Co/ZIF‑8 Heterometallic Nanoparticles: Control of Nanocrystal Size and Properties by a Mixed-Metal Approach

A mixed-metal approach has been used to control the size and physicochemical properties of heterometallic <b>Co</b>/<b>ZIF-8</b> nanomaterials. Intentional substitution of zinc with cobalt in a broad concentration range (from 0 to 100 molar percent with a 10% step) provided a series of <b>Co</b>/<b>ZIF-8</b> nanoparticles, whose sizes could be tuned in the range from 20 to over 500 nm in diameter. Zinc ions from the <b>ZIF-8</b> matrix were found to be uniformly substituted with the cobalt ions. The increase of nanoparticles size resulted in a change of their nitrogen sorption–desorption characteristics due to decreasing participation of the external surface area in the total surface area. Insights from UV–vis-NIR and IR spectroscopies, as well as remarks on nonlinear optical properties are also provided

Enhancement of Two-Photon Absorption Cross Section in CdSe Quantum Rods

Nonlinear optical properties of semiconducting CdSe quantum rods (QRs), with three various aspect ratios, were examined in a wide wavelength range using femtosecond Z-scan technique. The two-photon absorption cross section σ₂ was found to be as large as 164 000 GM at the wavelength of 750 nm: about 4 times larger than that expected for CdSe quantum dots of the same mass. On the basis of the obtained dispersion of the two-photon absorption cross section, we have selected wavelength ranges for optimal excitation of two-photon-induced emission. We have also studied the luminescence kinetics using degenerate pump–probe and time-correlated single-photon counting techniques. A strong influence of semiconducting CdSe rods morphology on their steady-state and time-resolved optical properties was found

Third-Order Nonlinear Optical Properties of Infrared Emitting PbS and PbSe Quantum Dots

The optical properties of small band gap, colloidal quantum dots are presented, with the special emphasis put on the measurements of their nonlinear optical properties in the infrared region of spectra. In particular, two types of colloidal quantum dots (PbS and PbSe), with the first exciton absorption band maxima in the near-infrared region of spectra, were investigated using a tunable femtosecond laser system and the <i>Z</i>-scan technique. The measurements of closed- and open-aperture <i>Z</i>-scan traces allowed for the calculation of real and imaginary parts of cubic nonlinearity, which were presented as appropriate cross sections used to characterize the nonlinear refractive and absorptive properties of the studied quantum dots. The maximum two-photon absorption cross section values taken for a single quantum dot were found to be ∼2400 GM (Goeppert-Mayer units) at 1300 nm and ∼15 500 GM at 1700 nm, for PbS and PbSe QDs, respectively. PbS quantum dots showed two-photon induced emission upon infrared excitation. The obtained results demonstrate the potential of IV–VI group colloidal quantum dots for low-cost photonic devices and two-photon absorbers in the near-infrared and infrared spectral ranges

Wavelength Dependence of the Complex Third-Order Nonlinear Optical Susceptibility of Poly(3-hexylthiophene) Studied by Femtosecond <i>Z</i>‑Scan in Solution and Thin Film

Comprehensive studies of the third-order susceptibility of regioregular poly­(3-hexylthiophene) in the wavelength range from 530 to 1600 nm for both thin film (with stacked chains interactions) and dilute solution (no chains interactions) were performed with the <i>Z</i>-scan technique. Negative nonlinear refraction was observed in the whole wavelength range of the measurements. The nonlinear absorption exhibits regions of two-photon and three-photon absorption, and on approaching the linear absorption region, a saturable absorption process is seen. It was found that for thin film the effective multiphoton absorption cross-sections σ₂ and σ₃ are 1.5 and 3 times bigger than for solution, respectively. Influence of the elongated conjugation length in the stacked polythiophene chains in thin film on the red shift of the multiphoton absorption cross-section spectra in comparison with those for polythiophene in solution is discussed

Third-Order Nonlinear Optical Properties of Aqueous Silver Sulfide Quantum Dots

Wide spectral wavelength range (500–1600 nm) measurements of nonlinear optical properties of silver sulfide (Ag2S, with 2- or 3-mercaptopropionic acid, 2 or 3MPA ligands) quantum dots (QDs) in aqueous colloidal solutions were performed using the Z-scan technique with tunable ∼55 fs laser pulses at 1 kHz. We have identified regions of the occurrence of various NLO effects including two-photon absorption, nonlinear refraction, as well as saturation of one-photon absorption. At the same time, we evaluated the relationship between the properties of the QDs and the variation of the material that covers their surface. The peak two-photon absorption cross section (σ2) values were determined to be 632 ± 271 GM (at 850 nm) for Ag2S-2MPA QDs and 772 ± 100 GM (at 875 nm) for Ag2S-3MPA QDs. The physicochemical factors influencing the three-dimensional self-organization of Ag2S QDs in water as well as their impact on spectroscopic properties were also investigated

Nonlinear-Optical Response of Prussian Blue: Strong Three-Photon Absorption in the IR Region

