Investigation of nanostructured hybrid organic/ semiconductor quantum dots in thin film and spatial distribution of the emission Hung-Ju Lin,1,2 François Flory,2,3 Judikael Le-Rouzo,2 and Cheng-Chung Lee1,* 1
Department of Optics and Photonics/Thin Film Technology Center, National Central University, 300 Chung-da Rd. Chung-Li, Taiwan
2
Aix-Marseille University, Institut Matériaux Microélectronique Nanosciences de Provence-IM2NP, CNRS-UMR 6242, Domaine Universitaire de Saint-Jérôme, Service 231, 13397 Marseille Cedex 20, France 3
Ecole Centrale Marseille, 38 rue Joliot Curie, 13451 Marseille Cedex 20, France *Corresponding author: [email protected]
Received 30 August 2013; revised 4 November 2013; accepted 4 November 2013; posted 5 November 2013 (Doc. ID 196794); published 16 December 2013
The optical properties of core-shell quantum dots (QDs) are important for optoelectronic devices and biological applications. In this study, we investigate the optical properties of core-shell CdSe/ZnS QDs embedded in PMMA polymer thin films. The luminescence from QD emission would be more applicable if the spatial distribution of the emission was controllable. We propose a method to control the emission distribution by modifying the nanostructure. A bi-periodic nanostructure was fabricated and characterized in hybrid QD thin films by a nano-imprint technique. The finite difference time domain method was used to simulate the electric field distribution in the measured structure. It is shown that the far-field distribution of the QD emission is controllable by manipulating the nanostructure of the hybrid QD thin films. © 2013 Optical Society of America OCIS codes: (310.6628) Subwavelength structures, nanostructures; (310.6860) Thin films, optical properties; (350.4600) Optical engineering. http://dx.doi.org/10.1364/AO.53.00A169
1. Introduction
Nano-objects such as quantum wires and quantum dots (QDs) exhibit quantum confinement effects, which generate attractive new optical and electronic properties. For example, the energy bandgap of semiconductor QDs can be tuned by changing the dot size because of the discrete energy levels from quantum confinement [1]. Models based on the effective mass approximation can be used to describe the electronic properties of semiconductor nanocrystals [2]. Semiconductor quantum objects have already found 1559-128X/14/04A169-06$15.00/0 © 2014 Optical Society of America
numerous applications in biological tagging [3], white LEDs [4], QLEDs [5], and solar cells, which have the potential to exceed the traditional limitations of sola-cell efficiency [6,7]. To understand the optical properties of photonic structures containing QDs, it is necessary to know the optical properties of the QDs in thin films. Hybrid organic polymer polymethyl methacrylate (PMMA) containing CdSe/ZnS semiconductor QDs was used in this study. Optical measurements, including absorption and luminescence, were performed. The fields of application for such luminescent layers could be widened if their light propagation could be controlled. For this purpose, we propose a method for the creation of nanostructures in hybrid QD films 1 February 2014 / Vol. 53, No. 4 / APPLIED OPTICS
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by means of nano-imprint lithography. This method was chosen because it is a cheap and fast reproduction technique [8]. 2. Properties of Hybrid Thin Films
