Photofluidic Near-Field Mapping of Electric-Field ... - ACS Publications

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Photofluidic Near-field Mapping of Electric Field Resonance in Plasmonic Meta Surface Assembled with Gold Nano Particles Minwoo Kim, Ji-Hyeok Huh, Joohyun Lee, Hwi Je Woo, Kwangjin Kim, DaeWoong Jung, Gi-Ra Yi, Mun Seok Jeong, Seungwoo Lee, and Young Jae Song J. Phys. Chem. Lett., Just Accepted Manuscript • DOI: 10.1021/acs.jpclett.7b01307 • Publication Date (Web): 27 Jul 2017 Downloaded from http://pubs.acs.org on July 29, 2017

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The Journal of Physical Chemistry Letters

Photofluidic Near-field Mapping of Electric Field Resonance in Plasmonic Meta Surface Assembled with Gold Nano Particles Minwoo Kim1, ‡, Ji-Hyeok Huh1, ‡, Joohyun Lee1, ‡, Hwi Je Woo1, Kwangjin Kim1, Dae-Woong Jung2, Gi-Ra Yi2, Mun Seok Jeong3,4, Seungwoo Lee1,2,5, *, and Young Jae Song1,4,5,6, * 1

SKKU Advanced Institute of Nano Technology (SAINT), Sungkyunkwan University (SKKU), Suwon 16419, Republic of Korea 2

School of Chemical Engineering, Sungkyunkwan University (SKKU), Suwon 16419, Republic of Korea 3

Department of Energy Science, Sungkyunkwan University (SKKU), Suwon 16419, Republic of Korea 4

Center for Integrated Nanostructure Physics (CINAP), Institute for Basic Science (IBS), Suwon 16419, Republic of Korea 5

School of Nano Engineering, Sungkyunkwan University (SKKU), Suwon 16419, Republic of Korea

6

Department of Physics, Sungkyunkwan University (SKKU), Suwon 16419, Republic of Korea

AUTHOR INFORMATION ‡

M.K., J.-H.H. and J.L. contributed equally to this work.

Corresponding Author *Email: [email protected] (S. Lee) *Email: [email protected] (Y. J. Song)

ACS Paragon Plus Environment

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Abstract We present a near-field mapping of electric fields from the individual super-spherical and ultrasmooth gold nanoparticles (AuNPs) and artificially-assembled AuNP nanostructures by measuring the reconfiguration of an azobenzene-containing polymer(azo-polymer) film. Various configurations of AuNPs and the azo-polymer were studied with AFM measurements and calculations. The interference was systematically studied with AuNP dimers of various gap distances and different embedding depth in the polymer film. Finally, we successfully demonstrated the interference of standing waves in artificially-assembled plasmonic meta surface.

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Plasmonic nanoparticles have been widely studied as they can provide various optical applications such as biosensing1–3, solar cells4,5, surface-enhanced Raman scattering6,7. Furthermore, it has been reported that plasmonic nanoparticles can be used as meta-atoms to develop metamaterials at optical freuqencies.8 For example, our group reported the highlyreliable and precisely-assembled gold nanoparticle clusters (i.e., metamolecules), which can artificially engineer its electromagnetic properties in a deterministic way by atomic force microscope (AFM)-enabled nanomanipulations.9 Recently, near-field optical properties of plasmonic nanoparticles and metamolecules have been investigated along with the advanced instrumental techniques. As a representative example, apertureless near-field scanning optical microscopy (aNSOM) was developed to characterize near-field optical properties (i.e., spatial distributions of electric field intensity and phase) of plasmonic nanoparticles and metamolecules.10–15 On the other hand, a photofluidic visualization of photochromic materials was proposed as an alternative approach to map the spatial gradient of the optical near-field intensities in various plasmonic nanoparticles or nanostructures such as nano-cubes16, nano-discs17, nanoantennae18 and hexagonal arrays of nano-triangles19. In general, among various photochromic materials, azobenzene-incorporated materials (e.g., azobenzene supramolecules and polymers) can become soften even at room temperature, when illuminated by light with an appropriate wavelength (i.e., athermal photofluidization).20 It has been believed that the photoisomerization between trans- and cis-conformations can drive such athermal photofluidization. More importantly, in stark contrast to thermally induced and isotropic fluidization, the photofluidization of azobenzene molecules is highly directional along the polarization of incoming light, because the azobenzene molecules under the light illumination get aligned


