High‐throughput Synthesis of Solution‐Processable van der Waals Heterostructures through Electrochemistry

2023-06-07T13:08:21+00:00June 7th, 2023|Categories: Publications|Tags: , , , , |

A general method is demonstrated for the preparation of high-quality van der Waals heterostructures in solution through an electrochemical strategy. The produced van der Waals heterostructures exhibit strong interlayer coupling, extraordinary structural integrity, large lateral dimension and good optoelectronic properties. Abstract Two-dimensional van der Waals heterostructures (2D vdWHs) have recently gained widespread attention because of their abundant and exotic properties, which open up many new possibilities for next-generation nanoelectronics. However, practical applications remain challenging due to the lack of high-throughput techniques for fabricating high-quality vdWHs. Here, we demonstrate a general electrochemical strategy to prepare solution-processable high-quality vdWHs, in which electrostatic forces drive the stacking of electrochemically exfoliated individual assemblies with intact structures and clean interfaces into vdWHs with strong interlayer interactions. Thanks to the excellent combination of strong light absorption, interfacial charge transfer, and decent charge transport properties in individual layers, thin-film photodetectors based on graphene/In2Se3 vdWHs exhibit great promise for near-infrared (NIR) photodetection, owing to a high responsivity (267 mA W−1), fast rise (72 ms) and decay (426 ms) times under NIR illumination. This approach enables various hybrid systems, including graphene/In2Se3, graphene/MoS2 and graphene/MoSe2 vdWHs, providing a broad avenue for exploring emerging electronic, photonic, and exotic quantum phenomena.

Published in: "Angewandte Chemie International Edition".

Magneto-optical Kerr effect in ferroelectric antiferromagnetic two-dimensional heterostructures. (arXiv:2304.04363v1 [cond-mat.mtrl-sci])

2023-04-11T10:19:18+00:00April 11th, 2023|Categories: Publications|Tags: , |

We study the magneto-optical Kerr effect (MOKE) of the two-dimensional heterostructure CrI3/In2Se3/CrI3 by using density functional theory calculations and symmetry analysis. The spontaneous polarization in the In2Se3 ferroelectric layer and the antiferromagnetic ordering in CrI3 layers break the mirror symmetry and the timereversal symmetry, thus activating MOKE. We show that the Kerr angle can be switched by either the polarization or the antiferromagnetic order parameter. Our results suggest that ferroelectric and antiferromagnetic 2D heterostructures could be exploited for ultra-compact information storage devices, where the information is encoded by the two ferroelectric or the two time-reversed antiferromagnetic states, and the read-out performed optically by MOKE.

Published in: "arXiv Material Science".

Phase Instability in van der Waals In2Se3 Determined by Surface Coordination

2023-03-03T13:07:58+00:00March 3rd, 2023|Categories: Publications|Tags: |

van der Waals In2Se3 has attracted significant attention for its room-temperature 2D ferroelectricity/antiferroelectricity down to monolayer thickness. However, instability and potential degradation pathway in 2D In2Se3 have not yet been adequately addressed. Using a combination of experimental and theoretical approaches, we here unravel the phase instability in both α- and β’-In2Se3 originating from the relatively unstable octahedral coordination. Together with the broken bonds at the edge steps, it leads to moisture-facilitated oxidation of In2Se3 in air to form amorphous In2Se3-3xO3x layers and Se hemisphere particles. Both O2 and H2O are required for such surface oxidation, which can be further promoted by light illumination. In addition, the self-passivation effect from the In2Se3-3 x O3 x layer can effectively limit such oxidation to only a few nanometer thickness. The achieved insight paves way for better understanding and optimizing 2D In2Se3 performance for device applications.

Published in: "Angewandte Chemie International Edition".

