In the current review paper, we provide the experimental evidences for the controlled structure of diamond cubic silicon nanocrystals (SiNCs) which enhances their photoluminescence quantum yields (PLQYs). Hydrogen-terminated Si provides a basic surface for further modification. Their enhancement was performed by a simple ligand exchange between the hydrogen atoms and hydrocarbon chains. On the basis of the systematic study on temperature dependence of PL properties along with relaxation dynamics, a long-accepted mechanism for enhancing absolute PL was recently revised. Specifically, combination of PL spectroscopic measurement from cryogenic to room temperature with structural characterization allows us to link the enhanced PLQYs with the notable difference in surface structure. The hydride-terminated surface suffers from the presence of a large amount of nonradiative relaxation channels whereas the passivation with alkyl monolayers suppresses the creation of the nonradiative relaxation channels to yield the high PLQY. This anchoring effect was responsible for the PLQYs as high as 56%. Tunability of PL bands was achieved in the wavelength ranging between 590 nm and 1064 nm. Next, the review considers the use of the highly emitting SiNCs for optoelectronic and biophotonic applications. The tunable light emitting diodes in which SiNCs serve as active layers are demonstrated. Si exhibits a high chemical affinity to covalent linkages with carbon, oxygen, and nitrogen, thereby producing almost unlimited variations in organic–Si architectures hybridized at the molecular level. Therefore, biomedical applications of SiNCs are attractive. Details of the biophotonic applications are not described in this review, but functional near-IR (NIR) emitting nanoparticles of SiNCs of narrow PL spectra having no long emission tails, continuously tunable over the 700–1000 nm window where the light absorption of water and the tissues including hemoglobin is minimal have a potential to become one of nontoxic biomarkers that is available in-vivo study. Finally, we provide perspective on the overall current status, challenges and potentials for this research field in near future.

Transparent polycrystalline ceramics, which can be fabricated through sintering processes, have received considerable attentions because those simultaneously possess optical and mechanical properties with relatively high productivity. For attaining the optical transmission in the polycrystalline ceramics, an important factor is to achieve dense and fine grained microstructure. This is because both the residual pores and grain boundaries (GB) become the dominant light scattering sources. Recently, in order to achieve highly transparent ceramics having dense and fine grained microstructures, several external field effects, such as electric, ultra high pressure and magnetic fields, have widely been utilized in the sintering processes. The former two effects can accelerate the powder densification, and hence, enable to attain dense and fine microstructures at relatively low temperatures. The third one can control crystal orientation of polycrystalline ceramics and decrease the light scattering at GB due to the textured microstructure. It is expected that although further improvement of each external field effects is necessary, the advanced sintering techniques that integrate each effect would enhance the development of novel transparent ceramics possessing excellent optical and mechanical properties.

In this study, systematic investigation using cellulose nanofiber (CellNF) as a carbon source for selective laser sintering (SLS) of Al₂O₃ was conducted. A composite powder that consisted of CellNF and Al₂O₃ was fabricated using electrostatic adsorption (EA) method in aqueous suspension. Heat-treatment temperature of 500 and 800°C was compared for the conversion of CellNF to carbon residue (CR) in the Al₂O₃ composite. After that, the parameters involving the volume fraction of CR and effect of Al₂O₃ particle size in the CR-Al₂O₃ composite particle were also investigated in order to obtain an optimized condition of the CR-Al₂O₃ composite particle for SLS. From the results obtained, 1.5 wt% CR-Al₂O₃ with alumina size of 0.8 μm that was heat-treated at 800°C exhibited optimum laser sinter ability owing to the good CR distribution over a higher surface area that promoted good laser adsorption for the alumina sintering.