Extended hybrid structures by self-assembly and in matrixes:
Disordered to ordered

Random networks of interconnected QDs, noble-metal NCs, and QD–NC mixtures can be assembled as highly porous three-dimensional aerogel monoliths or dense two-dimensional xerogel thin films (Figure 1a and 1c). ^{Reference Gacoin, Lahlil, Larregaray and Boilot12–Reference Korala, Li and Brock22} Controlled destabilization of presynthesized QD–NCs, by, for example, chemical or photochemical oxidation of the ligand stabilizers on the QD and/or NC surfaces, removal of surface ligands, or modification of electrostatic stabilization factors, causes QD–NCs to condense into gels. Gels are then dried of interstitial solvent supercritically to form ultralow-density aerogels or under ambient conditions to form less-porous xerogels. ^{Reference Gaponik, Herrmann and Eychmuller15} In general, nanoscale (e.g., quantum) properties are retained, whereas the direct particle–particle connections formed during condensation afford enhanced collective optical and electrical properties, such as enhanced electrical conductivity, increased QD–QD Dexter energy transfer (active at short distances, <2 nm) and Förster resonance energy transfer (FRET; active at longer distances, ∼2–10 nm), metal-induced quenching (by FRET or charge transfer), or plasmon-enhanced QD photoluminescence (PL), and the resultant shortening of nonradiative or radiative lifetimes. ^{Reference Gill and Hope-Weeks17,Reference Lesnyak, Wolf, Dubavik, Borchardt, Voitekhovich, Gaponik, Kaskel and Eychmuller19–Reference Hendel, Lesnyak, Kuhn, Herrmann, Bigall, Borchardt, Kaskel, Gaponik and Eychmuller21} Currently, in the case of mixed QD–NC aerogels, all of these processes could be active. Therefore, a key challenge in this field is to optimize the network structure to favor a targeted outcome, such as PL enhancement, by incorporating appropriate dielectric “spacers” to tune exciton–plasmon interactions for controlled semiconductor–metal spacing. ^{Reference Lesnyak, Wolf, Dubavik, Borchardt, Voitekhovich, Gaponik, Kaskel and Eychmuller19}

Nanoparticle colloids with carefully controlled size, shape, and surface properties can self-assemble into a wide array of ordered two- and three-dimensional nanocrystal superlattices (NCSLs) that mimic atomic lattices, and these can be single-component, binary, or ternary NCSLs (Figure 1b and 1d). ^{Reference Dong, Ye, Chen and Murray23–Reference Zeng, Li, Liu, Wang and Sun35} In the absence of significant Coulombic or van der Waals particle–particle interactions or strong ligand–ligand interactions, entropic contributions dominate, such that the nanoparticle colloids behave as “hard spheres” and pack in various high-density configurations, depending on the ratio of the radii of the constituents. Interparticle interactions play a significant role in the assembly of semiconductor/metal NCs, as a result of strong metal–metal van der Waals interactions, which favor close metal–metal associations. ^{Reference Bodnarchuk, Kovalenko, Heiss and Talapin29}

