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Orientation control of oxygen vacancy channel (OVC) is a highly desirable for tailoring oxygen diffusion as it serves fast transport channel in ion conductors, which is widespread exploited in solid-state fuel cells, catalysts, and ion-batteries. Direct observation of oxygen-ions hopping towards preferential vacant sites is a key to clarifying migration pathways. Here we report the anisotropic oxygen-ion migration mediated by strain in ultrathin cobaltites via in-situ thermal activation in an atomic-resolved transmission electron microscopy. Oxygen migration pathways are constructed on the basis of the atomic structure during the OVC switching, which is manifested as the vertical-to-horizontal OVC switching under tensile strain, but the horizontal-to-diagonal switching under compression. We evaluate the topotactic structural changes to OVC, determine the crucial role of tolerance factor for OVC stability and establish the strain-dependent phase diagram. Our work provides a practical guide for engineering OVC orientation that is applicable ionic-oxide electronics.

Protonation can be used to tune diverse physical and chemical properties of functional oxides. Although protonation of nickelate perovskites has been reported, details on the crystal structure of the protonated phase and a quantitative understanding of the effect of protons on physical properties are still lacking. Therefore, in this work, we select NdNiO

With high theoretical capacity and operating voltage, KVPO

Graphene on SiO2 enables fabrication of Si-technology-compatible devices, but a transfer of these devices from other substrates and direct growth have severe limitations due to a relatively small grain size or device-contamination. Here, we show an efficient, transfer-free way to integrate centimeter-scale, single-crystal graphene, of a quality suitable for electronic devices, on an insulating SiO2 film. Starting with single-crystal graphene grown epitaxially on Ru(0001), a SiO2 film is grown under the graphene by stepwise intercalation of silicon and oxygen. Thin (∼1 nm) crystalline or thicker (∼2 nm) amorphous SiO2 has been produced. The insulating nature of the thick amorphous SiO2 is verified by transport measurements. The device-quality of the corresponding graphene was confirmed by the observation of Shubnikov-de Haas oscillations, an integer quantum Hall effect, and a weak antilocalization effect within in situ fabricated Hall bar devices. This work provides a reliable platform for applications of large-scale, high-quality graphene in electronics.

Two-dimensional molybdenum disulfide (MoS2) is an emergent semiconductor with great potential in next-generation scaled-up electronics, but the production of high-quality monolayer MoS2 wafers still remains a challenge. Here, we report an epitaxy route toward 4 in. monolayer MoS2 wafers with highly oriented and large domains on sapphire. Benefiting from a multisource design for our chemical vapor deposition setup and the optimization of the growth process, we successfully realized material uniformity across the entire 4 in. wafer and greater than 100 μm domain size on average. These monolayers exhibit the best electronic quality ever reported, as evidenced from our spectroscopic and transport characterizations. Our work moves a step closer to practical applications of monolayer MoS2.

Liquid metal forms a thin layer of oxide skin via exposure to oxygen and this layer could be exfoliated by mechanical delamination or gas-injection/solvent-dispersion. Although the room-temperature fabrication of two-dimensional (2D) oxide through gas-injection and water-dispersion has been successfully demonstrated, a synthetic protocol in nonaqueous solvent at elevated temperature still remains as a challenge. Herein we report the mass-production of amorphous 2D SnOx nanoflakes with Bi decoration from liquid Sn-Bi alloy and selected nonaqueous solvents. The functional groups of the solvents play a key role in determining the final morphology of the product and the hydroxyl-rich solvents exhibit the best control toward 2D SnOx. The different solvent-oxide interaction that facilitates this phase-transfer process is further discussed on the basis of DFT calculation. Finally, the as-obtained 2D SnOx is evaluated in electrocatalytic CO2 reduction with high faradaic efficiency (>90%) of formic acid and stable performance over 10 h.

