Rapid Three-Dimensional Printing in Water Using Semiconductor

Letter pubs.acs.org/NanoLett

Rapid Three-Dimensional Printing in Water Using Semiconductor− Metal Hybrid Nanoparticles as Photoinitiators Amol Ashok Pawar,† Shira Halivni,† Nir Waiskopf,† Yuval Ben-Shahar,† Michal Soreni-Harari,‡ Sarah Bergbreiter,§ Uri Banin,*,† and Shlomo Magdassi*,† †

The Institute of Chemistry, the Hebrew University of Jerusalem, Edmond J. Safra campus, Givat Ram, Jerusalem, 91904, Israel Institute of Systems Research and §Department of Mechanical Engineering and Institute of Systems Research, University of Maryland, College Park, Maryland 20740, United States

‡

S Supporting Information *

ABSTRACT: Additive manufacturing processes enable fabrication of complex and functional three-dimensional (3D) objects ranging from engine parts to artificial organs. Photopolymerization, which is the most versatile technology enabling such processes through 3D printing, utilizes photoinitiators that break into radicals upon light absorption. We report on a new family of photoinitiators for 3D printing based on hybrid semiconductor−metal nanoparticles. Unlike conventional photoinitiators that are consumed upon irradiation, these particles form radicals through a photocatalytic process. Light absorption by the semiconductor nanorod is followed by charge separation and electron transfer to the metal tip, enabling redox reactions to form radicals in aerobic conditions. In particular, we demonstrate their use in 3D printing in water, where they simultaneously form hydroxyl radicals for the polymerization and consume dissolved oxygen that is a known inhibitor. We also demonstrate their potential for two-photon polymerization due to their giant two-photon absorption cross section. KEYWORDS: Semiconductor−metal hybrid nanoparticles, 3D printing, photopolymerization, photocatalysis

L

Semiconductor−metal HNPs manifest a synergistic property of light-induced charge separation at the semiconductor−metal interface that opened the path for their use as photocatalysts for water splitting and environmental applications.8,11−15 Recently, we reported that gold-tipped cadmium sulfide nanorods enable efficient light-induced generation of radicals, significantly enhanced compared to semiconductor nanocrystals.16 This is attributed to the efficient spatial charge separation combined with delayed competing electron−hole recombination and the catalytic function of the metal tip.16−18 Briefly, after absorption of a photon the formed excited electron and hole can drive reactions water and molecular oxygen to form reactive oxygen species (ROS), including hydroxyl and superoxide radicals.16 Our hypothesis was that the photogenerated ROS could initiate rapid polymerization of water-soluble monomers under low light intensity. Although semiconductor nanocrystals were reported to generate ROS in the presence of water and molecular oxygen,16,19−21 their utility in photopolymerization required high intensity UV lamps and irradiation for prolonged duration, unsuitable for 3D printing.21−25

ocalized photopolymerization in three-dimensional (3D) printing enables additive manufacturing of objects with micro nanoscale resolution.1−4 In water, such 3D printing is essential for fabrication of responsive hydrogels, bioscaffolds, and artificial organs.5−7 A powerful approach for 3D printing utilizes photoinitiators (PIs), organic molecules that upon light absorption break apart to form radicals for initiating the radical polymerization process. However, this process, although commonly used in nonaqueous systems, is limited in waterbased printing due to the absence of efficient water-soluble photoinitiators for the radical polymerization reaction and is also inhibited by the dissolved oxygen in water. We present a new type of photoinitiator, based on semiconductor−metal hybrid nanoparticles (HNPs)8,9 enabling rapid photopolymerization in aqueous systems. Unlike current water compatible photoinitiators (Supplementary Table S1), HNPs exhibit high absorption cross sections at operating wavelengths of commercially available digital light processing printers resulting in fast polymerization and printing. Their function as photoinitiators is uniquely based on a photocatalytic mechanism to form radicals while consuming dissolved oxygen enabling their use in 3D printing at ambient conditions. Unlike the common organic photoinitiators, the HNPs are not consumed during the light irradiation. Moreover, due to their giant two-photon absorption cross section10 they can be used in high-resolution 3D printing of submicron objects. © XXXX American Chemical Society

