ZnS core–shell quantum dots in plasmonic–photonic crystals

Materials Letters 93 (2013) 42–44

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Modified spontaneous emission from CdSe/ZnS core–shell quantum dots in plasmonic–photonic crystals G.Q. Liu a,n,1, Z.Q. Liu b,n,1, K. Huang a, Y.H. Chen a, L. Li a, F.L. Tang a, L.X. Gong a a b

College of Physics and Communication Electronics, Jiangxi Normal University, Nanchang 330022, China Department of Physics and National Laboratory of Solid State Microstructure, Nanjing University, Nanjing 210093, China

a r t i c l e i n f o

a b s t r a c t

Article history: Received 15 October 2012 Accepted 16 November 2012 Available online 27 November 2012

A plasmonic–photonic crystal was fabricated by self-assembling a three-dimensional photonic crystal (PC) on a gold film with a light-emitting film in between them. The attenuated total reflection (ATR) and photoluminescence (PL) measurements show that the coupling of surface plasmons to surface modes and additional guided modes yielded in plasmonic–photonic crystal result in two distinct reflectivity minima at 52.51 and 681 in its ATR curve and two emission peaks with different intensity at 590 nm and 620 nm in the PL spectrum. The introduced PC leads the reflectivity minimum at 50.41 for the gold film to shift to 52.51 for the plasmonic–photonic crystal due to the change in effective permittivity. & 2012 Elsevier B.V. All rights reserved.

Keywords: Optical materials and property Multilayer structure Spectroscopy

1. Introduction Since Yablonovitch investigated the inhibition of spontaneous emission (SE) in the photonic band gap (PBG) of photonic crystals (PCs) [1], significant modification of SE by PCs has been reported by infiltrating the emitters into the pre-fabricated PCs [2–4] or by placing them near the PC surface [5–9]. Recently, integrating PCs with metal films has been found to provide an excellent approach for studying the interaction of surface modes and surface plasmons (SPs) and further manipulating the photon behaviors [10,11]. For instance, more than one guided mode was excited when a surface grafting was placed on a metal film [12,13] and enhanced extraordinary transmission was achieved by depositing corrugated metal films on top of the three-dimensional (3D) PCs [14,15]. However, the preparation of high-quality 3D PCs on metal film with a light-emitting film in between them and the investigation of SE from emitters in such structures had not been performed. In this work, we propose a plasmonic–photonic crystal to integrate the functionalities of photonic and plasmonic components and to modify SE from QDs in the structure. Different from those reported in [12–14], our structure is composed of a gold film and a 3D PC with a light-emitting film in between them and the gold film locates at the bottom. Attenuated total reflection (ATR) and photoluminescence (PL) measurements were performed and some interesting phenomena

n

Corresponding authors. Tel.: þ 86 79188120370; fax: þ86 79188120370. E-mail addresses: [email protected] (G.Q. Liu), [email protected] (Z.Q. Liu). 1 These authors contributed equally to this work. 0167-577X/$ - see front matter & 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.matlet.2012.11.062

were found. The formation mechanisms of these phenomena are analyzed in detail.

2. Principles and experiments 2.1. Principles SPs are coherent charge oscillations that take place at metal/ dielectric interfaces. The propagation constant of SP waves is given by the following expression [10,16]: rffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi rffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi em ed 2p em ed ksp ¼ k0 ¼ em þ ed l0 em þ ed where em and ed are the real parts of dielectric constants of metal and dielectric material, respectively. SPs are excited only when the projection kx (x-component) of the wave vector of the incident photons matches that of surface polaritons at the gold/dielectric interface. The magnitude of kx is given by (2p/l)npsin(yint) where np is the refractive index of the prism, yint is the incidence angle, and l is the wavelength of incidence light. Since kx depends on yint, the resonance waveguide condition for the excitation of SPs can be achieved at 532 nm by varying yint, providing a means to probe the SP-photon interaction. 2.2. Fabrication of samples Our structure was prepared by self-assembly method [17,18] and the spin-coating method [19]. First, a gold film with 50 nm in thickness was deposited on a glass substrate. A SU8 photoresist (purchased from Microchem Cor., refractive index n ¼1.58, 16%)

