Synthesis of High Refractive Index and Shape Controllable Colloidal

Article pubs.acs.org/Macromolecules

Synthesis of High Refractive Index and Shape Controllable Colloidal Polymer Microspheres for Super-Resolution Imaging Haie Zhu,† Min Chen,† Shuxue Zhou,† and Limin Wu*,†,‡ †

Department of Materials Science and State Key Laboratory of Molecular Engineering of Polymers, Fudan University, Shanghai 200433, China ‡ Collaborative Innovation Center of Novel Organic Chemical Materials of Hubei Province, College of Chemistry and Chemical Engineering, Hubei University, Wuhan 430062, China S Supporting Information *

ABSTRACT: A conventional optical microscope fails to resolve features with sizes smaller than 200 nm due to the Abbe diffraction limit. Herein, we synthesize two kinds of high refractive index and shape controllable colloidal polymer microspheres, poly(9,9′-bis[4(2-acryloyloxyethyloxy)phenyl]fluorene) (poly(BAEPF)) and poly(2-phenylphenoxyethyl acrylate) (poly(OPPEA)), used for solid immersion lens. It is the high refractive index and close contact with the surface features that can make the colloidal polymer microspherebased microlens effectively collect evanescent information on the substrate and form a clear and magnified virtual image at certain position below the substrate surface. Thus, as small as 60 nm feature size on the substrate can be clearly resolved through the microlens under a visible light optical microscope, which is far beyond the resolution limit of a visible light microscope.

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INTRODUCTION Because of the Abbe diffraction limit, the far-field optical microscope cannot resolve objects with dimensions smaller than λ/2 (λ = the illumination wavelength),1 namely, 1.6) polymer microsphere-based SILs using “incomplete” suspension polymerization of 9,9′-bis[4-(2-acryloyloxyethyloxy)phenyl]fluorene (BAEPF) and 2-phenylphenoxyethyl acrylate (OPPEA) as the monomers, respectively, combined with postcuring by UV light. The “incomplete” suspension polymerization can make the polymer microspheres deform on the surfaces of the target objects for intimate contact with the microstructures, which benefits the effective coupling of evanescent wave into the SILs to increase super-resolution imaging contrast. Moreover, these partially solidified polymer microspheres can form SILs with tunable h/d ratios, which have an important impact on the super-resolution imaging behaviors. The optimum resolutions of 75 and 60 nm can be obtained by these polymer microsphere-based SILs under white and blue light microscopes, respectively.

Article

EXPERIMENTAL SECTION

Materials. 2-Phenylphenoxyethyl acrylate (OPPEA, n = 1.57) and 9,9′-bis[4-(2-acryloyloxyethyloxy)phenyl]fluorene (BAEPF, n = 1.62) were purchased from Nantong Manor Chemical Co., Ltd. (China). Lauroyl peroxide (LPO, 98%) and 1-hydroxycyclohexyl phenyl ketone (Photoinitiator-184) were from Aladdin Reagent Co., Ltd. (China). Poly(vinyl alcohol) (PVA1788) and chloroform (CHCl3) were obtained from Sinopharm Chemical Reagent Corp (China). The semiconductor chips containing subdiffraction-limited patterns of 104, 75, and 60 nm gaps were cut into small pieces, washed with ethanol, and dried for the super-resolution imaging experiments. Synthesis of Polymer Microspheres and Their SILs. Suspension polymerization was used to synthesize poly(OPPEA) and poly(BAEPF) colloidal microspheres. Typically, 0.07 g of PVA was added into 120 g of deionized water, heated to 65 °C for 10 min to obtain a 0.058 wt % PVA aqueous solution, and cooled down to room temperature. Then a homogeneous oil phase composed of 0.35 g of OPPEA (or BAEPF), 0.5 g of CHCl3, 0.005 g of LPO, and 0.006 g of photoinitiator-184 was poured into the above PVA aqueous solution. After being emulsified with a homogenizer at 6000 rpm for 3 min, CHCl3 was removed from the mixture by a rotary evaporator at 150 rpm and 40 °C for 40 min. Subsequently, the dispersion was poured into a 250 mL four-neck round-bottom flask equipped with a mechanical stirrer, a thermocouple, a reflux condenser, and a nitrogen gas inlet. The system was degassed by nitrogen for 30 min and then heated to 75 °C for polymerization at 250 rpm for 7 h to obtain polymer colloidal microspheres. Because of high viscous monomers, the monomer conversion was only around 60% monomer conversion. We refer to it as “incomplete” suspension polymerization. After a certain polymerization time (3−7 h), a drop of the microsphere suspension was taken from the reaction flask and spread onto the semiconductor chips. Thus, some microspheres rolled to the right position that above the patterns need to be imaged. Although these microspheres were stable enough on the surfaces of the target objects after dried under room temperature for 5 min, the residual monomer molecules would influence the imaging behavior. Therefore, these microspheres were subsequently further solidified via UV exposure in a sealed chamber under a nitrogen atmosphere for 25 min, and microsphere SILs were obtained. Super-Resolution Imaging of Polymer Microsphere SILs. Super-resolution imaging behavior of the polymer microsphere-based

