國
立 交 通 大 學
光電工程學系暨研究所
博
士 論 文
低維度氧化鋅奈米結構：製程、光學性質與染料敏化太陽電池
應用之研究
Low-Dimensional ZnO Nanostructures: Fabrication, Optical
Properties and Applications for Dye-Sensitized Solar Cells
研 究 生：鄭信民
指 導 教 授：謝文峰 教授
低維度氧化鋅奈米結構：製程、光學性質與染料敏化太陽電池
應用之研究
Low-Dimensional ZnO Nanostructures: Fabrication, Optical
Properties and Applications for Dye-Sensitized Solar Cells
研 究 生：鄭信民
Student：Hsin-Ming Cheng
指導教授：謝文峰 教授
Advisor：Prof. Wen-Feng Hsieh
國
立 交 通 大 學
光電工程學系暨研究所
博
士 論 文
A Dissertation
Submitted to Department of Photonics and 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 Doctor of Philosophy
In
Electro-Optical engineering
May 2011
誌
謝
『不曾長夜嚎哭過，不足以言人生。』我從沒想到，這六年心路歷程，會如 斯般辛苦。人生總是充滿驚奇，但也因此，值得細細去品味。回首博士研究的這 段時光，幫助及鼓勵我的人很多。首先，要感謝我的指導老師 謝文峰教授。無 論在知識和處世，謝老師皆是我的楷模。每每遇到心情挫折時，與老師談完總是 能受到鼓舞，重新出發，從老師身上學到的東西比這本論文要多得太多，我以當 謝老師的學生為榮。其次要感謝各位口試委員，除了論文的指正以及建議外，對 我選擇踏上學術之路的支持與勉勵，讓我更有勇氣堅持走自己的路。我也要感謝 雷射診斷實驗室的夥伴們，在研究及生活上交流，讓我心常保年輕。大學同窗兼 室友的摯友旭政，一直以他的經驗建議我最對的路；國峰、俊毅、楊松、小郭及 小布丁，在材料合成上的協助與經驗分享；偉豪、易慶在我跨足染敏電池研究時 的大力協助；智章學長、博濟在超快光學分析上的不吝指導；輝鴻、新翰、玫丹、 明容、穎書等學弟妹們，不嫌棄我老地邀我打羽球及花蓮出遊，均感激在心。 此外，還要感謝工研院材化所的洪健龍組長、劉文亮組長以及林麗娟主任， 舉薦我進修。永寬兄、一誠兄以及忠義大哥各位前輩，不嫌煩地引領小弟我執行 奈米線計畫；文蒂姐、秀連，教我溶凝膠方法；智仁、世莉協助TEM 上的分析； 宏勝與士欽兄在研究上的心得分享；廖博、松慰、秀芬在氧化鋅相關經驗的交流； 瑞雲、湘芸要常容忍我這個失職的XRD 管理員，均使我銘感五內。太電中心蔡 松雨組長、童永樑經理同意讓我使用無塵室；佳樺、坤穆、任安、佳音、文祥在 染敏電池上的經驗傳授，都讓我在嶄新領域上，不走過多冤枉路，亦要一併感謝。 最後，更要感謝我偉大的母親，無怨無悔地關懷與支持，幾乎給予我她的全 部；還有兩位阿姨，一直覺得我是家族的寶；未來的老婆淑惠，多年來的包容與 照顧。在天國的外公與外婆，也謝謝您們辛苦照顧我長大，卻沒能來得及讓您們 享受這一刻的喜悅。謹將我完成論文之喜悅與所有曾關心過我的人一同分享。低維度氧化鋅奈米結構：製程、光學性質與染料敏化太
陽電池應用之研究
研究生：鄭信民 指導教授：謝文峰 教授 國立交通大學光電工程學系暨研究所摘要
利用氣相沉積法搭配預鍍氧化鋅緩衝膜，氧化鋅奈米柱可成功的垂直成長於 玻璃、矽、碳化矽以及藍寶石基板上。氧化鋅奈米柱在與基板水平方向的磊晶性 質，與預鍍的氧化鋅膜的磊晶性息息相關，均受基板影響。同樣地，在氧化鋅奈 米柱的光激發光方面，亦受到選擇不同的基板而有所不同。此外，高密度、垂直 成長的氧化鋅奈米陣列亦可成功的磊晶於預鍍氮化鎵緩衝膜的藍寶石基板上。我 們發現氧化鋅奈米線成長於氮化鎵緩衝膜上會受到基板影響，而有水平方向 121.9 MPa 的雙軸壓縮應力。在共振拉曼頻譜上，我們發現 n 階 A1 與 E1 縱 模光學聲子強度比例（A1(nLO)/E1(nLO)）隨階數趨於增加，原因歸咎於聲子在 氧化鋅奈米線空間中受到空間侷限所造成的結果。氧化鋅奈米線於常溫及低溫下 的光激發光均與能帶附近激子的複合有相關連性。 我們利用溶膠-凝膠法，成功地合成具自組裝的奈米等級氧化鋅二次粒子。 藉由穿透式電子顯微鏡的分析，氧化鋅奈米二次粒子是由具相同晶面方向的微小 一次粒子所凝結而成。在共振拉曼分析上，我們發現聲子的頻譜有紅移現象，電 子與聲子的耦合強度比起經攝氏350以及500度熱處理過後樣品來得小。這種隨著 尺寸而變化的電子與聲子耦合，主要是受到 Fröhlich 交互作用所影響；這現象 在氧化鋅量子點系統也觀察到。另外，在氧化鋅量子點的吸收光譜與光激發光光譜研究中，發現譜峰有明顯的藍位移現象，再利用有效質量模型可粗略估計其量 子侷限效應在不同氧化鋅奈米晶粒尺寸下的結果。 在染料敏化太陽電池應用上，我們利用水熱法成長氧化鋅奈米線與分歧狀氧 化鋅奈米線於導電玻璃上，做為光電極。比起奈米粒子，一維奈米線結構更能減 少載子於傳輸過程中的所造成的躍遷損耗。而具分歧狀的氧化鋅奈米線於染料敏 化太陽電池的光電流以及效益表現上分別為4.27 mA/cm2 以及 1.51 %，均為單 純奈米線的兩倍。表面積增加提高了染料吸附是效益提升的主要原因。 我們亦嘗試利用具自組裝性的氧化鋅奈米二次粒子做為染料敏化太陽電池 之光電極試驗。我們發現這種具多層級的結構體，除了利用一次粒子維持了染料 的吸附效率外，二次粒子的結構並增加了光散射效益，間接提供了更多的光獲取 能力。我們也嘗試使用兩種吲哚啉染料，D149、D205，與氧化鋅光電極搭配， 分別達到具高效益的 4.95 % 以及 5.34 % 染料敏化太陽電池。在使用 D205染 料，我們發現開路電壓以及短路電流提升的原因主要是增加了長鏈的疏水官能 基，有效的抑制了電子與電解中液碘離子的再複合。對氧化鋅奈米粒子而言， D205同時也比D149染料具有更優越的電子傳輸率。我們亦利用交流阻抗分析 法，比較兩種染料所製備的染料敏化太陽電池，提供電子生命期長短的更直接證 據。
Low-Dimensional ZnO Nanostructures: Fabrication, Optical
Properties and Applications for Dye-Sensitized Solar Cells
Student: Hsin-Ming Cheng Advisor: Prof. Wen-Feng Hsieh
Department of Photonics and Institute of Electro-Optical Engineering National Chiao Tung University
Abstract
Vertically well-aligned ZnO nanorods were synthesized without employing any metal catalysts on various substrates including glass, Si (111), 6H-SiC (0001) and sapphire (0001), which were pre-coated with c-oriented ZnO buffer layers, by simple vapor phase deposition. The in-plane alignments of ZnO nanorods depend on the crystallographic alignment of pre-coated ZnO buffer layer. The photoluminescence of ZnO nanorods are basically related to the type of the substrates. In addition, high-density, vertically oriented arrays of ZnO nanowires were also successfully epitaxial grown on the GaN (0001)-buffered sapphire substrate. We demonstrated that the arrays of ZnO nanowires are well aligned along the c-axis and suffer a small biaxial compressive stress of 121.9 MPa. The increasing intensity ratio of nth-order longitudinal optical (LO) phonon (A1(nLO)/E1(nLO)) with increasing scattering order
in resonant Raman spectra (RRS) reveals the spatial phonon-confinement as shrinking the diameter of ZnO nanowires. The exciton-related recombinations near the band edge dominate the UV emissions at room temperature as well as at low temperature.
agglomeration of crystalline subcrystals, are successfully synthesized by a simple sol-gel method. TEM images display that one artificial cluster behaves in a single crystal like wurtzite structure owing to the fact that subcrystals coagulate at the same crystal orientation. Moreover, from the RRS measurement, the as-grown sample exhibits phonon redshift; meanwhile, the coupling strength between electron and longitudinal optical phonon, determined by the ratio of the second- to the first-order Raman scattering cross sections, diminishes compared with the samples after post-annealing at 350˚C and 500˚C. The size dependence of electron-phonon coupling is principally as a result of the Fröhlich interaction. ZnO quantum dots (QDs) of controlled sizes have been fabricated by a simple sol-gel method. The blueshift of room-temperature photoluminescence (PL) measurement from free exciton transition are observed decreasing with the QD size that is ascribed to the quantum-confinement effect. From the RRS, the coupling strength between electron and longitudinal optical phonon, deduced from the ratio of the second- to the first-order Raman scattering intensity, diminishes with reducing the ZnO QD diameter. The size dependence of electron-phonon coupling is principally a result of the Fröhlich interaction.
