氧化鋅量子點中不受激發功率影響之激子態研究

國

立

交

通

大

學

光電工程研究所

碩 士 論 文

氧化鋅量子點中不受激發功率影響之激子態研究

Invariable exciton states upon increasing

pumping in ZnO quantum dots

研 究 生：廖婉君

指導教授：謝文峰 教授

張振雄 教授

氧化鋅量子點中不受激發功率影響之激子態研究

Invariable exciton states upon increasing

pumping in ZnO quantum dots

研 究 生：廖婉君 Student: Wan-Jiun Liao

指導老師：謝文峰 教授 Advisors: Dr. Wen-Feng Hsieh

張振雄 教授 Dr. Chen-Shiung Chang

國立交通大學 光電工程研究所 碩士論文

A Thesis

Submitted to Institute of Electro-Optical Engineering College of Electrical Engineering and Computer Science

National Chiao Tung University In partial Fulfillment of the Requirements

for the Degree of Master

in

Electro-Optical Engineering June 2010

Hsinchu, Taiwan, Republic of China

Invariable exciton states upon increasing pumping

in ZnO quantum dots

Student: Wan-Jiun Liao Advisor: Dr. Wen-Feng Hsieh

Dr. Chen-Shiung Chang

Department of Photonics & Institute of Electro-Optical Engineering

National Chiao Tung University

Abstract

ZnO QDs are synthesized by a simple sol-gel method and the average size of QDs can be tailored under solution concentration. Size-dependent blue shifts of photoluminescence give the evidence of the quantum confinement effect. Furthermore, the unchanged spectral profiles of near-bandedge emission (NBE) for a fixed size of ZnO quantum dots (QDs) reveal no signature of biexciton formation and exciton-exciton scattering under excitation density covering three orders of magnitude. These results quite differ from that of the micrometer-sized powder. The intensities of NBE peaks attributed to free exciton and surface bound exciton states exhibit linear dependence on the excitation power density that confirms the invariable exciton states with no effect of two-exciton interaction in ZnO QDs upon increasing the pump intensity.

氧化鋅量子點中不受激發功率影響之激子態研究

研究生：廖婉君 指導老師：謝文峰 教授

張振雄 教授

國立交通大學光電工程研究所

摘要

利用溶膠-凝膠法藉由改變溶液濃度來合成不同晶粒大小的氧化鋅量子點， 並量測其光學特性。從低溫螢光光譜隨氧化鋅量子點尺寸變小而藍移的現象可觀 察到在氧化鋅量子點中存在量子侷限效應。然而，量測變功率螢光光譜發現，當 激發光強度改變了1000倍，除了光譜強度會隨激發強度增加而變大外，NBE的光 譜形狀幾乎都不變，且觀察不到雙激子的形成與激子與激子的散射現象，此情形 與氧化鋅塊材的特性相當不一樣。另外，自由激子與表面束縛激子的發光能量幾 乎不隨激發光強度改變及線性的發光強度與激發能量關係，表示當激發功率增 加，仍沒有激子間的交互作用，因此在氧化鋅量子點中，存在穩定的激子態。

誌謝

時間過得真快，一轉眼我的碩士生生活就要結束了。在這兩年的日子裡， 我學習與成長了很多，也讓我的想法與處事態度變得更成熟了。 研究所這段期間，我要謝謝我的指導教授謝文峰老師，謝謝老師在課業、 研究與待人處事上對我的指導與教誨，以及對我在生活上的關心，從老師的身上 我學習到很多。也要謝謝另外一位指導教授張振雄老師，提供我專業意見，以及 生活上的幫助。接下來要謝謝小豪帶我入門做實驗，雖然我常做出讓你啞口無言 的事，不過真的很謝謝你教了我很多事。還要謝謝小郭，當我研究上有問題時， 常常向你請教，謝謝你提供我很多想法與協助。也要謝謝楊松幫我打XRD與教我 SEM。以及謝謝鄭信民學長對我研究的關心與協助，還有歐博濟，雖然在碩二之 後才跟你比較有交集，常向你請教TRPL的問題，你也常會關心我的近況，論文或 找工作的進展如何，謝謝你。另外，智章學長、維仁學長、碧軒學長以及黃董偶 爾我有問題要請教你們，你們也都很熱心的幫我解答，謝謝你們。還有要謝謝黃 至賢，很多生活上的瑣碎事情常會去煩你，謝謝你啦，實驗室有了你讓我很不無 聊了XD 另外，謝謝蔡智雅學姐，課業上或是找工作等常找你幫忙或詢問你意見， 謝謝你；還要謝謝陳厚仁總是很大方的借我筆記與考古題，祝你跟學姐永遠甜蜜 幸福喔XD 還有黃冠智，常常會以你的經驗告訴我很多事情以及提供我建議，以 及林建輝，你真是一個奇妙的人，你的很多事情與想法常讓我們驚奇，也讓實驗 室更有趣了，還有黃棕儂，雖然你比較少來實驗室，但也很謝謝你在我還沒進實 驗室前對我的照顧，另外還有黎延垠你竟然拋棄我們提前畢業去了，剛進實驗室 時什麼都不懂，多虧你在，告訴與提醒我要做什麼，以及問你光電系的八卦，ㄎ

ㄎ，謝謝你們，雖然跟你們只相處了一年，但碩一的那一年裡，因為有你們讓我 覺得實驗室很溫馨也讓我能更快熟悉實驗室，當然也要謝謝和我當了兩年同學的 李柏毅，謝謝你對我生活上的關心。還有許家瑋，謝謝你很熱心想教我程式語言 但我還是沒學會= = 另外，還有實驗室的學弟們，雖然跟你們還沒有很熟，不過 也祝你們在實驗室的生活都能很愉快。 最後當然要謝謝一位特別的人，謝謝你的陪伴與鼓勵，讓我在心情不好沮 喪的時候得以抒發與得到安慰，未來還有很多挑戰等著我們，我們要一起加油!! 小布丁 于 九十九年六月

Contents

Abstract (in English)………..….I Abstract (in Chinese)……….II Acknowledgement………....…III Contents………...………...……...V List of Figures………..………...…VII List of Tables………...VIII

Chapter 1 Introduction………...….1

1.1 Basic properties of ZnO and significance of ZnO related photonic devices…...…1

1.2 General review of ZnO nanostructures………...……4

1.3 Motivations……….……….6

1.4 Organization of the thesis………8

Chapter 2 Theoretical background………..…………..9

2.1 Sol-gel method………...……..………...9

2.2 Quantum effect……….…………..…………...12

2.2.1 Quantum confinement effect……….………....13

2.2.2 Density of states………...………..…..……….16 2.3 X-ray diffraction….………...…19 2.3.1 Lattice parameters……….………..…..19 2.3.2 Debye-Scherer formula……….………....21 2.4 Photoluminescence characterization……….24 2.4.1 Fundamental transitions…….………...………..25

