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Electron-beam lithography of gold nanostructures for surface-enhanced Raman scattering

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2012 J. Micromech. Microeng. 22 125007

(http://iopscience.iop.org/0960-1317/22/12/125007)

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IOP PUBLISHING JOURNAL OF MICROMECHANICS AND MICROENGINEERING

J. Micromech. Microeng. 22 (2012) 125007 (9pp) doi:10.1088/0960-1317/22/12/125007

Electron-beam lithography of goldnanostructures for surface-enhancedRaman scatteringWeisheng Yue1, Zhihong Wang, Yang Yang, Longqing Chen, Ahad Syed,Kimchong Wong and Xianbin Wang

Advanced Nanofabrication and Imaging Core Lab, King Abdullah University of Science andTechnology, Thuwal 23955-6900, Saudi Arabia

E-mail: weisheng.yue@kaust.edu.sa

Received 17 September 2012Published 26 October 2012Online at stacks.iop.org/JMM/22/125007

AbstractThe fabrication of nanostructured substrates with precisely controlled geometries andarrangements plays an important role in studies of surface-enhanced Raman scattering(SERS). Here, we present two processes based on electron-beam lithography to fabricate goldnanostructures for SERS. One process involves making use of metal lift-off and the otherinvolves the use of the plasma etching. These two processes allow the successful fabrication ofgold nanostructures with various kinds of geometrical shapes and different periodicarrangements. 4-mercaptopyridine (4-MPy) and Rhodamine 6G (R6G) molecules are used toprobe SERS signals on the nanostructures. The SERS investigations on the nanostructuredsubstrates demonstrate that the gold nanostructured substrates have resulted in large SERSenhancement, which is highly dependent on the geometrical shapes and arrangements of thegold nanostructures.

(Some figures may appear in colour only in the online journal)

1. Introduction

Surface-enhanced Raman scattering (SERS) has triggered alot of research interest since its discovery in 1974 becauseof its suitability as an analytical tool for the ultrasensitivedetection of molecules [1, 2]. The SERS enhancementwith factors of as high as 1014 has been reported usingnoble metallic nanostructures as substrates [3]. Therefore,SERS is considered to be a promising method for thedetection of single molecules [4]. Localized surface plasmons(LSP) generated using metallic nanostructures have beenshown to provide a suitable basis for SERS measurements.The LSP results from electromagnetic oscillations at theinterface between a nanostructured metal and a dielectric,and it gives rise to an enhanced electric field in the areaof the metal. It is well known that the LSP resonance(LSPR) depends greatly on the size, shape and arrangementof nanostructures [5]. Therefore, the fabrication of SERS

1 Author to whom any correspondence should be addressed.

substrates with well-controlled surfaces plays an importantrole in SERS studies. A number of methods have beendeveloped to produce SERS substrates. These methodscan be classified as chemical methods and nanofabricationmethods. Chemical methods have an advantage of beingeasy to process and highly productive. The nanostructuresprepared by the chemical methods are normally aggregatesof nanoparticles, nanocubics and nanowires, and have aconsiderably high SERS enhancement. The enhancementfactors (EFs) of such chemically synthesized nanostructurescan be as much as 1010–1014 [2, 3, 6, 7]. However,the SERS effects of the substrates produced by chemicalmethods may be affected by incomplete reproducibility ofthe preparation process, missing homogeneity of the surfaces,or limited durability of the structures [8], and it is difficultto optimize the shape, dimensions and spacing of thenanostructures. When compared to nanostructures fabricatedwith chemical methods, the nanostructures fabricated with thenanofabrication methods have the advantage of being bothtunable and reproducible. With nanofabrication techniques,

0960-1317/12/125007+09$33.00 1 © 2012 IOP Publishing Ltd Printed in the UK & the USA

J. Micromech. Microeng. 22 (2012) 125007 W Yue et al

arbitrarily-shaped nanostructures with rational arrangementscan be fabricated. Electron-beam (e-beam) lithography,focused-ion-beam (FIB) lithography, interference lithography,nanosphere lithography and other nanofabrication methodshave been reported for the fabrication of nanostructures forSERS [9–11].

