Showing 1–50 of 62 results for author: Duan, X

Giant-exchange-driven Vectorial Control of a Minimal Topological Magnet in Eu3In2As4

Authors: Haonan Chen, Xunkai Duan, Guangyi Wang, Yuhan Du, Huayao Li, Jiayu Wang, Wenbin Wu, Zixuan Xu, Yingchao Xia, Jiaming Gu, Pengliang Leng, Lin Miao, Fengfeng Zhu, Xiang Yuan, Tong Zhou, Cheng Zhang

Abstract: The interplay between magnetism and band topology provides a route to controlling quantum states of matter, yet its realization in materials is often constrained by weak exchange coupling and complex electronic structures. Here, a giant exchange coupling is identified in the newly predicted topological magnet Eu3In2As4, giving rise to magnetization-dependent band shifts of up to 300 meV. Together… ▽ More The interplay between magnetism and band topology provides a route to controlling quantum states of matter, yet its realization in materials is often constrained by weak exchange coupling and complex electronic structures. Here, a giant exchange coupling is identified in the newly predicted topological magnet Eu3In2As4, giving rise to magnetization-dependent band shifts of up to 300 meV. Together with its intrinsically soft magnetic response, this strong cou-pling enables systematic tuning of topological phases by both the magnitude and orientation of applied magnetic fields. The magneto-topological phase diagram is mapped out in which an antiferromagnetic topological insulator ground state evolves, under modest fields, into a pro-posed intermediate 2/3-ferrimagnetic phase, and further into fully polarized ferromagnetic states predicted to host either Weyl or nodal-ring semimetals. Notably, the Weyl phase corresponds to a minimal model hosting a single pair of Weyl nodes. Quantum oscillations, anomalous Hall transport and magneto-infrared spectroscopy consistently reveal exchange-driven band recon-struction across these transitions. Rotation of the magnetization theoretically provides an effi-cient means to tune the momentum-space positions and separations of the Weyl nodes. These results establish Eu3In2As4 as a model system for exploring how strong exchange coupling can be used to control topological band structures with minimal complexity. △ Less

Submitted 6 August, 2026; originally announced August 2026.

Comments: 26 pages, 5 figures

Journal ref: Advanced Materials (2026): e74504

"Anomalous Solid Solution" in Ultra-High Melting Point Oxides: A New Strategy for Developing Ultra-High Temperature Thermal Protection Coatings

Authors: Yubo Wang, Hong Meng, Pengfei He, Shujun Hu, Chuan Sun, Ximing Duan, Xiaopeng Lu, Dingwang Yuan, Wangyu Hu, Xiubing Liang

Abstract: The high-temperature performance of ultra-high temperature ceramics (UHTCs) in atmospheric environment is fundamentally governed by their melting points of oxidation products. Typical high-melting-point oxides, such as ZrO2, undergo phase transformations at elevated temperatures, leading to structural instability. Although doping with rare-earth or transition-metal cations can suppress these trans… ▽ More The high-temperature performance of ultra-high temperature ceramics (UHTCs) in atmospheric environment is fundamentally governed by their melting points of oxidation products. Typical high-melting-point oxides, such as ZrO2, undergo phase transformations at elevated temperatures, leading to structural instability. Although doping with rare-earth or transition-metal cations can suppress these transformations, it often results in a reduction in melting point, thereby limiting practical service temperature. Here, ytterbia-stabilized zirconia (YbSZ) coatings are prepared via atmospheric plasma spraying, achieving a remarkable increase in the melting point of ZrO2 to approximately 2850 $^\circ\mathrm{C}$ and raising the ultimate plasma and oxyacetylene ablation temperature up to nearly 2780 $^\circ\mathrm{C}$ and 3200 $^\circ\mathrm{C}$, which is the highest temperature resistance property as reported. Notably, this performance enhancement originates from a synergistic mechanism of strengthened ionic-covalent mixed bonding and improved oxygen vacancy stability. Based on these findings, the concept of "anomalous solid solution" is firstly proposed to be used in the area of ultra-high temperature protection, which provides new insights into the compositional design of UHTC systems. △ Less

Submitted 30 July, 2026; originally announced July 2026.

Surface Functionalization Enables Two-Dimensional Altermagnetism and Giant Tunnel Magnetoresistance

Authors: Zhou Cui, Ziye Zhu, Bowen Hao, Xunkai Duan, Xuan Zhou, Yali Xie, Huali Yang, Baisheng Sa, Runwei Li, Tong Zhou

Abstract: Two-dimensional (2D) altermagnets (AMs) are highly desirable for ultrafast, stray-field-free spintronics because they combine compensated magnetic order and momentum-dependent spin splitting with the scalability, tunability, and interface compatibility of atomically thin materials. However, practical 2D AMs remain scarce. Rather than relying solely on the search for intrinsic 2D AMs, an appealing… ▽ More Two-dimensional (2D) altermagnets (AMs) are highly desirable for ultrafast, stray-field-free spintronics because they combine compensated magnetic order and momentum-dependent spin splitting with the scalability, tunability, and interface compatibility of atomically thin materials. However, practical 2D AMs remain scarce. Rather than relying solely on the search for intrinsic 2D AMs, an appealing route is to transform known 2D antiferromagnets (AFMs) into AMs through symmetry engineering. Here, we propose surface functionalization as a symmetry-guided, nonvolatile chemical switch for realizing this AFM-to-AM transformation. By breaking inversion and out-of-plane mirror symmetries while preserving the rotation symmetry connecting opposite-spin sublattices, single-sided functionalization lifts spin degeneracy and induces altermagnetic spin splitting. Using monolayer FeSe as a representative platform, first-principles calculations show that hydrogenation, oxidation, and fluorination convert spin-degenerate antiferromagnetic FeSe into a d-wave AM with pronounced momentum-dependent spin splitting. At the device level, our transport simulations reveal that the functionalized FeSe monolayer magnetic tunnel junctions exhibit giant tunnel magnetoresistance (TMR) up to $1.87\times10^3\%$, originating from momentum-selective spin filtering between parallel and antiparallel Néel-vector configurations. The strong dependence of TMR on functionalization geometry further demonstrates that surface chemistry provides an effective control knob for altermagnetic transport. Our work establishes a symmetry-to-chemistry-to-device strategy for engineering 2D AMs and developing high-performance altermagnetic spintronic devices. △ Less

Submitted 4 July, 2026; originally announced July 2026.

Comments: 7 pages, 4 figures

Proton-electron coupled catalyst for ionomer-free electrochemical energy conversion

Authors: Ao Zhang, Ran Wang, Mohammed O. Bazaid, Shiyi Wang, Ting-Jung Hsiao, Yibo Wang, Antonio Sorrentino, Yang Liu, Yu-Han Joseph Tsai, Boxuan Zhou, Bosi Peng, Zeyan Liu, Xiangfeng Duan, Adam Z. Weber, William A. Goddard III, Seung Soon Jang, Yu Huang

Abstract: Efficient electrochemical energy devices are vital to renewable energy technology, yet coordinating the effective flow of electrons, ions, and chemical species continues to be a major challenge. In conventional proton-exchange membrane fuel cell (PEMFC) catalyst layers, proton and electron transport are supplied separately through percolating carbon networks and ionomer binders, rendering the cata… ▽ More Efficient electrochemical energy devices are vital to renewable energy technology, yet coordinating the effective flow of electrons, ions, and chemical species continues to be a major challenge. In conventional proton-exchange membrane fuel cell (PEMFC) catalyst layers, proton and electron transport are supplied separately through percolating carbon networks and ionomer binders, rendering the catalyst largely passive and imposing fundamental trade-offs between reactant accessibility, ionic conductivity, and catalyst activity. Here, we introduce a one-dimensional proton-electron coupled catalyst (PECC) design, a transport-integrated electrocatalyst architecture in which the catalyst itself simultaneously supplies electronic and protonic transport to catalyst active sites. Using this PECC, PEMFCs can have an ionomer-free cathode catalyst layer (CCL), resulting in a dramatic 95% reduction in non-Fickian oxygen transport and boosting power density by 34% and 85% compared to traditional CCLs, with cathode Pt loadings of approximately 0.090 mg/cm^2 and 0.037 mg/cm^2, respectively. Meanwhile, PECC retains 65% of its mass activity and exhibits 32% higher power density than its ionomer-based CCL counterpart after 30k accelerated stressed test. Similar mass transport improvements have been observed in the electrochemical hydrogen pump (EHP) using PECC in the catalyst layers. Molecular dynamics simulations show the PECC's proton conductivity is 249% higher than Nafion. This PECC catalyst structure addresses core transport problems in PEMFCs, leading to almost 20% improvement in fuel efficiency and opens up new possibilities for designing high-performance, cost-effective electrochemical devices. △ Less

Submitted 23 May, 2026; originally announced May 2026.

Decoupling Intrinsic Molecular Efficacy from Platform Effects: An Interpretable Machine Learning Framework for Unbiased Perovskite Passivator Discovery

Authors: Jing Zhang, Ziyuan Li, Shan Gao, Zhen Zhu, Jing Wang, Xiangmei Duan

Abstract: Rational design of interface passivators for perovskite solar cells is hindered by the entanglement of intrinsic molecular efficacy with extrinsic platform-dependent performance - a confounding factor that obscures true chemical advances. Here, we present a generalizable, interpretable machine learning framework that decouples these effects via an asymptotic saturation model, enabling unbiased dis… ▽ More Rational design of interface passivators for perovskite solar cells is hindered by the entanglement of intrinsic molecular efficacy with extrinsic platform-dependent performance - a confounding factor that obscures true chemical advances. Here, we present a generalizable, interpretable machine learning framework that decouples these effects via an asymptotic saturation model, enabling unbiased discovery of molecules with genuine intrinsic gains. Trained on a curated dataset of 240 experimental entries, our model identifies hydrogen bond acceptor strength and electrostatic potential difference as key descriptors. Guided by these insights, we screened >121 million PubChem compounds using a hierarchical strategy integrating diversity clustering and uncertainty quantification. Five dual-functional candidates (e.g., TDZ-S, TZC-F) are identified, exhibiting superior predicted efficacy (surpassing experimental benchmarks) and high confidence. First-principles calculations confirm strong chemisorption (Eads<-1.7 eV), net electron donation, and optimized interfacial energetics. Crucially, our closed-loop "data-interpretation-screening-verification" pipeline establishes a transferable paradigm for rational materials design, extendable to other optoelectronic interfaces beyond perovskites. △ Less

Submitted 3 March, 2026; originally announced March 2026.

