INVESTIGATION OF FRICTION CHARACTERISTICS OF 2D MATERIALS USING ATOMIC FORCE MICROSCOPY
TRAN KHAC BIEN CUONG
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ABSTRACT Investigation of Friction Characteristics of 2D Materials Using Atomic Force Microscopy Tran Khac Bien Cuong School of Mechanical Engineering The Graduate School University of Ulsan Two-dimensional (2-D) crystal, including single or few atomic layers of molybdenum disulfide (MoS2) and graphene, have recently attracted a lot of interest for fundamental studies due to their remarkable electrical and mechanical properties. Based on the presence of the bandgap or the strong ambipolar electric effect, single and a few layers MoS2 and graphene have a great potential in micro- and nano-electromechanical systems (MEMS and NEMS). Furthermore, due to the low friction characteristics of MoS2 and graphite, they may have a great potential as a protective layer in MEMS and NEMS applications. However, fundamental friction characteristics of these materials are not clearly understood yet. Therefore, in order to understand friction characteristic of these 2D materials at nano-scale not only from the fundamental tribological point of view but also from their practical applications point of view, a more thorough study of friction characteristics of these atomically thin MoS2 and graphene under various conditions is needed. Raman microscopy has been a powerful technique useful for study of vibrational properties and electronic structure of nano materials, particularly for investigation of number of layer of MoS￢2 and graphene based on their “finger-print” peaks frequencies. Due to the thermal effect, the laser treatment parameters (laser power and laser treatment time) must be carefully taken into account for both data interpretation and specimen modification. However, thermal effect on these 2D materials is not completely explored, particularly on the friction characteristic. In this work, Raman microscopy is not only used for number of layer determination but also used for specimen treatment, by varying the laser excitation parameters, we created different thermal condition in the surface of the specimens. By doing so, we systematically investigated the thermal effects on friction characteristic of the atomically thin MoS2 and graphene by using the friction force microscopy (FFM). The experimental results show that both single layer MoS2 and graphene exhibited exceedingly low friction coefficients, of about 0.06 and 0.04 respectively, which make them promising candidates for protective layer application. It was also found that friction decreased with a factor of 0.4 as the number of layer increased from single layer to bulk (more than 20 layers), as expected. As for the thermal effect, formations of adsorbates on MoS2 are observed, depending on the laser treatment conditions. It is postulated that thermal conditions affect the formation of particle. As a consequence, the friction was significantly increased due to the formation of particles. The AFM manipulation of formed particles suggests that those are adsorbates from the atmospheric environment. However, such thermal effect is not significantly observed for graphene. These experimental results may help to gain a fundamental understanding of friction characteristic of atomically thin MoS2 and graphene and to understand the thermal effects on these 2D materials.