E-beam lithography is an ideal method for the fabricationof nanostructures. It is versatile and provides high resolutionand precise control over the geometry and separation ofnanostructures, guaranteeing a fabrication reproducibility andprecision down to the nanometer scale. With the e-beamlithography, it is possible to generate two-dimensional arraysof fields with different parameters on one substrate in a singleproduction cycle. This increases the usefulness of the e-beammethod in the fast development of optimum SERS substrates.In this work, we present two processes based on the e-beamlithography to a fabricated nanostructured SERS substrate.One process involves the use of lift-off and the other employsplasma etching. Several kinds of nanostructured gold surfacesare fabricated and the SERS effects on the substrates areinvestigated. At last, the SERS effects from the nanostructuresfabricated with the two methods were compared.

2. Experimental details

2.1. Electron-beam lithography

E-beam resists PMMA (polymethylmethacrylate, fromMicroChemical Corporation) or ma-N2400 (from MicroresistTechnology Gmbh) were spin coated to prepared siliconsubstrates. For the lift-off, the resist was spin coated on thecleaned surface of the silicon wafers. For the plasma etching,the e-beam resist was spin coated on the gold-coated siliconsubstrate. The silicon wafer was sputtered with a layer oftitanium (8 nm) followed by a layer of gold. The e-beamlithography work was carried out on a CRESTEC e-beamlithography system (CRESTEC CABLE 9500). The energyof the electron beam was 50 keV and the typical current was100 pA. After the e-beam lithography, the exposed PMMApattern was developed with a self-mixed PMMA developer,a mixture of isopropyl alcohol and deionized (DI) waterby a volume ratio of 7:3. The exposed ma-N2400 patternwas developed with developer a MaD525 (from MicroresistTechnology Gmbh).

2.2. Lift-off process

The PMMA resist was used for lift-off because PMMA hasgood solubility in acetone. After the e-beam exposure of thenanopatterns in the PMMA resist, the latent PMMA imageswere developed with an IPA-DI developer, which was amixture of isopropyl alcohol (IPA) and water in a volume ratioof IPA: DI = 7:3. The sample was then rinsed with DI water.Since the IPA is water soluble, it is cleared by rinsing. Thisenables good contact of the subsequently evaporated metallayer with the substrates. The substrate was first evaporatedwith a layer of 5 nm titanium and then a 30 nm gold layer usingan e-beam evaporator. The titanium layer serves as a bufferlayer to enhance the adhesion of the gold and silicon substrates.

The e-beam evaporator was used instead of the magnetronsputtering because the e-beam evaporation is more directionaland promotes the lift-off efficiency in the subsequent step. Thefinal step is to lift out the unwanted gold layer. The sample wasimmersed in acetone for 3 h and then agitated with ultrasonicbath. The unwanted gold layer was then removed with the resistlayer. The lift-off process is schematically shown in figure 1.

2.3. Plasma etching

The resist pattern works as a mask for the etching. For thePMMA resist, the e-beam exposed pattern was removed byetching, while for the ma-N2400, the exposed pattern wasprotected by the resist pattern. After the etching, the residualPMMA or ma-N2400 was removed with acetone followed byoxygen plasma cleaning. Therefore, PMMA is convenient forfabricating hole nanostructures, while ma-N2400 is convenientfor fabricating embossed nanostructures in the plasma etching.An etcher (Oxford PlasmaLab100) was used for etching. Theetching was carried out in two steps. The first step was theetching of the gold layer by introducing argon gas with flowof 30 sccm. The second step was then the cleaning of thetitanium layer with gases of argon (30 sccm), SF6 (10 sccm)and CF4 (20 sccm).

2.4. Raman measurements

A LabRAM HR800 (Horiba Jobin Yvon) micro-Raman systemwas used for Raman measurement. Raman spectra wereacquired in a backscattering configuration. A 50 × objectivewas used to focus light to the sample surface and collectscattered light from the substrate. The excitation source forthe measurement of the SERS spectra of the 4-MPy moleculeswas a 785 nm diode-pumped solid-state (DPSS) laser, and forthe measurement of the R6G SERS spectra, a 532 nm laserwas used. The acquisition time for the SERS spectra of the4-MPy was 30 s, and the time for the detection of the SERSspectra of the R6G molecules was 10 s.

