Project description:The integration of magnetic material with semiconductors has been fertile ground for fundamental science as well as of great practical interest toward the seamless integration of information processing and storage. We create van der Waals heterostructures formed by an ultrathin ferromagnetic semiconductor CrI3 and a monolayer of WSe2. We observe unprecedented control of the spin and valley pseudospin in WSe2, where we detect a large magnetic exchange field of nearly 13 T and rapid switching of the WSe2 valley splitting and polarization via flipping of the CrI3 magnetization. The WSe2 photoluminescence intensity strongly depends on the relative alignment between photoexcited spins in WSe2 and the CrI3 magnetization, because of ultrafast spin-dependent charge hopping across the heterostructure interface. The photoluminescence detection of valley pseudospin provides a simple and sensitive method to probe the intriguing domain dynamics in the ultrathin magnet, as well as the rich spin interactions within the heterostructure.
Project description:The generation and manipulation of spin polarization at room temperature are essential for 2D van der Waals (vdW) materials-based spin-photonic and spintronic applications. However, most of the high degree polarization is achieved at cryogenic temperatures, where the spin-valley polarization lifetime is increased. Here, we report on room temperature high-spin polarization in 2D layers by reducing its carrier lifetime via the construction of vdW heterostructures. A near unity degree of polarization is observed in PbI2 layers with the formation of type-I and type-II band aligned vdW heterostructures with monolayer WS2 and WSe2. We demonstrate that the spin polarization is related to the carrier lifetime and can be manipulated by the layer thickness, temperature, and excitation wavelength. We further elucidate the carrier dynamics and measure the polarization lifetime in these heterostructures. Our work provides a promising approach to achieve room temperature high-spin polarizations, which contribute to spin-photonics applications.
Project description:Semiconductor heterostructures form the cornerstone of many electronic and optoelectronic devices and are traditionally fabricated using epitaxial growth techniques. More recently, heterostructures have also been obtained by vertical stacking of two-dimensional crystals, such as graphene and related two-dimensional materials. These layered designer materials are held together by van der Waals forces and contain atomically sharp interfaces. Here, we report on a type-II van der Waals heterojunction made of molybdenum disulfide and tungsten diselenide monolayers. The junction is electrically tunable, and under appropriate gate bias an atomically thin diode is realized. Upon optical illumination, charge transfer occurs across the planar interface and the device exhibits a photovoltaic effect. Advances in large-scale production of two-dimensional crystals could thus lead to a new photovoltaic solar technology.
Project description:Unique electronic spin textures in topological states of matter are promising for emerging spin-orbit driven memory and logic technologies. However, there are several challenges related to the enhancement of their performance, electrical gate-tunability, interference from trivial bulk states, and heterostructure interfaces. We address these challenges by integrating two-dimensional graphene with a three-dimensional topological insulator (TI) in van der Waals heterostructures to take advantage of their remarkable spintronic properties and engineer proximity-induced spin-charge conversion phenomena. In these heterostructures, we experimentally demonstrate a gate-tunable spin-galvanic effect (SGE) at room temperature, allowing for efficient conversion of a non-equilibrium spin polarization into a transverse charge current. Systematic measurements of SGE in various device geometries via a spin switch, spin precession, and magnetization rotation experiments establish the robustness of spin-charge conversion in the Gr-TI heterostructures. Importantly, using a gate voltage, we reveal a strong electric field tunability of both amplitude and sign of the spin-galvanic signal. These findings provide an efficient route for realizing all-electrical and gate-tunable spin-orbit technology using TIs and graphene in heterostructures, which can enhance the performance and reduce power dissipation in spintronic circuits.
Project description:Materials that are simultaneously ferromagnetic and ferroelectric - multiferroics - promise the control of disparate ferroic orders, leading to technological advances in microwave magnetoelectric applications and next generation of spintronics. Single-phase multiferroics are challenged by the opposite d-orbital occupations imposed by the two ferroics, and heterogeneous nanocomposite multiferroics demand ingredients' structural compatibility with the resultant multiferroicity exclusively at inter-materials boundaries. Here we propose the two-dimensional heterostructure multiferroics by stacking up atomic layers of ferromagnetic Cr2Ge2Te6 and ferroelectric In2Se3, thereby leading to all-atomic multiferroicity. Through first-principles density functional theory calculations, we find as In2Se3 reverses its polarization, the magnetism of Cr2Ge2Te6 is switched, and correspondingly In2Se3 becomes a switchable magnetic semiconductor due to proximity effect. This unprecedented multiferroic duality (i.e., switchable ferromagnet and switchable magnetic semiconductor) enables both layers for logic applications. Van der Waals heterostructure multiferroics open the door for exploring the low-dimensional magnetoelectric physics and spintronic applications based on artificial superlattices.
