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Second-Harmonic Generation from Sub-5-nm Gaps by Directed SelfAssembly of Nanoparticles onto Template-Stripped Gold Substrates Zhaogang Dong, Mohamed ASBAHI, Jian Lin, Di Zhu, Ying Min Wang, Kedar Hippalgaonkar, Hong-Son Chu, Wei Peng Goh, FuKe Wang, Zhiwei Huang, and Joel K.W. Yang Nano Lett., Just Accepted Manuscript • DOI: 10.1021/acs.nanolett.5b02109 • Publication Date (Web): 13 Aug 2015 Downloaded from http://pubs.acs.org on August 15, 2015

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Second-Harmonic Generation from Sub-5-nm Gaps by Directed Self-Assembly of Nanoparticles onto Template-Stripped Gold Substrates Zhaogang Dong1, Mohamed Asbahi1, Jian Lin2, Di Zhu1,⊥, Ying Min Wang1, Kedar Hippalgaonkar1, Hong-Son Chu3, Wei Peng Goh1, Fuke Wang,1 Zhiwei Huang2,*, and Joel K. W. Yang4,1,* 1

Institute of Materials Research and Engineering, A*STAR (Agency for Science, Technology

and Research), 3 Research Link, Singapore 117602, Singapore 2

Department of Biomedical Engineering, Faculty of Engineering, National University of

Singapore, 117576, Singapore 3

Institute of High Performance Computing, A*STAR (Agency for Science, Technology and

Research), 1 Fusionopolis Way, #16-16 Connexis, 138632, Singapore 4

Singapore University of Technology and Design, 8 Somapah Road, Singapore 487372

⊥Current

Address: Department of Electrical Engineering and Computer Science, Massachusetts

Institute of Technology, 77 Massachusetts Avenue, Cambridge, Massachusetts 02139, USA.

*Address correspondence to [email protected], [email protected].

ACS Paragon Plus Environment

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Abstract Strong field enhancement and confinement in plasmonic nanostructures provide suitable conditions for nonlinear optics in ultra-compact dimensions. Despite these enhancements, second-harmonic generation (SHG) is still inefficient due to the centrosymmetric crystal structure of the bulk metals used, e.g. Au, and Ag. Taking advantage of symmetry breaking at the metal surface, one could greatly enhance SHG by engineering these metal surfaces in regions where the strong electric fields are localized. Here, we combine top-down lithography and bottom-up self-assembly to lodge single rows of 8-nm diameter Au nanoparticles into trenches in a Au film. The resultant “double gap” structures increase the surface-to-volume ratio of Au colocated with the strong fields in ~2-nm gaps to fully exploit the surface SHG of Au. Compared to a densely-packed arrangement of AuNPs on a smooth Au film, the double gaps enhance SHG emission by 4200-fold to achieve an effective second-order susceptibility χ(2) of 6.1 pm/V, making it comparable with typical nonlinear crystals. This patterning approach also allows for the scalable fabrication of smooth gold surfaces with sub-5-nm gaps, and presents opportunities for optical frequency up-conversion in applications that require extreme miniaturization.

Keywords: Sub-5-nm gap; Plasmonic nanostructures; Second-harmonic generation; Template stripping method; Directed self-assembly


