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Luminescence variation of organic Alq3 nanoparticles on surface of Au nanoparticles and graphene
Synthetic Metals 162 (2012) 1852–1857
Contents lists available at SciVerse ScienceDirect
Synthetic Metals journal homepage: www.elsevier.com/locate/synmet
Luminescence variation of organic Alq3 nanoparticles on surface of Au nanoparticles and graphene Jin Sun Jung a, Jin Woo Lee a, Mi Ri Seo b, Hyun Soo Lee c, Jeongyong Kim c,∗, Sang Wook Lee b,∗, Jinsoo Joo a,∗ a
Department of Physics, Korea University, Seoul 136-713, South Korea Department of Physics, Konkuk University, Seoul 143-701, South Korea c Department of Physics, University of Incheon, Incheon 406-772, South Korea b
a r t i c l e
i n f o
Article history: Received 5 July 2012 Accepted 7 August 2012 Available online 11 October 2012 Keywords: Photoluminescence Alq3 Nanoparticle Surface plasmon Graphene Energy transfer
a b s t r a c t We demonstrate the variation of photoluminescence (PL) intensity for tris(8-hydroxyquinoline) aluminum(III) (Alq3) nanoparticles (NPs) depending on the use of a thin layer of gold (Au) NPs or a graphene substrate. The high luminescence Alq3 NPs were fabricated by a re-precipitation method. Laser confocal microscope (LCM) PL spectra were measured and used to analyze the nanoscale light-emission characteristics of the Alq3 NPs with glass, a layer of Au NPs or graphene as a substrate. PL enhancement of the Alq3 NPs attached to a layer of Au NPs was observed, while a considerable reduction of PL intensity for Alq3 NPs when a single layer of graphene was used for the substrate was observed. Color charge-coupled device and LCM PL mapping images directly confirmed the variation of PL intensities. We observed that the Raman spectra of Alq3 NPs also changed according to the substrate used. The PL enhancement of Alq3 NPs on a layer of Au NPs originated from surface plasmon resonance coupling. On a single layer of graphene substrate, the PL quenching of the Alq3 NPs occurred due to a resonance energy transfer effect. © 2012 Elsevier B.V. All rights reserved.
1. Introduction -Conjugated polymers and small molecules, including their nanostructures, have been intensively studied in terms of their fundamental properties and potential application due to interest in their tunable characteristics depending on the synthetic conditions, dimensions, post-treatment, etc. [1,2]. These materials have been successfully applied to optoelectronic devices and photonics [3,4]. Tris(8-hydroxyquinoline) aluminum(III) (Alq3) is well known as an excellent light-emitting material, these characteristics were first reported for organic light emitting diodes (OLEDs) by Tang and VanSlyke in 1987 [5]. Alq3 belongs to a class of metal chelates comprising of a central Al3+ metal ion surrounded by multiple ligands, in this case three bidentate 8-hydroxyquinoline anions. The chelate net charge is zero and the cation coordination sites are all occupied. As a result, Alq3 is a remarkably stable material that can be sublimed without decomposition at temperatures up to 350 ◦ C [6]. Optoelectronic devices such as OLEDs, organic thin film
∗ Corresponding authors. Tel.: +82 2 3290 3103; fax: +82 2 927 3292. E-mail addresses: [email protected] (J. Kim), [email protected] (S.W. Lee), [email protected] (J. Joo). 0379-6779/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.synthmet.2012.08.005
transistors (OTFTs) and organic photovoltaic cells (OPVCs) have been developed using Alq3 to form a thin film [7–9]. For nanodevices using organic materials, Alq3 nanowires (NWs) were fabricated using vapor deposition and surfactant-assisted solution methods [10,11]. Mann and co-workers studied Alq3 nanoparticles (NPs) using the surfactant of sodium dodecyl sulfate (SDS) and cetyl trimethylammonium bromide (CTAB) [12]. The nanoscale characterization of the optical properties of a single unit of Alq3 nanostructure, such as the photoluminescence (PL) spectrum, have yet to be undertaken. This characterization could be important to understand the material’s intrinsic nature for nanoscale applications. The control of luminescence intensity in organic nanostructures will contribute to the development of highly efficient optoelectronic devices. The PL enhancement of organic light-emitting nanomaterials has been reported for their hybrid nanostructures with nanoscale metals [13–15]. The PL intensity of organic lightemitting nanomaterials hybridized with nanoscale metals was considerably enhanced by coupling with localized surface plasmon (SP). This is due to SP absorption energy being transferred to organic light-emitting materials when the SP absorption bands of the metals are closely matched with the energy band gap (i.e., photon excitation energy) of the light-emitting materials [16,17].
J.S. Jung et al. / Synthetic Metals 162 (2012) 1852–1857
The PL intensity of p-type organic light-emitting materials can be quenched by a junction with n-type semiconducting materials. Two-dimensional (2-D) graphene, which is a nearly transparent (letting in about 98% of incoming light) semi-metal, can induce variations of PL in organic nanostructures through resonance energy transfer [18]. A single layer of graphene has high conductivity, a flexible 2-D structure and high mobility, this can assist in the making of flexible and transparent electrodes for optoelectronic devices [19,20]. Hybrid nanostructures of inorganic and organic nanomaterials with graphene have exhibited new phenomena such as resonant energy transfer and energy storage in the battery field [18,21]. Therefore, it is possible to use these hybrid nanostructures with graphene for new functional optoelectronic nanomaterials. In this study, luminescent Alq3 NPs were fabricated by the re-precipitation method without surfactant. The Alq3 NPs were physically attached to a glass surface, a thin layer of gold (Au) NPs or a single layer of graphene. We observed that the PL intensity of the Alq3 single NPs varied with the type of substrate. The PL enhancement of Alq3 single NPs was observed when using a layer of Au NPs, this is because of SP resonance coupling and/or local field enhancement. However, using a single layer of graphene, the PL of Alq3 NPs was dramatically reduced by a resonant energy transfer effect.
