Efficient and accurate defect level modelling in monolayer MoS2_2 via GW+DFT with open boundary conditions

Within the framework of many-body perturbation theory (MBPT) integrated with density functional theory (DFT), a novel defect-subspace projection GW method, the so-called p-GW, is proposed. By avoiding the periodic defect interference through open boundary self-energies, we show that the p-GW can efficiently and accurately describe quasi-particle correlated defect levels in two-dimensional (2D) monolayer MoS$_2$. By comparing two different defect states originating from sulfur vacancy and adatom to existing theoretical and experimental works, we show that our GW correction to the DFT defect levels is precisely modelled. Based on these findings, we expect that our method can provide genuine trap states for various 2D transition-metal dichalcogenide (TMD) monolayers, thus enabling the study of defect-induced effects on the device characteristics of these materials via realistic simulations

Edge Contacts to Atomically Precise Graphene Nanoribbons.

Bottom-up-synthesized graphene nanoribbons (GNRs) are an emerging class of designer quantum materials that possess superior properties, including atomically controlled uniformity and chemically tunable electronic properties. GNR-based devices are promising candidates for next-generation electronic, spintronic, and thermoelectric applications. However, due to their extremely small size, making electrical contact with GNRs remains a major challenge. Currently, the most commonly used methods are top metallic electrodes and bottom graphene electrodes, but for both, the contact resistance is expected to scale with overlap area. Here, we develop metallic edge contacts to contact nine-atom-wide armchair GNRs (9-AGNRs) after encapsulation in hexagonal boron-nitride (h-BN), resulting in ultrashort contact lengths. We find that charge transport in our devices occurs via two different mechanisms: at low temperatures (9 K), charges flow through single GNRs, resulting in quantum dot (QD) behavior with well-defined Coulomb diamonds (CDs), with addition energies in the range of 16 to 400 meV. For temperatures above 100 K, a combination of temperature-activated hopping and polaron-assisted tunneling takes over, with charges being able to flow through a network of 9-AGNRs across distances significantly exceeding the length of individual GNRs. At room temperature, our short-channel field-effect transistor devices exhibit on/off ratios as high as 3 × 105 with on-state current up to 50 nA at 0.2 V. Moreover, we find that the contact performance of our edge-contact devices is comparable to that of top/bottom contact geometries but with a significantly reduced footprint. Overall, our work demonstrates that 9-AGNRs can be contacted at their ends in ultra-short-channel FET devices while being encapsulated in h-BN

Efficient and accurate defect level modeling in monolayer MoS via GW+DFT with open boundary conditions

Within the framework of many-body perturbation theory integrated with density functional theory (DFT), a novel defect-subspace projection GW method, the so-called p-GW, is proposed. By avoiding the periodic defect interference through open boundary self-energies, we show that the p-GW can efficiently and accurately describe quasi-particle correlated defect levels in two-dimensional (2D) monolayer MoS2. By comparing two different defect states originating from sulfur vacancy and adatom to existing theoretical and experimental works, we show that our GW correction to the DFT defect levels is precisely modeled. Based on these findings, we expect that our method can provide genuine trap states for various 2D transition-metal dichalcogenide (TMD) monolayers, thus enabling the study of defect-induced effects on the device characteristics of these materials via realistic simulations.ISSN:0038-110

Ab initio quantum transport simulations of defective devices based on 2-D materials via a projected-GW approach

We propose a novel ab inito defect modeling framework for devices based on two-dimensional (2-D) transition-metal dichalcogenide (TMDC) monolayers. The so-called projected (p)- GW method is combined with density functional theory and incorporated into the non-equilibrium Green’s function equations to efficiently and accurately investigate the influence of various defect types on the characteristics of 2-D field-effect transistors. Through quasi-particle correlated defect-level modeling, we show that one single defect located inside the channel under the gate is a main source to block the current flow, thus leading to a large performance degradation. Our variability study also confirms that defects inside transistors based on 2-D TMDC monolayers induce a significant threshold voltage shift and ON-state current variation

Dynamics of van der Waals charge qubit in two-dimensional bilayer materials: Ab initio quantum transport and qubit measurement

A van der Waals (vdW) charge qubit, electrostatically confined within two-dimensional (2D) vdW materials, is proposed as a building block of future quantum computers. Its characteristics are systematically evaluated with respect to its two-level anticrossing energy difference (Δ). Bilayer graphene (Δ≈ 0) and a vdW heterostructure (Δ≫ 0) are used as representative examples. Their tunable electronic properties with an external electric field define the state of the charge qubit. By combining density functional theory and quantum transport calculations, we highlight the optimal qubit operation conditions based on charge stability and energy-level diagrams. Moreover, a single-electron transistor design based on trilayer vdW heterostructures capacitively coupled to the charge qubit is introduced as a measurement setup with low decoherence and improved measurement properties. It is found that a Δ greater than 20 meV results in a rapid mixing of the qubit states, which leads to a lower measurement quantity, i.e., contrast and conductance. With properly optimized designs, qubit architectures relying on 2D vdW structures could be integrated into an all-electronic quantum computing platform.ISSN:2643-156

Exchange Interactions and Intermolecular Hybridization in a Spin-1/2 Nanographene Dimer

Phenalenyl is a radical nanographene with a triangular shape hosting an unpaired electron with spin S = 1/2. The open-shell nature of the phenalenyl is expected to be retained in covalently bonded networks. As a first step, we report synthesis of the phenalenyl dimer by combining in-solution synthesis and on-surface activation and its characterization on Au(111) and on a NaCl decoupling layer by means of inelastic electron tunneling spectroscopy (IETS). IETS shows inelastic steps that are identified as singlet–triplet excitation arising from interphenalenyl exchange. Spin excitation energies with and without the NaCl decoupling layer are 48 and 41 meV, respectively, indicating significant renormalization due to exchange with Au(111) electrons. Furthermore, third-neighbor hopping-induced interphenalenyl hybridization is fundamental to explaining the position-dependent bias asymmetry of the inelastic steps and activation of kinetic interphenalenyl exchange. Our results pave the way for bottom-up synthesis of S = 1/2 spin-lattices with large exchange interactions

Platinum contacts for 9-atom-wide armchair graphene nanoribbons

Creating a good contact between electrodes and graphene nanoribbons (GNRs) has been a long-standing challenge in searching for the next GNR-based nanoelectronics. This quest requires the controlled fabrication of sub-20 nm metallic gaps, a clean GNR transfer minimizing damage and organic contamination during the device fabrication, as well as work function matching to minimize the contact resistance. Here, we transfer 9-atom-wide armchair-edged GNRs (9-AGNRs) grown on Au(111)/mica substrates to pre-patterned platinum electrodes, yielding polymer-free 9-AGNR field-effect transistor devices. Our devices have a resistance in the range of 106-108 Ω in the low-bias regime, which is 2-4 orders of magnitude lower than previous reports. Density functional theory calculations combined with the non-equilibrium Green's function method explain the observed p-type electrical characteristics and further demonstrate that platinum gives strong coupling and higher transmission in comparison to other materials, such as graphene.</p