Vibrational spectroscopy (SERS and SERRS)

Journal of Molecular

Structure, 292 (1993)

17-28

17

Elsevier Science Publishers B.V., Amsterdam

VIBRATIONAL SPECTROSCOPY (SERS AND SERRS)

Ricardo Aroca Department of Chemistry and Biochemistry University of Windsor, Windsor, On., Canada N9B 3P4 Surface-enhanced Raman scattering (SERS) and surface-enhanced resonant Raman scattering (SERRS) are two analytical techniques that can be applied to the study of adsorbed molecules on a number of rough metal surfaces. A current problem in experimental vibrational spectroscopy is the characterization of molecular monolayers adsorbed on metal surfaces, in particular, Langmuir-Blodgett (LB) monomolecular layers deposited onto solid substrates. SERS and SERRS have been shown to be useful in the study of sequential LB monomolecular layers, diffusion through LB layers and selective adsorption phenomena. However, the problem of molecular orientation in monolayers has been arduous due to the depolarization effect of rough metal surfaces, and to the corresponding difficulty in the interpretation of polarized data. The natural complement to SERS is the infrared spectroscopy on smooth metal surfaces such as reflection-absorption infrared spectroscopy (RAIRS). Vibrational analysis using both techniques are illustrated here. The problem of order disorder in monolayers and the temperature dependence of the SERS signal are also discussed.

1. INTRODUCTION The absorption of infrared radiation and the inelastic scattering of light (Raman effect) are the classical complementary techniques to study vibrational properties of molecules in the ground or excited electronic state. Recently, an effort has been made to use the tools of vibrational spectroscopy for fundamental studies at surfaces and interfaces [l-4]. The motivation for a better understanding of surface phenomena is in the ultimate explanation of heterogeneous catalysis, corrosion, molecular organization, to name a few, among the many The outstanding issues in chemical physics. objective of the present communication is to emphasize the role of Raman and infrared techniques used in the study of the first monolayer or multilayer assemblies formed on certain metal surfaces. The highly reflecting metals (Al, Ag, Au, Cu) allow to carry out reflection experiments using infrared radiation (reflection-absorption) [5,6], or a laser beam to observe the vibrational modes in the Stokes inelastic light scattering [7,8]. The extremely small Raman scattering (RS) cross sections limits the application of RS to monolayers on reflecting

surfaces, and it had to wait until better detection systems such as CCD detectors became available. However, the rough surfaces of the same highly reflecting metals came to the rescue of the experimentalist by enhancing the Raman signal of adsorbates by as much as a millionfold. The effect, that gave rise to surface-enhanced spectroscopy and in particular to surface-enhanced (resonant) Raman scattering (SERS and SERRS), was first observed in electrochemical experiments [2-51. SERS and SERRS are formidable analytical tools for vibrational characterization of monolayer and submonolayer coverage on rough metal surfaces. In particular, SERS and SERRS have been applied to the study of Langmuir-Blodgett (LB) monomolecular layers 19,111. It was shown to be useful in the study of sequential LB monomolecular layers [12] and selective adsorption studies [ 131. LB interfaces, chemical reactions, diffusion in monolayers and intermolecular interactions on an LB assembly can be monitored using SERS (SERRS). Alternatively, the organized dielectric medium fabricated by LB technique has become a valuable asset in the study of SERS properties. For example, it provided an elegant way to probe chemisorption, physisorption,

0022-2860/93/$06.00 0 1993 Elsevier Science Publishers B.V. All rights reserved.

18

-8

I

Ul’d

300 nm’

Figure 1. SEM of Ag coated Sn spheres on glass (A), TEM of a 10 nm Ag island film (B) and 4 nm Au island film (C).

distance dependence, coverage dependence and polarization properties of the SERS signal [2]. However, there are limitations in the applicability of SERS. For instance, it has become evident in the SERS work on metal island films that the determination of molecular orientation is arduous and unreliable. The natural complement of SERS for a

complete vibrational analysis of a monolayer is the reflection-absorption infrared spectroscopy (RAIRS), and an example of the applications of both techniques to LB layer is presented here.