2.4.2 Influence of high excited light intensity…….……….………..31

Chapter 3 Experiment detail and analysis techniques………...…….34

3.1 Sample preparation………34

3.2 Microstructure and optical properties analysis………...…………..….37

3.2.1 X-ray diffraction………...……….37

3.2.2 Photoluminescence system………...……….38

Chapter 4 Results and Discussion……….40

4.1 Morphology and crystal structures………...………...……..40

4.1.1 Morphology………..…….40

4.1.2 X-ray diffraction measurement……..………..….41

4.2 Photoluminescence spectra………...……….42

4.2.1 Photoluminescence spectra for different sizes of ZnO particles……..…….42

4.2.2 Temperature-dependent PL for different sizes of ZnO particles………..….44

4.2.3 Power-dependent PL for different sizes of ZnO particles……….45

4.2.4 PL peak positions as a function of excitation power……….48

4.2.5 Integrated intensity as a function of excitation power………..……49

Chapter 5 Conclusions and Prospective ………..51

5.1 Conclusions………..…………...…………..51

5.2 Prospective………..……….….52

List of Figures

Fig. 1-1 The wurtzite structure model of ZnO………...3

Fig. 1.2 Representative scanning electron microscopy images of various ZnO nanostructure morphologies………..…...6

Fig. 2-1 Schematic of the rotes that one could follow within the scope of sol-gel processing………...…12

Fig. 2-2 Geometry used to calculate density of states in three, two and one dimensions………..17

Fig. 2-3 Variation in the energy dependence of the density of states…….……..……18

Fig. 2-4 X-ray scattering from a 2-dimendion periodic crystal………20

Fig. 2-5 The hexagonal unit cell………...21

Fig. 2-6 Grating treatment of line broadening by crystallite ………...21

Fig. 2-7 A pair excitation in the scheme of valence and conduction bands………...27

Fig. 2-8 Visualization of bound excitons………...28

Fig. 2-9 Radiative transition between a band and an impurity state………31

Fig. 2-10 The general scenario for many-particle effects in semiconductors……...32

Fig. 2-11 Schematic representation of the inelastic exciton-exciton scattering processes……….33

Fig. 3-1 Experiment equipment used for fabricating ZnO QDs..……….36

Fig. 3-2 A flow chart of fabricates ZnO QDs by sol-gel method……….36

Fig. 3-3 The x-ray spectrometer………...37

Fig. 3-4 PL detection systems………..39

Fig. 4-1 HRTEM image of the ZnO QDs fabricated using 0.04M Zn(OAc)2………..40

Fig. 4-2 XRD profiles of the ZnO QDs prepared with various concentration of Zn(OAc)2.………...41

Fig. 4-3 The PL spectra of the ZnO QDs measured at low temperature…….……….43 Fig. 4-4 PL spectra of different sizes of ZnO QD at low temperature…...….……….44 Fig. 4-5 Temperature-dependent PL spectra…...….……….45 Fig. 4-6 Power-dependent PL spectra…...….………..….47 Fig. 4-7 Theoretical fit to a PL spectrum of 6 nm ZnO QDs measured at 80K…...48 Fig. 4-8 The PL peak positions as a function of excitation power measured at 80K...49 Fig. 4-9 The integrated intensity of two decomposed peaks as a function of excitation

power measured at 80K…………..………50

List of Tables

Table 1-1 Comparison of properties of ZnO with those of other wide band gap

semiconductors………...1 Table 1-2 Properties of wurtzite ZnO ………2 Table 3.1 Shows that chemical reagent was used with sol-gel experiment process …35

Chapter 1 Introduction

1.1 Basic property of ZnO and significance of ZnO related photonic

devices

Zinc oxide (ZnO) is a promising material for a variety of practical applications [1]

such as piezoelectric transducers, optical waveguides, surface acoustic wave devices,

varistors, phosphors, solar cells, chemical and gas sensors, transparent electrodes, spin

functional devices, UV-light emitters [2] and ultraviolet laser diodes. The basic

properties of ZnO can be obtained by examining Table 1-1, which compares the

material properties of other relevant semiconductors [3].

 

Table 1-1. Comparison of properties of ZnO with those of other wide band gap semiconductors [3].

ZnO has attracted significant scientific and technological attention due to its

wide direct band gap (3.37 eV) that is suitable for photonic applications in the

ultraviolet (UV) or blue spectrum range [4]. In this regard, a large exciton binding

ZnS (39 meV) and other wide-gap semiconductors. The large exciton binding

energy allows stable existence of excitons and efficient excitonic emission at room

temperature (thermal energy 26 meV). The basic material parameters of ZnO are

also shown in Table 1-2. To realize any type of device technology, these parameters

are important to have control over the concentration of intentionally introduced

impurities (dopants), which are responsible for the electrical properties of ZnO. The

dopants determine whether the current (and, ultimately, the information processed by

the device) is carried by electrons or holes.

The structure of ZnO crystal is shown in Fig 1-1 which has a hexagonal wurtzite

structure (space group C6mc) with lattice parameters a = 0.3249 nm and c = 0.5207

nm. The structure of ZnO can be simply described as a number of alternating planes

composed of tetrahedrally coordinated O2- _{and Zn}2+ _{ions, stacked alternately along the}

c-axis, in which a1, a2, and c are the unit vectors in a unit cell, the large and small

circles denote the anion and cation atoms, respectively. The tetrahedral coordination

in ZnO results in non-central symmetric structure and consequently the development

of piezoelectricity and pyroelectricity.

1.2 General review of ZnO nanostructures

ZnO nanocrystals have recently attracted broad attention in fundamental studies

and technical applications [5] because of their distinguished performance in

electronics, optics and photonics. Therefore, in the last few decades, a variety of

ZnO nanostructure morphologies, such as nanowires [6], nanorods [7, 8], tetrapods

[9], nanoribbons/belts [9], and nanoparticles [10, 11] have been reported. Recently,

novel morphologies such as hierarchical nanostructures [12], bridge-/nail-like

nanostructures [13], tubular nanostructures [14], nanosheets [15], nanopropeller

arrays [16], nanohelixes [17], and nanorings [17] have, amongst others, been

demonstrated. These diverse ZnO nanostructures have been fabricated by various

methods, such as thermal evaporation [9], metal–organic vapor phase epitaxy

(MOVPE) [8], laser ablation, hydrothermal synthesis [7], sol-gel method [10, 11] and

template-based synthesis [6]. Several recent review articles have summarized progress in the growth and applications of ZnO nanostructures [4, 18]. Some of the

possible ZnO nanostructure morphologies are shown in Fig. 1.2.

Additionally, when the dimension of semiconductors are reduced from three

(bulk material) to the quasi-zero dimensional semiconductor structures such as

quantum dots (QDs), the optical properties of QDs are much different from the bulk

structure of nanostructures, i.e., the quantum confinement effect (QCE) [19] and

surface states [20]. These two mechanisms compete with each other to influence PL

spectra. For nanodots or nanostructures in ZnO system with diameters less than 10

nm, the QCE plays a dominant role as has been much reported [21]. On the other

hand, the surface-to-volume ratio also brings much influence on the system’s

Hamiltonian when the material size is reduced to the nanometer scale [22]. The

predominance of surface states is responsible for many novel physical features of

nanomaterials. In the past decade, various groups have devoted to produce ZnO

QDs and study the properties. For instance, Guo et al. [23] exhibited significantly

enhanced UV luminescence, diminished visible luminescence and excellent

third-order nonlinear optical response with poly vinyl pyrrolidone (PVP) modified

surface of ZnO nanoparticles. Pan et al. [24] predicted a significant increase in the

intensity ratio of the deep level to the near band edge emission is observed with

ever-increasing nanorod surface-aspect ratio.    Fonoberov et al. [25] have

theoretically investigated that, depending on the fabrication technique and ZnO QD

surface quality, the origin of UV photoluminescence (PL) in ZnO QDs is either

recombination of confined exciton or surface-bound ionized acceptor-exciton

complexs. Although there were many experiments to describe the behavior of ZnO

Fig. 1.2 Representative scanning electron microscopy images of various ZnO nanostructure morphologies [4].

