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Large-scale nanofabrication of designed nanostructures using Angled Nanospherical-Lens Lithography for Surface Enhanced Infrared Absorption Spectroscopy Yi-Hsin Chien, Chang-Han Wang, Chi-Ching Liu, Shih-Hui Chang, Kien Voon Kong, and Yun-Chorng Chang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b08994 • Publication Date (Web): 03 Jul 2017 Downloaded from http://pubs.acs.org on July 4, 2017

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ACS Applied Materials & Interfaces

Large-scale nanofabrication of designed nanostructures using Angled Nanospherical-Lens Lithography for Surface Enhanced Infrared Absorption Spectroscopy

Yi-Hsin Chien1, Chang-Han Wang1, Chi-Ching Liu1, Shih-Hui Chang2, Kien Voon Kong3 and Yun-Chorng Chang*1,2,4

1

2

Research Center for Applied Sciences, Academia Sinica, Taipei, Taiwan

Department of Photonics, National Cheng Kung University, Tainan, Taiwan

3

Department of Chemistry, National Taiwan University, Taipei, Taiwan

4

Department of Physics, National Taiwan University, Taipei, Taiwan

Corresponding Author’s Email: [email protected] : These authors contributed equally to the work 1 ACS Paragon Plus Environment


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ABSTRACT

Nanophotonics has been a focused research discipline for the past decade and has resulted in many novel concepts that promise to change human life. However, the actual penetration of this research into real products is severely limited mostly due to the slow development of economic nanofabrication. In this study, we have demonstrated a versatile and low-cost nanofabrication method referred to as “Angled Nanospherical-Lens Lithography (A-NLL)”, which is able to produce large-scale and periodic nano-patterns. The nano-patterns designed within a two-dimensional polar chart can be carefully fabricated on the substrate. The fabricated patterns easily cover an area larger than 1 cm2, which is beneficial as platforms for surface enhanced infrared absorption (SEIRA) where an expensive and bulky IR microscope is not required. We believe that the proposed A-NLL method is ideal for industrialization of future nanophotonic applications.

KEYWORDS: Nanosphere, Surface Plasmon, Nanofabrication, SEIRA, Lithography

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Introduction

Nanophotonics is a research field that studies the behavior of light at the nanometer scale.1 It can be branched into a wide variety of topics, including photonic crystals2, plasmonics3, metamaterials4, metasurface5, and quantum nano-photonics6. Due to the rapid advancement of precision nanofabrication methods, each of these topics has experienced a rapid development and led to several novel applications such as invisible cloak7, surface-enhanced Raman scattering (SERS)8, surface-enhanced infrared absorption9, plasmonic charge-coupled display10, meta-hologram11, and metalens12. The electromagnetic field is strongly enhanced when light is squeezed into such a tiny volume, and new optical phenomena can be applied to either break the current technological limits or obtain photonic devices with superior performance.

Despite the rapid pace of development, the penetration of these technologies into commercial products is actually quite limited. One of the limiting bottlenecks is related to the cost of fabricating the necessary nanostructures. Most of these nanostructures that demonstrated novel properties were fabricated using precise nanofabrication methods, such as electron-beam lithography (EBL) or focused-ion-beam milling (FIB). However, the high fabrication cost and low throughput associated with these methods have limited these nanostructures only for prototype demonstrations. After



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decades of research focused on finding novel applications, finding a low-cost nanofabrication method is very crucial for the continuous advancements of nanophotonics.

Several nanofabrication methods have been proposed as low-cost and high throughput methods. Nano-imprint lithography (NIL)13 is the most mature and is now commercially available. NIL can be used to reproduce the patterns that were designed on silicon hard molds. However, the hard molds are usually fabricated using either EBL or FIB, which makes the initial cost of NIL very high and the method time consuming. In addition, the multiple mechanical processes during NIL are not standard operation for current semiconductor manufacturing procedures and require additional developments for extra large-scale fabrication. Phase-shifting photolithography or related methods14-15 are also able to fabricate nanostructures on a large scale. However, the fabricated patterns are limited to certain patterns, which greatly limit the possible applications. Nanosphere Lithography (NSL) was demonstrated decades ago and has been adapted by researchers worldwide.16 The major limitation of NSL is that the fabricated patterns are limited to nano-triangles that are aligned in a honeycomb lattice. It has been previously demonstrated that oxygen plasma treatment can be used to tune the side-length of nano-triangles.17 However, it is still limited to producing triangular patterns. Based on NSL, a method referred as Nanospherical-Lens Lithography (NLL) is also developed using the nanospheres as nanoscale ball lenses to focus the incident ultraviolet (UV) light and expose the photoresist (PR) thin film underneath.

