CdS Nanosheets Using Acetate

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Colloidal Synthesis of PbS and PbS/CdS Nanosheets Using Acetate-Free Precursors Haitao Zhang, Benjamin H. Savitzky, Jun Yang, Jonathan T. Newman, Kaitlyn A Perez, Byung-Ryool Hyun, Lena F. Kourkoutis, Tobias Hanrath, and Frank W. Wise Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.5b03348 • Publication Date (Web): 02 Dec 2015 Downloaded from http://pubs.acs.org on December 6, 2015

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Chemistry of Materials

Colloidal Synthesis of PbS and PbS/CdS Nanosheets Using Acetate-Free Precursors

Haitao Zhang,*a Benjamin H. Savitzky,b Jun Yang,a Jonathan T. Newman,a Kaitlyn A. Perez,a ByungRyool Hyun,a Lena F. Kourkoutis,a,c Tobias Hanrath,d and Frank W. Wise a

a

School of Applied and Engineering Physics, b Department of Physics, c Kavli Institute at Cornell for Nanoscale Science, Chemical and Biomolecular Engineering, Cornell University, Ithaca, NY 14853.

d

School of

* To whom correspondence should be addressed. E-mail: [email protected].

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Abstract: New methods are developed for the synthesis and surface modification of colloidal PbS nanosheets. We find that residual acetate is a crucial reagent for the formation of PbS nanosheets by existing syntheses. The amount of acetate in reaction mixtures is, however, difficult to control, which substantially reduces the reproducibility of the synthesis. To solve this problem, we develop an acetate-free synthetic method to yield colloidal PbS nanosheets with lateral dimensions of several hundred nanometers. The thickness and shape of PbS nanosheets can be readily tuned by varying the synthetic parameters. As-synthesized nanosheets have photoluminescence quantum yield of around 6%. Time resolved photoluminescence spectroscopy shows that the carrier decay in nanosheets is not a single exponential, but involves surface defect states as well as multiple carrier interactions. The surfaces of PbS nanosheets can be modified by reaction with Cd(OA)2. Atomic-resolution electron microscopy reveals the formation of PbS/CdS core/shell nanostructures, and the photoluminescence quantum yield of PbS/CdS nanosheets reaches about 11%.


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Introduction Two-dimensional (2D) single-crystalline semiconductor nanosheets (NSs) have drawn growing interest within nanoscience research.1,2 In such 2D nanostructures, charge carriers are confined by finite thickness but can move freely in a plane. Thus, NSs may offer superior charge transport compared to assemblies of nanocrystals or nanowires, while still exhibiting size-dependent physical phenomena. These properties are highly desirable for the development of high performance optoelectronic devices. So far, research on colloidal semiconductor NSs has mainly focused on II-VI materials. In particular cadmium chalcogenide NSs with tunable thickness and optical properties have been well studied. 2-8 Lead chalcogenides (IV-VI) are attractive materials for optoelectronic applications in the near-infrared region. However, colloidal synthesis methods of lead chalcogenide NSs are still at the development stage; only a few examples have been reported.9-13 Schliehe et al. demonstrated the first colloidal PbS NSs.9 Lead acetate and thioacetamide (TAA) were used as precursors in their synthesis, and it was proposed that the 2D PbS NSs formed via oriented attachment of nanoparticles through the assistance of chlorine-containing co-solvents. This acetate-based synthetic method has been used in other studies to investigate mobility,14 carrier multiplication,15 and thickness control of PbS NSs.11,12 However, questions about the basic formation mechanism of PbS NSs remain unanswered. Further, the size-dependent optical properties of PbS NSs have not been well-characterized. Here we present new methods for the synthesis and surface modification of colloidal PbS NSs. Our studies on the previously-reported acetate-based synthetic method reveal that acetic acid (HAc), generated by the reaction of lead acetate and oleic acid (HOA), plays an important role in the formation of PbS NSs. However, the amount of acetic acid is difficult to control, 3 ACS Paragon Plus Environment


