Selective Preparation of Semiconducting Single-Walled Carbon Nanotubes: From Fundamentals to Applications By Jinghua Li Department of Chemistry Duke University _________________________ Approved: _________________________ Jie Liu, Supervisor _________________________ Desiree Plata _________________________ Benjamin J. Wiley _________________________ Weitao Yang Dissertation submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy in the Department of Chemistry in the Graduate School of Duke University 2016 ABSTRACT Selective Preparation of Semiconducting Single-Walled Carbon Nanotubes: From Fundamentals to Applications By Jinghua Li Department of Chemistry Duke University _________________________ Approved: _________________________ Jie Liu, Supervisor _________________________ Desiree Plata _________________________ Benjamin J. Wiley _________________________ Weitao Yang An abstract of a dissertation submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy in the Department of Chemistry in the Graduate School of Duke University 2016 Copyright by Jinghua Li 2016 Abstract Aligned single-walled carbon nanotubes (SWNTs) synthesized by the chemical vapor deposition (CVD) method have exceptional potential for next-generation nanoelectronics. Based on their electronic structure, SWNTs can be classified into semiconducting (s-) and metallic (m-) types. For application in logic gates, radio frequency devices and sensors, s-SWNTs are especially attractive owing to the presence of a sizable energy band gap. However, there are considerable challenges in the preparation of s-SWNTs with controlled properties (e.g., density, selectivity, band gap) for their application in solving real-world problems. This dissertation summarizes our efforts that aim to overcome the limitations by novel synthesis strategies and post-growth treatment, and research that demonstrates the application of as-prepared SWNTs as functional devices. The first part focuses on controlling SWNT properties through direct CVD synthesis. Chapter 2 describes a CVD strategy to decouple the conflict between density and selectivity of s-SWNTs in growth. Using this method, we have successfully obtained dense s-SWNTs (∼10 SWNTs/μm) over large areas and with high uniformity. We further investigated the importance of diameter control for the selective synthesis of s-SWNTs by using high temperature stable and uniform Fe–W nanoclusters as catalyst precursors (chapter 3). This study provides an effective strategy for increasing the purity of s-SWNTs via controlling the diameter distribution of SWNTs and adjusting the etchant concentration. By carefully comparing the chirality distributions of Fe–W-catalyzed and Fe-catalyzed SWNTs synthesized with different water vapor concentrations, the relationship between the diameter-dependent and electronic type-dependent etching iv mechanisms was summarized. In chapter 4, we demonstrated the selective synthesis of large diameter and highly conductive SWNTs through a thiophene-assisted CVD method. The second part of this dissertation is focused on optimizing properties of CVD- SWNTs via post-growth treatment and the subsequent applications as functional devices. Chapter 5 describes a facile and effective approach to selectively break all m-SWNTs by stacking two layers of horizontally aligned SWNTs to form crossbars and applying a voltage to the crossed SWNT arrays without gating. Systematic studies show that the on/off ratio can reach as high as 1.4 × 106 with a correspondingly high retention of on- state current compared to the initial current value before breakdown. Overall, this method provides important insight into transport at SWNT junctions and a simple route for obtaining pure s-SWNT thin film devices for broad applications. In chapter 6, we introduced a strain-release approach to enhance the density of horizontally aligned SWNTs. This method provides a novel way to obtain densely packed SWNT arrays and paves the way to achieve transistors with high performances for the next-generation nanoelectronics. Finally, we carried out studies that aim to investigate sensing mechanism of SWNT-based sensors by employing the SWNT crossbar device configuration (chapter 7). The results indicate that responses are mostly from the electrostatic doping effect on the whole nanotubes while the modification of the Schottky barrier at junctions only plays a minor part in the process, which motivated us to move beyond simple tube/tube crossbar junctions into more complex junction structures in order to take full advantage of the structure. v Contents Abstract .............................................................................................................................. iv List of Tables .......................................................................................................................x List of Figures .................................................................................................................... xi List of Abbreviations .........................................................................................................xx Acknowledgement .......................................................................................................... xxii Chapter 1: Introduction to Carbon Nanotubes .....................................................................1 1.1 Carbon nanotubes: overview ......................................................................................1 1.2 Discovery of carbon nanotubes ..................................................................................3 1.3 Structure and properties .............................................................................................5 1.3.1 Lattice structure ...................................................................................................5 1.3.2 Electronic structure ..............................................................................................8 1.3.3 Optical properties ..............................................................................................11 1.4 CNT synthesis ..........................................................................................................15 1.4.1 Arc discharge and laser ablation ........................................................................15 1.4.2 Chemical vapor deposition method for CNT synthesis .....................................18 1.4.3 Growth mechanism for CVD synthesis .............................................................20 1.5 Challenges in SWNT preparations ...........................................................................22 1.6 Application of SWNTs in nanoelectronics...............................................................23 1.6.1 Electronic device ...............................................................................................23 1.6.2 Sensors and sensing mechanism ........................................................................25 Chapter 2: Growth of High-Density-Aligned and Semiconducting-Enriched Single- Walled Carbon Nanotubes: Decoupling the Conflict between Density and Selectivity ....28 2.1 Introduction ..............................................................................................................28 2.2 Experimental section ................................................................................................30 vi 2.2.1 The improved multiple-cycle growth method ...................................................30 2.2.2 Characterization .................................................................................................32 2.2.3 Fabrication of FET device .................................................................................32 2.3 Results and discussion ..............................................................................................32 2.4 Conclusion ................................................................................................................51 Chapter 3: Importance of Diameter Control on Selective Synthesis of Semiconducting Single-Walled Carbon Nanotubes ......................................................................................53 3.1 Introduction ..............................................................................................................53 3.2 Experimental section ................................................................................................55 3.2.1 Preparation of nanocluster catalyst precusors ...................................................55 3.2.2 Growth of horizontally aligned SWNTs on quartz substrate ............................55 3.2.3 Characterization .................................................................................................56 3.2.4 Fabrication and measurement of back-gate FETs .............................................56 3.3 Results and discussion ..............................................................................................57 3.4 Conclusion ................................................................................................................77 Chapter 4: Selective Synthesis of Large Diameter and Highly Conductive Single-Walled Carbon Nanotubes by Thiophene-Assisted Chemical Vapor Deposition Method ............78 4.1 Introduction ..............................................................................................................78 4.2 Experimental section ................................................................................................80