J . Phys. Chem. 1989, 93, 6121-6125
6121
Plasma Modification of Mica: Forces between Fluorocarbon Surfaces in Water and a Nonpolar Liquid John L. Parker,**+Dong L. Cho, and Per M. Claesson Department of Physical Chemistry, Royal Institute of Technology, S-100 44 Stockholm, Sweden, and Institute f o r Surface Chemistry, Box 5607, S-114 86 Stockholm, Sweden (Received: November 7 , 1988; I n Final Form: February 6, 1989)
Mica has been exposed to low-temperature water vapor plasma. A combination of reaction and sputtering in the plasma environment alters the surface properties of mica and activates the surface toward reaction with chlorosilanes. The plasma treatment technique causes minimal alteration to the surface smoothness and oscillatory solvation forces were measured in octamethylcyclotetrasiloxane (OMCTS). Reaction of the plasma-treated mica with (tridecafluoro-l,l,2,2-tetrahydroocty1)-1-dimethylchlorosilane renders the surface hydrophobic. The oscillatory solvation force is removed by surface roughness, due to a low surface density of reacted silane (one molecule per 5 nm2 as estimated by ESCA) and the forces between these surfaces in OMCTS were attractive and in agreement with the force expected from van der Waals theory. The force between two silanated surfaces in water is in good agreement with the force calculated from DLVO theory at separations greater than 50 nm but an additional exponential attraction is observed at smaller separations. The magnitude and decay length of this attraction are in close agreement with those found previously between mica surfaces hydrophobed by Langmuir-Blodgett deposition of surfactant monolayers. Consequently, the hydrophobic interaction observed between surfactant-coated surfaces does not result from instabilities in the surfactant layer.
Introduction Over the past 15 years measurements of the forces between molecularly smooth sheets of muscovite mica have provided a wealth of knowledge on surface interactions.I4 Mica is ideal for these measurements because it is smooth, hard, and inert. However, the fact that mica is chemically inert is also a serious disadvantage because it is extremely difficult to covalently modify the surface by chemical reaction or treatment. Surfactant-coated mica surfaces produced by Langmuir-Blodgett deposition2 and adsorption from solution on mica3 were, until very recently, the only easily prepared alternative to pure mica suitable for investigation with the surface force technique. There have been many investigations of the interactions between mica surfaces coated with different surfactants but often interpretation of the measured force law is complicated by instabilities in the film or multilayer b ~ i l d u p . Recently ~ attention has been directed at producing surfaces with different properties from mica. Several successful attempts have been made at producing highly smooth metal films and the forces between these films have been r e p ~ r t e d . ~In ,~ another set of experiments sapphire crystals have been used successfully in place of mica.' However, the techniques used for preparation of these surfaces cannot be readily applied to the production of other types of surfaces. A broader understanding of the interaction between surfaces on a molecular level requires investigation of surfaces with many different properties. A general surface modification technique which can, in an easily controlled manner, produce surfaces which are highly smooth and stable is obviously desirable. Mica, which has no reactive groups on the surface, remains completely unreactive to functionalization with the common chlorosilanes. Polymerization of multifunctional silanes can produce poorly defined multilayer films but no covalent links between silane and mica can be formed. There is a large body of literature and many existing techniques for surface functionalization with silane compound^.*^^ A large array of these chemicals is commercially available and as a result surfaces with many different properties can be prepared with these compounds. When a gas at low pressure is exposed to a sufficiently strong electric field the molecules in the gas become ionized and a cold or low-temperature plasma is produced. Energetic particles or reactive species in the plasma can alter the surface chemistry of 'On leave from Department of Applied Mathematics, Research School of Physical Sciences, Australian National University, G.P.O. Box 4, Canberra, ACT 2601, Australia.
