Jing Zhang

A compact 1T‐MoS₂/graphene is prepared using a capillary tension strategy and employed as the anode for potassium‐ion batteries. The 1T‐MoS₂ with its high conductivity and wide layer spacing exhibits high electrochemical activity and fast kinetics for K⁺ storage. Based on this architectural construction, the superior gravimetric capacity (564 mAh g⁻¹) and volumetric capacity (512 mAh cm⁻¹) can be achieved simultaneously even after 100 cycles.

Abstract

The potassium‐ion battery (PIB) is an attractive energy storage device that possesses the potential advantages of high energy density and low cost. Herein, a pure 1T‐MoS₂ is synthesized on graphene oxide and assembled into a hydrogel. The hydrogel is further tightened to a compact 1T‐MoS₂/graphene (CTMG) bulk by a densifying strategy of capillary tension. When employed as an anode for PIBs, the CTMG electrode can store K⁺ through reversible intercalation and conversion electrochemistry, accompanied with fast kinetics since the 1T‐MoS₂ induces a pseudocapacitive storage mechanism and the extraordinary K⁺ transportation ability. Consequently, the CTMG electrode delivers the high and reversible rate capacities of 511 and 327 mAh g^‐1 at 0.1 and 1 A g^‐1, respectively. Moreover, the compact architecture reduces the electrode thickness by ≈33% enabling a high volumetric capacity (512 mAh cm^‐3 at 0.1 A g^‐1 after 100 cycles), as well as holding a promising application in thick electrode.

Noncommensurate 2D heterointerfaces hold great promise toward low friction and nanoelectromechanical applications since they should in principle enable superlubricity for all rotational configuration. In this work, it is shown that the origin for a high force barrier is a result of commensurability between the moiré structure and the contact geometry. Consequently, new geometries that can potentially overcome such commensurability are presented.

Abstract

Noncommensurate 2D interfaces hold great promise toward low friction and nanoelectromechanical applications. For identical constituents, the crystals interlock at specific rotational configurations leading to high barriers for slide. In contrast, nonidentical constituents comprising different lattice parameters should enable robust superlubricity for all rotational configurations. This is however not the case for gold–graphite interfaces, as both theory and experiments show scaling behavior of the sliding force as a function of the interface contact area. By simulating the sliding force for gold–graphite interfaces, this work shows that the origin for high force barriers at special angular configurations is a result of commensurability between the moiré structure and the contact geometry. Consequently, this paper suggests new geometries that can potentially overcome such commensurability effects to enable robust superlubricity.

A study of contact resistance tuning by self‐generated interlayer formation in organic field effect transistors is presented. The interlayer material is blended as an additive in the active layer and during metal deposition it migrates to the organic/metal interface. The generated interlayer of additive material modifies the device performance according to the tuning of the contact resistance.

Abstract

Contact resistance significantly limits the performance of organic field‐effect transistors (OFETs). Positioning interlayers at the metal/organic interface can tune the effective work‐function and reduce contact resistance. Myriad techniques offer interlayer processing onto the metal pads in bottom‐contact OFETs. However, most methods are not suitable for deposition on organic films and incompatible with top‐contact OFET architectures. Here, a simple and versatile methodology is demonstrated for interlayer processing in both p‐ and n‐type devices that is also suitable for top‐contact OFETs. In this approach, judiciously selected interlayer molecules are co‐deposited as additives in the semiconducting polymer active layer. During top contact deposition, the additive molecules migrate from within the bulk film to the organic/metal interface due to additive‐metal interactions. Migration continues until a thin continuous interlayer is completed. Formation of the interlayer is confirmed by X‐ray photoelectron spectroscopy (XPS) and cross‐section scanning transmission electron microscopy (STEM), and its effect on contact resistance by device measurements and transfer line method (TLM) analysis. It is shown that self‐generated interlayers that reduce contact resistance in p‐type devices, increase that of n‐type devices, and vice versa, confirming the role of additives as interlayer materials that modulate the effective work‐function of the organic/metal interface.

