Materials Research Bulletin 45 (2010) 699–704
Contents lists available at ScienceDirect
Materials Research Bulletin
journal homepage: www.elsevier.com/locate/matresbu
Selenium nanowires and nanotubes synthesized via a facile template-free
solution method
Huiyu Chen a, Dong-Wook Shin b, Jung-Gyu Nam c, Kee-Won Kwon d, Ji-Beom Yoo a,b,*
a
School of Advanced Materials Science & Engineering (BK21), Sungkyunkwan University, Suwon 440-746, Republic of Korea
SKKU Advanced Institute of Nanotechnology (SAINT), Sungkyunkwan University, Suwon 440-746, Republic of Korea
c
Display Lab, Samsung Advanced Institute of Technology (SAIT), Yongin 446-712, Republic of Korea
d
Department of Semiconductor Systems Engineering, Sungkyunkwan University, Suwon 440-746, Republic of Korea
b
A R T I C L E I N F O
A B S T R A C T
Article history:
Received 20 August 2009
Received in revised form 31 December 2009
Accepted 18 February 2010
Available online 25 February 2010
Trigonal selenium (t-Se) nanowires and nanotubes were successfully prepared on a large scale via an
environment-friendly synthetic process, in which no templates or surfactants were employed. These t-Se
nanowires having a width of 70–100 nm and length up to tens of micrometers were synthesized in
absolute ethanol at room temperature, while t-Se nanotubes with outer diameter ranging from 180 to
350 nm were obtained at 85 8C in water system. SEM and TEM analyses of the samples obtained at
different stages indicated that the formation of these t-Se 1D nanostructures was governed by a ‘‘solidsolution-solid’’ growth process. The amorphous Se (a-Se) nanoparticles were initially generated and then
would transform into crystal seeds for the subsequent growth of nanowires or nanotubes. Detailed
experiments found that temperature and solvents as well as concentrations of starting materials were
crucial to the formation of final morphology.
ß 2010 Elsevier Ltd. All rights reserved.
Keywords:
A. Nanostructures
A. Semiconductors
B. Chemical synthesis
B. Crystal growth
C. X-ray diffraction
1. Introduction
Over the past decade, one-dimensional (1D) nanostructures
such as nanorods, wires, belts and tubes, have been the focus of
intensive research owing to their unique physical and chemical
properties as well as fascinating potential applications in
electronic and photonic nanodevices [1–3]. Until now, a variety
of synthetic strategies including electrodeposition [4], laserassisted catalytic growth [5], template-assisted (anodic aluminum
oxide, carbon nanotubes, porous membranes, and surfactants) and
template-free synthesis [1], sonochemical fabrication [6], solutionphase route [7], and CVD (chemical vapor deposition) [8] have
been developed for the synthesis of 1D nanostructures. Among all
these methods, the solution-phase route seems more promising for
producing 1D nanomaterials in terms of low cost, process
simplicity and high-yield production.
As an important elemental semiconductor, selenium has been
extensively studied due to its particular physical properties such as
high photoconductivity (8 104 S cm1), piezoelectricity, thermoelectricity, and nonlinear optical responses [9]. In addition, it
* Corresponding author at: School of Advanced Materials Science & Engineering
(BK21), Sungkyunkwan University, Suwon 440-746, Republic of Korea.
Tel.: +82 31 290 7396; fax: +82 31 290 7410.
E-mail address: [email protected] (J.-B. Yoo).
0025-5408/$ – see front matter ß 2010 Elsevier Ltd. All rights reserved.
doi:10.1016/j.materresbull.2010.02.016
has a relatively low melting point (490 K) and glass-transition
temperature (304 K), high solubility in various solvents like CS2
and N2H4, and a high reactivity toward a variety of chemical
reagents [10]. Therefore, it has been widely used in many attractive
fields, such as solar cells, semiconductor rectifiers, medical
diagnostics, xerography, and also as starting material for the
fabrication of other functional materials [10,11].
