First-principles study of the oxidation susceptibility of WS2, WSe2, and WTe2 monolayers.

The environmental stability of two-dimensional (2D) transition metal dichalcogenide monolayers is of great importance for their applications in electronic, photonic, and energy storage devices. In this study, we focus on understanding the susceptibility of WS2, WSe2, and WTe2 monolayers to oxygen exposure in the form of atomic oxygen and O2 and O3 molecules, respectively. Calculations based on the van der Waals-corrected density functional theory predicted that O2 and O3 molecules are weakly adsorbed on these monolayers, although atomic oxygen prefers chemisorption accompanied by a significant charge transfer from the surface to oxygen. In the physisorbed molecular configurations consisting of O2 and O3, the partially oxidized monolayers retain their geometrical and electronic structures. The calculated transition path as the oxygen approaches the surface shows a high-energy barrier for all cases, thus explaining the photo-induced formation of the oxidized configurations in the experiments. Furthermore, oxidizing the WS2 monolayer is predicted to modify its electronic structure, reducing the band gap with increasing oxygen coverage on the surface. Overall, the calculated results predict the resilience of WS2, WSe2, and WTe2 monolayers against oxygen exposure, thus ensuring stability for devices fabricated with these monolayers.

Indirect Exchange Interaction Leads to Large Lattice Contribution to Magnetocaloric Entropy Change.

Materials with a large magnetocaloric response are highly desirable for magnetic cooling applications. It is suggested that a strong spin-lattice coupling tends to generate a large magnetocaloric effect, but no microscopic mechanism has been proposed. In this Letter, we use spin-lattice dynamics simulation to examine the lattice contribution to the magnetocaloric entropy change in bcc iron (Fe) and hcp gadolinium (Gd) with exchange interaction parameters determined from ab initio calculations. We find that indirect Ruderman-Kittel-Kasuya-Yosida (RKKY) exchange interaction in hcp Gd leads to longer-range spin-lattice coupling and more strongly influences the low-frequency long-wavelength phonons. This results in a higher lattice contribution toward the total magnetocaloric entropy change as compared to bcc Fe with short-range direct exchange interactions. Our analysis provides a framework for understanding the magnetocaloric effect in magnetic materials with strong spin-lattice couplings. Our finding suggests that long-range indirect RKKY-type exchange gives rise to a larger lattice contribution to the magnetocaloric entropy change and is, thus, beneficial for magnetocaloric materials.

Intrinsic and strain dependent ultralow thermal conductivity in novel AuX (X = Cu, Ag) monolayers for outstanding thermoelectric applications.

A large power factor and ultralow lattice thermal conductivity in 2D-monolayers of AuX (X = Cu and Ag) are achieved via first principles calculations. Low phonon frequency, small Debye temperature and high Gruneisen parameter limit the intrinsic thermal conductivity of both the studied materials. An ultra-low lattice thermal conductivity of 0.13 (0.30) W m-1 K-1 and 0.66 (1.59) W m-1 K-1 is obtained for unstrained AuCu and AuAg monolayers, respectively, at 700 (300) K, which further reduces to 0.04 (0.09) and 0.26 (0.63) W m-1 K-1 at 6% biaxial tensile strain. Such values of thermal conductivity are lower than the critical thermal conductivity for the state-of-art thermoelectric materials (kl < 2 W m-1 K-1). The peak values of ZT for unstrained monolayers are 2.20 and 1.40, which enhances to 3.61 and 2.91 at 6% strain for AuCu and AuAg monolayers, respectively. Interestingly pudding-mold band textures are found to be responsible for this unusual thermoelectric behaviour. The stability concerns (chemical/dynamic/mechanical) of these monolayers are ensured to stimulate experimental determinations for novel synthesis and possible applications.

Modulating the Combinatorial Target Power of MgSnN2 via RF Magnetron Sputtering for Enhanced Optoelectronic Performance: Mechanistic Insights from DFT Studies.