For further dye-sensitized solar cell (DSC) applications, the solvothermal method was utilized to fabricate the ZnO nanowires and branched nanowires on FTO substrates. The one-dimensional branched nanostructures can afford a direct conduction pathway instead of interparticle hopping while using nanoparticles. Furthermore, the short-circuit current density and the energy conversion efficiency of the branched ZnO nanowire DSCs are 4.27 mA/cm2 and 1.51 %, which are twice higher than the bare ZnO nanowire ones. The improvement was consequent on the enlargement of internal surface area within the photoelectrode and achieving higher dye adsorption to significantly enhance the performance of the DSCs.
Moreover, self-assembled ZnO secondary NPs have been fabricated as an effective photoelectrode for DSCs. The hierarchical architecture, which manifested the significant light-scattering, can provide more photon harvesting. In addition, dye-molecule adsorption retained sufficient due to enough internal surface area provided by the primary single nanocrystallites. Two indoline dyes, coded D149 and D205, were used as the sensitizers of ZnO DSCs with the optimal energy conversion efficiencies of 4.95% and 5.34%, respectively. The enhancement of Voc and Jsc for D205-sensertized ZnO DSCs was ascribed to the effective suppression of electron recombination by extending the alkyl chain on the terminal rhodanine moiety from ethyl to octyl. The higher charge-transfer rate and retardant fluorescence decay reveal that D205 has better electron injection dynamics for ZnO NPs as compared to D149. The further evidence is performed by the electrochemical impedance spectroscopy (EIS) which exhibits the longer electron lifetime for D205-sensitized ZnO DSC in comparison with D149-sensitized one.
Contents
Abstract in Chinese... I Abstract in English ... III Contents ...VI List of Tables ... X List of Figures...XI
Chapter 1: Introduction
1.1 General properties of ZnO ... 1
1.1.1 Prelude ... 1
1.1.2 Crystal Structure... 1
1.1.3 Optical Propertie... 4
1.1.4 Defects in ZnO ... 6
1.2 Low-Dimensional ZnO Nanostructures... 9
1.2.1 General of Nanostructures ... 9
1.2.2 One-Dimensional (1-D) ZnO Nanostructures ... 11
1.2.3 Zero-Dimensional (0-D) ZnO Nanostructures ... 12
1.3 Research Motivation ... 13
1.4 Organization of Dissertation ... 14
References... 15
Chapter 2: A Review of Growth and Characterization Techniques
2.1 Growth methods... 202.1.1 Growth of 1-D ZnO Nanowires/ Nanorods ... 20
2.1.2 Growth of 0-D ZnO Nanoparticles/ Quantum dots ... 35
2.2.1 X-Ray Diffraction... 41
2.2.2 Photoluminescence ... 43
2.2.3 Raman... 49
References... 52
Chapter 3: Experimental Procedures
3.1 Vapor-Solid Synthesis for 1-D ZnO Nanowires/ Nanorods ... 553.2 Hydrothermal Synthesis for 1-D ZnO Nanowires/ Nanorods... 56
3.3 Sol-Gel Synthesis for 0-D ZnO Nanoparticles/ Quantum dots... 57
3.4 Materials and Reagents ... 59
3.5 Characteristic Instruments ... 60
References... 62
Chapter 4: Growth and Structural Properties for One-Dimensional
ZnO Nanostructures
4.1 ZnO Nanostructures on Buffer Layers... 634.1.1 1-D ZnO Nanorods on ZnO Buffer Layers ... 63
4.1.2 1-D ZnO Nanowires on GaN Buffer Layers ... 75
4.2 ZnO Nanostructures from Solvothermal Method ... 88
4.2.1 ZnO Nanowires on FTO ... 88
4.2.2 Branched ZnO Nanowires ... 90
4.3 Summary ... 96
References... 97
Chapter 5: Growth and Structural Properties for Zero-Dimensional
ZnO Nanostructures
5.1 Secondary ZnO Nanoparticles ... 995.1.2 Raman Scattering of ZnO Nanoparticles... 104
5.1.3 Photoluminescence of ZnO Nanoparticles ... 110
5.2 Size-controlled ZnO Quantum Dots ... 112
5.2.1 Fabrication and Structural Properties of ZnO Quantum Dots ... 112
5.2.2 Band Gap Variation of ZnO Quantum Dots... 115
5.2.3 Electron-Phonon Coupling of ZnO Quantum Dots ... 119
5.3 Summary ... 122
References... 124
Chapter 6: Low-Dimensional ZnO Nanostructures for Dye-Sensitized
Solar Cell Applications
6.1 Introduction of Dye-Sensitized Solar Cells (DSCs) ... 1286.1.1 A History from Photography ... 128
6.1.2 Base Principles of DSCs... 130
6.1.3 ZnO Photoanodes for DSCs ... 134
6.1.4 Research Motivation... 136
6.2 Experimental Procedures ... 137
6.2.1 Cell Fabrication ... 137
6.2.2 Performance Characterization ... 139
6.2.2.1 The Solar Spectrum and Air Mass ... 139
6.2.2.2 Photocurrent-Voltage (I-V) Characterization ... 141
6.2.2.3 Incident Photon-to-Current Conversion Efficiency (IPCE)... 146
6.2.2.4 Electrochemical Impedance Spectroscopy (EIS)... 146
6.2.3 Characteristic Instruments ... 151
6.3 ZnO Nanostructures for DSSC ... 152
6.3.1 Branched ZnO Nanowires for DSSC ... 152
6.4 Summary ... 178 References... 180
Chapter 7: Conclusions
7.1 Conclusions... 187 7.2 Prospective... 189 References... 191CurriculumVitae
... 192List of Tables
TABLE 1-1. An appreciation of the potential of ZnO for optoelectronic applications can be obtained by examining this table, which compares the relevant material properties of ZnO with those of other wide band gap semiconductors…………..5 TABLE 1-2. Physical Properties of wurtzite ZnO………..8 TABLE 2-1. Summarizes some of the studies on solution growth and the resulting structures………..33 TABLE 2-2. Raman measurement configuration needed to observe the phonon
modes in hexagonal ZnO………..51 TABLE 5-1. Wave number, broadening and the ratio of n-LO phonons found in
RRS spectra. The assignments of bulk ZnO are also listed as a reference…..109 TABLE 6-1. Performances and electron transport properties of the bare ZnO
nanowire and the branched ZnO nanowire DSCs determined by photocurrent density-voltage (J-V) characteristics and electrochemical impedance spectroscopy (EIS) analysis………156 TABLE 6-2. Kinetic Parameters of the Idoline Dye Emission Decay Analysis….167 TABLE 6-3. Performances and electron transport properties of the D149- and
D205-sensitizered DSCs (27µm-thick ZnO photoelectrode) determined by J-V characteristics and EIS analysis……….175
List of Figures
Fig. 1-1. Stick-and-ball representations of ZnO crystal structures……...………2 Fig. 1-2. Primitive cell of the wurtzite-structure and schematic drawing of surfaces cut from a hexagonal single crystal with different surface planes…. …………...3 Fig. 1-3. Energy levels of native defects in ZnO. …...………..7 Fig. 1-4. Density of states in different dimensional systems. ...……….9 Fig. 2-1. Schematic illustration of vapor-solid growth mechanism………21 Fig. 2-2. In situ TEM images recorded during the process of nanowire growth…..22 Fig. 2-3. A schematic diagram of the horizontal tube furnace for growth of ZnO
nanostructures by the solid-vapor phase process……….23 Fig. 2-4. Scanning electron microscope images of ZnO nanowire arrays grown on sapphire substrates………24 Fig. 2-5. Side-view SEM images of ordered ZnO nanorod arrays and hexagonally ordered ZnO nanorod arrays grown by the VLS method on patterned Au-covered
GaN/Al2O3 substrates………24
Fig. 2-6. A collection of nanostructures of ZnO synthesized under controlled conditions by thermal evaporation of solid powders………26 Fig. 2-7. SEM images of the 6-fold ZnO nanostructures………...…...27 Fig. 2-8. ZnO nanorods grown using low-pressure MOVPE on Al2O3 (0001)
substrates………29 Fig. 2-9. FESEM image of 3-dimensional arrays of ZnO nanorods by using aqueous solution chemical method……….31 Fig. 2-10. ZnO nanowire array on a four-inch (ca. 10 cm) silicon wafer and ZnO
nanowire array on a two-inch (ca. 5 cm) PDMS substrate………..32 Fig. 2-11. Micrograph of an anodic alumina membrane (AAM) ……….34 Fig. 2-12. Schematic of the rotes that one could follow within the scope of sol-gel
processing……….37 Fig. 2-13. FESEM and HRTEM images of hierarchically ordered cone-shaped ZnO
nanocrystals. ……….39 Fig. 2-14. TEM of images OA-stabilized cone-shaped ZnO nanocrystals. …...…..39 Fig. 2-15. TEM images ZnO nanoparticles obtained in the presence of various
surfactants………..40 Fig. 2-16. X-ray diffraction from 2-dimensional periodic lattices………41 Fig. 2-17. Instrumental setup for high-resolution x-ray rocking curve……….43 Fig. 2-18. Three different concepts of excitons. Excitons ( bounded electron-hole pairs)
also can be viewed as the excited states of molecules………..45 Fig. 2-19. A pair excitation in the scheme of valence and conduction band in the
