Electrolyte

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Second-Order Nonlinear Optical Properties of the Ag(111)/ Electrolyte Interface in the Presence of Self-Assembled Monolayers Containing Conjugated π Systems. I. r-Functionalized Terthiophene Films on Ag(111) Elizabeth Santos* Facultad de Matema´ tica, Astronomıá y Fı´sica, Universidad Nacional de Co´ rdoba, M. Allende y Haya de la Torre, 5000 Co´ rdoba, Argentina

Daniel T. Schu¨hle,† Harold Jones, and Wolfgang Schmickler University of Ulm, 89069 Ulm, Germany Received February 7, 2005. In Final Form: April 25, 2005 The nonlinear optical properties of self-assembled monolayers obtained from bonding two different R-functionalized terthiophenes (R-T3) to (111) silver electrode surfaces have been investigated using second harmonic generation (SHG). The two (R-T3) compounds used were functionalized with alkane chains of different lengths (C8 and C4), and each was terminated with a thiol anchoring group. A nitrile group was attached to the terminal thiophene ring of the (R-T3) compound with the C4 chain. The orientation of the polarization of the incident beam was changed systematically and gradually between “p” and ”s” orientation and the SH signal (isotropic and anisotropic contributions) analyzed in both directions (“P” and “S”). The symmetry of the system was reduced by the presence of the adlayers from C3v to C3. The dependence on the applied potential and the incident wavelength has also been studied. The relative magnitudes and phases of the various second-order tensor elements have been estimated and compared with the values for a bare surface. A resonance process in the (R-T3) π moiety has been investigated, and from this, the effective “band-gap” energies of the organic semiconductor SAMs (i.e., the energy difference between the π-π* bands) have been estimated.

Introduction Thiophene and its derivatives have attracted considerable interest in recent years because of the possibility to obtain conducting polymers.1,2 The electrochemical polymerization can be achieved in organic solvents containing the monomer by applying a potential of about 1 V or higher versus Ag/AgCl. The electrodeposition is usually performed on Pt or Au substrates. It is possible by electrochemical doping to switch the conductivity from the insulating to the conducting state. This transformation can be performed by oxidation (called p-doping) or reduction (called n-doping). In a model of conjugated systems acting as organic semiconductors, the band gap is essentially the energy separation between the π and π* orbitals.1,2 The upper limit of the valence band corresponds to the highest occupied π bonding molecular orbital (HOMO) and the bottom of the conduction band to the lowest unoccupied π* antibonding molecular orbital (LUMO). Obviously, the width of these bands is relatively small with a high probability of large numbers of structural imperfections, such as sp3-bonded carbon, interrupting the conjugation.1 * Author to whom correspondence should be addressed. E-mail: [email protected]. † Undergraduate student program. (1) Evans, G. P. The Electrochemistry of Conducting Polymers. In Advances in Electrochemical Science and Engineering; Gerischer, H., Tobias, C. W., Eds.; VCH: New York, 1990; Vol. 1. (2) Baüerle, P. Sulfur-Containing Oligmers. In Electronic Materials: The Oligomer Approach; Mu¨llen, K., Wegner, G., Eds.; Wiley-VCH: New York, 1998; Chapter 2.

Another category of films which are intensively investigated are self-assembled monolayers (SAMs). Alkanethiols, thiophene or oligothiophenes, and R-alkanethiolterthiophenes (employed in this paper) can form SAMs on suitable substrates. However, we have to distinguish between the properties of the adlayers obtained from these different compounds. The properties of SAMs containing alkanethiols have now been under active investigation for two decades (for a review, see ref 3). It has been observed that highly organized and stable monolayers can be formed on both gold and silver. The molecules bind strongly to the substrate through a metal-sulfur bond. The films produced on these two metals differ in some structural details. On silver, the structures formed are more densely packed than those on gold. In the case of Au(111), the sulfur atoms from the adsorbate occupy the hollow sites and form (x3 × x3)R30° overlayers, whereas on Ag(111), they adopt either the (x7 × x7)R19.1° or (x7 × x7)R10.9° structure, whichever fits the geometry of this metal better.4-7 In this case, each hexagonal unit has one on-top site and two hollow sites occupied by adsorbates. The orientation of the alkanethiol forming the SAM was also found to be (3) Ulman, A. Chem. Rev. 1996, 96, 1533-1554. (4) Laibinis, P. E.; Whitesides, G. M.; Allara, D. L.; Tao, Y. T.; Parikh, A. N.; Nuzzo, R. G. J. Am. Chem. Soc. 1991, 113, 7152-7167. (5) Dhirani, A.; Hines, M. A.; Fisher, A. J.; Ismail, O.; Guyot-Sionnest, P. Langmuir 1995, 11, 2609-2614. (6) Mohtat, N.; Byloos, M.; Soucy, M.; Morin, S.; Morin, M. J. Electroanal. Chem. 2000, 484, 120-130. (7) Sellers, H.; Ulman, A.; Shnidman, Y.; Eilers, J. E. J. Am. Chem. Soc. 1993, 115, 9389-9401.

10.1021/la050351x CCC: $30.25 © 2005 American Chemical Society Published on Web 05/27/2005

R-Functionalized Terthiphene Films on Ag(111)

different for these two metals. The tilts of the chains were about 30° for gold and 12° for silver, and there is also a clear preference of an sp3 and sp hybridization of the sulfur on gold and silver, respectively.4-7 Quantum chemical calculations8 predict that there are no significant chemical interactions between thiophene and gold. However, SAMs obtained from thiophene, R-bithiophene, and terthiophene have been reported (see, for example, refs 9-11). These compounds spontaneously adsorb onto gold surfaces and form well-ordered SAMs. The SAM growth and molecular orientation are determined by a balance between metal-sulfur and metal-π electron interaction in the thiophene ring and thiophenethiophene interaction. Depending on the coverage, two phases with different molecular orientations have been observed. A transition occurs from a parallel to an upright orientation with respect to the gold surface. Similar studies on silver have still not been reported to our knowledge. More recently, the intriguing electrical properties of SAMs composed of molecules with an alkane chain and π-electron moieties at one end have been discussed12 and experimentally investigated.13,14 When bonded to a conducting or semiconducting substrate, structures may be formed with properties reminiscent of those present in field effect transistors.12 Under favorable circumstances, the electrons of the π systems are delocalized enough to form a semiconducting layer parallel to the substrate surface but insulated from it by the alkane chains. It has been shown that the conductivity of such a structure is highly anisotropic: the conductivity within the π-system layer is about 9 orders of magnitude larger than that perpendicular to this plane.13 Some of the properties of SAMs formed on a polycrystalline gold surface by the type of substance used in this work have been investigated by B. Liedberg et al.15 The SAMs formed have a layered structure similar to that discussed above for the alkanethiols, densely packed and highly organized. A model for the structure of these particular SAMs containing alkyl chains and π-conjugated terthiophene was proposed. The authors suggested a mechanism involving electronic coupling via the conjugated π system of the R-terthiophene units. Owing to the self-assembling of the adsorbates by binding to the surface through the thiol group, neighboring thiophene rings can interact producing some delocalization of the π electrons. During a polymerization process, a different mechanism occurs which involves lateral binding between the thiophene rings at the R positions. According to the structure of these adsorbates, this process should be hindered due to steric reasons. The SAMs formed with this type of substance on silver surfaces have so far not been investigated, but there is some evidence3-7 to suggest that the tilt angles could be (8) Elfeninet, F.; Fredricksson, C.; Sacher, E.; Selmoni, A. J. J. Chem. Phys. 1995, 102, 6153. (9) Matsuura, T.; Nakajima, M.; Shimoyama, Y. Jpn. J. Appl. Phys. 2001, 40, 6945-6950. (10) Ito, E.; Noh, J.; Hara, M. Jpn. J. Appl. Phys. 2003, 42, L852L855. (11) Matsuura, T.; Takamura, T.; Shimoyama, Y. Jpn. J. Appl. Phys. 1999, 38, 2874-2877. (12) Zhou, C.; Newns, D. M.; Misewich, J. A.; Pattnaik, P. C. Appl. Phys Lett. 1997, 70, 598. (13) Collet, J.; Lenfant, S.; Vuillaume, D.; Bouloussa, O.; Rondelez, F.; Gay, J. M.; Kham, K.; Chevrot, C. Appl. Phys. Lett. 2000, 76, 13391341. (14) Rong, H. T. Frey, S.; Yang, Y. J.; Zharnikov, M.; Buck, M.; Wu¨hn, M.; Wo¨ll, C.; Helmchen, G. Langmuir 2001, 17, 1582-1593. (15) Liedberg, B.; Yang, Z.; Engquist, I.; Wirde, M.; Gelius, U.; Go¨tz, G.; Baüerle, P.; Rummel, R.-M. Ziegler, C.; Go¨pel, W. J. Phys. Chem. B 1997, 101, 5951-5962.

