Synthetic Ligand Selection Affects Stoichiometry, Carrier Dynamics, and Trapping in CuInSe₂ Nanocrystals

Abstract

CuInSe2 nanocrystals exhibit tunable near-infrared bandgaps that bolster utility in photovoltaic applications as well as offer potential as substitutes for more toxic Cd- and Pb-based semiconductor compositions. However, they can present a variety of defect states and unusual photophysics. Here, we examine the effects of ligand composition (oleylamine, diphenylphosphine, and tributylphosphine) on carrier dynamics in these materials. Via spectroscopic measurements such as photoluminescence and transient absorption, we find that ligands present during the synthesis of CuInSe2 nanocrystals impart nonradiative electronic states which compete with radiative recombination and give rise to low photoluminescence quantum yields. We characterize the nature of these defect states (hole vs electron traps) and investigate whether they exist at the surface or interior of the nanocrystals. Carrier lifetimes are highly dependent on ligand identity where oleylamine-capped nanocrystals exhibit rapid trapping (<20 ps) followed by diphenylphosphine (<500 ps) and finally tributylphosphine (>2 ns). A majority of carrier population localizes at indium copper antisites (electrons), copper vacancies (holes), or surface traps (electrons and/or holes), all of which are nonemissive.

Mechanistic insights into the influence of surface ligands on quantum dots for photocatalysis

“…64 For ternary QDs, Schaller et al examined the effects of ligand composition (OAm, diphenylphosphine, and tributylphosphine) on the band gap of CuInSe 2 QDs, and they found that the band gap can be controlled between 1.27–1.43 eV by using these three ligands. 69…”

“…101 NMR spectroscopy is a convenient non-destructive-tool for determining the structure of ligands, assessing the coverage of ligands, and distinguishing between surface-bound and free ligands. 102,103 A variety of heteroatoms and functional groups in the ligand bound to the surface of the QDs can be detected by 1 H, 13 C and 31 P NMR, including phosphonic acids, 104 phosphines/phosphine oxides, 69,[104][105][106] amines, 69,89,105-107 fatty acids, 64,67,98,[108][109][110] amino acids, 111 carboxylic acids/ carboxylate 90,107,109,[112][113][114] and thiols. [114][115][116] For example, Herman et al reported the structure determination of capped ligands on the surface of CdSe QDs by 31 P and 1 H NMR.…”

“…64 For ternary QDs, Schaller et al examined the effects of ligand composition (OAm, diphenylphosphine, and tributylphosphine) on the band gap of CuInSe 2 QDs, and they found that the band gap can be controlled between 1.27-1.43 eV by using these three ligands. 69 The shi of the energy level of the CBM and VBM will further inuence the photocatalytic activity. For instance, using photodecomposition of organic dye methyl orange as a model reaction, Abargues et show a similar band edge energy to PbS-OAm, there is also a signicant difference in activity.…”

“…66,67 Therefore, a surface ligand is indispensable in the synthesis and preservation of QDs in solution. Different surface ligands have signicant effects on the energy level distribution, 68,69 amount of defect states, 70 charge separation efficiency [71][72][73] and stability 66,67 of QDs. Therefore, surface ligands can signicantly affect the photocatalytic performance of QDs.…”

Recent Advances in Metal Chalcogenide Quantum Dots: From Material Design to Biomedical Applications

“…[108] A recent study reported by Harvey et.al in 2021 showed that synthetic ligand selection affected the incorporation of defects within the crystalline lattice and passivation of the surfaces, which led to different stoichiometry, and carrier dynamics, and trapping in CuInSe 2 NCs. [109] Mei et al reported the microwave-assisted synthesis of Cu-In-S/ZnS QDs with dual ligands on their surfaces, and different ratios of thioglycolic acid (TGA) together with sodium citrate (SC) resulted in different shapes and fluorescence intensity of QDs, addressing the importance of ligand in morphology control and performance-boosting during the synthesis process. [44] The length of the ligand also has a significant impact on the performance of the synthesized MCQDs.…”