Selenium exists in several allotropic forms including trigonal,
monoclinic and amorphous phase. Among all the selenium
allotropes, trigonal selenium (t-Se) is one of the most thermodynamically stable phases and its anisotropic crystal structure
contains covalently bonded Se atoms that are helically arranged in
infinite chains. It has been reported that t-Se is apt to grow into 1D
nanostructure because of its unique chains of atom that is favor of
anisotropic growth [1]. A lot of efforts have been devoted to the
synthesis of selenium nanowires [6,7,9,12–17], nanorods [18–20],
nanotubes [21–27], nanobelts [14,28,29] and so on [30,31].
Typically, Xia’s group successfully employed solution refluxing
or sonochemical techniques to produce t-Se nanowires by the
reaction of selenium acid with excess hydrazine [9,17]. Xie and
co-workers reported the controlled synthesis of 1D t-Se microstructures using SeO2 and polyvinylpyrrolidone (PVP) by a
photothermal assisted method [21]. Li et al. obtained t-Se tubular
selenium by reducing selenious acid with hydrazine hydrate in
different solvents accompanying a sonochemical treatment [23].
However, most of these methods were involved in the use of
700
H. Chen et al. / Materials Research Bulletin 45 (2010) 699–704
product was collected quickly by centrifugation and washed
several times with de-ionized water, and finally dried at 40 8C in air
for 10 h.
surfactant as template and toxic reagents, or some specially
expensive instruments as well as complicated post-treatment
process. From the viewpoint of green chemistry and large-scale
production, it is thus necessary to develop a simple, efficient and
environment-benign route for the fabrication of Se nanostructures
with controllable phase and morphology.
In this work, we selected glucose as a safe reducing agent and
Na2SeO3 as selenium source to prepare amorphous selenium (aSe) particles and single crystal t-Se. These a-Se particles may
dissolve in absolute ethanol at room temperature, and then recrystallize with t-Se nuclei to grow into nanowires when the aging
time is enough. In water system and at high temperature, t-Se
nanotubes are the dominate product. No surfactant or template
was employed during the entire synthetic process, and ‘‘solidsolution-solid’’ mechanism for the formation of these Se nanowires
and nanotubes has been proposed based on the experimental
results.
Powder X-ray diffraction (XRD) measurements were performed
on a Bruker D8 focus diffractometer with Cu Ka radiation
(l = 0.15406 nm) at a scan rate of 48/min in the 2u range from
208 to 808. SEM images were obtained using a field emission
scanning electron microscope (FESEM, JEOL JSM6700F). Transmission electron microscopy (TEM and HRTEM) images as well as the
corresponding selected-area electron diffraction (SAED) patterns
were obtained using a JEOL JEM2100F instrument at an accelerating voltage of 200 kV. The Raman spectra were measured from 100
to 800 cm1 at room temperature using the 514-nm line of an Ar+
laser with a power level of 20 mW (RM1000-Invia, Renishaw).
2. Experimental
3. Results and discussion
2.1. Materials
The crystal structure of the obtained Se nanowires was
characterized by X-ray diffraction (XRD) and Raman spectroscopy.
Fig. 1a shows the typical XRD pattern. All the diffraction peaks
could be indexed as trigonal phase of selenium with lattice
constants of a = 4.367 Å and c = 4.956 Å, which were in good
agreement with the reported data (JCPDS card No. 06-0362,
a = 4.3662 Å and c = 4.9536 Å). At the same time no other peaks of
crystalline impurities were detected. Compared to the peaks in the
standard pattern of t-selenium, the intensities of the (1 0 0) and
(1 1 0) diffraction peaks were greatly enhanced, which indicated
that these Se nanowires had been preferentially grown along the
[0 0 1] direction. Fig. 1b presents the Raman spectrum of the asprepared Se nanowires. The strong resonance peak at 234 cm1 is a
characteristic signature of trigonal selenium, which can be
assigned to the stretching vibration of helical selenium chains
(A1 mode). The peak at 139 cm1 corresponds to the transverse
optical phonon mode (E mode). Another two peaks at 432 and
456 cm1 can be attributed to the second-order spectra of t-Se. In
comparison, the characteristic Raman resonance absorption band
for a-selenium and monoclinic selenium are centered at 264 and
256 cm1, respectively, which are not observed in this spectrum.
Combining the results of XRD and Raman spectrum analysis, it can
be concluded that well-crystallized t-Se was obtained via the
current synthetic route.
