Showing 1–23 of 23 results for author: Chai, Z

Quantum Acoustics Demystifies the Strange Metals

Authors: Eric J. Heller, Alhun Aydin, Anton M. Graf, Joost de Nijs, Yoel Zimmermann, Xiaoyu Ouyang, Shaobing Yuan, Zixuan Chai, Siyuan Chen, Jasper Jain, Mingxuan Xiao, Chenzheng Yu, Zhongling Lu, Joonas Keski-Rahkonen

Abstract: Phonons have long been thought to be incapable of explaining key phenomena in strange metals, including linear-in-\textit{T} Planckian resistivity from high to very low temperatures. We argue that these conclusions were based on static, perturbative approaches that overlooked essential time-dependent and nonperturbative electron-lattice physics. In fact ``phonons'' are not the best target for disc… ▽ More Phonons have long been thought to be incapable of explaining key phenomena in strange metals, including linear-in-\textit{T} Planckian resistivity from high to very low temperatures. We argue that these conclusions were based on static, perturbative approaches that overlooked essential time-dependent and nonperturbative electron-lattice physics. In fact ``phonons'' are not the best target for discussion, just like ``photons'' are not the best way to think about Maxwell's equations. Quantum optics connects photons and electromagnetism, as developed 60 years ago by Glauber and others. We have been developing the parallel world of quantum acoustics. Far from being only of academic interest, the new tools are rapidly exposing the secrets of the strange metals, revealing strong vibronic (vibration-electronic) interactions playing a crucial role forming polarons and charge density waves, linear-in-$T$ resistivity at the Planckian rate over thousands of degrees, resolution of the Drude peak infrared anomaly, and the absence of a $T^4$ low-temperature resistivity rise in 2D systems, and of a Mott-Ioffe-Regel resistivity saturation. We derive Planckian transport, polarons, CDWs, and pseudogaps from the Fröhlich model. The ``new physics'' has been hiding in this model all along, in the right parameter regime, if it is treated nonperturbatively. In the course of this work we have uncovered the generalization of Anderson localization to dynamic media: a universal Planckian diffusion emerges, a ``ghost'' of Anderson localization. Planckian diffusion is clearly defined and is more fundamental than the popular but elusive, model dependent concept of ``Planckian speed limit''. △ Less

Submitted 8 November, 2025; v1 submitted 3 November, 2025; originally announced November 2025.

Si-Substituted MAX Phases and In-Situ Formation of Si-coated MXene Composites via Chlorosilane Etching

Authors: Xudong Wang, Qian Fang, Mian Li, Zhifang Chai, Qing Huang

Abstract: Silicon-based MAX phases are a promising class of layered ceramics with superior thermal and chemical stability. However, their synthesis remains challenging due to inherent thermodynamic instability at high temperatures. Herein, we develop a general top-down strategy to synthesize a broad family of Si-substituted MAX phases (M = Ti, V, Nb, Ta, Cr; X = C, N) by reacting Al-based MAX precursors wit… ▽ More Silicon-based MAX phases are a promising class of layered ceramics with superior thermal and chemical stability. However, their synthesis remains challenging due to inherent thermodynamic instability at high temperatures. Herein, we develop a general top-down strategy to synthesize a broad family of Si-substituted MAX phases (M = Ti, V, Nb, Ta, Cr; X = C, N) by reacting Al-based MAX precursors with SiCl4 vapor. This approach not only circumvents traditional high-temperature limitations but also enables precise A-site defect engineering, resulting in phases with controlled vacancy concentrations (e.g., Nb2Si3/4C and Nb2Si1/2C). Furthermore, we introduce a redox potential-based model that rationalizes the reaction pathway. Using Tin+1AlXn etched with SiCl4 as an example, the process simultaneously forms Cl-terminated MXene (Mn+1XnCl2) and amorphous nano-Si, enabling the one-step synthesis of Si-coated MXene composites. This methodology provides new avenues for designing advanced MAX phases and MXene-based hybrids with tailored functionalities for applications in energy storage and catalysis. △ Less

Submitted 14 September, 2025; originally announced September 2025.

The Anisotropic Interface Continuum Solvation Model and the Finite-Element Anisotropic Poisson Solver

Authors: Ziwei Chai, Sandra Luber

Abstract: We propose an anisotropic interfacial continuum solvation (AICS) model to simulate the distinct in-plane and out-of-plane dielectric constants of liquids near solid-liquid interfaces and their spatial variations along the surface normal direction. In low-electron-density regions, each dielectric function in the diagonal components of a dielectric tensor varies monotonically with distance from the… ▽ More We propose an anisotropic interfacial continuum solvation (AICS) model to simulate the distinct in-plane and out-of-plane dielectric constants of liquids near solid-liquid interfaces and their spatial variations along the surface normal direction. In low-electron-density regions, each dielectric function in the diagonal components of a dielectric tensor varies monotonically with distance from the solid surface along the surface normal; in high-electron-density regions near the surface, each dielectric function adopts the electron-density-based formulation proposed by Andreussi et al. (J. Chem. Phys. 136, 064102 (2012)) The resulting dielectric tensor is continuously differentiable with respect to both electron density and spatial coordinates. We derived analytical expressions for electrostatic contributions to the KS potential and forces, and implemented AICS, including these analytical derivatives, into CP2K. To solve the anisotropic Poisson equations, we developed a parallel finite-element anisotropic Poisson solver (FEAPS) based on the FEniCSx platform and its interface with CP2K. Analytical forces were validated against finite-difference calculations, while electrostatic potentials computed under vacuum and isotropic solvent conditions using AICS and FEAPS were benchmarked against standard vacuum DFT and SCCS results, respectively. In the anisotropic solvent environment characterized by the enhanced in-plane and reduced out-of-plane dielectric functions near the Ag(111) surface, we calculated the resulting work functions and electrostatic potentials, and optimized the adsorption geometry for OH. Compared to the isotropic case, we observed more pronounced work function shifts and spatially modulated electrostatic profiles across different charge states. Our results also showed that OH tilted more towards the plane parallel to the surface under the anisotropic dielectric conditions. △ Less

