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Aakash Varambhia

Publications and source records attributed to Aakash Varambhia.

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An Interactive, Automated 4D-STEM data acquisition and analysis routine for Scanning Electron Nanobeam Diffraction and Ptychography experiments

Modern transmission electron microscopes are versatile instruments which have become indispensable tools for understanding structure and chemical composition at the nano- and atomic scale. In the physical sciences these instruments are still largely manually controlled, requiring significant operator expertise, limiting throughput, and precluding statistical analysis of large datasets. Recent technical advances in both hardware and in control software now allow for the interaction with almost every functionality of the microscope through a programming interface. This enables better experimental design and data collection automation while also reducing operator collection bias and required expertise. In this study, we present an automated data collection routine with machine-driven decision-making to enable the collection of hundreds of 4D-STEM nanobeam diffraction and ptychography data from a large distribution of size-selectively deposited Pt nanoparticles. We present a semi-automated data analysis workflow to extract pertinent information from the large volumes of collected data. For the nanobeam diffraction data, reducing each dataset to its azimuthal variance profile and combining automated crystal orientation mapping with per-particle morphology descriptors reveals the orientation, shape and phase distributions across the ensemble, including a weak {110} texture. For the ptychographic data, an automated screening pipeline identifies on-zone-axis particles and enables atomic-resolution phase imaging and lattice-strain mapping of individual grains. Together these demonstrate how automation turns instrument throughput into statistically meaningful, atomic-scale microstructural information.

physics.ins-det

Azetidinium as Cation in Lead Mixed Halide Perovskite Nanocrystals of Optoelectronic Quality

Previous theoretical calculations show azetidinium has the right radial size to form a 3D perovskite with lead halides [1], and has been shown to impart, as the A-site cation of ABX3 unit, beneficial properties to ferroelectric perovskites [2]. However, there has been very limited research into its use as the cation in lead halide perovskites to date. In this communication we report the synthesis and characterization of azetidinium-based lead mixed halide perovskite colloidal nanocrystals. The mixed halide system is iodine and chlorine unlike other reported nanocrystals in the literature where the halide systems are either iodine/bromine or bromine/chlorine. UV-visible absorbance data, complemented with photoluminescence spectroscopy, reveals an indirect-bandgap of about 1.96 eV for our nanocrystals. Structural characterization using TEM shows two distinct interatomic distances (2.98 +/- 0.15 Angstroms and 3.43 +/- 0.16 Angstroms) and non-orthogonal lattice angles (approximately 112 degrees) intrinsic to the nanocrystals with a probable triclinic structure revealed by XRD. The presence of chlorine and iodine within the nanocrystals is confirmed by EDS spectroscopy. Finally, light-induced electron paramagnetic resonance (LEPR) spectroscopy with PCBM confirms the photoinduced charge transfer capabilities of the nanocrystals. The formation of such semiconducting lead mixed halide perovskite using azetidinium as the cation suggests a promising subclass of hybrid perovskites holding potential for optoelectronic applications such as in solar cells and photodetectors.

physics.app-ph