Van Der Waals Planes Research Articles

Decoding the nanoscale porosity in serpentinites from multidimensional electron microscopy and discrete element modelling

Serpentinites, widespread in Earth's lithosphere, exhibit inherent nanoporosity that may significantly impact their geochemical behaviour. This study provides a comprehensive investigation into the characteristics, scale dependence, and potential implications of nanoporosity in lizardite-dominated serpentinites. Through a combination of multidimensional imaging techniques and molecular-dynamics-based discrete element modelling, we reveal that serpentinites function as nanoporous media with pore sizes predominantly less than 100 nm. Crystallographic relationships between olivine, serpentine, and nanoporosity are explored, indicating a lack of significant correlations. Instead, stochastic growth and random packing of serpentine grains within mesh cores may result in interconnected porosity. The analysis of pore morphology suggests that the irregular pore shapes align with the crystal form of serpentine minerals. Furthermore, the nanoporosity within brucite-rich layers at the serpentine-olivine interface is attributed to delamination along weak van der Waals planes, while pore formation within larger brucite domains likely results from low-temperature alteration processes. The fractal nature of the pore size distribution and the potential interconnectivity of porosity across different scales further support the presence of a pervasive nanoporous network within serpentinites. Confinement within these nanopores may introduce unique emergent properties, potentially influencing fluid transport, mineral solubility, and chemical reactions. As such, these processes may have profound implications for the geochemical evolution of serpentinites.

Atomic scale chemical ordering in franckeite—a natural van der Waals superlattice

The mineral franckeite is a naturally occurring van der Waals superlattice which has recently attracted attention for future applications in optoelectronics, biosensors and beyond. Furthermore, its stacking of incommensurately modulated 2D layers, the pseudo tetragonal Q-layer and the pseudo hexagonal H-layer, is an experimentally accessible prototype for the development of synthetic van der Waals materials and of advanced characterization methods to reveal new insights in their structure and chemistry at the atomic scale that is crucial for deep understanding of its properties. While some experimental studies have been undertaken in the past, much is still unknown on the correlation between local atomic structure and chemical composition within the layers. Here we present an investigation of the atomic structure of franckeite using state-of-the-art high-angle annular dark-field (HAADF) scanning transmission electron microscopy (STEM) and atom probe tomography (APT). With atomic-number image contrast in HAADF STEM direct information about both the geometric structure and its chemistry is provided. By imaging samples under different zone axes within the van der Waals plane, we propose refinements to the structure of the Q-layer and H-layer, including several chemical ordering effects that are expected to impact electronic structure calculations. Additionally, we observe and characterize stacking faults which are possible sources of differences between experimentally determined properties and calculations. Furthermore, we demonstrate advantages and discuss current limitations and perspectives of combining TEM and APT for the atomic scale characterization of incommensurately modulated von der Waals materials.

Progressive NO2 Sensors with Rapid Alarm and Persistent Memory‐Type Responses for Wide‐Range Sensing Using Antimony Triselenide Nanoflakes

AbstractAntimony triselenide (Sb2Se3) nanoflake‐based nitrogen dioxide (NO2) sensors exhibit a progressive bifunctional gas‐sensing performance, with a rapid alarm for hazardous highly concentrated gases, and an advanced memory‐type function for low‐concentration (<1 ppm) monitoring repeated under potentially fatal exposure. Rectangular and cuboid shaped Sb2Se3 nanoflakes, comprising van der Waals planes with large surface areas and covalent bond planes with small areas, can rapidly detect a wide range of NO2 gas concentrations from 0.1 to 100 ppm. These Sb2Se3 nanoflakes are found to be suitable for physisorption‐based gas sensing owing to their anisotropic quasi‐2D crystal structure with extremely enlarged van der Waals planes, where they are humidity‐insensitive and consequently exhibit an extremely stable baseline current. The Sb2Se3 nanoflake sensor exhibits a room‐temperature/low‐voltage operation, which is noticeable owing to its low energy consumption and rapid response even under a NO2 gas flow of only 1 ppm. As a result, the Sb2Se3 nanoflake sensor is suitable for the development of a rapid alarm system. Furthermore, the persistent gas‐sensing conductivity of the sensor with a slow decaying current can enable the development of a progressive memory‐type sensor that retains the previous signal under irregular gas injection at low concentrations.

