Structural, electronic and optical properties of BaRE6(Ge2O7)2(Ge3O10) (RE = Tm, Yb, Lu) compounds and BaYb6(Ge2O7)2(Ge3O10):Tm3+ and BaLu6(Ge2O7)2(Ge3O10):Yb3+,Tm3+ phosphors: potential applications in temperature sensing.

A new series of BaRE6(Ge2O7)2(Ge3O10) (RE = Tm, Yb, Lu) germanates and activated phases BaYb6(Ge2O7)2(Ge3O10):xTm3+ and BaLu6(Ge2O7)2(Ge3O10):12yYb3+,yTm3+ have been prepared using a solid-state reaction. An XRPD study has revealed that the compounds crystallize in the monoclinic system (space group P21/m, Z = 2). The crystal lattice consists of zigzag chains of edge-sharing distorted REO6 octahedra, bowed trigermanate [Ge3O10] units, [Ge2O7] groups, and eight-coordinated Ba atoms. The density functional theory calculations have confirmed a high thermodynamic stability of the synthesized solid solutions. According to the results of vibrational spectroscopy studies and diffuse reflectance measurements, the BaRE6(Ge2O7)2(Ge3O10) germanates are promising compounds for the creation of efficient lanthanide ion activated phosphors. Under 980 nm laser diode excitation, the BaYb6(Ge2O7)2(Ge3O10):xTm3+ and BaLu6(Ge2O7)2(Ge3O10):12yYb3+,yTm3+ samples exhibit upconversion luminescence corresponding to the characteristic 1G4 → 3H6 (455-500 nm), 1G4 → 3F4 (645-673 nm) and 3H4 → 3H6 (750-850 nm) transitions in Tm3+ ions. Heating of the BaLu6(Ge2O7)2(Ge3O10):12yYb3+,yTm3+ phosphor with the optimal composition up to 498 K leads to the enhancement of a broad band at 673-730 nm, caused by 3F2,3 → 3H6 transitions. It has been revealed that the fluorescence intensity ratio between this band and the band at 750-850 nm may be used for temperature sensing. The absolute and relative sensitivities in the studied temperature range reach 0.021% K-1 and 1.94% K-1, respectively.

Thermal Treatment Impact on the Mechanical Properties of Mg3Si2O5(OH)4 Nanoscrolls.

A group of phyllosilicate nanoscrolls conjoins several hydrosilicate layered compounds with a size mismatch between octahedral and tetrahedral sheets. Among them, synthetic Mg3Si2O5(OH)4 chrysotile nanoscrolls (obtained via the hydrothermal method) possess high thermal stability and mechanical properties, making them prospective composite materials fillers. However, accurate determination of these nano-objects with Young's modulus remains challenging. Here, we report on a study of the mechanical properties evolution of individual synthetic phyllosilicate nanoscrolls after a series of heat treatments, observed with an atomic force microscopy and calculated using the density functional theory. It appears that the Young's modulus, as well as shear deformation's contribution to the nanoscrolls mechanical behavior, can be controlled by heat treatment. The main reason for this is the heat-induced formation of covalent bonding between the adjacent layers, which complicate the shear deformation.

Nanotubes from the Misfit Layered Compound (SmS)1.19TaS2: Atomic Structure, Charge Transfer, and Electrical Properties.

