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Bioplasmonic Alloyed Nanoislands using Dewetting of Bilayer Thin Films Minhee Kang, Myeong-Su Ahn, Youngseop Lee, and Ki-Hun Jeong ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b10715 • Publication Date (Web): 26 Sep 2017 Downloaded from http://pubs.acs.org on September 27, 2017

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ACS Applied Materials & Interfaces

Bioplasmonic Alloyed Nanoislands using Dewetting of Bilayer Thin Films Minhee Kang1,2,§, Myeong-Su Ahn3,§, Youngseop Lee3 and Ki-Hun Jeong3,* 1

Smart Healthcare & Device Research Center, Samsung Medical Center, 81 Irwon-ro, Gangnam-

gu, Seoul 06351, Republic of Korea 2

Samsung Advanced Institute for Health Sciences & Technology (SAIHST), SungKyunKwan

University, 81 Irwon-ro, Gangnam-gu, Seoul 06351, Republic of Korea 3

Department of Bio and Brain Engineering, Korea Advanced Institute of Science and

Technology (KAIST), 291 Daehak-ro, Yuseong-gu, Daejeon 34141, Republic of Korea §

Both authors contributed equally to this work.

*

E-mail: [email protected]

KEYWORDS : Bioplasmonic substrates, Alloyed nanoislands, Solid state dewetting, Plasmonic nanostructures, AuAg alloy

ABSTRACT : Unlike monometallic materials, bimetallic plasmonic materials offer extensive benefits such as broadband tuning capability or high environmental stability. Here we report a

ACS Paragon Plus Environment

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broad range tuning of plasmon resonance of alloyed nanoislands by using solid state dewetting of gold and silver bilayer thin films. Thermal dewetting after successive thermal evaporation of thin metal double-layer films readily forms AuAg-alloyed nano-islands with a precise composition ratio. The complete miscibility of alloyed nanoislands results in programmable tuning of plasmon resonance wavelength in a broadband visible range. Such extraordinary tuning capability opens up a new direction for plasmonic enhancement in biophotonic applications such as surface enhanced Raman scattering or plasmon-enhanced fluorescence.

TEXT Extremely intense and highly confined electromagnetic fields near plasmonic nanostructures enhance spectral signatures for plasmonic biosensing such as surface-enhanced Raman scattering (SERS),1 plasmon enhanced fluorescence (PEF),2-3 or plasmon resonance energy transfer (PRET).4 This phenomenon mainly results from the interaction between light and free electrons of metal nanostructures, which induces localized surface plasmon resonance (LSPR). The resonant interaction occurs near plasmonic nanostructures and therefore the precise control of LSPR has become of considerable interest.5-8 For instance, SERS signal enhancement emanates from the extraordinary increase of both incident and scattered optical fields near biochemical molecules during the LSPR excitation. The maximum SERS gain takes place particularly when both excitation and scattering wavelengths from molecules approach the LSPR wavelength.5, 9-10 The LSPR wavelength depends strongly on the composition, size, shape, dielectric environment, and the separation distance between plasmonic nanostructures.11-14 Although considerable advances have been made in the rational design, they still hamper precise tuning of plasmon


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resonance.15

Metallic nanoislands derived from solid-state dewetting of thin metal film and its engineering techniques to tailor the plasmonic resonance become of tremendous practical importance in plasmonic applications such as plasmonic SERS16-17 or light trapping.18 A non-wetting solid surface of thin metal film is thermodynamically unstable in the as-deposited state and it readily de-wets or agglomerates to form the nanoislands during thermal dewetting.19-20 This often happens well far below the melting point of metal so that metal dewetting occurs while the film remains in the solid-state. The effect becomes more drastic as the film thickness in VolmerWeber (VW) mode decreases.21 Thin metal film breaks into islands with a characteristic length scale even down to the nanoscale regime. As a lithography-free nanofabrication technique, the solid-state dewetting of a thin metal film offers large-area fabrication of metallic nanoislands with controllable size, spacing,16, 22-23 and material composition24 in a reproducible and flexible manner.