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Metallic nanostructures with nanometer-scale gaps have demonstrated strong field enhancements arising from localized surface plasmon resonances.1 Within the classical electromagnetic limit, the field enhancement in tiny gaps increases exponentially as the sizes are reduced to ~1 nm.2 This phenomenon has been widely used for chemical sensing,3-5 nonlinear harmonic generation,6-8 plasmon-enhanced optical forces,5, 9, 10 and photovoltaics.11, 12 Selfassembly is a promising approach to achieve sub-5-nm gaps between close-packed particles, in which molecular ligands coating the particles act as precise dielectric spacers.13-16 Similarly, gaps can be defined with particles deposited onto a metal film,17-19 and with edge lithography using atomic-layer deposition.20 In comparison, top-down lithography defines gaps directly and deterministically, but resolution limits and surface roughness precludes patterning at sub-5-nm dimensions in a scalable manner. Hence, high-throughput approaches that achieve gaps of such dimensions in metals, and simultaneously produce low surface roughness to minimize scattering losses are currently still unavailable. Field enhancements in plasmonic nanostructures with sub-5-nm gaps have led to the discovery of new physical phenomena, such as charge-transfer plasmons via quantum tunneling,15, 21 nonlocal effects,18 and nonlinear plasmonics,6, 7, 22-24 enabling optical frequency conversion at the nanoscale.8, 25-30 Second-harmonic generation (SHG) emission from plasmonic nanostructures is attractive due to the advantageous field enhancements with simple illumination conditions, and ultra-small dimensions. In contrast, conventional nonlinear crystals are bulky and require phase matching conditions. However, SHG in plasmonic nanostructures without the presence of nonlinear materials still remain a challenge due to the centrosymmetric characteristic of bulk noble metals.31, 32 The crystalline symmetry of these metals is only broken at the surface to give rise to second-order surface susceptibilities.31, 33 Previous work has investigated enhanced


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SHG emission by aligning plasmonic resonances with the pump/SHG frequencies,34, 35 and breaking the local symmetry of the nanostructures.36, 37 However, the effect of increasing the surface-to-volume ratio of plasmonic nanostructures in regions of strong field enhancement has not been thoroughly investigated. In this paper, we present a scalable nanofabrication approach that combines top-down and bottom-up lithography to deterministically create plasmonic nanostructures with sub-5-nm “double gaps”. This structure was designed to enhance SHG emission due to both the coincidence of field enhancements within nanoscale gaps and the enhanced interaction with the second-order surface susceptibility of Au. Single rows of gold nanoparticles (AuNPs) were selfassembled into trenches on a template-stripped (TS) gold substrate. Sub-5-nm gaps were formed between the AuNPs and the inner walls of the trench as shown in Fig. 1(a). As the gaps are merely separated by the diameter of the nanoparticle, we refer to this structure as the “doublegap”. This structure has the following advantages: It increases the surface-to-volume ratio within the sub-5-nm plasmonic cavity, and significantly red shifts the plasmonic resonances of the bare TS gold substrate from the visible to the near-infrared regime to spectrally align with the pump laser. Compared to the sub-5-nm gaps formed between AuNPs on a flat gold film, the SHG emission from these double gaps is 4200-fold higher. As the interaction volume of the sample with the pump laser is limited to the sample surface in the setup, we measured a conversion efficiency of only 1.8x10-7. Nonetheless, its effective second-order susceptibility χ(2) is of 6.1 pm/V, which is comparable to typical nonlinear crystals.38 Technologically, our process provides a route for mass production of well-defined plasmonic nanostructures with sub-5-nm gaps. We show that these structures are promising for use in nonlinear optical applications within confined volumes.


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The heart of our nanostructure design for SHG enhancement is the double gap geometry as shown schematically in Fig. 1(a). Molecular ligands capping the particles act as dielectric spacers to create gaps of ~1-2 nm in size between particle and the trench side-walls. With an illumination with oscillation frequency ω, the optical fields will be enhanced in the sub-5-nm gaps. As a result, the incident optical field will have a strong interaction with the surfaces of gold within gaps leading to strong SHG emission. Figure 1(b) shows a representative scanning electron micrograph (SEM) of the sample with a single row of ~8 nm diameter AuNPs deposited inside the Au trench, where the gap sizes are measured to be ~2 nm.


