Anal. Chem. 1994,66, 261-270
Thin Ag Films: Influence of Substrate and Postdeposition Treatment on Morphology and Optical Properties Shane E. Roark and Kathy L. Rowlen' Department of Chemistry and Biochemistry, University of Colorado, Boulder, Colorado 80309-02 15
In an effort to understand the experimental parameters that influence thin metal f h morphology and optical characteristics, thin Ag films are examined with a combinationof atomic force microscopy (AFM), optical absorption, and surface-enhanced Raman spectroscopy (SERS).The morphology of 5 nm of Ag vapor deposited onto glass, derivatized glass, Formvar-coated glass, and mica is explored. The substrate is found to have a large effect on both Ag film surface morphology and optical properties. In addition, micrographs of a Ag film before and after exposure to solvent suggest solvent-inducedmorphological changes. Surface-enhanced Raman spectroscopy (SERS) has become an important spectroscopic tool in fields ranging from materials science to biochemistry.' The primary reasons for such widespread application of this technique are straightforward; SERS is extremely sensitive, is surface selective, and provides vibrational information with visible excitation. While SERS is typically used for probing molecules at rough metal surfaces, by taking advantage of long-range enhancement it is possible to probe molecules on a variety of dielectric surfaces as ell.^,^ Perhaps the chief limitation of SERS has been the lack of a universal, quantitative understanding of the relationship between metal substrate nanostructure, optical properties and the magnitude of e n h a n ~ e m e n t .Many ~ of the trends observed in SERS are not always reproducible; the literature contains many examples of conflicting Such discrepancies are most likely due to the difficulty in reproducing surface morphology. Thin metal films (TMFs) have played a central role in the current level of knowledge of the surface enhancement effect.8 TMFs are often the surface of choice for SERS due to the ease with which they are produced and their relative stability. TMFs are attractive for a variety of additional applications due to their unique optical and electronic properties. For example, the large enhancements of electromagnetic fields within TMFs have stimulated interest in their use in solar c e l l ~ . ~Thus, J ~ a thorough understanding of the relationship between T M F morphology and optical properties would have ramifications in other research fields as well.
* Author to whom correspondence should be addressed. (1) Otto, A.; Mrozek, I.; Grabhorn, H.; Akemann, W. J.Phys.: Condens.Matter 1992, 4, 1143. (2) Murray, C. A.; Allara, D. L.; Rhinewine, M. Phys. Rev. Lett. 1981, 46, 57. (3) Walls, D. J.; Bohn, P. W. J. Phys. Chem. 1989, 93, 2976. (4) Cotton, T. M.; Kim, J.; Chumanov, G. D. J. Raman Spectrosc. 1991,22,729. ( 5 ) Schlegel, V. L.; Cotton, T. M. Anal. Chem. 1991, 63, 241. (6) Van Duyne, R. P.; Hultecn, J. C.; Treichel, D. A. J. Chem. Phys. 1993, 99, 2101.
(7) Herne,T. M.;Ahern,A. M.;Garrell, R. L. J.Am. Chem. SOC1991,113,846. (E) Moskovits, M. Rev. Mod. Phys. 1985, 57, 783. (9) Hayashi, S.;Kozaru, K.; Yamamoto, K. SolidState Commun. 1991,79,763. (10) Anderson, W. A.; Delahoy, A. E.; Milano, R. A. Appl. Opt. 1976, 15, 1621.
0003~2700/94/0366-0261$04.50/0 0 1994 American Chemical Society
Numerous studies have been conducted on the optical properties and surface morphology of different TMF/substrate ~ o m b i n a t i o n s . ~ ~It- ~is5 well known that the unique characteristics of TMFs strongly depend on their surface morphology, which, in turn, is influenced by the substrate.lG20 For example, Gajdardziska-Josifovska et aZ.l 6 found significant qualitative differences in the optical, electrical, and microstructural properties of thin Ag films on amorphous (carbon and fusedsilica) and crystalline (NaCl) substrates. On the basis of scanning electron microscopy (SEM), the authors concluded that Ag films on amorphous substrates were qualitatively similar. However, Davis et aZ.18 compared both the optical properties and the microstructure of thin Ag films on the same amorphous substrates and noted that, while the microstructure was qualitatively similar on both substrates, substantial differences in optical transmittance were observed. Studies of other TMF/substrate combinations, such as Au on alkali halides,20 yield equally qualitative information about the influence of substrate on T M F morphology. In order to control T M F optical properties, a more quantitative understanding of surface morphology is required. TMFs have traditionally been characterized with electron microscopy. SEM can be utilized but typically requires coating the sample with a contiguous layer of conducting material and, as pointed out by Van Duyne et a1.,6 transfer of the SERS substrate into a vacuum may perturb the surface morphology. Transmission electron microscopy (TEM) offers better resolution than SEM and is therefore widely used to characterize T M F surface morphology. However, TEM requires that the film be deposited onto a surface transparent to electrons (e.g., Formvar). In addition, neither SEM nor TEM offer a direct measure of the height of surface features (Le., Z-axis measurements). For