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Langmuir 1991, 7, 1563-1566
Quantitative Analysis of Monolayer Composition by Sum-Frequency Vibrational Spectroscopy Colin D. Bain,' Paul B. Davies, T. Hui Ong, and Robert N. Ward Department of Chemistry, Cambridge University, Lensfield Road, Cambridge, U.K.
Murray A. Brown Unilever Research, Port Sunlight Laboratory, Bebington, Wirral, Merseyside, U.K. Received March 29, 1991. In Final Form: May 23, 1991 Infrared-visible sum-frequencygeneration has been used to obtain vibrational spectra of mixed monolayers of octadecanethiol and octadecanethiol-ds7adsorbed on gold and silver substrates. An analytical model is presented for deriving peak parameters from the sum-frequencyspectra. The derived intensity parameters for the symmetric and asymmetric methyl stretches were found to be directly proportional to the number density of octadecanethiol molecules in the monolayer, in agreement with theory.
Introduction Infrared-visible sum-frequency generation (SFG) is a novel, nonlinearoptical technique for obtainingvibrational spectra of molecules at interfaces.' In SFG, a fixed frequency, pulsed visible laser and a tunable, pulsed infrared laser are superimposed at an interface, and light emitted at the sum of the two incident frequencies is detected. When the infrared laser is in resonance with a vibrational mode that is both infrared and Raman active, a change in the intensity of the sum-frequency signal is observed. In sum-frequencyspectra complexpeak shapes often arise due to interference between the sum-frequency emission from the adsorbed molecules and the substrate (Figure 1).2 The frequency, intensity, and line width of a peak observed in a sum-frequency spectrum are not simply related to the intensity, frequency, and line width of the correspondingvibrational mode.3 Nevertheless,the complex lineshapes in sum-frequency spectra can be modeled theoretically. In this paper we exploit selfassembled monolayers to demonstrate that an intensity parameter derivable from sum-frequencyspectrais directly proportional to the number of molecules at an interface. Sum-frequency spectroscopy can thus be used for quantitative analysis of adsorbed films. The technique is particularly suited to in situ measurement of, for example, adsorption isotherms or the composition of films of mixed surfactants. Long-chain alkanethiols adsorb spontaneously from solution onto evaporated films of gold and ~ i l v e r .The ~ resulting monolayers are densely packed and highly oriented, with an average tilt of the hydrocarbon chains of - 2 7 O and -12O from the normal on gold and silver, respectively.6 Harris et studied monolayers of octadecanethiol on gold by SFG and detected three C-H stretching modes of the methyl group in the region 28W 3000 cm-l. By coadsorbing octadecanethiol-d3.r (ODTd37) and octadecanethiol (ODT) from solution, one can (1) Shen, Y. R. Nature 1989,337,519-525. (2) Miragliotta, J.; Polizotti, R. S.; Rabinowitz, P.; Cameron, S. D.; Hall, R. B. Chem. Phys. ISSO, 143, 123-130. (3) The same observation hold true in second-harmonic generation: In the presence of a nonresonant background one cannot assume a quadratic relationship between second-harmonic intensity and the concentration of moleculea at an interface. (4) Bain, C. D.; Whitaides, G. M. Angew. Chem., Znt. Ed. Engl. 1989, 28,606-512. (5) Laibinis, P. E.; Whiteaides, G. M.; Nuzzo, R. G.; Allara, D. L.; Tao, Y.-T.; Parikh, A. N. J. Am. Chem. SOC.,in preee. (6) Harris,A. L.; Chidsey, C. E. D.; Levinos, N. J.; Loiacono, D. N. Chem. Phys. Lett. 1987,141, 350-356.
0743-7463/91/ 2407-1563SQ2.5Q/O
control the number density of CH3 groups in a monolayer without causing significantperturbations in its room-temperature structure. If we assume that isotopic substitution has no effect on the process of monolayer formation, we can relate the number density of CH3 groups in the monolayer to the intensity of peaks in the sum-frequencyspectra and compare the results with the theoretical prediction. The validity of this assumption is discussed later.
