Resonant Raman Scattering Study of Strain and Defects in Chemical Vapor Deposition Grown MoS2 Monolayers.

The development of bottom-up synthesis routes for semiconducting transition metal dichalcogenides (TMDs) and the assessment of their defects are of paramount importance to achieve their applications. TMD monolayers grown by chemical vapor deposition (CVD) can be subjected to significant strain and, here, Raman and photoluminescence spectroscopies are combined to characterize strain in over one hundred MoS2 monolayer samples grown by CVD. The frequency changes of phonons as a function of strain are analyzed, and used to extract the Grüneisen parameter of both zone-center and edge phonons. Additionally, the intensity of the defect-induced longitudinal acoustic (LA) and transverse acoustic (TA) Raman bands are discussed in relation to strain and electronic doping. The experimental mode-Grüneisen parameters obtained are compared with those calculated by density functional theory (DFT), to better characterize the type of strain and its resulting effects on Grüneisen parameters. The findings indicate that the use of Raman spectra to determine defect densities in 2D MoS2 must be always conducted considering strain effects. To the best of the authors' knowledge, this work constitutes the first report on double resonance Raman processes studied as a function of strain in 2D-MoS2. The new approach to obtain the Grüneisen parameter from zone-edge phonons in MoS2 can also be extended to other 2D semiconducting TMDs.

Steric effects on single-molecule conductance in flat-lying phenanthrene.

A previous combined experimental and theoretical study found that the position of anchoring groups on a phenanthrene (PHE) backbone played a large role in determining the single-molecule conductance of the PHE derivative. However, a consistent 0.1 G0 feature was found across all PHE derivatives. To understand this, the previously investigated PHE derivatives were placed flat on a simulated Au substrate with a scanning tunneling microscope (STM) tip over PHE and conductance was calculated using the non-equilibrium Green's function technique in conjunction with density functional theory (NEGF-DFT). The location of the tip was varied to find the most conductive and most energetically favorable arrangements, which did not coincide. Furthermore, the variation in conductance found in erect junctions was not calculated to appear when PHE derivatives were lying flat, with all derivatives calculated to have conductance values around 0.1 G0.

A computational study of electron transport in dynamic tetrahydrofuran and ethylene carbonate solvents on a Ca metal anode.

Calcium-ion batteries offer many advantages to the current lithium-ion technology in terms of cost, sourcing materials, and potential for higher energy density. However, calcium-ion batteries suffer from lack of a stable electrolyte due to reduction from the anode. Building off of our recent work investigating the stability of two representative electrolyte solvents, tetrahydrofuran (THF) and ethylene carbonate (EC), we now use ab initio molecular dynamics (AIMD) and the non-equilibrium Green's function technique in conjunction with density functional theory (NEGF-DFT) to investigate charge transport as the solvent molecules dynamically interact with the anode surface. THF maintained a relatively consistent conductance throughout the trajectory, although some jumps in the conductance were attributed to THF molecular rearrangement. EC exhibited a large amount of molecular decomposition, and a corresponding decrease in conductance of several orders of magnitude was noted. Through this analysis, we show that molecular decomposition and early-stage solid-electrolyte interphase (SEI) formation plays a major role in the robustness of charge transport as the system evolves in time and with temperature.

Voltage prediction of vanadium redox flow batteries from first principles.

Global energy demand has been increasing for decades, which has created a necessity for large scale energy storage solutions for renewable energy sources. We studied the voltage of vanadium redox flow batteries (VRFBs) with density functional theory (DFT) and a newly developed technique usingab initiomolecular dynamics (AIMD). DFT was used to create cluster models to calculate the voltage of VRFBs. However, DFT is not suited for capturing the dynamics and interactions in a liquid electrolyte, leading to the need for AIMD, which is capable of accurately modeling such things. The molarities and densities of all systems were carefully considered to match experimental conditions. With the use of AIMD, we calculated a voltage of 1.23 V, which compares well with the experimental value of 1.26 V. The techniques developed using AIMD for voltage calculations will be useful for the investigation of potential future battery technologies or as a screening process for additives to make improvements to currently available batteries.

Modulation of Charge Transport through Single Molecules Induced by Solvent-Stabilized Intramolecular Charge Transfer.

