Transient and spectral photovoltage (PV) is investigated in nanocrystalline porous ${\mathrm{TiO}}_2,$ which belongs to a class of materials with a very low electrical conductivity, i.e., with a huge Maxwell relaxation time. The PV in such materials is caused by diffusion of excess charge carriers with different diffusion coefficients. Usually, the diffusion coefficient for electrons is larger than that for holes in porous ${\mathrm{TiO}}_2.$ The PV transients are strongly retarded in time with respect to the exciting light pulse. The retardation of the photovoltage transients becomes stronger with decreasing size of the interconnected ${\mathrm{TiO}}_2$ nanoparticles and generation of defect states. The band gap of porous ${\mathrm{TiO}}_2$ (rutile, anatase) and the preferential formation of electron traps below the band gap are analyzed.

Offering mild, non-invasive and deep cancer therapy modality, radio frequency (RF) radiation-induced hyperthermia lacks for efficient biodegradable RF sensitizers to selectively target cancer cells and thus avoid side effects. Here, we assess crystalline silicon (Si) based nanomaterials as sensitizers for the RF-induced therapy. Using nanoparticles produced by mechanical grinding of porous silicon and ultraclean laser-ablative synthesis, we report efficient RF-induced heating of aqueous suspensions of the nanoparticles to temperatures above 45-50 °C under relatively low nanoparticle concentrations (<1 mg/mL) and RF radiation intensities (1-5 W/cm(2)). For both types of nanoparticles the heating rate was linearly dependent on nanoparticle concentration, while laser-ablated nanoparticles demonstrated a remarkably higher heating rate than porous silicon-based ones for the whole range of the used concentrations from 0.01 to 0.4 mg/mL. The observed effect is explained by the Joule heating due to the generation of electrical currents at the nanoparticle/water interface. Profiting from the nanoparticle-based hyperthermia, we demonstrate an efficient treatment of Lewis lung carcinoma in vivo. Combined with the possibility of involvement of parallel imaging and treatment channels based on unique optical properties of Si-based nanomaterials, the proposed method promises a new landmark in the development of new modalities for mild cancer therapy.

We report new types of heterogeneous hydrogen-oxygen and silicon-oxygen branched chain reactions which have been found to proceed explosively after the filling of pores of hydrogen-terminated porous silicon (Si) by condensed or liquid oxygen in the temperature range of 4.2–90 K. Infrared vibrational absorption spectroscopy shows that, while initially Si nanocrystals assembling the layers have hydrogen-terminated surfaces, the final products of the reaction are ${\mathrm{SiO}}_2$ and ${\mathrm{H}}_2\mathrm{O}$. Time-resolved optical experiments show that the explosive reaction develops in a time scale of ${10}^{-6\mathrm{}}\mathrm{s}$. We emphasize the remarkable structural properties of porous Si layers which are crucial for the strong explosive interaction.

We demonstrate efficient resonant energy transfer from excitons confined in silicon nanocrystals to molecular oxygen (MO). Quenching of photoluminescence (PL) of silicon nanocrystals by MO physisorbed on their surface is found to be most efficient when the energy of excitons coincides with triplet-singlet splitting energy of oxygen molecules. The dependence of PL quenching efficiency on nanocrystal surface termination is consistent with short-range resonant electron exchange mechanism of energy transfer. A highly developed surface of silicon nanocrystal assemblies and a long radiative lifetime of excitons are favorable for achieving a high efficiency of this process.

Free charge carriers in mesoporous Si (meso-PS) consisting of Si nanocrystals of small dimensions of about 6–10 nm are investigated by the infrared-absorption technique. Adsorption of acceptor molecules or filling the pores with dielectric liquids are found to increase the concentration of free holes (p) in meso-PS up to the half of the doping level of the heavily boron-doped $p^{+}-\mathrm{Si}$ substrate $(p\sim 5\times {10}^{18}\hspace{0.5em}{\mathrm{cm}}^{-3})$ from which the meso-PS was made. Considering the value of p and the dc electrical conductivity $\sigma ,$ the hole mobility is determined as about $5\times {10}^{-4}$ and $5\times {10}^{-3}\hspace{0.5em}{\mathrm{cm}}^2\hspace{0.5em}{\mathrm{V}}^{-1}\hspace{0.5em}{\mathrm{s}}^{-1}$ for as-prepared meso-PS and meso-PS filled with a polar dielectric liquid, respectively. The activation energy is larger for $\sigma$ than for p giving evidence for thermal activation of the hole mobility. A model of the dielectric confinement for charge carriers and hydrogenic impurities is applied to explain the dependence of $\sigma$ and p on the dielectric constant of the ambience of the Si nanocrystals.