- Email: [email protected]
A nano-patterned organic light-emitting diode with high extraction efficiency
ARTICLE IN PRESS
Journal of Crystal Growth 288 (2006) 119–122 www.elsevier.com/locate/jcrysgro
A nano-patterned organic light-emitting diode with high extraction efficiency Benzhong Wanga,, Lin Kea, Soo-Jin Chuaa,b a
Institute of Materials Research and Engineering, 3 Research Link, Singapore 117602, Singapore b Centre for Optoelectronics, National University of Singapore, Singapore 119260, Singapore Available online 18 January 2006
Abstract In this paper, we report experimental results of photoluminescence (PL) of nanopatterned organic light-emitting diode (OLED). Nanosphere lithography is used to create the nanopatterns. Comparing the emission of the OLED from regions with and without the nanopatterns, the PL intensity resulting from the regions with the nanopatterns is increased by 43%. In addition, we also observed that the PL intensity is increased 6 times if the organic active layer is deposited on the nanopatterns and covered by a thin metal film of Al. This strong increase of PL intensity may be attributed to surface plasmon effects. r 2005 Published by Elsevier B.V. PACS: 78.66.Vs; 78.55.Kz Keywords: A1. Nanostructures; B2. Colloidal crystals; B2. Nanosphere lithography; B2. Photonic crystals; B3. Organic light emitting diodes
1. Introduction There have been various efforts to increase the extraction efficiency of organic light-emitting diodes (OLEDs). The photons emitted in the active region of OLEDs are either directly transmitted into the air or coupled into the glass and gone through the total internal reflection. The light is emitted in three types of modes: the external modes where the light escapes the substrate, the substrate waveguided modes, and the ITO/organic-waveguided modes [1,2] According to classical ray optics theory, the coupling efficiencies of the external, substrate, and ITO/organic modes are 18.9%, 34.2%, and 46.9%, respectively. There are lots of rooms for improving the coupling efficiency [1,2] One critical figure of merit for OLEDs is the external coupling efficiency, Zcp,ext, which links the external quantum efficiency photon/electron, Zext, to the internal quantum efficiency, Zint, by the relation Zext ¼ Zcp,ext Zint [3]. A number of methods have been suggested for increasing the output coupling efficiency. For example, Corresponding author. Fax: +65 68720785.
E-mail address: [email protected] (B. Wang). 0022-0248/$ - see front matter r 2005 Published by Elsevier B.V. doi:10.1016/j.jcrysgro.2005.12.039
random textures or ordered microlens arrays have been employed on the top surface of the glass substrate in order to minimize total internal reflection [4,5]. Photonic crystal (PC) patterns have previously been used with the aim of increasing the extraction efficiency of twodimensional (2D) slab InGaAs light-emitting structures [6]. Similarly, PC structure can be added to OLED devices. In this paper, photonics crystal patterns are well fabricated on the SiO2/glass substrate, and the effects of such nanostructures on the optical properties have been studied. In addition, metal coved nanostructured OLED has also been investigated. 6 times increase of photoluminescence (PL) emitted from this OLED has been observed which may attribute to surface plasmon effects of the metal covered OLED nanostructures. Self-assembly of nanospheres is a well-know method to fabricate large area nanostructures with sufficient control of size and shape, beside the advantages of low cost and high throughput. It has been demonstrated that both 2D and 3D crystalline structures can be obtained on substrate surface by the self-assembly of colloidal particles. These crystalline structures can be used as template for building nanostructures [7–12]. In this paper, we report primary
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experimental results regarding the fabrication of nanostructured OLED and its optical properties. It has been demonstrated theoretically that: for the triangular PC array of short dielectric posts, there is a wide range of structural parameters (a, the constant of PC lattice, and r, the radius of the posts) to be chosen to increase light extraction efficiency [13]. Therefore 600 nm constant of the nanopillar arrays has been chosen for the ease of fabrication to investigate experimentally the effects of such periodic nanostructures on the optical properties of the OLED. 2. Sample preparation The detailed procedure for fabricating large area periodic nanopillar arrays on a glass substrate is outlined in Fig. 1. Glass sheets coated with a SiO2 film of 500-nm thickness by PECVD are used as the substrates. The substrates are first coated with monodispersed polystyrene (PS) sphere solution to form large area hexagonally closepacked (hcp) structures on the surface. Depending on the concentration and speed of the spin-coater, monolayer or bilayer PS spheres can be obtained. The sphere suspensions were bought from Duke Scientific Corporation. The
Fig. 1. Schematic illustrations of the procedure for fabricating nanostructured OLED by nanosphere lithography: (a) arranging hexagonal closely packed PS spheres on a SiO2 film deposited by PECVD on a glass substrate as template; (b) thinning the template by reactive ion etching (RIE) with oxygen; (c) etching the SiO2 film through the left PS sphere arrays to form SiO2 pillars; (d) removing the PS template; (e) depositing organic active layer, and metal film.
