Nanoscale patterning of colloidal quantum dots for

Nanoscale patterning of colloidal quantum dots for surface plasmon generation 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, and Chang-Won Lee Frontier Research Lab., Samsung Advanced Institute of Technology, Yongin-si, Kyonggi-do, Korea, 446-712

ABSTRACT The patterning of colloidal quantum dots with nanometer resolution is essential for their application in photonics and plasmonics. Several patterning approaches, such as the use of polymer composites, molecular lock-and-key methods, inkjet printing, and microcontact printing of quantum dots, have limits in fabrication resolution, positioning and the variation of structural shapes. Herein, we present an adaptation of a conventional liftoff method for patterning colloidal quantum dots. This simple method is easy and requires no complicated processes. Using this method, we formed straight lines, rings, and dot patterns of colloidal quantum dots on metallic substrates. Notably, patterned lines approximately 10 nm wide were fabricated. The patterned structures display high resolution, accurate positioning, and well-defined sidewall profiles. To demonstrate the applicability of our method, we present a surface plasmon generator elaborated from quantum dots. Keywords: colloidal quantum dot, nanoscale patterning, electron beam lithography, lift-off process, surface plasmon generator, finite-difference time-domain simulation

1. INTRODUCTION Since its invention, colloidal quantum dots (QDs) have attracted interests because of its broad functionalities and applicabilities.1, 2 Owing to quantum confinement effect, their optical spectrum moves to higher energy with decreasing size of the material.3, 4, 5, 6 In addition, high internal radiative efficiency leads to promises for various photonic7, 8, 9, 10 and plasmonic applications.11, 12, 13 In spite of the numerous advantages of QDs, microfabrication of colloidal QDs into onedimensional (1D) or two-dimensional (2D) structures is still quite challenging because of its solubility issue and wetting property to oxide and metallic substrates. These difficulties result in restricted usage and application of QDs to diverse fields of photonics and plasmonics. To overcome those constraints, for example, researchers have developed methods of patterning mixture of photosensitive polymer and QDs.14, 15, 16, 17 However, this photo-curing patterning of nanocrystals suffers from fabrication resolution ( > 2 micron) and lower luminescence efficiency compared to that of pristine QDs. Another solution for patterning colloidal QDs is a microcontact printing using a polymer mold.18, 19, 20 This microcontact printing method has unique advantages of ease of use and high throughput and sub-micrometer scale resolution. However, it has a limit of alignment between the mold and pre-defined nanostructure and becomes disadvantageous in situations where only a fraction of the size of the mold has to be stamped, since the polymer mold is prone to be bent and leave unintended QDs on the substrate when the transferred regions inside a single mold are sparsely distributed. Recently, a few research groups came up with another method of patterning QDs with a self-assembled monolayer (SAM) treatment and molecule key-lock method.21, 22, 23, 24 This method requires 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, the excessive QDs are removed from the surface, leaving the patterned QDs only on top of the predefined SAM of key molecules. However, this method requires complicated surface treatment processes and is very sensitive to the chemical combination of the key-lock pair molecules. In this letter, we present a new patterning process for colloidal QDs similar to a conventional liftoff process. This approach facilitates an easy fabrication methods for defining high resolution 1D and 2D structures without a complicated

Advanced Fabrication Technologies for Micro/Nano Optics and Photonics VI, edited by Georg von Freymann, Winston V. Schoenfeld, Raymond C. Rumpf, Proc. of SPIE Vol. 8613, 861305 · © 2013 SPIE CCC code: 0277-786X/13/$18 · doi: 10.1117/12.2003055 Proc. of SPIE Vol. 8613 861305-1 Downloaded From: http://proceedings.spiedigitallibrary.org/ on 09/11/2013 Terms of Use: http://spiedl.org/terms

surface treatm ment process and/or a deliicate choice oof a key-lock k molecule paair. With our approach, arb bitrary shapess composed off QDs can be patterned p on designated d loccations. This method, unlik ke the microcoontact printing g method, cann be applied evven for definiing sparse pattterns. This oopens the possibility of usiing colloidal Q QDs as local photonic andd plasmonic soources in any planar geomeetry. To demoonstrate the no ovel approach h, we fabricatted 1D straigh ht line and 2D D ring structurees made of colloidal c QDss with linewiidths of a feew nanometerrs. Especiallyy, when nano ostructures aree fabricated onn a metallic pllane, we can observe o that ssurface plasm mons (SPs) can n be launchedd from these patterned p QDss efficiently wiithout the hellp of any oth her structures. In this case,, fabrication resolution r witth nanometer scale plays a significant role because thee generation and a detection oof SP are more efficient in nanometer n scaale structures.

