Jiuxiang Dai

Nature Electronics, Published online: 19 July 2021; doi:10.1038/s41928-021-00611-y

Author(s): Massimo Ostilli and Carlo Presilla

We study the ground state of a system of spinless electrons interacting through a screened Coulomb potential in a lattice ring. By using analytical arguments, we show that, when the effective interaction compares with the kinetic energy, the system forms a Wigner crystal undergoing a first-order qua...

[Phys. Rev. Lett. 127, 040601] Published Mon Jul 19, 2021

Nature Nanotechnology, Published online: 15 July 2021; doi:10.1038/s41565-021-00942-z

Author(s): M. Dupont, Y. O. Kvashnin, M. Shiranzaei, J. Fransson, N. Laflorencie, and A. Kantian

The monolayer halides CrX3 (X=Cl, Br, I) attract significant attention for realizing 2D magnets with genuine long-range order (LRO), challenging the Mermin-Wagner theorem. Here, we show that monolayer CrCl3 has the unique benefit of exhibiting tunable magnetic anisotropy upon applying a compressive ...

[Phys. Rev. Lett. 127, 037204] Published Fri Jul 16, 2021

Micromechanical exfoliation of borophene and transfer to arbitrary substrates are experimentally demonstrated, with molecular dynamics simulation providing further validation. While high-resolution electron imaging provides an atomistic glimpse of the crystallographic phases of borophene, Raman and XPS spectral data establish its chemical phase purity. When heterolayered with gold, borophene exhibits SERS-based molecular sensing. Borophene-based heterolayered excitonic devices are thus demonstrated.

Abstract

Borophene, the lightest among all Xenes, possesses extreme electronic mobility along with high carrier density and high Young's modulus. To accomplish device-quality borophene, novel approaches of realization of monolayers need to be urgently explored. In this work, micromechanical exfoliation is discovered to result in mono- and few-layered borophene of device quality. Borophene sheets are successfully fabricated down to monolayer thickness. Distinct crystallographic phases of borophene viz. XRD study reveals crystallographic phase transition from rhombohedral to several other eigen phases of borophene. The role of the destination substrates is held crucial in determining the final phase of the transferred sheet. The exfoliation energy is calculated by density functional theory. Molecular dynamics simulations are used to simulate the exfoliation process. Heterolayers of borophene, with black phosphorene (BP) or with molybdenum disulfide (MoS₂) atomic sheets, are found to result in photoexcited coupling quantum states. Gold-coated borophene bestows promising anchoring capability for surface-enhanced Raman spectroscopy (SERS). Successful demonstration of the electronic behavior of micromechanically exfoliated borophene and excitonic behavior of borophene-based heterolayers will guide future generation devices not only in electronics and excitonics, but also in thermal management, electronic packaging, hydrogen storage, hybrid energy storage, and clean energy solutions.

In article number 2010897, Yong Jin Jeong, Tae Kyu An, Insik In, Se Hyun Kim, and co-workers propose a clever strategy of engineering MXene inks for electrohydrodynamic jet printing to produce all printed logic circuits. This work could inspire further practical printed electronics by providing an option to manufacture electrodes in complex circuits via a simple technique.

The Fermi energy of the layered Dirac material EuMnBi₂ is widely tuned across the Dirac point by chemical substitution of the block layer. Since the high mobility is retained, the optimized thermopower results in an excellent power factor. Furthermore, the Nernst signal anomalously increases with decreasing carrier density beyond theoretical prediction, suggesting an impact of Eu local moments.

Abstract

Dirac/Weyl semimetals hosting linearly dispersing bands have received recent attention for potential thermoelectric applications, since their ultrahigh-mobility carriers could generate large thermoelectric and Nernst power factors. To optimize these efficiencies, the Fermi energy needs to be chemically controlled in a wide range, which is generally difficult in bulk materials because of disorder effects from the substituted ions. Here it is shown that the Fermi energy is tunable across the Dirac point for layered magnet EuMnBi₂ by partially substituting Gd³⁺ for Eu²⁺ in the insulating block layer, which dopes electrons into the Dirac fermion layer without degrading the mobility. Clear quantum oscillation observed even in the doped samples allows to quantitatively estimate the Fermi energy shift and optimize the power factor (exceeding 100 µW K⁻² cm⁻¹ at low temperatures) in combination with the first-principles calculation. Furthermore, it is shown that Nernst signal steeply increases with decreasing carrier density beyond a simple theoretical prediction, which likely originates from the field-induced gap reduction of the Dirac band due to the exchange interaction with the Eu moments. Thus, the magnetic block layer provides high controllability for the Dirac fermions in EuMnBi₂, which would make this series of materials an appealing platform for novel transport phenomena.

