Patterning of colloidal quantum dots for the ...

Patterning of colloidal quantum dots for the generation of surface plasmon Yeonsang Park Young-Geun Roh Un Jeong Kim Dae-Young Chung Hwansoo Suh Jineun Kim Sangmo Cheon Jaesoong Lee Tae-Ho Kim Kyung-Sang Cho Chang-Won Lee

J. Micro/Nanolith. MEMS MOEMS 12(4), 041202 (Oct–Dec 2013)

Patterning of colloidal quantum dots for the generation of surface plasmon Yeonsang Park Young-Geun Roh Un Jeong Kim Dae-Young Chung Hwansoo Suh Jineun Kim Sangmo Cheon Jaesoong Lee Tae-Ho Kim Kyung-Sang Cho Chang-Won Lee Samsung Advanced Institute of Technology Frontier Research Lab Yongin-si, Kyonggi-do 446-712, Republic of Korea E-mail: [email protected]

Abstract. Patterning of colloidal quantum dot (QD) of a nanometer resolution is important for potential applications in micro- or nanophotonics. Several patterning techniques such as polymer composites, molecular key-lock methods, inkjet printing, and the microcontact printing of QDs have been successfully developed and applied to various plasmonic applications. However, these methods are not easily adapted to conventional complementary metal-oxide semiconductor (CMOS)-compatible processes because of either limits in fabrication resolutions or difficulties in sub-100-nm alignment. Here, we present an adaptation of a conventional lift-off method for the patterning of colloidal QDs. This simple method can be later applied to CMOS processes by changing electron beam lithography to photolithography for building up photon-generation elements in various planar geometries. Various shapes formed by colloidal QD clusters such as straight lines, rings, and dot patterns with sub-100-nm size could be fabricated. The patterned structures show sub-10-nm positioning with good fluorescence properties and well-defined sidewall profiles. To demonstrate the applicability of our method, we present a surface plasmon generator from a QD cluster. © 2013 Society of Photo-Optical Instrumentation Engineers (SPIE) [DOI: 10.1117/1.JMM.12.4.041202]

Subject terms: colloidal quantum dot; nanoscale patterning; electron beam lithography; lift-off process; surface plasmon generator; finite-difference time-domain simulation. Paper 13022SSP received Mar. 11, 2013; revised manuscript received Apr. 26, 2013; accepted for publication May 7, 2013; published online Sep. 25, 2013.

1 Introduction Since its invention, the colloidal quantum dot (QD) has attracted interest due to its broad functionalities and applicabilities.1,2 Owing to the quantum confinement effect, the optical spectrum of QDs moves to a higher energy level upon a decrease in the size of the material.3–6 In addition, high internal radiative efficiency holds promise for various photonic7–10 and plasmonic applications.11–13 In spite of the numerous advantages of QDs, the microfabrication of colloidal QDs into one-dimensional (1-D) or two-dimensional (2-D) structures is still quite challenging because of solubility issues and the wetting properties on oxide and metallic substrates. These difficulties result in restricted usage and applications of QDs in diverse fields of photonics and plasmonics. To overcome these constraints, researchers have successfully developed methods of patterning mixtures of a photosensitive polymer and QDs.14–17 The photo-curing patterning of nanocrystals shows fabrication resolutions larger than 2 μm and reduced luminescence intensity per unit volume compared to that of pristine QDs. The microcontact printing method has shown unique advantages such as ease of use and high throughput with a sub-micrometer-scale resolution.18–20 However, it has difficulty in alignment between the mold and the predefined nanostructure and becomes disadvantageous in situations where only a fraction of the size of the mold has to be stamped. Recently, a few research groups devised method of patterning QDs with a self-assembled monolayer (SAM) treatment, called molecule 0091-3286/2013/$25.00 © 2013 SPIE

