Evidence in Support of Exciton to Ligand Vibrational Coupling in Colloidal Quantum Dots

Abstract

The Perspective focuses on the investigation of an unresolved conflict in semiconductor colloidal quantum dots (CQDs) research, concerning the influence of the immediate surrounding on the optical properties of the materials. Today's advanced synthetic colloidal procedures offer formation of a high-quality inorganic crystallite, capped with various organic/inorganic molecular ligands. The Perspective aims to clarify whether exciton recombination processes in CQDs are influenced by the type of crystallite-ligand bonding and, moreover, whether these excitonic processes experience direct coupling to the ligands' vibrational modes. Most ligands used have redox characteristics whose functional groups are added on to the CQDs' surface via coordination, covalent or ionic bonding. The surface-ligand bonding introduces electronic states either above or below the intraband/interband energy gap, resulting in electronic passivation or in creation of trapping states that affect intraband and interband relaxation processes. Furthermore, crystalline electronic states may have a direct coupling to molecular vibrational states via direct overlap of electronic wave functions or through a long-range energy-transfer process. Also, photoejected carriers resulting from an Auger process or ionization processes may diffuse temporarily onto a ligand site. These scenarios are discussed in the current publication with supporting theoretical and experimental observations.

Probing the Interaction of Quantum Dots with Chiral Capping Molecules Using Circular Dichroism Spectroscopy

“…32 The topic of exciton to ligand vibrational coupling was also discussed recently by Lifshitz. 33 However, in the current results we cannot evaluate the role of these mechanisms in the observed effect.…”

Quantum dot surface engineering: Toward inert fluorophores with compact size and bright, stable emission

“…Indeed even at room temperature, the rotation of alkyl chains on ligands is thermally accessible [43], and excitation of a QD with high energy light can theoretically heat a nanocrystal to 500 K, yielding energy that can be directly transferred to vibrate surface atoms and ligands [15, 117, 118]. This fluctuation occurs on the order of femtoseconds, much more rapidly than trapping or emission, so it could potentially dominate the emission dynamics.…”

Semiconductor quantum dots reveal dipolar coupling from exciton to ligand vibration

“…Potential vibroelectronic interactions between NCs and molecules must be taken into account and make the description of QDs more complex. In this context, most of the studies focus on the influence of ligands on the optoelectronic properties of NCs [30][31][32][33][34][35][36][37] . In particular, somespecific surface modifications with organic ligands proved to enhance light absorption and fluorescence emission [38][39][40] , which is a clear advantage for photovoltaic, photodetection and photoemission applications.…”