44 wileyonlinelibrary.com www.particle-journal.com www.MaterialsViews.com Optimizing the light-emitting effi ciency of silicon quantum dots (Si QDs) has been recently intensifi ed by the demand of the practical use of Si QDs in a variety of fi elds such as optoelectronics, photovoltaics, and bioimaging. It is imperative that an understanding of the optimum light-emitting effi ciency of Si QDs should be obtained to guide the design of the synthesis and processing of Si QDs. Here an investigation is presented on the characteristics of the photoluminescence (PL) from hydrosilylated Si QDs in a rather broad size region (≈2-10 nm), which enables an effective mass approximation model to be developed, which can very well describe the dependence of the PL energy on the QD size for Si QDs in the whole quantum-confi nement regime, and demonstrates that an optimum PL quantum yield (QY) appears at a specifi c QD size for Si QDs. The optimum PL QY results from the interplay between quantum-confi nement effect and surface effect. The current work has important implications for the surface engineering of Si QDs. To optimize the light-emission effi ciency of Si QDs, the surface of Si QDs must be engineered to minimize the formation of defects such as dangling bonds at the QD surface and build an energy barrier that can effectively prevent carriers in Si QDs from tunneling out.tuning of the optical properties of Si QDs by means of surface engineering. [24][25][26] Freestanding Si QDs that are either produced in liquid/gas phases [27][28][29][30] or released from solid matrices [31][32][33][34] have recently become popular largely because of their easily accessible surface. A standard surface modifi cation schemehydrosilylation [35][36][37][38][39] -has been developed to effectively disperse freestanding Si QDs in common solvents. It is found that hydrosilylation not only enables the formation of colloidal Si QDs, but also improves the effi ciency and stability of the light emission from Si QDs. Given the growing concern on the potential environmental impact of archetypal colloidal II/IV-VI QDs (e.g., CdSe and PbS QDs), [ 40 ] colloidal Si QDs hold great promise for the development of a variety of QD-based structures and devices.Since the Bohr radius of an exciton in Si is ≈5 nm, [ 17 ] the size regions of ≈5 and 5-10 nm approximately correspond to strong quantum confi nement and weak quantum confi nement for Si QDs, respectively. It has been observed that the photoluminescence (PL) energy of hydrosilylated Si QDs increases with the decrease of the QD size in both the strong and weak quantum-confi nement regimes, [ 35,41,42 ] consistent with the well-known quantum-confi nement effect. [ 43,44 ] Hessel et al. [ 41 ] have recently demonstrated that the PL quantum yield (QY) of hydrosilylated Si QDs increases with the decrease of the QD size in the weak quantum-confi nement regime. However, Jurbergs et al. [ 35 ] and Mastronardi et al. [ 42 ] previously showed that the PL QY of hydrosilylated Si QDs decreases with the decrease of the...

Colloidal silicon quantum dots (Si QDs) hold ever-growing promise for the development of novel optoelectronic devices such as light-emitting diodes (LEDs). Although it has been proposed that ligands at the surface of colloidal Si QDs may significantly impact the performance of LEDs based on colloidal Si QDs, little systematic work has been carried out to compare the performance of LEDs that are fabricated using colloidal Si QDs with different ligands. Here, colloidal Si QDs with rather short octyl ligands (Octyl-Si QDs) and phenylpropyl ligands (PhPr-Si QDs) are employed for the fabrication of LEDs. It is found that the optical power density of PhPr-Si QD LEDs is larger than that of Octyl-Si QD LEDs. This is due to the fact that the surface of PhPr-Si QDs is more oxidized and less defective than that of Octyl-Si QDs. Moreover, the benzene rings of phenylpropyl ligands significantly enhance the electron transport of QD LEDs. It is interesting that the external quantum efficiency (EQE) of PhPr-Si QD LEDs is lower than that of Octyl-Si QD LEDs because the benzene rings of phenylpropyl ligands suppress the hole transport of QD LEDs. The unbalance between the electron and hole injection in PhPr-Si QD LEDs is more serious than that in Octyl-Si QD LEDs. The currently obtained highest optical power density of ∼0.64 mW/cm from PhPr-Si QD LEDs and highest EQE of ∼6.2% from Octyl-Si QD LEDs should encourage efforts to further advance the development of high-performance optoelectronic devices based on colloidal Si QDs.

International audienceTremendous enhancement of optical emission efficiency was achieved in fluorosilicate glasses by growing lanthanide doped fluoride nanocrystals embedded in oxide glass matrix. The formation mechanism of the microstructure was elucidated by combining solid-state NMR, scanning TEM, EDX map, and large-scale molecular dynamics simulations. The results reveal that the growth of fluoride nanocrystals in fluorosilicate glass was originated from fluoride phase separation. Atomic level structures of phase separation of fluoride-rich regions in oxyfluoride glasses matrix were observed from both EDX maps and MD simulations, and it was found that, while silicon exclusively coordinated by oxygen and alkali earth ions and lanthanide mainly coordinated by fluorine, aluminum played the role of linking the two fluoride glass and oxide glass regions by bonding to both oxygen and fluoride ions. © 2016 American Chemical Society

Thermal quenching of photoluminescence represents a significant obstacle to practical applications such as lighting, display, and photovoltaics. Herein, a novel strategy is established to enhance upconversion luminescence at elevated temperatures based on the use of negative thermal expansion host materials. Lanthanide‐doped orthorhombic Yb2W3O12 crystals are synthesized and characterized by in situ X‐ray diffraction and photoluminescence spectroscopy. The thermally induced contraction and distortion of the host lattice is demonstrated to enhance the collection of excitation energy by activator ions. When the temperature is increased from 303 to 573 K, a 29‐fold enhancement of green upconversion luminescence in Er3+ activators is achieved. Moreover, the temperature dependence of the upconversion luminescence is reversible. The thermally enhanced upconversion is developed as a sensitive ratiometric thermometer by referring to a thermally quenched upconversion.

No abstract