Two widely used probe molecules in SERS studies, i.e.4-MPy and R6G, were used to probe SERS signals. The4-MPy has large Raman scattering cross-section at 785 nmexcitation [12] and the R6G has Raman scattering cross-section at a 532 nm excitation source [13]. The 4-MPysolutions with concentrations of 4 mM were prepared bydissolving the 4-MPy powder (from Sigma-Aldrich) into DIwater. The patterned substrate was cleaned with DI water,dried with a nitrogen gun and cleaned with oxygen plasma.After immersion for 3 h in a 4-MPy aqueous solution having aconcentration of 4 mM, the samples were taken out and rinsedwith DI water. On rinsing, the physically adsorbed moleculeson the surface of the nanostructures are removed, and onlychemically adsorbed molecules are left, which appear as onemonolayer of 4-MPy. R6G (from Sigma-Aldrich) was alsoused as a probe molecule for SERS detection. The R6Gsolution having a concentration of 1 × 10−3 M was preparedby dissolving R6G powders into DI water. The solution wasfurther diluted to 5 × 10−5 M and was used for SERSmeasurements. To ensure that the gold surface was free ofresiduals, the patterned substrate was further cleaned with

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Figure 1. Schematic of the two fabrication processes for the nanostructured SERS substrates.

DI water, dried with a nitrogen blower and cleaned withoxygen plasma before the measurements. The nanostructuredsubstrates were functionalized with R6G by immersing thesample into the R6G solution for 3 h. The substrates werethen rinsed with DI water and dried by a nitrogen blower. It isknown that R6G has an absorption band at ∼530 nm which isclose to the excitation wavelength (532 nm) [14]. Raman signalof R6G will be influenced by strong fluorescence backgroundwhen the R6G is excited by a 532 nm laser source. In this work,the concentration of the R6G solution is very low (10−5M) andfluorescence background is very low compared to the Ramanscattering intensities. Therefore, the background influence canbe neglected in this work.

3. Results and discussion

3.1. Results of nanofabrications

Both lift-off and plasma etching have enabled the successfulfabrication of the gold nanostructures. The metal lift-off isan easy and common method for fabricating fine metallicpatterns on substrates. During the lift-off process, the resistunder the film is removed with the solvent, taking the metalfilm with it, and leaving only the metal film on the substrate.Lift-off is an easy way to fabricate structures at micrometerscales, but it is of some difficulties to fabricate structures ofnanometer scales [15]. Higher requirements in resist material,resist thickness and adhesion properties of the resist to thesubstrate are raised when fabricating nanostructures by lift-off. The resist materials should have good adhesion to thesubstrate and be easily wet by the lift-off solvent. In addition,the resist film should be much thicker than the metallic layer(good thickness contrast) so that the solvent can seep into theresist. Keeping these requirements in mind, we used PMMAas the lift-off resist. PMMA is a high-resolution e-beam resist(∼10 nm) and has good adhesion to most substrate materials.In addition, PMMA has good solubility in acetone. We usedacetone as the lift-off solvent for PMMA.

Figure 2 shows scanning electron microscopy (SEM)pictures of nanostructures fabricated by the lift-off process.Figure 2(a) is parallel gold nanowires (nanograting). The linewidth is 60 nm and the period is 105 nm. Figure 2(b) is an

array of gold nanodiscs. The disc size is 110 nm, the arrayperiod is 250 nm and the thickness of the nanodisc is 30 nm.Figure 2(c) shows arrays of gold triangular dots. The edgelength of the triangle is 100 nm and the period of the array is500 nm. Figure 2(d) shows an array of nanophotonics split-ringresonators (SRRs). The size of a unit cell is around 160 nm andthe arm width is about 20 nm. This size is much smaller than thesize of the similar structures reported. The SRR structure is themost studied nanostructure in metamaterials, demonstratingboth negative permittivity and permeability [16]. The SRRsin the nanometer scale are relatively difficult to fabricate withthe lift-off process because the structure is more complex thandots and lines. The acetone cannot penetrate well into thecomplex structure. For such kinds of complex nanostructures,a longer ultrasonic agitation process with lower power helpsthe acetone to seep into the structure. Other kinds of goldnanostructures, like square dots and rhombus dots (images notshown) have also been successfully fabricated with the lift-off process. It is worth noting that the thickness differencebetween the gold layer and the PMM layer has a great influenceon the lift-off efficiency. The PMMA layer was 100 nm andthe gold layer was 30 nm, giving a thickness contrast (PMMAthickness/gold film thickness) greater than 3. The fabricationof gold nanostructures was not successful when we increasedthe gold thickness to 50 nm because the thickness contrast isnot sufficiently high. Fabrication of the gold nanostructureswith larger thickness is possible with a thicker PMMA layer.However, the increase of the PMMA thickness results inthe decrease of pattern resolution in the e-beam lithography.Therefore, the lift-off process has limitation in fabrication ofgold nanostructures with large thickness especially for finefeatures.