solids through creation of free radicals, formation of reactive groups, or sputtering of atoms or groups of atoms from the surface. Treatment of surfaces with this type of plasma is a powerful technique for surface modification of various materials.l*12 With certain gases and under the correct conditions, polymerization of the reactive species can take place at the surface and thin films are p r 0 d ~ c e d . l ~The major advantage of plasma treatment, in the current context, is that treatment does not alter the bulk properties of the substrate and only minor changes in the surface topology occur. Furthermore, plasma treatment is readily controlled by type of inlet gas, flow rate, reactor pressure, and energy input. The plasma treatment process is also affected by reactor design and the position of the substrate within the reactor. We have exposed mica to the plasma produced from water vapor. The resulting "activated" mica remains both smooth and hard but is highly reactive and lends itself to chemical modification as readily as silica. An immediately obvious application is hydrophobing the mica by reaction with chlorosilanes in the gas phase, in a manner analogous to that used by Rabinovich and Derjaguin in a recent study of the hydrophobic intera~ti0n.I~The unexpectedly long range attraction measured between hydrophobic surfaces has yet to be fully understood (however, see ref 15). We here present measurements of the forces between hydrophobed ~~
~
( I ) Israelachvili, J. N. Intermolecular and Surface Forces; Academic: New York, 1986. (2) Marra, J. J. Colloid Interface Sci. 1985, 107, 446. (3) Pashley, R. M.; Israelachvili, J. N. Colloids Surf. 1981, 2, 169. (4) Parker, J. L.; Christenson, H. K.;Ninham, B. W. J . Phys. Chem. 1988, 92, 4155. (5) Parker, J. L.; Christenson, H. K. J . Chem. Phys. 1988, 88, 8013. (6) Smith, C. P.; Maeda, M.; Atanasoska, L.; White, H. S.; McClure, D. J. J . Phys. Chem. 1988, 92, 199. (7) Horn, R. G.; Clarke, D. R.; Clarkson, M. T. J . Mater. Res. 1988, 3, 413. (8) Arkles, B. CHEMTECH 1977, 7, 766. (9) Leyden, D. E. Silanes Surfaces and Interfaces; Gordon Breach: New
York.
(IO) Hollaham, J. R., Bell, A. T., Eds. Techniques and Applications of Plasma Chemistry; Wiley: New York, 1974. ( 1 1) Boegin, H. V., Ed. Plasma Science and Technology; Cornell University Press: New York, 1982. (12) Yasuda, H. J . Macromol. Sci. Chem. 1966, AI0 (3). 15. (13) Yasuda, H. Plasma Polymerization; Academic Press: New York, 1985. (14) Rabinovich, Ya. I.; Derjaguin, B. V. Colloids Surf. 1988, 30, 243-25 1. (15) Eriksson, J. C.; Ljunggren, S.; Claesson, P. M. J. Chem. SOC.,Faraday Trans. 2 1989, 85, 163.
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6122 The Journal of Physical Chemistry, Vol. 93, No. 16, 1989
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ONTROL
Bare Mica
O(Auger)
ELECTRODES
40 Plasma
Figure 1. Schematic diagram of the plasma reactor used for preparing the surfaces. The chamber of the reactor is evaporated with a rotary vacuum pump. Water vapor gas is then introduced into the chamber and a rf voltage applied to the electrodes.
surfaces in water and a nonpolar liquid as well as activated mica in a nonpolar liquid.
Materials and Methods The plasma reactor (shown schematically in Figure 1) consists of a large glass vessel which is evacuated with a rotary vacuum pump to 1 O4 Torr. Two copper bands around the outside of the reactor are connected to a high-voltage rf generator (EN1 Power Systems Model HP G-2). The flow rate of the gas is controlled with a flow controller. Mica that has been cleaved and silvered on one side is glued to silica disks (silver side down) and then placed in the plasma reactor. After evacuation, water vapor is introduced and 20 W power is applied for 21/2 min. This was found to be the optimum condition.I6 The surfaces are then exposed to silane vapor in a preequilibrated vessel in a desiccator. The measured contact angle reaches its maximum after 3 min and the surfaces are removed and mounted in the surface force apparatus. Samples for ESCA analysis were treated in the same way as the disks for the experiments. A large sheet of freshly cleaved mica was used and the ESCA spectra were recorded on a Leybold Heraus spectrometer using A1 K a X-rays and a hemispherical analyzer. The number of atoms lost from the surface was quantified by the procedure described in ref 17. The thickness change in the mica after plasma and fluorocarbon silane treatment was determined as follows: Mica that had been glued to a silica disk (silver side down) was masked by placing another thin sheet of mica over half the surface. The disk was then treated and the mask removed leaving a disk with one half treated and the other half untreated. Another silvered piece of mica was then placed over the top of the disk (silver side up) and the difference in thickness between the treated and untreated parts was determined interferometrically. The surface forces apparatus has been described extensively elsewhere.I8 When the silica disks are mounted, with the mica surfaces facing each other, an optical cavity is formed and the mica-mica separation can be measured by a multiple-beam interferometry technique with a resolution of about 0.2 nm. The surfaces can be moved by expanding a piezoelectric ceramic tube or via a differential spring. Forces are measured with a spring device, whose deflection is calculated from a calibration of the movement in a region of zero force. The results of the force vs distance measurements are given as FIR which is the total measured force normalized by the mean radius of curvature of the surfaces. This quantity is equal to 2rE, where E is the corresponding interaction free energy per unit area (16) Parker, J. L.; Claesson, P. M.; Cho, D. L.; Ahlberg, A.; Tidblad, J.; Blomberg, E. J . Colloid Interface Sci., in press. (1 7) Herder, P. C.; Claesson, P. M.; Herder, C. E. J. Colloid Interface Sci.
