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Bright Colloidal PbS Nanoribbons Antara Debnath Antu, Zhoufeng Jiang, Shashini M. Premathilka, Yiteng Tang, Jianjun Hu, Ajit Roy, and Liangfeng Sun Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.8b00467 • Publication Date (Web): 08 May 2018 Downloaded from http://pubs.acs.org on May 8, 2018

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Chemistry of Materials

Bright Colloidal PbS Nanoribbons Antara Debnath Antu,† Zhoufeng Jiang,†, § Shashini M. Premathilka,†, § Yiteng Tang,†, § Jianjun Hu,# Ajit Roy,# Liangfeng Sun†, §, * †

Department of Physics and Astronomy, Bowling Green State University, Bowling Green, OH 43403,USA Center of Photochemical Sciences, Bowling Green State University, Bowling Green, OH 43403,USA # Materials and Manufacturing Directorate, Air Force Research Laboratory, Wright-Patterson Air Force Base, Ohio 45433, USA §

ABSTRACT: Colloidal lead sulfide (PbS) nanoribbons are synthesized using organometallic precursors with chloroalkane co-solvents. The few-atom-thick nanoribbons have a typical width 20 nm and a length more than 50 nm. Different from a nanosheet where the quantum confinement energy is mainly determined by the thickness, the narrow width of the nanoribbon has an additional contribution to the increase of energy gap. In contrast to nanosheets, the nanoribbons are much brighter. At room temperatures, wellpassivated nanoribbons have achieved more than 30% photoluminescence quantum yield in the infrared spectrum, competing with the well-developed colloidal lead chalcogenide quantum dots of the similar energy gap.

Colloidal quasi-two-dimensional (quasi-2D) nanoplatelets and nanosheets have attracted a broad interest due to the novel properties1-17 resulted from their anisotropic nanostructures. As an infrared material, quasi-2D PbS3 has its own unique properties12,18-31 due to its small energy gap, large exciton Bohr radius, and strong spin-orbit coupling. In contrast to quantum-dot films optimized for device performance,17-23 nanosheets have a large in-plane carrier mobility3,17,20 due to the lack of the charge-carrier scattering at the boundaries of quantum dots, which is important for optoelectronic devices demanding a high current density. Although a large lateral size is favorable for a large current density, it is usually not favorable for a high light-emitting efficiency. Since a larger nanosheet has more surface states, the photoluminescence quantum efficiency of nanosheets is low.15 The typical photoluminescence quantum yield of colloidal PbS nanosheets is around a few percent. Making a CdS shell on PbS core can reduce the surface states, improving the photoluminescence quantum yield to ~ 10%. 28 This is still far below the quantum yield of colloidal lead chalcogenide quantum dots.32 The low quantum yield limits their applications as a light-emitting material. To improve their optical properties, reducing the lateral size of the nanosheets is one of the solutions since the number of surface trap states per sheet can be significantly reduced. This will create the advantage to explore the intrinsic novel properties of the PbS nanosheets caused by the anisotropic confinement in the quasi-2D nanostructure, including enhanced optical radiative recombination4 and slow Auger recombination9,33,34 as having been demonstrated in colloidal CdS, CdSe and CdTe nanoplatelets. In this Article, we report the synthesis and the optical properties of colloidal PbS nanosheets with a reduced lateral size. These nanosheets exhibit more than 30% photoluminescence quantum yield at the wavelength beyond 1100 nm, competing with the well-developed colloidal PbS or PbSe quantum dots with a similar energy gap. Their photoluminescence lifetime is significantly longer than those reported in the literature,27,28 in-