Some fence-like gold residuals on the edges of the goldnanostructures are shown in figure 2. This happens becausethe sidewalls of PMMA holes are partly coated with gold, andthere is no gap between the sidewall gold and the substrategold layer. These kinds of residuals on the edges may bereduced by a bi-layer resists technique [17]. An under-cutprofile is formed in the e-beam exposure of bi-layer, whichhelps to produce gold nanostructures with clean edges. Wealso tried to fabricate nanostructures having closed structures(such as gold nano-rings) with the lift-off process, but this try

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(a) (b)

(c ) (d )

Figure 2. SEM images of gold nanostructures fabricated e-beam lithography in combination with the lift-off process.

was not successful. One possible reason was that the acetonewas unable to seep inside the circles. Thus, the lift-off processhas limitations in the fabrication of complex nanostructuresand closed nanostructures.

Another process employs plasma etching. Compared tothe lift-off process, the plasma etching has the advantageof reproducibility, especially for the complex nanostructures.In the plasma etching, the sample surface is etched by abombardment of argon ions, and the pattern on the mask istransferred to the substrate [18]. Both positive resist PMMAand negative resist ma-N2400 were used as masks for theplasma etching. The e-beam exposed pattern is etched intothe gold film as holes for the PMMA mask, while the e-beamexposed pattern is left for the ma-N2400 resist mask. It isconvenient to etch nanoholes in the gold film using the PMMAmask. Figure 3 shows SEM images of gold nanostructuresfabricated by e-beam lithography combined with the plasmaetching. Figures 3(a) and (b) show the SEM images ofrhombus nanoholes and circular nanoholes, respectively. Thethickness of the gold film is 50 nm, the edge length of therhombus nanohole is 100 nm and the diameter of the circularhole is 200 nm. It is clear that nanoholes can be well fabricatedusing the plasma etching method. In contrast, it is very difficultto fabricate gold nanoholes using the lift-off methods. For thefabrication of gold nanoholes with lift-off, a negative resist isrequired. The solvent cannot penetrate well into the holes toenable the peel-off of the resist.

A negative resist is convenient for fabricating embossedgold nanostructures using plasma etching. Figures 3(c) and (d)show the SEM images of gold nanogrids and SRRs fabricated

by plasma etching through ma-N2400 resist. Compared to thenanostructures fabricated with the lift-off process, the edgesof the nanostructures fabricated with plasma etching are freeof gold residuals. Because the resistance of resist masks to thebombardment of the argon ions is no as good as that of metals,a thick resist layer is necessary when fabricating thicker goldnanostructures. However, the increase of the resist layer willresult in the decrease of the feature size of the nanostructures.It is therefore difficult to achieve ultra-fine patterns (say lessthan 50 nm) with larger thickness (e.g., >50 nm) using plasmaetching.

Removal of post-etching residuals is a problem becausea polymer-like photo-resist gets cross-linked during plasmaetching [19]. To clean the residuals on the surface, thesamples were first soaked in acetone for 1 h, cleaned withIPA and DI water, and then cleaned with oxygen plasmausing a plasma cleaner (NanoPlas). After the oxygen plasmatreatment, the sample was cleaned with acetone, IPA andDI water again. After the cleaning procedure, the samplesurfaces were checked with electron-induced x-ray emission(EDX). The sample surfaces covered by both the positiveresist mask (PMMA) and negative resist mask (ma N2400)in the plasma etching were checked. Figure 4(a) shows EDXspectrum on the surface of gold nanoholes fabricated byetching of PMMA mask. Figure 4(b) shows EDX spectrumon the surface of the gold SRR structures that were fabricatedby etching of ma-N2400 resist mask. The red squares inthe SEM images indicate the regions used for acquiring thespectra. Only the elements Si and Au are seen in the EDXspectra. The element Si is from the silicon substrates and the

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(a) (b)

(c ) (d )

Figure 3. SEM images of gold nanostructures fabricated with e-beam lithography combined with plasma etching.

