Xingxing Zhang

2D nanomaterials with their unique nanosheet structure, large surface area, and extraordinary physicochemical properties have attracted tremendous interest for application in many different fields including nanomedicine in recent years. The recent progress in the development of different classes of 2D nanomaterials in biomedicine, especially for cancer theranostic applications, is summarized.

Abstract

2D nanomaterials with unique nanosheet structures, large surface areas, and extraordinary physicochemical properties have attracted tremendous interest. In the area of nanomedicine, research on graphene and its derivatives for diverse biomedical applications began as early as 2008. Since then, many other types of 2D nanomaterials, including transition metal dichalcogenides, transition metal carbides, nitrides and carbonitrides, black phosphorus nanosheets, layered double hydroxides, and metal–organic framework nanosheets, have been explored in the area of nanomedicine over the past decade. In particular, a large surface area makes 2D nanomaterials highly efficient drug delivery nanoplatforms. The unique optical and/or X‐ray attenuation properties of 2D nanomaterials can be harnessed for phototherapy or radiotherapy of cancer. Furthermore, by integrating 2D nanomaterials with other functional nanoparticles or utilizing their inherent physical properties, 2D nanomaterials may also be engineered as nanoprobes for multimodal imaging of tumors. 2D nanomaterials have shown substantial potential for cancer theranostics. Herein, the latest progress in the development of 2D nanomaterials for cancer theranostic applications is summarized. Current challenges and future perspectives of 2D nanomaterials applied in nanomedicine are also discussed.

Surface phonon polaritons in monolayer and bilayer hexagonal boron nitride are investigated by direct IR nanoimaging. Phonon polaritons in isolated monolayers are highly confined into the single atomic plane and exhibit dispersion properties at variance with hyperbolic phonon polaritons in bulk crystals.

Abstract

Phonon polaritons in van der Waals materials reveal significant confinement accompanied with long propagation length: important virtues for tasks pertaining to the control of light and energy flow at the nanoscale. While previous studies of phonon polaritons have relied on relatively thick samples, here reported is the first observation of surface phonon polaritons in single atomic layers and bilayers of hexagonal boron nitride (hBN). Using antenna‐based near‐field microscopy, propagating surface phonon polaritons in mono‐ and bilayer hBN microcrystals are imaged. Phonon polaritons in monolayer hBN are confined in a volume about one million times smaller than the free‐space photons. Both the polariton dispersion and their wavelength–thickness scaling law are altered compared to those of hBN bulk counterparts. These changes are attributed to phonon hardening in monolayer‐thick crystals. The data reported here have bearing on applications of polaritons in metasurfaces and ultrathin optical elements.

2D V‐V binary materials, as the internal combination of group‐VA elements (N, P, As, Sb, Bi), have become a popular research topic. Through the interaction among charge, orbital, lattice, and spin degrees of freedom, the favorable and superior properties in 2D V‐V binary materials can be further modulated, extending their application in novel electronic, optoelectronic, and energy devices.

Abstract

2D phosphorene, arsenene, antimonene, and bismuthene, as a fast‐growing family of 2D monoelemental materials, have attracted enormous interest in the scientific community owing to their intriguing structures and extraordinary electronic properties. Tuning the monoelemental crystals into bielemental ones between group‐VA elements is able to preserve their advantages of unique structures, modulate their properties, and further expand their multifunctional applications. Herein, a review of the historical work is provided for both theoretical predictions and experimental advances of 2D V‐V binary materials. Their various intriguing electronic properties are discussed, including band structure, carrier mobility, Rashba effect, and topological state. An emphasis is also given to their progress in fabricated approaches and potential applications. Finally, a detailed presentation on the opportunities and challenges in the future development of 2D V‐V binary materials is given.

Nature Communications, Published online: 26 July 2019; doi:10.1038/s41467-019-11328-0

Photodetectors based on two dimensional (2D) materials still suffer from low performance. Here, the authors tackle this issue by introducing a reconfigurable design enabled by locally tuning the doping of a 2D molybdenum disulfide film through the polarization of an underlying ferroelectric material.

