Interfacial Strength and Surface Damage Characteristics of Atomically Thin h‑BN, MoS₂, and Graphene

Surface damage characteristics of single- and multilayer hexagonal boron nitride (h-BN), molybdenum disulfide (MoS₂), and graphene films were systematically investigated via atomic force microscopy (AFM)-based progressive-force and constant-force scratch tests and Raman spectroscopy. The film-to-substrate interfacial strengths of these atomically thin films were assessed based on their critical forces (i.e., the normal force where the atomically thin film was delaminated from the underlying substrate), as determined from progressive-force scratch tests. The evolution of surface damage with respect to normal force was further investigated using constant-force tests. The results showed that single-layer h-BN, MoS₂, and graphene strongly adhere to the SiO₂ substrate, which significantly improves its tribological performance. Moreover, defect formation induced by scratch testing was found to affect the topography and friction force differently prior to failure, which points to distinct surface damage characteristics. Interestingly, the residual strains at scratched areas suggest that the scratch test-induced in-plane compressive strains were dominant over tensile strains, thereby leading to buckling in front of the scratching tip and eventually failure at sufficient strains. These trends represent the general failure mechanisms of atomically thin materials because of a scratch test. As the number of layers increased, the tribological performances of atomically thin h-BN, MoS₂, and graphene were found to significantly improve because of an increase in the interfacial strengths and a decrease in the surface damage and friction force. In all, the findings on the distinctive surface damage characteristics and general failure mechanisms are useful for the design of reliable, protective and solid-lubricant coating layers based on these materials for nanoscale devices

Friction characteristics of mechanically exfoliated and CVD-grown single-layer MoS2

Abstract In this work, the friction characteristics of single-layer MoS2 prepared with chemical vapor deposition (CVD) at three different temperatures were quantitatively investigated and compared to those of single-layer MoS2 prepared using mechanical exfoliation. The surface and crystalline qualities of the MoS2 specimens were characterized using an optical microscope, atomic force microscope (AFM), and Raman spectroscopy. The surfaces of the MoS2 specimens were generally flat and smooth. However, the Raman data showed that the crystalline qualities of CVD-grown single-layer MoS2 at 800 °C and 850 °C were relatively similar to those of mechanically exfoliated MoS2 whereas the crystalline quality of the CVD-grown single-layer MoS2 at 900 °C was lower. The CVD-grown single-layer MoS2 exhibited higher friction than mechanically exfoliated single-layer MoS2, which might be related to the crystalline imperfections in the CVD-grown MoS2. In addition, the friction of CVD-grown single-layer MoS2 increased as the CVD growth temperature increased. In terms of tribological properties, 800 °C was the optimal temperature for the CVD process used in this work. Furthermore, it was observed that the friction at the grain boundary was significantly larger than that at the grain, potentially due to defects at the grain boundary. This result indicates that the temperature used during CVD should be optimized considering the grain size to achieve low friction characteristics. The outcomes of this work will be useful for understanding the intrinsic friction characteristics of single-layer MoS2 and elucidating the feasibility of single-layer MoS2 as protective or lubricant layers for micro- and nano-devices

Laser-Induced Particle Adsorption on Atomically Thin MoS₂

Atomically thin molybdenum disulfide (MoS₂) shows great potential for use in nanodevices because of its remarkable electronic, optoelectronic, and mechanical properties. These material properties are often dependent on the thickness or the number of layers, and hence Raman spectroscopy is widely used to characterize the thickness of atomically thin MoS₂ due to the sensitivity of the vibrational spectrum to thickness. However, the lasers used in Raman spectroscopy can increase the local surface temperature and eventually damage the upper layers of the MoS₂, thereby changing the aforementioned material properties. In this work, the effects of lasers on the topography and material properties of atomically thin MoS₂ were systematically investigated using Raman spectroscopy and atomic force microscopy. In detail, friction force microscopy was used to study the friction characteristics of atomically thin MoS₂ as a function of laser powers from 0.5 to 20 mW and number of layers from 1 to 3. It was found that particles formed on the top surface of the atomically thin MoS₂ due to laser-induced thermal effects. The degree of particle formation increased as the laser power increased, prior to the thinning of the atomically thin MoS₂. In addition, the degree of particle formation increased as the number of MoS₂ layers increased, which suggests that the thermal behavior of the supported MoS₂ may differ depending on the number of layers. The particles likely originated from the atmosphere due to laser-induced heating, but could be eliminated via appropriate laser powers and exposure times, which were determined experimentally. The outcomes of this work indicate that thermal management is crucial in the design of reliable nanoscale devices based on atomically thin MoS₂

