Article pubs.acs.org/JPCC
Synthesis and Transformation of Zn-Doped PbS Quantum Dots Xingliang He,† Iraida N. Demchenko,‡ W. C. Stolte,§,∥,⊥ Anthony van Buuren,# and Hong Liang*,† †
Department of Mechanical Engineering, Texas A&M University, College Station, Texas 77843-3123, United States Institute of Physics, Polish Academy of Sciences, al. Lotników32/46, 02-668, Warsaw, Poland § Advanced Light Source, Lawrence Berkeley National Laboratory, Berkeley, California 94720, United States ∥ Department of Chemistry, and ⊥Harry Reid Center for Environmental Studies, University of Nevada, Las Vegas, Las Vegas, Nevada 89154, United States # Lawrence Livermore National Laboratory, Physical & Life Sciences Directorate, Condensed Matter & Materials Division, Livermore, California 94550, United States ‡
S Supporting Information *
ABSTRACT: A micelle-assisted wet-chemistry route is developed to synthesize pure and Zn-doped lead sulfide (PbS) quantum dots (QDs) and nanocrystals (NCs) under microwave irradiation. The formation mechanism includes three major steps, initialization of π-bonded complex, transformation into a micelle structure, and the dissipation of nanoparticles (NPs). The micelle structure plays an important role in PbS NCs and QDs transformation and formation. X-ray absorption near-edge structure (XANES) analysis confirms the quantum confinement in PbS QDs. The Burstein−Moss effect is responsible for the blue-shift of the absorption induced by Zn doping. This research opens a new way to prepare NCs and QDs that enables high-resolution analysis in quantum refinement and electronic structures. The NCs and QDs produced here have strong potential in applications in optical and electronic communication.
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electronic, optical, transport, and magnetic properties.19,20 It is reported that CdS and ZnS are prepared into diluted magnetic semiconductor QDs by Co2+ doping.21 Mn-doped CdS/ZnS core/shell NCs show a dependence of optical properties on the Mn radial positions.22 The color-tunable ZnSe NCs have been shown to cover most of the visible window after doping it with Cu or Mn.23 Indium doping shifts the Fermi level and provides access for electronic transportation.24 Different impurity concentration can induce transition between n- and p-type-doped semiconductor NCs.25,26 These results have shown that by using a dopant one can design and potentially fabricate QDs with superior properties. We have prepared Zn-doped PbS (Pb/Zn−S) QDs by incorporating Zn2+ ions into Pb2+ precursor solutions. The absorption blue-shifts of these Pb/Zn−S QDs and Burstein− Moss effects are investigated.
INTRODUCTION Recent development in visible light-emitting colloidal quantum dots (QDs)1,2 has attracted great interest in infrared (IR)-active QDs.3−5 Near-IR luminescence is in demand for telecommunication,6 in vivo fluorescence,7,8 electroluminescence,9,10 photodetection,11,12 and photovoltaics.12 Among the most studied IR luminescent materials, lead chalcogenide possesses unique properties such as large static dielectric constants, direct bandgap transitions, ferroelectric effects, large Bohr radii, and relatively small and approximately equal effective masses for electron and hole.3,4 Two methods have been frequently used to synthesize those materials. The first method derives from the synthesis of cadmium chalcogenides QDs in organic solvent.13,14 The other is a synthesis of the same but in an aqueous environment.15 Because it is costeffective, environmentally friendly, and scalable, microwave (MW) irradiation has recently become a widely used method in the preparation of NPs.16,17 In this research, we synthesize PbS NCs and QDs via a micelle-stabilized aqueous phase followed by MW irradiation. The mechanism for micelle-assisted formation of PbS QDs is proposed. XANES analysis focuses on the region of the X-ray absorption spectrum dominated by strong photoelectron scattering near the absorption edge.18 It confirms the quantum confinement of PbS QDs. Doping has been demonstrated to effectively enhance the performances of NCs by controlling their remarkable © 2012 American Chemical Society
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EXPERIMENTAL SECTION Materials. The following chemicals were purchased from Sigma-Aldrich: lead dichloride (PbCl2), 3-mercaptopropanoic acid (3-MPA), sodium hydroxide (NaOH), thioacetamide, and Received: May 15, 2012 Revised: September 6, 2012 Published: September 17, 2012 22001
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Figure 1. (a) XRD patterns for quartz substrate (black), PbS QDs (maroon), pure ZnS (green), and Zn-doped PbS QDs (blue). Inset shows careful scanning and the tiny shifts. (b) Comparison of XRD patterns of Pb/Zn−S QDs with standard. (c) EDS analysis of the Zn-doped PbS QDs.