19 DyPc:

GREEN LB FILM

UNREACTED

466 \

REACTED WITH Bra

1

300

I

400

1

500 WAVELENGTH(nm)

600

1

700

Figure 2. Electronic absorption spectra of octa-tert-butyl Dy bisphthalocyanine (DyPc,?. LB film before and after reaction with Br,.

2. EXPERIMENTAL CONSIDERATIONS The procedures for monolayer spreading and compression have been described in two recent In our laboratory, LB monographs [14,151. monolayers were prepared in a Lauda Langmuir film balance equipped with an electronically controlled dipping device, Lauda Filmlift FL-l. Two important variables for LB work are the quality of the water used and the temperature of the subphase. Better results have been obtained in our laboratory, for both the surface pressure-area (isotherms) studies and the monolayer transfer, by working with a subphase temperature of 15 “c. Cleaning and preparation of the substrate is the last step before LB fabrication. Finally, special attention should be paid to the type of molecule under investigation. The most common and best understood are amphiphilic molecules that are insoluble in water and contain at least one

hydrophillic end. A non-amphiphilic molecule is water insoluble and contains no hydrophillic ends. It has been pointed out that the interactive character of the isotherm obtained for the two types of molecules could have a very different nature [16]. Smooth metal films (Ag and Au) for reflection experiments and for metal island films can be prepared by vacuum evaporation. The thickness may be monitored with a quartz crystal oscillator. Scanning and transmission electron microscopies (SEM and ‘IEM) are routinely used for the study of surface morphology. Typical SEM and TEM of metal films are given in Figure 1. Raman shifts can be measured with a scanning spectrometer, or a spectrograph with a photodiode or a CCD detector. In our laboratory, infrared spectra were measured using a BOMEM DA3 FI-IR spectrometer equipped with a specular reflectance accessory. Raman shifts

20

DyPc; LB FILM ON Au 685

UNREACTED

GREEN FILM

REACTED WITH Bra

RED FILM 1542

500

1000

1500

WAVENUMBERS (cm-‘) Figure 3. SERRS spectra of an LB monolayer of DyPc: before and after exposure to Br,

double were measured with a Spex-1403 monochromator, a Ramanor U-100with microscope attachment or a THR 100 spectrometer and a Spex For data manipulation and CCD detector. mathematical analysis, spectral files could be imported to Spectra CalcTMsoftware available from Galactic Industries Corp.

SURFACE ENHANCED RAMAN SCATTERING.

3.

(RESONANT)

For a brief discussion of the SERS effect the nomenclature proposed by Metiu [16,17] has been adopted. Molecules coating a metal surface are excited by a monochromatic laser beam. The local electric field that drives the molecules and would generate the spectroscopic signal may be written as El = Ei + E, + E, + Et, , where E represents

the vector electric field. The first two terms form the primary field given by the electric field of the incident laser and the reflected field. For a flat metal surface, the primary field of the reflection experiment is given by Fresnel formulae 171. The E, may be enhanced by selecting a metal with the appropriate dielectric constants (e.g., Ag, Au, Cu) and by tailoring the shape of a metal surface. The enhancement is obtained by excitation of electromagnetic resonances that can couple with the incident field on rough metal surfaces. The electric field enhancement so obtained may be observed without the involvement of molecules [ 183, and it is clearly independent of the molecule adsorbed onto the metal surface. Coating the surface with molecules brings into play the last two terms E, + E, or secondary field [ 161, that depend on the magnitude of the induced dipoles. The last term called the Lorentz field represents the contribution of

21 1

j DyPc: LB FILM ON Au 685

GREEN FILM

RED FILM

REACTED WITH NO,

Id00

560

WAVENUMBERS(cm-‘) Figure 4. SERRS spectra of an LB monolayer of DyPci before and after exposure to NO,.

