Abstract
Non-photochemical laser-induced nucleation (NPLIN) is the formation of a new phase from a metastable phase by the action of light on matter. Using millijoule, nanosecond laser pulses at visible and near-infrared wavelengths, it is possible to form the new phase localized in the volume of the beam. In the case of nucleating molecular solids, the laser polarization may have an effect on the particular polymorph that is formed. Despite the huge potential for applications of NPLIN, there is uncertainty regarding the molecular-scale mechanism, and various possible scenarios may well be relevant to nucleation in general and not just NPLIN. In this Perspective, the discovery and phenomenology of NPLIN are described, putative mechanisms are outlined, and some observations on the broader class of nucleation phenomena are given.
Highlights
Nucleation is the starting point in the formation of a new phase of a substance from another, e.g., the formation of solid ice from liquid water
The early experimental indications of the requirement for aging, the alignment of urea needles, and polarization switching of glycine appeared to fit in well with both the two-step nucleation (TSN) model and the optical Kerr effect (OKE) mechanism proposed for Non-photochemical laser-induced nucleation (NPLIN)
The effect can be summarized with the following key observations: (i) a minimum, threshold laser power is required for nucleation; (ii) the probability of nucleation increases with supersaturation and scales linearly with low peak laser powers; (iii) the effect is not strongly wavelength dependent; (iv) a single, nanosecond laser pulse can induce nucleation; (v) the polarization of light affects the crystal product; (vi) aging of solutions appears to make NPLIN more effective, at least for some systems; and (vii) nanoparticle impurities increase the probability of NPLIN
Summary
Nucleation is the starting point in the formation of a new phase of a substance from another, e.g., the formation of solid ice from liquid water. A full discussion of CNT lies outside the scope of this Perspective, but in terms of mechanism, it describes the fate of a solid cluster formed randomly in a solution by aggregation of molecules. The number of such clusters is dictated by the free energy change of formation from solution, and this is expressed in terms of interfacial and bulk contributions. 1. Free energy of forming a spherical solid cluster of radius r from solution without (black solid line) and with (red dashed line) irradiation with laser light of intensity I, according to CNT and the dielectric polarization model (described in Sec. III B). This would have clear benefits in applications where the phase of a material is important, for example, in active pharmaceutical ingredients
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