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Nature Physics, Published online: 24 September 2019; doi:10.1038/s41567-019-0699-x

Author Correction: Enhancement of interlayer exchange in an ultrathin two-dimensional magnet

Recent advances in small‐molecule‐based organic field‐effect transistors for nonvolatile memory and artificial synapse are comprehensively reviewed. Working mechanisms and optimization approaches are discussed in detail, with a view toward inspiring more intriguing ideas on the rational design of materials and device structures.

Abstract

With the incorporation of tailorable organic electronic materials as channel and storage materials, organic field‐effect transistor (OFET)‐based memory has become one of the most promising data storage technologies for hosting a variety of emerging memory applications, such as sensory memory, storage memory, and neuromorphic computing. Here, the recent state‐of‐the‐art progresses in the use of small molecules for OFET nonvolatile memory and artificial synapses are comprehensively reviewed, focusing on the characteristic features of small molecules in versatile functional roles (channel, storage, modifier, and dopant). Techniques for optimizing the storage capacity, speed, and reliability of nonvolatile memory devices are addressed in detail. Insight into the use of small molecules in artificial synapses constructed on OFET memory is also obtained in this emerging field. Finally, the strategies of molecular design for improving memory performance in view of small molecules as storage mediums are discussed systematically, and challenges are addressed to shed light on the future development of this vital research field.

Nanohybrids with antagonistic properties (high capacitance and good conductivity) like pMoS₂‐nSNrGO are demonstrated among excellent electrode materials for ionic actuators. With a 670% bending improvement at a low voltage of 0.5 V and the ability to perform fast bending up to 15 Hz, the pMoS₂‐nSNrGO‐based actuators successfully act as soft fingers to touch fragile surfaces of smartphones to switch the flashlight.

Abstract

Future smart mobile electronics and wearable robotics that can perform delicate activities controlled by artificial intelligence can require rapid motion actuators working at low voltages with acceptable safety and improved energy efficiency. Accordingly, ionic soft actuators can have great potential over other counterparts because they exhibit gentle movements at low voltages, less than 2 V. However, these actuators currently show deficient performances at sub‐1 V voltages in the high‐frequency range because of the lack of electrode materials with the vital antagonistic properties of high capacitance and good conductivity. Herein, a mutually exclusive nanohybrid electrode (pMoS₂‐nSNrGO) is reported consisting of oxide‐doped p‐type molybdenum‐disulfide and sulfur‐nitrogen‐codoped n‐type reduced‐graphene‐oxide. The pMoS₂‐nSNrGO electrode derives high capacitance from MoS₂ and good charge transfer between the two components from p‐n nano‐junctions, resulting in excellent actuation performances (670% improvement compared with rGO electrode at 0.5 V and 1 Hz, together with fast responses up to 15 Hz). With such excellent performances, these actuators can be successfully applied to realize an artificial soft robotic finger system for delicately touching the fragile surfaces of smartphones and tablets. The mutually exclusive pMoS₂‐nSNrGO electrode can open a new way to develop high‐performance soft actuators for soft robotic applications in the future.

Nature Physics, Published online: 16 September 2019; doi:10.1038/s41567-019-0651-0

Few-layer magnetic materials sometimes show a different form of magnetism from their thicker equivalents. The authors contend that the mechanism is changes in the stacking order in the thin limit that modify the interlayer exchange interaction.

Aharonov–Bohm interferences in the quantum Hall regime are observed when electrons are transmitted between two edge channels. Such a phenomenon has been realized in 2D systems such as quantum point contacts, anti-dots and p-n junctions. Based on a theoretical investigation of the magnetotransport in stepped graphene, a new kind of Aharonov–Bohm interferometers is proposed herewith. Indeed, when a strong magnetic field is applied in a proper direction, oppositely propagating edge states can be achieved in both terrace and facet zones of the step, leading to the interedge scatterings and hence strong Aharonov–Bohm oscillations in the conductance in the quantum Hall regime. Taking place in the unipolar regime, this interference is also predicted in stepped systems of other 2D layered materials.

The doping of three‐dimensional (3D) graphene has emerged as a topic of interest because of attempts to combine its large available surface area and superior catalytic, structural, chemical, and biocompatible characteristics that can be induced by doping. This review provides an overview of the scalable chemical‐vapor‐deposition‐based growth of doped 3D graphene materials and their applications in various contexts.

