First Principle Study of Capping Energies and Electronic States in Stoichiometric and Nonstoichiometric PbSe Nanoclusters

Abstract

A large variety of PbSe nanoclusters have been modeled at the DFT level, to study their structure, their affinity for different ligands and their electronic properties, also depending on surface passivation. The clusters are extracted from the bulk rock salt structure with cubic, prism, truncated cubic, cuboctahedral and octahedral shape and they are fully relaxed, before computing the addition energies of methylamine and formate anions in different positions, to model the process of surface passivation. Then the density of states of all the clusters is computed, to study in particular the band gap and the behavior of the so-called intragap states, which affect the photophysical properties of the nanoparticles, also acting as trap states for charge carriers. We confirm the strong relationship between nanocluster off-stoichiometry and intragap states: such states can be localized on the surface, in the bulk or delocalized over the nanoparticle, according to the source of off-stoichiometry. The ability of different ligands to eliminate the intragap states are tested and discussed, also proposing nonstandard capping molecules.

Toward the Ultimate Limit of Connectivity in Quantum Dots with High Mobility and Clean Gaps

“…Note that the importance of stoichiometry control has been demonstrated by both experimental 8,9 and theoretical studies. 17,18,33 Yet our results indicate that for comparatively small CQD assemblies global charge balance itself is not sufficient to guarantee the absence of midgap states (see the panels in Figure 3a marked by "stoi=0"). In fact, accounting for the effects of connectivity increases the challenge of eliminating midgap states, in the sense that even the simplest bridging by an I atom fails to provide a trap-free band gap for 1.5 nm CQDs.…”

Implementation and Validation of Fully RelativisticGWCalculations: Spin–Orbit Coupling in Molecules, Nanocrystals, and Solids

“…The non-stoichiometric structure of the latter system has been found to lead to "trap" states that change the character of the HOMO and/or LUMO levels compared to stoichiometric nanocrystals; [125][126][127][128] non-stoichiometric ones have also been observed experimentally. 129 The geometries of the two lead chalcogenide clusters considered here are nearly cubic with (100) surfaces, which have been found to be the most stable surfaces for these materials.…”

“…127,128 The LUMO also has Pb character, and hence both HOMO and LUMO are expected to be affected by the inclusion of SOC.…”

Hydrogen Treatment as a Detergent of Electronic Trap States in Lead Chalcogenide Nanoparticles

“…Despite several reports confirming the presence of trap states in films of lead chalcogenide nanoparticles, − , their atomistic origin has remained elusive but for a few exceptions. − Ab initio calculations have suggested the presence of defect states in off-stoichiometric NPs and pointed out the possible use of ligands to achieve charge-orbital balance to heal trap states. ,, Charge-orbital balance is defined by assuming that the formal charge state of all NP constituents, including the ligands, add up to zero, thus leading in principle to a gap without any defect states (or clean gap). , However, recent calculations pointed out that even perfectly charge-balanced NPs may have states in their gap . For example, in the presence of charge balanced Se vacancies, deep unoccupied states may be present in the electronic gap of PbS NPs.…”

“…26−30 Ab initio calculations have suggested the presence of defect states in off-stoichiometric NPs and pointed out the possible use of ligands to achieve charge-orbital balance to heal trap states. 31,41,42 Charge-orbital balance is defined by assuming that the formal charge state of all NP constituents, including the ligands, add up to zero, thus leading in principle to a gap without any defect states (or clean gap). 31,43 However, recent calculations pointed out that even perfectly chargebalanced NPs may have states in their gap.…”