Temperature dependence of morphology and diameter of silicon nanowires synthesized by laser ablation

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

Temperature dependence of morphology and diameter

of silicon nanowires synthesized by laser ablation

Structure Research Laboratory, Center for Physical Sciences, University of Science and Technology of China, Hefei 230026, China

Received 15 March 2002

Abstract

The silicon nanowires (SiNWs) with different diameters and morphologies were synthesized by laser ablation of a target containing metals over a temperature range 910–1120°C The octopus-shaped wires of larger diameters were formed in lower temperature zone (910–960°C), while SiNWs and silicon nanoparticle chains of smaller diameters in higher temperature zone (960–1120°C) The distribution of the morphology and diameter of SiNWs as a function of growth temperature differs from that reported by thermal evaporation of SiO powders The study shows that the morphology and diameter of SiNWs synthesized by laser ablation not only correlate closely with the growth temperature of SiNWs, but also with the nature of a catalyst By change of nucleation temperature and critical nucleus size of nucleus droplets in vapor–liquid–solid (VLS) growth process, a catalyst can change relationships between the morphology, diameter, and growth temperature of SiNWs Ó 2002 Elsevier Science B.V All rights reserved

1 Introduction

One-dimensional nanostructured materials,

such as nanotubes [1,2], nanowires [3–8], and

nanobelts [9], are a burgeoning and intriguing

re-search area both for their fundamental scientific

issues in meso-physics phenomena and for

poten-tial nano-device application Silicon is one of the

most important electronic materials and holds

considerable technological promise for device

ap-plications Therefore much attention has been paid

recently to the investigation of SiNWs Until now,

a variety of techniques on synthesizing SiNWs have been developed, including lithography and etching techniques [10,11], scanning tunneling mi-croscopy [12], thermal evaporization [13], and laser ablation [3] However, controlling the morphology and size of as-grown SiNWs is still a challenging issue The recent studies [14,15] on SiNWs syn-thesized by thermal evaporation of SiO powders showed that the temperature of substrate for col-lecting SiNWs played a dominant role in control-ling the diameter of SiNWs and the formation of various kinds of silicon nanowire-related mor-phologies It was demonstrated that the diameters

of SiNWs decreased with the descending growth temperature and the morphology of SiNWs was different in different deposition temperature zone

Chemical Physics Letters 358 (2002) 396–400

www.elsevier.com/locate/cplett

*

Corresponding author Fax: +86-551-360-2803.

E-mail address: jghou@ustc.edu.cn (J.G Hou).

