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2021


Nitrogen-doped graphene on a curved nickel surface

O. Yu. Vilkov, A. V. Tarasov, K. A. Bokai, A. A. Makarova, M. Muntwiler, F. Schiller, J. E. Ortega, L. V. Yashina, D. V. Vyalikh, D. Yu. Usachov

Graphene growth and doping are well studied on flat surfaces of various materials. To further advance the technological implementation of graphene-based systems, fundamental studies need more appropriate model templates, whose surfaces would mimic substrates with non-trivial topography. Here, using electron and photoelectron diffraction and photoemission spectroscopy as well, we demonstrate how a curved tungsten crystal covered by a thin nickel film can properly be used as such platform, allowing the fabrication and comprehensive characterization of nitrogen-doped graphene. We show the way in which nitrogen impurities prefer to embed into the graphene matrix at different areas of the curved metallic surface with variable density of atomic steps. In particular, at atomically flat regions with a strong graphene-metal interaction, pyridinic configuration is the most abundant form of dopants, while graphitic nitrogen strongly dominates in places with a weak coupling of graphene to the substrate, i.e., in the vicinity of the surface irregularities. We recognize single crystals with curvilinear surfaces as versatile platforms for the studies of not only low-dimensional materials, but also molecular adsorption, chemical reactions and catalysis on surfaces with complex structure.

Carbon 183 (2021) 711-720


Mn-rich MnSb2Te4: A topological insulator with magnetic gap closing at high Curie temperatures of 45-50 K

S. Wimmer, J. Sánchez-Barriga, P. Küppers, A. Ney, E. Schierle, F. Freyse, O. Caha, J. Michalička, M. Liebmann, D. Primetzhofer, M. Hoffman, A. Ernst, M. M. Otrokov, G. Bihlmayer, E. Weschke, B. Lake, E. V. Chulkov, M. Morgenstern, G. Bauer, G. Springholz, O. Rader
Ferromagnetic topological insulators exhibit the quantum anomalous Hall effect, which is potentially useful for high-precision metrology, edge channel spintronics, and topological qubits. The stable 2+ state of Mn enables intrinsic magnetic topological insulators. MnBi2Te4 is, however, antiferromagnetic with 25 K Néel temperature and is strongly n-doped. In this work, p-type MnSb2Te4, previously considered topologically trivial, is shown to be a ferromagnetic topological insulator for a few percent Mn excess. i) Ferromagnetic hysteresis with record Curie temperature of 45–50 K, ii) out-of-plane magnetic anisotropy, iii) a 2D Dirac cone with the Dirac point close to the Fermi level, iv) out-of-plane spin polarization as revealed by photoelectron spectroscopy, and v) a magnetically induced bandgap closing at the Curie temperature, demonstrated by scanning tunneling spectroscopy (STS), are shown. Moreover, a critical exponent of the magnetization β ≈ 1 is found, indicating the vicinity of a quantum critical point. Ab initio calculations reveal that Mn–Sb site exchange provides the ferromagnetic interlayer coupling and the slight excess of Mn nearly doubles the Curie temperature. Remaining deviations from the ferromagnetic order open the inverted bulk bandgap and render MnSb2Te4 a robust topological insulator and new benchmark for magnetic topological insulators.

Advanced Materials (2021) 2102935


Origin of the band gap in Bi-intercalated graphene on Ir(111)

M. Krivenkov, D. Marchenko, J. Sánchez-Barriga, E. Golias, O. Rader and A. Varykhalov

Proximity to heavy sp-elements is considered promising for reaching a band gap in graphene that could host quantum spin Hall states. The recent report of an induced spin-orbit gap of 0.2 eV in Pb-intercalated graphene detectable by spin-resolved photoemission has spurred renewed interest in such systems (Klimovskikh et al 2017 ACS Nano 11, 368). In the case of Bi intercalation an even larger band gap of 0.4 eV has been observed but was assigned to the influence of a dislocation network (Warmuth et al 2016 Phys. Rev. B 93, 165 437). Here, we study Bi intercalation under graphene on Ir(111) and report a nearly ideal graphene dispersion without band replicas and no indication of hybridization with the substrate. The band gap is small (0.19 eV) and can be tuned by ±25 meV through the Bi coverage. The Bi atomic density is higher than in the recent report. By spin-resolved photoemission we exclude induced spin-orbit interaction as origin of the gap. Quantitative agreement of a photoemission intensity analysis with the measured band gap suggests sublattice symmetry breaking as one of the possible band gap opening mechanisms. We test several Bi structures by density functional theory. Our results indicate the possibility that Bi intercalates in the phase of bismuthene forming a graphene-bismuthene van der Waals heterostructure.

