Bilayer interference

enhanced

Raman spectroscopy

W. S. Bacsa and J. S. Lannin Department oj* Physics, PennsyIvania State University, University Park, Pennsylvania 16802

(Received 4 March 1992; accepted for publication 4 May 1992) A combination of the interference of incident and reflected coherent beams from a metal surface with a dielectric overlayer is calculated to yield an enhancement of the Raman signal of ultrathin adsorbed layers. The thickness of the dielectric layer is determined by optimizing the interference effect of the incident and reflected beams to enhance the electric field. In the case of SiOJAl an interference enhancement factor of 27 is found. An experimental confirmation of this enhancement is obtained by comparing the in situ multichannel Raman signals of one monolayer CI;c on an Al substrate and on a SiO,/AI bilayer. The use of the same bilayer substrate for a range of ultrathin films makes the metal-dielectric bilayer a versatile tool to investigate a number of nanoscale systems. The Raman process is associated with a low photon scattering efficiency, due to its second-order nature. Although multichannel photon detectors’ have enhanced Raman sensitivities, the technique is still limited by its low sensitivity in nanometer-scale systems such as ultrathin films and clusters where the signals may be orders of magnitude smaller than from bulk samples. Several attempts have been made to overcome this restriction. Greenler et al. ’ made use of multiple reflection between two parallel metal surfaces to investigate thin films analogous to surface infrared spectroscopy. However, the sensitivity to nanoscale adsorbates was limited. Another means to improve the signal of ultrathin adsorbates utilizes surface-enhanced Raman scattering (SERS) associated with island noble metal films such as Ag.3 While several factors (electromagnetic, chemical) contribute to SERS which change the transition matrix elements and Raman selection rules, the use of SERS to characterize nanoscale systems is complicated, and, in addition, is limited by adsorbate damping of the Ag surface plasmons.3 An alternative means of studying ultrathin films employs interference effects that enhance the Raman signal. Connell et aL” applied interference enhancement in Raman spectroscopy (IERS) by using a trilayer structure consisting of Al, SiO,, and a Te layer. The thickness of both the Te layer and a SiOZ layer were adjusted to give a low reflectivity. In order to obtain a low reflectivity, the top layer had to be sufficiently absorbing. The enhancement factor was estimated to be lO-lo3 depending on the optical properties of the top layer. Experimentally, the Raman signal was reported to be enhanced 20 times relative to thick films in the case of the Te layer. While employed in a number of studie@ to obtain typically one order of magnitude signal increase over bulk films, this trilayer geometry is restricted, however, by the required fixed top-layer thickness to obtain a low reflectivity. In addition, use of the trilayer to study ultrathin films and clusters requires the use of a top layer that may cause a large Raman background. In this letter the interference of the incident and reflected beam from a dielectric/metal bilayer is used to enhance the exciting electric field. Since the Raman cross section is proportional to the square of the electric field, the

Raman signal of an ultrathin layer deposited on an optimized SiO*/Al multilayer is found to be substantially enhanced. We apply this modified interference enhancement geometry to study the in situ Raman spectra of one monolayer of the fullerene’system, C&. The Al substrate layer ( 1 pm) was first evaporated in high vacuum ( lop6 Torr) onto a polished Si wafer followed by an amorphous (a-) SiOZ layer which was rf sputtered. The thickness of the SiO, layer (620 A) was measured by ellipsometry. This thickness lies in the range of maximum enhancement for a bilayer at 514.5 nm excitation. The Al layer as well as the SiO, layer were exposed to air before they were introduced into an ultrahigh vacuum (UHV) chamber ( 10-9-10-‘o Torr). Approximately one monolayer (ML) of Cbo was sublimed in a high vacuum (lo-* Torr) prechamber and directly transferred into the UHV chamber. The thickness of the CGowas calibrated by a quartz crystal monitor. The in situ Raman measurements were taken in the UHV chamber at room temperature utilizing 5 14.5 nm excitation with a Spex Triplemate (6 cm-i resolution) equipped with an ITT Mepsicron multichannel detector. Backscattering geometry was used with an angle of incidence of 40” and the polarization of the incident beam set parallel to the plane of incidence. As the Raman spectrum of Ceo is known to be sensitive to the incident laser power resulting in a number of spectral changes, the power density at the surface was reduced by use of a cylindrical lens and low incident laser radiation (10-20 mW). In order to optimize the effect of the multiple interference of the coherent incident and reflected beam, the electric field distribution of the SiO, and Al bilayer was calculated using the bulk optical constants of Al and SiO, the Fresnel equations, Snells law and a matrix formalism which relates the electric field of incident and reflected beams across layers and interfaces.’ Since a dielectric layer does not appreciably disturb the standing wave pattern of the interfering incident and reflected beams, the SiOZ layer is used as a dielectric spacer layer to access the increased electric field at the interference maximum. As a result, any adsorbate layer on top of the SiO, falls in the region of the enlarged electric field. The Raman signal of the adsorbate is thus enhanced by the high electric field at the SiO, in-