1.3 Motivations

Recently, the power-dependent photoluminescence (PL) of ZnO bulk associated

with biexciton recombination has been investigated by several research groups

[26-29]. Zhang et al. [26], who grew ZnO rods by metalorganic chemical vapor

deposition, indicated that the biexciton intensity is proportional to the 1.7th power of

the excitation density. Besides, we have observed the intensity of biexciton emission

in ZnO powder is proportional to the 1.86th power of the excitation power at T = 80

K, but it is close to unity exponent or even sub-linear when it is measured at the lower

temperature [30]. Acoustic and optical phonon scatterings playing key roles in

biexciton at various temperatures. At low temperature the acoustic phonon

scattering is the dominant mechanism for exciton thermalization while the optical

phonon scattering will participate in when the exciton kinetic energy approaches to

the energy of the lowest optical phonon about T = 80 K. The efficient cooling of

exciton with the assistance of optical phonon scattering allows effectively bounding

exciton pairs to form biexcitons.

However, Kim et al. [31] have reported the spectra of power-dependent PL in

ZnO nanorods synthesized by standard Schlenk techniques remain nearly unchanged

spectral profile as increasing the excitation intensity as compared with 40 meV red

shift for bulk crystal with their maximum excitation intensity. They attributed this

finding to the quantum confinement effects that can alter the properties of exciton

states and claimed in this study the exciton states of the nanorods are stable even

when the excitation intensity reaches the Mott density of the bulk crystal due to a

smaller exciton size and an enhanced exciton binding energy [32]. On the other

hand, Bagnall et al. [33] have observed the red-shifted PL peak of ~50 meV in ZnO

powder that may be attributed to the exciton-exciton scattering (or P band) rather than

electron-hole plasma (EHP or N band) with > 100 meV red shift.

The characterizations of ZnO QDs are complicated problems to be investigated.

important roles in stimulated emission and gain process in real photonic device

structures.

1.4 Organization of the thesis

This thesis is organized as follows. Chapter 2 covers the theoretical

background of experiments such as sol-gel method, X-ray diffraction (XRD),

photoluminescence (PL) characterization, and a general concept of quantum effect,

fundamental optical transitions and ZnO excitons-related emissions. In Chapter 3,

we describe the experimental details including the measurement apparatus and

processes. By means of the XRD and PL spectroscopy, the crystal structures and the

optical emission properties of ZnO QDs grown by the sol-gel method will be

investigated and discussed in Chapter 4. Finally, in Chapter 5, we conclude the

studies on the ZnO QDs and propose several topics of the future works.

                       

Chapter 2 Theoretical background

2.1 Sol-gel method

An aerosol is a colloidal suspension of particles in a gas (the suspension may be

called a fog if the particles are liquid and a smoke if they are solid) and an emulsion is

a suspension of liquid droplets in anther liquid. A sol is a colloidal suspension of

solid particles in a liquid, in which the dispersed phase is so small (~1-1000 nm) that

gravitational force is negligible and interactions are dominated by the short-range

forces, such as Van der Waals attraction and surface charge. All of these types of

colloids can be used to generate polymers or particles from which ceramic materials

can be made. A polymer is a huge molecule (also called a macromolecule) formed

from hundreds or thousands of units called monomers. If one molecule reaches

macroscopic dimensions so that it extends throughout the solution, the substance is

said to be gel.

Sol-gel synthesis has two ways to prepare solution. One way is the

metal-organic route with metal alkoxides in organic solvent; the other way is the

inorganic route with metal salts in aqueous solution. It is much cheaper and easier to

handle than metal alkoxides, but their reactions are more difficult to control. The

condensation of metal alkoxides M(OR)Z, where M = Si, Ti, Zr, Al, Sn, Ce, and OR is

an alkoxy group and Z is the valence or the oxidation state of the metal. First,

hydroxylation upon the hydrolysis of alkoxy groups:

ROH OH M O H OR M − + ₂ → − + . (2-1)

The second step, polycondensation process leads to the formation of branched

oligomers and polymers with a metal oxygenation based skeleton and reactive

residual hydroxyl and alkoxy groups. There are 2 competitive mechanisms:

(1) Oxolation-- formation of oxygen bridges:

XOH M O M M XO OH M − + − → − − + _{. (2-2)}

The hydrolysis ratio (h = H2O/M) decides X=H (h >> 2) or X = R (h < 2).

(2) Olation-- formation of hydroxyl bridges when the coordination of the metallic

center is not fully satisfied (N - Z > 0):

M OH M M HO OH M − + − → −( )₂ − , (2-3)

where X = H or R. The kinetics of olation is usually faster than those of oxolation.

Figure 2-1 presents a schematic of the routes that one could follow within the

scope of sol-gel processing [34]. In the sol-gel process, the precursors (starting

compounds) for preparation of a colloid consist of a metal or metalloid element

surrounded by various ligands. The precursors were mixed together and heated at

homogeneous product is obtained. Then, the materials have to be transformed into

the desired shape. For example, an alkyl is a ligand formed by removing one

hydrogen (proton) from an alkane molecule to produce, for example, methyl (‧CH3)

or ethyl (‧C2H5). An alcohol is a molecule formed by adding a hydroxyl (OH) group

to an alkyl (or other) molecule, as in methanol (CH3OH) or ethanol (C2H5OH).

Metal alkoxides are members of the family of metalorganic compounds, which

have an organic ligand attracted to a metal or metalloid atom. Metal alkoxides are

popular precursors because they react readily with water. The reaction is called

hydrolysis, because a hydroxy ion becomes attached to the metal atom. This type of

reaction can continue to build larger and larger molecules by the process of

polymerization. The gel point is the time (or degree of reaction) when the last bound

is formed that completes this giant molecule. It is generally found that the process

begins with the formation of fractal aggregates that begin to impinge on one another,

then those clusters link together as described by the theory of percolation. The gel

point corresponds to the percolation threshold, when a single cluster (call the

spanning cluster) appears that extends throughout the sol; the spanning cluster

coexists with a sol phase containing many smaller clusters, which gradually become

attached to the network. Gelation can occur after a sol is cast into a mold, in which

 

Fig. 2-1 Schematic of the rotes that one could follow within the scope of sol-gel processing [8].