Surface Properties of Laser-Treated Molybdenum Disulfide Nanosheets for Optoelectronic Applications

Transition metal dichalcogenide two-dimensional materials have attracted significant attention due to their unique optical, mechanical, and electronic properties. For example, molybdenum disulfide (MoS₂) exhibits a tunable band gap that strongly depends on the numbers of layers, which makes it an attractive material for optoelectronic applications. In addition, recent reports have shown that laser thinning can be used to engineer an MoS₂ monolayer with specific shapes and dimensions. Here, we study laser-thinned MoS₂ in both ambient and vacuum conditions via confocal μ-Raman spectroscopy, imaging X-ray photoelectron spectroscopy (i-XPS), and atomic force microscopy (AFM). For low laser powers in ambient environments, there is insufficient energy to oxidize MoS₂, which leads to etching and redeposition of amorphous MoS₂ on the nanosheet as confirmed by AFM. At high powers in ambient, the laser energy and oxygen environment enable both MoS₂ nanoparticle formation and nanosheet oxidation as revealed in AFM and i-XPS. At comparable laser power densities in vacuum, MoS₂ oxidation is suppressed and the particle density is reduced as compared to ambient. The extent of nanoparticle formation and nanosheet oxidation in each of these regimes is found to be dependent on the number of layers and laser treatment time. Our results can shed some light on the underlying mechanism of which atomically thin MoS₂ nanosheets exhibit under high incident laser power for future optoelectronic applications

Growth and Simultaneous Valleys Manipulation of Two-Dimensional MoSe2-WSe2 Lateral Heterostructure

The covalently bonded in-plane heterostructure (HS) of monolayer transition-metal dichalcogenides (TMDCs) possesses huge potential for high-speed electronic devices in terms of valleytronics. In this study, high-quality monolayer MoSe2-WSe2 lateral HSs are grown by pulsed-laser-deposition-assisted selenization method. The sharp interface of the lateral HS is verified by morphological and optical characterizations. Intriguingly, photoluminescence spectra acquired from the interface show rather clear signatures of pristine MoSe2 and WSe2 with no intermediate energy peak related to intralayer excitonic matter or formation of MoxW(1-x)Se2 alloys, thereby confirming the sharp interface. Furthermore, the discrete nature of laterally attached TMDC monolayers, each with doubly degenerated but nonequivalent energy valleys marked by (KM, K???M) for MoSe2 and (KW, K???W) for WSe2 in k space, allows simultaneous control of the four valleys within the excitation area without any crosstalk effect over the interface. As an example, KM and KW valleys or K???M and K???W valleys are simultaneously polarized by controlling the helicity of circularly polarized optical pumping, where the maximum degree of polarization is achieved at their respective band edges. The current work provides the growth mechanism of laterally sharp HSs and highlights their potential use in valleytronics

Growth and Simultaneous Valleys Manipulation of Two-Dimensional MoSe₂‑WSe₂ Lateral Heterostructure

The covalently bonded in-plane heterostructure (HS) of monolayer transition-metal dichalcogenides (TMDCs) possesses huge potential for high-speed electronic devices in terms of valleytronics. In this study, high-quality monolayer MoSe₂-WSe₂ lateral HSs are grown by pulsed-laser-deposition-assisted selenization method. The sharp interface of the lateral HS is verified by morphological and optical characterizations. Intriguingly, photoluminescence spectra acquired from the interface show rather clear signatures of pristine MoSe₂ and WSe₂ with no intermediate energy peak related to intralayer excitonic matter or formation of Mo_<i>x</i>W_{(1–<i>x</i>)}Se₂ alloys, thereby confirming the sharp interface. Furthermore, the discrete nature of laterally attached TMDC monolayers, each with doubly degenerated but nonequivalent energy valleys marked by (<i>K</i>_M, <i>K</i>′_M) for MoSe₂ and (<i>K</i>_W, <i>K</i>′_W) for WSe₂ in <i>k</i> space, allows simultaneous control of the four valleys within the excitation area without any crosstalk effect over the interface. As an example, <i>K</i>_M and <i>K</i>_W valleys or <i>K</i>′_M and <i>K</i>′_W valleys are simultaneously polarized by controlling the helicity of circularly polarized optical pumping, where the maximum degree of polarization is achieved at their respective band edges. The current work provides the growth mechanism of laterally sharp HSs and highlights their potential use in valleytronics