Carbon assisted synthesis of silicon 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

Carbon-assisted synthesis of silicon nanowires

Chemistry and Physics of Materials Unit and CSIR Centre of Excellence in Chemistry, Jawaharlal Nehru Centre for Advanced Scientific Research, Jakkur P.O., Bangalore 560 064, India

Received 18 September 2003 Published online: 4 November 2003

Abstract

Carbon-assisted synthesis of silicon nanowires has been accomplished with silicon powders as well as solid sub-strates The method involves heating an intimate mixture of silicon powder and activated carbon or a carbon coated solid substrate in argon at 1200–1350°C, and yields abundant quantities of crystalline nanowires Besides being simple, the method eliminates the use of metal catalysts

Ó 2003 Elsevier B.V All rights reserved

1 Introduction

There has been intense research activity in the

area of inorganic nanowires and nanotubes in the

last few years [1–3] Thus, nanowires of a variety

of inorganic materials including oxides, nitrides

and chalcogenides have been synthesized and

characterized In particular, silicon nanowires

(SiNWs) have received considerable attention and

several methods have been employed for their

synthesis These include thermal evaporation of Si

powder [4], vapor–liquid–solid method involving

liquid metal solvents with low solubility for Si [5],

laser ablation [6,7], and the use of silicon oxide in

mixture with Si [8,9] SiO2-sheathed crystalline

SiNWs have been obtained by heating Si–SiO2

mixtures [10] It has been recently reported that

enhanced yields of SiNWs are obtained by heating

a Si substrate coated with carbon nanoparticles at

1050 °C under vacuum [11] We consider the role

of carbon to be as in other carbothermal methods

of synthesizing nanowires of oxides, nitrides and other materials, involving a vapor–solid mecha-nism wherein carbon reacts with the oxide proba-bly producing a suboxide-type species As part of our program on the carbothermal synthesis of in-organic nanowires [12–14], we have been investi-gating carbon-assisted synthesis of SiNWs In this article, we report our important findings, which are of relevance to the vapor–solid and oxide-assisted growth of SiNWs

2 Experimental The synthesis of SiNWs has been carried out by employing the following procedures Procedure (i)

www.elsevier.com/locate/cplett

*

Corresponding author Fax: +91-80-8462760.

E-mail address: cnrrao@jncasr.ac.in (C.N.R Rao).

0009-2614/$ - see front matter Ó 2003 Elsevier B.V All rights reserved.