exciton picture for a direct gap semiconductor………46 Fig. 2-20. Visualization of an exciton bound to an ionized donor (D+X), a neutral
donor (D0X), and a neutral acceptor (A0X)………48
Fig. 2-21. Displacement patterns of the optical phonons of a lattice with wurtzite crystal structure………51 Fig. 3-1. A schematic diagram of the experimental apparatus for growth of ZnO nanowires……….……….56 Fig. 3-2. Raman detection systems. Light pass for triple additive configuration.…..61 Fig. 4-1. Top- and oblique-view SEM photographs of the vertically well-aligned ZnO nanorods fabricated on the various substrates:……….………64
Fig. 4-2. Magnified top-view SEM photographs of the vertically well-aligned ZnO nanorods fabricated on the Si (111), and sapphire (0001) substrates………...…65 Fig. 4-3. The EBSD image taken from two different position of the ZnO nanorods on the Si (111) substrate………66 Fig. 4-4. The inverse pole figures of the ZnO nanorods on the Si (111) substrate for the three orthogonal directions……….…66 Fig. 4-5. The EBSD image taken from two different position of the ZnO nanorods on the sapphire (0001) substrate………67 Fig. 4-6. The inverse pole figures of the ZnO nanorods on the sapphire (0001)
substrate for the three orthogonal directions………67 Fig. 4-7. Cross-sectional TEM images and the SAED images of ZnO nanorods grown on ZnO buffer layer……….………...…...68 Fig. 4-8. θ-2θ XRD profile of ZnO buffer layers on the various substrates………...70 Fig. 4-9. X-ray Φ-scan profiles of ZnO (20 2 2) plane from ZnO buffer layers on the various substrates………..………...71 Fig. 4-10. Typical room-temperature photoluminescence (RTPL) spectra of ZnO nanorods on the various substrates………...………..………..73 Fig. 4-11. SEM images of vertically well-aligned ZnO nanowires grown on
GaN-buffered sapphire substrate………..………75 Fig. 4-12. TEM image of wurtzite structure ZnO nanowire and the corresponding
electron diffraction pattern………...77 Fig. 4-13. θ-2θ XRD profile of ZnO nanowires on epi-GaN(001)/α-Al2O3(001)…....78 Fig. 4-14. Reciprocal space map of asymmetric (20 2 4) diffraction spots of a ZnO
Fig. 4-15. Conventional Raman spectrum (using frequency-doubled Yb:YAG laser (λ = 515 nm)) and resonant Raman scatterings (RRS) (using a He–Cd laser ( λ = 325 nm)) of ZnO nanowires on GaN buffer layer at room temperature……...82 Fig. 4-16. Fitting profile of RRS spectrum and its decomposition……….………..…84 Fig. 4-17. Room-temperature and low temperature photoluminescence (PL) spectrum of ZnO nanowires on GaN buffer layer……….….86 Fig. 4-18. Dependence of integrated PL band intensity of 3.343 eV and 3.364 eV on excitation laser intensity at T = 10K………87 Fig. 4-19. The SEM images of the ZnO nanowires and nanorods with different growth conditions…...…89 Fig. 4-20. The schematic growth procedure from the original ZnO nanowires to the
branched ZnO nanowires and their corresponding FESEM images…...91 Fig. 4-21. Low- and high-magnification FESEM images of the branched ZnO nanowires after second growth………...………...………...92 Fig. 4-22. The cross section FESEM images of successful branched ZnO nanowires
and failed branched ZnO nanowires. ………...93 Fig. 4-23. TEM image of a single branched ZnO nanowire………..94 Fig. 4-24. θ-2θ XRD profiles and conventional Raman spectra of the branched ZnO nanowires………95 Fig. 5-1. Large and local scale of scanning electron micrographs of various aging
time products synthesized using 10ml of primary supernatan..…………..…100 Fig. 5-2. Composition variation analysis by energy dispersive x-ray spectra (EDS) of different aging time products……….…….100 Fig. 5-3. TEM images of secondary ZnO NPs recognized of crystalline
Fig. 5-4. SEM micrographs of as-grown secondary ZnO NPs and after 1hour annealing at temperature 350˚C and 500˚C, respectively...…….103 Fig. 5-5. Normal Raman spectra of as-grown secondary ZnO NPs and after 1hour annealing at temperature 350˚C and 500˚C, respectively, using a frequency-doubled Yb:YAG laser ( = 515 nm)……….……….105 Fig. 5-6. Resonant Raman scatterings (RRS) of as-grown secondary ZnO NPs and
after 1hour annealing at temperature 350˚C and 500˚C, respectively, using a He–Cd laser ( = 325 nm)………107 Fig. 5-7. Normalized room-temperature PL spectra of the secondary ZnO NPs before and after heat treatment, respectively……….111 Fig. 5-8. TEM image of the ZnO QDs fabricated using 0.06M precursor with the inset of its corresponding selected area electron diffraction (SAED) pattern…113 Fig. 5-9. XRD profiles of the ZnO QDs prepared with various concentration of
Zn(OAc)2……….114
Fig. 5-10. Room temperature PL spectra and green emission of ZnO QDs with various sizes; and PL spectra of ZnO QDs (4.2 nm in diameter) as a function of laser power………..………...116 Fig. 5-11. The dependence of the band gap enlargement versus the ZnO QDs diameter as calculated from the effective mass model………..………118 Fig. 5-12. Resonant Raman scatterings of ZnO QDs with various particle sizes
measured at room temperature using a He–Cd laser (λ = 325 nm)…………...120 Fig. 5-13. Ratio between the second- and the first-order Raman scattering cross
section as a function of ZnO diameter…………..………..121 Fig. 6-1. Energy band diagram of a conventional p-n junction solar cell in the case of short circuit condition and charge separation under illuminations……….131 Fig. 6-2. Scheme of the operational principle of the DSCs. ………132
Fig. 6-3. Dynamics of different electron transfer processes in the conversion of light to electric power by a DSC……….133 Fig. 6-4. Flow chart of device assembly………...139 Fig. 6-5. Spectra of Black body 5250˚C AM0 and AM1.5, respectively…………. 141 Fig. 6-6. A typical I-V curve in the experiment……….………..………..143 Fig. 6-7. A simplified equivalent circuit for DSC..………145 Fig. 6-8. Equivalent circuit model for DSCs using TiO2 photoanode………….…..148 Fig. 6-9. Typical curves of impedance spectra for a DSC………..………150 Fig. 6-10. Current density against voltage (J-V) characteristics of the bare ZnO nanowires and the branched ZnO nanowire DSCs……….………153 Fig. 6-11. Nyquist plots and the fitting results of the bare ZnO nanowires and the
branched ZnO nanowire DSCs……….………..154 Fig. 6-12. The incident monochromatic photon to current conversion efficiency (IPCE)
of the bare ZnO nanowire and the branched ZnO nanowire DSCs………157 Fig. 6-13. Optical absorption of dye detached from the bare ZnO nanowire and the
branched ZnO nanowire substrates………158 Fig. 6-14. FESEM and TEM images for the self-assembled ZnO secondary
nanoparticles and the schematic multiple scattering of light within the hierarchical ZnO photoelectrode composed by self-assembled ZnO secondary nanoparticles……….160 Fig. 6-15. Diameter distribution for the ZnO secondary nanoparticles and the
corresponding optical absorption spectra of ZnO photoelectrodes with various film thicknesses, from 2µm to 12 µm……….161 Fig. 6-16. Molecular structures of indoline D149 and D205 dyes………..162
Fig. 6-17. Absorption and photoluminescence spectra of D149 and D205 dyes in tert-butyl alcohol/acetonitrile (1/1) solution and D149 and D205 dyes anchored on the 4µm-thick ZnO photoelectrodes………..163 Fig. 6-18. (a) Photoluminescence decay of D149 and D205 dyes in tert-butyl
alcohol/acetonitrile (1/1) solution and on ZnO photoelectrodes, respectively. (b) Schematic representation of the charge transfer of photo-excited indoline dyes anchored onto ZnO surfaces.……….……….166 Fig. 6-19. Photocurrent action spectra of ZnO DSCs constructed using D149 and D205,
with different photoelectrode thicknesses………..169 Fig. 6-20. Relationship between photovoltaic characteristics and photoelectrode
thickness of ZnO DSCs. Red circles and blue squares represent D149- and D205-sensitizered DSCs, respectively……….171 Fig. 6-21. Photovoltaic characteristics and Nyquist plots of DSCs with 27µm-thick
ZnO photoelectrodes and two different indoline dyes (D149 and D205)……172 Fig. 6-22. The equivalent circuit model of ZnO DSCs composed with hierarchical
nanoparticles………...174 Fig. 6-23. The cell behaviors of D205-sensertized DSCs composed with 23 µm-thick primary nanoparticles (via girding the secondary ZnO nanoparticles) with 4µm-thick commercial ZnO particles (Merck Ltd.) as a scattering layer on the top.……….……….177
Chapter 1 Introduction
1.1 General Properties of ZnO
1.1.1 Prelude
