Ultrafast growth of single crystalline si nanowires

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Ultrafast growth of single-crystalline Si nanowires J.B Chang, J.Z Liu, P.X Yan ⁎ , L.F Bai, Z.J Yan, X.M Yuan, Q Yang

Institute for Plasma and Metal Materials, Lanzhou University, Lanzhou 730000, China

Received 25 September 2005; accepted 22 December 2005

Available online 31 January 2006

Abstract

Silicon nanowires (SiNWs) have been catalytically synthesized by heat treatment of Si nanopowder at 980 °C The SiNWs comprise crystalline

Si nanoparticles interconnected with metal catalyst The formation mechanism of nanowires generally depends on the presence of Fe catalysts in the synthesis process of solid–liquid–solid (SLS) Although gas phase of vapor–liquid–solid (VLS) method can be used to produce various of different nanowire materials, growth model based on the SLS mechanism by heat treatment is more ascendant for providing ultrafast growth of single-crystalline Si nanowires and controlling the diameter of them easily The growth of single-crystalline SiNWs and morphology were discussed

© 2006 Elsevier B.V All rights reserved

Keywords: SiNWs

1 Introduction

Crystalline nanostructures offer unique access to

low-dimensional physics, and they can be used as nanotechnology

building blocks to reach higher device integration densities than

conventional fabrication methods and have more singularity

character One-dimensional (1D) structure with nanometer

diameters, such as carbon nanotubers and semiconductor

nanowires, has great potential for testing and understanding

fundamental concepts about the roles of dimensionality and

size, for example, optical, electrical, and mechanical properties

and for their potential applications in research and electronic

nanodevices[1] It's known that Si nanowires (SiNWs) have the

strong ability to confine photoenergy from visible light [2]

SiNWs are particularly attractive due to the central role of the

silicon semiconductor industry, which would allow SiNWs to

be implemented using existing technologies Because silicon

turns into a direct band-gap semiconductor at nanometer size

due to quantum confinement [3], it could be used in

optoelectronics SiNWs can be synthesized by laser ablation

[1], thermal evaporation of solid sources[4–6] and chemical

vapor deposition (CVD)[7] The various directional features of

these techniques were reported and the model proposed for

preferred SiNWs growth directions[8] These methods are often based on the vapor–liquid–solid (VLS idea [9]using various metals as catalysts, such as Au, Fe, Ti and Ga, to enhance the growth of SiNWs In this work, we demonstrate a simple method of growing SiNWs Si nanopowder was used in our work instead of the dangerous gas of silane as the Si source The synthesis of SiNWs was carried out using a mixture of Si nanoparticles and iron nitrate by thermal treatment at 980 °C in

an evacuated sealed quartz tube The key parameter necessary to induce nanowires formation is the temperature and catalyst

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⁎ Corresponding author Tel.: +86 931 8912661; fax: +86 931 8913554.

E-mail address: pxyan@lzu.edu.cn (P.X Yan) Fig 1 XRD spectrum of Si nanoparticles prepared by cathode arc plasma 0167-577X/$ - see front matter © 2006 Elsevier B.V All rights reserved.

doi: 10.1016/j.matlet.2005.12.085

2 Experiment

Before the preparation of producing SiNWs, Si nanoparticles were synthesized by cathode arc plasma which is one of the most powerful methods because of uniform particles and high efficiency, using a mixture of SiO2 and C with a molar proportion (1 : 1) in argon atmosphere The discharge voltage is

20 V and the current is 120 A The flux of hydrogen as protective gas was 15 standard cubic centimeters per minute Then the Si nanopowder was ultrasonically dispersed in the alcohol solution containing Fe(NO3)3, which was subsequently evaporated and the dried samples were calcined in an H2flow at

980 °C for 1 h X-ray diffraction (XRD) measurements and transmission electron microscope (TEM) were employed to

Fig 2 TEM image of Si nanoparticles by cathode arc plasma.

Fig 3 (a) TEM image of Si nanoparticles after heat treatment at 980 °C without Fe catalysts and (b) nanowires with Fe, (c) TEM image showing nanopartical catalysts

at the end of nanowires (d) TEM image of an individual smooth nanowire and corresponding SAED pattern.

investigate the structure and morphology of the Si nanoparticles

and nanowires

3 Results and discussion

The phase composition and phase structure of the as-synthesized

products were examined by X-ray diffraction (XRD, Siemens D-500

with Cu Ka radiation and a normal 2θ scan ).Fig 1shows a typical

XRD spectrum of the Si nanoparticles on different crystal planes

synthesized by cathode plasma discharge It can be seen from the

dominant diffraction peaks, as indexed in the spectrum, and originated

from cubic-structure Si, which can be readily indexed to face-centered

cell of Si (Joint Committee on Powder Diffraction Standard (JCPDS)

Card, No 05-0565) Their average diameter of 21.6 nm was calculated

using Scherrer equation, which is in good accordance with the TEM

observation (Fig 2)