doi:10.1016/j.cplett.2003.09.142

involved the solid state synthesis in which silicon

powder (Aldrich Chemicals) was finely ground

with activated carbon, keeping the molar ratio of

Si to C at 1:1 or 1:0.5 The activated carbon was

prepared by decomposing polyethylene glycol (600

units) in argon atmosphere at 700°C for 3 h The

finely ground mixture was taken in an alumina

boat and heated at 1200°C for 3 h in a mixture of

Ar (50 sccm; sccm, standard cubic centimeter per

minute) and H2 (20 sccm) The reaction was also

carried out under similar conditions in the absence

of carbon to verify whether carbon plays a role in

the formation of the nanowires Procedure (ii) was

similar to (i), except that the reactants were heated

in an Ar atmosphere (without any H2) The

product obtained was grey or white in color and

was collected as fine powders

In procedure (iii), a silicon substrate was used

as the source of silicon The Si(1 0 0) substrates

were cleaned by ultrasonication in distilled water

Amorphous carbon was sputtered on the

sub-strates using a JEOL JEE-400 vacuum evaporator,

with a sputtering time of 0.5–1 min The

carbon-coated Si substrates were heated to 1350°C for 3 h

in an atmosphere of Ar/H2 (25 sccm each) The

product formed as a layer on the substrate was

grey or white in color A blank run with the

sili-con substrate without any sputtered carbon was

carried out under similar conditions

X-ray diffraction (XRD) patterns of the

prod-ucts were recorded using a Seifert instrument with

Cu Ka radiation Scanning electron microscope

(SEM) images were obtained with a Leica S-440-i

microscope Transmission electron microscopic

(TEM) images were obtained with a JEOL (JEM

3010) operating with an accelerating voltage of 300

kV Powder samples for TEM were dispersed in

CCl4 using an ultrasonic bath, and a drop of the

suspension placed on a copper support grid

cov-ered with holey carbon film

3 Results and discussion

Heating silicon powder at 1200 °C, in the

ab-sence of any activated carbon, yields a small

pro-portion of SiNWs In Fig 1a, we show a typical

SEM image of the product of such a reaction to

illustrate the poor yield of SiNWs When the re-action was carried out in the presence of activated carbon (Si:C, 1:1) by procedure (i), we obtained nanowires in a good yield, as can be visualized in the SEM image in Fig 1b These have diameters ranging from 75–350 nm, with lengths of a few microns The XRD pattern of the product shown

in Fig 2a matches with that of bulk silicon of cubic structure (JCPDS file: 27-1702) There is a minor peak (with asterisk) which is attributed to the surface silicon oxide, since SiNWs undergo oxidation upon exposure in air Due to the high surface-to-volume ratio of the nanowires, a prominent surface oxide layer is generally present

We, however, see no reflections due to carbide and other impurity phases Along with the nanowires,

we also obtain Si nanojunctions, as shown in the low-magnification TEM image in Fig 3a The junction has a Y-shape, with arms of a uniform width of 200 nm, and a length of a few microns Careful studies of the TEM images and electron diffraction data may unravel the nature of the junction

In Fig 1c, we show the SEM image of the SiNWs obtained by procedure (i) with Si:C ratio of 1:0.5 The nanowires have diameters between 75 and 600 nm with lengths up to tens of microns The TEM image presented in Fig 3b reveals that the nanowires have a crystalline core and an amorphous sheath The diameter of the crystalline core is 40 nm and the thickness of the sheath is around 17 nm The amorphous sheath serves as a protective layer to the underlying crystalline sili-con core The amorphous sheath is of silica, formed by surface oxidation The selected area electron diffraction, given in the inset of Fig 3b, indicates the core to be of cubic silicon The XRD pattern of the product, given in Fig 2b, is char-acteristic of cubic silicon with a small impurity of silica

Reaction of silicon powder with activated car-bon in the absence of H2, by procedure (ii), yielded abundant quantities of SiNWs The product ob-tained consisted of grey and white portions The grey portion comprised SiNWs with diameters of

50 nm as shown in the SEM image in Fig 1d The length of the nanowires was several tens of microns Shown in the inset of Fig 1d is the SEM

image of the white portion of the product These nanowires have diameters ranging from 50 to 700

nm, with several tens of microns in length A low-magnification TEM image of the nanowires is shown in Fig 3c The nanowires are highly crys-talline as can be seen from the high-resolution transmission electron microscope (HREM) image

in Fig 3d The lattice spacing between the fringes

is 0.31 nm, corresponding to the (1 1 1) planes of silicon The crystallinity of the nanowires is con-siderably higher when only argon was used instead

of a mixture of argon and hydrogen The role of hydrogen in promoting the amorphization of sili-con is well-known [15,16]

In order to show the versatility of this method,

we have investigated the formation of SiNWs by heating silicon substrates coated with carbon, by procedure (iii) In the absence of carbon, we ob-tained very few SiNWs, as shown in the SEM

Fig 1 SEM images of (a) the product of the reaction of silicon powder obtained by procedure (i) in the absence of carbon, (b) SiNWs obtained by procedure (i) with a Si:C ratio of 1:1, (c) SiNWs obtained by procedure (i) with Si:C ratio of 1:0.5 and (d) SiNWs obtained

in the grey portion of the sample synthesized by procedure (ii) Inset shows the nanowires obtained in the white portion.