The morphology and microstructure of the Se nanowires was
investigated by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The typical SEM image is
shown in Fig. 2a, which demonstrates that these selenium
nanowires are produced with large quantities and have a width
of 70–100 nm as well as their length up to tens of micrometers. By
Sodium selenite (Na2SeO3) and glucose were purchased from
Sigma–Aldrich and used as received without further purification.
2.2. Preparation of t-selenium nanowires and nanotubes
In a typical procedure of synthesizing Se nanowires, 0.52 g
Na2SeO3 and 2 g glucose were dissolved in 320 mL water hosted in
a 500 mL beaker. After mixing for 20 min under vigorous magnetic
stirring, the beaker containing the mixture solution was sealed and
maintained in an electric oven with temperature set at 85 8C. Later
on, a hot turbid brick-red solution was obtained, indicating the
amorphous selenium generated. The hot solution was cooled down
by cold water in order to quench the reaction quickly. The product
was collected by centrifugation and washed several times with deionized water to remove the impurities. The final brick-red product
was redispersed in 10 mL absolute ethanol hosted in a small glass
bottle, and then sealed and stored in darkness for the further
growth of Se nanowires. After this dispersion was aged for one
week at room temperature, a sponge-like black-gray solid was
formed at the bottom and the color of upper solution changed to
colorless transparent. It was worth pointing out that this aging
process can be greatly accelerated using ultrasonic-assisted
techniques. The synthesis of Se nanotubes was performed under
different conditions: 1.03 g Na2SeO3 and 3 g glucose were
dissolved in 100 mL water hosted in a 250 mL beaker. After the
solution was under constant stirring for 20 min, the beaker
containing the mixture solution was sealed and then maintained at
85 8C for 4 h in an electric oven. After the reaction, the black-gray
2.3. Characterizations
Fig. 1. XRD pattern (a) and Raman spectrum (b) of the t-Se nanowires obtained in absolute ethanol solution with aging time of 7 days at room temperature in darkness. The
resonance peak centered at 234 cm1 in Raman spectrum is a characteristic stretching mode of t-selenium.
H. Chen et al. / Materials Research Bulletin 45 (2010) 699–704
701
Fig. 2. SEM (a) and TEM (b) images of the t-Se nanowires prepared in absolute ethanol with aging time of 7 days at room temperature in darkness. (c) HRTEM image of the t-Se
nanowires. The fringe spacing of 0.50 nm corresponds to (0 0 1) lattice planes of t-Se. (d) The related SAED pattern. The ½1 1̄ 0 zone axis of t-Se is identified and reveals the
growth along [0 0 1] direction.
careful observation, some of the Se nanowires have a slight bend
and seem to be flexible probably due to their remarkable long
length. Fig. 2b is a representative TEM image, which shows that
these Se nanowires are straight with smooth surface and uniform
with an average width of about 70 nm, consistent with the SEM
observation. In order to investigate the microstructure of the
nanowires, an individual selenium nanowire was selected for the
high-resolution TEM (HRTEM) observation and selected-area
electron diffraction (SAED) characterization. The HRTEM image
in Fig. 2c reveals that the fringe spacing of the crystal planes
perpendicular to the long axis direction of the nanowire is about
0.50 nm, which is close to the (0 0 1) lattice spacing of t-Se. The
discrete SAED spots (Fig. 2d) indicate a well-crystallized single
crystal, which can be well indexed to trigonal selenium along ½1 1̄ 0
zone axis. Both the HRTEM and SAED results indicate that the tselenium nanowires have a preferential [0 0 1] growth direction
along their long axis, which are in good agreement with the
analysis of the XRD pattern.
During the entire synthetic procedure, the less stable amorphous selenium (a-Se) colloidal nanoparticles were initially
generated in the solution. As the beaker containing the reactants
was heated in an oven and before the solution temperature was
increased to the target of 85 8C, the color of the solution changed to
brick-red, which indicated that the a-Se formed at this stage.
Fig. 3a and b shows the typical SEM and TEM images of the
obtained nanoparticles, from which we can find that these a-Se
colloidal nanoparticles exhibit perfect spherical shape with
monodispersed size of about 280 nm. The XRD pattern shown in
the inset of Fig. 3b further confirmed that all these selenium
colloidal nanoparticles were in the amorphous phase.
