Low-dimensional carbon and boron nitride nanomaterials - hexagonal boron nitride, graphene, boron nitride nanotubes and carbon nanotubes - remain at the forefront of advanced materials research. Catalytic chemical vapour deposition has become an invaluable technique for reliably and cost-effectively synthesising these materials. In this review, we will emphasise how a synergy between experimental and theoretical methods has enhanced the understanding and optimisation of this synthetic technique. This review examines recent advances in the application of CVD to synthesising boron nitride and carbon nanomaterials and highlights where, in many cases, molecular simulations and quantum chemistry have provided key insights complementary to experimental investigation. This synergy is particularly prominent in the field of carbon nanotube and graphene CVD synthesis, and we propose here it will be the key to future advances in optimisation of CVD synthesis of boron nitride nanomaterials, boron nitride - carbon composite materials, and other nanomaterials generally.

Chemical vapour deposition (CVD) growth of carbon nanotubes is currently the most viable method for commercial-scale nanotube production. However, controlling the 'chirality', or helicity, of carbon nanotubes during CVD growth remains a challenge. Recent studies have shown that adding chemical 'etchants', such as ammonia and water, to the feedstock gas can alter the diameter and chirality of nanotubes produced with CVD. To date, this strategy for chirality control remains sub-optimal, since we have a poor understanding of how these etchants change the CVD and nucleation mechanisms. Here, we show how ammonia alters the mechanism of methane CVD and single-walled carbon nanotube nucleation on iron catalysts, using quantum chemical molecular dynamics simulations. Our simulations reveal that ammonia is selectively activated by the catalyst, and this enables ammonia to play a dual role during methane CVD. Following activation, ammonia nitrogen removes carbon from the catalyst surface exclusively via the production of hydrogen (iso)cyanide, thus impeding the growth of extended carbon chains. Simultaneously, ammonia hydrogen passivates carbon dangling bonds, which impedes nanotube nucleation and promotes defect healing. Combined, these effects lead to slower, more controllable nucleation and growth kinetics.

Density functional theory is employed to demonstrate how ammonia-derived etchant radicals (H, NH, and NH2) can be used to promote particular (n,m) chirality single-walled carbon nanotube (SWCNT) caps during chemical vapour deposition (CVD) growth. We reveal that the chemical reactivity of these etchant radical species with SWCNTs depends on the SWCNT chirality. This reactivity is determined by the extent of disruption to the π-conjugation of the cap structure caused by reaction with the etchant species. H and NH2 attack single carbon atoms and preferentially react with near-zigzag SWCNT caps, whereas NH prefers to attack across CC bonds and selectively etches near-armchair SWCNT caps. We derive a model for predicting abundances of (n,m) SWCNTs in the presence of ammonia-derived radicals, which is consistent with (n,m) distributions observed in recent CVD experiments using ferrocene and ammonia. This model also demonstrates that chiral-selective etching of SWCNTs during CVD growth can be potentially exploited for achieving chirality-control using standard CVD synthesis.

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