At present, the actual mechanism of the photoluminescence (PL) of fluorescent carbon dots (CDs) is still an open debate among researchers. Because of the variety of CDs, it is highly important to summarize the PL mechanism for these kinds of carbon materials; doing so can guide the development of effective synthesis routes and novel applications. This review will focus on the PL mechanism of CDs. Three types of fluorescent CDs were involved: graphene quantum dots (GQDs), carbon nanodots (CNDs), and polymer dots (PDs). Four reasonable PL mechanisms have been confirmed: the quantum confinement effect or conjugated π-domains, which are determined by the carbon core; the surface state, which is determined by hybridization of the carbon backbone and the connected chemical groups; the molecule state, which is determined solely by the fluorescent molecules connected on the surface or interior of the CDs; and the crosslink-enhanced emission (CEE) effect. To give a thorough summary, the category and synthesis routes, as well as the chemical/physical properties for the CDs, are briefly introduced in advance.

The photoluminescence mechanism in carbon dots (graphene quantum dots, carbon nanodots, and polymer dots): current state and future perspective

The emerging graphene quantum dots (GQDs) and carbon dots (C-dots) have gained tremendous attention for their enormous potentials for biomedical applications, owing to their unique and tunable photoluminescence properties, exceptional physicochemical properties, high photostability, biocompatibility, and small size. This article aims to update the latest results in this rapidly evolving field and to provide critical insights to inspire more exciting developments. We comparatively review the properties and synthesis methods of these carbon nanodots and place emphasis on their biological (both fundamental and theranostic) applications.

Glowing Graphene Quantum Dots and Carbon Dots: Properties, Syntheses, and Biological Applications

A facile approach for preparation of photoluminescent (PL) carbon dots (CDs) is reported. The three resulting CDs emit bright and stable red, green and blue (RGB) colors of luminescence, under a single ultraviolet-light excitation. Alterations of PL emission of these CDs are tentatively proposed to result from the difference in their particle size and nitrogen content. Interestingly, up-conversion (UC)PL of these CDs is also observed. Moreover, flexible full-color emissive PVA films can be achieved through mixing two or three CDs in the appropriate ratios. These CDs also show low cytotoxicity and excellent cellular imaging capability. The facile preparation and unique optical features make these CDs potentially useful in numerous applications such as light-emitting diodes, full-color displays, and multiplexed (UC)PL bioimaging.

Red, Green, and Blue Luminescence by Carbon Dots: Full‐Color Emission Tuning and Multicolor Cellular Imaging

and Applications of Fullerenes, Carbon Dots, Nanotubes, Graphene, Nanodiamonds, and Combined Superstructures Vasilios Georgakilas,† Jason A. Perman,‡ Jiri Tucek,‡ and Radek Zboril*,‡ †Material Science Department, University of Patras, 26504 Rio Patras, Greece ‡Regional Centre of Advanced Technologies and Materials, Department of Physical Chemistry, Faculty of Science, Palacky University in Olomouc, 17 listopadu 1192/12, 771 46 Olomouc, Czech Republic

Broad family of carbon nanoallotropes: classification, chemistry, and applications of fullerenes, carbon dots, nanotubes, graphene, nanodiamonds, and combined superstructures.

To obtain graphene-based fluorescent materials, one of the effective approaches is to convert one-dimensional (1D) graphene to 0D graphene quantum dots (GQDs), yielding an emerging nanolight with extraordinary properties due to their remarkable quantum confinement and edge effects. In this review, the state-of-the-art knowledge of GQDs is presented. The synthetic methods were summarized, with emphasis on the top-down routes which possess the advantages of abundant raw materials, large scale production and simple operation. Optical properties of GQDs are also systematically discussed ranging from the mechanism, the influencing factors to the optical tunability. The current applications are also reviewed, followed by an outlook on their future and potential development, involving the effective synthetic methods, systematic photoluminescent mechanism, bandgap engineering, in addition to the potential applications in bioimaging, sensors, etc.

Focusing on luminescent graphene quantum dots: current status and future perspectives

Amino-functionalized graphene quantum dots (af-GQDs) with discrete molecular weights and specific edges were self-limitedly extracted from oxidized graphene sheet. Their optical properties can be precisely controlled only by the selective and quantitative functionalization at the edge sites. The af-GQDS exhibit bright colorful fluorescence under a single-wavelength excitation.

Optically tunable amino-functionalized graphene quantum dots

A nitrogen-containing graphene structure is provided with: a graphene structure configured from one graphene nanosheet or multiple layers of graphene nanosheets; and nitrogen introduced into the graphene structure. Preferably, the nitrogen-containing graphene structure is provided with a graphene structure configured from one graphene nanosheet or multiple layers of graphene nanosheets and having: a sheet portion including an armchair-shaped edge surface at the edge of the sheet; and a terminal six-membered ring which shares and bonds with only one side of the armchair-shaped edge surface. Preferably, the nitrogen-containing graphene structure is also provided with a nitrogen-containing functional group which bonds to at least one carbon atom selected from among a carbon atom (a) that does not bond to the armchair-shaped edge surface among the carbon atoms constituting the terminal six-membered ring, and a carbon atom (b) constituting the sheet portion (including the carbon atoms on the sheet portion sharing a side with the terminal six-membered ring). A phosphor dispersion liquid is formed by dispersing such a nitrogen-containing graphene structure into a solvent.

Nitrogen-containing graphene structure, and phosphor dispersion liquid

A nitrogen-containing graphene structure has a graphene structure including a monolayer or multilayer graphene nanosheet and nitrogen introduced into the graphene structure. The nitrogen-containing graphene structure preferably includes the above-described graphene structure having a sheet portion comprised of a monolayer or multilayer graphene nanosheet and containing, at an edge portion thereof, an armchair edge-face portion and a terminal six-membered ring bound to the armchair edge-face portion while sharing only one side therewith; and a nitrogen-containing functional group bound to any one or more carbon atoms selected from (a) the carbon atoms constituting the terminal six-membered ring but not bound to the armchair edge-face portion, and (b) the carbon atoms constituting the sheet portion (including the carbon atom on the side shared by the terminal six-membered ring). A phosphor dispersion is a dispersion of such a nitrogen-containing graphene structure in a solvent.

Nitrogen-containing graphene structure and phosphor dispersion

Diluters play a crucial role for reducing the rotational viscosity of liquid crystal (LC) materials for improving the response property of LC displays (LCDs). However, conventional diluters tend to deteriorate relatively easily under ultraviolet (UV) exposure, due to existence of vinyl groups. Hence, it was difficult to prepare polymer layers in the LC cells by exposing UV for polymerization regarding polymer-sustained vertically-aligned (PS-VA) and polyimide-free (PI-free) technologies. In this study, however, we succeeded to develop a high stable diluter, which shows a symmetrical structure and carries fluorinated groups for stable nematic phase and high affinity with other fluorinated LC compounds. With the use of the symmetrical-structured diluter, we developed the PS-VA and the PI-free in-plane switching cells with a fast response rate and high voltage holding ratio, by combining a reactive monomer including an azobenzene moiety.

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Kazuo Okamoto

Papers

Optically tunable amino-functionalized graphene quantum dots

Nitrogen-containing graphene structure, and phosphor dispersion liquid

Nitrogen-containing graphene structure and phosphor dispersion

New Diluter Solubilized in Liquid Crystal Compounds for High Stability and Fast Response Speed Liquid Crystal Displays