2028
Anal. Chem. 1984,56,2028-2033 1
”
”
1
”
Co 345.5 /Ni
”
[
Co/Fe could be the stability difference between Fe and Ni(Co) oxide films. In addition to the elements described above, Al and Cr were also studied as target materials. However, these emission lines were absent or very weak in a spectrum measured in low power glow discharges ( 15 W). Since several suitable emission lines, which should be observable even in low power operations, do exist in both A1 and Cr (15),it is assumed that the number of sputtered atoms was small. It is generally known that very stable and firm oxide films (A1,03 or Cr203)are formed on A1 or Cr surfaces. Thus, sputtering yields are being influenced by the properties of oxide films and these effects are more pronounced with an increase in the stability of oxide films on the target surfaces as were pointed out with X-ray photoelectron studies (23). Registry No. Ar, 7440-37-1;Ag, 7440-22-4;Cu, 7440-50-8;Ni, 7440-02-0; Co, 7440-48-4;Fe, 7439-89-6.
361.9 nm
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Flgure 9. Power dependence of the intensity ratio (Co I 345.5 nmlNi I 361.9 nm) normalized with Ar I1 and I emission lines denoted in Figure 7.
the same method as above are arranged in Figure 8 for Cu/Ni and in Figure 9 for Co/Ni. According to eq 2 and 3, the observed ratio of sputtering yields can be deduced from the results obtained in Figures 7-9 on the assumption that the proportionality factors which were estimated from calibration curves in binary alloys can be employed to analyze the results on pure elements and that the sputtering conditions for several target materials vary with input power in a similar manner. The observed ratios of sputtering yields are summarized in Table 111. Sputtering yields for several elements have been reported by theoretical or experimental methods in the earlier works (18-22). In comparison, the sputtering yield ratios calculated from these published values are also shown in Table 111. The observed ratios in our work are roughly in agreement with the reported values. Since the glow lamp was operated with Ar gas containing impurities, e.g., water and oxygen, these would be expected to have an influence on the sputtering. Especially, stability and strength of oxide films cause a major problem. A possible reason for the slight disagreements of Ni/Fe or
LITERATURE CITED (1) Grimm, W. Naturwlssenshaffen 1987, 5 4 , 588. (2) Grirnm, W. Specfrochin?. Acta, Part 8 1968, 2 3 8 , 443. (3) Berneron, R.; Charbonnier, J. C . S I A , Surf. Interface Anal. 1981, 3 , 134. (4) Ohashi, Y.; Yamamoto, Y.; Tsunoyama, K.; Kishidaka. H. S I A , Surf. Interface Anal. 1979, 1 , 53. (5) Waitlevertch M. E.; Hurwitz, J. D. Appl. Spectrosc. 1976, 3 0 , 510. (6) Belle, C. J.; Johnson, J. D. Appl. Spectrosc. 1973, 2 7 , 118. (7) Hirokawa, K. Bunko Kenkyu 1972, 2 2 , 317. (8) West, C. D.; Human, H. G. Specfrochin?. Acta, Part B 1976, 318, 81. (9) Boumans, P. W. J. M. Anal. Chem. 1972, 44, 1219. (IO) Dogan, M.; Laqua, K.; Massman, H. Spectrochin?. Acta, Part 8 1971, 2 6 8 , 631. (11) Wagatsuma, K.; Hirokawa, K. Anal. Chern. 1984, 5 6 , 908. (12) Wagatsuma, K.; Hirokawa, K. Anal. Chem. 1984, 56, 412. (13) Toyokawa, F.; Furuya, K.; Klkuchi, T. Surf. Scl. 1981, 110, 329. (14) Betz, G. Surf. Sci. 1960, 9 2 , 283. (15) Corliss, C. H.; Bozman, W. R. “Experimental Transition Probabilities for Spectral Lines of Seventy Elements”; US. Government Printing Office: Washington, DC, 1962; NBS Monograph 53. (16) Zaidel, A. N.; Prokof‘ev, V. K.; Raiskii, S. M. “Spektraltabellen”; VEB Veriag Technik: Berlin, 1961. (17) Moore, C. E. “Atomic Energy Levels“; NBS Circular 467, 1949. (18) Andersen, H. H.; Bay, H. L. Radiat. H f . 1973, 19, 63. (19) Laegreid, N.; Wehner, G. K. J . Appl. Phys. 1961, 32, 385. (20) Oechsner, H. 2.Phys. 1973, 261, 37. (21) Weijsenfeld, C. H.; Hoogendoorn, A,; Koedam. M. Physica (Amsterdam) 1961, 27, 763. (22) Andersen, H. H.; Bay, H. L. “Sputtering by Particle Bombardment I”; Behrish, R., Ed.; Springer-Verlag: Berlin, 1981. (23) Kim, K. S.;Winograd, N. Surf. Sci. 1974, 43, 625.