Evaluation of Pt, Rh, SnO2, (NH4)2Mo3S13, BaSO4 protection coatings on WSe2 photocathodes for solar hydrogen evolution

The stability of photoelectrodes is of prime importance, which determines the long-term efficiency of solar to hydrogen conversion. To this respect, the performances of Pt, Rh, SnO2, (NH4)2Mo3S13 (ATM), BaSO4 are compared as protection coatings for WSe2 photocathodes. It is found that Pt and Rh catalysts, selectively photodeposited at the active edges of WSe2 crystallites, are not able to sustain the maximum photocurrent densities for hydrogen evolution reaction due to surface oxidation. By comparison, the ATM layer effectively passivates the van der Waals planes of WSe2, however, it is oxidized with further amorphization, thus decreasing the photocurrent density by 82% after 3 h 10 min of PEC testing. The SnO2 layer on top of ATM/WSe2 stabilizes the photocurrent to some extent, but limits the maximum photocurrent density. Solution deposition of BaSO4 onto ATM/WSe2 photocathodes did not compromise the photocurrent density and resulted in significant improvement of the PEC stability. Herein, the photocurrent density is decreased by 44% after 3 h 10 min of testing, which is rather less than that of bare ATM/WSe2. The chemical states of all photocathodes are assessed by XPS analysis, before and after PEC testing, which identified that the BaSO4 suppresses photo-corrosion of the ATM.

Smart Bifunctional Sb2 Se3 Nanorods for Integrated Water Purification: Insoluble Liquid Separation and Photoelectrochemical Degradation.

Antimony selenide (Sb2 Se3 ) nanostructures enable bifunctional water purification by a single membrane through i) physical separation of water-insoluble oil and ii) photoelectrocatalytic degradation of water-soluble organic compounds. Sb2 Se3 nanorods with exposed surfaces of {h 0 0} and {h 0 l} planes exhibit superhydrophobicity (water contact angle of ≈159°) owing to extremely low surface energy of those dangling-bond-free van der Waals planes. Based on crystallographic understanding, superhydrophobic Sb2 Se3 nanorods were produced on a mesh-type substrate for utilization as a membrane for physical water/oil separation. Sb2 Se3 exhibited an optimal photocathodic response with p-type electrical conductivity under visible light along the longitudinal crystal direction. This indicated that the nanorods could be used as photoelectrocatalytic material for chemical water purification. A smart membrane with Sb2 Se3 nanostructures was proposed as a candidate for integrated water purification that can simultaneously accomplish water/oil separation and photoelectrocatalytic degradation of organic compounds in wastewater. Linear sweep voltammetry measurements of the Sb2 Se3 -membrane showed cathodic photocurrent generation (up to approximately 10 mA cm-2 at 0 V vs. reversible hydrogen electrode), which was enough to reduce O2 to an oxygen radical (O2 .- ) for degradation of methyl orange. Consequently, solar-driven integrated water purification was demonstrated for the first time by using a single material with a dual function of superhydrophobicity and photoactivity.

Топологические поверхностные состояния фермионов Дирака в термоэлектриках n-Bi-=SUB=-2-=/SUB=-Te-=SUB=-3-y-=/SUB=-Se-=SUB=-y-=/SUB=-

Topological surface states of Dirac fermions in n-Bi2Te3−ySey thermoelectrics L.N. Lukyanova, I.V. Makarenko, O.A. Usov, P.A. Dementev Ioffe Institute, 194021 St. Petersburg, Russia Abstract In n-Bi2Te3 and n-Bi2Te3−ySey thermoelectrics, the surface states of Dirac fermions of the interlayer van der Waals plane (0001) were studied by scanning tunneling microscopy (STM) and spectroscopy. The surface morphology and modulated line profiles of the images in tunneling microscopy is determined by local distortions of the surface electron states density and depend on the composition. The Dirac point ED of the studied compositions is localized in the energy gap, and it shifts to the top of the valence band with increase of Se content in the solid solutions. The dependence between Dirac surface state parameters (the Dirac point position, the Fermi velocity, the fermion surface concentration) and thermoelectric properties (Seebeck coefficient and the power factor) is established for the thermoelectrics studied.

