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7/27/2019 Laising Feom Single ZnO
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Journal of Luminescence 108 (2004) 385388
Room-temperature excitonic lasing from ZnO single nanobelts
Kazuki Bando*, Taiki Sawabe, Koji Asaka, Yasuaki Masumoto
Institute of Physics, University of Tsukuba, Tsukuba 305-8571, Japan
Abstract
Excitonic lasing from ZnO single nanobelts was observed at room temperature, which was due to excitonexciton
scattering processes appearing under intense light excitation. Morphologies of the nanobelts are rectangular shapes, so
that crystalline facets of the nanobelts acted as laser-cavity mirrors. Therefore, mode spacings corresponding to cavity
lengths of the respective nanobelts were observed in luminescence spectra. Lasing from ZnO single nanobelts at room
temperature and their lasing properties are reported.
r 2004 Elsevier B.V. All rights reserved.
Keywords: Nanowire; Nanobelt; Excitonic lasing; ZnO
1. Introduction
Nanocrystals of semiconducting oxide materials
have received much attention in recent years,
because it is expected that they can be applied to
short-wavelength optical devices and excitonic
devices operating at room temperature. In parti-
cular, the exciton binding energy in ZnO (59 meV)
is so large that ZnO can be applied to laser devices
based on excitonic processes. The excitonic pro-
cesses can result in lasing at lower threshold
than that of the electronhole plasma which is a
driving force of lasing for most semiconductor
lasers. To lower the threshold further, the use of
nanostructures can contribute to the confinement
of the excitons. One-dimensional nanostructures
have been recently investigated with respect to
oxide materials including ZnO. The nanowirewhich has a cylindrical morphology has been
already presented as a one-dimensional nanos-
tructure [1]. Belt-like nanostructures (the so-called
nanobelts), whose cross-sections are rectangular
shapes, have been recently grown [2]. Excitonic
lasing from ZnO nanowires at room temper-
ature has been reported [3,4] in which lasing
has occurred by an excitonexciton scattering
process. It is noteworthy that this lasing occurs
without an external cavity, because crystalline
facets of both ends of the nanowire act to form
a cavity. As the morphology of nanobelts is
different from that of the well-known nano-
wires, it can be expected that the cavity modes
formed in nanobelts would be different from
those in nanowires. In this work, we observed
excitonic lasing from the ZnO nanobelts at room
temperature arising from an excitonexciton scat-
tering process. In addition, we report on the
lasing-cavity mode formed and on the lasing
properties.
ARTICLE IN PRESS
*Corresponding author. Tel.: +81-29-853-4350; fax: +81-
29-853-6618.
E-mail address: [email protected]
(K. Bando).
0022-2313/$- see front matterr 2004 Elsevier B.V. All rights reserved.doi:10.1016/j.jlumin.2004.01.081
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2. Experimental
ZnO nanobelts can be grown by using various
methods [2,57]. In our work, the ZnO nanobeltswere simply grown by thermal evaporation of the
ZnO powder without catalysts, which is similar to
the method reported in Ref. [2]. The ZnO powder
was heated on ceramic boats in a furnace at
1450C for 3 h in flowing Ar gas at 50 sccm. The
aggregate of ZnO nanobelts, which is a white
wool-like product, adhered onto a ceramic tube
which was simultaneously inserted in the furnace.
In order to separate the aggregate into single
nanobelts, it was mixed in ethanol, treated by
sonication and dispersed onto copper substrates
by drop-casting the ethanol containing the nano-
belts. The nanobelts were observed by using
scanning electron microscopy (SEM) (HITACHI
S-4300). Fig. 1 shows a SEM image of the as-
synthesized wool-like products which were put
onto the copper substrates without dispersing. The
morphology of the products obtained from the
SEM observation agrees with that of nanobelts
which have been reported already [2,57]. Belt-like
morphological images were observed, and growth
of ZnO nanobelts was clearly confirmed.
