Air-stable PbS quantum dots synthesized with slow reaction kinetics via a PbBr2 precursor

PbS quantum dots have been synthesized using a PbBr2 precursor and the halide content has been examined. Slower reaction kinetics for quantum dots growth relative to the use of PbCl2 was observed for PbBr2, giving a possible route to increased control over quantum dot size with in situ passivation. Unambiguous determination of the surface conditions of nanomaterials is still a developing area of science, pushing the limits of current microscopy and analytical techniques. Contributions to a rigorous form of nanomaterial surface analysis are made here using X-ray photoelectron spectroscopy to analyse bonding in detail. Atomic resolution TEM is applied to produce energy dispersive X-ray spectroscopy maps with state of the art resolution. This analysis has been applied to air-stable halide terminated PbS nanoparticles, which is a nanomaterial of central importance for quantum confined solar cell applications. Chemical analysis from X-ray photoelectron spectroscopy is consistent with Br surface termination and high resolution energy dispersive X-ray spectroscopy (EDS) maps also show a positive spatial correlation for Br with quantum dot location. An observed excess Br content is attributed to the presence of bromine terminated PbS quantum dots nuclei in the final colloid.


Introduction
Semiconductor nanoparticles, sometimes as quantum dots (QDs), fabricated from lead chalcogenides are central to a number of advanced concepts in nanotechnology.PbS QDs have been used as a sensitizer and a light absorbing material in solar cells. 1 Perhaps most notable is the advantageous application of quantum conned PbS QDs in the area of colloidal quantum dot solar cells. 2,3The rst PbS quantum dot solar cell was fabricated in 2005. 4An efficiency greater than 1% was achieved in 2008 using a Schottky structure. 5Over the past three years, efficiencies in these cells have climbed steadily, [6][7][8][9] with the record efficiency currently standing at 9.2%. 10This suggests that there is substantial potential for these materials to produce low cost solution-processed solar cells employing novel quantum connement effects.][13] Synthesis of colloidal PbS quantum dots has been performed extensively and PbS colloidal synthesis recipes are increasingly mature. 14,15With the substantial interest in quantum connement effects, tunable bandgaps and modied photonic and phononic properties for photovoltaic and photonic applications, synthesis recipes for PbS quantum dots continue to be of signicant interest.PbS quantum dots can be synthesized with a number of different lead chemical precursors.7][18] The presence of halide atoms on the surface of the quantum dots has generally been inferred from the impressive air stability of these materials. 19,20However, these current methods have some disadvantages, such as the use of expensive TMS 2 S, the difficulties in synthesizing small particles with in situ passivation, or the requirements for large quantities of chemicals.
Synthesis with halide precursors comes with a few practical concerns, such as the removal of the unreacted portion of the commonly used Pb precursor from the PbS precipitate.Methods to address this have been demonstrated in the literature and continue to be developed.The level of residue can be assessed using conventional TEM where small ($1 nm) unreacted PbX 2 (X ¼ Cl, Br) particles are generally visible.Residue below the level of visible particulates is also generally difficult to assess.
Synthesis of halide-terminated nanoparticles has been performed recently, as well as nanoparticles that have multiple ligands. 17This trend toward increasingly complex surface terminations has potential to continue.However it is still very difficult to unambiguously assess the surface chemistry of nanomaterials, this article contributes to a methodology for more unambiguous determination of the surface chemistry of PbS quantum dots fabricated from a PbBr 2 precursor.The growth rates for particles using the PbBr 2 and PbCl 2 precursors were assessed and a measurable difference observed, with synthesis using the PbBr 2 precursor yielding approximately 30% smaller quantum dots with surface passivation under the same reaction conditions as that for the PbCl 2 precursor.It was of interest to determine the actual bromine content of the system and to attempt to determine the spatial dependence of the bromine distribution.Strong correlations of bromine content to the quantum dot locations would be expected for bromine surface termination.X-ray Photoelectron Spectroscopy (XPS) was used to assess both the content and bonding nature of the bromine in the nanomaterials.energy dispersive X-ray spectroscopy (EDS) was also used to map the spatial distribution of bromine using atomic resolution TEM.

