Reusable Inorganic Templates for Electrostatic Self-Assembly of

Letter pubs.acs.org/NanoLett

Reusable Inorganic Templates for Electrostatic Self-Assembly of Individual Quantum Dots, Nanodiamonds, and Lanthanide-Doped Nanoparticles Mingming Jiang,† Jonathan A. Kurvits,† Yao Lu,‡ Arto V. Nurmikko,†,§ and Rashid Zia*,†,§ †

Department of Physics, ‡Department of Chemistry, and §School of Engineering, Brown University, Providence, Rhode Island 02912, United States S Supporting Information *

ABSTRACT: In this paper, we present an electrostatic self-assembly method for the controlled placement of individual nanoparticle emitters based on reusable inorganic templates. This method can be used to integrate quantum emitters into nanophotonic structures over macroscopic areas and is applicable to a variety of patterning materials and emitter systems. By utilizing surface-charge-mediated self-assembly, highly ordered arrays of nanoparticle emitters were created. To illustrate the broad applicability of this technique, we demonstrate self-assembly using colloidal quantum dots (QD), nitrogen vacancy (NV) centers in diamond nanoparticles, and lanthanide-doped upconversion nanoparticles (UCNP). Placement of single QDs and NV centers was confirmed by performing photon antibunching measurements using a Hanbury-Brown Twiss setup. In addition, template reusability was demonstrated through daily redeposition experiments over a one month period. KEYWORDS: Colloidal quantum dots, diamond nitrogen-vacancy centers, isoelectric point, self-assembly, reusability, upconverting nanoparticles

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offer direct control over particle density down to the single emitter level.22 However, such polymer templates are not robust; they readily degrade and cannot be easily cleaned nor reused. While all of the aforementioned techniques can be applied (with varying difficulty and precision) to position quantum emitters near optical nanostructures, they all suffer from a common problem: they are single use methods. Once fabricated, the coupled emitter-structure patterns cannot be easily modified or reused. For example, if the quantum emitters photobleach, there is no way to easily replace them. Or if you want to investigate the coupling of different quantum emitters near the exact same nanostructure, additional nanofabrication steps or new samples are required. This process can be timeconsuming and wasteful, and more importantly, there will always be unknown variations between different samples. Thus, it can be difficult to perform statistical characterizations where

emiconductor QDs and NV centers in diamond nanoparticles have received considerable attention as robust solid-state single photon emitters1−4 in a wide range of applications from fundamental studies of light-matter interactions5−9 to emerging quantum information technology.10,11 The emission properties (e.g., spectral, angular, and polarization distributions) depend not only on the emitter’s intrinsic electronic structure but also its local optical environment. Therefore, a reliable and stable emitter-structure system is required to investigate these characteristics. Furthermore, for applications such as quantum information processing, one would like to position arrays of single photon sources, while maintaining their compatibility for integration with nanostructures. As a result, high specificity positioning of individual emitters near optical nanostructures has become a significant field of study. To address this issue, several techniques have been developed, including lithographic patterning,12−14 patterned chemical-functionalization,7,15 dip-pen techniques,16 capillarybased assembly,17,18 DNA/protein-mediated assembly,19,20 and polymer-based electrostatic assembly.21,22 For example, electrostatic self-assembly using polymer templates has been shown to © XXXX American Chemical Society

Received: March 13, 2015 Revised: July 23, 2015

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Figure 1. Electrostatic self-assembly of silica-clad QD arrays. (a) Schematic illustrating the self-assembly process. Inset shows a transmission electron microscope image of the silica-clad QDs. (b) SEM images of silica-clad QDs sitting on Al2O3 pads. Top three insets show 160 nm silica-clad QDs on different size pads and bottom inset shows 30 nm silica-clad QD on a 50 nm pad. (c,d,e) Dark-field microscope images of 160 nm silica-clad QDs on 160 nm pads with a 2 μm pitch composed of different dielectric materials, including Al2O3, Y2O3, and MgO. (f) Dark-field image showing the same deposition result as SEM image in panel (b).

