Jiuxiang Dai

Publication date: March 2023

Source: Materials Today, Volume 63

Author(s): Manzhang Xu, Jinpeng Xu, Lei Luo, Mengqi Wu, Bijun Tang, Lei Li, Qianbo Lu, Weiwei Li, Haoting Ying, Lu Zheng, Hao Wu, Qiang Li, Hanjun Jiang, Jun Di, Wu Zhao, Zhiyong Zhang, Yongmin He, Xiaorui Zheng, Xuetao Gan, Zheng Liu

Ferromagnetic EuCd₂As₂ exhibits anomalous Hall conductivity and Nernst effect, resulting from the non-zero Berry curvature close to the Fermi level. Scanning tunneling microscopy and angle-resolved photoemission spectroscopy are used to investigate its electronic structure which is in agreement with first principles calculations.

Abstract

Weyl semimetal is a unique topological phase with topologically protected band crossings in the bulk and robust surface states called Fermi arcs. Weyl nodes always appear in pairs with opposite chiralities, and they need to have either time-reversal or inversion symmetry broken. When the time-reversal symmetry is broken the minimum number of Weyl points (WPs) is two. If these WPs are located at the Fermi level, they form an ideal Weyl semimetal (WSM). In this study, intrinsic ferromagnetic (FM) EuCd₂As₂ are grown, predicted to be an ideal WSM and studied its electronic structure by angle-resolved photoemission spectroscopy, and scanning tunneling microscopy which agrees closely with the first principles calculations. Moreover, anomalous Hall conductivity and Nernst effect are observed, resulting from the non-zero Berry curvature, and the topological Hall effect arising from changes in the band structure caused by spin canting produced by magnetic fields. These findings can help realize several exotic quantum phenomena in inorganic topological materials that are otherwise difficult to assess because of the presence of multiple pairs of Weyl nodes.

Atomic-level regulated ReSe₂ nanosheets serve as the general platform to significantly advance the photocatalytic H₂ evolution on various semiconductors, such as TiO₂, CdS, ZnIn₂S₄, and C₃N₄. The outstanding activity is attributed to the existence of abundant Re/Se active sites and strongly coupled interface between atomic-level regulated ReSe₂ and the semiconductor photocatalyst.

Abstract

Solar hydrogen (H₂) generation via photocatalytic water splitting is practically promising, environmentally benign, and sustainably carbon neutral. It is important therefore to understand how to controllably engineer photocatalysts at the atomic level. In this work, atomic-level engineering of defected ReSe₂ nanosheets (NSs) is reported to significantly boost photocatalytic H₂ evolution on various semiconductor photocatalysts including TiO₂, CdS, ZnIn₂S₄, and C₃N₄. Advanced characterizations, such as atomic-resolution aberration-corrected scanning transmission electron microscopy (AC-STEM), synchrotron-based X-ray absorption near edge structure (XANES), in situ X-ray photoelectron spectroscopy (XPS), transient-state surface photovoltage (SPV) spectroscopy, and transient-state photoluminescence (PL) spectroscopy, together with theoretical computations confirm that the strongly coupled ReSe₂/TiO₂ interface and substantial atomic-level active sites of defected ReSe₂ NSs result in the significantly raised activity of ReSe₂/TiO₂. This work not only for the first time realizes the atomic-level engineering of ReSe₂ NSs as a versatile platform to significantly raise the activities on different photocatalysts, but, more importantly, underscores the immense importance of atomic-level synthesis and exploration on 2D materials for energy conversion and storage.

The reversible twinning-mediated pseudo-elasticity and variable electro-conductivity in Mo nanocrystals are shown by in situ tension and current measurement. Coherent and inclined twin boundary (TB) twinning mechanisms are uncovered in nanocrystals with smaller and larger diameters, respectively. The effects of size and TB types on the electrical conductivity are also quantified based on the experimental measurements and calculations.