Misfit layered compounds (MLCs) MX-TX2, where M, T = metal atoms and X = S, Se, or Te, and their nanotubes are of significant interest due to their rich chemistry and unique quasi-1D structure. In particular, LnX-TX2 (Ln = rare-earth atom) constitute a relatively large family of MLCs, from which nanotubes have been synthesized. The properties of MLCs can be tuned by the chemical and structural interplay between LnX and TX2 sublayers and alloying of each of the Ln, T, and X elements. In order to engineer them to gain desirable performance, a detailed understanding of their complex structure is indispensable. MLC nanotubes are a relative newcomer and offer new opportunities. In particular, like WS2 nanotubes before, the confinement of the free carriers in these quasi-1D nanostructures and their chiral nature offer intriguing physical behavior. High-resolution transmission electron microscopy in conjunction with a focused ion beam are engaged to study SmS-TaS2 nanotubes and their cross-sections at the atomic scale. The atomic resolution images distinctly reveal that Ta is in trigonal prismatic coordination with S atoms in a hexagonal structure. Furthermore, the position of the sulfur atoms in both the SmS and the TaS2 sublattices is revealed. X-ray photoelectron spectroscopy, electron energy loss spectroscopy, and X-ray absorption spectroscopy are carried out. These analyses conclude that charge transfer from the Sm to the Ta atoms leads to filling of the Ta 5d z 2 level, which is confirmed by density functional theory (DFT) calculations. Transport measurements show that the nanotubes are semimetallic with resistivities in the range of 10-4 Ω·cm at room temperature, and magnetic susceptibility measurements show a superconducting transition at 4 K.

Synthesis and Structure of Quasi-One-Dimensional Niobium Tetrasulfide NbS4.

The binary niobium sulfide NbS4 was synthesized as a crystalline phase. We showed that NbS4 can be formed from Nb metal, from defect niobium sulfide Nb1.14S2, or from some other niobium dichalcogenides in reactions with excess sulfur in an evacuated ampule at 440 °C. The crystal structure of NbS4 (monoclinic space group C2/c, a = 13.126(2) Å, b = 10.454(1) Å, c = 6.951(1) Å, ß = 111.939(5)°) is a packing of infinite chains {NbS4}1∞, analogous to VS4. In the chains, Nb atoms are in a tetragonal-antiprismatic coordination of sulfur atoms of disulfide groups (S2)2-; short Nb···Nb contacts (2.896 Å) alternating with longer ones (3.278 Å) appear within the chains at 150 K. According to density functional theory calculations, NbS4 is a thermodynamically stable compound, a nonmagnetic semiconductor. NbS4 is a new member of the family of quazi-one-dimensional compounds, group 5 metal polychalcogenides, well-known for their interesting electrophysical properties. The synthesis and crystal structure as well as the thermal stability and lattice dynamics of NbS4 are discussed here.

W Doping in Ni12P5 as a Platform to Enhance Overall Electrochemical Water Splitting.

Bifunctional electrocatalysts for efficient hydrogen generation from water splitting must overcome both the sluggish water dissociation step of the alkaline hydrogen evolution half-reaction (HER) and the kinetic barrier of the anodic oxygen evolution half-reaction (OER). Nickel phosphides are a promising catalysts family and are known to develop a thin active layer of oxidized Ni in an alkaline medium. Here, Ni12P5 was recognized as a suitable platform for the electrochemical production of γ-NiOOH─a particularly active phase─because of its matching crystallographic structure. The incorporation of tungsten by doping produces additional surface roughness, increases the electrochemical surface area (ESCA), and reduces the energy barrier for electron-coupled water dissociation (the Volmer step for the formation of Hads). When serving as both the anode and cathode, the 15% W-Ni12P5 catalyst provides an overall water splitting current density of 10 mA cm-2 at a cell voltage of only 1.73 V with good durability, making it a promising bifunctional catalyst for practical water electrolysis.

Asymmetric misfit nanotubes: Chemical affinity outwits the entropy at high-temperature solid-state reactions.

Asymmetric two-dimensional (2D) structures (often named Janus), like SeMoS and their nanotubes, have tremendous scope in material chemistry, nanophotonics, and nanoelectronics due to a lack of inversion symmetry and time-reversal symmetry. The synthesis of these structures is fundamentally difficult owing to the entropy-driven randomized distribution of chalcogens. Indeed, no Janus nanotubes were experimentally prepared, so far. Serendipitously, a family of asymmetric misfit layer superstructures (tubes and flakes), including LaX-TaX2 (where X = S/Se), were synthesized by high-temperature chemical vapor transport reaction in which the Se binds exclusively to the Ta atoms and La binds to S atoms rather than the anticipated random distribution. With increasing Se concentration, the LaS-TaX2 misfit structure gradually transformed into a new LaS-TaSe2-TaSe2 superstructure. No misfit structures were found for xSe = 1. These counterintuitive results shed light on the chemical selectivity and stability of misfit compounds and 2D alloys, in general. The lack of inversion symmetry in these asymmetric compounds induces very large local electrical dipoles. The loss of inversion and time-reversal symmetries in the chiral nanotubes offers intriguing physical observations and applications.

