Jiuxiang Dai

Orientation-controlled growth of 1T′ ReS₂ is explored via growing it on an Au substrate with different crystal facets. The strong ReS₂-Au interaction together with the reduction of lattice symmetry of the Au facet can effectively guide ReS₂ domains epitaxial growth toward single crystal with uniform lattice orientation. This is an important step for the orientation-controlled synthesis of low-symmetry 2D crystals.

Abstract

Low-symmetry 2D materials with strong in-plane anisotropy are ideal platforms for building multifunctional optoelectronic devices. However, the random orientations and easy formation of multidomain structures lead to the single-crystal synthesis of these materials remains a big challenge. Herein, for the first time, the orientation-controlled synthesis of ReS₂, a typical low-symmetry 2D material, is explored via interface engineering based on the strong interaction between the material and Au substrates with different symmetries. It is revealed that the lattice orientation and growth behavior of ReS₂ are closely relevant to the lattice symmetry of Au facets. Single crystal ReS₂ domains with two and even one orientations are acquired on the four-fold symmetry Au(001) facet and the two-fold symmetry Au(101) facet, respectively. Combined with density functional theory calculations, it is demonstrated that the synergy of ultra-strong ReS₂-Au interfacial coupling and reduction of symmetry of Au facet is critical to realizing its intrinsic anisotropic growth. Furthermore, great enhancement of electrical and photoelectrical performances are acquired on the well-aligned single crystal ReS₂ device. The progress achieved in this work provides significant guidance for the controllable synthesis of wafer-scale single crystals of low-symmetry 2D materials for their practical device applications.

Tuning the particle size of the 2D carbon nitride poly(heptazine imide) enables optimization of photocatalytic hydrogen evolution. It is shown that changes in the particle size affect the overall photocatalytic process in different ways, and the individual contributions of size-related variables on the photocatalytic activity are traced back. This multi-parameter analysis offers design strategies for next generation polymer photocatalysts.

Abstract

The carbon nitride poly(heptazine imide), PHI, has recently emerged as a powerful 2D carbon nitride photocatalyst with intriguing charge storing ability. Yet, insights into how morphology, particle size, and defects influence its photophysical properties are virtually absent. Here, ultrasonication is used to systematically tune the particle size as well as concentration of surface functional groups and study their impact. Enhanced photocatalytic activity correlates with an optimal amount of those defects that create shallow trap states in the optical band gap, promoting charge percolation, as evidenced by time-resolved photoluminescence spectroscopy, charge transport studies, and quantum-chemical calculations. Excessive amounts of terminal defects can act as recombination centers and hence, decrease the photocatalytic activity for hydrogen evolution. Re-agglomeration of small particles can, however, partially restore the photocatalytic activity. The type and amount of trap states at the surface can also influence the deposition of the co-catalyst Pt, which is used in hydrogen evolution experiments. Optimized conditions entail improved Pt distribution, as well as enhanced wettability and colloidal stability. A description of the interplay between these effects is provided to obtain a holistic picture of the size–property–activity relationship in nanoparticulate PHI-type carbon nitrides that can likely be generalized to related photocatalytic systems.

In article number 2101476, Jiahua Tao, Chunhu Zhao, Pingxiong Yang, and co-workers develop cost-efficient and high-throughput vapor transport deposition (VTD) to precisely control the growth of high quality Sb₂(S,Se)₃ absorbers. A high power conversion efficiency of 8.17% is achieved, which is the highest ever reported for Sb₂(S,Se)₃ solar cells fabricated via a VTD method. This work provides a prospective pathway for further improving the performance of Sb₂(S,Se)₃ solar cells.

In article number 2100936, Dongseob Kim, Jinkee Hong, Sangmin Lee and co-workers report a non-polar liquid lubricant submerged triboelectric nanogenerator which can generate high output and charge commercial batteries by effectively suppressing air breakdown due to the large Debye length of the liquid lubricant. In addition, it is able to reduce friction wear by the lubrication and rolling friction.

