୯ҥᆵεᏢᏢଣᔈҔނࣴز܌
ᅺγፕЎ
Graduate Institute of Applied Physics College of Science
National Taiwan University Master Thesis
ӣॣ൴ኳᄊቚம਼ϯᎋڼԯࢊޑᒿᐒႜ
Enhancement of Random Laser Action Assisted by Whispering-Gallery-Mode Resonance
શਯ௵
Tong-Ming Weng
ࡰᏤ௲Ǻഋ҉ޱ റγ Advisor: Yang-Fang Chen, Ph.D.
ύ҇୯ 102 ԃ 7 Д July, 2013
i
ᄔा!
ҁፕЎЬाҞޑӧܭࣴزճҔӣॣ൴ኳᄊቚம਼ϯᎋڼԯࢊޑᒿᐒႜǶࣁΑځύޑ
ၮբচǴךॺࣴزΑΒ਼ϯޖڼԯౚঅႬ਼ϯᎋڼԯጕޑႜ܄ǶךॺวӧΒ਼ϯޖڼ
ԯౚޑᔅշΠǴܫӀ္Ԗኧঁъଯቨλܭ0.3 ڼԯޑႜӾঢ়ǴԶᒿᐒႜޑ༾ϩໆηਏ
ΨቚуΑΎ७Ƕٰ৾অႬޑڼԯౚόՠёаࣁӣॣ൴ኳᄊޑӅਁ๚ǵቚமวӀமࡋǴΨ
ёаࣁঁණύЈǶၸךॺϡҹޑΒᆢޑཱུวӀቹႽǵ༝ॣ൴ኳᄊޑፕीᆉǵ
όӕελ਼ϯޖڼԯౚޑ٬ҔǴךॺӦᡍΑගрޑፕǶ೭ኬᐱޑႜ܄ஒёа
ճҔӧᇙբଯਏޑӀႝϡҹǶ
ii
Abstract
Whispering-gallery-mode (WGM) resonance enhanced random laser action has been
proposed and demonstrated. To illustrate the working principle, lasing characteristics of ZnO
nanorods decorated with SiO2 nanospheres have been investigated. It is found that with the
assistance of SiO2 nanospheres, the emission spectrum exhibits a very narrow background signal
with few sharp lasing peaks and a very small full width at half maximum of less than 0.3 nm. The
differential quantum efficiency (Șd) of random laser action can be greatly enhanced by up to 735
%. The decorated nanospheres not only enable to generate WGM resonance and enhance the
emission intensity, but also can serve as scattering centers. Cathodoluminescence mapping
images of nanorods decorated with nanospheres and theoretical calculation based on the spherical
cavity were utilized to confirm our proposed mechanism. The unique lasing behavior shown here
may open up a new approach for the creation of highly efficient optoelectronic devices.
iii
Contents
Chapter 1 Introduction……… ………1
Reference………...………...………6
Chapter 2 Background knowledge of experimental technique and studied nanomaterials……8
2.1 Theory of photoluminescence of semiconductors………...……...…….……….…8
2.2 ZnO nanorods……….…………..….……...……...…...13
Reference………....………16
Chapter 3 Experimental details, Theoretical background and Sample preparation……….17
3. 1 Scanning Electron Microscopy...……...……...……...……...……...……...…17
3. 2 Cathodoluminescence...24
3. 3 Photoluminescence arrangement………....……...……...……...……...……....……25
3. 4 Time-resolved photoluminescence………...……...……...……...……...………..27
3. 5 Absorption Spectroscopy………...……...……...…...……...……...……...……28
3. 6 Vapor-Soild (VS) Growth Mechanism of ZnO Nanorods…………...……..……...30
Reference………...………...……31
Chapter 4 Enhancement of Random Laser Action Assisted by Whispering-Gallery-Mode Resonance………...32
5. 1 Introduction...32
5. 2 Experiment……….…………...……...……...……...……...……...……...…35
iv
5. 3 Results and discussion………...……...……...……...……...……...36
5. 4 Summary………...……...……...……...……...……...……...……...43
Reference………...………...……52
Chapter 5 Conclusion………...……….55
v
List of Figures and Tables
Figure 1.1 Components of a typical laser: 1. Gain medium. 2. Laser pumping energy. 3. High
reflector 4. Output coupler 5. Laser beam………2
Figure 1.2 Schematic representation of random laser.………3
Figure 2.1 The schematic of photoluminescence. (a) An electron absorbs a photon and is promoted from the valence band to the conduction band. (b) The electrons cools down to the bottom of the conduction band. (c) The electron recombines with the hole resulting in the emission of light with energy hȞ……...9
Figure 2.2 Schematic diagram of common radiative transitions observable with photoluminescence transitions…….……….………..10
Figure 3.1 Signals that result from electron beam-specimen interaction………...18
Figure 3.2 The structural scheme of a typical scanning electron microscope………20
Figure 3.3 Photo of scanning electron microscopy (JSM-6500F, JEOL)………..………….……....21
Figure 3.4 The range and spatial resolution of backscattered electrons, secondary electrons, X-rays, and Auger electrons for electrons incident on a solid.………..23
Figure 3.5 Photo of the CL spectrometer.……….………….25
Figure 3.6 The photoluminescence experiment setup…..………...………….….26
Figure 3.7 The time-resolved PL setup……….28
Figure 3.8 The absorption spectroscopy setup………..29
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Figure 4.1 (a) Scanning electron microscope image of ZnO nanorods decorated with SiO2
nanospheres. (b) Enlarged SEM image of ZnO nanorods decorated with SiO2
nanospheres. (c) Statistical bar chart of the size distribution of SiO2 nanospheres...44
Figure 4.2 (a) and (b) Emission spectra of ZnO nanorods without and with the decoration of 120
nm SiO2 nanospheres.……….………45
Figure 4.3 (a) Plot of the emission intensity versus the pumping energy. Black boxes denote ZnO nanorods and red circles denote pristine ZnO nanorods with the decoration of 120 nm
SiO2 nanospheres. (b) Time-resolved photoluminescence decay spectra with fitting
curves for pristine ZnO nanorods and 120 nm SiO2 nanospheres/ZnO nanorods
monitored at the peak emission wavelength of 389 nm...46
Figure 4.4 Schematic illustration of the mechanisms responsible for the enhanced laser action in SiO2 nanospheres decorated ZnO nanorods. (a) SiO2 nanospheres serve as scattering
centers to assist light traveling in a randomly closed loop. (b) SiO2 nanosphere serves as
an excellent spherical cavity for the occurrence of whispering gallery made
resonance……….47
Figure 4.5 (a) and (b) Emission spectra of ZnO nanorods without and with the decoration of 190 nm and 250 nm SiO2 nanospheres under the same excitation power. The insets show
plots of the emission intensity versus the pumping energy. Black boxes denote pristine
ZnO nanorods and red circles denote ZnO nanorods decorated with the decoration of
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SiO2 nanospheres……….48
Figure 4.6 Plot of the diameter of spherical cavity versus TM-resonance peaks according to the therotical calculation given by Equation 1. For 120 nm SiO2 nanospheres, the
TM-resonance peak position by the theoretical calculation is approximately at 389 nm
for N=1. For 190 nm SiO2 nanospheres, the theoretical TM-resonance peak position is
approximately at 384 nm for N=2 (N=1 is far above 600 nm). And, for 250 nm SiO2
nanospheres, the theoretical TM-resonance peak position is at 378 nm for N=3...49
Figure 4.7 Scattering spectra of SiO2 nanospheres with three different sizes. It is clear that the transmittances for three different-size SiO2 nanospheres around 389 nm only exhibit a
slight difference………50
Figure 4.8 (a) Scanning electron microscope image of SiO2 nanospheres decorated ZnO nanorods.(b) The corresponding cathodoluminescence mapping image………51
1
Chapter 1
Introduction
Laser, as shown in Fig 1.1, which is usually constructed from a material that
provides optical gain through stimulated emission and an optical cavity that partially
traps the light. Once the total gain in this cavity is larger than losses, the system reaches
a threshold and begin lasing. Over the past decade, the random amplifying medium
which has been a topic of intense research. Mutiple scattering in disordered optical
materials is complex yet completely coherent, which means that the phase of each of the
optical wavelets undergoing a random walk is well de¿ned and interference eơects can
occur, even if a material is strongly disordered. Finally, laser physics and mutiple
scattering meet in the random laser : a mutiple scattering medium amplifies light to
reach a lasing threshold as shown in Fig 1.2.