2.2 Quantum effect

During the last decade, the growth of low-dimensional semiconductor structures

has made it possible to reduce the dimension from three (bulk material) to the

quasi-zero dimensional semiconductor structures usually called QDs. In these

nanostructures the quantum confinement effects become predominant and give rise to

many interesting electronic and optical properties. The electron energy will be

quantized and varies with dot sizes that cause the variation of band gap energy,

binding energy, and Bohr radius. The band gap and the density of states (DOS)

associated with a quantum-structure differ from that associated with bulk material,

2.2.1 Quantum confinement effect

Models explaining the confinement of charged particles in a three-dimensional

potential well typically involve the solution of Schrödinger’s wave equation using the

Hamiltonian [35] U V m m H h e + + ∇ − ∇ − = 2 2 2 2 ₀ 2 2 h h _{.      (2-4)}

Variation between treatments generally originates from differences in expressions

assigned to V0 for the confining potential well, which normally is accompanied by the

Coulombic interaction term U. Boundary conditions are imposed forcing the wave

functions describing the carriers to zero at the walls of the potential well.

Two regimes of quantization are usually distinguished in which the crystallite

radius R is compared with the Bohr radius of the excitons a_B or the related

quantities:

(1) Weak confinement regime for R≥a_B and

(2) Strong confinement regime for R<a_B.

In the weak confinement regime, the motion of center of exciton mass is

quantized while the relative motion of electron and hole given by the envelope

function )φ(r_e −r_h is hardly affected. In the strong confinement regime, however,

the Coulomb energy increases roughly with R-1_{, and the quantization energy with R}-2_,

Coulomb term can be neglected.

2.2.1.1 Weak confinement [36]

Coulomb-related correlation between the charged particles handled through the

use of variational approach involving higher-order wave function of the confined

particles, and we cannot neglect the electron hole Coulomb potential. The

Schrödinger equation may be written as

Ψ = Ψ − + + Ψ ∇ − − _{e h t} h e E r r U V m m 2 ) [ ( )] 2 ( h2 h2 2 _{0 . (2-5)} Taking r=re −rh and h e h h e e m m r m r m R + +

= , then the equation becomes

Ψ = Ψ + + ∇ − ∇ − _{R r} V R U r E_t M 2 ( ) ( )] 2 2 [ 2 2 2 μ h h with M =me +mh, h e h e m m m m + =

μ , and E as the total energy of the system. If we t

take Ψ=φ(R)ϕ(r) and consider Coulomb interaction first, then we get

) ( ) ( )] ( 2 [ 2 2 R E R R V M ∇R + φ = cφ − h , ) ( ) ( ) ( ) ( )] ( 2 [ 2 2 r E r E E r r U _{t c ex} r ϕ ϕ ϕ μ∇ + = − = − h .

Here E results from the inclusion of Coulomb interaction. Then we consider the _ex

confinement potential       B B a R R V a R R V > ∞ = ≤ = , ) ( , 0 ) ( . 

Thus, the energy is

... 3 , 2 , 1 , 2 2 2 2 2 = = n Ma n E_cn h π       (2-6)

ex g ex c t E Ma E E E E = + = + + = 2 2₂ 0 2 π ω h h          (2-7)

with M =m_e +m_h as the total mass of the electron and hole.

2.2.1.2 Strong confinement

The size quantization band states of the electron and hole dominate for the

kinetic energies of electron and hole are larger than the electron-hole Coulomb

potential, and the effect of the Coulomb attraction between the electron and hole can

be treated as a perturbation. Then the Schrödinger equation becomes

Ψ = Ψ + Ψ ∇ − − V E m m_{e e} 0 2 2 2 ) 2 2 ( h h .

The potential is defined as

B B a r r V a r r V > ∞ = ≤ = , ) ( , 0 ) (  

and the energy of an electron or a hole is

... 3 , 2 , 1 , 2 2 , 2 2 2 = = n a m n E h e cn π h  

The absorption energy of a photon is

μ π π ω₀ 2 ₂2 2₂2 2 ) 1 1 ( 2a m m E a E _g h e g h h h = + + = + (2-8) with h e h e m m m m + =

2.2.2 Density of states (DOS)

The concept of density of states (DOS) is extremely powerful and important

physical properties such as optical absorption, transport, etc., are intimately dependent

upon this concept. The density of states is the number of available electronic states

per unit volume per unit energy around an energy E. If we denote the density of

states by N(E), the number of states in an energy interval dE around an energy E is

N(E)dE. To calculate the density of states, we need to know the dimensionality of

the system and the energy vs. wave vector relation or the dispersion relation that the

electrons obey [37].

2.2.2.1 Density of states for a three-dimensional system

        In a three dimension system, the k-space volume between vector k and k + dk is

4πk2

dk (see Figure 2-2). We had shown above that the k-space volume per electron

state is_{(2 / )}3

L

π . Therefore, the number of states of electron in the region between k

and k+ dk are V dk k V dk k 2 2 3 2 2 8 4 π π π = . 

Denoting the energy and energy interval corresponding to k and dk as E and dE, we

see that the number of electron states between E and E+ dE per unit volume are

2 2 2 ) ( π dk k dE E N =  

and since m k E 2 2 2 h

= , then the equation becomes

3 2 1 2 3 2 2 h dE E m dk k = ,  which gives . 2 ) ( ₂2₃ 1 2 3 dE E m dE E N h π =  

        We must remember that the electron can have two states for a given k-value

since it can have a spin state of s = 1/2 or -1/2. Accounting for spin, the density of

states is . 2 ) ( _{2 3} 2 1 2 3 h π E m E N =

Fig. 2-2 Geometry used to calculate density of states in three, two and one dimensions.

2.2.2.2 Density of states for lower-dimensional systems

If we consider a 2-D system, a concept that has become a reality with use of

quantum wells, similar arguments tell us that the density of states for a parabolic band

2 ) ( h π m E N = . 

Finally, in a 1-D system or “quantum wire”, the density of states is

h π 2 1 2 1 2 ) ( − = m E E N . 

We notice that as the dimensionality of the system changes, the energy

dependence of the density of states also changes. In three-dimensional systems we

have E1/2_{-dependence as shown in Figure 2-3a, while in two-dimensional systems}

there is no energy dependence (Figure 2-3b). In one-dimensional systems, the

density of states has E-1/2_{-dependence and a peak at E = 0 (Figure 2-3c), and the}

density of states in zero-dimensional systems is shown in Figure 2-3d. The

variations related to dimensionality are extremely important and is a key driving force

to lower dimensional systems.

Fig. 2-3 Variation in the energy dependence of the density of states in a) three-dimensional b) two-dimensional c) one-dimensional d) zero-dimensional systems.

2.3 X-ray diffraction

2.3.1 Lattice parameters [38]

A crystal consists of orderly array of atoms, each of which can scatter

electromagnetic waves. A monochromatic beam of X-rays that falls upon a crystal

will be scattered in all directions inside it. However, owing to the regular

arrangement of the atoms, in certain directions the scattered waves will constructively

interfere with one another while in others they will destructively interfere. The peaks

of an x-ray diffraction pattern are made up of photons constructively interfere with

planes, this analysis was suggested in 1913 by W. L. Bragg. Consider an incident

monochromatic x-ray beam interacting with the atoms arranged in a periodic manner

as shown in 2-dimendion in Fig. 2-4. The atoms represented as circles in the graph

form different sets of plane in the crystal. To give a set of lattice planes with an

inter-plane distance of d, the condition for diffraction (peak) constructively interfere

to occur can be simply written as

λ θ n

dsin =

2 , n=1,2,3,..., (2.9) which is known as the Bragg’s law. In this equation, λ is the wavelength of the x-ray, θ is the diffraction angle, and n is an integer representing the order of the

 

Fig. 2-4 x-ray scattering from a 2-dimendion periodic crystal.

