Large scale synthesis, characterization and photoluminescence properties of amorphous silica nanowires by thermal evaporation of silicon monoxide

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

Large-scale synthesis, characterization and photoluminescence properties of amorphous silica nanowires by thermal evaporation of silicon monoxide

Sanjay K Srivastavaa, , P.K Singha, V.N Singhb, K.N Sooda, D Haranatha, Vikram Kumara

a

National Physical Laboratory, Dr K S Krishnan Marg, Pusa, New Delhi 110012, India

b Department of Physics, Indian Institute of Technology Delhi, Hauz Khas, New Delhi 110016, India

a r t i c l e i n f o

Article history:

Received 29 March 2009

Received in revised form

27 April 2009

Accepted 27 April 2009

PACS:

61.46.–w

81.07.–b

Keywords:

Silicon monoxide

Silicon oxide nanowires

Thermal evaporation

Photoluminescence

a b s t r a c t

A single step non-catalytic process based on thermal evaporation of silicon monoxide has been established for large-scale synthesis of silica nanowires Scanning electron microscopy, high-resolution transmission electron microscopy equipped with energy dispersive X-ray spectrometry (EDAX), X-ray diffractometry were used to characterize the morphology and structure of the material The as-synthesized nanowires had amorphous structures with diameters in the range 30–100 nm and hundreds of micrometers in length The EDAX analysis revealed that the nanowires consisted of mainly two elements Si and O in an atomic ratio of approximately 1:2 corresponding to silicon dioxide Photoluminescence spectra of the silica nanowires showed strong blue emission around 393 nm Nucleation and growth of silica nanowires has been discussed on the basis of tiny oxide cluster formation that acts as nucleation centers for the nanowires growth

&2009 Elsevier B.V All rights reserved

1 Introduction

During past two decades, a lot of attention has been paid to the

growth and characterization of one-dimensional (1-D)

nanos-tructures such as nanotubes, nanowires, nanobelts because of

their distinctive structure, unique properties and applications

[1–3] Silicon-based nanostructures have attracted significant

attention due to their potential applications in electronics and

opto-electronic devices [4] For example, silicon oxide (SiOx)

nanowires show intensive blue light emission, which may be a

candidate material for high-resolution optical heads of scanning

near-field optical microscopes, nanointerconnection integrated

optical devices, low-dimensional wave-guides, etc.[5–7] Several

methods such as laser ablation[5,8], thermal evaporation[7,9],

carbothermal reduction or carbon-assisted growth[6,10,11], direct

thermal oxidation of Si wafers [12–14] have been used to

synthesize SiOx nanowires However, most of these methods

employ metal catalysts such as Au[7,8,11,14], Ni[14,15], Fe[5,6],

Co[16], Ga[17,18], Cu[19], Sn[20]to assist the synthesis process

and consequently, the nanowires have significant presence of

embedded residual metallic impurities that may affect their

properties In the recent past, non-catalytic growth of silica

nanowires via carbothermal reduction of metal oxides such as MgO, CuO, WO3 has also been reported Despite considerable experi-mental efforts, the growth mechanism of silica nanowires is not well understood and indeed no consensus about the growth mechanisms has been achieved One school of thoughts believes that vapor– liquid–solid (VLS) [5,6,21]or solid–liquid–solid (SLS) [13,15] pro-cesses are the possible mechanisms in catalyst-assisted growth of amorphous SiOx(a-SiOx) nanowires Other school suggested differ-ent chemical reactions and sequences for the a-SiOx nanowires formation[10,11,17]to explain their experimental results Recently, Aharonovich and Lifshitz[22]found that metal catalyst is essential for SiOx nanowires growth and proposed an alternative catalyst-assisted mechanism based on preferential adsorption of SiOxon the catalyst droplet without penetration into it

In this paper, we report large-scale synthesis of pure silica (SiO2) nanowires by a non-catalytic approach based on thermal evaporation of SiO under argon ambient with traces of oxygen The process is simple yet elegant and involves only single process step wherein the SiO vapors are transported from hot zone (1200 1C) to downstream low-temperature zone where they are allowed to condense on a substrate The structure and photo-luminescence (PL) property of the as-deposited material has been investigated and the growth of SiO2nanowires is discussed on the basis of tiny SiO2 cluster formation via direct reaction of SiO vapors with O2 that subsequently acts as nucleation center for SiO2nanowire growth

Contents lists available atScienceDirect

journal homepage:www.elsevier.com/locate/physe

Physica E

1386-9477/$ - see front matter & 2009 Elsevier B.V All rights reserved.

doi: 10.1016/j.physe.2009.04.032



Corresponding author Tel.: +9111 4560 8617; fax: +9111 2572 6938.