2
Fig. 1.1 Components of a typical laser: 1. Gain medium.
2. Laser pumping energy. 3. High reflector 4. Output coupler 5. Laser beam.
In 1968, Letokhov1 theoretically predicted that laser-like emission from amplifying
disordered materials can be obtained by using non-resonant positive feedback via
multiple scattering of light. Unlike with the conventional laser action via Fabry-Perot
resonance, random laser cavities are self-formed to achieve coherent feedback in the
laser system with its low-cost and simple process technology. Moreover, random lasing
usually exhibits a very broad angular distribution which is ideal for display application.
3
Fig. 1.2 Schematic representation of a random laser.
So far, random lasing action has been found in many kinds of media such as
inorganic semiconductors, organic molecules, biological systems and cold atoms.2-5 It
has also been widely studied in nanoscale materials such as nanorod arrays and
nanocrystalline films.6,7 ZnO, with a wide bandgap of 3.37 eV , a high exciton binding
energy of 60 meV, and plenty kinds of nanostructures, is very suitable for the fabrication
of ultraviolet light-emitting diodes and laser devices with high efficiency.8-10 In addition,
due to a high refractive index in ultraviolet region (~ 2.5), the total internal reflection in
ZnO structures can be easily achieved. Based on these favorable properties,
conventional laser actions from ZnO nanostructures have been successfully
demonstrated by many types of resonators.11-13 However, according to previous
4
studies,14 the emission of ZnO nanorods (NRs) fabricated by vapor-solid (VS) growth
mechanism only shows a very broad spectrum which means random laser action still
needs improvement .7,15
Encouraged by the above mentioned novel properties arising from nanoscale
materials, in this thesis, we provide an alternative approach to enhance the random
lasing behavior arising from VS-ZnO NRs decorated by SiO2 nanospheres with the
assistance of WGM resonance which has been used to enhance the sensitivity of gas
sensors and be a cavity of lasing in many materials and circular structures.16-19 In this
thesis, we attempt to investigate and analyze the unique properties of our optical devices.
We found that after the SiO2 nanosphere decoration, the laser action of our devices can
be greatly improveded. Morover, through varying SiO2 nanosphere size, we found that
the laser action could be controlled by the sizs of nanosphere size.
In chapter 1, the development of random laser and the motivation of our work are
described. In chapter 2, fundamental concepts of photoluminescence (PL) and ZnO
materials are introduced. Chapter 3 presents how to prepare for our device and the
experimental setup of scanning electron microscopy (SEM), cathodoluminescence (CL),
PL, and time-resolved photoluminescence (TRPL). Chapter 4 shows the enhancement
of random laser action assisted by WGM resonance. We use the TRPL experiments,
varying the size of SiO2 nanospheres, CL mapping, and theoretical calculation to
5
confirm the underlying mechanism responsible for the enhanced random laser action in
SiO2 nanospheres decorated VS-ZnO NRs. The last chapter, chapter 5, decribes a brief
conclusion of this thesis.
6 Reference
1. V. S. Letokhov, Sov. Phys. JETP 1968, 26, 835.
2. H. D. Li, S. F. Yu, S. P. Lau, and E. S. P. Leong, Appl. Phys. Lett. 2006, 89, 021110.
3. M. Anni, S. Lattante, T. Stomeo, R. Cingolani, G. Gigli, G. Barbarella, and L.
Favaretto, Phys. Rev. B. 2004, 70, 195216
4. R. C. Polson, and Z. V. Vardeny, Appl. Phys. Lett. 2004, 85 1289–1291.
5. W. Guerin, N. Mercadier, F. Michaud, D. Brivio, L. S. Froufe-Pérez, R. Carminati, V.
Eremeev, A. Goetschy, S. E. Skipetrov, and R. Kaiser, J. Opt. 2010, 12, 024002.
6. S. F. Yu, C. Yuen, S. P. Lau, W. I. Park, and G. C. Yi, Appl. Phys. Lett. 2004, 84,
3241-3243.
7. H. Zhu, C. X. Shan, J. Y. Zhang, Z. Z. Zhang, D. X. Zhao, B. H. Li, B. Yao, D. Z.
Shen, X. W. Fan, Z. K. Tang, X. H. Huo, and K. L. Choy, Adv. Mater. 2010, 22,
1877-1881.
8. A. Tsukazaki, M. Kubota, A. Ohtomo, T. Onuma, K. Ohtani, H. Ohno, S. F.
Chichibu, and M. Kawasaki, Jpn. J. Appl. Phys. 2005, 44, L643-L645.
9. S. Chu, M. Olmedo, Z. Yang, J. Kong, and J. Liu, Appl. Phys. Lett. 2008, 93,
181106.
10. C. Zhang, F. Zhang, T. Xia, N. Kumar, J. I. Hahm, J. Liu, Z. L. Wang, and J. Xu,
Opt. Express 2009, 17, 7893-7900.
7 11. M. H. Huang, Science 2001, 292, 1897-1899.
12. D. J. Gargas, M. C. Moore, A. Ni, S. W. Chang, Z. Zhang, S.-L. Chuang, and P.
Yang, ACS Nano 2010, 4, 3270-3276.
