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CH 8 Characterization and Properties of nanomaterials (Structural…
CH 8 Characterization and Properties of nanomaterials
Structural Characterization
XRD
Bragg's Law
Peak broadening:
[Scherrer's formula]
Useful in characterizing nanoparticles, film thickness of epitaxial and highly textured thin-films
Disadvantage compare to SAED: low intensity of diffracted X-rays for low-Z materials (Thus more sensitive to high-Z materials)
SAXS
(Small angle X-ray scattering)
Strong diffraction peaks result from constructive interference of X-rays scattered from ordered arrays of atoms and molecule.
The amount and angular distribution of scattered intensity provides: size of very small particles, surface area per unit volume regardless whether the sample or particles are crystalline or amorphous
For spherical particles with uniform size:
Porod's law is observed
Two reasons deviate from the Porod's law:
1) The presence of smeared transition boundaries between the phases
2) The existence of electron density fluctuations in inhomogeneity regions, over distances exceeding interatomic ones
SEM
Resolution: ~nanometers Resolving power:
(NA=numerical aperture)
SE v.s. BSE
Surface v.s. content
EDS
TEM
SAD v.s. Imaging
No inherent ability to distinguish atomic species, electron scattering is sensitive to the target element and various spectroscopy are developed for the chemical composition analysis. (EDS, EELS)
SPM
(Scanning probe microscopy)
Provides 3D real-space images; allow spatially localized measurements of structures and properties. (STM and AFM, check CH7)
Gas adsoption
I: monolayer sorption in pores of molecular dimension; II, IV, V: multilayer sorption in highly porous materials with pores up to ~100 nm; III: multilayer sorption on a nonwetting material
Chemical Characterization
Optical spectroscopy
Absorption and emission
Photoluminescence: emission of light by a material throguh any proces other than blackbody radiation
Vibrational
IR and Raman
Ionic spectrometry
Electron spectrometry
Energy Dispersive X-ray spectroscopy, Auger electron spectroscopy, X-ray photoelectron spectroscopy, etc.
Relies on the unique energy levels of the emission of photons
Physical Properties of Nanomaterials
Melting points and lattice constants
Mechanical properties
Optical properties
Surface plasmon resonance
Quantum size effects
When the diameter of nanowires or nanorods reduces below the de Broglie wavelength, size confinement would also play an important role in determining the energy level just as for nanocrystals. For example, the edge of Si nanowires has a significant blue shift with sharp, discrete features and silicon nanowires also has shown relatively strong band-edge photoluminescence.
Large fraction of surface atoms; 2. large surface energy; 3. spatial confinement; 4. reduced imperfections
e.g.
low melting point or phase transition temp and reduced lattice constants;
mechanical properties of nanomaterials may reach the theoratical strength (higher than bulk) due to reduced probability of defects;
significantly different optical property (increased band gap);
conductivity decreases with a reduce dimension due to increased surface scattering (but could be enhanced by better ordering in microstrucutre);
magnetic properties are different (ferromagnetism may disappear and transfers to superparamagnetism in the nanometer scale due to the huge surface energy);
self-purification is an intrinsic thermodynamic property of nanostructures; reduced defects lead to chemical stability enhancement.
Electrical conductivity
Surface scattering
Change of electronic structure
Quantum transport
Ballistic conduction: the conducting channel contributes
to the total conductance
There exist no elastic scattering
The latter requires the absence of impurity and defects, when elastic scattering occurs, the transmission coefficients, and thus the electrical conductance will be reduced, which is then no longer precisely quantized
Coulomb blockade (charging): when the contact resistance is larrger thant he resistance of nanostructures in question and when the total capacitance of the object is so small that adding a single electron requires significant charging energy
(capacitance for a nanoparticle surrounded by a dielectric with a dielectric constant of (er))
The energy required to add a single charge to the particle is given by the charging energy:
Tunneling conduction (ch7)
Lower synthesis temp also favors a better alignment and thus a higher electrical conductivity
Ferroelectrics and dielectrics
Dielectric constant of ferroelectrics would increase with a decreasing grain size to 1micron in diameter and then decrease with further decrease in grain size or film thickness
Superparamagnetism
The magnetization curve must show no hysteresis since that is not a thermal equilibrium property
The magnetization curve for an isotropic sample must be temperature dependent to the extent that curves taken at different temps must approximately superimpose when plotted against H/T after correction for the temp dependence of the spontaneous magnetization