E-mail address: srivassk@mail.nplindia.ernet.in (S.K Srivastava).

2 Experimental

The growth process was carried out in a conventional

three-zone horizontal quartz tube furnace, the schematic of which is

shown in Fig 1 The requisite amount of source material, i.e.,

silicon monoxide (SiO) granules (purity 99.9%; Pure Tech Inc.,

New York, USA) was kept in an alumina boat that was placed in

the center of the quartz tube Ultrasonically cleaned silicon strips

of 2  2 cm2with and without Ni film were placed downstream in

the lower temperature zone of the furnace on an alumina sample

holder Thin Ni film (Ni powder, purity 99.99%; CERAC Inc., USA)

was deposited on cleaned silicon wafers by thermal evaporation

technique at a base pressure of 3.0  106Torr The source SiO and

the substrates were inserted in the tube and kept at locations at

1200 1C (center zone) and 1000 1C (in the direction of carrier

gas flow), respectively, identified earlier by temperature profiling

The quartz tube system was purged with a carrier gas (argon) flow

for three hours before heating up to 1200 1C under a constant

argon flow at the rate of 40 l/h The source SiO was then heated at

1200 1C for 1 h in argon ambient (99.95%) with traces of oxygen

and moisture Therefore, the growth time was 1 h After

completion of the growth process the system was cooled down

to room temperature under Ar flow A thick wool-like spongy,

white-colored material was deposited on the silicon substrates

(both on Ni coated and without Ni film) as well as on the alumina sample holder A thick white-colored material was also deposited

on the walls of the quartz tube in low-temperature zone To investigate the role of SiO as source, only silicon substrates were heated at 1000 1C (since the silicon substrates were placed at

1000 1C with source SiO at 1200 1C) for 1 h without SiO granules

at the center zone No deposition was found on the silicon substrates in this case

The as-deposited film on silicon substrates and the material collected from the quartz tube as well as from alumina sample holder were examined by scanning electron microscopy (SEM; LEO 440 VP) operating at 15 kV Structural analysis of the material was carried out by X-ray diffractometery (XRD) (Bruker, D-8 ADVANCE diffractometer) with CuKa radiation, high-resolution transmission electron microscopy (HRTEM) (FEI, Technai G20-stwin; 200 kV) equipped with energy dispersive X-ray spectro-scopy (EDAX) (EDAX company, USA) A part of the white film was delaminated from the Si wafer and was ultrasonically dispersed in ethanol for 10 min A few drops of the suspension were placed on carbon-coated copper (Cu) microgrid for TEM and HRTEM investigations PL measurement of the material was carried out

at room temperature using double monochromator-based spec-trometer (PerkinElmer LS55) with xenon flash lamp as excitation source

3 Results and discussion 3.1 Microstructural analysis

A low-magnification SEM micrograph of the as-deposited wool-like thick white film on Ni-coated silicon wafer and its magnified view is shown inFig 2(a) and (b), respectively, where high density of 1-D nanostructures in the form of wires having hundreds of micrometers length is clearly seen Fig 2(a) also reveals that several layers of nanowires were deposited one over another and total thickness of the film is estimated to be more

Ar

Outlet

3- Zone Tube Furnace

Quartz Tube

Si wafers

Fig 1 Schematic diagram of horizontal furnace set up for the synthesis of silica

nanowires.

10 µm

2 µm

25 µm

Fig 2 SEM micrographs of (a) nanowires deposited on Ni-coated Si wafers (low magnification), (b) magnified view of (a), (c) nanowires deposited on Si wafers without catalyst Ni film, (d) product collected from quartz tube magnified view of which is shown in the inset.

