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Characterization and application of XRD in two-dimensional materials


First, let's understand thin films. What kind of materials do we refer to as thin film materials?

  1. A thin film refers to a material layer with a relatively small thickness under certain conditions. Typically, the thickness of a thin film ranges from a few nanometers to several micrometers, making it thin compared to its width and length. The characteristics of thin films are primarily governed by materials exhibiting two-dimensional effects, and their properties on a two-dimensional scale differ from those on a larger scale. These properties may involve quantum effects, electronic behavior, and other phenomena distinct from macroscopic-scale properties.

  2. The scale of two-dimensional materials in the out-of-plane direction is much smaller than in-plane dimensions.


Epitaxial layer

Growing or depositing a single crystal layer with the same structure on a single crystal substrate is referred to as a single crystal epitaxial thin film.

A layer formed by atoms or molecules arranged in a specific, repetitive manner in three-dimensional space on a substrate crystal is characterized by unique physical and chemical properties such as hardness, transparency, conductivity, etc. The orientation of this thin film layer is typically determined by the substrate material, and the crystal orientation of the upper layer is influenced by the underlying crystal. In other words, the arrangement of the upper-layer crystals tends to align as closely as possible with the arrangement of the lower-layer crystals, determining the orientation of the upper-layer crystals in a specific direction. This direction can be along a certain axis (for example, a crystal can be oriented along a specific axis) or on a certain plane (for example, a crystal can be oriented on a specific plane).


Homoepitaxy: The thin film material and substrate material have the same crystal structure or lattice parameters. Heteroepitaxy: The thin film material and substrate material have significant differences in crystal structure or lattice parameters.

So, what are the types of structures for thin films? Generally, we categorize them into the following types:


Complete epitaxy:Single crystal layers grow perfectly on the substrate.

Near complete epitaxy:Both in-plane and out-of-plane directions are close to the substrate, but there are some embedded blocks or mosaic structures.

Textured polycrystalline:The out-of-plane direction corresponds to a layer that grows in a specific orientation, but the in-plane direction exhibits grains with random orientations.

Polycrystal:The thin film material is completely non-oriented, and the grains grow randomly.

Amorphous:The material itself lacks a long-range ordered three-dimensional structure.

From the grown thin film, we need to obtain specific information to understand the material's structure. The main points include:

  1. Crystalline state of the film layer
  2. Quality of the crystalline structure
  3. Presence of strain
  4. Defect structures
  5. Chemical composition
  6. Thickness
  7. Surface and interface roughness
  8. Density