Polymer-sorted high-density carbon nanotube (CNT) arrays have shown great potential to extend the silicon-based Moore's law. Imaging the CNT arrays on insulators like SiO2/Si using low-voltage scanning electron microscopy (LVSEM) to acquire array information like the alignment, density, and distribution of residual polymers is necessary. Such a task remains challenging due to the nanoscale CNT body (1-2 nm in diameter), nanoscale tube-to-tube separation (1-10 nm), the broadening of the apparent diameter, and the complex image contrast caused by the insulating substrate and polymer residues. In this study, the imaging mechanism for this system is investigated. Two methods are developed to separate the three dominant contrasts, i.e. topographic contrast, charge contrast, and material contrast, by selecting the take-off angle and energy of the emitted electrons as enabled by changing the working distance or the deceleration voltage. The contrast formation and separation mechanism is further confirmed by the dynamic contrast evolution due to the electron-beam-induced deposition of amorphous carbon. The contrast separation method is further applied to an individual CNT, reducing its apparent diameter from 36 nm to 6 nm. This result hints at the potential for LVSEM to count the density exceeding 150 CNTs/µm of CNT arrays. Finally, a comparative study of LVSEM and transmission electron microscopy confirms the failure of LVSEM to resolve CNTs in a bundle. The results suggest that the density of CNT arrays may be underestimated in reported SEM data. The proposed method can serve as a useful tool for further study and application of arrayed CNTs.
Hydrochemistry is a critical indicator of water quality. We analysed 146 hydrochemical parameters of groundwater and the hydrogeological background of the Kashi region in China, examined their spatial distributions, and elucidated the mechanisms of their occurrence. We used graphical and multivariate statistical methods to distinguish between high- and poor-quality water. In the Kizil-Cakmak River Basin of the northern Kashi region, the primary cations were Ca2+ and Mg2+, and the primary anion was SO42−; their concentrations were four times higher than the Class III water quality standard. Additionally, the salinity was high. In contrast, in the upstream and midstream parts of the Gez-Kushan River Basin in the southern Kashi region, the primary ions were HCO3− and SO42−, and the salinity was low. Downstream of the Gez-Kushan River Basin, the primary ion was Cl−, and the salinity was low. There was a wide-ranging spatial variability in the SO42−, Cl−, and Ca2+ contents throughout the study area. The present findings can be applied to locate suitable domestic, agricultural, and industrial water sources in the Kashi region and can provide theoretical guidance for the scientific development and management of groundwater in this region.
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