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Cite this: Nanoscale, 2014, 6, 7237
Received 7th March 2014 Accepted 4th May 2014
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Polarization-dependent enhanced photoluminescence and polarization-independent emission rate of quantum dots on gold elliptical nanodisc arrays† Qiangzhong Zhu, Shupei Zheng, Shijie Lin, Tian-Ran Liu and Chongjun Jin*
DOI: 10.1039/c4nr01261e www.rsc.org/nanoscale
We have fabricated gold (Au) elliptical nanodisc (ND) arrays via threebeam interference lithography and electron beam deposition of gold. The enhanced photoluminescence intensity and emission rate of quantum dots (QDs) near to the Au elliptical NDs have been studied by tuning the nearest distance between quantum dots and Au elliptical NDs. We found that the photoluminescence intensity is polarizationdependent with the degree of polarization being equal to that of the light extinction of the Au elliptical NDs, while the emission rate is polarization-independent. This is resulted from the plasmon-coupled emission via the coupling between the QD dipole and the plasmon nano-antenna. Our experiments fully confirm the evidence of the plasmophore concept proposed recently in the interaction of the QDs with metal nanoparticles.
Noble metal nanoparticles (NPs) and nanostructures exhibit fascinating plasmonic properties, which can concentrate light into a nanoscale spatial region. Considerable interest has been attracted towards the potential applications of these structures ranging from Raman scattering,1–3 sensing4–6 and information technology7,8 to uorescence enhancement.9–12 The excitation of localized surface plasmon resonance (LSPR) results in electric eld enhancement near the surface of the NPs,13,14 which has been widely studied for uorescence enhancement.15,16 However, when a nano-emitter is situated very close to the metal surface, uorescence quenching also occurs.17 There exists an optimized distance for the largest enhancement, and then this enhancement decreases with the increase of the distance.18–22 Therefore
State Key Laboratory of Optoelectronic Materials and Technologies, School of Physics and Engineering, Sun Yat-Sen University, Guangzhou, 510275, China. E-mail: [email protected]; Fax: +862084112282; Tel: +862084112282 † Electronic supplementary information (ESI) available: The thickness of the Al2O3 layer with different cycle numbers; SEM image of the Au ND array covered with QDs; the electric eld distribution of the Au elliptical ND at two LSPR wavelengths; the emission properties of the QD–ND hybrid system with the excitation light of different polarizations; and the emission properties of the QDs on a quartz slide deposited with an Al2O3 lm of 10 nm. See DOI: 10.1039/c4nr01261e
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the distance is a very important factor for analyzing the interaction between the nano-emitter and the metal NPs. The origin of uorescence enhancement in metal NPs is from two different ways based on the plasmon resonance wavelength: excitation enhancement and emission enhancement.23–25 If the wavelength of the excitation light is near to the plasmon resonance wavelength, the strong localized electric eld near the metal NP surface can incredibly strengthen the absorption of the incident light. This will enhance the photoluminescence (PL) of the quantum dots by increasing the absorption of the incident light. On the other hand, if the emission wavelength is near to the LSPR, it will generate another effect. In this case, the emission enhancement is normally attributed to the enhancement of the local density of optical states, which is in charge of the change of the emission rate of QDs, according to Fermi's “golden rule”.26–29 And the closer the plasmon resonance wavelength is from the emission wavelength, the higher the PL enhancement and the emission rate are.30 However the plasmon-enhanced uorescence sometimes carries the emission properties of the plasmon resonance,31–35 this could not be explained by semi-classical models.26 An excited-state uorophore or exciton can interact with a nearby metal NP to create plasmons. The uorophore-/ exciton-induced plasmons can radiate to the far-eld and create observable emission.36 This emission occurs rapidly, which is the origin of the decreased lifetimes. The emission retains the same spectrum of the uorophore or exciton, so it is perhaps best to think of the uorophore/exciton–metal complex as the emitting species. Lakowicz named it as plasmophore.37 This concept has been partially conrmed by the experiment of the uorescence near to the single nanorod38 as well as silicon QDs with a silver nanoparticle array,39 however, it is not clear whether the emission rate is polarization-dependent. In this letter, we fabricated square arrays of Au elliptical nanodiscs (NDs) via three-beam interference lithography and electron beam deposition. The extinction peak of the ND arrays was designed to be 668 nm which is near to the emission peak of the QDs at 655 nm. The polarization selectivity of PL
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enhancement was observed in this Au elliptical ND–QD hybrid system. The emission polarization evolved from the QD emission polarization to the ND antenna one as the distance between the QDs and Au elliptical NDs decreased, while the emission rate enhancement was polarization-independent, which is conicting to semiclassical models.26 This proves that the polarization selectivity of PL enhancement is determined by the plasmon resonance of the Au elliptical ND antenna, and the emission light mainly comes from the Au elliptical ND antenna when the distance between the QDs and Au elliptical NDs is small enough. Our experimental results prove the theory of the plasmophore.37 Patches of f 10 mm Au elliptical ND arrays were fabricated through interference lithography and electron beam deposition of gold on quartz substrates. First, a photoresist (PR, AR 3740, Allresist) layer was spin-coated onto a quartz substrate (25 mm 25 mm) with a solution of 10 wt% (Fig. 1a). The spin speed and the spin time were 2000 rpm and 35 s, the resulting thickness of the PR lm was about 250 nm. Then through the interference lithography method, we obtained a sample with a square periodic array of elliptical holes (Fig. 1b), the lattice constant is about 400 nm. A 30 s oxygen plasma etching was applied to make the wall of holes smooth. Aer that 5 nm nickel and 50 nm gold were sequentially deposited onto the sample (Fig. 1c) via electron beam deposition. The sample was than immersed in the acetone solution for 10 minutes to make sure that the PR was completely dissolved, and rinsed with ethanol, and then blown with nitrogen gas. The sample with an Au elliptical ND array (about f 10 mm) on the quartz substrate was obtained (Fig. 1d). Aer that, the Au elliptical ND arrays were deposited with different thicknesses of Al2O3 (Fig. 1e) by using an atomic layer deposition machine (ALD, Picosun SUNALE R-150 instrument). The temperature was set to be 300 C and the pulses of C3H9Al and H2O vapor precursors were alternatively introduced into the ALD chamber under a vacuum of 14 Pa. The pulse duration was 4 s for each precursor, and the pulses were separated by purging N2 gas for 8 s. The Al2O3 was slowly grown on the surface of the Au elliptical ND and the substrate, and the growth speed was about 0.1 nm per cycle (ESI, Fig. S1†), it is quite similar to the previous report.40 Thus the nearest distance between QDs and the surface of the Au elliptical NDs can be easily adjusted with a precision of 0.1 nm.