LaMnO₃/SrMnO₃ superlattices grown via a metalorganic aerosol deposition technique demonstrate a novel interfacial high‐temperature ferromagnetic phase, T _C ≈360 K, due to electron transfer from the LaMnO₃ into the SrMnO₃ layer. The charge transfer scale, λ_TF ≈2 unit cells, is determined via in situ growth monitoring by ellipsometry and confirmed by polarized neutron reflectometry and high resolution electron microscopy/spectroscopy.

Abstract

Heterostructures of strongly correlated oxides demonstrate various intriguing and potentially useful interfacial phenomena. LaMnO₃/SrMnO₃ superlattices are presented showcasing a new high‐temperature ferromagnetic phase with Curie temperature, T _C ≈360 K, caused by electron transfer from the surface of the LaMnO₃ donor layer into the neighboring SrMnO₃ acceptor layer. As a result, the SrMnO₃ (top)/LaMnO₃ (bottom) interface shows an enhancement of the magnetization as depth‐profiled by polarized neutron reflectometry. The length scale of charge transfer, λ_TF ≈2 unit cells, is obtained from in situ growth monitoring by optical ellipsometry, supported by optical simulations, and further confirmed by high resolution electron microscopy and spectroscopy. A model of the inhomogeneous distribution of electron density in LaMnO₃/SrMnO₃ layers along the growth direction is concluded to account for a complex interplay between ferromagnetic and antiferromagnetic layers in superlattices.

Modeling diffusion processes is a significant aspect of developing functional materials, e.g. ion conductors for energy applications. Commonly, modeling diffusion is done by computational science methodologies such as density functional theory and molecular dynamics. Modern approaches are taking advantage of artificial intelligence methods to accelerate research and to find heuristic models.

Abstract

Diffusion describes the stochastic motion of particles and is often a key factor in determining the functionality of materials. Modeling diffusion of atoms can be very challenging for heterogeneous systems with high energy barriers. In this report, popular computational methodologies are covered to study diffusion mechanisms that are widely used in the community and both their strengths and weaknesses are presented. In static approaches, such as electronic structure theory, diffusion mechanisms are usually analyzed within the nudged elastic band (NEB) framework on the ground electronic surface usually obtained from a density functional theory (DFT) calculation. Another common approach to study diffusion mechanisms is based on molecular dynamics (MD) where the equations of motion are solved for every time step for all the atoms in the system. Unfortunately, both the static and dynamic approaches have inherent limitations that restrict the classes of diffusive systems that can be efficiently treated. Such limitations could be remedied by exploiting recent advances in artificial intelligence and machine learning techniques. Here, the most promising approaches in this emerging field for modeling diffusion are reported. It is believed that these knowledge‐intensive methods have a bright future ahead for the study of diffusion mechanisms in advanced functional materials.

Nature Nanotechnology, Published online: 27 April 2020; doi:10.1038/s41565-020-0670-0

Magneto-optical interaction of light with magnetic metasurfaces can give rise to the photonic spin Hall effect such that the light trajectory depends on the polarization of the light. For disordered systems, the probability distribution of the spin-dependent trajectories is a sensitive tool to detect random nanoscale variations in the metasurface.

In ultra-pure materials electrons may exhibit a collective motion similar to the hydrodynamic flow of a viscous fluid, the phenomenon with far reaching consequences in a wide range of many body systems from black holes to high-temperature superconductivity. Yet the definitive detection of this intriguing behavior remains elusive. Until recently, experimental techniques for observing hydrodynamic behavior in solids were based on measuring macroscopic transport properties, such as the ‘nonlocal’ (or ‘vicinity’) resistance, which may allow alternative interpretation. Earlier this year two breakthrough experiments demonstrated two distinct imaging techniques making it possible to ‘observe’ the electronic flow directly. We demonstrate that a hydrodynamic flow in a long Hall bar (in the absence of magnetic field) exhibits a nontrivial vortex structure accompanied by a sign-alternating nonlocal resistance. An experimental observation of such unique flow pattern could serve a definitive...