Project description:Two-dimensional (2D) crystals offer a unique platform due to their remarkable and contrasting spintronic properties, such as weak spin-orbit coupling (SOC) in graphene and strong SOC in molybdenum disulfide (MoS2). Here we combine graphene and MoS2 in a van der Waals heterostructure (vdWh) to demonstrate the electric gate control of the spin current and spin lifetime at room temperature. By performing non-local spin valve and Hanle measurements, we unambiguously prove the gate tunability of the spin current and spin lifetime in graphene/MoS2 vdWhs at 300 K. This unprecedented control over the spin parameters by orders of magnitude stems from the gate tuning of the Schottky barrier at the MoS2/graphene interface and MoS2 channel conductivity leading to spin dephasing in high-SOC material. Our findings demonstrate an all-electrical spintronic device at room temperature with the creation, transport and control of the spin in 2D materials heterostructures, which can be key building blocks in future device architectures.
Project description:Excitons can be trapped by moiré potentials in van der Waals (vdW) heterostructures, forming ordered arrays of quantum dots. Excitons can also be trapped by defect potentials as single photon emitters. While the moiré and defect potentials in vdW heterostructures have been studied separately, their interplay remains largely unexplored. Here, we perform first-principles calculations to elucidate the interplay of the two potentials in determining the optoelectronic properties of twisted MoS2/WS2 heterobilayers. The binding energy, charge density, localization, and hybridization of the moiré excitons can be modulated by the competition and cooperation of the two potentials. Their interplay can also be tuned by vertical electric fields, which can either de-trap the excitons or strongly localize them. One can further tailor the interplay of the two potentials via defect engineering to create one-dimensional exciton lattices with tunable orientations. Our work establishes defect engineering as a promising strategy to realize on-demand optoelectronic responses.
Project description:To fully exploit van der Waals materials and their vertically stacked heterostructures, new mass-scalable production routes which are low cost but preserve the high electronic and optical quality of the single crystals are required. Here, we demonstrate an approach to realise a variety of functional heterostructures based on van der Waals nanocrystal films produced through the mechanical abrasion of bulk powders. We find significant performance enhancements in abraded heterostructures compared to those fabricated through inkjet printing of nanocrystal dispersions. To highlight the simplicity, applicability and scalability of the device fabrication, we demonstrate a multitude of different functional heterostructures such as resistors, capacitors and photovoltaics. We also demonstrate the creation of energy harvesting devices, such as large area catalytically active coatings for the hydrogen evolution reaction and enhanced triboelectric nanogenerator performance in multilayer films. The ease of device production makes this a promising technological route for up-scalable films and heterostructures.
Project description:Monolayers of semiconducting transition metal dichalcogenides exhibit intriguing fundamental physics of strongly coupled spin and valley degrees of freedom for charge carriers. While the possibility of exploiting these properties for information processing stimulated concerted research activities towards the concept of valleytronics, maintaining control over spin-valley polarization proved challenging in individual monolayers. A promising alternative route explores type II band alignment in artificial van der Waals heterostructures. The resulting formation of interlayer excitons combines the advantages of long carrier lifetimes and spin-valley locking. Here, we demonstrate artificial design of a two-dimensional heterostructure enabling intervalley transitions that are not accessible in monolayer systems. The resulting giant effective g factor of -15 for interlayer excitons induces near-unity valley polarization via valley-selective energetic splitting in high magnetic fields, even after nonselective excitation. Our results highlight the potential to deterministically engineer novel valley properties in van der Waals heterostructures using crystallographic alignment.
Project description:Two dimensional atomically thin crystals of graphene and its insulating isomorph hexagonal boron nitride (h-BN) are promising materials for spintronic applications. While graphene is an ideal medium for long distance spin transport, h-BN is an insulating tunnel barrier that has potential for efficient spin polarized tunneling from ferromagnets. Here, we demonstrate the spin filtering effect in cobalt|few layer h-BN|graphene junctions leading to a large negative spin polarization in graphene at room temperature. Through nonlocal pure spin transport and Hanle precession measurements performed on devices with different interface barrier conditions, we associate the negative spin polarization with high resistance few layer h-BN|ferromagnet contacts. Detailed bias and gate dependent measurements reinforce the robustness of the effect in our devices. These spintronic effects in two-dimensional van der Waals heterostructures hold promise for future spin based logic and memory applications.