2. E EXPERIM MENT 2.1 Sample p preparation For the experriment, we syynthesized CdSe/CdS/ZnS ccore-shell-sheell colloidal QDs Q by chemiccal processes..24 Ethanol (or an ethanol/m methanol mixtuure) are addeed into a QD//decane soluttion. Precipitaates are isolatted by a centtrifuge for tenn minutes. Afteer pouring offf the supernataant solution, tthe precipitatees are redisperrsed in cyclohhexane. After washing threee times, QDs arre finally disppersed into thee cyclohexanee. These synth hesized QDs emit e photolum minescence (PL L) with a peakk of around 6088 nm efficienttly, and have a full-width aat half-maxim mum (FWHM) of about 40 nnm. In order to t confirm thee feasibility off our method, we fabricateed straight linne patterns wiith various lin newidths in a few nanomeeter ranges onn metallic subsstrate. In plasm monic applicaations, metalliic surface plaane is essentiaal for the genneration and propagation p of plasmons.25 A 300 nm-thicck gold film iss deposited byy electron beam m evaporation n on sapphire substrate with h a 5 nm-thickk Ti layer usedd as an adhession layer. We W also fabricaated ring shaaped structures with a diam meter of 6 μm m and variouss linewidths in a few nanom meter ranges. Finally, F we maade dot pattern ns with variou us nanometer scale diameteers to show thee fine resolution of our liftofff method. 2.2 Fabricattion The whole faabrication proccedure is simiilar to the connventional lifttoff process. (F Fig. 1) The fiirst step is a spin-coating of electron beam m resist (ER) such as polym methylmethacrrylate (PMMA A) on prepareed substrate att 4,000 rpm fo or 40 secondss. The ER is baaked at 170 °C for 300 secconds. To deffine a structurre with nanom meter scale onn the substrattes, we used a JEOL JBX-99300FS machiine. After thee exposure, saamples were developed by y a 1:1 mixtur ure of methyliisobutylketonee (MIBK) and isopropyl alccohol (IPA) fo or 180 secondds. In order to o put QDs, QD D solution waas spin-coated d on the EBLpatterned reggion on substrrate at 2,000 rpm for 40 seeconds, and baked b at 90 °C for 5 minuutes to removee cyclohexanee solvent.

it

oating

Figure 1. Schematics of whole w fabrication procedure. R Red dots show colloidal c quantu um dots with reed fluorescence.

2.3 1D line p patterning 1D straight line patterns off QDs on metaal surface subsstrate are fabrricated. Opticaal microscopee image, taken n by a confocaal microscope ((Olympus LE EXT OLS30-S SU) as shown wn in Fig. 2(aa), shows straaight line of QDs clearly y. When thesee patterned sam mples are irraddiated by ultrraviolet (UV) light of 405 nm, we can observe o seaml mless line of flluorescence ass shown in Figg.2 (b). For innvulnerability check, we peerformed PL measurement before and aafter the liftofff process. Wee found no chan ange in PL speectra. From sccanning electroon microscop pe (SEM) imag ges of Fig. 2(cc), we found that t fabricatedd linewidths off QDs actuallyy ranges from 160 nm to 2990 nm. We ex xpect that thesse irregularitiees can be imprroved by EBL L process optim mization, spin coating of QD Ds, and liftofff conditions.

Proc. of SPIE Vol. 8613 861305-2 Downloaded From: http://proceedings.spiedigitallibrary.org/ on 09/11/2013 Terms of Use: http://spiedl.org/terms

(a)

(c)

(b)

Figure 2. 1-dimensional straight line patterns made of QDs using by liftoff method. (a) Optical microscope image of patterned QDs. The linewidth is designed to 100 nm and fabricated as 212 nm really. (b) Dark field image of patterned QDs (c) under ultraviolet excitation of 405 nm. Red luminescence from patterned QDs is shown well. (c) Scanning electron microscope image of straight line pattern of QDs.

2.4 2D ring patterning As an example of 2D patterns, we chose ring-type QD clusters with diameter of 6 micrometers with 500 nm width. Figure 3 shows ring shapes patterned of QDs by the liftoff method on metallic surface substrate. Same as the straight line patterns, we obtained optical microscope (Fig. 3 (a)), UV-irradiated (405 nm) dark-field (Fig. 3 (b)), and SEM (Fig. 3 (c)) images after the process. We found that the ring shape is well-defined by QD cluster and emit light similar to the pristine QDs. Additionally, SEM image of 2D ring patterns in Fig. 3(c) show a smooth interface of the QD ring pattern with no leftovers showing that the proposed liftoff method works well in the 2D geometry.