example, with TEM the average height of surface features is often estimated from the deposited mass.21 Atomic force microscopy (AFM) is an excellent alternative for surface studies of nonconducing surfaces, e.g., TMFs of less than 15-nm thickness.6J8 AFM provides the necessary (1 1) Sennett, R. S.; Scott, G. D. J. Opt. Soc. Am. 1950, 40, 203. (12) Johnson, P. B.; Christy, R. W. Phys. Rev. B 1972, 6, 4370. (13) Yamamoto, M.; Namioka, T. Appl. Opt. 1992, 31, 1612. (14) Shawki, G. S. A; El-Sherbiny, M. G.; Salem, F. B. Thin Solid Films 1981, 75, 29. (15) Yano, M.; Fukui, M.; Haraguchi, M.; Shintani, Y. Surf.Sci. 1990,227,129. (1 6) Gajdardziska-Josifovska,M.; McPhedran, R. C.; Cockayne, D. H.; McKenzie, D. R.; Collins, R. E. Appl. Opt. 1989, 28, 2736. (17) Gajdardziska-Josifovska, M.; McPhedran, R. C.; Cockayne, D. H.; Collins, R.E. Appl. Opt. 1989, 28, 2744. (18) Davis, C. A.; McKenzie, D. R.; McPhedran, R. C. Opt. Commun. 1991,85, 70. (19) Varnier, F.; Mayani, N.; Rasigni, G.; Rasigni, M.; Llebaria, A. J. Vac. Sei. Technol. A 1987, 5,1806. (20) Robins, J. L.; Donohoe, A. J. Thin Solid Films 1972, 12, 255. (21) Kovacs, G. J.; Loutfy, R. 0.;Vincett, P. S.;Jennings, C.; Aroca, R. fungmuir 1986, 2, 689.
Analytical Chemistty, Vol. 66,No. 2, January 15, 1994 281
advantage of probing the exact structure of nonconducting surfaces directly. No sample preparation is necessary, and a wide scan range is available. Individual surface features can be characterized in all three dimensions. Since the entire micrograph is digitized, distribution of surface features, such as height histograms, can be generated. These attributes offer a means to study T M F morphology in a manner not previously With the long-term goal of developing a precise theoretical relationship between surface morphology and resulting optical properties, including SERS, it seems clear that a thorough understanding of the parameters that affect surface morphology must be achieved. In order to further the theory of SERS, it is essential that the rough metal surface nanostructure be measured under the same conditions as the SERS experiment. In this work, the influenceof substrate and SERS sample preparation on the morphology of thin Ag films is quantitatively explored with a combination of atomic force microscopy, optical absorbance, and SERS. EXPERIMENTAL SECTION Slide Preparation. Precleaned microscope slides were soaked for 1-2 h in 3 M NaOH, followed by rinsing and sonicating four times in doubly deionized water. The slides were dried either by heating a t 120 "C for 2 h or by sonicating four times in dry methanol (Mallinkrodt) and dry acetone (Mallinkrodt) followed by heating at 120 "C for 0.5 h. Thin Formvar (Aldrich) layers were applied to the microscope slides by dip coating a t a controlled rate (gearhead motor, Edmund Scientific) of -4-5 cm/min from a solution of Formvar in chloroform (Mallinkrodt). Formvar is the commercial name for poly(viny1 formal), a transparent polymer with a refractive index of 1.5. The thickness of the Formvar layer was controlled by varying the concentration of the dip solution. Triphenylsilane derivatized glass was prepared by first refluxing the clean slides in dry acetone under nitrogen, followed by refluxing (2 h) with triphenylsilyl chloride (97% Aldrich) in toluene (under N2). The slides were then copiously rinsed with dry toluene and dry acetone. Derivatization was verified with UV-vis absorbance. Silver powder, 5-8 pm, was used for the preparation of Ag films (Aldrich, 99.9+%). Thin Ag films were deposited with a Denton Vacuum DV-502 vapor-phase depositor. The pressure inside the deposition chamber was -5 X Torr. The film thickness and the deposition rate were monitored with a factory calibrated, temperature-controlled, quartz crystal microbalance (Syncon Instruments STM-100). The microbalance crystal was placed adjacent to and in the same plane as the substrate. The substrate was positioned 22 cm above and parallel to the source. A deposition rate of 0.02 f 0.005 nm/s was used for all the depositions in this work. The substrate was shielded from the source until metal vaporization began. Throughout the deposition, adjustment of the source voltage was necessary to compensate for slight drift in the deposition rate. It should be recognized that this may be a significant source of error when interlaboratory comparisons are made. The temperature of the substrate within the vacuum chamber was monitored periodically over several depositions (over several months of operation) and was found to increase -5 "C during a deposition of 5 nm of 202
Analytical Chemistry, Vol. 66,No. 2, January 15, 1994
