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Abstract
Chemically derived graphene quantum dots (GQDs) to date have showed very broad emission linewidth due to many kinds of chemical bondings with different energy levels, which significantly degrades the color purity and color tunability. Here, we show that use of aniline derivatives to chemically functionalize GQDs generates new extrinsic energy levels that lead to photoluminescence of very narrow linewidths. We use transient absorption and time-resolved photoluminescence spectroscopies to study the electronic structures and related electronic transitions of our GQDs, which reveals that their underlying carrier dynamics is strongly related to the chemical properties of aniline derivatives. Using these functionalized GQDs as lumophores, we fabricate light-emitting didoes (LEDs) that exhibit green, orange, and red electroluminescence that has high color purity. The maximum current efficiency of 3.47 cd A−1 and external quantum efficiency of 1.28% are recorded with our LEDs; these are the highest values ever reported for LEDs based on carbon-nanoparticle phosphors. This functionalization of GQDs with aniline derivatives represents a new method to fabricate LEDs that produce natural color.
Highlights
Graphene quantum dots (GQDs) are graphene derivatives of nanometer size[1,2]; they form platelets that have an energy gap that is caused by either quantum confinement[3,4] or edge effects[5,6,7]
We find that the energy gap was related to the chemical properties of aniline derivatives, and its related electronic transitions were thoroughly studied by means of transient absorption and time-resolved photoluminescence spectroscopies
Bare graphene quantum dots (GQDs) were prepared by amidative cutting of graphite oxide as described previously with slight modification24, chemically functionalized using aniline derivatives: 6-aminoquinoline (1), 4-methoxyaniline (2), and 4-(methylthio)aniline (3) (Fig. 1a; details in Methods)
Summary
Graphene quantum dots (GQDs) are graphene derivatives of nanometer size[1,2]; they form platelets that have an energy gap that is caused by either quantum confinement[3,4] or edge effects[5,6,7]. Because almost all reduction processes have limited reducing power, GQDs are likely to preserve “extrinsic” chemical groups, especially oxides and nitrides Such chemical groups could have many kinds of bonding states with different energy levels, mostly related to nonbonding (n) and π orbitals. GQDs have been used as phosphors to convert monochromatic light (usually blue) to white light27 These results demonstrate the possibility of using GQDs in light-emitting devices, but a main challenge for practical applications is to improve color purity and to realize green and red light. We demonstrate LEDs that use GQDs functionalized with a series of aniline derivatives to produce green, orange, and red light that has excellent color purity
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