2. Experimental 2.1. Sample preparation The Alq3 powder was purchased from Sigma–Aldrich Co., and used without further purification. Alq3 NPs were prepared by a conventional re-precipitation method without surfactant [22]. The Alq3 powder was dissolved in tetrahydrofuran solvent and rapidly injected into a mixed solution of distilled water and acetone. The mixed solution was stirred for 10 min and then ultrasonicated for 10 min. The hydrophobic Alq3 molecules were self-aggregated into the form of NPs in distilled water. The Au NPs were synthesized through a reduction of gold (III) derivatives using Gelbart’s method [23]. HAuCl4 3H2 O aqueous solution was stirred and added to N(C8 H17 )4 Br in toluene. This mixture was added to a C12 H25 SH and NaBH4 solution. The color of the mixed solutions changed from orange to dark brown. The mixture was stirred for 12 h, and the produced Au NPs were filtered and cleaned with ethanol to remove the toluene. The diameter of the Au NPs was about 3–5 nm. After drying, the Au NPs were dispersed in chloroform solution and 100 l of this solution was spin-coated at 2000 rpm for 65 s on to glass. The thickness and roughness of the Au NPs layer measured by atomic force microscope (AFM; Nano-Focus Ltd., Albatross) were about 50 nm and 1.66 nm, respectively. A single layered graphene was prepared by mechanical exfoliation and placed on top of a SiO2 /Si substrate [24]. A micro-contact transfer technique was used for transferring the graphene to the cover glass [25]. Firstly, the single layered graphene was coated with poly(methyl methacrylate) (PMMA) 950 K C4 and baked at 180 ◦ C for 2 min. After the baking process, the thermal tape was attached to the spin-coated PMMA film to make handling of the PMMA easier for the graphene for transfer process. The PMMA/graphene/SiO2 /Si substrate was immersed in potassium hydroxide solution and then the PMMA/graphene layer was separated from the substrate. The PMMA layer with the graphene was transferred to the target position on the cover glass using a homemade micro-manipulator. After the graphene was transferred, the PMMA film was removed by washing the cover glass in acetone. To measure laser confocal microscope (LCM) Raman and PL spectra of the Alq3 single NPs, the solution with dispersed Alq3 NPs was dropped on to the glass, a layer of Au NPs and the graphene substrate. These samples were dried in a vacuum oven for 12 h.
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Fig. 1. Normalized UV/vis absorption (black curve) and PL (blue curve) spectra of Alq3 NPs. Top insets: SEM (left) and HR-TEM (right) images of Alq3 single NPs. Bottom inset: schematic chemical structure of the Alq3 molecule. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of the article.)
2.2. Measurements For the scanning electron microscope (SEM) and high resolution transmission electron microscope (HR-TEM) images, Hitachi S-4300 and JEOL JEM-2000EX II systems were employed, respectively. To study the optical properties, ultraviolet and visible (UV/vis) absorption and PL spectra (ex = 410 nm) of the Alq3 NPs dispersed in distilled water were measured using HP-8453 and HITACHI F-7000 fluorescence spectrophotometers, respectively. The LCM Raman spectra of the Alq3 single NPs on various substrates were measured with He-Ne (ex = 633 nm) and Nd-YAG lasers (ex = 532 nm). Diffraction gratings of 1200 and 600 groovs/mm were used for the Raman spectroscopy. Acquisition time per Raman spectrum was 60 s and the typical laser power was 6–8 mW. A home-made LCM system was used for the nanoscale PL spectrum of the single NPs. An unpolarized diode laser (ex = 405 nm) was used for PL excitation. The spot size of the focused laser beam was estimated to be about 200 nm. The PL signal was focused onto a multimode fiber that acted as a pinhole for confocal detection. The other end of the multimode fiber was connected to a photomultiplier tube for PL imaging and PL spectral measurements. Therefore, we were able to measure PL characteristics on a nanometer scale. The incident laser power on the sample and the acquisition time for each LCM PL spectrum was 20 W and 47.5 ms, respectively. For the LCM PL mapping, the synchronized input laser and output detector were scanned over a 20 m × 20 m region containing the Alq3 NPs on various substrates. A total 32,768 pixel data were obtained for the LCM PL mapping images of the Alq3 NPs (one pixel includes one LCM PL spectrum), these were analyzed using WITec project software. The details of the LCM PL measurements have been reported elsewhere [13]. 3. Results and discussion 3.1. Formation, UV/vis absorption and PL spectra of Alq3 NPs The SEM and HR-TEM images of the Alq3 NPs are shown in the left and right insets, respectively, of Fig. 1. The Alq3 NPs have a spherical shape and a diameter of about 50–100 nm. From the HRTEM image, the periodic stripe pattern associated with a crystalline structure was not observed for the Alq3 NPs, indicating an amorphous phase. Fig. 1 shows normalized UV/vis absorption (black curve) and PL (blue curve) spectra of the Alq3 NPs dispersed in distilled water. The UV/vis absorption characteristic peak of the Alq3
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Fig. 2. Schematic illustrations of Alq3 NPs on (a) cover glass, (b) a layer of Au NPs and (c) graphene. Reflectance images of Alq3 NPs on the surface of (d) cover glass and (e) a single layer of graphene. (f) Schematic illustration of the LCM PL measurement system.
