Multimodal Characterization of Nitrogen-Doped Carbon Quantum Dots

The weight-average (M_w) and number-average (M_n) molecular weights of the AL were determined using gel permeation chromatography (GPC, PL-GPC50, Agilent Technologies, USA). Two-dimensional heteronuclear single quantum coherence (2D-HSQC, III 400 MHz NMR, Bruker, Germany) spectra were acquired from AL samples of approximately 60 mg dissolved in 0.5 mL DMSO-d₆.^{25 (link)} Ultraviolet-visible (UV-vis) absorption spectra of the NCQDs were also recorded (TU-1950, Purkinje, China). The FL emissions of the NCQDs were assessed using an FL spectrophotometer (F-7000, Agilent Technologies, USA) equipped with a 150 W xenon lamp as the excitation source. The structures of the NCQDs were examined by transmission electron microscopy (TEM) and high-resolution TEM (HRTEM) using an instrument (JEM 2100, JEOL Ltd., Japan) at an accelerating voltage of 200 kV. Fourier transform infrared (FT-IR) spectra of the NCQDs were obtained (iS10, Nicolet, USA), and X-ray photoelectron spectroscopy (XPS) data were obtained (Escalab 250 XI, TERMO, USA) using Al Kα radiation. X-ray diffraction (XRD) patterns were obtained with an instrument (D8, Bruker AXS, Germany) with a Cu Kα radiation source.