Engineering topics

Cha, Judy J.

Publications and source records attributed to Cha, Judy J..

22 records · Page 2

DOE OSTI2021

Effects of growth substrate on the nucleation of monolayer MoTe 2

The discovery and characterization of two-dimensional (2D) materials beyond graphene has increased dramatically over the past decade with increasingly fine control over the growth dynamics of these materials. MoTe 2 represents a model material for studying phase change properties of 2D materials due to small energy differences between its 2H semiconducting phase and 1T' semimetallic phase. Although some substrates are demonstrated to be better than others for the wafer-scale growth of high quality MoTe 2 films, substrate effects on the nucleation and growth of MoTe 2 are still not well understood. Here, we grow monolayer MoTe 2 by converting MoO x thin films deposited on three different substrates: sapphire Al 2 O 3 (0001), amorphous SiO 2 , and amorphous AlO x , and examine the early stages of the conversion reaction to elucidate the substrate effects on the nucleation of MoTe 2 . We observe that the chemical composition of the substrate is more important than the surface topography and crystallinity of the substrate, with high quality monolayer 2H MoTe 2 formed on both Al 2 O 3 (0001) and AlO x in contrast to mixed phase 2H/1T' MoTe 2 formed on SiO 2 , as determined by Raman spectroscopy, X-ray photoelectron spectroscopy, and atomic force microscopy.

36 MATERIALS SCIENCE↗

DOE OSTI2020

Synthesis of Narrow SnTe Nanowires Using Alloy Nanoparticles

Topological crystalline insulator tin telluride (SnTe) provides a rich playground to examine interactions of correlated electronic states, such as ferroelectricity, topological surface states, and superconductivity. The study of SnTe nanowires may lead to even richer physics owing to the one-dimensional (1D) confinement effect and an increased contribution from the topological surface states. Thus, for transport measurements, SnTe nanowires must be synthesized with reduced diameters and high crystalline quality to ensure 1D confinement and phase coherence of the topological surface electrons. Here, we report a facile growth method to produce narrow SnTe nanowires with a high yield using alloy nanoparticles as growth catalysts. The average diameter of the SnTe nanowires grown using alloy nanoparticles is 85 nm, nearly a factor of three reduction compared to the average diameter of 240 nm when using gold nanoparticles as growth catalysts. Transport measurements reveal the effect of the nanowire diameter on the residual resistance ratio and magnetoresistance. Particularly, the ferroelectric transition temperature for SnTe evolves systematically with the nanowire diameter. In situ cryogenic cooling of narrow SnTe nanowires in a transmission electron microscope directly reveals the cubic to rhombohedral structural transition, which is associated with the ferroelectric transition. Thus, these narrow SnTe nanowires represent a model system to study electronic states arising from 1D confinement, such as 1D topological superconductivity and potential multiband superconductivity.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗

DOE OSTI2020

cm 2 -Scale Synthesis of MoTe 2 Thin Films with Large Grains and Layer Control

Owing to the small energy differences between its polymorphs, MoTe 2 can access a full spectrum of electronic states from the 2H semiconducting state to the 1T' semimetallic state and from the T d Weyl semimetallic state to the superconducting state in the 1T' and T d phase at low temperature. Thus, it is a model system for phase transformation studies as well as quantum phenomena such as the quantum spin Hall effect and topological superconductivity. Careful studies of MoTe 2 and its potential applications require large-area MoTe 2 thin films with high crystallinity and thickness control. Here, we present cm 2 -scale synthesis of 2H-MoTe 2 thin films with layer control and large grains that span several microns. Layer control is achieved by controlling the initial thickness of the precursor MoO x thin films, which are deposited on sapphire substrates by atomic layer deposition and subsequently tellurized. Despite the van der Waals epitaxy, the precursor–substrate interface is found to critically determine the uniformity in thickness and grain size of the resulting MoTe 2 films: MoTe 2 grown on sapphire show uniform films while MoTe 2 grown on amorphous SiO 2 substrates form islands. Furthermore, this synthesis strategy decouples the layer control from the variabilities of growth conditions for robust growth results and is applicable to growing other transition-metal dichalcogenides with layer control.

2D materials↗

DOE OSTI2020

Crossover between weak antilocalization and weak localization in few-layer W Te 2 : Role of electron-electron interactions

We report electron transport studies in an encapsulated few-layer WTe 2 at low temperatures and high magnetic fields. The magnetoconductance reveals a temperature-induced crossover between weak antilocalization and weak localization in the quantum diffusive regime. Here we show that the crossover clearly manifests coexistence and competition among several characteristic lengths, including the dephasing length, the spin-flip length, and the mean free path. In addition, low-temperature conductance increases logarithmically with the increase of temperature indicating an interplay of electron-electron interaction (EEI) and spin-orbit coupling (SOC). We demonstrate the existence and quantify the strengths of EEI and SOC which are considered to be responsible for gap opening in the quantum spin Hall state in WTe 2 at the monolayer limit.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