J . Phys. Chem. 1993, 97, 5080-5084
5080
Charge-Transfer Effect on Surface-Enhanced Second-Harmonic Generation Byoungchoo Park, M. S. Kim, and Y. D. Kim* Department of Physics, Sogang University, C.P.O. Box 1142, Seoul 121 -742, Korea
E. C. Jung Atomic Spectroscopy Department, Korea Atomic Energy Research Institute, P.O. Box 7, Taejon 302-606, Korea
C. S . Jung Department of Physics and Optical Engineering, Chongiu University, Chongiu 360-01 3, Korea Received: September 22, 1992; In Final Form: January 8, 1993
Surface-enhanced second-harmonic generation (SESHG) arising from electrochemically roughened silver surface adsorbed molecules has been investigated. The repetitions of electrode potential sweep in the nonfaradaicregion were observed to cause irreversible losses in the optical second-harmonic signal. The SESHG intensity profiles obtained in the presence of adsorbates were seen to depend on the electrode potential. We interpret the dependence of the SESHG intensities on the electrode potential as a charge-transfer effect. It is suggested that the resonant excitation of the charge-transfer transition enhances the second-harmonic generation intensities. A chargetransfer calculation is used to fit the data for the effect of electrode potential on the intensities. It is found that the second-harmonic intensity profile predicted in the calculation agrees very well with the experimental values.
Introduction For certain metallic surfaces in electrochemical systems, an enormous enhancement exists in the Raman and nonlinear optical signals resulting from the submonolayercoverageof the molecular adsorbates at the surface.’ It is well-known that the physical mechanism for such a large enhancement originates either from the electrodynamic effect, associated with a localized surfaceplasmon resonance,’or from the charge-transfer effect, involving an electron transition between the molecule and the metal surface.2 In the field of surface-enhanced Raman scattering (SERS), theoretical and experimental investigations related to the two effects are well e~tablished.~,~ The observed resonance shape of the SERS intensity profile can be uniquely described in terms of the potential-tuned resonance Raman effect, which is due to the charge-transfer (CT) transitions between the metal and the m o l e c ~ l e .On ~ ~the ~ other hand, considerably less work has been done on the CT effect with respect of surface-enhanced second harmonic generation (SESHG), although it is known that nonlinear optical properties can be enhanced by the CT effect.6 In the present work, detailed SESHG investigationshave been carried out for two different types of electrochemical systems. The major purpose of the present work is to study the behavior of the SESHG intensity when it is subjected to applied potentials on a silver electrode. We attempt to analyze our experimental data of the SESHG intensitieswith an extended version of chargetransfer theorye4 By doing this we can provide a better insight into the mechanism of surface-enhanced nonlinear spectroscopy. Experimental Section The preparation of electrochemicalsystems has been described by many authors.!-3 We used 0.05 M piperidine 0.1 M KCl electrolytes and 0.05 M pyridine + 0.1 M KCl electrolytes with deionized distilled water. We set three electrodes in the solution to form electrochemical cells. The three electrodes were the
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0022-365419312097-5080$04.00/0
working electrode, the saturated calomel reference electrode, and the Pt wire auxiliary electrode. The working electrode was composed of a polycrystallineAg plate (Aldrich 10 mm X 10 mm X 5 mm) and was pretreated by polishing (Linde Abrasive, 0.05pm A1203) and etching (H202 + NH40H) chemically. In order to remove impurities, the electrolyte was purged with N2 gas before the experiment. The potential of the electrode was controlled by a voltammograph (BAS Inc. CV-IB). All potentials were referenced to the saturated calomel electrode (SCE). Excitation for the SESHG was provided by a Q-switched NdYAGlaser (Chromatix, 1064nm, and 20-nsduration). Thepower of the Nd-YAG laser was kept near 0.4 mJ/pulse to avoid surface damage.’ The laser beam was incident on the Ag surface at an angle of 45O and collimated to an area of about 1 mm2 with p-polarization. As we were interested in the SESHG signals, we filtered the reflected and scattered 1064-nm radiation out and measured the SESHG intensities by the computer-controlled spectrometer. The SESHG signals were detected by a photomultiplier tube (PMT, RCA 1P28) and boxcar-averaging systems (EG&G Model 164). The SESHG signals were measured carefully during an electrode potential scan at 1 mV/s in the nonfaradaic potential region (the potential range in which no current flowsg). Results and Discussion Figure lshows the SESHG intensity profiles of 532 nm during the cathodic potential sweeps from -0.1 to -1.0 V,,, for the electrochemically roughened silver surface that was subjected to an electrochemical cycle in 0.1 M KCl + 0.05 M piperidine solution. The electrochemical cycle (oxidation reduction cycle; ORC) was initiated at -0.1 V,,,, reversed at +0.1 V,,, for 10 s, and then returned to -0.1 V,,,. Curve A in Figure 1 shows the intensity profile as a function of the electrode potential during the first cathodic sweep just after the complete reduction of the Ag electrode. In curve A, the 0 1993 American Chemical Society
Second-Harmonic Generation
The Journal of Physical Chemistry, Vol. 97,No. 19, 1993 5081
0
0
z
Z 00.0
0.2
0.4
0.6
Applied Potential
0.8
1.0
(-Vace)
Applied Potential
(-Vsce)
Figure 1. SESHG intensity profiles from the roughened silver surface with piperidine (C5Hl IN)adsorbateas a functionoftheelectrodepotential. Curve A SESHG intensity profileduring the first cathodicsweep.Curve B: SESHG intensity profile during the second cathodic sweep. Curve C: SESHG intensity profile during the third cathodic sweep. Curve D: SESHG intensity profile when the SESHG intensity did not decrease any more by repetitions of the potential sweep.
Figure 2. SESHG intensity profiles from the roughened silver surface with pyridine (C5H5N) adsorbate as a function of the electrode potential. Curve A: SESHG intensity profile during the first cathodicsweep.Curve B: SESHG intensity profile during the second cathodic sweep. Curve C: SESHG intensity profile during the third cathodic sweep. Curve D: SESHG intensity profile when the SESHG intensity did not decrease any more by repetitions of the potential sweep.
intensity of SESHG maximizesnear -0.25 V,, (V,,,, the potential at which the intensity reaches a maximum4) and gradually decreaseswith a decreasing potential of the silver electrode. During the second cathodic sweep from -0.1 to -1.0 V,,, immediately after the first potential sweep without another ORC, the SESHG intensity (curve B) reaches the maximum that is only a fraction of the original intensity near -0.25 V,,,. The SESHG intensity reaches an even smaller value for the third cathodic sweep (curve C). After another cathodic sweep was repeated several times without another ORC, decrement of the SESHG intensity was not observed any more. Curve D shows the SESHG intensity profile after the cathodic sweeps. The SESHG intensity of curve A strongly depends on the electrode potential, while the SESHG intensity of curve D does not depend on the electrode potential. In these measurements, the intensities of the SESHG profiles decrease considerably as the potential sweep is repeated. The decrement of SESHG intensity is neither an effect resulting from the change in surface coverage, which is a reversible and singlevalued function of the potential sweep, nor an electrodynamic effect because the large-scale roughnessdoes not alter as a function of the potential in the nonfaradaic region; in that case there will be no change in the electrodynamic effect. Thus, the electrodynamic effect clearly cannot provide an explanation for the decrement and the irreversible loss of the SESHG intensity. Therefore, the irreversible loss of the SESHG intensity can be explained uniquely by the effect resulting from the surface diffusion of adatoms that compose the surface complex? The irreversible loss of the SESHG intensity indicates strongly that surface complex plays an important role in the overall enhancement mechanism. When the SESHG intensity profile reached the minimum level, shown in curve D, the intensity profile did not change although we performed another cathodic sweep. Thus the intensity of curve D can be regarded as the effect of large-scale roughness becausethe irreversibleloss due to the reduction of surface complex does not appear any more. Therefore, the differenceof