Single Atomic Plane Research Articles

On the Emergence of Ferromagnetism in LaCoO3 Ultrathin Films

AbstractIt is well known that the properties of a crystal evolve as it increases in size from a single atomic plane to that of the bulk. Such size‐dependent transitions can stem from many different origins and depend on minute changes to crystal bonding and composition. A model example is that of LaCoO3, which is non‐magnetic in the bulk but can display ferromagnetism at the nanoscale. Here, the evolution of structure‐property relationships is studied in the LaCoO3−δ/SrTiO3 (001) system as the thickness of LaCoO3−δ is increased from a single plane to 10 unit cells. In situ synchrotron X‐ray studies are performed during and post‐deposition to probe changes in the interactions between structure, stoichiometry, and magnetic behavior. Structural quantification indicates that the oxygen octahedral rotation pattern evolves with thickness, due to inherent differences in crystal symmetry between the film and substrate. The change in rotation modifies the required energy barrier for the spin state transition via the Co–O bond length and Co–O–Co bond angle, affecting the appearance of ferromagnetism. Our results highlight the contributions of high spin Co2+ and/or high spin Co3+ to respective weak and robust ferromagnetism and the evolution of properties with size in ultrathin LaCoO3−δ heterostructures.

Graphene as a Lubricant Additive for Reducing Friction and Wear in Its Liquid-Based Form

Graphene is a single atomic plane of sp2-bound carbon that has attracted considerable interest in various technologies. On the basis of its unique physical, mechanical, and chemical properties, graphene is a potentially strong candidate as a lubricant additive in its liquid-based form to reduce friction and protect surfaces from degrading. Furthermore, graphene on wear performance acts as a heat dissipation source for liquid lubricants. This review explores and addresses the fundamental mechanisms illuminating the exceptional tribological behaviours of graphene family materials and their limitations. Although graphene additives were reported to improve friction coefficients and wear properties, several challenges remain a hindrance, such as production costs, dispersion stability, and lack of information regarding graphene optimisation. Thus, this review can provide a standard methodological framework for graphene additives in improving tribological performance. Moreover, this review provides an up-to-date review of current tribological experiments based on ultrafine particles incorporated with graphene as an additive for lubricating liquids.

GaN lateral polar junction arrays with 3D control of doping by supersaturation modulated growth: A path toward III-nitride superjunctions

We demonstrate a pathway employing crystal polarity controlled asymmetric impurity incorporation in the wide bandgap nitride material system to enable 3D doping control during the crystal growth process. The pathway involves polarity specific supersaturation modulated growth of lateral polar structures of alternating Ga- and N-polar GaN domains. A STEM technique of integrated differential phase contrast is used to image the atomic structure of the different polar domains and their single atomic plane boundaries. As a demonstration, 1 μm wide alternating Ga- and N-polar GaN domains exhibiting charge balanced and periodic domains for superjunction technology were grown. The challenges in characterizing the resulting 3D doping profile were addressed with atom probe tomography with atomic scale compositional resolution corroborating capacitance measurements and secondary-ion mass spectroscopy analysis.

Metal Sheet of Atomic Thickness Embedded in Silicon.

The controlled confinement of the metallic delta-layer to a single atomic plane has so far remained an unsolved problem. In the present study, the delta-type structure with atomic sheet of NiSi2 silicide embedded into a crystalline Si matrix has been fabricated using room-temperature overgrowth of a Si film onto the Tl/NiSi2/Si(111) atomic sandwich in ultrahigh vacuum. Tl atoms segregate at the growing Si film surface, and the 1.5-3.0 nm thick epitaxially crystalline Si layer forms atop the NiSi2 sheet. Confinement of the NiSi2 layer to a single atomic plane has been directly confirmed by transmission electron microscopy. The NiSi2 delta-layer demonstrates a p-type conductivity associated with the electronic transport through the two hole-like and one electron-like interface-state bands. The basic structural and electronic properties of the NiSi2 delta-layer remain after keeping the sample in air for one year.