Ag a t 0.02 nm/s. For a given Ag film thickness, Ag was deposited simultaneously onto both glass and Formvar substrates. SERS samples were prepared by dip coating the analyte onto the previously prepared Ag surfaces from a 1 X lo4 M solution of the analyte in acetone (Mallinkrodt). Benzoic acid (99% Mallinkrodt) and zinc tetraphenylporphine (ZnTPP, 98% Aldrich) were used as received. Measurements. Formvar layer thicknesses were determined with a Tencor Instruments a-Step profilometer. The profilometer was calibrated with a 850-nm step standard; from six measurements the mean and standard deviation were 850 f 10 nm. The Formvar solution %(wt/wt) concentrations and corresponding Formvar layer thicknesses for layers dip coated a t a controlled rate were as follows: 0.5% (86 f 2 nm), 1% (143 f 15 nm), 2% (294 f 48 nm), 3% (463 f 14 nm), 4% (725 f 7 5 nm), and 5% (910 f 45 nm) for 3-5 separate experiments each. For comparison, Formvar layers prepared a t an uncontrolled rate (hand dipping) gave the following: 0.5%(87*2nm), 1 % ( 1 1 3 f 5 n r n ) , 3 % ( 2 4 8 * 15nm),and 5% ( 1325 f 38 nm) for 3-5 separate experiments each. Handdipped samples were found to be less uniform over the length of the slide. Optical absorption data were obtained on a HewlettPackard 8452A diode array. The Raman instrument is comprised of an air-cooled argon ion laser (ILT), a premonochromator (OptometricsTGF-302), a half-wave plate (Melles Griot), a 40-cm focal length lens ( J M L Direct), a custombuilt back-scattering cell made from black Delrin, a f/l.7 collection lens, a holographic edge filter (Physical Optics Corp.), a f/4 focusing lens, a single 0.5 m-f/4 spectrograph with 1200 grooves/mm grating (Spex 500M), and a liquid nitrogen-cooled charge-coupled device (CCD) with a 384 X 576 pixel format (Princeton Instruments). The CCD was oriented such that the spectrum was collected over the long axis (Le., over 576 pixels). The 514.5-nm line was used for excitation, and the entrance slit width was 200 pm, The incident laser beam (parallel polarization) was oriented -65O to the sample surface normal. Using 514.5 nm of light, approximately 800 cm-1 can be sampled simultaneously on the CCD. AFM images were obtained in air using a Digital Instruments Nanoscope I11 scanning probe microscope with an etched single-crystal silicon probe tip (Digital Instruments). On average, the tips were 10-12 pm in length with an internal tip angle of 35O and had a tip radius of curvature of 10 nm and a spring constant of 0.29 N / m . All images shown were acquired in the constant force mode a t a scan rate of 6.8 Hz. The effect of tip size on surface feature measurements is assumed to be minor due to the oblate nature of the particles; the average angle between adjacent particles is approximately 137'. However, it is important to note that the morphology shown in the micrographs is a convolution of the true morphology and the tip function. Deconvolution represents a challenging problem due to uncertainty in the geometry of the scanning tip.22,23 The data shown in Table 1 were obtained from Fourier filtered (rejection of high-frequency noise) images. AI1 other (22) Burnham. N . A.; Colton. R. J.; Pollock. H.M. J . Vuc. Sci. Techno/. A . 1991. 9, 2548. 123) Arrifocts in S P M , Technical note, Topometrix Corp.. 1991
Table 1. Idand Size and Dkrtrfbutlon Values: Comparison of Sampllng Method’
height (nm) radius (nm) aspect ratio
glass (A)
Formvar (A) (86 nm)
5.8 f 1.1 15 f 3.2 0.4 f 0.06
6.8 f 1.4 14 f 3.4 0.5 f 0.2
5 nm of Ag on Formvar (A) (910nm) glass (B) 7.6 f 1.4 15 f 3.1 0.5 f 0.1
Formvar (B)(86 nm)
Formvar (B) (910nm)
5.7 f 1.1 11 f 1.4 0.5 f 0.1
6.7 f 1.2 12 f 2.2 0.6 f 0.07
4.8 f 1.2 12 f 2.3 0.4f 0.08
a 5 nm of Ag was simultaneously deposited on each of the three substrates. The values given on the left side (set A) of the table were obtained from the measurement of 30 particles within a 0.25 pm X 0.25pm area. The values on the right side (set B)were obtained from the measurement of 30 particles within a 0.75 pm X 0.75 pm area on the same micrographs.
data were obtained from unfiltered images. The micrographs shown are unfiltered, raw images. Quantitative AFM data were obtained by first selecting “counting boxes” on each image. The height and radius of individual islands were determined manually from cross-section line profiles. Manual measurements were made, rather than relying on manufacturer’s software analysis routines, in order to obtain welldefined measurements. For each image, three areas were chosen (-25% of the image) for line profile analysis. Within each area, random line profiles were selected, and height and radius measurements were obtained for all particles that were bisected by the line. For height measurements, the cursors were positioned at the maximum height of each particle and the lowest point on the line profile, such that all particle heights within that line were relative to the same base value. The base values for all line profiles within a given micrograph were approximately the same. For radius measurements, the cursors were positioned at the maximum height and the base of each particle. Ten particles were quantified within each of the three areas; thus, a total of 30 particles were randomly selected and quantified for each image. The aspect ratio was calculated for each particle. The distribution in particle parameters was calculated as f l a about the mean. The images were taken in random order and with different tips in order to verify that the observed trends were not due to instrumental artifacts. Although no top view of individual micrographs are presented here, the particles appear circular when viewed from above.24 Micrographs of bare glass and Formvar substrates were also obtained and revealed no discernible structure at the resolution employed for the Ag micrographs (1 pm X 1 pm scan area).
