Question 11

Question 1

q11a)

“Photonic balls” (PB) are obtained by the hierarchical self‐assembly of water‐stable polystyrene (PS) colloid dispersions emulsified in a continuous oil phase.

Describe their structural features and highlight their role in determining the optical properties of the photonic ball.

Answer 1

Photonic balls: assemblage of a polymer crystal (polystyrene colloids → hierarchical geometry) which exhibit grating scattering and Bragg diffraction. Synthesized with emulsion droplet flow synthesis.

Ordered crystalline layers (outside) → (1):
- structural color from Bragg diffraction (=constructive interference of light reflected at individual lattice planes) (defined by colloid size)
- no angular dependence
- the larger the colloid, the more red the color (intensity of Bragg reflection peak increases, crystallinity increases)

Ordered surface/periodic assemblies → (3):
- Grating effects (separating white light into its spectral components).
- For colloid size >400 nm
- gives bright, multichromatic color patches at outer regions of ball
- increasing size of colloids → grating diffraction dominates

Disordered core (by frustration) → (2):
- disorder and defects → nonresonant scattering → whitish appearance (can be decreased by gold nanoparticles)
- The smaller the ratio of the diameter of the ball and colloid (= more confinement) → the fewer ordered layer exist

Curvature of photonic balls:
- transmission of some light

Transmitted light: not inverse of reflection (only light above threshold wavelength is transmitted)

Question 2

q11b)

At the microscope, you notice that the photonic balls show a green coloration at the center.
What structural parameter brings to a brighter (higher reflected intensity) structural coloration?
How can you optimize such parameter experimentally?

Answer 2

Color: intense, uniform color in the center of the ball, outside: colorless rim
→ bc spherical nature of hierarchical assembly: only center reflects light that can be detected in the microscope (larger angles of incident/reflected light (blue shift) cannot be seen in the microscope.

For brighter structural color → increase the ball size → crystallinity increases of ball = more crystalline layers (more crisp color)

Optimize it experimentally: make bigger water droplets & higher colloid concentration in emulsion drop flow synthesis for less confinement and bigger balls

Question 3

q11c)

What is needed in order to obtain red (instead of green) coloration at the center of the PB when seen at the optical microscope?

Answer 3

• Change the colloid size (change Bragg reflection): small colloid is blue, medium colloid is green and large colloid is red.
• The smaller the colloid the more crystalline the ball.
  => use larger colloids

ratio dball/dcoll → the higher the ratio the higher the crystallinity

Question 4

q11d)

Name one strategy suitable to obtain mitigated scattering and “purify” the green appearance of PB dispersions obtained by the assembly of PS colloids with diameter of 225 nm

Explain your choice.

Answer 4

Gold nanoparticles (>12 nm) as subwavelength, spectrally selective absorbers into the photonic balls to suppress light scattering in spectrally unwanted regions -> purification of optical spectrum by suppression
- put in gold nanoparticles (don’t interfere with assembly of colloids) → dip in reflection (by plasmon resonance) → lowers reflected light intensity in red part of spectrum (put them in ordered part → increases color saturation)
- larger gold NP → absorbs longer wavelength = red (like quantum dots)

Critical thinking:
- In paper: used 12 nm gold nanoparticles to lower the reflected light intensity in blue/green part → red color more crisp → we don’t know if it works for green as well
- could also do even smaller gold NP to suppress the blue part?
- can also incorporate other NP with a different surface plasmon resonance → should absorb red (e.g. silver NP)