### Recent Submissions

• #### Comment on “Electrical Switch of Poisson’s Ratio in IV–VI Monolayers via Pseudophase Transitions”

(The Journal of Physical Chemistry Letters, American Chemical Society (ACS), 2022-04-28) [Article]
Significant modulation of Poisson's ratio of IV-VI semiconductor monolayers in an electric field was claimed to be discovered by first-principles calculations in The Journal of Physical Chemistry Letters, 2021, 12, 3217-3223. We show that these results are not correct because of improper modeling of the electric field.
• #### Unusual Activity of Rationally Designed Cobalt Phosphide/Oxide Heterostructure Composite for Hydrogen Production in Alkaline Medium

(ACS Nano, American Chemical Society (ACS), 2022-03-07) [Article]
Design and development of an efficient, nonprecious catalyst with structural features and functionality necessary for driving the hydrogen evolution reaction (HER) in an alkaline medium remain a formidable challenge. At the root of the functional limitation is the inability to tune the active catalytic sites while overcoming the poor reaction kinetics observed under basic conditions. Herein, we report a facile approach to enable the selective design of an electrochemically efficient cobalt phosphide oxide composite catalyst on carbon cloth (CoP-CoxOy/CC), with good activity and durability toward HER in alkaline medium (η10= -43 mV). Theoretical studies revealed that the redistribution of electrons at laterally dispersed Co phosphide/oxide interfaces gives rise to a synergistic effect in the heterostructured composite, by which various Co oxide phases initiate the dissociation of the alkaline water molecule. Meanwhile, the highly active CoP further facilitates the adsorption-desorption process of water electrolysis, leading to extremely high HER activity.
• #### Experimental identification of the second-order non-Hermitian skin effect with physics-graph-informed machine learning

(arXiv, 2022-03-01) [Preprint]
Topological phases of matter are conventionally characterized by the bulk-boundary correspondence in Hermitian systems: The topological invariant of the bulk in d dimensions corresponds to the number of (d − 1)-dimensional boundary states. By extension, higherorder topological insulators reveal a bulk-edge-corner correspondence, such that n-th order topological phases feature (d − n)-dimensional boundary states. The advent of nonHermitian topological systems sheds new light on the emergence of the non-Hermitian skin effect (NHSE) with an extensive number of boundary modes under open boundary conditions. Still, the higher-order NHSE remains largely unexplored, particularly in the experiment. We introduce an unsupervised approach – physics-graph-informed machine learning (PGIML) – to enhance the data mining ability of machine learning with limited domain knowledge. Through PGIML, we experimentally demonstrate the second-order NHSE in a two-dimensional non-Hermitian topolectrical circuit. The admittance spectra of the circuit exhibit an extensive number of corner skin modes and extreme sensitivity of the spectral flow to the boundary conditions. The violation of the conventional bulk-boundary correspondence in the second-order NHSE implies that modification of the topological band theory is inevitable in higher dimensional non-Hermitian systems.
• #### Damp heat-stable perovskite solar cells with tailored-dimensionality 2D/3D heterojunctions.

(Science (New York, N.Y.), American Association for the Advancement of Science (AAAS), 2022-02-17) [Article]
If perovskite solar cells (PSCs) with high power conversion efficiencies (PCEs) are to be commercialized, they must achieve long-term stability, which is usually assessed with accelerated degradation tests. One of the persistent obstacles for PSCs has been successfully passing the damp-heat test (85°C and 85% relative humidity), which is the standard for verifying the stability of commercial photovoltaic (PV) modules. We fabricated damp heat-stable PSCs by tailoring the dimensional fragments of two-dimensional perovskite layers formed at room temperature with oleylammonium iodide molecules; these layers passivate the perovskite surface at the electron-selective contact. The resulting inverted PSCs deliver a 24.3% PCE and retain >95% of their initial value after >1000 hours at damp-heat test conditions, thereby meeting one of the critical industrial stability standards for PV modules.
• #### BC6P Monolayer: Isostructural and Isoelectronic Analogues of Graphene with Desirable Properties for K-Ion Batteries