There has been a great deal of interest in zinc oxide (ZnO) semiconductor materials lately, as seen from a surge of a relevant number of publications. The interest in ZnO is fueled and fanned by its prospects in optoelectronics applications owing to its direct wide band gap (Eg ~3.3 eV at 300 K). Some optoelectronic applications of ZnO overlap with that of GaN, another wide-gap semiconductor (Eg ~3.4 eV at 300 K) which is widely used for production of green, blue-ultraviolet, and white light-emitting devices. However, ZnO has some advantages over GaN among which are the availability of fairly high-quality ZnO bulk single crystals and a large exciton binding energy (~60 meV, cf. ~25 meV for GaN). ZnO also has much simpler crystal-growth technology, resulting in a potentially lower cost for ZnO-based devices.
1.1.2 Crystal Structure
ZnO is an oxide of the group II metal zinc, and belongs to the P63mc space group in the Hermann–Mauguin notation [1]. ZnO is on the borderline between a semiconductor and an ionic material. Under most growth conditions, ZnO is an n-type semiconductor, though p-type conductivity of ZnO has also been reported for growth under certain conditions [2, 3]. The crystal structures shared by ZnO are wurtzite, zinc blende, and rocksalt (or Rochelle salt) as schematically shown in Fig. 1-1. Under ambient conditions, the thermodynamically stable phase is that of wurtzite symmetry. The zinc blende ZnO structure can be stabilized only by growth on cubic substrates,
and the rocksalt (NaCl) structure may be obtained at relatively high pressures. The wurzite crystal structure is shown in Fig. 1-2a. The lattice parameters of ZnO are a = 0.32495 nm and c = 0.52069 nm at 300K, with a c/a ratio of 1.602, which is close to the c/a = 8/3= 1.633 ratio of an ideal hexagonal close-packed (hcp) structure. In the direction parallel to the c-axis, the Zn-O distance is 0.1992 nm, and it is 0.1973 nm in all other three directions of the tetrahedral arrangement of the nearest neighbors. Even though it is tetrahedrally bonded, the bonds have a partial ionic character.
The lattice is composed of two interpenetrating hexagonal close-packed sublattices, each of which consists of one type of atom displaced with respect to each other along the threefold c-axis by the amount of u = 0.3825 (u = 3/8 = 0.375 for an ideal wurtzite structure) in fractional coordinates. The internal parameter u is defined as the length of the bond parallel to the c-axis (anion–cation bond length or the nearest-neighbor distance) divided by the c lattice parameter. The wurtzite-structure lattice is fourfold coordinated. That is, each atom has four nearest neighbor atoms. In
Figure 1-1. Stick-and-ball representations of ZnO crystal structures: (a) cubic rocksalt, (b) cubic zinc blende, and (c) hexagonal wurtzite. Shaded gray and black spheres denote Zn and O atoms, respectively.
and oxygen occupies the (0, 0, 0) and (0.6667, 0.3333, 0.5) positions [4, 5]. Figure 1-2(b) shows the cuts of different orientations of a hexagonal hcp structure.
The close-packed (0001) planes are made up of two subplanes (A and a), each consisting of either the cationic (Zn) or the anionic (O) species, respectively. The crystal can be considered to have the stacking sequence …AaBaAaBb… as compared to …AaBaCcAaBbCc… in diamond cubic silicon and sphalerite (GaAs). The result is a remarkable difference in the properties between (0001) and (0001) planes of ZnO, the former being Zn terminated and the later being O terminated. This structure does not possess a center of symmetry. The lack of inversion symmetry in ZnO leads to piezoelectricity. The polarity of the c-axis results in the Zn-terminated and O-terminated planes displaying extremely different properties.
The (0001) planes in ZnO are polar and hence, with no reconstructure or passivation, have the maximum surface energy among the low-index planes. This is in fact observed under most conditions during vapor phase growth. Crystals grown via
Figure 1-2. (a) Primitive cell (heavy lines) of the wurtzite-structure lattice placed within a hexagonal prism. a and c are the lattice constants. (b) Schematic drawing of surfaces cut from a hexagonal single crystal with different crystallographic orientations (surface planes).
the vapor phase usually are rod-shaped with hexagonal cross section. The crystals are elonged along [0001] direction and the prismatic sides of these crystals are usually the {1010} or {11 2 0} planes, implying that the (0001) plane has the highest energy. As a result, the growth rate along the c-axis is the highest.
1.1.3 Optical Properties
Optical properties and processes in ZnO as well as its refractive index were extensively studied many decades ago. Compendiums dealing with optical properties of ZnO and to some extent its alloys from far infrared to vacuum ultraviolet including phonons, plasmons, dielectric constant, and refractive indices are available in the literatures [6, 7]. The renewed interest in ZnO is fuelled and fanned by prospects of its applications in optoelectronics owing to its direct wide band gap (Eg~3.3 eV at 300 K), large exciton binding energy (~60 meV, Refs [8, 9]), and efficient radiative recombination. The large exciton binding energy paves the way for an intense near band-edge excitonic emission at room and even higher temperature, because this value is 2.4 times the room-temperature (RT) thermal energy (kBT = 25 meV).
Therefore, laser operation based on excitonic transitions, as opposed to electron–hole plasma, is expected. In this respect, there have also been a number of reports on laser emission from ZnO-based structures at room temperature and beyond. An appreciation of the potential of ZnO for optoelectronic applications can be obtained by examining Table 1-1, which compares the relevant material properties of ZnO with those of other wide band gap semiconductors.
The optical properties of a semiconductor have their genesis in both intrinsic and extrinsic effects. Intrinsic optical transitions take place between the electrons in the conduction band and the holes in the valence band, including excitonic effects caused
by the Coulomb interaction. Excitons are classified into the free and bound excitons. In high-quality samples with low impurity concentration, the excited states of free excitons can also be observed in addition to their ground-state transitions. Extrinsic properties are related to dopants/impurities or point defects and complexes, which usually create electronic states in the band gap and therefore influence both optical absorption and emission processes. The electronic states of the bound excitons strongly depend on the semiconductor material, in particular the band structure. In theory, excitons could be bound to neutral or charged donors and acceptors. A basic assumption in the description of the principal bound exciton states for neutral donors and acceptors is a dominant coupling of the like particles in the bound exciton states. For a shallow neutral donor-bound exciton, for example, the two electrons in the bound exciton state are assumed to pair off into a two-electron state with zero spin. The additional hole is then weakly bound in the net hole-attractive Coulomb potential set up by this bound two-electron aggregate. Similarly, shallow neutral acceptor bound excitons are expected to have a two-hole state derived from the topmost valence band and one electron interaction. These two classes of bound excitons are by far the most important cases of extrinsic processes. Other extrinsic transitions could TABLE 1-1. An appreciation of the potential of ZnO for optoelectronic applications can be obtained by examining this table, which compares the relevant material properties of ZnO with those of other wide band gap semiconductors. [10]
be seen in optical spectra such as free-to-bound (electron-acceptor), bound-to-bound (donor-acceptor), and the so-called yellow/green luminescence. The mostly observed green emission in ZnO luminescence spectra (manifesting itself as a broad peak around 500–530 nm), observed nearly in all samples regardless of growth conditions, is related to singly ionized oxygen vacancies by some and to residual copper impurities by others. A requisite consensus on this issue is still lacking.
