氨基功能化石墨烯量子点表面缺陷钝化及其发光增强

Surface Defect Passivation of Graphene Quantum Dots by Amino Functionalization and Photoluminescence Emission Enhancement

通过酸氧化法将氧化石墨烯进一步"切割"制备石墨烯量子点(GQDs),在100°C水热条件下,用氨水处理石墨烯量子点制备得到氨基功能化石墨烯量子点(N-GQDs)。傅里叶变换红外光谱证明NH_3可以有效地进攻环氧基碳和羧基碳,形成羟胺和酰胺基。原子力显微镜结果表明NH_3不仅能够有助于产生更小的量子点,还对石墨烯纳米片有致孔作用。氨基功能化之后,由于C―O―C相关的n-π*跃迁受到抑制,N-GQDs发光具有更弱的激发波长依赖性,并使其荧光量子产率从0.3%提高至9.6%。时间分辨发光光谱表明,相比含氧基团,含氮基团相关的局域电子激发态具有更长的荧光寿命和更弱的发射光谱依赖性。

In this work, graphene oxide sheets are cut into graphene quantum dots （GQDs） by acidic oxidation, then GQDs are hydrothermally treated with ammonia （NH3） at 100℃ to form amino-functionalized graphene quantum dots （N-GQDs）. Atomic force microscopy （AFM） shows smaller dots in ammonia treated GQDs, and holey graphene structure is directly observed. Fourier transform infrared （FTIR） spectra confirm that NH3 can effectively react with epoxy and carboxyl groups to form hydroxylamine and amide groups, respectively. The absorption and photoluminescence （PL） properties of the samples are determined by ultraviolet-visible-near infrared （UV-Vis-NIR） spectra and steady-state fluorescence spectra. Three PL excitation peaks occurring at around 250, 290, and 350 nm are attributed to C=C related π-π＊ transition, C-O-C and C=O related n-π＊ transitions, respectively. After amino functionalization, the C-O-C related n-π＊ transition is suppressed, and the PL emission spectrum of N-GQDs is less excitation wavelength. The fluorescence quantum yield of the N-GQDs is 9.6%, which is enhanced by 32 times compared with that of the unmodified GQDs （-0.3%）. Timeresolved PL spectra are also used to investigate the N-GQDs. The PL lifetimes depend on the emission wavelength and coincide with the PL spectrum, and are different from most fluorescent species. This result reveals the synergy and competition between defect derived photoluminescence and amino passivation of the N-GQDs. Compared with oxygen-related defects, nitrogen-related localized electronic states are expected to have a longer lifetime and enhanced radiative decay rates.