Jiuxiang Dai

Through low-temperature oxidization of polycrystalline Cu foils, rapid preparation of large-area single-crystal Cu(111) (up to 320 cm²) is achieved within 60 min, thanks to the transformation of the thin Cu _x O layer to a Cu(111) seed layer. Large-size high-quality graphene films are synthesized on the single-crystal Cu(111), and the graphene/Cu(111) composites exhibit enhanced thermal conductivity and ductility compared to their polycrystalline counterpart.

Abstract

The controlled preparation of single-crystal Cu(111) is intensively investigated owing to the superior properties of Cu(111) and its advantages in synthesizing high-quality 2D materials, especially graphene. However, the accessibility of large-area single-crystal Cu(111) is still hindered by time-consuming, complicated, and high-cost preparation methods. Here, the oxidization-temperature-triggered rapid preparation of large-area single-crystal Cu(111) in which an area up to 320 cm² is prepared within 60 min, and where low-temperature oxidization of polycrystalline Cu foil surface plays a vital role, is reported. A mechanism is proposed, by which the thin Cu _x O layer transforms to a Cu(111) seed layer on the surface of Cu to induce the formation of a large-area Cu(111) foil, which is supported by both experimental data and molecular dynamics simulation results. In addition, a large-size high-quality graphene film is synthesized on the single-crystal Cu(111) foil surface and the graphene/Cu(111) composites exhibit enhanced thermal conductivity and ductility compared to their polycrystalline counterpart. This work, therefore, not only provides a new avenue toward the monocrystallinity of Cu with specific planes but also contributes to improving the mass production of high-quality 2D materials.

Heterogeneous integration of oxides possessing multiferroicity is realized in a freestanding membrane. BaTiO₃/La_0.7Sr_0.3MnO₃ freestanding bilayer membrane is fabricated by using pulsed laser epitaxy. It exhibits room-temperature ferroelectric ferromagnetism along with magnetoelectric coupling. Plus, owing to the absence of substrate strain, the ferromagnetism in La_0.7Sr_0.3MnO₃ experiences the reorientation of the magnetic anisotropy.

Abstract

Transition metal oxides exhibit a plethora of electrical and magnetic properties described by their order parameters. In particular, ferroic orderings offer access to a rich spectrum of fundamental physics phenomena, in addition to a range of technological applications. The heterogeneous integration of ferroelectric and ferromagnetic materials is a fruitful way to design multiferroic oxides. The realization of freestanding heterogeneous membranes of multiferroic oxides is highly desirable. In this study, epitaxial BaTiO₃/La_0.7Sr_0.3MnO₃ freestanding bilayer membranes are fabricated using pulsed laser epitaxy. The membrane displays ferroelectricity and ferromagnetism above room temperature accompanying the finite magnetoelectric coupling constant. This study reveals that a freestanding heterostructure can be used to manipulate the structural and emergent properties of the membrane. In the absence of the strain caused by the substrate, the change in orbital occupancy of the magnetic layer leads to the reorientation of the magnetic easy-axis, that is, perpendicular magnetic anisotropy. These results of designing multiferroic oxide membranes open new avenues to integrate such flexible membranes for electronic applications.

Author(s): Junyi Ji, Guoliang Yu, Changsong Xu, and H. J. Xiang

Two atom-thick layers of the same crystalline material can be stacked on top of each other in ways that yield ferroelectricity.

