Session Z60: Modeling and Simulation of 2D Materials and Devices

An ab initio investigation of groups III-V monochalcogenides

Two-dimensional (2D) materials have attracted a great attention due to their unusual properties such as high degree of anisotropy and chemical functionality over relatively large areas. Since the graphene breakthrough, a variety of 2D materials have been predicted, synthesized, and characterized, the most prominent family being the transition-metal dichalcogenides, e.g., MoS₂, WSe₂, etc, representing semiconductors and topological semimetals. Inspired by the chemical flexibility and possible crystal structure variety, we consider the family of 2D monochalcogenides of the type MQ, where M = Al, Ga, In, Si, Ge, Sn, P, As, Sb; Q = S, Se, Te. Using density functional theory within the generalized gradient approximation (PBEsol) and hybrid functional calculations, we explore the possible crystal structure and electronic properties of this family of 2D MQ. For each compound we consider 12 crystal structures differing on space group, chemical bonding, and coordination, analyzing their electronic structure, band gaps, work function, and structural stability in terms of phonon spectra. The properties of these 2D materials are then compared to their 3D bulk parent structures, and a classification in terms of metallic versus semiconducting character, bonding and coordination is provided.   

Stability of layered In2Se3 phases from Diffusion Quantum Monte Carlo calculations

In₂Se₃ is a semiconductor material that can be found in different crystal structures, most of them forming two-dimensional layers stacked via van der Waals interactions. These layered crystal structures, composed of quintuple Se-In-Se-In-Se layers, differing on the stacking within the strongly bonded quintuple layer and across the van der Waals spacing, display electronic and optical properties that can be leveraged in a variety of device applications, including solar cells, photodetectors, and phase-change memory devices. However, the phase ordering and the transition between them remains unclear. Density functional theory and hybrid functional calculations show large variations in total energy differences between the different phases of In₂Se₃, dependent of the functional and the van der Waals correction used. Here we use diffusion Monte Carlo (DMC) calculations to determine the total energies and charge-density differences between the three most common layered structures of In₂Se₃, namely, α, α’, and β, which differ in bonding/coordination within the quintuple layer and stacking of quintuple layers. Effects of temperature were also included by calculating the Helmholtz free energy, where the temperature dependence is included through the vibrational entropy.   

Degenerate excitons in strongly spin-orbit coupled heterolayers

We study the optical response and exciton properties of two-dimensional topological systems with strong spin-orbit coupling, using ab-initio GW and Bethe-Salpeter equation (BSE) calculations. We develop a new method to derive group representations of exciton states directly from ab initio calculations without any assumption on the character of the envelope functions. Our analyses show that, for appropriate systems, it is possible for the two lowest-energy optically active excitons from a single valley to be degenerate. By construction of a smooth gauge, the envelope functions of these degenerate excitons exhibit 1s-like characters. Our finding suggests these material systems are very promising for many applications in optoelectronics.   

Ab initio calculations of exciton-phonon coupling in semiconducting systems

 One of the most important lossy mechanisms in solid state systems is the

scattering of excitonic states with phonons, contributing to the degradation

of optoelectronic performance. In this work we report a study of the calculation of exciton lifetimes in transition metal dichalcogenides (TMD) and

other band gap systems. TMD, in particular, are characterized by strong

Coulombic interactions, and as a consequence low screening coefficients.

That gives rise to tightly bound excitons that dominate the optical properties of the material. Phonon-induced interactions between bright and dark

excitonic states influence the coherence times of these systems. By using

a combination of density functional theory and many body perturbation

theory we can extract exciton lifetimes, and phonon linewidths for these

materials.   

BiAs as a novel 2D material

Finding two-dimensional semiconductors with large Rashba splitting is the vital step in the development of upcoming next generation spintronic technology as it assists the generation, detection and manipulation of spin current without magnetic field. Using first-principles calculations we find that BiAs is a stable layered semiconductors that crystalizes in a hexagonal honeycomb lattice geometry, with a narrow and indirect band gap. The inclusion of spin-orbit coupling in the calculations reveals the presence Rashba-Dresselhaus type spin splitting located around L-point of the Brillion zone, with an elliptical spin texture and Rashba energy E_R= 66 meV and coupling constant α_R = 4.32 eVÅ. We also studied BiAs in monolayer form, and find a direct band gap semiconductor with a circular spin texture around Γ point, and Rashba energy and coupling constant of  E_R = 17 meV and  α_R = 1.56 eVÅ, which are large spin-splittting parameter for a 2D material.  The effects of strain on the electronic structure of of the monolayer is also explored, and our results indicate that BiAs/AlN can potentially be used for the development novel field-effect transistors and spintronic devices.