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which hinders the reproducibility of the synthesis. To solve this problem, we have developed a new synthesis method which excludes acetic acid by using lead oxide (PbO) and bis(trimethylsilyl)sulfide (TMS) as precursors. The new syntheses produce PbS NSs with tunable thickness and shape by varying the synthetic parameters. The photoluminescence quantum yield (PL QY) of as-synthesized PbS NSs is about 6%. Inorganic surface passivation is an effective method to reduce surface trap states and improve the PL QY of colloidal NCs.16,17 We show that PbS/CdS core/shell NSs can be synthesized by reaction of PbS NSs with Cd(OA)2. Structural and chemical characterization confirms the presence of the CdS shell, and the PL QY of the PbS/CdS NSs reaches about 11%. Results and Discussion Effect of Acetate. In the acetate-based synthetic method developed by Schliehe et al.,9 PbS NSs are synthesized by the hot-injection of thioacetamide (TAA) solution (in TOP/DMF) into a Pb(OA)2 solution (in diphenyl ether/oleic acid), in the presence of a chlorine-containing cosolvent. The latter is found to be crucial for the formation of 2D nanostructures. Pb(OA)2 solution is prepared by reacting lead acetate trihydrate (Pb(CH3COO)2·3H2O) with oleic acid (HOA) in diphenyl ether: Pb(CH3COO)2·3H2O + 2HOA

Pb(OA)2 + 2CH3COOH + 3H2O

(1)

The solution is heated under vacuum to nominally remove the HAc and H2O produced in reaction (1) [Ref. 18] before adding the chlorine-containing co-solvents and sulfide precursor. We find that the conditions for the vacuum heating have dramatic effects on the reaction. Table 1 provides an overview of the synthesis conditions discussed below. We performed syntheses A and B to reveal how different vacuum heating times (at 100 oC/100 mTorr) impact


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the morphology of the PbS NSs. Synthesis A produces PbS NSs (with lateral dimensions of ~1 µm; Supporting Information, Fig. S1) at a yield of about 50%. However, when the vacuum heating time is extended to 60 minutes, synthesis B yields much smaller nanoplates (about 20-30 nm lateral dimensions; Supporting Information, Fig. S1) at low yield (< 1%). As the purpose of the vacuum heating is to remove the acetic acid (HAc) and H2O, these results imply that the synthesis may be sensitive to the residual HAc concentration. Table 1. Summary of varied reaction conditions in synthesis A-E a

a

Synthesis

Lead and Sulfide Reagents

Vacuum Time (minutes)

Molar Ratio of Added HAc/Lead

A

Pb(CH3COO)2·3H2O, TAA

20

N/A

B

Pb(CH3COO)2·3H2O, TAA

60

N/A

C

PbO, TAA

20

0

D

PbO, TAA

20

1/12

E

PbO, TAA

20

1/1

all other reaction conditions of synthesis A-E are the same. See details in Experimental Section.

Figure 1. TEM images of PbS nanocrystals produced by synthesis C (a) and D (b).



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To verify that acetic acid (HAc) affects the reaction, we performed several control experiments (syntheses C, D, and E) in which HAc was excluded from the reaction by using PbO as a starting material in the preparation of Pb(OA)2: PbO + 2HOA

Pb(OA)2 + H2O

(2)

With the PbO precursor, synthesis C only produces small nanoplates (20-30 nm, Figure 1a) at very low yield (< 1%). Varying the vacuum heating time (to remove H2O) of synthesis C produces the same products, which indicates that H2O doesn’t impact the formation of PbS nanosheets. Addition of HAc to the reaction solution (synthesis D) at a HAc/Pb molar ratio of 1:12 yields micron-size PbS NSs (Figure 1b) at a chemical yield of over 50%. However, increasing the ratio of HAc/Pb to 1:1 in synthesis E leads to the formation of aggregated nanocrystals with irregular forms (Supporting Information, Fig. S2). We also notice that the addition of HAc significantly increases the reactivity, based on the speed of the color changes of the reaction solution and the increase of the chemical yield. The color of the reaction solution remains light-brownish at the end of synthesis C, while it turns dark within one minute in synthesis D. These studies reveal that HAc is an important reagent for the formation of PbS NSs in the acetate-based synthetic method.9 While the detailed role of HAc in this synthesis remains unclear, it is evident that HAc increases reactivity and promotes the formation of 2D NSs. A certain concentration of HAc is required to yield high quality PbS NSs. However, this greatly hinders reproducibility, as it is difficult to control the residual HAc concentration in insufficiently dried reaction mixtures. We note that this observation is consistent with a previous report by Houtepen et al., who noted that residual acetate has critical impact on the size and shape of PbSe nanocrystals.18 To achieve reproducible PbS NS synthesis, an acetate-free method is called for. 6 ACS Paragon Plus Environment

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Acetate-Free PbS Nanosheets Synthesis. We developed an acetate-free synthesis method for colloidal PbS NSs based on PbO and bis(trimethylsilyl)sulfide (TMS) precursors. Compared to TAA, TMS has a higher reactivity toward metal salts. TMS precursors may therefore promote PbS NS formation in the absence of HAc as it addresses the high reactivity requirement discussed above.

