Jiuxiang Dai

Nature Communications, Published online: 10 June 2023; doi:10.1038/s41467-023-38556-9

In this Comment, the authors discuss the current status, the challenges, and potential technological impact of exciton transport in transition metal dichalcogenide (TMD) monolayers, lateral and vertical heterostructures as well as moiré excitons in twisted TMD heterostacks.

Atomically thin piezoelectrics suffer from weak piezoresponse along the thickness, which largely hinders their applications in a vertical crossbar architecture. Therefore, exploring new types of ultrathin materials with strong longitudinal piezoelectricity is highly desired. In this study, the monolayer CuInP2S6 is successfully exfoliated and confirmed to possessstrong longitudinal piezoelectricity with an effective d33eff of ≈5.12 pm V⁻¹, which is one or two orders of magnitude higher than that of most monolayer materials with intrinsic d33.

Abstract

Most atomically thin piezoelectrics suffer from weak piezoelectric response or current rectification along the thickness direction, which largely hinders their applications in a vertical crossbar architecture. Therefore, exploring new types of ultrathin materials with strong longitudinal piezoelectric coefficient and rectification is highly desired. In this study, the monolayer of van der Waals CuInP2S6 (CIPS) is successfully exfoliated and its strong piezoelectricity in the out-of-plane direction with an effective coefficient d33eff of ≈5.12 pm V⁻¹, which is one or two orders of magnitude higher than that of most existing monolayer materials with intrinsic d33, is confirmed. A prototype vertical device is further constructed and the current rectification is achieved through the flexoelectricity induced by the scanning tip force. The switching between low and high rectification states can be readily controlled by tuning the mechanical loads. These findings manifest that CIPS possesses promising application in vertical nanoscale piezoelectric devices and provides a novel strategy for achieving a good current rectification in ultrathin piezoelectrics.

A microfabrication approach using synthesized high-resolution and environmentally friendly photoresists is demonstrated. This approach allows the transfer of photoresists onto various substrates to enable reliable conformal manufacturing of thin-film flexible electronics. Various exemplary demonstrations suggest the potential of this technology for many emerging applications, including flexible electronics, smart devices, bioelectronics, as well as human-machine interfaces.

Abstract

Photolithographic techniques, which are widely used in the silicon-based semiconductor industry, enable the manufacture of high-yield and high-resolution features at the micrometer and nanometer scales. However, conventional photolithographic processes cannot accommodate the micro/nanofabrication of flexible and stretchable electronics. In this study, a microfabrication approach that uses a synthesized, environmentally friendly, and dry-transferable photoresist to enable the reliable conformal manufacturing of thin-film electronics is reported, which is also compatible with the existing cleanroom processes. Photoresists with high-resolution, high-density, and multiscale patterns can be transferred onto various substrates in a defect-free and conformal-contact manner, thus enabling multiple wafer reuses. Theoretical studies are conducted to investigate the damage-free peel-off mechanism of the proposed approach. The in situ fabrication of various electrical components, including ultralight and ultrathin biopotential electrodes, has been demonstrated, which offer lower interfacial impedance, durability, and stability, and the components are applied to collect electromyography signals with superior signal-to-noise ratio (SNR) and quality. Additionally, an exemplary demonstration of a human-machine interface indicates the potential of these electrodes in many emerging applications, including healthcare, sensing, and artificial intelligence.

Interlayer lattice and polarization coupling are explored for bilayers of in-plane-polarized ferroelectric Pb_1−xSr_xTiO₃. The cross-interface coupling can induce effects analogous to exchange bias in magnetism in the form of shifted hysteresis loops. In turn, such effects can be controlled via film thickness and chemistry to produce exotic effects including multistate polarization, exchange-spring-like function, and both coercivity softening and hardening.

Abstract

Interlayer coupling in materials, such as exchange interactions at the interface between an antiferromagnet and a ferromagnet, can produce exotic phenomena not present in the parent materials. While such interfacial coupling in magnetic systems is widely studied, there is considerably less work on analogous electric counterparts (i.e., akin to electric “exchange-bias-like” or “exchange-spring-like” interactions between two polar materials) despite the likelihood that such effects can also engender new features associated with anisotropic electric dipole alignment. Here, electric analogs of such exchange interactions are reported, and their physical origins are explained for bilayers of in-plane polarized Pb_1−xSr_xTiO₃ ferroelectrics. Variation of the strontium content and thickness of the layers provides for deterministic control over the switching properties of the bilayer system resulting in phenomena analogous to an exchange-spring interaction and, leveraging added control of these interactions with an electric field, the ability to realize multistate-memory function. Such observations not only hold technological promise for ferroelectrics and multiferroics but also extend the similarities between ferromagnetic and ferroelectric materials to include the manifestation of exchange-interaction-like phenomena.