Of the highly ordered NCSLs comprising more than one component, >20 binary NCSLs, ^{Reference Shevchenko, Talapin, Murray and O’Brien26,Reference Chen, Ye, Oh, Kikkawa, Kagan and Murray28} and only a few ternary NCSLs, ^{Reference Dong, Ye, Chen and Murray23,Reference Evers, Friedrich, Filion, Dijkstra and Vanmaekelbergh24,Reference Paik, Diroll, Kagan and Murray27} are known to date. These have been constructed from two metals, two or three semiconductors, two or three magnetic nanoparticles, or combinations thereof. Despite the structural diversity and sophistication, efforts to exploit new functionality resulting from ordered arrays of dissimilar but interacting nanoparticles remain limited to exchange-coupling and dipolar interactions in assemblies of magnetic nanoparticle pairs, ^{Reference Chen, Ye, Oh, Kikkawa, Kagan and Murray28,Reference Chen, Dong, Cai, Ye, Kang, Kikkawa and Murray30,Reference Dong, Chen, Ye, Kikkawa and Murray31,Reference Zeng, Li, Liu, Wang and Sun35} enhanced electronic conductivity in semiconductor mixtures, ^{Reference Urban, Talapin, Shevchenko, Kagan and Murray34} tunable plasmonic properties (e.g., strength of near-field couplings), ^{Reference Ye, Chen, Diroll and Murray36} structure-correlated catalytic activity, ^{Reference Kang, Ye, Chen, Qi, Diaz, Doan-Nguyen, Xing, Kagan, Li, Gorte, Stach and Murray37} and fluorescence quenching in the case of metal–semiconductor pairing. ^{Reference Shevchenko, Ringler, Schwemer, Talapin, Klar, Rogach, Feldmann and Alivisatos32} The last of these involved self-assembly of CdSe QDs and Au NCs, and fluorescence spectroscopy studies revealed PL quenching and shortening of QD PL lifetimes. Thus, as for the disordered-network assemblies described earlier, optimization of exciton–plasmon interactions in semiconductor–metal multicomponent NCSLs through nanoscale positioning and spacing will require the inclusion, for example, of nanospacers. ^{Reference Tripathi, Praveena and Basu38–Reference Zhang, Marocico, Lunz, Gerard, Gun’ko, Lesnyak, Gaponik, Susha, Rogach and Bradley40}

Hybrid excitonic–plasmonic pairs in single nanostructures:
Direct synthesis and molecular linkers

Single nanoconstructs exhibiting uninterrupted semiconductor–metal connections can be directly synthesized (Figure 2a and 2d). Results from this approach have been reviewed previously, ^{Reference Banin, Ben-Shahar and Vinokurov11} with examples ranging from symmetrical metal-core–semiconductor-shell nanostructures ^{Reference Guardia, Korobchevskaya, Casu, Genovese, Manna and Comin41,Reference Zhang, Tang, Lee and Ouyang42} to anisotropic “nanodumbbells” ^{Reference Costi, Cohen, Salant, Rabani and Banin43} and metal-tipped tetrapods. ^{Reference Mokari, Rothenberg, Popov, Costi and Banin44,Reference Mishra, Lian, Chakrabortty, Lin and Chan45} Structures comprising direct semiconductor–metal interfaces have been used to demonstrate plasmonics-enhanced nanoscale light–matter and light–matter–spin interactions; for example, a resonantly coupled (metal local surface-plasmon resonance [LSPR] overlapping with emitter first-exciton energy) gold core and CdSe shell afforded an enhanced optical Stark effect and ultrafast coherent spin manipulation. ^{Reference Zhang, Tang, Lee and Ouyang42} More commonly, semiconductor–metal nanoscale heterostructures have been studied in the context of light-induced charge separation and resulting applications in photocatalysis, ^{Reference Banin, Ben-Shahar and Vinokurov11} where this very feature—rapid charge transfer from semiconductor to metal—results in fluorescence quenching, such as in the case of metal- (gold-, silver-) tipped semiconductor (CdSe, CdS, ZnO) nanorods, ^{Reference Mokari, Rothenberg, Popov, Costi and Banin44,Reference Saunders, Popov and Banin46,Reference Pacholski, Kornowski and Weller47} and Au₂Cd alloy-core–CdSe-shell NCs. ^{Reference Guardia, Korobchevskaya, Casu, Genovese, Manna and Comin41}

Figure 2.