(Chemistry of Materials, American Chemical Society (ACS), 2021-12-03) [Article]
K-ion batteries are interesting alternatives to Li-ion batteries because of the earth-abundance of K and the similar chemistry between K and Li. However, a lack of high-performance anode materials is a major obstacle to the development of K-ion batteries. We show that the BC6P monolayer, which is isostructural and isoelectronic to graphene due to charge compensation between the constituent elements, can fill this gap. The capacity is found to be 1410 mAh/g (BC6PK6), i.e., about four times that of graphite. The diffusion barrier is as low as 0.13 eV and the average open-circuit voltage is as low as 0.35 V, ensuring high rate performance and high safety, respectively. Metallic states induced by K adsorption provide electrical conductivity during the battery cycle.
• #### Structure of monolayer 2$H$−TaS$_2$ on Au(111)

(Physical Review B, American Physical Society (APS), 2021-11-09) [Article]
We determined the structure of epitaxial 2H-TaS2 on Au(111) using the method of x-ray standing waves (XSW), supported by density functional theory (DFT) calculations and scanning tunneling microscopy (STM). The lattice mismatch between substrate and overlayer gives rise to a moiré superstructure, which modulates the structural and electronic properties. For a specific registry (S atoms directly above Au substrate atoms), local covalentlike bonds form, whereas globally weak van der Waals bonding prevails. Still, the TaS2 layer remains rather flat. Significant charge transfer from Au(111) into the conduction band of the two-dimensional material is found.
• #### Inducing Half-Metallicity in Monolayer MoSi2N4

(ACS Omega, American Chemical Society (ACS), 2021-11-04) [Article]
First-principles calculations are performed for the recently synthesized monolayer MoSi2N4 [Science 369, 670–674 (2020)]. We show that N vacancies are energetically favorable over Si vacancies, except for Fermi energies close to the conduction band edge in the N-rich environment, and induce half-metallicity. N and Si vacancies generate magnetic moments of 1.0 and 2.0 μB, respectively, with potential applications in spintronics. We also demonstrate that N and Si vacancies can be used to effectively engineer the work function.
• #### Molecular Dynamics Modeling of Kaolinite Particle Associations

(The Journal of Physical Chemistry C, American Chemical Society, 2021-10-18) [Article]
We developed a new procedure for calculating finite-size kaolinite particles, their associations with complex surface chemistry, and the natural flexibility of sheets within a particle using a large-scale atomic/molecular massively parallel simulator. For the first time, all possible particle associations previously described in the literature were obtained using an atomic method. The structural configurations obtained were shifted face-face, angular edge-edge, corner-corner, and shifted face-face-face booklet associations. The simulations showed that if the initial angle between two interacting particles is less than 45°, the particles will form layer-to-layer aggregates. If the angle is larger than 60°, the particles will form an angular arrangement. The densities of kaolinite arrangements with dense and loose packings were evaluated as a function of the structure. The densest structures, as expected, were the layered structures, with four and two layers. The density of the shifted face-face packing was about the same density as the two. The face-face-face association showed lower density, and the angular edge-edge association showed a 3 times lower density than the densest, four-layer structure.
• #### Control of spin–charge conversion in van der Waals heterostructures

(APL Materials, AIP Publishing, 2021-10-01) [Article]
The interconversion between spin and charge degrees of freedom offers incredible potential for spintronic devices, opening routes for spin injection, detection, and manipulation alternative to the use of ferromagnets. The understanding and control of such interconversion mechanisms, which rely on spin–orbit coupling, is therefore an exciting prospect. The emergence of van der Waals materials possessing large spin–orbit coupling (such as transition metal dichalcogenides or topological insulators) and/or recently discovered van der Waals layered ferromagnets further extends the possibility of spin-to-charge interconversion to ultrathin spintronic devices. Additionally, they offer abundant room for progress in discovering and analyzing novel spin–charge interconversion phenomena. Modifying the properties of van der Waals materials through proximity effects is an added degree of tunability also under exploration. This Perspective discusses the recent advances toward spin-to-charge interconversion in van der Waals materials. It highlights scientific developments which include techniques for large-scale growth, device physics, and theoretical aspects.
• #### Accordion-Like Carbon with High Nitrogen Doping for Fast and Stable K Ion Storage

