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Rapid Focused Ion Beam Milling Based Fabrication of Plasmonic Nanoparticles and Assemblies via “Sketch and Peel” Strategy Yiqin Chen, Kaixi Bi, Qian-Jin Wang, Mengjie Zheng, Qing Liu, Yunxin Han, Junbo Yang, Shengli Chang, Guanhua Zhang, and Huigao Duan ACS Nano, Just Accepted Manuscript • DOI: 10.1021/acsnano.6b06290 • Publication Date (Web): 21 Nov 2016 Downloaded from http://pubs.acs.org on November 22, 2016

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Rapid Focused Ion Beam Milling Based Fabrication of Plasmonic Nanoparticles and Assemblies via “Sketch and Peel” Strategy Yiqin Chen1#, Kaixi Bi1,2#, Qianjin Wang3, Mengjie Zheng1, Qing Liu1, Yunxin Han2, Junbo Yang2, Shengli Chang2, Guanhua Zhang4 and Huigao Duan4,5* 1 School of Physics and Electronics, State Key Laboratory of Advanced Design and Manufacturing for Vehicle Body, Hunan University, Changsha 410082, P. R. China 2 College of Science, National University of Defense Technology, Changsha 410073, P. R. China 3 College of Engineering and Applied Sciences, Nanjing University, Nanjing 210093, P. R. China 4 College of Mechanical and Vehicle Engineering, Hunan University, Changsha 410082, P. R. China 5 State Key Laboratory for Chemo/Biosensing and Chemometrics, Hunan University, Changsha 410082, P. R. China #These

authors contribute equally to this work.

*Corresponding author: [email protected].

ABSTRACT Focused ion beam milling (FIB-milling) is a versatile maskless and resistless patterning technique and has been widely used for the fabrication of

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inverse plasmonic structures such as nanoholes and nanoslits for various applications. However, due to its subtractive milling nature, it is an impractical method to fabricate isolated plasmonic nanoparticles and assemblies which are more commonly adopted in applications. In this work, we propose and demonstrate an approach to reliably and rapidly define plasmonic nanoparticles and their assemblies using FIB milling via a simple “Sketch and Peel” strategy. Systematic experimental investigations and mechanism studies reveal that the high reliability of this fabrication approach is enabled by a conformally formed sidewall coating due to the ion-milling-induced re-deposition. Particularly, we demonstrated that this strategy is also applicable to the state-of-the-art helium ion beam milling technology, with which high-fidelity plasmonic dimers with tiny gaps could be directly and rapidly prototyped. Because the proposed approach enables rapid and reliable patterning of arbitrary plasmonic nanostructures that are not feasible to fabricate via conventional FIB milling process, our work provides the FIB milling technology an additional nanopatterning capability and thus could greatly increase its utilization popularity for fundamental researches and device prototyping. KEYWORDS: nanofabrication, FIB milling, helium ion beam, plasmonics, Sketch and Peel.


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Reliable top-down fabrication of plasmonic nanostructures with controlled size, shape, arrangement and position is essential to the broad plasmonic applications in sensing,1,

2

integrated optical circuits and

optoelectronic devices.3-7 Due to the nanometric dimension and precision requirements,

the fabrication

of

plasmonic nanostructures

and

the

prototyping of plasmonic devices highly rely on high-resolution maskless lithographic techniques, mainly including electron-beam lithography (EBL) and focused ion beam (FIB, e.g. Ga+ and He+) milling.8-12 Among these two methods, EBL is more commonly used in practice because it has the flexibility to produce arbitrary planar plasmonic nanostructures including both particles and inverse structures by combining subsequent metallic lift-off or etching process.13, 14 Compared to EBL-based approach, FIB milling can directly create plasmonic nanostructures starting from a metallic layer and thus actually has several advantages for patterning metallic structures. First, it is an all-dry and resistless one-step fabrication process without involving any spin-coating, development and extra pattern transfer steps. Second, it can work on insulating substrates without charging effect that EBL usually suffers from. Considering most optical substrates such as quartz, CaF2 and MgF2 are insulating, this advantage is of particularly importance in practical applications. Third, with the current state-of-the-art focused helium ion beam, FIB milling has the capability to fabricate sub-10-nm features in plasmonic metallic structures that are not feasible or extremely difficult to fabricate with