RECEIVED for review March 19,1984. Accepted May 14,1984. We are grateful to Nissan Science Fundation for the financial support of our work.
Glow Discharge Sputtering of Chromium and Niobium Disk Cathodes in Argon Soo-Loong Tong’ and W. W. Harrison* Department of Chemistry, University of Virginia, Charlottesville, Virginia 22901 A glow dlscharge was used as an atomlzation/excitation source to study the Sputtering of chromium and nioblum dlsk cathodes with argon as the discharge gas. Voltage-current characterlstlc curves were determlned for both cathodes. The effect of dlscharge pressure and voltage on chromium and nlobium sputtered atom density was studied. Argon emission patterns were compared wlth chromium and nioblum emlsslons. On leave from t h e D e p a r t m e n t of Chemistry, U n i v e r s i t y of M a laya, Kuala Lumpur, Malaysia.
Although the glow discharge is one of the oldest spectroscopic sources, relatively few applications have developed of importance to analytical chemists. The most prominent of these has been its role as a hollow-cathode line source for atomic absorption spectroscopy ( I ) . In addition, the hollow cathode is one of several forms of the glow discharge which has been studied as an alternative excitation source for emission spectrochemical analysis in place of the conventional arc and spark sources (2). Both fundamental investigations and more applied studies have shown certain advantages of this technique, mainly because of the inherent stability of the
0003-2700/84/0356-2028$01.50/00 1984 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 56, NO. 12, OCTOBER 1984
discharge, compared to the arc and spark sources, for quantitative spectrochemical analysis (3). A modified configuration which consists of a small disk cathode and a hollow anode in close proximity forming an obstructed glow discharge was proposed by Grimm ( 4 ) . This structure overcame some difficulties associated with hollow cathode sampling and has been successfully employed in several laboratories (5, 6, 7 ) . The continued interest in versatile analytical spectroscopic sources for solids with multielement capability has led to the study of potential applications of the glow discharge as an atomization and ionization source. The glow discharge was first recognized as a useful atomization source for atomic absorption spectroscopy by Walsh (8)and several application reports have since appeared (9, 10, 11). Also, as a result of the significant electron impact and Penning ionization in a glow discharge environment associated with the sputtering atomization, its use as an analytical ion source was demonstrated by Coburn and co-workers (12). Further applications in analytical mass spectrometry have been reviewed recently (13). A consideration of the glow discharge reveals that there are many parameters which influence the sputtering atomization and processes in the discharge environment. These factors include the discharge pressure, current, voltage, type and flow rate of inert gas, and the geometry and type of electrode materials. Classical theory of the glow discharge describing the interrelationship of these parameters is well documented (14, 15). The recent rapid growth of literature on sputter deposition and plasma etching in the field of electronics and thin-film technology (16,17) has led to a better understanding of the sputtering process and the glow discharge environment (18). However, a simple extrapolation to discharge arrangements useful in analytical spectroscopy is somewhat hazardous in view of the complexity of the parameters involved, particularly since the pressure regions and electrode configurations are considerably different. Some fundamental studies, including certain analytical aspects, have been reported. These have been mainly directed toward the constricted hollow cathode and the obstructed Grimm-type discharge. These studies involved spectral line emission measurements pertaining to such areas as the voltage-current relationships, electron densities and plasma temperature estimations, and the influence of discharge parameters on atom and ion lines (3-7,19). Emission spectroscopic data are useful for deducing information concerning the excitation condition in the discharge plasma. The combination of optical spectroscopy and mass spectroscopy offers additional information which may be valuable in evaluating the glow discharge (20, 21). More detailed characterization studies of the sputtering atomization, excitation, and ionization in a representative regime of common analytical applications are needed to establish more optimum conditions of operations of the glow discharge and to assess the overall performance of this technique. To investigate some of the relevant aspects, the present study employs a water-cooled disk cathode in a diode arrangement of a dc discharge and relatively lower pressure and current density than utilized for constricted and obstructed discharges.
