Jiuxiang Dai

Large-area Bi₂S₃/ReS₂ heterostructure films are synthesized by two-step epitaxial growth method. The band alignment suggests the heterostructures belong to type-II heterostructures. Bi₂S₃/ReS₂ heterostructure film demonstrates excellent saturable absorption (SA) properties including high SA coefficient, large modulation depth, and low saturation intensity. Energy-level model based on charge transfer process is built to explain the enhancement of SA.

Abstract

2D van der Waals (vdW) heterostructure provides a novel platform to modulate linear and nonlinear optical (NLO) properties for optical devices by interface engineering. However, NLO properties and mechanisms based on vdW heterostructures are far from complete understanding. Herein, two-step vdW vapor epitaxial growth by either physical or chemical vapor deposition methods is successfully demonstrated to synthesize uniform Bi₂S₃/ReS₂ vdW heterostructures in large area. Type-II band-alignment of these heterostructures is confirmed by X-ray photoelectron spectroscopy combined with the UV–Vis spectra. The enhanced NLO response of Bi₂S₃/ReS₂ films is observed with high saturable absorption coefficient (≈−445 cm GW⁻¹), large modulation depth (≈10.5%), and low saturation intensity (≈105 GW cm⁻²). Energy level model based on type-II charge transfer process is used successfully to understand the optical physics process at the interface of the vdW heterostructures. Constructing heterostructures with two-step vdW vapor epitaxial growth provides a new method to design high-performance nonlinear photonic devices with 2D materials.

Photoluminescence (PL) of transition metal dichalcogenide (TMD) monolayers is strongly influenced by the dielectric environment. The defect states present in the substrate induces uncontrollable doping in the TMD monolayer and thereby modifies the PL spectra. There have been enormous efforts to tune and overcome the effect of inevitable substrate defects in PL spectra, but a proper understanding and a convenient way are still lacking. Here, we systematically studied the effect of surface defects by gradually increasing the separation between WS 2 monolayer and substrate. Hence, we could precisely modulate the exciton and trion contribution in the PL spectra of WS 2 . The excitation power dependant measurements on dielectric engineered and patterned substrates helped us to shed light on the mechanism of PL modulation in monolayer WS 2 . We have also studied the influence of the nature of the charge carried by substrate defects on the PL spectra. These ...

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Nature Communications, Published online: 22 September 2021; doi:10.1038/s41467-021-25912-w

The quantum anomalous Hall effect has so far been limited to temperature of the order of 20 mK. Here, Fijalkowski et al. report the existence of chiral edge channels up to the Curie temperature of bulk ferromagnetism of the magnetic topological insulator with a multi-terminal Corbino geometry.

A versatile technique for exfoliating and transferring electronic films and devices is presented. A variety of films as well as complex high-performance devices can be transferred, paving the way for increased system performance and functionality. Film and device characterization reveal unique strain-related challenges and solutions. This technique can potentially enable new heterogeneous integration electronics packaging strategies.

Abstract

Heterogeneous integration strategies are increasingly being employed to achieve more compact and capable electronics systems for multiple applications including space, electric vehicles, and wearable and medical devices. To enable new integration strategies, the growth and transfer of thin electronic films and devices, including III-nitrides, metal oxides, and 2D materials, using 2D boron nitride (BN)-on-sapphire templates are demonstrated. The van der Waals (vdW) BN layer, in this case, acts as a preferred mechanical release layer for precise separation at the substrate–film interface and leaves a smooth surface suitable for vdW bonding. A tensilely stressed Ni layer sputtered on top of the film induces controlled spalling fracture that propagates at the BN/sapphire interface. By incorporating controlled spalling, the process yield and sensitivity are greatly improved, owed to the greater fracture energy provided by the stressed metal layer relative to a soft tape or rubber stamp. With stress playing a critical role in this process, the influence of residual stress on detrimental cracking and bowing is investigated. Additionally, a back-end selected area lift-off technique is developed which allows for isolation and transfer of individual devices or arbitrary shapes.

A new growth mode of 2D van der Waals (vdW) heterostructures with distinctly different morphologies and growth rates from the conventional mode is observed using a custom-built system integrating physical vapor deposition with in situ optical microscopy/Raman spectroscopy. A model to explain these two growth modes is proposed, which appears to be applicable to 2D vdW heterostructures in general.