Structural and spectroscopic characterization of a new series of Ba2RE2Ge4O13 (RE = Pr, Nd, Gd, and Dy) and Ba2Gd2-xEuxGe4O13 tetragermanates.

A new series of Ba2RE2Ge4O13 (RE = Pr, Nd, Gd, Dy) germanates and Ba2Gd2-xEuxGe4O13 (x = 0.1-0.8) solid solutions have been synthesized using the solid-state reaction technique and characterized by X-ray powder diffraction. All compounds crystallize in the monoclinic system, space group C2/c, Z = 4. The crystal lattice consists of RE2O12 dimers, zigzag C2-symmetric [Ge4O13]10- tetramers, and ten-coordinated Ba atoms located in voids between polyhedra. The density-functional theory (DFT) calculations performed on a rich set of Ba2RE2Ge4O13 compounds have confirmed the high thermodynamic stability of monoclinic modification. Under ultraviolet (UV) light excitation Ba2Gd2-xEuxGe4O13 phosphors exhibit an orange-red emission corresponding to the characteristic f-f transitions in Eu3+ ions. The highest intensity of lines at 580 nm (5D0→7F0), 582-602 nm (5D0→7F1), 602-640 nm (5D0→7F2), 648-660 nm (5D0→7F3), and 680-715 nm (5D0→7F4) is observed for the samples with x = 0.4-0.6. The possibility of their application has been assessed by studying their color characteristics, quantum efficiency, and thermal stability. The obtained data indicate that Ba2Gd2-xEuxGe4O13 solids can be considered as promising materials for UV-excited phosphor-converted light-emitting diodes (LEDs) if an aluminum nitride substrate (λex = 255 nm) is used as a semiconductor chip.

YS-TaS2 and YxLa1-xS-TaS2 (0 ≤ x ≤ 1) Nanotubes: A Family of Misfit Layered Compounds.

We present the analysis of a family of nanotubes (NTs) based on the quaternary misfit layered compound (MLC) YxLa1-xS-TaS2. The NTs were successfully synthesized within the whole range of possible compositions via the chemical vapor transport technique. In-depth analysis of the NTs using electron microscopy and spectroscopy proves the in-phase (partial) substitution of La by Y in the (La,Y)S subsystem and reveals structural changes compared to the previously reported LaS-TaS2 MLC-NTs. The observed structure can be linked to the slightly different lattice parameters of LaS and YS. Raman spectroscopy and infrared transmission measurements reveal the tunability of the plasmonic and vibrational properties. Density-functional theory calculations showed that the YxLa1-xS-TaS2 MLCs are stable in all compositions. Moreover, the calculations indicated that substitution of La by Sc atoms is electronically not favorable, which explains our failed attempt to synthesize these MLC and NTs thereof.

Synthesis and characterization of quaternary La(Sr)S-TaS2 misfit-layered nanotubes.

Misfit-layered compounds (MLCs) are formed by the combination of different lattices and exhibit intriguing structural and morphological characteristics. MLC Sr x La1- x S-TaS2 nanotubes with varying Sr composition (10, 20, 40, and 60 Sr atom %, corresponding to x = 0.1, 0.2, 0.4 and 0.6, respectively) were prepared in the present study and systematically investigated using a combination of high-resolution electron microscopy and spectroscopy. These studies enable detailed insight into the structural aspects of these phases to be gained at the atomic scale. The addition of Sr had a significant impact on the formation of the nanotubes with higher Sr content, leading to a decrease in the yield of the nanotubes. This trend can be attributed to the reduced charge transfer between the rare earth/S unit (La x Sr1- x S) and the TaS2 layer in the MLC which destabilizes the MLC lattice. The influence of varying the Sr content in the nanotubes was systematically studied using Raman spectroscopy. Density functional theory calculations were carried out to support the experimental observations.