13. R. Chen, B. Ling, X. W. Sun, and H. D. Sun, Adv. Mater. 2011, 23, 2199-2204.
14. Y. T. Chen and Y. F. Chen, Opt. Express 2011, 19, 8728-8734.
15. H. Cao, Y. G. Zhao, S. T. Ho, E. W. Seelig, Q. H. Wang, and R. P. H. Chang, Phys.
Rev. Lett. 1999, 82, 2278-2281
16. C. W. Chen and Y. F. Chen, Appl. Phys. Lett. 2007, 90, 071104.
17. Y. Wu and P. T. Leung, Phys. Rev. A 1999, 60, 630-633.
18. T. J. Lin, H. L. Chen, Y. F. Chen, and S. Cheng, Appl. Phys. Lett. 2008, 93, 223903.
19. I. S. Grudinin, A. B. Matsko, and L. Maleki, Phys. Rev. Lett. 2009, 102, 043902.
8
Chapter 2
Background knowledge of experimental technique and studied nanomaterials
2.1 Theory of photoluminescence of semiconductors
Photoluminescence (PL) is a non-destructive optical technique used for the
characterization, investigation, and detection of point defects or for measuring the
band-gaps of materials. Photoluminescence involves the irradiation of the crystal to be
characterized with photons of energy greater than the band-gap energy of that material.
In the case of a crystal scintillator, the incident photons will create electron-hole pairs.
When these electrons and holes recombine, this recombination energy will transform
partly into non-radiative emission and partly into radiative emission.
As shown in Fig 2.1, we can briefly say photoluminescence process includes three
main phases: 1(1) Excitation: Electrons can absorb energy from external sources, such
as lasers, arc-discharge lamps, and tungsten-halogen bulbs, and be promoted to higher
energy levels. In this process electron-hole pairs are created. (2) Thermalization:
Excited pairs relax towards quasi-thermal equilibrium distributions. (3) Recombination:
The energy can subsequently be released, in the form of a lower energy photon, when
9
the electron falls back to the original ground state. This process can occur radiatively or
non-radiatively.
Fig. 2.1 The schematic of photoluminescence. (a) An
electron absorbs a photon and is promoted from the valence band to the conduction band. (b) The electrons cool down to the bottom of the conduction band. (c) The electron recombines with the hole resulting in the emission of light with energy hȞ.
When a semiconductor absorbs a photon of energy greater than the band gap, an
electron is excited from the valence band into the conduction band leaving behind a hole.
When the electron returns to its original state, its energy may be released through
radiative (release of a photon) or non-radiative (no photon production) recombination.
When the electron and hole recombine through radiative recombination, a photon is
emitted and the energy of the emitted photon is dependent on the change in energy state
10 of the electron-crystal system.
A simplified set of radiative transitions that lead to emission in semiconductors is
giving in Fig. 2.2.
Fig. 2.2 Schematic diagram of common radiative transitions observable with photoluminescence transitions.
Process (a) : Process (a) is the band to band transition, which dominates at room
temperature and can be used to estimate the material bandgap energy (Eg). For indirect
semiconductors, a band-to band recombination process is unlike because the electrons at
the bottom of the conduction band have a nonzero crystal momentum with respect to the
11
holes at the top of the valence band. Band to band transition contains the recombination
of free electrons and free holes. This transition occurs when an electron falls from its
conduction band state into the empty valence band state associated with the hole.
Band-to-band transition depends on the density of available electrons and holes, and the
probability is proportional to the absorption coefficient. The photon energy, which is
equal to the energy difference between the excited and ground states, released during
the process usually produces a photon and emits light in a semiconductor having a
direct band gap. It is given by the equation:
hȞ = Ef í Ei (2.1)
where Ef and Ei are, respectively, the final and initial state energies. In indirect
semiconductors, band-to-band recombination occurs with phonon contribution and
emitted photon energy is
hȞ = Ef í Ei Ʋ h (2.2)
Where h is the energy of phonon.(as shown in Fig2.3)
Process (b), (c), (d): At temperature for which kBT is greater than the ionization energy of shallow impurities, these impurities are ionized, hence band-to-band transition
dominate. At sufficiently low temperature the thermal energy of carriers becomes
smaller than the ionization energy of the impurities in which case are frozen to the
impurity. For example, in a p-type material containing NA acceptors per unit volume,
12
holes are trapped at the acceptor if kBT is smaller than EA, where EA is the ionization
energy of the acceptor. Process b represents the donor-to-free-hole transition as well as
process (c) represents the free-electron-to-acceptor transition. Process (d) is the
donor-acceptor pair (DAP) recombination. Donor is substitutional atom with a higher
number of valence electrons compared with the host atom, where as acceptor is a
substitutional atom with lower of valence electrons. Donor contributes excess electrons
to the crystal, while acceptor tends to capture electrons or equivalently donate holes.
Donor or acceptor may be electrically charged or neutral.2 As the neutral donor and the
neutral acceptor are brought closer together, the donor’s electron become increasingly
shared by the acceptor. In other words, the donor and the acceptor become increasingly
more ionized and form a pair.3 The energy of the DAP recombination emission in
relatively pure crystals can be expressed by:
hν(r)=Eg -(EA -ED)+e2/εr (2.3)
where EA and ED are the binding energies of the acceptor and donor, respectively, İ is
the dielectric constant, e is the electron charge, and r is the distance between the donor
and acceptor impurities which participate in the recombination. The last term arises
from the coulombic interaction of the carriers and depends on the separation r. the
radiative transition probability in this case is
P(r)=P(0)exp(-2r/a) (2.4)
13
where a is the Bohr radius of the less tightly bound center, and P(0) is the limiting
transition probability which is a constant as r~0.
Process (e): Process (e) is exciton transition. Coulombing attaction leads to the
formation of an excited state in which an electron and the hole remain bound to each
other in a hydrogen-like state, referred as a free exciton.4 The energy of the emitted
photon is
hȞ = Egí EX (2.5)
where Eg is the bandgap energy of the semiconductor and EX is the Coulomb energy of
the exciton. In reality, many semiconductor materials contain small amounts of natural
defects or impurities forming neutral donors and acceptors. Optically generated free
excitons can interact with those impurities and may become captured by them. They are
then called donor-bound or acceptor-bound excitons depending on whether the impurity
that the exciton attached is a donor or an acceptor.
2.2 ZnO nanorods
In the past decade global research interest in wide band gap semiconductors has
been attracted towards zinc oxide (ZnO) due to its excellent properties as a
semiconductor material. Zinc oxide (ZnO) is a direct band-gap (Eg = 3.37 eV)
semiconductor with a large exciton binding energy (60 meV), exhibiting near UV
14
emission, transparent conductivity and piezoelectricity.5 Furthermore, ZnO is bio-safe
and biocompatible, and may be used for biomedical applications without coating make
ZnO suitable for optoelectronics, transparent electronics, lasing, sensing, and wide
range of applications.5-8
Other basic characteristics of ZnO are the polar surfaces that are formed by
charged ions produced by positively charged Zn+ (0001) and negatively charged O-(000
聩) polar surfaces. It is responsible for the spontaneous polarization observed in ZnO.