18-20

Other researchers 4

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have

also

reported

similar

procedures

with

different

names,

such

as

Nanosphere

Photolithography.21-22 The combination of NLL with oblique angled Cr deposition and subsequent oxygen plasma treatments can be used to fabricate periodic metal nanodisks with diameters as small as 70 nm following standard semiconductor procedures.19 Illumination of UV light at a tilted angle can be used to produce various large patterns.20, 23-24 However, such angled exposure techniques are not able to fabricate complex small patterns. Another approach, hole-mask lithography (HML),25 is a method that takes advantages of the angled evaporation of materials into metal nanoholes created by nanospheres and obtain various designed nanodisk clusters. Both NLL and HML are low-cost and large-scale methods but both are limited to creating only round patterns because of the round metal holes inside the metal stencil thin film. The restriction on the pattern greatly limits the possible applications using these nanofabrication methods.

In this study, we propose to improve the conventional angled exposure NLL with Cr deposition at oblique angle and subsequent oxygen plasma treatments. The entire procedure, which is referred to as Angled Nanospherical-Lens Lithography (A-NLL), has demonstrated significant flexibility on pattern selections. Not only nanodisks but also curved lines are fabricated using A-NLL. The combination of nanodisks and curves results in various patterns that can be created. These fabricated nanostructures that cover large areas are especially useful as platforms for surface-enhanced infrared absorption (SEIRA). A simple C ring pattern, whose localized surface 5 ACS Paragon Plus Environment


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plasmon resonance (LSPR) is tuned to the infrared region, is also demonstrated. LSPR is a collective electron oscillation within a noble metal nanoparticle. When the nanoparticle is illuminated with a resonant light, the electromagnetic energy is highly localized near the metal-dielectric interface and is therefore very strong. This strong electric field at the surface is ideal for electric field-related applications such as surface-enhanced Raman scattering (SERS)26 or SEIRA27-29. SEIRA using resonant nanoantennae has been a hot research topic in the past few years.9,

30

Resonant

nanoantennae of various shapes have been fabricated using EBL31, Direct Laser Writing32, and Nano-Stencil Lithography33. Split-ring resonator, which is referred to as the C ring in this study, has been demonstrated to be a resonant nanoantenna for detecting single molecular monolayers.34 In this study, we demonstrate SEIRA of metal carbonyl clusters, which are robust and moisture stable compounds that have been used in the applications, including immunoassays, pharmaceuticals, CO-therapy, and bioimaging,35-44 that use the fabricated C-ring arrays without an optical microscope. In addition to SEIRA, we believe that the proposed A-NLL method can be useful for numerous applications in the field of Nanophotonics and Plasmonics.

Results and discussion

NLL is a technique that takes full advantage of the light focusing ability of the nanospheres. The vertical illuminated UV light is focused onto a sub-wavelength spot underneath the nanospheres,


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as illustrated in the simulate field energy distribution near a nanosphere (Fig. 1a). The diameter of the sphere is 1 µm. This vertically illuminated UV light is collimated and exposes only a cylindrical-shaped PR volume underneath the nanosphere. Reducing the UV exposure reduces the size of the PR hole but usually results in an unopened PR hole. The unopened PR holes are clearly visible in the cross-sectional SEM image of Fig. 1(c). This type of PR hole cannot be used further as lift-off PR for an additional metallization process. If the incident UV light is brought in at a certain angle, as illustrated in Fig. 1(b), the resulting PR hole will be a tiled hole with a shifted center location, as illustrated in the cross-sectional SEM image of Fig. 1(d). These tilted holes are not suitable for latter metal evaporation unless their hole diameters are close to or larger than the thickness of the PR thin film (~ 1 µm). These big structures severely limit the patterns that can be fabricated. It is not possible to obtain patterns that are separated by a distance of >100 nm, which also severely limits the functionalities of the fabricated nano-patterns. The large feather size and limited pattern selection associated with a conventional angled NLL are the major reasons why the NLL-based techniques have not garnered any attention in the nanotechnology community.