Several experiments with different aging time were carried out
to investigate the detailed growth process of t-Se nanowires. After
the amorphous Se colloidal nanoparticles were redispersed in the
absolute ethanol for 1 day, some spherical a-Se particles began to
collapse into irregular shapes (due to the dissolution process of aSe) as shown in Fig. 3c and d. Close inspection found that some
collapsed particles jointed together and a small fraction of
protruding granules generated on its surface. The SAED pattern
(Fig. 3e) demonstrated that these granules are crystalline. When
the dispersed solution was aged at room temperature for
prolonged time, amorphous Se nanoparticles partially dissolved
into the ethanol solution and generated high reactive selenium
atoms, which would re-crystallize and form crystalline t-Se nuclei
when the concentration of these free selenium atoms was high
enough. The nuclei could be precipitated out and deposited on the
surface of the collapsed a-Se colloidal particles, and this
dissolution process could continue until all the a-Se was
completely converted to more stable t-Se. Here, absolute ethanol
was served as an important medium for the phase transformation.
If water was used instead of ethanol for aging the a-Se for even
more than three weeks, no t-Se nanowires could be obtained and
the dispersion of a-Se preserved brick-red all the time. It might be
due to the much greater solubility of selenium in ethanol than in
water at room temperature [12,13].
When the aging time was prolonged to 3 days, some irregular
particles combined with some t-Se nanowires were obtained
(Fig. 3f). These nanowires have a mean width of 70–100 nm and
length of only a few micrometers. It was also observed that most of
the t-Se nanowires grow with one end on the surface of irregular
particles, where the t-Se seeds were produced and deposited in the
previous stage. It can be concluded that the nanowires grow in situ
where the crystalline seeds were located, and the continuous
feeding of selenium atoms onto the crystalline seeds led to the
formation of linear nanostructures due to its anisotropic crystal
structure. It has been reported that the diameter of these t-Se
nanowires was defined by the lateral dimensions of the
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H. Chen et al. / Materials Research Bulletin 45 (2010) 699–704
Fig. 3. (a) SEM image of a-Se nanoparticles obtained initially in 320 mL water containing 0.52 g Na2SeO3 and 2 g glucose. (b) TEM image of single a-Se nanoparticle. XRD
pattern in the inset demonstrates the amorphous phase. (c) SEM image and (d) TEM image of Se product prepared in absolute ethanol with aging time of 1 day at room
temperature. (e) The SAED pattern of the circled part in (d), suggesting the crystal structure of small t-Se granules. (f) SEM image of Se product prepared in absolute ethanol
with aging time of 3 days at room temperature.
nanocrystalline seeds and an increase in the growth time only led
to the formation of longer nanowires at the expense of a-Se
colloidal nanoparticles [9]. The same results were obtained in our
study. When the aging time of dispersed solution was increased to
one week, most of the amorphous selenium particles disappeared
and the t-Se nanowires became longer along their longitudinal
axis. Compared with the nanowires obtained with aging time of 3
days, the nanowires achieved at this stage almost preserved the
same width as before while the length could be up to tens of
micrometers (Fig. 2a). When the transformation from a-Se to t-Se
was complete, further extension of the aging time could not
obviously increase the length of the nanowires. At room
temperature, the process of phase transformation may proceed
slowly due to the insufficient energy supply. However, the
relatively slow reaction favors separating the growth from the
nucleation step, which, in turn, can provide enough time for the
anisotropic growth of the product and lead to the final formation of
t-Se nanowires.
Based on the results of a series of experiments, it was
interesting to find that Se nanotubes could be produced if we
aged the initial reactant system continuously at high temperature
(e.g., 85 8C). After the brick-red a-Se colloidal nanoparticles were
generated in the solution, the reaction was allowed to continue
instead of cooling down the reactants and collecting the a-Se
Fig. 4. (a) SEM image of Se nanowires and hemi-nanotubes obtained from the reaction at 85 8C for 15 h with 320 mL water containing 0.52 g Na2SeO3 and 2 g glucose. The
inset shows the typical hemi-tube structure and the scale bar of 300 nm. (b) Low-magnification and (c) magnified SEM images of Se nanotubes obtained from the reaction at
85 8C for 4 h with 100 mL water containing 1.03 g Na2SeO3 and 3 g glucose. White arrows in (c) and the magnified view of the white circled part in the inset show the open
tubular structure. (d) TEM image of the obtained t-Se nanotubes. (e) HRTEM image and (f) SAED pattern of single t-Se, suggesting the growth along [0 0 1] direction.