EXPERIMENTAL SECTION A low-pressureglow discharge source operating in the abnormal mode (15)was used. The design of the source (Figure 1)shows
a glass discharge chamber and a water-cooled cathode support. The glass chamber is made of a 4 cm i.d. glass joint (Kontes Glass Co.) with two quartz-windowed tubes (3.0 cm long, 2.5 cm i.d.) to provide an optical path for the radiation through the negative glow region of the discharge plasma. The main exhaust port (lateral) serves both for initial evacuation and for flow operation. The upper port consists of a brass tubing of 0.15-cm i.d. connected
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2029
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5
Flgure 1. Diode glow discharge source for optical spectrometry: (1) disk cathode; (2) macor shield; (3) aluminum cylinder for water cooling; (4) cathode lead; (5) water; (6) quartz window; (7) anode and inlet for inert gas; and (8) vacuum exhaust port.
to the glass chamber through a 0.63-cm Cajon Ultra Torr fitting. The brass tubing acts as the anode, as well as a continuous flow inlet of the discharge gas, centrally positioned at 2 cm above the surface of the cathode. The cathode is a replaceable chromium or niobium disk mounted onto the water-cooled aluminum cylinder. Machinable glass ceramic (Macor, Corning Glass Works, Corning, NY) provides proper insulation and shielding of the cathode and the aluminum cylinder support. Spacing of the ceramic shield and the cathode was designed to allow prolonged sputtering and to avoid subsequent arcing due to redeposition of the sputtered material. Thin-disk cathodes machined from Alfa Pure Metal of Cr (99.2%), 3 mm thick, 1.7 cm in diameter, and Nb (99.8%),0.127 mm thick, 1.7 cm in diameter, have been used. The surface was polished to a mirror finish, cleaned ultrasonicallywith acetone and water, rinsed with dilute HCl (10%)-distilled H20, and dried. The glow discharge source assembly was mounted on a vertically and transversely adjustable holder in the sample beam path of the Perkin-Elmer 372 double-beam atomic absorption spectrometer for absorption and emission measurements, without further modification of the instrument optical system. A 5-mm region immediatelyabove the disk cathode was sampled. Hollow cathode spectral line sources of Cr and Nb (Westinghouse) were used. Background non-atomic absorption was checked periodically and found t o be significant only at a very high current density discharges. The glow discharge source was powered by a Kepco Model BHK regulated power supply. Purified Ar gas, after passing through a gas purifier (Matheson Gas Purifier) and an ethanol-dry ice cold-trap, was allowed to flow through the source via a needle valve for controlling the flow rate (25-75 cm3 STP/min). The pressure was monitored on a side port of the glass chamber by a thermocouple gauge. Typical source pressure varied from 0.3 to 1.2 torr; discharge currents up to 40 mA and voltages of 400-1200 V were employed. In each experiment, the discharge torr before adassembly initially was pumped down to mitting the discharge gas.
