176Femtosecond (τ L ~ 100 fs) laser pulses incident on a metallic target are absorbed by conduction electrons in the skin layer with a depth of 10-20 nm. Owing to heat conduction, the energy absorbed by the electrons propagates into the target in the form of an electron heat wave [1][2][3][4][5]. Electronic energy is simultaneously transferred to the lattice through electron-phonon heat transfer. As a result, a heated layer with thickness d T ≈ 120 nm is formed in aluminum within the elec tron-ion relaxation time t eq = 2-3 ps. The lattice is melted when the energy flux of the femtosecond laser pulse exceeds the melting threshold for the given material. For aluminum, the absorbed energy at the melting threshold is estimated at ~15 mJ/cm 2 .The pressure in the heated layer increases strongly owing to the supersonic heat conduction for which the acoustic time t s = d T /c s ≈ 20 ps is much longer than the heating time t eq [1, 2, 5]. Acoustic decay of the heated and pressurized layer (d'Alembert's solution) and sub sequent reflection of acoustic waves at the vacuum interface are accompanied by generation of thermo mechanical tensile stresses [1,2,[5][6][7], where the amplitude of the tensile stresses increases with the energy density of the laser pulse F. Such motion results in deformation of the surface layer d T . When F exceeds the ablation threshold F abl , spallation (thermome chanical ablation) of part of the melted layer occurs as a result of the tensile stress exceeding the tensile strength of the liquid metal, and a crater forms on the surface. Our experiments and calculations indicate that, when the energy flux is slightly higher than the ablation threshold, the heated layer first expands to a certain value and then returns back with some residual deformation (see, e.g., Fig. 1).In the described experiments, the deformation of the surface of the target in the heated region was stud ied using femtosecond interference microscopy [8]. To analyze the structure of the surface layer after irradia tion near the ablation threshold, transmission electron microscopy was used.To heat and probe the surface of the target, 100 fs pulses generated by a femtosecond chromium-for sterite laser system were used. The surface of the target was heated by pulses having a fundamental wavelength of 1240 nm at an angle of incidence of 45°. The target was probed by the 620 nm second harmonic pulses with the measured time delay with respect to the heat ing pulse. The spatial distribution of the energy density in the focal spot had the Gaussian formIt has been revealed experimentally that nanocavities remain inside a surface layer of aluminum after action of a femtosecond laser pulse. This result is in agreement with numerical simulation. A detailed picture of melting, formation of expansion and compression waves, and bubble nucleation in the stretched melt has been reconstructed through atomistic simulation. It has been shown that the bubbles do not fully collapse but remain as frozen disk shaped nanocavities upon recrystallization ...
Enhancement of magnetocaloric effect (MCE) in nanostructured materials is important for refrigeration applications such as spot cooling in microelectromechanical system devices. Here we report the first investigation of MCE properties in ball-milled ZnFe2O4 particles. The MCE was obtained by measuring a family of M-H curves at set temperature intervals and calculating the entropy change (ΔS) for this system using the Maxwell relation. The surface structure of zinc ferrite particles is sensitive to ball milling conditions and we observed that these surface effects greatly impact the MCE and our observations could provide a route for its potential enhancement by controlled surface modification.
The photoluminescence of pure amorphous Si films and films with embedded Si quantum dots and large nano-crystallites is correlated with XRD and AFM measurements. Several PL bands in the IR spectral range with maxima at 0.90, 0.98, 1.18 and 1.39 eV have been revealed in studied samples. The 0.90-0.98 eV PL bands are attributed to band tail luminescence in Si nano-crystallites. Concurrently, the 1.18 and 1.39 eV PL bands are assigned to radiative transitions between quantum confined levels within Si quantum dots embedded into a-Si matrix. The bright visible and infrared (IR) PL of the nc-Si embedded into SiO x matrix was investigated and several models were proposed. These models include (i) the quantum confinement PL effect in small diameter (≤5 nm) Si quantum dots [2,3], (ii) radiative recombination via Si/SiO x surface states on Si nano-crystallites or via Si-based chemical species, and (iii) oxide defects at the Si/SiO x interface and/or in the SiO 2 layer [4][5][6][7][8]. In the quantum confinement model a strong dependence of the PL peak energy versus size of Si nano-crystallites is anticipated. However, in many cases size determination of Si nanocrystallites was not provided. Moreover, different dependencies of the PL peak positions on nc-Si sizes have been observed [9,10].Another PL model suggests the importance of the suboxide related defects at the Si / SiO x interface. The visible PL bands at 1.77, 2.05 and 2.30 eV were connected with oxide related defects at the interface or in the SiO x layer [11,12]. Substitution of the SiO 2 matrix by amorphous silicon (α-Si) may confirm the origin of visible PL bands and permit the investigation of the PL bands connected with quantum confined states in the nc-Si. Another motivation for using the amorphous Si versus SiO 2 as a matrix for nano-crystals is the possibility of the electric excitation of Si nano crystallite luminescence by electron/hole injection. This process has a low efficiency in the case of dielectric materials such as SiO 2 .This paper presents the results of photoluminescence and X-ray diffraction studies of amorphous Si films with embedded Si nano-or micro-crystallites. We combined PL spectroscopy with atomic force
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