Series of Selected Papers from Chun-Tsung Scholars,Peking University(2003)
Synthesis of quasi-one-dimensional semiconductor
nanomaterials and research of their growth
mechanisms.
Lei Ren
School of Physics
September 2003
Abstract
Synthesis of nanoscale materials plays a significant role in understanding
fundamental properties of small structures, creating nanostructured materials, and
developing nanotechnologies[1]. One-dimensional or quasi-one-dimensional
nanostructures, such as nanowires, rods, or tubes, have been the subject of intensive
research due to their potential use as active components or interconnects in fabricating
nanoscale electronic or electromechanical devices[2]. So far considerable kinds of
nanowires or nanorods have been fabricated by utilizing of various synthetic
technologies such as template, laser ablation, vapor growth, electrochemical
fabrication, solution-liquid-solid growth, solution synthesis, solvothermal method and
other methods. Among these methods, the vapor growth approach remains a valuable
technique for fabrication of low dimensional materials because of its simplicity and
low cost. Here I report my work of synthesis of silicon nanowires and selenium
nanowires separately, which were both produced by vapor growth.
PACS number(s): 61.46.+W, 81.07.Bc, 81.07.Nb, 81.10.Bk
701
Series of Selected Papers from Chun-Tsung Scholars,Peking University(2003)
Chapter 1
Synthesis of silicon nanowires and research of
their growth mechanisms
Introduction
Silicon Nanowire (SiNW) is a one-dimensional semiconductor material which can
be synthesized by laser ablation, simple physical evaporation and chemical vapor
deposition. We have synthesized the SiNWs through the thermal physical evaporation,
which is a typical approach of vapor growth. The morphology of SiNWs is dependent
on the growth conditions: the ambient pressure, the growth temperature, the sizes of
catalytic particles, the existence of the impurities, the flux of the buffer gas, the
position of the collector and etc. Based on the analysis of the growth condition and its
morphology, we suggested a growth model of the SiNWs. The key point of this model
is the vapor-liquid-solid (VLS) mechanism.
§1.1 The equipments for the synthesis of nanowires
The growth conditions of silicon nanowires and selenium nanowires can be
generalized as high temperature, ambient pressure, the flow of carrier gas and etc. The
equipments as rendered below can meet the above-mentioned conditions and our
fabrications of silicon nanowires and selenium nanowires were both conducted
through this set of equipments.
1. 1400℃ Quick raising Temp Tube electric Furnace (SSJ-IIH)
The controlling of temperature is conducted by an intellective high-precision
controller. According to practical measurement, its constant-temp zone can be up to
20cm.
2. Controller of gas flow (D08-4C/ZM)
Two kinds of gas can be introduced to the reaction tube through it as the carrier
gas.
3. Vacuum equipment
It consists of a quartz tube and sealing heads on both sides. Between the quartz
tube and one sealing head there is a rubber loop, which can be combined tightly with
the quartz tube and the sealing head because of the pressure caused by the vacuum
inside the tube and then lead to an airproof effect. The other end of the quartz tube is
connected to a mechanic pump which can adjust the vacuum level of the reaction
system by the needle valve between the pump and the system.
4. Collector
A cone copper(or non-corrosive steel) head is linked at the sealing head of the
gas-out end as a collector. It is cooled by water and extends into the quartz tube.
702
Series of Selected Papers from Chun-Tsung Scholars,Peking University(2003)
The equipments for the synthesis of nanowires as mentioned above is depicted in
Figure.1.1.1 and Figure.1.1.2
Figure.1.1.1 Sketch of the equipments for the synthesis of nanowires.
Figure.1.1.2 Our equipments for the synthesis of nanowires: 1400℃ Quick
raising Temp Tube electric Furnace (right) and Controller of gas flow (left)
703
Series of Selected Papers from Chun-Tsung Scholars,Peking University(2003)
§1.2 The equipments for the analysis of nanowires
In order to obtain the information about the morphology and the microscopic
structure of the as-prepared products, we usually use the scanning electron
microscope (SEM) and the transmission electron microscope (TEM). High resolution
transmission electron microscope (HRTEM) can show us the microscopic structure of
the products at atomic scale, combined that with the selected-area electron diffraction
(SAED) studies we can get the fundamental structure characters of the products. More
accurate structure information can be obtained through the X-ray diffraction (XRD).
The analysis of the chemical compositions is mainly conducted by the energy
dispersive spectroscopy (EDS). Now let’s take a view of the apparatus frequently used
in the investigations of my products of silicon nanowires and selenium nanowires.
1. SEM (Amray-1910)
It is a field emission scanning electron microscope which has a resolution of
1.5nm. Its magnification time can be up to 300,000 and its X-ray enegy dispersive
spectroscopy resolution is 150eV. It suits for the accurate analysis of the morphology
of nanomaterials and its taken-out images exceeds other ordinary SEMs’ in figure
quality. It is showed in Figure.1.2.1.
