Spatial Control of the Distribution of Nanogaps between Gold

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C: Surfaces, Interfaces, Porous Materials, and Catalysis

Patterning Nanogaps: Spatial Control of the Distribution of Nanogaps between Gold Nanoparticles and Gold Substrates Ly Thi Minh Huynh, and Sangwoon Yoon J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b08658 • Publication Date (Web): 29 Oct 2018 Downloaded from http://pubs.acs.org on November 2, 2018

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The Journal of Physical Chemistry

Patterning Nanogaps: Spatial Control of the Distribution of Nanogaps between Gold Nanoparticles and Gold Substrates Ly Thi Minh Huynh and Sangwoon Yoon* Department of Chemistry, Chung-Ang University, 84 Heukseok-ro, Dongjak-gu, Seoul 06974, Korea

Revision Submitted to J. Phys. Chem. C on October 24, 2018.

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ABSTRACT

Narrow nanogaps formed between nanostructures act as hot spots where the plasmonic properties are significantly enhanced. Consequently, the ability to create and control nanogaps is highly desirable for many plasmon-based applications. The nanoparticles-on-mirror (NPoM) is an attractive system that allows one to produce nanogaps on two-dimensional surfaces with great flexibility. NPoM is formed by adsorbing gold nanoparticles (AuNPs) on self-assembled monolayers (SAMs) of molecules on Au substrates. The properties of the resulting nanogaps are defined by the SAM molecular spacer and the shape and size of the adsorbed AuNPs. In this paper, we present a method for controlling the spatial distribution of the nanogaps with micrometer resolution. UV irradiation of the SAMs leads to desorption of the thiol molecules from the surface via photooxidation, which hinders the subsequent adsorption of AuNPs on the surface. By applying spatioselective irradiation, spatially controlled NPoM patterns is constructed. Furthermore, filling the irradiated regions with different type of molecules leads to patterned nanogaps with two different sets of properties on a single Au substrate. The gap properties are measured with dark field microscopy, scanning electron microscopy, and surface-enhanced Raman scattering. This method can be extended to the fabrication of more complex nanogap circuits with higher spatial resolution by applying advanced photolithography techniques.


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INTRODUCTION Plasmonic nanoparticles exhibit many fascinating properties mainly arising from localized surface plasmon resonance (LSPR), the collective oscillation of conduction band electrons in response to incident electromagnetic fields.1,2 LSPR is responsible for the extremely large absorption or scattering of light in the visible region by Au nanoparticles (AuNPs).3 LSPR also focuses the electric fields around the nanoparticles, which thus function as nanoantennae, overcoming the diffraction limit.4,5 The nonradiative relaxation of LSPR produces nonthermal hot charge carriers, which drive catalytic reactions at the surface of nanoparticles.6-12 The decay of LSPR ultimately leads to heating of the surrounding media. These local heaters are often used as photothermal therapy agents.13-15 The phenomena arising from LSPR are modified and enhanced at the narrow nanogaps formed between nanostructures. Changes in the nanogap distance can alter the optical resonance wavelength (and thus the visible color) of the nanoparticle systems.16-18 The intensity of the electric field increases greatly at the nanogaps, permitting significantly enhanced Raman scattering or fluorescence of the molecules residing in the gap junction.19-21 Thus, nanogaps are a requisite for surface-enhanced Raman scattering (SERS).22 Plasmondriven reactions are more effective at nanogaps because of spatial confinement.23 More heat is generated when nanoparticles form aggregates.24-26 Therefore, the creation and control of nanogaps is essential not only for the fundamental study of plasmonics, but also for the


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development of more advanced plasmonic materials by rational design of hierarchical structures. Among the various nanoparticle systems, the nanoparticles-on-mirror (NPoM) assembly is one of the simplest and the most useful as it enables control over the formation of nanogaps.2728