610
4
T
I
-I,,
,
,
,
,
,
,
Influence of Substrateon Morphology. Formuar. Sennett and Scott,” and more recently Schlegel and C o t t ~ n used ,~ TEMs of Ag films on Formvar and optical data of Ag films on glass to relate the surface morphology of Ag films to their optical properties. Our first step in developing a quantitative understanding of the influence of substrate on thin metal film morphology focuses on a comparison of Ag film morphology on glass and Formvar. In a previous study of the optical properties of Ag as a function of t h i c k n e s ~ we , ~ ~found the optical characteristics of Ag deposited simultaneously onto glass and Formvar-coated glass to be quite distinct. Optical. Figure 1 is a plot of the wavelength at maximum
optical density for a 5-nm Ag film deposited simultaneously onto glass and a range of Formvar layer thicknesses. As the Formvar layer thickness increases, there is a sharp blue-shift (Ax = 70 nm) that gradually levels off but remains significantly blue-shifted with respect to Ag on glass. Thelargest difference in absorption maxima is observed between Ag on glass and Ag on the thinnest Formvar layer (86 nm). While this trend was consistent over the four data sets represented in the figure, other experiments performed much later exhibit a slightly different trend, Le., Ag on 910 nm of Formvar is sometimes blue-shifted with respect to Ag on 86 nm of Formvar. However, the spectrum of Ag on Formvar is always substantially blue-shifted with respect to Ag on glass. It should be noted that the refractive index of Formvar is essentially the same as that of glass (i.e., 1S ) ;therefore, the shift in Am,, is not expected to be exclusively due to dielectric effects.26 Optical absorption data indicate that the Formvar substrate strongly affects the morphology of thin Ag films. Atomic Force Microscopy. A qualitative evaluation of atomic force micrographs for 5 nm of Ag simultaneously deposited on glass (Figure 2A) and 9 10 nm of Formvar (Figure 2B) indicates that Ag particles on glass are comparatively short with a large radiusof curvature. Ag particles on Formvar clearly have a larger aspect ratio.
(24) Roark, S. E.;Rowlen, K. L. Chem. Phys. Lett. 1992, 212, 50. (25) Roark, S.E.; Rowlen, K. L. Appl. Spectrosc. 1992, 46, 1759.
(26) Goudonnet, J. P.; Bijeon, J. L.; Warmack, R. J.; Ferrell, T. L. Phys. Rm. B. 1991, 43, 4605.
RESULTS AND DISCUSSION
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269
Flgure 2. Micrographs of 5 nm of Ag sirnuitaneousiy deposited onto (A) glass and (6) 910 nm of Formvar. Note that the Zaxis is 20 nmldiv.. whereas the X and Y axes are 200 nmldiv. The Z axis is exaggerated by a factor of 10 in order to aid in the visualization of the differences in surface features. Experimental conditions are detailed in the text. For Ag on glass. particle height. radius, and aspect ratio are 3.3 1.1, 12 3.0.and 0.3 0.7, respectively. For Ag on Formvar. height, radius, and aspect ratio are 7.6 f 2.1, 16 f 4, and 0.5 f 0.1, respectively.
*
*
Quantification. In order to evaluate thecapability of AFM to provide quantitative information about these particular surfaces, threecritical comparisons were made. Since particle size distributions are not necessarily G a ~ s s i a n ?the ~ method employed for particle sampling was examined. The effect of the number of particles sampled on the calculated average particle size was evaluated by comparing the mean and the relativeerror fromthreesamp1esets;eachsampleset contained 284
Analyilcal Chemlshy, Vol. 66, No. 2, Januaty 15, 1994
*
a specific number of particles randomly selected from a 0.75 pm X 0.75 pm area on a given micrograph. For 5 nm of Ag on glass (summarized in Table I), the percent relative error in particle height for 10,20,30,40,50,60,70, and 80 sampled particleswas 18.4,11.4,6.52,3.64,3.91,3.81,1.24,and0.99, respectively. Almost all of the error in measurement is associated with particle height; particle radius values deviate by less than 2% regardless of the number of particles sampled.
Tabk 2. Mean Idand Size Values: Comparkon of Depodtlons’ 5 nm of Ag on
glass (A) glass (B) glass (C) height (nm) radius (nm) aspectratio
4.61 12.6 0.38
3.33 11.8 0.28
4.83 11.6 0.42
x f u (8)
RE (% )
4.26 f 0.81 12.0 f 0.5 0.36 f 0.072
19 4 20
@Meanvalues were obtained from the quantification of 30 randomly selected particles within a 0.75 pm X 0.75 pm area of the respective micrographs. RE = relative error.