1987, 119, 155. (1 8 ) Israelachvili, J. N.; Adams, G. E. J . Chem. SOC.,Faraday Trans. I 1978, 74, 975.
L
After Silanation
'ti Kinetic Energy e V
0
I
1600
Figure 2. ESCA spectra recorded for untreated mica (top), H 2 0 plasma treated (20 W, 2.5 min) mica before (middle) and after (lower) silanation with the fluorocarbon silane.
between flat surfaces.I9 The separation is measured relative to the adhesive contact minimum of the surfaces in pure water and the most adhesive minima in octamethylcyclotetrasiloxane (OMCTS). The theoretical DLVO curves were calculated by using an exact numerical solution to the nonlinear PoissonBoltzmann20equation and a nonretarded van der Waals interaction J which is the same as with a Hamaker constant of 2.2 X that measured for mica across water.'* OMCTS was obtained from Aldrich and distilled twice before use. (Tridecafluoro- 1,1,2,2-tetrahydrooctyl)-1-dimethylchlorosilane, henceforth referred to as fluorocarbon silane, was obtained from Petrarch Systems and used without further purification. Potassium bromide was obtained from Merck. Water was purified with a Millipore water purification system.
Results Surface Characterization. The thickness of the mica was found to be essentially unaltered after treatment with H 2 0 plasma (20 W, 2.5 min exposure, 1.5 sccm) when inspected interferometrically. Exposure of the treated mica to the fluorocarbon silane for 3 min renders the surface hydrophobic with an advancing contact angle of 93-96" and a receding angle of 70-75'. The contact angle measured was independent of the concentration of the electrolyte in the measuring solution. (Full details will be reported elsewhere.I6) The silane layer was 1.O (*OS) nm thick as estimated by interferometry. The large uncertainty is due to difficulty in (19) Derjaguin, B. V. Kolloid-2. 1934, 69, 155.
(20) Chan, D. Y.;Pashley, R. M.; White, L. R. J . Colloid Interface Sci. 1980, 77, 283.
