APPLIED PHYSICS LETTERS 93, 063104 ͑2008͒
Y. Qin, X. N. Zhang,a͒ K. Zheng, H. Li, X. D. Han,a͒ and Z. Zhang
Institute of Microstructure and Property of Advanced Materials, Beijing University of Technology,
Beijing 100124, People’s Republic of China
͑Received 19 June 2008; accepted 7 July 2008; published online 14 August 2008͒
We report a catalyst-free epitaxial growth of silicon nanowires on polyhedral facets of mother Si
nanoparticles by thermal evaporation process. Single silicon nanowires and octopuslike silicon
nanowires ͑OSNWs͒ were synthesized under different temperatures. The OSNWs have several
directions including ͗112͘, ͗110͘, and the unusual directions of ͗100͘ and ͗111͘. A catalyst-free
temperature-dependent epitaxial growth model was suggested. Using the Wulff theory and first
principle calculations, these growth directions can be explained by the preferential selection of
temperature-dependent surface energies. It thus revealed an important but simple growth model in
which the growth directions could be delicately controlled through only determining temperature
and substrate orientation. © 2008 American Institute of Physics. ͓DOI: 10.1063/1.2967875͔
Silicon nanowires ͑Si NWs͒ have attracted much atten-
tion in the past ten years1,2 for their outstanding physical
properties and the central role of Si integrated circuit tech-
nology. Remarkable progress has been reported for the syn-
thesis of the Si NWs ͑Refs. 3–5͒ and applications to electron
devices6–9 such as diodes, logic gates, single electron tran-
sistors, and sensors. The different crystallographic oriented
Si NWs exhibited distinct properties due to the crystal struc-
ture anisotropy.10 Thus, it is important to control the growth
direction for the various applications to the nanodevices.
There are two typical growth mechanisms for Si NWs.
the second is oxide assistant growth ͑OAG͒ mechanism.4 By
VLS method, the growth direction of Si NWs was influenced
by the diameter.12–15 The growth direction is ͗110͘ for the
diameter less than 10 nm, while the growth direction is ͗111͘
for the diameter larger than 20 nm. In the OAG process, the
longitude directions are mainly ͗112͘ and ͗110͘, there were
scarcely ͗100͘ and ͗111͘ ͑Ref. 16͒ oriented Si NWs.
In this work, two morphologies of Si NWs were synthe-
sized by thermal evaporation. One is the freestanding single
Si nanowires ͑SSNWs͒ with highly preferential growth di-
rection of ͗110͘ while another is the octopuslike Si nano-
wires ͑OSNWs͒. Surprisingly, the OSNWs can possess un-
usual ͗100͘ and ͗111͘ growth directions, which have been
very rare for Si NWs synthesized by thermal evaporation.
The surface energy of typical low index faces was calculated
under different temperatures. The change in the growth di-
rection can be explained by the change in the relative surface
energy according to the Wulff theory. A catalyst-free epitax-
ial growth mechanism was suggested to interpret this un-
usual growth of Si NWs.
reached 5.4ϫ103 Pa. The SiO powder was heated to and
maintained at the temperature of 1260 °C for 10 min. After
growth, the silicon substrate surface coated with spongelike
product showed different colors, changing from light yellow
to yellow mixed with pink with temperature increasing.
Microstructure characterization was carried out by field-
emission scanning electron microscopy ͑SEM͒ ͑JEOL
6500F͒, transmission electron microscopy ͑TEM͒ ͑JEOL-
2010͒, and field-emission TEM ͑JEOL-2010F͒.
The surface energy of different crystallographic planes
was calculated by CASTEP and Discovery module in the
MATERIAL STUDIO® program. The ͗100͘, ͗110͘, and ͗111͘
slice models were built up with about 1.5 nm atom layer and
1.0 nm vacuum layer. To relax these three models and the
bulk silicon crystal, the molecular dynamics ͑MD͒ calcula-
tion with NPT ͑dynamics with a thermostat to maintain a
constant temperature and a barostat to maintain a constant
pressure͒ ensemble was performed. During the MD calcula-
tion, the pressure condition was set to 0 GPa, the tempera-
tures were set 273, 873, 1173, and 1573 K, respectively, and
the force filed was produced by COMPASS module automati-
cally. The relax time was set to 200 ps with 200 000 steps to
achieve the system balance that the fluctuation of the energy
and the temperature is less than 5%–10%.
The morphology of the as synthesized products was
strongly influenced by the temperature. Large quantities of
SSNWs were observed at low temperature about 900 °C, as
shown in Fig. 1͑a͒. This kind of NWs is long, straight, and
freestanding. Length of the SSNWs is about tens of microns
and the diameter distributes from 30 to 60 nm. At the high
temperature region around 1200 °C, the OSNWs were
formed. One or more branches share one particle ͓Fig. 1͑b͔͒.
Figure 1͑c͒ provides low magnified TEM image of a SSWN
with core-shell structure. The core is about 40 nm in diam-
eter and the shell is 10 nm in thickness. Selected area elec-
tron diffraction pattern ͑SAEDP͒ and high resolution TEM
͑HRTEM͒ results show that the core is crystal Si and the
shell is amorphous SiO2 ͓Figs. 1͑d͒ and 1͑e͔͒. Around 40
SSNWs were characterized, all of them have ͗110͘ growth
directions and no other directions were found. Figures
1͑f͒–1͑i͒ are the typical images of OSNWs, which show one
SiO powder of 0.3 g as thermal evaporation source ma-
terial was placed in the growth chamber firstly, and then the
chamber was closed. Si wafer was used as substrate. After
evacuation of the chamber to a pressure of 10−2 Pa, Ar gas
was introduced into the reaction chamber until the pressure
a͒
Authors to whom correspondence should be addressed. Electronic ad-
dresses:
Tel.:
86-10-67392281
and
xdhan@bjut.edu.cn. Tel.: 86-10-67396087.
0003-6951/2008/93͑6͒/063104/3/$23.00
93, 063104-1
© 2008 American Institute of Physics
130.113.111.210 On: Mon, 22 Dec 2014 05:39:50
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