Abstract

Understanding the growth mechanisms of 2D van der Waals (vdW) heterostructures is of great importance in exploring their functionalities and device applications. A custom-built system integrating physical vapor deposition and optical microscopy/Raman spectroscopy is employed to study the dynamic growth processes of 2D vdW heterostructures in situ. This allows the identification of a new growth mode with a distinctly different growth rate and morphology from those of the conventional linear growth mode. A model that explains the difference in morphologies and quantifies the growth rates of the two modes by taking the role of surface diffusion into account is proposed. A range of material combinations including CdI₂/WS₂, CdI₂/MoS₂, CdI₂/WSe₂, PbI₂/WS₂, PbI₂/MoS₂, PbI₂/WSe₂, and Bi₂Se₃/WS₂ is systematically investigated. These findings may be generalized to the synthesis of many other 2D heterostructures with controlled morphologies and physical properties, benefiting future device applications.

Low-dimensional black phosphorus (LDBP) opens up new opportunities for sensors development. An overview of LDBP for detection with focus on the structure, properties, protection strategies, preparation methods is presented. Furthermore, the relationship between the characteristics and sensing performance, along with the recent progress in various LDBP-based sensors are elaborated. Finally, the limitations and challenges are discussed.

Abstract

Low-dimensional black phosphorus (LDBP) materials have emerged, with considerable application potential in sensing due to their unique folded structures and remarkable physicochemical properties. These include anisotropies, layer-dependent and tunable band gaps, high carrier mobilities, high current switching ratios, and excellent electron donor capacities. As a type of supporting material that is favorable to signal transmission or reception, LDBP is widely researched in piezoelectric, flexible, chemical, and biomolecular sensors. This review summarizes the synthetic methods, properties, and modification strategies of LDBP, and then mainly focuses on the research progress in LDBP-based sensor applications, including physical and chemical sensors and biosensors. The major issues in LDBP-based sensor applications are also discussed. Finally, the prospects and challenges in the field of LDBP-based sensors are analyzed.

A spatially confined method is developed to grow single-walled carbon nanotube horizontal arrays in a floating solid catalyst chemical vapor deposition system. The trapping effect in the confined space and the design of titanium-based catalysts results in a high density of 65 tubes µm⁻¹ and high semiconducting purity of >95%.

Abstract

High-density horizontally aligned single-walled carbon nanotube (SWNT) arrays with high-purity semiconducting tubes are promising materials for energy-efficient electronics. However, realizing high density and high semiconducting purity through the direct-growth method is still a challenge. Here the problem is overcome through a spatially confined approach based on a floating solid catalyst chemical vapor deposition (FSCCVD) system. A confined space is designed to change the fluid dynamics in a chemical vapor deposition reactor, offering a trapping effect for both catalyst nanoparticles and carbon fragments. Therefore, densely and uniformly distributed catalyst nanoparticles are continuously deposited on substrates, greatly increasing the growth probability and efficiency of SWNTs. Besides, the solid catalyst TiC and the oxygen-vacancy-enriched catalyst TiO both facilitate the growth of semiconducting tubes. As a result, SWNT horizontal arrays with a high density of 65 tubes µm⁻¹ and a semiconducting purity of >95% are realized, which hold the potential for future application in carbon nanotube electronics.

Two-dimensional layered materials have attracted a lot of attention as building block in memristive devices owing to their high downscaling potential, easy stacking due to van der Waals forces and mechanical flexibility. In this study, memristive switching is explored in vertical device structures based on layered Sb 2 Te 3 . For this, epitaxial 2D-like Sb 2 Te 3 thin films with thicknesses of ∼20 nm were directly grown on conductive p-type Si (111) substrates by pulsed laser deposition. Analog programmability mimicking neuromorphic operation, stable multilevel retention and endurance performance with a memory window larger than one order of magnitude are achieved by utilizing Ag as electrode metal. However, Cu top electrodes lead to a memristive switching with generally smaller memory window and volatility of programmed states. Devices with both electrode metals offer forming-free operation and self-compliance. Structural and chemical charac...

Two-dimensional van der Waals (vdWs) materials have gathered a lot of attention recently. However, the majority of these materials have Curie temperatures that are well below room temperature, making it challenging to incorporate them into device applications. In this work, we synthesized a room-temperature vdW magnetic crystal Fe 5 GeTe 2 with a Curie temperature T ##IMG## [http://ej.iop.org/images/2053-1583/8/4/045030/tdmac2028ieqn1.gif] {$_c = 332$} K, and studied its magnetic properties by vibrating sample magnetometry (VSM) and broadband ferromagnetic resonance (FMR) spectroscopy. The experiments were performed with external magnetic fields applied along the c -axis ( H ##IMG## [http://ej.iop.org/images/2053-1583/8/4/045030/tdmac2028ieqn2.gif] {$\parallel$} c ) and the ab -plane ( H ##IMG## [http://ej.iop.org/images/2053-1583/8/4/045030/tdmac...] {$\parallel$}