The polar surfaces of ZnO have non-transferable and non-flowable ionic charges. The
interaction among the polar charges at the surface depends on their distribution.
Therefore the structure is arranged in such a way to minimize the electrostatic energy,
which is the main driving force for growing polar surface dominated nanostructures.
This effect results in a growth of various ZnO nanostructures such as nanorods,
nanosprings, nanocages, nanobelts, nanocombs, nanorings, and nanohelices.5
Additionally, in recent years, one-dimensional (1D) nanostructures such as rods,
wires, belts have attracted much attention due to their many unique properties and the
possibility that they may be expected to play an important role as both interconnects and
functional units in fabricating electronic, optoelectronic, electrochemical and
electromechanical nanodevices. It is generally accepted that 1D nanostructures are
useful materials for investigating the dependence of electrical and thermal transport or
15
mechanical properties on dimensionality and quantum confinement. In the recent years,
much effort has been devoted to developing various 1D semiconductor nanostructures.
Vapour–liquid–solid (VLS) and vapour–solid (VS) mechanisms for growth of whiskers
or fibres at high temperature are well recognized, and have been used to synthesize
various group III–V and II–VI compound semiconductor nanostructures. Intensive
research has been focused on fabricating 1D ZnO nanostructures and in correlating their
morphologies with their size-related optical and electrical properties. Many different
kinds of ZnO nanostructures have been developed, such as nanodots, nanorods,
nanowires, nanobelts, nanotubes, nanobridges and nanonails, nanowalls, nanohelixes,
and nanorings. Among these various types of nanostructure, ZnO nanorods and
nanowires have been the most widely studied because of their easy formation and
device applications.2, 9-10
16
Reference
1. Y. Yu, M. Cardona, Fundamentals of Semiconductors, pp.348, published by springer, 1990.
2. Quirk, M.; Serda, J. Semiconductor Manufacturing Technology, Prentice Hall, 2000.
3. Xu, Y.; Sheng, K.; Li, C.; Shi, G. ACS Nano 2010, 4, 4324-4330.
4. H. K. Schröeder, Semiconductor Material and Device Characterization, published by John Willey & Sons, 1998, 624,
5. Chen Z.; Ren, W.; Gao, L.; Liu, B.; Pei, S., Cheng, H. M. Nat. Mater. 2011, 10,
424-428.
6. O. Dulub, L. A. Boatner, and U. Diebold, Surf. Sci. 519, 201 (2002)
7. Z. L. Wang, J. Phy. Condens. Matter 16, 829 (2004)
8. T. Kogure, and Y. Bando, J. Electron Microsc. 47, 7903 (1993)
9. Park, S. Ruoff, R. S. Nature Nanotech. 2009, 4, 217-224.
10. Seung J. C.; Fethullah G.; Ki K. K.; Eun S. K.; Gang H. H.; Soo M. K.; Hyeon J.
S., Seon M. Y.; Jae Y. C.; Min H. P.; Cheol W. Y.; Didier P.; Young H. L.; Adv.
Mater. 2009, 21, 2328-2333.
17
Chapter 3
Experimental details, Theoretical background and Sample preparation
3.1 Scanning Electron Microscopy (SEM)
The scanning electron microscope (SEM) is a type of electron microscope that
images the sample surface by scanning it with a high-energy beam of electrons in a
raster scan pattern.1-2 With irradiating the sample with electron beam in a vaccum
chamber, secondary electrons (SE), backscattered electrons (BSE), characteristic x-rays
and other signals are generated as indicated in Fig 3.1. 3 The SEM mainly utilizes SE or
BSE signals to form an image. SE are produced near the sample surface, and SE image
obtained upon detecting these electrons reflects the fine topographical structure of the
sample. BSE are beam electrons that are reflected from the sample by elastic scattering.
BSE are often used in analytical SEM along with the spectra made from the
characteristic X-rays, and BSM image therefore reflects the compositional distribution
on the sample surface. Because the intensity of the BSE signal is strongly related to the
atomic number (Z) of the specimen, BSE images can provide information about the
distribution of different elements in the sample. Moreover, x-ray detector can be
equipped to the SEM, so the SEM is also applicable as an x-ray analyzer for
18
determining what elements are included in the sample.
Fig. 3.1 Signals that result from electron beam-specimen interaction.
The typical SEM instrument is made up of electron column, scanning system,
detector(s), display, vacuum system and electronics controls as shown in Fig. 3.2. 4 The
electron column of the SEM consists of an electron gun and two or more
electromagnetic lenses operating in vacuum. An electron beam is thermionically emitted
from an electron gun fitted with a tungsten filament cathode. Tungsten is normally used
19
in thermionic electron guns because it has the highest melting point and lowest vapour
pressure of all metals, thereby allowing it to be heated for electron emission, and
because of its low cost. The electron gun generates free electrons and accelerates these
electrons to energiesin the range 1-40 keV in the SEM. With the the electromagnetic
lenses (condenser and objective lenses), the electron beam generated by the electron gun
is coveraged into a fine beam in a high-vacuum column. Typically the electron beam is
defined by probe diameter, probe current and probe convergence. And by applying a
scan signal to the deflection coils, the electron beam is scanned along the sample
surface in X and Y direction. Objective lens is used to converge the electron beam into a
fine beam and focus it onto the sample surface. When the primary electron beam
interacts with the sample, the electrons lose energy by repeated random scattering and
absorption within a teardrop-shaped volume of the specimen known as the interaction
volume, which extends from less than 100 nm to around 5 μm into the surface. The
energy exchange between the electron beam and the sample results in the reflection of
high-energy electrons by elastic scattering, emission of secondary electrons by inelastic
scattering and the emission of electromagnetic radiation, each of which can be detected
by specialized detectors. A secondary electron detector for detecting signals produced
from the sample could converts signal to electric one. Finally the photomultiplier are
used to amplify the signals, the amplified electrical signal output is displayed as a
20
two-dimensional intensity distribution that can be viewed and photographed on a video
display, which are displayed as variations in brightness.
Fig. 3.2 The structural scheme of a typical scanning electron microscope.
When imaging in the SEM, samples must be electrically conductive at the surface,
and electrically grounded to prevent the accumulation of electrostatic charge at the
surface. Metal objects require cleaning and mounting on a specimen stub for SEM. It
21
tend to charge when scanned by the electron beam for nonconductive specimens, and
this causes scanning faults and other image artifacts especially in secondary electron
imaging mode. Therefore, they are usually coated with an ultrathin coating of
electrically conducting material by sputter coating or evaporation. Additionally, coating
could increase signal/noise ratio for samples which is composed of low atomic number
(Z) atoms. This improvement arises because secondary electron emission for high-Z
materials is enhanced. Figure 3.3 show a photo of the SEM (JEOL, JSM-6500F) image
used in this work.
