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Convection‐assisted chemical intercalation is introduced to realize uniform and dual‐face phase transition in layered MoTe₂ that provides a functional vertical heterophase structure. The vertical heterophase MoTe₂ is demonstrated as a new platform for three distinct applications, hybrid catalysis, automatic exfoliation and next‐generation electronic devices based on 2D materials.

Abstract

Phase engineering is a breakthrough for various electronic and energy device applications with transition metal dichalcogenides (TMDs). Chemical methods, such as lithium intercalation, are mostly used for phase engineering, which achieves atomically thin flakes and high catalytic performances in several group 6 TMDs including MoS₂. However, chemical methods cannot be applied to MoTe₂, a widely investigated group 6 TMD with intriguing semiconducting, topological, and catalytic properties. The lack of modifying MoTe₂ by chemical methods remains a puzzling issue considering the small energy difference between the polymorphs of MoTe₂. Here, a convection‐assisted lithium ion intercalation and phase transition is reported to achieve a vertical heterophase in a MoTe₂ crystal. The vertical heterophase in MoTe₂ reduces the Schottky barrier with metal electrodes down to 66 meV, enhancing the overall ion conductance for electrochemical hydrogen production. Moreover, the weakened adhesion of the 1T' phase layers on the top and bottom surfaces in the vertical heterophase, formed by the intercalation, enables a unique surface tension‐driven exfoliation of MoTe₂ flakes. The heterophase chemical engineering suggests a new platform for hybrid catalysts and next‐generation electronic devices based on 2D materials.

This work studies a novel, cost‐effective, energy‐efficient, and scalable surface modification of graphite felt electrodes based on the controlled electrochemical exfoliation in aqueous ammonium sulfate electrolyte to enhance the mass transfer and reaction kinetics of the electrode. This treatment induces sufficient oxygen groups, resulting in enhanced kinetics at the electrode‐electrolyte interface, larger surface area, and improved wettability, enabling better electrolyte accessibility to the electrode.

Abstract

A scalable and efficient process to modify electrodes with enhanced mass transfer and reaction kinetics is critical for redox flow batteries (RFBs). For the first time, this work introduces electrochemical exfoliation as a surface modification method of graphite felt (GF) to enhance the mass transfer and reaction kinetics in RFBs. Anion intercalation and subsequent gas evolutions at room temperature for one minute expand the graphite layers that increase the electrode surface area. Meanwhile, sufficient oxygen functional groups are introduced to the electrode, resulting in enhanced reaction kinetics and improved hydrophilicity. Further, spin‐polarized density functional theory is employed to reveal the role of oxygen functional groups in accelerating the vanadium redox reaction. Benefitting from sufficient oxygen groups, larger surface area, and superior wettability, the as‐prepared exfoliated GF (E‐GF) shows exceptional electrocatalytic activity with minimized overpotential, higher volumetric capacity, and improved energy efficiency. The redox flow battery assembled with the E‐GF electrode delivers voltage and energy efficiencies of 89.72% and 86.41% at the current density of 100 mA cm⁻², respectively. Remarkably, compared to the traditional GF treatment method, the elimination of the high temperature and long‐time treatment processes make this approach much more energy and time efficient, scalable, and affordable for large‐scale manufacturing.

A multicomponent magnetic phase diagram induced by biaxial strain is demonstrated in thin films of piezomagnetic Mn₃NiN. Confirming theoretical predictions, regions of high and low magnetization exist under compressive and tensile strain, respectively. Large and sharp magnetic changes can be achieved by traversing this rich phase diagram, opening the field to novel piezospintronic devices based on Mn₃ AN materials.