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considerably different for a silver surface than was the case for gold. Even without further structural details, it is quite clear that the molecules themselves have an overall bent geometry. Second harmonic generation (SHG) is a well-established method for investigating monolayers on surfaces.16-18 The SHG technique uses the second-order polarizability, always present at an interface between two centrosymmetric media, to generate the second harmonic frequency: 2ω 2ω Pinterface ) χinterface :EωEω

(1)

2ω is the interface nonlinear susceptibility where χinterface 2ω reflects tensor and Eω is the incident electric field. χinterface the symmetry of the interface, and information about the surface structure can be obtained by analyzing the different elements. Since the intensity of the signal depends on the second-order polarizability of the surface, this may be influenced by the application of external electrical fields. This can be achieved in an electrochemical system by applying a potential at the interface. The behavior of bare crystalline silver surfaces under such conditions have been investigated in detail in this19 and other laboratories20-24 by observing SH signals from silver electrodes in controlled electrochemical environments. The introduction of a monolayer onto a clean surface influences the polarizability characteristics of the surface25-29 and, depending on the environment, may also introduce extraneous electrical fields. The investigation of these effects in an electrochemical cell allows the electrical field at the surface to be controlled, so that the origin of the change in the properties of the surface may be definitely assigned. SHG is often used in a qualitative manner, with rather disappointing results. The purpose of the present study is to determine some of the nonlinear optical properties in an electrochemical environment to gain information on the structure of the SAM/Ag(111) interface, the electrical properties and the band-gap of this type of organic semiconductors. We test the capability of the SHG approach to obtain qualitative and quantitative information about these systems by varying different parameters such as wavelength, direction of polarization, azimuthal angle of the surfaces, and applied potential at the interfaces. We extend the existing models for metal/

(16) Heinz, T. F.; Tom, H. W. K.; Shen, Y. R. Phys. Rev. A 1983, 28, 1883-1885. (17) Shen, Y. R. Surf. Sci. 1994, 29, 551-562. (18) Shen, Y. R. Nature 1989, 337, 519-525. (19) Beltramo, G.; Santos, E.; Schmickler, W. J. Electroanal. Chem. 1998, 447, 71-80. (20) Richmond, G. L. Second Harmonic Generation as an In-situ Probe of Single-Crystal Electrode Surfaces. In Advances in Electrochemical Science and Engineering; Gerischer, H., Tobias, C. W., Eds., VCH: New York, 1992; Vol. 2. (21) Savinova, E. R. Scheybal, A.; Danckwerts, M.; Wild, U.; Pettinger, B.; Doblhofer, K.; Schlo¨gl, R.; Ertl, G. Faraday Discuss. 2002, 121, 181198. (22) Guyot-Sionnest, P.; Tadjeddine, A.; Liebsch, A. Phys. Rev. Lett. 1990, 64, 1678-1681. (23) Furtak, T. E.; Tang, Y.; Simpson, L. J. Phys. Rev. B 1992, 46, 1719-1724. (24) Li, C. M.; Urbach, L. E.; Dai, H. L. Phys. Rev. B 1994, 49, 21042112. (25) Santos, E.; Schu¨rrer, C.; A. Brunetti y W. Schmickler. Langmuir 2002, 18, 2771-2779. (26) Beltramo, G.; Santos, E.; Schmickler, W. Langmuir 2003, 19, 4723-4727. (27) Santos, E.; Schu¨rrer, C.; Beltramo, G.; Schmickler, W. J. Solid State Electrochem. 2003, 7, 567-571. (28) Corn, R. M.; Higgins, D. A. Chem. Rev. 1994, 94, 107-125. (29) Matranga, C.; Guyot-Sionnest, P. J. Chem. Phys. 2001, 115, 9503-9512.

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Figure 1. Physical model of the interface and possible orientations of the adsorbate molecule relative to the crystalline surface. The angles defined are a tilt angle, ξ, between the alkane chain and the perpendicular z axis, a torsional angle, F, between the Bulk all-trans alkane chain and the π moiety, a twist angle of the rings, η, and a tilt angle, ζ, for the π moiety. nBulk are the bulk M ,nS linear refractive indices for the metal and the electrolyte, respectively. The effective surface nonlinear polarizability χ(2ω) eff contains (2ω) all the nonlinear contributions of the interface: from the adsorbateχ(2ω) ads and from the substrate χmetal.

vacuum to metal/adlayer/solution interfaces giving the expressions for both isotropic and anisotropic parameters as a function of polarization and azimutal angles for different symmetries. Experimental Section The silver single-crystal electrodes, with surface orientation (111) within 1°, were provided by MaTeck GmbH with a lateral marking indicating the [21 h1 h ] direction. Before the surface was derivatized with the films, it was treated by annealing to 830 °C under an Ar atmosphere flux. After cooling, the coating with the film was carried out by submersion of the electrode in an approximately 2 mM solution of the derivative in ethanol for approximately 12 h. Two R-functionalized terthiophenes (R-Cn-T3) with different-length alkane chains (n ) 4 and 8 units) terminated with thiol groups were used to form SAMs on the (111) silver electrode surfaces. The terthiophene with a fourcarbon-atom chain was further modified by the substitution with a nitrile group onto the terminal thiophene ring (R-C4-T3-CN). In alcoholic solution, terthiophenes of the type used here exhibit a strong broad absorption transition, the π-π* transition mentioned above, in the region of 350 nm. In the case of the terthiophene with a nitrile group attached, the acceptor property of this group produces a red-shift of the absorption maximum of the terthiophene chromophore. Although so far no in-situ linear optical measurements have been carried out specifically with this film, the yellow coloration of the SAM/Ag(111) surface indicates that, in the adsorbed state, the absorption band also extends into the blue. The structure of the adsorbate molecules and their possible orientation relative to the crystalline surface are shown in Figure 1. The angles defined are a tilt angle, ξ,

between the alkane chain and the perpendicular z axis, a torsional angle, F, between the all-trans alkane chain and the π moiety, a twist angle of the rings, η, and a tilt angle, ζ, for the π moiety. B. Liedberg et al.15 have suggested that the carbon chains were tilted by ξ ) 35° from the perpendicular to the gold surface and the R-T3 unit was tilted relative to this z axis by ζ ) -14°, bringing it closer to the perpendicular. The coated crystal was positioned with the [21 h1 h ] plane oriented parallel to the incidence plane and was subsequently rotated around an axis perpendicular to the surface, and the SH-response containing the isotropic and anisotropic contributions was recorded. The in-situ SHG setup was similar to that used in the previous work,19,25-27 except that the wavelength of the incidence beam could be varied. The wavelengths employed were the fundamental of a Nd:YAG laser (Lumonics HY 1200) at 1064 nm and the output of a dye laser (LAS model LDL 205) working at points in the range 730-850 nm. The pulse duration was 10 ns at 1064 nm and about 5ns from the dye laser. Pulse energies on the order of 1-2 mJ were used from the dye laser, with a spot size on the order of 3.2 mm2 (i.e., power densities in the region of 104 kW/ cm2). In the case of the YAG fundamental, the spot size and pulse energy were controlled to bring the power density at the surface of the sample to within the same range as that used for the dye laser. However, the completely different mode structures of the two types of laser make it difficult to make quantitative comparison of the results produced using these two light sources. The experimental parameters which can be varied to gain information on the elements of the tensor and the symmetry of the surface are indicated in Figure 2. Important parameters are the incident wavelength, λF (second harmonic: λSH ) λF/2), direction of polarization relative to the plane of incidence


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Figure 2. Diagram of the different coordinates systems employed to calculate the elements of the second-order susceptibility tensor. Crystal coordinates: (x, y, z); beam coordinates: (s, κ, z). Angle of polarization: (γ ) 90°, s); (γ ) 0°, p). Angle of incidence θ ) 45°. Azimuthal angle of rotation: (φ ) 0° corresponds to the crystal axis [21 h1 h ].

