Tuning surface interactions to control shape and array behavior of diblock copolymer micelles on a silicon substrate

Surface Science 603 (2009) 625–631

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Tuning surface interactions to control shape and array behavior of diblock copolymer micelles on a silicon substrate I.R. Laskar a, S. Watanabe a,*, M. Hada a, H. Yoshida b, J. Li c, T. Iyoda c a

Department of Applied Science, Faculty of Science, Kochi University, 1-2-5 Akebono-cho, Kochi 780 8520, Japan Department of Applied Chemistry, Faculty of Urban Environmental Sciences, Tokyo Metropolitan University, 1-1 Minami-Osawa Hachiouji, Tokyo 192-0397, Japan c Chemical Resource Laboratory, Tokyo Institute of Technology, 1-25-4259, Nagatsuda, Midori Ku, Yokohama 226-8503, Japan b

a r t i c l e

i n f o

Article history: Received 4 July 2008 Accepted for publication 19 December 2008 Available online 29 December 2008 Keywords: Self-assembly Nanopatterning Surface energy Gold nanodot Block copolymer Liquid crystal Vacuum ultraviolet

a b s t r a c t The micellar shape of liquid crystalline diblock copolymers, PEOm-b-PMA(Az)n, consisting of high surface energy components was controlled by tuning surface interactions. On a fluorinated surface, the diblock copolymers formed ordered arrays of spherical micelles consisting of PEO cores surrounded by PMA(Az) coronas. Gold ions could be doped into the PEO cores by immersion in a solution of the gold ion. The Au3+doped micelles were subsequently etched and reduced by VUV radiation to form hexagonally ordered gold nanodots. Ó 2008 Elsevier B.V. All rights reserved.

1. Introduction The development of easy, rapid and efficient techniques for patterning surfaces on the nanometer scale is of great importance in modern science and technology. Block copolymers that spontaneously form a rich variety of nanoscale periodic patterns have received much attention as self-assembling nanotemplates [1–6]. In particular, the self-assembly of block copolymer micelles (BCMs) followed by doping and reduction of metal ions [7–12], as shown in Scheme 1, has distinct advantages over conventional self-assembly of surface-protected nanoparticles because it eliminates the multi-step synthesis of nanoparticles and avoids time consuming purification to remove free capping agents and surfactants [13]. However, this method is stringently limited to block copolymers which can form micelles with a well-defined shape on a substrate. If the interaction of either one or both blocks with the substrate becomes too strong, the BCM loses its well-defined structure, becoming unsuitable for the production of nanostructures with well-controlled morphologies. Therefore, controlling the shape of a wide variety of BCMs is of great current interest in nanostructure fabrication using such templates. The surface behavior of block copolymers depends on the interaction between solvents, substrate surfaces, and block components [14–16]. Optically and electrically functional components have a relatively high surface energy and

* Corresponding author. Tel.: +81 88 844 8301; fax: +81 88 844 8359. E-mail address: [email protected] (S. Watanabe). 0039-6028/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.susc.2008.12.027

BCMs consisting of such functional blocks tend to have a high affinity for substrate surfaces. For instance, amphiphilic diblock copolymers with azobenzene-based liquid crystalline side-chains, PEOmb-PMA(Az)n, shown in Scheme 1, provided a potential nanophase separated structure as the self-assembled nanotemplate [17,18]. The PMA(Az) block provided mesogenic properties such as long range orientational ordering and optical and electrical alignment and patterning [19], while the PEO block had the chemical functionality of binding to metal ions [20]. Such unique polymers are highly promising candidates for controlling micellar morphology and array behavior by external stimuli, which has never been achieved using simple block copolymers. However, highly functional diblock copolymers such as those studied here do not form well-defined spherical micelles on a normal Si substrate (vide infra). In this work, we demonstrate how surface interactions can be tuned to obtain regular arrays of micelles with well-defined shape using a BCM with relatively high surface energy. In addition, the BCMs of controlled shape accommodated metal ions M+, and subsequent etching and reduction of the M+-doped BCM generated surfaces patterned with a regular hexagonal array of metal nanodots. 2. Experimental 2.1. Materials The liquid crystalline block copolymer, PEO454-b-PMA(Az)80, was synthesized according to a method described in the literature