Materials with a quasi-one-dimensional stripy magnetic order often exhibit low crystal and magnetic symmetries, thus allowing the presence of various energy coupling terms and giving rise to macroscopic interplay between spin, charge, and phonon. In this work, we performed optical, electrical and magnetic characterizations combined with first-principles calculations on a van der Waals antiferromagnetic insulator chromium oxychloride (CrOCl). We detected the subtle phase transition behaviors of exfoliated CrOCl under varying temperature and magnetic field and clarified its controversial spin structures. We found that the antiferromagnetism and its air stability persist down to few-layer samples, making it a promising candidate for future 2D spintronic devices. Additionally, we verified the magnetoelastic coupling effect in CrOCl, allowing for the potential manipulation of the magnetic states via electric field or strain. These virtues of CrOCl provide us with an ideal platform for fundamental research on spin-charge, spin-phonon coupling, and spin-interactions.

Single-atom catalysts (SACs) exhibit great potential in heterogeneous catalysis. However, the achievement of obtaining high-loading SACs remains a bottleneck. Herein, we first demonstrate a unique gas-migration, trapping, and emitting strategy for building a kind of Cd-based SAC for CO2 reduction (CO2RR). The gas-migration and trapping processes (≤750 °C) endows the material with an ultrahigh Cd loading amount of 30.3 wt %, while the emitting process can facilely modulate the loading amount from 30.3 to 1.4 wt %. For the CO2RR, the Cd-NC SACs with a loading amount of 18.4 wt % exhibits the maximum Faraday efficiency of 91.4% for CO at -0.728 V. The operando infrared spectroscopy studies prove the presence of main intermediates *COO-, *COOH, and *CO on Cd-NC-5M SACs during the catalytic process, indicating that the CO2RR follows the proton-decoupled electron-transfer mechanism. Density functional theory simulations reveal that the Cd-N4 structure reduces the Gibbs free energy of the rate-determining step (the hydrogenation step of *COOH).

Manipulation of octahedral distortion at atomic scale is an effective means to tune the ground states of functional oxides. Previous work demonstrates that strain and film thickness are variable parameters to modify the octahedral parameters. However, selective control of bonding geometry by structural propagation from adjacent layers is rarely studied. Here we propose a new route to tune the ferromagnetism in SrRuO

Complementary to bulk synthesis, here we propose a designer lattice with extremely high magnetic frustration and demonstrate the possible realization of a quantum spin liquid state from both experiments and theoretical calculations. In an ultrathin (111) CoCr2O4 slice composed of three triangular and one kagome cation planes, the absence of a spin ordering or freezing transition is demonstrated down to 0.03 K, in the presence of strong antiferromagnetic correlations in the energy scale of 30 K between Co and Cr sublattices, leading to the frustration factor of ∼1000. Persisting spin fluctuations are observed at low temperatures via low-energy muon spin relaxation. Our calculations further demonstrate the emergence of highly degenerate magnetic ground states at the 0 K limit, due to the competition among multiply altered exchange interactions. These results collectively indicate the realization of a proximate quantum spin liquid state on the synthetic lattice.

The valley depolarization dynamics of free holes in monolayer transition-metal dichalcogenides are studied by solving the Boltzmann transport equation in real time fully ab inito. While monolayer MoSe2, WS2, WSe2, and MoTe2 possess long hole valley lifetimes due to the spin-valley locking effect, monolayer MoS2 unexpectedly shows ultrafast valley dynamics, with a hole valley lifetime two orders of magnitude shorter than those of the above four materials at room temperature. It is further revealed that the existence of the satellite Γ valley in MoS2 provides an additional hole relaxation path where the Γ valley acts as an intermediate in the hole relaxation between primary K' and K valleys, and moreover, the strong scattering between primary and satellite valleys ensures the ultrafast valley depolarization. By uncovering the pivotal role of the satellite valley, our results may have significant implications for finely controlling valley depolarization in the multivalley materials.

The greatest challenge for lithium-sulfur (Li-S) batteries application is the development of cathode hosts to address the low conductivity, huge volume change, and shuttling effect of sulfur or lithium polysulfides (LiPs). Herein, we demonstrate a composite host to circumvent these problems by confining sub-nanometric manganous oxide clusters (MOCs) in nitrogen doped mesoporous carbon nanosheets. The atomic structure of MOCs is well-characterized and optimized via the extended X-ray absorption fine structure analysis and density functional theory (DFT) calculations. Benefiting from the unique design, the assembled Li-S battery displays remarkable electrochemical performances including a high reversible capacity (990 mAh g-1 after 100 cycles at 0.2 A g-1) and a superior cycle life (60% retention over 250 cycles at 2 A g-1). Both the experimental results and DFT calculations demonstrate that the well-dispersed MOCs could significantly promote the chemisorption of LiPs, thus greatly improving the capacity and rate performance.

