Motivated by the extensive research on carbon nanotubes (CNTs), boron and its related nano-structures have attracted increasing interest for potential applications in nanodevices and nanotechnologies, due to their extraordinary properties. In this work, different types of B-related nanostructures were successfully grown on oxidized Si substrates with or without transition metal catalysts in CVD processes. The gas chemistry was monitored by in-situ mass-spectroscopy and optical emission spectroscopy. These helped to identify the gas reactive species during the deposition, creating thereby a controllable, optimum synthesis process and helping in identifying the growth mechanism.
The boron nitride nanotubes (BNNTs) were successfully synthesized at the low substrate temperatures of 600-800°C by a microwave plasma CVD process, using diborane and ammonia as the gas precursors. The optimum growth conditions of BNNTs were investigated by varying the experimental parameters, such as catalyst film thickness, substrate temperature, diborane flow rate, and growth time. The dense and crystalline BNNT deposits were obtained on 1nm nickel (Ni) or cobalt (Co) thin film coated oxidized Si (111) at a temperature of 800°C, a pressure of 15 torr, microwave power of 800 W, diborane flow rate (5 vol.% in hydrogen) of 5 sccm, ammonia flow rate of 27.5 sccm, hydrogen flow rate of 10sccm, and a deposition time of 1 hour. These nanotubes were either self-assembled in bundles or as a single tube with a diameter less than 10 nm. Raman spectra together with electron diffraction pattern indicated a hexagonal crystalline structure for these BN nanostructures. A growth mechanism of BNNTs involving dissolution-supersaturation- precipitation of BN in the metal catalysts was proposed. It was shown that the growth of BN nanostructures strongly depended on the catalyst and its film thickness, which resulted in the selective growth of BNNTs on the patterned catalyst islands. Ni dots with the diameters in the submicron range were used to synthesize aligned BNNTs. Fine BN nanostructures with a diameter around 10-20 nm and length up to 10 microns were grown and dispersed in the Ni dots. Nanosized Ni dots were suggested for the growth of the vertically aligned BNNTs.
Boron nanowires (BNWs) were also grown by the decomposition of diborane using a thermal CVD process at a temperature of 900°C, a pressure of 20 torr, diborane flow rate (5 vol.% in hydrogen) of 5 sccm, and nitrogen flow rate of 55 sccm. These BNWs had diameters in a range of 20-200 nanometers and lengths up to several tens of micrometers. Repeatable Raman spectra indicated icosahedra B12 to be the basic building units forming the B nanowires. Amorphous BNWs with rough surface were obtained without any catalysts on different substrates, such as Si wafer or ZrB2 powders. A vapor-solid (VS) growth was proposed for the amorphous BNWs, in which the solid phase precipitated directly from the vapor phase reactions. The amorphous BNWs were modified for size and composition using a plasma CVD process containing argon, ammonia and hydrogen. The diameters of these BNWs were reduced from 200 nm to several tens of nanometers, and a small amount of N was incorporated into BNWs after the plasma treatment. On the other hand, the metal catalyst proved to be effective for the growth of crystalline BNWs. Tetragonal BNWs with smooth surface were grown on thin Ni film (1 nm) coated Si substrates. Ni attachment was observed at the tip of the BNW for the first time, which indicated that the vapor-liquid-solid (VLS) growth mechanism can be used for synthesis of the BNW. The diameters of these BNWs were strongly dependent on the size of the metal particles encapsulated in the BNWs.
In summary, two boron-related nanostructures were synthesized by chemical vapor deposition (CVD) in this work. A new method was successfully developed to decrease the substrate temperature more than 400°C to fabricate boron nitride nanotubes in a microwave plasma enhanced chemical vapor deposition process using gas reactions of diborane and ammonia. The catalytic growth of BNNTs done in this work provided a novel way to selectively grow BNNTs in thin film form on Ni or Co coated Si substrates. For boron nanowires, the co-existence of two growth mechanisms was discovered having completely different morphology and crystallinity using the thermal CVD process. The metal catalyst assisted the growth of the crystalline BNWs by vapor-liquid-solid mechanism, which amorphous BNWs were produced without the use of the catalyst. These results are expected to open up more pathways to scale up the fabrication of vertically aligned BNNTs and BNWs for studies of their properties and applications.