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Recent Submissions

  • Effective Doping of Square/Octagon-Phase Arsenene by Adsorption of Organic Molecules

    Zhao, Ning; Schwingenschlögl, Udo (Advanced Theory and Simulations, Wiley, 2021-03-27) [Article]
    Adsorption of organic molecules can be a better choice than traditional chemical doping to achieve effective doping of 2D materials. The adsorption of organic molecules on square/octagon-phase (S/O-phase) arsenene is investigated and promoted the possibility of developing p–n junctions. In particular, it is found that adsorption of F4-TCNQ or TCNE molecules leads to effective p-doping. Adsorption of DMPD or TTF molecules leads to less effective n-doping. Interestingly, in the case of TTF adsorption strain engineering can be used to greatly improve the material properties. Therefore, both effective n- and p-doping of S/O-phase arsenene can be realized.
  • Two-Dimensional Tetrahex-GeC2: A Material with Tunable Electronic and Optical Properties Combined with Ultrahigh Carrier Mobility

    Zhang, Wei; Chai, Changchun; Fan, Qingyang; Sun, Minglei; Song, Yanxing; Yang, Yintang; Schwingenschlögl, Udo (ACS Applied Materials & Interfaces, American Chemical Society (ACS), 2021-03-19) [Article]
    Based on first-principles calculations, we propose a novel two-dimensional (2D) germanium carbide, tetrahex-GeC<sub>2</sub>, and determine its electronic and optical properties. Each Ge atom binds to four C atoms, in contrast to the known 2D hexagonal germanium carbides. Monolayer tetrahex-GeC<sub>2</sub> possesses a narrow direct band gap of 0.89 eV, which can be effectively tuned by applying strain and increasing the thickness. Its electron mobility is extraordinarily high (9.5 × 10<sup>4</sup> cm<sup>2</sup>/(V s)), about 80 times that of monolayer black phosphorus. The optical absorption coefficient is ∼10<sup>6</sup> cm<sup>-1</sup> in a wide spectral range from near-infrared to near-ultraviolet, comparable to perovskite solar cell materials. We obtain high cohesive energy (5.50 eV/atom), excellent stability, and small electron/hole effective mass (0.19/0.10 <i>m</i><sub>0</sub>). Tetrahex-GeC<sub>2</sub> turns out to be a very promising semiconductor for nanoelectronic, optoelectronic, and photovoltaic applications.
  • M2X Monolayers as Anode Materials for Li Ion Batteries

    Samad, Abdus; Schwingenschlögl, Udo (Physical Review Applied, American Physical Society (APS), 2021-03-09) [Article]
    Electrochemically efficient electrode materials are required for clean energy storage in Li ion batteries. We predict two-dimensional hexagonal metal nitrides, borides, and phosphides (Sc2B, Sc2N, Y2B, Y2N, and Y2P) and evaluate the feasibility of experimental realization. The materials combine excellent metallicity, as required for electrodes, with Li binding energies providing high storage capacity and a low average open-circuit voltage. In contrast to two-dimensional silicene, borophene, and SnS2, we observe negligible structural distortions during Li adsorption and extraction, which results in high reversibility and a long cycle life. Superionic Li diffusion enables fast charge or discharge of next-generation Li ion batteries.
  • Anisotropic Janus SiP2 Monolayer as a Photocatalyst for Water Splitting

    Yu, Tong; Wang, Cong; Yan, Xu; Yang, Guochun; Schwingenschlögl, Udo (The Journal of Physical Chemistry Letters, American Chemical Society (ACS), 2021-03-04) [Article]
    The design of materials meeting the rigorous requirements of photocatalytic water splitting is still a challenge. Anisotropic Janus 2D materials exhibit great potential due to outstandingly high photocatalytic efficiency. Unfortunately, these materials are scarce. By means of ab initio swarm-intelligence search calculations, we identify a SiP<sub>2</sub> monolayer with Janus structure (i.e., out-of-plane asymmetry). The material turns out to be semiconducting with an indirect band gap of 2.39 eV enclosing the redox potentials of water. Notably, the oxygen and hydrogen evolution half reactions can happen simultaneously at the Si and P atoms, respectively, driven merely by the radiation-induced electrons and holes. The carrier mobility is found to be anisotropic and high, up to 10<sup>-4</sup> cm<sup>2</sup> V<sup>-1</sup> s<sup>-1</sup>, facilitating fast transport of the photogenerated carriers. The SiP<sub>2</sub> monolayer shows remarkably strong optical absorption in the visible-to-ultraviolet range of the solar spectrum, ensuring efficient utilization of the solar energy.
  • Structure Prototype Outperforming MXenes in Stability and Performance in Metal-Ion Batteries: A High Throughput Study

