Surface-enhanced raman scattering from microlithographic silver particle surfaces

CHEMICAL

Volume 82, number 2

1 September 1981

PHYSICS LE-I-I’ERS

SURFACE-EJNHANCED RAMAN SCATTERING FROM MICROLITHOGRAPHIC

SILVER PARTICLE SURFACES

P.F. LIAO, J.G. BERGMAN, D.S. CHEMLA *, A. WOKAUN Bell Telephone Laboratories, Iiobnde& New Jersey 07733, USA J. MELNGAILIS,

A.M. HAWRYLUK

Research Laboratory for Electronic& MasvlchusettsInstitute Gzmbridge, Massachusetts 02139. USA

of Technology.

and N.P. ECONOMOU* Lincoln LaboratorieS Masmchusetts Institute of Technology, Lexington, Massachusetts 02173, Received 21 April 1981; in final

USA

form 18 May 1981

Surface-enhancedRaman scattering is observedfrom unirormly sized and shaped silverparticles produced by evaporation of silveronto a lithographicallyproduced microstructure. An excitation resonance is observed which can be shifted by changes in particle shape or surrounding dielectric constaat in agreement with particle plasmon theories of surface Raman enhancement

In this letter we report the first measurements of surface-enhanced Raman scattering (SERS) [l-15] from a lithographically produced silver particle surface. Our samples provide uniform and controlled “roughness” on a submicron scale which allows the clear observation of an excitation resonance as the incident laser wavelength is varied. At the excitation resonance peak we observe a Raman intensity for the 2144 cm-l stretch mode of CN molecules adsorbed on silver particles which is enhanced by e107. This resonance can be shifted by modifying the microparticle shape or by changing the dielectric constant of the surrounding medium and strongly supports particle plasmon theories [ 1 l-15) of enhanced Raman scattering. * Resident visitor from the Centre National d*Etude des T616communication, Bagneux, France. * The Lincoln Laboratory portion of this work was sponsored by the Department of the Air Force.

Raman cross sections of molecules adsorbed on metallic surfaces can be orders of magnitude larger than for the same molecule in the liquid or vapor phase [l-15]. Roughening of the metallic surface is required to obtain good enhancement [4,5,7-91. This observation, combined with the observation [9, 101 of enhancement for more than the first layer of molecules, has led to electromagnetic models for SEKS in which plasmon resonances of microscopic bumps on the surface act to increase the local field at the molecules and to amplify the re-radiated field of the Raman active molecule. By modeling the enhancing surface as a random array of metal spheroids, the magnitude and long range of the enhancement and the requiremept of surface roughness are predicted. The main features of these theories are easily seen by considering a single dielectric ellipsoid with an external laser field, EL directed along +Ze principal axis of the ellipsoid, and a nearby molecule also located

CHEMICAL PHYSICS LETTERS

Volume 82. number 2

on the principal axis. If the ellipsoid major (a) and minor (b) axis dimensions are such that a,b e h the problem can be solved in an electrostatic approximation [ 12-151. To further simplify the problem we replace the ellipsoid by a point dipole of magnitude [ 161 pE = c+E with

Here E and eO are the dielectric constants of the ellipsoid material and of the surrounding medium respectively; A IS a depolarization factor which has been tabulated [ 17]_ For a sphere, A = $; for a 3 : 1 aspect ratio ellipsoid, A = O_1. Eq. (1) shows a resonance at E = ~(1 - l/A). For a sphere this resonance occurs if E = -3 _eO, a 3 : 1 aspect ellipsoid is resonant if E = -5.25~ Sd-ver can satisfy these conditions for visrble lighht. The imaginary part of E is small (%0_3) while the real part decreases from -2 to -20 over the range 350-700 nm [ 18]_ At resonance the induced dlpolar field of the ellipsoid 1s large and produces a large Raman polarization and molecular dipole moment, pm oscillating at the Stokes frequency os, pm(os) = I?cQOIE(WL)E~/ eO r3_ Here WL 1s the laser frequency and r is the distance from the center of the ellipsoid to the molecule_ The field of the molecular dipole in turn polarlzes the ellipsoid to produce an ellipsoid dipole at the Stokes frequency. P~GJ~) = 2c~~~&,(o~)/~~ $. which is la‘rger than the usual Raman molecular dipole by the factor 4

f =s1

-

1 - E(Ws)/Eo

1 - E(WL)/E(J

[l - E(WS)/E()]A 1 -

[l - E(OL)/$)]A

X (ab’/r3)*_

(2)

The net enhancemem of the Raman intensity is given by IfI*. This expression is essentially the same as that given in refs. [ 12- 15]- Gersten and Nitzan [ 151 have solved the correct boundary value problem for the case of an ellipsoid and find in the dipole approximation f=,_,

1 [

I+-

eo- e(q) W(w,)

-

1+

veo_ I[ _

ql W+iJs)

4us) -

“E()

1’ (3)

where 1 - I? is an image enhancement 356

factor which

we shall neglect, W= Q&J~oQ&~).

