Jing Zhang

Nature Materials, Published online: 17 April 2023; doi:10.1038/s41563-023-01521-4

The authors demonstrate electrical on/off switching of interlayer interactions in tungsten diselenide/molybdenum disulfide heterobilayers, the phase diagram of which contains layer-dependent correlated regions that reveal the role of strong correlations in interlayer exciton dynamics.

Two types of spherical MoSe₂ 2H/1T hybrid nanoparticles are formed by ablating MoSe₂ powder in isopropyl alcohol with a femtosecond laser: onion-structured nanoparticles and polycrystalline nanoparticles. Different formation mechanisms are identified for the two types of nanoparticles and the spherical nanoparticles have high photothermal conversion efficiencies thanks to defects and structural disorder in the nanoparticles.

Abstract

MoSe₂ 2H/1T hybrid nanoparticles are prepared by femtosecond laser ablation of MoSe₂ powder in isopropyl alcohol with different laser powers and ablation times, and their formation mechanisms and photothermal conversion efficiencies (PTCEs) are studied. Two types of spherical nanoparticles are observed. The first type is onion-structured nanoparticles that are formed by nucleation on the surfaces of melted droplets followed by inward growth of {002} planes of MoSe₂. The second type is polycrystalline nanoparticles, formed by coalescence of crystalline nanoclusters fragmented from the powder during the laser ablation. The nanoparticle size in all samples shows a bimodal distribution, corresponding to different fragmentation mechanisms. The 2H-to-1T phase transition in the nanoparticles is likely caused by electron doping from the laser-induced plasma. The PTCEs of the nanoparticles increase with laser power and ablation time; the highest PTCE is around 38%. After examining the bandgaps and the Urbach energies of the nanoparticles, it is found that the high PTCEs are primarily attributed to defects and structural disorder in the laser-synthesized nanoparticles, which allow absorption of photons with energies smaller than the bandgap energy and facilitate non-radiative recombination of photoexcited carriers.

A high-performance oxidized MXene piezo-resonator with the single resonance response (3.37 MHz) is exhibited by mixing detecting technology, due to the interior Schottky effect. It avoids the dispute of indistinguishable vibratal states with the repeatable measurement of ultralow resolution (8 × 10⁻¹⁹ g) and other exceptional performances.

Abstract

Nanoelectromechanical systems (NEMS) of 2D nanomaterials are potent exploration devices for high-sensitive mechanical coupling, mass testing, and biosensing. Nevertheless, the internal interference from the multiple resonant states easily causes the deviation and overlap of the target signal. Here, an oxidized-MXene resonant system performs the unique response peak at the fundamental frequency f _0,1 of 3.37 ± 0.04 MHz within the ultrawide frequency up to 400 MHz, due to the ferroelectric-conductive structure. This unique resonant peak can effectively avoid the dispute of indistinguishable vibratal states. The resonator exhibits advanced performances with a large dynamic range of 70.41 ± 0.15 dB and low thermomechanical motion spectral density of 669 fmHz$669\;\frac{{{\rm{fm}}}}{{\sqrt {{\rm{Hz}}} }}$. The molecular sensing mechanisms of the oxidized-MXene system are systematically studied to achieve repeatable detection with high mass resolution (low to 8.00 ± 0.01 × 10^-19 g). These consequences can afford potential guidelines for the NEMS devices in terms of the credible and legible sensors for ultra-accurate and interference-free measurements.

Cell-surface adhesion strength is modulated with low (∼nA) power draw by applying low voltage (<1 V) across non-conductive, dielectrics of nanometric thickness. Adhesion strength, measured via microfluidic shear experiments, can be increased or decreased depending on the sign and magnitude of the applied voltage. Long term, 4 week cell patterning experiments demonstrate the effect is robust.

Abstract

Controlling cell adhesion to surfaces is an important, but difficult, problem. Current methods to control adhesion rely on surface functionalization, which have limited material choice to avoid cell toxicity and are typically cell specific. Herein, cell adhesion is modulated by using nanometric high-k dielectric films. Voltage is applied across the dielectric film, changing the film surface's zeta potential, ζ. High performance dielectrics, HfO₂ and SiO₂, enables a change in the ζ polarity and magnitude over large, 100 mV, ranges by applying ≈1 V across the dielectrics with ≈1nW power draw. Freshwater Chlorella vulgaris and saltwater Nannochloropsis oculata, which have a negative ζ, are used as model cells. Cell adhesion is observed to be inhibited when both surface and cell ζ are negative and enhanced when surface ζ is positive and cell ζ are negative using microfluidic experiments. Finally, millimetric scale cell patterning is demonstrated by spatially modulating ζ with no observed toxicity to cells over 4 weeks.

