The Synthesis Of Ⅲ Nitrides And AlN-based Diluted Magnetic Semiconductor Nanomaterials And Their High-pressure Studies

III-nitride compounds with various structures and morphologies have in recent years attracted increasing attention due to their significant applications in optoelectronic and field-emission devices. III-nitride nanostructures, in particular, are expected to have important applications in field emitters, flexible pulse-wave sensors and ultraviolet nanolasers, due to their excellent optical and electronic properties and large area surface. However, there are few reports on the synthesis of hierarchical nanostructures of group III nitride. Hence, the synthesis of AlN nanostructures with controlled shapes and sizes is an important topic worthy of exploration. Especially hierarchical structured nanomaterials, which have dual or multiple morphologies and structures, have attracted increasing interest because of their structural complexity and greater functionality.Aluminum nitride (AlN), as the largest band gap group III nitrides, has various unique properties such as excellent thermal conductivity, low dielectric loss, high melting point, high mechanical strength, and high piezoelectric response. It has attracted increasing attention in recent years. Recently, many efforts have been devoted to fabricating AlN based DMS due to the wide gap 6.2 eV of AlN. Magnetic transition metals TMs such as Mn, Cr, Co, and Fe are frequently used as magnetic dopants to fabricate AlN based DMS. Although, many efforts have been made to explore the synthesis and magnetic properties of AlN-based DMS bulk materials and films, there are very few research groups reported the synthesis of nanostructures. It has been reported that Al vacancies in AlN resulting in a ferromagnetic ground state with a magnetic moment of 3.0μB using ab initio calculation. However, they predicted sufficient Al vacancy concentration may be difficult to achieve in thermal equilibrium due to its high formation energy. Thus, viable and simple methods are highly desired for the synthesis of ferromagnetic semiconductors by reducing the formation energy of cation vacancies.In recent years, the high-pressure studies on nanomaterials have stimulated great enthusiasm in order to improve our understanding of the structural stability and the mechanical properties of nanomaterials. However, the high-pressure behavior of III nitrides nanomaterials is less known.In this paper, we report the synthesis of various morphologies of III nitrides and rare-earth doping III nitrides nanostructures by direct current arc discharge plasma method, including the analysis of their structures and growth mechanism and measurements of their photoluminescence and magnetic properties. In addition, we report a high-pressure study on the synthesized III nitrides nanostructures using an in situ synchrotron radiation X-ray diffraction technique in diamond anvil cell. The obtained main innovative results are as follow:1. We firstly synthesized a great deal of new nanostructures of AlN, such as nanobranches, nanourchins, microroses and nanonails, etc., by direct current arc discharge plasma method. These nanostructures can be controlled by the growth conditions include the gas pressure, direct current, reaction time and reaction time. The polar surface induced growth and the supersaturated Al vapor are proposed to explain the growth of AlN nanostructures, especially the synthesis AlN microroses. In this work, the adding NH3 plays an important role in the formation of AlN single-sided nanocombs and single-sided nanonails on nanowires.2. We firstly controlled fabrication of AlN branched nanostructures with tree shapes and sea urchin shapes through an improved direct current arc discharge plasma method without any catalyst and template. Differing from our previous setup, we designed a Mo plate as the upper collection substrate located near the tip of W cathode, and used Al anode as the nether substrate. As the Mo upper substrate is added on the W cathode, a small circulation of reactive vapors between the Mo upper substrate and Al nether substrate is formed, which results in high fluxes of Al and N vapors toward the Mo upper and Al nether substrates. In this work, we demonstrated that the improved DC arc discharge plasma method provides a simple route to fabricate branched nanostructures with various morphologies.3. We also studied the optical properties and the impact of defects of the as-synthesized novel nanostructures. A strong emission appears ranging from 550 to 570 nm. Those emissions are attributed to the the nitrogen vacancy and the radiative recombination of a photon- (or electron-) generated hole with an electron occupying the nitrogen vacancy. The emission intensity is related to AlN nanostructures. The AlN nanostructures have complex structures with large surface/volume ratio which