High frequency FTIR absorption of sio2 si 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

High-frequency FTIR absorption of SiO 2 /Si nanowires Quanli Hu *, Hiroshi Suzuki, Hong Gao, Hiroshi Araki, Wen Yang, Tetsuji Noda

National Institute for Materials Science, 1-2-1 Sengen, Tsukuba, Ibaraki 305-0047, Japan

Received 3 June 2003; in final form 29 July 2003 Published online: 26 August 2003

Abstract

An IR absorption measurement of SiO2/Si nanowires made by thermal evaporation was conducted In comparison with SiO2 nanoparticles, enhancement absorption of SiO2/Si nanowires around 1130 cm 1 was observed This en-hancement was considered to result from: (1) the interface effect of the open structure of chainlike SiO2/Si nanowires; (2) the vibration of an interstitial oxygen atom in a silicon single-crystalline core of nanowire; and The longitudinal optical (LO) modes of Si–O–Si stretching in an amorphous SiO2outer shell of SiO2/Si nanowires were also discussed

Ó 2003 Elsevier B.V All rights reserved

1 Introduction

IR absorption spectroscopy is useful for

un-derstanding the structural and compositional

properties of many kinds of oxides The IR

been studied for many years [1] Three major

ab-sorption bands centered at 460, 810, and 1070

These three absorption peaks reflect the rocking of

an oxygen atom about an axis through the two

silicon, the symmetrical stretching of an oxygen

atom along a line bisecting the axis through the

two silicon atoms and asymmetrical stretching of

an oxygen atom along a line parallel to the axis

through the two silicon atoms, respectively In

addition, an increase in the structural disorder could enhance the relative intensity of the ab-sorption band at a higher-frequency side [3] Among these absorption bands, the intensity of

high-frequency side even when the structural

different ways [4]

On the other hand, silicon nanowires have been grown using the VLS mechanism [5,6], STM [7], and laser ablation or thermal evaporation [8–10] Lee et al reported that the thermal evaporation method is useful for the large-scale synthesis of Si nanowires, which can be explained by a new oxide-assisted mechanism that involves the use of an oxide to promote nanowire growth Moreover, the double-layer structure of silicon nanowires is ob-served The TEM images of these nanowires indicate that each nanowire consists of an inner

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*

Corresponding author Fax: +81298592701.

E-mail address: HU.Quanli@nims.go.jp (Q Hu).

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

doi:10.1016/j.cplett.2003.07.015

(or SiO) amorphous layer The nanostructural

effect on the IR absorption properties may be

investigated by studying the vibration modes of

Si–O on the surface of the chain-like amorphous

How-ever, the detailed structural features of the surface

outer layer have not been clearly investigated

be-cause the fabrication of silicon nanowires with

different nanostructures is very difficult

The objectives of this work, after getting a

mount of nanowires by thermal physical

evapo-ration, are to study the IR absorption

character-istics of SiO2/Si nanowires and the nanostructural

effect on IR absorption characteristics at a higher

frequency

2 Experimental

Si powder (99.99 wt% purity, 300 mesh) was

used as raw material for the growth of silicon

nanowires After being ground in a mortar, the Si

nano-wires was conducted by a modified thermal

evap-oration process in a three-stage horizontal furnace

with three independent heating controllers

Semi-cut quartz tubes containing Si wafers were placed

along the downstream region in an alumina tube

to act as the substrate for the grown SiNWs

products Pre-sintered silicon powder was placed

for 2–3 h The pressure in the alumina tube was

maintained at normal values by flowing Ar gas at a

rate of 20 sccm The temperature distribution

along the alumina tube in the furnace was

con-trolled by temperature-setting values at three

points

Silicon nanowire products on a silicon wafer

and a quartz tube were examined by field-emission

scanning electron microscopy (FE-SEM,

JEOL-6700F) and energy dispersion (EDS) attached to

(TEM, JEOL-2010) was utilized to characterize

the detailed microstructure features of silicon

nanowires The IR transmission measurements

were conducted in a JEOL IR spectrometer model JIR-7000 with a Fourier transform infrared spec-trometer (FTIR) The sponge-like silicon nano-wires were mixed with high-purity KBr to make

a measurement pellet The spectral resolution in

3 Results and discussion After the fabrication of silicon nanowires by the thermal evaporation method, products with dif-ferent sizes of nanowires and surface states could

be obtained in different temperature ranges because the size of silicon nanowires can be controlled by the variation of ambient temperature and pressure [9] Three products were selected from different temperature ranges: (1) sample A: nanoparticles of silicon oxides (SiO2) with 40–60 nm average parti-cle size taken from the temperature region of

1000 K; (2) sample B: thick silicon nanowires (di-ameter: 800–1000 nm; length: 100–400 lm) taken from the temperature region of 1173 K; and (3) sample C: thin silicon nanowires (diameter: 50–150 nm; length: 40–100 lm) taken from the tempera-ture region of 1373 K Fig 1 shows FE-SEM im-ages of the three samples described above with different nanostructures In addition, the SAED (selected-area electron diffraction) analysis

the other hand, the single nanowire in samples B and C has a double-layer structure, which has a

outer shell (20–100 nm thickness)

