Jiuxiang Dai

Applied Physics Letters, Volume 122, Issue 12, March 2023.
Stacking order and strain are the key component in tuning magnetic property of two-dimensional (2D) van der Waals magnetic materials. In this work, we investigated the crystal structure stability of a 2D ferromagnetic Janus chromium dichalcogenide CrSTe bilayer in AB- and AC-stacking orders and found that AB-stacking CrSTe bilayers, which have a smaller layer spacing and a Curie temperature near room temperature, are more stable than AC-stacking one. The magnetic ground states, exchange coupling constant, and Curie temperature of the AB-stacking CrSTe bilayer can be tuned by strain. It is found that the magnetic ground states of the AB- and AC-stacking CrSTe bilayers are ferromagnetic and interlayer antiferromagnetic within a certain strain range, respectively, indicating that the CrSTe bilayers are expected to be used in the double spin filter. Our results demonstrated that the 2D Janus CrSTe bilayer has the potential in the application of spintronic devices with stable performance and low-power consumption at room temperature.

Applied Physics Letters, Volume 122, Issue 12, March 2023.
A recently developed technique of transmission-mode microwave impedance microscopy (T-MIM) has greatly extended the capabilities of standard reflection-mode MIM to novel applications, such as the in operando study of nanoscale electro-acoustic devices. As is common for new techniques, systematic design principles for boosting sensitivity and balancing bandwidth are lacking. Here, we show numerically and analytically that the T-MIM signal is proportional to the reflection-mode voltage enhancement factor η of the circuit, as long as the output impedance of the local voltage source is properly treated. We show that this proportionality holds in the currently achievable “weak sampling” regime and beyond, for which we demonstrate a realistic path with commercially available superconducting components and critically coupled impedance matching networks. We demonstrate that for these next-generation designs, the sensitivity is generally maximized at a slightly different frequency from the unloaded S11 resonance, which can be explained by the maximum power transfer theorem.

The polydisperse nature of graphene dispersions following liquid-phase exfoliation poses major manufacturing challenges since incumbent separation schemes rely on centrifugation, which is highly energy-intensive and possesses limited scalability. Here, cross-flow filtration is introduced as a centrifuge-free processing method that improves the throughput of graphene separation by two orders of magnitude in addition to substantially reducing environmental impact via inline recycling.

Abstract

Solution-processed graphene is a promising material for numerous high-volume applications including structural composites, batteries, sensors, and printed electronics. However, the polydisperse nature of graphene dispersions following liquid-phase exfoliation poses major manufacturing challenges, as incompletely exfoliated graphite flakes must be removed to achieve optimal properties and downstream performance. Incumbent separation schemes rely on centrifugation, which is highly energy-intensive and limits scalable manufacturing. Here, cross-flow filtration (CFF) is introduced as a centrifuge-free processing method that improves the throughput of graphene separation by two orders of magnitude. By tuning membrane pore sizes between microfiltration and ultrafiltration length scales, CFF can also be used for efficient recovery of solvents and stabilizing polymers. In this manner, life cycle assessment and techno-economic analysis reveal that CFF reduces greenhouse gas emissions, fossil energy usage, water consumption, and specific production costs of graphene manufacturing by 57%, 56%, 63%, and 72%, respectively. To confirm that CFF produces electronic-grade graphene, CFF-processed graphene nanosheets are formulated into printable inks, leading to state-of-the-art thin-film conductivities exceeding 10⁴ S m⁻¹. This CFF methodology can likely be generalized to other van der Waals layered solids, thus enabling sustainable manufacturing of the diverse set of applications currently being pursued for 2D materials.

Normally, the SHG signal intensity of nonlinear optical crystals is highly dependent on the angles between polarized light and crystal axis, resulting in big challenges in matching the laser polarization orientation. Hence, the polarization-independent SHG effect discovered for the first time in layered Kagome Nb₃SeI₇ crystals is of great significance for nonlinear optics.

Abstract

Second harmonic generation (SHG) of 2D crystals has been of great interest due to its advantages of phase-matching and easy integration into nanophotonic devices. However, the polarization-dependence character of the SHG signal makes it highly troublesome but necessary to match the laser polarization orientation relative to the crystal, thus achieving the maximum polarized SHG intensity. Here, it is demonstrated a polarization-independent SHG, for the first time, in the van der Waals Nb₃SeI₇ crystals with a breathing Kagome lattice. The Nb₃ triangular clusters and Janus-structure of each Nb₃SeI₇ layer are confirmed by the STEM. Nb₃SeI₇ flake shows a strong SHG response due to its noncentrosymmetric crystal structure. More interestingly, the SHG signals of Nb₃SeI₇ are independent of the polarization of the excitation light owing to the in-plane isotropic arrangement of nonlinear active units. This work provides the first layered nonlinear optical crystal with the polarization-independent SHG effect, providing new possibilities for nonlinear optics.