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EBL.9, 15, 16 With the fast improved performance of FIB technology based on new ion sources including He+, Be+, Si+ and Ne+,17-19 it is believed that FIB milling would be more and more accessible and popular for nanopatterning applications. Though FIB milling possesses many merits and is becoming highly accessible in common laboratories, unfortunately however, it is currently still far less used in patterning plasmonic structures compared to EBL-based process. The main reason is that FIB milling is intrinsically a subtractive fabrication technique which is only favorable for creating inverse structures such as nanoholes,20 nanoslits and nanotrenches.21,

22

To directly create

isolated plasmonic nanostructures which are more commonly adopted in practical applications due to their more tunable optical properties, removal of the majority of the starting metallic layer is required,3,

23

leading to

unacceptable long fabrication time in practice, as indicated by Figure 1a. Such a subtraction-fabrication nature significantly limits the functionality of FIB milling for broader applications because it makes FIB milling unfeasible to create isolated plasmonic particles. In this work, we propose and demonstrate a simple but highly robust approach to address the above challenge of FIB milling technology via the “Sketch and Peel” concept that has been recently developed in EBL.24 With this simple approach, arbitrary plasmonic nanoparticles and their arrays, which are not feasible to fabricate with conventional FIB milling, can be


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rapidly and reliably defined. Particularly, we demonstrate that the concept is also applicable to current state-of-the-art helium ion beam technology, enabling ultrafast prototyping of high-resolution plasmonic assemblies with tiny gaps. By providing FIB milling an additional capability, our work represents a significant progress for FIB direct patterning technology and to some degree revolutionizes the functionality of FIB for various applications.

RESULTS AND DISCUSSION Basic Concept. The basic concept of the “Sketch and Peel” fabrication strategy to obtain plasmonic nanoparticles is schematically shown in Figure 1b. In a typical process, starting from an initially evaporated metallic (e.g., Au) film without any adhesion layer (i) on a Si or SiO2 substrate, we first “sketch” the outline of the target particle using FIB milling to obtain a circular trench (ii & iii) which completely separates the particle and the rest part of the film. Afterwards, a transparent adhesive tape is conformally adhered onto the metallic surface (iv) and then peeled off from the substrate surface, resulting in isolated particles on the surface (v). The key of this fabrication approach is that the rest part of metallic film can be selectively peeled off while the isolated particle can well remain during the peeling process. The mechanism of this phenomenon will be discussed in Mechanism Analysis section. Note that the weak adhesion between the metallic film and the substrate is the prerequisite for this process. Compared to the conventional FIB milling


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method which requires point-by-point removal of almost the whole metallic film to obtain a particle (Figure 1a) and thus is actually impractical for real applications,3, 25 this “Sketch and Peel” strategy can fabricate isolated particles by only milling a very small part (i.e. the outline) of the metallic layer and thus the

fabrication

efficiency

is

dramatically

improved

with

mitigated

ion-bombardment-induced substrate damage.

Figure 1. Comparison of “Sketch and Peel” based FIB milling process with conventional FIB milling process to fabricate a plasmonic nanoparticle. (a) Conventional process, in which


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almost the whole starting film should be milled to obtain an isolated particle. (b) “Sketch and Peel” approach, in which only a narrow trench is required to mill to obtain a particle by selectively peeling off the majority of the metal film.

Concept Demonstration. As a proof of concept, Figure 2 demonstrates the fabrication of a gold disk array by Ga+-based FIB-milling via the “Sketch and Peel” approach. The diameter of the designed disks was 800 nm. Starting from a gold film with a thickness 30 nm on silicon substrate, Figure 2a-2d show the scenarios of the sample after FIB milling (Figure 2a), after pasting a tape (Scotch-810, tack value: 2.41 N/cm) that was commonly used for dry pattern transfer in some lithographic process such as nanosphere lithography and edge lithography (Figure 2b),26, 27 during the peeling process (Figure 2c) and after complete peeling (Figure 2d), respectively. The scanning electron microscopy (SEM) image of the as-FIB-milled structures is given by Figure 2e, showing that the target disks were isolated from the entire gold film by the well-defined outlines. After the peeling off process, the entire gold film was cleanly removed from the silicon substrate and transferred onto the adhesive tape (Figure 2d), and only the isolated gold disks remained, as depicted by Figure 2f. That demonstrated the reliable fabrication of plasmonic particles using the “Sketch and Peel” approach. Note that complete isolation of target particles from the entire gold film without any connections is necessary to enable reliable realization of this approach. Thus, the depth of milled outline (D) has to be larger than that of the starting gold film (T). If D is smaller than

T or there are any bridges connecting the inside disks with the rest gold film,


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the disks would have a high probability to be stripped away along with the entire gold film during the peeling process, as indicated by Figure S1 in the Supplementary Information.