Submitted 7 August, 2025; originally announced August 2025.

Phase stabilization of In2Se3 by disordered Ni intercalation and its enhanced thermoelectrical performance

Authors: Zengguang Shi, Yukun Xiao, Mian Li, Jianfeng Cai, Yanmei Chen, Jun Jiang, Xiaoping Ouyang, Zhifang Chai, Qing Huang

Abstract: Van der Waals (vdW) layered materials have gained significant attention owing to their distinctive structure and unique properties. The weak interlayer bonding in vdW layered materials enables guest atom intercalation, allowing precise tuning of their physical and chemical properties. In this work, a ternary compound, NixIn2Se3 (x = 0-0.3), with Ni randomly occupying the interlayers of In2Se3, was… ▽ More Van der Waals (vdW) layered materials have gained significant attention owing to their distinctive structure and unique properties. The weak interlayer bonding in vdW layered materials enables guest atom intercalation, allowing precise tuning of their physical and chemical properties. In this work, a ternary compound, NixIn2Se3 (x = 0-0.3), with Ni randomly occupying the interlayers of In2Se3, was synthesized via an intercalation route driven by electron injection. The intercalated Ni atoms act as anchor points within the interlayer of In2Se3, which effectively suppresses the phase transition of In2Se3 at elevated temperatures. Furthermore, the disordered Ni intercalation significantly enhanced the electrical conductivity of In2Se3 through electron injection, while reducing the thermal conductivity due to the interlayer phonon scattering, leading to an improved thermoelectric performance. For instance, the thermoelectric figure of merit (ZT) of Ni0.3In2Se3 increased by 86% (in-plane) and 222% (out-of-plane) compared to In2Se3 at 500 oC. These findings not only provide an effective strategy to enhance the performance of layered thermoelectric materials, but also demonstrate the potential of intercalation chemistry for expanding the application scope of van der Waals (vdW) layered materials. △ Less

Submitted 23 April, 2025; originally announced April 2025.

Organometallic-Inorganic Hybrid MXenes with Tunable Superconductivity

Authors: Qi Fan, Tao Bo, Wei Guo, Minghua Chen, Qing Tang, Yicong Yang, Mian Li, Ke Chen, Fangfang Ge, Jialu Li, Sicong Qiao, Changda Wang, Li Song, Lijing Yu, Jinghua Guo, Michael Naguib, Zhifang Chai, Qing Huang, Chaochao Dun, Ning Kang, Yury Gogotsi, Kun Liang

Abstract: Ti-based two-dimensional transition-metal carbides (MXenes) have attracted attention due to their superior properties and are being explored across various applications1,2. Despite their versatile properties, superconductivity has never been demonstrated, not even predicted, for this important group of 2D materials. In this work, we have introduced an electrochemical intercalation protocol to cons… ▽ More Ti-based two-dimensional transition-metal carbides (MXenes) have attracted attention due to their superior properties and are being explored across various applications1,2. Despite their versatile properties, superconductivity has never been demonstrated, not even predicted, for this important group of 2D materials. In this work, we have introduced an electrochemical intercalation protocol to construct versatile organometallic-inorganic hybrid MXenes and achieved tunable superconductivity in the metallocene-modified layered crystals. Through structural editing of MXene matrix at atomic scale and meticulously modulated intercalation route, Ti3C2Tx intercalated with metallocene species exhibits a superconductive transition temperature (Tc) of 10.2 K. Guest intercalation induced electron filling and strain engineering are responsible for the emerging superconductivity in this intrinsically non-superconducting material. Theoretically, simulated electron-phonon interaction effects further elucidate the nature of the changes in Tc. Furthermore, the Tc of crafted artificial superlattices beyond Ti-based MXenes have been predicted, offering a general strategy for engineering superconductivity and magnetism in layered hybrid materials. △ Less

Submitted 16 February, 2025; originally announced February 2025.