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perpendicularly to the light polarization. Thus, the locally concentrated electric field together with the field vector information can be directly visualized through spatially mapping the photofluidic movement of azobenzene-incorporated materials, which are already coated onto the plasmonic nanostructures.16–19,21 This photofluidic mapping of near-field optics could be advantageous over the conventional NSOM measurements for the following reasons. First, the near-field can be spatially mapped by a relatively versatile but robust AFM-enabled visualization of topographic change in contrast to the conventional NSOM measurement which requires quite complex and vibrational error-prone optical setups. Second, the high-fidelity mapping is available together with an ability to map the field vector, which would be difficult to achieve with the conventional NSOM. Furthermore, photofluidic mapping can be done without the disturbance of the near field from the tip-sample interaction which occurs inevitably when placing NSOM tips near the sample structure.20 In this work, we investigated photofluidic mapping of the electric field confined in the individual nanoparticles and artificially-designed nanostructures. Unlike the previous cases, the photofluidic mapping was done with super-spherical AuNPs on top of an azobenzene-containing polymer(azo-polymer) film. We, therefore, can avoid vacuum and chemical processes for e-beam lithography, and directly construct arbitrary nanostructures by programmable AFM-based manipulations. By varying the relative configurations of AuNPs and the azo-polymer with an additional thermal embedding process, the intensity and the wavelength of the electric field from single AuNPs were studied with the calculation of corresponding electric fields. AuNP dimers of various gap distances and different embedding depth were studied by measuring the pDR1 film reconfigured by the interference from each AuNP with corresponding calculations. Finally, circular corrals were fabricated with AuNPs by a vector manipulation of AFM based on the


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optimized conditions. The phase from each AuNP in this nanostructure could be modulated by the different symmetry between the circular structure from the linear polarization of light, i.e.

Figure 1. Photofluidic near-field mappings of electric fields from AuNPs with various configurations. (a) Schematic images of sample preparation and reconfiguration for different configurations of AuNPs and a pDR1 film, i.e., an on-top AuNP and an embedded AuNP. (b) Schematic images of modification and relaxation of pDR1 under the illumination of a 532 nm laser. (c)-(d) AFM images of 4 AuNPs after illumination of a 532 nm laser for On-top AuNP and Embedded AuNP respectively. (e) A height profile of AFM images from (c) and (d). Insets show the cropped AFM images of each corresponding AuNPs. Each blue and red mark indicates the peak position of AuNPs and the highest position of the pDR1 polymer for each. (f) A statistical plot of the reconfigured depth and the peak-to-peak distance for each case in (e). The reconfigured depth in the left ordinate is the height difference between the neighboring hill and valley of the pDR1 film, while the other ordinate is the distance from the center of AuNP and the first hill. The scale bars in (c) and (d) represent 2 µm, and the polarization of the incident light is indicated as a black arrow.


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plasmonic meta surface.22,23 AuNPs with a diameter of 60 nm were transferred on top of the spin-coated Poly[(methyl methacrylate)-co-(Disperse Red 1 acrylate)] (pDR1, Sigma-Aldrich) azo-polymer film as shown in Fig. 1(a), and then manipulated to fabricate artificially-designed nanostructures by AFM. A relatively flat substrate of a pDR1 film, therefore, could remain, and AuNPs could be fully exposed to the external laser without absorption by the polymer. Here, pDR1 polymer is consisted

of

Disperse

Red

1

acrylate

(DR1)

molecules

that

were

grafted

to

Poly(methylmethacrylate) (PMMA), and was used to induce the mass transport under illumination, as described in Fig. 1 (b). The 532 nm wavelength laser with linear and circular polarizations was used in this work to match with the absorption band of DR1 molecules (400600 nm).17 Conformation modification occurs from a trans form of pDR1 to a cis form under the illumination of a 532 nm laser, then the polymer mass is transferred to the direction of the incident light polarization after structural relaxation. The configuration of AuNPs and a pDR1 film can be further tuned with an additional thermal embedding process as shown in the bottom panel Fig. 1(a). Fig. 1(c) and (d) show AFM images of four AuNP monomers after the laser illumination on AuNPs (c) over and (d) embedded (~50 nm in depth) in the film respectively. As previously reported with the top-azopolymer-coated AuNPs,21 the AFM images show the development of electric field propagation, which was generated by dipole oscillations of AuNPs with the same polarization of the illuminated laser. As the AuNPs were highly uniform in its size and shape, the reconfigured patterns due to the electric field from each AuNPs were almost identical. The reconfigurations of the pDR1 film, however, show significant differences of the intensity and the wavelength of electric field, depending on the configurations between AuNPs