The modulation of charge transport through single molecules can be established by using the intrinsic characteristics of molecules and the physical properties of their environment. Therefore, the impact of the solvent on the electronic properties of molecules in the junction and their charge transport behavior are of great interest. Here, for the first time, we focused on charge transport through dimethylaminobenzonitrile (DMABN). This molecule shows unique behavior, specifically noticeable electronic structure modulations in bulk solvents, e.g., dual fluorescence in a polar environment. Using the scanning tunneling microscopy break junction (STM-BJ) technique, we find an order of magnitude increase in conductance along with a second conductance value in polar solvents over nonpolar solvents. Inspired by the twisted intramolecular charge transfer (TICT) explanation of the famous dual fluorescence of DMABN in polar solvents, we hypothesize stabilization of twisted DMABN molecules in the junction in more polar solvents. Ab initio molecular dynamics (AIMD) simulations using density functional theory (DFT) show that DMABN can twist in the junction and have a larger dipole moment compared to planar DMABN junction geometries, supporting the hypothesis. The nonequilibrium Green's function with the DFT approach (NEGF-DFT) is used to calculate the conductance throughout the AIMD trajectory, finding a significant change in the frontier orbitals and transmission function at large internal twisting angles, which can explain the dual conductance in polar solvents in STM-BJ experiments.

Exploring Calcium Manganese Oxide as a Promising Cathode Material for Calcium-Ion Batteries.

The dependence on lithium for the energy needs of the world, coupled with its scarcity, has prompted the exploration of postlithium alternatives. Calcium-ion batteries are one such possible alternative owing to their high energy density, similar reduction potential, and naturally higher abundance. A critical gap in calcium-ion batteries is the lack of suitable cathodes for intercalating calcium at high voltages and capacities while also maintaining structural stability. Transition metal oxide postspinels have been identified as having crystal structures that can provide low migration barriers, high voltages, and facile transport pathways for calcium ions and thus can serve as cathodes for calcium-ion batteries. However, experimental validation of transition metal oxide postspinel compounds for calcium ion conduction remains unexplored. In this work, calcium manganese oxide (CaMn2O4) in the postspinel phase is explored as an intercalation cathode for calcium-ion batteries. CaMn2O4 is first synthesized via solid-state synthesis, and the phase is verified with X-ray diffraction (XRD). The redox activity of the cathode is investigated with cyclic voltammetry (CV) and galvanostatic (GS) cycling, identifying oxidation potentials at 0.2 and 0.5 V and a broad insertion potential at -1.5 V. CaMn2O4 can cycle at a capacity of 52 mAh/g at a rate of C/33, and calcium cycling is verified with energy-dispersive X-ray spectroscopy (EDS) and X-ray photoelectron spectroscopy (XPS) and modeled with density functional theory (DFT) simulations. The results from the investigation concluded that CaMn2O4 is a promising cathode for calcium-ion batteries.

Reversible Electrochemical Anionic Redox in Rechargeable Multivalent-Ion Batteries.

Rechargeable multivalent-ion batteries are of significant interest due to the high specific capacities and earth abundance of their metal anodes, though few cathode materials permit multivalent ions to electrochemically intercalate within them. The crystalline chevrel phases are among the few cathode materials known to reversibly intercalate multivalent cations. However, to date, no multivalent-ion intercalation electrodes can match their reversibility and stability, in part due to the lack of design rules that guide how ion intercalation and electron charge transfer are coupled up from the atomic scale. Here, we elucidate the electronic charge storage mechanism that occurs in chevrel phase (Mo6Se8, Mo6S8) electrodes upon the electrochemical intercalation of multivalent cations (Al3+, Zn2+), using solid-state nuclear magnetic resonance spectroscopy, synchrotron X-ray absorption near edge structure measurements, operando synchrotron diffraction, and density functional theory calculations. Upon cation intercalation, electrons are transferred selectively to the anionic chalcogen framework, while the transition metal octahedra are redox inactive. This reversible electrochemical anionic redox, which occurs without breaking or forming chemical bonds, is a fundamentally different charge storage mechanism than that occurring in most transition metal-containing intercalation electrodes using anionic redox to enhance energy density. The results suggest material design principles aimed at realizing new intercalation electrodes that enable the facile electrochemical intercalation of multivalent cations.

Modified Born method for modeling melting temperature usingab initiomolecular dynamics.

The prediction of a material's melting point through computational methods is a very difficult problem due to system size requirements, computational efficiency and accuracy within current models. In this work, we have used a newly developed metric to analyze the trends within the elastic tensor elements as a function of temperature to determine the melting point of Au, Na, Ni, SiO2and Ti within ±20 K. This work uses our previously developed method of calculating the elastic constants at finite temperatures, as well as leveraging those calculations into a modified Born method for predicting melting point. While this method proves to be computationally expensive, the level of accuracy of these predictions is very difficult to reach using other existing computational methods.

