Enhance energy transfer between quantum dots by the surface plasmon of Ag island film

Hui Xu; Jiaming Liu; Xiaoyu Duan; Jiahua Li; Jiancai Xue; Xiye Sun; Yefan Cai; Zhang-Kai Zhou; Xuehua Wang

doi:10.1364/OME.4.002586

1. Introduction

The study of energy transfer (ET) in nanostructures has recently been the subject of intensive focus due to its various prospective applications ranging from quantum information processing to photoelectric devices. For instance, efficient ET from the nanoparticles to the quantum dots (QDs) strengthens nonlinear light-quantum emitter interactions at the nanoscale [1], the excitation ET semiconductor QDs is employed to demonstrate quantum operations [2], fluorescence ET under dipole–dipole coupling in Au nanodisks can be used as building blocks in modern nanoelectronics [3,4], and the dependence of ET between QDs with different size and metallic thin films with wrinkles provides an inexpensive option of improving performance of photovoltaic devices [5,6].

Among various ET processes, plasmon mediated ET has been attracted increasing interest. With the interaction between excitons in semiconductor nanoparticles (SNPs) and surface plasmons in metal nanoparticles (MNPs), a so-called plasmon-induce field enhancement effect, which amplifies both absorption and emission processes, leads to enhanced exciton emission, and the exciton energy can be transferred from SNPs to MNPs [7,8].The surface plasmons of the exquisitely designing and optimizing metal nanostructures are powerful tools to enhance the efficiency of both radiative and nonradiative energy transfers [9–13]. Many reports have shown the potential applications of plasmon enhanced energy transfer (PEET). For example, ET between near-band-edge emission in ZnO and surface plasmons in the Ni film is promising for future efficient optical emitters [14], the stimulated interactions between active optical dipoles and surface plasmons are used to generate plasmonic lasing [15–19], the Förster resonance energy transfer (FRET) process between surface plasmons exciton is highly sensitive to distance and has a narrow peak, which attracts interest in sensing application [20–24], and the effects of higher order plasmon modes of finite Ag nanoparticle chains on their collective optical properties offer distinct advantages for creating optical interconnects between other waveguides [25]. Moreover PEET is also proved to be useful in the field of molecular biology [26–30], nanophotonic devices [31–37] and LEDs [38–40].

In this letter, we realized PEET between quantum dots with the help of an Ag island film. The surface plasmons in Ag film enhanced the ET obviously, and the lifetimes of both QDs were shortened dramatically. The results demonstrate that great enhancement of the ET is achieved in our Ag island film nanostructure because of the plasmon coupling between donor and acceptor QDs, which can take advantage of plasmon controlled ET for light emission, light harvesting, or sensing applications. Furthermore, the Ag island film can be fabricated by the facile and controllable method of sputtering technique. With this method, we can prepare samples with large reaction area for interaction between Ag film and QDs. Also, the conditions and parameters required in the preparation can be easily controlled and repeated, which provides us obvious advantage than other preparing technology such as electron beam lithography (EBL) and focused ion beam (FIB). Our findings not only promote fundamental research, but also provide a new avenue for the design of functional nanodevices.

2. Experimental section

In our work, a nanostructure of Ag island film was built to enhance the properties of ET between the donor and acceptor QDs. To form such an island film, a silver sputtering process was carried out in argon atmosphere with a pressure of 0.05 MPa (Q150T ES, Quorum). The sputtering current and time were 20 mA and 55 s, respectively. In order to prepare a spacer layer to prevent plasmon induced fluorescence quenching, a thin Al₂O₃ layer was fabricated by atomic layer deposition (ALD) method (Picosun R-series). For the generation of Al₂O₃ layers, the sample was placed at the center of the ALD chamber with the temperature of 300 °C. Then, pulses of C₃H₉Al and H₂O vapor were introduced into the chamber, and the pulse duration was 4 s separated by a N₂ purging gas for 8 s. The Al₂O₃ was slowly grown on the surface of the Ag film, and the thickness of the Al₂O₃ can be controlled by the number of pulse cycles.

