Andrey N. Enyashin

Imogolite Nanotubes: Stability, Electronic, and Mechanical Properties

The aluminosilicate mineral imogolite is composed of single-walled nanotubes with stoichiometry of (HO)(3)Al(2)O(3)SiOH and occurs naturally in soils of volcanic origin. In the present work we study the stability and the electronic and mechanical properties of zigzag and armchair imogolite nanotubes using the density-functional tight-binding method. The (12,0) imogolite tube has the highest stability of all tubes studied here. Uniquely for nanotubes, imogolite has a minimum in the strain energy for the optimum structure. This is in agreement with experimental data, as shown by comparison with the simulated X-ray diffraction spectrum. An analysis of the electronic densities of states shows that all imogolite tubes, independent on their chirality and size, are insulators.

Defect-induced conductivity anisotropy in MoS2monolayers

Various types of defects in MoS2 monolayers and their influence on the electronic structure and transport properties have been studied using the Density-Functional based Tight-Binding method in conjunction with the Greens Function approach. Intrinsic defects in MoS2 monolayers significantly affect their electronic properties. Even at low concentration they considerably alter the quantum conductance. While the electron transport is practically isotropic in pristine MoS2, strong anisotropy is observed in the presence of defects. Localized mid-gap states are observed in semiconducting MoS2 that do not contribute to the conductivity but direction-dependent scatter the current, and that the conductivity is strongly reduced across line defects and selected grain boundary models.

Toward atomic-scale bright-field electron tomography for the study of fullerene-like nanostructures.

We present the advancement of electron tomography for three-dimensional structure reconstruction of fullerene-like particles toward atomic-scale resolution. The three-dimensional reconstruction of nested molybdenum disulfide nanooctahedra is achieved by the combination of low voltage operation of the electron microscope with aberration-corrected phase contrast imaging. The method enables the study of defects and irregularities in the three-dimensional structure of individual fullerene-like particles on the scale of 2-3 A. Control over shape, size, and atomic architecture is a key issue in synthesis and design of functional nanoparticles. Transmission electron microscopy (TEM) is the primary technique to characterize materials down to the atomic level, albeit the images are two-dimensional projections of the studied objects. Recent advancements in aberration-corrected TEM have demonstrated single atom sensitivity for light elements at subångström resolution. Yet, the resolution of tomographic schemes for three-dimensional structure reconstruction has not surpassed 1 nm3, preventing it from becoming a powerful tool for characterization in the physical sciences on the atomic scale. Here we demonstrate that negative spherical aberration imaging at low acceleration voltage enables tomography down to the atomic scale at reduced radiation damage. First experimental data on the three-dimensional reconstruction of nested molybdenum disulfide nanooctahedra is presented. The method is applicable to the analysis of the atomic architecture of a wide range of nanostructures where strong electron channeling is absent, in particular to carbon fullerenes and inorganic fullerenes.

