Tailored Band Edge Positions by Fractional Ligand Replacement of Nonconductive Colloidal Quantum Dot Films

Abstract

The tunable band edge position of colloidal quantum dot (CQD) films is a key part of efficient optoelectronic device design of various forms such as photovoltaics, light-emitting diodes, and photodetectors. An accurate estimation of shifts in the band edge position of CQD layers is still considered challenging, especially when the CQD films are nonconductive. Here, we investigate the effect of nonconductive CQD films on photoelectron spectroscopy (PES) and photoelectron yield spectroscopy (PYS). We demonstrate control of systematic band edge positions by fractional ligand replacement of nonconductive CQD film characterized with photoelectron yield spectroscopy and density functional theory calculations. As-synthesized CQDs with insulating oleate ligands were fractionally replaced with trans-3,5-difluorocinnamic acid molecules in a nonpolar solution. The fractionally replaced surface-bound ligands are quantitatively analyzed using 1H and 19F nuclear magnetic resonance (NMR) spectroscopy. We found that the energy levels of nonconducting CQD films shift linearly as a function of the number of bound trans-3,5-difluorocinnamates with specific dipole moments while retaining hydrophobic wettability.

Unveiling the Nanocluster Conversion Pathway for Highly Monodisperse InAs Colloidal Quantum Dots

“… 22 , 29 In Figure S7 , we demonstrate the removal of free oleic acid (OA) and 1-octadecene (ODE) by FTIR and 1 H NMR analysis. 30 In the FTIR spectra of purified nanoclusters, the broad –OH and sharp C=O peaks at 1705 cm –1 , typically associated with free OA, were not observed. Similarly, in the 1 H NMR spectra, the vinyl proton peaks of ODE at 5.0 ppm and free OA at 5.4–5.5 ppm disappeared, while the vinyl proton peak of bound oleate at 5.5 ppm remained in the purified nanoclusters.…”

“…For a more comprehensive characterization of the first-order conversion process and the resulting structure, we utilized InAs MSCs synthesized at 100 °C, a temperature at which they are rapidly formed without the formation of InAs CQDs, as depicted in Figure S6. Furthermore, purification of the InAs nanoclusters was performed using GPC to eliminate unreacted precursors. , In Figure S7, we demonstrate the removal of free oleic acid (OA) and 1-octadecene (ODE) by FTIR and 1 H NMR analysis . In the FTIR spectra of purified nanoclusters, the broad –OH and sharp CO peaks at 1705 cm –1 , typically associated with free OA, were not observed.…”

Ultrahigh-gain colloidal quantum dot infrared photodetectors: Unraveling the potential of electro-kinetically pumped charge multiplication

“…Elongation of the carbon chain of the thiol ligands led to a shift in the E F position toward the conduction band. This E F shift in the CQD lms could be in uenced by the reduced coverage with longer ligands 44 .…”

Semiconductor Nanocrystals: Unveiling the Chemistry behind Different Facets

“…We also studied a proton-assisted ligand exchange on (111) surfaces, a crucial process for trap-state passivation, 28 increasing conductivity, 46 and modulating energy levels 39 in NC films for solid-state devices. Choi et al 38 conducted fractional ligand exchange, systematically tailoring the band edge positions of PbS films through a proton-assisted 1:1 Xtype ligand exchange (Figure 3d). The results were directly observed through ambient photoelectron spectroscopy and further validated by a uniform dipole layer model based on our PbS model and first-principles density functional theory (DFT) calculations.…”

“…Those properties are determined by the surface–ligand interface, where surface atoms form structural disorders such as vacancies and dimerized or under-coordinated sites. Additionally, surface ligands also can directly influence optoelectronic properties such as the elimination of midgap trap states or the shift of absolute energy states of NCs through the formation of bonding (and antibonding) molecular orbitals or the generation of the electric field from dipole moments of ligand molecules (Figure C). , The complementary properties and functionalities, such as solubility, stability, or the ability to assemble NCs into ordered superlattices, are also achieved through ligand and surface chemistry, which is important for integrating NCs into solid films . Thus, extensive efforts have been focused on comprehensive understanding of the surface characteristics of colloidal nanocrystals at an atomistic level, which is essential for their precise regulation.…”