- Email: [email protected]
Surface-enhanced vibrational spectroscopy
Journal of
MOLECULAR STRUCTURE ELSEVIER
Journal of Molecular
Structure 4081409 (1997) 17-22
Surface-enhanced vibrational spectroscopy Ricardo Aroca, S. Rodriguez-Llorente Materials Surface Science Group, Department
of Chemistryand Biochemistry
Received 26 August
Universiq of Windsor, Windsor, Ont. N9B 3P4 Canada
1996; accepted 6 September
1996
Abstract
The nature and properties of surface-enhanced vibrational spectra of molecules adsorbed on surfaces that can enhance the absorption and the emission of electromagnetic radiation are discussed. Examples of surface-enhanced resonant Raman scattering, surface-enhanced Raman scattering in the near-infrared and surface-enhanced infrared are given. The rough surfaces used are metal island films coated with a nanometric vacuum evaporation. 0 1997 Elsevier Science B.V. Keywords:
organic film using the Langmuir-Blodgett
technique
or deposited
Surface-enhanced infrared spectroscopy; Surface-enhanced Raman spectroscopy; Langmuir-Blodgett
1. Introduction The study of molecular vibrations leads to the theory of infrared and Raman spectra [il. The basic theory of vibrational spectroscopy applies to gases and liquids. However, the theory has to be extended to include crystalline and non-crystalline matter. Twenty years after the discovery of surface-enhanced Raman scattering (SERS), it has become evident that the theory has extended further to include the special chapter of adsorbed molecules on surfaces and interfaces. Surface-enhanced vibrational spectroscopy (SEVS), is the study of molecular vibrations of adsorbates on surfaces and interfaces that can enhance the absorption or the emission of electromagnetic radiation. In parallel with the classical theory of molecular vibrations, SEVS comprises the theory of surfaceenhanced infrared and surface-enhanced Raman scattering. SEVS is a branch of the more general field of surface-enhanced spectroscopy (SES), a very active area of basic research with a broad range of analytical
films
applications. Two comprehensive references compile much of the initial work [2] and some of the developments that followed [3]. The initial period was characterized by a instructive debate about the origin of the several contributions that give rise to the observed SERS signal [4,5]. Experimentally, the initial SERS and SERRS studies were carried out with laser lines in the visible spectral region. However, an early report on the surface enhancement of infrared spectra by Hartstein et al. [6], using gold or silver islands was an indication of the possibilities for a complementary Raman and infrared enhanced vibrational spectroscopy. The use of near-infrared lasers for excitation of the inelastic fight scattering has brought about a renaissance of Raman spectroscopy and correspondingly an upsurge in near-infrared SERS. In particular, FT-Raman and FT-SERS at 1064 nm. Despite the loss in absolute intensity due to the fourth-power dependence of Raman inelastic scattering, the combined advantages of Fourier transform techniques and new detectors in
0022-2860/97/$17.00 0 1997 Elsevier Science B.V. All rights reserved PII SOO22-2860(96)09489-6
by
R. Aroca, S. Rodriguez-Llorenie/Journal of Molecular Structure 408/409 (1997) 17-22
18
the near-infrared have made IT-Raman one of the most successful analytical techniques for chemical and biochemical applications. The near-infrared region is also the most appropriate region for the SERS experiment, a spectral region where the excitation frequency is commonly far from the energy of the lowest electronic transition, i.e. an experimental condition that avoids preresonance or resonant Raman scattering. There exist, however, the possibility of a near-infrared absorption due to the formation of surface complexes with a charge-transfer electronic transition in this spectral region [7]. Excitation at 1064 nm that is in resonance with a charge-transfer or any other electronic transition in the near-infrared would give rise to FI-SERRS. The surface-enhanced resonant Raman scattering and the surface-enhanced fluorescence (SEF) are affected by damping, and careful consideration should be given to the perturbation of the optical properties of the excited state (I’& caused by the presence of the metal surface. For spectral analysis, the first step in the theory is the construction of a molecular model for the adsorbate, followed by group theory analysis and computation of vibrational states. Once the frequency analysis is completed, the interpretation of the observed intensities is next, followed by band shape analysis. It is now accepted to discuss the intensity enhancement as the results of two contributions: the electromagnetic enhancement mechanism produced by certain rough metal surfaces, and the perturbations in the optical parameters of the adsorbate (chemical effect). In addition, as previously mentioned, chemisorption may lead to charge-transfer electronic states that could be directly excited by the external radiation in the case of inelastic light scattering [S-11]. The charge transfer case follows within the SERRS treatment of intensities. The observed intensities in SEVS are proportional to these two main contributions, and may be written as follows: SERS m l~(~~)l~I~(~~)l~l~~/~Ql~ SEIR cx IA(
t8/.@Q]2
SERRS (and NI-SERRS) x h3cx/dQ~2/I’,ff
CClA(~~l*lA(qJl*