1.1.4 Defects in ZnO
Characteristically, defects represent one of the controversial areas of semiconductors, and ZnO is no exception, as the measurement techniques are not able to correlate electrical or optical manifestation of defects to their origin precisely. It is highly appropriate to say that the point defects in ZnO are not so well understood. While numerous assignments of the defect-related luminescence bands can be found in literature, only a few of them are trustworthy [11-13]. Over the years, oxygen vacancies were believed to be the dominant shallow donors in ZnO. Now it is becoming clear that these vacancies are formed in noticeable concentrations only after electron irradiation. As another vital misassignment till now, the green luminescence band in ZnO is commonly attributed to transitions from the oxygen vacancy (Vo) to the valence band. However, it is easy to show that such transition is highly unlikely in n-type ZnO. Problems in the identification of point defects are discussed from theoretical and experimental points of view in this section. It is also important to realize that ZnO is a relatively open structure, with a hexagonal close packed lattice where Zn atoms occupy half of the tetrahedral sites. All the octahedral sites are empty. Hence, there are plenty of sites for ZnO to accommodate intrinsic, namely Zn interstitials (Zni) defects and extrinsic dopants.
The electronic energy levels of native imperfections in ZnO are illustrated in Fig. 3. There are a number of intrinsic defects with different ionization energies. The Kröger Vink notation uses: i = interstitial site, Zn = zinc, O = oxygen and V = vacancy. The terms indicate the atomic sites, and superscripted terms indicate charges, where a dot indicates positive charge, a prime indicates negative charge, and a cross indicates zero charge, with the charges in proportion to the number of symbols. Figure 3 shows that there are a number of defect states within the bandgap of ZnO. The donor defects are: Zn•i•, Zn•i, Zn×i, V•o•, V•o, Vo and the acceptor defects are: V''
Zn, V
'
Zn. The defect ionization energies vary from ~0.05-2.8 eV. Zn interstitials and oxygen vacancies are known to be the predominant ionic defect types. However, both defects donate two electrons and so it is difficult to distinguish one from the other using the electrical measurements. As a result which defect dominates in native, undoped ZnO is still a matter of great controversy.
Table 1-2 shows a compilation of basic physical parameters for ZnO. It should be noted that there still exists uncertainty in some of these values. For example, there have few reports of p-type ZnO and therefore the hole mobility and effective mass are still in debate. Similarly, the values for thermal conductivity show some spread in values and this may be a result of the influence of defects such as dislocations. The values for carrier mobility will undoubtedly increase as more control is gained over compensation and defects in the material.
1.2 Low-Dimensional ZnO Nanostructures
1.2.1 General of Nanostructures
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. Generally, the crystal structure of a solid restricts the movement of carriers. In a semiconductor material, the outer electrons of the atoms are delocalized over the entire crystal, with the periodicity of the crystal structure limiting their movement. For certain electron energy, the carrier is allowed to move in one direction, but its motion in a different direction is restricted as a result of destructive interaction from the atomic lattice. This dependence of the electron energy on the momentum of the carrier results in a structure of energy bands where the carrier can exist. The electron energy of low-dimensional semiconductor structures becomes quantized and depends on the structural size. The band gap and density of states (DOS) associated with a quantum-structure differs from that associated with bulk material, determined from the magnitude of the three-dimension wave vector. Figure 1-4 illustrated the density of states in systems of differing dimensionality: bulk, quantum well, quantum wire, and quantum dot.
The degree of quantum confinement is determined by the interaction length over which the bond between an electron and a hole extends in an exciton, compared with the size of the materials. Take quantum dot for example, in weak quantum confinement, the interaction length is shorter than the dimensions of the quantum dot, resulting in very closely spaced quantum-confined energy states. In this case, the quantum dot seems more bulk-like because the electron and hole can separate beyond the exciton interaction length, thus breaking up the exciton and making the transition energy independent of the quantum dot size. In the strong quantum-confinement regime, the quantum dot size is smaller than the electron hole interaction length, resulting in widely spaced energy states. The quantum dot sizes can be so small that the energy spacing between the allowed states in the conduction and valence bands is large enough to cause a so-called phonon bottleneck.
In most semiconductors, excited electrons can relax down to the bottom of the conduction band by losing energy through carrier-carrier and carrier-phonon interactions. However, a phonon bottleneck occurs when the spacing between energy states is much larger than these interelectronic energies (as is the case in strong quantum confinement). In this case, the probability for a carrier in a higher energetic state to lose its energy to a lower energetic state through these interactions becomes very low. As a result, electrons cannot relax to the bottom of the conduction band, and so quantum dots with strong quantum confinement should not be expected to emit light from the normal transition across the band gap. However, this phonon bottleneck has been observed experimentally first in recent years. [16]
According to these above mentions, the quantum confinement effect come from nanostructures become predominant and give rise to many interesting electronic and optical properties.
1.2.2 One-Dimensional (1-D) ZnO Nanostructures
One-dimensional (1-D) nanostructures have attracted intensive attention because of their unique physical, optical and electrical properties resulting from their low dimensionality. The electron-hole interaction will have orders of magnitude enhancement in a nanostructure, due to the dramatically increased electron density of states near the van Hove singularity. ZnO has an effective electron mass of ~0.24 me, and a large exciton binding energy of 60 meV. Tremendous progress has been made to understand the quantum-size behavior and to investigate the size- and morphology-dependent properties. Compared with bulk ZnO, the reported 1-D ZnO nanowires or nanorods have the same lattice constant. However, the significant characteristics of 1-D ZnO nanostructures are their high surface-to-volume ratio and anisotropic carrier transport. With this respect great efforts and contribution from the fruitful group such as Yang’s [17] and Wang’s group [18] for ZnO nanostructures, have been made persistently to enrich the diversiform morphological world of nanostructures and show their possibilities for versatile utilizations, such as room temperature laser [19], waveguide [20] and piezoelectric nanogenerators [21, 22].
1-D ZnO nanowires and nanorods are also being intensively investigated because they possess a combination of attractively optical, electronic, mechanical, magnetic properties and so on. Extensively potential applications as light-emitting diodes [23, 24], UV photodetectors [25-27], field-effect transistors [28-31], field electron emitters [32-35] gas sensors [36-40], and solar cells [41-47] are continuously be evaluated. Moreover, the observations of quantum confinement [48] and the discrete energy levels [49] are demonstrated in ZnO/ZnMgO nanorod heterostructures. Therefore, the ZnO nanowires and nanorods have become one of the most promising elemental building blocks in nanotechnology applications. Other 1-D ZnO nanostructures such
as nanoneedle, nanopencil, nanobelt, nanotube, nanospiral, nanohelix, nanonail and dendritic nanowire have also been discovered [50, 51], but lack of applications at present merely interesting growth mechanisms have been emphatically demonstrated.
To date, most of the work on 1-D nanostructures ZnO has focused on the synthesis methods. For application of nano-photonics and electronics, it is needed to create ZnO nanowires that are selective area growth, highly aligned and orientation-ordered on substrates. The fundamental optical properties, including the origin of luminescence, carrier-carrier interaction, stimulated emission and lasing are also needed to understand. Furthermore, band-gap engineering, which is the process of controlling or altering the band gap, and fabrication of heterostructure or quantum-structures of ZnO-based nanowires and nanorods are important issues.
1.2.3 Zero-Dimensional (0-D) ZnO Nanostructures
Zero-dimensional (0-D) ZnO nanoparticles (NPs) or quantum dots (QDs) with several nanometers in diameter and large surface to volume ratio are becoming more and more attractive due to their quantum size effect and therefore unique properties different from the bulk ZnO materials. The physical properties in ZnO nanostructures changed dramatically as the diameter closes to the exciton Bohr radius (~2.43 nm) of bulk ZnO [52].
Accordingly, a great deal of attention has been paid to nanocrystalline materials for the study of the size-dependent quantum confinement effects. However, the ZnO nanocrystals have not been investigated as much as other semiconductor nanocrystals such as CdSe despite the possibility of UV/blue applications. The greater part of investigations for 0-D ZnO nanostructures focus on the fundamentally physical phenomenons, such as band edge emission [53-59], phonon quantum confinement
[60-64], random Laser [65-67] and nonlinear susceptibility [68, 69].
