학위논문 (석사)-- 서울대학교 대학원 : 재료공학부, 2014. 2. 김미영.Studying nucleation and growth has been one of the major goals in materials science. Fundamental understanding of initial growth is essential for fabrication of nanomaterials with desired physical properties. Consequently, atomic level investigation on as-grown nuclei and local atomic arrangements around defects is required. Such high-resolution study along with crystallographic analysis could be performed using transmission electron microscopy (TEM). Here, we report on atomic-resolution observation of initial growth behavior using TEM by growing nanomaterials directly on graphene. Graphene exhibiting excellent electron beam transparency and high mechanical strength is an ideal supporting layer for TEM measurements by minimizing the background signal from the underlying membrane. In addition to merely sustaining nanomaterials as a support, graphene can be further used as a substrate for nanomaterials growth. The crystalline nature of graphene along with its electron beam transparency ultimately enables direct imaging of nanomaterials and allows us to systematically investigate the initial growth mechanisms.
Using direct growth and imaging method, we could clearly observe the initial states of Zinc oxide (ZnO) nanomaterials. This enabled the observation of the transition in crystal structure of ZnO nuclei along with their orientational relationship with graphene. Furthermore, formation of various defects during nanomaterial growth could be clearly visualized with atomic-resolution. More generally, we believe that this simple technique may be readily expanded to investigate the growth mechanisms of many other nanomaterials on various two-dimensional layered substrates.Chapter 1 Introduction 1
1.1 Importance of studying the initial growth behavior of nanomaterials 1
1.2 Materials of interest 10
1.2.1 ZnO 10
1.2.2 Graphene 12
Chapter 2 Literature Survey 17
2.1 Direct crystal growth on prefabricated thin membranes for TEM measurements 17
2.2 Graphene as a supporting layer for TEM measurements 23
Chapter 3 Experimental Method 30
3.1 Growth of CVD graphene 30
3.2 Transfer of CVD graphene onto a TEM grid 31
3.3 Growth of ZnO nanomaterials on graphene placed on a TEM grid 35
3.4 Structural characterization of ZnO nanomaterials 35
3.4.1 SEM 35
3.4.2 TEM 36
3.5 First principles calculations 39
Chapter 4 Results and Discussions 40
4.1 Feasibility of the experimental technique for ZnO growth and TEM measurements 40
4.2 Overall growth behavior of ZnO nanomaterials on graphene 43
4.2.1 Nucleation and cluster growth stages 43
4.2.2 Postnucleation stages: coalescence 45
4.2.3 Postnucleation stages: formation of epitaxial relationship 46
4.3 Crystal structure of ZnO in the initial stage of growth 51
4.3.1 Formation of rocksalt ZnO structure 51
4.3.2 Nucleation barrier in heteroepitaxial growth 52
4.4 Disorder in the atomic arrangements around defects 58
4.5 Diverse applicability of the experimental technique for the initial growth study: ZnO nanomaterials on h-BN 63
4.5.1 Hexagonal boron nitride 63
4.5.2 Initial growth behavior of ZnO nanomaterials on h-BN 64
Chapter 5 Conclusions 73
References 74
Abstract 81Maste