G.Q. Liu et al. / Materials Letters 93 (2013) 42–44

doped with CdSe/ZnS core–shell quantum dots (QDs) (10  6 M) was then spin-coated on the gold film under the spin-coating speed and time of 3600 rpm and 60 s, respectively. The resulting sample (named as Au-SU8 (QDs) structure) was then baked at 90 1C for 30 min. Afterward, a face-centered cubic (fcc) PC composed of PS spheres with diameters of 260 nm (purchased from Duke Sci. Cor., polydispersion o3%, n ¼1.59) was selfassembled on the Au-SU8 (QDs) structure at 371 C and 45 mm Hg with a growth time of 5 h and the plasmonic–photonic crystal was thus formed. Last, the resulting sample was placed in a vacuum chamber for natural drying for 20 h to strengthen its

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mechanism. Fig. 1 shows a systematic diagram of the plasmonic– photonic crystal. 2.3. Experimental measurements The optical reflection mode of our structure was measured by a spectrophotometer with a halogen lamp as the light source. The ATR measurements were performed in the Kretschmann configuration (see Fig. 1) [10,20,21], wherein, a continuous wave (l ¼532 nm) is incident at the base of a hemispherical prism, onto which the glass was attached to the plasmonic–photonic crystal using an index matching fluid. As s-polarized light incident on metal films does not couple/induce SPs [10], only p-polarized light was chosen to act as the incident light in this work. The PL spectra was measured from the top of the PC by an inverted microscope (Zeiss observer A1) connected to a spectrometer. SE was excited with an Nd:YAG picosecond laser (Spectra Physics, Model 375B) operating at the wavelength of 532 nm and repetition rate of 82 MHz and having a pulse duration of 12 ps. The structure was roughly characterized by scanning electron microscopy (SEM, JEOL JSM-6700F) operating at 5.0 kV.

3. Experimental results and discussion Fig. 2a shows the SEM image of the plamonic–photonic crystal with its partial magnification image. A well-ordered fcc structure with only a little dislocation is observed, implying that the goldSU8 (QDs) structure does not influence the periodic characteristic of PC. Fig. 2b shows the reflection spectrum of the plasmonic– photonic crystal and a pronounced peak at 582 nm with a width of 25 nm (575–600 nm) is obtained, demonstrating that the PBG

Fig. 1. Systematic diagram of plasmonic–photonic crystal and ATR measurement configuration.

Reflection ratio (a. u.)

2.5 2.0 1.5 1.0 0.5 0.0 400

500

600

700

800

900

Wavelength (nm) Fig. 2. (a) SEM image of plamonic–photonic crystal with its magnification image (inset) and (b) Reflection spectrum of hybrid plasmonic–photonic crystal.

0.5

PL Intensity (a. u.)

Reflected intensity (a.u.)

1.0

θ1

0.4

θ2

0.3 Au Au-SU8(QDs) Au-SU8(QDs)-PC

0.2 30

35

40

45

Au-SU8(QDs)-PC Au-SU8(QDs) Pure QDs

0.8 0.6 0.4 0.2 0.0

50

55

60

Incident angle (degree)

65

70

550

600

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Wavelength (nm)

Fig. 3. (a) ATR curves of plasmonic–photonic crystal, gold film, and gold-SU8 (QDs) structure and (b) PL spectra of of plasmonic–photonic crystal, gold-SU8 (QDs) structure, and pure QDs.

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G.Q. Liu et al. / Materials Letters 93 (2013) 42–44

structure is essentially preserved and the gold film does not alter the diffraction of light throughout the depth of 3D PC. The ATR curves of the gold film, gold-SU8 (QDs) structure and plasmonic–photonic crystal are shown in Fig. 3a. The distinct reflectivity minimum found in the reflectivity curve of the gold film verifies that the SPs in gold films are efficiently excited by 532 nm lasers. When the light-emitting film was deposited on the gold film, the minimum still remains but a slightly angular shift. Surprisingly, when a high-quality 3D PC was self-assembled on the Au-SU8 (QDs) structure two novel physical phenomena are found. First, the minimum appearing at 50.41 for the gold film shifts to 51.51 for the gold-SU8 (QDs) structure and continuously shifts to 52.51 for the plasmonic–photonic crystal due to the change in effective permittivity of sample [12]. Second, at  681 another reflection dip appears. As is well known to all, the periodic modulation of dielectric materials of PCs acts as a reflection grating. Incorporation to the reflect mirror formed by the gold films, an optical resonant cavity is thus formed. When a single-frequency continuous wave incidents, it is separated into different orders in the resonant cavity. The reflection angle y(m) of r mth order may be related to the incidence angle as found from siny(m) ¼sinyint þmal/(2p) [12,13]. a is the lattice constant of PC. r Additional guided modes are thus occurred, enhanced by the surface modes, and coupled into the gold film at p-polarization, which can be observed as minima in the reflectivity curve of the sample [12,13]. The two minima observed in Fig. 3a correspond to the first- and second-order guided modes, and the subscripts of theta refer to the order. The PL spectra of plasmonic–photonic crystal, gold film and gold-SU8 (QDs) structure are shown in Fig. 3b. Slightly modification of SE is observed when the light-emitting film was deposited on the gold film. However, when a high-quality 3D PC was selfassembled on the gold-SU8 (QDs) structure, the PL spectrum changes significantly. Two distinct emission peaks are found at 590 nm and 620 nm, and the intensity of the peak at 620 nm is stronger than the peak at 590 nm by 30%. The latter just falls in the PBG while the former is far away from the PBG, indicating that the occurrence of the two emission peaks cannot be ascribed to the PBG of PC [1–4] but the energy coupling of photonic and plasmonic components within the light-emitting film, which leads to a significant change in the photon density of states (DOS) of the interlayer and as a result, the PL behaviors change largely.

