Hexagonal Boron Nitride as a Two-Dimensional Surfactant: Low-Density Flame-Resistant Composites Based on Boron Nitride Exfoliated by an Interface Trapping Technique

Hexagonal boron nitride (hBN) is a two-dimensional material isoelectric to graphene. It has a hexagonal structure with alternating boron and nitrogen atoms and is electrically insulating, thermally conductive, and chemically inert. However, like graphene, its use as a functional nanofiller requires exfoliation. Here, we present a method for exfoliating and dispersing hexagonal boron nitride (hBN) sheets within a polymer matrix. This is achieved by a solvent interface trapping method (SITM), where hBN exfoliates and spreads at a high-energy oil/water interface, separating the two phases and lowering the system's free energy. hBN thus acts as a surfactant, allowing us to form stable water-in-oil emulsions stabilized by films of overlapping hBN sheets. Polymerizing the continuous oil phase results in polymerized high internal phase emulsions (polyHIPE), with the foam cells lined with hBN. In the investigation presented here, we use acoustic spectroscopy, optical and electron microscopy, and image analysis to study the emulsion and polyHIPE formation mechanisms. We find that the amount of hBN used, the flake size of the hBN, the mixing time, the type of initiator used, and the oil-to-water ratio all play a role in the morphology of the final polyHIPE. Lastly, we demonstrate the flame-retardant properties of the hBN polyHIPEs and show that, at 1% loadings, the presence of hBN prevents dripping and relighting of the composites, even when the material is heated to a red glow.

Formation mechanisms of boron oxide films fabricated by large-area electron beam-induced deposition of trimethyl borate

Boron-containing materials are increasingly drawing interest for the use in electronics, optics, laser targets, neutron absorbers, and high-temperature and chemically resistant ceramics. In this article, the first investigation into the deposition of boron-based material via electron beam-induced deposition (EBID) is reported. Thin films were deposited using a novel, large-area EBID system that is shown to deposit material at rates comparable to conventional techniques such as laser-induced chemical vapor deposition. The deposition rate and stoichiometry of boron oxide fabricated by EBID using trimethyl borate (TMB) as precursor is found to be critically dependent on the substrate temperature. By comparing the deposition mechanisms of TMB to the conventional, alkoxide-based precursor tetraethyl orthosilicate it is revealed that ligand chemistry does not precisely predict the pathways leading to deposition of material via EBID. The results demonstrate the first boron-containing material deposited by the EBID process and the potential for EBID as a scalable fabrication technique that could have a transformative effect on the athermal deposition of materials.

Controlling Catalyst Bulk Reservoir Effects for Monolayer Hexagonal Boron Nitride CVD

Highly controlled Fe-catalyzed growth of monolayer hexagonal boron nitride (h-BN) films is demonstrated by the dissolution of nitrogen into the catalyst bulk via NH3 exposure prior to the actual growth step. This "pre-filling" of the catalyst bulk reservoir allows us to control and limit the uptake of B and N species during borazine exposure and thereby to control the incubation time and h-BN growth kinetics while also limiting the contribution of uncontrolled precipitation-driven h-BN growth during cooling. Using in situ X-ray diffraction and in situ X-ray photoelectron spectroscopy combined with systematic growth calibrations, we develop an understanding and framework for engineering the catalyst bulk reservoir to optimize the growth process, which is also relevant to other 2D materials and their heterostructures.

Generating electric current based on the solvent-dependent charging effects of defective boron nitride nanosheets

This work presents a method of generating electric current based on the defects of few-layer boron nitride nanosheets (BNNSs). The density functional theory calculations showed that the atomic charge of the B atom in acetone was more positive than in water. The electrostatic force microscopy measurements illustrated that the local electrical potential was 0.35 mV in acetone, while the potential signal was very difficult to capture when using water as the dispersant. This effect was further demonstrated by the performance of the acoustic energy-harvesting nanogenerator: the BNNSs were assembled into a film after being dispersed in acetone and then integrated into the generator device, generating average output current of ∼0.98 nA, which was much better than 0.2 nA, the average output current of another device with water as the dispersant. These results demonstrated that solvent effects made the as-prepared BNNSs carry net charges, which could be utilized to harvest acoustic energy and generate current.

Increasing the wear resistance by interstitial alloying with boron via chemical vapor deposition

The wear resistance of a Rh(111) surface can be strongly increased by interstitial alloying with boron atoms via chemical vapor deposition of trimethylborate [B(OCH3)3] at moderate temperatures of about 800 K. The fragmentation of the precursor results in single boron atoms that are incorporated in the fcc lattice of the substrate, as displayed by X-ray photoelectron diffraction. The penetration depth of the boron atoms is in the range of at least 100 nm with the boron distribution displaying a nearly homogeneous depth profile, as examined by combined X-ray photoelectron spectroscopy and Ar ion etching experiments. Compared to the bare Rh(111) surface, the wear resistance of the boron-doped Rh surface is increased to about 400%, as probed by the scratching experiments with atomic force microscopy. The presented synthesis route provides an easy method for case hardening of micro- or nanoelectromechanical devices (MEMS and NEMS, respectively) at moderate temperatures.