Using Surface Plasmon Resonance and the Quartz Crystal

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Using Surface Plasmon Resonance and the Quartz Crystal Microbalance to Monitor in Situ the Interfacial Behavior of Thin Organic Films Larry E. Bailey,† Dev Kambhampati,‡ Kay K. Kanazawa,† Wolfgang Knoll,†,§ and Curtis W. Frank*,† Department of Chemical Engineering, Stanford University, Stanford, California 94305-5025, Thermo Hybaid Interactiva, Sedanstrasse 10, D-89077 Ulm, Germany, and Max Planck Institute for Polymer Research, Ackermannweg 10, 55128, Mainz, Germany Received August 10, 2001 We have used a combination of surface plasmon resonance (SPR) and the quartz crystal microbalance (QCM) to monitor in situ the solution-phase adsorption of the perfluoropolyether lubricant Fomblin ZDOL onto a silver surface. This dual-probe technique was then extended in a novel way by the addition of electrochemical control and used to monitor the electrochemically induced solution-phase desorption of octadecanethiol (C18S) from a gold surface as well as the electrochemical polymerization of polypyrrole (PPY) on a gold surface. The experimental results obtained by the joint technique compare favorably with those obtained using SPR and QCM independently. The combination allows us to measure simultaneously the optical and acoustic properties of these materials as they interact with the metallic surface. While SPR and QCM have similar resolution and are both able to follow deposition in real time, there are distinct advantages to the simultaneous measurement. These advantages allow one to (1) test the validity of the governing equations often used to analyze data collected using the two techniques, bringing to light weaknesses in the assumptions inherent in these equations, (2) calculate interfacial density and refractive index values in a system where the bulk values are known and the physical state of the adsorbed material is similar to that of the bulk, (3) show that the viscoelastic properties of an adsorbed material change significantly as the material desorbs from an interface, and (4) observe the evolution in the electronic and chemical properties of a conducting polymer film as it is being deposited while precisely monitoring the mass of the deposited film.

Introduction Surface plasmon resonance (SPR) and the quartz crystal microbalance (QCM) are both well-established noninvasive techniques capable of providing a wealth of information about interfacial phenomena.1,2 Both SPR and QCM involve resonance phenomena that are perturbed by changes in the characteristics of an interfacial layer. These two techniques can be used to measure film thickness with resolution on the angstrom-to-nanometer level. Electrochemistry has been used in combination with both techniques to provide precise control of interfacial chemistry.3-5 SPR and QCM are often used to monitor thin-film deposition; however, each has its own specific strengths, weaknesses, and assumptions inherent in data collection and analysis. By combining these techniques to perform in situ measurements, we have been able to take advantage of the strengths of each while testing the validity of some assumptions inherent in data analysis, thus gaining a more complete understanding of the interfacial phenomena studied. * To whom correspondence should be addressed. E-mail: curt@ chemeng.stanford.edu. † Stanford University. ‡ Thermo Hybaid Interactiva. § Max Planck Institute for Polymer Research. (1) Knoll, W. Annu. Rev. Phys. Chem. 1998, 49, 569. (2) Czanderna, A. W.; Lu, C. In Applications of Piezoelectric Quartz Crystal Microbalances, Methods and Phenomena, 7th ed.; Lu, C., Czanderna, A. W., Eds.; Elsevier: New York, 1984. (3) Iwasaki, Y.; Horiuchi, T.; Morita, M.; Niwa, O. Sens. Actuators B 1998, 50, 145. (4) Ehler, T. T.; Walker, J. W.; Jurchen, J.; Shen, Y.; Morris, K.; Sullivan, B. P.; Now, L. J. J. Electroanal. Chem. 2000, 480, 94. (5) Buttry, D. A.; Schneider, T. W. J. Am. Chem. Soc. 1993, 115, 12391.

SPR is a noninvasive optical-measurement technique that probes the thickness and dielectric constant of thin films at a noble-metal surface. Thickness resolution is typically on the angstrom level, depending on the optical contrast between the film and the solvent,6 and films up to several hundred nanometers thick can be studied using this technique. The stability of SPR measurements is generally quite good, and sensitivity to environmental factors is minimal. However, there must be optical contrast between an adsorbed film and the solvent for the film to be seen by SPR. In addition, the refractive index of the film must be known independently if a simple measurement is to provide the thickness and mass of an adsorbed layer. SPR and QCM measurements have many similarities.7 Once again, thickness resolution is typically on the angstrom level. As with SPR, QCM is a noninvasive measurement. However, while SPR is sensitive to the optical properties of an adsorbed film, QCM is a mechanical measurement technique. The addition of material to the surface of the quartz perturbs the mechanical oscillator, leading to changes in the frequency and quality of its resonance. The frequency change depends on the mass of material on the quartz surface and the viscoelastic properties of the material. Thus, to sense an adsorbed film, there must be a viscoelastic or density contrast between the film and the surrounding medium. QCM is capable of subangstrom film-thickness resolution when used in air or vacuum. In liquid, the thickness (6) Tamada, K.; Ishida, T.; Knoll, W.; Fukushima, H.; Colorado, R.; Graupe, M.; Shmakova, O. E.; Lee, T. R. Langmuir 2001, 17, 1913. (7) Laschitsch, A.; Menges, B.; Johannsman, D. Appl. Phys. Lett. 2000, 77, 2252.

