J . Phys. Chem. 1985,89, 1843-1846
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Silver Particles on Stochastic Quartz Substrates Providing Tenfold Increase in Raman Enhancement M. Meier, A. Wokaun,* Physical Chemistry Laboratory, Swiss Federal Institute of Technology, ETH Zentrum, CH 8092 Zurich, Switzerland
and T. Vo-Dinh Health and Safety Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831 (Received: November 28, 1984)
Exceptionally strong Raman enhancements have been observed for molecules adsorbed on silver deposited onto etched quartz substrates. A stochastic arrangement of tall quartz posts, with an average spacing of N 110 nm, is obtained by plasma etching of S O t using a silver island film as an etch mask. Silver particles are produced by angle evaporation. Raman enhancement for adsorbed p-nitrobenzoic acid is maximized by optimizing the deposition parameters, which resdts in a tenfold enhancement compared to island films and crossed-gratingstructures. Observation of signals from several polyaromatic molecules demonstrates that the surface provides strong Raman enhancement for a variety of adsorbates.
1. Introduction
Surface enhanced Raman scattering (SERS)i-5 is presently being developed into an analytical tool for surface chemists and physicists. Problems studied include characterization and dynamics of adsorbed layers at electrodes$ conformational changes6 and charge-transfer kinetics' upon adsorption of organic molecules on colloidal metal particles, and conformational changes in adsorbates upon wetting* and application of p r e s ~ u r e . ~In heterogeneous catalysis, SERS promises to contribute to the understanding of catalytic mechanisms by in situ observation of surface species on the catalyst.iO*ii Recently, the potential of SERS for trace organic analysis has been investigated:I2 nanogram quantities of polynuclear aromatic molecules were detected by spotting the solution onto silver-coated Teflon or latex spheres. Common to these experiments is the need for a surface that provides strong Raman enhancements. The production of such surfaces is based on a large body of experimental and theoretical work,'-5 from which a consensus is emerging that at least two mechanisms are contributing to the enhancement. The important electromagnetic contributioniJ3includes amplified local fields close to the surface, which are due to excitation of surface plasmon modes, and modulation of the metallic reflectance due to adsorbate vibrations. The "chemical" contrib~tion'"~is based on the overlap of metal and adsorbate electronic wave functions, which leads to ground-state and light-induced charge-transfer processes. This (1) R. K. Chang and T. E. Furtak, Eds., "Surface Enhanced Raman Scattering", Plenum, New York, 1982. (2) A. Otto in "Light Scattering in Solids", Vol. 4, M. Cardona, Ed., Springer, Berlin 1983, Chapter 6. (3) R. P. Van Duyne in 'Chemical and Biochemical Applications of Lasers", Vol. 4, C. B. Moore, Ed., Academic Press, New York, 1979, p 101. (4) R. K. Chang and B. L. Laube, CRC CR't. Rev.Solid State Mater. Sci., 12, 1 (1984). (5) I. Pockrand, "Surface Enhanced Raman Vibrational Studies at Solid/Gas Interfaces", Springer, Berlin, 1984. (6) E. Koglin and J. M. SQuaris, J . Phys., Colloq., 44,C10-487 (1983). (7) C. J. Sandroff, D. A. Weitz, J. C. Chung, and D. R. Herschbach, J . Phys. Chem., 87, 2127 (1983). (8) C. J. Sandroff, S.Garoff, and K.P. Leung, Chem. Phys. Len., 96,547 (1983). (9) C. J. Sandroff, H. E. King, and D. R. Herschbach, J . Phys. Chem., in press. (10) S. K. Miller, A. Baiker, M. Meier, and A. Wokaun, J . Chem. Soc., Faraday Tram. I , 80, 1305 (1984). (1 1) A. Wokaun, A. Baiker, S.K. Miller, and W. Fluhr, J . Phys. Chem., submitted for publication. (12) T. Vo-Dinh, M. Y. K.Hiromoto, G. M. Begun, and R. L. Moody, Anal. Chem., 56, 1661 (1984). (13) A. Wokaun in "Solid State Physics", Vol. 38, H. Ehrenreich, F.Seitz, and D. Turnbull, Eds., Academic Press, New York, 1984, p 223.