NPoM is constructed by the adsorption of AuNPs on the surface of two-dimensional self-

assembled monolayers (SAMs) of thiol molecules on Au substrates. Thiol molecules are attached to the Au substrates via the formation of AuS covalent bond; the lateral van der Waals interaction between the backbone chains of the thiol molecules leads to the highly ordered, stable molecular monolayer structures.29 The AuNPs adsorb onto these SAM surfaces through the intermolecular interactions between the terminal group of the SAMs and the stabilizing ligands of the AuNPs. For example, the citrate-capped AuNPs adsorb on the protonated amine-, hydroxyl-, and methyl-terminated SAMs through the electrostatic attraction, hydrogen bonding, and van der Waals interactions, respectively.30,31 Then, narrow nanogaps are created between the AuNPs and the Au substrate. Thus, changing the SAM molecules or the AuNPs adsorbed on the SAMs modifies the nanogaps. The length of the SAM molecules defines the gap distance, while the size and shape of the adsorbed AuNPs determine the area and geometry, respectively, of the individual nanogaps.28,32-36 As these parameters are relatively easy to change in NPoM, this system has been widely used as a good platform to study plasmon coupling, SERS, and chemical reactivity in various nanogaps.28,37,38 The large-scale control of nanogaps is often required in device development. The overall density of nanogaps on Au substrates can be controlled by changing the number of AuNPs


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adsorbed on the SAMs. Control of the overall pattern is far more challenging. We have found that the AuNPs can be adsorbed on the surface in either aggregated or dispersed forms, leading to a different pattern, when SAM molecules with the same terminal group but different backbone structures are used.39,40 However, in all the cases mentioned above, nanogaps are formed over the entire Au substrate. Further localized spatial control of the adsorption of AuNPs cannot be achieved using the conventional method of immersing the SAM substrate in a colloidal AuNP suspension. Dropcasting of AuNPs onto SAM surfaces allows the NPoMs to be formed at the desired location, but with very poor controllability. The AuNPs usually diffuse toward the periphery of the cast droplets as the solvent vaporizes and form a circle of inhomogeneous aggregates (often called the “coffee ring effect” or “drying effect”).41-44 Fabrication of nanogap structures using an electron beam or scanning probe microscopy is expensive and requires special facilities and high level technical expertise. In this work, we try to put the final piece in the puzzle for the control of the nanogaps in NPoM. Can we control the spatial distribution of nanogaps? Can we create nanogaps at specifically desired locations? Can we fabricate two different AuNP adsorption patterns on the same Au substrate? In an attempt to answer these questions, we adopt a strategy based on the photooxidative desorption of thiol molecules from Au surfaces by UV irradiation. UV irradiation oxidizes the thiol molecules in SAMs to sulfonate, which then desorbs from the Au surfaces.45-48 Thus, UV irradiation of the SAMs, followed by the adsorption of AuNPs, can be used to prepare patterned nanogaps with micrometer spatial resolution. In addition, after irradiation, the introduction of a second thiol molecule onto the vacant areas created by


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irradiation allows the preparation of patterned mixed SAMs. The adsorption of AuNPs on the mixed SAMs then leads to an NPoM structure with two different gap properties on one substrate. Similar to photolithography in the semiconductor industry, this method can be extended to the fabrication of more complex and finer plasmonic architectures.

