1. The mechanism of light absorption in metals is mainly related to the absorption by free charge carriers in the conduction band. Since their concentration is high (1022 - 1023 cm-3), the absorption coefficient of light in the metal can reach 106 cm-1.

2. In the case of semiconductor light absorption process is more complicated than in metals, due to the change of mechanisms of absorption at the wavelength.  In semiconductors, having at room temperature a small concentration of free electrons, light absorption is determined mainly bound electrons (refers to intrinsic semiconductors). Strongly absorb radiation at frequency ω will be only those semiconductors for which the photon energy hω more bandgap.

Non-selective absorption of free electrons is clearly observed in a wide wavelength region of the spectrum (hω < Eg), due to electron transitions within the same area, such as the conduction band.

At frequencies below the fundamental absorption edge is also possible absorption of light, which is associated with the excitation of excitons, electronic transitions between the levels of impurities and allowed bands, as well as the absorption of light by lattice vibrations and free carries. Exciton bands are located in the semiconductor slightly below the bottom of the conduction band due to the exciton binding energy. Excitonic absorption spectra are hydrogen-structure energy levels.

Similarly impurities acceptors or donors create acceptor or donor levels lying in the forbidden zone. They significantly modify the absorption spectrum of the doped semiconductor.

 3. The quantum yield of the photoelectric effect of the metals in the visible and near-UV Y < 0,001

electron / photon. This is primarily due to the small escape depth of photoelectrons, which is significantly less than the depth of light absorption in the metal. Most of photoelectrons dissipates its energy before reaching the surface, and loses the opportunity to go into a vacuum. At photon energies near the threshold of the photoelectric effect, most of the photoelectrons excited below the vacuum level and does not give a contribution to the photoemission current. In addition, the reflection coefficient in the visible and near-UV is great and only a small part of the radiation is absorbed in the metal. These limits are partially removed in the far ultraviolet region of the spectrum where Y reaches 0.01 electron / photon at photon energies E> 10 eV.

4. High quantum yield of semiconductor materials have a p-type (p-type) and a small positive electron affinity χ

Most metal behave as mirror, meaning that no light is absorbed

on the other hand semiconductor do absorb high energy photons

1. The mechanism of light absorption in metals is mainly related to the absorption by free charge carriers in the conduction band. Since their concentration is high (1022 - 1023 cm-3), the absorption coefficient of light in the metal can reach 106 cm-1.

2. In the case of semiconductor light absorption process is more complicated than in metals, due to the change of mechanisms of absorption at the wavelength.  In semiconductors, having at room temperature a small concentration of free electrons, light absorption is determined mainly bound electrons (refers to intrinsic semiconductors). Strongly absorb radiation at frequency ω will be only those semiconductors for which the photon energy hω more bandgap.

Non-selective absorption of free electrons is clearly observed in a wide wavelength region of the spectrum (hω < Eg), due to electron transitions within the same area, such as the conduction band.

At frequencies below the fundamental absorption edge is also possible absorption of light, which is associated with the excitation of excitons, electronic transitions between the levels of impurities and allowed bands, as well as the absorption of light by lattice vibrations and free carries. Exciton bands are located in the semiconductor slightly below the bottom of the conduction band due to the exciton binding energy. Excitonic absorption spectra are hydrogen-structure energy levels.

Similarly impurities acceptors or donors create acceptor or donor levels lying in the forbidden zone. They significantly modify the absorption spectrum of the doped semiconductor.

 3. The quantum yield of the photoelectric effect of the metals in the visible and near-UV Y < 0,001

electron / photon. This is primarily due to the small escape depth of photoelectrons, which is significantly less than the depth of light absorption in the metal. Most of photoelectrons dissipates its energy before reaching the surface, and loses the opportunity to go into a vacuum. At photon energies near the threshold of the photoelectric effect, most of the photoelectrons excited below the vacuum level and does not give a contribution to the photoemission current. In addition, the reflection coefficient in the visible and near-UV is great and only a small part of the radiation is absorbed in the metal. These limits are partially removed in the far ultraviolet region of the spectrum where Y reaches 0.01 electron / photon at photon energies E> 10 eV.

4. High quantum yield of semiconductor materials have a p-type (p-type) and a small positive electron affinity χ

Dear Leonid ,

In metal nanoparticles absorption is strong while luminescence (emission) is weak unlike in semiconductors nanoparticles. Why is this the case?

1. Semiconductors

Luminescent properties of nanoparticles strongly depend on a condition of their surface. It is connected, first of all, by that intensity of a luminescence of semiconductor nanoparticles is caused by processes of transmission of energy of excitement to the center of a luminescence. Thus also process of dissipation of this energy from a nanoparticle surface in environment is possible. In view of the fact that for nanoparticles existence of the developed surface is characteristic, i.e. high value of the relation of the area to volume, this process has a high probability. Suppression of a luminescence results, up to a total disappearance, and also distortion of its spectral characteristics. For reduction of these effects seek to stabilize a condition of a surface, and also to cover a surface of nanoparticles with dielectric with bigger, than at material of nanoparticles, width of the forbidden zone.

2. Metals

Optical characteristics of metal nanoparticles, in particular, silver nanoparticles, are defined by existence the plasmon oscillations in the frequency range lying in visible spectrum. These vibrations induce strong local electromagnetic fields in the vicinity of nanoparticle surface, thereby significantly determine the kinetics of molecular processes in the art. Thus, deactivation of the excited state of the fluorescent molecules near the surface of the metal is due to nonradiative processes.

These non-radiative processes include: a) nonradiative energy transfer to the metal, b) intermolecular interaction modulated metal, resulting in quenching of luminescence, c) charge transfer.

The effect of surface plasmon resonance (SPR) of nanoscale metal nanoparticles on fluorophores manifested in the fact that the intensity of the latter may vary substantially near the metal surface.

It is difficult to talk of light absorption or emission from metal or semiconductor surface, in general terms.

One needs to consider the light spectrum, temperature, angle of incidence, refractive index, direct/indirect band-gap, and the quantum-mecanical selection rules. 

Metals absorb strongly, but also reflect strongly, and the reflection depends upon the refractive index and the surface roughness (angle of incidence also). Coal, graphite are among the strongest light absorbers. For infrared light, metals will certainly be stronger absorber than semiconductors. But for ultraviolet light, there may not be much difference, if the semiconductor is direct band-gap. Direct band-gap semiconductors will absorb more strongly than the indirect band-gap semiconductors because of the easier QM selection rules. 4.5 nm NbN has been observed to absorb light 100% because of no reflection. 

Emission from semiconductors will depend upon several additional factors. All strong visible light emitters are semiconductor junctions. The strongest light emitter is the semiconductor high-low-high hetero-junction, because of Einstein's population inversion. Population inversion is not possible in metals; metals can emit strongly only when heated.