10.1021/la0112716 CCC: $22.00 © 2002 American Chemical Society Published on Web 12/22/2001

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resolution is somewhat less than that in air or vacuum, but it is still on the angstrom level. QCM can accurately measure films up to several micrometers thick, depending on the viscoelastic properties of the deposited material.8 However, because of the nature of the quartz oscillator, it is also very susceptible to drift due to temperature fluctuations, pressure changes, or mechanical disturbances.2 In addition, the calculation of film thicknesses from frequency shifts obtained when the quartz is used in a liquid medium is somewhat questionable. For thin films, simple relations such as the Sauerbrey equation often hold.9-11 For thicker or more viscous films, more detailed analyses are necessary.8,12,13 Conveniently, both SPR and QCM measure the properties of a film adsorbed on a metallic electrode. Thus, the addition of electrochemical control to a measurement cell that utilizes these two techniques is quite simple, requiring only counter and reference electrodes. However, this addition opens the door to the study of a vast number of systems. There are many benefits to the combination SPR-QCM measurement. Using SPR and QCM independently to monitor interfacial processes in situ, we are able to obtain two measures of a process that rely on fundamentally different principles of physics. By carefully considering the nature of each measurement, our ability to interpret the results obtained from either technique is greatly enhanced. In addition, the weaknesses of assumptions inherent to data analysis become apparent, and differences measured by each technique can often be reconciled through theoretical considerations. Specific to the systems presented herein, use of the dualprobe technique has allowed us to (1) test the validity of the Sauerbrey equation for interpreting QCM frequency shifts in terms of film thickness when the QCM is exposed to a liquid medium, (2) calculate interfacial density and refractive index values in a system where the bulk values are known and the physical state of the adsorbed material is similar to that of the bulk, (3) show that the viscoelastic properties of an adsorbed material change significantly as the material desorbs from an interface, and (4) observe evolution in the electronic and chemical properties of a conducting polymer film as it is being deposited while precisely monitoring the mass of the deposited film. None of these observations would have been possible using either SPR or QCM independently. Materials Fomblin ZDOL Adsorption. Fomblin ZDOL was purchased from Ausimont and fractionated by Phasex Corporation. Its molecular structure is shown below:

HO-CH2-CF2-O-(CF2CF2O)p-(CF2O)q-CF2-CH2-OH This material is a linear, random copolymer of perfluoromethylene and perfluoroethylene repeat units with two hydroxyl end groups. It has an average molecular weight of 2740 g/mol and a polydispersity of 1.08. The ratio of p to q is typically around 1.0, which yields average p and q values of 14.1. For this molecular weight, the length of the fully extended chain is approximately 100 Å. The bulk refractive index of ZDOL is 1.29, and the bulk (8) Voinova, M. V.; Rodahl, M.; Jonson, M.; Kasemo, B. Phys. Scr. 1999, 59, 391. (9) Sauerbrey, G. Z. Phys. 1959, 155, 206. (10) Weerawarden, A.; Drummond, C. J.; Caruso, F.; McCormick, M. Langmuir 1998, 14, 575. (11) Niwa, M.; Date, M.; Higashi, N. Macromolecules 1996, 29, 3681. (12) Lu, C. S.; Lewis, O. J. Appl. Phys. 1972, 43, 4385. (13) White, C. C.; Schrag, J. L. J. Chem. Phys. 1999, 111, 11192.

Bailey et al. density is 1.81 g/mL.14 ZDOL physisorption takes place on a vacuum-evaporated silver electrode from 1,1,2-trichlorotrifluoroethane (FC113) solvent (Aldrich, 99.8%), which has a refractive index of 1.355. Octadecanethiol Desorption. 1-Octadecanethiol (98%) was purchased from Aldrich and used without further purification. The molecular structure is shown below:

H-S-(CH2)17-CH3. The length of the fully extended C18S chain is approximately 23 Å, which we would expect to be the upper bound of the thickness of a well-formed, single C18S monolayer. The refractive index of octadecanethiol films on gold surfaces varies widely, depending on the formation conditions, film thickness, and solvent, with values between 1.4 and 1.48 reported in the literature.15 We have assumed that the refractive index of our films is approximately 1.45. In addition, the density of self-assembled monolayers of octadecanethiol is not well established. Although we have assumed the bulk density to be 0.847 g/mL, the actual density may be much higher because of the ordered nature of the C18S film. The electrochemically induced cathodic C18S desorption takes place from a vacuum-evaporated gold electrode in a solution of 0.1 M tetraethylammonium perchlorate (Acros) in acetonitrile (Aldrich), which has a refractive index of 1.344. Pyrrole Polymerization. Pyrrole (98%) was purchased from Aldrich and used without further purification. Its molecular structure, in monomeric and polymerized forms, is shown below:

All polypyrrole (PPY) depositions take place from a solution of 0.1 M pyrrole and 0.1 M sodium perchlorate (E.M. Science) in ultrapure water. Pyrrole is electrochemically polymerized on a vacuum-evaporated gold electrode. The refractive index of PPY is known to vary, depending on the oxidation state of the polymer.16 The refractive index of the aqueous pyrrole solution is 1.339. The density of PPY is assumed to be 1.5 g/mL.17

Experimental Section To ensure that the combination of SPR and QCM measurement techniques does not perturb the system, we first measured the three interfacial processes studied herein using SPR and QCM independently. These data (not shown) suggest that the systems are not affected by the combination measurement. We then performed the joint measurements. In this section, we report the preparation and geometry of the substrate, followed by a description of the sample cell. The system hardware, electronics, and relevant theory for each individual technique are then described. Substrate Preparation. Blank quartz crystals (1 in. diameter AT-cut wafers from Maxtek, Inc.) are carefully cleaned and coated on one side with a layer of photoresist (Shipley Microposit), which has a nominal thickness of 100 nm. A holographic grating is written into the resist and then transferred to the quartz substrate by reactive ion-beam etching,7,18 providing a corrugated surface as shown in Figure 1a. Next, the grated quartz crystals are coated with 10 nm of chromium (Plasmaterials, 99.99% purity), followed by silver or gold (Plasmaterials, 99.99% purity) deposited by thermal evaporation in a commercial instrument (Edwards Auto 306). The metallic layer is approximately 250 nm thick, and the electrode configuration is shown in Figure 1b. (14) Fomblin ZDOL product literature, Ausimont USA, Inc: Thorofare, NJ. (15) Peterlinz, K. A.; Georgiadis, R. Langmuir 1996, 12, 4731. (16) Diaz, A. F.; Castillo, J. I.; Logan, J. A.; Lee, W. Y. Electroanal. Chem. 1981, 129, 115. (17) Ayad, M. M. J. Appl. Polym. Sci. 1994, 53, 1331. (18) Knobloch, H.; Knoll, W. J. Chem. Phys. 1991, 94, 835.

