To understand how small a nanometer is, we typically compare the nanoscale to objects that we know how big or small they are. Another way to help understand the minuteness of a nanometer is to examine objects on a size scale. Nanoscale is defined in the range of 1-100 nm to cover almost the entire critical length of characteristic physical phenomena.
Photography, which was developed in the 18th and 19th centuries, provides a more recent example of the use of silver nanoparticles. In the early twentieth century, Gustav Mie introduced the Mie theory, which is a mathematical treatment of light scattering that describes the relationship between the size of metal colloids and the optical properties of the solutions containing them. From these discussions, it can be concluded that nanoscale materials are not new at all.
Important properties of materials, such as electrical, optical, thermal, and mechanical properties, are determined by how molecules and atoms assemble at the nanoscale into larger structures. Nanotechnology deals with the manipulation, control and integration of atoms and molecules to form nanoscale materials, structures, components, devices and systems with their industrial and commercial applications. These materials are created from building blocks that have at least one dimension in the nanoscale range of 1-100 nm.
It is a solid particle that is a single crystal with all three dimensions in the nanometer.
Surface Effect
Volume of the layer or shell with a thickness 𝛿𝛿 on the surface of a spherical particle with a radius 𝑅𝑅 can be determined from the following equation,. The ratio of the volume of the surface layer or shell to total volume of a particle can be determined by. Nanoparticles have a "structural magic number", that is, the optimum number of atoms that leads to a stable configuration while maintaining a specific structure.
If we know the crystal structure, we can calculate the number of atoms per particle. The energy required to create a new surface containing 𝑁𝑁𝑏𝑏 atoms is given by 𝑁𝑁2𝑏𝑏𝑢𝑢, where 𝑢𝑢 is the bond energy between two atoms. Due to the reduced number of neighbors on each surface of the atom, the force 𝑓𝑓 acts perpendicular to the surface.
The amount of surface energy per particle 𝑢𝑢𝑠𝑠𝑢𝑢𝑟𝑟𝑠𝑠𝑎𝑎𝑐𝑐𝑠𝑠 is equal to 𝛾𝛾𝐴𝐴, where 𝛾𝛾 is the specific surface energy and 𝐴𝐴 is the surface area of one particle. From the equation, the surface energy per mole increases by 1/d, and in some cases, especially those involving very small particles, this can have dramatic consequences.
Quantum confinement effect 58
Quantum confinement effect 59
Reduced imperfections 60
Reduced imperfections 61
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Reduced imperfections 65
Reduced imperfections 66
Crystal properties 68
Crystal properties 69
Crystal properties 70
Crystal properties 71
Crystal properties 72
Crystal properties 73
Mechanical properties 74
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Mechanical properties 77
Mechanical properties 78
Thermal properties 79
Thermal properties 80
Thermal properties 81
Thermal properties 82
Thermal properties 83
Thermal properties 84
Optical properties 85
Optical properties 86
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Optical properties 88
Optical properties 89
O PTICAL P ROPERTIES 90
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O PTICAL P ROPERTIES
O PTICAL P ROPERTIES 95
O PTICAL P ROPERTIES 96
E LECTRICAL P ROPERTIES
S URFACE S CATTERING 98
S URFACE S CATTERING 99
- Magnetic properties 100
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- Chemical properties 105
The particle size decreases, the coercivity increases to a maximum and then decreases towards zero. When the size of single-domain particles is further reduced below a critical diameter, the coercivity becomes zero and such particles become superparamagnetic. When the size of NPs decreases, they become a single domain, and thus magnetization reversal occurs via coherent rotation, which has a high energy cost and leads to an increase in the coercive field.
If the size of the NPs is too small, thermal agitation is not negligible and induces fluctuations and,.
