Session S44: 2D Moiré Materials: Theory and First-principles Calculations

Application of the Faddeev technique to the bound states of three charged particles in two dimensions

In this work, we present the application of the Faddeev technique for the exact numerical solution of the Schrödinger equation for three-charged particle bound systems in two dimensions. Three coupled Faddeev integral equations for the most general bound state of three different mass particles interacting with different pair interactions are formulated in momentum space depending on the magnitude of Jacobi momenta and the angle between them. We discuss the numerical implementation and present our results for the binding energy and wave function of a trion, a three-body system consisting of electrons and holes, in a MoS2 layer.   

Mottness and magnetism in moiré bilayer transition metal dichalcogenides: A cluster dynamical mean-field study

Moiré bilayer transition metal dichalcogenides (TMDs) have created much excitement recently due to their tunability of the relative magnitude between bandwidth and interaction strength. Examples include the heterobilayer MoTe₂/WSe₂ and the homobilayer WSe₂ exhibiting continuous Mott transitions and quantum criticality. The moiré Hubbard model (MHM) on a triangular lattice is believed to be a good candidate for a low-energy model of TMDs. In this talk I present a combined cluster dynamical mean-field study in real space (cellular dynamical mean-field theory) and reciprocal space (dynamical cluster approximation) of the MHM. The interplay of nesting properties with (quantum and spatial) correlations and the application of an external Zeeman field gives rise to the intriguing metal-insulator crossovers as well as rich magnetic phases observed in experiments.

    

Lithium ion intercalation induced dynamic evolution of moiré superlattices in graphene on SiC (0001)

    Ion intercalation in moiré superlattices in van der Waals layered materials has gained significant interest recently. However, there is little knowledge of the dynamic intercalation process and resultant spatiotemporal evolution of the topological domain walls (TWDs) networks, owing to limited experimental techniques and molecular modelling. In this work, we employed density functional theory calculations and molecular dynamics simulations to understand the Li⁺ ion intercalation dynamics in bilayer graphene on SiC (0001) substrate observed in state-of-the-art in-situ low-energy electron microscope experiments. Combined with experimental results, we unravel a novel mechanism: coupled dynamics of Li⁺ ion intercalation and graphene stacking sliding, to understand the spatiotemporal evolution of TDWs upon Li⁺ intercalation in two types of moiré patterns: zebraic patterns and triangular patterns. The obtained knowledge lays grounds to understand the intercalation dynamics in other vdW materials and enables approaches to manipulate the layered vdW heterostructure systems for novel properties and applications.

    

Generalized force field methodology for moiré heterostructures

In this work, a generalized force field methodology for the relaxation of large moiré heterostructures is proposed. The force field parameters are optimized to accurately reproduce the structural degrees of freedom of some small and manageable cells found using density functional theory (DFT). The parameters can then be used to tackle larger systems. We specialize to the case of transition-metal dichalcogenide (TMD) homo- and heterobilayers using a combination of the Stillinger-Weber (SW) intralayer- and the Kolmogorov-Crespi (KC) interlayer-potential. The results show excellent agreement in terms of band structure and effective masses between DFT and SW+KC force field relaxation. Using the relaxed structures, a simplified and systematic scheme for the extraction of moiré potential is presented. The results show the importance of in plane and out of plane relaxation effects on the moiré potential which is made both deeper and wider after relaxation. An interpolation based methodology for the calculation of the correct binding energy is also proposed. Finally, we glimpse into the formation rate of domains induced by atomic reconstruction, which can be managed with our force-field parametrization.   

Computational Studies on the Electrochemical Performance of Dopedand Substituted Ti3C2Tx (T = O,OH) MXene

Using Density functional theory (DFT) in conjunction with a solvation model, we have investigated the phenomenon of electrode-

electrolyte interaction at the electrode surface and its consequences on the electrochemical properties like the charge storage and total

capacitance of doped and substituted functionalized Ti₃C₂T_x (T = O, OH) supercapacitor electrode. We have studied nitrogen-doped,

nitrogen substituted, and molybdenum substituted MXenes in acidic electrolyte H₂SO₄ solution. By considering nitrogen doping at

different sites, we found that the greatest capacitance is obtained for doping at functional sites. Our results agree well with the available

experiment. We also found that the enhancement in capacitances due to nitrogen doping is due to amplifications in the

pseudocapacitance. We propose that the primary mechanism leading to the enhanced value of the capacitances due to nitrogen doping

is surface redox activity. The performances of substituted systems, on the other hand, are degraded compared to the pristine ones. This

suggests that better storage capacities in Ti₃C₂T_x electrode can be obtained by doping only. We provide insights into the reasons behind

contrasting behavior in doped and substituted systems and suggest ways to further improve the capacitances in the doped system.   