Abstract

Graphene doping principally commenced to compensate for its inert nature and create an appropriate bandgap. Doping of 3D graphene has emerged as a topic of interest because of attempts to combine its large available surface area—arising from its interconnected porous architecture—with superior catalytic, structural, chemical, and biocompatible characteristics that can be induced by doping. In light of the latest developments, this review provides an overview of the scalable chemical vapor deposition (CVD)‐based growth of doped 3D graphene materials as well as their applications in various contexts, such as in devices used for energy generation and gas storage and biosensors. In particular, single‐ and multielement doping of 3D graphene by various dopants (such as nitrogen (N), boron (B), sulfur (S) and phosphorous (P)), the doping configurations of the resultant materials, an overview of recent developments in the field of CVD, and the influence of various parameters of CVD on graphene doping and 3D morphologies are focused in this paper. Finally, this report concludes the discussion by mentioning the existing challenges and future opportunities of these developing graphitic materials, intending to inspire the unveiling of more exciting functionalized 3D graphene morphologies and their potential properties, which can hopefully realize many possible applications.

Various nanoparticles can be regulated by four different strategies including control of interparticle interactions (noncovalent bonds, magnetic, electrostatic, van der Waals forces, etc.), ligand engineering (ligand exchange and ligand collapse), temporal regulation (such as lifetime of active species), and spatial regulation (such as distribution of active species). The final products have potential applications in multiplexing, analyte‐detection, photocatalysis, etc.

Abstract

Nanoparticles (NPs) have been studied for several decades, and outstanding advancements have been made involving fabrication methods and various properties for many different applications. NP assemblies exhibit collective properties that are superior to the properties of individual NPs, and their assembly behavior is significantly affected by interparticle interactions and surrounding layers. The temporal/spatial regulation of NPs and the active species present in NP systems are crucial for achieving desirable performances. Here, the interparticle interactions, surrounding materials (especially ligands), and temporal and spatial regulation are the main topics discussed. The principles and classical examples of these regulation strategies are provided, and the resulting NPs regulated by these strategies exhibit remarkable properties and have great potential for various applications. Finally, the future prospects of the NPs are outlined with respect to the surface modification, temporal and spatial regulation, as well as the binary cooperative complementary principle.

A biomimetic leaf‐like nanostructure composed of 1D AgNWs skeleton (vein) and 2D MXene as the lamina is fabricated via vacuum‐assisted layer‐by‐layer assembly for electromagnetic interference (EMI) shielding, humidity monitoring, and self‐derived hydrophobicity. (MA₁)₁₀ silk presents exceptional EMI shielding effectiveness of ≈90 dB at 12.4 GHz at a thickness of 480 µm, and MXene‐coated textile induces a hydrophilic‐to‐hydrophobic transition, generating a large contact angle of >140°.

Abstract

Although flexible and multifunctional textiles are promising for wearable electronics and portable device applications, the main issue is to endow textiles with multifunctionalities while maintaining their innate flexible and porous features. Herein, a vacuum‐assisted layer‐by‐layer assembly technique is demonstrated to conformally deposit electrically conductive substances on textiles for developing multifunctional and flexible textiles with superb electromagnetic interference (EMI) shielding performances, superhydrophobicity, and highly sensitive humidity response. The formed leaf‐like nanostructure is composed of silver nanowires (AgNWs) as the highly conductive skeleton (vein) and transition metal carbide/carbonitride (MXene) nanosheets as the lamina. The presence of MXene protects AgNWs from oxidation and enhances the combination of AgNWs with the fabric substrate, and the transformation of its functional groups leads to self‐derived hydrophobicity. The flexible and multifunctional textile exhibits a low sheet resistance of 0.8 Ω sq⁻¹, outstanding EMI shielding efficiency of 54 dB in the X‐band at a small thickness of 120 µm, and highly sensitive humidity responses, while retaining its satisfactory porosity and permeability. The self‐derived hydrophobicity with a large contact angle of >140° is achieved by aging the hydrophilic MXene coated silk. The wearable multifunctional textiles are highly promising for applications in intelligent garments, humidity sensors, actuators, and EMI shielding.

Controlling the dynamic responses of different ordered phases has stimulated extensive research interests. Here, the implementation of a charge‐density‐wave (CDW) for terahertz detection is reported and its extremely high sensitivity driven by CDW distortion is demonstrated. The collective excitation combined with the strong interaction with long‐wavelength photons opens up novel feasibility toward realistic exploration of many‐body states for imaging and sensing applications.

