
materials We believe that the pure crystallineb-SiC nanowires
with a small diameter described herewith will express excellent in
fields of high mechanical strength material, and will be of use for
application in electronic circuits, in light-emitting devices and in
other advanced blocks of nanodevices
Acknowledgements
This research work is supported by the Creative Research
Group of National Science Foundation of China (Grant no
50721003), the Foundation of the Ministry of Education of China
for Returned Scholars (Grant no 2005383) and the National Basic
Research Program of China (Grant no 2006CB933000)
References
[1] Z.L Wang, Adv Mater 12 (2000) 1295.
[2] Y Xia, P Yang, Y Sun, Y Wu, B Mayers, B Gates, Y Yin, F Kim, H Yan, Adv.
Mater 15 (2003) 353.
[3] W.M Zhou, Z.X Yang, F Zhu, Y.F Zhang, Physica E 31 (2006) 9.
[4] W.M Zhou, L.J Yan, Y Wang, Y.F Zhang, Appl Phys Lett 89 (2006) 13105.
[5] Z Pan, H.L Lai, F.C.K Au, X Duan, W Zhou, W Shi, N Wang, C.S Lee, N.B Wong, S.T Lee, S Xie, Adv Mater 12 (2000) 1186.
[6] A.M Morales, C.M Lieber, Science 279 (1998) 208.
[7] D.P Yu, C.S Lee, I Bello, X.S Sun, Y.H Tang, G.W Zhou, Z.G Bai, Z Zhang, S.Q Feng, Solid State Commun 105 (1998) 403.
[8] Z.S Wu, S.Z Deng, N.S Xu, C Jian, J Zhou, C Jun, Appl Phys Lett 80 (2002) 3829.
[9] B.Q Wei, J.W Ward, R Vajtai, P.M Ajayan, R Ma, G Ramanath, Chem Phys Lett 354 (2002) 264.
[10] J Wei, K.Z Li, H.J Li, Q.G Fu, L Zhang, Chem Phys 95 (2006) 140 [11] K Saulig.Wenger, D Cornu, F Chassagneux, G Ferro, T Epicier, P Miele, Solid State Commun 124 (2002) 157.
[12] L.Z Pei, Y.H Tang, Y.W Chen, C Guo, X.X Li, Y Yuan, Y Zhang, J Appl Phys 99 (2006) 114306.
[13] L.Z Pei, Y.H Tang, X.Q Zhao, Y.W Chen, J Mater Sci 42 (2007) 5068.
[14] X.T Zhou, N Wang, F.C.K Au, H.L Lai, H.Y Peng, I Bello, C.S Lee, S.T Lee, Mater Sci Eng A 286 (2000) 119.
[15] T.E Bogart, S Dey, K.K Lew, S.E Mohney, J.M Redwing, Adv Mater 17 (2005) 114.
[16] Z Li, J Zhang, A Meng, J Guo, J Phys Chem B 110 (2006) 22382 [17] Z Zhang, X.D Han, Y.F Zhang, K Zheng, X.N Zhang, Y.J Hao, X.Y Guo, J Yuan, Z.L Wang, Nano Lett 7 (2007) 452.
[18] H.W Shim, H Huang, Appl Phys Lett 90 (2007) 083106.
[19] S Perisanu, P Vincent, A Ayari, M Choueib, S.T Purcell, M Bechelany,
D Cornu, Appl Phys Lett 90 (2007) 043113.
[20] H.W Shin, H.C Huang, Nanotechnology 18 (2007) 335607.
[21] H von Berlepsch, C Bottcher, A Ouart, C Burger, S Dahne, S Kirstein, J Phys Chem B 104 (2000) 5255.
[22] M Il-Shik, C Gyoujin, Mater Sci Eng C 24 (2004) 301.
[23] H.K Seong, H.J Choi, S.K Lee, J.I Lee, D.J Choi, Appl Phys Lett 85 (2004) 1256.
[24] X.M Liu, K.F Yao, Nanotechnology 16 (2005) 2932.
[25] H.J Choi, H.K Seong, J Cryst Growth 269 (2004) 472.
[26] H Ye, N Titchenal, Y Gogotsi, F Ko, Adv Mater 17 (2005) 1531.
[27] S.Z Deng, Z.B Li, W.L Wang, N.S Xu, Z Jun, X.G Zheng, H.T Xu, C Jun, J.C She, Appl Phys Lett 89 (2006) 23118.
[28] B Park, Y Ryu, K Yong, Surf Rev Lett 11 (2004) 373.