Figure 2. a) TEM image of purified PbS NSs. b) ED pattern of an isolated PbS NS. c) HAADF STEM image of a PbS NS showing an edge parallel to a direction.

In a typical synthesis, a TMS (0.16 mmol) solution in TOP (930 µL, 2.0 mmol) and DMF (70 µL, 0.90 mmol) is injected into a Pb(OA)2 solution in diphenyl ether/1,1,2-trichloroethane at 90 oC. Pb(OA)2 is prepared by reacting PbO (2.3 mmol) with oleic acid (HOA, 5.1 mmol) at a PbO/HOA molar ratio of 1:2.2. The reaction is allowed to proceed for 20 minutes before nanocrystal products are isolated by centrifugation. This synthesis produces a mixture of 2D NSs and nanoparticles with a bimodal size distribution (~ 5 nm and 15 nm, respectively; Supporting Information, Fig. S3). The addition of 1,1,2-trichloroethane and DMF is important for the formation of PbS NSs. Only nanoparticles are produced in reactions without these two reagents. Previous studies revealed that both 1,1,2-trichloroethane and DMF can facilitate the assembly of 2D nanostructures.9,19 Although we expect that the growth of nanoparticles could be suppressed by carefully choosing reaction solvents and surfactants, our current synthesis has not been able to



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avoid the formation of nanoparticle byproducts. Pure PbS NSs (about 200 nm × 50 nm, Figure 2a) are obtained at a chemical yield of about 50% by washing the mixture products in toluene followed by centrifugation. The synthesis with longer reaction time of 60 minutes produces thicker PbS NSs (this will be discussed below) at a higher chemical yield of about 80%, but nanoparticles still exist as a byproduct at the end of the reaction. Electron diffraction (ED) and high-angle annular dark-field (HAADF) STEM studies confirm the single-crystalline nature of the PbS NSs (Figure 2b,c). HAADF images reveal that the NS edges terminate on {110} planes of the PbS rock salt structure and suggest that the top surface of the NS is a {100} plane. Based on this structural analysis and the elongation of the nanosheets in the direction, we infer that the sheets grow along the direction (inset of Figure 2b). This is consistent with the previous report that PbS NSs are formed via oriented attachment of nanoparticles on {110} facets.9 The thickness of the PbS NSs can be tuned by varying the reaction time. We measured the thickness of the PbS nanosheets by AFM. The long-chain OA ligands were replaced with shorter ethanedithiol (EDT) ligands to obtain a better understanding of the NS thickness independent of the thickness of the ligand layers.11 With the other synthetic conditions being the same as those described above, the average thickness of the PbS NSs increases from 2.0±0.3 nm (about 7 atomic layers) to 3.6 ± 0.2 nm (about 12 atomic layers) when the reaction time is extended from 20 to 60 minutes. Thicker PbS nanosheets can be obtained by further extending the reaction time, but this causes severe agglomeration of NSs in the reaction solution. The shape of the PbS NSs can be tuned by the amount of oleic acid (HOA) and reaction temperature. As shown in Figure 3, PbS NSs with different length/width ratios are synthesized by changing these synthesis parameters. The “belt-like” PbS NSs with a length:width ratio of 8 ACS Paragon Plus Environment

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about 10:1 are produced at relatively low temperature and low HOA/Pb molar ratio (Figure 3a). Increasing the reaction temperature and HOA/Pb ratio leads to the formation of more-square PbS NSs (Figure 3b,c).

Figure 3. TEM images of PbS NSs of different shapes synthesized by varying the amount of oleic acid and reaction temperature: a) about 200nm × 20nm, 80 oC, HOA/Pb = 2.2; b) about 200nm × 50nm, 90 oC, HOA/Pb = 2.2; c) about 200nm × 100nm, 90 oC, HOA/Pb = 4.4. Scale bars are 200 nm.