Figure.1.2.1. SEM (Amray-1910)
2. TEM (JOEL-200CX)
It has a point resolution of 0.45nm and the acceleration potential of its electron
gun is 200kV. Its EDS can detect elements ranging from Na to U. It can be used for
the investigation of the microscopic morphology, the electron diffraction and the
704
Series of Selected Papers from Chun-Tsung Scholars,Peking University(2003)
analysis of the chemical compositions of solid materials such as metals, minerals,
semiconductors, superconductors, compounds and etc. It is showed in Figure.1.2.2.
3. HRTEM (Hitachi9000NAR)
The acceleration potential of its electron gun can be up to 300KV. Its main
accessories are the X-ray energy dispersive spectroscopy (DX-4X) of PHILIPS
company, STEM system (Hitachi), and figure intensifier (GATAN). The point
resolution and the line resolution are 0.18nm and 0.10nm respectively. It is showed in
Figure.1.2.3.
Figure.1.2.3. HRTEM (Hitachi9000NAR)
Figure.1.2.2. TEM (JOEL-200CX)
4. Strata DB235 FIB
The field emission Strata DB235 FIB working at scanning microscope mode
(SEM) can be used for the ion beam erosing, the deposition of nanoparticles, the
fabrication of sub-micrometer structures by electron beam exposure, the observation
and the supervision of the process of fabrication, and etc. Its main indices are: the
resolution of electron beam is 3nm (1kV, 2mm), 2nm (5kV, 5mm), 1nm (30kV, 7mm);
the resolution of ion beam is 7nm (30kV, 1pA); the line accuracy of the electron beam
exposure is 100nm. Now, it is mainly used in the fabrications of the nanometer
quantum devices and the 2-dimensional photon crystals. It is showed in Figure.1.2.4.
5. TEM (Tecnai F30)
The field emission TEM was imported by our university through the national 985
projects. It has a point resolution of 0.205nm (300kV) and a line resolution of
705
Series of Selected Papers from Chun-Tsung Scholars,Peking University(2003)
0.102nm (300kV). The energy resolution of its EDS is 130.8eV. The element ranges
of the EDS and the EELS are
5
B
92
U
and
1
H
92
U
respectively. The
magnification time ranges from 51 to 1,000,000. It is showed in Figure.1.2.5.
Figure.1.2.4. Strata DB235 FIB
Figure.1.2.5. TEM (Tecnai F30)
§1.3 The synthesis of silicon nanowires
So far, the SiNWs can be synthesized through a few approaches such as laser
ablation[3][4], thermal physical evaporation[5], chemical vapor deposition[6] and etc.
According to some laboratory results, we noticed that there was no remarkable
difference between laser ablation and thermal physical evaporation. We have produced
the SiNWs through thermal physical evaporation by utilizing of the equipments
depicted in §1.1.
The mixed powders of silicon and ferrum at a mass ratio of 95%:5% was put into
a quartz boat. Then they were posited at the constant-temp zone of the quartz tube for
reaction. One end of the quartz tube was connected to the vacuum equipment and
carrier gas was introduced from the other end. At the meantime, the cooling collector
extended into the tube at the gas-out end. During the whole reaction, a constant flow
of argon gas (99.9%, 100 sccm) was introduced into the system through the controller
(D08-4C/ZM). The system was evacuated by a mechanic pump and the pressure
inside the quartz tube was kept at 200 Torr. The temperature of the system was
elevated to the reaction temperature of 1220℃. The whole reaction lasted for 10 hours.
After cooling down to room temperature, the collector was found covered with a
706
Series of Selected Papers from Chun-Tsung Scholars,Peking University(2003)
yellow sticky layer, which was proved products of SiNWs after further investigation.
Both the growth and the morphology of the SiNWs would be influenced by the
reaction conditions such as temperature, ambient pressure, flow rate of argon gas and
etc.
§1.4 The analysis of the structures of the silicon nanowires
The as-grown samples were then dispersed in alcohol solution by sonication and
dripped onto carbon-coated holey Cu grids destined for TEM investigations. The
observation and analysis of the SiNWs’ morphologies, defects, and the shapes and
positions of the catalytic particles (Fe) would offer a great deal of useful information
for suggesting the growth model of SiNWs. Figure.1.4.1 is the TEM image of our
as-grown SiNWs (taken by JOEL-200CX ). As shown in it, the average diameter of
the SiNWs is 20  5nm . The length of the wires can be up to several micrometers.
The (111) crystalline planes ( d111  0.31nm ) parallel the axis direction of the SiNWs.
50nm
Figure.1.4.1 TEM image of as-grown SiNWs. The inset picture is the electron
diffraction pattern of them, which shows their single crystalline nature.