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 0 6 7 1 - 1

Laser ablation synthesis of nanowires is quite

popular, by which long, uniform-sized, and

single-crystal SiNWs can be readily fabricated in bulk

quantities [16] To our knowledge, there is no

re-port to date on the temperature dependence of the

morphology of SiNWs synthesized by laser

abla-tion In this Letter, we present the results on this

project Our results show that the morphology and

diameter of SiNWs synthesized by laser ablation

not only correlate closely with the growth

tem-perature of SiNWs, but also with the nature of a

catalyst

2 Experimental

The experimental apparatus used for the

pre-sent work is similar to the one described previously

[3,17] An alumina tube was mounted inside a

horizontal tube furnace A target was made by

compressing Si powders (purity 99.99%) with 5

mol% Zr (purity P 92%; impurities, Mg, Fe, Ge,

Ca, Cl) The target was placed at the center inside

the furnace A strip-like Si substrate (68 mm in

length and 20 mm in width) was placed at the

outlet end, near a cooling copper finger for

col-lecting the deposited products There existed a

temperature gradient from center to the gas outlet

end of the furnace A PtRh–Pt thermocouple was

used to measure the temperature distribution in

the alumina tube After the tube had been

evacu-ated to 0.01 Torr by a mechanical vacuum pump,

5% H2–Ar gas mixture, as a carrier gas, was

in-troduced and kept flowing at a flow rate of 50

sccm The pressure in the tube was controlled at

300 Torr Then the furnace was heated to 1200°C

at the central region After the temperature and

pressure in the tube had been stabilized, pulsed

laser beam from an Nd:YAG laser (wavelength

532 nm, pulse width 7–8 nm, frequency 10 Hz,

average power 1.7 W) ablated the target for 1 h

When ablation was over, the fluffy as-deposited

products with different colors and appearances

were found on the surface of the Si substrate The

morphologies and electron diffraction patterns of

the as-deposited products were investigated by

transmission electron microscopy (TEM) The

chemical composition was analyzed by an energy

dispersive X-ray spectrometry (EDS) attached to JEOL-2010 high-resolution TEM

3 Results and discussion Fig 1 shows the typical bright-field TEM im-ages of the products, which are, respectively, cor-responding to the different growth temperature zones, A, B1, B2, and C, on the Si substrate (shown in Fig 2) The appearances and colors of as-deposited products on the Si substrate are dis-tinctly different, just by observing with the naked eyes A thin dark yellow gauze-like product was formed in zone A, a thick dark yellow fluffy product in Zone B, and a thin light yellow powder-like product in zone C As can be seen in Fig 1, both morphology and diameter of as-grown SiNWs are different in different growth tempera-ture zones In zone A (1080–1120 °C), the nano-wires had a uniform diameter of about 35 nm The presence of nanoparticles at the tips of the nano-wires implies that growth mechanism of the nanowires in this zone is vapor–liquid–solid (VLS) growth [18] Energy dispersive spectroscopy (EDS) analysis indicated that the nanoparticles at the tips

of the nanowires only contained silicon and oxy-gen Within the detection limit of EDS measure-ments (
0:5%), no evidence of existence of Zr or any other elements was detectable on the tip Even though the metal silicide was not found, it is be-lieved that the function of these Si tips of the nanowires is analogous to that of the metal silicide catalyst in VLS growth process [15,16] Namely, melt Si nanoparticles may act as a nucleus for the nanowires In zone B (960–1080 °C), a large quality of what is called silicon nanoparticle chains were formed (Figs 1b,c) Clearly, the diameter of the silicon nanoparticle chain decreases with re-ducing the growth temperature Lee et al [16] have intensively studied the growth mechanism of the silicon nanoparticle chains They proposed that nucleation and growth occurring alternatively re-sulted in the formation of chains of silicon nano-particles The formation of the kinks of silicon nanoparticle chains resulted from a change of growth direction of the SiNWs We examined the kinks by using energy dispersive spectroscopy and

confirmed that the kinks only contained silicon

and oxygen

It should be noted that, from zone A to zone

B, the temperature dependence of the

morphol-ogy and diameter of the SiNWs is similar to that

by thermal evaporation, reported recently by Peng et al [14] It can be seen that the diameter

of SiNWs decreases remarkably with a decrease

in growth temperature They proposed that the variation of the diameter resulted from the

Fig 2 Schematic diagram of the different growth temperature zones on the silicon substrate.

Fig 1 TEM images of the typical morphologies of SiNWs grown in: (a) zone A (1120–1080 °C); (b) zone B 1 (1080–1020 °C); (c) zone

B 2 (1020–960 °C); and (d) zone C (960–910 °C) Inset is electron diffraction patterns of the SiNWs.

