Phase transformation of PbSe/CdSe nanocrystals from core-shell to Janus structure studied by photoemission spectroscopy

Abstract

Photoelectron spectroscopic measurements have been performed, with synchrotron radiation on PbSe/CdSe heteronanocrystals that initially consist of core-shell structures. The study of the chemical states of the main elements in the nanocrystals shows a reproducible and progressive change in the valence-band and core-level spectra under photon irradiation, whatever the core and shell sizes are. Such chemical modifications are explained in light of transmission electron microscopy observations and reveal a phase transformation of the nanocrystals: The core-shell nanocrystals undergo a morphological change toward a Janus structure with the formation of semidetached PbSe and CdSe clusters. Photoelectron spectroscopy gives new insight into the reorganization of the ligands anchored at the surface of the nanocrystals and the modification of the electronic structure of these heteronanocrystals.

Quantum Dot Acceptors in Two-Dimensional Epitaxially Fused PbSe Quantum Dot Superlattices

“…To study the nature of the defects which are located at the surface of the defective QDs and able to generate such multiple in-gap states, we carried out X-ray photoemission spectroscopic (XPS) measurements. While the examination of the Se 3d core level for the superlattice yields a branching ratio consistent with the literature 42 and does not show any sign of oxidation (Figure 6a), fitting the Pb 5d core level requires three doublets (Figure 6b). Based on previous XPS analyses of PbSe QDs, 42 we identify the three components as follows: the central peak corresponds to the bulk component of Pb atoms bound to Se; the small shoulder visible at low binding energy is caused by excess of unbound surface Pb atoms; finally, the broad shoulder at high binding energy arises from PbO species.…”

“…While the examination of the Se 3d core level for the superlattice yields a branching ratio consistent with the literature 42 and does not show any sign of oxidation (Figure 6a), fitting the Pb 5d core level requires three doublets (Figure 6b). Based on previous XPS analyses of PbSe QDs, 42 we identify the three components as follows: the central peak corresponds to the bulk component of Pb atoms bound to Se; the small shoulder visible at low binding energy is caused by excess of unbound surface Pb atoms; finally, the broad shoulder at high binding energy arises from PbO species. These species also appear in the XPS spectrum of the O 1s core level in three different forms (Figure 6c).…”

Atomistic understanding of cation exchange in PbS nanocrystals using simulations with pseudoligands

“…The dissolved S anions may also recrystallize on other parts of the same NC with the cations in the solution, thus enabling massive migration of anions and reconstruction of PbE/CdE HNCs. This phenomenon becomes more important at high temperatures and/or with active ligands, in some extreme cases PbE NCs or PbE/CdE core/shell HNCs may transform to a Janus-like heterostructure 26 29 33 .…”

Elucidating the Electronic Cross-Talk Dynamics across the Heterointerface of Janus CdSe/PbSe Nanocrystals

“…In this regard, Cd and Pb chalcogen especially CdSe and PbSe based HNCs score considerable attention representing a unique system where two structurally immiscible materials with very less crystal mismatch, extremely different charge carrier effective masses, and different band alignments coexist forming a sharp heterointerface. Both the symmetric and asymmetric structures are possible depending on the reaction conditions employed providing fundamental information about the reaction mechanisms and tunable properties. − Anisotropic structures are derived either from restructuring the core/shell NCs or direct Pb 2+ (Cd 2+ ) to Cd 2+ (Pb 2+ ) cation exchange with judicious change in the reaction conditions. − Partial cation exchange on presynthesized CdSe NCs often leads to anisotropic Janus HNCs where one part is composed of CdSe and the other is PbSe. , For Janus HNCs both CdSe and PbSe are surface exposed in contrast to the core/shell NCs where the shell creates a barrier for charge transfer interactions. In CdSe/PbSe HNCs due to quasi type-II band alignment the hole localizes in the PbSe region while the electron is spread throughout the NC. ,, Thus, Janus HNCs are advantageous as both electron and hole are specially decoupled and accessible to the surface beneficial for photocatalytic reactions and photovoltaics.…”