EXPERIMENTAL SECTION Preparation of SAMs (SAM/Au). The two major components of an NPoM are the SAMs on the Au substrates and the adsorbing AuNPs. The Au substrates for the SAMs were fabricated at the National Nanofab Center (NNFC, Daejeon, Korea). Au films (150 nm thickness) were deposited onto Ti (10 nm)-coated 4-inch silicon wafers by e-beam evaporation. The Au substrates were cut into 1 cm × 1 cm pieces, cleaned with Piranha solution (H2SO4:H2O2 = 3:1) for 5 min, rinsed with water and ethanol repeatedly, and then dried with N2 gas. The clean Au substrates were immersed in an ethanol solution of the thiol (1 mM, 2 mL) for 16 h for the formation of thermodynamically stable SAMs.29 The SAM structures on the Au substrates are hereafter referred to as SAM/Au. The SAM/Au system was washed with ethanol and dried with N2. All the thiol compounds used in the experiments were purchased from Aldrich and used without further purification (see Supporting Information for a full list of chemicals). After its preparation, SAM/Au was placed on a stage for irradiation. UV irradiation. A Hg-Xe lamp (LC8, Hamamatsu) was used for irradiation of the SAM/Au. We removed the cutoff filter to fully utilize all the UV lines from the lamp below 455 nm. The UV light from the lamp was delivered to the surface of SAM/Au using a light guide. The beam


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size at the sample was ~1.2 cm. The lamp power measured at the sample using a power meter (Orion TH, Ophir) was 1.6 W/cm2. SAM/Au was illuminated from the light guide either directly or through a shadow mask. We used a TEM grid (G400-C3, Gilder) with a bar width of 25 µm and a hole width of 37 µm as the mask. Preparation of AuNPs and NPoMs. The AuNPs were synthesized using the seeded growth method.49 Detailed procedures are provided in the Supporting Information. Briefly, reduction of HAuCl4 with citrate produced the seed nanoparticles.50 The seed particles grew into larger particles via five cycles of repeated reduction of Au3+ by citrate.49 The size and optical properties of the AuNPs were measured using a transmission electron microscope (TEM, JEM2100, JEOL) and a UV-Vis spectrometer (Lambda 25, PerkinElmer). The final AuNPs had an average diameter of 59 ± 6 nm (N = 300) and showed an LSPR band at 538 nm (Figure S1). The NPoM was prepared by immersing the irradiated or unirradiated SAM/Au substrates in an AuNP solution (108 pM, 2 mL) for 3 h. After the adsorption of the AuNPs was complete, the substrates were washed with water and dried with N2 prior to measurement. Measurements. The adsorption of AuNPs on SAM/Au was visualized using dark field microscopy (DFM) and scanning electron microscopy (SEM). A dark field objective (100×, NA 0.90) was mounted on an upright microscope (BX-51, Olympus) to measure the scattering of light from the AuNPs. SEM (Sigma, Carl Zeiss) provided direct images of the adsorbed AuNPs. The Raman spectra of the NPoM were obtained using a Raman microscope (Raman Microprobe, Kaiser Optical Systems). For measurement of the Raman spectra, a 785 nm, 30 mW laser was focused on the sample through a 50× objective (NA 0.75). The Raman scattering


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from the sample was collected by the same objective and guided to the spectrometer (f/1.8, resolution 5 cm-1) through an optical fiber. The total exposure time for a single acquisition of a Raman spectrum was 3 s. For Raman intensity mapping, a 100 μm x 100 μm area was scanned in 3 μm steps. At each spot, Raman spectra were obtained for a total exposure time of 1 s.

RESULTS AND DISCUSSION In Figure 1a, we depict the general concept for the spatial control of nanogap formation. First, SAMs of thiol molecules on an Au substrate are prepared using either the alkyl thiol (1octanethiol, C8SH) or the aromatic thiol (4-methylbenzenethiol, MBT). A Hg-Xe lamp is then used to illuminate the area of the surface over which the nanogap formation is to be blocked. The UV irradiation induces photooxidation of the thiol SAMs and subsequent desorption of the thiol molecules from the surface.45-48 In the following step, the immersion of the substrate in an AuNP solution leads to the selective adsorption of AuNPs on the unirradiated area of the SAM/Au.


