### Recent Submissions

• #### MXene-Modulated Electrode/SnO2 Interface Boosting Charge Transport in Perovskite Solar Cells

(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

(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

(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

(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

(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

(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.
• #### Janus Monolayers of Magnetic Transition Metal Dichalcogenides as an All-in-One Platform for Spin-Orbit Torque

(arXiv, 2020-07-15) [Preprint]
We theoretically predict that vanadium-based Janus dichalcogenide monolayers constitute an ideal platform for spin-orbit-torque memories. Using first principles calculations, we demonstrate that magnetic exchange and magnetic anisotropy energies are higher for heavier chalcogen atoms, while the broken inversion symmetry in the Janus form leads to the emergence of Rashba-like spin-orbit coupling. The spin-orbit torque efficiency is evaluated using optimized quantum transport methodology and found to be comparable to heavy nonmagnetic metals. The coexistence of magnetism and spin-orbit coupling in such materials with tunable Fermi-level opens new possibilities for monitoring magnetization dynamics in the perspective of non-volatile magnetic random access memories.
• #### Flexible C6BN Monolayers As Promising Anode Materials for High-Performance K-Ion Batteries

(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

(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

(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

(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.
• #### B2P6: A Two-Dimensional Anisotropic Janus Material with Potential in Photocatalytic Water Splitting and Metal-Ion Batteries

(Chemistry of Materials, American Chemical Society (ACS), 2020-05-27) [Article]
Both anisotropic and Janus two-dimensional materials are known for extraordinary properties. We predict a two-dimensional material, B2P6, which combines anisotropy with the Janus geometry, based on evolutionary search and first-principles calculations. High stability of the material is demonstrated in terms of the cohesive energy, phonon spectrum, and melting point. B2P6 turns out to be an indirect band gap semiconductor with anisotropic electronic transport and strong absorption of solar radiation. Importantly, its Janus structure results in an intrinsic electric field, which significantly suppresses the recombination of photogenerated carriers. We demonstrate high efficiency of photocatalytic water splitting with a solar-to-hydrogen efficiency of 28.2%, by far in excess of the conventional theoretical limit of 18%. The structural anisotropy is found to be promising for application in metal-ion batteries. We observe directional diffusion with Li, Na, and K diffusion barriers of only 0.07, 0.04, and 0.03 eV, respectively, suggesting ultrafast charge/discharge characteristics.
• #### Direct Pyrolysis of Supermolecules: An Ultrahigh Edge-Nitrogen Doping Strategy of Carbon Anodes for Potassium-Ion Batteries

Most reported carbonaceous anodes of potassium-ion batteries (PIBs) have limited capacities. One approach to improve the performance of carbon anodes is edge-nitrogen doping, which effectively enhances the K-ion adsorption energy. It remains challenging to achieve high edge-nitrogen doping due to the difficulty in controlling the nitrogen dopant configuration. Herein, a new synthesis strategy is proposed to prepare carbon anodes with ultrahigh edge-nitrogen doping for high-performance PIBs. Specifically, self-assembled supermolecule precursors derived from pyromellitic acid and melamine are directly pyrolyzed. During the pyrolysis process, the amidation and imidization reactions between pyromellitic acid and melamine before carbonization enable the successful carbonization of pyromellitic acid-melamine supermolecule. The obtained 3D nitrogen-doped turbostratic carbon (3D-NTC) possesses a 3D framework composed of carbon nanosheets, turbostratic crystalline structure, and an ultrahigh edge-nitrogen-doping level up to 16.8 at% (73.7% of total 22.8 at% nitrogen doping). These features endow 3D-NTCs with remarkable performances as PIB anodes. The 3D-NTC anode displays a high capacity of 473 mAh g-1 , robust rate capability, and a long cycle life of 500 cycles with a high capacity retention of 93.1%. This new strategy will boost the development of carbon anodes for rechargeable alkali-metal-ion batteries.
• #### Finely Tuned Submicroporous Thin-Film Molecular Sieve Membranes for Highly Efficient Fluid Separations

Polymeric membranes with increasingly high permselective performances are gaining a significant role in lowering the energy burden and improving the environmental sustainability of complex chemical separations. However, the commercial deployment of newly designed materials with promising intrinsic properties for fluid separations has been stalled by challenges associated with fabrication and scale up of low-cost, high-performance, defect-free thin-film composite (TFC) membranes. Here, a facile method to fabricate next-generation TFC membranes using a bridged-bicyclic triptycene tetra-acyl chloride (Trip) building block with a large fraction of finely tuned structural submicroporosity (pore size < 4 Å) is demonstrated. The TFCs exhibit superb potential for removal of small (≈200 g mol−1) organic microcontaminants from organic solvent streams by showing both improved rejection and permeance in organic systems compared to current state-of-the-art commercial membranes. The TFCs also display unprecedented properties for desalination applications with performance located far above the current water permeance/sodium chloride rejection trendline. The strategy of using highly contorted triptycene building blocks with well-defined interconnected internal free volume elements establishes a scalable, generalized approach to fabricate highly selective, submicroporous TFC membranes for a wide variety of challenging energy-intensive fluid separations.
• #### Confined variational calculation of o -Ps-He scattering properties

