On the Electronic, Mechanical and Optical Properties of Superhard Cross-Linked Carbon Nanotubes (Tubulanes)

We have investigated the electronic, optical, and mechanical
properties
of six structures belonging to the Tubulanes-cross-linked carbon nanotube
family, using Density Functional Theory (DFT) with the Generalized
Gradient Approximation (GGA) and the Perdew–Burke–Ernzerhof
(PBE) functional. Our results highlight the remarkable anisotropic
mechanical behavior of these materials, distinguishing them from isotropic
structures, such as diamond. Notably, the 8-tetra-22 structure has
a higher Young’s modulus (YM) along
the z-direction compared to diamond. Unlike diamonds,
the mechanical properties of Tubulanes are direction-dependent, exhibiting
significant variations in Young’s Modulus (2.3 times). Additionally,
the Poisson’s ratio is highly anisotropic, with at least one
direction exhibiting a value close to zero. The inherent anisotropy
of these materials enables tunable mechanical properties that depend
on the direction of applied stress. Regarding their electronic properties,
all Tubulane structures studied possess indirect electronic band gaps,
dominated by 2p orbitals. The band dispersion is relatively high,
with band gaps ranging from 0.46 to 2.74 eV, all of which are smaller
than that of diamond. Notably, the 16-tetra-22 structure exhibits
the smallest bandgap (0.46 eV), making it particularly interesting
for electronic applications. Additionally, these structures exhibit
porosity, which offers advantages over denser materials, such as diamond.
Given recent advances in the synthesis of 3D carbon-based materials,
the synthesis of tubulane-like structures is within current technological
capabilities.