(Advanced Energy Materials, Wiley, 2021-09-24) [Article]
Potassium ion battery (PIB) is a potential candidate for future large-scale energy storage. A key challenge is that the (de)potassiation stability of graphitic carbon anodes is hampered by the limited (002) interlayer spacing. Amorphous carbon with a hierarchical structure can buffer the volume change during repeated (de)potassiation and enable stable cycling. Herein, a direct pyrolysis approach is demonstrated to synthesize a highly nitrogen-doped (26.7 at.%) accordion-like carbon anode composed of thin carbon nanosheets and a turbostratic crystalline structure. The hierarchical structure of accordion-like carbon is endowed by a self-assembly process during pyrolysis carbonization. The hierarchical nitrogen-doped accordion structure enables a high reversible capacity of 346 mAh g−1 and superior cycling stability. This work constitutes a general synthesis methodology that can be used to prepare hierarchical carbon anodes for advanced PIBs.
• #### Lattice-matched III-nitride structures comprising BAlN, BGaN, and AlGaN for ultraviolet applications

(Materials Research Express, IOP Publishing, 2021-08-18) [Article]
The optical properties of BAlN, BGaN and AlGaN ternary alloys are investigated using hybrid density functional for the design of lattice-matched optical structures in the ultraviolet spectrum. The calculated AlGaN properties agree well with previous reports, validating the model. A peculiar non-monotonic behavior of the refractive index as a function of the boron composition is found. The results of this calculation are interpolated to generate a three-dimensional dataset, which can be employed for designing a countless number of lattice-matched and –mismatched heterostructures. These heterostructures could span a range of operating wavelength well into the deep ultraviolet with refractive indices ranging from 1.98 to 2.41 for AlN at 0 eV and GaN near the GaN bandgap, respectively. An example is shown where a lattice-matched heterostructure, AlN/B0.108Ga0.892N, is applied for DBR applications with a large index difference. A DBR comprising the AlN/B0.108Ga0.892N heterostructure at the UV wavelength of 375 nm is found to exceed 93% peak reflectivity with only 10 pairs and reaches 100% reflectivity with 35 pairs. For a chosen design with 25 pairs, the DBR has a peak reflectivity of 99.8% and a bandwidth of 26 nm fulfilling the requirements of most devices especially ultraviolet vertical-cavity surface emitting lasers.
• #### Ultrahigh Carrier Mobility in the Two-Dimensional Semiconductors B8Si4, B8Ge4, and B8Sn4

(Chemistry of Materials, American Chemical Society (ACS), 2021-08-11) [Article]
Based on evolutionary search and first-principles calculations, we predict for B8Si4 structural stability in terms of cohesive energy, phonon spectrum, and melting point. The size of the indirect band gap is similar to that of bulk Si, and the electronic transport turns out to be highly anisotropic for both holes and electrons. The predicted structure prototype is shared by B8Ge4, B8Sn4, and B8Pb4. B8Ge4 is an indirect band gap semiconductor, with the hole mobility similar to that of B8Si4. B8Sn4 is an indirect band gap semiconductor with the gap size similar to that of bulk Ge. The hole mobility of B8Sn4 turns out to be as high as ∼106 cm2 V–1 s–1 and the electron mobility as high as ∼105 cm2 V–1 s–1, exceeding the performance of graphene (2 × 105 cm2 V–1 s–1). B8Pb4 is found to be metallic.
• #### Modeling of n -Alkanes on Calcite/Dolomite by Molecular Dynamics Simulations and First-Principles Calculations