Nature, Published online: 22 March 2023; doi:10.1038/s41586-023-05797-z

The epitaxial synthesis of high-density, vertically aligned arrays of two-dimensional (2D) fin-oxide heterostructures is described, enabling the fabrication of 2D fin field-effect transistors with high electron mobility and desirable low-power specifications.

Nature, Published online: 22 March 2023; doi:10.1038/s41586-023-05819-w

A two-dimensional field-effect transistor made of indium selenide is shown to outperform state-of-the-art silicon-based transistors, operating at lower supply voltage and achieving record high transconductance and ballistic ratio.

An ultrathin crystalline ferroelectric polymer based on trichloromethyl (CCl₃)-terminated poly(vinylidene difluoride-co-trifloroethylene) (P(VDF-TrFE)) is developed on AlO _X to realize negative-capacitance metal–oxide–semiconductor field-effect transistors (MOSFETs) with a MoS₂ channel. Systematic adjustment of the thickness ratio between the polymer and dielectric AlO _X achieves a hysteresis-free FET with a minimum subthreshold swing of 28 mV dec⁻¹, operating at less than 2 V.

Abstract

Negative-capacitance field-effect transistors (NC-FETs) have gathered enormous interest as a way to reduce subthreshold swing (SS) and overcome the issue of power dissipation in modern integrated circuits. For stable NC behavior at low operating voltages, the development of ultrathin ferroelectrics (FE), which are compatible with the industrial process, is of great interest. Here, a new scalable ultrathin ferroelectric polymer layer is developed based on trichloromethyl (CCl₃)-terminated poly(vinylidene difluoride-co-trifloroethylene) (P(VDF-TrFE)) to achieve the state-of-the-art performance of NC-FETs. The crystalline phase of 5–10 nm ultrathin P(VDF-TrFE) is prepared on AlO _X by a newly developed brush method, which enables an FE/dielectric (DE) bilayer. FE/DE thickness ratios are then systematically tuned at ease to achieve ideal capacitance matching. NC-FETs with optimized FE/DE thickness at a thickness limit demonstrate hysteresis-free operation with an SS of 28 mV dec⁻¹ at ≈1.5 V, which competes with the best reports. This P(VDF-TrFE)-brush layer can be broadly adapted to NC-FETs, opening an exciting avenue for low-power devices.

Ti₃C₂T_x MXene films are integrated into GaN high electron mobility transistors (HEMTs) as gate contacts, wherein van der Waals heterojunctions are formed between MXene films and GaN without direct chemical bonding. The Schottky gate GaN HEMTs with enhanced gate controllability exhibit a record high I _ON/I _OFF current ratio of ≈10¹³ and a near-ideal subthreshold swing of 61 mV dec⁻¹.

Abstract

Gate controllability is a key factor that determines the performance of GaN high electron mobility transistors (HEMTs). However, at the traditional metal-GaN interface, direct chemical interaction between metal and GaN can result in fixed charges and traps, which can significantly deteriorate the gate controllability. In this study, Ti₃C₂T_x MXene films are integrated into GaN HEMTs as the gate contact, wherein van der Waals heterojunctions are formed between MXene films and GaN without direct chemical bonding. The GaN HEMTs with enhanced gate controllability exhibit an extremely low off-state current (I _OFF) of 10⁻⁷ mA mm⁻¹, a record high I _ON/I _OFF current ratio of ≈10¹³ (which is six orders of magnitude higher than conventional Ni/Au contact), a high off-state drain breakdown voltage of 1085 V, and a near-ideal subthreshold swing of 61 mV dec⁻¹. This work shows the great potential of MXene films as gate electrodes in wide-bandgap semiconductor devices.

The study highlights the enhanced moiré potential in twisted trilayer homostructures of transition metal dichalcogenides. Adjusting the number of stacked layers leads to multiple exciton resonances with different circular polarization and g-factor, indicating controllable moiré potential. This discovery presents a new preparation method for moiré potentials, which paves the way for exploring highly correlated quantum phenomena.

Abstract

The exploration of moiré superlatticesholds promising potential to uncover novel quantum phenomena emerging from the interplay of atomic structure and electronic correlation . However, the impact of the moiré potential modulation on the number of twisted layers has yet to be experimentally explored. Here, this work synthesizes a twisted WSe₂ homotrilayer using a dry-transfer method and investigates the enhancement of the moiré potential with increasing number of twisted layers. The results of the study reveal the presence of multiple exciton resonances with positive or negative circularly polarized emission in the WSe₂ homostructure with small twist angles, which are attributed to the excitonic ground and excited states confined to the moiré potential. The distinct g-factor observed in the magneto-optical spectroscopy is also shown to be a result of the confinement of the exciton in the moiré potential. The moiré potential depths of the twisted bilayer and trilayer homostructures are found to be 111 and 212 meV, respectively, an increase of 91% from the bilayer structure. These findings demonstrate that the depth of the moiré potential can be manipulated by adjusting the number of stacked layers, providing a promising avenue for exploration into highly correlated quantum phenomena.