The interface between 2D materials and soft, stretchable polymeric substrates is dominated by weak van der Waals forces. Under mechanical loading, slippage, and decoupling of the 2D material on the polymer is observed, which leads to extensive damage propagation in the 2D lattice. Here, graphene is functionalized through mild introduction of defects for an increase in adhesion at the graphene–polymer interface. Under in situ cyclic loading, the increased adhesion inhibits damage initiation and interfacial fatigue propagation within graphene.

Abstract

The interface between two-dimensional (2D) materials and soft, stretchable polymeric substrates is a governing criterion in proposed 2D materials-based flexible devices. This interface is dominated by weak van der Waals forces and there is a large mismatch in elastic constants between the contact materials. Under dynamic loading, slippage, and decoupling of the 2D material is observed, which then leads to extensive damage propagation in the 2D lattice. Herein, graphene is functionalized through mild and controlled defect engineering for a fivefold increase in adhesion at the graphene-polymer interface. Adhesion is characterized experimentally using buckling-based metrology, while molecular dynamics simulations reveal the role of individual defects in the context of adhesion. Under in situ cyclic loading, the increased adhesion inhibits damage initiation and interfacial fatigue propagation within graphene. This work offers insight into achieving dynamically reliable and robust 2D material-polymer contacts, which can facilitate the development of 2D materials-based flexible devices.

A convenient, reliable method for forming an ideal vdW metal contact and eliminating Fermi level pinning and contact resistance in 2D semiconductors is required to develop potential applications of 2D materials in various high-performance devices. The methods for forming metal contacts on 2D semiconductors is explored, and the method that produced the most stable and ideal vdW contact.

Abstract

High-performing 2D electrical and optical devices can be realized by forming an ideal van der Waals (vdW) metal contact with weak interactions and stable interface states. However, the methods for applying metal contacts while avoiding damage from metal deposition present challenges in realizing a uniform, stable vdW interface. To overcome this problem, this study develops a method for forming vdW contacts using a sacrificial Se buffer layer. This study explores this method by investigating the difference in the Schottky barrier height between the vdW metal contact deposited using a buffer layer, a transferred metal contact, and a conventional directly deposited metal contact using rectification and photovoltaic characteristics of a Schottky diode structure with graphite. Evidently, the Se buffer layer method forms the most stable and ideal vdW contact while preventing Fermi-level pinning. A tungsten diselenide Schottky diode fabricated using these vdW contacts with Au and graphite as the top and bottom electrodes, respectively, exhibits excellent operation with an ideality factor of ≈1, an on/off ratio of > 10⁷, and coherent properties. Additionally, when using only the vdW Au contact, the electrical and optical properties of the device can be minutely modulated by changing the structure of the Schottky diode.

Perforated Carbon Nanotube Films

In article number 2300766, Run Shi, Kai Liu, and co-workers report a large-scale growth of uniform monolayer MoS₂ by evenly distributing gas flows of precursors through a perforated carbon nanotube film. The as-grown monolayer MoS₂ shows quite good uniformity in geometry, density, structure, and electrical properties.

This work systematically investigates the working mechanisms of molecules as van der Waals (vdW) dielectrics, and thus reveals two important new insights. An important advantage of molecules-based vdW dielectrics over conventional dielectric materials is that defects hardly affect their insulating properties. As₂O₃ is an unprecedentedly competitive candidate as a vdW dielectric for 2D vdW semiconductors-based complementary metal-oxide-semiconductor applications.

Abstract

Sb₂O₃ molecules offer unprecedented opportunities for the integration of a van der Waals (vdW) dielectric and a 2D vdW semiconductor. However, the working mechanisms underlying molecules-based vdW dielectrics remain unclear. Here, the working mechanisms of Sb₂O₃ and two Sb₂O₃-like molecules (As₂O₃ and Bi₂O₃) as dielectrics are systematically investigated by combining first-principles calculations and gate leakage current theories. It is revealed that molecules-based vdW dielectrics have a considerable advantage over conventional dielectric materials: defects hardly affect their insulating properties. This shows that it is unnecessary to synthesize high-quality crystals in practical applications, which has been a long-standing challenge for conventional dielectric materials. Further analysis reveals that a large thermionic-emission current renders Sb₂O₃ difficult to simultaneously satisfy the requirements of dielectric layers in p-MOS and n-MOS, which hinders its application for complementary metal-oxide-semiconductor (CMOS) devices. Remarkably, it is found that As₂O₃ can serve as a dielectric for both p-MOS and n-MOS. This work not only lays a theoretical foundation for the application of molecules-based vdW dielectrics, but also offers an unprecedentedly competitive dielectric (i.e., As₂O₃) for 2D vdW semiconductors-based CMOS devices, thus having profound implications for future semiconductor industry.

