Synthesis and characterization of well-aligned quantum

silicon nanowires arrays

Mei Lua, Meng-Ke Lia, Ling-Bing Konga, Xin-Yong Guob, Hu-Lin Lia,*

aDepartment of Chemistry, Lanzhou University, Lanzhou 730000, People’s Republic of ChinabLab of Special Functional Materials, Henan University, Kaifeng 475001, People’s Republic of China

Received 6 January 2003; accepted 11 March 2003

Abstract

Quantum silicon nanowires (SiNWs) arrays have been synthesized by chemical vapor deposition template method without catalyst. The

results of SEM and TEM reveal clear alignment of the SiNWs and each nanowire with perfect lattices is a single crystal. The growth

mechanism of SiNWs without catalyst is discussed based on VLS mechanism. The unusual pattern in the Raman spectrum may be a unique

characteristic of low-dimensional nano-scale materials. Enhanced photoluminescence properties may be associated with the quantum

confinement effect and the formation of ordered arrays. Field emission from SiNWs arrays under various anode–cathode distances are

analyzed based on Fowler–Nordheim theory. The superior field emission behavior is believed to originate from the oriented growth and the

sharp tips of SiNWs.

q 2003 Elsevier Ltd. All rights reserved.

Keywords: B. Defects; D. Electron microscopy; E. Chemical vapor deposition; Silicon nanowires

1. Introduction

Silicon quantum-wires as a special form of crystalline

silicon have attracted much interest due to its unusual

quantum-confinement effects as well as potentially useful

electrical, optical, mechanical, and chemical properties

[1–4]. It has been suggested that they may be used for

developing one-dimensional quantum-wires, high-speed

field effect transistors and miniature microwave generators.

These applications usually require controlled growth of the

nanostructure in orientation and size in order to be capable

of being incorporated effectively into devices. To date,

Silicon nanowires (SiNWs) have been successfully syn-

thesized by different methods [2–7], such as laser ablation,

lithography and scanning tunneling microscopy. The key

factor in these methods is metal catalyst that is required for

the nucleation and growth of SiNWs. However, SiNWs

produced by most of these methods are of random

orientation and twisting each other, which restrict their

nanoelectronic applications.

In this paper, we prepared well-aligned SiNWs arrays by

chemical vapor deposition template method without catalyst

and investigated their interesting field emission properties.

This method demonstrated to be an efficient approach to the

production of highly ordered and isolated nanowires arrays

over large areas [8–10]. Compared with the high density of

defects near the tip of SiNWs prepared by previous

methods, SiNWs produced in this way have sharp tips and

perfect lattices, which might be promising materials for

future nano optic-electronic devices and superior field

emitters.

2. Experimental section

2.1. SiNWs synthesis

Alumina template was prepared by anodic oxidation of

electropolished aluminum plate at a cell voltage of 20 V in

0.5 M phosphoric acid at 25 8C for 1.5 h. After anodization,

the alumina membrane was separated from aluminum

substrate using the voltage-decreasing method [11]. Finally,

the membrane was rinsed thoroughly with distilled

water and dried by pure nitrogen blowing. Subsequently,

1359-8368/03/$ - see front matter q 2003 Elsevier Ltd. All rights reserved.

doi:10.1016/S1359-8368(03)00048-9

Composites: Part B 35 (2004) 179–184

www.elsevier.com/locate/compositesb

* Corresponding author. Tel.: þ86-931-891-2517; fax: þ86-931-891-

2582.

E-mail address: lihl@lzu.edu.cn (H.-L. Li).

the membrane was placed in a quartz boat and then inserted

into the center of a quartz tube reactor winded with heating

tungsten filament. Atmosphere in the reactor was purged

with a mechanical vacuum pump. A flow of H2 (10 ml/min)

and Ar (30 ml/min) was passed for 0.5 h to replace the other

remaining gas and the reactor was heated to reaction

temperature, 900 8C. Then a flow of SiH4 was introduced at

the same rate with H2 (10 ml/min) for 1 h. After deposition,

the sample was cooled to room temperature in an Ar

atmosphere. The deposits on one surface of the alumina

membrane were removed by polishing with alumina power.

