Fully Atomistic Reactive Molecular Dynamics simulations of mechanical properties of Gold Nanotips Encapsulated with Carbon Nanocones

Ductility and malleability are two fundamental aspects of plasticity. A solid is malleable if it easily deforms under compressive stress, while it can be considered ductile if it easily deforms under tensile stress. Gold besides being a very good conductor of electricity, it is also ductile and malleable. Due to these properties its use as nanotips in some applications (such is, in enhanced Raman spectroscopy) is limited because it easily deforms and breaks. In order to overcome some of these difficulties, we have experimentally manipulated multi-walled carbon nanocones (MWNCs) to encapsulate chemically etched produced gold nanotips. MWNCs exhibit unique structural, mechanical, chemical, and electronic properties, and they are stiffer than carbon nanotubes. Their conical shape makes them ideal to encapsulate conical-like metallic tips. The mechanical properties of these composite systems (MWNCs plus gold tips) were studied through atomic force microscopy (AFM) and in situ Raman spectroscopy experiments, which provided real time information about their structural stability. In order to gain further insights about their structural stability and fracture dynamics, we have also carried out fully atomistic reactive molecular dynamics. We have considered single and multi-walled carbon nanocones encapsulating gold nanotips. The system was then dynamically pushed against a substrate and the mechanical stability, energy, force profiles, and fracture patterns analyzed. The fracture patterns extracted form the simulations can help to explain the increased mechanical stability. We observed that besides protecting the gold tip from direct contact with the substrates/samples, the cones efficiently accumulate and dissipate the strain/stress, thus expanding the elastic regime of the gold tip, preserving its physical integrity 1. We also observed the formation of carbon chain originated from the cone fractures. These chains can easily react with hydrogen, oxygen and nitrogen atoms present in the air. These results are consistent with the signatures of these chemical bonds experimentally observed in the Raman spectra 1.

1 A. G. Cano-Marques et al., Nature Scientific Reports 5, 10408 (2015).