### Recent Submissions

• #### Experimental Identification of the Second-Order Non-Hermitian Skin Effect with Physics-Graph-Informed Machine Learning.

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, higher-order topological insulators reveal a bulk-edge-corner correspondence, such that nth order topological phases feature (d - n)-dimensional boundary states. The advent of non-Hermitian 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. An unsupervised approach-physics-graph-informed machine learning (PGIML)-to enhance the data mining ability of machine learning with limited domain knowledge is introduced. Through PGIML, the second-order NHSE in a 2D non-Hermitian topoelectrical circuit is experimentally demonstrated. 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.
• #### Excitons and light-emission in semiconducting MoSi2X4 two-dimensional materials

(npj 2D Materials and Applications, Springer Science and Business Media LLC, 2022-11-07) [Article]
Semiconducting two-dimensional materials with chemical formula MoSi2X4 (X = N, P, or As) are studied by means of atomistic ground- and excited-state first-principles simulations. Full-fledged quasi-particle bandstructures within the G0W0 approach substantially correct the electronic bandgaps previously obtained with hybrid-functional density functional theory and highlight the absence of lateral valleys close in energy to the conduction band minimum. By solving the Bethe–Salpeter equation, we show that the optical properties are dominated by strongly bound excitons with the absorbance and maximum short-circuit current densities of MoSi2P4 and MoSi2As4 comparable to those of transition metal dichalcogenides. Due to the presence of the outer SiX layers, the exciton binding energies are smaller than those generally found for transition metal dichalcogenides. Long radiative lifetimes of bright excitons, over 10 ns at room temperature for MoSi2As4, and the absence of band-nesting are very promising for application in efficient ultra-thin optoelectronic devices.
• #### Supermolecule-mediated defect engineering of porous carbons for zinc-ion hybrid capacitors

(Nano Energy, Elsevier BV, 2022-09-23) [Article]
Zinc ion hybrid capacitors hold great potential for future energy storage that requires both high energy density and high power capability. However, the charge storage mechanism of porous carbon cathode is ambiguous in Zn2+ ion-containing aqueous solutions, albeit porous carbon usually stores charge by electric double-layer capacitance. Herein, we developed a supermolecule-mediated direct pyrolysis carbonization strategy to convert sustainable sodium lignosulfonate resources into three-dimensional highly heteroatom-doped porous carbons with large mesopores. Through this strategy, we obtained lignin-derived porous carbons with high heteroatom dopings (14.9 at% nitrogen and 4.7 at% oxygen) and relatively high specific surface areas. Furthermore, the nitrogen doping configurations were mainly edge-nitrogen dopants even under high pyrolysis temperatures (> 900 °C). Lignin-derived nitrogen-doped porous carbon showed a high gravimetric specific capacitance of 266 F g−1 with high rate capability, which is endowed by the increased surface pseudocapacitance. First-principles calculations and molecular dynamics simulations indicate that the edge nitrogen and oxygen dopants contribute to the reversible adsorption/desorption of zinc ions and protons. Pores size less than 6.8 Å can cause a significant diffusion energy barrier for the hydrated zinc ions, thus degrading the capacitance and rate capability.
• #### In situ grown oxygen-vacancy-rich copper oxide nanosheets on a copper foam electrode afford the selective oxidation of alcohols to value-added chemicals

(Communications Chemistry, Springer Science and Business Media LLC, 2022-09-12) [Article]
Selective oxidation of low-molecular-weight aliphatic alcohols like methanol and ethanol into carboxylates in acid/base hybrid electrolytic cells offers reduced process operating costs for the generation of fuels and value-added chemicals, which is environmentally and economically more desirable than their full oxidation to CO2. Herein, we report the in-situ fabrication of oxygen-vacancies-rich CuO nanosheets on a copper foam (CF) via a simple ultrasonication-assisted acid-etching method. The CuO/CF monolith electrode enables efficient and selective electrooxidation of ethanol and methanol into value-added acetate and formate with ~100% selectivity. First principles calculations reveal that oxygen vacancies in CuO nanosheets efficiently regulate the surface chemistry and electronic structure, provide abundant active sites, and enhance charge transfer that facilitates the adsorption of reactant molecules on the catalyst surface. The as-prepared CuO/CF monolith electrode shows excellent stability for alcohol oxidation at current densities >200 mA·cm2 for 24 h. Moreover, the abundant oxygen vacancies significantly enhance the intrinsic indicators of the catalyst in terms of specific activity and outstanding turnover frequencies of 5.8k s−1 and 6k s−1 for acetate and formate normalized by their respective faradaic efficiencies at an applied potential of 1.82 V vs. RHE.
• #### Lattice Instability and Ultralow Lattice Thermal Conductivity of Layered PbIF