Abstract

Deformation twinning merits attention because of its intrinsic importance as a mode of energy dissipation in solids. Herein, through the atomistic electron microscopy observations, the size-dependent twinning mechanisms in refractory body-centered cubic molybdenum nanocrystals (NCs) under tensile loading are shown. Two distinct twinning mechanisms involving the nucleation of coherent and inclined twin boundaries (TBs) are uncovered in NCs with smaller (diameter < ≈5 nm) and larger (diameter > ≈5 nm) diameters, respectively. Interestingly, the ultrahigh pseudo-elastic strain of ≈41% in sub-5 nm-sized crystals is achieved through the reversible twinning mechanism. A typical TB cross-transition mechanism is found to accommodate the NC re-orientation during the pseudo-elastic deformation. More importantly, the effects of different types of TBs on the electrical conductivity based on the repeatable experimental measurements and first-principles calculations are quantified. These size-dependent mechanical and electrical properties may prove essential in advancing the design of next-generation flexible nanoelectronics.

Applied Physics Letters, Volume 122, Issue 8, February 2023.
Anharmonicity as a fundamental issue inspires numerous interesting phenomena in phase transition, electronic structure, thermal transport, and so on. Here, we find that the peculiar [math] phonon mode of in-plane rotational vibration of group-IB-atom ring introduces the anharmonicity into the s(I) and s(II) phases of two-dimensional group-IB chalcogenides. Compared to the high-symmetry s(I) phase, the [math] phonon mode is always active and the anharmonicity is stronger in the symmetry-breaking s(II) phase by releasing the strain energy. The temperature-hardened [math] mode stabilizes the s(I) phase and reduces the lattice thermal conductivity by strengthening the anharmonicity. The strain-softened [math] mode drives the s(II)-to-s(I) phase transition and enhances the lattice thermal conductivity by weakening the anharmonicity. We also establish the relationships of the anharmonicity vs the band structure and Poisson's ratio. As the anharmonicity is weakened during the strain-induced s(II)-to-s(I) phase transition, the bandgap significantly increases. Meanwhile, the weaker anharmonicity implies the lower Poisson's ratio, which further drops much faster with the strain. Our work realizes the tuning of anharmonicity by the peculiar phonon mode in 2D group-IB chalcogenides, which provides a useful guidance for further understanding the anharmonic effect.

Applied Physics Letters, Volume 122, Issue 8, February 2023.
Antiferroelectric AgNbO3 ceramic is investigated with a focus on the effects of uniaxial compressive stress on dielectric response and phase transitions as well as its frequency-dependent ferroelastic behavior. The application of uniaxial compressive stress leads to diffused phase transitions, higher phase transition temperatures, and increased permittivity parallel to the stress application direction for low-temperature phase regions (MI, MIIa). The stress-dependent permittivity response at different phase regions reveals the influence of stress on domain wall motion and phase changes. Additionally, loading rate-dependent stress–strain measurements demonstrate easier ferroelastic domain switching under a lower loading frequency, where the coercive stress increases with frequency initially while getting saturated above 5 mHz. This study reveals the impact of external stress, which can alter the dielectric response and affect domain wall movement at different extents depending on the loading frequency and shift phase boundaries of AgNbO3, implying positive prospects of property engineering of energy storage materials by stress application.

Nature Communications, Published online: 24 February 2023; doi:10.1038/s41467-023-36565-2

Plateau’s law describes the configuration (joint angles ~120°) of soap bubbles, foams and cellular structures, but its applicability to solid films has not been explored. Here, the authors report the observation of a variant Plateau’s law in networks of nanobubbles made of 2D semiconductors, measuring largely varying joint angles due to the thickness-dependent effective surface tension.

The controllable preparation of ultrathin monoclinic ZrO₂ (m-ZrO₂) single crystals (EOT≈0.29 nm, EOT is equivalent oxide thickness) via thermal oxidation of ZrS₂ is achieved. By using K₃[Fe(CN)₆] as seeding, the large-scale parent ZrS₂ is easily grown. The grown m-ZrO₂ presents a high dielectric constant of ≈19 and a breakdown voltage of ≈7.22 MV cm⁻¹. Using m-ZrO₂ as top gate dielectric, the MoS₂ field effect transistor (FET) shows excellent device performance.