SiOH + OH−⇌ SiO− + H2O).24,25 As a result, the negatively charged silica-clad QDs are attracted to the positively charged Al2O3 pads (and repelled from the native oxide elsewhere on the wafer). This combined effect of pad attraction and substrate repulsion allows for very selective and precise positioning of silica-clad QDs at desired locations. To demonstrate this principle, both custom synthesized (CdSe-core CdS/Zn0.5Cd0.5S/ZnS-multishell, emission peak ∼600 nm) and commercial (CdSe/ZnS, emission peak ∼620 nm, NNLabs) QDs were encapsulated by silica shells of different thicknesses using a water-in-oil microemulsion26−28 growth technique (see Supporting Information). Such silicaclad QDs were previously combined with positively charged polymer templates to achieve large ordered arrays of single photon emitters.22 Here, in this study arrays of ∼20 nm thick Al2O3 pads were fabricated on silicon and quartz substrates by e-beam lithography, Al2O3 sputtering, and subsequent lift-off. The sample was immersed into the silica-clad QD solution (2.5 mg/L) for 8 h to ensure good coverage on the Al2O3 pads. Finally, the sample was dried with nitrogen, during and after which the positions of the silica-clad QDs were maintained by van der Waals forces.29 A schematic for the process is shown in Figure 1a (for more details, see Figure S2 in Supporting Information). The silica-clad QD self-assembly results were checked for different pad sizes by scanning electron microscopy (SEM), as shown in Figure 1b. When the Al2O3 pad diameter is close to the silica-clad QD diameter (160 nm), each pad has only one silica sphere, containing a single QD. As the pad size increases, the average number of silica-clad QDs on each Al2O3 pad also increases, as shown in the inset of Figure 1b. Thus, by matching the pad size with the diameter of a given silica sphere, it is

variations in the emission can be deconvolved from variations of the nanostructures themselves. As a result, there is a need for iterative placement of different quantum emitters on the same nanostructures. Here, we present an inorganic, reusable electrostatic selfassembly method that allows for precise positioning and easy integration of individual nanoparticle emitters with large-scale nanostructure arrays. By taking advantage of the opposite surface charge in solution of silica-clad QDs and other oxide materials (e.g., Al2O3, Y2O3, and MgO), we demonstrate scalable self-assembly of colloidal QDs on predefined pad arrays. Then, we demonstrate the reusability of these Al2O3 templates by redepositing new silica-clad QDs on the same sample multiple times while maintaining precise single QD placement. To verify the single QD placement, we perform photon antibunching measurements using an HBT setup. We then illustrate how this approach can be readily extended to other emitter systems, including NV centers in diamond nanoparticles and lanthanide-doped UCNPs. Reusable Electrostatic Self-Assembly for Arrays of Silica-Clad QDs. The isoelectric point (IEP) of a material is defined to be the pH value at which the material’s surface has no net electrical charge in solution. If a material’s IEP is greater than the pH of the solution, its surface charge is positive, whereas the surface charge is negative if the IEP is below the solution’s pH. For example, the typical IEP of SiO2 is 1.7−3.5, whereas the IEP of Al2O3 is 8−9.23 Thus, when a silicon substrate coated with Al2O3 pads is dipped into a primarily ethanol silica-clad QD solution that has an effectively neutral pH value, Al2O3 acquires a positive surface charge (AlOH +H+ ⇌ AlOH+2 ),24 while the silica-clad QDs and native oxide on the silicon wafer both acquire a negative surface charge ( B

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Figure 2. Reusability experiments for QD self-assembly on Al2O3 pad array. (a) Optical dark-field image of pad array sample without QDs. (b−h) Dark-field images demonstrating QD coverage. All eight images are for the same array on the same sample. (d) Day 8 has a low QD coverage due to the buildup of contaminants. (i) QD deposition results for the one-month-long experiment.

Figure 3. Photon antibunching experiment for QDs on Al2O3 pad array. (a) Confocal image for a 40 μm × 40 μm area showing QD arry with 2 μm pitch. (b) Confocal image for a 12 μm × 12 μm area. The 25 bright sites are marked by green (number of QDs is 1), blue (number of QDs is more than 1), and red (insufficient signal to fit) circles. (c) g(2)(t) curves for all the bright sites in the scan image, g(2)(0) < 0.5 is shown in green frames and ≥0.5 is shown in blue ones. (The 0.5 threshold is indicated by the dashed black line.) Red frames had insufficient counts for analysis, and black frames indicate dark spot where no measurement was performed. The temporal span of each histogram is 118 ns.