Molecular Beam Epitaxy of Twin-Free Bi2Se3 and Sb2Te3 on In2Se3/InP(111)B Virtual Substrates. (arXiv:2302.10985v1 [cond-mat.mtrl-sci])

2023-02-23T02:29:28+00:00February 23rd, 2023|Categories: Publications|Tags: , |

Three-dimensional topological insulators (3D-TIs) are a new generation of materials with insulating bulk and exotic metallic surface states that facilitate a wide variety of ground-breaking applications. However, utilization of the surface channels is often hampered by the presence of crystal defects, such as antisites, vacancies and twin domains. For terahertz device applications, twinning is shown to be highly deleterious. Previous attempts to reduce twins using technologically important InP(111) substrates have been promising, but have failed to completely suppress twin domains while preserving high structural quality. Here we report growth of twin-free molecular beam epitaxial Bi2Se3 and Sb2Te3 structures on ultra-thin In2Se3 layers formed by a novel selenium passivation technique during the oxide desorption of smooth, non-vicinal InP(111)B substrates, without the use of an indium source. The formation of un-twinned In2Se3 provides a favorable template to fully suppress twin domains in 3D-TIs, greatly broadening novel device applications in the terahertz regime.

Published in: "arXiv Material Science".

Edge-based 2D alpha-In2Se3-MoS2 ferroelectric field effect device. (arXiv:2301.00568v1 [cond-mat.mtrl-sci])

2023-01-03T02:29:27+00:00January 3rd, 2023|Categories: Publications|Tags: , , |

Heterostructures based on two dimensional (2D) materials offer the possibility to achieve synergistic functionalities which otherwise remain secluded by their individual counterparts. Herein ferroelectric polarization switching in alpha-In2Se3 has been utilized to engineer multilevel non-volatile conduction states in partially overlapping alpha-In2Se3-MoS2 based ferroelectric semiconducting field effect device. In particular, we demonstrate how the intercoupled ferroelectric nature of alpha-In2Se3 allows to non-volatilely switch between n-i and n-i-n type junction configurations based on a novel edge state actuation mechanism, paving the way for sub-nanometric scale non-volatile device miniaturization. Furthermore the induced asymmetric polarization enables enhanced photogenerated carriers separation resulting in extremely high photoresponse of 1275 AW-1 in the visible range and strong non-volatile modulation of the bright A- and B- excitonic emission channels in the overlaying MoS2 monolayer. Our results show significant potential to harness the switchable polarization in partially overlapping alpha-In2Se3-MoS2 based FeFETs to engineer multimodal non-volatile nanoscale electronic and optoelectronic devices.

Published in: "arXiv Material Science".

Non-volatile Electric Control of Magnetic and Topological Properties of MnBi2Te4 Thin Films. (arXiv:2212.14331v1 [cond-mat.mtrl-sci])

2023-01-02T02:30:22+00:00January 2nd, 2023|Categories: Publications|Tags: , |

In this letter, we propose a mechanism to control the magnetic properties of topological quantum material (TQM) by using magnetoelectric coupling: this mechanism uses a heterostructure of TQM with two-dimensional (2D) ferroelectric material, which can dynamically control the magnetic order by changing the polarization of the ferroelectric material and induce possible topological phase transitions. This concept is demonstrated using the example of the bilayer MnBi2Te4 on ferroelectric In2Se3 or In2Te3, where the polarization direction of the 2D ferroelectrics determines the interfacial band alignment and consequently the direction of the charge transfer. This charge transfer, in turn, enhances the stability of the ferromagnetic state of MnBi2Te4 and leads to a possible topological phase transition between the quantum anomalous Hall (QAH) effect and the zero plateau QAH. Our work provides a route to dynamically alter the magnetic ordering of TQMs and could lead to the discovery of new multifunctional topological heterostructures.

Published in: "arXiv Material Science".