(a)

(c)

b

o

0

CO 148.0 nm

120.0

(b)

(d)

i- P,ofila,

100.0

80.0 60.0 40.0 20.0 0.0

Figure 3. 2-dimensional ring patterns made of QDs using by liftoff method (a) Optical microscope image of patterned QDs. The diameter of ring and the linewidth of circumference are designed to 6 mm and 60 nm respectively. (b) Dark field image under ultraviolet excitation. Arrays of ring patterns composed of QDs are observed well. (c) SEM image of circumference in ring-patterned QDs. The linewidth of circumference was measured as approximately 600 nm. (d) Sectional AFM data for the circumferential pattern along the white line in (c).

It is worth noting that the sidewall of patterned structure is important for the application of photonic and plasmonic devices. The sidewall of a patterned structure defines the radiation-to-surface plasmon conversion efficiency and the photonic mode formation along the QD dielectric. We measured atomic force microscopy along the white line in Fig. 3 (c). Figure 3 (d) shows the depth profile of the pattern showing abrupt wall formation.

Proc. of SPIE Vol. 8613 861305-3 Downloaded From: http://proceedings.spiedigitallibrary.org/ on 09/11/2013 Terms of Use: http://spiedl.org/terms

2.5 0D dot patterning Dot patterns with a few nanometer sizes are fabricated as the extreme examples of patterning QDs. The dot structures are designed to have diameter with from 100 nm to 350 nm. Figure 4 (a) and (b) represent optical microscope and dark field images, respectively. We observed the red-colored dot patterns under UV irradiation. Compared to the above cases, emission of dot-patterned structures is very weak because of smaller volume of QD of each cluster. (We note that the noise level of Fig. 4(b) is higher than dark field images of straight line and ring patterns.) At present, emission intensities from dot clusters are not uniform due to the non-uniform patterning of QDs. We expect this will be improved in the near future by optimizing processes further. To measure the size a QD dot pattern, we obtained SEM image as shown in Fig. 4(c). The diameter of patterned dot is measured as 220 nm.

(a)

(b)

(c)

Figure 4. 0-dimensional dot patterns made of QDs using by liftoff method (a) Optical microscope image of patterned QDs. The diameter of dot pattern is designed to 200 nm and fabricated as about 220 nm according to the measurement of SEM image. (b) Dark field image on metallic surface substrate under ultraviolet excitation of 405 nm. (c) SEM image of dotpatterned QDs.

2.6 Surface plasmon generation For application of QD patterning technology, we can use a patterned QD line on a metallic structure as a local SP source. SPs can only be generated with additional in-plane momentum either by a prism or groove / slit pattern on a metallic surface plane.26, 27 However, if emissive materials like QDs are patterned, the edges of structure can play roles of momentum supplier to SP so that there is no need for other metal patterns or prism. Figure 5 (a) shows a simulation of SP generation from rectangular structure located on a metallic surface plane calculated by a method of finite-difference time-domain (FDTD).28, 29 Experimental far-field observation requires conversion of SP to radiation, therefore, we need another momentum-aid structure. To see the output coupling of propagated SP on metal surface, a slit with 150 nm linewidth was etched by the focused ion beam (FIB) machine at 5 μm away from the patterned QDs. We used transmission measurement geometry for imaging the scattered radiation at the slit. Figure 5 (b) represents a schematic of transmission measurement geometry where the patterned QDs as a local source of SPs is located at the opposite side of observation. The laser at 405 nm excites the QD line structure. The incident light is focused only to QDs and blocked by metallic surface layer deposited on substrate as well as a long pass filter. Therefore, we can observe the radiation energy from the SPs propagated from the patterned QDs and coupled to the slit. The fluorescence from the patterned QDs scatter in every direction, and coupled to SP mode due to the edge of structure. Then, these SPs propagate until they reach the slit at 5 μm away. Figure 5 (c) shows the optical image obtained at the substrate side. As shown in Fig. 5 (c), we successfully observe the light transmitted through the slit with QD luminescence color, which tells us the straight line pattern made of QDs works well as a local SP source.