Graphene is often used as an acceptor in highly efficient energy transfer processes between its electrons and neighbouring optical emitters such as quantum dots, fluorescent molecules and color centres in crystals. Here we demonstrate that graphene can act not only as an acceptor in energy transfer processes, but also an acceptor of charge donated by photoexcited quantum emitters. Specifically, we use heterostructures comprised of graphene and hexagonal boron nitride (hBN) to demonstrate a reversible charge transfer process from quantum emitters in hBN to graphene. The process acts as a controllable, energy-resolved filter that quenches quantum emitters with ground states located above the Fermi level of graphene. Our findings shed light on the positions of hBN defect states within the bandgap of hBN, and are important for the design of devices based on 2D heterostructures, opening new avenues to technologies based on electrical excitation, manipulation, and readout of the quant...

The nitrogen doping mechanisms in Ti₃C₂ MXene are successfully revealed, using a combination of density functional theory simulations and experiment characterization. Three possible sites are found to accommodate the nitrogen dopants: lattice substitution, function substitution, and surface absorption, all favorable for improving the capacitance of Ti₃C₂ electrodes.

Abstract

Nitrogen doping has been proven to be a facile modification strategy to improve the electrochemical performance of 2D MXenes, a group of promising candidates for energy storage applications. However, the underlying mechanisms, especially the positions of nitrogen dopants, and its effect on the electrical properties of MXenes, are still largely unexplored. Herein, a comprehensive study is carried out to disclose the nitrogen doping mechanism in Ti₃C₂ MXene, by employing theoretical simulation and experimental characterization. Three possible sites are found in Ti₃C₂T _x (T = F, OH, and O) to accommodate the nitrogen dopants: lattice substitution (for carbon), function substitution (for –OH), and surface absorption (on –O). Moreover, electrochemical test results confirm that all the three kinds of nitrogen dopants are favorable for improving the specific capacitance of the Ti₃C₂ electrode, and the underlying factors are successfully distinguished. By revealing the nitrogen doping mechanisms in Ti₃C₂ MXene, this work provides theoretical guidelines for modulating the electrochemical properties of MXene materials for energy storage applications.

The electrocatalytic performance of the emerging 2D MC₂ structures is explored by performing an unbiased structural search and first‐principles calculations. A series of excellent catalysts are identified, where NbC₂ is used for hydrogen evolution reaction (HER), TaC₂ for bifunctional catalysis in HER/ oxygen reduction reaction (ORR), and MoC₂ for multifunctional catalysis in HER/ oxygen evolution reaction (OER)/ORR. Additionally, the catalytic mechanisms for different reactions are investigated in depth.

Abstract

Searching for highly efficient, stable, and cost‐effective electrocatalysts for water splitting and oxygen reduction reaction (ORR) is critical for renewable energies, yet it remains a great challenge. Here, by performing an unbiased structural search and first‐principles calculations, the electrocatalytic performance of the emerging 2D transitional‐metal carbides, MC₂ (M represents the transition metal of Ti, V, Nb, Ta, and Mo, C is carbon), is systematically investigated. Owing to their super stability and outstanding electronic conductivity, fast charge transfer kinetics is allowed during catalysis. Specifically, NbC₂, TaC₂, and MoC₂ possess excellent hydrogen evolution reaction (HER) performance under the reaction by the Volmer‐Heyrovsky mechanism. Moreover, taking advantage of the dual‐active‐site catalytic mechanism for oxygen evolution reaction (OER) and ORR over traditional single‐active‐site mechanism, TaC₂ presents promising bifunctional electrocatalytic activity with a low overpotential of 0.06 and 0.37 V for HER and ORR, respectively. Meanwhile, the low overpotential endows MoC₂ remarkable multifunctional electrocatalytic performance in overall water splitting (0.001 V for HER, 0.45 V for OER) and ORR (0.47 V). These intriguing results demonstrate the robust applicability of MC₂ monolayers as effective electrocatalysts.

Quasi‐2D perovskites are engineered in terms of defect passivation and exciton confinement to reach high emitting efficiency. Compared with preparing the precursor solution with stoichiometric quasi‐2D compositions, simply adding a large organic halide salt into the 3D perovskite precursor ensures not only defect passivation but also effective formation of quasi‐2D perovskite domains, avoiding unfavorable appearance of low‐order domains.

