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2020


Hybrid h-BN–Graphene Monolayer with B–C Boundaries on a Lattice-Matched Surface

K.A. Bokai, A.V. Tarasov, V.O. Shevelev, O.Yu. Vilkov, A.A. Makarova, D. Marchenko, A.E. Petukhov, M. Muntwiler, A.V. Fedorov, V.Yu. Voroshnin, L.V. Yashina, C. Laubschat, D.V. Vyalikh, and D.Yu. Usachov

In-plane heterostructures of hexagonal boron nitride (h-BN) and graphene (Gr) have recently appeared in the focus of material science research owing to their intriguing and tunable electronic properties. However, disclosure of the atomic structure and properties of one-dimensional heterojunctions between Gr and h-BN domains remains a largely unexplored and challenging task. Here, we report an approach to obtain a perfectly oriented and atomically thin hybrid h-BN–Gr heterolayer on the Co(0001) surface. A perfect matching of the lattice parameters ensures an epitaxial growth of both Gr and h-BN on the close-packed Co surface. High crystalline quality of the resulting interface allowed us to uncover the structural and electronic properties of the lateral h-BN/Gr heterojunctions by means of complementary microscopic and spectroscopic techniques. In particular, we established the coexistence of two types of zigzag boundaries made of B–C bonds, while the boundaries with N–C bonds were found to be unfavorable. Observation of spin-polarized edge states at the C-zigzag edges of Gr domains allowed us to determine the atomic structure of C-BN heterojunctions with scanning tunneling microscopy.

Chem. Mater.  32, 3 (2020) 1172–1181


Crystalline and amorphous calcium carbonate as structural components of the Calappa granulata exoskeleton

Katsikini M., Proiou E., Vouroutzis N., Pinakidou, F., Paloura E.C., Smirnov, D., Brzhezinskaya M., Ves S.

The exoskeleton of crustaceans consists of chitin biopolymers where the embedded inorganic biominerals, mainly CaCO3, affect strongly its mechanical properties. Raman and Near Edge X-ray Absorption Fine Structure (NEXAFS) spectroscopies and Transmission Electron Microscopy (TEM) are applied to investigate the CaCO3 structure in various parts of the Calappa granulata crab exoskeleton. The shape of the main Raman peak of CaCO3 reveals the presence of two phases which are identified as calcite and amorphous calcium carbonate (ACC). The relative concentration of the two phases in various parts of the exoskeleton is determined from the area ratio under the corresponding peaks. The results of the Ca L3,2-edge NEXAFS analysis are in line with the Raman findings, since the energy separation of peaks that appear in the lower frequency region of the main L2 and L3 peaks due to crystal field splitting, is directly related to the percentage of the ACC phase in the total CaCO3 mineral content. The C K-edge spectra are used for the determination of the extent of calcification of the exoskeleton. Furthermore, dark and bright field TEM images reveal the presence of nanocrystallites with an average size of 20 nm. The structure of the nanocrystallites, as derived from the Selected Area Electron Diffraction patterns, is calcite. The results suggest that ACC plays a structural role in the exoskeleton of Calappa granulata.

Journal of Structural Biology 211 (2020) 107557(9).


Preferred attachment of fluorine near oxygen-containing groups on the surface of double-walled carbon nanotubes,

Yu.V. Fedoseeva, L.G. Bulusheva, V.O. Koroteev, J.-Y. Mevellec, B.V. Senkovskiy, E. Flahaut, A.V. Okotrub

Two samples of double-walled carbon nanotubes (DWCNTs), one with well-graphitized nanotube walls and another containing oxygen at outer nanotube surfaces, were fluorinated at room temperature using gaseous BrF3. The products were comprehensively studied using transmission electron microscopy, Raman scattering, X-ray photoelectron, and near-edge X-ray absorption fine structure spectroscopies. The experimental data found twice the concentration of sidewall fluorine in the oxygenated DWCNTs. Quantum chemical modeling supported the experimental results revealing the preferable development of CF areas near the carbon atoms bonded with oxygen-containing groups. This observation demonstrates that tuning of the physical and chemical properties of carbon nanotubes can be achieved via the controlled co-modification by fluorine and oxygen functional groups.