Figure 1. Molecular structure diagram of (a) OPPEA and (b) BAEPF.

Figure 2. (a) SEM images of the chip with 104 nm gaps between 150 nm wide stripe. (b, c) The corresponding optical images observed by optical microscope under white and blue lights. (d) SEM image of the chip with 60 and 75 nm gaps between 115 and 145 nm wide stripes. (e, f) The corresponding optical images observed by an optical microscope under white and blue lights. The dotted boxes of (b, c) and (e, f) correspond to the SEM images of (a) and (d), respectively. B

DOI: 10.1021/acs.macromol.6b02626 Macromolecules XXXX, XXX, XXX−XXX

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Macromolecules

Figure 3. Super-resolution imaging of the chip shown in Figure 2a through poly(BAEPF) SILs with h/d ratios of 0.35 (a1, a2), 0.40 (b1, b2), 0.55 (c1, c2), 0.87 (d1, d2), 0.89 (e1, e2), and 0.92 (f1, f2) under white light (a1−f1) and blue light (a2−f2). Insets: the side SEM images of the poly(BAEPF) SILs with h/d ratios of 0.35, 0.40, 0.55, 0.87, 0.89, and 0.92, but the same diameter of ∼13 μm, which were obtained at polymerization time of 2.92, 3.25, 3.50, 4.17, 4.58, and 5.25 h, respectively, and then cured by UV exposure. All the inserted SEM images have the same scale bars as that of (a1), which are 5 μm. SIL was characterized by an Olympus optical microscope (BX63) fitted with an objective lens of 100× and numerical aperture NA of 0.8 in the reflection mode. A halogen light source (λpeak ∼ 550 nm) was used for white light illumination, and a blue (λpeak ∼ 470 nm) light was obtained by using a blue filter. These SILs can be removed from the surfaces after imaging without destroying the surfaces of the objects. Characterization. The morphologies of the SILs and surface microstructures of the chip were observed by a Philips XL30 scanning electron microscope (SEM).

are shown in Figure 1. The fabrication of SILs can be divided into two steps: one is partially solidified poly(OPPEA) and poly(BAEPF) microspheres synthesized through suspension polymerization, and another is further solidification process via UV exposure when they are transferred to the chips. Before further solidification, the partially solidified polymer microspheres would undergo a gravity-induced deformation which makes SILs intimately contact with the surface patterns of the objects. Super-Resolution Imaging of Polymer Microsphere SILs. In order to study the super-resolution imaging of these polymer microsphere-based SILs, semiconductor chips were used as the target objects, whose surfaces have subdiffraction patterns of 104, 75, and 60 nm gaps between specific stripes of

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RESULTS AND DISCUSSION Synthesis of Polymer Microspheres for SILs. Two kinds of high refractive index monomers OPPEA and BAEPF were used to prepare polymer microsphere SILs, whose structures C

DOI: 10.1021/acs.macromol.6b02626 Macromolecules XXXX, XXX, XXX−XXX

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Figure 4. Super-resolution imaging of the chips under white light (a1−d1) and blue light (a2−d2) through poly(BAEPF) SILs (d ∼ 14 μm) with various h/d ratios of 0.61 (a1, a2), 0.75 (b1, b2), 0.79 (c1, c2), and 0.84 (d1, d2). The white arrows indicate 75 nm gaps, while the red arrow indicates the 60 nm gaps that can be resolved by the SIL. Insets: the side SEM images of the poly(BAEPF) SILs (d ∼ 14 μm) with h/d ratios of 0.61−0.84. All the inserted SEM images have the same scale bars as that of (a1), which are 5 μm.

