Lead Chalcogenide Nanoparticles and Their Size-Controlled Self

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Lead Chalcogenide Nanoparticles and Their Size-Controlled Self Assemblies for Thermoelectric and Photovoltaic Applications Caleb K Miskin, Swapnil D Deshmukh, Venkata Vasiraju, Kevin Bock, Gaurav Mittal, Angela Dubois-Camacho, Sreeram Vaddiraju, and Rakesh Agrawal ACS Appl. Nano Mater., Just Accepted Manuscript • DOI: 10.1021/acsanm.8b02125 • Publication Date (Web): 01 Feb 2019 Downloaded from http://pubs.acs.org on February 12, 2019

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ACS Applied Nano Materials

Lead Chalcogenide Nanoparticles and Their Size-Controlled Self Assemblies for Thermoelectric and Photovoltaic Applications Caleb K. Miskin,†,§ Swapnil D. Deshmukh,†,§ Venkata Vasiraju,‡ Kevin Bock,† Gaurav Mittal,† Angela DuboisCamacho,† Sreeram Vaddiraju,‡ and Rakesh Agrawal*,† † ‡

Davidson School of Chemical Engineering, Purdue University, West Lafayette, IN 47907, USA. Department of Chemical Engineering, Texas A&M University, College Station, TX 77843, USA.

§

These authors contributed equally to this work *Corresponding Author: [email protected] Keywords: lead chalcogenides, nanoparticles, self-assembly, solution-processing, thermoelectrics

Abstract We report a facile, room temperature synthesis of PbS, PbSe, PbS xSe1-x, and PbTe nanoparticles and their microscale assemblies by combining a chalcogen solution and a lead halide solution in select thiol-amine mixtures. Selection of appropriate thiol-amine pair and/or the use of appropriate amine to thiol ratio has demonstrated a size control on nanoparticle self-assemblies ranging from nano to microscale. Proper washing of these particles has yielded phase pure and compositionally uniform material with the minimal or no presence of any carbonaceous ligands on the particle surface, making it attractive for electronic device fabrication. The resulting PbS material exhibits bandgap in the range of 0.6eV to as high as 1.2eV for various assembly sizes. These optical bandgaps confirm the retention of quantum confinement of PbS material even in self-assembled nano/micro structures, which could be an interesting phenomenon for future photovoltaic development. Along with carbon-free, quantum-confined self-assemblies, this chemistry also provides a room temperature and instantaneous reaction route to synthesize individually dispersed PbS and PbSe particles with long chain ligand capping similar to traditional synthesis routes. The PbSe material synthesized from this route shows the ability to alloy with PbS at room temperature in the entire composition range and also demonstrates thermoelectric performance comparable to existing undoped PbSe literature.

Introduction Lead chalcogenides (e.g. PbS, PbSe, PbTe, and related alloys) have emerged as excellent materials for quantum dot (QD) photovoltaics as well as thermoelectric devices. Solar cells made from PbS quantum dots are some of the most efficient QD solar cells with efficiencies crossing 11%.1 In addition, lead chalcogenides, especially PbSe, PbTe and their alloys, are considered excellent candidates for thermoelectric devices and are among the top performing high temperature thermoelectric materials.2–6 Recent understanding revealing the effect of material length scale i.e. from bulk to nano-scale on various optoelectronic properties has led to an improvement in thermoelectric as well as photovoltaic performance of the materials.7,8 Traditionally, the growth of these thermoelectric materials is accomplished from high-temperature melt processes, yielding materials with bulk properties. Such fabrication processes are also costly and not highly scalable for wide spread application of these technologies. On the other hand, developments in solution chemistry provide greater flexibility in tailoring the properties of the materials and films when compared with traditional solid-state chemistry. Recently, low-temperature methods have been investigated through solution-based colloidal techniques, which provide the ability to synthesize nanoscale materials and utilize material properties in the quantum regime. Such solution chemistries facilitate the use of printing, slot-die coating, and other scalable, roll-to-roll fabrication methodologies to reduce the cost of production for these devices. Various solution processing routes have been developed to synthesize lead chalcogenide materials at different reaction conditions. Because of the insolubility of chalcogens like Se and Te in many traditional solvents, the most commonly used synthesis route for lead chalcogenides involve use of the reactive solvent like alkylphosphines (e.g Trioctylphosphine-TOP), which can dissolve Se and Te by making TOPSe and TOPTe complexes respectively.9–11 These reactions are carried out at elevated temperatures and in the presence of long chain organic compounds like oleic acid, to control the size and shape of the particles. Researchers have also demonstrated cubic, spherical, needle and star shaped nanoparticles for lead chalcogenide system by utilizing various surfactant chemistries.9,12 In order to achieve quantum confined nanoparticle sizes, most of these reaction pathways require low concentrations, elevated

