Shell Nanocrystals


nescence quantum yields,5 reduced non-radiative Auger processes,6,7 and larger Stokes shifts,8 as compared to plain core nanocrystals. PbS nanocrystal...
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Synthesis and Characterization of PbS/ZnS Core/Shell Nanocrystals Janice E. Boercker, Danielle L Woodall, Paul D. Cunningham, Diogenes Placencia, Chase T Ellis, Michael H. Stewart, Todd H. Brintlinger, Rhonda M. Stroud, and Joseph G. Tischler Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.8b01421 • Publication Date (Web): 01 Jun 2018 Downloaded from http://pubs.acs.org on June 1, 2018

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Synthesis and Characterization of PbS/ZnS Core/Shell Nanocrystals Janice E. Boercker*, Danielle L. Woodall, Paul D. Cunningham, Diogenes Placencia, Chase T. Ellis, Michael H. Stewart, Todd H. Brintlinger, Rhonda M. Stroud, Joseph G. Tischler United States Naval Research Laboratory, Washington, D.C. 20375, United States ABSTRACT: We demonstrate a synthetic method to add a ZnS shell, with controlled thickness, to PbS nanocrystals using Zn oleate and thioacetamide as Zn and S precursors. The ZnS shell reaction is self-limiting and deposits approximately a monolayer of ZnS per shell reaction without causing the PbS nanocrystals to Ostwald ripen. The reaction is self-limiting because the sulfur precursor, thioacetamide, is less reactive towards the PbS/ZnS core/shell nanocrystal surface as compared to the Zn oleate precursor present in the reaction solution. To increase the ZnS shell thickness beyond a monolayer, subsequent ZnS shell reactions are modified by adding the thioacetamide 10 minutes before the Zn oleate. This gives the thioacetamide time to react at the PbS/ZnS core/shell nanocrystal surface before the Zn oleate is added. High angle annular dark field scanning transmission electron microscopy (HAADF-STEM) shows most ZnS shells lack crystalline order. However, select core/shell nanocrystals have epitaxial crystalline (zinc-blende) ZnS shells or crystalline (zinc-blende) shells with no obvious epitaxial relationship to the PbS core. The PbS core 1Sh-1Se absorbance and photoluminescence peak energies redshift upon shell addition due to relief of a ligand-induced tensile strain and wave function leakage into the shell. The photoluminescence quantum yield decreases after ZnS shell addition likely due to non-radiative defect states at the core/shell interface.

Introduction 1

Core/shell nanocrystals are attractive for optoelectronic applications because they have superior properties such as improved chemical stability,2-4 increased photoluminescence quantum yields,5 reduced non-radiative Auger processes,6,7 and larger Stokes shifts,8 as compared to plain core nanocrystals. PbS nanocrystals coated with Pb9 or Cd2,10 chalcogenide shells have been studied recently due to their optical absorption in the shortwave-infrared (SWIR) which makes them attractive for applications such as photovoltaics,11 light-emitting diodes (LEDs),12-14 and deep tissue biological imaging.15-18 These core/shell materials have been implemented into devices and have shown improved device performance as compared to their plain core counterparts. For example, solar cells based on PbS/CdS core/shell nanocrystals have demonstrated increased open-circuit voltages (Voc) relative to plain PbS nanocrystals, due to reduced non-radiative recombination as a result of the CdS shell.11 High-performance SWIR LEDs have been made from PbS/CdS nanocrystals and have shown an increase in performance as compared to PbS only nanocrystals due to the CdS shell increasing the photoluminescence quantum yield.12,13 Additionally, PbS/CdS/ZnS core/shell/shell nanocrystals have been shown to be more stable in aqueous solutions, brighter, and potentially less toxic than plain PbS nanocrystals.2,15 Furthermore, PbS nanocrystals coated with both zincblende and wurtzite CdS have demonstrated dual emission in the visible/near-infrared regions, which could be useful for ratiometric nanocrystal sensors for high spatial resolution pH and temperature measurements.19-23 While PbS nanocrystals with Pb and Cd chalcogenide shells have

been extensively studied and found to have improved properties, to the best of our knowledge no other shell materials have been deposited directly onto PbS nanocrystals. ZnS shells have greatly increased the photoluminescence quantum yields of several nanocrystal cores, such as CdSe and CdS, 3,4,24,25 and are less toxic and more earthabundant than Cd chalcogenide shells.26 Thus, a ZnS shell, applied directly to a PbS nanocrystal could be advantageous. Additionally, since the energy level alignment between PbS and ZnS in nanocrystal core/shells has not been determined experimentally, it is interesting scientifically to study PbS/ZnS core/shells. If the core/shell energy level alignment is type-I with confined carriers in the core, then an increase in photoluminescence is expected.1 Whereas, type-II energy level alignment could lead to an interband transition energy further into the infrared, below the 1Sh-1Se transition of the core PbS nanocrystal.1 This could result in a narrow gap, earth-abundant material, of which there are few.2,10,26 Finally, a ZnS shell could potentially mechanically stabilize the PbS surface thereby reducing the softening of the surface phonon modes and consequently the non-radiative recombination due to electron-phonon coupling.27 One reason PbS/ZnS core/shell nanocrystals have not been made before is that common ZnS shell deposition techniques occur at temperatures (~140-300 °C)3,4 at which PbS nanocrystals Ostwald ripen.2 Therefore, as we show in this work, the key to depositing a ZnS shell on PbS nanocrystals is to find a shell reaction which occurs at a low enough temperature (80 °C, Figure S1) such that the PbS nanocrystals do not Ostwald ripen. In this paper

