THERMAL CONDUCTIVITY OF COMPLEX CRYSTALS, HIGH TEMPERATURE MATERIALS AND TWO DIMENSIONAL LAYERED MATERIALS By XIN QIAN B.S. Huazhong University of Science and Technology, 2014 A thesis submitted to the Faculty of the Graduate School of the University of Colorado in partial fulfillment of the requirement for the degree of Doctor of Philosophy Department of Mechanical Engineering 2019 i This thesis entitled: Thermal Conductivity of Complex Crystals, High Temperature Materials and Two Dimensional Layered Materials written by Xin Qian has been approved for the Department of Mechanical Engineering Prof. Ronggui Yang, Chair Prof. Baowen Li Date: The final copy of this thesis has been examined by the signatories, and we find that both the content and the form meet acceptable presentation standards of scholarly work in the above mentioned discipline. ii ABSTRACT Xin Qian (Ph.D, Mechanical Engineering) Thermal Conductivity of Complex Crystals, High Temperature Materials and Two Dimensional Layered Materials Thesis directed by Professor Ronggui Yang Thermal conductivity is a critical property for designing novel functional materials for engineering applications. For applications demanding efficient thermal management like power electronics and batteries, thermal conductivity is a key parameter affecting thermal designs, stability and performances of the devices. Thermal conductivity is also the critical material metrics for applications like thermal barrier coatings (TBCs) in gas turbines and thermoelectrics (TE). Therefore, thermal conductivities of various functional materials have been investigated in the past decade, but most of the materials are simple and isotropic crystals at low temperature. This is because the first-principles calculation is limited to simple crystals at ground state and few experimental methods are only capable of measuring thermal conductivity along a single direction. The objective of this thesis is to develop first-principles based atomistic modeling tools to study thermal conductivity and phonon properties of complex crystals, high temperature materials, as well as and ultrafast laser based pump-probe techniques to characterize anisotropic thermal conductivity of layered two-dimensional materials. In the first part of this thesis, an integrated density functional theory and molecular dynamics (DFT-MD) method is developed to model the thermal conductivity and phonon properties of hybrid organic-inorganic crystals, a special kind of complex crystals integrating both organic molecules and inorganic frameworks. This DFT-MD method first develops an empirical potential iii field from first-principles DFT calculations, then predicts thermal conductivity using MD simulation. We applied this method to predict thermal conductivities of II-VI based hybrid crystals and organometal halide perovskites. An ultralow thermal conductivity (0.6 W/mK) is predicted in the perovskite CH3NH3PbI3, agreeing well with experimental measurements. In the second part, instead of using empirical functional forms, a data driven machine learning algorithm is used to develop high-fidelity potential field for phonon modeling. We demonstrated that the machine learning based potential is a powerful tool for modeling phonons at high temperature, even for dynamically unstable high-temperature phases, which is a challenging problem for both empirical potential based MD and static first-principles calculations. Using a simple machine learning algorithm called Gaussian process regression, we developed potential field that can effectively capture the stabilization of BCC phase of Zirconium at 1188 K, which is predicted to be unstable using static first-principles calculations. In the third part, a varied laser spot size technique based on time-domain thermoreflectance (TDTR) is developed to characterize anisotropic thermal conductivity. This method is applied to measure both the thermal conductivity parallel to the basal planes as well as the through-plane thermal conductivity of transition metal dichalcogenides, a group of layered two dimensional materials. Interestingly, the through-plane thermal conductivity is observed to decrease with the increasing heating frequency (modulation frequency of the pump laser) from 0.6 to 10 MHz, due to the non-equilibrium transport between different phonon modes. A two channel thermal model is developed to capture the non-equilibrium transport and to derive the thermal conductivity at local equilibrium. This finding suggest that in electronic devices working at a few GHz, the material could tend to become much more thermally insulating than steady state, raising great challenges for near junction thermal management. iv ACKNOWLEGEMENT When taking a look back at this my PhD journey, indeed I wish to thank various people for their support and company. First and Foremost, I would like to offer my deep and sincere gratitude to my advisor Prof. Ronggui Yang for his continuous support during my PhD journey, for his vision and enthusiasm in scientific research and his immense knowledge. From him I learned how to probe and think critically of scientific problems, which will be invaluable for my future career. In addition to my advisor, I would like to thank the rest of my thesis committee: Prof. Baowen Li, Prof. David Marshall, Prof. Margret Murnane and Prof. Kurt Maute, for their encouragement and insightful comments and being supportive during the last five years. I thank my fellow labmates and friends in our group for their encouragement and company: Xiaokun Gu, Dongliang Zhao, Puqing Jiang, Xinpeng Zhao, Rongfu Wen, Shanshan Xu, Ablimit Aili and Tianzhu Fan. I owe Xiaokun deeply since he always remained available and supportive whenever I have difficulties and problems in my research. Xiaokun is like my “second advisor” especially in my first two years. From him I learned not only research skills but also perseverance and patience. I enjoyed discussing scientific problems with him so much that we remained in frequent touch till today. Thanks to Donliang and Xinpeng for their support, knowledge and discussions, and of course, their sense of humor which brought me lots of happiness. I thank Dr. Puqing Jiang for his help on the pump-probe experiments and lots of collaborated work with him. I thank Prof. Xiaobo Yin in our department and Prof. Jun Liu at North Carolina State University for various scientific discussions and their advice and help on bringing back the pump-probe setup when the laser system was down. During the visiting time of Prof. Congliang Huang and Prof. Xu Ji, the time I spent with them is helpful and joyful. Thanks to my friends: Junling Long, Haoran Jiang, Lu Ma, Duanfeng Gao, Baochen Wu, Lili Feng and so on for their encouragement and v support during the past five years. I thank the fellows playing soccer with me who are great resources to refresh myself during the weekends. I will miss the time spent with you guys. Finally, I would like to thank my parents and grandparents for their unconditional love and understanding and support throughout my life, without which I can achieve nothing. (I would not forget to thank my girlfriend who never appeared in my entire life, allowing me to fully dedicate myself to research.) vi Table of Contents CHAPTER I INTRODUCTION ..................................................................................................... 1 I.1 Advances and Challenges in Modeling and Characterizing Thermal Conductivity .............. 1 I.2 Thermal Transport in Hybrid Organic-Inorganic Complex Crystals ..................................... 4 I.3 Phonon and Thermal Properties of High Temperature Materials ........................................... 8 I.4 Thermal Transport in Two-Dimensional Layered Materials .................................................. 9 I.5 Objectives of this Thesis....................................................................................................... 11 I.6 Organization of this Thesis ................................................................................................... 13 CHAPTER II THERMAL CONDUCTIVITY MODELING OF HYBRID ORGANIC- INORGANIC CRYSTALS ........................................................................................................... 15 II.1 Introduction ......................................................................................................................... 15 II.2 Simulation Strategy ............................................................................................................. 17 II.3 II-VI based Hybrid Organic-Inorganic Crystals .................................................................. 23 II.4 Organometal Halide Perovskites ......................................................................................... 34 II.5 Summary of this Chapter ..................................................................................................... 41 CHAPTER III MACHINE LEARNING DRIVEN ATOMISTIC MODELING ON PHONON DIPSERSION STABILITY OF ZIRCONIUM ............................................................................ 42 III.1 Introduction ........................................................................................................................ 42 III.2 Methodology of Building Machine Learning Potential ..................................................... 46 III.2.1 Fitting Potential Energy Surface using GAP Method ..................................................... 46 III.2.2 Generation of Training Database ...................................................................................