0022-365418512089-1843$01.50/0
enhancement contribution depends on the adsorption site (importance of adatoms,2 the geometry of bonding, and the energy levels of the adsorbate considered; enhancements may be large for specific molecules. Although the chemisorption-induced enhancement is a source of information on metal-adsorbate interactions, it is restricted due to its specificity and cannot provide a basis for a generally applicable surface analytical technique. Our aim, therefore, is to increase the electromagnetic enhancement contribution to a level where this mechanism alone is sufficiently strong to provide observable signals for arbitrary adsorbates. This study presents a novel type of substrate, where a tenfold increase of adsorbate Raman signals compared to the most strongly enhancing known silver surfaces has been observed. This progress was achieved by utilizing present understanding of the enhancement dependence on surface geometry. In section 2 the design principle of our substrates is compared with other surfaces that have been used in SERS experiments. Substrate preparation is described in section 3. Metal deposition was optimized by using enhanced Raman scattering from p-nitrobenzoic acid (PNBA) as described in section 4. Results from the adsorption of several test molecules and a concluding evaluation are presented in section 5.
2. Stochastic Particle Surfaces Electromagnetic field amplification at the surface, which gives rise to Raman enhancement, is due to excitation of surface plasmons.' Three types of experimental configurations have been successfully used: grating structures on continuous metallic surfaces which support extended surface p1asmons;l discontinuous particle surfaces exhibiting localized particle plasmon res~nance;'*~*~ electrodes subject to oxidation-reduction cyclesi4 and etched metallic surfacesi0 which incorporate features of both the above types. Discrete particles of the coinage metals may be produced in the form of colloids,14"cold f i l m ~ " , ~orJ ~room temperature island film^.^,'^ As island films usually feature a wide distribution of particle sizes and shapes, only a fraction of the islands is resonant with a given excitation ~ a v e l e n g t h , ' ~ due J * to the strong shape (14) J. A. Creighton, C. G. Blatchford, and M. G. Albrecht, J . Chem. Soc., Faraday Trans. 2, 75, 790 (1979). (15) T. H. Wood, D. A. Zwemer, C. V. Shank, and J. E. Rowe, Chem. Phys. Lett., 82, 5 (1981). (16) C. Y. Chen and E. Burstein, Bull. Am. Phys. Soc., 25, 24 (1980). (17) M. Meier, A. Wokaun, and P. F. Liao, J . Opr. SOC.Am. B, in press. (18) The metal islands are coupled by their dipolar interactions, which contribute to shift and broadening of the resonances. See ref 17 and references cited therein.
0 1985 American Chemical Society
1844 The Journal of Physical Chemistry, Vol. 89, No. 10, 1985
dependence of the resonance condition.’J3 To produce particles of uniform shape, crossed-grating substrates have been prepared by lithographic techniq~es.’~Angle evaporation of Ag onto these substrates yields square arrays of silver particles, the eccentricity of which can be varied by changing evaporation conditions. Although the particles appear quite uniform,I9 their dimensions are tied to the grating period, which always exceed half the wavelength used for the holographic exposures. Thus electron micrographs typically show long-axis dimensions of 200-300 nm. Theoretical calculation^^^^^ indicate that the enhancement is strongly reduced on such particles due to radiation damping. Optimum particle sizes are determined by taking into account two loss mechani~ms?~J~ surface scattering of conduction electrons for small particles and radiation damping for large particles. For silver spheres, an optimum diameter of -35 nm is found (ref 21, Figure 1); for 4:l ellipsoids enhancement is maximum at a long-axis dimension of -80 nm. It is difficult to produce periodic structures of such dimensions over large areas by lithographic techniques. The procedure described below avoids this difficulty by using an island film as an etch mask on a S i 0 2 substrate. Reactive ion etching produces a surface which is densely covered with a stochastic arrangement of needles, -60 nm in diameter and 100 nm tall, and spaced 110 nm apart on average. By angle evaporation of metal onto this substrate the needles shade each other in the same way as the posts on crossed grating samp l e ~ . ’Thereby ~ a surface of highly uniform metal particles is produced.
