[Paper Review] Ab initio simulation of amorphous graphite

Rajendra Thapa, Chinonso Ugwumadu+3 more|PubMed|Feb 22, 2022

Graphene research and applications4 citations

TL;DR

This study uses ab initio molecular dynamics to demonstrate the formation of amorphous graphite (a-G), a layered carbon material composed of disordered monolayer graphene planes (with pentagons, hexagons, and heptagons) separated by ~3.1 Å. The transition occurs at ~3000 K and densities of 2.2–2.8 g/cm³, with interlayer cohesion driven by delocalized π-electrons rather than van der Waals forces, and electronic transport significantly reduced due to topological ring disorder.

ABSTRACT

An amorphous graphite material has been predicted from molecular dynamics simulation using ab initio methods. Carbon materials reveal a strong proclivity to convert into a sp^{2} network and then layer at temperatures near 3000 K within a density range of ca. 2.2-2.8 g/cm^{3}. Each layer of amorphous graphite is a monolayer of amorphous graphene including pentagons and heptagons in addition to hexagons, and the planes are separated by about 3.1 Å. The layering transition has been studied using various structural and dynamical analyses. The transition is unique as one of partial ordering (long range order of planes and galleries, but topological disorder in the planes). The planes are quite flat, even though monolayer amorphous graphene puckers near pentagonal sites. Interplane cohesion is due partly to non-Van der Waals interactions. The structural disorder has been studied closely, especially the consequences of disorder to electronic transport. It is expected that the transition elucidated here may be salient to other layered materials.

Motivation & Objective

• To investigate the formation mechanism of a novel amorphous graphite (a-G) phase from amorphous carbon or random configurations under high-temperature annealing.
• To determine the structural, energetic, and electronic properties of a-G, particularly its layered architecture and interlayer cohesion.
• To analyze the impact of topological ring disorder (pentagons, heptagons) on in-plane electronic transport and charge distribution.
• To clarify the role of electronic interactions—beyond van der Waals—in stabilizing the layered structure of a-G.
• To compare the electronic structure and conductivity of a-G with crystalline graphite and defective graphene.

Proposed method

• Ab initio molecular dynamics (AIMD) simulations using the PBE functional and Nosé-Hoover thermostat at 3000 K were performed on carbon systems with densities of 2.2–2.8 g/cm³.
• NVT simulations were conducted on multiple models (M1–M6) with varying system sizes, initial states (amorphous or random), and exchange-correlation functionals (PBE, PBE+vdW, GAP-ML).
• Conjugate gradient relaxation was applied post-simulation to confirm structural stability and minimize energy.
• Electronic structure analysis included density of states (DoS), charge density mapping, and band-decomposed charge density projections to visualize π- and π*-orbital contributions.
• Space-projected conductivity (SPC) based on the Kubo-Greenwood formula was computed to evaluate conduction pathways and quantify electronic transport anisotropy.
• Structural analysis included radial distribution functions (RDF), layer identification via atomic position clustering, and visualization of atomic configurations over time.

Experimental results

Research questions

• RQ1 Can amorphous carbon or random carbon configurations spontaneously form a layered structure resembling graphite under high-temperature annealing?
• RQ2 What is the role of topological ring disorder (pentagons, heptagons) in the structural and electronic properties of the resulting amorphous graphite?
• RQ3 How does interlayer cohesion in a-G differ from van der Waals forces, and what electronic mechanisms stabilize the layered structure?
• RQ4 To what extent does ring disorder reduce in-plane electronic conductivity compared to crystalline graphite?
• RQ5 Can the electronic structure of a-G be characterized by delocalized π-electrons in the interlayer galleries, and how does this affect charge distribution?

Key findings

• Amorphous graphite (a-G) forms robustly in a temperature/density window of ~3000 K and 2.2–2.8 g/cm³, with layers of disordered monolayer graphene separated by ~3.1 Å.
• The total energy of a-G is only 0.32 eV/atom above crystalline graphite, indicating thermodynamic stability despite topological disorder.
• Interlayer cohesion is primarily due to delocalized π-electrons forming bonding orbitals that extend into the galleries, not van der Waals forces.
• Charge density maps show a low-density, delocalized electron gas in the galleries, with >2% of the maximum charge density on isolated layers, indicating significant electronic coupling.
• The electronic density of states (DoS) of a-G shows a broad peak at the Fermi level, lacking the semi-metallic character of crystalline graphite.
• Space-projected conductivity (SPC) reveals that electronic transport in a-G is reduced by a factor of ~10⁻² compared to graphite, with conduction paths avoiding ring defects such as pentagons and heptagons.