Statistical electrical measurements of 2D memristors show the extraordinary improvement of yield and endurance by optimizing the metal deposition and MoS₂ condition. The intriguing convergence of switching voltages and resistance ratio is revealed. An “effective switching layer” model compatible with both monolayer and few-layer MoS₂, is proposed to understand the reliability improvement and the convergence of switching metrics.

Abstract

2D memristors have demonstrated attractive resistive switching characteristics recently but also suffer from the reliability issue, which limits practical applications. Previous efforts on 2D memristors have primarily focused on exploring new material systems, while damage from the metallization step remains a practical concern for the reliability of 2D memristors. Here, the impact of metallization conditions and the thickness of MoS₂ films on the reliability and other device metrics of MoS₂-based memristors is carefully studied. The statistical electrical measurements show that the reliability can be improved to 92% for yield and improved by ≈16× for average DC cycling endurance in the devices by reducing the top electrode (TE) deposition rate and increasing the thickness of MoS₂ films. Intriguing convergence of switching voltages and resistance ratio is revealed by the statistical analysis of experimental switching cycles. An “effective switching layer” model compatible with both monolayer and few-layer MoS₂, is proposed to understand the reliability improvement related to the optimization of fabrication configuration and the convergence of switching metrics. The Monte Carlo simulations help illustrate the underlying physics of endurance failure associated with cluster formation and provide additional insight into endurance improvement with device fabrication optimization.

The formation of clean and defect-free van der Waals stackings at the Sb–PdSe₂ heterointerfaces boosts the transport characteristics of few-layer PdSe₂ field-effect transistors, including low contact resistance down to 0.55 kΩ µm, high on-current density reaching 96 µA µm⁻¹, and high electron mobility of 383 cm² V⁻¹ s⁻¹ at room temperature and 2,184 cm² V⁻¹ s⁻¹ at 10 K.

Abstract

Even though atomically thin 2D semiconductors have shown great potential for next-generation electronics, the low carrier mobility caused by poor metal–semiconductor contacts and the inherently high density of impurity scatterings remains a critical issue. Herein, high-mobility field-effect transistors (FETs) by introducing few-layer PdSe₂ flakes as channels is achieved, via directly depositing semimetal antimony (Sb) as drain–source electrodes. The formation of clean and defect-free van der Waals (vdW) stackings at the Sb–PdSe₂ heterointerfaces boosts the room temperature transport characteristics, including low contact resistance down to 0.55 kΩ µm, high on-current density reaching 96 µA µm⁻¹, and high electron mobility of 383 cm² V⁻¹ s⁻¹. Furthermore, metal–insulator transition (MIT) is observed in the PdSe₂ FETs with and without hexagonal boron nitride (h–BN) as buffer layers. However, the layered h–BN/PdSe₂ vdW stacking eliminates the interference of interfacial disorders, and thus the corresponding device exhibits a lower MIT crossing point, larger mobility exponent of γ ∼ 1.73, significantly decreased hopping parameter of T ₀, and ultrahigh electron mobility of 2,184 cm² V⁻¹ s⁻¹ at 10 K. These findings are expected to be significant for developing high mobility 2D-based quantum devices.

Full recyclability, room-temperature healability, and multimodal rapid actuation capabilities (e.g., bending, twisting, folding/deploying, and rolling locomotion) are effectively integrated into a single magnetic soft robot. Full recyclability allows soft robots to be recycled as raw materials at the end of their life and on-demand reprocessed into new robots with reprogrammable geometry and functions for further diverse applications.

Abstract

Magnetic soft robots capable of wirelessly controlled programmable deformation and locomotion are desirable for diverse applications. Such multi-variable actuation ideally requires a polymer matrix with a well-defined range of softness and stretchability (Young's modulus of 0.1–10 MPa, high stretchability >200%). However, this defined mechanical range excludes most polymer candidates, leaving only a limited number of available polymers (e.g., PDMS, Ecoflex) with covalently cross-linked networks that may lead to non-recyclable robots and further potential threats to environment. Herein, based on the synergistic effects of reduced cross-linking density and intermolecular hydrogen bonding, a dynamic covalent polyimine is newly designed as polymer matrix and magnetic microparticles as fillers, and integrate defined softness and stretchability, full chemical recyclability, rapid room-temperature healability and multimodal actuation into a single magnetic soft robot. The polyimine is soft and stretchable enough to process soft robots in various geometries by simple laser cutting, without the need to pre-design the geometry to suit target scenarios. Through a cyclic depolymerization/repolymerization, this full recycling restores 100% of the robots’ mechanical properties and rapid deformability/mobility to their original level within seconds and heals quickly within minutes when damaged, facilitating ideal cyclic material economy for soft robots in diverse scenarios.