Optical Characterizations. The absorption edges and PL emission peaks of the PbS NSs shift to longer wavelengths with extended reaction times, which also indicates an increasing thickness (Figure 4). The absorption spectra of PbS NS thin films are measured using an integrating sphere to minimize light scattering artifacts (see Figure S4 for the absorption spectrum of PbS NS measured without using an integrating sphere). The absorption edge is not sharp due to the thickness polydispersity of PbS NSs. Similar absorption spectra have been reported in other colloidal PbS NSs studies.11,15 The absorption edge is determined by the position of the lowest energy peak in the second derivative of the absorption curve. As shown in Figure 4a, the absorption edge of PbS NSs shifts from approximately 1400 nm to 1600 nm when the reaction time is extended from 20 minutes to 60 minutes. The corresponding PL peaks exhibit similar red shifts, from 1450 nm to 1650 nm (Figure 4b). This suggests that the PL of PbS NSs is indeed coming from the radiative recombination of photoexcited states at the band 9 ACS Paragon Plus Environment


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edge. The PL quantum yield (QY) of the as-synthesized PbS NSs is about 6%, measured using the standard procedure.20 (see the method section and Supporting Information Figure S6 for details)

Figure 4. Absorption (a) and PL (b) spectra of PbS NSs synthesized using different reaction times: I, 20 minutes; II, 40 minutes; III, 60 minutes. Absorption edges: I, 1400 nm; II, 1500 nm; III, 1600 nm. PL peaks: I, 1450 nm; II, 1560 nm; III, 1650 nm.

Time resolved PL spectroscopy provides some insight into carrier recombination dynamics in the NSs. The recombination dynamics are more complicated than in 0D quantum dots due to the larger volume and surface area, which significantly increases the possibility of carrier-carrier interaction, as well as surface defects. We measure the temporal evolution of the PL intensity at different wavelength at different pump intensities, for PbS NSs suspended in tetrachloroethylene (TCE). Figure 5a shows the PL spectrum for the highest pump intensity. Each spectrum is fitted to a Gaussian shape. The wavelength-integrated intensity is plotted in Figure 5b, and center wavelength in Figure 5c. There are a few things worth noticing. First, the PL decay is highly non-exponential, even at the lowest pump intensity (Figure 5b). This could be due to carriercarrier interactions, but could also be an indication of a large distribution of surface defect states that serve as non-radiative recombination centers. We believe both effects contribute. Second, the PL decay is highly dependent on pump intensity. At higher pump intensity, the recombination is an order of magnitude faster than at low intensity. This indicates that we are


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seeing second-order, or even higher-order, recombination. Theoretical calculations show that the binding energy for excitons in NSs in organic solvent is on the order of 50-100 meV,21 larger than the thermal energy at room temperature. Therefore what we are seeing is most likely the exciton-exciton recombination. On the other hand, at longer time, carriers decay at the same rate, independent of the initial excitation intensity. This is more clearly seen in Figure 5d, where PL intensities at different excitation intensity are scaled, and the long-time components overlap very well. Third, at low excitation intensity, the PL spectrum red-shifts as time evolves. This suggests exciton migration from high energy sites to low energy sites. The thickness of the NSs is not atomically uniform, which could induce inhomogeneity of the local energy landscape, and excitons would migrate over time from the thinner area (higher energy) to the thicker area (lower energy). At high excitation intensity, the PL shifts to the blue before undergoing a shift to the red. The exact mechanism is unknown at the moment. Fourth, the recombination depends on history. This is shown in Figure 5d (inset), where we plot the PL intensity traces together, but shifted horizontally (temporally) so that the initial decay point in each trace overlaps with the previous trace. If the non-exponential decay is solely due to higher-order recombination, the recombination rate should be a function of the instantaneous carrier density, which is also a function of the instantaneous PL intensity. Clearly, the traces do not overlap with each other, which shows that the recombination rate is not a function of carrier density alone, but depends on the history of the recombination. This indicates that trap states are involved in the recombination.



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Figure 5. (a) PL spectral evolution of PbS NSs synthesized with 20 mins reaction time. Excitation is 800 nm at 1200 μJ/cm. Each spectrum is fitted to a Gaussian shape to extract the total intensity and center wavelength. (b) Extracted total PL intensity for different excitation intensity. (c) Extracted PL center wavelength for different excitation intensity. (d) Same data as in (b) but all traces are scaled so that the long time component overlaps. (inset, same data as in (b) but all traces are horizontally shifted so that the initial point in the next trace overlaps with the previous trace. Colors in (b),(c),(d) are consistent with each other.