Thermoelectric properties and prospects of <i>MAX</i> phases and derived <i>MX</i>ene phases

Thermoelectric materials, a kind of new energy material, can directly convert heat energy into electric energy, and vice versa, without needing any other energy conversion. However, the present development status of thermoelectric materials severely restricts their engineering applications in thermoelectric devices. Improving the thermoelectric performances of existing thermoelectric materials and exploring new thermoelectric materials with excellent performance are eternal research topics in thermoelectricity field. In recent years, the <i>MAX</i> phases and their derived <i>MX</i>ene phases have gradually received the attention of researchers due to their unique microstructures and properties. The crystal structure of <i>MAX</i> phases is comprised of <i>M</i><sub><i>n</i>+1</sub><i>X</i><sub><i>n</i></sub> structural units and the single atomic plane of A stacked alternately. The two-dimensional<i> MX</i>ene phase derived can be prepared after the atoms in the <i>A</i>-layer of <i>MAX</i> have been etched. The <i>MAX</i> phases and their derived <i>MX</i>ene phases have both metal feature and ceramic feature, and also have good thermal conductivity and electric conductivity, and they are anticipated to be the promising thermoelectric materials. In this paper, the present development status of the preparation technology and the thermoelectric properties of <i>MAX</i> phases and <i>MX</i>ene are reviewed. Finally, some feasible schemes to improve the thermoelectric properties of <i>MAX</i> and its derived <i>MX</i>ene phase materials are proposed, and the development direction and prospect of <i>MAX</i> phases and <i>MX</i>ene are prospected as well.

Comparing Graphene Oxide and Reduced Graphene Oxide as Blending Materials for Polysulfone and Polyvinylidene Difluoride Membranes

Graphene is a single atomic plane of graphite, and it exhibits unique electronic, thermal, and mechanical properties. Exfoliated graphene oxide (GO) contains various hydrophilic functional groups, such as hydroxyl, epoxide, and carboxyl groups, that can modify the hydrophobic characteristics of a membrane surface. Though reduced graphene oxide (rGO) has fewer functional groups than GO, its associated sp2 structures and physical properties can be recovered. A considerable amount of research has focused on the use of GO to obtain a pristine graphene material via reduction processes. In this study, polysulfone (PSf) and polyvinylidene fluoride (PVDF) membranes that were blended with GO and rGO, respectively, were fabricated by using the immersion phase inversion method and an n-methylpyrrolidone (NMP) solvent. Results showed that the graphene nanomaterials, GO and rGO, can change the pore morphology (size and structure) of both PSf and PVDF membranes. The optimum content of both was then investigated, and the highest flux enhancement was observed with the 0.10 wt% GO-blended PSf membrane. The presence of functional groups in GO within prepared PSf and PVDF membranes alters the membrane characteristics to hydrophilic. An antifouling test and rejection efficiency evaluation also showed that the 0.10 wt% membrane provided the best performance.

Plane and plane-radial discrete breathers in fcc metals

Discrete breather (DB) is a time-periodic, spatially localized vibrational mode in a perfect nonlinear lattice. In this study, two new types of DBs are reported in fcc metals (Al, Cu and Ni), based on the molecular dynamics simulations using standard embedded atom method interatomic potentials. All calculations are performed at a zero temperature in a three-dimensional computational cell with the use of periodic boundary conditions. A plane DB is excited in a single (111) atomic plane by displacing the atoms from their equilibrium lattice sites according to a specific pattern corresponding to a delocalized vibrational mode in a two-dimensional triangular lattice. This plane DB is delocalized in two dimensions and localized in one dimension, normal to the excited (111) plane. It is shown that in all studied metals the plane DBs have maximal lifetimes of 17–22 ps in the range of initial amplitudes of 0.15–0.30 Å. Herewith the studied mode demonstrates a hard type of nonlinearity, i.e. its frequency increases with amplitude. The second new type of DB is the plane-radial DB obtained by imposing a radial localizing function on the plane DB. This disk-type DB is localized in all three dimensions. The time evolution of the atomic displacement amplitudes and the kinetic energy are studied. The DBs of this type can exist for 9 and 4 ps in Cu and Ni, respectively, and then they decay by dissipating their vibrational energy onto neighboring atoms. The long-lived plane-radial DB in Al is not found.

Stacking control in graphene-based materials: A promising method for fascinating physical properties

Graphene, defined as a single atomic plane of graphite, is a semimetal with small overlap between the valence and the conduction bands. The stacking of graphene up to several atomic layers can produce diverse physical properties, depending on the stacking way. The bilayer graphene is also a semimetal, adopting the AB-stacked (or Bernal-stacked) structure or the rare AA-stacked structure. The trilayer or a few layer graphene (FLG) can be semimetal or semiconductor, depending on whether it takes Bernal (ABA) stacking or rhombohedral (ABC) stacking. We will give a perspective on the recent two mild approaches to control the stacking via local transition from ABC stacking into ABA stacking. It is believed that with the rapid development of graphene-based materials, these techniques for stacking control can be used for more complex structure to fulfill fascinating properties and devices.