Applied Surface Science 504 (2020) 144357


Overview of the transformation of spongin scaffolds to a carbonized 3D structure at 1200°C - enlarged view

Overview of the transformation of spongin scaffolds to a carbonized 3D structure at 1200°C. (A) Typical cellular and hierarchical morphology of Hippospongia communis demosponge organic skeleton after purification remains unchanged during the process of carbonization in spite of a decrease in volume by up to 70%. (B) Carbonized 3D scaffold can be sawn into 2-mm-thick slices (C). Stereomicroscopy (D and E) images of carbonized spongin network confirm its structural integrity, typical for sponge-like constructs.(F) NEXAFS C1s K-edge spectra of native and carbonized spongin heated at different temperatures, HOPG, and nanocomposite MWCNT/Cr2O3.

Extreme biomimetics: Preservation of molecular detail in centimeter-scale samples of biological meshes laid down by sponges

 Ia. Petrenko, A.P. Summers, P. Simon, S. Żółtowska-Aksamitowska, M. Motylenko, C. Schimpf, D. Rafaja, F. Roth, K. Kummer, E. Brendler, O.S. Pokrovsky, R. Galli, M. Wysokowski, H. Meissner, E. Niederschlag, Y. Joseph, S. Molodtsov, A. Ereskovsky, V. Sivkov, S. Nekipelov, O. Petrova, O. Volkova, M. Bertau, M. Kraft, A. Rogalev, M. Kopani, T. Jesioniowski, and H. Ehrlich

A chemical procedure for modification of double-walled carbon nanotubes (DWCNTs) to enhance their response to humidity was developed. The DWCNTs walls were etched by hot concentrated sulfuric acid, after what the edge carbon sites were saturated by chlorine via reaction with CCl4 vapor. This treatment increases the dispersibility of DWCNTs in solvents, removes oxygen groups, and produces chlorine-decorated holes in the outer walls. Networks of chlorinated holey DWCNTs showed a high repeatable response to humid environment and a good reversible behavior after the sensor purging by dry air. The density functional theory calculations predict enhanced polarization of the DWCNTs when they contain chlorine-decorated holes in the outer walls and physisorption of H2O molecules near chlorine atoms. These two effects are the cause of an intense low-noise signal to gaseous H2O and easy sensor recovery.

Science Advances 5 (2019) eaax2805.


Chlorinated holey double-walled carbon nanotubes for relative humidity sensors

L.G. Bulusheva, V.I. Sysoev, E.V. Lobiak, Yu.V. Fedoseeva, A.A. Makarova, M. Dubois, E. Flahaut, and A.V. Okotrub

A chemical procedure for modification of double-walled carbon nanotubes (DWCNTs) to enhance their response to humidity was developed. The DWCNTs walls were etched by hot concentrated sulfuric acid, after what the edge carbon sites were saturated by chlorine via reaction with CCl4 vapor. This treatment increases the dispersibility of DWCNTs in solvents, removes oxygen groups, and produces chlorine-decorated holes in the outer walls. Networks of chlorinated holey DWCNTs showed a high repeatable response to humid environment and a good reversible behavior after the sensor purging by dry air. The density functional theory calculations predict enhanced polarization of the DWCNTs when they contain chlorine-decorated holes in the outer walls and physisorption of H2O molecules near chlorine atoms. These two effects are the cause of an intense low-noise signal to gaseous H2O and easy sensor recovery.

Carbon 148 (2019) 413.


Milling-induced chemical decomposition of the surface of EuBaCo2O5.5 powders studied by means of soft X-ray absorption spectroscopy

V.R. Galakhov, M.S. Udintseva, V.V. Mesilov, B.A. Gizhevskii, S.V. Naumov, S.V. Telegin, and D.A. Smirnov

We present results of soft X-ray absorption spectroscopy studies of EuBaCo2O5.5 powders subjected to mechanical impact (milling in a ball mill). We show that in the near-surface region (5-10 nm) of particles of the powder, EuBaCo2O5.5 decomposes into Co3O4, BaCO3, and EuCoO3. A knowledge of the low-spin state of Co3+ ions in EuCoO3 and Co3O4 and possibilities of surface-sensitive soft X-ray absorption spectroscopy have allowed to determine the composition of the EuBaCo2O5.5 sample subjected to mechanical impact near its surface, which is inaccessible to standard X-ray phase analysis.