In Fig 2, the absorption spectra of sample A taken from the temperature region of 1000 K are presented Here, only the position of the absorp-tion peak is investigated It shows the well-known transversal optical (TO) resonances, the Si–O–Si

the strongest absorption peak locates at the

absorp-tion spectra of samples B and C taken from the temperature regions of 1173 and 1300 K are also shown in Fig 2 Compared with that of sample A,

the absorption bands of samples B and C centered

vari-ations Namely, the strongest absorption peak

been observed before from either bulk or film

samples, and there is a shoulder of absorption

spectra at the higher-frequency side around 1170–

in-fluence of Si–O bond stretching on the shape and

intensity of the spectra of samples B and C at a higher frequency, a detailed analysis of IR spectra

This analysis shows that IR absorption spectra in

in four absorption bands with a symmetrical Gaussian shape centered at about 1070, 1130,

areas of the four absorption bands above, which reflect the relative distribution for each stretching mode, is shown in Table 1 Table 1 indicates that the TO asymmetric stretching modes of the Si–O

sample A Two other weak modes, longitudinal optical (LO) asymmetric stretching modes at 1160

If we only consider the effect of nanoscale size,

we should find similar enhancement on higher frequency absorption of IR spectrum in nanopar-ticle and nanowire Because either nanoparnanopar-ticles or nanowires have a strong surface tension to cause the distortion and the shortening of Si–O bond length And this will produce more intensity at higher frequency However if we further consider the structural characteristics of nanoparticles and nanowires, the differences on IR spectrum are very obvious First of all, the difference comes from effect of crystalline field of silicon single crystal core In SiO2/Si nanowires, the crystalline field of silicon core could bring some influence on the Si–O

kind of influence may increase the energy gap be-tween excited state and ground state for Si–O vi-bration absorption to cause the increasing of higher frequency absorption in nanowires Oppo-sitely, the crystalline field effect does not exist in

single crystal core Second, the difference comes

nano-wires In SiO2/Si nanowires, the interface between silicon core and SiO2outer shell has a large ratio in the structure of nanowires body And there are a lot of point defects such as vacancy and broken bonds of Si–O on these interfaces Therefore, these could also bring the strong absorption intensity at higher frequency

Fig 1 The FE-SEM images of samples A, B, and C.

In another aspect, themselves of SiO2/Si

wires with micron-meter order length and

nano-meter-order diameter bring the disorder in some

extent in the measurement pellet According to

Gaskell [11], the intensity of the absorption band

main stretching mode can be enhanced in samples

with a large degree of structure disorder increased

by different means, for instance, by ion

bom-bardment In the present work, the mixing of

obvi-ously results in a more open structure with free

volume and surface to produce disordered effect,

which cannot be achieved by the ordinary

me-chanical-grounding method This is one of reasons

for the enhanced intensity of the band around

Furthermore, the absorption band centered at

ox-ygen atom dissolved in silicon nanowires cannot

reacts with molten silicon, especially on the

during the formation of silicon nanowires [12] The oxygen in turn may dissolve in silicon single crystalline core to a certain extent Some of them may form interstitial oxygen atoms The intersti-tial oxygen atoms are assumed to be bound to two neighboring silicon atoms in regular lattice sites [13] This means that two neighboring silicon atoms give up their covalent bond and engage with an interstitial oxygen atom instead, forming

an isosceles triangle with Si–O–Si at the corners

and single crystalline silicon can be clearly ob-served And the point defects and broken bonds

to be found on the interface also provide an evidence for above discussion

Moreover, in samples B and C, the

which the LO modes of Si–O–Si stretching con-tribute, is observed Usually, in an infinite bulk

Fig 2 The IR absorption spectra around 400–2000 cm 1 for the samples A, B, and C.

Table 1

Relative distribution for each stretching mode from the fitting

analysis

(cm 1 ) (cm 1 ) (cm 1 ) (cm 1 )

only be observed by using polarized light with

some angles of incidence because the

electro-magnetic wave such as infrared wave cannot

in-teract with longitudinal phonons For example, Berreman carried out experiments in oblique in-cidence with p-polarized light (with the electric field vector parallel to the plane of incidence) [14] But the excitation of longitudinal optical reso-nances is possible if the film is sufficiently thin compared to the incoming wavelength In the

was attributed to the fact that the thickness of

than the wavelength of the infrared optical reso-nances (2500–25 000 nm) In addition, the

very high ratio to produce many chance that may cause considerable absorption of LO modes, too These also cause the strong signals in the region

of the LO vibration modes

Finally, in sample A, most of the nanoparticles have no silicon single-crystalline core or chainlike structure; the influence from the interstitial oxygen vibration mode is either weak or zero Therefore, the strongest absorption peak locates at 1082

stretching modes The shoulder of the absorption

co-existence of the LO and the TO resonances of Si–O–Si asymmetric stretching modes

Further work is underway to investigate the

outer-Fig 4 The HRTEM image of one part of sample C.

Fig 3 The deconvolution in four absorption bands with

symmetrical Gaussian shape centered at about 1070, 1130,

1160, and 1200 cm 1 for samples A, B, and C.

shell dependence of IR absorption intensity and

position

4 Conclusion

Enhancement of the relative intensity of IR

nanowires was observed The interface effect and

characteristics were suggested to result in the

en-hancement of above vibration modes In addition,

found, which depend on the nanoscale size effect of

SiO2 outer layer in SiO2/Si nanowires

Acknowledgements

This study was financially supported, partially,

by the Budget for Nuclear Research of the

Min-istry of Education, Culture, Sports, Science, and

Technology, with screening and counseling by the

Atomic Energy Commission of Japan

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