Spatially limited hybrid nanoconstructs. (a) Schematic depicting semiconductor nanoemitter (e.g., shell, rod) (red) and metal plasmonic nanocrystal (NC) (yellow) single nanostructures containing direct semiconductor–metal interfaces. (b) Schematic of a metal-planet–quantum dot (QD)-satellite nanostructure, wherein the two components are separated by a spacer (e.g., SiO₂, polymer, DNA). (c) Schematics of QD-core–metal-shell and QD-planet–metal-satellite nanostructures, wherein the two components are separated by a spacer (e.g., SiO₂, polyelectrolyte layers, DNA). (d) (Top) Optical Stark effect (OSE): Time-resolved linear optical absorption spectra with subresonant excitation energy (1.892 eV at 1.45 GW cm⁻²), where the subresonant and probe laser beams are co-polarized. (Bottom) Complete quenching of the photoluminescence (PL) of CdS nanorods (dotted line) upon growth of gold tips (solid line). (e) (Top) PL spectra of CdSe/ZnS QDs and hybrid silver/SiO₂/QDs as a function of the SiO₂ spacer thickness. (Bottom) PL spectra of bare CdTe QDs and gold/SiO₂/QDs as a function of spacer thickness. (f) (Top) Multifunctionality afforded by CdSe/CdS giant-QD/SiO₂/gold-shell hybrid constructs: Simultaneous excitation of the QD (ultraviolet excitation) and gold-shell (near-infrared excitation) components affords simultaneous optical readout (QD PL), thermometry (QD PL is temperature-sensitive), and photothermal heating (gold shell). (Bottom) PL intensity as a function of time (binning time, 10 ms) for CdSe/CdS QDs (red trace) and QD/SiO₂/gold-shell hybrid nanostructures (orange trace), revealing radiative efficiences of neutral and charged states becoming almost equal with Au-shell addition. (d) Adapted with permission from Reference 42 (top). © 2010 Macmillan Publishers Ltd. Nature, and adapted with permission from Reference 46 (bottom). © 2006 American Chemical Society. (e) Adapted with permission from Reference 49 (top). © 2013 American Chemical Society, and adapted with permission from Reference 50 (bottom). © 2013 OSA Publishing. (f) Adapted with permission from Reference 59 (top). © 2015 Royal Society of Chemistry, and adapted with permission from Reference 57 (bottom). © 2015 Macmillan Publishers Ltd: Nature Nanotechnology.

To avoid the quenching observed for direct-junction semiconductor–metal nanoconstructs, dielectric spacers have been synthetically added (Figure 2b and 2e). ^{Reference Liu, Prall and Klimov48–Reference Tang, Xu, Guo, Zhuang, Tian, Wang, Duan and Pan50} Here, the core is a metal nanocrystal (gold ^{Reference Liu, Prall and Klimov48–Reference Tang, Xu, Guo, Zhuang, Tian, Wang, Duan and Pan50} or silver ^{Reference Naiki, Masuhara, Masuo, Onodera, Kasai and Oikawa49} ) overcoated with a thickness-tunable SiO₂ layer using a modified Stöber process. ^{Reference Liu, Prall and Klimov48–Reference Liz-Marzan, Giersig and Mulvaney51} The SiO₂ surface is subsequently activated for QD (CdSe, ^{Reference Liu, Prall and Klimov48} CdSe/ZnS, ^{Reference Naiki, Masuhara, Masuo, Onodera, Kasai and Oikawa49} CdTe ^{Reference Tang, Xu, Guo, Zhuang, Tian, Wang, Duan and Pan50} ) binding by the addition of aminosilane coupling agents, ^{Reference Liu, Prall and Klimov48–Reference Tang, Xu, Guo, Zhuang, Tian, Wang, Duan and Pan50} giving a “planet–satellite” configuration. The added control afforded by this synthesis over semiconductor–metal spacing has yielded tunable effects ranging from nonradiative damping (quenching) to field-enhanced PL (either excitation ^{Reference Naiki, Masuhara, Masuo, Onodera, Kasai and Oikawa49} or emission ^{Reference Tang, Xu, Guo, Zhuang, Tian, Wang, Duan and Pan50} enhancement) as a function of SiO₂ thickness. Both quenching and enhancement have been realized for similar constructs in which the SiO₂ layer is effectively replaced by molecular spacers or rigid DNA linkers. ^{Reference Lee, Govorov and Kotov52–Reference Sun, Tian, Zhang, Xu, Sfeir, Cotlet and Gang56} For example, Lee et al. demonstrated a “nanoscale thermometer” using a poly(ethylene glycol) (PEG) spacer, where PEG’s responsiveness to heat (thermal swelling) afforded QD PL intensity tuning through dynamic control of the CdTe QD–Au distance. For this case of overlapping PL and LSPR energies, PL enhancement occurred at the shortest distances. ^{Reference Lee, Govorov and Kotov52} In contrast, in QD–Au constructs connected by rigid DNA linkers, QD PL was quenched with a distance dependence characteristic of FRET (1/d ^{Reference Grzelczak, Vermant, Furst and Liz-Marzan6} dependence, where d is the distance), ^{Reference Gueroui and Libchaber53} or of either FRET or a nanometal surface energy transfer mechanism (1/d ^{Reference Hennessy, Badolato, Tamboli, Petroff, Hu, Atatüre, Dreiser and Imamoglu4} dependence), depending on the Au NC size and the extent of PL–LSPR overlap. ^{Reference Li, Cushing, Wang, Shi, Hornak, Hong and Wu55}