The recent improvement has been actively made for conventional plasmonic materials such as Au and Ag due to their plasmonic properties within visible wavelength range.25 In particular, Ag has a high scattering cross-section for exceptional plasmonic enhancement shorter than 520 nm in wavelength.26 However, the Ag nanostructures tend to readily degrade in oxidizing environment and thus this corrosion apparently decreases the plasmonic extinction and even shifts the plasmon resonance wavelength (PRW).27 In contrast, Au has great stability and facilitates many plasmon-based applications in corrosive environment while it hardly activates surface plasmons at an excitation wavelength shorter than ca. 520 nm due to its interband


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transition in mid-visible ranges.28-30 Recently, synthesizing bimetallic nanostructures of two distinct metal elements has become of considerable interests to tailor their properties on demand.31-33 The performance of bimetallic nanostructures even surpasses the individual properties associated with single counterparts and exhibits beneficial properties often explained by synergistic effects of bimetallic materials.34-35 The synergistic effects of bimetals are generally observed in a homogeneous conformation rather than a phase-separated conformation (i.e. coreshell structures), which exhibits two extinction peaks from the core and shell metals, respectively. As the shell metal becomes thick enough, the core-shell structures show a relatively constant peak that arises from the shell metals only, regardless of metal composition.36 In contrast, the bimetallic nanostructures with complete miscibility rather than phase-separated hybrid nanostructures exhibit a single and composition-sensitive optical responses,37 which facilitates precise tuning between individual responses from bimetals by changing metal compositions. Some facile methods for fabricating bimetallic nanostructures of Au and Ag have been mostly achieved by using solution-phase nanoparticle synthesis but they suffer from the intrinsic limitations such as particle aggregation and co-location, which lead to poor reproducibility and repeatability.38-41 Some efforts to overcome such limitations have been accomplished by introducing templated

growth of self-assembled nanostructures,42 and electron-beam

lithography43 to form regular arrays of bimetallic nanostructures on a flat substrate. However, they still need additional chemical modification, removal of excess reagents,44 and other driving forces such as electrostatic and hydrodynamic interaction45-46 to assemble bimetallic nanostructures on the substrate.

Here we report plasmonic alloyed nanoislands for biosensing applications. Alloyed nanoislands


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with well-defined sizes and shapes can be simply obtained by using successive thermal evaporation of thin Au and Ag films and low temperature thermal dewetting (Figure 1a). The complete miscibility of Au and Ag enables the precise tuning of PRW over a broadband visible wavelength region by altering the thickness ratio of individual thin films (Figure 1b). Besides, the precise control of total film thickness also allows broadband tuning of PRW because the total film thickness determines the average diameter of AuAg alloyed nanoislands after thermal dewetting. Based on the calculated results for PRW of AuAg alloyed nanostructures, the alloy composition obviously dominates the PRW rather than the nanoisland diameter (Figure 1c and see the Experimental Section). Note that the effective dielectric function is assumed to be the composition-weighted average of wavelength-dependent dielectric function for pure Au and Ag47 during the calculation.

Figure 1. (a) Schematic illustration of simple nanofabrication for AuAg alloyed nanoislands by using successive thin film evaporation and thermal dewetting. The individual and total film thicknesses of Au and Ag determine the Au and Ag fractions as well as the average diameter of AuAg alloyed nanoislands. (b) The complete miscibility of Au and Ag leads to the


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programmable tailoring of plasmon resonance wavelength depending on the individual film thickness of thin Au and Ag films. (c) Calculated plasmon resonance wavelength of AuAg alloyed nanoislands as a function of Au fraction and diameter.

The alloyed nanoislands were simply fabricated by using successive thin films evaporation and thermal dewetting. Thin Ag and Au films were thermally evaporated on a 4-inch quartz substrate in series and then thermally annealed in a box furnace. The purity of Au and Ag source material was over 99.9% and the substrate temperature was kept at room temperature during thermal evaporation. The deposition rate for Au and Ag ranges from 0.1 to 0.5 Å/s. The total film thickness and the deposition rate were carefully monitored with a quartz crystal microbalance. The Ag and Au fractions were controlled by adjusting the thickness ratio of individual film thickness under a constant total film thickness of 14 nm and thus three different alloy compositions (Au0.36, Au0.5, and Au0.64) from the thickness fraction of Au were prepared. Finally, the bimetal layers were thermally annealed in a box furnace. Thin metal film often melts at a much lower temperature than its bulk melting point.48 Depending on melting temperature of thin films on their thickness, thermal dewetting temperature was set at 700 oC at which both Ag and Au melt in order to perform sufficient dewetting.49 The temperature was constantly increased by 20 oC/min and kept constant at 700 oC for 1 hour. The samples were slowly cooled down by 5 oC/min in order to minimize the thermal stress between the nanoislands and the substrate (see supporting information (SI) Figure S1). The atmosphere in the box furnace was sustained without any gas control during the whole process for annealing. The shape and size of AuAg alloyed nanoislands were observed by using FE-SEM and the size distributions were also calculated from the binary SEM images. The inset images indicate the optical images for wafer-level fabrication (Figure 2a). The bimetallic films were successfully


