Atomic-level structural analysis elucidates the "mystery" of carbon materials - Success in identifying the origin of "mystery" peaks derived from defects, guiding precise design of next-generation materials

A research team led by Associate Professor Yasuhiro Yamada of the Graduate School of Engineering, Chiba University, has succeeded in elucidating, at the atomic level, the structural origin of "mystery" peaks derived from defects that appear in spectra from carbon materials, such as Raman spectroscopy (Note 1) and X-ray photoelectron spectroscopy (XPS) (Note 2), which have long been considered a "mystery." This achievement, which clarifies the influence of specific atomic structures like cyclic ethers and non-hexagonal rings (Note 3) on spectra through the fusion of spectroscopic analysis and simulation, has enabled the analysis of carbon material structures with unprecedented accuracy. As a new evaluation platform for accurately grasping the nature of defects, it is expected to accelerate the development of next-generation materials and be applied to the precise design of innovative carbon materials for applications ranging from aerospace engineering to fuel cells and insulation. This research was published online in the academic journal Journal of Materials Science on June 29, 2026. (Paper here: 10.1007/s10853-026-12911-9)

Background of the Research

Carbon materials, indispensable in all fields from aerospace engineering to fuel cells and insulation, depend heavily on their structural properties, particularly the types and distribution of defects, for their performance and specific applications. To date, Raman spectroscopy, infrared spectroscopy (IR), and XPS have been primarily used for the analysis of carbon materials. However, due to the diversity of structural conditions and inconsistencies in interpretation, accurately assigning specific peaks to local chemical structures has been extremely difficult, and understanding complex defect structures at the atomic level has been a long-standing "black box" in carbon materials science (References).

Figure: Elucidation of the structural origin of "defect peaks" in carbon materials. The red peak in the figure is the peak derived from defects. (Image: Gemini Pro)

Key Research Findings

Using carbon fibers (Note 4) as an analytical model, the research team constructed 34 graphene models including oxygen-containing functional groups, non-hexagonal rings, and vacancy defects, and performed detailed structural analysis through experimental evaluation and advanced spectral calculations (Figure).

1 Elucidation of the true origin of the 285 eV peak in XPS: It was discovered that the peak around 285 eV, widely believed to be due to sp3 hybridized carbon (Note 5), is actually derived from carbon atoms surrounded by three rings, including vacancy defects such as seven-membered and eight-membered rings.

2 Identification of the structural origin of the mystery peak in Raman spectroscopy: It was elucidated that the peak at 1500-1550 cm−1 originates from C=C bonds within six-membered rings influenced by nearby non-hexagonal rings and oxygen-containing functional groups such as cyclic ethers.

Future Outlook

This research has elucidated the influence of various defects, such as non-hexagonal rings and cyclic ethers, on Raman and XPS spectra, establishing a framework for accurate interpretation of spectral data. The precise atomic-level origin of specific peaks derived from these defects has been a long-standing mystery. These research findings provide a crucial foundation for evaluating the structure of various carbon materials with unprecedented accuracy. Understanding the precise nature of defects is essential for improving the mechanical, thermal, electrical, and chemical properties of materials, paving the way for the precise engineering of next-generation carbon materials for advanced industrial and environmental applications. In the future, we will proceed to elucidate different types of defect structures.

Glossary

Note 1) Raman spectroscopy: An analytical technique that investigates molecular vibrational states and crystal structure disorder by measuring light scattered (Raman scattered light) when a substance is irradiated with laser light. In carbon materials, it is widely used as an extremely common method for evaluating the degree of crystallinity and defects.

Note 2) X-ray photoelectron spectroscopy (XPS): An analytical technique that investigates the types of elements and chemical bonding states on the surface of a substance by measuring the energy of emitted electrons when the substance is irradiated with X-rays.

Note 3) Non-hexagonal ring: While the carbon network of normal graphene and other carbon planes is composed of a hexagonal (six-membered ring) network, it refers to anomalous structures such as pentagons (five-membered rings), heptagons (seven-membered rings), and octagons (eight-membered rings) that occur within that structure.

Note 4) Carbon fiber: A common carbon material prepared at high temperatures.

Note 5) sp3 hybridized carbon: A type of carbon atom bonding state where it forms four single bonds with its surroundings, creating a tetrahedral structure in three dimensions. Because of its extremely strong properties, it has generally been interpreted as the cause of peaks around 285 eV in XPS analysis of carbon materials.

Paper Information

Title: Unveiling origins of defect peaks in carbon materials by analyzing oxygen and non‐hexagonal rings in isotropic pitch‐based carbon fiber using Raman, infrared, X‐ray photoelectron spectroscopy, and density functional theory calculations

Authors: Y. Yamada*, M. Morimoto, T. Senda, K. Kondo, S. Sato, S. Kubo, T. Sogabe

Journal: Journal of Materials Science

DOI: 10.1007/s10853-026-12911-9

References

Title: Structural control and structural analyses of defects in carbon materials

Journal: Carbon Reports

DOI: 10.7209/carbon.030302