dicating a great suppression of surface and defect states. To distinguish them from “traditional” nanosheets,3 we call them “nanoribbons” thereafter due to their long and narrow shape. The synthesis of colloidal PbS nanoribbons is similar to the synthesis of PbS nanosheets developed earlier. 35 The lead precursor is prepared by dissolving 506 mg lead oxide (lead oxide PbO is toxic which should be handled properly according to the material safety data sheet) into 10 mL diphenyl ether and 1.8 mL oleic acid in a 3-neck flask. Then the mixture is heated to 100 °C and degassed under a high vacuum until it turns pale yellow. A chloroalkane solvent (1 mL) is added to the solution at the temperature 4 °C lower than its boiling point. Separately, the sulfur precursor is prepared by dissolving thioacetamide (12 mg) into N,N- dimethylformamide (70 µL) and trioctylphosphine (930 µL) under nitrogen environment. After both precursors are ready, the sulfur precursor is then injected into the flask containing the lead precursor at a certain temperature while the solution is vigorously stirred. After a few minutes, the reaction solution is then slowly cooled down to room temperature. The final solution is washed twice with toluene and then precipitated by centrifuging. The precipitated nanoribbons are then re-dispersed in toluene. The nanoribbons have a width ~ 20 nm and a length more than 50 nm as shown in the Z-contrast transmission electron microscopy (TEM) image (Figure 1a). Their morphology is similar to the rectangular PbS nanosheets made by Acharya et al, 21 except that the lateral size of the nanoribbons is about ten times smaller. The stacked nanoribbons in the TEM image show moiré patterns (Figure 1b), indicating a good crystallinity of each nanoribbon. The face-view HRTEM image (Figure 1b, bottom inset) of a single nanoribbon show an array of atoms forming a square lattice. The distances between every other layer of atoms in the two orthogonal directions are 0.59 nm, matching up with the lattice constant of PbS. The selected-area electron diffraction (SAED) pattern from multiple nanoribbons oriented nearly in the same direction shows a sharp squared pattern made of bright dots (Figure 1b, top inset), further confirming the cubic crystal structure. These nanoribbons tend to form

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clusters which appear like a flower (Figure 1c). Some petals of the “nano-flower” are lying down flatly on the TEM substrate while some are standing up on their edges. The latter makes it possible to image the edge of the nanoribbon27 to measure its thickness. Under high-resolution TEM (HRTEM), the array of atoms at the edge of the nanoribbon were imaged (Figure 1d), showing that the separation distance between the neighbor atoms along the edge is different from these across the edge. By doing the same analysis as we did for PbS nanosheets, 27 we found that the edge of the nanoribbon is also made of {110} facets. The HRTEM image of the nanoribbon edges also reveals the number of atomic layers in the thickness direction of the nanoribbon. For instance, the PbS nanoribbons synthesized at 130 oC show the thickness of 8 layers of atoms (Figure 1d).

Figure 1. (a) A Z-contrast TEM image showing some freestanding nanoribbons circled in red. (b) A TEM image showing stacked PbS nanoribbons. Each nanoribbon has a width ~ 20 nm. The stacked nanoribbons show moiré patterns, indicating good crystallinity of each nanoribbon. Top inset, SAED pattern of the nanoribbons, the electron beam spot on the sample is about 100 nm in diameter; bottom inset, face-view HRTEM image of the nanoribbon. (c) Aggregation of PbS nanoribbons forms a flower-like structure. (d) HRTEM images showing edges of the nanoribbons synthesized at 130 oC. The improved optical properties of nanoribbons are demonstrated by the measurements of absolute photoluminescence

quantum yields (Supporting Information A) and photoluminescence lifetimes (Supporting Information B). The absolute quantum yield of each sample was measured using an integrating sphere, which is based on the design proposed by Friend et al. 36 The sample is loaded into a small glass cuvette or drop-casted onto a glass slide and then mounted on a sample holder that allows the sample to intercept or pass the excitation light beam. Three spectra are taken (Figure 2a): (1) excitation light beam only, (2) sample in integrating sphere but off the excitation light beam, and (3) sample in the excitation light beam. In each photon-number spectrum (intensity * wavelength versus wavelength),36 five parameters are calculated, L1, L2, L3 (area under excitation light spectrum), P2 and P3 (area under PL spectrum), where the subscripts 1, 2, and 3 denote the different measurements aforementioned. The quantum yield is then calculated by the following equation,36 (𝐿2 ∗ 𝑃3 ) − (𝐿3 ∗ 𝑃2 ) 𝜂= . 𝐿1 ∗ (𝐿2 − 𝐿3 ) The absolute quantum yield of typical PbS nanosheets with energy gap around 0.92 eV is about 3.4±0.2% (Supporting Information C); while the nanoribbons with a similar energy gap achieve 5.7±0.7% photoluminescence quantum yield (Supporting Information D). The moderate improvement of the absolute quantum yield indicates the reduction of surface or defect states in the nanoribbons. These states can be further reduced by treating the nanoribbons using trioctylphosphine (TOP) (Supporting Information E). Neither noticeable morphology change nor significant spectrum shift is observed from the nanoribbons after the treatment (Supporting Information E). However, the absolute quantum yield of the nanoribbons reaches 34% (Figure 2b) after 30-days TOP treatment. It is three times more than the highest quantum yield of PbS nanosheets reported in the literature,28 catching up with the visible-light-emitting CdSe4 and CdSe/ZnS37 nanoplatelets. The TOP treatments on other nanoribbons of different energy gaps also result in improved photoluminescence quantum yields. The 34% quantum yield was obtained by a typical TOP treatment of the nanoribbons without further optimization. It is expected that a higher quantum yield can be obtained after optimizing the concentration of the TOP and the treatment time. The improved optical properties are also confirmed by the time-resolved photoluminescence measurements. The e-folding photoluminescence lifetime of the as-synthesized nanoribbons is 96 nanoseconds, nearly twice of the photoluminescence lifetime of the nanosheets (Figure 2c). The e-folding photoluminescence lifetime of the PbS nanoribbons after TOP passivation increases to 372 ns, while the maximum photoluminescence lifetime of PbS nanosheets capped by CdS shells has only 190 ns, as reported in the literature. 28 A simplified crystal structure of a nanoribbon has two large {100} facets and four small {110} facets (Figure 3b) as revealed by HRTEM (Figure 1). This is the same as the crystal structure of PbS nanosheets reported earlier.27 Calculations show that the {100} facets of colloidal PbS quantum dots are self-passivated.38 At {110} facets of a PbS quantum dot, the lead (Pb) atoms are usually well-passivated by oleic-acid ligands39 while the sulfur atoms are left unpassivated. The unpassivated sulfur atoms allow for possible hole traps, quenching photoluminescence. It has been demonstrated that the surface sulfur atoms of a PbS quantum dot can be passivated by TOP either during the synthesis40 or after the synthesis,41 where the photoluminescence efficiencies are improved by a factor of 2 ~ 3. For colloidal PbS nanoribbons, the photoluminescence quantum yield is