This work demonstrates room temperature laser‐like emission at 1319 nm, with single‐mode operation. The combination of hexagonal boron nitride encapsulated MoTe₂ and an unmembraned photonic crystal nanobeam cavity gives rise to a robust light‐emitting device. This design offers a feasible approach to realize an integrated light source on silicon platform.

Abstract

Molybdenum ditelluride (MoTe₂) has recently shown promise as a gain material for silicon photonics. Reliable single‐mode operation and material stability remain two of the major issues that need to be addressed to advance this exciting technology, however. Here, laser‐like emission from a sandwiched MoTe₂ heterostructure on a silicon single‐mode resonator is reported. The heterostructure consists of a layer of MoTe₂ sandwiched between thin films of hexagonal boron nitride. It is known that tellurium compounds are sensitive to oxygen exposure, which leads to rapid degradation of the exposed layers in air. By encapsulating the MoTe₂ gain material, much improved environmental stability is observed. Using a recently introduced single‐mode resonator design, better control over the mode spectrum of the cavity is exercised and single‐mode operation with a wide free spectral range is demonstrated. At room temperature, a Q‐factor of 4500 and a threshold of 4.2 kW cm⁻² at 1319 nm wavelength are achieved. These results lend further support to the paradigm of 2D material‐based integrated light sources on the silicon platform.

Lateral bilayer (LBL) WS₂–MoS₂ heterostructures have been successfully synthesized via a two‐step CVD growth method. The photodetector device, based on the LBL WS₂‐MoS₂ heterostructure, exhibits ultrahigh photoresponsivity and detectivity (6.72 × 10³ A W^‐1 and 3.09 × 10¹³ Jones for 457 nm laser light), orders of magnitude higher than those of MoS₂ and WS₂ monocrystals.

Abstract

2D heterostructures combining different layered semiconductors have received great interest due to their intriguing electrical and optical properties. However, the arbitrary growth of layers in a lateral heterostructure remains a challenge. Here, the synthesis of large‐scale lateral bilayer (LBL) WS₂–MoS₂ heterostructures is reported by a two‐step chemical vapor deposition route. Raman, photoluminescence, and second‐harmonic generation images show the sharp boundaries between WS₂ and MoS₂ domains in the heterostructure. Atomically resolved scanning transmission electron microscopy further reveals that sharp boundaries are formed by seamless connections via a lateral zigzag epitaxy between WS₂ and MoS₂. Notably, the photodetector device based on the LBL WS₂–MoS₂ heterostructure exhibits ultrahigh photoresponsivity and detectivity (6.72 × 10³ A W⁻¹ and 3.09 × 10¹³ Jones for 457 nm laser light, respectively), orders of magnitude higher than those of MoS₂ and WS₂ monocrystals. These excellent performances render LBL WS₂–MoS₂ heterostructures as promising candidates for next‐generation optoelectronics.

Controlled oxygen plasma exposure of multilayer WSe 2 can modulate the carrier type of WSe 2 field effect transistors. XPS shows with increased exposure, the top WSe 2 layer(s) oxidizes to amorphous WO ( x −3) layer-by-layer with a high degree of controllability as both single or 2 layers can be transformed. A systematic study of Raman and photoluminescence signatures clearly demonstrate the processing times necessary to selectively oxidized the top 1 or 2 layers of exfoliated WSe 2 flakes. WSe 2 devices exposed in the channel regions experience an increase in p-type conduction and large layer-dependent n-type suppression. Devices exposed in the contact region prior to metalization show slight increases of both p-type and n-type conduction. Devices made on flakes, which were completely exposed show suppression of p-type conduction and layer dependent suppression of n-type conduction. Based on these results we were ...

Recent progress on 2D superlattices prepared by solution‐phase strategies using diverse genuine unilamellar nanosheets as building blocks is summarized. A flocculation strategy is considered an efficient method for large‐scale synthesis of these 2D superlattices. High‐performance devices enabled by these 2D superlattices for energy storage and conversion are presented together with a discussion on challenges and perspectives.

