Using nanosphere lithography for fabrication of a multilayered system of ordered gold nanoparticles

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1 PACS n Semiconductor Physics, Quntum Electronics & Optoelectronics, V. 20, N 2. P Using nnosphere lithogrphy for friction of multilyered system of ordered gold nnoprticles V.I. Styopkin, V.R. Likhovetskyi, V.I. Rudenko Institute of Physics, NAS of Ukrine, 46, prospect Nuky, Kyiv, Ukrine, E-mil: vis49@mil.ru; lykh@iop.kiev.u; vl@iop.kiev.u Astrct. New modifiction of nnosphere lithogrphy hs een relized to otin multilyered systems of ordered gold nnoprticles (NP). NP hve een formed using vcuum deposition of 5 60-nm lyer of gold on ionic etched multilyered regulr coting consisted of severl lyers of 200-nm polystyrene spheres. Opticl study shows tht spectr of NP depend on their thickness nd my e chnged y het tretment. Incresing the NP thickness within the 5 20-nm rnge leds to shortwve displcement of the plsmon resonnce pek position, while the longwve shift is oserved in nm rnge. Het tretment of NP rings nrrowing nd displcement of spectrl nds, rising the extinction. It hs een supposed tht vrition of the NP shpe is the most sustntil fctor for chnges of opticl properties in the 5 20 nm thickness region, while electromgnetic coupling etween NP in different lyers ecomes more importnt for thicknesses lrger thn 40 nm. Opticl properties inherent to the otined system of NP cn e tuned y chnging the polystyrene spheres dimeter, extent of etching, thickness of gold lyer nd with the het tretment. It my e used in design of nnophotonic devices. Keywords: nnosphere lithogrphy, gold nnoprticles, opticl properties, het tretment, ionic etching. Mnuscript received ; revised version received ; ccepted for puliction ; pulished online Introduction Ordered nole metl nnoprticles (NP) re promising mterils for photonic mterils, SERS, ner-field optics, etc. Different lithogrphic techniques re used to fricte these nnostructures. The high resolution of NP production cn e otined with electron em nd focused ion em lithogrphy. However, these methods re seril nd expensive, which confines high volume mnufcture of NP. An lterntive method for production of ordered NP with determined shpe is the nnosphere lithogrphy (NSL) [1-3]. NSL is powerful process to produce vrious rrys of periodic structures tht my e used in the field of nno- nd microfriction. It my e pplied in devices of nnophotonics, such s surfce enhnced Rmn scttering sustrtes, iosensors, design of ordered nnostructures for ultrhigh density mgnetic recording. In simplest version, NSL is two-step process. The one-lyer coting of ordered polystyrene spheres (PS) is formed on the sustrte. Then, desired mteril is deposited on this sustrte through holes in the PS lyer. In this wy, fter PS removl one otins system of ordered NP of this mteril. But coverge of the sustrte with NP is low nd mounts 7.2% [4]. NP in the form of metl film on surfce of PS my e used to increse volume of otined nnostructures [5, 6]. But the closeness of these NP to ech other 240

Ion etching the PS in coting llows controlling their size nd distnces etween them [7, 8].