J. Micro/Nanolith. MEMS MOEMS

key-lock method.21–24 This method uses a SAM chemical treatment of a region to be patterned followed by a QD coating of the surface. Once a key-lock pair is formed between the patterned SAM molecule and the ligand molecule of the QDs, excessive QDs can be easily removed from the surface, leaving only the patterned QDs on top of the predefined SAM of key molecules. However, this method requires additional chemical combinations of the key-lock pair molecules for more than two types of QDs. In this letter, we present a simple patterning process for colloidal QDs similar to a conventional lift-off process. This approach facilitates fabrication processes for defining highresolution 1-D and 2-D structures without a complicated surface treatment process and/or a delicate choice of a key-lock molecule pair. With our approach, arbitrary shapes composed of QDs can be patterned onto a designated location. This method, unlike the microcontact printing method, can be applied even for defining sparse patterns. Our method opens up the possibility of acquiring complementary metaloxide semiconductor (CMOS) compatibility with colloidal QDs as local photonic and plasmonic sources on a planar metal or oxide surface. We successfully fabricated 1-D straight-line and 2-D ring structures made of colloidal QDs with linewidths of a few hundreds of nanometers. As an extreme case of patterning, we fabricated zero-dimensional (0-D) dot with a few tens of a nanometer diameter. The QD nanostructures fabricated on a metallic plane could be used for surface plasmons (SP) launchers where fabrication resolution on the sub-100-nm scale plays a significant role.

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Park et al.: Patterning of colloidal quantum dots for the generation of surface plasmon

2 Experiment 2.1 Sample Preparation We synthesized CdSe/CdS/ZnS core-shell-shell colloidal QDs by a chemical process.24 In this process, ethanol (or an ethanol/methanol mixture) is initially added to a QD/decane solution. The precipitates are isolated using a centrifuge for 10 min. After pouring off the supernatant solution, the precipitates are redispersed in cyclohexane. After washing three times, QDs are finally dispersed into the cyclohexane. These synthesized QDs emit photoluminescence (PL) with a peak around 608 nm and have a full-widthat-half-maximum of about 40 nm [Fig 1(a)]. In order to confirm the feasibility of our method, we fabricated straight line patterns with various linewidths with a few hundred nanometers range on metallic substrates. For plasmonic applications, a 300-nm-thick gold film is deposited by electron beam evaporation on a sapphire substrate with a 5-nmthick Ti layer as an adhesion layer.25 We also fabricated ring-shaped structures with a diameter of 6 μm and various linewidths with a few hundred nanometers. Finally, we made dot patterns with a few tens of nanometer diameters in order to study the patterning resolution of our lift-off method. 2.2 Fabrication The entire fabrication procedure is similar to the conventional lift-off process [Fig. 1(b)]. The first step is the spin-coating of an electron beam resist (ER) such as polymethylmethacrylate on a prepared substrate at 4000 rpm for 40 s. The ER is baked at 170°C for 300 s. To define a structure, we used JEOL JBX-9300FS. After the exposure,

samples were developed using a 1:1 mixture of methyl isobutyl ketone and isopropyl alcohol for 180 s. In order to place QDs, a QD solution was spun-coat onto the electron beam lithography (EBL)-patterned region on the substrate at 2000 rpm for 40 s and then baked at 90°C for 5 min to remove the cyclohexane solvent. From Fig. 1(a), we observe that PL spectral shape of the patterned QDs is not different from that of pristine QDs except for the intensity decrease owing to volume shrinkage. 2.3 1-D Line Patterning One-dimensional straight-line patterns of QDs on an Au surface are fabricated. An optical microscope image is taken by a confocal microscope (Olympus LEXT OLS30-SU) as shown in Fig. 2(a). When these patterned samples are irradiated by ultraviolet (UV) light at 405 nm, seamless lines of fluorescence are observed as shown in Fig. 2(b). To check potential damage on the QDs during the fabrication process, we compared PL spectra before and after the lift-off process and found no changes except for the intensity decrease due to the reduced number of the QDs per unit volume. From the scanning electron microscope (SEM) images shown in Fig. 2(c), we found a slight nonuniform top surface profile of the QD lines. 2.4 2-D Ring Patterning As an example of 2-D patterns, we fabricated ring-type QD clusters with a diameter of 6 μm with 500-nm width. Figure 3 shows ring-shaped QD clusters on an Au surface substrate. As in the straight-line patterns, we obtained optical microscope [Fig. 3(a)], UV-irradiated (405 nm) dark-field

Fig. 1 (a) Photoluminescence spectra of QDs. The spectra show a peak of around 608 nm and full-width at half-maximum (FWHM) of approximately 40 nm before (black line) and after (red line) liftoff process. A weak peak at 650 nm comes from the 1st order diffraction of He-Cd pumping laser with 325 nm. (b) Schematic of entire fabrication procedure. The red dots show colloidal quantum dots with red fluorescence.


