Spin-Stabilization by Coulomb Blockade in a Vanadium Dimer in WSe2.

Charged dopants in 2D transition metal dichalcogenides (TMDs) have been associated with the formation of hydrogenic bound states, defect-bound trions, and gate-controlled magnetism. Charge-transfer at the TMD-substrate interface and the proximity to other charged defects can be used to regulate the occupation of the dopant's energy levels. In this study, we examine vanadium-doped WSe2 monolayers on quasi-freestanding epitaxial graphene, by high-resolution scanning probe microscopy and ab initio calculations. Vanadium atoms substitute W atoms and adopt a negative charge state through charge donation from the graphene substrate. VW-1 dopants exhibit a series of occupied p-type defect states, accompanied by an intriguing electronic fine-structure that we attribute to hydrogenic states bound to the charged impurity. We systematically studied the hybridization in V dimers with different separations. For large dimer separations, the 2e- charge state prevails, and the magnetic moment is quenched. However, the Coulomb blockade in the nearest-neighbor dimer configuration stabilizes a 1e- charge state. The nearest-neighbor V-dimer exhibits an open-shell character for the frontier defect orbital, giving rise to a paramagnetic ground state. Our findings provide microscopic insights into the charge stabilization and many-body effects of single dopants and dopant pairs in a TMD host material.

Two-dimensional materials-based probabilistic synapses and reconfigurable neurons for measuring inference uncertainty using Bayesian neural networks.

Artificial neural networks have demonstrated superiority over traditional computing architectures in tasks such as pattern classification and learning. However, they do not measure uncertainty in predictions, and hence they can make wrong predictions with high confidence, which can be detrimental for many mission-critical applications. In contrast, Bayesian neural networks (BNNs) naturally include such uncertainty in their model, as the weights are represented by probability distributions (e.g. Gaussian distribution). Here we introduce three-terminal memtransistors based on two-dimensional (2D) materials, which can emulate both probabilistic synapses as well as reconfigurable neurons. The cycle-to-cycle variation in the programming of the 2D memtransistor is exploited to achieve Gaussian random number generator-based synapses, whereas 2D memtransistor based integrated circuits are used to obtain neurons with hyperbolic tangent and sigmoid activation functions. Finally, memtransistor-based synapses and neurons are combined in a crossbar array architecture to realize a BNN accelerator for a data classification task.

Heterogeneous Integration of Atomically Thin Semiconductors for Non-von Neumann CMOS.

Atomically thin, 2D, and semiconducting transition metal dichalcogenides (TMDs) are seen as potential candidates for complementary metal oxide semiconductor (CMOS) technology in future nodes. While high-performance field effect transistors (FETs), logic gates, and integrated circuits (ICs) made from n-type TMDs such as MoS2 and WS2 grown at wafer scale have been demonstrated, realizing CMOS electronics necessitates integration of large area p-type semiconductors. Furthermore, the physical separation of memory and logic is a bottleneck of the existing CMOS technology and must be overcome to reduce the energy burden for computation. In this article, the existing limitations are overcome and for the first time, a heterogeneous integration of large area grown n-type MoS2 and p-type vanadium doped WSe2 FETs with non-volatile and analog memory storage capabilities to achieve a non-von Neumann 2D CMOS platform is introduced. This manufacturing process flow allows for precise positioning of n-type and p-type FETs, which is critical for any IC development. Inverters and a simplified 2-input-1-output multiplexers and neuromorphic computing primitives such as Gaussian, sigmoid, and tanh activation functions using this non-von Neumann 2D CMOS platform are also demonstrated. This demonstration shows the feasibility of heterogeneous integration of wafer scale 2D materials.

Spin-dependent vibronic response of a carbon radical ion in two-dimensional WS2.

Atomic spin centers in 2D materials are a highly anticipated building block for quantum technologies. Here, we demonstrate the creation of an effective spin-1/2 system via the atomically controlled generation of magnetic carbon radical ions (CRIs) in synthetic two-dimensional transition metal dichalcogenides. Hydrogenated carbon impurities located at chalcogen sites introduced by chemical doping are activated with atomic precision by hydrogen depassivation using a scanning probe tip. In its anionic state, the carbon impurity is computed to have a magnetic moment of 1 µB resulting from an unpaired electron populating a spin-polarized in-gap orbital. We show that the CRI defect states couple to a small number of local vibrational modes. The vibronic coupling strength critically depends on the spin state and differs for monolayer and bilayer WS2. The carbon radical ion is a surface-bound atomic defect that can be selectively introduced, features a well-understood vibronic spectrum, and is charge state controlled.

Substrate Modification during Chemical Vapor Deposition of hBN on Sapphire.