RESULTS AND DISCUSSION Presputtering Effects. Upon initiation of the glow discharge, an equilibration period is required during which time the cathode surface is sputter cleaned and gases adsorbed on the surfaces of the source are desorbed. The time required
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ANALYTICAL CHEMISTRY, VOL. 56, NO. 12, OCTOBER 1984
for such an induction period depends on the nature of the cathode material, the type of impurity gases, and the degree of vacuum attained. For most materials, a few minutes will suffice, but for strong oxide formers such as chromium and niobium, the presence of water vapor, which dissociates in the discharge, can extend the induction period considerably. We have followed the relative populations of several discharge species during this induction period with the aim of correlating this with discharge processes. Beginning at discharge initiation, argon atoms and ions were monitored by emission, showing only relatively minor and similar changes with time. This might be expected as the discharge gas is constantly replenished in a flow mode. Scanning of regions of oxygen emission showed no detectable oxygen presence. Previous studies have shown that the rise in sputtered atom population in a glow discharge coincided with a rapid decrease in the H30+signal (21). These results are in line with those of other investigators in showing the significant effect of water vapor (16,22) on sputter yield. One reason for the low sputtering rate of the target during the presputtering period, as reflected by the low absorbance of ground-state chromium atoms, may be the formation of an inert surface oxide layer. Another possible contribution may be that the percentage of discharge current carried by lighter ions such as H+, OH+,and H30+ can detract from the sputter production of the heavier Ar+. Although formation of hydrogen in the discharge plasma has been reported before, attributed to the dissociation of adsorbed water vapor (22-25) the mode of this evolution was not clear. We find that the appearance of hydrogen, as measured by atomic emission, does not follow the same outgassing pattern of the adsorbed water. Knewstubb (22) has attempted to account for the principal mode of formation of H30+ in the negative glow, suggesting that this formation could have involved some excited state of water, possibly with a considerable lifetime, via the following reaction:
H20* + H20*
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600
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DISCHARGE VOLTAGE, V 50
B
Nb
H30* + OH40
which is energetically more favorable as compared to
H20+ + H 2 0
3
4
H30+ + OH
H30++ e-
This is consistent with the high initial water concentrations in the discharge and the fact that H30+is often the dominant mass spectral peak in the induction period (21). Voltage-Current Characteristics. Voltage-current characteristic curves were determined for both chromium and niobium at several operating pressures as shown in Figures 2A,B. It is possible to control the operating voltage at a given current by appropriate adjustment of the discharge gas pressure. As the pressure decreases, the operating voltage must rise to maintain a given discharge current. The characteristic curves for chromium are similar to those observed for nickel ( 4 , 6) as well as aluminum and iron ( 5 ) in a Grimm-type discharge a t about 10 torr. They also resemble those obtained for platinum (26) in a DC glow discharge in the millitorr range. The response for niobium, however, is distinctly different in that the discharge current is less influenced by the discharge voltage than by the operating pressure of the discharge, suggesting a lesser dependency of ion yield on sputter ion energy. The threshold discharge current is seen to increase sharply with discharge pressure. Sputtered Atom Density. By measuring the atomic absorbance of chromium, an indication of the relative sputtered atom density in the negative glow region can be obtained. Figure 3 shows the absorbance at several pressures; also plotted are constant voltage responses. At constant pressure,
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I
900
Flgure 2. Voltage-current characteristic curves for (A) chromium and (B) niobium at various discharge pressures.
the atomic density-and, presumably, the sputter rateincreases rapidly and almost linearly as a function of discharge current. As the current is increased at constant pressure, one is confronted with two reinforcing effects. Not only is the
ANALYTICAL CHEMISTRY, VOL. 56, NO. 12, OCTOBER 1984 1.2
2031 I
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Cr I I Cr abs Cr II b A1 I 0
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-
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0.6
-
0.4
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Figure 3. Sputtered chromium atomic absorbance as a function of the discharge currents at various discharge pressures and voltages; 425.4 nm. 0.4 I
I
1
I
1.0 DISCHARGE PRESSURE, Torr
0.6
0.8
I
J
1.2
Figure 5. Comparison of chromium atomic absorption and emission and argon emission as a function of discharge pressure (current, 10 mA): (a) Cr(1) emission at 425.4 nm; (b) Cr absorbance at 425.4 nm; (c) Cr(I1) emission at 205.6 nm; (d) argon(1) emission at 750.4 nm; and (e) Ar(I1) emission at 461.0 nm 1s
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20 30 40 DISCHARGE CURRENT, mA
50
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0.2
Figure 4. Sputtered niobium atomic absorbance as a function of the discharge currents at various discharge pressures and voltages; 408.0 nm.
