Direct Observation of the Outermost Surfaces of Mesoporous Silica Thin Films by High Resolution Ultralow Voltage Scanning Electron Microscopy

Abstract

The properties of the outermost surfaces of mesoporous silica thin films are critical in determining their functions. Obtaining information on the presence or absence of silica layers on the film surfaces and on the degree of mesopore opening is essential for applications of surface mesopores. In this study, the outermost surfaces of mesoporous silica thin films with 3-dimensional orthorhombic and 2-dimensional hexagonal structures were observed using ultralow voltage high resolution scanning electron microscopy (HR-SEM) with decelerating optics. SEM images of the surfaces before and after etching with NHF were taken at various landing voltages. Comparing the images taken under different conditions indicated that the outermost surfaces of the nonetched mesoporous silica thin films are coated with a thin layer of silica. The images taken at an ultralow landing voltage (i.e., 80 V) showed that the presence or absence of surface silica layers depends on whether the film was etched with an aqueous solution of NHF. The mesostructures of both the etched and nonetched films were visible in images taken at a conventional landing voltage (2 kV); hence, the ultralow landing voltage was more suitable for analyzing the outermost surfaces. The SEM observations provided detailed information about the surfaces of mesoporous silica thin films, such as the degree of pore opening and their homogeneities. AFM images of nonetched 2-dimensional hexagonal mesoporous silica thin films show that the shape of the silica layer on the surface of the films reflects the curvature of the top surface of the cylindrical mesochannels. SEM images taken at various landing voltages are discussed, with respect to the electron penetration range at each voltage. This study increases our understanding of the surfaces of mesoporous silica thin films, which may lead to potential applications utilizing the periodically arranged mesopores on these surfaces.

Ellipsometry‐Based Approach for the Characterization of Mesoporous Thin Films for H2 Technologies

“…Synthesis of Mesoporous SiO 2 Films: Mesoporous SiO 2 films were synthesized according to the syntheses reported in the literature. [20] For a cubic pore structure, TEOS, HCl, H 2 O (MillliQ water, 18 MOhm cm), and EtOH were stirred in a vial for 30 min in a water bath at 60 C. F127 was then added and mixed for another 30 min at room temperature (RT). The final molar ratio of the solution was TEOS:F127:EtOH:H 2 O: HCl ¼ 1:0.004:20:4.9:0.15.…”

“…This process was performed and optimized based on the method described in ref. [20]. Other materials such as mesoporous TiO 2 (see Figure 3c and Figure 4) can also serve as a reference material, where properties such as pore size and ordering as well as layer thickness can also be variably adjusted.…”

Direct Observation Techniques Using Scanning Electron Microscope for Hydrothermally Synthesized Nanocrystals and Nanoclusters

“…In this report, we focused on the observation of extremely low impact electron energy to collect information from the surface. The mean free path of an electron with an energy of 100 eV in a solid sample is below 1 nm [14]. Therefore, the observation of the surface morphology at low electron energies is favorable.…”

“…One of the drawbacks of SEM is the spatial resolution of images. However, with the development of observation techniques at lower accelerating voltages and high voltage specimen bias, the resolution of SEM has been dramatically improved to directly observe the morphology and surface of nanocrystals, together with improvements in the lens detector [11][12][13][14][15][16]. In this study, we report the use of low-voltage high-resolution SEM and ultra-high solid angle energy-dispersive X-ray analyzer (EDS) to understand the 10-50 nm sized CeO 2 nanocrystals and nanoclusters that were synthesized by a hydrothermal reaction.…”

High spatial resolution scanning electron microscope: evaluation and structural analysis of nanostructured materials

“…Сканирование с нанометровым пространственным разрешением позволяет получать информацию о топографии поверхности образца, ее составе, кристаллической структуре и химических связях [1,2]. Уменьшая энергию первичных электронов, можно минимизировать повреждения образца в результате облучения, а также повысить качество изображения диэлектрических образцов с возможностью выборочного получения информации о поверхности [3].…”