我們在水相中以一步合成法成功合成出小顆硫化鉛奈米粒子，所合成出的正方體的邊長為21奈米、正八面體邊長為57奈米，並深入探討其成核與晶體成長的機制，借由了解這些機制我們可以掌握控制其大小及形貌的關鍵因素，也因此我們得以以較簡單的方式大量生產這些極小顆且具有均一形狀的硫化鉛奈米粒子；此外，我們也因此得以製造出較大顆的硫化鉛多面體使得單顆硫化鉛粒子的導電性量測得以被實現。
先前我們已觀測到氧化亞銅晶面導向的導電性，而在本實驗中我們又再次借由量測面凸正方體、截角截邊正方體、正八面體與截角截邊正八面體硫化鉛粒子的I-V曲線來展示硫化鉛粒子的晶面導向導電性，其中我們發現{111}晶面幾乎不導電，{100}與{110}導電性相較之下極好，又以{110}最好，與{111}晶面差了約二至三個數量級。這也證實了晶面導向導電性是廣泛存在於半導體之中的，我們認為這些導電性的差異來自于半導體界面上不同晶面所造成的能帶扭曲幅度不同。

We have fabricated small PbS nanocrystals (cubes: 21 nm; octahedra: 57 nm) in aqueous solution using an one-step approach. We gain knowledge in manipulating their shape and size by clear exploration of the relation between nucleation and crystal growth. This enables us to scale up the production using a more intuitive method. Also, the refined seeding growth method allows us to enlarge the size of PbS polyhedra, fulfilling single particle electrical conductivity measurements.
Facet-dependent electrical conductivity of Cu2O crystals has been previously revealed. In this study, we have again successfully demonstrated the facet-dependent electrical property of PbS using face-raised cubes, edge-and corner-truncated cubes, edge- and corner-truncated octahedra and pristine octahedra with the size of several hundreds of nanometers. Two tungsten probes are manipulated to contact the opposite faces of a single particle. The {110} facets showed the highest electrical conductivity, followed by the {100} facets and the {111} facets were observed to be the least conductive. The {111} facets are 500 times less conductive than the {110} faces at an applied voltage of 5 V. This study suggests that facet-dependent electrical conductivity should be regarded as an intrinsic property for semiconductors.

TABLE OF CONTENTS

Abstract -----------------------------------------------------------------------------------------------------------i
論文摘要 ---------------------------------------------------------------------------------------------------------ii
謝誌---------------------------------------------------------------------------------------------------------------iii
Table of Contents------------------------------------------------------------------------------------------------v
List of Figures---------------------------------------------------------------------------------------------------vii
List of Schemes-------------------------------------------------------------------------------------------------viii

Chapter 1 One Step Synthesis of Small PbS Nanocrystals with Shape Control and Their Growth Mechanism

1.1 Introduction --------------------------------------------------------------------------------------------------1
1.2 Experimental-------------------------------------------------------------------------------------------------3
1.2.1 Reagents -----------------------------------------------------------------------------------------3
1.2.2 Instrumentation---------------------------------------------------------------------------------3
1.2.3 One-pot synthesis of small PbS cubes ------------------------------------------------------3
1.2.4 One-pot synthesis of small PbS octahedra -------------------------------------------------4
1.2.5 Characterizations ------------------------------------------------------------------------------4
1.2.5.1 Electron microscopy -----------------------------------------------------------------4
1.2.5.2 Element analysis by EDS and XPS ------------------------------------------------6
1.2.5.3 Powder X-ray diffraction ------------------------------------------------------------8
1.2.5.4 Zeta potential measurements --------------------------------------------------------9
1.2.5.5 Scaling up the production-----------------------------------------------------------10
1.3 Results and discussion ------------------------------------------------------------------------------------11
1.3.1 Previous hypothesis: reaction rate determines shape ------------------------------------11
1.3.2 Nucleation versus crystal growth-----------------------------------------------------------12
1.3.3 The size effect of seeds in seeding growth method --------------------------------------13
1.3.4 Influence of reagent introduction sequence -----------------------------------------------20
1.4 Conclusion--------------------------------------------------------------------------------------------------25
Reference--------------------------------------------------------------------------------------------------------26
Appendix --------------------------------------------------------------------------------------------------------28
i. measurements of particle size and its distribution -------------------------------------------28
ii. XPS data of PbS octahedra and cubes --------------------------------------------------------29

Chapter 2 One-Step Synthesis of Large PbS Nanocrystals with Well-Defined Facets and Their Facet-Dependent Electrical Conductivity Measurements