variation of the diameter of the droplet nucleated

at different temperature It is recognized that the

melting point of nanoparticles decreases with the

reduction of their size When metal silicide

nanoparticles of different sizes in the carrier gas

were present above the Si substrate, the larger

nanoparticles with higher melting points

con-densed on the higher temperature position of the

substrate, and the smaller nanoparticles with

lower melting points on the lower temperature

position of the substrate

According to the above discussion,

nanostruc-tured morphology with smaller size should have

been anticipated in zone C (910–960°C) However,

in addition to some aggregative fine particles, we

observed that a large quantity of silicon wires of

rather larger diameters (100–150 nm), called

oc-topus-shaped silicon nanowires, were formed in

zone C, a relatively lower temperature zone (Fig

1d) It is interesting that the octopus-shaped

sili-con nanowires synthesized by thermal evaporation

of SiO powders were found at higher growth

temperature of more than 1230°C [15] Focusing

on the octopus-shaped structure, it can be seen

that two or more branches share the same tip,

which suggests that the tip might act as the

nu-cleation site for two or more branches when the

diameter of tip was large enough Moreover, a

bifurcation phenomenon of the silicon nanowires

was also observed, which may be attributed to

renucleation of the crystal silicon in growth

pro-cess On the other hand, the diameter of each

branch of the octopus-shaped nanowires decreases

gradually as the distance from the tip increases

TEM investigations revealed that there existed two

kinds of structures in the entire length of the

branch One was a crystal silicon core sheathed

with a thick amorphous outer layer of silicon

ox-ide, which originated from the tip and terminated

at reaching a certain length The other, the rest of

the branch, was a complete amorphous silicon

oxide Fig 3 is the HRTEM image of the interface

(arrow in Fig 1d) of two kinds of structures along

the axis of the branch It can be seen that the

growth direction of the crystal silicon wash1 1 1i,

which is consistent with the fact that the growth

direction of SiNWs synthesized by metal catalyzed

VLS growth is predominantly h1 1 1i [3,14] The

peculiar feature of the branch suggests that there may exist a competitive growth between crystal silicon core and outer layer of silicon oxide When the forming rate of outer layer of silicon oxide exceeded the growth rate of crystal silicon, outer layer of silicon oxide will surround the crystal sil-icon As a result, the growth of crystal silicon ceased and silicon oxide of outer layer coalesced together and extended

In order to investigate the reason why the SiNWs of such larger diameters were deposited in zone C, we analyzed the chemical composition of the tip of the octopus-shaped wires in the zone C

by EDS EDS analysis showed that the tip con-tained Si, Mg, Ge, and O (oxygen came from the outer layer of the tip), as shown in Fig 4 From

Fig 3 HRTEM image of the interface between the crystal silicon and amorphous silicon oxides,along the axis of the branch of the octopus-shaped wires.The arrow shown in Fig 1d indicates the interface.

Fig 4 EDS spectrum taken from the tip of the octopus-shaped SiNWs.

Mg–Si, Si–Ge, and Mg–Ge binary phase diagrams

[19,20], it is evident that addition of Mg and Ge

into Si can reduce the melting point of the silicon

solid solution Moreover, the melting points of

nanoparticles are usually lower than the

corre-sponding bulk material All of these factors

col-lectively could effectively reduce the melting points

of the tips of larger diameter in zone C This then

explains why these octopus-shaped wires could be

deposited in zone C, a relatively lower temperature

zone Furthermore, it is known that the diameters

of the nanowires formed in VLS process have

re-lation to the critical diameter dc of the liquid

droplets nucleated in VLS process Droplets larger

than dcwill become stable nuclei, whereas droplets

smaller than dc will disappear gradually The

crit-ical nucleus size can be expressed as r¼

ð2cÞ=DFv; where c is the specific interfacial free

energy of the condensate–vapor interface and DFv

is the bulk free energy change per unit volume

[21,22] We assume that the addition of Mg and Ge

into Si nanoparticles would increase interfacial

free energy c Therefore r becomes larger, which

means that the droplets with larger diameters

(> r) can grow to form nanowires This may be

the reason why the diameters of nanowires in zone

C are larger than those in zone A or zone B

4 Conclusions

In summary, the diameter and morphology of

SiNWs synthesized by laser ablation not only

correlate closely with growth temperature, but also

with the nature of a catalyst The nature of a

catalyst has a direct influence upon the nucleation

temperature and critical nucleus size of the

drop-lets in VLS growth process The addition of Mg

and Ge into Si tips gave rise to the deposition of

octopus-shaped SiNWs of larger diameters in the

lower temperature zone Furthermore, it is

sug-gested in our research that there is a corresponding

correlation between morphology and diameter of

nanowires Nanowires of larger diameter (100–150

nm) were inclined to be octopus-shaped, while

nanowires of smaller diameter (10–15 nm) were inclined to be nanoparticle-chain-shaped

Acknowledgements This work was supported by the NSF of China (59972036, 10074059, and 19904012) and the ICQS of Chinese Academy of Sciences

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