(Physical Review A, American Physical Society (APS), 2020-04-21) [Article]
High-precision quantum-mechanical calculations have been developed to investigate positronium (Ps) scattering. Positronium scattering experiments are a powerful tool to study positronium-matter interactions, but the theoretical description of these experiments needs better accuracy. We have developed an ab initio confined variational approach that can reach higher collision energy, includes higher orbital momenta and uses small confining radii. Calculation of the Ps-He momentum-transfer cross section shows that the experimental Doppler broadening spectroscopy results are questionable. The energy dependence of the pickoff annihilation rate is also calculated, demonstrating an important role of the so far neglected P-wave contribution.
• #### Gas Sensing Performance of Pristine and Monovacant C6BN Monolayers Evaluated by Density Functional Theory and the Nonequilibrium Green’s Function Formalism

(The Journal of Physical Chemistry C, American Chemical Society (ACS), 2020-03-04) [Article]
The application potential of pristine and monovacant C6BN for sensing gaseous pollutants (CO, CO2, NO, NO2, NH3, H2S, and SO2) is investigated using density functional theory with van der Waals dispersion correction. The adsorption sites and distances are determined. In addition to applying widely used theoretical approaches (adsorption energy, charge transfer, and work function) to evaluate gas sensing properties, the current−voltage characteristics are calculated before and after gas adsorption, using the nonequilibrium Green’s function formalism. The reliability of the approaches is analyzed. From a material point of view, we observe that all molecules under investigation physisorb on pristine C6BN. However, it turns out that pristine C6BN cannot be used for sensitive sensing, which we attribute to tiny charge transfers and band gap changes. On the other hand, we find that monovacancies in C6BN improve the adsorption energy and, in turn, enhance the sensitivity
• #### Identification and Resolution of Unphysical Multielectron Excitations in the Real-Time Time-Dependent Kohn-Sham Formulation

(Physical Review Letters, American Physical Society (APS), 2020-01-15) [Article]
We resolve a fundamental issue associated with the conventional Kohn-Sham formulation of real-time time-dependent density functional theory. We show that unphysical multielectron excitations, generated during time propagation of the Kohn-Sham equations due to fixation of the total number of Kohn-Sham orbitals and their occupations, result in incorrect electron density and, therefore, wrong predictions of physical properties. A new formulation is proposed in that the number of Kohn-Sham orbitals and their occupations are updated on the fly, the unphysical multielectron excitations are removed, and the correct electron density is determined. The correctness of the new formulation is demonstrated by simulations of Rabi oscillation, as analytical results are available for comparison in the case of noninteracting electrons.
• #### Complex three-dimensional graphene structures driven by surface functionalization

(Nanoscale, Royal Society of Chemistry (RSC), 2020) [Article]
<p>A self-folding approach inspired by the origami technique is developed to form complex three-dimensional graphene structures using pattern-based surface functionalization.</p>
• #### Molecular engineering of high-performance nanofiltration membranes from intrinsically microporous poly(ether-ether-ketone)

(Journal of Materials Chemistry A, Royal Society of Chemistry, 2020) [Article]
Poly(ether-ether-ketone) has received increased attention due to its high thermal and chemical stability, and high performance in various applications. However, it suffers from semi-crystalline morphology, low fractional free volume, and poor processability, requiring the use of harsh acidic solvents, which leads to undesired sulfonation. In this work, three intrinsically microporous poly(ether-ether-ketone) (iPEEK), incorporating spirobisindane, Tröger’s base, and triptycene contorted structures, were developed for organic solvent nanofiltration. Molecular dynamics simulations have assisted the molecular engineering of the polymers and the understanding of the improved membrane performance through the binding energies between solvents and polymers. Application of the design principles of polymers of intrinsic microporosity has led to a paradigm shift with a notable enhancement in both the polymer properties and the subsequently fabricated nanofiltration membranes’ performance. The iPEEKs showed excellent solution processability, high surface area of 205–250 m2 g-1, and excellent thermal stability. Mechanically flexible nanofiltration membranes were prepared from N-methyl-2-pyrrolidone dope solution at iPEEK concentrations of 19–35 wt%. The molecular weight cutoff of the membranes was fine-tuned in the range of 450–845 g mol-1 displaying 2–6 fold higher permeance (3.57–11.09 L m-2 h-1 bar-1) than previous reports. The long-term stabilities were demonstrated by a 7-day continuous cross-flow filtration.
• #### Mechanism of wettability alteration of the calcite {101̄4} surface

(Physical Chemistry Chemical Physics, Royal Society of Chemistry (RSC), 2020) [Article]
<p>We propose that formation of Na$^{+}$ hydrates plays an important role in the wettability alteration of the calcite {101̄4} surface.</p>