Jiuxiang Dai

This perspective presents experimental results and theoretical predictions of surface functionalization of 2D materials to induce magnetism. Surface functionalization using molecules with tailored magnetic properties is an exciting technique, as it makes available the whole spectrum of 2D materials as magnetic systems, which are currently limited to a small subset of 2D materials that are intrinsically magnetic.

Abstract

Surface functionalization with organic molecules has proven effective to modulate the electronic properties of 2D materials. More recently, surface functionalization has been shown to induce magnetic behaviors in nonmagnetic 2D materials. If magnetism can be imparted to 2D materials via surface functionalization by molecules with tailored magnetic properties, it would make available the whole spectrum of 2D materials as magnetic systems, which are currently limited to a small subset of 2D materials that are intrinsically magnetic. This perspective will present experimental results and theoretical predictions of surface functionalization of 2D materials to induce magnetism. Key experimental studies on graphene, layered topological insulators, and transition metal dichalcogenides functionalized with single-molecule magnets, adatoms, and nonmagnetic molecules will be discussed to advocate for utilizing surface functionalization to induce magnetism in 2D materials. The modulation of magnetic properties of 2D materials via surface functionalization is still in its infancy, presenting exciting opportunities for new ideas and discoveries. The perspective will conclude with the future outlook and challenges for this topic.

Zr(HPO₄)₂ nanosheets are used for the first time as nanofillers for dielectric energy storage composites. The incorporation of merely 1.6 wt% nanosheets into the composite results in remarkably enhanced energy density of 11.22 J cm⁻³ at 650 kV mm⁻¹, which is about 165% that of the composite without nanosheets. Meanwhile, a high discharge efficiency of 89.8% is maintained.

Abstract

Dielectric film capacitors have aroused considerable attention on account of the fast development of pulsed power systems. However, enhanced energy density is always acquired at the cost of deteriorated charge/discharge efficiency. Herein, well balanced energy density and efficiency are achieved in a series of reasonably designed bilayer composites consisting of a ferroelectric layer and a paraelectric layer at the meantime. It is interesting to find that, when merely 1.6 wt% Zr(HPO₄)₂ nanosheets are introduced into the ferroelectric layer, a substantially improved energy density of 11.22 J cm⁻³, which is about 165% that of the bilayer composite without Zr(HPO₄)₂ nanosheets, is achieved at 650 kV mm⁻¹. Meanwhile, a high charge/discharge efficiency of 89.8% and a low loss tangent of 0.024@10 kHz which is much lower than the pristine ferroelectric polymer layer (0.058@10 kHz) is maintained. Furthermore, finite element simulation reveals that the electric breakdown paths will develop along the macroscopical-interfaces between adjoining layers and the microcosmic-interfaces between the Zr(HPO₄)₂ nanosheets and polymer matrix, which can effectively increase the length of breakdown paths and contribute to improved breakdown strength. This work demonstrates that the Zr(HPO₄)₂ nanosheets can be promising fillers for other high-performance dielectric composites.

Thermoelectric conversion has been extensively studied in low-dimensional materials where quantum confinement and spin textures can largely modulate thermopower generation. In this review, the thermoelectric microscopy-based quantum sensing of thermopower from classical, spin-polarized electron transport, collective spin dynamics, and relativistic effect is summarized. Investigating the microscopic nature of thermopower in quantum materials provide insights for the future thermoelectric applications.

Abstract

Thermoelectric power, has been extensively studied in low-dimensional materials where quantum confinement and spin textures can largely modulate thermopower generation. In addition to classical and macroscopic values, thermopower also varies locally over a wide range of length scales, and is fundamentally linked to electron wave functions and phonon propagation. Various experimental methods for the quantum sensing of localized thermopower have been suggested, particularly based on scanning probe microscopy. Here, critical advances in the quantum sensing of thermopower are introduced, from the atomic to the several-hundred-nanometer scales, including the unique role of low-dimensionality, defects, spins, and relativistic effects for optimized power generation. Investigating the microscopic nature of thermopower in quantum materials can provide insights useful for the design of advanced materials for future thermoelectric applications. Quantum sensing techniques for thermopower can pave the way to practical and novel energy devices for a sustainable society.

