Jiuxiang Dai

The emerging Ni-based infinite-layer superconducting oxide thin films mark the birth of the Ni age of superconductivity, which may differ from well-known high-temperature superconducting cuprates and Fe-based superconductors because no magnetic order is known in their bulk parent compounds. However, it has now been surprisingly discovered that long-range antiferromagnetism coexists with superconductivity in infinite-layer nickelate thin films.

Abstract

Due to the lack of any magnetic order down to 1.7 K in the parent bulk compound NdNiO₂, the recently discovered 9–15 K superconductivity in the infinite-layer Nd_0.8Sr_0.2NiO₂ thin films has provided an exciting playground for unearthing new superconductivity mechanisms. Herein, the successful synthesis of a series of superconducting Nd_0.8Sr_0.2NiO₂ thin films ranging from 8 to 40 nm is reported. The large exchange bias effect is observed between the superconducting Nd_0.8Sr_0.2NiO₂ films and a thin ferromagnetic layer, which suggests the existence of the antiferromagnetic order. Furthermore, the existence of the antiferromagnetic order is evidenced by X-ray magnetic linear dichroism measurements. These experimental results are fundamentally critical for the current field.

Combining atomic-resolution images from annular dark-field scanning transmission electron microscopy with reactive molecular modelling, it is revealed that the folding edge formation has statistical preferences under geometric conditions based on the orientation mismatch. It is further investigated how loading directions and strong interlayer friction, interplay with WS₂ lattice's crack preference, govern the deformation and fracture pattern around folding edges.

Abstract

Triangular nanovoids in 2D materials transition metal dichalcogenides have vertex points that cause stress concentration and lead to sharp crack propagation and failure. Here, the atomistic mechanics of back folding around triangular nanovoids in monolayer WS₂ sheets is examined. Combining atomic-resolution images from annular dark-field scanning transmission electron microscopy with reactive molecular modelling, it is revealed that the folding edge formation has statistical preferences under geometric conditions based on the orientation mismatch. It is further investigated how loading directions and strong interlayer friction, interplay with WS₂ lattice's crack preference, govern the deformation and fracture pattern around folding edges. These results provide fundamental insights into the combination of fracture and folding in flexible monolayer crystals and the resultant Moiré lattices.

Two dimensional van der Waals ferromagnets with honeycomb structures are expected to host the bosonic version of Dirac particles in their magnon excitation spectra. Using inelastic neutron scattering, we study spin wave excitations in polycrystalline CrCl 3 , which exhibits ferromagnetic honeycomb layers with antiferromagnetic stackings along the c -axis. For comparison, polycrystal samples of CrI 3 with different grain sizes are also studied. We find that the powder-averaged spin wave spectrum of CrCl 3 at T  = 2 K can be adequately explained by the two dimensional spin Hamiltonian including in-plane Heisenberg exchanges only. The observed excitation does not exhibit noticeable broadening in energy, which is in remarkable contrast to the substantial broadening observed in CrI 3 . Based on these results, we conclude that the ferromagnetic phase of CrCl 3 hosts massless Dirac magnons and is thus not topological.

Single-walled carbon nanotube (SWCNT) electronics are shown to be capable of maintaining advances in computing power and energy efficiency in the more Moore era. Further development of SWCNT electronics for next-generation high-performance and low-power integrated circuits (ICs) is still facing many daunting challenges from the material preparation, device fabrication, and circuit design and device–circuit system optimization.

Abstract

Single-walled carbon nanotubes (SWCNTs) have been considered as one of the most promising electronic materials for the next-generation electronics in the more Moore era. Sub-10 nm SWCNT-field effect transistors (FETs) have been realized with several performances exceeding those of Si-based FETs at the same feature size. Several industrial initiatives have attempted to implement SWCNT electronics in integrated circuit (IC) chips. Here, the recent advances in SWCNT electronics are reviewed from in-depth understanding of the fundamental electronic structures, the carrier transport mechanisms, and the metal/SWCNT contact properties. In particular, the subthreshold switching properties are highlighted for low-power, energy-efficient device operations. State-of-the-art low-power SWCNT-based electronics and the key strategies to realize low-voltage and low-power operations are outlined. Finally, the essential challenges and prospects from the material preparation, device fabrication, and large-scale ICs integration for future SWCNT-based electronics are foregrounded.

In this work, an efficient new strategy is presented for the processing of wastewater utilizing an accessible redox reaction with MoSe₂ nanoflowers, which shows a strong oxidizing ability and permits the decomposition of dye molecules in dark environments without the need of an external power source.

