Quantum Dots

Quantum Optical Signature of Plasmonically Coupled Nanocrystal Quantum Dots Feng Wang, Niladri S. Karan, Hue Minh Nguyen, Benjamin D. Mangum, Yagnaseni Ghosh, Chris J. Sheehan, Jennifer A. Hollingsworth, and Han Htoon* Semiconductor quantum dots coupled through charge tunneling or dipole–dipole interaction are considered to be the key building blocks for solid state quantum computation architectures.[1–8] They also lie at the heart of many quantum photonic/plasmonic devices such as single-photon transistors[9,10] and routers[11] that are needed for the removal of optical-electronic-optical conversion bottlenecks in quantum and classical communication networks. Modification of photonic density of states that occurs in the vicinity of plasmonic nanostructures has been predicted to enhance the dipole–dipole interaction and induce coupling of two or more spatially separated quantum dots (QDs).[12–14] However, quantum optical signatures of such coupled quantum dots (CQDs) have so far not been observed in any experiments conducted on multiple quantum emitters-plasmonic structure coupled systems.[15] Here we show that small clusters of two to three silica-coated nanocrystals that are coupled to plasmonic gap-bar antennas can exhibit complete photon antibunching behavior, a characteristic of single quantum emitters. Through a detailed analysis of their photoluminescence (PL) emission characteristics, we show that the observed photon antibunching is, indeed, the evidence of CQD formation resulting from the plasmonic enhancement of dipole–dipole interaction. Our work opens new possibilities for investigating other properties of CQD such as entanglement generation[13,14] and single photon switching.[9] CQDs can be formed via either electron/hole tunneling or dipole–dipole interaction of excitons.[1–8] Multiple studies have shown that key signatures of tunnelling-coupled QDs, such as level anticrossing[8] and entanglement between quantum states of two coupled QDs, can also be realized in the case of dipole-coupled QDs.[1,4–7] Such systems have, therefore, been proposed for use as coupled qubits in a manner similar to tunnelling-coupled QDs.[1,4–7] Because

Dr. F. Wang, Dr. N. S. Karan, Dr. H. M. Nguyen, Dr. B. D. Mangum, Dr. Y. Ghosh, C. J. Sheehan, Dr. J. A. Hollingsworth, Dr. H. Htoon Center for Integrated Nanotechnologies, Materials Physics & Applications Division Los Alamos National Laboratory Los Alamos, NM 87545, USA E-mail: [email protected] DOI: 10.1002/smll.201500823


both charge-tunnelling and dipole interactions are short range (≈a few nm), strong coupling between a pair of QDs with separation distance >10 nm can hardly be induced and the emission from each QD is completely independent (Figure 1a). Formation of CQDs with controlled geometry, size (i.e., number of constituent QDs), and density (i.e., QD to QD separation) demands sophisticated fabrication and synthesis approaches.[1] In the case of epitaxially grown selfassembled quantum dots, vertical stacking[2] and droplet epitaxy[1] growth approaches were employed to achieve necessary coupling. Studies of these CQDs revealed photon statistical characteristics of CQDs, such as photon antibunching in cross-correlation between the emissions of two QDs.[16,17] Coherent phenomena of CQDs (i.e., Rabi oscillation, level anticrossing of exciton dressed states, etc.),[3,18] as well as quantum entanglement among exciton and spin states of CQDs,[19] have also been demonstrated. Whereas, in the case of colloidal nanocrystal quantum dots, the success in formation of CQD is quite limited, and no quantum optical experiments on CQDs have yet been reported.[20–22] Because dipole–dipole interactions can be influenced by the photon density of states (PDOS), optical cavities, which are capable of providing high PDOS have been utilized to enhance the interaction and induce coupling of distant dipoles.[23,24] Recent studies have shown that plasmonic structures are also capable of coupling and entangling two separate QDs through the same mechanisms.[12–14] Plasmonic enhancement of Förster resonance energy transfer (FRET) between different types of donor/acceptor pairs, which can be regarded as a precursor to such formation of CQD, has been demonstrated in recent ensemble-level spectroscopy studies.[25–27] Experimental efforts of a nanowire-quantum emitter coupled system has also led to the demonstration of exciton-plasmon-photon conversion,[28] single plasmon generation, routing and detection,[29] and controlled coupling of single as well as multiple quantum emitters to nanowire waveguides.[15,30] However, Hanbury Brown–Twiss (HBT) experiments conducted on a QD pair coupled to the nanowire so far have produced incomplete photon-antibunching consistent with the emission of two independent QDs.[15] When two close-packed QDs with slightly different energy level structures are coupled via dipole–dipole interaction, exciton created in the high energy QD will be transferred to the lower energy QD (Figure 1b). When strong coupling between two identical QDs can be induced, the lowest energy level