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and the pDR1 film. While AuNPs embedded in the film (Fig. 1(d)) show shallow depth of the reconfigured pDR1 film nearby AuNPs, AuNPs over the film (Fig. 1(c)) show higher contrast with longer propagation length. Fig. 1(e) compares the height profile of the reconfigured pDR1 film of Fig. 1(c) and (d). The wavelength from the embedded AuNP (marked with red triangles) is shorter than that from AuNP over the film (marked with blue triangles), due to the increased wave number while the propagation of electric field occurs in the surrounding pDR1 film. Weak intensity of electric field from the embedded AuNPs is confirmed by measuring the depth of reconfigured pDR1 film near AuNPs. These clear correlations of intensity and wavelength for embedding depth of AuNPs are statistically summarized in the Fig. 1(f). The shorter wavelength and the weaker intensity for the embedded AuNPs can be understood by the higher refractive index (n=1.694+0.015i) of the surrounding pDR1 film as shown in FEM simulations (Fig. 2).24 Two different mechanisms were reported to explain the reconfiguration of azo-polymer for external polarized optical excitations.17,18,25 In this work, as the external light was illuminated normally onto the substrate to excite AuNPs laterally and vertically on the substrate, conventional terminologies will be used to avoid the confusion based on the orientation of the substrate as follows; an out-of-plane (in-plane) direction is denoted for a vertical (lateral) direction from the substrate as a Z axis (X and Y axis). As clearly seen in our AFM images and each corresponding FEM calculations in Fig. 2, the pDR1 film moves laterally on the surface, showing the intensity modulations of electric field from AuNPs. The mass of the pDR1 film are transferred away from the maximum positions of electric field intensity, and piled up on the inflection position of the electric field. On the other hand, the previous studies revealed that a vertical component of electric field lifts up the pDR1 film on top of metallic nanostructures.16,18,25 Thus, unlike the lateral component, the height of a pDR1 film increases


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where the vertical component exists. With this method, it has been reported that the spatial confinement of electric field could be visualized in a sub-15 nm scale when a thin pDR1 film was coated on top of gold nanocubes.16 Different configurations (denoted in Fig. 1(a)) between AuNPs and pDR1 films were systematically investigated with AFM images and corresponding FEM simulations as shown in Fig. 2. More careful attention is required when interpreting or comparing AFM images with FEM simulations, as the mass-transport mechanism of a pDR1 film is not fully understood

Figure 2. FEM calculations of electric fields near a AuNP for different configurations. (a) An AFM image after illumination of a 532 nm laser for the AuNP on a pDR1 film. (b)-(e) are the simulated results of electric fields in a top view for the total intensity |E| (b) and the components for a x direction | | (c), a y direction | | (d) and a z direction | | (e). (f)-(j) are the corresponding AFM image (f) and the calculated intensities of a total electric field (g), a x component (h), a y component (i) and a z component (j) for the embedded AuNP in a pDR1 film. (k)-(l) are the simulated results of electric fields in a side view for out-of-plane components | | for both AuNP on the pDR1 film (k) and the embedded AuNP (l). All the scale bars are 300 nm, and all the polarizations of an incident light are indicated as a black arrow. The axis for a top view and a side view are denoted in (a) and (k) respectively. All the intensities of electric fields in a top view were calculated on the plane above 5 nm from the pDR1 film, and plotted with normalizations.