than 100mm Fig 2(b) gives an idea of nanowires diameter

distribution which, indeed, is quite uniform The nanowires

formation is straight, free from wrinkles and chain-like

morphology It is to be noted that a thick white layer was

deposited over the entire area of the Ni-coated Si wafers even on

the side edges To study the role of Ni on the growth of nanowires,

we performed identical experiment on silicon wafers without Ni

film as well as on alumina substrate holder A high density

of nanowires was also observed on Si wafers without Ni film (see

Fig 2(c)) similar to that observed with Ni film This clearly

indicates the non-catalytic growth of nanowires, which was

further confirmed by the investigations of the white spongy

material deposited on the inner surface of the quartz tube and also on the alumina sample holder Fig 2(d) shows the low-magnification SEM image of the substance collected from the quartz tube, which shows agglomerated clusters consisting

of high density of nanowires The magnified view of a single cluster is depicted in the inset of Fig 2(d) These observations suggest that growth of nanowires in the present process is probably not governed by the VLS or SLS mechanism, which essentially is a catalytic process The present results are different from that of Aharonovich et al [8,22] where metal catalyst (Au or Ni) was found to be essential for the SiOx nanowires growth and also different from bi-cycle chain-like morphology

of silica nanowires observed by Kar and Chaudhuri [14]

during carbon-assisted non-catalytic growth It is also to be remarked here that no nanowire growth was observed on silicon substrates heated at 1000 1C without SiO source at center of tube (1200 1C), which clearly indicates that the nanowires growth in the present process does not take place due to silicon substrates heating in presence of oxygen traces or moisture The growth of thick nanowire film on alumina substrate holder is also evidence that SiO is the main source for silicon nanowires growth

Fig 3(a) shows a typical TEM micrograph of nanowires deposited on Ni-coated Si wafers The nanowires have diameter

in the range 30–100 nm with center of the distribution at 50 nm The diameters remain nearly constant throughout the length of the nanowires The nanowires have remarkably clean and smooth surface It is important to note here that the nanowires have circular cross-section revealing the cylindrical nature (shown by circles inFig 3(a)) and no metal particles are seen at either end of the wires The TEM investigations of the material further confirm our view that growth of nanowires is essentially non-catalytic in the present process

3.2 Structural and compositional analyses The XRD patterns (not shown here) revealed amorphous character of the deposited nanowires film, which was further confirmed by the HRTEM (Fig 3(b)) study The selected area electron diffraction (SAED) pattern (shown in the inset ofFig 3(b)) recorded from a single nanowire where only diffusive rings reveal the amorphous nature of the nanowires No lattice fringes could

be resolved in the HRTEM across the diameter of the nanowires (Fig 3(b)) Furthermore, no Si-SiO2core-shell kind of structure as seen by Park and Yong[13]and Zhang et al.[23]was observed This reveals that the nanowires have uniform amorphous structure across the length and diameter EDAX spectra of the nanowires were also recorded during TEM investigation to examine their chemical composition The EDAX spectrum shown

in Fig 4 for a single nanowire reveals presence of only two elements Si and O with an atomic ratio of approximately 1:2 (strong C and Cu signals are attributed to the carbon-coated Cu microgrid) Based on the above observations, we may conclude that the nanowires are amorphous silicon dioxide (SiO2) 3.3 Photoluminescence

Fig 5 shows the PL spectrum of SiO2 nanowires recorded at room temperature with 241 nm excitation A strong blue luminescence is observed with peak position at 393 nm (3.15 eV) The PL properties of silica nanowires have been investigated before showing single or two PL bands depending

on their structural properties [5,7,24] For example, Yu et al [5]

observed two broad PL peaks of SiO2 nanowires at 470 nm (2.65 eV) and 420 nm (3.0 eV), whereas Zhu et al [24]

100 nm

5 nm

Fig 3 (a) TEM micrographs of the nanowires showing metal-free ends with

circular cross-section (indicated by circles), (b) HRTEM micrograph of a nanowire

showing amorphous structure The SAED pattern of the nanowire is shown in the

inset of (b).

reported that two broad PL peaks of SiOxwere at around 570 nm

(2.2 eV) and 430 nm (2.88 eV) On the other hand, Wang et al

[7] observed single broad PL peak at 446 nm (2.78 eV) from

SiOx nanowires These emissions have been attributed to the

structural defects related to oxygen deficiency in the silica

nanowires that act as radiative recombination centers

Nishikawa et al.[25]have observed several luminescence bands

in the range 1.9–4.3 eV in various types of high-purity silica

glasses where the band at 3.1 eV was attributed to some intrinsic

diamagnetic defect centers, such as twofold coordinated silicon

lone pair centers (O–Si–O) caused by high oxygen deficiency in

the samples Therefore, the observed blue light emission from the

silica nanowires in the present study could have its origin to the

structural defects such as oxygen deficiency, which might have

been generated during the nanowires growth

3.4 Growth mechanism The growth mechanism in 1-D nanostructures has been explained by the screw dislocation model and the VLS model in the past The former is not appropriate in case of amorphous nanowires whereas the latter is based on the three-step mechan-ism involving (i) diffusion of Si/SiOx vapors into the metal particles, (ii) formation of liquid droplet of metal and Si/SiOx and (iii) the precipitation in the form of solid nanowires after super-saturation of liquid metal–Si/SiOx droplet at the liquid– solid interface Thus a metal nanoparticle at one end of the nanowires is usually considered as the evidence for the operation