    Sun, Minglei; Schwingenschlögl, Udo (Advanced Energy Materials, Wiley, 2021-03-03) [Article]
    While the MXene Ti3C2 is well known for its extraordinary material properties with wide applications, it is demonstrated here that it is not the most stable 2D titanium carbide. Evolutionary search and first-principles calculations are employed to predict for Ti3C3 a novel structure prototype with P4/mmm symmetry and tetragonal sandwich structure. The cohesive energy, phonon dispersion, and melting point demonstrate high stability of Ti3C3. The mechanical properties are found to be even better than those of graphene in terms of Young's modulus and fracture strength. The metallicity of Ti3C3 indicates potential in metal-ion batteries. The diffusion barriers for Li, Na, K, and Ca atoms are found to be as low as 0.15, 0.04, 0.002 (record among the known 2D materials), and 0.14 eV, respectively, suggesting the possibility to realize fast charge and discharge. Importantly, the discovered structure prototype gives rise to a whole family of 2D materials. For example, six thermally and dynamically stable materials with metallic properties, Ti3X3 (X = B, Si, Ge, N, P, and As) are identified. The family is promising not only in the fields of nano-mechanics and metal-ion batteries but also can guide the search for further 2D structure prototypes.
  • Multivalley Band Structure and Phonon-Glass Behavior of TlAgTe

    Shafique, Aamir; Sharma, Sitansh; Sajjad, Muhammad; Schwingenschlögl, Udo (ACS Applied Energy Materials, American Chemical Society (ACS), 2021-02-17) [Article]
    We show that the extraordinary crystal structure of TlAgTe results in a phonon-glass electron-crystal behavior. The material’s electronic transport properties are evaluated by first-principles calculations and Boltzmann transport theory. We find a multivalley conduction band (n-doping) and low dispersion at the valence band edge (p-doping), which results in a high power factor. Since the mean free path of a large number of phonon modes is found to fall below the Ioffe–Regel limit, semiclassical Boltzmann transport theory cannot describe the phonon transport, but a two-channel model must be applied. The lattice thermal conductivity turns out to be as low as 0.43 W m–1 K–1 because of strong lattice anharmonicity (originating from Tl 6s2 lone pairs) and low group velocities (originating from loose bonding of the Tl atoms), which renders TlAgTe to be a highly promising thermoelectric material.
  • Confined variational calculation of positronium-hydrogen scattering below the positronium excitation threshold

    Wu, M. S.; Zhang, Junyi; Qian, Y.; Varga, K.; Schwingenschlögl, Udo; Yan, Z.-C. (Physical Review A, American Physical Society (APS), 2021-02-15) [Article]
    The confined variational method is used to study S-, P-, and D-wave positronium-hydrogen scattering with the scattering energy between 0.068 and 3.333 eV. Accurate phase shifts and S-wave scattering lengths are calculated and compared with other theoretical methods. Existing discrepancies for the D-wave phase shifts are resolved. According to collision momentum, total orbital angular momentum, and spin configuration, the distortion effects of positronium are quantitatively studied. Finally, the finite nuclear mass effects are investigated.
  • Unique Omnidirectional Negative Poisson’s Ratio in δ-Phase Carbon Monochalcogenides

    Sun, Minglei; Schwingenschlögl, Udo (The Journal of Physical Chemistry C, American Chemical Society (ACS), 2021-02-12) [Article]
    Auxetic materials (negative Poisson’s ratio) are of exceptional importance for nanomechanical applications. Using first-principles calculations, we propose two-dimensional δ-phase carbon monochalcogenides (δ-CS, δ-CSe, and δ-CTe) with very strong auxeticity. In contrast to the known two-dimensional materials, we find that Poisson’s ratio is negative in all crystal directions. In addition, the δ-phase carbon monochalcogenides turn out to be direct or quasi-direct bandgap semiconductors with impressive absorption of solar radiation.
  • Strain-attenuated spin frustration in double perovskite Sr2FeOsO6