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Letters
1600
E
+
.-
(19) P.F. Liao, J. G. Bergman, D. S. Chemla, A. Wokaun, J. Melngailis, A. M. Hawryluk, and N. P.Econornou, Chem. Phys. Lett., 82, 355 (1981). (20) M.Kerker, D. S. Wang, and H. Chew, Appl. Opt., 19,4159 (1980). (21) A. Wokaun, J. P. Gordon, and P. F. Liao, Phys. Rev. Letr., 48,957 (1982). (22) P.W.Barber, R. K. Chang, and H. Massoudi, Phys. Rev. B, 27,7251 (1983). (23) M. Meier and A. Wokaun, Opt. k t t . , 8, 581 (1983). (24) E. J. Zeman and G. C. Schatz, Jerusalem Symp. Quantum Chem. Biochem. 17th, 1984, in press. (25) M.C . Buncick, R.J. Warmack, J. W. Little, and T. L. Ferrell, Bull. Am. Phys. SOC.,29, 129 (1984). (26) P. D.Enlow and T.Vo-Dinh, submitted for publication.
800L
I
In
C
01 * c
0 X
+ +
‘O0I
K
01
I
I
40
I
80 120 silver thickness ( n m )
I
160
Figure 1. Raman enhancement as a function of silver thickness deposited onto an etched quartz substrate. The intensity of the 1597-cm-’ line of adsorbed PNBA is shown; an excitation wavelength of 514.5 nm was used. The symbols correspond to different angles 0 of evaporation (X, 0’; 40°; 0 , 55’; A, 65’; 78’).
.,
+,
3. Substrate Preparation The substrates were prepared a t Oak Ridge National Laboratory by etching fused quartz plates in a CHF3 p l a ~ m a . ~A~ + ~ ~ 500-nm layer of S i 0 2was thermally evaporated onto fused quartz at a rate of 0.1-0.2nm/s. The resulting crystalline quartz was annealed to the fused quartz at about 950 O C for 45 min. The SiOz layer was subsequently covered by a 5-nm layer of silver using thermal evaporation. The substrate was then flash-heated to about 500 O C for 20 s. Upon heating, the silver beads up into small globules which serve as etch masks. Final etching of the substrate was performed in a CHF3 plasma to produce submicron prolate S i 0 2 posts which are stochastically distributed over the surface area. 4. Silver Deposition and Raman Enhancements The size and shape of the metal particles can be influenced by varying (i) the thickness of metal deposited (as measured by a quartz crystal monitor perpendicular to the source) and (ii) the angle 0 between evaporation direction and surface normal, which influences the mutual shading of the posts. In spite of the stochastic nature of the substrate, the particles are relatively uniform in shape (see electron micrographs below). Silver is evaporated a t a rate of 1 1 . 5 nm/s in a vacuum of IO” Torr. Subsequently the surface is wetted with a few drops of 10-2 M ethanolic solution of PNBA, and the excess liquid is blown off by nitrogen. PNBA has been frequently used as a standard in SERS experiments. The high concentration in the solution ensures that the Ag surface is completely covered by -10 monolayers of PNBA. Approximate coverages can be determined by redissolving the adsorbate in ethanol and determining the number of molecules from the absorption using Beer’s law. This measurement has been performed for the present stochastic post samples, Ag island films,
X 0
1
61 8
i
: m
* i , I1
0
14 000
18 000
22 000
excitation energy (cm-1)
Figure 2. Wavelength dependence of the Raman enhancement. 100 nm of Ag was deposited onto an etched quartz substrate at evaporation angles The scattering cross section for the of 40’ (+), 55O (e), or 78’ (). 1597-cm-’ line of adsorbed PNBA is plotted as a function of excitation energy. For comparison data from the same thickness of Ag deposited onto as 260-nm period crossed grating structurez9are also included (A); solid line is a guide to the eye.