Theoretical and experimental comparative study of the stability and phase transformations of sesquichalcogenides M2Q3 (M = Nb, Mo; Q = S, Se).

The extensive family of transition metal chalcogenides has been comprehensively investigated owing to their diverse useful properties. However, even among them, there are ones that have received comparatively less attention; in particular, these are molybdenum and niobium sulfides and selenides with the composition of M : Q = 2 : 3 (M = Mo, Nb; Q = S, Se). Mo or Nb chalcogenides with this stoichiometry may adopt one of two structures: (i) sesquichalcogenides M2Q3, where important structural elements are infinite metal chains, or (ii) self-intercalated compounds M1.33Q2, in which extra M atoms are inserted between MQ2 layers. Depending on the M-Q combination, in practice, either none, one, or both of them may exist. The reasons for chemical dissimilarity in the series of seemingly related compounds haven't been addressed until the present work. Here, we present the first generalized comparative study of these chalcogenides by quantum-chemical computations verified by laboratory experiments. High-temperature phases of Mo2S3 and Nb2Se3 may be stably isolated at room temperature, while "Nb2S3" and "Mo2Se3" had not been obtained, nor were they expected to exist from DFT data. The structure-determining motifs of sesquichalcogenides M2Q3 are metallic chains, and thus, apparently, if metal's electron deficiency (or excess) prevents the formation of M-M chains, then the M2Q3-type structure cannot form. If the metal has an adequate electron density and the structure does form at high temperature (as it happens for Mo2S3 and Nb2Se3), then it can be kinetically stabilized by quenching, and stored under laboratory conditions for long times. However, if Nb2Se3 is left to cool down slowly, it undergoes phase transition to iso-stoichiometric intercalate Nb1.333Se2, in good agreement with DFT predictions of the close values of their free energies. Isostructural intercalate Nb1.333S2 is found to be the only experimental product in the Nb-S system, in full accordance with DFT prediction. Effective stabilization of self-intercalated phases is provided by significant charge transfer from intercalated Nb atoms to the NbQ2 layers, as confirmed by DFT. The obtained data may serve to get insight into polymorphism of some less-studied transition metal chalcogenides and to promote their use for future functional materials.

Nd3+,Ho3+-Codoped apatite-related NaLa9(GeO4)6O2 phosphors for the near- and middle-infrared region.

The apatite-like NaLa9(GeO4)6O2:Nd3+,Ho3+ phosphor is prepared using the solid-state method. Rietveld refinement of high-resolution time-of-flight neutron powder diffraction measurements indicate that this compound crystallizes in the hexagonal system with space group P63/m, Z = 1 and unit cell parameters a = 9.88903(6) Å, c = 7.25602(5) Å, V = 614.521(7) Å3 at room temperature. The 4f sites are statistically occupied by La, Nd and Na, while 6h sites are occupied by La and Nd. Luminescence in the near- and middle-IR range caused by the transitions in neodymium and holmium ions is excited under 808 nm laser diode radiation. The highest emission intensity in NaLa9-x-yNdxHoy(GeO4)6O2 is attained at trace amounts of holmium, and it decreases sharply when y increases to 0.01. The IR phosphors have a good thermal stability and exhibit a very weak upconversion emission in the red spectral range upon 808 nm excitation. A scheme of excitation and emission pathways involving ground/excited state absorption, energy transfer, cross-relaxation, nonradiative multiphonon relaxation processes in Nd3+ and Ho3+ ions has been proposed. The data analysis indicates that Nd3+ ions serve as sensitizers for Ho3+ ions in these compounds, stimulating intense 2.1 µm and 2.7 µm emissions. These apatite-related germanate phosphors are promising materials for near- and middle-infrared solid-state lighting applications.

