Photoluminescence Properties of Graphene versus Other Carbon Nanomaterials

Abstract

Photoluminescent nanomaterials continue to garner research attention because of their many applications. For many years, researchers have focused on quantum dots (QDs) of semiconductor nanocrystals for their excellent performance and predictable fluorescence color variations that depend on the sizes of the nanocrystals. Even with these advantages, QDs can present some major limitations, such as the use of heavy metals in the high-performance semiconductor QDs. Therefore, researchers continue to be interested in developing new QDs or related nanomaterials. Recently, various nanoscale configurations of carbon have emerged as potential new platforms in the development of brightly photoluminescent materials. As a perfect π-conjugated single sheet, graphene lacks electronic bandgaps and is not photoluminescent. Therefore, researchers have created energy bandgaps within graphene as a strategy to impart fluorescence emissions. Researchers have explored many experimental techniques to introduce bandgaps, such as cutting graphene sheets into small pieces or manipulating the π electronic network to form quantum-confined sp(2) "islands" in a graphene sheet, which apparently involve the formation or exploitation of structural defects. In fact, defects in graphene materials not only play a critical role in the creation of bandgaps for emissive electronic transitions, but also contribute directly to the bright photoluminescence emissions observed in these materials. Researchers have found similar defect-derived photoluminescence in carbon nanotubes and small carbon nanoparticles, dubbed carbon "quantum" dots or "carbon dots". However, they have not systematically examined the emissions properties of these different yet related carbon nanomaterials toward understanding their mechanistic origins. In this Account, we examine the spectroscopic features of the observed photoluminescence emissions in graphene materials. We associate the structural characteristics in the underlying graphene materials with those emission properties as a way of classifying them into two primary categories: emissions that originate from created or induced energy bandgaps in a single graphene sheet and emissions that are associated with defects in single- and/or multiple-layer graphene. We highlight the similarities and differences between the observed photoluminescence properties of graphene materials and those found in other carbon nanomaterials including carbon dots and surface defect-passivated carbon nanotubes, and we discuss their mechanistic implications.

Origin and Perspectives of the Photochemical Activity of Nanoporous Carbons

“…Their photoluminescence characteristics are linked to the sizes and ratio of sp 2 and sp 3 domains. The mechanism is considered complex and involves 1) defects' state emission/surface energy traps and 2) intrinsic state emissions, which include electron–hole recombination, quantum size effects/zigzag states 170, 171, 172, 173, 174. The emission characteristics were linked to surface chemistry and thus to alterations in the bandgap 175.…”

Sensitivity to Heavy-Metal Ions of Unfolded Fullerene Quantum Dots

“…In particular, innovative carbon-based nanomaterials such as graphene quantum dots (GQDs) and graphene oxide quantum dots (GOQDs) have attracted much interest as stable, nontoxic, photoluminescent nanomaterials with possible applications in chemical and biological sensors, drug delivery, bio-imaging and energy conversion [7][8][9][10][11][12][13]. The physical origin of the photoluminescence (PL) from these carbonbased nanoparticles is still quite controversial and different mechanisms have been suggested, possibly acting together, mainly based on quantum confinement in sp 2 domains with a strong influence from edge and oxygen-containing functional groups [14][15][16][17][18]. Nonetheless, several groups have reported a characteristic fluorescence with uniform and stable features in aqueous solution of GOQDs prepared by a variety of methods, either based on bottom-up or on top-down synthesis from graphite [19][20][21][22][23][24][25][26][27].…”

Towards efficient photoinduced charge separation in carbon nanodots and TiO2 composites in the visible region

“…[1][2][3][4][5][6][7][8] Especially, CDs, owing to their superior performance in terms of water solubility, stability, toxicity, resistance to photobleaching, and biocompatibility, have recently drawn significant attention. [9][10][11][12][13][14][15] The electron injection from CDs to TiO 2 was demonstrated to be feasible and the CD-sensitized TiO 2 photoelectrodes have been applied in photocatalysis and PVs.…”