Two-Photon Excitation Spectroscopy of Silicon Quantum Dots and Ramifications for Bio-Imaging

Two-photon excitation in the near-infrared (NIR) of colloidal nanocrystalline silicon quantum dots (nc-SiQDs) with photoluminescence also in the NIR has the potential to open up new opportunities in the field of deep biological imaging. Spectra of the degenerate two-photon absorption (2PA) cross section of colloidal nc-SiQDs are measured using two-photon excitation over a spectral range 1.46λ>650 nm) above the two-photon band gap E(QD)g/2, and at a representative photon energy ℏω=0.99 eV (λ=1250 nm) below this gap. Two-photon excited photoluminescence (2PE-PL) spectra of nc-SiQDs with diameters d=1.8±0.2 and d=2.3±0.3 nm, each passivated with 1-dodecene and dispersed in toluene, are calibrated in strength against 2PE-PL from a known concentration of Rhodamine B dye in methanol. The 2PA cross section is observed to be smaller for the smaller diameter nanocrystals and the onset of 2PA is observed to be blueshifted from the two-photon indirect band gap of bulk Si, as expected for quantum confinement of excitons. The efficiencies of nc-SiQDs for bio-imaging using 2PE-PL are simulated in various biological tissues and compared to other quantum dots and molecular fluorophores and found to be superior at greater depths.This research was funded by Robert A. Welch Foundation Grants F-1038 and F-1464, and partially supported by the National Science Foundation through the Center for Dynamics and Control of Materials; an NSF MRSEC under Cooperative Agreement No. DMR-1720595. B. Mendoza acknowledges support from Consejo Nacional de Ciencia y Tecnología, México (Grant No. A1-S-9410).Center for Dynamics and Control of Material

Enhanced Photovoltaic Performance of Mesoscopic Perovskite Solar Cells by Controlling the Interaction between CH3NH3PbI3 Films and CsPbX3 Perovskite Nanoparticles

We report the incorporation of all-inorganic highly stable CsPbX3 (X = I, Br) based perovskite nanoparticles (NPs) on top of a bulk CH3NH3PbI3perovskite thin film. This design utilizes the photogeneration ability of perovskite NPs and also improves the interfacial charge transport which happens to be a critical factor in deciding the photovoltaic performance of any solar cell device. With variation in the lead halide (PbX2, X = I, Br, Cl) content, the synthesized CsPbX3 NPs shows tunable band-edge position and fluorescence characteristics. The interaction of all inorganic NPs with the bulk perovskite resulted in improved hole injection and electron blocking characteristics leading to enhanced light harvesting efficiency. The CsPbBr3 and CsPbI3 perovskite NPs were used for fabricating the bulk-NP structure due to their better absorption and valence band edge characteristics. The inclusion of CsPbI3 NPs on top of the bulk perovskite showed a significant increment in the power conversion efficiency of 28%, in comparison with a reference sample without NPs, due to significant improvements in current density, open circuit voltage, and fill factor

Linear and nonlinear optical properties of a quadrupolar carbo-benzene and its benzenic parent: The carbo-merization effect

International audienceHerein, the optical properties of thiophene-functionalized quadrupolar carbo-benzenes and a benzenic parent, of generic structure Th–Ctriple bondC–[core]–Ctriple bondC–Th, Th = R2C4HS, are comparatively investigated. Beyond the previously unknown dioctylthienylethynylbenzene (core = p-C6H4, R = nOct), two bis-dialkylthienylethynyl-carbo-benzenes (core = C18Ph4, R = nOct, nBu) are envisaged for the unique "carbo-aromatic" character of the C18 macrocycle. The three targets were synthesized from the corresponding ethynylthiophenes in 47, 20 and 10% yield, respectively, then characterized by classical methods such as NMR spectroscopy, and X-ray crystallography for one of the carbo-benzenes. Regarding linear and nonlinear optical properties, our results show that the carbo-merization induces a significant shift to lower energies of the one-photon electronic excitations accompanied by an 8-fold increase of the molar extinction coefficient compared to the parent molecule. Intriguingly, these excitations lead to a broad band of photoluminescence comprising decay transitions of the type S1 → S0 but also of the type S2 → S0. This phenomenon of emission from higher excited states, which is contrary to Kasha's rule, is assigned to - or revealed by - a reduction of the internal conversion efficiency between S2 and S1. Two-photon induced transitions are also enhanced, the two-photon absorption cross-section (σ2PA) being in average five times larger for the carbo-benzenes than for their benzene parent in the wavelength range 650–950 nm, with a maximum of σ2PA = 1430 GM (1 GM = 10−50 cm4 s/photon). Beyond a moderate nonlinearity, this comparative study provides quantitative insights about the way carbo-merization or insertion of a π-conjugated macrocycle between chromophoric functions (here thiophene rings) can tune optical properties of organic molecules. The optical properties of the bis-dialkylthienylethynyl-carbo-benzenes are also discussed in regard of recent reports on organic chromophores based on other types of π-conjugated macrocyclic cores

Enhanced Photovoltaic Performance of Mesoscopic Perovskite Solar Cells by Controlling the Interaction between CH₃NH₃PbI₃ Films and CsPbX₃ Perovskite Nanoparticles