where A(@,_) is the enhanced-absorption term at the frequency wL, and A(q) is the enhanced-emission factor at the frequency 0s. The nomenclature permits one to see that the interpretation of the observed intensity for the vibrational modes of a given adsorbate is a difficult task. The enhanced vibrational intensities are determined by the electromagnetic enhancement (surface roughness and optical constants) and surface induced change on the molecular optical parameters. The electromagnetic contribution in the observed intensities has been rationalized in terms of the propensity rules [ 121. The contribution from the change in transition moments is difficult to include, since the optical parameters for most polyatomic molecules are not known. Presently, high level of theory ab initio techniques are providing IR and Raman intensities for small polyatomic molecules, and the work could be extended to adsorbates. If the perturbation of the optical parameters is negligible the propensity rules would provide an adequate description of the observed intensities. The propensity rules are not expected to be closely followed in the case of SERRS produced either by molecular resonances or charge-transfer resonances. The state of affairs is much closer to the intensity theory of crystals, where a set of distinctly different spectra are expected for a combination of crystal orientations and light polarization. In addition, the SERS intensities are affected by the relative position of the excitation frequency with respect to plasmon absorption of the rough metal surface [ 13,141. The chapter for the surface selection rules for SERS and SEIR has not been completed. The difficulties encountered in the spectral interpretation have not prevented an outburst of activity in SEVS: SERS active surface development and analytical applications of SERS, SERRS and SEIR. In particular, the SERRS effect that combines the multiplicative effect of two resonances has inspired the search for “single molecule detection” [ 151. The approach to surface-enhanced vibrational spectroscopy describe above, is illustrated here with three examples: SERRS spectra of Langmuir-Blodgett (LB) monolayers of N-hexyl-perylene tetracarboxylic diimide (HPTCNH), and the FI-SERS of an LB
R. Aroca, S. Rodriguez-Llorente/Journal
monolayer of 7-methyl-8,13-benzo[a]naphthacenedione (BQD). Finally, SEIR is illustrated using perylene tetracarboxylic dianhydride (PTCDA) vacuum evaporated onto silver and gold island films.
19
of Molecular Structure 408/409 (1997) 17-22
a liquid nitrogen cooled MCT detector. The reflection spectra were obtained by using a Spectra-Tech variable angle reflectance accessory and an incidence angle of 75”. The FI-SERS spectra was recorded with a Bruker RFSlOO equipped with an IT-Raman microscope.
2. Experimental LB mixed monolayers of HPTCNH [ 161 with arachidic acid [ 171, and the LB of BQD were prepared in a Lauda Langmuir film balance equipped with an electronically controlled dipping device (Lauda Filmlift- 1). Double distilled water purified through a MilliQ system (Millipore) with a measured resistivity of 18.2 Ma cm was used as the subphase. For visible absorption and fluorescence work, the substrates were Coming 7059 glass slides. The silver island films were prepared under high vacuum (10” mbar) by thermal evaporation to Coming 7059 glass slides fixed at 200°C for SERRS and IT-SERS, and onto IR transparent substrates for SEIR. Film thickness was monitored by a XTC Inficon quartz crystal oscillator. The absorption spectra were recorded using a Response single beam spectrophotometer interfaced to an IBM microcomputer. The fluorescence and SERRS spectra were recorded using a Spex 1403 double beam monochromator equipped with a photomultiplier tube. The SERRS spectra were also recorded the backscattering geometry using a microscope attachment and a x 100 objective on a THR 1000 spectrograph with a liquid nitrogen cooled CCD detector. The excitation source was the 514.5 nm line of an argon ion laser. The infrared spectra were recorded using a Bomem DA3 FTIR equipped with
3. Results and discussion 3.1. Surface-enhanced HPTCNH
d
460
450
560 550 Wavelengthlnm
600
E
Fig. I. HPTCNH absorption spectra. 10” M solution in chloroform and mixed LB monolayer on glass.
of
The absorption spectrum of a 10-b M solution of HPTCNH in chloroform, and the absorption spectrum of the LB film are shown in Fig. 1. The absorption of HPTCNH in solution at 580 nm is due to the formation of dimers, and is also part of the broad absorption in the LB film. It can be seen that the 514.5 nm laser line is in resonance with the electronic absorption and will give rise to RRS for the neat material and to SERRS for the LB on silver island film, since the 514.5 nm line is also in resonance with the broad plasmon resonance in 500 nm region. The SERRS and the RRS spectra of HPTCNH are shown in Fig. 2. The RRS spectrum of the solid sample, was recorded using a CsI pellet of HPTCNH. The most prominent RRS bands are seen at 1573 cm-‘, 1378 cm-‘, 1300 cm-’ and 544 cm-‘. These bands are also present in the SERRS spectrum, however their relative intensity is different. The 544 cm-’ band is seen besides a stronger band at 554 cm-‘. The 1300 cm-’ becomes a shoulder of the strong 1292 cm-’ band. The 1378 cm-’ is practically
1000
350
resonant Raman scattering
HPTCNH Mixed LB 1:lO AA 554
500
1000 Wavenumbedcm
1500
’
Fig. 2. Resonant Raman scattering spectrum of neat HPTCNH and SERRS of a single mixed monolayer on silver islands.