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temperatures and longer durations. Although chemicals like TOP, long chain organic compounds and surfactants provide great control over the shape and size of the particles, the ultimate device fabricated using these particles contains significant carbon residue originating from the ligands on the nanoparticle surface. Such carbon residue in the films adversely affects the films’ electrical properties, necessitating an additional step commonly known as ligand exchange to improve the electrical performance of the film.13–15 In this work we have developed a room temperature route to synthesize lead chalcogenide materials ranging from individually dispersed particles of a few nanometers in size to their nano-/microscale assemblies using a facile, rapid reaction pathway with almost 100% yield. Surprisingly, the assemblies formed from nanoparticles via this route still exhibit their nanoscale material properties. Such nano-/microscale assemblies with quantum scale properties could easily be scaled up for synthesizing materials on large scale. The particles synthesized from this route are also free of any carbonaceous ligands making it even more attractive for thermoelectric and photovoltaic applications. Further understanding of this reaction system could also be extended to fabrication of other semiconducting materials like Bi2Se3, Bi2Te3, etc. The solvent system which enables us to develop such a versatile route of synthesizing nanoparticles, involves thiol and amine compounds. Recently, thiol-amine mixtures have shown great promise for the fabrication of inorganic semiconducting thin films due to the mixtures’ ability to dissolve and form stable solutions of a host of metal salts, chalcogens, and even pure metals.16–21 However, thiol-amine mixtures do not form stable solutions for combinations of lead salts and chalcogens, resulting in a reaction of the lead and chalcogen solutes instead. Interestingly, while thiolamine mixtures can dissolve many metal and metalloid chalcogenides, lead chalcogenides show almost no solubility in most of these solution mixtures, facilitating this reaction. Similar findings have been reported by the Brutchey group for the dissolution of lead oxides and their combination with chalcogen solution.22 Herein, we demonstrate a novel, room-temperature technique to synthesize phase pure PbS, PbSe, and PbTe nanoparticles and their assemblies through combination of a chalcogen solution and a lead halide solution in select thiol-amine mixtures. We further show how this chemistry can be utilized to control the sizes of these particles ranging from quantum scale (in case of PbS, PbSe) to micron scale assemblies (for PbS, PbSe, PbTe). We also demonstrate the ability to produce uniform PbSxSe1-x alloy nanoparticles via this technique. The synthesized materials are also anticipated to have promising thermoelectric and photovoltaic applications.

Result and Discussion Characterization of PbS, PbSe and PbTe: Figure 1a, 1b and 1c show SEM micrographs of the resulting particles obtained by injecting chalcogen-thiolamine solution into PbI2-thiol-amine solution for PbS, PbSe and PbTe, respectively. The corresponding lead chalcogenide nucleated immediately upon injection. Similar results were obtained when PbBr2 and PbCl2 were used instead of PbI2 under identical reaction conditions. In case of PbS particle synthesis, monoamine-monothiol mixture was used for reaction and the PbS nanoparticles obtained were observed to self-assemble into highly uniform microscale spheres, in spite of some irregularity in terms of the primary particle size and shape that can be seen in the magnified surface image given in the inset to Figure 1a. During analysis, some of these assemblies were observed to fracture under the high voltage electron beam, which revealed their particulate nature throughout (Figure S1). This nature is also confirmed from the Scherrer size obtained using X-ray diffraction (XRD) data which ranges from 1020 nm for such microscale assemblies. On the other hand, PbSe and PbTe syntheses were carried out in diaminemonothiol mixture and the microscale assemblies observed for these particles were highly faceted as compared to PbS. This difference between PbS and PbSe/PbTe microstructures could be a result of the amine used for the reactions. The presence of diamine could form different chalcogen complex as compared to monoamine affecting the reactivity and hence the surface morphology of the particles. Unlike Te, which specifically requires diamine-monothiol for dissolution, Se can be dissolved in monoamine-monothiol solution as well. So, to study the effect of amine on particle morphology, Se was dissolved in monoamine and diamine solution separately along with ethanethiol. It has been shown that dissolution of Se in thiol-amine solution forms Se anion which is stabilized by ammonium cation in the solution.21 As diamine and monoamine molecules will form cations with double and single ammonium moiety respectively, the Se anion would experience different environment in both these solutions. This change in the cation environment could alter the way Se anion reacts with lead molecules and also the way PbSe nanoparticles approach each other for self-assembly. When monoamine was used to synthesize PbSe particles using this route, phase pure PbSe particles precipitated out of the reaction mixture but no microscale assemblies were observed with those particles


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(Figure S2). This difference in PbSe morphologies with monomaine and dimanie solutions strengthens our hypothesis for amine’s impact on reaction pathway and assemblies.

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Figure 1: SEM micrographs of (a) PbS, (b) PbSe, (c) PbTe and corresponding PXRD patterns shown below each (d-f). The insets to (a), (b) and (c) show a detailed view of the surface morphology. ICSD collection code numbers 38293, 62196, 96500, and 96502 were used for the PbS, PbSe, PbTe, and Te standards, respectively.

X-ray diffraction patterns for the particles are given in Figure 1d-f, which implies that no undesired crystalline phases are present in the particles. Average Scherrer sizes for the PbS, PbSe, and PbTe particles were calculated to be approximately 18 nm, 35 nm, and 53 nm, respectively. The additional small peak between the (111) and (200) peaks in the PbTe sample (Figure 1f), is due to incomplete filtering of the Cu Kβ signal from the X-ray source. This peak is not observed when operating the diffractometer in parallel beam mode, which confirms this interpretation. The relatively small Scherrer size of these particles compared with the observed crystallite size suggests the microscale assemblies to be polycrystalline with smaller crystallite domains, as opposed to single crystals. Although these crystalline domains are not highly uniform in sizes, especially for PbTe system, and also not ideal from a crystal quality point-of-view, the presence of such nano-domains may enhance the phonon scattering in the material due to nano-structure effect, resulting in lower thermal conductivity and potentially enhancement of thermoelectric performance of the system. The lead chalcogenide particles formed through this route were also found to be stable over a period of at least 6 months when analyzed using XRD and SEM. Controlling Microscale Assembly and Primary Particle Size: Based on our current understanding of the system, the particle synthesis could be altered to vary the size of the particles by changing the reactivity and solubility of lead complexes in the solution. For reactive crystallization processes, higher reactivity of reactants will lead to higher extent of nucleation, resulting in smaller size particles. 23 Previous detailed analysis performed on dissolution of copper chlorides in thiol-amine solution by Murria et al. suggests the formation of copper thiolate complexes in the solution along with various ammonium chloride species. 24 In case of lead salts, similar dissolution mechanism is assumed and is shown in Scheme 1, according to which, lead cation is expected to form lead thiolates species in the solution which will react with chalcogen ammonium complexes from chalcogen dissolution to form lead chalcogenide particles. Many other metal salts form a stable solution when it is dissolved with chalcogen in thiol-amine solution. This suggests that the lead thiolate complexes formed in thiolamine solution have higher formation energy compared to the corresponding lead chalcogenide in the same thiolamine solution at room temperature, resulting in the precipitation of lead chalcogenides. Depending on the thiol used