Abstract

Water pollution is one of the leading causes of death and disease worldwide, yet mitigating it remains a challenge. This paper presents an efficient new strategy for the processing of wastewater utilizing an accessible redox reaction with MoSe₂ nanoflowers, which shows a strong oxidizing ability and permits the decomposition of dye molecules in dark environments without the need for an external power source. This reaction can treat wastewater at a decomposition rate above 0.077 min⁻¹, even when interacting with organic pollutants at concentrations up to 1500 ppm. Theoretical calculations by Dmol³ simulation elucidates that the reactions proceed spontaneously, and the kinetic constant (k _obs) for this redox reaction with 10 ppm RhB dye is 0.53 min⁻¹, which is 65 times faster than the titanium dioxide photocatalytic wastewater treatment. More importantly, the residual waste solution can be further utilized as a precursor to reconstruct the MoSe₂ nanoflowers. To demonstrate the effectiveness and reusability, the treated effluent is directly used as the sole source of irrigated water for plants with no adverse effect. This method offers an eco-friendly and more accessible way to treat industrial wastewater with zero-discharge.

2D van der Waals (vdW) semiconductor Cr _x Ga_{1− x} Te with highly tunable intrinsic room-temperature ferromagnetism is realized and shows robust room-temperature ferromagnetism with the 2D quantum confinement effect, enabling T _C record-high 314.9–329 K for 2D semiconductors. The M _sat of vdW Cr _x Ga_{1− x} Te crystals can be effectively tuned up to ≈5.4 times by controlling Cr concentration and up to ≈75.9 times by changing the thickness.

Abstract

The combination of semiconductivity and tunable ferromagnetism is pivotal for electrical control of ferromagnetism and next-generation low-power spintronic devices. However, Curie temperatures (T _C) for most traditional intrinsic ferromagnetic semiconductors (≤200 K) and recently discovered two-dimensional (2D) ones (<70 K) are far below room temperature. 2D van der Waals (vdW) semiconductors with intrinsic room-temperature ferromagnetism remain elusive considering the unfavored 2D long-range ferromagnetic order indicated by Mermin–Wagner theorem. Here, vdW semiconductor Cr _x Ga_{1− x} Te crystals exhibiting highly tunable above-room-temperature ferromagnetism with bandgap 1.62–1.66 eV are reported. The saturation magnetic moment (M _sat) of Cr _x Ga_{1− x} Te crystals can be effectively regulated up to ≈5.4 times by tuning Cr content and ≈75.9 times by changing the thickness. vdW Cr _x Ga_{1− x} Te ultrathin semiconductor crystals show robust room-temperature ferromagnetism with the 2D quantum confinement effect, enabling T _C 314.9–329 K for nanosheets, record-high for intrinsic vdW 2D ferromagnetic semiconductors. This work opens an avenue to room-temperature 2D vdW ferromagnetic semiconductor for 2D electronic and spintronic devices.

The field-effect transistor based on thermally fragile layered molybdenum ditelluride (MoTe₂) is systematically studied by in situ electro-thermal measurements complemented by detailed material characterization up to high ambient temperatures. The results, besides analyzing thermal endurance, identify optimum post-fabrication annealing temperature for MoTe₂. Furthermore, MoTe₂ devices exhibit hole doping at high temperatures by utilizing ambient oxygen as a dopant.

Abstract

Functional 2D material-based devices are likely subjected to high ambient temperatures when integrated into miniaturized circuits for practical applications, which may induce irreversible structural changes in materials and impact the device performance. However, majority of 2D devices’ studies focus on room temperature or low-temperature operation conditions. Here, the high-temperature (up to 673 K) electro-thermal response of molybdenum ditelluride (MoTe₂)-based field-effect transistors is investigated. The optimal annealing temperature of around 500–525 K for the multilayer MoTe₂ devices with two-fold enhancement in maximum current level, field-effect mobility, and current on-off ratio is identified. Furthermore, MoTe₂ devices show the transition of electrical response from gate modulation to the degenerately p-doped (hole dominant) characteristics when the operation temperature increases to ≈600 K. The gate-dependent electro-thermal measurements complemented by surface chemistry analysis confirm the near range hopping transport in the MoTe₂ channel at high temperature induced by thermally triggered oxidation of MoTe₂. These results not only provide the thermal endurance limits of MoTe₂ for practical applications, but also indicate the possible applications of MoTe₂ for thermal sensing applications.