2 influences strongly on their properties. Moreover, the size of NP is limited y the used sort of PS. To overcome these disdvntges, in this work NSL method ws used to produce ordered NP on the surfces of etched PS in multilyer coting. Ion etching the PS in coting llows controlling their size nd distnces etween them [7, 8]. Besides, in the cse of multilyered coting etching PS in the top lyer opens those in lower lyers nd increse numer of the otined NP. Deposition of metl on the surfce of etched PS cretes complex structure of ordered NP tht re locted in different lyers. The im of the work ws to otin nnostructures, consisted of NP formed on the surfce of etched PS in multilyered cotings, nd to study there opticl properties. 2. Experiment 200-nm PS (stndrd devition 6 nm) were used to fricte the multilyered cotings. PS were supplied y Sigm-Aldrich (2% queous suspension). The cotings were otined on glss sustrtes. Before PS deposition, the sustrtes were clened nd hydrophilized. They were mintined in Pirnh solution (3:1 H 2 SO 4 :H 2 O 2 ) t 70 C for 1 hour. Then, the sustrtes were rinsed in fresh distilled wter nd sonicted for 30 min in 5:1:1 H 2 O:NH 4 OH:H 2 O 2 solution. Before PS deposition, the sustrtes were rinsed gin in fresh distilled wter. Diluted with distilled wter to 0.1% content PS suspension ws drop-coted onto sustrte nd dried t 35 C. The volume of deposited PS suspension ws djusted to otin cotings with the thickness of three lyers of PS on the sustrte. After drying, the coting ws formed on the sustrte, the min prt of which contins three nd more lyers of PS. Lesser prt of sustrte ws covered with one nd two lyers of PS. The PS cotings were etched in JFC-1100 Ion Sputtering Device (JEOL) under the ir pressure close to 0.2 Torr. A degree of PS etching depends on the pplied node-cthode voltge, ir pressure in the chmer, durtion of process, using of ion strem concentrtor, etc. The process conditions were djusted to otin PS dimeters nm in the upper lyer of coting. Etched PS were covered with gold y using therml evportion in vcuum. The weight thickness of gold lyer vried from 5 up to 60 nm. Thus, it ws otined multilyered system of gold NP on the surfce of etched PS. To enhnce stility, the NP were thermlly treted. 5-min het tretment ws done in ir t C. It should e noted tht uncovered PS soften t C for 5 min, ut the gold coting mkes it possile to sufficiently rise the temperture of het tretment. Morphology of this multilyer system of gold NP ws studied using JSM-35 scnning electron microscope (SEM). Opticl spectr of the NP system were otined with MDR-6 spectrometer within the rnge 400 to 800 nm with non-polrized incident light em t room temperture Results SEM imge of prt of the multilyered PS coting is shown in Fig. 1. Two PS re lcking in the upper lyer nd PS of the second lyer ecome visile t these plces. Scheme of PS pcking in such coting is shown in Fig. 1. After ion etching, PS in upper lyers reduce nd PS of the second nd third lyers pper. SEM imge of the multilyered PS coting fter ion etching nd deposition of the gold lyer re shown in Fig. 2. PS in the upper lyer decrese down to nm, while the size of visile prt of PS in the second lyer reduces down to nm. The scheme of PS pcking fter etching is shown in Fig. 2. Het tretment reduces the size of PS nd NP on the PS, correspondingly. The PS nd NP sizes reduction cn e oserved on SEM imge of the system fter het tretment (Fig. 3). This reduction will give n increse of NP thickness. Fig. 1. SEM imge of the multilyered PS coting with defects in the upper lyer () nd the scheme of this coting ().

Opticl density spectr of the multilyered system of etched PS fter deposition of gold lyer of different thickness nd the spectrum of the initil PS coting re shown in Fig. 4.

The position of spectr mximum displces to the lower wvelengths with thickening the gold lyer up to 40 nm.

4 shows tht het tretment of NP leds to nrrowing the spectrl nds, rising the extinction nd displcing the pek position. The vlue nd sign of this displcement depends on the gold lyer thickness.

3 Opticl density spectr of the multilyered system of etched PS fter deposition of gold lyer of different thickness nd the spectrum of the initil PS coting re shown in Fig. 4. The spectr of gold NP on etched PS revel rod peks. The extinction rises with the incresing of gold lyer thickness. The position of spectr mximum displces to the lower wvelengths with thickening the gold lyer up to 40 nm. But the resonnce pek of 60-nm NP shifts to longer wvelengths in comprison with tht of 40-nm NP. Fig. 4 shows tht het tretment of NP leds to nrrowing the spectrl nds, rising the extinction nd displcing the pek position. The vlue nd sign of this displcement depends on the gold lyer thickness. As result, the wvelength of spectr mximum lowers with thickening the gold lyer within the rnge 5 20 nm, while within the rnge nm of gold thickness the wvelength rises. Thus, het tretment gives different sign of displcement for the spectr in different regions of gold NP thicknesses. Fig. 2. SEM imge of the multilyered PS coting fter ion etching : 1 PS in the upper lyer, 2 PS in the second lyer, 3 PS in the third lyer (); scheme of PS pcking fter etching (). Fig. 3. SEM imge of the multilyered NP system fter het tretment. Fig. 4. Opticl density spectr of PS coting nd of multilyered system of gold NP with different thicknesses on the etched PS efore () nd fter het tretment (). 242

