Part 3 - Characterization of nanoparticles Flashcards
How can light interact with matter?
- Scattering
- Transmission
- Absorption
- Interference
- Reflection
- Refraction
- Difraction
What types of light scattering exists? Elastic or inelastic?
- Rayleigh - elastic
- Raman - inelastic
- Mie - elastic
What types of Raman scattering are there?
Stokes (scattered light have a smaller frequency than incident) and Anti-Stokes (opposite)
What is the difference between Rayleigh and Mie scattering?
Dependent on particle size (Mie for larger particles). Mie scattering scatters light forward a lot more than Rayleigh.
Write down the Beer-Lambert law.
A = - log (I1 / I0) = alpha * c * l
I0 and I1 are incident and transmitted light.
alpha is the extinction coefficient.
c is the molar concentration
l is the length of medium the light must pass through.
What assumptions is the Beer-Lambert law based on?
- Absorbers must act independently of eachother.
- The absorber medium is homogenous.
- Medium must not be turbid, ie. the solute should not percipitate in larger agglomerates which scatter light.
- The incident light should not influence the atoms or molecules under study.
What is the difference between localized surface plasmons and surface plasma polaritons?
LSP: localized plasmon oscillation in nanoparticles
SP: oscillating electric dipoles propagating along surface in a waveguide-like fashion until released at some distance from origin.
Why are optical properties of nanomaterials interesting?
Because they change based on both intrinsic (changes due to surface-to-volume ratio) and extrinsic (cluster size) so they are highly tunable.
Name some fields where LSPR can be utilized.
Catalysts, targeted drug-delivery, optical switches and amplifiers, biosensors, biomedical diagnostics, non-linear optics.
What does the LSPR depend on?
- Metal itself
- Size and shape
- Surface functionalities
- Chemical environment.
What is measured in dynamic light scattering (DLS)?
The movement of the particles. Larger particles -> moves slower -> intensity varies slowly. Smaller particles the opposite.
What does the auto-correlation function measure?
How a signal is dependent on itself at increasing time intervals.
How does the auto-correlation vary for small and large particles?
Autocorrelation diminishes more quickly for smaller particles.
Write down the Siegert relation for monodisperse particles.
g(s,t) = exp(-s^2 Dt_d)
s = 4pi / lambda * sin(theta / 2)
Magnitude of scattering vector
Write down the Siergert relation for polydisperse particles.
ln g(s,t) = - Ds^2 t_d + sigma^2 s^4 t_d^2 / 2! + ….
D is the average diffusion coefficient.
sigma is the standard deviation of D distribution
Write down the Stokes-Einstein equation
D = kT / 6 pi eta R_h
eta = viscocity R_h = hydrodynamic radius
How can we write the probabilty of a particle being at position r at time t when Brownian motion is assumed?
P(r,t | 0,0) = (4piDt)^-3/2 exp (-r^2/4Dt)
How does the peaks of different particle sizes in DLS-measurements look like for equal numbers of particles of both sizes?
The number distribution is equal. The volume distribution is skewed as the volume goes like r^3. The intensity of Rayleigh scattering goes like r^6, so for equal numbers the peak for the larger particle will be much larger.
What is the basic principle of differential scanning calorimetry?
It measures the energy adsorbed or produces as a function of time. It uses a sample pan and a reference pan, and look at the differential heat flow (heat / time). The heat flow is a function of sample temeprature, so the in order to keep the pan at the same temperature different energies are needed when some process occurs.
Give examples of some exothermic and endothermic processes that can be seen in DSC.
Exothermic:
Crystallization
Cross-linking
Endothermic:
Glass transition
Melting
Give the fundamental principle of XPS.
X-ray is kicks out photoelectrons. The binding energy is calculated by looking at the difference in kinetic energy and energy of incoming light.
E_bind = hv - E_kin - phi
Phi is the work function of the detector.
What are the depth XPS penetrates to?
About 10 nm.
How can XPS be used to determine the binding types?
The binding energies of the electrons are shifted based on the chemical environment (different binding energy for C-electrons bound to F than H, due to different electronegativity).
