APPLIED PHYSICS LETTERS 89, 043115 共2006兲
Low-temperature growth and blue luminescence of SnO2 nanoblades
Yung-Chiun Hera兲 and Jer-Yau Wu
Department of Materials Engineering, National Chung Hsing University, Taichung, Taiwan 402,
Republic of China
Yan-Ru Lin and Song-Yeu Tsai
Photovoltaics Technology Center, ITRI, Hsinchiu, Taiwan 31040, Republic of China
共Received 24 March 2006; accepted 31 May 2006; published online 27 July 2006兲
Large-scale SnO2 nanoblades have been synthesized on a glass substrate covered with a
100-nm-thick SnO2 buffer layer in a controlled aqueous solution at temperatures below 100 ° C.
Typical widths of the nanoblades were about 100– 300 nm and the lengths were up to 10 ␮m,
depending on the growth temperature. The thicknesses were about a few tens of nanometers.
Transmission electron microscopy data, x-ray diffraction patterns, and x-ray photoelectron
spectroscopy spectral analyses confirmed that the as-grown nanoblades had the phase structure of
the rutile form of SnO2 growing along the 关110兴 direction. No other impurities, such as elemental
Sn and tin oxides, were detected. An intense blue luminescence centered at a wavelength of 445 nm
with a full width at half maximum of 75 nm was observed in the as-grown SnO2 nanoblades, which
is different from the yellow-red light emission observed in SnO2 nanostructures prepared by other
methods. It is believed that the strong blue luminescence from the as-grown SnO2 nanoblades is
attributed to oxygen-related defects that have been introduced during the growth process. © 2006
American Institute of Physics. 关DOI: 10.1063/1.2235925兴
Transparent and semiconducting tin oxide 共SnO2兲 has
been widely used for optoelectronic devices,1,2 chemical
sensors,3,4 dye-based solar cells,5 photoconductors,6 and
transistors7 because of its excellent optical and electrical
properties and chemical stability. As one-dimensional 共1D兲
and quasi-one-dimensional 共quasi-1D兲 nanostructures are expected to dramatically improve the desired properties of materials, considerable efforts have thus been devoted to the
synthesis of SnO2 nanowires, nanorods, nanotubes, and
nanobelts via various routes. These include vapor-liquidsolid 共VLS兲 catalytic growth,8 laser ablation,9 and thermal
evaporation.10 However, these methods usually require high
reaction temperature 共⬎1000 ° C兲 or complicated processing
steps so that they are highly incompatible with low-cost flexible substrates such as polymers. In addition, SnO2 nanostructures prepared by those methods are frequent mixtures
of several distinct morphologies and are randomly oriented
due to prolonged exposure to high temperatures and the isothermal growing environment. Accordingly, a solution-based
growth process at a relatively low temperature might be a
better choice for the fabrication of 1D SnO2 nanostructures.
Recently, growth of highly ordered and crystalline SnO2 nanorods of 500 nm in length and 50 nm in width has been
reported by Vayssieres and Graetzel using a one-step, aqueous, low-temperature growth process.11 In this letter, we synthesized large-scale SnO2 nanoblades with widths of
100– 300 nm, thickness of a few tens of nanometers, and
lengths up to 10 ␮m in an aqueous solution at a temperature
as low as 70 ° C. The photoluminescence 共PL兲 spectra of the
as-synthesized SnO2 nanoblades were analyzed and exhibited an intense blue luminescence at a wavelength of 445 nm
共⬃2.8 eV兲.