Ultrathin 2D PtSe₂ nanosheets with various thickness are synthesized by a CVD method. The thickness dependent electrical and magnetoelectrical properties suggest that PtSe₂ is an intriguing platform for both developing novel functional electronics and conducting fundamental 2D magnetism studies. The existence of magnetic order in this intrinsically non-magnetic system will define a new class of magnetic material and stimulate further study.

Abstract

Thickness-dependent chemical and physical properties have gained tremendous interest since the emergence of two-dimensional (2D) materials. Despite attractive prospects, the thickness-controlled synthesis of ultrathin nanosheets remains an outstanding challenge. Here, a chemical vapor deposition (CVD) route is reported to controllably synthesize high-quality PtSe₂ nanosheets with tunable thickness and explore their thickness-dependent electronic and magnetotransport properties. Raman spectroscopic studies demonstrate all E_g , A _{1 g} , A _{2 u} , and E_u modes are red shift in thicker nanosheets. Electrical measurements demonstrate the 1.7 nm thick nanosheet is a semiconductor with room temperature field-effect mobility of 66 cm² V⁻¹ s⁻¹ and on/off ratio of 10⁶. The 2.3–3.8 nm thick nanosheets show slightly gated modulation with high field-effect mobility up to 324 cm² V⁻¹ s⁻¹ at room-temperature. When the thickness is over 3.8 nm, the nanosheets show metallic behavior with conductivity and breakdown current density up to 6.8 × 10⁵ S m^–1 and 6.9 × 10⁷ A cm⁻², respectively. Interestingly, magnetoresistance (MR) studies reveal magnetic orders exist in this intrinsically non-magnetic material system, as manifested by the thickness-dependent Kondo effect, where both metal to insulator transition and negative MR appear upon cooling. Together, these studies suggest that PtSe₂ is an intriguing system for both developing novel functional electronics and conducting fundamental 2D magnetism study.

As flexible device have become vital in the forefront of technology, the maintenance of multifunctional performances under high flexion is highly desired. Here, the synthesis of super-flexible freestanding BiMnO₃ membranes with stable ferroelectricity and ferromagnetism simultaneously by pulsed laser deposition using recently promising water-soluble Sr₃Al₂O₆ as the sacrificial layer is reported.

Abstract

Multiferroic materials with flexibility are expected to make great contributions to flexible electronic applications, such as sensors, memories, and wearable devices. In this work, super-flexible freestanding BiMnO₃ membranes with simultaneous ferroelectricity and ferromagnetism are synthesized using water-soluble Sr₃Al₂O₆ as the sacrificial buffer layer. The super-flexibility of BiMnO₃ membranes is demonstrated by undergoing an ≈180° folding during an in situ bending test, which is consistent with the results of first-principles calculations. The piezoelectric signal under a bending radius of ≈500 µm confirms the stable existence of electric polarization in freestanding BiMnO₃ membranes. Moreover, the stable ferromagnetism of freestanding BiMnO₃ membranes is demonstrated after 100 times bending cycles with a bending radius of ≈2 mm. 5.1% uniaxial tensile strain is achieved in freestanding BiMnO₃ membranes, and the piezoresponse force microscopy (PFM) phase retention behaviors confirm that the ferroelectricity of membranes can survive stably up to the strain of 1.7%. These super-flexible membranes with stable ferroelectricity and ferromagnetism pave ways to the realizations of multifunctional flexible electronics.

Publication date: December 2021

Source: Materials Today, Volume 51

Author(s): Yu Lei, Srimanta Pakhira, Kazunori Fujisawa, He Liu, Cynthia Guerrero-Bermea, Tianyi Zhang, Archi Dasgupta, Luis M. Martinez, Srinivasa Rao Singamaneni, Ke Wang, Jeff Shallenberger, Ana Laura Elías, Rodolfo Cruz-Silva, Morinobu Endo, Jose L. Mendoza-Cortes, Mauricio Terrones

Author(s): Zhuoliang Ni, Huiqin Zhang, David A. Hopper, Amanda V. Haglund, Nan Huang, Deep Jariwala, Lee C. Bassett, David G. Mandrus, Eugene J. Mele, Charles L. Kane, and Liang Wu

We have developed a sensitive cryogenic second-harmonic generation microscopy to study a van der Waals antiferromagnet MnPS3. We find that long-range Néel antiferromagnetic order develops from the bulk crystal down to the bilayer, while it is absent in the monolayer. Before entering the long-range a...