Theory Sum-frequency emission arises from the nonlinear induced in polarization at the sum-frequency, P(2)(u,u,), the surface layer by the electric fields of the two lasers7
P (2)(08,)
= x ( ~ ) ( ~ , , = ~ ~ + ~ ~ ) (1) :E~E~
x ( ~is ) the second-order nonlinear susceptibility of the surface layer, and E~ and EIRare the local electric fields. This oscillating polarization emits radiation at wau, with an intensity proportional to I X ( ~ ) I ~ . x ( ~contains ) contributions from both the metal-monolayer interface and the monolayer itself. The nonlinear susceptibility of the substrate8 can be characterized by a frequency-independent magnitude and phase, Ixgleie.g For molecules that do not absorb light at the sumfrequency (such as the hydrocarbon films considered in this paper), the molecular nonlinear response, xg), is negligible unless the frequency of the infrared radiation is in resonance with a vibration of the molecule. Perturbation theory' yields the following expression for '!x
where w, is the vibrational frequency and r, is the transverse relaxation rate, which describes the homogeneous broadening of the vibrational resonance. (7) Shen, Y. R. Principles of Nonlinear Optics; Wiley: New York, 1984. (8) Guyot-Sionnest,P.; Chen, W.; Shen, Y. R. Phys. Rev. B Condenu. Matter 1986,33,8254-8263. Over a large frequency range, or in spectral regions containing substrate absorptions, x(') will vary with frequency. (9) Evaporatedf i i of Au and Ag on silico%ave a strong (111)texture (ref 5). x& of (111) surfacea is azimuthally anisotropic with the form x(') = a + c(a)co8 34, where a and c(s)are complex parameters (Sipe, J. fioee, D. J.; van Driel, H.M. Phys. Rev. B Condens. Matter 1987, 35,1124-1141). In our experimente,the (111) domaina are believed to be randomly oriented and much smaller than both the irradiated area and the coherence length of lasers. Consequently,only the inotropic part of x& will contribute to the observed sum-frequencyspectra.
0 1991 American Chemical Societv
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1564 Langmuir, Vol. 7,No. 8, 1991
incorporate inhomogeneous broadening, we assume a Gaussiandistribution of vibrational frequencies,w,, about a central frequency w,O, with standard deviation u,
This choice of distribution is arbitrary but results in a tractable functional form that is similar to the Voigt profile used in modeling the lineshapes of high-resolutionspectra of small molecules.12 Inclusion of inhomogeneousbroadening did not produce a significant improvement in the fit to the SF spectra from substrates with c 7r/2 (e.g. Au) but did improve the fit around w = w, in spectra obtained from ~i1ver.l~ With the inclusion of inhomogeneous broadening, eq 4 can be expressed in the form
-
21x&lBu( --$)1’2[Re(w,(z))
sin t - Im(w,(z)jcos e] (7)
where 2850
2900
2950
Infrared Wavenumber (cm-’) Figure 1. Representative lineshapes in sum-frequency spectra. = 1cm-1, and I’= 3 cm-1. For each curve, o,= 2900 cm-’, JB/xNR[ The phase of the nonlinear susceptibility is (a) c = r/2, (b) t = 0, and (c) c = -r/4.
The intensity factor, B,,can be written as the product of the infrared transition moment, A, and the Raman transition moment, M,averaged over the orientations of the molecules in the monolayer
On reflective substrates the zzz component of B and x can be determined most readily. We willomit thezzz subscript henceforth for clarity. The orientational average in eq 3 will be different for monolayersadsorbed on gold and silver. In mixed monolayers of deuterated and nondeuterated thiols on the same metal, however, the term within the summation will be constant for a given C-H vibrational mode. B, is then directly proportional to the number, N, of nondeuterated molecules per unit area in the monolayer. While this linear relationship has generally been assumed,1° it has not been verified experimentally. The lineshapes observed in sum-frequency spectra are determined by the relative magnitude and phase of the nonresonant and resonant susceptibilities”
Where two or more resonances are close together in frequency, additional cross-terms have to be considered (5) Y
The effect of vibrational dephasing and relaxation on the line width is incorporated through r in eq 2. To (10) Guyot-Sionnest,P.;Superfine, R.;Hunt, J. H.; Shen,Y.R.C h m .
Phys. Lett. 1988, 144, 1-5.
(11) Our approach parallels that of Hall (Miagliotta,J.; Polhotti, R. S.; Rabinowitz, P.;Cameron, 5.D.; Hall, R.B. Appl. Ph 8. A 1990,51, 221-2251 and Guyot-Sionneat(Guyot-Sionnest,P.;Tadjdine, A. Chem. Phys. Lett. 1990,172,341-346) with the exception that we include in-
homogeneous broadening explicitly in our model.
and w,(z) is the complex error function defined by
Extension to multiple peaks is accomplished by replacing w,(d in eq 7 by CYWAZ). The proportionality constant in eq 7 is difficult to determine accurately,as it depends on the overlapintegral of the electric fields of the two lasers, the linear and nonlinear Fresnel factors of the surface, and the efficiency of the detectors. The substrate signal, however, provides us with an internal reference: for a given sample, the ratio &/XNR is independent of all these instrumental factors-only the polarization and angles of incidence of is reproducible the two laser beams need be fiied.14 B,/XNR to within a few percent from sample to sample16and from day to day and should be reproducibleamong laboratories.