Before analysis of the nanostructured hybrid thin films, the fabrication and properties of the films without any structure must be investigated. It is important to choose a proper polymer matrix that will not interact with semiconductor QDs such as the CdSe, CdTe, and CdSe/ZnS. In a previous work [9], it has been shown that PMMA is an appropriate material for this purpose because of its good chemical stability. The high transparency of PMMA in the visible wavelength range [10] also makes it quite suitable for the fabrication of hybrid thin films because any change in the optical properties will be contributed from the embedded QDs. The quantum confinement effect induces significant changes in the optical properties such as the production of an absorption spectrum with excitonic features and high luminescence efficiency [11]. To obtain strong luminescence generation relating to high quantum yield performance, core-shell-type QDs were chosen [12,13]. CdSe/ZnS QDs coated with hydrophobic organic molecules provided by PlasmaChem GmbH were mixed into a soluble solvent— in this case, chloroform—and then mechanically stirred. TEM measurement was performed to confirm the size of the CdSe/ZnS QDs, which was around 3 nm in diameter, as shown in Fig. 1. Thanks to the high solubility of the PMMA polymer and CdSe/ZnS QDs, both dissolved well in the chloroform solvent to form a hybrid composition. Thin films are more useful than the bulk materials for applications in optoelectronic devices such as OLEDs and solar cells. The thin films were deposited using the spin-coating technique. For the deposition process, PMMA powder was dissolved in 1 ml of chloroform and then stirred for 4 h. At the same time, the CdSe/ZnS QDs were also dissolved in another bottle of chloroform and
Fig. 1. TEM image of core-shell CdSe/ZnS QDs with diameters of around 3 nm. A170
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mechanically stirred for the same period of time. Following an ultrasonic dispersion treatment, sufficient amounts of solution containing specific quantities of PMMA and QDs were extracted to form a homogeneous composition to give different weight ratio percentages (w.r.) of QDs with respect to the PMMA. In this research, a weight ratio percentage of 16.6% QDs was applied. Before spinning to form a thin film, the hybrid solution still needed to be mechanically stirred to prevent aggregation. The rotation speed for spin coating was adjusted to between 1000 and 2000 rpm to ensure the film thicknesses close to 2 μm. All of the above preparation and fabrication processes were performed in a glove box with a low-oxygen atmosphere within 2–0.1 ppm in order to avoid any oxidation contamination to the QDs. To obtain a hybrid layer with strong luminescence, we deposited a 16.6% QD layer with an increased thickness of around 2.49 μm, as measured by the Talystep and m-line technique. The absorption spectra of the PMMA layers containing semiconductor QDs are shown in Fig. 2. The absorption was deduced from the measured transmittance (T%) and reflectance (R%) spectra using the relation of 1- T%- R%. The principle absorption peak is close to 545 nm and accompanied by other absorption peaks at 505 and 445 nm, which might be induced by the transitions between different discrete levels in the quantum confinement of CdSe/ZnS QDs. PMMA has a large bandgap at around 5.6 eV, [14] so this does not affect the absorption in the near-UV and visible spectral range. Therefore, the optical absorption in this spectral range is attributable only to the QDs. Thanks to the wide absorption band, the CdSe/ZnS QDs can be excited in a wide wavelength range below the wavelength of 560 nm. Many high-power light sources can be used to achieve this QD excitation; for example, an Ar laser (514.5 nm) or He-Cd laser (325 nm). In this study, both lasers were used to obtain the same emission peak position. The measured photoluminescence curve with a narrow FWHM around 23 nm is shown in Fig. 2. It indicates the
Fig. 2. Absorption and luminescence spectra of hybrid thin film layers of PMMA/core-shell CdSe/ZnS QDs.
Fig. 3. Nano-imprint method applied to obtain the nanostructured polymer films. (a) Sketch of a silicon mold with a periodic structure and (d) cross-sectional view. (b) Principal step in the thermal imprinting lithography process and (e) cross-sectional view. (c) Expected nanostructured hybrid polymer with (f) cross-sectional view.