all polarized molecules at the point where the local field is calculated. Notably, the Lorentz field will introduce a coverage dependence in the effects of the secondary field. The secondary field would include an emission enhancement which is produced by the radiation emitted from the polarized solid caused the oscillating dipole (adsorbed molecule). Similarly, if the coupling takes place with nomadiative electromagnetic resonances, the emission is very effectively quenched. At this point, the molecular problem should be added, i.e., to what extent the molecular polarizability has been changed by adsorption to the metal surface. Finding the effect of molecule-metal interactions on the optical response of adsorbates is a very arduous task, and the magnitude of the effect would depend on the nature of the interaction. A recent review on the subject has been written by Otto [ 191. For the interpretation of SERS spectra it should be remembered that the

enhancement of various mechanisms is multiplicative. Therefore, a modest enhancement factor for a number of contributions could give rise to a large enhancement of the measured intensity. In summary, the SERS signal encompasses contributions that are clearly related to the proximity of the surface and to its macroscopic optical properties (dielectric constant and shape) and contributions due to a change in the optical response of the molecule (polarizability changes). The fact that the contributions are multiplicative increases sensitivity and makes the Raman experiment advantageous for analytical applications. However, the weakness of SERS will be evident in applications were the exact nature of the enhancement should be known. There is one general case in which the molecular problem could be ignored: physisorption. In the examples presented here, physisorbed molecules on metal surfaces only are considered, and

22 SERRS

of

PPTCDP

1383

One LB on 6 nm Ag at 298 K

SERRS at 120 K

I

400

600

800

1000 Wavenumbers

Figure 5.

1200 (cm

-1

1400

I

1600

)

SERRS spectra of PPTCDP on Ag recorded at 298 K (upper trace) and at 120 K (lower trace).

the effect of the surface on polarizability may be neglected.

the

molecular

3.1 SERRS of Langmuir-Blodgett monolayers The fabrication of Langmuir-Blodgett monolayers of electroactive organic materials has been one of the main objectives in our laboratory. A driving force for studies in this field is the potential of LB for applications in basic research and the development of new technologies [ 14,151. Most molecules under investigation are not amphiphile, and forming a stable floating Langmuir monolayer could present a challenge. Recently, a series of rare complexes octa-tert-butylbisthalocyanine earth (LnP~~ were tested for LB fabrication and spectroscopic studies. The synthesis of the LnPc: [20] compounds begin with the formation of a monophthalocyanine complex which converts to a green material of general formula LnPc;. The green

material of molecular formula DyPc$ was used in the SERRS experiments presented below. The electronic absorption spectrum of an LB film of DyPc: on glass is given in Figure 2. Typical bands such as Soret and Q-bands observed in all green materials can be seen in the spectrum of DyPc; at 318 nm and 670 nm respectively. The radical anion band of the green material is also seen at 466 nm. Figure 3 (upper trace) illustrates the SERRS spectrum of one LB monolayer of DyPc; on a Au island film. It is important to point out that the SERRS spectrum and the resonant Raman spectrum of DyPc; in a KBr pellet were identical; i.e., the analyte may be considered to be physisorbed on the metal surface. The most characteristic vibrational fundamentals (by frequency and relative intensity) were recorded at 685 cm-’ macrocycle breathing, 747 cm-’ macrocycle stretching, 1327 and 1524 cm-’ pyrrole ring stretching. A complete spectroscopic characterization

sERRS of PPTCDP 514.5

nm Laser

Line

1 1.11on 6 nm Ag 0.8 s Aquisition

Time

Figure 6. SERRS of one LB monolayer of PPTCDP on Ag detected with a CCD detector and recorded with two different acquisition times.

is the first step in adsorption studies of certain gases on thin solid films of PC molecules. The subject is of interest in basic research [13], and PC materials satisfy the requirements for applications as gas sensors [15]. LB film of DyPc,’ of a very distinct green colour was exposed to NO, and Br, vapours. The changes in the LB films were followed visually. The green film exposed to gases changed into a red film and recovered its original colour after desorption. The effect of gas can be observed in the electronic spectrum as shown in Figure 2 (lower trace). The red shift in the Q-band seems to be characteristic of the oxidized form of the rare earth bisphthalocyanine 1211. Note that EPR experiments with the green DyPc; material in solid and in solution show a strong signal of a free radical typical of the organic moiety. The green material became silent with the