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Figure 1. (a) Illustration of the basic method for the spatial control of nanogap formation between the AuNPs and Au surfaces. UV irradiation from a Hg-Xe lamp removes the thiol SAMs through photooxidative desorption and thus the AuNPs adsorb only on the unirradiated area. (b) Photographs of a sample after each step presented in (a). (c) SEM images of unirradiated (red square) and irradiated (yellow square) regions of the C8SH SAMs on the Au substrate after the adsorption of the AuNPs. (d) SEM images of a similar sample with MBT SAMs. The photographs and SEM images clearly show that AuNPs adsorb only on the unirradiated region.

The results presented in Figure 1b-1d clearly show that our scheme works. Figure 1b displays the photographic images of the substrates after each step. The Au substrate does not change its color after the formation of the SAMs. The color remains the same throughout the substrate


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upon irradiation of the lower half of the SAM/Au substrate for 1 h. However, when the substrate is immersed in an AuNP solution for 3 h, we see a distinct color difference between the irradiated and unirradiated area. The AuNPs adsorb only on the unirradiated area (the upper half) where thiol SAMs remain intact, resulting in a red color. Van der Waals interaction of the methyl terminal group of the SAMs with the citrate-capped AuNPs leads to the weak binding of the AuNPs to the SAM surfaces.30 In marked contrast, the AuNPs do not adsorb on the irradiated area, suggesting that the thiol SAMs have been removed by UV irradiation. The presence or absence of AuNPs on SAM/Au is confirmed by SEM. In Figure 1c and 1d, we display the representative SEM images obtained from the irradiated and unirradiated regions of the samples with C8SH (Figure 1c) and MBT (Figure 1d) SAMs. The SEM images clearly show that the AuNPs adsorb only on the unirradiated region, whereas their adsorption is effectively blocked by the removal of the thiol SAMs using UV. It is evident from the SEM images that this scheme applies to both alkyl and aromatic thiols. A closer examination reveals that there is a subtle difference between C8SH and MBT SAMs in the AuNP adsorption patterns in the unirradiated regions. The AuNPs adsorb on the C8SH SAM as aggregates, while they are present in the dispersed form on the MBT SAM. These adsorption patterns on methylterminated SAMs are consistent with previous observations.39,40 In our previous studies, we revealed that the difference in surface roughness and the consequent hydrophobicity between the C8SH and MBT SAMs determines the adsorption pattern of citrate-capped AuNPs onto the SAMs.39,40 Scanning tunneling microscopy (STM) and contact angle measurements show that the C8SH SAM surfaces are more corrugated than the MBT SAM surfaces, rendering the


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surfaces of the former more hydrophobic. The more hydrophobic C8SH SAM surfaces directly interact with the citrate-capped AuNPs when immersed in an aqueous solution of the AuNPs, which results in the stripping of the citrate layers of the AuNPs and thus the aggregation of the AuNPs. In contrast, water mediates the adsorption of the AuNPs onto the less hydrophobic MBT SAM surfaces. The remaining citrate anions on the AuNPs keep the AuNPs dispersed during the adsorption process through electrostatic repulsion, leading to the dispersed adsorption. The results in Figure 1 clearly indicate that UV irradiation removes the SAMs from the Au substrates, which prevents the AuNPs from adsorbing on the irradiated areas. X-ray photoelectron spectroscopy and mass spectrometry studies revealed that UV irradiation oxidizes the thiol SAMs to sulfonate (–SO3).45-48 SERS measurements also confirmed that the exposure of SAMs to UV irradiation leads to C–S bond fission, followed by sulfur oxidation.48 The oxidized sulfonate species are weakly bound to the surface and thus readily desorbed.45 Here we measure the amount of irradiation time required to remove the thiol SAMs by varying the UV irradiation time on the C8SH SAMs and measuring the number of AuNPs adsorbed on the remaining SAMs using SEM. Figure 2 shows that the number of adsorbed AuNPs decreases as the irradiation time increases. No AuNPs are adsorbed on the Au substrates after 1 h, indicating that at least 1 h is required to completely remove the C8SH SAM using UV. As a control experiment, we also count the number of AuNPs in the unirradiated area, which remains largely the same. Assuming that desorption occurs almost instantly after oxidation, the decrease in the number of AuNPs with irradiation time should closely follow first-order


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kinetics. Fitting the decay to an exponential function yields a photooxidation rate constant of 0.055 ± 0.013 min–1, which is consistent with previously reported values.45

Figure 2. Average number of adsorbed AuNPs per unit area (μm2) in the irradiated (red filled circle) and unirradiated (red open square) regions with increase in irradiation time.

