Features
Until spring 2008 the following features of organic nanofibers have been demonstrated:
- Polarized light emission
- Anisotropic light emission and random lasing
Optically pumped nanofiber: small pumping rate (a), high pumping rate (b,lasing), anisotropic light emission (c).
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Emission of different colors, UV, blue, green, red
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Guiding of electromagnetic waves
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Optical frequency doubling
Spectrum of emitted light from nanofibers after infrared excitation at two different wavelengths. At 830 nm only second harmonic generation (SHG) appears (blue light at 415 nm), whereas at 770 nm a luminescence spectrum is emitted in addition to the SHG.
- Electrical transport
- Mechanical manipulation is possible, two-dimensional nanoarchitecture
These features are very promising for implementation into future security labels:
- Nanofibers are only visible when excited with UV light (money checker level) and they can emit different colors of light.
- Nanofibers are extremely well polarized since they consist of slabs of materials with very well oriented molecular emitters. I.e., a nanofiber feature is changing appearance when looking at it through a polarizer.
- Nanofibers have anisotropic luminescence, i.e. most light is coming from the ends of the nanofibers and also in-plane. This means a nanofiber feature will appear brighter when looking at it from the side than from the top. Also the polarization effect mentioned above is visible without the use of a polarizer (just not as bright) if one looks at the sample from the side and turns it. Hence even in the passive mode without any further optical enhancement the nanofiber mark emits in-plane light, directed along pre-defined directions.
- If nanofiber labels are inspected through a slightly magnifying microscope, the rod-like structure of the label is easily seen.
An intriguing feature is that nanofibers have all the above mentioned features without any additional techniques or tricks applied. Additional features can be induced easily by, e.g. structuring the underlying substrate, placing the nanofibers on top of a grating structure etc.
Stability and toxicity
- Nanofibers made from para-phenylenes are stable against most chemicals.
- They are inert.
- They are non-toxic.
- They can safely be heated up to 100oC.
- They do not bleach except after strong illumination with UV light (wavelength below 400 nm).
- They are mechanically stable if not attacked by a sharp (micron-scaled) object.
If arrays of nanofibers are transferred without a coating layer, disadvantages are:
- Water is under most circumstances removing the stamped area.
- Very sharp objects can destroy the structure.
- Permanent strong illumination with UV light will photobleach the sample.