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we show that a ZnS shell can be added to PbS nanocrystals, with approximately monolayer control and without Ostwald ripening, by using Zn oleate and thioacetamide as Zn and S precursors and a reaction temperature of 80 °C. Due to the thioacetamide being less reactive towards the Zn-terminated core/shell nanocrystal surface as compared to free Zn oleate in the reaction solution, the ZnS shell reaction is self-limiting to approximately one monolayer per shell reaction. In order to increase the ZnS shell thickness beyond a monolayer, the ZnS shell reaction had to be modified by adding a 10 minute time delay between the thioacetamide and Zn oleate injections. This delay allowed the thioacetamide time to react at the Znterminated nanocrystal surface before the Zn oleate was added. However, the precursors had to be added simultaneously for the first ZnS monolayer as the addition of thioacetamide without Zn oleate present causes the PbS nanocrystals to precipitate. High angle annular dark field scanning transmission electron microscopy (HAADFSTEM) shows that, within the same batch of PbS/ZnS core/shells, most of the ZnS shells are amorphous. However, select core/shell nanocrystals have epitaxial crystalline (zinc-blende) ZnS shells or crystalline (zinc-blende) shells with no obvious epitaxial relationship to the PbS core. We also find that upon ZnS shell addition the PbS core 1Sh-1Se absorbance and photoluminescence peak energies redshift due to the relaxation of a ligand-induced tensile strain as well as wave function leakage into the ZnS shell. Furthermore, the photoluminescence quantum yield and exciton lifetimes decrease after shell addition, likely due to non-radiative defects present at the core/shell interface.

Experimental Section General Considerations. Standard Schlenk-line techniques were used unless otherwise noted. Oleic acid (90%), 1-octadecene (90%), lead oxide (99.999%), bis(trimethylsilyl)sulfide (synthesis grade), thioacetamide (≥99%), zinc acetate (99.99%), N,N-dimethylformamide (anhydrous, 99.8%), toluene (anhydrous 99.8%), acetonitrile (anhydrous, 99.8%), tetrachloroethylene (anhydrous, ≥99%), and chloroform (anhydrous, ≥99%) were all purchase from Sigma Aldrich. All chemicals were used as received except for the 1-octadecene, which was dried by heating it to 110 °C under vacuum for two hours and placing it over activated 3 Å molecular sieves in a glovebox. This dried 1-octadecene was used for the ZnS shell reaction. However, the 1-octadecene used for the bis(trimethylsilyl)sulfide stock solutions was just dried via heating under vacuum, and the 1-octadecene used to make the Pb oleate solution for the synthesis of the PbS cores was taken directly from the bottle and dried during the Pb oleate formation. PbS Core Synthesis. PbS nanocrystal cores were made using two similar methods. The three smallest ( 4 nm) were made using a method recently published by Lee et al.,29 which is an adaptation of the synthesis by Hines et al.