Layer-dependent optically-induced spin polarization in InSe. (arXiv:2212.05423v1 [cond-mat.mtrl-sci])

2022-12-13T02:29:40+00:00December 13th, 2022|Categories: Publications|Tags: , |

Optical control of spin in semiconductors has been pioneered using nanostructures of III-V and II-VI semiconductors, but the emergence of two-dimensional van der Waals materials offers an alternative low-dimensional platform for spintronic phenomena. Indium selenide (InSe), a group-III monochalcogenide van der Waals material, has shown promise for opto-electronics due to its high electron mobility, tunable direct bandgap, and quantum transport. There are predictions of spin-dependent optical selection rules suggesting potential for all-optical excitation and control of spin in a two-dimensional layered material. Despite these predictions, layer-dependent optical spin phenomena in InSe have yet to be explored. Here, we present measurements of layer-dependent optical spin dynamics in few-layer and bulk InSe. Polarized photoluminescence reveals layer-dependent optical orientation of spin, thereby demonstrating the optical selection rules in few-layer InSe. Spin dynamics are also studied in many-layer InSe using time-resolved Kerr rotation spectroscopy. By applying out-of-plane and in-plane static magnetic fields for polarized emission measurements and Kerr measurements, respectively, the $g$-factor for InSe was extracted. Further investigations are done by calculating precession values using a $textbf{k} cdot textbf{p}$ model, which is supported by textit{ab-initio} density functional theory. Comparison of predicted precession rates with experimental measurements highlights the importance of excitonic effects in InSe for understanding spin dynamics. Optical orientation of spin is an important prerequisite for opto-spintronic phenomena and devices, and these first demonstrations of layer-dependent optical excitation of spins in InSe lay the foundation for combining layer-dependent spin properties with advantageous electronic properties found in this material.

Published in: "arXiv Material Science".

Phase and polarization modulation in two-dimensional In2Se3 via in situ transmission electron microscopy | Science Advances

2022-10-21T22:36:30+00:00October 21st, 2022|Categories: Publications|Tags: |

Abstract Phase transitions in two-dimensional (2D) materials promise reversible modulation of material physical and chemical properties in a wide range of applications. 2D van der Waals layered In 2 Se 3 with bistable out-of-plane ferroelectric (FE) α phase and antiferroelectric (AFE) β′ phase is particularly attractive for its electronic

Published in: "Science Advances".

Ferroelectricity controlled chiral spin textures and anomalous valley Hall effect in the Janus magnet-based multiferroic heterostructure. (arXiv:2209.11394v1 [cond-mat.mes-hall])

2022-09-26T04:30:52+00:00September 26th, 2022|Categories: Publications|Tags: , , |

Realizing effective manipulation and explicit identification of topological spin textures are two crucial ingredients to make them as information carrier in spintronic devices with high storage density, high data handling speed and low energy consumption. Electric-field manipulation of magnetism has been achieved as a dissipationless method compared with traditional regulations. However, the magnetization is normally insensitive to the electric field since it does not break time-reversal symmetry directly, and distribution of topological magnetic quasiparticles is difficult to maintain due to the drift arising from external fluctuation, which could result in ambiguous recognition between quasiparticles and uniform magnetic background. Here, we demonstrate that electric polarization-driven skyrmionic and uniform ferromagnetic states can be easily and explicitly distinguished by transverse voltage arising from anomalous valley Hall effect in the Janus magnet-based multiferroic heterostructure LaClBr/In2Se3. Our work provides an alternative approach for data encoding, in which data are encoded by combing topological spin textures with detectable electronic transport.

Published : "arXiv Mesoscale and Nanoscale Physics".