To monitor the enhancement of ET between the donor QDs (QDs_D) and acceptor QDs (QDs_A), we recorded the photoluminescence (PL) spectra and decay dynamics of the donor-only, acceptor-only and donor-acceptor samples. The involved QDs_D and QDs_A are commercial carboxyl CdSe/ZnS core/shell QDs (Invitrogen Corp.). Their emission peaks are located at around 605 and 655 nm for the donor and acceptor ones, respectively. For PL emission and decay dynamics measurements, a mixture solution of QDs_D (27 nM) and QDs_A (53 nM) at a volume ratio of 2:1 in deioned water was prepared. It is worth noting that a volume ratio of 2:1 is chosen in order to make QDs_D and QDs_A on the quartz substrate exhibit almost the same PL intensity. These three types of QDs solutions were then spin-coated on the Al₂O₃–coated Ag film or a clean silica substrate at 2000 r.p.m. for 150 s to make sure a uniform distribution of the QDs on the surface of the nanostuctured film or the quartz substrate. After these treatments, the PL emission spectra and decay dynamics of the QDs-loaded samples were recorded.

The PL emissions from the QDs-loaded samples were collected under the reflection configuration. In our optical measurement, a mode-locked Ti: sapphire laser (MaiTai, Spectra Physics) was used to generate an s–polarized laser with a pulse width of ~100 fs and a repetition rate of 79 MHz. To launch the localized surface plasmon in the Ag films and excite the QDs, the wavelength of laser beam was tuned to 400 nm with a power of 100 μW, and the individual exposure lasted for 100 ms. The scattering noise was filtered using a band-pass filter (BPF), and the PL from the sample was collected by a focusing lens and a long-wave-pass filter (LWPF) with a cutoff wavelength of 550 nm before entering the liquid-nitrogen-cooled CCD (SPEC-10, Princeton Instruments). The time-resolved PL decay traces were recorded using a time-correlated single-photon counting system (PicoQuant GmbH).

3. Results and discussions

Figure 1(a) shows the top-view scanning electron microscopy (SEM, Zeiss Auriga) image of an Ag island film on the silica substrate. The inset is the measured corresponding absorption spectrum, which was taken on an ultraviolet-visible-near infrared spectrometer (Lambda 950, PerkinElmer) at normal incidence. From the SEM image, one can see the Ag film shows a semicontinuous network nanostructure with numerous narrow gaps below length scale of 10 nm. These nanogaps support large surface plasmon electric field enhancements (discussed later) which can increase PL or promote ET of nanoemitters. The island film is also called percolating film characterized by a flat absorption curve at the near infrared region [41,42], just as shown in the inset picture of Fig. 1(a). Figure 1(b) exhibits pulse cycle dependence of Al₂O₃ layer’s thickness, in which results of 5 samples with pulse cycle n = 25, 50, 200, 500, 1000 were presented. Since the measured thickness for these 5 samples was respectively 3, 5, 20, 50 and 100 nm, it can be estimated that the growth thickness of per cycle is 0.1 nm, which is in accordance with previous report [43,44]. In order to study the distance–dependence of the PEET process, the Al₂O₃ layer was applied to serve as the interval spacer for modulating the distance between QDs and Ag island film, and the thicknesses d of the alumina layers were chosen to be 4 nm, 8 nm, 12 nm and 16 nm respectively. Figure 1(c) shows the tilt-view SEM image of an Ag island film coated with 4 nm Al₂O₃ layer, from which island-like surface morphology with narrow nanogaps can also be found. After analyzing the inset picture of Fig. 1(c), one can see the combined QDs are well dispersed with a density of about 3000 μm⁻², which is similar to our previous result [45].

Fig. 1 (a) SEM image of an Ag island film. Inset is the measured absorption spectrum of the Ag island film at normal incidence;(b) Pulse cycle dependence of Al₂O₃ layer’s thickness (c) SEM image of an Ag island film with 4 nm Al₂O₃ spacer layer. Inset is the SEM image of the 4 nm Al₂O₃ coated Ag island film with combined QDs spin dispersed on the surface. The scale bar of inset is 50 nm.

