1、Simulation of Raman Enhancement in SERS-Active Substrates with Au Layer Considering Different Geometry of Nanoparticles Hui-Wen Cheng1 and Yiming Li1,2,3,*Institute of Communications Engineering, National Chiao Tung University, 1001 Ta- Hsueh Road, Hsinchu 300, Taiwan 2Department of Electrical Engin

2、eering, National Chiao Tung University, 1001 Ta-Hsueh Road, Hsinchu 300, Taiwan 3National Nano Device Laboratories, Hsinchu 300, Taiwan *Tel: +886 3 5712121 ext 52974; Fax: +886 3 5726639; E-mail: .twAbstract-In this work, we study surface enhanced Raman identification of Rhdamin

3、e 6G (R6G) are examined. This paper spectroscopy (SERS) active substrates for the detection of is organized as follows. In Sec. II, we introduce the fabrication Rhodamine 6G. To examine the electromagnetic enhancement, process and computational technique for the SERS-active with different shape of n

4、anoparticle, we apply the finite- substrates. In Sec. III, the local field enhancements of difference timedomain (FDTD) algorithm to analyze the nanoparticle with different shapes are calculated by thres-ructures by solving a set of coupled Maxwell' sequations in dimensional (3D) finite-differen

5、ce time-domain (FDTD) differential form. The field enhancements are investigated in the numerical simulation. Finally, we draw the conclusions and visible regime with the wavelength of 633 nm. In the suggest the future work.experimental measurement, the surface enhanced Raman scattering signals from

6、 the surface of substrates with 12-hourII. FABRICATION AND COMPUTATIONAL TECHNIQUE hydrothermal treatment and without treatment are performed and compared. Through the three-dimensional (3D) FDTD For the flow of fabrication, as shown in Fig. 1, first, calculation, the enhancements with different sha

7、pe of buffered oxide etchant (BOE) and standard RCA cleaning arenanoparticle are tested and obtained which are nanoparticle, carried out to prepare clean silicon substrates (Boron-dopedAuTi Sigold nanocage and gold/silver alloy for spherical, cubic and pyramidical shapes. The results show that the e

8、nhancement of(i)spherical and cubic shapes can be much improved by nanocage and gold/siliver alloy structures.Keywords- Surface-Enhanced Raman spectroscopy (SERS), electromagnetic enhancement, nanoparticle, gold nanocage, gold/silver alloy, finite-difference timedomain, hydrothermally treated substr

9、ate.(ii)1. INTRODUCTIONSurface-enhanced Raman Scattering (SERS) is one of the characterization techniques, which is sensitive to the enhanced electromagnetic fields 1-6. SERS-active substrates have recently attracted a great deal of attention for rapid identification of chemical and bacterial sample

10、s 5-7. The fabricated nanostructures for both bottom-up and top-down approaches have been reported. And, the degree of Raman enhancement is strongly dependent on the morphology of formulated nanostructures 8. Recently, a top-down approach for the fabrication of SERS-active substrate was proposed 9-1

11、2. However, the expensive substrate, equipments and complicated process are needed. Therefore, a low cost, environment friendly and simple fabrication for SERS-active substrates will be of great interest for basic and clinical researchers as well as for biotechnologies. In this study, we experimenta

12、lly and computationally study the local field enhancementsof nanoparticles on hydrothermally roughened SERS-active substrates, where the effects of shape and size of Au particles and application of the fabricated samples in(iii)(iv)Figure 1. Schematic representation for the fabrication of SERS-activ

13、e substrate. First, silicon wafers were cleaned by BOE and standard RCA cleaning procedures. Then, Ti films were deposited on the pre-cleaned silicon wafers using reactive DC magnetron sputtering system. The asdeposited samples were cleaved and treated under hydrothermal conditions for various durat

14、ions. Subsequently, Au was thermal evaporated onto the hydrothermally roughened substrates for sensing.1.0。2,00 umFigure. 2 (a) The AFM image of titanium thin films treated under hydrothermal condition for 12 hours treatment duration. (b) The plot of simulated substrate which is part of real substra

15、te, where the matrix of nanoparticles is 3 x 5 due to periodical property of the simulated structure.p<100>). Then, 100-nm-thick titanium films are deposited on the pre-cleaned silicon wafers using reactive DC magnetron sputtering system. The as-deposited sample is cleaved into 0.5 cm x 1 cm s

16、quares and rinsed with ethanol, and de-ionized water. Subsequently, the sample is put into a 23 mL Teflon-lined stainless steel autoclave filled with 20 mL distilled water, which is sealed, and heated at 2000c for 2, 4, 6, 8, 10, and 12 hours, respectively. Then the treated sample is cooled to room

17、temperature naturally, washed with distilled water for several times, and dried with a stream of cylinder air. For example, the image of Fig. 2(a) shows the AFM images represent titanium thin films treated under hydrothermal conditions for 12 hours treatment duration.The image of Fig. 2(b) shows the

18、 plane view of the gold-coated nanoparticular structure, where the matrix of nanoparticles is 3 x 5 due to periodical property of the simulated structure. Numerical simulation using a 3D FDTD method is conducted to investigate the local field enhancement of substrate 13-5. The Maxwell ' s curl e

19、quations in linear, isotropic, nondispersive, lossy materials are ?BKKK?=- ? X E, (1) ?EKKt?t=-J1KK £ + £?汨，(2) ? ? BKNNbiteration=0, (3)'s equations.Figure 3. The simulation procedure of solving the Maxwell ? K? EK=p£,(4)where EK and BKpermittivity and JK andare the vectors of el