NPs was observed at 379 nm, this is similar to that of amorphous Alq3 films [26]. This peak originated from the 1 La electronic transition state of the Alq3 molecule [27]. The PL spectrum of the Alq3 NPs in distilled water exhibited a peak at 520 nm, which was also observed in surfactant-stabilized aqueous Alq3 colloids [12]. 3.2. Schematic illustrations and reflectance images of Alq3 NPs on various substrates Fig. 2(a)–(c) shows schematic illustrations of the Alq3 NPs on glass, a layer of Au NPs and a single layer of graphene, respectively. Fig. 2(d) shows a reflectance image of the Alq3 NPs on cover glass, in which the homogeneously dispersed Alq3 NPs are represented by the dark spots. Fig. 2(e) shows the reflectance image of Alq3 NPs on the layer of graphene, here the dotted line represents a single layer of graphene and the dark spots represent the Alq3 NPs. From this reflectance image, we can see a homogeneous dispersion of Alq3 NPs on a single layer of transparent graphene. Fig. 2(f) shows a schematic diagram of the LCM PL measurement system that characterizes the nanoscale optical properties of the Alq3 single NPs. 3.3. LCM Raman spectra of Alq3 NPs on various substrates Fig. 3(a) and (b) shows the LCM Raman spectra of the Alq3 NPs on various substrates using 633 nm and 532 nm excitation lasers,
respectively. The characteristic vibration modes of the Alq3 NPs on glass were observed at 754, 1058, 1388 and 1596 cm−1 , as shown in Fig. 3(a) and listed in Table 1. The peak at 754 cm−1 originated from the ring breathing mode. The CH bend and ring stretching modes were observed at 1058 and 1596 cm−1 , respectively. The peak at 1388 cm−1 was attributed to mixed modes that contain contributions from C C and C N stretching of the pyridyl side of the quinolate ligands as well as C H in-plane bending motions [28]. Raman characteristic peaks of the Alq3 NPs on the layer of Au NPs were clearly observed at 754, 1058, 1388 and 1596 cm−1 , these coincide with those seen on the glass substrate. The peak at 1279 cm−1 due to the CH bend mode was enhanced for the Alq3 NPs on the layer of Au NPs. Also, the peak at 1488 cm−1 for the ring stretching mode was increased. We believe that the enhancement of CH bend and ring stretching modes of Alq3 NPs was due to electromagnetic field enhancement through coupling with SPs of the metallic Au NPs [29]. The Raman spectra of the Alq3 NPs on the graphene substrate were different depending on the excitation wavelength, as shown in Fig. 3(a) and (b). When the 633 nm laser was used for the Alq3 NPs with a single layer of graphene, the unique vibration modes of graphene, i.e., the G band and 2D band peaks, were dominant at 1583 and 2641 cm−1 , respectively (see SI) [30]. The Raman peak intensity of 2D band was ∼1.8 times higher than that of G band, indicating a single layer of graphene. The Raman characteristic peaks of Alq3 NPs at 754, 1058 and 1388 cm−1 were quenched, as shown
Table 1 Assignment of Raman characteristic vibration modes to Alq3 NPs on three types of substrates. Modes
Ring breathing Ring deformation Ring deformation + Al N stretching CH bend CH bend D-band CH bend + ring stretching Ring stretching CH2 bend Ring stretching C C stretching G-band Ring stretching 2D-band
Substrates Glass
Layer of Au NPs
Graphene (ex = 633 nm)
Graphene (ex = 532 nm)
754 804 918 1058 – – 1388 1426 – – – – 1596 –
754 806 – 1058 1279 – 1388 1422 – 1488 – – 1596 –
– – – – – 1337 1388 – 1447 – 1526 1583 – 2641
– 804 919 1066 – – 1388 – – – – 1588 – 2668
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in Fig. 3(a). The peaks at 1447 and 1526 cm−1 were observed in the region of CH2 bending and C C stretching modes of carbon based materials, respectively. Using the 532 nm laser, the Raman characteristic peaks of the Alq3 NPs were observed at 804, 919, 1066 and 1388 cm−1 simultaneously with strong graphene characteristic peaks at 1588 and 2668 cm−1 (see SI), as shown in Fig. 3(b) and listed in Table 1. The Raman peak intensity of 2D band was comparable to that of G band, indicating that several layers of graphene were used for the substrate. The Raman characteristic peak at 919 cm−1 corresponding to ring deformation and Al N stretching modes of the Alq3 NPs was observed when using the glass and several layers of graphene substrates. As the excitation wavelength of the 532 nm laser for Raman spectra was closely matched with the –* transition of Alq3, some resonance enhancement might occur and Raman characteristic peaks of the Alq3 NPs could be observable. Also, the observation of Raman characteristics for the Alq3 NPs in Fig. 3(b) might be due to the use of several layers of graphene, not a single layer of graphene. 3.4. Luminescence characteristics of Alq3 NPs on various substrates
Fig. 3. LCM Raman spectra of Alq3 NPs on glass, a layer of Au NPs and the layer of graphene. The excitation wavelength (ex ) for Raman spectra was (a) 633 nm and (b) 532 nm.
The luminescence intensities of the Alq3 NPs on various substrates were directly compared through color CCD images, as shown in Fig. 4. The Alq3 NPs on glass exhibited green light emission. For the Alq3 NPs on a layer of Au NPs, the yellow-green light emission of the Alq3 NPs was much brighter than that from the glass, in the same conditions of the CCD experiments (Fig. 4(b)). Fig. 4(c) and (d) shows the color CCD and microscope images of the Alq3 NPs on a single layer of graphene, respectively. The dotted boundaries in Fig. 4(c) and (d) represent the graphene. The intensity of light emission of Alq3 NPs on the graphene was dramatically reduced in comparison with those in the outer regions (i.e., Alq3 NPs without the graphene substrate), even when the exposure time for the CCD image was set to be 10 times longer than in the other two cases (on glass and on a layer of Au NPs). Therefore, these results suggest that the luminescence intensities of Alq3 NPs were changed by the type of substrate used.
Fig. 4. Color CCD images of Alq3 NPs on the surface of (a) cover glass, (b) a layer of Au NPs and (c) graphene. (d) Microscope image of Alq3 NPs on the graphene. The dotted boundaries represent the graphene. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of the article.)
