Xingxing Zhang

An intercalation‐assisted method is developed to obtain multi‐metal‐doped H‐MoS₂, which is proven to be a universal method. The doping of adjacent cobalt and palladium monomers on MoS₂ greatly enhances the HER catalytic activity. The overpotential at 10 mA cm⁻¹ and Tafel slope of Co‐Pd‐MoS₂ are 49.3 mV and 43.2 mV dec⁻¹, respectively.

Abstract

Lack of effective strategies to regulate the internal activity of MoS₂ limits its practical application for hydrogen evolution reactions (HERs). Doping of heteroatoms without forming aggregation or an edge enrichment is still challenging, and its effect on the HER needs to be further explored. Herein, a two‐step method is developed to obtain multi‐metal‐doped H‐MoS₂, which includes intercalation of the layered MoO₃ precursor with a following sulfurization. Benefiting from the capability of the intercalation method to uniformly and simultaneously introduce different elements into the van der Waals gap, this method is universal to obtain multi‐heteroatoms co‐doped MoS₂ without forming clusters, phase separation, and an edge enrichment. It is demonstrated that the doping of adjacent cobalt and palladium monomers on MoS₂ greatly enhances the HER catalytic activity. The overpotential at 10 mA cm⁻² and Tafel slope of Co and Pd co‐doped MoS₂ is found to be 49.3 mV and 43.2 mV dec⁻¹, respectively, representing a superior acidic HER catalytic activity. This intercalation‐assisted method also provides a new and general strategy to synthesize uniformly doped transition metal dichalcogenides for various applications.

Transition metal dichalcogenides (TMDs) are considered to be promising candidates over noble metal catalysts for their electrochemical hydrogen production. The basic mechanism for the hydrogen evolution reaction (HER) is introduced, followed by a description of the different synthesis methods and modulation approaches to enhance the catalytic performance of TMD‐based catalysts toward HER.

Abstract

Hydrogen has been deemed as an ideal substitute fuel to fossil energy because of its renewability and the highest energy density among all chemical fuels. One of the most economical, ecofriendly, and high‐performance ways of hydrogen production is electrochemical water splitting. Recently, 2D transition metal dichalcogenides (also known as 2D TMDs) showed their utilization potentiality as cost‐effective hydrogen evolution reaction (HER) catalysts in water electrolysis. Herein, recent representative research efforts and systematic progress made in 2D TMDs are reviewed, and future opportunities and challenges are discussed. Furthermore, general methods of synthesizing 2D TMDs materials are introduced in detail and the advantages and disadvantages for some specific methods are provided. This explanation includes several important regulation strategies of creating more active sites, heteroatoms doping, phase engineering, construction of heterostructures, and synergistic modulation which are capable of optimizing the electrical conductivity, exposure to the catalytic active sites, and reaction energy barrier of the electrode material to boost the HER kinetics. In the last section, the current obstacles and future chances for the development of 2D TMDs electrocatalysts are proposed to provide insight into and valuable guidelines for fabricating effective HER electrocatalysts.

A liquid metal establishes an ultrasmooth liquid–liquid interface that catalyses the dissociation of organic precursors into interfacial graphitic carbon films. The electrochemical synthesis of these 2D graphitic films is accomplished at room temperature (with only a small energy input, an onset voltage of 0.45 V) and they self‐exfoliate from the nonpolar surface of the liquid metal by applying higher voltages.

Abstract

Room‐temperature synthesis of 2D graphitic materials (2D‐GMs) remains an elusive aim, especially with electrochemical means. Here, it is shown that liquid metals render this possible as they offer catalytic activity and an ultrasmooth templating interface that promotes Frank–van der Merwe regime growth, while allowing facile exfoliation due to the absence of interfacial forces as a nonpolar liquid. The 2D‐GMs are formed at low onset potential and can be in situ doped depending on the choice of organic precursors and the electrochemical set‐up. The materials are tuned to exhibit porous or pinhole‐free morphologies and are engineered for their degree of oxidation and number of layers. The proposed liquid‐metal‐based room‐temperature electrochemical route can be expanded to many other 2D materials.

Microscopy data of nanomaterials often contains rich yet complicated information that reflects the material properties, but is mostly overlooked by researchers. Deep learning is an ideal approach to finding these highly correlated and non‐linear features. As a case study, a neural network model called “2DMOINet” is trained for optical identification and characterization of exfoliated 2D materials.