Near-infrared photodetectors based on a CH₃NH₃PbI₃ quantum dots (MQDs)/Ta₂NiSe₅ mixed-dimensional van der Waals heterostructure (MvdWHs) with a responsivity of 2.4 × 10² A W⁻¹ and a detectivity of 6.0 × 10² Jones is reported, indicating that the optoelectrical properties of two-dimensional ternary Ta₂NiSe₅ can be significantly optimized via hybridization with MQDs and will accelerate the evolution of MvdWHs-based devices.

Abstract

2D materials and their van der Waals heterostructures are of great fundamental interest and have potential applications in emerging electronics devices. However, atomically thin photodetector applications have suffered from performance limitations because of low optical absorption. Here, a near-infrared photodetector based on methylammonium lead halide perovskite quantum dots (MQDs), combined with Ta₂NiSe₅ van der Waals heterostructures, is fabricated to take advantage of their energy band alignment. An ultrasensitive photoresponse with a detectivity of 6.0 × 10¹² Jones for 800 nm illumination is demonstrated. Furthermore, the field-effect mobility and responsivity are enhanced by factors of 2 and 7.4, respectively, by interfacing Ta₂NiSe₅ and the MQDs. These results indicate that the optoelectrical properties of 2D ternary Ta₂NiSe₅ can be significantly optimized via hybridization with MQDs and can accelerate the evolution of MQD-van der Waals heterostructure-based devices.

Nature Electronics, Published online: 10 December 2021; doi:10.1038/s41928-021-00699-2

Elaborately designed WSe₂ flakelets decorated on the N-doped graphene (WSe₂/NG) with an abundant active site can not only act as a redox accelerator to promote the bidirectional conversion of lithium polysulfides (LiPSs) and significantly alleviate the shuttling effect, but also as a regulator for enabling uniform Li plating/stripping to mitigate the growth of Li dendrite.

Abstract

The practical application of lithium–sulfur batteries is still limited by the lithium polysulfides (LiPSs) shuttling effect on the S cathode and uncontrollable Li-dendrite growth on the Li anode. Herein, elaborately designed WSe₂ flakelets immobilized on N-doped graphene (WSe₂/NG) with abundant active sites are employed to be a dual-functional host for satisfying both the S cathode and Li anode synchronously. On the S cathode, the WSe₂/NG with a strong interaction towards LiPSs can act as a redox accelerator to promote the bidirectional conversion of LiPSs. On the Li anode, the WSe₂/NG with excellent lithiophilic features can regulate the uniform Li plating/stripping to mitigate the growth of Li dendrite. Taking advantage of these merits, the assembled Li−S full batteries exhibit remarkable rate performance and stable cycling stability even at a higher sulfur loading of 10.5 mg cm⁻² with a negative to positive electrode capacity (N/P) ratio of 1.4 : 1.

Author(s): Jiacheng Zhu, Tingxin Li, Andrea F. Young, Jie Shan, and Kin Fai Mak

We demonstrate a mechanism for magnetoresistance oscillations in insulating states of two-dimensional (2D) materials arising from the interaction of the 2D layer and proximal graphite gates. We study a series of devices based on different 2D systems, including mono- and bilayer Td−WTe2, MoTe2/WSe2 m...

[Phys. Rev. Lett. 127, 247702] Published Fri Dec 10, 2021

Nature Communications, Published online: 10 December 2021; doi:10.1038/s41467-021-27531-x

This article discusses the state of the art of using 2D materials in optoelectronic integration. The typical optoelectronic devices based on 2D materials including light sources, optical modulators, photodetectors, field-effect transistors, and logic circuits are summarized. It also provides general advice for the future development of optoelectronic integration based on 2D materials.