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On the contrary, the LCM PL intensity of the Alq3 NPs on a single layer of graphene was dramatically reduced in comparison with the intensities on glass or on a layer of Au NPs (Fig. 5(d)). Similar results were observed using an air-gap lens for the LCM PL measurements. That is, when the focused laser first irradiated the Alq3 NPs and then passed through the single layer of graphene, the LCM PL intensity of the Alq3 NPs on the single layer of graphene was also reduced. The average PL intensity of the Alq3 NPs on a single layer of graphene was about 15 times lower than that on glass, as shown in Fig. 5(d) and in the inset. A single layer of graphene has been known as a PL quencher, and the photo-induced excitons of the Alq3 NPs were reduced in number due to resonance energy transfer to the graphene layer, this is the reverse case of the energy transfer in the SPR coupling between the Alq3 NPs and the layer of Au NPs [18,33–35]. 4. Conclusion
Fig. 5. LCM PL mapping images of Alq3 NPs on the surface of (a) cover glass, (b) a layer of Au NPs and (c) a single layer of graphene. (d) Comparison of LCM PL spectra of Alq3 NPs on glass, a layer of Au NPs and a single layer of graphene. Inset: magnification of LCM PL spectrum of Alq3 NPs on a single layer of graphene. (For interpretation of the references to color in text, the reader is referred to the web version of the article.)
Fig. 5(a)–(c) shows the LCM PL mapping images of the Alq3 NPs on the surface of glass, a layer of Au NPs and a single layer of graphene, respectively. Using the LCM PL system with a high spatial resolution (about 200 nm), one can obtain the nanoscale PL characteristics of single nano-units. The color scale bars on the right in Fig. 5(a)–(c) represent the PL intensity in units of photon count. The brightness of the Alq3 NPs in LCM PL images was enhanced when the layer of Au NPs was used for the substrate in comparison to the brightness on glass. However, the brightness of Alq3 NPs was considerably reduced when a single layer of graphene was used for the substrate (Fig. 5(c)). Therefore, the results of the LCM PL mapping images qualitatively agree with those of the color CCD images. From the LCM PL mapping experiments, LCM PL spectra were measured from 10 ∼ 30 Alq3 NPs on each substrate, then the average LCM PL spectra were obtained and shown in Fig. 5(d). The LCM PL peak of the Alq3 NPs on a layer of Au NPs was observed at 532 nm. The LCM PL peaks of the Alq3 NPs on a single layer of graphene and on glass were observed at 521 nm. There was no shift of PL peak positions indicating no chemical interaction between the Alq3 NPs and the single layer of graphene. The Alq3 NPs were just physically adsorbed on the single layer of graphene. We observe that the LCM PL peak of the Alq3 NPs on a layer of Au NPs was red-shifted and enhanced about 5 times compared to the intensity on glass (Fig. 5(d)). The PL enhancement of the Alq3 NPs on a layer of Au NPs originated from the energy transfer effect in surface plasmon resonance (SPR) coupling and/or local field enhancement between the Au NPs (i.e., the metal nano-gap effect) [15,31]. The SP absorption band of the Au NPs is usually observed at 450–550 nm [32], this is closely matched by the PL band of Alq3 NPs (at about 520 nm). Therefore, the energy transfer between the layer of Au NPs and the Alq3 NPs in SPR coupling could occur, resulting in more excitons for PL enhancement. The local electric field enhancement between the Au NPs as a nano-gap effect could also contribute to the increase of the PL of the Alq3 NPs.
Luminescent Alq3 NPs with a diameter of 50–100 nm were fabricated by a re-precipitation method without surfactant. PL enhancements and reductions of the Alq3 NPs were observed when a layer of Au NPs and a single layer of graphene were used as substrates, respectively. Raman characteristic peaks of the Alq3 NPs varied with the type of substrates. The LCM PL enhancement of the Alq3 NPs on a layer of Au NPs originated from the energy transfer effect on SPR coupling and/or local field enhancement between the Au NPs. The reduction of LCM PL intensity from the Alq3 NPs on a single layer of graphene was due to the resonance energy transfer effect. These results were directly confirmed by color CCD images. Acknowledgement This research was supported by a National Research Foundation of Korea (NRF) grant funded by the Korean government (MEST) (No. R0A-2007-000-20053-0). Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.synthmet. 2012.08.005. References [1] S. Mann, Nature Materials 8 (2009) 781. [2] F.S. Kim, G. Ren, S.A. Jenekhe, Chemistry of Materials 23 (2011) 682. ´ S.R. Forrest, M.E. Thompson, Science 276 [3] Z. Shen, P.E. Burrows, V. Bulovic, (1997) 2009. [4] P. Peumans, S. Uchida, S.R. Forrest, Nature 425 (2003) 158. [5] C.W. Tang, S.A. VanSlyke, Applied Physics Letters 51 (1987) 913. [6] W. Zhao, W. Wei, J. Lozano, J.M. White, Chemistry of Materials 16 (2004) 750. [7] W.H. Koo, S.M. Jeong, F. Araoka, K. Ishikawa, S. Nishimura, T. Toyooka, H. Takezoe, Nature Photonics 4 (2010) 222. [8] C.-A. Di, G. Yu, Y. Liu, X. Xu, D. Wei, Y. Song, Y. Sun, Y. Wang, D. Zhu, Advanced Functional Materials 17 (2007) 1567. [9] Q.L. Song, H.B. Yang, Y. Gan, C. Gong, C.M. Li, Journal of the American Chemical Society 132 (2010) 4554. [10] C.-P. Cho, C.-A. Wu, T.-P. Perng, Advanced Functional Materials 16 (2006) 819. [11] W. Chen, Q. Peng, Y. Li, Advanced Materials 20 (2008) 2747. [12] A.M. Collins, S.N. Olof, J.M. Mitchels, S. Mann, Journal of Materials Chemistry 19 (2009) 3950. [13] D.H. Park, M.S. Kim, J. Joo, Chemical Society Reviews 39 (2010) 2439. [14] T. Tamai, M. Watanabe, Y. Hatanaka, H. Tsujiwaki, N. Nishioka, K. Matsukawa, Langmuir 24 (2008) 14203. [15] M.S. Kim, D.H. Park, E.H. Cho, K.H. Kim, Q.-H. Park, H. Song, D.-C. Kim, J. Kim, J. Joo, ACS Nano 3 (2009) 1329. [16] M.-K. Lee, T.G. Kim, W. Kim, Y.-M. Sung, Journal of Physical Chemistry C 112 (2008) 10079. [17] J. Joo, D.H. Park, M.-Y. Jeong, Y.B. Lee, H.S. Kim, W.J. Choi, Q.-H. Park, H.-J. Kim, D.-C. Kim, J. Kim, Advanced Materials 19 (2007) 2824.
