Fundamental gap of fluorographene by many-body GW and fixed-node diffusion Monte Carlo methods

Mn2C MXene functionalized by oxygen is a semiconducting antiferromagnet and an efficient visible light absorber

Manganese-based MXenes are promising two-dimensional materials due to the broad palette of their magnetic phases and the possibility of experimental preparation because the corresponding MAX phase was already prepared. Here, we systematically investigated geometrical conformers and spin solutions of oxygen-terminated Mn2C MXene and performed subsequent many-body calculations to obtain reliable electronic and optical properties. Allowing energy-lowering using the correct spin ordering via supercell magnetic motifs is essential for the Mn2CO2 system. The stable ground-state Mn2CO2 conformation is antiferromagnetic (AFM) with zigzag lines of up and down spins on Mn atoms. The AFM nature is consistent with the parent MAX phase and even the clean depleted Mn2C sheet. Other magnetic states and geometrical conformations are energetically very close, providing state-switching possibilities in the material. Subsequent many-body GW and Bethe-Salpeter equation (BSE) calculations provide indirect semiconductor characteristics of AFM Mn2CO2 with a fundamental gap of 2.1 eV (and a direct gap of 2.4 eV), the first bright optical transition at 1.3 eV and extremely strongly bound (1.1 eV) first bright exciton. Mn2CO2 absorbs efficiently the whole visible light range and near ultraviolet range (between 10 and 20%).

Solvent Controlled Generation of Spin Active Polarons in Two-Dimensional Material under UV Light Irradiation

Polarons belong to a class of extensively studied quasiparticles that have found applications spanning diverse fields, including charge transport, colossal magnetoresistance, thermoelectricity, (multi)ferroism, optoelectronics, and photovoltaics. It is notable, though, that their interaction with the local environment has been overlooked so far. We report an unexpected phenomenon of the solvent-induced generation of polaronic spin active states in a two-dimensional (2D) material fluorographene under UV light. Furthermore, we present compelling evidence of the solvent-specific nature of this phenomenon. The generation of spin-active states is robust in acetone, moderate in benzene, and absent in cyclohexane. Continuous wave X-band electron paramagnetic resonance (EPR) spectroscopy experiments revealed a massive increase in the EPR signal for fluorographene dispersed in acetone under UV-light irradiation, while the system did not show any significant signal under dark conditions and without the solvent. The patterns appeared due to the generation of transient magnetic photoexcited states of polaronic character, which encompassed the net 1/2 spin moment detectable by EPR. Advanced ab initio calculations disclosed that polarons are plausibly formed at radical sites in fluorographene which interact strongly with acetone molecules in their vicinity. Additionally, we present a comprehensive scenario for multiplication of polaronic spin active species, highlighting the pivotal role of the photoinduced charge transfer from the solvent to the electrophilic radical centers in fluorographene. We believe that the solvent-tunable polaron formation with the use of UV light and an easily accessible 2D nanomaterial opens up a wide range of future applications, ranging from molecular sensing to magneto-optical devices.

Benchmarking fundamental gap of Sc2C(OH)2 MXene by many-body methods

Sc₂C(OH)₂ is a prototypical non-magnetic member of MXenes, a promising transition-metal-based 2D material family, with a direct bandgap. We provide here a benchmark of its fundamental gap Δ obtained from many-body GW and fixed-node diffusion Monte Carlo methods. Both approaches independently arrive at a similar value of Δ ∼ 1.3 eV, suggesting the validity of both methods. Such a bandgap makes Sc₂C(OH)₂ a 2D semiconductor suitable for optoelectronic applications. The absorbance spectra and the first exciton binding energy (0.63 eV), based on the Bethe-Salpeter equation, are presented as well. The reported results may serve to delineate experimental uncertainties and enable selection of reasonable approximations such as density functional theory functionals, for use in modeling of related MXenes.