The error in height may be associated with the tip function, i.e., the tip may be too large to accurately sample “low spots” on the micrograph. This seems likely since many of the measured height values are less than the thickness measured by a quartz crystal microbalance (Le., 5 nm). Assuming that the height value from sampling 90 particles (4.98 nm) represents the true mean, the value from 30 particles (5.06 f 0.33 nm) encompasses the true mean. Considering the tradeoff between sampling error and analysis time, a sampling of 30 particles was deemed adequate. Development of a reliable, automated analysis routine would eliminate constraints on sampling the entire micrograph. The method of particle sampling was also examined by comparing the values obtained from 30 particles randomly selected within a small region (0.25 pm X 0.25 pm) of the micrograph and those from 30 particles selected from a larger region (0.75 pm X 0.75 pm). The mean values for particle height, radius, and aspect ratio, as well as the distribution in these parameters, are given in Table 1. In this case, the deviation about the mean is not exclusively measurement error, rather it is representative of the distribution of sizes. While the absolute values of the measurements made over the two areas are different, it is important to note that the trends are exactly the same. For example, the aspect ratio of Ag on glass, 86 nm of Formvar, and 910 nm of Formvar increases in both sampling cases. For the particular deposition summarized in Table 1, the mean radius of Ag particles on glass and 910 nm of Formvar is essentially the same. The mean height increases on going from glass to 910 nm of Formvar. The relative differences in each of the measurements between the two sampling areas range from 10% to 27%. Intuitively, sampling from a larger area, while more time-consuming, should provide a more representative measurement of the surface. The third comparison was conducted in order to examine the reproducibility in particle characteristics between various deposition experiments. The mean size values for 5 nm of Ag on glass are shown in Table 2 for three separate experiments carried out over a period of several months. Based on the results summarized above, 30 particles within an area of (0.75 pm X 0.75 pm) were sampled. Note that the relative error in size characteristics ranges from -4% to 20%. The lowest error is associated with radius, and the greatest error is associated with height. Propagating a relativeerror in particle height of 6.5%, as determined from sampling 3 X 30 particles on glass, it is estimated that 18% of the relative error in particle height is due to irreproducibility between experiments. A g on Glass and 910 nm of Formuar. Ag on Formvar consistently forms larger aspect ratio particles than Ag on
-
-
Table 3. Idand Slze and Dlstrlbullon on Varlous Substrates’
glass island height 4.6 f 1.4 (nm) island radius 13 f 3 (nm) aspect ratio 0.4 f 0.1 A, (nm) 580
nm of Aa on phenyl5 glass mica ~~
3.1 f 0.9 15 f 3 0.2 f 0.07 556
7.0 f 1.6 17 f 3 0.4 f 0.06 598
Formvar 7.6 f 2.1 16f4 0.5 f 0.1 558
5 nm of Ag simultaneouslydeposited onto each substrate. The column heading phenyl glass represents glass derivatized with triphenylsilane. The mica was cleavedand cleaned prior to deposition. Formvar thickness = 910 nm.
glass; a factor of 1.5 greater for the deposition presented in Table 1 and a factor of 1.7 greater for the deposition shown in Figure 2. The height of Ag particles on Formvar is also consistently higher than for Ag on glass; a factor of 1.4greater for the deposition presented in Table 1 and a factor of 2.3 greater for the deposition shown in Figure 2. However, the radius of Ag on Formvar varies from approximately equal to that of Ag on glass to a factor of 1.3 greater than that of Ag on glass. Cross-sectional views of the micrographs at a given percentage of the base-to-peak height (bph) allow for the evaluation of particle density. The number of particles contained within an area of 0.25 pm X 0.25pm wasdetermined both by manually counting features that were clearly distinguishable from background and by computer analysis. For the data shown in Table 1, at 75% bph the particles are clearly separated, and a manual count reveals 70 and 62 particles on glass and Formvar, respectively. For comparison, software analysis yielded 74 and 67 particles on glass and Formvar, respectively. At 50% bph there are 62 (63) and 40 (39) Ag particles on glass and 9 10 nm Formvar, respectively. A manual count from the three-dimensional images results in 82 and 66 particles for Ag on glass and 910 nm Formvar, respectively. Regardless of the counting process, there are consistently more particles on glass. Ag on 86 nm of Formvar. The intermediate case of 5 nm of Ag on an 86-nm layer of Formvar (over glass) is more difficult to quantify; the surface is far more heterogeneous. It is speculated that this “thin” layer of Formvar results in either incomplete coverage or, at least, the generation of energetically distinct sites, which may account for the differences in both aspect ratio and optical properties. In summary, AFM confirms, as indicated by optical absorption,*s that a Formvar substrate strongly influences the surface morphology of vapor-deposited Ag. This result indicates that the correlation of surface features, as measured by TEM, with optical properties may be somewhat misleading. Relationship between Morphology and Optical Properties. Before considering additional substrates, it is useful to summarize the relationship between morphology and optical properties for Ag on Formvar. Although in most of the cases presented here optical and AFM measurements were not made on the same sample, on the basis of the comparisons made in the previous section, it is assumed that the relative trends observed for Ag on glass and 910 nm of Formvar are representative. Comparison can be made between the observed optical and morphological trends and those predicted by Zeman Analytical Chemistry, Vol. 66,No. 2, January 15, 1994
265
0'
lo
1' 1 -
0
2
1'
260 360 460 560 660 760
Wavelength (nrn)
360 460 560 660 760 Wavelength (nrn)
Flgure 3. PanelA shows the opticalabsorption spectrum for a particular sample of 5 nm of Ag on glass before (highest 0.d.) and after dipping in acetone (lowest 0.d.). Panel B shows the optical absorption spectra for 5 nm of Ag on glass (lowest o.d.), 86 nm of Formvar (middle o.d.), and 910 nm of Formvar (highest 0.d.) after dipping in acetone.
