In-plane Disorder Research Articles

Simultaneous Characterization of In-Plane and Cross-Plane Resistivities in Highly Anisotropic 2D Layered Heterostructures.

Understanding and characterizing the intrinsic properties of charge carrier transport across the interfaces in van der Waals heterostructures is critical to their applications in modern electronics, thermoelectrics, and optoelectronics. However, there are very few published cross-plane resistivity measurements of thin samples because these inherently 2-probe measurements must be corrected for contact and lead resistances. Here, we present a method to extract contact resistances and metal lead resistances by fitting the width dependence of the contact end voltages of top and bottom electrodes of different contact widths to a model based on current crowding. These contributions are then subtracted from the total 2-probe cross-plane resistance to obtain the cross-plane resistance of the material itself without needing multiple devices and/or etching steps. This approach was used to measure cross-plane resistivities of a (PbSe)1(VSe2)1 heterostructure containing alternating layers of PbSe and VSe2 with random in-plane rotational disorder. Several samples measured exhibited a 4 order of magnitude difference between cross-plane and in-plane resistivities over the 6-300 K temperature range. We also reported the observation of charge density wave transition in the cross-plane transport of the (PbSe)1(VSe2)1 heterostructure. The device fabrication process is fully liftoff compatible, and the method developed enables the straightforward measurement of the resistivity anisotropy of most thin film materials with nm thicknesses.

Surface topography and microstructure changes of ultrafine-grained graphite prepared from filler of onion-like carbon spheres by Xe ions irradiation

Onion-like carbon spheres (OLC) were doped into the traditional graphite raw material system to decrease the pore diameter to ∼400 nm, with potential for use in molten salt reactors. The obtained graphite (OLCG) prepared from OLC was subjected to 7 MeV Xe26+ irradiation in this work, and a systematic investigation was conducted to verify its irradiation behavior. The surface morphology and microstructural evolution in graphite samples under different irradiation conditions were studied with different characterization methods. The surface roughness of OLCG increased and the hill appeared at lower irradiation doses, which was similar to that of IG-110, but the filler particles of OLCG gradually become spherical at 2.5 dpa. The analysis by X-ray diffraction reveals that OLCG's graphitization degree is approaching but slightly worse compared to IG-110. Raman studies indicate that the irradiation causes an increase in defect density and in-plane disorder both in graphite. The OLC filler exhibits a greater sensitivity to radiation, leading to OLCG being slightly sensitive to radiation as compared to IG-110. Neutron irradiation of the resulting graphite should be continued as OLCG showed good irradiation resistance at low irradiation doses and had a much smaller pore diameter.

Low temperature relaxor, polarization dynamics and energy storage properties of Ca0.28Ba0.72Nb2O6 tungsten bronze ceramics

The development of modern electronic devices requires dielectrics with high energy storage density, high power density and excellent working stability as the dielectric layer of multilayer ceramic capacitors. Herein, Ca0.28Ba0.72Nb2O6-based tungsten bronze ceramics doped by ions with various radii were prepared, and the energy storage performance was improved by adjusting the relaxor behavior. Rare double relaxor behavior was confirmed based on macroscopic phenomenological model and characterization of domain structure, and the low-temperature relaxor behavior was found to be sensitive to the in-plane polarization displacement disorder of Nb2 sites. Meanwhile, a significantly improved energy storage performance was obtained: Wre ∼ 2.59 J/cm3, η ∼ 85.3 %, PD ∼ 130.78 MW/cm3. The excellent temperature stability of the energy storage performance is explained by the polarization reversal dynamics via Vopsaroiu model. This work highlights the potential of tungsten bronze as energy storage devices, and provides an additional feasible paradigm of pulse power capacitors with excellent thermal stability.

