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We study the effects of electron irradiation on suspended graphene monolayers and graphene supported on SiO2 substrates in the range 5.0 × 1015–4.3 × 1017 electrons/cm2. The suspended graphene monolayers are exfoliated over SiO2 substrates containing micrometer-sized holes, with graphene completely covering the hole, and are referred to as graphene drums. The irradiation was performed using a scanning electron microscope at 20–25 keV electron energy. We observe a two-stage behavior for the ID/IG, ID′/IG, and ID/ID′ ratios as a function of the average distance between defects, LD, where ID, IG, and ID′ are the intensities of the Raman D, G, and D′ peaks, respectively. Good fits to the dependence of the ratios on LD are obtained using the local activation model equation. The fits are used to characterize the defects at high defect densities. We also carried out annealing studies of samples irradiated to the first stage and used an Arrhenius plot to measure activation energies for defect healing, Ea. We measured Ea = 0.90 eV for the graphene drums, consistent with the hydroxyl groups; for supported graphene, we measured Ea = 0.36 eV, consistent with hydrogen adsorbates. We also studied the surface of the drums using atomic force microscopy and found no observable holes after irradiation and annealing. Our results show that the local activation model is useful in characterizing the defects in graphene drums. 

Using a scanning electron microscope, we irradiate graphene drums with electrons at an energy of 20 keV and a dosage of about 1.58 × 1017 electrons/cm2. The drums consist of graphene exfoliated in ambient air over holes having a diameter of 4.6 μm and etched into an SiO2 substrate. After irradiation, we observe that the drum’s suspended monolayer (ML) region has a ratio of the Raman D peak height, ID, to the Raman G peak height, IG, as high as 6.3. In contrast, the supported ML on the SiO2 substrate has an ID/IG ratio of 0.49. Previous studies have shown that graphene drums containing air can leak in a vacuum at a low rate. We attribute the high ID/IG ratio of the suspended ML to the air that may be in the drums. We propose that the air produces much adsorbed water on the ML, resulting in a high average defect density during irradiation. We present Raman maps of the full-width-at-half maximum, position, and height of the G, 2D, D, and D’ peaks before and after irradiation and maps of ID/IG and ID/ID’. We anneal the drums at temperatures from 50 to 215 °C and find that ID/IG significantly reduces to 0.42. The annealing data are analyzed using an Arrhenius plot. We also find that ID/ID’ depends on annealing temperature and has values ≥8, in the range expected for sp3 defects, for ID/IG ≤ 3.9. This irradiation method may help achieve high average defect densities in ML graphene, imparting novel and potentially valuable properties. 

Abstract Monolayer (ML) molybdenum disulfide (MoS₂) is a novel 2-dimensional (2D) semiconductor whose properties have many applications in devices. Despite its potential, ML MoS₂ is limited in its use due to its degradation under exposure to ambient air. Therefore, studies of possible degradation prevention methods are important. It is well established that air humidity plays a major role in the degradation. In this paper, we investigate the effects of substrate hydrophobicity on the degradation of chemical vapor deposition (CVD) grown ML MoS 2 . We use optical microscopy, atomic force microscopy (AFM), and Raman mapping to investigate the degradation of ML MoS 2 grown on SiO 2 and Si 3 N 4 that are hydrophilic and hydrophobic substrates, respectively. Our results show that the degradation of ML MoS₂ on Si 3 N 4 is significantly less than the degradation on SiO 2 . These results show that using hydrophobic substrates to grow 2D transition metal dichalcogenide ML materials may diminish ambient degradation and enable improved protocols for device manufacturing.