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O CH3(OCH2CH2)mO

PEOn

Br n

PMA(Az)n O O N CH2(CH2)9CH2O N

CH2CH2CH2CH3

PEOm-b-PMA(Az)n

a

PEO core

PMA(Az) corona

Self-organization

b Si shrinkage

PEOm-b-PMA(Az)n

Si

solvent evaporation

collapse

e

M0

Etching and Reduction

d

M+

Si

Si

M+ doping

c Si Block Copolymer Micelles (BCMs)

Scheme 1. Schematic depiction of the templated self-assembly of gold nanodot arrays.

[17]. HAuCl4  3H2O was purchased from Wako Pure Chemical Industries Ltd., Japan. (3,3,4,4,5,5,6,6,6-nonafluorohexyl)trimethoxysilane (FAS9) was bought from Gelest Inc. All other reagents and solvents were purchased from WAKO, Inc. or NAKALAI Tesque, Inc. and used without further purification. Milli-Q water with resistivity >18 MX cm was used throughout these experiments. Si(1 0 0) wafers (intrinsic or antimony doped) were purchased from Fujimi Fine Technology (Japan), and were cut into 1  1 cm2 pieces. 2.2. Dynamic light scattering (DLS) DLS measurements were performed with a DLS-6500 (Otsuka Electronics Co., Ltd.) instrument, consisting of a goniometer and a 10 mW He–Ne laser (k = 632.8 nm). Sample solutions in the concentration range of 104–102 wt% were prepared and filtered through 0.45 lm syringe filters (Whatman Puradisk , 13 mm PTFE membrane). All measurements were carried out in sealed cylindrical scattering cells at four different scattering angles (30°, 60°, 90°, and 120°) at 25 °C. The size and size distribution of micelles were estimated by the cumulant method [21].

(iv) FSP treated. A commercially available surface-treating agent (NovecTM EGC-1720, Sumitomo 3M Ltd.) was mixed with three times its volume of ethanol. About 50 lL of this solution were spin-coated on a 1  1 cm2 Si wafer at 2000 rpm for 10 s, and the substrate was annealed at 70 °C overnight.

2.4. Monomicellar film preparation About 6 lL of the polymer solution (0.002 wt% in toluene) were dropped on a wafer. The wafer was immediately placed at 20 °C in a tightly closed glass container (100 mL) saturated with toluene vapor to allow slow evaporation of the solvent.

2.5. Gold nanodot deposition

TM

2.3. Pretreatment of Si wafers (i) VUV-treated. Si wafers were placed in a vacuum chamber evacuated to 1.0  103 Pa by a rotary pump. The wafers were exposed for 15 min to VUV light generated from an excimer lamp (Ushio Inc., UER20-172 V, k = 172 nm and 10 mW cm2). (ii) HF-treated. Silicon wafers were immersed in a 48% HF solution for 30 min, thoroughly rinsed with water and dried in vacuo. (iii) FAS treated. All Si wafers were exposed to a VUV lamp for 15 min immediately prior to deposition. The Si wafers were immersed in a 2 wt% solution of FAS9 in ethanol overnight. The FAS-coated wafers were then rinsed with perfluorohexane and dried in vacuo.

The monomicellar film was immersed in 1 M HAuCl4 (aq) for 30 min at 20 °C, rinsed with water, and dried with nitrogen. This Au3+-doped film was placed in a vacuum chamber evacuated to 1.2  103 Pa by a rotary pump, and exposed to excimer VUV radiation for 15 min.