KH560 modification is exploited to successfully prepare a scalable crosslinked polyvinyl-butyral-based solid polymer electrolyte (PVB-SPE) foil with remarkable ionic conductivity and superior mechanical, optical, and thermal performances. The laminated WO₃–NiO device fabricated using the modified PVB-SPE is functional at temperatures ranging from −20 to 80 °C, thereby showing promise for realizing commercially viable large-area electrochromic devices.

Abstract

Polyvinyl butyral (PVB) is a well-established polymer interlayer material that has been used in laminated safety glass panels for over 80 years. However, its intrinsically poor ionic conductivity (σ) severely restricts its widespread application as a solid polymer electrolyte (SPE) for laminated WO₃–NiO electrochromic devices (ECDs). Here, a new strategy for significantly improving the σ of PVB via a cross-linking reaction with 3-glycidoxypropyltrimethoxysilane (KH560) is presented. The cross-linked PVB-SPE with 10 wt.% KH560 exhibits the highest room-temperature σ value among the investigated samples (1.51 × 10⁻⁴ S cm⁻¹), which is also higher than that of previously reported PVB-based SPEs (10⁻⁵–10⁻⁷ S cm⁻¹). Additionally, the prepared SPE exhibits comprehensive optical, mechanical, and thermal performances, including a high visible transmittance (>91%), relatively high adhesive strength (2.13 MPa), and superior thermal stability (up to 150 °C). Laminated WO₃–NiO ECDs with dimensions of 5 × 5 cm² and 20 × 20 cm², fabricated by leveraging the aforementioned properties of the electrolyte, operate stably at temperatures ranging from −20 to 80 °C, underscoring the potential of the PVB-SPE for realizing commercially viable large-area ECDs.

Wafer-scale low-power hetero-CMOS inverters are realized by integrating monolayer MoS₂ and SWCNT networks. An ultralow standby power consumption of ≈5 pW at a reduced supply voltage of 0.25 V, high NMs (>70%), and dynamic analysis in a push-pull configuration are achieved. It paves the way toward the wafer-scale integration of low-dimensional materials for low-power nanoelectronics.

Abstract

Scalable nanoelectronics with energy-efficient logic technology is crucial for next-generation edge devices. Low-dimensional semiconductors, such as transition metal dichalcogenides and single-walled carbon nanotubes (SWCNTs), have tunable properties with reduced short-channel effects. The unique properties of each material can be utilized owing to the heterogeneous integration of multiple semiconducting channels to form complementary metal-oxide-semiconductor (CMOS) logic. However, the integration remains challenging. This study reveals the realization of low static power hetero-CMOS inverters by the integration of n-type monolayer MoS₂ and p-type SWCNT networks. The balanced inverter exhibits a large peak gain of ≈67 at a supply voltage of 2 V with the customized design of the wafer-scale synthetic process and channel integration. An ultralow standby power consumption of ≈5 pW and a practical peak gain of ≈7 at a reduced supply voltage of 0.25 V are achieved. A high noise margin (>70%) validates the circuit's tolerance to external noises and the dynamic analysis of the inverting amplifier in push–pull configuration exhibits a large AC gain. This work paves the way toward the wafer-scale integration of low-dimensional materials for low-power nanoelectronics.

Nature Communications, Published online: 13 April 2023; doi:10.1038/s41467-023-37692-6

Low-frequency quantum oscillations in LaRhIn₅: Dirac point or nodal line?

Nature Photonics, Published online: 10 April 2023; doi:10.1038/s41566-023-01188-y

A stack of longitudinal 2D light sheets provides 3D holographic images with improved depth perception.

Large-area structure-selective synthesis of monolayer and spiral MoSe₂ is demonstrated using a flux-controlled chemical vapor deposition method. Under low, medium, and high MoSe₂ flux conditions, monolayer, spiral, and thick spiral MoSe₂ are synthesized, respectively. Nonlinear optical (NLO) signals of spiral MoSe₂ are enhanced due to its broken inversion symmetry. Additionally, resonance effects with excitons are confirmed through broadband NLO responses.

