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1.
Nanoscale ; 14(35): 12918-12927, 2022 Sep 15.
Artigo em Inglês | MEDLINE | ID: mdl-36043425

RESUMO

As a semiconductor ferroelectric, GeTe has become a focus of renewed attention due to the recent discovery of giant Rashba splitting. It already has a wide range of applications, from thermoelectricity to data storage. Its stability in ambient air, as well as the structure and properties of an oxide layer, define the processing media for device production and operation. Here, we studied a reaction between the GeTe (111) surface and molecular oxygen for crystals having solely inversion domains. We evaluated the reaction kinetics both ex situ and in situ using NAP XPS. The structure of the oxide layer is extensively discussed, where, according to HAADF-STEM and STEM-EDX, nanoscale phase separation of GeO2 and Te is observed, which is unusual for semiconductors. We believe that such behaviour is closely related to the ferroelectric properties and the domain structure of GeTe.

2.
Phys Chem Chem Phys ; 23(30): 16077-16088, 2021 Aug 04.
Artigo em Inglês | MEDLINE | ID: mdl-34291782

RESUMO

Gibbs free energies for Li+ solvation in water, methanol, acetonitrile, DMSO, dimethylacetamide, dimethoxyethane, dimethylformamide, gamma-butyrolactone, pyridine, and sulfolane have been calculated using the cluster-continuum quasichemical theory. With n independent solvent molecules S initial state forming the "monomer" thermodynamic cycle, Li+ solvation free energies are found to be on average 14 kcal mol-1 more positive compared to those from the "cluster" thermodynamic cycle where the initial state is the cluster Sn. We ascribe the inconsistency between the "monomer" and "cluster" cycles mainly to the incorrectly predicted solvation free energies of solvent clusters Sn from the SMD and CPCM continuum solvation models, which is in line with the earlier study of Bryantsev et al., J. Phys. Chem. B, 2008, 112, 9709-9719. When experimental-based solvation free energies of individual solvent molecules and solvent clusters are employed, the "monomer" and "cluster" cycles result in identical numbers. The best overall agreement with experimental-based "bulk" scale lithium cation solvation free energies was obtained for the "monomer" scale, and we recommend this set of values. We expect that further progress in the field is possible if (i) consensus on the accuracy of experimental reference values is achieved; (ii) the most recent continuum solvation models are properly parameterized for all solute-solvent combinations and become widely accessible for testing.

3.
J Phys Chem Lett ; 11(24): 10511-10518, 2020 Dec 17.
Artigo em Inglês | MEDLINE | ID: mdl-33285062

RESUMO

Lithium plating-one of the critical processes in the desired high-energy lithium metal batteries-is accompanied by lithium whisker growth, which causes several problems that prevent the employment of metallic lithium anodes in rechargeable systems. They include low coulombic efficiency, electrolyte consumption, and the risk of short circuits, which can lead to thermal runaway of the battery. In recent years several strategies were suggested to mitigate whisker growth. The mechanism of this process, however, still lacks understanding. Here, we reveal the importance of surface diffusion along grain boundaries in solid lithium. We show that, at first, the plating of lithium onto a lithium substrate is possible as bulk crystal growth with a planar crystallization front for the Li grains with oblique (nonperpendicular to the surface) grain boundaries. Further, the developed compressive stress makes lithium diffusion to the grain base unfavorable, and new grains nucleate at the surface. The latter are the cause of the whisker growth from their roots. These findings indicate that the control of grain-boundary diffusion and grain size and structure paves the way to overcome the nonuniform morphology of plated lithium.

4.
ACS Nano ; 2020 Nov 02.
Artigo em Inglês | MEDLINE | ID: mdl-33136362

RESUMO

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.

5.
Phys Chem Chem Phys ; 22(28): 16184-16192, 2020 Jul 22.
Artigo em Inglês | MEDLINE | ID: mdl-32643737

RESUMO

The metallic lithium electrode has major concerns such as extremely high reactivity and nonuniform needle-like electrodeposition, limiting its wide application as a negative electrode in secondary batteries. Its reactions with the electrolyte leading to solid electrolyte interphase (SEI) formation play an important role, and controlling its composition and properties can help to overcome both difficulties. Even though solid electrolyte interphase chemistry and properties seem to be well known, many surface chemistry experiments reported are not perfect with respect to the purity needed for Li studies and can be interpreted alternatively. Here, we studied reactions between lithium and propylene carbonate and ethylene carbonate in model reactions realized in an ultra-high vacuum. In addition to the already reported reaction pathway yielding lithium carbonate and semicarbonate, our theoretical (DFT) modeling confirms the preference of alternative routes. Along with the most beneficial final lithium carbonates, dilithium 1,2-dialkoxide (DD) can form barrierlessly as a final product by two-electron transfer. Experimental XPS/NEXAFS studies of gas phase and solid-gas model reactions revealed that in both cases DD is the main reaction product. Understanding of the discovered reaction pathway can also be essential for reactions in liquid electrolytes, although the low electric conductivity of the SEI makes it less probable.

