Strain-induced Mn valence state variation in CaMnO3-δ/substrate interfaces: electronic reconstruction versus oxygen vacancies.

This study investigates the nanoscale crystalline and electronic structures of the interfaces between CaMnO3-δ and substrates such as SrTiO3 (001) and LaAlO3 (001) by employing advanced transmission electron microscopy and electron energy loss spectroscopy techniques. The objective is to comprehend the influence of different strains on the Mn valence state. Our findings reveal that the Mn valence state remains relatively stable in the region of a weakly tensile-strained interface, whereas it experiences a significant decrease from Mn4+ to Mn2.3+ in the region of a strongly tensile-strained interface. Although this reduction in valence appears to be consistent with the electron reconstruction scenario, the observed increase in the out-of-plane lattice constant at the interface implies the accumulation of oxygen vacancies at the interface. Consequently, the present study offers a comprehensive understanding of the intricate relationships among the Mn valence state, local structure, and formation of oxygen vacancies in the context of two distinct strain cases. This knowledge is essential for tailoring the interface properties and guiding future developments in the field of oxide heterostructures.

Thermally Driven Structural Order of Oligo(Ethylene Glycol)-Terminated Alkanethiol Monolayers on Au(111) Prepared by Vapor Deposition.

To probe the effects of deposition temperature on the formation and structural order of self-assembled monolayers (SAMs) on Au(111) prepared by vapor deposition of 2-(2-methoxyethoxy)ethanethiol (CH3O(CH2)2O(CH2)2SH, EG2) for 24 h, we examined the surface structure and electrochemical behavior of the resulting EG2 SAMs using scanning tunneling microscopy (STM) and cyclic voltammetry (CV). STM observations clearly revealed that EG2 SAMs vapor-deposited on Au(111) at 298 K were composed of a disordered phase on the entire Au surface, whereas those formed at 323 K showed improved structural order, showing a mixed phase of ordered and disordered phases. Moreover, at 348 K, uniform and highly ordered EG2 SAMs on Au(111) were formed with a (2 × 3√3) packing structure. CV measurements showed sharp reductive desorption (RD) peaks at -0.818, -0.861, and -0.880 V for EG2 SAM-modified Au electrodes formed at 298, 323, and 348 K, respectively. More negative potential shifts of RD peaks with increasing deposition temperature are attributed to an increase in van der Waals interactions between EG2 molecular backbones resulting from the improved structural quality of EG2 SAMs. Our results obtained herein provide new insights into the formation and thermally driven structural order of oligo(ethylene glycol)-terminated SAMs vapor-deposited on Au(111).

Vertically-Oriented WS2 Nanosheets with a Few Layers and Its Raman Enhancements.

Vertically-oriented two-dimensional (2D) tungsten disulfide (WS2) nanosheets were successfully grown on a Si substrate at a temperature range between and 550 °C via the direct chemical reaction between WCl6 and S in the gas phase. The growth process was carefully optimized by adjusting temperature, the locations of reactants and substrate, and carrier gas flow. Additionally, vertically-oriented 2D WS2 nanosheets with a few layers were tested as a surface-enhanced Raman scattering substrate for detecting rhodamine 6G (R6G) molecules where enhancement occurs from chemical enhancement by charge transfer transition from semiconductor). Raman spectra of R6G molecules adsorbed on vertically-oriented 2D WS2 nanosheets exhibited strong Raman enhancement effects up to 9.2 times greater than that on the exfoliated WS2 monolayer flake sample. From our results, we suggest that the WS2 nanosheets can be an effective surface-enhanced Raman scattering substrate for detecting target molecules.

Relation between work function and structural properties of triangular defects in 4H-SiC epitaxial layer: Kelvin probe force microscopic and spectroscopic analyses.