Applied Surface Science 493 (2019) 1048.


enlarged view

SEM micrographs of: (a) SiNW-15 nanostructured silicon surface using 15 s silver deposition in the first MAWCE etching step; (b) SiNW-45 nanostructured silicon surface using 45 s silver deposition in the first MAWCE etching step; (c) typical cross sectional view for MAWCE SiNWs array; (d) in-situ mechanically modified nanostructured silicon surface. (e) XANES Si L2,3 spectra for the references (from down to top) crystalline silicon c-Si, amorphous silicon a-Si, silicon suboxides SiO1.3 and SiO1.7, thermally grown 40 nm film of silicon dioxide SiO2; (f) XANES Si L2,3 registered from the initial arrays obtained under different etching time (15 and 45 sec) and their in-situ mechanically modified surface parts. Arrows indicate the presence of differently pronounced dip.

Surface deep profile synchrotron studies of mechanically modified top-down silicon nanowires array using ultrasoft X-ray absorption near edge structure spectroscopy

S. Yu. Turishchev, E. V. Parinova, A. K. Pisliaruk, D. A. Koyuda, D. Yermukhamed, T. Ming, R. Ovsyannikov, D. Smirnov, A. Makarova, and V. Sivakov

Atomic, electronic structure and composition of top-down metal-assisted wet-chemically etched silicon nanowires were studied by synchrotron radiation based X-ray absorption near edge structure technique. Local surrounding of the silicon and oxygen atoms in silicon nanowires array was studied on as-prepared nanostructured surfaces (atop part of nanowires) and their bulk part after, first time applied, in-situ mechanical removal atop part of the formed silicon nanowires. Silicon suboxides together with disturbed silicon dioxide were found in the composition of the formed arrays that affects the electronic structure of silicon nanowires. The results obtained by us convincingly testify to the homogeneity of the phase composition of the side walls of silicon nanowires and the electronic structure in the entire length of the nanowire. The controlled formation of the silicon nanowires array may lead to smart engineering of its atomic and electronic structure that infuences the exploiting strategy of metal-assisted wet-chemically etched silicon nanowires as universal matrices for different applications.

Scientific Reports 9 (2019) 8066.


enlarged view

Morphological and structural characterization of the 1% LSO sample. a) TEM image of 1% LSO. b) HRTEM image of 1% LSO, from the inset rectangle in (a). c,d) The SAED pattern and identities of the different diffraction rings, corresponding to the layered, spinel, and Li2SnO3 structures, respectively. e-i) STEM image and EDS maps of O, Mn, Ni, and Sn.
XAS Spectra of the four samples in TEY Mode. k) Ni L2,3 edge. l) Mn L2,3 edge. m) O K edge.

Tuning Anionic Redox Activity and Reversibility for a High-Capacity Li-Rich Mn-Based Oxide Cathode via an Integrated Strategy

Q. Li, D. Zhou, L. Zhang, D. Ning; Z. Chen, Z. Xu, R. Gao, X. Liu, D. Xie, G. Shcumacher, and X. Liu

When fabricating Li-rich layered oxide cathode materials, anionic redox chemistry plays a critical role in achieving a large specific capacity. Unfortunately, the release of lattice oxygen at the surface impedes the reversibility of the anionic redox reaction, which induces a large irreversible capacity loss, inferior thermal stability, and voltage decay. Therefore, methods for improving the anionic redox constitute a major challenge for the application of high-energy-density Li-rich Mn-based cathode materials. Herein, to enhance the oxygen redox activity and reversibility in Co-free Li-rich Mn-based Li1.2Mn0.6Ni0.2O2 cathode materials by using an integrated strategy of Li2SnO3 coating-induced Sn doping and spinel phase formation during synchronous lithiation is proposed. As an Li+ conductor, a Li2SnO3 nanocoating layer protects the lattice oxygen from exposure at the surface, thereby avoiding irreversible oxidation. The synergy of the formed spinel phase and Sn dopant not only improves the anionic redox activity, reversibility, and Li+ migration rate but also decreases Li/Ni mixing. The 1% Li2SnO3-coated Li1.2Mn0.6Ni0.2O2 delivers a capacity of more than 300 mAh g-1 with 92% Coulombic efficiency. Moreover, improved thermal stability and voltage retention are also observed. This synergic strategy may provide insights for understanding and designing new high-performance materials with enhanced reversible anionic redox and stabilized surface lattice oxygen.