Electron passivation in CaF2 on calcium metal anodes.

Current electrolytes in calcium-ion batteries suffer from a lack of stability and degradation caused by reduction from the anode. The solid-electrolyte interphase (SEI) that forms on the anodes during operation stems the flow of electrons from the anode to the electrolyte. CaF2 is a common inorganic compound found in the SEI, and is derived from electrolyte salts such as Ca(PF6)2. CaF2 can exist in crystalline, polycrystalline, and amorphous phases in the SEI, and as recent work has shown, different phases of the same compound can have vastly different electronic conductivities. Using the non-equilibrium Green's function technique with density functional theory (NEGF-DFT), we find that amorphous phase systems enhance electron tunneling in thin CaF2 films by 1-2 orders of magnitude when compared to crystalline and polycrystalline CaF2 systems. Transport through several amorphous structures was considered showing that, despite their random structures, their conductance properties are similar. Through analysis of the decay constant ß and the low-bias conductance of each system, we show that crystalline and polycrystalline CaF2 offer greater protection of the electrolyte than amorphous CaF2.

Investigating the role of structural water on the electrochemical properties of α-V2O5 through density functional theory.

The α polymorph of V2O5 is one of the few known cathodes capable of reversibly intercalating multivalent ions such as Mg, Ca, Zn and Al, but suffers from sluggish diffusion kinetics. The role of H2O within the electrolyte and between the layers of the structure in the form of a xerogel/aerogel structure, though, has been shown to lower diffusion barriers and lead to other improved electrochemical properties. This density functional theory study systematically investigates how and why the presence of structural H2O within α-V2O5 changes the resulting structure, voltage, and diffusion kinetics for the intercalation of Li, Na, Mg, Ca, Zn, and Al. We found that the coordination of H2O molecules with the ion leads to an improvement in voltage and energy density for all ions. This voltage increase was attributed to the extra host sites for electrons present with H2O, thus leading to a stronger ionization of the ion and a higher voltage. We also found that the increase in interlayer distance and a potential "charge shielding" effect drastically changes the electrostatic environment and the resulting diffusion kinetics. For Mg and Ca, this resulted in a decrease in diffusion barrier from 1.3 eV and 2.0 eV to 0.89 eV and 0.4 eV, respectively. We hope that our study motivates similar research regarding the role of water in both V2O5 xerogels/aerogels and other layered transition metal oxides.

Phenalenyls as tunable excellent molecular conductors and switchable spin filters.

Phenalenyl-based radicals are stable radicals whose electronic properties can be tuned readily by heteroatom substitution. We employ density functional theory-based non-equilibrium Green's function (NEGF-DFT) calculations to show that this class of molecules exhibits tunable spin- and charge-transport properties in single molecule junctions. Our simulations identify the design principles and interplay between unusually high conductivity and strong spin-filtering.

Electron leakage through heterogeneous LiF on lithium-metal battery anodes.

The solid-electrolyte interphase (SEI) that forms on lithium ion battery (LIB) anodes prevents degradation-causing transfer of electrons to the electrolyte. Grain boundaries (GBs) between different SEI components, like LiF, have been suggested to accelerate Li+ transport. However, using the non-equilibrium Green's function technique with density functional theory (NEGF-DFT), we find that GBs enhance electron tunneling in thin LiF films by 1-2 orders of magnitude, depending on the bias. Extrapolating to thicker films using the Wentzel-Kramers-Brillouin (WKB) method emphasizes that safer batteries require passivation of GBs in the SEI.

Correlated Energy-Level Alignment Effects Determine Substituent-Tuned Single-Molecule Conductance.

The rational design of single-molecule electrical components requires a deep and predictive understanding of structure-function relationships. Here, we explore the relationship between chemical substituents and the conductance of metal-single-molecule-metal junctions, using functionalized oligophenylenevinylenes as a model system. Using a combination of mechanically controlled break-junction experiments and various levels of theory including non-equilibrium Green's functions, we demonstrate that the connection between gas-phase molecular electronic structure and in-junction molecular conductance is complicated by the involvement of multiple mutually correlated and opposing effects that contribute to energy-level alignment in the junction. We propose that these opposing correlations represent powerful new "design principles" because their physical origins make them broadly applicable, and they are capable of predicting the direction and relative magnitude of observed conductance trends. In particular, we show that they are consistent with the observed conductance variability not just within our own experimental results but also within disparate molecular series reported in the literature and, crucially, with the trend in variability across these molecular series, which previous simple models fail to explain. The design principles introduced here can therefore aid in both screening and suggesting novel design strategies for maximizing conductance tunability in single-molecule systems.

