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The oxidation mechanism of atomically thin molybdenum disulfide (MoS₂) plays a critical role in its nanoelectronics, optoelectronics, and catalytic applications, where devices often operate in an elevated thermal environment. In this study, we systematically investigate the oxidation of mono- and few-layer MoS₂flakes in the air at temperatures ranging from 23 °C to 525 °C and relative humidities of 10%–60% by using atomic force microscopy (AFM), Raman spectroscopy and x-ray photoelectron spectroscopy. Our study reveals the formation of a uniform nanometer-thick physical adsorption layer on the surface of MoS₂, which is attributed to the adsorption of ambient moisture. This physical adsorption layer acts as a thermal shield of the underlying MoS₂lattice to enhance its thermal stability and can be effectively removed by an AFM tip scanning in contact mode or annealing at 400 °C. Our study shows that high-temperature thermal annealing and AFM tip-based cleaning result in chemical adsorption on sulfur vacancies in MoS₂, leading to p-type doping. Our study highlights the importance of humidity control in ensuring reliable and optimal performance for MoS₂-based electronic and electrochemical devices and provides crucial insights into the surface engineering of MoS₂, which are relevant to the study of other two-dimensional transition metal dichalcogenide materials and their applications.

 

The vibration-assisted atomic force microscope (AFM)-based nanomachining offers a promising opportunity for low-cost nanofabrication with high tunability. However, critical challenges reside in advancing the throughput and the quality assurance of the process due to extensive offline experimental investigations and characterizations, which in turn hinders the wide industry applications of current AFM-based nanomachining process. Hence, it is necessary to create an in-process monitoring for the nanomachining to allow real-time inspection and process characterizations. This paper reports a sensor-based analytic approach to allow real-time estimations of the AFM-based nanomachining process. The temporal-spectral features of collected acoustic emission (AE) sensor signals are applied to predict the depth morphology of nanomachined trenches under different machining conditions. The experimental case study suggests that the most significant frequency domain information from AE sensor can accurately predict (R-squared value around 92%) the nanomachined depth profile. It, therefore, breaks the current limitation of characterization tools at the nanoscale precision level, and opens up an opportunity to allow real-time estimation for quality inspection of vibration-assisted AFM-based nanofabrication process. 

The atomic force microscope (AFM)-based nanomachining has the potential for highly customized nanofabrication due to its low cost and tunability. However, the low productivity and issues related to the quality assurance for AFM-based nanomachining impede it from large-scale production due to the extensive experimental study for turning process parameters with time-consuming offline characterizations. This work reports an analytic approach to capturing the AE spectral frequency responses from the nanopatterning process using vibration-assisted AFM-based nanomachining. The experimental case study suggests the presented approach allows characterizations of subtle variations on the AE frequency responses during the nanomachining processes (with overall 93% accuracy), which opens up the chance to explain the variations of the nano-dynamics using the senor-based monitoring approach for vibration-assisted AFM-based nanomachining. 

Continuum mechanics break down in bending stiffness calculations of mono- and few-layered two-dimensional (2D) van der Waals crystal sheets, because their layered atomistic structures are uniquely characterized by strong in-plane bonding coupled with weak interlayer interactions. Here, we elucidate how the bending rigidities of pristine mono- and few-layered molybdenum disulfide (MoS 2 ), graphene, and hexagonal boron nitride (hBN) are governed by their structural geometry and intra- and inter-layer bonding interactions. Atomic force microscopy experiments on the self-folded conformations of these 2D materials on flat substrates show that the bending rigidity of MoS 2 significantly exceeds those of graphene or hBN of comparable layers, despite its much lower tensile modulus. Even on a per-thickness basis, MoS 2 is found to possess similar bending stiffness to hBN and is much stiffer than graphene. Density functional theory calculations suggest that this high bending rigidity of MoS 2 is due to its large interlayer thickness and strong interlayer shear, which prevail over its weak in-plane bonding. 

Abstract Atomic force microscope (AFM)-based nanolithography is a cost-effective nanopatterning technique that can fabricate nanostructures with arbitrary shapes. However, existing AFM-based nanopatterning approaches have limitations in the patterning resolution and efficiency. Minimum feature size and machining performance in the mechanical force-induced nanofabrication process are limited by the radius and sharpness of the AFM tip. Electric-field-assisted atomic force microscope (E-AFM) nanolithography can fabricate nanopatterns with features smaller than the tip radius, but it is very challenging to find the appropriate input parameter window. The tip bias range in E-AFM process is typically very small and varies for each AFM tip due to the variations in tip geometry, tip end diameter, and tip conductive coating thickness. This paper demonstrates a novel electric-field and mechanical vibration-assisted AFM-based nanofabrication approach, which enables high-resolution (sub-10 nm toward sub-5 nm) and high-efficiency nanopatterning processes. The integration of in-plane vibration with the electric field increases the patterning speed, broadens the selectable ranges of applied voltages, and reduces the minimum tip bias required for nanopatterning as compared with E-AFM process, which significantly increases the versatility and capability of AFM-based nanopatterning and effectively avoids the tip damage. 

Abstract Electric-field-assisted atomic force microscope (E-AFM) nanolithography is a novel polymer-patterning technique that has diverse applications. E-AFM uses a biased AFM tip with conductive coatings to make patterns with little probe-sample interaction, which thereby avoids the tip wear that is a major issue for contact-mode AFM-based lithography, which usually requires a high probe-sample contact force to fabricate nanopatterns; however, the relatively large tip radius and large tip-sample separation limit its capacity to fabricate high-resolution nanopatterns. In this paper, we developed a contact mode E-AFM nanolithography approach to achieve high-resolution nanolithography of poly (methyl methacrylate) (PMMA) using a conductive AFM probe with a low stiffness (~0.16 N/m). The nanolithography process generates features by biasing the AFM probe across a thin polymer film on a metal substrate. A small constant force (0.5-1 nN) applied on the AFM tip helps engage the tip-film contact, which enhances nanomachining resolution. This E-AFM nanolithography approach enables high-resolution nanopatterning with feature width down to ~16 nm, which is less than one half of the nominal tip radius of the employed conductive AFM probes.