Monitoring the Interface Behavior of Thin Films

Figure 1. Substrate for the joint SPR-QCM experiments. (a) Grated quartz crystal. (b) Electrode configuration on the front and back of the quartz crystals.

Figure 2. Top-down schematic cross-section of the SPR-QCM cell for simultaneous surface plasmon resonance and quartz crystal microbalance measurements. The front electrode serves as the active interface for our adsorption, desorption, and polymerization measurements as well as the working electrode for electrochemical control. The front electrode is wrapped to the back of the crystal, allowing us to make all electrical contacts on the back using a small amount of conducting silver paint (G.C. Electronics). Care is taken to ensure that the silver paint does not contact the interior of the sample cell. The rear, keyhole-shaped electrode serves as the second electrode for QCM excitation. Measurement Cell. A schematic of the custom sample cell is shown in Figure 2. It consists of a Teflon tube with O-ring groves machined in each end. On the left side of the tube, an optical window allows laser light to enter the cell, bounce off the grated quartz crystal, and leave through the window, providing the means for SPR measurements. The angle between the surface normal and the incident laser beam, θ, is defined in the Figure and can be varied from 4 to 28°. On the right side of the tube, the grated quartz crystal that was described above is mounted, with the grated front electrode facing the cell. Electrical contact between the two quartz electrodes and the QCM electronics is made using spring-loaded pins (ETC) pressed against the right side of the quartz, with the

Langmuir, Vol. 18, No. 2, 2002 481 front electrode grounded and the back electrode receiving the QCM drive potential, ensuring that the rf fields are contained within the quartz. To facilitate the electrochemical measurements, a loop of platinum wire is used as a counter electrode and placed near the left side of the cell. A reference electrode is placed between the counter electrode and the quartz crystal. For C18S desorption, a platinum quasi-reference is used because the experiments are performed in an organic solvent. For PPY deposition in an aqueous solvent, a KCl-saturated Ag/AgCl reference electrode (Cypress Systems) is used. The front surface of the quartz crystal serves as the working electrode. Viton O-rings are placed in the O-ring grooves, and the entire assembly is clamped together to form a tight seal. The cell is contained within an aluminum block that has channels machined in it. Temperature-controlled water is circulated through these channels to maintain an isothermal environment within the cell and minimize measurement drift associated with thermal fluctuations. Surface Plasmon Resonance. Surface plasmon resonance is a highly selective diagnostic tool for detecting interfacial events. A plasmon resonance is generated by coupling plane-polarized light to the freely oscillating electrons that are present within a metal surface (typically gold or silver). Efficient momentum matching between the photon and electron wavevectors can be achieved by using either a prism or a grating configuration.1 The coupling results in an evanescent wave that decays exponentially, perpendicular to the metal surface. It is this wave that is used to probe interfacial properties. The scan and kinetic SPR measurement modes are widely used to describe events occurring at the metal interface.1 In the scan mode, reflectivity changes are monitored as a function of the angle of incidence of the incoming laser beam. Two points are worth noting. For angles smaller than the critical angle, a steady rise in the reflectivity is observed, while for angles larger than the critical angle, a steady drop in reflectivity is observed until reaching a certain angle at which the reflectivity signal reaches a minimum. This angle is called the resonance angle. For values higher than the resonance angle, a steady increase in the reflectivity value is observed. The resonance angle is strongly dependent on the dielectric constant of the medium surrounding the metal interface, the laser wavelength, and the dielectric properties of the metal. Any changes in the dielectric properties of the medium will result in a shift in the resonance angle, which is usually the case when some material binds to or dissociates from the metal surface. These events can be monitored in real-time using the kinetic measurement mode. In this mode, reflectivity changes occuring at a specific incident angle are monitored as a function of time. Assuming that the thickness of the dielectric film is much less than the wavelength of the probe laser, we can estimate the film thickness (df) from19

df )

(

)

sλx-m(s - m) f m + s 2 ∆(sin θr) 2π (f - s)(f - m) ms (1)

where λ is the wavelength of the incident laser (632.8 nm), f is the dielectric constant of the film, s is the dielectric constant of the solvent, m is the real part of the dielectric constant of the metal, and θr is the resonance angle of the plasmon resonance curve. This equation is a first-order approximation of the plasmon behavior, and a more rigorous numerical solution of Fresnel’s equations was actually used for data analysis.1 However, eq 1 provides us with the general dependence of the SPR data on various system parameters and will be useful for comparisons to the QCM data and the governing equation. The optical configuration and electronics used to collect the SPR data are shown in Figure 3. Light from a 632.8 nm HeNe laser is passed through two crossed polarizers (Newport) to select polarization and intensity. This polarized light beam then passes through a beam chopper (Stanford Research Systems, Inc.) that is connected to a lock-in amplifier (Stanford Research Systems). Next, the beam passes to the custom Teflon sample cell of Figure (19) Pockrand, I. Surf. Sci. 1978, 72, 577.

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Figure 3. Schematic of the surface plasmon resonance optics and electronics.

Figure 4. Plasmon resonance curves collected before and after ZDOL adsorption.

2 where it is coupled to the metal using the surface grating.1 The reflected laser light is monitored by a photodetectorthat is also connected to the lock-in amplifier. The angle between the incident laser beam, sample surface, and photodetector is varied using a pair of goniometers (Huber). Data acquisition and control of system electronics are accomplished using custom software. Quartz Crystal Microbalance. The resonance frequency of the QCM changes in a well-defined manner as mass is added to or removed from its surface, with a mass resolution of approximately 50 ng/cm2 in our system. To interpret the observed frequency shifts in terms of an adsorbed mass or of lubricant layer thickness, we have used the well-known Sauerbrey equation2,9

mf )

-∆f Fqvq 2fq2

(2)

where mf is the mass density of material adsorbed on the surface of the quartz (kg/m2), ∆f is the frequency change of the quartz (Hz), fq is the initial frequency of the quartz (nominally 5 MHz), Fq is the density of the quartz crystal (2650 kg/m3), and vq is the shear wave velocity within the quartz (3340 m/s for AT-cut crystals). Substituting the above values into eq 2 and dividing by the film density, we obtain the following simple relationship between the film thickness (tf , Å) and the frequency change of the crystal