Controlled Doping of MS2 (M=W, Mo) Nanotubes and Fullerene‐like Nanoparticles

Doping of semiconductor nanocrystals and nanowires with minute amounts of foreign atoms plays a major role in controlling their electrical, optical, and magnetic properties. In the case of carbon nanotubes, subsequent doping with oxygen and potassium leads to a p-type and n-type behavior, respectively. In another work, VOx nanotubes were transformed from spin-frustrated semiconductors to ferromagnets by doping with either electrons or holes. Calculations indicated that nand p-type doping of multiwall MoS2 nanotubes (INT) could be accomplished by substituting minute amounts of the Mo lattice atoms with either Nb (p-type) and Re (n-type), respectively. Substituting (< 0.1 at%) molybdenum by rhenium atoms and sulfur by halogen atoms was shown to produce n-type conductivity in MoS2 crystals. To synthesize rhenium-doped nanoparticles (NP) and nanotubes both in situ and subsequent doping methods were used. Figure 1a shows the quartz reactor used for in situ synthesis of rhenium doped MoS2 NP with fullerene-like structure (Re:IF-MoS2). The formal Re concentration was varied from 0.02 to 0.7 at%. The precursor RexMo1 xO3 (x< 0.01) powder was prepared in a specially designed auxiliary reactor (see Supporting Information). Evaporation of this powder takes place in area 1 at 770 8C (Figure 1 a). The oxide vapor reacts with hydrogen gas in area 2 (Figure 1a) at 800 8C which leads to a partial reduction of the vapor and its condensation into Re-doped MoO3 y nanoparticles. The resulting NP react with H2/H2S gas in area 3 at 810–820 8C to produce reduced oxide nanoparticles engulfed with a few closed layers of Re:MoS2, which protect it against ripening into bulk 2H-MoS2. [7] To complete this oxide to sulfide conversion a long (25–35 h) annealing process at 870 8C in the presence of H2S and forming gas (H2 10 wt %; N2) was performed. At the end of this diffusion-controlled process a powder of Re-doped MoS2 NP with a fullerene-like (IF) structure (Re:IF-MoS2) was obtained. In addition, doping of IF-WS2 NP and INT-WS2 was subsequently carried out by heating the pre-prepared IF/INT in an evacuated quartz ampoule also containing ReO3, or ReCl3 and iodine. In the case of ReCl3, both the rhenium and the chlorine atoms (substitution to sulfur atoms) served as ntype dopants. Typical high-resolution scanning electron microscopy (HRSEM) and transmission electron microscopy (HRTEM) micrographs of the Re-doped fullerene-like NP are shown in Figure 1b. The Re:IF-MoS2 consists of about 30 closed (concentric) MoS2 layers. No impurity, such as oxides, or platelets (2 H) of MoS2 could be found in the product powder. The line profile and the Fourier analyses (FFT) (inset of Figure 1b) show an interlayer spacing of 0.627 nm (doped). Furthermore, the layers seem to be evenly folded and closed with very few defects and cusps, demonstrating the Re-doped NP to be quite perfectly crystalline. HRTEM did not reveal any structural changes even for the samples with high Re concentration (0.71 at%). However, owing to its quasispherical shape and size, this analysis cannot completely rule-out the presence of a small amount of the ReS2 phase in the IF NP. Figure 1c shows a typical TEM image of Re(Cl) (post synthesis) doped multiwall WS2 nanotube. There is no [*] L. Yadgarov, Dr. R. Rosentsveig, Dr. A. Albu-Yaron, Prof. R. Tenne Department of Materials and Interfaces, Weizmann Institute Rehovot 76100 (Israel) E-mail: reshef.tenne@weizmann.ac.il

MoS2 Hybrid Nanostructures: From Octahedral to Quasi‐Spherical Shells within Individual Nanoparticles

MoS2, a layered compound with tribological and catalytic applications, is known to form a range of hollow closed nanostructures and nanoparticles, including graphene-like structures. These have been demonstrated experimentally through high-temperature synthesis and pulsed laser ablation (PLA), and theoretically with quantum chemical calculations. The smallest allowed structures are nanooctahedra of 3 to 8 nm size. Nicknamed the “true inorganic fullerene” in analogy to carbon fullerenes, they differ from larger multiwalled MoS2 fullerene-like nanoparticles both in their morphology and predicted electronic properties. The larger fullerene-like particles are quasi-spherical (polyhedral) or nanotubular, typically with diameters of 20 to 150 nm. Above a few hundred nm in size, these nanoparticles transform into 2H-MoS2 platelets. Fullerene-like particles have been recognized as superior solid lubricants with numerous commercial applications, and MoS2 nanooctahedra may have catalytic applications. Understanding the fundamental commonality of these two morphologies might prove essential in the development of new materials. The research on hollow MoS2 nanostructures of minimal size (< 10 nm in diameter) was initiated in 1993 upon the first independent proposal of the formation of nanooctahedra of MoS2 [3,5] (and BN) with six rhombi in their corners. In 1999, it was demonstrated that twoto four-walled MoS2 nanooctahedra, 3–5 nm in size and up to ca. 10 atoms, could be obtained by PLA. Similar results were subsequently reported in Ref. [1, 2] as illustrated in Figure 1a. Recent studies of high energy density methods such as laser ablation and arc– discharge resulted in small structures with only a limited number of atoms: Mo–S clusters or double-walled nanooctahedra.

Inorganic Nanotubes and Fullerene-Like Structures (IF)

Back in 1992 it was proposed that nanoparticles of layered compounds will beunstable against folding and will close up into fullerene-like structures (IF) andnanotubes. In the years that followed nanotubes and fullerene-like structureswere synthesized from numerous compounds with layered structure. Morerecently, crystalline and noncrystalline nanotubes of compounds with a 3D, i.e.,quasi-isotropic lattice have been intensively investigated. In view of their eminentapplications potential, much effort and substantial progress has been achieved inthe scaling-up of the synthesis of inorganic nanotubes and fullerene-likenanoparticles of WS2 and MoS2 and also other compounds. Early on it wassuggested that hollow nano-octahedra consisting of a few hundred MoS2moieties make the true analogs of C60, etc. This notion has been advancedconsiderably in recent years through a combined experimental–theoreticaleffort.Substantial progress has been accomplished in the use of such nanoparticlesfor tribological applications and lately for impact resilient nanocomposites.These tests indicated that IF-MoS2 and IF-WS2 are heading for large-scaleapplications in the automotive, machining, aerospace, electronics, defense, medicaland numerous other kinds of industries. A few products based on thesenanoparticles have been recently commercialized by “ApNano Materials, Inc”(“NanoMaterials, Ltd.”, see also www.apnano.com). Most recently, a manufacturingfacility for the commercialization of these nanomaterials has been erectedand sales of the product started. Novel applications of inorganic nanotubesand fullerene-like nanoparticles in the fields of catalysis; microelectronics;Li rechargeable batteries; medical and optoelectronics will be discussed.

