Preparation and photoluminescence properties of amorphous silica nanowires

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

Preparation and photoluminescence properties of amorphous

silica nanowires

Laboratory of Internal Friction and Defects in Solids, Institute of Solid State Physics, Academia Sinica, P.O Box 1129,

Hefei 230031, People's Republic of China Received 10 October 2000; in ®nal form 4 January 2001

Abstract

Bulk-quantity amorphous silica nanowires (SiONWs) have been synthesized by carbothermal reduction reaction between silicon dioxide and active carbons Transmission electron microscopy (TEM) image shows the formation of the nanowires at a diameter of 60±110 nm and a length up to hundreds micrometers Besides most smooth-surface polyp-shaped nanowires, two other forms of nanowires, named amoeba-polyp-shaped and frog-egg-polyp-shaped nanowires, have also been observed The nanowires can emit stable and high brightness blue light at 435 nm (2.85 eV) under excitation at 260

nm (4.77 eV) The formation of the nanowires into di€erent shapes may be explained by the vapor±liquid±solid (VLS) mechanism Ó 2001 Published by Elsevier Science B.V

1 Introduction

One-dimensional quantum wires are of great

scienti®c interest due to their great potential for

testing and understanding fundamental concepts

about the roles of dimensionality in mesoscopic

physics and for applications in nanodevices [1,2]

For instance, nanotweezers made of carbon

na-notubes can be used to manipulate submicron

clusters and nanowires [3] GaAs and InAs

nano-wires have found applications in developing

one-dimensional high-speed ®eld e€ect transistor, or

laser working at low-threshold current density and

high gain [4] GaN nanowires may be fabricated

into one-dimensional nanoscale luminescence

di-odes [5] With the development of mesoscopic

science and the advances in integrated optical

technology, it is important to synthesize nanowires with optical properties that can meet the demands

of further applications Silica as photolumines-cence (PL) materials has long been concerned for, and people have continually reported PL bands with peak energies around 1.9±4.3 eV for bulk SiO2 or SiO2 ®lms [6±12] Recently, Yu et al [13] have also reported fabrication and intensive blue light emission phenomenon of silica nanowires (SiONWs), and pointed out their potential ap-plications in high-resolution optical heads of scanning near-®eld optical microscope or nanoin-terconnections in future integrated optical devices Therefore, SiO2 nanowires are a promising one-dimensional luminescence materials, but they have only been synthesized by using excimer laser ab-lation In this Letter, we report the carbothermal reduction synthesis and characterization and PL properties of the nanowires Their growth process has also been discussed

9 March 2001

Chemical Physics Letters 336 (2001) 53±56

www.elsevier.nl/locate/cplett

* Corresponding author Fax: +86-551-5591434.

E-mail address: jjdu@mail.issp.ac.cn (X.C Wu).

0009-2614/01/$ - see front matter Ó 2001 Published by Elsevier Science B.V.

PII: S 0 0 0 9 - 2 6 1 4 ( 0 1 ) 0 0 0 6 3 - X

2 Experimental

The mixtures of SiO2…4 g†, Fe…NO3†3
9H2O

(250 mg) and active carbons (4 g) were ball-milled

for 20 h in ethanol media After desiccated, they

were pressed into several circular pellets

(U1 cm  0:5 cm) under 10 Mpa The pellets were

placed at the center of conventional horizontal

furnace with a sintered alumina tube (U2:5 cm 

100 cm) and calcined at 1350°C for 3 h in ¯owing

argon (40 ml/min) A white product was found to

deposit on the surface of the pellets and the

ther-malcouple TEM images of the products were taken

with a JEM-200CX transmission electron

micro-scope The composition of SiONWs was

deter-mined by the X-ray photoemission spectra (XPS),

which were recorded on a VGESCALAB MKII

X-ray photoelectron spectrometer XPS data were

collected in the constant analyzer energy (CAE)

mode at 20 eV Mg Ka (hm ˆ 1253:6 eV) radiation

was employed as excitation source with an anode

voltage of 12 KV and an emission current of

20 mA PL spectrum was measured in a Hitachi 850

¯uorescence spectrophotometer with a Xe lamp at

room temperature The excitation wavelength was

260 nm, and the ®lter wavelength was 310 nm

3 Results and discussion

TEM micrography shows the general

morphol-ogy and dimension of SiONWs The nanowires

shown in Fig 1(a) look like polyp with trunks and

branches, but their surface is smooth The trunk

has 110 nm in diameter and hundreds micrometers

in length by scanning throughout the sample The

brunches have the diameter about 60 nm and a

length up to 5 lm Electron di€raction

micro-graphs (inset) show that the nanowires are

amor-phous Fig 1(b) indicates the amoeba-shape of the

nanowires, with the diameter of about 70 nm and a

length up to tens of micrometers Fig 1(c) shows

frog-egg morphology of the nanowires, which is

similar to wire-like silica nanosphere agglomerates

[14] Circular nanoparticles are connected via

necks to form pearl-like chains The largest

parti-cles have the diameter about 100 nm, while the

smallest particles have the diameter about 60 nm

Due to low reaction temperature, no silicon car-bide nanowires were observed The reactants were suciently ball-milled to accelerate chemical reaction If the time grinding the reactants were reduced, we could still obtain the SiONWs Further evidence for the formation of SiO2

nanowires can obtained through XPS The two strong peaks at 103.35 and 532.65 eV as shown in

Fig 1 TEM morphologies of SiONWs (a) polyp-shaped SiONWs; (b) amoeba-shaped SiONWs; (c) frog-egg-shaped SiONWs.