Ferroelectricity controlled chiral spin textures and anomalous valley Hall effect in the Janus magnet-based multiferroic heterostructure. (arXiv:2209.11394v1 [cond-mat.mes-hall])

2022-09-26T02:29:26+00:00September 26th, 2022|Categories: Publications|Tags: , , |

Realizing effective manipulation and explicit identification of topological spin textures are two crucial ingredients to make them as information carrier in spintronic devices with high storage density, high data handling speed and low energy consumption. Electric-field manipulation of magnetism has been achieved as a dissipationless method compared with traditional regulations. However, the magnetization is normally insensitive to the electric field since it does not break time-reversal symmetry directly, and distribution of topological magnetic quasiparticles is difficult to maintain due to the drift arising from external fluctuation, which could result in ambiguous recognition between quasiparticles and uniform magnetic background. Here, we demonstrate that electric polarization-driven skyrmionic and uniform ferromagnetic states can be easily and explicitly distinguished by transverse voltage arising from anomalous valley Hall effect in the Janus magnet-based multiferroic heterostructure LaClBr/In2Se3. Our work provides an alternative approach for data encoding, in which data are encoded by combing topological spin textures with detectable electronic transport.

Published in: "arXiv Material Science".

Ferroelectric tuning of superconductivity and band topology in a two-dimensional heterobilayer. (arXiv:2203.03082v1 [cond-mat.supr-con])

2022-03-08T04:30:43+00:00March 8th, 2022|Categories: Publications|Tags: , |

Realization of tunable superconductivity with concomitant nontrivial band topology is conceptually intriguing and highly desirable for superconducting devices and topological quantum computation. Based on first-principles calculations, here we present the first prediction of simultaneously tunable superconducting transition temperature (Tc) and band topology in a superconducting IrTe2 overlayer on a ferroelectric In2Se3 monolayer. We first demonstrate that the Tc is substantially enhanced from that of IrTe2 nanoflakes (Tc ~3 K) due to significant charge repartitioning around the Fermi level. More importantly, the Tc is shown to sensitively depend on the In2Se3 polarization, with the higher Tc of ~(8-10) K attributed to enhanced interlayer electron-phonon coupling when the polarization is downward. The band topology is also switched from trivial to nontrivial as the polarization is reversed from upward to downward. These findings provide physically realistic platforms for simultaneously tuning superconductivity and band topology in two-dimensional heterobilayers and related heterostructures using a reversible and nonvolatile approach.

Published : "arXiv Mesoscale and Nanoscale Physics".

Large perpendicular magnetic anisotropy of transition metal dimers driven by polarization switching of two-dimensional ferroelectric In2Se3 substrate. (arXiv:2202.13726v1 [cond-mat.mtrl-sci])

2022-03-01T05:29:22+00:00March 1st, 2022|Categories: Publications|Tags: , |

Large perpendicular magnetic anisotropy (MA) is highly desirable for realizing atomic-scale magnetic data storage which represents the ultimate limit of the density of magnetic recording. In this work, we studied the MA of transition metal dimers Co-Os, Co-Co and Os-Os adsorbed on two-dimensional ferroelectric In2Se3 (In2Se3-CoOs, In2Se3-OsCo, In2Se3-CoCo and In2Se3-OsOs) by first-principles calculations. It is found that the Co-Os dimer in In2Se3-CoOs has large total perpendicular magnetic anisotropy energy (MAE) of ~ 40 meV. In particular, the MAE arising from Os atom is up to ~ 60 meV. The large MAE is attributed to the high spin-orbit coupling constant and the onefold coordination of Os atom. In addition, the MA of the dimers can be tuned by the polarization reversal of In2Se3. When the polarization is upward, the easy-axis directions of MA in In2Se3-OsCo, In2Se3-CoCo and In2Se3-OsOs are all in-plane, while the directions become perpendicular as the polarization is switched to downward. For the In2Se3-CoOs, switching polarization from upward to downward enhance the perpendicular MA from ~ 20 meV to ~ 40 meV. Based on the second-order perturbation theory, we confirm that the exchange splitting of dxy/dx2-y2 and dxz/dyz orbitals as well as the occupation of dz2 orbital at the vicinity of Fermi level play important roles in the changes of MA with the reversal of FE polarization of In2Se3.

Published in: "arXiv Material Science".