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The three smallest PbS cores were made as follows:28 To start, a 0.151 M stock solution of bis(trimethylsilyl)sulfide in dried 1-octadecence was made. Then, lead oxide (0.361 g) was mixed with oleic acid and 1-octadecene. The oleic acid to lead oxide ratio was used to vary the size of the nanocrystals with ratios of 2.1 (0.95 g oleic acid, 11.9 g 1octadecene), 3.3 (1.5 g oleic acid, 11.4 g 1-octadecene), and 6.2 (2.85 g oleic acid, 10.2 g 1-octadecene) resulting in PbS nanocrystal diameters of 3.1 ± 0.4, 3.3 ± 0.4 and 3.5 ± 0.7 nm, respectively. Note that as the quantity of oleic acid was increased, the quantity of 1-octadecene added was lowered such that the total volume of oleic acid and 1octadecene was constant for each reaction. The lead oxide, oleic acid, 1-octadecene mixture was heated under vacuum to 110 °C for one hour at which point the yellow/orange lead oxide powder had completely reacted with the oleic acid to make Pb oleate and the solution was colorless. This Pb oleate solution was then back filled with argon and heated to 130 °C, at which point 5 mL of the 0.151 M bis(trimethylsilyl)sulfide stock solution was swiftly injected. The solution immediately turned a dark brown color and after 2 minutes the reaction mixture was cooled to room temperature using a liquid nitrogen bath. The nanocrystals were separated from unreacted precursors and byproducts by adding ~20 mL of toluene and ~16 mL of acetonitrile and centrifuging at 5 krpm for 3-5 minutes. After centrifugation, a dark precipitate formed at the bottom of the centrifuge tubes and the supernatants were discarded via decanting. This process was repeated with 4-8 mL each of toluene and acetonitrile and the product was dried (excess solvent/nonsolvent removed) and stored in a glovebox. Approximations are given for the toluene and acetonitrile volumes as they varied with nanocrystal size, this is true for subsequent purification procedures as well. The PbS nanocrystals larger than 4 nm were synthesized as follows:29 First, two different stock solutions of bis(trimethylsilyl)sulfide in dried 1-octadecene were made (93 mM and 39 mM). Then lead oxide (0.446 g) was mixed with 1-octadecene (50mL) and oleic acid (3.8 mL) and heated under vacuum to 110 °C for one hour to form a colorless Pb oleate solution. This solution was then cooled to 100 °C and 3 mL of the 93 mM bis(trimethylsilyl)sulfide stock solution was swiftly injected. The solution turned dark brown over the first 50 seconds of the reaction. In order to tune the PbS nanocrystal size without nucleating more particles, 3 mL of the lower concentration (39 mM) bis(trimethylsilyl)sulfide stock solution were injected at 6-7 minute intervals after the initial injection of the 93 mM stock solution. The PbS core diameter was tuned from 4.1 ± 0.6 nm to 6.4 ± 0.7 nm by changing the number of 39 mM stock solution injections from 0 to 5. The reaction was continued for 6-7 minutes after the last injection, at which time 10 mL of room temperature toluene was injected and the reaction was cooled to room temperature with a liquid nitrogen bath. These larger nanocrystals were separated from unreacted precursors and byproducts by adding 44-50 mL of toluene and 22-28 mL acetonitrile and centrifuging at 5

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krpm for 3-5 minutes. A dark precipitate formed at the bottom of the tubes after centrifuging and the supernatants were discarded via decanting. This process was repeated with 4-6 mL each of toluene and acetonitrile, and the product was dried (excess solvent/nonsolvent removed) and stored in a glovebox. Synthesis of Zn oleate. Zn acetate and oleic acid were mixed together and heated to 110 °C for 2 hours under vacuum. During the heating process, the solution changed from cloudy and white and to clear and colorless as the Zn acetate reacted with the oleic acid to make Zn oleate. The oleic acid to Zn acetate molar ratios used were between three and five. As the solution was allowed to cool, it was opened to air and poured into two centrifuge tubes before reaching room temperature. The solution was left overnight at which point it became a sticky, white solid. Continuing under air, acetone was added to the solid and shaken such that a milky white suspension formed. Next, the solution was centrifuged at 5 krpm for 3-5 minutes, which caused the Zn oleate to separate from the excess oleic acid and acetone and deposit on the bottom of the centrifuge tube. The acetone and oleic acid were decanted, leaving the Zn oleate. This acetone cleaning process was repeated three times in order to ensure the removal of excess oleic acid. The Zn oleate was then placed in a vacuum oven at room temperature for two to three hours to remove any remaining acetone and free water. Finally, the oven was backfilled with argon and the white powdery Zn oleate was stored in a glove box. First Monolayer ZnS Shell Reaction. This ZnS shell reaction, which will be referred to as the “first monolayer ZnS shell reaction”, takes inspiration from the PbTe/PbS core shell synthesis created by Ibáñez et al.30 and is similar to the CdSe/CdS “flash” synthesis developed by Cirillo et al.31 Zn oleate was mixed with 1-octadecene and heated to 90 °C under Ar until the Zn oleate dissolved and the solution became clear, at which point the solution was cooled to 50 °C. Separately, thioacetamide was mixed with N,N-dimethylformamide and the PbS nanocrystal cores were mixed with toluene. Next, both the room temperature thioacetamide and PbS nanocrystal solutions were injected simultaneously into the 50 °C Zn oleate solution. The reaction was then heated to 80 °C over ~5 min and held there for a total reaction time of ~ 30 minutes (including the 5 minutes of heating from 50 °C to 80 °C), at which point the reaction was cooled to room temperature using a liquid nitrogen bath. The core/shell nanocrystals were separated from unreacted precursors and byproducts by adding 12-18 mL of toluene and 5-8 mL of acetonitrile and centrifuging at 5 krpm for 3-5 minutes. A dark precipitate formed at the bottom of the tubes after centrifuging and the supernatants were discarded via decanting. This process was repeated with 1-2 mL each of toluene and acetonitrile. The product was stored in toluene in a glovebox. A typical ZnS shell reaction contained 25-35 mg of PbS cores, 0.21-0.38 g of Zn oleate, 0.025-0.045 g thioacetamide, 6 mL of dried 1-octadecene, 1 mL of N,Ndimethylformamide and 1 mL of toluene. The weight of