The unique structural features of many ternary nitride materials with strong chemical bonding and band gaps above 2.0 eV are limited and are experimentally unexplored. It is important to identify candidate materials for optoelectronic devices, particularly for light-emitting diodes (LEDs) and absorbers in tandem photovoltaics. Here, we fabricated MgSnN2 thin films, as promising II-IV-N2 semiconductors, on stainless-steel, glass, and silicon substrates via combinatorial radio-frequency magnetron sputtering. The structural defects of the MgSnN2 films were studied as a function of the Sn power density, while the Mg and Sn atomic ratios remained constant. Polycrystalline orthorhombic MgSnN2 was grown on the (120) orientation within a wide optical band gap range of ∼2.20-2.17 eV. The carrier densities of 2.18× 1020 to 1.02 × 1021 cm-3, mobilities between 3.75 and 2.24 cm2/Vs, and a decrease in resistivity from 7.64 to 2.73 × 10-3 Ω cm were confirmed by Hall-effect measurements. These high carrier concentrations suggested that the optical band gap measurements were affected by a Burstein-Moss shift. Furthermore, the electrochemical capacitance properties of the optimal MgSnN2 film exhibited an areal capacitance of 152.5 mF/cm2 at 10 mV/s with high retention stability. The experimental and theoretical results showed that MgSnN2 films were effective semiconductor nitrides toward the progression of solar absorbers and LEDs.

Electronic Structure-, Phonon Spectrum-, and Effective Mass- Related Thermoelectric Properties of PdXSn (X = Zr, Hf) Half Heuslers.

We hereby discuss the thermoelectric properties of PdXSn(X = Zr, Hf) half Heuslers in relation to lattice thermal conductivity probed under effective mass (hole/electrons) calculations and deformation potential theory. In addition, we report the structural, electronic, mechanical, and lattice dynamics of these materials as well. Both alloys are indirect band gap semiconductors with a gap of 0.91 eV and 0.82 eV for PdZrSn and PdHfSn, respectively. Both half Heusler materials are mechanically and dynamically stable. The effective mass of electrons/holes is (0.13/1.23) for Zr-type and (0.12/1.12) for Hf-kind alloys, which is inversely proportional to the relaxation time and directly decides the electrical/thermal conductivity of these materials. At 300K, the magnitude of lattice thermal conductivity observed for PdZrSn is 15.16 W/mK and 9.53 W/mK for PdHfSn. The highest observed ZT value for PdZrSn and PdHfSn is 0.32 and 0.4, respectively.

Ozonation of Group-IV Elemental Monolayers: A First-Principles Study.

Environmental effect on the physical and chemical properties of two-dimensional monolayers is a fundamental issue for their practical applications in nanoscale devices operating under ambient conditions. In this paper, we focus on the effect of ozone exposure on group-IV elemental monolayers. Using density functional theory and the climbing image nudged elastic band approach, calculations are performed to find the minimum energy path of O3-mediated oxidation of the group-IV monolayers, namely graphene, silicene, germanene, and stanene. Graphene and silicene are found to represent two end points of the ozonation process: the former showing resistance to oxidation with an energy barrier of 0.68 eV, while the latter exhibit a rapid, spontaneous dissociation of O3 into atomic oxygens accompanied by the formation of epoxide like Si-O-Si bonds. Germanene and stanene also form oxides when exposed to O3, but with a small energy barrier of about 0.3-0.4 eV. Analysis of the results via Bader's charge and density of states shows a higher degree of ionicity of the Si-O bond followed by Ge-O and Sn-O bonds relative to the C-O bond to be the primary factor leading to the distinct ozonation response of the studied group-IV monolayers. In summary, ozonation appears to open the band gap of the monolayers with semiconducting properties forming stable oxidized monolayers, which could likely affect group-IV monolayer-based electronic and photonic devices.

Orientation-dependent mechanical response of graphene/BN hybrid nanostructures.

Graphene-based hybrid van der Waals structures have emerged as a new class of materials for novel multifunctional applications. In such a vertically-stacked heterostructure, it is expected that its mechanical strength can be tailored by the orientation of the constituent monolayers relative to each other. In this paper, we explore this hypothesis by investigating the orientation dependence of the mechanical properties of graphene/h-BN heterostructures together with that of graphene and h-BN bilayers. The calculated results simulating the pull-out experiment show a noticeable dependence of the (out-of-plane) transverse mechanical response, which is primarily governed by the interlayer strength, on the stacking configurations. The degree of the dependence is directly related to the nature of the interlayer interactions, which change from covalent to covalent polar in going from graphene bilayer to graphene/BN to BN bilayer. In contrast, molecular dynamics simulations mimicking nanoindentation experiments predict that the in-plane mechanical response, which mainly depends on the intra-layer interactions, shows little or no dependence on the stacking-order. The BN monolayer is predicted to fracture before graphene regardless of the stacking pattern or configuration in the graphene/BN heterostructure, affirming the mechanical robustness of graphene. Thus, the graphene-based hybrid structures retain both stiffness and toughness required for a wide range of optoelectromechanical applications.