Received: May 4, 2017 Revised: June 8, 2017

A

DOI: 10.1021/acs.nanolett.7b01870 Nano Lett. XXXX, XXX, XXX−XXX

Letter

Nano Letters

Figure 1. The 3D printing using HNPs as photoinitiators. (a) Absorption spectra of CdS−Au HNPs (black), bare CdS rods (red) and I2959 (blue). Inset: TEM image of CdS−Au having rod dimensions of 37 × 4 nm (length × diameter) with 1.5 nm diameter gold tip. (b) Polymerization degree under UV light at 385 nm with intensity of 20 mW/cm2 using CdS−Au (black), CdS (red), and I2959 (blue) as photoinitiators. The CdS−Au and CdS are in the same concentration (0.4 μM), whereas the I2959 photoinitiator is in its optimized concentration for polymerization (0.02 M). The polymerization degree of acrylamide was calculated from the decrease of the Fourier transform infrared (FTIR) absorption peaks of methylene group vibrations at 988 cm−1 (assigned to the out-of-plane bending mode of the C−H unit) normalized to the CO stretch peak at 1654 cm−1 as an internal standard. The error bars are assigned to be the maximum error as extracted from triplicate measurements. (c−e) Images of a 3D printed Buckyball using CdS−Au HNPs as the photoinitiators and CdSe/CdS seeded nanorods as fluorescent markers. (c) Regular light photo. (d) Fluorescence image under 365 nm excitation. (e) Scanning electron microscopy image of the dried structure.

The HNPs enabled much faster and more complete photopolymerization of acrylamide compared with bare CdS nanorods and I2959. The 0.85 polymerization degree of the HNPs after 60 s at submicromolar concentrations is also the highest among reported water-compatible PIs (Supplementary Table S1).30 The superior polymerization degree of the HNPs in comparison with I2959 is directly related to their high absorption coefficient at the excitation wavelength compared with negligible absorption coefficient of the commercial I2959 PI. While bare CdS nanorods manifest nearly similar absorption as the HNPs, the superior performance of the latter is attributed to the electron−hole charge separation induced by the metal tip accompanied by its catalytic function as is analyzed in detail below. Additionally, polymerization using HNPs was also successfully performed under radiation in the visible (vis) range (450 nm), demonstrating the ability to utilize the HNPs as photoinitiators using a wide range of irradiation wavelengths (Supplementary Figure S2). Additional control experiments confirmed the action of the HNPs as the photoinitiators; aqueous monomer solutions without HNPs were studied with (a) PEI and (b) gold nanoparticles (1.5 nm diameter; same as the HNPs gold tip diameter). No polymerization was observed under the same conditions and even after prolonged UV exposure at higher intensity (300 s, 250 mW/cm2) for both systems (Supplementary Figure S3a, b). Aging of the monomer solutions with HNPs did not show any polymerization without light exposure (Supplementary Figure S3c).

Cadmium sulfide (CdS) nanorods with gold tips were synthesized using hot-injection method followed by selective metal growth (see Supporting Information).8,26 These hybrid nanorods were dispersed in water using polyethylenimine (PEI) as the dispersant,27 which enhances their photocatalytic activity allowing functionality in ROS formation.16,28 Figure 1a inset shows transmission electron microscopy (TEM) images of the matchstick-like hybrid nanorods composed of CdS nanorods (37 × 4 nm) with 1.5 nm diameter single gold tips (stronger contrast) grown selectively on their apex. Ideal PIs for common digital light processing (DLP) 3D printers should absorb strongly at 385−405 nm and generate free radicals efficiently. In order to study the photoinitiation performance of the CdS−Au HNPs, light absorption, and polymerization kinetics were studied and compared to CdS nanorods and the most commonly reported commercial watersoluble PI (Irgacure 2959, I2959). As shown in Figure 1a, unlike the commercial PI I2959, the CdS−Au HNPs and bare CdS rods both absorb at the wavelengths of interest. The molar extinction coefficients of both HNPs and CdS rods at 385 nm are similar, ∼107 M−1 cm−1 (1.65 Lg−1 cm−1) compared with the negligible value of I2959, ε = 0.0003 M−1 cm−1 (≈1.3 × 10−6 Lg−1 cm−1). This is also 4 orders of magnitude larger than recently reported water-compatible PIs of salts of phosphine oxides, ε = 256 M−1 cm−1 (≈0.58 Lg−1 cm−1 at 383 nm).29 The superior functionality of the HNPs is manifested in Figure 1b, comparing the polymerization kinetics of acrylamide monomers in aqueous solution (Supplementary Figure S1). B