Photoluminescence (PL) from the single nano-belts under weak and intense excitation conditions
were obtained by using either a continuous wave
(CW) HeCd laser (325 nm) or the fourth-harmo-
nic generation output (266 nm) of a pulsed
Nd:YAG laser (8 ns, 10 Hz) as excitation sources,
respectively. PL images of the single nanobelts
were enlarged by using a long-distance microscope
to detect PL from only a single nanobelt fromthose dispersed on the substrate, and the PL was
detected by using a monochromator and a liquid
nitrogen-cooled CCD. In order to measure the PL
spectra, single nanobelts whose crystalline facets
were of good quality, were selected, because it was
expected that the nanobelts with good crystalline
facets could act as self-formed cavities and result
in successful lasing.
3. Results and discussion
Fig. 2 shows the excitation-intensity dependence
of PL spectra at room temperature. The inset
shows the SEM image of the single nanobelt whose
PL spectra are shown in Fig. 2. The dimensions of
the selected nanobelt are about 10 mm in length,
ARTICLE IN PRESS
Fig. 1. SEM image of the aggregate of the ZnO nanobelts on
the substrate.
3.0 3.1 3.2 3.3 3.4
PS
P
2
3
5
5
I0
0.75I0
0.69I0
0.64I0
0.59I0
He-Cd Laser
RT Single Nanobelt
Nd:YAG LaserFHG (266nm)
I0
~ 500kW/cm2
LuminescenceIntensity(a.u.)
Photon Energy (eV)
Fig. 2. PL spectra of a ZnO single nanobelt. The inset shows
the SEM image of the single nanobelt whose PL spectra were
measured. The lowest PL spectrum was obtained for weak He
Cd cw laser excitation. The others were obtained by intense
Nd:YAG laser excitation. For this intense excitation, PL from
both spontaneous (P) and stimulated (PS) emissions due to the
excitonexciton scattering process were observed.
K. Bando et al. / Journal of Luminescence 108 (2004) 385388386
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2mm in width, and B100 nm in thickness. A
relatively wide nanobelt was selected, because the
PL intensity from the wider nanobelts is strong
enough to detect. In addition, the length of theselected nanobelt was relatively short compared to
that of ordinary nanobelts. It appears that long
nanobelts were broken by the dispersing process of
the sonication in the ethanol and cleaved facets
were accidentally formed at both ends of these
nanobelts. In Fig. 2, the lowest PL spectrum was
obtained under weak cw excitation and is a broad
PL band which consists of PL bands due to free
excitons and their phonon replicas. The PL
spectrum just above was obtained by the weakest
of the pulsed excitations and the PL peak position
is observed at lower energy than that observed for
weak cw excitation. The PL band appears around
3.25 eV which is lower than the exciton resonance
(B3.31eV) by the exciton binding energy of
59 meV, indicating that spontaneous emission (P)
due to exciton-recoil processes of the exciton
exciton scattering appears under intense excita-
tion. With increasing excitation intensity, three
sharp peaks (PS) abruptly appear in the lower
energy tail (B3.2 eV) of the spontaneous emission
above an excitation threshold of I=0.69I0
(B350 kW/cm2
). Considering that the PL peaksappear above a certain threshold power, it is
concluded that stimulated emission occurs. Upon
increasing the excitation level further, the intensity
of the sharp peaks superlinearly increases and the
number of peaks increases on the lower-energy
side. These peaks comprise several sharp peaks
whose energy spacings are constant, which clearly
indicates that lasing occurs in the nanobelts. It is
considered that the self-formed cavity of the
nanocrystal brings about lasing in the cavity
modes formed in the nanobelt. The cavity modesappear as the several peaks with constant energy
spacing in the PL spectra. An important aspect is
that this lasing action occurs without external
mirrors and the nanolaser cavity is formed by the
crystalline facets alone. It is noted that this PL
from the nanobelt was detected not along the
length of the nanobelt, but in the vertical direction
of the substrate. Although it is considered that the
cavity is formed along the length of the nanobelt, it
is considered that the light emitted from the
nanobelt was scattered in every direction due to
the roughness on the surface of the substrate. In
actuality, the lasing was also observed along the
length of the nanobelt.In order to examine the relation between the
cavity and its lasing action further, PL measure-
ments were performed with respect to several
nanobelts whose lengths differed from each other.