[Phys. Rev. Lett. 130, 146801] Published Tue Apr 04, 2023

Abstract

Two-dimensional van der Waals (2D vdW) magnets have attracted great attention recently and possess the unprecedented advantages of incorporating high-quality vdW heterostructures and homostructures into spintronic devices, and exploring various physical phenomena or technologies. Among them, Fe₅GeTe₂ (F5GT) has ferromagnetic order close to room temperature, however the magnetic properties near its intrinsic transitions and F5GT-based 2D devices remain mostly unexplored. Here, we systematically demonstrate the peculiar magnetic properties of Fe₅GeTe₂ nanoflakes near its intrinsic transition temperature (T_p) which is far lower than its Curie temperature (T_C) of ∼ 265 K, and firstly discover anomalous magnetoresistance effect in F5GT homo-junctions by magneto-transport measurements. The strongest anomalous Hall effect occurs around T_p which is located in a temperature range from 130 to 160 K for the F5GT nanoflakes with different thicknesses. Furthermore, negative magnetoresistance (N-MR) and butterfly-shaped magnetoresistance (B-MR) are observed in F5GT homo-junction devices, and they appeared only in an intermediate temperature range from 110 to 160 K, noticeably showing the maxima near the T_p rather than the lowest temperature. Our experimental results clearly reveal the significant influence of intrinsic transitions on magnetic properties of F5GT and magnetoresistance effect in F5GT homo-junction devices, which imply a new strategy to achieve high-performance 2D spintronic devices by tuning intrinsic magnetic or structural transitions in 2D vdW magnets.

Proceedings of the National Academy of Sciences, Volume 120, Issue 15, April 2023.

Nature Physics, Published online: 04 April 2023; doi:10.1038/s41567-023-02015-5

The combination of magnetic and non-magnetic layers in (MnBi2Te4)(Bi2Te3) is predicted to produce topologically protected states on the surface. Experiments now show that the nature of the topmost layer controls the location of these states.

Applied Physics Letters, Volume 122, Issue 14, April 2023.
We report on the growth and lasing characteristics of 2.3%-compressive-strained InGaAsSb multiple-quantum-well (MQW) lasers on InP substrates with emission wavelengths near 2.2 μm. MQW structures with four wells were grown by metalorganic molecular beam epitaxy at 500 °C. X-ray diffraction and photoluminescence results showed that the InGaAsSb well layers were grown with the assistance of the Sb surfactant effect. The emission wavelengths of the MQW lasers with well thicknesses of 6.4 and 8.4 nm were 2.190 and 2.278 μm, respectively. For the MQW laser with the well thickness of 8.4 nm, the threshold current under continuous-wave operation was 22 mA at 15 °C, and the characteristic temperature was estimated to be 53 K in the temperature region from 15 to 35 °C and 42 K in the region from 35 to 55 °C. The laser with the 8.4-nm-thick well had an emission wavelength about 90 nm longer than that of the one with the 6.4-nm-thick well, but the lasing characteristics of the two were comparable.

Applied Physics Letters, Volume 122, Issue 14, April 2023.
We have demonstrated the fabrication process for a lateral p-type Schottky barrier diode (SBD) with the annealed Mg ohmic contact layer on a MOVPE-grown p-GaN wafer and measured the electrical characteristic of the diode. Because of the selective-area ohmic contact, the interface between the Schottky electrode and p-type GaN is well protected from any damage introduced by dry-etching or regrowth. The ideality factor of the forward current–voltage characteristic is as low as 1.09 at room temperature and an on–off ratio above 109 is also achieved. Various metals are deposited as the Schottky electrode and the work function dependence of the Schottky barrier height is confirmed with a pinning factor of 0.58. The temperature dependence of the current–voltage characteristic indicates that the GaN p-type SBD still fits the thermionic emission mode at 600 K with an ideality factor of 1.1. The reverse current of the p-SBD is also studied with the Poole–Frenkel emission model, and the trap energy level in the p-GaN is confirmed.

An intriguing photodelamination phenomenon that arises from ordered band alignment and resultant energy-funneling effect is observed in nonuniform van der Waals semiconductors, which holds potential as a noninvasive but convenient atomic-layer etching strategy for thickness uniformity management in 2D van der Waals semiconductor wafers for electronic applications.