    Rout, Paresh Chandra; Schwingenschlögl, Udo (Physical Review B, American Physical Society (APS), 2021-01-15) [Article]
    Using density functional theory together with Monte Carlo simulations, we demonstrate that epitaxial strain, both compressive and tensile, attenuates the spin frustration of double perovskite Sr2FeOsO6 to significantly enhance the critical temperature to 310 K, enabling room-temperature applications. We discover under tensile strain a tetragonal (I4/m)-to-monoclinic (P21/n) structural transition concomitant with an antiferromagnetic-to-ferrimagnetic transition. Furthermore, an indirect-to-direct band gap transition is observed with the valence and conduction states localized on different transition metal sublattices, opening a route to efficient electron-hole separation upon photoexcitation.
  • Scaling-up perovskite solar cells on hydrophobic surfaces

    Isikgor, Furkan Halis; Subbiah, Anand Selvin; Eswaran, Mathan Kumar; Howells, Calvyn Travis; Babayigit, Aslihan; de Bastiani, Michele; Yengel, Emre; Liu, Jiang; Furlan, Francesco; Harrison, George T.; Zhumagali, Shynggys; Khan, Jafar Iqbal; Laquai, Frédéric; Anthopoulos, Thomas D.; McCulloch, Iain; Schwingenschlögl, Udo; De Wolf, Stefaan (Nano Energy, Elsevier BV, 2020-11-25) [Article]
    Despite impressive power conversion efficiencies (PCEs) reported for lab-scale perovskite solar cells (PSCs), obtaining large-area devices with similar performance remains challenging. Fundamentally, this can largely be attributed to a polarity mismatch between the perovskite-precursor solution and the underlying hydrophobic contact materials, resulting in perovskite films of insufficient quality for scaled devices. Specifically, for p-i-n devices, the commonly used DMF/DMSO co-solvent has a significant polarity mismatch with its underlying hole-transporting layer, PTAA. Here, the role of MAPbI3•solvent adduct interaction with the PTAA surface towards the formation of micro- and nano-scale pinholes is elucidated in detail. Replacing DMSO with NMP in the co-solvent system changes the binding energy profoundly, enabling uniform and dense films over large areas. The PCE of DMF/NMP ink-based devices drops slightly with increasing active device area, from 21.5% (0.1 cm2) to 19.8% (6.8 cm2), in comparison with conventional DMF/DMSO ink. This work opens a pathway towards the scalability of solution-processed perovskite optoelectronic devices.
  • MXene-Modulated Electrode/SnO2 Interface Boosting Charge Transport in Perovskite Solar Cells

    Wang, Yunfan; Xiang, Pan; Ren, Aobo; Lai, Huagui; Zhang, Zhuoqiong; Xuan, Zhipeng; Wan, Zhenxi; Zhang, Jingquan; Hao, Xia; Wu, Lili; Sugiyama, Masakazu; Schwingenschlögl, Udo; Liu, Cai; Tang, Zeguo; Wu, Jiang; Wang, Zhiming; Zhao, Dewei (ACS Applied Materials & Interfaces, American Chemical Society (ACS), 2020-11-17) [Article]
    Interface engineering is imperative to boost the extraction capability in perovskite solar cells (PSCs). We propose a promising approach to enhance the electron mobility and charge transfer ability of tin oxide (SnO2) electron transport layer (ETL) by introducing a two-dimensional carbide (MXene) with strong interface interaction. The MXene-modified SnO2 ETL also offers a preferable growth platform for perovskite films with reduced trap density. Through a spatially resolved imaging technique, profoundly reduced non-radiative recombination and charge transport losses in PSCs based on MXene-modified SnO2 are also observed. As a result, the PSC achieves an enhanced efficiency of 20.65% with ultralow saturated current density and negligible hysteresis. We provide an in-depth mechanistic understanding of MXene interface engineering, offering an alternative approach to obtain efficient PSCs.
  • Cyclized polyacrylonitrile anode for alkali metal ion batteries

    Zhang, Wenli; Sun, Minglei; Yin, Jian; Abou-Hamad, Edy; Schwingenschlögl, Udo; Da Costa, Pedro M. F. J.; Alshareef, Husam N. (Angewandte Chemie International Edition, Wiley, 2020-11-16) [Article]
    Alkali metal (Li, Na, and K) ion batteries are vital in portable and large-scale stationary energy storage. Recently, organic anodes have attracted increasing attention for alkali metal ion batteries due to their chemical diversity and potential high capacity. In this work, we discovered that cyclized polyacrylonitrile (cPAN) can serve as a superior anode for alkali metal ion batteries. Remarkably, upon activation cycling, as an anode of lithium-ion battery, cPAN exhibits a reversible capacity as high as 1238 mAh g-1 under a current density of 50 mA g-1. Based on electrochemical experiments and first-principles calculations, it is demonstrated that the hexagonal carbon ring, piperidine ring, and pyridine nitrogen in ladder cPAN are the main active sites for lithium-ion storage. In addition, we show that cPAN displays a unique potential-dependent solid electrolyte interphase formation from 0.1 to 0.01 V vs. Li/Li+. Furthermore, cPAN displays decent performance as an anode in SIBs and PIBs. The discovery of cPAN anode could pave the way for the future development of organic anodes for alkali metal ion batteries.
  • Graphene origami structures with superflexibility and highly tunable auxeticity

