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Research Update: Preserving the photoluminescence efficiency of near infrared emitting nanocrystals when embedded in a polymer matrix Olga Solomeshch and Nir Tessler Citation: APL Mater. 4, 040702 (2016); doi: 10.1063/1.4947570 View online: http://dx.doi.org/10.1063/1.4947570 View Table of Contents: http://scitation.aip.org/content/aip/journal/aplmater/4/4?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Optimization of structural and dielectric properties of CdSe loaded poly(diallyl dimethyl ammonium chloride) polymer in a desired frequency and temperature window J. Appl. Phys. 119, 014108 (2016); 10.1063/1.4939162 Ag +12 ion induced modifications of structural and optical properties of ZnO-PMMA nanocomposite films AIP Conf. Proc. 1512, 394 (2013); 10.1063/1.4791077 Analysis of the forward and reverse bias I-V and C-V characteristics on Al/PVA:n-PbSe polymer nanocomposites Schottky diode J. Appl. Phys. 111, 074513 (2012); 10.1063/1.3698773 Dispersion of Cd X ( X = Se , Te ) nanoparticles in P3HT conjugated polymer J. Renewable Sustainable Energy 1, 023107 (2009); 10.1063/1.3101815 Size-tunable infrared (1000–1600 nm) electroluminescence from PbS quantum-dot nanocrystals in a semiconducting polymer Appl. Phys. Lett. 82, 2895 (2003); 10.1063/1.1570940 Reuse of AIP Publishing content is subject to the terms at: https://publishing.aip.org/authors/rights-and-permissions. Download to IP: 109.67.251.14 On: Mon, 25 Apr 2016 12:49:03

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Research Update: Preserving the photoluminescence efficiency of near infraredemitting nanocrystals when embedded in a polymer matrixOlga Solomeshch and Nir Tessler Citation: APL Mater. 4, 040702 (2016); doi: 10.1063/1.4947570 View online: http://dx.doi.org/10.1063/1.4947570 View Table of Contents: http://scitation.aip.org/content/aip/journal/aplmater/4/4?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Optimization of structural and dielectric properties of CdSe loaded poly(diallyl dimethyl ammonium chloride)polymer in a desired frequency and temperature window J. Appl. Phys. 119, 014108 (2016); 10.1063/1.4939162 Ag +12 ion induced modifications of structural and optical properties of ZnO-PMMA nanocomposite films AIP Conf. Proc. 1512, 394 (2013); 10.1063/1.4791077 Analysis of the forward and reverse bias I-V and C-V characteristics on Al/PVA:n-PbSe polymernanocomposites Schottky diode J. Appl. Phys. 111, 074513 (2012); 10.1063/1.3698773 Dispersion of Cd X ( X = Se , Te ) nanoparticles in P3HT conjugated polymer J. Renewable Sustainable Energy 1, 023107 (2009); 10.1063/1.3101815 Size-tunable infrared (1000–1600 nm) electroluminescence from PbS quantum-dot nanocrystals in asemiconducting polymer Appl. Phys. Lett. 82, 2895 (2003); 10.1063/1.1570940

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APL MATERIALS 4, 040702 (2016)

Research Update: Preserving the photoluminescenceefficiency of near infrared emitting nanocrystalswhen embedded in a polymer matrix

Olga Solomeshch and Nir TesslerMicroelectronic and Nanoelectronic Centers, Electrical Engineering Department, TechnionIsrael Institute of Technology, Haifa 32000, Israel

(Received 25 February 2016; accepted 14 April 2016; published online 21 April 2016)

Near infrared light emitting nanocrystals are known to lose efficiency whenembedded in a polymer matrix. One of the factors leading to reduced efficiency isthe labile nature of the ligands that may desorb off the nanocrystal surface whenthe nanocrystals are in the polymer solution. We show that adding trioctylphos-phine to the nanocrystal-poly(methylmethacrylate) solution prior to film castingenhances the photoluminescence efficiency. The solid films’ photoluminescencequantum efficiency values are reduced by less than a factor of two in the solidform compared to the solution case. We demonstrate record efficiency values of25% for lead sulfide nanocrystals solid films emitting at 1100 nm. C 2016 Au-thor(s). All article content, except where otherwise noted, is licensed under a CreativeCommons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).[http://dx.doi.org/10.1063/1.4947570]