[Phys. Rev. Lett. 127, 187201] Published Fri Oct 29, 2021

Aggregated suspensions of 2D materials with high-enough concentrations can be processed into homogenous gel inks for various printing and coating methods. The stability of the inks, their rheological properties, and film formation behavior mainly depend on their interflake van der Waals interactions, eliminating the need for addition of the conventional additives and enabling their room-temperature processing.

Abstract

Processing 2D materials into printable or coatable inks for the fabrication of functional devices has proven to be quite difficult. Additives are often used in large concentrations to address the processing challenges, but they drastically degrade the electronic properties of the materials. To remove the additives a high-temperature post-deposition treatment can be used, but this complicates the fabrication process and limits the choice of materials (i.e., no heat-sensitive materials). In this work, by exploiting the unique properties of 2D materials, a universal strategy for the formulation of additive-free inks is developed, in which the roles of the additives are taken over by van der Waals (vdW) interactions. In this new class of inks, which is termed “vdW inks”, solvents are dispersed within the interconnected network of 2D materials, minimizing the dispersibility-related limitations on solvent selection. Furthermore, flow behavior of the inks and mechanical properties of the resultant films are mainly controlled by the interflake vdW attractions. The structure of the vdW inks, their rheological properties, and film-formation behavior are discussed in detail. Large-scale production and formulation of the vdW inks for major high-throughput printing and coating methods, as well as their application for room-temperature fabrication of functional films/devices are demonstrated.

Although studied for decades, 2D materials have often struggled to surpass small scales. Recent progress in the larger-scale synthesis of graphene represents an exciting paradigm shift. An overview of scalable graphene powder and film production is provided, addressing industrial and academic trends, and specifically highlighting flash Joule heating methods.

Abstract

In the past 17 years, the larger-scale production of graphene and graphene family materials has proven difficult and costly, thus slowing wider-scale commercial applications. The quality of the graphene that is prepared on larger scales has often been poor, demonstrating a need for improved quality controls. Here, current industrial graphene synthetic and analytical methods, as well as recent academic advancements in larger-scale or sustainable synthesis of graphene, defined here as weights more than 200 mg or films larger than 200 cm², are compiled and reviewed. There is a specific emphasis on recent research in the use of flash Joule heating as a rapid, efficient, and scalable method to produce graphene and other 2D nanomaterials. Reactor design, synthetic strategies, safety considerations, feedstock selection, Raman spectroscopy, and future outlooks for flash Joule heating syntheses are presented. To conclude, the remaining challenges and opportunities in the larger-scale synthesis of graphene and a perspective on the broader use of flash Joule heating for larger-scale 2D materials synthesis are discussed.

2D all-inorganic NbWO₆ perovskite nanosheets with thicknesses down to 1.5 nm are synthesized for the fabrication of gas sensors, and the fabricated few-layer NbWO₆-based semiconducting sensor exhibits fast H₂S sensing speed with high selectivity and sensitivity at low temperature.

Abstract

Hydrogen sulfide (H₂S) detection with high selectivity and low working temperature is of great significance due to its strong toxicity both to the environment and to humans and also as an endogenous signaling molecule existing in various physiological processes. 2D perovskites with high carrier mobility are promising candidates for gas sensing; however, the development of stable and nontoxic 2D perovskites nanosheets still remains a challenge. Herein, 2D all-inorganic NbWO₆ perovskite nanosheets with thicknesses down to 1.5 nm are synthesized by liquid exfoliation, and the gas-sensing performance based on these ultrathin nanosheets is investigated. A few-layer NbWO₆-based sensor exhibits fast H₂S sensing speed (<6 s) with high selectivity and sensitivity (S = 12.5 vs 50 ppm) at low temperature (150 °C). A small variation of H₂S concentration (<0.5 ppm) can be detected with a fully reversible resistance signal. This work sheds light on the development of high-performance gas sensors working in ambient conditions based on low-dimensional, nontoxic, and wide-bandgap perovskite semiconductors. The high carrier mobility, ultrathin structure, and soft nature make this type of 2D perovskite semiconductor an ideal material candidate for the fabrication of flexible, transparent, and wearable sensing devices in the future.