Titanium Dichalcogenides as Nanoreactors for Magnetic High-Anisotropy Phases.

Composite single crystals consisting of nanoscaled Fe3Se4 inclusions encapsulated into the interlayer space of TiSe2 matrix were obtained by the decay of homogeneous Fe0.5TiSe2 intercalation compound. These composites have a high magnetic anisotropy due to the coherent bond between inclusions and the host lattice of TiSe2. The influence of selenium pressure over the composite surface on the composition of the inclusions is studied, and the possibility of controlling their content is demonstrated. The thermodynamic stability of the composite with a small excess of selenium in comparison with the stoichiometric material is established based on theoretical calculations. The estimated energy of the chemical bond between components of the composite is close to the van der Waals bond energy. A method to control the orientation and defects within the inclusions in the host lattice is proposed.

Single Walled BiI3 Nanotubes Encapsulated within Carbon Nanotubes.

Inorganic nanotubes are morphological counterparts of carbon nanotubes (CNTs). Yet, only graphene-like BN layer has been readily organized into single walled nanotubes so far. In this study, we present a simple route to obtain inorganic single walled nanotubes - a novel ultrathin morphology for bismuth iodide (BiI3), embedded within CNTs. The synthesis involves the capillary filling of BiI3 into CNT, which acts as a nanotemplate, by annealing the BiI3-CNT mixture above the melting point of BiI3. Aberration corrected scanning/transmission electron microscopy is used in characterizing the novel morphology of BiI3. A critical diameter which enables the formation of BiI3 nanotubes, against BiI3 nanorods is identified. The relative stability of these phases is investigated with the density functional theory calculations. Remarkably, the calculations reveal that the single walled BiI3 nanotubes are semiconductors with a direct band gap, which remain stable even without the host CNTs.

XPS experimental and DFT investigations on solid solutions of Mo1-xRexS2 (0 < x < 0.20).

The synthesis, characterization, experimental X-ray photoelectron spectra (XPS) and density-functional theory (DFT) investigations on solid solutions of Mo1-xRexS2 (x = 0.05, 0.10, 0.15 and 0.20) are reported herein. It is shown that even at a low concentration of dopant Re atoms, clustering occurs. At an Re concentration of 5% the formation of dimer-like impregnations is observed. An increase in the dopant concentration leads to an increase in the amount of clustered rhenium atoms and to the formation of rhombic clusters. The absence of magnetism within the studied Mo1-xRexS2 solid solutions allowed us to suggest a mechanism for the distribution of rhenium inside molybdenum disulphide through the initial formation of rhenium disulphide and its subsequent spreading.

Diameter-dependent wetting of tungsten disulfide nanotubes.

The simple process of a liquid wetting a solid surface is controlled by a plethora of factors-surface texture, liquid droplet size and shape, energetics of both liquid and solid surfaces, as well as their interface. Studying these events at the nanoscale provides insights into the molecular basis of wetting. Nanotube wetting studies are particularly challenging due to their unique shape and small size. Nonetheless, the success of nanotubes, particularly inorganic ones, as fillers in composite materials makes it essential to understand how common liquids wet them. Here, we present a comprehensive wetting study of individual tungsten disulfide nanotubes by water. We reveal the nature of interaction at the inert outer wall and show that remarkably high wetting forces are attained on small, open-ended nanotubes due to capillary aspiration into the hollow core. This study provides a theoretical and experimental paradigm for this intricate problem.

Structural and chemical analysis of gadolinium halides encapsulated within WS2 nanotubes.

