Letter pubs.acs.org/JPCL
Tuning of Coupling and Surface Quality of PbS Nanocrystals via a Combined Ammonium Sulfide and Iodine Treatment Haitao Zhang,*,† Jun Yang,† Jiun-Ruey Chen,‡ James R. Engstrom,‡ Tobias Hanrath,‡ and Frank W. Wise† †
School of Applied and Engineering Physics and ‡School of Chemical and Biomolecular Engineering, Cornell University, Ithaca, New York 14853, United States S Supporting Information *
ABSTRACT: Surface states of colloidal nanocrystals are typically created when organic surfactants are removed. We report a chemical process that reduces surface traps and tunes the interparticle coupling in PbS nanocrystal thin films after the surfactant ligands have been stripped off. This process produces PbS/PbI2 core/shell nanocrystal thin films via a combined ammonium sulfide and iodine treatment. These all-inorganic nanocrystal thin films are air-stable and exhibit bright emission with optimum photoluminescence quantum yield close to that of pristine PbS nanocrystals passivated by oleate ligands. Interparticle coupling of post-treatment nanocrystal thin films is continuously tunable by varying the iodine treatment process. Optical studies reveal that this method can produce PbS nanocrystal thin films superior in both coupling and surface quality to nanocrystals linked by small molecules such as ethanedithiol or 3-mercaptopropionic acid.
C
olloidal semiconductor nanocrystals (NCs) have emerged as promising new optoelectronic materials due to their size-tunable properties and low-cost solution-processing fabrication.1−6 Optoelectronic devices require facile charge transport, but bulky organic surfactant ligands create insulating barriers and block electronic communication between NCs. Tremendous advances have been made in replacing these longchain ligands with small molecules to increase interparticle coupling.6−13 Such surface modifications, however, typically yield electronic trap states that hamper charge transport. Our previous studies show that (NH4)2S treatment can effectively remove insulating organic surfactants and create allinorganic-connected NC assemblies (Figure 1).13 Such
and PbS (Figure 1). Photoluminescence (PL) quantum yield of the treated PbS NC thin films can reach 5%, close to that of pristine PbS NC thin films with oleate surfactant ligands. We use the ratio of PL intensity in thin films of large-bandgap NCs that contain a small concentration of small-bandgap NCs to characterize the interparticle coupling19 and use the PL lifetime of isolated small-bandgap NCs to characterize the NC surface quality. The results reveal that the (NH4)2S/I2 treatment can produce PbS NC thin films better in both interparticle coupling and surface quality than NCs linked by small molecules that have hydrocarbon chains with fewer than six carbons. Moreover, the interparticle coupling of PbS NC thin films produced by this method is tunable by controlling the I2 treatment process. Thus, this method can be tailored for different applications, which may require trade-offs between exciton dissociation and recombination. The chemical treatment of NCs involves two steps. First, the surfactant ligands are removed by our previously developed method with modifications.13 In a typical experiment, a spincoated PbS NC film (with oleate surfactant ligands; ∼40 nm thick) on a substrate (Si or glass slide) is dipped into a 0.2 mM (NH4)2S CH3CN/methanol (4000/1, in volume) anhydrous solution for 30 s. This is followed by washing in CH3CN for 30 s and in CCl4 for another 30 s to remove residue consisting of (NH4)2S and replaced organic ligand. FTIR studies confirm the removal of organic surfactant ligands (Figure S1, Supporting Information). This ligand removal process quenches the PL of PbS NC thin films,13 which can be attributed to a combination
Figure 1. Schematic of PbS NCs surface modification by reacting with (NH4)2S and I2.
connected NC assemblies exhibit strong interparticle coupling but suffer from a high density of surface defects. The formation of inorganic shells such as CdS14 has been used to provide better passivation for colloidal lead chalcogenide NCs. Halide ions and molecules are also used to treat PbS NCs in both solutions and thin films to passivate surface defects,6,15−17 and among those iodide (I−) exceeds others in performance.18 Here we report a chemical process that reduces the density of surface trap states in (NH4)2S-treated PbS NCs by converting surface PbS into PbI2, through the redox reaction between I2 © XXXX American Chemical Society
Received: December 17, 2015 Accepted: January 25, 2016
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DOI: 10.1021/acs.jpclett.5b02813 J. Phys. Chem. Lett. 2016, 7, 642−646
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The Journal of Physical Chemistry Letters
Figure 2. (a) Selected PL spectra of (NH4)2S-treated PbS NCs during I2 treatment. (b) Summarized PL peak intensity (blue) and center energy (red) during I2 treatment.