a continuous Ag film, and a glass slide that had all been coated from the same loL2M PNBA solution. The coverage was found to be independent of the type of substrate to within 15% and corresponds to 10 monolayers. Consequently, the measured Raman intensities represent the surface enhancement integrated over the first few molecular layers. Distance dependence studies’ have established that the enhancement is in general quite small outside the tenth adsorbed layer. Raman spectra are recorded for various argon ion and dye laser excitation wavelengths, using the setup described earlier.” Figure 1 shows the intensity of the 1597-cm-’ band of PNBA as a function of silver thickness deposited onto the stochastic post substrates, at a fixed excitation wavelength (514.5nm). Data for various evaporation angles 0 ranging from Oo to 7 8 O are included in the figure. The Raman intensity is seen first to increase with deposited silver thickness dAg;it reaches a maximum at dAg= 100 nm and then decreases. This behavior is relatively independent of the evaporation angle used.
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The Journal of Physical Chemistry. Vol. 89. No. 10, 1985 1845
Letters
400
600
800
wavelength (nm) 3. Transmission of etched quartz substrate with 100 nm of Ag deposited at an angle of No.An electron micrograph of the sample is shown in the inM (a). The ellipsoidal shape of the prticlra is clearly visible in the higher magnification of i w t (b) where the evaporation angle was 5 5 O .
The wavelength dependence was investigated while keeping deposition thickness constant at the optimum value dM = !00 nm. Results from IO samples, at three angles of evaporation, are collected in Figure 2. For identical experimental parameters, sample to sample variation was *30%, while on a given sample intensities were reproducible to f l W . In Figure 2 data from all samples corresponding to one evaporation angle were therefore normalized to the average value at 19430-cm-’ excitation. To obtain Raman scattering cross sections. the raw data had to be divided by q,,,,,,: and corrected for the variation of detection sensitivity with wavelength. Both these factors were taken into account by dividing by the measured wavelengthdependent Raman intensity of a standard (liquid cyclohexane). The Raman enhancement is seen (Figure 2) to exhibit a h d maximum for excitation energies centered around 170C10-18M)o cm-’. Again we find that the wavelength dependence is largely uninfluenced by the evaporation angle used. To interpret the data we have measured the normal incidence transmission which is shown in Figure 3. The sample is completely absorbing in the blue spectral region; the transmission rises to = 10% in the red. The strong absorptive losses for high photon energies correlate with the deerease of the Raman enhancement in this region. For a quantitative correlation a more complete characterization of the optical properties is in progress using the procedure of Bergman et al?’ Electron micrographs of the silver-coated samples have been taken; as a representative example an evaporation of 100 nm of Ag at 0 = 5 5 O is shown in the inset of Figure 3. The surface is uniformly covered with silver particles that are quite similar in size and shape. It is evident that upon further increase of the d e p i t e d silver thickness the particles would start to coalesce. This explains the relatively steep decrease in the enhancement seen in Figure 1 for da > 100 nm. From the micrographs, average half-axis lengths of the silver spheroids have been measured as a e 60 nm, b u c u 30 nm. Particles of this size exhibit plasmon reponanas which are strongly broadened by radiation damping.”J’-’ This effect can account for the width of the enhancement curve in Figure 2. The reponance wavelength is red-shifted from the isolated particle value due to dipolar interactions (cf. ref 17 and references cited therein). A more detailed analysis will be given elsewhere.”