Fig. 3.3 Photo of scanning electron microscopy (JSM-6500F, JEOL)
22
The image details and resolution in the SEM are determined not by the size of the
electron probe by itself but rather by the size and characteristics of the interaction
volume, which is the area of the sample excited by the electron beam to produce a
signal. When the accelerated beam electrons strike a specimen they penetrate inside it to
depths of about 1 μm and interact both elastically and inelastically with the solid,
forming a limiting interaction volume from which various types of radiation emerge,
including BSE, SE, characteristic and brehmsstrahlung x-rays, and
cathodoluminescence in some materials. The combined effect of elastic and inelastic
scattering controls the penetration of the electron beam into the solid. The resulting
region over which the incident electrons interact with the sample is known as interaction
volume. The interaction volume has several important characteristics, which determine
the nature of imaging in the SEM. The energy deposition rate varies rapidly throughout
the interaction volume, being greatest near the beam impact point. The interaction
volume has a distinct shape as shown in Fig 3.4. For low-Z target it has distinct pear
shape. For intermediate and high-Z number materials the shape is in the form of
hemi-sphere. The interaction volume increases with increasing incident beam energy
and decreases with increasing average atomic number of the specimen. For secondary
electrons the sampling depth is from 5 to 50 nm and diameter equals the diameter of the
area emitting backscattered electrons. BSE are emitted from much larger depths
23
compared to SE. Finally the resolution in the SEM is controlled by the size of the
interaction volume.
The effective interaction volume can be calculated using the electron range (R): 5
ൌͲǤͲʹܣܧଵǤ
ߩܼǤ଼ଽ ሺߤ݉ሻ
Where A is the atomic weight (in g/mole), Z is the atomic number,Ȱis the density
(in g/cm3), and E0 is the energy of electron beam (in KeV)
Fig. 3.4 The range and spatial resolution of backscattered electrons, secondary electrons, X-rays, and Auger electrons for electrons incident on a solid.
24 3.2 Cathodoluminescence (CL)
When an electron is promoted into the conduction band, the electron and hole
become free and they can move independently in corresponding bands.6 The major
electron-hole recombination pathway between the conduction and valence bands
involve donor and/or acceptor levels, recombination via deep level traps, and
recombination at the surface. Electron beam excitation in general leads to emission by
all the luminescence mechanisms present in the material. Photoluminescence emission
may strongly depend on the excitation hv, which can be used for selective excitation of
particular emission processes. Cathodoluminescence analysis of materials, on the other
hand, can provide depth-resolved information by varying the electron beam energy as
shown in Fig 3.4. In general, electron beam energy of upon to 30 keV can be used.
Cathodolumiscence analysis enables one to assess various properties of the material
with a spatial resolution down to 1 ƶm or less. Spectroscopic CL and monochromatic
imaging can be used in identification and measurement of luminescence center
concentration and distributions, as well as in the determination of the composition of
compound materials.
Here, the CL spectrometer (Gatan, MonoCL3) as shown in Figs. 3.5 is adapted to
the SEM with the energy of electron beam from 1kVto 30 kV. PMT and detectors are
used to gathered the visible and IR emission spectra.
25
Fig. 3.5 Photo of the CL spectrometer.
3.4 Photoluminescence (PL) arrangement
Fig 3.6 illustrates the photoluminescence setup. In this research work, PL measurements were performed at room temperature by using laser lines with a wavelength of266 nm from a pulsed laser as an excitation source. The PL measurement has three main steps, in the first step, the semiconductor is optically excitated to create electron-hole pairs. Different kinds of lasers such as He-Cd laser and Ar+ laser with wave lengths of 266 nm are used for the excitations in
26
ZnO. The laser beam is then projected on the semiconductor sample with the help of a setup as shown in the schematic diagram. In the second step, the excited electron-hole pairs recombine radiatively and emit light. In the final step, the emitted light is detected and dispersed by a double grating monochromator and photomultiplier detectors. The final spectrum is collected and analyzed in a computer.
The intensity of the PL signal apparently depends on the material’s quality, the system throughout, and detector sensitivity. The resolution of the system, its ability to accurately measure energy, is determined by the focal length of the monochromator, where the grating spacing sets the wavelength coverage. At long wavelength a PMT with a GaAs or other composite cathode is useful. Gemanium photodiodes are good for the near-infrared range.
Figure 3.6: The photoluminescence experiment setup.
27 3.5 Time-resolved photoluminescence
Time resolved photoluminescence (TRPL) is used to investigate the carrier
dynamics which is a sensitive and powerful technique to record the transient PL decay
curve. Unlike the steady-state PL measurement, time-resolved data frequently contain
more information than is available from the steady-state data. In a TRPL measurement
the sample is excited with a very short and intense light pulse, and the luminescence is
measured with the necessary temporal resolution. Luminescence intensities of the
transitions depend on the corresponding recombination rates. Because the
recombination rate depends on the carrier populations in the participating states, the
carrier populations in diơerent states can be calculated as a function of time. Therefore,
also the transition processes between the participating states can be studied.
Immediately after the excitation pulse even more rapid processes have a major role, but
due to the limitations of the detection system and the rate-equation model these
processes were not studied in this thesis.
Figure 3.7 shows schematically the TRPL measurement system used in this work.
The basic optics, beam focusing, sample cooling, luminescence collimating and
monochromator were the same as in the continuous wave PL (CWPL) measurement
system. The excitation pulse source was a mode locked titanium sapphire laser
operating at the wavelength of 266 nm. The photon counting system uses a
28
multichannel plate photomultiplier as a detector. Excitation laser pulses are detected by
a silicon photodiode. The system measures the time between the detected PL photon
and the next laser pulse. The system assumes, that only one photon is detected between
two pulses. The temporal resolution of the photon counting system is about 30 ps. It is
mainly determined by the photo multiplier tube.
Figure 3.7: The time-resolved PL setup
3.6 Absorption Spectroscopy
Absorption spectroscopy is a optical technique that measures the absorbance of the
incident light, as a function of wavelength, due to the interaction between the
electromagnetic field and the sample. Absorption spectroscopy is employed as an
analytical tool to determine the band gap of semiconductor, energy level of the excited
29
states of nanoparticles, resonant frequency of surface plasmon of noble metals and etc.
In general, the infrared and ultraviolet-visible spectroscopy is used in academic
researches. There are many approaches to measuring absorption spectra. The most
common arrangement is to focus a beam of light on a sample and detect the intensity of
the incident light that passes through it. As shown in Fig. 3.8, a Xe lamp and Spectra
Pro 300i monochromator were used as a tunable wavelength of incident light source.
The white light emitted from the Xe lamp can be separated different single wavelength
of incident light source by the monochromator. In order to detect a specific transmission
signal, the signal was recorded by a Si/Ge detector.
Figure 3.8: The absorption spectroscopy setup.