§1.5 Research of the growth mechanism of the silicon
nanowires
To explain the key role played by the metal catalyst (Fe), we propose a
mechanism which adopts the concepts of the vapor-liquid-solid (VLS) model
707
Series of Selected Papers from Chun-Tsung Scholars,Peking University(2003)
introduced in the 1960s to explain the growth of silicon whiskers[7]. This model, first
proposed by Saito et al.[8] to explain the formation of sea-urchin-like structures, is
extended here to the formation of SiNWs. The morphology of the SiNWs is very
similar to that of the traditional whisker materials except smaller lengths of diameters.
In our experiment, the influence of the temperature, the effect of the Fe catalyst and
the emergence of catalytic particles in the SiNW products have all proved that the
vapor growth of SiNWs through thermal physical evaporation accords with the VLS
model.[9]
Figure.1.5.1 presents a simplified sketch of the proposed scenario. The first step of
the process is the formation of a liquid nanoparticle of Fe supersaturated with silicon
(Fig.1.5.1Ⅰ). These nanoparticles originate from vapor condensation in the moderate
temperature zone of the quartz tube for reaction. The nanoparticle size is determined
by the reaction parameters such as temperature gradients, gas pressure, gas flux, etc.
The supersaturation is generated by decomposition and absorption of silicon
structures on the surface of the nanoparticles. During the synthesis the liquid
nanoparticle is able to incorporate a large amount of silicon (the Fe-Si phase diagram
gives a maximum silicon solubility of 60% ). Upon cooling (near the cold collector ),
the solubility limit of silicon decreases and therefore silicon atoms start to segregate
towards the surface. At this stage, there is a competition between the formation of an
amorphous silicon sheet and the nucleation of nanowires.
Once the nanowire nuclei are formed, growth should proceed through further
incorporation of silicon at the root (the Fe nanoparticle), as shown in Fig.1.5.1Ⅱ. In
order to obtain long nanowires, the growth process should continue for a sufficiently
long time, until local temperatures are too low, leading to the solidification of the
nanoparticles.
Figure.1.5.1 A simplified sketch of the VLS model for the
growth of SiNWs
708
Series of Selected Papers from Chun-Tsung Scholars,Peking University(2003)
§1.6 The influence of the reaction conditions on the
morphology of silicon nanowires
(1) Temperature
According to some research[7][10], the growth based on VLS model needs a high
temperature. Reactions can hardly take place below the temperature. Reseaches also
show that the higher the temperature is, the bigger the diameters’ sizes of the SiNWs
are. As the temperature increases, the atoms of the reactants become more active and
the rate of the reaction will increase accordingly, leading to a more facile growth of
SiNWs and a larger average diameter of the wires. That accords with the
Gibbs-Thompson’s formula.
v
rc  kT
P
ln
Ps
where Ps is the saturation pressure of the vapor, P is the pressure of the reaction
site,  is the surface specific energy of the nucleus, v is the molecular volume, T
is the temperature and k is the Boltzman constant. From this equation, it is clear that
the critical radius of nucleus rc decreases as the temperature increases. This
indicates that the solid phase is easier to form at a higher temperature and the number
of the nuclei is larger than that at low temperature. Hence, the coalescence of the
nuclei is dominated so that the burgeoning nuclei can accumulate to form a larger
growth seed and the mean diameters become larger at high temperature.
(2) Sizes of the catalytic particles
The diameter of one SiNW decreases as the size of its catalytic particle decreases
and the average diameter of SiNWs relies on the liquid Fe-Si particles[10]. This is in
good agreement with the theory of VLS model.
(3) Ambient pressure[11]
The pressure inside the reaction system influences the sizes of the SiNWs greatly.
It is found that the average size of the SiNWs increases with increasing ambient
pressure. This is for the reason that the size of the liquid Fe-Si particle can be changed
by the gas pressure, which accords with the the Gibbs-Thompson’s formula as
mentioned above.
References
[1] Jiang, Z.Y.; Xie, Z.X.; Xie, S.Y.; Zhang, X.H.; Huang, R.B.; Zheng, L.S.
Chem.Phys.Lett. 2003, 368, 425
[2] Gates, B.; Mayers, B.; Cattle, B.; Xia, Y. N. Adv. Funct. Mater. 2002, 12, 219
709
Series of Selected Papers from Chun-Tsung Scholars,Peking University(2003)
[3] Yu, D.P.; Lee, C.S.; Bello, I.; Sun, X.S.; Tang, Y.H. et.al. Solid State Commn.
