Ferroelectric Modulation of Quantum Emitters in Monolayer WS.

Quantum photonics promises significant advances in secure communications, metrology, sensing, and information processing/computation. Single-photon sources are fundamental to this endeavor. However, the lack of high-quality single photon sources remains a significant obstacle.

Observation of interlayer plasmon polaron in graphene/WS heterostructures.

Harnessing electronic excitations involving coherent coupling to bosonic modes is essential for the design and control of emergent phenomena in quantum materials. In situations where charge carriers induce a lattice distortion due to the electron-phonon interaction, the conducting states get "dressed", which leads to the formation of polaronic quasiparticles. The exploration of polaronic effects on low-energy excitations is in its infancy in two-dimensional materials.

Enhancing Single Photon Emission Purity via Design of van der Waals Heterostructures.

Quantum emitters are essential components of quantum photonic circuitry envisioned beyond the current optoelectronic state-of-the-art. Two dimensional materials are attractive hosts for such emitters. However, the high single photon purity required is rarely realized due to the presence of spectrally degenerate classical light originating from defects.

Highly Efficient Spin-Orbit Torque Switching in BiSe/FeGeTe van der Waals Heterostructures.

Topological insulators (TIs) have shown promise as a spin-generating layer to switch the magnetization state of ferromagnets via spin-orbit torque (SOT) due to charge-to-spin conversion efficiency of the TI surface states that arises from spin-momentum locking. However, when TIs are interfaced with conventional bulk ferromagnetic metals, the combination of charge transfer and hybridization can potentially destroy the spin texture and hamper the possibility of accessing the TI surface states. Here, we fabricate an all van der Waals (vdW) heterostructure consisting of molecular beam epitaxy grown bulk-insulating BiSe and exfoliated 2D metallic ferromagnet FeGeTe (FGT) with perpendicular anisotropy.

Interlayer Exciton-Phonon Bound State in BiSe/Monolayer WS van der Waals Heterostructures.

The ability to assemble layers of two-dimensional (2D) materials to form permutations of van der Waals heterostructures provides significant opportunities in materials design and synthesis. Interlayer interactions can enable desired properties and functionality, and understanding such interactions is essential to that end. Here we report formation of interlayer exciton-phonon bound states in BiSe/WS heterostructures, where the BiSe A surface phonon, a mode particularly susceptible to electron-phonon coupling, is imprinted onto the excitonic emission of the WS.

Enhancing the Purity of Deterministically Placed Quantum Emitters in Monolayer WSe.

We present a method utilizing an applied electrostatic potential for suppressing the broad defect bound excitonic emission in two-dimensional materials (2DMs) which otherwise inhibits the purity of strain induced single photon emitters (SPEs). Our heterostructure consists of a WSe monolayer on a polymer in which strain has been deterministically introduced via an atomic force microscope (AFM) tip. We show that by applying an electrostatic potential, the broad defect bound background is suppressed at cryogenic temperatures, resulting in a substantial improvement in single photon purity demonstrated by a 10-fold reduction of the correlation function (0) value from 0.

Emergent Moiré Phonons Due to Zone Folding in WSe-WS Van der Waals Heterostructures.

Bilayers of 2D materials offer opportunities for creating devices with tunable electronic, optical, and mechanical properties. In van der Waals heterostructures (vdWHs) where the constituent monolayers have different lattice constants, a moiré superlattice forms with a length scale larger than the lattice constant of either constituent material regardless of twist angle. Here, we report the appearance of moiré Raman modes from nearly aligned WSe-WS vdWHs in the range of 240-260 cm, which are absent in both monolayers and homobilayers of WSe and WS and in largely misaligned WSe-WS vdWHs.

Room-Temperature Oxygen Transport in Nanothin BiOSe Enables Precision Modulation of 2D Materials.

Oxygen conductors and transporters are important to several consequential renewable energy technologies, including fuel cells and syngas production. Separately, monolayer transition-metal dichalcogenides (TMDs) have demonstrated significant promise for a range of applications, including quantum computing, advanced sensors, valleytronics, and next-generation optoelectronics. Here, we synthesize a few-nanometer-thick BiOSe compound that strongly resembles a rare 3 bismuth oxide (BiO) phase and combine it with monolayer TMDs, which are highly sensitive to their environment.