Having found that UV irradiation on SAMs hinders the formation of nanogaps, we then apply this principle to the creation of NPoM patterns with micrometer spatial resolution. When we directly illuminate a region of the SAM/Au substrate as shown in Figure 1a, the resulting NPoM area is limited by the beam size and shape of the UV light, which is typically on a scale of centimeters. Simply using a mask with micrometer-scale patterns, we can lower the spatial resolution down to the order of micrometers and can create desired NPoM patterns (Figure 3a). Exposure of the SAM/Au substrate to UV irradiation through a mask selectively removes the SAM in the hole area of the mask. Subsequent adsorption of AuNPs leads to the formation of NPoM along the mask pattern. This scheme is conceptually similar to UV


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photolithography widely used in the semiconductor industry. Thus, spatial resolution even below a micrometer could be achieved using more advanced UV photolithography. As a proof of concept, we used a TEM grid as a mask (hole width 37 µm, bar width 25 µm). Figure 3 shows that the AuNPs adsorb only on the unirradiated areas where the grid bars block the UV light. The DFM images display bright scattering corresponding to the adsorbed AuNPs in the unirradiated areas, reproducing the bar pattern, while the irradiated areas remain dark, indicating that very few AuNPs are adsorbed (Figure 3b). SEM images obtained at 3000× and 20000× magnification clearly show that AuNPs are adsorbed only in the bar area (Figure 3c).

Figure 3. (a) Scheme of the controlled nanogap patterning using a mask. UV illumination through a mask removes the SAMs only in the hole regions. Adsorption of AuNPs in the


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subsequent step leads to the formation of NPoM that reproduces the mask pattern. (b) DFM and (c) SEM images of the resulting samples when a TEM grid (hole width 37 µm, bar width 25 µm) is used as a mask. NPoM is formed along the bar pattern of the grid. Very few AuNPs are found in the hole regions. (d) Raman spectra obtained from the bar (red line) and hole (blue line) region. (e) Raman intensity mapping with respect to the 1076 cm–1 mode. The nanogaps formed along the bar pattern produce SERS spectra of MBT confined between the AuNPs and Au substrates.

These results suggest that the nanogaps between the AuNPs and the Au substrates are formed preferentially along the bar pattern of the TEM grid. Because SERS is a phenomenon that occurs only at nanogaps, we confirm the formation of patterned nanogaps by comparing the Raman spectra from the bar and hole areas. Figure 3d shows that a strong Raman signal corresponding to the SAM molecules (MBT) is observed from the bar area, whereas no Raman signal is obtained from the hole area. Raman mapping for the peak at 1076 cm–1 (ν7a mode, a combination band of the CCC in-plane bend, and CS stretch of MBT)51,52 beautifully reproduces the TEM grid pattern, confirming that nanogaps form along the bar pattern of the mask (Figure 3e). The area where the SAMs have been removed by UV irradiation could conceivably be filled with different molecules from the existing molecules in the unirradiated region to produce nanogaps with different gap properties. We explored this possibility (Figure 4a). First, a SAM


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of C8SH is prepared; the C8SH/Au substrate is irradiated through a TEM grid mask for 2 h. UV irradiation removes the C8SH SAM in the hole areas of the grid mask. After irradiation, the substrate is immersed in an MBT solution (1 μM, 2 mL) for 30 min. This leads to the formation of a mixed SAM of C8SH and MBT along the TEM grid pattern on the Au substrate. AuNPs are then adsorbed onto SAM/Au.

