No abstract

The basal plane of graphene has been known to be less reactive than the edges, but some studies observed vacancies in the basal plane after reaction with oxygen gas. Observation of these vacancies has typically been limited to nanometer-scale resolution using microscopic techniques. This work demonstrates the introduction and observation of subnanometer vacancies in the basal plane of graphene by heat treatment in a flow of oxygen gas at low temperature such as 533 K or lower. High-resolution transmission electron microscopy was used to directly observe vacancy structures, which were compared with image simulations. These proposed structures contain C═O, pyran-like ether, and lactone-like groups.

C1s X-ray photoelectron spectroscopy (XPS) spectra of graphene with two to eight pentagons and fullerene pentagons were simulated using density functional theory calculation. Peak shifts and full width at half maximum (FWHM) of calculated C1s spectra were compared with those of actual C1s spectra.

Graphene nanoribbons (GNRs) have recently emerged as alternative 2D semiconductors owing to their fascinating electronic properties that include tunable band gaps and high charge-carrier mobilities. Identifying the atomic-scale edge structures of GNRs through structural investigations is very important to fully understand the electronic properties of these materials. Herein, we report an atomic-scale analysis of GNRs using simulated X-ray photoelectron spectroscopy (XPS) and Raman spectroscopy. Tetracene with zigzag edges and chrysene with armchair edges were selected as initial model structures, and their XPS and Raman spectra were analyzed. Structurally expanded nanoribbons based on tetracene and chrysene, in which zigzag and armchair edges were combined in various ratios, were then simulated. The edge structures of chain-shaped nanoribbons composed only of either zigzag edges or armchair edges were distinguishable by XPS and Raman spectroscopy, depending on the edge type. It was also possible to distinguish planar nanoribbons consisting of both zigzag and armchair edges with zigzag/armchair ratios of 4:1 or 1:4, indicating that it is possible to analyze normally synthesized GNRs because their zigzag to armchair edge ratios are usually greater than 4 or less than 0.25. Our study on the precise identification of GNR edge structures by XPS and Raman spectroscopy provides the groundwork for the analysis of GNRs.

Epoxide is one of the simplest functional groups on fullerenes, but mechanisms for migration of oxygen atoms and formations of CO and CO 2 gases upon heat treatment are still unclear. In this work, epoxidized fullerenes were heated in helium gas up to 673 K and the pyrolyzed structures of epoxidized fullerenes were analyzed using X-ray photoelectron spectroscopy, infrared spectroscopy, direct-injection mass spectrometry, elemental analysis, and density functional theory calculation. Functional groups such as CO and lactone groups were formed by heat treatment of the epoxidized fullerenes at 523 K. At 673 K, lactone groups were decomposed into CO and CO 2 gases. The amount of the CO 2 gas was more than that of the CO gas. This suggests that a formation of CO 2 gas from lactone groups is energetically more favorable than that of CO gas. Moreover, the ratio of CO 2 gas to CO gas increased, as the amount of oxygen atoms on fullerenes increased. The formation of CO and CO 2 gases at 673 K indicates that the presence of carbene sites with either vacancy defects or ether groups on fullerenes.