Developments in Interferometric Techniques for In-situ Observation of Surface Kinetics of Crystals in Solutions and Three-dimensional Analysis of Transport Phenomena

Abstract

Importance of optical imaging techniques for mapping surface features of crystals growing from their aqueous solution at a nanoscopic level and the associated transport phenomena has been realized over the past few decades. The spatial resolution in the measurements of growth rates and detection of surface irregularities as provided by scanning probe microscopic (SPM) techniques such as AFM and STM has been found to be better than the conventional interferometric techniques. However, with recent developments in the field of light sources and optical components, new advanced interferometers have attained nearly the same resolution as scanning microscopes. The advantages of using optical interferometers can be summarized as follows: (1) they can be used not only at room temperature but also at elevated temperatures as high as ∼2000 K; (2) their ability to resolve time is much better, making them more suitable for observing rapidly growing crystals; (3) coupled with the principles of tomography, they are capable of providing an insight into the effects of three‐dimensional transport phenomena on the surface kinetics of the growing crystals in a strict non‐invasive sense. Against this background, the present article is primarily structured into two parts: the first part is intended to cover the key aspects of conventional as well as advanced interferometric techniques for the measurements of crystal growth rates and observation of surface micro‐morphologies, such as growth steps and hillocks, with resolutions comparable to that of the scanning probe microscopic techniques. The potential of recently developed interferometric techniques, such as Laser Confocal Phase‐Shift Interferometry (LCPSI) for imaging mono‐molecular growth steps of sub‐millimeter sized crystals, such as protein crystals and measurements of growth and/or dissolution rates as low as 10−5 nm/s (1μm/year) would be demonstrated. Second part of the article emphasizes the importance of basic interferometric technique in mapping the three‐dimensional distribution of concentration and/or temperature field using the principles of tomography in the vicinity of the growing crystal surface at a sub‐millimeter scale. The salient features of direct as well as iterative tomography algorithms have been briefly discussed. Representative results in the form of interferograms, path‐integrated concentration contours and three‐dimensional concentration profiles over select horizontal planes above the growing crystal are presented.