Mobility Edges in Two-Dimensional Aperiodic Potentials

Authors: Si-Yuan Chen, Zixuan Chai, Chenzheng Yu, Anton M. Graf, Joonas Keski-Rahkonen, Eric J. Heller

Abstract: In 1958, Anderson proposed a new insulating mechanism in random lattices, now known as Anderson localization. It has been shown that a metal-insulating transition occurs in three dimensions, and that one-dimensional disordered systems can be solved exactly to show strong localization regardless of the strength of disorders. Meanwhile, the two-dimensional case was known to be localizing from a scal… ▽ More In 1958, Anderson proposed a new insulating mechanism in random lattices, now known as Anderson localization. It has been shown that a metal-insulating transition occurs in three dimensions, and that one-dimensional disordered systems can be solved exactly to show strong localization regardless of the strength of disorders. Meanwhile, the two-dimensional case was known to be localizing from a scaling argument. Here, we report that there exists a mobility edge in certain random potentials which separate the extended-like states from short-ranged localized states. We further observe that the location of the mobility edge depends on the typical wavelength of the potential, and that the localization length are are related to the energy of an eigenstate. Finally, we apply a renormalization group theory to explain the localization effects and the existence of mobility edge and propose an experimental scheme to verify the mobility edge in photonic crystals. △ Less

Submitted 9 December, 2024; originally announced December 2024.

Sublayers Editing of Covalent MAX Phase for Nanolaminated Early Transition Metal Compounds

Authors: Ziqian Li, Ke Chen, Xudong Wang, Kan Luo, Lei Lei, Mian Li, Kun Liang, Degao Wang, Shiyu Du, Zhifang Chai, Qing Huang

Abstract: Two-dimensional transition metal carbides and nitrides (MXenes) have gained popularity in fields such as energy storage, catalysis, and electromagnetic interference due to their diverse elemental compositions and variable surface terminations (T). Generally, the synthesis of MXene materials involves etching the weak M-A metallic bonds in the ternary layered transition metal carbides and nitrides (… ▽ More Two-dimensional transition metal carbides and nitrides (MXenes) have gained popularity in fields such as energy storage, catalysis, and electromagnetic interference due to their diverse elemental compositions and variable surface terminations (T). Generally, the synthesis of MXene materials involves etching the weak M-A metallic bonds in the ternary layered transition metal carbides and nitrides (MAX phase) using HF acid or Lewis acid molten salts, while the strong M-X covalent bonds preserve the two-dimensional framework structure of MXenes. On the other hand, the MAX phase material family also includes a significant class of members where the A site is occupied by non-metal main group elements (such as sulfur and phosphorus), in which both M-A and M-X are covalent bond-type sublayers. The aforementioned etching methods cannot be used to synthesize MXene materials from these parent phases. In this work, we discovered that the covalent bond-type M-A and M-X sublayers exhibit different reactivity with some inorganic materials in a high-temperature molten state. By utilizing this difference in reactivity, we can structurally modify these covalent sublayers, allowing for the substitution of elements at the X site (from B to Se, S, P, C) and converting non-metal A site atoms in non-van der Waals (non-vdW) MAX phases into surface atoms in vdW layered materials. This results in a family of early transition metal Xide chalcogenides (TMXCs) that exhibit lattice characteristics of both MXenes and transition metal chalcogenides. Using electron-donor chemical scissors, these TMXC layered materials can be further exfoliated into monolayer nanosheets. The atomic configurations of each atom in these monolayer TMXCs are the same as those of conventional MXenes, but the oxidation states of the M-site atoms can be regulated by both X-site atoms and intercalated cations. △ Less

Submitted 2 December, 2024; originally announced December 2024.

Semiconductive and Ferromagnetic Lanthanide MXenes Derived from Carbon Intercalated Two-dimensional Halides

Authors: Qian Fang, Liming Wang, Kai Chang, Hongxin Yang, Pu Yan, Kecheng Cao, Mian Li, Zhifang Chai, Qing Huang

Abstract: Two-dimensional (2D) magnetic semiconductors are a key focus in developing next-generation information storage technologies. MXenes, as emerging 2D early transition metal carbides and nitrides, offer versatile compositions and tunable chemical structures. Incorporating lanthanide metals, with their unique role of 4f-electrons in engineering physical properties, into MXenes holds potential for adva… ▽ More Two-dimensional (2D) magnetic semiconductors are a key focus in developing next-generation information storage technologies. MXenes, as emerging 2D early transition metal carbides and nitrides, offer versatile compositions and tunable chemical structures. Incorporating lanthanide metals, with their unique role of 4f-electrons in engineering physical properties, into MXenes holds potential for advancing technological applications. However, the scarcity of lanthanide-containing ternary MAX phase precursors and the propensity of lanthanides to oxidize pose significant challenges to obtain lanthanide MXenes (Ln2CT2) via the top-down etching method. Here, we propose a general bottom-up methodology for lanthanide MXenes, that derive from carbon intercalated van der Waals building blocks of 2D halides. Compared to conventional MXenes conductors, the synthesized Ln2CT2 exhibit tunable band gaps spanning 0.32 eV to 1.22 eV that cover typical semiconductors such as Si (1.12 eV) and Ge (0.67 eV). Additionally, the presence of unpaired f-electrons endows Ln2CT2 with intrinsic ferromagnetism, with Curie temperatures ranging between 36 K and 60 K. Theoretical calculations reveal that, in contrast to traditional MXenes, the number of d-electrons states around the Fermi level are largely diminishes in bare Ln2C MXenes, and the halogen terminals can further exhaust these electrons to open band gaps. Meanwhile, the Ln-4f electrons in Ln2CT2 are highly localized and stay away from the Fermi level, contributing to the spin splitting for the observed ferromagnetic behavior. Lanthanide MXenes hold immense promise for revolutionizing future applications in spintronic devices. △ Less

Submitted 23 October, 2024; originally announced October 2024.

