Techniques for Analyzing Solid Dispersed Systems (P2): Scanning Electron Microscopy (SEM) & Transmission Electron Microscopy (TEM)

Electron microscopy is an advanced imaging technique with high magnification capabilities, using an electron beam as the light source instead of visible light, as in optical microscopy. In electron microscopy, the focused electron beam interacts with the sample surface, leading to various forms of interaction between the beam and the specimen. Each interaction generates a distinct signal, which is used to create high-resolution images.

Unlike optical microscopy, electron microscopy can provide images with much higher magnification, clarifying surface details and internal structures of the sample. Today, electron microscopy is applied in many fields, from quality control to the study of atomic structures in advanced materials.

Scanning Electron Microscopy (SEM) and Transmission Electron Microscopy (TEM) are the two most common types of electron microscopy. Although both techniques are based on the same fundamental principles, they differ significantly in equipment structure and the signals analyzed.

• • Scanning Electron Microscopy (SEM): Produces surface images by analyzing secondary electron (SE) and backscattered electron (BSE) signals, providing detailed information about the morphology and topography of the sample surface.
  • Transmission Electron Microscopy (TEM): Detects electrons transmitted through the sample to produce projection images, enabling observation of internal structures with higher spatial resolution.

This article will delve into these two popular electron microscopy techniques and provide a brief comparison of their key differences in operation and practical applications.

Common Features of Electron Microscopy

To effectively compare SEM and TEM, it is essential to first understand the common components and functions that all types of electron microscopes share. The core components of any electron microscope include the following:

1. 1. Electron Source: Generates the electron beam, the primary light source for illuminating the sample.
   2. Condenser Lenses: : These lenses focus and direct the electron beam toward the sample, ensuring precise illumination.
   3. Objective Lens:
      • In TEM: This lens creates the image by using electrons transmitted through the specimen.
      • In SEM: This lens produces the final focused beam for scanning the sample surface, generating high-resolution images.
   4. Sample Chamber: The area where the sample is placed. The size of this chamber determines the maximum size of the sample that can be analyzed.
   5. Detectors: Collect signals to generate images of the sample.

Scanning Electron Microscopy (SEM)

Scanning Electron Microscopy (SEM) is an important tool in pharmaceutical sciences, especially in the development of formulations and product quality control. One of the main applications of SEM is the analysis of particle size, morphology, and surface properties of components in pharmaceutical formulations. This technique provides highly detailed images with high resolution, enabling the evaluation of particle size distribution and surface characteristics such as roughness or coating structures.

In addition to providing qualitative information, SEM also supports quantitative analysis through precise size measurements. This is particularly useful in studying the effects of technological processes (e.g., hot-melt extrusion, spray drying) on the properties of the final product. For instance, SEM can be used to compare particle morphology before and after spray drying or to examine surface changes during hot-melt extrusion, thereby optimizing production processes and improving formulation quality.

Principle of Operation

SEM uses a focused electron beam to scan the surface of the sample. Signals generated at each point on the sample are collected to build a magnified image pixel by pixel. Scanning coils, located beneath the condenser lens, ensure that the beam is precisely directed across the sample surface in the X-Y plane.

Depending on the magnification, which can reach up to 2 million times, the beam scans a field of view ranging from a few micrometers to several millimeters. Large samples can be analyzed directly without additional preparation if they fit within the sample chamber.

Key Signals in SEM

The primary signals used to create SEM images include backscattered electrons (BSEs) and secondary electrons (SEs). Both types of signals are generated when the electron beam interacts with the subsurface of the sample. SEs originate from a shallower region than BSEs, allowing SEM to produce detailed images of the sample’s surface. These images help observe:

• • Surface morphology
  • Surface composition
  • Surface topography

If the sample is not completely flat, SEM images may exhibit a 3D effect, providing a deeper and more vivid perspective.

