The cooperation between scientists of the Institute of Nuclear Physics Polish Academy of Sciences and ASTRA beamline resulted in a better understanding of the mechanism of phase formation in pulsed laser irradiation of suspension (PLIS) for submicron heterostructure formation.
According to Shakeri, et al., irradiation of suspended nanoparticles using laser pulses results in formation submicron particles by the following mechanism.
Suspended agglomerates are heated by the energy absorption of a laser pulse according to their absorption efficiency, depending on the wavelength-dependent absorption cross section calculated by Mie theory. The absorbed energy is directly proportional to the laser fluence and the surface area of the agglomerate. On the other hand, the pulse factors influence the temperature development, so that the pulse shape is responsible for heat conservation, while the pulse duration controls the heat release. Cooling of the particles and re-agglomeration of the irradiated particles occurs in the time between pulses by synergistic agitation of the suspension. In addition, pulse repetition affects particle growth by changing the probability of the probe. The heating-cooling process is repeated depending on the probability of the probe in the system, resulting in alternative particle growth limited by the melting possibility of agglomerates. More importantly, the temperature of agglomerates not only determines their thermodynamically stable phases but also affects the rate of solvent dissociation during interfacial solvent-particle interactions. Although the solvent bath is at ambient temperature, due to the transparency to the laser beam, the kinetic energy of the molecules coming into contact with the surface of the heated particles is very high, resulting in the bond breaking/formation at the interface between the dissociated solvent and the heated particles. The dissociated species diffuse into the sphere where the oxidation/reduction reactions take place according to their nature.
EXAFS spectra measured in ASTRA beamline of SOLARIS were initially used for invesigation of oxidation evaluation of samples. Linear combination fitting (LCF) of EXAFS spectra also used using reference spectra for measurement of exact composition of samples. Chemical composition variations detected by LCF helped the scientists to detect chemical/physical type of phase transition during PLIS phase formation. Gaussian deconvolution of XANES spectra showed the probable constituent transitions, which were later used for fitting of various phases in the system derived by calculated XANES spectra using Feff code.
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Image: Figure 1. Mechanism of particle formation during PLIS.