Search NASA⌕ Search

Engineering topics

Zhang, Kunyan

Publications and source records attributed to Zhang, Kunyan.

6 records
All sourcesNASA NTRSDOE OSTI
DOE OSTI2025

Achieving High-Yield Conversion of Janus Transition Metal Dichalcogenides on Diverse Substrates

Janus transition metal dichalcogenides (TMDCs) with intrinsic broken mirror symmetry and vertical dipole moment provide an additional degree of freedom to manipulate material symmetry down to atomic-layer thickness. However, despite advances in synthesis strategies, fundamental understanding of this atomic substitution process remains limited, which has impeded their implementation in advanced devices. Here, by using a room-temperature atomic-layer substitution (RT-ALS) strategy, we systematically investigate the critical factors facilitating the high-yield conversion of Janus TMDCs on diverse substrates. Combining Raman spectroscopy probes, X-ray photoelectron spectroscopy (XPS) measurements, and density functional theory (DFT) calculations, we demonstrate that substrates with enhanced electron doping or larger surface polarity substantially benefit the conversion of Janus TMDCs reaching a near-unity yield. Intriguingly, the strong affinity between Janus TMDCs and substrates (e.g., Au) brings about abnormal Raman spectroscopic phenomena. These findings highlight the significance of substrates in achieving the reliable synthesis of Janus two-dimensional materials with improved homogeneity on various substrates. In addition, this takes us one step closer to utilizing Janus TMDCs as a versatile platform in next-generation optoelectronic devices, sensors, and quantum technologies.

Doping↗
DOE OSTI2023

Step engineering for nucleation and domain orientation control in WSe 2 epitaxy on c -plane sapphire

In this study, epitaxial growth of two-dimensional transition metal dichalcogenides on sapphire has emerged as a promising route to wafer-scale single-crystal films. Steps on the sapphire act as sites for transition metal dichalcogenide nucleation and can impart a preferred domain orientation, resulting in a substantial reduction in mirror twins. Here we demonstrate control of both the nucleation site and unidirectional growth direction of WSe 2 on c-plane sapphire by metal–organic chemical vapour deposition. The unidirectional orientation is found to be intimately tied to growth conditions via changes in the sapphire surface chemistry that control the step edge location of WSe 2 nucleation, imparting either a 0° or 60° orientation relative to the underlying sapphire lattice. The results provide insight into the role of surface chemistry on transition metal dichalcogenide nucleation and domain alignment and demonstrate the ability to engineer domain orientation over wafer-scale substrates.

36 MATERIALS SCIENCE↗
DOE OSTI2021

Designing artificial two-dimensional landscapes via atomic-layer substitution

Significance Manipulating materials with atomic-scale precision is essential for the development of a next-generation material design toolbox. Tremendous efforts have been made to advance the compositional, structural, and spatial accuracy of material deposition and patterning. Here, we presented a new reaction pathway to implement the conversions of two-dimensional materials within the atomic-layer thickness at room temperature for electrical dipole manipulation. Not only could various Janus monolayer transition metal dichalcogenides with vertical dipole be realized, but also some heterostructures, including the dipole-nondipole heterostructures (MoS 2 -MoSSe) and multiheterostructures (MoS 2 -MoSSe-MoSeS-MoSe 2 ) within the same monolayer host structure are developed, in which the dipoles can be selectively patterned to be zero (MoS 2 , MoSe 2 ), positive (MoSSe), and negative (MoSeS).

Guo, Yunfan↗
DOE OSTI2020

Anisotropic Fano resonance in the Weyl semimetal candidate LaAlSi

Topological Weyl semimetal (WSM) is a solid-state realization of chiral Weyl fermions, whose phonon behaviors provide in-depth knowledge of their electronic properties. In this work, anisotropic Fano resonance is observed in a type-II WSM candidate LaAlSi by polarized Raman spectroscopy. The asymmetric line shape occurs for the $B^2_1$ phonon mode of LaAlSi only for 488- and 532-nm laser excitations but not for 364-, 633-, and 785-nm excitations, suggesting the excitation selectivity. The asymmetry, frequency, and linewidth of the $B^2_1$ phonon mode, along with the spectral background, all show fourfold rotational symmetry as a function of the polarization angle in the polarized Raman spectra. While the shift of Raman frequency in a metal or semimetal is typically attributed to Kohn anomaly, here we show that the anisotropic frequency shift in LaAlSi cannot be explained by the effect of Kohn anomaly, but potentially by the anisotropic scattering background of Fano resonance. Origins of the excitation-energy dependence and anisotropic behavior of the Fano resonance are discussed by the first-principles calculated electronic band structure and phonon dispersion.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