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The PEET process was investigated firstly by PL measurements, and the results are presented in Fig. 2. Figure 2(a) shows the PL spectra from the QDs_D and QDs_A on the Ag films, with emission peaks located at 605 and 655 nm, respectively. The PL spectra from the QDs on the silica substrate are indicated by the black solid (QDs_D) and dash (QDs_A) curves as a reference. As shown in Fig. 2(a), the two emission peaks both increase obviously as the thickness of Al₂O₃ layer is decreased since plasmonic field enhancement caused by Ag film increases with the approaching of QDs. Figure 2(b) presents the PL spectra from the mixed donor-acceptor QDs on the Ag films, from where the PL spectra exhibits two emission peaks located at 605 and 655 nm simultaneity. It should be noticed that the existence of Al₂O₃ layer effectively avoids the plasmon induced PL quenching of QDs, for an obvious PL intensity decline can be found if the QDs are dispersed on the pristine Ag island film (see the dash black line in Fig. 2(b)).

Fig. 2 (a) and (b) PL spectra of the pure and mixed QDs on Ag island film and silica substrate. The emissions around 605 and 655 nm are attributed to the QDs_D and QDs_A, respectively. The black solid and dash curves in (a) represent the PL spectra of the QDs_D and QDs_A on the silica substrate as a reference, respectively. The dash black line of (b) is the PL spectrum of QDs on pristine Ag island film. (c) The PL plasmon enhancement factors of QDs. (d) The PL intensity ratio of QDs_A to QDs_D for pure QDs (blue) and mixed QDs (red).

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From the spectra analysis based on the PL data from Fig. 2(a) and 2(b), an enhanced energy transfer phenomena can be found. As it is shown in Fig. 2(c), when the distance between QDs and Ag film is below 8 nm, the PL plasmon enhancement factor (I_Ag/I_SiO2) of QDs_D is larger than that of QDs_A for pure QDs coated on Ag film (blue and red solid curves); however, in the condition that mixed QDs loaded on Ag film, the PL enhancement factor of QDs_D becomes much smaller than that of QDs_D (blue and red dash curves). The obvious increment and decline of PL enhancement factors of QDs_A and QDs_D can be attributed to the fact that the energy transfer from donor to acceptor QDs is largely enhanced by the existence of surface plasmon supported by Ag island film. We also calculated the PL intensity ratio of QDs_A to QDs_D (I_{QDs_A}/I_{QDs_D}), and the results are presented in Fig. 2(d). Comparing the two curves (red and blue) which represented the values of I_{QDs_A}/I_{QDs_D} for mixed QDs and pure QDs spin-coated on Ag film, a clearly rising of I_{QDs_A}/I_{QDs_D} can be found when the distance between the mixed QDs and Ag film is smaller than 8 nm. The sharp PL enhancement of QDs_A stems from the energy transfer from the QDs_D, which confirms the efficient process of PEET in our proposed nanosystem composed of mixed QDs and Ag island film (the origin will be discussed later).

In the PEET process, accompanied by the PL enhancement of the QDs with metallic nanostructures, a modified emission rate from the acceptor and donor QDs is also expected. Therefore, a time-correlated single-photon counting system was also applied to further study the dynamics of the PEET between quantum dots. The normalized time-resolved decay dynamics of the donor and accepter QDs are shown in Fig. 3(a) and 3(b).The decay curves of the donor and accepter QDs on silica follow a single exponential function with a lifetime of 5.2 and 9.8 ns, respectively. On the other side, the time-resolved PL decay traces of QDs coated on the Ag films exhibit a two-component exponential behavior with the form:

I_{PL} (t) = A_{f} e^{- t / t_{f}} + A_{s} e^{- t / t_{s}}

where A_f and A_s are the weight factors of the fast and slow decay processes, respectively. t_f and t_s are the corresponding lifetimes (emission rate τ = 1/t).

Fig. 3 (a) and (b) Normalized time-resolved PL spectra of the QDs_D and QDs_A on the quartz substrate and Ag island films with varying thicknesses of the Al₂O₃ spacer layer. (c) and (d) The calculated emission rate (blue) and normalized weight factors of the fast decay component of the QDs_D and QDs_A on Ag island films with varying thicknesses of the Al₂O₃ spacer layer. As the thickness of the spacer layers decreasing from 16 nm to 4 nm, the fast emission rate of the QDs_D increases from 0.45 to 0.51 ns⁻¹ and the corresponding normalized A_f rate rises from 47.3% to 94.8%; the fast emission rate of the QDs_A accelerates from 0.32 to 0.38 ns⁻¹ and the corresponding normalized A_f rate enhances from 79.8% to 95.4%.