SEM and dark-field images were used to validate this technique. For example, Figure 1f shows a 5 × 5 array that is missing only one silica-clad QD on the central pad, which is consistent with the SEM image in Figure 1b. This self-assembly method is fairly general as any dielectric material can be used for the patterned pads as long as the pH of the solvent is between the IEP of SiO2 and that of the dielectric pattern. To demonstrate this, we made three patterned samples with identical (160 nm) pad diameters using several different dielectric materials, Al2O3, Y2O3 (IEP 7−9), and MgO (IEP 12−13),23 and dipped them into the same silica-clad QD (160 nm diameter) solution. Optical dark-field images in Figure 1c−

possible to achieve controllable placement of individual QDs with very high accuracy. Since the QDs are centered in the silica spheres, the positioning resolution of this technique is limited by the diameter of the silica cladding and the size of pads. Here, we have fabricated silica spheres as small as 30 nm on 50 nm Al2O3 pads. Note that the size of Al2O3 pads used here is limited by our fabrication equipment and methods (i.e., sputtering Al2O3 complicates lift-off for smaller structure dimensions). Dark-field microscopy, as shown in Figure 1c−f, was also used to check the deposition results for pads composed of Al2O3 as well as other materials. Side-by-side comparisons of C

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Figure 4. Silica cladding of lanthanide-doped UCNPs. (a) SEM image of silica-clad UCNPs showing single shell occupancy. (b) Dark-field microscope image of a 40 μm × 40 μm array of self-assembled silica-clad UCNPs showing high pad occupancy. (c) Confocal scan image of a 12 μm × 12 μm array of UCNPs.

avalanche photodiodes (PicoQuant, τ-SPAD) by a 50/50 beam splitter and time-correlated single-photon counts were recorded with a PicoHarp 300 (PicoQuant). The confocal scan image of fluorescence from a 40 μm × 40 μm array with a 2 μm pitch is shown in Figure 3a demonstrating high QD coverage. The differences in fluorescence intensity at each pad are most likely from variations in bare QD size and crystal orientation30 or empty/multiple QD occupied silica spheres. To obtain the photon antibunching results, a smaller area (12 μm × 12 μm) of the QD array was scanned, as shown in Figure 3b. Twentyfive sites (marked with circles) were chosen to perform antibunching measurements, while 11 sites were determined to be too dim. Then the stage was moved to focus on each bright site, after which QDs were pumped for 10 minutes and intensity correlation histograms were obtained. After normalizing the histograms by the average photon counts at long times, all traces were fit to a two-level model31

e show the resulting deposition in which all three materials yield almost 100% coverage. After the QD-on-pad structure is formed, electrostatic attraction is strong enough to adhere the silica-clad QDs to the pads for optical characterization. However, the silica-clad QDs can be easily removed from the Al2O3 pads by sonication in a solvent (e.g., acetone or ethanol). Once cleaned, the patterned substrate can be dipped back into the QD solution for redeposition on the same Al2O3 pads and high pad occupancy is recovered. To demonstrate this reusability, a onemonth-long QD redeposition experiment was carried out. A new Al2O3 template was fabricated with the same pad size (160 nm diameter and 20 nm thickness) as the one shown in Figure 1b. This sample was dipped into the same silica-clad QD solution overnight for 12 h to ensure high coverage. Following self-assembly, QD coverage was determined from dark-field images, as shown in Figure 2b−g. The sample was then sonicated in acetone to remove the silica-clad QDs, which was again confirmed by dark-field microscopy, Figure 2a. After cleaning with ethanol, the sample was submerged again in the QD solution overnight for 12 h. This process of QD deposition and cleaning was repeated daily for one month. Note that on days 8 and 9, QD coverage dropped to almost zero, as shown in Figure 2d. This was due to both the contamination of the QD solution from repeated use and buildup of contaminants on the sample surface. However, ∼100% coverage could be recovered by a more thorough solvent cleaning process (5 min in acetone, 5 min in methanol, 5 min in ethanol, all with sonication) and replacement of the QD solution. After day 9, the same thorough cleaning process and solution replacement was repeated every 3 days. The result of this one month deposition process, shown in Figure 2i, demonstrates very high (>95%) coverage for most trials over this long time span. To show the long-term stability of this template, we redeposited on the same sample after 6 months and still obtained high coverage, as shown in Figure 2h. Photon Antibunching Experiments. To verify that there were single QDs on each pad in the arrays made from the above method, room-temperature antibunching measurements were performed using an HBT setup combined with a scanning confocal microscope. The QD arrays on a quartz sample were mounted on a piezo-electric nanopositioning stage (Mad City Labs, Nano-View/M) and were pumped by a continuous wave 532 nm laser (Coherent, Verdi) with incident power of 20 μW which was focused by a 100×, 1.3 NA oil-immersion objective. The fluorescent emission was collected by the same objective and coupled into a 105 μm diameter multimode fiber. The emission was directed onto two identical single photon

