Separation of Spectroscopically Uniform Nanographenes

Area of Science:

• Materials Science
• Nanotechnology
• Photochemistry

Background:

• Graphene quantum dots (GQDs) exhibit excitation-dependent photoluminescence (PL), a property that hinders their application in photofunctional materials.
• Uniform PL properties in GQDs are highly desirable for consistent performance in various applications.
• The structural origins of excitation-dependent PL in GQDs are linked to the sp² domains on their surfaces.

Purpose of the Study:

• To establish a protocol for separating spectroscopically uniform lipophilic GQDs from a mixture.
• To investigate the relationship between GQD size, sp² domain structure, and photoluminescence properties.
• To differentiate between GQDs with excitation-independent and excitation-dependent PL.

Main Methods:

• Synthesis of GQD-1 mixture via reaction of p-methoxybenzylamine with GQD-2 (derived from graphite).
• Separation of GQD-1 into GQD-1a (large) and GQD-1b (small) using size-exclusion chromatography.
• Characterization of PL properties with respect to excitation wavelengths for both GQD fractions.

Main Results:

• Successful separation of GQD-1a (19.8 nm) and GQD-1b (4.9 nm) with distinct PL characteristics.
• Large GQD-1a exhibited PL largely independent of excitation wavelength.
• Small GQD-1b displayed excitation-dependent PL, attributed to variations in sp² domain sizes and band gaps.

Conclusions:

• The study successfully isolated GQDs with uniform PL properties (GQD-1a).
• GQD size and the nature of surface sp² domains are critical determinants of PL behavior.
• Large, well-developed sp² domains in larger GQDs lead to excitation-independent PL, while smaller domains in smaller GQDs result in excitation-dependent PL.