2.1 Preface-------------------------------------------------------------------------------------------------------35
2.2 Experimental------------------------------------------------------------------------------------------------36
2.2.1 Reagents ----------------------------------------------------------------------------------------36
2.2.2 Instrumentation --------------------------------------------------------------------------------36
2.2.3 Preparation of large PbS nanocrystals------------------------------------------------------37
2.2.3.1 Synthesis and characterization of face-raised cubes and edge- and corner- truncated cubes -----------------------------------------------------------------------37
2.2.3.2 Synthesis of octahedra and edge- and corner-truncated octahedra -----------38
2.3 Single particle I-V curve measurements -----------------------------------------------------------------40
2.4 Results and discussion -------------------------------------------------------------------------------------42
2.4.1 I-V curve measurements of {111}-{111} --------------------------------------------------42
2.4.2 {100}-{100} electrical conductivity with different particle shapes and sizes---------44
2.4.3 {110}-{110} electrical conductivity with different particle shapes --------------------46
2.4.4 Comparing the electrical conductivity of the three basic facets and illustration of the modified band diagram of PbS and W probe ----------------------------------------------48
2.4.5 Probing different facets -----------------------------------------------------------------------50
2.5 Conclusion---------------------------------------------------------------------------------------------------52
Reference --------------------------------------------------------------------------------------------------------53




LIST OF FIGURES

Chapter 1
One Step Synthesis of Small PbS Nanocrystals with Shape Control and Their Growth Mechanism
Figure 1.1 SEM, TEM images and SAED patterns of PbS nanocrystals --------------------------------5
Figure 1.2 Atomic sturcture of three basic facets of PbS --------------------------------------------------7
Figure 1.3 PXRD of PbS cubes and octahedra --------------------------------------------------------------8
Figure 1.4 Zeta potential measurements of synthesized PbS ----------------------------------------------9
Figure 1.5 Images of scaled up production------------------------------------------------------------------10
Figure 1.6 SEM images polyhedra prepared using seeding growth method----------------------------11
Figure 1.7 SEM images of synthesized PbS nanocrystal--------------------------------------------------14
Figure 1.8 SEM images of synthesized PbS nanocrystal--------------------------------------------------15
Figure 1.9 SEM images of synthesized PbS nanocrystal--------------------------------------------------17
Figure 1.10 SEM images of synthesized PbS nanocrystal ------------------------------------------------19
Figure 1.11 SEM images of the result of changing reagent introduction sequence--------------------21
Figure 1.12 Monitoring the pH value of the whole process-----------------------------------------------23
Figure 1.13 Monitoring the process of different reagent introduction sequence by UV-vis spectra 24
Figure 1.14 UV-vis absorption spectrum of PbS nanocrystals -------------------------------------------24

Chapter 2
One-step Synthesis of Large Lead Sulfide Nanocrystals with Well-Defined Facet and Their Facet-dependent Electrical Conductivity

Figure 2.1 SEM, TEM images and SEAD patterns of synthesized PbS nanocrystal------------------39
Figure 2.2 PXRD patterns of synthesized PbS nanocrystal-----------------------------------------------39
Figure 2.3 SEM image of single layer of edge- and corner-truncated PbS octahedra-----------------40
Figure 2.4 I-V characteristics of {111}-{111}--------------------------------------------------------------43
Figure 2.5 I-V characteristics of {100}-{100}--------------------------------------------------------------45
Figure 2.6 I-V characteristics of {110}-{110}--------------------------------------------------------------47
Figure 2.7 Comparison of I-V characteristics of the three basic facets of PbS crystals ---------------48
Figure 2.8 Proposed band diagram of PbS and W probe--------------------------------------------------49
Figure 2.9 I-V characteristics of probing {110}-{111} ---------------------------------------------------51
Figure 2.10 Proposed band diagram contacting two different facets ------------------------------------51

LIST OF SCHEMES


Chapter 1
One Step Synthesis of Small PbS Nanocrystals with Shape Control and Their Growth Mechanism
Scheme 1.1 Seeding growth method developed by Jian-Kwan Wu --------------------------------------11
Scheme 1.2 Seeding growth method using well-defined PbS cubes as seeds --------------------------13
Scheme 1.3 Seeding growth method using well-defined PbS octahedra as seeds----------------------15
Scheme 1.4 Seeding growth method using well-defined PbS cubes as seeds---------------------------17
Scheme 1.5 Seeding growth method using well-defined PbS octahedra as seeds----------------------19
Scheme 1.6 Changing reagent adding sequence ------------------------------------------------------------21

Chapter 2
One-step Synthesis of Large Lead Sulfide Nanocrystals with Well-Defined Facet and Their Facet-dependent Electrical Conductivity

Scheme 2.1 Layout of experimental setup-------------------------------------------------------------------41

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