Synthesis of PbS Quantum Dots

Lead chalcogenide nanocrystals are of increasing importance due to their potential in

4.2 Results and Discussion

an emitting wavelength in the infrared. PbS nanocrystals were synthesized to study if the surface initiated Suzuki-Miyaura coupling polymerization can be extended to different types of nanocrystals.

For these experiments, a narrow particle size distribution was considered important, especially regarding potential centrifugation experiments after the formation of hybrid particles.

The synthesis of PbS QDs was adopted from two literature known procedures by Cademartiri et al.¹⁶⁰ and Moreels et al.¹⁶¹, respectively. Both synthesis protocols are based on the hot injection method and have in common that only the weakly binding L-type ligand oleyl amine is employed, allowing for a ligand exchange process after nanoparticle formation. This enables the introduction of nanoparticle ligands that are suitable precursors for the formation of Suzuki-Miyaura initiator complexes.

PbS nanocrystals were synthesized under variation of temperature, reaction time, Pb to S ratio and precursor to ligand ratio, with the aim of obtaining monodisperse QDs.

Table 5. Varied parameters for the synthesis of PbS nanocrystals.

Entry Nucleation and

a According to Cademartiri et al.: 0.25 mL of a 0.15 M sulfur precursor solution in oleyl amine were rapidly injected into 2.38 mL of a 1.5 M PbCl2 precursor solution in oleyl amine at 120 °C. ^b According to Moreels et al.: 2.25 mL of a 0.33 M sulfur precursor solution in oleyl amine were rapidly injected into 7.5 mL of a 0.4 M PbCl2 solution in oleyl amine at a

temperature between 120 °C - 160 °C. ^c Addition of 1 g of 4-bromobenzoic acid during nanocrystal synthesis.

The first two batches of PbS quantum dots were synthesized according to the publication of Cadematiri et al.¹⁶⁰, in which the influence of the PbCl2:oleyl amine ratio and of the Pb:S ratio on the particle size dispersity of the resulting quantum dots is investigated (Table 5, Entry 1 & 2). They

However, attempts to synthesize monodisperse PbS QDs by this protocol failed (Table 5, Entry 1 & 2) and only polydisperse nanocrystals were obtained. An additional problem that emerged was the removal of excess PbCl2, which turned out to be difficult and very time consuming. For this reason, further experiments were conducted according to the publication of Moreels et al.¹⁶¹ In this protocol, the Pb:S ratio is significantly lower (4:1 instead of 10:1), which should result in a higher conversion of PbCl2. The first two experiments were conducted at 125 °C with different reaction times (10 min., Entry 3, 30 min., Entry 4). The QDs obtained in both experiments were analyzed by TEM and undefined polydisperse structures were observed. For this reason, the reaction time was increased to 90 minutes to increase the time for size focusing and the temperature was varied (125 °C, Entry 5, 160 °C, Entry 6). After 90 minutes at 125 °C, mostly cubic shaped PbS QDs were isolated from the reaction mixture (Figure 45, left). Increasing the reaction temperature to 160 °C to promote the size focusing resulted in the formation of QDs with a bimodal size distribution (Figure 45, center).

The PbS QDs obtained under the conditions described in Entry 5 with the temperature set to 125 °C and a reaction time of 90 minutes were considered to be of sufficient quality and were employed in surface initiated polymerization experiments after subsequent functionalization with appropriate ligands (Chapter 4.2.10).

Figure 45. TEM images of PbS quantum dots synthesized according to the conditions described in Table 5 Entry 5 (left) and Entry 6 (center). The PbS QDs depicted in the right TEM images were synthesized in the presence of

4-bromobenzoic acid (Entry 7). The inlets depict particle size histograms.

For the grafting of conjugated polymers from nanocrystals, the latter need to be functionalized with appropriate ligands that feature a functional group that binds to the nanocrystal surface and a haloaryl-moiety with which a Pd(0) precursor can react to form a surface bound initiator complex.

The addition of a haloaryl ligand directly into the synthesis of the nanocrystals renders a ligand exchange step superfluous. For this reason, the synthesis of PbS QDs in the presence of 4-bromobenzoic acid was studied. Synthesis conditions were adapted from the experiment described in Table 5, Entry 5, as these resulted in tolerably monodisperse PbS nanocrystals. 1 g of

4-4.2 Results and Discussion

bromobenzoic acid was introduced into the reaction mixture. The obtained nanocrystals were analyzed by TEM (Figure 45, right). The introduction of this small ligand into the synthesis unexpectedly results in the formation of highly monodisperse nanocrystals and the mean particle diameter is decreased from 12.1 nm to 8.0 nm with σ decreasing from 1.2 nm to 0.71 nm. The significant change in mean particle diameter indicates a strong influence of 4-bromobenzoic acid on particle growth.

The nanocrystals were analyzed by ¹H-NMR spectroscopy to study if the newly introduced ligand is actually bound to the nanocrystal surface. The ¹H-NMR spectra of 4-bromobenzoic acid (bottom), of oleyl amine (center) and of the nanoparticles after work-up (top) are depicted in Figure 46.

Figure 46. ¹H-NMR spectra recorded in THF-d8 at room temperature of 4-bromobenzoic acid (bottom), of oleyl amine (center) and of PbS QDs synthesized in the presence of oleyl amine and 4-bromobenzoic acid (top), after work-up.

Surprisingly, in the spectrum of the nanoparticles after work-up, signals that can be assigned to

synthesis, the major part of the functional ligand is hereby removed, resulting in a large excess of oleyl amine (20:1 oleyl amine:4-bromobenzoic acid).

Although 4-bromobenzoic acid is not the major ligand after particle synthesis, its use during nanocrystal synthesis is still beneficial, as highly monodisperse PbS nanocrystals were obtained.

Additionally, carboxylic acids promote the precipitation of PbCl2162, and the introduction of 4-bromobenzoic acid strongly reduced the effort for removal of excess PbCl2.

4.2.10 Hybrid Particle Synthesis by Surface Initiated Suzuki-Miyaura Coupling Polymerization