the PbS nanocrystals was taken as the weight of the PbS nanocrystals including the oleic acid ligands and set such that the PbS nanocrystal concentration was ~30 mg per 8.2 mL of reaction solution. The concentrations of Zn oleate and thioacetamide were set so that there was enough Zn and S in the reaction such that if all the Zn and S were to deposit on the PbS cores it would result in a 4 to 5 monolayer thick ZnS shell. Additionally, the molar ratio of Zn oleate to thioacetamide was kept at 1:1. The amounts of Zn oleate and thioacetamide required per monolayer for a particular core size were calculated assuming spherical cores and the addition of uniform 0.2673 nm thick ZnS shells (half the lattice constant of zinc-blende ZnS, see below) per monolayer. Note that because the oleic acid ligands were included in the PbS nanocrystal weight the estimated total possible shell monolayers, for a particular Zn oleate and thioacetamide concentration, is a lower bound. Additional Monolayer ZnS Shell Reaction. Additional ZnS monolayers beyond the first can be added to the PbS core by repeating what we will refer to as the “additional monolayer ZnS shell reaction”. The “additional monolayer ZnS shell reaction” was the same at the “first monolayer ZnS shell reaction” described above except that the thioacetamide and PbS/ZnS core/shell nanocrystals were mixed first and allowed to react for 10 minutes before the Zn oleate was added. In the “additional monolayer ZnS shell reaction” 4 mL of 1-octadecene was heated to 50 °C and then the PbS/ZnS core/shell nanocrystals in 1 mL of toluene and the thioacetamide in 1 mL of N,Ndimethylformamide were injected. The reaction solution was then heated to 80 °C over ~5 minutes and held at 80 °C. Meanwhile, the Zn oleate was mixed with 2 mL 1octadecene and heated with a heat gun until the solution was clear and colorless. Then the Zn oleate was injected into the reaction mixture 10 minutes after the thioacetamide and PbS/ZnS core/shells were injected. The reaction was continued at 80 °C for a total reaction time of ~30 min (including the 5 minutes of heating from 50 °C to 80 °C) at which point the reaction was cooled to room temperature using a liquid nitrogen bath and cleaned up and stored just like in the “first monolayer ZnS shell reaction”. It is important that the ZnS nanocrystals, which homogeneously nucleated in the previous ZnS shell reaction, be removed before the subsequent shell reaction. Sizeselective precipitation using toluene and acetonitrile as the solvent/nonsolvent pair was used to remove these smaller ZnS nanocrystals from the PbS/ZnS core/shell nanocrystals. The core/shell nanocrystals were suspended in toluene (~2-4 mL) and a small amount of acetonitrile (~100-500 μL) was added such that after centrifugation (5 krpm for 3-5 minutes) there was still a brown color in the supernatant in addition to the dark precipitate. While this resulted in some loss of PbS/ZnS core/shells to the supernatant it removed the ZnS nanocrystals, which was confirmed by transmission electron microscopy (TEM) and energy-dispersive x-ray spectroscopy (EDS) (see below and Figure S8).

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In order to have enough material for characterization after each “additional monolayer ZnS shell reaction” we scaled up the “first monolayer ZnS shell reaction” by a factor of three. We saved 1/3 of the reaction yield from this reaction and the remaining 2/3 was used for the first “additional monolayer ZnS shell reaction” which was scaled up by a factor of two. Finally, half of the yield from this reaction was saved and half was used for the second “additional monolayer ZnS shell reaction” which was done as stated above without any upscaling. Optical Characterization. Room temperature photoluminescence was measured using a Bruker Vertex 80v Fourier-transformed infrared spectrometer (FTIR) equipped with a UV/vis/NIR CaF2 beamsplitter and a Hamamatsu PbS/Si detector. The samples were excited with 153 mW from a 660 nm diode laser perpendicular to the collection axis. The concentrations of nanocrystals, suspended in tetrachloroethylene, and IR-140 dye, dissolved in 200 proof ethanol, were adjusted to 90 ± 0.5% transmission at 660 nm so as to minimize multiexcitons32 as well as reabsorption33 (Figure S2). The photoluminescence quantum yield of the nanocrystals was found by first taking spectra of the nanocrystals and the IR-140 dye which has a known photoluminescence quantum efficiency of (16.7 ± 1%).33 Next, a spectrum of a white light source, with a known intensity versus energy profile found using a spectroradiometer, was taken and used to normalize for energy dependent efficiency differences in the FTIR. Finally, the photoluminescence quantum yield of the nanocrystals was calculated by multiplying the photoluminescence quantum yield of the IR-140 dye (16.7 ± 1%)33 by the ratio of the integrated photoluminescence of the nanocrystals to that of the IR-140 dye. Absorbance spectra of the nanocrystals were obtained using a Perkin-Elmer Lambda 750 spectrometer equipped with PMT and PbS detectors, and deuterium and tungsten lamps. The full width at half maximum (FWHM) of the 1Sh-1Se absorbance and photoluminescence peaks, as well as the area under the photoluminescence peaks, were found by fitting the peaks to a Gaussian function using the peak analyzer feature in OriginPro 8.5. However, the 1Sh-1Se absorbance peaks of the smaller core/shells were not fit well with a Gaussian function, so the FWHM were estimated by hand. The PbS and PbS/ZnS photoluminescence dynamics were measured using time-correlated single photon counting. The system was based on a variable repetition rate 1 ps pulsed 586 nm Rhondamine 6G dye laser. Nanocrystal solutions were placed in 1 mm path quartz spectrophotometer cells. Backscattered emission was collected and sent to an InGaAs single photon avalanche photodiode with a ~250 ps instrument response function. The excitation power and nanocrystal concentration were carefully selected in order to ensure artifact free results (Figure S3). An excitation density of 1 x 1011 photons/cm2 minimized second order recombination. In order to maintain a reasonable photon counting rate (>300 photons/second), the nanocrystal concentration was chosen