1998,105,403
[4] Morales, A.M.; Lieber, C.M. Science 1998,279,208
[5] Yu, D.P.; Bai, Z.G.; Ding, Y.; Hang, Q.L.; Zhang, H.Z. et.al. Appl.Phys.Lett. 1998,
72,26,3458
[6] Hu, J.T.; Ouyang, M.; Yang, P.D.; Lieber, C.M. Nature 1999, 399, 48
[7] Wagner, R. S.; Ellis, W. C. Appl. Phys. Lett.1964, 4, 89
[8] Saito, Y. et al. Jpn. J. Appl. Phys.1994, 33, 526; Carbon 1995, 33, 979
[9] Gavillet, J.; Loiseau, A.; Journet, C.; Willaime, F.; Ducastelle, F.; Charlier, J.-C.
Phys.Rev.Lett. 2001, 87, 275504-1
[10] Givargrgizov, E.I. J. Cryst. Growth 1975, 31, 20
[11] Zhang, H. Z.; Yu, D. P.; Ding, Y.; Bai, Z. G.; Hang, Q. L.; Feng, S. Q. Appl.
Phys. Lett., 1998, 73, 3396.
710
Series of Selected Papers from Chun-Tsung Scholars,Peking University(2003)
Chapter 2
Hexagonal selenium nanowires synthesized via
vapor phase growth
Lei Ren, Hongzhou Zhang, Pingheng Tan§, Yaofeng Chen, Zhensheng Zhang,
Yongqin Chang, Jun Xu, Fuhua Yang§, and Dapeng Yu*
School of Physics, State Key Laboratory for Mesoscopic Physics, and Electron
Microscopy Laboratory, Peking University, Beijing 100871, P.R.China
§
National Laboratory for Superlattices and Microstructures, Institute of
Semiconductors, Chinese Academy of Sciences, Beijing 100083, P.R.China
Abstract Hexagonal Se nanowires were synthesized using a simple vapor phase
growth with the assistance of the silicon powder as a source material, which turned
out to be very important in the growth of the Se nanowires. The morphology,
microstructure, and chemical compositions of the nanowires were characterized using
various means (XRD, SEM, TEM, XPS, and Raman spectroscopy). The possible
growth mechanism of the Se nanowires was explained. The as–grown Se nanowires
may find wide applications in biology, and optoelectronics.
Keywords
Selenium nanowires, Vapor phase growth, TEM, Raman
.*Author to whom correspondence should be addressed: E-mail: yudp@pku.edu.cn.
Tel: +86-010-62759474. Fax: +86-010-62759474
Introduction
Selenium is known as an essential element for life due to its great nutritional effect
and many fascinating properties in biology and chemistry: for example, protecting
cells against the effects of free radicals that are produced during normal oxygen
metabolism, reinforcing the normal functions of the immune system and thyroid gland,
localizing malfunction in the pancreas, and the safe uses of selenium in agriculture. 1
However, selenium attracts even more attention in the physical sciences than in the
biosciences 1 because it exhibits many more promising physical properties, such as
the anisotropy of the thermoconductivity, superconductivity of metallic selenium
blow 6.7K, 2 a high photoconductivity (8  104 Scm-1), hydration and oxidation
catalytic activity, high piezoelectric, thermoelectric, and nonlinear optical responses. 3
It thus has been of tremendous importance in extensive applications, for example,
electrophotographic materials, 4 and Xerox copying machines and rectifiers.3 On the
other hand, quasi-one-dimensional (1D) materials have attracted much attention for
711
Series of Selected Papers from Chun-Tsung Scholars,Peking University(2003)
both their importance in mesoscopic physics and potential applications in fabricating
novel nanoelectronic, optoelectronic, electrochemical, and electromechanical devices.
5, 6
Recently, selenium nanowires have been fabricated through a few approaches, e.g.,
laser ablation, 7 reduction method, 4 catalytic growth 8 and solution phase approach. 3,
9
So far most of the reported methods for Se nanowire growth are chemical routines.
Compared with the above-mentioned methods, the physical evaporation approach
remains a valuable technique for fabrication of low dimensional materials because of
its simplicity and low cost. Here we report a vapor phase growth of hexagonal
selenium nanowires by physical evaporation of selenium powder using silicon
powders as catalyst.
§2.1
Experimental Section
Our fabrication of Se nanowires was based on a thermal evaporation process of
Se powder with Si powder as catalyst. The synthesis apparatus has been described in
our previous papers. 10 The synthesis process was carried out in a tube furnace. The
selenium powder (  99.95% ) in a quartz crucible was placed in the front of the quartz
tube. A long quartz plate covered with silicon powder (99%) was placed at the central
zone of the quartz tube. During the experiment, a constant flow of argon gas (99.9%,
140 sccm) was introduced into the system. The system was evacuated by a mechanic
pump and the pressure inside the quartz tube was kept at 150 Torr. The temperature of
the system was elevated to the reaction temperature. The temperature near the Se
source and the Si covered quartz plate was around 680C and 950C, respectively.
