Jiuxiang Dai

A step toward the optical generation and control of coherent THz spin and lattice dynamics in 2D antiferromagnets. A coherent hybrid phonon–magnon mode is triggered in the 2D antiferromagnet FePS₃ by pumping below the band gap in the presence of an external magnetic field.

Abstract

Coherent THz optical lattice and hybridized phonon–magnon modes are triggered by femtosecond laser pulses in the antiferromagnetic van der Waals semiconductor FePS₃. The laser-driven lattice and spin dynamics are investigated in a bulk crystal as well as in a 380 nm-thick exfoliated flake as a function of the excitation photon energy, sample temperature and applied magnetic field. The pump-probe magneto-optical measurements reveal that the amplitude of a coherent phonon mode oscillating at 3.2 THz decreases as the sample is heated up to the Néel temperature. This signal eventually vanishes as the phase transition to the paramagnetic phase occurs, thus revealing its connection to the long-range magnetic order. In the presence of an external magnetic field, the optically triggered 3.2 THz phonon hybridizes with a magnon mode, which is utilized to excite the hybridized phonon–magnon mode optically. These findings open a pathway toward the optical control of coherent THz photo–magnonic dynamics in a van der Waals antiferromagnet, which can be scaled down to the 2D limit.

Publication date: March 2023

Source: Progress in Materials Science, Volume 133

Author(s): Xuelian Wu, Hui Ling Tan, Chaohua Zhang, Zhenyuan Teng, Zailun Liu, Yun Hau Ng, Qitao Zhang, Chenliang Su

A perylene-based chiral nanographene has been synthesized through helical π-extension of highly emissive chromophores whilst maintaining the frontier molecular orbital distributions. This nanographene exhibits excellent luminescence properties, with a record high Φ _F of 93 % among chiral nanographenes and a remarkable CPL brightness of 32 M⁻¹ cm⁻¹.

Abstract

Chiral nanographenes with both high fluorescence quantum yields (Φ _F) and large dissymmetry factors (g _lum) are essential to the development of circularly polarized luminescence (CPL) materials. However, most studies have been focused on the improvement of g _lum, whereas how to design highly emissive chiral nanographenes is still unclear. In this work, we propose a new design strategy to achieve chiral nanographenes with high Φ _F by helical π-extension of strongly luminescent chromophores while maintaining the frontier molecular orbital (FMO) distribution pattern. Chiral nanographene with perylene as the core and two dibenzo[6]helicene fragments as the wings has been synthesized, which exhibits a record high Φ _F of 93 % among the reported chiral nanographenes and excellent CPL brightness (B _CPL) of 32 M⁻¹ cm⁻¹.

Nature Communications, Published online: 25 November 2022; doi:10.1038/s41467-022-35077-9

npj 2D Materials and Applications, Published online: 24 November 2022; doi:10.1038/s41699-022-00359-9

2D molybdenum compounds, by virtue of their high surface-to-volume ratio, unique electronic structure, and physicochemical properties, show great potential in electrocatalytic energy conversion applications. This review provides a comprehensive overview of the strategies for the synthesis and modulation of 2D molybdenum compounds and their applications in various electrocatalytic reactions that involve the cycles of water, carbon, and nitrogen.

Abstract

The development of advanced nanomaterials is urgent for electrocatalytic energy conversion applications. Recently, 2D nanomaterial-derived heterogeneous electrocatalysts have shown great promise for both fundamental research and practical applications owing to their extremely high surface-to-volume ratio and tunable geometric and electronic properties. Because of their unique electronic structure and physicochemical properties, molybdenum (Mo)-based 2D nanomaterials are emerging as one of the most attractive candidates among the nonprecious materials for electrocatalysts. This review provides a comprehensive overview of the recent advances in the synthesis and modulation of 2D Mo compounds for applications in electrocatalytic energy conversion. The categories based on different compositions and corresponding synthetic approaches of 2D Mo compounds are first introduced. Subsequently, various atomic/plane/synergistic engineering strategies, along with catalytic optimization in the electrochemical process that involves the cycles of water, carbon, and nitrogen, are discussed in detail. Finally, the current challenges and future opportunities for the development of 2D Mo-based electrocatalysts are proposed with the goal of shedding light on these promising 2D nanomaterials for electrocatalytic energy conversion.

It is demonstrated that the formation and properties of 2D oxides at the epitaxial graphene/silicon carbide (EG/SiC) interface is dependent on the graphene buffer layer properties prior to element intercalation (Ga, In, and Sn). The graphene/X/SiC (X = 2D Ga or Ga₂O₃) junction is tunable from Ohmic to a Schottky or tunnel barrier depending on the interface species. Finally, using vertical transport measurements and electron energy loss spectroscopy analysis, the bandgap of crystalline bilayer Ga₂O₃ with ferroelectric wurtzite structure is identified as 6.6 ± 0.6 eV, significantly larger than that of bulk β-Ga₂O₃ (≈4.8 eV).

