Root-Growth Mechanism for Single-Walled Boron Nitride Nanotubes in Laser Vaporization Technique

Abstract

We present a detailed study of the growth mechanism of single-walled boron nitride nanotubes synthesized by laser vaporization, which is the unique route known to the synthesis of this kind of tube in high quantities. We have performed a nanometric chemical and structural characterization by transmission electron microscopy (high-resolution mode (HRTEM) and electron energy loss spectroscopy) of the synthesis products. Different boron-based compounds and other impurities were identified in the raw synthesis products. The results obtained by the TEM analysis and from the synthesis parameters (temperature, boron, and nitrogen sources) combined with phase diagram analysis to provide identification of the fundamental factors determining the nanotube growth mechanism. Our experiments strongly support a root-growth model that involves the presence of a droplet of boron. This phenomenological model considers the solubility, solidification, and segregation phenomena of the elements present in this boron droplet. In this model, we distinguish three different steps as a function of the temperature: (1) formation of the liquid boron droplet from the decomposition of different boron compounds existing in the hexagonal boron nitride target, (2) reaction of these boron droplets with nitrogen gas present in the vaporization chamber and recombination of these elements to form boron nitride, and (3) incorporation of the nitrogen atoms at the root of the boron particle at active reacting sites that achieves the growth of the tube.

One-dimensional surface phonon polaritons in boron nitride nanotubes

“…Hexagonal boron nitride (h-BN) has a strong phonon resonance that results in a negative permittivity band on the high frequency side of the resonance 18 , and has recently been found to be capable of supporting mid-IR SPhPs 19 . The multiwall boron nitride nanotubes (BNNTs) [20][21][22] , are a one-dimensional (1D) version of boron nitride, with geometry that can be controlled through chemical synthesis. BNNTs are chemically inert and mechanically strong with diameters as small as 2 nm.…”

Very long single- and few-walled boron nitride nanotubes via the pressurized vapor/condenser method

“…BNNTs form in the deposits of the liberated vapors. The energy is input by laser [2][3][4][5][6] or by arc discharge [7][8][9] . Only small quantities (mg's) of material have been produced by this method, but the tubes are high quality.…”

“…These maps show, as expected, that the tubes contain both boron and nitrogen but also confirm that the observed droplets are boron. Also of note--the boron particles are coated with a uniform layer of boron nitride, a layer that has been theorized to play an important role in BNNT formation [6].…”

Modeling of Laser Vaporization and Plume Chemistry in a Boron Nitride Nanotube Production Rig

“…Nanotubes are most often seen extending from these droplets suggesting that liquid boron droplet sites are critical to the nucleation of BNNT nanotubes. In the process described by Arenal et al 27 and Lee et al 20 it is assumed that liquid boron assists in the catalysis of molecular nitrogen to build the BN nanotube from the root. The simulations here suggest that a supersaturated state of BN gas in the plume provides a reservoir from which extremely rapid condensation of nanotubes progresses when a "suitable" nucleation site is encountered.…”