In this study, we have improved the conventional angled NLL with several new processes. The detailed process flow is described in the Methods section. With the addition of rotational Cr deposition at an oblique angle immediately after the PR development, the evaporated Cr will only cover the top flat area and leave the PR holes uncovered. The subsequent Oxygen plasma treatment 7 ACS Paragon Plus Environment


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will clean the residual PR and open the unopened holes, as shown in Fig. 1(e). Using these two processes, we can simultaneously obtain an open hole with a very small hole diameter. These small Cr holes can be used to fabricate very small (D < 100 nm) nanodisks.19 In addition, the processes can also help fabricate two holes that are very close to each other. Fig. 1(f) illustrated the cross-sectional SEM image of dimer hole arrays with a separation that is approximately 100 nm. It should be noted that it is possible to obtain an even smaller gap than the gap shown in Fig. 1(f). The gap separation of < 50 nm can be repeatedly fabricated but it is not sufficiently mechanically stable to be observed in the cross-sectional SEM image.

Results from Fig. 1 indicate that the exposed PR nano-pattern can be preserved as long as these PR holes are as shallow as 300 nm and they are not required to be open holes. The following oblique-angle deposition of Cr can fix the size and location of the PR holes, and the subsequent oxygen plasma can clean underneath PR to open the PR holes. The incorporated processes remove the restriction between the size and depth of the PR holes and thus significantly enhance the flexibility of pattern selection. It is now possible to fabricate more complex patterns using A-NLL. To create a systematical way to bridge the design and fabrication, a two-dimensional polar chart is proposed as illustrated in Fig. 2(a). There are three controllable parameters that are associated with A-NLL. The first is the tilt angle α, which is linearly proportional to the shift distance between the centers of the nanosphere to the off-centered exposed spot. The linear relationship is clearly 8 ACS Paragon Plus Environment

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observed in the experimental results shown in Fig. 2(b). The open circle is the experimentally observed shift distance when the tilt angle varies. The diameter of nanosphere for this experiment is 1 µm. The sample can be rotated to different azimuth angles (β) to expose different spots on the same concentric circle using the same tilt angle. Therefore, various combinations of α and β effectively cover the entire area around each nanosphere. The third parameter is the exposure duration (τ) where the longer duration results in a larger spot. It should be noted that the same exposure duration for a higher tilt angle results in a smaller hole. Therefore, the expose durations for different tilt angles need to be calibrated in advance.

Once the desired pattern is drawn on the design chart, the patterns can be reproduced using a proper combination of the three parameters indicated by the design chart. For example, the inset of Fig. 2(c) illustrates a design of 4 nanodisks using the same tilt angle. These 4 different nanodisks occupy different azimuth angles and with decreasing diameters clockwise. The diameter for each is approximately 320, 300, 260 and 200 nm. Then, we can reproduce it precisely using a nanosphere with diameters of 2 µm. The tilt angle is 15°. Using the same tilt angle, we can fabricate not only a series of nanodisks but also a continuous curve, as shown in Fig. 2(d). The inset of Fig. 2(d) reveals the design using the same tilt angle at 30°. We expose the patterns while rotating the sample at a certain tilt angle. The azimuth angles of the exposure are between 0° and 320°. The rotational speed is related to the linewidth. The diameter of nanosphere used in this design is 1 µm. The designed 9 ACS Paragon Plus Environment


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patterns can also include both curve and disk, as illustrated in Figs. 2(e) and 2(f). In Fig. 2(e), the smiling face design includes a pair of nanodisks and a C ring, which are fabricated using the same tilt angle of 9°. The nanodisks and the C ring can also be fabricated using a different tilt angle to construct a crying face pattern shown in Fig. 2(f). The mouth is exposed at a tilt angle of 9° while the two eyes are exposed at an angle of 28°. Noted in both the smiling face and crying face designs, the gaps between the eyes and the mouth are all smaller than 100 nm. By varying the design, we can fabricate several common patterns that have been demonstrated to exhibit interesting optical properties in the field of nanophotonics. In Fig. 2(g), we used a careful design so that the location of nanodisks follows the equation of ‫ݎ‬ሺ߶ሻ = ‫ݎ‬଴ + ‫ݎ‬௦ ∙ ሺߚ/2ߨሻ in the polar chart. This results in nanodisks arranged in an Archimedes’ spiral. The Archimedes’ spiral arrangement of nanoholes has been demonstrated to convert the spin angular momentum to orbital angular momentum, which can be used to rotate tiny microspheres.45-46 In Fig. 2(h), we have fabricated arrays of nanodisk heptamer using two different tilt angles. The center nanodisk is exposed at the tilt angle of 0°, and the surrounding 6 disks are exposed at a tilt angle of 13°. A plasmonic heptamer can be used for ultrasensitive index sensing with its sharp Fano resonance responses.47

