[论文解读] Raman Scattering by sp$^2$ Amorphous Carbons

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

Carbon Nanotubes in Composites参考文献 70被引用 4

一句话总结

本文通过识别稳定石墨烯分子作为基本结构单元（BSUs），提出了对sp²非晶碳（sp² ACs）中拉曼光谱的分子级解释，揭示了其独特的强制断裂非晶性。该研究将G-D-2D拉曼谱图与分子-晶体二重性关联，并将谱带展宽及双声子特征归因于电子密度离域化与键长分散，从而实现从分子模型到准粒子声子模型的尺寸依赖性转变。

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.

研究动机与目标

• 理解sp²非晶碳中拉曼光谱的起源，超越传统晶体模型。
• 识别这些材料中非晶性的结构基础，提出一种新类型：强制断裂非晶性。
• 建立一阶与双声子拉曼光谱的分子近似，将光谱特征与电子及几何畸变关联。
• 追踪随着尺寸增加，从分子到准粒子声子行为的转变，实现石墨烯中光学声子平均自由程的估算。

提出的方法

• 作者采用分子近似分析sp²非晶碳的拉曼光谱，将基本结构单元（BSUs）视为孤立的石墨烯分子。
• 通过在BSUs中引入因化学取代或应变引起的C=C键长分散，模拟一阶声子光谱。
• 通过电致非谐性解释双声子光谱，其归因于蜂窝晶格中高度离域的电子密度。
• 实验观测到从分子到准粒子声子模型的尺寸依赖性交叉，从而实现对光学声子平均自由程的估算。
• 分析基于G-D-2D拉曼谱图作为石墨烯材料中分子-晶体二重性的特征标志。
• 研究使用30页、9幅图、3张表及87篇参考文献，支持理论与光谱框架。

实验结果

研究问题

• RQ1 sp²非晶碳中非晶性的真正结构起源是什么？它与传统非晶化有何不同？
• RQ2 石墨烯分子单元中的键长分散与电子离域化如何影响观测到的拉曼光谱？
• RQ3 G-D-2D拉曼谱图在多大程度上可由分子级行为解释，而非扩展的晶体模型？
• RQ4 随着尺寸增加，这些材料中从分子到准粒子声子描述的转变过程如何发生？
• RQ5 电致非谐性在塑造sp²非晶碳双声子拉曼特征中起什么作用？

主要发现

• sp²非晶碳的基本结构单元（BSUs）被识别为稳定、孤立的石墨烯分子，由于强制断裂而形成基于分子的非晶结构。
• 标准G-D-2D拉曼谱图归因于碳原子的扩展蜂窝结构组成，并反映了石墨烯材料中的分子-晶体二重性。
• G带与D带的谱带展宽源于BSUs中C=C键长的显著分散，由化学或机械形变引起。
• 电致非谐性，由高电子密度离域化驱动，被确定为双声子拉曼特征形成的关键因素。
• 观测到尺寸驱动的从分子到准粒子声子模型的转变，从而实现对石墨烯中光学声子平均自由程的估算。
• 本研究首次将分子近似应用于石墨烯体系中的双声子拉曼光谱，揭示了电子结构与振动响应之间的直接关联。