variations in QD size are less than 5% [15]. The inset to Fig. 2 shows the strong photoluminescence observed in the green range. This photoluminescence was generated homogeneously within the hybrid thin films since the QDs were homogeneously embedded within the films. For application in optoelectronic devices such as LED, the optical far field of the luminescent light produced by the QDs needs to be directional and controllable. This important issue is dealt with in the next section. 3. Hybrid Thin Films with a Periodic Structure
In order to ensure the emitted luminescent light is directional, we constructed a bi-periodic grating structure in the hybrid thin film. Electron beam lithography can be used to directly make nanostructures; however, the high energy of the electron beam could cause degradation in the luminescence efficiency of the QDs [16]. Hence it was necessary to use an indirect structure transferring method, in this case, the nano-imprint method. The nano-imprint method has many advantages such as fast production and repeatability of fabrication, which helps to keep the cost down [17]. The silicon mold patterned by electron beam lithography with a 2D periodic structure is shown in Fig. 3(a). A cross-sectional image is shown in Fig. 3(d) with a period of 1.6 μm. The same process was used for the preparation of the hybrid thin film layer of PMMA/core-shell CdSe/ZnS QDs on a glass substrate as that mentioned in Section 2. Due to the glass transition temperature of the PMMA being between 100°C and 110°C [18,19], the patterned silicon mold had to be preheated to above 110°C on a metal-top hot plate for 5 min before imprinting. After preheating, the mold must be heated
homogeneously. During the imprinting process, a homogeneous pressure of greater than 50 psi was applied to the samples for 15 min, while the heat treatment was maintained on the silicon mold, as shown in Figs. 3(b) and (3(e). After imprinting, the sample was cooled down to room temperature prior to demolding. A good transfer of the structure to the hybrid thin film could be achieved, as illustrated in Fig. 3(c) and the cross section in Fig. 3(f). The samples, including the silicon mold, were characterized by noncontact optical probe mapping (STIL Micromeasure2). From the analysis of the morphology, it was found that the original silicon mold had a period of 1.6 μm and a hole diameter of 1.2 μm, as shown in Fig. 4(a). After imprinting, the hybrid QD-polymer layers exhibited the same periodic structure (1.6 μm), which had been nicely transferred, as shown in Fig. 4(b). 4. Optical Field Simulation
Since the period of the structured hybrid thin film and its emission spectra have been characterized, the diffraction angle can be estimated based on the classical diffraction theory. The diffraction angle, θm , from a periodic grating of period d can be expressed by Eq. (1) for the wavelength where m is the diffraction order: θm sin−1
mλ . d
(1)
As shown in Fig. 5, the first-order diffraction angles (m 1) versus different propagation wavelengths from 300 to 700 nm were determined for a period grating of 1.6 μm. Since the QDs emission wavelength is 1 February 2014 / Vol. 53, No. 4 / APPLIED OPTICS
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Fig. 5. Calculated first-order diffraction angle for a grating period of 1.6 μm versus wavelength. For QDs, the light emission occurs at a wavelength of 560 nm, and the diffraction angle is expected to be 20.5°.
Fig. 4. (a) Patterned silicon mold with its periodic structure [it is the step (a) in Fig. 3]. (b) Nanostructured hybrid thin film with QDs made by nanoimprint technique [step (c) in Fig. 3].
560 nm, the first-order diffraction angle is predicted to be 20.5°0.67°. A finite difference time-domain (FDTD) simulation was carried out with Lumerical Solutions software in order to further investigate the distribution of the electric field of the periodic nanostructured thin film. It can be seen in a cross section of the structure considered in the simulation illustrated in the left part of Fig. 6 that the period is 1.6 μm. The height of each cylindrical structure is 0.6 μm, and the diameter of each cylindrical structure is 1.2 μm, according to the morphology measurement. The light emitted from the QDs under laser illumination was assumed to be beneath the periodic structure and homogeneously distributed. The emitted light in the nanostructured portion was decomposed in thin slices. All the slices were assumed to be the same. Thus each slice could be treated as a plane wave emission source emitting in the Z direction. The considered emission wavelength was 560 nm. As can be seen in Fig. 6, there was significant redirection of the QD emission source propagating out of the periodic structure. The electric field distribution on the top of the biperiodic structure surface was also calculated (see the left part of Fig. 7). The calculation was performed A172
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for a localized square region delimited by the dashed line in the left part of Fig. 7 and repeated periodically in the X and Y directions. The electric field distribution in this region was calculated using a local fielddetecting sensor in front of the structure. It can be seen in the right part of Fig. 7 that the electric field is strongly concentrated in the four corners corresponding to the cylindrical positioning. Utilizing the above calculation, the FDTD method can be used solve for the electric field distribution throughout the periodic structure. In practice, it also would be interesting to understand the field far away from the modeled region. Information about the electric field distribution was recorded by the local field detector. The field would then propagate to any position in the half-space above the detecting sensor. The local field detector means this detector is located close to the sample surface, within 1 μm in our study. The technique for transformation from the local field to the far field is based on the equivalence principle. The equivalence principle means the equivalent property of field, which can exist in the frequency domain or in the time domain. It would be correct if the detected surface is a unit cell, and periodicity is assumed in the far-field propagation. In the propagation space, it is assumed that the medium (air) is homogeneous and isotropic. Therefore, the field can
Fig. 6. Left: cross section of the considered structure. Right: electric field simulation at 560 nm.