Electronic Nature Transition and Magnetism Creation in Vacancy-Defected Ti2CO2 MXene under Biaxial Strain: A DFTB + U Study

The structural, electronic, and magnetic properties of vacancy defect in Ti₂CO₂ MXene and the effect of strain have been investigated using the density functional tight-binding (DFTB) approach including spin-polarization with Hubbard onsite correction (DFTB + U). The band gap of pure Ti₂CO₂ is ∼1.3 eV, which decreases to ∼0.4 and ∼1.1 eV in the case of C- and O-vacancies, respectively, i.e., the semiconducting behavior is retained. In contrast, Ti₂CO₂ undergoes semiconductor-to-metal transition by the introduction of a single Ti-vacancy. This transition is the result of introduced localized states in the vicinity of the Fermi level by the vacancy. Both Ti- and O-vacancies have zero net magnetic moments. Interestingly, the nonmagnetic (NM) ground state of semiconducting Ti₂CO₂ turns into a magnetic semiconductor by introducing a C-vacancy with a magnetization of ∼2 μ_B/cell. Furthermore, we studied the effect of strain on the electronic structure and magnetic properties of Ti-, C-, and O-vacant Ti₂CO₂. The nature of the band gap in the presence of single O-vacancy remains indirect in both compression and tensile strain, and the size of the band gap decreases. Compression strain on Ti-vacant Ti₂CO₂ changes metal into a direct semiconductor, and the metallic character remains under tensile biaxial strain. In opposition, a semiconductor-to-metal transition occurs by applying a compressive biaxial strain on C-vacant Ti₂CO₂. We also find that the magnetism is preserved under tensile strain and suppressed under compression strain on V_C-Ti₂CO₂. Moreover, we show that double C-vacancies maintain magnetism. Our findings provide important characteristics for the application of the most frequent MXene material and should motivate further investigations because experimentally achieved MXenes always contain point defects.

Novel First-Principles Insights into Graphene Fluorination

Fluorination of graphene sheets with Xenon difluoride leads to the formation of the widest band gap Gr derivative, namely Fluorographene. Accurate experimental observations distinguish a two stages mechanism in the fluorination procedure, the half-fluorination stage wherein one side of the Gr sheet is rapidly fluorinated and then the full-fluorination stage involving much slower fluorination of the opposite side of the sheet [Nature Comm. 5, 1 (2014)]. Here, we perform comprehensive density functional calculations to illustrate accurate microscopic insights into the much slower rate of the full-fluorination stage, compared with the half-fluorination one. The calculated minimum energy paths for the half- and full-fluorination processes demonstrate much enhanced fluorine adsorption after the half-fluorination stage, which sounds inconsistent with the experimental picture. This ambiguity is explained in terms of significant chemical activation of the graphene sheet after half-fluorination, which remarkably facilitates the formation of chemical contaminants in the system and thus substantially slows down the full-fluorination procedure. After considering the binding energy and durability of the relevant chemical species, including hydrogen, oxygen, and nitrogen molecules and xenon atom, it is argued that oxygen-fluorine ligands are the most likely chemical contaminants opposing the complete fluorination of a graphene sheet. Then, we propose an oxygen desorption mechanism to carefully explain much enhanced rate of the full-fluorination procedure at elevated temperatures. The potential photocatalytic application of the pristine and defected samples in water splitting and carbon dioxide reduction reactions are also discussed.

Electronic, Optical, and Elastic Properties of CaFI Monolayer and Acoustic Phonon Dispersion at Hypersonic Frequencies Using Density Functional Theory and beyond with Random Phase Approximation and Bethe-Salpeter Equation

The extraordinary properties of graphene have motivated us to investigate a novel 2D compound. In this framework, we study the structural, vibrational, electronic, optical, and elastic properties of a new two-dimensional CaFI monolayer, using DFT, GW, RPA, and BSE methodologies. The phonon dispersion curve of the CaFI monolayer exhibited no unstable phonon modes, confirming that this 2D sheet is dynamically stable. Our GW calculations show that the indirect bandgap energy value of CaFI is 6.52 eV. Interestingly, the bandgap rapidly decreased by improving the electric field value. Our BSE computations indicate that this monolayer becomes translucent when the incident light frequency exceeds the plasma frequency (6.50 eV). Also, we have computed the second and third elastic constants of CaFI by combining the DFT and RPA approaches with the homogeneous deformation method. Additionally, the longitudinal acoustic phonon dispersion of CaFI was studied. We have determined that the longitudinal acoustic wave velocity in our sheet is higher than the LA wave velocity of germanium measured using Brillouin or ultrasonic techniques.