Photoelectron holographic evidence for the incorporation site of Se and suppressed atomic displacement of the conducting layer of La(O,F)BiSSe

La(O,F)BiS2-x Se x is a layered material that is considered to be a candidate exotic superconductor as well as a promising thermoelectrical material. We performed soft X-ray photoelectron holography to study the Se incorporation site and the local atomic arrangement of the conducting layer. A comparison of the experimental holograms with the simulated holograms indicates that Se atoms preferentially occupy the S sites in the conducting Bi–S plane of La(O,F)BiS2. A comparison between the state-of-the-art holographic reconstructions of La(O,F)BiSSe and La(O,F)BiS2 suggests that Se substitution suppresses the displacement of S atoms in La(O,F)BiS2. These results provide photoelectron holographic evidence for the Se incorporation site and the Se-induced suppression of in-plane disorder.

Topography changes and microstructural evolution of nuclear graphite (IG-110) induced by Xe26+ irradiation

AbstractThe microstructure of nuclear graphite, a key material in nuclear reactors, is affected by the high-flux irradiation. The damage to the graphite by irradiation is important for reactor safety. To understand the damage of nuclear graphite by irradiation, IG-110 nuclear graphite, a representative nuclear graphite, was chosen to investigate the change in morphology and microstructure caused by 7 MeV Xe26+ irradiation with peak damage doses of 0.1-5.0 displacements per atom (dpa) for samples of a size of 40.0 mm×40.0 mm×2.0 mm. The topography and microstructure of IG-110 were characterized by SEM, AFM, grazing incidence XRD, Raman spectroscopy and nano-indentation. Results indicate that after 7 MeV Xe26+ irradiation at a dose of 0. 11 dpa, a ridge-like structure appears on the surface of the IG-110 graphite, mainly in the binder region, and the surface roughness increases slightly. With a further increase of the irradiation dose, the ridge-like structure also appears in the filler region. At a dose of 0.55 dpa, pore shrinkage increases accompanied by pore closure, and the surface roughness also increases. The changes in topography and microstructure caused by irradiation are attributed to the expansion of graphite along the c-axis direction. With increasing irradiation dose the defect density and the degree of in-plane disorder in the graphene sheets increases, while the modulus and hardness of the graphite first increase and then decrease. Their increase is caused by dislocation pinning and closure of fine pores, while their decrease is attributed to an increase in porosity and the generation of an amorphous structure.

Evolution of carbon nanostructure during pyrolysis of homogeneous chitosan-cellulose composite fibers

Chitosan-cellulose composite fibers spun using a Lyocell technology are characterized by a homogeneous distribution and a close packing of the two biopolymers inside the fibrous matrix. Due to the intimate contact of cellulose and chitosan, synergistic effects can be observed during the pyrolysis of the composite fibers. In this study, the catalytic role of chitosan in altering the cellulose pyrolysis pathway in the composite fibers at moderate treatment temperatures up to 900 °C is confirmed. Analyses of the evolved gases during pyrolysis revealed that chitosan promoted cellulose dehydration and substantially decreased the formation of levoglucosan, explaining the higher char yield. The enhanced dehydration reaction is associated with the formation of intermolecular crosslinks due to the incorporation of nitrogen from chitosan in the resulting carbon structures. Nitrogen could also contribute to the in-plane disorder in the aromatic clusters when the pyrolysis is carried out at 500–700 °C, although the in-plane disorder is less noticeable from 700 to 900 °C. Nevertheless, the size of the aromatic cluster continues to grow when the composite fibers are pyrolyzed in a temperature range of 500–900 °C.

On-Surface Polymerization of In-Plane Highly Ordered Carbon Nitride Nanosheets toward Photocatalytic Mineralization of Mercaptan Gas.