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By fitting the measured decay curves, we obtained the t_s of QDs on Ag film are the same with that of QDs deposited on silica substrate, while the t_f values are much shorter and emission rates (τ_f = 1/t_f) accelerate with the thickness decrease of Al₂O₃ spacer layer (Fig. 3(c) and 3(d)). Take the sample with d = 4 nm as an example, the measured t_f are 1.98 and 2.65 ns for QDs_D and QDs_A, with calculated τ_f being 0.51 and 0.38 ns⁻¹, respectively. Furthermore, the normalized weight factors of fast decay process (A_f/(A_f + A_s)) for both QDs_D and QDs_A also dramatically increase as QDs getting closer to the Ag island film. Since the existence of surface plasmon induced electric field enhancements and the ET process bring to the emission rate acceleration phenomenon, the fast decay contribution is from the QDs which interact with Ag film and involved in energy transfer, while the slow decay process can be attribute to the QDs which participate in the interaction weakly. Considering that the normalized weight factors of fast decay component nearly reach 1 (~0.95) as the thickness of Al₂O₃ spacer layer reducing from 16 to 4 nm, it is clear to see that excition-plasmon coupling in our nanosystem gets to a very strong degree.

In order to reveal the underlying physical mechanism of the PEET, we carried out computational analysis to simulate the electric field distribution of the Ag island film by using the finite difference time domain (FDTD) method. The size of the basic simulation unit cell is x = 200 nm, y = 200 nm, with the mesh of 0.5 nm. In Fig. 4(a), the gray substrate is silica substrate, and the average thicknesses of Ag film and Al₂O₃ spacer layer are ~20 and 4 nm, respectively. The enhancement of the electric field is calculated with different z values at wavelength of 400 and 605 nm. The PL intensity of the acceptor ( $I_{D - A}^{A}$ ) in the donor-acceptor sample has the relationship as [46–48]:

\begin{array}{l} I_{(D-A)}^{A} \propto N^{(D)} \cdot {| f^{(D)} (λ_{e x c}) |}^{2} \cdot n E T R \cdot N^{(A)} \\ n E T R = \frac{{| {\vec{n}}_{A} \cdot \overset{\leftrightarrow}{G} ({\vec{r}}_{A}, {\vec{r}}_{D}, ω) \cdot {\vec{n}}_{D} |}^{2}}{{| {\vec{n}}_{A} \cdot {\overset{\leftrightarrow}{G}}_{v a c} ({\vec{r}}_{A}, {\vec{r}}_{D}, ω) \cdot {\vec{n}}_{D} |}^{2}} = \frac{{| {\vec{n}}_{A} \cdot {\vec{E}}_{D} ({\vec{r}}_{A}, ω) |}^{2}}{{| {\vec{n}}_{A} \cdot {\vec{E}}_{D, v a c} ({\vec{r}}_{A}, ω) |}^{2}} \end{array}

where

N_{}^{(D)}

and

N_{}^{(A)}

are the number of the donors and acceptors, respectively.

| f_{}^{(D)} (λ_{e x c}) |^{2}

is the field enhancement factor of the donors at the excitation wavelength; nETR is the normalized energy transfer rate, which means normalized to the case in vacuum (without Ag island film). From Fig. 4(b), one can clearly see the electric field enhancements at 400 nm, which makes the estimated maximum

| f_{}^{(D)} (λ_{e x c} = 400 n m) |^{2}

to be 36. Also, we can obtain enhanced electric field at the wavelength of 605 nm, which is the reason for decay rate accelerations (also be seen in Fig. 3). Moreover, it is easy to tell that electric field shows larger enhancements when we set the calculation region closer to the surface, and this fact is in accordance with the results shown by the distance dependence of PL and time-resolved PL measurements discussed in Fig. 2 and 3.