M of NaOH solution was used to titrate this turbid solution until the white floc totally disappeared, resulting in a clear and transparent Pb2+ precursor solution. Titration was stopped when the pH value of the solution increased to ∼11.6. The resulting optically transparent solution was protected from light and stored at room temperature until needed. Simultaneously, a S2− source stock solution was prepared by dissolving thioacetamide in DI water. Next, 5 mL of this S2− solution was swiftly injected into 35 mL of the Pb2+ precursor solution under vigorous stirring. The final concentration of S2− for this
zinc chloride (ZnCl2). All chemicals were used without further purification. Synthesis of PbS NCs and QDs. Our Pb2+ precursor solution was prepared by mixing a solution of PbCl2 and a stabilizer agent, 3-MPA, in deionized (DI) water. We fixed the concentration of Pb2+ and varied the concentration of 3-MPA to find the proper ratio for synthesis of QDs. Typically, 0.01914 g (∼0.07 mmol) of PbCl2 and 0.0349 mL (∼0.4 mmol) of 3MPA were distributed in 35 mL of DI water. A white floc appeared as soon as the 3-MPA was added. Subsequently, a 1 22002
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three of those match the three strongest peaks of cubic zincblende ZnS (JCPDS file no. 65-0309). The lattice tension originating from the lattice mismatch between PbS and ZnS27−29 can shift the XRD peaks. Inside the Pb/Zn−S QDs, the Zn dopant can thus form a ZnS-like structure in the PbS lattice, marked as Zn−S in Figure 1a. The presence of sulfate/ sulfite and thiosulfate species is confirmed via the following XANES analysis. Other peaks on the blue curve of Figure 1a can be assigned to sulfate/sulfite and thiosulfate components. Figure 1c shows an EDS spectrum for the Pb/Zn−S QDs. The relatively strong additional Cu peaks are a result of the Cu TEM grid. In addition to the appearances of Pb and S, the weak Zn intensity can be attributed to the relatively small amount of Zn adopted when doping PbS. Crystalline structures of asprepared PbS QDs and its Zn dopant are confirmed now. In the TEM images as shown in Figure 2, there are two visible size distributions. The aging process enhanced the
typical synthesis was 1 mM. In addition, the ratios of 10:8, 10:6, 10:4, and 10:2 of Pb2+ to S2− were also selected to modulate the QDs. The thoroughly mixed solution was finally placed in a MW oven (with a frequency of 2450 MHz and work power of 960 W) and irradiated for a total of 40 s with intervals of 10 s to prevent the vessel from overheating. After MW irradiation, a light gray-green primary QDs solution was prepared. A further aging process was necessary for the growth of well-crystallized PbS. After 10 days of aging, primary PbS colloids would change into layer-separated PbS solutions, which contained PbS QDs in the top clear solution layer and PbS NCs in a dark solution at the bottom. After separating and centrifugalizing the dark bottom and clear top solutions at 7000 rpm for 30 min, we obtained PbS NCs and QDs, respectively. For Pb/Zn−S QDs, Zn was doped using a Pb2+/Zn2+ precursor solution with the subsequent procedure being the same as that for PbS QDs. A mole ratio of 80:1 was selected for Pb to Zn. Characterizations for the NCs and QDs. The ultraviolet−visible−near IR (UV−vis−NIR) absorption spectra were recorded with a UV−vis−NIR spectrometer (Hitachi U4100). The X-ray diffraction (XRD) patterns of the PbS before and after doping were collected with a scanning rate of 0.02 s−1 in the 2θ range 20°−70° using a Bruker, D8 advance X-ray diffractometer with Cu K radiation operating at 40 kV, 50 mA. XANES analysis for bulk PbS and PbS QDs was performed near the sulfur K-edge (∼2472 eV) at ambient conditions on beamline 9.3.1 at the Advanced Light Source, Lawrence Berkeley National Laboratory. XANES spectra were collected with an energy resolution of ∼0.4 eV using the detection modes of total fluorescence yield (TFY), total electron yield (TEY), and transmission (TR). The TFY is known to be bulk sensitive that uses a large area Si-photodiode (Hamamatsu model S358409). The energy dispersive X-ray spectra (EDS) and the selected area electron diffraction (SAED) patterns were recorded with the instruments connected with a high-resolution transmission electron microscope (HRTEM, JEOL 2010, accelerating voltage of 200 kV). The samples for the TEM (JEOL 1200, accelerating voltage of 100 kV) study were prepared by slow evaporation of a drop of the colloidal solution on carbon-coated copper (Cu) grids at room temperature. The samples for XRD and XANES analysis were prepared by placing 2 mL of colloidal QDs solutions on quartz substrates that were subsequently dried on a hot plate (at a level of 100 °C) for 15 min.
