Controlled surface for enhanced luminescence quantum yields of silicon nanocrystals

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. ©2019 Japan Society of Powder and Powder Metallurgy