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Chemistry of Materials improved from 5.7% to 34% after TOP passivation, as mentioned earlier. This is about 6 times improvement. Based on the aforementioned analysis on PbS quantum dot, we speculate that this improvement is due to the passivation of the sulfur atoms by TOP at the {110} facets.

about two orders of magnitude larger than the quantum dot in comparison. It implies that the defect density in a nanoribbon after TOP passivation is much less than in a quantum dot. It is likely due to the low area ratio of {110} facets to {100} facets. In a typical nanoribbon of 2  20  50 nm (thick  width  length), the surface area of {110} facets counts for 14% of the total surface area while the rest are self-passivated {100} facets. In a typical PbS quantum dot, the unpassivated {110} facets and {111} facets can count more than 50% of the total surface area (Figure 3a), resulting in a higher surface defect density. The reduction of defective edge facets in CdSe quantum belts has also been proposed to explain the improvement of the photoluminescence quantum yield.42,43

Figure 3. (a) Facets of a PbS quantum dot which has a shape of a truncated cuboctahedron.3 (b) Facets of a PbS nanoribbon. Lead (sulfur) atoms are indicated by grey (yellow) balls.

Figure 2. (a) Schematics showing the three measurements to obtain the absolute quantum yield. (b) Photon-number spectra of the three measurements for PbS nanoribbons capped with TOP. The three curves represent the spectra when the sample intercepts the excitation beam (dashed line), is off the excitation beam a (solid line) and excitation beam only (dotted line). The excitation wavelength ranges from 800 nm to 1000 nm while the photoluminescence wavelength ranging from 1100 nm to 1500 nm. (c) Photoluminescence (PL) decay traces for TOPpassivated nanoribbons (solid diamonds), as-synthesized nanoribbons (solid circles) and nanosheets (solid squares). τ: photoluminescence e-folding lifetime, η: photoluminescence quantum yield. The photoluminescence quantum yield of the TOP-capped nanoribbons is nearly the same as the well-developed colloidal PbS and PbSe quantum dots of a similar energy gap. 32 This is remarkable since the surface area of a typical nanoribbon is

It should be noted that the PbS nanosheets of larger lateral sizes after the same passivation do not show such significant improvements in optical properties. In the control experiment, the nanosheets with lateral sizes larger than 200 nm by 200 nm were treated by TOP in the same way. The photoluminescence quantum yield increased from 3.4% to 4.0%, while the photoluminescence lifetime increased from 55 ns to 67 ns (Supporting Information C). The limited improvements might indicate that either there are still many surface defects on the PbS nanosheets after passivation or there are defects underneath the nanosheet surfaces which cannot be passivated using surface ligands. The optical absorption and emission spectra from the nanoribbons can be tuned in a broad range by changing the reaction temperature. As the reaction temperature increases from 70 oC to 150 oC, the absorption edge as well as the corresponding photoluminescence peak shifts from 1200 nm to 1900 nm (Figure 4). The higher reaction temperature results in larger PbS quantum dots at the beginning of the reaction, which finally form thicker nanoribbons through two-dimensional oriented attachment as occurred in nanosheets.3,22,25 The thicker nanoribbons have a smaller energy gap due to a weaker quantum confinement, showing redshifted optical absorption and emission spectra. The full width at half maximum (FWHM) of the photoluminescence peak ranges from 0.13 eV to 0.18 eV while the FWHM to the central photon energy ratio is around 17%, indicating a narrow distribution of thickness of the nanoribbons. For each nanoribbon, the absorption edge (defined as the point of the sharpest increase of the absorbance in the spectrum) nearly overlaps with the photoluminescence peak, showing a negligible Stokes shift. The exciton peaks of the PbS nanoribbons are not as strong as these in PbS quantum dots. Actually, the curve of the absorption spectrum near the band gap is more like a step than a peak. This is similar to what is currently attainable with PbS nanosheets.3,12,22,28 One reason could be that the exciton binding energy in the 2D nanoribbons is smaller than zero-dimensional quantum dots. The other reason could be that the thickness inhomogeneity of the nanoribbons is still large so that the inhomogeneous broadening smears out the excitonic peaks