Abstract

The quantum behavior of carriers in solid is the foundation of modern electronic and optoelectronic technology, but it is still facing huge challenges within inherited single‐particle quantum processes working at the millimeter wave/terahertz (THz) band. Here, a straightforward strategy for the direct detection of millimeter wave/THz photons in a sub‐wavelength metal‐TaSe₂‐metal structure under strong interaction with a localized field of surface plasmon is proposed. By breaking the inversion symmetry under the perturbations of electric field and atomic reconstruction from van der Waals integration, the nonequilibrium electronic states under a radiant field can be manipulated in a collective fashion, leading to a large photocurrent responsivity over 40 A W⁻¹ and noise equivalent power less than 1 pW Hz^−1/2 even at room temperature. A more than 40‐fold enhancement in responsivity is achieved when transitioning from the normal phase to the CDW phase. The findings shed fresh light on the understanding of the delicate balance in the charge‐ordered phase, and facilitate the exploitation of a correlated electron system for optoelectronic applications in fields of security, remote sensing, and imaging.

This review discusses the progress of key experimental and theoretical components of scanning thermal microscopy including thermal probes, experimental methods, heat transfer mechanisms, and calibration strategies and highlights the recent applications to novel materials and devices, with emphasis on thermoelectric, biological, phase change, and 2D materials.

Abstract

As the size of materials, particles, and devices shrinks to nanometer, atomic, or even quantum scale, it is more challenging to characterize their thermal properties reliably. Scanning thermal microscopy (SThM) is an emerging method to obtain local thermal information by controlling and monitoring probe–sample thermal exchange processes. In this review, key experimental and theoretical components of the SThM system are discussed, including thermal probes and experimental methods, heat transfer mechanisms, calibration strategies, thermal exchange resistance, and effective heat transfer coefficients. Additionally, recent applications of SThM to novel materials and devices are reviewed, with emphasis on thermoelectric, biological, phase change, and 2D materials.

A flexible photocatalyst is successfully prepared by growing N‐WO₃/Ce₂S₃ nanotube bundles onto a flexible carbon textile. The resultant photocatalyst exhibits a flexible interwoven 3D conductive architecture, a large interfacial area, excellent light absorption, and superior separation efficiency of photoinduced electron–hole pairs, delivering remarkable photocatalytic degradation of organic compounds in both air and water media.

Abstract

The availability of robust, versatile, and efficient photocatalysts is the main bottleneck in practical applications of photocatalytic degradation of organic pollutants. Herein, N‐WO₃/Ce₂S₃ nanotube bundles (NBs) are synthesized and successfully immobilized on a carbon textile, resulting in a flexible and conducting photocatalyst. Due to the large interfacial area between N‐WO₃ and Ce₂S₃, the interwoven 3D carbon architecture and, more importantly, the establishment of a heterojunction between N‐WO₃ and Ce₂S₃, the resultant photocatalyst exhibits excellent light absorption capacity and superior ability to separate photoinduced electron–hole pairs for the photocatalytic degradation of organic compounds in air and water media. Theoretical calculations confirm that the strong electronic interaction between N‐WO₃ and Ce₂S₃ can be beneficial to the enhancement of the charge carrier transfer dynamics of the as‐prepared photocatalyst. This work provides a new protocol for constructing efficient flexible photocatalysts for application in environmental remediation.

In article number https://doi.org/10.1002/adfm.2019037381903738, Hongkun Tian, Feng Zhu, and co‐workers present fully integrated microscale organic field‐effect transistors (micro‐OFETs). By mastering the local growth of molecular semiconductors on pre‐defined terraces and developing nondestructive photolithographic processes, micro‐OFET arrays based on single‐crystal quasi‐two‐dimensional molecular layers are created, providing potential applications in high‐density monolithic integration of low‐cost circuits and large‐area flexible displays.

A novel approach for Mott transition property tuning has been achieved by introducing metal phases into a vanadium dioxide (VO₂) matrix to form metal‐VO₂ nanocomposite thin films. Broad range tuning has been achieved via controlling the metal phase dimension. The tuning effects are attributed to the energy band structure reconstruction around metal‐VO₂ phase boundaries.

