The developed devices use compact, continuously graded QDs with suppressed Auger recombination incorporated into a pulsed, high-current-density charge-injection structure supplemented by a low-loss photonic waveguide. Here we resolve these challenges and achieve amplified spontaneous emission (ASE) from electrically pumped colloidal QDs. However, the implementation of such devices has been hampered by fast Auger recombination of gain-active multicarrier states 1, 8, poor stability of QD films at high current densities 9, 10 and the difficulty to obtain net optical gain in a complex device stack wherein a thin electroluminescent QD layer is combined with optically lossy charge-conducting layers 11, 12, 13. Therefore, the current study could in large perspective be extended to unfold the intricate optical nature of lattice mismatched core/shell QDs and even doped QDs, for rational design of advanced light-emitting nanomaterials.Colloidal quantum dots (QDs) are attractive materials for realizing solution-processable laser diodes that could benefit from size-controlled emission wavelengths, low optical-gain thresholds and ease of integration with photonic and electronic circuits 1, 2, 3, 4, 5, 6, 7.
In conclusion, the strain-improved EMA model yields conclusive and constructive insights into band alignment shifts caused by strain, the consequent effects on carrier localization and recombination kinetics of core/shell nanocrystals, and the resulting impact on PL QY. But fast growth of CdS shell will substantially decrease the PL QY due to the strong extension of electron wave function into the shell in consequence of strain, allowing the underlying non-radiative recombination to compete with the slowed radiative recombination. As a result, the PL emission peak is shifted over a large wavelength range. The most important findings based on the agreement between theoretical and experimental results can be summarized below: 1) the compressive strain on core and tensile strain on shell largely alter the conduction band offset rather than valance band offset of CdTe/CdS QDs, 2) strain largely varies the localization of electron wave function of CdTe/CdS QDs with thick CdS shell (~ 3 monolayers). The theoretical results were then carefully compared with experimentally derived data. In detail, through considering band deformation potential, the core/shell band alignments caused by strain were rectified and then included EMA simulations on electronic level, carrier spatial distribution, and electron-hole wave function overlap. Herein, we report our recent investigations on theoretical modeling of CdTe/CdS core/shell QDs by a strain-modified effective mass approximation (EMA). The CdTe/CdS core/shell QDs represent a typical strained system due to the substantial lattice mismatch between CdTe and CdS. At the present, quantitative influences of strain on the electronic structure of QDs remain to be unfold, which is not only fundamentally but also practically interesting. Nevertheless, the formation of such core/shell structure often leads to strain due to the lattice mismatch between core and shell. Construction of semiconductor/semiconductor core/shell structures has been demonstrated to be one of the most effective ways to improve the photoluminescence (PL) efficiency and tune the PL emission of QDs as well.
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