C tf ) - ‚∆f Ff

(3)

where Ff is the film density (g/mL), and C is a material constant for the quartz resonator (1.77 (Å‚g)/(mL‚Hz)). A Hewlett-Packard ES5100A network analyzer is used to drive the quartz crystal at its resonance frequency. The conductivity versus frequency sweeps are routed via a GPIB interface to a PC running HP BASIC, where the data are curve-fit and the crystal frequency at maximum conductivity is calculated. Using this procedure, we are able to obtain a frequency resolution of a few Hertz in liquid. Electrochemistry. Current is passed through the solution from the counter electrode to the vacuum-evaporated gold working electrode. All potential measurements are made relative to the reference. A Hokuto Denko HB-111 function generator controls voltage transients, while an Intertech Systems PGS151 potentiostat/galvanostat supplies the electrical power to the cell. A PC equipped with a National Instruments data acquisition card and LabVIEW software collects the data.

Results ZDOL Adsorption. For this experiment, a crystal with a silver front electrode is used to maximize sensitivity1 because of the extremely thin nature of the adsorbing

Figure 5. Comparison of ZDOL adsorption kinetics as measured by SPR and QCM.

film. The sample cell is filled with FC113 and allowed to stabilize for at least 30 min. The SPR and QCM electronics are then used to monitor the system as described below. First, a plasmon resonance is scanned: the intensity of reflected laser light is measured as a function of the incident angle to characterize the optical properties of the interface before adsorption. This plasmon resonance is shown as the dark line in Figure 4. The angle between the sample and the laser beam is then adjusted to the linear region of the plasmon resonance curve, which is 12.44° relative to the surface normal, and the intensity at this angle is monitored as a function of time to generate the SPR kinetic data. Concurrently, the oscillation frequency of the quartz is monitored as a function of time to generate the QCM kinetic data. After collecting a kinetic baseline for 500 s with both SPR and QCM, we added a solution of ZDOL in FC113 to the cell ([ZDOL]final ) 1.92 × 10-6 moles/mL) and recorded the responses from SPR and QCM. The adsorption kinetics for the two techniques, normalized by the final thicknesses to facilitate comparison, are shown in Figure 5. The small spike in the SPR data immediately after the ZDOL solution addition is caused by refractive index variations within the cell due to incomplete mixing. As the lubricant diffuses and the solution becomes well mixed, this perturbation disappears. Within experimental error, the normalized adsorption kinetics are identical for the two techniques. Once the system reaches equilibrium, the kinetic measurements are stopped, and a second plasmon reso-


nance is scanned. For this system, the addition of ZDOL to the surface of the quartz served to shift the resonance curve to lower angles, leading to the second curve in Figure 4. Analysis of the frequency and plasmon resonance curve shifts yields apparent equilibrated ZDOL film thicknesses of 18 ( 1 and 13 ( 2 Å as determined by SPR and QCM, respectively, assuming a refractive index of 1.29 and a density of 1.81 g/mL. We will address the apparent discrepancy in thicknesses in the Discussion. Octadecanethiol Desorption. For this experiment, crystals with a gold front electrode are used. While the SPR sensitivity is lower because of broader plasmon resonance curves, gold provides an inert and well-studied substrate for C18S attachment and electrochemical measurements. Changing the metal should have little or no impact on the QCM data. After the gold is vacuumevaporated onto the front electrode, the crystals are immediately transferred to a solution of 10 mM octadecanethiol in acetonitrile.5 Then, the C18S self-assembled monolayer is allowed to form on the gold surface for varying periods of time. After a C18S film is formed, the crystals are removed from the deposition solution and mounted within the sample cell. The cell is filled with a solution of 0.1 M tetraethylammoniumperchlorate (TEAP) in acetonitrile and allowed to stabilize while being held at a potential of -0.3 V versus a platinum quasi-reference. Plasmon resonance curves collected before and after C18S desorption for a system in which the C18S film is allowed to self-assemble for 160 min are shown in Figure 6a. The resonance curves for a system in which the C18S film is allowed to self-assemble for 465 min are shown in Figure 6b. Adsorption kinetics are measured at an angle of 17° for both runs. Once the system is stable and the initial plasmon resonance has been measured, the kinetic SPR and QCM data are collected. A baseline is measured for 25 s, followed by a cathodic step change in potential from -0.3 to -2.0 V, causing the electrochemically induced desorption of the C18S molecules.5 The desorption kinetics for both 160and 465-min equilibration times, normalized by the change at long times, are shown in Figure 7. For the shorter equilibration time, the SPR kinetics differ significantly from those of the QCM, while for the longer equilibration time, the two techniques produce nearly identical results. For the 160-min equilibration-time run, the apparent film thickness values, on the basis of the assumed density and refractive index values given above, are 30 ( 10 and 20 ( 5 Å as determined by SPR and QCM, respectively. For the 465-min equilibration-time run, the apparent thickness values are 30 ( 10 and 43 ( 5 Å as determined by SPR and QCM, respectively. The large error bars on the thickness values are due to the width of the plasmon resonance curves, which makes precise angular-shift determinations difficult, and the range of possible density and refractive index values for the C18S film at the gold surface. These factors have a direct impact on the thickness calculated by each technique, as shown in eqs 1 and 3. We consider this point further in the Discussion. While we have used the combined SPR-QCM technique to monitor the solution-phase adsorption of thiol onto the gold electrodes, this phenomenon is not discussed in detail herein. The adsorption experiment is more complicated in the context of the dual measurement, requiring the thiol adsorption to be monitored just after the solution surrounding the front face of the crystal is changed from a thiol-free to a thiol-containing solution. This change leads to significant fluctuations in both the SPR and the QCM signals, making it difficult to track the adsorption. In the

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Figure 6. Plasmon resonance curves collected before and after C18S desorption for (a) 160 min of self-assembled monolayer formation time and (b) 465 min of self-assembled monolayer formation time.