It should be noted that the sample configuration immediately before the metal evaporation, as shown in Fig. 1(e), is very similar to the HML immediately before the angled metal evaporation. This indicates that the angled metal evaporation can also be applied with our proposed method to 10 ACS Paragon Plus Environment

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fabricate more complicated nano-patterns. To demonstrate this capability, a pentamer pattern is designed, as illustrated in Fig. 3(a). Following the normal procedures of A-NLL, the carefully designed pentamer Cr hole array is clearly observed in Fig. 3(b), which represents the top-view SEM image of the sample immediately after the OBL deposition of Cr. The subsequent oxygen plasma treatment is able to clean the residual PR inside the hole, which can be observed from the cross-sectional SEM image shown in Fig. 3(c). The clear space below the suspended Cr thin film is a perfect configuration for angled metal evaporation. The arrows marked in Fig. 3(c) indicate the evaporation direction for the subsequent angled evaporation. In regular HML, the angled evaporation is usually performed at a fixed azimuth angle. To obtain a complex nano-pattern, an expensive vacuum-grade rotation setup is required. In our demonstration, we only use a rotating fan for our angled deposition. Using this configuration, a single Cr hole will create a circular pattern after angled metal deposition. For the pentamer hole pattern, the angled deposition will result in a nanoscale olympic logo, as illustrated in Fig. 3(d). The SEM images are shown in both Figs. 2 and 3 and demonstrate the fabrication capabilities of our proposed method. By carefully designing the patterns on the metal stencil film when fabricating with A-NLL and the parameters during angled evaporation, we can achieve almost any patterns that can be designed within a two-dimensional polar chart. In addition, the nanostructures are fabricated using a standard metal lift-off process. The top surface is usually very flat, and the sidewalls are usually smooth. The smooth sidewall can be 11 ACS Paragon Plus Environment


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observed in the cross-sectional SEM images of the C ring array before lifting off the metal, as shown in Fig. S1(a). Figs. S1(b) and S1(c) are the images taken via an atomic force microscope (AFM). The height profile, shown in Fig. S1(d), also reveals flat surface and smooth sidewalls along a designated direction, which is indicated by a red line in Fig. S1(c).

We would like to note that these demonstrated nanostructures do not have as high fidelity as the nanostructures produced using FIB, which are precisely cut from a continuous metal film. EBL and our techniques share a similar limitation that the evaporation process always limits the fidelity of the nanostructure. It is also difficult to produce a pair of round nanodisks with a very short distance in between using EBL. Our technique is not as powerful for fabricating arbitrary nanostructures for all nanophotonic applications. However, it can be used in many nanophotonic applications that require less precise nanostructures but large sampling area, such as surface-enhanced infrared absorption (SEIRA) spectroscopy. This proposed technique is capable of fabricating periodic patterns that cover a large area. Such a large area will make it an easier measurement for SEIRA. Researchers around the world have proposed various designs of nano-antennae that demonstrate SEIRA. Most of these nano-antennae are usually fabricated with precise nanofabrication techniques. The fabricated nano-antenna usually only covers a square area with its side-length of a few hundreds of microns. To measure infrared absorption from such area, an optical IR microscope is required. The IR microscope helps align the invisible IR light on the small sampling area with the help of a 12 ACS Paragon Plus Environment

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reference visible laser that coincide with the IR light which cannot be accomplished using a simple lens. The complicated optical design for the IR microscope makes it both bulky and expensive, which is not suitable for portable commercial applications. In addition, the proposed technique is also a very fast technique. It usually takes less than 10 minutes to align single-layered nanospheres to cover a 1 cm × 1 cm sample. The length and width of the area can be easily scaled up with a wider spreader and longer translator. The following UV exposure and photoresist development both take a few minutes to complete. Thus, we can finish the entire lithography process within 1 hour including all of the preparation time. It takes less than 3 hours to complete the fabrication from the bare substrate to the finishing nanostructures, which is a very fast process compared with many nanofabrication techniques.