20
R. Aroca,
of Molecular
S. Rod: iguez-Llorente/Journal
unchanged at 1380 cm-‘. The 1573 cm-’ band is the same in both spectra. However, the band at 1586 cm-’ in the RRS spectrum is resolved into two components at 1582 cm-’ and 1592 cm-’ respectively. From Raman studies of perylene tetracarboxylic derivatives, it is known that the 1292-1300 cm-’ and 544-554 cm-’ are commonly observed pairs. The relative intensity with which they are recorded seems to depend on molecular orientation and degree of aggregation. However, the relative intensities in the SERRS spectrum are also affected by molecular orientation. The broad fluorescence background at low wavenumbers is typical of the monomer fluorescence and has been seen in mixed LB layers of perylene tetracarboxylic derivatives. The interpretation of the observed intensities in the SERRS spectrum can be carried out with the help of the following considerations. (1) Characteristic Raman bands of the chromophore are commonly observed in RRS and SERRS. This fact is the key to analytical applications of SERRS. For example, as it can be seen in Fig. 2, the characteristic modes of HPTCNH are present in both the RRS and the SERRS spectra. (2) Chemisorption versus physisorption. The minor changes in wavenumbers observed for HPTCNH can be attributed to band narrowing, and it is concluded that the monolayer on rough silver produces the spectrum of a physisorbed molecule. (3) Molecular orientation at the metal surface. There are directional effects in both the electromagnetic and electro optical components of the observed intensities. The discussion would be possible if a series of spectra are recorded varying
Structure
408/409
(I 997)
I7-22
the polarization of the incident radiation. Finally, in the particular case of dyes such as HPTCNH, the formation of aggregates may also affect the relative intensities and band shapes of the observed spectra. 3.2. Su$ace-enhanced Raman scattering infrared region: FT-SERS
in the near-
The first report on FT-SERS of LB monolayers using the 1064 nm laser excitation and gold island films was published in 1992 [18]. Here, the FTSERS of a single monolayer of naphthacenedione deposited onto a 6 nm silver island film was obtained and is shown in Fig. 3. It can be seen that the FTRaman of the bulk and that of the monolayer are similar. There are only minor differences in relative intensities, however the observed wavenumbers are identical. It is also a case of physisorption. This example represent the ideal case for analytical applications, where the FT-SERS is simply the enhanced version of the unenhanced reference spectrum. It is fair to say that the near-infrared region provides one of the best spectral windows for analytical applications of SERS. A simple calculation using the electromagnetic interpretation predicts that for a prolate 38 nrn/33 nm, a number of metals could provide a hundredfold enhancement at 1064 nm. For example, Fe, MO, Ni, Rh, Pt and Ag. The study of different metals in this spectral region is another avenue for new developments in SERS
1300
20 nm film on silver (RAIRS) w
_
20 nm film on KBr
-_-Jq
FT-RAMAN, so”’ sample
200
400
600
600
1000
1200
1400
I
1200
1600
Wavenumberlcm”
Fig. 3. The FT-Raman and IT-SERS 7-methyl-8,13&enzo[a]naphthacenedione
I
1300
I
Fig. 4. Pictorial demonstration spectra of a monolayer on silver islands.
of
20 nm PTCDA film, provided
absorption
lT-IR
I
I
1100 1000 900 WAVENUMBERlcm-’
spectra.
I
800
of average molecular alignment in a by the transmission and reflection-
R. Aroca, S. Rodriguez-Llorente/Journal
of Molecular
Structure 408/409
(1997)
21
17-22
SEIR on Silver nm PTCDA on 6 nm Ag
20 nm PTCDA on Id nm Ag Ii00
12bo
llb0
ldO0
800
960
WAVENUMBERkm-’ Fig. 5. SEIR spectra of PTCDA on silver island in the transmission
3.3. Surface-enhanced
infrared
The enhancement of the vibrational intensities in the mid-infrared contains two factors. The enhanced absorption cross section produced by changes in the metal response to electromagnetic radiation and the electro optical parameters of the adsorbate. The first experimental observations were achieved on silver and gold island films using the ATR configuration [6]. The effect has also been observed in transmission and reflection-absorption infrared experiments [ 19221. SEIR is illustrated here using vacuum evaporated films of perylene tetracarboxylic dianhydride (PTCDA) on silver and gold island films. The vibrational spectra of thin solid films of perylene tetracarboxylic dianhydride (PTCDA) have been reported, and the vibrational assignments of characteristic fundamental frequencies was given [23]. The surface-enhanced infrared (SEIR) spectra have been recorded for samples prepared by vacuum evaporation of PTCDA onto rough silver and gold films of 6, 10 and 14 nm mass thicknesses. Reflection absorption and transmission FIIR spectra of a thin solid film deposited on smooth silver and KBr respectively, were also recorded for comparison with the SEIR spectra, and are shown in Fig. 4. In the transmission IT-IR spectrum the out-of-plane C-H vibrations at 734 cm-‘, 809 cm-’ and 862 cm-’ have negligible relative intensity. This is a demonstration of a face-on average molecular orientation in the film. In the reflection-absorption spectrum, the out-ofplane C-H vibrations are seen with strong relative
geometry.