number of bombarding ions increasing with current, thus producing a higher sputter rate, but simultaneously the ions are striking with a higher energy due to the increased discharge voltage, thus further enhancing sputter yield. By maintaining the voltage constant, that is by changing the pressure as the current is varied, one can attempt to define the effect of current alone on sputtered atom density. It must be recognized, that a pressure change is not an isolated event in that the mean free paths of discharge species are influenced. However, Davis and Vanderslice (26) have shown that the energy distribution of an ion in a n abnormal glow discharge is determined by the ratio between the mean free path for symmetrical charge transfer with ambieht gas molecules and the cathode dark space. If the voltage is kept constant, the product of the pressure and dark space distance is relatively constant (15),indicating that the effect of gas pressure on ion energy distribution is small. When the pressure is maintained constant, it can be seen from Figure 3 that the atomic density does not rise as rapidly and even begins to level off, perhaps due to the increased redeposition rate. From an analytical standpoint, it is desirable to operate at low pressures (within stability limitations) to enhance elemental sensitivity. Figure 4 shows the same type of plot for niobium in which the effects are similar but more exaggerated, and there appears to be a threshold current not observed with chromium.
C
10
20 30 DISCHARGE CURRENT, mA
40
Figure 6. Effect of discharge current on atomic absorbance and emission at two different voltages, 600 V (-) and 800 V (---). Wavelengths: see Figure 5.
Other Discharge Species. While the atomic absorption values of chromium and niobium provide a relative measure of the sputtered atom densities and net sputter rates, there exists with this source the possibility of monitoring other important species arising from the sample or the discharge environment. Figure 5 shows a comparison of argon emission and chromium atomic emission and absorption, as well as chromium ion emission, all as a function of discharge pressure. Argon atomic emission shows relatively little change with pressure at constant current density because the effects due to increasing pressure and those arising from the attendant reduction in discharge voltage tend to be somewhat compensatory. The three chromium species plotted in the same figure show that the ground-state atoms, the excited atoms, and the ions are all similarly influenced by the pressure increase, yielding a rapid decrease in population as a result of the lower discharge voltage. In a complementary experiment, the discharge voltage was held constant with increasing discharge current by adjustment of the pressure. Under these
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ANALYTICAL CHEMISTRY, VOL. 56, NO. 12, OCTOBER 1984
4
8
12
16
20
DISCHARGE CURRENT, mA
Flgure 7. Niobium emission as a function of discharge current at various discharge pressure and voltages; 408.0 nm.
constant voltage conditions, the chromium atomic absorption increases initially, up to a current of about 10-15 mA and then shows little additional increase with current, as indicated in Figure 6. However, the emission of both chromium and argon shows uniform increase with discharge current. Redeposition of sputtered atoms at the higher currents may limit the atomic absorption. An example of the emission of niobium as a function of discharge current is shown in Figure 7 for several different operating pressures. At constant pressure (dashed lines), the emission increases rapidly with current with the greater intensity at low pressures. The constant voltage curves attempt to demonstrate the effect of increasing current alone, without the attendent higher ion and electron energies. Again, as with chromium, the emission continues to increase after the absorption (Figure 4) has essentially reached a maximum. Penning ionization has been shown to be one of the ionization modes in a glow discharge, although the extent of this contribution may depend on discharge conditions. In any systematic variation of discharge parameters, it would be useful to know the effect of such factors on the populations of metastable argon species, Arm*. The glow discharge creates by electron impact many excited states of argon, some of which decay to produce metastable species. T w o 4p 4s transitions which lead to argon metastables result in emission a t 696.5 nm (3Pz)and 794.8 nm (3P0). We have followed the 696.5-nm radiation as a function of increasing discharge current in Figure 8A. Data taken at six different discharge voltages show a consistent and similar increase in emission intensity with current. Figure 8B shows that discharge voltage is not a significant variable. Emission resulting from transitions to the metastable state cannot be taken as necessarily representing rate of metastable formation since other pathways also exist and may predominate over this step, but such decay is suggested as a major mode (27).
