| Boron nitride (BN) compounds have recently attracted much research attention. Similar to carbon (C) solids, BN is a multi-phase material with analogous lattice structures in either hexagonal phase or cubic phase. Based on experiences of discovering new phases for C such as C60 and carbon nanotubes (CNTs), we speculate that energetic growth species and catalyst nanoparticles are some of the key factors for the growth of metastable BN phases. This hypothesis has prompted our long-term investigation of fundamental factors that enable phase control of BN compounds including thin films and nanostructures. For the first time, we have succeeded in controlling BN phases using a same synthesis technique: RF plasma-enhanced pulsed-laser deposition (PE-PLD). We have succeeded in transforming hexagonal phase BN films (h-BN) into cubic phase BN (c-BN) films and finally, boron nitride nanotubes (BNNTs). In addition, we have managed to use chemical reactions and catalysts to energize the growth of BNNTs by thermal chemical vapor deposition (CVD) techniques and discovered the growth of single crystalline boron nitride nanowires (BNNWs).; The cBN phase has desirable properties such as extreme hardness comparable to diamond, high thermal conductivity, wide band gap (6eV) and being chemically more inert than diamond to oxidation and ferrous metals, etc. As reported in literatures, Ar or other massive ions (Kr, Xe) bombardment was usually used in order to grow cBN. Structural damages on cBN occurred due to implantation and bombardments of these massive ions with high kinetic energies. In this work, we attempted to grow cBN films in pure N2 plasma with reduced ion mass and kinetic energies. This would reduce the level of defects and help to improve the adhesion of cBN films. Furthermore, we even attempted to grow cBN films in a vacuum without any auxiliary ion bombardment. In our PE-PLD technique, we use RF plasmas to induce DC substrate bias voltages that can create bombardments of positive ions on the BN growth surfaces. We found that BN films tended to grow with the hBN phase at low bias voltages while the growth of cBN films required higher bias voltages. At even higher bias voltages, we found that the growth of hBN and cBN films are suppressed and lead to the so-called total resputtering condition. We discovered that the use of Fe catalysts at this condition can induced the growth of BNNTs at low temperatures.; BNNTs are structurally similar to CNTs and also exhibit extraordinary mechanical properties. However, unlike CNTs, BNNTs possess uniform electronic properties that are insensitive to their diameters, the number of walls, and chiralities. Theoretically, the band gaps (∼5eV) of BNNTs are tunable and can even be eliminated by transverse electric fields through the giant DC Stark effect. In addition, BNNTs demonstrate high oxidation resistance up to ∼1100°C, excellent piezoelectricity, and present potential material for room temperature hydrogen storage. However, growing BNNTs is challenging. In the last ten years, BNNTs were grown by are discharge, laser ablation, and substitution reactions from CNTs, ball-milling and chemical vapor deposition (CVD), at temperatures from 1100 to 3000°C. These BNNTs were dominated by impurities including amorphous boron nitride (a-BN) powders and other solid-state by-products. In this project, we have succeeded for the first time in growing multi-walled BNNTs (MWBNNTs) directly on substrates at 600°C by our PE-PLD approach. Scanning electron microscopy (SEM) indicates that multiple MWBNNTs grown from adjacent Fe catalyst particles tend to form vertical bundles. Long and straight tubular structures were detected by transmission electron microscopy (TEM). These BNNTs are found to have square-like cross-section caps, which are typical for high-quality BNNTs. Tunneling spectroscopy indicates that their band gap ranges from 4.4 to 4.9 eV. Details of these results and a phase selective growth... |
