Dangling bond
When speaking of a dangling bond, one is generally referring to the state described above, containing one electron and thus leading to a neutrally charged atom.
It has also been found in experiments that Electron Paramagnetic Resonance (EPR) spectra of amorphous hydrogenated silicon (a-Si:H) do not differ significantly from the deuterated counterpart, a-Si:D, suggesting that there is hardly any backbonding to the silicon from hydrogen on a dangling bond.
[3] Both free and immobilized radicals display very different chemical characteristics from atoms and molecules containing only complete bonds.
Creating dangling bonds with unpaired electrons can, for example, be achieved by cutting or putting large mechanical strain on a polymer.
This allows for absorption and emission at longer wavelengths, because electrons can take smaller energy steps by moving to and from this extra level.
The mechanism of this is thought to be as follows: The photon energy is transferred to the system which causes the weak Si-Si bonds to break, leading to the formation of two bound radicals.
The free electrons being localized and being very close together is an unstable state, so hydrogen atoms “move” to the site of the breakage.
The diffusion of hydrogen plays a key role in the process and explains why long illumination is required.
[9] Light can also induce dangling bond formation in materials with intimately related valence alternation pairs (IVAP), such as a-As2S3.
[10] Clean cleaved surfaces of such materials form paired electron localized states on alternate sites resulting in a very weak to no EPR signal.
The gas molecules can get trapped and, when staying close to a spin center, affect the EPR signal.
They are often the only defect sites present on atomic semiconductors, which provide such "soft centers" for molecules to adsorb to.
Hydrogen introduced to the silicon during the synthesis process is well known to saturate most dangling bonds, as are other elements such as oxygen, making the material suitable for applications (see semiconductor devices).
[12] In doped semiconductors, surface properties are still dependent on the dangling bonds, since they occur in a number density of around 1013 per square centimeter, compared to dopant electrons or holes with a number density of 1014 to 1018 per cubic centimeter which are thus much less abundant on the material surface.
By definition, passivation is a treatment process of the surface of the layers to reduce the effects of the surrounding environment.
This strategy is based on the formation of a dielectric layer (mostly silicon dioxide SiO2, aluminum oxide Al2O3, or silicon nitride (SiNx) on the top of the c-Si substrate be the mean of thermal oxidation or other deposition techniques such as atomic layer deposition (ALD).
This repletion assures reducing one type of the charge carriers concentration at the interface wherein the recombination decreases.
[16] In experiments by Yunteng Qu et al., dangling bonds on graphene oxide were used to bind single metal atoms (Fe, Co, Ni, Cu) for applications in catalysis.
The resulting catalyst had a high density of catalytic centers and showed high activity, comparable to other non-noble metal catalysts in oxygen reduction reactions while maintaining stability in a wide range of electrochemical potential, comparable to Pt/C electrodes.
[17] An example of an organic ferromagnetic polymer is presented in an article by Yuwei Ma et al.: by cutting with ceramic scissors or stretching a piece of Teflon tape, a network of strongly coupling dangling bonds arises on surfaces where the polymer was broken (from cutting or in strain-induced cavities).
