Cathodoluminescence in single and multiwall WS2 nanotubes: Evidence for quantum confinement and strain effect

Abstract

For nanoparticles with sub-10 nm diameter, the electronic bandgap becomes size dependent due to quantum confinement; this, in turn, affects their electro-optical properties. Thereby, MoS 2 and WS 2 monolayers acquire luminescent capability, due to the confinement-induced indirect-to-direct bandgap transition. Rolling up of individual layers results in single wall inorganic nanotubes (SWINTs). Up to the present study, their luminescence properties were expected to be auspicious but were limited to theoretical investigations only, due to the scarcity of SWINTs and the difficulties in handling them. By optimizing the conditions in the plasma reactor, relatively high yields of WS 2 SWINTs 3-7 nm in diameter were obtained in this work, compared to previous reports. A correlative approach, transmission electron microscopy coupled with a scanning electron microscope, was adapted to overcome handling obstacles and for testing individual nanotubes by lowtemperature cathodoluminescence. Clear cathodoluminescence spectra were obtained from WS 2 -SWINTs and compared with those of WS 2 multiwall nanotubes and the corresponding bulk material. Uniquely, the optical properties of INTs acquired from cathodoluminescence were governed by the opposite impact from quantum size effect and strain in the bent triple S-W-S layers. The experimental findings were confirmed by the Density Functional and Time-Dependent Density Functional theoretical modeling of monolayer and bilayer nanotubes of different chiralities and diameters. This study provides experimental evidence of the quantum confinement effect in WS 2 SWINTs akin to WS 2 monolayer. The ability to tune the electronic structure with morphology or number of layers may be exploited toward photoelectrochemical water splitting with WS 2 catalysts, devising field effect transistors, photodetectors, and so on.

Nanotubes from Ternary WS2(1–x)Se2x Alloys: Stoichiometry Modulated Tunable Optical Properties

“…Furthermore, the folding energy of WS 2 and MoS 2 plane is about 10 times larger than that of the graphitic layers into carbon nanotubes . To comply with the large elastic energy of folding, WS 2 (MoS 2 ) nanotubes adopt larger radii than their carbon counterparts and generally come in multiwall structures; however, single-walled WS 2 nanotubes were generated by high energy plasma radiation …”

“…6 To comply with the large elastic energy of folding, WS 2 (MoS 2 ) nanotubes adopt larger radii than their carbon counterparts and generally come in multiwall structures; 5 however, single-walled WS 2 nanotubes were generated by high energy plasma radiation. 7 Several chemical strategies were conceived to facilitate the folding of the layers and form pure phases of nanotubes from inorganic layered compounds. One important strategy is nanotemplating.…”

MoS2 and WS2 Nanotubes: Synthesis, Structural Elucidation, and Optical Characterization

“…39 In a recent study, cathodoluminescence (CL) of single-and multiwall WS 2 nanotubes was investigated. 40 While the CL signal at room temperature was extremely weak and marred by the noise, strong spectrally resolved CL was obtained at −140 °C. Clear blue shifts of the spectral features associated with the A and B excitons were observed in the singlewall tubes compared to multiwall nanotubes.…”

“…Strong light–matter coupling was also observed in ∼60 nm thick WS 2 nanodisks . In a recent study, cathodoluminescence (CL) of single- and multiwall WS 2 nanotubes was investigated . While the CL signal at room temperature was extremely weak and marred by the noise, strong spectrally resolved CL was obtained at −140 °C.…”

WS2 Nanotube-Embedded SiOC Fibermat Electrodes for Sodium-Ion Batteries

“…Examples include nanoflakes, nanosheets, nanohoneycombs, nanotubes, and nanoflowers of WS 2 to enhance the performance of energy storage devices, specifically lithium-ion batteries (LIBs). Unique structures of WS 2 have rarely been studied for sodium-ion batteries (NIBs). , Although their counterparts, namely MoS 2 nanotubes, have been extensively studied for electrochemical energy storage, electrochemical hydrogen storage, and used as field effect transistors, up-to-date WS 2 nanotubes (WS 2 NTs) have only been studied in LIBs and NIBs to a muchlesser extent, even though they establish stable cycling over a wide voltage range. ,, Theoretical calculations indicated that single-wall WS 2 NTs are an exclusive type of semiconductor with an explicit band gap ranging from 0.1 to 1.9 eV, depending on their diameter. , They become both direct (in the case of a zigzag) and indirect (in the case of an armchair) band gap semiconductors controlled by chirality, making them useful for various technological applications such as next-generation electronic and optoelectronic devices and high-quality factor nanoelectromechanical systems. , However, the electronic properties of multiwall WS 2 NTs, which are currently produced in large amounts, are not that different from those of the bulk material, that is, having an indirect band gap of ∼1.9 eV . Such a small bandgap make them suitable for the next generation electrochemical energy storage devices.…”