Carbon-based nanoparticles, which are usually referred to as carbon dots (CDs), are popular due to their strong photoluminescence (PL). Light-emitting applications of CDs rely on their optical properties in the solid state, which are often overlooked. Here, we investigate the structure and optical properties of CDs, not only in different solvents, but also in several solid matrices. Additionally, the suitability of CDs for use in light-emitting applications is examined.<br/>The CDs are synthesized by a solvothermal method using citric acid and 1,5-diaminonaphthalene, and purified by dialysis. TEM images show that the average size of the CDs is 19.5 ± 6.7 nm. XRD patterns and high-resolution scanning TEM images of the CDs indicate the amorphous structure of the CDs. FTIR and NMR spectroscopy analyses show that the CDs are composed of aromatic as well as aliphatic carbon. The CDs show two distinct excitation-wavelength-independent PL emission bands: one at ~400 nm and another green emission band at ~520 nm. Comprehensive structural analysis and density functional theory calculations show that different emission colors arise from the two different molecular fluorophores: the violet emission arises from aliphatic molecules that are most likely fragments of citric acid, whereas the green-emitting fluorophores have aromatic structure and are identified as N4,N11-dimethyldibenzo[a,h]phenazine-4,11-diamine.<br/>Detailed optical characterization of the CDs are carried out in different organic solvents. It is observed that as solvent polarity decreases PL quantum yield (QY) increases (e.g., PL QY in methanol is 3.5 %, while in benzene it increases to 53.6%). Finally, the CDs are dispersed in different solid matrices (optical epoxy, poly(methyl methacrylate) (PMMA), and polystyrene (PS)) and films are prepared. It is observed that the PL QY of CDs in solid matrices strongly depends on the concentration (wt. %) of CDs. Increasing the concentration of CDs results in strong reabsorption and self-quenching. The PL QY of the films with optimum concentration of CDs is 33.5 %, 30 % and 22.5 % in PMMA, epoxy, and PS, respectively. In addition to promising PL QY values, the CDs films show broadband, white color PL emission with CIE (International Commission on Illumination) coordinates of (0.34, 0.42) in PMMA and (0.33, 0.35) in epoxy. The white PL emission, high PL QY of the films suggest that they can be excellent candidates for UV-pumped light-emitting device applications.

Structure and optical properties of white emitting carbon dots in solution and solid states

Fall 2022 abstracts, posters, and presentations will launch in mid-September.

ACS Fall 2022

The official archive and repository for all ACS Meetings & Expositions abstracts and select posters or presentations.