Large-scale SnO2 nanoblades were synthesized by a
hydrothermal process. An aqueous solution consisting of
0.001M of tin 共IV兲 chloride pentahydrate 共SnCl4 · 5H2O兲 and
a兲
Electronic mail: [email protected]
0.1M of urea 共NH2兲2CO in the presence of 0.1M of NaOH
was prepared and contained in a closed Pyrex bottle with an
autoclavable screw cap. A 100-nm-thick crystalline SnO2
buffer layer was deposited on Corning Eagle 2000 glass substrates by an ion beam assisted deposition system. Then the
glass substrates were placed standing against the walls of the
closed Pyrex bottle. Thereafter, the bottle was placed in a
regular oven and heated at a constant temperature of 70 or
90 ° C for 24 h. The as-synthesized thin films were subsequently dried in air. The microscopic morphologies of the
resultant films were observed with a field-emission scanning
electron microscope 共FE-SEM兲. The structures and constituent phases of the as-synthesized films were characterized by
an x-ray diffractometer 共XRD兲 using Cu K␣ radiation, a
transmission electron microscope 共TEM兲, and an x-ray photoelectron spectroscopy 共XPS兲 using Mg radiation. The photoluminescence spectra were measured at room temperature
in the spectral range of 350– 800 nm using a He–Cd laser
with a wavelength of 325 nm as the excitation source.
Figures 1共a兲 and 1共b兲 show typical FE-SEM images of
the as-synthesized films prepared at 90 and 70 ° C for 24 h,
respectively. Clearly, large-scale and bladelike 1D SnO2
nanostructures 共nanoblades兲 were easily grown on glass
substrates covered with a crystalline SnO2 buffer layer at
temperatures below 100 ° C. All the SnO2 nanoblades had
similar thicknesses on the order of tens of nanometers, while
the aspect ratios of nanoblades were found to strongly
depend on the synthesis temperature. At 90 ° C, SnO2 nanoblades with typical lengths of ⬃1 ␮m and widths of
100– 200 nm were obtained, and the aspect ratio was thus in
the range of 5–10. As the synthesis temperature was reduced
to 70 ° C, the typical lengths and widths of SnO2 nanoblades
were found to increase to ⬃10 ␮m and ⬃300 nm, respectively, giving an aspect ratio of ⬃33. Apparently, SnO2 nanoblades synthesized at a lower temperature exhibited a faster
growth rate than those synthesized at a higher temperature.
According to the energy dispersive x-ray spectra of the as-
0003-6951/2006/89共4兲/043115/3/$23.00
89, 043115-1
© 2006 American Institute of Physics
Downloaded 27 Jul 2006 to 140.120.134.56. Redistribution subject to AIP license or copyright, see http://apl.aip.org/apl/copyright.jsp
043115-2
Her et al.
Appl. Phys. Lett. 89, 043115 共2006兲
FIG. 3. Typical TEM image and SAED pattern of the SnO2 nanoblades.
FIG. 1. Typical FE-SEM images of the as-synthesized films prepared at 共a兲
90 and 共b兲 70 ° C for 24 h.
synthesized SnO2 nanoblades measured with the same SEM
system, the elemental composition was verified to be Sn and
O with an approximate atomic ratio of 1:2, which confirms
that the nanoblades are primarily SnO2.
The structures of the as-grown SnO2 nanoblades synthesized at 90 and 70 ° C for 24 h were determined by XRD, as
shown in Fig. 2. All the diffraction peaks for both sets of
samples can be indexed to the tetragonal rutile structur
e of SnO2 with lattice parameters of a = 0.4741 nm and
c = 0.3182 nm, which agree well with the standard data file
共ICDD-PDF card No. 41-1445兲.12 An intense diffraction
peak due to the 共110兲 plane suggests that SnO2 crystal grew
along the 关110兴 direction. No characteristic peaks peculiar to
impurities, such as elemental Sn and other tin oxides, were
observed, which indicates that the level of impurity in the
sample is lower than the resolution limit of XRD
共⬃5 at. % 兲. Figure 3 shows a typical TEM image of the
SnO2 nanoblades. A selected area electron diffraction
共SAED兲 pattern of a nanoblade taken perpendicular to its
longitudinal axis confirms the high single crystallinity. Regular diffraction spots could be indexed for the 关11̄1兴 zone axis
of single-crystalline tetragonal rutile SnO2.
