Jiuxiang Dai

Blue phosphorene is a new emerging 2D semiconducting material with potential applications. Herein, an evidence of the existence of Dirac fermions in blue phosphorene grown on Cu(111) substrate, similar to graphene, is presented. Therefore, all the expectations held for graphene, such as high-speed electronic devices based on ballistic transport at room temperature, may also be applied to blue phosphorene.

Abstract

2D materials beyond graphene and in particular 2D semiconductors have raised interest due to their unprecedented electronic properties, such as high carrier mobility or tunable bandgap. Blue phosphorene is an allotrope of black phosphorene that resembles graphene as it presents a honeycomb structure. However, it is known to have semiconductor character and the crucial point is to determine whether this hexagonal phase of phosphorene presents Dirac fermions as in graphene. Here, the first compelling experimental evidence of Dirac fermions in blue phosphorene layer grown on Cu(111) surface is presented. The results highlight the formation of a highly ordered blue phosphorene sheet with a clear Dirac cone at the high symmetry points of the Brillouin Zone. The charge carriers behave as massless relativistic particles. Therefore, all the expectations held for graphene, such as high-speed electronic devices based on ballistic transport at room temperature, may also be applied to blue phosphorene.

An carbazole deriative based ultra-long room temperature phosphorescence (ULRTP) material is developed with a phosphorescence quantum yield of 64%, which contains dual component of 9-(4-bromo-3-methylphenyl)-9H-carbazole and (5-(4-bromo-3-methylphenyl)-5H-benzo[b]carbazole)(BCzPMB). While the BCzPMB is a new impurity different from the one reported previously. The ULRTP is likely from defect trap mechanisms but not heavy atom effects caused by molecular packing.

Abstract

Ultra-long room temperature phosphorescence (ULRTP) materials show valuable applications in encryption, biological imaging, and many other fields. Amazingly, the concomitant impurities from raw materials that are normally ignored contribute dramatically to the ULRTP. In this study, CzPMB [9-(4-bromo-3-methylphenyl)-9H-carbazole] with phosphorescent quantum efficiency of 64% is prepared from commercial carbazole, but the phosphorescent efficiency is drastically reduced to < 2% once trace impurity (5-(4-bromo-3-methylphenyl)-5H-benzo[b]carbazole) is separated. HPLC studies demonstrated the separated impurity is a byproduct derived from trace benzocarbazole in commercial carbazole. Subsequently, the ULRTP for the CzPMB synthesized from lab-made carbazole is totally unobserved, strongly confirming the dramatic impact of impurity. A defect trapping mechanism in multicomponent system rather than heavy atom effect is proposed for highly efficient ULRTP after carefully analyzing the crystal packings and molecular energy levels. Inspired by this discovery, a series of effective ULRTP bi-component systems with the highest phosphorescence efficiency of 64.1% are reproduced by directed host-guest doping. This strategy paves a viable path for the design of organic materials with highly efficient ULRTP.

This work investigates the implications of liquid V₂O₅ precursor on the liquid-assisted growth of VO₂ and further phase engineering of single-crystal VO₂ beams between the M1-, M2-, and T-phases by manipulation of oxygen stoichiometry, enabling various multiphase structures in single VO₂ beams and enriching the deformation modes for their actuation applications.

Abstract

The study of VO₂ flourishes due to its rich competing phases induced by slight stoichiometry variations. However, the vague mechanism of stoichiometry manipulation makes the precise phase engineering of VO₂ still challenging. Here, stoichiometry manipulation of single-crystal VO₂ beams in liquid-assisted growth is systematically studied. Contrary to previous experience, oxygen-rich VO₂ phases are abnormally synthesized under a reduced oxygen concentration, revealing the important function of liquid V₂O₅ precursor: It submerges VO₂ crystals and stabilizes their stoichiometric phase (M1) by isolating them from the reactive atmosphere, while the uncovered crystals are oxidized by the growth atmosphere. By varying the thickness of liquid V₂O₅ precursor and thus the exposure time of VO₂ to the atmosphere, various VO₂ phases (M1, T, and M2) can be selectively stabilized. Furthermore, this liquid precursor-guided growth can be used to spatially manages multiphase structures in single VO₂ beams, enriching their deformation modes for actuation applications.

2D monolayer transition metal dichalcogenides (TMDCs) are seen as potentially promising candidates for novel light-emission applications, owing to their unique characteristics. This review summarizes the state-of-the-art progress made in the development of 2D TMDC-based light-emitting devices and highlights the remaining challenges.