(Advanced Theory and Simulations, Wiley, 2021-08-08) [Article]
Using a combination of molecular dynamics simulations and first-principles calculations, the interaction of n-alkanes with {101⎯⎯4} calcite/dolomite is investigated. It is observed that the n-alkane molecules align preferentially parallel to the interface, despite interaction by weak physisorption, and give rise to distinct adsorption layers. The ordering turns out to be more pronounced on calcite than dolomite due to a smaller average velocity of the n-alkane molecules. The observations are explained in terms of adsorption energies and charge transfers. The results show that functionalization is no prerequisite of structural ordering and a distinct mass density profile perpendicular to the interface.
• #### Large Magnetocrystalline Anisotropy and Giant Coercivity in the Ferrimagnetic Double Perovskite Lu2NiIrO6

(Nano Letters, American Chemical Society (ACS), 2021-08-03) [Article]
We discover that large uniaxial magnetocrystalline anisotropy driven by the simultaneous presence of spin–orbit coupling and structural distortions is the origin of the giant coercivity observed experimentally in the double perovskite Lu2NiIrO6. The magnetic easy axis turns out to be the monoclinic b-axis with an anisotropy constant as high as 1.9 × 108 erg/cm3. The predicted coercive field of 50 kOe and Curie temperature of 220 K agree with the experimentally observed values and point to potential of Lu2NiIrO6 in spintronics applications. We find that the spin–orbit coupling induces a rare Ir4+Jeff = 1/2 Mott insulating state, suggesting that Lu2NiIrO6 provides a playground to study the interplay between spin–orbit coupling and electronic correlations in a 5d transition metal oxide. The spin–orbit coupling also results in a direct band gap with the valence and conduction states localized on different transition metal sublattices, i.e., efficient electron–hole separation upon photoexcitation and low electron–hole recombination.
• #### Chiral Helimagnetism and One-Dimensional Magnetic Solitons in a Cr-Intercalated Transition Metal Dichalcogenide

(Advanced Materials, Wiley, 2021-07-24) [Article]
Chiral magnets endowed with topological spin textures are expected to have promising applications in next-generation magnetic memories. In contrast to the well-studied 2D or 3D magnetic skyrmions, the authors report the discovery of 1D nontrivial magnetic solitons in a transition metal dichalcogenide 2H-TaS2 via precise intercalation of Cr elements. In the synthetic Cr1/3TaS2 (CTS) single crystal, the coupling of the strong spin–orbit interaction from TaS2 and the chiral arrangement of the magnetic Cr ions evoke a robust Dzyaloshinskii–Moriya interaction. A magnetic helix having a short spatial period of ≈25 nm is observed in CTS via Lorentz transmission electron microscopy. In a magnetic field perpendicular to the helical axis, the helical spin structure transforms into a chiral soliton lattice (CSL) with the spin structure evolution being consistent with the chiral sine-Gordon theory, which opens promising perspectives for the application of CSL to fast-speed nonvolatile magnetic memories. This work introduces a new paradigm to soliton physics and provides an effective strategy for seeking novel 2D magnets.
• #### Two Phases of Monolayer Tantalum Sulfide on Au(111)

(ACS Nano, American Chemical Society (ACS), 2021-07-23) [Article]
We prepared monolayers of tantalum sulfide on Au(111) by evaporation of Ta in a reactive background of H2S. Under sulfur-rich conditions, monolayers of 2H-TaS2 formed, whereas under sulfur-poor conditions TaS2–x with 0 ≤ x ≤ 1 were found. We identified this phase as TaS, a structure that can be derived from 2H-TaS2 by removal of the bottom S layer.
• #### Concurrent cationic and anionic perovskite defect passivation enables 27.4% perovskite/silicon tandems with suppression of halide segregation