Abstract

High-κ materials that exhibit large permittivity and band gaps are needed as gate dielectrics to enhance capacitance and prevent leakage current in downsized technology nodes. Among these, monoclinic ZrO₂ (m-ZrO₂) shows good potential because of its inertness and high-κ with respect to SiO₂, but a method to produce ultrathin single crystal is lacking. Here, the controllable preparation of ultrathin m-ZrO₂ single crystals via the in situ thermal oxidation of ZrS₂ is achieved. As-grown m-ZrO₂ presents an equivalent oxide thickness of ≈0.29 nm, a high dielectric constant of ≈19, and a breakdown voltage (E _BD) of ≈7.22 MV cm⁻¹. MoS₂ field effect transistor (FET) by using m-ZrO₂ as a dielectric layer shows comparable mobility to that using SiO₂ dielectric. The ultraclean interface of m-ZrO₂/MoS₂ and high crystalline quality of m-ZrO₂ lead to negligible hysteresis in transfer curves. Single crystal m-ZrO₂ dielectric shows potential application in digital complementary metal oxidesemiconductor (CMOS) logic FET.

Applied Physics Letters, Volume 122, Issue 8, February 2023.
Antiferromagnetic transition metal oxides are an established and widely studied materials system in the context of spin-based electronics, commonly used as passive elements in exchange bias-based memory devices. Currently, major interest has resurged due to the recent observation of long-distance spin transport, current-induced switching, and THz emission. As a result, insulating transition metal oxides are now considered to be attractive candidates for active elements in future spintronic devices. Here, we discuss some of the most promising materials systems and highlight recent advances in reading and writing antiferromagnetic ordering. This article aims to provide an overview of the current research and potential future directions in the field of antiferromagnetic insulatronics.

Author(s): Viviana Villafañe, Bianca Scaparra, Manuel Rieger, Stefan Appel, Rahul Trivedi, Tongtong Zhu, John Jarman, Rachel A. Oliver, Robert A. Taylor, Jonathan J. Finley, and Kai Müller

We demonstrate that semiconductor quantum dots can be excited efficiently in a resonant three-photon process, while resonant two-photon excitation is highly suppressed. Time-dependent Floquet theory is used to quantify the strength of the multiphoton processes and model the experimental results. The…

[Phys. Rev. Lett. 130, 083602] Published Thu Feb 23, 2023

Applied Physics Letters, Volume 122, Issue 8, February 2023.
Traditional surface engineering, as a means of manufacturing triboelectric nanogenerator (TENG), is complex and expensive. The yield of traditional polymer process is low, which leads to the high cost and low stability of traditional TENGs and greatly limits their practical applications. Moreover, it is worth noting that with the miniaturization and integration of electronic devices, generators need to provide higher current in parallel circuits. In this study, we report the performance of the enhanced Cu/P-type GaN TENG contacts in centimeter scale. Considering the high surface mechanical strength and surface structure characteristics of GaN wafers, we propose using molten KOH to etch the Ga polar GaN surface to form more interface electrons and dangling bonds without destroying the surface structure. Our experimental results show that the generator performance has been drastically improved (the short circuit current increases from 9 to 80 μA, and the open circuit voltage increases from 8 to 29 V). The maximum load electric power density of ∼0.28 W/m2 was obtained. We also compared the open circuit current density with the reported different type TENGs based on Schottky contact at the centimeter-level. The Cu/P-type GaN TENGs achieved in this work exhibit excellent open circuit current density of ∼36 μA/cm2. Thus, we provide insight into surface engineering for future generation TENG devices.

Applied Physics Letters, Volume 122, Issue 8, February 2023.
Atomically thin two-dimensional (2D) materials make it possible to create a variety of van der Waals (vdW) heterostructures with different physical features and attributes, which enables the growth of innovative electronics and optoelectronics applications. The band alignment and charge transfer play a crucial role in the physical and optoelectrical properties of the vdW heterostructure. Here, we design a vdW heterojunction device comprising low-symmetric CrOCl to induce a stable anti-ambipolar behavior and polarization-sensitive photodetection performance. 2D CrOCl exhibits strong in-plane anisotropy and linear dichroism, and an anti-ambipolar transport behavior is observed in a MoTe2 channel due to the gate-tunable band bending and charge transfer at MoTe2/CrOCl interface. The devices also exhibit well photodetection performance with a responsivity of 1.05 A/W and a temporal response of 970 μs. Owing to the anisotropic CrOCl serving as a photosensitizing layer, the device achieves the capability of polarization-sensitive photodetection with a photocurrent dichroic ratio up to ∼6. This work offers a valid device model and design strategy to realize the versatile optoelectronics, including the anti-ambipolar transistor and polarimetric photodetectors.