The nonlinear-optical properties of Prussian Blue nanoparticles have been evaluated with the use of femtosecond Z-scan measurements in the 1350–1750 nm range. This well-known inorganic pigment having interesting magnetic and electrochemical properties was found to be an efficient near-IR three-photon absorber. The maximum of the effective three-photon cross section is as high as 4.5 × 10^–78 cm⁶ s² at 1375 nm. By a comparison of the three-photon molar-mass-normalized merit factors, σ₃/<i>M</i>, we show that this material is a competitive multiphoton absorber, especially in comparison to semiconductor quantum dots

Tip Enhancement of Upconversion Photoluminescence from Rare Earth Ion Doped Nanocrystals

We present tip-enhanced upconversion photoluminescence (PL) images of Er³⁺- and Yb³⁺-doped NaYF₄ nanocrystals on glass substrates with subdiffraction spatial resolution. Tip–sample distance dependent measurements clearly demonstrate the near-field origin of the image contrast. Time-resolved PL measurements show that the tip increases the spontaneous emission rate of the two emission channels of Er³⁺ in the visible region. Very efficient enhancement of upconversion PL is discussed in the context of the two-photon nature of the excitation process and homoenergy transfer between the ions within the nanocrystals. Comparison between different nanocrystals and tips shows a strong influence of the tip shape on the image contrast that becomes particularly relevant for the larger dimensions of the investigated nanocrystals

Core/Shell Quantum Dots Encapsulated in Biocompatible Oil-Core Nanocarriers as Two-Photon Fluorescent Markers for Bioimaging

Highly fluorescent quantum dots (QDs)-loaded nanocapsules, intended for fluorescent cell imaging, were prepared via an emulsification/solvent-evaporation method. CdSe/ZnS core/shell quantum dots were applied as cargo; Poloxamer 403 as the polymer component; Cremophor EL as the nonionic surfactant; and mineral oil, oleic acid, or silicone oil were applied as the oil phases. Transmission electron microscopy, atomic force microscopy, dynamic light scattering, and zeta potential measurements were used to characterize the novel QDs-labeled nanoparticles by particle size, distribution, and morphology, as well as by ζ-potential and physical stability. The fabricated long-lasting nanocapsules exhibit good luminescence properties upon both one-photon and two-photon excitation. The potential of the encapsulated QDs for fluorescent imaging was evaluated in cytotoxicity studies as well as in imaging of intracellular localization, accumulation, and distribution of QDs delivered to well-characterized human cancer cell linesdoxorubicin-sensitive breast (MCF-7/WT) and alveolar basal epithelial (A549)as well as on normal human umbilical vein endothelial (HUVEC) cells, as investigated by confocal laser scanning microscopy (CLSM). The colloidal CdSe/ZnS-loaded nanocapsules are shown to exhibit strong two-photon-induced luminescence upon excitation in the NIR optical transmission window spectral range, making them ideal markers for bioimaging application. The total two-photon cross section of a single nanocapsule was determined to be about 4.1 × 10⁶ GM at 800 nm

Beyond Single-Wavelength SHG Measurements: Spectrally-Resolved SHG Studies of Tetraphosphonate Ester Coordination Polymers

Powder second-harmonic generation (SHG) efficiencies are usually measured at single wavelengths. In the present work, we provide a proof of concept of spectrally resolved powder SHG measured for a newly obtained series of three non-centrosymmetric coordination polymers (CPs). CPs are constructed from tetrahedral linker–tetraphenylmethane-based tetraphosphonate octaethyl ester and cobalt­(II) ions of mixed, octahedral (<i>O</i>_<i>h</i>), and tetrahedral (<i>T</i>_<i>d</i>), geometries and different sets of donors (CoO₆ vs CoX₃O). Isostructurality of the obtained materials allowed for the determination of anion-dependent tunability of SHG optical spectra and their relationship with solid-state absorption spectra

Uniting Ruthenium(II) and Platinum(II) Polypyridine Centers in Heteropolymetallic Complexes Giving Strong Two-Photon Absorption

New trinuclear RuPt₂ and heptanuclear RuPt₆ complex salts are prepared by attaching Pt^II 2,2′:6′,2″-terpyridine (tpy) moieties to Ru^II 4,4′:2′,2″:4″,4‴-quaterpyridine (qpy) complexes. Characterization includes single crystal X-ray structures for both polymetallic species. The visible absorption bands are primarily due to Ru^II → qpy metal-to-ligand charge-transfer (MLCT) transitions, according to time-dependent density functional theory (TD-DFT) calculations. These spectra change only slightly on Pt coordination, while the orange-red emission from the complexes shows corresponding small red-shifts, accompanied by decreases in intensity. Cubic molecular nonlinear optical behavior has been assessed by using Z-scan measurements. These reveal relatively high two-photon absorption (2PA) cross sections σ₂, with maximal values of 301 GM at 834 nm (RuPt₂) and 523 GM at 850 nm (RuPt₆) when dissolved in methanol or acetone, respectively. Attaching Pt^II(tpy) moieties triples or quadruples the 2PA activities when compared with the Ru^II-based cores