intensity compared with the in-plane vibrations. The experiment shows that the PTCDA film on smooth silver also has face-on molecular orientation. The SEIR spectra of PTCDA on silver island films are shown in Fig. 5. The observed wavenumbers are unchanged from those observed in the reference films. Therefore, PTCDA is physically adsorbed on the rough metal surface. The observed enhancement peaks at a film mass thickness of 10 nm, as has been previously observed. The relative intensities of the in-plane and out-ofplane vibrations in SEIR, do not follow the RAIRS surface selection rules. The relative intensities in the SEIR spectrum are closer to the FT-IR of the KBr pellet [23]. Similarly, the SEIR spectrum of PTCDA on gold shown in Fig. 6 is closely related to the infrared spectrum of the PTCDA in a
1300
I
1800
2.5 “m PTCDA on 10 nm Au
I
I
I
I
I
1600
1400
1200
1000
800
Wavenumberlcm.’ Fig. 6. SEIR of PTCDA geometry.
on gold islands
in the transmission
I
22
R. Aroca, S. Rodriguez-Llorente/Journal of Molecular Structure 408/409 (1997) 17-22
KBr pellet. The SEIR of PTCDA follow the RAIRS selection rules.
on gold does not
4. Conclusions The study of molecular vibrations of adsorbates on rough surfaces that can enhance the absorption and emission of radiation is the subject of surfaceenhanced Raman scattering and surface-enhanced infrared. The surface selection rules of the electromagnetic enhancement and the change in the molecular optical parameters will determine the observed vibrational intensities. One SERS active substrate, such as silver islands, can be used to obtain the complete vibrational spectra: SERS plus SEIR. The nearinfrared region seems to offer optimum conditions for new SERS application.
Acknowledgements We would like to acknowledge J. Hellman from Bruker Canada for FT-Raman measurements. NSERC of Canada is gratefully acknowledged for financial support.
References [I] Molecular
Vibrations, E.B. Wilson, Jr., J.C. Decius, Cross, Dover Publications, Inc., N.Y. 1980.
P.C.
[2] R.K. Chang and T.E. Furtak (eds.), Surface Enhanced Raman Scattering, Plenum, New York, 1982. [3] M. Kerker (ed.), Selected Papers on Surface-enhanced Raman Scattering, SPIE Milestone Series, 1990. [4] M. Moskovits, Rev. Mod. Phys., 57 (1985) 783. [5] A. Otto, J. Raman Spectrosc., 22 (1991) 743. A. Otto, I. Mrozek, H. Grabhom and W. Akeman, J. Phys.: Condens. Matter, 4 (1992) 1143. [6] A. Hartstein, J.R. Kirtley and J.C. Tsang, Phys. Rev. Let., 45 (1980) 201. [7] P. Hildebrandt, S. Keller, A. Hoffmann, F. Vanhecke and B. Schrader, 3. Raman Spectrosc., 24 (1993) 791. [8] H. Metiu, Prog. Surf. Sci., 17 (1984) 153. [9] D.A. Weitz, S. Garoff, J.I. Gersten and A. Nitzan, J. Chem. Phys., 78 (1983) 5324. [IO] P.W. Barber, R.K. Chang and H. Massoudi, Phys. Rev., B, 27 (1983) 7251. [I 11 M. Xu. and M.J. Dignam, J. Chem. Phys., 99 (1993) 2307. [12] M. Moskovits, J. Chem. Phys., 77 (1982) 4406. [13] M. Moskovits and J.S. Suh, J. Phys. Chem., 88 (1984) 142. [ 141 J.R. Menendez, A. Obuchowska and R. Aroca, Spectrochim. Acta, A52 (1996) 392. [15] K. Kneipp, Y. Wang, R.R. Dasari and M.S. Feld, Appl. Spectrosc., 49 (1995) 780. [16] E. Johnson and R. Aroca, Applied Spectros., 49 (1995) 472. [ 171 A. Ulman, Ultrathin Organic Films, Academic Press, New York, 1991. [18] CA. Jennings, G.J. Kovacs and R. Aroca, J. Phys. Chem., 96 (1992) 1340. [19] Y. Nishikawa, K. Fujiwara, K. Ataka and M. Osawa, Anal. Chem., 65 (1993) 556. [20] M. Osawa, K. Ataka, K. Yoshii and Y. Nishikawa, Appl. Spectrosc., 47 (1993) 1497. [21] Y. Nishikawa, K. Fujiwara and T. Shima, Appl. Spectrosc., 45 (1991) 747. [22] E. Johnson and R. Aroca, J. Phys. Chem., 99 (1995) 9325. [23] K. Akers, R. Aroca, A.M. Hor and R.O. Loutfy, J. Phys. Chem., 91 (1987) 2954.