2D Materials 8 (2021) 035007/1-15


Engineering of Numerous Moiré Superlattices in Twisted Multilayer Graphene for Twistronics and Straintronics Applications

M. Brzhezinskaya, O. Kononenko, V. Matveev, A. Zotov, I. I. Khodos, V. Levashov, V. Volkov, S. I. Bozhko, S. V. Chekmazov, and D. Roshchupkin

Because of their unique atomic structure, 2D materials are able to create an up-to-date paradigm in fundamental science and technology on the way to engineering the band structure and electronic properties of materials on the nanoscale. One of the simplest methods along this path is the superposition of several 2D nanomaterials while simultaneously specifying the twist angle between adjacent layers (θ), which leads to the emergence of Moiré superlattices. The key challenge in 2D nanoelectronics is to obtain a nanomaterial with numerous Moiré superlattices in addition to a high carrier mobility in a stable and easy-to-fabricate material. Here, we demonstrate the possibility of synthesizing twisted multilayer graphene (tMLG) with a number of monolayers NL = 40–250 and predefined narrow ranges of θ = 3–8°, θ = 11–15°, and θ = 26–30°. A 2D nature of the electron transport is observed in the tMLG, and its carrier mobilities are close to those of twisted bilayer graphene (tBLG) (with θ = 30°) between h-BN layers. We demonstrate an undoubtful presence of numerous Moiré superlattices simultaneously throughout the entire tMLG thickness, while the periods of these superlattices are rather close to each other. This offers a challenge of producing a next generation of devices for nanoelectronics, twistronics, and neuromorphic computing for large data applications.

ACS Nano 15 (2021) 12358-12366


Modulating nitrogen species via N-doping and post annealing of graphene derivatives: XPS and XAS examination

M. K. Rabchinskii, S. D. Saveliev, D. Yu. Stolyarova, M. Brzhezinskaya, D. A. Kirilenko, M. V. Baidakova, S. A. Ryzhkov, V. V. Shnitov, V. V. Sysoev, P. N. Brunkov

Here, we have thoroughly studied the effect of chemistry of graphene derivatives on the composition of N-species after N-doping with the help of core-level spectroscopy techniques. The modulation of the N-species by tailoring the functionalization and atomic structure of graphene derivatives prior to chemical N-doping is experimentally demonstrated for the first time. The large extent of non-terminated or phenol-functionalized graphene edges is found to facilitate the formation of pyridinic nitrogen with its relative content exceeding 72%. In turn, the predominant decoration by the pyrazolic moieties is shown for the perforated and carboxyl-derived graphene layers. The thermal annealing at moderate temperatures of ca.345 °C is shown to equally readjust the composition of N-species in graphene derivatives regardless of their chemistry, nanostructure, and the initial distribution of the N-species. Further examination of N K-edge X-ray absorption spectra (XAS) pointed out that the oxidation of the graphene layer governs the manifestation of the π∗ resonances and configuration of the σ∗ resonance. As a result, a set of facile methods to synthesize graphene derivatives with the desired type of the embedded nitrogen species for the optoelectronic and catalytic applications are proposed and crucial features of their identification using core-level spectroscopy techniques are emphasized.

Carbon 182 (2021) 593-604


Silicon Suboxides as Driving Force for Efficient Light‐Enhanced Hydrogen Generation on Silicon Nanowires

Tingsen Ming, Sergey Turishchev, Alexander Schleusener, Elena Parinova, Dmitry Koyuda, Olga Chuvenkova, Martin Schulz, Benjamin Dietzek and Vladimir Sivakov

Efficient light‐stimulated hydrogen generation from top–down produced highly doped n‐type silicon nanowires (SiNWs) with silver nanoparticles (AgNPs) in water‐containing medium under white light irradiation is reported. It is observed that SiNWs with AgNPs generate at least 2.5 times more hydrogen than SiNWs without AgNPs. The authors’ results, based on vibrational, UV–vis, and X‐ray spectroscopy studies, strongly suggest that the sidewalls of the SiNWs are covered by silicon suboxides, by up to a thickness of 120 nm, with wide bandgap semiconductor properties that are similar to those of titanium dioxide and remain stable during hydrogen evolution in a water‐containing medium for at least 3 h of irradiation. Based on synchrotron studies, it is found that the increase in the silicon bandgap is related to the energetically beneficial position of the valence band in nanostructured silicon, which renders these promising structures for efficient hydrogen generation.