54 X.C Wu et al / Chemical Physics Letters 336 (2001) 53±56

Fig 2(b) and (c) correspond to the binding ener-gies of Si(2p) and O(1s) for SiO2, respectively No obvious Si peaks (Si2p 98.7 eV in Si) are observed The quanti®cation of the peaks reveals that atomic ratio of Si to O is 1:2.41 Obviously, the observa-tion of oxygen must be due to adsorpobserva-tion and surface contamination of the sample The survey spectrum in Fig 2(a) also displays C(1s) (at 284.65 eV) peak, which can be attributed to a small amount of the residual graphite (284.3 eV for C1s

in graphite)

As is shown in Fig 3, a stable and strong blue light emission is revealed at 435 nm (2.85 eV) at room temperature under excitation at 260 nm while ultraviolet and blue light emission at 350 nm (3.54 eV), 420 nm (3.0 eV), and 465 nm (2.7 eV) can also be observed Compared with [13], intensive peak at 420 nm in [13] changes into shoulder in Fig 3 while shoulder at 435 nm into intensive peak The shoulder at 465 nm in Fig 3 approaches the peak at 470 nm in [13] Ultraviolet light emission at

350 nm was observed in oxidized porous silicon and annealed SiO2 [15] The PL spectrum is also di€erent from that of oxidized Si nanowires [16] The growth process of the nanowires can be explained by the vapor±liquid±solid (VLS) mech-anism, since little droplets can be seen at the tops

of the nanowires [17] The growth of the nanowires

Fig 2 XPS of the sample (a) survey spectrum of the sample;

(b) Si2p binding energy spectrum; (c) O1s binding energy

spectrum.

Fig 3 PL spectrum of the SiONWs at room temperature under excitation at 260 nm.

X.C Wu et al / Chemical Physics Letters 336 (2001) 53±56 55

could be divided into three steps The ®rst step is

that silica is reduced by active carbon to silicon

and silicon monoxide, and then silicon reacts with

iron to form FeSi2 [18] The second step is that

FeSi2 is evaporated on the surface of the pellets

and the thermalcouple to become liquid-phase

catalytic growth center The third step is that the

vapor of silicon and silicon monoxide is

trans-ported to the catalytic center to form SiO and Si

nanowires by VLS mechanism while both silicon

and silicon monoxide are all oxidized to

amor-phous SiO2 during cooling In the above-growth

model, the nucleation step can be further divided

into monocentric and polycentric nucleation, and

growth step into periodic stable growth and

peri-odic unstable growth [19] Thus the combination

of di€erent nucleation and growth processes can

give rise to di€erent forms of SiONWs, which is

similar to the growth model of Si nanowires [20]

The formation process of polyp-shaped SiONWs

is considered to be due to the coexistence of

monocentric and polycentric nucleation and of the

periodic stable growth on the basis of the trunks

and the branches with even diameters When

trunks stably grow in monocentric nucleus, some

FeSi2 nanoparticles deposit on the surface of the

trunks to become many new growth centers,

namely, polycentre, resulting in the formation of

branch-shaped nanowires The branch-shaped

nanowires can grow stably, but the stability is only

relative Amoeba-shaped nanowires are attributed

to monocentric nucleation and periodic unstable

growth to exhibit a typical periodic instability of

diameter The block dots of the polyp-shaped and

amoeba-shaped SiONWs are the sites of nuclei

However, the periodicity is not strict and di€ers

for various nanowires Frog-egg-shaped nanowires

are due to wire-like arrangement of deposited

sil-icon oxide nanoparticles by surface tension, and

the necks are formed between nanoparticles at

high temperatures, so its growth process also

be-longs to that of polycentric nucleation

4 Conclusions

Amorphous SiONWs have been successfully

synthesized on large scale using a carbothermal

reduction approach at 1350°C in a ¯owing argon atmosphere Three types of di€erent shapes of SiONWs have been observed The periodic stable growth and unstable growth of the nanowires co-exist in the product

Acknowledgements This work was supported by the Ministry of Science and Technology of China (NKBRSF-G19990646), National Science Foundation under contract NSF 59872043, the Fundamental Science Bureau, Academia Sinica

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