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Zhu, Wenguang

Publications and source records attributed to Zhu, Wenguang.

At least 19 records
All sourcesNASA NTRSDOE OSTI
DOE OSTI2024

Atomic-scale manipulation of polar domain boundaries in monolayer ferroelectric In 2 Se 3

Domain boundaries have been intensively investigated in bulk ferroelectric materials and two-dimensional materials. Many methods such as electrical, mechanical and optical approaches have been utilized to probe and manipulate domain boundaries. So far most research focuses on the initial and final states of domain boundaries before and after manipulation, while the microscopic understanding of the evolution of domain boundaries remains elusive. In this paper, we report controllable manipulation of the domain boundaries in two-dimensional ferroelectric In 2 Se 3 with atomic precision using scanning tunneling microscopy. We show that the movements of the domain boundaries can be driven by the electric field from a scanning tunneling microscope tip and proceed by the collective shifting of atoms at the domain boundaries. Our density functional theory calculations reveal the energy path and evolution of the domain boundary movement. The results provide deep insight into domain boundaries in two-dimensional ferroelectric materials and will inspire inventive applications of these materials.

36 MATERIALS SCIENCE↗
DOE OSTI2023

Direct Imaging of Antiferromagnet-Ferromagnet Phase Transition in van der Waals Antiferromagnet CrSBr

The advent of van der Waals (vdW) ferromagnetic (FM) and antiferromagnetic (AFM) materials offers unprecedented opportunities for spintronics and magneto-optic devices. Combining magnetic Kerr microscopy and density functional theory calculations, the AFM-FM transition is investigated and a surprising abnormal magneto-optic anisotropy in vdW CrSBr associated with different magnetic phases (FM, AFM, or paramagnetic state) is discovered. This unique magneto-optic property leads to different anisotropic optical reflectivity from different magnetic states, permitting direct imaging of the AFM Néel vector orientation and the dynamic process of the AFM-FM transition within a magnetic field. Using Kerr microscopy, not only the domain nucleation and propagation process is imaged but also the intermediate spin-flop state in the AFM-FM transition is identified. In conclusion, the unique magneto-optic property and clear identification of the dynamics process of the AFM-FM phase transition in CrSBr demonstrate the promise of vdW magnetic materials for future spintronic technology.

36 MATERIALS SCIENCE↗
DOE OSTI2017

Theoretical Design of Robust Ferromagnetism and Bipolar Semiconductivity in Graphene-Based Nanoroads

The search for graphene-based materials for spintronics applications has intensified in recent years, and numerous designs have been proposed based on various modifications to pristine graphene. Despite the tremendous progress made in the past, finding a design that can be realized in practice remains a challenging task. Encouraged by recent experimental breakthroughs, here we propose a feasible scheme to realize graphene-based magnetic nanoroads. This new material consists of a half-hydrogenated graphene nanoroad embedded in a fully hydrogenated graphene sheet. Using first-principles density functional theory calculations, we demonstrate that such a design can convert nonmagnetic pristine graphene into a bipolar ferromagnetic semiconductor. More importantly, as a result of areal magnetization enabled by half-hydrogenation, the overall magnetism of such a nanoroad is very robust against a variation of either its width or orientation. We also propose a simple design of an all-electric controlled device based on the new material for the generation and regulation of a fully spin-polarized electric current.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2017

Prediction of intrinsic two-dimensional ferroelectrics in In 2 Se 3 and other III 2 -VI 3 van der Waals materials

Interest in two-dimensional (2D) van der Waals materials has grown rapidly across multiple scientific and engineering disciplines in recent years. However, ferroelectricity, the presence of a spontaneous electric polarization, which is important in many practical applications, has rarely been reported in such materials so far. Here we employ first-principles calculations to discover a branch of the 2D materials family, based on In 2 Se 3 and other III 2 -VI 3 van der Waals materials, that exhibits room-temperature ferroelectricity with reversible spontaneous electric polarization in both out-of-plane and in-plane orientations. The device potential of these 2D ferroelectric materials is further demonstrated using the examples of van der Waals heterostructures of In 2 Se 3 /graphene, exhibiting a tunable Schottky barrier, and In 2 Se 3 /WSe 2 , showing a significant band gap reduction in the combined system. Lastly, these findings promise to substantially broaden the tunability of van der Waals heterostructures for a wide range of applications.

