Ultrathin Boron Growth onto Nanodiamond Surfaces via Electrophilic Boron Precursors

Diamond as a templating substrate is largely unexplored and the unique properties of diamond including its large bandgap, thermal conductance and lack of cytotoxicity make it versatile in emergent technologies in medicine and quantum sensing. Surface termination of an inert diamond substrate and its chemical reactivity are key in generating new bonds for nucleation and growth of an overlayer material. Oxidized high-pressure high-temperature (HPHT) nanodiamonds NDs are largely terminated by alcohols that act as nucleophiles to initiate covalent bond formation when an electrophilic reactant is available. In this work we demonstrate a templated synthesis of ultrathin boron on ND surfaces using trigonal boron compounds. Boron trichloride (BCl3), boron tribromide (BBr3) and borane (BH3) were found to react with ND substrates at room temperature in inert conditions. BBr3 and BCl3 were highly reactive with the diamond surface and sheet-like structures were produced and was verified with electron microscopy. Surface sensitive spectros-copies were used to probe the molecular and atomic structure of the ND constructs’ surface and quantification showed the boron shell was less than 1 nm thick after 1-24 hour reaction protocols. Observations of the reaction supports a self-terminating mechanism, similar to atomic layer depo-sition growth and is likely due to the quenching of alcohols on the diamond surface. The boron-diamond nanostructures were found to aggregate in dichloromethane and were dispersed in various solvents and characterized with dynamic light scattering for future cell imaging or cancer therapy applications using boron neutron capture therapy (BNCT). The unique templating mech-anism based on nucleophilic alcohols and electrophilic trigonal precursors allows for covalent bond formation and will be of interest to researchers using diamond for quantum sensing, additive manufacturing and BNCT.