Abstract

Advanced microscopy and/or spectroscopy tools play indispensable roles in nanoscience and nanotechnology research, as they provide rich information about material processes and properties. However, the interpretation of imaging data heavily relies on the “intuition” of experienced researchers. As a result, many of the deep graphical features obtained through these tools are often unused because of difficulties in processing the data and finding the correlations. Such challenges can be well addressed by deep learning. In this work, the optical characterization of 2D materials is used as a case study, and a neural‐network‐based algorithm is demonstrated for the material and thickness identification of 2D materials with high prediction accuracy and real‐time processing capability. Further analysis shows that the trained network can extract deep graphical features such as contrast, color, edges, shapes, flake sizes, and their distributions, based on which an ensemble approach is developed to predict the most relevant physical properties of 2D materials. Finally, a transfer learning technique is applied to adapt the pretrained network to other optical identification applications. This artificial‐intelligence‐based material characterization approach is a powerful tool that would speed up the preparation, initial characterization of 2D materials and other nanomaterials, and potentially accelerate new material discoveries.

The strain‐engineered anisotropic optical and electrical properties in solution‐grown, sub‐millimeter‐size 2D Te are systematically investigated through designing and introducing a controlled buckled geometry in its intriguing chiral‐chain lattice. The results suggest the potential of 2D Te as a promising candidate for designing and implementing flexible and stretchable devices with strain‐engineered functionalities.

Abstract

Atomically thin materials, leveraging their low‐dimensional geometries and superior mechanical properties, are amenable to exquisite strain manipulation with a broad tunability inaccessible to bulk or thin‐film materials. Such capability offers unexplored possibilities for probing intriguing physics and materials science in the 2D limit as well as enabling unprecedented device applications. Here, the strain‐engineered anisotropic optical and electrical properties in solution‐grown, sub‐millimeter‐size 2D Te are systematically investigated through designing and introducing a controlled buckled geometry in its intriguing chiral‐chain lattice. The observed Raman spectra reveal anisotropic lattice vibrations under the corresponding straining conditions. The feasibility of using buckled 2D Te for ultrastretchable strain sensors with a high gauge factor (≈380) is further explored. 2D Te is an emerging material boasting attractive characteristics for electronics, sensors, quantum devices, and optoelectronics. The results suggest the potential of 2D Te as a promising candidate for designing and implementing flexible and stretchable devices with strain‐engineered functionalities.

A thermomechanical lithography technique for direct nanocutting of 2D materials is demonstrated. A heated scanning nanotip performs the cutting of the 2D material by thermomechanically cleaving the chemical bonds in concert with the rapid sublimation of the polymer layer underneath. A resolution of 20 nm is obtained in monolayer MoTe₂, MoS₂, and MoSe₂.

Abstract

Atomically thin materials, such as graphene and transition metal dichalcogenides, are promising candidates for future applications in micro/nanodevices and systems. For most applications, functional nanostructures have to be patterned by lithography. Developing lithography techniques for 2D materials is essential for system integration and wafer‐scale manufacturing. Here, a thermomechanical indentation technique is demonstrated, which allows for the direct cutting of 2D materials using a heated scanning nanotip. Arbitrarily shaped cuts with a resolution of 20 nm are obtained in monolayer 2D materials, i.e., molybdenum ditelluride (MoTe₂), molybdenum disulfide (MoS₂), and molybdenum diselenide (MoSe₂), by thermomechanically cleaving the chemical bonds and by rapid sublimation of the polymer layer underneath the 2D material layer. Several micro/nanoribbon structures are fabricated and electrically characterized to demonstrate the process for device fabrication. The proposed direct nanocutting technique allows for precisely tailoring nanostructures of 2D materials with foreseen applications in the fabrication of electronic and photonic nanodevices.

Broadband and fast hybrid photodetectors are constructed based on the vertically stacked heterostructures of inorganic molecular crystal Sb₂O₃/monolayer MoS₂ grown by a two‐step chemical vapor deposition method. A remarkably high responsivity and detectivity is achieved via optimizing the coverage ratio of Sb₂O₃ atop the channel of monolayer MoS₂.