TABLE E h r m n h t d t k . fw tlr ISW-cm-’ Band of PNBA. substrate intensity, cu s-’ mW-l C a F l roughened filmb 40* IO island filmc so* IO c r d grating I90 40 etched quartf 1450 100
* *
aAdswbcd from lW2 M ethanolic solution; excited at 514.5 nm. b54 nm of Ag deposited on top of IMX) nm of CaF,. evaporated onto a glass slide. ‘ 5 nm of & dcposited onto glass slide. rate 0.05 nm/s. 100 nrn of Ag deposited onto 260-nm period crossed quartz grating? evaporation angle 0 = SOo. rate 1.5 nm/r. * I 0 0 nm of Ag deposited onto etched quartz substrates (this work); evaporation angle 0 = 4O0 7 8 O . rate 1.5 nm/s. Also included in Figure 2 are data from Ag particles deposited onto a crossed grating sample of 2600-A period.” A 100-nm layer of Ag was evaporated along the diagonal of the grating, at a polar angle 0 = 80”. In the 1600(t20000-cm-‘ region the enhancement is smaller by a factor of -10 compared to the etched quartz substrates. The sensitivity of the measured Raman spectra is given by the product of the enhanad scattering class Section. the b‘ facta, and the detection sensitivity. For our spectrometer/GaAs photomultiplier combination this instrumental sensitivity drops strongly toward the red spearal region. Maximum Raman signals are therefore obtained when using 514.5- or 488.0-nm excitation. Measured intensities for PNBA for four different samples are compared in Table 1. It is seen that the grating structuret9 is superior to silver island films or CaF, roughened films by factors of 3-5 and that the optimized particles on stochastic quartz substrates exceed the grating by an additional factor of 10. Measurements in electrochemical environments are not directly comparable as there the solid/electrolyte interface is observed. Roughened electrodes removed from the electrolyte under p tentiostatic control have not been tested as substrates in this study. Table I demonstrates that the stochastic post substrates, which are easily produced and inexpensive, can provide a full order of magnitude enhancement increase compred to the d - g r a t i n g structures, which are previously known to be the most strongly enhancing ~ u r f a c e s . ’ ~ . ~ ~ (28) A. Woknun.
cation. (21) J. 0. Bcrgmen. D.S. Chcmla. P. F. Liao, A. M.O b , A. F i n a u t R. M. Hart. and D.H. Olson. Opt. Lcrr.. 6.33 (1981).
M. Mc~N,and T. Vo-Dinh. to bc submitted for publi-
(29) H. W. Lcbmann. R.Widmer, M. E b n a t k , A. Wcbun. M.M ~ N . and S. K. Miller. J. Voc. Sei. Tmhnd. 8.1. 1207 (1983).
J . Phys. Chem. 1985, 89, 1846-1849
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TABLE II: Raman Intensities for Various Molecules on Ag Particles Dewsited onto Etched Ouartz Substrates intensity,'
adsorbent solution no adsorbentb M PNBA M phenanthrene M benzoquinoline lo-' M coronene
cts s-l m W 100 1450 120 20 370
"Excited at 514.5 nm; strongest Raman Bands. b"Cathedral" peaks2 due to graphitic surface impurities.
5. Applications and Conclusions To test whether the silver surfaces developed in this study are of general applicability, we have detected adsorbed layers of polynuclear aromatic molecules, i.e., benzoquinoline, phenanthrene, and coronene. The signal intensities measured are given in Table 11. The success in detecting three molecules which have been chosen because of their relevance for trace analysis, and have not been selected because of favorable SERS properties, demonstrates that an advance has been achieved toward developing SERS as a general surface analytical tool. A novel substrate preparation procedure has been described which allows one to produce surfaces of uniform metal particles with 50-100-nm dimensions. The method avoids the relatively involved lithographic substrate patterning19s29by using an island film as an etch mask. The surfaces yield a tenfold increase in
enhancement and are therefore useful in trace analysis30 for any molecule that can be brought into proximity of the surface. Furthermore these surfaces are of interest for fundamental studies, as they consist of stochastic arrangements of uniformly shaped particles. They allow an experimental test of a recent theoretical treatment'' of particle dipolar interactions on stochastic and regular particles surfaces. The influence of particles interactions on Raman enhancement is being investigated both on stochastic and crossed grating substrates; this will be the subject of a forthcoming publication.2s Acknowledgment. The authors thank H. W. Lehmann for providing the crossed grating structure; they thank K. T. Carron for preparing the CaFz roughened films and for determining the PNBA coverages. We are grateful to U. P. Wild and R. R. Ernst for their continued interest and encouragement of this work. T. Vo-Dinh thanks U. P. Wild for his hospitality during a sabbatical stay at ETH. This work was sponsored by the Swiss National Science Foundation, the Branco Weiss Foundation, the U.S. Department of the Army (Interagency Agreement No. DOE 40-1294-82/ARMY 331 1-1450), and the Office of Health and Environmental Research, U.S. Department of Energy, under Contract DE-AC05-840R21400 with Martin Marietta Energy Systems, Inc. (30) T. Vo-Dinh, M. Meier, and A. Wokaun, to be submitted for publication.