In an inverse scenario compared to these metal-core–semiconductor-shell nanostructures, metal shells have been grown or assembled around semiconductor cores (Figure 2c and 2f). ^{Reference Ji, Giovanelli, Habert, Spinicelli, Nasilowski, Xu, Lequeux, Hugonin, Marquier, Greffet and Dubertret57–Reference Karan, Keller, Sampat, Roslyak, Arefin, Hanson, Casson, Desireddy, Ghosh, Piryatinski, Iyer, Htoon, Malko and Hollingsworth59} Here, rather than quantum semiconductor shells serving the role of the emitter, more traditional and potentially better performing colloidal QDs comprise the emitter, whereas the gold shell imparts multifunctionality (combined QD PL and gold scattering or absorption-mediated photothermal heating) ^{Reference Jin and Gao58,Reference Karan, Keller, Sampat, Roslyak, Arefin, Hanson, Casson, Desireddy, Ghosh, Piryatinski, Iyer, Htoon, Malko and Hollingsworth59} or Purcell effects. ^{Reference Ji, Giovanelli, Habert, Spinicelli, Nasilowski, Xu, Lequeux, Hugonin, Marquier, Greffet and Dubertret57}

Jin and Gao reported a novel approach for gold-shell synthesis that used a peptide “gold deposition template,” circumventing the need for presynthesized gold nanoclusters as structural templates and affording a new opportunity for growing ultrathin shells (<5 nm⁵⁸ versus 5–20 nm^60,61). In addition, by adding one to two polyelectrolyte-bilayer spacers, they decreased the QD PL quenching compared to that of direct-contact hybrid structures (45–75% versus ∼100% quenching). Following a similar approach, but modifying and elucidating the highly pH-dependent gold-reduction process, Karan et al. employed thicker spacers in the form of SiO₂ layers of controlled thickness (10–17.5 nm). ^{Reference Karan, Keller, Sampat, Roslyak, Arefin, Hanson, Casson, Desireddy, Ghosh, Piryatinski, Iyer, Htoon, Malko and Hollingsworth59} They also used thick-shell or “giant” CdSe–CdS core–shell QDs (g-QDs) ^{Reference Chen, Vela, Htoon, Casson, Werder, Bussian, Klimov and Hollingsworth62–Reference Vela, Htoon, Chen, Park, Ghosh, Goodwin, Werner, Wells, Casson and Hollingsworth64} as the emitter, such that the combination of a thicker spacer layer and a more stable QD could afford complete suppression of gold-mediated quenching. ^{Reference Karan, Keller, Sampat, Roslyak, Arefin, Hanson, Casson, Desireddy, Ghosh, Piryatinski, Iyer, Htoon, Malko and Hollingsworth59} In addition, it was demonstrated both experimentally and theoretically that the interplay of the thicknesses of the SiO₂ spacer and gold shell allows precise tuning of the LSPR from the visible to the near-infrared range, as well as large gold absorption cross-sections despite a compact total particle size (40–60-nm “inorganic” diameter). ^{Reference Karan, Keller, Sampat, Roslyak, Arefin, Hanson, Casson, Desireddy, Ghosh, Piryatinski, Iyer, Htoon, Malko and Hollingsworth59}

