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• W851014295 abstract "In this paper, the possibility of encapsulating Au metallic nanoparticles (MNPs) non-epitaxially in a semiconducting and insulating shell was tested in order to explore their potential application in photovoltaic thin films. The proposed synthesis of growing Ag onto Au, doping it with sulfur and ion exchanging the Ag with Zn or Cd was shown to be successful through Transmission Electron Microscopy (TEM) and UV/VIS spectra. The final products of the synthesis showed a ~50 – 100nm red shift of the Au MNP plasmon peak from its benchmark wavelength of ~525nm. Introduction The modern world is experiencing an ever increasing demand for energy sources. Fossil fuels, such as coal, oil and natural gas, are able to meet these demands for the time being, but are finite resources. Fossil fuels are burned to heat water to create steam, which is then used to drive a turbine, generating electricity. But, by-products of nitroxides, sulfoxides (contributors to acid rain) and carbon dioxide are produced in massive amounts, contributing to global warming and other undesirable environmental outcomes. Solar energy represents a renewable, almost infinite source of clean energy. Every day the Earth is bombarded with enough energy to meet global energy demands for an entire year, and the Sun will be around for millions of years, identifying it as an essentially infinite source of energy. Solar Panels produce energy by using light sensitive materials to produce excitons, excited electron-hole pairs. Connecting positive and negative electrodes will cause excited electrons and their positive holes to migrate. Excited electrons will exit the light sensitive material, enter a circuit, deposit its energy and then re-enter the material at the opposite electrode, recombining with a hole. However, solar panel technology suffers from low efficiency and high production costs. Currently, monocrystalline silicon solar panels can reach efficiencies of about 17%, but are expensive to produce. Part of the problem arises from the Thick/Thin Paradox of photovoltaics: the thicker the film, the more absorbance, but less efficiency (and vice-versa). This is caused by the distance excitons must travel to reach the electrodes; as the distance increases, recombination becomes more probable. Recent advances in nano-technology have revealed that semiconducting Quantum Dots (QDs) could be a viable alternative. One reason is their nano-scale size yields a drastic reduction in production cost (significantly less materials are needed). QDs are nanocrystals that are known to behave like, and be modeled as a Particle in a Box (PB). This model states that as a crystal is made smaller, its electronic energy levels become farther spaced apart. This occurs because the energy spectrum of the crystal changes from a continuous stream to one that is more discreet. This is known as quantum confinement. Excitons produced in QDs can also be harnessed by allowing them to migrate through the tunneling effect. Tunneling is the possibility of something traveling between two points through a space that classical physics models predict to be impossible. When choosing a particular type of QD, it is very important that one with high chargecarrier mobility is chosen. This is the ability for excitons to travel from one QD to the next. If the excitons cannot travel properly, they will be prone to recombination (electrons relaxing back into a hole). If this happens, the overall efficiency will be severely limited. PbS QDs have already been shown to be easily applied to photovoltaic thin films, but they still suffer from a lack of high efficiency. Today, they can reach efficiencies of about 5%. However, it has been shown that the efficiency of QD thin films can be increased when combined with phenomena of surface plasmons, thus addressing the Thick/Thin Paradox (thinner, but still absorbance and efficiency increase) iii . This is the second reason thin films of QDs are a viable alternative. Surface plasmon resonance is a feature of metallic nanoparticles (MNPs). Recently, they have become a widely explored topic on their effects of QDs for lasers iv , light-emitting diodes v and solar fuel production iii . Surface plasmons of MNPs are known to couple with the excitons of QDs and affect them vi through two distinct mechanisms: (1) plasmon-exciton energy transfer and (2) modification of the local radiation field in semi-conducting domains vi . (2) occurs when the electrons at the surface of a MNP begin to resonate with incoming electromagnetic waves. This causes an induced electromagnetic field to be produced by the MNP running parallel to the surface of its thin film. Simply stated, incoming light is absorbed and re-emitted parallel to the thin film. Typically, light that is not absorbed by a solar cell will just bounce off and away, but MNPs cause light to be trapped along the surface and therefore dramatically increasing the chances for absorption. MNPs do follow the PB model, but they are different from QDs. QDs have well defined electronic structures, so they are capable of producing excitons. But, MNPs retain the delocalized 'sea of electrons' feature metals exhibit, so they are incapable of producing excitons. The induced electromagnetic field of MNPs decays exponentially as it travels away from its origin, so it is very important that the QDs the plasmons are coupling with are spatially close enough to properly increase the efficiency. Weak coupling regimes have been fabricated and estimated to increase efficiency by 80% under normal conditions. Placing MNPs and QDs in exceptionally close proximity is known as strong coupling; when QDs and MNPs are placed close together, the effect of the plasmon-coupling increases non-linearly vii, . Achieving a strong coupling regime suggests that the QDs must be directly bound to the MNPs, but this is a problem, because a bleeding effect would occur, where excitons will escape form one medium into the other and recombine. Quenching of fluorescent QDs when placed closely to MNPs has already been demonstrated in recent studies ix, x, . However, it has been suggested that a core/shell method will allow for a strong coupling regime between the MNPs and QDs, without the bleeding effect and maintaining quantum confinement. The core/shell method is taking a MNP core and encapsulating it non-epitaxially, in an inorganic, insulating shell. This method allows MNPs to be placed in close proximity to the QDs and provide strong plasmonic coupling without or very little of the bleeding effect taking place. This method has already been explored extensively using semiconducting shells xii, xiii, . Using short chained, organic ligands, such as MPA (3-mercaptopropionic acid), will allow for appreciable charge mobility and close proximity to the MNPs. Thus, a strong regime is yielded, while preserving exciton production, efficiency enhancement and quantum confinement. This theory will be tested by attempting to grow a silver shell around a Au MNP, dope it with sulfur and then ion exchange with Zn or Cd to create a core/shell Au/ZnS or Au/CdS structure. For the rest of this paper, X/Y refers to a core/shell structure." @default.
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• W851014295 date "2013-01-01" @default.
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• W851014295 title "A Strong Plasmon Coupling Quantum Dot Photo Voltaic Thin Film" @default.
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