    Ho, Duc Tam; Kim, Sung Youb; Schwingenschlögl, Udo (Physical Review B, American Physical Society (APS), 2020-11-11) [Article]
    The two-dimensional structure of graphene makes it difficult to realize flexibility and auxeticity (negative Poisson’s ratio) in graphene-based structures. Using molecular dynamics simulations, we demonstrate for graphene origami structures effective tuning of both the flexibility and Poisson’s ratio through the geometry, including the potential to combine superflexibility with a highly tunable negative Poisson’s ratio in contrast to any existing graphene-based structure. Auxeticity even can be achieved under large applied strain, both tensile and compressive.
  • Monolayer Ag2S: Ultralow Lattice Thermal Conductivity and Excellent Thermoelectric Performance

    Sharma, Sitansh; Shafique, Aamir; Schwingenschlögl, Udo (ACS Applied Energy Materials, American Chemical Society (ACS), 2020-10-15) [Article]
    For efficient thermoelectric materials, high power factor and low lattice thermal conductivity are desired properties. Therefore, the high lattice thermal conductivity of two-dimensional materials limits their usage in thermoelectric applications. We employ first-principles calculations along with semiclassical Boltzmann transport theory for the electron and phonon dynamics to investigate the thermoelectric properties of nonmetal-shrouded monolayer Ag2S. We show that the simultaneous presence of flat and dispersive bands in the vicinity of the conduction band edge leads to a high power factor, while close proximity of the acoustic and optical bands in the phonon dispersion results in low thermal conductivity. With moderate electron doping, a high in-plane thermoelectric figure of merit is achieved. Our results demonstrate great potential of nonmetal-shrouded monolayer Ag2S in thermoelectric applications.
  • Beryllene: A Promising Anode Material for Na- and K-Ion Batteries with Ultrafast Charge/Discharge and High Specific Capacity

    Sun, Minglei; Yan, Yuan; Schwingenschlögl, Udo (The Journal of Physical Chemistry Letters, American Chemical Society (ACS), 2020-10-12) [Article]
    We predict two-dimensional Be materials, α- and β-beryllene. In α-beryllene each Be atom binds to six other Be atoms in a planar scheme, whereas β-beryllene consists of two stacked α-beryllene monolayers. Both α- and β-beryllene are found to be highly stable, as demonstrated by high cohesive energies close to that of bulk Be, an absence of imaginary phonon modes, and high melting points. Both materials are metallic, indicating potential applications in Na-ion and K-ion batteries, which are explored in detail. The diffusion barriers of Na (K) on α- and β-beryllene are found to be only 9 (3) and 4 (5) meV, respectively. In particular, the diffusion barrier of K on α-beryllene exhibits the lowest ever recorded value in two-dimensional materials, suggesting the possibility of ultrafast charge/discharge. As the theoretical specific capacities of Na/K on α- and β-beryllene are found to be 1487/1322 and 743/743 mA h g–1, respectively, the storage capacity is ultrahigh.
  • Quantum dots in AA-stacked bilayer graphene

    Qasem, H. S.; Abdullah, H. M.; Shukri, M. A.; Bahlouli, H.; Schwingenschlögl, Udo (Physical Review B, American Physical Society (APS), 2020-08-13) [Article]
    While electrostatic confinement in single-layer graphene and AA-stacked bilayer graphene is precluded by Klein tunneling and the gapless energy spectrum, we theoretically show that a circular domain wall that separates domains of single-layer graphene and AA-stacked bilayer graphene can provide bound states. Solving the Dirac-Weyl equation in the presence of a global mass potential and a local electrostatic potential, we obtain the energy spectrum of these states and the corresponding radial probability densities. Depending on the mass potential profile, regular bound states can exist inside the quantum dot and topological bound states at the domain wall. Controlling the electrostatic potential inside the quantum dot enables the simultaneous presence of both types of states. We find that the number of nodes of the radial wave function of the regular bound states inside the quantum dot is equal to the radial quantum number. The energy spectra of the bound states display anticrossings, reflecting coupling of electron- A nd holelike states.
  • Flexible C6BN Monolayers As Promising Anode Materials for High-Performance K-Ion Batteries