In this study, we choose a simple C ring pattern to demonstrate SEIRA in the common signature region of IR between 1500 to 3000 cm-1. The experimentally obtained transmission spectra for the C-ring arrays with various diameters are shown in Figs. 4(a)-4(b). Two distinct peaks in each spectrum reveal when the arrays are excited with an X-polarized light. The absorption signal located between 1500 to 2000 cm-1 shifts to a smaller wavenumber with an increasing C ring outer diameter (OD). The absorption signal near 3300 cm-1 also shifts slightly with an increasing C ring diameter. Only one peak, near 3300 cm-1, in each spectrum reveals when the arrays are excited with a Y-polarized light. The absorption signal also slightly redshifts with increasing C ring diameters. To 13 ACS Paragon Plus Environment


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analyze the origin of these three absorption signals, electromagnetic simulations are performed using the 3-dimensional finite-difference time-domain (3D-FDTD) method. The simulated transmission spectra are shown in Figs. 4(c)-4(d). Fig. 4(c) illustrates the transmission spectra when incident with an X-polarized light. Two distinct absorption signals are clearly presented near 3300 cm-1 and between 1500 to 2000 cm-1. Figs. 4(e)-4(f) demonstrate the charge and field energy distributions of the three corresponding modes. The distributions enclosed by either red, blue or green dashed boxes correspond to the spectra response enclosed by a dash box with the same color shown in Figs. 4(c)-4(d). From the simulation results, the first signal between 1500 to 2000 cm-1 corresponds to the localized surface plasmon dipole mode, and the second signal near 3300 cm-1 should originate from the quadruple mode. In addition, the quadruple mode is also affected by the grating effect. The 1-µm periodicity in both X and Y direction of the C ring array on Si substrate (n= 3.4) leads to the first grating order at approximately 3000 cm-1. The asymmetric Fano-like transmission spectrum of the quadruple mode due to the compression of the grating effect can be easily distinguished from the symmetric Lorentz shape of the dipole mode. Fig. 4(d) illustrates the simulated transmission spectra when incident with a Y-polarized light. Only one response near 3300 cm-1 is observed. The corresponding charge and field energy distributions indicate that this signal originate from the dipole mode along the Y-direction. Its asymmetric Fano-like spectrum is also affected by the grating effect near 3000 cm-1. It should be noted that the tiny oscillations in the simulated spectra are due to the 14 ACS Paragon Plus Environment

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observation points, which are used for obtaining the energy flux, being too close to the nanostructures so that the energy is not fully dissipated before the simulation is terminated. Moving the observation point further away from the nanostructures and increasing the simulation time points can further suppress this oscillation. For an unpolarized incident light, all these three modes will show up in the spectral range between 1500 to 3000 cm-1, which covers most of the interesting signature region of molecular vibration. Thus, these simple C ring arrays with tunable diameters should be ideal platforms for SEIRA. An ideal platform for SEIRA must exhibit homogeneous substrates. Otherwise, molecular vibrations will be enhanced differently at different locations. It is important to fabricate C ring arrays with uniform diameters so the LSPR of the fabricated area does not vary across the entire sampling area. Fig. S2 illustrates the uniformity of the fabricated C ring arrays. Fig. S2(a) demonstrates the transmission spectra from 9 different locations on the same sample, indicated by the inset of Fig. S2(a). The 9 different curves only vary slightly with their transmission intensity but remain at a constant center resonance location. This indicates that our fabricated C ring array should be very uniform in size across the entire sample, which is further verified by the top-view SEM images shown in Fig. S2(b). The outer diameters (OD) of the C ring for all 9 different locations are very close to 600 nm with very little standard deviation.

SEIRA of osmium carbonyl clusters using the fabricated C-ring arrays is investigated. The periodicity of the array is 1 µm. The C ring made from Au is attached with osmium carbonyl clusters. 15 ACS Paragon Plus Environment


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It was reported that the incubation of Os3(CO)10(µ-H)2 with gold leads to the formation of Os3(CO)10(µ-H)(µ-Au).48-50 The standard transmission spectrum of osmium carbonyl clusters is shown in green in Fig. 5(a) and reveals double peaks near 2000 cm-1. The inset of Fig. 5(a) shows the molecular structure of Os3(CO)10(µ-H)(µ-Au). These sets of peaks correspond to various carbonyl stretching vibrations.51 Fig. 5(b) shows the measured transmission spectra of osmium carbonyl clusters attached to the C ring arrays on top of a silicon substrate when excited with the X-polarized (red curve) and Y-polarized (blue curve) light. When excited with an X-polarized light, the double peaks at approximately 2000 cm-1 are still visible. As a comparison, the double peaks are less visible in the transmission spectrum excited with a Y-polarized light. To evaluate the vibrational signal strength, the measured curves are accurately baseline-corrected using an adapted version of the asymmetric least squares smoothing algorithm proposed by Eilers52 from the corresponding experimental results. The baseline-corrected spectra omit the narrow molecular vibrational signals and only represent the bare plasmonic response from the C ring arrays. The baseline-corrected signals for both polarizations from the osmium carbonyl clusters attached on the C ring arrays are illustrated as the dashed black and gold curves in Fig. 5(b). The strength of the signal is defined by dividing the measured curve with the baseline curve.31 In Fig. 5(c), the red and blue curves represent the strength of the signals when excited with an X- and Y- polarized light. It is clearly observed that the red curves exhibit a larger signal enhancement compared with the blue curves. This indicates that 16 ACS Paragon Plus Environment