of the VLS model[26] But no metal particles were found at either end of the nanowires in the present study as confirmed by SEM and TEM results Therefore, VLS or SLS mechanism is not pertinent

to explain the present experimental observations Further, growth

of SiO2 nanowires on silicon wafers without Ni film, alumina holder and quartz tube rules out the metal catalyst-assisted growth mechanism by Aharonovich and Lifshitz [22] In the present case, the growth of the SiO2 nanowires cannot be explained using the oxide-assisted growth (OAG) process pro-posed by Lee et al.[26–28]for the crystalline Si nanowires The OAG process presumes an oxide cluster rather than a metal particle after the VLS mechanism to assist the formation of the nanowires wherein an outer oxide shell formation is essential to prevent the growth along the lateral direction that results in 1-D Si-SiOxcore-shell nanowire[26]formation However, the nano-wires grown in the present study are amorphous across the diameter instead of crystalline Si core and amorphous SiO2outer shell structure as confirmed by HRTEM image and SAED pattern

On the other hand, the formation of crystalline Si–SiO2core-shell nanowires first and then complete oxidation of the structure to result SiO2nanowires is also not practically possible as discussed

by Buttner and Zacharias [29] Therefore, what could be the driving force for the formation of 1-D amorphous SiO2nanowires? Since no crystalline Si embedded in SiO2 outer shell was observed unlike some earlier reports on growth of Si nanowires by SiO evaporation[23], the concept that SiO first disproportionate into Si and SiO2to form Si nanowires with SiO2outer layer may not be applied Therefore, the following reaction mechanism may

be proposed to explain our observations The SiO vapors, generated at temperature 1200 1C, are transported downstream towards the substrates by the carrier gas and during traversal they get converted into SiO2 molecules directly by reacting with O2 according to the following equation:

2SiOðgÞ þ 2O2ðgÞ ! 2SiO2ðsÞ

It may be remarked here that in the present experiment no special arrangement was used to remove residual O2 from the carrier gas or from the process chamber (either by vacuum or hydrogen gas or O2traps) consequently, the traces of residual O2

in the chamber could be present The SiO2 molecules so formed condense on the substrate or quartz tube wall in the low-temperature zone (1000 1C) to form SiO2nanoclusters that then act as nucleation center for the growth of SiO2 nanowires Subsequently, SiO2 nanoclusters may aggregate to induce 1-D SiO2 nanostructures to minimize its systemic energy [30] The proposed mechanism may find theoretical support by Zhang and Zhang [31] who found that growth of energetically favored anisotropic 1-D silica nanowires may occur without metal catalyst template by short-range ordering of building blocks such as (SiO2)8 clusters They showed that silica cluster (SiO2)8 is geometrically highly symmetric structure, energetically highly stable with high chemically reactive ends of SiQO groups, and thus make it easy to be assembled into larger linearly extended

Fig 4 EDAX spectrum of a nanowire (shown in the inset) showing Si and O as

main detected elements The quantitative data is also shown in the inset.

300

393 nm

Wavelength (nm)

Fig 5 Room temperature PL spectrum of the SiO 2 nanowires recorded with

241 nm excitation.

clusters and hence into 1-D silica nanowires The present study

shows that no preferential site or morphology of the substrate is

required for the formation of the SiO2 nanowires Though exact

formation mechanism of silica nanowires from SiO vapors, factors

controlling the diameters of the nanowires in the present approach,

remains still unclear and needs further detail investigations, the

present work is useful due to its simplicity and the low cost

4 Conclusions

Amorphous silicon dioxide nanowires of several hundred

microns in length and tens of nanometers in diameter have been

synthesized in bulk by a non-catalytic single step process using

thermal evaporation of silicon monoxide under argon atmosphere

with traces of oxygen The nanowires were free from metal

contaminations and showed blue photoluminescence at room

temperature It is proposed that in-situ formation of SiO2vapors

via reaction of SiO vapors with O2leads to the formation of SiO2

nanoclusters, which consequently results in the formation of large

nanowires The present simple and low-cost process of producing

pure silica nanowires (free from metallic contaminations) in bulk

may lead to potential applications in nanoelectronics and optical

devices

Acknowledgements

The authors wish to thank Ms Manisha and Dr S.K Halder for

XRD measurements of the samples and the Director, NPL for his

permission to publish this work

References

[1] C.N.R Rao, F.L Deepak, G Gundiah, A Govindaraj, Prog Solid State Chem 31

(2003) 5.