Abstract

The newly emerged 2D materials heterostructures, including layered and nonlayered structures, are regarded as the building blocks for future high performance optoelectronic devices. However, it still remains a great challenge to directly synthesize 2D heterostructures for realizing broadband detection in photodetectors. In this work, the growth of vertically stacked inorganic molecular Sb₂O₃/monolayer MoS₂ heterostructures through a two‐step chemical vapor deposition method is demonstrated, and high performance ultraviolet/near‐infrared photodetectors based on the achieved heterostructures are further developed. Excellent responsivity of 5.3 × 10⁴ A W⁻¹ and detectivity of 2.0 × 10¹⁵ Jones are obtained under 457 nm illumination. Additionally, the photodetection range can be extended to near‐infrared region. Maximum responsivity of 7.8 A W⁻¹, detectivity of 3.4 × 10¹¹ Jones, and fast response speed (<60 ms) are obtained under 1064 nm laser illumination at room temperature, which is far superior to those of the previously reported ultra‐thin 2D van de Waals heterostructures. The inorganic molecular Sb₂O₃/monolayer MoS₂ heterostructures enrich the family of 2D materials heterostructures, showing potential applications in high performance functional electronics and optoelectronics.

Highly sensitive temperature dependent second harmonic generation (SHG) response is shown in atomically thin layers of transition‐metal dichalcogenides (TMDs) through multiphoton microscopy. A significant temperature dependent SHG enhancement and SHG quenching is observed for single layer and few odd layers (3L, 5L, 7L, etc.), respectively, for TMDs. This research enables novel applications of TMDs in nonlinear optical devices.

Abstract

Atomically thin transition metal dichalcogenides (TMDs) are important semiconducting materials because of their interesting layer dependent properties. Recently, optical second harmonic generation (SHG) is used to probe layer number, lattice orientation, phase variation, and strain vector in ultrathin TMDs. Here, it is demonstrated that SHG response of ultrathin TMDs is highly sensitive to temperature modulation. Furthermore, temperature dependent SHG is found to show opposite trends for single layer and few odd layers (3L, 5L, 7L, etc.) of TMDs. A remarkable temperature dependent SHG enhancement (25.8%) is found in single layer molybdenum diselenide (MoSe₂) using 900 nm laser excitation whereas few odd layers show significant temperature dependent SHG quenching which is found to be ‐55.2%, ‐31.02%, and ‐18.4% in case of 3L, 5L, and 7L of MoSe₂. Temperature dependent SHG investigation with other TMDs, like MoS₂, WS₂, and WSe₂, shows the similar trend which reveals an important structural characteristic for TMDs. Second order nonlinear susceptibility calculations considering weak van der Waal forces during thermal expansion in ultrathin TMDs show good agreement with the experimental findings. The results show SHG as a powerful and sensitive approach to investigate thermal variation in ultrathin TMDs.

All‐optically controlled MoTe₂/Si THz modulators based on liquid exfoliated MoTe₂ are experimentally investigated. The MoTe₂/Si modulators present excellent performance with ultrasensitive and broadband modulation. The modulation depth is as large as 99.9% even under a very low illumination power of 300 mW and the modulation bandwidth covers the wide frequency range from 0.3 to 2.0 THz.

Abstract

Despite the impressive progresses in terahertz (THz) sources and detection, there is still a big challenge for high performance active optoelectronic THz devices such as THz modulators. All‐optically controlled THz modulators with large modulation depth (MD) and wide modulation bandwidth are of great importance for the THz technology. Herein, a MoTe₂/Si van der Waals (vdWs) heterostructure is rationally designed as all‐optical THz modulator by taking advantage of their similar band alignment and easy integration with Si complementary metal‐oxide‐semiconductor (CMOS). The MoTe₂/Si modulator presents an ultrasensitive THz modulation performance with a MD of 99.9% under a low illumination power of 300 mW at 1064 nm. This is a record MD for TMDCs‐based all‐optically controlled THz modulators to date. Moreover, the MoTe₂/Si modulator exhibits broadband modulation performance with a wide frequency range from 0.3 to 2.0 THz. The high modulation performance under low illumination power is beneficial for practical application with low energy consumption and easy heat dissipation, which is advantageous to modulator chip. This work validates a facile protocol for fabricating high performance THz modulators and paves the way for their practical applications in THz technology.

