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구형 및 평면 원자현미경 탐침에 대한 2차원 소재의 마찰 특성
Bien-Cuong Tran Khac,정구현 한국트라이볼로지학회 2019 한국트라이볼로지학회지 (Tribol. Lubr.) Vol.35 No.4
Two-dimensional materials such as graphene, h-BN, and MoS2 have attracted increased interest as solid lubricant and protective coating layer for nanoscale devices owing to their superior mechanical properties and low friction characteristics. In this work, the frictional properties of single-layer graphene, h-BN, and MoS2 are experimentally investigated under various normal forces using atomic force microscope (AFM) tips with a spherical and flat end, with the aim to gain a better understanding of frictional behaviors. The nonlinear relationship between friction and normal force friction was clearly observed for single-layer graphene, h-BN, MoS2 specimens slid against the spherical and flat AFM tips. The results also indicate that single-layer graphene, h- BN, MoS2 exhibit low frictional properties (e.g., friction coefficient below 0.1 under 70~100 nN normal force). In particular, graphene is found to be superior to h-BN and MoS2 in terms of frictional properties. However, the friction of single-layer graphene, h-BN, MoS2 against the flat tip is larger than that against the spherical tip, which may be attributed to the relatively large adhesion. Furthermore, it is shown that the fluctuation of friction is more significant for the flat tip than the spherical tip. The resutls of this study may be helpful to elucidate the feasibility of using two-dimensional materials as solid lubricant and protective coating layer for nanoscale devices.
Tran Khac, Bien-Cuong,DelRio, Frank W.,Chung, Koo-Hyun American Chemical Society 2018 ACS APPLIED MATERIALS & INTERFACES Vol.10 No.10
<P>Surface damage characteristics of single- and multilayer hexagonal boron nitride (h-BN), molybdenum disulfide (MoS<SUB>2</SUB>), 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, MoS<SUB>2</SUB>, and graphene strongly adhere to the SiO<SUB>2</SUB> 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, MoS<SUB>2</SUB>, 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.</P> [FIG OMISSION]</BR>
Laser-Induced Particle Adsorption on Atomically Thin MoS<sub>2</sub>
Tran Khac, Bien Cuong,Jeon, Ki-Joon,Choi, Seung Tae,Kim, Yong Soo,DelRio, Frank W.,Chung, Koo-Hyun American Chemical Society 2016 ACS APPLIED MATERIALS & INTERFACES Vol.8 No.5
<P>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.</P>
Khac, Bien-Cuong Tran,Chung, Koo-Hyun American Scientific Publishers 2016 Journal of Nanoscience and Nanotechnology Vol.16 No.5
<P>Atomically thin layered materials such as MoS2 and graphene have attracted a lot of interest as protective coating layers for micro- and nano-electromechanical devices based on their superior mechanical properties and chemical inertness. In this work, the frictional characteristics of single layer MoS2 and graphene prepared by the mechanical exfoliation method were quantitatively investigated using atomic force microscopy. The results showed that both MoS2 and graphene exhibited relatively low friction forces of 1 similar to 3 nN under normal forces ranging from 1 to 30 nN. However, a higher increase in the friction force as the normal force increased was observed in the case of MoS2. The differences in the adhesion characteristics and mechanical properties of atomically thin layered materials may influence the puckering of the layer, which in turn influences the frictional behavior.</P>