Surface Modification of PbS Nanosheets. Our motivation to synthesize PbS NSs derives from the expected optoelectronic properties of 2D semiconductor nanostructures, such as superior charge transport compared to assemblies of nanoparticles and size-tunable optical properties. However, the PL measurements indicate that a significant density of trap states exist in the NSs.

These are generally attributed to defects resulting from incomplete surface

passivation, and could degrade the charge transport and fluorescence emission. In spherical (i.e., 12 ACS Paragon Plus Environment

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0D) nanoparticles and 1D nanowires,16,17,22,23 the impact of surface trap states can be mitigated by creating core/shell nanostructures. Among quantum dots, PbS/CdS core/shell nanostructures have been synthesized to improve the surface quality of PbS nanoparticles.17 We pursued a similar core/shell synthesis strategy to create heterostructured PbS/CdS core/shell NSs. We adopted a cation exchange strategy that has been used in PbS/CdS core/shell nanoparticle synthesis.17 In this method, a reaction between Cd(OA)2 and PbS nanocrystals converts the surface PbS into CdS: PbS + Cd(OA)2

CdS + Pb(OA)2

(3)

This reaction is carried out by combining PbS NSs (30 mg) and Cd(OA)2 (0.6 mmol) in 10 mL of toluene and stirring at room temperature. We confirmed the core/shell PbS/CdS structure of the NSs by HAADF STEM imaging and electron energy loss spectroscopic (EELS) mapping after 12 hours of Cd(OA)2 treatment. In assynthesized PbS NSs, the termination of the sheet edges is atomically abrupt, generally on PbS {110} planes (Figure 2c). After Cd(OA)2 treatment, a layer about 1 nm thick forms at the NS edges. This layer is amorphous in most areas (Figure 6b), but shows hexagonal ordering in several regions (Figure 6c). The lattice parameter of the hexagonally-ordered region is 4.16±0.02 Å, in agreement with the bulk lattice parameter (4.15 Å) of wurtzite CdS. A map of the Cd-M4,5 edge (Figure 6d) shows the presence of Cd across the NS and an increased Cd concentration at the edges, consistent with projection through a uniform surface layer. The simultaneouslyrecorded HAADF STEM image in Figure 6e can be interpreted as an approximate map of Pb in the NS because the heavy Pb atoms dominate the HAADF signal. The overlay of the Cd map and the Pb map (Figure 6f) shows that Pb is present only in the NS interior, while a layer of Cd 13 ACS Paragon Plus Environment


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without Pb is present at the edges. This is consistent with the formation of a PbS/CdS core/shell structure. Note that the periodicities apparent in Figure 6 d-f are Moiré fringes from finite sampling of the underlying atomic lattice.

Figure 6. HADDF STEM studies on Cd(OA)2 treated PbS NS in a showing an amorphous layer in most edge regions (b), and hexagonal ordering in several regions (c). Spectroscopic mapping using the cadmium M4,5 EELS edge (d) on the NS in a. The simultaneously recorded HADDF STEM image in e can be perceived as a map of Pb. The overlay of d and e shows that Pb is present only in the interior, while a layer of Cd with no Pb is present at the edge (f).

We monitored the optical properties of the resulting PbS/CdS NSs over different reaction times. As shown in Figure 7a, the PL peak of the PbS NSs gradually shifts to shorter wavelength during the reaction. The absorption edge of PbS NSs exhibits a similar blue-shift (Supporting 14 ACS Paragon Plus Environment

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Information, Figure S5). These observations suggest that Cd(OA)2 treatment decreases the thickness of PbS NSs by converting the surface PbS into CdS, thereby producing a PbS/CdS core/shell structure. Time-resolved PL was also measured for varying treatment times (Figure 7b) at low excitation intensity. Clearly the ratio of the amplitudes of the fast component to the slow component decreases as the reaction goes on. This is consistent with our previous results that the fast component in the PL decay is due to surface defects, and our surface treatment reduces the loss of carriers to these defects. Meanwhile, the fluorescence quantum yield of the PbS NSs increases from 6% to 11% (Figure S6). The intensity dependence and wavelength dependence of the PL after Cd(OA)2 treatment (Figure S7) show the same trends as before treatment. The only difference is that at low pump intensity, the fast component is smaller after the treatment.