Stack of Graphene/Copper Foils/Graphene by Low-Pressure Chemical Vapor Deposition as a Thermal Interface Material

Although there are several kinds of thermal interfacial materials used in the electronic semiconductor industry, such as thermal grease, thermal glue, thermal gap filler, thermal pad and thermal adhesive, the problem of heat dissipation still remains a challenge. In this context, chemical vapor deposition of graphene on copper foils in vacuum has recently become considered as a wonderful hybrid material (graphene/copper/graphene) for more demanding thermal management applications, thanks to the unique properties of graphene in comparison with other materials. We found that the thermal properties of copper films change as graphene is deposited on top of the copper surface. Especially, a single atomic plane of graphene can significantly increase the film’s thermal conductivity. Our graphene on copper foil was analyzed and measured by optical microscopy, Raman spectroscopy, scanning electron microscopy and heat transfer technique. This stack of graphene/copper/graphene materials may play a very important role as a potential material with superior thermal conductivity to replace traditional copper shim thermal pads in current electronic devices.

In Situ Atomic-Scale Studies of the Formation of Epitaxial Pt Nanocrystals on Monolayer Molybdenum Disulfide.

Pt-nanocrystal:MoS2 hybrid materials have promising catalytic properties for hydrogen evolution, and understanding their detailed structures at the atomic scale is crucial to further development. Here, we use an in situ heating holder in an aberration-corrected transmission electron microscope to study the formation of Pt nanocrystals directly on the surface of monolayer MoS2 from a precursor on heating to 800 °C. Isolated single Pt atoms and small nanoclusters are observed after in situ heating, with two types of preferential alignment between the Pt nanocrystals and the underlying monolayer MoS2. Strain effects and thickness variations of the ultrasmall Pt nanocrystal supported on MoS2 are studied, revealing that single atomic planes are formed from a nonlayered face-centered cubic bulk Pt configuration with a lattice expansion of 7-10% compared to that of bulk Pt. The Pt nanocrystals are surrounded by an amorphous carbon layer and in some cases have etched the local surrounding MoS2 material after heating. Electron beam irradiation also initiates Pt nanocrystal etching of the local MoS2, and we study this process in real time at atomic resolution. These results show that the presence of carbon around the Pt nanocrystals does not affect their epitaxial relationship with the MoS2 lattice. Single Pt atoms within the carbon layer are also immobilized at high temperature. These results provide important insights into the formation of Pt:MoS2 hybrid materials.

Epitaxial Templating of Two-Dimensional Metal Chloride Nanocrystals on Monolayer Molybdenum Disulfide.

We demonstrate the formation of ionic metal chloride (CuCl) two-dimensional (2D) nanocrystals epitaxially templated on the surface of monolayer molybdenum disulfide (MoS2). These 2D CuCl nanocrystals are single atomic planes from a nonlayered bulk CuCl structure. They are stabilized as a 2D monolayer on the surface of the MoS2 through interactions with the uniform periodic surface of the MoS2. The heterostructure 2D system is studied at the atomic level using aberration-corrected transmission electron microscopy at 80 kV. Dynamics of discrete rotations of the CuCl nanocrystals are observed, maintaining two types of preferential alignments to the MoS2 lattice, confirming that the strong interlayer interactions drive the stable CuCl structure. Strain maps are produced from displacement maps and used to track real-time variations of local atomic bonding and defect production. Density functional theory calculations interpret the formation of two types of energetically advantageous commensurate superlattices via strong chemical bonds at interfaces and predict their corresponding electronic structures. These results show how vertical heterostructured 2D nanoscale systems can be formed beyond the simple assembly of preformed layered materials and provide indications about how different 2D components and their interfacial coupling mode could influence the overall property of the heterostructures.

Raman scattering of few-layers MoTe2

We report on room-temperature Raman scattering measurements in few-layer crystals of exfoliated molybdenum ditelluride (MoTe2) performed with the use of 632.8 nm (1.96 eV) laser light excitation. In agreement with a recent study reported by Froehlicher et al (2015 Nano Lett.) we observe a complex structure of the out-of-plane vibrational modes , which can be explained in terms of interlayer interactions between single atomic planes of MoTe2. In the case of low-energy shear and breathing modes of rigid interlayer vibrations, it is shown that their energy evolution with the number of layers can be well reproduced within a linear chain model with only the nearest neighbor interaction taken into account. Based on this model the corresponding in-plane and out-of-plane force constants are determined. We also show that the Raman scattering in MoTe2 measured using 514.5 nm (2.41 eV) laser light excitation results in much simpler spectra. We argue that the rich structure of the out-of-plane vibrational modes observed in Raman scattering spectra excited with the use of 632.8 nm laser light results from its resonance with the electronic transition at the M point of the MoTe2 first Brillouin zone.