Abstract

Transition metal dichalcogenides (TMDCs) have various electronic and optical properties depending on their structure, so they can be used as a fascinating material in various applications including photonics, electronics, optoelectronics, and valleytronics. In particular, spiral TMDCs grown through the formation of screw dislocations exhibit novel electronic and optical properties different from layer-by-layer TMDCs. However, large-area structure-selective synthesis of TMDCs remains challenging. Here, this work reports for the first time the large-area structure-selective synthesis of monolayer MoSe₂ and spiral MoSe₂ using a flux-controlled chemical vapor deposition method. Under a low MoSe₂ flux condition, monolayer MoSe₂ is synthesized, whereas thick spiral MoSe₂ is synthesized under a high flux condition. Under a medium flux condition, both monolayer and spiral MoSe₂ are synthesized. In addition, through the nonlinear optical (NLO) signal analysis of monolayer MoSe₂ and spiral MoSe₂, the giant enhancement of NLO signals induced by the combined effect of breaking inversion symmetry and the excitonic resonance effects in the synthesized MoSe₂ is confirmed. Monolayer MoSe₂ and spiral MoSe₂ synthesized using this method are expected to be used as advanced optical materials for novel electronics, optoelectronics, and NLO applications.

The success of quantum technologies relies on the capability of producing efficient sources of single photons. 2D materials offer the unique opportunity of having such sources on an atomically thin surface from which photons can be extracted very efficiently. It is shown that building micrometric domes of 2D materials provides spatially ordered and scalable arrays of quantum emitters.

Abstract

Monolayers (MLs) of transition-metal dichalcogenides host efficient single-photon emitters (SPEs) usually associated to the presence of nanoscale mechanical deformations or strain. Large-scale spatial control of strain would enhance the scalability of such SPEs and allow for their incorporation into photonic structures. Here, the formation of regular arrays of strained hydrogen-filled one-layer-thick micro-domes obtained by H-ion irradiation and lithography-based approaches is reported. Typically, the H₂ liquefaction for temperatures T<32 K causes the disappearance of the domes preventing their use as potential SPEs. Here, it is shown that the dome deflation can be overcome by hBN heterostructuring, that is by depositing thin hBN flakes on the domes. This leads to the preservation of the dome structure at all temperatures, as found by micro-Raman and micro-photoluminescence (µ-PL) studies. Eventually, spatially controlled hBN-capped WS₂ domes show the appearance, at 5 K, of intense emission lines originating from localized excitons, which are shown to behave as quantum emitters here. The electronic properties of the emitters are addressed by time-resolved µ-PL yielding time decays of 1–10 ns, and by magneto-µ-PL measurements. The latter provide an exciton magnetic moment a factor of two larger than the value observed in planar strain-free MLs.

A strategy of graphene-enhanced integration for realizing strong van der Waals contacts between a variety of 2D semiconductors and metals with atomically flat and ultra-clean interface for high-performance 2D electronics.

Abstract

2D semiconductors have shown great potentials for ultra-short channel field-effect transistors (FETs) in next-generation electronics. However, because of intractable surface states and interface barriers, it is challenging to realize high-quality contacts with low contact resistances for both p- and n- 2D FETs. Here, a graphene-enhanced van der Waals (vdWs) integration approach is demonstrated, which is a multi-scale (nanometer to centimeter scale) and reliable (≈100% yield) metal transfer strategy applicable to various metals and 2D semiconductors. Scanning transmission electron microscopy imaging shows that 2D/2D/3D semiconductor/graphene/metal interfaces are atomically flat, ultraclean, and defect-free. First principles calculations indicate that the sandwiched graphene monolayer can eliminate gap states induced by 3D metals in 2D semiconductors. Through this approach, Schottky barrier-free contacts are realized on both p- and n-type 2D FETs, achieving p-type MoTe₂, p-type black phosphorus and n-type MoS₂ FETs with on-state current densities of 404, 1520, and 761 µA µm⁻¹, respectively, which are among the highest values reported in literature.

Antisolvent engineering is proposed to realize high-quality dominantly oriented perovskite film by the antisolvent of isopropyl alcohol (IPA). The interaction between IPA and PbI₂ leads to the direct crystallization of (111)-α-FAPbI₃ at room temperature, sidestepping the intermediates of PbI₂•DMSO, FA₂Pb₃I₈•4DMSO, and δ-FAPbI₃. Solar cells based on (111)-α-FAPbI₃ demonstrate improved performance compared to the randomly oriented perovskite films treated by other antisolvents.