6.
Langmuir ; 36(30): 8716-8722, 2020 Aug 04.
Artigo em Inglês | MEDLINE | ID: mdl-32640797

RESUMO

The development of high specific energy Li-O2 batteries faces a problem of poor cycling as a result of passivation of the positive electrode by both the discharge product (Li2O2) and side products (Li2CO3, etc.). The latter are the result of oxidation of the electrode materials or electrolyte components primarily by discharge intermediate superoxide anions (O2-) and, in less degree, by Li2O2. We report cyclic voltammetry studies of the electrode passivation in different relatively stable solvents. We found that slower passivation is observed for the electrolytes based on high donor number solvents or solvents with high viscosity. Moreover, such behavior is reproduced for three different electrode materials [glassy carbon (GC), TiC, and TiN] that pinpoints the primary role of different oxygen reduction reaction mechanisms (Li2O2 surface deposition or solution growth) influenced by Li+ solvation energy and solvent viscosity. The chemistry of interaction between LiO2/Li2O2 and the electrode/solvent turns out to be less important. Additionally, we found that, for the electrode made of GC and TiN in all electrolyte solutions, the passivation by side products suppresses oxygen reduction after a certain number of cycles. In contrast, for TiC after several cycles, further passivation does not happen as a result of the formation of a thin and stable TiO2 layer in high donor number solvents.

7.
Nanoscale ; 12(7): 4591-4601, 2020 Feb 21.
Artigo em Inglês | MEDLINE | ID: mdl-32043506

RESUMO

The development of high-energy lithium-oxygen batteries has significantly slowed by numerous challenges including capacity limitations due to electrode surface passivation by the discharge product Li2O2. Since the passivation rate and intensity are dependent on the deposit morphology, herein, we focus on the mechanisms governing Li2O2 formation within the porous cathode. We report evidence of homogeneous nucleation of Li2O2 crystallites and their further assembly in bulk of the electrolyte solution in DMSO, which possesses a high donor number. After careful estimation of the superoxide ion concentration distribution within a phenomenological model, it was found that the high stability of superoxide ions formed during the ORR towards disproportionation and sufficient diffusivity of (0.5-1.2) × 10-6 cm2 s-1 enabled Li2O2 nucleation and crystallization not only at the surface but also in the electrolyte, and the reaction zone spread throughout the internal space of the porous electrode. High initial supersaturation promoted the homogeneous nucleation of Li2O2 nanoplates, which instantly assembled into mesocrystals also in the solution bulk. These results were supported by operando SAXS/WAXS and morphology observations. Thus, although homogeneous nucleation is not dominant, it is important for achieving a high capacity in Li-O2 batteries.

8.
RSC Adv ; 10(27): 16118-16124, 2020 Apr 21.
Artigo em Inglês | MEDLINE | ID: mdl-35493665

RESUMO

Solid electrolytes are of high interest for the development of advanced electrochemical energy storage devices with all-solid-state architectures. Here, we report the fabrication of the electrolyte membranes based on LiTFSI (LiN(CF3SO2)2) and PEO-PVDF blends with improved properties. We show that addition of PVDF enables preparation of free-standing films of the compositions within the so called "crystallinity gap" of the LiTFSI-PEO system known to provide high ion conductivity. We show that optimal PVDF content enables preparation of the films with reasonable elastic modulus and high ionic conductivity of about 0.3 mS cm-1 at 60 °C and about 0.1 mS cm-1 at room-temperature. Combining FTIR spectroscopy, XRD and DSC measurements we show that a noticeable fraction of PVDF remains crystalline and enhances the mechanical properties of the material, and at the same time it additionally promotes LiTFSI dissociation and disordering. Density functional theory calculations showed that the Li+-PEO-PVDF complexation energy magnitude is almost as high as that of Li-PEO complexes, thus the salt dissociation ability can be retained in spite of the introduction of the substantial amounts of PVDF required for mechanical stability.