DOI: 10.1021/acs.nanolett.7b01870 Nano Lett. XXXX, XXX, XXX−XXX

Letter

Nano Letters

Figure 2. Mechanism of polymerization using HNPs as photoinitiators. (a) A scheme of the photocatalytic polymerization of HNPs as PIs. Left: the possible routes for photocatalytic radical formation by HNPs upon light excitation in aqueous solution. The holes react with the water and hydroxide to form hydroxyl radicals, while the electrons consume the oxygen to form superoxide and hydroxyl radicals. Right: The radical polymerization reaction of acrylamide by the hydroxyl radicals. The inhibition reaction by oxygen is also presented. (b) Polymerization degree of acrylamide versus time using HNPs as photoinitiators in water (black), in the presence of sulfide/sulfite (red), or ethanol (blue) as hole acceptors. The addition of both leads to a decay in the polymerization rate of the monomers, indicating that the holes and hydroxyl radicals are essential to the polymerization. (c) Polymerization degree versus time for various HNPs concentrations: 0.1 μM (black) 0.2 μM (red), 0.3 μM (blue), 0.45 μM (orange), 1.8 μM (pink). The error bars are assigned to be the maximum error as extracted from triplicate measurements.

and hydroxide by holes thus producing hydroxyl radicals and (2) reduction of molecular oxygen by the electrons producing superoxide and/or hydrogen peroxide that can later decompose to hydroxyl radicals via photolysis.16 The importance and contribution of each route and of the different reactive oxygen species to the polymerization of aqueous monomer solutions was studied by adding of selective hole scavengers. Use of scavengers for affecting bulk polymerization behavior was previously reported for TiO2 nanoparticles.31 In our case of HNPs photoinitiators, hole scavengers are known to increase the efficiency of photocatalytic processes involving the excited electrons because they reduce the electron−hole back-recombination rate in the HNPs.28 Upon the addition of two types of hole acceptors, a mixture of sulfide/sulfite and ethanol, the polymerization rate slowed down (Figure 2b). These results point to the importance of the valence band holes and hydroxyl radicals to the polymerization process. Ethanol has a more pronounced effect although it is a weaker hole acceptor, consistent with its additional known role as a hydroxyl radical scavenger. The polymerization rate and degree were found to increase with the HNPs concentration (Figure 2c) while complete polymerization was observed at concentrations above 0.45 μM. Furthermore, we tested the polymerization following degassing of oxygen by purging with argon. Gelling occurred within 30 s at HNPs concentrations as low as 1 nM (Supplementary Figure S4). This effect of oxygen is known with common PIs because oxygen scavenges the radicals.32 The removal of oxygen during polymerization was previously reported for organic PIs by applying different techniques such as inert gas purging, covering of the resin with a solid or liquid oxygen barrier, changing the reaction temperature, insertion of chemical additives, and even

These HNPs were used for the 3D printing of hydrogels by a DLP printer (Freeform Plus 39, Asiga) with a UV-LED light source (385 nm; 17.5 mW/cm2, for printing parameters see Supplementary Table S2). Aqueous ink was prepared by dissolving 2.22 g of acrylamide and 2.22 g of cross-linker PEGylated diacrylate (PEGDA; SR 610) in 2.22 mL of TDW. Ten milliliters of HNPs aqueous dispersion (final concentration 0.5 μM) was added as photoinitiators. Twenty microliters of CdSe/CdS nanorods dispersion (final concentration 0.2 nM) was added as fluorescent markers to feature the additional capability of adding emissive functionality to the 3D printed objects. As seen in Figure 1c, a spherical C180 Buckyball hydrogel object containing 73% w/w water was successfully printed. The printed hydrogel fluoresced under UV light due to the presence of the CdSe/CdS nanorods that retained their emission throughout the 3D printing process (Figure 1d). The printed buckyball retained its structure even after drying as observed in the scanning electron microscope image (Figure 1e). A control experiment attempting similar printing with a commercial PI (Irgacure 2959) at 0.02 M, 5 orders of magnitude higher than the HNPs concentration has failed, even after increasing the irradiation time to 1.5 min per layer. An additional control experiment performed using similar concentration of CdS nanorods without the Au tip also failed, indicating the essential role of the synergistic effects akin to the HNP structure. Next, we address the mechanism of operation of HNP as novel PIs. Photoexcitation of HNPs results in formation of an electron−hole pair and fast intrinsic charge separation. In aqueous solutions, this subsequently leads to formation of freeradicals by two main routes (Figure 2a): (1) oxidation of water C