in the absorption spectra. Nevertheless, the thinner nanoribbons show more prominent absorption peaks (Figure 4a), which is likely due to the reduced dielectric screening to the Coulomb interaction between the electron and the hole of an exciton in thinner nanoribbons.

sample, more than 50 edges were measured to obtain the average and the standard deviation of the thickness (Supporting Information G). The X-ray diffraction pattern of the film consisting nanoribbons was also measured to estimate the thickness by using Scherrer equation (Supporting Information H), as has been done for PbS nanosheets.3,44 The results for PbS nanoribbons show qualitatively the increase of thickness at a higher reaction temperature (Supporting Information H). However, the results are not quantitatively accurate due to the random orientation of the nanoribbons when they are deposited on the glass substrate (Supporting Information H). Therefore, only the thicknesses measured by TEM are used for the calculation thereafter.

Figure 4. (a) Optical absorption spectra of nanoribbons synthesized at different reaction temperatures with 1,1,2-Trichloroethane (TCA). The curves are vertically shifted for clarity. (b) Photoluminescence (PL) spectra of PbS nanoribbons synthesized at different reaction temperatures with TCA co-solvents. Each spectrum is normalized to its maximum intensity. Different chloroalkane co-solvents have been used for the synthesis of nanoribbons, resulting in the same morphology of the nanoribbons. For the same reaction temperature, the optical absorption and emission spectra of the nanoribbons synthesized with different chloroalkane co-solvents are nearly the same, as shown in Figure 4 (co-solvent TCA) and the figure in Supporting Information F (co-solvent chloroform). This indicates that the type of chloroalkane has a minor effect on the thickness of a nanoribbon. The thickness is mainly determined by the reaction temperature, as observed in the synthesis of PbS nanosheets.3,22,44 To understand the relation between the energy gap of the nanoribbon and its thickness, we measured its thickness using TEM and derived its energy gap from its photoluminescence peak. The thickness increases from 1.4 nm to 2.8 nm when the reaction temperature increases from 70 oC to 150 oC (Figure 5a). The thickness was obtained by measuring the edge of the nanoribbon standing up on its edge on the TEM substrate. For each

Figure 5. (a) Dependence of the nanoribbon thickness on the reaction temperature. Vertical error bars indicate the ± standard deviation of the thickness. (b) The relation between the energy gap and the thickness of PbS nanoribbons (solid squares) and the one-dimensional quantum confinement model derived for PbS nanosheets22 (solid line). Vertical error bars indicate the errors of the energy gap derived from photoluminescence peaks. Horizontal error bars indicate the ± standard deviation of the thickness. The energy gaps of the nanoribbons are derived from the photoluminescence peak shown in Figure 4b. The dependence of the energy gap on the thickness of the nanoribbon is shown in Figure 5b. The energy gap increases when the thickness of the nanoribbon decreases as the consequence of quantum confinement. For comparison, the dependence of the energy gap on the thickness of the PbS nanosheets is also the plotted in the same figure. For the same thickness, the energy gap of nanoribbons is larger than nanosheets. This difference is likely caused by the