Abstract

Atomically thin 2D van der Waals semiconductors are promising candidate materials for post-silicon electronics. However, it remains challenging to attain completely uniform monolayer semiconductor wafers free of over-grown islands. Here, the observation of the energy-funneling effect and ambient photodelamination phenomenon in inhomogeneous few-layer WS₂ flakes under low-illumination fluencies down to several nW µm⁻² and its potential as a noninvasive atomic-layer etching strategy for selectively stripping the local excessive overlying islands are reported. Photoluminescent tracking on the photoetching traces reveals relatively fast etching rates of around 0.3–0.8 µm min⁻¹ at varied temperatures and an activation energy of 1.7 eV. By using crystallographic and electronic characterization, the noninvasive nature of the low-power photodelamination and the highly preserved lattice quality are also confirmed in the as-etched monolayer products, featuring a comparable density of atomic defects (≈4.2 × 10¹³ cm⁻²) to pristine flakes and a high electron mobility of up to 80 cm² V⁻¹ s⁻¹ at room temperature. This approach opens a noninvasive postetching route for thickness uniformity management in 2D van der Waals semiconductor wafers for electronic applications.

Bidirectional (excitatory and inhibitory) optoelectronic synapses are successfully prepared by using 2D molecular crystal heterojunctions. Superior contrast ratios (CR) of 1.53 × 10³ are demonstrated, transcending previous optoelectronic synapses, and the devices are applied for motion detection with an accuracy exceeding 90%.

Abstract

Light-stimulated optoelectronic synaptic devices are fundamental compositions of the neuromorphic vision system. However, there are still huge challenges to achieving both bidirectional synaptic behaviors under light stimuli and high performance. Herein, a bilayer 2D molecular crystal (2DMC) p-n heterojunction is developed to achieve high-performance bidirectional synaptic behaviors. The 2DMC heterojunction-based field effect transistor (FET) devices exhibit typical ambipolar properties and remarkable responsivity (R) of 3.58×10⁴ A W⁻¹ under weak light as low as 0.008 mW cm⁻². Excitatory and inhibitory synaptic behaviors are successfully realized by the same light stimuli under different gate voltages. Moreover, a superior contrast ratio (CR) of 1.53×10³ is demonstrated by the ultrathin and high-quality 2DMC heterojunction, which transcends previous optoelectronic synapses and enables application for the motion detection of the pendulum. Furthermore, a motion detection network based on the device is developed to detect and recognize classic motion vehicles in road traffic with an accuracy exceeding 90%. This work provides an effective strategy for developing high-contrast bidirectional optoelectronic synapses and shows great potential in the intelligent bionic device and future artificial vision.

The synthesis of two-dimensional (2D) nickel-doped cobalt monoxide (Ni-doped CoO) nanosheets with a high substitution doping concentration is proposed for the first time. 2D Ni-doped CoO exhibits obvious ferromagnetism with a Curie temperature of ≈180 K, in sharp contrast to the pristine non-ferromagnetic 2D CoO, opening up a prospective way for tuning the magnetism of 2D nonlayered oxide semiconductors.

Abstract

Element doping has become an effective strategy to engineer the magnetic properties of two-dimensional (2D) materials and is widely explored in van der Waals layered transition metal dichalcogenides. However, the high-concentration substitution doping of 2D nonlayered metal oxides, which can preserve the original crystal texture and guarantee the homogeneity of doping distribution, is still a critical challenge due to the isotropic bonding of closed-packed structures. In this work, the synthesis of high-quality 2D nonlayered nickel-doped cobalt monoxide nanosheets via in situ atmospheric pressure chemical vapor deposition method is reported. High-resolution transmission electron microscopy confirmed that nickel atoms are doped at the intrinsic cobalt atom sites. The nickel doping concentration is stable at ≈15%, superior to most magnetic dopants doping in 2D materials and metal oxides. Magnetic measurements showed that pristine cobalt monoxide is nonferromagnetic, whereas nickel-doped cobalt monoxide exhibits robust ferromagnetic behavior with a Curie temperature of ≈180 K. Density functional theory calculations reveal that nickel atoms can improve the internal ferromagnetic correlation, giving rise to significant ferromagnetic performance of cobalt monoxide nanosheets. These results provide a valuable case for tuning the competing correlated states and magnetic ordering by substitution doping in 2D nonlayered oxide semiconductors.