30
3.7 Vapor-Soild (VS) Growth Mechanism of ZnO Nanorods
The VS process grows to form wirelike or beltlike nanostructure with the oxide
vapor higher temperature region. It cannot be explained that how atoms and building
blocks can be rationally assembled into one-dimension wirelike or beltlike
nanostructure. The studied ZnO NRs in this work were fabricated by the VS growth
mechanism.7 A sapphire substrate was placed on the top of alumina boat loaded with a
high purity Zn powder (99.99%), and the whole alumina boat was located at the center
of a tube furnace. Subsequently, the reaction chamber was evacuated and kept at a
pressure of 10 Torr when argon and oxygen with a high purity of 99.9% were
introduced into the reaction chamber at a flow rate of 200 sccm and 5 sccm, respectively.
In addition, the growth temperature was maintained at 620°C and the dwell time was
one hour. After the fabrication, VS-ZnO NRs were formed uniformly over the entire
substrate.
31
References
1. Online resource, http://en.wikipedia.org/wiki/Scanning_electron_microscope.
2. D. McMullan, Scanning, 2006, 17, 175.
3. Online source, http://www4.nau.edu/microanalysis/Microprobe-SEM/Signals.html
4. S. Fatikow, “Nanostructuring and Nanobonding by EBID”. Automated
nanohandling by Microrobts, 2007, Chapter 1.
5. K. Kanaya, S. Okayama, J. Appl. Phys. 1972, 5, 43
6. B. G. Yacobi and D. B. Holt, Cathodoluminescence microscopy of inorganic solids,
Plenum Press, New York and London (1990)
7. A. Umar, S. H. Kim, Y. S. Lee, K. S. Nahm, and Y. B. Hahn, J. Cryst. Growth , 2005,
282, 131-136.
32
Chapter 4
Enhancement of Random Laser Action Assisted by Whispering-Gallery-Mode Resonance
4.1 Introduction
In recent years, random lasing is a phenomenon that has been extensively
investigated in some disordered media for its unique properties and potential
applications, since the original idea proposed by Letokhov.1-5 Comparing with the
conventional laser action due to Fabry-Perot resonance, random lasing necessitates no
mirror cavities to achieve coherent feedback in the laser system with its low-cost and
simple process technology. Moreover, random lasing usually exhibits a very broad
angular distribution which is ideal for display application. In random lasers, cavities are
self-formed, and coherent feedback is provided by scattering events in the random
medium. As the close-loop is formed in the cavities and the gain exceeds the loss,
random laser action can be achieved. A high gain medium and efficient light scattering
centers are therefore required for the accomplishment of random lasing.
Recently many nanoscale optoelectronic devices have been extensively developed
because of their low dimensionality and quantum confinement effect, such as
33
transistors,6,7 photodetectors,8,9 and emitters.10,11 Random lasing has also been widely
studied in nanoscale materials such as nanorod arrays and nanocrystalline films.12,13
ZnO, with a wide bandgap of 3.37 eV , a high exciton binding energy of 60 meV, and
plenty kinds of nanostructures, is very suitable for the fabrication of ultraviolet
light-emitting diodes and laser devices with high efficiency.14-16 In addition, due to a
high refractive index in ultraviolet region (~ 2.5), the total internal reflection in ZnO
structures can be easily achieved. Based on these favorable properties, conventional
laser actions from ZnO nanostructures have been successfully demonstrated by
Fabry-Perot cavities, whispering-gallery-mode (WGM) cavities and different types of
resonators.17-19 However, according to previous studies,20 the emission of ZnO nanorods
(NRs) fabricated by vapor-solid (VS) growth mechanism exhibits a very broad spectrum
and the quantum efficiency of random laser action is rather poor.5,13
Here, we provide an alternative approach to enhance the random lasing behavior
arising from VS-ZnO NRs decorated by SiO2 nanospheres with the assistance of WGM
resonance. WGM resonance has been used to enhance the sensitivity of gas sensors,
such as the detection of CO2 and H2O, and WGM lasing has also been demonstrated in
many materials and circular structures.21-24 In this study, it is found that after the
nanosphere decoration, the differential quantum efficiency can be greatly enhanced and
the emission spectra show only very sharp peaks with a full width at half maximum
34
(FWHM) less than 0.3 nm and a very narrow background signal. Time-resolved
photoluminescence (TRPL) experiments have also been performed to verify the induced
laser action. Through varying SiO2 nanosphere size, cathodoluminescence (CL)
mapping and theoretical calculation, we firmly confirm that WGM is indeed the
underlying mechanism responsible for the enhanced random laser action in SiO2
nanospheres decorated VS-ZnO NRs. Our result shown here should be very useful for
the future development of highly efficient light emitting devices.
35
4.2 Experiment
The studied ZnO NRs in this work were fabricated by the VS growth mechanism.25
A sapphire substrate was placed on the top of alumina boat loaded with a high purity Zn
powder (99.99%), and the whole alumina boat was located at the center of a tube
furnace. Subsequently, the reaction chamber was evacuated and kept at a pressure of 10
Torr when argon and oxygen with a high purity of 99.9% were introduced into the
reaction chamber at a flow rate of 200 sccm and 5 sccm, respectively. In addition, the
growth temperature was maintained at 620°C and the dwell time was one hour. After the
fabrication, VS-ZnO NRs were formed uniformly over the entire substrate.
To investigate random laser action, the samples were optically excited by a
Q-switched Nd: yttrium aluminum garnet laser (266 nm, 3–5 ns pulse, 10 Hz) focused
to a beam size about 200 ȝm in diameter. TRPL experiment was performed at room
temperature excited by a 260 nm pulse laser. The cathodoluminescence (CL) mapping
images were carried out on the same SEM instrument equipped with Gatan-Mono-CL3
operating at 10 kV. All measurements were performed at room temperature.
36
4.3 Results and Discussion
For the studied devices, a droplet of 2 ȝL SiO2 nanospheres (10 ȝM in ethanol) was
deposited on VS-ZnO NRs. Three different sizes of SiO2 nanospheres with diameter of
120 nm, 190 nm, and 250 nm were used. The morphology of the composite consisting
of VS-ZnO NRs and SiO2 nanospheres was characterized by scanning electron
microscopy (SEM) (JSM 6500, JEOL). It is clearly seen in Figure 4.1 that VS-ZnO
NRs have lengths about 5 ~ 8 ȝm and diameters ranging between 100 nm and 500 nm.
As shown in Figure 4.1a, the SiO2 nanospheres with a diameter about 120 nm are
randomly deposited on VS-ZnO NRs.
The emission spectra of pristine VS-ZnO NRs under different pumping energy
illuminated with 266 nm pulsed laser are shown in Figure 4.2a. We only observe a very
broad spontaneous emission spectrum at around 389 nm with a FWHM of about 12 nm
which is similar to previous report.20 As the pumping energy increases, the emission
intensity increases gradually without any indication of sharp peaks which can be used to
identify as the occurrence of laser action.