2.6. Optical contact angle measurement and calculation of surface free energy of the substrates Static contact angle measurements were carried out on the surface of the Si substrates using a contact angle measurement system (FTA125, First Ten Angstroms, Inc.). The measurements were made in air at room temperature by the sessile drop technique, water (hH2 O ) and diiodomethane (hCH2 I2 ) were the wetting liquids, and a drop volume of 7 lL was used. At least three measurements were performed on every sample, placing the liquid drops on different parts of the Si substrate surface. The surface free energy was calculated using the Owens–Wendt equation [22]:

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100

nominal tip radius of <10 nm and a spring constant in the range of 2.5–10 N/m were oscillated at their fundamental frequencies, which ranged between 100 and 200 kHz. A scan field of view was set to 2000  2000 nm, 1000  1000 nm, or 500 nm  500 nm with a scan rate of 0.5–0.8 Hz and 512 scanning lines.

Rh /nm

80 60

2.8. XPS

40

The XPS spectrum was taken with an ESCA3200 (Shimadzu Co. Ltd.) using a photon energy of 1253.6 eV (Mg–Ka). The pressure in the analysis chamber was 108 Torr during measurements. To compensate for surface charging effects, all binding energies were referenced to the C 1 s neutral carbon peak at 284.3 eV.

20 0 0.001

0.01

0.1

1

2.9. UV–vis

[PEO 454-b -PMA(Az)80] /wt% Fig. 1. Hydrodynamic radii (Rh) of copolymer aggregates measured at various concentrations of PEO454-b-PMA(Az)80 in toluene.

UV–vis spectra were measured with JASCO V-560 UV–vis spectrophotometer. 3. Results and discussion

p p ð1 þ cos hÞcL ¼ 2 cDS cDL þ 2 cPS cPL where h was the contact angle of the liquid used in the experiment; cL was the surface free energy of the liquid; cDS and cPS were the dispersion and polar components, respectively, of the surface free energy for the substrate; and cDL and cPL were the dispersion and polar components, respectively, of the free energy for the liquid. The two solvents, water (cL = 72.8 mN m1, cDL = 21.8 mN m1, cPL = 51.0 mN m1) and diiodomethane (cL = 50.8 mN m1, cDL = 48.5 mN m1, cPL = 2.3 mN m1), were used to calculate the surface free energy of the substrates. 2.7. AFM AFM was performed in the tapping mode with a SPI3800 N probe station and an SPA400 unit (SII NanoTechnology Inc.) Commercial silicon cantilever probes coated with Al, each with a

The properties of the amphiphilic diblock copolymer PEOm-bPMA(Az)n in solution were studied first. In order to determine the critical micelle concentration (CMC) and to measure micelle size, dynamic light scattering (DLS) measurements were carried out on the solution of PEO454-b-PMA(Az)80. Since toluene was a good solvent for PMA(Az) and a nonsolvent for PEO, PEOm-bPMA(Az)n formed reverse micelles with a hydrophilic PEO core and a hydrophobic PMA(Az) corona in hydrophobic organic solvents [23]. Fig. 1 shows the hydrodynamic radius (Rh) of the reverse micelles measured at various concentrations of PEO454-bPMA(Az)80 in toluene. The radius of the BCMs rose sharply at a concentration of about 0.008 wt%, which was identified as the CMC of the solutions. Above the CMC, the hydrodynamic radius stayed constant at 59.5 nm, and was nearly independent of the scattering angle over the range h = 30–120°, suggesting a spherical morphology for the micelles.

Hydrophobicity

Low

a

High

c

b 5.9

4.0

15.6

/nm

/nm

/nm

0

0

0

High

γs /mJ m θH O /deg

–2

2

73.1

55.7

41.8

<5

44

86

γs

d 8.7

/nm

Low

0 Fig. 2. AFM topographic images (2000  2000 nm2) of PEO454-b-PMA(Az)80 micelles deposited from a 0.002 wt% solution on surface-modified Si substrates: (a) VUV-treated Si substrate, (b) untreated Si substrate with its native oxide, (c) HF-treated Si substrate, and (d) FAS9-coated Si substrate. The surface energy (cs) and contact angle of water (hH2 O ) for each surface are shown under the AFM images. The insets in (a–d) show higher resolution (500  500 nm2) images.