Figure 2 ． Experimental demonstration of the fabrication process flow for obtaining

plasmonic nanoparticles using FIB milling via the “Sketch and Peel” strategy. (a-d) Four photographs showing the samples during the fabrication process: after “sketching” the outlines with FIB milling on a 30-nm-thick gold film (a), after pasting a transparent scotch tape (b), peeling off the tape (c) and after complete peeling off (d). (e, f) SEM images showing the fabricated structures before (e) and after (f) the peeling step, respectively. The periodic gold disks were defined on a 1 cm × 1 cm silicon die with a diameter of 800 nm and a pitch of 2.5 µm. Scale bar: 2 µm.

Arbitrary Geometries. Such a simple approach demonstrated extremely high fidelity in patterning plasmonic particles with arbitrary geometries, as shown in Figure 3, in which all structures were obtained with the Ga+-based FIB milling process via the “Sketch and Peel” approach. Figure 3a-3c


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demonstrate the fabrication of three types of plasmonic structures with different shapes including nanosquares, nanotriangles and nanorods. Together with the nanodisks, these nanostructures represent the most commonly used plasmonic building blocks due to their tunable optical properties. From the SEM images, we can see that the designed structural geometries such as the sharp corners of the nanotriangles were well preserved. To demonstrate the reliability of this peeling approach is not relevant to the peeling direction and the geometrical symmetry of the structures, we designed and fabricated “L”-shaped chiral plasmonic structures which are sensitive to circularly polarized light,28 as shown by Figure 3d. All of the designed structures were well defined, indicating the isolated structures have high stability during the peeling process. It should be noted that over-milling was always conducted to ensure the complete isolation of the designed plasmonic nanoparticles from the gold film, which resulted in obvious trenches around the plasmonic particles. Though such self-aligned trenches are byproducts in the over-milling process, they may actually benefit many plasmonic applications in surface enhanced spectroscopy and sensing by selectively filling the trenches with extra active media. From the figures, we also notice that the edge roughness of the structures was a bit high. There were two possible reasons leading to the high edge roughness. On the one hand, as-deposited gold film was polycrystalline and the film had many grain boundaries. The milling efficiency was supposed to depend on the crystal


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facets during the milling process, resulting in edge roughness of the structures.29 On the other hand, the re-deposition effect during the milling process may also cause increased edge roughness. To mitigate the edge roughness, a feasible choice is to coat a protective layer (e.g. SiO2 or spin-on glass) on the top of gold film, as discussed by other researchers.30, 31 To further confirm the versatility of this “Sketch and Peel” approach, we also demonstrated that particles with multiscale geometry were able to be fabricated with one peeling process. Figure 3e and Figure S2 show a series of fabricated gold disks with varied diameter from 100 nm to 120 µm, indicating the generality of this approach for fabricating structures with different scales. In addition, with this “Sketch and Peel” strategy, FIB milling exhibits the excellent capability in prototyping components for optoelectronic and electronic devices as well. As an example, Figure S2f-2g demonstrate the rapid and direct fabrication of nanogap electrodes. Particularly, by combining the direct milling advantage of FIB with the current “Sketch and Peel” approach, functional structures that are not feasible or extremely challenging to fabricate by any other methods can be readily fabricated. As an example, Figure 3f shows an array of pads in each of which a nanohole array was fabricated. Such a multiscale plasmonic array has demonstrated intriguing optical properties for sensing and lasing applications,32, 33 but its direct maskless fabrication remains a challenge for long time because both common EBL and FIB has difficulties to realize structures including densely-packed complementary


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elements. One possible limit of this process is that it would be difficult to directly obtain closely packed periodic metallic structures. As demonstrated in Figure S3, dense structures with a too narrow space (< 100 nm) were not realized because the narrow gold nanostrips tended to break due to the reduced mechanical strength to support the complete stripping. To avoid this limit, one feasible approach is to design shared boundaries for milling to replace the space between the final structures, as demonstrated in our previous work to obtain densely-packed plasmonic structures.24, 34