2D carbon nitride nanosheets have attracted ever-increasing interest in photocatalysis due to their unique structural advantages. However, the nanosheets synthesized by the traditional methods, such as post oxidation and liquid exfoliation, have suffered from in-plane disorder with abundant structural defects, which seriously counteracts their structural benefits for photocatalysis. Herein, it is demonstrated that polymer carbon nitride nanosheets with in-plane highly ordered structure (PCNNs-IHO) can be successfully prepared by on-surface polymerization of melamine on NaCl crystal surface at elevated temperatures. The NaCl crystals with relative high surface energy not only facilitate the adsorption and activation of melamine to undergo condensation reaction, but also function as unique substrates to orientate the assembly of 2D nanosheet structure. In addition, NaCl also acts as a reactant to provide Na+ doping into carbon nitride matrix, affording PCNNs-IHO with robust structural base sites. Benefiting from this structural basicity, PCNNs-IHO exhibits superior photocatalytic performance toward CH3 SH mineralization under visible light irradiation.

Bulk O2 formation and Mg displacement explain O-redox in Na0.67Mn0.72Mg0.28O2

Summary O-redox in compounds with Li on the transition-metal layers (TML) has recently been attributed to the formation of molecular O2 on charge, trapped in the lattice. Here, we show that a similar process occurs for P2-Na0.67[Mn0.72Mg0.28]O2, which contains Mg2+ on the TML. The molecular O2 is identified by high-resolution RIXS and quantified by magnetometry, showing that it equates to the charge passed. This O2 is trapped in voids that are formed by Mg2+ out-of-plane displacement and Mn4+ in-plane disordering and is then reduced on discharge associated with a large voltage hysteresis. In contrast to compounds containing Li+ in the TML, in which the honeycomb ordering and the high-voltage plateau are irreversibly lost after the first cycle, in P2-Na0.67[Mn0.72Mg0.28]O2, the plateau reappears partially on the second charge due to the partial reversibility of Mn in-plane and Mg out-of-plane migration and the local reformation of the honeycomb ordering.

TEM and DFT study of basal-plane inversion boundaries in SnO2-doped ZnO

In our recent study (Ribic et al. 2020) we reported the structure of inversion boundaries (IBs) in Sb2O3-doped ZnO. Here, we focus on IBs that form in SnO2-doped ZnO. Using atomic resolution scanning transmission electron microscopy (STEM) methods we confirm that in SnO2-doped ZnO the IBs form in head-to-head configuration, where ZnO4 tetrahedra in both ZnO domains point towards the IB plane composed of a close-packed layer of octahedrally coordinated Sn and Zn atoms. The in-plane composition is driven by the local charge balance, following Pauling's principle of electroneutrality for ionic crystals, according to which the average oxidation state of cations is 3+. To satisfy this condition, the cation ratio in the IB-layer is Sn4+: Zn2+=1:1. This was confirmed by concentric electron probe analysis employing energy dispersive spectroscopy (EDS) showing that Sn atoms occupy 0.504 ? 0.039 of the IB layer, while the rest of the octahedral sites are occupied by Zn. IBs in SnO2-doped ZnO occur in the lowest energy, IB3 translation state with the cation sublattice expansion of ?IB(Zn-Zn) of +91 pm with corresponding O-sublattice contraction ?IB(O-O) of -46 pm. Based on quantitative HRTEM and HAADF-STEM analysis of in-plane ordering of Sn and Zn atoms, we identified two types of short-range distributions, (i) zigzag and (ii) stripe. Our density functional theory (DFT) calculations showed that the energy difference between the two arrangements is small (~6 meV) giving rise to their alternation within the octahedral IB layer. As a result, cation ordering intermittently changes its type and the direction to maximize intrinsic entropy of the IB layer driven by the in-plane electroneutrality and 6-fold symmetry restrictions. A long-range in-plane disorder, as shown by our work would enhance quantum well effect to phonon scattering, while Zn2+ located in the IB octahedral sites, would modify the bandgap, and enhance the in-plane conductivity and concentration of carriers. Keywords

Highly oriented lamellar polyaniline with short-range disorder for enhanced electrochromic performance