The remarkable property portfolio of a wide range of chalcogenides is attributed to their unconventional bonding, coined metavalent bonding. This mechanism explains application‐relevant properties, crucial for thermoelectrics, phase change materials, topological insulators, or as building blocks for active photonic components. The concept of metavalent bonding provides optimization schemes for different applications and gives rise to interesting effects in reduced dimensions.

Abstract

A unified picture of different application areas for incipient metals is presented. This unconventional material class includes several main‐group chalcogenides, such as GeTe, PbTe, Sb₂Te₃, Bi₂Se₃, AgSbTe₂ and Ge₂Sb₂Te₅. These compounds and related materials show a unique portfolio of physical properties. A novel map is discussed, which helps to explain these properties and separates the different fundamental bonding mechanisms (e.g., ionic, metallic, and covalent). The map also provides evidence for an unconventional, new bonding mechanism, coined metavalent bonding (MVB). Incipient metals, employing this bonding mechanism, also show a special bond breaking mechanism. MVB differs considerably from resonant bonding encountered in benzene or graphite. The concept of MVB is employed to explain the unique properties of materials utilizing it. Then, the link is made from fundamental insights to application‐relevant properties, crucial for the use of these materials as thermoelectrics, phase change materials, topological insulators or as active photonic components. The close relationship of the materials' properties and their application potential provides optimization schemes for different applications. Finally, evidence will be presented that for metavalently bonded materials interesting effects arise in reduced dimensions. In particular, the consequences for the crystallization kinetics of thin films and nanoparticles will be discussed in detail.

A theory‐guided synthesis approach is employed to achieve unexplored quasi‐binary TMDC alloys through computationally predicted stability maps. The synthesized alloys can be exfoliated into 2D‐structures, and some of them exhibit: i) outstanding thermal stability tested up to 1230 K, ii) exceptionally high electrochemical activity for CO₂ reduction reaction, iii) excellent energy efficiency in a high‐rate Li–air battery, and iv) high break‐down current‐density.

Abstract

Transition metal dichalcogenide (TMDCs) alloys could have a wide range of physical and chemical properties, ranging from charge density waves to superconductivity and electrochemical activities. While many exciting behaviors of unary TMDCs have been demonstrated, the vast compositional space of TMDC alloys has remained largely unexplored due to the lack of understanding regarding their stability when accommodating different cations or chalcogens in a single‐phase. Here, a theory‐guided synthesis approach is reported to achieve unexplored quasi‐binary TMDC alloys through computationally predicted stability maps. Equilibrium temperature–composition phase diagrams using first‐principles calculations are generated to identify the stability of 25 quasi‐binary TMDC alloys, including some involving non‐isovalent cations and are verified experimentally through the synthesis of a subset of 12 predicted alloys using a scalable chemical vapor transport method. It is demonstrated that the synthesized alloys can be exfoliated into 2D structures, and some of them exhibit: i) outstanding thermal stability tested up to 1230 K, ii) exceptionally high electrochemical activity for the CO₂ reduction reaction in a kinetically limited regime with near zero overpotential for CO formation, iii) excellent energy efficiency in a high rate Li–air battery, and iv) high break‐down current density for interconnect applications. This framework can be extended to accelerate the discovery of other TMDC alloys for various applications.

Heterostructured NiSe₂/CoSe₂ nanohybrids with different interfacial densities are synthesized via an innovative strategy of successive ion injection. Advanced synchrotron techniques and theoretical calculations demonstrate that the dense nanointerface structure can enhance the performance for oxygen electrocatalysis via increasing the intrinsic reactivity of metallic atoms and introducing a synergistic effect with surface electrochemically in‐situ‐formed oxides/hydroxides.

Abstract

Constructing heterostructures with abundant interfaces is essential for integrating the multiple functionalities in single entities. Herein, the synthesis of NiSe₂/CoSe₂ heterostructures with different interfacial densities via an innovative strategy of successive ion injection is reported. The resulting hybrid electrocatalyst with dense heterointerfaces exhibits superior electrocatalytic properties in an alkaline electrolyte, superior to other benchmarks and precious metal catalysts. Advanced synchrotron techniques, post structural characterizations, and density functional theory (DFT) simulations reveal that the introduction of atomic‐level interfaces can lower the oxidation overpotential of bimetallic Ni and Co active sites (whereas Ni²⁺ can be more easily activated than Co²⁺) and induce the electronic interaction between the core selenides and surface in situ generated oxides/hydroxides, which play a critical role in synergistically reducing energetic barriers and accelerating reaction kinetics for catalyzing the oxygen evolution. Hence, the heterointerface structure facilitates the catalytic performance enhancement via increasing the intrinsic reactivity of metallic atoms and enhancing the synergistic effect between the inner selenides and surface oxidation species. This work not only complements the understanding on the origins of the activity of electrocatalysts based on metal selenides, but also sheds light on further surface and interfacial engineering of advanced hybrid materials.