MOLECULAR STRUCTURE ELSEVIER
Journal of Molecular
Structure 4081409 (1997) 17-22
Surface-enhanced vibrational spectroscopy Ricardo Aroca, S. Rodriguez-Llorente Materials Surface Science Group, Department
of Chemistryand Biochemistry
Received 26 August
Universiq of Windsor, Windsor, Ont. N9B 3P4 Canada
1996; accepted 6 September
1996
Abstract
The nature and properties of surface-enhanced vibrational spectra of molecules adsorbed on surfaces that can enhance the absorption and the emission of electromagnetic radiation are discussed. Examples of surface-enhanced resonant Raman scattering, surface-enhanced Raman scattering in the near-infrared and surface-enhanced infrared are given. The rough surfaces used are metal island films coated with a nanometric vacuum evaporation. 0 1997 Elsevier Science B.V. Keywords:
organic film using the Langmuir-Blodgett
technique
or deposited
Surface-enhanced infrared spectroscopy; Surface-enhanced Raman spectroscopy; Langmuir-Blodgett
1. Introduction The study of molecular vibrations leads to the theory of infrared and Raman spectra [il. The basic theory of vibrational spectroscopy applies to gases and liquids. However, the theory has to be extended to include crystalline and non-crystalline matter. Twenty years after the discovery of surface-enhanced Raman scattering (SERS), it has become evident that the theory has extended further to include the special chapter of adsorbed molecules on surfaces and interfaces. Surface-enhanced vibrational spectroscopy (SEVS), is the study of molecular vibrations of adsorbates on surfaces and interfaces that can enhance the absorption or the emission of electromagnetic radiation. In parallel with the classical theory of molecular vibrations, SEVS comprises the theory of surfaceenhanced infrared and surface-enhanced Raman scattering. SEVS is a branch of the more general field of surface-enhanced spectroscopy (SES), a very active area of basic research with a broad range of analytical
films
applications. Two comprehensive references compile much of the initial work [2] and some of the developments that followed [3]. The initial period was characterized by a instructive debate about the origin of the several contributions that give rise to the observed SERS signal [4,5]. Experimentally, the initial SERS and SERRS studies were carried out with laser lines in the visible spectral region. However, an early report on the surface enhancement of infrared spectra by Hartstein et al. [6], using gold or silver islands was an indication of the possibilities for a complementary Raman and infrared enhanced vibrational spectroscopy. The use of near-infrared lasers for excitation of the inelastic fight scattering has brought about a renaissance of Raman spectroscopy and correspondingly an upsurge in near-infrared SERS. In particular, FT-Raman and FT-SERS at 1064 nm. Despite the loss in absolute intensity due to the fourth-power dependence of Raman inelastic scattering, the combined advantages of Fourier transform techniques and new detectors in
0022-2860/97/$17.00 0 1997 Elsevier Science B.V. All rights reserved PII SOO22-2860(96)09489-6
by
R. Aroca, S. Rodriguez-Llorenie/Journal of Molecular Structure 408/409 (1997) 17-22
18
the near-infrared have made IT-Raman one of the most successful analytical techniques for chemical and biochemical applications. The near-infrared region is also the most appropriate region for the SERS experiment, a spectral region where the excitation frequency is commonly far from the energy of the lowest electronic transition, i.e. an experimental condition that avoids preresonance or resonant Raman scattering. There exist, however, the possibility of a near-infrared absorption due to the formation of surface complexes with a charge-transfer electronic transition in this spectral region [7]. Excitation at 1064 nm that is in resonance with a charge-transfer or any other electronic transition in the near-infrared would give rise to FI-SERRS. The surface-enhanced resonant Raman scattering and the surface-enhanced fluorescence (SEF) are affected by damping, and careful consideration should be given to the perturbation of the optical properties of the excited state (I’& caused by the presence of the metal surface. For spectral analysis, the first step in the theory is the construction of a molecular model for the adsorbate, followed by group theory analysis and computation of vibrational states. Once the frequency analysis is completed, the interpretation of the observed intensities is next, followed by band shape analysis. It is now accepted to discuss the intensity enhancement as the results of two contributions: the electromagnetic enhancement mechanism produced by certain rough metal surfaces, and the perturbations in the optical parameters of the adsorbate (chemical effect). In addition, as previously mentioned, chemisorption may lead to charge-transfer electronic states that could be directly excited by the external radiation in the case of inelastic light scattering [S-11]. The charge transfer case follows within the SERRS treatment of intensities. The observed intensities in SEVS are proportional to these two main contributions, and may be written as follows: SERS m l~(~~)l~I~(~~)l~l~~/~Ql~ SEIR cx IA(
t8/.@Q]2
SERRS (and NI-SERRS) x h3cx/dQ~2/I’,ff
CClA(~~l*lA(qJl*
