Abhishek Misra

Van der Waals heterostructures, which are composed of layered two-dimensional materials, offer a platform to investigate a diverse range of physical phenomena and could be of use in a variety of applications. Heterostructures containing two-dimensional ferromagnets, such as chromium triiodide (CrI3), have recently been reported, which could allow two-dimensional spintronic devices to be developed. Here we study tunnelling through thin ferromagnetic chromium tribromide (CrBr3) barriers that are sandwiched between graphene electrodes. In devices with non-magnetic barriers, conservation of momentum can be relaxed by phonon-assisted tunnelling or by tunnelling through localized states. In contrast, in the devices with ferromagnetic barriers, the major tunnelling mechanisms are the emission of magnons at low temperatures and the scattering of electrons on localized magnetic excitations at temperatures above the Curie temperature. Magnetoresistance in the graphene electrodes further suggests induced spin-orbit coupling and proximity exchange via the ferromagnetic barrier. Tunnelling with magnon emission offers the possibility of spin injection.

Two-dimensional (2D) materials and their hetero structures are promising for memristive applications due to their extreme scalability and high performance.1 Threshold switching in 2D-Transition Metal Dichalcogenide (TMDC) memristors has been previously identified for neuromorphic applications.2 On the other hand, accurate control of defect concentration in 2D-TMDC films is necessary for optimized performance of the memristive devices. In this work, we explore a chemical route to control defect concentration in 2D-MoS2 films. We demonstrate that the defect concentration in 2D-MoS2 can be tuned by H2O2 treatment. We then optimize the resistance switching behavior of Au/MoS2/Ag/Au memristors to obtain reliable threshold resistance switching with high on/ off ratio, low operating voltages and self-compliance behavior. This work offers promise for a low-cost, scalable approach to develop 2D-TMDC based high-performance neuromorphic hardware.

Synthesizing a material with the desired polymorphic phase in a chemical vapor deposition (CVD) process requires a delicate balance among various thermodynamic variables. Here, we present a methodology to synthesize rhombohedral (3R)-phase MoS2in a well-defined sword-like geometry having lengths up to 120 μm, uniform width of 2-3 μm and thickness of 3-7 nm by controlling the carrier gas flow dynamics from continuous mode to pulsed mode during the CVD growth process. Characteristic signatures such as high degree of circular dichroism (∼58% at 100 K), distinct evolution of low-frequency Raman peaks and increasing intensity of second harmonic signals with increasing number of layers conclusively establish the 3R-phase of the material. A high value (∼844 pm/V) of second-order susceptibility for few-layer-thick MoS2swords signifies the potential of MoS2to serve as an atomically thin nonlinear medium. A field effect mobility of 40 cm2/V-s and Ion/Ioffratio of ∼106further confirm the electronic-grade standard of this 3R-phase MoS2. These findings are significant for the development of emerging quantum electronic devices utilizing valley-based physics and nonlinear optical phenomena in layered materials.

The work function (WF) of a material governs the back and forth movement of the charge carriers across the hetero-interface of two materials. Therefore, for optimum device performance, precise knowledge of the WF is prerequisite while employing any new material in electronic devices. In this work, using metal oxide semiconductor capacitors, we experimentally determine the WF of layered van der Waals topological semimetals (TSMs) 1T′-MoTe2, 1T-PtSe2, and Td-WTe2 as 4.87, 5.05, and 4.82 eV, respectively. The experimentally obtained results are corroborated with density functional theory calculations. Furthermore, by analyzing the vertical current transport across the metal oxide semiconductor stack using Fowler-Nordheim tunneling formalism, the barrier height between the TSMs and the gate insulator (SiO2) is experimentally calculated. The obtained barrier heights are also following the same trend as that of WF for three TSMs. These TSMs host unique topological nontrivial phases potentially useful for the development of emerging quantum technologies, and therefore, the findings of this study are significant for designing the future quantum devices.

The original version of this Article contained an error in the spelling of the author Matthew Holwill, which was incorrectly given as Mathew Holwill. This has now been corrected in both the PDF and HTML versions of the Article.

Semiconducting ferromagnet-nonmagnet interfaces in van der Waals heterostructures present a unique opportunity to investigate magnetic proximity interactions dependent upon a multitude of phenomena including valley and layer pseudospins, moiré periodicity, or exceptionally strong Coulomb binding. Here, we report a charge-state dependency of the magnetic proximity effects between MoSe2 and CrBr3 in photoluminescence, whereby the valley polarization of the MoSe2 trion state conforms closely to the local CrBr3 magnetization, while the neutral exciton state remains insensitive to the ferromagnet. We attribute this to spin-dependent interlayer charge transfer occurring on timescales between the exciton and trion radiative lifetimes. Going further, we uncover by both the magneto-optical Kerr effect and photoluminescence a domain-like spatial topography of contrasting valley polarization, which we infer to be labyrinthine or otherwise highly intricate, with features smaller than 400 nm corresponding to our optical resolution. Our findings offer a unique insight into the interplay between short-lived valley excitons and spin-dependent interlayer tunneling, while also highlighting MoSe2 as a promising candidate to optically interface with exotic spin textures in van der Waals structures.

Despite a rich choice of two-dimensional materials, which exists these days, heterostructures, both vertical (van der Waals) and in-plane, offer an unprecedented control over the properties and functionalities of the resulted structures. Thus, planar heterostructures allow p-n junctions between different two-dimensional semiconductors and graphene nanoribbons with well-defined edges; and vertical heterostructures resulted in the observation of superconductivity in purely carbon-based systems and realisation of vertical tunnelling transistors. Here we demonstrate simultaneous use of in-plane and van der Waals heterostructures to build vertical single electron tunnelling transistors. We grow graphene quantum dots inside the matrix of hexagonal boron nitride, which allows a dramatic reduction of the number of localised states along the perimeter of the quantum dots. The use of hexagonal boron nitride tunnel barriers as contacts to the graphene quantum dots make our transistors reproducible and not dependent on the localised states, opening even larger flexibility when designing future devices.