In contrast to the thin-gold-shell examples, Ji et al. established a true “plasmonic resonator” effect in the synthetic construction of a QD enveloped in a thick SiO₂ shell (35 nm) coated with a thick gold shell (20 nm), which afforded overlap of the QD PL and the gold plasmon resonance peak. ^{Reference Ji, Giovanelli, Habert, Spinicelli, Nasilowski, Xu, Lequeux, Hugonin, Marquier, Greffet and Dubertret57} In this way, they targeted strong QD–Au coupling, which they observed in the form of neutral and charged states having comparable radiative efficiencies resulting from Purcell enhancement of radiative recombination rates compared to nonradiative Auger recombination, similar to effects observed for CdSe–ZnS QDs dispersed on nano-roughened gold films. ^{Reference Shimizu, Woo, Fisher, Eisler and Bawendi65}

Finally, it is worth noting that structures comprising single QDs encircled by multiple Au NCs can be synthesized using various modifications of the DNA linker approach ^{Reference Fu, Micheel, Cha, Chang, Yang and Alivisatos66} or, more recently, “DNA origami” scaffolds (Figure 2c). ^{Reference Schreiber, Do, Roller, Zhang, Schuller, Nickels, Feldmann and Liedl67} The latter approach takes advantage of the ability of DNA to self-assemble into designed two- and three-dimensional nanostructures by the interaction between a long DNA “scaffold” strand with many short DNA “staple” strands. ^{Reference Rothemund68–Reference Douglas, Marblestone, Teerapittayanon, Vazquez, Church and Shih70} Adoption of the DNA origami approach to the assembly of nanomaterials, including the creation of semiconductor–metal hybrid nanostructures, affords an important opportunity to tune separation distances from ∼5 to 200 nm in planet–satellite configurations, with possible implications for surface plasmon amplification by stimulated emission of radiation technologies and self-assembled metamaterials. ^{Reference Schreiber, Do, Roller, Zhang, Schuller, Nickels, Feldmann and Liedl67}

Plasmonic and antenna effects on emission characteristics: Directionality, polarization, collection efficiency

The previous section has shown that even simple antenna geometries allow for very large spontaneous emission enhancements. The high symmetry of these antennas suitable for the enhancement of the spontaneous emission rate does not redistribute the angular emission pattern sufficiently to improve the photon collection efficiency. However, several structures have been devised to achieve a high degree of emission directionality. For example, QDs have been successfully coupled to miniaturized Yagi-Uda antennas. ^{Reference Curto, Volpe, Taminiau, Kreuzer, Quidant and van Hulst83} Furthermore, QDs and color centers have been coupled to plasmonic nanowires that convert the common dipolar emission pattern to guided modes of a subwavelength waveguide. ^{Reference Schell, Kewes, Hanke, Leitenstorfer, Bratschitsch, Benson and Aichele86–Reference Akimov, Mukherjee, Yu, Chang, Zibrov, Hemmer, Park and Lukin88} Unfortunately, the intrinsic losses associated with plasmonic structures prevent very high collection efficiencies. Thus, there exist efforts to replace plasmonic structures with dielectric ones, ^{Reference Yalla, Le Kien, Morinaga and Hakuta89–Reference Lund-Hansen, Stobbe, Julsgaard, Thyrrestrup, Sunner, Kamp, Forchel and Lodahl91} with planar antennas reaching a collection efficiency of 99%. ^{Reference Hennessy, Badolato, Tamboli, Petroff, Hu, Atatüre, Dreiser and Imamoglu4,Reference Lee, Chen, Eghlidi, Kukura, Lettow, Renn, Sandoghdar and Götzinger92,Reference Chu, Brenner, Chen, Ghosh, Hollingsworth, Sandoghdar and Götzinger93} Using such antenna devices could lead to a number of very exciting developments in single-photon technologies, where individual photons could be obtained on demand and with near-unity certainty.