concentration of the purchased suspension with mean diameter 900 nm and 60076 nm was 10% w/w, which were used as received. Spin coating of the PS spheres was carried out at the speed of 800 rpm. At these conditions, most area of the substrate was covered with monolayered spheres. Diameters of the 2D hcp arranged PS spheres were then reduced by dry etching with oxygen reactive ions etching (RIE) as shown in Fig. 1B at these conditions: O2 flow, 20 sccm, RF power, 200 W, chamber pressure, 8 Torr. The left PS spheres with thinned diameter are served as masks to create nanostructures onto the SiO2 film. Inductive coupled plasma (ICP) with mixed CF4 and O2 was employed to etch the SiO2 film at these conditions: CF4 flow, 20 sccm, O2 flow, 10 sccm, RF power, 100 W, ICP power, 500 W, chamber pressure, 8 Torr. Finally, the wafer with 2D ordered SiO2 nanopillars was cleaned by ultrasonic agitation in toluene solution to remove any left PS material. For comparing the effects of 2D ordered nanostructures on the optical properties of the nanostructured OLED, we prepared a sample of which only half of the substrate was coved by monolayer close-packed nanospheres. The SiO2 nanopillars were then fabricated by the processes as depicted in Fig. 1. Fig. 2(a) shows a photonic micrograph of the monolayer hcp arranged nanospheres. As seen in the image, a uniform arrangement of the spheres with single domain is obtained in a large area although line and point defects were also observed in somewhere. Fig. 2(b) shows a SEM image of the SiO2 nanopillar array obtained by the processes described in Fig. 1. As observed, the hcp lattice is quite uniform. The lattice constant is 615 nm, which corresponds to the diameter of the PS spheres used here. Several vacancies and dislocations mainly due to spheres with large or small diameters are observed. Fig. 2(c) shows a high magnification SEM image of the SiO2 nanopillars. It is clearly observed that the side wall of the pillars is not vertical. The tilted wall of the nanopillars should be due to the fact that diameters of the PS spheres were reduced during ICP dry etching. The mean diameter of the SiO2 pillars is about 300 nm measured on the top of the pillars. The separation of the pillars is 150 nm. The height of the pillars is estimated to be 350 nm by cross-section view of SEM. The Alq3 material with thickness of 75 nm was then deposited on the SiO2 nanopillar structures by evaporating. A 5 A˚ lithium fluoride (LiF) and 200 nm thick Al film was deposited on the top of the Alq3 active layer. The devices formed by this method were then sealed by another glass substrate with epoxy to protect the devices. To investigate the metal film effects on the optical properties of the nanostructured OLED devices, half of the substrate was masked. The micro-PL was used to investigate the light emitting properties of the nanostructured OLED devices at room temperature. He–Ge laser with 325 nm line was used as excitation source. The laser was incident at back side of the devices studied here. The beam spot of the laser is estimated to be 5 mm.
ARTICLE IN PRESS B. Wang et al. / Journal of Crystal Growth 288 (2006) 119–122
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Fig. 3. (a) PL spectra of the sample with (solid line) and without (dashed line) SiO2 nanopillar arrays. (b) PL spectra of the sample with (solid line) and without (dashed line) covering of Al film on the nanostructured devices.