DOI: 10.1021/acs.nanolett.7b01870 Nano Lett. XXXX, XXX, XXX−XXX

Letter

Nano Letters chemical modification of the monomers structure.33 Uniquely, in our case the HNPs themselves actually consume the oxygen by the reduction processes of the excited electrons.16 Therefore, HNPs as PIs have a dual function of efficient production of hydroxyl radicals as well as oxygen scavenging, both leading to superior photopolymerization at aerobic conditions. Furthermore, unlike commonly used PIs, the HNPs initiate the polymerization reaction as photocatalysts that are not consumed during the process. A unique feature of semiconductor nanorods is their giant two-photon absorption cross section (∼107 GM),10,34,35 significantly larger than organic PIs for two-photon polymerization (the highest reported two-photon absorption cross section of photoinitiators used for 3D printing of hydrogels by two-photon polymerization is 200 GM at the wavelength of 780 nm, Supplementary Table S3).36 This should enable polymerization by two-photon absorption of the HNPs and hence is of direct relevance for two-photon polymerization 3D printers, which provides the highest resolution in 3D voxel printing.37 To study two-photon polymerization with HNPs, a short pulse laser (at 840 nm, 77 MHz, 150 fsec pulse duration, 670 mW, spot size radius 20 μm) was used to irradiate the solution containing monomers. One hundred microliters of monomer solution (10 g of acrylamide, 5 g of TDW, and 1 g of ethoxylated trimethylolpropane triacrylate) was mixed with 200 μL of HNPs (final concentration 1 μM), and 20 μL of green fluorescent CdSe/CdS nanorods as markers (final concentration 0.2 nM). Prior to irradiation, the aqueous mixture was purged with argon for 5 min to remove excess of oxygen and then exposed to the ultrafast laser for a duration of 10 min. The two-photon fluorescence of the marker nanorods could be visualized as presented in Figure 3a. Upon laser exposure, a polymerized hydrogel was formed within the cuvette, as demonstrated in Figure 3c,d.

In control experiments without the HNPs, or with ethanol as a hole and radical scavenger, no polymerization occurred under the same conditions. Actual two-photon printing using the HNPs as photoinitiators was performed using a NanoScribe Photonics Professional (GT) tool (Supporting Information and methods for details). The resultant spiral pattern with micrometer features is shown in Figure 3b. We have demonstrated the functionality of hybrid semiconductor−metal nanoparticles as a new type of photocatalytic photoinitiators. These nanoparticles are highly efficient in inducing photopolymerization reactions and enable the application of 3D printing in water. They also show high potential for two-photon printing owing to the unique quantum confined characteristics of the semiconductor component. Their function is photocatalytic unlike the commonly used PIs in 3D printing, which are disintegrated to radicals by photon absorption. Moreover, HNPs are highly versatile and tunable systems allowing for selective tuning of their functionality as PIs.11,38 The semiconductor and metal segments can be tuned in terms of their composition, size, shape, and relative location toward optimal performance in photopolymerization and in particular in 3D printing. This includes the following: (1) controlling the semiconductor band gap and its one and two-photon absorption characteristics to match the specific printing requirements such as wavelength of irradiation. (2) Tuning the relative energetics of the semiconductor and metal components to optimize the charge separation characteristics. (3) Designing the system toward selective efficient radical formation to achieve the best performance. (4) Flexible adjustment of the nanoparticle surface coating allows for its dispersibility in diverse matrices and solvents as needed in different printing scenarios. Therefore, we envision high potential for further realization of HNPs as PIs UV curable inks and in 3D printing applications.