• Highly oriented lamellar PANI with in-plane short-range disorder was prepared. • Every PANI layer serves as an independent electrochromic film for ion transmission. • This structure simultaneously improves ion diffusion capability and reaction area. • pTSA-PANI displays high contrast ratio, coloration efficiency and fast response. Conjugated organic polymers (COP) are fundamental for the future development of electrochromic devices. Despite the progress of composite materials, the electrochromic performance of pristine COP is still limited by dense three-dimensional (3D) structure. This work reports the fabrication of high optical contrast polyaniline (PANI) films with fast switching speed by electrochemical polymerization. Benefitting from the stereo-structure and size of p-toluenesulfonic acid (pTSA), highly oriented lamellar polyaniline with nanoscale large inter-layer spacing and a disordered in-plane structure is obtained. These structural factors simultaneously improve ion diffusion capability and optimise reaction area, resulting in a high optical contrast of above 70% at a wavelength of 720 nm, a fast response time (0.8 s for coloring and 4.2 s for bleaching), and remarkable coloration efficiency (328.5 cm 2 C −1 ). In addition, the pTSA-PANI quasi-solid-state electrochromic device displayed good electrochromic performance with optical contrast of 82%, suggesting an inherent suitability for use in electrochromic fields. Furthermore, the synthetic method developed here represents a new and simple technique to modulate photoelectronic properties.

The Superconducting Phase Diagram of ThFe1−xNixAsN

Differently from most of the other 1111-type iron-based superconductors (IBSCs), ThFeAsN without an antiferromagnetic order shows superconductivity at 30 K in the absence of chemical doping and other treating. In order to enhance its superconductivity and understand its peculiar superconducting behaviour, it is necessary to investigate the evolution of the superconducting phase through electron doping, especially heavy electron doping. Chemically, the doping of Ni is an effective way to realize heavy electron doping, as Ni atoms have two more 3d electrons than Fe atoms and electrons could be doped directly into the FeAs planes. As is known that ThFeAsN is a kind of unconventional superconductor, while ThNiAsN is a kind of conventional superconductor, then if ThFeAsN is doped with Ni to form ThFe1−xNixAsN, some intriguing behaviours such as the phase transition from unconventional superconductivity to conventional superconductivity may be observed. Here, we have synthesized ThFe1−xNixAsN with 0.01 ≤ x ≤ 1. It is found that, with doping of Ni, the superconductivity of ThFeAsN is suppressed quickly. For ThNiAsN resynthesized by us, its Tc is 4.8 K, which is higher than the previously reported value of Tc = 4.3 K. The superconducting phase diagram of ThFe1−xNixAsN is drawn to summarize the doping results. Considering the other electron doping results of ThFeAsN1−xOx and ThFe1−xCoxAsN, it can be concluded that the electron doping is not an effective way to increase the Tc of ThFeAsN. The hole doping result is expected. Being associated with the previous results, it can be concluded that the superconductivity of ThFeAsN is much more sensitive to in-plane disorder than that of doped LaFeAsO, which reveals again that their superconductivity mechanisms are really different.

Influence of out-of-plane disorder formed by low-temperature sintering on carbohydrate-doped MgB2 wire

Structural disorder within grains formed by carbon doping is widely known to enhance the in-field transport critical current performance of MgB2 wire. The structural formation usually requires high-temperature sintering above 600 °C due to low reactivity of the dopant. In this study, we selected a carbohydrate (malic acid) that has a low-decomposition temperature as the carbon dopant and performed low-temperature sintering (below 600 °C) to introduce greater disorder within the MgB2 grains. We revealed that the doping and low-temperature sintering process cause severe disorder, especially along the out-of-plane (c-axis) direction. The disordered formation, in addition to the in-plane disorder combined with well-connected grains, eventually raised the in-field transport performance.