2.2. SiNWs characterization

The SiNWs were released from AAO template in 6 M

NaOH for 24 h, and then thoroughly washed with distilled

water. The samples were ultrasonically dispersed in acetone

and placed on a Cu supporting grid destined for immediate

TEM examination. Conventional TEM analysis and high-

resolution transmission electron microscope (HRTEM)

were both performed using a JEOL-2010 microscope at

200 kV with a point-to-point resolution of about 0.14 nm

equipped with link-ISIS energy dispersive spectroscopy

(EDS) elemental composition analyzer. The SEM images of

SiNWs were obtained as followers: the SiNWs/AAO

membranes were glued (using epoxy) to a metallic support

with the cross-section up and then immersed into 6 M

NaOH solution for 20 min in order to dissolve AAO

membranes. Three same samples were sputtered with

,10 nm of Au prior to imaging (JSM-5600LV electron

microscope). For XRD study, the membrane was trans-

formed on to the stand silicon supporter and the spectrum

was obtained by using D/MAX-2400X-ray diffractometer.

Raman scattering measurement was carried out at room

temperature using a Renishaw System 1000 (Renishaw plc.,

UK) in a backscattering configuration. Excitation was done

with 623.8 nm line of a He–Ne laser source. The PL spectra

of the SiNWs were measured by Shimadzu RF-530/PC

spectrofluorophotometer. The 396 nm emission line from an

argon-ion laser was used to excite the luminescence.

2.3. Field emission measurements

The field emission measurements were carried out in a

vacuum chamber at a pressure of about 1027 Torr at room

temperature. Before the beginning of measurements, the

surface of the samples was cleaned in ultrahigh vacuum by

heating, which usually considerably increased the stability

of the emission current. The sample was used as the cathode,

while a copper sheet polished serves as an anode. The

distance between the anode and the sample (cathode)

surface was controlled by the thickness of a mica spacer

containing a hole (,1 mm2) in the center. Voltages up to

3 kV were applied to the anode and the emission current was

detected with a microamperometer.

3. Results and discussion

3.1. Electron microscopy

Fig. 1(a) and (b) shows the low magnification cross-

section and the high magnification surface SEM images of

the SiNWs arrays by dissolving alumina, respectively. It is

found that the nanowires were very straight and in a good

alignment. A sharp tip was also found at the end of each

nanowire, which will be beneficial to a field emitter. In the

mean time, there is a silicon surface film at the bottom of the

SiNWs, which is always obtained in other nanomaterials

synthesized by template method. TEM image in Fig. 2(a)

shows the SiNWs are parallel each other when alumina is

partially dissolved, whose situations within the pores of

the template are kept. The EDS spectra collected from the

middle part of the SiNWs arrays (Fig. 2(b)) show the

presence of silicon in addition to Al and O. The exist of

a small amount of P further confirmed the anion ions of

anodizing electrolytes are incorporated in the formation

of alumina template during the anodization procedure [12].

A single SiNW is shown in Fig. 2(c), in which alumina is

dissolved completely. The selected-area electron diffraction

Fig. 1. (a) Low magnification SEM image of SiNWs arrays; (b) high

magnification SEM image of SiNWs arrays.

M. Lu et al. / Composites: Part B 35 (2004) 179–184180

(SAED) pattern taken from this SiNW is shown in Fig. 2(d).

It can be seen that the diffraction spots are organized in an

almost precise hexagon or parallelogram, indicating that the

diamond lattice structure of bulk Si is also preserved in the

SiNWs. According to the geometry analyses of electron

diffraction, the cubic indices of the diffraction spots are

demarcated. To confirm the results, other SiNWs were

selected to perform the SAED experiments and the same

results were obtained. Therefore, it can be concluded that

each single SiNW is a single crystal.

HRTEM image in Fig. 3 shows the representative micro-

structural characteristics of individual SiNWs. The incident

electron beam is parallel to the [110] zone axis. It is clear

that the straight SiNW has smooth surface and the change in

diameter along its length is seldom observed. Also, the

SiNWs can be made virtually defect free and demonstrate

no kink, dislocation and small angle boundaries because of

the periodic change growth direction along the length of the

SiNWs. The growth plane is one of the (111) planes and the

fast growth direction is along the [2211] axis of the SiNWs.