[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] C.N.R Rao, A Govindaraj, S.R.C Vivekchand, Annu Rep Prog Chem Sect A

102 (2006) 20.

[4] L.J Chen, J Mater Chem 17 (2007) 4639.

[5] D.P Yu, L Hang, Y ding, H.Z Zhang, Z.G Bai, J.J Wang, Y.H Zou, W Qian, G.C Xiong, S.Q Feng, Appl Phys Lett 73 (1998) 3076.

[6] X.C Wu, W.H Song, K.Y Wang, T Hu, B Zhao, Y.P Sun, J.J Du, Chem Phys Lett 336 (2001) 53.

[7] Y.W Wang, C.H Liang, G.W Meng, X.S Peng, L.D Zhang, J Mater Chem 12 (2002) 651.

[8] I Aharonovich, S Tamir, Y Lifshitz, Nanotechnology 19 (2008) 065608 [9] C.H Liang, L.D Zhang, G.W Meng, Y.W Wang, Z.Q Chu, J Non-Crystalline Solids 277 (2000) 63.

[10] Y.C Lin, W.T Lin, Nanotechnology 16 (2005) 1648.

[11] S.H Li, X.F Zhu, Y.P Zhao, J Phys Chem B 108 (2004) 17032.

[12] J.Q Yu, Y Jiang, X.M Meng, C.S Lee, S.T Lee, Chem Phys Lett 367 (2003) 339 [13] B.T Park, K Yong, Nanotechnology 15 (2004) S365.

[14] S Kar, S Chaudhuri, Solid State Commun 133 (2005) 151.

[15] H.F Yan, Y.J Xing, Q.L Hang, D.P Yu, Y.P Wang, J Xu, Z.H Xi, S.Q Feng, Chem Phys Lett 323 (2000) 224.

[16] H Takikawa, M Yatsuki, T Sakakibara, Jpn J Appl Phys 38 (1999) L401 [17] B Zheng, Y Wu, P Yang, J Liu, Adv Mater 14 (2002) 122.

[18] Z Pan, S Dai, D.B Beach, D.H Lowndes, Nano Lett 3 (2003) 629.

[19] H.W Kim, S.H Shim, J.W Lee, Physica E 37 (2007) 163.

[20] S.H Sun, G.W Meng, M.G Zhang, Y.T Tian, T Xie, L.D Zhang, Solid State Commun 128 (2003) 287.

[21] 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) 405.

[22] I Aharonovich, Y Lifshitz, Appl Phys Lett 90 (2007) 263109.

[23] Y.F Zhang, Y.H Tang, C Lam, N Wang, C.S Lee, I Bello, S.T Lee, J Cryst Growth 212 (2000) 115.

[24] Y.Q Zhu, W.B Hu, W.K Hsu, M Terrones, N Grobert, T Karali, H Terrones, J.P Hare, P.D Townsend, H.W Kroto, D.R.M Walton, Adv Mater 11 (1999) 844.

[25] H Nishikawa, T Shiroyama, R Nakamura, Y Ohki, Phys Rev B 45 (1992) 586 [26] R.Q Zhang, Y Lifshitz, S.T Lee, Adv Mater 15 (2003) 635.

[27] N Wang, Y.F Zhang, Y.H Tang, C.S Lee, Phys Rev B 58 (1998) R16024 [28] S.T Lee, Y.F Zhang, N Wang, Y.H Tang, I Bello, C.S Lee, Y.W Chung, J Mater Res 14 (1999) 4503.

[29] C.C Buttner, M Zacharias, Appl Phys Lett 89 (2006) 263106.

[30] Y Zhang, N Wang, R He, J Liu, X Zhang, J Zhu, J Cryst Growth 233 (2001) 803.

[31] D Zhang, R.Q Zhang, J Phys Chem B 110 (2006) 1338.