111 (D-1). Q UANTUM SIZE EFFECTS
These changes arise through systematic transformations in the density of electronic energy levels as a function of size, and these changes result in strong changes in optical and electrical properties with size. In the case of metals, where the Fermi level lies in the center of a band and the corresponding energy level spacing is very small, the electronic and optical properties more closely resemble those of the continuum, even relatively so. In semiconductors, the Fermi level lies between the two bands, so the band edges dominate the low-energy optics and.
For insulators, the band gap between two bands in bulk form is already too large. The quantum size effect is most pronounced for semiconductor nanoparticles, where the bandgap increases as size decreases, shifting the interband transition to higher frequencies. In a semiconductor, the energy separation is on the order of a few electrovolts and increases rapidly.
For metal nanoparticles, to observe the localization of the energy levels, the size must be well below 2 nm, (since the level spacing must exceed the thermal energy. The discrete electronic energy level has been observed in far-infrared absorption measurements of gold In addition to the size- limitation, light emitted by nanowires is highly polarized along their longitudinal directions.
116 (D-2). S URFACE PLASMON RESONANCE
The increase in both absorption wavelength and peak width with increasing particle size: extrinsic size effects. For example, conduction electrons in silver and gold have an average free path of 40-50 nm and will be constrained by the particle surfaces in 20 nm particles. If the electrons scatter with the surface in an elastic but totally random way, the coherence of the overall plasmon oscillation is lost.
In this region, the absorption wavelength increases, but the peak width decreases with increasing particle size: Intrinsic size effect. Longitudinal modes can be easily tuned to cover the spectral region from visible to near-infrared by controlling their aspect ratios. Gold nanorods with an aspect ratio of 2-5.4 could fluoresce with a quantum yield more than a million times greater than that of the bulk metal.
Electrical conductivity in metals or (Ohmic conductivity) can be described by the different electron scattering, the total resistance, ρT, is a combination of individual and independent scattering, known as Matthiessen's rule：. However, the defect scattering makes a small contribution, and therefore has a very small influence on the electrical resistance. If the mean free electron path, λs, is the smallest, it will dominate the total electrical resistance:.
In elastic, also called specular scattering, the electron does not lose its energy and its momentum or velocity along the direction parallel to the surface, the electrical conductivity remains the same. When scattering is completely inelastic, non-specular or diffuse, the scattered electron loses its velocity in the direction parallel to the surface or conduction direction, and the electrical direction. The above model is based on the assumption that all surface scattering is inelastic and in terms of classical physics.
The fraction of elastic scattering on a surface is very difficult to determine experimentally: it is known that surface impurity and roughness favor inelastic scattering. Increased electrical resistivity of metallic nanowires with reduced diameters due to surface scattering has been widely reported. In a polycrystalline material, as the crystallite size becomes smaller than the electron mean free path, grains contribute to electrical resistivity.
132 (E-2). C HANGE OF ELECTRONIC STRUCTURE
133 (E-3). Q UANTUM TRANSPORT
Coulomb blockade (or Coulomb charging or Coulomb staircase) occurs when the contact resistance is greater than the resistance of. For a nanoparticle surrounded by a dielectric with a dielectric constant of εr, the capacitance of the nanoparticle depends on its size as:. Energy required to add a single charge to the particle is given by the charge energy:.
Tunneling: transport of charge through an insulating medium that separates two conductors that are very closely spaced.
C). T UNNELING CONDUCTION
137 (D). E FFECT OF MICROSTRUCTURE
Basic Concepts
THE TEMPERATURE DEPENDENCE OF CARRIER CONCENTRATION
Forward bias: A large number of charge carriers flow across the semiconductor as seen in a. Reverse bias: The junction area is relatively free of mobile charge carriers, the junction is highly insulating. At high reverse bias voltages, of the order of several hundred volts, a large number of charge carriers (electrons and holes) are generated.
ELECTRICAL RESISTIVITY OF MTALS
ELECTRICAL CONDUCTIVITY
The electrical properties are a result of its electron band structure: the arrangement of the outermost electron bands and the way in which they are filled with electrons. (Electrical conduction only occurs when there are available.