Journal of Applied Physics, Volume 133, Issue 11, March 2023.
Twisted 2D bilayers of van der Waals materials, a new class of quantum materials, offer pioneering advances in the field of nanoelectronics and photonics. As these layered materials can have various preferential stacking configurations with varying electronic behavior, it is important to have a characterization technique that can unambiguously probe the stacking order and interlayer interactions in 2D materials and twisted 2D homobilayers. In this work, we show that by using Raman spectroscopy, we can probe variations in the interlayer coupling of bilayer WSe[math] stacked at different twist angles. The interlayer interactions are weakest at a twist angle of 30[math], and the twisted bilayer system is almost equivalent to two decoupled monolayers of WSe[math]. Also we demonstrate Raman mapping as a quick imaging tool with capabilities of clear distinction between 2H and 3R polytypes of bilayer WSe[math] and can be used to study various kirigami structures and bilayer nucleation centers commonly observed during chemical vapor deposition-based growth of WSe[math]. This work proves to be beneficial in the characterization of twisted bilayers of 2D materials and offer key insights into the optoelectronic properties of 2D materials and heterostructures.

Journal of Applied Physics, Volume 133, Issue 11, March 2023.
Recently, two-dimensional (2D) magnets have drawn substantial attention from researchers for their fascinating properties and great application potential in the fields of biomedicine, data storage, signal transfer, and energy conversion. However, the low Curie/Néel temperature of 2D magnets hinders their application. In this Perspective, we present some physical insights into enhancing the magnetic stability of 2D magnets. First, the microscope theoretical model of 2D magnets is introduced. Then, we review and analyze several effective and commonly used methods for enhancing the magnetic stability of 2D magnets. Finally, we present the perspective and summary. This Perspective presents the advanced understanding of magnetic stability in 2D materials, which can provide new opportunities for further advancement in a wide variety of applications.

Rechargeable Mg batteries are expected to achieve high volumetric energy densities only if the excess Mg is limited. Herein, the proof-of-concept of anode-free Mg batteries is first proposed using Mg₂Mo₆S₈-MgS composite cathode. The anode-free Mg₂Mo₆S₈-MgS/Cu batteries deliver a capacity of 190 mAh g⁻¹ with capacity retention of 92% after 100 cycles, corresponding to the energy density of 420 Wh L⁻¹.

Abstract

The desperate pursuit of high gravimetric specific energy leads to the ignorance of volumetric energy density that is one of the basic requirements for batteries. Due to the high volumetric capacity, less-prone formation of dendrite, and low reduction potential of Mg metal, rechargeable Mg batteries are considered with innately high volumetric energy density. Nevertheless, the substantial elevation in energy density is compromised by extremely excessive Mg metal anode. Herein, the proof-of-concept of anode-free Mg₂Mo₆S₈-MgS/Cu batteries is proposed, in which MgS as the premagnesiation additive constantly decomposes to replenish Mg loss by electrolyte corrosion over cycling, while both Mg₂Mo₆S₈ and MgS acts as the active material to reversibly provide high capacities. Besides, Mg₂Mo₆S₈ shows superior catalytic activity on the decomposition of MgS and provides the strong affinity to polysulfides to restrain their dissolution. Consequently, the anode-free Mg₂Mo₆S₈-MgS/Cu batteries deliver a high reversible capacity of 190 mAh g⁻¹ with the capacity retention of 92% after 100 cycles, corresponding to the highly competitive energy density of 420 Wh L⁻¹. The proposed anode-free Mg battery here spotlights the great promise of Mg batteries in achieving high volumetric energy densities, which will significantly expedite the advances of Mg batteries in practice.

A previously unreported orthorhombic Sn₃O₄ polymorph was synthesized via a facile hydrothermal method and characterized carefully by empirical and calculational methods.

Abstract

An unexplored tin oxide crystal phase (Sn₃O₄) was experimentally synthesized via a facile hydrothermal method. After tuning the often-neglected parameters for the hydrothermal synthesis, namely the degree of filling of the precursor solution and the gas composition in the reactor head space, an unreported X-ray diffraction pattern was discovered. Through various characterization studies, such as Rietveld analysis, energy dispersive X-ray spectroscopy, and first-principles calculations, this novel material was characterized as orthorhombic mixed-valence tin oxide with the composition Sn^II ₂Sn^IVO₄. This orthorhombic tin oxide is a new polymorph of Sn₃O₄, which differs from the reported conventional monoclinic structure. Computational and experimental analyses showed that orthorhombic Sn₃O₄ has a smaller band gap (2.0 eV), enabling greater absorption of visible light. This study is expected to improve the accuracy of hydrothermal synthesis and aid the discovery of new oxide materials.