2D magnetoelectric multiferroics in MnSTe/In2Se3 heterobilayer with ferroelectrically controllable skyrmions. (arXiv:2202.13069v1 [cond-mat.mtrl-sci])

2022-03-01T05:29:14+00:00March 1st, 2022|Categories: Publications|Tags: |

The magnetoelectric effect and skyrmions are two fundamental phenomena in the field of condensed-matter physics. Here, using first-principles calculations and Monte-Carlo simulations, we propose that strong magnetoelectric coupling can be demonstrated in a multiferroic heterobilayer consisting of two-dimensional (2D) MnSTe and {alpha}-In2Se3. As the electric polarization in ferroelectric {alpha}-In2Se3 is switched, the creation and annihilation of topological magnetic phase can be achieved in this multiferroic heterobilayer, giving rise to the intriguing ferroelectrically controllable skyrmions. This feature is further revealed to be closely related to the physical quantity of D2/|KJ|, which is generally applicable for describing the required conditions of such physics. Moreover, the evaluations of their topological magnetic phases with temperature are systematically discussed. These insights not only greatly enrich the research on 2D magnetoelectric multiferroics, but also pave a promising avenue to realize new skyrmionic device concepts.

Published in: "arXiv Material Science".

Designing Ultra-Flat Bands in Twisted Bilayer Materials at Large Twist Angles without specific degree. (arXiv:2202.13791v1 [physics.comp-ph])

2022-03-01T04:30:26+00:00March 1st, 2022|Categories: Publications|Tags: , , , |

Inter-twisted bilayers of two-dimensional (2D) materials can host low-energy flat bands, which offer opportunity to investigate many intriguing physics associated with strong electron correlations. In the existing systems, ultra-flat bands only emerge at very small twist angles less than a few degrees, which poses challenge for experimental study and practical applications. Here, we propose a new design principle to achieve low-energy ultra-flat bands with increased twist angles. The key condition is to have a 2D semiconducting material with large energy difference of band edges controlled by stacking. We show that the interlayer interaction leads to defect-like states under twisting, which forms a flat band in the semiconducting band gap with dispersion strongly suppressed by the large energy barriers in the moire superlattice even for large twist angles. We explicitly demonstrate our idea in bilayer alpha-In2Se3 and bilayer InSe. For bilayer alpha-In2Se3, we show that a twist angle -13.2 degree is sufficient to achieve the band flatness comparable to that of twist bilayer graphene at the magic angle -1.1 degree. In addition, the appearance of ultra-flat bands here is not sensitive to the twist angle as in bilayer graphene, and it can be further controlled by external gate fields. Our finding provides a new route to achieve ultra-flat bands other than reducing the twist angles and paves the way towards engineering such flat bands in a large family of 2D materials.

Published : "arXiv Mesoscale and Nanoscale Physics".

Data-driven discovery of high performance layered van der Waals piezoelectric NbOI2. (arXiv:2202.01373v1 [cond-mat.mtrl-sci])

2022-02-04T02:29:22+00:00February 4th, 2022|Categories: Publications|Tags: , |

Using high-throughput first-principles calculations to search for layered van der Waals materials with the largest piezoelectric stress coefficients, we discover NbOI2 to be the one among 2940 monolayers screened. The piezoelectric performance of NbOI2 is independent of thickness, and its electromechanical coupling factor of near unity is a hallmark of optimal interconversion between electrical and mechanical energy. Laser scanning vibrometer studies on bulk and few-layer NbOI2 crystals verify their huge piezoelectric responses, which exceed internal references such as In2Se3 and CuInP2S6. Furthermore, we provide insights into the atomic origins of anti-correlated piezoelectric and ferroelectric responses in NbOX2 (X = Cl, Br, I), based on bond covalency and structural distortions in these materials. Our discovery that NbOI2 has the largest piezoelectric stress coefficients among 2D materials calls for the development of NbOI2-based flexible nanoscale piezoelectric devices.

Published in: "arXiv Material Science".

