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7: Molecular and Solid State Structure
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7.1: Crystal Structure
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In any sort of discussion of crystalline materials, it is useful to begin with a discussion of crystallography: the study of the formation, structure, and properties of crystals. A crystal structure is defined as the particular repeating arrangement of atoms (molecules or ions) throughout a crystal. Structure refers to the internal arrangement of particles and not the external appearance of the crystal. However, these are not entirely independent.
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7.2: Structures of Element and Compound Semiconductors
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A single crystal of either an elemental (e.g., silicon) or compound (e.g., gallium arsenide) semiconductor forms the basis of almost all semiconductor devices. The ability to control the electronic and opto-electronic properties of these materials is based on an understanding of their structure. In addition, the metals and many of the insulators employed within a microelectronic device are also crystalline.
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7.3: X-ray Crystallography
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The significance of this for chemistry is that given this fact, crystalline solids will be easily identifiable once a database has been established. Much like solving a puzzle, crystal structures of heterogeneous compounds could be solved very methodically by comparison of chemical composition and their interactions.
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7.4: Low Energy Electron Diffraction
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Low energy electron diffraction (LEED) is a very powerful technique that allows for the characterization of the surface of materials. Its high surface sensitivity is due to the use of electrons with energies between 20-200 eV, which have wavelengths equal to 2.7 – 0.87 Å (comparable to the atomic spacing). Therefore, the electrons can be elastically scattered easily by the atoms in the first few layers of the sample. Its features, such as little penetration of low–energy electrons have positione
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7.5: Neutron Diffraction
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The first neutron diffraction experiment was in 1945 by Ernest O. Wollan using the Graphite Reactor at Oak Ridge. Along with Clifford Shull they outlined the principles of the technique. However, the concept that neutrons would diffract like X-rays was first proposed by Dana Mitchell and Philip Powers. They proposed that neutrons have a wave like structure, which is explained by the de Broglie equation.
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7.6: XAFS
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X-ray absorption fine structure (XAFS) spectroscopy includes both X-ray absorption near edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) spectroscopies. The difference between both techniques is the area to analyze and the information each technique provides.
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7.7: Circular Dichroism Spectroscopy and its Application for Determination of Secondary Structure of Optically Active Species
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Circular dichroism (CD) spectroscopy is one of few structure assessmet methods that can be utilized as an alternative and amplification to many conventional analysis techniques with advatages such as rapid data collection and ease of use. Since most of the efforts and time spent in advancement of chemical sciences are devoted to elucidation and analysis of structure and composition of synthesized molecules or isolated natural products rather than their preparation, one should be aware of all the
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7.8: Protein Analysis using Electrospray Ionization Mass Spectroscopy
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Electrospray ionization-mass spectrometry (ESI-MS) is an analytical method that focuses on macromolecular structural determination. The unique component of ESI-MS is the electrospray ionization. The development of electrospraying, the process of charging a liquid into a fine aerosol, was completed in the 1960’s when Malcolm Dole.
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7.9: The Analysis of Liquid Crystal Phases using Polarized Optical Microscopy
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Liquid crystals are a state of matter that has the properties between solid crystal and common liquid.
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