Au is from the nanostructures. It is known that the majorelement components for PMMA (C5O2H8) are C and O. Thema-N2400 resist consists of phenolic resin (C6H5OH) andbisazide (N3CH2CH2(OCH2CH2)N3) and its major elementalcomponents are C, O and N [20]. However, the major elementC composing the resists was not detected by the EDX analysis.As a result, the resist residuals on the sample surfaces werecleared by the cleaning procedure.

It is generally accepted that SERS is caused by the stronglocalized electromagnetic fields that are associated with thelocalized surface plasma on the nanostructures. To observethe localized electrical fields on the gold nanostructures,3D-FDTD (three-dimensional finite-difference time-domain)simulations of electromagnetic fields of the nanostructureswere performed. The commercial software CST MicrowaveStudio [21] was used for the simulation. Figure 5 showsexamples of the electric-field distribution on the surface of thegold nanodisc (see figure 2(b)), gold triangular nanodots (seefigure 2(c)) and the gold SRR (see figure 3(d)). The electricfield was monitored at the frequency equivalent to 532 nm,which is the Raman excitation wavelength. Clearly, thedifferent gold nanostructures demonstrated different localizedfield distribution models under illumination. For the goldnanodisc, the highest electric fields distribute at the disc edges,with two lobes in the polarization direction of the excitationsource. For the triangular nanodot, the highest electric fieldsare concentrated at the two sharp tips in the polarizationdirection. For the gold SRR, high electric-field intensity isobserved at the two tips, outside of the four corners and thebottom part. These differences in the electric-field distribution

are the cause of different SERS properties observed onnanostructures.

3.2. SERS properties of nanostructures

The SERS properties of the gold nanostructures wereinvestigated. Greatly enhanced Raman scattering was observedon these nanostructures. The SERS spectra vary with thegeometries, sizes and arrangement of the nanostructures.Figure 6 shows examples of SERS spectra obtained on some ofthe nanostructures. Each Raman spectrum was obtained aftersubtracting the background baseline and was normalized tothe acquisition time. Figure 6(a) shows the SERS spectra ofthe 4-MPy molecules on the gold nanogratings. The highestRaman intensity occurs in the 1000 cm−1 band. This vibrationband originated from the ring-breathing mode correspondingto the band at 988 cm−1 in the Raman spectrum of the bulk4-MPy. The ring-breathing mode at 988 cm−1 in the solid4-MPy is blueshifted to 1000 cm−1 in the SERS spectra due tothe variation of hydrogen bonds when the Au atoms are in thesame plane as the 4-MPy molecules [22]. The line width ofthe gold nanograting is 60 nm, and the period varies from 105to 305 nm with increments of 100 nm. The SERS intensitydecreases with the increase the period (line distance). Thisis because the LSP of the gold nanolines interacts betweenadjacent gold nanolines, and the coupling effect decrease withthe increase of line distance [23]. As the gold nanogratingis fabricated on gold thin film, propagation surface plasma(SPP) exists in the film [24]. The SPP modulated by thegrating couples to the incident light and thus enhance the fieldamplitude above the grating surface. Consequently, the total

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(a)

(b)

Figure 4. EDX spectrum obtained from the surface of gold nanoholes (a) and from the gold SRRs (b). The gold nanoholes are fabricated byetching of PMMA mask and the SRRs are fabricated by etching of ma-N2400 resist mask.

SERS enhancement effect was the combination contributionof LSPR and SPP on the gold nanogratings. Figure 6(b) showsSERS spectra of the R6G on the arrays of gold nanodiscs.The size (diameter) of the gold nanodisc was 110 nm and theperiod of the arrays vary from 160 to 310 nm with incrementsof 50 nm. The dominant peaks are located at 1281, 1360, 1508and 1650 cm−1, which are assigned to aromatic stretchingvibrations [25]. SERS enhancement variations that are similarthat are the grating were observed. It was also observed thatthe SERS enhancement increases greatly with the decreaseof the array period, while maintaining the nanodisc size. Thisobservation is consistent with the results of Yu et al [26].SERS properties of the gold nanostructures fabricated with theplasma etching process were also investigated. In figure 6(c),SERS spectra of the R6G molecules on the array of gold nano-

SRRs are shown that were fabricated with the plasma etchingprocess. Clearly, the Raman scattering of the R6G molecules issignificantly enhanced by the gold nano-SRRs. The SRRs arewidely used photonics structures for metamaterials. This workdemonstrates that SRRs can also be used for the detection ofsingle molecules as the SERS substrate. Such an applicationhas been demonstrated by Cubukcu et al using a circular SRR-based sensor [27].