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Contents lists available at SciVerse ScienceDirect
Synthetic Metals journal homepage: www.elsevier.com/locate/synmet
Luminescence variation of organic Alq3 nanoparticles on surface of Au nanoparticles and graphene Jin Sun Jung a, Jin Woo Lee a, Mi Ri Seo b, Hyun Soo Lee c, Jeongyong Kim c,∗, Sang Wook Lee b,∗, Jinsoo Joo a,∗ a
Department of Physics, Korea University, Seoul 136-713, South Korea Department of Physics, Konkuk University, Seoul 143-701, South Korea c Department of Physics, University of Incheon, Incheon 406-772, South Korea b
a r t i c l e
i n f o
Article history: Received 5 July 2012 Accepted 7 August 2012 Available online 11 October 2012 Keywords: Photoluminescence Alq3 Nanoparticle Surface plasmon Graphene Energy transfer
a b s t r a c t We demonstrate the variation of photoluminescence (PL) intensity for tris(8-hydroxyquinoline) aluminum(III) (Alq3) nanoparticles (NPs) depending on the use of a thin layer of gold (Au) NPs or a graphene substrate. The high luminescence Alq3 NPs were fabricated by a re-precipitation method. Laser confocal microscope (LCM) PL spectra were measured and used to analyze the nanoscale light-emission characteristics of the Alq3 NPs with glass, a layer of Au NPs or graphene as a substrate. PL enhancement of the Alq3 NPs attached to a layer of Au NPs was observed, while a considerable reduction of PL intensity for Alq3 NPs when a single layer of graphene was used for the substrate was observed. Color charge-coupled device and LCM PL mapping images directly confirmed the variation of PL intensities. We observed that the Raman spectra of Alq3 NPs also changed according to the substrate used. The PL enhancement of Alq3 NPs on a layer of Au NPs originated from surface plasmon resonance coupling. On a single layer of graphene substrate, the PL quenching of the Alq3 NPs occurred due to a resonance energy transfer effect. © 2012 Elsevier B.V. All rights reserved.
1. Introduction -Conjugated polymers and small molecules, including their nanostructures, have been intensively studied in terms of their fundamental properties and potential application due to interest in their tunable characteristics depending on the synthetic conditions, dimensions, post-treatment, etc. [1,2]. These materials have been successfully applied to optoelectronic devices and photonics [3,4]. Tris(8-hydroxyquinoline) aluminum(III) (Alq3) is well known as an excellent light-emitting material, these characteristics were first reported for organic light emitting diodes (OLEDs) by Tang and VanSlyke in 1987 [5]. Alq3 belongs to a class of metal chelates comprising of a central Al3+ metal ion surrounded by multiple ligands, in this case three bidentate 8-hydroxyquinoline anions. The chelate net charge is zero and the cation coordination sites are all occupied. As a result, Alq3 is a remarkably stable material that can be sublimed without decomposition at temperatures up to 350 ◦ C [6]. Optoelectronic devices such as OLEDs, organic thin film
∗ Corresponding authors. Tel.: +82 2 3290 3103; fax: +82 2 927 3292. E-mail addresses: [email protected] (J. Kim), [email protected] (S.W. Lee), [email protected] (J. Joo). 0379-6779/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.synthmet.2012.08.005
transistors (OTFTs) and organic photovoltaic cells (OPVCs) have been developed using Alq3 to form a thin film [7–9]. For nanodevices using organic materials, Alq3 nanowires (NWs) were fabricated using vapor deposition and surfactant-assisted solution methods [10,11]. Mann and co-workers studied Alq3 nanoparticles (NPs) using the surfactant of sodium dodecyl sulfate (SDS) and cetyl trimethylammonium bromide (CTAB) [12]. The nanoscale characterization of the optical properties of a single unit of Alq3 nanostructure, such as the photoluminescence (PL) spectrum, have yet to be undertaken. This characterization could be important to understand the material’s intrinsic nature for nanoscale applications. The control of luminescence intensity in organic nanostructures will contribute to the development of highly efficient optoelectronic devices. The PL enhancement of organic light-emitting nanomaterials has been reported for their hybrid nanostructures with nanoscale metals [13–15]. The PL intensity of organic lightemitting nanomaterials hybridized with nanoscale metals was considerably enhanced by coupling with localized surface plasmon (SP). This is due to SP absorption energy being transferred to organic light-emitting materials when the SP absorption bands of the metals are closely matched with the energy band gap (i.e., photon excitation energy) of the light-emitting materials [16,17].
J.S. Jung et al. / Synthetic Metals 162 (2012) 1852–1857
The PL intensity of p-type organic light-emitting materials can be quenched by a junction with n-type semiconducting materials. Two-dimensional (2-D) graphene, which is a nearly transparent (letting in about 98% of incoming light) semi-metal, can induce variations of PL in organic nanostructures through resonance energy transfer [18]. A single layer of graphene has high conductivity, a flexible 2-D structure and high mobility, this can assist in the making of flexible and transparent electrodes for optoelectronic devices [19,20]. Hybrid nanostructures of inorganic and organic nanomaterials with graphene have exhibited new phenomena such as resonant energy transfer and energy storage in the battery field [18,21]. Therefore, it is possible to use these hybrid nanostructures with graphene for new functional optoelectronic nanomaterials. In this study, luminescent Alq3 NPs were fabricated by the re-precipitation method without surfactant. The Alq3 NPs were physically attached to a glass surface, a thin layer of gold (Au) NPs or a single layer of graphene. We observed that the PL intensity of the Alq3 single NPs varied with the type of substrate. The PL enhancement of Alq3 single NPs was observed when using a layer of Au NPs, this is because of SP resonance coupling and/or local field enhancement. However, using a single layer of graphene, the PL of Alq3 NPs was dramatically reduced by a resonant energy transfer effect.