To understand the relationship between the work function and structural properties of sufficiently expanded triangular defects (size: ∼250 µm) in the 4H-SiC epitaxial layer, Kelvin probe force microscopy (KPFM) and spectroscopic [micro-Raman spectroscopy and photoluminescence (PL)] analyses were performed. Spectroscopic analysis demonstrated that the triangular defects mostly comprise the 3C polytypes and that it experiences internal stress, defects, and defect-induced carrier generation. The distinguishable areas in the triangular defects had surface potential values different from those of the 4H-SiC matrix; this could be explained by the work function difference, which arises from variations in the electron affinity of the 3C polytype as well as the positional variations of the Fermi energy level in terms of electron concentration. In addition, tensile-stress-induced surface disorder leading to variations in electron affinity was discussed. The mechanical properties of the triangular defects measured by a nanoindenter were significantly deteriorated because of many dislocation arrays and stacking faults with many broken and/or strained bonds.

Modulation of metal-insulator transitions of NdNiO3/LaNiO3/NdNiO3 trilayers via thickness control of the LaNiO3 layer.

Over the last few decades, manipulating the metal-insulator (MI) transition in perovskite oxides (ABO3) via an external control parameter has been attempted for practical purposes, but with limited success. The substitution of A-site cations is the most widely used technique to tune the MI transition. However, this method introduces unintended disorder, blurring the intrinsic properties. The present study reports the modulation of MI transitions in [10 nm-NdNiO3/t-LaNiO3/10 nm-NdNiO3/SrTiO3 (100)] trilayers (t = 5, 7, 10, and 20 nm) via the control of the LaNiO3 thickness. Upon an increase in the thickness of the LaNiO3 layer, the MI transition temperature undergoes a systematic decrease, demonstrating that bond disproportionation, the MI, and antiferromagnetic transitions are modulated by the LaNiO3 thickness. Because the bandwidth and the MI transition are determined by the Ni-O-Ni bond angle, this unexpected behavior suggests the transfer of the bond angle from the lower layer into the upper. The bond-angle transfer eventually induces a structural change of the orthorhombic structure of the middle LaNiO3 layer to match the structure of the bottom and the top NdNiO3, as evidenced by transmission electron microscopy. This engineering layer sequence opens a novel pathway to the manipulation of the key properties of oxide nickelates, such as the bond disproportionation, the MI transition, and unconventional antiferromagnetism with no impact of disorder.

Bi-Stability and Orientation Change of a Thin α-Fe2O3 Layer on a ε-Fe2O3 (004) Surface.

This study reports the key ingredients that influence the orientation and stability of a α-Fe2O3 layer that grows on a metastable ε-Fe2O3 during pulsed laser deposition. Depending on the substrate temperature, two different α-Fe2O3 orientations arise on the ε-Fe2O3 (004) surface. At 800 °C, (2-10)α-oriented α-Fe2O3 is stabilized, whereas at 700 °C, (006)α orientation occurs. The (2-10)α-oriented α-Fe2O3 layer possesses an interface with densely packed Fe ions with presumably considerable number of oxygen vacancies. On the other hand, the (006)α-oriented α-Fe2O3 layer is stabilized, as in the case of the YSZ (100) substrate, due to the domain pattern with an in-plane rhombic shape, which is known to become an effective nucleation site. Growth with the unexpected (2-10)α orientation can be understood based on a model that takes into account the surface energy as the dominant factor, which mainly stems from the presence of dangling bonds on the surface and the atomic vibration of the surface atoms. As the surface is one of the critical elements related to the specific functionality of a material, the present study will offer valuable insights into the designs of functional devices with novel surface properties.

Side chain engineering in DTBDT-based small molecules for efficient organic photovoltaics.