(ACS Applied Materials & Interfaces, American Chemical Society (ACS), 2022-09-02) [Article]
Understanding the interplay between various design strategies (for instance, bonding heterogeneity and lone pair induced anharmonicity) to achieve ultralow lattice thermal conductivity (κl) is indispensable for discovering novel functional materials for thermal energy applications. In the present study, we investigate layered PbXF (X = Cl, Br, I), which offers bonding heterogeneity through the layered crystal structure, anharmonicity through the Pb2+ 6s2 lone pair, and phonon softening through the mass difference between F and Pb/X. The weak interlayer van der Waals bonding and the strong intralayer ionic bonding with partial covalent bonding result in a significant bonding heterogeneity and a poor phonon transport in the out-of-plane direction. Large average Grüneisen parameters (≥2.5) demonstrate strong anharmonicity. The computed phonon dispersions show flat bands, which suggest short phonon lifetimes, especially for PbIF. Enhanced Born effective charges are due to cross-band-gap hybridization. PbIF shows lattice instability at a small volume expansion of 0.1%. The κl values obtained by the two channel transport model are 20–50% higher than those obtained by solving the Boltzmann transport equation. Overall, ultralow κl values are found at 300 K, especially for PbIF. We propose that the interplay of bonding heterogeneity, lone pair induced anharmonicity, and constituent elements with high mass difference aids the design of low κl materials for thermal energy applications.
• #### A family of LixBy monolayers with a wide spectrum of potential applications

(Applied Surface Science, Elsevier BV, 2022-08-24) [Article]
Using first-principles calculations and evolutionary structure search, we predict a series of kinetically and thermodynamically stable LixBy monolayers. While the Li2B2 monolayer (metallic) is found to be isotropic, the Li4B8 (metallic), Li2B6 (indirect bandgap semiconductor with a gap size similar to that of bulk Si), and Li2B12 (metallic) monolayers are anisotropic. Notably, the Li2B2 monolayer has a small Poisson's ratio, the Li2B6 monolayer achieves a high hole mobility of 6.8 × 103 cm2·V−1·s−1 (as required for high-speed electronic devices), and the Li2B12 monolayer is auxetic.
• #### Tuneable Poisson's ratio of monolayer GeS and Ge2SSe

(Extreme Mechanics Letters, Elsevier BV, 2022-08-19) [Article]
A tuneable Poisson's ratio is desirable to extend the applications of 2D materials. We demonstrate that Poisson's ratio of monolayer GeS depends approximately linear on the applied strain and electric field strength. In contrast, monolayer Ge2SSe is subject to abrupt switching between positive and negative values (sign reversal) at particular critical values of the strain and electric field strength. The tuneability of Poisson's ratio is attributed to the interplay of different bending stiffnesses. A structural transition is identified as origin of the sign reversal in the case of monolayer Ge2SSe.
• #### Large Spin Coherence Length and High Photovoltaic Efficiency of the Room Temperature Ferrimagnet Ca2FeOsO6 by Strain Engineering

The influence of epitaxial strain on the electronic, magnetic, and optical properties of the distorted double perovskite Ca2FeOsO6 is studied. These calculations show that the compound realizes a monoclinic structure with P21/n space group from −6% to +6% strain. While it retains ferrimagnetic ordering with a net magnetic moment of 2 μB per formula unit at low strain, it undergoes transitions into E-antiferromagnetic and C-antiferromagnetic phases at −5% and +5% strain, respectively. It is shown that spin frustration reduces the critical temperature of the ferrimagnetic ordering from the mean field value of 600–350 K, in excellent agreement with the experimental value of 320 K. It is also shown that the critical temperature can be tuned efficiently through strain and that the spin coherence length surpasses that of Sr2FeMoO6 under tensile strain. An indirect-to-direct bandgap transition is observed at +5% strain. Localization of the valence and conduction states on different transition metal sublattices enables efficient electron–hole separation upon photoexcitation. The calculated spectroscopic limited maximum efficiency of up to 33% points to excellent potential of Ca2FeOsO6 in solar cell applications.
• #### Efficient and stable perovskite-silicon tandem solar cells through contact displacement by MgF x

(Science, American Association for the Advancement of Science (AAAS), 2022-06-23) [Article]
The performance of perovskite solar cells with inverted polarity (p-i-n) is still limited by recombination at their electron extraction interface, which also lowers the power conversion efficiency (PCE) of p-i-n perovskite-silicon tandem solar cells. A ~1 nm thick MgFx interlayer at the perovskite/C60 interface through thermal evaporation favorably adjusts the surface energy of the perovskite layer, facilitating efficient electron extraction, and displaces C60 from the perovskite surface to mitigate nonradiative recombination. These effects enable a champion Voc of 1.92 volts, an improved fill factor of 80.7%, and an independently certified stabilized PCE of 29.3% for a ~1 cm2 monolithic perovskite-silicon tandem solar cell. The tandem retained ~95% of its initial performance following damp-heat testing (85 Celsius at 85% relative humidity) for > 1000 hours.
• #### Observation of cnoidal wave localization in non-linear topolectric circuits