Considering the super-resolution imaging of poly(BAEPF) microsphere-based SILs is quitely clear for 104 nm microstructure which is considerably far beyond our expectation, we further used them to observe these finner feature on the chips as shown in Figure 2d. As shown in Figure 4, under white light, the 75 nm gaps (indicated by white arrow) of the chip cannot be resolved unless the SILs of h/d ≥ 0.75 are used. With the increase of h/d ratio from 0.61 to 0.84, both focus depth Z and image magnification factors M increase, and the image resolution increases first and then decreases (Figure 4a1−d1). Also, blue light can make these SILs obtain much clear images (Figure 4a2−d2) and even resolve 60 nm gaps indicated by red arrow at h/d = 0.79 (Figure 4c2). We also used another high refractive index monomer, 2-phenylphenoxyethyl acrylate, to prepared poly(OPPEA) microsphere-based SILs using the same procedure as above. Compared to BAEPF, OPPEA has only one CC bond and needs longer polymerization time before its partially sodified microspheres are transferred onto the chips to form poly(OPPEA) SILs with similar h/d ratios to those of poly(BAEPF) SILs. The side SEM images of the as-prepared poly(OPPEA) SILs shown in the insets of Figure 5 present a slightly shrinking surface topography, which probably resulted from the linear oligomers in the poly(OPPEA) microspheres, which are prone to shrink under vacuum. As shown in Figures 5a1−d1 and 5a2−d2, the poly(OPPEA) microsphere-based SILs have similar superresolution imaging behaviors to poly(BAEPF) ones: The increase in h/d ratio from 0.65 to 0.89 increases both focus depth Z and image magnification factors M. When poly(OPPEA) SILs with h/d ≥ 0.74 are used, 75 nm features on the chips can be resolved under white light, while under blue light the super-resolution images are more clearly. The influence of diameter of the SILs on their superresolution imaging behaviors has also been investigated using poly(OPPEA) SILs with the same h/d of 0.79. As shown in Figure 6, all these superhemispherical SILs can clearly resolve the 75 nm gaps on the chip under white light and 60 nm gaps

150, 145, and 115 nm wide, respectively, as shown in Figure 2a,d. According to the Abbe diffraction limit, the resolution limit of an optical microscope is 0.5λ/NA. Thus, the resolution limit of the optical microscope used here should be 344 and 294 nm under white and blue lights, respectively, which is obviously unable to resolve these tiny features on the chips as shown in Figure 2b,c,e,f. The poly(BAEPF) microspheres obtained at various polymerization times had various d of 4−35 μm and h/d ratios of 0.35−0.92 (see Figure S1). Figure 3a displays the their superresolution imaging behaviors of the poly(BAEPF) microspheres with the same diameter d of ∼13 μm but various h/d ratios, as shown in the inserted SEM images of Figure 3, for observation of chips with subdiffraction structure shown in Figure 2a. The parameter h/d ratio of the SILs has an important influence on their super-resolution imaging behaviors, including focus depth (Z: defined as the distance of image plane from the chip surface; when the image plane is above the chip surface, Z has a positive value and a real image is obtained; when the image plane is below the chip surface, Z has a negative value and a virtual image is obtained), image magnification factors (M), and image resolution. As h/d increases from 0.35 to 0.92, the super-resolution imaging displays the following characteristics: (1) The image plane is moving from above the chip to near its surface (real image obtained at h/d ≤ 0.40), then downward, and far away from the surface (virtual image obtained at h/d ≥ 0.55). (2) The image magnification factor M increases from 1.4× to 3.9×. (3) The image resolution is increasing. As shown in Figure 3a1−e1, when h/d ≤ 0.35, the 104 nm features on the chips cannot be resovled (Figure 3a1). When h/d increases from 0.40 to 0.89, these features can not only be discerned but also become more clearly. However, at too large h/d ratio, e.g., 0.92, although these features can still be clearly resolved (Figure 3f1), a slightly decreasing intensity is observed owing to the decreased illumination cross-sectional area.25 Blue light can make these SILs display much higher clarity due to its shorter wavelength (Figure 3a2−f2). D