A new small-molecule donor with a dithieno[2,3-d:2',3'-d']-benzo[1,2-b:4,5-b']-dithiophene (DTBDT) core and both alkyl and alkylthio substituents is designed and synthesized to improve the miscibility between DTBDT-based small molecules and [6,6]-phenyl-C71-butyric acid methyl ester (PC71BM). The alkyl substituent on the 4-position and the alkylthio substituent on the 5-position of the substituted thiophene are expected to improve intermolecular interactions and prevent severe aggregation of the small molecules. The new small molecule, DTBDT-S-C8-TTR, exhibits a homogenous blend morphology with small domains and edge-on-oriented crystalline structures in blends with PC71BM, and give a maximum power conversion efficiency (PCE) of 8.43%. To recover the crystallinity of the DTBDT-S-C8-TTR small molecules weakened after being blended with PC71BM, a solvent vapor annealing (SVA) treatment is performed. The SVA-treated blend films reveal well-developed crystalline domains with interconnected fibrillar structures. This blend morphology allows efficient charge carrier transport in blends and leads to increased PCEs. The maximum PCE of 9.18% achieved using DTBDT-S-C8-TTR suggests that substituting both alkylthio and alkyl groups into DTBDT can yield small-molecule-based organic photovoltaics (OPVs) displaying improved photovoltaic performances.

Correction: Single phase of spinel Co2RhO4 nanotubes with remarkably enhanced catalytic performance for the oxygen evolution reaction.

Correction for 'Single phase of spinel Co2RhO4 nanotubes with remarkably enhanced catalytic performance for the oxygen evolution reaction' by So Yeon Kim et al., Nanoscale, 2019, 11, 9287-9295.

Single phase of spinel Co2RhO4 nanotubes with remarkably enhanced catalytic performance for the oxygen evolution reaction.

We report the effective crystal growth for a unique single phase of spinel cobalt rhodium oxide (Co2RhO4) nanotubes via the electrospinning process combined with the thermal annealing process. In the spinel structure of the electrospun Co2RhO4 nanotubes, Co3+ cations and Rh3+ cations randomly occupy the octahedral sites, while the remaining half of the Co2+ cations occupy the centres of the tetrahedral sites as proved by microscopic and spectroscopic observations. Furthermore, electrospun spinel Co2RhO4 nanotubes exhibit excellent catalytic performances with the least positive onset potential, greatest current density, and low Tafel slope which are even better than those of the commercial Ir/C electrocatalyst for the oxygen evolution reaction (OER) in alkaline solution. Our demonstration of significantly enhanced OER activity with a single phase of electrospun spinel Co2RhO4 nanotubes thus opens up the broad applicability of our synthetic methodology for accessing new OER catalysis.

Ordered Mesoporous C3 N5 with a Combined Triazole and Triazine Framework and Its Graphene Hybrids for the Oxygen Reduction Reaction (ORR).

Mesoporous carbon nitrides (MCN) with C3 N4 stoichiometry could find applications in fields ranging from catalysis, sensing, and adsorption-separation to biotechnology. The extension of the synthesis of MCN with different nitrogen contents and chemical structures promises access to a wider range of applications. Herein we prepare mesoporous C3 N5 with a combined triazole and triazine framework via a simple self-assembly of 5-amino-1H-tetrazole (5-ATTZ). We are able to hybridize these nanostructures with graphene by using graphene-mesoporous-silica hybrids as a template to tune the electronic properties. DFT calculations and spectroscopic analyses clearly demonstrate that the C3 N5 consists of 1 triazole and 2 triazine moieties. The triazole-based mesoporous C3 N5 and its graphene hybrids are found to be highly active for oxygen reduction reaction (ORR) with a higher diffusion-limiting current density and a decreased overpotential than those of bulk g-C3 N4 .

Urine IP-10 as a biomarker of therapeutic response in patients with active pulmonary tuberculosis.