Spin-Sensitive Epitaxial InSe Tunnel Barrier in InSe/BiSe Topological van der Waals Heterostructure.

Current-generated spin arising from spin-momentum locking in topological insulator (TI) surface states has been shown to switch the magnetization of an adjacent ferromagnet (FM) via spin-orbit torque (SOT) with a much higher efficiency than heavy metals. However, in such FM/TI heterostructures, most of the current is shunted through the FM metal due to its lower resistance, and recent calculations have also shown that topological surface states can be significantly impacted when interfaced with an FM metal such as Ni and Co. Hence, placing an insulating layer between the TI and FM will not only prevent current shunting, therefore minimizing overall power consumption, but may also help preserve the topological surface states at the interface.

Laser-Patterned Submicrometer BiSe-WS Pixels with Tunable Circular Polarization at Room Temperature.

Characterizing and manipulating the circular polarization of light is central to numerous emerging technologies, including spintronics and quantum computing. Separately, monolayer tungsten disulfide (WS) is a versatile material that has demonstrated promise in a variety of applications, including single photon emitters and valleytronics. Here, we demonstrate a method to tune the photoluminescence (PL) intensity (factor of ×161), peak position (38.

Stacking-dependent optical properties in bilayer WSe.

The twist angle between the monolayers in van der Waals heterostructures provides a new degree of freedom in tuning material properties. We compare the optical properties of WSe homobilayers with 2H and 3R stacking using photoluminescence, Raman spectroscopy, and reflectance contrast measurements under ambient and cryogenic temperatures. Clear stacking-dependent differences are evident for all temperatures, with both photoluminescence and reflectance contrast spectra exhibiting a blue shift in spectral features in 2H compared to 3R bilayers.

Direct-Write of Nanoscale Domains with Tunable Metamagnetic Order in FeRh Thin Films.

We have directly written nanoscale patterns of magnetic ordering in FeRh films using focused helium-ion beam irradiation. By varying the dose, we pattern arrays with metamagnetic transition temperatures that range from the as-grown film temperature to below room temperature. We employ transmission electron microscopy, X-ray diffraction, and temperature-dependent transport measurements to characterize the as-grown film, and magneto-optic Kerr effect imaging to quantify the He irradiation-induced changes to the magnetic order.

Direct observation of minibands in a twisted graphene/WS bilayer.

Stacking two-dimensional (2D) van der Waals materials with different interlayer atomic registry in a heterobilayer causes the formation of a long-range periodic superlattice that may bestow the heterostructure with properties such as new quantum fractal states or superconductivity. Recent optical measurements of transition metal dichalcogenide (TMD) heterobilayers have revealed the presence of hybridized interlayer electron-hole pair excitations at energies defined by the superlattice potential. The corresponding quasiparticle band structures, so-called minibands, have remained elusive, and no such features have been reported for heterobilayers composed of a TMD and another type of 2D material.

Twist Angle-Dependent Atomic Reconstruction and Moiré Patterns in Transition Metal Dichalcogenide Heterostructures.

Van der Waals layered materials, such as transition metal dichalcogenides (TMDs), are an exciting class of materials with weak interlayer bonding, which enables one to create so-called van der Waals heterostructures (vdWH). One promising attribute of vdWH is the ability to rotate the layers at arbitrary azimuthal angles relative to one another. Recent work has shown that control of the twist angle between layers can have a dramatic effect on TMD vdWH properties, but the twist angle has been treated solely through the use of rigid-lattice moiré patterns.

Emergent electric field control of phase transformation in oxide superlattices.

Electric fields can transform materials with respect to their structure and properties, enabling various applications ranging from batteries to spintronics. Recently electrolytic gating, which can generate large electric fields and voltage-driven ion transfer, has been identified as a powerful means to achieve electric-field-controlled phase transformations. The class of transition metal oxides provide many potential candidates that present a strong response under electrolytic gating.

Synthesis of High-Quality Monolayer MoS by Direct Liquid Injection.

We report the synthesis of high-quality single monolayer MoS samples using a novel technique that utilizes direct liquid injection (DLI) for the delivery of precursors. The DLI system vaporizes a liquid consisting of a selected precursor dissolved in a solvent into small, micron-sized droplets in an expansion chamber maintained at a selected temperature and pressure, before delivery to the deposition chamber. We demonstrate the synthesis of monolayer MoS on SiO/Si substrates using the DLI technique with film quality superior to exfoliated samples or those grown by traditional tube furnace chemical vapor deposition (CVD) methods.