Fig. 1
Schematic diagram for the fabrication of Au elliptical nanodisc
arrays.
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In our case, the thicknesses of the Al2O3 lms were 1, 2, 3, 5, 10 and 15 nm respectively, the corresponding cycles were 10, 20, 30, 50, 100, and 150 respectively. Finally, the commercial carboxyl CdSe/ZnS quantum dots (Invitrogen Corporation) were dispersed in deionized water and subsequently drop-cast onto the Au elliptical ND array covered with an Al2O3 layer (Fig. 1f). It is very important to note that we dropped the same amount of QD solution (5 mL) on each sample. The QDs are homogeneously distributed onto the sample (see ESI Fig. S2†). Fig. 2a shows a scanning electron microscopy (SEM) image of a Au elliptical ND array on a quartz substrate. A Zeiss Auriga-3934 electron microscope operating at an accelerating voltage of 5 kV was used for SEM. In order to see the Au elliptical NDs clearly, we sputtered 5 nm gold onto the sample. The lattice constant of the ND array is 400 nm, the major and minor axes of the Au elliptical NDs are 360 nm and 200 nm respectively. To characterize the optical properties of the Au elliptical ND array, we measured the zero-order transmission spectra at normal incidence when the electric eld of the incident waves is parallel to the minor axis (4 ¼ 0 ) and major axis (4 ¼ 90 ) respectively. The transmittance spectra were measured using a UV-VIS-NIR spectrophotometer (PerkinElmer Lambda950). The plotted extinction is dened as 1 T, where T is the transmission of the array. A strong peak at the wavelength of 668 nm appears at the extinction spectrum of the elliptical ND array when 4 ¼ 0 , and it moves to 930 nm when 4 ¼ 90 (Fig. 2b to c). The peaks slightly redshi when an Al2O3 layer was grown onto the NDs and the shi becomes larger as the thickness of the Al2O3 layer increases. The peaks can be contributed to the LSPR of the Au elliptical NDs along the minor and major axes, respectively. Large electric eld enhancement mainly occurs at the surface of the Au elliptical NDs because of the strong localized surface plasmon resonance (ESI Fig. S3†), and it could enhance the
Fig. 2 (a) Scanning electron microscopy image of a Au elliptical nanodisc array. (b) and (c) show the measured extinction spectra of the Au elliptical ND array with different thicknesses of the Al2O3 layer when the polarization of the incident light is parallel to the minor and major axes respectively. (d) Experimental (Exp) and simulated (Sim) extinction spectra of the Au elliptical ND array with a 3 nm thickness Al2O3 layer for two polarizations.