formed by the input (lowercase symbols) and output beams (uppercase symbols): parallel (“p”, “P” γ, Γ ) 0°) and perpendicular (“s”, “S” γ, Γ ) 90°) to this plane. Another parameter is the rotational angle, φ, of the sample around a “z” axis perpendicular to the surface. Under these circumstances, we are working with three different coordinate systems: The beam coordinates (s, κ, z), the crystal coordinates (x, y, z) and the microscopic molecular coordinates of the adsorbate with the orientation angles (F, ζ, ξ, η) defined in Figure 1. A further parameter is the applied potential at the interface. In addition to the second harmonic measurements, impedance spectra were measured in a frequency range of 100 mHz to 100 kHz for both SAMs using the experimental procedure described previously.25 The impedance spectra have been recorded employing an EG&G Potentiostat model 263A and a lock-in amplifier model 5210. The electrolytes used in all the measurements were aqueous solutions of 50 mM KClO4. The reference electrode was a homemade Ag/AgCl separated from the main compartment of the cell through a Pt wire. However, all potential values were referred to a saturated calomel electrode (SCE). Second Harmonic Generation from a Covered Surface. The second-order polarization tensor in the presence of an adsorbate, χ(2ω) eff , reflects the overall properties of the surface, and it is usually described as the sum of at least three contributions (cf. Figure 1 and refs 25,27-29): (2w) (2ω) (2ω) χ(2ω) eff ) χmetal + χads + ∆χint

(2)

The subscripts denote the second-order susceptibility of the bare metal (metal) and that of an isolated monolayer of adsorbed species (ads). The latter term represents the changes induced by the interaction adsorbate-substrate. As shown (2ω) ) can be also considered to in Figure 1, the second term (χads be composed of partial contributions from the different parts of the adsorbate molecules. This term is related to the molecular nonlinear polarizability tensor, β, through the average of the molecular orientation distribution function, F(F,ξ,ζ,η).25,27-28 All the contributions to the overall susceptibility are dependent on the wavelength of the incident beam. This dependence is given

by the following equation28,31-34

χ(2ω) (ω) ) -Ne3 i

〈o|ri|q〉〈q|rj|p〉〈p|rk|o〉

∑ (2pω - E

o,p,q

qo

- ipfqo)(pω - Epo - ipfpo)

(3)

where ω ) c/λF, ri,j,k are the Cartesian coordinate operators which describe the polarizability of the medium and |o>, |p>,|q>, represent the initial, intermediate, and upper states, respectively. Thus, the matrix elements in the numerator of eq 3 arise from the two single photon processes (one between the lower state |o> and the intermediate state |p> and a second between |p> and the upper state |q>) and a process at 2pω between |q> and |o>. SHG is most frequently performed under nonresonant conditions, and in this case, the states |p> and |q> are formally described as being virtual states. However, as shown in eq 3, the is actually determined by a summation of the magnitude of χ(2ω) i off-resonant contributions from all real states. The degree of off-resonance appears in the denominator of eq 3. The damping factors, f, are included here to allow for the existence of relaxation processes and are usually insignificant compared to the degree of off-resonance. However, in the event of a resonance occurring with either the process at pω ) pc/λF or 2pω ) pc/λSH, i.e., when (2pω - Eqo) or (pω - Epo) approach zero, only the small imaginary terms in the denominator of eq 3 remain and the contribution from this single resonant event to the overall sum becomes dominant. More-detailed investigation of this process reveals33,34 that, for single molecules, on crossing a resonance, χ(2ω) displays a dispersive-type line shape. As an exact resonance is reached, the sign of the real component of χ(2ω) passes through zero and inverts abruptly. The magnitude of χ(2ω) (and the SH signal) increases dramatically and this effect produces a 180° shift in the phase of the signal. As already pointed-out in previous work,25 for a complex interface like that under discussion here, it is difficult to distinguish between the different contributions to the overall susceptibility indicated in eq 1. In the event of a resonance, (30) Guyot-Sionnest, P.; Chen, W.; Shen, Y. R. Phys. Rev. B 1986, 33, 8254-5263. (31) Urbach, L. E.; Percival, K. L.; Hicks, J. M.; Plummer, E. W.; Dai, H.-L. Phys. Rev. B 1992, 45, 3769-3772. (32) Shen, Y. R. The Principles of Nonlinear Optics; Wiley: New York, 1984. (33) Boyd, R. W. Nonlinear Optics; Academic Press: New York, 1992. (34) Higgins, D. A.; Abrams, M. B.; Byerly, S. K.; Corn, R. M. Langmuir 1992, 8, 1994-2000.

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Figure 3. Structures showing the symmetries for a bare (111) surface and for one with two different overlayers, each with 3-fold symmetry, i.e., (x3 × x3)R30° and (x7 × x7)R19.1°. σv are the reflection planes. First (b), second (Y), and third (O) layer of a (111) surface, and adsorbate (×). considerable enhancement of the SH signal is expected. Under these circumstances, the contribution from the surface component in which the resonance occurs may be dominant and the SHG signal effectively arises from either the (modified) metal surface or from the (modified) adsorbate surface. However, the situation becomes complicated because electronic transitions also cause variations of the linear optical properties. These changes may cause either enhancement or attenuation of the SHG signal. An interesting discussion about this topic concerning bare Ag(111) surfaces has been given by C. M. Li et al.24 Analysis of the Surface Second-Order Susceptibility Tensor Elements. As a result of the complicated expressions required to describe the linear and nonlinear properties, the quantitative analysis of the SH response is rarely simple. The comparison of the experimental results by different groups is made particularly difficult since different notations are in use. Also, several important publications contain typographical and other errors. Some typographical errors have produced misunderstanding, and after 10 years, there are still discussions about the right expressions (for example, see the comments in the refs 29 (2001), 19 (1998), and 23 (1992) about expression 1 in ref 22 (1990) for the isotropic contribution under “pP” condition for the polarizations). In an attempt to clarify the situation, a moredetailed description of the analysis performed in this work is given in the Appendix. We have corrected a number of omissions in previous work and unified the different notations. We have also extended the treatment to a general case, taking into account the presence of the electrolyte and different possible geometries of the adsorbate. The SH response of an interface between two centrosymmetric materials is mainly attributed to the second-order nonlinearity of a surface layer. It is usually assumed32 that the bulk linear refractive indices of the two adjoining phases extend right up to the interface but that its nonlinear optical susceptibility is different from that of the bulk, which vanishes in the electric dipole approximation. According to Y. R. Shen,32 the surface layer has little effect on the linear wave propagation. This is also valid for the presence of a thin-film layer at the interface. Since the thickness of a monolayer is much smaller than the input wavelengths used (a few nanometers compared to λF ) 1064730 nm), it would seem that the overall effects could be described within the model of a single surface rather than an extended interface. Thus, we will assume, as a first approximation, a physical model of the interface consisting of two isotropic adjoining media with the linear refractive index values of the Bulk bulk (nBulk ), and an effective surface nonlinear polarizM ,n S (2ω) ability, χeff , which contains all the nonlinear contributions of the interface (Figure 1). In this analysis, we consider the usual reflection geometry used in surface SHG which implies the geometry, I, in the phenomenological treatment proposed by V. Mizrahi and J. E. Sipe for the metal/vacuum interface (see Figure 3 of ref 35).

Table 1. Tensor Elements Contained in the “q” coefficients of Eq 5 for Two Different Symmetry Groups C3v symmetry iV qf

aiss

aicc

aisc

biss

bicc

bisc

ciss

cicc

cisc

0 0 χxxz -χxxx χxxx 0 0 0 -χxxx χzxx χzzz, χxxz, 0 0 0 χxxx -χxxx χxxx 0 χzxx

S P

C3 symmetry iV qf S p

aiss

aicc

aisc

biss

bicc

bisc

ciss

cicc

cisc

0 χxyz χxxz -χxxx χxxx χyyy -χyyy χyyy -χxxx χzxx χzzz, χxxz, -χxyz χyyy -χyyy χxxx -χxxx χxxx χyyy χzxx

When the sample is rotated, the beam coordinates are also rotated relative to the crystal coordinates. In the case of the (111) silver surface used in this work, on rotating the sample through 360°, the intensity of the second harmonic signal reflects the 3-fold symmetry of the surface in either the presence or the absence of an adsorbate. However, in the presence of the SAM, depending on the structure of the overlayer and its relative orientation to the substrate, the 3-fold symmetry itself may be reduced, and in the case under discussion, the change is from C3v to C3 symmetry as will be discussed later. This is illustrated in Figure 3 which shows the symmetries for a bare (111)-surface and for one with two different overlayers, each with 3-fold symmetry, i.e., (x3 × x3)R30° and (x7 × x7)R19.1°. In the first overlayer, the rotation by 30° with respect to the substrate does not change the C3v symmetry, but in the second case, the reflection planes, σv, are absent. The general expression for the SH signal contains an isotropic contribution and an anisotropic one, which depends on the relative position of the crystal to the [21h 1 h ] plane through the azimuthal angle, φ, and is given by