(arXiv, 2022-06-20) [Preprint]
We observe a localized cnoidal (LCn) state in an electric circuit network. Its formation derives from the interplay of non-linearity and the topology inherent to a Su-Schrieffer-Heeger (SSH) chain of inductors. Varicap diodes act as voltage-dependent capacitors, and create a non-linear on-site potential. For a sinusoidal voltage excitation around midgap frequency, we show that the voltage response in the non-linear SSH circuit follows the Korteweg-de Vries equation. The topological SSH boundary state which relates to a midgap impedance peak in the linearized limit is distorted into the LCn state in the non-linear regime, where the cnoidal eccentricity decreases from edge to bulk.
• #### The metallic C6S monolayer with high specific capacity for K-ion batteries

(Materials Today Chemistry, Elsevier BV, 2022-06-20) [Article]
K-ion batteries (KIBs) attract considerable attention due to the abundance of K, high-working voltages, and chemical similarity with Li, enabling the utilization of mature Li-ion technology. However, shortage of high-performance anode materials is a critical obstacle for the development of KIBs. Through first-principles swarm-intelligence structural search, we identify a potential anode material, the C6S monolayer, which provides not only a remarkably high specific capacity of 1546 mAh/g but also a low diffusion barrier of 0.11 eV and a low open-circuit voltage of 0.21 V. Inherent metallicity originates from delocalized π electrons.
• #### Protected valley states and generation of valley- and spin-polarized current in monolayer MA2Z4

(Physical Review B, American Physical Society (APS), 2022-05-31) [Article]
The optical selection rules obeyed by two-dimensional materials with spin-valley coupling enable the selective excitation of carriers. We show that several members of the monolayer MA2Z4 (M=Mo and W;A=C, Si, and Ge; Z=N, P, and As) family are direct band-gap semiconductors with protected valley states and that circularly polarized infrared light can induce valley-selective interband transitions. Therefore, they are able to generate a close to 100% valley- and spin-polarized current under an in-plane bias and circularly polarized infrared light, which can be exploited to encode, process, and store information.
• #### Excellent Thermoelectric Performance of the Metal Sulfide CuTaS3

(ACS Applied Energy Materials, American Chemical Society (ACS), 2022-05-27) [Article]
We use the ab initio scattering and transport method for computing the electronic transport properties of the ternary compound CuTaS3. Both the valence and conduction band edges show band convergence, resulting in high p- and n-type thermoelectric power factors, respectively. We find that polar optical phonon scattering and ionized impurity scattering significantly reduce the carrier mobilities. The lattice thermal conductivity is low due to low phonon group velocities. The combination of the high power factors with the low lattice thermal conductivity leads to high p- and n-type thermoelectric figures of merit.
• #### Regulating the redox reversibility of zinc anode toward stable aqueous zinc batteries

(Nano Energy, Elsevier BV, 2022-05-16) [Article]
Aqueous zinc batteries are among the most promising large-scale energy storage technologies but their practical application is hindered by the low redox reversibility of Zn anode in aqueous electrolytes. In this work, an indium-coated carbon-zinc composite (ICZ) anode is demonstrated with outstanding cyclic stability in classic aqueous zinc sulfate electrolytes. Compared with Zn and the carbon-zinc composite (CZ) anodes, the ICZ anode reduces the high overpotential required for Zn nucleation, and prevents high-rate HER and its related parasitic reactions, endowing high redox reversibility of the ICZ anode. Consequently, the ICZ anode enhances the cyclic stability of zinc ion batteries and zinc ion capacitors. The practical potential of the ICZ anode is demonstrated by an ICZ//porous carbon pouch cell delivering a high areal capacity of 4.7 mAh cm−2 at 6 mA cm−2. Our work provides an effective redox-kinetics regulation strategy for Zn anodes in aqueous zinc batteries.
• #### 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.
• #### 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.
• #### Morphology-Control Growth of Graphene Islands by Nonlinear Carbon Supply

(2022) [Article]
Controlling the morphology of graphene and other two-dimensional (2D) materials in chemical vapor deposition (CVD) growth is crucial because the morphology reflects the crystal quality of as-synthesized nanomaterials in a certain way, and consequently it indirectly represents the physical properties of 2D materials such as band gap, selective ion transportation, and impermeability. However, precise control of the morphology is limited by the complex formation mechanism and sensitive growth-environment factors of graphene. Therefore, the CVD synthesis of single-crystal hexagonal-shaped graphene islands with specific sizes is challenging. Herein, an unconventional nonlinear carbon supply growth strategy is proposed to realize controllable CVD growth of desired hexagonal graphene islands with specific sizes on Cu substrates. Large-area graphene films of isolated islands with desired densities, sizes, and distances between the islands were successfully synthesized. Subsequently, the direct growth of a planar-tunnel-junction structure based on two parallel gapped graphene islands was achieved by specific adjustment of the growth and etching processes of graphene CVD synthesis. We therefore demonstrated that the nonlinear carbon supply growth strategy is a reliable method for the synthesis of high-quality graphene and can facilitate the direct growth of graphene-based nanodevices in the future.
• #### 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.