[Paper Review] A DFT Study of the Electronic, Optical, and Mechanical Properties of a Recently Synthesized Monolayer Fullerene Network

Raphael M. Tromer, Luiz Antônio Ribeiro+1 more|arXiv (Cornell University)|Jul 4, 2022

Fullerene Chemistry and Applications35 references70 citations

TL;DR

This study employs density functional theory (DFT) to investigate the electronic, optical, and mechanical properties of a recently synthesized monolayer quasi-hexagonal C60 network (qHPC60). The results reveal a direct bandgap of 0.9 eV, excellent agreement with the experimental optical bandgap of 1.6 eV after scissor correction, anisotropic mechanical properties with elastic moduli between 50–62 GPa, and strong UV absorption with low reflectivity and refractive index >1, indicating potential as a UV collector and optoelectronic material in the 1.5–5.5 eV range.

ABSTRACT

Closely packed quasi-hexagonal and quasi-tetragonal crystalline phase of C$_{60}$ molecules (named qHPC$_{60}$) was recently synthesized. Here, we used DFT simulations to investigate the electronic, optical, and mechanical properties of qHPC$_{60}$ monolayers. qHPC$_{60}$ has a moderate direct electronic bandgap, with anisotropic mechanical properties. Their elastic modulus ranges between 50 and 62 GPa. The results for optical properties suggest that qHPC$_{60}$ can act as UV collectors for photon energies until 5.5 eV since they present low reflectivity and refractive index greater than one. The estimated optical bandgap (1.5-1.6 eV) is in very good agreement with the experimental one (1.6 eV).

Motivation & Objective

• To investigate the electronic, optical, and mechanical properties of the recently synthesized monolayer quasi-hexagonal C60 network (qHPC60) using DFT.
• To resolve the lack of detailed theoretical analysis on the physical properties of qHPC60 despite its experimental realization.
• To validate the experimental bandgap of 1.6 eV through DFT simulations with a scissor operator correction.
• To explore the potential of qHPC60 as a semiconductor for optoelectronic applications, particularly in UV and visible light harvesting.
• To analyze the anisotropic mechanical and electronic behavior arising from the unique topological structure of the qHPC60 lattice.

Proposed method

• Performed DFT calculations using the SIESTA code with the PBE-GGA functional and norm-conserving Troullier-Martins pseudopotentials.
• Applied van der Waals corrections to the exchange-correlation functional to account for weak intermolecular interactions.
• Used a double-zeta plus polarization (DZP) basis set and a 250 Ry kinetic energy cutoff for numerical convergence.
• Calculated electronic band structure along the Γ–X–U–Y–Γ path and projected density of states (PDOS) to analyze orbital contributions.
• Applied a 0.6 eV scissor operator to correct the DFT bandgap and improve agreement with experimental optical transitions.
• Calculated optical coefficients (absorption, refractive index, reflectivity) from the complex dielectric function using Fermi’s golden rule and Kramers-Kronig relations.

Experimental results

Research questions

• RQ1 What is the electronic band structure and bandgap of the qHPC60 monolayer, and how does it compare to the experimental value of 1.6 eV?
• RQ2 How do the mechanical properties of qHPC60, particularly its elastic modulus and anisotropy, depend on crystal orientation and strain?
• RQ3 What are the optical absorption characteristics of qHPC60 across the UV–visible spectrum, and can it act as an efficient UV collector?
• RQ4 How do the frontier molecular orbitals (HOMO and LUMO) influence electron delocalization and mobility in qHPC60?
• RQ5 To what extent do the optical properties (refractive index, reflectivity) exhibit anisotropy depending on the light polarization direction?

Key findings

• The qHPC60 monolayer exhibits a direct bandgap of 0.9 eV, which, after a 0.6 eV scissor correction, aligns closely with the experimental optical bandgap of 1.6 eV.
• The calculated formation energy of -9.14 eV/atom confirms thermodynamic stability, comparable to other 2D carbon allotropes.
• The elastic modulus ranges from 50 to 62 GPa, with clear in-plane anisotropy, showing higher mechanical resilience along the y-direction.
• Optical absorption begins at approximately 1.5–1.6 eV, matching the experimental onset, with a first peak at ~2.2 eV in the visible range due to HOMO–LUMO transitions.
• The material exhibits low reflectivity (up to ~30% at 3.3–3.5 eV) and refractive index >1 across the UV–visible range, indicating strong UV light collection potential.
• The HOMO and LUMO frontier orbitals are delocalized across C–C bonds, suggesting high electron mobility and favorable transport properties.