Abstract

Multicomponent magnetic phase diagrams are a key property of functional materials for a variety of uses, such as manipulation of magnetization for energy efficient memory, data storage, and cooling applications. Strong spin‐lattice coupling extends this functionality further by allowing electric‐field‐control of magnetization via strain coupling with a piezoelectric. Here this work explores the magnetic phase diagram of piezomagnetic Mn₃NiN thin films, with a frustrated noncollinear antiferromagnetic (AFM) structure, as a function of the growth induced biaxial strain. Under compressive strain, the films support a canted AFM state with large coercivity of the transverse anomalous Hall resistivity, ρ_xy , at low temperature, that transforms at a well‐defined Néel transition temperature (T _N) into a soft ferrimagnetic‐like (FIM) state at high temperatures. In stark contrast, under tensile strain, the low temperature canted AFM phase transitions to a state where ρ_xy is an order of magnitude smaller and therefore consistent with a low magnetization phase. Neutron scattering confirms that the high temperature FIM‐like phase of compressively strained films is magnetically ordered and the transition at T _N is first‐order. The results open the field toward future exploration of electric‐field‐driven piezospintronic and thin film caloric cooling applications in both Mn₃NiN itself and the broader Mn₃ AN family.

Recently, transistor‐based artificial synapses have received much attention due to their good stability, relatively controllable test parameters, and clear operating mechanisms. In addition, they can perform concurrent learning, in which synaptic weight can be performed without interrupting the signal transmission process. This review summarizes recent advances in transistor‐based artificial synapses.

Abstract

Simulating biological synapses with electronic devices is a re‐emerging field of research. It is widely recognized as the first step in hardware building brain‐like computers and artificial intelligent systems. Thus far, different types of electronic devices have been proposed to mimic synaptic functions. Among them, transistor‐based artificial synapses have the advantages of good stability, relatively controllable testing parameters, clear operation mechanism, and can be constructed from a variety of materials. In addition, they can perform concurrent learning, in which synaptic weight update can be performed without interrupting the signal transmission process. Synergistic control of one device can also be implemented in a transistor‐based artificial synapse, which opens up the possibility of developing robust neuron networks with significantly fewer neural elements. These unique features of transistor‐based artificial synapses make them more suitable for emulating synaptic functions than other types of devices. However, the development of transistor‐based artificial synapses is still in its very early stages. Herein, this article presents a review of recent advances in transistor‐based artificial synapses in order to give a guideline for future implementation of synaptic functions with transistors. The main challenges and research directions of transistor‐based artificial synapses are also presented.

The structural evolutions of MoS₂ during the electrochemical lithiation/delithiation process are systematically investigated using synchrotron X‐ray absorption spectroscopy and Raman spectroscopy. It is revealed that amorphous MoS₂ nanograins are generated after delithiation, and the fully lithiated products involve additional Mo‐S related phases besides the known Mo and Li₂S.

Abstract

Molybdenum disulfide (MoS₂) is a promising high‐capacity anode for lithium‐ion batteries. However, the conversion reaction mechanism of MoS₂ (the delithiation pathway in particular) has been controversial, which limits the rational optimization of its electrochemical performance. The main challenge is how to precisely identify the amorphous nanomaterials generated during lithiation/delithiation. Here, the structural evolutions of MoS₂ during lithiation/delithiation are systematically investigated using synchrotron X‐ray absorption spectroscopy at Mo K‐edge and S K‐edge and Raman spectroscopy. It is revealed that amorphous MoS₂ nanograins rather than sulfur as previously suggested, are formed after delithiation, and that the fully lithiated MoS₂ electrode contains additional Mo‐S related phases besides the known Mo and Li₂S. Density functional theory simulations suggest that the Mo nanoparticles formed during lithiation are very reactive with Li₂S, thus enabling the regeneration of MoS₂ upon delithiation. These findings deepen the understanding of the lithiation/delithiation mechanism of MoS₂, which will pave the way for the rational design of advanced MoS₂‐based electrodes.

This work reports a controlled one‐step synthesis of high‐quality 2D InSe thin films via a chemical vapor transport method. The hexagonal boron nitride‐encapsulated InSe flakes show high mobilities and the ability to observe the quantum Hall effect in a directly synthesized van der Waals semiconductor. This work provides a scalable and cost‐efficient technique to produce high‐quality 2D semiconductors for device applications with high mobility.