Advanced Functional Materials (2019) 1806706.


enlarged view

hBN monolayer on curved Ni(1 1 1). Top, STM and, bottom, LEED for a monolayer of hBN homogeneously covering the curved Ni crystal sketched in the center. The STM images have been taken at the positions roughly indicated over the sample. LEED patterns correspond to the center ((1 1 1) plane), midway (vicinal angle α = ±7°), and densely stepped edges (α = ±15°) of the sample and have been acquired using 63 eV electron impinging parallel to the [1 1 1] direction in all cases. Insets in STM images belong to the indicated line profiles, which prove the presence of hBN-covered microfacets.

Boron nitride monolayer growth on vicinal Ni(111) surfaces systematically studied with a curved crystal

L. Fernandez, A. Makarova, C. Laubschat, D. V. Vyalikh, D. Yu. Usachov, J.E. Ortega, and F. Schiller

The structural and electronic properties of hexagonal boron nitride (hBN) grown on stepped Ni surfaces are systematically investigated using a cylindrical Ni crystal as a tunable substrate. Our experiments reveal homogeneous hBN monolayer coating of the entire Ni curved surface, which in turn undergoes an overall faceting. The faceted system is defined by step-free hBN/Ni(1 1 1) terraces alternating with strongly tilted hBN/Ni(1 1 5) or hBN/Ni(1 1 0) nanostripes, depending on whether we have A-type or B-type vicinal surfaces, respectively. Such deep substrate self-organization is explained by both the rigidity of the hBN lattice and the lack of registry with Ni crystal planes in the vicinity of the (1 1 1) surface. The analysis of the electronic properties by photoemission and absorption spectroscopies reveal a weaker hBN/Ni interaction in (1 1 0)- and (1 1 5)-oriented facets, as well as an upward shift of the valence band with respect to the band position at the hBN/Ni(1 1 1) terrace.

2D Matter (2019).


enlarged view

(a) Large scale STM topograph of MoS2 islands on fully covered Gr/Ir(1 1 1) grown in two successive growth cycles and subsequently imaged in situ. The inset shows a zoomed in part of the image marked with a blue box. The arrows in the inset mark different features of the topograph. Green: monolayer MoS2 island, black: bilayer MoS2 island, white: mirror twin boundary, blue: metallic edge state. Image information: tunneling voltage -1.5 V, tunneling current 20 pA, image size 250 × 250 nm2. (b) Corresponding LEED pattern at 86 eV. (c) XPS spectra of the Mo 3d peak. The doublet peak is split into Mo 3d5/2 and Mo 3d3/2 components. The red color traces show the peak for elemental Mo, green color for MoS2 which is shifted by 0.95 eV to a higher binding energy. The additional peak in the green spectrum around 226.5 eV binding energy is due to the S 2s core level. The spectra were recorded using an excitation energy of 370 eV. (d) S 2p doublet.

Narrow photoluminescence and Raman peaks of epitaxial MoS2 on graphene/Ir(1 1 1)

N. Ehlen, J. Hall, B.V. Senkovskiy, M. Hell, J. Li, A. Herman, D. Smirnov, A. Fedorov, V.Yu. Voroshnin, G. Di Santo, L. Petaccia, T. Michely, and A. Grüneis

We report on the observation of photoluminescence (PL) with a narrow 18 meV peak width from molecular beam epitaxy grown MoS2 on graphene/Ir(1 1 1). This observation is explained in terms of a weak graphene-MoS2 interaction that prevents PL quenching expected for a metallic substrate. The weak interaction of MoS2 with the graphene is highlighted by angle-resolved photoemission spectroscopy and temperature dependent Raman spectroscopy. These methods reveal that there is no hybridization between electronic states of graphene and MoS2 as well as a different thermal expansion of both materials. Molecular beam epitaxy grown MoS2 on graphene is therefore an important platform for optoelectronics which allows for large area growth with controlled properties.

2D Mater. 6 (2019) 011006.


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