Fig. 7. Left: top view of the structure. Right: electric field simulation of the left-hand region.
and the classical diffraction theory have both been used to investigate the directional emission from the QDs. The calculated light distributions 1 m away from the sample are in agreement with the results of the two calculations. This shows that it is possible to control the propagation of the luminescence of thin film QDs. Experiments conducted for confirmation also showed a critical effect from the roughness of the structure that could lead to strong divergence and even diminish the emission to be undetectable. After improvement of the structural surface, this technique is expected to find many practical applications in such various scientific fields such as QDs in solar cells, in LEDs, and for biological tagging. The authors gratefully acknowledge the assistance with funding provided by the National Science Council of Taiwan under Contract No. NSC 102-2221E-008 -097 and the French Institute of Taipei. In addition, the authors would like to thank Claude Alfonso (DENO Team of IM2NP) for her help with the TEM measurement. References
Fig. 8. Far-field simulation of diffraction pattern distribution for an emission wavelength of 560 nm.
be projected as a function of angle into the far-field space above the detecting surface. For a hemispherical surface with a 1 m radius away from the sample, the far field can be then obtained on this curved surface. The field profile projected on a planar surface is shown in Fig. 8. In the figure, the emitted light is then mostly distributed as first-order diffraction patterns at angles located at around 20°, which is in agreement with the previous calculation of diffraction by a grating (Fig. 5). 5. Conclusions
Hybrid organic/semiconductor QDs, PMMA thin film containing CdSe/ZnS QDs successfully has been fabricated. It was found that the absorption spectrum in the visible range is defined by the QDs, thanks to the high transparency of the PMMA in this wavelength range. A strong peak in luminescence was obtained around the wavelength of 560 nm due to quantum confinement in the CdSe/ZnS QDs. For applications, it is important to show that the emitted light direction can be controlled. We demonstrate that such hybrid films can be structured by a nano-imprint technique. There is good transfer of a pattern with a 1.6 μm bi-periodicity and a 1.2 μm hole diameter produced by e-beam lithography on a silicon mold. FDTD simulations for the electric field distribution
1. A. I. Ekimov, A. L. Efros, and A. A. Onushchenko, “Quantum size effect in semiconductor microcrystals,” Solid State Commun. 56, 921–924 (1985). 2. S. V. Gaponenko, Optical Properties of Semiconductor Nanocrystals (Cambridge University, 1998). 3. M. Bruchez, M. Moronne, P. Gin, S. Weiss, and A. P. Alvistos, “Semiconductor nanocrystals as fluorescent biological labels,” Science 281, 2013–2016 (1998). 4. S. Nizamoglu, G. Zengin, and H. V. Demir, “Color-converting combinations of nanocrystal emitters for warm-white light generation with high color rendering index,” Appl. Phys. Lett. 92, 031102 (2008). 5. S. Coe, W. K. Woo, M. Bawendi, and V. Bulovic, “Electroluminescence from single monolayers of nanocrystals in molecular organic devices,” Nature 420, 800–803 (2002). 6. P. V. Kamat, “Quantum dot solar cells. Semiconductor nanocrystals as light harvesters,” J. Phys. Chem. C 112, 18737–18753 (2008). 7. W. Shockley and J. J. J. Queisser, “Efficiency loss mechanisms: theory and characterization,” J. Appl. Phys. 32, 510–519 (1961). 8. Q.-C. Hsu, J.-J. Hsiao, T.-L. Ho, and C.-D. Wu, “Fabrication of photonic crystal structures on flexible organic light-emitting diodes using nanoimprint,” Microelectron. Eng. 91, 178–184 (2012). 9. L. Zhou, C. Gao, and W. Xu, “Simultaneous photoluminescence import and mechanical enhancement of polymer films using silica-hybridized quantum dots,” J. Mater. Chem. 20, 5675–5681 (2010). 10. D. Sun and H.-J. Sue, “Tunable ultraviolet emission of ZnO quantum dots in transparent poly(methyl methacrylate),” Appl. Phys. Lett. 94, 253106 (2009). 11. J. M. Costa-Fernández, R. Pereiro, and A. Sanz-Medel, “The use of luminescent quantum dots for optical sensing,” TrAC, Trends Anal. Chem. 25, 207–218 (2006). 12. S. F. Wuister, C. de Mello Donegá, and A. Meijerink, “Localfield effects on the spontaneous emission rate of CdTe and CdSe quantum dots in dielectric media,” J. Chem. Phys. 121, 4310–4315 (2004). 13. A. M. Smith and S. Nie, “Semiconductor nanocrystals: structure, properties, and band gap engineering,” Acc. Chem. Res. 43, 190–200 (2010). 14. D. I. Son, C. H. You, J. H. Jung, and T. W. Kim, “Carrier transport mechanisms of organic bi-stable devices fabricated utilizing colloidal ZnO quantum dot-polymethylmethacrylate 1 February 2014 / Vol. 53, No. 4 / APPLIED OPTICS
