The electronic, optical and chemical properties of two-dimensional transition metal dichalcogenides strongly depend on their three-dimensional atomic structure and crystal defects. Using Re-doped MoS
...as a model system, here we present scanning atomic electron tomography as a method to determine three-dimensional atomic positions as well as positions of crystal defects such as dopants, vacancies and ripples with a precision down to 4 pm. We measure the three-dimensional bond distortion and local strain tensor induced by single dopants. By directly providing these experimental three-dimensional atomic coordinates to density functional theory, we obtain more accurate electronic band structures than derived from conventional density functional theory calculations that relies on relaxed three-dimensional atomic coordinates. We anticipate that scanning atomic electron tomography not only will be generally applicable to determine the three-dimensional atomic coordinates of two-dimensional materials, but also will enable ab initio calculations to better predict the physical, chemical and electronic properties of these materials.
The development of efficient non-noble metal electrocatalysts for the oxygen reduction reaction (ORR) is still highly desirable before non-noble metal catalysts can replace platinum catalysts. ...Herein, we have synthesized a new type of ORR catalyst, Co 3 (PO 4 ) 2 C-N/rGOA, containing N-coordinated cobalt phosphate, through the thermal treatment of a phosphonate-based metal–organic framework (MOF). Co 3 (PO 4 ) 2 C-N/rGOA exhibits not only a comparable onset potential and half-wave potential but also superior stability to the commercial Pt/C catalyst for the ORR in alkaline solutions (0.1 and 1.0 M KOH). A combination of structural characterization ( e.g. , XPS, HRTEM, XANES, and EXAFS) and electrochemical analysis shows that the high ORR activity of the Co 3 (PO 4 ) 2 C-N/rGOA catalyst should be attributed to the co-existence of N-doped graphitic carbon and the cobalt phosphate with Co–N species that boost the activity of the cobalt phosphate. These findings open an avenue for exploring the use of phosphonate-based MOFs for energy conversion and storage applications.
Nucleation plays a critical role in many physical and biological phenomena that range from crystallization, melting and evaporation to the formation of clouds and the initiation of neurodegenerative ...diseases
. However, nucleation is a challenging process to study experimentally, especially in its early stages, when several atoms or molecules start to form a new phase from a parent phase. A number of experimental and computational methods have been used to investigate nucleation processes
, but experimental determination of the three-dimensional atomic structure and the dynamics of early-stage nuclei has been unachievable. Here we use atomic electron tomography to study early-stage nucleation in four dimensions (that is, including time) at atomic resolution. Using FePt nanoparticles as a model system, we find that early-stage nuclei are irregularly shaped, each has a core of one to a few atoms with the maximum order parameter, and the order parameter gradient points from the core to the boundary of the nucleus. We capture the structure and dynamics of the same nuclei undergoing growth, fluctuation, dissolution, merging and/or division, which are regulated by the order parameter distribution and its gradient. These experimental observations are corroborated by molecular dynamics simulations of heterogeneous and homogeneous nucleation in liquid-solid phase transitions of Pt. Our experimental and molecular dynamics results indicate that a theory beyond classical nucleation theory
is needed to describe early-stage nucleation at the atomic scale. We anticipate that the reported approach will open the door to the study of many fundamental problems in materials science, nanoscience, condensed matter physics and chemistry, such as phase transition, atomic diffusion, grain boundary dynamics, interface motion, defect dynamics and surface reconstruction with four-dimensional atomic resolution.
Abstract Cu-oxide-based catalysts are promising for CO 2 electroreduction (CO 2 RR) to CH 4 , but suffer from inevitable reduction (to metallic Cu) and uncontrollable structural collapse. Here we ...report Cu-based rock-salt-ordered double perovskite oxides with superexchange-stabilized long-distance Cu sites for efficient and stable CO 2 -to-CH 4 conversion. For the proof-of-concept catalyst of Sr 2 CuWO 6 , its corner-linked CuO 6 and WO 6 octahedral motifs alternate in all three crystallographic dimensions, creating sufficiently long Cu-Cu distances (at least 5.4 Å) and introducing marked superexchange interaction mainly manifested by O-anion-mediated electron transfer (from Cu to W sites). In CO 2 RR, the Sr 2 CuWO 6 exhibits significant improvements (up to 14.1 folds) in activity and selectivity for CH 4 , together with well boosted stability, relative to a physical-mixture counterpart of CuO/WO 3 . Moreover, the Sr 2 CuWO 6 is the most effective Cu-based-perovskite catalyst for CO 2 methanation, achieving a remarkable selectivity of 73.1% at 400 mA cm −2 for CH 4 . Our experiments and theoretical calculations highlight the long Cu-Cu distances promoting *CO hydrogenation and the superexchange interaction stabilizing Cu sites as responsible for the superb performance.
Getting a grip on the switching mechanism in nanoionic resistive memories: the bipolar electrochemical mechanism for mass transfer of Ag in nanoscale SiO2 is disclosed. The in‐situ atomic‐level ...experiments provide detailed evidence of the mass‐transfer process under external electric fields. The mass transfer of Ag directly leads to conductive filament formation and disruption, which is responsible for the switching mechanism in nanoionic resistive memories.