The Journal of Physical Chemistry, Vol. 93, No. 16, 1989 6123
Plasma Modification of Mica
103 h
-k
5 lo2
s
10 0
0
2
4
6
8
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Y0=130mV
10 15 20 25 30
6
120
IO‘
10 --200
80
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D (nm) 5
40
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20
40
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80
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Figure 3. Force as a function of separation between H20 plasma treated
(20 W, 2.5 min) mica both before and after silanation with the fluorocarbon silane in octamethylcyclotetrasiloxane (OMCTS). The force is normalized by the radius of curvature of the surfaces and is proportional to the free energy of interaction between flat surfaces. The upper curve (A) shows the force before silanation; the dashed lines indicate inaccessible regions of the force curve. The dotted line gives the force envelope for untreated mica in OMCTS.24The lower curve (B) shows the force between similar surfaces after silanation with the fluorocarbon silane. The solid line is the theoretically expected nonretarded continuum van der Waals force with a Hamaker constant of A = 1.4 X J (the value expected for mica across OMCTS).26
focusing the optical system. The ESCA spectra for mica, activated mica, and silanated mica are shown in Figure 2. It is clear from the spectra that the amounts of potassium, aluminum, and oxygen are reduced after activation whereas the silicon signal remains nearly the same. The decrease in the potassium and aluminum signals corresponds to a loss of 1.7 X lOI4 and 3.7 X lOI4 atoms cm-2, respectively. For comparison, the number of potassium and aluminum atoms in the surface layer of mica is 2.1 X 1014. The ESCA spectrum after treatment of activated mica with the fluorocarbon silane shows a large fluorine peak. The peak area indicates that 2.7 X 10l5 molecules are bound per cm2 and this gives a surface density of one silane molecule per 45 A2. Force Measurements. The force measured between activated mica in dry OMCTS is shown in Figure 3A. The force is a decaying oscillatory function of surface separation (for comparison the dotted line shows the “force envelope”, obtained by joining adjacent maxima and adjacent minima, measured between bare mica surfaces).24 The measured force has a period of oscillation of 0.8 nm. The surfaces come to molecular contact (D = 0 in Figure 3A) and the pull-off force is 40 & 5 mN m-I. On all occasions the force measured after separation from molecular contact was strongly repulsive and surface damage was observed in the fringe pattern. All the points shown in (Figure 3A) were measured before the surfaces were brought into molecular contact (D = 0). The force measured between activated mica after silanation in dry OMCTS is shown in Figure 3B. The advancing contact angle of OMCTS on a similarly prepared surface was found to be 15’. The measured force is everywhere attractive and the surfaces jump (when the gradient of the force exceeds the spring constant) into contact from a separation of 4.5 f 0.2 nm. The pull-off force from this position was found to be 12 mN m-l. The surfaces were not damaged on separation. (21) Christenson, H. K. J . Phys. Chem. 1986, 90, 4.
D (nm) Figure 4. Force as a function of separation between mica after
H20 plasma treatment (20 W, 2.5 min) and silanation with the fluorocarbon silane in water (A) and at M KBr (B). The upper solid line in the top figure is force calculated from DLVO theory with constant charge boundary conditions; the middle line is with constant potential (73 mV) boundary conditions. The lower line is calculated from DLVO theory with an additional exponential attraction (0.8 (mN m-l) exp(-D/12 nm)). The lower figure (B) shows the force between similar surfaces in M KBr solution. The solid line is the calculated DLVO force with constant potential boundary conditions and an additional exponential attraction (3.7 (mN m-l) exp(-D/8 nm)). The forces between silanated mica surfaces immersed in water and M KBr are shown in Figure 4. In water the force remains repulsive down to surface separations of 40 nm; the force then begins to deviate from that predicted by DLVO theory and the surfaces jump into contact from a separation of 35 nm. The pull-off force from the contact position was large, 400-500 mN m-I, and occasionally separation damaged the contact region. The decay length of the measured force in lo4 M KBr ( K - I = 22 nm) M). This is due is shorter than expected ( K - I = 30 nm for to a combination of experimental error and a large background electrolyte concentration (5 X lo-’ M). The magnitude of the repulsive force and the adhesion between the surfaces varied from experiment to experiment and was found to depend critically on the surface preparation conditions. The forces were very similar for different sets of surfaces with similar contact angles. The results were highly reproducible for experiments with one set of sheets even at different contact positions. In a few experiments vapor cavities were observed between the surfaces in contact.22 This was not investigated in detail and no cavities were observed away from contact or during force measurement.
Discussion Surface Structure. The absence of any measurable change in the thickness of mica after activation does not, by itself, indicate that the surface is smooth. The interferometric technique is insensitive to changes in roughness which occur over a region in size less than the wavelength of light. This means that large undulations may exist with the average thickness unchanged. The surfaces must, however, be highly smooth because oscillatory solvation forces were measured. The reduction in potassium and aluminum observed in the ESCA spectra after plasma treatment indicates that these atoms are selectively sputtered from the surface. The structure of muscovite mica is shown in Figure 5. The mineral consists of (22) Christenson, H. K.; Claesson, P. M. Science 1988, 239, 390.