Nanoscale inorganic materials display unique size, shape, and composition dependent elec-tronic and optical properties. These tunable properties have been implemented in nanocrystal (NC)based devices made of a single component,1,2 as a combination of different nanocrystals or compo-sitions,3 or in hybrid organic-inorganic devices where nanocrystals were interfaced with semicon-ducting polymers.4–10 Despite the progress made in many aspects of nanocrystal’s synthesis anddevices made of them, there seems to remain an issue that holds back some applications. There aremany reports that although the photoluminescence (PL) efficiency of near infrared (NIR) emittingNCs in solution could be 20% or above, the quantum efficiency (QE) measured for solid films isin the range of 1% and often below it.7 To maintain above 1% efficiency, a higher gap shell isintroduced11 and an almost 4%,12 or 12% for low NC loading films,13 is considered very high.

Our studies of the role of ligands in affecting the energy levels of nanocrystals2,3,10 have empha-sized the role of ligand exchange which is based on the fact that most ligands would dynamicallydetach and reattach to the nanocrystal’s surface. This has led us to postulate that diluting thenanocrystals in a matrix also dilutes the ligands and thus favors the situation where part of thenanocrystal’s surface becomes non-passivated thus dropping the PL efficiency. Ligand desorptionin dilute solutions and the role of surface coverage in affecting the emission quantum yield havebeen reported in Refs. 14–16. Here we report that adding excess ligands to the solution of, commer-cially available, lead sulfide PbS nanocrystal results is in PL efficiency, of NCs embedded in apoly(methylmethacrylate) (PMMA) matrix, of 25% and 12% for nanocrystals emitting at 1100 nmand 1500 nm, respectively.

For this study, we purchased two types of commercially available colloidal lead sulfide (PbS)nanocrystals (NCs) series C (SCR) from CAN GmbH (www.can-hamburg.com) as solutions intoluene. The NCs size distribution was slightly improved through size selection procedures carriedout by the manufacturer. A polymer host, poly(methylmethacrylate) PMMA solution in toluene,and additives of trioctylphosphine (TOP) and/or aniline, both purchased from Aldrich, were usedto prepare free standing films from polymer-NCs blends. The use of relatively thick films was toensure sufficient absorption in the NIR to ensure accurate efficiency measurements (see methodbelow).

2166-532X/2016/4(4)/040702/5 4, 040702-1 ©Author(s) 2016.

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040702-2 O. Solomeshch and N. Tessler APL Mater. 4, 040702 (2016)

To characterize the as-received NCs, we used a 1 mm quartz cuvette to hold the NCs toluenesolutions. Near-infrared (NIR)–visible absorption spectra were measured using UV-3101 PC spec-trophotometer (Shimadzu Scientific Instruments, Inc.) in 700–1700 nm region. Photoluminescence(PL) and photoluminescence quantum efficiency (PL QE) measurements were performed using theintegrated system, based on the FS920 fluorimeter (Edinburgh Instruments Ltd., UK), equippedwith liquid nitrogen cooled germanium photo-detector with lock-in amplification and an integratingsphere (Labsphere, Inc. IS-040-SL with UV-VIS-NIR reflectance coating). The labsphere was fibercoupled to the FS920 and excited by the monochromatic xenon lamp (450 W) light at 886 nm.The entire system response was normalized by a calibrated detector (Newport 818 IR) and amulti-function optical meter (Newport 1835C) in 800–1700 nm region. The PL QE was performedfollowing the procedure described in Ref. 17.