The emission spectrum of SiO2 nanospheres decorated VS-ZnO NRs is shown in
Figure 4.2b. Quite interestingly, after decorating nanospheres, the FWHM of whole
emission spectrum is reduced to 1.9 nm and the light intensity is greatly enhanced. As
the pumping energy increases, several sharp laser-like emission peaks superposed on the
37
broad spontaneous emission. We also found that the position and intensity of the sharp
peaks randomly change at different moment, which is a signature of the inherent nature
of random lasing behavior.5,12 The emission characteristic of the randomly assembled
VS-ZnO NRs as shown in Figure 2b can therefore be attributed to random laser action,
which is achieved when specific frequencies of light are multiply amplified by
stimulated emission in randomly closed loop paths.
To further examine the laser action, Figure 4.3a shows the analysis of the
dependence of the emission intensity on pumping energy. We can clearly see that there
is an abrupt change of the slope, which provides a signature for the occurence of
stimulated emission. The value of the lasing threshold is approximately 52 ȝJ, impling
that the laser action can be easily achieved compared with previous reports.20 The
differential quantum efficiency (Șd) defined as photons emitted per radiative
electron-hole pair recombination above threshold, can be determined by Șd = PO/PI,26
where PO and PI are the output and input pumping power, respectively. It is found that
the differential quantum efficiency (Șd) of ZnO NRs decorated with 120 nm SiO2
nanospheres is about 7.3 times larger than the efficiency without 120 nm SiO2
nanospheres as shown in Figure 3a. Moreover, TRPL experiments of the spontaneous
emission (by pristine ZnO nanorods) and stimulated emission (by SiO2/ZnO composite)
monitored at 389 nm are shown in Figure 4.3b. While the spontaneous emission exhibits
38
a biexponential decay with time constants of 798 ps and 2.56 ns, the stimulated
emission shows shorter decay times with 516 ps and 2.09 ns which means the excitons
recombine with higher recombination rate.. This result provides an additional evidence
to support the laser action in SiO2 nanospheres decorated ZnO NRs according to
previous reports.2,27
Let us now try to understand the hidden mechanisms for the induced laser action
and high differential quantum efficiency as observed above. There are two main
possible mechanisms for the improved lasing characteristics arising from the decoration
of SiO2 nanospheres. First, there is a large contrast in refractive index between SiO2
nanospheres and air, and the surface of ZnO NRs becomes rougher after the SiO2
nanosphere deposition. The emitted light beam can thus be strongly scattered by SiO2
nanospheres, which makes light travel more randomly. Therefore, random laser action is
more easily achieved, and the threshold pumping energy is reduced. Second, due to the
total internal reflections of light at the circular boundary, the spherical-shaped dielectric
cavity could support WGM resonance. Once ZnO NRs is pumped by laser, the emissive
light will prefer to incident in SiO2 nanospheres than air because the refractive index of
SiO2 is nearer to it of ZnO NRs. After resonating in the SiO2 nanospheres, the intensity
of light with the specific frequency will be greatly enhanced with the reduced width of
emission spectrum, and it will induce more stimulated emission when the enhanced
39
light propagates with a closed-loop path among ZnO NRs with higher recombination
rate. Consequently, the emission spectrum is much narrower and the random laser action
is achieved. Figure 4.4 illustrates the underpinned mechanisms responsible for the
enhanced laser action assisted by the decoration of SiO2 nanospheres.
To explore the possibility that WGM resonance is indeed responsible for the
enhanced laser action, firstly we examine the Q factor which is an important parameter
to describe laser cavity. From the experimental data, the Q factor is estimated to be 760
by the definition Q = Ȝ/ǻȜ, where Ȝ is the peak wavelength and ǻȜ is the line-width of
the peak. Considering the WGM in a spherical cavity, the Q factor can be determined by
the following equation: 28
(1)
where D is the diameter of SiO2 nanospheres, m is an integer, R is the reflectivity
of the boundary, n is the refractive index. If the experimentally obtained Q factor and
m=6 (for WGM) is inserted into the equation, it can be deduced that the reflectivity is
about 99.6% for a WGM cavity, which is reasonable for the total internal reflection on
the boundary of SiO2 nanospheres.
Secondly, we theoretically calculate the resonant WGMs due to nanoscale SiO2
/4 /2
3 2
2 (1 mm)
sin( ),
DmnR R m
Q
π π=
λ −40
spherical cavity. 29 The main idea is that a light wave interferes with itself when having
completed one full circulation within the resonator of SiO2 nanospheres. In order to
generate the constructive interference, the total phase shift of the wave along its path
has to be an integer multiple of 2ʌ. Taking into account of the polarization-dependent
negative phase shift that occurs during the process of total internal reflection, we obtain
the following equation by considering the condition of spherical cavity: 23
(2)
The factors m1 and m2 are the reflective indices of air and SiO2 nanospheres
respectively. ȕ depends on the polarization of light wave, i.e., for the TM polarization
and for the TE polarization . Ri is the geometric parameter, which is the diameter of SiO2 nanospheres. și is the angle of the incidence of the circulating light.
According to the theoretical calculation, the WGM for the TE polarization does not
exist, which is consistent with the previous investigation of laser action in cylindrical
cavity.23 For 120 nm SiO2 nanospheres, the TM-resonance peak position obtained by the
theoretical calculation is approximately at 389 nm for N=1. It is in good agreement with
the emission peak of SiO2 nanospheres decorated ZnO NRs as observed in Figure 2.
In order to further confirm the above proposed mechanisms for the induced laser
( )
1 2 2 2 2
i 2 i 1 i
2 i
Ȝ 2
R N tan ȕ m tan ș m csc ș .
m ʌ ș
ª − º
= « + − »
¬ ¼
1
TM 2
ȕ =m− ȕTE=m2
41
action, different sizes of SiO2 nanospheres decorated ZnO NRs have been investigated.
Figures 4.5a and (b) show the emission spectra of the pristine ZnO NRs and ZnO NRs
decorated by 190 nm and 250 nm SiO2 nanospheres under the same pumping energy.