Abstract

2D materials show wide-ranging physical properties with their electronic bandgaps varying from zero to several electronvolts, offering a rich platform to explore novel electronic and optoelectronic functions. Notably, atomically thin 2D materials are well suited for integration in optoelectronic circuits, because of their ultrathin body, strong light–matter interactions, and compatibility with the current silicon photonic technology. In this paper, an overview of the state of the art of using 2D materials in optoelectronic devices and integration is provided. The optoelectronic properties of 2D materials and their typical electronic and optoelectronic applications including light sources, optical modulators, photodetectors, field-effect transistors, and logic circuits are summarized. The device configurations, operation mechanisms, and device figures-of-merit are introduced and discussed. By discussing the recent advances, future trends, and existing challenges of 2D materials and their optoelectronic devices, this review has provided an insight into the perspectives of 2D materials for optoelectronic integration and may guide the development of this field within the research community.

A new ferroelectric phase is observed in 2D defective semiconductor α‑Ga₂Se₃ by introducing Ga vacancies into asymmetrical sites and their movement at neighboring sites. Switched polarization can occur in nanoflakes of ≈4 nm with a high switching temperature of 450 K. This work provides a 2D ferroelectric system-defective semiconductor that enables more functionality and more flexible modulation.

Abstract

2D ferroelectrics with robust polar order in the atomic-scale thickness at room temperature are needed to miniaturize ferroelectric devices and tackle challenges imposed by traditional ferroelectrics. These materials usually have polar point group structure regarding as a prerequisite of ferroelectricity. Yet, to introduce polar structure into otherwise nonpolar 2D materials for producing ferroelectricity remains a challenge. Here, by combining first-principles calculations and experimental studies, it is reported that the native Ga vacancy-defects located in the asymmetrical sites in cubic defective semiconductor α‑Ga₂Se₃ can induce polar structure. Meanwhile, the induced polarization can be switched in a moderate energy barrier. The switched polarization is observed in 2D α‑Ga₂Se₃ nanoflakes of ≈4 nm with a high switching temperature up to 450 K. Such polarization switching could arise from the displacement of Ga vacancy between neighboring asymmetrical sites by applying an electric field. This work removes the point group limit for ferroelectricity, expanding the range of 2D ferroelectrics into the native defective semiconductors.

Nature Nanotechnology, Published online: 09 December 2021; doi:10.1038/s41565-021-01058-0

A high-efficient and flexible technique is proposed to fabricate polystyrene sphere self-assembly monolayers at air/water interface via ultrasonic spray. Compared with other self-assembly techniques based on air/water interface, this “non-contact” injection method shows several advantages: 1) large-area capability; 2) high efficiency and quality; 3) excellent adaptability and flexibility; and 4) strong fault tolerance.

Abstract

Colloidal lithography provides a rapid and low-cost approach to construct 2D periodic surface nanostructures. However, an impressive demonstration to prepare large-area colloidal template is still missing. Here, a high-efficient and flexible technique is proposed to fabricate self-assembly monolayers consisting of orderly-packed polystyrene spheres at air/water interface via ultrasonic spray. This “non-contact” technique exhibits great advantages in terms of scalability and adaptability due to its renitent interface dynamic balance. More importantly, this technique is not only competent for self-assembly of single-sized polystyrene spheres, but also for binary polystyrene spheres, completely reversing the current hard situation of preparing large-area self-assembly monolayers. As a representative application, hexagonal-packed silver-coated silicon nanorods array (Si-NRs@Ag) is developed as an ultrasensitive surface-enhanced Raman scattering (SERS) substrate with very low limit-of-detection for selective detection of explosive 2,4,6-trinitrotoluene down to femtomolar (10⁻¹⁴ m) range. The periodicity and orderliness of the array allow hot spots to be designed and constructed in a homogeneous fashion, resulting in an incomparable uniformity and reproducibility of Raman signals. All these excellent properties come from the Si-NRs@Ag substrate based on the ordered structure, open surface, and wide-range electric field, providing a robust, consistent, and tunable platform for molecule trapping and SERS sensing for a wide range of organic molecules.

The disc-shaped bismuth oxychloride, BiOCl, is sensitive to illumination to drive self-redox (self-photocorrosion and recovery). Smart new solar–metal–air batteries (SMABs) based on BiOCl hydrogel film electrodes are assembled. The SMABs fully powered by solar energy, can act as an energy storage device with an open circuit voltage of 0.38 V.

