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QED Radiation as the Source of Excitons at the Nanoscale
Solar radiation lacking the photon energy at the macroscale to create excitons by photoexcitation is superseded at the nanoscale as absorbed radiation is induced by QED to produce the high energy photons that create the excitons
By: Thomas Prevenslik
Excitons as free carriers at the macroscale are created by photoexcitation, the free carriers comprising unbound electrons in the conduction band leaving holes in the valence band. If the dynamics of the Coulomb attraction between the electron and hole is negligible, then the pairs are free carriers. But at the nanoscale, the dynamics of the exciton are strongly influenced by the physical size and shape of the material. Therefore, a deciding property of excitons in nanoscale systems is that the exciton size is dictated not by the electron–hole Coulomb interaction, but by the spatial confinement of the physical dimensions of the nanostructure that accentuates many of interesting physical properties, exposing them for examination. The theorist may conclude that perhaps we should not try to force-fit classical theory to model excitons in nanostructures as they often carry with them assumptions that we have forgotten, or that we erroneously ignore. Nanostructures thus provide both a test-bed and an inspiration for new quantum mechanical approaches to the theories of photoexcitation. See Nature Review Article – “Excitons in Nanoscale Systems”, at http://www.nanoqed.org , 2011.
Although the claim is made excitons in nanoscale systems depend on the physical dimensions of the nanostructure, and not by electron–hole Coulomb interaction, the confinement mechanism by which this occurs is not identified. Instead, confinement of carbon nanotubes (CNTs) and quantum dots (QDs) is described only by their characteristic dimension, i.e. their diameter and radius, respectively Ibid.
At the macroscale, exciton energies are no grater than a few 100 meV. But at the nanoscale, exciton energies are much higher, although absorption spectra show energies that extend far beyond the ultraviolet (UV). For example, CNT exciton binding energies are from 0.3 to 1 eV while absorption spectra are increasing beyond 3 eV as shown in Fig. 3(a); while excitons in QDs have energies from 0.7 to 1.5 eV while absorption spectra are still increasing beyond 3.5 eV as shown in Figs. 5(a) and 4(a), respectively Ibid. Exciton energies simply cannot explain the absorption spectra of CNTs and QDs.
QED induced radiation is proposed as the source of high absorption beyond the UV observed in absorption spectra for CNTs and QDs, thereby allowing excitons to be created by photoexcitation. QED stands for quantum electrodynamics. Indeed, QED radiation is the consequence of QM that requires the heat capacity of the nanostructure to vanish under the TIR confinement. QM stands for quantum mechanics and TIR for total internal reflection.
Unlike classical physics that allows the atom to have thermal kT energy at the nanoscale, QM requires the heat capacity to vanish. Lacking heat capacity, absorbed energy cannot be conserved by an increase in temperature, and therefore conservation proceeds by the QED induced creation of photons inside the nanostructure. TIR confinement constrains the absorbed energy tangential to the surface rather than in the volume of the nanostructure because of the high surface to volume ratios in nanostructures. For illustrative purposes, the tangential TIR mode is illustrated in the above figure for a hypothetical hexagonal shaped QD or CNT.
Typically, lasers are used to excite many CNTs and QDs in a transparent liquid, but QED radiation is induced any time energy of any form is absorbed at the nanoscale, including Joule heating of individual CNTs and QDs. Details are given for a diverse range of applications of QED radiation. See http://www.nanoqed.org 2009-2011.
1. Excitons are created in nanostructures by photoexcitation under high energy QED radiation. By including nanostructures such as CNTs and QDs in photocells, low level solar radiation is absorbed and induced by QED to create higher energy photons that improve efficiency by multi-exciton generation (MEG).
2. QED radiation comprising high energy photons explains why the absorption spectrum of CNTs and QDs significantly increases beyond the UV. Visible photons are observed as the high energy QED photons absorbed by the nanostructure material undergo photoluminescence. Excitons simply cannot explain the high absorption spectra of nanostruct ures beyond the UV.
3. Unlike the lack of a physical mechanism to explain the confinement of excitons, QED radiation relies on the TIR confinement of absorbed energy to create QED photons tangential to the surface of the nanostructure. TIR confinement of photons is only momentary upon the absorption of energy and vanishes as the energy leaks to the surroundings, or is no longer supplied to the nanostructure, i.e., if no energy is absorbed, there is no TIR confinement, QED photons, and excitons.
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About QED Indcued EM Radiation: Classically, absorbed EM energy is conserved by an increase in temperature. But at the nanoscale, temperature increases are forbidden by quantum mechanics. QED radiation explains how absorbed EM energy is conserved at the nanoscale by the emission of nonthermal EM radiation.