Fig. 2. (a) Photograph shows arrangement of the PS spheres, (b) low magnification, and (c) 201 tilted-view high magnification SEM images of periodic ordered SiO2 nanopillar arrays.
3. Results and discussion Strong PL is observed for all measured devices by eyes implying the quality of the devices is quite good. Fig. 3(a) shows PL results of the sample with and without the SiO2 nanopillar arrays. The emitting wavelength locates at around 520 nm. It is clearly observed that the PL intensity
of the sample with the SiO2 nanopillar arrays is increased about 2 times comparing with that without these nanostructures. The increase of PL for the sample with the nanostructures may be explained by two mechanisms. One is the roughness of the surface formed by the nanostructures of the sample. As discussed above, the side walls of the nanopillars are not vertical. Therefore, some active materials could be deposited on the side walls. The light emitted from the side walls should do not suffer the total internal reflection, which results more light extraction. In the other hand, 2D PC may also play an important role for extracting light from the periodic ordered structures, the 2D photonic band gap (PBG) effect. Light extraction in this case is enhanced in two possible ways [14,15]. (1) because multiple scattering of photons by lattice of periodically varying refractive indices in the PCs acts to form PBGs in which lateral propagation of the Bloch guided modes is prohibited, light generated in the band gap region can couple only to radiation modes and is radiated outward. (2) the refractive index periodicity creates a cutoff frequency for guided modes. Guided modes are folded by the PCs at the Brillouin zone boundaries, allowing
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phase matching to the radiation modes that lie above this cut-off frequency [16,17]. The guided modes that phase match to the radiation modes become leaky resonances of the PCs which Bragg scatter the light emitted from of the active region. Fig. 3(b) shows typical PL results of the samples with and without metal film covering. It is clearly observed that PL intensity is increased about 6 times. A small increase in the luminescence intensity might be expected for the metal coated sample because the metal reflects pump light back through the active layer, doubling the effective path of the incident light. However, the large increase of 6 times in the PL intensity for the sample with metal coatings must have other mechanism. One possible explanation is the effect of surface plasmon. It has been demonstrated that surface plasmons can increase the density of states and spontaneous emission rate in the semiconductor [18–20], and lead to the enhancement of light emission by surface plasmon-quantum well coupling [21,22]. However, if the metal/semiconductor interface were perfectly flat, it would be difficult to extract light from the surface plasmon mode, a non-propagating evanescent wave. In our case, the PL enhancement may be attributed to strong interaction with the surface plasmons. Electron–hole pairs excited within the Alq3 active layer couple to electron vibrations at the interface of metal/active layer when the energies of electron–hole pairs in the active layer and of the metal surface plasmon are similar. Then, electron–hole recombination produce surface plasmons instead of photons, and this new recombination path increases the spontaneous recombination rate. On the other hand, the small periodic ordered structures can efficiently scatter the surface plasmons as light to increase the PL intensity. 4. Conclusion Nanosphere lithography is successfully used to create the nanopatterns on SiO2/glass substrates. Significant enhancement of PL intensity has been obtained for the OLED formed on the nanopatterns. In addition, we also observed
that the PL intensity is increased 6 times if the organic active layer is deposited on the nanopatterns and covered by a thin metal film of Al. This strong increase of PL intensity may be attributed to surface plasmon effects. Right now, we just obtained primary results of the nanostructured OLED. To fully understand the optical properties of the devices, more investigations are need.