Abstract

Herein, a BiOCl hydrogel film electrode featuring excellent photocorrosion and regeneration properties acts as the anode to construct a novel type of smart solar–metal–air batteries (SMABs), which combines the characteristics of solar cells (direct photovoltaic conversion) and metal–air batteries (electric energy storage and release interacting with atmosphere). The cyclic photocorrosion processes between BiOCl (Bi³⁺) and Bi can simply be achieved by solar light illumination and standing in the dark. Upon illumination, the device takes open-circuit configuration to charge itself from the sunlight. Notably, in this system, the converted solar energy can be stored in the SMABs without the need of external assistance. In the discharging process in the dark, Bi⁰ spontaneously turns back to Bi³⁺ producing electrons to induce the oxygen reduction reaction. With an illumination of 15 min, the battery with an electrode area of 1 cm² can be continuously discharged for ≈3000 s. Taking elemental Bi as the calculation object, the theoretical capacity of the SMABs is 384.75 mAh g^-1, showing its potential application in energy storage. This novel type of SMABs is developed based on the unique photocorrosive and self-oxidation reaction of BiOCl to achieve photochemical energy generation and storage.

New photovoltaic materials and devices have been a research hotspot for recent years. Ferroelectric materials with multiple effects have great potential for applications in the fields of photoelectric detection, photoelectric storage, etc. This manuscript summarizes briefly the mechanism, performance improvement and applications of ferroelectric photovoltaic materials and devices.

Abstract

Ferroelectric materials have been a focus of much research over the last few decades for their unique piezoelectric and optoelectronic properties. Conventional solar cells have been devised based on the photovoltaic effect of semiconductor p–n junctions, with their photogenerated voltage being influenced by the bandgap of the semiconductors, limiting their further development. Ferroelectric photovoltaics have attracted attention for their unusual photovoltaic effect and controllability. The photogenerated voltage that is independent of bandgap along the polarization direction can be generated in ferroelectric materials, undoubtedly making up for the lack of solar cells. Ferroelectric materials have been used in a wide range of piezoelectric, storage, sensor, and optoelectronic because of their unique optical and electrical properties. However, the small photogenerated current of ferroelectric photovoltaic devices is one of the challenges that need to be overcome. Researchers have shown that the photogenerated current of ferroelectric photovoltaic devices can be significantly improved by cation doping and heterostructure construction, reigniting the enthusiasm for the investigation of ferroelectric photovoltaics. This paper reviews a variety of ferroelectric photovoltaic materials, the mechanism of ferroelectric photovoltaics, approaches for improving ferroelectric photovoltaic performance, and the applications and future prospects for ferroelectric materials.

Perovskite-Inspired Materials

In article number 2106295, Vincenzo Pecunia and co-workers investigate self-powered photodetectors comprising antimony-based perovskite derivatives. 2D embodiments—consisting of sheets of corner-sharing metal-halide octahedra—more readily convert light into mobile charges. This enables self-powered photodetectors with considerably higher performance compared to lower-dimensional perovskite derivatives. Tuning the structural dimensionality is thus key to advancing perovskite-inspired optoelectronics toward self-powered, ubiquitous light sensing.

Only a few dozen 2D crystals have been successfully prepared, though many more—including metal iodides—are predicted to exist. A major reason for this disparity is their instability under ambient conditions. A wet-chemical method to synthesize 2D-CuI, stabilizing it by graphene encapsulation in a one-step process, is designed, and its chemical structure is imaged.

Abstract

Heterostructures composed of 2D materials are already opening many new possibilities in such fields of technology as electronics and magnonics, but far more could be achieved if the number and diversity of 2D materials were increased. So far, only a few dozen 2D crystals have been extracted from materials that exhibit a layered phase in ambient conditions, omitting entirely the large number of layered materials that may exist at other temperatures and pressures. This work demonstrates how such structures can be stabilized in 2D van der Waals (vdw) stacks under room temperature via growing them directly in graphene encapsulation by using graphene oxide as the template material. Specifically, an ambient stable 2D structure of copper and iodine, a material that normally only occurs in layered form at elevated temperatures between 645 and 675 K, is produced. The results establish a simple route to the production of more exotic phases of materials that would otherwise be difficult or impossible to stabilize for experiments in ambient.