Chalcogen powder is pre-melted and resolidified to give a stable chalcogen precursor for the chemical vapor deposition growth of transition metal dichalcogenides. The obtained monolayer tungsten disulfide is uniform and shows high optical and structural quality.

Abstract

Two-dimensional (2D) semiconductors including transition metal dichalcogenides (TMDCs) have gained attention in optoelectronics for their extraordinary properties. However, the large amount and locally distributed lattice defects affect the optical properties of 2D TMDCs, and the defects originate from unstable factors in the synthesis process. In this work, we develop a method of pre-melting and resolidification of chalcogen precursors (sulfur and selenium), namely resolidified chalcogen, as precursor for the chemical vapor deposition growth of TMDCs with ultrahigh quality and uniformity. Taking WS₂ as an example, the monolayer WS₂ shows uniform fluorescence intensity and a small full-width at half-maximum of photoluminescence peak at low temperatures with an average value of 13.6±1.9 meV. The defect densities at the interior and edge region are both low and comparable, i.e., (9±3)×10¹² cm⁻² and (10±4)×10¹² cm⁻², indicating its high structural quality and uniformity. This method is universal in growing high quality monolayer MoS₂, WSe₂, MoSe₂, and will benefit their applications.

Electrocatalytic carbon dioxide reduction (CO₂ ECR) over metal phosphorous trichalcogenide (MPCh₃) nanosheets was systematically investigated by Dong Wang, Chen Shen, Dan Ren, Hongguang Wang, and co-workers in their Research Article (e202217253). Unlike the layered CoPS₃ and NiPS₃ nanosheets, the active Sn atoms tend to be exposed on the surfaces of nonlayered SnPS₃ nanosheets, which exhibit clearly improved formic acid selectivity.

A “posterior” interfacial polymerization (p-IP) strategy is developed to fabricate a graphene oxide (GO) framework membrane with a highly positively charged surface for ion sieving. The generated electrostatic repulsion of ions from the positively charged surface, in conjunction with reinforced size exclusion from the p-IP-formed polyamide network, endows the membrane with outstanding performance for ion separation.

Abstract

Two-dimensional graphene oxide (GO) membranes are gaining popularity as a promising means to address global water scarcity. However, current GO membranes fail to sufficiently exclude angstrom-sized ions from solution. Herein, a de novo “posterior” interfacial polymerization (p-IP) strategy is reported to construct a tailor-made polyamide (PA) network in situ in an ultrathin GO membrane to strengthen size exclusion while imparting a positively charged membrane surface to repel metal ions. The electrostatic repulsion toward metal ions, coupled with the reinforced size exclusion, synergistically drives the high-efficiency metal ion separation through the synthesized positively charged GO framework (PC-GOF) membrane. This dual-mechanism-driven PC-GOF membrane exhibits superior metal ion rejection, anti-fouling ability, good operational stability, and ultra-high permeance (five times that of pristine GO membranes), enabling a sound step towards a sustainable water-energy-food nexus.

Nature, Published online: 05 April 2023; doi:10.1038/s41586-023-05848-5

A single-element ferroelectric state is observed in a black phosphorus-like bismuth layer, in which the ordered charge transfer and the regular atom distortion between sublattices happen simultaneously and ferroelectric switching is further visualized experimentally.

Applied Physics Letters, Volume 122, Issue 14, April 2023.
We report deterministic control over a moiré superlattice interference pattern in twisted bilayer graphene by implementing designable device-level heterostrain with process-induced strain engineering, a widely used technique in industrial silicon nanofabrication processes. By depositing stressed thin films onto our twisted bilayer graphene samples, heterostrain magnitude and strain directionality can be controlled by stressor film force (film stress × film thickness) and patterned stressor geometry, respectively. We examine strain and moiré interference with Raman spectroscopy through in-plane and moiré-activated phonon mode shifts. Results support systematic C3 rotational symmetry breaking and tunable periodicity in moiré superlattices under the application of uniaxial or biaxial heterostrain. Experimental results are validated by molecular statics simulations and density functional theory based first principles calculations. This provides a method not only to tune moiré interference without additional twisting but also to allow for a systematic pathway to explore different van der Waals based moiré superlattice symmetries by deterministic design.

