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Raja, Archana

Publications and source records attributed to Raja, Archana.

7 records
All sourcesNASA NTRSDOE OSTI
DOE OSTI2025

Local Interface Effects Modulate Global Charge Order and Optical Properties of 1T–TaS2/1H–WSe2 Heterostructures

1T-TaS2 is a layered charge density wave (CDW) crystal exhibiting sharp phase transitions and associated resistance changes. These resistance steps could be exploited for information storage, underscoring the importance of controlling and tuning the CDW states. Given the importance of out-of-plane interactions in 1T-TaS2, modulating interlayer interactions by heterostructuring is a promising method for tailoring CDW phase transitions. In this work, we investigate the optical and electronic properties of heterostructures comprising 1T-TaS2 and monolayer 1H-WSe2. By systematically varying the thickness of 1T-TaS2 and its azimuthal alignment with 1H-WSe2, we find that intrinsic moiré strain and interfacial charge transfer introduce CDW disorder in 1T-TaS2 and modify the CDW ordering temperature. Furthermore, our studies reveal that the interlayer alignment impacts the exciton dynamics in 1H-WSe2, indicating that heterostructuring can concurrently tailor the electronic phases in 1T-TaS2 and the optical properties of 1H-WSe2. This work presents a promising approach for engineering the optoelectronic behavior of heterostructures that integrate CDW materials and semiconductors.

charge density wave↗
DOE OSTI2025

Long-lived photoinduced polar states in metal halide perovskites

Ferroic polarization in hybrid perovskites is crucial for enhancing photovoltaic performance and developing potential electronic applications. Controlling ferroic polarization with an optical field enables probing of ferroic polarization without the unwanted interface ionic effects caused by electronic contacts. This study employs ultrafast near-infrared photoexcitation to control dynamic structural transitions in soft single crystalline hybrid Cu (II) halide perovskites, achieving a long-lived polar state (beyond 104 s) at room temperature. We probe reversible long-lived polar domains under near-infrared photoexcitation using in-situ second harmonic generation microscopy. Theoretical calculation informs the polar lattice microstrain likely induced by anisotropic structure deformation in octahedral copper halide under near-infrared photoexcitation. The reversible slow structure deformation is further confirmed by in-situ photo-induced X-ray diffraction measurement. This work provides a material platform for understanding, controlling, and probing polarization under photoexcitation. Our methodology enables the identification of previously undiscovered polar phases in ferroelectric halide perovskites.

Dou, Yixuan↗
DOE OSTI2025

Graphene-driven correlated electronic states in one dimensional defects within WS2

Tomonaga-Luttinger liquid (TLL) behavior in one-dimensional systems has been predicted and shown to occur at semiconductor-to-metal transitions within two-dimensional materials. Reports of one-dimensional defects hosting a Fermi liquid or a TLL have suggested a dependence on the underlying substrate, however, unveiling the physical details of electronic contributions from the substrate require cross-correlative investigation. Here, we study TLL formation within defectively engineered WS2 atop graphene, where band structure and the atomic environment is visualized with nano angle-resolved photoelectron spectroscopy, scanning tunneling microscopy and spectroscopy, and non-contact atomic force microscopy. Correlations between the local density of states and electronic band dispersion elucidated the electron transfer from graphene into a TLL hosted by one-dimensional metal (1DM) defects. It appears that the vertical heterostructure with graphene and the induced charge transfer from graphene into the 1DM is critical for the formation of a TLL.

Rossi, Antonio↗
DOE OSTI2024

Difficult Measurements of Materials Systems at Cryogenic Temperatures: Cryo-EELS and Cryo-4D-STEM

Scanning/transmission electron microscopy (S/TEM) in materials science has traditionally been accomplished at room temperature due to their solid state in this temperature range and relative insensitivity to radiolysis damage. Cooling to cryogenic temperatures tended to introduce instabilities such as vibration and drift and can lead to the buildup of carbon contamination or ice reducing contrast. The need to expand TEM experimental techniques into new fields such as quantum and battery materials led to new efforts to reduce these instabilities while achieving temperatures at or well below liquid nitrogen (77 K). Further, developments in detector and spectrometer technology brought new capabilities for high resolution electron energy loss spectroscopy (EELS) and scanning nanodiffraction (4D-STEM). These experimental modalities can require even tighter controls of drift, vibration, and exposure time. Here this presentation will discuss accomplishing difficult experiments at cryogenic temperatures in honor of the late Dr. Lena Kourkoutis who inspired and led many such experiments.

36 MATERIALS SCIENCE↗
DOE OSTI2024

Anomalous Interlayer Exciton Diffusion in WS 2 /WSe 2 Moiré Heterostructure

Stacking van der Waals crystals allows for the on-demand creation of a periodic potential landscape to tailor the transport of quasiparticle excitations. We investigate the diffusion of photoexcited electron-hole pairs, or excitons, at the interface of WS 2 /WSe 2 van der Waals heterostructure over a wide range of temperatures. We observe the appearance of distinct interlayer excitons for parallel and antiparallel stacking and track their diffusion through spatially and temporally resolved photoluminescence spectroscopy from 30 to 250 K. While the measured exciton diffusivity decreases with temperature, it surprisingly plateaus below 90 K. Our observations cannot be explained by classical models like hopping in the moiré potential. A combination of ab initio theory and molecular dynamics simulations suggests that low-energy phonons arising from the mismatched lattices of moiré heterostructures, also known as phasons, play a key role in describing and understanding this anomalous behavior of exciton diffusion. Our observations indicate that the moiré potential landscape is dynamic down to very low temperatures and that the phason modes can enable efficient transport of energy in the form of excitons.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2024

Interlayer Coupling Controlled Ordering and Phases in Polar Vortex Superlattices

The recent discovery of polar topological structures has opened the door for exciting physics and emergent properties. There is, however, little methodology to engineer stability and ordering in these systems, properties of interest for engineering emergent functionalities. Notably, when the surface area is extended to arbitrary thicknesses, the topological polar texture becomes unstable. Here we show that this instability of the phase is due to electrical coupling between successive layers. We demonstrate that this electrical coupling is indicative of an effective screening length in the dielectric, similar to the conductor–ferroelectric interface. Controlling the electrostatics of the superlattice interfaces, the system can be tuned between a pure topological vortex state and a mixed classical-topological phase. This coupling also enables engineering coherency among the vortices, not only tuning the bulk phase diagram but also enabling the emergence of a 3D lattice of polar textures.

3D ordering↗