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emission property of QDs. Fig. 2d shows the measured and simulated extinction spectra of the Au elliptical ND array with a 3 nm thickness Al2O3 layer. The nite-difference time-domain (FDTD) method (FDTD Solutions, Lumerical Solutions) is used to calculate the extinction spectra. Permittivity of gold was taken from the literature of Johnson and Christy.41 In the numerical calculations, the mesh size was 2 nm, periodic boundary conditions in the x and y directions were employed. In order to avoid reections from the edges of the computational window, we imposed perfectly matching layer (PML) boundary conditions in the z direction. A very good agreement between the simulated and experimental data was observed when 4 ¼ 0 and 90 , respectively. However, the experimental spectra present broadened resonance features compared with the simulated extinction spectra. This may be caused by the deviation of the size of the Au elliptical NDs, the minor imperfections of the Au elliptical ND's surface, while this is not considered in the simulation. It is clear that if the peak of QD's uorescence is longer than 620 nm, the interaction of the QD's uorescence with gold nanodiscs is attributed to the interaction of the QD's dipole and LSPRs of gold nanodiscs, because there are only two LSPRs at this wavelength region along minor and major axes. This means that this interaction can be still thought as an interaction of the QDs with a single gold nanoparticle except the deviation of nanodiscs, because there is no lattice plasmon when the wavelength is longer than 620 nm. This experimental design helps us to reduce the difficulty of the alignment in the research of the interaction of the QDs with a single metal nanoparticle. The PL measurements were taken in a homemade confocal laser microscopy system. The excitation laser with a wavelength of 400 nm was generated by a BBO crystal pumped with a mode locked Ti:sapphire laser (Mai Tai, spectra Physics) with a pulse duration of 120 fs and a repetition rate of 79 MHz. The collected PL was coupled to a spectrometer of Acton SP2750. Then a liquid-nitrogen-cooled CCD (SPEC-10, Princeton) was used as a power detector. The pump laser (500 mW) was focused onto the sample's surface through a microscope objective (50/ 0.65) to a 5 mm diameter spot (corresponding to an area containing 120 NDs). According to the laser power and spot size, the laser power density was about 0.25 104 W cm2. This high power density was used to ensure that the PL is near the saturated pump power regime, where the PL intensity is limited by the radiative decay rate and is independent of the internal quantum efficiency. The polarization of the pump laser was kept constant during all measurements. The polarization properties of photoluminescence of the QDs remain the same whatever the polarization of the pump laser is parallel to the major or minor axes (ESI Fig. S4†). Because the excitation wavelength is far away from the LSPR wavelength of the nanodisc array, there is no excitation enhancement in the ND–QD system.39 We also measured the emission properties of the QDs without NDs, to make sure that the emission polarization of the QDs in the absence of the NDs is isotropic (ESI Fig. S5a†). The emission peak of the QDs locates at about 655 nm, and the average diameter of the QDs is measured to be about 13 nm.42 The PL components with different polarizations were analyzed
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using a polarizer, and recorded using a liquid-nitrogen-cooled CCD detector. Mainly we measured two polarized PL components with their polarizations parallel to the major (q ¼ 90 ) and minor (q ¼ 0 ) axes, respectively. The excitation laser was eliminated using a lter that absorbs light below 450 nm. The polarized PL component (q ¼ 0 ) spectra of the samples covered by an Al2O3 layer with different thicknesses (t ¼ 1, 2, 3, 5, 10 and 15 nm) are shown in Fig. 3a. The PL intensity increases with the decrease of the thickness t in the range of 3–15 nm, and it increases signicantly from t ¼ 5 nm to t ¼ 3 nm, which means that the interaction between the QDs and LSPR becomes obviously stronger. This could be proved later by the timeresolved PL spectra shown in Fig. 4. However the PL intensity drops down as we further decrease the value of t. This means that the uorescence quenching plays an important role in the PL process. The emission peak of the QDs slightly moves from 655 nm to 659 nm (t ¼ 3 nm). This is caused by the coupling between the QDs and the LSPR of Au elliptical NDs at 668 nm.39 Because the LSPR is at the longer wavelength of the uorescence peak of QDs on a quartz substrate, the coupling near to the peak of the LSPR is stronger, which results in the redistribution of the uorescence of the QDs on the Au elliptical ND–QD samples. A more detailed analysis of the polarization dependence of the PL intensity and enhancement is shown in Fig. 3b and c. In Fig. 3b, it is clear that the intensity of PL components with its polarization parallel to the minor axis (q ¼ 0 ) is the strongest, then it drops down with its polarization changing along the major axis (q ¼ 90 ). Fig. 3c shows the polar plot of the measured PL enhancement on the Au elliptical ND array with various thicknesses of the Al2O3 layer as a function of the polarization angle. It is found that the PL enhancement reaches 4.88 times for q ¼ 0 and 3.26 times for q ¼ 90 in the sample with t ¼ 3 nm. When the value of t changes from 15 nm to 3 nm, the PL enhancement curves gradually change from the circular curve to the elliptical curve in Fig. 3c. The ellipticity remains the same from t ¼ 3 nm to t ¼ 1 nm. Finally the PL enhancement curve turns to be almost the same in appearance as the extinction curve of the Au elliptical ND array at 659 nm (Fig. 3d). This means that the emission polarization evolves from the QD emission polarization to the ND antenna one as the value of t decreases. In this case, the emission process is composed of three steps:38 (1) the excitation of QDs from the ground state to the excited state, (2) the energy exchange from the QD dipole to the plasmon polariton, and (3) the emission of the Au elliptical ND antenna. Because the rst and last steps are very fast, the second step determines the emission rate, this means that the emission rate should be polarization-independent, this could be obtained from the latest plasmophore theory.37 However, it was never experimentally conrmed before. To conrm the above assumption, we rst measured the time-resolved PL component of QDs on the quartz substrate and various Au elliptical ND arrays, as shown in Fig. 4a for the case of the emission polarization along the minor axis. The timeresolved PL decay traces were recorded by a time-correlated single-photon counting system (PicoQuant GmbH). The PL decay of the QDs on the SiO2 substrate with the Al2O3 layer (t ¼ 10 nm) follows a single exponential function with a lifetime Nanoscale, 2014, 6, 7237–7242 | 7239
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Fig. 3 (a) The PL spectra of an Au elliptical ND–QD hybrid system with different thicknesses of the Al2O3 layer when q ¼ 0 . (b) The PL intensity spectra of the Au elliptical ND–QD hybrid system with the 3 nm Al2O3 layer at different emission polarization angles q. (c) Polar plots of the QD PL enhancement as a function of q at the peak wavelength, for different values of t. (d) Polar plots of the QD PL enhancement and extinction at wavelength of 659 nm when t ¼ 3 nm. The pump power is 500 mW for all experiments.