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for dissolution, different lead thiolate species will form in the solution. These species are expected to have different solubility and formation energies, which would affect the nucleation for lead chalcogenide particles. Utilizing this information of the thiol-amine solution system, several solutions of lead salt and chalcogens were prepared in a similar manner as described in the experimental section using five different thiols. For PbS synthesis, PbI2 solutions of 0.05 M concentration were prepared using BA and various thiols with carbon chain lengths varying from 2 (Ethanethiol–ET ) up to 12 (Dodecanethiol-DDT). Corresponding 0.05 M S solutions were also prepared using the same pair of amine and thiol with 1:1 volumetric ratio. In the case of PbSe and PbTe, ethylenediamine (EN) was used for dissolution instead of BA and analogous reactions were performed. This variation of thiol chain lengths showed no substantial effect on the yield of the reactions but it influenced the size of the particles significantly. Scheme 1. Schematic representation of proposed lead halide and chalcogen dissolution in thiol-amine mixture followed by its reaction to form lead chalcogenide material. A = chalcogen. Amine-Thiol protonation & deprotonation

R-NH3+ + R-S-

R-NH2 + R-SH Pb halide dissolution

2R-NH3+ X-

PbX2 + R-NH2 + R-SH

+

(R-S-)2 Pb2+ Lead Thiolate

Chalcogen dissolution

A

+

(R-NH3+)2 A2- + R-S-S-R

R-NH2 + R-SH

Chalcogen ammonium complex Pb chalcogenide precipitation

(R-S-)2 Pb2+ + (R-NH3+)2 A2-

PbA

+

2R-NH3+ + 2R-S-

Lead Chalcogenide

As these particles assemble into sub-micron scale structures, the assembly sizes were determined using SEM while the primary particle size was derived from XRD analysis. As shown in Figure 2a, PbS shows a gradual drop in the size of spherical assemblies as the carbon chain length on the thiol is increased, starting from about 900nm for ET to around 50-80nm for DDT (Figure S3a shows TEM image of 50-80 nm size PbS self-assemblies). Similarly, for PbSe, the assembly sizes vary from about 1.1 µm for ET to around 20nm for DDT (Figure 2b). PbTe on the other hand, does not show much variation in size when the thiol chain length is varied from ET to pentanethiol (PenT), but the DDT synthesized PbTe particles (about 200 nm) are almost 10 times smaller in size when compared with ET synthesized PbTe particles (Figure 2c). X-ray diffraction performed on these particles shows phase pure material in all cases. Using the average FWHM obtained from (111), (200) and (220) peaks for all 3 systems, the Scherrer size of particles is calculated and shown in Figure 2d. Similar to the assemblies, the size of the primary particles also dropped gradually for PbS and PbSe with increasing carbon chain length on thiols and sizes of around 6.5 nm and 9 nm were achieved for PbS and PbSe nanoparticles respectively with DDT based synthesis. For PbTe, similar to microscale assemblies, we did not observe much variation in the Scherrer size of the primary particles when thiol was varied from ET to PenT but it dropped by almost 20% when DDT was used for synthesis.


Propylamine (C3)

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Pentylamine (C55)

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Ethanethiol (C2)2) Ethanethiol (C

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Butylamine (C4)

10 ET

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Thiols Used for Reactions

Figure 2: SEM micrographs showing effect of thiol carbon chain length on (a) PbS, (b) PbSe, (c) PbTe and (d) a plot of Scherrer size variation for PbS, PbSe and PbTe with respect to thiol carbon chain length

Along with XRD, Raman spectroscopy was used to characterize the lead chalcogenide assemblies and primary particles that were formed with different thiol reactions (Figure 3). According to previous reports, lead chalcogenides are quite sensitive to photo degradation and it was necessary to limit the laser power to no more than 2 mW to avoid beam damage and corresponding photo degradation peaks.25 For micron size PbTe, the data collected from the instrument had very poor signal to noise ratio which limited us from performing any curve fitting on that particular data set. In general, for all lead chalcogenide materials, the signal to noise ratio obtained for nanoscale assemblies synthesized using DDT was better than the one obtained for microscale assemblies synthesized using ET. Along with improved signal to noise ratio, the data obtained for nanoscale assemblies show peaks corresponding to higher order phonon scattering. The wavenumber shift in first order longitudinal-optical LO(Γ) Raman peak with decreasing nanoparticle size was previously observed and associated with the shape of the LO phonon dispersion in various sized nanoparticles.26 For PbSe, the shift of LO phonon from 142 cm-1 to 137 cm-1 agrees with the reduced Scherrer size of these particles when synthesized using DDT vs ET. Also, for PbS, the peak transition from 452 cm-1 to 432 cm-1 confirms the size variation when compared with the similar trend reported by Baranov et. al.27