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Fig. 2 One-dimensional straight-line patterns made of QDs using by a lift-off method: (a) optical microscope image of patterned QDs. The linewidth of the fabricated lines are 212 nm. The green scale bar corresponds to 15 μm. (b) Dark-field image of patterned QDs under ultraviolet excitation of 405 nm. The red luminescence lines from the patterned QDs are shown well. (c) Scanning electron microscope image of a straight-line pattern of QDs.

[Fig. 3(b)], and atomic force microscope (AFM) [Fig. 3(c)] images after the entire process. We found that the ring shape is well defined by the QD cluster and no spectral shape change. The AFM image of the 2-D ring patterns in Fig. 3(c) shows the smooth interface of the QD ring pattern with no leftovers, showing that the proposed lift-off method works well in 2-D geometry. It is worth noting that the sidewall of a patterned structure is important for the application of photonic and plasmonic devices. The sidewall of a patterned structure affects the radiation-to-SP conversion efficiency and the photonic mode formation along the QD dielectric. We used AFM for the surface topography along the white line in Fig. 3(c). Figure 3(d) shows the depth profile of the pattern, showing keen edge formation. The thickness of patterned QDs is found 70 nm according to the depth profile shown in Fig. 3(d). 2.5 Sub-100-nm Scale Dot Patterning We designed and fabricated several dot patterns with sub-100-nm diameters ranging from 10 to 100 by 20 nm

steps. The undercut shape of resist profile was found very important to make dot patterns of QDs as designed. We found the edge sharpness of ER patterns strongly affect the uniform size distribution of developed ER patterns and the resultant size distributions of patterned QDs. By optimizing the dose and the focal depth of electron beam in EBL, we could obtain well-defined patterns of ER with uniform size in nanometer scale [Fig. 4(a)] and dark-field image [Fig. 4(b)]. From the SEM image of patterned QDs as shown in Fig. 4(c), we confirm that the dot pattern with approximately 50 nm is well-formed and our technique for QD patterning can indeed be applicable to sub-100nm scale successfully. 2.6 SP Generation For the application of QD patterning technology, we apply a patterned QD line on a metallic structure as a local SP source. SPs can only be generated with additional inplane momentum either by a prism or groove/slit pattern on a metallic surface plane.26,27 However, if emissive

Fig. 3 Two-dimensional ring patterns made of QDs via a lift-off method: (a) optical microscope image of the patterned QDs. The diameter of the ring and the circumference of the linewidth are designed as 6 μm and 60 nm, respectively. The green scale bar corresponds to 15 μm. (b) Dark-field image under ultraviolet irradiation. Arrays of circular fluorescence from the QDs are clearly observed. (c) Atomic force microscopy (AFM) image of the circumference in ring-patterned QDs. The measured linewidth was approximately 600 nm. (d) Sectional AFM data for the circumferential pattern along the white line in (c).


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Fig. 4 Dot patterns with sub-100-nm scale made of QDs: (a) SEM image of patterned electron beam resist (ER); (b) dark-field image on a metallic surface substrate under ultraviolet excitation of 405 nm; (c) SEM images of dot-patterned QDs with 50-nm diameter. The white scale corresponds to 50 nm.

Fig. 5 Surface plasmon launching experiment using transmission geometry: (a) finite-difference time-domain (FDTD) simulation results show that SPs are generated at the edge of the patterned QDs and propagate well through the plane geometry of the metallic surface. The green box represents region of surface plasmon propagation. (b) Schematic diagram of transmission measurement geometry. (c) Transmitted light image taken on the side of the substrate. The light emitted from the patterned QDs propagates and scatters to the slit. This means that the patterned QDs work well as a local SP generation source.

