Auger Recombination in Chemical Vapor Deposition-Grown Monolayer WS₂

Reduced dimensionality and strong Coulombic interactions in monolayer semiconductors lead to enhanced many-body interactions. Here, we report Auger recombination, i.e., exciton–exciton annihilation, in large-area chemical vapor deposition-grown monolayer WS₂. Using ultrafast spectroscopy, we experimentally determine the Auger rate to be 0.089 ± 0.001 cm²/s at room temperature, which is an order of magnitude greater than the bulk value. This nonradiative recombination pathway dominates, regardless of excitation energy, for exciton densities greater than 8.0 ± 0.6 × 10¹⁰ cm^–2 and below the Mott density. Higher-energy excitation above the A exciton resonance may initially produce a hot electron–hole gas that precedes exciton formation. Therefore, we use resonant excitation of the A exciton to ensure accuracy and avoid artifacts associated with other photogenerated species

Photoinduced Bandgap Renormalization and Exciton Binding Energy Reduction in WS₂

Strong Coulomb attraction in monolayer transition metal dichalcogenides gives rise to tightly bound excitons and many-body interactions that dominate their optoelectronic properties. However, this Coulomb interaction can be screened through control of the surrounding dielectric environment as well as through applied voltage, which provides a potential means of tuning the bandgap, exciton binding energy, and emission wavelength. Here, we directly show that the bandgap and exciton binding energy can be optically tuned by means of the intensity of the incident light. Using transient absorption spectroscopy, we identify a sub-picosecond decay component in the excited-state dynamics of WS₂ that emerges for incident photon energies above the A-exciton resonance, which originates from a nonequilibrium population of charge carriers that form excitons as they cool. The generation of this charge-carrier population exhibits two distinct energy thresholds. The higher threshold is coincident with the onset of continuum states and therefore provides a direct optical means of determining both the bandgap and exciton binding energy. Using this technique, we observe a reduction in the exciton binding energy from 310 ± 30 to 220 ± 20 meV as the excitation density is increased from 3 × 10¹¹ to 1.2 × 10¹² photons/cm². This reduction is due to dynamic dipolar screening of Coulomb interactions by excitons, which is the underlying physical process that initiates bandgap renormalization and leads to the insulator–metal transition in monolayer transition metal dichalcogenides

Electrical Characterization of Discrete Defects and Impact of Defect Density on Photoluminescence in Monolayer WS₂

Transition-metal dichalcogenides (TMDs) are an exciting class of 2D materials that exhibit many promising electronic and optoelectronic properties with potential for future device applications. The properties of TMDs are expected to be strongly influenced by a variety of defects which result from growth procedures and/or fabrication. Despite the importance of understanding defect-related phenomena, there remains a need for quantitative nanometer-scale characterization of defects over large areas in order to understand the relationship between defects and observed properties, such as photoluminescence (PL) and electrical conductivity. In this work, we present conductive atomic force microscopy measurements which reveal nanometer-scale electronically active defects in chemical vapor deposition-grown WS₂ monolayers with defect density varying from 2.3 × 10¹⁰ cm^–2 to 4.5 × 10¹¹ cm^–2. Comparing these defect density measurements with PL measurements across large areas (>20 μm distances) reveals a strong inverse relationship between WS₂ PL intensity and defect density. We propose a model in which the observed electronically active defects serve as nonradiative recombination centers and obtain good agreement between the experiments and model

Graphene As a Tunnel Barrier: Graphene-Based Magnetic Tunnel Junctions

Graphene has been widely studied for its high in-plane charge carrier mobility and long spin diffusion lengths. In contrast, the out-of-plane charge and spin transport behavior of this atomically thin material have not been well addressed. We show here that while graphene exhibits metallic conductivity in-plane, it serves effectively as an insulator for transport perpendicular to the plane. We report fabrication of tunnel junctions using single-layer graphene between two ferromagnetic metal layers in a fully scalable photolithographic process. The transport occurs by quantum tunneling perpendicular to the graphene plane and preserves a net spin polarization of the current from the contact so that the structures exhibit tunneling magnetoresistance to 425 K. These results demonstrate that graphene can function as an effective tunnel barrier for both charge and spin-based devices and enable realization of more complex graphene-based devices for highly functional nanoscale circuits, such as tunnel transistors, nonvolatile magnetic memory, and reprogrammable spin logic

