Local Optical Properties in CVD-Grown Monolayer WS2 Flakes

Unveiling the power of MXenes: Solid lubrication perspectives and future directions

The interaction between two surfaces leads to the generation of friction and wear of material. Friction and wear are some of the major challenges that may readily be overcome by the third part of tribology called lubrication. Utilizing solid lubricants including polymers, carbon-based materials, soft metals, transition metal dichalcogenides, along with their potential benefits and drawbacks in dry environments can reduce friction. Recently, an emerging class of two-dimensional (2D) transition metal nitrides, carbides or carbonitrides commonly known as MXenes have emerged as an attractive alternative for solid lubrication because of their ability to establish wear-resistant tribo layers and well as low friction and shear strength. Furthermore, the inherent hydrophilic nature of these substances has led to limited dispersion stability and phase compatibility when combined with pure base oils. As a result, their potential use as solid lubricants and lubricant additives has been impeded. To address this issue and enhance the applicability of MXenes as solid lubricants, their surface modification can be an attractive tool. Therefore, this review provides a succinct summary of the current state-of-the-art in surface functionalization of MXenes, a subject that has not yet been thoroughly addressed. Further, the mechanical behavior of MXenes and composites has been discussed, followed by the potential of MXenes as a solid lubricant at micro- and macro-scale. Finally, the existing opportunities and challenges of the research area have been discussed with possible future research directions. We believe, this article will be a valuable resource for MXenes and opens the door to improve the chemical, physical and mechanical properties of MXenes in various applications, such as solid lubrication.

Determination and investigation of defect domains in multi-shape monolayer tungsten disulfide

Single-layer tungsten disulfide (WS2) is among the most widely investigated two-dimensional materials. Synthesizing it over large areas would enable the exploitation of its appealing optical and electronic properties in industrial applications. However, defects of different nature, concentration and distribution profoundly affect the optical as well as the electronic properties of this crystal. Controlling the defect density distribution can be an effective way to tailor the local dielectric environment and therefore the electronic properties of the system. In this work we investigate the defects in single-layer WS2, grown in different shapes by liquid phase chemical vapor deposition, where the concentration of certain defect species can be controlled by the growth conditions. The properties of the material are surveyed by means of optical spectroscopy, photoelectron spectroscopy and Kelvin probe force microscopy. We determine the chemical nature of the defects and study their influence on the optical and electronic properties of WS2. This work contributes to the understanding of the microscopic nature of the intrinsic defects in WS2, helping the development of defect-based technologies which rely on the control and engineering of defects in dielectric 2D crystals.

Fast Twist Angle Mapping of Bilayer Graphene Using Spectroscopic Ellipsometric Contrast Microscopy

Twisted bilayer graphene provides an ideal solid-state model to explore correlated material properties and opportunities for a variety of optoelectronic applications, but reliable, fast characterization of the twist angle remains a challenge. Here we introduce spectroscopic ellipsometric contrast microscopy (SECM) as a tool for mapping twist angle disorder in optically resonant twisted bilayer graphene. We optimize the ellipsometric angles to enhance the image contrast based on measured and calculated reflection coefficients of incident light. The optical resonances associated with van Hove singularities correlate well to Raman and angle-resolved photoelectron emission spectroscopy, confirming the accuracy of SECM. The results highlight the advantages of SECM, which proves to be a fast, nondestructive method for characterization of twisted bilayer graphene over large areas, unlocking process, material, and device screening and cross-correlative measurement potential for bilayer and multilayer materials.

Local dielectric function of hBN-encapsulated WS2flakes grown by chemical vapor deposition

Hexagonal boron nitride (hBN), sometimes referred to as white graphene, receives growing interest in the scientific community, especially when combined into van der Waals (vdW) homo- and heterostacks, in which novel and interesting phenomena may arise. hBN is also commonly used in combination with two-dimensional (2D) semiconducting transition metal dichalcogenides (TMDCs). The realization of hBN-encapsulated TMDC homo- and heterostacks can indeed offer opportunities to investigate and compare TMDC excitonic properties in various stacking configurations. In this work, we investigate the optical response at the micrometric scale of mono- and homo-bilayer WS₂grown by chemical vapor deposition and encapsulated between two single layers of hBN. Imaging spectroscopic ellipsometry is exploited to extract the local dielectric functions across one single WS₂flake and detect the evolution of excitonic spectral features from monolayer to bilayer regions. Exciton energies undergo a redshift by passing from hBN-encapsulated single layer to homo-bilayer WS₂, as also confirmed by photoluminescence spectra. Our results can provide a reference for the study of the dielectric properties of more complex systems where hBN is combined with other 2D vdW materials into heterostructures and are stimulating towards the investigation of the optical response of other technologically-relevant heterostacks.

Heteroatomic stitching of broken WS2 monolayer with enhanced surface potential

Sub-nanometre thick two-dimensional (2D) materials suffer from severe cracking during the high temperature chemical vapour deposition growth process. The cracking can be utilised to generate more active edges. These active edges can be stitched with a homo- or hetero-material. While the direct growth of 2D-heterostructures is mostly limited to a small fraction of outer edges of monolayer flakes, the cracked monolayers can be utilised to produce a large fraction of heterostructures. Heterostructures are important for developing multifunctional components for nanoscale electronics and optoelectronics. In this work, we demonstrate the formation of WS₂-MoS₂ heterostructures in large fractions by atomic stitching of cracked WS₂ monolayers with the sequential growth of MoS₂. Aberration-corrected scanning transmission electron microscopy and Raman spectroscopy have been utillised to probe fine details on the stitched interface between WS₂ and MoS₂. Growth of MoS₂ domains were observed to take place at the terminating edge of cracked WS₂, which then merged to form MoS₂ multilayers at the cracked site. Growth of MoS₂ at the opposite edges of WS₂ eventually results in a MoS₂-MoS₂ junction with a Σ = 7 tilt boundary. Kelvin probe force microscope (KPFM) measurement from the stitched region revealed that the work function of WS₂ is ∼32.46 meV higher than that of MoS₂, which also closely matches with the Fermi energy difference between these two materials. With the aid of an atomic force microscope and KPFM, the surface potential width at the stitched region was found to be ∼5 times higher than the actual width of the interface, confirming the modulation of properties near the interface.

