why is the color of 20 nm gold nanoparticles red? What happens when two or three gold particles agglomerate together?
Dear Mr Rahdar, Good Time
What actually happens in metals when subjected to electromagnetic radiation is that the nanostructure will be polarized due to the electric field. The electric dipole moment will then oscillate. The natural frequency of the oscillation depends on type and dimensions of the nanomaterial. If this natural frequency equals the frequency of the incident light then the dipole will oscillate in its resonance mode. This is called Plasmon resonance. Agglomeration of metal nanoparticles and therefore changing their dimensions will change the frequency of plasmon resonance and in accordance changes the light adsorption characteristics of the nanostructure. Increase in the dimensions of metal nanoparticles will lower the plasmon resonance and therefore longer wavelenghts will appear.For instance, Au in Colloidal form (less than 2 nm) is light pink while when Au nanoparticles are agglomerated they will become red. Therefore changing the diameter of nanoparticles will change their light adsorption properties and therefore their color.
Regards
Pashai
Dear Mr Rahdar, Good Time
What actually happens in metals when subjected to electromagnetic radiation is that the nanostructure will be polarized due to the electric field. The electric dipole moment will then oscillate. The natural frequency of the oscillation depends on type and dimensions of the nanomaterial. If this natural frequency equals the frequency of the incident light then the dipole will oscillate in its resonance mode. This is called Plasmon resonance. Agglomeration of metal nanoparticles and therefore changing their dimensions will change the frequency of plasmon resonance and in accordance changes the light adsorption characteristics of the nanostructure. Increase in the dimensions of metal nanoparticles will lower the plasmon resonance and therefore longer wavelenghts will appear.For instance, Au in Colloidal form (less than 2 nm) is light pink while when Au nanoparticles are agglomerated they will become red. Therefore changing the diameter of nanoparticles will change their light adsorption properties and therefore their color.
Regards
Pashai
Dear Dr. Rahdar:
In fact, gold particle in solutions have not a specific color, but exhibits an apparent color being that the apparent color can be engineered by the tailoring of nanoparticle size and its shape, as well smoothness of surface. However, important effects can be exerted under “color phenomenon” from Local refractive index and aggregation state since that in a broad sense further specific properties are expected steaming from a monodisperse and unaggregate state.
The phenomenon of colors at nanoparticles of Gold is ascribed to another two major phenomena. The first phenomenon is called of Size-effect of the gold nanoparticle, typically mentioned as a function of diameter of gold nanoparticle. In a general away, distinct synthesis method of gold nanoparticle gives “distinct colors”, since nanoparticles with distinct sizes are prepared. Of course, a same method of chemical synthesis can be engineered to prepare a broad set of sizes of nanoparticles (providing several colors between blue and red). The second phenomenon is the existence of the Plasmon phenomenon at surface of gold nanoparticles called of Surface Plasmon Resonance.
As a function of size (diameter) of gold nanoparticles, the visible light with distinct wavelengths can be absorbed. In this sense, some wavelengths will satisfy fulfill condition for the absorption of specific wavelengths by a s-p free electron (conduction) develops a resonant character. This resonance phenomenon involving collective oscillating of free electrons at surface of metal characterizes the Plasmon phenomena. From this point, color theory is necessary, as a whole the color changes from blue to red. The observed color is a function of wavelengths, in fact is not a specific wave but a kind of portion of spectral interval, depending on the size of nanoparticle there is a balance between both phenomena of absorption and reflexation of regions of the visible spectra ascribed to the blue or blue-green wavelengths and red wavelengths. Large nanoparticles tend to absorb in the red region giving further reflexation condition at blue-green region starting (450-520 nm). Counterclockwise, small gold nanoparticle (at around 30 nm) tends to absorb in the region of the blue-green reflecting light at around 700 nm. Here, typically, a blue tonality is reached instead of defined and intense blue as bluewish or blue-reddish.
In above sense, the observed color expected at agglomerate is a color which tends to red but with further contribution of blue since average size of agglomerate undergone further contribution of highly non-smooth surface. Seems that this find is in according to approach provided by the Dr. Pashai.
kind regards
Marcos Nobre
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