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Figure 3: Raman spectrum of (a) PbS using BA+ET (b) PbSe using EN+ET, (c) PbTe using EN+ET, (d) PbS using BA+DDT, (e) PbSe using EN+DDT, (f) PbTe using EN+DDT

This variation in particle size could arise from either the change in reaction kinetics of different thiolate species or reduction in the saturation of lead chalcogenides in final solutions in various thiol-amine pairs. As the thiolate molecules were different between different reaction systems, one could argue that the smaller size particles could arise because of the different reactivity arising from different chain lengths associated with the thiolate species rather than the saturation differences between the solutions. To resolve this, we conducted an experiment by using a same thiolate molecule for the reaction but changing the saturation extent for lead chalcogenides in the solution. The proposed dissolution of precursors in thiol-amine solution starts with deprotonation of thiol by protonating the corresponding amine in the solution as shown in Scheme 1. The higher extent of this proton exchange yields higher solubility of the precursors. This extent of protonation and deprotonation reaction could also be determined by measuring the conductivity of thiol-amine solutions. It has already been demonstrated that shorter carbon chain amines/thiols give higher conductivity than longer carbon chain amines/thiols, indicating higher solubility of precursors in the shorter carbon chain compounds.28 As PbS has finite solubility in EN-monothiol solution as well as monoamine-ethanedithiol (EDT) solution, this trend was tested with different carbon chain length compounds. Table S1 in SI summarizes this effect of carbon chain length on PbS solubility confirming our hypothesis of higher saturation with longer carbon chain compounds. So, assuming the similar trend with monoamine-monothiol solutions, the saturation of lead chalcogenides in thiol-amine solution was altered by changing the amine used for the reaction. PbS synthesis was performed using various amines with carbon chain lengths varying from 3 (Propylamine-PA) up to 18 (Oleylamine-OLA) while keeping the same thiol (ET) for dissolution. Figure S3b in SI summarizes the effect of various amine carbon chain lengths on PbS assemblies. The results show the expected trend of smaller particles as well as smaller assemblies for long chain amines, confirming the size dependence on degree of saturation. In conclusion, lead chalcogenides have decreased solubility in longer chain thiols. This reduced solubility of lead chalcogenides results in an instantaneously higher saturation condition in the solution when the reactants are mixed. Higher saturation at the start of crystallization yields a higher nucleation rate giving smaller individual particles. The large number of small particles formed in the beginning of the crystallization creates higher saturation of surface energy in solution resulting in an increased driving force for self-assembly to minimize this energy. In essence we have a saturation of surface sites on which to initiate the self-assembly. This increased driving force for reducing the surface energy creates more sites for self-assembly process, which reduces the size of final self-assembled microspheres.


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Controlling the nanoparticle synthesis from a small chain metal thiolate species still requires use of long chain amine compounds to control the saturation of lead chalcogenides. These long chain amines are known to act as a ligand for various nanoparticle systems including lead chalcogenides.29,30 The presence of such long chain compounds has shown adverse effects on the electrical properties of the films prepared from such particles and an additional step of ligand exchange to remove excess carbon is often performed for improved electrical performance.13–15 To achieve control over the size of the particles without introducing any long chain compounds in the system, we determined to alter the solubility and saturation via another route. In particular, we accomplished this by varying the ratio of thiol to amine using small chain compounds. As described earlier, thiol-amine dissolutions are interdependent on amines and thiols for protonation and deprotonation reactions. Significantly limiting the supply of either amine or thiol would change the dissolution capability and saturation of the overall system. As thiol reacts with the metal cation forming metal thiolates, reducing the quantity of thiol while increasing the amount of amine to keep the total concentration constant, should reduce the solubility of metal chalcogenides in the solution. This reduced solubility will ultimately result in higher saturation yielding smaller individual particles. To verify this hypothesis, syntheses were performed as described in the Methods section, with the exception of a change in the amine to thiol ratio in solution preparation. Briefly, a 0.05 M PbI2 solution was prepared in BA and propanethiol (PT) solvents at 4 different mole ratios of BA:PT (1, 5, 25, 100). Sulfur solution was prepared in BA without any thiol being used for dissolution. When characterized, the SEM micrographs showed a similar trend for the size of assemblies. As no thiol was used in sulfur dissolution, assemblies formed in 1:1 BA:PT reactions were only 300 nm in size. When XRD analysis was performed on these particles, average Scherrer sizes of 8.6 nm, 6.8 nm, 5.9 nm and 3.1 nm were observed with BA:PT ratio of 1, 5, 25 and 100 respectively (Figure S4a). Although all of these Scherrer sizes of PbS particles were smaller than the Bohr radius of PbS which is around 15 nm, the sizes of the assemblies were in sub-micron range (Figure S4b). UV-vis absorption spectroscopy performed on these particles by casting a particle film on fluorine doped tin oxide (FTO) coated soda lime glass substrates showed interesting results for these assemblies. The collected absorption spectrum is transformed into a tauc plot and is shown in Figure 4a. As the band gap of bulk PbS material is 0.37eV, all particles synthesized here show different extent of quantum confinement depending on the amine to thiol ratio used for the synthesis even with assembled structures. The bandgap values of 0.6eV, 0.7eV, 1.0eV and 1.2eV are obtained for particles with Scherrer size of 8.6nm, 6.8nm, 5.9nm and 3.1nm respectively. Based on the empirical relation, E = 0.41+ (0.0252d2 + 0.283d)-1, provided by Moreels et al. for PbS quantum dots,31 the values obtained for the above mentioned Scherrer size particles are 0.64eV, 0.73eV, 0.8eV and 1.3eV, respectively, which are close to the values obtained experimentally. Similar to PbS, the thiol variation’s impact on PbSe and PbTe particles was also studied with EN-ET system. The resulting particles with amine to thiol ratio of 100 are shown in Figure S5 and the quantum confinement observed for PbSe particles is shown in Figure 4b. PbS BA:PT-1 BA:PT-5 BA:PT-25 BA:PT-100