Highly (001)-textured p-type WSe2 Thin Films as Efficient Large-Area Photocathodes for Solar Hydrogen Evolution

Highly (001)-textured, photoactive WSe2 thin films have been prepared by an amorphous solid-liquid-crystalline solid process promoted by palladium. By increasing the thickness of the Pd promoter film (≥10 nm) the structure and texture of the WSe2 films can be improved significantly. However, these as-crystallized WSe2 films are only weakly photoactive in a 0.5 М H2SO4 electrolyte under AM 1.5 solar irradiation which we attribute to an inefficient photogenerated charge transfer across the WSe2/electrolyte interface via the prevailing van der Waals planes of the WSe2 crystallites. In this work photochemically deposited platinum on the p-type WSe2 photocathode is used for an efficient electron transfer thus inducing the hydrogen evolution reaction. Upon illuminating the WSe2 photocathodes in a Pt-ion containing electrolyte, the photogenerated electrons reduce Pt+ to Pt leading to the precipitation of Pt islands, preferentially at edge steps of the WSe2, i.e. at the grain boundaries of the WSe2 crystallites. The increasing amount of Pt islands at the grain boundaries linearly enhances the photocurrent density up to 2.5 mA cm−2 at 0 VRHE in sulfuric acid, the highest reported value up to now for WSe2 thin films.

Structure and magnetic properties of cobalt-intercalated layered InSe crystals

We study the structure and magnetic properties of CoxInSe layered crystals electrochemically intercalated by cobalt in a constant magnetic field. It is found that impurity clusters consisting of cobalt nano-particles with the fcc structure are formed in the intercalates under investigation on the Van der Waals planes in the space between the layers. Intercalates Co0.1InSe obtained by implantation in a magnetic field exhibit a change in their magnetic properties (dependence of the magnetic moment in the magnetic field strength has the form of a hysteresis loop, which is typical of ferromagnetic materials).

Reactive magnetron sputtering of tungsten disulfide (WS2−x) films: Influence of deposition parameters on texture, microstructure, and stoichiometry

Tungsten disulfide (WS2−x) films (0.07⩽x⩽0.7) were prepared by reactive magnetron sputtering from a tungsten target in rare gas/H2S atmospheres and at substrate temperatures up to 620°C. The nucleation and growth of the films were investigated by in situ energy dispersive x-ray diffraction (EDXRD) and by ex situ techniques such as electron microscopy, elastic recoil detection analysis, and x-ray reflectivity. From the EDXRD analysis it was found that the films always nucleate with the (001) planes, i.e., the van der Waals planes, parallel to the substrate surface. For high deposition rates and/or low substrate temperatures a texture crossover from the (001) to the (100) crystallite orientation occurs during the growth. High deposition rates, low substrate temperatures, or low sputtering pressures lead to a significant lattice expansion of the crystallites in the c direction (up to 3%). This is most probably caused by a disturbed or turbostratic film growth induced by the energetic bombardment during film deposition. Reflected and neutralized energetic ions (Ar0,S0) from the tungsten target and negative ions (S−) accelerated in the cathode dark space constitute the main sources of the energetic bombardment leading to crystallographic defects. The energy of these particles can be tailored by (i) thermalization between target and substrate in the sputtering gas or (ii) by a reduction of the discharge or target voltage, respectively, by high frequency excitation of the plasma. Films deposited under favorable conditions with respect to low particle energies and at substrate temperatures higher than 200°C exhibit a significant sulfur deficiency of up to about 5at.% compared to the stoichiometric composition of WS2. This is ascribed to an energetic particle bombardment-induced sulfur desorption from the growing films.

Temperature dependence anisotropic photoconductivity in 2H-MoSe 2 single crystals

Single crystals of 2H-MoSe 2 were investigated by the anisotropic photoconductivity (PC) measurements as a function of temperature in the range of 12–300 K. A significant shift towards lower energy has been observed in the PC spectra of the edge plane with respect to the van der Waals plane. The anisotropy in the indirect energy gap is being attributed to crystal anisotropy. The parameters that describe the temperature dependence of the anisotropic indirect energy gap are evaluated and discussed utilizing the Varshni and Manoogian–Woolley semi-empirical relationships. The Debye temperature of the sample has also been estimated by the fitted values.

Synthesis of type-II textured tungsten disulfide thin films with bismuth interfacial layer as a texture promoter

Highly textured tungsten disulfide (WS 2) thin films have been obtained by sulfurization of tungsten trioxide. The properties of WS 2 thin films prepared with bismuth interfacial layer as texture promoter has been studied. The WS 2 thin films were found to have predominant type-II orientation. The stacking of 2 H-WS 2 crystallites observed with scanning electron microscopy was not reported hitherto. The films can be pictured as an assembly of WS 2 hexagonal crystallites. The energy dispersive X-ray analysis and X-ray photoelectron spectroscopy (XPS) studies confirm that the films are stoichiometric. The XPS analysis described the local environment of the tungsten atoms and the formal oxidation states of the tungsten and sulfur atoms were + 4 and − 2. Together with the high degree of crystallinity and excellent texture of the film, a relatively smooth morphology, on submicron scale, is revealed through atomic force microscopy study. The conditions for the desired textured growth with the van der Waals planes parallel to the substrate surface are reported.