Interlayer excitons in 2D transition-metal dichalcogenide heterostructures have attracted great attention recently, and are creating their identities and impacts. A brief overview of the enticing properties of interlayer excitons in strong quantum confinement, broadband and tunable spectrum, ultrafast dynamics, and electrical manipulation is given, illustrating the state-of-the-art developments and their grand potential in ultralow-threshold lasing emission, broadband photodetection, valleytronics, and future excitonic transistors.

Abstract

Optoelectronic materials that allow on-chip integrated light signal emitting, routing, modulation, and detection are crucial for the development of high-speed and high-throughput optical communication and computing technologies. Interlayer excitons in 2D van der Waals heterostructures, where electrons and holes are bounded by Coulomb interaction but spatially localized in different 2D layers, have recently attracted intense attention for their enticing properties and huge potential in device applications. Here, a general view of these 2D-confined hydrogen-like bosonic particles and the state-of-the-art developments with respect to the frontier concepts and prototypes is presented. Staggered type-II band alignment enables expansion of the interlayer direct bandgap from the intrinsic visible in monolayers up to the near- or even mid-infrared spectrum. Owing to large exciton binding energy, together with ultralong lifetime, room-temperature exciton devices and observation of quantum behaviors are demonstrated. With the rapid advances, it can be anticipated that future studies of interlayer excitons will not only allow the construction of all-exciton information processing circuits but will also continue to enrich the panoply of ideas on quantum phenomena.

The rapidly developing field of curvilinear magnetism and superconductivity is reviewed. The covered topics range from novel physical concepts for material science through first proof-of-concept device demonstrations to established technologies of flexible magnetoelectronics. The review is relevant for a broad community of physicists, material scientists, and engineers, targeting fundamental research and technology transfer activities.

Abstract

Traditionally, the primary field, where curvature has been at the heart of research, is the theory of general relativity. In recent studies, however, the impact of curvilinear geometry enters various disciplines, ranging from solid-state physics over soft-matter physics, chemistry, and biology to mathematics, giving rise to a plethora of emerging domains such as curvilinear nematics, curvilinear studies of cell biology, curvilinear semiconductors, superfluidity, optics, 2D van der Waals materials, plasmonics, magnetism, and superconductivity. Here, the state of the art is summarized and prospects for future research in curvilinear solid-state systems exhibiting such fundamental cooperative phenomena as ferromagnetism, antiferromagnetism, and superconductivity are outlined. Highlighting the recent developments and current challenges in theory, fabrication, and characterization of curvilinear micro- and nanostructures, special attention is paid to perspective research directions entailing new physics and to their strong application potential. Overall, the perspective is aimed at crossing the boundaries between the magnetism and superconductivity communities and drawing attention to the conceptual aspects of how extension of structures into the third dimension and curvilinear geometry can modify existing and aid launching novel functionalities. In addition, the perspective should stimulate the development and dissemination of research and development oriented techniques to facilitate rapid transitions from laboratory demonstrations to industry-ready prototypes and eventual products.

We demonstrate the structure evolution of hexagonal boron nitride (hBN) flakes grown on molten Cu in atmospheric pressure chemical vapor deposition by regulating the flux of precursor. We found that under lower precursor flux, tuned by temperature that controls the sublimation rates, the hBN grains change from triangle to truncated triangle shape with additional B-terminated edges, which could be understood through kinetic Wulff construction, while under higher flux, they form circular shape following deposition-controlled growth and predicted by a phase field modeling. In addition to the monolayer morphology from a single nucleation, adlayer patterns with centered aggregation and diffusive features at high precursor flux are observed and simulated by a two-dimensional (2D) diffusion-reaction model, where the random diffusion and deposition are revealed to be the dominating kinetics. The nucleation density and growth velocity could also be modulated by the ammonia borane heating...

State-of-the-art fabrication and characterisation techniques have been employed to measure the thermal conductivity of suspended, single-crystalline MoS 2 and MoS 2 /hBN heterostructures. Two-laser Raman scattering thermometry was used combined with real time measurements of the absorbed laser power. Measurements on MoS 2 layers with thicknesses of 5 and 14 nm exhibit thermal conductivity in the range between 12 Wm −1 K −1 and 24 Wm −1 K −1 . Additionally, after determining the thermal conductivity of the latter MoS 2 sample, an hBN flake was transferred onto it and the effective thermal conductivity of the heterostructure was subsequently measured. Remarkably, despite that the thickness of the hBN layer was less than a hal of the thickness of the MoS 2 layer, the heterostructure showed an almost eight-fold increase in the thermal conductivity, being able to dissipate more than ten times the ...