2. Experimental 2.1. Sample preparation The Alq3 powder was purchased from Sigma–Aldrich Co., and used without further purification. Alq3 NPs were prepared by a conventional re-precipitation method without surfactant [22]. The Alq3 powder was dissolved in tetrahydrofuran solvent and rapidly injected into a mixed solution of distilled water and acetone. The mixed solution was stirred for 10 min and then ultrasonicated for 10 min. The hydrophobic Alq3 molecules were self-aggregated into the form of NPs in distilled water. The Au NPs were synthesized through a reduction of gold (III) derivatives using Gelbart’s method [23]. HAuCl4 3H2 O aqueous solution was stirred and added to N(C8 H17 )4 Br in toluene. This mixture was added to a C12 H25 SH and NaBH4 solution. The color of the mixed solutions changed from orange to dark brown. The mixture was stirred for 12 h, and the produced Au NPs were filtered and cleaned with ethanol to remove the toluene. The diameter of the Au NPs was about 3–5 nm. After drying, the Au NPs were dispersed in chloroform solution and 100 l of this solution was spin-coated at 2000 rpm for 65 s on to glass. The thickness and roughness of the Au NPs layer measured by atomic force microscope (AFM; Nano-Focus Ltd., Albatross) were about 50 nm and 1.66 nm, respectively. A single layered graphene was prepared by mechanical exfoliation and placed on top of a SiO2 /Si substrate [24]. A micro-contact transfer technique was used for transferring the graphene to the cover glass [25]. Firstly, the single layered graphene was coated with poly(methyl methacrylate) (PMMA) 950 K C4 and baked at 180 ◦ C for 2 min. After the baking process, the thermal tape was attached to the spin-coated PMMA film to make handling of the PMMA easier for the graphene for transfer process. The PMMA/graphene/SiO2 /Si substrate was immersed in potassium hydroxide solution and then the PMMA/graphene layer was separated from the substrate. The PMMA layer with the graphene was transferred to the target position on the cover glass using a homemade micro-manipulator. After the graphene was transferred, the PMMA film was removed by washing the cover glass in acetone. To measure laser confocal microscope (LCM) Raman and PL spectra of the Alq3 single NPs, the solution with dispersed Alq3 NPs was dropped on to the glass, a layer of Au NPs and the graphene substrate. These samples were dried in a vacuum oven for 12 h.
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Fig. 1. Normalized UV/vis absorption (black curve) and PL (blue curve) spectra of Alq3 NPs. Top insets: SEM (left) and HR-TEM (right) images of Alq3 single NPs. Bottom inset: schematic chemical structure of the Alq3 molecule. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of the article.)
2.2. Measurements For the scanning electron microscope (SEM) and high resolution transmission electron microscope (HR-TEM) images, Hitachi S-4300 and JEOL JEM-2000EX II systems were employed, respectively. To study the optical properties, ultraviolet and visible (UV/vis) absorption and PL spectra (ex = 410 nm) of the Alq3 NPs dispersed in distilled water were measured using HP-8453 and HITACHI F-7000 fluorescence spectrophotometers, respectively. The LCM Raman spectra of the Alq3 single NPs on various substrates were measured with He-Ne (ex = 633 nm) and Nd-YAG lasers (ex = 532 nm). Diffraction gratings of 1200 and 600 groovs/mm were used for the Raman spectroscopy. Acquisition time per Raman spectrum was 60 s and the typical laser power was 6–8 mW. A home-made LCM system was used for the nanoscale PL spectrum of the single NPs. An unpolarized diode laser (ex = 405 nm) was used for PL excitation. The spot size of the focused laser beam was estimated to be about 200 nm. The PL signal was focused onto a multimode fiber that acted as a pinhole for confocal detection. The other end of the multimode fiber was connected to a photomultiplier tube for PL imaging and PL spectral measurements. Therefore, we were able to measure PL characteristics on a nanometer scale. The incident laser power on the sample and the acquisition time for each LCM PL spectrum was 20 W and 47.5 ms, respectively. For the LCM PL mapping, the synchronized input laser and output detector were scanned over a 20 m × 20 m region containing the Alq3 NPs on various substrates. A total 32,768 pixel data were obtained for the LCM PL mapping images of the Alq3 NPs (one pixel includes one LCM PL spectrum), these were analyzed using WITec project software. The details of the LCM PL measurements have been reported elsewhere [13]. 3. Results and discussion 3.1. Formation, UV/vis absorption and PL spectra of Alq3 NPs The SEM and HR-TEM images of the Alq3 NPs are shown in the left and right insets, respectively, of Fig. 1. The Alq3 NPs have a spherical shape and a diameter of about 50–100 nm. From the HRTEM image, the periodic stripe pattern associated with a crystalline structure was not observed for the Alq3 NPs, indicating an amorphous phase. Fig. 1 shows normalized UV/vis absorption (black curve) and PL (blue curve) spectra of the Alq3 NPs dispersed in distilled water. The UV/vis absorption characteristic peak of the Alq3
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Fig. 2. Schematic illustrations of Alq3 NPs on (a) cover glass, (b) a layer of Au NPs and (c) graphene. Reflectance images of Alq3 NPs on the surface of (d) cover glass and (e) a single layer of graphene. (f) Schematic illustration of the LCM PL measurement system.