9.
Nanoscale ; 11(14): 6838-6845, 2019 Apr 04.
Artigo em Inglês | MEDLINE | ID: mdl-30912561

RESUMO

Many obstacles impede the development of Li-air batteries for practical applications. In particular, there is lack of understanding of the dynamics of processes occurring in porous air electrodes during discharge, including oxygen transport limitations, pore clogging and electrode passivation by both insulating discharge and parasitic reaction products. Here, using small-angle neutron scattering, which provides information on the whole electrode adequate to electrochemical data, we uncover the mechanisms limiting the Li-O2 porous carbon electrode capacity by analysis of the cathode pore filling in highly and poorly solvating media - dimethyl sulfoxide and acetonitrile. The results obtained allowed us suppose that in both cases the cell death is mainly triggered by blocking of oxygen transport pathways inside carbon black particle agglomerates. Total discharge capacities are, indeed, higher in highly solvating solutions due to a higher discharge intermediate lifetime and longer diffusion distance, which enable Li2O2 formation outside the carbon black agglomerates, which are, as we demonstrated, in fact mesocrystals that are confirmed by the appearance of a diffraction peak in scattering curves.

10.
Nanoscale ; 10(48): 22810-22817, 2018 Dec 28.
Artigo em Inglês | MEDLINE | ID: mdl-30488051

RESUMO

Embedding foreign atoms in graphene and interchanging the underlying substrate are proved to be efficient methods for manipulating the properties of graphene. Combining ARPES experiments with DFT calculations we show that boron-doped graphene (B-graphene) grown on a Co(0001) substrate by chemical vapor deposition (CVD) becomes hole doped and its Fermi surface near the K-point reveals strongly spin-polarized states. The latter stems from the spin-polarized mini Dirac cone that is an intrinsic two-dimensional feature of the graphene/Co(0001) interface and is formed by a mixture of C 2pz and Co 3d states. Since the CVD method allows the achievement of up to 20 at% of incorporated B atoms, this provides a certain flexibility for handling the spin-polarized properties of the system. We also show that the bonding of the B-graphene layer to the Co(0001) substrate can be released by intercalation of Li into the interface. This allows the exploration of the doping effect in detail. Finally, our ARPES data indicate a gap opening in the Dirac cone as a result of the highly unbalanced boron concentrations in the two graphene sublattices.

11.
Chemistry ; 24(71): 18952-18962, 2018 Dec 17.
Artigo em Inglês | MEDLINE | ID: mdl-30238511

RESUMO

Composites of WS2 nanotubes (NT-WS2 ) and gold nanoparticles (AuNPs) were prepared using aqueous HAuCl4 solutions and subjected to surface analysis. The obtained materials were jointly characterized by X-ray photoelectron (XPS), Raman scattering (RSS), and ultraviolet photoelectron (UPS) spectroscopies. Optical extinction spectroscopy and electron energy loss spectroscopy in the scanning transmission electron microscopy regime (STEM-EELS) were also employed to study plasmon features of the nanocomposite. It was found that AuNPs deposition is accompanied by a partial oxidative dissolution of WS2 , whereas Au-S interfacial species could be responsible for the tight contact of metal nanoparticles and the disulfide. A remarkable sensitivity of n-type resistance of NT-WS2 and Au-NT-WS2 to the adsorption of NO2 gas was also demonstrated at room temperature using periodical illumination by a 530 nm light-emitting diode. Au-NT-WS2 nanocomposites are found to possess a higher photoresponse and enhanced sensitivity in the 0.25-2.0 ppm range of NO2 concentration, as compared to the pristine NT-WS2 . This behaviour is discussed within the physisorption-charge transfer model to explore sensing properties of the nanocomposites.

12.
Nanoscale ; 10(25): 12123-12132, 2018 Jul 05.
Artigo em Inglês | MEDLINE | ID: mdl-29915820

RESUMO

Recrystallization of bulk materials is a well-known phenomenon, which is widely used in commercial manufacturing. However, for low-dimensional materials like graphene, this process still remains an unresolved puzzle. Thus, the understanding of the underlying mechanisms and the required conditions for recrystallization in low dimensions is essential for the elaboration of routes towards the inexpensive and reliable production of high-quality nanomaterials. Here, we unveil the details of the efficient recrystallization of one-atom-thick pure and boron-doped polycrystalline graphene layers on a Co(0001) surface. By applying photoemission and electron diffraction, we show how more than 90% of the initially misoriented graphene grains can be reconstructed into a well-oriented and single-crystalline layer. The obtained recrystallized graphene/Co interface exhibits high structural quality with a pronounced sublattice asymmetry, which is important for achieving an unbalanced sublattice doping of graphene. By exploring the kinetics of recrystallization for native and B-doped graphene on Co, we were able to estimate the activation energy and propose a mechanism of this process.

