Nanotubes from Transition Metal Dichalcogenides: Recent Progress in the Synthesis, Characterization and Electrooptical Properties.

Inorganic layered compounds (2D-materials), particularly transition metal dichalcogenide (TMDC), are the focus of intensive research in recent years. Shortly after the discovery of carbon nanotubes (CNTs) in 1991, it was hypothesized that nanostructures of 2D-materials can also fold and seam forming, thereby nanotubes (NTs). Indeed, nanotubes (and fullerene-like nanoparticles) of WS2 and subsequently from MoS2 were reported shortly after CNT. However, TMDC nanotubes received much less attention than CNT until recently, likely because they cannot be easily produced as single wall nanotubes with well-defined chiral angles. Nonetheless, NTs from inorganic layered compounds have become a fertile field of research in recent years. Much progress has been achieved in the high-temperature synthesis of TMDC nanotubes of different kinds, as well as their characterization and the study of their properties and potential applications. Their multiwall structure is found to be a blessing rather than a curse, leading to intriguing observations. This concise minireview is dedicated to the recent progress in the research of TMDC nanotubes. After reviewing the progress in their synthesis and structural characterization, their contributions to the research fields of energy conversion and storage, polymer nanocomposites, andunique optoelectronic devices are being reviewed. These studies suggest numerous potential applications for TMDC nanotubes in various technologies, which are briefly discussed.

Highly Stable CsPbBr3@MoS2 Nanostructures: Synthesis and Optoelectronic Properties Toward Implementation into Solar Cells.

Halide perovskites (HPs) have gained significant interest in the scientific and technological sectors due to their unique optical, catalytic, and electrical characteristics. However, the HPs are prone to decomposition when exposed to air, oxygen, or heat. The instability of HP materials limits their commercialization, prompting significant efforts to address and overcome these limitations. Transition metal dichalcogenides, such as MoS2, are chemically stable and are suitable for electronic, optical, and catalytic applications. Moreover, it can be used as a protective media or shell for other nanoparticles. In this study, a novel CsPbBr3@MoS2 core-shell nanostructure (CS-NS) is successfully synthesized by enveloping CsPbBr3 within a MoS2 shell for the first time. Significant stability of CS-NSs dispersed in polar solvents for extended periods is also demonstrated. Remarkably, the hybrid CS-NS exhibits an absorption of MoS2 and quenching of the HP's photoluminescence, implying potential charge or energy transfer from HPs to MoS2. Using finite difference time domain simulations, it is found that the CS-NSs can be utilized to produce efficient solar cells. The addition of a MoS2 shell enhances the performance of CS-NS-based solar cells by 220% compared to their CsPbBr3 counterparts. The innovative CS-NS represents important progress in harnessing HPs for photovoltaic and optoelectronic applications.

Size-Dependent Control of Exciton-Polariton Interactions in WS2 Nanotubes.

Multiwall WS2 nanotubes (and fullerene-like nanoparticles thereof) are currently synthesized in large amounts, reproducibly. Other than showing interesting mechanical and tribological properties, which offer them a myriad of applications, they are recently shown to exhibit remarkable optical and electrical properties, including quasi-1D superconductivity, electroluminescence, and a strong bulk photovoltaic effect. Here, it is shown that, using a simple dispersion-fractionation technique, one can control the diameter of the nanotubes and move from pure excitonic to polaritonic features. While nanotubes of an average diameter >80 nm can support cavity modes and scatter light effectively via a strong coupling mechanism, the extinction of nanotubes with smaller diameter consists of pure absorption. The experimental work is complemented by finite-difference time-domain simulations, which shed new light on the cavity mode-exciton interaction in 2D materials. Furthermore, transient absorption experiments of the size-fractionated nanotubes fully confirm the steady-state observations. Moreover, it is shown that the tools developed here are useful for size control of the nanotubes, e.g., in manufacturing environment. The tunability of the light-matter interaction of such nanotubes offers them intriguing applications such as polaritonic devices, in photocatalysis, and for multispectral sensors.

