Flower like silicon nanostructures

Physica E 38 (2007) 27–30
Flower-like silicon nanostructures
Zhihong Liu
a
, Jia Sha
a,b
, Qing Yang
a
, Zixue Su
a,b
, Hui Zhang
a
, Deren Yang
a,Ã
a
State Key Laboratory of Silicon Materials, Zhejiang University, Zheda Road 38, Hangzhou 310027, PR China
b
Department of Physics, Zhejiang University, Zheda Road 38, Hangzhou 310027, PR China
Available online 17 December 2006
Abstract
In this paper we present a flower-like silicon nanostructure grown by combining the oxidation-assisted growth (OAG) mechanism and
the vapor–liquid–solid (VLS) growth mechanism. It is found that the flower-like silicon nanostructures are nucleated initially via the VLS
mechanism and then grown on silicon wafer via the OAG mechanism. Furthermore, light emission was observed, which is considered to
be the enhanced photothermal effect.
r 2007 Elsevier B.V. All rights reserved.
PACS: 68.65.Àk; 78.20.Nv; 78.67.Àn
Keywords: Silicon; Flower-like; Photothermal; Nanowires
1. Introduction
During recent years, silicon nanostructures have attracted
much attention due to their potential applications in
interconnection and basic blocks for future nanoscale

electronic and optoelectronics devices [1–3],andsensor
applications [4–6]. There are two major synthesis methods:
vapor–liquid–solid (VLS) and oxide-assisted growth (OAG).
The VLS process was originally developed by Wagner [7]
and co-workers, recently Lieber, Yang, and many other
research groups used it to generate silicon nanowires and
other n anowires [8–11]. The VLS process is a well-controlled
method, while the OAG method [12–15] can produce large
quantity SiNWs by the simple thermal evaporation of
silicon monoxide powders. Furthermore, different morphol-
ogies such as wires, rods, chains, coaxial cables, and ribbon
structures can be produced in the OAG process.
In this paper we present a flower-like silicon nanos-
tructures grown by combining the thermal evaporation of
silicon monoxide (SiO) powders with the VLS growth
mechanism. Enhanced photothermal effect is observed.
2. Experiment
The flower-like silicon nanostructure was synthesized by
thermal evaporation SiO powders at 1100 1Cinan
evacuated quartz tube. SiO powders (99.99%, Shanghai
Chemical Co.) were put in the center of the furnace, and
several n-type (1 1 1) silicon wafers with a resistivity of
about 0.01 O cm were placed in the downstream. The
silicon wafers were cleaned by the standard RCA process,
then covered with a 10 nm thick Au film. The furnace was
evacuated to 30 Pa by a mechanical pump, meanwhile the
temperature of the furnace was raised up to 1100 1Cata
heating rate of 20 1C/s. Then a mixed gas of Ar (80%) and
H
2

(20%) at 10
4
Pa was kept flowing at a flow rate of 250
standard cubic centimeters per minute (sccm) through the
tube. After 3 h of growth, the flower-like silicon nanos-
tructure was formed on the silicon substrates of the
downstream at the area of about 700 1C. For comparison,
the silicon wafers without Au film deposition were
also prepared, and the same experiments were
carried out. The as-grown specimens were analyzed by a
field emission scanning electronic microscope (FESEM,
FEI, Sirion), a transmission electronic microscope (TEM,
JEM-2010, JEO L), and a fiber optical spectrometer
(Tensail TS100A).
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doi:10.1016/j.physe.2006.12.028
Ã
Corresponding author. Tel.: +86 571 87951667;
fax: +86 571 87952322.
E-mail address: (D. Yang).
3. Result and discussion
Fig. 1 is the SEM image of an as-grown specimen. The
flower-like nanostructures with the diameter of
500–1000 nm and the length of 3–5 mm on the silicon
substrate can be observed. A typical flower is composed of
an intertwined stem, a bulbous head consisting of a tight
bundle of several nanowires, and a single catalyst particle
attached at the top end of the flower-like nanostructures.