    Xiang, Pan; Sharma, Sitansh; Wang, Zhiming M.; Wu, Jiang; Schwingenschlögl, Udo (ACS Applied Materials & Interfaces, American Chemical Society (ACS), 2020-06-25) [Article]
    K-ion batteries attract extensive attention and research efforts because of the high energy density, low cost, and high abundance of K. Although they are considered suitable alternatives to Li-ion batteries, the absence of high-performance electrode materials is a major obstacle to implementation. On the basis of density functional theory, we systematically study the feasibility of a recently synthesized C6BN monolayer as anode material for K-ion batteries. The specific capacity is calculated to be 553 mAh/g (K2C6BN), i.e., about twice that of graphite. The C6BN monolayer is characterized by high strength (in-plane stiffness of 309 N/m), excellent flexibility (bending strength of 1.30 eV), low output voltage (average open circuit voltage of 0.16 V), and excellent rate performance (diffusion barrier of 0.09 eV). We also propose two new C6BN monolayers. One has a slightly higher total energy (0.10 eV) than the synthesized C6BN monolayer, exhibiting enhanced electronic properties and affinity to K. The other is even energetically favorable due to B-N bonding. All three C6BN monolayers show good dynamical, thermal, and mechanical stabilities. We demonstrate excellent cyclability and improved conductivity by K adsorption, suggesting great potential in flexible energy-storage devices.
  • Graphene Origami with Highly Tunable Coefficient of Thermal Expansion

    Ho, Duc Tam; Park, Harold S.; Kim, Sung Youb; Schwingenschlögl, Udo (ACS Nano, American Chemical Society (ACS), 2020-06-15) [Article]
    The coefficient of thermal expansion, which measures the change in length, area, or volume of a material upon heating, is a fundamental parameter with great relevance for many applications. Although there are various routes to design materials with targeted coefficient of thermal expansion at the macroscale, no approaches exist to achieve a wide range of values in graphene-based structures. Here, we use molecular dynamics simulations to show that graphene origami structures obtained through pattern-based surface functionalization provide tunable coefficients of thermal expansion from large negative to large positive. We show that the mechanisms giving rise to this property are exclusive to graphene origami structures, emerging from a combination of surface functionalization, large out-of-plane thermal fluctuations, and the three-dimensional geometry of origami structures.
  • Effects of gas adsorption on monolayer Si2BN and implications for sensing applications

    Babar, Vasudeo Pandurang; Murat, Altynbek; Schwingenschlögl, Udo (Journal of Physics: Condensed Matter, IOP Publishing, 2020-06-15) [Article]
    Using density functional theory, we investigate the adsorption behavior of CO, NH3, and NO molecules on monolayer Si2BN. The energetically favorable structural configurations along with their adsorption energies, charge transfers, and electronic properties are discussed. The CO and NH3 molecules show physisorption with moderate adsorption energies, whereas the NO molecule is subject to chemisorption. We further calculate the current–voltage characteristics using the non-equilibrium Green's function formalism. Significant anisotropy is observed for the armchair and zigzag directions, consistent with the anisotropy of the electronic band structure. Pronounced enhancement of the resistivity upon gas adsorption indicates that monolayer Si2BN is promising as gas sensing material.
  • Tunable magnetic anisotropy in Cr–trihalide Janus monolayers

    Albaridy, Rehab; Manchon, Aurelien; Schwingenschlögl, Udo (Journal of Physics: Condensed Matter, IOP Publishing, 2020-05-29) [Article]
    Achieving a two-dimensional material with tunable magnetic anisotropy is highly desirable, especially if it is complemented with out-of-plane electric polarization, as this could provide a versatile platform for spintronic and multifunctional devices. Using first principles calculations, we demonstrate that the magnetic anisotropy of Cr-trihalides become highly sensitive to mechanical strain upon structural inversion symmetry breaking through the realization of Janus monolayers. This remarkable feature, absent in pristine Cr-trihalide monolayers, enables mechanical control of the direction of the easy axis: biaxial compressive/tensile strain supports in-plane/out-of-plane orientation of the easy axis. The magnetic exchange itself shows higher sensitivity to compressive than to tensile strain, while in general the Janus monolayers maintain ferromagnetic ordering in the studied range of strain.

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