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such that the optical density of the 1Sh-1Se exciton was approximately 0.3. Transmission Electron Microscopy (TEM). TEM samples were created by drop casting a small volume (4 nm) but increases appreciably for the smaller PbS cores, after the addition of a monolayer of ZnS (Figure 4b and Figure

Absorbance (a.u.)

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Figure 4. Normalized absorbance (a) and photoluminescence (b) of 6.5 ± 0.5 nm PbS cores (green) and corresponding PbS/ZnS core/shells (blue) made from one “first monolayer ZnS shell reaction”. The ZnS shell thickness is 0.21 ± 0.06 nm. Upon ZnS shell addition the 1Sh-1Se absorbance peak redshifts 42 meV and photoluminescence peak redshifts 41 meV. c) Redshift of the 1Sh-1Se absorbance peak (blue open squares) and photoluminescence peak (green circles) after one “first monolayer ZnS shell reaction” as a function of core PbS diameter. The redshift is relative to plain PbS cores.

seen in the smaller cores is due to the smaller cores experiencing more wave function leakage into the ZnS shell than the larger cores. XRD of the initial PbS cores and corresponding PbS/ZnS core/shell nanocrystals was used to partially explain the origin of the 1Sh-1Se absorbance and photoluminescence peak redshifts as well as their dependence on PbS core size. Figure 5 shows the XRD spectra of 5.1 ± 0.8 nm PbS cores and the corresponding PbS/ZnS core/shells after

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one “first monolayer ZnS shell reaction”. All the peaks in the PbS core spectrum are shifted slightly to lower angles as compared to bulk rock-salt PbS (PDF # 9013403), which indicates that the lattice constant of the plain PbS cores is larger than bulk PbS. After the ZnS shell is added, however, the PbS peaks shift to higher angles and match up with bulk PbS. This shows that the ZnS shell causes the PbS core lattice constant to decrease to the bulk PbS value. Also, there are no peaks from the ZnS shell present in the XRD spectrum even though HAADF-STEM indicates that select shells are crystalline zinc-blende ZnS. The lack of ZnS peaks is actually not surprising because the ZnS peaks are expected to be much broader and weaker than the PbS peaks. This is because the small ZnS shell thickness (0.18

on the nanocrystals. They further suggest that this affect is greater for smaller nanocrystals and that is why smaller nanocrystals are under higher tensile strain. As shown in Figure 6a, when a monolayer of ZnS is added to the PbS cores the tensile strain is removed and the PbS core lattice constant decreases, within error, to the bulk PbS value. We hypothesize that the ZnS shell lowers the PbS core lattice constant because it relieves the ligand-induced tensile strain by replacing the oleic acid ligands on the PbS surface. However, because the ZnS lattice constant is 10% lower than that of PbS the tensile stress may also be relieved due to chemical pressure from the ZnS shell adding a compressive strain to the PbS core. Furthermore, as the smaller PbS cores start with a larger tensile stress, they have more stress to relieve, so the smaller cores experience a larger change in lattice

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Chemistry of Materials

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2θ (degrees) Figure 5. XRD spectra of 5.1 ± 0.8 nm PbS cores (green) and PbS/ZnS core/shells (blue) after one “first monolayer ZnS shell reaction” which added a 0.18 ± .06 nm ZnS shell. The vertical purple lines indicate the diffraction pattern of rocksalt PbS (PDF # 9013403) and the vertical red lines indicate that of zinc-blende ZnS (PDF #5000088). The solid portion of the PDF line is proportional to the PDF intensity and the dotted portion is a guide of the eye. The inset shows the (220) reflections demonstrating the shift to larger angles and a better match to bulk PbS after the addition of a monolayer of ZnS