Figure 7. (a) PL spectra and (b) time-resolved PL of as-synthesized PbS NSs (I) and PbS NSs after reacting with Cd(OA)2 for different time (hours): II, 2; III, 4; IV, 12; V, 36; VI, 60. PL peak positions (nm): I, 1470; II, 1380; III, 1350; IV, 1350; V, 1330; VI, 1320. PL 1/e lifetime (ns): I, 40; II, 120; III,130; IV, 160; V, 190; VI, 190.



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Conclusion We report new methods for the synthesis and surface modification of colloidal PbS NSs. We identify acetic acid as a crucial reagent for the formation of colloidal PbS NSs in the previously reported synthetic method, which may hinder reproducibility. We develop an acetatefree synthesis method which yields colloidal PbS NSs with lateral dimensions of several hundred nanometers. The thickness and shape of PbS NSs can be tuned by varying reaction time, temperature, and HOA concentration. We modify the PbS NS surface by exposing the NSs to Cd(OA)2 to form PbS/CdS core/shell nanostructures, and characterize these structures by atomic resolution STEM/EELS. Improvements in the surface passivation of PbS NSs after Cd(OA)2 treatment are confirmed by PL lifetime and quantum yield results. We expect that this work will benefit the development of high quality 2D semiconductor nanostructures for optoelectronic applications. Experimental Section Synthesis. All of the manipulations were carried out in a dinitrogen atmosphere by employing standard Schlenk line and glove box techniques unless otherwise noted. Lead(II) acetate trihydrate (99.999%), thioacetamide (≥98.5%), lead oxide(II) (≥99%), bis(trimethylsilyl)sulfide (≥99%), cadmium oxide (99.99%), diphenyl ether (≥99%), oleic acid (90%), 1,1,2-trichloroethane (96%), trioctylphosphine (97%), and dimethyl formamide (99.8%) were purchased from Aldrich and used as received. Syntheses A and B were carried out according to the literature method which used lead acetate and thioacetamide (TAA) as starting materials.9 In a typical synthesis, a mixture of 860 mg (2.3 mmol) lead(II) acetate trihydrate, 3.5 mL (10.0 mmol) oleic acid, and 10 mL diphenyl ether was heated to 110°C under nitrogen until the solution turned clear. The reaction flask was heated under vacuum (100 oC/100 mTorr) for 20 minutes for A and 60 minutes for B. Then 1.0 mL (10.8 mmol) 1,1,2-trichloroethane was added to reaction solution and the solution was reheated to 130°C in a nitrogen environment. A solution of 12 mg (0.16 mmol) thioacetamide (TAA) in 930 µL (2.0 mmol) trioctylphosphine (TOP) and 70 µL (0.90 mmol) dimethyl formamide (DMF) was injected into the reaction solution at 130°C. The reaction was allowed to proceed for 5 minutes. The reaction solution was then cooled by removing heat source. 16 ACS Paragon Plus Environment

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Nanocrystal products were separated by centrifugation and then washed 3-5 times by dispersing in toluene and centrifuging. The purified nanocrystals were dispersed in toluene. Syntheses C, D, and E were carried out using PbO (506 mg, 2.3 mmol) as the lead precursor. The other reaction conditions were the same as in synthesis A, except that after 20 minutes of heating under vacuum, 11 µL (0.19 mmol) and 132 µL (2.3 mmol) acetic acid were added to the reaction solution of D and E, respectively. Acetic Acid-Free Synthesis. In a typical synthesis, a mixture of 506 mg (2.3 mmol) PbO, 1.8 mL (5.1 mmol) oleic acid, and 10 mL diphenyl ether was heated to 110°C under nitrogen until the solution turned clear. The reaction flask was heated under vacuum (100 oC/100 mTorr) for 20 minutes. Then 1 mL (10.8 mmol) 1,1,2-trichloroethane was added to the reaction solution and the solution temperature was stabilized at 90°C. A solution of bis(trimethylsilyl)sulfide (TMS, 0.16 mmol) in 930 µL (2.0 mmol) TOP and 70 µL (0.90 mmol) DMF was injected into the reaction solution at 90°C. The reaction was allowed to proceed for 20 minutes. The reaction solution was then cooled down by water bath. Nanocrystal products were separated by centrifugation and then washed 3-5 times by dispersing in toluene and centrifuging. The purified PbS NSs were dispersed in toluene. The PbS NSs solution is not completely transparent, indicating the formation of large aggregates. The chemical yield of PbS NSs is about 50% (Chemical yield is calculated by