Abstract

2D genuine unilamellar nanosheets, that are, the elementary building blocks of their layered parent crystals, have gained increasing attention, owing to their unique physical and chemical properties, and 2D features. In parallel with the great efforts to isolate these atomic‐thin crystals, a unique strategy to integrate them into 2D vertically stacked heterostuctures has enabled many functional applications. In particular, such 2D heterostructures have recently exhibited numerous exciting electrochemical performances for energy storage and conversion, especially the molecular‐scale heteroassembled superlattices using diverse 2D unilamellar nanosheets as building blocks. Herein, the research progress in scalable synthesis of 2D superlattices with an emphasis on a facile solution‐phase flocculation method is summarized. A particular focus is brought to the advantages of these 2D superlattices in applications of supercapacitors, rechargeable batteries, and water‐splitting catalysis. The challenges and perspectives on this promising field are also outlined.

Recent progress in graphene‐based fibers (GBFs), including advances in synthetic techniques, optimization strategies, and novel applications, are reviewed. A perspective on future research directions is also presented to fully explore the great potential of GBFs in flexible and wearable electronics.

Abstract

Graphene‐based fibers (GBFs) are macroscopic 1D assemblies formed by using microscopic 2D graphene sheets as building blocks. Their unique structure exhibits the same merits as graphene such as low weight, high specific surface area, excellent mechanical/electrical properties, and ease of functionalization. Furthermore, the fibrous nature of GBFs is intrinsically compatible with existing textile technologies, making them suitable for applications in flexible and wearable electronics. Recently, novel synthetic methods have endowed GBFs with new structures and functions, further improving their mechanical and electrical properties. These improvements have rapidly bridged the gaps between laboratory demonstrations and real‐life applications in fiber‐shaped batteries, supercapacitors, and electrochemical sensors. Recent advances in the fabrication, optimization, and application of GBFs are systematically reviewed and a perspective on their future development is given.

A new crystal structure of WS₂ , 2M, is reported. It belongs to the 1T′‐phase family, members of which exhibit W–W zigzag chains along the b axis. Superconductivity with T _c of 8.8 K is reported in these 2M WS₂ crystals. Moreover, calculations show that a topological surface state exists on the their surface, making them potential candidates for topological superconductors.

Abstract

Recently the metastable 1T′‐type VIB‐group transition metal dichalcogenides (TMDs) have attracted extensive attention due to their rich and intriguing physical properties, including superconductivity, valleytronics physics, and topological physics. Here, a new layered WS₂ dubbed “2M” WS₂, is constructed from 1T′ WS₂ monolayers, is synthesized. Its phase is defined as 2M based on the number of layers in each unit cell and the subordinate crystallographic system. Intrinsic superconductivity is observed in 2M WS₂ with a transition temperature T _c of 8.8 K, which is the highest among TMDs not subject to any fine‐tuning process. Furthermore, the electronic structure of 2M WS₂ is found by Shubnikov–de Haas oscillations and first‐principles calculations to have a strong anisotropy. In addition, topological surface states with a single Dirac cone, protected by topological invariant Z₂, are predicted through first‐principles calculations. These findings reveal that the new 2M WS₂ might be an interesting topological superconductor candidate from the VIB‐group transition metal dichalcogenides.

Ultraclean transfer of synthesized monolayers is developed for artificially stacked monolayers with tunable twisting and a hetero‐interface. Diverse Moiré electronic superlattices are directly visualized and studied with scanning tunneling microscopy (STM), which will open up opportunities for diverse correlated properties in artificial 2D lattices.

Abstract

Twisting between two stacked monolayers modulates periodic potentials and forms the Moiré electronic superlattices, which offers an additional degree of freedom to alter material property. Considerable unique observations, including unconventional superconductivity, coupled spin‐valley states, and quantized interlayer excitons are correlated to the electronic superlattices but further study requires reliable routes to study the Moiré in real space. Scanning tunneling microscopy (STM) is ideal to precisely probe the Moiré superlattice and correlate coupled parameters among local electronic structures, strains, defects, and band alignment at atomic scale. Here, a clean route is developed to construct twisted lattices using synthesized monolayers for fundamental studies. Diverse Moiré superlattices are predicted and successfully observed with STM at room temperature. Electrical tuning of the Moiré superlattice is achieved with stacked TMD on graphite.