ISHG ) |N[A(θ,γ,Γ,λ) + B(θ,γ,Γ,λ) sin(3φ) + C(θ,γ,Γ,λ) cos(3φ)]|2 (4) where N is a normalization factor, A the coefficient for the isotropic contribution, and B and C the coefficients for the anisotropic contribution. These coefficients depend on the incident wavelength, λF, the incident angle, θ, and the polarization angles, γ and Γ, for the incident and second harmonic beams, respectively, through the Fresnel coefficients. The dependence on the polarization angle of incidence is of the form (see also the Appendix):

Qγi ) qΓsc sin(γ) cos(γ) + qΓcc cos2(γ) + qΓss sin2(γ)

(5)

with Q ) A,B,C; q ) a,b,c; Γ ) S,P. The “q” coefficients also


Langmuir, Vol. 21, No. 14, 2005 6411

Table 2. Optical Constant and Fresnel Coefficients Contained in the “q” Coefficients of Eq 5

contain different elements of the susceptibility tensor χ(2ω) eff as shown in Table 1. The Fresnel coefficients are those given by Sipe35,36 except that in our case they contain also the dielectric constant of the electrolyte (cf. Table 2). In addition to the dependence on incident angle, θ, just as is the case with secondorder susceptibility, since the dielectric constants of both the metal and water could be complex, they are also wavelength dependent. The corresponding values for the wavelength used here have been taken from P. B. Johnson et al.37 and are also included in Table 2. All isotropic and anisotropic contributions to the SH signal can in principle be determined from the experimental data by Fourier analysis. The coefficients A,B, and C are related to the Fourier coefficients Re/Im Fi by the following expressions:

{Re}F0 ) |A|2 +

|B|2 |C|2 + 2 2

(6a)

{Im}F0 ) 0

(6b)

Figure 4. Cyclic voltammograms of bare (...) and covered Ag(111) electrodes by both R-terthiophene compounds: R -C8-T3 SAM (- ‚ -) and R-C4-T3-CN SAM (s). Sweep rate: 50 mV/s.

{Re}F3 ) |A| |C| cos(ψC - ψA)

(6c)

Results and Discussion

{Im}F3 ) -|A| |B| cos(ψB - ψA)

(6d)

{Re}F6 ) {Im}F6 ) -

1 (|C|2 - |B|2) 4

1 |B| |C| cos(ψB - ψC) 2

(6e)

(6f)

where ψA, ψB, and ψC are the phases of the complex coefficients A, B, and C, respectively. The corresponding absolute values of these phases cannot be determined with our experimental setup. Only the relative values: (ψB - ψA), (ψC - ψA), and (ψB - ψC)can be obtained for every direction of polarization and potential value. Equation 5 has been employed in various previous publications18,38 to obtain information about the molecular orientation of adsorbates exclusively at isotropic substrates where only the isotropic term A contributes to the second harmonic signal. We have extended this procedure to the case where the anisotropic terms B and C play also an important role, such as SAMs on single-crystal surfaces. The expressions for the different coefficients A, B, and C as a function of the input polarization angle, γ, are given in the Appendix. (35) Mizrahi, V.; Sipe, J. E. J. Opt. Soc. Am. 1988, B5, 660-667. (36) Sipe, J. E.; Moss, D. J.; van Driel, H. M. Phys. Rev. 1987, B35, 1129-1141. (37) Johnson, P. B.; Christy, R. W. Phys. Rev. B 1972, 6, 4370-4379. (38) Eisert, F. Dannenberger, O.; Buck, M. Phys. Rev. B 1998, 58, 10860-10870.

Electrochemical Measurements. Figure 4 shows cyclic voltammograms (CV) recorded with a Ag(111) electrode covered with either investigated films, i.e., those obtained from R-Cn-T3 or R-Cn-T3-CN. The current in the CVs depends linearly on the potential sweep rate, indicating that it is a charging process, and it is dramatically lower in comparison to the bare surface. The oxidation current observed at potential values higher than 0.2 V can be attributed to a destructive removal of the films. However, the monolayers seem to be quite stable. The positive limit of the potential scan must be shifted to more positive values and it takes many cycles in order to obtain the voltammetric response of a bare surface. The electrical properties of the films can be better characterized by means of electrochemical impedance measurements. A simple model to represent the system is an equivalent circuit consisting of a resistance (electrolyte) and capacitance (film) connected in series. Usually, the capacitance is obtained directly from the imaginary part of the impedance at a constant frequency of 20 Hz. However, this evaluation at one frequency is not satisfactory, and an analysis of the whole spectrum is preferable. Figure 5a shows a typical complex permittivity plot associated with these films (squares) in comparison with the results obtained with the bare surface (crosses). The impedance spectra have been measured in a frequency range of 100 mHz to 100 kHz. Two different equivalent circuits have been employed to fit the experimental data.

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Figure 5. (a) Impedance spectra for bare (+) and covered (9) Ag(111) surfaces. The imaginary versus real components of the dielectrical function are shown in a frequency range between 100 mHz and 10 kHz. Amplitude of the superimposed a.c. potential signal: 10 mV. (- - -) best fit with circuit 1; (s) best fit with circuit 2. (b) Film capacity as a function of the applied potential obtained from: (0, O) real component of the dielectrical function at a constant frequency of 20 Hz.; (9, b) fit with circuit 1; ($, Q) fit with circuit 2. Here, the lines are just guides to the eye.

The feature of the complex permittivity plot should be for the series connection of resistance and capacitance (circuit 1 in Figure 5a) a semicircle with the intersection at the real axis (when ω f 0) giving the capacitance value. The result of this fit is shown as a dashed line. The experimental points show a distortion of this behavior at low frequencies. The data are better fit (full line) employing a more-complicated equivalent circuit which includes a resistance and a constant phase element39 (Rp and Q in circuit 2 of Figure 5a) parallel to the capacitance. The constant phase element can account for inhomogeneity of the film. The values obtained for this element indicate that irregularities represents less than 5% of the total surface. The addition of chloride (10 mM KCl) has no effect

on Cp, but produces an increase of Q, which we attribute to the adsorption of this anion on the defect sites of the film. However, the typical sharp peak corresponding to the disorder-order phase transition usually observed on bare surfaces40 is absent. This indicates that the defects are randomly distributed and the presence of the SAM produces a screening of long-range forces. The parallel capacity curves obtained employing these three different methods for the electrode covered with films obtained from both R-terthiophene compounds are shown in Figure 5b. The parallel capacity and resistivity values (not shown) seem to be nearly independent of the potential in the region for which the layers are stable. The capacity values are also much smaller than those observed with a bare surface

(39) Ross Macdonald, J. Impedance Spectroscopy; Wiley: New York, 1987.

(40) Beltramo, G.; Santos, E. J. Electroanal. Chem. 2003, 556, 127136.


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Figure 6. Second harmonic signal intensity of both SAM/Ag(111) and bare surfaces at “P” and “S” direction of polarization as a function of the rotational angle obtained using the 1064-nm fundamental wavelength at “p” direction. (a) Bare Ag(111) surface. (b) Ag(111) covered with R-C8-T3 SAM (without substituent). (c) Ag(111) covered with R-C4-T3-CN SAM (nitrile substituent). Applied potential USCE ) -0.115 V. At the right, the real (9) and imaginary (0) components of the Fourier coefficients are also shown for the different curves.