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transformed into hemispherical nanoislands that still maintain comparable sizes and packing densities for different fractions of Ag and Au (Figure 2d and see supporting information (SI) Figure S2-3). The composition of AuAg alloyed nanostructures were characterized by using a high-resolution scanning transmission electron microscope (HR-STEM, Carl Zeiss LIBRA 200FE-EF TEM) equipped with both high angle annular dark field (HAADF) scanning transmission electron microscopy and energydispersive x-ray spectroscopy (EDXS) (Figure 2b). The HAADF images clearly exhibit both the compositional homogeneity and the structural conformation. Resulting from electron interaction near the nuclei of Ag and Au, the high angle scattering is strongly associated with the atomic number (Z). The HAADF image contrast is proportional to the 1.7 power of the average atomic number in the atomic column.50 As the Z of Ag (47) and Au (79) are amply different, the Z-contrast imaging method was utilized for investigating both the inhomogeneity of chemical composition and structural conformation within a single AuAg alloyed nanoisland.51 The elemental maps recorded with EDX fully supports the homogeneous distribution of Au and Ag elements in a single alloyed nanoisland (Figure 2b). Note that Au and Ag elements can readily merge each other and also form a face centered cubic lattice (FCC) under sufficient thermal energy (see supporting information (SI) Figure S4) due to the similar lattice constant (4.08 Å for Au and 4.09 Å for Ag) and the high inter-diffusion rate.52 The line-scanned profiles of Ag and Au, extracted along the center of individual nanoisland (marked by dashed line in Figure 2b), explain a well-defined alloy formation and its homogeneous spatial distribution (Figure 2c), compared with a heterogeneous distribution such as core-shell structures.53 Element-specific analysis based on the linescanned profiles shows that the ratio of Au signal to total signal remains constant along the centerline of nanoisland with different Au fractions, which further elucidates the homogeneous atomic distribution of Ag and Au element (Figure 2c and see supporting information (SI) Figure S5). The EDX spectra also reveal both the presence and the compositional fraction of individual Au and Ag elements (see


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supporting information (SI) Figure S6). While the origin of the carbon, oxygen, and copper emissions is related to the carbon film on the grid surface, the obtained Au fractions are 0.53, 0.59, and 0.78 for Au0.36, Au0.5, and Au0.64, respectively (Figure 2e).

Figure 2. (a) Average diameter and its distribution of AuAg alloyed nanoislands for (i) Au0.36 (ii) Au0.5 and (iii) Au0.64. The inset images indicate the optical images for wafer level fabrication (scale bar: 200 nm, inset scale bar: 2 cm). (b) High angle annular dark field (HAADF) images (left) and Energy-dispersive x-ray (EDX) mapping of Au and Ag elements. The HAADF images confirm the compositional homogeneity and the EDX spectra verify the presence of Au and Ag and the compositional fraction of each element. The red and green color corresponds to Au (right-top) and Ag (left-bottom) for the EDX mapping images, respectively (scale bar: 50 nm). The alloyed compositions are overlapped in the right-bottom. (c) The line


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profiles of cross-sectional composition for Au0.36 and the ratio of element-specific signal (Au) to total signal (Au+Ag). Constant ratio along the single nanoislands indicates homogeneous conformation of alloys. (d) The effective diameters and the packing densities of AuAg alloyed nanoislands from SEM images. (e) Au fraction from Au thickness fraction and EDX data for Au0.36, Au0.5 and Au0.64, respectively.

The combination of Au and Ag allows tuning of PRW over a broadband visible wavelength region due to their composition-sensitive plasmonic properties. In this experiment, the composition ratio was precisely controlled by changing the individual film thicknesses of Au and Ag under a constant total thickness of 14 nm. The PRW of alloyed nanoislands becomes red-shifted from that of monometallic Ag nanoislands, i.e., Au 0, in proportion to the film thickness fraction of Au (Figure 3a). Depending on the composition ratio, the nanoislands exhibit a single peak of plasmon resonance, which also supports the Au and Ag elements are fully alloyed as previously demonstrated from the HAADF results. The experimental results well agree with the simulation results, which were calculated by using the finite difference time domain (FDTD) method (Figure 3b and Figure S7 in supporting information). The configuration of AuAg alloyed nanoislands modeled in simulation was determined by measuring the size distribution and fill factor based on the SEM images of alloyed nanoislands. The plasmonic properties of AuAg alloy were mathematically calculated for the FDTD calculation by using dielectric functions (see the Experimental Section). The LSPR varies linearly with the Au fraction in both the calculated and experiment results, which further confirms the homogeneous distribution of alloyed nanoislands. The extinction of AuAg alloyed nanostructures decreases as the Au film thickness increases because Ag has higher scattering cross-section than Au in a visible range.54 The AuAg alloyed nanoislands not only tailor the PRW but also substantially enhance the environmental stability against corrosion. This environmental stability of