Ultrathin nonlayered 2D wide‐bandgap CuBr nanoflakes are controllably synthesized by self‐confined chemical vapor deposition, and possess abundant room‐temperature exciton behavior and novel nonlinear optical phenomena. Self‐driven and solar‐blind ultraviolet detectors based on the Ag–CuBr Schottky junction are constructed and display outstanding properties of high photoresponsivity, external quantum efficiency, and detectivity (D*) with fast response rate.

Abstract

2D planar structures of nonlayered wide‐bandgap semiconductors enable distinguished electronic properties, desirable short wavelength emission, and facile construction of 2D heterojunction without lattice match. However, the growth of ultrathin 2D nonlayered materials is limited by their strong covalent bonded nature. Herein, the synthesis of ultrathin 2D nonlayered CuBr nanosheets with a thickness of about 0.91 nm and an edge size of 45 µm via a controllable self‐confined chemical vapor deposition method is described. The enhanced spin‐triplet exciton (Z _f, 2.98 eV) luminescence and polarization‐enhanced second‐harmonic generation based on the 2D CuBr flakes demonstrate the potential of short‐wavelength luminescent applications. Solar‐blind and self‐driven ultraviolet (UV) photodetectors based on the as‐synthesized 2D CuBr flakes exhibit a high photoresponsivity of 3.17 A W⁻¹, an external quantum efficiency of 1126%, and a detectivity (D*) of 1.4 × 10¹¹ Jones, accompanied by a fast rise time of 32 ms and a decay time of 48 ms. The unique nonlayered structure and novel optical properties of the 2D CuBr flakes, together with their controllable growth, make them a highly promising candidate for future applications in short‐wavelength light‐emitting devices, nonlinear optical devices, and UV photodetectors.

Diverse material structures for stretchable inorganic electronics are summarized, covering both functional devices and soft substrates, with a focus on the fundamental principles, design approaches, and system demonstrations. Strategies that allow spatial integration of 3D stretchable device configurations are also highlighted. Finally, perspectives on remaining challenges and open opportunities are provided.

Abstract

Over the past decade, the area of stretchable inorganic electronics has evolved very rapidly, in part because the results have opened up a series of unprecedented applications with broad interest and potential for impact, especially in bio‐integrated systems. Low modulus mechanics and the ability to accommodate extreme mechanical deformations, especially high levels of stretching, represent key defining characteristics. Most existing studies exploit structural material designs to achieve these properties, through the integration of hard inorganic electronic components configured into strategic 2D/3D geometries onto patterned soft substrates. The diverse structural geometries developed for stretchable inorganic electronics are summarized, covering the designs of functional devices and soft substrates, with a focus on fundamental principles, design approaches, and system demonstrations. Strategies that allow spatial integration of 3D stretchable device layouts are also highlighted. Finally, perspectives on the remaining challenges and open opportunities are provided.

In article number https://doi.org/10.1002/adfm.2019012401901240, Tianfeng Chen and co‐workers design bioinspired 2D MoSe₂ nanosheets with high photothermal conversion efficiency to achieve efficient photothermal‐triggered cancer immunotherapy, by activating cytotoxic T lymphocytes, reprogramming tumor associated macrophages to a tumoricidal M1 phenotype, and inactivation of the PD‐1/PD‐L1 pathway to avoid immunologic escape.

A single‐crystalline gallium nitride (GaN) film with atomic‐step terraces is realized on a complementary metal‐oxide‐semiconductor‐compatible Si(100) substrate by using a one‐atom‐thick single‐crystalline graphene buffer layer. The monolayer single‐crystalline graphene provides an in‐plane driving force for the uniform alignment of nitrides domains. This approach can also enable the growth of wafer‐scale hexagonal single‐crystalline films on amorphous or flexible substrates.