20、ectric and magnetic fields, respectively? and 仙 are permeability andp are the current density vefcrtorgandensity. For aglobally defined curvilinear space, Maxwell's equations are easily implemented in theirdifferential form, where Faraday The FDTD method solves Maxwell's law is Eq. (1) and A

21、mpere's law is Eq. (2).'s equations by first discretiziequOltions via's mesh and0.01 um, where the time stepH fields at subsequentcentral differences in time and space. Then, based upon a 3D Yee components of the electric and magnetic fields at points, the discretized spacing in the x, y

22、, and z directions adopted in our simulation are |x| = 0.01 um, |y| = 0.01 um and |z| =A t is 0.0004 and the time duration T is 3 in units offemtoseconds. The discretized equations are iteratively solved in a leapfrog manner, alternating between computing the E andAvtaEsjnas shown in Fig. 3. Notably

23、, we employ the perfectly matched layer as the simulation domain boundaries in which both electric and magnetic conductivities are introduced in such a way that wave impedance remains constant, absorbing the energy withoutinducing reflections. III. RESULTS AND DISCUSSIONIn order to have less light a

24、bsorption, the larger scattering of substrate is better to achieve larger field enhancement. For chemical sensing, the hydrothermally roughened substrates are treated with aqueous solutions of 10-4 M R6G. The The chemical structure of R6G is shown in Fig. 4(a). Fig. 4(b) shows that the characteristi

25、c Raman vibrational modes of R6G immobilized on the substrate with or without hydrothermal treatment. The substrate with hydrothermal treatment shows(r*c> -c，>ity20X403 L unieated>80X103 -3 三-20x103 111/inn Qnn -lonndwww _ w w - wRaman shift (cm")Figure4. (a) Chemical structure of Rhod

26、amine 6G (R6G). The molecule is widely used for SERS measurements. (b) The Raman spectra for R6G (10-4 M) immobilized on hydrothermally untreated (blue) and treated (orange)QP0ll,tBEgoUELIULJJP®IZOPOBEJAu NsnacageAu/Agsubstrates.AuAgFigure 5. Gold nanoparticle, gold nanocage and gold/silver all

27、oy (from left to right) for spherical, cubic and pyramidical shapes, respectively.larger intensity than that without hydrothermal treatment due to the roughness on the surface 16. According to the Beckmann-Kirchhoff theory, the roughened surface has larger scattering on the surface of substrate so t

28、hat the intensity can be enhanced. Through using the FDTD simulation, the evaluation of electric field on the substrates is carried out by the directing light with a wave length of 633 nm.Notably the nanosensor also can be fabricated by other synthesis methods to achieve different shape of nanoparti

29、cles.14 1.31211 10 0 9 0.8 0 7 0 6SampleFigure 6. The plot of electric field enhancement factor versus different samples.Figure. 7. The top view of electric field distribution with spherical shape of (a) Au nanoparticle, (b) Au nanocage and (c) Au/Ag alloy, respectively.Here, we consider gold nanopa

30、rticle, gold nanocage and gold/silver alloy (from left to right) for spherical, cubic and pyramidical shapes, as shown in Fig 5. The simulation results show that the electric field (Ex) enhancement of nanoparticle with cubic shape is larger than that with spherical and pyramid shapes, as shown in Fi

31、g. 6. To improve the enhancement, the structure is considered to fabricate by different synthesized structures for spherical, cubic and pyramidical shapes, respectively. The synthesized structures are illustrated in Fig. 5, which are the gold nanocage (middle one) and gold/silver alloy with empty an

32、d silver inside, respectively. From the results of Fig. 6, the Au/Ag alloy and gold nanocage are adopted for spherical and cubic shapes because the enhancement is much improved. For pyramid, the Emetal alloy is almost the same. These results can be explained x enhancement of nanocage or by distribut

33、ion of electric field. The corresponding distributions of electric field are shown in Fig. 7, 8 and 9, respectively. For spherical shape, the enhancement of Au nanoparticle is locallyFigure. 8. Top views of electric field distribution with the cubic shape of (a)Au nanoparticle, (b) Au nanocage, (c)a

34、nd Au/Ag alloy, respectively.Figure. 9. Top views of electric field distribution with the pyramidical shape of (a) Au nanoparticle, (b) Au nanocage, and (c) Au/Ag alloy, respectively. increased, as shown in Fig. 7(a). Considering the nanocage structure, the enhancement of whole plane is increased, c

35、ompared with nanopartilce structure, due to empty inside, as shown in Fig. 7(b). With silver inside, the enhancement can be much improved by different materials, as shown in Fig. 7(c). For cubic shape, the distributions of nanoparticle and nanocage are quite similar, as shown in Figs. 8(a) and 8(c).

36、 It is obviously that the larger enhancement can be obtained by nanocage due to tips on the corners, as shown in Fig. 8(b). For the pyramidical shape, the electric field can not be improved by synthesized structures so that the distributions are quite similar, as shown in Fig. 9.IV. CONCLUTIONSIn co

37、nclusion, we have successfully prepared SERS-active substrates with low background for the detection of both Rhodamine 6G. The enhancement could be controlled by tuning the surface roughness of the substrates through varying treatment duration. Through FDTD simulation, the field enhancement of spher

38、ical and cubic shape nanoparticles can be enhanced by using Au/Ag alloy and naocage samples, where the different shape of nanoparticles also can be fabricated by other synthesis method for local field enhancement in diverse nanosensor applications. ACKNOWLEDGMENTThis work was supported in part by Na

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