The products were collected by a copper collector cooled by water. The whole
reaction lasted for 3 hours. After cooling down to room temperature, the collector was
found covered with a brick-red layer, which was scratched off for investigation.
The morphology of the samples was analyzed by using a field emission Strata
DB235 FIB working at scanning microscope mode (SEM), a transmission electron
microscope (TEM, Tecnai F30) equipped with energy dispersive spectroscopy (EDS).
Raman scattering measurements were conducted to further analyze the structures of
the products, while X-ray photoelectron spectroscopy (XPS) depicted the chemical
compositions of the samples.
§2.2
Results and Discussion
Figure.2.2.1 shows the XRD spectrum of the as-grown samples. The peak
positions and their relative intensities are well consistent with a hexagonal selenium
phase of with random crystallographic orientation. The lattice constants calculated
from the diffraction patterns are a = 0.437 nm, c = 0.495 nm. This crystal structure
with D36 symmetry is a very stable crystalline form of selenium, which consists of
parallel spiral chains of selenium atoms terminating at the corners and center of a
regular hexagon. The bonding within the chains is much stronger than the bonding
712
Series of Selected Papers from Chun-Tsung Scholars,Peking University(2003)
between the chains. 2, 3 The as-grown samples were then dispersed in alcohol solution
by sonication and dripped onto silicon wafers and carbon-coated holey Cu grids
destined for SEM and TEM investigations, respectively. The SEM image illustrated in
Figure.2.2.2 reveals the general morphology of the Se nanowires grown with Si as a
catalyst. It is seen that the Se nanowires are over several micrometers in length and
50~100 nm in diameter. In contrast, only micro-sized bubbles are observed if no
silicon powder was added to the source materials, as is shown in the SEM image in
Figure.2.2.3, which reveals the crucial role that silicon plays in the growth of the Se
nanowires.
Intensity
101
100
012
110
20
30
40
2 (degree)
111
201
003
50
60
Figure.2.2.1 XRD spectrum from bulk quantity of the products. The
peak positions and relative intensities confirm that the products are of a
hexagonal Se phase.
Figure.2.2.2 SEM image revealing the
general morphology of the Se nanowires
Figure.2.2.3 SEM image showing that
only micro-sized bubbles are observed in
case no silicon powder was added to the
source materials.
713
Series of Selected Papers from Chun-Tsung Scholars,Peking University(2003)
The microstructure of the as-grown samples is further analyzed using TEM and
HRTEM. As shown in Figure.2.2.4(a), the typical TEM image discloses the general
morphology of the samples. The Se nanowires exhibit straight rod shapes. The length
of the wires, as shown in Figure.2.2.4(a), can be up to several micrometers. The finest
nanowire is around 20 nm. The diameter of the Se nanowires is not uniform, varying
in the range of 20~300 nm. One single nanowire is magnified in Figure.2.2.4(b). The
bright and dark strips on the nanowire reveal the single crystalline nature of the
samples. The chemical composition corresponding to the single nanowire is
investigated by EDS and depicted in Figure.2.2.4(c). Only Se peaks are observed from
the samples, confirming that the products are Se nanowires, while the C and Cu peaks
originate from the supporting carbon membrane and copper grids, respectively.
HRTEM image in Figure.2.2.5 can give further insight into the details of the structure.
The lattice fringes can be clearly distinguished. The spacing between two fringes is
about 0.16 nm, which corresponds to the interplanar spacing of the (003) planes, that
is parallel to the c-axis direction. The continuous fringes demonstrate that the Se
nanowires have a low defect density. An amorphous layer with 2 nm in thickness is
observed sheathing the Se nanowires, as is indicated with a black arrow. The fast
Fourier transform (FFT) corresponding to the HRTEM image is shown as the inset of
Figure.2.2.5. The nanowires turned out to be very sensitive to longer time electron
beam irradiation, in particular at high magnification.
(a)
(b)
Se
Intensity (a.u.)
(c)
Se
500 nm
Cu
0
2
8
10
Energy (KeV)
Figure.2.2.4 (a). TEM image of the Se nanowires. The diameters of the Se nanowires
vary in the range between 20~300 nm. (b). Magnified TEM image of a single Se
nanowire. (c). The corresponding EDS spectrum of the single Se nanowire. Only Se is
detected, confirming the nanowires are composed of selenium, while the C and Cu
peaks come from the holey carbon film grids.
714
12
Series of Selected Papers from Chun-Tsung Scholars,Peking University(2003)
c direction
5 nm
Figure.2.2.5 HRTEM image showing the microstructure details of a single Se nanowire.
The wire axis is found parallel to the c-axis direction of the hexagonal Se crystal.