DFTB investigations of the electronic and magnetic properties of fluorographene with vacancies and with adsorbed chemical groups

The electronic and magnetic properties of fluorographene (CF) in the presence of F-vacancy defects and/or chemical groups (-OH, -CN, or -NH₂) were computationally investigated within the framework of the density functional tight-binding (DFTB) method. The current method parameterization allowed us to perform accurate electronic structure calculations (at the ab initio level of many-body methods in the particular case of CF) for hundreds of atoms in the computational cell. We show that the F-vacancy and/or chemical groups influence the magnetic structure, which depends on the number of defects and their distribution between the two sides of the graphene plane. Interestingly, we pointed out a possibility of imprinting local magnetism not only via F-vacancy and -OH combinations, but also using F-vacancies and -CN or -NH₂ groups. In such structures, the magnetic ordering and the total magnetic moments depend on their adsorption sites and their presence in the same or on opposite sides of the graphene plane. We devote particular attention to the interacting chemical group with the F-vacancies. The interaction between the adsorbed chemical group and the unpaired spins associated with the F-vacancies in CF gives rise to interesting magnetic structures. Finally, the zigzag-like direction is shown as the most preferred for the defluorination of CF. Stable ferrimagnetic zigzag chains with interesting properties are considered to be basic magnetic features in perturbed CF. Our work provides new guidelines for engineering multifunctional spintronic components using CF as a base material. We believe, in particular, that the magnetism is predominantly controlled by the F-vacancies, and the ferromagnet can ideally be regulated via the adsorption of a chemical group on a defective CF supercell.

The electronic and optical properties of III–V binary 2D semiconductors: how to achieve high precision from accurate many-body methods

We tested the precision of accurate many-body GW and BSE methods on seven hexagonal 2D III–V binary semiconductors (BN, BP, BAs, AlN, GaN, GaP, and GaAs), and we provided benchmark electronic and optical properties.

Noncovalent Interactions by the Quantum Monte Carlo Method: Strong Influence of Isotropic Jastrow Cutoff Radii

We present a paradigmatic example of a strong effect of Jastrow cutoff radii setup on the accuracy of noncovalent interaction energy differences within one-determinant Slater-Jastrow fixed-node diffusion Monte Carlo (1FNDMC) simulations using isotropic Jastrow terms and effective-core potentials. Analysis of total energies, absolute and relative errors, and local energy variance of energy differences vs the reference results suggests a simple procedure to marginalize the related biases. The presented data showcase improvements in dispersion-bounded systems within such a 1FNDMC method.

Frontiers of stochastic electronic structure calculations

In recent years there has been a rapid growth in the development and application of new stochastic methods in electronic structure. These methods are quite diverse, from many-body wave function techniques in real space or determinant space to being used to sum perturbative expansions. This growth has been spurred by the more favorable scaling with the number of electrons and often better parallelization over large numbers of central processing unit (CPU) cores or graphical processing units (GPUs) than for high-end non-stochastic wave function based methods. This special issue of the Journal of Chemical Physics includes 33 papers that describe recent developments and applications in this area. As seen from the articles in the issue, stochastic electronic structure methods are applicable to both molecules and solids and can accurately describe systems with strong electron correlation. This issue was motivated, in part, by the 2019 Telluride Science Research Center workshop on Stochastic Electronic Structure Methods that we organized. Below we briefly describe each of the papers in the special issue, dividing the papers into six subtopics.