References [1] M.-H. Lu, J.C. Sturm, Appl. Phys. Lett. 78 (2001) 1927. [2] N.C. Greenham, R.H. Friend, D.D.C. Bradley, Adv. Mater. 6 (1994) 491. [3] C.F. Madigan, M.-H. Lu, J.C. Sturm, Appl. Phys. Lett. 76 (2000) 1650. [4] I. Schnitzer, E. Yablonovitch, Appl. Phys. Lett. 63 (1993) 2174. [5] M.-H. Lu, J.C. Sturm, Appl. Phys. Lett. 78 (2001) 1927. [6] M. Boroditsky, T.F. Krauss, R. Coccioli, R. Vrijen, R. Bhat, E. Yablonovitch, Appl. Phys. Lett. 75 (1999) 1036. [7] P. Jiang, J.F. Bertone, V.L. Colvin, Science 291 (2001) 453. [8] X. Chen, Z. Chen, N. Fu, G. Lu, B. Yang, Adv. Mater. 15 (2003) 1413. [9] U.C. Fischer, H.P. Zingsheim, J. Vac. Sci. Technol. B 19 (1981) 881. [10] F. Burmeister, C. Schafle, B. Keilhofer, C. Bechinger, J. Boneberg, P. Leiderer, Adv. Mater. 10 (1998) 495. [11] C.L. Haynes, R.P. Van Duyne, J. Phys. Chem. B 105 (2001) 5599. [12] I.W. Hamley, Angew. Chem. Int. Ed. 42 (2003) 1692. [13] S. Fan, P.R. Villeneuve, J.D. Joannopoulos, Phys. Rev. Lett. 78 (1997) 3294. [14] M. Boroditsky, R. Vrijen, T.F. Krauss, R. Coccioli, R. Bhat, E. Yablonovitch, J. Lightwave Technol. 17 (1999) 2096. [15] T.N. Oder, J. Shakya, J.Y. lin, H.J. Jiang, Appl. Phys. Lett. 83 (2003) 1231. [16] A.A. Erchak, D.J. Ripin, S.H. Fan, P. Pakich, J.D. Joannoupoulos, E.P. Ippen, G.S. Petrich, L.A. Kolodziejski, Appl. Phys. Lett. 78 (2001) 563. [17] V.N. Astratov, I.S. Culashaw, R.M. Stevenson, D.M. Whittaker, M.S. Skolnick, T.F. Krauss, R.M. De La Rue, J. Lightwave Technol. 17 (1999) 2050. [18] N.E. Hecher, R.A. Hopfel, N. Sawaki, Physica E 2 (1998) 98. [19] W.L. Barnes, J. Lightwave Technol. 17 (1999) 2170. [20] S. Gianordoli, et al., Appl. Phys. Lett. 77 (2000) 2295. [21] I. Gontijo, Phys. Rev. B 60 (1999) 11564. [22] A. Neogi, et al., Phys. Rev. B 66 (2002) 153305.
Journal of Crystal Growth 288 (2006) 119–122 www.elsevier.com/locate/jcrysgro
A nano-patterned organic light-emitting diode with high extraction efficiency Benzhong Wanga,, Lin Kea, Soo-Jin Chuaa,b a
Institute of Materials Research and Engineering, 3 Research Link, Singapore 117602, Singapore b Centre for Optoelectronics, National University of Singapore, Singapore 119260, Singapore Available online 18 January 2006
Abstract In this paper, we report experimental results of photoluminescence (PL) of nanopatterned organic light-emitting diode (OLED). Nanosphere lithography is used to create the nanopatterns. Comparing the emission of the OLED from regions with and without the nanopatterns, the PL intensity resulting from the regions with the nanopatterns is increased by 43%. In addition, we also observed that the PL intensity is increased 6 times if the organic active layer is deposited on the nanopatterns and covered by a thin metal film of Al. This strong increase of PL intensity may be attributed to surface plasmon effects. r 2005 Published by Elsevier B.V. PACS: 78.66.Vs; 78.55.Kz Keywords: A1. Nanostructures; B2. Colloidal crystals; B2. Nanosphere lithography; B2. Photonic crystals; B3. Organic light emitting diodes
1. Introduction There have been various efforts to increase the extraction efficiency of organic light-emitting diodes (OLEDs). The photons emitted in the active region of OLEDs are either directly transmitted into the air or coupled into the glass and gone through the total internal reflection. The light is emitted in three types of modes: the external modes where the light escapes the substrate, the substrate waveguided modes, and the ITO/organic-waveguided modes [1,2] According to classical ray optics theory, the coupling efficiencies of the external, substrate, and ITO/organic modes are 18.9%, 34.2%, and 46.9%, respectively. There are lots of rooms for improving the coupling efficiency [1,2] One critical figure of merit for OLEDs is the external coupling efficiency, Zcp,ext, which links the external quantum efficiency photon/electron, Zext, to the internal quantum efficiency, Zint, by the relation Zext ¼ Zcp,ext Zint [3]. A number of methods have been suggested for increasing the output coupling efficiency. For example, Corresponding author. Fax: +65 68720785.
