[Paper Review] Raman Scattering by sp$^2$ Amorphous Carbons

E. F. Sheka, Ye. A. Golubev+1 more|arXiv (Cornell University)|Feb 23, 2020

Carbon Nanotubes in Composites70 references4 citations

TL;DR

This paper proposes a molecular-level interpretation of Raman spectra in sp² amorphous carbons (sp² ACs) by identifying stable graphenous molecules as the basic structural units (BSUs), which exhibit a unique enforced fragmentation amorphicity. The study links the G-D-2D Raman pattern to molecular-crystalline duality and attributes spectral broadening and two-phonon features to electron density delocalization and bond length dispersion, enabling a size-dependent transition from molecular to quasi-particle phonon models.

ABSTRACT

The paper presents a cooperative consideration of Raman spectra of $^2$ amorphous carbons as well as the nature and type of their amorphicity. The latter was attributed to the amorphization of a new type named as enforced fragmentation. The fragments are stable graphenous molecules, which are the basic structural units (BSUs) of the solids, determining them as amorphics with molecular structure. Due to weak intermolecular interaction, BSUs, once aggregated, are the main defendants for IR absorption and Raman scattering of the solids, just justifying the consideration of them at molecular level. The standard G-D-2D pattern of Raman spectra of polycyclic aromatic hydrocarbons, $^2$ amorphous carbons, graphene and/or graphite crystal is attributed to extended honeycomb composition of carbon atoms and are suggested as manifestation of molecule-crystal dualism of graphenous materials. The molecular approximation, applied to the analysis of one-phonon spectra of the studied $^2$ ACs, makes it possible to trace a direct connection of the G-D spectra image as well as their broadband structure with a considerable dispersion of the C=C bond lengths within BSUs honeycomb structure, caused by the influence of chemical action, deformation, etc. This approximation, applied to the interpretation of two-phonon spectrum of graphenous molecules for the first time, reveals a particular role of electrical anharmonicity in the spectra formation and attributes this effect to a high degree of the electron density delocalization. A size-stimulated transition from molecular to quasi-particle phonon consideration of Raman spectra was experimentally traced, which allowed evaluation of a free path of optical phonons in graphene crystal.

Motivation & Objective

• To understand the origin of Raman spectra in sp² amorphous carbons beyond traditional crystalline models.
• To identify the structural basis of amorphicity in these materials, proposing a new type: enforced fragmentation.
• To establish a molecular approximation for one- and two-phonon Raman spectra, linking spectral features to electronic and geometric distortions.
• To trace the transition from molecular to quasi-particle phonon behavior with increasing size, enabling estimation of optical phonon mean free path in graphene.

Proposed method

• The authors analyze Raman spectra of sp² amorphous carbons using a molecular approximation, treating the basic structural units (BSUs) as isolated graphenous molecules.
• They model one-phonon spectra by incorporating C=C bond length dispersion within BSUs due to chemical substitution or strain.
• Two-phonon spectra are interpreted via electrical anharmonicity, attributed to high electron density delocalization in the honeycomb lattice.
• A size-dependent crossover from molecular to quasi-particle phonon models is experimentally observed, allowing estimation of optical phonon free path.
• The analysis draws on the G-D-2D Raman pattern as a signature of molecule-crystal duality in graphenous materials.
• The study uses 30 pages, 9 figures, 3 tables, and 87 references to support the theoretical and spectroscopic framework.

Experimental results

Research questions

• RQ1 What is the true structural origin of amorphicity in sp² amorphous carbons, and how does it differ from conventional amorphization?
• RQ2 How do bond length dispersion and electron delocalization in graphenous molecular units influence the observed Raman spectra?
• RQ3 To what extent can the G-D-2D Raman pattern be explained by molecular-level behavior rather than extended crystalline models?
• RQ4 How does the transition from molecular to quasi-particle phonon description occur with increasing size in these materials?
• RQ5 What role does electrical anharmonicity play in shaping the two-phonon Raman features of sp² amorphous carbons?

Key findings

• The basic structural units (BSUs) of sp² amorphous carbons are identified as stable, isolated graphenous molecules, forming a molecular-based amorphous structure due to enforced fragmentation.
• The standard G-D-2D Raman pattern is attributed to the extended honeycomb composition of carbon atoms and reflects a molecule-crystal duality in graphenous materials.
• Spectral broadening in the G and D bands arises from significant dispersion in C=C bond lengths within BSUs, induced by chemical or mechanical deformation.
• Electrical anharmonicity, driven by high electron density delocalization, is identified as the key factor in the formation of two-phonon Raman features.
• A size-stimulated transition from molecular to quasi-particle phonon models is experimentally observed, enabling the estimation of the optical phonon mean free path in graphene.
• The study provides the first application of molecular approximation to two-phonon Raman spectra in graphenous systems, revealing a direct link between electronic structure and vibrational response.