DOI: 10.1021/acs.macromol.6b02626 Macromolecules XXXX, XXX, XXX−XXX

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Figure 5. Super-resolution imaging of the chips under white light (a1−d1) and blue light (a2−d2) through poly(OPPEA) (d ∼ 15 μm) with various h/d ratios of 0.65 (a1, a2), 0.74 (b1, b2), 0.85 (c1, c2), and 0.89 (d1, d2). The white arrows indicate 75 nm gaps in the images. Insets: the side SEM images of the poly(OPPEA) SILs with h/d ratios of 0.65−0.89. All the inserted SEM images have the same scale bars as that of (a1), which are 5 μm.

Figure 6. Super-resolution imaging of the chips through poly(OPPEA) SILs (h/d ∼ 0.79) with various diameters under white light (a1−c1) and blue light (a2−c2). The polymer microsphere-based SILs used in pictures (a1, a2), (b1, b2), and (c1, c2) have 7.9, 10.5, and 12.8 μm diameters, respectively. The white arrows indicate 75 nm gaps while the red arrows indicate the 60 nm gaps that can be resolved by these SILs.

TiO2 and ZrO2 nanoparticles. However, there is no inorganic component in the polymer microsphere-based SILs here. We guess there may exist crystalline domains in these polymer microspheres, which have been later confirmed by XRD (see Figure S2). Thus, this possible light interaction with crystals could help to collect the microstructure information on objects to increase resolution of the SIL.

under blue light. When the diameter increases from 7.9 to 12.8 μm, both the field of view and focus depth Z increase while the image magnification factor reveals little change. Super-Resolution Imaging Mechanism. This superresolution imaging of poly(BAEPF) and poly(OPPEA) SILs cannot be explained by classical solid immersion mechanism. Based on λ/2nSIL under an optical microscope with λ= 550 nm white light and 470 nm blue light illumination, the highest resolutions of the SIL made of poly(BAEPF) are 170 and 145 nm, while those of poly(OPPEA) are 175 and 150 nm, respectively, which are significantly lower than our 75 and 60 nm resolutions obtained here. Their working mechanism is not very clear at present. Based on our previous study,23,24 there may exist a light interaction with densely packed metamaterial media formed by three-dimensional stacking of high-index

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CONCLUSION We have demonstrated the synthesis of two kinds of high refractive index and shape controllable colloidal polymer microspheres using incomplete suspension polymerization combined with postcuring by UV light. The incompletely polymerized microspheres can form SILs with tunable h/d ratios when E

DOI: 10.1021/acs.macromol.6b02626 Macromolecules XXXX, XXX, XXX−XXX

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transferred onto the target substrates. This deformable morphology combined with high refractive index can guarantee the SILs closely contact with the surface features to effectively collect evanescent information on the substrate and form a clear and magnified virtual image, and the parameters such as h/d ratios and diameters of these SILs have important influences on their super-resolution imaging behaviors. At optimum parameters, as small as 75 and 60 nm features on semiconductor chips can be clearly resolved by the pure polymer colloidal microspherebased solid immersion lens with an optical microscope under white light and blue light illumination, respectively, which vastly excels the 200 nm resolution limit for visible light optical microscopes and also exhibits considerably enhanced resolution compared to the previously reported pure polymer spheres as SILs (130 nm resolution). The polymer microsphere-based SILs can not only be used for super-resolution imaging but may also find applications in the fields of optical memory storage, spectral signal enhancement, near-field lithography, and so on.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.6b02626. Figures S1 and S2 (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (L.W.). ORCID

Min Chen: 0000-0002-6422-9942 Limin Wu: 0000-0001-8495-8627 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Financial support of this research from the National Natural Science Foundation of China (51133001, 21374018 and 51673045) and Collaborative Innovation Center Foundation of Novel Organic Chemical Materials of Hubei Province is appreciated.

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REFERENCES

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DOI: 10.1021/acs.macromol.6b02626 Macromolecules XXXX, XXX, XXX−XXX