Comments: 31 pages, 5 figures and 1 table

Symmetry-Driven Unconventional Magnetoelectric Coupling in Perovskite Altermagnets: From Bulk to the Two-Dimensional Limit

Authors: Zhou Cui, Ziye Zhu, Xunkai Duan, Bowen Hao, Xianzhang Chen, Jiayong Zhang, Tong Zhou

Abstract: The emergence of altermagnets establishes a new paradigm for multiferroics. Unlike conventional multiferroics relying on direct magnetoelectric coupling, multiferroic altermagnets host a crystal-symmetry-mediated magnetoelectric interaction that is intrinsically more efficient and robust. Among candidate material platforms, layered perovskites are particularly appealing owing to their structural d… ▽ More The emergence of altermagnets establishes a new paradigm for multiferroics. Unlike conventional multiferroics relying on direct magnetoelectric coupling, multiferroic altermagnets host a crystal-symmetry-mediated magnetoelectric interaction that is intrinsically more efficient and robust. Among candidate material platforms, layered perovskites are particularly appealing owing to their structural diversity and synthetic versatility. However, magnetoelectric properties at the two-dimensional scale remain largely unexplored, hindering their applicability in miniaturized, highly integrated devices. Here, we systematically investigate the dimensional evolution of ferroelectric polarization and magnetism in perovskite systems through symmetry analysis. We demonstrate that altermagnetism can persist in the two-dimensional limit, yet is strongly constrained by the magnetic configuration-with only C-type antiferromagnetic order supporting it. Based on mode-decomposition calculations, we further reveal that symmetry-restricted multimode couplings simultaneously govern ferroelectric polarization and altermagnetic spin splitting. Finally, combined with first-principles calculations, we propose several strategies to lift the magnetic-configuration constraint, extending the range of viable altermagnetic systems. These results underscore the critical role of dimensionality in symmetry-driven magnetoelectric coupling in perovskite altermagnets and pave the way toward next-generation electrically controlled spintronic and multiferroic devices. △ Less

Submitted 9 January, 2026; originally announced January 2026.

Comments: 7 pages, 4 figures

Defect Engineering for Stabilizing Magnetic and Topological Properties in Mn(Bi1-xSbx)2Te4

Authors: Haonan Chen, Jiayu Wang, Huayao Li, Xunkai Duan, Yuxiang Wang, Zixuan Xu, Yingchao Xia, Wenhao He, Zehao Jia, Xiangyu Cao, Yicheng Mou, Xiangyu Jiang, Jiaming Gu, Pengliang Leng, Fengfeng Zhu, Changlin Zheng, Xiang Yuan, Faxian Xiu, Tong Zhou, Lin Miao, Cheng Zhang

Abstract: MnBi2Te4 is a versatile platform for exploring diverse topological quantum states, yet its potential is hampered by intrinsic antisite defects. While Sb substitution has been employed to tune the Fermi level towards the charge neutral point, it exacerbates the formation of Mn-Sb antisite defects. Here, we address this challenge by combining first-principles calculations with strategic synthesis to… ▽ More MnBi2Te4 is a versatile platform for exploring diverse topological quantum states, yet its potential is hampered by intrinsic antisite defects. While Sb substitution has been employed to tune the Fermi level towards the charge neutral point, it exacerbates the formation of Mn-Sb antisite defects. Here, we address this challenge by combining first-principles calculations with strategic synthesis to systematically investigate and control antisite defects in Mn(Bi1-xSbx)2Te4. Our calculations reveal that increasing antisite defect density progressively destroys the field-forced magnetic Weyl state, eventually driving the system into a trivial magnetic insulator. Motivated by these findings, we develop an optimized chemical vapor transport method, yielding high-quality Mn(Bi1-xSbx)2Te4 crystals with significantly reduced antisite defect density. The emergence of strong Shubnikov-de Haas oscillations in the forced ferromagnetic state and a pronounced anomalous Hall effect near charge neutrality, with opposite signs for n- and p-type samples, confirms the type-II Weyl semimetal nature. These findings underscore the critical role of antisite defects in determining the magnetic and topological properties of Mn(Bi1-xSbx)2Te4 and establish defect engineering via optimized synthesis as a crucial strategy for realizing its exotic magnetic topological states. △ Less

Submitted 25 December, 2025; originally announced December 2025.

Comments: 26 pages, 8 figures

Altermagnetoelectric Spin Field Effect Transistor

Authors: Ziye Zhu, Xianzhang Chen, Xunkai Duan, Zhou Cui, Jiayong Zhang, Igor Zutic, Tong Zhou

Abstract: Spin field-effect transistors (SFETs) are promising candidates for low-power spin-based electronics, yet existing realizations that rely on spin-orbit coupling are constrained by limited material choices and short spin-coherence lengths. Here we propose a different operating principle based on multiferroic altermagnets, in which spin splitting is tuned by an electric field through symmetry control… ▽ More Spin field-effect transistors (SFETs) are promising candidates for low-power spin-based electronics, yet existing realizations that rely on spin-orbit coupling are constrained by limited material choices and short spin-coherence lengths. Here we propose a different operating principle based on multiferroic altermagnets, in which spin splitting is tuned by an electric field through symmetry control rather than conventional spin-orbit physics. Using an effective model combined with quantum transport simulations, we show that the conductance is determined by the degree of matching between the electrically controlled spin texture of the channel and the fixed spin polarization of ferromagnetic contacts, enabling clear ON and OFF states. Remarkably, we also address a long-standing challenge in multiferroic device design: spintronic channels require metallic carriers, whereas ferroelectricity is usually suppressed in metals. We resolve this conflict by imprinting multiferroic altermagnetism into highly conductive materials via the proximity effect. First-principles calculations for graphene on multiferroic vanadium sulfide halides confirm that graphene acquires a ferroelectrically switchable spin splitting while retaining its metallic character. These results establish a practical route to SFET implementation and identify multiferroic altermagnets as a versatile platform for next-generation spintronic devices. △ Less

Submitted 2 December, 2025; originally announced December 2025.

Comments: 6 pages, 4 figures

Identifying the Catalytic Descriptor of Single-Atom Catalysts in Nitrate Reduction Reaction: An Interpretable Machine-Learning Method

Authors: Zhen Zhu, Shan Gao, Jing Zhang, Xuxin Kang, Shunfang Li, Xiangmei Duan

Abstract: Elucidating the catalytic descriptor that accurately characterizes the structure-activity relationships of typical catalysts for various important heterogeneous catalytic reactions is pivotal for designing high-efficient catalytic systems. Here, an interpretable machine learning technique was employed to identify the key determinants governing the nitrate reduction reaction ($\rm NO_3RR$) performa… ▽ More Elucidating the catalytic descriptor that accurately characterizes the structure-activity relationships of typical catalysts for various important heterogeneous catalytic reactions is pivotal for designing high-efficient catalytic systems. Here, an interpretable machine learning technique was employed to identify the key determinants governing the nitrate reduction reaction ($\rm NO_3RR$) performance across 286 single-atom catalysts (SACs) with the active sites anchored on double-vacancy $\rm BC_3$ monolayers. Through Shapley Additive Explanations (SHAP) analysis with reliable predictive accuracy, we quantitatively demonstrated that, favorable $\rm NO_3RR$ activity stems from a delicate balance among three critical factors: low $\rm N_V$, moderate $\rm D_N$, and specific doping patterns. Building upon these insights, we established a descriptor ($ψ$) that integrates the intrinsic catalytic properties and the intermediate O-N-H angle ($θ$), effectively capturing the underlying structure-activity relationship. Guided by this, we further identified 16 promising catalysts with predicted low limiting potential ($U_{\rm L}$). Importantly, these catalysts are composed of cost-effective non-precious metal elements and are predicted to surpass most reported catalysts, with the best-performing Ti-V-1N1 is predicted to have an ultra-low $U_{\rm L}$ of $-0.10$ V. △ Less

Submitted 22 October, 2025; originally announced October 2025.

Comments: 10 pages, 8 figures, 74 references

Altermagnetic Proximity Effect

Authors: Ziye Zhu, Richang Huang, Xianzhang Chen, Zhou Cui, Xunkai Duan, Jiayong Zhang, Igor Zutic, Tong Zhou

Abstract: Proximity effects not only complement the conventional methods of designing materials, but also enable realizing properties that are not present in any constituent region of the considered heterostructure. Here we reveal an unexplored altermagnetic proximity effect (AMPE), distinct from its ferromagnetic and antiferromagnetic counterparts. Using first-principles and model analyses of van der Waals… ▽ More Proximity effects not only complement the conventional methods of designing materials, but also enable realizing properties that are not present in any constituent region of the considered heterostructure. Here we reveal an unexplored altermagnetic proximity effect (AMPE), distinct from its ferromagnetic and antiferromagnetic counterparts. Using first-principles and model analyses of van der Waals heterostructures based on the prototypical altermagnet V$_2$Se$_2$O, we show that its hallmark momentum-alternating spin splitting can be directly imprinted onto adjacent nonmagnetic layers -- a process we term altermagnetization. This is demonstrated in a monolayer PbO through characteristic band splitting and real-space spin densities, with systematic dependence on interlayer spacing and magnetic configuration. We further predict broader AMPE manifestations: Valley-selective spin splitting in a monolayer PbS and a topological superconducting phase in monolayer NbSe$_2$, both inheriting the alternating $k$-space spin texture of the altermagnet. These results establish AMPE not only as a distinct proximity mechanism, but also as a powerful method of using altermagnetism in designing emergent phenomena and versatile applications. △ Less

Submitted 16 March, 2026; v1 submitted 8 September, 2025; originally announced September 2025.

Comments: 7 pages, 4 figures; PRL in press

Emergent Multiferroic Altermagnets and Spin Control via Noncollinear Molecular Polarization

Authors: Ziye Zhu, Yuntian Liu, Xunkai Duan, Jiayong Zhang, Bowen Hao, Su-Huai Wei, Igor Zutic, Tong Zhou

Abstract: Altermagnets, with spin splitting and vanishing magnetization, have been attributed to many fascinating phenomena and potential applications. In particular, integrating ferroelectricity with altermagnetism to enable magnetoelectric coupling and electric control of spin has drawn significant attention. However, its experimental realization and precise spin manipulation remain elusive. Here, by focu… ▽ More Altermagnets, with spin splitting and vanishing magnetization, have been attributed to many fascinating phenomena and potential applications. In particular, integrating ferroelectricity with altermagnetism to enable magnetoelectric coupling and electric control of spin has drawn significant attention. However, its experimental realization and precise spin manipulation remain elusive. Here, by focusing on molecular ferroelectrics, the first discovered ferroelectrics renowned for their highly controllable molecular polarizations and structural flexibility, we reveal that these obstacles can be removed by an emergent multiferroic altermagnets with tunable spin polarization in a large class of fabricated organic materials. Using a symmetry-based design and a tight-binding model, we uncover the underlying mechanism of such molecular ferroelectric altermagnets and demonstrate how noncollinear molecular polarization can switch the spin polarization on and off and even reverse its sign. From the first-principles calculations, we verify the feasibility of these materials in a series of well-established hybrid organic-inorganic perovskites and metal-organic frameworks. Our findings bridge molecular ferroelectrics and altermagnetic spintronics, highlighting an unexplored potential of multifunctional organic multiferroics. △ Less

Submitted 8 September, 2025; v1 submitted 9 July, 2025; originally announced July 2025.