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Figure 1. Schematic of the double-gap structure and its fabrication process. (a) AuNPs assembled in a single row within Au trenches effectively convert a single large gap into two sub5-nm gaps. Strong field enhancement of the incident beam occurs within these gaps resulting in


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SHG emissions. (b) Top-down scanning electron micrograph (SEM) image of a single row of ~8 nm diameter AuNPs assembled within a trench in a gold film. Gaps defined by the ligand are measured to be ~2 nm. (c)-(e) Process flow for a scalable fabrication of plasmonic nanostructures with sub-5-nm gaps. (c) Metal evaporation of gold (e.g. 150 nm) onto the patterned silicon template with a fin width of ~8 nm and template stripping of the Au film attached to a glass substrate using optical-adhesive (OA) glue. The patterned silicon template is cleaned and reused to make multiple samples. (d) Template-stripped (TS) gold substrates with characteristic smooth surfaces and narrow trenches. (e) Deposition of gold nanoparticles (AuNPs) by the directed selfassembly method to achieve sub-5-nm gaps. The fabrication process presented here addresses the following outstanding challenges in plasmonic nanostructure fabrication: (1) the reliance on beam-based approaches to replicate sub10-nm features in metals, (2) large surface roughness of metal films that causes scattering losses in the resonances, and (3) the lack of deterministic self-assembly approaches in defining lateral gaps. While template stripping methods are known to result in smooth metal surfaces,39-41 it has not been employed to achieve sub-10-nm structures with aspect ratios larger than unity, due to the potential for template fracture. Here, we developed a process that extends template stripping to sub-10-nm dimensions, and demonstrate the robustness of templates in crystalline silicon with aspect ratios of ~5. In addition to shrinking down the gap size, the combination of top-down lithography and bottom-up assembly used here enables two ~2-nm gaps to be fabricated within 10 nm of each other in the double-gap geometry. Plus, it gives us the flexibility study different material combinations, e.g. by substituting AuNP with other nanoparticle types. Figure 1(c)-(e) presents the process flow for fabricating plasmonic nanostructures with sub-5-nm gaps. Templates were fabricated by pattern transfer of hydrogen silsesquioxane (HSQ)


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lines, defined by electron beam lithography (EBL), into silicon substrates via reactive ion etching. The patterned templates consisted of sub-10-nm wide mesh of lines covering an area of ~400 µm2. As shown in Fig. 1(c), 150 nm of Au was then evaporated onto the patterned silicon substrates. UV cured optical-adhesive (OA) glue (NOA61, Norland Products) was used to bond a glass slide to the gold film, as shown in Fig. 1(d). The template stripping process involves the mechanical separation of the gold film off the silicon template. The silicon template can be reused to produce multiple template-stripped (TS) gold structures (details in the supporting information). The TS gold substrates are shown in Fig. 1(e) with trenches that were ~11 nm wide. Using a recently-developed directed self-assembly approach of sub-10 nm nanoparticles (DSA-n),16, 42 oleylamine-coated gold nanoparticles of ~8 nm core diameter were selectively deposited into the trenches to form sub-5-nm gaps on either side of the particle as shown in Fig. 1(e). Figure 2 illustrates high-resolution electron micrographs of the sub-10-nm silicon templates and corresponding inverse patterns in gold. Figure 2(a)-(b) show SEM images of the patterned Si templates with square and triangle network of fins. The width of the fins was ~8 nm. Atomic force microscopy (AFM) was used to measure the surface roughness and height profile of the etched silicon template as shown in Fig. S3 in the supporting information. The root-meansquare (RMS) value for the surface roughness was ~0.42 nm across an area of 1 µm2 and the silicon fin was ~55 nm tall as seen in the line-scan profile in Fig. S3(b). Figures 2(c)-(d) show SEM images of the square and triangle pattern of trenches in Au, with widths of ~12 nm. Figure 2(e) presents the cross-sectional TEM image of the template (see the details on sample preparation in the supporting information). The cross-sectional image shows an average gap size of ~10 nm. Remarkably, template stripping was performed without


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fracturing the silicon fins despite their high aspect-ratio of ~5. This high aspect-ratio is important to achieve the localized gap plasmon resonances as shown in the finite-difference time-domain (FDTD) simulations in Fig. S5. Note the characteristic smooth surfaces of template stripped films, as compared to the as-deposited rougher Au surface in the TEM images in Fig. 2(e). The lateral offset between the trench on the top surface and the bump at the bottom surface could either be due to an unintentional angled deposition during the evaporation process, or shear forces during template stripping, similar to demolding in nanoimprint.43 This slight lateral offset will not affect the optical characteristic of the sample as the 150 nm Au layer was sufficiently thick to prevent any optical effects from the bottom surface.