The constituent phases of the as-synthesized nanoblades
can be further confirmed by the core-level spectra of Sn 3d
and O 1s, as shown in Figs. 4共a兲 and 4共b兲. All peak positions
were corrected by the C 1s peak at 284.5 eV, which was due
to contamination on the grown film. For the nanoblades prepared at 90 ° C, the Sn 共3d5 / 2兲 and O 共1 s兲 peaks were located at 486.6 and 531.6 eV, respectively. As the preparing
temperature was reduced to 70 ° C, the Sn 共3d5 / 2兲 and O
共1s兲 peaks were found to slightly shift to 486.5 and
530.6 eV, respectively. Generally, the Sn 共3d5 / 2兲 peak in
SnOx can be built up of the following three components:
Sn4+ 共486.58 eV兲, Sn2+ shifted with respect to Sn4+ towards
FIG. 2. XRD patterns of the as-grown films prepared at 90 and 70 ° C for
FIG. 4. XPS spectra of 共a兲 Sn 3d and 共b兲 O 1s for the as-grown SnO2
24 h.
nanoblades prepared at 70 and 90 ° C for 24 h.
Downloaded 27 Jul 2006 to 140.120.134.56. Redistribution subject to AIP license or copyright, see http://apl.aip.org/apl/copyright.jsp
043115-3
Appl. Phys. Lett. 89, 043115 共2006兲
Her et al.
FIG. 5. Room-temperature PL spectra of the as-grown SnO2 nanoblades
synthesized at 90 and 70 ° C for 24 h and the as-deposited SnO2 thin film
with thickness of 100 nm on glass substrates.
the lower binding energy by 0.7 eV, and Sn0 shifted with
respect to Sn4+ towards the lower binding energy by 2.2 eV.
The O 共1s兲 peak in SnOx can be built up of the following two
components: O – Sn4+ 共530.37 eV兲 and O – Sn2+ shifted with
respect to O – Sn4+ towards the lower binding energy by
0.6 eV.13 As the Sn 共3d5 / 2兲 and O 共1s兲 peaks in our nanoblades were built up by only Sn4+ and O – Sn4+ components,
respectively, and the peaks corresponding to Sn2+ and
O – Sn2+ were not detected, the constituent phases of the asgrown nanoblades can be confirmed to be SnO2.
Figure 5 shows room-temperature PL spectra of the asgrown SnO2 nanoblades synthesized at 90 and 70 ° C for
24 h and the as-deposited SnO2 thin film with a thickness of
100 nm on glass substrates, where the excited wavelength is
325 nm. It can be seen that the as-deposited SnO2 thin film
had no luminescence at room temperature, while an intense
blue luminescence centered at 445 nm 共⬃2.8 eV兲 with a full
width at half maximum 共FWHM兲 of 75 nm 共0.47 eV兲 was
observed in the spectra for the as-grown SnO2 nanoblades
prepared at 70 and 90 ° C. Moreover, the SnO2 nanoblades
synthesized at 70 ° C exhibited a broadened peak and a lower
luminescence intensity. The FWHM increased by about
7 nm, and the intensity decreased by around 37%, compared
with the SnO2 nanoblades synthesized at 90 ° C. It should be
noted that the strong blue luminescence from SnO2 nanostructures has never been reported before, although yellowred light emissions with a peak at ⬃605 nm were observed
in SnO2 ribbons grown by a laser-ablation technique and
beaklike SnO2 nanorods synthesized by a vapor-liquid-solid
approach.9,14 Up to now, the photoluminescence mechanism
for SnO2 is still not clear. However, similar photoluminescence bands with different peak energies from 1.9 to 4.3 eV
have been observed in high-purity silica glasses, and the
mechanism for each luminescence band has been thoroughly
studied.15 It is believed that preexisting defects, or impurities
introduced during sample preparation, are responsible for
these luminescence bands, and the blue luminescence at
2.7 eV is due to a triplet-to-ground transition of a neutral-
oxygen-vacancy defect based on ab initio molecular-orbital
calculations. Considering that the energy gap of bulk SnO2 is
3.62 eV, the blue luminescence from the as-synthesized
SnO2 nanoblades can be attributed to oxygen-related defects
that have been introduced during growth. As the SnO2 nanoblades were synthesized in an aqueous solution at low temperatures, a high density of oxygen vacancies and tin vacancies might be expected. The interactions between oxygen
vacancies and interfacial tin vacancies would lead to the formation of a significant number of trapped states, which form
a series of metastable energy levels within the band gap, and
result in a strong PL signal at room temperature.