The hollow cavities of nanotubes serve as templates for the growth of size- and shape-confined functional nanostructures, giving rise to novel materials and properties. In this work, considering their potential application as MRI contrast agents, gadolinium halides are encapsulated within the hollow cavities of WS2 nanotubes by capillary filling to obtain GdX3@WS2 nanotubes (where X = Cl, Br or I and @ means encapsulated in). Aberration corrected scanning/transmission electron microscopy (S/TEM) and spectroscopy is employed to understand the morphology and composition of the GdI3@WS2 nanotubes. The three dimensional morphology is studied with STEM tomography but understanding the compositional information is non-trivial due to the presence of multiple high atomic number elements. Therefore, energy dispersive X-ray spectroscopy (EDS) tomography was employed revealing the three dimensional chemical composition. Molecular dynamics simulations of the filling procedure shed light into the mechanics behind the formation of the confined gadolinium halide crystals. The quasi-1D system employed here serves as an example of a TEM-based chemical nanotomography method that could be extended to other materials, including beam-sensitive soft materials.

Solar Synthesis of PbS-SnS2 Superstructure Nanoparticles.

We report the synthesis and supporting density-functional-theory computations for a closed-cage, misfit layered-compound superstructure from PbS-SnS2, generated by highly concentrated sunlight from a precursor mixture of Pb, SnS2, and graphite. The unique reactor conditions created in our solar furnace are found to be particularly conducive to the formation of these nanomaterials. Detailed structural and chemical characterization revealed a spontaneous inside-out formation mechanism, with a broad range of nonhollow fullerene-like structures starting at a diameter of ∼20 nm and a wall thickness of ∼5 layers. The computations also reveal a counterintuitive charge transfer pathway from the SnS2 layers to the PbS layers, which indicates that, in contrast to binary-layered compounds where it is principally van der Waals forces that hold the layers together, polar forces appear to be as important in stabilizing superstructures of misfit layered compounds.

Atomic-scale evolution of a growing core-shell nanoparticle.

Understanding the atomic-scale growth at solid/solution interfaces is an emerging frontier in molecular and materials chemistry. This is particularly challenging when studying chemistry occurring on the surfaces of nanoparticles in solution. Here, we provide atomic-scale resolution of growth, in a statistical approach, at the surfaces of inorganic nanoparticles by state-of-the-art aberration-corrected transmission electron microscopy (TEM) and focal series reconstruction. Using well-known CdSe nanoparticles, we unfold new information that, for the first time, allows following growth directly, and the subsequent formation of CdS shells. We correlate synthetic procedures with resulting atomic structure by revealing the distribution of lattice disorder (such as stacking faults) within the CdSe core particles. With additional sequential synthetic steps, an ongoing transformation of the entire structure occurs, such that annealing takes place and stacking faults are eliminated from the core. The general strategy introduced here can now be used to provide equally revealing atomic-scale information concerning the structural evolution of inorganic nanostructures.

Optical Properties of Triangular Molybdenum Disulfide Nanoflakes.

The results from calculations of optical and electronic properties of triangular MoS2 nanoflakes with edge lengths ranging from 1.6 to 10.4 nm are presented. The optical spectra were calculated using the time-dependent extension of the density-functional tight-binding method (TD-DFTB). The size effect in the optical absorption spectra is clearly visible. With decreasing length of the nanoflakes edges, the long-wavelength absorption in the range of visible light is shifted toward short-wavelength absorption, confirming a quantum-confinement-like behavior of these flakes. In contrast, the edges of the nanoflakes exhibit a distinct metallic-like behavior. The relation of the absorption properties to the observed photoluminescence of MoS2 nanoflakes is discussed in a qualitative manner.

On the crystallization of polymer composites with inorganic fullerene-like particles.

The effect of a sulfide fullerene-like particle embedded into a polymer has been studied by molecular dynamics simulations on the nanosecond time scale using a mesoscopic Van der Waals force field evaluated for the case of a spherical particle. Even in this approach, neglecting the atomistic features of the surface, the inorganic particle acts as a nucleation agent facilitating the crystallization of the polymeric sample. A consideration of the Van der Waals force field of multi-walled sulfide nanoparticles suggests that in the absence of chemical interactions the size of the nanoparticle is dominating for the adhesion strength, while the number of sulfide layers composing the cage does not play a role.