Results and discussion
Recently Weidman and co-workers have reported that a large excess of PbCl 2 precursor (Pb : S ratio of 24 : 1) results in the formation of highly monodisperse PbS quantum dots due to the delaying of the onset of Ostwald ripening. 16In this work, a much smaller PbBr 2 precursor to sulphur (Pb : S) ratio of 4 : 1 was used.The primary interest in the work was to attempt to track the location and concentration of any halides in the system in the nal washed material rather than maximize a monodisperse particle size.Nonetheless, the degree of monodispersity offered by the 4 : 1 precursor ratio was sufficient for this.The dependence of growth rates of the PbS quantum dots as a function of the reaction conditions was studied.PbS quantum dots were synthesized by using PbBr 2 at three different, 80 C, 120 C and 160 C, PbCl 2 was used at 80 C as a reference.
Fig. 1 shows the transmission electron microscopy (TEM) images as well as a histogram of the sizes of the PbS quantum dots synthesized.The size has been estimated by measuring more than a hundred nanoparticles in the high resolution images.Compared to the quantum dots synthesized via PbCl 2 as shown in Fig. 1(a), quantum dots synthesized using PbBr 2 (Fig. 1(b)) were observed to have approximately a 30% smaller diameter than that from PbCl 2 under the same reaction conditions, i.e. 80 C for 1 min.Fig. 2(a) shows a dramatic blue shi from 1336 nm to 954 nm in the absorbance peak of PbS quantum dots when changing the lead precursor from PbCl 2 to PbBr 2 , corresponding to an approximate size decrease from 5.7 to 4.1 nm according to Weidman's model. 16This suggests that the reaction via the PbBr 2 is slower so as to suppress Ostwald ripening and to provide better control on quantum dot synthesis, particularly promising for ultra-small wide bandgap quantum dots.
By using the "hard-so acid-base" theory, 21 we can explain this phenomenon.According to this theory, "so" acid can bind with "so" base easier than "hard" base, and Pb is a kind of "so" acid, while Br is a "soer" base than Cl, so Pb-Br bond requires higher energy to be break which leads to a much slower reaction speed and longer nuclei period.
Higher temperatures for hot injection result in larger quantum dots as shown in Fig. 1(b)-(d), as expected.The resulting quantum dot sizes are listed in Table 1.From highresolution TEM (HRTEM) imaging lattice fringes were observed suggesting that the quantum dots are highly crystalline.
Smaller quantum dots were fabricated using a shorter reaction time, i.e. 10 s at different temperatures.The smallest size obtained was 2.9 nm, which was synthesized at 80 C for 10 s.
Fig. 2(b) shows well-dened absorption peaks from 878 nm to 1336 nm.The quantum dot size has been estimated using the model proposed by Weidman.As shown in Table 1, these estimates are in good agreement with the sizes measured from TEM images.These quantum dots have shown strong PL emission.A PL spectrum of the PbS synthesized at 120 C for 1 min is shown in Fig. 2(b) inset.The Stoke shi of PL peak from the absorbance is $150 nm which is consistent to other published work.Also a 190 nm full width at half max (FWHM), very similar to that of the absorbance peak, indicates a good size distribution of quantum dot size.
We have evaluated the crystallinity and purity of the quantum dots by X-ray diffraction (XRD).Fig. 3 shows the XRD pattern of the quantum dots aer ve washes.The peaks match with the PbS rock salt crystalline structure (ICDD PDF number 00-005-0592).There is no additional peak observed from the sample, which suggests that the unreacted PbBr 2 residue was removed effectively.The typical fringe spacings measured from the high resolution TEM images are 3.0 and 3.5 A, which correspond to the {200} and {101} lattice planes of the rock salt structure PbS, respectively.
The air stability of the PbS quantum dots (synthesized at 120 C for 1 min) was assessed under ambient conditions for over one month and the change in the position of absorbance peak was measured.From Fig. 4, there was no observable shi between peaks measured at different times, indicating that the quantum dots were stable against ambient oxygen due to passivation of Pb atom with stable Br atoms.Similar work using PbCl 2 precursor has been demonstrated by Weidman. 16he other advantage of using halide atom to passivate quantum dot surface is a higher quantum yield can be obtained.These halide atoms can be expected to bind with Pb atoms, where the risk of oxidation is high.By reducing the exposed surface, the halide passivated PbS quantum dots could achieve a higher quantum yield. 17Moreover, oleic acid ligand may be removed during the washing process while halide atoms can still bind with quantum dots. 7aving obtained bromine terminated PbS quantum dots, it was of interest to obtain more information about the physical and chemical environment of the halide atoms aer the application of the established washing procedure.While halide surface passivation can be inferred from air stability measurements such as those shown in Fig. 4, establishing precedents for more directly probing the halide content would be of substantial utility, particularly as nanomaterials are being fabricated with increasingly complex surface terminations.