(by a factor of about 10). These results indicate that the alkane chains of the SAM form an insulating barrier that prevents the penetration of water and ions. At higher positive potentials, a decrease in the resistivity has been observed, indicating probably a disruption of the monolayer. The increase in the capacity at higher potential when the constant frequency of 20 Hz was employed to calculated this value is an artifact due to the bad fit. The numerical values obtained for the capacitance are somewhat higher than those reported for functionalized alkanethiols on gold by Sondag-Huethorst et al.41 (approximately 1.2 and 2.3 µF/cm2 for CH3-(CH2)11-S-Au and CN-(CH2)11-S-Au, respectively). The larger values observed here may be attributed to a shorter alkyl chain in the present case but also to the presence of the conjugated π system, which almost certainly results in a relative increase in the value of the dielectric constant. The results shown in Figure 5 for the two R-terthiophene compounds would seem to be consistent with the molecular (41) Sondag-Huethorst, J. A. M.; Fokkink, L. G. J. Langmuir 1995, 11, 2237-2241.

structures of the two compounds. The R-T3-CN compound has the shortest alkane chain and the SAM it forms displays a lower resistivity than the other compound. The presence of the nitrile group would also be expected to have a polarizing effect on the SAM, and this could also make a contribution to a higher value of the capacitance. Second Harmonic Measurements. Configuration of the SAM. The second harmonic signal intensity of the SAM/Ag(111) surface as a function of the rotational angle obtained using the 1064-nm fundamental wavelength at a constant potential of -0.115 V is shown in Figure 6. With the incident beam maintained in the “p” polarization direction, the SH signal at a wavelength of 532 nm was measured with the analyzer set at both, “P” and “S” directions. Figure 6b corresponds to Ag(111) covered with a R -C8-T3 SAM without a substituent, while Figure 6c shows the results with the R-C4-T3-CN SAM. The corresponding Fourier coefficients for every curve are shown at the right of the figure. The response of a bare surface is also shown for comparison (Figure 6a). The 3-fold symmetry is evident for the three systems: bare surface and surfaces covered with either film. The

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measurements performed with bare Ag(111) surfaces agree with results obtained by Richmond et al.42,43 Fourier analysis of the signals shows an appreciable contribution of the F0, F3, and F6 coefficients over F1, F2, F4, F5, and others of higher order. The ratio between the magnitudes of F0 and F3 in the case of “pP” configuration is about 10 times higher for the R -C8-T3 SAM than for the R-C4-T3CN SAM. Covered surfaces give higher values of F3 relative to F6 in the case of the “pP” configuration in comparison to bare surfaces, where the coefficient F6 is predominant. Both covered surfaces show negative values for Re F3, indicating a phase difference between isotropic and anisotropic contributions near 180° according to eq 6c. Similar results have been obtained for SAMs obtained from both alkanethiols and thiols containing phenyl rings.25,27 In contrast, the 6-fold pattern displayed by bare surfaces indicates a phase difference between isotropic and anisotropic contributions near 90°, as also observed by Richmond et al.42,43 They attribute this value to the Fresnel coefficients, the susceptibility elements remaining real (nonresonance). Only at about 2pω ) 3.82 eV (λF ) 650 nm) a peak in the wavelength dependence of the element χxxx is observed which is attributed to a resonance with an interband transition. The values of the other tensor elements (χzxx, χzzz, χxxz) as function of the wavelength are almost constant. Near the spectral region where the peak is observed, a large change of the relative phase from π/2 to 0° between the CpP and ApP parameters occurs. In our case, the phase difference observed for all covered surfaces at the long-wave range could indicate the presence of resonance behavior. This resonance may be attributed to the localized sulfurssilver bond together with some coupling to delocalized free electrons because this effect extends to a large frequency range. A distinction between C3 and C3v symmetry should be possible if the anisotropic response is analyzed in greater detail. Particularly for the “pS” configuration, the experimental data are better reproduced (solid lines in Figure 6) assuming C3 symmetry rather than the usual C3v symmetry of a bare Ag(111) surface. As can be seen from Table 1, eqs 6, and the appendix (eq A13), in contrast to the situation with C3v symmetry, the isotropic coefficient ApS is different from zero for C3 symmetry (it contains the tensor element χxyz). In this case, the angular dependence of the SH intensity is an alternating 3-fold pattern (both F6 and F3 coefficients different from zero), whereas for C3v symmetry, eq 5 predicts a uniform 6-fold pattern (only the real component of F6 coefficient is nonzero). Also, in the case of C3 symmetry for the “pS” configuration, the anisotropic parameter CpS

(eq A15) is different from zero (it contains the tensor element χyyy), giving a imaginary component of the F6 coefficient. These facts are especially evident for the R-C4T3-CN SAM (see at right of Figure 6c). A similar reduction of the surface symmetry by the presence of an adsorbed layer has been observed by M. L. Lynch et al.44 for iodine, carbon monoxide, and hydrogen adsorbed on Pt(111). B. Pettinger et al.45 also found a reduction from a C3v to a Cs symmetry due to the reconstruction of the surface layer of Au(111) from a (1 × 1) to a (x3 × 23) structure. The Cs symmetry group contains an overall 1-fold rotational axis and a vertical mirror plane, which can be considered as a mixture of symmetry elements for 1-fold, 2-fold, and 3-fold rotational axes. The corresponding expression for the susceptibility tensor is more complicated and is given elsewhere.45 However, as already mentioned, the Fourier analysis of our result did not indicate an appreciable contribution of the F1 and F2 terms, which should be the case if 1-fold and 2-fold symmetry elements were present. The SH-response with the “pP” configuration also shows a broadening or a splitting of the peaks as a consequence of a reduced surface symmetry. These effects are more evident with the R-C4-T3-CN SAM. The C3v symmetry must yield zero values for both imaginary components of F3 and F6 (BpS ) 0, see eqs 6d, 6f, and A14) with these conditions of polarization, and this is not the case. These results can be explained by assuming that the films themselves form a structure with 3-fold symmetry on a Ag(111) surface, but with the whole structure rotated relative to the substrate by an angle different from nπ/6, with integral n, so that the C3v symmetry is not preserved. This observation is consistent with the report that alkanethiols form usually structure of the type (x7 × x7)R19.1° on Ag(111)1,2 which shows C3 symmetry. Probably, the contribution of the film itself to the effective total susceptibility depends on the relative orientation of the electrical field to the surface, as can be inferred from the different results obtained from different direction of the analyzed polarization. The values of the tensor elements χxyz and χyyy could be estimated relative to the element χxxx from the experimental curves. However, eqs 6 give four different solutions for every parameter A, B, and C and consequently also four different values for χxyz and χyyy. The results obtained from Figure 6 are shown in Table 3. It is difficult to draw some quantitative conclusions. The relative values of element χyyy obtained with the “pP” configuration are higher than those with the “pS” configuration. It is particularly extreme for the R-C4-T3-CN SAM where the magnitude differs by about an order and the phase differences shift by up to 90°. One should notice (see Figure 6) that for the “pP” configuration the contribution of F6 and for the “pS” configuration the contribution of F3 close to almost over the noise level. Thus, the use of eqs 6 to estimate the magnitude and phase for the different parameters under these conditions of polarization for this system is inaccurate. At intermediate polarization angles for the incident beam, the contributions of both F3 and F6 are appreciable and the error incurred by using eqs 6 should be lower. Thus, a systematic variation of the polarization angle of the incident beam (i.e., varying the polarization direction continuously from the “p” (γ ) 0°) to “s” (γ ) 90°) direction)

(42) Wong, E. K. L.; Richmond, G. L. J. Chem. Phys. 1993, 99, 55005507. (43) Bradley, R. A.; Georgiadis, R.; Kevan, S. D.; Richmond, G. L. J. Chem. Phys. 1993, 99, 5535-5546.

(44) Lynch, M. L.; Barner, B. J.; Corn, R. M. J. Electroanal. Chem. 1991, 300, 447-465. (45) Pettinger, B.; Lipkowski, J.; Mirwald, S.; Friedrich, A. J. Electroanal. Chem. 1992, 329, 289-311.

Table 3. Different Solutions for the Ratio of the Different Tensor Susceptibility Elements Obtained from the Data Shown in Figure 6 R -C8-T3 SAM χyyy

χxyz

R-C4-T3-CN SAM χyyy

solution 1 solution 2 solution 3 solution 4

from “pS “configuration 0.058 e+i70 3.0 e+i178 0.186 e+i3 0.058 e-i70 3.0 e+i1.7 0.186 e-i3 0.597 e+i89 0.187 e+i163 0.50 e+i74 0.597 e-i89 0.187 ei17 0.50 e-i74

solution 1 solution 2 solution 3 solution 4

from “pP “configuration 0.070 e+i45 1.40 e+i84 0.070 e-i45 1.40 e-i84 0.989 e+i89 1.20 e+i87 0.989 e-i89 1.20 e-i87

χxyz 2.2 e+i89 2.2 e-i9 0.267 e+i87 0.267 e-i93


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Figure 7. 3-D Plots of the SH signal for (a) “S” (Γ ) 90°) and (b) “P” (Γ ) 0°) direction of polarization as a function of the rotation angle, φ, and of the polarization angle, γ, of the fundamental beam obtained with a film of R-C8-T3 SAM without a nitrile group. Applied potential USCE ) -0.315 V. Table 4. Ratio of the Different Susceptibiliy Elements of the Corresponding Tensor Ag(111)-S-C8-T3 bare Ag(111) (from ref 43)