Table 4. Optlcal Parameters before and after Solvent Exposure' 5 nm Ag on
glass A, before 626 f 2 A-after 601 f 6
86 nm of
Formvar
ALu
-25 f 6
599 f 2 597 f 2 -2 f 3
au before auafter Aau
0.52 f 0.12 0.41 f 0.02 -0.11 f 0.12
0.47 f 0.01 0.46 f 0.02 -0.01 f 0.02
910 nm of
Formvar
584 f 2 586 f 3 f 2 f4 0.45 f 0.02 0.47 f 0.02
+0.02 f 0.03
glass 591 f 6 567 f 9
-24 f 11 0.54 f 0.02 0.45 f 0.02 -0.09 f 0.03
a The abbreviation au represents absorbance units. The reported values are the mean f u for two (average f 0.5 ran e) to five measurements. The first three data columns (glass a n f Formvar) are optical measurements before and after dip coating in acetone a t a rate of -4.5 cmimin. Ag was simultaneously deposited onto these substrates. The last column is data from optical measurements made before and after dip coating in 10-4 M benzoic acid in acetone.
and SchatzS2' For a given semimajor axis, A,, of the surface plasmon should red-shift and increase in magnitude as the aspect ratio decreases. A direct comparison with optical theory is possible for the data presented in Table 1, since Ag on glass and Ag on 910 nm of Formvar exhibit the same mean radius, and Ag on glass has a lower aspect ratio. The optical data (see Figure 1) clearly show that the surface plasmon for Ag on glass is red-shifted and of greater magnitude, as predicted. Influence of Substrate on Morphology. Additional Substrates. An AFM study of 5 nm of Ag simultaneously deposited on a variety of surfaces was conducted in an attempt to sort out the important contributions to particle growth. Table 3 summarizes particle size characteristics for Ag on glass, derivatized glass, mica, and Formvar. Micrographs of each substrate prior to deposition revealed no discernible features within the scan range used (1 pm X 1 pm). If surface chemistry plays an important role in Ag particle growth, one would predict that derivatizing a glass surface with an olefinic or aromatic molecule might result in better wetting of the surface by Ag; thus, wider and shorter particles. As can be seen in Table 3, Ag on derivatized glass forms wider particles (27) Zeman, E. J.; Schatz, G. C. J. Phys. Chem. 1987, 91, 634.
266
Analytical Chemistry, Vol. 66,No. 2, January 15, 1994
with a much lower aspect ratio than for Ag on bare glass. As previously noted, Ag on Formvar forms much higher aspect ratio particles that are more isolated than on glass. In an attempt to estimate the influence of substrate roughness, Ag was also deposited onto atomically flat mica. While the size values indicate a moderate aspect ratio, close examination of the micrograph reveals that most particles are in contact and that there are large spaces with apparently no Ag. These attributes may be due to tip displacement of Ag particles. If however they are real, the true aspect ratio would be greater than the measured value, due to an artificially large measured radius. It is interesting to consider the possibility of a relationship between the Ag particle formation and the mobility of Ag on the surface. Robins and DonohoeZ0indicate that the mobility of metal nuclei may be a dominant contributor to particle formation. With the substrates studied here, one might predict reduced Ag atom mobility on glass derivatized with triphenylsilane due to chemical interaction between Ag and the phenyl rings (e.g., ir back-bonding). Since lower aspect ratio particles are observed for Ag on the derivatized glass, one might speculate that reduced mobility results in wider, lower aspect ratio particles. A Monte Carlo simulation of the deposition process is being developed in order to compare the effect of a variety of experimental parameters, including a metal atom-substrate interaction term, with observed trends.28 Influence of Solvent Exposure on Morphology. Optical. In order to correlate optical properties, surface morphology, and surface enhancement of Raman spectra, it is necessary to probe the surface under the conditions of the SERS experiment. Typically, the molecule of interest is deposited onto the SERS active surface via dip c ~ a t i n g or ~ .spin ~~ coating.6 The change in optical absorption observed upon the addition of a sample has been attributed t o dielectric effect^.^^^^^^^ While the dielectric of the sample undoubtedly affects the optical properties of TMFs, the very act of depositing t h e s a m p l e could p e r t u r b s u r f a c e morphology-resulting in a perturbation of the optical properties. We have attempted to quantify the degree of surface perturbation induced by dip coating. In all cases, the sample was exposed to air for 5 min to allow the solvent to evaporate prior to measurement. Figure 3A shows the UV-vis absorption spectrum for 5 nm on glass before and after dip coating in acetone. The effect of dip coating is drastic, A,, blue-shifts 26 f 6 nm and the extinction decreases. Table 4 summarizes the changes observed in optical parameters as a result of dip coating in acetone. Dip coating 5 nm of Ag on glass in methanol affects the surface to an even greater extent, blueshifting Amax -20% more than dip coating in acetone. However, dip coating 5 nm of Ag on glass in a 1 X lo4 M benzoic acid solution (acetone) has essentially the same effect on the optical properties as dip coating in acetone. While it is known that molecular transitions can couple with surface plasmons to affect a change in the optical absorbance of SERS substrate^,^^ this effect is limited to molecules whose transition
-
(28) Venalainen, 0.;Heinio, J.; Kaski, K. Phys. Scr. 1991, T38, 66. (29) Garoff. S.; Stephens, R . 9.;Hanson, C. D.; Sorenson, G . K . Opt. Commun. 1982, 4 1 , 257. (30) Kim, J.; Cotton, T M . ; Uphaus, R. A,; Mobius, D. J . Phys. Chem. 1989,93, 3713. (31) Glass, A . M.; Laio, P. F.; Bergman, J. G.; Olson, D. H. Opt. Lert. 1980, 5, 368
Figure 4. Micrograph of the Ag on glass sample in Figure 2A. before (A) and after (E) dipping in acetone. Particle size values ( A ) are summallzed In Table 5.
