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What is Scanning Electron Microscopy (SEM)

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What is Scanning Electron Microscopy (SEM)

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Scanning Electron Microscopy SEM represents one of the powerful techniques of visualization in providing a high-resolution imaging of sample surface morphologies. Working principles of scanning electron microscopy include a focused electron beam being scanned across the surface of a specimen to create signals containing comprehensive details about topography, composition, and crystal structure.

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MUHAMMAD HAIQAL ISKANDAR BIN AZHAR (22301654)

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MUHAMMAD HAIQAL ISKANDAR BIN AZHAR (22301654)

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Generates a beam of electrons.

Electrons are typically generated from tungsten filament or a field-emission source.

Electrons are thermionically emitted by heating or field-emitted under high voltage.

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Restricts the diameter of the diameter of the electron beam to control its size and prevent divergence.

It helps improve image resolution by ensuring a sharp beam.

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Focuses the electron beam into a smaller spot size.

Reduces the beam diameter to match the size required for scanning fine details.

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Deflect the focused electron beam in a raster pattern across the sample surface.

This movement allows point-by-point imaging of the sample.

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Focus the electron beam further just before it interacts with the sample.

Plays a crucial role in final resolution and depth of focus.

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The object being analyzed.

Needs to be conductive or coated with a conductive material to avoid charging under the electron beam.

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Secondary electrons are emitted when the electron beam interacts with the atoms of the sample surface.

These low-energy electrons are used to form the high-resolution surface morphology image.

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Collects the emitted secondary electrons and converts them into an electrical signal.

The intensity of the signal determines the brightness of the image and it helps in revealing the surface details.

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The process starts with the generation and acceleration of electrons. Usually, a filament of tungsten or lanthanum hexaboride emits electrons by thermionic emission. These electrons are accelerated by an anode to high energies, typically in the range of 1-30 keV, to form a focused electron beam. This high-energy beam is crucial for subsequent interactions with the sample.

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The resulting electron beam is then carefully focused by a set of electromagnetic lenses. These lenses act similarly to optical microscope lenses and control the direction of the electron beam to modify its size and shape. This focusing is required for achieving high resolution, enabling the acquisition of a detailed picture of the sample's surface.

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The focused electron beam scans across the surface of the sample in a raster pattern, similar to the way in which a television screen scans. Scanning is achieved by the deflection of the electron beam from electromagnetic coils. Because of this very accurate scanning, it is possible to collect the signal at defined points on the sample surface, and a high-resolution 2D image can be generated.

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Signals such as secondary electrons produced during the interaction between the high-energy electron beam and the specimen are low-energy electrons ejected from the atoms in the surface of the specimen; these are the SEs that carry the important topographic information. Other examples are the backscattered electrons, BSE, which are high-energy electrons that are scattered backward from the atoms in the specimen. They give compositional information since the intensity depends on the atomic number of the elements involved. Further, characteristic X-rays are produced by the ionization of atoms in the sample by incident electrons, enabling the analysis of elements.

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Special detectors then collect these emitted signals carefully: SE, BSE, and X-rays. A 2D image is then developed on the computer screen based on the intensity of the detected signal at each point in the area scanned. The different detectors may visualize different features of the sample, such as topography, composition, or crystal structure, enabling a full understanding of the characteristics of the sample.

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Advantages of Scanning Electron Microscopy(SEM)

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Advantages of Scanning Electron Microscopy(SEM)

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Scanning Electron Microscopy (SEM), boasts of remarkable capabilities such as a superior resolution that can go down to the nanometer scale, thus enabling detailed visualization of surface structures at the nanoscale. It also provides exceptional depth of field for 3-D-like imaging of surfaces, which is invaluable in analyzing complex topographies. Besides that, SEM is extremely versatile, enabling the analysis of metals, ceramics, polymers, and biological samples. Notably, it is possible to equip SEMs with energy-dispersive X-ray spectroscopy detectors that enable elemental analysis of a sample to be performed, combining imaging and topographical mapping in one system.