Abstract

Vanadium dioxide (VO₂) is a well‐studied Mott‐insulator because of the very abrupt physical property switching during its semiconductor‐to‐metal transition (SMT) around 341 K (68 °C). In this work, through novel oxide‐metal nanocomposite designs (i.e., Au:VO₂ and Pt:VO₂), a very broad range of SMT temperature tuning from ≈323.5 to ≈366.7 K has been achieved by varying the metallic secondary phase in the nanocomposites (i.e., Au:VO₂ and Pt:VO₂ thin films, respectively). More surprisingly, the SMT T _c can be further lowered to ≈301.8 K (near room temperature) by reducing the Au particle size from 11.7 to 1.7 nm. All the VO₂ nanocomposite thin films maintain superior phase transition performance, i.e., large transition amplitude, very sharp transition, and narrow width of thermal hysteresis. Correspondingly, a twofold variation of the complex dielectric function has been demonstrated in these metal‐VO₂ nanocomposites. The wide range physical property tuning is attributed to the band structure reconstruction at the metal‐VO₂ phase boundaries. This demonstration paved a novel approach for tuning the phase transition property of Mott‐insulating materials to near room temperature transition, which is important for sensors, electrical switches, smart windows, and actuators.

A versatile water‐assisted printing technique is demonstrated to fabricate single‐ and multilayer graphene ink based devices onto 3D objects of different shapes, curvatures, and textures.

Abstract

Printing has drawn a lot of attention as a means of low per‐unit cost and high throughput patterning of graphene inks for scaled‐up thin‐form factor device manufacturing. However, traditional printing processes require a flat surface and are incapable of achieving patterning onto 3D objects. Here, a conformal printing method is presented to achieve functional graphene‐based patterns onto arbitrarily shaped surfaces. Using experimental design, a water‐insoluble graphene ink with optimum conductivity is formulated. Then single‐ and multilayered electrically functional structures are printed onto a sacrificial layer using conventional screen printing. The print is then floated on water, allowing the dissolution of the sacrificial layer, while retaining the functional patterns. The single‐ and multilayer patterns can then be directly transferred onto arbitrarily shaped 3D objects without requiring any postdeposition processing. Using this technique, conformal printing of single‐ and multilayer functional devices that include joule heaters, resistive deformation sensors, and proximity sensors on hard, flexible, and soft substrates, such as glass, latex, thermoplastics, textiles, and even candies and marshmallows, is demonstrated. This simple strategy promises to add new device and sensing functionalities to previously inert 3D surfaces.

Knittable and washable MXene‐coated cellulose yarns are developed via a two‐step dip coating process for use in wearable applications. These conductive yarns, which combine the versatile chemistry and promising electrical and electrochemical properties of MXenes with existing cellulose‐based yarns, can offer a platform technology for various textile‐based devices by allowing tunability in performance for the building blocks of textiles.

Abstract

Textile‐based electronics enable the next generation of wearable devices, which have the potential to transform the architecture of consumer electronics. Highly conductive yarns that can be manufactured using industrial‐scale processing and be washed like everyday yarns are needed to fulfill the promise and rapid growth of the smart textile industry. By coating cellulose yarns with Ti₃C₂T _x MXene, highly conductive and electroactive yarns are produced, which can be knitted into textiles using an industrial knitting machine. It is shown that yarns with MXene loading of ≈77 wt% (≈2.2 mg cm⁻¹) have conductivity of up to 440 S cm⁻¹. After washing for 45 cycles at temperatures ranging from 30 to 80 °C, MXene‐coated cotton yarns exhibit a minimal increase in resistance while maintaining constant MXene loading. The MXene‐coated cotton yarn electrode offers a specific capacitance of 759.5 mF cm⁻¹ at 2 mV s⁻¹. A fully knitted textile‐based capacitive pressure sensor is also prepared, which offers high sensitivity (gauge factor of ≈6.02), wide sensing range of up to ≈20% compression, and excellent cycling stability (2000 cycles at ≈14% compression strain). This work provides new and practical insights toward the development of platform technology that can integrate MXene in cellulose‐based yarns for textile‐based devices.

Ultrathin Pt porous nanosheets are created for the first time and applied to oxygen reduction reaction (ORR) electrocatalysis. Nanosurface engineering with simultaneous high surface distortion and Pt utilization achieves preeminent ORR performance.