Abstract

Quasi‐2D metal halide perovskite films are promising for efficient light‐emitting diodes (LEDs), because of their efficient radiative recombination and suppressed trap‐assisted quenching compared with pure 3D perovskites. However, because of the multidomain polycrystalline nature of solution‐processed quasi‐2D perovskite films, the composition engineering always impacts the emitting properties with complicated mechanisms. Here, defect passivation and domain distribution of quasi‐2D perovskite films prepared with various precursor compositions are systematically studied. As a result, in perovskite films prepared from stoichiometric quasi‐2D precursor compositions, large organic ammonium cations function well as passivators. In comparison, precursor compositions of simply adding large organic halide salt into a 3D perovskite precursor ensure not only the defect passivation but also the effective formation of quasi‐2D perovskite domains, avoiding unfavorable appearance of low‐order domains. Quasi‐2D perovskite films fabricated with a well‐designed precursor composition achieve a high photoluminescence quantum yield of 95.3% and an external quantum efficiency of 14.7% in LEDs.

Black phosphorus (BP) is developed as a metal‐free 2D catalyst for the photoelectrochemical synthesis of ammonia at ambient conditions. The ammonia yield rate and Faradaic efficiency in acid electrolyte is determined as 102.4 µg h⁻¹ mg_cat. ⁻¹ and 23.3%, respectively. The excellent catalytic properties of BP are attributed to the synergistic effects of photoexcitation enhanced electrocatalysis and external bias promoted photocatalysis.

Abstract

Efficient production of ammonia using environmentally friendly techniques under ambient conditions is crucial to renewable energy storage and industrial applications, and catalysts with new reaction pathways are highly desirable. In this work, black phosphorus (BP) is used as a metal‐free 2D catalyst for the photoelectrochemical (PEC) nitrogen reduction reaction (NRR). The electrode is fabricated by layer‐by‐layer assembly of BP nanosheets on an indium tin oxide substrate. The PEC NRR activity in the N₂ saturated aqueous electrolyte without a sacrificial agent is excellent, as exemplified by an ammonia yield rate of 102.4 µg h⁻¹ mgcat.⁻¹ and Faradaic efficiency of 23.3% at −0.4 V, which are the best among nonmetal catalysts for synthesis of ammonia by photocatalysis and electrocatalysis. Furthermore, the BP electrode shows excellent stability after 6 consecutive cycles. The excellent PEC catalytic properties are attributed to the light excitation enhanced electrocatalytic process and that the external bias promoted photocatalytic process improves ammonia production synergistically. The results not only demonstrate the great potential of BP in PEC catalysis, but also identify a promising technique to produce ammonia under ambient conditions using solar energy and electric energy.

Edge1T‐MoS2/edgeNi(OH)2 heterostructures are highly active toward alkaline hydrogen evolution reaction (HER). The prepared 1T–MoS₂ quantum sheet/Ni(OH)₂ catalyst with rich edge1T‐MoS2/edgeNi(OH)2 sites efficiently dissociates H₂O to produce H₂ in alkaline media in a bi‐functional manner, delivering excellent alkaline HER activity and stability surpassing that of Pt/C at high HER current densities (up to 500 mA cm⁻²).

Abstract

Large‐scale production of hydrogen from water‐alkali electrolyzers is impeded by the sluggish kinetics of hydrogen evolution reaction (HER) electrocatalysts. The hybridization of an acid‐active HER catalyst with a cocatalyst at the nanoscale helps boost HER kinetics in alkaline media. Here, it is demonstrated that 1T–MoS₂ nanosheet edges (instead of basal planes) decorated by metal hydroxides form highly active edge1T‐MoS2/edgeNi(OH)2 heterostructures, which significantly enhance HER performance in alkaline media. Featured with rich edge1T‐MoS2/edgeNi(OH)2 sites, the fabricated 1T–MoS₂ QS/Ni(OH)₂ hybrid (quantum sized 1T–MoS₂ sheets decorated with Ni(OH)₂ via interface engineering) only requires overpotentials of 57 and 112 mV to drive HER current densities of 10 and 100 mA cm⁻², respectively, and has a low Tafel slope of 30 mV dec⁻¹ in 1 m KOH. So far, this is the best performance for MoS₂‐based electrocatalysts and the 1T–MoS₂ QS/Ni(OH)₂ hybrid is among the best‐performing non‐Pt alkaline HER electrocatalysts known. The HER process is durable for 100 h at current densities up to 500 mA cm⁻². This work not only provides an active, cost‐effective, and robust alkaline HER electrocatalyst, but also demonstrates a design strategy for preparing high‐performance catalysts based on edge‐rich 2D quantum sheets for other catalytic reactions.