NPs was observed at 379 nm, this is similar to that of amorphous Alq3 films [26]. This peak originated from the 1 La electronic transition state of the Alq3 molecule [27]. The PL spectrum of the Alq3 NPs in distilled water exhibited a peak at 520 nm, which was also observed in surfactant-stabilized aqueous Alq3 colloids [12]. 3.2. Schematic illustrations and reflectance images of Alq3 NPs on various substrates Fig. 2(a)–(c) shows schematic illustrations of the Alq3 NPs on glass, a layer of Au NPs and a single layer of graphene, respectively. Fig. 2(d) shows a reflectance image of the Alq3 NPs on cover glass, in which the homogeneously dispersed Alq3 NPs are represented by the dark spots. Fig. 2(e) shows the reflectance image of Alq3 NPs on the layer of graphene, here the dotted line represents a single layer of graphene and the dark spots represent the Alq3 NPs. From this reflectance image, we can see a homogeneous dispersion of Alq3 NPs on a single layer of transparent graphene. Fig. 2(f) shows a schematic diagram of the LCM PL measurement system that characterizes the nanoscale optical properties of the Alq3 single NPs. 3.3. LCM Raman spectra of Alq3 NPs on various substrates Fig. 3(a) and (b) shows the LCM Raman spectra of the Alq3 NPs on various substrates using 633 nm and 532 nm excitation lasers,
respectively. The characteristic vibration modes of the Alq3 NPs on glass were observed at 754, 1058, 1388 and 1596 cm−1 , as shown in Fig. 3(a) and listed in Table 1. The peak at 754 cm−1 originated from the ring breathing mode. The CH bend and ring stretching modes were observed at 1058 and 1596 cm−1 , respectively. The peak at 1388 cm−1 was attributed to mixed modes that contain contributions from C C and C N stretching of the pyridyl side of the quinolate ligands as well as C H in-plane bending motions [28]. Raman characteristic peaks of the Alq3 NPs on the layer of Au NPs were clearly observed at 754, 1058, 1388 and 1596 cm−1 , these coincide with those seen on the glass substrate. The peak at 1279 cm−1 due to the CH bend mode was enhanced for the Alq3 NPs on the layer of Au NPs. Also, the peak at 1488 cm−1 for the ring stretching mode was increased. We believe that the enhancement of CH bend and ring stretching modes of Alq3 NPs was due to electromagnetic field enhancement through coupling with SPs of the metallic Au NPs [29]. The Raman spectra of the Alq3 NPs on the graphene substrate were different depending on the excitation wavelength, as shown in Fig. 3(a) and (b). When the 633 nm laser was used for the Alq3 NPs with a single layer of graphene, the unique vibration modes of graphene, i.e., the G band and 2D band peaks, were dominant at 1583 and 2641 cm−1 , respectively (see SI) [30]. The Raman peak intensity of 2D band was ∼1.8 times higher than that of G band, indicating a single layer of graphene. The Raman characteristic peaks of Alq3 NPs at 754, 1058 and 1388 cm−1 were quenched, as shown
Table 1 Assignment of Raman characteristic vibration modes to Alq3 NPs on three types of substrates. Modes
Ring breathing Ring deformation Ring deformation + Al N stretching CH bend CH bend D-band CH bend + ring stretching Ring stretching CH2 bend Ring stretching C C stretching G-band Ring stretching 2D-band
Substrates Glass
Layer of Au NPs
Graphene (ex = 633 nm)
Graphene (ex = 532 nm)
754 804 918 1058 – – 1388 1426 – – – – 1596 –
754 806 – 1058 1279 – 1388 1422 – 1488 – – 1596 –
– – – – – 1337 1388 – 1447 – 1526 1583 – 2641
– 804 919 1066 – – 1388 – – – – 1588 – 2668
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in Fig. 3(a). The peaks at 1447 and 1526 cm−1 were observed in the region of CH2 bending and C C stretching modes of carbon based materials, respectively. Using the 532 nm laser, the Raman characteristic peaks of the Alq3 NPs were observed at 804, 919, 1066 and 1388 cm−1 simultaneously with strong graphene characteristic peaks at 1588 and 2668 cm−1 (see SI), as shown in Fig. 3(b) and listed in Table 1. The Raman peak intensity of 2D band was comparable to that of G band, indicating that several layers of graphene were used for the substrate. The Raman characteristic peak at 919 cm−1 corresponding to ring deformation and Al N stretching modes of the Alq3 NPs was observed when using the glass and several layers of graphene substrates. As the excitation wavelength of the 532 nm laser for Raman spectra was closely matched with the –* transition of Alq3, some resonance enhancement might occur and Raman characteristic peaks of the Alq3 NPs could be observable. Also, the observation of Raman characteristics for the Alq3 NPs in Fig. 3(b) might be due to the use of several layers of graphene, not a single layer of graphene. 3.4. Luminescence characteristics of Alq3 NPs on various substrates
Fig. 3. LCM Raman spectra of Alq3 NPs on glass, a layer of Au NPs and the layer of graphene. The excitation wavelength (ex ) for Raman spectra was (a) 633 nm and (b) 532 nm.
The luminescence intensities of the Alq3 NPs on various substrates were directly compared through color CCD images, as shown in Fig. 4. The Alq3 NPs on glass exhibited green light emission. For the Alq3 NPs on a layer of Au NPs, the yellow-green light emission of the Alq3 NPs was much brighter than that from the glass, in the same conditions of the CCD experiments (Fig. 4(b)). Fig. 4(c) and (d) shows the color CCD and microscope images of the Alq3 NPs on a single layer of graphene, respectively. The dotted boundaries in Fig. 4(c) and (d) represent the graphene. The intensity of light emission of Alq3 NPs on the graphene was dramatically reduced in comparison with those in the outer regions (i.e., Alq3 NPs without the graphene substrate), even when the exposure time for the CCD image was set to be 10 times longer than in the other two cases (on glass and on a layer of Au NPs). Therefore, these results suggest that the luminescence intensities of Alq3 NPs were changed by the type of substrate used.
Fig. 4. Color CCD images of Alq3 NPs on the surface of (a) cover glass, (b) a layer of Au NPs and (c) graphene. (d) Microscope image of Alq3 NPs on the graphene. The dotted boundaries represent the graphene. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of the article.)
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On the contrary, the LCM PL intensity of the Alq3 NPs on a single layer of graphene was dramatically reduced in comparison with the intensities on glass or on a layer of Au NPs (Fig. 5(d)). Similar results were observed using an air-gap lens for the LCM PL measurements. That is, when the focused laser first irradiated the Alq3 NPs and then passed through the single layer of graphene, the LCM PL intensity of the Alq3 NPs on the single layer of graphene was also reduced. The average PL intensity of the Alq3 NPs on a single layer of graphene was about 15 times lower than that on glass, as shown in Fig. 5(d) and in the inset. A single layer of graphene has been known as a PL quencher, and the photo-induced excitons of the Alq3 NPs were reduced in number due to resonance energy transfer to the graphene layer, this is the reverse case of the energy transfer in the SPR coupling between the Alq3 NPs and the layer of Au NPs [18,33–35]. 4. Conclusion
Fig. 5. LCM PL mapping images of Alq3 NPs on the surface of (a) cover glass, (b) a layer of Au NPs and (c) a single layer of graphene. (d) Comparison of LCM PL spectra of Alq3 NPs on glass, a layer of Au NPs and a single layer of graphene. Inset: magnification of LCM PL spectrum of Alq3 NPs on a single layer of graphene. (For interpretation of the references to color in text, the reader is referred to the web version of the article.)
