Wu Junxiao, Wang Peijie, Wang Fuhe, Fang Yan
The Beijing Key Laboratory for Nano-photonics and Nano-structure, Department of Physics, Capital Normal University, Beijing 100048, China.
Nanomaterials (Basel). 2018 Oct 22;8(10):864. doi: 10.3390/nano8100864.
Photoluminescence (PL) is the most significant feature of graphene quantum dots (GQDs). However, the PL mechanism in GQDs has been debated due to the fact that the microstructures, such as edge and in-plane defects that are critical for PL emission, have not been convincingly identified due to the lack of effective detection methods. Conventional measures such as high-resolution transmission electron microscopy and infrared spectroscopy only show some localized lattice fringes of GQDs and the structures of some substituents, which have little significance in terms of thoroughly understanding the PL effect. Here, surface-enhanced Raman spectroscopy (SERS) was introduced as a highly sensitive surface technique to study the microstructures of GQDs. Pure GQDs were prepared by laser ablating and cutting highly oriented pyrolytic graphite (HOPG) parallel to the graphite layers. Consequently, abundant SERS signals of the GQDs were obtained on an Ag electrode in an electrochemical environment for the first time. The results convincingly and experimentally characterized the typical and detailed features of GQDs, such as the crystallinity of sp² hexagons, the quantum confinement effect, various defects on the edges, sp³-like defects and disorders on the basal planes, and passivated structures on the periphery and surface of the GQDs. This work demonstrates that SERS is thus by far the most effective technique for probing the microstructures of GQDs.
光致发光（PL）是石墨烯量子点（GQDs）最为显著的特性。然而，由于缺乏有效的检测方法，对于GQDs中光致发光的微观结构，如对PL发射至关重要的边缘和面内缺陷，尚未得到令人信服的识别，因此GQDs中的PL机制一直存在争议。诸如高分辨率透射电子显微镜和红外光谱等传统方法仅显示了GQDs的一些局部晶格条纹以及一些取代基的结构，这对于全面理解PL效应意义不大。在此，表面增强拉曼光谱（SERS）作为一种高灵敏度的表面技术被引入，用于研究GQDs的微观结构。通过激光烧蚀并切割与石墨层平行的高度取向热解石墨（HOPG）制备了纯GQDs。结果，首次在电化学环境中的银电极上获得了GQDs丰富的SERS信号。这些结果令人信服地通过实验表征了GQDs的典型和详细特征，如sp²六边形的结晶度、量子限制效应、边缘的各种缺陷、基面的类sp³缺陷和无序以及GQDs外围和表面的钝化结构。这项工作表明，SERS是迄今为止探测GQDs微观结构最有效的技术。