To further clarify the chemical situations of the elements, we measured XPS of the
samples and the Se 3d peaks are shown in Figure.2.2.6. There are two peaks
positioned at 59.2eV and 55.7eV, which arise from Se-O and Se-Se bonds,
respectively. The percentage of Se-O bonds is much lower than that of the Se-Se
bonds, suggesting that a very small amount of Se is in the oxidized state. According to
our HRTEM results (Figure.2.2.5), the Se-O bonds are most likely from the
amorphous layer covering the Se nanowires. Therefore, we can conclude that our
samples consist of Se and they are well crystallized and sheathed by a thin layer of
amorphous oxide materials.
Raman scattering has proven to be a versatile technique to characterize
nanostructured materials.11 In order to further investigate the crystal quality of
as-prepared Se nanowires, we measured the Raman spectrum of the as-prepared Se
nanowires along with that of Se powder for comparison, as shown in Figure.2.2.7. An
intensive peak at 237.3 cm-1 is observed in the Raman spectra of Se powder, which is
attributed to the Raman scattering of the A1 mode of hexagonal selenium.2 Moreover,
two 2nd-order modes at about 440 and 458 cm-1 are also observed. They correspond to
the combination mode and overtone of the phonons at the edge point (M) of the
Brillouin zone.12 The Raman spectrum of as-prepared Se nanowires shows a similar
spectral feature as that of Se powder. The peak frequency (237.3 cm-1) and narrow
715
Series of Selected Papers from Chun-Tsung Scholars,Peking University(2003)
half-width at maximum (9.0 cm-1) of the A1 mode in Se nanowires indicate that the
as-prepared Se nanowires have a high crystal quality, and because the diameter of Se
nanowires is large enough, the quantum confinement effect on the peak frequency can
be ignored. Furthermore, no observation of any signals of the 256-cm-1 peak for
monoclinic selenium and of the 264-cm-1 peak for amorphous selenium 3 indicates
that almost no monoclinic selenium or amorphous selenium is contained in the
prepared samples. Although the Se nanowires are covered with an amorphous oxide
layer of a thickness about 2 nm, they appear to be weak in the Raman spectra because
of small amount of amorphous Se oxide in the prepared samples.
Se-Se(55.7eV)
Intensity (a.u.)
Experiment
Se-O
Se-Se
Se-O(59.2eV)
45
50
55
60
65
Binding Energy(eV)
200
458.1
439.6
Intensity (arb. units)
237.3
Figure.2.2.6 XPS spectrum of the Se nanowires. It is visible that Se-Se bonds
dominate in the products, and a small portion of Se-O bonds was also revealed.
Se nanowires
x5
Se powder
x5
300
400
Raman Shift (cm-1)
500
600
Figure.2.2.7 Raman spectrum of the Se nanowires in comparison with that of the bulk Se powder.
716
Series of Selected Papers from Chun-Tsung Scholars,Peking University(2003)
A series of experiments were done to explore the growth mechanism of the Se
nanowires. Different catalysts such as Cu, Fe, Ni, and Si were added into the source
materials, and we find that only silicon is effective to the growth of the Se nanowires.
In fact, only large sphere-shaped bubbles were formed if no Si was used
(Figure.2.2.3). Therefore, a preliminary model was proposed to explaind the growth
of the Se nanowires, in particular, the role that the Si may play in the growth of the Se
nanowires. There exist a reaction during the growth:
2Se  Si  SiSe2
(1)
Silicon and selenium can react to form SiSe2 , which will decrease the
concentration of Se vapor and as a result prevent the formation of large Se bubbles,
which is in favor of the nanowire growth. The SiSe2 is solid and remained at the site
of Si source. That can explain how the Si was involved in the growth of the Se
nanowires, and why the SiSe2 compound was not observed in the final nanowires.
The growth temperature has a significant effect on the diameters of the nanowires,
and the mean diameter of the Se nanowires increased as the increasing of the
temperature. Systematic work has revealed that the mean diameters of the nanowires
grown at 750C, 850C, and 950C were statistically around 75 nm, 83 nm, and 90
nm, respectively. This is typical for vapor phase nucleation. According to the basic
thermodynamics, the sizes of the critical nuclei are given by Gibbs-Thompson’s
formula
v
rc  kT
P
ln
Ps
(2)
where Ps is the saturation pressure of the vapor, P is the pressure of the reaction
site,  is the surface specific energy of the nucleus, v is the molecular volume, T
is the temperature, and k is the Boltzman constant. From this equation, it is evident
that the critical radius of nucleus rc decreases as the temperature increases. This
indicates that the solid phase is easier to form at a higher temperature and the number
of the nuclei is larger than that at low temperature. Hence, the coalescence of the
nuclei is dominated so that the burgeoning nuclei can accumulate to form a larger
growth seed and the mean diameters become larger at high temperature, which is in
good agreement with our experimental results. There are two vital factors for the
one-dimensional growth of the Se nanowires. One is the steep temperature gradient,
which provides an extrinsic driving force for the growth. The other is the peculiar
717
Series of Selected Papers from Chun-Tsung Scholars,Peking University(2003)
crystal structure of the Se, which is the intrinsic reason for the preferable 1-D growth.