The splitting of quasi-Fermi levels (QFLs) represents a key concept utilized to describe finite-bias operations of semiconductor devices, but its atomic-scale characterization remains a significant challenge. Herein, the nonequilibrium QFL or electrochemical potential profiles within single-molecule junctions obtained from the first-principles multispace constrained-search density-functional formalism are presented. Benchmarking the standard...

High‐gyrotropy garnet on silicon is obtained despite the presence of a magnetic dead layer at the substrate thin‐film interface. A near‐ideal stoichiometry and compensation temperature in sputter‐deposited cerium‐doped terbium iron garnet (Ce:TbIG) suggests a lack of cation redistribution. A low room‐temperature magnetization, high coercivity, large remnant magnetization, and anisotropy‐ aided preferential in‐plane magnetization are beneficial for magnetless non‐reciprocal mode conversion optical isolators.

Abstract

One of the best magneto‐optical claddings for optical isolators in photonic integrated circuits is sputter deposited cerium‐doped terbium iron garnet (Ce:TbIG) which has a large Faraday rotation (≈−3500° cm⁻¹ at 1550 nm). Near‐ideal stoichiometry Ce + TbFe = 0.57 of Ce_0.5Tb_2.5Fe_4.75O₁₂ is found to have a 44 nm magnetic dead layer that can impede the interaction of propagating modes with garnet claddings. The effective anisotropy of Ce:TbIG on Si is also important, but calculations using bulk thermal mismatch overestimate the effective anisotropy. Here, X‐ray diffraction measurements yield highly accurate measurements of strain that show anisotropy favors an in‐plane magnetization in agreement with the positive magnetostriction of Ce:TbIG. Upon doping TbIG with Ce, a slight decrease in compensation temperature occurs which points to preferential rare‐earth occupation in dodecahedral sites and an absence of cation redistribution between different lattice sites. The high Faraday rotation, large remanent ratio, large coercivity, and preferential in‐plane magnetization enable Ce:TbIG to be an in‐plane latched garnet, immune to stray fields with magnetization collinear to direction of light propagation.

The device processing and handling of air‐sensitive stacked silicanes as a conductive channel for FET fabrication are proposed. The transistor exhibits a room temperature current on/off ratio of ≈2 and a hole mobility of 1.8 cm² V⁻¹ s⁻¹ and the gate bias modulation depends on the stacking defects on the channel with a thickness of 3 nm.

Abstract

2D silicon nanomaterials have unique potential for use in applications owing to their many different exotic electronic properties. Field‐effect transistors are fabricated based on free‐standing silicanes through a solution process. Owing to the sensitive surface and the nanometer thickness, the devices require the use of fabrication conditions similar to those of lithium‐ion batteries to prevent oxidation of the sheets. Reliable transistor performance is observed at room temperature in a channel thinner than 3 nm, as drain voltage dependent transfer curves current modulation, depending on the edge effect of the silicane, although the transistor property is modest (hole mobility of 1.8 cm² V⁻¹ s⁻¹). The results suggest the feasibility of other air‐sensitive 2D nanomaterials for applications in nanoelectronic devices.

Systematic tailoring of remanent magnetization is realized by spin‐orbit torque (SOT) in Pt/Co/IrMn stacks, which show double‐biased hysteresis loops. The evolution of remanence after SOT switching is illustrated by anomalous Hall resistance via magnetotransport measurements and domains via magneto–optical Kerr microscope measurements. This work may have potential applications in multilevel storage and neuromorphic computing.