Figure 3. Plasmonic particles with arbitrary geometries fabricated by Ga+-FIB-milling based

on the “Sketch and Peel” strategy. (a-c) Commonly-used periodic plasmonic structures with square (a), triangle (b) and rod (c) shape. The pitch of the structures was 800 nm. The average edge length of square and triangle was 273 nm and 185 nm, respectively. The length and width of each rod were 320 nm and 90 nm, respectively. (d) “L”-shaped chiral structures with a pitch of 2 µm. (e) Gold disks with varied diameter from 100 nm to 1 µm. (f) Arrayed


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circular gold pads embedded with nanoholes. The diameter of each gold pad was 4.5 µm. The diameter of the embedded nanoholes was set to be 300 nm. Note that the elliptical geometry was induced by the astigmatism of the ion beam during our fabrication process. The starting gold film was 30 nm. All structures were defined on silicon substrate with a 285-nm SiO2 layer. The black edges in each SEM image were ascribed to the relative lower intensity of secondary-electron signal in the over-milled trenches during the imaging. All scale bars: 1 µm.

Mechanism Analysis. It is of great interest that a simple “Sketch and Peel” approach can significantly extend the patterning capability of FIB milling technology. While the complete stripping of a continuous gold film or gold nanostructures on a substrate with weak adhesion is understandable and has been widely used for various applications,35-38 the key to understand the mechanism of the current fabrication approach is to explain why FIB-milled isolated gold particles are highly stable against the peeling. It is well-known that re-deposition is a common and unavoidable phenomenon during the ion milling or sputtering process.39 Such re-deposition phenomenon becomes more pronounced in milling deep trenches, which can even result in a conformal coating of substrate materials on the sidewall of the final structures due to the excessive milling of the substrate.40 In our system, the gold film was deposited on a silicon substrate. As mentioned in Figure 1, during the outline “Sketch” process, over milling is required to completely isolate the inside particles from the entire gold film. Over milling to the substrate is supposed to lead to the sputtering of silicon atoms, and some of the sputtered silicon atoms will re-deposit onto the sidewalls of the formed trenches, as indicated by Figure 4a-i. With the


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increased over milling of the substrate by increasing the ion-milling dose, thicker silicon coating would form (Figure 4a-ii) and completely wrap the isolated gold particles (Figure 4a-iii). Such a coating, if it really exists, can effectively protect the particles against the peeling force. On the one hand, the conformal coating that tightly wraps the inside particles can improve the mechanical stability of inside particles. On the other hand, the coating can act as a perfect fence to prohibit the formation of breakages or notches at the gold-substrate interface. Without breakages and notches, the peeling force required to strip the gold film would be extremely high, as discussed by many literatures.41, 42

Figure 4. Mechanism analysis of the “Sketch and Peel” FIB milling process. (a) The 2D and


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3D schematics showing the formation of sidewall coating due to ion-milling-induced re-deposition. (b) The topographic atomic-force-microscopy image of arrayed patterns after removing of the fabricated gold disks with 500-nm diameter and 30-nm thickness, demonstrating the existence of sidewall coatings. (c, d) Tilted-view SEM micrograph of an as-fabricated gold disk (a) and its morphology after stripping the inside gold disk (d), further demonstrating the formed sidewall coating. The thickness of the defined gold structures was 100 nm. (e) Series of intuitive 3D schematics showing the detailed morphological evolution during the peeling process to define a gold disk. Scale bar: 100 nm.

To verify above hypothesis about the formation of a protective sidewall coating, we removed the final 500-nm-diameter gold disk with 30-nm thickness on the sample and then measured the morphology of the resultant surface via atomic force microscopy (AFM). Figure 4b shows the three-dimensional AFM image, in which the re-deposited sidewall coating (indicated by the circularly protruded rings) can be clearly seen. The average height was measured to be ~ 13 nm, which was sufficiently high to wrap and stabilize the inside gold particles. To further demonstrate the formation of sidewall coating during the FIB milling, we executed the “Sketch and Peel” process on 100-nm-thick gold film which was supposed to lead to thicker and higher sidewall coating. Figure 4c shows the SEM image of a prepared gold disk after peeling, from which the re-deposited sidewall coating at the outer sidewall of the trench can be seen, as highlighted by the arrow. Such a re-deposited sidewall coating can be more clearly seen at the inner sidewall of the trench after removing the gold disk, as shown by the SEM image in Figure 4d.


