Himanshu Jain

Piezoresponse force microscopy (PFM) techniques, which probe nanoscale electromechanical coupling, are often difficult to interpret. For ferroelectric mixed conductors useful for computation elements, this becomes more difficult due to non-piezoelectric electromechanical effects. The measured PFM signal can have tip-local and nonlocal contributions associated with multiple coupling mechanisms, and it is often unclear what material property is truly being probed. Here, using antimony sulfide (Sb2S3) as a model system, an emerging phase change material for optical and electrical memory and computation elements, we show that the PFM signal can be separated into a fast, capacitive contribution, and a slow, ionic contribution whose time-domain characteristics match well with diffusion-controlled kinetics. This ionic contribution, rather than ferroelectric switching, leads to electromechanical hysteresis in Sb2S3, with no clearly measurable ferroelectric switching effects. Uniquely, electromechanical behavior has a very clear spatial contrast that is decoupled from surface topography, showing that the signal arises from surface charging that reflects bulk-like properties and allowing a self-consistent interpretation in terms of dielectric behavior. This protocol establishes a clear interpretation of a non-piezoelectric signal, often mischaracterized as an indication of the ferroelectric state. Additionally, because all electromechanical experiments show a strong, low-frequency electric relaxation in crystalline Sb2S3 while the response of glassy Sb2S3 is static, we conclude that the crystalline phase of Sb2S3 is a mixed conductor, while the amorphous phase remains strictly insulating.

Recent results showed that during growth, a crystal seed embedded in glass experiences nonuniform forces from the host matrix. This result raised the possibility that during devitrification of glass, the growth of a crystal can be affected by the presence of neighboring crystals. To investigate the nature of any such interdependence of growth of neighboring crystals, atomic scale morphology of two seeds growing in a glass matrix was simulated as a function of their separation and relative misorientation. The results show a coupled rotation of seeds driven by (1) non-uniform forces from the surrounding glass, and (2) mutual interactions between the seeds. At small separations and low misorientation, each seed rotates toward perfect alignment, ultimately forming a single crystal. The onset and rate of alignment, as well as subsequent crystal growth, are strongly dependent on the growth temperature. In contrast, at larger separations and higher misorientation, the seeds can not achieve full alignment before coalescing, resulting in polycrystalline structures.

Rotation of crystal seed during the early stages of growth in a glass matrix has been observed due to some torque, contradicting the expectations from the isotropic, uniform structure of the surrounding amorphous matrix. We establish an atomistic origin of this new phenomenon from molecular dynamics simulations using LiNbO3 and LiNbO3-SiO2 glasses as model systems. Effectively, it arises due to non-uniform forces on the seed from the surrounding glass, which appears inhomogeneous and anisotropic on the scale of glass-crystal interface. The seeded crystal growth (SCG) at higher temperatures amplifies this effect due to enhanced atomic dynamics. Silica, when added to LiNbO3 glass, reduces the crystal growth rate due to increased viscosity and restricted atomic mobility across the growth interface, but has minimal effect on the crystal rotation. These findings challenge a general assumption that glass is an isotropic material, especially during the early stage of its crystallization, and provide insights for tailoring the microstructure of widely used glass-ceramics.

Single crystals fabricated in glass by localized heating can develop uniquely deformed lattices stabilized by the surrounding amorphous medium. The development of lattice curvature appears to be intrinsic to the crystal growth process in some systems, while the result of the locally changing crystallography in others. In this work, a model laser‐fabricated rotating lattice Sb₂S₃ crystal grown in stoichiometric glass is used to demonstrate fabrication of novel macroperiodic metastructures that utilize intrinsic lattice curvature superimposed with subtle crystallographic influences. The limited availability of slip systems drives the lattice curvature magnitude to vary with crystal growth direction, maximizing for lattices aligned with the predominant Burgers vector along with corresponding increases in dislocation density. Misaligned lattice orientations form smaller secondary lattice curvatures arising from misaligned Burgers vectors with further elastic contributions. Over extended crystal growth, these secondary components align the lattice to rotate about either the or crystal axes forming repeating metastructures of lattice orientation with periodicity 20–160 microns in length. The mechanistic approach used in this work may be expanded to other systems with known slip systems to better understand and design macroperiodic metastructures.