In summary, we have synthesized large-scale SnO2
nanoblades with thickness of a few tens of nanometers and
aspect ratios up to 33, in an aqueous solution at temperatures
as low as 70 ° C. The structure of the as-grown nanoblades
was confirmed to be the tetragonal rutile structure of SnO2,
and the SnO2 nanoblades grew along the 关110兴 direction. The
Sn 共3d5 / 2兲 and O 共1s兲 peaks in the XPS spectrum for the
SnO2 nanoblades were built up by only the Sn4+ and the
O – Sn4+ components, respectively. This indicates that impurities, such as elemental Sn and other tin oxides, did not exist
in the as-grown nanoblades. The PL spectra of the assynthesized SnO2 nanoblades exhibited an intense blue luminescence at a wavelength of 445 nm with FWHM of 75 nm,
which may be attributed to oxygen-related defects that have
been introduced during the growth process.
This work was sponsored by the Industrial Technology
Research Institute 共ITRI兲 under Grant No. F445D51110 and
by the National Science Council of Taiwan under Grant No.
NSC93-2216-E005-005. The authors would like to thank Edward Young for reviewing the manuscript.
A. Aoki and H. Sasakura, Jpn. J. Appl. Phys. 9, 582 共1970兲.
C. Tatsuyama and S. Ichimura, Jpn. J. Appl. Phys. 15, 843 共1976兲.
3
E. Comini, G. Faglia, G. Sberveglieri, Z. W. Pan, and Z. L. Wang, Appl.
Phys. Lett. 81, 1869 共2002兲.
4
A. Kolmakov, Y. X. Zhang, G. S. Cheng, and M. Moskovits, Adv. Mater.
共Weinheim, Ger.兲 15, 997 共2003兲.
5
S. Ferrere, A. Zaban, and B. A. Gregg, J. Phys. Chem. B 101, 4490
共1997兲.
6
S. Mathur, S. Barth, H. Shen, J. C. Pyun, and U. Werner, Small 1, 713
共2005兲.
7
Y. N. Xia, P. D. Yang, Y. G. Sun, Y. Y. Wu, B. Mayers, B. Gates, Y. D.
Yin, F. Kim, and H. Q. Yan, Adv. Mater. 共Weinheim, Ger.兲 15, 353 共2003兲.
8
Y. Chen, X. Cui, K. Zhang, D. Pan, S. Zhang, B. Wang, and J. Hou, Chem.
Phys. Lett. 369, 16 共2003兲.
9
J. Q. Hu, Y. Bando, Q. L. Liu, and D. Golberg, Adv. Funct. Mater. 13, 493
共2003兲.
10
Z. R. Dai, J. L. Gole, J. D. Stout, and Z. L. Wang, J. Phys. Chem. B 106,
1274 共2002兲.
11
L. Vayssieres and M. Graetzel, Angew. Chem., Int. Ed. 43, 3666 共2004兲.
12
ICDD-PDF Card No. 41-1445 共unpublished兲.
13
J. Szuber, G. Czempik, R. Larciprete, D. Koziej, and B. Adamowicz, Thin
Solid Films 391, 198 共2001兲.
14
J. H. He, T. H. Wu, C. L. Hsin, K. M. Li, L. J. Chen, Y. L. Chueh, L. J.
Chou, and Z. L. Wang, Small 2, 116 共2006兲.
15
H. Nishikawa, T. Shiroyama, R. Nakamura, Y. Ohki, K. Nagasawa, and Y.
Hama, Phys. Rev. B 45, 586 共1992兲.
1
2
Downloaded 27 Jul 2006 to 140.120.134.56. Redistribution subject to AIP license or copyright, see http://apl.aip.org/apl/copyright.jsp