(Joule, Elsevier BV, 2021-06-16) [Article]
Stable and efficient perovskite/silicon tandem solar cells require defect passivation and suppression of light-induced phase segregation of the wide-band-gap perovskite. Here, we report how molecules containing both electron-rich and electron-poor moieties, such as phenformin hydrochloride (PhenHCl), can satisfy both requirements, independent of the perovskite’s surface chemical composition and its grain boundaries and interfaces. PhenHClpassivated wide-band-gap ( 1.68 eV) perovskite p-i-n single-junction solar cells deliver an open-circuit voltage (VOC) 100 mV higher than control devices, resulting in power conversion efficiencies (PCEs) up to 20.5%. These devices do not show any VOC losses after more than 3,000 h of thermal stress at 85C in a nitrogen ambient. Moreover, PhenHCl passivation improves the PCE of textured perovskite/silicon tandem solar cells from 25.4% to 27.4%. Our findings provide critical insights for improved passivation of metal halide perovskite surfaces and the fabrication of highly efficient and stable perovskite-based single-junction and tandem solar cells.
• #### Sustained Solar-Powered Electrocatalytic H2 Production by Seawater Splitting Using Two-Dimensional Vanadium Disulfide

(ACS Sustainable Chemistry & Engineering, American Chemical Society (ACS), 2021-06-15) [Article]
Robust and stable electrodes made from earth-abundant materials have gained widespread interest in large-scale electrocatalytic water splitting toward hydrogen energy technologies. In this study, the vanadium disulfide (VS2)/amorphous carbon (AC) heterostructure was employed as an electrode for direct seawater splitting. Two-dimensional VS2 nanoparticles were deposited on AC with a high degree of uniformity via a well-optimized one-step chemical vapor deposition approach. The VS2/AC heterostructure electrode was found to possess rich active sulfur sites, near-zero Gibbs free energy, a large surface area, and exceptional charge transfer toward the electrolyte, resulting in enhanced hydrogen evolution reaction (HER) performance with a low onset potential and low overpotential of 11 and 61 mV (vs reversible hydrogen electrode (RHE)), respectively. The electrode also sustained robust stability throughout the 50 h of chronoamperometry studies under acidic electrolyte conditions. Interestingly, the VS2/AC electrocatalyst accomplished an exceptional HER performance under natural seawater conditions in the absence of an external electrolyte with an onset potential of 56 mV vs RHE and attained η200 at an overpotential of 0.53 V vs RHE. In spite of this, the heterostructure exhibited superior stability over 21 days at a high current density of 250 mA/cm2 under both indoor and solar-powered outdoor conditions. Overall, this VS2/AC heterostructure may open a new pathway toward direct seawater splitting for long-term, stable, large-scale hydrogen generation.
• #### Designing graphene origami structures with a giant isotropic negative coefficient of thermal expansion

(Extreme Mechanics Letters, Elsevier BV, 2021-06-02) [Article]
Materials with an isotropic negative coefficient of thermal expansion (CTE) of the order of K−1 are rare, and almost all of them are porous. Using molecular dynamics simulations, we show that graphene origami structures obtained by pattern-based hydrogenation can exhibit a negative CTE. The magnitude and anisotropy of the CTE can be controlled by parameters of the pattern-based hydrogenation that determine the stiffness and Poisson ratio, respectively. We achieve an isotropic CTE of K−1, which is an enhancement by three orders of magnitude as compared to reports for other graphene-based structures and comes close to the record of all known materials.
• #### First principles calculations of the structural, electronic, magnetic, and thermodynamic properties of the Nd2MgGe2 and Gd2MgGe2 intermetallic compounds

(Scientific Reports, Springer Science and Business Media LLC, 2021-05-25) [Article]
AbstractIn recent years the intermetallic ternary RE2MgGe2 (RE = rare earth) compounds attract interest in a variety of technological areas. We therefore investigate in the present work the structural, electronic, magnetic, and thermodynamic properties of Nd2MgGe2 and Gd2MgGe2. Spin–orbit coupling is found to play an essential role in realizing the antiferromagnetic ground state observed in experiments. Both materials show metallicity and application of a Debye-Slater model demonstrates low thermal conductivity and little effects of the RE atom on the thermodynamic behavior.