Abstract

Unlike the well‐established shape/composition control, surface distortion is a newly emerged yet largely unexplored nanosurface engineering for boosting electrocatalysis. Tapping into the novel electrocatalysts for taking full use of the distortion effect is therefore of importance but remains a formidable challenge. Here, an approach to designing highly distorted porous Pt nanosheets (NSs) by electrochemical erosion of ultrathin PtTe₂ NSs is reported. The inherent ultrathin feature and massive leaching of Te have conspired to produce a highly distorted structure. As a result, the generated Pt NSs exhibit a much‐enhanced oxygen reduction reaction (ORR) mass and specific activity of 2.07 A mg_Pt ⁻¹ and 3.1 mA cm⁻² at 0.90 V versus reversible hydrogen electrode, 9.8 and 10.7 times higher than those of commercial Pt/C. The highly distorted Pt NSs can endure 30 000 cycles with negligible activity decay and structure variation. Density functional theory calculations reveal that the electrochemical corrosion induced nanopores, boundaries, and vacancies consist of Pt sites with substantially low coordination numbers deviating from the one of pristine Pt (111) surface. These Pt sites actively act as electron‐depleting centers for highly efficient electron transfer toward the adsorbing O‐species. This study opens a new design for fully using the distortion effect to promote ORR performance and beyond.

The thermal conductivity of thermoelectric (PbTe–PbSe) nanostructured thin films grown by atomic layer deposition is investigated in this work. The compositional variation and resulting defects in the nanostructures are effective in reducing the thermal conductivity below that of the control PbTe and PbSe films, showing a pathway to engineer materials for further improvement of the thermoelectric figure of merit (ZT).

Abstract

This work studies the thermal conductivity and phonon scattering processes in a series of n‐type lead telluride‐lead selenide (PbTe–PbSe) nanostructured thin films grown by atomic layer deposition (ALD). The ALD growth of the PbTe–PbSe samples in this work results in nonepitaxial films grown directly on native oxide/Si substrates, where the Volmer–Weber mode of growth promotes grains with a preferred columnar orientation. The ALD growth of these lead‐rich PbTe, PbSe, and PbTe–PbSe thin films results in secondary oxide phases, along with an increase microstructural quality with increased film thickness. The compositional variation and resulting point and planar defects in the PbTe–PbSe nanostructures give rise to additional phonon scattering events that reduce the thermal conductivity below that of the corresponding ALD‐grown control PbTe and PbSe films. Temperature‐dependent thermal conductivity measurements show that the phonon scattering in these ALD‐grown PbTe–PbSe nanostructured materials, along with ALD‐grown PbTe and PbSe thin films, are driven by extrinsic defect scattering processes as opposed to phonon–phonon scattering processes intrinsic to the PbTe or PbSe phonon spectra. The implication of this work is that polycrystalline, nanostructured ALD composites of thermoelectric PbTe–PbSe films are effective in reducing the phonon thermal conductivity, and represent a pathway for further improvement of the figure of merit (ZT), enhancing their thermoelectric application potential.

Bilayer graphene (BLG) attracts great interest in both scientific research and potential applications. To achieve integrated applications of functional devices, synthesis of large-scale, single-crystalline BLG with a controlled stacking structure on insulating substrates is highly desired. However, so far, only polycrystalline BLG has been fabricated on insulating substrates, with single-crystalline domains limited to about half a millimeter. Here, we demonstrate successful fabrication of centimeter-scale, single-crystalline AB-stacked BLG on insulating substrates. First, we epitaxially grow single-crystalline BLG on Ru(0 0 0 1) and then transfer it to insulating substrates by an oxygen-intercalation-assisted electrochemical approach. The oxygen atoms intercalated between the BLG and Ru effectively decouple the BLG from Ru, enabling its successful transfer to a SiO 2 substrate, which makes it readily available to Si-based technologies. Low-energy electron diffraction, ...

Among two-dimensional (2D) transition metal dichalcogenides (TMDs), platinum diselenide (PtSe 2 ) stands at a unique place in the sense that it undergoes a phase transition from type-II Dirac semimetal to indirect-gap semiconductor as thickness decreases. Defects in 2D TMDs are ubiquitous and play crucial roles in understanding and tuning electronic, optical, and magnetic properties. Here we investigate intrinsic point defects in ultrathin 1T-PtSe 2 layers grown on mica through the chemical vapor transport (CVT) method, using scanning tunneling microscopy and spectroscopy (STM/STS) and first-principles calculations. We observed five types of distinct defects from STM topography images and obtained the local density of states (LDOS) of the defects. By combining the STM results with the first-principles calculations, we identified the types and characteristics of these defects, which are Pt vacancies at the topmost and next monolayers, Se vacancies in the topmos...