Because the hexagonal Se is asymmetric and exhibits a chain-structure along the (001)
c-axis direction, it has a much faster growth along the c-axis direction, 3 as is revealed
in the HRTEM image in Figure.2.2.5. Above-discussed model can explain some of
the experimental evidences, however, the real growth mechanism of the Se nanowires
still remains a problem for further investigation.
§2.3
Conclusions
Se nanowires were synthesized through a simple physical evaporation by using Si
as an assistant source material. XRD patterns of the samples demonstrate the
nanowires have a pure hexagonal Se phase. EDS and XPS results confirmed the
nanowires consist of Se, while a little amount of Se is in the state of Se-O bonds
sheathing the nanowires. TEM and SEM revealed the length of nanowires is up to
several micrometers and the diameter of nanowires can be as thin as 20 nm. HRTEM
analysis depicted the detailed crystalline structures of the Se nanowires . Raman
spectra of Se nanowires were compared with that of the bulk Se powder.
Acknowledgment
This project was financially supported by the National Natural Science Foundation
of China (NSFC, No. 50025206, 20151002, 60071014), the Scientific Research
Foundation for the Returned Overseas Chinese Scholars, State Education Ministry,
and national 973 projects (No. 2002CB613505, MOST, P. R. China). We sincerely
give our thanks to them. D. P. Yu is obliged to the financial support from Cheung
Kong Scholar program, and L. Ren is grateful to the Chun-Tsung Fund, Peking
University, for the program of the research experience for undergraduates.
References and Notes
(1) Muth, O.H.; Oldfield, J. E. ; Weswig, P. H. Symposium: Selenium in
Biomedicine, 7-26.
(2) Cooper, W. C. The physics of selenium and tellurium, 3-20.
(3) Gates, B.; Mayers, B.; Cattle, B.; Xia, Y. N. Adv. Funct. Mater. 2002, 12,
219-227.
(4) Ibragimov, N. I.; Abutalibova, Z. M.; Agaev, V. G. Thin Solid Films 2000,
359,125-126.
(5) (a) Morales, M.; Lieber, C. M. Science 1998, 279, 208-211, (b) Yu, D. P.; Lee,
C. S.; Bello, I.; Sun, X. S.; Zhou, G. W.; Bai, Z .G.; Zhang, Z.; Feng, S. Q. Solid
718
Series of Selected Papers from Chun-Tsung Scholars,Peking University(2003)
State Commu. 1998, 105, 403-407. (c) Mayers, B.; Gates, B.; Yin, Y. D., Xia, Y. N.
Adv. Mater. 2001, 13, 1380-1384.
(6). (a) Yu, D. P.; Bai, Z. G.; Wang, J. J.; Zou, Y. H.; Qian, W.; Zhang, H. Z.; Ding,
Y.; Xiong, G. C.; Feng, S. Q. Phys. Rev. B 1999, 59, R2498-R2501.; (b) Yu, D. P.;
Bai, Z. G.; Ding, Y.; Hang, Q. L.; Zhang, H. Z. Appl. Phys. Lett. 1998, 72,
3458-3460. (c) Prokes, S. M.; Wang, K.L. MRS Bull. 1999, 24, 13-19;
(7) Jiang, Z.Y.; Xie, Z. X.; Xie, S. Y.; Zhang, X. H.; Huang, R. B;. Zheng, L. S.
Chem. Phys. Lett. 2003, 368, 425-429.
(8) Lickes, J. P.; Dumont, F. C.; Buess-Herman, C. Colloids Surf A, 1996, 118,
167-170.
(9) Abdelouas, A.; Gong, W. L.; Lutze, W.; Shelnutt, J. A.; Franco, R.; Moura, I.
Chem. Mater. 2000, 12, 1510-1512.
(10) Zhang, H. Z.; Yu, D. P.; Ding, Y.; Bai, Z. G.; Hang, Q. L.; Feng, S. Q. Appl.
Phys. Lett., 1998, 73, 3396-3398.
(11) Tan, P. H.; Brunner, K.; Bougeard, D.; Abstreiter, G. Phys. Rev. B, 2003 , 68,
in press.
(12) Martin, R. M.; Lucovsky, G.; Helliwell, K. Phys. Rev. B 1976, 13, 1383-1395.