Atomic-Level Sculpting of Crystalline Oxides: Toward Bulk Nanofabrication with Single Atomic Plane Precision.

The atomic-level sculpting of 3D crystalline oxide nanostructures from metastable amorphous films in a scanning transmission electron microscope (STEM) is demonstrated. Strontium titanate nanostructures grow epitaxially from the crystalline substrate following the beam path. This method can be used for fabricating crystalline structures as small as 1-2 nm and the process can be observed in situ with atomic resolution. The fabrication of arbitrary shape structures via control of the position and scan speed of the electron beam is further demonstrated. Combined with broad availability of the atomic resolved electron microscopy platforms, these observations suggest the feasibility of large scale implementation of bulk atomic-level fabrication as a new enabling tool of nanoscience and technology, providing a bottom-up, atomic-level complement to 3D printing.

High-performance planar nanoscale dielectric capacitors

We propose a model for planar nanoscale dielectric capacitor consisting of a single layer, insulating hexagonal boron nitride (BN) stripe placed between two metallic graphene stripes, all forming commensurately a single atomic plane. First-principles density functional calculations on these nanoscale capacitors for different levels of charging and different widths of graphene - BN stripes mark high gravimetric capacitance values, which are comparable to those of supercapacitors made from other carbon based materials. Present nanocapacitor model allows the fabrication of series, parallel and mixed combinations which offer potential applications in 2D flexible nanoelectronics, energy storage and heat-pressure sensing systems.

Liquid-phase exfoliated graphene: functionalization, characterization, and applications.

The development of chemical strategies to render graphene viable for incorporation into devices is a great challenge. A promising approach is the production of stable graphene dispersions from the exfoliation of graphite in water and organic solvents. The challenges involve the production of a large quantity of graphene sheets with tailored distribution in thickness, size, and shape. In this review, we present some of the recent efforts towards the controlled production of graphene in dispersions. We also describe some of the chemical protocols that have provided insight into the vast organic chemistry of the single atomic plane of graphite. Controlled chemical reactions applied to graphene are expected to significantly improve the design of hierarchical, functional platforms, driving the inclusion of graphene into advanced functional materials forward.

Thermal Properties of Graphene–Copper–Graphene Heterogeneous Films

We demonstrated experimentally that graphene-Cu-graphene heterogeneous films reveal strongly enhanced thermal conductivity as compared to the reference Cu and annealed Cu films. Chemical vapor deposition of a single atomic plane of graphene on both sides of 9 μm thick Cu films increases their thermal conductivity by up to 24% near room temperature. Interestingly, the observed improvement of thermal properties of graphene-Cu-graphene heterofilms results primarily from the changes in Cu morphology during graphene deposition rather than from graphene's action as an additional heat conducting channel. Enhancement of thermal properties of graphene-capped Cu films is important for thermal management of advanced electronic chips and proposed applications of graphene in the hybrid graphene-Cu interconnect hierarchies.

Influence of thin Slip Bands on Grain Boundary Stress Fields and Microcrack Initiation: Analytical and Numerical Approaches

Strain localization is often observed in single and poly-crystals, for instance forming clear bands or persistent slip bands respectively during post-irradiation tensile loading or cyclic loading. This concerns FCC, BCC or HCP crystallographic structures. At the intersection of the so-called slip bands (SBs) and grain boundaries (GBs), stress concentration areas emerge contributing to intergranular microcrack initiation. Indeed, GB stress fields show singularities due to the SB plastic shearing of the surrounding elastic matrix. Since Stroh's work [[?]], GB normal stress fields have been computed using pile-up theory assumptions. However, as these approaches assume that slip occurs along only one single atomic plane, they overestimate GB normal stress fields because SBs generally display finite thickness, from a few ten nm to a few μm. Crystalline finite elements (FE) computations are carried out using the Cast3M software. Meshed microstructure includes an elastoplastic SB within a main elastic grain which which is embedded at free surface of an elastic matrix. Results show in fact that using the pile-up formalism leads to large GB stress field overestimations. Therefore, an analytical formula describing the normal GB stress fields is proposed in the current paper, extend- ing the pile-up solution in the neighbourhood of the SB corner. The relationships between SB length and thickness and stress singularities are established in the light of the theory of matching asymptotic expansions. A double fracture criterion allows the prediction of intergranular microcrack initiation based on the computed stress singularities. Another analytical model based on the solution of the thermoelastic problem allow to propose a more general formula of the grain boundary stress fields accounting for the unloading effects. Using only one stress field computed by FE, the prefactors are computed and both analytical formulae are shown to be validated for all the range of grain size, SB thickness and remote applied stress.