Abstract

Recently, 2D electron gases have been observed in atomically thin semiconducting crystals, enabling the observation of rich physical phenomena at the quantum level within the ultimate thickness limit. However, the observation of 2D electron gases and subsequent quantum Hall effect require exceptionally high crystalline quality, rendering mechanical exfoliation as the only method to produce high‐quality 2D semiconductors of black phosphorus and indium selenide (InSe), which hinder large‐scale device applications. Here, the controlled one‐step synthesis of high‐quality 2D InSe thin films via chemical vapor transport method is reported. The carrier Hall mobility of hexagonal boron nitride (hBN) encapsulated InSe flakes can be up to 5000 cm² V⁻¹ s⁻¹ at 1.5 K, enabling to observe the quantum Hall effect in a synthesized van der Waals semiconductor. The existence of the quantum Hall effect in directly synthesized 2D semiconductors indicates a high quality of the chemically synthesized 2D semiconductors, which hold promise in quantum devices and applications with high mobility.

In article number https://doi.org/10.1002/adfm.2019022161902216, SungWoo Nam and co‐workers report a colorimetric strain sensor with electrical quantification based on an integrated system of colloidal photonic crystals and a crumpled graphene photo‐transducer. The developed sensor enables direct visual readout and a 100‐fold improved strain sensing over plain crumpled graphene strain sensors in applications including body motion monitoring.

For the first time, a 2D material–based photodetector is reported using ionic glass as the electrostatic gating method, choosing MoSe₂ over LaF₃ ionic glass as an archetypal system. The wider possibilities offered by this architecture are unveiled, and a careful analysis of its unique optoelectronic properties is provided.

Abstract

Modulating the carrier density of 2D materials is pivotal to tailor their electrical properties, with novel physical phenomena expected to occur at a higher doping level. Here, the use of ionic glass as a high capacitance gate is explored to develop a 2D material–based phototransistor operated with a higher carrier concentration up to 5 × 10¹³ cm⁻², using MoSe₂ over LaF₃ as an archetypal system. Ion glass gating reveals to be a powerful technique combining the high carrier density of electrolyte gating methods while enabling direct optical addressability impeded with usual electrolyte technology. The phototransistor demonstrates I _ON/I _OFF ratio exceeding five decades and photoresponse times down to 200 µs, up to two decades faster than MoSe₂ phototransistors reported so far. Careful phototransport analysis reveals that ionic glass gating of 2D materials allows tuning the nature of the carrier recombination processes, while annihilating the traps' contribution in the electron injection regime. This remarkable property results in a photoresponse that can be modulated electrostatically by more than two orders of magnitude, while at the same time increasing the gain bandwidth product. This study demonstrates the potential of ionic glass gating to explore novel photoconduction processes and alternative architectures of devices.

Light‐assisted charge propagation and switching between nonconductive and conductive states is demonstrated in networks of self‐assembled and self‐aligned organic nanostructures on hexagonal boron nitride. By employing the inherent anisotropy of the organic nanostructures' optical properties, it is possible to allow selective charge propagation only in one direction within the crystallite networks controlled by the light polarization direction.

Abstract

Introducing organic semiconductors as additional building blocks into heterostructures of 2D materials widens the horizon of their applications. Organic molecules can form self‐assembled and self‐aligned crystalline nanostructures on 2D materials, resulting in well‐defined interfaces that preserve the intrinsic properties of both constituents. Thus, organic molecules add unique capabilities to van der Waals heterostructures that have no analogues in inorganic matter. This study explores light‐assisted charge propagation in organic semiconductor networks of quasi‐1D needle‐like crystallites, epitaxially grown on insulating hexagonal boron nitride. Electrostatic force microscopy is employed to demonstrate that upon external illumination it is possible to change the conductivity of organic crystallites by more than two orders of magnitude. Furthermore, by exploiting the highly anisotropic optical properties of the organic nanoneedles, a selective charge propagation along the crystallites is triggered that matches the orientation of the molecular backbones with the incident light's polarization direction. These results demonstrate the possibility to use a “light‐gate” to switch on the conductivity of organic nanostructures and even to guide the charge propagation along desired directions in self‐assembled crystallite networks.