Figure 3. X-ray photoelectron spectra of (a) I(3d) and (b) Pb(4f) regions for PbS NC thin film treated in 0.005 mM I2/CCl4 solution for 720 min.
peak (Figure S2a, Supporting Information), which confirms that the NCs retain quantum confinement. Scanning electron microscopy (SEM) studies also show that the NCs retain their shapes after reacting with iodine (Figure S2b, Supporting Information). The post-treated PbS NC thin film exhibits good stability in air. Exposing the thin film to air for 10 h does not cause detectable changes on PL intensity and peak position. The etching of PbS NCs is faster in higher concentration iodine solutions. An etching speed of ∼0.6 nm/h is observed when PbS NC thin film is treated in a 0.05 mM I2/CCl4 solution, but the maximum PL quantum yield of PbS NC thin film in this treatment is 6) exhibit slightly better surface quality than (NH4)2S/I2-treated NCs with the same coupling. In addition, our method provides versatile tunability on the optoelectronic properties of NC thin
REFERENCES
(1) Talapin, D. V.; Murray, C. B. PbSe Nanocrystal Solids for n- and p-Channel Thin Film Field-Effect Transistors. Science 2005, 310, 86− 89. (2) Liu, Y.; Tolentino, J.; Gibbs, M.; Ihly, R.; Perkins, C. L.; Liu, Y.; Crawford, N.; Hemminger, J. C.; Law, M. PbSe Quantum Dot FieldEffect Transistors with Air-Stable Electron Mobilities above 7 cm2 V−1 s−1. Nano Lett. 2013, 13, 1578−1587. (3) Wise, F. W. Lead Salt Quantum Dots: the Limit of Strong Quantum Confinement. Acc. Chem. Res. 2000, 33, 773−780. (4) Ip, A. H.; Thon, S. M.; Hoogland, S.; Voznyy, O.; Zhitomirsky, D.; Debnath, R.; Levina, L.; Rollny, L. R.; Carey, G. H.; Fischer, A.; et al. Hybrid Passivated Colloidal Quantum Dot Solids. Nat. Nanotechnol. 2012, 7, 577−582. (5) Sun, L.; Choi, J. J.; Stachnik, D.; Bartnik, A. C.; Hyun, B.-R.; Malliaras, G. G.; Hanrath, T.; Wise, F. W. Bright Infrared QuantumDot Light-Emitting Diodes through Inter-Dot Spacing Control. Nat. Nanotechnol. 2012, 7, 369−373. (6) Tang, J.; Kemp, K. W.; Hoogland, S.; Jeong, K. S.; Liu, H.; Levina, L.; Furukawa, M.; Wang, X.; Debnath, R.; Cha, D.; et al. Colloidal-Quantum-Dot Photovoltaics Using Atomic-Ligand Passivation. Nat. Mater. 2011, 10, 765−771. 645
DOI: 10.1021/acs.jpclett.5b02813 J. Phys. Chem. Lett. 2016, 7, 642−646
Letter
The Journal of Physical Chemistry Letters (7) Kovalenko, M. V.; Scheele, M.; Talapin, D. V. Colloidal Nanocrystals with Molecular Metal Chalcogenide Surface Ligands. Science 2009, 324, 1417−1420. (8) Nag, A.; Kovalenko, M. V.; Lee, J.