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polymer nanocomposites,” Appl. Phys. Lett. 97, 013304 (2010). 15. M. Hong, M. Guo-Hong, W. Wen-Jun, G. Xue-Xi, and M. Hong-Liang, “Size-dependent optical properties and carriers dynamics in CdSe/ZnS quantum dots,” Chin. Phys. B 17, 1280 (2008). 16. V. Nandwana, C. Subramani, Y.-C. Yeh, B. Yang, S. Dickert, M. D. Barnes, M. T. Tuominen, and V. M. Rotello, “Direct patterning of quantum dot nanostructures via electron beam lithography,” J. Mater. Chem. 21, 16859–16862 (2011).
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17. J. Damon Hoff, L.-J. Cheng, E. Meyhofer, L. J. Guo, and A. J. Hunt, “Nanoscale protein patterning by imprint lithography,” Nano Lett. 4, 853–857 (2004). 18. G. Wang, Z. Lu, C. Yuan, and Y. Wei, “Influence of the surface pressure of Langmuir–Blodgett films on the efficiency of organic electroluminescence,” Thin Solid Films 288, 334–336 (1996). 19. C. Martin, L. Ressier, and J. P. Peyrade, “Study of PMMA recoveries on micrometric patterns replicated by nanoimprint lithography,” Physica E 17, 523–525 (2003).
Department of Optics and Photonics/Thin Film Technology Center, National Central University, 300 Chung-da Rd. Chung-Li, Taiwan
2
Aix-Marseille University, Institut Matériaux Microélectronique Nanosciences de Provence-IM2NP, CNRS-UMR 6242, Domaine Universitaire de Saint-Jérôme, Service 231, 13397 Marseille Cedex 20, France 3
Ecole Centrale Marseille, 38 rue Joliot Curie, 13451 Marseille Cedex 20, France *Corresponding author: [email protected]
Received 30 August 2013; revised 4 November 2013; accepted 4 November 2013; posted 5 November 2013 (Doc. ID 196794); published 16 December 2013
The optical properties of core-shell quantum dots (QDs) are important for optoelectronic devices and biological applications. In this study, we investigate the optical properties of core-shell CdSe/ZnS QDs embedded in PMMA polymer thin films. The luminescence from QD emission would be more applicable if the spatial distribution of the emission was controllable. We propose a method to control the emission distribution by modifying the nanostructure. A bi-periodic nanostructure was fabricated and characterized in hybrid QD thin films by a nano-imprint technique. The finite difference time domain method was used to simulate the electric field distribution in the measured structure. It is shown that the far-field distribution of the QD emission is controllable by manipulating the nanostructure of the hybrid QD thin films. © 2013 Optical Society of America OCIS codes: (310.6628) Subwavelength structures, nanostructures; (310.6860) Thin films, optical properties; (350.4600) Optical engineering. http://dx.doi.org/10.1364/AO.53.00A169
1. Introduction
Nano-objects such as quantum wires and quantum dots (QDs) exhibit quantum confinement effects, which generate attractive new optical and electronic properties. For example, the energy bandgap of semiconductor QDs can be tuned by changing the dot size because of the discrete energy levels from quantum confinement [1]. Models based on the effective mass approximation can be used to describe the electronic properties of semiconductor nanocrystals [2]. Semiconductor quantum objects have already found 1559-128X/14/04A169-06$15.00/0 © 2014 Optical Society of America
numerous applications in biological tagging [3], white LEDs [4], QLEDs [5], and solar cells, which have the potential to exceed the traditional limitations of sola-cell efficiency [6,7]. To understand the optical properties of photonic structures containing QDs, it is necessary to know the optical properties of the QDs in thin films. Hybrid organic polymer polymethyl methacrylate (PMMA) containing CdSe/ZnS semiconductor QDs was used in this study. Optical measurements, including absorption and luminescence, were performed. The fields of application for such luminescent layers could be widened if their light propagation could be controlled. For this purpose, we propose a method for the creation of nanostructures in hybrid QD films 1 February 2014 / Vol. 53, No. 4 / APPLIED OPTICS