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Chemistry of Materials additional quantum confinement due to the small lateral size of the nanoribbon. The width of the nanoribbon is close to the Bohr radius (~ 20 nm) of the exciton in PbS, resulting in an additional confinement energy that increases the energy gap. In contrast, the PbS nanosheet has a typical lateral size (both width and length) more than 100 nm, which has a negligible effect on the energy gap. Because of this, nanoribbons are not a strict 2D system with solely one-dimensional confinement since there are still nontrivial lateral quantum confinements. The mechanism of the growth of PbS nanoribbons instead of nanosheets is not fully understood yet. Our study shows that the PbO impurity introduced during the reaction is necessary to obtain nanoribbons. In the synthesis of PbS nanoribbons, the lead oleate is degassed at a higher vacuum than in the synthesis of nanosheets.20,22 In consequence, the Pb precursors turn pale yellow instead of colorless (Supporting Information I). The photoluminescence spectroscopy of this lead precursor shows a broad emission peak around 550 nm. This spectrum is nearly the same as the spectrum of PbO solution (Figure 6). PbO can form through partial backward conversion of lead oleate as reported earlier in the synthesis of colloidal PbSe nanocrystals,45 where the existence of PbO was confirmed by using the optical absorption spectroscopy. It is likely that the pale yellow impurity mixed with the lead oleate is PbO, which causes the formation of nanoribbons instead of nanosheets.

1b. However, their high photoluminescence efficiency indicates that each nanoribbon within the cluster is still well separated from each other by the capping ligands. Otherwise, the photoluminescence will be significantly quenched. In summary, few-atom-thick colloidal PbS nanoribbons have been synthesized using the one-pot organometallic route. The smaller surface area of a nanoribbon than a nanosheet results in improved optical properties. Surface passivation of the nanoribbons using TOP improves further their photoluminescence quantum yield to more than 30%, competing with the colloidal lead chalcogenide quantum dots emitting at the same wavelength. Even higher photoluminescence quantum yields are expected after careful optimization of the passivation. By changing the reaction temperature, the thickness of the nanoribbons can be tuned from 1.4 nm to 2.8 nm, resulting in tunable energy gap between 1.0 eV and 0.7 eV. These high-quality and energygap tunable PbS nanoribbons are favorable for further research to explore their unique intrinsic electronic and optical properties as well as for applications in infrared photonics and optoelectronics.

ASSOCIATED CONTENT Supporting Information. Synthesis of PbS nanoribbons, absolute quantum-yield measurement, photoluminescence lifetime measurement, characteristics of PbS nanosheets, trioctylphosphine treatment, optical absorption and emission spectra of nanoribbons synthesized with chloroform, thickness statistics, X-ray-diffraction measurements and calculations of the thickness, colors of the lead precursors and other solutions. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author * [email protected]

Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

ACKNOWLEDGMENT Figure 6. Photoluminescence (PL) spectra of lead oxide (PbO) in diphenyl ether and oleic acid solution (dashed line), lead precursors after normal degassing (solid line) and lead precursors after additional degassing (dotted line). The PbO impurity in PbS causes the mismatch of the crystal lattice which could be the reason that some nanoribbons are slightly curved when observed from their edges (Figure 1c). It is interesting that the nanoribbons with PbO impurities have even higher photoluminescence quantum yield than the nanosheets that do not have such impurities. It implies that PbO impurities do not quench photoluminescence. Density-functional-theory calculations have predicted that the substitutional atomic oxygen defects (where an oxygen atom replaces a sulfur atom, both at the surface and interior) do not produce in-gapstates.46 In experiments, it has been reported that the surface oxide has a negligible effect on the fluorescence properties of the colloidal PbSe nanocrystals.47 These may explain why the PbO impurity does not have a negative effect on the optical properties of the nanoribbons. On the other hand, the nanoribbons aggregate to form big clusters – nano-flowers – as shown in Figure

The work is partially supported with funding provided by the Office of the Vice President for Research & Economic Development, Bowling Green State University. Acknowledgment is made to the donors of the American Chemical Society Petroleum Research Fund for partial support of this research. L. Sun acknowledges the funding provided by the U.S. Air Force Summer Faculty Fellowship Program. We thank Charles Codding (manager of the machine shop) and Doug Martin (manager of the electronic shop) for their technical assistance at Bowling Green State University. The authors thank Dr. Marilyn Louise Cayer for her help on the TEM measurements using the Microscopy Core Facility at Bowling Green State University.

ABBREVIATIONS HRTEM, high resolution transmission electron microscopy; TEM, transmission electron microscopy; 2D, two-dimensional; TOP, trioctylphosphine; selected-area electron diffraction, SAED; full width at half maximum, FWHM; photoluminescence, PL; 1,1,2Trichloroethane, TCA.

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TOC

Colloidal lead sulfide (PbS) nanoribbons are synthesized using organometallic precursors with chloroalkane co-solvents. Wellpassivated nanoribbons have achieved more than 30% photoluminescence quantum yield in the infrared spectrum, competing with the well-developed colloidal lead chalcogenide quantum dots of the similar energy gap.


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Bright Colloidal PbS Nanoribbons - Chemistry of Materials (ACS

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