Abstract

2D monolayer transition metal dichalcogenides (TMDCs) show great promise for the development of next-generation light-emitting devices owing to their unique electronic and optoelectronic properties. The dangling-bond-free surface and direct-bandgap structure of monolayer TMDCs allow for near-unity photoluminescence quantum efficiencies. The excellent mechanical and optical characteristics of 2D TMDCs offer great potential to fabricate TMDC-based light-emitting diodes (LEDs) featuring good flexibility and transparency. Great progress has been made in the fabrication of bright and efficient LEDs with varying device structures. In this review, the aim is to provide a comprehensive summary of the state-of-the-art progress made in the construction of bright and efficient LEDs based on 2D TMDCs. After a brief introduction to the research background, the preparation of 2D TMDCs used for LEDs is briefly discussed. The requirements and the corresponding challenges to achieve bright and efficient LEDs based on 2D TMDCs are introduced. Thereafter, various strategies to enhance the brightness of monolayer 2D TMDCs are described. Following that, the carrier-injection schemes enabling bright and efficient TMDC-based LEDs along with the device performance are summarized. Finally, the challenges and future prospects regarding the accomplishment of TMDC-LEDs with ultimate brightness and efficiency are discussed.

The synthesis of 2D single-element cobalt nanosheets with a sub-millimeter scale via van der Waals epitaxy is demonstrated for the first time, which is guaranteed by the synergistic effect between van der Waals interactions and surface energy minimization. Cobalt nanosheets exhibit significant in-plane magnetic anisotropy and magnetoresistance effects, opening up exciting opportunities for the exploration of 2D single-element magnetism.

Abstract

2D single-element materials, which are pure and intrinsically homogeneous on the nanometer scale, can cut the time-consuming material-optimization process and circumvent the impure phase, bringing about opportunities to explore new physics and applications. Herein, for the first time, the synthesis of ultrathin cobalt single-crystalline nanosheets with a sub-millimeter scale via van der Waals epitaxy is demonstrated. The thickness can be as low as ≈6 nm. Theoretical calculations reveal their intrinsic ferromagnetic nature and epitaxial mechanism: that is, the synergistic effect between van der Waals interactions and surface energy minimization dominates the growth process. Cobalt nanosheets exhibit ultrahigh blocking temperatures above 710 K and in-plane magnetic anisotropy. Electrical transport measurements further reveal that cobalt nanosheets have significant magnetoresistance (MR) effect, and can realize a unique coexistence of positive MR and negative MR under different magnetic field configurations, which can be attributed to the competition and cooperation effect among ferromagnetic interaction, orbital scattering, and electronic correlation. These results provide a valuable case for synthesizing 2D elementary metal crystals with pure phase and room-temperature ferromagnetism and pave the way for investigating new physics and related applications in spintronics.

Superconducting quantum interferometer devices are important in investigating magnetic properties in quantum materials, but cannot probe micro-scale samples with small magnetic signals. Based on the superconducting micro-magnetometer, the density of vortex pinning centers in Bi₂Sr₂CaCu₂O₈ nanoflake can be determined as 1.1 × 10¹² m⁻². This superconducting micro-magnetometer sheds light on the detection of small magnetic signals of nanoflakes.

Abstract

Superconducting quantum interferometer device (SQUID) plays a key role in understanding electromagnetic properties and emergent phenomena in quantum materials. The technological appeal of SQUID is that its detection accuracy for the electromagnetic signal can precisely reach the quantum level of a single magnetic flux. However, conventional SQUID techniques normally can only be applied to a bulky sample and do not have the capability to probe the magnetic properties of micro-scale samples with small magnetic signals. Herein, it is demonstrated that, based on a specially designed superconducting nano-hole array, the contactless detection of magnetic properties and quantized vortices in micro-sized superconducting nanoflakes is realized. An anomalous hysteresis loop and a suppression of Little–Parks oscillation are observed in the detected magnetoresistance signal, which originates from the disordered distribution of the pinned vortices in Bi₂Sr₂CaCu₂O_8+δ. Therefore, the density of pinning centers of the quantized vortices on such micro-sized superconducting samples can be quantitatively evaluated, which is technically inaccessible for conventional SQUID detection. The superconducting micro-magnetometer provides a new approach to exploring mesoscopic electromagnetic phenomena of quantum materials.