4. Conclusions In conclusion, we have described a photonic and plasmonic platform in which the 3D PC plays a crucial role in the formation

of significant optical properties of such a structure. We therefore emphasize that the observed reflectivity minima in the ATR curve and the significant modification of SE are, mostly, a result of the coupling of SPs to surface modes and additional guided modes yielded in the resonant cavity. Our study provides a convenient and low-cost method to fabricate large-area metal-based PC structures with a nonbreaking PBG characteristic. More importantly, the rich variety of the optical phenomena of such structure can be exploited to enable further understanding of SP-photon interaction and applications for the design of PC-based light sources, resonant cavity and light trapping in photovoltaic devices.

Acknowledgements Work was funded by the National Natural Science Foundation of China (NSFC) (Nos. 11004088, 11264017), Natural Science Foundation of Jiangxi Province (Nos. 2010GQW0025, 20122BAB202006), and Science-Technology Support Project of Jiangxi Province (No. 20112BBE50033). References [1] Yablonovitch E. Phys Rev Lett 1987;58:2059–62. [2] Bjorknas K, Raynes P, Gilmour S. J Mater Sci Mater Electron 2003;14: 397–401. ¨ [3] Romanov SG, Maka T, Sotomayor Torres CM, Muller M, Zentel R. Appl Phys Lett 1999;75:1057–9. ¨ [4] Romanov SG, Chigrin DN, Solovyev VG, Maka T, Gaponik N, Eychmuller A, et al. Phys Status Solidi A 2003;197:662–72. [5] Meade RD, Brommer KD, Rappe AM, Joannopoulos JD. Phys Rev B 1991;44: 10961–4. [6] Koenderink AF, Kafesaki M, Soukoulis CM, Sandoghdar V. Opt Lett 2005;30: 3210–2. [7] Liu GQ, Li L, Gong LX, Tang FL, Wang Q. Mater Lett 2012;66:466–8. [8] Ganesh N, Zhang E, Mathias PC, Chow E, JANT Soares, Malyarchuk V, et al. Nat Nanotechnol 2007;2:515–20. [9] Liu Z, Feng T, Dai Q, Wu L, Lan S, Ding C, et al. Chin Phys B 2010;19:114210. [10] Aslan K, Previte MJR, Zhang Y, Geddes CD. Anal Chem 2008;80:7304–12. [11] Dionne JA, Sweatlock LA, Atwater HA. Phys Rev B 2005;72:075405. [12] Gitsas A, Yameen B, Lazzara TD, Steinhart M, Duran H, Knoll W. Nano Lett 2010;10:2173–6. [13] Lau KHA, Duran H, Knoll W. J Phys Chem B 2009;113:3179–89. ¨ L, Brodoceanu D, Bauerle ¨ [14] Landstrom D, Garcia-Vidal FJ, Rodrigo SG, MartinMoreno L. Opt Express 2009;17:761–72. [15] Farcau C, Atilean S. J Opt A: Pure Appl Opt 2007;9:S345–9. [16] Liu GQ, Tang FL, Li L, Gong LX, Ye ZQ. Mater Lett 2011;65:1998–2000. [17] Liu GQ, Wang ZS, Ji YH. Thin Solid Films 2010;518:5083–90. [18] Guo W, Wang M, Xia W, Dai L. J Mater Res 2012;27:1663–71. [19] Liu GQ, Wang ZS, Liao YB, Hu HH, Chen Y. Appl Opt 2009;48:2480–4. [20] Go´mez DE, Vernon KC, Mulvaney P, Davis TJ. Nano Lett 2010;10:274–8. [21] Aslan K, Zhang Y, Geddes CD. Anal Chem 2009;81:3801–8.