Optical activity of surface lattice resonances is demonstrated by Nicolas Vogel and co‐workers in article number https://doi.org/10.1002/adma.2020013302001330. The controlled arrangement of chiral crescent‐shaped nanoantennae allows a selective response of the surface lattice resonances to the handedness of incident circularly polarized light. Modified colloidal lithtography enables the fabrication of cm²‐scale substrates using a self‐assembled poly(N ‐isopropylacrylamide)–silica core–shell system as particle masks.

Ferromagnetism is successfully introduced in superconducting TaS₂ molecular superlattices with single Co atom doping, which is achieved by an interlayer‐space‐confined chemical design (ICCD). This ICCD approach can be applied to various metal ions, opening up new avenues for designing artificial 2D material superlattices with exotic phases and desired functionalities.

Abstract

Ferromagnetism and superconductivity are two antagonistic phenomena since ferromagnetic exchange fields tend to destroy singlet Cooper pairs. Reconciliation of these two competing phases has been achieved in vertically stacked heterostructures where these two orders are confined in different layers. However, controllable integration of these two phases in one atomic layer is a longstanding challenge. Here, an interlayer‐space‐confined chemical design (ICCD) is reported for the synthesis of dilute single‐atom‐doped TaS₂ molecular superlattice, whereby ferromagnetism is observed in the superconducting TaS₂ layers. The intercalation of 2H‐TaS₂ crystal with bulky organic ammonium molecule expands its van der Waals gap for single‐atom doping via co‐intercalated cobalt ions, resulting in the formation of quasi‐monolayer Co‐doped TaS₂ superlattices. Isolated Co atoms are decorated in the basal plane of the TaS₂ via substituting the Ta atom or anchoring at a hollow site, wherein the orbital‐selected p–d hybridization between Co and neighboring Ta and S atoms induces local magnetic moments with strong ferromagnetic coupling. This ICCD approach can be applied to various metal ions, enabling the synthesis of a series of crystal‐size TaS₂ molecular superlattices.

A light‐induced switching of magnetism is reported in porphyrin functionalized ultrathin FeS nanosheets. Under light‐irradiation, a large number of photo‐generated carriers take part in a singlet‐to‐triplet conversion through intersystem crossing of porphyrin molecules decorated on the FeS. The generated triplet carriers make spin flips occur at FeS surface, finally leading to a magnetic structure transition from ferromagnetic to antiferromagnetic configuration.

Abstract

The intrinsically weak photomagnetic interaction makes it difficult to realize effectively light‐controlled magnetic structure transformation. Here unprecedented light‐induced switching of magnetism is reported in porphyrin functionalized ultrathin FeS nanosheets. Contrary to the antiferromagnetic (AFM) bulk, the troilite FeS nanosheets demonstrate a ferromagnetic (FM) behavior due to superficial symmetry breaking. Under light irradiation, a large number of photo‐generated carriers take part in a singlet‐to‐triplet conversion through intersystem crossing of tetra(4‐carboxyphenyle)porphyrin molecules decorated on the nanosheets. The generated triplet carriers make spin flips occur at FeS surface, finally leading to a magnetic structure transition from FM to AFM configuration. The findings not only shed new light on steering arrangement of spin electrons but also open up almost unlimited possibilities for spintronics and optical detection of spin‐dependent physiological reactions.

Recently, various synthesis techniques are explored to fabricate large‐area high‐quality thin‐film black phosphorus (BP). Benefiting from the superior optoelectronic properties, such as tunable bandgap, strong photon absorption, and high carrier mobility, thin‐film BP is widely investigated as mid‐infrared (MIR) absorber material. BP‐based MIR photodetectors with various configurations are successfully demonstrated with decent photoresponse, broad detection waveband, and high polarization sensitivity.