Recently, several applications such as photocatalytic activity [70, 71] and cell labeling [72] have been reported owing to successfully faceted control and surface functionality of ZnO NPs and QDs, respectively. Organization of ZnO NPs and QDs building blocks into premeditated one- (1-D), two- (2-D), and three-dimensional (3-D) architectural systems should be considered as one of the key challenges in today’s science and engineering.
1.3 Research Motivation
The synthesis of an array of well-aligned ZnO nanowires and nanorods is of great interest because it is an imperative step to realize nanophotonic devices, which include light-emitting diodes and laser diodes. Various methods have been reported for fabricating arrays of well-aligned 1-D ZnO nanostructures that include vapor transports and condensation methods, template methods, metal-organic source vapor deposition methods, and buffer layer pre-coated methods. The conductive ZnO buffer layer behaves as the active nucleus for growth of ZnO nanorods on different substrate and patterned area for selective growth of ZnO arrays. Accordingly, the buffer layer pre-coated method is a promising candidate for photonic device applications. In order to speed up the practical use of such ZnO nanorod arrays, studies on the growth behavior of vertically well-aligned ZnO nanorods on various substrates for different applications seem more and more imperative. Furthermore, chemical approaches towards such 1-D ZnO nanostructures would also be of interest since they are easy to perform and allow for a facile scale-up procedure. In this respect, a solution-based approach will also be attempted.
confinement of carrier and phonon leads not only continuous tuning of the optoelectronic properties but also improvement in device performance. In this respect, optical properties examined from Raman scattering and photoluminescence (PL) of ZnO NPs with different particle sizes obtained via sol-gel method will be investigated. It is therefore important to realize the size-dependent quantum confinement effect and exciton-phonon interaction within the 0-D ZnO nanostructures from both the fundamental scientific research and photonic application points of view.
1.4 Organization of Dissertation
This dissertation is organized as follow. After Chapter of Introduction, Chapter 2 presents a brief review of growth and characterization techniques. The statement of experimental procedures for all ZnO nanostructures in my present work is illustrated in Chapter 3. The growth of the ZnO nanowires on various substrates via vapor-solid transport with pre-coated buffer layers is then demonstrated in Chapter 4. The structures and optical properties of well-aligned ZnO nanowires/nanorods on different substrate are compared. I also explain the growth mechanism and the epaxitial relationship between the 1-D ZnO nanostructures and the substrates. In Chapter 5, I describe self-assembled secondary ZnO nanoparticles (NPs), and size-controll ZnO quantum dots (QDs) synthesized by a simple sol-gel method. The coupling strength between electron and longitudinal optical phonon is carried out by resonant Raman spectroscopy (RRS). In Chapter 6, the dye-sensitized solar cells (DSC) with hierarchical architecture composed of various low-dimensional ZnO nanostructures are discussed.
Finally, in Chapter 7, I conclude the results and benefits on the low-dimensional ZnO nanostructures. The several issues for further works are also proposed.
References and Notes
[1] Hadis Morkoç and Ümit Özgür “Zinc Oxide: Fundamentals, Materials and Device Technology” (Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim, 2009). [2] K. Kim, H. Kim, D-K. Hwang, J-H. Lim and S-J. Park, Appl. Phys. Lett. 83, 63
(2003).
[3] Y. R. Ryu, T. S. Lee and, H. W. White, Appl. Phys. Lett., 83, 87 (2003).
[4] W. B. Person, “A handbook of Lattice Spacings and Structures of Metala and Alloys”, Vol.2, (New York: Pergamon Press, 1967).
[5] “International Tables for X-Ray Crystallography”, (The International Union of Crystallography, 1965).
[6] C. Bundesmann, R. Schmidt-Grund, And M. Schubert, Far-infrared to vacuum-ultra-violet optical properties of ZnO: phonons, plasmons, dielectric constants, refractive indices, band-toband transitions, and excitons, in Transparent Conductive Zinc Oxide, Springer Series in Materials Science, Vol. 104 (eds K. Ellmer, A. Klein and B. Rech). (Springer, Berlin, 2008).
[7] C. Bundesmann, “Phonons and plasmons in ZnO-based alloy and doped ZnO thin films studied by infrared spectroscopic ellipsometry and Raman scattering spectroscopy”, Ph.D. thesis, Universität Leipzig, 2005 (Shaker, Aachen, 2006) [8] D.C. Look, Mater. Sci. Eng. B, 80, 381(2001).
[9] Reynolds, D.C., Look, D.C., Jogai, B., Litton, C.W., Cantwell, G. and Harsch, W.C. Phy. Rev. B 60, 2340 (1999).
[10] Y. F. Chen, D. M. Bagnall, H. J. Koh, K. T. Park, K. Hiraga , Z. Q. Zhu, T. Yao, J. Appl. Phys. 84 3912 (1998).
[11] B. Lin, Z. Fu, Y. Jia, Appl. Phys. Lett. 79, 943 (2001).
S. Fung, W. M. Kwok, W. K. Chan, D. L. Phillips, L. Ding, and W. K. Ge, J. Phys. Chem. B 110, 20865 (2006).
[13] P. Jiang, J. Zhou, H. F. Fang, C. Y. Wang, Z. L. Wang, and S. S. Xie, Adv. Funct. Mater. 17, 1303 (2007).
[14] L. Schmidt-Mende and J. L. MacManus-Driscoll, Materials Today, 10 (11), 40 (2007).
[15] S. J. Pearton, D. P. Norton, K. Ip, Y. W. Heo, and T. Steiner., Prog. Mater. Sci. 50, 293 (2005).
[16] J. Urayama, T. B. Norris, J. Singh and P. Bhattacharya, Phys. Rev. Lett. 86, 4930 (2001).
[17] M. H. Huang, S. Mao, H. Feick, H. Yun, Y. Wu, H. Kind, E. Weber, R. Russo, and P. D.Yang. Science 292, 1897 (2001).
[18] Z. W. Pan, Z. R. Dai, and Z. L. Wang, Science 291, 1947 (2001).
[19] P. Yang, H. Yan, S. Mao, R. Russo, J. Johnson, R. Saykally, N. Morris, J. Pham, R. He, and H.-J Choi, Adv. Funct. Mater. 12, 323 (2002).
[20] J. C. Johnson, H. Yan, P. Yang, and R. J. Saykally, J. Phy.Chem. B 107, 8816 (2003).
[21] Z. L. Wang and J. Song, Science 312, 242 (2006).
[22] Y. Qin, X. Wang and Z. L. Wang1, Nature 451, 809 (2008).
[23] Y. C. Kong, D. P. Yu, B. Zhang, W. Fang, and S. Q. Feng, Appl. Phys. Lett. 78, 407 (2001).
[24] C. H. Liu, J. A. Zapien, Y. Yao, X. M. Meng, C. S. Lee, S. S. Fan, Y. Lifshitz, and S. T. Lee, Adv. Mater. 15, 838 (2003).
[25] H. Kind, H. Yan, B. Messer, M. Law, and P. Yang, Adv. Mater. 14, 158 (2002). [26] C. Soci, A. Zhang, B. Xiang, S. A. Dayeh, D. P. R. Aplin, J. Park, X. Y. Bao, Y. H.
Lo, and D. Wang, Nano Lett. 7, 1003 (2007).
[27] L. W. Ji, S. M. Peng, Y. K. Su, S. J. Young, C. Z. Wu, and W. B. Cheng, Appl.
Phys. Lett. 94, 203106 (2009).
[28] J. Goldberger, D. J. Sirbuly, M. Law, and P. Yang, J. Phys. Chem. B 109, 9 (2005).
[29] Z. Fan and J. G. Lu, Appl. Phys. Lett. 86, 032111 (2005).
[30] J. I. Sohn, S. S. Choi, S. M. Morris, J. S. Bendall, H. J. Coles, W-K. Hong, G. Jo, T. Lee, and M. E. Welland, Nano Lett. 10, 4316 (2010).
[31] H. T. Ng, J. Han, T. Yamada, P. Nguyen, Y. P. Chen, M. Meyyappan, Nano Lett. 4, 1247 (2004).
[32] C. J. Lee, T. J. Lee, S.C. Lyu, Y. Zhang, H. Ruh, H. J. Lee, Appl. Phys. Lett. 81, 3648 (2002).
[33] Y. W. Zhu, H. Z. Zhang, X. C. Sun, S. Q. Feng, J. Xu, Q. Zhao, B. Xiang, R. M. Wang , D. P. Yu, Appl. Phys. Lett. 83, 144 (2003).
[34] Y. K. Tseng, C. J. Huang, H. M. Cheng, I. N. Lin, K. S. Liu, I. C. Chen, Adv. Fun. Mater. 13 811 (2003).
