Size-Dependent PbS Quantum Dot Surface Chemistry Investigated via Gel Permeation Chromatography

Abstract

We report the first comprehensive study on the purification of PbS quantum dots (QDs) using gel permeation chromatography (GPC). GPC enables the efficient and repeatable separation of unbound small molecules from QDs in a homogeneous solvent environment. This separation allows us to gain further insight into the surface capping layer of PbS QDs and measure the ligand density on QDs of different sizes. We find that small oleate-capped PbS QDs are stable in solution in toluene after purification by GPC with negligible free ligand concentrations. However, purification of larger-diameter QDs reveals two oleate populations that persist after multiple GPC purifications: one with a broad olefin resonance characteristic of strongly bound ligands in smaller QDs, and the other with a narrower peak suggesting dynamic exchange with freely diffusing species. Variable-temperature NMR indicates a reversible equilibrium between the two populations, which is also observed in samples purified through precipitation with a polar solvent. The observation of two oleate populations following GPC purification is maintained even when the large-diameter QDs are formed using a strictly anhydrous procedure that should eliminate the presence of water, hydroxide, or oleic acid in as-synthesized samples. In conducting this strictly anhydrous synthesis, we incidentally observe that water plays a role in determining the resultant size of PbS QDs prepared by traditional methods compared to their anhydrous analogues. Understanding how the surfaces of PbS QDs are terminated should aid in interpreting their behavior in subsequent ligand exchange steps as necessary for diverse applications.

Study and Comparison on Purification Methods of Multicolor Emission Carbon Dots

“…Gel permeation chromatography (GPC), a type of size exclusion chromatography (SEC), is an alternative approach to isolate CDs into fractions of different particle size [17,18] . When CDs are allowed to move through a column containing the gel, the small‐sized substances are able to penetrate into the gel particle pores while the large‐sized substances are excluded from the pores and pass directly through the column.…”

Directed Ligand Exchange on the Surface of PbS Nanocrystals: Implications for Incoherent Photon Conversion

“…Such a procedure is critical to the optimization of PbS NCs for optoelectronic applications such as photovoltaics, photodetectors, and incoherent photon conversion. − PbS NCs are critical materials in this field due to many factors, especially their strong performance in devices, reliable synthesis, and a size-tunable bandgap reaching from the visible into the short-wave infrared . Two surface facets are preferred in PbS NCs, and both are usually present in varying proportions on the intermediate-sized particles (3–8 nm) that are closely studied for these applications. , A lead-rich (111) plane is the preferred surface for the smallest particles, which are octahedral, and a stoichiometric (100) facet becomes progressively favored for larger particles that are increasingly cubic. − The variety of local coordination environments that arise from the differing planes, edges, and vertexes that form these surfaces has been associated with the observed size-dependence of NC reactivities and ligand-exchange procedures. − ,− …”

Simple cubic self-assembly of PbS quantum dots by finely controlled ligand removal through gel permeation chromatography

“…GPC with a size-exclusion effect for the purication and separation of colloidal QDs has been reported to remove residual long alkyl ligands and high-boiling solvents such as octadecene aer synthesis. [31][32][33][34][35] On the other hand, the GPC method in this study controls ligand coverage of the QDs and does not show any sizeexclusion effect on the size of the QDs. This GPC process enables the so, quantitative, and continuous control over the ligand coverage of the QDs, which retain sufficient solubility for processability and luminescence properties.…”

“…32,41 The peak intensity of the bound OA was calibrated by subtracting the overlapping free OA peak, and the molar concentration of the bound OA in the NMR measurement was obtained via the internal standard. 32 The molar concentration of the QDs was estimated using the empirical equation of Moreels et al 39 The obtained results for the ligand density (GPC-1: 0.7 nm À2 ; GPC-2: 1.2 nm À2 ; GPC-3: 1.6 nm À2 ; GPC-4: 2.8 nm À2 ; GPC-5: 5.3 nm À2 ) (Table 1 and Fig. 2) are consistent with the ligand density obtained from the TGA measurements.…”