× 100%. The actual yield is estimated by

assuming that weight percentage of surfactant ligands in dried nanocrystal products is 20%. Theoretical yield is calculated based on the amount of limiting reagent (TMS).). The thickness and shapes of PbS NSs can be tuned by varying the amount of oleic acid, reaction temperature, and reaction time. Surface Modification of PbS NSs. A typical reaction was carried out by mixing assynthesized PbS NSs (30 mg) and a Cd(OA)2/oleic acid solution (containing 0.6 mmol Cd(OA)2) in 10 mL toluene and stirring at room temperature. Cd(OA)2/oleic acid solution was prepared by the reaction of CdO (400 mg, 3.1 mmol) and oleic acid (6 mL, 17.0 mmol) at 250 oC. The solution was heated under vacuum (100 oC/100 mTorr) for 20 minutes to remove H2O. Ethanedithiol (EDT) Treatment: Before the AFM measurements, drop casted PbS nanosheets (from 1 mg/mL toluene solution) on a silicon wafer were dipped into a 0.1 M EDT acetonitrile solution for 1 minute. This was followed by washing in acetonitrile for 30 s and in toluene for another 30 s, to remove organic ligand residues. HAADF STEM Imaging and Electron Energy Loss Spectroscopic (EELS) Mapping. HAADF STEM data was acquired on an aberration-corrected Nion UltraSTEM, with geometric probe aberrations corrected up to and including fifth order. An accelerating voltage of 60 kV and a convergence semi-angle of 30 mrad were used. To increase signal to noise and average out the scan noise, up to 6 successive HAADF STEM images, each recorded at 16 microseconds per pixel, were cross-correlated and averaged in Figs 2c and 5c. Electron energy loss spectroscopy 17 ACS Paragon Plus Environment


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was performed on the Cd-M4,5 edge, and spectra were background-subtracted using a linear combination of power laws (LCPC) combined with local background averaging (LBA) fitting.24 Optical Characterizations. PL Lifetime. PbS nanosheets (NSs) were excited by pulse laser light (1 kHz, 800 nm). The emission was collected and wavelength resolved by a monochromator, before been detected by an InGaAs PMT (instrument response 1 ns). For the intensity dependent measurement, in order to increase the dynamic range of our detector, we put an additional tunable neutral density filter right in front of the detector, so that the amount of light going into the detector is similar. The amount of attenuation is accounted for in the final data. Absorption Using Integrating Sphere. An integrating sphere was used in the absorption spectroscopy to reduce the effect of scattering. Briefly, broad-band white light from a quartz tungsten halogen was focused into the small input port of the integrating sphere. Light from the output port was collected using a large core optical fiber and directed into a monochromator for wavelength discrimination. Two back-to-back measurements were done, with and without the PbS NSs. The ratio of the intensity of the output light corresponds to the absorption of the PbS NSs. PL Quantum Yield Measurement. PL Quantum yield was measured using an integrating sphere (Labsphere) following previous method.20 Briefly, three measurements were done (a) without the sample in the sphere, (b) with sample in the sphere out of the direct laser beam, and (c) with sample in the sphere in the direct laser beam. The light was collected from a small port of the integrating sphere, coupled into a large core optical fiber and guided into the spectrometer. The sensitivity of the setup was independently calibrated with a quartz tungsten halogen lamp. Both the spectrum of the laser La,b,c and spectrum of photoluminescence Pb,c were collected, and quantum yield was calculated as =

!"# $ !

where % = 1 − . Raw data and more detailed #

analysis are presented in the Supporting Information.

Acknowledgements Research was supported by the U.S. Department of Energy, Office of Basic Energy Sciences, Division of Materials Sciences and Engineering, under Award DESC0006647, and by the Cornell Center for Materials Research (CCMR), with funding from the Materials Research Science and Engineering Center program of the National Science Foundation (cooperative agreement DMR 1120296). Supporting Information Available: Additional TEM images, optical absorption spectra, raw data for the quantum yield measurement as well as time resolved PL for Cd treated PbS NSs. This material is available free of charge via the Internet at http://pubs.acs.org. 18 ACS Paragon Plus Environment

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Graphical Table of Contents


CdS Nanosheets Using Acetate

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