Accelerating Voltage in SEM

The accelerating voltage in SEM typically ranges from 1 keV to 30 keV:

• • Low Voltage (1-5 keV): Produces a “soft” beam, suitable for analyzing electron-sensitive or insulating samples.
  • High Voltage (20-30 keV): Enhances spatial resolution.

For example, a Field Emission SEM (FE-SEM) using an SE detector can resolve features smaller than 2 nm, enabling detailed observation of extremely fine sample characteristics.

Applications in Solid Dispersed Systems

Particle Morphology and Size Analysis: SEM is used to observe the shape, size, and size distribution of particles in solid dispersed systems.

System Homogeneity Evaluation: SEM combined with EDS (Energy Dispersive X-ray Spectrometer) can analyze the distribution of components in solid dispersed systems, thereby assessing system homogeneity.

Crystallization Studies: SEM can be used to observe the formation and growth of crystals in solid dispersed systems, helping evaluate system stability.

Transmission Electron Microscopy (TEM)

TEM provides high spatial resolution images and is a useful analytical technique for studying solid dispersed systems. Electron diffraction and atomic lattice imaging can detect crystalline volumes and are also used to identify different polymorphic forms.

Principle of Operation:

TEM creates magnified images of the sample by projecting a broad electron beam through the specimen and capturing the transmitted electrons in a single frame. Unlike SEM, TEM does not require scanning coils to move the beam across the sample. Instead, a broad beam (commonly referred to as “parallel illumination“) is produced by the illumination system.

The objective lens in TEM plays a crucial role in producing high-resolution images from electrons transmitted through the sample’s exit surface.

TEM is a destructive technique, particularly for organic compounds, and the amount of electron dose absorbed by the sample must be controlled to prevent sample alteration during analysis. Irradiating the sample with a high-energy electron beam (up to 300 keV) generates secondary electrons, which in turn create free radicals and ions. These highly reactive species cause bond breaking, known as the radiation damage process.

Magnification of TEM

The magnification of TEM images can exceed 50 million times, allowing direct observation of crystal structures at the atomic level. However, the sample must be extremely thin (usually less than 100 nm) for the electron beam to pass through.

Types of Transmitted Electrons Analyzed in TEM

Transmitted electrons can be classified into Direct Beam and Diffracted Beams.

Intermediate lenses and apertures beneath the sample can be adjusted to form images from specific diffracted beams. This helps to better understand the crystal structure and defects in the sample.

Accelerating Voltage in TEM

The accelerating voltage in TEM typically ranges from 30 kV to 300 kV, significantly higher than in SEM. This high voltage enables the creation of images with extremely high resolution.

A TEM with aberration correction can achieve spatial resolution below 1 Å (angstrom), allowing the observation of the smallest details in materials, such as the atomic structure of nanoparticles.

Applications in Solid Dispersed Systems

Detecting Low Levels of Crystallinity: TEM is more sensitive than techniques like pXRD, FTIR, and DSC in detecting crystallinity in solid dispersed systems.

Polymorph Identification: TEM can be used to determine different polymorphic forms of pharmaceutical substances in solid dispersed systems by analyzing electron diffraction.

Recrystallization Mechanism Studies: TEM can provide information about the location, size, and shape of crystals formed during recrystallization, helping to better understand recrystallization mechanisms.

Comparison of SEM and TEM

Conclusion

SEM and TEM are two complementary techniques in pharmaceutical research. SEM is ideal for surface and morphological analysis, while TEM is suitable for studying structure and composition at the atomic level. The choice between the two methods depends on the specific research objective and the type of specimen to be analyzed.

References

[1] Kính hiển vi điện tử quét (SEM) và kính hiển vi điện tử truyền qua (TEM) để xác định đặc tính vật liệu

[2] Đặc điểm của các chất phân tán rắn vô định hình và xác định mức độ tinh thể thấp bằng kính hiển vi điện tử truyền qua

[3] Sự khác biệt giữa SEM và TEM

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