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It astonished me when I learned that at nanoscale, gold is no longer "gold",rather it's red. Other elements' nanoparticles also have a different color than their macro counterparts.

Why does this phenomenon occur?

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Disclaimer: I am not an expert on this

Gold nanoparticles interaction with light is strongly dictated by their environment, size and physical dimensions.

Small (~30nm) monodisperse gold nanoparticles absorb light in the blue-green portion of the spectrum (~450 nm) while red light (~700 nm) is reflected, yielding a rich red color. This absorption is due to surface plasmon resonance.

From wikipedia

Surface plasmon resonance (SPR) is the resonant oscillation of conduction electrons at the interface between a negative and positive permittivity material stimulated by incident light. The resonance condition is established when the frequency of incident photons matches the natural frequency of surface electrons oscillating against the restoring force of positive nuclei

As particle size increases, the wavelength of absorption shifts towards longer, redder wavelengths. Red light is then absorbed, and blue light is reflected, yielding solutions with a pale blue or purple colour.

As you approach the bulk limit, the surface plasmon resonance wavelengths move into the IR portion of the spectrum.

The surface plasmon resonance phenomenon is not only sensitive to size, but also to the morphology and shape of the nanoparticles. This can be used to tune the optical properties of the gold nanoparticles.

For other kinds of nanoparticles, for example, semi-conductor nanoparticles, quantum confinement (e.g. quantum dots) might come into play, and this affects the wavelength of absorption of electromagnetic radiation, and hence the perceived colour.

The matrix the nanoparticles are in itself will also play a role in the perceived colour. Light scattering events taking place at the nanoparticle interface, vis-à-vis single or multiple light scattering events also need to be considered. To learn more about light scattering phenomenon read Mie theory and T-matrix method (again, size and shape of the scattering particles influence the solution of maxwell's equations).

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  • $\begingroup$ Why this shift in wavelength occurs? Is it because when size gets bigger there are more positive nuclei to attract the electron thus there oscillations is more constrained and thus frequency decease? $\endgroup$ – Mockingbird Oct 22 '16 at 1:40
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    $\begingroup$ The energy levels themselves and hence the difference in energy between them depends on the size of the particle, just as in a particle in a box. In this case its a particle in a cube or sphere etc. but the principle is the same. The smaller the dimension the bigger the energy gaps and the bluer the absorption, bigger dimensions, smaller energy gaps red absorption. Of course there are selection rules to follow and particular geometries/symmetry will have a big effect on energy levels but the overall principle should remain. $\endgroup$ – porphyrin Oct 22 '16 at 10:38
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There is a fairly simple explanation of why small-enough particles are different from bulk materials. Once the particle becomes similar in size to the wavelengths of light involved then quantum effects start to matter for how the particle behaves.

The actual mechanisms that give specific colour may vary, but the main point is that the size of the particle comes to dominate the bulk properties of the material. In a bulk semiconductor, for example, the electronic properties depend on the band gap between occupied electron levels and unoccupied electron levels. When the particle is small enough, however, the electrons are more constrained and become more like an electron confined to a small box. This can mean that the possible energy levels of the electrons are determined by the physical size of the box not the bulk properties of the material. Hence any emission or absorption of light will be different to the bulk material. The additional physical constraints impose a different set of energy levels on the electrons.

The actual details can be quite complicated but the basic idea is that when particles are small compared to the wavelengths of light, you have to take into account the extra constraints on the system when calculating possible energy levels involved in electron transitions which are what causes colour.

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