case of SPR, the optical properties of a 10 mM octadecanethiol and 0.1 M TEAP solution in acetonitrile are significantly different than the optical properties of either pure acetonitrile or 0.1 M TEAP in acetonitrile. These differences cause sharp shifts in the resonance curve when the solution is changed; therefore, it is difficult to follow the adsorption kinetics. In the case of QCM, completely changing the solution leads to thermal drift that often takes tens of minutes to cease. This drift then complicates interpretation of the adsorption results. These difficulties could eventually be overcome; however, the desorption experiment is simple and eloquent by comparison, requiring only a change in the electrode potential to induce thiol desorption from the interface and producing only a minimal perturbation to the system. Thus, thiol desorption is presented in detail herein. Polypyrrole Surface Reaction. For this experiment, crystals with a gold front electrode are used. After electrode preparation, the crystals are mounted in the sample cell that is then filled with a solution of 0.1 M pyrrole and 0.1 M sodium perchlorate in ultrapure water. The system is allowed to stabilize with no applied current. Representative plasmon resonance curves collected before and after PPY deposition are shown in Figure 8. For this deposition, there is a significant change in both the resonance angle and the shape of the plasmon resonance curve. Adsorption kinetics are measured at 13°.

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Figure 9. Comparison of PPY deposition kinetics as measured by SPR and QCM.

time, reaching a plateau level hundreds of seconds after the current is turned off, while the QCM shows a plateau with only a slight decrease in film thickness after 720 s. Discussion

Figure 7. Comparison of C18S desorption kinetics as measured by SPR and QCM for (a) 160 min of self-assembled monolayer formation time and (b) 465 min of self-assembled monolayer formation time.

Figure 8. Plasmon resonance curves collected before and after PPY deposition.

A baseline is collected for 500 s, followed by a step change in the current from 0.0 to 8.0 µA/cm2. This current is maintained for 720 s, at which point it is returned to 0.0 mA. The PPY deposition kinetics measured by SPR and QCM, normalized by their values at 720 s, are shown in Figure 9. Both techniques show a flat baseline and nearly linear growth in film thickness with time; however, when the current is turned off, the kinetic measurements diverge significantly. SPR appears to show the film growing with

Film Thickness Comparisons. The validity of the Sauerbrey equation for interpretation of QCM frequency changes in a viscous medium must be demonstrated, as it assumes that the viscoelastic properties of the adsorbed film are similar to those of the quartz and that the viscoelastic properties of the film and the solution do not change over the course of the experiment.2 For very thin adsorbed films that interact minimally with the solvent, this assumption should not prove problematic, as the viscoelastic property changes will be minimal for the oscillating system as a whole. However, for thicker films or films that are able to form strong interactions with the solvent, more complicated relationships must be used to interpret the response of the QCM to mass and liquid loading.20-22 The agreement between the normalized SPR and QCM kinetic data for both ZDOL and the long equilibration-time C18S system suggests that for the ultrathin films studied here we are operating in the region of linear response for both techniques. Thus, there are no significant deviations from the ideal behavior described by the Sauerbrey equation. However, the variation in apparent thickness measured by the two techniques illustrates the need to know the material properties of adsorbed films accurately if quantitative information about film thickness is to be obtained. These differences in apparent film thickness can be thought of in two different ways. In the case of ZDOL adsorption, we have independently measured both the density and the refractive index of the bulk material. In addition, we expect the perfluoropolyether backbone of ZDOL to be relatively disordered at the interface. Thus, the interactions between ZDOL molecules should not change significantly between the bulk and adsorbed states. These two facts suggest that we may be able to use the different thickness values to obtain additional information about the conformation of the lubricant at the interface. If we assume that there is a systematic error in our use of bulk refractive index and density values for an adsorbed (20) Martin, S. J.; Granstaff, V. E.; Frye, G. C. Anal. Chem. 1991, 63, 2272. (21) Martin, S. J.; Spates, J. J.; Wessendorf, K. O.; Schneider, T. W.; Huber, R. J. Anal. Chem. 1997, 69, 2050. (22) Kanazawa, K. K. Manuscript in preparation.


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ZDOL film, we can use the information obtained from the two measurement techniques to correct for this error. From eq 1, we see that to first order the film thickness calculated by SPR is inversely proportional to the square of the film’s refractive index (df ∝ 1/f , f ) nf2). From eq 3, we see that the film thickness obtained from QCM is inversely proportional to the density of the film (tf ∝ 1/F). In both cases, the bulk value for these key material parameters was used in calculations. However, theories governing thin polymer films at interfaces suggest that the density of the film, which is composed of trains on the substrate with loops and tails protruding into the solution, is lower than the bulk density value.23,24 In addition, the refractive index of a material is proportional to its density, leading to a systematic error in our calculations that we can address theoretically. The refractive index (n) of a material is related to its density by25

n)1+

2πLRF M

(4)

where L is Avogadro’s number, R is the polarizability volume, M is the molar mass, and F is the density. Substituting the bulk ZDOL refractive index of 1.29 and the bulk density of 1.81 g/mL into eq 4, assuming that all other material parameters in eq 4 are constant, we obtain the relationship

n ) 1 + 0.160F

(5)

Using eq 5 to relate film density to film refractive index, combined with the first-order relationship between refractive index and film thickness for SPR (tf ∝ 1/n2) and the relationship between density and film thickness for QCM (tf ∝ 1/F), we are able to calculate an effective thickness that forces the results obtained from the two techniques to agree. This thickness is found to be 22 Å, which corresponds to a film density of 1.08 g/mL and a refractive index of 1.175. This average density, which is approximately 40% lower than the bulk density, is very reasonable if we assume that the loop, train, and tail model is a valid description of the polymer interface. Our analysis illustrates the strong role that surfaces can play in changing the material properties of a thin film and the dangers inherent in assuming bulk material properties are applicable to this type of system. While a general investigation of polymer conformation at surfaces is not the specific focus of our work, this example illustrates that the combined SPR-QCM measurement shows great promise in monitoring physical property changes of polymer chains at interfaces. For the C18S desorption experiments, the refractive index and density of the adsorbed film are not known precisely and are expected to be significantly different from those of bulk C18S because of the formation of a highly ordered, self-assembled monolayer. This constraint, coupled with the fact that the film structure varies significantly depending on the deposition solvent and conditions,15 has traditionally made it very difficult to obtain film thicknesses using either SPR or QCM without making broad assumptions about material properties or employing elaborate, ex situ film-characterization techniques.6,15 Thus, many authors have been satisfied with measuring only adsorption and desorption kinetics for (23) Adamson, A. W.; Gast, A. P. Physical Chemistry of Surfaces; Wiley & Sons: New York, 1997. (24) Di Marzio, E. A. Physics of Polymer Surfaces and Interfaces; Butterworth-Heinemann: London, 1992; p 73. (25) Atkins, P. W. Molecular Quantum Mechanics; Oxford University Press: New York, 1983; p 358.