Nano-“Squeegee” for the Creation of Clean 2D Material Interfaces

Two-dimensional (2D) materials exhibit many exciting phenomena that make them promising as materials for future electronic, optoelectronic, and mechanical devices. Because of their atomic thinness, interfaces play a dominant role in determining material behavior. In order to observe and exploit the unique properties of these materials, it is therefore vital to obtain clean and repeatable interfaces. However, the conventional mechanical stacking of atomically thin layers typically leads to trapped contaminants and spatially inhomogeneous interfaces, which obscure the true intrinsic behavior. This work presents a simple and generic approach to create clean 2D material interfaces in mechanically stacked structures. The operating principle is to use an AFM tip to controllably squeeze contaminants out from between 2D layers and their substrates, similar to a “squeegee”. This approach leads to drastically improved homogeneity and consistency of 2D material interfaces, as demonstrated by AFM topography and significant reduction of photoluminescence line widths. Also, this approach enables emission from interlayer excitons, demonstrating that the technique enhances interlayer coupling in van der Waals heterostructures. The technique enables repeatable observation of intrinsic 2D material properties, which is crucial for the continued development of these promising materials

Charge Trapping and Exciton Dynamics in Large-Area CVD Grown MoS₂

There is keen interest in monolayer transition metal dichalcogenide films for a variety of optoelectronic applications due to their direct band gap and fast carrier dynamics. However, the mechanisms dominating their carrier dynamics are poorly understood. By combining time-resolved terahertz (THz) spectroscopy and transient absorption, we are able to shed light on the optoelectronic properties of large area CVD grown mono- and multilayer MoS₂ films and determine the origins of the characteristic two-component excited state dynamics. The photoinduced conductivity shows that charge carriers, and not excitons, are responsible for the subpicosecond dynamics. Identical dynamics resulting from sub-optical gap excitation suggest that charge carriers are rapidly trapped by midgap states within 600 fs. This process complicates the excited state spectrum with rapid changes in line-width broadening in addition to a red-shift due to band gap renormalization and simple state-filling effects. These dynamics are insensitive to film thickness, temperature, or choice of substrate, which suggests that carrier trapping occurs at surface defects or grain boundaries. The slow dynamics are associated with exciton recombination and lengthen from 50 ps for monolayer films to 150 ps for multilayer films indicating that surface recombination dominates their lifetime. We see no signatures of trions in these MoS₂ films. Our results imply that CVD grown films of MoS₂ hold potential for high-speed optoelectronics and provide an explanation for the absence of trions in some CVD grown MoS₂ films

Enhancing the Purity of Deterministically Placed Quantum Emitters in Monolayer WSe₂

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 WSe2 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 g(2)(0) value from 0.73 to 0.07. In addition, we see a 2-fold increase in the intensity of the SPEs as well as the ability to activate/deactivate the emitters at certain wavelengths. Finally, we present an increase in the operating temperature of the SPE up to 110 K, a 50 K increase when compared with the results when no electrostatic potential is present

Room-Temperature Spin Filtering in Metallic Ferromagnet–Multilayer Graphene–Ferromagnet Junctions

We report room-temperature negative magnetoresistance in ferromagnet–graphene–ferromagnet (FM|Gr|FM) junctions with minority spin polarization exceeding 80%, consistent with predictions of strong minority spin filtering. We fabricated arrays of such junctions <i>via</i> chemical vapor deposition of multilayer graphene on lattice-matched single-crystal NiFe(111) films and standard photolithographic patterning and etching techniques. The junctions exhibit metallic transport behavior, low resistance, and the negative magnetoresistance characteristic of a minority spin filter interface throughout the temperature range 10 to 300 K. We develop a device model to incorporate the predicted spin filtering by explicitly treating a metallic minority spin channel with spin current conversion and a tunnel barrier majority spin channel and extract spin polarization of at least 80% in the graphene layer in our structures. The junctions also show antiferromagnetic coupling, consistent with several recent predictions. The methods and findings are relevant to fast-readout low-power magnetic random access memory technology, spin logic devices, and low-power magnetic field sensors

Double Indirect Interlayer Exciton in a MoSe₂/WSe₂ van der Waals Heterostructure

An emerging class of semiconductor heterostructures involves stacking discrete monolayers such as transition metal dichalcogenides (TMDs) to form van der Waals heterostructures. In these structures, it is possible to create interlayer excitons (ILEs), spatially indirect, bound electron–hole pairs with the electron in one TMD layer and the hole in an adjacent layer. We are able to clearly resolve two distinct emission peaks separated by 24 meV from an ILE in a MoSe₂/WSe₂ heterostructure fabricated using state-of-the-art preparation techniques. These peaks have nearly equal intensity, indicating they are of common character, and have <i>opposite</i> circular polarizations when excited with circularly polarized light. <i>Ab initio</i> calculations successfully account for these observations: they show that both emission features originate from excitonic transitions that are indirect in momentum space and are split by spin–orbit coupling. Also, the electron is strongly hybridized between both the MoSe₂ and WSe₂ layers, with significant weight in both layers, contrary to the commonly assumed model. Thus, the transitions are not purely interlayer in character. This work represents a significant advance in our understanding of the static and dynamic properties of TMD heterostructures