Abstract
AlQ3 amorphous nanoparticles and nanowires can be fabricated under a cold trap of liquid nitrogen by vapor condensation in inert gases. Both diameter and size distribution of nanoparticles decrease with decreasing the He pressure. Longer, thinner and higher congregate nanowires are obtained at lower Ar pressure. Larger specific surface area of smaller nanoparticles and higher density of nanowires lead to a stronger PL intensity than those of commercial powder and thin film. Quasi-amorphous AlQ3 thin film with nanoprotrusions on the surface can also be fabricated by vapor condensation in vacuum and exhibits a low turn-on field of 2 V/μm to 12 V/μm. Amorphous nanowires also exhibit a low turn-on field of 3 V/μm to 20 V/μm. Both larger thickness of nanostructured AlQ3 and a smaller diameter of nanoprotrusions lead to a larger geometric enhancement for field emission. Compared with most inorganic 1D and diamond film emitters, nanostructured AlQ3 shows a superior field emission characteristic and stands for a promising cathode emitter. A linear relationship between calculated and actual radius of nanoprotrusions demonstrates that surface roughness of thin film has a significant influence on the efficiency of field emission. By a one-step heat treatment, crystalline AlQ3 nanowires can grow directly from amorphous nanoparticles and film. Systematic heat treatment demonstrates that temperatures between 150oC and 190oC are the most appropriate to form fine and long nanowires. Both higher heating temperature and longer heating time promote the growth and lead to more complete transformation. The crystalline nanowires are α-phase predominant, and the growth can be explained by nucleation and molecular migration that is dictated by anisotropic nature of α-AlQ3. Two phase transitions are observed in the transformation process. The first one is a transition from amorphism to γ□phase and the second one is a transition from γ□to α□phase. By Kissinger’s method, the activation energies for the two phase transitions are first time calculated to be 9.7 and 12.1 kJ/mol, respectively. The crystallization below the glass transition temperature and the lowering of phase transition temperatures are attributed to larger surface energy of amorphous nanoparticles. A blue shift and enhanced photoluminescence after heat treatment can be ascribed to more efficient Rayleigh scattering and preferentially formed α phase. Heat treatment is detrimental to field emission property because the random growth, unequal distribution of nanowires and an uneven and scraggly surface reduce effective emitters and cause non-uniform electron emission. The decreased grain boundaries after heat treatment reduce the number of transport channels for emitting electrons, resulting in worse field emission.