Graphene on SiO

Two-dimensional molybdenum disulfide (MoS

Lack of appropriate cathodes severely restrains the development of high-energy Mg batteries. In this work, we proposed joint cationic and anionic redox chemistry of transition-metal (TM) sulfides as the most promising way out. A series of solid-solution pyrite Fe

Liquid metal forms a thin layer of oxide skin via exposure to oxygen and this layer could be exfoliated by mechanical delamination or gas-injection/solvent-dispersion. Although the room-temperature fabrication of two-dimensional (2D) oxide through gas-injection and water-dispersion has been successfully demonstrated, a synthetic protocol in nonaqueous solvent at elevated temperature still remains as a challenge. Herein we report the mass-production of amorphous 2D SnO

Nano-thick metallic transition metal dichalcogenides such as VS$_{2}$ are essential building blocks for constructing next-generation electronic and energy-storage applications, as well as for exploring unique physical issues associated with the dimensionality effect. However, such 2D layered materials have yet to be achieved through either mechanical exfoliation or bottom-up synthesis. Herein, we report a facile chemical vapor deposition route for direct production of crystalline VS$_{2}$ nanosheets with sub-10 nm thicknesses and domain sizes of tens of micrometers. The obtained nanosheets feature spontaneous superlattice periodicities and excellent electrical conductivities (~3$\times$10$^{3}$ S cm$^{-1}$), which has enabled a variety of applications such as contact electrodes for monolayer MoS$_{2}$ with contact resistances of ~1/4 to that of Ni/Au metals, and as supercapacitor electrodes in aqueous electrolytes showing specific capacitances as high as 8.6$\times$10$^{2}$ F g$^{-1}$. This work provides fresh insights into the delicate structure-property relationship and the broad application prospects of such metallic 2D materials., 23 pages, 5 figues

Artificial complex-oxide heterostructures containing ultrathin buried layers grown along the pseudocubic [111] direction have been predicted to host a plethora of exotic quantum states arising from the graphene-like lattice geometry and the interplay between strong electronic correlations and band topology. To date, however, electronic-structural investigations of such atomic layers remain an immense challenge due to the shortcomings of conventional surface-sensitive probes with typical information depths of a few angstroms. Here, we use a combination of bulk-sensitive soft X-ray angle-resolved photoelectron spectroscopy (SX-ARPES), hard X-ray photoelectron spectroscopy (HAXPES), and state-of-the-art first-principles calculations to demonstrate a direct and robust method for extracting momentum-resolved and angle-integrated valence-band electronic structure of an ultrathin buckled graphene-like layer of NdNiO

A heterostructure photovoltaic diode featuring an all-solid-state TiO2/graphene/dye ternary interface with high-efficiency photogenerated charge separation/transport is described here. Light absorption is accomplished by dye molecules deposited on the outside surface of graphene as photoreceptors to produce photoexcited electron-hole pairs. Unlike conventional photovoltaic conversion, in this heterostructure both photoexcited electrons and holes tunnel along the same direction into graphene, but only electrons display efficient ballistic transport toward the TiO2 transport layer, thus leading to effective photon-to-electricity conversion. On the basis of this ipsilateral selective electron tunnelling (ISET) mechanism, a model monolayer photovoltaic device (PVD) possessing a TiO2/graphene/acridine orange ternary interface showed ∼86.8% interfacial separation/collection efficiency, which guaranteed an ultrahigh absorbed photon-to-current efficiency (APCE, ∼80%). Such an ISET-based PVD may become a fundamental device architecture for photovoltaic solar cells, photoelectric detectors, and other novel optoelectronic applications with obvious advantages, such as high efficiency, easy fabrication, scalability, and universal availability of cost-effective materials.