Plasmonic effects on multiexciton processes and photon emission statistics

Beyond impacting emitter exciton behavior, interactions with plasmonic nanostructures can modify the radiative and nonradiative recombination processes of multiexciton states. ^{Reference Cheng, Yuan, Wang, Lin, Shen, Hung, Tang and Tseng94–Reference Wang, Karan, Nguyen, Ghosh, Sheehan, Hollingsworth and Htoon107} Normally, because of Auger-mediated processes, multiexciton emission is suppressed, ^{Reference Klimov, Mikhailovsky, McBranch, Leatherdale and Bawendi108} such that a single emitter only produces one photon in a pulsed-excitation cycle. As a result, the second-order photon-correlation function (g ⁽²⁾, which represents the distribution histogram of two successive photon-detection events) acquired under pulsed excitation exhibits little contribution from the zero-time-delay peak, that is, a low probability of detecting two photons during one excitation cycle. This condition is called photon “anti-bunching.” In contrast, recent g ⁽²⁾ studies of QDs coupled to various metal surfaces ^{Reference Cheng, Yuan, Wang, Lin, Shen, Hung, Tang and Tseng94,Reference LeBlanc, McClanahan, Jones and Moyer98,Reference Naiki, Masuo, Machida and Itaya102} and nanostructures ^{Reference Dey, Zhou, Tian, Jenkins, Chen, Zou and Zhao97,Reference Yuan, Wang, Cheng, Wang, Kuo, Shih and Tang103,Reference Park, Ghosh, Xu, Mack, Wang, Hollingsworth and Htoon105,Reference Wang, Karan, Nguyen, Ghosh, Sheehan, Hollingsworth and Htoon107} have revealed dramatic strengthening of the zero-time-delay peak (i.e., photon “bunching”) ( Figure 4 ). At low pump fluence, the area ratio of the center and side peaks of the g ⁽²⁾ function (g ⁽²⁾ ₍₀₎/g ⁽²⁾ _(T)) reflects the ratio of biexciton to single-exciton quantum yields (Q _2X/Q _1X), ^{Reference Nair, Zhao and Bawendi109,Reference Park, Malko, Vela, Chen, Ghosh, Garcia-Santamaria, Hollingsworth, Klimov and Htoon110} so these results indicate a strong enhancement of biexciton emission efficiency relative to that of single-exciton emission.

Figure 4.

(a) CdSe/ZnS quantum dots (QDs) dispersed on a nano-roughened gold surface ^{Reference Liu, Kumar, Hu, Cheng and Irudayaraj100} and (b) CdSe/CdS giant-QDs (g-QDs) dispersed on a silver nanoflake film ^{Reference Chu, Brenner, Chen, Ghosh, Hollingsworth, Sandoghdar and Götzinger93} show strong enhancement in the degree of photon bunching (peak area ratio, g ⁽²⁾ ₍₀₎/g ⁽²⁾ _(T)), with the center peak increasing to ∼0.5 in the case of standard QDs and exceeding 1 (maximum ∼2.0) in the g-QD example. (a) Adapted with permission from Reference 98. © 2013 American Chemical Society. (b) Adapted with permission from Reference 104. © 2013 American Physical Society.

This enhancement is commonly interpreted as resulting from plasmonic enhancement of bi- and higher-order multiexciton radiative recombination processes. ^{Reference LeBlanc, McClanahan, Jones and Moyer98,Reference Masuo, Naiki, Machida and Itaya101,Reference Naiki, Masuo, Machida and Itaya102} However, the fact that a similar enhancement of Q _2X/Q _1X can also be observed for QDs spread on an indium tin oxide substrate ^{Reference Cheng, Yuan, Wang, Lin, Shen, Hung, Tang and Tseng94} —where no plasmonic enhancement of any radiative recombination process is expected—suggests that apparent plasmonic enhancement of bi-/multiexciton efficiencies can, instead, result simply from a reduction of single-exciton quantum yields (QYs) due to the nonradiative processes discussed previously—direct charge tunneling or FRET to the metal. Thus, these results should be interpreted carefully.