致 谢
首先，要感谢李政道先生及其夫人秦惠 女士设立的“ 政基金”， 为北大
学子提供了这样一个可以施展才华的机会， 使我也有幸能够在大学本科知识的
学习中涉足科学研究。感谢“ 政基金”办公室的各位老师和领导，感谢王海欣
老师认真负责的工作。感谢班主任连贵君老师和物理学院副院长刘玉鑫教授对我
工作方面大力的支持。
衷心感谢我的导师俞大鹏教授。俞老师的悉心指导，使我顺利地进入到纳米
材料及电子显微学这样一个前沿的科研领域。俞老师勤奋敬业的精神和宽以待人
的品格给了我莫大的启迪。衷心感谢冯孙齐教授的指导和帮助，冯先生严谨的作
风和广博的学识令人敬仰。
衷心感谢张洪洲博士。一年来，张洪洲师兄从治学态度到日常实验的具体安
排都给予了我细致入微的指导，总是用微笑来回答我所遇到的问题。他使我深刻
体会到物理学是一门严谨的学科以及在科研的过程中理论与实验紧密结合的重
要性。大师兄忘我的工作热情和无比严格的要求将永远是我的榜样。
感谢北大电子显微镜实验室的徐军、张会珍、王荣明、汪裕萍、盖秀贞、尤
力平、张小平、陈晶等老师。在他们的帮助和指导下，我逐渐地熟悉了电镜的操
作。
感谢中科院半导体所超晶格及显微结构实验室的杨富华教授和谭平恒博士
在工作方面的大力合作。谭师兄一丝不苟的治学态度也使我受益不浅。
感谢我的两位同级同学贾宏博和孙笑晨，我们在同一个实验室里克服了无数
719
Series of Selected Papers from Chun-Tsung Scholars,Peking University(2003)
的困难，与贾宏博同学在学习和工作中结下的兄弟般的情谊将使我永生难忘。实
验室良好的工作氛围对我影响至深，感谢张晔博士、常永勤博士和向斌、陈喜红、
廖志敏、赵清、徐向宇、陈耀锋、罗旭辉、张振生、宋学峰，以及已经毕业的金
薇、王莹、朱炎武、向杰等师兄师姐的合作和有益的讨论。
作者简介：
任磊，男，1982 年 4 月出生于河南省郑州市。高中就读于郑州一中，曾获
全国数学联赛河南省一等奖、全国物理竞赛河南省一等奖，2000 年保送入北京
大学物理学院物理系学习。在大学期间学习成绩优秀，社会活动广泛，曾获北京
大学“社会工作奖”，入选过物理学院第一界学生会主席团，担任学生会学术实
践部部长。大二上学期曾于基础物理实验室在张洁天教授的指导下进行二极管电
学性质的深入研究，大二下学期受“ 政基金”的资助进入俞大鹏教授的实验室
从事纳米材料方面的科研工作。
感想与寄语：
“I believe I can fly!” 篮球巨星迈克尔·乔丹的一句名言，不仅是对其惊人的
弹跳能力和超群的篮球技艺的真实写照，更是对他在强敌面前不低头、在困境之
中不屈服、能够在败境中寻得胜机的超人能力的最佳诠释。
“ 政”活动的十八个月，一个个悬梁刺股的深夜，一次次闻鸡起舞的黎明，
一滴滴失败后的泪水，一缕缕重新振作后充满期待和希望的眼神，最终浇铸了成
功的喜悦和自豪。得到的结果固然可喜，获得的专业知识和能力也非常重要，但
从中锻炼出来的积极思考、勇于实践、不畏惧并能够坦然接受失败、不逃避并能
够扛得住压力的意志更为宝贵，这是成功者所必备的素质。
“雄关漫道真如铁，而今迈步从头越。”对科学的探求是永无止境的，面前还
有很长的路要走，从“ 政”活动中收获的一切都将是我进一步发展的基石。
“ 政”，感谢你，想说爱你不容易！
指导教师简介：
俞大鹏，男，1959 年 3 月出生，教授，博士研究生导师，长江学者，电子
显微镜实验室主任。1982 年本科毕业于上海华东理工大学材料科学与工程系，
1985 年硕士毕业于中科院上海硅酸盐研究所，1993 年博士毕业于法国南巴黎大
学固体物理实验室，1995 年中科院北京电镜实验室博士后并进入北京大学物理
系任副教授，1999 年 7 月破格晋升为教授。2003 年被聘为“长江学者”。
主要研究方向是：准一维纳米结构的制备、生长机理、显微结构；准一维纳
米电子学、量子尺寸限制效应、量子输运等物理性能研究；电子显微学在凝聚态
物理中的应用。在纳米硅量子线研究方面做了开创性研究，取得重要突破，在世
界上首次成功获得了纳米硅量子线；深入研究了纳米硅量子线的微观结构和一维
择优生长机制，和与低维结构相关联的量子尺寸限制效应；成功制备和研究了半
导体（ZnSe、ZnO、GaSe、GaN）纳米线、金属纳米线；制备了单原子层碳纳米
管，BN 纳米管；研究了单层纳米碳管的场发射效应。
720