Tuning the electronic structure of twisted transition metal dichalcogenides heterotrilayer with an applied electric field

Moiré superlattices generated by twisting two or more layers of semiconducting transition metal dichalcogenides (TMDs) have attracted great attention due to their ability to host strongly correlated states, such as Mott insulators at half-filling (one hole per moiré cell) and generalised Wigner crystals at fractional fillings. The top valence bands in these systems are derived either from the monolayer states in the K^-/K⁺ valley or alternatively from monolayer states in the Γ valley. Here, we demonstrate using density-functional theory calculations that the order of K^-/K⁺-derived and Γ-derived valence bands in a heterotrilayer consisting of a 2H-WSe₂ bilayer stacked on a MoSe₂ layer with a twist angle of 3.1° can be controlled by a perpendicular electric field. Interestingly, the wavefunctions of the Γ-derived bands are delocalized over the three layers and realize a honeycomb lattice at the moiré scale with inequivalent A and B sites, whereas K^-/K⁺- derived bands give rise to a triangular lattice and are fully localized on specific layers. From these ab initio insights, we construct an effective Hamiltonian to study the correlated hole states that arise when the system is doped. Our results show that twisted heterotrilayer TMDs are an ideal platform to investigate Hubbard models on both triangular and honeycomb lattices in the same system and are in agreement with experimental measurements.

    

Spin-polarization anisotropy in bent tungsten diselenide nanoribbons and excitonic states

Spin-polarization anisotropy in bent tungsten diselenide nanoribbons and excitonic states*

Hong Tang, Santosh Neupane, Li Yin, Jason M. Breslin, and Adrienn Ruzsinszky

Department of Physics, Temple University, Philadelphia, PA 19122

 

A WSe2 monolayer shows many interesting properties due to its spin-orbit coupling induced spin splitting in bands around the Fermi level and the spin-valley configuration [1]. In this work, we studied the WSe2 nanoribbons under different bending curvatures and doping with density functional theory and their optical absorption and excitonic states with many-body perturbation GW and BSE (Bethe-Salpeter equation) methods. We show that the WSe2 nanoribbons can exhibit an enhanced SOC effect and a spatially varying spin polarization in bands around the Fermi level under bending [2]. The spin-polarization can show an anisotropy (or asymmetry) in those nearly degenerate bands, leading to a controllable magnetism with bending.  Electron or hole doping to the nanoribbon suggests a potential application in compact and controllable magnetic nanodevices and spintronics. The optical absorption spectrum of the nanoribbon presents a large tunability with bending within the near infrared region of about 0.4 to 1.5 eV, showing an enhanced absorption at a large bending condition. The exciton states generally show mixed or various spin configuration in the electron and hole pairs, potentially useful for applications in spin-based quantum information processes.   

 

*Work supported by Office of Science, Department of Energy, DE-SC0021263

1 .  Xiao-Xiao Zhang, Ting Cao, Zhengguang Lu, Yu-Chuan Lin, Fan Zhang, Ying Wang, Zhiqiang Li, James C. Hone, Joshua A. Robinson, Dmitry Smirnov, Steven G. Louie and Tony F. Heinz, Magnetic brightening and control of dark excitons in monolayer WSe2, Nature Nanotechnology volume 12, pages883–888 (2017).

2.   Hong Tang, Santosh Neupane, Li Yin, Jason M. Breslin, and Adrienn Ruzsinszky, Spin-polarization anisotropy in bent tungsten diselenide nanoribbons and excitonic states, arXiv    ????

 

    

Scaffold-guided crystallization of perovskite nanowire arrays

Solution-processable metal halide perovskites are being actively explored for optoelectronic devices, including active matrix displays, photodetectors, and solar cells. A critical challenge facing the widespread adoption of these materials is control over crystallization outcomes during film formation. Here, we introduce a method to grow metal halide perovskite crystals as vertical nanowire arrays with large surface areas for photophysical processes, such as exciton dissociation. The crystals are grown via a two-step process in which precursors are deposited into the cylindrical pores of anodized titanium oxide scaffolds in the first step and after immersing the infiltered scaffold in the second precursor solution, perovskite crystals start to nuclei. After 1 min of immersion, nanowires begin to form and longer immersion times will increase the nanowire’s dimension. The advantage of using this two-step method is that crystal nucleation is confined inside the TiO₂ pores, resulting in vertical growth of the nanowire arrays. FA/BAPbI₃ perovskite was chosen in this study. X-ray diffraction and photoluminescence analysis confirmed the formation of perovskite photoactive alfa polymorph. To analyze the optoelectronic properties of perovskite nanowire arrays, solar cell devices are fabricated with the porous TiO₂ and Spiro-MeOTAD as electron transport layers respectively.