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a

hydrophilic PEO core

Hydrophilic Surface Si

b

hydrophobic PMA(Az) corona

Hydrophobic Surface Si

c Fluorinated Surface Si Scheme 2. Schematic depiction of the change in shape of BCMs induced by changing the chemical properties of the substrate surface.

The investigations of the shape of PEOm-b-PMA(Az)n micelles were carried out on untreated and three different treated Si substrates. A hydrophilic Si substrate was prepared by exposure to vacuum ultraviolet (VUV) radiation, which converted most of the surface oxides to hydroxides [24], causing the substrate surface to become highly hydrophilic ðhH2 O < 5 Þ. A 0.002 wt% solution of PEO454-b-PMA(Az)80 in toluene was applied dropwise to the hydrophilic Si substrate, which was immediately placed in a container saturated with toluene vapor to allow slow evaporation of the solvent. An AFM image showed that irregular micellar blobs were formed all over the resulting surface (Fig. 2a). In order to determine if loaded BCMs could be used as nanoreactors for preparing metal nanodots, as-formed BCMs were immersed in a 1 M HAuCl4 (aq) solution for 30 min. The AFM image taken after this showed that the height of the BCMs remained the same as before immersion, indicating that were incapable of absorbing gold ions. This result was further confirmed by subsequent VUV etching and reduction of the Au3+-treated BCMs. There was no trace of gold nanodots

a

deposited on the underlying substrate (see Supporting Information S1(1c)). As shown in Scheme 2a, the PEO core of the BCM adhered strongly to the hydrophilic surface and had no ability to absorb gold ions, suggesting that the surface character of the substrate needed to be modified to reduce the interaction between the PEO core and the substrate surface. The native oxide-covered Si substrate was observed to have less hydrophilic character (hH2 O ¼ 40 Þ than the VUV-treated substrate. Fig. 2b shows an AFM image of the BCMs loaded on this substrate. A few BCMs had become spherical because of the reduced interaction of their PEO cores with the substrate surface, but they did not absorb gold ions either, and there still existed strong interactions between the PEO cores and the relatively less hydrophilic substrate surface (see Supporting Information S1(2c)). Therefore, the Si substrate was next rendered hydrophobic by etching the native oxide with HF ðhH2 O ¼ 86 Þ. Upon immersion in 1 M HAuCl4 (aq), the BCMs loaded on this hydrophobic Si substrate could accommodate gold ions. Furthermore, the doped gold ions were transformed into metallic gold upon exposure to VUV radiation (see Supporting Information S1(3c)). It was surmised that the interaction of the PEO block with the hydrophobic surface was significantly reduced. As shown in Fig. 2c, however, the loaded BCMs were quite structureless and essentially disordered, implying that, as shown in Scheme 2b, the hydrophobic PMA(Az) corona rather than the hydrophilic PEO core interacted strongly with the hydrophobic surface. Micelle-substrate interactions depend not only on the chemical properties of the substrate surface but also on the surface energy cs [14]. The surface energy of the HF-treated hydrophobic substrate was 41.8 mJ m2, lower than that of the VUV-treated surface (73.1 mJ m2) and of the untreated Si substrate (55.7 mJ m2). The extent of interaction of both the PEO core and the PMA(Az) corona with the substrate surface could be lowered by further reducing the surface energy of the Si substrate. It is well documented that the incorporation of fluorine onto a substrate reduces the surface energy due to its small atomic radius and high electronegativity [25]. Si substrates coated with nonafluorohexyltrimethoxysilane (FAS9) exhibited a lower cs (18.6 mJ m2) than any of the preceding substrates. Fig. 2d shows the AFM image of the BCMs on this fluorinated substrate. All of these BCMs possessed a well-defined spherical shape, and readily absorbed gold ions by simple immersion in 1 M HAuCl4 (aq). Consequently, the subsequent VUV etching and reduction of the Au3+-doped BCMs resulted in

c

b

d

13.3

67.2

10.7

15.6

/nm

/nm

/nm

/nm

0

0

0

0

e

g

f

h

5.2

3.4

5.9

6.6

/nm

/nm

/nm

/nm

0

0

0

0

Fig. 3. AFM topographic images (2000  2000 nm2) of PEO454-b-PMA(Az)80 micelles deposited from a 0.01 wt% solution on surface-modified Si substrates before (upper) and after (lower) VUV etching: (a and f) VUV-treated Si substrate; (b, c, g and h) untreated Si substrate with its native oxide; and (d and f) HF-treated Si substrate. The insets in (a– d) show higher resolution (500  500 nm2) images.