1 September 1981

V=

f&(.$0)/

Qi(F;l)_ The Legendre polynomials of the second kind, Q, are functions of E. = ~/(a* - b*)l/* and .$I = r/(a2 - b2)lj2- Although eqs. (2) and (3) predict resonant behavior as a function of excitation wavelength, experiments have generally shown only a slowly increasing enhancement [2] as the excitation wavelength is varied from 450 to 650 nm. Since the resonant frequency is shape dependent through the factor A, this non-resonant behavior can be attributed to the wide range of shapes and sizes of the metal protrusions which are found on a rough surface. To obtain a comparison with the particle plasmon resonance theories we have used microlithographic techniques to produce a regular array of isolated, rtnifonnlj sized silver particles of 100 nm dimension and variable shapes. Lithography has been used by Tsang et al. [5] to produce grating structures which demonstrate enhancement by surface plasmons, and Crelghton et al. [6] have shown a connection between the plasmon absorption resonance and the Raman enhancement of solutions containing sliver sols. The substrate for our experiments consists of Si02 posts on a silicon wafer, 500 nm high and 100 nm in diameter. They are arranged in a square lattice of 300 m-n dimensions, and are fabricated as follows. A silicon wafer with 500 nm thick oxide layer is coated with 300 a of chrome and 1000 A of photoresist. A crossed grating pattern is produced by holographic exposures with a 325 nm He-Cd laser. After development, the resulting array of resist posts is used as a mask for argon ion milling a chrome pattern. Chrome makes an excellent mask for the final highly directional reactive plasma etching [ 191 of the SiO, in a CHF, plasma. Erosion of the chrome mask produces the slightly conical shape posts. The substrates are quite durable and can be chemically cleaned and reused many times. By evaporating silver at grazing incidence onto this substrate along a non-channeling direction so that the posts shadow each other, we obtain isolated silver particles on the tops of the posts as shown in fig. 1. They are quite uniform in size and as a first approximation can be considered as ellipsoids with an =3 : 1 aspect ratio. Other aspect ratios can be obtained by varying the angle of evaporation and hence the shadowing of the posts. A monolayer [4] of CN

Volume 82, number 2

CHEMICAL

1 September 1981

PHYSICS LETTERS

Fig. 1. Electron micrograph of silver particle array produced by evaporating silver onto sides of SiOz posts The calibration bar is 1.0 pm in length.

molectdes 1s absorbed onto the silver by exposing the sample to HCN vapor. Raman light of the 2144 cm-1 CN stretch mode is then excited with 20-l 00 mW of laser light from either an argon ion or Rh6G dye laser, and measured through a monochromator set for 8 cm-1 resolution. The samples are mounted at 60” in the p-polarized configuration; sample orientation was found not to be a particularly sensitive parameter. In fig. 2 the variation of the measured normalized peak Raman intensity versus excitation photon energy is shown for the sample in a nitrogen atmosphereThe normalization eliminates the a4 density of states factor in the expression [2] for the Raman cross section, the laser intensity and the detector sensitivity. Data are shown for two different aspect ratio particies. The open circles correspond to the ~3 : 1 aspect particles shown in fig. 1, while the solid points were taken with particles with an a2 : 1 aspect ratio. In each case one clear resonance is observed. Evaluating eq. (3) using experimental values [ 181 for e(o), two narrow peaks separated by atib = 0.27 eV are predicted. However, if one broadens the theoretical resonances by increasing e2

INCIDENT 700 1

I

WAVELENGTH

600 I

I

I

I

18

20 INCIDENT

(nm)

500

I

2.2

I

I

I

2.4

26

26

PHOTON ENERGY -IeV)

Fig- 2. Dependence of the Raman signal on the aspect ratio of the silver ellipsoids. The normalized Raman intensity of the CN (2144 cm-‘) viiration in nitrogen is shown as a function of incident photon energy. (a) 3 : 1 aspect ratio ellipsoids (fig. 1); (b) 2 : 1 aspect ratio ellipsoids.

357

CHEMICAL PHYSICS LETTERS

Volume 82, number2

to 2.0, or if one assumes a slight distribution (*12%)

of aspect ratios, the two peaks coalesce into one, and the width, location and shift of the peak can be fit to the data yielding an aspect ratio of 3.9 : 1 for the open circle data and 3.2 : 1 for the solid points. These values compare favorably with the observed ratios, especially considering that we have neglected the effects of interactions between particles [ZO] , the presence of the SiO, posts, and deviations from ellipsoidal shape. Furthermore the =I00 mn size of the particles is only moderately small compared to X. Hence the dipolar approximation is not strictly valid. Mie resonances of small particles are known to broaden as particle size increases [ 12]_ Reflectivity measurements at near normal incidence also show the broad plasmon resonance. A complete study of optical properties using transparent microstructures is in progress and will be reported elsewhere. Calibration of the absolute detection sensitivity of our apparatus using the known output of a standard tungsten lamp indicates an ~10~ enhancement of the CN Raman cross section at the resonant peak (assuming a Raman cross section of 3 X 10W30 cm2 INCIDENT WAVELENGTH 600