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by means of nano-imprint lithography. This method was chosen because it is a cheap and fast reproduction technique [8]. 2. Properties of Hybrid Thin Films
Before analysis of the nanostructured hybrid thin films, the fabrication and properties of the films without any structure must be investigated. It is important to choose a proper polymer matrix that will not interact with semiconductor QDs such as the CdSe, CdTe, and CdSe/ZnS. In a previous work [9], it has been shown that PMMA is an appropriate material for this purpose because of its good chemical stability. The high transparency of PMMA in the visible wavelength range [10] also makes it quite suitable for the fabrication of hybrid thin films because any change in the optical properties will be contributed from the embedded QDs. The quantum confinement effect induces significant changes in the optical properties such as the production of an absorption spectrum with excitonic features and high luminescence efficiency [11]. To obtain strong luminescence generation relating to high quantum yield performance, core-shell-type QDs were chosen [12,13]. CdSe/ZnS QDs coated with hydrophobic organic molecules provided by PlasmaChem GmbH were mixed into a soluble solvent— in this case, chloroform—and then mechanically stirred. TEM measurement was performed to confirm the size of the CdSe/ZnS QDs, which was around 3 nm in diameter, as shown in Fig. 1. Thanks to the high solubility of the PMMA polymer and CdSe/ZnS QDs, both dissolved well in the chloroform solvent to form a hybrid composition. Thin films are more useful than the bulk materials for applications in optoelectronic devices such as OLEDs and solar cells. The thin films were deposited using the spin-coating technique. For the deposition process, PMMA powder was dissolved in 1 ml of chloroform and then stirred for 4 h. At the same time, the CdSe/ZnS QDs were also dissolved in another bottle of chloroform and
Fig. 1. TEM image of core-shell CdSe/ZnS QDs with diameters of around 3 nm. A170
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mechanically stirred for the same period of time. Following an ultrasonic dispersion treatment, sufficient amounts of solution containing specific quantities of PMMA and QDs were extracted to form a homogeneous composition to give different weight ratio percentages (w.r.) of QDs with respect to the PMMA. In this research, a weight ratio percentage of 16.6% QDs was applied. Before spinning to form a thin film, the hybrid solution still needed to be mechanically stirred to prevent aggregation. The rotation speed for spin coating was adjusted to between 1000 and 2000 rpm to ensure the film thicknesses close to 2 μm. All of the above preparation and fabrication processes were performed in a glove box with a low-oxygen atmosphere within 2–0.1 ppm in order to avoid any oxidation contamination to the QDs. To obtain a hybrid layer with strong luminescence, we deposited a 16.6% QD layer with an increased thickness of around 2.49 μm, as measured by the Talystep and m-line technique. The absorption spectra of the PMMA layers containing semiconductor QDs are shown in Fig. 2. The absorption was deduced from the measured transmittance (T%) and reflectance (R%) spectra using the relation of 1- T%- R%. The principle absorption peak is close to 545 nm and accompanied by other absorption peaks at 505 and 445 nm, which might be induced by the transitions between different discrete levels in the quantum confinement of CdSe/ZnS QDs. PMMA has a large bandgap at around 5.6 eV, [14] so this does not affect the absorption in the near-UV and visible spectral range. Therefore, the optical absorption in this spectral range is attributable only to the QDs. Thanks to the wide absorption band, the CdSe/ZnS QDs can be excited in a wide wavelength range below the wavelength of 560 nm. Many high-power light sources can be used to achieve this QD excitation; for example, an Ar laser (514.5 nm) or He-Cd laser (325 nm). In this study, both lasers were used to obtain the same emission peak position. The measured photoluminescence curve with a narrow FWHM around 23 nm is shown in Fig. 2. It indicates the
Fig. 2. Absorption and luminescence spectra of hybrid thin film layers of PMMA/core-shell CdSe/ZnS QDs.