[35] S. Y. Li, P. Lin, C. Y. Lee, T. Y. Tseng, J. Appl. Phys. 95, 3711 (2004).
[36] Z. Fan, D. Wang, P. Chang, W. Tseng, and J. G. Lu, Appl. Phys. Lett. 85, 5923 (2004).
[37] Z. Fan and J. G. Lu, Appl. Phys. Lett. 86, 123510 (2005)
[38] H. T. Wang, B. S. Kang, F. Ren, L. C. Tien, P. W. Sadik, D. P. Norton, S. J. Pearton, and J. Lin, Appl. Phys. Lett. 86, 243503 (2005).
[39] B.S. Kang, Y.W. Heo, C. Tien, D.P. Norton, F. Ren, B.P. Gila, S.J. Pearton, Appl. Phys. A 80 1029 (2005).
[41] M. Law, L. E. Greene, J. C. Johnson, R. Saykally, and P. D. Yang, Nat. Mater. 4 455 (2005).
[42] J. B. Baxter and E. S. Aydil, Appl. Phys. Lett. 86, 053114 (2005).
[43] A. D. Pasquier, H. Chen, and Y. Lu, Appl. Phys. Lett. 89, 253513 (2006).
[44] J. J. Wu, G. R. Chen, H. H. Yang, C. H. Ku, and J. Y. Lai, Appl. Phys. Lett. 90, 213109 (2007).
[45] Y. F. Hsu,1 Y. Y. Xi,1 A. B. Djurišić,1,a and W. K. Chan, Appl. Phys. Lett. 92,
133507 (2008).
[46] Y. Y. Lin, Y. Y. Lee, L. Chang,J. J. Wu, and C. W. Chen, Appl. Phys. Lett. 94, 063308 (2009).
[47] A. L. Briseno, T. W. Holcombe, A. I. Boukai, E. C. Garnett, S. W. Shelton, J. J. M. Fréchet, and P. Yang, Nano Lett. 10, 334 (2010).
[48] W. I. Park, G. C. Yi, M. Kim, S. J. Pennycook, Adv. Mater. 15, 526 (2003). [49] T. Yatsui, J. Lim, M. Ohtsu, S. J. An, G. C. Yi, Appl. Phys. Lett. 85, 727 (2004). [50] Z. L. Wang, J. Phys.: Condens. Matter 16 R829 (2004).
[51] Z. L. Wang,, Materials Today, 7 (6), 26 (2004).
[52] R. T. Senger, and K. K. Bajaj, Phys. Rev. B 68, 045313 (2003).
[53] D. W. Bahnemann, C. Kormann, and M. R. Hoffmann, J. Phys. Chem. 91 3789 (1987).
[54] L. Guo, S. Yang, C. Yang, P. Yu, J. Wang, W. Ge, G. K. L. Wong, Appl. Phys. Lett. 76 2901 (2000).
[55] E. A. Meulenkamp, J. Phys. Chem. B 102 5566 (1998).
[56] E. M. Wong, J. E. Bonevich, P. C. Searson, J. Phys. Chem. B 102 7770 (1998). [57] S. Sakohara, M. Ishida, M. A. Anderson J. Phys. Chem. B 102 10169 (1998). [58] S. Mahamuni, K. Borgohain, B. S. Bendre, V. J. Leppert, S. H. Risbud, J. Appl.
Phys. 85 2861 (1999).
[59] H. Zhou, H. Alves, D. M. Hofmann, W. Kriegseis, B. K. Meyer, G. Kaczmarczyk, A. Hoffmann, Appl. Phys. Lett. 80 210 (2002).
[60] V. A. Fonoberov, and A. Balandin, Appl. Phys. Lett. 85, 5971 (2004). [61] V. A. Fonoberov, and A. Balandin, Phys. Rev. B 70, 195410 (2004).
[62] K. A. Alim, V. A. Fonoberov, and A. Balandin, Appl. Phys. Lett. 86, 053103 (2005).
[63] K. A. Alim, V. A. Fonoberov, M. Shamsa, and A. Balandin, J. Appl. Phys. 97, 124313 (2005).
[64] V. A. Fonoberov, K. A. Alim, and A. Balandin, Phys. Rev. B 73, 165317 (2006). [65] H. Cao, Y.G. Zhao, H. C. Ong, S. T. Ho, J. Y. Dai, J. Y. Wu, and R.P.H. Chang,
Appl. Phys. Lett. 73, 3656 (1998).
[66] H. Cao, Y.G. Zhao, S. T. Ho, E. W. Seelig, Q. H. Wang, R. P. H.Chang, Phys. Rev. Lett. 82, 2278 (1999).
[67] X. Wu, A. Yamilov, H. Noh, H. Cao, E. W.Seelig, R. P. H.Chang, J. Opt. Soc. Am. B 21, 159 (2004).
[68] L. Guo, S. Yang, C. Yang, P. Yu, J. Wang, W. Ge, G. K. L. Wong, Appl. Phys. Lett. 76 2901 (2000).
[69] Y. Zhu, H. I. Elim, Y. L. Foo, T. Yu, Y. Liu, W. Ji, J. Y. Lee, Z. Shen, Andrew T. S. Wee, J. T. L. Thong, and C. H. Sow, Adv. Mater. 18 587 (2006).
[70] A. Mclaren, T. Valdes-Solis, G. Li, and S. C. Tsang, J. AM. CHEM. SOC. 131, 12540 (2009).
[71] L. Zhang, L. Yin, C.Wang, N. Lun and Y. Qi, ACS Appl. Mater. Interface 2 1769 (2010).
Chapter 2 A Review of Growth and
Characterization Techniques
2.1 Growth Methods
2.1.1 Growth of 1-D ZnO Nanowires/ Nanorods
Vapor-Liquid-Solid (VLS) Method. Among all vapor based methods, the VLS
method, which is a growth mechanism based on chemical vapor transport, seems to be the most successful for fabricating nanowires with single crystalline structures and in relatively large quantities. This process was first developed by Wagner et al. to produce Si single crystalline micro-whiskers in 1960s [1], and recently re-examined successfully by Lieber [2] and Yang [3]. The key factor is needed to deposit metal clusters such as Fe, Co, Ni, Cu, Sn and Au as the catalysts. A typical VLS process starts with the dissolution of gaseous reactants into nano-sized liquid droplets of catalyst metal while the liquid droplets are supersaturated with the guest material, followed by nucleation and growth of single crystalline nanorods and then nanowires. The 1D growth is mainly induced and dictated by the liquid droplets, whose size remains essentially unchanged during the entire process of nanowire growth. In the sense, each of liquid droplets serves as a soft template to strictly limit the lateral growth of an individual nanowire. As a major requirement, there should exist a good solvent capable of forming liquid alloy with the target material, ideally they should be able to form eutectic compounds. All of the major steps involved in a VLS process for a Ge nanowire case are schematically illustrated in Fig. 2-1 [3]. In the beginning Ge and Au form liquid alloys when the temperature is raised above the eutectic point. Once the liquid droplet is supersaturated with Ge, growth of nanowire takes place at the solid-liquid interface. The vapor pressure of Ge in the chemical-vapor-deposition
Figure 2-1. Schematic illustration of vapor-solid growth mechanism including three stages (I) alloying, (II) nucleation and (III) axial growth. Three stages are projected onto the coventional Au-Ge phase diagram; (b) shows the compositional and phase evolution during the nanowire growth process. [3]
system has to be kept sufficiently low so that the second ordinary nucleation will be completely suppressed. Figure 2-2 shows a sequence of real-time TEM images during the growth of a Ge nanowire.
Both physical methods (thermal evaporation and laser ablation) and chemical methods (chemical vapor transport and deposition) have been employed to generate the vapor species required for the growth of nanowires, and no significant difference was found in the quality of nanowires produced by these methods.
Figure 2-2. In situ TEM images recorded during the process of nanowire growth. (a) Au nanoclusters in solid state at 500 °C; (b) alloying initiated at 800 °C, at this stage Au exists mostly in solid state; (c) liquid Au/Ge alloy; (d) the nucleation of Ge nanocrystal on the alloy surface; (e) Ge nanocrystal elongates with further Ge condensation and eventually forms a wire (f).[3]
The VLS processes are usually carried out in a horizontal tube furnace, as shown in Fig. 2-3. In this schematic, the carrier gas, Ar, is introduced from the left end of the alumina tube and is pumped out from the right end. The source material is loaded on an alumina boat and positioned at the center of the highest temperature zone in the alumina tube. The substrate temperature usually drops with the distance from the position of the source material(s). The local temperature where the substrate is situated (usually 500-700 °C) determines the type of product that will be obtained. To reduce the decomposition temperature, ZnO powder is usually mixed with graphite powder to form the source mixture. At temperatures 800-1100 °C, graphite reacts with ZnO to form Zn, CO, and CO2 vapors, which then react on the substrate to form ZnO nanostructures.