■

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.nanolett.7b01870. Experimental details, and the following figures: Figure S1 shows FTIR spectrum of acrylamide used for the polymerization degree calculation. Figure S2 shows the polymerization degree of acrylamide using HNPs as PIs at different excitation wavelengths. Figure S3 shows the results of the following control experiments with only PEI ligands, with small diameter gold nanoparticles, and with no light radiation. Figure S4 shows the polymerization of acrylamide using HNPs in low concentration upon the removal of oxygen from the system (PDF)

■

AUTHOR INFORMATION

Corresponding Authors

Figure 3. Two-photon polymerization using HNPs as photoinitiators. (a) The near-infrared (NIR) excitation beam path can be visualized by the green fluorescence following two-photon excitation of the marker nanorods. (b) A spiral printed structure (120 × 120 × 16 μm) by a two-photon polymerization printer. Image taken in the Nanoscribe following laser writing. (c,d) A sample polymerized within the cuvette placed upside down, viewed under regular light (c) and under UV light (d).

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Shira Halivni: 0000-0002-6217-1871 Uri Banin: 0000-0003-1698-2128 Shlomo Magdassi: 0000-0002-6794-0553 D

DOI: 10.1021/acs.nanolett.7b01870 Nano Lett. XXXX, XXX, XXX−XXX

Letter

Nano Letters Author Contributions

(19) Ipe, B. I.; Lehnig, M.; Niemeyer, C. M. Small 2005, 1, 706−709. (20) Yamamoto, M.; Oster, G. J. Polym. Sci., Part A-1: Polym. Chem. 1966, 4, 1683−1688. (21) Zhang, D.; Yang, J.; Bao, S.; Wu, Q.; Wang, Q. Sci. Rep. 2013, 3, 1399. (22) Stroyuk, O. L.; Kuchmiy, S. Y.; Andryushina, N. S. Khim., Fiz. Tekhnol. Poverkhni 2015, 6, 67−84. (23) Liu, X. F.; Ni, X. Y.; Wang, J.; Yu, X. H. Nanotechnology 2008, 19, 485602. (24) Stroyuk, A. L.; Granchak, V. M.; Korzhak, A. V.; Kuchmii, S. Y. J. Photochem. Photobiol., A 2004, 162, 339−351. (25) Lobry, E.; Bt Bah, A. S.; Vidal, L.; Oliveros, E.; Braun, A. M.; Criqui, A.; Chemtob, A. Macromol. Chem. Phys. 2016, 217, 2321− 2329. (26) Mokari, T.; Sztrum, C. G.; Salant, A.; Rabani, E.; Banin, U. Nat. Mater. 2005, 4, 855−863. (27) Nann, T. Chem. Commun. 2005, 2005, 1735−1736. (28) Ben-Shahar, Y.; Scotognella, F.; Kriegel, I.; Moretti, L.; Cerullo, G.; Rabani, E.; Banin, U. Nat. Commun. 2016, 7, 10413. (29) Benedikt, S.; Wang, J.; Markovic, M.; Moszner, N.; Dietliker, K.; Ovsianikov, A.; Grützmacher, H.; Liska, R. J. Polym. Sci., Part A: Polym. Chem. 2016, 54, 473−479. (30) Pawar, A. A.; Saada, G.; Cooperstein, I.; Larush, L.; Jackman, J. A.; Tabaei, S. R.; Cho, N. J.; Magdassi, S. Sci. Adv. 2016, 2, e1501381. (31) Ye, J.; Ni, X. Y.; Dong, C. J. Macromol. Sci., Part A: Pure Appl.Chem. 2005, 42 (10), 1451−1461. (32) Bhanu, V. A.; Kishore, K. Chem. Rev. 1991, 91, 99−117. (33) Ligon, S. C.; Husar, B.; Wutzel, H.; Holman, R.; Liska, R. Chem. Rev. 2014, 114 (1), 557−589. (34) Feng, X. B.; Ji, W. Opt. Express 2009, 17, 13140−13150. (35) Li, X. P.; van Embden, J.; Chon, J. W. M.; Gu, M. Appl. Phys. Lett. 2009, 94, 103117. (36) Xing, J. F.; Liu, L.; Song, X. Y.; Zhao, Y. Y.; Zhang, L.; Dong, X. Z.; Jin, F.; Zheng, M. L.; Duan, X. M. J. Mater. Chem. B 2015, 3, 8486− 8491. (37) Xing, J. F.; Zheng, M. L.; Duan, X. M. Chem. Soc. Rev. 2015, 44, 5031−5039. (38) Cozzoli, P. D.; Pellegrino, T.; Manna, L. Chem. Soc. Rev. 2006, 35, 1195−1208.