Minimum tracking linear response Hubbard and Hund corrected Density Functional Theory in CP2K

Authors: Ziwei Chai, Rutong Si, Mingyang Chen, Gilberto Teobaldi, David D. O'Regan, Li-Min Liu

Abstract: We present the implementation of the Hubbard ($U$) and Hund ($J$) corrected Density Functional Theory (DFT+$U$+$J$) functionality in the Quickstep program, which is part of the CP2K suite. The tensorial and Löwdin subspace representations are implemented and compared. Full analytical DFT+$U$+$J$ forces are implemented and benchmarked for the tensorial and Löwdin representations. We also present th… ▽ More We present the implementation of the Hubbard ($U$) and Hund ($J$) corrected Density Functional Theory (DFT+$U$+$J$) functionality in the Quickstep program, which is part of the CP2K suite. The tensorial and Löwdin subspace representations are implemented and compared. Full analytical DFT+$U$+$J$ forces are implemented and benchmarked for the tensorial and Löwdin representations. We also present the implementation of the recently proposed minimum-tracking linear-response method that enables the $U$ and $J$ parameters to be calculated on first principles basis without reference to the Kohn-Sham eigensystem. These implementations are benchmarked against recent results for different materials properties including DFT+$U$ band gap opening in NiO, the relative stability of various polaron distributions in TiO$_2$, the dependence of the calculated TiO$_2$ band gap on +$J$ corrections, and, finally, the role of the +$U$ and +$J$ corrections for the computed properties of a series of the hexahydrated transition metals. Our implementation provides results consistent with those already reported in the literature from comparable methods. We conclude the contribution with tests on the influence of the Löwdin orthonormalization on the occupancies, calculated parameters, and derived properties. △ Less

Submitted 24 July, 2024; v1 submitted 18 July, 2024; originally announced July 2024.

Journal ref: J. Chem. Theory Comput. 2024, 20, 20, 8984-9002

Chemical-scissor-mediated structural editing of layered transition metal carbides

Authors: Haoming Ding, Youbing Li, Mian Li, Ke Chen, Kun Liang, Guoxin Chen, Jun Lu, Justinas Palisaitis, Per O. A. Persson, Per Eklund, Lars Hultman, Shiyu Du, Zhifang Chai, Yury Gogotsi, Qing Huang

Abstract: Intercalation of non-van der Waals (vdW) layered materials can produce new 2D and 3D materials with unique properties, but it is difficult to achieve. Here, we describe a structural editing protocol for 3D non-vdW layered ternary carbides and nitrides (MAX phases) and their 2D vdW derivatives (MXenes). Gap-opening and species-intercalating stages were mediated by chemical scissors and guest interc… ▽ More Intercalation of non-van der Waals (vdW) layered materials can produce new 2D and 3D materials with unique properties, but it is difficult to achieve. Here, we describe a structural editing protocol for 3D non-vdW layered ternary carbides and nitrides (MAX phases) and their 2D vdW derivatives (MXenes). Gap-opening and species-intercalating stages were mediated by chemical scissors and guest intercalants, creating a large family of layered materials with unconventional elements and structures in MAX phases, as well as MXenes with versatile termination species. Removal of surface terminations by metal scissors and stitching of carbide layers by metal atoms leads to a reverse transformation from MXenes to MAX phases, and metal-intercalated 2D carbides. This scissor-mediated structural editing may enable structural and chemical tailoring of other layered ceramics. △ Less

Submitted 28 July, 2022; originally announced July 2022.

Synthesis and thermal expansion of chalcogenide MAX phase Hf2SeC

Authors: Xudong Wang, Ke Chen, Erxiao Wu, Yiming Zhang, Haoming Ding, Nianxiang Qiu, Yujie Song, Shiyu Du, Zhifang Chai, Qing Huang

Abstract: Thermal expansion of MAX phases along different directions tended to be different because of the anisotropy of hexagonal crystals. Herein, a new Hf2SeC phase was synthesized and confirmed to be relatively isotropic, whose coefficients of thermal expansion (CTEs) were determined to be 9.73 μK-1 and 10.18 μK-1 along a and c directions. The strong M-S bond endowed Hf2SC and Zr2SC lower CTEs than Hf2S… ▽ More Thermal expansion of MAX phases along different directions tended to be different because of the anisotropy of hexagonal crystals. Herein, a new Hf2SeC phase was synthesized and confirmed to be relatively isotropic, whose coefficients of thermal expansion (CTEs) were determined to be 9.73 μK-1 and 10.18 μK-1 along a and c directions. The strong M-S bond endowed Hf2SC and Zr2SC lower CTEs than Hf2SeC and Zr2SeC. A good relationship between the thermal expansion anisotropy and the ratio of elastic stiffness constant c11 and c33 was established. This straightforward approximation could be used to roughly predict the thermal expansion anisotropy of MAX phases. △ Less

Submitted 16 November, 2021; originally announced November 2021.