Ab initio investigation of the temperature-dependent elastic properties of Bi, Te and Cu.

Using density functional theory and ab initio molecular dynamics, we have investigated the elastic properties of Bi, Te and Cu as a function of temperature. We compare calculated quantities which can be used to determine the effectiveness of our proposed method, such as the bulk (K), shear (G), and Young's (E) moduli. We also computed Poisson's ratio (ν) and the Pugh ratio (γ) for each of these materials at different temperatures to investigate changes in ductility. We have used the elastic moduli to calculate the Debye temperature θ D and minimum thermal conductivity k min of these materials as a function of temperature. We found that the elastic properties calculated in this work are in good agreement with experimental work. The inclusion of temperature effects has allowed for the proper prediction of ductility for each of these materials, a feat that standard density functional theory calculations has previously been unable to accomplish for Bi and Te.

Reassessing destructive quantum interference in azulene-based devices.

Quantum interference (QI) effects have recently attracted increased interest in electron transport studies of single molecular junctions. Although QI effects have been explained in a variety of molecular devices by different chemical rules, such as orbital-based prediction, the graphical scheme, and cross-conjugated states, recently, experimental and theoretical reports have claimed to have reached a better understanding of QI features. In particular, azulene molecule derivatives present an insightful case study where these simple rules of thumb can fail. Here, we explore the validity of graphical rules and the effects of closed loops in the azulene molecular structure. The electron transport behavior through an azulene core with different moieties (thiol, ethynyl-thiol, phenyl-thiol, and ethynyl-phenyl-thiol) was investigated with first-principles calculations combined with the non-equilibrium Green's function (NEGF) technique. The transmission spectra at zero bias show that the graphical rules are not sufficient to predict and explain the destructive QI effect in these azulene derivatives. Instead, closed-loop diagrams should be taken into account to properly describe the transport properties in those systems, but the presence of a closed-loop does not necessarily lead to the absence of destructive QI in the transmission spectrum. Our results indicate that the destructive QI effect is found when the azulene core is coupled at the 4,7Az-, 5,7Az- and 1,3Az-positions with ethynyl-phenyl-thiol moieties, while no obvious destructive QI effect is observed in the other azulene derivatives, either with the thiol, ethynyl-thiol or phenyl-thiol anchoring groups. We also demonstrated that the I-V curves depend more strongly on anchoring groups than the coupling position.

First principles study of graphene on metals with the SCAN and SCAN+rVV10 functionals.

Integrating graphene into electronic devices requires support by a substrate and contact with metal electrodes. Ab initio calculations at the level of density functional theory are performed on graphene-fcc-metal(111) [Gr/M(111)] (M = Ni, Cu, Au) systems. The strongly constrained and appropriately normed (SCAN) and SCAN with the revised Vydrov-van Voorhis (SCAN+rVV10) functionals are relatively new approximations to the exchange-correlation (xc) energy shown to account for van der Waals (vdW) interactions which many non-empirical semi-local functionals fail to include. Binding energies and distances as well as electronic band structures are calculated with SCAN, SCAN+rVV10, Perdew-Burke-Ernzerhof (PBE), and PBE-D3 with and without Becke-Johnson damping, Bayesian error estimation functional with van der Waals correlation (BEEF-vdW), and optB86b-vdW. SCAN and SCAN+rVV10 succeed in describing chemisorption and physisorption in the Gr/Ni(111) system and physisorption in the Gr/Cu(111) and Gr/Au(111) systems. Incorrectly, the physisorption is found to be more favorable than chemisorption in the Gr/Ni(111) system with SCAN, but the result is reversed when the experimental bulk Ni lattice parameter is used as opposed to the SCAN calculated lattice parameter. The SCAN+rVV10 functional produces binding energies and distances comparable to those calculated using the random phase approximation as well as the experiment. The SCAN based functionals produce the highest spin magnetic moments in the bulk Ni and Gr/Ni(111) systems compared to the rest of the functionals investigated, overestimating the experiment by at least ∼0.18 µB. Also, in contrast to the rest of the functionals, the induced spin magnetic moment in graphene is found to be larger in magnitude in the physisorption region than the chemisorption region. The pristine graphene band structure is preserved in the physisorbed systems but with a shift in the Dirac point away from the Fermi energy causing graphene to become n-doped in the Gr/Cu(111) system and p-doped in the Gr/Au(111) system. Chemisorption occurs in the Gr/Ni(111) system where carbon pz states mix with the nickel d states causing a gap to form at the K point, destroying the Dirac point and conical dispersion.