According to Figure 4.5a, we found that the laser action can still be induced with the
position of lasing peaks centered approximately at 384 nm after the decoration of 190
nm SiO2 nanospheres. However, according to Figure 4.5b, after the decoration of 250
nm SiO2 nanospheres, even though there exist several random lasing-like peaks
compared to the pristine VS-ZnO NRs, they are not as pronounced as those of ZnO NRs
decorated with 120 nm and 190 nm SiO2 nanospheres. To investigate the effects of the
size of decrated SiO2 nanospheres on the laser action of ZnO NRs, we have examined
the resonant WGMs based on Equation 2. As shown in Figure 4.6, it is found that for
the decoration of 190 nm SiO2 nanospheres, the theoretical TM-resonance peak position
for N=1 is far above 600 nm, which is not in the range of our interest. But, for N=2 the
wavelength of WGM resonance is approximately at 384 nm, which is consistent with
the position of the induced lasing peaks shown in Figure 4.5a. Because of a slight
difference between the peak position of WGM mode and ZnO emission spectrum, there
is a less pronounced enhancement of the lasing action compared to that of the 120 nm
SiO2 nanospheres decorated ZnO NRs. For the decoration of 250 nm SiO2 nanospheres,
the nearest theoretical TM-resonance peak position to the ZnO emission spectrum at
42
389 nm is 378 nm with N=3, which is too far away from the main emission peak to have
a significant influence. Therefore, the blurred lasing-like peaks shown in Figure 4.5b
can be attributed to the scattering effect induced by the decoration of nanospheres. We
thus can see that the random laser action is mainly determined by the condition of
WGM resonance. Moreover, the normalized transmittance spectra have been performed
for different-size SiO2 nanospheres with the same number density deposited on glass
substrates as shown in Figure 4.7. It is found that the scattering only has a slight
difference among three different-size nanospheres around 389 nm, implying the
scattering effect is very similar. It supports the fact that WGM resonance plays a more
important role than scattering effect in the induced laser action due to the decoration of
SiO2 nanospheres.
Finally, Figure 4.8 shows the corresponding CL mapping image for ZnO NRs
decorated with 120 nm SiO2 nanospheres, in which the emission at 389 nm is selected
as the mapping wavelength. Compared with the SEM image shown in Figure 4.8a and
the CL mapping image shown in Figure 4.8b, it is clear that the bright emission comes
from the location where the SiO2 nanospheres were decorated on ZnO NRs. It therefore
provides a firm evidence that SiO2 nanospheres do play a very important role for the
enhancement of light emission and the intensity of 389 nm radiation is indeed amplified
by the cavity of SiO2 nanospheres with WGM resonance.
43
4.3 Summary
In conclusion, a novel approach to enhance random laser action based on WGM
resonance has been demonstrated. It is found that the lasing characteristics can be
improved significantly, including narrower emission spectra, sharper lasing peaks,
smaller lasing threshold, and higher differential quantum efficiency. Through the
variation of the size of decorated nanospheres, CL mapping images, and the theoretical
calculation, we have firmly confirmed that the induced laser action in nanospheres
decorated ZnO NRs arises from the assistance of WGM resonance. Our approach could
be extended to many other composites consisting of nanoparticles and light emitting
materials. It therefore can open up a new route for the creation of highly efficient
optoelectronic devices.
44
Figure 4.1. (a) Scanning electron microscope image of ZnO nanorods decorated with SiO2 nanospheres. (b) Enlarged SEM image of ZnO nanorods decorated with SiO2
nanospheres. (c) Statistical bar chart of the size distribution of SiO2 nanospheres.
45
375 380 385 390 395 400
0 500 1000 1500 2000
Emission intensity (a.u.)
Wavelength (nm)
71 μJ 77 μJ 85 μJ 93 μJ
(a)
Figure 4.2. (a) and (b) Emission spectra of ZnO nanorods without and with the decoration of 120 nm SiO2 nanospheres.
375 380 385 390 395 400
0 2000 4000 6000 8000 10000
Emission intensity (a.u.)
Wavelength (nm)
52 μJ 56 μJ 64 μJ 73 μJ
(b)
46
0 2 4 6
0.1 1
Normalized Counts
Time (ns)
Pristine ZnO NRs ZNO+SiO2
(b)
Figure 4.3. (a) Plot of the emission intensity versus the pumping energy. Black boxes
denote ZnO nanorods and red circles denote pristine ZnO nanorods with the decoration
of 120 nm SiO2 nanospheres. (b) Time-resolved photoluminescence decay spectra with
fitting curves for pristine ZnO nanorods and 120 nm SiO2 nanospheres/ZnO nanorods
monitored at the peak emission wavelength of 389 nm.
40 50 60 70 80 90
0 2000 4000 6000 8000 10000 12000 14000 16000
ZnO
ZnO/120 nm SiO2
Emission Intensity(a.u.)
Pump Intensity(μJ) 52 μJ
(a)
47
Figure 4.4. Schematic illustration of the mechanisms responsible for the enhanced laser
action in SiO2 nanospheres decorated ZnO nanorods. (a) SiO2 nanospheres serve as
scattering centers to assist light traveling in a randomly closed loop. (b) SiO2
nanosphere serves as an excellent spherical cavity for the occurrence of whispering
gallery made resonance.
48
Figure 4.5. (a) and (b) Emission spectra of ZnO nanorods without and with the
decoration of 190 nm and 250 nm SiO2 nanospheres under the same excitation power.
The insets show plots of the emission intensity versus the pumping energy. Black boxes
denote pristine ZnO nanorods and red circles denote ZnO nanorods decorated with the
decoration of SiO2 nanospheres.
375 380 385 390 395 400
-500 0 500 1000 1500 2000 2500 3000
Emission Intensity (a.u.)
Wavelength (nm)
ZnO/190nm Si02 ZnO
(a)
Pumping energy : 85μJ
375 380 385 390 395 400
0 500 1000 1500
Emission intensity(a.u.)
Wavelength (nm)
ZnO/250 nm SiO2
(b)
ZnO50 60 70 80 90 100
0 1000 2000 3000 4000
Pristine ZnO NWs ZnO/190 nm SiO2
Emission Intensity(a.u.)
Pump Intensity(μJ)
50 60 70 80 90 100
0 500 1000 1500 2000 2500
Pristine ZnO NWs ZnO/250 nm SiO2
Emission Intensity(a.u.)
Pump Intensity(μJ)
49
Figure 4.6. Plot of the diameter of spherical cavity versus TM-resonance peaks
according to the therotical calculation given by Equation 1. For 120 nm SiO2
nanospheres, the TM-resonance peak position by the theoretical calculation is
approximately at 389 nm for N=1. For 190 nm SiO2 nanospheres, the theoretical
TM-resonance peak position is approximately at 384 nm for N=2 (N=1 is far above 600
nm). And, for 250 nm SiO2 nanospheres, the theoretical TM-resonance peak position is
at 378 nm for N=3.
370 375 380 385 390 395
100 150 200 250
D:250nm λ :378nm
D:120nm λ :389nm D:190nm
λ :384nm
D iam et e r ( n m )
Wavelength (nm)
N=1 N=2 N=3
50
350 400 450 500 550 600
0.04 0.06 0.08 0.10 0.12 0.14 0.16 0.18 0.20
389 nm
Normalized Scattering (a.u.)
Wavelength (nm)
120 nm 190 nm 250 nm
Figure 4.7. Scattering spectra of SiO2 nanospheres with three different sizes. It is clear that the transmittances for three different-size SiO2 nanospheres around 389 nm
only exhibit a slight difference.
51
Figure 4.8. (a) Scanning electron microscope image of SiO2 nanospheres decorated ZnO nanorods. (b) The corresponding cathodoluminescence mapping image.