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As previously discussed, with the protection of the sidewall coating, the inside gold particles would be extremely stable against the peeling force. In this case, the evolution of the sample during the peeling process is illustrated by Figure 4e, which intuitively demonstrates the selective stripping of the exterior gold layer with the help of continuously propagated interfacial breakage and also demonstrates the stability of the inside particle that was protected by the sidewall coating. The final gold particles were also demonstrated to be highly stable against additional peeling by adhesive tape, as revealed by Figure S4a-4c. No loss or displacement of the particles was found during the 2nd and even the 3rd peeling. In contrast, upon the removal the sidewall coating via wet chemical etching, these defined gold particles can be easily stripped or even washed away in solution, as shown by Figure S4d, indicating the key role of the sidewall coating in protecting the inside particles. Application to Helium Ion Beam Milling. Recent developments show that focused Helium Ion Beam (He+-FIB) has superior capability for direct patterning of plasmonic nanostructures down to sub-10-nm scale and is becoming a popular and high-end fabrication technique in fundamental researches.17,

43

Unfortunately, He+-FIB suffers from the extremely low

throughput due to the decreased ion-beam current and milling yield. Here, we demonstrate that the “Sketch and Peel” strategy is also applicable to He+-FIB, which makes He+-FIB an excellent technique for direct and reliable


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fabrication plasmonic nanostructures with tiny gaps and smooth edges. As an example, plasmonic dimers on insulated quartz substrate were defined for the demonstration.


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Figure 5. Demonstration of “Sketch and Peel” strategy using focused helium ion beam milling for patterning isolated plasmonic dimers. (a, b) SEM images of an array of plasmonic dimers fabricated by He+-FIB milling before (a) and after (b) peeling. The fabrication started from a perfect 30-nm-thick gold film on 1-mm-thick quartz substrate. The pitch of array was


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set to be 500 nm, and the average diameter of resultant disks was ~ 85 nm. (c) SEM image of an array of 5×5 plasmonic dimers with a pitch of 3 µm for single-particle dark-field scattering measurement. (d) The dark-field image of the corresponding plasmonic dimers in panel (c). (e) Enlarged SEM image of a selected plasmonic dimer in panel (c). (f) Polarization-resolved dark-field scattering spectra obtained from the selected plasmonic dimer. Scale bar: (a, b) 200 nm; (c, d) 3 µm; (f) 100 nm.

Similar with the fabrication concept for a single gold nanoparticle, the outlines of the plasmonic dimers were first “sketched” and milled on a gold film. The overlapped part of outlines was replaced by a shared boundary. Due to the extremely high resolution patterning capability, the structures could be fabricated with much smaller size and smooth edges. Figure 5a shows an array of plasmonic dimers before peeling of exterior gold layer. The milled trenches had an average width of ~ 20 nm and the diameter of the final nanodisks was ~ 85 nm. After peeling by a Scotch tape, the entire exterior gold layer was completely removed, resulting in well-defined plasmonic dimers with a gap size of ~ 20 nm on the quartz substrate. Such kind of plasmonic dimers with uniform size and gap on insulating substrate, which has been demonstrated to possess promising potential in various applications, is not feasible to fabricate by any existing techniques even with state-of-the-art EBL-based processes, indicating the advantage of this “Sketch and Peel”-based He+-FIB process for nanopatterning. To evaluate the optical properties of the fabricated structures, we prepared a 5 × 5 array of sparsely-distributed plasmonic dimers with a pitch of 3 µm for single-particle dark scattering measurements, as depicted in


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Figure 5c. To obtain smaller gaps, the dose of the He+-FIB milling was further optimized. By doing a quick estimation, the patterning efficiency of such an array of sparse plasmonic structures was more than 2500 times higher compared to conventional He+-FIB milling process with the same patterning parameters. Figure 5d shows the corresponding dark-field image of the fabricated array of plasmonic dimers. The uniform scattering intensity and identical scattering colors from the structures indicate that the plasmonic dimers fabricated by He+-FIB had uniform dimension due to its high-resolution patterning capability. Figure 5e gives the enlarged SEM image of a selected dimer, confirming the well-defined geometry and tiny gap of the structures. Further single-particle dark-field scattering measurement reveals that this particle with a 15-nm gap had strong polarization-dependent scattering due to the plasmon coupling. As shown in Figure 5f, a 43-nm redshift was observed by comparing the x-polarized and y-polarized scattering signal. With the high-fidelity of geometry and dimension, the measured spectra of the particle almost quantitatively agree with the numerically calculated spectra in terms of both spectral positions at two polarizations and the

lineshape

of

the

resonance

peaks

(Figure

S5).