Fig. 3 also shows the tip of the SiNWs is generally sharp and

has perfect lattices. It is different from the previous SiNWs

that are generally round and contain a high density of

stacking faults and micro-twins. Obviously, the sharp tip

suggests a distinctly different formation mechanism than

that based on classical VLS mechanism.

3.2. X-ray diffraction

The XRD spectrum of the SiNWs arrays in Fig. 4

contains seven peaks, which are identified to match well

with the (111), (220), (311), (400), (331), (422) and (511)

diffraction peaks of the diamond lattice structure of bulk

silicon. The other unmarked peaks are judged to result from

alumina. Calculated from the interplanar spacing of the

most intense (111) peak (d ¼ 0:3147 nm), the lattice

parameter of the SiNWs can be obtained as aSiNWs ¼

0:5451 nm; which is 0.387% larger than the standard value

aSi ¼ 0:5430 nm for bulk silicon, revealing there is a slight

lattice expansion and distortion in the SiNWs structure.

From the XRD results, the arrays of SiNWs show

polycrystalline structure as if the result conflict with the

results of SAED above. Considering it is the statistical

results obtained by the XRD pattern and the diffraction

pattern of different grains indicate different orientations, it

can be proposed that these individual SiNW is essentially

single crystal and SiNWs in an array has a different crystal

orientation.

Fig. 2. (a) TEM image of SiNWs arrays; (b) EDS spectrum of SiNWs

arrays; (c) TEM image of a single SiNW; (d) SAED pattern of the single

SiNW, the inset data is the cubic indices of the diffractive spots.

Fig. 3. Typical HRTEM image of SiNWs with sharp tip. Fig. 4. XRD spectrum of SiNWs arrays.

M. Lu et al. / Composites: Part B 35 (2004) 179–184 181

3.3. Possible mechanism

It is clear that the conventional VLS mechanism [13,14]

could not explain the growth of SiNWs, because catalyst is

no longer required in the deposition. We consider it is

probably because there are a large of Lewis acid nature of

surface sites in amorphous and transition alumina and these

sites have the intrinsic catalytic activity of transition

alumina in front of the decomposition of SiH4 [15]. It

should be concluded that the internal pore surface within

alumina has a catalytic behavior in addition to its template

effect. Moreover, there are high density of dangling bonds at

the surface of atomic Si, which leads to the bonding with

each other between atomic Si and a continuous diffusion

into the pores of alumina. On the other hand, the carrier gas

Ar will collide with the pore surface and the atomic Si has

absorbed and exchanged energy and momentum with the

atom, causing overcooling at the surface. Because the

precipitation, nucleation and growth of SiNWs always

occurred at the area near the cold fringer, such an

overcooling is important for providing temperature gradient

used as an external driving force for nanowire growth.

Although, it hasn’t been made clear the reason for the

formation of sharp tip on the SiNWs, we believe further

work in the future should be done to interpret this

phenomenon.

3.4. Raman spectroscopy

Fig. 5 shows Raman spectrum of the prepared SiNWs

arrays. The most impressive Raman feature is the peak

located at ,513 cm21, with a shoulder at 487 cm21, which

is ascribed to the scattering of the first order optical phonon

mode (TO). Comparing the TO mode of c-Si [16], the

corresponding Raman peak of SiNWs is shifted to 513 cm21

from 520 cm21, the full width at half maximum (FWHM) of

the TO mode is broadened to ,18 cm21 from 2.8 cm21,

and its line shape becomes asymmetric. The downshift,

larger FWHM and asymmetry may be associated with the

quantum confinement effect caused by the small diameters,

unique shapes and high surface-to-volume ratio of Si

nanocrystals. In addition, two broad peaks at ,286 and

920 cm21 can be observed, which are assigned to the

scattering of the second-order transverse acoustic phonon

mode (2TA) and the second-order optical phonon mode

(2TO), respectively. It is clear for the two broad peaks the

frequencies shift towards lower a lot and the relative

intensities increase much as compared with those of 2TA

and 2TO modes of c-Si. These typical characteristics of

SiNWs arrays are always expected to appear in porous

silicon, nanocrystalline silicon and freestanding nanowires.