g(2)(t ) = 1 −

1 −|t | / τ e n

(1)

which allowed us to obtain the number of QDs per silica sphere. Here, g(2)(t) is the intensity correlation function, n is the number of quantum emitters, τ is the emitter lifetime, and t is the time difference between detection events at the two SPADs. The green and blue circles in Figure 3b denote cases where single or multiple QDs were measured. Note that some of the silica-clad QDs blinked off or became photobleached when being pumped continuously during the 10 minutes-long measurements, as marked by red circles in Figure 3b. These QDs did not give sufficient signal to provide a meaningful g(2). The result of all fits are displayed in Figure 3c in which 16 of them are clearly single QDs (i.e., exhibit g(2)(0) < 0.5) and 5 have multiple QDs. Extension of Technique to Other Nanoparticle Emitters. In addition to QDs, there are a wide variety of other emitter nanoparticles that are used in nanophotonic applications. Importantly, the method presented here is quite general and can be readily extended to other emitter systems. Note that there are two ways to apply this technique to other systems. For high precision control of individual particles, one can use the full encapsulation and self-assembly method described above. Alternatively, for nanoparticle emitters with an intrinsically low or high IEP (or a surface coating that gives them an effectively low or high IEP), it is also possible to perform self-assembly without silica encapsulation. As examples of these two approaches, we demonstrate self-assembly with two canonical emitter systems, lanthanide-doped upconverting D

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Figure 5. Reusability experiment for diamond nanoparticles containing NV centers. (a) SEM image showing electrostatic self-assembly on Al2O3 pad arrays. Inset shows nanodiamonds on pads of different diameters (70, 100, 200, and 300 nm). (b−e) Dark-field images showing four redeposition cycles of nanodiamonds on Al2O3 pad arrays. Top images show arrays after deposition and bottom ones show them after sonication in acetone.

Figure 6. Photon antibunching experiment for NV centers on predefined Al2O3 pad arrays. (a) Confocal image for a 40 μm × 40 μm area showing NV center array with 2 μm pitch. (b) Confocal for a 12 μm × 12 μm area. The 29 bright sites are marked by green (number of NVs is 1) and blue (number of NVs is more than 1). (c) g(2)(t) curves for all the bright sites in the scan image, g(2)(0) < 0.5 is shown in green frames and ≥0.5 is shown in blue ones. (The 0.5-threshold is indicated by the dashed black line.) Black frames indicate the dark spot where no measurement was performed. The temporal span of each histogram is 100 ns.

nanoparticles (UCNPs) and diamond nanoparticles with luminescent nitrogen vacancy (NV) centers. Because of the shielded 4f valence structure of lanthanide ions, dopants in UCNPs often behave as isolated, atomic-like systems that exhibit visible emission upon near-infrared excitation. As a result, UCNPs have garnered recent interest for low-background, high-contrast biological imaging as well as integration with optical nanostructures.32−39 To demonstrate self-assembly of UCNPs, commercially available oleic acid (OA) coated NaYF4, Er3+, and Yb3+ doped nanoparticles (Mesolight LLC) were encapsulated with silica using the same process as was used for QDs. Note that due to their larger size, initial attempts at encapsulation lead to significant clumping

and multiparticle occupancy. However, single particle occupancy was obtained by aggressive sonication in hexane prior to the silica growth process. These results are shown in Figure 4a, where single nanoparticle encapsulation is clearly demonstrated. Al2O3 pad arrays on silicon substrates were then dipped in the silica-clad UCNP solution overnight and self-assembly was then confirmed via dark-field imaging and confocal scans. As can be seen in Figure 4b,c, high particle coverage was achieved, similar to the results for silica-clad QDs. However, encapsulation is not always required, because many solid-state emitters have an intrinsically low IEP. For example, negatively charged NV centers have become an important test bed for quantum information processing, where single particle E