Fig. 4 (a) The time-resolved PL spectra of Au elliptical ND–QD hybrid systems for different values of t when q ¼ 0 . (b) The emission rate
and normalized Af of Au elliptical ND–QD hybrid systems as a function of t when q ¼ 0 . (c) The emission rate and normalized Af of Au elliptical ND–QD hybrid systems as a function of q when t ¼ 3 nm.
of 21.2 ns, which means that the aggregation effects caused by the assembling of QDs can be neglected.43 Meanwhile, the emission rate of QDs is also polarization-independent (ESI Fig. S5b†). The time-resolved PL curves of QDs on the Au elliptical ND array with Al2O3 layer decay in the two-component exponential formula: IPL(t) ¼ Afet/tf + Aset/ts
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(1)
where Af and As are the weight factors of the fast and slow decay processes, respectively, and tf and ts are the corresponding lifetimes. The lifetimes of the slow process extracted from the measured intensity decay curves for different samples are the same as those of QDs deposited onto the quartz substrate with the Al2O3 layer, while the fast process exhibits a much shorter lifetime (tf). The fast lifetime is caused by the QDs near to the Au elliptical NDs. Fig. 4b shows the changes of the QD emission
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rate (inverse of fast lifetime, sf ¼ 1/tf) and the normalized Af rate (Af/(Af + As)) with the thickness of Al2O3. When the thickness of the Al2O3 (t) is 3 nm, sf ¼ 0.36 109 s1 and Af/(Af + As) ¼ 0.89. Compared with the emission rate on the quartz substrate with the Al2O3 layer (s0 ¼ 1/ts ¼ 0.047 109 s1), the enhancement factor of the PL decay rate (sf/s0) reaches as high as 7.6, which means that there exists a strong coupling between the QDs and the Au elliptical NDs. The value of sf gets higher when t is smaller than 3 nm, however the uorescence quenching also occurs, which can be taken into account by introducing an additional nonradiative decay. As we increase the value of t when t > 3 nm, the QD emission rate and the normalized Af rate drop down in the same trend. This indicates that the interaction between the QDs and the Au elliptical NDs becomes weaker with the increase in t. We also measured the time-resolved PL curves of QDs on the Au elliptical ND array with different polarizations for t ¼ 3 nm. The emission rate and normalized Af were found to be the same for different polarizations (Fig. 4c). In other words, the emission rate is polarization-independent. This means that the polarization dependence of the PL emission enhancement could not be explained by the semiclassical model, which denes the PL emission enhancement by the equation: f ¼ sf/s0.26 This conrms that the emission with a fast lifetime originates from the plasmonic nanoantenna (Fig. 3d). So we could reason that the QDs close to the Au elliptical NDs can transfer energy to the NDs once they are excited, then the photons emitted from the plasmonic nanoantenna, which is a F¨ orster energy resonance transfer. In detail, according to the F¨ orster energy resonance transfer theory, the emission rate has a relationship with the relative position, orientations between the donor and the acceptor, and the overlapping region between the normalized emission spectra of the donor and the extinction spectra of the acceptor.38 Because we detected thousands of QDs in the experiment, the statistical orientations of the quantum dots should be isotropic on the Al2O3 lm without gold NDs (ESI Fig. S5†). Similarly, the orientation of the quantum dots should have no inuence on the energy transfer process due to the statistical effect for the sample of QDs on gold NDs. Most of the QDs couple with the plasmon resonance of the nanoantenna along the minor axis (q ¼ 0 ), because the extinction peak overlaps with the emission peak of the QDs, while the coupling of QDs with the plasmon resonance of the nanoantenna along the major axis (q ¼ 90 ) is trivial, because the emission peak of QDs and extinction peak have almost no overlap. One might be wondering why we still see the emitted light with fast lifetime and polarization along the major axis, according to the energy transfer theory, even the QD's dipole is vertical to the minor axis of the NDs, the dipole could also couple to plasmon through the gold ND antenna along the minor axis,38 then the energy emits from the nanoantenna. This means that the emission rate for both polarizations is determined by the statistical energy transfer process from the QDs to the nanoantenna along the minor axis, the emission rate is polarization-independent. In conclusion, we have fabricated square periodic arrays of Au elliptical NDs through interference lithography, electron beam evaporation and li-off processes. With the help of atomic
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layer deposition, we can tune the nearest distance between quantum dots and the Au elliptical NDs. Polarization-dependent enhanced photoluminescence and polarization-independent emission rate of quantum dots on the Au elliptical ND array have been observed. The emission rate has a relationship with the F¨ oster energy transfer process between the QDs and the Au elliptical NDs, which is dependent on the nearest distance between QDs and NDs. The emission polarization evolves from the QD emission polarization to the ND antenna one as the decrease of the nearest distance between the QDs and Au elliptical NDs. Our experimental results concretely prove the theory of plasmophore.
Acknowledgements The authors acknowledge the nancial support from the National Natural Science foundation of China (11174374, 11374376), the Key project of DEGP (no. 2012CXZD0001). The work is partially supported by the Chinese National Key Basic Research Special Fund (2010CB923201).
