Nanomaterials Properties Lecture 2024/2025 PDF

By Dr. Bishoy Samy 2024/2025  PROPERTIES OF NANO-CRYSTALS 1- Optical properties 2- Electrical properties 3- Thermal properties 4- Magnetic properties 5- Mechanical properties  PROPERTIES OF NANO-CRYSTALS 1- Optical Properties, The optical properties of na...

By Dr. Bishoy Samy 2024/2025  PROPERTIES OF NANO-CRYSTALS 1- Optical properties 2- Electrical properties 3- Thermal properties 4- Magnetic properties 5- Mechanical properties  PROPERTIES OF NANO-CRYSTALS 1- Optical Properties, The optical properties of nanomaterials are very interesting to study because of their nanoscale dimension, these properties are strongly influenced by a number of factors such as: 1- size 2- shape 3- surface functionalization 4- doping (doping is the intentional introduction of impurities into an intrinsic semiconductor for the purpose of modulating its electrical, optical and structural properties). 5- The interactions with other materials, etc.  SIZE EFFECT ON OPTICAL PROPERTIES OF NANOMATERIALS 1. Band gap (semiconductors): The optical band gap increases with the decrease in particle size, especially for the semiconductor nanomaterials. Thus the colloidal quantum dots solutions produce different colors for different sizes, especially in the range of 1–10 nm.  SIZE EFFECT ON OPTICAL PROPERTIES OF NANOMATERIALS 2. Light attenuation size dependence: When light is incident on a nanoparticle, it can be scattered or absorbed. In nanoparticles, the fraction of light that is scattered or absorbed can vary greatly depending on the particle diameter. (At sizes˂ 20 nm, mainly absorption. Above 100nm, mainly scattering) The optimal amount of scattering and absorption can be achieved through a proper particle design. Aggregate formation can also affect light absorption/scattering by nanoparticles (can increase the effective size of a nanoparticle resulting in an increase in scattering). Ex, a solution of 20 nm diameter silica particles is clear but precipitating, drying and re-suspending the silica particles (aggregated) will give a milky white color (scattering). Aggregated  SIZE EFFECT ON OPTICAL PROPERTIES OF NANOMATERIALS 3. Plasmon Resonances for Small Spherical Particles (metals): Dipole plasmon resonance: When a small spherical metallic nanoparticle is irradiated by light (electromagnetic radiation), the oscillating electric field polarize the particle causing the conduction electrons to oscillate coherently. When the electron cloud is displaced relative to the nuclei, a restoring force arises from Coulomb attraction between electrons and nuclei that results in oscillation of the electron cloud relative to the nuclear framework. The wavelength at which e-s resonate depends on NPs size.  SIZE EFFECT ON OPTICAL PROPERTIES OF NANOMATERIALS 3. Plasmon Resonances for Small Spherical Particles (metals): Quadrapole plasmon resonance (shape effect): Higher modes of plasmon excitation where half of the electron cloud moves parallel to the applied field and half moves antiparallel ex, nanorods. 2- Electrical Properties Electrical properties of materials are based on the movement of electrons and the spaces, or “holes,” they leave behind and on the crystal ordering as well. These properties are based on the chemical and physical structure of the material. In bulk materials, electrical conductivity decreases with a reduced dimension due to increased surface scattering. On the other hand, nanomaterials have been found to have some interesting electrical properties and they can show even better electrical conductivity as a result of a better atomic ordering in the nanoscale. ✓ Example: Polymeric fibers 2- Electrical Properties Carbon nanotubes: ✓ Unique electrical properties. ✓ Their electrical properties change with Diameter Twist (direction in which the sheet was rolled up) Number of walls ✓ They can be either conducting or semi-conducting in their electrical behavior. If it's rolled so that its hexagons line up straight along the tube's axis, the nanotube acts as a metal. If it's rolled on the diagonal, so the hexagons spiral along the axis, it acts as a semiconductor  3- Thermal Properties 1. Melting point: Nanoparticles are found to have lower melting temperatures when the particle size decreases below 100nm. ✓ The relationship between the change in melting point (∆𝜽) and nanoparticle radius (r) is given by: ∆𝜽 = 𝟐𝑻𝒐 𝝈/𝝆𝑳𝒓 Particle radius Deviation in melting point Bulk melting point Particle density Latent heat of fusion Surface tension coefficient Au NPs 3- Thermal Properties 1. Melting point: Nanoparticles are found to have lower melting temperatures when the particle size decreases below 100nm. ✓ Lowering of the melting point is proportional to 1/r. ✓ ∆𝜽 can be as large as couple of hundred degrees when the particle size gets below 10 nm! ✓ Most of the time, 𝝈 the surface tension coefficient is unknown; by measuring the melting point as a function of radius, 𝝈 can be estimated. 