-S.; Liu, W.; Spokoyny, B.; Talapin, D. V. Metal-free Inorganic Ligands for Colloidal Nanocrystals: S2‑, HS‑, Se2‑, HS‑, Te2‑, HTe−, TeS32‑, OH−, and NH2‑ as Surface Ligands. J. Am. Chem. Soc. 2011, 133, 10612−10620. (9) Dong, A.; Ye, X.; Chen, J.; Kang, Y.; Gordon, T.; Kikkawa, J. M.; Murray, C. B. A Generalized Ligand-Exchange Strategy Enabling Sequential Surface Functionalization of Colloidal Nanocrystals. J. Am. Chem. Soc. 2011, 133, 998−1006. (10) Law, M.; Luther, J. M.; Song, Q.; Hughes, B. K.; Perkins, C. L.; Nozik, A. J. Structural, Optical, and Electrical Properties of PbSe Nanocrystal Solids Treated Thermally or with Simple Amines. J. Am. Chem. Soc. 2008, 130, 5974−5985. (11) Luther, J. M.; Law, M.; Song, Q.; Perkins, C. L.; Beard, M. C.; Nozik, A. J. Structural, Optical, and Electrical Properties of SelfAssembled Films of PbSe Nanocrystals Treated with 1,2-Ethanedithiol. ACS Nano 2008, 2, 271−280. (12) Dirin, D. N.; Dreyfuss, S. b.; Bodnarchuk, M. I.; Nedelcu, G.; Papagiorgis, P.; Itskos, G.; Kovalenko, M. V. Lead Halide Perovskites and Other Metal Halide Complexes As Inorganic Capping Ligands for Colloidal Nanocrystals. J. Am. Chem. Soc. 2014, 136, 6550−6553. (13) Zhang, H.; Hu, B.; Sun, L.; Hovden, R.; Wise, F. W.; Muller, D. A.; Robinson, R. D. Surfactant Ligand Removal and Rational Fabrication of Inorganically Connected Quantum Dots. Nano Lett. 2011, 11, 5356−5361. (14) Pietryga, J. M.; Werder, D. J.; Williams, D. J.; Casson, J. L.; Schaller, R. D.; Klimov, V. I.; Hollingsworth, J. A. Utilizing the Lability of Lead Selenide to Produce Heterostructured Nanocrystals with Bright, Stable Infrared Emission. J. Am. Chem. Soc. 2008, 130, 4879− 4885. (15) Bae, W. K.; Joo, J.; Padilha, L. A.; Won, J.; Lee, D. C.; Lin, Q.; Koh, W.-k.; Luo, H.; Klimov, V. I.; Pietryga, J. M. Highly Effective Surface Passivation of PbSe Quantum Dots through Reaction with Molecular Chlorine. J. Am. Chem. Soc. 2012, 134, 20160−20168. (16) Jähnig, F.; Bozyigit, D.; Yarema, O.; Wood, V. Comparison of Salt- and Molecular-Based Iodine Treatments of PbS Nanocrystal Solids for Solar Cells. APL Mater. 2015, 3, 020701. (17) Lan, X.; Voznyy, O.; Kiani, A.; Garcia de Arquer, F. P.; Abbas, A. S.; Kim, G. H.; Liu, M.; Yang, Z.; Walters, G.; Xu, J.; et al. Passivation Using Molecular Halides Increases Quantum Dot Solar Cell Performance. Adv. Mater. 2016, 28, 299−304. (18) Ning, Z.; Voznyy, O.; Pan, J.; Hoogland, S.; Adinolfi, V.; Xu, J.; Li, M.; Kirmani, A. R.; Sun, J.-P.; Minor, J.; et al. Air-Stable n-Type Colloidal Quantum Dot Solids. Nat. Mater. 2014, 13, 822−828. (19) Zhitomirsky, D.; Voznyy, O.; Hoogland, S.; Sargent, E. H. Measuring Charge Carrier Diffusion in Coupled Colloidal Quantum Dot Solids. ACS Nano 2013, 7, 5282−5290. (20) Nefedov, N. I.; Salyn, Ya. V.; Keller, H. X-ray Electron Studies of Lead and Mercury Compounds. Zh. Neorg. Khim. 1979, 24, 2564− 2566. (21) Liu, Y.; Gibbs, M.; Puthussery, J.; Gaik, S.; Ihly, R.; Hillhouse, H. W.; Law, M. Dependence of Carrier Mobility on Nanocrystal Size and Ligand Length in PbSe Nanocrystal Solids. Nano Lett. 2010, 10, 1960−1969. (22) Choi, J. J.; Lim, Y.-F.; Santiago-Berrios, M. E. B.; Oh, M.; Hyun, B.-R.; Sun, L.; Bartnik, A. C.; Goedhart, A.; Malliaras, G. G.; Abruna, H. D.; et al. PbSe Nanocrystal Excitonic Solar Cells. Nano Lett. 2009, 9, 3749−3755.
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