Publication date: December 2021

Source: Materials Today, Volume 51

Author(s): Liusi Yang, Dashuai Wang, Minsu Liu, Heming Liu, Junyang Tan, Zhongyue Wang, Heyuan Zhou, Qiangmin Yu, Jingyun Wang, Junhao Lin, Xiaolong Zou, Ling Qiu, Hui-Ming Cheng, Bilu Liu

The intercalation of a molecule or ion in a layered structure is key to enhancing energy storage, material conductivity, intercalant structural ordering, and the formation of two-dimensional (2D) superconducting states. The process of intercalation modifies the vibrational energy of the host, which can be monitored non-invasively by Raman spectroscopy. However, the detected Raman spectral shifts may originate from a variety of phenomena, generally making the technique an indirect means of identifying intercalation success. Here, we discuss newly discovered low-frequency (LF) (<100 cm −1 ) Raman features due to the formation of unique 2D polar metals (Ag, Cu, Pb, Bi, Ga, In) or metal alloys (In x Ga 1− x ) intercalated at an epitaxial graphene (EG)/silicon carbide (SiC) interface and demonstrate that 2D-Ag and 2D-Ga can have spatially distinct phases with their own unique Raman responses. Additionally, we establish that the 2D-Ga exhi...

Publication date: December 2021

Source: Materials Today, Volume 51

Author(s): Saif Siddique, Chinmayee Chowde Gowda, Solomon Demiss, Raphael Tromer, Sourav Paul, Kishor Kumar Sadasivuni, Emmanuel Femi Olu, Amreesh Chandra, Vidya Kochat, Douglas S. Galvão, Partha Kumbhakar, Rohan Mishra, Pulickel M. Ajayan, Chandra Sekhar Tiwary

By refining the synthesis of high-quality thin films, a superconducting dome is observed in La_{1− x} Sr _x NiO₂. Perhaps more surprisingly, undoped LaNiO₂ also shows a strong indication of superconductivity. A generalized superconducting dome can be obtained by combining the La/Pr/Nd systems, revealing systematic shifts in the unit cell volume and the relative electron–hole population across the lanthanides.

Abstract

The occurrence of unconventional superconductivity in cuprates has long motivated the search for manifestations in other layered transition metal oxides. Recently, superconductivity is found in infinite-layer nickelate (Nd,Sr)NiO₂ and (Pr,Sr)NiO₂ thin films, formed by topotactic reduction from the perovskite precursor phase. A topic of much current interest is whether rare-earth moments are essential for superconductivity in this system. In this study, it is found that with significant materials optimization, substantial portions of the La_{1− x} Sr _x NiO₂ phase diagram can enter the regime of coherent low-temperature transport (x = 0.14 - 0.20), with subsequent superconducting transitions and a maximum onset of ≈9 K at x = 0.20. Additionally, the unexpected indication of a superconducting ground state in undoped LaNiO₂ is observed, which likely reflects the self-doped nature of the electronic structure. Combining the results of (La/Pr/Nd)_{1− x} Sr _x NiO₂ reveals a generalized superconducting dome, characterized by systematic shifts in the unit cell volume and in the relative electron-hole populations across the lanthanides.

2D magnetic crystals constitute ideal platforms to experimentally access the fundamental physics of magnetism and achieve magneto-optical and magneto-electric devices for future electronics. In this work, a new generalized synthetic concept for producing wafer-scale epitaxial thin films of 2D ferromagnetic ternary chalcogenide crystals is described. The growth process utilizes commercial wafers (Ge, Si) as a metal source as well as epitaxy provider.

Abstract

Following the first experimental realization of intrinsic ferromagnetism in 2D van der Waals (vdW) crystals, several ternary metal chalcogenides with unprecedented long-range ferromagnetic order have been explored. However, the synthesis of large-area 2D ternary metal chalcogenide thin films is a great challenge, and a generalized synthesis has not been demonstrated yet. Here, a quick and scalable synthesis of epitaxially aligned ferromagnetic ternary metal chalcogenide thin films (Cr₂Ge₂Te₆, Cr₂Si₂Te₆, Mn₃Si₂Te₆) is reported. The synthesis is based on the flux-controlled surface diffusion of Te on metal (Cr, Mn)-deposited wafer (Ge, Si) substrates. Magnetic anisotropy study of the epitaxial ternary thin films reveals the intrinsic magnetic easy axis; out-of-plane direction for Cr₂Ge₂Te₆ and Cr₂Si₂Te₆, and in-plane direction for Mn₃Si₂Te₆. In addition to the synthesis, this work creates an opportunity for transfer-free device fabrication for realizing magnetoelectronics based on the electrical control of both charge and spin degrees of freedom in 2D ferromagnetic semiconductors.