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the vibrational signals are significantly enhanced when they match well the localized surface plasmon resonance of the nanoantennae. The signal enhancement in SEIRA is a result of the resonant coupling between the broad plasmonic and narrow vibrational absorption, which results in an asymmetric or Fano-type of line shape for the molecular vibrational signals. The line shape also changes with a different ratio of vibrational frequency to the plasmonic resonance frequency. Fig. S3(a) represents the experimental transmission spectra of C ring arrays with different outer diameters and modified with Os3(CO)10(µ-H)2 clusters. The baseline for each curve is also obtained and is represented with a dashed line as shown in Fig. S3(a). The strength of the signal for each sample is shown in Fig. S3(b). It is clearly observed that the line shape varies for the different ratio between the vibrational frequencies to the plasmonic resonance frequency, which further confirms that this signal enhancement is a result of SEIRA. Therefore, our fabricated C ring arrays are able to significantly enhance the FTIR sensitivity and determine the existence of a single layer of osmium carbonyl clusters on the sample surface. These FTIR results indicate that we can take advantages of the SEIRA from our fabricated C ring arrays for more ultrasensitive biosensing in the future. It was also reported that a rough Au surface fabricated by evaporation a very thin layer (~8 nm) of Ag on the surface can be used to enhance the molecular vibrational signal in FTIR measurements.27 For comparison, we have prepared a Au thin film (~ 8 nm thick) on top of silicon substrate and modified it with Os3(CO)10(µ-H)2 clusters. The measured transmission spectrum is shown as the blue curve in 17 ACS Paragon Plus Environment


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Fig. S4(a). We cannot measure any response in the signature region near 2000 cm-1. By comparison, the C ring arrays modified with Os3(CO)10(µ-H)2 clusters reveal a strong double peaks near 2000 cm-1. Red and blue curves represent the corresponding strength of the signals, respectively. This further reveals that the enhancement from the rough Au surface depends strongly on how it is prepared, which is not suitable for daily SEIRA applications.

Conclusion

We have demonstrated a versatile and low-cost nanofabrication method that is able to produce various periodic nano-patterns that cover a large area. The technique is based on multiple angled exposures of Nanospherical-Lens Lithography with improved manufacturing processes. Using the proposed method, we can fabricate many patterns that can be designed on a two-dimensional polar chart. The fabricated patterns easily cover an area larger than 1 cm2, which will very easily integrate into future commercial applications. We have demonstrated the use of a simple C-ring array as a platform for surface enhanced infrared absorption (SEIRA). SEIRA of osmium carbonyl clusters is also demonstrated. It should be noted that SEIRA from our nano-patterns does not required an expensive and bulky microscope for IR measurements, which opens up the possibility for portable IR devices. We believe that the results of this study will serve as


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an important foundation for the industrialization of many novel nanophotonic concepts, which will be more appealing for the researchers worldwide and attract greater industrial attention.

METHODS

Double side polished and highly resistive silicon wafers were cut into smaller square pieces with a side length of 2 cm. The samples were ultrasonically cleaned in isopropyl alcohol, rinsed with deionized water, and blown-dried with nitrogen. In the beginning, a 1-µm thick PR film was spin-coated on top of the substrate followed by soft-baking at 100 °C for 3 minutes. The nanospheres used in this study were polystyrene (PS) spheres with various diameters purchased from Polyscience. A convective self-assembly method is used to align the PS nanospheres to form a hexagonal and close-pack array on top of the un-exposed photoresist (PR) thin film (Fig. 6a) with a typical area of 1 cm × 1 cm. The defects, including point defects and grain boundary, do exist within the nanosphere array. The typical area size for perfect alignment can be as large as 20 µm × 20 µm. Therefore, this method is not suitable for applications that require long-range perfect crystal alignments. The light source for the subsequent exposure of UV light is a commercial Hg-Xe lamp (Newport) equipped with a 365 nm bandpass filter. For conventional Nanospherical-Lens Lithography, the sphere arrays were used as a lithography phase mask for the incident UV light illuminated vertically from the top. An array of PR holes is revealed after the PR development. The



