Synthesis of Mn²⁺-Doped PbS Quantum Dots and Their Spectroscopic Properties

Quantum dots are semiconductor nanocrystals that have size-dependent properties, unlike bulk materials, as indicated by the well-studied CdSe system.¹ They are characterized by a band gap, and corresponding to the band gap there is an absorption in the UV-VIS region and an emission in the visible region followed by excitation in the band gap. It is well established that the emission spectrum is sensitive to trace levels of dopants such as Mn²⁺, as has recently been demonstrated for ZnS quantum dots.² The goal of the authors’ research program is to achieve a fundamental understanding of the synthesis and spectroscopic properties of quantum dots other than the CdSe system.

The authors conducted a number of studies with the ZnS quantum dots. One of their research objectives is to be able to dope the quantum dots with magnetic metal ions such as Mn²⁺ to impart to them interesting and useful spectroscopic and magnetic properties that could lead to practical applications. In this regard, the authors are investigating quantum dots that can be doped with trivalent lanthanide metal ions, which may lead to doped materials with valuable spectroscopic and magnetic properties. The systems of interest for doping with lanthanide metal ions are the PbS and CdS quantum dots, in which the ionic radii of Pb²⁺ and Cd²⁺ are close to those of trivalent lanthanides. Doping these quantum dots with trivalent lanthanide ions is an ongoing project in the authors’ laboratory and these results will be published separately. In contrast, in the ZnS quantum dot lattice, the ionic radius of Zn²⁺ is much smaller than those of the trivalent lanthanides.

This paper discusses preliminary studies on the synthesis and characterization of PbS quantum dots that are doped and undoped with Mn²⁺. To the authors’ knowledge, the synthetic procedure reported here is the first to employ an aqueous phase synthesis at ambient temperatures to obtain undoped and doped PbS quantum dots. PbS quantum dots, reported in the literature, are in thin film and other matrices and in nonaqueous solvents.^3–14

Materials and methods

All compounds employed in the synthesis were acquired from Aldrich Chemical Co. (Milwaukee, WI) and were used without further purification. A typical synthesis of PbS quantum dots doped with Mn²⁺ is described here, and the nanocrystals without Mn²⁺ were synthesized using the same procedure. The stabilizer sodium polyphosphate Na(PO₃)_n (10.2 g) was dissolved in 70 mL of MilliQ water (Millipore Corp., Billerica, MA). A solution of 3.31 g Pb(NO₃)₂ in 10 mL of MilliQ water was added to the stabilizer and stirred at room temperature for 90 min. Following the addition of Pb²⁺, a white precipitate formed, indicating a complex formation between the Pb²⁺ and (PO₃)_n. After 90 min, the solution was filtered, after which 1 mL of 1.80 g Mn(NO₃)₂ dissolved in 10 mL of MilliQ water was added to it, corresponding to 10% doping with Mn²⁺. This was immediately followed by the dropwise addition of a 10-mL solution of 2.40 g Na₂S in MilliQ water. The mixture was stirred at room temperature for 1 hr. The solution was centrifuged for 10 min and the supernatant was decanted. The slurry of black Mn²⁺-doped PbS quantum dots was washed twice with MilliQ water. The first wash was discarded and the second was saved. The second wash was used to examine the UV-VIS absorbance, emission, and transmission electron microscopy (TEM) image of the quantum dots. The remaining slurry was dried and used for electron paramagnetic resonance (EPR) analysis.

The UV-VIS spectra were obtained with a Lambda 20 spectrophotometer and the emission spectra were recorded with an LS 50B spectrofluorimeter (both from PerkinElmer, Shelton, CT). The EPR spectra of the solid PbS samples were recorded at room temperature with a JEOL JES-TE100 (JEOL-USA, Peabody, MA) at a microwave frequency of 9.0559 GHz over a sweep width of 300 mT (T = tesla).

Results and discussion

Figure 1 - TEM image of PbS quantum dots, indicating an average size of 100 nm and nanoparticles that are fused.

Figure 2 - UV-VIS spectrum of a dispersion of PbS in water, exhibiting a shoulder at 350 nm corresponding to the band gap of the nanocrystals.

The TEM image of the PbS quantum dots doped with Mn²⁺ is displayed in Figure 1. The extent of doping is estimated to be about 1% from prior studies with ZnS quantum dots, even though the concentration of Mn²⁺ in the synthesis is 10% of the Zn²⁺ concentration.² The average size is about 100 nm, with nanoparticles that are both smaller and continued larger than this size (see Figure 1). In comparison, ZnS yielded nanoparticles that were 5 nm on average under the same conditions.² The UV-VIS spectrum of a dispersion of PbS (the supernatant from the wash of the nanocrystals) is shown in Figure 2, with a shoulder at 350 nm. This corresponds to the band gap of the PbS semiconductor nanocrystals; it is similar to those reported in the literature^5–8 and is at a lower energy than the ZnS quantum dots, which have a shoulder at 300 nm. When this solution was examined for emission by scanning the excitation and emission spectra, no measurable emission could be observed.

Figure 3 - EPR spectrum of the Mn²⁺-doped PbS quantum dots, indicating the expected six lines. Doping occurs predominantly on the surface of the nanocrystals.

Extended spectral accumulations and signal averaging by excitation at 350 nm corresponding to the band gap also yielded no measurable emission from 400 to 900 nm. This is in agreement with previous observations in the literature, where the emission of PbS in the aqueous phase is quenched by water and can be observed only in nonaqueous media with PbS nanoparticles coated with an organic compound such as thiol or in thin films^{.6–8,10,12,13} The EPR of the PbS quantum dots doped with Mn²⁺ is shown in Figure 3.

To the authors’ knowledge, this paper is the first to report on the EPR of PbS nanoparticles doped with magnetic nuclei. The EPR demonstrates the expected six peaks for Mn²⁺, which are not well separated. This pattern, as shown in the case of ZnS quantum dots doped with Mn²⁺, indicates that in the case of PbS quantum dots the ions are present predominantly on the surface of the nanocrystals. As a result, extensive interaction occurs between the Mn²⁺ ions, yielding an unresolved six-line pattern.

Conclusion

PbS quantum dots of 100 nm average size doped with Mn²⁺ were prepared in aqueous phase at room temperature using polyphosphate as the stabilizer. The doping of the Mn²⁺ ions was shown to occur predominantly on the surface of the nanocrystals by EPR.

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The authors are with the Department of Chemistry and the Nanotechnology Research and Computation Center (NRCC), Western Michigan University, Kalamazoo, MI 49008, U.S.A.; tel.: 269-387-3656; fax: 269-387-2909; e-mail: [email protected]. Support for this research from the W.M. Keck Foundation (Los Angeles, CA) is gratefully acknowledged.