    

First-principles calculation of the non-equilibrium quasi-Fermi levels in defective WSe2 p-n junctions

While the two-dimensional p-n junctions have been extensively studied for electronic and optoelectronic devices, the semiclassical approaches without considering atomistic details are still insufficient to describe its electronic structures, such as long depletion width and the role of defects. To overcome such limitations, we combine the multi-space constrained-search density functional theory (MS-DFT) formalism [1] together with the simulated doping method [2] for describing the doped p-n junction under finite-bias conditions. By calculating the lateral WSe₂ p-n junctions, we find that the charge density profile in the depletion region calculated within the first-principles approach is more diffuse than the analytical assumption, leading to a longer depletion width than the analytical expression. We then introduce Se (W) vacancy in the depletion region and show that as the forward bias voltage increases, the depletion width of the p-doped (n-doped) WSe₂ decreases faster than that of the n-doped (p-doped) WSe₂. Thanks to the MS-DFT that uniquely allows plotting quasi-Fermi level (QFL) profiles within the first-principles calculation, we also extract the QFL profiles in the defective WSe₂ p-n junction. Careful analysis of the QFL profiles shows that the asymmetrically varying depletion width in defective WSe₂ p-n junction originates from the asymmetrically penetrated QFL profiles mediated by the defect. Our findings highlight the importance of the first-principles approaches for 2D p-n junction devices in terms of the design of next-generation 2D p-n junction devices.

[1] J. Lee, H. S. Kim, and Y.-H. Kim, Adv. Sci. 7, 2001038 (2020); J. Lee, H. Yeo, Y.-H. Kim, Proc. Natl. Acad. Sci. U.S.A. 19, 10142-10148 (2020).

[2] O. Sinai and L. Kronik, Phys. Rev. B 87, 235305 (2013).   

Hydrogen Evolution Reaction on BP Monolayer and MoS2/BP van der Waals Heterostructure from First-principles

Molecular hydrogen is a sustainable energy carrier. One of the methods for its production is water splitting, whereby electrochemical hydrogen evolution reaction (HER) is efficiently catalyzed by Pt. Owing to its high cost, this field is being actively explored for earth-abundant low-cost electrocatalysts like MoS₂. MoS₂ is a promising acid-stable catalyst; however, its applicability is limited by poor electrical transport and inefficient charge transfer at the interface. Therefore, the present work examines its bilayer van der Waals heterostructure (vdW HTS), which incorporates spatial separation of e^--h⁺ on two layers. As per existing literature, the second constituent monolayer BP is advantageous as an electrode material, owing to its chemical stability in both oxygen and water environments. We have performed first-principles based calculations under the framework of density functional theory (DFT) for hydrogen evolution reaction in an electrochemical double layer on the BP monolayer and MoS₂/BP vdW HTS. The climbing image nudged elastic band method have been employed to determine the minimum energy pathways. The comparative study has been undertaken by analyzing their electrostatic potential and work function in Heyrovsky and Tafel reaction intermediates. Finally, we observe the Heyrovsky reaction path to be favorable.   

Extraordinary thermal conductivity of gold sulfide monolayers

Gold sulfide monolayers (α-, β-Au₂S  and α-, β-, γ-AuS) have emerged as a new class of two-dimensional (2D) materials with appealing properties such as high thermal and dynamical stability, oxidation resistibility, and excellent electron mobility. However, their thermal properties are still unexplored. In this study, based on first-principles calculations and the Peierls-Boltzmann transport equation, we report the thermal conductivity (κ) and related phonon thermal properties of all members of this family. Our results show that gold sulfide monolayers have ultralow thermal conductivity within the range of 0.04 W m^-1K^-1 to 10.62 W m^-1K^-1, with different levels of anisotropy. Particularly, our results demonstrate that β-Au₂S, having κ_aa= 0.06 W m^-1K^-1 and κ_bb= 0.04 m^-1K^-1 along the principal in-plane directions, has one of the lowest κ values that have been reported for a 2D material. This extremely low thermal conductivity can be attributed to its flattened phonon branches and low phonon group velocity, high anharmonicity, and short phonon lifetimes. Our results may provide insight into the application of gold sulfide monolayers as thermoelectric materials, and motivate future κ measurements of gold sulfide monolayers.    

Intervalley excitonic hybridization, optical selection rules, and imperfect circular dichroism in monolayer hBN

We perform first-principles GW plus Bethe-Salpeter equation calculations to investigate the photophysics of monolayer hexagonal boron nitride (hBN), revealing excitons with novel k-space characteristics. The excitonic states forming the first and third peaks in its absorption spectrum are s-like, but those of the second peak is notably p-like, a first finding of strong co-occurrence of bright s-like and bright p-like states in an intrinsic 2D material. Moreover, even though the k-space wavefunction of these excitonic states are centered at the K- and K'-valleys like in monolayer transition metal dichalcogenides, the envelope functions of the basis excitons at one valley have significant extents to the basin of the other valley. As a consequence, the optical response of monolayer hBN exhibits a lack of circular dichroism, as well as a coupling that induces an intervalley mixing between s- and p-like states.   