Figure 4. (a) Scheme for creating patterned nanogaps with different gap properties on a single Au substrate. UV irradiation removes the C8SH SAMs along the hole pattern of the mask. Immersion of the substrate in an MBT solution fills in the hole regions with MBT SAMs. Adsorption of the AuNPs leads to NPoMs with C8SH and MBT SAMs under the AuNPs in bar and hole areas, respectively. (b) DFM and (c) SEM images of the resulting NPoM structure. Magnified SEM images show different AuNP distribution patterns for C8SH and MBT SAMs,


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consistent with previous results. (d) Raman mapping with respect to 1076 cm–1, the 7a mode of MBT. High intensity in the hole regions indicates the formation of NPoM with MBT SAMs in the nanogaps while the bar regions are made up of NPoM with C8SH SAMs.

Figure 4b shows the DFM image of the resulting NPoM substrate. A strong greenish colored scattering is observed from the hole area, which is clearly different from the dark background in Figure 3b. This indicates that the AuNPs are adsorbed on the hole areas that have been irradiated and filled with MBT. SEM images also strongly support the formation of patterned nanogaps with two distinctively different gap molecules. The SEM image in Figure 4c depicts the uniform distribution of the AuNPs on the substrate, both on the hole and bar areas. Upon a closer look, however, it is evident that the AuNP adsorption patterns differ between the hole and bar areas; this division is indicated by the dotted line in the figure. The AuNPs are in a dispersed form in the hole region (MBT SAM) whereas they are aggregated in the bar area (C8SH SAM).39,40 This result suggests that NPoM is constructed on an Au substrate, but with two different gap properties patterned along the shape of the grid mask. SERS measures the chemical nature of those nanogaps (Figure 4d). Scanning the substrate with respect to the Raman peak intensity at 1076 cm–1 (MBT, ν7a mode) shows high intensity in the hole regions, indicating that the MBT SAMs are newly formed in the region after UV irradiation while the C8SH SAM remains in the bar area. SERS mapping clearly shows that two different types of nanogaps can be patterned on a single Au substrate with micrometer spatial selectivity.


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This study provides new physical concepts to produce nanogaps with different gap properties (e.g., gap distance, conductance, and molecular packing density) in a spatially controlled pattern using photooxidation of thiol molecules and adsorption of gold nanoparticles on molecular layers. Such patterned plasmonic nanogaps can be applied to SERS-based microfluidic sensors, color displays, and photonic devices.

CONCLUSIONS In this work, we demonstrated the production of patterned nanogap distributions on twodimensional Au surfaces. The application of UV irradiation through a mask removed the thiol molecule SAMs, hindering the AuNPs from adsorbing on the surface. Subsequent immersion of the substrates in an AuNP solution permitted the formation of NPoMs along the blocked pattern. SERS mapping indicated that nanogaps were formed only in the unirradiated areas. Filling the irradiated region with a different molecule produced mixed NPoM patterns with two different gap properties. The combination of photooxidation of SAMs and adsorption of AuNPs on SAMs allows one to create nanogaps readily in a spatially controlled pattern. This method can be adopted to the fabrication of more complex nanogap circuits with higher spatial resolution by applying advanced photolithography techniques. The patterned nanogaps can be implemented in microfluidic sensors, color displays, and photonic devices.

SUPPORTING INFORMATION.


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Details of the synthesis and characterization of Au nanoparticles. This material is available free of charge via the Internet at http://pubs.acs.org.

CORRESPONDING AUTHOR *[email protected]

ACKNOWLEDGEMENTS We gratefully acknowledge the support of this work by the National Research Foundation (NRF) of Korea (2016R1A2B2007259). This research was also supported by the Chung-Ang University Research Grants (2016).

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Spatial Control of the Distribution of Nanogaps between Gold

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