Yunhao Lu

Topological properties determined by atomic buckling in self-assembled ultrathin Bi(110).

Topological insulators (TIs) are a new type of electronic materials in which the nontrivial insulating bulk band topology governs conducting boundary states with embedded spin-momentum locking. Such edge states are more robust in a two-dimensional (2D) TI against scattering by nonmagnetic impurities than in its three-dimensional (3D) variant, because in 2D the two helical edge states are protected from the only possible backscattering. This makes the 2D TI family a better candidate for coherent spin transport and related applications. While several 3D TIs are already synthesized experimentally, physical realization of 2D TI is so far limited to hybrid quantum wells with a tiny bandgap that does not survive temperatures above 10 K. Here, combining first-principles calculations and scanning tunneling microscopy/spectroscopy (STM/STS) experimental studies, we report nontrivial 2D TI phases in 2-monolayer (2-ML) and 4-ML Bi(110) films with large and tunable bandgaps determined by atomic buckling of Bi(110) films. The gapless edge states are experimentally detected within the insulating bulk gap at 77 K. The band topology of ultrathin Bi(110) films is sensitive to atomic buckling. Such buckling is sensitive to charge doping and could be controlled by choosing different substrates on which Bi(110) films are grown.

Effects of strain on electronic and optic properties of holey two-dimensional C2N crystals

A two-dimensional (2D) material, the holey 2D C2N (h2D-C2N) crystal, has recently been synthesized. Here, we investigate the strain effects on the properties of this material by first-principles calculations. We show that the material is quite soft with a small stiffness constant and can sustain large strains ≥12%. It remains a direct gap semiconductor under strain, and the bandgap size can be tuned in a wide range as large as 1 eV. Interestingly, for biaxial strain, a band crossing effect occurs at the valence band maximum close to a 8% strain, leading to a dramatic increase of the hole effective mass. Strong optical absorption can be achieved by strain tuning with absorption coefficient ∼106 cm−1 covering a wide spectrum. Our findings suggest the great potential of strain-engineered h2D-C2N in electronic and optoelectronic device applications.Shan Guan, 2 Yingchun Cheng, Chang Liu, Junfeng Han, Yunhao Lu, Shengyuan A. Yang, ∗ and Yugui Yao † Research Laboratory for Quantum Materials, Singapore University of Technology and Design, Singapore 487372, Singapore School of Physics, Beijing Institute of Technology, Beijing 100081, China Institute of Advanced Materials (IAM), Nanjing Tech University, Nanjing 211816, China School of Materials Science and Engineering, Zhejiang University, Hangzhou 310027, China

Strain-Induced Isostructural and Magnetic Phase Transitions in Monolayer MoN2

The change of bonding status, typically occurring only in chemical processes, could dramatically alter the material properties. Here, we show that a tunable breaking and forming of a diatomic bond can be achieved through physical means, i.e., by a moderate biaxial strain, in the newly discovered MoN2 two-dimensional (2D) material. On the basis of first-principles calculations, we predict that as the lattice parameter is increased under strain, there exists an isostructural phase transition at which the N-N distance has a sudden drop, corresponding to the transition from a N-N nonbonding state to a N-N single bond state. Remarkably, the bonding change also induces a magnetic phase transition, during which the magnetic moments transfer from the N(2p) sublattice to the Mo(4d) sublattice; meanwhile, the type of magnetic coupling is changed from ferromagnetic to antiferromagnetic. We provide a physical picture for understanding these striking effects. The discovery is not only of great scientific interest in exploring unusual phase transitions in low-dimensional systems, but it also reveals the great potential of the 2D MoN2 material in the nanoscale mechanical, electronic, and spintronic applications.

Identifying individual n- and p-type ZnO nanowires by the output voltage sign of piezoelectric nanogenerator

Based on a comparative study between the piezoelectric outputs of n-type nanowires (NWs) and n-core/p-shell NWs along with the previous study (Lu et al 2009 Nano. Lett. 9 1223), we demonstrate a one-step technique for identifying the conductivity type of individual ZnO nanowires (NWs) based on the output of a piezoelectric nanogenerator without destroying the sample. A negative piezoelectric output voltage indicates an NW is n-type and it appears after the tip scans across the center of the NW, while a positive output voltage reveals p-type conductivity and it appears before the tip scans across the central line of the NW. This atomic force microscopy based technique is reliable for statistically mapping the majority carrier type in ZnO NWs arrays. The technique may also be applied to other wurtzite semiconductors, such as GaN, CdS and ZnS.