BACKGROUND: Prior to clinical trials of new TB drugs or therapeutic vaccines, it is necessary to develop monitoring tools to predict treatment outcomes in TB patients. Urine interferon gamma inducible protein 10 (IP-10) is a potential biomarker of treatment response in chronic hepatitis C virus infection and lung diseases, including tuberculosis. In this study, we assessed IP-10 levels in urine samples from patients with active TB at diagnosis, during treatment, and at completion, and compared these with levels in serum samples collected in parallel from matched patients to determine whether urine IP-10 can be used to monitor treatment response in patients with active TB. METHODS: IP-10 was measured using enzyme-linked immunosorbent assays in urine and serum samples collected concomitantly from 23 patients with active TB and 21 healthy adults (44 total individuals). The Mann-Whitney U test and Wilcoxon matched-pairs signed rank test were used for comparisons among healthy controls and patients at three time points, and LOESS regression was used for longitudinal data. RESULTS: The levels of IP-10 in urine increased significantly after 2 months of treatment (P = 0.0163), but decreased by the completion of treatment (P = 0.0035). Serum IP-10 levels exhibited a similar trend, but did not increase significantly after 2 months of treatment in patients with active TB. CONCLUSIONS: Unstimulated IP-10 in urine can be used as a biomarker to monitor treatment response in patients with active pulmonary TB.

A critical role of catalyst morphology in low-temperature synthesis of carbon nanotube-transition metal oxide nanocomposite.

The effect of the catalyst morphology on the growth of carbon nanotubes (CNT) on nanostructured transition metal oxides was investigated to study a novel low-temperature synthetic route to functional CNT-transition metal oxide nanocomposites. Among several nanostructured manganese oxides with various morphologies and structures, only exfoliated 2D nanosheets of layered MnO2 acted as an effective catalyst for the chemical vapor deposition of CNT at low temperatures of 400-500 °C, which emphasizes the critical role of the catalyst morphology in CNT growth. Heat treatment of the MnO2 nanosheets under a C2H2 flow induced the deposition of CNT, as well as a phase transition to a 2D ordered assembly of MnO nanoparticles. The resulting CNT-MnO nanocomposites displayed excellent functionalities in Li-ion electrodes with huge discharge capacities and good rate characteristics, which highlights the usefulness of the present method for studying functional CNT-metal oxide nanocomposites. Electron microscopy and density functional theory calculations propose a formation mechanism via the efficient adsorption of carbon on the MnO2 nanosheets followed by the surface diffusion of carbon. It is of prime importance that the substitution of Fe for layered MnO2 nanosheets remarkably improved the efficiency of the formation of CNT by enhancing the surface adsorption of carbon species. This is the first report of the efficient growth of CNT at a very low temperature of 400 °C. The universal merit of the 2D nanosheet morphology was confirmed by the successful synthesis of a CNT-TiO2 nanocomposite with exfoliated titanate nanosheets. The present study demonstrates that employing exfoliated transition metal oxide nanosheets as catalysts provides an efficient low-temperature synthetic route to functional CNT-transition metal oxide nanocomposites.

Chemical Vapor Deposition of Bernal-Stacked Graphene on a Cu Surface by Breaking the Carbon Solubility Symmetry in Cu Foils.

The synthesis of Bernal-stacked multilayer graphene over large areas is intensively investigated due to the value of this material's tunable electronic structure, which makes it promising for use in a wide range of optoelectronic applications. Multilayer graphene is typically formed via chemical vapor deposition onto a metal catalyst, such as Ni, a Cu-Ni alloy, or a Cu pocket. These methods, however, require sophisticated control over the process parameters, which limits the process reproducibility and reliability. Here, a new synthetic method for the facile growth of large-area Bernal-stacked multilayer graphene with precise layer control is proposed. A thin Ni film is deposited onto the back side of a Cu foil to induce controlled diffusion of carbon atoms through bulk Cu from the back to the front. The resulting multilayer graphene exhibits a 97% uniformity and a sheet resistance of 50 Ω sq-1 with a 90% transmittance after doping. The growth mechanism is elucidated and a generalized kinetic model is developed to describe Bernal-stacked multilayer graphene growth by the carbon atoms diffused through bulk Cu.

