Lena Yadgarov

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

Dependence of the Absorption and Optical Surface Plasmon Scattering of MoS2 Nanoparticles on Aspect Ratio, Size, and Media

The optical and electronic properties of suspensions of inorganic fullerene-like nanoparticles of MoS2 are studied through light absorption and zeta-potential measurements and compared to those of the corresponding microscopic platelets. The total extinction measurements show that, in addition to excitonic peaks and the indirect band gap transition, a new peak is observed at 700-800 nm. This spectral peak has not been reported previously for MoS2. Comparison of the total extinction and decoupled absorption spectrum indicates that this peak largely originates from scattering. Furthermore, the dependence of this peak on nanoparticle size, shape, and surface charge, as well as solvent refractive index, suggests that this transition arises from a plasmon resonance.

Observation of a Burstein–Moss Shift in Rhenium-Doped MoS2 Nanoparticles

We investigated the optical properties of rhenium-doped MoS2 nanoparticles and compared our findings with the pristine and bulk analogues. Our measurements reveal that confinement softens the exciton positions and reduces spin-orbit coupling, whereas doping has the opposite effect. We model the carrier-induced exciton blue shift in terms of the Burstein-Moss effect. These findings are important for understanding doping and finite length scale effects in low-dimensional nanoscale materials.

Lubricating Medical Devices with Fullerene-Like Nanoparticles

In the present work, MoS2 nanoparticles with fullerene-like structure, and most particularly those doped with minute amounts of rhenium atoms, are used as additive to medical gels in order to facilitate their entry into constricted openings of soft material rings. This procedure is used to mimic the entry of endoscopes to constricted openings of the human body, like urethra, etc. It is shown that the Re-doped nanoparticles reduce the traction force used to retrieve the metallic lead of the endoscope from the soft ring by a factor close to three times with respect to the original gel. The mechanism of the mitigation of both friction and adhesion forces in these systems by the nanoparticles is discussed.

Synthesis of core–shell single-layer MoS2 sheathing gold nanoparticles, AuNP@1L-MoS2

Nanoparticles, and more specifically gold nanoparticles (AuNPs), have attracted much scientific and technological interest in the last few decades. Their popularity is attributed to their unique optical, catalytic, electrical and magnetic properties when compared with the bulk. However, one of the main problems with AuNPs is their long-term stability. Two-dimensional materials like MoS2 (WS2) are semiconductors that exhibit a combination of properties which make them suitable for electronic, optical and (photo)catalytic devices. Few-layer MoS2 (WS2) nanoparticles (NPs), and in particular single-layer ones, show intriguing optical and electrical properties which are very different from those of the bulk compounds. Here we demonstrate the synthesis of AuNPs sheathed by a single layer of MoS2 (WS2), i.e. a core-shell nanostructure (AuNP@1L-MoS2). The hybrid NPs exhibit optical properties that are different from those of either constituent and are amenable for modulation via their chemistry, offering a myriad of applications.

Optical properties of multilayer films of nanocomposites based on WS2 nanotubes decorated with gold nanoparticles

Multilayer films of WS2 nanotubes decorated with gold nanoparticles are prepared for the first time using nanocomposite assemblage on the water-heptane interface and film transition onto optically transparent or semiconducting surfaces. The film morphology resembles a mosaic structure of 10-25 square micron areas with in-plane textured nanotubes. Optical properties demonstrate several features around 490, 545, and 675 nm connected either with excitonic transitions or scattering guided by high anisotropy of the nanotubes and their texturing peculiarities in the films.