could result in the enhancement of emission intensity.4. We firstly fabricated GaN porous nanostructure using direct current arc discharge plasma method. This nanostructure has large surface/volume ratio and many holes. PL spectra of the GaN porous nanostructures were measured under an ultraviolet excitation from He-Cd laser at 325 nm at room temperature. A strong emission centered at 678.6 nm appears ranging from 550 to 750 nm as well as a weak emission around 391.2 nm. The emission band at 391.2 nm is attributed to band edge emission of GaN. The emission band at 678.6 may be correspond to the nitrogen vacancy in surface or subsurface of GaN porous nanostructures. In the formation of GaN porous nanostructures, lower N2 pressure was used than that for the growth of AlN hierarchical nanostructures. The lower N2 pressure could induce small quality of evaporation of Ga.5. We firstly synthesized Fe-doped AlN nanowires and 6-fold-symmetrical hierarchical nanostructures using direct current arc discharge plasma method without introducing any catalysts and templates. In this work, metal nanoparticles were found on the tips of the nanowires. Therefore, the formation mechanism of these nanostructures should be related to the VS mechanism. The plot of the magnetization (M) versus magnetic field strength (H), measured by VSM at 300 K, clearly indicated ferromagnetism at room temperature. The saturation magnetization and the coercive fields (Hc) of the AlN:Fe nanostructures are about 0.256 emu g-1 and 131 Oe, respectively. The origin of ferromagnetism in AlN:Fe could be from the Fe3+-VN-Fe3+ groups and Fe.6. We firstly fabricated Sc and Y-doped AlN hexagonal nanoprisms and 6-fold-symmetrical hierarchical nanostructures though direct current arc discharge plasma method. These hexagonal nanoprisms have diameters around 100-180 nm and lengths around 150-200 nm. The plot of the magnetization (M) versus magnetic field strength (H), measured by VSM at 300 K, clearly indicated ferromagnetism at room temperature. The saturation magnetization and coercive fields (Hc) of the AlN:Sc hexagonal nanoprisms are measured to be 0.049 emu g-1 and 299 Oe respectively. The saturation magnetization and the coercive fields (Hc) of the AlN:Sc nanostructures are about 0.04 emu g-1 and 200 Oe, respectively. The saturation magnetization and coercive fields (Hc) of the AlN:Y hexagonal nanoprisms are about 0.050 emu g-1 and 101 Oe respectively. Our first-principles calculations have established that the observed ferromagnetism in AlN:Sc and AlN:Y hexagonal nanoprisms is not from Sc and Y atoms, but from the Al vacancies. The calculated formation energy of Al vacancies in AlN:Sc (3.556 eV) is much smaller than that in pure AlN (6.405 eV), implying that doping Sc has significantly reduced the formation energy of Al vacancies. The calculated formation energy of Al vacancies in AlN:Y is 4.876 eV is bigger than that in AlN:Sc due to the bigger radius of Y3+ ions. All this results together point out that the doping nonmagnetic element, such as Sc and Y, in semiconductors can induce an enhancement magnetic property resulting from high cation vacancy concentration that is a visible method to develop a novel class DMSs.7. We firstly reported a series of high-pressure studies on AlN nanocrystals and nanowires (in a single diamond anvil cell), GaN nanowires and InN nanocrystals using in situ synchrotron radiation X-ray diffraction technique in diamond anvil cell. As for AlN nanocrystals and nanowires in a single diamond anvil cell, an onset pressure of 21.5 GPa from the wurtzite to rocksalt phase transition is observed in AlN nanocrystals and nanowires, respectively. Furthermore, the transition to the rocksalt phase is completed up to 27.8 GPa, which is very swift due to the effect of hydrostaticity. Such same high pressure behaviors in AlN nanocrystals and nanowires might be attributed to them with the similar size and diameter. The bulk modulus of rocksalt phase of AlN nanocrystals and nanowires are B0=312.6±22.7 GPa, and B0=324.9±15.8 GPa, respectively. Therefore, this indicates that the reduction of particle size can significantly lead to an enhancement of the bulk modulus. As for GaN nanowires, an onset pressure of 44.3 GPa from the wurtzite to rocksalt phase transition, the transition to the rocksalt phase is completed up to 52.7 GPa. The bulk modulus of wurtzite and rocksalt phases of GaN nanowires are B0=175±7 GPa, and B0=263.6±5 GPa, respectively. As for InN nanocrystals, an onset pressure of 12.6 GPa from the wurtzite to rocksalt phase transition, the transition to the rocksalt phase is completed up to 17.4 GPa. The bulk modulus of wurtzite and rocksalt phases of GaN nanowires are B0=161.8±9 GPa, and B0=263.6±5 GPa, respectively. These findings further confirmed the reduction of particle size can significantly lead to an enhancement of the bulk modulus.