4 Fig. 5 shows the opticl density spectr of multilyered system of gold NP with the thickness vlues 20 nd 40 nm efore nd fter het tretment. In the cse of 20 nm, the thickness position of spectrum mximum chnges from 604 to 570 nm fter het tretment, while for the thickness 40 nm from 557 to 579 nm. 4. Discussion Opticl spectr of nole metl NP re minly controlled y loclized surfce plsmon resonnce (LSPR). Chrcteristics of this resonnce depend on the size, shpe, properties of sustrte. They my lso e influenced y electromgnetic coupling etween the djcent NP. In this work, opticl properties of the multilyered NP system show good correltion with dt of gold NP in other works. The spectr of rectngulr gold NP re clculted in [9]. Pek of dipole LSPR is otined t 545 nm for the system of NP with 200-nm size, 50-nm thickness nd interprticle distnce of 200 nm. In our work, NP with 150-nm size nd 60-nm thickness show the pek t 575 nm. This 30-nm displcement my e cused y difference in the NP shpe nd influence of sustrte. Fig. 5. Opticl density spectr of the multilyered system of gold NP on the etched PS efore nd fter het tretment with the thickness of gold 20 nm () nd 40 nm (). The conditions of NP formtion in [10] re similr to those of this work. The gold NP with the thickness 30 nm were otined on 170-nm PS on glss sustrte. Resulting NP system shows spectr with the pek t 600 nm. This is lso close to the results of our work. To explin chnges in the spectr with vrition of NP thickness nd fter het tretment, only dipole resonnce hs een considered in this work. We supposed tht opticl properties of multilyered NP system minly depend on the NP thickness nd interction of electromgnetic fields of nerest NP. In the region of smll NP thickness, its vrition will not chnge the interprticle distnce of the NP in the sme lyer. NP of the different lyers re plced t quite lrge distnce, nd electromgnetic coupling of NP is low. Thus, vrition of distnce etween these NP will not influence sufficiently on their properties, nd opticl spectr of NP system minly depend on vrition of single NP properties. Thickening the NP will chnge their shpe. This effect ws considered for NP with the shpe of spheroids [11] nd rods [12]. Such chnging is lso studied for vrition of spect rtio of rectngulr gold NP [9]. Clcultions show long wve shift of pek position for length increse of long side of NP. This displcement is explined y the influence of phse retrdtion of incident rdition nd y electrodynmics oundry effects relted with the prticle shpe. The effect of phse retrdtion is reveled t reltively lrge sizes of NP, when the induced polriztion is not ll in-phse for plne-wve excittion due to the finite wvelength. In our cse, NP sizes re out 150 nm, nd effect of phse retrdtion my e oserved. But the increse in the NP thickness does not chnge the NP size in the plne of incident light polriztion, nd phse retrdtion cnnot cuse evident chnges in the opticl spectr. To greter extent, it should influence vrition of the NP spect rtio. Thickening the NP increses the rtio of thickness to size of NP tht is in the plne of incident light polriztion. Thus, the resonnt pek position will shift to the short wve side. A similr shortwve shift with NP thickening ws otined for resonnce in Ag nnodisks [12], where it ws shown tht such shift is more significnt t smll thickness. On the other hnd, electromgnetic coupling etween NP will exhiit sufficient effect t lrger NP thickness. This interction etween prticle fields involves oth very short-distnce interctions due to evnescent fields nd long-rnge interctions, minly due to propgting dipolr fields. It lso cuses displcement of the resonnt pek. The vlue nd sign of this displcement depend on orienttion of light polriztion reltive to intercting NP. Study of electromgnetic coupling of gold NP pirs [13] shows tht the decrese in distnce etween NP leds to longwve shift of resonnce in the cse of prllel orienttion of light polriztion to the long xis of prticle pirs. In the cse of orthogonl orienttion of 243

