• Energy Research

The luminescence efficiency of lanthanide-doped upconversion nanoparticles is of particular importance for their embodiment in biophotonic and photonic applications. Here, we show that the upconversion luminescence of typically used NaYF4:Yb3+30%/Tm3+0.5% nanoparticles can be enhanced by ~240 times through a hierarchical active core/active shell/inert shell (NaYF4:Yb3+30%/Tm3+0.5%)/NaYbF4/NaYF4 design, which involves the use of directed energy migration in the second active shell layer. The resulting active core/active shell/inert shell nanoparticles are determined to be about 11 times brighter than that of well-investigated (NaYF4:Yb3+30%/Tm3+0.5%)/NaYF4 active core/inert shell nanoparticles when excited at ~980 nm. The strategy for enhanced upconversion in Yb3+/Tm3+-codoped NaYF4 nanoparticles through directed energy migration might have implications for other types of lanthanide-doped upconversion nanoparticles.

In this work, we report on efficient visible and near-IR upconversion emissions in colloidal hexagonal-phase core/shell NaYF4:Er(3+)/NaYF4 nanoparticles (∼38 nm) under IR laser excitation at 1523 nm. Varying amounts of Er(3+) dopants were introduced into the core NaYF4:Er(3+) nanoparticles, revealing an optimized Er(3+) concentration of 10% for the highest luminescent efficiency. An inert epitaxial shell layer of NaYF4 grown onto the core of the NaYF4:Er(3+) 10% nanoparticle increased its upconversion emission intensity fivefold due to suppression of surface-related quenching mechanisms, yielding the absolute upconversion efficiency to be as high as ∼3.9±0.3% under an excitation density of 18 W/cm(2). The dependence of the intensity of upconversion emission peaks on laser excitation density in the core/shell nanoparticle displayed "saturation effects" at low excitation density in the range of 1.5-18 W/cm(2), which again demonstrates high upconversion efficiency.

Abstract This work demonstrates the effectiveness of interfacial engineering and defect passivation in the bulk of mixed perovskite absorber to achieve high performance perovskite solar cells (PSCs). It is found that the extent of I–V hysteresis is most significant in PSCs with a single-layer electron transport layer (ETL) composed of SnO2 quantum dots (QD-SnO2) or SnO2 nanoparticles (NP–SnO2). The hysteresis of the PSCs can be effectively suppressed by adopting a multilayer structure for the ETL to optimize the ETL/perovskite interface. Furthermore, the performance of the PSCs can be enhanced by incorporating a controlled amount of an organic cross linker, 2,2′-(ethylenedioxy)bis (ethylammonium iodide) (EDAI), in the mixed perovskite absorber layers. The experimental results consistently show that incorporation of an optimal amount of EDAI is effective in passivating defect states in the bulk of mixed perovskite as well as the grain boundaries and film surface, while excessive EDAI significantly degrades the photovoltaic (PV) performance of PSCs due to generation of defects and electrically insulating properties of EDAI itself. Owing to the synergistic effect contributed from optimized ETL/perovskite interface and the mixed perovskite thin film, the PSC with an active area of 0.06 cm2 exhibits a champion power conversion efficiency (PCE) of ~19.0% with negligible hysteresis and improved stability. The proposed strategies were also applied to fabricate larger area devices from 0.15 cm2 to 0.85 cm2, exhibiting the PCEs of ~12%–18% with negligible hysteresis.

Applications in Theranostics Guanying Chen,*,†,‡ Hailong Qiu,†,‡ Paras N. Prasad,*,‡,§ and Xiaoyuan Chen* †School of Chemical Engineering and Technology, Harbin Institute of Technology, Harbin, Heilongjiang 150001, China ‡Department of Chemistry and the Institute for Lasers, Photonics, and Biophotonics, University at Buffalo, State University of New York, Buffalo, New York 14260, United States Department of Chemistry, Korea University, Seoul 136-701, Korea Laboratory of Molecular Imaging and Nanomedicine, National Institute of Biomedical Imaging and Bioengineering, National Institutes of Health, Bethesda, Maryland 20892-2281, United States

Quantum dots (QDs) are semiconductor nanocrystals with peculiar optoelectronic properties. Their wide application in light-emitting diodes, solar cells, and the medical and defense fields makes them a potential candidate in the area of photonics and biophotonics. In this feature issue of Optical Materials Express, together with Optics Express we focus on different aspects of semiconducting nanocrystals research, especially on the advances in the synthesis, physical properties, and application of QDs.

Quantum dots are known for their superior optical properties; however, when transferred into aqueous media, their luminescent properties are frequently compromised. When encapsulated in micelles for bioimaging applications, luminescent silicon quantum dots can lose as much as 50% of their luminescence depending on the formulation used. Here, we create an energy transfer micelle platform that combines silicon quantum dots with an anthracene-based dye in the hydrophobic core of 1,2-distearoyl-sn-glycero-3-phosphoethanolamine-N-[methoxy(polyethylene glycol)-2000] (DSPE-PEG) micelles. These phospholipid micelles are water dispersible, stable, and surrounded by a PEGylated layer with modifiable functional groups. The spectroscopic properties of energy transfer between the anthracene donors and silicon quantum dot acceptors were analyzed based on the observed dependence of the steady-state emission spectrum on concentration ratio, excitation wavelength, pH, and temperature. The luminescence of silicon quantum dots from the core of a 150 nm micelle is enhanced by more than 80% when the anthracene dye is added. This work provides a simple yet readily applicable solution to the long-standing problem of luminescence enhancement of silicon quantum dots and can serve as a template for improving the quantum dot emission yield for biological applications where luminescence signal enhancements are desirable and for solar applications where energy transfer plays a critical role in device performance.

This paper presents the observation of asymmetric behavior between the forward and backward stimulated emission, generated in multiphoton active dye solutions, through three- or four-photon excitation of subpicosecond laser pulses. At a pump energy level considerably higher than the lasing threshold value, the peak wavelengths of the forward stimulated emission are $20--30\text{\ensuremath{-}}\mathrm{nm}$ shorter than those of the backward stimulated emission for the two investigated stilbazolium dye solutions (PRL-L3 and PRL-L10). This obvious spectral asymmetry can be explained by the following three considerations: (i) the difference of spatial/temporal sequences between the forward and backward stimulated emission pulses; (ii) blueshift of the peak wavelength of transient gain experienced by the forward stimulated emission pulse; and (iii) saturation of reabsorption at the forward lasing wavelength range. These proposed explanations are verified by a specially designed pump-probe experiment, utilizing a white-light continuum as the probe beam and the $\ensuremath{\sim}1300\text{\ensuremath{-}}\mathrm{nm}$ laser radiation as the pump beam for three-photon excitation. The experimental results have clearly shown the existence of the saturation effect of reabsorption and the gain-peak blueshift effect as well as their transient features.