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The Journal of Physical Chemistry, Vol. 93, No. 16, 1989
1
H 2 0 PLASMA TREATMENT
0 Oxygen
@ Potassium
0
Silicon
0
Aluminum
0
Hydrogen
Figure 5. Crystal structure of mica, shown schematically, before plasma treatment and a hypothetical structure after treatment. The plasmatreated structure shows the potassium and aluminum removed in the same ratio obtained from ESCA analysis.
aluminosilicate layers held together by ionic bonds to potassium ions. Mica cleaves along the potassium-oxygen layer because the ionic bonds are much weaker than the covalent bonds within the lower layer. The surface exposed in this manner consists of oxygen atoms which form covalent bonds to silicon or aluminum with a 3:l ratio. The negative lattice charge of mica which is neutralized by the potassium ions is due to the 1 in 4 substitution of aluminum for silicon. During plasma treatment loss of potassium through sputtering takes place because this outermost layer is only ionically bound. The ESCA spectra indicate that all the potassiums are removed from the surface layer after 2.5 min of treatment. Loss of potassium from the surface would create a negative charge in the surface layer. This unfavorable situation can be avoided by loss of oxygen or aluminum oxide from the surface or by the addition of a proton. Silicon is not removed because of its low sputter yield. The mechanism by which this sputtering takes place is unclear, but the result is a largely silicon oxide like surface layer. Hydroxyl groups or other species from the plasma may be incorporated in the crystal. The fact that the treated mica is reactive to chlorosilanes provides circumstantial evidence for this incorporation. A likely structure for activated mica is shown in Figure 5. The figure shows the potassium ions removed and several aluminum atoms removed from the crystal. Silanation with the fluorocarbon silane produces a surface layer with a thickness which is in reasonable agreement with the extended length of the molecule (as estimated from molecular models). The observed contact angle hysteresis, although smaller than that found with Langmuir-Blodgett films on suggests that the surface is not completely homogeneous. Forces in OMCTS. The forces measured between activated mica surfaces in OMCTS are qualitatively similar to those found between untreated mica surfaces and the adhesion in molecular (23) Claesson, P. M.; Christenson, H. K. J . Phys. Chem. 1988, 92, 1650.
contact is only slightly lower.24 The periodicity of the oscillations is close to the molecular diameter of the OMCTS molecule. The magnitude of the repulsive part of the force law is, however, considerably smaller for activated mica. It has been reasonably well established that surface roughness affects the range and strength of these oscillatory solvation forces.21 For example, the forces between mica surfaces coated by deposition of a double-chain surfactant, dioctadecyldimethylammonium bromide (DDOAB) in 0MCTS:I are much smaller in magnitude and range when compared with the force found between bare mica surfaces and the forces observed here. Similarly, the forces between one metal surface and a single mica surface also show a shorter range and reduced strength despite the fact that one surface is molecularly ~ m o o t h . The ~ forces between surfaces coated with CTAB by adsorption from solution were purely attractive and showed no oscillations.21 It was argued that this difference in behavior between the two surfactant surfaces was due to the difference in roughness. The packing density for the deposited DDOAB (0.28 nm-2 per hydrocarbon chain) is much higher than for CTAB (0.55 nm-2) adsorbed from solution. This difference in packing density causes a change in roughness and allows penetration of the solvent into the layer. The forces between the silanated mica surfaces are qualitatively similar to the forces found between CTAB surfaces prepared by adsorption from solutionthe force remains attractive and shows no oscillatory behavior. This is also likely to be due to surface roughness which results from the relatively low surface coverage of fluorocarbon silane. Despite being highly hydrophobic, these surfaces are