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Figure 4: Tauc plot of (a) PbS, (b) PbSe particles synthesized with different amine:thiol ratio, confirming quantum confinement at higher amine:thiol ratios

After confirming the quantum confinement of the particles, Fourier transformed infrared spectroscopy (FTIR) was performed on PbS assemblies synthesized in BA-ET system and PbSe/PbTe assemblies synthesized in EN-ET system for surface analysis. FTIR data collected in transmission mode is shown in Figure 5. In case of PbS, the stretches observed in the range of 1330 cm-1 to 1430 cm-1 could arise from surface oxidation caused due to alcohol washing. This was confirmed when PbS particles, washed with hexane without any alcohol, showed absence of such



stretches. For all three systems, the collected spectra show no stretches in the range of 2700 cm-1 to 3100 cm-1, which corresponds to the C-H bonds, suggesting absence of carbonaceous ligand on the particles surface. This was a surprising finding, as thiols are known to act as a ligand for lead chalcogenide nanoparticles. So, to further analyze the particles for ligands, we performed 1H NMR, which is highly sensitive technique for hydrocarbon detection, on PbS microspheres suspended in DMSO-d6 solvent. The data obtained from NMR is presented in Fig S6 and it also suggests absence (or presence below the detection limit of the instrument) of any ligand on the PbS particles. Based on these analysis, we believe, the initial nanoparticle synthesis in thiol-amine system is still governed by ligands in the solution (most likely thiols), but once the lead chalcogenide material has formed, the ligands on the particle are being displaced as the particles self-assemble. Although this process is not completely understood and is still under investigation, the current hypothesis is based on similar nature of agglomeration reported by Warner, where oleic acid ligand on PbS nanoparticles were stripped by adding superhydride solution in the PbS dispersion, which destabilized Pb-oleic acid bond resulting in self-assembled PbS nano-rod structures free of oleic acid ligands.32

Transmission (a.u.)

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PbTe washed with alcohol PbSe washed with alcohol PbS washed with alcohol PbS washed with hexane Absence of C-H stretches

S=O stretches

4000 3500 3000 2500 2000 1500 1000 Wavenumber (cm-1) Figure 5: FTIR Spectra showing absence of any C-H stretches for PbS, PbSe and PbTe particles and presence of S=O stretches for PbS particles washed with alcohols

While the syntheses we have described provide a novel method to synthesize carbon-free, quantum-confined nanoparticle assemblies, the thiol-amine system can also be used to obtain individual dispersions of lead chalcogenide particles similar to other traditional synthesis routes but at room temperature. This can be achieved by combining the two pathways of achieving smaller particles, i.e. use of long carbon chain compound and reduced thiol quantity. OLA with 18 carbon chain length was used as a ligand species for achieving such individual particle dispersions. PbI2 was dissolved in OLA-ET solution at 0.05 M concentration with minimal thiol yielding OLA:ET volume ratio of 200. For PbS reaction, S was dissolved in OLA without any thiol addition, while for PbSe reaction, Se was dissolved in OLAET solution with OLA:ET volume ratio of 500 to keep the thiol usage at minimal quantity. Both reactions were carried out in similar manner as explained in Methods section. For washing, instead of 6 washes only 3 washes were performed to retain a good suspension of the particles, and the final OLA capped particles were dispersed in toluene for preparing the TEM samples. Figure 6 shows the TEM images for individually dispersed PbS and PbSe particles with size ranging between 4-5 nm along with HRTEM image of PbS particle with (200) planes.


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Figure 6:(a) TEM (b) HRTEM image of PbS nanoparticles and (c) TEM image of PbSe nanoparticles synthesized using OLA and reduced ET, showing individually dispersed particles

Characterization of PbSxSe1-x nanoparticles: Along with pure lead chalcogenides, mixed chalcogen lead alloys have also been shown to have excellent thermoelectric properties. The thiol-amine solvent system also provides a route to synthesize PbSxSe1-x alloy nanoparticles at room temperature. Highly compositionally-controlled PbSxSe1-x nanoparticle alloys were obtained by co-dissolving S and Se at various ratios and injecting this into the Pb solution as described in the experimental section. Due to the finite solubility of PbS in diamine-monothiol solutions, monoamine-monothiol solution was used for these alloy syntheses, which resulted in smaller particle size as well as the absence of self-assembly for the particles containing Se as a chalcogen. Table 1 shows the summary of reactions performed for five target S:Se compositions along with the measured SEM-EDS S:Se compositions, which very closely matches the intended compositions. Table 1. Summary of PbSxSe1-x reactions and measured compositions by SEM-EDS.