XPS measurements were performed on thin lms of the Brterminated PbS quantum dots and analyzed in some detail to obtain information about the relative content and bonding of the halide ions.Fig. 5(a)-(c) show the high-resolution XPS spectra of S 2p, Br 3d, and Pb 4f peaks respectively of PbS quantum dots synthesized at 120 C for 1 min.The S 2p peak at 163.5 eV corresponds to the binding energy of Pb-S while the Br 3d at 68.6 eV corresponds to the binding energy of Br with Pb. 22,23 Both of the tted peaks clearly show large splitting due to spin orbit coupling.The Pb 4f signal contains two peaks corresponding to the spin orbit interactions for the 4f 7/2 and 4f 5/2 core electronic states.They are clearly both broad and asymmetric, with shoulders corresponding to peak splitting due to perturbations from the local chemical bonding.The 4f 7/2 peak was tted with two components centred at 137.5 eV and 138.5 eV.The former corresponds to the binding energy of Pb-S, the latter corresponds to Pb-Br bonds.The bonds of Pb and oleic acid ligands may also contribute to the peak at 138.5 eV. 22PS measurements have been used to positively correlate the presence of certain species on the surface of quantum dots.The spectra shown in Fig. 5 are consistent with Br surface termination.However, while XPS can show that lead is bonded to sulphur as well as bromide in the system, it cannot determine whether or not the Pb-Br bond is at the surface of the nanomaterial or whether it appears as a result of PbBr 2 residue.Additionally, the XPS analysis indicates an excess of Br in the quantum dot lms.The ratio of Pb atoms on the surface to that of the core of an approximately 4.7 nm PbS quantum dot is about 0.56.The ratio of Pb atoms bonded to bromine/oleic acid to those bonded to sulphur obtained from the XPS spectra is 0.66, which is slightly larger than the ratio of 0.56 expected ideally from the PbS quantum dots terminated by Br and oleic acid.This suggests that aer an acceptable washing procedure and in the absence of PbBr 2 clusters any Br content outside the quantum dots is at very low level.
To complement the XPS analysis, high-resolution EDS mapping has been performed using a large area JEOL Centurio SDD EDS detector that allows elemental mapping with atomic resolution.Probing the precise surface chemistry of nanomaterials is generally quite difficult.Here an attempt is made to do this with reasonable resolution by taking advantage of the atomic resolution available in this particular TEM.Fig. 6 shows a dark-eld scanning TEM (STEM) image PbS quantum dots and the corresponding maps of S, Pb, and Br.As suggested by the EDS maps, the positions of S, Pb and Br atoms are highly localized to individual pixels and have signicant overlap with the dark eld STEM image of the PbS quantum dots.
Pixel by pixel analysis of normalized EDS images was performed using the OpenCV image processing library.By masking 44 of the quantum dots occupying 39% of the image, the relative elemental compositions and their spatial overlap were assessed both inside and outside the quantum dots.Pb and S signals from inside the quantum dot regions being 66% and 70% respectively, consistent with a slight excess of S in the material leading to as-grown p-type doping.The Br signal is atter, with a measured 57% of the signal coming from the quantum dot regions.Careful visual inspection of the dark-eld HRTEM image shows very small crystallites outside what are nominally the PbS quantum dots that are not readily apparent in bright eld images.The rms value of the product of the signal strength at each pixel in two normalized EDS maps for different elements was summed and used as an index to compare elemental spatial overlap.The index ranges from 0 to 1, with 1 obtained from a comparison between identical images.The relative spatial overlap of the elements outside the quantum dot regions was 0.64, 0.4 and 0.63 for Pb-S, Pb-Br and S-Br respectively.The relatively strong correlations between the S and Br elements, as well as Pb and S, outside the quantum dot regions suggests that the signal is coming from something other than PbBr 2 residue.Sulphur has high solubility in the solvents used in the washing steps and S content outside the quantum dots is expected to be relatively small.
From the lightly higher than expected Br content of the quantum dot lms measured by XPS and the spatial correlation between Pb and S as well as Br and S outside of the quantum dot regions observed in the EDS maps, we conclude that the small crystallites apparent in the dark eld HRTEM image are nuclei of bromine terminated PbS quantum dots.During the growth phase, these nuclei are typically consumed by larger quantum dots.However the 10 second to 1 min reaction times in typical PbS syntheses are relatively short.Unlike precursor chemicals but similar to quantum dots, these nuclei survived the washing procedure and were present in the nal colloid.These nuclei would have surface area to volume ratios that are quite large, Fig. 4 Absorbance comparison of PbS quantum dots synthesized at 120 C for 1 min after storage under ambient conditions.There is no observable shift in the absorption peaks over the period of measurement as expected from halide surface passivation.The curves are vertically shifted for the sake of clarity.leading to higher Br to Pb and S ratios consistent with that observed in XPS.Equivalently these nuclei can be thought of as partially reacted non-stoichiometric atomic clusters that could easily contain increased Br content relative to crystalline PbS quantum dots.

