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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.

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*Corresponding author. Tel.: +81-29-853-4350; fax: +81-

29-853-6618.

E-mail address: bando@sakura.cc.tsukuba.ac.jp

(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,

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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.

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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)}.

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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.

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