Applied Physics Letters, Volume 122, Issue 12, March 2023.
Ferroelectric materials have been explored for a long time for easy integration with state-of-the-art semiconductor technologies. Doped wurtzite nitrides have been reported as promising candidates due to their high stability, compatibility, and scalability. We investigate doping effects on ferroelectric properties of Sc-doped AlN (AlScN) and B-doped AlN (AlBN) by first-principles methods. The energy barrier against polarization switching is observed to decrease with increasing doping concentration at low concentration ranges, which is the origin of the emerging ferroelectricity in doped AlN. Further increasing the doping concentration to a critical value, the ferroelectric wurtzite phase transforms into paraelectric phases (a rock salt phase for AlScN and a zinc blende phase for AlBN), making it invalid to decrease the coercivity by increasing the doping concentration. Furthermore, it is revealed that different nonpolar structures (a hexagonal phase for AlScN and a [math]-BeO phase for AlBN) appear in the ferroelectric switching pathway, generating different switching features in doped AlN. Our results give a microscopic understanding of the ferroelectricity in doped wurtzite materials and broaden the route to improve their ferroelectric properties.

Applied Physics Letters, Volume 122, Issue 12, March 2023.
One hotspot of integrated optics is how to realize a highly integrated and high-gain on-chip amplification system in a thin film of lithium niobate on insulator (TFLNOI). Here, a low erbium-doped TFLNOI waveguide amplifier with shorter length is demonstrated using the photolithography-assisted oblique-reactive ion etching technique. A maximum net internal gain of 5.4 dB in the small-signal-gain regime is measured at the peak emission wavelength of 1531.35 nm for a waveguide length of 1.5 mm and an erbium-doped concentration of 0.1 mol. %, indicating a gain per unit length of 36 dB cm−1. This work paves the way for the monolithic integration of diverse active and passive photonic components on the TFLNOI platform.

Applied Physics Letters, Volume 122, Issue 12, March 2023.
This Perspective discusses and compares several different approaches to the design of high-bandwidth, low-voltage electro-optic devices, such as Mach–Zehnder modulators, made using thin-film lithium niobate (TFLN) and strategies for their incorporation as part of a larger photonic integrated circuit (PIC).

Author(s): Haoying Sun, Jiahui Gu, Yongqiang Li, Tula R. Paudel, Di Liu, Jierong Wang, Yipeng Zang, Chengyi Gu, Jiangfeng Yang, Wenjie Sun, Zhengbin Gu, Evgeny Y. Tsymbal, Junming Liu, Houbing Huang, Di Wu, and Yuefeng Nie

The increasing miniaturization of electronics requires a better understanding of material properties at the nanoscale. Many studies have shown that there is a ferroelectric size limit in oxides, below which the ferroelectricity will be strongly suppressed due to the depolarization field, and whether…

[Phys. Rev. Lett. 130, 126801] Published Tue Mar 21, 2023

Nature Materials, Published online: 20 March 2023; doi:10.1038/s41563-023-01492-6

This Review discusses the field of antiferromagnetic spintronics with a focus on coherent effects.

Nature Electronics, Published online: 20 March 2023; doi:10.1038/s41928-023-00941-z

The Curie temperature of Fe5+xGeTe2 thin films can be modulated from 260 to 380 K via iron doping, allowing the two-dimensional material to be used to create planar spiral inductors and low-pass Butterworth filters.

Nature Nanotechnology, Published online: 20 March 2023; doi:10.1038/s41565-023-01349-8

Differential phase contrast scanning transmission electron microscopy probes the electric field distribution across a GaN-based semiconductor heterointerface.

Nature Nanotechnology, Published online: 20 March 2023; doi:10.1038/s41565-023-01342-1

A semirigid stamp and a standard photolithography mask-aligner enable a reliable and scalable pickup and release process for van der Waals materials integration at the wafer scale.

Nature Nanotechnology, Published online: 20 March 2023; doi:10.1038/s41565-023-01340-3

Multiple alternating layers of two-dimensional materials and epilayers are grown on III–N and III–V substrates in a single growth run. Then, each epilayer is harvested by mechanical exfoliation, producing multiple freestanding membranes from a single wafer.

Nature Nanotechnology, Published online: 20 March 2023; doi:10.1038/s41565-023-01343-0

By integrating split ferroelectric capacitors with complementary characteristics in the same memory cell, this architecture tackles the problem of conflicting memory requirements for training and inference, which has long plagued edge intelligence applications.