Synthesis, Characterization and Property Study on InAs Nanowires
|Course||Condensed Matter Physics|
|Keywords||InAs Nanowire Nanocrystal Growth Kinetics Micro-Raman scattering|
As one of the III-V group materials, InAs （indium arsenide） is an important narrow direct band semiconductor. It has high electron mobility, low electron effective mass and a large exciton Bohr radius. InAs nanowires are now exploring its potential in application and science, such as nano-electronic devices, nano-photonic devices, quantum devices, bio-sensors, nano-physics and nano-chemistry. Up to date, there are three main methods that have been developed to prepare InAs nanowires. They are metalorganic vapor phase epitaxy （MOVPE）, laser-assisted catalytic growth （LCG） and solution methods. Although they have some special merits in some aspects, there are some limits unavoidable. For example, they all need relatively high temperature （MOVPE: 400-700℃; LCG:～800℃; solution: 240-360℃）, deadly toxic or severely handled arsenic sources （AsH3, As[Si（CH3）3]3 or As[N（CH3）2]3）, nano catalyst particles which often remains in products or at the ending of nanowires. One of the requirements for developing nano devices is that the structure and morphology of nanomaterials must be technically controllable. However, the morphology evolution mechanisms of nanowires in vapor or liquid phase are not fully understood yet. Also, it is most important to use different computation and testing methods to study the InAs nanowire’s physical properties, which will be the foundation for deepening the knowledge in nanoscale dimension and developing new nano devices.Aiming at the three aspects described above, this thesis presents the corresponding work as follows. Firstly, a solution route to prepare InAs nanowires was developed, which was featured in low temperature （120-180℃）, high efficiency, low cost, catalyst nanoparticle free and relatively safe. Secondly, a totally new kinetic model was built, which described the nanocrystals’ morphology evolution in nonequilibrium growth conditions. Lastly, special Raman scattering phenomenon in InAs nanowires was studied.In this dissertation, the main research and results are outlined as fellows:（1） InAs nanowires were successfully synthesized by using InCl3, As2O3 and NaBH4 as reactants and polyethylene glycol （PEG） as main solvents. This method is featured in low reaction temperature （120-180℃）, controllable average diameter between 15 to 60 nm, relatively safe and green, cheap reactants and solution system, catalyst nanoparticle free, short reaction time （5 min - 10 h）, high efficiency （>80%） and mass productive.（2） Characterization of the InAs nanowires showed that, the products were composed of zinc blende structured InAs nanowires grown in  direction and small amount of wurtzite structured InAs nanowires grown in  direction. The twin crystals structure was universal for the zinc blende InAs nanowires. The formation mechanism of these structures was discussed. A new InAs nanowire growth mechanism, the ligand-aided solution-solid （LASS） growth mechanism, was supposed. The InAs nanowire’s spindle shape evolution mechanism was discussed.（3） The thermal stability of the InAs nanowire’s structure in vacuum and normal pressure was studied. It was pointed out that even in low vacuum condition, enough attention should be paid to the thermal stability of InAs nanowires. It was found that when capped with SiO2 layer, the wires thermal stability is greatly improved. The electron beam sublimation of InAs nanowires was studied. Using InAs nanowires as templates, electron beam induced thin walled （1-3 nm） organic nanotubes was prepared.（4） A new kinetic model was built, which described the nanocrystals’ morphology evolution in nonequilibrium growth conditions. The 1D-3D growth transition was explained for the first time by a model. The kinetic origination of the nanowire’s diameter focusing was also found for the first time. How different injection strategies and different morphologies of nanocrystal nuclei affect the nanowire morphology evolution, together with the maximum aspect ratio that could be reached in nonequilibrium growth were studied. The characteristics and difference of the nanocrystals independent growth and co-growth were studied. It was found in co-growth process, a morphology focusing with mass defocusing phenomenon appeared. The mechanism is that, although the bigger nanocrystals grow faster, they consume excess monomer in the 1D growth stage, which accelerates the smaller ones entering the 3D growth stage earlier. Compared with the independent growth, the final length of the nanowire was less sensitive to the length change of the nanocrystal nucleus, and the final diameter and length distribution were narrower.（5） Micro-Raman scattering was applied to study the as prepared InAs nanowires with different diameters. Strong low frequency branch of LO-phonon-plasmon mode （L-） was observed. The longitudinal optic （LO） mode was found to be dependent on diameter of the nanowires. The nanowire diameter related wave vector uncertainty, carrier distribution in InAs nanowires and the carrier concentration related Fermi-Thomas screening effect were proposed to play important roles for this phenomenon.