Further

polarization-resolved Raman measurements (Figure S6) indicate that such gold

disk

dimers

could

serve

as

substrates

surface-enhanced Raman spectroscopy (SERS).


for

high-performance

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CONCLUSIONS We have proposed and demonstrated a “Sketch and Peel” process that enables FIB milling applicable for direct and rapid fabrication of plasmonic nanoparticles and assemblies. Systematic mechanism study reveals that the re-deposited sidewall coating plays a key role in stabilizing the target plasmonic particles and assemblies during the selective removal of the unwanted exterior metallic layer. Considering that FIB milling technology holds many advantages for direct patterning of plasmonic structures but is only available for fabricating inverse structures in the past, the current work significantly extends the capability of FIB milling for broader applications in which both particle and inverse plasmonic structures can be efficiently fabricated by FIB milling. Particularly, we demonstrated that this “Sketch and Peel” strategy is also applicable to state-of-the-art helium ion-beam milling technology, with which densely-spaced plasmonic nanoparticles such as dimers can be fabricated with high fidelity and resolution. Compared to commonly used EBL, this FIB-based patterning concept provides an extra functionality to fabricate plasmonic structure in a resistless and solvent-free way with enhanced geometric control and resolution. Such all-dry process is demanded for patterning highly reactive magnesium (Mg) structures for smart plasmonics.44 Along with the increasing popularity of FIB milling technology in the research and industrial labs, we believe that the “Sketch and Peel” process, which to some degree revolutionizes the current capability of


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FIB milling, will find a variety of applications in nanophotonics, nanoplasmonics and metasurfaces. In addition, the process can also be utilized for patterning large metallic (e.g. Au, Ag, Pt) structures with tiny gaps for nanoelectronic applications. Though the current peeling process only works for metals with weak adhesion, we may extend it to the fabrication of other metallic structures with strong film-substrate adhesion (e.g. Pd and Al) by modifying the surface energy of the substrates through introducing an anti-adhesion coating or underlying pre-deposited gold buffer layer.45, 46 With the anti-adhesion buffer layer, this fabrication concept may even be applicable to indium tin oxide (ITO) and alloy metals that are usually deposited by sputtering process.

METHODS (1) Gold deposition All of the starting gold films on substrates for FIB milling were deposited by an electron-beam evaporator system (Lab-Line, Kurt J. Lesker Company). The evaporated gold has an extreme high purity of 99.999%. Three different types of substrates including prime silicon wafer, silicon wafer with a top 285-nm SiO2 layer and transparent quartz were used for gold deposition. The working pressure was kept at the level of 1 × 10-5 Torr during the evaporation. The gold film thickness was monitored by a quartz-crystal microbalance (QCM). All gold films were deposited at an evaporation rate of 1 Å/s. No


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adhesion layer was used for all gold films to enable their weak adhesion with the substrate for complete peeling.

(2) Focused Ga+ ion beam milling The resultant gold nanoparticles were prepared using a focused ion beam microscopic system (Ga+ ion, Helios NanoLab 600i Dual Beam, FEI) equipped with a digital pattern generator. All structures were fabricated by carefully optimizing the patterning parameters. In the whole fabrication process, an ion beam with 16 nm nominal spot size at 30 kV and 10 pA beam current was used for the patterning. A 2-nm isometric step size was used, which enabled a 93.75% beam overlap. A dose matrix was used to optimize the dosage for obtaining the best possible plasmonic structures.