Fig. 3. Nano-imprint method applied to obtain the nanostructured polymer films. (a) Sketch of a silicon mold with a periodic structure and (d) cross-sectional view. (b) Principal step in the thermal imprinting lithography process and (e) cross-sectional view. (c) Expected nanostructured hybrid polymer with (f) cross-sectional view.
variations in QD size are less than 5% [15]. The inset to Fig. 2 shows the strong photoluminescence observed in the green range. This photoluminescence was generated homogeneously within the hybrid thin films since the QDs were homogeneously embedded within the films. For application in optoelectronic devices such as LED, the optical far field of the luminescent light produced by the QDs needs to be directional and controllable. This important issue is dealt with in the next section. 3. Hybrid Thin Films with a Periodic Structure
In order to ensure the emitted luminescent light is directional, we constructed a bi-periodic grating structure in the hybrid thin film. Electron beam lithography can be used to directly make nanostructures; however, the high energy of the electron beam could cause degradation in the luminescence efficiency of the QDs [16]. Hence it was necessary to use an indirect structure transferring method, in this case, the nano-imprint method. The nano-imprint method has many advantages such as fast production and repeatability of fabrication, which helps to keep the cost down [17]. The silicon mold patterned by electron beam lithography with a 2D periodic structure is shown in Fig. 3(a). A cross-sectional image is shown in Fig. 3(d) with a period of 1.6 μm. The same process was used for the preparation of the hybrid thin film layer of PMMA/core-shell CdSe/ZnS QDs on a glass substrate as that mentioned in Section 2. Due to the glass transition temperature of the PMMA being between 100°C and 110°C [18,19], the patterned silicon mold had to be preheated to above 110°C on a metal-top hot plate for 5 min before imprinting. After preheating, the mold must be heated
homogeneously. During the imprinting process, a homogeneous pressure of greater than 50 psi was applied to the samples for 15 min, while the heat treatment was maintained on the silicon mold, as shown in Figs. 3(b) and (3(e). After imprinting, the sample was cooled down to room temperature prior to demolding. A good transfer of the structure to the hybrid thin film could be achieved, as illustrated in Fig. 3(c) and the cross section in Fig. 3(f). The samples, including the silicon mold, were characterized by noncontact optical probe mapping (STIL Micromeasure2). From the analysis of the morphology, it was found that the original silicon mold had a period of 1.6 μm and a hole diameter of 1.2 μm, as shown in Fig. 4(a). After imprinting, the hybrid QD-polymer layers exhibited the same periodic structure (1.6 μm), which had been nicely transferred, as shown in Fig. 4(b). 4. Optical Field Simulation
Since the period of the structured hybrid thin film and its emission spectra have been characterized, the diffraction angle can be estimated based on the classical diffraction theory. The diffraction angle, θm , from a periodic grating of period d can be expressed by Eq. (1) for the wavelength where m is the diffraction order: θm sin−1
mλ . d
(1)
As shown in Fig. 5, the first-order diffraction angles (m 1) versus different propagation wavelengths from 300 to 700 nm were determined for a period grating of 1.6 μm. Since the QDs emission wavelength is 1 February 2014 / Vol. 53, No. 4 / APPLIED OPTICS
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Fig. 5. Calculated first-order diffraction angle for a grating period of 1.6 μm versus wavelength. For QDs, the light emission occurs at a wavelength of 560 nm, and the diffraction angle is expected to be 20.5°.
Fig. 4. (a) Patterned silicon mold with its periodic structure [it is the step (a) in Fig. 3]. (b) Nanostructured hybrid thin film with QDs made by nanoimprint technique [step (c) in Fig. 3].