Abstract

Fabricating perovskite films with a dominant crystal orientation is an effective path to realizing quasi-single-crystal perovskite film, which can eliminate the fluctuation of the electrical properties in films arising from grain-to-grain variations, and improve the performance of perovskite solar cells (PSCs). Perovskite (FAPbI₃) films based on one-step antisolvent methods usually suffer from chaotic orientations due to the inevitable intermediate phase conversion from intermediates of PbI₂•DMSO, FA₂Pb₃I₈•4DMSO, and δ-FAPbI₃ to α-FAPbI₃. Here, a high-quality perovskite film with (111) preferred orientation ((111)-α-FAPbI₃) using a short-chain isomeric alcohol antisolvent, isopropanol (IPA) or isobutanol (IBA), is reported. The interaction between IPA and PbI₂ leads to a corner-sharing structure instead of an edge-sharing PbI₂ octahedron, sidestepping the formation of these intermediates. With the volatilization of IPA, FA⁺ can replace IPA in situ to form α-FAPbI₃ along the (111) direction. Compared to randomly orientated perovskites, the dominantly (111) orientated perovskite ((111)-perovskite) exhibits improved carrier mobility, uniform surface potential, suppressed film defects and enhanced photostability. PSCs based on the (111)-perovskite films show 22% power conversion efficiency and excellent stability, which remains unchanged after 600 h continuous working at maximum power point, and 95% after 2000 h of storage in atmosphere environment.

Across-layer sliding ferroelectricity that arises from the asymmetry of next-neighbor interlayer couplings is proposed, where the vertical polarizations of intercalated centro-symmetric 2D materials like graphene bilayer can be switched via multilayer sliding, which may exist in a series of other heterolayers, including molecular systems. It may clarify a previously unexplained phenomenon and be utilized for high-density data storage.

Abstract

Although the monolayers of most 2D materials are non-ferroelectric with highly symmetric lattices, symmetry breaking may take place in their bilayers upon some stacking configuration, giving rise to so-called sliding ferroelectricity where the vertical polarizations can be electrically reversed via interlayer translation. However, it is not supposed to appear in systems like graphene bilayer with centro-symmetry at any stacking configuration, and the origin of the recently reported ferroelectricity (Nature 2020, 588, 71) in graphene bilayer intercalated between h-BN remains mysterious. Here, a type of across-layer sliding ferroelectricity that arises from the asymmetry of next-neighbor interlayer couplings is proposed. The first-principles evidence is shown that the vertical polarizations in intercalated centro-symmetric 2D materials like graphene bilayer can be switched via multilayer sliding, which is likely to be the origin of the observed ferroelectric hysteresis. Moreover, such ferroelectricity may exist in a series of other heterolayers with quasi-degenerate polar states, like graphene bilayer or trilayer on BN substrate, or even with a molecule layer on surface where each molecule can store 1-bit data independently, resolving the bottleneck issue of sliding ferroelectricity for high-density data storage.

An interface-engineering causing paramagnetic-to-ferromagnetic transition for CaRuO₃ sublayers in the CaRuO₃/SrTiO₃ superlattices is demonstrated. The strong symmetry-mismatch between CaRuO₃ and SrTiO₃ produces a large tuning to the coupled octahedra at heterointerfaces, resulting in distinct interfacial ferromagnetic phases. This is the first report that interfacial ferromagnetism stems exclusively from tilting/rotating oxygen octahedra in a paramagnetic oxide.

Abstract

By modifying the entangled multi-degrees of freedom of transition-metal oxides, interlayer coupling usually produces interfacial phases with unusual functionalities. Herein, a symmetry-mismatch-driven interfacial phase transition from paramagnetic to ferromagnetic state is reported. By constructing superlattices using CaRuO₃ and SrTiO₃, two oxides with different oxygen octahedron networks, the tilting/rotation of oxygen octahedra near interface is tuned dramatically, causing an angle increase from ≈150° to ≈165° for the RuORu bond. This in turn drives the interfacial layer of CaRuO₃, ≈3 unit cells in thickness, from paramagnetic into ferromagnetic state. The ferromagnetic order is robust, showing the highest Curie temperature of ≈120 K and the largest saturation magnetization of ≈0.7 µ _B per formula unit. Density functional theory calculations show that the reduced tilting/rotation of RuO₆ octahedra favors an itinerant ferromagnetic ground state. This work demonstrates an effective phase tuning by coupled octahedral rotations, offering a new approach to explore emergent materials with desired functionalities.