where A(@,_) is the enhanced-absorption term at the frequency wL, and A(q) is the enhanced-emission factor at the frequency 0s. The nomenclature permits one to see that the interpretation of the observed intensity for the vibrational modes of a given adsorbate is a difficult task. The enhanced vibrational intensities are determined by the electromagnetic enhancement (surface roughness and optical constants) and surface induced change on the molecular optical parameters. The electromagnetic contribution in the observed intensities has been rationalized in terms of the propensity rules [ 121. The contribution from the change in transition moments is difficult to include, since the optical parameters for most polyatomic molecules are not known. Presently, high level of theory ab initio techniques are providing IR and Raman intensities for small polyatomic molecules, and the work could be extended to adsorbates. If the perturbation of the optical parameters is negligible the propensity rules would provide an adequate description of the observed intensities. The propensity rules are not expected to be closely followed in the case of SERRS produced either by molecular resonances or charge-transfer resonances. The state of affairs is much closer to the intensity theory of crystals, where a set of distinctly different spectra are expected for a combination of crystal orientations and light polarization. In addition, the SERS intensities are affected by the relative position of the excitation frequency with respect to plasmon absorption of the rough metal surface [ 13,141. The chapter for the surface selection rules for SERS and SEIR has not been completed. The difficulties encountered in the spectral interpretation have not prevented an outburst of activity in SEVS: SERS active surface development and analytical applications of SERS, SERRS and SEIR. In particular, the SERRS effect that combines the multiplicative effect of two resonances has inspired the search for “single molecule detection” [ 151. The approach to surface-enhanced vibrational spectroscopy describe above, is illustrated here with three examples: SERRS spectra of Langmuir-Blodgett (LB) monolayers of N-hexyl-perylene tetracarboxylic diimide (HPTCNH), and the FI-SERS of an LB
R. Aroca, S. Rodriguez-Llorente/Journal
monolayer of 7-methyl-8,13-benzo[a]naphthacenedione (BQD). Finally, SEIR is illustrated using perylene tetracarboxylic dianhydride (PTCDA) vacuum evaporated onto silver and gold island films.
19
of Molecular Structure 408/409 (1997) 17-22
a liquid nitrogen cooled MCT detector. The reflection spectra were obtained by using a Spectra-Tech variable angle reflectance accessory and an incidence angle of 75”. The FI-SERS spectra was recorded with a Bruker RFSlOO equipped with an IT-Raman microscope.
2. Experimental LB mixed monolayers of HPTCNH [ 161 with arachidic acid [ 171, and the LB of BQD were prepared in a Lauda Langmuir film balance equipped with an electronically controlled dipping device (Lauda Filmlift- 1). Double distilled water purified through a MilliQ system (Millipore) with a measured resistivity of 18.2 Ma cm was used as the subphase. For visible absorption and fluorescence work, the substrates were Coming 7059 glass slides. The silver island films were prepared under high vacuum (10” mbar) by thermal evaporation to Coming 7059 glass slides fixed at 200°C for SERRS and IT-SERS, and onto IR transparent substrates for SEIR. Film thickness was monitored by a XTC Inficon quartz crystal oscillator. The absorption spectra were recorded using a Response single beam spectrophotometer interfaced to an IBM microcomputer. The fluorescence and SERRS spectra were recorded using a Spex 1403 double beam monochromator equipped with a photomultiplier tube. The SERRS spectra were also recorded the backscattering geometry using a microscope attachment and a x 100 objective on a THR 1000 spectrograph with a liquid nitrogen cooled CCD detector. The excitation source was the 514.5 nm line of an argon ion laser. The infrared spectra were recorded using a Bomem DA3 FTIR equipped with
3. Results and discussion 3.1. Surface-enhanced HPTCNH
d
460
450
560 550 Wavelengthlnm
600
E
Fig. I. HPTCNH absorption spectra. 10” M solution in chloroform and mixed LB monolayer on glass.
of
The absorption spectrum of a 10-b M solution of HPTCNH in chloroform, and the absorption spectrum of the LB film are shown in Fig. 1. The absorption of HPTCNH in solution at 580 nm is due to the formation of dimers, and is also part of the broad absorption in the LB film. It can be seen that the 514.5 nm laser line is in resonance with the electronic absorption and will give rise to RRS for the neat material and to SERRS for the LB on silver island film, since the 514.5 nm line is also in resonance with the broad plasmon resonance in 500 nm region. The SERRS and the RRS spectra of HPTCNH are shown in Fig. 2. The RRS spectrum of the solid sample, was recorded using a CsI pellet of HPTCNH. The most prominent RRS bands are seen at 1573 cm-‘, 1378 cm-‘, 1300 cm-’ and 544 cm-‘. These bands are also present in the SERRS spectrum, however their relative intensity is different. The 544 cm-’ band is seen besides a stronger band at 554 cm-‘. The 1300 cm-’ becomes a shoulder of the strong 1292 cm-’ band. The 1378 cm-’ is practically
1000
350
resonant Raman scattering
HPTCNH Mixed LB 1:lO AA 554
500
1000 Wavenumbedcm
1500
’
Fig. 2. Resonant Raman scattering spectrum of neat HPTCNH and SERRS of a single mixed monolayer on silver islands.