01
0
I
I
10
20
30
DISCHARGE CURRENT, mA
B \ 0.80 Torr
-
CONCLUSIONS Our results indicate that optimum operating conditions may be different for ground-state atoms (for atomic absorption spectroscopy) vs. excited-state atoms (for emission spectroscopy). Increasingly high discharge pressure or current (not necessarily the discharge voltage) leads to the achievement of greater sensitivity in emission spectroscopic studies. For atomic absorption measurements, though sputtered atomic density is also enhanced by increased discharge current, it appears that greater efficiency of the sputtering process is
0.48
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I
0.38 0
*-0.26 0
10
1
1
I
500
700
900
DISCHARGE VOLTAGE , V
Figure 8. Measure of the argon emission to a metastable level (696.5 nm) as a function of (A) discharge current and (B) voltage.
Anal. Chem. 1984,56,2033-2035
obtained in high-voltage-low-pressure discharges.
ACKNOWLEDGMENT We are grateful to the Department of Energy, Division of Chemical Sciences, for research support. Registry No. Nb, 7440-03-1;Cr, 7440-47-3. LITERATURE CITED Walsh, A. Spectrochim. Acta 1955, 7 , 108. McNally, J. R.; Harrison, G. R.; Rowe, E. J . Opt. Soc. Am. 1947, 3 7 , 93. Slevin, P. J.; Harrison, W. W. Appl. Spectrosc. Rev. 1975, 70, 201. Grimm, W. Spectrochlm. Acta 1986, 238, 443. Dogan, M.; Laqua, K.; Massmann, H. Spectrochim. Acta, Part B 1971, 268, 631. Boumans. P. W. J. M. Anal. Chem. 1072, 4 4 , 1219. Ferreira, N. P.; Human, H. G. C. Spectrochim. Acta, Part B 1981, 368, 215. Waish, A. i n Proceedings of the Xth Colloqulm Spectroscopium Internationale; Washington, DC, 1962; p 127. Gandrud, B. W.; Skogerboe, R. K. Appl. Spectrosc. 1971, 2 5 , 243. Gough, D. S. Anal. Chem. 1976, 48, 1926.
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(11) (12) (13) (14) (15)
Bruhn, C. G.; Harrison, W. W. Anal. Chem. 1978, 50, 16. Coburn, J. W. Rev. Sci. Insfrum. 1970, 4 7 , 1219. Coburn, J. W.; Harrlson, W. W. Appl. Spectrosc. Rev, 1981, 77, 95. Von Engel, A. “Ionized Gases“; Ciarendon Press: Oxford, 1965. Llewelyn-Jones, F. ”The Glow Discharge”; Methuen, Wiley: London, New York, 1966. (16) Westwood, W. D. Prog. Surf. Sci. 1976, 7 , 71. (17) Coburn, J. W.; Winters, H. F. J. Vac. Sci. Techno/. 1979, 76, 391. (18) Winters, H. F. “Topics in Current Chemistry-Plasma Chemistry 111”; Veprek, S., Venugopaian, M., Eds.; Springer-Verlag: Berlin, 1980; pp 71-125
(19) L’vov,B. V. “Atomic Absorption Spectrochemical Analysis”; Adam Hilger: London, 1970. (20) Loving, T. J.; Harrison, W. W. Anal. Chem. 1983, 5 4 , 1523. (21) Loving, T. J.; Harrison, W. W. Anal. Chem. 1983, 5 4 , 1526. (22) Knewstubb, P. F.; Tilkner, A. W. J. Chem. Phys. 1962, 36, 684. (23) Stern, E.; Caswell, H. L. J . Vac. Sci. Techno/. 1966, 4 , 128. (24) Westwood, W. D.; Boynton, R. J. J . Appl. Phys. 1973, 4 4 , 2610. (25) Westwood, W. D.; Boynton, R. J. J . Appl. Phys. 1972, 4 3 , 2691. (26) Davis, W. D.; Vanderslice, T. A. Phys. Rev. 1963, 137, 219. (27) Strauss, J. A,; Ferreira, N. P.; Human, H. G. C. Spectrochim. Acta, Part B 1982, 376, 947.