Comments: 7 pages, 4 figures

Journal ref: Sci. China-Phys. Mech. Astron. 68,127562 (2025)[Cover, Editor's Focus]

Two-Dimensional Ferroelectric Altermagnets: From Model to Material Realization

Authors: Ziye Zhu, Xunkai Duan, Jiayong Zhang, Bowen Hao, Igor Zutic, Tong Zhou

Abstract: Multiferroic altermagnets offer new opportunities for magnetoelectric coupling and electrically tunable spintronics. However, due to intrinsic symmetry conflicts between altermagnetism and ferroelectricity, achieving their coexistence, known as ferroelectric altermagnets (FEAM), remains an outstanding challenge, especially in two-dimensional (2D) systems. Here, we propose a universal, symmetry-bas… ▽ More Multiferroic altermagnets offer new opportunities for magnetoelectric coupling and electrically tunable spintronics. However, due to intrinsic symmetry conflicts between altermagnetism and ferroelectricity, achieving their coexistence, known as ferroelectric altermagnets (FEAM), remains an outstanding challenge, especially in two-dimensional (2D) systems. Here, we propose a universal, symmetry-based design principle for 2D FEAM, supported by tight-binding models and first-principles calculations. We show that lattice distortions can break spin equivalence and introduce the necessary rotation-related symmetry, enabling altermagnetism with electrically reversible spin splitting. Guided by this framework, we identify a family of 2D vanadium oxyhalides and sulfide halides as promising FEAM candidates. In these compounds, pseudo Jahn-Teller distortions and Peierls-like dimerization cooperatively establish the required symmetry conditions. We further propose the magneto-optical Kerr effect as an experimental probe to confirm FEAM and its electric spin reversal. Our findings provide a practical framework for 2D FEAM and advancing electrically controlled spintronic devices. △ Less

Submitted 21 May, 2025; v1 submitted 8 April, 2025; originally announced April 2025.

Comments: 7 pages, 4 figures

The 2D Materials Roadmap

Authors: Wencai Ren, Peter Bøggild, Joan Redwing, Kostya Novoselov, Luzhao Sun, Yue Qi, Kaicheng Jia, Zhongfan Liu, Oliver Burton, Jack Alexander-Webber, Stephan Hofmann, Yang Cao, Yu Long, Quan-Hong Yang, Dan Li, Soo Ho Choi, Ki Kang Kim, Young Hee Lee, Mian Li, Qing Huang, Yury Gogotsi, Nicholas Clark, Amy Carl, Roman Gorbachev, Thomas Olsen , et al. (48 additional authors not shown)

Abstract: Over the past two decades, 2D materials have rapidly evolved into a diverse and expanding family of material platforms. Many members of this materials class have demonstrated their potential to deliver transformative impact on fundamental research and technological applications across different fields. In this roadmap, we provide an overview of the key aspects of 2D material research and developme… ▽ More Over the past two decades, 2D materials have rapidly evolved into a diverse and expanding family of material platforms. Many members of this materials class have demonstrated their potential to deliver transformative impact on fundamental research and technological applications across different fields. In this roadmap, we provide an overview of the key aspects of 2D material research and development, spanning synthesis, properties and commercial applications. We specifically present roadmaps for high impact 2D materials, including graphene and its derivatives, transition metal dichalcogenides, MXenes as well as their heterostructures and moiré systems. The discussions are organized into thematic sections covering emerging research areas (e.g., twisted electronics, moiré nano-optoelectronics, polaritronics, quantum photonics, and neuromorphic computing), breakthrough applications in key technologies (e.g., 2D transistors, energy storage, electrocatalysis, filtration and separation, thermal management, flexible electronics, sensing, electromagnetic interference shielding, and composites) and other important topics (computational discovery of novel materials, commercialization and standardization). This roadmap focuses on the current research landscape, future challenges and scientific and technological advances required to address, with the intent to provide useful references for promoting the development of 2D materials. △ Less

Submitted 28 April, 2025; v1 submitted 28 March, 2025; originally announced March 2025.

Comments: 97 pages, to be published in 2D Materials

A Printed Microscopic Universal Gradient Interface for Super Stretchable Strain-Insensitive Bioelectronics

Authors: Kaidong Song, Jingyuan Zhou, Chen Wei, Ashok Ponnuchamy, Md Omarsany Bappy, Yuxuan Liao, Qiang Jiang, Yipu Du, Connor J. Evans, Brian C. Wyatt, Thomas O'Sullivan, Ryan K. Roeder, Babak Anasori, Anthony J. Hoffman, Lihua Jin, Xiangfeng Duan, Yanliang Zhang

Abstract: Stretchable electronics capable of conforming to nonplanar and dynamic human body surfaces are central for creating implantable and on-skin devices for high-fidelity monitoring of diverse physiological signals. While various strategies have been developed to produce stretchable devices, the signals collected from such devices are often highly sensitive to local strain, resulting in inevitable conv… ▽ More Stretchable electronics capable of conforming to nonplanar and dynamic human body surfaces are central for creating implantable and on-skin devices for high-fidelity monitoring of diverse physiological signals. While various strategies have been developed to produce stretchable devices, the signals collected from such devices are often highly sensitive to local strain, resulting in inevitable convolution with surface strain-induced motion artifacts that are difficult to distinguish from intrinsic physiological signals. Here we report all-printed super stretchable strain-insensitive bioelectronics using a unique universal gradient interface (UGI) to bridge the gap between soft biomaterials and stiff electronic materials. Leveraging a versatile aerosol-based multi-materials printing technique that allows precise spatial control over the local stiffnesses with submicron resolution, the UGI enables strain-insensitive electronic devices with negligible resistivity changes under a 180% stretch ratio. We demonstrate various stretchable devices directly printed on the UGI for on-skin health monitoring with high signal quality and near perfect immunity to motion artifacts, including semiconductor-based photodetectors for sensing blood oxygen saturation levels and metal-based temperature sensors. The concept in this work will significantly simplify the fabrication and accelerate the development of a broad range of wearable and implantable bioelectronics for real-time health monitoring and personalized therapeutics. △ Less

Submitted 31 October, 2024; originally announced November 2024.

Topological surface state dominated nonlinear transverse response and microwave rectification at room temperature

Authors: Qia Shen, Jiaxin Chen, Bin Rong, Yaqi Rong, Hongliang Chen, Tieyang Zhao, Xianfa Duan, Dandan Guan, Shiyong Wang, Yaoyi Li, Hao Zheng, Xiaoxue Liu, Xuepeng Qiu, Jingsheng Chen, Longqing Cong, Tingxin Li, Ruidan Zhong, Canhua Liu, Yumeng Yang, Liang Liu, Jinfeng Jia

Abstract: Nonlinear Hall effect (NLHE) offers a novel means of uncovering symmetry and topological properties in quantum materials, holding promise for exotic (opto)electronic applications such as microwave rectification and THz detection. The BCD-independent NLHE could exhibit a robust response even at room temperature, which is highly desirable for practical applications. However, in materials with bulk i… ▽ More Nonlinear Hall effect (NLHE) offers a novel means of uncovering symmetry and topological properties in quantum materials, holding promise for exotic (opto)electronic applications such as microwave rectification and THz detection. The BCD-independent NLHE could exhibit a robust response even at room temperature, which is highly desirable for practical applications. However, in materials with bulk inversion symmetry, the coexistence of bulk and surface conducting channels often leads to a suppressed NLHE and complex thickness-dependent behavior. Here, we report the observation of room-temperature nonlinear transverse response in 3D topological insulator Bi2Te3 thin films, whose electrical transport properties are dominated by topological surface state (TSS). By varying the thickness of Bi2Te3 epitaxial films from 7 nm to 50 nm, we found that the nonlinear transverse response increases with thickness from 7 nm to 25 nm and remains almost constant above 25 nm. This is consistent with the thickness-dependent basic transport properties, including conductance, carrier density, and mobility, indicating a pure and robust TSS-dominated linear and nonlinear transport in thick (>25 nm) Bi2Te3 films. The weaker nonlinear transverse response in Bi2Te3 below 25 nm was attributed to Te deficiency and poorer crystallinity. By utilizing the TSS-dominated electrical second harmonic generation, we successfully achieved the microwave rectification from 0.01 to 16.6 GHz in 30 nm and bulk Bi2Te3. Our work demonstrated the room temperature nonlinear transverse response in a paradigm topological insulator, addressing the tunability of the topological second harmonic response by thickness engineering. △ Less

Submitted 29 October, 2024; originally announced October 2024.

Antiferroelectric Altermagnets: Antiferroelectricity Alters Magnets

Authors: Xunkai Duan, Jiayong Zhang, Ziye Zhu, Yuntian Liu, Zhenyu Zhang, Igor Zutic, Tong Zhou

Abstract: Magnetoelectric coupling is crucial for uncovering fundamental phenomena and advancing technologies in high-density data storage and energy-efficient devices. The emergence of altermagnets, which unify the advantages of ferromagnets and antiferromagnets, offers unprecedented opportunities for magnetoelectric coupling. However, electrically tuning altermagnets remains an outstanding challenge. Here… ▽ More Magnetoelectric coupling is crucial for uncovering fundamental phenomena and advancing technologies in high-density data storage and energy-efficient devices. The emergence of altermagnets, which unify the advantages of ferromagnets and antiferromagnets, offers unprecedented opportunities for magnetoelectric coupling. However, electrically tuning altermagnets remains an outstanding challenge. Here, we demonstrate how this challenge can be overcome by using antiferroelectricity and ferroelectricity to modulate the spin splitting in altermagnets, employing a universal, symmetry-based design principle supported by an effective model. We introduce an unexplored class of multiferroics: antiferroelectric altermagnets (AFEAM), where antiferroelectricity and altermagnetism coexist in a single material. From first-principles calculations, we validate the feasibility of AFEAM in well-established van der Waals metal thio(seleno)phosphates and perovskite oxides. We reveal the design of AFEAM ranging from two-dimensional monolayers to three-dimensional bulk structures. Remarkably, even a weak electric field can effectively toggle spin polarization in the AFEAM by switching between antiferroelectric and ferroelectric states. Our findings not only enrich the understanding of magnetoelectric coupling but also pave the way for electrically controlled spintronic and multiferroic devices. △ Less

Submitted 14 March, 2025; v1 submitted 8 October, 2024; originally announced October 2024.

Comments: PRL Editors' Suggestion and Featured in Physics (https://physics.aps.org/articles/v18/58)

Journal ref: Phys. Rev. Lett. 134 (2025) 106801

Origin of Interstitial Doping Induced Coercive Field Reduction in Ferroelectric Hafnia

Authors: Tianyuan Zhu, Liyang Ma, Xu Duan, Shi Liu

Abstract: Hafnia-based ferroelectrics hold promise for nonvolatile ferroelectric memory devices. However, the high coercive field required for polarization switching remains a prime obstacle to their practical applications. A notable reduction in coercive field has been achieved in ferroelectric Hf(Zr)$_{1+x}$O$_2$ films with interstitial Hf(Zr) dopants [Science 381, 558 (2023)], suggesting a less-explored… ▽ More Hafnia-based ferroelectrics hold promise for nonvolatile ferroelectric memory devices. However, the high coercive field required for polarization switching remains a prime obstacle to their practical applications. A notable reduction in coercive field has been achieved in ferroelectric Hf(Zr)$_{1+x}$O$_2$ films with interstitial Hf(Zr) dopants [Science 381, 558 (2023)], suggesting a less-explored strategy for coercive field optimization. Supported by density functional theory calculations, we demonstrate the $Pca2_1$ phase, with a moderate concentration of interstitial Hf dopants, serves as a minimal model to explain the experimental observations, rather than the originally assumed rhombohedral phase. Large-scale deep potential molecular dynamics simulations suggest that interstitial defects promote the polarization reversal by facilitating $Pbcn$-like mobile 180$^\circ$ domain walls. A simple pre-poling treatment could reduce the switching field to less than 1 MV/cm and enable switching on a subnanosecond timescale. High-throughput calculations reveal a negative correlation between the switching barrier and dopant size and identify a few promising interstitial dopants for coercive field reduction. △ Less

Submitted 3 July, 2024; originally announced July 2024.

