Ultrafine and uniform silicon nanowires grown with zeolites

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Ultrafine and uniform silicon nanowires grown with zeolites

C.P Li a, X.H Sun a,b, N.B Wong a,b, C.S Lee a, S.T Lee a,*, Boon K Teo c,1

a Department of Physics and Materials Science, Center of Super-Diamond and Advanced Films,

The City University of Hong Kong, 83 Tat Chee Avenue, Kowloon, Hong Kong SAR, China

b Department of Biology and Chemistry, The City University of Hong Kong, Hong Kong SAR, China

c Department of Chemistry, University of Illinois at Chicago, 845 W Taylor Street, Chicago, IL 60607, USA

Received 21 June 2002; in final form 21 August 2002

Abstract

Ultrafine and uniform silicon nanowires (SiNWs), with a Si crystalline core of 1–5 nm (average 3 nm) in diameter and a SiO2 outer layer of 10–20 nm thick, were synthesized by the oxide-assisted growth method via the dispropor-tionation of thermally evaporated SiO using zeolite as a template/precursor From transmission and secondary electron microscopic characterizations, we deduced that the zeolite acted to limit the lateral growth of the Si crystalline core and supply the excess oxide to form the thick oxide outer layer The ultrafine SiNWs exhibited strong photoluminescence that peaked at 720 nm

Ó 2002 Elsevier Science B.V All rights reserved

1 Introduction

Since the discovery of Si whiskers [1], silicon

nanowires (SiNWs) have attracted much attention

in mesoscopic research and device applications, as

well as in the fundamental research because of

their highly interesting optical and electrical

properties [2–14] The metal-catalyst

vapor–liq-uid–solid (VLS) reaction has been used to grow

SiNWs of different diameters [2] Other growth

methods and/or strategies include the

oxide-as-sisted growth [3–7] It is obvious that the control

of the diameter and uniformity of SiNWs is a crucial factor in the design and fabrication of nanoscale devices In this Letter, we report a new method for the preparation of very fine (1–5 nm) and uniform SiNWs using zeolites as templates and/or precursors

SiNWs were prepared by thermal evaporation

of pure SiO powder (Aldrich, 325 mesh, 99.9%) at

1250°C in an evacuated alumina tube The zeolite (Zeolite Y, a mixture of SiO2, Al2O3, and Na2O) substrate was packed was held by quartz wool in

an inner alumina tube, through which the carrier gas exited The carrier gas consisted of 95% Ar and 5% H2 with a flow rate of 50 SCCM (standard cubic cm per min) was forced to flow through the zeolites and the whole system was kept at a pres-sure of 400 mbar The zeolite substrate changed from a white powder to small green pallets The

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* Corresponding author Fax: +852-27844696.

E-mail addresses: apannale@cityu.edu.hk (S.T Lee),

boonkteo@uic.edu (B.K Teo).

1 Also corresponding author.

0009-2614/02/$ - see front matter Ó 2002 Elsevier Science B.V All rights reserved.

PII: S 0 0 0 9 - 2 6 1 4 ( 0 2 ) 0 1 3 7 5 - 1

products were first examined with a scanning

electron microscope (SEM) (Philips XL 30 FEG),

which was equipped with energy dispersive X-ray

spectroscopy (EDS) The SiNWs samples from the

surface of the beads were dispersed onto ÔholeyÕ

carbon TEM grids The nanostructure of the

samples were then characterized by

high-resolu-tion transmission electron microscope (HRTEM) (Philips CM200 at 200 kV) A micro-Raman spectrometer (Renishaw 2000 micro-Raman spec-trometer) was used to characterize the PL prop-erties of the sample at room temperature The 514.5 nm emission from argon ion laser was used

to excite the luminescence

Fig 1 (a) A typical SEM image of the SiNWs (b) A zoom-out image of (a).