High performance ternary organic phototransistors with a photoresponse up to 2600 nm at room temperature are developed. The device exhibits a photoresponsivity of 2.75 × 10⁶ A W⁻¹ at 2000 nm. Moreover, the functions as synaptic devices are achieved. The overall performance is superior to that of previously reported organic infrared phototransistors.

Abstract

Infrared (IR) photodetection is important for light communications, military, agriculture, and related fields. Organic transistors are investigated as photodetectors. However, due to their large band gap, most organic transistors can only respond to ultraviolet and visible light. Here high performance IR phototransistors with ternary semiconductors of organic donor/acceptor complex and semiconducting single-walled carbon nanotubes (SWCNTs), without deep cooling requirements are developed. Due to both the ultralow intermolecular electronic transition energy of the complex and charge transport properties of SWCNTs, the phototransistor realizes broadband photodetection with photoresponse up to 2600 nm. Moreover, it exhibits outstanding performance under 2000 nm light with photoresponsivity of 2.75 × 10⁶ A W⁻¹, detectivity of 3.12 × 10¹⁴ Jones, external quantum efficiency over 10⁸%, and high I _photo/I _dark ratio of 6.8 × 10⁵. The device exhibits decent photoresponse to IR light even under ultra-weak light intensity of 100 nW cm⁻². The response of the phototransistor to blackbody irradiation is demonstrated, which is rarely reported for organic phototransistors. Interestingly, under visible light, the device can also be employed as synaptic devices, and important basic functions are realized. This strategy provides a new guide for developing high performance IR optoelectronics based on organic transistors.

Being a flexible wide band gap semiconductor, hexagonal boron nitride (h-BN) has great potential for technological applications like efficient deep ultraviolet light sources, building block for two-dimensional heterostructures and room temperature single photon emitters in the ultraviolet and visible spectral range. To enable such applications, it is mandatory to reach a better understanding of the electronic and optical properties of h-BN and the impact of various structural defects. Despite the large efforts in the last years, aspects such as the electronic band gap value, the exciton binding energy and the effect of point defects remained elusive, particularly when considering a single monolayer. Here, we directly measured the density of states of a single monolayer of h-BN epitaxially grown on highly oriented pyrolytic graphite, by performing low temperature scanning tunneling microscopy (STM) and spectroscopy (STS). The observed h-BN electronic band gap on defect-free regio...

We investigate, using a systematic computational approach, the possibility of the existence of two-dimensional quasicrystalline phases of binary metal-oxides. Our approach relies on the construction of the complete two-dimensional binary phase diagram through the use of unbiased global structural prediction methods. We then identify, in the low-energy periodic phases, structural elements that can be used to generate quasicrystalline phases through an inflation process. In this way we obtain chemically consistent two-dimensional quasicrystal approximants of both barium and titanium oxides. In the proposed structures, the metallic sites occupy the vertices of the aperiodic square-triangle tiling, while the oxygen atoms decorate the interior of the polygons. We then study the properties of the approximants, both free-standing and deposited on a metallic substrate. Finally, we discuss in which circumstances the formation of these phases seems to be favored.

Quantifying anisotropy in the chemical reactions of mesoscopic crystals has mostly resorted on the combination of electron microscopy and diffraction. In this work, we established crystal-facet-resolved kinetic measurements of oxidation reactions in two-dimensional (2D) transition metal dichalcogenides (TMDs) using optical second-harmonic generation spectroscopy and scanning probe microscopy. We show the in-plane anisotropy of their bond-breaking reactions is governed by their structure and strongly material-dependent among four TMDs. The facet-resolved analysis directly revealed that the reactions proceed fastest (slowest) for chalcogen (metal)-terminated zigzag edges with armchair edges in the middle. The degree of the anisotropy inducing trigonal oxidation patterns was much higher in MoS 2 and MoSe 2 than WS 2 and WSe 2 . Kinetic Wulff construction based on edge-specific reaction rates verified the material-dependent mesoscopic reaction ...

npj 2D Materials and Applications, Published online: 07 July 2021; doi:10.1038/s41699-021-00244-x

2D-MoS₂ goes 3D: transferring optoelectronic properties of 2D MoS₂ to a large-area thin film

Author(s): Amir A. Pahlavan, Lisong Yang, Colin D. Bain, and Howard A. Stone

Droplets made of a mixture of two liquids with different volatilities take on a pancake shape.