19 Appl. Phys. Lett. 61 (l), 6 July 1992 0003-6951/92/260019-03$03.00 @ 1992 American Institute of Physics 19 Downloaded 26 Oct 2007 to 130.120.231.106. Redistribution subject to AIP license or copyright, see http://apl.aip.org/apl/copyright.jsp

4 % ‘flsft t 0 t

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Onit

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3 n il

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ilt

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2000 L

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0 Raman

FIG. 1. Calculated normalized electric field component parallel to the interfaces in the Al and SiOz layers and vacuum for angles of incidence of 60” (rectangles), 40” (pluses), 20” (rectangles), and 0” (pluses). The polarization of the incident beam lies parallel to the plane of incidence.

terface. This condition holds as long as the thickness of the topmost, adsorbate ultrathin film layer is not too large. In most cases, depending on the optical properties of the adsorbed layer, the electric field distribution does not change appreciably if the thickness is varied over 3t 5 A. For layers thicker than 10 A, its effect on the field distribution has to be included by adding another layer in the calculation and adjusting the thickness of the SiOz layer accordingly. Although the optical constants of ultrathin layers are not well known, an effective medium theory with variable void fraction can be used to estimate the optical constants of the topmost ultrathin layer. As the thin SiO, layer has only a weak Raman signal and the neighboring Al layer has no first-order Raman active modes, the combination of the two layers turns out to be a very suitable substrate to study ultrathin layers by Raman spectroscopy. Figure 1 shows the square of the normalized electric field distribution perpendicular to the 620~A-thick SiO, and Al layers for the electric field vector parallel to the plane of incidence. The interference of the incident and reflected beams results in strong oscillations of the electric field perpendicular to the surface and shows standing wave behavior. This interference pattern near a metal surface has been experimentally observed by oscillations of the vibrational Raman signal of physisorbed N2 and O2 on Ag ( 111) with layer thickness.* In Fig. 1 it is seen that the electric field decreases appreciably in the SiO, layer while in the metallic Al layer, the electric field is strongly damped within a few hundred angstroms. As Raman scattering in the pseudobackscattering geometry is performed with non-normal angle of incidence, Fig. 1 also shows the electric field distribution as a function 20

Appl. Phys. Lett., Vol. 61, No. 1, 6 July 1992

I

I

600 Shift

I

1200 [cm-']

I

1

1800

FIG. 2. Comparison of the unpolarized Raman spectra of 1 ML C&, on Al (unenhanced) and SiOr/Al (enhanced) and assignment of the symmetry allowed modes.