During the heat treatment at 980 °C, Si nanoparticles couldn't be

transformed into nanowires if there was no iron catalysts as shown in

Fig 3(a) When introduced iron catalysts, Si nanowires came out.Fig

3(b) shows the TEM images of SiNWs with tens of nanometer in

diameter and several hundreds of micrometers in length Fe

nanoparticles as catalysts are embedded in SiNWs as shown inFig 3

(c) Selected area electron diffraction (SAED) indicates that the

nanowires are made of crystalline silicon inFig 3(d)

After heat treatment, when H2 flow was closed and air was

introduced into the quartz tube during the cooling procedure, SiOx

nanotubes would be produced deriving from SiNWs oxidated (Fig 4)

The ring in the SAED image inserted inFig 4(b) is from the reflection

of SiO2(400)

The mechanism of SiNWs growth is explained now At high

temperature, Fe(NO3)3 deposited into iron oxide and momentarily

deoxidized into iron nanodroplets in H2 atmosphere In the

metal-catalyzed SLS technique, a liquid metal cluster or catalyst acts as the energetically favored site for the adsorption fusing Si nanoparticles of solid-phase reactants, to function as Si reservoir by eutectic liquid formation, and to become supersaturated with Si The present of a nanopartical catalyst at one end of the nanowires is the essential feature

of SLS growth As shown by the arrow inFig 3(c), SiNWs terminated

at one end in a nanoparticle with a diameter 1∼1.2 times that of the connected nanowire.Fig 3(d) shows the TEM image of an individual smooth nanowire and the corresponding selected area diffraction (SAED) pattern The d-spacings of the nanocrystals calculated from the two diffraction dots of the SAED pattern are consistent with those of Si (200) and (400) It is well established that Si nanowires grown by the metal-catalyzed SLS technique usually have a growth direction along (200) and are single-crystalline [10] Therefore, the (200) growth direction may be regarded as a typical feature of the metal-catalyzed SLS process We surprisedly find that nanotubes appear in this experiment inFig 4(a) It is obvious that nanopartical catalysts are at the end of nanotubes as shown by the arrow Fig 4(b) shows TEM image of an individual nanotube and the corresponding SAED pattern (diffraction ring), in which the ring is from reflection of SiO2(400) When inpouring air into the evacuated sealed quartz tube at high temperature, the surface of SiNWs was immediately oxided[11]and meanwhile the Si inside was melted and subsequently evaporated leaving a SiO2tube[2]

4 Propose mechanism

The formation mechanism of nanowires generally depends

on the presence or absence of metal catalysts in the synthesis process, i.e., SLS and VLS except for oxide-assisted growth

[12] Unlike the well-developed VLS, the detail of SLS process for silicon nanowires is not expatiated In this paper, we

Fig 4 (a) TEM image showing catalysts at the end of nanotubes, (b) TEM image of an individual nanotube and SAED pattern.

Fig 5 Schematic figure of nanowires growth process.

expound SiNWs formation mechanism The solid–liquid–solid

(SLS) nanowires growth mechanism is illustrated in the case of

nanowires growth process in Fig 5 The SiNWs have been

synthesized by heat treatment of Si nanoparticles In the

metal-catalyzed SLS technique, a liquid metal cluster or catalyst acts

as the energetically favored site for the adsorption of

liquid-phase reactants The heat treatment can be acted as a kind of

sinter In this experiment, the primary growth mode is that

agglomeration incorporates particles into nanowires In heat

treatment process, particles begin to melt and the adsorbability

gradually augments with the temperature increasing Because of

the driving force of thermodynamics, the Si atoms dissolve in

the Fe nanocrystal to form a liquid FeSi seed droplet

Thereupon, a tiny cervix between Fe particle and Si particle

comes into being The cervix is filled and leveled up through

diffusion of the surface The seed droplet reaches the eutectic

composition Si diffuses from the liquid molten alloy phase ball

and grows epitaxially at the liquid/solid interface Simultaneity,

Fe catalysts remove ahead and continue to absorb other Si

nanoparticles resulting in the production of long SiNWs

The nanowires are able to grow when the FeSi alloy eutectic

temperature and the concentration of crystallizing material can

be exceeded The current interest in the physics and possible

applications of Si nanostructures and the need to develop

techniques to fabricate such structures made it appropriate to

take another look at the SLS technique as a means of Si

nanowires ultrafast growth of fabrication

5 Conclusion

In summary, Si nanowires were catalytically synthesized by

calcining Si nanopowder containing Fe(NO3)3 in an H2 A

pathway of the growth of SiNWs was presented based on SLS mechanism Under the conditions used to grow the nanowires, diffusion of Si through or around a solid FeSi nanoparticle appears to be rapid enough to transport Si away from the surface

to the growing wires In addition, rapid oxidation of SiNWs could lead to SiOxnanotubes We further discussed the growth mechanism

Acknowledgements

We thank for Engineer Shuang Wang and Youxiang Li, Testing and analytic center, Gansu Academy of Science, who give a lot of help

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