The lattice constant can be found by means of the diffraction pattern. For 

hexagonal unit cell (Figure 2-5), which is characterized by lattice parameters a and c,

the plane spacing equation for hexagonal structure is  

2 2 2 2 2 2 ₃ 4 1 c l a k hk h d ⎟⎟_⎠+ ⎞ ⎜⎜ ⎝ ⎛ + + = , (2.10) whereh,k, and l are the Mill’s indices.

Combining the Bragg’s law (λ=2dsinθ ) with (2-2), we can get:

. sin 4 3 4 1 2 2 2 2 2 2 2 2 _λ θ = + ⎟⎟ ⎠ ⎞ ⎜⎜ ⎝ ⎛ + + = c l a k hk h d (2-11) Rearranging (2-3) gives } 3 4 { 4 sin2 2 2 ₂ 2 2₂ c l a k hk h ₊ ⎟⎟ ⎠ ⎞ ⎜⎜ ⎝ ⎛ + + =λ θ , (2-12) thus the lattice parameters can be estimated from Eq. (2-12).

 

Fig. 2-5 The hexagonal unit cell.

2.3.2 Deby-Scherer formula [39]

Considering the path difference between the successive planes when the incident

beam remains fixed at the Bragg angle θ, but with the diffracted ray leaving at an

angle θ+Δθ, corresponding to intensity I in the spectrum line an distance Δθ away

from the peak as Figure 2-6.

Fig. 2-6 Grating treatment of line broadening by crystallite.

The path difference between waves from successive planes is now

= d sinθ+ d sinθcosΔθ+d cosθsinΔθ (2.13)

where BC and CE is the path difference between incident ray of the successive planes,

and d is the interplanar distance. If Δθ is very small, we can write cosΔθ ≈ 1 and

sinΔθ ≈ Δθ, in which case

BC + CE = 2dsinθ+ d cosθΔθ. (2.14)

Combining Bragg’s law Eq. (2.1)

BC + CE = 2dsinθ+ d cosθΔθ= nλ + d cosθΔθ, (2.15) where λ is the wavelength of the incident x-ray. Therefore the phase difference δ per interplanar distance or “aperture” is

δ = θ θ λ π λ π λ 2 +2 dcos •Δ n = 2nπ + θ θ λ π _•Δ cos 2 d (2.16)

Since a phase difference of 2π n produces the same effect as a zero phase, we can write the effective phase difference per aperture as

δ = θ θ λ π _•Δ cos 2 d (2.17)

We obtain the result that the distribution of intensity of I in a spectrum line a

distance R from the grating is effectively

δ δ 2 1 sin 2 1 sin ) ( 2 2 2 N R I = Φ (2.18)

2 2

max ( ) N

R

I = Φ . (2.19)

where Φ is the amplitude at unit distance from the graying, and N is the total number

of grating aperture.

Diving Eq. (2.10) by Eq. (2.11), we obtain

2 2 2 max ) 1 ( 2 1 sin 2 1 sin N N I I δ δ = . (2.20) Since 2 1

Nδ will change much faster than 2 1

δ, the function will reach its first

minimum before 2 1

δ is very large. We can therefore replace δ 2 1 sin by 2 1 δ, and we will get 2 max ) 2 1 2 1 sin ( δ δ N N I I ₌ . (2.21) The ratio max I I will fall to 2 1 when δ δ N N 2 1 2 1 sin = 2 1 . (2.22)

The solution to the equation (2.14) yields the required phase difference

corresponding to the half maximum. It may be obtained

δ

N

2 1

= 1.39, (2.23)

Since, according to equation (2.9), δ = θ θ

λ π _•Δ cos 2 d , and D = Nd, we obtain θ θ λ _{= 2Δ} cos 89 . 0 D . (2.24)

Let B be taken as the full width at half maximum from Δθ to –Δθ, hence B = 2Δθ= θ λ cos 89 . 0 D (radians), (2.25)

where λ is the wavelength of x-ray and D is the average size of the particles.

2.4 Photoluminescence characterization [35, 40]

Photoluminescence (PL) is a powerful and noninvasive optical analysis

technology for the semiconductor industry. It has high sensitivity to reveal the band

structure and the carrier transportation behaviors in a material. From PL spectrum

the defect or impurity can also be found in the compound semiconductors, which

affect material quality and device performance. A given impurity produces a set of

characteristic spectral features. The fingerprint identifies the impurity type, and

often several different impurities can be seen in a single PL spectrum. In addition,

the full width at half maximum of the PL peak is an indication of sample’s quality

[41]. So the PL could be a judgment of the material quality and be a key technology

of the development of nano-technology.

PL is the optical radiation emitted by a physical system (in excess the thermal

equilibrium blackbody radiation) resulting from excitation to a nonequilibrium state

by illuminating with light. Three processes can be distinguished: (i) creation of

electron-hole pairs, and (iii) escape of the recombination radiation from the sample.

2.4.1 Fundamental Transitions

An electron is excited from the valence band to the conduction band by

absorption of a photon. In this sense an optical excitation is a two-particle transition.

The same is true for the recombination process. An electron in the conduction band

can return radiatively or nonradiatively to the valance band only if there is a free

space, i.e., a hole available. Two quasi-particles annihilate in the recombination

process. What we need for the understanding of the optical properties of the

electronic system of a semiconductor is therefore a description of the excited states of

the N-particle problem. We will consider the fundamental transitions, those

occurring at or near the band edges.

1. Free excitons (Wannier-Mott excitons)

An electron in the conduction band and a hole in the valence band are created at

the same point in space and can attract each other through their Coulomb interaction.

Using the effective mass approximation, the Coulomb interaction between electron

and hole leads to a hydrogen-like problem with a Coulomb potential term

h e r r e − − ε πε₀ 2 4 .

Here we will consider the so-called Wannier-Mott excitons more specifically.

This type of excitons has a large Bohr radius (i.e., the mean distance between electron

and hole) that encompasses many atoms, and they are delocalized states that can move

freely throughout the crystal; hence the alternative name of free excitons.

Indeed excitons in semiconductors form, to a good approximation, a hydrogen-

or positronium-like series of states below the gap. For a simple parabolic band in a

direct-gap semiconductor one can separate the relative motion of electron and hole

and the motion of the center of mass. This leads to the dispersion relation of exciton

as shown in Fig. 2-7. M K n Ry E K n E B g B ph 2 1 ) , ( _{= −} * _{2 +} h2 2 _{, (2.26)} where nB=1,2,3,... is the principal number, )

1 ( 6 . 13 ₂ 0 * ε μ m Ry = eV is exciton binding

energy, M =m_e +m_h and K =k_e+k_h are translational mass and wave vector of

the exciton, respectively. The series of exciton states in (2.26) has an effective

Rydberg energy *

Ry modified by the reduced mass of electron and hole and the

dielectric constant of the medium in which these particles move. The radius of the

exciton equals the Bohr radius of H atom again modified by ε and μ. Using the

material parameters for typical semiconductors one finds that the orbits of electron

and hole around their common center of mass average over many unit cells and this in

excitons are called Wannier-Mott excitons.