[Phys. Rev. Lett. 127, 024501] Published Wed Jul 07, 2021

Nature, Published online: 07 July 2021; doi:10.1038/s41586-021-03585-1

Programmable quantum simulation of two-dimensional antiferromagnets is achieved with up to 196 neutral atoms, and the capability of the platform is demonstrated on square and triangular arrays.

In article 2100185, Mario Lanza and co-workers use 2D multilayer hexagonal boron nitride (h-BN) to fabricate devices that exhibit stable random telegraph noise current signals, and employ them in true random number generator circuits for advanced data encryption. The currents are very stable because they are confined across the few-atoms-wide defects embedded within the crystalline and highly stable h-BN structure, which avoids their lateral propagation.

New synthesis strategies of fluoride-free MXenes beyond Ti₃C₂T _x are imperative to extend 2D MXenes. In article number 2101015, Pulickel M. Ajayan, Li Song, and co-workers simulate the feasibility of etching various MAX materials in HCl solution influenced by temperature and pressure. Fluoride-free Mo₂CT_x MXene is then obtained with high quality and efficiency via a controllable HCl hydrothermal etching method, which exhibits a modulated energy-storage mechanism.

With lithography-free, direct-laser-writing-controlled MoS₂/MoS_{2− x} O _δ grain boundaries, synaptic devices exhibit short-term and long-term plasticity characteristics that are responsive to electric and light stimulation simultaneously, as well as low energy consumption that is over 40 times lower than that of conventional complementary metal–oxide–semiconductor (CMOS) devices.

Abstract

Synaptic devices based on 2D-layered materials have emerged as high-efficiency electronic synapses and neurons for neuromorphic computing. Lateral 2D synaptic devices have the advantages of multiple functionalities by responding to diverse stimuli, but they consume large amounts of energy, far more than the human brain. Moreover, current lateral devices employ several mechanisms based on conductive filaments and grain boundaries (GBs), but their formation is random and difficult to control, also hindering their practical applications. Here, four-terminal, lateral synaptic devices with artificially engineered GBs are reported, which are made from monolayer MoS₂. With lithography-free, direct-laser-writing-controlled MoS₂/MoS_{2− x} O _δ GBs, such synaptic devices exhibit short-term and long-term plasticity characteristics that are responsive to electric and light stimulation simultaneously. This enables detailed simulations of biological learning and cognitive processes as well as image perception and processing. In particular, the device exhibits low energy consumption, similar to that of the human brain and much lower than those of other lateral 2D synaptic devices. This work provides an effective way to fabricate lateral synaptic devices for practical application development and sheds light on controllable electrical state switching for neuromorphic computing.

Current-controlled topological skyrmion–bubble and skyrmion–stripe transformations are achieved in a reversible and reproducible way by tuning the nanosecond pulsed current amplitude on the order of ≈10¹⁰ A m⁻², which enables future reliable and energy-efficient manipulation of electromagnetic properties in topological spintronic devices.

Abstract

Topological magnetic charge Q is a fundamental parameter that describes the magnetic domains and determines their intriguing electromagnetic properties. The ability to switch Q in a controlled way by electrical methods allows for flexible manipulation of electromagnetic behavior in future spintronic devices. Here, the room-temperature current-controlled topological magnetic transformations between Q = −1 skyrmions and Q = 0 stripes or type-II bubbles in a kagome crystal Fe₃Sn₂ are reported. It is shown that reproducible and reversible skyrmion–bubble and skyrmion–stripe transformations can be achieved by tuning the density of nanosecond pulsed current of the order of ≈10¹⁰ A m⁻². Further numerical simulations suggest that spin-transfer torque combined with Joule thermal heating effects determine the current-induced topological magnetic transformations.

Oxygen atoms in situ substitute the intrinsic sulfur monovacancies in the WS₂ monolayer, resulting in robust photoluminescence. The enhancement is ascribed to the suppression of nonradiative recombination and the conversion from trion to neutral exciton. Notably, gratifying stability in the ambient condition is achieved due to the repaired saturated coordination bonds at sulfur vacancy sites, which provides more possibilities for specific applications.