Strong light-matter interaction in tungsten disulfide nanotubes.

Transition metal dichalcogenide materials have recently been shown to exhibit a variety of intriguing optical and electronic phenomena. Focusing on the optical properties of semiconducting WS2 nanotubes, we show here that these nanostructures exhibit strong light-matter interaction and form exciton-polaritons. Namely, these nanotubes act as quasi 1-D polaritonic nano-systems and sustain both excitonic features and cavity modes in the visible-near infrared range. This ability to confine light to subwavelength dimensions under ambient conditions is induced by the high refractive index of tungsten disulfide. Using "finite-difference time-domain" (FDTD) simulations we investigate the interactions between the excitons and the cavity mode and their effect on the extinction spectrum of these nanostructures. The results of FDTD simulations agree well with the experimental findings as well as with a phenomenological coupled oscillator model which suggests a high Rabi splitting of ∼280 meV. These findings open up possibilities for developing new concepts in nanotube-based photonic devices.

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.

Spectroscopic determination of phonon lifetimes in rhenium-doped MoS2 nanoparticles.

We investigated the infrared vibrational properties of pristine and Re-substituted MoS2 nanoparticles and analyzed the extracted phonon lifetimes in terms of multiple scattering events. Our measurements reveal both size- and doping-dependent changes that we attribute to grain boundary scattering and charge and mass effects, respectively. By contrast, Born charge is affected only by size. These findings illustrate the utility of reaching beyond traditional bulk semiconductors and quantum dots to explore how doping and confinement impact carrier-phonon interactions in low-dimensional semiconducting nanomaterials.

Mechanism of WS2 Nanotube Formation Revealed by in Situ/ex Situ Imaging.

Multiwall WS2 nanotubes have been synthesized from W18O49 nanowhiskers in substantial amounts for more than a decade. The established growth model is based on the "surface-inward" mechanism, whereby the high-temperature reaction with H2S starts on the nanowhisker surface, and the oxide-to-sulfide conversion progresses inward until hollow-core multiwall WS2 nanotubes are obtained. In the present work, an upgraded in situ SEM µReactor with H2 and H2S sources has been conceived to study the growth mechanism in detail. A hitherto undescribed growth mechanism, named "receding oxide core", which complements the "surface-inward" model, is observed and kinetically evaluated. Initially, the nanowhisker is passivated by several WS2 layers via the surface-inward reaction. At this point, the diffusion of H2S through the already existing outer layers becomes exceedingly sluggish, and the surface-inward reaction is slowed down appreciably. Subsequently, the tungsten suboxide core is anisotropically volatilized within the core close to its tips. The oxide vapors within the core lead to its partial out-diffusion, partially forming a cavity that expands with reaction time. Additionally, the oxide vapors react with the internalized H2S gas, forming fresh WS2 layers in the cavity of the nascent nanotube. The rate of the receding oxide core mode increases with temperatures above 900 °C. The growth of nanotubes in the atmospheric pressure flow reactor is carried out as well, showing that the proposed growth model (receding oxide core) is also relevant under regular reaction parameters. The current study comprehensively explains the WS2 nanotube growth mechanism, combining the known model with contemporary insight.

Plasmon-Enhanced Perovskite Solar Cells Based on Inkjet-Printed Au Nanoparticles Embedded into TiO2 Microdot Arrays.