E-mail address: [email protected] (B. Wang). 0022-0248/$ - see front matter r 2005 Published by Elsevier B.V. doi:10.1016/j.jcrysgro.2005.12.039
random textures or ordered microlens arrays have been employed on the top surface of the glass substrate in order to minimize total internal reflection [4,5]. Photonic crystal (PC) patterns have previously been used with the aim of increasing the extraction efficiency of twodimensional (2D) slab InGaAs light-emitting structures [6]. Similarly, PC structure can be added to OLED devices. In this paper, photonics crystal patterns are well fabricated on the SiO2/glass substrate, and the effects of such nanostructures on the optical properties have been studied. In addition, metal coved nanostructured OLED has also been investigated. 6 times increase of photoluminescence (PL) emitted from this OLED has been observed which may attribute to surface plasmon effects of the metal covered OLED nanostructures. Self-assembly of nanospheres is a well-know method to fabricate large area nanostructures with sufficient control of size and shape, beside the advantages of low cost and high throughput. It has been demonstrated that both 2D and 3D crystalline structures can be obtained on substrate surface by the self-assembly of colloidal particles. These crystalline structures can be used as template for building nanostructures [7–12]. In this paper, we report primary
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experimental results regarding the fabrication of nanostructured OLED and its optical properties. It has been demonstrated theoretically that: for the triangular PC array of short dielectric posts, there is a wide range of structural parameters (a, the constant of PC lattice, and r, the radius of the posts) to be chosen to increase light extraction efficiency [13]. Therefore 600 nm constant of the nanopillar arrays has been chosen for the ease of fabrication to investigate experimentally the effects of such periodic nanostructures on the optical properties of the OLED. 2. Sample preparation The detailed procedure for fabricating large area periodic nanopillar arrays on a glass substrate is outlined in Fig. 1. Glass sheets coated with a SiO2 film of 500-nm thickness by PECVD are used as the substrates. The substrates are first coated with monodispersed polystyrene (PS) sphere solution to form large area hexagonally closepacked (hcp) structures on the surface. Depending on the concentration and speed of the spin-coater, monolayer or bilayer PS spheres can be obtained. The sphere suspensions were bought from Duke Scientific Corporation. The
Fig. 1. Schematic illustrations of the procedure for fabricating nanostructured OLED by nanosphere lithography: (a) arranging hexagonal closely packed PS spheres on a SiO2 film deposited by PECVD on a glass substrate as template; (b) thinning the template by reactive ion etching (RIE) with oxygen; (c) etching the SiO2 film through the left PS sphere arrays to form SiO2 pillars; (d) removing the PS template; (e) depositing organic active layer, and metal film.
concentration of the purchased suspension with mean diameter 900 nm and 60076 nm was 10% w/w, which were used as received. Spin coating of the PS spheres was carried out at the speed of 800 rpm. At these conditions, most area of the substrate was covered with monolayered spheres. Diameters of the 2D hcp arranged PS spheres were then reduced by dry etching with oxygen reactive ions etching (RIE) as shown in Fig. 1B at these conditions: O2 flow, 20 sccm, RF power, 200 W, chamber pressure, 8 Torr. The left PS spheres with thinned diameter are served as masks to create nanostructures onto the SiO2 film. Inductive coupled plasma (ICP) with mixed CF4 and O2 was employed to etch the SiO2 film at these conditions: CF4 flow, 20 sccm, O2 flow, 10 sccm, RF power, 100 W, ICP power, 500 W, chamber pressure, 8 Torr. Finally, the wafer with 2D ordered SiO2 nanopillars was cleaned by ultrasonic agitation in toluene solution to remove any left PS material. For comparing the effects of 2D ordered nanostructures on the optical properties of the nanostructured OLED, we prepared a sample of which only half of the substrate was coved by monolayer close-packed nanospheres. The SiO2 nanopillars were then fabricated by the processes as depicted in Fig. 1. Fig. 2(a) shows a photonic micrograph of the monolayer hcp arranged nanospheres. As seen in the image, a uniform arrangement of the spheres with single domain is obtained in a large area although line and point defects were also observed in somewhere. Fig. 2(b) shows a SEM image of the SiO2 nanopillar array obtained by the processes described in Fig. 1. As observed, the hcp lattice is quite uniform. The lattice constant is 615 nm, which corresponds to the diameter of the PS spheres used here. Several vacancies and dislocations mainly due to spheres with large or small diameters are observed. Fig. 2(c) shows a high magnification SEM image of the SiO2 nanopillars. It is clearly observed that the side wall of the pillars is not vertical. The tilted wall of the nanopillars should be due to the fact that diameters of the PS spheres were reduced during ICP dry etching. The mean diameter of the SiO2 pillars is about 300 nm measured on the top of the pillars. The separation of the pillars is 150 nm. The height of the pillars is estimated to be 350 nm by cross-section view of SEM. The Alq3 material with thickness of 75 nm was then deposited on the SiO2 nanopillar structures by evaporating. A 5 A˚ lithium fluoride (LiF) and 200 nm thick Al film was deposited on the top of the Alq3 active layer. The devices formed by this method were then sealed by another glass substrate with epoxy to protect the devices. To investigate the metal film effects on the optical properties of the nanostructured OLED devices, half of the substrate was masked. The micro-PL was used to investigate the light emitting properties of the nanostructured OLED devices at room temperature. He–Ge laser with 325 nm line was used as excitation source. The laser was incident at back side of the devices studied here. The beam spot of the laser is estimated to be 5 mm.