Near-Room-Temperature Ferromagnetic Behavior of Single-Atom-Thick 2D Iron in Nanolaminated Ternary MAX Phases

Authors: Youbing Li, Jinghua Liang, Haoming Ding, Jun Lu, Xulin Mu, Pengfei Yan, Xiao Zhang, Ke Chen, Mian Li, Per O. A. Persson, Lars Hultman, Per Eklund, Shiyu Du, Hongxin Yang, Zhifang Chai, Qing Huang

Abstract: Two dimensional (2D) ferromagnetic materials have attracted much attention in the fields of condensed matter physics and materials science, but their synthesis is still a challenge given their limitations on structural stability and susceptibility to oxidization. MAX phases nanolaminated ternary carbides or nitrides possess a unique crystal structure in which single-atom-thick A sublayers are inte… ▽ More Two dimensional (2D) ferromagnetic materials have attracted much attention in the fields of condensed matter physics and materials science, but their synthesis is still a challenge given their limitations on structural stability and susceptibility to oxidization. MAX phases nanolaminated ternary carbides or nitrides possess a unique crystal structure in which single-atom-thick A sublayers are interleaved by two dimensional MX slabs, providing nanostructured templates for designing 2D ferromagnetic materials if the non-magnetic A sublayers can be substituted replaced by magnetic elements. Here, we report three new ternary magnetic MAX phases (Ta2FeC, Ti2FeN and Nb2FeC) with A sublayers of single-atom-thick 2D iron through an isomorphous replacement reaction of MAX precursors (Ta2AlC, Ti2AlN and Nb2AlC) with a Lewis acid salts (FeCl2). All these MAX phases exhibit ferromagnetic (FM) behavior. The Curie temperature (Tc) of Ta2FeC and Nb2FeC MAX phase are 281 K and 291 K, respectively, i.e. close to room temperature. The saturation magnetization of these ternary magnetic MAX phases is almost two orders of magnitude higher than that of V2(Sn,Fe)C MAX phase whose A-site is partial substituted by Fe. Theoretical calculations on magnetic orderings of spin moments of Fe atoms in these nanolaminated magnetic MAX phases reveal that the magnetism can be mainly ascribed to intralayer exchange interaction of the 2D Fe atomic layers. Owning to the richness in composition of MAX phases, there is a large compositional space for constructing functional single-atom-thick 2D layers in materials using these nanolaminated templates. △ Less

Submitted 13 May, 2021; originally announced May 2021.

MAX Phase Zr2SeC and Its Thermal Conduction Behavior

Authors: Ke Chen, Xiaojing Bai, Xulin Mu, Pengfei Yan, Nianxiang Qiu, Youbing Li, Jie Zhou, Yujie Song, Yiming Zhang, Shiyu Du, Zhifang Chai, Qing Huang

Abstract: The elemental diversity is crucial to screen out ternary MAX phases with outstanding properties via tuning of bonding types and strength between constitutive atoms. As a matter of fact, the interactions between M and A atoms largely determine the physical and chemical properties of MAX phases. Herein, Se element was experimentally realized to occupy the A site of a MAX phase, Zr2SeC, becoming a ne… ▽ More The elemental diversity is crucial to screen out ternary MAX phases with outstanding properties via tuning of bonding types and strength between constitutive atoms. As a matter of fact, the interactions between M and A atoms largely determine the physical and chemical properties of MAX phases. Herein, Se element was experimentally realized to occupy the A site of a MAX phase, Zr2SeC, becoming a new member within this nanolaminated ternary carbide family. Comprehensive characterizations including Rietveld refinement of X-ray Diffraction and atom-resolved transmission electron microscopy techniques were employed to validate this novel MAX phase. The distinct thermal conduction behaviors emerged are attributed to the characteristic interactions between Zr and Se atoms. △ Less

Submitted 4 February, 2021; originally announced February 2021.

Molten salt synthesis of a nanolaminated Sc2SnC MAX Phase

Authors: Youbing Li, Yanqing Qin, Ke Chen, Lu Chen, Xiao Zhang, Haoming Ding, Mian Li, Yiming Zhang, Shiyu Du, Zhifang Chai, Qing Huang

Abstract: The MAX phases are a family of of ternary layered material with both metal and ceramic properties, and it is also precursor ma-terials for synthesis of two-dimensional MXenes. The theory predicted that there are more than 600 stable ternary layered MAX phases. At present, there are more than 80 kinds of ternary MAX phases synthesized through experiments, and few reports on MAX phases where M is a… ▽ More The MAX phases are a family of of ternary layered material with both metal and ceramic properties, and it is also precursor ma-terials for synthesis of two-dimensional MXenes. The theory predicted that there are more than 600 stable ternary layered MAX phases. At present, there are more than 80 kinds of ternary MAX phases synthesized through experiments, and few reports on MAX phases where M is a rare earth element. In this study, a new MAX phase Sc2SnC with rare earth element Sc at the M sites was synthesized through the reaction sintering of Sc, Sn, and C mixtures. Phase composition and microstructure of Sc2SnC were confirmed by X-ray diffraction, scanning electron microscopy and X-ray energy spectrum analysis. And structural stability, mechanical and electronic properties of Sc2SnC was investigated via density functional theory. This study open a door for ex-plore more unknown ternary layered rare earth compounds Ren+1SnCn (Re=Sc, Y, La-Nd, n=1) and corresponding rare earth MXenes. △ Less

Submitted 20 October, 2020; originally announced October 2020.