Conclusion
The synthesis via PbBr 2 precursor has showed observably slower growth rates leading to quantum dots of smaller size relative to those made using PbCl 2 precursor.We attribute this to the higher Pb-Br bonding energy according to "hard-so acid-base" theory.This can lead to increased control over the size of particles without any loss of passivation.The XPS and EDS analysis of PbS quantum dot surfaces yields information on the halide content and location in substantial detail aer the quantum dots have been subjected to the thorough washing process required to remove Pb-halide precursors.Nuclei of bromine terminated PbS quantum dots are observed in dark eld HRTEM images and indicate an excess Br content conrmed by both the XPS and EDS measurements.Given the impressive air-stability of halide terminated Pb chalcogenide quantum dots, increased understanding of the halide incorporation is bound to be an important aspect of applications of these materials to nextgeneration photovoltaic cells.

Methods
In a typical synthesis of PbS quantum dots synthesized via PbBr 2 , 1.5 mmol (564 mg) of PbBr 2 (Sigma Aldrich, 98%) was mixed with 7.5 ml oleylamine in a three-necked ask, then heated to 120 C under N 2 ow aer degassing for 20 min.0.375 mmol (12 mg) of sulphur was mixed with 2.25 ml oleylamine, heated to 120 C under N 2 ow and kept at this temperature for 20 min before being cooled to room temperature.The Pboleylamine solution was adjusted to the desired temperature and then 2.25 ml of the S-oleylamine solution was injected.The temperature of the solution was maintained for the time desired for the reaction then heating was stopped.The ask was then submerged in a cold water bath to allow the product solution to cool quickly and stop the reaction.
Aer synthesis, 20 ml ethanol (Sigma Aldrich, 99.8%) was added to the as-synthesized product solution followed by centrifugation to precipitate the particles.Aer centrifugation, the supernatant was discarded and the precipitate was redispersed in 10 ml of hexane (Scharlau, 96%). 2 ml of oleic acid was added to the solution aer another centrifugation.The solution was stirred overnight to complete the ligand exchange.Aer ligand exchange, 40 ml of methanol (RCI Labscan, 99%) was added to precipitate the particles and remove the excess oleic acid.Aer centrifugation, the supernatant was discarded and the precipitate was re-dispersed by adding 10 ml of hexane.This process was repeated twice.All processes were performed in ambient conditions with centrifugation at 4000 rpm for 3 min.
PbS quantum dots synthesized via PbCl 2 were made by following recipes which have been published previously by other researchers. 16In a typical synthesis, 1.5 mmol (417 mg) of PbCl 2 (Alfa Aesar, 99%) was mixed with 7.5 ml oleylamine (Sigma Aldrich, 70%) in a three-necked ask.The solution was degassed for 20 min before being heated to 120 C in nitrogen.0.375 mmol (12 mg) of sulphur (Sigma Aldrich, 99.5%) was mixed with 2.25 ml oleylamine, heated to 120 C under N 2 ow and then cooled to room temperature aer 20 min.The Pboleylamine solution was cooled to 80 C and 2.25 ml of the Soleylamine solution was injected.The combined solution was kept at 80 C for 1 min before quenching the reaction by submerging the ask into a water bath.The washing process and ligand exchange process was same as the process via PbBr 2 precursor.
Transmission electron microscopy (TEM) was performed on a Phillips CM200 and a JEOL JEM-ARM200F with a voltage of 200 kV.Energy-dispersive X-ray spectroscopy (EDS) was performed on the JEOL JEM-ARM200F TEM using a large area JEOL Centurio SDD EDS detector that allows elemental mapping with atomic resolution.The sample was prepared by drop-casting the quantum dots solution in hexane onto a copper TEM grid with an amorphous carbon support.Analysis of the TEM image and the particle size calculations were performed using ImageJ soware.
The absorbance spectra of the PbS quantum dots were measured using a Perkin Elmer LAMBDA 1050 UV-VIS-NIR spectrophotometer.The quantum dots were dispersed in hexane for this measurement.X-ray diffraction (XRD) was performed on a PANalytical Xpert Materials Research Diffractometer (MRD) with a Cu anode.Samples were prepared by drop-casting the solution of quantum dots in hexane onto a silicon wafer and drying under ambient conditions.
X-ray photoelectron spectroscopy (XPS) was performed on a Thermo Scientic ESCALAB250Xi, with a mono-chromated Al Xray source.The power was 150 W and spot size was 500 micrometres.Samples were prepared by drop-casting the concentrated quantum dot solution in hexane onto silicon substrates.
Photoluminescence (PL) spectra was detected by an InGaAs photodetector (Electro Optical Systems, IGA-030-E) using a lockin amplier (Stanford Research Systems SR510) and optical chopper.The sample solutions in hexane were excited with an argon ion laser (Coherent Innova 70) emitting at a wavelength of 514.5 nm.