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details of the conventional NLL fabrication process can be found in our previous publications18-20. To create a PR hole at a shifted location, the incident UV light is illuminated at a tilted angle (Fig. 6b). Dimer holes can be fabricated after two separate angled exposures while rotating the samples in between the exposures (Fig. 6c). Usually, the resulting PR holes are not open holes (Fig. 6d), and a thin layer of Cr is deposited at a rotational oblique angle (Fig. 6e). The deposited Cr will only cover the surface area and not inside the hole. The following O2 plasma treatment will etch away the remaining PR and clean the un-opened PR holes (Fig. 6f). Following this step, a metal thin film is evaporated onto the surface of the substrate, and metal nanostructures are revealed after lifting off the PR (Fig. 6h). A tabletop scanning electron microscope (SEM; PhenomWorld Phenom Pure) and a field emission SEM (FEI Nova 200) were used to analyze the corresponding nanostructures after each step. The thickness of the nanostructures was analyzed using a commercial atomic force microscope (Asylum MFP-3D). Infrared absorption spectra of the fabricated arrays were measured using a commercial Fourier transform Infrared (FTIR) spectrometer (Bruker VERTEX 70) that can be evacuated to vacuum.

Electromagnetic simulations were performed using the three-dimensional finite-difference time-domain (3D-FDTD) method, using a freely available software package.53 The boundaries are periodic in the x- and y-directions and are perfectly matched layers in the z-direction. The incident light source is plane wave propagating along the z-direction. The side length of the grid cell for all 20 ACS Paragon Plus Environment

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dimensions is 4 nm. The total length of the grid cell along the z-direction is 3.2 µm, and the object is located near the center (z=0). The light source is a plane wave located near one end and the transmission flux is monitored at the other end along the z-direction. The plasma frequency and damping constant of Au are set at 1.32×1016 Hz and 0.68×1014 Hz, respectively.

The preparation of osmium carbonyl clusters. The Os3(CO)10(µ-H)2 clusters were prepared according to the reported procedures.54 Briefly, the clusters are made by reacting Os3(CO)10(NCMe)2 in 1,1-dichloroethane by bubbling H2 through the solution. The compound was characterized using its well-known IR spectrum [vco = 2074 (s). 2061 (m), 2023 (vs), 2008 (s), 1986 (w) cm-1].

Supporting Information:

Cross-sectional view SEM and AFM images of the C ring arrays (Figure S1); Nanostructure Uniformity Test across the entire sample (Figure S2); SEIRA on C ring arrays with different outer diameters (Figure S3); Compare the signal enhancements between C ring arrays and rough Au film (Figure S4)

ACKNOWLEDGMENTS

We thank the financial support from the Minister of Science and Technology, Taiwan under Grant Number (MOST 103-2112-M-001 -037 -MY3). The National Center for High-Performance



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Computing of Taiwan and the Computer is also acknowledged for the use of high performance computing facilities. The valuable suggestions from Professor Yung-Chiang Lan from the Department of Photonics, National Cheng Kung University are also much appreciated.

FIGURE CAPTION:

Figure 1: Simulated field energy distribution when UV light illuminates (a) vertically or (b) at a tilted angle on top of a nanosphere (D = 1000 nm). (c) and (d) represent the corresponding cross-sectional view SEM images of the unopened PR holes immediately after PR development for the cases (a) and (b), respectively. (e) and (f) reveal the opened PR holes for the single and dimer hole arrays after oblique-angle deposition of Cr and subsequent O2 plasma treatment. The scale bar in each SEM image represents 1 µm.

Figure 2: (a) Two-dimensional polar design chart with three controllable variables (α, β, τ). (b) The linear relationship between the tilt angle (α) and shift distance between the center and the dot location. Various periodic patterns that can be designed and fabricated, including (c) Self-similar nanodisk chain, (d) C ring, (e) Smiling face, (f) Crying face, (g) Archimedes Spiral, and (h) Nanodisk heptamer. The scale bar in each SEM image represents 1 µm.


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Figure 3: (a) Pattern design chart of A-NLL (b) Top-view SEM image of the fabricated Cr stencil film (c) Cross-sectional SEM image of the sample after oxygen plasma treatment. The orange arrow (top) indicates the angled deposition, and the pink arrow (bottom) indicates the sample rotation during deposition. (d) Top-view SEM image of the fabricated array of Olympic 5-ring logo. All scale bars indicate 1 µm.