Fig 1a is an overview of the SEM image of the

SiNWs A large quantity of SiNWs was found on

the surface of the zeolite pallet In the zoom-out

image (Fig 1b), we observed that the SiNWs were

attached to the surface of the zeolite pallet EDS

results show that SiNWs are composed of mainly

Si, O, and a small amount of Al The small amount

of Al came from zeolite and provides strong

evi-dence for the proposed growth mechanism to be

described later Fig 2 is the TEM image of a single

SiNW In the TEM results, we found each SiNW

has a very fine crystalline silicon core and a thick

amorphous silicon dioxide outer layer The

diam-eters of the Si cores range from 1 to 5 nm, with the

dominant diameter of 3 nm These Si cores are

very fine and uniform in diameter throughout the

entire length (1 micron or longer) of each wire

The diameter of the amorphous SiO2 layer of the

SiNWs ranges from 20 to 40 nm and is also quite

uniform throughout the entire length of the wires

A central Si core of 1.3 nm in diameter and a

relative thick SiO2outer layer of 20 nm in diameter

were observed in our samples Assuming a Si–Si

bond length of 0.235 nm, this fine nanowire of 1.3

nm in diameter contains only six to seven silicon

atoms across the short dimension To the best of our knowledge, this is the finest SiNWsynthesized

to date The amorphous oxide surface of this wire

is quite rough This phenomenon can be found in other SiNWs with the Si core less than 2 nm in diameter Due to the very fine SiNWs, the selected area electron diffraction (SAED) revealed only the amorphous structure of the outside SiO2 layer, as well as the carbon film in background The inset of Fig 2 shows a HRTEM image of the same SiNW

It confirms that the core is crystalline silicon with 3.1 AA d-spacing The SiO2layer has a thickness of

12 nm on both sides of the center SiNW The overall diameter of the nanowire is 30 nm Fig 3 shows a proposed mechanism for the formation of these very fine and uniform SiNWs The growth of the SiNWs is similar to the previ-ously described oxide-assisted growth mechanism [15] The SiO powders were firstly sublimated at

1250 °C and formed nanoclusters in the vapor phase In the present experiment, the zeolites were positioned downstream from the SiO starting material where the temperature was about 930°C The SiO nanoclusters in the vapor subsequently deposited on the surface of zeolites and some dif-fused into the channels of the zeolites as shown schematically in Fig 3a At that temperature re-gime, SiO nanoclusters disproportionated to form

Si and SiO2 and resulted in the precipitation of silicon nanoparticles (the nuclei of Si nanowires) surrounded by shells of silicon oxide In the channels of zeolites, the nucleation process was limited by the openings of the channels and a large quantity of SiO2 in zeolites retarded the dispro-portionation of SiO Therefore, the core of SiNWs

Fig 2 The TEM image of a typical single SiNWwith a Si core

diameter of 3 nm covered with a SiO 2 layer of 28 nm The inset

is the HRTEM image of the same SiNW.

Fig 3 Proposed growth mechanism for the very fine and uniform SiNWs.

was limited to 1–3 nm in diameter at the

nucle-ation stage, while the zeolite supplied additional

silicon oxide to form the shell of the SiNW,

re-sulting in an oxide layer much thicker than that in

normal SiNWs, as depicted in Fig 3b The finding

of Al in SiNWs provides strong evidence that the

oxide layer of SiNWs comes partly from the

zeo-lite Because the silicon oxide outer layer plays a

key role in the growth process of SiNWs, the

thicker oxide layer limited the lateral growth of the

Si nucleation core At this point, the

Ôoxide-as-sistedÕ growth process became operative, with the

ÔoxideÕ being primarily supplied Ôin situÕ by the

zeolites The increased SiO2 local concentration

(from the zeolite) at the Si–SiO2 interface again

limits the growth of the wires to larger diameter

The net result is a very fine (1–5 nm in diameter)

and uniform SiNWsheathed by a thick and

uni-form oxide layer (20–40 nm in diameter) and each

SiNWhas a ÔrootÕ in the zeolite, as shown in Fig

3c This growth process is consistent with the SEM

results (see Fig 1b) showing that the SiNWs were

attached to the surface of zeolites

The photoluminescence (PL) of these very fine

SiNWs was measured at room temperature Very

weak PL intensity was obtained from the normal

SiNWsample of 20–50 nm in diameter (curve a in

Fig 4) The PL peak centers at around 600 nm

However, the very fine SiNWsamples reported in

this Letter exhibited very strong (at least one order

of magnitude higher) photoluminescence in the PL measurement (curve b Fig 4) The PL peak centers around 720 nm The strong PL intensity probably arises from the quantum size effect of ultrafine Si core (<5 nm in diameter) in association with the interface between the ultrafine silicon core and the sheathing silicon oxide layer [16]