Technology advancements in history have often been propelled by material innovations. In recent years, two-dimensional (2D) materials have attracted substantial interest as an ideal platform to ...construct atomic-level material architectures. In this work, we design a reaction pathway steered in a very different energy landscape, in contrast to typical thermal chemical vapor deposition method in high temperature, to enable room-temperature atomic-layer substitution (RT-ALS). First-principle calculations elucidate how the RT-ALS process is overall exothermic in energy and only has a small reaction barrier, facilitating the reaction to occur at room temperature. As a result, a variety of Janus monolayer transition metal dichalcogenides with vertical dipole could be universally realized. In particular, the RT-ALS strategy can be combined with lithography and flip-transfer to enable programmable in-plane multiheterostructures with different out-of-plane crystal symmetry and electric polarization. Various characterizations have confirmed the fidelity of the precise single atomic layer conversion. Our approach for designing an artificial 2D landscape at selective locations of a single layer of atoms can lead to unique electronic, photonic, and mechanical properties previously not found in nature. This opens a new paradigm for future material design, enabling structures and properties for unexplored territories.
Properties of semiconductors are largely defined by crystal imperfections including native defects. Van der Waals (vdW) semiconductors, a newly emerged class of materials, are no exception: defects ...exist even in the purest materials and strongly affect their electrical, optical, magnetic, catalytic and sensing properties. However, unlike conventional semiconductors where energy levels of defects are well documented, they are experimentally unknown in even the best studied vdW semiconductors, impeding the understanding and utilization of these materials. Here, we directly evaluate deep levels and their chemical trends in the bandgap of MoS
, WS
and their alloys by transient spectroscopic study. One of the deep levels is found to follow the conduction band minimum of each host, attributed to the native sulfur vacancy. A switchable, DX center - like deep level has also been identified, whose energy lines up instead on a fixed level across different hosts, explaining a persistent photoconductivity above 400 K.
Abstract Multiferroic materials, which simultaneously exhibit ferroelectricity and magnetism, have attracted substantial attention due to their fascinating physical properties and potential ...technological applications. With the trends towards device miniaturization, there is an increasing demand for the persistence of multiferroicity in single-layer materials at elevated temperatures. Here, we report high-temperature multiferroicity in single-layer CuCrSe 2 , which hosts room-temperature ferroelectricity and 120 K ferromagnetism. Notably, the ferromagnetic coupling in single-layer CuCrSe 2 is enhanced by the ferroelectricity-induced orbital shift of Cr atoms, which is distinct from both types I and II multiferroicity. These findings are supported by a combination of second-harmonic generation, piezo-response force microscopy, scanning transmission electron microscopy, magnetic, and Hall measurements. Our research provides not only an exemplary platform for delving into intrinsic magnetoelectric interactions at the single-layer limit but also sheds light on potential development of electronic and spintronic devices utilizing two-dimensional multiferroics.
Piezotronics is a new field integrating piezoelectric effect into nanoelectronics, which has attracted much attention for the fundamental research and potential applications. In this paper, the ...piezotronic effect of zinc oxide (ZnO) nanowires, including the response of the electrical transport and photoconducting behaviors on the nanowire bending, has been investigated by in situ transmission electron microscopy (TEM), where the crystal structure of ZnO nanowires were simultaneously imaged. Serials of consecutively recorded current‐voltage (I–V) curves along with an increase of nanowire bending show the striking effect of bending on their electrical behavior. With increasing the nanowire bending, the photocurrent of ZnO nanowire under ultraviolet illumination (UV) drops dramatically and the photo response time becomes much shorter. In addition, the dynamic nanomechanics of ZnO nanowires were studied inside TEM. These phenomena could be attributed to the piezoelectric effect of ZnO nanowires, and they suggest the potential applications of ZnO nanowires on piezotronic devices.
A mechanism for the piezotronic effect of zinc oxide nanowires is discussed. A piezoelectric field is built in zinc oxide nanowires due to immobile ionic charges at the surface when the nanowire is bent. In the bent nanowire, the piezo‐induced electric field lowers the surface recombination barrier. This is the origin of the piezotronic effect of zinc oxide nanowires.
Defect engineering in graphene is important for tailoring graphene's properties thus applicable in various applications such as porous membranes and ultra‐capacitors. In this paper, we report a ...general route towards defect‐ and pore‐ engineering in graphene through remote plasma treatments. Oxygen plasma irradiation was employed to create homogenous defects in graphene with controllable density from a few to ≈103 (μm−2). The created defects can be further enlarged into nanopores by hydrogen plasma anisotropic etching with well‐defined pore size of a few nm or above. The achieved smallest nanopores are ≈2 nm in size, showing the potential for ultra‐small graphene nanopores fabrication.
A general route towards defect‐ and pore‐ engineering in graphene through remote plasma treatments is reported. Oxygen plasma irradiation is employed to create homogenous defects in graphene with controllable density. The created defects can be further enlarged into nanopores by hydrogen plasma anisotropic etching with well‐defined pore size. The achieved smallest nanopores are ≈2 nm in size.
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