materials such as QDs are patterned, the edges of the structure serve as a momentum supplier to the SP such that there is no need for other metal patterns or a prism. Figure 5(a) shows a simulation of SP generation from a rectangular structure located on a metallic surface plane as calculated by a finite-difference time-domain (FDTD) method.28,29 To observe the output coupling of the propagated SP on the metal surface, a slit with a 150-nm linewidth was etched by a focused ion beam (FIB) 5 μm away from the patterned QDs. We used transmission measurement geometry to image the scattered radiation at the slit. Figure 5(b) represents a schematic of the transmission measurement geometry, where the patterned QDs as a local source of SPs are located on the opposite side of the observed area. A laser at 405 nm was used to excite the QD line structure. The incident light is focused on the QDs but blocked by the metallic surface layer deposited on the substrate and by a long-pass filter. The SPs that propagates from the patterned QDs becomes coupled to the slit. Figure 5(c) shows an optical image J. Micro/Nanolith. MEMS MOEMS

obtained on the substrate side. In this way, we successfully observed the light transmitted through the slit with QD luminescence color, which tells us that the straight-line pattern made of QDs works well as a local SP source. 3 Conclusion In conclusion, we present a lift-off method for patterning colloidal QDs with resolutions of a few tens of nanometers on a metallic surface plane. By making a 1-D straight line, a 2-D ring, and dot patterns, we show that our method is indeed feasible in numerous cases. This advance will serve a basic technology in nano-optical devices where QDs can be used as a local source or detector. In addition, our approach has the advantage of freedom in the creation of pattern shapes and fine resolutions during CMOS-compatible fabrication if EBL is replaced by conventional photolithography. We also confirmed that the spectra of the patterned QDs are unchanged to those of pristine QDs so that the fabricated QD clusters are robust during the entire lift-off

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process. Finally, we observe that SPs from a QD cluster can propagate well enough to transfer energy to another slit 5 μm away. From the results of our study, we expect that the proposed method can be indeed used in photonic and plasmonic applications. References 1. C. B. Murray, D. J. Norris, and M. G. Bawendi, “Synthesis and characterization of nearly monodisperse CdE (E ¼ sulfur, selenium, tellurium) semiconductor nanocrystallites,” J. Am. Chem. Soc. 115(19), 8706–8715 (1993). 2. C. B. Murray, C. R. Kagan, and M. G. Bawendi, “Synthesis and characterization of monodisperse nanocrystals and close-packed nanocrystal assemblies,” Ann. Rev. Mater. Res. 30(1), 545–610 (2000). 3. R. Rossetti, S. Nakahara, and L. E. Brus, “Quantum size effects in the redox potentials, resonance Raman spectra, and electronic spectra of CdS crystallites in aqueous solution,” J. Chem. Phys. 79(2), 1086–1088 (1983). 4. L. E. Brus, “Electron-electron and electron-hole interactions in small semiconductor crystallites: the size dependence of the lowest excited electronic state,” J. Chem. Phys. 80(9), 4403–4409 (1984). 5. R. Rossetti et al., “Size effects in the excited electronic states of small colloidal CdS crystallites,” J. Chem. Phys. 80(9), 4464–4469 (1984). 6. M. A. Reed et al., “Observation of discrete electronic states in a zero-dimensional semiconductor nanostructure,” Phys. Rev. Lett. 60(6), 535–537 (1988). 7. V. L. Colvin, M. C. Schlamp, and A. P. Alivisatos, “Light-emittingdiodes made from cadmium selenide nanocrystals and a semiconducting polymer,” Nature 370(6488), 354–357 (1994). 8. B. O. Dabbousi et al., “Electroluminescence from CdSe quantum‐dot/ polymer composites,” Appl. Phys. Lett. 66(11), 1316–1318 (1995). 9. K.-S. Cho et al., “High-performance crosslinked colloidal quantum-dot light-emitting diodes,” Nat. Photon. 3(6), 341–345 (2009). 10. T.-H. Kim et al., “Full-colour quantum dot displays fabricated by transfer printing,” Nat. Photon. 5(3), 176–182 (2011). 11. A. V. Akimov et al., “Generation of single optical plasmons in metallic nanowires coupled to quantum dots,” Nature 450(7168), 402–406 (2007). 12. H. Wei et al., “Propagating surface plasmon induced photon emission from quantum dots,” Nano Lett. 9(12), 4168–4171 (2009).


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