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a

30

mean = 8.37 σ = 0.99

Frequency /%

nm

7.8

25 20 15 10 5 6

0

0 5

7

8

9

10

11

12

Hight of micelles /nm

b

30

mean = 11.9 σ = 1.0

Frequency /%

nm

14.8

25 20 15 10 5

0

0 8

9

10

11

12

13

14

15

Hight of annealed micelles /nm

c

30

mean = 20.6 σ = 1.1

Frequency /%

nm

22.3

25 20 15 10 5

0

0

18

20

22

24

Hight of Au-loaded micelles /nm

d

30

mean = 3.77 σ = 0.77

Frequency /%

nm

5.4

25 20 15 10 5 0

0

0

1

2

3

4

5

6

7

Hight of Au nanodots /nm

Fig. 4. AFM topographic images (500  500 nm2) of BCMs templated on an FSP-coated Si substrate: (a) self-assembled monolayer of PEO454-b-PMA(Az)80 micelles, (b) the micelles after annealing at 140 °C for 2 h, (c) the gold-doped micelles obtained by immersion in 1 M HAuCl4 (aq), and (d) the gold nanodots deposited by VUV etching and reduction of the Au3+-doped micelles. The histograms show the height distribution corresponding to each BCM.

the formation of gold nanodots (see Supporting Information S1(4c)). This indicated that the PEO cores were uniformly surrounded by the PMA(Az) coronas, and were unattached to the substrate surface, as shown in Scheme 2c. Control over the shape of the BCMs allowed their chemical functionality to be tuned. Solutions with higher concentrations of PEOm-b-PMA(Az)n (0.01 wt% > CMC) resulted in spherical micellar blobs (Fig. 3a), micellar aggregates (Fig. 3b), and interconnected micellar networks (Fig. 3c and d). However, there were no monomicellar films produced from the micellar solution. The formation of irregular blobs and interconnected networks was nicely demonstrated by Brus

et al. [26] based on a coarse-grained model of nanoparticle selfassembly that explicitly included the dynamics of the evaporating solvent. In the case of heterogeneous evaporation, the solvent remained metastable on the surface. Throughout, the driving force for micelle assembly varied with position on the surface. If the micelles were sufficiently mobile to track the drying fronts of the micellar fluid, their aggregate patterns were shaped by the structural history of evaporation. Specifically, the locations of micellar domains at long times roughly traced the intersection lines of the micellar fluid, leading to network morphologies. Strong interactions of the substrate surfaces with either the PEO or the PMA(Az)