± 0.06 nm) results in substantial Scherrer broadening, and the small ZnS mass fraction (~10%) combined with the lower ZnS x-ray scattering factor (as compared to PbS)47 results in low peak intensities. Figure 6a shows the lattice constants for PbS nanocrystals of various sizes. The lattice constants are all greater than bulk PbS and are larger for smaller nanocrystals. This shows that the plain PbS cores start under tensile strain and that this tensile strain is larger for smaller nanocrystals. This trend is similar to that found by Bertolotti et al.48 (compare green to red points in Figure 6a) who have suggested that this tensile strain is due to the ligand packing density not matching the adsorption sites

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Chemistry of Materials

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Figure 6. (a) PbS core (green squares) and corresponding PbS/ZnS core/shell lattice constants (blue circle) as a function of core diameter. PbS core lattice constant as a function of core diameter from reference #48 (red circles). The lattice constants in this work where found using powder XRD and the lattice constants in reference #48 were found using synchrotron x-ray total scattering measurements. (b) Variation of the change in PbS core lattice constant after one “first monolayer ZnS shell reaction” with PbS core diameter. The change is lattice constant is the result from subtracting the PbS core lattice constants from their respective Pb/ZnS core/shell lattice constants. (c) Comparison of the observed 1Sh-1Se absorbance peak redshifts, relative to plain PbS cores, to those calculated by multiplying the PbS lattice constant changes by (dEg/da)T meV/Å values found from literature (see Supporting Information for more details).

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constant upon ZnS shell addition as demonstrated in Figure 6b. The release of the tensile strain in the PbS nanocrystals, by the ZnS shell, redshifts the 1Sh-1Se exciton peak energy of the PbS core. This is because PbS has a negative pressure coefficient,49 (dEg/dP)T < 0, which means that the 1Sh1Se exciton peak energy decreases when the lattice constant decreases. We estimated the contribution of the decrease in lattice constant to the redshift by first estimating the change in 1Sh-1Se absorbance peak energy with change in lattice constant, (dEg/da)T, from literature values, see Supporting Information S12 for details. Next, we multiplied this value by the observed changes in lattice constant (Figure 6b), the results of which are shown in Figure 6c, along with the observed redshifts of the 1Sh-1Se absorbance peak energy. We compare the calculated redshifts to the observed 1Sh-1Se absorbance peak energy redshifts because the (dEg/da)T values estimated from literature come from experimental (dEg/dP)T values found from the change in 1Sh-1Se absorbance peak energies with applied pressure.49 For the larger PbS cores with lower 1Sh1Se absorbance peak energies, the experimentally observed redshift can be mostly explained by the lattice constant decrease. However, because (dEg/da)T decreases with decreasing PbS core size the redshift of the smaller cores is only partially explained by the lattice constant change. Nevertheless, it is interesting to note that because the relief of the tensile strain does explain some of the redshift upon shell addition for all core sizes, another consequence of the ligand-induce tensile strain48 on the PbS nanocrystals is a blue shift of the absorbance and photoluminescence peak energies. We hypothesize that the remaining redshift, which cannot be explained by the relief of tensile strain, is due to the PbS wave function leaking into the ZnS shell. This is reasonable due to the large Bohr radius of PbS (~20 nm)50 and thus the large quantum confinement of the PbS cores. Furthermore, the smaller PbS cores experiencing a larger absorbance and photoluminescence peak broadening after ZnS shell addition, relative to the larger PbS cores (Figure S10), also suggests that there is some wave function leakage into the ZnS shell. This is because if the wave function leaks into the ZnS shell the increased size polydispersity, due to the shell, will broaden the 1Sh-1Se absorbance and photoluminescence peaks of the smaller cores more than the larger cores. This is due to smaller nanocrystals being more sensitive to size changes because of the inverse relationship between transition energy and nanocrystal size.3,51 Additionally, there will be more wave function leakage out of the smaller PbS cores than the larger cores because the carriers experience a larger amount of quantum confinement in the smaller cores.34 The increased wave function leakage into the ZnS shell in the smaller cores is why there is a larger redshift upon shell addition to the smaller core relative to the larger cores. The lattice constant change with the number of ZnS shell reactions for 5.5 ± 0.6 nm PbS cores is shown in Figure 7a. There is a large change in lattice constant after the