The TEM image of a flower-like silicon nanostructure on
a holey carbon grid is given in Fig. 2. The upper right inset
of Fig. 2 is the selected area electron diffraction (SAED)
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Fig. 2. TEM image of a flower-like nanostructure. The upper right inset is the SAED pattern of the stem, which is marked with an arrow b.
Fig. 1. SEM image of an as-grown specimen. The surface of the silicon substrate is covered with flower-like silicon nanostructures. The white arrow
reveals a single catalyst particle attached at the top end of a flower-like nanostructure.
Z. Liu et al. / Physica E 38 (2007) 27–3028
pattern of the stem, which is marked as b with an arrow. It
indicates that the flower-like silicon nanostructures are
crystalline in nature. Figs. 3a and b are the corresponding
energy dispersive X-ray spectroscopy (EDX) data of the
head (marked as a with an arrow in Fig. 2) and the stem
(marked as b with an arrow in Fig. 2) of the flower-like
silicon nanostructure. The EDX spectra show that the stem
only contains Si and O elements, while the head contains
2.8 at% Au besides Si and O.
On the basis of the results of SEM and TEM, it is
considered that the growth of those flower-like silicon
nanostructures is via both of the VLS process and the
OAG process. In the initial stage, the Au film is dissolved
as liquid drops on silicon substrate, and then those Au
drops act as catalysts to enhance the nucleation of silicon
nanostructures, which is described as the VLS process [8].
Later, silicon nanostructures are grown up on the
nucleation sites via SiO vapor, which is the so-called
OAG process [16]. The growth may take place at the
periphery of the Au liquid drops. As a result, the Au ball is
pushed away from the silicon substrate and lifts upward by
the growing nanowires. In the silicon substrate without Au

film, no flower-like structures were observed besides
individual nanowires.
Under the irradiation of a 980 nm laser of 100 mW, the
as-grown specimen emits a visible light. Fig. 4 is the optical
spectrum. It can be seen that the wavelength of the light is
mainly from 520 to 860 nm. It is believed that the emitting
of the light is due to the enhanced photothermal effect of
silicon nanostructures which was report ed by N. Wang et
al. [17]. The special structure of flower-like silicon
nanostructures enhances the optical absorption, and raises
the temperature of the nanostructures, so that the silicon
nanostructures emit visible light.
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Element
Weight%
Spectrum 3
Atomic%
O K 31.04 52.12
Si K 44.31 42.37
Ca K 4.03 2.70
Au M 20.62 2.81
Element Weight% Atomic%
O K 34.31 47.83
Si K 52.17
0
Ca
O
Au
Si
a

b
Au
Au
Au
Au
Au
Ca
Ca
Full Scale 368 cts Cursor: -0.102 keV (0 cts)
12345678
910
keV
0
O
Si
Full Scale 368 cts Cursor: -0.102 keV (0 cts)
12345678910
keV
65.69
Spectrum 2
Fig. 3. (a) is the corresponding EDX data of the head (marked as a with an arrow in Fig. 2) and (b) is the corresponding EDX data of the stem (marked as
b with an arrow in Fig. 2). The upper right insets in the spectra are the element rate table, respectively.
Z. Liu et al. / Physica E 38 (2007) 27–30 29
4. Conclusions
We have demonstrated the synthesis of flower-like silicon
nanostructures. The synthesis can be controlled at optimal
experimental condition consistent with a VLS growth
mechanism. The optical characterization of the as-pr epared
flower-like silicon nanostructures has been carried out, and
light emission into visible range was observed, which is

believed to be due to the enhanced photothermal effect.
Acknowledgments
The authors would like to thank the Natural Science
Foundation of China (Grant No. 60225010) for financial
support.
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400 600 800
0
100
200
300
400
intensity (a.u.)
λ nm
Fig. 4. Optical spectrum of flower-like nanostructure excited by a 980 nm
laser under a power of 100 mW.
Z. Liu et al. / Physica E 38 (2007) 27–3030