Manifestation of fluorophore segmental motion in carbon dots in steady-state fluorescence experiments.

The steady state fluorescence anisotropy of carbon dot solutions of different viscosities η and its variation with temperature T has been investigated. The dependence of the anisotropy on T/η is shown to be described by the Perrin equation, which implies that Brownian rotational motion of carbon dots in solution is a basic mechanism of fluorescence depolarization. Peculiarities of the Perrin plot testify that the luminous entity ("fluorophore") responsible for carbon dot fluorescence displays noticeable segmental motions, which are independent of the overall rotational diffusion of the dots. The Perrin model fit to the experimental data yields the effective volumes of the fluorophore VF = 0.35 ± 0.15 nm3 and of the carbon dot as a whole VC = 10.5 ± 1.8 nm3. The rotational motions of the fluorophores are shown to be limited and spectrally dependent. A feasible nature of the fluorophores in question is discussed.

Effect of dark states on the fluorescence of carbon nanodots.

The nonlinear fluorescence properties of colloidal carbon dot solutions in glycerol were investigated at T = 298 K and 77 K. It was first found that the fluorescence intensity depended sublinearly on optical excitation intensity even at moderate excitation levels. With increasing excitation, the fluorescence signal shows a trend toward saturation, the latter being most pronounced at liquid nitrogen temperature. The relation between the fluorescence intensity and the excitation density was shown to be well approximated by the hyperbola equation. The behavior of the fluorescence intensity was theoretically described by a 3-level model that involves the singlet fluorescent state S1 and optically "dark" triplet state T1 of a carbon dot. The triplet states are almost inactive optically and manifest themselves in a very weak phosphorescence of the carbon dots that is several orders weaker than the fluorescence of the dots. The lifetimes of the triplet state were measured to be 0.75 ms at room temperature and 0.25 s at T = 77 K. Within the model used, the intersystem crossing (S1 → T1) lifetime in the carbon dots was estimated to be τIST ∼ 10-5 s at T = 77 K and τIST ∼ 10-6 s at T = 298 K.

Monodisperse spherical mesoporous silica particles: fast synthesis procedure and fabrication of photonic-crystal films.

A procedure for the synthesis of monodisperse spherical mesoporous silica particles (MSMSPs) via the controlled coagulation of silica/surfactant clusters into spherical aggregates with mean diameters of 250-1500 nm has been developed. The synthesis is fast (taking less than 1 h) because identical clusters are simultaneously formed in the reaction mixture. The results of microscopic, x-ray diffraction, adsorption and optical measurements allowed us to conclude that the clusters are ∼15 nm in size and have hexagonally packed cylindrical pore channels. The channel diameters in MSMSPs obtained with cethyltrimethylammonium bromide and decyltrimethylammonium bromide as structure-directing agents were 3.1 ± 0.15 and 2.3 ± 0.12 nm, respectively. The specific surface area and the pore volume of MSMSP were, depending on synthesis conditions, 480-1095 m(2) g(-1) and 0.50-0.65 cm(3) g(-1). The MSMSP were used to grow opal-like photonic-crystal films possessing a hierarchical macro-mesoporous structure, with pores within and between the particles. A selective filling of mesopore channels with glycerol, based on the difference between the capillary pressures in macro- and mesopores, was demonstrated. It is shown that this approach makes it possible to control the photonic bandgap position in mesoporous opal films by varying the degree of mesopore filling with glycerol.

Ordered porous diamond films fabricated by colloidal crystal templating.

We have developed a colloidal crystal templating method for preparation of diamond films with 2D and 3D ordered porous structures. The technological process involved breaks down into (a) impregnation into the pores of silica colloidal crystal (opal) films of detonation nanodiamond (DND) particles from their hydrosol; (b) microwave plasma-enhanced chemical vapor deposition (MWPECVD) regrowth with diamond of pores with high DND filling; (c) Ar(+) ion dry etching of fragments of shells of coalesced diamond crystallites which form in the course of MWPECVD on the surface of the SiO(2) beads making up the outer surface of a film and (d) wet etching of the SiO(2) template in aqueous HF solution. The final samples are either connected to the substrate or free-standing films of various thicknesses having 2D or 3D ordered porous structures. The morphology of the diamond films fabricated by this method replicates the pore network of the opal template. Raman measurements confirm the diamond structure of the synthesized ordered porous material.

Ultrafast optical switching in three-dimensional photonic crystals.

We present the first experimental investigation of ultrafast optical switching in a three-dimensional photonic crystal made of a Si-opal composite. Ultrafast (30 fs) changes in reflectivity around the photonic stop band up to 1% were measured for moderate pump power (70 microJ/cm(2)). Short-lived photoexcited carriers in silicon induce changes in the dielectric constant of Si and diminish the constructive interference inside the photonic crystal. The results are analyzed within a model based on a two-band mixing formalism.

Magnetic ordering and phase transition in MnO embedded in a porous glass.

We present the results of a neutron diffraction study of the antiferromagnet MnO embedded in a porous glass. The type of magnetic ordering and the structural distortion are similar to those of the bulk, but the ordered magnetic moment of 3.84(4)muB/ion is strongly reduced and the Néel temperature is enhanced. The magnetic transition is second order, in contrast to the first order transition of the bulk. The size of the magnetic region is smaller than the average size of the nanoparticles. The reasons for this behavior are discussed.