Fig 2 XRD patterns of SiNWs obtained by procedure (i) with

a Si:C ratio of (a) 1:1 and (b) 1:0.5.

image in Fig 4a On carrying out the reaction with

sputtered carbon, the yield of SiNWs improves

considerably, as can be seen from the SEM image

in Fig 4b The nanowires have diameters in the

range of 50–300 nm

The formation of SiNWs in the presence of

carbon can be explained as follows Silicon is

generally covered by an oxide layer The oxide

layer gets reduced by carbon into silicon monoxide

by the reaction

SixO2þ C ! SixOþ CO ðx > 1Þ ð1Þ

Crystalline silicon, formed in step (3), nucleates and grows perpendicular to the (1 1 1) direction to form the nanowires Similar reactions have been proposed for the oxide-assisted synthesis of SiNWs [7], although the monoxide type species is generated by other means

4 Conclusions SiNWs have been obtained by reacting silicon powder or silicon substrates with carbon in an inert atmosphere Carbothermal reduction of the silica layer covering Si generates crystalline SiNWs with high aspect ratios The method is convenient and inexpensive for the synthesis of Si nanowires, devoid of metallic impurities

Fig 3 (a) TEM image of a Si nanojunction obtained by procedure (i) with Si:C ratio of 1:1 (b) TEM image of a nanowire obtained by procedure (i) with Si:C ratio of 1:0.5 Inset is the SAED pattern (c) TEM image of the white portion of the sample obtained by procedure (ii) (d) HREM image of a single nanowire obtained in the white portion of the sample synthesized by procedure (ii) The white arrow indicates the direction of growth of the nanowire.

[1] P Yang, Y Wu, R Fan, Int J Nanosci 1 (2002) 1 [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, M Nath, Dalton Trans (2003) 1.

[4] D.P Yu, Z.G Bai, Y Ding, Q.L Hang, H.Z Zhang, J.J Wang, Y.H Zou, W Qian, G.C Xiong, H.T Zhou, S.Q Feng, Appl Phys Lett 72 (1998) 3458.

[5] M.K Sunkara, S Sharma, R Miranda, G Lian, E.C Dickey, Appl Phys Lett 79 (2001) 1546.

[6] A.M Morales, C.M Lieber, Science 279 (1998) 208 [7] N Wang, Y.H Tang, Y.F Zhang, C.S Lee, S.T Lee, Phys Rev B 58 (1998) R16024.

[8] N Wang, Y.F Zhang, Y.H Tang, C.S Lee, S.T Lee, Appl Phys Lett 73 (1998) 3902.

[9] Y.F Zhang, Y.H Tang, N Wang, C.S Lee, I Bello, S.T Lee, J Cryst Growth 197 (1999) 136.

[10] J.L Gole, J.D Stout, W.L Rauch, Z.L Wang, Appl Phys Lett 76 (2000) 2346.

[11] S Botti, R Gardi, R Larciprete, A Goldoni, L Gregoratti,

B Kaulich, M Kiskinova, Chem Phys Lett 371 (2003) 394 [12] G Gundiah, A Govindaraj, C.N.R Rao, Chem Phys Lett 351 (2002) 189.

[13] G Gundiah, G.V Madhav, A Govindaraj, M.M Seikh, C.N.R Rao, J Mater Chem 12 (2002) 1606.

[14] F.L Deepak, K Mukhopadhyay, C.P Vinod, A Govind-araj, C.N.R Rao, Chem Phys Lett 353 (2002) 345 [15] Y.J Xing, D.P Yu, Z.H Xi, Z.Q Xue, Appl Phys A 76 (2003) 551.

[16] H Fritzsche (Ed.), Amorphous Silicon and Related Ma-terials, World Scientific, Singapore, 1989.

Fig 4 SEM images of SiNWs obtained with a Si substrate by

procedure (iii) (a) in the absence of carbon and (b) with carbon

sputtered on the surface.