A 2D Metal Oxide Nanosheet as an Efficient Additive for Improving Na-Ion Electrode Activity of Graphene-Based Nanocomposites.

An efficient way to improve the Na-ion electrode activity of graphene-based nanocomposite is developed by employing exfoliated metal oxide nanosheet as an additive. The titanate-nanosheet-incorporated Na-SnS2 -reduced graphene oxide (rG-O) nanocomposites can be synthesized by the electrostatically derived restacking of the colloidal mixture of SnS2 , rG-O, and titanate nanosheets with the Na+ cation. The incorporation of titanate into the Na-SnS2 -rG-O nanocomposites is effective in improving the nanoscale mixing of component nanosheets and the porosity of the composite structure. The resulting nanocomposites deliver superior discharge capacities and rate properties to the titanate-free nanocomposite. The universal applicability is further confirmed by MoS2 -rG-O nanocomposites upon the addition of titanate. This study highlights that the exfoliated metal oxide nanosheet can be used as an efficient additive for graphene-based nanocomposites to explore Na-ion electrode materials.

Highly Efficient Silver-Cobalt Composite Nanotube Electrocatalysts for Favorable Oxygen Reduction Reaction.

This paper reports the synthesis and characterization of silver-cobalt (AgCo) bimetallic composite nanotubes. Cobalt oxide (Co3O4) nanotubes were fabricated by electrospinning and subsequent calcination in air and then reduced to cobalt (Co) metal nanotubes via further calcination under a H2/Ar atmosphere. As-prepared Co nanotubes were then employed as templates for the following galvanic replacement reaction (GRR) with silver (Ag) precursor (AgNO3), which produced AgCo composite nanotubes. Various AgCo nanotubes were readily synthesized with applying different reaction times for the reduction of Co3O4 nanotubes and GRR. One hour reduction was sufficiently long to convert Co3O4 to Co metal, and 3 h GRR was enough to deposit Ag layer on Co nanotubes. The tube morphology and copresence of Ag and Co in AgCo composite nanotubes were confirmed with SEM, HRTEM, XPS, and XRD analyses. Electroactivity of as-prepared AgCo composite nanotubes was characterized for ORR with rotating disk electrode (RDE) voltammetry. Among differently synthesized AgCo composite nanotubes, the one synthesized via 1 h reduction and 3 h GRR showed the best ORR activity (the most positive onset potential, greatest limiting current density, and highest number of electrons transferred). Furthermore, the ORR performance of the optimized AgCo composite nanotubes was superior compared to pure Co nanotubes, pure Ag nanowires, and bare platinum (Pt). High ethanol tolerance of AgCo composite nanotubes was also compared with the commercial Pt/C and then verified its excellent resistance to ethanol contamination.

Structural Stability and Phase Transitions of Octanethiol Self-Assembled Monolayers on Au(111) in Ultrahigh Vacuum.

To understand the structural stability of as-prepared octanethiol (OT) self-assembled monolayers (SAMs) with a fully covered c(4 x 2) phase on Au(111) in ultrahigh vacuum (UHV) conditions of 3 x 10(-7) Pa at room temperature, we examined OT SAM samples obtained as a function of storage period using scanning tunneling microscopy (STM). STM imaging revealed that phase transition of OT SAMs after storage in UHV for 3 days occurs from the c(4 x 2) phase to the mixed phase containing ordered c(4 x 2) and disordered phases. It was also observed that the disordered phase was mainly located at around vacancy islands and near step edges of Au(111) terraces, implying that desorption of OT molecules chemisorbed on Au(111) in UHV occurs more quickly in these regions compared with in the closely packed and ordered domains. After a longer storage in UHV for 6 days, OT SAMs with the c(4 x 2) phase were changed to the disordered phase containing a partially ordered domain with a row structure. From this study, we clearly demonstrated that OT molecules in SAMs on Au(111) are desorbed spontaneously in UHV at room temperature, resulting in the formation of disordered and row phases.