Journal ref: Physical Review Letters 134, 056802 (2025)

Emergent superconductivity in doped ferroelectric hafnia

Authors: Xu Duan, Shi Liu

Abstract: Superconductivity and ferroelectricity,representing two distinct forms of ordered states, are typically not found together in the same system, making it even more difficult to create a connection between them. Here, supported by first-principles calculations, we propose that Anderson-Blount's ferroelectric-like metal can be manifested in electron-doped ferroelectric Pca2$_1$ HfO$_2$. In this syste… ▽ More Superconductivity and ferroelectricity,representing two distinct forms of ordered states, are typically not found together in the same system, making it even more difficult to create a connection between them. Here, supported by first-principles calculations, we propose that Anderson-Blount's ferroelectric-like metal can be manifested in electron-doped ferroelectric Pca2$_1$ HfO$_2$. In this system, polar phonons and consequently ferroelectricity are not affected by the presence of itinerant electrons. We find that a nonpolar optical phonon, being strongly coupled to doped electrons, can acquire a pronounced electron-phonon coupling strength to activate conventional Bardeen-Cooper-Schrieffer superconductivity. The displacements of polar oxygen atoms in Pca2$_1$ HfO$_2$ create a link between ferroelectricity and superconductivity, enabling a tunable superconducting temperature ranging approximately from 10 to 30 Kelvin. Owing to hafnia's compatibility with silicon, we suggest HfO$_2$-based ferroelectric superconductors present an opportunity to construct high-performing hybrid integrated systems utilizing switchable quantum states. △ Less

Submitted 5 June, 2023; v1 submitted 3 June, 2023; originally announced June 2023.

Comments: 14 pages, 4 figures

Journal ref: Phys. Rev. B 108, L241114(2023)

Signatures of Chiral Superconductivity in Chiral Molecule Intercalated Tantalum Disulfide

Authors: Zhong Wan, Gang Qiu, Huaying Ren, Qi Qian, Dong Xu, Jingyuan Zhou, Jingxuan Zhou, Boxuan Zhou, Laiyuan Wang, Yu Huang, Kang L. Wang, Xiangfeng Duan

Abstract: Chiral superconductors, a unique class of unconventional superconductors in which the complex superconducting order parameter winds clockwise or counter-clockwise in the momentum space, represent a topologically non-trivial system with direct implications for topological quantum computing. Intrinsic chiral superconductors are extremely rare, with only a few arguable examples including heavy fermio… ▽ More Chiral superconductors, a unique class of unconventional superconductors in which the complex superconducting order parameter winds clockwise or counter-clockwise in the momentum space, represent a topologically non-trivial system with direct implications for topological quantum computing. Intrinsic chiral superconductors are extremely rare, with only a few arguable examples including heavy fermion metals (UTe$_2$, UPt$_3$) and perovskite superconductor Sr$_2$RuO$_4$. Chiral molecules with neither mirror nor inversion symmetry have been widely investigated, in which the spin degeneracy may be lifted by the molecular chirality. Thus, a combination of superconductivity with chiral molecules may lead to a spin-polarized ground state for realizing chiral superconductivity. Herein we report the first investigation of unconventional superconductivity in chiral molecule intercalated tantalum disulfide (TaS$_2$) and reveal key signatures of chiral superconductivity. Little-Parks measurements demonstrate a robust and reproducible half-flux quantum phase shift in both left- and right-handed chiral molecule intercalated TaS$_2$, which is absent in pristine TaS$_2$ or achiral molecule intercalated TaS$_2$, highlighting the essential role of molecular chirality in inducing unconventional superconductivity. The robust half-flux quantum phase shift demonstrates unconventional superconductivity and constitutes strong evidence supporting a chiral superconducting ordering parameter. Critical current measurements at lower temperature reveal a peculiar asymmetric phase shift under opposite supercurrent, with a relative phase difference approaching the unity of π at below 0.5 K, further supporting topologically non-trivial superconductivity. Our study signifies the potential of hybrid superlattices with intriguing coupling between the crystalline atomic layers and the self-assembled molecular layers. △ Less

Submitted 10 February, 2023; originally announced February 2023.

Comments: 22 Pages, 5 figures, 6 Extended Data figures

How to Report and Benchmark Emerging Field-Effect Transistors

Authors: Zhihui Cheng, Chin-Sheng Pang, Peiqi Wang, Son T. Le, Yanqing Wu, Davood Shahrjerdi, Iuliana Radu, Max C. Lemme, Lian-Mao Peng, Xiangfeng Duan, Zhihong Chen, Joerg Appenzeller, Steven J. Koester, Eric Pop, Aaron D. Franklin, Curt A. Richter

Abstract: Emerging low-dimensional nanomaterials have been studied for decades in device applications as field-effect transistors (FETs). However, properly reporting and comparing device performance has been challenging due to the involvement and interlinking of multiple device parameters. More importantly, the interdisciplinarity of this research community results in a lack of consistent reporting and benc… ▽ More Emerging low-dimensional nanomaterials have been studied for decades in device applications as field-effect transistors (FETs). However, properly reporting and comparing device performance has been challenging due to the involvement and interlinking of multiple device parameters. More importantly, the interdisciplinarity of this research community results in a lack of consistent reporting and benchmarking guidelines. Here we report a consensus among the authors regarding guidelines for reporting and benchmarking important FET parameters and performance metrics. We provide an example of this reporting and benchmarking process for a two-dimensional (2D) semiconductor FET. Our consensus will help promote an improved approach for assessing device performance in emerging FETs, thus aiding the field to progress more consistently and meaningfully. △ Less

Submitted 4 August, 2022; v1 submitted 30 March, 2022; originally announced March 2022.

Comments: 15 pages, 3 figures

Journal ref: Nature Electronics 5 (2022) 416-423

Stabilizing $γ$-MgH$_2$ at Nanotwins in Mechanically Constrained Nanoparticles

Authors: Jochen A. Kammerer, Xiaoyang Duan, Frank Neubrech, Rasmus R. Schröder, Na Liu, Martin Pfannmöller

Abstract: Reversible hydrogen uptake and the metal/dielectric transition make the Mg/MgH$_2$ system a prime candidate for solid state hydrogen storage and dynamic plasmonics. However, high dehydrogenation temperatures and slow dehydrogenation hamper broad applicability. One promising strategy to improve dehydrogenation is the formation of metastable $γ$-MgH$_2$. A nanoparticle (NP) design, where $γ$-MgH… ▽ More Reversible hydrogen uptake and the metal/dielectric transition make the Mg/MgH$_2$ system a prime candidate for solid state hydrogen storage and dynamic plasmonics. However, high dehydrogenation temperatures and slow dehydrogenation hamper broad applicability. One promising strategy to improve dehydrogenation is the formation of metastable $γ$-MgH$_2$. A nanoparticle (NP) design, where $γ$-MgH$_2$ forms intrinsically during hydrogenation is presented and a formation mechanism based on transmission electron microscopy results is proposed.Volume expansion during hydrogenation causes compressive stress within the confined, anisotropic NPs, leading to plastic deformation of $β$-MgH$_2$ via (301) $β$ twinning. It is proposed that these twins nucleate $γ$-MgH$_2$ nanolamellas, which are stabilized by residual compressive stress. Understanding this mechanism is a crucial step toward cycle-stable, Mg-based dynamic plasmonic and hydrogen-storage materials with improved dehydrogenation. It is envisioned that a more general design of confined NPs utilizes the inherent volume expansion to reform $γ$-MgH$_2$ during each rehydrogenation △ Less

Submitted 30 April, 2021; originally announced April 2021.

Journal ref: Adv. Mater. 33, 2008259 (2021)

Two-dimensional multiferroic metal with voltage-tunable magnetization and metallicity

Authors: Xu Duan, Jiawei Huang, Bin Xu, Shi Liu

Abstract: We design a multiferroic metal that combines seemingly incompatible ferromagnetism, ferroelectricity, and metallicity by hole doping a two-dimensional (2D) ferroelectric with high density of states near the Fermi level. The strong magnetoelectric effect is demonstrated in hole-doped and arsenic-doped monolayer α-In2Se3 using first-principles calculations. Taking advantage of the oppositely charged… ▽ More We design a multiferroic metal that combines seemingly incompatible ferromagnetism, ferroelectricity, and metallicity by hole doping a two-dimensional (2D) ferroelectric with high density of states near the Fermi level. The strong magnetoelectric effect is demonstrated in hole-doped and arsenic-doped monolayer α-In2Se3 using first-principles calculations. Taking advantage of the oppositely charged surfaces created by an out-of-plane polarization, the 2D magnetization and metallicity can be electrically switched on and off in an asymmetrically doped monolayer. The substitutional arsenic defect pair exhibits an intriguing electric field-tunable charge disproportionation process accompanied with an on-off switch of local magnetic moments. The charge ordering process can be controlled by tuning the relative strength of on-site Coulomb repulsion and defect dipole-polarization coupling via strain engineering. Our design principle relying on no transition metal broadens the materials design space for 2D multiferroic metals. △ Less

Submitted 27 June, 2021; v1 submitted 18 March, 2021; originally announced March 2021.

Comments: 23 pages,5 figures

Journal ref: Materials Horizons, 8, 2316-2324 (2021)

On-demand quantum spin Hall insulators controlled by two-dimensional ferroelectricity

Authors: Jiawei Huang, Xu Duan, Sunam Jeon, Youngkuk Kim, Jian Zhou, Jian Li, Shi Liu

Abstract: The coexistence of ferroelectric and topological orders in two-dimensional (2D) atomic crystals allows non-volatile and switchable quantum spin Hall states. Here we offer a general design principle for 2D bilayer heterostructures that can host ferroelectricity and nontrivial band topology simultaneously using only topologically trivial building blocks. The built-in electric field arising from the… ▽ More The coexistence of ferroelectric and topological orders in two-dimensional (2D) atomic crystals allows non-volatile and switchable quantum spin Hall states. Here we offer a general design principle for 2D bilayer heterostructures that can host ferroelectricity and nontrivial band topology simultaneously using only topologically trivial building blocks. The built-in electric field arising from the out-of-plane polarization across the heterostrucuture enables a robust control of the band gap size and band inversion strength, which can be utilized to manipulate topological phase transitions. Using first-principles calculations, we demonstrate a series of bilayer heterostructures are 2D ferroelectric topological insulators (2DFETIs) characterized with a direct coupling between band topology and polarization state. We propose a few 2DFETI-based quantum electronics including domain-wall quantum circuits and topological memristor. △ Less

Submitted 2 May, 2021; v1 submitted 20 January, 2021; originally announced January 2021.