The XRD data in Figure 7a shows a clear transition from pure PbS to pure PbSe in gradual steps, while a plot of the average lattice parameter as calculated from each peak in the XRD pattern (Figure 7b) shows a linear trend indicating the system follows Vegard’s law. One standard deviation is given as the error bar, which results from the small discrepancies in the parameter calculation depending on the peak used. Note that the increased noise in the pure PbSe sample, which can be attributed in part to the smaller particle size, results in the larger uncertainty in the lattice parameter. SEM imaging found that the alloy nanoparticles were not self-assembled in this case, showing a similar behavior to PbSe particles synthesized in BA-PT system, emphasizing the role of amine-chalcogen species on selfassemblies. To analyze the distribution of elements in the alloy nanoparticles, STEM-EDS was performed on particles with targeted composition of PbS0.75Se0.25 and is presented in supporting information (Figure S7). This analysis shows uniform distribution of S and Se across the entire particle suggesting formation of homogeneous alloy.


a

x = 0 (PbSe) x = 0.25 x = 0.5 x = 0.75 x = 1 (PbS)

PbSe Standard PbS Standard

Intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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Lattice Parameter (Angstroms)


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6.15 6.10 6.05 6.00 5.95 5.90 0.0

0.2

0.4

0.6

0.8

1.0

Measured S/(S+Se) (x)

Degrees 2

Figure 7: (a) XRD data of PbSxSe1-x alloy nanoparticles synthesized using a co-dissolved S/Se precursor at various S to Se ratios and (b) lattice parameter variation of PbSxSe1-x alloy at various S to Se ratio confirming Vegard’s law for the system

As mentioned in the Experimental section, 33% excess sulfur was needed in solution to obtain the desired S/Se ratio. One possible reason for using 33% excess sulfur for the reaction is that the S is more soluble than Se in BA-PT and may be less likely to react, preferring to stay in solution at a higher concentration. We have also observed a gas evolution while dissolving S, which we believe is removing sulfur from the system. This is supported by the fact that when lead acetate paper is suspended in the headspace above the dissolving sulfur, it is observed to discolor and turn light brown. This is not observed when suspended over either PT or the PT-BA mixture. H2S is believed to be the species leaving the system as this would result in PbS formation on the lead acetate paper. H 2S is also known to be produced when sulfur is dissolved in amines.33 The rate at which this gas is evolved may have implications for the shelf life of S-Se solutions and hence alloy composition reproducibility. Alloys of the form PbSexTe1-x and PbSxTe1-x were also targeted by injecting a co-dissolved Se-Te solution and STe solution in EN-PT system. Interestingly, in these cases compositionally uniform particles are not obtained. Instead, a range of compositions is obtained. XRD analysis of these particles show distinct peaks for individual lead chalcogenides. We hypothesize that the difficulty in synthesizing the alloy tellurium compounds results from either the increased metallic nature of Te compared to S and Se and its comparatively higher reactivity resulting in nonuniform incorporation or the role of EN in the reaction which results in highly varied reactivity for different chalcogens. More work is underway to understand these systems and control the chemistry. Thermoelectric Properties of PbSe: To study the quality of material synthesized using thiol-amine solutions, thermoelectric measurements were performed on PbSe particles synthesized with the BA-PT system. Prior to measurements, the PbSe pellet was annealed at 900 K in H2 atmosphere. While this is a high temperature anneal, it is more than 500 K less than the conditions under which Bridgman growth is performed34 and avoids the additional hassle and cost of the typical ball milling step. Although this annealing step could result in sintering of the particles to some extent, it was required for fabrication of the pellet which was used for thermoelectric property measurements. Furthermore, thermoelectric performance parameters were measured up to 700K and a prior heating to 900K provided conditioning of the material to avoid any hysteresis effect. The measured Seebeck coefficients of PbSe pellets in the temperature range of 300-700 K were found to be negative, indicating n-type semiconductivity (Figure 8a). The magnitude of the Seebeck coefficients increased with temperature from 300 to 625 K, followed by a slight decrease from 625 to 700 K. A maximum absolute value of the Seebeck coefficient of -147 µV/K was observed at 625 K. Both the n-type behavior of PbSe and the measured values of Seebeck coefficients were as expected, especially at low temperatures. However, the high temperature Seebeck coefficients are found to be lower than expected. The electrical conductivity of the pellet also increased with temperature. A peak electrical conductivity of 44 S/cm was observed at 675 K. One should note that the measured electrical conductivity values are for PbSe pellet and not directly for nanoparticles. The annealing and palletization process could induce sintering of the particles which could have a significant effect on electrical conductivity when compared to nanoparticle films. The measured electrical conductivities for the pellets were observed to be on par with those reported in undoped PbSe samples.2 However, they were lower by an order of magnitude relative to those observed in doped PbSe samples.34 The high temperature data in Log(electrical


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conductivity) vs 1/T plot (Figure 8b) suggests intrinsic behavior of material while the slow increase at lower temperature caused due to increase in carriers suggest extrinsic behavior with non-degenerately doped characteristics of the material. The thermal conductivity of the pellets ranged from 2.2 W/(m∙K) to 1.8 W/(m∙K) in the 300-600 K temperature regime (Figure 8c). Thermal conductivities of the pellets decreased with increased temperature due to increased phonon scattering induced by higher lattice vibrations. A minimum thermal conductivity of 1.8 W/(m∙K) is achieved in this pellet at 573 K. Due to instrument limitation, the thermal conductivity was measured only until 573 K. For predicting the zT of the samples at higher temperatures, the thermal conductivity value of 1.8 W/(m∙K) was employed for all temperatures above 573 K. As the thermal conductivity is expected to decrease with increase in temperature this extrapolation is considered conservative and within error.

