Abstract
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.
Highlights
Graphene quantum dots (GQDs), as a new type of zero-dimensional quantum dot, have drawn intense attention due to their unique properties, such as chemical stability, low toxicity, dispersibility in water, controllable size, and wavelength tenability [1,2,3,4,5]
The PL emission of GQDs excited at a wavelength of 400 nm showed a peak centered at 473 nm without chemical passivation additives
To study the SERS of GQDs, pure GQDs without chemical additives, residues and stabilizers that usually exist in conventionally chemically synthesized GQDs were fabricated by ablating highly oriented pyrolytic graphite (HOPG) with a pulsed laser beam along the orientation parallel to the graphite layers in deionized water
Summary
Graphene quantum dots (GQDs), as a new type of zero-dimensional quantum dot, have drawn intense attention due to their unique properties, such as chemical stability, low toxicity, dispersibility in water, controllable size, and wavelength tenability [1,2,3,4,5]. The GQD domains have a band gap that can be tuned by changing their size and shape [14,15,16]. The periphery and surface of the GQD domains can be chemically passivated by electron-donating substituents. GQDs exhibit photoluminescence (PL) due to the quantum confinement effect and surface passivation effect [17,18,19,20,21,22]
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