Organic two-dimensional (2D) crystals are fundamentally important for development of future devices. Despite that more than a half of man-made products contain polymers, 2D crystals consisting of long linear chains have yet to be explored. Here we report on the fabrication of 2D polyaniline (PANI) crystals via rational electrochemical polymerization followed by liquid-phase exfoliation. The 2D PANI is molecularly thin (∼0.8 nm) and composed of PANI chains with a number-average molecular weight of ∼31 000. The chains are parallel to each other with the benzene rings standing almost vertically to the surface, implying a face-to-face arrangement of the neighboring chains held together by abundant π–π interactions augmented with hydrogen bonds. The 2D PANI can be readily transferred to various solid surfaces and exhibit interesting electrical and optical properties, suggesting that they would be potentially useful in photoelectronic devices and other applications.

In the current research project, we have prepared a novel Sb@C nanosphere anode with biomimetic yolk–shell structure for Li/Na-ion batteries via a nanoconfined galvanic replacement route. The yolk–shell microstructure consists of Sb hollow yolk completely protected by a well-conductive carbon thin shell. The substantial void space in the these hollow Sb@C yolk–shell particles allows for the full volume expansion of inner Sb while maintaining the framework of the Sb@C anode and developing a stable SEI film on the outside carbon shell. As for Li-ion battery anode, they displayed a large specific capacity (634 mAh g–1), high rate capability (specific capabilities of 622, 557, 496, 439, and 384 mAh g–1 at 100, 200, 500, 1000, and 2000 mA g–1, respectively) and stable cycling performance (a specific capacity of 405 mAh g–1 after long 300 cycles at 1000 mA g–1). As for Na-ion storage, these yolk–shell Sb@C particles also maintained a reversible capacity of approximate 280 mAh g–1 at 1000 mA g–1 after 200 cycles.

Via designing a facile microscale all-solid-state lithium-oxygen battery system constructed in an environmental scanning electron microscope, direct visualization of discharge and charge processes of the lithium-oxygen battery is achieved. Different morphologies of the discharge product are observed, including a sphere, conformal film, and red-blood-cell-like shape, with a particle size up to 1.5 μm; whereas upon charge, the decomposition initiates at their surface and continues along a certain direction, instead of from the contact point at the electrode. These new findings indicate that the electron and lithium ion conductivities of Li2O2 could support the growth and decomposition of the discharge product in our system. In addition, our results indicate that various morphologies of Li2O2 arise from the different current density and surface chemistry of CNT, and the growth and decomposition of the particle are related to the uneven distribution of the ionic and electronic conductivities of Li2O2.

Red phosphorus (P) have been considered as one of the most promising anode material for both lithium-ion batteries (LIBs) and (NIBs), because of its high theoretical capacity. However, natural insulating property and the large volume expansion of red P during cycling lead to poor cyclability and low rate performance, which prevents its practical application. Here, we significantly improves both lithium storage and sodium storage performance of red P by confining nanosized amorphous red P into the mesoporous carbon matrix (P@CMK-3) using a vaporization-condensation-conversion process. The P@CMK-3 shows a high reversible specific capacity of ∼ 2250 mA h g(-1) based on the mass of red P at 0.25 C (∼ 971 mA h g(-1) based on the composite), excellent rate performance of 1598 and 624 mA h g(-1) based on the mass of red P at 6.1 and 12 C, respectively (562 and 228 mA h g(-1) based on the mass of the composite at 6.1 and 12 C, respectively) and significantly enhanced cycle life of 1150 mA h g(-1) based on the mass of red P at 5 C (500 mA h g(-1) based on the mass of the composite) after 1000 cycles for LIBs. For Na ions, it also displays a reversible capacity of 1020 mA h g(-1) based on the mass of red P (370 mA h g(-1) based on the mass of the composite) after 210 cycles at 5C. The significantly improved electrochemical performance could be attributed to the unique structure that combines a variety of advantages: easy access of electrolyte to the open channel structure, short transport path of ions through carbon toward the red P, and high ionic and electronic conductivity.

Convective transportation of materials in the solid state occurring in a prototype solid bilayer system of Al and Si with negligible mutual solubility has been directly imaged in real time at nanoscale using a valence energy-filtered transmission electron microscope. Such solid-state convection is driven by the stress gradient developing in the bilayer system due to the amorphous to crystalline phase transformation of the Si sublayer. The process is characterized by compression experienced in the Si phase crystallizing within the Al sublayer, as well as by the development of mushroom-shaped "plumes" of Al nanocrystals in the Si sublayer as a result of compressive stress relaxation and discrete, new nucleation of crystalline Al. The real-time, atomistic observation and the thus-obtained fundamental understanding of solid-state convection enable highly sophisticated applications of such a complex process in advanced fabrication and processing of nanomaterials and solid-state devices.

