2019my supervisor, Prof. Guoqiang Hu, for his invaluable guidance and continuous support throughout my Ph.D study. His insightful suggestions and generous help have enlightened my research journey. Under his guidance, my thinking towards problems becomes deeper and broader. I also learned a lot from his high standard and rigorous attitude toward research, which will benefit me in all my life.My sincere thanks to my lab mates for their friendship, helpful discussions, and encouragement. Thanks to my friends who play badminton and do sports together with me at NTU. They made my study and life more meaningful and enjoyable.Last but not least, special thanks to my beloved parents and sister, for their endless love, encouragement, and support all the time.