ARTICLE IN PRESS B. Wang et al. / Journal of Crystal Growth 288 (2006) 119–122
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Fig. 3. (a) PL spectra of the sample with (solid line) and without (dashed line) SiO2 nanopillar arrays. (b) PL spectra of the sample with (solid line) and without (dashed line) covering of Al film on the nanostructured devices.
Fig. 2. (a) Photograph shows arrangement of the PS spheres, (b) low magnification, and (c) 201 tilted-view high magnification SEM images of periodic ordered SiO2 nanopillar arrays.
3. Results and discussion Strong PL is observed for all measured devices by eyes implying the quality of the devices is quite good. Fig. 3(a) shows PL results of the sample with and without the SiO2 nanopillar arrays. The emitting wavelength locates at around 520 nm. It is clearly observed that the PL intensity
of the sample with the SiO2 nanopillar arrays is increased about 2 times comparing with that without these nanostructures. The increase of PL for the sample with the nanostructures may be explained by two mechanisms. One is the roughness of the surface formed by the nanostructures of the sample. As discussed above, the side walls of the nanopillars are not vertical. Therefore, some active materials could be deposited on the side walls. The light emitted from the side walls should do not suffer the total internal reflection, which results more light extraction. In the other hand, 2D PC may also play an important role for extracting light from the periodic ordered structures, the 2D photonic band gap (PBG) effect. Light extraction in this case is enhanced in two possible ways [14,15]. (1) because multiple scattering of photons by lattice of periodically varying refractive indices in the PCs acts to form PBGs in which lateral propagation of the Bloch guided modes is prohibited, light generated in the band gap region can couple only to radiation modes and is radiated outward. (2) the refractive index periodicity creates a cutoff frequency for guided modes. Guided modes are folded by the PCs at the Brillouin zone boundaries, allowing
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phase matching to the radiation modes that lie above this cut-off frequency [16,17]. The guided modes that phase match to the radiation modes become leaky resonances of the PCs which Bragg scatter the light emitted from of the active region. Fig. 3(b) shows typical PL results of the samples with and without metal film covering. It is clearly observed that PL intensity is increased about 6 times. A small increase in the luminescence intensity might be expected for the metal coated sample because the metal reflects pump light back through the active layer, doubling the effective path of the incident light. However, the large increase of 6 times in the PL intensity for the sample with metal coatings must have other mechanism. One possible explanation is the effect of surface plasmon. It has been demonstrated that surface plasmons can increase the density of states and spontaneous emission rate in the semiconductor [18–20], and lead to the enhancement of light emission by surface plasmon-quantum well coupling [21,22]. However, if the metal/semiconductor interface were perfectly flat, it would be difficult to extract light from the surface plasmon mode, a non-propagating evanescent wave. In our case, the PL enhancement may be attributed to strong interaction with the surface plasmons. Electron–hole pairs excited within the Alq3 active layer couple to electron vibrations at the interface of metal/active layer when the energies of electron–hole pairs in the active layer and of the metal surface plasmon are similar. Then, electron–hole recombination produce surface plasmons instead of photons, and this new recombination path increases the spontaneous recombination rate. On the other hand, the small periodic ordered structures can efficiently scatter the surface plasmons as light to increase the PL intensity. 4. Conclusion Nanosphere lithography is successfully used to create the nanopatterns on SiO2/glass substrates. Significant enhancement of PL intensity has been obtained for the OLED formed on the nanopatterns. In addition, we also observed
that the PL intensity is increased 6 times if the organic active layer is deposited on the nanopatterns and covered by a thin metal film of Al. This strong increase of PL intensity may be attributed to surface plasmon effects. Right now, we just obtained primary results of the nanostructured OLED. To fully understand the optical properties of the devices, more investigations are need.