Halogenated MXenes with Electrochemically Active Terminals for High Performance Zinc Ion Batteries

Authors: Mian Li, XinLiang Li, Guifang Qin, Kan Luo, Jun Lu, Youbing Li, Guojin Liang, Zhaodong Huang, Lars Hultman, Per Eklund, Per O. A. Persson, Shiyu Du, Zhifang Chai, Chunyi Zhi, Qing Huang

Abstract: The class of two-dimensional metal carbides and nitrides known as MXenes offer a distinct manner of property tailoring for a wide range of applications. The ability to tune the surface chemistry for expanding the property space of MXenes is thus an important topic, although experimental exploration of new surface terminals remains a challenge. Here, we synthesized Ti3C2 MXene with unitary, binary… ▽ More The class of two-dimensional metal carbides and nitrides known as MXenes offer a distinct manner of property tailoring for a wide range of applications. The ability to tune the surface chemistry for expanding the property space of MXenes is thus an important topic, although experimental exploration of new surface terminals remains a challenge. Here, we synthesized Ti3C2 MXene with unitary, binary and ternary halogen terminals, e.g. -Cl, -Br, -I, -BrI and -ClBrI, to investigate the effect of surface chemistry on the properties of MXenes. The electrochemical activity of Br and I element result in the extraordinary electrochemical performance of the MXenes as cathodes for aqueous zinc ion batteries. The -Br and -I containing MXenes, e.g. Ti3C2Br2 and Ti3C2I2, exhibit distinct discharge platforms with considerable capacities of 97.6 mAh g-1 and 135 mAh g-1. Ti3C2(BrI) and Ti3C2(ClBrI) exhibit dual discharge platforms with capacities of 117.2 mAh g-1 and 106.7 mAh g-1. In contrast, the previously discovered MXenes Ti3C2Cl2 and Ti3C2(OF) exhibit no discharge platforms, and only ~50% of capacities and energy densities of Ti3C2Br2. These results emphasize the effectiveness of the Lewis-acidic-melt etching route for tuning the surface chemistry of MXenes, and also show promise for expanding the MXene family towards various applications. △ Less

Submitted 16 June, 2020; originally announced June 2020.

Experimental sensing quantum atmosphere of a single spin

Authors: Kehang Zhu, Zhiping Yang, Qing-Dong Jiang, Zihua Chai, Zhijie Li, Zhiyuan Zhao, Ya Wang, Fazhan Shi, Chang-Kui Duan, Xing Rong, Jiangfeng Du

Abstract: Understanding symmetry-breaking states of materials is a major challenge in the modern physical sciences. Quantum atmosphere proposed recently sheds light on the hidden world of these symmetry broken patterns. But the requirements for exquisite sensitivity to the small shift and tremendous spatial resolution to local information pose huge obstacles to its experimental manifestation. In our experim… ▽ More Understanding symmetry-breaking states of materials is a major challenge in the modern physical sciences. Quantum atmosphere proposed recently sheds light on the hidden world of these symmetry broken patterns. But the requirements for exquisite sensitivity to the small shift and tremendous spatial resolution to local information pose huge obstacles to its experimental manifestation. In our experiment, we prepare time-reversal-symmetry conserved and broken quantum atmosphere of a single nuclear spin and successfully observe their symmetry properties. Our work proves in principle that finding symmetry patterns from quantum atmosphere is conceptually viable. It also opens up entirely new possibilities in the potential application of quantum sensing in material diagnosis. △ Less

Submitted 12 January, 2020; originally announced January 2020.

The role of Hume-Rothery's rules play in the MAX phases formability

Authors: Yiming Zhang, Zeyu Mao, Qi Han, Youbing Li, Mian Li, Shiyu Du, Zhifang Chai, Qing Huang

Abstract: MAX phases are a family of layered, hexagonal-structure ternary carbides or nitrides of a transitional metal and an A-group element. What makes this type of material fascinating and potentially useful is their remarkable combinations of metallic and ceramic characteristics; as well as the indispensable role in 'top-down' synthesis of their 2D counterparts, MXenes. To enhance the efficiency in the… ▽ More MAX phases are a family of layered, hexagonal-structure ternary carbides or nitrides of a transitional metal and an A-group element. What makes this type of material fascinating and potentially useful is their remarkable combinations of metallic and ceramic characteristics; as well as the indispensable role in 'top-down' synthesis of their 2D counterparts, MXenes. To enhance the efficiency in the successful search for potential novel MAX phases, the main efforts could go toward creating an informationprediction system incorporating all MAX phases' databases, as well as generally valid principles and the high-quality regularities. In this work, we employ structure mapping methodology, which has shown its merit of being useful guides in materials design, with Hume-Rothery parameters to provide guiding principles in the search of novel MAX phases. The formable/non-formable data on MAX phases can be ordered within a twodimensional plot by using proposed expression of geometrical and electron concentration factors. △ Less

Submitted 3 July, 2020; v1 submitted 20 November, 2019; originally announced November 2019.