Journal of Applied Physics, Volume 133, Issue 7, February 2023.
Oxygen vacancy is crucial to the optical properties in In[math]O[math], however, the single oxygen vacancy model fails to explain the observed multi-peak emission in the experiment. Herein, we have theoretically investigated the diversity of oxygen vacancy distribution, revealing the relationship between the defect configurations and the optical properties. Combining the first-principles calculations and bayesian regularized artificial neural networks, we demonstrate that the structural stability can be remarkably enhanced by multi-oxygen vacancy aggregation, which will evolve with the defect concentration and temperature. Notably, our results indicate that the single oxygen vacancy will induce the emission peaks centered at 1.35 eV, while multi-peak emission near 2.35 eV will be attributed to the distribution of aggregated double oxygen vacancies. Our findings provide a comprehensive understanding of multi-peak emission observed in In[math]O[math], and the rules of the vacancy distribution may be extended for other metal oxides to modulate the optical properties in practice.

Journal of Applied Physics, Volume 133, Issue 7, February 2023.
Edge kinetics in 2D structures has been a key to understanding the growth. In this paper, the effect of hydrogen passivation on the growth of hexagonal boron nitride (h-BN) was studied. Without hydrogen, the filling process of the gap on bare edges of h-BN is difficult because of the formation of dimers that distorts the edge. With hydrogen passivation, such difficulty can be largely reduced. In addition, hydrogen passivation can reduce the edge bending to the substrate. In summary, the amount of hydrogen passivation during the growth is the long-ignored parameter and can be the key to a good crystal quality.

Journal of Applied Physics, Volume 133, Issue 7, February 2023.
Reflective metasurfaces for arbitrary wave-front control require unit cells to achieve both [math] phase rotation and unity amplitude in reflection waves, and such requirements are a fundamental challenge for ultra-thin metasurfaces without the use of a metallic plate. We analytically show that in two coupled resonators, tuning the resonance frequency with the external decay rate for one of the resonators enables near [math] phase rotation and unity amplitude in the reflection wave, where no reflector is required. We implement the mechanism on a reflective graphene metasurface with its thickness being less than a [math] free space wavelength. As an illustration, we numerically demonstrate that in a wireless communication scenario, the actively tunable graphene metasurface is able to reflect an incident wave to a receiver or be transparent for an incident wave, which is the significant advantage arising from the structure without a metallic plate. In addition, the loss effect of the metasurface on the performance is discussed in terms of the conductive loss of graphene and the deviation of the reflection phase from a desired distribution. Our results open up opportunities for reflective metasurfaces without a metallic plate.

Nature Materials, Published online: 20 February 2023; doi:10.1038/s41563-023-01483-7

Cubic materials such as GeTe have low lattice thermal conductivity, thought to arise from a non-cubic local structural transition. Here, using a variable-shutter pair distribution function method, the average structure is shown to be crystalline but with anisotropic dynamics at higher temperatures.

Vapor-growth of organic single-crystal patterns for large-scale field-effect transistors integration is realized based on microspacing in-air sublimation. To overcome inconsistent in-plane orientation of the discrete anisotropic single-crystals, surface wettability patterns with inter-connecting motifs are designed to induce a homogeneous orientation among the whole patterned single-crystalline film.

Abstract

Fabrication of single-crystalline organic semiconductor patterns is of key importance to enable practical applications. However due to the poor controllability on nucleation locations and the intrinsic anisotropic nature of single-crystals, growth of single-crystal patterns with homogeneous orientation is a big challenge especially by the vapor method. Herein a vapor growth protocol to achieve patterned organic semiconductor single-crystals with high crystallinity and uniform crystallographic orientation is presented. The protocol relies on the recently invented microspacing in-air sublimation assisted with surface wettability treatment to precisely pin the organic molecules at desired locations, and inter-connecting pattern motifs to induce homogeneous crystallographic orientation. Single-crystalline patterns with different shapes and sizes, and uniform orientation are demonstrated exemplarily by using 2,7-dioctyl[1]benzothieno[3,2-b][1]benzothiophene (C8-BTBT). Field-effect transistor arrays fabricate on the patterned C8-BTBT single-crystal patterns show uniform electrical performance: a 100% yield with an average mobility of 6.28 cm² V⁻¹ s⁻¹ and in a 5 × 8 array. The developed protocols overcome the uncontrollability of the isolated crystal patterns in vapor growth on non-epitaxial substrates, making it possible to align the anisotropic electronic nature of single-crystal patterns in large-scale devices integration.

Polymorphic AgCuS is mounted on a printed circuit board and applied to a temperature gradient of 333 to 368 K. A pn-junction is created during the β-AgCuS to α-AgCuS phase transition at ≈363 K, resulting in the formation of a one-compound diode. The overlying U/I curve illustrates the electronic response upon polarization.