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Figure 2. Scanning electron micrograph (SEM) and dark field transmission electron microscope (TEM) images of the sub-10-nm silicon templates and template-stripped (TS) gold substrates. (a)-(b) Silicon templates after the dry etching process using inductively-coupled-plasma reactive ion etching (ICP RIE). (c)-(d) TS gold substrates with square and triangular shapes. (e) TEM cross-sectional image of the square TS gold substrates (with a pitch of 920 nm). The TEM image shows that the trenches in gold have an aspect ratio of ~5.


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We investigated the reusability of the sub-10-nm silicon templates. No observable degradation was found after repeated use of the silicon template, as reported in Fig. S4. In addition, the surface roughness of the fabricated plasmonic nanostructures after each template stripping process remained consistent with an averaged RMS roughness of ~0.6 nm. A detailed study of the maximum number of cycles that a single template can be used requires further investigation. Nonetheless, we have to-date used a single template to make up to 6 samples without any observable degradation. Next, we employed a recently-developed directed selfassembly approach (see supporting information),16, 42 to selectively position AuNPs into the trenches. The AuNPs are capped by oleylamine ligands with a length of ~2 nm,44 where the corresponding SEM image is shown in Fig. 1(b). An overview SEM image of the whole TS sample with AuNPs was shown in Fig. S6, where the number of AuNPs deposited into the groove region depends on its lateral dimension. We performed 3D finite-difference time-domain (FDTD) simulations to investigate the resonant mode of this double-gap structure, and estimate the field enhancement. Figure 3(a) shows the simulation schematic for an idealized sample with a single row of AuNPs within the trenches. The width of the trenches was 12 nm with a periodicity of 310 nm. The normally incident light is polarized across the gaps. The refractive index of the oleylamine ligand is 1.46,45 where more details of the FDTD simulations are in the methods section. Figure 3(b) presents the simulated relative reflectance spectra referenced to a flat gold film, for gaps was varied from 2 nm to 0.5 nm (i.e. AuNP diameter varying from 8 nm to 11 nm). The introduction of AuNPs into the trenches caused a large redshift in the resonance from visible to the near-infrared, where Fig. S5 presents the FDTD simulation of the bare template, before the deposition of AuNPs. Figure 3(c) is a plot of the electric field intensity distribution |E|2 at the resonance wavelength λ0


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of 873 nm and a gap size of 2 nm (i.e. AuNP with a diameter of 8 nm), showing an enhancement in the optical intensity ~2000 times in the gap region. The corresponding charge distribution for this resonant mode is shown in Fig. 3(d).

Figure 3. Finite-difference time-domain (FDTD) simulation of the template-stripped (TS) gold substrates with assembled AuNPs within a single trench. (a) Schematic of the cross-section of the double-gap structure used in the simulations. Oleylamine ligands from the self-assembly process are indicated in green, which defines the gap “g” between the AuNP and trench sidewall. (b) Simulated relative reflectance spectra of TS gold substrate with deposited AuNPs, with


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respect to a flat gold film. The gap size “g” was varied from 2 nm to 0.5 nm. (c)-(d) Electric field intensity distribution |E|2 and charge distribution ρ at the resonance wavelength λ0 of 873 nm with a gap size of ~2 nm. Experiments show that the double-gap structures with a pitch size of 310 nm exhibit plasmon resonances at ~880 nm as corroborated by numerical simulations. Reflectance measurements for the double-gap structures and close-packed AuNPs on a gold film are shown in Figure 4(a). Comparison was also made with bare TS substrates, i.e. before AuNPs, showing a resonance red-shift of ~250 nm (cf. Fig. S7). Despite the formation of nanogaps, close-packed AuNPs on an Au film have a similar resonance to bare TS substrates at ~636 nm, indicating that it is the combined structure of the particle in the trench that provides the large redshift. The new resonant mode has a close alignment with the pump laser wavelength of 878 nm used in the experiment.

