The melting behavior of 0.1-10-nm-thick discontinuous indium films formed by evaporation on amorphous silicon nitride is investigated by an ultrasensitive thin-film scanning calorimetry technique. The films consist of ensembles of nanostructures for which the size dependence of the melting temperature and latent heat of fusion are determined. The relationship between the nanostructure radius and the corresponding melting point and latent heat is deduced solely from experimental results ͑i.e., with no assumed model͒ by comparing the calorimetric measurements to the particle size distributions obtained by transmission electron microscopy. It is shown that the melting point of the investigated indium nanostructures decreases as much as 110 K for particles with a radius of 2 nm. The experimental results are discussed in terms of existing melting point depression models. Excellent agreement with the homogeneous melting model is observed.

The radius dependences of the melting and freezing temperatures, Tm(R) and Tf(R), respectively, associated with size-polydisperse and dilute sets of spherical Bi nanoparticles embedded in a sodium-borate glass, were previously determined by applying an experimental method based on combined and simultaneous use of small-angle X-ray scattering (SAXS) and wide-angle X-ray scattering (WAXS) techniques [1]. This procedure combines size information from analyses of SAXS curves and determinations of temperature-dependent fractions of crystalline volume derived from integral values of Bragg peaks in WAXS patterns. For dilute sets of nanoparticles with approximately same size, an alternative and simpler procedure can be applied, which requires only SAXS measurements of several samples each of them containing nanoparticles with different average radius [2]. Previous determinations of the radius dependences of melting and freezing temperatures of Bi referred to nanoparticles with radii varying from 1 nm up to 4 nm circa [1]. The present study aims at obtaining additional and stronger evidences regarding the conclusions derived from previous work [1]. This purpose was expected to be achieved by determining the radius dependences of the melting and freezing temperatures of spherical Bi nanoparticles over a radius range much wider than in previous work. Thus, we have studied a series of samples containing dilute and size-polydisperse sets of spherical Bi nanoparticles embedded in a sodium-borate glass, the sets of nanoparticles in each sample having different average radius, and wide-partially overlapping-radius distributions. SAXS and WAXS measurements were conducted at the Brazilian National Synchrotron Light Laboratory (LNLS). The simultaneous SAXS and WAXS measurements were performed in situ, on sample heating and cooling cycles, using a specially designed high-temperature chamber [3] and two independent position sensitive X-ray detectors. By combining the results derived from SAXS and WAXS measurements referring to seven different samples, we have determined the radius dependences of the melting and freezing temperatures of spherical Bi nanoparticles with radii ranging from 1 up to 10 nm, that is, over a radius range much wider than in previous work. The results of SAXS/WAXS studies of a series of samples covering a wide radius range provided additional and stronger evidences supporting previous main conclusions, namely (i) the temperatures of melting and freezing of spherical Bi nanoparticles both decrease linearly for increasing reciprocal radius (1/R), and (ii) the effect of undercooling is absent for Bi liquid nanodroplets with radius smaller than a critical value Rc=1.8nm. The observed agreements of Tm(R) and Tf(R) functions derived from measurements corresponding to different samples, with partially overlapping radius ranges, indicated a good reproducibility of the experimental results and, consequently, established the robustness of the method that combines information derived from SAXS and WAXS. This SAXS/WAXS pr...

We report a study of the thermodynamic properties of indium clusters on a SiN (x) surface during the early stages of thin film growth using a sensitive nanocalorimetry technique. The measurements reveal the presence of abnormal discontinuities in the heat of melting below 100 degrees C. These discontinuities, for which temperature separation corresponds to a spatial periodicity equal to the thickness of an indium monolayer, are found to be related to the atomic "magic numbers," i.e., the number of atoms necessary to form a complete shell of atoms at particle surface.

No abstract

No abstract