Giant Magnetoelectric Coupling in Multiferroic PbTi1-x V x O3 from Density Functional Calculations.

Giant magnetoelectric coupling is a very rare phenomenon that has gained much attention in the past few decades due to fundamental interest as well as practical applications. Here, we have successfully achieved giant magnetoelectric coupling in PbTi1-x V x O3 (x = 0-1) using a series of generalized gradient-corrected GGA (generalized gradient approximation), including on-site Coulomb repulsion (U)-corrected spin-polarized calculations based on accurate density functional theory. Our total energy calculations show that PbTi1-x V x O3 stabilizes in C-type antiferromagnetic ground state for x > 0.123. With the substitution of V into PbTiO3, the tetragonal distortion is highly enhanced accompanied by a linear increase in polarization. In addition, our band structure analysis shows that for lower x values, the tendency to form two-dimensional magnetism of PbTi1-x V x O3 decreases. The orbital magnetic polarization was calculated with self-consistent field method by including orbital polarization correction in the calculation as well as from the computed X-ray magnetic dichroism spectra. A nonmagnetic metallic ground state is observed for the paraelectric phase for V concentration (x) = 1 competing with a volume change of 10% showing a large magnetovolume effect. Our orbital-projected density of states as well as orbital ordering analysis suggest that the orbital ordering plays a major role in the magnetic-to-nonmagnetic transition when going from ferroelectric to paraelectric phase. The calculated magnetic anisotropic energy shows that the direction [110] is the easy axis of magnetization for x = 1 composition. The partial polarization analysis shows that the Ti/V-O hybridization majorly contributes to the total electrical polarization. The present study adds a new series of compounds to the magnetoelectric family with rarely existing giant coupling between electric- and magnetic-order parameters. These results show that such kind of materials can be used for novel practical applications where one can change the magnetic properties drastically (magnetic to nonmagnetic, as shown here) with external electric field and vice versa.

Large Negative Thermal Expansion Induced by Synergistic Effects of Ferroelectrostriction and Spin Crossover in PbTiO3-Based Perovskites.

The discovery of unusual negative thermal expansion (NTE) provides the opportunity to control the common but much desired property of thermal expansion, which is valuable not only in scientific interests but also in practical applications. However, most of the available NTE materials are limited to a narrow temperature range, and the NTE effect is generally weakened by various modifications. Here, we report an enhanced NTE effect that occurs over a wide temperature range α‾V=-5.24×10-5∘C-1,25-575∘C, and this NTE effect is accompanied by an abnormal enhanced tetragonality, a large spontaneous polarization, and a G-type antiferromagnetic ordering in the present perovskite-type ferroelectric of (1-x)PbTiO3-xBiCoO3. Specifically, for the composition of 0.5PbTiO3-0.5BiCoO3, an extensive volumetric contraction of ~4.8 % has been observed near the Curie temperature of 700 °C, which represents the highest level in PbTiO3-based ferroelectrics. According to our experimental and theoretical results, the large NTE originates from a synergistic effect of the ferroelectrostriction and spin crossover of cobalt on the crystal lattice. The actual NTE mechanism is contrasted with previous functional NTE materials, in which the NTE is simply coupled with one ordering such as electronic, magnetic, or ferroelectric ordering. The present study sheds light on the understanding of NTE mechanisms, and it attests that NTE could be simultaneously coupled with different orderings, which will pave a new way toward the design of large NTE materials.

Metamagnetism stabilized giant magnetoelectric coupling in ferroelectric xBaTiO3-(1 - x)BiCoO3 solid solution.

