Role of Fluorescent By-Products, Structure and Optical Properties of White Emitting Carbon Dots

Carbon-based luminescent nanoparticles, which are usually referred to as carbon dots (CDs), are popular for light-emitting applications due to their fascinating optical properties and low-cost synthesis. In recent years, a desire to improve the photoluminescence (PL) quantum yield (QY) of CDs has led to the popularity of bottom-up synthesis methods. However, recent studies highlight the presence of fluorescent by-products in CD dispersions, especially in CDs synthesized by bottom-up methods. Here, the effects of fluorescent by-products and the structure of CDs are investigated. 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-diaminonaphthlene. The as-synthesized product (unpurified CDs) shows a strong blue PL emission in addition to a weak green PL emission. Transmission electron microscopy images show that the average size of as-synthesized CDs is ~23 ± 9.7 nm. A significant increase in particle size is observed as a function of time after synthesis when the dispersion is stored in a water and ethanol mixture. The average size of the CDs increases from 23 nm to 63 nm over a period of 13 months after the synthesis. With storage time, the blue emission is substantially reduced; PL QY decreases from 30% to 3% after 13 weeks. From these observations it is concluded that the blue emission is mostly from free-floating molecular fluorophores that slowly attach to the CDs and cause an increase in particle size [1]. Furthermore, the free-floating fluorophores are removed by dialysis and a detailed characterization of purified CDs is also carried out. X-ray photoelectron spectroscopy shows that the CDs also contain oxygen (31%) and nitrogen (9%) in addition to carbon (60%). Fourier-transform infrared and nuclear magnetic resonance spectroscopy analysis shows that the CDs are composed of aromatic as well as aliphatic carbon. Purified CDs show two distinct excitation-wavelength-independent PL emission bands: one at ~400 nm and another green emission band at ~520 nm.<br/>As in polar solvents, like water and ethanol, fluorophores can adsorb on CDs and cause particle size growth, the CDs in low polarity solvents can unravel into individual fluorophores. Detailed optical characterization of the CDs are carried out in different organic solvents. As solvent polarity decreases more fluorophores release from the CDs and redissolve in the solvent. It is observed that as solvent polarity decreases PL QY increases (e.g., PL QY in methanol is 3.5 % while in benzene it increases to 53.6%). PL lifetime also decreases with polarity of the solvents. Finally, the CDs are dispersed in two different solid matrices (epoxy and poly (methyl methacrylate) (PMMA)) and films are prepared. The PL QY of the films is 15.3 % and 8.0 % in PMMA and epoxy, 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. White color PL emission, high PL QY of the films, and dominating aromatic structure of the CDs suggest that they can be excellent candidates for light emitting device applications. Electroluminescence measurements and device fabrication will also be discussed.<br/>[1] N. Javed and D. M. O'Carroll, Nanoscale Adv., 2021,3, 182-189