this type of system, and quantitative discussions of film thickness have typically been subject to assumed material properties.1,5,26,27 These issues make it difficult to discuss the quantitative differences in C18S film thickness obtained by SPR and QCM. In lieu of making an analysis similar to that performed for ZDOL, we must be satisfied with the observation that within experimental uncertainty film thicknesses measured independently by SPR and QCM are identical. However, the joint SPR-QCM measurement illustrates the importance of recognizing this uncertainty and the danger associated with simply using material properties to make quantitative comparisons between different systems. In addition, the joint measurement does provide additional insight into the kinetics of C18S desorption, as will be discussed below. Simple Adsorption and Desorption Kinetics. As shown in Figure 5, the ZDOL adsorption kinetics measured independently by SPR and QCM are in excellent agreement with each other. Either measurement, used independently, would be sufficient to understand the kinetics of ZDOL adsorption onto a silver surface. However, as discussed above, the use of two techniques has allowed us to infer information about the actual density, refractive index, and conformation of ZDOL molecules at the solidliquid interface. In contrast, the C18S system appears to be slightly more complicated than the ZDOL system, showing SPR desorption kinetics that differ significantly depending on the film formation time prior to desorption. The normalized SPR kinetic response curves are compared in Figure 10a, while the QCM response curves for the two desorption events are compared in Figure 10b. The SPR curves are quite different, while the QCM response is nearly identical for the two desorption events. This observation, taken together with the fact that for long film-formation times SPR and QCM show identical desorption kinetics, suggests that the physical properties of the system that are probed by the two techniques differ at short film-formation times but become similar at longer times. These kinetic differences raise two interesting questions. First, why should we expect the behavior of the C18S film to vary with film-formation time? Second, what is physically different between the two measurement techniques that could allow them to produce different kinetic traces? Thiol film properties that depend on the film-formation time have been seen previously. Schneider and Buttry noted large variations in the QCM frequency change upon electrochemical desorption of decylthiol films that had been allowed to equilibrate in acetonitrile solutions for varying periods of time.5 They attribute these variations to multilayer formation at the gold surface and note that even after equilibration for 10 h in a solution of 10 µM decylthiol and 0.1 M tetraethylammoniumperchlorate in acetonitrile, the desorbed mass is greater than would be expected for a single, saturated, self-assembled monolayer of decylthiol. Their electrochemical QCM data support a model in which a partially formed, chemisorbed thiol layer at the gold surface is coated with additional layers of physisorbed material. They note that the equilibration time has a significant effect on the film thickness, which would be related to the level of organization within the film, but do not present a systematic study of film properties or thickness as a function of equilibration time. (26) Schessler, H. M.; Karpovich, D. S.; Blanchard, G. J. J. Am. Chem. Soc. 1996, 118, 9645. (27) Scho¨nherr, H.; Kremer, F. J. B.; Kumar, S.; Rengo, J. A.; Wolf, H.; Ringsdorf, H.; Jaschke, M.; Butt, H.-J.; Bamberg, E. J. Am. Chem. Soc. 1996, 118, 13051.

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Figure 11. Model system for investigation of the length scales probed by SPR and QCM as a thin uniform film moves away from the gold surface.

Figure 12. Model SPR response to the presence of the thin model film in solution.

Figure 10. Comparison of C18S desorption kinetics for the two different self-assembled monolayer formation times as measured by (a) SPR and (b) QCM.

In the context of their data and model, it is reasonable to assume that the physical properties of the film that are probed by SPR and QCM could vary significantly with equilibration time. To first order, both SPR and QCM can measure the thickness of thin films adsorbed at an interface. Thus, we would expect these measurements to be redundant. However, as was shown above, the measured film thickness is highly dependent on the physical properties of the film. In the same way, these techniques rely on different principles of physics to characterize the kinetic evolution of films at interfaces. The differences between the kinetics measured by the two techniques could be due to a number of factors. The most noteworthy factor would be a difference in the length scales that are probed by SPR and QCM in a direction perpendicular to the surface. To investigate the length scales probed by the two techniques, we have simulated SPR and QCM responses to an idealized, uniform 1 nm film in the vicinity of the metal surface. The model system is shown in Figure 11. It is very important to note that this model is a highly idealized picture of the interfacial phenomena that we are studying. In the actual C18S system, we would expect the C18S molecules to desorb individually or in small groups, diffusing away from the gold surface with time. However, our simple model allows us to investigate a worst-case scenario (simultaneous desorption and delamination of a full monolayer) for the impact of the C18S

molecules on our measurement techniques as the molecules diffuse away from the gold surface and into solution. If one technique probes longer length scales than the other, we would expect this technique to “see” the thiol molecules for a longer period of time as they diffuse away from the gold surface. The distance, x, between the film and the metal is varied between 1.0 nm and 1.0 µm. For simulation purposes, the thiol film is assumed to have a density of 0.847 g/mL and a refractive index of 1.45, as presented above. The film viscosity, which is unknown, is varied between 0.34 and 1000 cP. The solvent is assumed to have a density of 0.786 g/mL, a refractive index of 1.344, and a viscosity of 0.34 cP, as presented above. The complex dielectric function of the gold substrate is assumed to be -12.3 + 1.29i, a typical value measured in our experiments. The angular shift in the plasmon resonance curve due to the presence of this film, obtained by solving Fresnel’s equations for the four-layer model system, is shown in Figure 12. In the Figure, the angular shift is plotted against the distance between the metal surface and the desorbed thin film. For short distances less than a few nanometers, there is very little angular change. As the distance increases, there is a monotonic decrease in the shift due to the film being displaced from the gold substrate, with the shift reaching 1 % of its initial value at 370 nm. The frequency shift of the QCM data of the same system, obtained through the use of a four-layer QCM model of Kanazawa,22 is shown in Figure 13. In this Figure, the frequency shift due to the presence of the film is plotted as a function of the distance between the film and metal surface for various film viscosities. Once again, for distances less than a few nanometers, there is very little change in the QCM response. There is also a monotonic increase in the crystal frequency as the film moves farther


