Optical properties of doped and undoped 9-armchair graphene nanoribbons [data]

Graphene nanoribbons are one-dimensional stripes of graphene with width- and edge-structure-dependent electronic properties. They can be synthesized bottom-up to obtain precise ribbon geometries. Here we investigate the optical properties of solution-synthesized 9-armchair graphene nanoribbons (9-aGNRs) that are stabilized as dispersions in organic solvents and further fractioned by liquid cascade centrifugation (LCC). Absorption and photoluminescence spectroscopy reveal two near-infrared absorption and emission peaks whose ratios depend on the LCC fraction. Similarly, the Raman D/G-mode ratios vary with fraction and indicate a higher defect density for fractions obtained at higher centrifugal forces. Low-temperature single-nanoribbon photoluminescence spectra suggest the presence of two different nanoribbon species. Based on density functional theory (DFT) and time-dependent DFT calculations, pristine 9-aGNRs are assigned to the lowest energy transitions and 9-aGNRs with edge-defects, introduced by an incomplete graphitization, are assigned to more blue-shifted transition peaks. Hole doping of 9-aGNRs dispersion with the electron acceptor F4TCNQ leads to concentration dependent bleaching of the main absorption bands and redshifted, charge-induced absorption features but no new emission features, thus, indicating the formation of polarons instead of trions (charged excitons) in charged 9-aGNRs.

石墨烯纳米带（graphene nanoribbons）是一类一维石墨烯条带，其电子特性由条带宽度与边缘结构共同决定，可通过自下而上法合成以获得精准可控的几何形貌。本研究针对溶液合成的9-扶手椅型石墨烯纳米带（9-armchair graphene nanoribbons, 9-aGNRs）展开光学特性探究：该类纳米带以分散液形式稳定存在于有机溶剂中，并通过液相级联离心法（liquid cascade centrifugation, LCC）完成分级分离。
吸收光谱与光致发光光谱测试结果显示，体系存在两处近红外吸收与发射峰，其峰强度比值随液相级联离心分级组分的不同而变化。类似地，拉曼D/G模强度比也随分级组分发生改变，且更高离心力下获得的分级组分具有更高的缺陷密度。
低温下单纳米带光致发光光谱表明，体系中存在两种不同的纳米带物种。基于密度泛函理论（density functional theory, DFT）与含时密度泛函理论（time-dependent DFT）的计算结果，本征9-aGNRs对应最低能量跃迁，而因石墨化不完全引入边缘缺陷的9-aGNRs则对应更蓝移的跃迁峰。
利用电子受体F4TCNQ对9-aGNRs分散液进行空穴掺杂后，主吸收带出现浓度依赖的光谱漂白现象，同时伴随红移的电荷诱导吸收特征，但未产生新的发射峰。上述结果表明，带电9-aGNRs中形成的是极化子（polarons）而非三重子（charged excitons）。