Ethylene Carbonate-Based Electrolyte Decomposition and Solid-Electrolyte Interphase Formation on Ca Metal Anodes.

The formation of a solid-electrolyte interphase (SEI) in multivalent ion batteries, resulting from the decomposition of organic solvents at the anode interface, is a major bottleneck to their development as it prevents ionic diffusion and reversible stripping and plating. To gain insight into SEI formation in these systems, we investigate the decomposition of pure ethylene carbonate (EC) and an EC/Ca(ClO4)2 electrolyte on a Ca metal surface using density functional theory and ab initio molecular dynamics calculations. We first find that CaCO3, CaO, and Ca(OH)2 are all primary inorganic SEI components. We then investigate the reaction mechanisms of this decomposition, finding that although a fast two-electron reduction producing CO32- and C2H4 is thermodynamically and kinetically favorable, a reaction producing C2H4O22- and CO dominates when multiple EC molecules are considered. Finally, we find similar results upon the inclusion of Ca(ClO4)2 salt.

Ba4B8TeO19: A UV Nonlinear Optical Material.

We report a new noncentrosymmetric barium tellurium borate, Ba4B8TeO19 that has potential ultraviolet (UV) nonlinear optical (NLO) applications. Ba4B8TeO19 was synthesized by a flux method and crystallizes in the noncentrosymmetric space group Cc. The material exhibits a framework structure of [B8O17]∞ double layers connected to distorted TeO6 octahedra. Second harmonic generation (SHG) measurements at 1064 and 532 nm on polycrystalline Ba4B8TeO19 indicate that the title compound is phase-matchable (type I) with a moderate SHG response (1 × KH2PO4 at 1064 nm and 0.2 × ß-BaB2O4 at 532 nm). In addition, a short absorption edge (210 nm) was measured. Using density functional theory calculations, we show that the SHG response originates from contributions from O 2p and Te 5s states at the valence and conduction band edges. Finally, by computing the linear optical properties, we find that this compound displays a moderate birefringence of 0.055 at 1064 nm and 0.059 at 532 nm, necessary conditions for phase-matching in UV NLO materials.

Hybrid density functional theory modeling of Ca, Zn, and Al ion batteries using the Chevrel phase Mo6S8 cathode.

Hybrid density functional theory (DFT) is used to study the Chevrel phase Mo6X8 (X = S, Se, Te) as a promising cathode material intercalated with various metal ions (M = Li, Na, Be, Mg, Ca, Sr, Ba, Zn, Al). Electronic properties and voltages are calculated for each case. Ca ions are predicted to produce a voltage output ranging from 1.8-2.1 V, comparable to the voltage calculated for Li ions while providing two electrons per transferred ion. The highest voltage is determined to result when the chalcogen X in Mo6X8 is S, over Se or Te. Additionally, a comparison of the local-density approximation (LDA), the Perdew-Burke-Ernzerhof (PBE), the Hubbard U corrected GGA-PBE (PBE+U), the meta-GGA modified Becke-Johnson (mBJ), and the hybrid Heyd-Scuseria-Ernzerhof (HSE) functionals are made. The electronic structure determined with HSE is taken as the most reliable, and PBE and LDA can provide reasonable approximations. The PBE+U approach yields an erroneous band gap and should be avoided. The voltages calculated with HSE are in excellent agreement with available experimental data.

Towards graphyne molecular electronics.

α-Graphyne, a carbon-expanded version of graphene ('carbo-graphene') that was recently evidenced as an alternative zero-gap semiconductor, remains a theoretical material. Nevertheless, using specific synthesis methods, molecular units of α-graphyne ('carbo-benzene' macrocycles) can be inserted between two anilinyl (4-NH2-C6H4)-anchoring groups that allow these fragments to form molecular junctions between gold electrodes. Here, electrical measurements by the scanning tunnelling microscopy (STM) break junction technique and electron transport calculations are carried out on such a carbo-benzene, providing unprecedented single molecule conductance values: 106 nS through a 1.94-nm N-N distance, essentially 10 times the conductance of a shorter nanographenic hexabenzocoronene analogue. Deleting a C4 edge of the rigid C18 carbo-benzene circuit results in a flexible 'carbo-butadiene' molecule that has a conductance 40 times lower. Furthermore, carbo-benzene junctions exhibit field-effect transistor behaviour when an electrochemical gate potential is applied, opening the way for device applications. All the results are interpreted on the basis of theoretical calculations.