Charge carrier mobility (μ_int) and series resistance (R _SD) are the essential aspects of field-effect transistors. However, a proper technique to characterize these parameters from a standalone 2D transistor is still lacking. This study successfully demonstrates a universal method to accurately extract μ_int and R _SD in various elemental and transition metal dichalcogenide channels with different gate configurations.

Abstract

2D semiconductor field-effect transistors (2D FETs) have emerged as a promising candidate for beyond-silicon electronics applications. However, its device performance has often been limited by the metal-2D semiconductor contact, and the non-negligible contact resistance (R _SD) not only deteriorates the on-state current but also hinders the direct characterization of the intrinsic properties of 2D semiconductors (e.g., intrinsic charge carrier mobility, μ_int). Therefore, a proper extraction technique that can independently characterize the metal-2D semiconductor contact behavior and the intrinsic properties of a 2D semiconducting layer is highly desired. In this study, a universal yet simple method is developed to accurately extract the critical parameters in 2D FETs, including characteristic temperature (T _o), threshold voltage (V _T), R _SD, and μ_int. The practicability of this method is extensively explored by characterizing the temperature-dependent carrier transport behavior and the strain-induced band structure modification in 2D semiconductors. Technology computer aided design simulation is subsequently employed to verify the precision of R _SD extraction. Furthermore, the universality of the proposed method is validated by successfully implementing the extraction to various 2D semiconductors, including black phosphorus, indium selenide, molybdenum disulfide, rhenium disulfide, and tungsten disulfide with top- and bottom-gated configurations.

Metal–organic framework nanosheets display the high surface area and aspect ratio of 2D materials but possess a modular structure that allows for systematic tuning of their chemical and optoelectronic properties, and the introduction of new surface functionalities. Here, the progress that has so far been made in four key application areas are discussed and common opportunities and challenges are identified.

Metal–organic framework nanosheets (MONs) have recently emerged as a distinct class of 2D materials with programmable structures that make them useful in diverse applications. In this review, the breadth of applications that have so far been investigated are surveyed, thanks to the distinct combination of properties afforded by MONs. How: 1) The high surface areas and readily accessible active sites of MONs mean they have been exploited for a variety of heterogeneous, photo-, and electro-catalytic applications; 2) their diverse surface chemistry and wide range of optical and electronic responses have been harnessed for the sensing of small molecules, biological molecules, and ions; 3) MONs tunable optoelectronic properties and nanoscopic dimensions have enabled them to be harnessed in light harvesting and emission, energy storage, and other electronic devices; 4) the anisotropic structure and porous nature of MONs mean they have shown great promise in a variety of gas separation and water purification applications; are discussed. The aim is to draw links between the uses of MONs in these different applications in order to highlight the common opportunities and challenges presented by this promising class of nanomaterials.

The phase-controlled synthesis of FeTe nanosheets has been achieved by tuning the atom ratio of Fe precursor and Te precursor in atmosphere. Tetragonal FeTe nanoplates are easier to be obtained in Fe-rich conditions, while hexagonal FeTe nanoplates tend to grow in Te-rich conditions. Density functional theory simulations verify the experimental results theoretically.