700

(nm) 500 7

\

1 September1981

and one monolayer coverage [4] of 1015 cm-2). Using a 12% distribution in aspect ratios and the dielectric function of bulk silver 1181, we calculate an enhancement of 1.3 X lo* with eq. (3) in reasonable agreement with the measured value. A further test of particle plasmon models can be made by varying the dielectric constant of the surrounding medium. As ~0 is increased, the resonance should shift to the red. In fig. 3 we see that this behavior is indeed verified as the sample of fig. 1 was immersed in either water (e. = 1.77) or cyclohexane (ec = 2.04), although our dye laser could not be tuned sufficiently to completely resolve the resonances. The predicted resonance for 3.9 : 1 aspect ellipsoids with a 12% distribution would be at 1.9 eV in water and 1.5 eV in cyclohexane. The relative amplitudes of the three sets of data could be repeatedly checked by removing the samples from the liquids into nitrogen atmosphere. We have observed enhancements of order lo7 of Raman scattering from CN molecules absorbed on microlithographically prepared silver particle surfaces. The dependence on particle shape and surrounding dielectric constant is found to agree with recent theories of enhancement by particle plasmon resonances. By using lithographic techniques to precisely determine surface morphology it should be possible to achieve very large enhancements. Gersten and Nitzan [ 1 S] have predicted enhancements of 1O1l for correctly shaped and sized silver particles. The microstructure substrates were prepared during a visit of P-F. Liao to the submicron structures laboratory at M.I.T. He wishes to thank HI. Smith and members of the laboratory for their aid and support. The authors also acknowledge discussions and help received from P.A. Wolff and H. Craighead, P. Grabbe, R. Hart, R. Howard, E. Hu, L. Jackel, RA- Lemons and C-V_ Shank of Bell Laboratories.

References 1.8

2.0

2 2

2.4

2.6

INCIDENT PHOTON ENERGY-(eV)

Fig. 3. Dependenceof the CN Raman enhancemer.ton the dielectricconstant(Q) of the surroundingmedium, for 3 : 1 eIIipsoids.(a) Nitrogen(eo = 1); (b) Hz0 (GO= 1.77); and (c) cyclohexane(q = 204).

358

VI M. FIeischman,P.J. Hendra and A.J. McQuilIan,Chem. Phys. Letters 26 (1974) 163_

PI R.P. van Duyne, in: Chemicaland biochemik applica-

tions of lasers,VoL 4, ed. C.B. Moore (Academic Press, New York, 1978).

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CHEMICALPHYSICS

[3] T-E. Furtak, Solid State Commun. 28 (1978) 903. 143LG. Bergman,J-P. Heritage,A. Pinczulc,J-M. Worloek and J-H. McFee, Chem. Phys Letters 68 (1979) 412. [S] J.C. Tsang, JR. RirUey and J.k Bradley, Phys Rev. Letters 43 (1979) 772; J.C. Tsang, J.R. Kirtley and T.N. Theia, Solid State Commun 35 (1980) 667. [6] J.A. Creighton,C-G. Blat&ford and M-G. AIbrecht, J- Chem. SOG Faraday Trans 1175 (1979) 790. [7] T-H. Wood and M.V. Klein, J. Vacuum Sot 16 (1979) 459. [8] C.Y. Chen, E. Burstehrand S. Lundquist, Solid State Commun 32 (1979) 63. [9] J-E. Rowe, C-V. Shank, D.A. Zwemer andC.A. Murray, Phys Rev. Letters 44 (1980) 1770. [lo] CA. Murray, D.L. ABara and M. Rhinewine, to be published.

LETTERS

1 September 1981

[ll] M. Moscwits, J. Chem Phys 69 (1978) 4159. [ 121 S.L. McCall, P.M. Platzman and P-A. Wolff, Phys Letters 77A (1980) 381. 113J D.S. Warg, H. Chew and M. Rerker, AppL Opt 19 (1980) 2256. [ 141 CY- Chen and E Burstehr,Phys Rev. Letters 45 (19SO) 1287. 1151J.I. Gemtenand A. Nitzan, J. Chem. Phys 73 (198Oj 3023. [16] C.J.F. Bo?tcher,Theory of eIectricpolarization,Vol. 1 (Elsevier,Amsterdam, 1973) p_ 79. [17] EC. Stoner, Phil Mag. 36 (1945) 803. [18] P.B. Johnson and R.W. Chris@, Phys Rev. B6 (1972) 4370. [ 191 H-W. Lehmann and R Widmer, AppL Phys Letters 32 (1978) 163. 1201J-G- Bergman,D.S. Chemla, P.F. Liao, A.M. Glass, A. Pinczuk,RM. Hart and D.H. Olson, to be published.

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