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(a)ÅÅÅÅ

PbS Lattice Constant ( )

first ZnS monolayer, but with subsequent monolayers the lattice

5.96 5.95 5.94 5.93

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60

50 1Sh-1Se Absorbance Photoluminescence

40 1

2

3

ZnS Shell Reaction Number Figure 7. (a) PbS core lattice constant variation with number of ZnS shell reactions. “0” ZnS shell reactions is the plain 5.5 ± 0.6 nm PbS cores. The first shell reaction is a “first monolayer ZnS shell reaction” and all subsequent are “additional monolayer ZnS shell reactions”. There are two points for the first and second shell reactions because two shell reactions were done on one portion of the PbS cores and three on another portion. (b) Redshift of the 1Sh-1Se absorbance peak energy (blue open squares) and the photoluminescence peak energy (green circles) with number of ZnS shell reactions. The redshift is relative to the plain PbS cores which were the same cores as in (a).

constant remains, within error, at the bulk value. We hypothesize that the first ZnS monolayer replaces the ligands and relieves the ligand-induced tensile stress thus bringing the PbS nanocrystal lattice constant down to the bulk value. As the tensile stress has already been removed, subsequent ZnS shell layers do not affect the PbS cores lattice constant and it remains, within error, around the bulk value. While this data does suggest ZnS monolayers beyond the first have no effect on the PbS core lattice constant, the largest shell we have investigated is only three monolayers thick. Thus, further research is needed to determine if ZnS shells thicker than three monolayers add additional compressive stain, due to the 10% lattice mismatch between ZnS and PbS, which results in PbS core lattice constants smaller than the bulk value. While

the core lattice constant appears not to change with continued shell growth beyond the first monolayer, the 1Sh1Se absorbance peak and the photoluminescence peak do redshift slightly with continued shell addition up to three monolayers Figure 7b (full spectra in Figure S13). This additional redshift is likely due to wave function leakage into the progressively thicker ZnS shell. Photoluminescence of PbS/ZnS Core/Shell Nanocrystals. To determine if the 1Sh-1Se absorbance and photoluminescence peak energy redshifts were due to type II energy level alignment we measured the photoluminescence decay dynamics of the PbS cores and corresponding PbS/ZnS core/shell pairs. An increase in photoluminescence lifetime upon shell addition would indicate type II alignment due to the spatial separation of the electron and holes in the core/shell nanocrystals. The photoluminescence decay of 5.1 ± 0.7 nm PbS cores and the PbS/ZnS core/shells made from these cores is shown in Figure 8a and is a typical result. The photoluminescence decay of the plain PbS cores is a single exponential with a time constant of 1.08 ± 0.02 µs, which is similar to literature reports on the exciton lifetime in PbS nanocrystals.52,53 After the addition of one monolayer of ZnS the photoluminescence decays faster and is best fit by a biexponential with time constants of 90 ± 10 ns and 620 ± 20 ns. Figure 8b shows the ratio of the exciton lifetime of the PbS/ZnS core/shells to the exciton lifetime of the corresponding PbS cores, where the exciton lifetime of the core/shell is taken as a weighted average of the two time constants that describe the decay. The ratio of the two lifetimes is smaller than one for all of the samples, which shows that the exciton lifetime decreases upon shell addition for all the PbS cores explored. We hypothesize that the faster time constants seen in the core/shell photoluminescence decay are due to non-radiative traps introduced by interface defects caused by the 10% lattice mismatch between rock-salt PbS and zinc-blende ZnS. As the ZnS shell seems to remove the ligand-induced tensile strain,48 but not add additional compressive strain, it is likely that the stress due to the lattice mismatch is released by the creation of interface defects.3 These defects likely result in non-radiative traps which increase the decay rate and obscure the photoluminescence decay profile. Therefore, it is not possible to determine the energy level alignment accurately from time-resolved photoluminescence with these traps present. In order to determine the band alignment using photoluminescence decay dynamics, further research will be needed to remove these traps which can perhaps be done by annealing the core/shell34,54 or by adding some Pb oleate to the ZnS shell reaction in order to intentionally grade the chemical composition at the PbS ZnS interface.55 Figure 8c shows the photoluminescence quantum yields for the PbS cores and the corresponding PbS/ZnS core/shells. For all of the PbS cores studied the photoluminescence quantum yield decreased after the addition of a ZnS shell. This is likely due to the non-radiative traps created by the ZnS shell. A similar drop in photoluminescence quantum yield due to interface defects was seen by

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Chemistry of Materials

(a)

Normalized Intensity

Sagar et al.34 when they deposited a CdS shell onto PbS nanocrystals using colloidal atomic layer deposition. Removing these non-radiative traps, via annealing or composition

1

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Photoluminecence Quantum Yield (%)