Fig. 2-7 A pair excitation in the scheme of valence and conduction bands (a) in the exciton picture for a direct gap semiconductor (b).

2. Biexcitons

It is well known that two hydrogen atoms with opposite electron spins can bind

to form a hydrogen molecule. In the same sense, it has been calculated that two

positronium-like atoms can form a positronium-like molecule as a bound state. So

the idea is not far away that two excitons could bind to form a new quasiparticle, the

so-called biexciton or excitonic molecule. It has been found theoretically that the

biexciton should form a bound state for all ratios of effective electron and hole masses

and dimensionalities of the sample.

The dispersion relation is given in the simplest case by

ex b biex b ex g biex M k E E E k E 4 ) ( 2 ) ( 2 2 h + − − = , (2-27) assuming that the effective mass of the biexciton is just twice that of the exciton.

3. Bound excitons

Similar to the way that free carriers can be bound to defects, it is found that

excitons can also be bound to defects. Some of these defects can bind an exciton

resulting in a bound exciton complex (BEC). In Figure 2-8 we visualize exciton

bound to an ionized donor (D+_{X), a neutral donor (D}0_{X), and a netural axxeptor (A}0_X).

An ionized acceptor does not usually bind an exciton since a neutral acceptor and a

free electron are energetically more favorable. The binding energy of an exciton (X)

is the highest for a neutral acceptor (A0_{X complex), the lower for a neutral donor}

(D0X) and the lower still for an ionized donor (D+X). The binding energy b

E of

exciton to the complex usually increases according to b X A b X D b X D E E E + < 0 < 0 . The binding energy is defined as the energetic distance from the lowest free exciton state

at k = 0 to the energy of the complex. There is a rule of thumb, known as Hayne’s

rule, which relates the binding energy of the exciton to the neutral complex with the

binding of the additional carrier to the point defect.

4. Surface-bound exciton

The surface-to-volume ratio brings much influence on the system’s Hamiltonian

when the material size is reduced to the nanometer scale [22]. From the calculation

of exciton states at the QD surface, we can know the exciton is bound to the

surface-located acceptor [25]. Unlike the acceptor, the donor does not bind the

exciton. Because the hole is much heavier than the electron, which makes the

surface donor a shallow impurity, while the surface acceptor a deep impurity.

Therefore, excitons can be effectively bound only to surface acceptors.

5. Two-Electron Satellites (TES) [42]

Two-electron satellite (TES) transitions involve radiative recombination of an

exciton bound to a neutral donor, leaving the donor in the excited state. In the

effective mass approximation, the energy difference between the ground-state neutral

donor bound excitons and their excited states (TES) can be used to determine the

donor binding energies [43] (the donor excitation energy from the ground state to the

first excited state equals to 3/4 of the donor binding energy, E_D) and catalog the

6. Donor-Acceptor Pairs (DAP)

Donors and acceptors can form pairs and act as stationary molecules imbedded in

the host crystal. The coulomb interaction between a donor and an acceptor results in

a lowering of their binding energies. In the donor-acceptor pair case it is convenient

to consider only the separation between the donor and the acceptor level:

r q E E E E_{pair g D A} ε 2 ) ( + + − = , (2-28) where r is the donor-acceptor pair separation, E_DandE_Aare the respective ionization

energies of the donor and the acceptor as isolated impurities.

7. Deep transitions

By deep transition we shall mean either the transition of an electron from the

conduction band to an acceptor state or a transition from a donor to the valence band

in Fig. 2-9. Such transition emits a photon hν =E_g −E_ifor direct transition and

p i g E E

E

hν = − − if the transition is indirect and involves a phonon of energyE . _p

Hence the deep transitions can be distinguished as ( ) conductionⅠ -band-to-acceptor

transition, which produces an emission peak at hν =E_g −E_A , and ( ) Ⅱ

donor-to-valence-band transition which produces emission peak at the higher photon

C

V

D

A

 

Fig. 2-9 Radiative transition between a band and an impurity state.

2.4.2 Influence of high excited light intensity

The PL conditions as mentioned above are excited by low excitation light

intensity. At low excitation light intensity (low density regime in Fig. 2-10), the PL

properties are determined by single electron-hole pairs, either in the exciton states or

in the continuum. Higher excitation intensity (intermediate density regime in Fig.

2-10) makes more excitons; such condition would lead to the exciton inelastic

scattering processes and form the biexciton. The scattering processes may lead to a

collision-broadening of the exciton resonances and to the appearance of new

luminescence bands, to an excitation-induced increase of absorption, to bleaching or

to optical amplification, i.e., to gain or negative absorption depending on the

excitation conditions. If we pump the sample even harder, we leave the intermediate

and arrive at the high density regime in Fig. 2-10, where the excitons lose their

is known as the electron-hole plasma (EHP).

 

Fig. 2-10 The general scenario for many-particle effects in semiconductors. [35]

1. Electron-Hole Plasma

In this high density regime, the density of electron-hole pairs np is at least in

parts of the excited volume so high that their average distance is comparable to or

smaller than their Bohr radius, i.e., we reach a “critical density” c p

n in an EHP, given

to a first approximation by 3 c _≈1

p Bn

a . We can no longer say that a certain electron is

bound to a certain hole; instead, we have the new collective EHP phase. The

transition to an EHP is connected with very strong changes of the electronic

excitations and the optical properties of semiconductors.

2. Scattering Processes

In the inelastic scattering processes, an exciton is scattered into a higher excited

state, while another is scattered on the photon-like part of the polariton dispersion and

leaves the sample with high probability as a luminescence photon, when this

photon-like particle hits the surface of the sample. This process is shown

schematically in Fig. 2-11 and the photons emit in such a process having energies En

given by Ref. [44] kT n E E E_{n ex b}ex 2 3 1 1 ₂ ⎟− ⎠ ⎞ ⎜ ⎝ ⎛ − − = , (2-29) where n = 2, 3, 4,…, ex b

E = 60 meV is the binding energy of the free exciton of ZnO, 

and kT is the thermal energy. The resulting emission bands are usually called

P-bands with an index given by n.

energy Wave vector E_g Eexciton P2 P∞ 1 2 ∞ n_B= continuum  

Chapter 3 Experiment details and analysis techniques

3.1 Sample preparation

We fabricate ZnO QDs by sol-gel method. Sol-gel method was chosen due to

its simple handling, low cost, and narrow size distribution. In particular, it has the

potential to produce samples with large areas and complicated forms on various

substrates.