Fig. 2
Fig. 2 (a) Comparison of absorbance of PbS quantum dots synthesized via PbBr 2 and PbCl 2 precursor under same condition (80 C, 1 min).(b) Absorbance of PbS quantum dots synthesized via PbBr 2 under different conditions, the inset shows PL and absorbance spectrum of PbS QDs which was synthesized at 120 C for 1 min.(c) HRTEM image of PbS QDs which were synthesized at 80 C for 10 s, the image shows a clear fringe which indicates a high crystallinity.

Fig. 5
Fig. 5 XPS spectra of PbS quantum dots synthesized at 120 C for 1 min: (a) S 2p, (b) Br 3d and (c) Pb 4f.All spectra show corresponding peak doublets from spin orbit coupling.The peak for the Br 3d 5/2 state at 68.6 eV corresponds to Br bound to Pb.The Pb 4f 7/2 peak shows further decomposition into two peaks as a result of perturbations to the core electronic states from the bonding of Pb to S and Pb to Br and oleic acid ligands.

Fig. 3
Fig. 3 XRD pattern of PbS quantum dots synthesized via PbBr 2 .The red bars are the reference pattern of PbS rock salt structure (ICDD-5-0592).The inset is HRTEM image of PbS quantum dot which was synthesized at 160 C for 10 s.

Fig. 6 A
Fig.6A dark-field STEM image and the corresponding atomic resolution EDS maps of PbS quantum dots synthesized using a PbBr 2 precursor (120 C, 1 min).Each pixel in each of the maps corresponds to an area of approximately 9 A 2 .

Table 1
Estimated quantum dot mean diameter for different synthesis condition