addition of NO,. The adsorption of both gases is reversible and by the effect in the visible spectrum it should be treated as a chemisorption. The effect of gases may also be seen in the SERRS spectra of single monolayer deposited on Au island film as illustrated in Figure 3 and Figure 4. The effect of gas adsorption followed the pattern observed in the case of unsubstituted bisphthalocyanine materials (LnPcJ [13,221. Changes in the relative intensities of the macrocycle breathing, the pyrrole stretching and the increase in the intensity of the stretching C=N mode of the aza groups strongly indicate a polarization effect of the adsorbed gas at the centre of the macrocycle. However, it is proposed that the coordination of the gas molecule takes place through the metal atom. It should be noticed that the effect of both gases on the SERRS spectrum of the monolayer is essentially the same, and this

24

LIMITING AREA= 40 A2

20

40

80

60

-Area/Molecule

(A” )

Figure 7. Surface pressure-area isotherm of PPTCDP.

corresponds to the effect of an electrowithdrawing group. 3.2 Low temperature experiments and molecular orientation The present section focuses on experimental results that could yield molecular orientation from LB-SERRS with polarized light. It is known that SERRS spectra measured on metal island films are normally depolarized. Depolarized spectra in SERS means that the same relative intensity of the vibrational fundamentals is observed in the SP (Sincident, P-analyzed light) and the SS spectra. Further, experimental results would depend upon the efficiency of TM (P-polarized) or TE (S-polarized) KW. Efforts have been made to use SERS measurements in media of differing refractive indexes to determine molecular orientation [24]. However, the experiments are arduous and the results

are not conclusive. There exists another concealed variable in the task of detennining molecular orientation: the effect of the laser on the organization of the fiit layer. To illustrate this problem, the SERRS results obtained for a monolayer of a red dye N’-pentyl-3,4:9,10_peryleneN-pentyl, bis(dicarboximide) (PPTCDP) are given. The SERRS spectra for one monolayer of PPTCDP on a 6 nm Ag island film recorded at room temperature and at 120 K are shown in Figure 6. Two important qualities of the low temperature spectrum are seen. First, a band narrowing which indicates a large inhomogeneous broadening in the room temperature spectrum. Second, the overall intensity (peak heights) of the low temperature spectrum is higher. The intensity is explained by the contribution of the Boltzmann factor and the band narrowing due to the partial elimination of the inhomogeneous broadening. The spectra shown in Figure 5 are steady state

25 FT-IR

Spectra

Absorption:

Absorption (mixed

of PPTCDP

KBr Pellet

of 56 LBs on ZnS

LB 3:l

PPTCDPAA)

Reflection/Absorption (mixed

LB 3:l

of 7 LBs on

120

nm

Ag

PPTCDP:AA)

I

1300

1400

1700

1600

1500 Wavenumbers

(cm

-1

1800

)

Figure 8. Absorption and Reflection-absorption IT-IR spectra of PPTCDP. The absorption spectrum of PPTCDP in a KBr pellet (upper trace) is the rCferenCe spectrum.

spectra; i.e., they were recorded using a scanning instrument and the total acquisition time was about 30 minutes. Using a CCD detector and the THRlOO spectrometer, the spectra of the same sample were recorded at room temperature and are presented in Figure 6. With acquisition times of 0.1 second or less, the observed spectrum was characterized by narrow bands (lower trace) and higher peak height intensities. Larger acquisition times produced the

well known “steady state” spectrum given in Figure 5. It is proposed that the laser beam induces a drastic change in the molecular organization of the monolayer. The changes do not affect the molecular structure; they clearly disrupt the molecular orientation on the metal surface. Therefore, the SERRS experiment changes the orientation that the polarization measurements are trying to determine. It is concluded that SERS and SERRS is a very