20
R. Aroca,
of Molecular
S. Rod: iguez-Llorente/Journal
unchanged at 1380 cm-‘. The 1573 cm-’ band is the same in both spectra. However, the band at 1586 cm-’ in the RRS spectrum is resolved into two components at 1582 cm-’ and 1592 cm-’ respectively. From Raman studies of perylene tetracarboxylic derivatives, it is known that the 1292-1300 cm-’ and 544-554 cm-’ are commonly observed pairs. The relative intensity with which they are recorded seems to depend on molecular orientation and degree of aggregation. However, the relative intensities in the SERRS spectrum are also affected by molecular orientation. The broad fluorescence background at low wavenumbers is typical of the monomer fluorescence and has been seen in mixed LB layers of perylene tetracarboxylic derivatives. The interpretation of the observed intensities in the SERRS spectrum can be carried out with the help of the following considerations. (1) Characteristic Raman bands of the chromophore are commonly observed in RRS and SERRS. This fact is the key to analytical applications of SERRS. For example, as it can be seen in Fig. 2, the characteristic modes of HPTCNH are present in both the RRS and the SERRS spectra. (2) Chemisorption versus physisorption. The minor changes in wavenumbers observed for HPTCNH can be attributed to band narrowing, and it is concluded that the monolayer on rough silver produces the spectrum of a physisorbed molecule. (3) Molecular orientation at the metal surface. There are directional effects in both the electromagnetic and electro optical components of the observed intensities. The discussion would be possible if a series of spectra are recorded varying
Structure
408/409
(I 997)
I7-22
the polarization of the incident radiation. Finally, in the particular case of dyes such as HPTCNH, the formation of aggregates may also affect the relative intensities and band shapes of the observed spectra. 3.2. Su$ace-enhanced Raman scattering infrared region: FT-SERS
in the near-
The first report on FT-SERS of LB monolayers using the 1064 nm laser excitation and gold island films was published in 1992 [18]. Here, the FTSERS of a single monolayer of naphthacenedione deposited onto a 6 nm silver island film was obtained and is shown in Fig. 3. It can be seen that the FTRaman of the bulk and that of the monolayer are similar. There are only minor differences in relative intensities, however the observed wavenumbers are identical. It is also a case of physisorption. This example represent the ideal case for analytical applications, where the FT-SERS is simply the enhanced version of the unenhanced reference spectrum. It is fair to say that the near-infrared region provides one of the best spectral windows for analytical applications of SERS. A simple calculation using the electromagnetic interpretation predicts that for a prolate 38 nrn/33 nm, a number of metals could provide a hundredfold enhancement at 1064 nm. For example, Fe, MO, Ni, Rh, Pt and Ag. The study of different metals in this spectral region is another avenue for new developments in SERS
1300
20 nm film on silver (RAIRS) w
_
20 nm film on KBr
-_-Jq
FT-RAMAN, so”’ sample
200
400
600
600
1000
1200
1400
I
1200
1600
Wavenumberlcm”
Fig. 3. The FT-Raman and IT-SERS 7-methyl-8,13&enzo[a]naphthacenedione
I
1300
I
Fig. 4. Pictorial demonstration spectra of a monolayer on silver islands.
of
20 nm PTCDA film, provided
absorption
lT-IR
I
I
1100 1000 900 WAVENUMBERlcm-’
spectra.
I
800
of average molecular alignment in a by the transmission and reflection-
R. Aroca, S. Rodriguez-Llorente/Journal
of Molecular
Structure 408/409
(1997)
21
17-22
SEIR on Silver nm PTCDA on 6 nm Ag
20 nm PTCDA on Id nm Ag Ii00
12bo
llb0
ldO0
800
960
WAVENUMBERkm-’ Fig. 5. SEIR spectra of PTCDA on silver island in the transmission
3.3. Surface-enhanced
infrared
The enhancement of the vibrational intensities in the mid-infrared contains two factors. The enhanced absorption cross section produced by changes in the metal response to electromagnetic radiation and the electro optical parameters of the adsorbate. The first experimental observations were achieved on silver and gold island films using the ATR configuration [6]. The effect has also been observed in transmission and reflection-absorption infrared experiments [ 19221. SEIR is illustrated here using vacuum evaporated films of perylene tetracarboxylic dianhydride (PTCDA) on silver and gold island films. The vibrational spectra of thin solid films of perylene tetracarboxylic dianhydride (PTCDA) have been reported, and the vibrational assignments of characteristic fundamental frequencies was given [23]. The surface-enhanced infrared (SEIR) spectra have been recorded for samples prepared by vacuum evaporation of PTCDA onto rough silver and gold films of 6, 10 and 14 nm mass thicknesses. Reflection absorption and transmission FIIR spectra of a thin solid film deposited on smooth silver and KBr respectively, were also recorded for comparison with the SEIR spectra, and are shown in Fig. 4. In the transmission IT-IR spectrum the out-of-plane C-H vibrations at 734 cm-‘, 809 cm-’ and 862 cm-’ have negligible relative intensity. This is a demonstration of a face-on average molecular orientation in the film. In the reflection-absorption spectrum, the out-ofplane C-H vibrations are seen with strong relative
geometry.