Figure 4. Optical measurements of the template-stripped (TS) gold substrates with the deposited AuNPs. (a) Measured relative reflectance spectra of the double-gap structure and “AuNPs on flat gold film”. The minima in the reflectance spectra are the positions of the respective plasmon resonances. The deposited AuNPs onto TS gold substrates have plasmon resonances in the nearinfrared regime (i.e. ~878 nm), which has a better spectral overlap with the pump laser


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wavelength in the SHG experiment. The dotted lines in the schematic inset represent the gold surfaces involved in the SHG emission process. (b) Log-scale plot of second-harmonic generation (SHG) emission intensities from different samples. A ~320-fold enhancement was obtained from the double-gap structures over that from the AuNPs on flat gold film. In addition, since the areal coverage ratio of AuNPs in double-gap structure is only 7.6%, the enhancement factor of SHG emission from AuNPs is estimated to be ~4200 (see detailed discussion in the main text and Fig. S6). The pump laser power was 3 mW and the integration time of the spectrometer was 5 s. (c) Log-log plot of the intensity relationship between SHG emission and pump laser power showing a slope of ~2, as expected for the SHG process. Figure 4(b) presents the measured second-harmonic generation (SHG) intensities. Details of the experimental setup are shown in Fig. S9. The beam diameter of the pump laser was 1.2 µm and the peak power intensity was 35.4 GW/cm2. As shown in Fig. S10, no SHG emission from flat gold film was observed. Due to the field enhancements in small gaps, close-packed AuNPs on a flat Au film also emit at the SHG wavelength as seen in Fig. 4(b). In comparison, the double-gap structures exhibited a ~320x higher emission, notably with a lower density of particles. Interestingly, the anti-Stokes photoluminescence was also enhanced by the sub-5-nm double-gap structures. These results are in agreement with previous reports on the importance of the spectral alignment between the incident pump wavelength with the plasmon resonance of the nanostructures.23, 30 The two-photon process for SHG emission was further confirmed by the characteristic slope of 2 in the log-log intensity relationship between the pump laser and the SHG emission, shown in Fig. 4(c). To obtain a more representative estimate of the SHG enhancement, we corrected for the areal coverage of Au particles in the two samples, where the detailed definition and calculation


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of this areal ratio is shown in Fig. S6. AuNPs in double-gap structure cover only ~7.6% of the area covered by close-packed AuNPs on a flat gold film shown in Fig. S8(a). Correcting for the areal coverage, the effective enhancement factor of SHG emission from the double-gap structure is ~4200 times larger than that from “AuNPs on flat gold film”. At the intensity level of the pump laser and SHG emission,34 we calculated the SHG conversion efficiency to be ~1.8x10-5 %, and the effective second-order susceptibility χ(2) ~6.1 pm/V, which is comparable to typical nonlinear crystals, e.g. potassium dihydrogen phosphate (KDP) and aluminum nitride have χ(2) typical values of ~0.86 pm/V and 14.8 pm/V respectively, at a pump laser wavelength of 1064 nm.38 Although conventional nonlinear crystals are able to achieve SHG conversion efficiencies close to unity, it requires a large interaction volume ≫ λ0

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and specific illumination

configuration to satisfy the phase-matching condition. The enhanced SHG emission is ascribed to the enhanced optical interaction between the fundamental pump laser and the second-order surface nonlinear susceptibility. The dominant component of second-order surface nonlinear susceptibility in noble metals is χnnn, where “n” refers to the normal direction to the metal film surface.31,