Abstract

Pnp-switchable semiconductor materials are capable of switching their electronic properties from p- to n-type conduction. Observed in the handful of discovered compounds, this behavior is usually accompanied by a temperature-dependent phase transition. During this transition, the dynamical rearrangement of a certain substructure enables the change of the predominant charge carrier type. Considering the immense demand for compact and flexible electronic components, one possible approach is the construction of unconventional one-compound diodes using these pnp-switchable materials. In this study, pnp-switchable AgCuS is applied to realize a functional one-compound diode. AgCuS is accessible in large quantities as bulk material in a simple and short timeframe. Featuring an addressable pnp-switch at 364 K, this material is suitable for diode generation and usage in varied applications. The diode properties of AgCuS devices are reported and illustrate its reversibility and flexibility for diode operation. The material is fully characterized with regards to its electrical and thermal properties, as well as its diode performance. Properties of AgCuS are discussed in relation to the pnp-switchable material Ag₁₈Cu₃Te₁₁Cl₃, which is successfully used to fabricate the first one-compound diode operating close to room temperature.

2D materials possess a range of unique and exclusive properties due to their ultrathin feature. This minireview summarizes the specific steps and principles of blowing method, focuses on the recent progress in preparing different categories of 2D materials, and further proposes an outlook for future opportunities in this growing field.

Abstract

2D materials possess a range of unique and exclusive properties due to their ultrathin feature, leading their forefront role in several research areas, including electronics, photonics, catalysis and renewable energy. To ensure a wider application for high-quality 2D materials, there is a global pursuit to find new “industrial scale” methodologies for the facile fabrication of advanced 2D materials. Blowing method is a general and scalable strategy for synthesizing ultrathin materials with layered or nonlayered structures, which opens up a new avenue for economic and massive preparation of good-quality nonlayered 2D nanosheets. This minireview summarizes the specific steps and principles of blowing method, focuses on the recent progress in preparing different categories of 2D materials, and further proposes an outlook for future opportunities in this growing field.

This is the first time that a free-standing ultra-thin 2D ionic salt supported with strong hydrogen-bonding assisted ionic interaction is successfully synthesized without any substrates, which is of great significance for the research of which kind of bonds can maintain the free-standing existence of a 2D nanosheets.

Abstract

2D materials have attracted great interest since the report of graphene. However, because of the fragile stability of ultra-thin nanosheets, most studies are restricted to sheets maintained by strong covalent or coordination bonds. The research on which kind of bonds can maintain the free-standing existence of 2D nanosheets is still of great significance. Recently, 2D ionic salts are successfully synthesized on substrates, but whether 2D ionic salts can free-stand is still a problem. Herein this problem is addressed by a free-standing 2D ionic salt (thickness: ≈2 nm) exfoliated from a 4,4′-bipyridinium hydrochloride salt crystal. The stability of this 2D salt is supported by a strong NH···Cl hydrogen (H)-bonding assisted ionic interaction (17.99 kcal mol⁻¹), which is verified by density functional theory calculation and natural bond orbital analysis. The salt crystal has strong air-stable radicals inside and the 2D ionic salt exhibits red fluorescence in solution and in solid-state, especially in solution the stokes shifts are very large (≈ 386 nm). This breakthrough work is not only beneficial for the construction of novel 2D materials but also for the understanding of H-bonding interactions.

Nature Reviews Materials, Published online: 17 February 2023; doi:10.1038/s41578-023-00544-2

High-performance ferroelectric materials are used in many applications, ranging from actuators to capacitors. Now, high entropy is emerging as an effective and flexible strategy for enhancing the physical properties of ferroelectrics via the delicate design of local polarization configurations.

Chlorine is an indispensable base chemical that is used to produce numerous materials. To enable the chlorine industry to harvest renewable energies, a safe chlorine storage is of central importance. More recently, trichlorides, [Cl₃]⁻, have evolved to a key technique for the storage of chlorine and for important chlorination reactions. In this minireview, the potential of trichlorides for academic research and the chlorine industry are outlined.

Abstract

Chlorine plays a central role for the industrial production of numerous materials with global relevance. More recently, polychlorides have been evolved from an area of academic interest to a research topic with enormous industrial potential. In this minireview, the value of trichlorides for chlorine storage and chlorination reactions are outlined. Particularly, the inexpensive ionic liquid [NEt₃Me][Cl₃] shows a similar and sometimes even advantageous reactivity compared to chlorine gas, while offering a superior safety profile. Used as a chlorine storage, [NEt₃Me][Cl₃] could help to overcome the current limitations of storing and transporting chlorine in larger quantities. Thus, trichlorides could become a key technique for the flexibilization of the chlorine production enabling an exploitation of renewable, yet fluctuating, electrical energy. As the loaded storage, [NEt₃Me][Cl₃], is a proven chlorination reagent, it could directly be employed for downstream processes, paving the path to a more practical and safer chlorine industry.