Abstract

Phase controllable synthesis of 2D materials is of significance for tuning related electrical, optical, and magnetic properties. Herein, the phase-controllable synthesis of tetragonal and hexagonal FeTe nanoplates has been realized by a rational control of the Fe/Te ratio in a chemical vapor deposition system. Using density functional theory calculations, it has been revealed that with the change of the Fe/Te ratio, the formation energy of active clusters changes, causing the phase-controllable synthesis of FeTe nanoplates. The thickness of the obtained FeTe nanoplates can be tuned down to the 2D limit (2.8 nm for tetragonal and 1.4 nm for hexagonal FeTe). X-ray diffraction pattern, transmission electron microscopy, and high resolution scanning transmission electron microscope analyses exhibit the high crystallinity of the as-grown FeTe nanoplates. The two kinds of FeTe nanoflakes show metallic behavior and good electrical conductivity, featuring 8.44 × 10⁴ S m⁻¹ for 9.8 nm-thick tetragonal FeTe and 5.45 × 10⁴ S m⁻¹ for 7.6 nm-thick hexagonal FeTe. The study provides an efficient and convenient route for tailoring the phases of FeTe nanoplates, which benefits to study phase-sensitive properties, and may pave the way for the synthesis of other multiphase 2D nanosheets with controllable phases.

An atomic-switch-type artificial synapse fabricated on Ti₃C₂T_X MXene nanosheets with lots of surface functional groups successfully mimics the dynamics of biological synapse. The synaptic dynamics originate from the gradual formation and annihilation of the conductive metallic filaments on the MXene surface with distributed functional groups. Subsequently, its feasibility for an HW-NN with learning ability is demonstrated using a convolutional neural network composed MXene synapse devices.

Abstract

MXenes, an emerging class of two-dimensional (2D) transition metal carbides and nitrides, have attracted wide attention because of their fascinating properties required in functional electronics. Here, an atomic-switch-type artificial synapse fabricated on Ti₃C₂T_x MXene nanosheets with lots of surface functional groups, which successfully mimics the dynamics of biological synapses, is reported. Through in-depth analysis by X-ray photoelectron spectroscopy, transmission electron microscopy, and energy dispersive X-ray spectroscopy, it is found that the synaptic dynamics originated from the gradual formation and annihilation of the conductive metallic filaments on the MXene surface with distributed functional groups. Subsequently, via training and inference tasks using a convolutional neural network for the Canadian-Institute-For-Advanced-Research-10 dataset, the applicability of the artificial MXene synapse to hardware neural networks is demonstrated.

A hydrophobic ammonium salt, 4-(trifluoromethyl) benzylamine, is introduced to form a quasi-2D hybrid perovskite by a one-step spin-coating method. Due to the relatively low surface energy of fluorinated molecules, an upper gradient low-dimensional structure is formed spontaneously from top to bottom, and more stable devices are obtained with a power conversion efficiency of 17.07%.

Abstract

Highly efficient and stable quasi-2D hybrid perovskite solar cells (PSCs) using hydrophobic 4-(trifluoromethyl) benzylamine (4TFBZA) as the spacer cation are successfully demonstrated. It is found that the incorporation of hydrophobic 4TFBZA into MAPbI₃ can effectively induce a spontaneous upper gradient 2D (SUG-2D) structure, passivate the trap states, and restrain the ion motion. Meanwhile, the strong hydrogen bonding of F···HN between 4TFBZA ions and methylamine ions can effectively suppress the decomposition of perovskite, which gives the device a better thermal stability. Besides, due to the SUG-2D structure with hydrophobic 4TFBZA, the device also exhibits a better moisture stability. The SUG-2D-structure-based device exhibits a power conversion efficiency of 17.07% with a high open-circuit voltage of 1.10 V and a notable fill factor of 71%. This work provides a new strategy for constructing efficient and stable quasi-2D PSCs, and it is an inspiration for the packaging strategy of perovskites.

To overcome the challenge that conductive tissue engineering scaffolds are normally not biodegradable, biodegradable and conductive germanium phosphide nanosheets are incorporated into an injectable and adhesive hyaluronic acid-graft-dopamine hydrogel to construct a biohybrid hydrogel scaffold. The in vivo implanted conductive scaffold can activate endogenous neural stem cell neurogenesis and improve the recovery of motor function in rat spinal cord injury.