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Cite this: Nanoscale, 2014, 6, 7237
Received 7th March 2014 Accepted 4th May 2014
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Polarization-dependent enhanced photoluminescence and polarization-independent emission rate of quantum dots on gold elliptical nanodisc arrays† Qiangzhong Zhu, Shupei Zheng, Shijie Lin, Tian-Ran Liu and Chongjun Jin*
DOI: 10.1039/c4nr01261e www.rsc.org/nanoscale
We have fabricated gold (Au) elliptical nanodisc (ND) arrays via threebeam interference lithography and electron beam deposition of gold. The enhanced photoluminescence intensity and emission rate of quantum dots (QDs) near to the Au elliptical NDs have been studied by tuning the nearest distance between quantum dots and Au elliptical NDs. We found that the photoluminescence intensity is polarizationdependent with the degree of polarization being equal to that of the light extinction of the Au elliptical NDs, while the emission rate is polarization-independent. This is resulted from the plasmon-coupled emission via the coupling between the QD dipole and the plasmon nano-antenna. Our experiments fully confirm the evidence of the plasmophore concept proposed recently in the interaction of the QDs with metal nanoparticles.
Noble metal nanoparticles (NPs) and nanostructures exhibit fascinating plasmonic properties, which can concentrate light into a nanoscale spatial region. Considerable interest has been attracted towards the potential applications of these structures ranging from Raman scattering,1–3 sensing4–6 and information technology7,8 to uorescence enhancement.9–12 The excitation of localized surface plasmon resonance (LSPR) results in electric eld enhancement near the surface of the NPs,13,14 which has been widely studied for uorescence enhancement.15,16 However, when a nano-emitter is situated very close to the metal surface, uorescence quenching also occurs.17 There exists an optimized distance for the largest enhancement, and then this enhancement decreases with the increase of the distance.18–22 Therefore
State Key Laboratory of Optoelectronic Materials and Technologies, School of Physics and Engineering, Sun Yat-Sen University, Guangzhou, 510275, China. E-mail: [email protected]; Fax: +862084112282; Tel: +862084112282 † Electronic supplementary information (ESI) available: The thickness of the Al2O3 layer with different cycle numbers; SEM image of the Au ND array covered with QDs; the electric eld distribution of the Au elliptical ND at two LSPR wavelengths; the emission properties of the QD–ND hybrid system with the excitation light of different polarizations; and the emission properties of the QDs on a quartz slide deposited with an Al2O3 lm of 10 nm. See DOI: 10.1039/c4nr01261e
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the distance is a very important factor for analyzing the interaction between the nano-emitter and the metal NPs. The origin of uorescence enhancement in metal NPs is from two different ways based on the plasmon resonance wavelength: excitation enhancement and emission enhancement.23–25 If the wavelength of the excitation light is near to the plasmon resonance wavelength, the strong localized electric eld near the metal NP surface can incredibly strengthen the absorption of the incident light. This will enhance the photoluminescence (PL) of the quantum dots by increasing the absorption of the incident light. On the other hand, if the emission wavelength is near to the LSPR, it will generate another effect. In this case, the emission enhancement is normally attributed to the enhancement of the local density of optical states, which is in charge of the change of the emission rate of QDs, according to Fermi's “golden rule”.26–29 And the closer the plasmon resonance wavelength is from the emission wavelength, the higher the PL enhancement and the emission rate are.30 However the plasmon-enhanced uorescence sometimes carries the emission properties of the plasmon resonance,31–35 this could not be explained by semi-classical models.26 An excited-state uorophore or exciton can interact with a nearby metal NP to create plasmons. The uorophore-/ exciton-induced plasmons can radiate to the far-eld and create observable emission.36 This emission occurs rapidly, which is the origin of the decreased lifetimes. The emission retains the same spectrum of the uorophore or exciton, so it is perhaps best to think of the uorophore/exciton–metal complex as the emitting species. Lakowicz named it as plasmophore.37 This concept has been partially conrmed by the experiment of the uorescence near to the single nanorod38 as well as silicon QDs with a silver nanoparticle array,39 however, it is not clear whether the emission rate is polarization-dependent. In this letter, we fabricated square arrays of Au elliptical nanodiscs (NDs) via three-beam interference lithography and electron beam deposition. The extinction peak of the ND arrays was designed to be 668 nm which is near to the emission peak of the QDs at 655 nm. The polarization selectivity of PL
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enhancement was observed in this Au elliptical ND–QD hybrid system. The emission polarization evolved from the QD emission polarization to the ND antenna one as the distance between the QDs and Au elliptical NDs decreased, while the emission rate enhancement was polarization-independent, which is conicting to semiclassical models.26 This proves that the polarization selectivity of PL enhancement is determined by the plasmon resonance of the Au elliptical ND antenna, and the emission light mainly comes from the Au elliptical ND antenna when the distance between the QDs and Au elliptical NDs is small enough. Our experimental results prove the theory of the plasmophore.37 Patches of f 10 mm Au elliptical ND arrays were fabricated through interference lithography and electron beam deposition of gold on quartz substrates. First, a photoresist (PR, AR 3740, Allresist) layer was spin-coated onto a quartz substrate (25 mm 25 mm) with a solution of 10 wt% (Fig. 1a). The spin speed and the spin time were 2000 rpm and 35 s, the resulting thickness of the PR lm was about 250 nm. Then through the interference lithography method, we obtained a sample with a square periodic array of elliptical holes (Fig. 1b), the lattice constant is about 400 nm. A 30 s oxygen plasma etching was applied to make the wall of holes smooth. Aer that 5 nm nickel and 50 nm gold were sequentially deposited onto the sample (Fig. 1c) via electron beam deposition. The sample was than immersed in the acetone solution for 10 minutes to make sure that the PR was completely dissolved, and rinsed with ethanol, and then blown with nitrogen gas. The sample with an Au elliptical ND array (about f 10 mm) on the quartz substrate was obtained (Fig. 1d). Aer that, the Au elliptical ND arrays were deposited with different thicknesses of Al2O3 (Fig. 1e) by using an atomic layer deposition machine (ALD, Picosun SUNALE R-150 instrument). The temperature was set to be 300 C and the pulses of C3H9Al and H2O vapor precursors were alternatively introduced into the ALD chamber under a vacuum of 14 Pa. The pulse duration was 4 s for each precursor, and the pulses were separated by purging N2 gas for 8 s. The Al2O3 was slowly grown on the surface of the Au elliptical ND and the substrate, and the growth speed was about 0.1 nm per cycle (ESI, Fig. S1†), it is quite similar to the previous report.40 Thus the nearest distance between QDs and the surface of the Au elliptical NDs can be easily adjusted with a precision of 0.1 nm.