Fig. 4 (a) 3D model of disordered film with 4 nm isolated layer on silica substrate. (b) Electric field distribution of Ag film with different height from the surface of silica substrate at the wavelength of 400 and 605 nm. (c) The calculated normalized energy transfer rate of QDs on Ag island film coated by Al₂O₃ layer with varying thicknesses.

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In the calculation of nETR, ${\vec{n}}_{A}$ and ${\vec{n}}_{D}$ are the unit vectors along the directions of the dipole moments of the acceptor and donor, respectively; ω is the transition frequency, $\overset{\leftrightarrow}{G} ({\vec{r}}_{A}, {\vec{r}}_{D}, ω)$ is the dyadic Green function, ${\vec{E}}_{D} ({\vec{r}}_{A}, ω)$ is the electric field at the position of the acceptor induced by the donor dipole in the our structures, while ${\overset{\leftrightarrow}{G}}_{v a c} ({\vec{r}}_{A}, {\vec{r}}_{D}, ω)$ and ${\vec{E}}_{D, v a c} ({\vec{r}}_{A}, ω)$ correspond to the case in vacuum. The calculations of the electric field induced by the dipole are performed by the finite element method with the COMSOL Multiphysics software. To simplify the calculation model, we consider 9 Ag spheres [41] with a diameter of 30 nm and an interval of 7 nm (data achieved from Fig. 1(c)). The substrate is set as SiO₂ with refractive index of 1.5. An Al₂O₃ layer with thickness of d is coated on the Ag spheres. The donor with wavelength of 605 nm is set to above the central Ag sphere with a distance of 2 nm to the Al₂O₃ layer’s surface, while the acceptor with the same wavelength is set to above the adjacent Ag sphere and with the same height of the donor. Both the directions of the dipole moments of the donor and acceptor are set to be perpendicular to the Al₂O₃ layer. The perfectly matched layer (PML) is applied for the absorption boundary. As shown in Fig. 4(c), the value of nETR increases with the decreasing of d, which suggests that the existence of Ag film facilitates the energy transfer between QDs, and larger nETR can be anticipated when the QDs approach closer to the Ag film. In addition, it is easy to find the curve of nETR presents a sharp increment when the value of d is below 8 nm. This result can well uncover the underlying physical origin of the dramatic energy transfer enhancement shown by Fig. 2 (c) and 2(d). It should be mentioned that although our theoretical model is comparatively simple, the excellent agreement between the theoretical and experimental results clearly indicates the viability of our theoretical model.

4. Conclusions

In this letter, we reported plasmon enhanced energy transfer between donor (605 nm) and acceptor (655 nm) quantum dots with the help of Ag island film. A series of samples with the distances between QDs and Ag film varying from 4 to 16 nm were fabricated, in order to systematically investigate the PEET processes. PL measurements clearly show the fluorescence intensity of QDs_A sharply increases as the mixed QDs are approaching to the surface of Ag film (especially under 8 nm), which suggests that near field electric enhancements of the Ag island make obvious PEET process occur. Furthermore, as demonstrated by the time-resolved PL spectra, the decay rates of both donor and acceptor quantum dots were accelerated: the decay rate increased from 0.19 to 0.51 ns⁻¹ for QDs_D, and from 0.10 to 0.38 ns⁻¹ for QDs_A. At the end, the underlying physical origin was analyzed by FDTD and COMSOL calculations, and large near field and QD energy transfer rate enhancements had been observed at the surface of Ag island film, which promoted the PEET of quantum dots. Based on the findings mentioned above, we clearly exhibit the efficient ET behavior in the nanosystem of QDs and Ag island film; since the Ag island film can be fabricated with high reproducibility as well as large scale, it may give rise to various applications in the design of functional nanodevices for nano-optics and quantum science.

Acknowledgments

This work was supported in part by National Basic Research Program of China (2010CB923200), the National Natural Science Foundation of China (11204385), the Fundamental Research Funds for the Central Universities (Grant 12lgpy45), and the Fund of Education department of Guangdong Province (2012LYM_0011)

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Enhance energy transfer between quantum dots by the surface plasmon of Ag island film

Abstract

1. Introduction

2. Experimental section

3. Results and discussions

4. Conclusions

Acknowledgments

References and links

Cited By

Figures (4)

Equations (2)

Optical Materials Express