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Figure 1. Schematics representing photon emissions of a) two independent QDs and b,c) CQDs. d) Schematic of gap bar antenna structure. e) TEM image of the silica coated CdSe/16CdS core/shell g-NQDs.

will be split into two states that are shared by both QDs (Figure 1c).[4] As the lowest energy state for both cases behaves as a quantum two-level system, its emission should exhibit complete photon antibunching provided that the emissions of higher energy states are separated. Aiming to observe such signatures of CQD formation, we investigated photon statistics of small clusters of silica-coated nanocrystals placed on top of the plasmonic gap-bar antenna using a two-step e-beam lithography approach. The antennas composed of two parallel bars with length, width, and thickness of ≈900, ≈145, ≈50 nm separated by a 40–60 nm-wide gap are fabricated on top of a ≈20 nm thick Au film (Figure 1d). The dimensions are selected to achieve a plasmon resonance that overlaps directly on PL emission band of the QDs (640 nm). This overlap, confirmed by the measured scattering spectra,[31] ensures strong enhancement to the dipole–dipole interaction. While any antenna in resonances with the emission of QDs can in principle be utilized, we chose this geometry to make placement of nanocrystal quantum dots (NQDs) easier. Additionally, this structure provides a uniform plasmonic field along the length of the bar, which is favorable to induce coupling of multiple QDs. Our nanocrystals are CdSe/CdS core (4 nm)/thick shell (16 monolayer) NQDs (also known as giant NQDs or g-NQDs).[32] These g-NQDs are characterized by blinkingfree PL emission and relatively long PL lifetimes (40–80 ns).[32,33] Our prior studies of g-NQDs spread on the rough silver films[34,35] and graphene[36] revealed that formation of interfacial states and energy transfer, respectively, can induce significant PL quenching. To prevent these effects, the g-NQDs are over-coated with 10 nm-thick silica shells. The resulting g-NQDs have total diameters of >30 nm (Figure 1e), and the direct coupling of g-NQDs via dipole–dipole interaction, therefore, becomes impossible. To directly correlate optical characteristics of our g-NQD clusters with their exact size, shape, and alignment to gapbar antennas, we performed advanced single nanostructure small 2015, 11, No. 38, 5028–5034

optical spectroscopy studies and high resolution scanning electron microscopy (SEM) imaging on the same cluster. The SEM images and spectroscopy data obtained from clusters of two and three silica-coated g-NQDs are displayed as Figure 2a–d and Figure 2e–h, respectively. PL time traces (black traces of Figure 2b,f) show blinking-free PL emission typically observed for g-NQDs. The PL decay curves (blue traces of Figure 2b,f) can be fitted with the longest lifetime components in the range of 8–15 ns, indicating four to eight times enhancement in PL decay relative to ≈65 ns average lifetime of g-NQD on glass. Because similar enhancement factors were also observed on single silica-coated g-NQDs coupled to the same antenna structure with virtually no change in PL emission intensity (Figure S2, Supporting Information) compared to reference g-NQDs on glass, we attributed this enhancement to the plasmonic enhancement of radiative decay rates. These enhancement factors are in good agreement with results of our finite integration technique simulation (Figure S3, Supporting Information). Second order photon correlation traces (top trace of Figure 2c,g) show a degree of photon antibunching (i.e., the ratio of areas of center and side peaks), R = g(2)(0)/g(2)(T) of 0.45 and 0.5 for clusters of two and three g-NQDs respectively. Nonzero R values are expected for clusters of independent quantum dots. However, it is important to note that because these g(2) traces were extracted from all the detected photons without applying either spectral or temporal selection, R has two contributions, namely, the emission of single photons from independent quantum dots and the emission of bi-excitons formed within individual QDs.[37,38] These processes are reflected by the two terms of the following equation R = Q2 X /( mQ1 X ) + ( m − 1)/ m


where m and Q1x/Q2x represent number of independent quantum emitters (NQDs) and average quantum yields of

