Fluorescent carbon dots are nontoxic nanoparticles composed of carbon, exhibiting advantageous properties for applications in bioimaging and functional materials. We present a methodology for synthesizing fluorescent nitrogen-doped carbon dots (N-CDs) using starch, a biopolymer, and urea as the sources of nitrogen, via the microwave-assisted hydrothermal method. Furthermore, the dependence of the fluorescence spectra and fluorescence quantum yield of N-CDs on the initial concentration of urea in the reactant solution was examined, thereby providing a comprehensive understanding of the influence of nitrogen doping on the CDs. The fluorescence of N-CDs was tunable by varying the excitation wavelength. Stronger fluorescence intensity was observed for a moist phosphate salt/N-CD composite, in contrast to the weaker fluorescence exhibited by a dried one. Fluorescence lifetime measurements revealed that the change in fluorescence intensity can be attributed to the suppression of the non-radiative deactivation process. This observation highlights the critical importance of the interaction between water molecules and surface functional groups in controlling the photophysics of the excited state of N-CDs.

Tetracyanoquinodimethane (TCNQ) is an important constituent of organic conductors and a versatile electron acceptor. TCNQ exhibited thermally activated delayed fluorescence and an unusually long fluorescence lifetime. In this study, we studied the Stark effect on the absorption spectrum of TCNQ using electroabsorption spectroscopy to gain insights into its photophysics. The electroabsorption spectrum was simulated using multiple absorption bands for different electronic states, which were characterized by different dipole moments and polarizabilities. These electronic states are identified as a locally excited (LE) state with a high oscillator strength and zero dipole moment, and an intramolecular charge transfer (ICT) state with a nonzero dipole moment. The mixing of the LE state with the ICT state is augmented when the molecule is perturbed by an electric field. We provide tangible experimental evidence establishing the key role of mixing between the emissive LE and nonemissive ICT states in the deactivation pathway of electronically excited TCNQ. The dipole moment of the ICT state suggests symmetry breaking of the structure belonging to the D_2h point group.

Perylenetetracarboxylic diimide (PTCDI) is an n-type organic semiconductor molecule that has been widely utilized in numerous applications such as photocatalysis and field-effect transistors. Polarizability and dipole moment, which are inherent properties of molecules, are important parameters that determine their responses to external electric and optical fields, physical properties, and reactivity. These parameters are fundamentally important for the design of innovative materials. In this study, the effects of external electric fields on absorption and fluorescence spectra were investigated to obtain the PTCDI parameters. The PTCDI substituted by an octyl group (N,N′-Dioctyl-3,4,9,10-perylenedicarboximide) dispersed in a polymethyl methacrylate (PMMA) matrix was studied in this work. The features of vibronic progression in the absorption spectrum were analogous to those observed in solution. The red shift of the absorption band caused by the Stark effect was mainly observed in the presence of an external electric field. Changes in parameters such as the dipole moment and polarizability between the ground and the Franck–Condon excited states of the PTCDI monomer were determined. The fluorescence spectrum shows a contribution from a broad fluorescence band at wavelengths longer than the monomer fluorescence band. This broad fluorescence is ascribed to the excimer-like fluorescence of PTCDI. The effects of the electric field on the fluorescence spectrum, known as the Stark fluorescence or electrofluorescence spectrum, were measured. Fluorescence quenching is observed in the presence of an external electric field. The change in the polarizability of the monomer fluorescence band is in good agreement with that of the electroabsorption spectrum. A larger change in the polarizability was observed for the excimer-like fluorescence band than that for the monomer band. This result is consistent with exciton delocalization between PTCDI molecules in the excimer-like state.

This study introduces an innovative experimental method that combines real-time heating and three-dimensional (3D) observation techniques to examine the sintering process of copper nanoparticles (Cu NPs) using scanning transmission electron microscopy (STEM). This study visualizes the time-resolved 3D dynamics through in situ heating and electron tomography (ET), minimizing contamination by employing a novel MEMS-based heating holder and optimized high-resolution time-resolved 3D data acquisition. Cu NPs (approximately 150 nm in diameter) were suspended in ethanol, placed on MEMS heating chips within an argon-filled glove box, and transferred to TEM without exposure to air. Low-dose HAADF-STEM imaging was used to minimize beam-induced damage and contamination. The block-matching and 3D filtering (BM3D) method was utilized for eliminating noise from images, and the iterative series reduction (ISER) method was employed in order to make certain that the 3D reconstruction was accurate. In situ two-dimensional (2D) observations indicated neck development and grain densification with rising temperatures, whereas three-dimensional (3D) reconstructions provided quantitative measurements of neck width, interparticle distances, particle radii, and aspect ratios. This study illustrates how sophisticated imaging techniques can help us understand nanoscale sintering dynamics, provide us with insightful experimental data for computational models, and set up a solid framework for investigating nanoscale material properties.