References [1] M.-H. Lu, J.C. Sturm, Appl. Phys. Lett. 78 (2001) 1927. [2] N.C. Greenham, R.H. Friend, D.D.C. Bradley, Adv. Mater. 6 (1994) 491. [3] C.F. Madigan, M.-H. Lu, J.C. Sturm, Appl. Phys. Lett. 76 (2000) 1650. [4] I. Schnitzer, E. Yablonovitch, Appl. Phys. Lett. 63 (1993) 2174. [5] M.-H. Lu, J.C. Sturm, Appl. Phys. Lett. 78 (2001) 1927. [6] M. Boroditsky, T.F. Krauss, R. Coccioli, R. Vrijen, R. Bhat, E. Yablonovitch, Appl. Phys. Lett. 75 (1999) 1036. [7] P. Jiang, J.F. Bertone, V.L. Colvin, Science 291 (2001) 453. [8] X. Chen, Z. Chen, N. Fu, G. Lu, B. Yang, Adv. Mater. 15 (2003) 1413. [9] U.C. Fischer, H.P. Zingsheim, J. Vac. Sci. Technol. B 19 (1981) 881. [10] F. Burmeister, C. Schafle, B. Keilhofer, C. Bechinger, J. Boneberg, P. Leiderer, Adv. Mater. 10 (1998) 495. [11] C.L. Haynes, R.P. Van Duyne, J. Phys. Chem. B 105 (2001) 5599. [12] I.W. Hamley, Angew. Chem. Int. Ed. 42 (2003) 1692. [13] S. Fan, P.R. Villeneuve, J.D. Joannopoulos, Phys. Rev. Lett. 78 (1997) 3294. [14] M. Boroditsky, R. Vrijen, T.F. Krauss, R. Coccioli, R. Bhat, E. Yablonovitch, J. Lightwave Technol. 17 (1999) 2096. [15] T.N. Oder, J. Shakya, J.Y. lin, H.J. Jiang, Appl. Phys. Lett. 83 (2003) 1231. [16] A.A. Erchak, D.J. Ripin, S.H. Fan, P. Pakich, J.D. Joannoupoulos, E.P. Ippen, G.S. Petrich, L.A. Kolodziejski, Appl. Phys. Lett. 78 (2001) 563. [17] V.N. Astratov, I.S. Culashaw, R.M. Stevenson, D.M. Whittaker, M.S. Skolnick, T.F. Krauss, R.M. De La Rue, J. Lightwave Technol. 17 (1999) 2050. [18] N.E. Hecher, R.A. Hopfel, N. Sawaki, Physica E 2 (1998) 98. [19] W.L. Barnes, J. Lightwave Technol. 17 (1999) 2170. [20] S. Gianordoli, et al., Appl. Phys. Lett. 77 (2000) 2295. [21] I. Gontijo, Phys. Rev. B 60 (1999) 11564. [22] A. Neogi, et al., Phys. Rev. B 66 (2002) 153305.