The phase transition of ThFe1-xCoxAsN from superconductor to metallic paramagnet

Differently from most of the other 1111-type iron-based superconductors, ThFeAsN itself shows superconductivity at 30 K without antiferromagnetism, even in the absence of chemical doping and other treating. In order to understand its peculiar behavior better, it is necessary to investigate the evolution of the superconducting phase through electron doping. Chemically, Co doping is a more effective way to introduce electrons, as carriers are doped directly into the FeAs planes. It also could provide information on how well the ThFeAsN tolerates in-plane disorder. Here we have substituted Co for Fe to synthesize ThFe1-xCoxAsN. It is found that the superconductivity of ThFeAsN parent compound is quickly suppressed upon Co doping. With a doping amount of ), the superconductivity of ThFe1-xCoxAsN vanishes. ThCoAsN has been synthesized and characterized. It shows no superconductivity at 1.8 K. As both its crystal structure and transport behaviour are similar to those of itinerant ferromagnets LaCoAsO and LaCoPO, it is expected that ThCoAsN would be a kind of itinerant ferromagnet. However, until 1.8 K, the expected itinerant ferromagnetism could not be confirmed. The experimental data support that ThCoAsN is a kind of metallic paramagnet above 1.8 K.

Material Development and Physical Properties of BiS2-Based Layered Compounds

In 2012, we discovered superconductivity in Bi4O4S3 and LaO1-xFxBiS2, whose crystal structures are composed of alternate stacks of a BiS2-type conduction layer and a blocking layer. Since the discovery, many related superconductors and functional materials have been synthesized. Furthermore, the possibility of unconventional superconductivity has been proposed in recent theoretical and experimental studies. In addition, notable correlations between local structure and physical properties have been revealed in the BiS2-based systems. In this review article, we summarize the material development (probably all the compounds) and physical properties of BiS2-related materials developed in the last six years. The key parameters for the emergence of bulk superconductivity in the BiS2-based compounds are carrier doping to the parent phase (band insulator), pressure-induced structural transition, in-plane chemical pressure, and the suppression of in-plane disorder. We systematically review those parameters and the method of designing new superconductors with BiS2 layers. In addition, studies of BiS2-based compounds as thermoelectric materials are briefly reviewed.

Improvement of superconducting properties by high mixing entropy at blocking layers in BiS2-based superconductor REO0.5F0.5BiS2

To investigate the interlayer interaction in the recently synthesized high-entropy-alloy-type (HEA-type) REO0.5F0.5BiS2 superconductors (RE: rare earth), we have systematically synthesized two sets of samples with close lattice parameters (close to those of PrO0.5F0.5BiS2 or CeO0.5F0.5BiS2) but having different mixing entropy (ΔSmix) for the RE site. The crystal structure was investigated using synchrotron X-ray diffraction and Rietveld refinement. For the examined samples having different ΔSmix, the increase in ΔSmix does not largely affect the bond lengths and the bond angle of the BiS2 conducting layer but clearly suppresses the in-plane disorder at the in-plane S1 site, which is the parameter essential for the emergence of bulk superconductivity in the REO0.5F0.5BiS2 system. Bulk nature of superconductivity is improved by the increase in ΔSmix for the present samples. The results of this work clearly show that the increase in mixing entropy at the blocking layer can positively affect the emergence of bulk superconductivity by modifying the local structure of the conducting layer. This is the evidence of the interaction between the high entropy states of the blocking layers and the physical properties of the conducting layers.

Following the in-plane disorder of sodiated hard carbon through operando total scattering

Mechanisms of ion insertion in hard carbon and the corresponding structural impact on the graphene layer.