Abstract

Multilevel remanence states have potential applications in ultra‐high‐density storage and neuromorphic computing. Continuous tailoring of the multilevel remanence states by spin‐orbit torque (SOT) is reported in perpendicularly magnetized Pt/Co/IrMn heterostructures. Double‐biased hysteresis loops with only one remanence state can be tuned from the positively or negatively single‐biased loops by SOT controlled sign of the exchange‐bias field. The remanence states associated with the heights of the sub‐loops are continually changed by tuning the ratio of the positively and negatively oriented ferromagnetic domains. The multilevel storage cells are demonstrated by reading the remanent Hall resistance through changing the sign and/or the magnitude of current pulse. The synaptic plasticity behaviors for neuromorphic computing are also simulated by varying the remanent Hall resistance under the consecutive current pulses. This work demonstrates that SOT is an effective method to tailor the remanence states in the double‐biased heavy metal/ferromagnetic/antiferromagnetic system. The multilevel‐stable remanence states driven by SOT show potential applications in future multilevel memories and neuromorphic computing devices.

Photothermal conversion defines the direct translation of solar illumination into thermal energy, which enables 2D transition metal carbides/nitrides (MXenes) to find applications in diverse fields, such as solar steam generation and biomedicals. Synthetic strategies of MXenes and MXene‐based nanocomposites are summarized, and the mechanism and current applications of photothermal MXenes are introduced in detail.

Abstract

Since their discovery in 2011, 2D transition metal carbide/nitride (MXene) materials have received extensive interest due to their unique planar structure, chemical diversity, and superior physiochemical features. Very recently, MXenes have demonstrated outstanding photothermal conversion by virtue of excellent electromagnetic wave absorption capacity and a localized surface plasmon resonance effect. Photothermal conversion is an efficient way to utilize solar energy that allows the transformation of solar illumination into thermal energy, thus enabling MXenes to be applied in various fields, such as solar steam generation and biomedicals. However, the light‐to‐heat capability of MXenes has been paid much less attention until now. Recent progress in photothermal MXenes is reviewed to provide a comprehensive understanding of their photothermal conversion mechanism and applications. First, synthetic strategies of MXenes and their nanocomposites will be briefly summarized, and the discussion of the photothermal conversion mechanism and, most importantly, current advances in their photothermal applications will follow. It is highly anticipated that 2D MXenes, through elaborate material design and interdisciplinary approach, will become one of the mainstream photothermal materials and their application fields will also be expanded in the near future.

High magnetic fields have revealed a surprisingly small Fermi surface in underdoped cuprates, possibly resulting from Fermi-surface reconstruction due to an order parameter that breaks translational symmetry of the crystal lattice. A crucial issue concerns the doping extent of such a state and its relationship to the principal pseudogap and...

Nature Nanotechnology, Published online: 02 March 2020; doi:10.1038/s41565-020-0647-z

This Review highlights the progress made towards the development of neuromorphic devices and architectures enabled by low-dimensional nanomaterials

Nature, Published online: 05 May 2021; doi:10.1038/d41586-020-00502-w

Nature waded through the literature on the coronavirus — and summarized key papers as they appeared.

A new phase of group‐IV monochalcogenides is predicted to exist in the 2D limit. They exhibit a camel's back band structure, which induces a distinct excitonic spectrum and strong excitonic absorption. Intriguingly, high‐temperature exciton gas to electron–hole liquid phase transition can be achieved with a much lower excitation power density compared to that observed in 2D group‐VI transition metal dichalcogenides.

Abstract

Different dispersion near the electronic band edge of a semiconductor can have great influence on its transport, thermoelectric, and optical properties. Using first‐principles calculations, it is demonstrated that a new phase of group‐IV monochalcogenides (γ‐MX, M = Ge, Sn; X = S, Se, or Te) can be stabilized in monolayer limit. γ‐MXs are shown to possess a unique band dispersion—that is, camel's back like structure—in the top valence band. The band nesting effect near the camel's back region induces a large excitonic absorbance and significantly different exciton behaviors from other 2D materials. Importantly, the small effective mass and the indirect characteristics of lowest‐energy exciton render it advantageous for the generation of electron–hole liquid state. After careful evaluation of the electron–hole dissociation temperature and the Mott critical density, it is predicted that a high‐temperature exciton gas to electron–hole liquid phase transition can be achieved in these materials with a low excitation power density. The findings open up new opportunities for both the fundamental research on exciton physics and design of excitonic devices based on 2D materials with distinct band dispersion.

