Molecular Tailoring Approach (MTA) Assisted Density Functional Theory Study for Large Core and Core-Shell Nanocluster Quantum Dots

Molecular fragmentation-based method, viz. molecular tailoring approach (MTA) was employed with density functional framework to investigate prototype medium and large sized semiconductor quantum dots (QDs) of (CdSe)_n, n = 33, 66, 99, 146, and 185 typically of size 1.5-2.7 nm. The trends computed for structural parameters and the band gap energies of prototype structures show a good agreement with those reported earlier for n ≤ 99. In order to study the effect of surface passivation, we use a representative model where 2-coordinated surface atoms of (CdSe)_n QDs were saturated by hydrogen atoms. The prototype for passivated nanoclusters shows enhancement of band gap energy over their bare (CdSe)_n counterparts. We further highlight the strength of our approach by extending the study for the modeled nanoclusters larger than 2.2 nm size, viz. (CdSe)₁₄₆ (diameter d = 2.5 nm), (CdSe)₁₈₅ (d = 2.7 nm), and a prototype core-shell (CS) QD, viz. (CdSe)₆₆/(ZnS)₁₁₉, (d = 2.8 nm), which is otherwise an arduous task on off-the-shelf contemporary hardware. MTA-assisted density functional method offers a reliable and rapid approach for initial steps of geometry optimization of medium and large nanoclusters. The geometrical features viz. surface reorganization through self-healing, selective localization of molecular orbitals, and size dependency of band gap are also retained by MTA. The present approach coupled with a dedicated high performance computer cluster thus has the potential of extending the limits of density functional framework to handle the nanoclusters larger than 3.5 nm (~few hundred atoms).