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Raman spectra (488 nm excitation) of microcrystalline MoS 2 powder and a natural MoS 2 crystal. The data were collected in an air atmosphere at room temperature. The plots have been shifted vertically for clarity and do not have the same scale. The additional peak corresponds to the 501.7 nm argon-ion laser line
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Molybdenum disulfide (MoS2) and molybdenum trioxide are investigated using Raman spectroscopy with emphasis on the application to tribological systems. The Raman vibrational modes were investigated for excitation wavelengths at 632.8 and 488 nm using both micro-crystalline MoS2 powder and natural MoS2 crystals. Differences are noted in the Raman sp...
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... 2 spectra were also collected with a micro-Raman system using a 488 nm excitation source. The same MoS 2 powder and natural crystal samples as presented above were investigated. The spectra for each are presented in Fig. 6. The microcrystalline powder reveals two clearly distinct peaks located at 400 cm -1 (A 1g ) and 375 cm -1 (E 2g 2 ), while the crystal sample reveals the same peaks, although slightly shifted, at 406 cm -1 (A 1g ) and 381 cm -1 (E 2g 2 ), along with a sometimes apparent broad, weak structure centered around 450 cm -1 . The overall ...
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Citations
... Windom et al. [12] utilised Raman spectroscopy to study molybdenum disulfide and trioxide, demonstrating their applications in tribological systems. Rauscher et al. [13] highlighted the need for improved detector technologies for spectroscopic biosignature characterisation in space telescopes, underscoring the significance of advanced detection techniques. ...
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... After defects are introduced in MoS2, remarkable changes in peaks can be observed: (i) the positions of two prominent Raman-active peaks shift in opposite directions (Usually caused by vacancies with E′ peak redshift, A′1 peak blueshift) [50], (ii) two prominent Raman-active peaks will broaden (Usually caused by both vacancies and implantations) [51][52][53], (iii) defectactivated peaks might appear [53]. Substitution in TMDs can also be distinguished by observing Raman peaks shift [54] (can be treated as vacancies since substitution atoms cannot fully restore the bonding strength in pristine material). For example, in Parkin et al.'s experiment [50], monolayer MoS2 was electron beam irradiated to generate defects. ...
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... "In contrast, the spectrum generated with 632.8-nm excitation is striking insofar as it consists of the same first-order bands and those due to harmonics and combination modes". The absorption spectrum of MoS2 is characterized by peaks at "1.9 eV (653 nm) and 2.1 eV (590 nm) related to d-to-d orbital transitions split by spin-orbit coupling and designated A1 and B1 excitons, respectively" (Tuschel, 2015, mentioning Stacy and Hodul, 1985, and Windom et al., 2011. Using the 632.8-nm excitation, it "couples into the A1 transition producing a resonance enhancement of all of the additional modes observed in Figure 1" (given by Tuschel). ...
... "All of the bands in the spectrum excited with 632.8-nm light can be accounted for and have previously been assigned (Tuschel and references therein). The article by Windom et al., 2011, is proposing the spectra of microcrystalline powder and natural crystal samples. ...
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Graphical Abstract
... Figure 1(b) illustrates the Raman spectrum of the Mo-O-S nanostructure, which was excited by a 532 nm laser line at room temperature. The Raman bands corresponding to the crystalline orthorhombic α-MoO 3 are associated with phonon frequency peaks at 194 cm - of Mo and S in opposite directions [77,78]. Furthermore, the phonon frequency peaks at 108 cm -1 , 123 cm -1 , 143 cm -1 , and 232 cm -1 are attributed to the crystal orthormiclinic Mo 2 S 3 phase [79]. ...
... 77]. The Raman active modes of α-MoO 3 can be traced back to calculations based on k = 0 for the D h 2 16 (Pbmm) space group, resulting in seven distinct intense modes, including 3A g (212, 334, and 819 cm -1 ), 2B 2g (194, 282 cm -1 ), 2B 1g (994 cm -1 ), and B 3g (664 cm -1 ). ...
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... The low energy Raman spectrum exhibits two major peaks at 372.7 cm −1 (in-plane E 2 2g ) and 401.1 cm −1 (out-of-plane A 1g ). These two sharp peaks are in good agreement with the MoS 2 powder samples [33]. The frequency difference between two modes is 28.4 cm −1 which diagnosis the number of layers of MoS 2 and indicating that it consists of 7 or more layers. ...
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... This demonstrated that a straightforward, one-step hydrothermal approach is capable of producing MoS 2 in the 1T phase. Other peaks are the extra second-order lines that occur from the Raman effect's resonance type, which alters the behavior of the 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 crystalline structure and causes an emission of light that is deviated from the first-order modes [44]. ...
