Also, the geometric separation of excitons in the nanostructures results in inhibited relaxation processes (exciton cooling and multi-exciton recombination), which gives dual-emission 15. The primary reason for dual colour emission is believed to be the size of the shell, the less non-radiative transfer of the shell’s hole towards the core, enhancing the probability of radiative recombination of the shell’s exciton into dual emission 14. These are DiB’s (Dot in bulk) semiconductors, which is due to the large size of the shell as compared to the core exhibits dual emission. Recently, the hetero-structuring idea has been implemented by various groups for actuating NCs capability of emitting multi-colour light because of radiative recombination of excitons confined into core/shell domains. Investigating the formation and relaxation of the CT state provides information about the excitation and de-excitation processes 13. The charge separation state can be achieved either by direct excitation of the CT state or by photoexcitation of an exciton followed by hole transfer through the core-shell boundary. The spatial separation of the electron and hole wave functions within such semiconductor heterostructure 5, 6 results in a prolonged charge transfer (CT) states which favours desirable characteristics for applications, such as light emitters, lasers, photo-catalysts and photovoltaic devices 7, 8, 9, 10, 11, 12. At strong confinement regime (size below the Bohr’s exciton radius) allows for controlling the spatial distribution of carrier wave functions across various domains of a hetero-nanostructure QDs which shows size-dependent absorption and photoluminescence (PL) spectra 2, 3.Within a study carried out, the calculated exciton’s binding energy approximates to the reported Bohr’s exciton radius of CdS single crystal (2.5 nm) 4 at the shell thickness of 2.4 nm, as obtained in our model. The spacing between the valence band and conduction band can be varied by changing the size or shape of the nanocrystals which gives more monochromatic emission and near unity quantum yield by increasing shell thickness. This quantizes the conduction band (CB) and the valence band (VB) energy levels, nearly continuous for bulk semiconductors. The effect is present when the dimensions of the semiconductor are roughly below the Bohr’s exciton radius of that specific enclosure. Quantum dots primarily exhibits the confinement effect that leads to the spatial enclosure of the electronic charge carriers within the nanocrystals (NCs) 1. Thus, the understanding of the motion of e-h in core-shell QDs is essential for photovoltaic, LEDs, etc. The findings are a close approximation to the experimental evidences. However, an intermediate state appeared as pseudo Type II excitons, in which holes are co-localized in the shell as well core whereas electrons are confined in core only, resulting in a dual absorption band (excitation energy), carried out by the analysis of the overlap percentage using the Hartree-Fock method. As a result, there is a jump in the transition energy towards the higher side (blue shift). Upon increasing shell thickness (e.g., from 0.25–3.25 nm) of core-shell QDs, the radial distribution function (RDF) of hole shifts towards the shell suggesting the confinement region switched from Type-I to Type-II excitons. The quantum confinement effect allows controlling the spatial distribution of the charge carriers in the core-shell quantum dots (QDs). Nanostructured semiconductors have the unique shape/size-dependent band gap tunability, which has various applications.
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