Part 3 - Characterization of nanoparticles

Question 1

How can light interact with matter?

Answer 1

1. Scattering
2. Transmission
3. Absorption
4. Interference
5. Reflection
6. Refraction
7. Difraction

Question 2

What types of light scattering exists? Elastic or inelastic?

Answer 2

1. Rayleigh - elastic
2. Raman - inelastic
3. Mie - elastic

Question 3

What types of Raman scattering are there?

Answer 3

Stokes (scattered light have a smaller frequency than incident) and Anti-Stokes (opposite)

Question 4

What is the difference between Rayleigh and Mie scattering?

Answer 4

Dependent on particle size (Mie for larger particles). Mie scattering scatters light forward a lot more than Rayleigh.

Question 5

Write down the Beer-Lambert law.

Answer 5

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.

Question 6

What assumptions is the Beer-Lambert law based on?

Answer 6

• 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.

Question 7

What is the difference between localized surface plasmons and surface plasma polaritons?

Answer 7

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.

Question 8

Why are optical properties of nanomaterials interesting?

Answer 8

Because they change based on both intrinsic (changes due to surface-to-volume ratio) and extrinsic (cluster size) so they are highly tunable.

Question 9

Name some fields where LSPR can be utilized.

Answer 9

Catalysts, targeted drug-delivery, optical switches and amplifiers, biosensors, biomedical diagnostics, non-linear optics.

Question 10

What does the LSPR depend on?

Answer 10

1. Metal itself
2. Size and shape
3. Surface functionalities
4. Chemical environment.

Question 11

What is measured in dynamic light scattering (DLS)?

Answer 11

The movement of the particles. Larger particles -> moves slower -> intensity varies slowly. Smaller particles the opposite.

Question 12

What does the auto-correlation function measure?

Answer 12

How a signal is dependent on itself at increasing time intervals.

Question 13

How does the auto-correlation vary for small and large particles?

Answer 13

Autocorrelation diminishes more quickly for smaller particles.

Question 14

Write down the Siegert relation for monodisperse particles.

Answer 14

g(s,t) = exp(-s^2 Dt_d)

s = 4pi / lambda * sin(theta / 2)
Magnitude of scattering vector

Question 15

Write down the Siergert relation for polydisperse particles.

Answer 15

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

Question 16

Write down the Stokes-Einstein equation

Answer 16

D = kT / 6 pi eta R_h

eta = viscocity
R_h = hydrodynamic radius

Question 17

How can we write the probabilty of a particle being at position r at time t when Brownian motion is assumed?

Answer 17

P(r,t | 0,0) = (4piDt)^-3/2 exp (-r^2/4Dt)

Question 18

How does the peaks of different particle sizes in DLS-measurements look like for equal numbers of particles of both sizes?

Answer 18

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.

Question 19

What is the basic principle of differential scanning calorimetry?

Answer 19

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.

Question 20

Give examples of some exothermic and endothermic processes that can be seen in DSC.

Answer 20

Exothermic:
Crystallization
Cross-linking

Endothermic:
Glass transition
Melting

Question 21

Give the fundamental principle of XPS.

Answer 21

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.

Question 22

What are the depth XPS penetrates to?

Answer 22

About 10 nm.

Question 23

How can XPS be used to determine the binding types?

Answer 23

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).