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Figure 2. a,e) SEM images, PL time traces (black trace of b,f)), decay curves (blue trace of b,f)), c,g) g(2) traces extracted at gate time delays indicated in plot, and d,h) R versus gate time delay showing exponential decay of R for two clusters with two and three silica-coated g-NQDs positioned inside the antenna gap. i–l) are respectively the SEM image, decay curve (blue)/PL time trace(gray), g(2) at different gate time delays, and experimental (blue dots)/ simulated curve (black line) for R versus gate time delay of a reference cluster with two g-NQDs placed far away from the gap-bar antenna.

single/bi-exciton states, respectively (see Supporting Information S4 for derivation of this expression).[39] In fact, measurements performed on the single g-NQDs trapped in the same antenna structure yielded R values in the range of 0.3–0.8 (Figure S2, Supporting Information), indicating that R values of our clusters could have more contribution from bi-excitons emission than from uncorrelated single excitons in multiple quantum dots. Since bi-exciton emission decays at a rate much faster than that of a single exciton, the contribution of bi-exciton emission and multiple emitters can be separated by rejecting the photons detected during early time delays.[39] Here we apply this approach to determine the number of independent emitters formed within the clusters presented in Figure 2a,e. The g(2) traces reconstructed by rejecting the photon detected in the first 2, 4, 14 (12) ns of time delay (denoted as gate time delay GTD) are displayed as second, third, and fourth g(2) traces in Figure 2c,g. The data clearly show the center peak


of the g(2) decay with the increase of GTD and a complete photon antibunching with R ≈ 0.1 at 14 (12) ns. The plot of R extracted at different GTDs (Figure 2d,h) shows a clear exponential decay consistent with the analytical expression derived considering the different decay rates of single and bi-exciton states (see Supporting Information S4). A nearly identical behavior was also observed in a single g-NQDs antenna coupled system (Figure S2, Supporting Information). On the other hand, measured R values of a 2-g-NQD cluster deposited away from the antenna (Figure 2i) decay only to ≈0.5 after GTD of 50 ns (blue data points of Figure 2l), which is consistent with the existence of two independent g-NQDs. The decay curve of R can be further verified by our Monte Carlo simulation (black curve of Figure 2l) with the experimentally obtained lifetime (78 ns) and bi-exciton quantum yield (0.3) (see Supporting Information S4). These data together lead to the conclusion that high R values of the clusters seen in Figure 2 result primarily from the bi-exciton

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Figure 3. a,e) SEM images, PL time traces (black trace of b,f)), decay curves (blue trace of b,f)), c,g) g(2) traces extracted at gate time delays indicated in plot, and d,h) R versus gate time delay showing exponential decay of R of two clusters composed of five (a-d) and eight (e-h) silica-coated g-NQDs, respectively.

emission, and most important, the emission of the single exciton state of the clusters exhibits a complete photon antibunching, a characteristic of single quantum emitters. This result is very surprising because SEM images clearly show that clusters are composed of two and three g-NQDs, respectively. (See Supporting Information S5 for further validation of time gated g(2) experiment.) Analysis performed on data obtained from clusters of four or more g-NQDs revealed similar results. Specifically, g(2) traces (Figure 3c,g) and R versus GTD curves (Figure 3d,h) exhibit exponential decay of R very similar to that observed in smaller clusters of Figure 2. However, instead of decaying to the background level (≈0.1), R values of five and eight g-NQD clusters decay only to the values of 0.61 and 0.65 at GTD of 15 and 20 ns, respectively. Since the bi-exciton emission has no contribution to the photon emission at these GTDs, the residual values of R can be attributed to the independent emitters in clusters. The relation Rresidual = (m − 1)/m yields m of 2.27 and 2.67 for clusters of Figure 3a,e, respectively. These m values together with those of other clusters (Figure S5, Supporting Information) are plotted as the function of the number of g-NQDs in the cluster (N) revealed by the SEM images in Figure 4a. The plot shows that m is factor of two to four times smaller than N and the clusters with very similar N can exhibit significantly different m values. Provided a g-NQD can couple to one to three of its nearest neighbors via plasmon-enhanced near-field interactions, clusters of two to four g-NQD should behave as a single emitter exhibiting complete photon antibunching.[12,13] small 2015, 11, No. 38, 5028–5034

