Last but not least, I wish to express my deepest gratitude for my family: my mom, my dad, who took care my little daughter during three years of my research and study. They sacrificed their happiness for my happiness, their days for my days. I would like to thank for my sisters and my brother for your effort to look after my daughter during my absence. Above all, I would express my special thanks to my husband Cao D.V and my daughters, Lan Chi and Bao An, for your endless love, understanding and support. You are the most important persons in my life and have been my greatest motivation.