Small 17(8) (2021) 2007650-1-6


NiPS3

Correlations in the Electronic Structure of van der Waals NiPS3 Crystals: An X-ray Absorption and Resonant Photoelectron Spectroscopy Study

M. Yan, Y. Jin, Z. Wu, A. Tsaturyan, A. Makarova, D. Smirnov, E. Voloshina, and Yu. Dedkov

The electronic structure of high-quality van der Waals NiPS3 crystals was studied using near-edge X-ray absorption spectroscopy (NEXAFS) and resonant photoelectron spectroscopy (ResPES) in combination with density functional theory (DFT) approach. The experimental spectroscopic methods, being element specific, allow one to discriminate between atomic contributions in the valence and conduction band density of states and give direct comparison with the results of DFT calculations. Analysis of the NEXAFS and ResPES data allows one to identify the NiPS3 material as a charge-transfer insulator. Obtained spectroscopic and theoretical data are very important for the consideration of possible correlated-electron phenomena in such transition-metal layered materials, where the interplay between different degrees of freedom for electrons defines their electronic properties, allowing one to understand their optical and transport properties and to propose further possible applications in electronics, spintronics, and catalysis.

J. Phys. Chem. Lett. 2021, 12, 9, 2400–2405


Hole-matrixed carbonylated graphene: Synthesis, properties, and highly-selective ammonia gas sensing

M. K. Rabchinskii, A. S. Varezhnikov, V. V. Sysoev, M. A. Solomatin, S. A. Ryzhkov, M. V. Baidakova, D. Yu. Stolyarova, V. V. Shnitov, S. I. Pavlov, D. A. Kirilenko, A. V. Shvidchenko, E. Yu. Lobanova, M. V. Gudkov, D. A. Smirnov, V. A. Kislenko, S. V. Pavlov, S. A. Kislenko, N. S. Struchkov, I. I. Bobrinetskiy, A. V. Emelianov, P. Liang, Z. Liu, P. N. Brunkov

Here, the synthesis of holey carbonylated (C-ny) graphene derivative and its application for gas sensing is demonstrated. The carbonylation of graphene oxide leads to the 3-fold increase in the concentration of carbonyl groups’ up to 9 at.% with a substantial elimination of other oxygen functionalities. Such a chemical modification is accompanied by the perforation of the graphene layer with the appearance of matrices of nanoscale holes, leading to corrugation of the layer and its sectioning into localized domains of the π-conjugated network. Combined with the predominant presence of carbonyls, granting the specificity in gas molecules adsorption, these features result in the enhanced gas sensing properties of C-ny graphene at room temperature with a selective response to NH3. Opposite chemiresistive response towards ammonia when compared to other analytes, such as ethanol, acetone, CO2, is demonstrated for the C-ny graphene layer both in humid and dry air background. Moreover, a selective discrimination of all of the studied analytes is further approached by employing a vector signal generated by C-ny multielectrode chip. Comparing the experimental results with the calculations performed in framework of density functional theory, we clarify the effect of partial charge transfer caused by water and ammonia adsorption on the chemiresistive response.

Carbon 172 (2021) 236-247

2020


frolov

Atomic and Electronic Structure of a Multidomain GeTe Crystal

A. S. Frolov, J. Sánchez-Barriga, C. Callaert, J. Hadermann, A. V. Fedorov, D. Yu. Usachov, A. N. Chaika, B. C. Walls, K. Zhussupbekov, I. V. Shvets, M. Muntwiler, M. Amati, L. Gregoratti, A. Yu. Varykhalov, O. Rader, and L. V. Yashina

Renewed interest in the ferroelectric semiconductor germanium telluride was recently triggered by the direct observation of a giant Rashba effect and a 30-year-old dream about a functional spin field-effect transistor. In this respect, all-electrical control of the spin texture in this material in combination with ferroelectric properties at the nanoscale would create advanced functionalities in spintronics and data information processing. Here, we investigate the atomic and electronic properties of GeTe bulk single crystals and their (111) surfaces. We succeeded in growing crystals possessing solely inversion domains of ∼10 nm thickness parallel to each other. Using HAADF-TEM we observe two types of domain boundaries, one of them being similar in structure to the van der Waals gap in layered materials. This structure is responsible for the formation of surface domains with preferential Te-termination (∼68%) as we determined using photoelectron diffraction and XPS. The lateral dimensions of the surface domains are in the range of ∼10–100 nm, and both Ge- and Te-terminations reveal no reconstruction. Using spin-ARPES we establish an intrinsic quantitative relationship between the spin polarization of pure bulk states and the relative contribution of different terminations, a result that is consistent with a reversal of the spin texture of the bulk Rashba bands for opposite configurations of the ferroelectric polarization within individual nanodomains. Our findings are important for potential applications of ferroelectric Rashba semiconductors in nonvolatile spintronic devices with advanced memory and computing capabilities at the nanoscale.

ACS Nano 14, 12 (2020) 16576–16589


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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