intensities between curves A and D can be considered as the pure enhancement effect of the surface complex. Once the SESHG intensity decreases, the intensity can be only restored by another ORC. Comparing the SESHG result of curve A with that of curve D, it is clear that the second-harmonic intensities of curve A are more enhanced and more remarkably dependent on the applied potential of the electrode than those of curve D. Using the same potential sweep method, we observed the irreversible loss and the potential dependence of the SESHG intensity profiles for the roughened silver surface in 0.1 M KCl 0.05 M pyridine solution as shown in Figure 2. The intensity
of SESHG for the surfaceincreases after -0.6 V,,,and maximizes near 4 9 Vse. The intensitiesof the SESHG profile also decrease as the potential sweep is repeated. Comparing the SESHG result of the first cathodic potential sweep (curve A) with that of the last potential sweep (curve D), it is clear that the second-harmonic intensities of curve A are more enhanced and more dependent on the potential of the silver electrodethan those of curve D, although the large-scale roughness on the silver surface does not change. Similarly with the consideration in the piperidine-Ag system, we also consider the difference of intensities between curves A and D as the enhancement effect of a pyridine surface complex. On the basis of these results, we speculate that the potential dependence of the SESHG intensity from the surfaces-adsorbed molecules is due to the CT transition process of surface complex. (See also discussions of Figure 4 later in this section.) The resonant excitation of the charge-transfer transition can enhancethe secondharmonic generation intensity. Considering the interaction between the molecule and the metal surface, we may expect that the surface complex has a noncentrosymmetric structure and produces very large nonlinear responses. We shall denote the ground state of the molecule by lg) and its excited state by In). The states Im) are to represent the conduction band of the metal and lie between lg) and In). To determine the surface intensity, we consider that the electromagneticfields are perpendicular to the surface (TM mode) m d that the orientations of the pyridineI0 and piperidine" molecules are also perpendicular to the surface through the lone pair electron of the ring nitrogen. These considerations have been made before in theoretical models of SERS,I0 and the orientations of the adsorbed molecules on the surface have been proved experimental1y.l' Assuming the surface of the electrode as the xy-plane, the dominant second-order susceptibility is fizz,, where z is the direction perpendicular to the surface. In the intramolecular charge-transfer case, the major contribution to /3 is given byI2
+
where (rzgn)2 is the oscillator strength of the transition, AP,,the change in dipole displacements between the ground and excited state (Ar,,,= P,,,,- r z g g ) , wngthe transition frequency, and wo the frequency of the incident field. In eq 1, we have not considered damping factors, which will be phenomenologically introduced
Park et al.
5082 The Journal of Physical Chemistry, Vol. 97, No. 19, 1993 VACUUM LEVEL
-
I
\
In. r
1 9 WfglI 'il .............
METAL
E,
Ig>
MOLECULE
(b) (c) (a) Figure 3. (a) Energy level scheme for the molecule-metal system. The (discrete) molecular levels are g and n. The (continuous) metal levels of the conduction band are shown on the left. W, well depth; a, work function; wf, Fermi energy; Ef, Fermi level; Ig), ground molecular state; In), excited molecular state; wng, energy difference between lg) and In); E., affinity energy of molecule;eV, applied electrode potential;wnf, energy difference between the Fermi level of metal and the excited molecular state; wfe, energy difference between the Fermi level of metal and the molecular ground state. (b) The scheme for molecule-to-metal charge-transfer transitions between the ground molecular state and unfilled levels of the metal. (c) The scheme for metal-to-molecule charge-transfer transitions between filled levels of the metal and the excited molecular state.
later since the damping term only makes the calculation complicated at this stage. We now consider the condition in which the frequency of the exciting field is less than that of the molecular transition (WO