The Fe intercalation on the interstitial sites in high Curie temperature (T_c) Fe₃GeTe₂ may be responsible for the local antiferromagnetic coupling that gives rise to the exchange bias effect, and that the interlayer exchange paths greatly contribute to the enhancement of T _c.

Abstract

Fe₃GeTe₂ have proven to be of greatly intrigue. However, the underlying mechanism behind the varying Curie temperature (T _c) values remains a puzzle. This study explores the atomic structure of Fe₃GeTe₂ crystals exhibiting T _c values of 160, 210, and 230 K. The elemental mapping reveals a Fe-intercalation on the interstitial sites within the van der Waals gap of the high-T _c (210 and 230 K) samples, which are observed to have an exchange bias effect by electrical transport measurements, while Fe intercalation or the bias effect is absent in the low-T _c (160 K) samples. First-principles calculations further suggest that the Fe-intercalation layer may be responsible for the local antiferromagnetic coupling that gives rise to the exchange bias effect, and that the interlayer exchange paths greatly contribute to the enhancement of T _c. This discovery of the Fe-intercalation layer elucidates the mechanism behind the hidden antiferromagnetic ordering that underlies the enhancement of T _c in Fe₃GeTe₂.

In this study, the multiscale mechanical degradation of graphene assemblies is characterized through practical microstructure-guided multiscale modeling. Three representative models are developed to study the mechanical behaviors of graphene assemblies at different lengthscales. The structure-strength relation of graphene assemblies is given, and practical strategies are proposed followed by experimental realization, to significantly improve the mechanical properties of graphene-based nanocomposites.

Abstract

Exploring and understanding the structure-mechanical property relations in hierarchical graphene assemblies is crucial for optimizing their mechanical properties and developing new functionalities, as the tensile strength is two orders of magnitude degradation from pristine graphene to graphene assemblies. Yet, quantifying the strength degradation across multiscale is a challenge due to the complex hierarchical structures. Thus, key structures and dominated factors at different lengthscales that affect the mechanical properties of graphene assemblies should be extracted for the reasonable unveiling of this problem. In this study, the multiscale mechanical degradation of graphene assemblies through practical microstructure-guided multiscale modeling is characterized. Combining with experimental observations, three representative models are developed to study the mechanical behaviors of graphene assemblies at different lengthscales. Then, the dominated factors affecting the strength at these lengthscales are identified, that is the defects in monolayer graphene, tension-shear load transfer for stacked graphene, and uniformity of graphene assemblies. Based on the simulation results, the structure-strength relation of graphene assemblies is given, and practical strategies are proposed followed by experimental realization, to significantly improve the mechanical properties of graphene-based nanocomposites.

A facile one-step PMMA-encapsulated electrode transfer method applicable for air-sensitive 2D materials is developed, showing great advantages of damage-free electrode patterning process and in situ PMMA protection from air exposure. With this method, the intrinsic electrical properties of highly ambient-sensitive SmTe_{2 a} nanosheets grown by chemical vapor deposition can be readily investigated, showing ultralow contact resistance and high signal/noise ratio.

Abstract

Owing to rapid property degradation after ambient exposure and incompatibility with conventional device fabrication process, electrical transport measurements on air-sensitive 2D materials have always been a big issue. Here, for the first time, a facile one-step polymer-encapsulated electrode transfer (PEET) method applicable for fragile 2D materials is developed, which showed great advantages of damage-free electrodes patterning and in situ polymer encapsulation preventing from H₂O/O₂ exposure during the whole electrical measurements process. The ultrathin SmTe₂ metals grown by chemical vapor deposition (CVD) are chosen as the prototypical air-sensitive 2D crystals for their poor air-stability, which will become highly insulating when fabricated by conventional lithographic techniques. Nevertheless, the intrinsic electrical properties of CVD-grown SmTe₂ nanosheets can be readily investigated by the PEET method instead, showing ultralow contact resistance and high signal/noise ratio. The PEET method can be applicable to other fragile ultrathin magnetic materials, such as (Mn,Cr)Te, to investigate their intrinsic electrical/magnetic properties.