Implementing Room-Temperature Multiferroism by Exploiting Hexagonal-Orthorhombic Morphotropic Phase Coexistence in LuFeO3 Thin Films.

Room-temperature multiferroism in LuFeO3 (LFO) films is demonstrated by exploiting the orthorhombic-hexagonal (o-h) morphotrophic phase coexistence. The LFO film further reveals a magnetoelectric coupling effect that is not shown in single-phase (h- or o-) LFO. The observed multiferroism is attributed to the combination of sufficient polarization from h-LFO and net magnetization from o-LFO.

A Conductive Hybridization Matrix of RuO2 Two-Dimensional Nanosheets: A Hybrid-Type Photocatalyst.

A universal methodology to efficiently improve the photocatalyst performance of semiconductors was developed by employing exfoliated RuO2 two-dimensional nanosheets as a conducting hybridization matrix. The hybridization with a RuO2 nanosheet is easily achieved by crystal growth or electrostatically derived anchoring of semiconductor nanocrystals on the RuO2 nanosheet. An enhanced chemical interaction of inorganic semiconductor with hydrophilic RuO2 nanosheet is fairly effective in optimizing their photocatalytic activity and photostability by the enhancement of charge separation and charge mobility. The RuO2 -containing nanohybrids show much better photocatalyst functionalities than do the graphene-containing ones. The present study clearly demonstrates that hydrophilic RuO2 nanosheets are superior hybridization matrices, over the widely used hydrophobic graphene nanosheets, for exploring new efficient hybrid-type photocatalysts.

Carrier trapping and confinement in Ge nanocrystals surrounded by Ge3N4.

We investigated the optical properties of Ge nanocrystals surrounded by Ge3N4. The broad emission ranging from infrared to blue is due to the dependence on the crystal size and preparation methods. Here, we report high resolution Photoluminescence (PL) attributed to emission from individual Ge nanocrystals (nc-Ge) spatially resolved using micro-photoluminescence and detailed using temperature and power-dependent photoluminescence studies. The measured peaks are shown to behave with excitonic characteristics and we argue that the spread of the nc-Ge peaks in the PL spectrum is due to different confinement energies arising from the variation in size of the nanocrystals.

Unusually Huge Charge Storage Capacity of Mn3O4-Graphene Nanocomposite Achieved by Incorporation of Inorganic Nanosheets.

Remarkable improvement in electrode performance of Mn3O4-graphene nanocomposites for lithium ion batteries can be obtained by incorporation of a small amount of exfoliated layered MnO2 or RuO2 nanosheets. The metal oxide nanosheet-incorporated Mn3O4-reduced graphene oxide (rGO) nanocomposites are synthesized via growth of Mn3O4 nanocrystals in the mesoporous networks of rGO and MnO2/RuO2 2D nanosheets. Incorporation of metal oxide nanosheets is highly effective in optimizing porous composite structure and charge transport properties, resulting in a remarkable increase of discharge capacity of Mn3O4-rGO nanocomposite with significant improvement of cyclability and rate performance. The observed enormous discharge capacity of synthesized Mn3O4-rGO-MnO2 nanocomposite (∼1600 mA·h·g(-1) for the 100th cycle) is the highest value among reported data for Mn3O4-rGO nanocomposite. Despite much lower electrical conductivity of MnO2 than RuO2, the MnO2-incorporated nanocomposite at optimal composition (2.5 wt %) shows even larger discharge capacities with comparable rate characteristics compared with the RuO2-incorporated homologue. This finding underscores that the electrode performance of the resulting nanosheet-incorporated nanocomposite is strongly dependent on its pore and composite structures rather than on the intrinsic electrical conductivity of the additive nanosheet. The present study clearly demonstrates that, regardless of electrical conductivity, incorporation of metal oxide 2D nanosheet is an effective way to efficiently optimize the electrode functionality of graphene-based nanocomposites.