Two-dimensional transition metal dichalcogenides are of great interest due to their extraordinary electrical and optical characteristics for possible optoelectronic applications like photodetectors. In the current work, MoS2 film has been deposited by a simple and economical hydrothermal route in an alkaline solution in one pot. Raman and X-ray diffraction analyses verified the growth of the dominant 1T-phase, which provides more active sites on the film. Electron microscope images exhibit the formation of long MoS2 nanowires on top of skein-like microspheres. Each microsphere is composed of curly nanosheets. A growth mechanism is suggested for the formation of the achieved morphology. The energy bandgap of the film was calculated to be around 1.72 eV, using the optical absorbance spectrum. The photosensitivity tests showed a 26% response of the fabricated photosensor toward incident light (450 nm) and confirmed that the as-grown MoS2 has high response stability. The recorded response time of 187 s and the recovery time of 210 s showed that the sample responds quickly to incident light. The obtained results showed the effect of the 1T-MoS2 phase and nanowire structure on improving the performance of photodetectors and presented a simple method for the synthesis of MoS2 films with nanowire morphology.
... Each of these initial bands stems from vibrational modes occurring within the S-Mo-S layer. Therefore, Raman peaks corresponding to E 1 2g and A 1g modes confirm the presence of MoS 2 [53,54]. The G band, positioned around 1584 cm −1 , is a distinctive feature in carbon allotropes, signifying in-plane C-C vibrations. ...
In the present work, surface modification of Ti-6Al-4V alloy was carried out using Laser surface texturing (LST). The investigation aims to explore the synergetic impact of LST and molybdenum disulfide (MoS2), as a solid lubricant, on the tribological properties of Ti-6Al-4V alloy. Three types of LST textures (Circular, triangular, and square textures) were created on the Ti6Al4V alloy. Subsequently, tribological tests were conducted on a universal tribometer (ball-on-disc), at a load of 20 N with a frequency of 15 Hz. The worn surface was analyzed using various methods, including optical microscopy, 3D-profilometer, FESEM, EDAX analysis, and Raman spectroscopy. The study compared the coefficient of friction (COF) and wear behavior of un-textured surfaces (U TS ) with those of textured surfaces (T S ) under both dry sliding conditions (D SC ) and lubricated sliding conditions (L SC ). The results demonstrated a significant reduction in the COF and wear coefficients on the T S . Specifically, the circular texture exhibited significant results with a 10.30%, 10.42%, and 28.80% decrease in COF and a remarkable reduction of 39.22%, 47.51%, and 77.90% in wear coefficients compared to the U TS tested under D SC and L SC (using PAO-4 and PAO-4 + 1% wt MoS 2 ). Severe abrasion, adhesion, and delamination as the primary wear mechanisms were observed on the U TS and T S under D SC , while the L SC was characterized by mild adhesive and delamination wear on the U TS and T S . The carbon layer formation and concentration of Mo and S particles resulted in lower friction and wear coefficients for the U TS , and T S under LSC (PAO-4 + 1% wt MoS 2 ). The analysis indicated that the use of LST and solid lubricant nanoparticles on a Ti-6Al-4V alloy would result in improved service life and better endurance in cutting tools and tribo-mating parts.
... Atmospheric O 2 molecules have a small effect on the tribological properties of MoS 2 films at room temperature [65]. However, at high temperatures, they oxidize the edge active sites and grain boundaries, generating MoO 2 and MoO 3 , which destroys the shear-prone laminar slip structure of MoS 2 , leading to high friction and wear [66,67]. Increasing temperature increases the oxidation rate, and tribological properties deteriorate significantly. ...
... However, some recent experimental studies [57,65,67,76,77] and simulations [76,78] have shown that water does not promote MoS 2 oxidation at room temperature. In a humid N 2 environment, no O signal was detected on the wear surface of the film, suggesting that water did not cause film oxidation [57]. ...
... This result is consistent with the Raman spectroscopy analysis by Windom et al. Humid environments have little effect on oxidation compared to dry air or O 2 environments [67]. In addition, an appropriate increase in temperature in humid air improves the tribological properties of the films, which suggests that water adsorption/desorption is a reversible process, i.e., water is physically adsorbed rather than chemically adsorbed [57,77]. ...
Superlubricity is an ideal state with zero contact friction between two frictional interfaces. It has become a hot research topic for many scientists in the past 20 years, and the field spans the complex hot research directions of physics, chemistry, mechanics, and materials. The concept of superlubricity was introduced in 1990, and the understanding of the process of realizing superlubricity is vital for controlling the tribological properties of materials and promoting the development of tribology. This review focuses on the fundamental properties of molybdenum disulfide (MoS 2 ) films and the influence of the environment on affecting MoS 2 films. As a result, some methods for realizing superlubricity by MoS 2 films are proposed. The key to achieving superlubricity with MoS 2 is summarized. Finally, an outlook on the application of MoS 2 films is given.
