Aditya Singh

Crystals often have complex structural domains, but a general method to remove or deterministically control such local heterogeneity is lacking. The resulting heterogeneity in crystal orientations obscures our understanding of material properties and can reduce the reliability and performance of related applications. Here, using shear stress from an atomic force microscope tip, we ferroelastically write local crystal orientations in oxide thin films. Applying this deterministic and reversible control to SrRuO₃ and (La_0.7Sr_0.3)(Mn_0.9Ru_0.1)O₃ films, we realize twin-free single crystals and design specific crystal-orientation domain textures at the nanoscale. Furthermore, through magnetoelastic coupling, we can mechanically manipulate the local magnetic anisotropy, and thereby write and erase functional nanoscale magnetic textures unattainable by conventional methods. Thus, pure mechanical force emerges as a means to control structural heterogeneity on demand and may make it possible to program electronic and spintronic functionalities.

Quantized lattice vibrations (i.e., phonons) in solids are robust and unambiguous fingerprints of crystal structures and of their symmetry properties. In metals and semimetals, strong electron-phonon coupling may lead to so-called Kohn anomalies in the phonon dispersion, providing an image of the Fermi surface in a nonelectronic observable. Kohn anomalies become prominent in low-dimensional systems, in particular, in graphene, where they appear as sharp kinks in the in-plane optical phonon branches. However, in spite of intense research efforts on electron-phonon coupling in graphene and related van der Waals heterostructures, little is known regarding the links between the symmetry properties of optical phonons at and near Kohn anomalies and their sensitivity towards the local environment. Here, using inelastic light scattering (Raman) spectroscopy, we investigate a set of custom-designed graphene-based van der Waals heterostructures, wherein dielectric screening is finely controlled at the atomic-layer level. We demonstrate experimentally and explain theoretically that, depending exclusively on their symmetry properties, the two main Raman lines of graphene react differently to the surrounding environment. While the 2D line, which is due to near-zone-edge optical phonons, undergoes changes due to the neighboring dielectric environment, the in-plane, zone-center optical phonons are symmetry protected from the influence of the latter. These results shed new light on the unique electron-phonon coupling properties in graphene and related systems and provide invaluable guidelines to characterize dielectric screening in van der Waals heterostructures and moiré superlattices. Published by the American Physical Society 2025

Layer transfer offers enormous potential for the industrial implementation of two-dimensional (2D) material technology platforms. However, the transfer method used must retain the as-grown uniformity and cleanliness in the transferred films for the fabrication of 2D material-based devices. Additionally, the method used must be capable of large-area transfer to maintain wafer-scale fabrication standards. Here, a facile route to transfer centimeter-scale synthesized 2D transition metal dichalcogenides (TMDCs) (3L MoS₂, 1L WS₂) onto various substrates such as sapphire, SiO₂/Si, and flexible substrates (mica, polyimide) has been developed using a water-soluble layer (Na₂S/Na₂SO₄) underneath the as-grown film. The developed transfer process represents a fast, clean, generic, and scalable technique to transfer 2D atomic layers. The key strategy used in this process includes the dissolution of the Na₂S/Na₂SO₄ layer due to the penetration of NaOH solution between the growth substrate and hydrophobic 2D TMDC film. As a proof-of-concept device, a broadband photodetector has been fabricated onto the transferred 3L MoS₂, which shows photoresponse behavior for a wide range of wavelengths ranging from near-infrared (NIR) to UV. The enhancement in photocurrent was found to be 100 times and 10 times the dark current in the UV and visible regions, respectively. The fabricated photodetector shows a higher responsivity of 8.6 mA/W even at a low applied voltage (1.5 V) and low power density (0.6 μW/mm²). The detector enables a high detectivity of 2.9 × 10¹¹ Jones. This work opens up the pathway toward flexible electronics and optoelectronics.