-100

4.8

a

Log (electrical conductivity)

Seebeck Coefficient (V/K)

The zT values of the pellets increased with temperature in the 300-700 K temperature regime (Figure 8d). A maximum zT value of 0.26 is estimated for the PbSe pellets. The observed zT values are slightly lower than those reported for undoped PbSe.2,34,35 As no annealing or doping optimization was performed for this report, significant opportunity exists to increase the zT values further.

-110 -120 -130 -140 -150

b

4.6 4.4 4.2 4.0 3.8 3.6

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zT

Thermal Conductivity (W/m-K)

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2.0

0.15 0.10

1.9

0.05

1.8

0.00 300

400

500

600

Temperature (K)

700

300

400

500

Temperature (K)

Figure 8: Thermoelectric performance parameters of PbSe pellet including (a) Seebeck coefficient, (b) electrical conductivity, (c) thermal conductivity, and (d) zT. The open, black points correspond to extrapolated values obtained by using the thermal conductivity at 550 °C for all points thereafter to give a conservative estimate of zT at higher temperatures.

With promising results on PbSe thermoelectric properties, future work will involve looking at the thermoelectric properties of different sizes of PbSe, PbTe and possibly mixtures of PbSe and PbTe. Future work will also involve fabricating films of these materials and testing the properties of the film by annealing at lower temperatures which would be of more interest in term of commercialization of these materials on large scale.

Conclusion: We have successfully developed a novel route to synthesize lead chalcogenide particles at room temperature. Lead chalcogenides with highly uniform assemblies were fabricated via a facile, one-step mixing of lead halide solution and chalcogen solution in thiol-amine solvent mixtures. Microscale assembly size along with individual primary nanoparticle size were controlled by altering solution solubility, saturation levels and nucleation rates via selection of various thiol-amine pairs. Lead chalcogenide nanoparticles within the Bohr radii of the material system were synthesized by reducing the thiol in solution, which exhibited promising quantum confinement effects on self-



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assembled structures. Resulting particles also show minimal or no presence of carbon residues from ligands implying promising applicability for device fabrication. Additionally, this chemistry demonstrated highly compositioncontrolled room temperature alloying of PbSxSe1-x nanoparticles. Thermoelectric measurements on PbSe nanoparticles synthesized from this route showed promising results when compared with undoped PbSe material synthesized from other routes. In general, the choice of thiol-amine and ratios thereof provides excellent control of the solution chemistry giving control over lead chalcogenide material synthesis. Further understanding of this solvent system will allow us to extend its applicability to different material systems and apply them to device applications.

Experimental: Materials: Lead (II) iodide (PbI2, 99.999%), lead (II) bromide (PbBr 2, 99.99%), sulfur powder (S, 99.98%), n-propylamine (PA, >99%), n-butylamine (BA, 99.5%), n-pentylamine (PenA, 99%), n-hexylamine (HA, 99%), oleylamine (OLA, 70%, primary amines >98%), ethanethiol (ET, 99+%), 1-propanethiol (PT, 99%), 1-butanethiol (BT, 99+%), 1pentanethiol (PenT, 98%), 1-dodecanethiol (DDT, >98%), 1,2-ethylenediamine (EN, ≥99%), 1,2-ethanedithiol (EDT, ≥98%), selenium powder (Se, 99.99%) and tellurium powder (Te, 99.997%) were purchased from Sigma-Aldrich and used without further purification except OLA, which was degassed using a freeze pump thaw process prior to use. Lead (II) chloride (PbCl2, 99.999%) was obtained from Alfa Aesar. We also sourced BA from Acros Organics (99+%) and found that similar results were obtained. Safety Note: Amines and thiols are both moderately toxic and both (especially thiols) may have a strong, offensive odor. We have found that some mixtures of amines and thiols will degrade nitrile and latex rubber gloves within a short period of exposure, so caution is urged. The reader is referred to the ESI† for additional details on material compatibility of thiol-amine mixtures. Synthesis of Pb chalcogenide nanoparticles: Elemental sulfur was dissolved in either an amine or thiol-amine mixture by first adding the amine and then the thiol depending on the experiment. Sulfur forms a bright red/orange color solution when dissolved in just an amine and turns colorless after the addition of thiol. In a representative synthesis, 0.1 M S solution was prepared in 1:1 volumetric mixtures of BA and PT. A lead precursor solution was prepared by dissolving PbI 2 at a concentration of 0.1 M in a 1:1 volumetric mixture of BA and PT to obtain a clear, light yellow solution. For PbSe and PbTe syntheses, Se and Te were dissolved in 1:1 volumetric mixtures of EN and ET instead of BA and PT. At 0.1 M concentration, the resulting solutions were dark orange and dark red respectively for Se and Te. For these two reactions, PbI 2 was also dissolved in 1:1 volumetric mixture of EN and ET at 0.1 M concentration. All preparations, including powder weighing and subsequent dissolutions, and reactions were performed in a nitrogen filled glovebox with oxygen and water concentrations maintained below 0.1 ppm. The lead chalcogenide synthesis was conducted by injecting 4 mL of the chalcogen solution into 4 mL of the PbI2 solution at room temperature inside the glovebox. The reaction content was then added to a 25 mL centrifuge tube and topped with ~16 mL of 1:1 BA-PT solvent mixture for PbS and 1:1 EN-ET solvent mixture for PbSe and PbTe. This was done to avoid the precipitation of any unreacted precursors or byproducts that may only be soluble in thiol-amine mixture prior to centrifugation. To separate the nanoparticles from the reaction byproducts, the mixture was centrifuged for 3 mins at 14000 rpm and the supernatant was discarded. Subsequent washes were performed as follows: first, ~2 mL of hexane was added to the centrifuge tube and vortexed until the particles were well dispersed; second, ~23 mL of methanol/isopropanol were added, and the centrifuge tube was vortex mixed again; third, the particles were again separated by centrifuging at 14000 rpm for 3 mins. This process was repeated 6 times and the supernatant was discarded each time. In the case of PbTe, after the first centrifuge process the supernatant was discarded and the particles were re-suspended in EN-ET solution for one more time to dissolve unreacted Tellurium from the particles. As tellurium is comparatively unstable in thiol-amine solution and could precipitate from solution after any air exposure while transferring from the glove box to the centrifuge, this extra EN-ET wash is important to avoid any Te impurity in final particles. Note that other thiols, amines, and ratios of thiol to amine were used to obtain particle size control as later discussed, but the reaction methodology as just described was the same in all cases. Synthesis of PbSxSe1-x alloy nanoparticles:


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Elemental sulfur and selenium were co-dissolved in 1:1 volumetric mixtures of BA and PT, resulting in dark red solutions. To obtain target PbSxSe1-x nanoparticle compositions of x = 0.25, 0.5, and 0.75, we found that 33% excess sulfur was needed to be dissolved compared to the target composition. Hence for x = 0.25, a S-Se solution was prepared with 0.033 M S and 0.075 M Se; for x = 0.5, a 0.067 M S and 0.05 M Se solution was prepared; and for x = 0.75, a 0.1 M S and 0.025 M Se solution was prepared. The lead precursor solution was prepared as before by dissolving PbI 2 at a concentration of 0.1 M in a 1:1 volumetric mixture of BA and PT to obtain a clear, light yellow solution. PbSxSe1x nanoparticles were synthesized by rapidly injecting 5 mL of the S-Se solution into 5 mL of the PbI 2 solution. The resulting particles were washed in a manner similar to the PbS particles described above. Material Characterization: X-ray diffractograms (XRD) were obtained using a Rigaku Smart Lab diffractometer in Bragg-Brentano mode, using a Cu Kα (λ = 1.5406 Å) source operating at 40kV/44mA. Scanning electron microscopy (SEM) images were collected using an FEI Nova NanoSEM at an accelerating voltage of 5 kV, 3.0 spot size, and working distance of ~23 mm using the TLD (Through Lens Detector). The energy dispersive spectroscopy (EDS) measurements were performed using an FEI Quanta 3D at 20 kV. EDS spectra were obtained using an Oxford INCA Xstream-2 silicon drift detector with Xmax80 window and analyzed using Aztec software. Transmission electron microscopy (TEM) images were collected using Tecnai G2 20 TEM with an accelerating voltage of 200 kV. STEM-EDS data was collected on Talos 200X TEM using SiN grid. Absorption data for particles was collected using Perkin Elmer Lambda 950 UV/Vis spectrophotometer in transmission mode on FTO coated glass substrate. Raman spectra were collected on Horiba/Jobin-Yvon HR800 microscope with an excitation laser wavelength of 632.8 nm. All the Raman spectra were analyzed by performing appropriate Lorentzian fitting on a data obtained by subtracting a 4 th order polynomial background from collected data. FTIR spectra were collected on Thermo-Nicolet Nexus 670 FTIR unit in transmission mode using NaCl crystals. 1H NMR spectra were collected using Bruker AV-III-400-HD instrument. Thermoelectric property measurement of PbSe: Gram quantities of PbSe were obtained by scaling the reaction to 43.4 mL solutions of 0.2 M PbI2 and 0.2 M Se, each in 1:1 volume ratio of BA and PT. The washed particles were pressed into pellets of 12.5 mm diameter and 1-2 mm thickness using hot uniaxial pressing for 1 hour in nitrogen via a boron nitride (BN) coated steel die at 300 MPa and 700 K. The pellets were further annealed in an atmosphere of hydrogen at a temperature of 900 K for 2 hours. To characterize the thermoelectric performance of the annealed pellets, the variation of their Seebeck coefficient, electrical conductivity, and thermal conductivity as a function of temperature was measured. The analogue subtraction method and 4-point probe measurement were respectively employed for measuring the Seebeck coefficients and electrical conductivities of the pellets.36 For the determination of the pellet thermal conductivity (κ), the following were measured experimentally: pellet density (ρ) using Archimedes principle, heat capacity (Cp) using differential scanning calorimetry (DSC), and thermal diffusivity (α) using laser flash measurement. Based on these measurements, κ was determined using the relationship, κ=αρCp.37–39 All measurements were performed in a vacuum of 1 mTorr or lower. Finally, the thermoelectric figure of merit (zT) of the pellets was calculated using the well-known relationship, zT= (S2 σT)⁄κ.37–40

Supporting Information: Observations on material compatibility for thiol-amine mixtures, additional SEM and TEM images for lead chalcogenide nanoparticles synthesized at various conditions, tabulated data on solubility limit of PbS in various thiolamine pairs, STEM-EDS data on PbS0.75Se0.25 alloy particle, 1H NMR data on PbS microsphere.

Acknowledgement: CKM is grateful to be supported by an NSF graduate research fellowship (DGE-0833366). The authors also gratefully acknowledge the funding of NSF under grant #1534691-DMR (DMREF). VV gratefully acknowledges the financial support provided by Texas A&M Institute of Advanced Study via the HEEP Fellowship. We also express appreciation to Dr. Ruihong Zhang for helpful discussions.

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