Figure 4: (a)-(b) Experimental and (c)-(d) simulated polarized transmission spectra of C ring arrays with various outer diameters (OD). For the X-polarization, two distinct absorption peaks observed correspond to the dipole (~ 1500 cm-1) and quadruple (~ 3200 cm-1) modes. For the Y-polarization, the absorption peak near 3000 cm-1 corresponds to the dipole mode. The experimental spectra match well with the simulation for both polarizations. (e) and (f) are the charge and field energy distributions of the three modes, respectively. The dashed color boxes in (e) and (f) correspond to the enclosed transmission spectra in (c) and (d). All of these modes redshift when the C rings become larger.

Figure 5: (a) Green curve represents the standard transmission spectrum of Os3(CO)10(µ-H)2 with two distinct peaks at approximately 2000 cm-1. Inset shows the molecular structure of the Os3(CO)10(µ-H)2 cluster. (b) Red and blue curves are the measured transmission spectra of the C ring array with Os3(CO)10(µ-H)2 clusters when it is excited with the X- and Y- polarized light. The



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dashed black and gold curves represent the iteratively estimated baseline from the experimental results using the AsLSS algorithm52.The inset shows the definition of Ex and Ey polarizations. (c) The red and blue curves represent the baseline-corrected vibrational strength when it is excited with Ex and Ey polarizations.

Figure 6: (a) - (h) Schematic illustrations of the fabrication procedures using Angled Nanospherical-Lens Lithography.


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Graphical Abstract 451x338mm (72 x 72 DPI)



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Figure 1: Simulated field energy distribution when UV light illuminates (a) vertically or (b) at a tilted angle on top of a nanosphere (D = 1000 nm). (c) and (d) represent the corresponding cross-sectional view SEM images of the unopened PR holes immediately after PR development for the cases (a) and (b), respectively. (e) and (f) reveal the opened PR holes for the single and dimer hole arrays after oblique-angle deposition of Cr and subsequent O2 plasma treatment. The scale bar in each SEM image represents 1 µm. 122x156mm (300 x 300 DPI)


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Figure 2: (a) Two-dimensional polar design chart with three controllable variables (α, β, τ). (b) The linear relationship between the tilt angle (α) and shift distance between the center and the dot location. Various periodic patterns that can be designed and fabricated, including (c) Self-similar nanodisk chain, (d) C ring, (e) Smiling face, (f) Crying face, (g) Archimedes Spiral, and (h) Nanodisk heptamer. The scale bar in each SEM image represents 1 µm. 189x72mm (299 x 299 DPI)



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Figure 3: (a) Pattern design chart of A-NLL (b) Top-view SEM image of the fabricated Cr stencil film (c) Cross-sectional SEM image of the sample after oxygen plasma treatment. The orange arrow (top) indicates the angled deposition, and the pink arrow (bottom) indicates the sample rotation during deposition. (d) Topview SEM image of the fabricated array of Olympic 5-ring logo. All scale bars indicate 1 µm. 90x81mm (300 x 300 DPI)


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Figure 4: (a)-(b) Experimental and (c)-(d) simulated polarized transmission spectra of C ring arrays with various outer diameters (OD). For the X-polarization, two distinct absorption peaks observed correspond to the dipole (~ 1500 cm-1) and quadruple (~ 3200 cm-1) modes. For the Y-polarization, the absorption peak near 3000 cm-1 corresponds to the dipole mode. The experimental spectra match well with the simulation for both polarizations. (e) and (f) are the charge and field energy distributions of the three modes, respectively. The dashed color boxes in (e) and (f) correspond to the enclosed transmission spectra in (c) and (d). All of these modes redshift when the C rings become larger. 140x106mm (300 x 300 DPI)



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Figure 5: (a) Green curve represents the standard transmission spectrum of Os3(CO)10(µ-H)2 with two distinct peaks at approximately 2000 cm-1. Inset shows the molecular structure of the Os3(CO)10(µ-H)2 cluster. (b) Red and blue curves are the measured transmission spectra of the C ring array with Os3(CO)10(µ-H)2 clusters when it is excited with the X- and Y- polarized light. The dashed black and gold curves represent the iteratively estimated baseline from the experimental results using the AsLSS algorithm52.The inset shows the definition of Ex and Ey polarizations. (c) The red and blue curves represent the baseline-corrected vibrational strength when it is excited with Ex and Ey polarizations. 82x110mm (300 x 300 DPI)


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Figure 6: (a) - (h) Schematic illustrations of the fabrication procedures using Angled Nanospherical-Lens Lithography. 90x155mm (300 x 300 DPI)


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