13.
Sci Rep ; 8(1): 485, 2018 01 11.
Artigo em Inglês | MEDLINE | ID: mdl-29323213

RESUMO

Helically spin-polarized Dirac fermions (HSDF) in protected topological surface states (TSS) are of high interest as a new state of quantum matter. In three-dimensional (3D) materials with TSS, electronic bulk states often mask the transport properties of HSDF. Recently, the high-field Hall resistance and low-field magnetoresistance indicate that the TSS may coexist with a layered two-dimensional electronic system (2DES). Here, we demonstrate quantum oscillations of the Hall resistance at temperatures up to 50 K in nominally undoped bulk Bi2Se3 with a high electron density n of about 2·1019 cm-3. From the angular and temperature dependence of the Hall resistance and the Shubnikov-de Haas oscillations we identify 3D and 2D contributions to transport. Angular resolved photoemission spectroscopy proves the existence of TSS. We present a model for Bi2Se3 and suggest that the coexistence of TSS and 2D layered transport stabilizes the quantum oscillations of the Hall resistance.

14.
Phys Chem Chem Phys ; 19(45): 30520-30532, 2017 Nov 22.
Artigo em Inglês | MEDLINE | ID: mdl-29115310

RESUMO

To realize spintronic devices based on topological insulators (TIs), well-defined interfaces between magnetic metals and TIs are required. Here, we characterize atomically precisely the interface between the 3d transition metal Fe and the TI Bi2Te3 at different stages of its formation. Using photoelectron diffraction and holography, we show that after deposition of up to 3 monolayers Fe on Bi2Te3 at room temperature, the Fe atoms are ordered at the interface despite the surface disorder revealed by our scanning-tunneling microscopy images. We find that Fe occupies two different sites: a hollow adatom deeply relaxed into the Bi2Te3 quintuple layers and an interstitial atom between the third (Te) and fourth (Bi) atomic layers. For both sites, our core-level photoemission spectra and density-functional theory calculations demonstrate simultaneous chemical bonding of Fe to both Te and Bi atoms. We further show that upon deposition of Fe up to a thickness of 20 nm, the Fe atoms penetrate deeper into the bulk forming a 2-5 nm interface layer containing FeTe. In addition, excessive Bi is pushed down into the bulk of Bi2Te3 leading to the formation of septuple layers of Bi3Te4 within a distance of ∼25 nm from the interface. Controlling the magnetic properties of the complex interface structures revealed by our work will be of critical importance when optimizing the efficiency of spin injection in TI-based devices.

15.
Langmuir ; 33(37): 9378-9389, 2017 09 19.
Artigo em Inglês | MEDLINE | ID: mdl-28636393

RESUMO

The charge-transfer kinetics of lithium ion intercalation into LixMn2O4 cathode materials was examined in dilute and concentrated aqueous and carbonate LiTFSI solutions using electrochemical methods. Distinctive trends in ion intercalation rates were observed between water-based and ethylene carbonate/diethyl carbonate solutions. The influence of the solution concentration on the rate of lithium ion transfer in aqueous media can be tentatively attributed to the process associated with Mn dissolution, whereas in carbonate solutions the rate is influenced by the formation of a concentration-dependent solid electrolyte interface (SEI). Some indications of SEI layer formation at electrode surfaces in carbonate solutions after cycling are detected by X-ray photoelectron spectroscopy. The general consequences related to the application of superconcentrated electrolytes for use in advanced energy storage cathodes are outlined and discussed.

16.
ACS Nano ; 11(6): 6336-6345, 2017 06 27.
Artigo em Inglês | MEDLINE | ID: mdl-28494148

RESUMO

Regardless of the widely accepted opinion that there is no Raman signal from single-layer graphene when it is strongly bonded to a metal surface, we present Raman spectra of a graphene monolayer on Ni(111) and Co(0001) substrates. The high binding energy of carbon to these surfaces allows formation of lattice-matched (1 × 1) structures where graphene is significantly stretched. This is reflected in a record-breaking shift of the Raman G band by more than 100 cm-1 relative to the case of freestanding graphene. Using electron diffraction and photoemission spectroscopy, we explore the aforementioned systems together with polycrystalline graphene on Co and analyze possible intercalation of oxygen at ambient conditions. The results obtained are fully supported by Raman spectroscopy. Performing a theoretical investigation of the phonon dispersions of freestanding graphene and stretched graphene on the strongly interacting Co surface, we explain the main features of the Raman spectra. Our results create a reliable platform for application of Raman spectroscopy in diagnostics of chemisorbed graphene and related materials.