also charged in water. This indicates that parts of the underlying mica substrate must be accessible to the aqueous environment. Forces in Water. In pure water fluorocarbon and hydrocarbon surfaces prepared by Langmuir-Blodgett deposition show no repulsion but a strong attractive f o r ~ ewhich ~ ~ , is~ well ~ described (in the regime 30-90 nm) by an exponential force law F / R (mN m-l) = -A exp(-D/X) where A = 2 mN m-l and X = 12-16 nm. In the presence of salt the measured force law can be well fitted by adding an exponential attraction to the double layer force obtained by fitting the data a t large ~ e p a r a t i o n . ~ ~ The attractive force between silanated mica surfaces in water is much larger than the attraction calculated from van der Waals theory. (The effect of the silane layer is to reduce the effective Hamaker constant at small separations.) The forces shown in Figure 4 are well fitted by adding an exponential attraction to the double layer force obtained by fitting the data at large separation. The parameters used are similar to those required to fit the force between fluorocarbon surfactant coated surfaces at 1O4 M tetrapentylammonium bromide25( A = 2 mN m-l and X = 12 nm) which, considering the large difference in surface preparation techniques, is comparable to the values of A = 3.7 mN m-l and X = 8 nm required to fit the data in Figure 4. The surface potential for the plasma prepared surfaces is slightly higher than the potential measured between the fluorocarbon surfactant surfaces in the presence of salt. Conclusions Mica, which is normally chemically inert, can be activated toward reaction with chlorosilanes by treatment with water vapor plasma. These activated surfaces maintain a high degree of smoothness and oscillations are measured in OMCTS. Hydrophobic surfaces prepared by silanation of activated mica are very stable and the forces in water show an attraction characteristic of the hydrophobic interaction. Hence, the hydrophobic interaction observed in surfactant systems cannot be due to instabilities of the surfactant films. Finally, it should be emphasized that the technique described here for the first time is a general technique. Plasma-treated mica is reactive to a large number of different (24) Christenson, H. K.; Blom, C. E. J . Chem. Phys. 1987,86,419. (25) Christenson,H. K.; Claesson, P. M.; Berg, J.; Herder, P. C. J . Phys. Chem. 1989, 93, 1472. (26) Horn, R. G.; Israelachvili, J. N. J . Chem. Phys. 1981, 75, 1400.
J . Phys. Chem. 1989, 93, 6125-6128 silanes. It should be possible to produce stable and well-defined surfaces with almost any desired surface property for study with the surface force technique. Acknowledgment. We thank H. K. Christenson for his help
6125
in preparation of the manuscript and J. C. Eriksson and P. Stenius for their interest and support of this work. Registry No. OMCTS, 556-67-2; (tridecafluoro-l,1,2,2-tetrahydrooctyl)-I-dimethylchlorosilane,102488-47-1.
Surface-Enhanced Raman Scattering and Surface-Enhanced Resonant Raman Scattering Studies of Perylenetetracarboxylic Derivatives on Ag-Coated Sn Spheres and Ag and Au Island Films U. Guhathakurta-Ghosh and Ricardo Aroca* Department of Chemistry and Biochemistry, University of Windsor, Windsor, Canada N9B 3P4 (Received: November 8, 1988; In Final Form: February 21, 1989)
Surface-enhanced Raman scattering (SERS) and surface-enhanced resonant Raman scattering (SERRS) of perylenetetracarboxylic dianhydride and diimide have been studied on three different SERS-active surfaces. SERS and SERRS spectra obtained on metal island films of Ag and Au, with well-defined plasmon resonances in the visible region of the spectrum, are compared with the surface-enhanced scattering obtained on Ag-coated Sn spheres. It is shown that the latter surface is SERS active in a wide spectral region encompassing that of the Ag and Au island films. A negative dependence for the SERRS intensity with the increase of surface coverage emerged from the experimental data. Molecular spectra obtained by SERRS at submonolayer surface coverage provided the required information for a definite vibrational interpretation of multiplets observed in the Raman spectra of thin solid films of both compounds.