SERS EFs on the nanostructured substrates werecalculated. The EF is defined as EF = (ISERSNbulk)/

(IbulkNSERS) [2], where ISERS is the SERS intensity, NSERS isthe number of molecules illuminated by the laser sourceon the SERS substrates, Ibulk is the normal Raman intensityof the solid samples and Nbulk is the number of molecules inthe laser excitation volume in the solid samples. For the SERS

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(a)

(b)

(c)

Figure 5. Electromagnetic-field distribution on the surface on thegold nanodisc (a), triangular nanodot (b) and gold SRR (c).

of 4-MPy, the SERS intensity of the band at 1092 cm−1 wasfor calculation. For the SERS of R6G, the SERS intensity ofband at 1650 cm−1 was used. The calculated EFs for differentnanostructures are listed in table 1.

The SERS effects of the gold nanostructures fabricatedwith the two methods were compared. Two types of

(a)

(b)

(c )

Figure 6. SERS spectra on different nanostructured substrates.(a) SERS spectra of the 4-MPy on gold nanogratings: the gratingperiod varies from 105 nm to 305 nm and the line width is 60 nmfixed. (b) SERS spectra of R6G on the array of gold nanodisc: theperiod of the array is 160–310 nm and the disc size is 110 nm.(c) SERS spectrum of R6G on the array of gold SRRs.

gold nanostructures, i.e. gold nanodiscs and gold triangularnanodots, were fabricated on silicon substrates and used forcomparison. The geometrical sizes and lattice constants arethe same as the SEM images shown in figures 2(b) and(c). Raman measurements were performed with detection ofR6G molecules. Figures 7(a) and (b) show, respectively, theSERS spectra of R6G on the substrates gold nanodiscs and

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(a)

(b)

Figure 7. Comparison of the SERS spectra acquired from goldnanostructures fabricated with the lift-off process and plasmaetching process: (a) gold nanodisc and (b) gold triangular nanodots.

Table 1. Enhancement factors of SERS on different nanostructuredsubstrates.

Sample EFs ( × 104) Probe molecule

Nanogratings 105 nm period 36.4 4-MPy205 nm period 13.6 4-MPy305 nm period 7.6 4-MPy

Nanodisc 160 nm period 3.8 R6G210 nm period 2.8 R6G260 nm period 2.2 R6G310 nm period 1.4 R6G

SRR 500 nm period 3.1 R6G

gold nanotriangles. While the peak shape and peak positionof the SERS spectra on the nanostructures are very similar,the gold nanostructures fabricated with lift-off demonstratedhigher SERS enhancement than that fabricated with the plasmaetching. A possible explanation was that the edges of thenanostructures made by lift-off were relatively sharper thanthat of the nanostructures made by the plasma etching. Theedges of the nanostructures were blunted by the bombardmentof Ar ions during the etching process. Many research works

have shown that sharp edges tend to produce a higherelectric field and thus a higher SERS intensity [28]. It wasreasonable that higher SERS enhancement was observed onthe gold nanostructures made by the lift-off method. For thesame reason, larger SERS enhancement was observed on thetriangular nanodots than on the nanodiscs.

4. Conclusion

In conclusion, we have presented two processes for thefabrication of SERS substrates with e-beam lithographycombined with lift-off and plasma etching. The e-beamlithography has demonstrated high ability in the control ofthe geometry and spacing of nanostructures, which are animportant consideration in the design of SERS substrates.Using these two processes, a variety of nanostructures,disks, holes, gratings and other complex nanostructures werefabricated. Large SERS enhancement was observed on thegold nanostructures and the SERS properties were shown tobe highly dependent on the geometries and arrangement ofthe nanostructures. In addition, the SERS enhancement on thegold nanostructures fabricated with the lift-off process washigher than that on the same nanostructures fabricated withthe plasma etching.

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