36 MATERIALS SCIENCE↗
DOE OSTI2016

Generation and the role of dislocations in single-crystalline phase-change In 2 Se 3 nanowires under electrical pulses

Using single crystalline In2Se3 nanowires as a platform, we have studied the RESET switching (from low to high electrical resistance) in this phase-change material under electric pulses. Particularly, we correlated the atomic-scale structural evolutions with local electrical resistance variations, by performing transmission electron microscopy and scanning Kelvin probe microscopy on the same nanowires. By coupling the experimental results with density functional theory calculations, we show that the immobile dislocations generated via vacancy condensations are responsible for the RESET switching and that the material maintains the single crystallinity during the process. This new mechanism is fundamentally different from the crystalline-amorphous transition, which is commonly understood as the underlying process for the RESET switching in similar phase-change materials.

Environmental Molecular Sciences Laboratory↗
DOE OSTI2015

Atomistic mechanisms for bilayer growth of graphene on metal substrates

Epitaxial growth on metal substrates has been shown to be the most powerful approach in producing large-scale high-quality monolayer graphene, yet it remains a major challenge to realize uniform bilayer graphene growth. Here we carry out a comparative study of the atomistic mechanisms for bilayer graphene growth on the (111) surfaces of Cu and Ni, using multiscale approaches combining first-principles calculations and rate-equation analysis. We first show that the relatively weak graphene-Cu interaction enhances the lateral diffusion and effective nucleation of C atoms underneath the graphene island, thereby making it more feasible to grow bilayer graphene on Cu. In contrast, the stronger graphene-Ni interaction suppresses the lateral mobility and dimerization of C atoms underneath the graphene, making it unlikely to achieve controlled growth of bilayer graphene on Ni. We then determine the critical graphene size beyond which nucleation of the second layer will take place. Intriguingly, the critical size exhibits an effective inverse "Ehrlich-Schwoebel barrier" effect, becoming smaller for faster C migration from the Cu surface to the graphene-Cu interface sites across the graphene edge. Lastly, these findings allow us to propose a novel alternating growth scheme to realize mass production of bilayer graphene.

36 MATERIALS SCIENCE↗
DOE OSTI2014

Correlation effects in (111) bilayers of perovskite transition-metal oxides

We investigate the correlation-induced Mott, magnetic, and topological phase transitions in artificial (111) bilayers of perovskite transition-metal oxides LaAuO 3 and SrIrO 3 for which the previous density-functional theory calculations predicted topological insulating states. Using the dynamical-mean-field theory with realistic band structures and Coulomb interactions, LaAuO 3 bilayer is shown to be far away from a Mott insulating regime, and a topological-insulating state is robust. On the other hand, SrIrO 3 bilayer is on the verge of an orbital-selective topological Mott transition and turns to a trivial insulator by an antiferromagnetic ordering. Oxide bilayers thus provide a novel class of topological materials for which the interplay between the spin-orbit coupling and electron-electron interactions is a fundamental ingredient.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2013

Electrical tuning of valley magnetic moment through symmetry control in bilayer MoS2

Crystal symmetry governs the nature of electronic Bloch states. For example, in the presence of time-reversal symmetry, the orbital magnetic moment and Berry curvature of the Bloch states must vanish unless inversion symmetry is broken1. In certain two-dimensional electron systems such as bilayer graphene, the intrinsic inversion symmetry can be broken simply by applying a perpendicular electric field2,3. In principle, this offers the possibility of switching on/off and continuously tuning the magnetic moment and Berry curvature near the Dirac valleys by reversible electrical control4,5. Here we investigate this possibility using polarization-resolved photoluminescence of bilayer MoS2, which has the same symmetry as bilayer graphene but has a bandgap in the visible spectrum6,7 allowing direct optical probing5,8 12. We find that in bilayer MoS2 the circularly polarized photoluminescence can be continuously tuned from 15% to 15% as a function of gate voltage, whereas in structurally non-centrosymmetric monolayer MoS2 the photoluminescence polarization is gate independent. The observations are well explained as resulting from the continuous variation of orbital magnetic moments between positive and negative values through symmetry control.

Wu, Sanfeng↗
DOE OSTI2012

Atomic scale control of catalytic process in oxidation of Pb thin films

Using scanning tunneling microscopy (STM), we demonstrate that oxidation on Pb films is greatly enhanced by atomic Cs catalysts. With only a minute concentration of isolated Cs substitutional atoms in the surface layer (0.004 ML coverage), surface oxidation rates are greatly enhanced. First-principles density functional theory (DFT) calculations reveal that a substitutional Cs atom strongly increases O2 binding on the surface. Then, with additional oxygen exposure this substitutional Cs-initiated oxidation process results in growth of PbO layers in an auto-catalytic manner. Furthermore, we investigate the role of temperature in the oxidation of the Pb films with and without Cs, and we explore the overall morphology of the resultant oxide layers.