A hybrid biosensor based on a graphene resistor functionalized with self-assembled Graphene-AuNPs (Gold Nanoparticles) is demonstrated for the real-time detection of hepatitis B surface antigen (HBsAg). The hybrid biosensor consists of a ssDNA sequence attached to a graphene resistor device via π – π stacking interactions in combination with a ssDNA functionalized AuNP. The ssDNA has complementary sequences which through hybridization, yield the graphene-AuNP hybrid biosensor. Real-time 2-point resistance measurements, performed using varying concentrations of HBsAg, show a linear dependence of resistance change against the logarithm of HBsAg concentration (log[HBsAg]). A limit of detection of 50 pg ml −1 was observed. Moreover, the hybrid biosensor platform has potential to be applied to any biomarker of interest.

We review theoretical and experimental highlights in transport in two-dimensional topological materials over the last five years. Topological materials comprise topological insulators, Weyl semimetals and topological superconductors. This review focuses on key developments in the understanding of transport phenomena involving surfaces and interfaces of two-dimensional topological materials that have not been covered elsewhere. The review is structured around the following general topics: (i) topological insulators are finding applications in magnetic devices, while controversy continues to surround Hall transport in doped samples and the general issue of topological protection; (ii) state-of-the-art experiments on transition metal dichalcogenides have revealed new valley-dependent electrical and optical phenomena which have spin-dependent counterparts in topological insulators; (iii) in Weyl semimetals the manifestations of Fermi arcs in transport are actively investigated as we...

Surface‐enhanced Raman scattering is used to monitor metabolic perturbations in the tumor extracellular environment in a label‐free and noninvasive manner, providing new readouts in the secretion of kynurenine, tryptophan, and purine derivative metabolites by cancer cells.

Abstract

The composition and intercellular interactions of tumor cells in the tissues dictate the biochemical and metabolic properties of the tumor microenvironment. The metabolic rewiring has a profound impact on the properties of the microenvironment, to an extent that monitoring such perturbations could harbor diagnostic and therapeutic relevance. A growing interest in these phenomena has inspired the development of novel technologies with sufficient sensitivity and resolution to monitor metabolic alterations in the tumor microenvironment. In this context, surface‐enhanced Raman scattering (SERS) can be used for the label‐free detection and imaging of diverse molecules of interest among extracellular components. Herein, the application of nanostructured plasmonic substrates comprising Au nanoparticles, self‐assembled as ordered superlattices, to the precise SERS detection of selected tumor metabolites, is presented. The potential of this technology is first demonstrated through the analysis of kynurenine, a secreted immunomodulatory derivative of the tumor metabolism and the related molecules tryptophan and purine derivatives. SERS facilitates the unambiguous identification of trace metabolites and allows the multiplex detection of their characteristic fingerprints under different conditions. Finally, the effective plasmonic SERS substrate is combined with a hydrogel‐based three‐dimensional cancer model, which recreates the tumor microenvironment, for the real‐time imaging of metabolite alterations and cytotoxic effects on tumor cells.

Function follows form: In the realm of nanomaterials, influencing fundamental properties by manipulating the dimensionality is a challenging but highly rewarding task. This vision is realized by exploiting the flexibility of colloidal synthesis to sculpt the crystal and to alter the electrical properties of lead‐sulfide nanostructures from semiconducting to metallic and superconducting.