Abstract

Fabricating single‐crystalline gallium nitride (GaN)‐based devices on a Si(100) substrate, which is compatible with the mainstream complementary metal‐oxide‐semiconductor circuits, is a prerequisite for next‐generation high‐performance electronics and optoelectronics. However, the direct epitaxy of single‐crystalline GaN on a Si(100) substrate remains challenging due to the asymmetric surface domains of Si(100), which can lead to polycrystalline GaN with a two‐domain structure. Here, by utilizing single‐crystalline graphene as a buffer layer, the epitaxy of a single‐crystalline GaN film on a Si(100) substrate is demonstrated. The in situ treatment of graphene with NH₃ can generate sp³ CN bonds, which then triggers the nucleation of nitrides. The one‐atom‐thick single‐crystalline graphene provides an in‐plane driving force to align all GaN domains to form a single crystal. The nucleation mechanisms and domain evolutions are further clarified by surface science exploration and first‐principle calculations. This work lays the foundation for the integration of GaN‐based devices into Si‐based integrated circuits and also broadens the choice for the epitaxy of nitrides on unconventional amorphous or flexible substrates.

In this work, stacking defects in hexagonal Ge₄Se₃Te, GaSe, and Sb₂Te₃ are characterized experimentally, followed by an investigation of the influence of observed and hypothetical stacking defects on optical and electronic properties by theoretical means. A connection between observed defects and the bonding situation is then drawn and related to the presence of van der Waals and metavalent bonding in chalcogenides.

Abstract

Phase‐change materials for high‐density data storage traditionally exploit the amorphous‐to‐crystalline phase transition. A number of these compounds are organized in blocks, separated by van der Waals‐like gaps. Such layered chalcogenides are attracting interest due to their unique material properties and the possibility to change their properties upon local rearrangements at the gap, giving rise to novel applications. To better understand the behavior of layered chalcogenides, the connection between structural defects, physical properties, and the bonding situation is highlighted here using electron microscopy, X‐ray diffraction, and density functional theory. In particular, stacking defects in hexagonal Ge₄Se₃Te, GaSe, and Sb₂Te₃ are characterized experimentally, followed by an investigation of the influence of observed and hypothetical stacking defects on optical and electronic properties by theoretical means. Then, a connection between observed defects and the bonding situation in these materials is drawn and related to the presence of van der Waals and metavalent bonding in chalcogenides. Finally, additional experiments are performed to validate the conclusions for other metavalently bonded layered chalcogenides. Transmission electron microscopy provides a powerful tool for direct detection of defects and, when combined with diffraction experiments and ab initio theory, it facilitates the precise investigation of the bonding mechanisms in layered chalcogenides.

Ambipolar transistors represent transistors that allow synchronous transport of electrons and holes and their accumulation within semiconductors. This review provides a comprehensive summary of recent advances in various semiconducting materials realized in ambipolar transistors and their functional memory, synapse, logic, as well as light‐emitting applications.

Abstract

Ambipolar transistors represent a class of transistors where positive (holes) and negative (electrons) charge carriers both can transport concurrently within the semiconducting channel. The basic switching states of ambipolar transistors are comprised of common off‐state and separated on‐state mainly impelled by holes or electrons. During the past years, diverse materials are synthesized and utilized for implementing ambipolar charge transport and their further emerging applications comprising ambipolar memory, synaptic, logic, and light‐emitting transistors on account of their special bidirectional carrier‐transporting characteristic. Within this review, recent developments of ambipolar transistor field involving fundamental principles, interface modifications, selected semiconducting material systems, device structures, ambipolar characteristics, and promising applications are highlighted. The existed challenges and prospective for researching ambipolar transistors in electronics and optoelectronics are also discussed. It is expected that the review and outlook are well timed and instrumental for the rapid progress of academic sector of ambipolar transistors in lighting, display, memory, as well as neuromorphic computing for artificial intelligence.