Comparison between Pile-Up Singularities and Stress Fields Induced by Thin Slip Bands. Application to the Prediction of Grain Boundary Microcrack Nucleation

Slip localization is widely observed in metallic polycrystals after tensile deformation, cyclic deformation or pre-irradiation followed by tensile deformation. Such strong deformation localized in thin slip bands induces local stress concentrations in the quasi-elastic matrix around, at the intersections between slip bands (SBs) and grain boundaries (GBs) where microcrack initiation is often observed. Since the work of Stroh, such stress fields have been mostly modeled using the dislocation pile-up theory which leads to stress singularities similar to the LEFM ones. The Griffith criterion has then been widely applied, leading usually to strong underestimations of the macroscopic stress to GB crack initiation. In fact, slip band thickness is finite: 20nm-1000nm depending on material, temperature and loading conditions. Then, many slip planes are plastically activated through the thickness, and not only one single atomic plane. To evaluate more realistic stress fields, numerous crystalline finite element (FE) computations have been carried out using microstructure inputs (slip band aspect ratio, crystal and GB orientation...). A strong influence of slip band thickness close to the slip band corner has been highlighted, which is not accounted for by the pile-up theory. But far away, the thickness has a negligible effect and the predicted stress fields are close to the one predicted by the pile-up theory. Closed-form expressions are deduced from the numerous FE computation results allowing a straightforward prediction of GB stress fields. Slip band plasticity parameters, such as length and thickness, as well as crystal orientation, GB plane and remote stress are taken into account. The dependence with respect to the various parameters can be understood in the framework of matching expansions usually applied to cracks with V notches of finite thickness. As the exponent of the GB stress close-field is only about one-half of the pile-up or LEFM crack one, the Griffith criterion may not be used for GB microcrack prediction in case of finite thickness. That is why finite crack fracture mechanics is used together with both energy and stress criteria. Taking into account SB finite thickness, t>0, leads to predicted remote stresses to GB microcrack initiation three to six times lower than the ones predicted using the to pile-up theory, in agreement with experimental data.

Prediction of grain boundary stress fields and microcrack initiation induced by slip band impingement

Slip localization is widely observed in metallic polycrystals undergoing cyclic deformation or post-irradiation tensile deformation, whatever their crystallographic structure. Hence, strong strain localization occurs in thin slip bands (SBs) inducing by the way local stress concentrations at their intersections with grain boundaries (GBs). Many GB stress field formulae based on the dislocation pile-up theory have been proposed since the pionnering work of Stroh and others. These allow the use of the Griffith criterion for prediction GB fracture initiation. However, recent observations show that assuming that slip is localized on a single atomic plane leads to unrealistic results. In fact, a large number of slip planes are plastically activated and then finite slip band thickness should be accounted for. Numerous crystalline finite element (FE) computations have been carried out using considering a slip bands with low critical resolved shear stress embedded in an elastic matrix. The computed GB normal and shear stress fields: are considerable lower than the pile-up ones and exhibit strong dependency on the slip band thickness close to the SB corner but are in fair agreement with the solution predicted by the pile-up theory far away Since the pile-up theory leads to the overestimation of the local GB stress fields, the main goal of the current paper is to perform analytical model of GB stress components based upon FE calculations. The effect of various parameters can be understood in the framework of matching asymptotic expansions which is usually applied to cracks with V notches of finite thickness. Finally, the predicted remote stresses to GB fracture in pre-irradiated austenitic stainless steels subjected to tensile loading in various environment are compared to experimental data and the pile-up based predictions.

The new science of oxide interfaces

Over the past decade, a huge range of emergent properties have been demonstrated at the interfaces between complex oxides. Such properties represent a paradigm of ‘greater than the sum of parts’, with radically different properties to the parent materials being obtained. Hence, oxide interfaces offer the potential to create entirely new functionality, and, in many cases, they do this at room temperature. The most notable example of the recent years is one where electronic reconstruction occurs at interfaces leading to many novel phenomena: two-dimensional electron gases with remarkable properties, superconducting states where the conductivity is confined to a single atomic plane, improper ferroelectric states and interfacial magnetism.