In summary, we have demonstrated that zeolite can be used as a template/precursor to grow very fine and uniform SiNWs via the disproportiona-tion reacdisproportiona-tion of SiO by thermal evaporadisproportiona-tion The diameter of the Si core ranges from 1 to 5 nm with

an average of 3 nm sheathed by a thick and uni-form oxide layer of 20–40 nm in diameter The SiNWs show unusually strong photoluminescence

Acknowledgements The authors would like to dedicate this Letter

to Mrs Anna Lee in her memory This work was supported in part by the Research Grants Council

of Hong Kong (CityU 1063/01P) and the Strategic Research Grants of the City University of Hong Kong (No 7001175) as well as by a grant from the National Science Foundation, USA (to B.K Teo) B.K Teo would like to express his most sincere gratitude for the kind hospitality Prof S.T Lee and his colleagues at COSDAF extended to him during his visit to the center in the summer of

2001, during which this work was performed

References

[1] R.S Wagner, W.C Ellis, Appl Phys Lett 4 (1964) 89 [2] A.M Morales, C.M Lieber, Science 279 (1998) 208 [3] Y.F Zhang, Y.H Tang, N Wang, D.P Yu, C.S Lee,

I Bello, S.T Lee, Appl Phys Lett 72 (1998) 1835 [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 283 (1998) 3458.

[5] N Wang, Y.H Tang, Y.F Zhang, D.P Yu, C.S Lee,

I Bello, S.T Lee, Chem Phys Lett 283 (1998) 368 [6] W.S Shi, H.Y Peng, Y.F Zheng, N Wang, N.G Shang, Z.W Pan, C.S Lee, S.T Lee, Adv Mater 12 (2000) 1343 [7] Y.H Tang, Y.F Zhang, N Wang, C.S Lee, X.D Han,

I Bello, S.T Lee, J Appl Phys 85 (1999) 7981 [8] F.C.K Au, K.W Wong, Y.H Tang, Y.F Zhang, I Bello, S.T Lee, Appl Phys Lett 75 (1999) 1700.

[9] S.G Volz, G Chen, Appl Phys Lett 75 (1999) 2056.

Fig 4 The PL spectra from (a) normal SiNWs of 20–50 nm in

diameters (b) very fine and uniform SiNWs of 1–5 nm in

di-ameters synthesized with zeolites.

[10] S.T Lee, N Wang, Y.F Zhang, Y.H Tang, MRS Bull 36

(1999).

[11] Y Cui, X Duan, J Hu, C.M Lieber, J Phys Chem B 104

(2000) 5213.

[12] Y Cui, C.M Lieber, Science 291 (2001) 851.

[13] Y.F Zhang, L.S Liao, W.H Chan, S.T Lee, R

Sammy-naiken, T.K Sham, Phys Rev B 61 (2000) 8296.

[14] X.H Sun, H.Y Peng, Y.H Tang, W.S Shi, N.B Wong, C.S.

Lee, S.T Lee, T.K Sham, J Appl Phys 89 (2000) 6396.

[15] Y.F Zhang, Y.H Tang, C Lam, N Wang, C.S Lee,

I Bello, S.T Lee, J of Cryst Growth 212 (2000) 115 [16] In a separate experiment, we measured the photolumines-cence (PL) of Al doped (5–10%) as-prepared SiNWs of 20

nm in diameter and observed no significant enhancement in the PL over undoped SiNWs, suggesting that the-order-of magnitude enhancement in the PL of our sample doesnÕt arise primarily from the Al in the silicon oxide layer of SiNWs prepared from zeolites.