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Nano Letters placement is often essential.40−43 Carboxylated diamond nanoparticles have an IEP ∼ 344 and therefore are negatively charged in a neutral solution and will be attracted to Al2O3 pads without silica cladding. To demonstrate this, we performed selfassembly using commercially available diamond nanoparticles (Adamas Nanotechnologies), which had an average diameter of 40 nm and nominally contained between 1 and 4 NV centers. SEM images from this self-assembly process are shown in Figure 5a. Similar to the silica-clad QDs, we performed a multicycle reusability experiment with these unencapsulated diamond nanoparticles, as shown in Figure 5b−e, and antibunching experiments were used to approximate the number of NV centers on each pad (Figure 6). Because of the irregular shape and size of the nanodiamonds, some pads attracted more than one nanoparticle; nevertheless, we observed from SEM images that 48% of the 70 nm pads had a single diamond nanoparticle. Consistent with this coverage and the nominal 1−4 NVs per particle, the antibunching measurements in Figure 6 confirm single photon emission from ∼19% of the pads. As shown in Figure 5, this inorganic template can be reused multiple times for self-assembly of diamond nanoparticles. However, after each cleaning and redeposition, we observed some erosion and thinning of the Al2O3 pads, as can be seen by their decreased intensity in darkfield images (bottom of Figure 5b−e). After five cycles, while high coverage was still obtained for deposition on large pads, the templates were eroded enough to significantly reduce coverage on pads with initial diameters smaller than 100 nm. This was likely because, unlike the QDs that were dispersed in ethanol, the NV centers were dispersed in DI-water, which may have slowly eroded any oxygen-deficient alumina in the pads. Additionally, due to their smaller size (∼40 nm) and nonspherical shape, the diamond nanoparticles have a much larger surface area to volume ratio. Therefore, their removal required longer sonication periods, which may have made the patterns more susceptible to damage. Nevertheless, good coverage and removal of NV containing diamond nanoparticles was still obtained, and a less corrosive dispersal solution could be used to increase the longevity of the patterned templates. As can be seen from these examples, this inorganic selfassembly technique is readily applicable to a large range of nanoparticle emitters either through silica encapsulation or by leveraging particles with an intrinsically low IEP. With the proper choice of substrate and patterning material, this technique can also be applied to high IEP nanoparticles. For example, MgO nanocubes have an intrinsically high IEP,23 and studies of transition-metal-doped MgO nanocubes have demonstrated sharp photoluminescence lines that could be useful for nanophotonic applications.45,46 By using a high IEP sapphire substrate patterned with low IEP SiO2 pads, the electrostatic self-assembly process described here could be used for precise positioning of MgO nanocubes. Thus, in principle this technique could be applied to any nanoparticle and patterning template pair with sufficiently different IEP values. Summary. In conclusion, we have developed a reusable, inorganic, electrostatic self-assembly method that allows for the repeatable and scalable placement of individual nanoparticle emitters. This process has been demonstrated with silica-clad QDs to illustrate that single photon sources can be positioned on the same array multiple times by simple cleaning and redeposition. We also demonstrated that this approach could be applied to other low-IEP emitters, including unencapsulated diamond nanocrystals and silica-clad lanthanide-doped UCNPs.

Furthermore, with the appropriate choice of substrate and patterning material, this technique can be used on any nanoparticle emitter (without silica encapsulation) as long as they have a sufficiently high or low IEP. The ability to iteratively position individual emitter nanoparticles on a solid substrate and near the same set of optical nanostructures could help facilitate large-scale statistical studies of emitter-nanostructure interactions. As a simple example, we include a brief study of QD emission near gold nanorod antennas in the Supporting Information. Given the simplicity of this technique as well as the practical and scientific advantages of being able to reuse and reexamine the same nanostructures with multiple emitters, we anticipate that this method could be of interest to the broad nanophotonics community, including those working in the fields of metamaterials, plasmonics, and photonic crystals. The scalability of this approach may also be advantageous for more distant applications with large arrays of single photon sources.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.nanolett.5b01009. Details about the synthesis of silica cladding on the colloidal QDs, fabrication of Al2O3 pad arrays, and fluorescence lifetime measurements for QDs positioned near gold nanorod antennas. (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank S. Cueff, C. M. Dodson, S. Karaveli, D. Li, and D. Stein for helpful discussions. Financial support for this work was provided by the Air Force Office of Scientific Research (FA9550-10-1-0211 and FA9550-12-1-0488).