The exceptional property of plasmonic materials to localize light into sub-wavelength regimes has significant importance in various applications, especially in photovoltaics. In this study, we report the localized surface plasmon-enhanced perovskite solar cell (PSC) performance of plasmonic gold nanoparticles (AuNPs) embedded into a titanium oxide (TiO2) microdot array (MDA), which was deposited using the inkjet printing technique. The X-ray (XRD) analysis of MAPI (methyl ammonium lead iodide) perovskite films deposited on glass substrates with and without MDA revealed no destructive effect of MDA on the perovskite structure. Moreover, a 12% increase in the crystallite size of perovskite with MDA was registered. Scanning electron microscopy (SEM) and high-resolution transmission electron microscopy (HR-TEM) techniques revealed the morphology of the TiO2_MDA and TiO2-AuNPs_MDA. The finite-difference time-domain (FDTD) simulation was employed to evaluate the absorption cross-sections and local field enhancement of AuNPs in the TiO2 and TiO2/MAPI surrounding media. Reflectance UV-Vis spectra of the samples comprising glass/TiO2 ETL/TiO2_MDA (ETL-an electron transport layer) with and without AuNPs in TiO2_MDA were studied, and the band gap (Eg) values of MAPI have been calculated using the Kubelka-Munk equation. The MDA introduction did not influence the band gap value, which remained at ~1.6 eV for all the samples. The photovoltaic performance of the fabricated PSC with and without MDA and the corresponding key parameters of the solar cells have also been studied and discussed in detail. The findings indicated a significant power conversion efficiency improvement of over 47% in the PSCs with the introduction of the TiO2-AuNPs_MDA on the ETL/MAPI interface compared to the reference device. Our study demonstrates the significant enhancement achieved in halide PSC by utilizing AuNPs within a TiO2_MDA. This approach holds great promise for advancing the efficiency and performance of photovoltaic devices.

Improving the Stability of Halide Perovskite Solar Cells Using Nanoparticles of Tungsten Disulfide.

Halide perovskites-based solar cells are drawing significant attention due to their high efficiency, versatility, and affordable processing. Hence, halide perovskite solar cells have great potential to be commercialized. However, the halide perovskites (HPs) are not stable in an ambient environment. Thus, the instability of the perovskite is an essential issue that needs to be addressed to allow its rapid commercialization. In this work, WS2 nanoparticles (NPs) are successfully implemented on methylammonium lead iodide (MAPbI3) based halide perovskite solar cells. The main role of the WS2 NPs in the halide perovskite solar cells is as stabilizing agent. Here the WS2 NPs act as heat dissipater and charge transfer channels, thus allowing an effective charge separation. The electron extraction by the WS2 NPs from the adjacent MAPbI3 is efficient and results in a higher current density. In addition, the structural analysis of the MAPbI3 films indicates that the WS2 NPs act as nucleation sites, thus promoting the formation of larger grains of MAPbI3. Remarkably, the absorption and shelf life of the MAPbI3 layers have increased by 1.7 and 4.5-fold, respectively. Our results demonstrate a significant improvement in stability and solar cell characteristics. This paves the way for the long-term stabilization of HPs solar cells by the implementation of WS2 NPs.

Beneficial effect of Re doping on the electrochemical HER activity of MoS2 fullerenes.

Electrochemical generation of hydrogen by non-precious metal electrocatalysts at a lower overpotential is a focus area of research directed towards sustainable energy. The exorbitant costs associated with Pt-based catalysts is the major bottleneck associated with commercial-scale hydrogen generation. Strategies for the synthesis of cost-effective and stable catalysts are thus key for a prospective 'hydrogen economy'. In this report, we highlight a novel and general strategy to enhance the electrochemical activity of molybdenum disulfide (MoS2) in a fullerene structure (IF-). In particular, pristine (undoped) and rhenium-doped nanoparticles of MoS2 with fullerene-like structures (IF-MoS2) were studied, and their performance as catalysts for the hydrogen evolution reaction (HER) was compared to that of 2H-MoS2 particles (platelets). The current density of the IF-MoS2 was higher by one order of magnitude than that of few-layer (FL-) MoS2, due to the enhanced density of the edge sites. Furthermore, Re doping of as low as 100 ppm in IF-MoS2 decreased the onset potential by 60-80 mV and increased the activity by 60 times compared with that of the FL-MoS2. The combined synergistic effect of Re doping and the IF structure not only changes the intrinsic nature of the MoS2 but also increases its reactivity. This strategy highlights the potential use of the IF structure and Re doping in electrocatalytic hydrogen evolution using MoS2-based catalysts.

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.