χyyy/χxxx

χxyz/χxxx

χzxx/χxxx

χxxz/χxxx

χzzz/χxxx

0.271e-i52

2.26ei75°

0.319e-121°

4.98ei8°

0.324

3.89

52.5ei200° 62

-

-

allows a better understanding of the system and a more precise assignment to a symmetry group. The 3-D Plots of Figure 7 show this angular response for “S” (Γ ) 90°) and “P” (Γ ) 0°) direction of the SH signal obtained with a film of R-C8-T3 SAM without a nitrile group at a constant potential value of -0.315 V. Values for the parameters AγΓ, BγΓ, and CγΓ have been obtained by Fourier analysis

using the eqs 6. In this case, the better fit obtained by eqs 5 and A13-18 allows selecting between the different possible solution, and a better assignment of the parameters AγΓ, BγΓ, and CγΓ can be done. The dependence of the magnitude of these parameters on the angle of polarization of the incident beam and the corresponding best fit are shown in Figure 8. Since it is difficult to measure absolute

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Figure 8. Dependence on the angle of polarization, γ, of the incident beam of the magnitude of parameters AγΓ, BγΓ, and CγΓ obtained by Fourier analysis from data shown in Figure 7 using eqs 6. The lines are the best fit with eqs 4-5 and (s) A13,16, (- - -) A14,17, and (‚‚‚) A15,18.

Figure 9. Second harmonic signal intensity at “P” direction of polarization for both R-Cn-T3 SAMs at two different wavelengths (λF ) 1064 and 800 nm) for the incident beam with “p” polarization. Applied potential USCE ) -0.115 V.

values of the SH intensity, the parameters were normalized to C45S (corresponding to γ ) 45° and Γ ) 0°). As can be seen from this figure, despite the relatively experimental uncertainty in the values of some of the parameters, the data is definitely more consistent with the C3 symmetry.

Santos et al.

The ratio of the different susceptibility elements relative to the χxxx element obtained from the best fit of the data shown in Figures 7-8 are given in Table 4. These results are compared with those of E. K. L. Wong and G. L. Richmond42 for a bare Ag(111) at the potential of zero charge. They perform measurements employing three different combinations of incident and SH polarizations (‘pP’, ‘sP’, and ‘mS’). Obviously, the elements χyyy and χxyz are absent at the bare Ag(111) surface because this system has C3v symmetry. The relative magnitudes of the elements χxxz, χzxx, and χzzz for the covered surface are similar to those of the bare surface. Imaginary components different from zero have been obtained in the presence of the adlayer for the relative susceptibility elements χzxx, χyyy, and χxyz, as can be inferred from the phase values. This result indicates therefore the presence of a resonance phenomenon. This point will be discussed in the next section. Measurements at Different Wavelengths. Resonance Effects and the “Band-Gap”. As can be seen from Figure 1, the configuration of the interfaces investigated in the present work is rather complicated. We can determine only an effective susceptibility tensor, which contains all the contributions. The experimental facts indicate that the second harmonic signal responds to the overall symmetry of the system, i.e., the surface structure resulting from the adsorbed layer and the three first atoms layer of the substrate. These results could indicate that (2ω) both terms, χ(2ω) ads and χmetal contribute to the total signal. The SH signals observed for both SAMs when the incident wavelength is changed from λF ) 1064 nm (pω ) 1.17 eV, 2pω ) 2.34 eV) to λF ) 800 nm (pω ) 1.55 eV, 2pω ) 3.10 eV) are shown in Figure 9. In these measurements, only the laser wavelength was changed, i.e., the orientation of the SAM/Ag(111) electrode surface on the rotational angle scale was identical at the two wavelengths. As can be seen from the Figure 9, there is an obvious difference between the responses of the two SAMs to the change in wavelength. In the case of the R-T3 unsubstituted SAM (Figure 9a), the 3-fold waveform is qualitatively similar at both wavelengths. However, as can be seen in Figure 9b, with the R-C4-T3-CN SAM, the signal at λF ) 800 nm is shifted by almost 60° relative to that observed at λF ) 1064 nm. These observations can be explained very well by taking into account the effects which were discussed in the Introduction (eq 3) with regards to the changes in χ(2ω) that a resonance can produce. It is reasonable to assume that this resonance is essentially a property of the terthiophene moiety. In the case of polythiophene, the π-π* interband transition energy was found to be about 2.5 eV.1 The lack of intense coloration in the case of the R-C8-T3 compound indicates that this transition lies at shorter wavelengths than λSH ) 400 nm (2pω ) 3.10 eV). As already mentioned, the π-π* transition of terthiophenes lies nominally in the region of 350 nm (i.e., Eπ-π* ) 3.55 eV). It seems highly probable that the effects observed here involve this transition and its second harmonic frequency. The R-C8-T3 SAM, at λF ) 800 nm (λSH ) 400 nm, 2pω ) 3.10 eV), is still to the low-energy side of resonance. However, as shown by the intense yellow coloration produced, substitution of the nitrile acceptor group onto the terthiophene moiety produces a red-shift of the absorption band to wavelengths longer than 400 nm. Thus, in the case of R-C4-T3-CN SAM, on changing the wavelength from λF ) 1064 nm (2pω ) 2.34 eV) to λF ) 800 nm (2pω ) 3.10 eV), we probably are crossing the


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Figure 10. Real and imaginary components of the Fourier coefficients, Fi, obtained from SHG measurements using eqs 6a-f with both R-Cn-T3 SAMs as a function of the applied potential at the interface for the two different wavelengths of the incident beam investigated.

resonance. Preliminary measurements46,47 carried out at λF ) 730 nm (2pω ) 3.40 eV) on both SAMs confirm this interpretation of the observations. In addition, the results on the R-C8-T3 SAM, although not as dramatic as with the other SAM, are in agreement with this explanation. Measurements at both λF ) 1064 nm and λF ) 800 nm are in keeping with both SH wavelengths being on the longwavelength side of resonance (Eπ-π* > 2pω). These results may indicate that at λSH ) 365 nm we are still on the long-wavelength side of resonance but also that we are close to the crossover. In conclusion, these observations indicate that the π*-π transition for the R-C4-T3-CN SAM lies between 532 and 400 nm (3.10 eV > Eπ-π* > 2.33 eV) and for the R-C8-T3 SAM it lies close to 365 nm (Eπ-π* J 3.40 eV). A morequantitative discussion in connection with these aspects is given in the next section. Measurements at Different Potentials. Electric Field Effects. Fourier analysis yields more details about the difference of behavior with potential and wavelength for both covered surfaces. Figure 10 shows real and imaginary components of Fourier coefficients as a function of the applied potential at the two wavelengths for electrodes covered with either film. Since, as mentioned in the Experimental Section, it is difficult to compare the intensities between both wavelengths, we have normalized the different coefficients with the value of the F0 at -0.315 V for each wavelength. The selection of this value for the normalization is arbitrary and facilitates only the comparison. (46) Santos, E.; Jones, H.; Schmickler, W.; Go¨tz, G.; Baüerle, P., in preparation. (47) Beltramo, G.; Jones, H.; Santos, E. 1st Spring Meeting of the International Society of Electrochemistry; Alicante, Spain, March, 2003.

Although the interrelation between anisotropic and isotropic parameters (see eqs 6) makes it difficult to distinguish the individual effects, some remarks can be clearly drawn. For the film prepared from the unsubstituted terthiophene, no large differences in the Fourier coefficients with the external electrical field can be appreciated for the two wavelengths. The coefficient F0 remains almost constant by varying the potential for surfaces covered with either film at the lower wavelength, but at 800 nm, it shows the opposite dependence on potential. In the case of the adlayer containing the nitrile group (an electron acceptor), F0 decreases about 50% with increasing potential; for the R-C8-T3 SAM adlayer, it increases. The imaginary component of F3, although small, is also present and has a different sign for both films. The appearance of a contribution of the components of F6 at the shorter wavelength is noticeable, the imaginary for the R-C8-T3 SAM and the real for the R-C8-T3-CN SAM. The first could be explained, according to eq 6f, through an increment of the magnitude of anisotropic contributions BpP and/or CpP or a decrease of the phase difference between both anisotropic coefficients. Anyway, it is a change in the anisotropic part of the SH signal. At the shorter wavelength for the R-C4-T3-CN SAM, an increase of the Re F6 coefficient with the potential and also a change on the sign occur around -0.2 V. According to eq 6e, that means an increase (decrease) of CpP (BpP) with the potential. At potentials lower than -0.2 V, it implies |CpP| < |BpP| and at potentials higher than -0.2 V |CpP| > |BpP|. This could indicate a change from C3 to C3v symmetry caused by a transformation of the adlayer. However, no changes are observed at the lower wavelength. The parameters CpP and BpP contain the tensor elements χxxx and χyyy, respectively, concerning the “parallel” electrical

