14.2 Characterization Techniques: XRD, SEM, and TEM

X-ray diffraction and electron microscopy are powerful tools for studying materials at the atomic level. These techniques reveal crystal structures, surface features, and internal compositions, providing crucial insights into the properties and behavior of inorganic compounds.

XRD uses X-rays to analyze crystal structures, while SEM and TEM use electron beams to create high-resolution images. Together, these methods offer a comprehensive view of materials, from atomic arrangements to surface topography, essential for understanding and developing new inorganic materials.

X-ray Diffraction (XRD)

Principles and Applications of XRD

• X-ray diffraction utilizes X-rays to determine crystal structures of materials
• Involves bombarding a sample with X-rays and analyzing the resulting diffraction pattern
• Provides information about atomic and molecular structure of crystalline materials
• Widely used in materials science, chemistry, and structural biology
• Helps identify unknown substances by comparing diffraction patterns to databases

Bragg's Law and Diffraction Mechanics

• Bragg's Law describes the conditions for constructive interference of scattered X-rays
• Expressed mathematically as $n\lambda = 2d\sin\theta$
• $n$ represents the order of reflection (integer)
• $\lambda$ denotes the wavelength of incident X-rays
• $d$ signifies the interplanar spacing in the crystal lattice
• $\theta$ indicates the angle between the incident ray and the scattering planes
• Constructive interference occurs when the path difference equals an integer multiple of the wavelength
• Produces bright spots or peaks in the diffraction pattern

Types of XRD Techniques

• Powder XRD analyzes polycrystalline or powdered samples
• Involves grinding the sample into a fine powder to ensure random orientation of crystallites
• Produces a diffraction pattern with concentric rings (Debye-Scherrer rings)
• Used for phase identification, quantitative analysis, and determination of unit cell parameters
• Single-crystal XRD examines individual crystals of sufficient size and quality
• Provides more detailed structural information compared to powder XRD
• Allows determination of complete crystal structures, including atomic positions and bond lengths
• Requires careful sample preparation and alignment of the crystal

Electron Microscopy

Scanning Electron Microscopy (SEM)

• SEM uses a focused beam of electrons to scan the surface of a sample
• Produces high-resolution images of surface topography and composition
• Electrons interact with atoms in the sample, generating various signals
• Secondary electrons provide information about surface topography
• Backscattered electrons offer compositional contrast based on atomic number
• Typical resolution ranges from 1-20 nm, depending on the instrument and sample
• Samples must be conductive or coated with a conductive material (gold, carbon)

Transmission Electron Microscopy (TEM)

• TEM transmits a beam of electrons through an ultra-thin sample
• Produces high-resolution images of internal structure and composition
• Enables visualization of features at the atomic scale (resolution < 1 Å)
• Requires extremely thin samples (typically < 100 nm thick)
• Bright-field imaging shows mass-thickness contrast
• Dark-field imaging highlights specific crystallographic orientations
• High-resolution TEM (HRTEM) can resolve individual atomic columns

Advanced Electron Microscopy Techniques

• Electron diffraction provides information about crystal structure and orientation
• Produces diffraction patterns similar to XRD but with higher spatial resolution
• Selected Area Electron Diffraction (SAED) analyzes specific regions of a sample
• Energy-dispersive X-ray spectroscopy (EDS) enables elemental analysis
• Detects characteristic X-rays emitted when the electron beam interacts with the sample
• Allows mapping of elemental distributions across a sample
• Can be used in both SEM and TEM

Sample Preparation and Microscope Operation

• Sample preparation techniques vary depending on the material and analysis method
• SEM samples may require conductive coating, drying, or fixation
• TEM samples demand specialized thinning techniques (ion milling, ultramicrotomy)
• Focused Ion Beam (FIB) allows precise sample preparation for both SEM and TEM
• Resolution affected by factors such as electron source, lens quality, and sample characteristics
• Magnification in SEM typically ranges from 10x to 500,000x
• TEM magnification can exceed 1,000,000x, allowing visualization of atomic structures