In order to establish the correlation between the magnetoelectric coupling and magnetic instability, we have studied the structural, magnetic, and ferroelectric properties of BaTiO3 modified BiCoO3i.e. xBaTiO3-(1 - x)BiCoO3 as a function of BaTiO3 concentration (x) and volume from a series of general-gradient-corrected (GGA), GGA plus onsite Coulomb repulsion (U), full potential, spin-density-functional band-structure calculations within the framework of density functional theory along with synchrotron X-ray diffraction and magnetic measurement studies. G-type antiferromagnetic ordering was found to be energetically favorable among all the considered magnetic configurations for x < 0.45 and higher concentrations stabilize with nonmagnetic (NM) states. We observe metamagnetic spin state transitions associated with paraelectric to ferroelectric transitions as a function of volume and x using synchrotron diffraction and computational studies, indicating a strong magnetoelectric coupling. Specifically for x = 0.33 composition, a pressure induced high spin (HS) to low spin (LS) transition occurs when the volume is compressed below 2.5%. Our orbital-projected density of states show a HS state for Co3+ in the ferroelectric ground state for x < 0.45 and the corresponding paraelectric phase is stable in the NM state due to the stabilization of LS state as evident from our fixed-spin-moment calculations and magnetic measurements. The nature of chemical bonding has been studied using partial density of states, electron localization function, and Born effective charge analysis. High values of spontaneous ferroelectric polarizations are predicted for lower x values which inversely vary with x because of the reduction of tetragonality (c/a) with increase in x which indicates the presence of both spin-lattice and ferroelectricity-lattice coupling. Our partial polarization analysis shows that not only the lone pair at Bi sites but also the d0-ness of Ti4+ ions contribute to the net polarization. Moreover, we find that the HS-LS transition point and magnetoelectric coupling strength can be varied by x.

Electronic and Magnetic Structures of Hole Doped Trilayer La4-xSrxNi3O8 from First-Principles Calculations.

The magnetic and electronic properties of trilayer La4Ni3O8, similar to hole-doped cuprates, are investigated by performing full-potential linearized augmented plane wave method-based spin-polarized calculations with LDA and GGA functionals including Hubbard U parameters to account for strong correlation effects. On the basis of these calculations, we found that La4Ni3O8 is a C-type anti-ferromagnetic (C-AFM) Mott insulator in agreement with previous experimental and theoretical observations. Our calculations suggest that the two crystallographically nonequivalent nickel atoms Ni1 and Ni2 are found to be in high-spin state with an average valency of +1.33. Intermediate band-gap states are originated from dz2 electrons of both types of Ni ions after including the strong correlation effects. To understand the role of hole doping on electronic structure, phase stability, and magnetic properties of La4Ni3O8, similar calculations were performed for La4-xSrxNi3O8 as a function of x, using the supercell approach. We found that the hole doping brings insulator-to-metal transition without changing the C-AFM ordering, though the magnetic moment is enhanced at both Ni sites. Moreover, these Ni atoms are always in an average valence state irrespective of hole doping or volume change. So the electronic properties of hole-doped La4Ni3O8 cannot be compared with hole-doped cuprates that are high-TC superconductors.

Theoretical and experimental investigation on structural, electronic and magnetic properties of layered Mn5O8.

We have investigated the crystal, electronic, and magnetic structures of Mn5O8 by means of state-of-the-art density functional theory calculations and neutron powder diffraction (NPD) measurements. This compound stabilizes in the monoclinic structure with space group C2/m where the Mn ions are in the distorted octahedral and trigonal prismatic coordinations with oxygen atoms. The calculated structural parameters based on total energy calculations are found to be in excellent agreement with low temperature NPD measurements when we accounted for the correct magnetic structure and Coulomb correlation effect in the computation. Using fully relativistic generalized-gradient approximation with Hubbard U (GGA+U) we found that the magnetic ordering in Mn5O8 is A-type antiferromagnetic and the direction of the easy axis is [1 0 0] in agreement with susceptibility and NPD measurements. However, the calculation without the inclusion of Hubbard U leads to ferrimagnetic half metal as a ground state contradictory to experimental findings, indicating the presence of a strong Coulomb correlation effect in this material. The GGA calculation without the Coulomb correction effect itself is sufficient to reproduce the experimentally observed magnetic moments in various Mn sites. We found that Mn in this material exhibits mixed valence behavior with 2+ and 4+ oxidation states reflecting different magnetic moments in the Mn sites. We explored the electronic band characteristics using total, site-, and orbital-projected density of states which emphasized the mixed-valent nature of Mn. A dominant Mn 3d character of the density of states at Fermi energy is the origin for the metallic behavior of Mn5O8. The bond strength analysis based on the crystal orbital Hamiltonian population between constituents indicates strong anisotropy in the bonding behavior which results from the layered nature of its crystal structure. Our bonding analysis shows that there is a noticeable covalent bond between Mn 3d-O 2p states which stabilizes the observed low symmetric structure. Our experimental findings and theoretical predictions suggest that Mn5O8 can be classified as a strongly correlated mixed valent antiferromagnetic metal.