See Membrane Characterisation Equipment

Scanning electron microscopy: SEM and FE-SEM with EDX

SEM and FE-SEM (with EDX)

Scanning Electron Microscopy (SEM), Field Emission SEM (FE-SEM) and Energy Dispersive X-ray (EDX) Spectroscopy


SEM analysis is a powerful investigative tool which uses a focused beam of electrons to produce detailed, high magnification and high resolution images of a sample's surface topography. An Energy Dispersive X-Ray Analyzer (EDX or EDA) is also used to provide elemental identification and quantitative compositional information.

Scanning electron microscopy is a method for highresolution imaging of surfaces. The SEM uses electrons for imaging, much as a light microscope uses visible light. The advantages of SEM over light microscopy include much higher magnification (>300,000X) and greater depth of field up to 100 times that of light microscopy. While light microscopy is used to observe details up to 0.25 µm, SEM can reveal details up to 0.4 nm.

The principle of SEM is based on the interactions between the specimen and a focused beam of electrons. At the interface of the sample, various signals are generated containing information about the surface’s topography and composition. Typical types of signals are: secondary electrons (SE), backscattered electrons (BE), characteristic X-rays, light (cathode luminescence), specimen current and transmitted electrons. Most commonly used and available detectors are the secondary electron (SEI) detector, backscattered electron (BEI) detector and Energy Dispersive X-ray Spectrometry (EDS) detector. 


SEM’s are roughly divided into two categories. The first type uses a thermionic emission gun to generate the electron beam and uses either a tungsten filament or a LaB6 cathode filament. The maximum resolution is about 3 nm at a magnification of 300,000 X. The second type is the Field Emission SEM (FESEM). This type uses a sharpened, tungsten, metallic tip and a conducting fluorescent screen enclosed in ultra-high vacuum. Here the sample is held at a large negative potential relative to the fluorescent screen resulting in a strong electrostatic field which generates the electron beam. The maximum resolution is 0.4 nm at a magnification up to 1,000,000 times.

EDS detection makes use of the X-ray spectrum emitted electrons to obtain a localized chemical analysis. In principle, all elements from atomic number 4 (Be) to 92 (U) can be detected. The detection limit of EDS analysis in the SEM depends on the composition of the sample being analyzed, but is in the range 0.1-0.5 wt%. It is an effective technique for major and minor element analysis, but lacks the sensitivity for trace-element analysis.

The Membrane Science and Technology cluster has both types a SEM and FE-SEM available; the JEOL JSM-6010LA and the JEOL JSM6000F. The main difference between the two machines is the resolution and the energy necessary to visualise the microscopic structure. The JSM-6010LA has a resolution of 4 nm at 20 kV. The JSM7610F has a resolution of 0.6 nm at 30 kV. 

One important extra option of the JSM-6010LA is the ability to investigate samples in the low vacuum mode, which does not require additional sample preparation. Normally, a SEM sample is first dried thoroughly and subsequently coated with a, thin conductive layer. This layer is needed to protect the sample for beam damaging and charging. When a microscope is equipped with a low vacuum option, it has the possibility to investigate non-conductive samples without applying the conductive coating, even when the samples are not dried. The second extra option of the JSM-6010LA is the possibility to do elemental analysis (EDS) of the specimen. The EDS has two operational modes: Spectrum A plot of X-ray detected versus their energies. The characteristics X-rays allow the elements present in the sample to be identified. Mapping An image showing how the distribution of elements is divided over the sample surface. In a mapping image every element has its own specific color.

Examples To illustrate the most striking difference between the two microscopes, images were recorded of an identical sample.

 

Notice the difference in detail and the difference in accelerating voltage 15 kV versus 3.0 kV. At a much lower accelerating voltage, the JSM7610F outperforms the JSM6010LA with respect to image detail. Another advance of FESEM is the reduced beam damage using sensitive materials because of the lower energy input.

 

In figure 3, a specimen is first observed in low vacuum-mode (JSM6010LA) without a conductive coating, resulting in the image on the left side. After recording this image, the specimen was coated with a thin layer of Platinum, resulting in the image on the right side. The main difference between the two images is the low vacuum in the specimen chamber (30 Pa) and the application of the BEI detector. The application of the BEI detector at low vacuum is necessary because the SEI detector cannot operate in the low vacuum mode without destroying it. In figure 4 examples of high resolution images are shown using the FESEM. Images A-D show cross sections and surfaces of ultrafiltration membranes. A and B are pristine membranes. C-D are nanoparticles fouled membranes.


SEM and FESEM samples require a protective coating in the high vacuum mode (2 to 5 nm) to promote heat conduction otherwise the energy originating from the electron beam melts/burns down and charges the specimen. At lower magnifications often a gold sputter coated layer is applied. A gold sputter coating is relatively easy to apply and is very useful for low and moderate magnifications. The drawback of a gold coating is that the layer is relatively thick before a smooth layer is obtained. At low magnifications an extremely thin coating layer can be applied. The drawback of a thin coating layer however is a more uneven distribution of the protecting layer which may results in charging, image drift and beam damage of the membrane. On the other hand a thick coating layer obscures details by covering, bridging or clogging nano gaps and pores. For this reason a thin surface coating with smaller sized platinum grains is preferred when high magnifications, as is common in FESEM imaging, are needed to reveal surface details. 

In-house: JEOL JSM-6010LA and JEOL JSM7610F