A comparison of hexagonal boron nitride (hBN) layers grown by chemical vapor deposition on C-plane (0001) versus A-plane (112̅0) sapphire (α-Al2O3) substrate is reported. The high deposition temperature (>1200 °C) and hydrogen ambient used for hBN deposition on sapphire substantially alters the C-plane sapphire surface chemistry and leaves the top layer(s) oxygen deficient. The resulting surface morphology due to H2 etching of C-plane sapphire is inhomogeneous with increased surface roughness which causes non-uniform residual stress in the deposited hBN film. In contrast to C-plane, the A-plane of sapphire does not alter substantially under a similar high temperature H2 environment, thus providing a more stable alternative substrate for high quality hBN growth. The E2g Raman mode full width at half-maximum (FWHM) for hBN deposited on C-plane sapphire is 24.5 ± 2.1 cm-1 while for hBN on A-plane sapphire is 24.5 ± 0.7 cm-1. The lesser FWHM standard deviation on A-plane sapphire indicates uniform stress distribution across the film due to reduced undulations on the surface. The photoluminescence spectra of the hBN films at 300 and 3 K, obtained on C-plane and A-plane sapphire exhibit similar characteristics with peaks at 4.1 and 5.3 eV reported to be signature peaks associated with defects for hBN films deposited under lower V/III ratios. The dielectric breakdown field of hBN deposited on A-plane sapphire was measured to be 5 MV cm-1, agreeing well with reports on mechanically exfoliated hBN flakes. Thus, under the typical growth conditions required for high crystalline quality hBN growth, A-plane sapphire provides a more chemically stable substrate.

Interface Transparency and Rashba Spin Torque Enhancement in WSe2 Heterostructures.

Rashba spin current generation emerges in heterostructures of ferromagnets and transition metal dichalcogenides (TMDs) due to an interface polarization and associated inversion symmetry breaking. Recent work exploring the synthesis and transfer of epitaxial films on the top of low layer count 2D materials reveals that atomic potentials from the underlying substrate interface are not completely screened. The extension of this transparency effect to other interfacial phenomena, such as the Rashba effect and associated spin torques, has not yet been demonstrated. Here, we report enhanced spin transfer torques from the Rashba spin current in heterostructures of permalloy (Py) and WSe2. We show that insertion of up to two monolayers of WSe2 enhances the spin transfer torques in a Rashba system by up to 3×, without changing the fieldlike Rashba spin-orbit torque (SOT), a measure of interface polarization. Our results indicate that low layer count TMD films can be used as an interfacial "scattering promoter" in heterostructure interfaces without quenching the original polarization.

Scalable Substitutional Re-Doping and its Impact on the Optical and Electronic Properties of Tungsten Diselenide.

Reliable, controlled doping of 2D transition metal dichalcogenides will enable the realization of next-generation electronic, logic-memory, and magnetic devices based on these materials. However, to date, accurate control over dopant concentration and scalability of the process remains a challenge. Here, a systematic study of scalable in situ doping of fully coalesced 2D WSe2 films with Re atoms via metal-organic chemical vapor deposition is reported. Dopant concentrations are uniformly distributed over the substrate surface, with precisely controlled concentrations down to <0.001% Re achieved by tuning the precursor partial pressure. Moreover, the impact of doping on morphological, chemical, optical, and electronic properties of WSe2 is elucidated with detailed experimental and theoretical examinations, confirming that the substitutional doping of Re at the W site leads to n-type behavior of WSe2 . Transport characteristics of fabricated back-gated field-effect-transistors are directly correlated to the dopant concentration, with degrading device performances for doping concentrations exceeding 1% of Re. The study demonstrates a viable approach to introducing true dopant-level impurities with high precision, which can be scaled up to batch production for applications beyond digital electronics.

Single- versus Dual-Ion Conductors for Electric Double Layer Gating: Finite Element Modeling and Hall-Effect Measurements.

Electric double layer (EDL) gating using a single-ion conductor is compared to a dual-ion conductor using both finite element modeling and Hall-effect measurements. Modified Nernst-Planck Poisson (mNPP) equations are used to calculate the ion density per unit area in a parallel plate capacitor geometry with a bulk ion concentration of 215 ≤ cbulk ≤ 1782 mol/m3. With electrodes of equal size at a 2 V potential difference, the EDL ion density of the single-ion conductor is ∼7 × 1013 ions/cm2, which is approximately 50% of the ion density induced in the dual-ion conductor. However, this difference is reduced to 8% when the electrode at which the cationic EDL forms is 10 times smaller than the counter electrode. Thus, for a field-effect transistor gated by a single-ion conductor, it is especially important to have a large gate-to-channel size ratio to achieve strong ion doping. The modeled ion densities are validated by Hall-effect measurements on graphene Hall bars gated by a polyethylene oxide (PEO)-based single-ion conductor. The sheet carrier density, nS, is ∼2 × 1013 cm-2 at Vg = 2 V, which is 3.5 times smaller than the predicted value and has the same order of magnitude as the ns measured for a PEO-based, dual-ion conductor on the same graphene. The numerical modeling results can be approximated by a simple analysis of capacitors in series, where the EDLs are modeled as capacitors with thickness estimated by the sum of the Debye screening length and the Stern layer. The series of capacitor estimate agrees with the numerical modeling of the dual-ion conductor to within 10% and the single-ion conductor to within 30% from 0.25 to 2 V (cbulk = 925 mol/m3); similar agreement is observed in the concentration range of 353-1650 mol/m3 for both single- and dual-ion conductors.