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Figure 2. (a) TEM image of PbS NCs; (b) TEM image of PbS QDs; (c) HRTEM image of PbS QDs taken over (b); (d) corresponding SAED pattern for PbS QDs; and (e) size distribution of PbS QDs.
RESULTS AND DISCUSSION Transformation from PbS NCs to PbS QDs. Experimentally, in a micelle-assisted wet-chemistry routine, PbS NCs and QDs were made by varying the ion-to-molar ratio and doping Zn into PbS QDs using mixed Pb2+/Zn2+ precursor solution. In XRD characterizations, an amorphous quartz substrate was used to avoid interference, as shown in the black curve of Figure 1a. Most of the observed peaks in the maroon colored XRD pattern agree well with diffractions appearing in face-centered cubic (fcc) PbS (JCPDS file no. 659496). Two fcc sodium chloride (NaCl) XRD peaks (JCPDS file no. 05-0628) are also observed as a minor product of chemical reactions. As a reference, the XRD pattern of pure ZnS (Sigma-Aldrich, product no. 244627) was obtained that is shown as the green spectrum in Figure 1a. In addition to the diffraction patterns that have been determined for PbS and NaCl, the blue XRD plot reveals more peaks after Zn was doped into PbS. Besides tiny shifts as shown in Figure 1a and b,
differentiation of the size difference. Following the synthesis routine of Figure 4a, the bottom dark part from the aged PbS colloidal solution contains only PbS NCs with a size around 50 nm (see Figure 2a). From the top clear part, only small QDs (∼5 nm) are obtained (Figure 2b and e). From the same image, a narrow size distribution of QDs is observed. Figure 2c is the HRTEM image of Figure 2b. It confirms the existence of monocrystalline PbS QDs. The spacing between two crystal planes matches the strongest PbS diffraction (200) peak in the XRD pattern. Figure 2d is the SAED pattern of the same sample over a larger area. For our PbS samples, different monocrystalline QDs have different crystalline orientations. In accordance with the XRD result, three main fcc diffraction patterns are also observed: PbS (200), PbS (111), and PbS (220). 22003
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bonding.32,33 We also note that the 3-MPA molecule has a carbonyl group in which the oxygen is more electronegative than the carbon. The polarity of CO bond can then be increased because the oxygen pulls the electron density away from the carbon. The carbonyl carbon becomes electrophilic, and thus more reactive with Pb.34,35 As a whole, a relatively strong π-bonded complex will be formed between the metallic cation, Pb2+, and organic anions from carboxyl group. For electrons involved in π-bonding, anti-π (π*) bonding, and nonbonding states, UV irradiation can stimulate their transitions from the highest occupied molecular orbital to the lowest unoccupied molecular orbital.36 The green peak appearing on the edges of the UV region (∼390 nm) in Figure 4b can be explained as the excitation of nonbonding electrons (n-e−) to higher π* orbitals. The charge transfer in πconjugated complex molecules is predicted to be a general effect.37,38 Charge transfer happened when two components contributed as the electron donor and electron acceptor, respectively. In our case, the Pb2+ ion and carboxyl group can, respectively, play the role of electron donor and acceptor. Another green absorption peak in Figure 4b, located near 850 nm, is a result of this electron transfer from the donor to the orbitals associated with the acceptor. In terms of both hue and shade, white is the extreme end of the visible spectrum, this peak then explains why our Pb2+ floc looks white. This πconjugated Pb2+ complex, white floc, is drawn in the upper image of Figure 4c. The asymmetry in this sharp peak at 850 nm is a result of the detector in the UV−vis−NIR spectrometer changing wavelength at 850 nm. As the hydrophobic thiol (−SH) groups39,40 are exposed to the aqueous surrounding, the white floc can then be precipitated out from the resulting water solution. To obtain a well-stabilized precursor solution, we titrated the turbid white Pb2+ floc with 1 M of NaOH until it disappeared totally, resulting in a clear and transparent solution as shown in the lower-right inseted picture of Figure 4b. In the appearance of 3-MPA [HS(CH2)2COOH], the higher the pH value, the more carboxylate anions [HS(CH2)2COO−] will be released from it. It is reported that 3-MPA has an acid dissociation constant value (pKa) of 10.3.41 We found that the clear and transparent solution could form at a pH value above 11. A hydrophilic “head” that is apt to be in close contact with the surrounding solvent is the precondition to form a typical micelle in aqueous solution.42 Here, we have a much higher 3MPA concentration (10 mM) than Pb salt (1.75 mM) in our solution. So, once the pH value goes above the pKa of 3-MPA, excess HS(CH2)2COO− is generated. Micelle is then formed due to the interaction between Pb2+ and HS(CH2)2COO−. The hydrophilic HS(CH2)2COO− also facilitates dissolving the micelles into the aqueous surroundings. This is why no precipitations are observed from the micelle solution. The blue curve of Figure 4b also shows the absorption spectrum for the Pb2+ micelle solution. When comparing this curve to the green curve, there is no visible absorbance. Similar to the discussion above, the blue peak located in the UV region near 350 nm accounts for n-e− excitation to the π* states. These electron transitions happen inside the micelle between metallic cations (Pb2+) and HS(CH2)2COO−. In the middle image of Figure 4c, we draw a stable Pb2+ micelle that is distributed in water solution. The key to synthesizing PbS QDs is the control of its nucleation and the following growth process. Proper concentration of 3-MPA can prompt a suitable nucleation condition.