3- Thermal Properties 2. Other phase transition temperatures: ✓ Other phase transitions have similar size dependence. ✓ For example, the ferroelectric-paraelectric transition temperatures, or the Curie temperatures (temperature above which, ferromagnetic materials lose their permanent magnetic properties) for barium titanate( BaTiO3) : Bulk material: 130°C Below 200nm: decreases drastically (75°C at size120nm). 3- Thermal Properties 2. Lattice constants: ✓ The change of crystal structure may occur when the dimension of materials is sufficiently small. For example : Lattice constants of CdS nanopaticles decrease linearly with an increasing reciprocal particle radius. Surface modified Bare 3- Thermal Properties 2. Lattice constants: ✓ Another example: Self annealing in thin films Bulk material are usually annealed to induce phase transitions into more favorable crystal structure. Cu, before annealing Cu, annealed at high temp. At the nanoscale, amorphous Cu thin films can crystallize into the more thermodynamically favored fcc-Cu simply by aging at room temperature.  SIZE EFFECT ON MAGNETIC PROPERTIES OF NANOMATERIALS Magnetic properties of nano materials are different from that of bulk materials. In bulk materials: Paramagnetic materials: Do not retain their magnetic properties after removal of external magnetic field. Ferromagnetic materials: consist of magnetic domains in which, large numbers of atom's moments (1012 to 1015) are aligned so that the magnetic force within the domain is strong. When a ferromagnetic material is in the unmagnetized state, the domains are nearly randomly organized and the net magnetic field for the part as a whole is zero. When a magnetizing force is applied, the domains become aligned to produce a strong magnetic field and retain its magnetization after removing the external magnetic field.  SIZE EFFECT ON MAGNETIC PROPERTIES OF NANOMATERIALS In Nanomaterials: At nanoscale, when the size of a ferromagnetic material decrease below a critical size, there will be no more domain formation and it will behave as a single atom (but with much larger magnitude) that can flip their magnetization freely under the effect of thermal activation (at room temperature) Superparamagnetic material ✓ Behaves as paramagnetic material but with huge magnetization (sum of the magnetic moments of all atoms forming the nanoparticle) ✓ Don’t retain their magnetic moment after removing the magnetic field (no longer behave as ferromagnetic materials. For applications requiring permanent magnets: ✓ Larger (ferromagnetic) nanoparticles are required. Ex, memory storage, Ferrofluids For applications requiring enhanced magnetic responses: Smaller (superparamagnetic) nanoparticles are required. Ex, contrasting agents for magnetic resonance imaging (MRI), Drug delivery systems.  MECHANICAL PROPERTIES, The measurable properties that allow a metal to resist external forces without failing are its mechanical properties (mechanical strength, hardness, brittleness, ductility, toughness,…. ). Calculated values for mechanical strength of ideal crystals are usually 100 to 1000X the experimental values. This is usually explained in terms of the defects inside the crystal as well as surface defects. It was noted that mechanical properties of materials are enhanced with decreasing size. NOT ALWAYS TRUE !!! Ex, Whiskers and Nanowires: ✓ A whisker has mechanical values approaching theoretical ones. ✓ This is noticed only if the diameter is less than 10 microns. The enhancement in mechanical strength starts in the micrometer scale.  MECHANICAL PROPERTIES, Enhancement in mechanical properties of nanomaterials is related to: 1. High internal perfection: Imperfections in crystals are highly energetic and small size makes elimination of imperfections possible. Some imperfections in bulk materials, such as dislocations are often created to accommodate stresses generated in the synthesis and processing of bulk materials due to temperature gradient and other inhomogeneities. Such stresses are unlikely to exist in small structures. 2. Perfection of the side facets: In general, smaller structures have less surface defects.  MECHANICAL PROPERTIES, Examples: 1. Carbon nanotubes: One of the strongest materials in nature. v v 2. Composite materials: ✓ Adding 3 wt/% nano-SiO2 to concrete can improve its compressive strength, bending strength, and splitting tensile strength. ✓ Mechanical properties of polymeric materials can be increased by the addition of nano-fillers. Note: There is no solid understanding on the size dependence of the mechanical properties !!