Formation of amorphous carbon multi-walled nanotubes from random initial configurations

TL;DR

This paper reports the simulation of amorphous carbon nanotubes with up to four walls, analyzing their structure, defects, electronic properties, and thermal conductivity, using machine learning and density functional validation.

Contribution

It introduces a method to simulate a-CNTs from random configurations and predicts key variables with machine learning, validated by density functional calculations.

Findings

Identified defect configurations and electronic delocalization in a-CNTs.

Calculated vibrational density of states and thermal conductivity.

Observed low-frequency radial breathing modes.

Abstract

Amorphous carbon nanotubes (a-CNT) with up to four walls and sizes ranging from 200 to 3200 atoms have been simulated, starting from initial random configurations and using the Gaussian Approximation Potential [Phys. Rev. B 95, 094203 (2017)]. The important variables (like density, height, and diameter) required to successfully simulate a-CNTs, were predicted with a machine learning random forest technique. The models were validated $\textit{ex post facto}$ using density functional codes. The a-CNT models ranged from 0.55 nm - 2 nm wide with an average inter-wall spacing of 0.31 nm. The topological defects in a-CNTs were discussed and new defect configurations were observed. The electronic density of states and localization in these phases were discussed and delocalized electrons in the $\pi$ subspace were identified as an important factor for inter-layer cohesion. Spatial projection of the electronic conductivity favors axial transport along connecting hexagons, while non-hexagonal parts of the network either hinder or bifurcate the electronic transport. A vibrational density of states was calculated and is potentially an experimentally testable fingerprint of the material and the appearance of a low-frequency radial breathing mode was discussed. The thermal conductivity at 300 K was calculated using the Green-Kubo formula.