I have used laser ablation method to prepare tin oxide nanoparticle. The UV-VIs absorbance spectrum show decay in absorbance concentration when i have changed energy from 60mJ to 180mJ
Briefly, this is related to the inertia (mass) of the particles absorbing the energy; as frequency increases beyond a certain limit, the non-vanishing mass of particles (i.e. electrons and ions) increasingly prevents them from reacting to the external stimulus (here, a laser beam), whereby absorption decreases. For this reason, the leading-order term in the asymptotic series expansion of the relative dielectric function ε(q;ν) of a material system as frequency ν increases towards infinity, is 1, the relative dielectric constant of the vacuum.
It is related to surface plasmons, but also to bulk plasmons (which have higher energies), and in general to any excitation process in the system that can absorb energy from the incident light wave. I should add that absorbance is not the same as absorption, however since beyond a certain frequency also reflectance diminishes for increasing value of the frequency of the incident light, its contribution to absorbance diminishes accordingly: the more transparent a piece of material becomes for increasing frequencies of the incident light, the less it reflects of this light (the reflectivity is proportional to the absolute value of the refractive index minus 1, and beyond a certain frequency the refractive index steadily approaches 1, a fact that is directly related to the dielectric function ε(q;ν) approaching 1 for |ν| → ∞; see my earlier response on this page).
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Since I am not experimentalist, I am not in a position to answer your last question with certainty. I hope some experimentalist with experience in the field will come along and give an authoritative answer to this question. For now, agglomeration is indeed likely to decrease the gap (since it affords the system of nano-particles a macroscopic character -- overlap of the wave functions of the neighbouring nano particles gives rise to formation of bands, bending the energy levels of the localised states in the nano-particles, thus leading to a smaller excitation gap). By the same mechanism, one will get increased accumulation of energy levels in some windows of energy. For this reason, absorption will increase in these windows of energy. It is also likely that surface contamination plays a significant role in your case, as it gives rise to additional electronic states.
Note that the size of the energy gap is related to the lowest energy below which no absorption takes place; it is not directly related to the quantitative amount of absorption over the energy regions where absorption takes place. As a matter of fact, at the edges of a band gap one in general has a singular density of states.
Lastly, I shall not go into details, but suffice to mention that the optical gap is not equal to the quasi-particle gap, but the two are related. To lowest order of perturbation theory, optical gap is 3/2 = 1.5 times the quasi-particle gap.
Hi I have started working on PLAL .. Do you know how to reduce the splashing problem during the ablation process??
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