H. Chen et al. / Materials Research Bulletin 45 (2010) 699–704
703
Fig. 5. SEM images of Se product obtained before the temperature increases to 85 8C (a), 100 min (b), 2.5 h (c), and 4 h (d). White arrows in (b) and magnified view in the inset
of (d) show the open tubular structure, and scale bar represents 500 nm in the inset of (d). The reaction system includes 100 mL water containing 1.03 g Na2SeO3 and 3 g
glucose.
nanoparticles. As the SEM image is shown in Fig. 4a, when the
sample was synthesized at 85 8C for 15 h with the concentrations
of starting materials unchanged, the products included Se
nanowires and scroll-like hemi-nanotubes, combined with a few
particles. Generally, the reaction rate can be greatly enhanced
when the concentrations of reactants are substantially increased.
In order to complete the reaction in shorter time, we both
increased the concentration of Na2SeO3 and reduced the solvent
volume simultaneously as described in the synthetic process of tSe nanotubes in the experimental section. In such case, largescaled Se nanotubes could be obtained with the reaction time of 4 h
and typical SEM images are shown in Fig. 4b and c. The tubular
characteristics of the 1D nanostructure can be clearly seen in an
enlarged SEM image (Fig. 4c). By careful observations, these
nanotubes have a diameter ranging from 180 to 350 nm. The TEM
image (Fig. 4d) shows that the Se hollow nanotubes are
approximately 80–120 nm in thickness of walls and 250–
320 nm in outer diameter. The SAED pattern (Fig. 4e) taken from
an individual nanotube indicates its single crystallinity. The fringe
spacing observed in the HRTEM image (Fig. 4f) is about 0.50 nm,
consistent well with interplanar spacings of the (0 0 1) lattice
planes of t-Se, which implies that the Se nanotube is a single crystal
and also grows along the [0 0 1] direction.
To understand the entire growth process of such nanotubes,
series of experiments on various growth stages of t-Se nanotubes
were carried out in details. As shown in Fig. 5a, a-Se colloidal
nanoparticles with various sizes (100–400 nm) were generated
before the temperature of the solution increased to 85 8C. The
broad size distribution probably results from the high concentration of initial reactants. These a-Se colloidal nanoparticles would
partially dissolve in the following stage with increasing temperature owing to their instable phase, and produced t-Se crystal seeds
for the subsequent growth of 1D nanostructure. As a matter of fact,
the phase transformation energy from amorphous selenium to
trigonal selenium is only 6.63 kJ/mol, hence, a slightly increase in
temperature will lead to achieve the phase conversion [32]. As
clearly shown in Fig. 5b, some partially dissolved nanoparticles
with rough surface and a few nanotubes with length within 1.5 mm
and different diameters could be obtained when the reaction time
was prolonged to 100 min. It was reasonable that much more a-Se
was obtained when the concentrations of original reactants were
obviously increased, and thus, more t-Se seeds with hexagonally or
trigonally facetted shapes could be transformed from the less
stable a-Se particles. Also, some of the t-Se seeds might join
together leading to the increase in their lateral dimensions, which
influenced and determined the diameters of the 1D nanostructure.
Therefore, the diameters of the resultant 1D Se nanostructure
exhibited various values. The continuous addition of Se atoms to
the t-Se seeds would preferentially occur at the circumferential
edges of hexagonally or trigonally facetted seeds, resulting in the
formation of t-Se nanotubes [14]. In comparison, when the
concentration of t-Se seeds was not high enough, only a small
amount of them partially joined together and probably produced
some scroll-like hemi-nanotubes (Fig. 4a). This result also confirms
that the long t-Se nanotubes develop from very short ones, instead
of plane-scroll mechanism. By increasing the reaction time to 2.5 h
or above, longer Se nanotubes could be obtained accompanied with
the gradual disappearance of nanoparticles (Fig. 5c and d). Similar
to the preparation of t-Se nanowires in the room temperature, the
nanotubes would not grow longer once the amorphous selenium
nanoparticles were completely converted into trigonal selenium.