Abstract

Developing biodegradable conductive hydrogels is of great importance for the repair of electroactive tissues, such as myocardium, skeletal muscle, and nerves. However, conventional conductive phase incorporation in composite hydrogels, such as polypyrrole, polyaniline, carbon nanotubes, graphene, and gold nanowires, which are non-degradable materials, will exist in the body as foreign matter. Herein, an injectable hydrogel based on the integration of conductive and biodegradable germanium phosphide (GeP) nanosheets into an adhesive hyaluronic acid-graft-dopamine (HA-DA) hydrogel matrix is explored, and the successful application of this biohybrid hydrogel in spinal cord injury (SCI) repair is demonstrated. The incorporation of polydopamine (PDA)-modified GeP nanosheets (GeP@PDA) into HA-DA hydrogel matrix significantly improves the conductivity of HA-DA/GeP@PDA hydrogels. The conductive HA-DA/GeP@PDA hydrogels can accelerate the differentiation of neural stem cells (NSC) into neurons in vitro. In a rat SCI complete transection model, the in vivo implanted HA-DA/GeP@PDA hydrogel is found to improve the recovery of locomotor function significantly. The immunohistofluorescence investigation suggests that the HA-DA/GeP@PDA hydrogels promote immune regulation, endogenous angiogenesis, and endogenous NSC neurogenesis in the lesion area. The strategy of integrating conductive and biodegradable GeP nanomaterials into an injectable hydrogel provides new insight into designing advanced biomaterials for SCI repair.

Multiple bits are successfully created on non-volatile memory based on vdW heterostructure floating-gate memory (FGM) by systematically tuning the dimensions of the 2D materials. In particular, a fingerprint mechanism is established that links the bit number and dimensions of 2D crystals on vdW heterostructures. This approach could enable the precise generation of the desired number of bits in layered-material-based vdW FGMs.

Abstract

Van der Waals (vdW) heterostructures with 2D materials have shown that atomically thin non-volatile memories are advantageous in terms of integration, while offering high performance and excellent stability. The non-volatile memory behavior of 2D materials has mainly been studied for single-bit operation, and there is growing interest in expanding to multi-bit operation to enhance the storage capacities of memory devices. However, the conditions or rules for generating the desired number of bits in 2D-based multi-bit memory remain to be identified. In this study, multiple bits are successfully created on non-volatile memory based on vdW heterostructure floating-gate memory (FGM) by systematically tuning the dimensions of the 2D materials. In particular, a fingerprint mechanism is established that links the bit number and dimensions of 2D crystals on vdW heterostructures. This approach could enable the precise generation of the desired number of bits in layered-material-based vdW FGMs.

The electronic and magnetic properties of Dy metal–organic networks are inspected on a coinage surface. It is revealed that the coordination environment re-orientates the easy axis of magnetization and tunes the magnetic anisotropy of Dy centers compared to isolated atoms, thus opening new avenues for tailoring the magnetic properties of lanthanides in 2D materials.

Abstract

Taming the magnetic anisotropy of lanthanides through coordination environments is crucial to take advantage of the lanthanides properties in thermally robust nanomaterials. In this work, the electronic and magnetic properties of Dy-carboxylate metal–organic networks on Cu(111) based on an eightfold coordination between Dy and ditopic linkers are inspected. This surface science study based on scanning probe microscopy and X-ray magnetic circular dichroism, complemented with density functional theory and multiplet calculations, reveals that the magnetic anisotropy landscape of the system is complex. Surface-supported metal–organic coordination is able to induce a change in the orientation of the easy magnetization axis of the Dy coordinative centers as compared to isolated Dy atoms and Dy clusters, and significantly increases the magnetic anisotropy. Surprisingly, Dy atoms coordinated in the metallosupramolecular networks display a nearly in-plane easy magnetization axis despite the out-of-plane symmetry axis of the coordinative molecular lattice. Multiplet calculations highlight the decisive role of the metal–organic coordination, revealing that the tilted orientation is the result of a very delicate balance between the interaction of Dy with O atoms and the precise geometry of the crystal field. This study opens new avenues to tailor the magnetic anisotropy and magnetic moments of lanthanide elements on surfaces.

The electronic properties of stacked few-layer 2D materials are highly dependent on their precise structural arrangement. In article number 2100388, featured on the front cover, Michael J. Zachman, Miaofang Chi, and co-workers describe a four-dimensional scanning transmission electron microscopy technique that utilizes interference between Bragg discs to enable high-resolution mapping of picometer-scale structural reconstructions and measurement of average interlayer spacings in these materials.

Publication date: July–August 2021

Source: Materials Today, Volume 47

Author(s): Laurie Donaldson