Nature Communications, Published online: 23 February 2023; doi:10.1038/s41467-023-36749-w

Understanding high piezoelectricity in relaxors is challenging due to the heterogeneous structures. Here, the authors demonstrate that the competing local polar order-disorder state with balanced length and direction randomness is the key.

Nature, Published online: 22 February 2023; doi:10.1038/d41586-023-00314-8

A system of ultracold rubidium atoms confined by two misaligned laser-beam arrays has been used to simulate remarkable structures called twisted-bilayer materials. The atomic technology exhibits phenomena such as superfluidity — the frictionless flow of atoms — typically observed in these materials.

Author(s): Chenmu Zhang, Ruoyu Wang, Himani Mishra, and Yuanyue Liu

Two-dimensional semiconductors have demonstrated great potential for next-generation electronics and optoelectronics, however, the current 2D semiconductors suffer from intrinsically low carrier mobility at room temperature, which significantly limits their applications. Here we discover a variety o…

[Phys. Rev. Lett. 130, 087001] Published Wed Feb 22, 2023

Applied Physics Letters, Volume 122, Issue 8, February 2023.
Type-II heterostructures as active layers for semiconductor laser devices combine the advantages of a spectrally broad, temperature stable, and efficient gain with the potential for electrical injection pumping. Their intrinsic charge carrier relaxation dynamics limit the maximum achievable repetition rates beyond any constraints of cavity design or heat dissipation. Of particular interest are the initial build up of gain after high-energy injection and the gain recovery dynamics following depletion through a stimulated emission process. The latter simulates the operation condition of a pulsed laser or semiconductor optical amplifier. An optical pump pulse injects hot charge carriers that eventually build up broad spectral gain in a model (Ga,In)As/GaAs/Ga(As,Sb) heterostructure. The surplus energies of the optical pump mimic the electron energies typical for electrical injection. Subsequently, a second laser pulse tuned to the broad spectral gain region depletes the population inversion through stimulated emission. The spectrally resolved nonlinear transmission dynamics reveal gain recovery times as fast as 5 ps. These data define the intrinsic limit for the highest laser repetition rate possible with this material system in the range of 100 GHz. The experimental results are analyzed using a microscopic many-body theory identifying the origins of the broad gain spectrum.

Applied Physics Letters, Volume 122, Issue 8, February 2023.
Spin–orbit interaction and spin-relaxation mechanisms of a shallow InAs quantum well heterostructure are investigated by magnetoconductance measurements as a function of an applied top-gate voltage. The data are fit using a Iordanskii–Lyanda-Geller–Pikus model and two distinct transport regimes are identified. The spin–orbit interaction splitting energy is extracted from the fits to the data, which also displays two distinct regimes. The different regimes exhibit different spin-scattering mechanisms, the identification of which is of relevance for device platforms of reduced dimensionality which utilize the spin–orbit interaction.

Applied Physics Letters, Volume 122, Issue 8, February 2023.
Combining a two-dimensional (2D) antiferromagnetic (AFM) material, MnPS3 and a 2D ferroelectric material, Sc2CO2, we propose 2D van der Waals (vdW) heterostructure multiferroics to realize strong magnetoelectric coupling, which is important for designing high-performance magnetoelectric devices. By using first-principles simulations, it is found that the transition from an AFM state to a ferromagnetic (FM) state of a MnPS3 layer could be realized by reversing the polarization direction of a Sc2CO2 layer. We further reveal that such strong magnetoelectric effects originate from the large inter-layer charge transfer due to the competitive interaction between the difference of the interface work functions between MnPS3 and Sc2CO2 and the strong electronegativity of the O atom interface in the Sc2CO2 layer. Our results suggest a feasible scheme for constructing 2D vdW heterostructure multiferroics with very strong inter-layer magnetoelectric coupling effect.