Langmuir, Vol. 18, No. 2, 2002 487

away from the surface of the quartz, with the shift reaching 1 % of its initial value at 340 nm. On the basis of the analysis presented above, the length scales probed by the two techniques are comparable. Both are on the order of hundreds of nanometers, and the slight differences between the length scales are not large enough to account for the observed differences in C18S desorption kinetics. However, looking more closely at the model QCM data of Figure 13, we see that the QCM response is strongly dependent on the viscoelastic properties of the film. For systems in which the film viscosity is identical to that of the solvent, the impact of the film on the quartz frequency is minimal. In contrast, at short distances between the film and the quartz and at high film viscosities, the QCM frequency shift approaches the ideal Sauerbrey value of 4.8 Hz. While the viscosity of the C18S film on the surface of the quartz immediately after desorption is unknown, the literature strongly suggests that the viscosity of thin adsorbed films can be much higher than that of bulk materials because of the surface confinement of the film.28-30 Tribological studies of thin films at interfaces suggest that the effective viscosity of the film is many orders of magnitude higher than the bulk viscosity.28,29 In addition, evaporation studies suggest that the entropy of thin lubricant layers decreases significantly as the layer thickness is decreased, lowering the mobility of the molecules and confining them to two dimensions.30 This increase in rigidity due to surface confinement is the fundamental physical phenomenon that allows the Sauerbrey equation to relate QCM frequency changes to an adsorbed mass or thickness with relative accuracy in the thin-film regime. This calculation is possible because it is assumed that the film is rigid, with physical properties similar to those of the quartz crystal itself. We expect the act of desorption to lead to an enormous decrease in the rigidity of the C18S film. This behavior would, in turn, instantaneously shift the QCM response from the high-rigidity curve in Figure 13 to one of the lower-viscosity curves. Because the concentration of C18S is extremely small, we suggest that the viscosity of the C18S-rich layer in solution quickly approaches that in the bulk solvent. Once this viscosity change happens, QCM will no longer be able to detect the presence of the material in solution. This picture of the system is consistent with

the observation that the QCM kinetic response is independent of film equilibration time, despite such dependence of the SPR kinetic data. To first order, QCM is sensitive only to the actual desorption event, and we do not expect the kinetics of sulfur-gold bond cleavage to be significantly affected by the presence, absence, or structure of physisorbed C18S molecules attached to those that are chemisorbed. In contrast, Figure 12 shows that the SPR kinetic response will be affected by the behavior of the molecules after they desorb from the gold surface. As discussed previously, SPR is sensitive to the refractive index of the material being investigated. It is likely that there will be some change in the refractive index of the C18S molecules themselves upon desorption because of the loss of molecular-level order within the system. This change does make it difficult to quantitatively interpret SPR data without knowing the refractive index precisely, as discussed above. However, these changes will be relatively small. Once the molecules are in solution, they will slowly diffuse away from the gold surface until they are no longer detected by SPR; however, Figure 12 clearly shows that they will be seen by SPR for several hundred nanometers. Under conditions in which the amount of physisorbed C18S is relatively small and the molecules desorb individually, they are expected to have diffusion coefficients on the order of 1 × 10-5 cm2/s.31 With diffusion coefficients this large, the time needed for an average molecule to diffuse 400 nm through the solution, a distance that moves it outside the window of detection for SPR, is approximately 80 µs. This diffusion time is much shorter than the time resolution of our SPR measurement, supporting the observation that for long equilibration times, SPR and QCM show identical desorption kinetics. However, under conditions in which there is a large amount of physisorbed material, the molecules may desorb as aggregates. They must break up in solution after desorbing, and we expect their diffusion away from the surface to be much slower than the diffusion rate of an average molecule. This behavior leads to film breakup and diffusion kinetics that are observable on the time scale of our measurements and explains the delayed kinetic response for C18S films with a shorter equilibration time. Pyrrole Electropolymerization Kinetics. The PPY electropolymerization kinetics measured by SPR and QCM are quite different. SPR deposition kinetics in Figure 9 show a flat baseline until 520 s, followed by a nearly linear deposition with time until the current is turned off at 720 s. After 720 s, SPR shows an increase in adsorbed mass with time that plateaus hundreds of seconds after the deposition current is stopped. In contrast, QCM deposition kinetics in Figure 9 show that the mass deposition of polypyrrole is strictly governed by electrochemical control, with the crystal frequency changing appreciably only while current is applied to the cell.32 The small decrease in QCM frequency after the polymerization reaction is stopped is caused by anions leaving the film after the polymerization potential is removed.32,33 At this point, we must once again recall that SPR and QCM rely on fundamentally different material parameters to monitor film deposition. In the case of SPR, the film refractive index is vitally important to the measurement, and any changes in this key material parameter will be

(28) Luengo, G.; Schmitt, F. J.; Hill, R.; Israelachvili, J. Macromolecules 1997, 30, 2482. (29) Dhinojwala, A.; Cai, L.; Granick, S. Langmuir 1996, 12, 4537. (30) Waltman, R. J.; Tyndall, G. W.; Pacansky, J. Langmuir 1999, 15, 6470.

(31) CRC Handbook of Chemistry and Physics, 72nd ed.; Lide, D. R., Ed.; CRC Press: Boston, 1991. (32) Wurm, D. B.; Kim, Y.-T. Langmuir 2000, 16, 4533. (33) Reynolds, J. R.; Pyo, M.; Qui, Y.-J. J. Electrochem. Soc. 1994, 141, 35.

