A general methodology that utilizes confined mesoporous silica as template for preparing highly ordered mesostructured nanowires and nanowire arrays is developed. The prepared Ag, Ni, and Cu 2 O nanowires, with unprecedented mesostructures of coaxially multilayered helical, and stacked-donut structures, have the unique features of hierarchical organization, modulated surface morphology, high surface area, and chirality. Surface-enhanced Raman spectra from a silver mesostructured-nanowire bundle are presented.
This study solves a more than two-decades-long "MoS 2 Nanotubes" synthetic enigma: the futile attempts to synthesize inorganic nanotubes (INTs) of MoS 2 via vapor−gas−solid (VGS) reaction. Among them was replication of the recently reported pure-phase synthesis of the analogous INT-WS 2 . During these years, successful syntheses of spherical nanoparticles of WS 2 and MoS 2 were demonstrated as well. All these nanostructures were obtained by VGS reaction of corresponding oxides with H 2 /H 2 S gases, at elevated temperatures (>800 °C), in a fluidized bed reactor (FBR) and a one-pot process. This success and apparent similarity between the two compounds "hid" from us the option of looking for the INT-MoS 2 reaction parameters in entirely different regimes. The main challenge in the synthesis of INT-MoS 2 via VGS was the instability of the in situ prepared suboxide nanowhiskers against over-reduction and recrystallization at high temperatures. The elucidated growth mechanism dictates separation of the reaction into five steps, as properties of the intermediate products are not consistent with a single process and require individual conditions for each step. A horizontal reactor with a porous-quartz reaction cell, which creates proper quasi-static (contrary to the FBR) conditions for the reaction involving sublimation, was imperative for the effective nanofabrication of INT-MoS 2 . These findings render a reproducible synthetic route for the production of highly crystalline pure-phase MoS 2 nanotubes via a multistep VGS process, without the assistance of a catalyst and in a scalable fashion. Being a semiconductor, flexible, and strong, INT-MoS 2 offers a platform for much research and numerous potential applications, particularly in the field of optoelectronics and reinforcement of polymer composites.
We present a comprehensive multiphonon Raman and complementary infrared analysis for bulk and monolayer MoS 2 . For the bulk the analysis consists of symmetry assignment from which we obtain a broad set of allowed second-order transitions at the high symmetry M, K and Γ Brillouin zone (BZ) points. The attribution of about 80 transitions of up to fifth order processes are proposed in the low temperature (95 K) resonant Raman spectrum measured with excitation energy of 1.96 eV, which is slightly shifted in energy from the A exciton. We propose that the main contributions come from four phonons:The last three are single degenerate phonons at M with an origin of the E 1 2g (Γ) and E 2 2g (Γ) phonons. Among the four phonons, we identify in the resonant Raman spectra all (but one) of the second-order overtones, combination and difference-bands and many of the third order bands. Consistent with the expectation that at the M point only combinations with the same inversion symmetry (g or u) are Raman-allowed, the contribution of combinations with the LA(M) mode can not be considered with the above four phonons. Although minor, contributions from K point and possibly Γ-point phonons are also evident. The '2LA band', measured at ~ 460 cm -1 is reassigned. Supported by the striking similarity between this band, measured under off-resonant conditions, and recently published two phonon density of states, we propose that the lower part of the band, previously attributed to 2LA(M), is due to a van Hove singularity between K and M. The higher part, previously attributed exclusively to the A 2u (Γ) phonon, is mostly due to the LA and LA' phonons at M. For the monolayer MoS 2 the second-order phonon processes from the M and Γ Brillouin zone points are also analyzed and are discussed within similar framework to that of the bulk.2
The chemistry of methyl bromide on Ru(001) has been studied utilizing work function change (Δφ) measurements and temperature-programmed desorption (TPD) in the crystal temperature range of 82−1350 K. Employing a Δφ-TPD mode, chemical changes in the adsorbed state could be detected at temperatures below the onset for desorption. A decrease in work function of 2.15 ± 0.02 V has been measured at the completion of a monolayer coverage, which has been determined to consist of (3.6 ± 0.3) × 1014 molecules/cm2, equivalent to CH3Br/Ru = 0.22 ± 0.02. The onset for C−Br bond cleavage near 125 K was observed. 50% of the initial 1 monolayer methyl bromide molecules decompose to adsorbed methyl and bromine. A low-temperature increase in work function was found to precede dissociation or desorption as coverage increases. This change in work function is discussed in terms of several possible mechanisms, including multiple sites population, molecular rearrangement, and tilt angle, that change with coverage and surface temperature. A thermally activated flipping mechanism in which a fraction of the adsorbate molecules rearrange to adsorb with the methyl group facing the surface is found to be most consistent with the observed results. Sequential dehydrogenation of the methyl fragments, competing with minor methane production at higher coverages, was directly observed by employing the differential work function measurements. The corresponding surface temperature window for each of these decomposition steps has been determined, and the detailed reaction mechanism is discussed. Bromine atoms on Ru(001) were found to decrease the work function by 320 mV at a coverage Br/Ru = 0.3, indicating a complex charge redistribution upon adsorption. Deuterium preadsorption, which significantly passivates the surface, has been employed to better understand the various reactivity steps of the hydrocarbon fragments. Finally, work function measurements indicate the presence of strong interactions of the methyl bromide molecules with the metal surface up to the third layer. Alternating contributions to the work function of the first three layers are observed. This is understood in terms of an opposite adsorption geometry in which bromine faces the surface in the first layer, methyl in the second, and bromine again in the third. Upon heating, the third and fourth layers rearrange in a bilayer-like structure before the completion of the fourth layer, leading to higher stability of the combined two layers compared with the third alone. This structure is rather similar to that of methyl bromide in its molecular crystal.
Cobalt-ferrite nanocrystals were synthesized using a high-temperature organometallic decomposition scheme in the presence of surfactant molecules. The influence of the addition of cosurfactant molecules of polyol type on the resulting nanocrystals was examined. The properties of the nanocrystals were studied using electron microscopy, and magneto-optical and Raman spectroscopies. The addition of the cosurfactants was found to influence the growth mechanism of the nanocrystals, resulting in a significant reduction in the concentration of the Co 2+ ions incorporated into the ferrite lattice up to a factor of 4 and an increase in the size of the synthesized nanocrystals. In addition, control over the occupation of octahedral versus tetrahedral coordination sites by the cobalt ions was demonstrated.
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