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surface defects and the additional defects, caused by the ZnS shell, only slightly affect the quantum yield. Whereas, the PbS cores with the higher initial photoluminescence quantum yields likely have more pristine surfaces which are affected to a greater extent by the addition of the ZnS shell, and thus experience a more dramatic drop in photoluminescence quantum yield. The photoluminescence quantum yields of the PbS cores made using the method by Hines et al.28 (open green symbols in Figure 8c) are lower than those of the smallest nanocrystals made from the Lee et al.29 method (closed green symbols in Figure 8c). Since smaller nanocrystals should have higher quantum yields,56-59 this indicates that the Hines et al. method produces nanocrystals with more defects than those from the Lee et al. method. Since both methods use the same precursors we hypothesize that the high quality nanocrystals made using the Lee et al. method could be due to the longer reactions times which allow for the annealing of the nanocrystal surface and subsequent removal of the surface defects. This is a reasonable assumption as Hines et al. did find that heating the PbS nanocrystals smoothed the nanocrystal surface and removed faceting. Thus annealing could also result in the removal of surface defects and therefore an increase in photoluminescence quantum yield.34,60 Another major difference between the two synthesis methods is that in the Lee et al. method multiple precursor injections are used to increase the size of the nanocrystals, whereas in the Hines et al. method the oleic acid to Pb oleate ratio is used to control the nanocrystal size. While nanocrystal quality could depend on this difference, this is not the case because PbS cores with the highest quantum yield were made using the Lee et al. method with only one precursor injection and an oleic acid to lead oleate ratio of six, which is similar to that used in the Hines et al. method.

Conclusions Figure 8. (a) Time-resolved photoluminescence decay for 5.1 ± 0.7 nm diameter PbS cores (green) and PbS/ZnS core/shells (blue) made from those cores with one “first monolayer ZnS shell reaction”. The ZnS shell thickness is 0.18 ± 0.06 nm. (b) Ratio of exciton lifetime of the PbS/ZnS core/shells, made with one “first monolayer ZnS shell reaction”, to the PbS cores as a function of core diameter. The core/shell lifetime is a weighted average of the two time constants. (c) Photoluminescence quantum yield variability with PbS core diameter for PbS cores (green squares) and their corresponding PbS/ZnS core/shells (blue circles) made with one “first monolayer ZnS shell reaction”. The PbS cores were made using 28 the synthesis methods by Hines et al. (open symbols) and 29 Lee et al. (closed symbols).

grading, may improve the PbS/ZnS core/shell photoluminescence quantum yields (Figure S15). Additionally, the photoluminescence quantum yield decrease upon shell addition is larger for PbS cores that initially have higher quantum yields (Figure 8c and Table S1), such as the smaller cores synthesized by the Lee et al. method.29 This is likely because the surface of PbS cores with the lower photoluminescence quantum yields may already have

We developed a solution synthesis method for PbS/ZnS core/shell nanocrystals with approximately monolayer control which does not cause the nanocrystals to Ostwald ripen. Due to the sulfur precursor, thioacetamide, preferentially reacting with Zn oleate rather than at the PbS/ZnS core/shell nanocrystal surface the shell reaction was found to be self-limiting. In order to increase the ZnS shell beyond one monolayer the ZnS shell reaction was modified such that the thioacetamide was allowed to react with the PbS/ZnS core/shell surface for 10 minutes before the Zn oleate was added. Bright-field TEM, HAADF-STEM, and STEM-EDS mapping shows that a monolayer of ZnS is deposited onto the PbS cores during each shell reaction and that most of the ZnS shells, within a PbS/ZnS core/shell batch, are amorphous. However, select PbS/ZnS core/shell nanocrystals have epitaxial crystalline (zinc-blende) ZnS shells or crystalline (zincblende) ZnS shells with no obvious epitaxial relationship between the core and the shell. Using absorption and photoluminescence spectroscopy, the 1Sh-1Se absorbance and photoluminescence peak energies were found to redshift after ZnS shell addition, due to relief of a ligand-

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Chemistry of Materials

induced tensile strain and wave function leakage into the shell. While photoluminescence lifetime and quantum yield decrease after ZnS shell addition, likely due to nonradiative trap states at the core/shell interface caused by the lattice mismatch, it may be possible to remove these traps by annealing the nanocrystals or compositionally grading the interface. Removing these traps will allow for the determination of the energy level alignment between the PbS cores and the ZnS shell using time-resolved photoluminescence and may lead to improved optical properties as compared to PbS nanocrystals.

ASSOCIATED CONTENT Supporting Information. Additional synthesis and characterization details, bright-field TEM images, ZnS shell thickness error bar estimation, additional HAADF-STEM images, further details on how homogenously nucleated ZnS nanocrystals limit ZnS shell growth, additional absorbance and photoluminescence spectra, redshift as a function of PbS core 1Sh-1Se absorbance peak energy, details of estimating (dEg/da)T meV/Å from literature, photoluminescence quantum yields before and after annealing, FTIR of core and core/shell nanocrystals. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author * [email protected]

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT The authors are grateful to S. Brittman for helpful discussions. The Office of Naval Research (ONR) is gratefully acknowledged for their financial support of this work. D.L.W. acknowledges the National Research Council postdoctoral program.

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