Kim, Jisun↗
DOE OSTI2012

Strain tuning of topological band order in cubic semiconductors

We theoretically explore the possibility of tuning the topological order of cubic diamond/zinc-blende semi- conductors with external strain. Based on a simple tight-binding model, we analyze the evolution of the cubic semiconductor band structure under hydrostatic or biaxial lattice expansion, by which a generic guiding princi- ple is established that biaxial lattice expansion can induce a topological phase transition of small band-gap cubic semiconductors via a band inversion and symmetry breaking at point. Using density functional theory cal- culations, we demonstrate that a prototype topological trivial semiconductor, InSb, is converted to a nontrivial topological semiconductor with a 2% 3% biaxial lattice expansion.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2012

Local fields in conductor surface electromigration: A first-principles study in the low-bias ballistic limit

Utilizing first-principles quantum transport calculations, we investigate the role of local fields in conductor surface electromigration. A nanometer-thick Ag(100) thin film is adopted as our prototypical conductor, where we demonstrate the existence of intense local electric fields at atomic surface defects under an external bias. It is shown that such local fields can play an important role in driving surface electromigration and electrical breakdown. The intense fields originate from the relatively short (atomic-scale) screening lengths common to most elemental metals. This general short-range screening trend is established self-consistently within an intuitive picture of linear response electrostatics. The findings shed new light on the underlying physical origins of surface electromigration and point to the possibility of harnessing local fields to engineer electromigration at the nanoscale.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2012

Quantum oscillation of Rashba spin splitting in topological insulator Bi2Se3 induced by the quantum size effects of Pb adlayers

Basedon rst-principlescalculationswithindensityfunctionaltheory,we ndthatPbadlayerscaninduceagiantRashbaspinsplittinginthequantumwellstatesofanunderlyingBi2Se3 lm.AsthethicknessofthePbadlayersvaries,thedistancebetweenthePbadlayerandtheBi2Se3 lm,thechargedensityattheinterface,andthebindingbetweenthePbadlayersexhibitanoscillatorybehaviorduetothequantumsizeeffects,whichinturnmodulatesthemagnitudeoftheRashbaspinsplittingofthequantumwellstatesintheBi2Se3 lm.

Yang, Hong↗
DOE OSTI2012

Intrinsic spin Hall effect in monolayers of group-VIdichalcogenides: A rst-principles study

Using rst-principlescalculationswithindensityfunctionaltheory,weinvestigatetheintrinsicspinHalleffectinmonolayersofgroup-VItransition-metaldichalcogenidesMX2(M=Mo,WandX=S,Se).MX2monolayersaredirectband-gapsemiconductorswithtwodegeneratevalleyslocatedatthecornersofthehexagonalBrillouinzone.Becauseoftheinversionsymmetrybreakingandthestrongspin-orbitcoupling,chargecarriersinoppositevalleyscarryoppositeBerrycurvatureandspinmoment,givingrisetobothavalley-Hallandaspin-Halleffect.WealsoshowthattheintrinsicspinHallconductivityininversion-symmetricbulkdichalcogenidesisanorderofmagnitudesmallercomparedtomonolayers.Ourresultdemonstratesmonolayerdichalcogenidesasanidealplatformfortheintegrationofvalleytronicsandspintronics.

Feng, Wanxiang↗
DOE OSTI2012

Tailoring Magnetic Doping in the Topological Insulator Bi2Se3

WetheoreticallyinvestigatethepossibilityofestablishingferromagnetisminthetopologicalinsulatorBi2Se3viamagneticdopingof3dtransitionmetalelements.Theformationenergies,chargestates,bandstructures,andmagneticpropertiesofdopedBi2Se3arestudiedusing rst-principlescalculationswithindensityfunctionaltheory.OurresultsshowthatBisubstitutionalsitesareenergeticallymorefavorablethaninterstitialsitesforsingleimpurities.DetailedelectronicstructureanalysisrevealsthatCrandFedopedmaterialsarestillinsulatinginthebulkbuttheintrinsicbandgapofBi2Se3issubstantiallyreducedduetothestronghybridizationbetweenthedstatesofthedopantsandthepstatesoftheneighboringSeatoms.ThecalculatedmagneticcouplingsuggeststhatCrdopedBi2Se3ispossibletobebothferromagneticandinsulating,whileFedopedBi2Se3tendstobeweaklyantiferromagnetic.

Zhang, Jian-Min↗