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2019 Highlights

2DCC User Research Highlights 2019

The strongly spin-momentum coupled electronic states in topological insulators (TI) have been extensively pursued to realize efficient magnetic switching. However, previous studies show a large discrepancy of the charge-spin conversion efficiency. Moreover, current-induced magnetic switching with TI can only be observed at cryogenic temperatures. We report spin-orbit torque switching in a TI-ferrimagnet heterostructure with perpendicular magnetic anisotropy at room temperature. The obtained effective spin Hall angle of TI is substantially larger than the previously studied heavy metals. Our results demonstrate robust charge-spin conversion in TI and provide a direct avenue towards applicable TI-based spintronic devices.

LANL CINT scientists and their collaborators at UConn, AFRL, Penn State, and U. Oregon have discovered a method to create spatially localized quantum emission sites in a wafer-scale transition metal dichalcogenide film, WSe2, synthesized at the 2D Crystal Consortium (2DCC) facility. The team’s objective was to determine the role of strain in creating localized quantum emission sites in order to learn how to control their properties through strain.

Because the WSe2 was very thin, it conformed to the radius of ultra-sharp tips with sub-10 nanometer diameters, and bent towards the substrate. The resulting strain was enough to change the electronic structure, but only at the tips. The affected area emitted light that was fundamentally different in nature than that from the rest of the WSe2 film, where photons were ejected one-by-one, achieving what is referred to as deterministic quantum emission.

An undergraduate REU student at UConn participated in the research as is a co-author on the paper.

The team published the results as a Featured Article in the May 27th issue of Applied Physics Letters.

Link to LANL press release:

https://www.lanl.gov/discover/news-release-archive/2019/May/0529-quantum-information.php

Monolayer 2D transition metal dichalcogenides (TMDs) have been a focus of increasing interest due to their unique properties but the development of device technologies has been hampered by difficulties in synthesizing large area monolayer and few layer films. We developed a multi-step process involving nucleation, ripening and preferential lateral growth to achieve epitaxial WSe2 monolayers on sapphire by gas source chemical vapor deposition. This process enables control of the nucleation density of WSe2 on the sapphire and the lateral growth rate of domains resulting in coalescence of monolayer regions over wafer-scale areas with minimal multilayer growth. The process also provides insights into the fundamental kinetic and thermodynamic factors that impact monolayer growth.

Broader Impacts Highlights 2019

The Thin Films and In Situ Characterization Facility in the 2D Crystal Consortium – Materials Innovation Platform (2DCC-MIP) user facility includes a multimodule ultra-high vacuum system for the synthesis and study of two dimensional (2D) chalcogenide films by molecular beam epitaxy (MBE) [upper panel].  The lower left panel shows a high resolution cross-sectional transmission electron microscope image of a complex 2D chalcogenide film synthesized in this facility. Such 2D films are characterized in their pristine state by transferring them in vacuo to surface sensitive characterization tools. The in vacuo environment avoids the modification of the inherent properties of these 2D films by exposure to ambient atmosphere. The electronic energy surface states in these 2D films are probed using angle resolved photoemission spectroscopy (ARPES) and scanning tunneling microscopy. The right lower panel shows ARPES measurements of a topological insulator thin film.

STEPFORWARD is a geospatial tool built on Google Maps API to identify minority serving institutions (MSIs), primarily undergraduate institutions (PUIs) and non-R1 institutions within a geographical radius of travel. Higher Education R&D data highlights institutions active in research fields of interest.

 

The tool enables 2DCC researchers and the broader community to identify a diverse range of institutions for outreach visits and research interactions. The 2DCC supports visits to these institutions as part of a travel extension program.

The 2DCC completed its 4th year of sponsorship of the Graphene and Beyond Workshop.  To date, >650 individuals from academia, government and industry across all career levels have attended.  The 2DCC has broadened participation by sponsoring travel and registration for dozens of users and potential users particular from MSIs and PUIs. The workshop enhances community synergy toward a strong future in 2D crystal science and technology. The workshop consists of tutorials and research talks focused on 2D material synthesis, properties, theory and optic/electronics as well as recent results from the 2DCC and facilitated discussion on commercialization opportunities for 2D materials.

In 2019, the 2DCC partnered with the 5th annual US/EU 2D Materials Workshop for a joint workshop that included speakers from across the globe including the Graphene Flagship.

LiST is a web-based data management software tool developed by the 2DCC to capture, organize and curate experimental and theory/simulation data produced by the facility and external users.

Under LiST, data follow the sample from substrate preparation to synthesis protocol, integrated UHV characterization, ex situ characterization, modeling, delivery, and publication, with fine-grained access control and community-accessible tools and data.

In-House 2DCC Research Highlights 2019

2-dimensional materials hold promise for next-generation electronics. However, in order to realize 2D-based technologies, key milestones must be identified and met. This article identifies areas of research which are fundamentally required for achieving electronic grade 2D materials and brings together experts in these respective areas to discuss key challenges. As a result, this article serves as a roadmap for the realization of 2D materials which can be used in the development of technology toward industries including healthcare, internet of things, high-performance computing, and economical solar energy.

 

This article can serve as a guide, and a source of inspiration and motivation for researchers and members of the 2D community. Additionally, it communicates the importance of 2D research to the greater scientific community, as well as industry players, demonstrating the  current and potential impact of these materials on technology and society.

A graphene lattice remains the same under a rotation of 180º. This special symmetry is lost in monolayer transition metal dichalcogenides (TMDs), e.g. every horizontal Mo-S bond would end up as S-Mo bonds. While this lack of inversion symmetry brings out fascinating properties such as spin-coupled valley degrees of freedom, it also means that when two grains related by 180º rotation merge, the two lattices cannot smoothly stitch together, leaving an unavoidable scar – an inversion domain boundary (IDB) – that degrades transport properties.

 

In two recent 2DCC publications, joint experimental and theory efforts identified a general method to achieve orientational selectivity, originating from a localized defect pair that amplifies the energetic distinction between the two orientations. For monolayer MoS2 or WSe2 grown on hexagonal boron nitride (hBN), the defect pair is a generalization of a so-called Frenkel pair – a interstitial-vacancy pair shared between adjacent layers of a 2D heterostack. MoS2 and WSe2 domains initiated from such defects exhibit an orientation uniformity of over 80%, leading to suppression IDBs upon coalescence, as confirmed by aberration-corrected scanning/transmission electron microscopy. Fully coalesced 2D chalcogenide monolayer films synthesized using this approach demonstrated superior optical and electrical properties compared to films grown on sapphire under similar conditions. The results motivate further efforts focused on the development of single crystal hBN substrates and epilayers for synthesis of wafer-scale single crystal TMD films.

The strongly spin-momentum coupled electronic states on the surfaces of topological insulators (TIs) exist because of time-reversal symmetry. The theoretical description of these states is fundamentally analogous to a picture used to describe particles known as ‘axions’ theoretically postulated to exist in Nature but never observed. Demonstrating and understanding this conceptual analogy is important for gaining new insights into how our universe works. By interfacing the two opposite surfaces of a TI thin film with different magnetic materials and measuring the magnetic field dependence of the electrical conductance,  we show that the ‘axion’ analogy is robust and leads to a new phase of matter known as an ‘axion insulator.’

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