According to the above detailed investigations about the
growth process of Se nanowires and nanotubes, a ‘‘solidsolution-solid’’ growth process was proposed as shown in Fig. 6.
Firstly, the a-Se colloidal nanoparticles were generated in the
synthetic system (step a or b). In fact, according to Stranski rule,
during the crystal growth process, the least stable phase usually
precipitates first because it is favored kinetically, then this
metastable phase is transformed into a more thermodynamically
stable phase [33]. Based on our observations in the series of
experiments, it is understood that the a-Se nanoparticles would be
transformed into t-Se seeds, which served for the subsequent
704
H. Chen et al. / Materials Research Bulletin 45 (2010) 699–704
Fig. 6. Schematic diagram of the possible growth process for t-Se nanowires and nanotubes. The a-Se nanoparticles generate initially and the size distribution varies according
to the starting materials with (a) low and (b) high concentrations. (c) The uniform a-Se nanoparticles are dispersed in absolute ethanol and stored at room temperature in
darkness, resulting in the gradual dissolution of a-Se and formation of t-Se seeds, and t-Se nanowires thereafter. (d) The t-Se nanowires and some hemi-nanotubes are
produced in H2O at 85 8C with the low concentration of reactants and reaction time of 15 h. (e) Various sizes of a-Se nanoparticles with high concentration dissolve at 85 8C
leading to the formation of small tubular t-Se, and long nanotubes thereafter.
growth of 1D nanostructures. At room temperature, the solubility
of a-Se in the ethanol is much larger than that in pure water and
crystal growth rate is slow, leading to final formation of t-Se
nanowires (step c and thereafter). While at higher temperature of
85 8C and with higher concentrations of initial reactants, t-Se
nanowires combined with some scroll-like hemi-nanotubes (step
d) or nanotubes (step e and thereafter) were formed.
In addition, the glucose maybe also plays a vital role in the
formation of such nanotubes or scroll-like hemi-nanotube in this
study, because glucose serves as not only a reducing reagent but
also a structure-directing reagent. Our present understanding of
the detailed reason to the formation of different final products
related to the various concentrations of t-Se seeds is still limited,
and further investigation is in progress.
4. Conclusions
In summary, t-Se nanowires with width of 70–100 nm and
length up to tens of micrometers were synthesized in absolute
ethanol at room temperature, while t-Se nanotubes with outer
diameter ranging from 180 to 350 nm could be obtained at 85 8C in
water system. HRTEM and SAED analyses indicate that these 1D tSe nanostructures grow along [0 0 1] direction. A ‘‘solid-solutionsolid’’ growth process was proposed based on the SEM and TEM
observations of the samples obtained at different stages. Amorphous Se nanoparticles were generated initially and then would
transform into crystal seeds for the subsequent growth of
nanowires or nanotubes. At room temperature, a-Se nanoparticles
dispersed in ethanol can transform into t-Se nanowires owing to
the higher solubility of a-Se and slower growth rate of t-Se. In
contrast, at 85 8C and with higher concentrations of reactants, t-Se
nanotubes are easily achieved probably because of the faster
growth speed and the in-situ formation from t-Se seeds with
hexagonally or trigonally facetted morphology.
Acknowledgments
The authors gratefully acknowledge the financial support from
the BK21 Project through School of Advanced Materials Science &
Engineering and from Samsung Advanced Institute of Technology
(SAIT) through SKKU Advanced Institute of Nanotechnology
(SAINT).
References
[1] Y.N. Xia, P.D. Yang, Y.G. Sun, Y.Y. Wu, B. Mayers, B. Gates, Y.D. Yin, F. Kim, H.Q. Yan,
Adv. Mater. 15 (2003) 353.
[2] Z.L. Wang, J. Nanosci. Nanotechnol. 8 (2008) 27.
[3] J.Y. Chen, B. Wiley, Y.N. Xia, Langmuir 23 (2007) 4120.
[4] D.S. Xu, Y.J. Xu, D.P. Chen, G.L. Guo, L.L. Gui, Y.Q. Tang, Adv. Mater. 12 (2000) 520.
[5] A.M. Morales, C.M. Lieber, Science 279 (1998) 208.
