Impact of Bis(imino)pyridine Ligands on Mesoscale Properties of CdSe/ZnS Quantum Dots

Abstract

We investigate the effect of surface modification of CdSe/ZnS quantum dots (QDs) with bis­(imino)­pyridine (BIP) ligands. BIPs are a class of redox noninnocent ligands known to facilitate charge transfer in base metals on the molecular scale, but their behavior in nano- to mesoscale systems has been largely unexplored. Using electron microscopy, crystallography, and ultrafast spectroscopy, we reveal that structure-specific π–π stacking of the BIP molecules alters interdot separation in QD films, thereby leading to changes in optical and electronic properties. The three variations used are unsubstituted (BIP-H), dimethyl (BIP-Me), and diisopropyl (BIP-Ipr) BIP, and when compared with the native octadecylamine ligand, we find that both energy and charge transfer efficiencies between QDs are increased postligand exchange, the highest achieved through BIP-Ipr despite its larger unit cell volume. We further investigate charge transfer from QD films to conducting (indium tin oxide, ITO) and semiconducting (zinc oxide, ZnO) substrates using time-resolved spectroscopy and determine that the influence of the ligands is QD band gap-dependent. In QDs with a large band gap (2.3 eV), the BIP ligands facilitate charge transfer to both ITO and ZnO substrates, but in dots with a small band gap (1.9 eV), they pose a hindrance when ZnO is used, resulting in reduced recombination rates. These results highlight the importance of investigating multiple avenues in order to optimize surface modification of QDs based on the end goal. Finally, we verify that BIP ligands hasten the rate of QD photobrightening under continuous illumination, allowing the ensemble to achieve stable emission faster than in their native configuration. Our study sets the stage for novel charge transfer systems in the meso- and nanoscale, yielding a diverse selection of new surface ligands for applications such as conductive materials and energy production/storage devices employing QDs.

Tuning three-dimensional nano-assembly in the mesoscale via bis(imino)pyridine molecular functionalization

“…First, we sought to ascertain whether the steric ligand parameters affect the mean shell size. Unit cell structures of the BIP ligands, obtained from crystallization data 28 , reveal that BIP-H has the smallest volume, while BIP-IPr the largest, which does not correlate to shell size, since BIP-Me functionalized QDs lead to the smallest shells. Packing density of QDs in the shell walls was next suspected to determine shell size, but with BIP-IPr QDs having the smallest inter-dot separation, this physical property was also not in agreement with observed shell size variation 28 .…”

“…Unit cell structures of the BIP ligands, obtained from crystallization data 28 , reveal that BIP-H has the smallest volume, while BIP-IPr the largest, which does not correlate to shell size, since BIP-Me functionalized QDs lead to the smallest shells. Packing density of QDs in the shell walls was next suspected to determine shell size, but with BIP-IPr QDs having the smallest inter-dot separation, this physical property was also not in agreement with observed shell size variation 28 . This lack of a relationship to any physiochemical property (lattice dimensions, torsional angles, sterics, lattice volumes) prompted an investigation of the thermodynamic effect of the ligands on the host 5CB using differential scanning calorimetry (DSC) in Fig.…”

“…One is that the mesogenic ligand used in driving the assembly are highly cytotoxic; the other is the lack of control in tuning shell sizes, leading to structures too large to be suitable for in vitro applications 26 , 27 . In this current work, we move away from mesogenic ligands and focus on the bis(imino)pyridine (BIP) family of ligands 28 which, in addition to being biocompatible, allow structural modularity and offer a route to fundamentally understand and thereby manipulate the assembly process to controllably tune the shell sizes. Using CdSe/ZnS core/shell quantum dots (QDs) functionalized with BIP molecules and dispersed in 5CB, we demonstrate that subtle changes to the BIP structure results in a tunable assembly process, yielding shells of predictable sizes.…”

Virtual Issue on Ultrafast Charge Separation in Energy Research

“…Subpicosecond interfacial charge separation and slow excitonic recombination (on the order of nanoseconds) are exhibited by graphene quantum dots in the presence of an electron-rich organic molecule . Heterogeneous charge transfer efficiency in QDs connected by redox noninnocent ligands can be modulated by tuning the interdot separation, which can be achieved through substitutional changes in the ligands . Passivation of QDs with organic molecules affects the energy gap and electron–vibrational interactions, and it can favor charge separation and suppress charge recombination .…”

“…27 Heterogeneous charge transfer efficiency in QDs connected by redox noninnocent ligands can be modulated by tuning the interdot separation, which can be achieved through substitutional changes in the ligands. 28 Passivation of QDs with organic molecules affects the energy gap and electron−vibrational interactions, and it can favor charge separation and suppress charge recombination. 29 In InP/ZnS QDs, photoexcited carriers injected into ZnS relax to the alloyed layer and then to the InP core on a picosecond time scale, recombining finally in nanosecond time scales.…”