of the angle of incidence. The longer optical path through the multilayer and decreased phase shift with increasing angle of incidence broadens the interference maximum and shifts it slightly farther away from the Al interface. However, the electric field at the vacuum/SiO, interface does not change substantially with different angles of incidence. Finally, it is important to bear in mind that the enhanced electric field implies a larger power density at the vacuum/ SiO, interface which might lead to unwanted heating effects. Apart from field enhancement, other important characteristics are exhibited by the dielectric-metal bilayer. The interference of the incident and reflected beam causes a change in the polarization in the case of a linearly polarized incident beam which is polarized neither parallel nor perpendicular to the plane of incidence. The polarization at the vacuum/SiO, interface is then elliptical. In order to estimate the enhancement obtained by increasing the electric field, the calculated amplitudes at a vacuum/Al interface are compared to those at the vacuum/SiO, interface. This yields a theoretical intensity increase by a factor of 19-27 depending on the angle of incidence. Maximal enhancement is obtained at the Brewster angle where the reflection losses at the vacuum/SiO, are smallest. A comparison with the theoretical enhancement factor has also been obtained experimentally by observing the Raman spectra of 1 ML Ceo on Al without enhancement to that of a SiO,/Al bilayer. Figure 2 compares the unenhanced and bilayer enhanced Raman spectra obtained for an angle of incidence of 40”. Apart from two breathing modes at 495 cm-’ W. S. Bacsa and J. S. Lannin

20

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[AA,(l)] and 1465 cm-’ [A,(2)], only very weak features are seen from the 1 ML C,, spectrum on the Al substrate. As the Al film was exposed to air, the C6c molecules are adsorbed here on a thin ( - 20 A) nonreactive Al,Os layer. The ability to observe Raman scattering from a single C6,, monolayer without interference enhancement is attributed to resonant Raman coupling.’ In contrast to this weak signal, the spectrum of C6s on a SiO,/Al bilayer yields a signal that is found to be strongly enhanced, with the Raman spectrum also showing a number of distinct weaker features. A comparison of the intensity of the strongest scattering A,(2) mode in the spectra on SiOJAl and Al reveals an enhancement factor of -20, which is close to the theoretically estimated value. It is interesting to note that a large enhancement is experimentally observed despite a larger background signal in the case of SiOJAl. This could be due to possible hydrocarbon contamination and luminescence from the sputtered SiOz layer. The signal of the SiO, layer itself is found to be very small and not significant for the observed C,, spectrum. In contrast to the bilayer IERS, the use of a third layer in trilayers implies in most cases a large nonuniform background signal. Furthermore, the exciting electric field at the vacuum interface is smaller for trilayers, resulting in an -2-4 times smaller enhancement factor. The major Raman peaks of Cc0 on SiOJAl of Fig. 2 can be assigned to the 2 A, and 8 Hg Raman allowed modes of an isolated, truncated icoshedral C6c molecule.” However, a number of weaker not allowed lines and line broadening effects reflect the different behavior of 1 ML C,s on the sputtered SiOz layer compared to thick C,, layers. These differences are attributed to the lower symmetry of the ChOmolecule on the Si02 surface and to interactions of the molecules with their neighbors. A more detailed discussion of the vibrational spectra of l-ML-thick C6c will be given elsewhere.

21

The combination of field enhancement and the possibility to discriminate the signal against the surrounding dielectric layer makes the simple SiO,/AI bilayer an ideal tool to study ultrathin layers and clusters by Raman spectroscopy. The SiO,/Al multilayer furthermore has the advantage that only the thickness of the SiOz layer has to be adjusted if the thickness of the layer under investigation and its optical constants are known. In addition, the thickness of the topmost layer can be varied by - 10 A during measurement without loss of enhancement. For ultrathin layers the optical SiO, thickness lies in the range /2/4-J/8. The enhancement from a bilayer is found to be 24 times larger than a trilayer. A single dielectric layer on a metal substrate thus turns out to be a simple and very effective means to use the coherence of the laser beam to enhance the Raman signal of a very large number of nanoscale systems. This work was supported by NSF Grant No. DMR 89223051.

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21 W. S. Bacsa and J. S. Lannin Appl. Phys. Lett., Vol. 61, No. 1, 6 July 1992 Downloaded 26 Oct 2007 to 130.120.231.106. Redistribution subject to AIP license or copyright, see http://apl.aip.org/apl/copyright.jsp