Dr. Lena Yadgarov
School of Physics and Astronomy, School of Chemistry, Tel Aviv University ,
Inorganic layered compounds like WS2 or MoS2, with low dimensionality have strong (covalent) bonds in the layer (a-b plane) and weak van der Waals forces along the c-axis hold the layers together. Hence, the high energy stored in the dangling bonds at the periphery of these nanoscopic sheets may induce formation of closed-caged inorganic nanotubes and fullerene-like nanoparticles (INT and IF) . Due to their unique properties and promising applications, the study of these nanostructures is a rapidly growing field.
In the present work, the existing synthetic methods for production of INT-WS2 method was substantially modified and improved [2-5]. Specifically, WO3-2.82 nanoparticles (NP) were inserted into horizontal furnace in H2S under reducing atmosphere. The yield in this case was ~ 90% and the entire process is quite simple and short (~ 2 h). Interestingly, after careful study of the synthetic proses, it was found that by slight changes of the conditions, such as temperature, pressure or gas flows rate, one can get a variety of different nanostructures (i.e. WO2.78 nanowires, WO2.78@WS2core shell nanowires, WS2 scrolls, IF nanoparticles, thin sheets, spear-like nanotubes and more).
Controllable electronic and optical properties are of great importance for fabrication of nanoscale devices and multifunctional materials. Since the IF/INT-MS2 (M=Mo, W) are semiconductors, their properties can be controlled by doping. Indeed, it was shown that doping of the IF/INT with a few hundred ppm of Re atoms leads to their increased conductivity and to substantial decrease in friction and wear . Study of the optical properties of the Re doped IF-MoS2 revealed that these NPs undergo Burstein-Moss optical shift due to band filling .
Inspired by the fascinating properties of the doped IF/INT, the optical and electronic properties of the undoped counterparts were reconsidered. Herein, it was found that the IF-MoS2, INT-WS2 maintains not only the excitonic structure of the bulk, but also the plasmonic scattering (which does not exist in the bulk). Furthermore, the interaction of INT with light is dominated by a plexcitonic state, i.e. exhibiting strong exciton-surface plasmon coupling . The plasmonic resonance occurs in the near-infrared spectral region, such that its higher energy part overlaps the A and B excitonic features of the semiconductor. This finding is of particular interest since it reveals this fundamental electromagnetic phenomenon in an individual nanotube, rather than in a hybrid system, like Au NP coupled to CdSe quantum dots. The strong light-matter interaction in the IF/INT-MS2 suggests that they can operate as a plasmonic device at room temperature, which opens up new possibilities for their applications, such as saturable absorbers in a mode-locked laser, optical tracking during medical diagnostics (or drug delivery) and more.