We report the incorporation of all-inorganic highly stable CsPbX₃ (X = I, Br) based perovskite nanoparticles (NPs) on top of a bulk CH₃NH₃PbI₃perovskite thin film. This design utilizes the photogeneration ability of perovskite NPs and also improves the interfacial charge transport which happens to be a critical factor in deciding the photovoltaic performance of any solar cell device. With variation in the lead halide (PbX₂, X = I, Br, Cl) content, the synthesized CsPbX₃ NPs shows tunable band-edge position and fluorescence characteristics. The interaction of all inorganic NPs with the bulk perovskite resulted in improved hole injection and electron blocking characteristics leading to enhanced light harvesting efficiency. The CsPbBr₃ and CsPbI₃ perovskite NPs were used for fabricating the bulk-NP structure due to their better absorption and valence band edge characteristics. The inclusion of CsPbI₃ NPs on top of the bulk perovskite showed a significant increment in the power conversion efficiency of 28%, in comparison with a reference sample without NPs, due to significant improvements in current density, open circuit voltage, and fill factor

Linear and nonlinear optical properties of a quadrupolar carbo-benzene and its benzenic parent: The carbo-merization effect

International audienceHerein, the optical properties of thiophene-functionalized quadrupolar carbo-benzenes and a benzenic parent, of generic structure Th–Ctriple bondC–[core]–Ctriple bondC–Th, Th = R2C4HS, are comparatively investigated. Beyond the previously unknown dioctylthienylethynylbenzene (core = p-C6H4, R = nOct), two bis-dialkylthienylethynyl-carbo-benzenes (core = C18Ph4, R = nOct, nBu) are envisaged for the unique "carbo-aromatic" character of the C18 macrocycle. The three targets were synthesized from the corresponding ethynylthiophenes in 47, 20 and 10% yield, respectively, then characterized by classical methods such as NMR spectroscopy, and X-ray crystallography for one of the carbo-benzenes. Regarding linear and nonlinear optical properties, our results show that the carbo-merization induces a significant shift to lower energies of the one-photon electronic excitations accompanied by an 8-fold increase of the molar extinction coefficient compared to the parent molecule. Intriguingly, these excitations lead to a broad band of photoluminescence comprising decay transitions of the type S1 → S0 but also of the type S2 → S0. This phenomenon of emission from higher excited states, which is contrary to Kasha's rule, is assigned to - or revealed by - a reduction of the internal conversion efficiency between S2 and S1. Two-photon induced transitions are also enhanced, the two-photon absorption cross-section (σ2PA) being in average five times larger for the carbo-benzenes than for their benzene parent in the wavelength range 650–950 nm, with a maximum of σ2PA = 1430 GM (1 GM = 10−50 cm4 s/photon). Beyond a moderate nonlinearity, this comparative study provides quantitative insights about the way carbo-merization or insertion of a π-conjugated macrocycle between chromophoric functions (here thiophene rings) can tune optical properties of organic molecules. The optical properties of the bis-dialkylthienylethynyl-carbo-benzenes are also discussed in regard of recent reports on organic chromophores based on other types of π-conjugated macrocyclic cores

Linear and nonlinear optical properties of a quadrupolar carbo-benzene and its benzenic parent: The carbo-merization effect

International audienceHerein, the optical properties of thiophene-functionalized quadrupolar carbo-benzenes and a benzenic parent, of generic structure Th–Ctriple bondC–[core]–Ctriple bondC–Th, Th = R2C4HS, are comparatively investigated. Beyond the previously unknown dioctylthienylethynylbenzene (core = p-C6H4, R = nOct), two bis-dialkylthienylethynyl-carbo-benzenes (core = C18Ph4, R = nOct, nBu) are envisaged for the unique "carbo-aromatic" character of the C18 macrocycle. The three targets were synthesized from the corresponding ethynylthiophenes in 47, 20 and 10% yield, respectively, then characterized by classical methods such as NMR spectroscopy, and X-ray crystallography for one of the carbo-benzenes. Regarding linear and nonlinear optical properties, our results show that the carbo-merization induces a significant shift to lower energies of the one-photon electronic excitations accompanied by an 8-fold increase of the molar extinction coefficient compared to the parent molecule. Intriguingly, these excitations lead to a broad band of photoluminescence comprising decay transitions of the type S1 → S0 but also of the type S2 → S0. This phenomenon of emission from higher excited states, which is contrary to Kasha's rule, is assigned to - or revealed by - a reduction of the internal conversion efficiency between S2 and S1. Two-photon induced transitions are also enhanced, the two-photon absorption cross-section (σ2PA) being in average five times larger for the carbo-benzenes than for their benzene parent in the wavelength range 650–950 nm, with a maximum of σ2PA = 1430 GM (1 GM = 10−50 cm4 s/photon). Beyond a moderate nonlinearity, this comparative study provides quantitative insights about the way carbo-merization or insertion of a π-conjugated macrocycle between chromophoric functions (here thiophene rings) can tune optical properties of organic molecules. The optical properties of the bis-dialkylthienylethynyl-carbo-benzenes are also discussed in regard of recent reports on organic chromophores based on other types of π-conjugated macrocyclic cores