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yet.16,25,26 Fig. 2(a) is an AFM data for an AuNP on a pDR1 film, which clearly shows the reconfiguration of the pDR1 film due to the localized electric field from the AuNP after illumination. Fig. 2(b)-(e) are the calculated images of total, a X component, a Y component and a Z (vertical) component for electric field in a top view. It has been already reported in the previous work16, where the vertical component lifted up the azobenzene molecules when the gold nano cube was coated with the pDR1 film. In our case, however, AuNPs are located on top of the pDR1 film so the overall reconfiguration pattern recorded on the surface is dominated by the inplane components of the electric field from the AuNP. The reconfiguration occurs mainly on the X direction where it matches with the polarization of the incident light. As reported in the previous work21, there also exists the electric field in Y direction (the orthogonal direction to the polarization of the incident light) after illumination on metallic nanostructures, and it deforms the pDR1 film in a Y direction. The induced motion of azobenzene molecules in the pDR1 film, thus, moves and creates dips near the AuNP in X and Y directions. As indicated in the AFM image of the embedded AuNP after illumination (Fig. 2(f)), the reconfiguration pattern is less clear, and no reconfiguration in a Y direction is observed. The calculated field distribution of total electric field (Fig. 2(g)) and lateral components (Fig. 2(h) and (i)) show the decreased intensity of electric fields after the embedding process. Unlike the AuNP on the pDR1 film, the field intensity of a Y component is much weaker than that of the X component, which results in no notable change in a Y direction of the AFM image. A vertical component of the embedded AuNP (Fig. 2(j)) shows the electric field near the AuNP but with lower intensity. The calculation of electric field distributions of a Z component was also plotted in a side view for the AuNP on the pDR1 film (Fig. 2(k)) and the embedded AuNP in the pDR1 film (Fig. 2(l)). It clearly indicates that the most intensity of the vertical component is confined between the bottom of the


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AuNP and the pDR1 film. As shown in the height profile of Fig. 1(e), the height of both AuNPs were increased after light illumination when compared with the flat pDR1 film surface. As AuNPs on the pDR1 film were fully exposed to an external light without additional absorption by polymer, the pDR1 film under the AuNPs could be reconfigured and lifted further with stronger electric fields in the Z direction, compared to the embedded AuNPs (Fig. S1). The configurations of AuNPs with the pDR1 film as well as the different mechanism from in-plane and out-of-plane components, therefore, should be considered to understand the reconfiguration of the pDR1 film.

Figure 3. Electric field interference of AuNP dimers with various interparticle gaps. AFM images of AuNP dimers on a pDR1 film having the interparticle gap of 285 nm (a), 335 nm (b), 400 nm (c), 450 nm (d) and 500 nm (e) after illumination of a laser. (f) is a plot of reconfigured depth of a pDR1 film at the center of each AuNP dimer on a pDR1 film. AFM images of AuNP dimers embedded in a pDR1 film having the interparticle gap of 285 nm (g), 335 nm (h), 400 nm (i), 450 nm (j) and 500 nm (k) after illumination of a laser. (l) is a plot of reconfigured depth of a pDR1 film at the center of the embedded AuNP dimers. All the AFM image sizes are 2×2 µm2, and the polarizations of an incident light are indicated as a white arrow. Insets in (a)-(e) and (g)(k) are the corresponding calculated intensities of electric field.


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AuNP dimers of various interparticle distances were systematically investigated to study the AuNP’s movements and the interference of the radiative electric fields, depending on different AuNPs configurations in this photofluidic measurement. The AuNP dimers of various gap distances (285 nm to 500 nm of the center-to-center distance) were fabricated on the pDR1 film by vector manipulations with AFM. Then an external laser was illuminated with a wellaligned polarization along the long axis of the AuNP dimer as shown in Fig. 3(a)-(e). On the other hand, Fig. 3(g)-(k) show the embedded AuNP dimers, which were achieved with vector manipulations of AFM followed by gentle annealing as before. As discussed in Fig. 1, the reconfigured pattern of the pDR1 film near AuNPs in AFM images can be clearly distinguished depending on the relative configuration between AuNPs and pDR1 films. Unlike AuNPs that are separated more than 3 µm shown in Fig. 1(d) and (e), the reconfigured pattern of pDR1 can be affected when AuNPs are more closely located. This can be further analyzed with the plot of the depressed depth at the center of the dimers for different gap distances in Fig. 3(f). As could be reasoned by the characteristic length and depth of the reconfigured pattern from the single AuNP (Fig. 1(e) and (f)), the depth at the center position of AuNP dimers is increasing at the gap distance of

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