Abstract

As one of the most promising materials for two-dimensional transition metal chalcogenides (2D TMDs), molybdenum diselenide (MoSe₂) has great potential in photodetectors due to its excellent properties like tunable bandgap, high carrier mobility, and excellent air stability. Although 2D MoSe₂-based photodetectors have been reported to exhibit admired performance, the large-area 2D MoSe₂ layers are difficult to be achieved via conventional synthesis methods, which severely impedes its future applications. Here, we present the controllable growth of large-area 2D MoSe₂ layers over 3.5-inch with excellent homogeneity by a simple post-selenization route. Further, a high-quality n-MoSe₂/p-Si van der Waals (vdW) heterojunction device is in-situ fabricated by directly growing 2D n-MoSe₂ layers on the patterned p-Si substrate, which shows a self-driven broadband photoresponse ranging from ultraviolet to mid-wave infrared with an impressive responsivity of 720.5 mA·W⁻¹, a high specific detectivity of 10¹³ Jones, and a fast response time to follow nanosecond pulsed optical signal. In addition, thanks to the inch-level 2D MoSe₂ layers, a 4 × 4 integrated heterojunction device array is achieved, which has demonstrated good uniformity and satisfying imaging capability. The large-area 2D MoSe₂ layer and its heterojunction device array have great promise for high-performance photodetection and imaging applications in integrated optoelectronic systems.

Abstract

Understanding and control of many-body collective phenomena such as charge density wave (CDW) and superconductivity in atomically thin crystals remains a hot topic in material science. Here, using first-principles calculations, we find that 1T-HfTe₂ possessing no CDWs in the bulk form, unexpectedly shows a stable 2 × 2 CDW order in the monolayer form, which can be attributed to the enhancement of electron–phonon coupling (EPC) in the monolayer. Meanwhile, the CDW induces a metal-to-insulator transition in monolayer 1T-HfTe₂ through the accompanying lattice distortion. Remarkably, Ising superconductivity with a significantly enhanced in-plane critical field can emerge in centrosymmetric monolayer 1T-HfTe₂ after the CDW is suppressed by electron doping. The Ising paring is revealed to be protected by the spin–orbital locking without the participation of the inversion symmetry breaking which is a must for conventional 2H-NbSe₂-like Ising superconductors. Our results open a new window for designing and controlling novel quantum states in two-dimensional (2D) matter.

Nature Synthesis, Published online: 08 June 2023; doi:10.1038/s44160-023-00342-2

Nearly all phosphorus-containing chemicals are prepared from phosphate ores via hazardous, energy-intensive, multi-step procedures that feature toxic, pyrophoric white phosphorus (P4) as an intermediate. Using a different approach, many of these products can be prepared from phosphates in two simple steps free from P4.

Nature Electronics, Published online: 08 June 2023; doi:10.1038/s41928-023-00971-7

Arrays of thin-film transistors can be fabricated on the 5-inch wafer scale using solution-based processing of molybdenum disulfide and sodium-embedded alumina inks for the semiconductor and gate dielectric, respectively, yielding devices with room-temperature mobilities of up to 80 cm2 V−1 s−1.

Real-time switching studies in wurtzite ferroelectrics reveal that an antipolar metastable state mediates polarization inversion.

Abstract

Recently, great efforts have been made to explore high-performance microwave absorbing (MA) materials, which aim to reduce the booming electromagnetic wave (EMW) pollution caused by electronic devices. MXenes (i.e., two-dimensional (2D) transition metal carbide/nitride), an emerging family of 2D nanomaterials, demonstrate superiority to conventional nanomaterials, such as tunable electrical conductivity, unique layer structure, abundant terminal groups, and great physico-chemical properties, becoming potential candidate materials for high-efficiency MA. By hybridizing MXenes with other lossy mediums and designing novel microstructures, MA properties can be further optimized and enhanced for practical applications. Furthermore, MXene-based materials, with smartly designed multi-functionality, become a research priority to develop new or advanced applications for the coming “intelligent era”. This review focuses on using MXenes to effectively absorb EMW energy. In the first part, the fabricating methods of MXenes are systematically introduced followed by a second part on the progress of MXene-based materials regarding the MA performances, as well as a summarization on their next-generation smart multifunctional materials. In the final part, opinions on the future perspectives and opportunities of MXene-based MA materials are presented. It is believed that this review will pave the way for further flourishing the development of MXene-based materials in MA field.