Optical reflectance imaging reveals interlayer coupling in mechanically stacked MoS2 and WS2 bilayers

Optical reflectance imaging is a popular technique for characterizing 2D materials, thanks to its simplicity and speed of data acquisition. The use of this method for studying interlayer phenomena in stacked 2D layers has, however, remained limited. Here we demonstrate that optical imaging can reveal the nature of interlayer coupling in stacked MoS₂ and WS₂ bilayers through their observed reflectance contrast versus the substrate. Successful determination of interlayer coupling requires co-optimization of the illumination wavelength and the thickness of an underlying SiO₂ film. Our observations are supported by multilayer optical calculations together with an analysis of the effect of any interlayer gap. This approach promises quick characterization of constructed 2D material systems.

Photodetection Properties of MoS2, WS2 and MoxW1-xS2 Heterostructure: A Comparative Study

Layered transition metals dichalcogenides such as MoS₂ and WS₂ have shown a tunable bandgap, making them highly desirable for optoelectronic applications. Here, we report on one-step chemical vapor deposited MoS₂, WS₂ and Mo_xW_1-xS₂ heterostructures incorporated into photoconductive devices to be examined and compared in view of their use as potential photodetectors. Vertically aligned MoS₂ nanosheets and horizontally stacked WS₂ layers, and their heterostructure form Mo_xW_1-xS₂, exhibit direct and indirect bandgap, respectively. To analyze these structures, various characterization methods were used to elucidate their properties including Raman spectroscopy, X-ray diffraction, X-ray photoelectron spectrometry and high-resolution transmission electron microscopy. While all the investigated samples show a photoresponse in a broad wavelength range between 400 nm and 700 nm, the vertical MoS₂ nanosheets sample exhibits the highest performances at a low bias voltage of 5 V. Our findings demonstrate a responsivity and a specific detectivity of 47.4 mA W^-1 and 1.4 × 10¹¹ Jones, respectively, achieved by Mo_xW_1-xS₂. This study offers insights into the use of a facile elaboration technique for tuning the performance of Mo_xW_1-xS₂ heterostructure-based photodetectors.

Visualization of Band Shifting and Interlayer Coupling in WxMo1-xS2 Alloys Using Near-Field Broadband Absorption Microscopy

Beyond-diffraction-limit optical absorption spectroscopy provides in-depth information on the graded band structures of composition-spread and stacked two-dimensional materials, in which direct/indirect bandgap, interlayer coupling, and defects significantly modify their optoelectronic functionalities such as photoluminescence efficiency. We here visualize the spatially varying band structure of monolayer and bilayer transition metal dichalcogenide alloys by using near-field broadband absorption microscopy. The near-field spectral and spatial information manifests the excitonic band shift that results from the interplay of composition spreading and interlayer coupling. These results enable us to identify, notably, the top layer of the bilayer alloy as pure WS₂. We also use the aberration-free near-field transmission images to demarcate the exact boundaries of alloyed and pure transition metal dichalcogenides. This technology can offer valuable insights on various layered structures in the era of "stacking science" in the quest of quantum optoelectronic devices.

Optical Response of CVD-Grown ML-WS2 Flakes on an Ultra-Dense Au NP Plasmonic Array

The combination of metallic nanostructures with two-dimensional transition metal dichalcogenides is an efficient way to make the optical properties of the latter more appealing for opto-electronic applications. In this work, we investigate the optical properties of monolayer WS2 flakes grown by chemical vapour deposition and transferred onto a densely-packed array of plasmonic Au nanoparticles (NPs). The optical response was measured as a function of the thickness of a dielectric spacer intercalated between the two materials and of the system temperature, in the 75–350 K range. We show that a weak interaction is established between WS2 and Au NPs, leading to temperature- and spacer-thickness-dependent coupling between the localized surface plasmon resonance of Au NPs and the WS2 exciton. We suggest that the closely-packed morphology of the plasmonic array promotes a high confinement of the electromagnetic field in regions inaccessible by the WS2 deposited on top. This allows the achievement of direct contact between WS2 and Au while preserving a strong connotation of the properties of the two materials also in the hybrid system.

Excitonic absorption and defect-related emission in three-dimensional MoS2 pyramids

MoS2 micro-pyramids have demonstrated interesting properties in the fields of photonics and non-linear optics. In this work, we show the excitonic absorption and cathodoluminescence (CL) emission of MoS2 micro-pyramids grown by chemical vapor deposition (CVD) on SiO2 substrates. The excitonic absorption was obtained at room and cryogenic temperatures by taking advantage of the cathodoluminescence emission of the SiO2 substrate. We detected the CL emission related to defect intra-gap states, localized at the pyramid edges and with an enhanced intensity at the pyramid basal vertices. The photoluminescence and absorption analysis provided the Stokes shift of both the A and B excitons in the MoS2 pyramids. This analysis provides new insights into the optical functionality of MoS2 pyramids. This method can be applied to other 3D structures within the 2D materials family.