Designing Xenes with Two-Dimensional Triangular Lattice

Authors: Xu Duan, Zhao Liu, Brendan M. Hanrahan, Wei Zhu, Shi Liu

Abstract: Xenes, graphene-like two-dimensional (2D) monoelemental crystals with a honeycomb symmetry, have been the focus of numerous experimental and theoretical studies. In comparison, single-element 2D materials with a triangular lattice symmetry have not received due attention. Here, taking Pb as an example, we investigate the triangular-lattice monolayer made of group-IV atoms employing first-principle… ▽ More Xenes, graphene-like two-dimensional (2D) monoelemental crystals with a honeycomb symmetry, have been the focus of numerous experimental and theoretical studies. In comparison, single-element 2D materials with a triangular lattice symmetry have not received due attention. Here, taking Pb as an example, we investigate the triangular-lattice monolayer made of group-IV atoms employing first-principles density functional theory calculations. The flat Pb monolayer supports a mirror-symmetry-protected spinless nodal line in the absence spin-orbit coupling (SOC). The introduction of an out-of-plane buckling creates a glide mirror, protecting an anisotropic Dirac nodal loop. Both flat and buckled Pb monolayers become topologically trivial after including SOC. A large buckling will make the Pb sheet a 2D semiconductor with symmetry-protected Dirac points below the Fermi level. The electronic structures of other group-IV triangular lattices such as Ge and Sn demonstrate strong similarity to Pb. We further design a quasi-3D crystal PbHfO$_2$ by alternately stacking Pb and 1T-HfO$_2$ monolayers. The new compound PbHfO$_2$ is dynamically stable and retains the properties of Pb monolayer. By applying epitaxial strains to PbHfO$_2$, it is possible to drive an insulator-to-metal transition coupled with an anti-ferroelectric-to-paraelectric phase transition. Our results suggest the potential of the 2D triangular lattice as a complimentary platform to design new type of broadly-defined Xenes. △ Less

Submitted 3 September, 2020; originally announced September 2020.

Journal ref: Phys. Rev. Materials 4, 124003 (2020)

Coexistence of nontrivial topological properties and strong ferromagnetic fluctuations in $A_2$Cr$_3$As$_3$ ($A$=Na, K, Rb and Cs)

Authors: Chenchao Xu, Ninghua Wu, Guo-Xiang Zhi, Bing-Hua Lei, Xu Duan, Fanlong Ning, Chao Cao, Qijin Chen

Abstract: Superconductivity in crystals without inversion symmetry has received extensive attention due to its unconventional pairing and possible nontrivial topological properties. Using first-principles calculations, we systemically study the electronic structure of noncentrosymmetric superconductors $A_2$Cr$_3$As$_3$ ($A$=Na, K, Rb and Cs). Topologically protected triply degenerate points connected by on… ▽ More Superconductivity in crystals without inversion symmetry has received extensive attention due to its unconventional pairing and possible nontrivial topological properties. Using first-principles calculations, we systemically study the electronic structure of noncentrosymmetric superconductors $A_2$Cr$_3$As$_3$ ($A$=Na, K, Rb and Cs). Topologically protected triply degenerate points connected by one-dimensional arcs appear along the $C_{3}$ axis, coexisting with strong ferromagnetic (FM) fluctuations in the non-superconducting state. Within random phase approximation, our calculations show that strong enhancements of spin fluctuations are present in K$_2$Cr$_3$As$_3$ and Rb$_2$Cr$_3$As$_3$, and are substantially reduced in Na$_2$Cr$_3$As$_3$ and Cs$_2$Cr$_3$As$_3$. Symmetry analysis of spin-orbit coupling $g_{k}$ suggests that the arc surface states might remain stable in the superconducting state, giving rise to possible nontrivial topological properties. △ Less

Submitted 3 April, 2020; v1 submitted 10 September, 2019; originally announced September 2019.

Comments: We have some typos corrected and add discussion about the effect of ferromagnetic fluctuations on the band topology. We also add discussion on the topological property in the superconducting state with the symmetry indicators method recently proposed

Journal ref: npj Computational Materials 6,30 (2020)

Hopping conductance and macroscopic quantum tunneling effect in three dimensional Pb$_x$(SiO$_2$)$_{1-x}$ nanogranular films

Authors: Xiu-Zhi Duan, Zhi-Hao He, Yang Yang, Zhi-Qing Li

Abstract: We have studied the low-temperature electrical transport properties of Pb$_x$(SiO$_2$)$_{1-x}$ ($x$ being the Pb volume fraction) nanogranular films with thicknesses of $\sim$1000 nm and $x$ spanning the dielectric, transitional, and metallic regions. It is found that the percolation threshold $x_c$ lies between 0.57 and 0.60. For films with $x$$\lesssim$0.50, the resistivities $ρ$ as functions of… ▽ More We have studied the low-temperature electrical transport properties of Pb$_x$(SiO$_2$)$_{1-x}$ ($x$ being the Pb volume fraction) nanogranular films with thicknesses of $\sim$1000 nm and $x$ spanning the dielectric, transitional, and metallic regions. It is found that the percolation threshold $x_c$ lies between 0.57 and 0.60. For films with $x$$\lesssim$0.50, the resistivities $ρ$ as functions of temperature $T$ obey $ρ\propto\exp(Δ/k_BT)$ relation ($Δ$ being the local superconducting gap and the $k_B$ Boltzmann constant) below the superconducting transition temperature $T_c$ ($\sim$7 K) of Pb granules. The value of the gap obtained via this expression is almost identical to that by single electron tunneling spectra measurement. The magnetoresistance is negative below $T_c$ and its absolute value is far larger than that above $T_c$ at a certain field. These observations indicate that single electron hopping (or tunneling), rather than Cooper pair hopping (or tunneling) governs the transport processes below $T_c$. The temperature dependence of resistivities shows reentrant behavior for the 0.50$<$$x$$<$0.57 films. It is found that single electron hopping (or tunneling) also dominates the low-temperature transport process for these films. The reduction of the single electron concentration leads to an enhancement of the resisivity at sufficiently low temperature. For the 0.60$\lesssim$$x$$\lesssim$0.72 films, the resistivities sharply decrease with decreasing temperature just below $T_c$, and then show dissipation effect with further decreasing temperature. Treating the conducting paths composed of Pb particles as nanowires, we have found that the $R(T)$ data below $T_c$ can be well explained by a model that includes both thermally activated phase slips and quantum phase slips. △ Less

Submitted 16 October, 2018; originally announced October 2018.

Comments: 8 pages, 6 figures

Journal ref: Phys. Rev. B 99, 094204 (2019)

Harmonic field in knotted space

Authors: Xiuqing Duan, Zhenwei Yao

Abstract: Knotted fields enrich a variety of physical phenomena, ranging from fluid flows, electromagnetic fields, to textures of ordered media. Maxwell's electrostatic equations, whose vacuum solution is mathematically known as a harmonic field, provide an ideal setting to explore the role of domain topology in determining physical fields in confined space. In this work, we show the uniqueness of a harmoni… ▽ More Knotted fields enrich a variety of physical phenomena, ranging from fluid flows, electromagnetic fields, to textures of ordered media. Maxwell's electrostatic equations, whose vacuum solution is mathematically known as a harmonic field, provide an ideal setting to explore the role of domain topology in determining physical fields in confined space. In this work, we show the uniqueness of a harmonic field in knotted tubes, and reduce the construction of a harmonic field to a Neumann boundary value problem. By analyzing the harmonic field in typical knotted tubes, we identify the torsion driven transition from bipolar to vortex patterns. We also analogously extend our discussion to the organization of liquid crystal textures in knotted tubes. These results further our understanding about the general role of topology in shaping a physical field in confined space, and may find applications in the control of physical fields by manipulation of surface topology. △ Less

Submitted 25 April, 2018; originally announced April 2018.

Comments: 5 pages, 2 figures

Journal ref: Physical Review E 97, 040702(R) (2018)

Large-Scale Fabrication of RF MOSFETs on Liquid-Exfoliated MoS2

Authors: Kuanchen Xiong, Lei Li, Asher Madjar, James C. M. Hwang, Zhaoyang Lin, Yu Huang, Xiangfeng Duan, Alexander Goritz, Matthias Wietstruck, Mehmet Kaynak

Abstract: For the first time, thousands of RF MOSFETs were batch-fabricated on liquid-exfoliated MoS2 below 300 °C with nearly 100% yield. The large-scale fabrication with high yield allowed the average performance instead of the best performance to be reported. The DC performance of these devices were typical, but the RF performance, enabled by buried gates and on the order of 100 MHz, was reported for the… ▽ More For the first time, thousands of RF MOSFETs were batch-fabricated on liquid-exfoliated MoS2 below 300 °C with nearly 100% yield. The large-scale fabrication with high yield allowed the average performance instead of the best performance to be reported. The DC performance of these devices were typical, but the RF performance, enabled by buried gates and on the order of 100 MHz, was reported for the first time for liquid-exfoliated MoS2. To resolve the dilemma of thin vs. thick films, gate recess was used on 20-nm thick films to improve the gate control while keeping the contact resistance lower than that on 10-nm films. These innovations may enable thin-film transistors to operate in the microwave range. △ Less

Submitted 27 March, 2018; originally announced March 2018.

DNA nanotechnology-enabled chiral plasmonics: from static to dynamic

Authors: Chao Zhou, Xiaoyang Duan, Na Liu

Abstract: In this Account, we discuss a variety of static and dynamic chiral plasmonic nanostructures enabled by DNA nanotechnology. In the category of static plasmonic systems, we first show chiral plasmonic nanostructures based on spherical AuNPs, including plasmonic helices, toroids, and tetramers. To enhance the CD responses, anisotropic gold nanorods with larger extinction coefficients are utilized to… ▽ More In this Account, we discuss a variety of static and dynamic chiral plasmonic nanostructures enabled by DNA nanotechnology. In the category of static plasmonic systems, we first show chiral plasmonic nanostructures based on spherical AuNPs, including plasmonic helices, toroids, and tetramers. To enhance the CD responses, anisotropic gold nanorods with larger extinction coefficients are utilized to create chiral plasmonic crosses and helical superstructures. Next, we highlight the inevitable evolution from static to dynamic plasmonic systems along with the fast development of this interdisciplinary field. Several dynamic plasmonic systems are reviewed according to their working mechanisms. △ Less

Submitted 18 March, 2018; originally announced March 2018.

Comments: 7 figures

Journal ref: Acc. Chem. Res. 50, 2906 (2017)

Plasmonic Toroidal Metamolecules Assembled by DNA Origami

Authors: Maximilian J. Urban, Palash K. Dutta, Pengfei Wang, Xiaoyang Duan, Xibo Shen, Baoquan Ding, Yonggang Ke, Na Liu

Abstract: We demonstrate hierarchical assembly of plasmonic toroidal metamolecules, which exhibit tailored optical activity in the visible spectral range. Each metamolecule consists of four identical origami-templated helical building blocks. Such toroidal metamolecules show stronger chiroptical response than monomers and dimers of the helical building blocks. Enantiomers of the plasmonic structures yield o… ▽ More We demonstrate hierarchical assembly of plasmonic toroidal metamolecules, which exhibit tailored optical activity in the visible spectral range. Each metamolecule consists of four identical origami-templated helical building blocks. Such toroidal metamolecules show stronger chiroptical response than monomers and dimers of the helical building blocks. Enantiomers of the plasmonic structures yield opposite circular dichroism spectra. The experimental results agree well with the theoretical simulations. We also demonstrate that given the circular symmetry of the structures, distinct chiroptical response along their axial orientation can be uncovered via simple spin-coating of the metamolecules on substrates. Our work provides a new strategy to create plasmonic chiral platforms with sophisticated nanoscale architectures for potential applications such as chiral sensing using chemically-based assembly systems. △ Less

Submitted 18 March, 2018; originally announced March 2018.