RECEIVED for review October 12,1983. Accepted May 9,1984.
Surface-Enhanced Raman Scattering from Copper and Zinc Phthalocyanine Complexes by Silver and Indium Island Films Carol Jennings and Ricardo Aroca* Department of Chemistry and Erindale College, University of Toronto, Mississauga, Ontario, Canada L5L IC6 Ah-Mee Hor and Rafik 0. Loutfy Xerox Research Centre of Canada, 2660 Speakman Drive, Mississauga, Ontario, Canada L5K 2Ll
Metal island fHms of silver and Indium have been used to study the surface-enhanced Ramarl scattering (SERS) of copper and zinc phthalocyaninecomplexes. The spectral lines do not show any significant shifts when compared wlth the spontaneous Raman scattering. Metal isiarrd films are emerglng as a powerful spectroscopic technique for materlai identification of very thin films and monolayers.
Surface-enhanced Raman scattering ( I ) is a new physical phenomenon and a potentially powerful analytical tool. The prevailing view in the understanding of the origin of the enhanced Raman intensities is that although the electromagnetic enhancement pldys a predominant role, enhancements due to specific chemical effects are also important. Experimentally, there is no question concerning the observation of substantial SERS when molecules are in contact with rough metal surfaces (2-5). In particular, the so-called “extremely rough surfaces” !uch as metal island films and metal colloids are emerging as standard techniques, not solely to study the specific characteristic of SERS but simply to observe vibrational spectra of thin films, monolayers, and also submonolayers deposited on rough metal surfaces. In the present work we have studied the SERS of copper and zinc phthalocyanine (Pc) complexes on silver and indium island films. In previous communications the SERS from metal-free Pc by silver films (6)and the luminescence enhancement by indium films (7) were reported. EXPERIMENTAL SECTION Cu and Zn Pc from Eastman Kodak were purified by the train-sublimation technique. The silver and the indium films were vacuum evaporated (p C lo4 torr) onto tin oxide glass slides and then coated with Cu and Zn Pc complexes in a Varian NRC 3115 torr. Metal film vacuum system at a base pressure of 5 X
thickness was about 7.5 nm in all samples, as determined by an Inficon XTM quartz crystal oscillator thickness monitor, and phthalocyanine coatings ranged from 200 to 7.5 nm. The spectra Physics Ar ion laser (Model 164) was used for spectral excitation. Raman spectra given in Figures 1 and 2 were obtained using 150 mW of the 514-nm laser line, filtered with an interference filter. The incident radiation was S polarized and the polarization of the scattered light was not analyzed. Scattered light was dispersed by using a Spex 1403 double monochromator with 1800 lines/mm holographic gratings and detected by a cooled ITT FW30 photomultiplier. The spectral band-pass was 5 cm-l. The optimal experimental geometry was attained when the film and the incident laser line were forming a 30’ angle (41= 60’ in Figure 3).
RESULTS AND DISCUSSION In a previous report (6) the SERS of metal-free Pc has been observed when a Pc film initially deposited onto a glass subtrate was then overlaid with a silver film. Experiments in which Pc molecules were vacuum deposited onto a continuous silver film failed to show similar enhancement. Considering the important role that surface roughness plays in the enhancement of Raman scattering (RS), the metal island films only were used in this work. The nucleation and growth of evaporated silver films have been extensively studied (8). I t has been shown that slowly formed films (total evaporation time of 1.5 min or more) tend to grow more in height, whereas “fast” films (total evaporation time 2 s) become continuous at a lower mass thickness. Films used here were slowly formed films with an evaporation rate of 0.1 nm/s (75 s total time), and 7.5 nm thickness. Transmission eletron micrographs of these films indicate that metal islands are obtained with an average island size less than 20 nm. Glass slides with silver and indium island films were then removed from the vacuum and later overcoated with 7.5 nm and 25 nm thick phthalocyanine films. Because of the
0003-2700/84/0356-2033$01.50/0 0 1984 American Chemical Society