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Figure 11. Phase differences between isotropic and anisotropic parameters obtained with both R -Cn-T3 SAMs as a function of the applied potential at the two different wavelengths investigated.

field of the incident and SH beams, E B (ω) B (2ω) and x and E x (ω) (2ω) E B y and E B y . It is amazing that the applied potential, which implies an electrical field in the direction z (perpendicular to the surface) could have an influence on the second-order polarization components in either the x or y direction (parallel to the surface). It is interesting to analyze the behavior of the real component of F3 and the imaginary components of F3 and F6. The sign of these coefficients gives information about the relative phases between the isotropic and anisotropic contributions A, B, and C. The relative phases between these vectors in the complex plane as a function of potential for both wavelengths are shown in the Figure 11. A has been arbitrary fixed at the real axis. The real component of F3, the more important contribution after F0, practically does not show any change with the potential for all the cases. However, very significant for the interpretation of the observations is the change of sign of this coefficient for the R-C8-T3-CN SAM when the wavelength is changed from 1064 to 800 nm, which points to a shift of the phase between A and C by about 180° (see eq 6c and Figure 11). According to the discussion of the previous section in relation to eq 3, this is a clear indication that a characteristic resonance frequency has been crossed between 1064 and 800 nm. A decrease at more positive charge than the pzc of the resonance peak observed for the χxxx element of bare Ag(111) was attributed to the lower position of the electronic level relative to the Fermi level.44

Santos et al.

The opposite potential dependence observed for F0 at the shorter wavelength with both R-terthiophene films could also be due to different positions of the electronic levels responsible for the resonance. According to eq 3, an abrupt change in the SH intensity should be observed if the Fermi level crossed the position of the electronic level responsible for the resonance. In our measurements however, an almost smooth dependence of the coefficients on the potential was observed. In the case of molecular monolayers containing conjugated π systems, such as those investigated in this work, the delocalized electrons are not bound to specific atomic sites but distributed over relatively large dimensions. The electrical conductivity of oligoterthiophenes is due to large-scale delocalization of electrons, and the SAMs formed are frequently assumed to have semiconductor characteristics.1,2,15 We will consequently assume, to a first approximation, that the films do not behave as a conducting polymer but as an organic semiconductor, with electronic band-like structures (see Figure 12). Under these circumstances, the differing response of the SHG signal to an external electric field in the two types of SAMs investigated here (Figure 10) can be rationalized as follows. In the case of the R-C8-T3 SAM, more-negative values of the applied potential displace the Fermi level relative to the electronic levels (or bands) of the molecules to produced an increased occupation of the upper electronic band (π* antibonding orbitals). As a consequence of this, the probability of the resonant second harmonic transition between the two quasi-bands of the SAM chromophore is reduced and thus smaller values for the second-order susceptibility are produced. In the case of the R-C4-T3 -CN-SAM, the nitrile group acts similar to an acceptor introduced into an inorganic semiconductor and it extracts electrons from the π orbital. Thus, the effect of a negative external potential is to promote the occupation of the lower band (π bonding orbitals) and in this case increasing the probability of the resonance transition. The determination of the molecular orientation on isotropic substrates has been carried out by D. A. Higgins et al.34 for p-nitrophenol on fused silica and by F. Eisert et al.38 for the p-nitroaniline moiety on polycrystalline gold. They assume that the main contribution to the second harmonic signal comes from the resonance with electronic levels of the adsorbate and neglect any contribution from the substrate. Recently, Y. Rao et al.48 have developed an approach to determine the orientational order of molecular monolayer, but also they do not consider the anisotropy of the substrate. In the present case, the data do not allow definitive conclusions to be drawn. Since we observe also the contribution of anisotropic terms to eqs 6, this provides us with more information. The investigated systems correspond to a C3 symmetry, which indicates that the SH signal is an effect that results from the adsorbed layer and the last three atomic layers of the substrate. On the other hand, it is not likely that the molecular tensor, β, may be simplified to only one dominant element, βzzz. It is also possible that multiple electronic transitions in the adsorbate could contribute to resonances at the fundamental and second harmonic wavelengths. Conclusions and Perspectives It has been shown15 that SAMs formed by the two types of terthiophenes under investigation here form a monolayer essentially free of defects on gold surfaces. In an (48) Rao, Y.; Yi. Tao Wang, H. J. Chem. Phys. 2003, 119, 5226-5236.


Langmuir, Vol. 21, No. 14, 2005 6419

Figure 12. Possible energy diagrams showing a schematic “bandlike” structure for the conjugated π bonding and π* antibonding orbitals of the adsorbates. EF is the Fermi level, and its position relative to the electronic levels of the film shifts with the applied potential.

electrochemical environment, where the metal surface serves as an electrode, the alkyl chains of the SAM forms an insulating barrier. This situation is confirmed in our measurements with a (111) silver electrode, where, with potential differences in the range -600 to +200 mV, the current measured was in the region of a few microamps, the parallel capacity is practically constant, and the contribution from inhomogeneity lower than 5%. The presence of the adsorbed layer modifies the symmetry of the surface from C3v to C3, indicating that the SAM forms a rotated structure respect to the substrate (either the (x7 × x7)R19.1° or (x7 × x7)R10.9° structure). The sulfur-silver bond produces changes in the SH response in comparison to bare surfaces due to a modification of the electronic properties of the interface. This effect is similar for all the SAMs and is also observed at longer wavelengths. The phase difference between isotropic and anisotropic contributions changes from about 90° for bare surfaces to about 0° for surfaces covered with SAMs.

In addition, from the observation of changes in both the magnitude and phase of the SH signal under resonant or near-resonant conditions, one is able to localize resonance frequencies which are essentially the band-gap of the organic semiconductor/metal electrode SAM given by the lateral interactions of the π moiety. The phase difference between isotropic and anisotropic contributions changes from about 0° at longer wavelengths to about 180° for at shorter wavelengths. The effect of the potential is more evident at λF ) 800 nm than at 1064 nm. The Fourier coefficient, F0, at λ F ) 800 nm shows an opposite potential effect: while in the case of the adlayer containing the CN group (electronacceptor), they decrease with increasing potential, for the other adlayer, they increase. Particularly amazing is the change of sign in the real component of F3 when the fundamental wavelength is changed from 1064 to 800 nm, which clearly indicates that a characteristic resonance frequency has been crossed. A working hypothesis to explain this effect could be given by considering a bandlike behavior of the π-conjugated molecular orbitals of the

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Santos et al.

film and the electron acceptor nitrile group acting similar to an acceptor impurity in a crystalline inorganic semiconductor. The present work has shown that an exhaustive analysis of wavelength-dependent SHG measurements employing different conditions of polarization and varying the applied potential at the interface can contribute to a better understanding of the electronic interactions between substrate and adsorbates. However, more experimental data are necessary in order to drawn definitive conclusions. Other parameters are necessary to be varied in order to test other physical models. Finally, to understand the parameters involved in determining the electrical and optical properties of the SAMs on metal surfaces, a systematic variation of the chemical structure of the adsorbate is required. In this sense, the investigation of different R-thiophene substituted with different groups with different acceptor and donor properties is in progress.46 Appendix We give here a short description of the procedure and a list of variables and parameters involved in the analysis of the SH response. In the nomenclature employed in this paper, sub index ‘1’ refers to electrolyte and ‘2’ to metal side of the interface. The beam coordinates are (s, κ, z), and the crystal coordinates are (x, y, z). The rotation angle between both coordinates systems is defined by φ. The effective tensor can be obtained for the C3v symmetry, applying the appropriate symmetry operations, following the phenomenological model of Sipe et al.35,36

χ(2ω) eff ) -χxxx sin(3φ) χxxx sin(3φ) 0 -χxxx cos(3φ) χxxz 0 χxxz -χxxx cos(3φ) χxxx cos(3φ) 0 χxxx sin(3φ) 0 χzxx χzxx χzzz 0 0 0 (A1)

(

)

Also extending this treatment for the C3 symmetry we obtain

(

χ(2ω) eff ) -χxxx sin(3φ) χxxx sin(3φ) -χyyy cos(3φ) +χyyy cos(3φ) -χxxx cos(3φ) χxxx cos(3φ) +χyyy sin(3φ) +χyyy sin(3φ) χzxx