A.A.P. and S.H. contributed equally to this work. A.A.P. prepared the monomer solutions, optimized 3D printing ink, and performed 3D printing. S.H. prepared the HNPs and together with N.W. designed and performed the experiments and collected and analyzed the data. Y.B.S. synthesized and characterized the semiconductor nanoparticles and HNPs; M.S.H. and S.B. planned and performed two-photon printing experiments; U.B. and S.M. led and guided the project. All authors discussed and interpreted the results and were involved in the writing. Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS This work was financially supported in part by the Israel Science Foundation (U.B., Grant 1560/13) and in part by the National Research Foundation of Singapore under the CREATE program (S.M.). We acknowledge E. Blayvas of the unit for nanocharacterization for assistance in the environmental scanning electron microscopy imaging performed at The Hebrew University Center for Nanoscience and Nanotechnology, Edmond J. Safra Campus, Jerusalem, Israel. We thank Dr. Y. Levi-Kalisman for providing small diameter Au nanocrystals for control experiments.

■

REFERENCES

(1) MacDonald, E.; Wicker, R. Science 2016, 353, aaf2093. (2) Kawata, S.; Sun, H. B.; Tanaka, T.; Takada, K. Nature 2001, 412, 697−698. (3) Tumbleston, J. R.; Shirvanyants, D.; Ermoshkin, N.; Janusziewicz, R.; Johnson, A. R.; Kelly, D.; Chen, K.; Pinschmidt, R.; Rolland, J. P.; Ermoshkin, A.; Samulski, E. T.; DeSimone, J. M. Science 2015, 347, 1349−1352. (4) Melchels, F. P. W.; Feijen, J.; Grijpma, D. W. Biomaterials 2010, 31, 6121−6130. (5) Murphy, S. V.; Atala, A. Nat. Biotechnol. 2014, 32, 773−785. (6) He, Y.; Yang, F. F.; Zhao, H. M.; Gao, Q.; Xia, B.; Fu, J. Z. Sci. Rep. 2016, 6, 29977. (7) Hockaday, L. A.; Kang, K. H.; Colangelo, N. W.; Cheung, P. Y. C.; Duan, B.; Malone, E.; Wu, J.; Girardi, L. N.; Bonassar, L. J.; Lipson, H.; Chu, C. C.; Butcher, J. T. Biofabrication 2012, 4, 035005. (8) Mokari, T.; Rothenberg, E.; Popov, I.; Costi, R.; Banin, U. Science 2004, 304, 1787−1790. (9) Costi, R.; Saunders, A. E.; Banin, U. Angew. Chem., Int. Ed. 2010, 49, 4878−4897. (10) Scott, R.; Achtstein, A. W.; Prudnikau, A.; Antanovich, A.; Christodoulou, S.; Moreels, I.; Artemyev, M.; Woggon, U. Nano Lett. 2015, 15, 4985−4992. (11) Banin, U.; Ben-Shahar, Y.; Vinokurov, K. Chem. Mater. 2014, 26, 97−110. (12) Simon, T.; Bouchonville, N.; Berr, M. J.; Vaneski, A.; Adrovic, A.; Volbers, D.; Wyrwich, R.; Doblinger, M.; Susha, A. S.; Rogach, A. L.; Jackel, F.; Stolarczyk, J. K.; Feldmann, J. Nat. Mater. 2014, 13, 1013−1018. (13) Wu, K. F.; Lian, T. Chem. Soc. Rev. 2016, 45, 3781−3810. (14) Wu, K.; Chen, J.; McBride, J. R.; Lian, T. Science 2015, 349, 632−635. (15) Han, Z. J.; Qiu, F.; Eisenberg, R.; Holland, P. L.; Krauss, T. D. Science 2012, 338, 1321−1324. (16) Waiskopf, N.; Ben-Shahar, Y.; Galchenko, M.; Carmel, I.; Moshitzky, G.; Soreq, H.; Banin, U. Nano Lett. 2016, 16, 4266−4273. (17) Ben-Shahar, Y.; Scotognella, F.; Waiskopf, N.; Kriegel, I.; Dal Conte, S.; Cerullo, G.; Banin, U. Small 2015, 11, 462−471. (18) Amirav, L.; Alivisatos, A. P. J. Am. Chem. Soc. 2013, 135, 13049− 13053. E

DOI: 10.1021/acs.nanolett.7b01870 Nano Lett. XXXX, XXX, XXX−XXX