Depending on the density and geometry, g-NQDs forming larger clusters would split into multiple molecules of two to four g-NQDs and exhibit photon statistics with m characterizing the number of CQDs. Thus, our observations can be interpreted as evidence of plasmon-assisted CQD formation. On the other hand, if only one out of two to four g-NQDs in a cluster were emitting and the rest were nonemissive, the clusters would exhibit exactly the same photon statistical behavior as we observed in Figures 2 and 3. To distinguish between these two scenarios, we conducted a systematic analysis comparing PL intensity (i.e., average count rate) of clusters and single g-NQDs acquired at identical pump fluence and collection conditions. Our analysis showed that while PL intensity of 20 single g-NQDs deposited on plasmonic nanoantennas are distributed in the range of 1.5–3.5 kHz count rate with the average of 2.1 kHz (Figure 4b), clusters of Figure 2a,e emit at 5 and 6 kHz, which is higher by more than a factor of 2. Clusters in Figure 3, characterized with m = 2.27 and 2.67, are emitting at very high levels of 9 and 20 kHz, yielding a PL intensity per independent emitters (I/m) value of 3.8 and 8.7 kHz, respectively. The other six clusters of various sizes also emit at high intensity levels with I/m values in the rage of 3.75 to 10 kHz. I/m values of all the clusters (blue bars) distributed in the range significantly higher than that of single g-NQDs (red bars). By contrast, PL intensity per number of g-NQDs (I/N) values are distributed in the identical range (green bars) as the single g-NQDs. These two facts together indicate that nearly all the g-NQDs

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Figure 4. a) Plot of the number of independent emitters (m) versus the number of g-NQDs appearing in SEM image (N). Two dashed lines correspond to m = N/2 and N/4. b) Upper histogram shows the distribution of PL count rates (I) for single g-NQDs coupled to plasmonic structures. Blue and green bars of the lower histogram show the distribution of I/m and I/N of ten clusters.

of clusters are actively participating in the PL emission. Therefore, the second scenario, in which only one out of two to four g-NQDs of the clusters are emitting, is unlikely to take place.

To provide further support of this point, we analyzed pump-dependent PL saturation behaviors of clusters (red points of Figure 5a,b) in comparison with those of individual g-NQDs coupled to plasmonic structure (blue data points of

Figure 5. a–b) Red data points: PL of clusters composed of three and eight g-NQDs plotted as the function of average pump power. Green and blue data points are for two single g-NQDs deposited on glass and plasmonic structures. Black traces are fitted curves used to extract absorption cross-section. c) Distribution of constant A providing measure of absorption cross-section for reference QDs (red bars), single g-NQDs (green bars), and g-NQD clusters (blue bars). d) Plot of fσ versus N/m showing that fσ ≈N/m for most of the clusters.


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Figure 5b) and deposited on a glass substrate (green points of Figure 5a). Based on our prior work,[38] we developed a Poissonian distribution model to describe PL saturation behavior of the g-NQDs in term of bi-exciton quantum yield (Q2X) and a fitting constant A that provides a direct measure of g-NQD absorption cross-section σ (Supporting Information S7). The fit to the saturation data of the g-NQDs on the glass substrate shown in Figure 5a (green data points and black curve) yields the A value of 7.5. The fits to the other 15 g-NQDs on glass produce A values distributed between ref ) of 7 and 10 (red bars of Figure 5c) with the average (Aavg 8.3. The fits to two single g-NQDs that are coupled to the antennas (Figure 5b blue points and Figure S6a, Supporting Information) also give A values of 8.6 and 9.5, respectively, indicating that coupling to the antennas does not provide any enhancement to σ at the laser excitation wavelength of 405 nm. This finding is consistent with the results of our simulated and experimentally measured light scattering spectra, both of which show no enhancement at 405 nm.[31] The fit to PL saturation curves of clusters (red points of Figure 5a,b) yields Acluster values of 18 and 30. As displayed by the blue bars of Figure 5c, the Acluster values of all the clusters are distributed between 17 and 32. In this case, Acluster represents an average σ of independent emitters, and the ref ratio Acluster/Aavg can be interpreted as an absorption crosssection enhancement factor (fσ). The fσ are distributed in the range of two to four (Figure 5d). Because σ of single g-NQD coupled to the same antenna structures were not enhanced, we can conclude that this enhancement of σ could not have originated from plasmonic enhancement of laser excitation field but instead resulted from coupling of two to four g-NQDs via plasmon-enhanced near-field interaction. This conclusion is further confirmed by the fact that fσ for most clusters coincides with the numbers of g-NQDs per independent emitter or CQD given by N/m (Figure 5d). Together, these results present clear evidence that a strong plasmonic field of the gap-bar antenna can facilitate formation of two and three g-NQD CQDs that are capable of exhibiting photon statistics of a single quantum emitter (Figure 2). These CQDs present a very exciting opportunity to further investigate other quantum optical as well as coherent properties of CQD, such as entanglement generation[13,14] and nonlinear Fano effects.[40] These CQDs also open the path to realize CQD-based quantum plasmonic devices, such as single photon switches and routers.[9,11] We are currently improving the design and fabrication processes to push plasmon-CQD interaction to a strong interaction regime where these phenomena are expected to manifest.