Phase Intergrowth and Structural Defects in Organic Metal Halide Ruddlesden–Popper Thin Films

Organic metal halide Ruddlesden–Popper layered perovskite phases combine the excellent optoelectronic properties of three-dimensional, bulk hybrid perovskites with superior material stability under ambient conditions. However, the thin film structure of these layered perovskites is still poorly understood, as phase purity is typically determined solely by specular X-ray diffraction. The thin film structure of these Ruddlesden–Popper phases was examined by increasingly local characterization techniques. From the comparison of grazing-incidence wide-angle X-ray scattering patterns of cast films to expected scattering from single-crystal structures, significant in-plane disorder was observed. Spatially localized photoluminescence measurements show that films do not phase separate on the micrometer scale. Selected area electron diffraction measurements show the intergrowth of different phases within the same thin film, consistent with previous observations seen in epitaxially grown Ruddlesden–Popper complex o...

Critical Role of the Chemical Environment of Interlayer Na Sites: An Effective Way To Improve the Na Ion Electrode Activity of Layered Titanate.

The chemical environments of the interlayer Na sites of layered titanate are finely controlled by the intercalation of n-alkylamine with various alkyl chain lengths to explore an effective way to improve its electrode functionality for sodium-ion batteries (SIBs). The n-alkylamine intercalation via ion-exchange and exfoliation-restacking routes allows the modification of in-plane structures of layered titanate to be tuned. Among the present n-alkylamine-intercalates, the n-pentylamine-intercalated titanate shows the largest discharge capacity with the best rate characteristics, underscoring the critical role of optimized intracrystalline structure in improving the SIB electrode performance of layered titanate. The creation of turbostratic in-plane structure degrades the SIB electrode performance of layered titanate, indicating the detrimental effect of in-plane structural disorder on electrode activity. 23Na magic-angle spinning nuclear magnetic resonance spectroscopy demonstrates that the n-alkylamine-intercalated titanates possess two different interlayer Na+ sites near ammonium head groups/titanate layers and near alkyl chains. The intercalation of long-chain molecules increases the population of the latter site and the overall mobility of Na+ ions, which is responsible for the improvement of electrode activity upon n-alkylamine intercalation. The present study highlights that the increased population of interlayer metal sites remote from the host layers is effective in improving the electrode functionality of layered metal oxide for SIBs and multivalent ion batteries.

Evolution of Anisotropic Displacement Parameters and Superconductivity with Chemical Pressure in BiS2-Based REO0.5F0.5BiS2 (RE = La, Ce, Pr, and Nd)

In order to understand the mechanisms behind the emergence of superconductivity by the chemical pressure effect in REO0.5F0.5BiS2 (RE = La, Ce, Pr, and Nd), where bulk superconductivity is induced by the substitutions with a smaller-radius RE, we performed synchrotron powder X-ray diffraction, and analyzed the crystal structure and anisotropic displacement parameters. With the decrease of the RE3+ ionic radius, the in-plane disorder of the S1 sites significantly decreased, very similar to the trend observed in the Se-substituted systems: LaO0.5F0.5BiS2-xSex and Eu0.5La0.5FBiS2-xSex. Therefore, the emergence of bulk superconductivity upon the suppression of the in-plane disorder at the chalcogen sites is a universal scenario for the BiCh2-based superconductors. In addition, we indicated that the amplitude of vibration along the c-axis of the in-plane chalcogen sites may be related to the Tc in the BiCh2-based superconductors.

Synchrotron powder X-ray diffraction and structural analysis of Eu0.5La0.5FBiS2-xSex

Eu0.5La0.5FBiS2-xSex is a new BiS2-based superconductor system. In Eu0.5La0.5FBiS2-xSex, electron carriers are doped to the BiS2 layer by the substitution of Eu by La. Bulk superconductivity in this system is induced by increasing the in-plane chemical pressure, which is controlled by the Se concentration (x). In this study, we have analysed the crystal structure of Eu0.5La0.5FBiS2-xSex using synchrotron powder diffraction and the Rietveld refinement. The precise determination of the structural parameters and thermal factors suggest that the emergence of bulk superconductivity in Eu0.5La0.5FBiS2-xSex is achieved by the enhanced in-plane chemical pressure and the decrease in in-plane disorder.