Direct observation of delithiated structures of LiCoO2 at atomic scale has been achieved using spherical aberration-corrected scanning transmission electron microscopy (STEM) with high-angle annular-dark-field (HAADF) and annular-bright-field (ABF) techniques. The ordered Li, Co, and O columns for LiCoO2 nanoparticles are clearly identified in ABF micrographs. Upon the Li ions extraction from LiCoO2, the Co-contained (003) planes distort from the bulk to the surface region and the c-axis is expanded significantly. Ordering of lithium ions and lithium vacancies has been observed directly and explained by first-principles simulation. On the basis of HAADF micrographs, it is found that the phase irreversibly changes from O3-type in pristine LiCoO2 to O1-type LixCoO2 (x ≈ 0.50) after the first electrochemical Li extraction and back to O2-type LixCoO2 (x ≈ 0.93) rather than to O3-stacking after the first electrochemical lithiation. This is the first report of finding O2-LixCoO2 in the phase diagram of O3-LiCoO...

Direct observation of delithiated structures of LiCoO(2) at atomic scale has been achieved using spherical aberration-corrected scanning transmission electron microscopy (STEM) with high-angle annular-dark-field (HAADF) and annular-bright-field (ABF) techniques. The ordered Li, Co, and O columns for LiCoO(2) nanoparticles are clearly identified in ABF micrographs. Upon the Li ions extraction from LiCoO(2), the Co-contained (003) planes distort from the bulk to the surface region and the c-axis is expanded significantly. Ordering of lithium ions and lithium vacancies has been observed directly and explained by first-principles simulation. On the basis of HAADF micrographs, it is found that the phase irreversibly changes from O3-type in pristine LiCoO(2) to O1-type Li(x)CoO(2) (x ≈ 0.50) after the first electrochemical Li extraction and back to O2-type Li(x)CoO(2) (x ≈ 0.93) rather than to O3-stacking after the first electrochemical lithiation. This is the first report of finding O2-Li(x)CoO(2) in the phase diagram of O3-LiCoO(2), through which the two previously separated LiCoO(2) phases, i.e. O2 and O3 systems, are connected. These new investigations shed new insight into the lithium storage mechanism in this important cathode material for Li-ion batteries.

We demonstrate that the electronic devices built on patterned graphene nanoribbons (GNRs) can be made with atomic-perfect-interface junctions and controlled doping via manipulation of edge terminations. Using first-principles transport calculations, we show that the GNR field effect transistors can achieve high performance levels similar to those made from single-walled carbon nanotubes, with ON/OFF ratios on the order of 10(3)-10(4), subthreshold swing of 60 meV per decade, and transconductance of 9.5 x 10(3) Sm-1.

We describe the synthesis of novel monocrystalline FeCo nanowires encapsulated inside multiwalled carbon nanotubes (MWNTs). These FeCo nanowires exhibit homogeneous Fe and Co concentrations and do not contain an external oxide layer due to the presence of insulating nanotube layers. The method involves the aerosol thermolysis of toluene-ferrocene-cobaltocene solutions in inert atmospheres. The materials have been carefully characterized using state-of-the-art high-resolution transmission electron microscopy (HRTEM), electron-energy-loss spectroscopy (EELS), scanning electron microscopy (SEM), energy-dispersive X-ray analysis (EDX), electron diffraction, HREELS-STM elemental mapping, X-ray powder diffraction, and SQUID magnetometry. We noted that the formation of FeCo alloys occurs at relatively low pyrolytic temperatures (e.g., 650-750 degrees C). These single-crystal nanowires, which have not been reported hitherto, always exhibit the FeCo (110) plane parallel to the carbon nanotube axis. The FeCo nanomaterials have shown large coercive fields at room temperature (e.g., 900 Oe). We envisage that these aligned ferromagnetic nanowires could be used in the fabrication of high-density magnetic storage devices and magnetic composites.