Atom by atom: HRTEM insights into inorganic nanotubes and fullerene-like structures

The characterization of nanostructures down to the atomic scale is essential to understand some physical properties. Such a characterization is possible today using direct imaging methods such as aberration-corrected high-resolution transmission electron microscopy (HRTEM), when iteratively backed by advanced modeling produced by theoretical structure calculations and image calculations. Aberration-corrected HRTEM is therefore extremely useful for investigating low-dimensional structures, such as inorganic fullerene-like particles and inorganic nanotubes. The atomic arrangement in these nanostructures can lead to new insights into the growth mechanism or physical properties, where imminent commercial applications are unfolding. This article will focus on two structures that are symmetric and reproducible. The first structure that will be dealt with is the smallest stable symmetric closed-cage structure in the inorganic system, a MoS2 nanooctahedron. It is investigated by means of aberration-corrected microscopy which allowed validating the suggested DFTB-MD model. It will be shown that structures diverging from the energetically most stable structures are present in the laser ablated soot and that the alignment of the different shells is parallel, unlike the bulk material where the alignment is antiparallel. These findings correspond well with the high-energy synthetic route and they provide more insight into the growth mechanism. The second structure studied is WS2 nanotubes, which have already been shown to have a unique structure with very desirable mechanical properties. The joint HRTEM study combined with modeling reveals new information regarding the chirality of the different shells and provides a better understanding of their growth mechanism.

Modeling of the electronic structure, chemical bonding, and properties of ternary silicon carbide Ti3SiC2

Layered ternary carbides and nitrides of d and p elements exhibit a unique combination of properties characteristic of both metals and ceramics. This determines their high technological potential as materials for high-temperature ceramics, protective coatings, sensors, electrical contacts and also attracts attention to a detailed investigation of the nature of their properties. Along with remarkable achievements in the synthesis, studies of functional characteristics, and solutions of materials science problems, great progress in the description and prediction of fundamental physicochemical properties has recently been achieved with the use of first principle (ab initio) methods. By the example of titanium silicon carbide Ti3SiC2 (a prototype of the MAX phase family) the possibilities of current ab initio methods are considered for the analysis and prediction of structural, cohesive, mechanical properties, non-stoichiometry and doping effects, the description of electronic characteristics and features of the chemical bonding in nanolaminates. The data on the quantum chemical studies of the Ti3SiC2 surface states as well as its hypothetical nanotubular forms are discussed.

Fullerene‐like Mo(W)1−xRexS2 Nanoparticles

Inorganic fullerene-like (IF) Mo(1-x)Re(x)S(2) and W(1-x)Re(x)S(2) nanoparticles have been synthesized by a gas-phase reaction involving the respective metal halides with H(2)S. The IF-Mo(W)(1-x)Re(x)S(2) nanoparticles, containing up to 5 % Re, were characterized by a variety of experimental techniques. Analyses of the X-ray powder diffraction and different electron microscopy techniques show that the Re is doped in the MoS(2) host lattice. Interestingly, Re-doped MoS(2) nanotubes are present as well, although in small quantities ( approximately 5 %). XPS results confirm the nanoparticles to be more n-type arising from the effect of Re doping. Additionally, density-functional tight-binding (DFTB) calculations support the observed n-type behavior.

C28 fullerites—structure, electronic properties and intercalates

Mechanical and electronic properties of hypothetical carbon nanostructures, on the basis of C28 building blocks, hyperdiamond and hyperlonsdaleite, have been investigated with DFT based methods. The low mass density and large internal surface suggest applications as catalyst, nanosieve and gas storage material. We estimate the active volume accessible by H2. Special emphasis is given to the possibility to tune their properties by endo- and exohedral intercalation with Zn, Ti and K. While endohedral intercalation with Zn does not affect the overall structure, endohedral Ti intercalation has different consequences on the structural stability of the two allotropes. Exohedral intercalation with K leads to an ionic fullerite phase with metallic conductivity.