Abstract

Novel confinement techniques facilitate the formation of non-layered 2D materials. Here it is demonstrated that the formation and properties of 2D oxides (GaO_x, InO_x, SnO_x) at the epitaxial graphene (EG)/silicon carbide (SiC) interface is dependent on the EG buffer layer properties prior to element intercalation. Using 2D Ga, it is demonstrated that defects in the EG buffer layer lead to Ga transforming to GaO_x with non-periodic oxygen in a crystalline Ga matrix via air oxidation at room temperature. However, crystalline monolayer GaO₂ and bilayer Ga₂O₃ with ferroelectric wurtzite structure(FE-WZ') can then be formed via subsequent high-temperature O₂ annealing. Furthermore, the graphene/X/SiC (X = 2D Ga or Ga₂O₃) junction is tunable from Ohmic to a Schottky or tunnel barrier depending on the interface species. Finally, using vertical transport measurements and electron energy loss spectroscopy analysis, the bandgap of 2D gallium oxide is identified as 6.6 ± 0.6 eV, significantly larger than that of bulk β-Ga₂O₃ (≈4.8 eV), suggesting strong quantum confinement effects at the 2D limit. The study presented here is foundational for development of atomic-scale, vertical 2D/3D heterostructure for applications requiring short transit times, such as GHz and THz devices.

The 1T’ Re _x Mo_{1− x} S₂–2H MoS₂ lateral heterostructure is reported to boost hydrogen evolution reaction (HER). At the 1T’ Re _x Mo_{1− x} S₂–2H MoS₂ heterojunction, a Mo-rich heterojunction with high structural stability is formed, the electron transfer from 2H MoS₂ to 1T’ Re _x Mo_{1− x} S₂, and the HER performance is greatly improved with the combination of increased density of states near the Fermi level and optimal ΔG _H* of the Mo-rich heterojunction.

Abstract

The imperfect interfaces between 2D transition metal dichalcogenides (TMDs) are suitable for boosting the hydrogen evolution reaction (HER) during water electrolysis. Here, the improved catalytic activity at the spatial heterojunction between 1T’ Re _x Mo_{1− x} S₂ and 2H MoS₂ is reported. Atomic-scale electron microscopy confirms that the heterojunction is constructed by an in-situ two-step growth process through chemical vapor deposition. Electrochemical microcell measurements demonstrate that the 1T’ Re _x Mo_{1− x} S₂–2H MoS₂ lateral heterojunction exhibits the best HER catalytic performance among all TMD catalysts with an overpotential of ≈84 mV at 10 mA cm⁻² current density and 58 mV dec⁻¹ Tafel slope. Kelvin probe force microscopy shows ≈40 meV as the work function difference between 2H MoS₂ and 1T’ Re _x Mo_{1− x} S₂, facilitating the electron transfer from 2H MoS₂ to 1T’ Re _x Mo_{1− x} S₂ at the heterojunction. First-principles calculations reveal that Mo-rich heterojunctions with high structural stability are formed, and the HER performance is improved with the combination of increased density of states near the Fermi level and optimal ΔG _H* as low as 0.07 eV. Those synergetic effects with many electrons and active sites with optimal ΔG _H* improve HER performance at the heterojunction. These results provide new insights into understanding the role of the heterojunction for HER.

Solution-processed high-performance p-type WSe₂ thin-film transistor is successfully fabricated by Br₂-doping with a field-effect hole mobility of more than 27 cm² V⁻¹ s⁻¹, and a high on/off current ratio of ≈10⁷. The resulting complementary inverters with patterned p-type WSe₂ and n-type MoS₂ layered films reaches an ultra-high gain of 1280 under a driving voltage (V _DD) of 7 V.

Abstract

Semiconducting ink based on 2D single-crystal flakes with dangling-bond-free surfaces enables the implementation of high-performance devices on form-free substrates by cost-effective and scalable printing processes. However, the lack of solution-processed p-type 2D semiconducting inks with high mobility is an obstacle to the development of complementary integrated circuits. Here, a versatile strategy of doping with Br₂ is reported to enhance the hole mobility by orders of magnitude for p-type transistors with 2D layered materials. Br₂-doped WSe₂ transistors show a field-effect hole mobility of more than 27 cm² V⁻¹ s⁻¹, and a high on/off current ratio of ≈10⁷, and exhibits excellent operational stability during the on-off switching, cycling, and bias stressing testing. Moreover, complementary inverters composed of patterned p-type WSe₂ and n-type MoS₂ layered films are demonstrated with an ultra-high gain of 1280 under a driving voltage (V _DD) of 7 V. This work unveils the high potential of solution-processed 2D semiconductors with low-temperature processability for flexible devices and monolithic circuitry.

Metavalent bonding (MVB) is defined in the property space and its origin and distinction from resonant bonding and hyperconjugation are unclear. The essential electronic mechanisms of MVB are uncovered that involve bonding and antibonding pairwise interactions alternating along linear chains of at least five atoms, facilitating long-range electron transfer in response to polar fields that cause exotic properties.