3.5. Photoluminescence spectroscopy

The typical room temperature PL spectrum of the SiNWs

and bulk Si are shown in Fig. 6(a) and (b), which are

dominated by sharp PL spectral features with the peaks

centers at 596 and 591 nm, respectively. It can be seen that

the PL intensity of SiNWs has been increased by about four

times. The spectral blueshift of SiNWs is attributed to the

large PL energy because of the quantum size effect. The

enhanced PL intensity is considered to be originated not

only from the direct band gap of SiNWs, but also from the

formation of highly ordered arrays of SiNWs compared with

previous porous silicon and freestanding SiNWs. In

addition, breadth of the PL peak reported by Zhou et al.

[17] was not observed for the SiNWs arrays, indicating the

diameters uniformity of the SiNWs.

3.6. Field emission properties

It is well known that nanotubes and nanowires with

sharp tip are promising materials for applications as cold

cathode field emission devices [18]. Fig. 7 shows the

curve of current vs voltage (I –V curve) for SiNWs

arrays. It can be seen that the robustness of the emission

Fig. 5. Raman spectrum of SiNWs arrays.

Fig. 6. Photoluminescence spectrum of SiNWs arrays (a) and the bulk

silicon (b).

M. Lu et al. / Composites: Part B 35 (2004) 179–184182

process from the emitter. The turn-on field for electron

emission, defined as the macroscopic fields needed to

produce a current density of 0.01 mA/cm2 is ,14 V/mm,

which is comparable with those for other field emitters

including carbon nanotubes, diamond and SiC. The I –V

data analyzed by the Fowler–Nordheim theory is

presented in the inset of Fig. 7. According to the FN

theory [19], the field emission current I can be expressed

as a function of the fraction of the emitting area A; the

applied electric field E; the local work function of the

emission tip F and a field enhancing factor b: That

is, I / Aðb2=FÞE2 exp½27 £ 107F3=2=ðbEÞ�: Thus I –V

characteristics of the field electron emission in FN

plots ½lnðI=E2Þ vs ðE21Þ� are presented by straight lines.

As can be seen, almost straight line is obtained,

indicating that the field emission from SiNW arrays is

a barrier tunneling quantum mechanical process. The

superior field emission behavior is believed to originate

from the sharp tips and oriented growth of SiNWs.

The I –V data for SiNWs arrays under different

anode– cathode distances are presented in Fig. 8.

Relatively smooth and consistent I –V curves are

obtained, revealing that there is a good agreement in

the characteristics of the samples as a function of

anode–cathode distances. Specifically, the turn-on field

values are similar for different anode–cathode distances.

The inset in Fig. 8 shows the I –V curve for one sample

in which the anode is in intimate contact with the

sample. This helps to provide a more complete

characterization of the SiNWs arrays by comparing the

values of the current and applied voltage. The compari-

son of the I –V curves shows the Ohmic behavior of

the sample when the anode and cathode are in contact

and the nonlinear behavior when there is a separation of

the anode and cathode.

The stability and durability of SiNWs arrays is remark-

able. During 24 h of continuous operation at 1.5 mA/cm2,

the current fluctuation was as low as ^5% and the average

current did not decrease over this period.

4. Conclusions

In conclusion, the morphologies and microstructures of

well-aligned SiNWs arrays synthesized by CVD template

method were investigated by electron microscopy. It is

found from the results of SAED and XRD that each

nanowire is a single crystal and has different orientation in

an array. The growth mechanism of SiNWs without catalyst

was discussed based on VLS mechanism. Peculiar optical

properties may be associated with the quantum confinement

effect and the formation of ordered arrays. It is demonstrated

that SiNWs with oriented growth and sharp tips are good

field emitters. It is reasonable to speculate that SiNWs

arrays produced by this method would be very useful for

electro-optic and other nanoelectronic device applications.

Acknowledgements

This work is supported by the National Natural Science

Foundation of China (Grant No. 60171004).

Fig. 7. Current–voltage characteristics of SiNWs arrays (Inset: Fowler–

Nordheim plot of SiNWs arrays).

Fig. 8. Current–voltage characteristics of SiNWs arrays at different anode–

cathode separations (Inset: I –V curve for the anode touching the sample).

M. Lu et al. / Composites: Part B 35 (2004) 179–184 183

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