The ZnO colloidal spheres were produced by one-stage reaction process, and

reactions were described as the following equations:

COOH xCH COO CH OH Zn O xH COO CH Zn( ₃ )₂ + ₂ ⎯⎯→Δ ( −)_x( ₃ −)_2−x + ₃ , (3-1) COOH CH x O H x ZnO COO CH OH Zn( −)_x( ₃ −)_2−x ⎯⎯→Δ +( −1) ₂ +(2− ) ₃ . (3-2)

Equation (3-1) is the hydrolysis reaction for Zn(OAc)2 to form metal complexes. We

increased the temperature of reflux from RT to 160o_{C and maintained for aging. The}

zinc complexes will dehydrate and remove acetic acid to form pure ZnO as Eq. (3-2)

during the aging time. Actually, the two reactions described above proceed

simultaneously while the temperature is over 110o_C.

All chemicals used in this study were reagent grade and employed without

further purification. A typical reaction was listed in Table 3-1, stoichiometric zinc

diethylene glycol [99.5% DEG, ethylenediamine-tetra-acetic acid (EDTA)] to make

0.1 M, 0.05 M, 0.01 M solutions. The first thing we notice is that we can control the

QDs size with domination concentration of zinc acetate in the solvent (DEG). Then

the temperature of reaction solution was increased to 160℃, and white colloidal ZnO

was formed in the solution that was employed as the primary solution. The primary

solutions were put separately in a centrifuge operating at 3000 rpm for 30 minutes.

The supernatant was decanted off and saved, and the polydisperse powder was

discarded. Finally, the supernatant was then dropped onto Si (100) substrates and

dried at 150℃ for further characterization. The experiment equipment and a flow

chart of fabricate ZnO QDs by sol-gel method was shown in Figure 3-1&3-2.

Chemical reagent Molecular formula Degree of purity

Source

Zinc acetate dehydrate Zn(CH3COOH)2‧2H2O 99.5% Riedel-deHaen

Diethylene glycol C4H10O3 99.5% EDTA

Table 3-1 Shows that chemical reagent was used with sol-gel experiment process.

 

 

 

Fig. 3-1 Experiment equipment used for fabricating ZnO QDs.

                       

Fig. 3-2 A flow chart of fabricate ZnO QDs by sol-gel method.

Zn(CH3COOH)2‧2H2O

Diethylene-glycol (DEG)

Counter flow apparatus

White colloidal formed

Centrifuge

Clear solution

drop or spin coating on SiO2/Si(001)

varying solution concentration

heating up to 160℃

3.2 Microstructure and optical properties analysis

3.2.1 X-ray diffraction

The crystal structures of the as-grown powder were inspected by using XRD

(model: MAC Sience, MXP18) at room temperature equipped with CuK X-ray source

(λ=1.5405Å) in National Synchrotron Radiation Research Center (NSRRC), Taiwan.

Data were recorded between the angle range of 20° < 2θ < 80° with steps of 0.02°and

rate of scanning is 40°/min. The operation voltage of the system is 50 kV and the

operation current is 200 mA. The essential features of x-ray spectrometer are shown

in Figure 3-3. X-rays from the tube T are incident on a crystal C which may be set at

any desired angle to the incident beam by rotation about an axis through O, the center

of the spectrometer circle. D is a detector which measures the intensity of the

diffraction x-rays.

 

The sizes of the nanocrystallites can be determined by X-ray diffraction using the

measurement of the full width at half maximum (FWHM) of the X-ray diffraction

lines. The average diameter is obtained by

θ λ cos 89 . 0 B

D= , where D is the average

diameter of the nanocrystallite, λ is the wavelength of the X-ray source, and B is the

FWHM of X-ray diffraction peak at the diffraction angle θ.

3.2.2 Photoluminescence system

PL provides a non-destructive technique for the determination of certain

impurities in semiconductors. The shallow-level and the deep-level of impurity

states were detected by PL system. It was provided radiative recombination events

dominate nonradiative recombination.

In the PL measurements, we used a He-Cd cw laser (325 nm) [Kimmon

IK5552R-F] as the excitation light. Light emission from the samples was collected

into the TRIAX 320 spectrometer and detected by a photomultiplier tube (PMT). As

shown in Fig. 3-4, the diagram of PL detection system includes mirror, focusing and

collecting lens, the sample holder and the cooling system. The excitation laser beam

was directed normally and focused onto the sample surface with power being varied

with an optical attenuator. The spot size on the sample is about 100 μm.

into a 0.32 cm focal-length monochromator (TRIAX 320) with a 1200 lines/mm

grating, then detected by either an electrically cooled CCD (CCD-3000) or a

photomultiplier tube (PMT-HVPS) detector. The temperature-dependent PL

measurements were carried out using a closed cycle cryogenic system. A closed

cycle refrigerator was used to set the temperature anywhere between 15 K and 300 K.

And the power-dependent PL spectra were used for monitoring the characteristic of

excitons and thermalization effect at the different excitation density.

Chapter 4 Results and Discussion

 

4.1 Morphology and crystal structures

4.1.1 Morphology

Shown in Fig. 4-1 is a typical high-resolution transmission electron microscope

(HRTEM) image of the ZnO nanoparticles. Nanoparticles aged at 160 °C for 1 hr

and solution concentration of 0.04 M was selected for particle size determination by

HRTEM. The shape of particles is predominantly spherical, and the nanoparticles

are clearly well separated and essentially have some aggregation. The average

diameter of the number-weighted particles obtained from a colloid aged at 160 °C for

1 hr (0.04 M) was determined to be 6.18 ± 0.3 nm.

4.1.2 X-ray diffraction measurement

The XRD profiles of the ZnO QDs with various concentrations of Zn(OAc)2 are

shown in Fig. 4-2. The diffraction pattern and interplanar spacings match nicely

with the standard diffraction pattern of wurtzite ZnO, demonstrating the formation of

wurtzite ZnO nanocrystals. All of the samples present similar XRD peaks that can

be indexed as the wurtzite ZnO crystal structure with lattice constants a = 3.253 Å and

c = 5.213 Å, which are consistent with the value in the standard card (JCPDS

89-1397).

Fig. 4-2 XRD profiles of the ZnO QDs prepared with various concentration of Zn(OAc)2. The

crystalline size can be approximately estimated to be 16 and 7 nm, respectively (top to bottom), for concentrations of 0.16 and 0.06 M.

No diffraction peaks of other species could be detected that indicates all the

formed. The full width at half maximum (FWHM) of the diffraction peaks increases,

that is, the average crystalline size decreases, as the concentration of zinc precursor

reduces. The average size is calculated from the width of diffraction peak using the

Debye-Scherer formula: [45, 46]D = 0.89λ /(wcos θ ), where D is the average

crystalline diameter of the particles, λ is the wavelength of the x-ray source, w is the

linewidth at half maximum in excess of the instrumental broadening, and θ is the

diffraction angle. The crystalline size can be estimated to be 16 nm and 7 nm, for

concentrations of 0.16 M, and 0.06 M, respectively.

4.2 Photoluminescence spectra

4.2.1 Photoluminescence spectra for different sizes of ZnO particles

The PL spectra of ZnO QDs with average crystalline sizes of 16 and 7 nm at 13

K are shown in Fig. 4-3. For the 16-nm ZnO QDs, a strong UV emission peak at

3.388 eV with FWHM of ~ 96 meV was observed accompanied with very broad weak

visible emission with the slightly strong blue emission and rather weak yellow

emission bands. And only a strong UV emission peak at 3.411 eV with FWHM of ~

174 meV was observed unaccompanied with visible emission for the 7 nm ZnO QDs.