Fig. 1
Schematic diagram for the fabrication of Au elliptical nanodisc
arrays.
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In our case, the thicknesses of the Al2O3 lms were 1, 2, 3, 5, 10 and 15 nm respectively, the corresponding cycles were 10, 20, 30, 50, 100, and 150 respectively. Finally, the commercial carboxyl CdSe/ZnS quantum dots (Invitrogen Corporation) were dispersed in deionized water and subsequently drop-cast onto the Au elliptical ND array covered with an Al2O3 layer (Fig. 1f). It is very important to note that we dropped the same amount of QD solution (5 mL) on each sample. The QDs are homogeneously distributed onto the sample (see ESI Fig. S2†). Fig. 2a shows a scanning electron microscopy (SEM) image of a Au elliptical ND array on a quartz substrate. A Zeiss Auriga-3934 electron microscope operating at an accelerating voltage of 5 kV was used for SEM. In order to see the Au elliptical NDs clearly, we sputtered 5 nm gold onto the sample. The lattice constant of the ND array is 400 nm, the major and minor axes of the Au elliptical NDs are 360 nm and 200 nm respectively. To characterize the optical properties of the Au elliptical ND array, we measured the zero-order transmission spectra at normal incidence when the electric eld of the incident waves is parallel to the minor axis (4 ¼ 0 ) and major axis (4 ¼ 90 ) respectively. The transmittance spectra were measured using a UV-VIS-NIR spectrophotometer (PerkinElmer Lambda950). The plotted extinction is dened as 1 T, where T is the transmission of the array. A strong peak at the wavelength of 668 nm appears at the extinction spectrum of the elliptical ND array when 4 ¼ 0 , and it moves to 930 nm when 4 ¼ 90 (Fig. 2b to c). The peaks slightly redshi when an Al2O3 layer was grown onto the NDs and the shi becomes larger as the thickness of the Al2O3 layer increases. The peaks can be contributed to the LSPR of the Au elliptical NDs along the minor and major axes, respectively. Large electric eld enhancement mainly occurs at the surface of the Au elliptical NDs because of the strong localized surface plasmon resonance (ESI Fig. S3†), and it could enhance the
Fig. 2 (a) Scanning electron microscopy image of a Au elliptical nanodisc array. (b) and (c) show the measured extinction spectra of the Au elliptical ND array with different thicknesses of the Al2O3 layer when the polarization of the incident light is parallel to the minor and major axes respectively. (d) Experimental (Exp) and simulated (Sim) extinction spectra of the Au elliptical ND array with a 3 nm thickness Al2O3 layer for two polarizations.