is close to that of the metal substrate. This is not the case for benzoic acid. For 5 nm of Ag on Formvar there is essentially no change in the optical parameters upon dipping in acetone; Ag on Formvar is much less susceptible to perturbation by solvent. Assuming that the changes in optical absorption for Ag on glassaredue toa solvent-inducedchangein thelocaldielectric, this is an intriguing observation since one would expect the samechangein dielectric for both surfaces. The lackof solvent influenceon Ag on Formvar may indicate that Agon Formvar is less susceptible to solvent-induced morphological changes.
Figure 3 8 shows the UV-vis absorption spectra, after dip coating in acetone, for 5 nm of Ag simultaneously deposited onto glass, 86 nm of Formvar, and 910 nm of Formvar. Note that Amax is essentially the same for all three surfaces. Atomic Force Microscopy. Figure 4 shows the micrograph ofAgonglass beforeandafterdipcoating inacetone. Figure 5shows themicrographsofAgonglass beforeandafterdipping in a 10-4 M acetone solution of benzoic acid. An attempt was made to image the same area on the sample after dip coating. Just as with the optical experiments, the sample was allowed to 'dry" for - 5 min prior to imaging. However, AFM data Anatyiimi Chemistry, Vol. 66, No. 2,January 15, 1994
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Figure 5. Micrograph of 5 nm 01 Ag on glass before dipping ( A ) and after dipping (5) in lo-‘ M benzoic acid in acetone. Sire are summarized in Table 5.
for the “after dip coating” samples should be regarded with caution since a small amount of liquid may influence measurements.” With that caveat. these micrographs (Figures4 and 5 ) suggest that the dip-coatingprocedure perturbs surfacestructure. Table5 summarizes thesizecharacteristics o f & on glass before and after dipping in acetone and in an acetonesolution of benzoicacid. While theoptical trends are the same for both dip-coating baths, the trends in particle aspect ratio are opposite. In the acetone case, particle height (32) Rugar. D.:Hansma. P. Phyr. Today 1990, 43, Ocl. 23.
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characteristics
and aspect ratio both increase, but the mean particle radius remains fairly constant. If the optical properties are interpreted in terms of morphological features, the trends are consistent with optical theory; a blue-shift in for an increase in aspect ratio with long axis. N ~ in ~ Table 5 that the distribution in particle height and radius decreases, From a visual comparison of the micrographs in Figure 4, one would readily concur that, after dipping, Ag particle sizecharacteristics appear moreuniform. Apparently, free Ag atoms, or small clusters of Ag atoms, are transported
~
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during the dipping procedure and deposited in larger cluster formations. For the benzoic acid case, the mean particle radius is much larger after dipping, while particle height has increased only slightly; the net result is a decrease in aspect ratio. Close examination of the Ag islands in Figure 5B,after dipping in benzoic acid, indicates that a large number of Ag islands have been "shoved" together, forming larger islands with distinct peaks. However, it is difficult to draw conclusions from this micrograph since quantification of surface features in this case may be strongly influenced by the presence of benzoic acid molecules. Assuming that surface tension holds a solution layer of 100-pm thickness at the surface during the dipping procedure, 6 X 10-l8 mol of benzoic acid would be deposited within a 0.75 pm X 0.75 pm area. Assuming a density of 1 g/cm3 implies that the thickness of the benzoic acid layer would be 1 nm. Although this is a crude estimate of the amount of benzoic acid deposited, 1 nm is sufficient thickness to affect the quantification of surface features. For example, if benzoic acid fills in some of the space between Ag particles, this would result in a superficially low height value for Ag particles; the aspect ratio would also be superficially low. Additional studies, with increased resolution, may provide the means to sort out the influence of deposited molecules. In summary, sample deposition on T M F surfaces appears to affect surface morphology. A more detailed study is necessary to sort out dielectric and morphological contributions to the solvent-induced changes in optical properties. Substrate Influenceon SERS. Formvar. Figure 6 displays the spectra of benzoic acid (dip coated from acetone solution) on 5 nm of Ag as a function of Formvar thickness. Once again, the samples were exposed to air for -5 min prior to measurement. It should be noted that the Formvar background was extremely weak and did not interfere with the Raman signal from the analyte. The inset in Figure 6 shows the relative intensity of the ring breathing mode at 1005 cm-I as a function of Formvar thickness. The maximum enhancement is obtained for 5 nm of Ag on 86 nm of Formvar. Both the optical properties and size characteristics of Ag on 86 nm of Formvar are not particularly reproducible; however, the trend in relative enhancement shown in Figure 6 is very reproducible (four separate experiments)-benzoic acid always exhibits maximum enhancement on the 86 nm of Formvar substrate, and the enhancement of glass and 910 nm of Formvar is approximately the same. The trend in SERS enhancement cannot be explained by relative differences between the excitation wavelength and,,A since the surface plasmon resonance of Ag, after dip coating, has maximum extinction at nearly the same wavelength for each surface (see Figure 3B).