    

First-principles study on the defect properties of two-dimensional materials

Recently, Defects in two-dimensional(2D) materials are revealed to be crucial to apprehend the various properties of materials. For example, they play the role of color centers, motivating electron transitions between the defect states. In addition, they have a large potential to be single photon emitters, which could be employed as a fundamental component of qubits. To exploit defects as a platform for future technology, we need to first appreciate the electronic structures of defects. It is, however, difficult to identify atomic defects especially when they are composed of vacancies and/or atoms with similar atomic numbers to those of host atoms, e.g., carbon or oxygen in hexagonal boron nitride (hBN). We perform first-principles calculations based on the density functional theory (DFT) to investigate the structural and electronic properties of defects in hBN and WSe₂, two prototypical 2D systems. As a first step, we examine the structural stabilities of defects in hBN and WSe₂ by evaluating the formation energies. Then we scrutinize their structural and electronic structures, focusing on levels of the defect states to estimate transition energies between the mid-gap states within HSE06, which is well-known for its great accuracy for the optical bandgap. We further consider the interaction between the defects in the hBN and WSe₂ with a few combinatory defects in an hBN/WSe₂/hBN heterostructure, which could affect the local geometry of defects, electronic structures, and their magnetic properties.

    

Identification and Manipulation of Defects in Black Phosphorus

Black phosphorus and black phosphorene are attractive for device applications because of their thickness-dependent band gap. We identify and manipulate commonly occurring defects in black phosphorus, combining density functional theory (DFT) calculations with STM experiments. The nature of a ubiquitous defect, imaged at negative bias as a bright dumbbell extending over several nanometers, has been the subject of much debate. By comparing simulated and experimental STM images at both negative and positive bias, this feature is shown to arise from a substitutional Sn impurity in the second P sublayer. Similarly, another frequently observed defect type is identified as arising from an interstitial Sn atom; this defect can be switched to a more stable configuration consisting of a Sn substitutional defect + P adatom, by application of an electrical pulse via the STM tip. DFT calculations show that this pulse-induced structural transition switches the system from a non-magnetic configuration to a magnetic one. We show that an unambiguous identification of defect features requires a comparison of simulated and experimental STM images; identifications based on electronic charge densities alone can be misleading. Moreover, these comparisons should be carried out at multiple bias voltages. We introduce States Projected Onto Individual Layers (SPOIL) quantities which provide information about atom-wise and orbital-wise contributions to bias-dependent features observed in STM images.   

Donor doping of corundum aluminum gallium oxide alloys

Designs of electronic devices using aluminum gallium oxide (ALGO) alloys as the barrier layer and gallium oxide (Ga₂O₃) as the active layer are being considered. The success of these devices is predicated in part on the ability to achieve controlled doping of the ALGO barrier layer. We use hybrid density functional theory calculations to examine the prospects for n-type doping high-Al content corundum ALGO alloys with H, Si, Ge, Sn, Hf, Zr, and Ta.  We also consider the detrimental role of cation vacancies as compensating acceptors.  Based on this investigation we identify Si as the most promising n-type dopant for corundum ALGO alloys.

    

The Effect of Strain on the Band Offsets in Monoclinic β-(AlxGa1-x)2O3 Alloys

Alloys of gallium oxide and aluminum oxide offer a tunable ultrawide bandgap reaching far into the ultraviolet-C spectral region and permit device architectures with potentially very large breakdown fields, thus are promising for applications in high power electronic devices. The volume of the unit cell of Al₂O₃ is smaller than that of Ga₂O₃. As a result, the incorporation of  aluminum into Ga₂O₃ leads to a shrinking of the lattice. In the case of pseudomorphic heteroepitaxial growth, when the epitaxial layer adopts the interfacial lattice spacing of the template, the epitaxial layer is under strain. The alignment of energy bands between the substrate and the strained epitaxial film remains an open question. We propose a model for such band offsets in the specific case of β-(Al_xGa_1-x)₂O₃ alloys grown on beta Ga₂O₃ substrates. Density functional theory (DFT) calculations of the band structure of both, β-Ga₂O₃ and θ-Al₂O₃, under various strain scenarios, in combination with Vegard’s rule allow us to construct a linear model on how the branch-point energy depends on the four  independent components of the monoclinic strain tensor across the entire composition range. We apply this model to predict the alignment of the energy bands for specific strain patterns associated with the pseudomorphic growth of β-(Al_xGa_1-x)₂O₃ alloys on, for example, (010) or (-201) faces of Ga₂O₃.   

Extrinsic Polarons: Engineering Coexistence Between Free and Trapped Carriers

The coexistence of high and low conductance states in semiconducting material systems is of great interest, to both the scientific and technological research communities, due to its enormous utility within computing applications. In this talk, we present an innovative approach for designing extrinsic polaron systems in which the coexistence between delocalized and localized states may be tuned through extrinsic polaron dopants. As a model system to explore these physics we have selected Ti-doped SnO2. Through comprehensive first-principles calculations, we demonstrate how strain and alloying can be employed to engineer the coexistence properties within this archetypal extrinsic polaron system. Moreover, we are able to further crystalize these first-principles findings through a compact tight-binding model. Our results indicate that lattice strain and alloying concentration are the two primary means by which the activation barrier between localized and delocalized states can be tuned in extrinsic polaron materials. Overall, this study presents an exciting route toward engineering carrier localization within computing applications and probing its behavior in strongly correlated materials.