(3) Helium ion beam milling Focused helium ion beam milling (He+-FIB) was carried out using a Zeiss ORION NanoFab system, operated at 25 kV acceleration voltage with a beam current of 4.5 pA. Beam control for He+-FIB was realized using a modified pattern generator (Fibics NPVE v4.2). The pattern generator allowed computer-based pattern design with dosage variations. Test writing was performed on quartz substrate. The circular ring in layout was draw by the torus elements having a given width of 0.25 nm. Initial dose exposures indicated that a dose of 224 nC/µm2 was an optimal initial setting for experiments.


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(4) Stripping gold film For completely stripping of the gold films, we cut a piece of Scotch tape (Scotch-810, 3M) with the size larger than the sample. Subsequently, the adhesive surface of the Scotch tape was conformally and tightly pasted onto the gold film. Finally, we used a tweezer to clamp one side of Scotch tape and slowly peeled the gold film off.

(5) Etching A mixed aqueous solution of KI/I2/H2O (weight ratio = 4:1:10) was used as the etchant for gold etching to investigate the sidewall coating. The duration time for gold etching was 3 minutes. The SiO2 etching was conducted using HF aqueous solution (1% weight ratio) for 2 min. Following the etching process, DI water was used to wash the residue etchant solution. Finally, the sample was dried with a steady nitrogen gas flow.

(6) Scanning electron microscopy The morphology of gold structures was imaged by a field-emission scanning electron microscopic system (SIGMA-HD, Carl Zeiss). The imaging on conductive substrate was carried out with an accelerating voltage of 10 kV and working distance of 7 mm. To obtain better SEM image for the structures on insulating quartz substrate, 1 kV accelerating voltage was used to mitigate the charging effect and the working distance was set 3 mm.


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(7) Dark-field scattering measurements The dark-field imaging was executed on a Carl-Zeiss AXIO-10 microscopy equipped with a colorful CCD camera. The halogen lamp (100 W, 3150 K) was used as the light source and scattering light was collected by an objective (Zeiss Epiplan, 50 ×, N.A. = 0.75). The spectra measurement was performed on a WITec confocal microscopic system equipped with a high-resolution piezoelectric stage. A light source (Halogen, 3150K) with 100 W power was illuminated on the structures by a dark-field objective. During the whole measurement process, we kept the light source same intensity. To obtain the maximal scattering signal, the spatial position of the measured structure was optimized to the focal point of the incident light by carefully adjusting the piezoelectric stage. The scattering light was collected by an objective lens (Zeiss Epiplan, 50 ×, N.A. = 0.75) and detected by an electric-cooled CCD (Andor, DV 401A-BV-352). The scattering light was polarized by an analyzer (200 nm – 4 µm). Scattering signals of the particle and substrate were accumulated with the integration time of 10 s for three times. The signal of light source and background noise was integrated with the time of 0.02 s for 50 times The calculation of dark-field scattering spectrum was referred to the formula as follow: ூ

ିூ

‫ܫ‬௦௖௔௧ = ூ ೛ೌೝ೟೔೎೗೐ିூ ೞೠ್ , ೓ೌ೗೚೒೐೙

೏ೌೝೖ

where Iscat(λ) was the final scattering signal from a single gold nanostructure;


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Iparticle and Isub were the collected signals from the measured particle and its adjacent substrate, respectively; Ihalogen and Idark were the light source signal and background count of the measurement system, respectively.

ACKNOWLEDGEMENTS We gratefully acknowledge the financial support from the National Natural Science Foundation of China (Grant nos.11274107, 61204109 and 11574078), the Foundation for the authors of National Excellent Doctoral Dissertation of China (201318), and the Natural Science Foundation of Hunan Province (2015JJ1008, 2015RS4024). The authors thank Ms. Xuejiao Wang for AFM characterization.

Supporting Information Available: the stripping result in case that the outline was not completely milled, SEM images of gold disks with varied sizes, alignment mark array and electrode pair defined by “Sketch and Peel”-based FIB milling, the process window of spacing parameter in periodic structures fabricated by “Sketch and Peel” strategy, SEM images of the defined structures under different processes, polarized-dependent Raman measurement of gold disk dimer, the calculated dark-field scattering spectra and near-field distribution of a gold dimer at two different polarizations. This material is available free of charge via the Internet at http://pubs.acs.org.


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Table of Contents Graphic The following graph will be used for graphical Table of Contents: Before stripping

FIB Defining Outline

Pasting a Tap After stripping Stripping


Rapid Focused Ion Beam Milling Based ... - ACS Publications

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