A general Lewis acidic etching route for preparing MXenes with enhanced electrochemical performance in non-aqueous electrolyte

Authors: Youbing Li, Hui Shao, Zifeng Lin, Jun Lu, Per O. A. Persson, Per Eklund, Lars Hultman, Mian Li, Ke Chen, Xian-Hu Zha, Shiyu Du, Patrick Rozier, Zhifang Chai, Encarnacion Raymundo-Piñero, Pierre-Louis Taberna, Patrice Simon, Qing Huang

Abstract: Two-dimensional carbides and nitrides of transition metals, known as MXenes, are a fast-growing family of 2D materials that draw attention as energy storage materials. So far, MXenes are mainly prepared from Al-containing MAX phases (where A = Al) by Al dissolution in F-containing solution, but most other MAX phases have not been explored. Here, a redox-controlled A-site-etching of MAX phases in L… ▽ More Two-dimensional carbides and nitrides of transition metals, known as MXenes, are a fast-growing family of 2D materials that draw attention as energy storage materials. So far, MXenes are mainly prepared from Al-containing MAX phases (where A = Al) by Al dissolution in F-containing solution, but most other MAX phases have not been explored. Here, a redox-controlled A-site-etching of MAX phases in Lewis acidic melts is proposed and validated by the synthesis of various MXenes from unconventional MAX phase precursors with A elements Si, Zn, and Ga. A negative electrode of Ti3C2 MXene material obtained through this molten salt synthesis method delivers a Li+ storage capacity up to 738 C g-1 (205 mAh g-1) with high-rate performance and pseudocapacitive-like electrochemical signature in 1M LiPF6 carbonate-based electrolyte. MXene prepared from this molten salt synthesis route offer opportunities as high-rate negative electrode material for electrochemical energy storage applications. △ Less

Submitted 29 September, 2019; originally announced September 2019.

Multielemental single-atom-thick A layers in nanolaminated V2(Sn, A)C (A=Fe, Co, Ni, Mn) for tailoring magnetic properties

Authors: Youbing Li, Jun Lu, Mian Li, Keke Chang, Xianhu Zha, Yiming Zhang, Ke Chen, Per O. A. Persson, Lars Hultman, Per Eklund, Shiyu Du, Zhifang Chai, Zhengren Huang, Qing Huang

Abstract: Tailoring of individual single-atom-thick layers in nanolaminated materials offers atomic-level control over material properties. Nonetheless, multielement alloying in individual atomic layers in nanolaminates is largely unexplored. Here, we report a series of inherently nanolaminated V2(A'xSn1-x)C (A'=Fe, Co, Ni and Mn, and combinations thereof, with x=1/3) synthesized by an alloy-guided reaction… ▽ More Tailoring of individual single-atom-thick layers in nanolaminated materials offers atomic-level control over material properties. Nonetheless, multielement alloying in individual atomic layers in nanolaminates is largely unexplored. Here, we report a series of inherently nanolaminated V2(A'xSn1-x)C (A'=Fe, Co, Ni and Mn, and combinations thereof, with x=1/3) synthesized by an alloy-guided reaction. The simultaneous occupancy of the four magnetic elements and Sn, the individual single-atom-thick A layers in the compound constitute high-entropy-alloy analogues, two-dimensional in the sense that the alloying exclusively occurs in the A layers. V2(A'xSn1-x)C exhibit distinct ferromagnetic behavior that can be compositionally tailored from the multielement A-layer alloying. This two-dimensional alloying provides a structural-design route with expanded chemical space for discovering materials and exploit properties. △ Less

Submitted 10 August, 2019; originally announced August 2019.

Synthesis of MAX Phases Nb2CuC and Ti2(Al0.1Cu0.9)N by A-site Replacement Reaction in Molten Salts

Authors: Haoming Ding, Youbing Li, Jun Lu, Kan Luo, Ke Chen, Mian Li, Per O. A. Persson, Lars Hultman, Per Eklund, Shiyu Du, Zhengren Huang, Zhifang Chai, Hongjie Wang, Ping Huang, Qing Huang

Abstract: New MAX phases Ti2(AlxCu1-x)N and Nb2CuC were synthesized by A-site replacement by reacting Ti2AlN and Nb2AlC, respectively, with CuCl2 or CuI molten salt. X-ray diffraction, scanning electron microscopy, and atomically-resolved scanning transmission electron microscopy showed complete A-site replacement in Nb2AlC, which lead to formation of Nb2CuC. However, the replacement of Al in Ti2AlN phase w… ▽ More New MAX phases Ti2(AlxCu1-x)N and Nb2CuC were synthesized by A-site replacement by reacting Ti2AlN and Nb2AlC, respectively, with CuCl2 or CuI molten salt. X-ray diffraction, scanning electron microscopy, and atomically-resolved scanning transmission electron microscopy showed complete A-site replacement in Nb2AlC, which lead to formation of Nb2CuC. However, the replacement of Al in Ti2AlN phase was only close to complete at Ti2(Al0.1Cu0.9)N. Density-functional theory calculations corroborated the structural stability of Nb2CuC and Ti2CuN phases. Moreover, the calculated cleavage energy in these Cu-containing MAX phases are weaker than in their Al-containing counterparts, indicating that they are precursor candidates for MXene derivation. △ Less

Submitted 23 July, 2019; v1 submitted 19 July, 2019; originally announced July 2019.