Fig. 5(a)–(c) shows the LCM PL mapping images of the Alq3 NPs on the surface of glass, a layer of Au NPs and a single layer of graphene, respectively. Using the LCM PL system with a high spatial resolution (about 200 nm), one can obtain the nanoscale PL characteristics of single nano-units. The color scale bars on the right in Fig. 5(a)–(c) represent the PL intensity in units of photon count. The brightness of the Alq3 NPs in LCM PL images was enhanced when the layer of Au NPs was used for the substrate in comparison to the brightness on glass. However, the brightness of Alq3 NPs was considerably reduced when a single layer of graphene was used for the substrate (Fig. 5(c)). Therefore, the results of the LCM PL mapping images qualitatively agree with those of the color CCD images. From the LCM PL mapping experiments, LCM PL spectra were measured from 10 ∼ 30 Alq3 NPs on each substrate, then the average LCM PL spectra were obtained and shown in Fig. 5(d). The LCM PL peak of the Alq3 NPs on a layer of Au NPs was observed at 532 nm. The LCM PL peaks of the Alq3 NPs on a single layer of graphene and on glass were observed at 521 nm. There was no shift of PL peak positions indicating no chemical interaction between the Alq3 NPs and the single layer of graphene. The Alq3 NPs were just physically adsorbed on the single layer of graphene. We observe that the LCM PL peak of the Alq3 NPs on a layer of Au NPs was red-shifted and enhanced about 5 times compared to the intensity on glass (Fig. 5(d)). The PL enhancement of the Alq3 NPs on a layer of Au NPs originated from the energy transfer effect in surface plasmon resonance (SPR) coupling and/or local field enhancement between the Au NPs (i.e., the metal nano-gap effect) [15,31]. The SP absorption band of the Au NPs is usually observed at 450–550 nm [32], this is closely matched by the PL band of Alq3 NPs (at about 520 nm). Therefore, the energy transfer between the layer of Au NPs and the Alq3 NPs in SPR coupling could occur, resulting in more excitons for PL enhancement. The local electric field enhancement between the Au NPs as a nano-gap effect could also contribute to the increase of the PL of the Alq3 NPs.
Luminescent Alq3 NPs with a diameter of 50–100 nm were fabricated by a re-precipitation method without surfactant. PL enhancements and reductions of the Alq3 NPs were observed when a layer of Au NPs and a single layer of graphene were used as substrates, respectively. Raman characteristic peaks of the Alq3 NPs varied with the type of substrates. The LCM PL enhancement of the Alq3 NPs on a layer of Au NPs originated from the energy transfer effect on SPR coupling and/or local field enhancement between the Au NPs. The reduction of LCM PL intensity from the Alq3 NPs on a single layer of graphene was due to the resonance energy transfer effect. These results were directly confirmed by color CCD images. Acknowledgement This research was supported by a National Research Foundation of Korea (NRF) grant funded by the Korean government (MEST) (No. R0A-2007-000-20053-0). Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.synthmet. 2012.08.005. References [1] S. Mann, Nature Materials 8 (2009) 781. [2] F.S. Kim, G. Ren, S.A. Jenekhe, Chemistry of Materials 23 (2011) 682. ´ S.R. Forrest, M.E. Thompson, Science 276 [3] Z. Shen, P.E. Burrows, V. Bulovic, (1997) 2009. [4] P. Peumans, S. Uchida, S.R. Forrest, Nature 425 (2003) 158. [5] C.W. Tang, S.A. VanSlyke, Applied Physics Letters 51 (1987) 913. [6] W. Zhao, W. Wei, J. Lozano, J.M. White, Chemistry of Materials 16 (2004) 750. [7] W.H. Koo, S.M. Jeong, F. Araoka, K. Ishikawa, S. Nishimura, T. Toyooka, H. Takezoe, Nature Photonics 4 (2010) 222. [8] C.-A. Di, G. Yu, Y. Liu, X. Xu, D. Wei, Y. Song, Y. Sun, Y. Wang, D. Zhu, Advanced Functional Materials 17 (2007) 1567. [9] Q.L. Song, H.B. Yang, Y. Gan, C. Gong, C.M. Li, Journal of the American Chemical Society 132 (2010) 4554. [10] C.-P. Cho, C.-A. Wu, T.-P. Perng, Advanced Functional Materials 16 (2006) 819. [11] W. Chen, Q. Peng, Y. Li, Advanced Materials 20 (2008) 2747. [12] A.M. Collins, S.N. Olof, J.M. Mitchels, S. Mann, Journal of Materials Chemistry 19 (2009) 3950. [13] D.H. Park, M.S. Kim, J. Joo, Chemical Society Reviews 39 (2010) 2439. [14] T. Tamai, M. Watanabe, Y. Hatanaka, H. Tsujiwaki, N. Nishioka, K. Matsukawa, Langmuir 24 (2008) 14203. [15] M.S. Kim, D.H. Park, E.H. Cho, K.H. Kim, Q.-H. Park, H. Song, D.-C. Kim, J. Kim, J. Joo, ACS Nano 3 (2009) 1329. [16] M.-K. Lee, T.G. Kim, W. Kim, Y.-M. Sung, Journal of Physical Chemistry C 112 (2008) 10079. [17] J. Joo, D.H. Park, M.-Y. Jeong, Y.B. Lee, H.S. Kim, W.J. Choi, Q.-H. Park, H.-J. Kim, D.-C. Kim, J. Kim, Advanced Materials 19 (2007) 2824.
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