Moire flat bands in twisted 2D hexagonal vdW material. (arXiv:2110.07962v1 [cond-mat.mtrl-sci])

2021-10-18T02:29:24+00:00October 18th, 2021|Categories: Publications|Tags: , , , |

Moire superlattices in twisted bilayer graphene (TBG) and its derived structures can host exotic correlated quantum phenomena because the narrow moire flat minibands in those systems effectively enhance the electron-electron interaction. Correlated phenomena are also observed in 2H-transitional metal dichalcogenides moire superlattices. However, the number of moire systems that have been explored in experiments are still very limited. Here we theoretically investigate a series of two-dimensional (2D) twisted bilayer hexagonal materials (TBHMs) beyond TBG at fixed angles of 7.34 and 67.34 degree with 22 2D van der Waals (vdW) layered materials that are commonly studied in experiments. First-principles calculations are employed to systemically study the moire minibands in these systems. We find that flat bands with narrow bandwidth generally exist in these systems. Some of the systems such as twisted bilayer In2Se3, InSe, GaSe, GaS and PtS2 even host ultra-flat bands with bandwidth less than 20 meV even for such large angles, which make them especially appealing for further experimental investigations. We further analysis the characters of moire flat bands and provides guidance for further exploration of 2D moire superlattices that could host strong electron correlations.

Published in: "arXiv Material Science".

Atomic-Scale Visualization and Manipulation of Domain boundaries in 2D Ferroelectric In2Se3. (arXiv:2109.06100v1 [cond-mat.mes-hall])

2021-09-14T02:30:15+00:00September 14th, 2021|Categories: Publications|Tags: , |

Domain boundaries in ferroelectric materials exhibit rich and diverse physical properties distinct from their parent materials and have been proposed for novel applications in nanoelectronics and quantum information technology. Due to their complexity and diversity, the internal atomic and electronic structure of domain boundaries that governs the electronic properties as well as the kinetics of domain switching remains far from being elucidated. By using scanning tunneling microscopy and spectroscopy (STM/S) combined with density functional theory (DFT) calculations, we directly visualize the atomic structure of domain boundaries in two-dimensional (2D) ferroelectric beta’ In2Se3 down to the monolayer limit and reveal a double-barrier energy potential of the 60{deg} tail to tail domain boundaries for the first time. We further controllably manipulate the domain boundaries with atomic precision by STM and show that the movements of domain boundaries can be driven by the electric field from an STM tip and proceed by the collective shifting of atoms at the domain boundaries. The results will deepen our understanding of domain boundaries in 2D ferroelectric materials and stimulate innovative applications of these materials.

Published in: "arXiv Material Science".

Broadband Photocurrent Spectroscopy and Temperature Dependence of Band-gap of Few-Layer Indium Selenide. (arXiv:2104.04877v1 [cond-mat.mes-hall])

2021-04-13T04:30:23+00:00April 13th, 2021|Categories: Publications|Tags: , |

Understanding broadband photoconductive behaviour in two dimensional layered materials are important in order to utilize them for a variety of opto-electronic applications. Here we present our results of photocurrent spectroscopy measurements performed on few layer Indium Selenide (InSe) flakes. Temperature (T) dependent (40 K < T < 300 K) photocurrent spectroscopy was performed in order to estimate the band-gap energies E_g(T) of InSe at various temperatures. Our measurements indicate that room temperature E_g value for InSe flake was ~ 1.254 eV, which increased to a value of ~ 1.275 eV at low temperatures. The estimation of Debye temperatures by analysing the observed experimental variation of E_g as a function of T using several theoretical models is presented and discussed.

Published : "arXiv Mesoscale and Nanoscale Physics".

Two-dimensional multiferroic metal with voltage-tunable magnetization and metallicity. (arXiv:2103.10238v1 [cond-mat.mtrl-sci])

2021-03-19T02:29:24+00:00March 19th, 2021|Categories: Publications|Tags: |

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 {alpha}-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.

Published in: "arXiv Material Science".

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