Ciccarino, Christopher J.

DOE OSTI2022

Mid- to Far-Infrared Anisotropic Dielectric Function of HfS 2 and HfSe 2

The far-infrared (far-IR) remains a relatively underexplored region of the electromagnetic spectrum extending roughly from 20 to 100 µm in free-space wavelength. Research within this range has been restricted due to a lack of optical materials that can be optimized to reduce losses and increase sensitivity, as well as by the long free-space wavelengths associated with this spectral region. Here the exceptionally broad Reststrahlen bands of two Hf-based transition metal dichalcogenides (TMDs) that can support surface phonon polaritons (SPhPs) within the mid-infrared (mid-IR) into the terahertz (THz) are reported. In this vein, the IR transmission and reflectance spectra of hafnium disulfide (HfS 2 ) and hafnium diselenide (HfSe 2 ) flakes are measured and their corresponding dielectric functions are extracted. These exceptionally broad Reststrahlen bands (HfS 2 : 165 cm -1 ; HfSe 2 : 95 cm -1 ) dramatically exceed that of the more commonly explored molybdenum- (Mo) and tungsten- (W) based TMDs (≈5–10 cm -1 ), which results from the over sevenfold increase in the Born effective charge of the Hf-containing compounds. Further, this work therefore identifies a class of materials for nanophotonic and sensing applications in the mid- to far-IR, such as deeply sub-diffractional hyperbolic and polaritonic optical antennas, as is predicted via electromagnetic simulations using the extracted dielectric function.

36 MATERIALS SCIENCE↗

DOE OSTI2021

Capturing 3D atomic defects and phonon localization at the 2D heterostructure interface

The three-dimensional (3D) local atomic structures and crystal defects at the interfaces of heterostructures control their electronic, magnetic, optical, catalytic, and topological quantum properties but have thus far eluded any direct experimental determination. Here, we use atomic electron tomography to determine the 3D local atomic positions at the interface of a MoS 2 -WSe 2 heterojunction with picometer precision and correlate 3D atomic defects with localized vibrational properties at the epitaxial interface. We observe point defects, bond distortion, and atomic-scale ripples and measure the full 3D strain tensor at the heterointerface. By using the experimental 3D atomic coordinates as direct input to first-principles calculations, we reveal new phonon modes localized at the interface, which are corroborated by spatially resolved electron energy-loss spectroscopy. We expect that this work will pave the way for correlating structure-property relationships of a wide range of heterostructure interfaces at the single-atom level.

42 ENGINEERING↗