Highly crystalline single-domain magnetite Fe3O4 nanoparticles (NPs) are important, not only for fundamental understanding of magnetic behaviour, but also for their considerable potential applications in biomedicine and industry. Fe3O4 NPs with sizes of 10–300 nm were systematically investigated to reveal the fundamental relationship between the crystal domain structure and the magnetic properties. The examined Fe3O4 NPs were prepared under well-controlled crystal growth conditions using a large-scale liquid precipitation method. The crystallite size of cube-like NPs estimated from X-ray diffraction pattern increased linearly as the particle size (estimated by transmission electron microscopy) increased from 10 to 64.7 nm, which indicates that the NPs have a single-domain structure. This was further confirmed by the uniform lattice fringes. The critical size of approximately 76 nm was obtained by correlating particle size with both crystallite size and magnetic coercivity; this was reported for the first time in this study. The coercivity of cube-like Fe3O4 NPs increased to a maximum of 190 Oe at the critical size, which suggests strong exchange interactions during spin alignment. Compared with cube-like NPs, sphere-like NPs have lower magnetic coercivity and remanence values, which is caused by the different orientations of their polycrystalline structure.

Correlations between crystallite/particle size and the luminescent characteristics of submicrometer phosphors were investigated. Spray pyrolyzed europium doped yttrium oxide (Y2O3:Eu3+) particles were selected as a model material. Crystallite size and the particle size were controlled independently. The morphology and crystallite structure were characterized by field-emission scanning electron microscopy, high-resolution transmission electron microscopy, X-ray diffraction, and selected area electron diffraction. Photoluminescence (PL) properties were examined by spectrofluorophotometry and an absolute PL quantum efficiency (QE) measurement system. Chemical analyses and elemental mapping were conducted by Fourier transform infrared spectrophotometry and STEM equipped with energy dispersive spectroscopy, respectively. The results revealed that the PL properties were strongly dependent on crystallite size, particle size, surface chemistry, and the distribution of europium inside the phosphor particles. The PL intensities and QE increased with increasing crystallite size and particle size. The effect of crystallite size on PL properties played a more important role than that of particle size.

Graphene quantum dots (GQDs) containing N atoms were successfully synthesized using a facile, inexpensive, and environmentally friendly hydrothermal reaction of urea and citric acid, and the effect of the GQDs’ C–N configurations on their photoluminescence (PL) properties were investigated. High-resolution transmission electron microscopy (HR-TEM) images confirmed that the dots were spherical, with an average diameter of 2.17 nm. X-ray photoelectron spectroscopy (XPS) analysis indicated that the C–N configurations of the GQDs substantially affected their PL intensity. Increased PL intensity was obtained in areas with greater percentages of pyridinic-N and lower percentages of pyrrolic-N. This enhanced PL was attributed to delocalized π electrons from pyridinic-N contributing to the C system of the GQDs. On the basis of energy electron loss spectroscopy (EELS) and UV-Vis spectroscopy analyses, we propose a PL mechanism for hydrothermally synthesized GQDs.

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Pyrrolic-N-rich carbon dots (CDs) that exhibit an absorption peak in the first near-infrared (NIR) window region were developed using a one-step microwave-assisted hydrothermal synthesis. A high concentration of urea enabled the introduction of a large amount of pyrrolic-nitrogen on the CD surfaces. Upon optimization of the experimental conditions, the absorption peak of the CDs red-shifted from 550 to 650 nm. The resulting pyrrolic-N-rich CDs exhibited photothermal effects with high NIR photothermal efficiency (54.3%) and photoluminescence. The prepared CDs, which show a first NIR window absorption peak, photoluminescence, and negative surface charge, have the potential to be used as multifunctional nanocarriers for cell imaging and drug delivery and as photothermal agents in cancer therapy.

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Mesopore-free hollow silica particles with a spherical shape, smooth surface, and controllable diameter (from 80 to 300 nm) and shell thickness (from 2 to 25 nm) were successfully prepared using an additive-free synthesis method. Different from other hollow particle developments, a mesopore-free shell was produced because of the absence of additive. Although common reports pointed out the importance of the additional additive in pasting and growing silica on the surface of a template, here we preferred to exploit the effect of the template charge in gaining the silica coating process. To form the silica, basic amino acid (i.e., lysine) was used as a catalyst to replace ammonia or hydrazine, which is harmless and able to control the silica growth and produce hollow particles with smooth surfaces. Control of the particle diameter was drastically achieved by altering the size of the template. The flexibility of the process in controlling the shell thickness was predominantly attained by varying the compositions of the reactants (i.e., silica source and catalyst). The present mesopore-free hollow particles could be efficiently used for various applications, especially for thermal insulator and optical devices because of their tendency not to adsorb large molecules, as confirmed by adsorption analysis.

A facile, economic and environmentally friendly one-step approach for the preparation of highly luminescent graphene quantum dots (GQDs) was developed using a hydrothermal reaction between citric acid and urea.