26

sensitive and chemically specific method for LB characterization. However, it is not an appropriate technique for the determination of molecular orientation on metal island films. The natural complement for SERS is infrared and, in particular, reflection-absorption infrared spectroscopy on metal surfaces that allows the determination of molecular orientation in LB layers. Finally, a note on SERS experiments on colloidal systems. Surface selection rules for molecules adsorbed on colloidal silver have been shown to work in a number of SERS experiments [24]. We have carried out comparative studies for a given molecule in the form of an LB film on Ag islands and in solution on colloidal silver. Although the results of this work are reported separately, the bandwidth of the spectra obtained on colloidal silver were notably comparable to the low temperature spectra. The laser effect (broadening) was not prominent in the colloidal experiments, and the polarization measurements could help to determine the molecular orientation on the metal surface. 4. REFLECTION-ABSORPTION SPECTROSCOPY

INFRARED

The red dye PPTCDP used in the SERRS demonstrations was also employed in the RAIRS experiments. Langmuir monolayers of PPTCDP were prepared at 15oC in a Langmuir balance and the recorded surface pressure/area isotherm is shown in Figure 7. The limiting area indicates a tilted edge-on packing of molecule on the water surface. In order to improve the transfer of monolayers to smooth metal surfaces a mixed layer was prepared with a 3:l = PPICDP: arachidic acid mol ratio. Langmuir-Blodgett films were fabricated on a 120 nm evaporated film of Ag. Multilayers were also formed on ZnS for transmission infrared experiments. A Bomem DA4 FT-IR spectrometer with an MTC liquid nitrogen cooled detector was used to record IR spectra. A spectral reflectance accessory was used to obtain RAIR spectra with an angle of incidence of 75”. The IR spectrum of the red dye in a KBr pellet is given as a reference spectrum in Figure 8. The discussion will now be limited to the fundamental vibrational modes due to the C=O stretching vibrations in PPTCDP molecule. The last two bands seen in the KBr pellet spectrum are at 1655 cm-’ and 1696 cm-’ are clearly assigned

to the symmetric C=O stretching vibration and to the antisymmetric C=O stretching vibration respectively. The absorption spectrum of a multilayer on ZnS and the reflection-absorption spectrum on Ag are also shown in Figure 8. For and edge-on molecular orientation of PPTCDP, the dynamic dipole of the symmetric mode would be parallel to the surface and, correspondingly, the antisymmetric mode would be perpendicular to the metal surface. The geometry of the transmission experiment favours the interaction between the electric field of the infrared radiation and the symmetric vibration as observed in the transmission experiment for the LB film on ZnS. A similar result should have been found for a flat-on organization. However, the results exclude a headon organization. In the RAIRS experiments, solely the P incident light can interact with the adsorbate, and only molecular . vibrations with a finite component of their dynamic dipole perpendicular to the metal surface are observable [5]. The RAIRS spectrum (bottom spectrum) showed a reverse trend when compared with the transmission experiment. The antisymmetric C=O vibration was seen with higher relative intensity than the symmetric mode. The results exclude the flat-on molecular orientation and confirm the edge-on organization in the LB film. The relative intensity of the symmetric mode in the RAIRS spectrum indicates that the long molecular axis in the edge-on orientation forms an angle with the metal surface. The angles formed between the long axis (and the short axis) and the parallel to the metal surface may be estimated from the set of experiments [26]. A complete report of these measurements will be given separately [27]. In conclusion, SERS (SERRS) and RAIRS are powerful complementary techniques for vibrational studies of molecules on metal surfaces. SERS and particularly SERRS are impressive analytical tools for vibrational studies of LB monolayers, LB interfaces, chemical reactions and diffusion in monolayers. SERS and RAIRS are sensitive, chemically specific methods for LB characterization. SERS should be complemented with RAIRS for a complete characterization and determination of molecular organization of monolayers on metal surfaces.

27 Acknowledgements I would like to acknowledge the contribution to this work of Dr. A. Maiti, Dr. L. Tomilova and my students Dr. D. Bat&i, E. Johnson and I. Gobernado-Mitre. Financial support from NSERC of Canada and NATO Scientific Affairs Division (CGR 900582) is gratefully acknowledged.

10. 11. 12 ’ 13. 14.

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