Ultrafast, Highly Reversible, and Cycle-Stable Lithium Storage Boosted by Pseudocapacitance in Sn-Based Alloying Anodes

Boosting power density is one of the primary challenges that current lithium ion batteries face. Alloying anodes that possess suitable potential windows stand at the forefront in pursuing ultrafast and highly reversible lithium storage to achieve high power/energy lithium ion batteries. Herein, ultrafast lithium storage in Sn-based nanocomposite anodes is demonstrated, which is boosted by pseudocapacitance benefitting from a high fraction of highly interconnected interfaces of Fe/Sn/Li2 O. By tailoring the voltage window in the range of 0.005-1.2 V for the alloying/dealloying reactions, such Sn-based nanocomposite anodes achieve simultaneous ultrahigh rate capability, superlong cycling performance, and close-to-100% Coulombic efficiency. The nanocomposite anode delivers a high reversible capacity (≈420 mAh g-1 ) at 1 A g-1 for more than 1200 cycles, corresponding to only 0.016% per cycle of capacity decay. A reversible capacity of 350 mAh g-1 can be maintained at an ultrahigh current density of 80 A g-1 , with 67.3% capacity retention relative to the capacity at 1 A g-1 . This combination of pseudocapacitive lithium storage and spatially confined electrochemical reactions in Sn-based nanocomposite anode materials may pave the way for the development of high power/energy and long life lithium ion batteries.

Artificial gravity field, astrophysical analogues, and topological phase transitions in strained topological semimetals

Effective gravity and gauge fields are emergent properties intrinsic for low-energy quasiparticles in topological semimetals. Here, taking two Dirac semimetals as examples, we demonstrate that applied lattice strain can generate warped spacetime, with fascinating analogues in astrophysics. Particularly, we study the possibility of simulating black-hole/white-hole event horizons and gravitational lensing effect. Furthermore, we discover strain-induced topological phase transitions, both in the bulk materials and in their thin films. Especially in thin films, the transition between the quantum spin Hall and the trivial insulating phases can be achieved by a small strain, naturally leading to the proposition of a novel piezo-topological transistor device. Possible experimental realizations and analogue of Hawking radiation effect are discussed. Our result bridges multiple disciplines, revealing topological semimetals as a unique table-top platform for exploring interesting phenomena in astrophysics and general relativity; it also suggests realistic materials and methods to achieve controlled topological phase transitions with great potential for device applications.Condensed matter: Creating black holes in materialsA material that mimics the behavior of a black hole is developed by researchers in China and Singapore. Yugui Yao from the Beijing Institute of Technology and colleagues show that mechanical strain in a material known as Dirac semimetal can imitate the warping of space–time predicted by general relativity. Simulations of the Universe predict a wide range of counter-intuitive phenomenon. But many of these are beyond state-of-the-art technology to detect. Instead, scientists can engineer materials that are governed by equations similar to those that define astrophysical phenomena. Yao et al. investigate Dirac semimetals whose electronic bandstructure gives rise to massless quasiparticles that resemble relativistic particles. They show that altering the uniaxial strain enables control over these quasiparticles so that they emulate the behavior associated with black and white holes, event horizons and gravitational lensing.

Donor/acceptor doping and electrical tailoring in ZnO quantum dots

The authors report on donor/acceptor doping in ZnO quantum dots (QDs) grown by a metal-organic chemical vapor deposition method. The Ga donor and N acceptor, as identified by x-ray photoelectron spectroscopy (XPS), are introduced into ZnO QDs. They demonstrate, with a combination of valence band XPS and scanning tunneling microscopy, that the electrical properties as well as Fermi level of the ZnO QDs can be well tuned by the donor/acceptor doping. In addition, photoluminescence from the ZnO QDs with quantum confinement effect is observed.