Figure 1 shows the absorption (dashed line) and emission (full line) spectra for the 1100 nm(Figure 1(a)) and the 1500 nm (Figure 1(b)) batches. The first excitonic peak absorption is foundto be at ∼1000 nm and ∼1400 nm and the PL emission peak is found to be at about 100 nm longerwavelength. The PL QE measurements described above yielded QE in solution of 45% and 20% forthe 1100 nm and 1500 nm batches, respectively.

Next, we moved to examine the PL properties of the NCs when embedded within a polymermatrix. As oxygen may affect the NC surface and lead to ligand desorption,18,19 all film preparationswere carried out inside an inert glove box (<1 ppm O2 and H2O). Free standing films were producedfrom blends of PMMA solution in toluene (60 mg/ml) with ∼4 wt. % (∼1 vol. %) of NCs and whereapplies also ∼5 vol. % of additives. The blend was prepared in a small glass Petri dish and stirred for1 h using a magnetic stirrer. The blend was kept inside the glove box during the solvent evaporationand once dry the film was easily removed from a glass and finally dried for 3 h in the vacuum ovenat 60 ◦C. Thicknesses of prepared films were in the range 0.2–0.3 mm.

The empty round symbols in Figure 2 show the absorption (dashed line) and emission (fullline) spectrum of the 1100 nm (Figure 2(a)) and 1500 nm (Figure 2(b)) batches in a PMMA matrix.We note that, compared to Figure 1, the excitonic peak in the absorption is broadened and lesspronounced and that the emission spectrum is red shifted. The PL QE was measured to be 10%and 6% for the 1100 nm and 1500 nm PMMA based films, respectively. The ∼4 fold reduction inPL QE together with the smearing of the excitonic peak and its red shift suggests that the surfaceof the nanocrystals was damaged and that despite the low volume fraction, aggregates might haveformed. The labile nature of the ligands is often used to perform ligand exchange2,20 and it is knownthat the process propagates as the original ligand desorbs and the excess amount of the new ligand

FIG. 1. Absorption (dashed line) and emission (full line) spectrum of the ∼1100 nm and the ∼1500 nm NCs measuredas solution in toluene. The corresponding quantum efficiency was 45% and 20% for the 1100 nm and 1500 nm batches,respectively.

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040702-3 O. Solomeshch and N. Tessler APL Mater. 4, 040702 (2016)

FIG. 2. Absorption (dashed line) and emission (full line) spectrum of the ∼1100 nm (a) and the ∼1500 nm (b) NCs measuredas solid films. The empty symbols are for PbS NCs in PMMA matrix and the full symbols are for the case where TOP isadded to the NC-PMMA solution prior to film casting. The quantum efficiency values of the TOP containing films were 25%and 12% for the 1100 nm and 1500 nm batches, respectively.

ensures that the adsorption would be of the new ligand. It is also known that the presence, or lackof, ligands affects the nanocrystal’s optical properties and that at least some of the ligands coveringthe NC may be removed in dilute solutions.14 Considering the labile nature of the ligands and theexcess amount of PMMA, it is most likely that at least some of the ligands were detached from theNCs surface and dispersed in the polymer solution and/or matrix. We thus attribute the decrease inthe NCs properties to this ligand desorption mechanism.

Assuming that our postulation of the ligand desorption is correct, one could follow the samelogic used in the ligand-exchange procedures and provide excess amount of free ligands such thatwhen a ligand is desorbed, there would be another ligand, as a reservoir, to take its place andre-passivate the surface. To this end, we repeated the film preparation procedure and this time added5 vol. % of TOP to the NC-PMMA solution. We chose TOP as it was shown that synthesis resultingin TOP capping produced highly luminescent PbS nanocrystals.21 The full circles in Figure 2 showthe absorption (dashed line) and emission (full line) spectrum of the 1100 nm (Figure 2(a)) and1500 nm (Figure 2(b)) batches in a PMMA+TOP matrix. Note that both the absorption and theemission characteristics are largely recovered following the addition of TOP. The PL QE measure-ments yielded efficiencies of 25% and 12% for the 1100 nm and 1500 nm batches on PMMA+TOPmatrix, respectively. Such efficiencies for NIR emitting NCs in a solid matrix are clearly recordvalues.