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AuAg alloyed nanoislands has been also investigated for one month by measuring the changes of the extinction spectra for Au0.0, Au0.5, and Au1.0 depending on the exposed time in air (Figure 3c). The extinction values of Au0.0 drastically decreases with the exposed time and the PRW becomes shifted toward longer wavelengths due to oxidation of Ag nanoisland.55-56 In contrast, the extinction values of Au0.5 and Au1.0 were slightly changed over the time (see supporting information (SI) Figure S8). The experimental results successfully demonstrate the AuAg alloyed nanoislands have much higher corrosion resistance and environmental stability than pure Ag nanoislands.

Figure 3. (a) The measured extinction spectra depending on Au fraction in AuAg alloyed nanoislands. (b) The FDTD calculated and measured LSPR wavelengths of AuAg nanoislands depending on the Au fraction. (c) The stability of pure Ag, pure Au and Au0.5 alloyed nanoislands at different air-exposed times. The extinction of pure Ag decreases with the exposed time while the extinction of pure Au and alloys exhibit small changes. The LSPR wavelength of


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alloys also barely changes with the times, whereas the pure Ag shows red-shifting of LSPR wavelength due to oxidation of Ag nanoisland.

The precise wavelength tuning of PRW significantly affects the signal enhancement of surface enhanced Raman scattering (SERS). In this experiment, SERS signals of benzenethiol (BT) molecules were measured at an excitation wavelengths of 488 nm by using AuAg alloyed nanoislands of five different compositions under a constant total film thickness of 14 nm (Figure 4a, and see the supporting information Figure S9). Monometallic nanoislands of Au1.0 and Au0.0 (Ag1.0) exhibit the plasmon resonance at 451 and 563 nm, respectively, whereas the alloyed nanoislands of Au0.36, Au0.5, and Au0.64 show the plasmon resonance at between 451 nm and 563 nm. The measured SERS signals apparently vary with the Au fraction of alloyed nanoislands with different PRWs. The main SERS peak of BT molecules is located at 1081 cm-1, corresponding to 515 nm on wavelength scale under 488 nm excitations. The plasmonic enhancement in SERS is strongly associated with the extinction product Λ(λex)·Λ(λRS).5 Using the measured extinction spectra of alloyed nanoislands with different Au fractions, the extinction products were obtained by multiplying both the individual extinction values at the excitation wavelength and the Raman scattering wavelengths. The experimental results clearly demonstrate that the main SERS peak of BT molecules has the maximum value for the Au0.5 nanoislands of 505 nm in PRW due to the maximum extinction product. Besides, the spectral overlap between the LSPR wavelength and fluorescence leads to an enhancement or quenching of fluorescence emission. In this experiment, the fluorescence intensities of Congo red molecules were measured by using five different AuAg alloyed nanoislands whose plasmon resonance lies within 451-563 nm. Congo red absorbs light within a range from 400 nm to 560 nm but the closer the wavelength is to 500 nm the


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greater the absorbance. The experimental results show that the fluorescence intensity of Congo red becomes maximized for the Au0.5 of PRW = 505 nm (Figure 4b). The spectral overlap of the plasmon resonance and the fluorescence absorption band leads to an enhancement of fluorescence emission. If the LSPR wavelength is overlapped with the fluorescence emission, an enhancement or quenching of the emission intensity is possible.57

Figure 4. (a) SERS signals of benzenethiol (BT) molecules at 1081 cm-1 with 488 nm laser excitation for AuAg alloyed nanostructures. The SERS peak of BT molecules has the maximum value for the Au0.5 nanoislands of 505 nm in PRW due to the maximum extinction product. (b) Fluorescence intensity of


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Congo red molecules depending on AuAg alloy fraction in AuAg alloyed nanostructures. Note that Congo red absorbs light within a range from 400 nm to 560 nm but the closer the wavelength is to 500 nm the greater the absorbance. The experimental results show that the fluorescence intensity of Congo red becomes maximized for the Au0.5 of 505 nm in PRW. The spectral overlap of the PRW and fluorescence absorption band leads to an enhancement of fluorescence emission.