Energy storage performance of nanocomposites is improved through the use of parallel 2D nanosheets fabricated via Langmuir–Blodgett deposition. Specially, the introduction of seven-layered Sr_1.8Bi_0.2Nb₃O₁₀ nanosheets with ultrathin thickness (14 nm) effectively inhibits electrical treeing and improves energy storage performance.

Abstract

Polymer-based nanocomposites are desirable materials for next-generation dielectric capacitors. 2D dielectric nanosheets have received significant attention as a filler. However, randomly spreading the 2D filler causes residual stresses and agglomerated defect sites in the polymer matrix, which leads to the growth of an electric tree, resulting in a more premature breakdown than expected. Therefore, realizing a well-aligned 2D nanosheet layer with a small amount is a key challenge; it can inhibit the growth of conduction paths without degrading the performance of the material. Here, an ultrathin Sr_1.8Bi_0.2Nb₃O₁₀ (SBNO) nanosheet filler is added as a layer into poly(vinylidene fluoride) (PVDF) films via the Langmuir–Blodgett method. The structural properties, breakdown strength, and energy storage capacity of a PVDF and multilayer PVDF/SBNO/PVDF composites as a function of the thickness-controlled SBNO layer are examined. The seven-layered (only 14 nm) SBNO nanosheets thin film can sufficiently prevent the electrical path in the PVDF/SBNO/PVDF composite and shows a high energy density of 12.8 J cm⁻³ at 508 MV m⁻¹, which is significantly higher than that of the bare PVDF film (9.2 J cm⁻³ at 439 MV m⁻¹). At present, this composite has the highest energy density among the polymer-based nanocomposites under the filler of thin thickness.

Transforming liquid metals into 2D sheets using the vortex fluidic device enables substrate and surfactant-free processing, as established in preparing 2D sheets of gallium oxide encasing subvalent indium from a eutectic melt of the two metals. The 2D sheets display reduced resistance between a metal and a semiconductor, demonstrating their potential application in silicon-based devices.

Abstract

Reducing resistance in silicon-based devices is important as they get miniaturized further. 2D materials offer an opportunity to increase conductivity whilst reducing size. A scalable, environmentally benign method is developed for preparing partially oxidized gallium/indium sheets down to 10 nm thick from a eutectic melt of the two metals. Exfoliation of the planar/corrugated oxide skin of the melt is achieved using the vortex fluidic device with a variation in composition across the sheets determined using Auger spectroscopy. From an application perspective, the oxidized gallium indium sheets reduce the contact resistance between metals such as platinum and silicon (Si) as a semiconductor. Current‒voltage measurements between a platinum atomic force microscopy tip and a Si−H substrate show that the current switches from being a rectifier to a highly conducting ohmic contact. These characteristics offer new opportunities for controlling Si surface properties at the nanoscale and enable the integration of new materials with Si platforms.

Applied Physics Letters, Volume 122, Issue 14, April 2023.
The heterogeneous integration of III-nitride materials with other semiconductor systems for electronic devices is attractive because it combines the excellent electrical properties of the III-nitrides with other device platforms. Pursuing integration through metalorganic chemical vapor deposition (MOCVD) is desirable because of the scalability of the technique, but the high temperatures required for the MOCVD growth of III-nitrides (>1000 °C) are incompatible with direct heteroepitaxy on some semiconductor systems and fabricated wafers. Thus, the MOCVD growth temperature of III-nitride films must be lowered to combine them with other systems. In this work, 16 nm-thick Si:GaN films were grown by MOCVD at 550 °C using a flow modulation epitaxy scheme. By optimizing the disilane flow conditions, electron concentrations up to 5.9 × 1019 cm−3 were achieved, resulting in sheet resistances as low as 1070 Ω/□. Film mobilities ranged from 34 to 119 cm2 V−1 s−1. These results are promising for III-nitride integration and expand device design and process options for III-nitride-based electronic devices.