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block froze the micelle-substrate interface, preserving the network nanostructures. Slightly weaker binding on the surface led to less interconnected nanostructures like the irregular blobs of micellar aggregates. A facile, broadly applicable preparation of a fluorinated substrate for tuning the surface interaction was thought to be critical for the practicality of this method. A fluorosilane polymer (FSP) was deemed more desirable than FAS9 because the former could be coated over large areas of versatile substrates by simple spincoating. The FSP-coated Si substrate had a high water contact angle ðhH2 O ¼ 90 Þ and low surface energy (cs = 18.7 mJ m2), as did the FAS9-coated substrate. AFM measurements on as-coated substrates showed that the BCMs self-assembled on the FSP-coated Si substrate into a hexagonal close-packed array (Fig. 4a). Individual micelles formed hemispherical bumps 8.4 nm high, attributed to collapse of both cores and coronas of the BCMs. An apparent diameter (not corrected for tip convolution) of 56.5 nm was measured with the AFM, much smaller than the 119-nm diameter of the BCMs in solution. The micellar structures observed for low surface energy surfaces could be interpreted as having arisen from multi-step processes (Scheme 1). Initially, the regular monolayers of the spherical BCMs were self-organized during solvent evaporation, with the BCMs immobilized in a close-packed arrangement (Scheme 1a). Upon solvent evaporation, the BCMs released solvents from their swollen coronas and collapsed (Scheme 1b and c). The void spaces between the BCMs had been originally occupied by the swollen PMA(Az) corona, and consequently, the periodicity of the micellar array was expected to correlate well with the 119 nm diameter of the BCMs in solution, and was actually 116 nm. Subsequent annealing at 140 °C for 2 h formed more spherical BCMs, whose average height increased from 8.4 to 11.9 nm (Fig. 4b). Gold ions could be doped into these BCMs by immersion in 1 M HAuCl4 (aq) for 30 min, upon which the average height of the BCMs increased further from 11.9 to 20.6 nm, indicating absorption of gold ions by the PEO cores (Fig. 4c), while the organization of the Au3+-doped BCMs remained intact. Finally, etching of the BCMs and simultaneous reduction of the doped gold ions by VUV irradiation resulted in a regular hexagonal array of gold nanodots (ave. height 3.8 nm; Fig. 4d). Fig. 5 shows X-ray photoelectron spectra of Au3+ doped into the BCMs after VUV etching of the polymer. After exposure to VUV light, the two distinctive peaks of Au 4f7/2 and Au 4f5/2 photoelectrons were observed at 84.0 and 87.6 eV, respectively. These two peaks were in good

10

4f7/2

4f5/2 87.6

4

1 x 10 /counts s

-1

8

4

2

0 90.0

(b)

(c)

250

350

450 550 650 Wavelength /nm

750

Fig. 6. Extinction spectra of HAuCl4-doped BCMs on fluorinated quartz plates before (a) and after (b) VUV etching of the polymers. (c) The absorption spectrum of HAuCl4 in an aqueous solution.

agreement with those reported for Au0, which was substantially lower than that of AuI and other oxidized Au compounds (Au2O3, Au(OH)3 etc) [8a,27–29]. In addition, the change of extinction spectra of HAuCl4-doped BCMs on fluorinated quartz plates by VUV etching of the polymers is shown in Fig. 6. Before exposure to VUV light, a broad absorption band of the gold ions and the azobenzene molecules [17] in PEO454-b-PMA(Az)80 was observed in the UV–vis region. After VUV-treatments, new extinction band appeared at 564 nm, corresponding to the plasmon resonance band of metallic gold nanoparticles [30]. These data indicate that upon VUV etching of the BCM template, the gold ions doped into the PEO core were spontaneously reduced to metallic gold. 4. Conclusion We have demonstrated that a block copolymer consisting of highly functional components with high surface energy formed a self-assembled monolayer of well-defined spherical micelles on a hydrophobic fluorinated substrate. In addition, the BCMs displayed their chemical functionality (absorbing metal ions) because the micellar shape could be strictly controlled. The subsequent etching and reduction of the Au3+-doped BCMs by excimer VUV radiation deposited a regular hexagonal array of gold nanodots on the underlying substrate. Thus, surface nanopatterning could be achieved in only three steps by such BCM templating. Our techniques for controlling the micellar shape of a block copolymer by tuning its surface interactions will allow the use of a wide variety of block copolymers for broad application as BCM templates.

84.0

6

(a)

Extinction /a.u.

630

Acknowledgments

87.0

84.0

81.0

Binding energy /eV Fig. 5. XPS spectra of gold nanodots deposited onto a fluorinated Si substrate by VUV etching and reduction of Au3+-doped BCMs.

This work was financially supported by Japan Society for the Promotion of Science (JSPS) and Core Research for Evolutional Science and Technology (CREST) of Japan Science and Technology Corporation (JST), Grant-in-Aids for Scientific Research (S) (No. 18101005) and a Special Research Grant for Green Science from Kochi University.

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