Comments: 4 figures

Journal ref: J. Am. Chem. Soc. 138, 5495 (2016)

Granular-composite-like electrical transport properties of polycrystalline cubic TaN$_{x}$ thin films prepared by rf sputtering method

Authors: Ran Li, Xiu-Zhi Duan, Xin Zhu, Yang Yang, Ding-Bang Zhou, Zhi-Qing Li

Abstract: We have systematically investigated the electrical transport properties of polycrystalline TaN$_x$ ($0.83$$\lesssim$$x$$\lesssim$1.32) films with rocksalt structure from 300 down to 2 K. It is found that the conductivity varies linearly with $\ln T$ from $\sim$6 K to $\sim$30 K, which does not originate from the conventional two dimensional weak-localization or electron-electron interaction effect… ▽ More We have systematically investigated the electrical transport properties of polycrystalline TaN$_x$ ($0.83$$\lesssim$$x$$\lesssim$1.32) films with rocksalt structure from 300 down to 2 K. It is found that the conductivity varies linearly with $\ln T$ from $\sim$6 K to $\sim$30 K, which does not originate from the conventional two dimensional weak-localization or electron-electron interaction effects, but can be well explained by the intergrain Coulomb effect which was theoretically proposed in the granular metals. While the fluctuation-induced tunneling conduction process dominates the temperature behaviors of the conductivities (resistivities) above $\sim$60 K. Normal state to superconductive state transition is observed in the $x$$\gtrsim$1.04 films in low temperature regime. The superconductivity can still be retained at a field of 9 T. The upper critical field increases linearly with decreasing temperature in the vicinity of the superconductive transition temperature, which is the typical feature of granular superconductors or dirty type-II superconductors. The granular-composite-like electrical transport properties of the polycrystalline TaN$_x$ films are related to their microstructure, in which the TaN$_x$ grains with high conductivity are separated by the poorly conductive amorphous transition layers (grain boundaries). △ Less

Submitted 14 March, 2018; originally announced March 2018.

Comments: 7 pages. 4 figures

Tunable Electronic Structure and Topological Properties of $LnPn$ ($Ln$=Ce, Pr, Gd, Sm, Yb; $Pn$=Sb, Bi)

Authors: Xu Duan, Fan Wu, Jia Chen, Peiran Zhang, Yang Liu, Huiqiu Yuan, Chao Cao

Abstract: We have performed systematic first principles study of the electronic structure and band topology properties of $LnPn$ compounds ($Ln$=Ce, Pr, Gd, Sm, Yb; $Pn$=Sb, Bi). Assuming the $f$-electrons are well localized in these materials, both hybrid functional and modified Becke-Johnson calculations yield electronic structure in good agreement with experimental observations, while generalized gradien… ▽ More We have performed systematic first principles study of the electronic structure and band topology properties of $LnPn$ compounds ($Ln$=Ce, Pr, Gd, Sm, Yb; $Pn$=Sb, Bi). Assuming the $f$-electrons are well localized in these materials, both hybrid functional and modified Becke-Johnson calculations yield electronic structure in good agreement with experimental observations, while generalized gradient approximation calculations severely overestimate the band inversions. From Ce to Yb, a systematic reduction of band inversion with respect to the increasing $Ln$ atomic number is observed, and $\mathcal{Z}_2$ for Ce$Pn$ and Yb$Pn$ are [1;000] and [0;000], respectively. In both hybrid functional and modified Becke-Johns calculations, a topologically nontrivial to trivial transition is expected around SmSb for the antimonides and around DyBi for the bismuthides. Such variation is related with lanthanide contraction, but is different from simple pressure effect. △ Less

Submitted 7 November, 2018; v1 submitted 13 February, 2018; originally announced February 2018.

Comments: Updated version

Journal ref: Communications Physics 1, 71 (2018)

Liquid phase mass production of air-stable black phosphorus/phospholipids nanocomposite with ultralow tunneling barrier

Authors: Qiankun Zhang, Yinan Liu, Jiawei Lai, Shaomian Qi, Chunhua An, Yao Lu, Xuexin Duan, Wei Pang, Daihua Zhang, Dong Sun, Jian-Hao Chen, Jing Liu

Abstract: Few-layer black phosphorus (FLBP), a recently discovered two-dimensional semiconductor, has attracted substantial attention in the scientific and technical communities due to its great potential in electronic and optoelectronic applications. However, reactivity of FLBP flakes with ambient species limits its direct applications. Among various methods to passivate FLBP in ambient environment, nanoco… ▽ More Few-layer black phosphorus (FLBP), a recently discovered two-dimensional semiconductor, has attracted substantial attention in the scientific and technical communities due to its great potential in electronic and optoelectronic applications. However, reactivity of FLBP flakes with ambient species limits its direct applications. Among various methods to passivate FLBP in ambient environment, nanocomposites mixing FLBP flakes with stable matrix may be one of the most promising approaches for industry applications. Here, we report a simple one-step procedure to mass produce air-stable FLBP/phospholipids nanocomposite in liquid phase. The resultant nanocomposite is found to have ultralow tunneling barrier for charge carriers which can be described by an Efros-Shklovskii variable range hopping mechanism. Devices made from such mass-produced FLBP/phospholipids nanocomposite show highly stable electrical conductivity and opto-electrical response in ambient conditions, indicating its promising applications in both electronic and optoelectronic applications. This method could also be generalized to the mass production of nanocomposites consisting of other air-sensitive two-dimensional materials, such as FeSe, NbSe2, WTe2, etc. △ Less

Submitted 22 January, 2018; originally announced January 2018.

Ultrafast Epitaxial Growth of Metre-Sized Single-Crystal Graphene on Industrial Cu Foil

Authors: Xiaozhi Xu, Zhihong Zhang, Jichen Dong, Ding Yi, Jingjing Niu, Muhong Wu, Li Lin, Rongkang Yin, Mingqiang Li, Jingyuan Zhou, Shaoxin Wang, Junliang Sun, Xiaojie Duan, Peng Gao, Ying Jiang, Xiaosong Wu, Hailin Peng, Rodney S. Ruoff, Zhongfan Liu, Dapeng Yu, Enge Wang, Feng Ding, Kaihui Liu

Abstract: A foundation of the modern technology that uses single-crystal silicon has been the growth of high-quality single-crystal Si ingots with diameters up to 12 inches or larger. For many applications of graphene, large-area high-quality (ideally of single-crystal) material will be enabling. Since the first growth on copper foil a decade ago, inch-sized single-crystal graphene has been achieved. We pre… ▽ More A foundation of the modern technology that uses single-crystal silicon has been the growth of high-quality single-crystal Si ingots with diameters up to 12 inches or larger. For many applications of graphene, large-area high-quality (ideally of single-crystal) material will be enabling. Since the first growth on copper foil a decade ago, inch-sized single-crystal graphene has been achieved. We present here the growth, in 20 minutes, of a graphene film of 5 x 50 cm2 dimension with > 99% ultra-highly oriented grains. This growth was achieved by: (i) synthesis of sub-metre-sized single-crystal Cu(111) foil as substrate; (ii) epitaxial growth of graphene islands on the Cu(111) surface; (iii) seamless merging of such graphene islands into a graphene film with high single crystallinity and (iv) the ultrafast growth of graphene film. These achievements were realized by a temperature-driven annealing technique to produce single-crystal Cu(111) from industrial polycrystalline Cu foil and the marvellous effects of a continuous oxygen supply from an adjacent oxide. The as-synthesized graphene film, with very few misoriented grains (if any), has a mobility up to ~ 23,000 cm2V-1s-1 at 4 K and room temperature sheet resistance of ~ 230 ohm/square. It is very likely that this approach can be scaled up to achieve exceptionally large and high-quality graphene films with single crystallinity, and thus realize various industrial-level applications at a low cost. △ Less

Submitted 8 July, 2017; originally announced July 2017.

Curvature-driven stability of defects in nematic textures over spherical disks

Authors: Xiuqing Duan, Zhenwei Yao

Abstract: Stabilizing defects in liquid-crystal systems is crucial for many physical processes and applications ranging from functionalizing liquid-crystal textures to recently reported command of chaotic behaviors of active matters. In this work, we perform analytical calculations to study the curvature driven stability mechanism of defects based on the isotropic nematic disk model that is free of any topo… ▽ More Stabilizing defects in liquid-crystal systems is crucial for many physical processes and applications ranging from functionalizing liquid-crystal textures to recently reported command of chaotic behaviors of active matters. In this work, we perform analytical calculations to study the curvature driven stability mechanism of defects based on the isotropic nematic disk model that is free of any topological constraint. We show that in a growing spherical disk covering a sphere the accumulation of curvature effect can prevent typical +1 and +1/2 defects from forming boojum textures where the defects are repelled to the boundary of the disk. Our calculations reveal that the movement of the equilibrium position of the +1 defect from the boundary to the center of the spherical disk occurs in a very narrow window of the disk area, exhibiting the first-order phase-transition-like behavior. For the pair of +1/2 defects by splitting a +1 defect, we find the curvature driven alternating repulsive and attractive interactions between the two defects. With the growth of the spherical disk these two defects tend to approach and finally recombine towards a +1 defect texture. The sensitive response of defects to curvature and the curvature driven stability mechanism demonstrated in this work in nematic disk systems may have implications towards versatile control and engineering of liquid crystal textures in various applications. △ Less

Submitted 27 June, 2017; originally announced June 2017.

Comments: 11 pages, 5 figures

Journal ref: Phys. Rev. E 95, 062706, 2017

Enhanced interlayer neutral excitons and trions in trilayer van der Waals heterostructures

Authors: Chanyeol Choi, Jiahui Huang, Hung-Chieh Cheng, Hyunseok Kim, Abhinav Kumar Vinod, Sang-Hoon Bae, V. Ongun Ozcelik, Roberto Grassi, Jongjae Chae, Shu-Wei Huang, Xiangfeng Duan, Kristen Kaasbjerg, Tony Low, Chee Wei Wong

Abstract: Vertically stacked van der Waals heterostructures constitute a promising platform for providing tailored band alignment with enhanced excitonic systems. Here we report observations of neutral and charged interlayer excitons in trilayer WSe2-MoSe2-WSe2 van der Waals heterostructures and their dynamics. The addition of a WSe2 layer in the trilayer leads to significantly higher photoluminescence quan… ▽ More Vertically stacked van der Waals heterostructures constitute a promising platform for providing tailored band alignment with enhanced excitonic systems. Here we report observations of neutral and charged interlayer excitons in trilayer WSe2-MoSe2-WSe2 van der Waals heterostructures and their dynamics. The addition of a WSe2 layer in the trilayer leads to significantly higher photoluminescence quantum yields and tunable spectral resonance compared to its bilayer heterostructures at cryogenic temperatures. The observed enhancement in the photoluminescence quantum yield is due to significantly larger electron-hole overlap and higher light absorbance in the trilayer heterostructure, supported via first-principle pseudopotential calculations based on spin-polarized density functional theory. We further uncover the temperature- and power-dependence, as well as time-resolved photoluminescence of the trilayer heterostructure interlayer neutral excitons and trions. Our study elucidates the prospects of manipulating light emission from interlayer excitons and designing atomic heterostructures from first-principles for optoelectronics. △ Less

Submitted 16 September, 2018; v1 submitted 19 June, 2017; originally announced June 2017.