To conclude, we have successfully demonstrated the novel and straightforward fabrication of AuAg alloyed nanoislands with broadband tunability. Successive thin films evaporation and thermal dewetting enable the large-area nanofabrication of AuAg alloyed nanoislands with welldefined sizes and shapes. The complete miscibility of Au and Ag leads to programmable tailoring of plasmon resonance over the extraordinary broadband wavelength region by simply changing the film thickness ratio. This novel material can serve as an excellent candidate for not only bioplasmonic applications such as SERS or PEF but also many other applications such as plasmonic solar cells and plasmon-driven catalysis, where high plasmonic performance and environmental stability are important.


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ASSOCIATED CONTENT Supporting Information. Experimental methods, Figures S1-S9 (PDF) AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] Author Contributions M. Kang and M. Ahn contributed equally to this work. In addition, the manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

Notes The authors declare no competing financial interest. ACKNOWLEDGMENT This work was supported by the National Research Foundation of Korea, funded by the Ministry of Science, ICT & Future Planning (2016919189, 2016013061, 2016919193, 2016924609), and a grant of the Korea Health Technology R&D Project through the Korea Health Industry Development Institute (KHIDI), funded by Ministry of Health & Welfare, Republic of Korea (HI13C2181, HI16C1111).


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Table of Contents (TOC) Graphic


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Figure 1. (a) Schematic illustration of simple nanofabrication for AuAg alloyed nanoislands by using successive thin film evaporation and thermal dewetting. The individual and total film thicknesses of Au and Ag determine the Au and Ag fractions as well as the average diameter of AuAg alloyed nanoislands. (b) The complete miscibility of Au and Ag leads to the programmable tailoring of plasmon resonance wavelength depending on the individual film thickness of thin Au and Ag films. (c) Calculated plasmon resonance wavelength of AuAg alloyed nanoislands as a function of Au fraction and diameter. 209x111mm (150 x 150 DPI)


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Figure 2. (a) Average diameter and its distribution of AuAg alloyed nanoislands for (i) Au0.36 (ii) Au0.5 and (iii) Au0.64. The inset images indicate the optical images for wafer level fabrication (scale bar: 200 nm, inset scale bar: 2 cm). (b) High angle annular dark field (HAADF) images (left) and Energy-dispersive x-ray (EDX) mapping of Au and Ag elements. The HAADF images confirm the compositional homogeneity and the EDX spectra verify the presence of Au and Ag and the compositional fraction of each element. The red and green color corresponds to Au (right-top) and Ag (left-bottom) for the EDX mapping images, respectively (scale bar: 50 nm). The alloyed compositions are overlapped in the right-bottom. (c) The line profiles of cross-sectional composition for Au0.36 and the ratio of element-specific signal (Au) to total signal (Au+Ag). Constant ratio along the single nanoislands indicates homogeneous conformation of alloys. (d) The effective diameters and the packing densities of AuAg alloyed nanoislands from SEM images. (e) Au fraction from Au thickness fraction and EDX data for Au0.36, Au0.5 and Au0.64, respectively. 247x181mm (150 x 150 DPI)



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Figure 3. (a) The measured extinction spectra depending on Au fraction in AuAg alloyed nanoislands. (b) The FDTD calculated and measured LSPR wavelengths of AuAg nanoislands depending on the Au fraction. (c) The stability of pure Ag, pure Au and Au0.5 alloyed nanoislands at different air-exposed times. The extinction of pure Ag decreases with the exposed time while the extinction of pure Au and alloys exhibit small changes. The LSPR wavelength of alloys also barely changes with the times, whereas the pure Ag shows red-shifting of LSPR wavelength due to oxidation of Ag nanoisland. 183x133mm (150 x 150 DPI)


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Figure 4. (a) SERS signals of benzenethiol (BT) molecules at 1081 cm-1 with 488 nm laser excitation for AuAg alloyed nanostructures. The SERS peak of BT molecules has the maximum value for the Au0.5 nanoislands of 505 nm in PRW due to the maximum extinction product. (b) Fluorescence intensity of Congo red molecules depending on AuAg alloy fraction in AuAg alloyed nanostructures. Note that Congo red absorbs light within a range from 400 nm to 560 nm but the closer the wavelength is to 500 nm the greater the absorbance. The experimental results show that the fluorescence intensity of Congo red becomes maximized for the Au0.5 of 505 nm in PRW. The spectral overlap of the PRW and fluorescence absorption band leads to an enhancement of fluorescence emission. 133x186mm (150 x 150 DPI)


Bioplasmonic Alloyed Nanoislands Using ... - ACS Publications

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