While adding TOP resulted in improved optical properties, we noted that the film became lessuniform and there was a clear vertical phase segregation. We tried using aniline instead of TOPand while the film properties were recovered, the optical properties were almost as if no ligand wasintroduced (PL QE was 14%). We next tested a 50:50 mix of TOP:aniline and found that both thefilm properties and the optical properties were improved although not as high as TOP only films.The PL QE of TOP:aniniline PMMA films was 20% for the 1100 nm batch. The PL spectrum takenas part of this TOP/aniline study is shown in Figure 3 clearly indicating the importance of TOP.

Finally, we need to ascertain that the role of TOP involves an attachment of TOP to thenanocrystals and at least partial exchange of the oleic acid ligands. To do so, we have followed theprotocol described in Ref. 2 where it was shown that partial exchange of ligands leads to stark shiftof the absorption peak. Knowing the wt. % of the nanocrystals in the solution, one can estimatethe total weight of ligands that are attached to the nanocrystals (assuming ideal coverage). Thena weighted amount of TOP is added to create the relative percentage shown in Figure 4. Figure 4shows the absorption spectrum around the peak of the first exciton peak for two PbS solutions. Thefirst (solid line) is for the as-received solution with all the nanocrystals being covered by oleic acid.The second (dashed line) is for a solution where TOP was added to form 20% of the total weight of

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040702-4 O. Solomeshch and N. Tessler APL Mater. 4, 040702 (2016)

FIG. 3. Emission spectrum of the ∼1100 nm NCs batch in a PMMA matrix and for three different additives. The roundsymbols are for TOP additive, the diamonds are for aniline, and the squares are for 50:50 TOP: aniline.

FIG. 4. Absorption spectrum of the first excitonic peak for as-received solution of nanocrystals and for a solution whereminute amount of TOP was added to. The inset shows the position of the peak’s center (following Gaussian fit) as a functionof the amount of TOP added.

ligands. Figure 4 shows a clear red shift and the inset shows that this shift increased between 0%,11%, and 20% in qualitative agreement with previously reported results for CdSe.2

To conclude, we have demonstrated the record PL efficiency values for nanocrystal based solidfilms emitting above 1000 nm. The commonly reported low efficiency values are attributed to liganddesorption that may be mediated by extrinsic presence of oxygen18,19 or by the labile nature theligands where the NCs are diluted in a, polymer, solution.14 By processing the films in inert atmo-sphere, we achieved PL QE of 10% and 6% for the 1100 nm and 1500 nm batches in PMMA, whichis similar to the values in Ref. 13. Adding the trioctylphosphine (TOP) to the NC-PMMA solutionprior to film casting enhanced the PL QE to record values of 25% and 12% for PbS nanocrystals

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040702-5 O. Solomeshch and N. Tessler APL Mater. 4, 040702 (2016)

solid films emitting at ∼1100 nm and ∼1500 nm, respectively. We expect that studies of differentligands (as hexadecylamine, HDA)14 may further boost the PL QE of solid films. As the method isgeneral, it should also improve the NCs performance when the polymer is a semiconducting one andthus lead to higher efficiency LEDs and possibly also to better photo-cells.22

Olga Solomeshch is grateful for the support by the Center for Absorption in Science of theMinistry of Immigrant Absorption and the Committee for Planning and Budgeting of the Councilfor Higher Education under the framework of the KAMEA Program.1 C. Piliego, L. Protesescu, S. Z. Bisri, M. V. Kovalenko, and M. A. Loi, Energy Environ. Sci. 6(10), 3054 (2013).2 N. Yaacobi-Gross, M. Soreni-Harari, M. Zimin, S. Kababya, A. Schmidt, and N. Tessler, Nat. Mater. 10(12), 974 (2011).3 N. Yaacobi-Gross, N. Garphunkin, O. Solomeshch, A. Vaneski, A. S. Susha, A. L. Rogach, and N. Tessler, ACS Nano 6(4),