Proc. of SPIE Vol. 8613 861305-4 Downloaded From: http://proceedings.spiedigitallibrary.org/ on 09/11/2013 Terms of Use: http://spiedl.org/terms

(a)

(b)

(c)

laser excitation

^D

-

SP propagation

-,,

substrate

I

ight transmissio

Figure 5. Surface plasmoon generation measurement m usiing transmissio on geometry (a) FDTD simulattion results show w that SP o patterned QD Ds and propagatted well through the plane geo ometry of metallllic surface. (b) Schematic is generatted at the edge of diagram oof transmission measurement geometry. g (c) Trransmitted ligh ht image taken on o the side of suubstrate. The em mitted light from m patterned QDss is propagated and scattered too slit. This meaans that the patterned QDs worrk well as the lo ocal SP generationn source.

3. C CONCLUS SION In conclusionn, we present a liftoff meth hod for patternning colloidall QDs with a few nanometeer resolutionss on a metallicc surface planee. By making 1D straight line, 2D ring, aand dot patterrns respectivelly, we show oour method in ndeed providess feasibility forr versatile caases. The sideewall of patteerned QDs arre smooth enough comparred to other QD Q patterningg technology suuch as polymer composite, molecule keyy-lock method d, and microccontact printinng method. Th his will play a role on decreeasing opticall loss at the in nterface of a structure wheen QDs are used u as the loocal source off photonic andd plasmonic deevices. Additiionally, our ap pproach has aan advantage of freedom in i pattern shaapes and fine resolutions inn fabrication. A After the proccess, we confi firmed that thee patterned QDs Q emit PL like the pristiine QDs and the fabricatedd structures aree very robust on o post-processs after liftofff. Finally, we observe o that SPs S from QDss can propagatte well enoughh to transfer ennergy to anothher slit at 5 μm m away. From m the results of o our study, we w expect thaat the suggesteed method cann be extensivelyy used to the field f of photonic and plasm monic applicatiions.

RE REFERENCE ES [1] Murrray, C. B., Noorris, D. J., Baawendi, M. G G., "Synthesis and a characteriization of neaarly monodisperse CdE (E = sulfuur, selenium, tellurium) t sem miconductor nnanocrystallitees," J. Am. Chem. Soc. 115((19), 8706–87 715 (1993). [2] Murrray, C. B., Kagan, K C. R., Bawendi, M. G., "Synthessis and characcterization of monodispersee nanocrystalss and close-packed nanocrystal assemblies," a A Ann. Rev. Matter. Res. 30, 54 45–610 (20000). [3] Rosssetti, R., Nakaahara, S., Brus, L. E., "Quaantum size efffects in the red dox potentialss, resonance Raman R spectraa, and electronic speectra of CdS crystallites in aaqueous solutiion," J. Chem. Phys. 79(2), 1086-1088 (1 1983). [4] Bruss, L. E., "Eleectron-electron n and electroon-hole interaactions in small semiconduuctor crystallites: The sizee depeendence of thee lowest exciteed electronic sstate," J. Chem m. Phys. 80(9)), 4403-4409 (1984) [5] Rosssetti, R., Ellisson, J. L., Gib bson, J. M., B Brus, L. E., "Size " effects in the excitedd electronic states s of smalll collooidal CdS crystallites," J. Chem. C Phys. 800(9), 4464-4469 (1984) [6] Reedd, M. A., Randall, J. N., Ag ggarwal, R. J.,, Matyi, R. J., Moore, T. M., Wetsel, A. E E., "Observation of discretee electtronic states inn a zero-dimensional semicconductor nanostructure," Phys. Rev. Letttt. 60(6), 535– –537 (1988) [7] Colvvin, V. L., Schlamp, S M. C., Alivisatoos, A. P., "L Light-Emitting g-Diodes madde from cadm mium selenidee nanoocrystals and a semiconducting polymer, " Nature 370, 354-357 (199 94) [8] Dabbbousi, B. O., O Bawendi, M. G., Onnitsuka, O., Rubner, M. F., "Electrooluminescencee from CdSee quanntum‐dot/polyymer composittes," Appl. Phhys. Lett. 66(11), 1316-1318 8 (1995) [9] Cho, K. -S., Lee, E. K., Joo, W. W -J., Jang, E.,, Kim, T. -H., Lee, S. J., Kw won, S. -J., Haan, J. Y., Kim m, B. -K., Choii, B. L L., Kim, J. M.,, "High-perforrmance crossllinked colloid dal quantum-dot light-emittiing diodes," Nat. N Photon. 3, 341--345 (2009) [10] Kim m, T. -H., Cho,, K. –S., Lee, E. K., Lee, S. J., Chae, J., Kim, K J. W., Kiim, D. H., Kw won, J. –Y., Amaratunga, G., G Lee,, S. Y., Choi, B. L., Kuk, Y., Y Kim, J. M.., Kim, K., "F Full-colour quaantum dot disp splays fabricatted by transfer printting," Nat. Phhoton. 5, 176-1 182 (2011)