Rapid crystallization of WS2films assisted by a thin nickel layer: Anin situenergy-dispersive X-ray diffraction study

By rapid thermal crystallization of an amorphous WS3+x film, deposited by reactive magnetron sputtering at temperatures below 150 °C, layer-type semiconducting tungsten disulfide films (WS2) were grown. The rapid crystallization was monitored in real-time by in situ energy-dispersive X-ray diffraction. The films crystallize very fast (>40 nm/s), provided that a thin nickel film acts as nucleation seeds. Experiments on different substrates and the onset of the crystallization only at a temperature between 600 and 700 °C points to the decisive role of seeds for the textured growth of WS2, most probably liquid NiSx drops. The rapidly crystallized WS2 films exhibit a pronounced (001) texture with the van der Waals planes oriented parallel to the surface, leading to photoactive layers with a high hole mobility of about 80 cm2/Vs making such films suitable as absorbers for thin film solar cells. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Electrical Anisotropy of W-Doped ReSe2 Crystals

Single crystals of W-doped have been grown by chemical vapor transport process with bromine as the transporting agent. Single crystalline platelets up to surface area and in thickness were obtained. From the X-ray diffraction patterns, the doped crystals are found to crystallize in the triclinic-layered structure. The electrical anisotropy has been investigated along and perpendicular to the -axis on the van der Waals plane by temperature-dependent conductivity and Hall effect measurements. The influence of the dopant will be compared and discussed.

Temperature dependence of absorption edge anisotropy in 2H- MoSe 2 layered semiconductors

The absorption-edge anisotropy of 2H- MoSe 2 was studied by photoconductivity (PC) measurements as a function of temperature in the range of 12–300 K. A significant shift towards lower energies has been observed in the PC spectra on the edge plane with respect to those corresponding to the van der Waals (VdW) plane. The parameters that describe the temperature dependence of the absorption edges are evaluated by the Bose–Einstein empirical expression. Effective phonon energy was estimated from the temperature dependence of the Urbach energy. The estimated effective phonon energy for the VdW and edge planes, respectively, can be correlated to the observed Raman active E 1g and A 1g modes.

The electrical and optical anisotropy of rhenium-dopedWSe2 single crystals

Single crystals of rhenium-dopedWSe2 with large edge plane have been grown by the chemical vapour transport method usingbromine as the transporting agent. From the x-ray diffraction patterns, boththe doped and undoped crystals are found to crystallize in the 2H structure.The role of rhenium in affecting both the electrical and optical properties of theWSe2 crystals is examined.The thicker Re-doped WSe2 samples enable easier exploration of the electrical and optical anisotropiesof the materials both parallel and perpendicular to the crystalc-axis. The samples are n-type in nature from Hall measurements andthe conductivity anisotropy decreases drastically as a result of doping.Photovoltage measurements revealed that both undoped and Re-dopedWSe2 are indirect semiconductors. The indirect band gap of the doped sample showed considerable redshift compared with the undoped sample and also exhibited anisotropy along and perpendicular tothe c-axis. The anisotropy in the band gap is attributed to crystal anisotropy, which was alsostudied via the polarization-dependent electrolyte electroreflectance measurements takenalong the van der Waals plane (; ) and the as-grown edge plane (, ; ). The strong dependence of excitonic transitions A and B on polarization compared withthat of the undoped samples.

Reactive magnetron sputtering of molybdenum sulfide thin films: In situ synchrotron x-ray diffraction and transmission electron microscopy study