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An example of Scanning Electron Microscopy (SEM) imaging for surface morphology analysis could be the examination of the surface of a fractured polymer sample. In this case, the SEM would allow for high-resolution imaging to observe the fracture surface's microstructure, revealing features such as:

Surface roughness: The size and distribution of surface irregularities.

Morphological features: Such as cracks, voids, and inclusions that may have formed during processing or due to external stresses.

Fracture patterns: For example, ductile or brittle fracture surfaces, which may appear as a smooth, shiny surface or rough, jagged patterns depending on the material's properties.

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Metal Surface Examination: SEM can be used to inspect the surface of a metal alloy such as steel or aluminum after it has undergone a fatigue test. The SEM would capture the surface's fine details, showing features like:

Pits or cracks formed due to cyclic stress.

Grain boundaries and microstructural phases within the metal.

Evidence of corrosion or wear patterns, such as pitting, crevice corrosion, or abrasion marks.

Polymer Blends: For polymer blends such as thermoplastic elastomers or composite materials, SEM imaging can provide insights into the dispersion of filler particles like carbon black or glass fibers within the polymer matrix. The SEM would reveal:

The distribution of reinforcing agents.

Interfacial bonding between the polymer and fillers.

Phase separation if incompatible polymers were used in the blend.

Coatings and Thin Films: SEM is widely used to study the surface morphology of thin coatings or thin films applied to various substrates such as metal, glass, or ceramics. This could include:

Coating uniformity and thickness.

The adhesion between the coating and the substrate.

The presence of defects such as bubbles, pores, or delamination.

Biological Samples: SEM imaging is also useful in analyzing the surface structures of biological samples like plant leaves, insect exoskeletons, or human tissue. It can reveal:

Surface texture, such as microstructures like trichomes on plant leaves or the surface of insect wings.

Surface morphology of cells, tissues, or bacteria for research in medical or biological studies.

Fractography of Ceramics: SEM is often used to study fractures in ceramics. The imaging might show:

The crack propagation path.

Fracture origins and the size of microcracks that contribute to failure.

Features indicating brittle fracture, like clean and smooth surfaces or ductile fracture in evidence of plastic deformation at the crack tip.

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Example of the SEM imaging for surface morphology analysis

An example of the SEM imaging for surface morphology analysis is ilmenute surface analysis. In surface morphology analysis of ilmenite, SEM images reveal cracks in the sample, which are crucial for understanding the mechanical behavior of ilmenite. Many of the surface furrows that indicate operations or environmental influences on the mineral represent mechanical degradation or weathering. In addition, the characteristics of ilmenite particles and their potential uses are clarified by examining the particle shape through SEM and considering its mineralogical properties.

A) Specimen preparation
First, an appropriate sample is selected to investigate such characteristics as cracks, shape of particles and furrows. The sample is then attached to the stub by using the conductive carbon double sided tape to provide the conduction path through the pads on the SEM stubs. In the case of ilmenite, no preparation is necessary, but for other non-conductive materials, gold or platinum coatings may be necessary to eliminate charging effects. After that, the sample is dried and washed to eliminate moisture that may interfere with image formation. Last but not the least, depending on the nature and type of the sample the sample may be broken or reduced mechanically by fracturing, trimming or cutting to allow direct interaction of the electron beam with the surface features of interest.

B) Imaging process in SEM

i) Secondary Electron Imaging (SEI):
SEI measures secondary electrons produced from the surface of ilmenite. It is widely applied in the analysis of topographical relief, roughness and other surface irregularities such as cracks for which SEI assists in determination of the depth and orientation of cracks on the surface. For particle shape and morphology, gives geometrical description of the ilmenite particles.

ii) Backscattered Electron Imaging (BSEI):
BSEI collects electrons which are back scattered because of their interaction with the sample. It offers information about compositional contrast where materials with high atomic number such as titanium and iron in ilmenite will be bright while those with low atomic number will be dark. After that, morphology and roughness, BSEI images are especially helpful in studying the roughness and variations in the chemical composition that may be unnoticed in SEI.

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SEM imaging for surface morphology analysis on ilmenite

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SEM imaging for surface morphology analysis on ilmenite

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SEM imaging for surface morphology analysis on ilmenite

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SEM imaging for surface morphology analysis on ilmenite