Introduction Resonant Raman scattering (RRS) and IR spectra of thin solid films of 3,4,9,1O-perylenetetracarboxylicdianhydride (PTCDA), and 3,4,9,10-perylenetetracarboxylic N,N'-bis(methy1)diimide (PTCDIMe) have been previously reported.'-* Both molecules show electronic absorptions in the visible region, in resonance with laser frequencies found, for instance, in the Ar+ and Kr+ CW lasers. Metal island films of Ag and Au with plasmon absorptions in the same spectral region can be prepared by controlled metal evaporation. Therefore, the Ar+ or Kr' laser lines can excite the lowest order particle plasmon for electromagnetic e n h a n ~ e m e n t ~ , ~ of the scattering cross section of molecules adsorbed at the particle surface and produce resonant Raman enhancement due to excitation in resonance with the molecular electronic absorption (SERRS). The double resonance could provide Raman signals for molecular identification in the femtomole region,5 with enormous potential for analytical applications. In the search for a SERS-active surface with a wide range of applications we have tested a well-characterized structure formed by the evaporation of Sn onto a warm glass substrate and coated with Ag.6 It has been shown7q8that Sn films grown at the appropriate substrate temperature form a two-dimensional distribution of spherical islands, a basic structure that is preserved by the Ag coating. In Ag and Au island films, spheroidal particles with dimensions in the range 10-100 nm have been found to produce the strongest enhancemenkg The average diameter of Sn spheres formed in a 100-nm Sn film is ca. 530 nm. However, the surface of the Sn spheres was found to be rough6 and comparable to an aggregated metal colloid, a well-known SERS-active surface in sols experi~~
( I ) Akers, K.; Aroca, R.; Hor, A. M.; Loutfy, R. 0.J . Phys. Chem. 1987, 91, 2954. ( 2 ) Akers, K.; Aroca, R.; Hor, A. M.; Loutfy, R. 0. Spectrochim. Acta 1988, 44A, 1129. (3) Surface Enhanced Raman Scattering; Chang, R. K., Furtak, T. E., Eds.; Plenum: New York, 1982. (4) Moskovits, M. Rev. Mod. Phys. 1985, 57, 783. (5) Zeman, E. J.; Carron, K. T.; Schatz, G.C.; Van Duyne, R. P. J . Chem. Phys. 1987,87, 4189. (6) Aroca, R.; Kovacs, G. J . J . Mol. Struct. 1988, 174, 53. (7) Murr, L. E. Thin Solid Films 1974, 20, 81. (8) Kovacs, G. J.; Vincett, P. S. Thin Solid Films 1984, 1 1 1 , 65. (9) Wokaun, A. Mol. Phys. 1985, 56, 1.
ments. Boyd et al.l0 have recently shown for Ag, Cu, and Au that the effect of surface roughness on the photoinduced luminescence enhancement was due to local field enhancement in the rough surface protrusions. Similarly, the Ag-coated Sn spheres with a rough surface seem to enhance the local optical fields via localized plasmon resonances, within a wide range of the visible spectrum. Finally, SERS and SERRS were used here to obtained the molecular spectra of PTCDA and PTCDIMe at submonolayer coverage of the dye, in order to resolve the problem posed by the RRS spectra of thin solid films where a number of doublets were believed to be due to solid-state splitting.
Experimental Section Metal island films were prepared by slow vacuum evaporation (0.1 nm/s) of the metal onto warm substrates (glass slides kept at 200 "C), and the thickness was monitored with a quartz crystal oscillator. Control of the substrate temperature during deposition permitted the growth of a film with a highly homogeneous distribution of particle shapes. The plasmon absorptions of the IO-nm Ag and 4-nm Au films were broad and centered at 490 and 600 nm, respectively. Ag-coated Sn spheres6 were formed by evaporating 100-nm mass thickness of Sn at a rate of 0.5 nm/s onto a glass substrate heated at 120 O C . The spherical-island Sn structures formed on the glass substrate were then overlaid with 100 nm of Ag holding the substrate at room temperature. Transmission electron microscopy of the Ag-coated Sn surface (see Figure 1) have shown that the Ag overcoating preserved the surface details of the Sn spheres. In a separate evaporator used for preparation of organic films, overlayers of 1- and 5-nm mass thickness of PTCDA and PTCDIMe were evaporated onto the SERS-active surfaces. The 488- and the 647.1-nm lines of the Ar+ and Kr' ion laser were used, and all four polarized spectra were routinely measured: SS, SP, PP, and PS. In Porto's notation this is Z ( Y Y ) X = S S , Z( Y Z ) X = SP, Z ( X Y ) X = PS, Z ( X Z ) X = PP. For example, for a right-angle scattering the notation Z ( Y Y ) X indicates incident light propagating in the Z direction and polarized in the Y direction, which is scattered into the X direction with Y polarization. In the experimental geometry the ZX plane was the plane of incidence and the scattering plane and (IO) Boyd, G. T.; Yu, Z. H.; Shen, Y . R. Phys. Rev. 1986, B33, 7923.
0022-3654/89/2093-6125$01.50/00 1989 American Chemical Society
