Growth Control of WS2: From 2D Layer by Layer to 3D Vertical Standing Nanowalls

Wafer-scale controlled growth of MoS2by magnetron sputtering: from in-plane to inter-connected vertically-aligned flakes

Recently, Molybdenum disulfide (MoS₂) has attracted great attention due to its unique characteristics and potential applications in various fields. The advancements in the field have substantially improved at the laboratory scale however, a synthesis approach that produces large area growth of MoS₂on a wafer scale is the key requirement for the realization of commercial two-dimensional (2D) technology. Herein, we report tunable MoS₂growth with varied morphologies via radio frequency magnetron sputtering by controlling growth parameters. The controlled growth from in-plane to vertically-aligned (VA) MoS₂flakes has been achieved on a variety of substrates (Si, Si/SiO₂, sapphire, quartz, and carbon fiber). Moreover, the growth of VA MoS₂is highly reproducible and is fabricated on a wafer scale. The flakes synthesized on the wafer show high uniformity, which is corroborated by the spatial mapping using Raman over the entire 2-inch Si/SiO₂wafer. The detailed morphological, structural, and spectroscopic analysis reveals the transition from in-plane MoS₂to VA MoS₂flakes. This work presents a facile approach to directly synthesize layered materials by sputtering technique on wafer scale. This paves the way for designing mass production of high-quality 2D materials, which will advance their practical applications by integration into device architectures in various fields.

Tailoring the Vertical and Planar Growth of 2D WS2 Thin Films Using Pulsed Laser Deposition for Enhanced Gas Sensing Properties

In this study, pulsed laser deposition has been utilized for the controllable synthesis of WS₂ thin films with growth orientation ranging from vertically to horizontally aligned layers, and the effect of growth parameters has been investigated. The growth of thin films on SiO₂ substrates at three different pressures (30, 50, and 70 mTorr) and three different temperatures (400, 500, and 600 °C) has been studied. Detailed characterizations carried out on the as-grown layers clearly show the formation of the 2H-WS₂ phase and its morphological evolution with deposition conditions. Atomic force microscopy and cross-sectional transmission electron microscopy have been used to deduce the growth mechanism of the vertical and planar films with different deposition parameters. The samples grown with a combination of lower temperatures and higher pressures exhibit a vertical flake-like growth with a flake thickness of ∼2-5 nm. However, at higher temperatures and lower pressures, the film growth is observed to be rather planar. The gas sensing parameters and the underlying mechanism have been observed to be quite different for vertically and horizontally grown layers. The vertical layers showed a selective response toward NO₂ gas at room temperature (RT) with a limit of detection less than 50 ppb. In comparison, a very subdued and poor gas sensing response was recorded for the planar film at RT. A large specific area and abundance of active edge sites along with the flat basal plane present in the vertically grown layers seem to be responsible for efficient gas sensing toward NO₂.

Highly Responsive Pd-Decorated MoO3 Nanowall H2 Gas Sensors Obtained from In-Situ-Controlled Thermal Oxidation of Sputtered MoS2 Films

Among transition metal oxides, MoO₃ is a promising material due to its layered structure and different oxidation states, making it suitable for different device applications. One of the methods used to grow MoO₃ is radio frequency magnetron sputtering (RFMS), which is the most compatible method in industry. However, obtaining nanostructures by RFMS for metal oxides is challenging because of compact morphology film formation. In this study, α-MoO₃ with vertical nanowalls is obtained by a two-step process; deposition of magnetron-sputtered MoS₂ vertical nanowalls and postoxidation of these structures without changing the morphology. In situ transmittance and electrical measurements are performed to control the oxidation process, which shed light on understanding the oxidation of MoS₂ nanowalls. The transition from MoS₂ to α-MoO₃ is investigated with partially oxidized MoS₂/MoO₃ samples with different thicknesses. It is also concluded that oxidation starts from nanowalls perpendicular to the substrate and lasts with oxidation of basal planes. Four different thicknesses of α-MoO₃ nanowall samples are fabricated for H₂ gas sensors. Also, the effect of Pd deposition on the H₂-sensing properties of sensors is deeply investigated. An outstanding response of 3.3 × 10⁵ as well as the response and recovery times of 379 and 304 s, respectively, are achieved from the thinnest Pd-loaded sample. Also, the gas-sensing mechanism is explored by gasochromic measurements to investigate the sensor behaviors under the conditions of dry air and N₂ gas as the carrier gas.

Wafer-sized WS2 monolayer deposition by sputtering

We demonstrate that tungsten disulphide (WS₂) with thicknesses ranging from monolayer (ML) to several monolayers can be grown on SiO₂/Si, Si, and Al₂O₃ by pulsed direct current-sputtering. The presence of high quality monolayer and multilayered WS₂ on the substrates is confirmed by Raman spectroscopy since the peak separations between the A_1g-E_2g and A_1g-2LA vibration modes exhibit a gradual increase depending on the number of layers. X-ray diffraction confirms a textured (001) growth of WS₂ films. The surface roughness measured with atomic force microscopy is between 1.5 and 3 Å for the ML films. The chemical composition WS_x (x = 2.03 ± 0.05) was determined from X-ray Photoelectron Spectroscopy. Transmission electron microscopy was performed on a multilayer film to show the 2D layered structure. A unique method for growing 2D layers directly by sputtering opens up the way for designing 2D materials and batch production of high-uniformity and high-quality (stochiometric, large grain sizes, flatness) WS₂ films, which will advance their practical applications in various fields.

Abnormal temperature-dependent photoluminescence characteristics of ReS2nanowalls

Two samples with [001] orientated rhenium disulfide (ReS₂) nanowalls (NWs) grown above and in front of precursor (NH₄ReO₄) by chemical vapor deposition were investigated. The temperature-dependent photoluminescence (PL) indicated that the PL peak exhibited linear blue-shift at a rate of ∼0.24 meV K^-1with increasing the temperature from 10 to 300 K, while the linewidth monotonically increased due to the exciton-phonon interaction. This abnormal blue-shift of PL emission energy, which is explained by a competition between the band gap shrinkage and the energy level degeneracy with respect to the increase of temperature and lattice constant, enables ReS₂NWs to possess great potential for development of thermal sensors. In addition, exciton localization effect in the ReS₂NWs from abundant edges and weak interlayer interaction was also observed to be related to the height and density of ReS₂NWs. These results not only enrich the understanding for exciton dynamics in ReS₂NWs, but also help to exploit ReS₂NWs for device applications.