Indeed, Park et al. showed that core/thick-shell or “giant” QDs (g-QDs) coupled to silver “nanoflakes” can exhibit strong photon bunching with the center peak of the g ⁽²⁾ distribution reaching two times the area of the side peaks, which requires that the biexciton QY (Q _2X) exceeds the single-exciton QY (Q _1X) (Figure 4b). ^{Reference Park, Ghosh, Chen, Piryatinski, Xu, Mack, Wang, Klimov, Hollingsworth and Htoon104,Reference Park, Ghosh, Xu, Mack, Wang, Hollingsworth and Htoon105} This condition cannot arise from simple enhancement of biexciton radiative decay rate alone but requires an overall “quenching” (g-QD/metal PL ∼1/10th g-QD PL).

Plasmon-assisted coupling between individual emitters

In addition to enhancing the radiative recombination rate of a quantum emitter, polarization of a metal nanostructure can result in near-field electric interactions that are known to induce coupling between nearby nanoemitters. As such, plasmonics is a potentially powerful tool for enhancing FRET-based energy funneling in the context of photovoltaic and sensing applications and for inducing entanglement between the quantum states of two or more nanoemitters for a wide variety of optical-communication and quantum-information technologies.

Provided that donor and acceptor emitters are located in close proximity to a metal NC, the energy transfer facilitated by the surface plasmon modes can be interpreted semiclassically. The electric field associated with the transition dipoles of the donor and acceptor emitters (i.e., QDs) polarizes the metal NC, creating oscillating images. The near-field interactions between the donor and acceptor dipole images open an additional pathway for energy flow. This pathway—because of the strong polarization of the metal NC—can dominate the total energy-transfer rate, significantly increasing the rate values. ^{Reference Durach, Rusina, Klimov and Stockman111} Details on the interplay between different plasmon-assisted pathways to the total FRET rate have been previously considered. ^{Reference Pustovit and Shahbazyan112,Reference Govorov, Bryant, Zhang, Skeini, Lee, Kotov, Slocik and Naik113}

Experimentally, plasmonic enhancement of FRET between different types of donor–acceptor pairs was demonstrated in recent ensemble-level studies. ^{Reference Zhang, Marocico, Lunz, Gerard, Gun’ko, Lesnyak, Gaponik, Susha, Rogach and Bradley40,Reference Zhou, Li, Yang, Peng, Su, Zhang, Li, Kim, Yu, Zhou, Hao and Wang114,Reference Wu and Wang116} In particular, by systematically analyzing PL decay rates, emission spectra, and intensities of layered structures comprising Au NCs, acceptor–donor QD layers, and spacer layers, Ozel et al. demonstrated the ability to enable/disable coupled plasmon–exciton formation selectively at the donor or acceptor site of their choice. ^{Reference Ozel, Hernandez-Martinez, Mutlugun, Akin, Nizamoglu, Ozel, Zhang, Xiong and Demir115}

Furthermore, using a similar acceptor-QD/metal/donor-QD three-layer structure with varying spacer-layer thicknesses, Zhang et al. investigated the distance dependence of the local-surface-plasmon-coupled FRET process. ^{Reference Zhang, Marocico, Lunz, Gerard, Gun’ko, Lesnyak, Gaponik, Susha, Rogach and Bradley40} Their analysis revealed that the Förster radius defining the range of FRET processes is increased from 4.2 nm for typical donor–acceptor pairs to values exceeding 10 nm for plasmon-coupled FRET.

Whereas FRET coupling between ensembles of donor and acceptor QDs can be utilized as an effective exciton-transport mechanism in light-harvesting and -sensing applications, the coupling of a few individual QDs into “molecule-like” quantum states might afford important building blocks for solid-state quantum computation architecture ^{Reference Wu and Wang116–Reference Stinaff, Scheibner, Bracker, Ponomarev, Korenev, Ware, Doty, Reinecke and Gammon119} and many quantum-photonic–plasmonic devices such as single-photon transistors ^{Reference Neumeier, Leib and Hartmann120,Reference Chang, Sorensen, Demler and Lukin121} and routers ^{Reference Hoi, Wilson, Johansson, Palomaki, Peropadre and Delsing122} that are needed for realization of quantum-communication networks.

