Structural origins of carbon quantum dot luminescence by synchrotron x-ray spectroscopy

Abstract

A carbon quantum dot (CQD) sample series was synthesized from citric acid and varying concentrations of thiourea. The highest (sample 1) and lowest (sample 2) concentrations of thiourea exhibited unique visual effects and electronic structures. X-ray excited optical luminescence (XEOL) along with UV-visible spectroscopy provided unique insight into the absorption and emission mechanisms of samples 1 and 2, where only sample 2 emitted XEOL. Sample 1 exhibited the commonly observed aggregation caused quenching (ACQ) effects in the solid state. While sample 2 displayed unique aggregation induced emissions (AIE) effects upon exciting the sample above the C K edge. The AIE and ACQ sample differences were suspected to be from S moiety differences arising from the varying thiourea concentrations during synthesis. Furthermore, x-ray absorption spectroscopy (XAS) in modes of total electron yields (TEY) and partial fluorescence yields (PFY) allowed the identification of specific core and surface states of the CQDs. It was discovered that thiophene moieties were uniquely formed in the AIE sample’s surface and not anywhere in the ACQ CQD sample. The thiophene surface functionality is believed to be a significant contributor to the AIE effects seen in the XEOL studies. Understanding and preventing the common CQD ACQ mechanism allows the application of CQDs in solid lighting applications.