Non-van-der-Waals 2D CuCrSe₂ nanosheets with thickness down to monolayer are produced using a redox-controlled exfoliation route. Differing from its bulk form with antiferromagnetic order, the CuCrSe₂ nanosheets exhibit intriguing even–odd-layer-dependent magnetism evolution, which can be attributed to the orbital shift of Cr layers due to Cu-induced centrosymmetry breaking.

Abstract

Van der Waals (vdW) layered materials with strong magnetocrystalline anisotropy have attracted significant interest as the long-range magnetic order in these systems can survive even when their thicknesses is reduced to the 2D limit. Even though the interlayer coupling between the neighboring magnetic layers is very weak, it has a determining effect on the magnetism of these atomic-thickness materials. Herein, a new 2D ferromagnetic material, namely, non-vdW CuCrSe₂ nanosheets with even–odd-layer-dependent ferromagnetism when laminated from an antiferromagnetic bulk is reported. Monolayer and even-layer CuCrSe₂ exhibit the anomalous Hall effect and a significantly enhanced magnetic ordering temperature of more than 125 K. In contrast, the linear Hall effect exists in the odd-layer samples. Theoretical calculations indicate that the layer-dependent magnetic coupling is attributable to the orbital shift of the Cr atoms in the CrSe₂ layers owing to the Cu-induced breaking of the centrosymmetry. Thus, this work sheds light on the exotic magnetic properties of layered materials that exhibit phenomena beyond weak interlayer interactions.

Ternary metal sulfide/oxide (ZnIn₂S₄ and InVO₄) is chosen for the construction of efficient and stable Z-scheme heterojunction photocatalyst. The optimized heterojunction presents remarkable overall water splitting and excellent stability. The unique Z-scheme modulated charge transfer is proved to promote the spatial separation of photoexcited charges and strengthen the anti-photocorrosion capability of the system.

Abstract

The charge transfer within heterojunction is crucial for the efficiency and stability of photocatalyst for overall water splitting (OWS). Herein, InVO₄ nanosheets have been employed as a support for the lateral epitaxial growth of ZnIn₂S₄ nanosheets to produce hierarchical InVO₄@ZnIn₂S₄ (InVZ) heterojunctions. The distinct branching heterostructure facilitates active site exposure and mass transfer, further boosting the participation of ZnIn₂S₄ and InVO₄ for proton reduction and water oxidation, respectively. The unique Z-scheme modulated charge transfer, visualized by simulation and in situ analysis, has been proved to promote the spatial separation of photoexcited charges and strengthen the anti-photocorrosion capability of InVZ. The optimized InVZ heterojunction presents improved OWS (153.3 µmol h⁻¹ g⁻¹ for H₂ and 76.9 µmol h⁻¹ g⁻¹ for O₂) and competitive H₂ production (21090 µmol h⁻¹ g⁻¹). Even after 20 times (100 h) of cycle experiment, it still holds more than 88% OWS activity and a complete structure.

Fe₄Se₅ ultrathin films are experimentally demonstrated to host a pair-checkerboard antiferromagnetic (AFM) ground state with in-plane magnetization, evident with magnetic-field-dependent spin contrasts in real-space by spin-polarized scanning tunneling microscopy. The AFM order is modulated by a spin–lattice coupling and exhibits three types of nanoscale AFM domains, which are subject to thermal fluctuations into short-ranged patches at elevated temperatures.

Abstract

Unraveling the magnetic order in iron chalcogenides and pnictides at atomic scale is pivotal for understanding their unconventional superconducting pairing mechanism, but is experimentally challenging. Here, by utilizing spin-polarized scanning tunneling microscopy, real-space spin contrasts are successfully resolved to exhibit atomically unidirectional stripes in Fe₄Se₅ ultrathin films, the plausible closely related compound of bulk FeSe with ordered Fe-vacancies, which are grown by molecular beam epitaxy. As is substantiated by the first-principles electronic structure calculations, the spin contrast originates from a pair-checkerboard antiferromagnetic ground state with in-plane magnetization, which is modulated by a spin–lattice coupling. These measurements further identify three types of nanoscale antiferromagnetic domains with distinguishable spin contrasts, which are subject to thermal fluctuations into short-ranged patches at elevated temperatures. This work provides promising opportunities in understanding the emergent magnetic order and the electronic phase diagram for FeSe-derived superconductors.