Comments: 25 pages, 5 figures(Maintext). 9 pages, 7 figures(Supplementary Information). - Accepted for publication in npg: 2D materials and applications and reformatted to its standard. - Updated co-authors and references. - Title and abstract are modified for clarity. - Errors have been corrected, npg: 2D materials and applications (2018)

Monte Carlo simulation of electrostatic interactions in inhomogeneous dielectric media: Correct sampling for the local lattice simulation algorithm

Authors: Xiaozheng Duan, Issei Nakamura, Zhen-Gang Wang

Abstract: We present a lattice Monte Carlo algorithm based on the one originally proposed by Maggs and Rossetto for simulating electrostatic interactions in inhomogeneous dielectric media. The original algorithm is known to produce attractive interactions between particles of the same dielectric constant in the medium of different dielectric constant. We demonstrate that such interactions are spurious, caus… ▽ More We present a lattice Monte Carlo algorithm based on the one originally proposed by Maggs and Rossetto for simulating electrostatic interactions in inhomogeneous dielectric media. The original algorithm is known to produce attractive interactions between particles of the same dielectric constant in the medium of different dielectric constant. We demonstrate that such interactions are spurious, caused by incorrectly biased statistical weight arising from particle motion during the Monte Carlo moves. We propose a simple parallel tempering algorithm that corrects this unphysical bias. The efficacy of our algorithm is tested on a simple binary mixture and on an uncharged polymer in a solvent, and applied to salt-doped polymer solutions. △ Less

Submitted 10 May, 2017; originally announced May 2017.

Gate-induced insulator to band-like transport transition in organolead halide perovskite

Authors: Dehui Li, Hung-Chieh Cheng, Hao Wu, Yiliu Wang, Jian Guo, Gongming Wang, Yu Huang, Xiangfeng Duan

Abstract: Understanding the intrinsic charge transport in organolead halide perovskites is essential for the development of high-efficiency photovoltaics and other optoelectronic devices. Despite the rapid advancement of the organolead halide perovskite in photovoltaic and optoelectronic applications, the intrinsic charge carrier transport in these materials remains elusive partly due to the difficulty of f… ▽ More Understanding the intrinsic charge transport in organolead halide perovskites is essential for the development of high-efficiency photovoltaics and other optoelectronic devices. Despite the rapid advancement of the organolead halide perovskite in photovoltaic and optoelectronic applications, the intrinsic charge carrier transport in these materials remains elusive partly due to the difficulty of fabricating electrical devices and obtaining good electrical contact. Here, we report the fabrication of organolead halide perovskite microplates with monolayer graphene as low barrier electrical contact. A systematic charge transport studies reveal an insulator to band-like transport transition. Our studies indicate that the insulator to band-like transport transition depends on the orthorhombic-to-tetragonal phase transition temperature and defect densities of the organolead halide perovskite microplates. Our findings are not only important for the fundamental understanding of charge transport behavior but also offer valuable practical implications for photovoltaics and optoelectronic applications based on the organolead halide perovskite. △ Less

Submitted 9 October, 2016; originally announced October 2016.

Comments: 18 pages, 5 figures

High Current Density Vertical Tunneling Transistors from Graphene/Highly-Doped Silicon Heterostructures

Authors: Yuan Liu, Jiming Sheng, Hao Wu, Qiyuan He, Hung-Chieh Cheng, Muhammad Imran Shakir, Yu Huang, Xiangfeng Duan

Abstract: Graphene/silicon heterostructures have attracted tremendous interest as a new platform for diverse electronic and photonic devices such as barristors, solar cells, optical modulators, and chemical sensors. The studies to date largely focus on junctions between graphene and lightly-doped silicon, where a Schottky barrier is believed to dominate the carrier transport process. Here we report a system… ▽ More Graphene/silicon heterostructures have attracted tremendous interest as a new platform for diverse electronic and photonic devices such as barristors, solar cells, optical modulators, and chemical sensors. The studies to date largely focus on junctions between graphene and lightly-doped silicon, where a Schottky barrier is believed to dominate the carrier transport process. Here we report a systematic investigation of carrier transport across the heterojunctions formed between graphene and highly-doped silicon. By varying the silicon doping level and the measurement temperature, we show that the carrier transport across the graphene/p++-Si heterojunction is dominated by tunneling effect through the native oxide. We further demonstrate that the tunneling current can be effectively modulated by the external gate electrical field, resulting in a vertical tunneling transistor. Benefited from the large density of states of highly doped silicon, our tunneling transistors can deliver a current density over 20 A/cm2, about two orders of magnitude higher than previous graphene/insulator/graphene tunneling transistor at the same on/off ratio. △ Less

Submitted 28 December, 2015; originally announced December 2015.

Wafer-scale growth of large arrays of perovskite microplate crystals for functional electronics and optoelectronics

Authors: Gongming Wang, Dehui Li, Hung-Chieh Cheng, Yongjia Li, Chih-Yen Chen, Anxiang Yin, Zipeng Zhao, Zhaoyang Lin, Hao Wu, Qiyuan He, Mengning Ding, Yuan Liu, Yu Huang, Xiangfeng Duan

Abstract: Methylammonium lead iodide perovskite has attracted intensive interest for its diverse optoelectronic applications. However, most studies to date have been limited to bulk thin films that are difficult to implement for integrated device arrays because of their incompatibility with typical lithography processes. We report the first patterned growth of regular arrays of perovskite microplate crystal… ▽ More Methylammonium lead iodide perovskite has attracted intensive interest for its diverse optoelectronic applications. However, most studies to date have been limited to bulk thin films that are difficult to implement for integrated device arrays because of their incompatibility with typical lithography processes. We report the first patterned growth of regular arrays of perovskite microplate crystals for functional electronics and optoelectronics. We show that large arrays of lead iodide microplates can be grown from an aqueous solution through a seeded growth process and can be further intercalated with methylammonium iodide to produce perovskite crystals. Structural and optical characterizations demonstrate that the resulting materials display excellent crystalline quality and optical properties. We further show that perovskite crystals can be selectively grown on prepatterned electrode arrays to create independently addressable photodetector arrays and functional field effect transistors. The ability to grow perovskite microplates and to precisely place them at specific locations offers a new material platform for the fundamental investigation of the electronic and optical properties of perovskite materials and opens a pathway for integrated electronic and optoelectronic systems. △ Less

Submitted 16 October, 2015; originally announced October 2015.

Comments: 8 pages, 4 figures

Journal ref: Science Advances. 1, e1500613, 2015

Van der Waals heterojunction devices based on organohalide perovskites and two-dimensional materials

Authors: Hung-Chieh Cheng, Gongming Wang, Dehui Li, Qiyuan He, Anxiang Yin, Yuan Liu, Hao Wu, Mengning Ding, Yu Huang, Xiangfeng Duan

Abstract: The recently emerged organohalide perovskites (e.g., CH3NH3PbI3) have drawn intense attention for high efficiency solar cells. However, with a considerable solubility in many solvents, these perovskites are not typically compatible with conventional lithography processes for more complicated device fabrications that are important for both fundamental studies and technological applications. Here we… ▽ More The recently emerged organohalide perovskites (e.g., CH3NH3PbI3) have drawn intense attention for high efficiency solar cells. However, with a considerable solubility in many solvents, these perovskites are not typically compatible with conventional lithography processes for more complicated device fabrications that are important for both fundamental studies and technological applications. Here we report the creation of novel heterojunction devices based on perovskites and two-dimensional (2D) crystals by taking advantage of the layered characteristic of lead iodide (PbI2) and vapor phase intercalation. We show a graphene/perovskite/graphene vertical stack can deliver a highest photoresponsivity of ~950 A/W and photoconductive gain of ~2200, and a graphene/WSe2/perovskite/graphene heterojunction can display a high on/off ratio (~10^6) transistor behavior with distinct gate-tunable diode characteristics and open-circuit voltages. Such unique perovskite-2D heterostructures have significant potential for future optoelectronic research and can enable broad possibilities with compositional tunability of organohalide perovskites and the versatility offered by diverse 2D materials. △ Less

Submitted 25 September, 2015; originally announced September 2015.

Conductance Discontinuity on the Surface of a Topological Insulator with Magnetic Electrodes

Authors: Xiaopeng Duan, Xi-Lai Li, Yuriy G. Semenov, Ki Wook Kim

Abstract: Asymmetric electrical conductance is theoretically demonstrated on the surface of a topological insulator (TI) in the limit of infinitesimally small forward and reverse biases between two spin selective electrodes. The discontinuous behavior relies on the spin-momentum interlocked nature of TI surface electrons together with the resulting imbalance in the coupling coefficients between the electrod… ▽ More Asymmetric electrical conductance is theoretically demonstrated on the surface of a topological insulator (TI) in the limit of infinitesimally small forward and reverse biases between two spin selective electrodes. The discontinuous behavior relies on the spin-momentum interlocked nature of TI surface electrons together with the resulting imbalance in the coupling coefficients between the electrodes and TI surface states. The analysis is based on a transmission matrix model that, in combination with a phenomenological treatment for the diffusive limit, accounts for both ballistic and scattered paths simultaneously. With the estimated conductance asymmetry over a factor of 10, implementation in the ratchet-like applications and low-voltage rectification circuits appears practicable. △ Less

Submitted 15 September, 2015; originally announced September 2015.

Glass formation in a mixture of hard disks and hard ellipses

Authors: Wen-Sheng Xu, Xiaozheng Duan, Zhao-Yan Sun, Li-Jia An

Abstract: We present an event-driven molecular dynamics study of glass formation in two-dimensional binary mixtures composed of hard disks and hard ellipses, where both types of particles have the same area. We demonstrate that characteristic glass-formation behavior appears upon compression under appropriate conditions in such systems. In particular, while a rotational glass transition occurs only for the… ▽ More We present an event-driven molecular dynamics study of glass formation in two-dimensional binary mixtures composed of hard disks and hard ellipses, where both types of particles have the same area. We demonstrate that characteristic glass-formation behavior appears upon compression under appropriate conditions in such systems. In particular, while a rotational glass transition occurs only for the ellipses, both types of particles undergo a kinetic arrest in the translational degrees of freedom at a single density. The translational dynamics for the ellipses is found to be faster than that for the disks within the same system, indicating that shape anisotropy promotes the translational motion of particles. We further examine the influence of mixture's composition and aspect ratio on the glass formation. For the mixtures with an ellipse aspect ratio of $k=2$, both translational and rotational glass transition densities decrease with increasing the disk concentration at a similar rate and hence, the two glass transitions remain close to each other at all concentrations investigated. By elevating $k$, however, the rotational glass transition density diminishes at a faster rate than the translational one, leading to the formation of an orientational glass for the ellipses between the two transitions. Our simulations imply that mixtures of particles with different shapes emerge as a promising model for probing the role of particle shape in determining the properties of glass-forming liquids. Furthermore, our work illustrates the potential of using knowledge concerning the dependence of glass-formation properties on mixture's composition and particle shape to assist in the rational design of amorphous materials. △ Less

Submitted 25 May, 2015; originally announced May 2015.