Continuous Wave Sum Frequency Generation and Imaging of Monolayer and Heterobilayer Two-Dimensional Semiconductors.

We report continuous-wave second harmonic and sum frequency generation from two-dimensional transition metal dichalcogenide monolayers and their heterostructures with pump irradiances several orders of magnitude lower than those of conventional pulsed experiments. The high nonlinear efficiency originates from above-gap excitons in the band nesting regions, as revealed by wavelength-dependent second order optical susceptibilities quantified in four common monolayer transition metal dichalcogenides. Using sum frequency excitation spectroscopy and imaging, we identify and distinguish one- and two-photon resonances in both monolayers and heterobilayers.

Resonant optical Stark effect in monolayer WS.

Breaking the valley degeneracy in monolayer transition metal dichalcogenides through the valley-selective optical Stark effect (OSE) can be exploited for classical and quantum valleytronic operations such as coherent manipulation of valley superposition states. The strong light-matter interactions responsible for the OSE have historically been described by a two-level dressed-atom model, which assumes noninteracting particles. Here we experimentally show that this model, which works well in semiconductors far from resonance, does not apply for excitation near the exciton resonance in monolayer WS.

Chemical Identification of Interlayer Contaminants within van der Waals Heterostructures.

van der Waals heterostructures (vdWHs) leverage the characteristics of two-dimensional (2D) material building blocks to create a myriad of structures with unique and desirable properties. Several commonly employed fabrication strategies rely on polymeric stamps to assemble layers of 2D materials into vertical stacks. However, the properties of such heterostructures frequently are degraded by contaminants, typically of unknown composition, trapped between the constituent layers.

Electrical detection of current generated spin in topological insulator surface states: Role of interface resistance.

Current generated spin polarization in topological insulator (TI) surface states due to spin-momentum locking has been detected recently using ferromagnet/tunnel barrier contacts, where the projection of the TI spin onto the magnetization of the ferromagnet is measured as a voltage. However, opposing signs of the spin voltage have been reported, which had been tentatively attributed to the coexistence of trivial two-dimensional electron gas states on the TI surface which may exhibit opposite current-induced polarization than that of the TI Dirac surface states. Models based on electrochemical potential have been presented to determine the sign of the spin voltage expected for the TI surface states.

Spatially Selective Enhancement of Photoluminescence in MoS by Exciton-Mediated Adsorption and Defect Passivation.

Monolayers of transition-metal dichalcogenides (TMDs) are promising components for flexible optoelectronic devices because of their direct band gap and atomically thin nature. The photoluminescence (PL) from these materials is often strongly suppressed by nonradiative recombination mediated by midgap defect states. Here, we demonstrate up to a 200-fold increase in PL intensity from monolayer MoS synthesized by chemical vapor deposition (CVD) by controlled exposure to laser light in the ambient.

Quantum Calligraphy: Writing Single-Photon Emitters in a Two-Dimensional Materials Platform.

We present a paradigm for encoding strain into two-dimensional materials (2DMs) to create and deterministically place single-photon emitters (SPEs) in arbitrary locations with nanometer-scale precision. Our material platform consists of a 2DM placed on top of a deformable polymer film. Upon application of sufficient mechanical stress using an atomic force microscope tip, the 2DM/polymer composite deforms, resulting in formation of highly localized strain fields with excellent control and repeatability.

Electrical Detection of Charge-to-spin and Spin-to-Charge Conversion in a Topological Insulator BiTe Using BN/AlO Hybrid Tunnel Barrier.

One of the most striking properties of three-dimensional topological insulators (TIs) is spin-momentum locking, where the spin is locked at right angles to momentum and hence an unpolarized charge current creates a net spin polarization. Alternatively, if a net spin is injected into the TI surface state system, it is distinctively associated with a unique carrier momentum and hence should generate a charge accumulation, as in the so-called inverse Edelstein effect. Here using a Fe/AlO/BN tunnel barrier, we demonstrate both effects in a single device in BiTe: the electrical detection of the spin accumulation generated by an unpolarized current flowing through the surface states, and that of the charge accumulation generated by spins injected into the surface state system.