The outstanding thermoelectric performance of pristine half‐filled p‐bonded chalcogenides with octahedral arrangement can be understood from a chemical bonding perspective, where different bonding mechanisms can be separated in a map depicting the electrons transferred and/or shared between adjacent atoms. Metavalent bonding is responsible for the large band degeneracy, the band anisotropy, and the low lattice thermal conductivity, giving rise to a promising thermoelectric performance.

Abstract

Thermoelectric materials have attracted significant research interest in recent decades due to their promising application potential in interconverting heat and electricity. Unfortunately, the strong coupling between the material parameters that determine thermoelectric efficiency, i.e., the Seebeck coefficient, electrical conductivity, and thermal conductivity, complicates the optimization of thermoelectric energy converters. Main‐group chalcogenides provide a rich playground to alleviate the interdependence of these parameters. Interestingly, only a subgroup of octahedrally coordinated chalcogenides possesses good thermoelectric properties. This subgroup is also characterized by other outstanding characteristics suggestive of an exceptional bonding mechanism, which has been coined metavalent bonding. This conclusion is further supported by a map that separates different bonding mechanisms. In this map, all octahedrally coordinated chalcogenides with good performance as thermoelectrics are located in a well‐defined region, implying that the map can be utilized to identify novel thermoelectrics. To unravel the correlation between chemical bonding mechanism and good thermoelectric properties, the consequences of this unusual bonding mechanism on the band structure are analyzed. It is shown that features such as band degeneracy and band anisotropy are typical for this bonding mechanism, as is the low lattice thermal conductivity. This fundamental understanding, in turn, guides the rational materials design for improved thermoelectric performance by tailoring the chemical bonding mechanism.

Focused ion beam irradiation of monolayer transition metal dichalcogenides is performed to produce single atom to 50 nm defects and the spatial distribution of defects caused during ion beam raster is investigated for the first time. Parameters such as material (MoS₂ or WS₂), device configuration (suspended or supported), and irradiation dose (10¹³–10¹⁶ ions cm⁻²) are used to engineer defect density and average defect area.

Abstract

Manipulation and structural modifications of 2D materials for nanoelectronic and nanofluidic applications remain obstacles to their industrial‐scale implementation. Here, it is demonstrated that a 30 kV focused ion beam can be utilized to engineer defects and tailor the atomic, optoelectronic, and structural properties of monolayer transition metal dichalcogenides (TMDs). Aberration‐corrected scanning transmission electron microscopy is used to reveal the presence of defects with sizes from the single atom to 50 nm in molybdenum (MoS₂) and tungsten disulfide (WS₂) caused by irradiation doses from 10¹³ to 10¹⁶ ions cm⁻². Irradiated regions across millimeter‐length scales of multiple devices are sampled and analyzed at the atomic scale in order to obtain a quantitative picture of defect sizes and densities. Precise dose value calculations are also presented, which accurately capture the spatial distribution of defects in irradiated 2D materials. Changes in phononic and optoelectronic material properties are probed via Raman and photoluminescence spectroscopy. The dependence of defect properties on sample parameters such as underlying substrate and TMD material is also investigated. The results shown here lend the way to the fabrication and processing of TMD nanodevices.

The use of scanning tunneling microscopy to lithographically pattern hydrogen terminated silicon enables atomic precision imbedding of phosphorus for quantum devices and materials. Single electron transistors including single and few atom transistors fabricated using this technique are demonstrated. Transport measurements reveal well‐defined and highly stable quantum behavior including Coulomb diamonds and an artificial hydrogenic atom in silicon.