Along these lines, recent theoretical studies have shown that plasmon-coupled FRET can also induce strong coupling and even entanglement between multiple QDs coupled to a plasmonic structure. ^{Reference Martin-Cano, Martín-Moreno, García-Vidal and Moreno123} One of the goals in employing surface plasmon modes is to achieve an entanglement of excitonic qubits at distances exceeding the operating wavelength, requiring the use of plasmonic waveguide structures capable of supporting long-range plasmon–polariton modes.

In the laboratory, QDs (and other quantum emitters) have been coupled to silver nanowires, and exciton–plasmon–photon conversion has been demonstrated, ^{Reference Heeres, Kouwenhoven and Zwiller124} as well as single-plasmon generation, routing, and detection. ^{Reference Li, Wei and Xu125} Coupling of multiple QDs to a silver nanowire through “by-chance” deposition was also recently shown. ^{Reference Kolesov, Grotz, Balasubramanian, Stoehr, Nicolet, Hemmer, Jelezko and Wrachtrup126} Similar to previous studies, ^{Reference Kolesov, Grotz, Balasubramanian, Stoehr, Nicolet, Hemmer, Jelezko and Wrachtrup126,Reference Gonzalez-Tudela, Martin-Cano, Moreno, Martin-Moreno, Tejedor and Garcia-Vidal127} in this case, it was demonstrated that single photons emitted by individual quantum emitters are converted into plasmons that propagate along the length of the metal nanowire and are then converted back into single photons. By performing g ⁽²⁾ measurements, the researchers found that two QDs spaced sufficiently close to be excited simultaneously (∼200 nm) exhibited photon statistics characteristic of two independent emitters (g ⁽²⁾ _{(T =0)} ≈ 0.5) ( Figure 5a). ^{Reference Kolesov, Grotz, Balasubramanian, Stoehr, Nicolet, Hemmer, Jelezko and Wrachtrup126} In contrast, Wang et al. recently demonstrated that small clusters of two to three SiO₂-coated g-QDs coupled to a plasmonic (gold) gap-bar antenna can exhibit complete photon antibunching—a characteristic of single quantum emitters, despite multiple optically active g-QDs being present (Figure 5b). ^{Reference Wang, Karan, Nguyen, Ghosh, Sheehan, Hollingsworth and Htoon128} Through a detailed analysis of their emission characteristics, they further showed that the observed photon antibunching resulted from plasmonics-enhanced dipole–dipole interactions (i.e., metal-induced formation of a coupled QD–QD emissive state). In this case, the presence of the SiO₂ shell yielded a QD–QD separation distance of at least 30 nm, rendering alternative interpretations of the data, such as conventional dipole–dipole interaction, not physically possible. Furthermore, they additionally concluded that the observed complete antibunching behavior resulted not from the existence of nonemissive QDs in the clusters but from the formation of novel coupled-QD “molecules,” an important step toward entangled-photon generation.

Figure 5.

(a) (Top) Scanning electron microscope (SEM) image of two quantum dots (QDs) (labeled A and B) coupled to a silver nanowire (C to D) with a separation of less than 200 nm afford (bottom) a trace of the second-order photon-correlation function (g ⁽²⁾(t)) characteristic of two independent QDs. (b) (Left) SEM image of three SiO₂-coated giant-QDs positioned within the gap of a gold gap-bar antenna exhibit (bottom right) nearly complete photon antibunching in the single-exciton (SX) g ⁽²⁾ trace, which was reconstructed from the (top right) raw g ⁽²⁾ trace by rejecting photons of the biexciton (BX) emission process that arrive at early time delays. (a) Adapted with permission from Reference 125. © 2014 American Chemical Society. (b) Adapted with permission from Reference 128. © 2015 John Wiley and Sons.