peak ≈0.15), and 8 mL of cyclohexane were stirred for 15 min in a 20 mL vial. Then 100 µL of NH4OH was added to the mixture, followed by an addition of 80 µL of TEOS. The mixture was then stirred for 48 h at room temperature. The resulting g-NQD/SiO2 nanoparticles were precipitated using ethanol and collected by centrifugation at 10 000 rpm for 10 min. The product was redispersed in 10 mL nanopure water (18 MΩ) with a final concentration of ≈5 × 10−9 M. Later, silica-coated g-NQDs were functionalized with (3-aminopropyl) triethoxysilane (APTES) using approach described in ref. [43] and redispersed in ethanol/water mixtures. Fabrication of g-NQDs/Nanoantenna Coupled Structures: Standard e-beam lithography and lift-off process was performed to fabricate gap-bar antenna composed of 150 × 900 nm parallel gold bars separated by a 40–60 nm wide gap. A set of markers was also fabricated together with the antennas so that the locations where the PL measurements are performed could be tracked in later correlated SEM investigations. The second step of e-beam lithography was performed to make 80 nm diameter holes in the 150 nm thick PMMA film on top of the gap-bar antennas for g-NQDs attachment. After the lithography processes, the sample was then immersed into the diluted solution of APTES functionalized silica-coated g-NQDs for 3 h. Next, the sample was rinsed with acetone for 1 min and ethanol for another 1 min and then blown dry with nitrogen gas. Correlated Optical Spectroscopy-Scanning Electron Microscopy Studies: In order to prevent electron beam induced charging, we performed an optical study first and located the same clusters in the SEM study using reference markers. A conventional confocal microscope system with a 100×, 0.85 NA objective is used to confocally excite the sample and collect the PL signal. The g-NQDs were excited with 80 ps laser pulses at 405 nm and at a 1 MHz repetition rate. We split the photon stream into two independent detectors (≈350 ps time resolution) and performed time-tagged, time-correlated photon counting to acquire PL time traces, PL decays, and g(2)s simultaneously using Hydraharp electronics. We then employed a time-gate photon selection approach reported in ref. [32] to decouple the contribution of bi-exciton emission in determining the numbers of independent emitters. For our PL saturation experiments, a liquid crystal intensity modulator is used to sweep pump power at the rate of 10 Hz. The measurement is repeated for ten cycles, and all the data points were plotted in saturation curves of Figure 5. The spread of the data points resulted from the intensity fluctuation. The fit to the saturation data was performed for the data points in the top 5% of intensity data since those points most likely represent the emission of neutral excitons that are known to have near unity quantum yield. An FEI Magellan 400 system was used for obtaining the high-resolution SEM pictures of QDs with 1 kV acceleration voltage and 25 pA current. The low operating voltages and the Au film ground allow the clear imaging of QDs without conductive coatings.

Experimental Section Growth of Silica-Coated g-NQDs: CdSe/CdS core (4 nm)/ thick shell (16 monolayer) g-NQDs were synthesized following standard high temperature colloidal synthetic procedure using SILAR approach.[41] Silica coating of hydrophobic g-NQDs was done via a reverse microemulsion method.[20,42] In brief, 0.5 mL Igepal CO-520, 450 µL g-NQD solution (optical density of CdSe 1S small 2015, 11, No. 38, 5028–5034

Supporting Information Supporting Information is available from the Wiley Online Library or from the author.

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Acknowledgements This work was supported by a Single Investigator Small Group Research Grant (2009LANL1096), Division of Materials Science and Engineering (MSE), Office of Basic Energy Sciences (OBES), Office of Science (OS), U.S. Department of Energy (DOE), and conducted at the Center for Integrated Nanotechnologies (CINT), a U.S. DOE, OBES Nanoscale Science Research Center and User Facility.

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© 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Received: March 24, 2015 Revised: May 23, 2015 Published online: July 3, 2015

small 2015, 11, No. 38, 5028–5034

Quantum Optical Signature of Plasmonically Coupled Nanocrystal Quantum Dots.

Small clusters of two to three silica-coated nanocrystals coupled to plasmonic gap-bar antennas can exhibit photon antibunching, a characteristic of s...
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