Abstract

Black phosphorus (BP), a van der Waals (vdW) layered material, has been intensively studied in recent years since the rediscovery of its thin‐film form in 2014. It is considered as a promising material for mid‐infrared (MIR) photodetection, due to its intrinsic narrow bandgap, tunable band properties, decent optical absorption, high room‐temperature mobility, and high compatibility with silicon‐based technology. Here, the recent advances in the synthesis techniques, the novel optoelectronic properties, and applications of thin‐film BP flake in MIR photodetection are reviewed. Over 17 synthesis techniques of BP films, as well as their merits and drawbacks, are summarized and discussed. The recently discovered strain‐, electric‐field‐, and chemical‐doping‐induced bandgap tuning effects in BP effectively extend its optical absorption cutoff wavelength into regime with longer wavelength (>4 µm). In addition, the establishment of BP‐based vdW heterostructures paves a new way to design novel high‐performance MIR photodetectors. BP MIR photodetectors enabled by various photocurrent generation mechanisms (photoconductive, photogating, and photovoltaic effect) and device configurations (transistor‐type, waveguide‐coupled, Schottky‐junction‐type, and heterojunction‐type devices) are summarized and compared.

Novel nonlayered 2D Fe₃O₄ nanosheets with cubic inverse‐spinel structure deliver an ultrabroadband photoresponse from the ultraviolet to the long‐wavelength infrared (LWIR) range. The distinguished LWIR detection performance with ultrahigh photoresponsivity of 561.2 A W⁻¹, external quantum efficiency of 6.6 × 10³%, and detectivity of 7.42 × 10⁸ Jones can be attributed to a synergistic effect of the photoconductive effect and the bolometric effect.

Abstract

The ultrabroadband spectrum detection from ultraviolet (UV) to long‐wavelength infrared (LWIR) is promising for diversified optoelectronic applications of imaging, sensing, and communication. However, the current LWIR‐detecting devices suffer from low photoresponsivity, high cost, and cryogenic environment. Herein, a high‐performance ultrabroadband photodetector is demonstrated with detecting range from UV to LWIR based on air‐stable nonlayered ultrathin Fe₃O₄ nanosheets synthesized via a space‐confined chemical vapor deposition (CVD) method. Ultrahigh photoresponsivity (R ) of 561.2 A W⁻¹, external quantum efficiency (EQE) of 6.6 × 10³%, and detectivity (D *) of 7.42 × 10⁸ Jones are achieved at the wavelength of 10.6 µm. The multimechanism synergistic effect of photoconductive effect and bolometric effect demonstrates the high sensitivity for light with any light intensities. The outstanding device performance and complementary mixing photoresponse mechanisms open up new potential applications of nonlayered 2D materials for future infrared optoelectronic devices.

Ferromagnetism is successfully introduced in superconducting TaS₂ molecular superlattices with single Co atom doping, which is achieved by an interlayer‐space‐confined chemical design (ICCD). This ICCD approach can be applied to various metal ions, opening up new avenues for designing artificial 2D material superlattices with exotic phases and desired functionalities.

Abstract

Ferromagnetism and superconductivity are two antagonistic phenomena since ferromagnetic exchange fields tend to destroy singlet Cooper pairs. Reconciliation of these two competing phases has been achieved in vertically stacked heterostructures where these two orders are confined in different layers. However, controllable integration of these two phases in one atomic layer is a longstanding challenge. Here, an interlayer‐space‐confined chemical design (ICCD) is reported for the synthesis of dilute single‐atom‐doped TaS₂ molecular superlattice, whereby ferromagnetism is observed in the superconducting TaS₂ layers. The intercalation of 2H‐TaS₂ crystal with bulky organic ammonium molecule expands its van der Waals gap for single‐atom doping via co‐intercalated cobalt ions, resulting in the formation of quasi‐monolayer Co‐doped TaS₂ superlattices. Isolated Co atoms are decorated in the basal plane of the TaS₂ via substituting the Ta atom or anchoring at a hollow site, wherein the orbital‐selected p–d hybridization between Co and neighboring Ta and S atoms induces local magnetic moments with strong ferromagnetic coupling. This ICCD approach can be applied to various metal ions, enabling the synthesis of a series of crystal‐size TaS₂ molecular superlattices.