The nucleation and growth of magnetron sputtered MoSx films has been investigated by in situ energy dispersive x-ray diffraction, electron microscopy, and elastic recoil detection analysis. The MoSx films (0.5⩽x⩽2) were prepared by reactive magnetron sputtering from a molybdenum target in an argon–hydrogen sulfide mixture at substrate temperatures up to 700 °C. Using time-resolved in situ x-ray diffraction it was found that the films start to grow with (001) orientation where the van der Waals planes are parallel to the substrate surface. Depending on the deposition conditions a crossover of texture to the (100) orientation occurs, which leads to very rough surfaces. This texture crossover occurs earlier at low substrate temperatures and/or high deposition rates and/or high energetic particle bombardment of the growing films. The MoSx films exhibit significant lattice strain (up to 4%) in the c direction, i.e., perpendicular to the van der Waals planes, which decreases at high substrate temperatures and/or low deposition rates. This lattice expansion is not caused by film stress. Instead, it seems to be connected with disturbed or turbostratic growth due to crystallographic defects induced by energetic bombardment of the films. Also, intercalation of hydrogen could be responsible for lattice expansion, since significant amounts of hydrogen were detected by elastic recoil detection analysis. The sulfur deficiency (up to 20%) found in films that were deposited at temperatures higher than 200 °C could have been initiated by the reducing effect of the atomic hydrogen in our Ar/H2S plasma.

The chemical synthesis in solution and characterization of transition metal dichalcogenideMX2 (M = Mo,W; X = S, Se) nanoparticles

MX2 nanoparticles,with M = Mo,W and X = S or Se, were synthesized via a chemical reaction between molybdenum carbonyl (ortungsten carbonyl) and chalcogen dissolved in para-xylene solution. X-ray diffractionand high-resolution transmission electron microscopy analysis showed that theMoS2,WS2,MoSe2 and WSe2 powders were constituted of rounded amorphous nanoparticles. Upon annealing at550 °C the nanoparticles crystallized, losing their round shape. The four materialssynthesized under similar conditions showed that the annealed selenide nanoparticles(MoSe2 and WSe2) are well crystallized, with aligned stacks of 5–10 layers (002 vander Waals (vdW) planes), while the annealed sulfide nanoparticles(MoS2 and WS2) exhibit curved and tangled 002 planes, randomly distributed in the particle.

Chemical synthesis of molybdenum disulfide nanoparticles in an organic solution

Molybdenum disulfide, MoS2, nanoparticles can be synthesized either at high temperature as mono- and polycrystalline materials or at low temperature using various (electro)chemical routes. In the work presented, MoS2 nanoparticles were obtained using a low temperature (140 °C) method via a chemical solution reaction route between the organometallic precursor Mo(CO)6 and sulfur in p-xylene. As obtained, the MoS2 nanoparticles of 10–30 nm diameter were mostly amorphous with a rounded shape. Upon annealing at 550 °C under vacuum, the nanoparticles lost their rounded shape and became slightly crystallized with curved (002) basal van der Waals planes (2H hexagonal structure). An increase of 2–4% in the d(002) spacing of the annealed MoS2 nanoparticles compared to polycrystalline MoS2 was observed. The size and shape of these nanoparticles play an important role for their properties, e.g. in catalysis and lubrication.

Thermomechanical Characterization of Bismuth Telluride Based Thermoelectric Materials

ABSTRACTThe thermoelectric properties of bismuth telluride based thermoelectric (TE) materials are well-characterized, but comparatively little has been published on the mechanical and thermomechanical properties of these materials. In this paper, we present the initial dynamic mechanical analysis (DMA) data for n-type and p-type bismuth telluride based TE materials. The materials' tan δ values, indicative of viscoelastic energy dissipation modes, approach that of glassy or crystalline polymers and are greater than ten times the tan delta of structural metals. TE samples measured perpendicular to the van der Waals planes have higher tan δ values. Thermal scans in the DMA compressive mode showed changes in mechanical properties versus temperature with clear hysteresis effects. These changes were correlated to differential scanning calorimetry (DSC) thermal transitions. The expected anisotropy was shown in flexural 3-point bending results for one n-type material that showed a storage modulus of 0.10 to 0.45 GPa in the direction parallel to the van der Waals planes and 0.07 to 0.2 GPa in the perpendicular direction.

Influence of the electrochemical deposition parameters on the microstructure of MoS2 thin films

Thin films molybdenum dichalcogenide, MoS2, were prepared by cathodic electrochemical deposition from aqueous and non-aqueous solutions of tetrathiomolybdate ions, at different temperatures. The films were X-ray amorphous as deposited. They consist of an amorphous matrix in which quantum sized nanocrystallites or clusters were embedded. Upon annealing at high temperatures, the films obtained from aqueous solutions become crystalline and highly texturized having their van der Waals planes oriented parallel to the substrate, whereas, those obtained from ethylene glycol solutions kept on the amorphous matrix, with slightly larger sizes MoS2 nanoparticles embedded, than before annealing. Difference in the mechanism of the electrodeposition in aqueous and ethylene glycol solutions is discussed.