Abstract

The photoluminescence quantum yield (PLQY) of the chemical vapor deposition (CVD) grown transition-metal dichalcogenides (TMDs) films is often much lower than their mechanically exfoliated counterparts, making the coexistence of large-area and high PLQY in TMDs monolayer a huge challenge. Here, an in situ defect engineering strategy is reported to fundamentally dilutes the impact of intrinsic sulfur vacancy on tungsten disulfide (WS₂) monolayer. By ingeniously incorporating oxygen atoms in the sulfur vacancy sites of WS₂ lattice via the CVD method, oxygen doped WS₂ monolayer exhibits remarkably improved optical properties. The PLQY is uniformly enhanced by nearly two orders and can reach up to 9.3%, which is even higher than mechanically exfoliated counterparts. Besides, strong W-O bonds endow materials with superior environment stability, and the high PLQY could persist with an endurance of up to 3 months under ambient conditions without any protection. More in-depth insights from the first-principle calculations illustrate that the enhancement mechanism is the synthetic action of the suppression of nonradiative recombination and conversion from trion to neutral, and the excellent stability arises from repaired saturated coordination bonds at sulfur vacancy sites. This method opens up more possibilities for both fundamental exciton physics and optoelectronics applications.

By comprehensive characterizations and analysis on crystal and electronic structures of La_{2− x} Sr _x Co₂O_{6− δ} perovskite oxides, the underlying physics for the relevance of magnetism with oxygen evolution reaction is revealed. A facile descriptor of Curie/Neel temperature is developed, which can easily be obtained from magnetization curves.

Abstract

Developing accurate descriptors for oxygen evolution reaction (OER) is of great significance yet challenging, which roots in and also boosts the understanding of its intrinsic mechanisms. Despite various descriptors are reported, it still has limitations in the facile prediction, given that complicated analytical techniques as well as time-consuming modeling and calculations are indispensable. In the present work, strong correlation of magnetic property with OER performance is revealed by in-depth investigations on the crystal and electronic structures. A facile descriptor of Curie/Neel temperature (T _C/N) is developed for La_{2− x} Sr _x Co₂O_{6− δ} perovskite oxides, based on the inference that both magnetism and OER are rooted in the electron exchange interaction. Specifically, both the T _C/N and OER activity are proportional to the degree of p-d orbital hybridization, which increases with enlarged bond angle of Co─O─Co and/or increased oxidation of Co. This finding reveals that T _C/N from magnetic characterizations is an effective descriptor in designing novel OER electrocatalysts, and interdisciplinary researches are advantageous for revealing the controversial mechanisms of OER process.

We report chiral mesostructured Fe₃O₄ films exhibiting the chirality-dependent electrical conductivity. Below 30 K, as the half-metallic Fe₃O₄ is completely spin polarized and the chiral mesostructured Fe₃O₄ films can act as a spin filter, the transport of spins is chirality-dependent. Thus, the antipodal chiral mesostructured Fe₃O₄ films exhibited asymmetric responses to the spin transport and resulted in different electrical resistance.

Abstract

Half-metallic materials are theoretically predicted to be metallic and insulating, which have not been confirmed experimentally, and the predictions are still in doubt. We report the resistance-chiral anisotropy (R-ChA), i.e., chirality-dependent electrical conductivity, in chiral mesostructured Fe₃O₄ films (CMFFs) grown on the substrates via a hydrothermal method using amino acids as symmetry-breaking agents. Two levels of chirality exist in the CMFFs: primary distortion of the crystal lattice forms twisted nanoflakes, and secondary helical stacking of nanoflakes forms fan-shaped nanoplates. At temperatures below 30 K, the CMFFs exhibited metallic conductivity and insulation for one handedness and the other, respectively. The chirality-dependent effective magnetic fields were speculated to stabilize the opposite spin in the antipodal chiral frame, which led to the free transport of electrons in one handedness of the chiral structure and immobility for the other handedness.