Abstract

Atomically precise fabrication has an important role to play in developing atom‐based electronic devices for use in quantum information processing, quantum materials research, and quantum sensing. Atom‐by‐atom fabrication has the potential to enable precise control over tunnel coupling, exchange coupling, on‐site charging energies, and other key properties of basic devices needed for solid‐state quantum computing and analog quantum simulation. Using hydrogen‐based scanning probe lithography, individual dopant atoms are deterministically placed relative to atomically aligned contacts and gates to build single electron transistors, single atom transistors, and gate‐controlled quantum sensing devices. The key steps required to fabricate and demonstrate the essential building blocks needed for spin selective initialization/readout and coherent quantum manipulation are described.

Focused ion beam irradiation of monolayer transition metal dichalcogenides is performed to produce single atom to 50 nm defects and the spatial distribution of defects caused during ion beam raster is investigated for the first time. Parameters such as material (MoS₂ or WS₂), device configuration (suspended or supported), and irradiation dose (10¹³–10¹⁶ ions cm⁻²) are used to engineer defect density and average defect area.

Abstract

Manipulation and structural modifications of 2D materials for nanoelectronic and nanofluidic applications remain obstacles to their industrial‐scale implementation. Here, it is demonstrated that a 30 kV focused ion beam can be utilized to engineer defects and tailor the atomic, optoelectronic, and structural properties of monolayer transition metal dichalcogenides (TMDs). Aberration‐corrected scanning transmission electron microscopy is used to reveal the presence of defects with sizes from the single atom to 50 nm in molybdenum (MoS₂) and tungsten disulfide (WS₂) caused by irradiation doses from 10¹³ to 10¹⁶ ions cm⁻². Irradiated regions across millimeter‐length scales of multiple devices are sampled and analyzed at the atomic scale in order to obtain a quantitative picture of defect sizes and densities. Precise dose value calculations are also presented, which accurately capture the spatial distribution of defects in irradiated 2D materials. Changes in phononic and optoelectronic material properties are probed via Raman and photoluminescence spectroscopy. The dependence of defect properties on sample parameters such as underlying substrate and TMD material is also investigated. The results shown here lend the way to the fabrication and processing of TMD nanodevices.

Light‐assisted charge propagation and switching between nonconductive and conductive states is demonstrated in networks of self‐assembled and self‐aligned organic nanostructures on hexagonal boron nitride. By employing the inherent anisotropy of the organic nanostructures' optical properties, it is possible to allow selective charge propagation only in one direction within the crystallite networks controlled by the light polarization direction.

Abstract

Introducing organic semiconductors as additional building blocks into heterostructures of 2D materials widens the horizon of their applications. Organic molecules can form self‐assembled and self‐aligned crystalline nanostructures on 2D materials, resulting in well‐defined interfaces that preserve the intrinsic properties of both constituents. Thus, organic molecules add unique capabilities to van der Waals heterostructures that have no analogues in inorganic matter. This study explores light‐assisted charge propagation in organic semiconductor networks of quasi‐1D needle‐like crystallites, epitaxially grown on insulating hexagonal boron nitride. Electrostatic force microscopy is employed to demonstrate that upon external illumination it is possible to change the conductivity of organic crystallites by more than two orders of magnitude. Furthermore, by exploiting the highly anisotropic optical properties of the organic nanoneedles, a selective charge propagation along the crystallites is triggered that matches the orientation of the molecular backbones with the incident light's polarization direction. These results demonstrate the possibility to use a “light‐gate” to switch on the conductivity of organic nanostructures and even to guide the charge propagation along desired directions in self‐assembled crystallite networks.

Nature Nanotechnology, Published online: 05 August 2019; doi:10.1038/s41565-019-0514-y

Nature Physics, Published online: 05 August 2019; doi:10.1038/s41567-019-0602-9

Nature Physics, Published online: 05 August 2019; doi:10.1038/s41567-019-0606-5