Optical effects in nano-dimensional stuctures

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Optical effects in nano-dimensional stuctures
Benny Hallam Paper Technology
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Outline

• What impact could nano-technology have on the
  optical properties of paper?
• Does embracing nano-technology require a fresh
  approach to thinking about paper?
• How do we define and use the building blocks we
  have?
• What results might we expect? Observations of
  nano-dimensional structures in Nature may give us
  an indication....

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Paper optics
Paper coating
Base paper
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Paper optics
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Paper optics
Nano-dimensional structure
Kubuelka-Munk considers the paper sheet as a
black box Light is reflected, absorbed or
transmitted according to S and K
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But what about the detail inside the box?
Nano-dimensional structure
Structural colour is not inherent in the
material, it is a result of its detailed
arrangement
Some images supplied by P Vukusic, University of
Exeter
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Biomimicry

• K Watanabe et al., Jpn J. of Appl. Phys., 44,
  L48-L50 (2005)
• Fabricated by FIB-CVD

• H Xu et al., Materials Lett., 58 27-28, 3419-3423
  (2004)

• H Xu et al., Materials Lett., 58 27-28, 3419-3423
  (2004)

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What is the purpose?

• What might we gain through exploitation of
  nano-dimensional structures?
• Case studies
• Optical efficiency - leading to high performance
  in light weight papers
• A new approach to achieving paper shade in white
  and coloured papers

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1. Optical efficiency in ultra-thin structures
P Vukusic, B Hallam and J Noyes, Brilliant
whiteness in ultrathin beetle scales, Science,
315, 348 (2007)

• South-east Asian beetle (Cyphochilus spp.) is
  covered with a white surface structure
• Underneath is black

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Cyphochilus spp.

• Surface covered by small scales
• Scales contain a random lattice of interwoven
  fibrils c.200nm diameter
• Only 5 micron thick, making it interesting for
  understanding optical performance/weight
  relationship
• Void volume is ca. 30, which is high compared to
  typical paper coating layers (5-10 for 100 clay
  and 10-20 for 100 CO3)

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Optical efficiency

• Beetle outperforms mineral coating layers at same
  coat weight
• Requires twice the thickness of PCC to match
  beetle's optical performance

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Reflectivity spectra for 5 micron thick films

• The reflection spectra show very different
  behaviour patterns
• Pores in coating structure are too small, giving
  wavelength dependent scatter

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Cyphochilus spp.
Steepness (D30/D70) 100
D70
D30

• How can we introduce greater void volume into
  coating structure?
• Moving to narrower psd pigments increases coating
  steepness....and with fewer fines....increases
  the size of the voids in the coating structure

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Cyphochilus spp.

• Moving to steeper pigment systems increases pore
  size and improves scatter
• But care must be taken as we are also changing
  average particle size, and number density of
  pores not just pore volume - can we do more?

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2. Absorptive vs structural colour

• Violet dye has been added to hand sheets to
  provide a close match to Morpho Didius butterfly
• Match is best at approximately 4.0 dye loading

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Absorptive colour

• Dyes provide colour through subtractive means
• Clariants Cartazine Violet RN dye was chosen for
  this study
• Hand sheets 80/20 PGW/Kraft, 70/30 Imax60/IC85
  (at 30.2 loading level), 52gsm
• Sheets 100 micron thick

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Morpho - structural colouration

• The Morpho spp. of butterfly is characterised by
  high reflectivity
• This is a result of structural colour, not
  absorption - in practice this means selective
  reflection

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Morpho - multilayer reflection
3mm
100mm
Source P. Vukusic, Exeter University

• Wings comprise overlapping scales, similar to
  tiles on a roof
• Iridescence comes from within the scale
• Interesting microstructure observed in
  cross-section
• Structure is only 5 microns thick (20x thinner
  than the hand sheets)

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Morpho - multilayer reflection

• Christmas tree structure approximates a
  multi-layer stack
• Reflections from each interface interfere to
  provide the reflected appearance
• Only certain colours will interfere
  constructively, depending on n and d

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Comparison of colour mechanisms
To achieve a similar colour purity and shade, the
Morpho demonstrates much higher luminance despite
being 20x thinner

• Paper CIE L decreases with dye addition
• The paper starts with a higher L than the
  butterfly, unsurprising since it is scattering
  the full spectrum
• At similar colour purity and shade, the
  structural colour gives 25 greater luminance
  despite being 20x thinner

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Comparison - brightness
Similar results are seen in brightness
measurements

• Similar results seen in Brightness measures
• These results suggest that it is not possible to
  obtain saturated colour without significantly
  sacrificing the intensity of the visual
  experience through subtractive colouration
• It may be impossible to obtain highly saturated
  colours at all without considering
  nano-dimensional structuring

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What are the available building blocks?

• The wealth of different minerals may offer the
  scope for development of novel architectures in
  the future with suitable structuring

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Conclusions

• Controlling structural photonics through
  nano-dimensional architectures offers a new path
  for advancing paper optics
• Visual benefits may include
• opacity
• shade
• speciality grades (coloured papers, etc)
• anti-counterfeit/document security
• enhanced performance in LWC
• Structures may also be considered for the UV -
  such as enhancing fluorescent emission from FWAs
• Infra-red compatible structures may also be
  considered for reducing drying times