Abstract

In the realm of colloidal nanostructures, with their immense capacity for shape and dimensionality control, the form is undoubtedly a driving factor for the tunability of optical and electrical properties in semiconducting or metallic materials. However, influencing the fundamental properties is still challenging and requires sophisticated surface or dimensionality manipulation. Such a modification is presented for the example of colloidal lead‐sulfide nanowires. It is shown that the electrical properties of lead sulfide nanostructures can be altered from semiconducting to metallic with indications of superconductivity, by exploiting the flexibility of the colloidal synthesis to sculpt the crystal and to form different surface facets. A particular morphology of lead sulfide nanowires is prepared through the formation of {111} surface facets, which shows metallic and superconducting properties in contrast to other forms of this semiconducting crystal, which contain other surface facets ({100} and {110}). This effect, which is investigated with several experimental and theoretical approaches, is attributed to the presence of lead‐rich {111} facets. The insights promote new strategies for tuning the properties of crystals and new applications for lead sulfide nanostructures.

The role of the crystal lattice for the electronic properties of cuprates and other high-temperature superconductors remains controversial despite decades of theoretical and experimental efforts. While the paradigm of strong electronic correlations suggests a purely electronic mechanism behind the insulator-to-metal transition, recently the mutual enhancement of the electron–electron and the...

A two‐step chemical functionalization method is developed to enhance the contact behavior of metal/MoS₂ interfaces. After the two‐step chemical treatment, the unintentional defect states at the metal/MoS₂ junctions are removed, resulting in facilitated injection of electron from metal to channels. Moreover, the present chemical method employs simple processes, enabling integration of this functionalization into conventional semiconductor processing.

Abstract

The performance of electronic/optoelectronic devices is governed by carrier injection through metal–semiconductor contact; therefore, it is crucial to employ low‐resistance source/drain contacts. However, unintentional introduction of extrinsic defects, such as substoichiometric oxidation states at the metal–semiconductor interface, can degrade carrier injection. In this report, controlling the unintentional extrinsic defect states in layered MoS₂ is demonstrated using a two‐step chemical treatment, (NH₄)₂S(aq) treatment and vacuum annealing, to enhance the contact behavior of metal/MoS₂ interfaces. The two‐step treatment induces changes in the contact of single layer MoS₂ field effect transistors from nonlinear Schottky to Ohmic behavior, along with a reduction of contact resistance from 35.2 to 5.2 kΩ. Moreover, the enhancement of I _ON and electron field effect mobility of single layer MoS₂ field effect transistors is nearly double for n‐branch operation. This enhanced contact behavior resulting from the two‐step treatment is likely due to the removal of oxidation defects, which can be unintentionally introduced during synthesis or fabrication processes. The removal of oxygen defects is confirmed by scanning tunneling microscopy and X‐ray photoelectron spectroscopy. This two‐step (NH₄)₂S(aq) chemical functionalization process provides a facile pathway to controlling the defect states in transition metal dichalcogenides (TMDs), to enhance the metal‐contact behavior of TMDs.

Salt‐assisted synthesis can be divided into the salt‐template method and molten salt method based on the state of the salts at different temperatures. The roles of salts in salt‐assisted methods comprise the hard template effect, lattice match mechanism, ion‐induced effect, and liquid exfoliation.

Abstract

2D materials have demonstrated good chemical, optical, electrical, and magnetic characteristics, and offer great potential in numerous applications. Corresponding synthesis technologies of 2D materials that are high‐quality, high‐yield, low‐cost, and time‐saving are highly desired. Salt‐assisted methods are emerging technologies that can meet these requirements for the fabrication of 2D materials. Herein, the recent process for the salt‐assisted synthesis of 2D materials and their typical applications are summarized. First, the properties of salt crystals and molten salts are briefly introduced, and then some examples of 2D materials synthesis with the assistance of salt as well as their representative applications are presented. The underlying mechanisms of salts with different states on the formation of 2D morphology are discussed to aid in the rational design of synthetic route of 2D materials. At last, the challenges and future perspectives for salt‐assisted methods are briefly described. This review provides guidance for the controllable synthesis of 2D materials based on the salt‐assisted approaches.

Most digital information today is encoded in the magnetization of ferromagnetic domains. The demand for ever-increasing storage space fuels continuous research for energy-efficient manipulation of magnetism at smaller and smaller length scales. Writing a bit is usually achieved by rotating the magnetization of domains of the magnetic medium, which relies...