Direct Probe Fourier Transform Far-Infrared Study of Cations in [NH,]Na-Y [Pt(NH,),]Na-Y Zeolites and Their in Situ Deammination Reactions
and
Geoffrey A. Ozin,* Mark D. Baker, Kate Helwig, and John Godber Lash Miller Chemistry Laboratories, University of Toronto, Toronto, Ontario, Canada M5S I A1 (Received: December 4, 1984; In Final Form: March 20, 1985)
The ammonium, NH4+,and tetrammineplatinum(II), [Pt(NH3)4]2+,cation vibrational modes in self-supporting wafers of [NH4]Na-Y and [Pt(NH3)4]Na-Y have been directly probed by in situ Fourier transform far-infrared (FT-far-IR) spectroscopy in the 300-30-cm-' range. These a-cage cations are found to absorb in the far-IR region at room temperature, around 169 and 42 cm-I, respectively. By subjecting these materials to an in situ vacuum thermal treatment, it is possible to observe (by mid- and far-IR spectroscopy) the dehydration/deammination processes occurring (a) in [NH4]Na-Y, which lead ultimately to the acid zeolite NaH-Y, and (b) in [Pt(NH3)4]Na-Y, which involves an autoreduction step of the Pt2+ions so formed, resulting in highly dispersed Pt: clusters immobilized in NaH-Y. Furthermore, the site locations of the residual Na+ ions generated in this way can be pinpointed by their characteristic far-IR vibrational signatures; moreover, the observation of Si/Al-induced cation vibrational frequency shifts (Si/Al = 1.0, 1.25, 2.50, 3.8) proves to be a valuable method for directly probing metal-support effects.
Introduction The first step in the production of high-dispersion platinum metal group-zeolite catalysts invariably requires ion exchange of either an alkali, alkaline earth, or ammonium form of the zeolite with a cationic transition metal ammine complex of the form of [M(NH3),]q+. Depending on the choice of transition metal and the subsequent thermal/chemical treatment, one can obtain a wide range of metal zeolite compositions, possessing metal cations, metal atoms and/or metal clusters having various site locations, populations, and states of dispersi0n.I On the other hand, vacuum thermal pretreatment of the ammonium form of the original zeolite leads to the respective acid form which also represents an important class of zeolite catalysts.* Many different physicochemical (1) P. Gallezot, Sur/. Sci., 106, 459 (1981). and references cited therein. (2) D. W. Breck in "Zeolite Molecular Sieves", Wiley, New York, 1974, and references cited therein.
methods have been apDlied to the characterization of the above types of materials with iarying degrees of success. With the recent demonstration of the ability of FT-far-IR spectroscopy to detect metal atoms, cations, and clusters in zeolites,) the question arises as to the potential of the method for directly probing the more complex NH4+ and [M(NH3),]q+ cations in zeolites, as well as the outcome of subsequent reaction^.^ (3) G. A. Ozin, J. Godber, and M. D. Baker, J . Phys. Chern., 88, 4902 (1984); M. D. Baker, G. A. Ozin, and J. Godber, J. Phys. Chem., 89, 305 (1985); G. A. Ozin, J. Godber, and M. D. Baker, J. Phys. Chem., in press; G. A. Ozin, M. D. Baker, and J. Godber, J . Am. Chem. Soc., 107, 1995 (1985); 'Heterogeneous Catalysis", B. Shapiro, Ed., Texas A & M University Press, College Station, TX, 1984; G. A. Ozin, J. M. Parnis, and M. D. Baker, Angew. Chem., 1075 (1983); G. A. Ozin, J. Godber, and M. D. Baker, Card. Rev.-Sei. Eng., in press. (4) We note that the far-IR spectrum of [NHJNa-Y has been previously reported, and the assignments are in agreement with the results of the present study. T. Stock, D. Dombrowski, J. Fruwert, and H. Ratajczak, J . Chem. Soc., Farday Trans. 1, 79, 2773 (1983).
0022-3654/85/2089-1846$01.50/0 0 1985 American Chemical Society
