China
Africa
Explore chapters and articles related to this topic
Biological and Clinical Perspectives of Nano Quantum Dots for Cancer Theranostics
Published in Cherry Bhargava, Amit Sachdeva, Pardeep Kumar Sharma, Smart Nanotechnology with Applications, 2020
Bakul Tikoo, Gagandeep Singh, Ashok Kumar Yadav, Rajiv Kumar, Gurpal Singh, Ashish Suttee
An intrinsic band gap is a known property of semiconducting materials, which is inversely proportional to the size of QDs. The electrons can be excited by absorbing incident light from the valence to the conduction band, leaving behind a hole. Thus, an exciton is formed by bond formation between the electron and the hole. Upon recombination (i.e. the return of the excited electron to its ground state) of this exciton, a photon is emitted at a longer wavelength. This phenomenon is called fluorescence. When the synthesized QDs are too small in size to be compared with the wavelength of the electron, the Quantum Confinement Effect is observed (Figure 14.6). Once these confined excitons are excited by a light beam, they re-emit light (i.e. fluoresce) with a narrow and symmetrical emission spectrum that depends directly on the nanocrystal size [27]. The summarized advantages/ disadvantages are given in Table 14.3.
Renewable Energy through Nanotechnology
Published in Cherry Bhargava, Amit Sachdeva, Nanotechnology, 2020
W. Nada, S. Dania, Sharon Santhosh, Asha Anish Madhavan
The four stages involved in the working of an OSC are: Light absorption and exciton formation. This depends on the absorption coefficient of the material.Exciton diffusion occurs due to differences in the concentration of charges present in the donor and acceptor layer of the OSC. The diffusion length of excitons also plays a key role in this process.Exciton dissociation occurs when the work function produced by the electrodes is sufficiently large enough to overcome the exciton binding energy and separate the electron-hole pair.Charge transport occurs to the respective electrodes if recombination is reduced to a minimum.
Coherent Optical Measurements in ND
Published in Thomas C. Weinacht, Brett J. Pearson, Time-Resolved Spectroscopy, 2018
Thomas C. Weinacht, Brett J. Pearson
As a final example of 2D spectroscopy, we consider dynamics of excitons in natural QDs coupled to excitons in a surrounding quantum well (QW).7 An exciton is a bound state of an electron and a hole, where the electron is bound to the hole by static Coulomb attraction and has insufficient energy to escape as a free electron (similar to the way an electron is bound to a proton in a hydrogen atom). An exciton is generally free to move in a bulk semiconductor, but can be confined in one dimension in a QW, or all three dimensions in a QD. The confinement results in discrete states just below the bandgap of the bulk semiconductor. A QW is produced by growing a thin semiconductor layer (on the order of 10 nm) between two other materials. For example, a QW can be created by sandwiching GaAs between AlxGa1−xAs, where x is a number between 0 and 1. A QD can be formed naturally in a QW if there are local monolayer fluctuations in the QW thickness. In this case, the QD is strongly coupled to the surrounding QW, and the interaction between the dot and well can be thought of in terms of a typical system-bath interaction.
Dependence of nonlinear optical properties on electrostatic interaction in an excitonic parabolic quantum dot in a static magnetic field
Published in Journal of Modern Optics, 2021
In quantum dots, confined excitons illustrate a motivating model for studying nonlinear optical properties [21,22]. An exciton is a bound electron-hole pair that is formed by the Coulomb interaction (electrostatic interaction) between the electron and hole. Theoretically, this system is similar to the hydrogen atom and also has discrete eigen-energies. An exciton is made up of an electron and a hole with more closely matched effective masses and confinement of charge carriers leads to the higher value of exciton binding energy in low-dimensional systems. The transitions between confined exciton states generally fall in the meV range that can easily get manipulated with the fields [23,24]. The excitons, therefore, illustrate a motivating model for studying nonlinear optical properties in quantum dots and can have great applicability in optical devices [25].
LCA study of photovoltaic systems based on different technologies
Published in International Journal of Green Energy, 2018
Weslley M. Soares, Daniel D. Athayde, Eduardo H.M. Nunes
A strategy used for the development of third-generation PV systems is the use of materials where the creation of multiple excitons is obtained after exposure to sunlight. As already discussed, an exciton is created when an electron is promoted from the valence band to the conduction one by the absorption of a photon. This photon should exhibit energy larger than the band gap of the semiconductor material. The electron promoted to the valence band can be used to store energy when a battery is coupled, for instance, to a p-n junction. The creation of multiple excitons, also called carrier multiplication (CM), allows a single photon to generate more than one exciton, increasing the PV cell efficiency. A variety of nanocrystalline materials shows CM (Cirloganu et al. 2014; Midgett et al. 2013). However, as the more stable condition is that one which occurs the recombination of the electron–hole pair, the exciton created upon sunlight illumination should display a short lifetime. In attempting to inhibit this recombination reaction, the extraction of multiple excitons has been carried out in third-generation PV cells. According to Cate et al. (2015), the coupling of materials for promoting the exchange of free mobile charge is a promising strategy to develop PV systems with high performances. Moreover, effects of quantum confinement should also be taken into consideration for obtaining an efficient CM, which can be reached by using quantum dots (Stewart et al. 2013; Ten Cate et al. 2015). Nonetheless, further improvements in the fabrication methods are required before the commercial use of third-generation solar cells.
Single step green synthesis of nickel and nickel oxide nanoparticles from Hordeum vulgare for photocatalytic degradation of methylene blue dye
Published in Inorganic and Nano-Metal Chemistry, 2020
Muhammad Imran Din, Maria Tariq, Zaib Hussain, Rida Khalid
Figure 1 shows the UV/VIS absorption spectrum of Ni NPs and its formation due to reduction of aqueous metal ions during exposure of Hordeum vulgare extract. The absorption peak obtained at 235 nm corresponds to the absorption of nickel ions and peak sharpness suggests the formation of well dispersed or stable nanoparticles with no aggregation. Anyhow, the results obtained from UV-Visible spectroscopy showed the metallic peak at 235 nm. The characteristic absorbance peak of Ni nanoparticles exists in the range of 230 nm to 400 nm. Thus UV-visible spectroscopy confirms the formation of Ni and NiO nanoparticles. Additionally, like other semiconductors, NiO NPs show an excitonic absorption peak corresponding to their band gap at ∼3.6 eV. An exciton is a bound state of an electron and electron hole which attract each other through columbic interactions. An exciton is produced when the semiconductor absorbs a photon having energy corresponding to its direct band gap and as a result of this, transition of the electron occurs from the valence band to conduction band. A hole is created in the valence band whose all properties mimic that of a missing electron. Since the electron is negatively charged and hole is positively charged an attractive force exists between them and they behave as electron and electron hole pair. This interaction provides a stabilizing energy balance and as a result of this the exciton has slightly less energy than the unbound electron and hole. When the excited electron de-excites to valence band then the exciton is annihilated along with the emission of photon, this process is called radiative recombination. Within the crystal, the exciton has a finite size defined by the Bohr exciton diameter which is the distance in an electron hole pair and can vary from 1 nm to ≤100 nm depending on the nature of material.