560 nm, the first-order diffraction angle is predicted to be 20.5°0.67°. A finite difference time-domain (FDTD) simulation was carried out with Lumerical Solutions software in order to further investigate the distribution of the electric field of the periodic nanostructured thin film. It can be seen in a cross section of the structure considered in the simulation illustrated in the left part of Fig. 6 that the period is 1.6 μm. The height of each cylindrical structure is 0.6 μm, and the diameter of each cylindrical structure is 1.2 μm, according to the morphology measurement. The light emitted from the QDs under laser illumination was assumed to be beneath the periodic structure and homogeneously distributed. The emitted light in the nanostructured portion was decomposed in thin slices. All the slices were assumed to be the same. Thus each slice could be treated as a plane wave emission source emitting in the Z direction. The considered emission wavelength was 560 nm. As can be seen in Fig. 6, there was significant redirection of the QD emission source propagating out of the periodic structure. The electric field distribution on the top of the biperiodic structure surface was also calculated (see the left part of Fig. 7). The calculation was performed A172
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for a localized square region delimited by the dashed line in the left part of Fig. 7 and repeated periodically in the X and Y directions. The electric field distribution in this region was calculated using a local fielddetecting sensor in front of the structure. It can be seen in the right part of Fig. 7 that the electric field is strongly concentrated in the four corners corresponding to the cylindrical positioning. Utilizing the above calculation, the FDTD method can be used solve for the electric field distribution throughout the periodic structure. In practice, it also would be interesting to understand the field far away from the modeled region. Information about the electric field distribution was recorded by the local field detector. The field would then propagate to any position in the half-space above the detecting sensor. The local field detector means this detector is located close to the sample surface, within 1 μm in our study. The technique for transformation from the local field to the far field is based on the equivalence principle. The equivalence principle means the equivalent property of field, which can exist in the frequency domain or in the time domain. It would be correct if the detected surface is a unit cell, and periodicity is assumed in the far-field propagation. In the propagation space, it is assumed that the medium (air) is homogeneous and isotropic. Therefore, the field can
Fig. 6. Left: cross section of the considered structure. Right: electric field simulation at 560 nm.
Fig. 7. Left: top view of the structure. Right: electric field simulation of the left-hand region.
and the classical diffraction theory have both been used to investigate the directional emission from the QDs. The calculated light distributions 1 m away from the sample are in agreement with the results of the two calculations. This shows that it is possible to control the propagation of the luminescence of thin film QDs. Experiments conducted for confirmation also showed a critical effect from the roughness of the structure that could lead to strong divergence and even diminish the emission to be undetectable. After improvement of the structural surface, this technique is expected to find many practical applications in such various scientific fields such as QDs in solar cells, in LEDs, and for biological tagging. The authors gratefully acknowledge the assistance with funding provided by the National Science Council of Taiwan under Contract No. NSC 102-2221E-008 -097 and the French Institute of Taipei. In addition, the authors would like to thank Claude Alfonso (DENO Team of IM2NP) for her help with the TEM measurement. References
Fig. 8. Far-field simulation of diffraction pattern distribution for an emission wavelength of 560 nm.
be projected as a function of angle into the far-field space above the detecting surface. For a hemispherical surface with a 1 m radius away from the sample, the far field can be then obtained on this curved surface. The field profile projected on a planar surface is shown in Fig. 8. In the figure, the emitted light is then mostly distributed as first-order diffraction patterns at angles located at around 20°, which is in agreement with the previous calculation of diffraction by a grating (Fig. 5). 5. Conclusions
Hybrid organic/semiconductor QDs, PMMA thin film containing CdSe/ZnS QDs successfully has been fabricated. It was found that the absorption spectrum in the visible range is defined by the QDs, thanks to the high transparency of the PMMA in this wavelength range. A strong peak in luminescence was obtained around the wavelength of 560 nm due to quantum confinement in the CdSe/ZnS QDs. For applications, it is important to show that the emitted light direction can be controlled. We demonstrate that such hybrid films can be structured by a nano-imprint technique. There is good transfer of a pattern with a 1.6 μm bi-periodicity and a 1.2 μm hole diameter produced by e-beam lithography on a silicon mold. FDTD simulations for the electric field distribution
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