Proc. of SPIE Vol. 8613 861305-5 Downloaded From: http://proceedings.spiedigitallibrary.org/ on 09/11/2013 Terms of Use: http://spiedl.org/terms

[11] Akimov, A. V., Mukherjee, A., Yu, C. L., Chang, D. E., Zibrov, A. S., Hemmer, P. R., Park, H., Lukin, M. D., "Generation of single optical plasmons in metallic nanowires coupled to quantum dotsNature," 450, 402-406 (2007) [12] Wei, H., D., Ratchford, D., Li, X. E., Xu, H., Shih, C.-K., "Propagating surface plasmon induced photon emission from quantum dots," Nano Lett. 9(12), 4168-4171 (2009) [13] Jun, Y. C., Pala, R., Brongersma, M. L., "Strong modification of quantum dot spontaneous emission via gap plasmon coupling in metal nanoslits," J. Phys. Chem. C 114(16), 7269-7273 (2010) [14] Vossmeyer, T., Jia, S., DeIonno, E., Diehl, M. R., Kim S.-H., Peng, X., Alivisatos, A. P., Heath, J. R., "Combinatorial approaches toward patterning nanocrystals," J. Appl. Phys. 84(7), 3664-3670 (1998) [15] Wang, Y., Tang, Z., Correa-Duarte, M. A., Liz-Marzan, L. M., Kotov, N. A., "Multicolor luminescence patterning by photoactivation of semiconductor nanoparticle films," J. Am. Chem. Soc. 125(10), 2830-2831 (2003) [16] Jun, S., Jang, E., Park, J., Kim, J., "Photopatterned semiconductor nanocrystals and their electroluminescence from hybrid light-emitting devices," Langmuir 22(6), 2407-2410 (2006) [17] Kim, W. J., Kim, S. J., Lee, K. -S., Samoc, M., Cartwright, A. N., Prasad, P. N., "Robust microstructures using UV photopatternable semiconductor nanocrystals," Nano Lett. 8(10), 3262-3265 (2008) [18] Yan, X., Yao, J., Lu, G., Chen, X., Zhang, K., Yang, B., "Microcontact printing of colloidal crystals," J. Am. Chem. Soc. 126(34), 10510-10511 (2004) [19] Wu, X. C., Chi, L. F., Fuchs, H., "Patterning of semiconductor nanoparticles via microcontact printing," Eur. J. Inorg. Chem. 18, 3729–3733 (2005) [20] Kim, L. A., Anikeeva, P. O., Coe-Sullivan, S. A., Steckel, J. S., Bawendi, M. G., Bulovic, V., "Contact printing of quantum dot light-emitting devices," Nano Lett. 8 (12), 4513-4517 (2008) [21] Harnett, C. K., Satyalakshmi, K. M., Craighead, H. G., "Low-energy electron-beam patterning of aminefunctionalized self-assembled monolayers," Appl. Phys. Lett. 76(17), 2466-2468 (2000) [22] Harnack, O., Raible, I., Yasuda, A., Vossmeyer, T., "Lithographic patterning of nanoparticle films selfassembled from organic solutions by using a water-soluble mask," Appl. Phys. Lett. 86, 034108 (2005) [23] Kramer, R. K., Pholchai, N., Sorger, V. J., Yim, T. J., Oulton, R., Zhang, X., "Positioning of quantum dots on metallic nanostructures," Nanotechnology 21(14), 145307 (2010) [24] Reiss, P., Protiere, M., Li, L., "Core/shell semiconductor nanocrystals," Small 5(2), 154-168 (2009) [25] Schuller, J. A., Barnard, E. S., Cai, W., Jun, Y. C., White, J. S., Brongersma, M. L., "Plasmonics for extreme light concentration and manipulation," Nature Mater. 9, 193–204 (2010) [26] Raether, H., [Surface Plasmons], Springer, Berlin, (1988) [27] Barnes, W. L., Dereux, A., Ebessen, T. W., "Surface plasmon subwavelength optics," Nature 424, 824-830 (2003) [28] Taflove, A. and Hagness, S. C., [Computational Electrodynamics: The Finite-Difference Time-Domain Method], Artech House Publishers, (2005) [29] Johnson, S. G., "MEEP," (14 October 2005).

Proc. of SPIE Vol. 8613 861305-6 Downloaded From: http://proceedings.spiedigitallibrary.org/ on 09/11/2013 Terms of Use: http://spiedl.org/terms