Abstract

A distinct type of metavalent bonding (MVB) is recently proposed to explain an unusual combination of anomalous functional properties of group IV chalcogenide crystals, whose electronic mechanisms and origin remain controversial. Through theoretical analysis of evolution of bonding along continuous paths in structural and chemical composition space, emergence of MVB in rocksalt chalcogenides is demonstrated as a consequence of weakly broken symmetry of parent simple-cubic crystals of Group V metalloids. High electronic degeneracy at the nested Fermi surface of parent metal drives spontaneous breaking of its translational symmetry with structural and chemical fields, which open up a small energy gap and mediate strong coupling between conduction and valence bands making metavalent crystals highly polarizable, conductive, and sensitive to bond-lengths. Stronger symmetry-breaking structural and chemical fields, however, transform them discontinuously to covalent and ionic semiconducting states. MVB involves bonding-antibonding pairwise interactions alternating along linear chains of at least five atoms, which facilitate long-range electron transfer in response to polar fields causing unusual properties. The precise picture of MVB predicts anomalous second-order Raman scattering as an addition to set off their unusual properties, and will guide in design of new metavalent materials with improved thermoelectric, ferroelectric and nontrivial electronic topological properties.

Directional transport of liquid metal (LM) droplets is demonstrated via magnetically active asymmetric microwall arrays. Gradual bending of microwall generates height gradient, which makes the LM droplet to roll-off the microwall by gravitational force. Opposite-directional transport of two LM droplets is achieved on the microwall arrays with pre-programmed bending directions.

Abstract

Eutectic gallium indium liquid metal (LM) is a promising conductive liquid for various electronic applications. In particular, the directional transport of LM droplets has potential applications in soft electronics to control electrical conductivity. Existing methods transport LM droplets by applying an electric field to generate an interfacial tension difference within the LM droplet due to nonuniform ionic distribution of the electrical double layer. However, these methods require confined channels and tethered systems to apply the electric field. In this study, channel-free wireless transport of LM droplets is demonstrated via on-demand magnetomechanical actuation of asymmetric microwall arrays comprising vertically aligned ferromagnetic iron particles embedded in polydimethylsiloxane. Asymmetric microwall of two different widths is designed to generate an asymmetric bending stiffness at a given magnetic field. The asymmetric microwall bends gradually in response to a linear external magnetic field perpendicular to the alignment axis of the iron particles. Therefore, nonuniform magnetomechanical bending induces a local height gradient along the microwall, causing gravitational-force-driven roll-off motion of the LM droplet. Transport direction of LM droplet is modulated by varying the geometric parameters. Finally, the opposite-directional transport of two LM droplets is demonstrated under a linear magnetic field by pre-programming the asymmetric bending direction of each microwall array.

Nature, Published online: 23 November 2022; doi:10.1038/s41586-022-05381-x

Publication date: February 2023

Source: Progress in Materials Science, Volume 132

Author(s): Ashraful Azam, Jack Yang, Wenxian Li, Jing-Kai Huang, Sean Li

Author(s): Kyung-Su Kim (김경수), Chaitanya Murthy, Akshat Pandey, and Steven A. Kivelson

The two-dimensional Wigner crystal (WC) occurs in the strongly interacting regime (rs≫1) of the two-dimensional electron gas (2DEG). The magnetism of a pure WC is determined by tunneling processes that induce multispin ring-exchange interactions, resulting in fully polarized ferromagnetism for large…

[Phys. Rev. Lett. 129, 227202] Published Tue Nov 22, 2022

This article introduces various synthetic approaches and physical properties of amorphous carbon (a-C) thin films and discusses the potential applications including hardmasks, EUV pellicles, diffusion barriers, deformable electrodes and interconnections, sensors, active layers, electrodes for energy, micro-supercapacitors, batteries, nanogenerators, EMI shielding, and nanomembranes.

Abstract

While various crystalline carbon allotropes, including graphene, have been actively investigated, amorphous carbon (a-C) thin films have received relatively little attention. The a-C is a disordered form of carbon bonding with a broad range of the CC bond length and bond angle. Although accurate structural analysis and theoretical approaches are still insufficient, reproducible structure–property relationships have been accumulated. As the a-C thin film is now adapted as a hardmask in the semiconductor industry and new properties are reported continuously, expectations are growing that it can be practically used as active materials beyond as a simple sacrificial layer. In this perspective review article, after a brief introduction to the synthesis and properties of the a-C thin films, their potential practical applications are proposed, including hardmasks, extreme ultraviolet (EUV) pellicles, diffusion barriers, deformable electrodes and interconnects, sensors, active layers, electrodes for energy, micro-supercapacitors, batteries, nanogenerators, electromagnetic interference (EMI) shielding, and nanomembranes. The article ends with a discussion on the technological challenges in a-C thin films.