Figure 13. Model QCM response to the presence of the thin model film in solution.

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obvious in our data. The optical properties of polypyrrole films are known to change with time as the film is deposited.34 They also vary significantly depending on the oxidation state of the film.16 These changes are likely to account for the change in the shapes of the plasmon resonance curves before and after pyrrole deposition, as shown in Figure 8, These changes in curve shapes make quantitative interpretation of our SPR data using eq 1 difficult because we should not necessarily expect a simple linear relationship between film thickness and the reflectivity measured at a constant angle to exist. However, the similar linear responses of both the SPR and the QCM kinetics in Figure 9 suggest that these changes in plasmon resonance curve shapes do not have a large impact on the measured deposition kinetics. By jointly analyzing the SPR and QCM data, we can quantitatively discuss the optical property changes of the polypyrrole film as a function of film thickness. We will assume that the QCM kinetic response provides a relatively accurate description of film deposition kinetics because its behavior is consistent with the literature and our understanding of the PPY electropolymerization process. Thus, the deviations in the SPR kinetic response must be due to evolution of the film’s optical properties after deposition. To first order, these data show that the refractive index of the film increases with time after deposition, with the changes taking place over the course of hundreds, if not thousands, of seconds. This increase in refractive index, which is due to evolution in the electronic structure of the film and may be related to the anion diffusion discussed above, causes an apparent increase in the thickness of the film with time, as measured by SPR. Unlike conditions in the ZDOL adsorption experiment, the refractive index and density of PPY at the surface are expected to be significantly different from those of bulk monomeric pyrrole because the chemical structures of the surface polymer and the bulk monomer are different. Thus, we cannot simply follow the ZDOL analysis to calculate the actual surface density and refractive index of PPY. However, we can use our combined data sets to quantify the changes in refractive index. For the purposes of these calculations, we will assume that PPY has a constant density of exactly 1.5 g/mL at the interface,17 allowing us to use the QCM data to calculate the film thickness as a function of time during the deposition process. We then use eq 1, with ∆(sin θ) obtained from the SPR data and df obtained from the QCM data, to calculate the film refractive index as a function of time. Rearranging eq 1 and solving the resulting quadratic, we obtain the following relationship between the film refractive index and the relevant system parameters:

nf )

[〈

∆(sin θr) 1 m + s + + 2 A‚df

x(

〉

∆(sin θr) m + s + A‚df

)]

1/2

2

- 4‚sm

(6)

]

(7)

where

A)

[

ms sλx-m(s - m) m + s 2‚π

2

and the other parameters are defined above. (34) Kim, Y.-T.; Collins, R. W.; Vedam, K.; Allara, D. L. J. Electrochem. Soc. 1991, 138, 3266.

Figure 14. PPY film refractive index as a function of time. Deposition takes place between 520 and 720 s, with film equilibration thereafter.

The calculated film refractive index as a function of time, obtained using eq 6, is shown in Figure 14. The range of refractive index values in the Figure (∼1.351.70) is quite reasonable and agrees with a reported value of 1.52 for this system.34 Although we do not understand all of the details of the refractive-index evolution, the general trends are noteworthy. Initially, there is a sharp increase in the refractive index as material is added to the surface, starting at 520 s. This behavior is expected because the conjugation length of the conducting polymer increases with time, leading to an increase in both the electron mobility within and the polarizability of the polymer molecules, thus increasing the refractive index. Eventually, the refractive index plateaus to a relatively constant value throughout most of the deposition process. There is a slight dip in the refractive index just before the polymerization is stopped, which we cannot explain at this time. After stopping the polymerization at 720 s, there is a systematic increase in the film refractive index with time. As discussed above, this final increase is probably due to changes in the oxidation state of the PPY film after the deposition potential is removed. Conclusions We have used a combination of surface plasmon resonance and the quartz crystal microbalance with electrochemical control to monitor in situ the interfacial behavior of three types of thin films. Our results illustrate the power of this combination of techniques, allowing us to more fully understand these systems and to calculate material properties that would be inaccessible using either SPR or QCM independently. The ability to monitor interfacial phenomena using two fundamentally different measurement techniques greatly increases the amount of information known about the system, aids in data interpretation, and illustrates weaknesses in the assumptions inherent in data analysis. The density and refractive index values of the perfluoropolyether lubricant Fomblin ZDOL adsorbed on a silver surface are much smaller than the bulk values. These observations are readily explained in the context of the accepted loop, train, and tail model for polymer adsorption. While our results aid our understanding of the conformation of this system at the interface, they also illustrate the dangers associated with using bulk-material properties to analyze thin-film data.


Our C18S desorption kinetic measurements support the assertion that the conformation of a thiol film formed in acetonitrile evolves over the course of hundreds of minutes. In addition, the differences in the kinetic traces obtained by SPR and QCM for different equilibration times have prompted us to look more closely at changes in the physical properties of the C18S layer upon desorption. These changes suggest that the viscoelastic properties of the film change dramatically as it desorbs from the surface and that it is important to use accurate material-property values if the SPR and QCM measurements are to be interpreted quantitatively. Finally, our pyrrole electropolymerization data show that SPR and QCM measure dramatically different deposition kinetics. Taken together, we are able to use the joint SPR-QCM measurement to calculate the refractive index of the PPY film throughout and after polymerization to show that there is a pronounced increase in this parameter both as a function of film thickness

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during polymerization and as a function of the film oxidation state after polymerization. Acknowledgment. We thank Bernhard Menges of the Max Planck Institute for Polymer Research for supplying the grated quartz surfaces and for assistance in interpretation of the plasmon resonance data, Heraldo Botelho of Stanford University for programming the QCM data acquisition software, and Professor John Reynolds of the University of Florida at Gainesville for assistance with the polypyrrole system. Financial support for L.E.B. by the National Science Foundation through its Graduate Research Fellowship program and Stanford University through its Stanford Graduate Fellowship program is also greatly appreciated. This work was partially supported by the NSF-MRSEC program through the Center on Polymer Interfaces and Macromolecular Assemblies (CPIMA) under DMR 9808677. LA0112716

Using Surface Plasmon Resonance and the Quartz Crystal

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