| This thesis contains two parts, the first one involves ZnO-based semiconductor p-type doping and its related device, ZnO-based diluted semiconductor; the second part is the in situ TEM analysis of the nano-lithiumion battery.ZnO is an important compound semiconductor ofⅡ-Ⅵgroup, which exhibits excellent optical and electrical properties with a wide direct bandgap of 3.37 eV and a large exciton binding energy of 60 meV at room temperature, far above the thermal motion energy of 26 meV. With favorable properties, ZnO is expected to fabricate violet or UV light-emitting diodes (LED). Intrinsic ZnO is a natural n-type semiconductor, and there exists many donor defects like zinc interstitials and oxygen vacancies. In order to realize stable p-type conversion, ZnO needs to be doped with acceptor. However, the solid solubility of acceptor impurity is relatively low and its stability in ZnO is relatively bad, which limit the development of ZnO based light emitting LED device. The alloy Zn1-xMgxO also plays an important role in the research of ZnO, in which Mg ions replace the positions of Zn ions. Zn1-xMgxO alloy keeps the ZnO wurtzite structure, owns a small mismatch with ZnO and can modify the band gap from 3.3eV to 4.3eV. Moreover, ZnO/ZnMgO superlattices and quantum wells can be used to improve the emission efficiency of ZnO based LED. In order to realize the Zn1-xMgxO alloy application, it is critical to prepare stable p-type Zn1-xMgxO which has attracted many researchers. Most of them focus on doping with group V elements on the substitution O positions, such as N,P,As etc. However, the p-type property of Zn1-xMgxO fabricated by using these methods is still unstable, although it has been studied for a long time. Recently, group IA elements have attracted much attention. By theoretical calculation, group IA elements are considered as relative shallow acceptors. The ionization energy of LiZn, NaZn, Kzn is 0.11eV, 0.16eV,0.29eV respectively. Referring to ion radius, Li ion is too small and it prefers staying in the interstitial sites and acting as an donor. So we only discuss the Na and K doped p-type Zn1-xMgxO and related LED devices in chapter 4. The Na doped p-type Zn1-xMgxO thin film fabricated by PLD shows a resistivity of 1.08Ω·cm and a hole concentration of 1.21×1019 cm-3. Then we fabricated a p-Zn1-xMgxO:Na/n-Zn0 LED by depositing the Na doped p-type Zn1-xMgxO thin film on the n-type ZnO single crystal substrate and realized its electroluminescence at room temperature. The K doped Zn0.95Mg0.05O thin film realized the p-type conversion as well, with a resistivity of 15.21Ω·cm, and a hole concentration of 5.54×1018 cm-3.Since Dietl et al. theoretically predicted that ZnO would be able to realize diluted magnetic semiconductor (DMS) when doping with transitional metal ions, ZnO based DMS has attracted much attention. ZnO based DMS refers to the ZnO doped with transitional metal ions, behaving both semiconductor and magnetic properties. They show excellent magneto-optical, magnetic-electrical and magnetic properties, with potential application in electronics, information and etc.. The success of fabricating DMS will perform a profound impact on the IT industry, and bring huge economic and social benefits. In Chapter five, we will discuss the properties of Mn, Co, Cu doped ZnO and investigated them in detail to make clear the origin of ferromagnetic at room temperature (RTFM). It is found that the RTFM of Mn doped ZnO is favor to be achieved in a p-type environment, and the magnetic moment per Mn atom for Mn:ZnO films can be further enhanced by improving its hole concentration. The RTFM of Co doped ZnO can be realized in a n-type environment with sufficient oxygen vacancies. The fundamental mechanism for realizing the RTFM in Cu doped ZnO is the interaction between the Cu2+ ions, with the oxygen defects playing as the medium. The p-type doping is detrimental to the RTFM of Co or Cu doped ZnO.With the depletion of traditional energy such as oil, coal and etc., it is urgent to develop some green energy such as Lithium-ion battery. The lithium-ion battery behaves high voltage, small size, light weight, high charging and discharging speed, pollution-free, self-discharge slow, high energy density, no memory effect, and long cycle life is attracting more and more attention. However during charging and discharging process, the electrode material in lithium-ion will occur significant volume expansion, and these changes bring in a high density of dislocations, macro-defects and reduce battery performance and life time. Although it has been studied for many years, the detailed mechanisms of strain-induced plasticity and strain accommodation in electrode during charging and discharging are still unknown. How to improve the battery efficiency is still poorly understood as well. So it is urgent to find a way to make a real-time observation of microscopic changes in the process of charging and discharging. In Chapter 7, we have successfully constructed a nanoscale electrochemical device to meet such need. It consists of a single nanowire as an anode, an ionic liquid-based electrolyte (ILE), and a cathode of LiCoO2 bulk material or metal Lithium and makes a nano-battery inside a high-resolution transmission electron microscope (TEM). It is the first time that we made a real-time observation of SnO2 nanowires micro-structure and phase changes during charging and discharging using in situ TEM method; using carbon, aluminum, copper and other coating structures can improve the conductivity of SnO2 nanowires, enhance its charging rate, limit its expansion along the radial direction, and reduce the dislocation zone forming in the reaction front; there are two different mechanisms of Sn dendrite formation in the charging and discharging process of SnO2 nanowires; the phosphorus doping combined with carbon coating leads to a ultra-high charging rate in Si nanowires. This method shows significant importance for the design of advanced lithium batteries.
|
PDF Full Text Request