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emission property of QDs. Fig. 2d shows the measured and simulated extinction spectra of the Au elliptical ND array with a 3 nm thickness Al2O3 layer. The nite-difference time-domain (FDTD) method (FDTD Solutions, Lumerical Solutions) is used to calculate the extinction spectra. Permittivity of gold was taken from the literature of Johnson and Christy.41 In the numerical calculations, the mesh size was 2 nm, periodic boundary conditions in the x and y directions were employed. In order to avoid reections from the edges of the computational window, we imposed perfectly matching layer (PML) boundary conditions in the z direction. A very good agreement between the simulated and experimental data was observed when 4 ¼ 0 and 90 , respectively. However, the experimental spectra present broadened resonance features compared with the simulated extinction spectra. This may be caused by the deviation of the size of the Au elliptical NDs, the minor imperfections of the Au elliptical ND's surface, while this is not considered in the simulation. It is clear that if the peak of QD's uorescence is longer than 620 nm, the interaction of the QD's uorescence with gold nanodiscs is attributed to the interaction of the QD's dipole and LSPRs of gold nanodiscs, because there are only two LSPRs at this wavelength region along minor and major axes. This means that this interaction can be still thought as an interaction of the QDs with a single gold nanoparticle except the deviation of nanodiscs, because there is no lattice plasmon when the wavelength is longer than 620 nm. This experimental design helps us to reduce the difficulty of the alignment in the research of the interaction of the QDs with a single metal nanoparticle. The PL measurements were taken in a homemade confocal laser microscopy system. The excitation laser with a wavelength of 400 nm was generated by a BBO crystal pumped with a mode locked Ti:sapphire laser (Mai Tai, spectra Physics) with a pulse duration of 120 fs and a repetition rate of 79 MHz. The collected PL was coupled to a spectrometer of Acton SP2750. Then a liquid-nitrogen-cooled CCD (SPEC-10, Princeton) was used as a power detector. The pump laser (500 mW) was focused onto the sample's surface through a microscope objective (50/ 0.65) to a 5 mm diameter spot (corresponding to an area containing 120 NDs). According to the laser power and spot size, the laser power density was about 0.25 104 W cm2. This high power density was used to ensure that the PL is near the saturated pump power regime, where the PL intensity is limited by the radiative decay rate and is independent of the internal quantum efficiency. The polarization of the pump laser was kept constant during all measurements. The polarization properties of photoluminescence of the QDs remain the same whatever the polarization of the pump laser is parallel to the major or minor axes (ESI Fig. S4†). Because the excitation wavelength is far away from the LSPR wavelength of the nanodisc array, there is no excitation enhancement in the ND–QD system.39 We also measured the emission properties of the QDs without NDs, to make sure that the emission polarization of the QDs in the absence of the NDs is isotropic (ESI Fig. S5a†). The emission peak of the QDs locates at about 655 nm, and the average diameter of the QDs is measured to be about 13 nm.42 The PL components with different polarizations were analyzed
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using a polarizer, and recorded using a liquid-nitrogen-cooled CCD detector. Mainly we measured two polarized PL components with their polarizations parallel to the major (q ¼ 90 ) and minor (q ¼ 0 ) axes, respectively. The excitation laser was eliminated using a lter that absorbs light below 450 nm. The polarized PL component (q ¼ 0 ) spectra of the samples covered by an Al2O3 layer with different thicknesses (t ¼ 1, 2, 3, 5, 10 and 15 nm) are shown in Fig. 3a. The PL intensity increases with the decrease of the thickness t in the range of 3–15 nm, and it increases signicantly from t ¼ 5 nm to t ¼ 3 nm, which means that the interaction between the QDs and LSPR becomes obviously stronger. This could be proved later by the timeresolved PL spectra shown in Fig. 4. However the PL intensity drops down as we further decrease the value of t. This means that the uorescence quenching plays an important role in the PL process. The emission peak of the QDs slightly moves from 655 nm to 659 nm (t ¼ 3 nm). This is caused by the coupling between the QDs and the LSPR of Au elliptical NDs at 668 nm.39 Because the LSPR is at the longer wavelength of the uorescence peak of QDs on a quartz substrate, the coupling near to the peak of the LSPR is stronger, which results in the redistribution of the uorescence of the QDs on the Au elliptical ND–QD samples. A more detailed analysis of the polarization dependence of the PL intensity and enhancement is shown in Fig. 3b and c. In Fig. 3b, it is clear that the intensity of PL components with its polarization parallel to the minor axis (q ¼ 0 ) is the strongest, then it drops down with its polarization changing along the major axis (q ¼ 90 ). Fig. 3c shows the polar plot of the measured PL enhancement on the Au elliptical ND array with various thicknesses of the Al2O3 layer as a function of the polarization angle. It is found that the PL enhancement reaches 4.88 times for q ¼ 0 and 3.26 times for q ¼ 90 in the sample with t ¼ 3 nm. When the value of t changes from 15 nm to 3 nm, the PL enhancement curves gradually change from the circular curve to the elliptical curve in Fig. 3c. The ellipticity remains the same from t ¼ 3 nm to t ¼ 1 nm. Finally the PL enhancement curve turns to be almost the same in appearance as the extinction curve of the Au elliptical ND array at 659 nm (Fig. 3d). This means that the emission polarization evolves from the QD emission polarization to the ND antenna one as the value of t decreases. In this case, the emission process is composed of three steps:38 (1) the excitation of QDs from the ground state to the excited state, (2) the energy exchange from the QD dipole to the plasmon polariton, and (3) the emission of the Au elliptical ND antenna. Because the rst and last steps are very fast, the second step determines the emission rate, this means that the emission rate should be polarization-independent, this could be obtained from the latest plasmophore theory.37 However, it was never experimentally conrmed before. To conrm the above assumption, we rst measured the time-resolved PL component of QDs on the quartz substrate and various Au elliptical ND arrays, as shown in Fig. 4a for the case of the emission polarization along the minor axis. The timeresolved PL decay traces were recorded by a time-correlated single-photon counting system (PicoQuant GmbH). The PL decay of the QDs on the SiO2 substrate with the Al2O3 layer (t ¼ 10 nm) follows a single exponential function with a lifetime Nanoscale, 2014, 6, 7237–7242 | 7239
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Fig. 3 (a) The PL spectra of an Au elliptical ND–QD hybrid system with different thicknesses of the Al2O3 layer when q ¼ 0 . (b) The PL intensity spectra of the Au elliptical ND–QD hybrid system with the 3 nm Al2O3 layer at different emission polarization angles q. (c) Polar plots of the QD PL enhancement as a function of q at the peak wavelength, for different values of t. (d) Polar plots of the QD PL enhancement and extinction at wavelength of 659 nm when t ¼ 3 nm. The pump power is 500 mW for all experiments.