5 light polriztion, the shortwve displcement is oserved. In the considered multilyered system, NP re locted in different lyers t vrious distnces. So, electromgnetic coupling etween NP is very complicted. The scheme of NP rrngement in different lyers is shown in Fig. 6 (side view). The schemtic chrge distriution in NP is lso given for the excittion with light hving polriztion prllel to xis pssing the centers of 1 nd 2 NP. Incresing the NP thickness does not chnge the distnce etween NP 1 nd 2 locted in one lyer. Distnce etween NP 3 in the second lyer nd NP 1 nd 2 in upper lyer decreses. Thus, thickening the NP will influence minly on the electromgnetic coupling etween NP in different lyers. Fig. 7 shows the scheme of NP rrngement in different lyers (top view) nd for chrge distriution in them under two orienttions of light polriztion. In the cse of light polriztion in Fig. 7, electromgnetic coupling of NP 3 nd 4 gives longwve shift of resonnce. Chrges in NP 1 nd 2 re rrnged lmost symmetriclly reltive to chrges in NP 3 nd coupling etween these prticles will not chnge resonnce noticely. In the cse of light polriztion in Fig. 7, electromgnetic coupling of NP 3 nd 4 leds to shortwve shift of resonnce, while coupling of NP 3 with prticles 1 nd 2 gives longwve shift. One my guess tht interction of NP 3 with two prticles (1 nd 2) gives lrger effect thn tht with one NP (4). Thus, it my e supposed tht in our multilyered NP system electromgnetic interction of NP in different lyers will led to longwve shift of resonnt pek. But this interction will give noticele effect on plsmonic resonnce only t the smll distnce etween NP. Results of studying the gold NP [13, 14] show tht for 100-nm NP interction coupling revels sufficient effect on opticl spectr t the interprticle distnce smller thn nm. Fig. 6. Scheme of rrngement of NP in different lyers (side view) nd chrge distriution in them under excittion with light. Fig. 7. Scheme of rrngement of NP in different lyers (top view) nd chrge distriution in them under two orienttions of light polriztion. From chnges in the PS size fter etching, it my e supposed tht the distnce etween djcent NP in different lyers mounts out 100 nm. Incresing the NP thickness up to nm will lower the interprticle distnce down to nm. Results of [14, 15] give the longwve shift of resonnce 5 10 nm for these distnces. Therefore, the resonnce spectr will revel shortwve shift with the NP thickening minly due to vrition of single NP properties up to 40 nm thickness. Susequent rise of the thickness will increse electromgnetic coupling of NP t different lyers nd led to longwve shift of resonnce. The sme resons my explin the chnges in spectr fter het tretment. As it ws shown erlier, het tretment reduces the size of PS nd NP nd increses thickness of NP. The ove discussion leds to supposition tht within the rnge 0 40 nm the thickness incresing fter het tretment will shift resonnce to shortwve prt of spectr, while for thicknesses lrger 40 nm to the longwve prt. 5. Conclusions In this pper, we hve shown tht using NSL in comintion with the ionic etching llows to otin multilyered system of ordered metl NP. Opticl spectr of the initil NP system nd thermlly treted NP were studied. It hs een shown tht opticl properties of the multilyered NP system my e tuned y vrying the NP thickness nd het tretment. Thickening the NP shifts LSPR spectr, vlue nd sign of displcement depends on the NP thickness. The het tretment gives enlrging the NP thickness nd chnge opticl properties, too. Considering the influence of NP thickness on spectr hs shown two min resons for resonnce modifiction. Chnging the NP shpe is the most sustntil fctor in the thickness rnge 5 20 nm, while the electromgnetic coupling etween NP in different lyers ecomes more importnt for thicknesses lrger thn 40 nm. The proposed system of ordered NP my e used to design nnophotonic devices. 244

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