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ABBREVIATIONS QD, quantum dot; IEP, isoelectric point; HBT, HanburyBrown Twiss; NV, nitrogen vacancy; SEM, scanning electron microscope; UNCP, upconverting nanoparticle

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REFERENCES

(1) Lounis, B.; Bechtel, H.; Gerion, D.; Alivisatos, P.; Moerner, W. Chem. Phys. Lett. 2000, 329, 399−404. (2) Brokmann, X.; Giacobino, E.; Dahan, M.; Hermier, J. Appl. Phys. Lett. 2004, 85, 712−714. (3) Beveratos, A.; Brouri, R.; Gacoin, T.; Poizat, J.-P.; Grangier, P. Phys. Rev. A: At., Mol., Opt. Phys. 2001, 64, 061802. (4) Kurtsiefer, C.; Mayer, S.; Zarda, P.; Weinfurter, H. Phys. Rev. Lett. 2000, 85, 290−293. (5) Novotny, L.; van Hulst, N. Nat. Photonics 2011, 5, 83−90. (6) Beams, R.; Smith, D.; Johnson, T. W.; Oh, S.-H.; Novotny, L.; Vamivakas, A. N. Nano Lett. 2013, 13, 3807−3811. (7) Curto, A. G.; Volpe, G.; Taminiau, T. H.; Kreuzer, M. P.; Quidant, R.; van Hulst, N. F. Science 2010, 329, 930−933. (8) Empedocles, S. A.; Norris, D.; Bawendi, M. Phys. Rev. Lett. 1996, 77, 3873−3876. (9) Empedocles, S.; Bawendi, M. Science 1997, 278, 2114−2117.