Fig. 4 (a) The time-resolved PL spectra of Au elliptical ND–QD hybrid systems for different values of t when q ¼ 0 . (b) The emission rate
and normalized Af of Au elliptical ND–QD hybrid systems as a function of t when q ¼ 0 . (c) The emission rate and normalized Af of Au elliptical ND–QD hybrid systems as a function of q when t ¼ 3 nm.
of 21.2 ns, which means that the aggregation effects caused by the assembling of QDs can be neglected.43 Meanwhile, the emission rate of QDs is also polarization-independent (ESI Fig. S5b†). The time-resolved PL curves of QDs on the Au elliptical ND array with Al2O3 layer decay in the two-component exponential formula: IPL(t) ¼ Afet/tf + Aset/ts
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(1)
where Af and As are the weight factors of the fast and slow decay processes, respectively, and tf and ts are the corresponding lifetimes. The lifetimes of the slow process extracted from the measured intensity decay curves for different samples are the same as those of QDs deposited onto the quartz substrate with the Al2O3 layer, while the fast process exhibits a much shorter lifetime (tf). The fast lifetime is caused by the QDs near to the Au elliptical NDs. Fig. 4b shows the changes of the QD emission
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rate (inverse of fast lifetime, sf ¼ 1/tf) and the normalized Af rate (Af/(Af + As)) with the thickness of Al2O3. When the thickness of the Al2O3 (t) is 3 nm, sf ¼ 0.36 109 s1 and Af/(Af + As) ¼ 0.89. Compared with the emission rate on the quartz substrate with the Al2O3 layer (s0 ¼ 1/ts ¼ 0.047 109 s1), the enhancement factor of the PL decay rate (sf/s0) reaches as high as 7.6, which means that there exists a strong coupling between the QDs and the Au elliptical NDs. The value of sf gets higher when t is smaller than 3 nm, however the uorescence quenching also occurs, which can be taken into account by introducing an additional nonradiative decay. As we increase the value of t when t > 3 nm, the QD emission rate and the normalized Af rate drop down in the same trend. This indicates that the interaction between the QDs and the Au elliptical NDs becomes weaker with the increase in t. We also measured the time-resolved PL curves of QDs on the Au elliptical ND array with different polarizations for t ¼ 3 nm. The emission rate and normalized Af were found to be the same for different polarizations (Fig. 4c). In other words, the emission rate is polarization-independent. This means that the polarization dependence of the PL emission enhancement could not be explained by the semiclassical model, which denes the PL emission enhancement by the equation: f ¼ sf/s0.26 This conrms that the emission with a fast lifetime originates from the plasmonic nanoantenna (Fig. 3d). So we could reason that the QDs close to the Au elliptical NDs can transfer energy to the NDs once they are excited, then the photons emitted from the plasmonic nanoantenna, which is a F¨ orster energy resonance transfer. In detail, according to the F¨ orster energy resonance transfer theory, the emission rate has a relationship with the relative position, orientations between the donor and the acceptor, and the overlapping region between the normalized emission spectra of the donor and the extinction spectra of the acceptor.38 Because we detected thousands of QDs in the experiment, the statistical orientations of the quantum dots should be isotropic on the Al2O3 lm without gold NDs (ESI Fig. S5†). Similarly, the orientation of the quantum dots should have no inuence on the energy transfer process due to the statistical effect for the sample of QDs on gold NDs. Most of the QDs couple with the plasmon resonance of the nanoantenna along the minor axis (q ¼ 0 ), because the extinction peak overlaps with the emission peak of the QDs, while the coupling of QDs with the plasmon resonance of the nanoantenna along the major axis (q ¼ 90 ) is trivial, because the emission peak of QDs and extinction peak have almost no overlap. One might be wondering why we still see the emitted light with fast lifetime and polarization along the major axis, according to the energy transfer theory, even the QD's dipole is vertical to the minor axis of the NDs, the dipole could also couple to plasmon through the gold ND antenna along the minor axis,38 then the energy emits from the nanoantenna. This means that the emission rate for both polarizations is determined by the statistical energy transfer process from the QDs to the nanoantenna along the minor axis, the emission rate is polarization-independent. In conclusion, we have fabricated square periodic arrays of Au elliptical NDs through interference lithography, electron beam evaporation and li-off processes. With the help of atomic
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layer deposition, we can tune the nearest distance between quantum dots and the Au elliptical NDs. Polarization-dependent enhanced photoluminescence and polarization-independent emission rate of quantum dots on the Au elliptical ND array have been observed. The emission rate has a relationship with the F¨ oster energy transfer process between the QDs and the Au elliptical NDs, which is dependent on the nearest distance between QDs and NDs. The emission polarization evolves from the QD emission polarization to the ND antenna one as the decrease of the nearest distance between the QDs and Au elliptical NDs. Our experimental results concretely prove the theory of plasmophore.
Acknowledgements The authors acknowledge the nancial support from the National Natural Science foundation of China (11174374, 11374376), the Key project of DEGP (no. 2012CXZD0001). The work is partially supported by the Chinese National Key Basic Research Special Fund (2010CB923201).
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