Applied Physics Letters, Volume 122, Issue 14, April 2023.
The recent discovery of HfO2-based and nitride-based ferroelectrics that are compatible to the semiconductor manufacturing process has revitalized the field of ferroelectric-based nanoelectronics. Guided by a simple design principle of charge compensation and density functional theory calculations, we discover that HfO2-like mixed-anion materials, TaON and NbON, can crystallize in the polar [math] phase with a strong thermodynamic driving force to adopt anion ordering spontaneously. Both oxynitrides possess large remnant polarization, low switching barriers, and unconventional negative piezoelectric effect, making them promising piezoelectrics and ferroelectrics. Distinct from HfO2 that has a wide bandgap, both TaON and NbON can absorb visible light and have high charge carrier mobilities, suitable for ferroelectric photovoltaic and photocatalytic applications. This class of multifunctional nonperovskite oxynitride containing economical and environmentally benign elements offers a platform to design and optimize high-performing ferroelectric semiconductors for integrated systems.

A carrier modulation method for WSe₂ transistors has been demonstrated by tuning the thickness of the interfacial hexagonal boron nitride dielectric layer. The carrier type of the WSe₂ channel can be tuned from holes to electrons, contributing to the construction of versatile two-surface-channel WSe₂ transistors and high area-efficiency logic gates and circuits.

Abstract

2D materials with atomic thickness display strong gate controllability and emerge as promising materials to build area-efficient electronic circuits. However, achieving the effective and nondestructive modulation of carrier density/type in 2D materials is still challenging because the introduction of dopants will greatly degrade the carrier transport via Coulomb scattering. Here, a strategy to control the polarity of tungsten diselenide (WSe₂) field-effect transistors (FETs) via introducing hexagonal boron nitride (h-BN) as the interfacial dielectric layer is devised. By modulating the h-BN thickness, the carrier type of WSe₂ FETs has been switched from hole to electron. The ultrathin body of WSe₂, combined with the effective polarity control, together contribute to the versatile single-transistor logic gates, including NOR, AND, and XNOR gates, and the operation of only two transistors as a half adder in logic circuits. Compared with the use of 12 transistors based on static Si CMOS technology, the transistor number of the half adder is reduced by 83.3%. The unique carrier modulation approach has general applicability toward 2D logic gates and circuits for the improvement of area efficiency in logic computation.

Hexagonal magnetic semiconducting α-MnTe nanoribbons with an anisotropic (100) surface are controllably synthesized via salt-assisted chemical vapor deposition (CVD) method. Due to highly anisotropic (100) surface, the α-MnTe nanoribbons as photodetector exhibit attractive sensitivity to polarization.

Abstract

2D materials with low symmetry are explored in recent years because of their anisotropic advantage in polarization-sensitive photodetection. Herein the controllably grown hexagonal magnetic semiconducting α-MnTe nanoribbons are reported with a highly anisotropic (100) surface and their high sensitivity to polarization in a broadband photodetection, whereas the hexagonal structure is highly symmetric. The outstanding photoresponse of α-MnTe nanoribbons occurs in a broadband range from ultraviolet (UV, 360 nm) to near infrared (NIR, 914 nm) with short response times of 46 ms (rise) and 37 ms (fall), excellent environmental stability, and repeatability. Furthermore, due to highly anisotropic (100) surface, the α-MnTe nanoribbons as photodetector exhibit attractive sensitivity to polarization and high dichroic ratios of up to 2.8 under light illumination of UV-to-NIR wavelengths. These results demonstrate that 2D magnetic semiconducting α-MnTe nanoribbons provide a promising platform to design the next-generation polarization-sensitive photodetectors in a broadband range.