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We present an accurate online method for the study of size-dependent evaporation of free nanoparticles allowing us to detect a size change of 0.1 nm. This method is applied to Ag nanoparticles. The linear relation between the onset temperature of evaporation and the inverse of the particle size verifies the Kelvin effect and predicts a surface energy of 7.2 J/m(2) for free Ag nanoparticles. The surface energy of nanoparticles is significantly higher as compared to that of the bulk and is essential for processes such as melting, coalescence, evaporation, growth, etc., of nanoparticles.

The size-dependent evaporation of free-spherical PbS nanoparticles has been investigated by in-flight sintering of size-classified aerosols. The temperature (T(ev)) at which the particle size decreases due to evaporation is found to be size dependent and decreases with decreasing particle size. A linear relationship between the evaporation temperature and the inverse of the particle size is obtained as is the case with size-dependent melting of nanoparticles. This gives a direct evidence of the Kelvin effect and allows one to estimate the surface energy of nanoparticles. The surface energy of PbS nanoparticles has been found to be 2.45 J m(2).

An effective mass approximation (EMA) with finite-depth square-well potential is used to investigate the size-dependent band gap (BG) of PbS and CuBr nanoparticles embedded in different matrices. These two materials are interesting from the theoretical point of view as PbS is a low-BG material with smaller effective masses and larger dielectric constants, whereas CuBr is a wide-BG material with larger effective masses and smaller dielectric constants. Comparing the experimental BGs with our theoretical calculations, it is shown that EMA provides a better description of the experimental data, especially for CuBr, when the Coulomb interaction having the size-dependent dielectric constant is included in the calculation. Further, comparing the change in the BG of spherical nanoparticle, nanowire and thin film, it is predicted that the effective dimensionality of semiconductor nanoparticles can be increased by embedding them in another semiconducting matrix.

Processes such as coalescence, sintering, and evaporation depend on the surface tension. We have evaluated the surface tension of free gold nanoparticles from the size-dependent evaporation data. A value of 8.78 N/m has been obtained, which is in very good agreement with other theoretical models, and the values deduced from the size-dependent lattice parameter. The value of surface tension on the sintering process of gold nanoparticles is discussed.

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We report about the band-gap tuning of PbS nanoparticles by in-flight sintering of size-classified aerosols. Band gaps in the range of 0.5–2.0 eV were obtained. The band gap first decreases and then increases upon increasing the sintering temperature. The decrease in the band gap is associated with the sintering of primary particles in aggregates leading to larger crystalline domains. The increase in the band gap is associated with the partial evaporation of the particles, which sets in after quasispherical and monocrystalline particles were formed by sintering. The same band gap is found for large spherical particles and aggregates of smaller primary particles, suggesting that an intimate contact between primary particles by means of sinter necks leads to a weaker quantum confinement effect than when the particles are merely touching each other.