17.
Nano Lett ; 17(2): 805-810, 2017 02 08.
Artigo em Inglês | MEDLINE | ID: mdl-28005367

RESUMO

The structural organization of compounds in a confined space of nanometer-scale cavities is of fundamental importance for understanding the basic principles for atomic structure design at the nanolevel. Here, we explore size-dependent structure relations between one-dimensional PbTe nanocrystals and carbon nanotube containers in the diameter range of 2.0-1.25 nm using high-resolution transmission electron microscopy and ab initio calculations. Upon decrease of the confining volume, one-dimensional crystals reveal gradual thinning, with the structure being cut from the bulk in either a <110> or a <100> growth direction until a certain limit of ∼1.3 nm. This corresponds to the situation when a stoichiometric (uncharged) crystal does not fit into the cavity dimensions. As a result of the in-tube charge compensation, one-dimensional superstructures with nanometer-scale atomic density modulations are formed by a periodic addition of peripheral extra atoms to the main motif. Structural changes in the crystallographic configuration of the composites entail the redistribution of charge density on single-walled carbon nanotube walls and the possible appearance of the electron density wave. The variation of the potential attains 0.4 eV, corresponding to charge density fluctuations of 0.14 e/atom.

18.
Sci Rep ; 6: 27483, 2016 06 07.
Artigo em Inglês | MEDLINE | ID: mdl-27270569

RESUMO

Low-field magnetotransport measurements of topological insulators such as Bi2Se3 are important for revealing the nature of topological surface states by quantum corrections to the conductivity, such as weak-antilocalization. Recently, a rich variety of high-field magnetotransport properties in the regime of high electron densities (∼10(19) cm(-3)) were reported, which can be related to additional two-dimensional layered conductivity, hampering the identification of the topological surface states. Here, we report that quantum corrections to the electronic conduction are dominated by the surface states for a semiconducting case, which can be analyzed by the Hikami-Larkin-Nagaoka model for two coupled surfaces in the case of strong spin-orbit interaction. However, in the metallic-like case this analysis fails and additional two-dimensional contributions need to be accounted for. Shubnikov-de Haas oscillations and quantized Hall resistance prove as strong indications for the two-dimensional layered metallic behavior. Temperature-dependent magnetotransport properties of high-quality Bi2Se3 single crystalline exfoliated macro and micro flakes are combined with high resolution transmission electron microscopy and energy-dispersive x-ray spectroscopy, confirming the structure and stoichiometry. Angle-resolved photoemission spectroscopy proves a single-Dirac-cone surface state and a well-defined bulk band gap in topological insulating state. Spatially resolved core-level photoelectron microscopy demonstrates the surface stability.

19.
Nano Lett ; 16(7): 4535-43, 2016 07 13.
Artigo em Inglês | MEDLINE | ID: mdl-27248659

RESUMO

The implementation of future graphene-based electronics is essentially restricted by the absence of a band gap in the electronic structure of graphene. Options of how to create a band gap in a reproducible and processing compatible manner are very limited at the moment. A promising approach for the graphene band gap engineering is to introduce a large-scale sublattice asymmetry. Using photoelectron diffraction and spectroscopy we have demonstrated a selective incorporation of boron impurities into only one of the two graphene sublattices. We have shown that in the well-oriented graphene on the Co(0001) surface the carbon atoms occupy two nonequivalent positions with respect to the Co lattice, namely top and hollow sites. Boron impurities embedded into the graphene lattice preferably occupy the hollow sites due to a site-specific interaction with the Co pattern. Our theoretical calculations predict that such boron-doped graphene possesses a band gap that can be precisely controlled by the dopant concentration. B-graphene with doping asymmetry is, thus, a novel material, which is worth considering as a good candidate for electronic applications.

20.
Inorg Chem ; 54(23): 11542-9, 2015 Dec 07.
Artigo em Inglês | MEDLINE | ID: mdl-26574775

RESUMO

Inorganic clathrate materials are of great fundamental interest and potential practical use for application as thermoelectric materials in freon-free refrigerators, waste-heat converters, direct solar thermal energy converters, and many others. Experimental studies of their electronic structure and bonding have been, however, strongly restricted by (i) the crystal size and (ii) essential difficulties linked with the clean surface preparation. Overcoming these handicaps, we present for the first time a comprehensive picture of the electronic band structure and the chemical bonding for the Sn(24-x-δ)InxAs(22-y)I8 clathrates obtained by means of photoelectron spectroscopy and complementary quantum modeling.

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