Pb/Zn−S QDs with a narrow size distribution are shown in the TEM image of Figure 3a and size distribution of Figure 3e.
Figure 3. (a) TEM image of Pb/Zn−S QDs; (b,c) HRTEM images of Pb/Zn−S QDs; (d) the corresponding SAED patterns of (a); and (e) size distribution of Pb/Zn−S QDs.
The HRTEM images in Figures 3b and c provide evidence of their crystallized structure. The crystal lattices indicate that the colloidal solution is composed of different monocrystalline Pb/ Zn−S QDs. Because of the random growth distribution, different QDs will be developed into different single-crystalline structures. This is why abundant crystalline information can be received at the nanoscale. In HRTEM characterization, a wellknow, “Moiré fringe”, that is, arrays of fake atomic dimension due to layered structures and small particles, is often observed.30,31 In Figure 3c, the Moiré fringes, highlighted in green, exist when two or more QDs are stacked against each other with close contact. The formation of the fringe patterns is attributed to the different orientation between two monocrystalline QDs. Figure 3d shows the corresponding SAED pattern for Pb/Zn−S QDs. As compared to Figure 2d, we observe that more electron diffraction patterns appear due to Zn doping. Three of those can be attributed to the diffractions from zincblende-like crystalline planes, Zn−S (111), Zn−S (220), and Zn−S (311) marked in green. Mechanisms of Micelle-Assisted Synthesis. We propose a new mechanism for the synthesis using the micelle-assistant approach. The summary of the synthesis routine is shown in Figure 4a. First, a turbid solution containing white floc, shown in the upper-left inseted picture of Figure 4b, was formed after adding 3-MPA stabilizer into the Pb2+ precursor solution. Among the elements used for synthesis, Pb, S, O, and C have lone electron pairs on 6p, 3p, 2p, and 2p orbitals, respectively. These lone pair electrons from p orbitals usually engage in π22004
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Figure 4. (a) Brief summary for synthesis of PbS QDs. (b) UV−vis−NIR absorption spectra of Pb2+floc solution (green) and Pb2+ micelle solution (blue). Inseted pictures are taken, respectively, for turbid white floc (upper left) and clear micelle (lower right). (c) Schematic representation of the mechanism for micelle-assisted formation of PbS QDs: π-conjugated Pb2+ complex (upper), stable Pb2+ micelle (middle), and example PbS QDs (bottom).
indicate surface oxidation. The very intense resonance at ∼2.482 keV and the pronounced feature at ∼2.478 keV that appeared in the TEY spectrum are dramatically reduced in the TFY signal. A comparison of the XANES spectra for PbSO4 and Na2SO3 standards43 indicates the feature at ∼2.478 keV in our bulk PbS XANES spectrum can be attributed to the absorption of sulfur in the form of sulfite (SO32−), whereas the second resonance at ∼2.482 keV is mainly in the form of sulfate (SO42−). From Figure 5b, the blue-shift of the absorption threshold (inflection point) of 1.67 eV for PbS QDs with respect to the bulk sample indicates a decrease of number of minimum p-S states in the conduction band contribution at ∼2.4698 keV (see the shoulder marked by arrow). This decrease leads to the transitions into the states with higher energies, which means the increase of bandgap too. The absorption onset positions for PbS bulk and PbS QDs obtained under the technique mentioned by Lee et al.44 makes it possible to conclude that this shift is on the ∼electronvolt level. Electrons can “feel” the presence of the particle boundaries when the wavelength of the electronic wave is comparable with the QDs’ size (