800 IO00 1200 Raman Shift (cm") Flgure 6. SERS spectra of benzoic acki on varlous thlcknesses of Formvar: 0,87 f 2, 113 f 5,248 f 15, and 1325 f 38 nm. A 90-s Integration time was used for the acqulstbn of each spectrum. For visualization purposes, the spectra are spaced by a constant, with Increasing Formvar thlckness from the bottom (Ag on glass) to the top spectrum. The Inset shows the relathre Intensity of the 1005-cm-1 band for a single experiment. Four separate experiments were conducted over a perlod of several months. Whlie the trend was extremely reproducible, absolute lntenslties varled greatly from experiment to experiment. 400
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Raman Shift fcm-') Figure 7. Surfaceenhanced resonance Raman spectra of ZnTPP on various thlcknesses of Formvar (glven in the caption for Flgure 6). A 30-s Integration time was used for the acqulstion of each spectrum. Formvar thickness increases from the top (Ag on glass) to the bottom spectrum. The Inset shows the relative Intensity of the 1356cm-' band for a single experiment. Four separate experlments were conducted over a period of several months. Whlle the trend was extremely reproducible, absolute lntenslties varled greatly from experiment to experiment.
Goudonnet et a1.26 observed a similar trend for benzoic acid on a thin Ag layer deposited onto a layer of Si02 of variable thickness. The thickness of the Si02 layer, which was deposited onto Si, was varied from 0 to 50 nm. The authors found a distinct maximum in the Raman enhancement as a function of Si02 thickness (at 10 nm of Si02 for 514.5nm excitation). Through quantitative comparison with optical theory, the trend was attributed to differences in the local dielectric. The resulting dielectric having contributions from Si, SiOz, and the deposited molecule. Analytical Chemishy, Vol. 66, No. 2, Januery 15, 1994
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Formvar thickness (x) yields a line: 3.7 (k0.15)- 0.0039 (*O.O004)x, R = 0.99. Figure 8 displays the optical spectra of 5 nm of Ag simultaneously deposited onto glass, 86 nm of Formvar, and 910 nm of Formvar after dip coating in 10-4 M ZnTPP (acetone). For Ag on glass, the optical absorption spectrum after dip coating is complicated by interactions with electronic transitions in ZnTPP.5J5 Intriguingly, the absorption spectra of ZnTPP and Ag on Formvar appear to be simply additive. The exact nature of substrate influence on SERS spectra is unclear. Undoubtedly, substrate affects both surface morphology and the local dielectric and, therefore, optical properties. Deposition of solvent (sample) affects the local dielectric and appears to affect surface morphology and, therefore, optical properties. A more detailed study, in progress, is required to understand the individual contributions to the optical properties of TMFs.
Wavelength (nm)
Flgure 8. Optlcal absorption spectra for 5 nm of Ag after dip coating in a lo-' M acetone solution of ZnTPP. The Ag samples were simultaneously deposited onto glass (solid line with lowest 0.d. at 420 nm), 86 nm of Formvar (dashed line), and 910 nm of Formvar (solid line with highest 0.d. at 420 nm).
For comparison with benzoic acid, Figure 7 shows the spectrum of ZnTPP on 5 nm of Ag as a function of Formvar thickness. The inset is the relative enhancement of the 1356cm-1 band. In contrast to benzoic acid, ZnTPP exhibits maximum enhancement on glass. The enhancement decreases exponentially;linear regression of log (relative intensity) versus
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ACKNOWLEDGMENT We thank K. A. Douglas for use of the atomic force microscope. K.L.R. wishes to thank the Beckman Foundation for a Young Investigator Award and the Society of Analytical Chemists of Pittsburgh for a Starter Grant. This work was funded in part by a grant from the National Science Foundation (CHE-1533849). Received for review June 10, 1993. Accepted October 5, 1993." Abstract published in Aduonce ACS Abszrocts, November 15, 1993.