Comments: author name corrected

Experimental observation of dynamical bulk-surface correspondence for topological phases

Authors: Ya Wang, Wentao Ji, Zihua Chai, Yuhang Guo, Mengqi Wang, Xiangyu Ye, Pei Yu, Long Zhang, Xi Qin, Pengfei Wang, Fazhan Shi, Xing Rong, Dawei Lu, Xiong-Jun Liu, Jiangfeng Du

Abstract: We experimentally demonstrate a dynamical classification approach for investigation of topological quantum phases using a solid-state spin system through nitrogen-vacancy (NV) center in diamond. Similar to the bulkboundary correspondence in real space at equilibrium, we observe a dynamical bulk-surface correspondence in the momentum space from a dynamical quench process. An emergent dynamical topo… ▽ More We experimentally demonstrate a dynamical classification approach for investigation of topological quantum phases using a solid-state spin system through nitrogen-vacancy (NV) center in diamond. Similar to the bulkboundary correspondence in real space at equilibrium, we observe a dynamical bulk-surface correspondence in the momentum space from a dynamical quench process. An emergent dynamical topological invariant is precisely measured in experiment by imaging the dynamical spin-textures on the recently defined band-inversion surfaces, with high topological numbers being implemented. Importantly, the dynamical classification approach is shown to be independent of quench ways and robust to the decoherence effects, offering a novel and practical strategy for dynamical topology characterization, especially for high dimensional gapped topological phases. △ Less

Submitted 18 April, 2019; originally announced April 2019.

Comments: 6 pages, 4 figures

Journal ref: Phys. Rev. A 100, 052328 (2019)

Tin+1Cn MXene with fully saturated and thermally stable Cl terminations

Authors: J. Lu, I. Persson, H. Lind, M. Li, Y. Li, K. Chen, J. Zhou, S. Du, Z. Chai, Z. Huang, L. Hultman, J. Rosen, P. Eklund, Q. Huang, P. O. Å. Persson

Abstract: MXenes are a rapidly growing family of 2D materials that exhibit a highly versatile structure and composition, allowing for significant tuning of the material properties. These properties are, however, ultimately limited by the surface terminations, which are typically a mixture of species, including F and O that are inherent to the MXene processing. Other and robust terminations are lacking. Here… ▽ More MXenes are a rapidly growing family of 2D materials that exhibit a highly versatile structure and composition, allowing for significant tuning of the material properties. These properties are, however, ultimately limited by the surface terminations, which are typically a mixture of species, including F and O that are inherent to the MXene processing. Other and robust terminations are lacking. Here, we apply high-resolution scanning transmission electron microscopy (STEM), corresponding image simulations and first-principles calculations to investigate the surface terminations on MXenes synthesized from MAX phases through Lewis acidic melts. The results show that atomic Cl terminates the synthesized MXenes, with mere residual presence of other termination species. Furthermore, in situ STEM-electron energy loss spectroscopy (EELS) heating experiments show that the Cl terminations are stable up to 750 °C. Thus, we present an attractive new termination that widely expands the MXenes functionalization space and enable new applications. △ Less

Submitted 16 January, 2019; originally announced January 2019.

Comments: 14 pages, 4 figures, 5 pages supplemental info

Element Replacement Approach by Reaction with Lewis Acidic Molten Salts to Synthesize Nanolaminated MAX Phases and MXenes

Authors: Mian Li, Jun Lu, Kan Luo, Youbing Li, Keke Chang, Ke Chen, Jie Zhou, Johanna Rosen, Lars Hultman, Per Eklund, Per O. Å. Persson, Shiyu Du, Zhifang Chai, Zhengren Huang, Qing Huang

Abstract: Nanolaminated materials are important because of their exceptional properties and wide range of applications. Here, we demonstrate a general approach to synthesize a series of Zn-based MAX phases and Cl-terminated MXenes originating from the replacement reaction between the MAX phase and the late transition metal halides. The approach is a top-down route that enables the late transitional element… ▽ More Nanolaminated materials are important because of their exceptional properties and wide range of applications. Here, we demonstrate a general approach to synthesize a series of Zn-based MAX phases and Cl-terminated MXenes originating from the replacement reaction between the MAX phase and the late transition metal halides. The approach is a top-down route that enables the late transitional element atom (Zn in the present case) to occupy the A site in the pre-existing MAX phase structure. Using this replacement reaction between Zn element from molten ZnCl2 and Al element in MAX phase precursors (Ti3AlC2, Ti2AlC, Ti2AlN, and V2AlC), novel MAX phases Ti3ZnC2, Ti2ZnC, Ti2ZnN, and V2ZnC were synthesized. When employing excess ZnCl2, Cl terminated MXenes (such as Ti3C2Cl2 and Ti2CCl2) were derived by a subsequent exfoliation of Ti3ZnC2 and Ti2ZnC due to the strong Lewis acidity of molten ZnCl2. These results indicate that A-site element replacement in traditional MAX phases by late transition metal halides opens the door to explore MAX phases that are not thermodynamically stable at high temperature and would be difficult to synthesize through the commonly employed powder metallurgy approach. In addition, this is the first time that exclusively Cl-terminated MXenes were obtained, and the etching effect of Lewis acid in molten salts provides a green and viable route to prepare MXenes through an HF-free chemical approach. △ Less

Submitted 15 March, 2019; v1 submitted 15 January, 2019; originally announced January 2019.

Comments: Title changed; experimental section and discussion revised

Journal ref: J. Am. Chem. Soc. 2019, 141, 11, 4730-4737