χzxx

0

0

-χxxx cos(3φ) +χyyy sin(3φ) χxxx sin(3φ) +χyyy cos(3φ)

χzzz 0

χxxz

χxyz

-χxyz χxxz

0

0

)

(A2)

We have to consider four different wave vectors. The wave vector for the downward (sign ‘-’) wave of the fundamental incident beam coming from the electrolyte oscillating at frequency ω: ω b ν 1-

)b κ 1 - k1zzˆ

(A3a)

The wave vector for the downward wave of the fundamental beam propagating inside the metal oscillating at frequency ω: ω )b κ 2 - k2zzˆ b ν 2-

(A3b)

The corresponding two second harmonic waves originating

from the polarization sheet and propagating upward (in the electrolyte) and downward (inside the metal) from the interface and oscillating at frequency 2ω: 2ω b ν 1+ )K B 1 + K1zzˆ

(A3c)

2ω )K B 2 - K2zzˆ b ν 2-

(A3d)

We have split the wave vector of the various beams into components parallel (κ bi,K B i) and perpendicular (kiz, Kiz) to the surface. zˆ is the unit vector normal to the interface. According to the usual boundary conditions of continuity,49-51 the total tangential component of the electrical field E must be continuous across the interface. Thus,

B 1+ ) zˆ × E B 2zˆ × E B 1- + zˆ × E

(A4a)

Consequently, it is required that the individual frequency components, at ω and 2ω, are separately continuous across the boundary. To satisfy this condition for all points on the boundary simultaneously, all the components of the wave vector parallel to the interface must be equal in both media:30,35,36,49-51

b|(2 ) kκ and |K B |(1 ) |K B |(2 ) 2kκ (A5) |κ b|(1 ) |κ On the other hand, the components perpendicular to the interface for the fundamental and the second harmonic beams in medium ‘i’, kiz and Kiz, respectively, are a function of the incident angle, θin, and of dielectric constant of the medium ‘i’:

kκ )

ω ω sin(θin) cx 1 2

2 ) k2κ + k1z

(ωc )

2 ) k2κ + k2z

(ωc )

(A6a)

ω 1

(A6b)

ω 2

(A6c)

2 ) 4k2κ + K1z

(2ωc )

2ω 1

(A6d)

2 4k2κ + K2z )

(2ωc )

2ω 2

(A6e)

2

2

2

According to V. Mizrahi and J. E. Sipe,35 the expression for the induced nonlinear polarization sheet sitting in the interface at z ) 0+ is

b,t) ) P2ω(r χ:eωeω|Ein|2 exp(2ikκx) exp(-i2ωt)δ(z - 0+) (A7) where χ is the surface second-order susceptibility tensor and eω is the unit vector of the incident electrical field beam for the fundamental (oscillating at frequency ω and with an amplitude |Ein|). The corresponding expression for the second harmonic electrical field to be detected is (49) Hecht, E. Optics; Addison-Wesley Publ.: Reading, MA, 1987. (50) Brevet, P. F. Surface second harmonic generation; Presses Polytechniques et universitaires romandes: Lausanne, France, 1997. (51) Bloembergen, N.; Pershan, P. S. Phys. Rev. B 1962, 128, 606622.


E2ω(r b,t) )

Langmuir, Vol. 21, No. 14, 2005 6421

2πi(2ω)2 2ω ω ω e χ:e e |Ein|2 × 2 K1zc exp(2ikκκˆ + K1zzˆ ) exp(-i2ωt) (A8)

The particular form of eω and e2ω depends on which geometry was used for the interface. In our case, they are35

e ) ω

(sˆ ts12sˆ

+

pˆ 2-tp12pˆ 1-)eˆ in

ˆ (1 + Rs12)S ˆ +P ˆ s+(P ˆ s+ + Rp12P ˆ s-)] e2ω ) eôut[S

(A9a) (A9b)

Equation A8 contains two contributions, i.e., the directly generated upward propagating wave which is given in terms of the nonlinear polarization (eq A7) and the downward propagating wave given by eq A3d reflected upward by the interface at z ) 0. The unit vectors specifying the ‘s’ and ‘p’ polarization components for the fundamental and second harmonic are given by

pî( )

sˆ ) S ˆ ) κˆ × zˆ

(A10a)

(kκzˆ - kizκˆ ) (2kκzˆ - Kizκˆ ) ; P î( ) (ω/c)xiω (2ω/c)xi2ω

(A10b)

AγS )

{

eˆ

in

(A11a)

out

(A11b)

P ˆ 1+ ) cos(Γ); eˆ

out

S ˆ ) sin(Γ)

tp12

)

Rs12

Rp12 )

2k1zxω1 xω2 k1zω2 + k2zω1 K1z - K2z ) K1z + K2z

2ω K1z2ω 2 - K2z1 2ω K1z2ω 2 + K2z1

2tp12ts12xω2 x2ω 1 sin(γ) cos(γ)χxxz

ω2 s 2 (t12 ) (ω/c)

x2ω 1 sin2(γ)χ

xxx

kκ

(A12a)

[

CγS ) k2 2ω 2 k 2ω ω p 2 2zx 1 p s 2zx 1 x 2 - (t12 ) cos (γ)χyyy - 2t12 t12 cos(γ) sin(γ)χxxx + kκ (ω/c)kκ ω2 x2ω 1 sin2(γ)χyyy kκ

s 2 (t12 ) (ω/c)

{

(A12c)

{

(A12d)

The expressions for the isotropic and anisotropic coefficients for the different combination of direction of polarization for the fundamental and second harmonic beams can be obtained combining the tensor elements of eq A2 given in Table 1 with the coefficients given in Table 2 according to eqs 4 and 5. These can be resumed as follows for the C3 symmetry:

]

×

s (1 + R12 ) (A15)

{

Γ) 0(P): p (t12 )

2

2

(ω/c)

[

2 k2z χzxx + k2κ χzzz -

2ω s p 1 t12 t12

x

ω2

(ω/c)2ω 2

]

12ω k2zK2zχxxz cos2(γ) 22ω

K2z sin(γ) cos(γ)χxyz +

s 2 ω (t12 ) 2

2

sin (γ)χzxx

}

p (1 + R12 )

2 2ω 1 k2zK2z p 2 - (t12 ) cos2(γ)χyyy + 2(ω/c)22ω 2 kκ

x

2ω ω2 s p 1 t12 t12 2ω 2

k2zK2z (ω/c)kκ

sin(γ) cos(γ)χxxx -

2ωωK s 2 1 2 2z (t12 ) 22ω 2 kκ

2

sin (γ)χyyy

(A16)

}

×

p (1 + R12 ) (A17)

CγP )

(A12b)

}

×

s (1 + R12 ) (A14)

BγP )

where γ ) (or Γ)) 0 means ‘p’ (or ‘P’), and γ ) (or Γ)) π/2 corresponds to ‘s’ (or ‘S’) polarized light. The transmission and reflection Fresnel coefficients appearing in eqs A9a-b are the usual

2k1z ts12 ) k1z + k2z

}

(1 + Rs12) (A13)

k2 2ω 2 k 2ω ω p 2 2zx 1 p s 2zx 1 x 2 - (t12 ) cos (γ)χxxx + 2t12 t12 cos(γ) sin(γ)χyyy + kκ (ω/c)kκ

AγP )

eˆ pˆ 1- ) cos(γ); eˆ sˆ ) sin(γ)

2(tp12)2k2zx2ω 1 cos2(γ)χxyz + (ω/c)

-

BγS )

The definition of the plane polarization properties of the fundamental and detected SH beams are given by in

{

Γ ) (1/2)π(S):

2 2ω 1 k2zK2z p 2 - (t12 ) cos2(γ)χxxx + 2(ω/c)22ω 2 kκ s p t12 t12

ω 2ω 1 x2 k2zK2z

2ω 2

2ωωK s 2 1 2 2z sin(γ) cos(γ)χyyy + (t12 ) sin2(γ)χxxx (ω/c)kκ 22ω 2 kκ

}

×

p (1 + R12 ) (A18)

Acknowledgment. The authors are indebted to Professor Peter Bauërle and Dr. Gu¨nter Go¨tz for supplying the samples of R-terthiophene compounds and giving advice on the production of SAMs. This work is supported by CONICET, DLR-SePCyT, Volkswagen Stiftung, and in part by Fonds der Chemischen Industrie. E.S. thanks DAAD for the support obtained through the re-invitation program for former scholarship holders. LA050351X

Electrolyte

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