If the nanolaser cavity is formed by crystalline end
facets, the peak energy spacings should be related
to the energy spacings between modes formed in
the nanobelts, and therefore should depend on the
length of the nanobelts. In Fig. 3, the spacings are
plotted as a function of the reciprocal nanobelt
length 1/L. As the FabryP!erot cavity model
should apply to the nanobelt lasing cavity, the
relationship between the mode spacing and the
cavity length can be evaluated by using DE=hc/
2nL, where DE is mode spacing, h is Plancks
constant, c the speed of light and L is the length of
each nanobelt. Here, n is the effective refractive
index expressed by n=n+Edn/dE, where the
energy dispersion of the refractive index n(E) was
given by Park and Schneider [8]. The solid line in
Fig. 3 shows the relationship calculated by this
equation. As shown in Fig. 3, the peak energy
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0.00 0.02 0.04 0.06 0.08 0.100.000
0.002
0.004
0.006
0.008
0.010
0.012
0.014
ModeSpacingE
(eV)
1/L (m)
E= hc/2L{n+E(dn/dE)}
Fig. 3. The relation between the peak energy spacing and the
reciprocal nanobelt length 1/L. The solid line is the relationship
given by the equation DE=hc/2L{n+E(dn/dE)}.
K. Bando et al. / Journal of Luminescence 108 (2004) 385388 387
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spacings observed in the PL spectra of the
respective nanobelts agree quantitatively with
mode spacings estimated from the equation.
Therefore, it is concluded that the cavity modesare formed between the two end facets of the
nanobelts.
In summary, we observed excitonic lasing from
ZnO single nanobelts at room temperature. The
nanobelts were simply grown by a thermal
evaporation process in a furnace. Under cw
excitation, PL from the free exciton was observed
at room temperature. With an increasing level of
pulsed-laser excitation, PL from an exciton
exciton scattering process appeared. Stimulated
emission consisting of several sharp peaks which
abruptly appeared with a further increase of
the level of excitation. In other words, lasing
occurred within the self-formed cavity of the
nanobelt. By measuring the sharp PL peaks
of the lasing from the several single nanobelts,
it was concluded that the cavity modes were
formed between the two end facets of the
nanobelts.
Acknowledgements
This work was partly supported by the Sasaka-
wa Scientific Research Grant from The JapanScience Society and Research Project and Na-
noscience Special Project of University of Tsuku-
ba. The authors are grateful to Professor Y.
Ootuka of the university for the use of the SEM.
References
[1] M.H. Huang, Y. Wu, H. Feick, N. Tran, E. Weber, P.
Yang, Adv. Mater. 13 (2001) 113.
[2] Z.W. Pan, Z.R. Dai, Z.L. Wang, Science 291 (2001) 1947.
[3] M.H. Huang, S. Mao, H. Feick, H. Yan, Y. Wu, H. Kind,E. Weber, R. Russo, P. Yang, Science 292 (2001) 1897.
[4] J.C. Johnson, H. Yan, R.D. Schaller, L.H. Haber, R.J.
Saykally, P. Yang, J. Phys. Chem. B 105 (2001) 11387.
[5] J. Zhang, W. Yu, L. Zhang, Phys. Lett. A 299 (2002) 276.
[6] B.D. Yao, Y.F. Chan, N. Wang, Appl. Phys. Lett. 81 (2002)
757.
[7] Y.B. Li, Y. Bando, T. Sato, K. Kurashima, Appl. Phys.
Lett. 81 (2002) 144.
[8] Y.S. Park, J.R. Schneider, J. Appl. Phys. 39 (1968) 3049.
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