Figure 2-3. A schematic diagram of the horizontal tube furnace for growth of ZnO nanostructures by the solid-vapor phase process.
[4]. As mentioned above, selective nanowire growth could be readily achieved by patterning the Au thin film before growth. Typical scanning electron microscope (SEM) images of nanowire arrays grownon a-plane sapphire (112 0) substrates with patterned Au thin film are presented in Fig. 2-4. By adjusting the growth time, nanowires could be grownup to 10 mmin length. The diameters of these wires range from 20 to 150nm, although more than 95% of themhave diameters between 70 and 100nm.
Klingshirn’s group also identified well-defined lasing modes under optical excitation of ZnO nanorods grown in a similar fashion using catalyst-assisted VLS technique by employing self-organized polystyrene spheres on GaN substrates as a mask during Au evaporation [5]. Ordered arrays of [0001]-oriented ZnO nanorods with 200nm diameter and 4.7 mm length were obtained with 500nm rod-to-rod spacing as shown in Fig. 2-5. As alluded earlier, well-aligned ZnO nanorods provide a perfect gain medium as well as act as waveguides and Fabry-Perot resonators with well-defined cavity ends.
Figure 2-4. Scanning electron microscope images of ZnO nanowire arrays grown on sapphire substrates (a–e). A top view of the well-faceted hexagonal nanowire tips is shown in (e). (f) High-resolution TEM image of an individual ZnO nanowire showing its [0001] growth direction.
Figure 2-5. (a) 45˚ side-view SEM images of ordered ZnO nanorod arrays and (b) hexagonally ordered ZnO nanorod arrays grown by the VLS method on patterned Au-covered GaN/Al2O3 substrates. The inset is the top view of the nanorod arrays.
Vapor-Solid (VS) Method. The vapor-solid (VS) method is another chemical vapor transport mechanism compared with VLS, depending on the presence of a metal catalyst. VS growth also holds for the growth of 1D ZnO nanomaterials. In this process, evaporation, chemical reduction or gaseous reaction first generates the vapor. The vapor is subsequently transported and condensed onto a substrate. The VS method has been used to prepare whiskers of oxide, therefore, possible to synthesize the 1D nanostructures if one can control the nucleation and the subsequent growth process. According to the difference on nanostructure formation mechanisms, the extensively used vapor transport process can be categorized into the catalyst free VS process and catalyst assisted VLS process. Synthesis utilizing VS process is usually capable of producing a rich variety of nanostructures, including nanowires, nanorods, nanobelts, nanohelix and other complex structures.[6-7]
Using a solid state thermal sublimation process and controlling the growth kinetics (VS growth mechanism), local growth temperature, and the chemical composition of the source materials, a wide range of nanostructures of ZnO have been synthesized by Wang’s group, as shown in Fig. 2-6 [7]. The two important characteristics of the wurtzite structure are the noncentral symmetry and polar surfaces. The structure of ZnO, for example, can be described as a number of alternating planes composed of tetrahedrally coordinated O2- and Zn2+ ions, stacked alternately along the c-axis. The oppositely charged ions produce positively charged (0001)-Zn and negatively charged (0001)-O polar surfaces, resulting in a normal dipole moment and spontaneous polarization along the c-axis, as well as a divergence in surface energy. As a result, the formation of a self-coiled, coaxial, multilooped nanoring structure is spontaneous, which means that the self-coiling along the rim proceeds as the nanobelt grows under the driving force of stacking the polar surfaces.
Figure 2-6. A collection of nanostructures of ZnO synthesized under controlled conditions by thermal evaporation of solid powders. [7]
A variety of novel hierarchical nanostructures with 6-, 4-, and 2-fold symmetries have been successfully grown by a vapor transport and condensation technique from Ren’s group, as shown in Fig. 2-7 [8] . It was found that the major core nanowires are single-crystal In2O3 with 6, 4, and 2 facets, and the secondary nanorods are single-crystal hexagonal ZnO and grow either perpendicular on or slanted to all the facets of the core In2O3 nanowires. Because no catalyst is used in the system, the In2O3 nanowire growth should be based on the vapor-solid mechanism. On the other hand, it is more difficult to define the ZnO nanorod growth mechanism. Probably ZnO nanorods also grow based on the vapor-solid mechanism because the In2O3 core is covered by a ZnO layer that can be the base for further ZnO nanorod growth. Compared to the aligned ZnO grown by the vapor-liquidsolid mechanism [4] with source temperature around 900 °C, the metal and/or metal oxide vapor pressure here
Figure 2-7. SEM images of the 6-fold ZnO nanostructures. (a) SEM image showing the abundance of the 6S-fold symmetry. Scale bar, 10 µm. (b) SEM image showing the 6M-fold symmetry. (c) High magnification SEM image of the 6S-fold symmetry. (d) High magnification SEM image of the 6M-fold symmetry. (e) Head on look at a 6S-fold symmetry to show the hexagonal nature of the major core nanowire. [8]
is much higher. This high vapor pressure is necessary for the growth of the hierarchical structures. The growth conditions such as temperature, pressure, and source component ratios are correlated to affect the supersaturation rate and the structure formed.
Metal-Organic Chemical Vapor Deposition (MOCVD) Method. For device fabrication, heteroepitaxial growth with control over impurities and thickness down to nanometer scale is required. VLS method is limited and cannot completely meet these requirements. Growth of complex structures for device applications can only be accomplished by advanced epitaxial methods such as metal-organic chemical vapor deposition (MOCVD) or metal-organic vapor phase epitaxy (MOVPE) and molecular
beam epitaxy (MBE). Particularly, MOCVD has been proven to be a powerful technique for large-scale production with accurate control over doping and thickness.
For MOCVD growth of ZnO nanorods, usually diethylzinc and oxygen are employed as the reactants and argon as the carrier gas [9]. N2O as the oxygen source and nitrogen as the carrier gas have also been used [10]. Typical growth temperatures range from 400 to 1050 °C. The growth occurs without a catalyst, and flat terraces and steps are observed at the nanorod tips resulting from the layer-by-layer growth mode, instead of the metallic nanoparticles characteristic to catalyst-assisted VLS processes.
Figure 2-8 shows ZnO nanorods grown by Park et al. [9] using low-pressure MOVPE on Al2O3 (0001) substrates at 400 °C without any metal catalysts. Very thin ZnO buffer layers were deposited at a low temperature before the nanorod growth. The mean diameter of nanorods obtained by MOVPE was as small as 25 nm, smaller than the typical diameters of 50-100nm for those prepared by other deposition methods [4]. Furthermore, ZnO nanorods were well aligned vertically, showing uniformity in their diameters, lengths, and densities as revealed from electron microscopy. These ZnO nanorods were epitaxially grown with homogeneous in-plane alignment as well as c-axis orientation. The room-temperature PL spectra of the nanorods showed strong and narrow excitonic emission with a dominant peak at 3.29 eV and an extremely weak deep level emission at 2.5 eV, indicating the high optical quality of the nanorods. Free exciton emission lines were still clearly visible at 10 K, and no quantum confinement effect was evident for the nanorods with diameters exceeding 20nm [11].
For ZnO nanorods grown at relatively higher temperatures (700-1050 °C), vertical alignment of the c-axis-oriented nanorods was observed only on a-plane sapphire substrates, whereas the use of c-plane sapphire, Si (111), SrTiO3 (100), and
Figure 2-8. (a) plan-view and (b) tilted images of ZnO nanorods with a mean diameter of 25nmand (c) tilted and (d) cross-sectional images of ZnO nanorods with a mean diameter of 70 nm. In (c), hexagon-shaped pyramids with flat terraces and steps are seen at the ends of the nanorods. [9]
SrTiO3 (111) substrates resulted in rather random alignment [10]. To test the possibility of a catalyst-assisted process, nanorods were also grown on a-plane sapphire substrates partially coated with a thin (1-3 nm) gold layer. Close to 100% vertical orientation of ZnO nanorods with a diameter of 50±5nm and a length of several micrometers was observed in areas without gold metallization, while growth with no preferential direction occurred in the areas coated with gold, demonstrating that MOVPE growth of ZnO nanorods is different from the VLS process.
![Figure 2-12. Schematic of the rotes that one could follow within the scope of sol-gel processing [38]](https://thumb-ap.123doks.com/thumbv2/9libinfo/8143240.166757/57.892.140.757.117.488/figure-schematic-rotes-follow-scope-sol-gel-processing.webp)