The sharp UV emission peak is attributed to the near band edge emission (NBE) [27,

oxygen vacancy (green emission) or oxygen interstitial (red emission). The intensity

of the UV and visible peak ratio increases with decreasing the QD size. The strong

UV emission and weak visible emission in PL spectra indicates that the ZnO QDs

have a good crystal quality.

Fig. 4-3 The PL spectra of the ZnO QDs measured at low temperature.

Figure 4-4 demonstrates the PL spectra of ZnO QDs with different average

crystalline sizes measured at T = 13 K. The near band edge UV emission is

attributed to the free exciton (FX) emission [32], which shifts (solid line) to the higher

energy (from 3.386 eV to 3.42 eV) as the QD size decreases (16 nm – 6 nm) due to

the quantum confinement effect (QCE). Besides, the FWHM increases as the

average QDs size decreases that may be caused by the contribution of surface-optical

Since the Bohr radius of the exciton in bulk ZnO is about 2.34 nm [49], we must

consider the electron hole Coulomb interaction in our samples and the particles are in

the moderate to weak confinement regime.

Fig. 4-4 PL spectra of different sizes of ZnO QD at low temperature. The line indicates the shifts of FX peak energy.

4.2.2 Temperature-dependent PL for different sizes of ZnO particles

Figure 4-5 displays the temperature-dependent PL of different sizes of ZnO, it

reveals only single one band for T = 8 ~ 300 K in QDs system. Due to small binding

energy of D0X, it will be ionized as T > 100 K, so we can easily attribute the single

band to the FX emission. We also find that the peak energy difference of FX in QDs

system between 8 K and 300 K decrease with the particle size decreasing. It is

result of Coulomb interaction.

Fig. 4-5 Temperature-dependent PL spectra of (a) 16 nm, (b) 7 nm, (c) 6 nm.

4.2.3 Power-dependent PL for different sizes of ZnO particles

The power-dependent PL spectra of different sizes of ZnO QDs are shown in Fig.

4-6. These spectral shapes are almost the same except that the spectral intensity

49 kW/cm2_{), covering a range of three orders of magnitude, which show the exciton}

states remain invariable.

The character in ZnO QDs gives a sharp contrast to the ZnO powder (~ 1 μm),

in which the relative peak intensities of biexciton (BX) and P-band (exciton-exciton

scattering) emissions increase as the excitation power increasing [30]. For

quantitative analysis, the spectral shape of NBE emission measured in 6-nm ZnO QDs

at 80 K is decomposed into two peaks and theoretical fitting results are shown in Fig.

4-7. For comparison, the inset of Fig. 4-7 shows the fit of the PL spectrum in ZnO

powder at 80 K. The dashed curves denote the various emissions and the solid curve

corresponds to the sum of the theoretical fit, which shows good agreement with the

experimental data marked as the open dots. The PL spectra of QDs and that of

powder are very diverse. The BX emission can be observed in ZnO powder,

nevertheless, in ZnO QDs only the emission of surface bound exciton (SX) [51] rather

Fig. 4-6 Power-dependent PL spectra of (a) 16 nm at 15 K, (b) 16 nm at 80 K, (c) 7 nm at 15 K, (d) 7 nm at 80 K, (e) 6 nm at 8 K, (f) 6 nm at 80 K.

Fig. 4-7 Gaussian fit to a PL spectrum of 6 nm ZnO QDs (0.04M) measured at 80K. The inset shows the spectral fitting for ZnO powder. The fitted line shapes are shown separately in dash lines. Solid lines correspond to the fit and dots represent the data.

4.2.4 PL peak positions as a function of excitation power

The peak positions of these emissions with increasing the excitation power in

6-nm ZnO QDs at 80 K are plotted in Fig. 4-8. The FX is observed at 3.418 eV and

the PL emission peak at 3.325 eV may be attributed to the emission of SX. The peak

positions of FX emission and SX emission are hardly shifted as the excitation power

Fig. 4-8 The PL peak positions as a function of excitation power measured at 80K.

4.2.5 Integrated intensity as a function of excitation power

In order to further understand the characteristic of these two peaks, the integrated

PL intensities of these peaks as a function of excitation power in 6-nm ZnO QDs at

80K are depicted in Fig. 4-9. Both of the integrated intensities exhibit nearly linear

dependence on the excitation power with both exponents close to 0.96. However,

the exponent of BX (with binding energy of ~15 meV) on the excitation power in

ZnO powder comes near the theoretical value of 2 having 1.86 at 80K. It is due to

efficient cooling via participation of optical phonons when the exciton kinetic energy

approaches to the energy of the lowest optical phonon [30]. However, reducing the

exciton-LO phonon coupling in ZnO QDs [32] causes the lack of the efficient cooling

of exciton with assistance of optical phonon scattering or the so-called phonon

bottleneck. Therefore, it is hardly to allow effectively bounding exciton pairs to

In addition, the P-band emission is hardly found in Fig. 4-6. This finding

means that the probability of exciton-exciton scattering is also very low. For that

reason, we suspect the other reason might be that there is only one exciton existing in

a QD within the exciton lifetime even with the achievable high power CW excitation,

so that neither exciton-exciton scattering nor BX can be observed in ZnO QDs.

Therefore, the increase of excitation intensity provides the larger chance of a single

exciton being excited or the larger excitation rate of single exciton. Thus the peak is

hardly shifted and the PL intensity is linear dependence with excitation intensity

corroborating the invariable exciton states upon increasing pumping in ZnO QDs.

Fig. 4-9 The integrated intensity of two decomposed TES and FX peaks as a function of excitation power measured at 80K. The inset shows the excitation power dependent emission intensity of biexciton in ZnO powder measured at 80 K. The power factor is 1.86.

Chapter 5 Conclusions and Prospective

 

5.1 Conclusions

In summary, we have measured XRD to inspect the crystal structures and the

average size of ZnO QDs synthesized by a simple sol-gel method. The average sizes

of ZnO QDs were verfied by TEM. The diffraction pattern and interplanar spacings

indicate the formation of wurtzite ZnO nanocrystals, and no diffraction peaks of other

species could be detected that indicates all the precursors have been completely

decomposed and no other crystal products were formed. We have measured

temperature-dependent and power-dependent PL spectra of ZnO QDs with different

sizes to investigate the optical properties. From Size-dependence of efficient UV

photoluminescence from low temperature PL gives evidence for quantum

confinement effect. The ZnO QDs exhibit strong UV emission and weak visible

emission, indicating very good crystal quality. Moreover, the biexciton and P-band

emissions in ZnO QDs are hardly observed and unchanged shapes of

power-dependent PL spectra show the exciton states remain invariable in ZnO QDs.

The intensities of emission peaks associated with these invariable free exciton and

5.2 Prospective

In order to further investigate the optical properties of ZnO QDs, we will

measure the time resolved photoluminescence (TRPL) to inspect the lifetime of ZnO

QDs. Through measuring the lifetime of ZnO QDs, we can identify the different PL

origins of physical mechanisms and the process of exciton recombination. Besides,

we will do single-photon measurement to study the optical characterizations in a