Broadly, the Shahani group will use synchrotron- and laboratory-based methods, including X-ray tomography and diffraction, to peer into the growth dynamics of crystals in realtime. Such studies would not be possible without the great strides in data sampling and reconstruction, computer hardware and storage, and algorithms for processing Big Data in a massively parallel environment. It is anticipated that our in situ and multimodal approach will provide a fresh lens for solving age-old conundrums in the field of crystal growth. Below, we outline our particular interests.
Growth of semiconductor crystals
Our research in this area concerns the flux and directional growth of single-element semiconductor crystals (e.g., Si, left). The incorporation of defects during solidification has important consequences for the electronic and mechanical properties of the material. For instance, electron-hole recombination rates depend strongly on the character of the grain boundaries. Thus, a fundamental understanding of the structure of semiconductor crystals during the growth process will provide the strategic link between solidification microstructure and underlying materials behavior.
While a few groups have succeeded in tracking the growth of semiconductor crystals from a liquid or gas phase, several open questions remain: how and why do the defects form in the first place? What are the thermophysical factors that govern the spacing and distribution of defects during crystal growth? We seek to identify the driving forces for defect formation in semiconductor crystals using a combination of realtime imaging and post mortem analyses.
Growth of nature's forbidden crystals
Quasicrystals are sometimes called "nature's forbidden crystals" because their structure is ordered but aperiodic. This means that quasicrystalline patterns (e.g., Penrose tiling shown at left) fill all available space, but in such a way that the pattern of its atomic arrangement never repeats. Perhaps unsurprisingly, the nucleation and growth of quasicrystals from a liquid is a topic that is as controversial as their inherent crystallography: multiple growth models have been proposed, nearly all of which lack experimental verification. The central issue in the field has been to understand whether quasicrystals are energetically stabilized or entropically stabilized via defects.
We have much to learn in this realm. For instance, is the growth of quasicrystals in some ways analogous to the growth of semiconductors, discussed above? If so, to what extent can classical theories of crystal growth be applied to the study of quasicrystal growth? Using synchrotron-based live imaging, we have viral the unique opportunity to watch the growth of quasicrystals from a liquid. The three-dimensional reconstructions will help us identify whether structural defects (e.g., dislocations and phasons) stabilize the quasicrystalline lattice, and how they might influence the morphology of the solid-liquid interfaces.
Growth of eutectic patterns
In certain alloy systems, a liquid of a fixed composition freezes to form a mixture of two different solid phases. Such a mixture is commonly known as a eutectic. These materials often exhibit outstanding mechanical and electrical properties because their microstructures act as natural or in situ composite materials. In order to tune the eutectic patterns (e.g., left, a eutectic spiral courtesy Jianrong Gao) to technological demands, we are exploring the crystallization of eutectic alloys. The morphology of the eutectic phases depends on the driving force for growth, the anisotropy in interfacial energy and interfacial mobility, as well as the chemical environment of the parent liquid phase.
In the latter case, it is well known that trace impurities (e.g., Na) drastically change the morphology of the eutectic phases (e.g., Si), from a coarse to fine structure. Although the effects of eutectic modification have been well documented, particularly for Si-based eutectics, there is no accepted explanation for the mechanism by which the microstructure changes so much upon adding trace amounts of a metal. Thus, our goal is to provide a unified picture of eutectic growth and modification.
Growth of abnormal grains in polycrystals
Most engineering materials are commonly found in polycrystalline form. The sizes, orientations, and boundaries of the grains influence many of the solid’s properties. During thermal processing, normal grain growth occurs, wherein some grains enlarge while others shrink and disappear. A second possibility, termed abnormal grain growth, is that a very few grains grow at a much faster rate than the others and eventually consume the sample volume. Abnormal grain growth has been observed in a vast array of metallic and semi-metallic alloys, yet its origins have remained an enigma for at least the past seventy years. Interestingly, a vast majority of these alloys also contain second-phase particles; thus, an open question concerns the role of the particles on abnormal grain growth.
To tackle the problem of abnormal grain growth, the Shahani group will employ a unique multimodal imaging platform: laboratory-based diffraction tomography together with absorption tomography will allow for the visualization of both grain boundaries and particles, respectively, as grain growth proceeds. An example of our approach is shown at left, where the two datasets have been superimposed.
The group is looking for hardworking and motivated individuals at the graduate level. Currently, there are projects and opportunities for multiple students in the areas of eutectic transformations and abnormal grain growth.