intensity compared with the in-plane vibrations. The experiment shows that the PTCDA film on smooth silver also has face-on molecular orientation. The SEIR spectra of PTCDA on silver island films are shown in Fig. 5. The observed wavenumbers are unchanged from those observed in the reference films. Therefore, PTCDA is physically adsorbed on the rough metal surface. The observed enhancement peaks at a film mass thickness of 10 nm, as has been previously observed. The relative intensities of the in-plane and out-ofplane vibrations in SEIR, do not follow the RAIRS surface selection rules. The relative intensities in the SEIR spectrum are closer to the FT-IR of the KBr pellet [23]. Similarly, the SEIR spectrum of PTCDA on gold shown in Fig. 6 is closely related to the infrared spectrum of the PTCDA in a
1300
I
1800
2.5 “m PTCDA on 10 nm Au
I
I
I
I
I
1600
1400
1200
1000
800
Wavenumberlcm.’ Fig. 6. SEIR of PTCDA geometry.
on gold islands
in the transmission
I
22
R. Aroca, S. Rodriguez-Llorente/Journal of Molecular Structure 408/409 (1997) 17-22
KBr pellet. The SEIR of PTCDA follow the RAIRS selection rules.
on gold does not
4. Conclusions The study of molecular vibrations of adsorbates on rough surfaces that can enhance the absorption and emission of radiation is the subject of surfaceenhanced Raman scattering and surface-enhanced infrared. The surface selection rules of the electromagnetic enhancement and the change in the molecular optical parameters will determine the observed vibrational intensities. One SERS active substrate, such as silver islands, can be used to obtain the complete vibrational spectra: SERS plus SEIR. The nearinfrared region seems to offer optimum conditions for new SERS application.
Acknowledgements We would like to acknowledge J. Hellman from Bruker Canada for FT-Raman measurements. NSERC of Canada is gratefully acknowledged for financial support.
References [I] Molecular
Vibrations, E.B. Wilson, Jr., J.C. Decius, Cross, Dover Publications, Inc., N.Y. 1980.
P.C.
[2] R.K. Chang and T.E. Furtak (eds.), Surface Enhanced Raman Scattering, Plenum, New York, 1982. [3] M. Kerker (ed.), Selected Papers on Surface-enhanced Raman Scattering, SPIE Milestone Series, 1990. [4] M. Moskovits, Rev. Mod. Phys., 57 (1985) 783. [5] A. Otto, J. Raman Spectrosc., 22 (1991) 743. A. Otto, I. Mrozek, H. Grabhom and W. Akeman, J. Phys.: Condens. Matter, 4 (1992) 1143. [6] A. Hartstein, J.R. Kirtley and J.C. Tsang, Phys. Rev. Let., 45 (1980) 201. [7] P. Hildebrandt, S. Keller, A. Hoffmann, F. Vanhecke and B. Schrader, 3. Raman Spectrosc., 24 (1993) 791. [8] H. Metiu, Prog. Surf. Sci., 17 (1984) 153. [9] D.A. Weitz, S. Garoff, J.I. Gersten and A. Nitzan, J. Chem. Phys., 78 (1983) 5324. [IO] P.W. Barber, R.K. Chang and H. Massoudi, Phys. Rev., B, 27 (1983) 7251. [I 11 M. Xu. and M.J. Dignam, J. Chem. Phys., 99 (1993) 2307. [12] M. Moskovits, J. Chem. Phys., 77 (1982) 4406. [13] M. Moskovits and J.S. Suh, J. Phys. Chem., 88 (1984) 142. [ 141 J.R. Menendez, A. Obuchowska and R. Aroca, Spectrochim. Acta, A52 (1996) 392. [15] K. Kneipp, Y. Wang, R.R. Dasari and M.S. Feld, Appl. Spectrosc., 49 (1995) 780. [16] E. Johnson and R. Aroca, Applied Spectros., 49 (1995) 472. [ 171 A. Ulman, Ultrathin Organic Films, Academic Press, New York, 1991. [18] CA. Jennings, G.J. Kovacs and R. Aroca, J. Phys. Chem., 96 (1992) 1340. [19] Y. Nishikawa, K. Fujiwara, K. Ataka and M. Osawa, Anal. Chem., 65 (1993) 556. [20] M. Osawa, K. Ataka, K. Yoshii and Y. Nishikawa, Appl. Spectrosc., 47 (1993) 1497. [21] Y. Nishikawa, K. Fujiwara and T. Shima, Appl. Spectrosc., 45 (1991) 747. [22] E. Johnson and R. Aroca, J. Phys. Chem., 99 (1995) 9325. [23] K. Akers, R. Aroca, A.M. Hor and R.O. Loutfy, J. Phys. Chem., 91 (1987) 2954.