3128 (2012).4 V. L. Colvin, M. C. Schlamp, and A. P. Alivisatos, Nature 370(6488), 354 (1994).5 N. Tessler, V. Medvedev, M. Kazes, S. H. Kan, and U. Banin, Science 295(5559), 1506 (2002).6 S. Coe, W. K. Woo, M. Bawendi, and V. Bulovic, Nature 420(6917), 800 (2002).7 J. S. Steckel, S. Coe-Sullivan, V. Bulovic, and M. G. Bawendi, Adv. Mater. 15(21), 1862 (2003).8 D. S. Koktysh, N. Gaponik, M. Reufer, J. Crewett, U. Scherf, A. Eychmuller, J. M. Lupton, A. L. Rogach, and J. Feldmann,

ChemPhysChem 5(9), 1435 (2004).9 S. A. McDonald, G. Konstantatos, S. G. Zhang, P. W. Cyr, E. J. D. Klem, L. Levina, and E. H. Sargent, Nat. Mater. 4(2),

138 (2005).10 M. Soreni-Harari, N. Yaacobi-Gross, D. Steiner, A. Aharoni, U. Banin, O. Millo, and N. Tessler, Nano Lett. 8(2), 678 (2008).11 M. J. Panzer, V. Wood, S. M. Geyer, M. G. Bawendi, and V. Bulovic, J. Disp. Technol. 6(3), 90 (2010).12 P. Moroz, G. Liyanage, N. N. Kholmicheva, S. Yakunin, U. Rijal, P. Uprety, E. Bastola, B. Mellott, K. Subedi, L. F. Sun,

M. V. Kovalenko, and M. Zamkov, Chem. Mater. 26(14), 4256 (2014).13 T.-W. F. Chang, A. Maria, P. W. Cyr, V. Sukhovatkin, L. Levina, and E. H. Sargent, Synth. Met. 148(3), 257 (2005).14 G. Kalyuzhny and R. W. Murray, J. Phys. Chem. B 109(15), 7012 (2005).15 M. Greben, A. Fucikova, and J. Valenta, J. Appl. Phys. 117(14), 144306 (2015).16 Z. Ning, O. Voznyy, J. Pan, S. Hoogland, V. Adinolfi, J. Xu, M. Li, A. R. Kirmani, J.-P. Sun, J. Minor, K. W. Kemp, H.

Dong, L. Rollny, A. Labelle, G. Carey, B. Sutherland, I. Hill, A. Amassian, H. Liu, J. Tang, O. M. Bakr, and E. H. Sargent,Nat. Mater. 13(8), 822 (2014).

17 J. C. deMello, H. F. Wittmann, and R. H. Friend, Adv. Mater. 9(3), 230 (1997).18 T. C. Kippeny, M. J. Bowers, A. D. Dukes, J. R. McBride, R. L. Orndorff, M. D. Garrett, and S. J. Rosenthal, J. Chem. Phys.

128(8), 084713 (2008).19 R. Ihly, J. Tolentino, Y. Liu, M. Gibbs, and M. Law, ACS Nano 5(10), 8175 (2011).20 M. Soreni-Harari, D. Mocatta, M. Zimin, Y. Gannot, U. Banin, and N. Tessler, Adv. Funct. Mater. 20(6), 1005 (2010).21 K. A. Abel, J. Shan, J.-C. Boyer, F. Harris, and F. C. J. M. van Veggel, Chem. Mat. 20(12), 3794 (2008).22 M. Tabachnyk, B. Ehrler, S. Gélinas, M. L. Böhm, B. J. Walker, K. P. Musselman, N. C. Greenham, R. H. Friend, and A.

Rao, Nat. Mater. 13(11), 1033 (2014).

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