[6] B.T. Mayers, K. Liu, D. Sunderland, Y.N. Xia, Chem. Mater. 15 (2003) 3852.
[7] B. Gates, Y.D. Yin, Y.N. Xia, J. Am. Chem. Soc. 122 (2000) 12582.
[8] M.H. Huang, S. Mao, H.N. Feick, H.Q. Yan, Y.Y. Wu, H. Kind, E. Weber, R. Russo, P.D.
Yang, Science 292 (2001) 1897.
[9] B. Gates, B. Mayers, B. Cattle, Y.N. Xia, Adv. Funct. Mater. 12 (2002) 219.
[10] D.M. Chizhikov, V.P. Shchastlivyi, Selenium and Selenides, Collets Publishing,
London, 1968.
[11] L.I. Berger, Semiconductor Materials, CRC Press, Boca Raton, FL, 1997.
[12] W. Zhu, H. Xu, W. Wang, J. Shi, Appl. Phys. A 83 (2006) 281.
[13] Q. Li, V.W.W. Yam, Chem. Commun. (2006) 1006.
[14] Y.R. Ma, L.M. Qi, W. Shen, J.M. Ma, Langmuir 21 (2005) 6161.
[15] X.Y. Gao, T. Gao, L.D. Zhang, J. Mater. Chem. 13 (2003) 6.
[16] B. Zhang, X.C. Ye, W. Dai, W.Y. Hou, F. Zuo, Y. Xie, Nanotechnology 17 (2006)
385.
[17] B. Gates, B. Mayers, A. Grossman, Y.N. Xia, Adv. Mater. 14 (2002) 1749.
[18] S.Y. Xie, C.F. Wang, X.H. Zhang, Z.Y. Jiang, Z.X. Xie, Z.Q. Tian, R.B. Huang, L.S. Zheng,
J. Mater. Chem. 13 (2003) 1447.
[19] Z.W. Quan, P.P. Yang, C.X. Li, X.M. Zhang, J. Yang, J. Lin, Cryst. Growth Des. 8 (2008)
3834.
[20] K. Mondal, P. Roy, S.K. Srivastava, Cryst. Growth Des. 8 (2008) 1580.
[21] B. Zhang, W. Dai, X.C. Ye, F. Zuo, Y. Xie, Angew. Chem. Int. Ed. 45 (2006) 2571.
[22] Y.R. Ma, L.M. Qi, J.M. Ma, H.M. Cheng, Adv. Mater. 16 (2004) 1023.
[23] X.M. Li, Y. Li, S.Q. Li, W.W. Zhou, H.B. Chu, W. Chen, I.L. Li, Z.K. Tang, Cryst. Growth
Des. 5 (2005) 911.
[24] M.H. Chen, L. Gao, Chem. Phys. Lett. 417 (2006) 132.
[25] K. Tang, D.B. Yu, F. Wang, Z.R. Wang, Cryst. Growth Des. 6 (2006) 2159.
[26] G.C. Xi, K. Xiong, Q.B. Zhao, R. Zhang, H.B. Zhang, Y.T. Qian, Cryst. Growth Des. 6
(2006) 577.
[27] S.Y. Zhang, Y. Liu, X. Ma, H.Y. Chen, J. Phys. Chem. B 110 (2006) 9041.
[28] Q. Xie, Z. Dai, W.W. Huang, W. Zhang, D.K. Ma, X.K. Hu, Y.T. Qian, Cryst. Growth
Des. 6 (2006) 1514.
[29] Q.Y. Lu, F. Gao, S. Komarneni, Chem. Mater. 18 (2006) 159.
[30] S.L. Xiong, B.J. Xi, W.Z. Wang, C.M. Wang, L.F. Fei, H.Y. Zhou, Y.T. Qian, Cryst.
Growth Des. 6 (2006) 1711.
[31] J.M. Song, Y.J. Zhan, A.W. Xu, S.H. Yu, Langmuir 23 (2007) 7321.
[32] R.A. Zingaro, W.C. Cooper, Selenium, Van Nostrand-Reinhold, New York, 1974.
[33] J.P. Jolivet, M. Henry, J. Livage, Metal Oxide Chemistry and Synthesis—From
Solution to Solid State, E. Bescher, Translator, John Wiley and Sons, Ltd., Chichester, UK, 2000, p. 47.