Comments: 10 pages, 12 figures, accepted for publication in J. Chem. Phys

Suppression of space broadening of exciton polariton beams by Bloch oscillation effects

Authors: Xudong Duan, Bingsuo Zou, Yongyou Zhang

Abstract: We theoretically study the transport of exciton polaritons under different applied photon potentials. The relation between the photon potentials and the thickness of the cavity layer is calculated by the finite element simulation. The theoretical analysis and numerical calculation indicate that the cavity photon potential is proportional to the thickness of the cavity layer with the coefficient be… ▽ More We theoretically study the transport of exciton polaritons under different applied photon potentials. The relation between the photon potentials and the thickness of the cavity layer is calculated by the finite element simulation. The theoretical analysis and numerical calculation indicate that the cavity photon potential is proportional to the thickness of the cavity layer with the coefficient being about $1.8$ meV/nm. Further, the periodic and linear photon potentials are considered to control the transport of the exciton polaritons in weak- and strong-field pump situations. In both situations the periodic potential cannot by itself effectively suppress the scatterings of the disorder potentials of the cavity photons and excitons and the nonlinear exciton-exciton interaction. When the linear potential is added to the cavity photons, the polariton transport exhibits the Bloch oscillation behavior. Importantly, the polariton Bloch oscillation can strongly suppress the space broadening due to the disorder potentials and nonlinear exciton-exciton interaction, which is beneficial for designing the polariton circuits. △ Less

Submitted 30 May, 2015; v1 submitted 21 May, 2015; originally announced May 2015.

Comments: 7 pages, 8 figures

Electric field induced strong enhancement of electroluminescence in multi-Layer MoS2

Authors: Dehui Li, Rui Cheng, Hailong Zhou, Chen Wang, Anxiang Yin, Yu Chen, Nathan O. Weiss, Yu Huang, Xiangfeng Duan

Abstract: The layered transition metal dichalcogenides (TMDs) have attracted considerable interest due to their unique electronic and optical properties. Here we report electric field induced strong electroluminescence in multi-layer MoS2 and WSe2. We show that GaN-Al2O3-MoS2 and GaN-Al2O3-MoS2-Al2O3-graphene vertical heterojunctions can be created with excellent rectification behaviour. Electroluminescence… ▽ More The layered transition metal dichalcogenides (TMDs) have attracted considerable interest due to their unique electronic and optical properties. Here we report electric field induced strong electroluminescence in multi-layer MoS2 and WSe2. We show that GaN-Al2O3-MoS2 and GaN-Al2O3-MoS2-Al2O3-graphene vertical heterojunctions can be created with excellent rectification behaviour. Electroluminescence studies demonstrate prominent direct bandgap excitonic emission in multi-layer MoS2 over the entire vertical junction area. Importantly, the electroluminescence efficiency observed in multi-layer MoS2 is comparable to or even higher than that in monolayers, corresponding to a relative electroluminescence enhancement factor of >1000 in multi-layer MoS2 when compared to its photoluminescence. This striking enhancement of electroluminescence can be attributed to the high electric field induced carrier redistribution from low energy points (indirect bandgap) to high energy points (direct bandgap) of k-space, arising from the unique band structure of MoS2 with a much higher density of states at high energy points. The electric field induced electroluminescence is general for other TMDs including WSe2, and can provide a fundamental platform to probe the carrier injection, population and recombination in multi-layer TMDs and open up a new pathway toward TMD based optoelectronic devices. △ Less

Submitted 16 October, 2015; v1 submitted 11 January, 2015; originally announced January 2015.

Comments: 8 pages, 5 figures

Journal ref: Nature Communications, 6, 7509, 2015

Towards barrier free contact to MoS2 using graphene electrodes

Authors: Yuan Liu, Hao Wu, Hung Chieh Cheng, Sen Yang, Enbo Zhu, Qiyuan He, Mengning Ding, Dehui Li, Jian Guo, Nathan O Weiss, Yu Huang, Xiangfeng Duan

Abstract: The two-dimensional (2D) layered semiconductors such as MoS2 have attracted tremendous interest as a new class of electronic materials. However, there is considerable challenge in making reliable contacts to these atomically thin materials. Here we present a new strategy by using graphene as back electrodes to achieve Ohmic contact to MoS2. With a finite density of states, the Fermi level of graph… ▽ More The two-dimensional (2D) layered semiconductors such as MoS2 have attracted tremendous interest as a new class of electronic materials. However, there is considerable challenge in making reliable contacts to these atomically thin materials. Here we present a new strategy by using graphene as back electrodes to achieve Ohmic contact to MoS2. With a finite density of states, the Fermi level of graphene can be readily modified by gate potential to ensure a nearly perfect band alignment with MoS2. We demonstrate, for the first time, a transparent contact can be made to MoS2 with essentially zero contact barrier and linear output behaviour at cryogenic temperatures (down to 1.9 K) for both monolayer and multilayer MoS2. Benefiting from the barrier-free transparent contacts, we show that a metal-insulator-transition (MIT) can be observed in a two-terminal MoS2 device, a phenomenon that could be easily masked by Schottky barrier and only seen in four-terminal devices in conventional metal-contacted MoS2 system. With further passivation y born nitride encapsulation, we demonstrate a record high extrinsic (two-terminal) field effect mobility over 1300 cm2/Vs in MoS2. △ Less

Submitted 24 December, 2014; originally announced December 2014.

Structural instabilities and mechanical properties of U$_2$Mo from first principles calculations

Authors: Ben-Qiong Liu, Xiao-Xi Duan, Guang-Ai Sun, Jin-Wen Yang, Tao Gao

Abstract: We perform detailed first principles calculations of the structural parameters at zero pressure and high pressure, the elastic properties, phonon dispersion relation, and ideal strengths of U$_2$Mo with $C11_b$ structure. In contrast to previous theoretical studies, we show that this $I4/mmm$ structure is indeed a mechanically and dynamically unstable phase, which is confirmed by the negative elas… ▽ More We perform detailed first principles calculations of the structural parameters at zero pressure and high pressure, the elastic properties, phonon dispersion relation, and ideal strengths of U$_2$Mo with $C11_b$ structure. In contrast to previous theoretical studies, we show that this $I4/mmm$ structure is indeed a mechanically and dynamically unstable phase, which is confirmed by the negative elastic constant $C_{66}$ as well as the imaginary phonon modes observed along the $Σ_1$-N-P line. The calculations of ideal strengths for U$_2$Mo are performed along [100], [001], and [110] directions for tension and on (001)[010] and (010)[100] slip systems for shear load. The ideal shear strength is about 8.1 GPa, much smaller than tension of 18-28 GPa, which indicates that the ductile U$_2$Mo alloy will fail by shear rather than by tension. △ Less

Submitted 3 November, 2014; originally announced November 2014.

Comments: 7 pages, 8 figures, 4 tables

Thickness Scaling Effect on Interfacial Barrier and Electrical Contact to Two-Dimensional MoS2 Layers

Authors: Song-Lin Li, K. Komatsu, Shu Nakaharai, Yen-Fu Lin, M. Yamamoto, X. F. Duan, K. Tsukagoshi

Abstract: Understanding the interfacial electrical properties between metallic electrodes and low dimensional semiconductors is essential for both fundamental science and practical applications. Here we report the observation of thickness reduction induced crossover of electrical contact at Au/MoS2 interfaces. For MoS2 thicker than 5 layers, the contact resistivity slightly decreases with reducing MoS2 thic… ▽ More Understanding the interfacial electrical properties between metallic electrodes and low dimensional semiconductors is essential for both fundamental science and practical applications. Here we report the observation of thickness reduction induced crossover of electrical contact at Au/MoS2 interfaces. For MoS2 thicker than 5 layers, the contact resistivity slightly decreases with reducing MoS2 thickness. By contrast, the contact resistivity sharply increases with reducing MoS2 thickness below 5 layers, mainly governed by the quantum confinement effect. It is found that the interfacial potential barrier can be finely tailored from 0.3 to 0.6 eV by merely varying MoS2 thickness. A full evolution diagram of energy level alignment is also drawn to elucidate the thickness scaling effect. The finding of tailoring interfacial properties with channel thickness represents a useful approach controlling the metal/semiconductor interfaces which may result in conceptually innovative functionalities. △ Less

Submitted 15 February, 2015; v1 submitted 7 October, 2014; originally announced October 2014.

Comments: First to observe the quantum confinement effect on interfacial barrier in 2D materials

Journal ref: ACS Nano, 8 (2014) 12836-12842

Spin Logic via Controlled Correlation in Nanomagnet-Dirac Fermion Heterostructures

Authors: Xiaopeng Duan, Yuriy G. Semenov, Ki Wook Kim

Abstract: A hybrid structure combining the advantages of topological insulator (TI), dielectric ferromagnet (FM), and graphene is investigated to realize the electrically controlled correlation between electronic and magnetic subsystems for low-power, high-functional applications. Two-dimensional Dirac fermion states provide an ideal environment to facilitate strong coupling through the surface interactions… ▽ More A hybrid structure combining the advantages of topological insulator (TI), dielectric ferromagnet (FM), and graphene is investigated to realize the electrically controlled correlation between electronic and magnetic subsystems for low-power, high-functional applications. Two-dimensional Dirac fermion states provide an ideal environment to facilitate strong coupling through the surface interactions with proximate materials. The unique properties of FM-TI and FM-graphene interfaces make it possible for active "manipulation" and "propagation", respectively, of the information state variable based solely on the spin logic platform through electrical gate biases. Our theoretical analysis verifies the feasibility of the concept for logic application with both current-driven and current-less interconnect approaches. The device/circuit characteristics are also examined in realistic conditions, suggesting the desired low-power performance with the estimated energy consumption for COPY/NOT as low as the \textit{attojoule} level. △ Less

Submitted 10 July, 2014; originally announced July 2014.

Electroluminescence and photocurrent generation from atomically sharp WSe2/MoS2 heterojunction p-n diodes

Authors: Rui Cheng, Dehui Li, Hailong Zhou, Chen Wang, Anxiang Yin, Shan Jiang, Yuan Liu, Yu Chen, Yu Huang, Xiangfeng Duan

Abstract: The p-n diodes represent the most fundamental device building block for diverse optoelectronic functions, but are difficult to achieve in atomically thin transition metal dichalcogenides (TMDs) due to the inability to selectively dope them into p- or n-type semiconductors. Here we report the first demonstration of an atomically thin and atomically sharp heterojunction p-n diode by vertically stack… ▽ More The p-n diodes represent the most fundamental device building block for diverse optoelectronic functions, but are difficult to achieve in atomically thin transition metal dichalcogenides (TMDs) due to the inability to selectively dope them into p- or n-type semiconductors. Here we report the first demonstration of an atomically thin and atomically sharp heterojunction p-n diode by vertically stacking p-type monolayer tungsten diselenide (WSe2) and n-type few-layer molybdenum disulfide (MoS2). Electrical measurement demonstrates excellent diode characteristics with well-defined current rectification behaviour and an ideality factor of 1.2. Photocurrent mapping shows fast photoresponse over the entire overlapping region with a highest external quantum efficiency up to 12 %. Electroluminescence studies show prominent band edge excitonic emission and strikingly enhanced hot electron luminescence. A systematic investigation shows distinct layer-number dependent emission characteristics and reveals important insight about the origin of hot-electron luminescence and the nature of electron-orbital interaction in TMDs. We believe that these atomically thin heterojunction p-n diodes represent an interesting system for probing the fundamental electro-optical properties in TMDs, and can open up a new pathway to novel optoelectronic devices such as atomically thin photodetectors, photovoltaics, as well as spin-/valley-polarized light emitting diodes and on-chip lasers. △ Less

Submitted 15 September, 2014; v1 submitted 13 March, 2014; originally announced March 2014.

Comments: 27 pages, 7 figures