Intercalation of Lithium inside Bilayer Buckled Borophene: A First-Principles Prospective

It is the keen wish of scientists to develop anode materials having low volume expansion and large capacity with high mobility. Therefore, lithium (Li) has been intercalated in bilayer buckled borophene to improve the adsorption energy, theoretical capacity, open-circuit voltage (OCV), diffusion barrier, and structural stability. Here, we investigated the bilayer b-borophene as anode material for Li-ion batteries using first-principle calculations. The intercalation of Li preserved the metallic nature of borophene and no volume expansion was found for a fully lithiated structure. This theoretical capacity of 1859 mAh/g, diffusion barrier 80 eV, and OCV 0.08 V indicate that intercalation improves said parameters compared to commercially used Graphene and prove it as a potential candidate for anode material in Li-ion batteries.   

A new material for Lithium and Sodium ion batteries: monolayer BPt2

Li-ion and Na-ion battery technologies have been one of the most important studies recently due to their high efficiency in energy storage field. But still, the battery life is far from satisfying the users because of intrinsic issues of materials especially on electrolyte decomposition at high voltage and capacity attenuation through cycling. This is also restricting the further developments of electric devices. Therefore more effective materials are needed. We discovered a new two-dimensional (2D) material BPt₂ with extremely high storage capacity for Li-ion and Na-ion battery applications. This robust metallic monolayer does not lose its metallic character ,which is desirable for battery applications, even under external conditions such as strain and charging. The calculated low diffusion barrier of Li and Na on BPt₂ monolayer makes it a promising candidate for next generation battery electrode material. Calculated open circuit voltages  for multilayer structures using cluster expansion formalism and density functional theory simulations indicate that upto 3 layers of Li/Na can be stored between BPt₂ layers. Therefore BPt₂ layers promise to be an effective material for  Li-ion and Na-ion battery electrode applications.   

Quantum capacitance of vertical tunnel field-effect transistors: A first-principles study

The vertical two-dimensional (2D) van der Waals (vdW) heterostructure has been intensively studied for the application of tunnel field-effect transistor (TFET) devices. Despite the similarities between TFET and capacitor architectures, the correlations between quantum capacitance and quantum transport characteristics have been rarely discussed. Carrying out first-principles finite-bias non-equilibrium TFET simulations within the multi-space constrained-search density functional theory (MS-DFT) formalism we have recently developed [1], we elucidate the quantum transport and quantum capacitance properties of the graphene-based TFET in atomistic details. We show that the total capacitance of graphene-based TFET significantly deviates from the classical geometric capacitance due to the low quantum capacitance of graphene electrodes. Under applying the gate-bias, we extract electrode-specific quantum capacitances and find that electrodes exhibit negative quantum capacitances raising the total capacitance at the resonant-tunneling regime. Finally, we extend the study for the defective channel case and study how a point defect introduced within the inner channel region affects the capacitance and transport properties. Our findings provide fundamental insight into the non-equilbrium device characteristics of low-dimensional quantum devices and point towards a future direction for the design of 2D vdW heterojunction devices.   

Spatial mapping of disordered 2D systems: the conductance Sudoku

Motivated by recent advances on local conductance measurement techniques at the nanoscale, timely questions are being raised about what possible information can be extracted from a disordered graphene sheet by selectively interrogating its transport properties. We demonstrate how an inversion technique originally developed to identify the number of scatterers in a quantum device can be adapted to a multi-terminal set up in order to provide detailed information about the spatial distribution of impurities on the surface of graphene, as well as other 2D material systems. The methodology input is conductance readings (for instance, as a function of the chemical potential) between different electrode pairs, the output being the spatially resolved impurity density. We show that the obtained spatial resolution depends on the number of such readings. Furthermore, by separating the impurity locations into partitions arranged in a grid-like geometry, this inversion procedure resembles a Sudoku puzzle in which the compositions of different sectors of a device are found by imposing that they must add up to specific constrained values established for the grid rows and columns. We argue that this technique may be used with other quantities besides the conductance, paving the way to alternative new ways of extracting information from a disordered material through the selective probing of local quantities.    

Direct and converse flexoelectricity in two-dimensional materials

Flexoelectricity, the generation of macroscopic polarization or voltage in response to a strain gradient, is expected to play a prominent role in two-dimensional (2D) crystals due to their extreme flexibility. Several attempts have been carried out to calculate the flexoelectric response of a monolayer (or few layers) due to a flexural deformation, but generally with remarkable disagreement in reported values. Here, building on recent developments in electronic-structure methods, we define and calculate the flexoelectric response of two-dimensional materials fully from first principles. In particular, we show that the open-circuit voltage response to a flexural deformation is a fundamental linear-response property of the crystal that can be calculated within the primitive unit cell of the flat configuration. Applications to graphene, silicene, phosphorene, BN and transition-metal dichalcogenide monolayers reveal that two distinct contributions exist, respectively of purely electronic and lattice-mediated nature. Within the former, we identify a key \emph{metric} term, consisting in the quadrupolar moment of the unperturbed charge density. We propose a simple continuum model to connect our findings with the available experimental measurements of the converse flexoelectric effect.