Applied Physics Letters, Volume 122, Issue 8, February 2023.
Thin-film lithium niobate (TFLN) photonic integrated circuits (PICs) have emerged as a promising integrated photonics platform for the optical communication, microwave photonics, and sensing applications. In recent years, rapid progress has been made on the development of low-loss TFLN waveguides, high-speed modulators, and various passive components. However, the integration of laser sources on the TFLN photonics platform is still one of the main hurdles in the path toward fully integrated TFLN PICs. Here, we present the heterogeneous integration of InP-based semiconductor lasers on a TFLN PIC. The III–V epitaxial layer stack is adhesively bonded to a TFLN waveguide circuit. In the laser device, the light is coupled from the III–V gain section to the TFLN waveguide via a multi-section spot size converter. A waveguide-coupled output power above 1 mW is achieved for the device operating at room temperature. This heterogeneous integration approach can also be used to realize on-chip photodetectors based on the same epitaxial layer stack and the same process flow, thereby enabling large-volume, low-cost manufacturing of fully integrated III–V-on-lithium niobate systems for next-generation high-capacity communication applications.

The effect of birefringence on the polarized Raman response of anisotropic layered materials is investigated by selecting transparent α-MoO₃ as an example. It is revealed that birefringence modulates the response in a weak manner, which facilitates crystal orientation identification. The physical origin of the intrinsic responses is further studied by combining the atomic vibrational patterns and bond polarizability model.

Abstract

Optical anisotropy, which is quantified by birefringence (Δn) and linear dichroism (Δk), can significantly modulate the angle-resolved polarized Raman spectroscopy (ARPRS) response of anisotropic layered materials (ALMs) by external interference. This work studies the separate modulation of birefringence on the ARPRS response and the intrinsic response by selecting transparent birefringent crystal α-MoO₃ as an excellent platform. It is found that there are several anomalous ARPRS responses in α-MoO₃ that cannot be reproduced by the real Raman tensor widely used in non-absorbing materials; however, they can be well explained by considering the birefringence-induced Raman selection rules. Moreover, the systematic thickness-dependent study indicates that birefringence modulates the ARPRS response to render an interference pattern; however, the amplitude of modulation is considerably lower than that by linear dichroism as occurred in black phosphorous. This weak modulation brings convenience to the crystal orientation determination of transparent ALMs. Combining the atomic vibrational pattern and bond polarizability model, the intrinsic ARPRS response of α-MoO₃ is analyzed, giving the physical origins of the Raman anisotropy. This study employs α-MoO₃ as an example, although it is generally applicable to all transparent birefringent ALMs.

Layer-controlled growth is difficult due to the higher nucleation potential on the surface of 2D materials. Herein, a seeding solution approach is demonstrated to change the nucleation pattern of seed crystals on the substrate surface by precisely controlling the concentration of metal precursors and grow H-VS₂ nanosheets with tunable layer numbers by temperature-controlled kinetic and thermodynamic synergy.

Abstract

The discovery of layered magnetic semiconductor materials has stimulated a search for the magnetic order of atomically thin-layered materials. However, layer-controlled growth is difficult because of the high nucleation barrier of the material surface during the few-layer formation of 2D materials. Herein, a seeding solution method is demonstrated that changes the nucleation mode of seed crystals on the substrate surface by precisely controlling the concentration of metal precursors and promoting the formation of cluster seed nuclei to ensure a sufficient source of metal for subsequent reactions. It is studied that the kinetic and thermodynamic synergy through temperature control is readily to grow VS₂ with a tunable layer number. Monolayer VS₂ exhibits strong ferromagnetic ordering with a saturation magnetization strength (M_s) of 37 emu per cc and a coercivity (H_c) of 135 Oe at 300 K. Notably, the ferromagnetism of VS₂ has layer-dependent performance of the saturation magnetization and coercivity decreases with increasing number of layers. Monolayer VS₂ exhibits typical semiconductor properties in Hall devices. This study broadens the chemical pathway for the tunable synthesis of 2D layered magnetic materials and provides the possibility for the construction of novel spintronic and magnetoelectronic devices.