F

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Letter

Nano Letters (10) Imamog, A.; Awschalom, D. D.; Burkard, G.; DiVincenzo, D. P.; Loss, D.; Sherwin, M.; Small, A. Phys. Rev. Lett. 1999, 83, 4204. (11) Fernée, M. J.; Rubinsztein-Dunlop, H. Phys. Rev. B: Condens. Matter Mater. Phys. 2006, 74, 115321. (12) Mentzel, T. S.; Wanger, D. D.; Ray, N.; Walker, B. J.; Strasfeld, D.; Bawendi, M. G.; Kastner, M. A. Nano Lett. 2012, 12, 4404−4408. (13) Manfrinato, V. R.; Wanger, D. D.; Strasfeld, D. B.; Han, H.-S.; Marsili, F.; Arrieta, J. P.; Mentzel, T. S.; Bawendi, M. G.; Berggren, K. K. Nanotechnology 2013, 24, 125302. (14) Junkin, M.; Watson, J.; Geest, J. P. V.; Wong, P. K. Adv. Mater. 2009, 21, 1247−1251. (15) Pattantyus-Abraham, A. G.; Qiao, H.; Shan, J.; Abel, K. A.; Wang, T.-S.; van Veggel, F. C.; Young, J. F. Nano Lett. 2009, 9, 2849− 2854. (16) Wang, Y.; Zhang, Y.; Li, B.; Lü, J.; Hu, J. Appl. Phys. Lett. 2007, 90, 133102. (17) Yin, Y.; Lu, Y.; Gates, B.; Xia, Y. J. J. Am. Chem. Soc. 2001, 123, 8718−8729. (18) Cui, Y.; Björk, M. T.; Liddle, J. A.; Sönnichsen, C.; Boussert, B.; Alivisatos, A. P. Nano Lett. 2004, 4, 1093−1098. (19) Kramer, R.; Pholchai, N.; Sorger, V.; Yim, T.; Oulton, R.; Zhang, X. Nanotechnology 2010, 21, 145307. (20) Abramson, J.; Palma, M.; Wind, S.; Hone, J. Adv. Mater. 2012, 24, 2207−2211. (21) Zhao, Y.; Thorkelsson, K.; Mastroianni, A. J.; Schilling, T.; Luther, J. M.; Rancatore, B. J.; Matsunaga, K.; Jinnai, H.; Wu, Y.; Poulsen, D.; Frechet, J. M. J.; Alivisatos, A. P.; Xu, T. Nat. Mater. 2009, 8, 979−985. (22) Zhang, Q.; Dang, C.; Urabe, H.; Wang, J.; Sun, S.; Nurmikko, A. Opt. Express 2008, 16, 19592−19599. (23) Kosmulski, M. Chemical properties of material surfaces; CRC Press: Boca Raton, FL, 2001. (24) Goyne, K. W.; Zimmerman, A. R.; Newalkar, B. L.; Komarneni, S.; Brantley, S. L.; Chorover, J. J. Porous Mater. 2002, 9, 243−256. (25) Behrens, S. H.; Grier, D. G. J. Chem. Phys. 2001, 115, 6716− 6721. (26) Darbandi, M.; Thomann, R.; Nann, T. Chem. Mater. 2005, 17, 5720−5725. (27) Pak, J.; Yoo, H. J. Mater. Chem. A 2013, 1, 5408−5413. (28) Li, Z.; Zhang, Y.; Jiang, S. Adv. Mater. 2008, 20, 4765−4769. (29) Hiemenz, P. C.; Rajagopalan, R. Principles of Colloid and Surface Chemistry, revised and expanded; CRC Press: Boca Raton, FL, 1997; Vol. 14. (30) Empedocles, S. A.; Neuhauser, R.; Shimizu, K.; Bawendi, M. G. Adv. Mater. 1999, 11, 1243−1256. (31) Kitson, S.; Jonsson, P.; Rarity, J.; Tapster, P. Phys. Rev. A: At., Mol., Opt. Phys. 1998, 58, 620. (32) Wu, S.; Han, G.; Milliron, D. J.; Aloni, S.; Altoe, V.; Talapin, D. V.; Cohen, B. E.; Schuck, P. J. Proc. Natl. Acad. Sci. U. S. A. 2009, 106, 10917−10921. (33) Chan, E. M.; Gargas, D. J.; Schuck, P. J.; Milliron, D. J. J. Phys. Chem. B 2012, 116, 10561−10570. (34) Gargas, D. J.; Chan, E. M.; Ostrowski, A. D.; Aloni, S.; Altoe, M. V. P.; Barnard, E. S.; Sanii, B.; Urban, J. J.; Milliron, D. J.; Cohen, B. E. Nat. Nanotechnol. 2014, 9, 300−305. (35) Saboktakin, M.; Ye, X.; Chettiar, U. K.; Engheta, N.; Murray, C. B.; Kagan, C. R. ACS Nano 2013, 7, 7186−7192. (36) Schietinger, S.; Aichele, T.; Wang, H.-Q.; Nann, T.; Benson, O. Nano Lett. 2010, 10, 134−138. (37) Sun, Q.-C.; Mundoor, H.; Ribot, J. C.; Singh, V.; Smalyukh, I. I.; Nagpal, P. Nano Lett. 2014, 14, 101−106. (38) Saboktakin, M.; Ye, X.; Oh, S. J.; Hong, S.-H.; Fafarman, A. T.; Chettiar, U. K.; Engheta, N.; Murray, C. B.; Kagan, C. R. ACS Nano 2012, 6, 8758−8766. (39) Yuan, P.; Lee, Y. H.; Gnanasammandhan, M. K.; Guan, Z.; Zhang, Y.; Xu, Q.-H. Nanoscale 2012, 4, 5132−5137. (40) Jelezko, F.; Wrachtrup, J. Phys. Status Solidi A 2006, 203, 3207− 3225.

(41) Doherty, M.; Manson, N.; Delaney, P.; Jelezko, F.; Wrachtrup, J.; Hollenberg, C. Phys. Rep. 2013, 528, 1−45. (42) Taylor, J.; Cappellaro, P.; Childress, L.; Jiang, L.; Budker, D.; Hemmer, P.; Yacoby, A.; Walsworth, R.; Lukin, M. Nat. Phys. 2008, 4, 810−816. (43) Huck, A.; Kumar, S.; Shakoor, A.; Andersen, U. Phys. Rev. Lett. 2011, 106, 096801. (44) Chakrapani, V.; Angus, J. C.; Anderson, A. B.; Wolter, S. D.; Stoner, B. R.; Sumanasekera, G. U. Science 2007, 318, 1424−1430. (45) Altman, I.; Pikhitsa, P.; Choi, M.; Song, H.-J.; Nasibulin, A.; Kauppinen, E. Phys. Rev. B: Condens. Matter Mater. Phys. 2003, 68, 125324. (46) Altman, I.; Pikhitsa, P.; Choi, M.; Jeong, J.; Song, H.-J.; Agranovski, I.; Bostrom, T. Appl. Phys. Lett. 2003, 83, 3689.

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