Multiple unpinned Dirac points in group-Va single-layers with phosphorene structure

Emergent Dirac fermion states underlie many intriguing properties of graphene, and the search for them constitutes one strong motivation to explore two-dimensional (2D) allotropes of other elements. Phosphorene, the ultrathin layers of black phosphorous, has been a subject of intense investigations recently, and it was found that other group-Va elements could also form 2D layers with similar puckered lattice structure. Here, by a close examination of their electronic band structure evolution, we discover two types of Dirac fermion states emerging in the low-energy spectrum. One pair of (type-I) Dirac points is sitting on high-symmetry lines, while two pairs of (type-II) Dirac points are located at generic k-points, with different anisotropic dispersions determined by the reduced symmetries at their locations. Such fully-unpinned (type-II) 2D Dirac points are discovered for the first time. In the absence of spin-orbit coupling (SOC), we find that each Dirac node is protected by the sublattice symmetry from gap opening, which is in turn ensured by any one of three point group symmetries. The SOC generally gaps the Dirac nodes, and for the type-I case, this drives the system into a quantum spin Hall insulator phase. We suggest possible ways to realise the unpinned Dirac points in strained phosphorene. An international research team has predicted the existence of an exotic particle in two-dimensional materials. Two-dimensional materials can host particles known as Dirac fermions that have an electrical charge but effectively no mass and are pinned at specific momentum points. Yunhao Lu from China’s Zhejiang University, with co-workers in China, Singapore, Japan and Russia, has shown that new unpinned Dirac particles could exist in single-atom-thick materials with a wrinkled structure. Lu’s team studied the electronic properties of a single layer of atoms from group Va of the periodic table, which includes phosphorus, antimony and bismuth. They found that these materials support two types of Dirac fermions rather than the one found in graphene. Due to the special wrinkled lattice, these new fermions can freely move in the momentum space and have strongly anisotropic dynamics.

Interfacial Multiferroics of TiO2/PbTiO3 Heterostructure Driven by Ferroelectric Polarization Discontinuity

Novel phenomena appear when two different oxide materials are combined together to form an interface. For example, at the interface of LaAlO3/SrTiO3, two-dimensional conductive states form to avoid the polar discontinuity, and magnetic properties are found at such an interface. In this work, we propose a new type of interface between two nonmagnetic and nonpolar oxides that could host a magnetic state, where it is the ferroelectric polarization discontinuity instead of the polar discontinuity that leads to the charge transfer, forming the interfacial magnetic state. As a concrete example, we investigate by first-principles calculations the heterostructures made of ferroelectric perovskite oxide PbTiO3 and nonferroelectric polarized oxide TiO2. We show that charge is transferred to the interfacial layer forming an interfacial ferromagnetic ordering that may persist up to room temperature. Especially, the strong coupling between bulk ferroelectric polarization and interface ferromagnetism represents a new type of magnetoelectric effect, which provides an ideal platform for exploring the intriguing interfacial multiferroics. The findings here are important not only for fundamental science but also for promising applications in nanoscale electronics and spintronics.

Two-dimensional ferroelectricity and switchable spin-textures in ultra-thin elemental Te multilayers

New ferroelectric materials with satisfactory performance at the nanoscale are critical for the ever-developing microelectronics industry. Here, we report two-dimensional (2D) ferroelectricity in elemental tellurium multilayers, which exhibit spontaneous in-plane polarization due to the interlayer interaction between lone pairs. The magnitude of the polarization reaches about 1.02 × 10−10 C m−1 per layer, which can be detected by current experimental technology as recently done for the 2D FE compound SnTe. The spontaneous ferroelectric polarization can be preserved for the bilayer Te film even above room temperature. Also, we show that due to the strong spin–orbit coupling of Te, there appear nontrivial valley-dependent spin-textures for the hole carriers, and the textures are coupled with the direction of FE polarization, which is tunable by an external electric field. Our findings not only introduce the concept of ferroelectricity in elemental systems, but also broaden the family of the 2D ferroelectric materials and offer a promising platform for novel electronic and spintronic applications.