46, 47

As a result, the polarization

density at the generated second harmonic, Pn(2ω) is given by χnnn En(ω)2, integrated over the surface of the plasmonic cavity.33 Therefore, SHG emission can be increased by increasing En(ω) and/or the total area over which the integration is performed. In the double gap structures, the introduction of nanoparticles into the trenches achieves an increase in En(ω) due to the reduction in gap size to ~2 nm, and good spectral alignment with the 878-nm pump laser. In addition, it approximately doubles the surface area within the volume of high electric field enhancement. This multi-fold effect is what leads to a strong SHG emission. We further compared flat gold


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films with and without oleylamine ligands and observed no SHG emissions from either sample, suggesting that the SHG contribution from oleylamine ligands was minimal. In summary, we have presented a route to produce a large number of samples from a single lithographically-defined template that creates sub-5-nm double gap structures in a regular and controllably patterned array. The directed self-assembly of AuNPs onto template-stripped gold substrates provide a versatile approach for engineering the SHG enhancement in nanogaps between metal structures. The spectral alignment of the resonant mode with the pump laser, strong field enhancement in sub-5-nm gaps, and the doubling of available metal surfaces at the location of enhanced fields act in concert to provide the 3 orders of magnitude enhancement observed. Further enhancement is to be expected by additionally aligning higher-order resonances of the structures to the SHG emission wavelength, as recently demonstrated.30 Due to the regular nature of the patterns, and the availability of different types of nanoparticles, we believe this approach could lead to the directional emission of SHG, electro-optic modulators,48 optical switches,7 and even lasing in the extreme ultraviolet (EUV) regime.6

ASSOCIATED CONTENT Supporting Information Method for the experiments and simulations; Reusability test of silicon templates for the template stripping process; FDTD simulation of TS gold substrate; Characterization of the TS substrate with AuNPs deposited by using directed self-assembly; AuNPs on flat gold film by directed self-assembly; Experimental setup for measuring SHG emission; SHG emissions from flat Au film; FDTD Simulation on the influence of trench depth; TEM image of the template stripped substrate with deposited AuNP.


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AUTHOR INFORMATION Corresponding Authors *E-mail: [email protected], [email protected]. Notes The authors declare no competing financial interests.

ACKNOWLEDGEMENTS We would like to acknowledge the funding support from Agency for Science, Technology and Research (A*STAR) Young Investigatorship (grant number 0926030138), SERC (grant number 092154099), and National Research Foundation grant award No. NRF-CRP 8-2011-07. J. L. and Z. H. would like to acknowledge the funding support from Ministry of Education Singapore on the Academic Research Fund (AcRF)-Tier 2 with a grant number MOE2014-T2-1-010. In addition, Z. D. would like to acknowledge template-stripping pointers from W. Du and C. A. Nijhuis from Department of Chemistry, National University of Singapore. H.-S. C. would like to acknowledge the support of the A*STAR Computational Resource Centre through the use of its high performance computing facilities.


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For Table of Contents Use Only

Title: Second-Harmonic Generation from Sub-5-nm Gaps by Directed Self-Assembly of Nanoparticles onto Template-Stripped Gold Substrates Authors: Zhaogang Dong, Mohamed Asbahi, Jian Lin, Di Zhu, Ying Min Wang, Kedar Hippalgaonkar, Hong-Son Chu, Wei Peng Goh, Fuke Wang, Zhiwei Huang, and Joel K. W. Yang

The table of contents graphic depicts the sample schematic of sub-5-nm double gap nanostructures, which were fabricated by combining top-down lithography and bottom-up selfassembly. Lodging chemically-synthesized nanoparticles into the trenches provides the following advantages to the enhanced second-harmonic generation (SHG). First, the gap size of the trench has been reduced to create two ~2 nm-wide gaps, which were defined by the chemical ligand length deterministically. Second, it significantly redshifts the resonance of the underlying structure to align with the pump laser. Third, it increases the surface area of metal within regions of high electric fields to exploit the χ(2) nonlinear susceptibility of Au surfaces. Such a precise engineering of surfaces at the regions of strong field enhancement will give rise to an enhanced SHG emission by 4200-fold in the gap.


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Second-Harmonic Generation from Sub-5 nm ... - ACS Publications

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