Experimental Section Spectrometer. Tunable IR radiation was generated by difference-frequencymixing in LiNbOs (Spectra-Physics, WEX1) of the NdYAG fundamental at 1064 nm (Spectra-Physics, DCR-3,8 ns, 10 Hz)with the output of a near-infrared dye laser (Spectra-Physics, PDL-2) pumped by the frequency-doubled output of the NdYAG laser. The long wavelength cutoff of the LiNbOs crystal (4 pm) does not permit us to obtain spectra of the CDI,modes. A fraction of the frequency-doubled NdYAG output at 532 nm was used as the visible beam. The two laser beams were incident on the sample in a counterpropagating (12) T h e line widths of the methyl modes in the infrared spectra of crystalline alkanes are dominated by homogeneous broadeningand are therefore cloeer to a Lorentzian than a Gawian form (MacPhail,R. A.; Snyder, R. G.; Straw, H. L.J. Chem. Phys. 1982,77,1118-1137). Inhomogeneous broadeningdue to differencen in the local environment is generallytaken to be cloeer to a Gaussianthan a Lorentzian form (Gadzuk, J. W.; Luntz, A. C. Surf. Sci. 1984,144,429-450). We do not believe that the detailed form of the inhomogeneous broadening w i l l have a significant effect on the principal conclusione in thin paper. (13) For e # +*/2, the peak profde far from remnauce is determined by e and B,, independentof r and u. Near resonance, the peak-&trough intensity and separation can be fit with the two parameters r and u. (14) BJxm ha^ the same unite as w. and r. (15) For monolayers of ODT on Au the standard deviation in B/xm a
2%.
Langmuir, Vol. 7, No.8, 1991 1566
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1 Au
-
h
"
I
A
I
I
I
I
I
x
aC
3 10% \ 0%
Infrared Wavenumber (cm-' )
Figure 2. Sum-frequency spectra of mixed monolayers of ODT
/,.
and ODT-dm on gold (upper figure) and silver (lower figure). The intensity of the sum-frequency signal is plotted against the wavenumber of the infrared laser. For each spectrumthe percent fraction of ODT in the adsorption solution is shown. The spectra have been offset vertically for clarity of presentation. geometry, with incident angles of 6 5 O for the infrared beam and 60° for the green beam. The respective powers were typically 600 pJ/pulse at 3000 cm-1 and 1-1.5 mJ/pulse at 532 nm (below the measured damage threshold of each sample). The infrared beam was slightly focused onto the surface to give an area of overlap of the two beams of -1 mm2. The mid-infrared laser was calibrated to h0.5cm-' by photoacoustic spectroscopy of the methane US fundamental band. The observed line width of the laser was 98%) was available from previous studies. Octadecanethiol-dmwas synthesized by P. Laibinii (Harvard University). Adsorption solutions were prepared by dissolving the two thiols, in varying ratios, in absolute ethanol to a total concentration of 0.5 mM.lB Random errors in the composition of the solutions lie within the data points in Figure 3. Preparation of Samplee. Metal substrates were prepared by evaporating -2000 A of gold or silver onto polished, singlecrystal, silicon (100)wafers (MEMC) which had been cut into 1 cm X 3 cm slides and precleaned with "piranha" eolution.1' For the gold mirror substrates, a thin layer of chromium was evaporated onto the silicon wafers to act as an adhesion promoter. The slides were removed from the evaporator and immersed immediately in the adsorption solutions. After adsorption of the monolayer was complete, the samples were removed from (16) For details, we Bain, C. D.; Evall, J.; Whiteaides, C. M. J. Am. Chem. Soe. 1989,111,7165-7164. (17) Bain, C. D.; Troughton, E. B.; Tao, Y.-T.; Evall, J.; Whitesid-, C. M.J. Am. Chem. SOC.1989,111,321-336.
1/87
0.
0
I
2 0 4 0
I
1
1
8 0 8 0 1 0 0
'YOODT in solution
Figure 3. Intensity of sum-frequency peaks normalized to the nonresonant background JB/xmI(cm-9. The abscissa represents the percent fraction of ODT in the ODTIODT-dn mixtures in solution. Key: (a) r+, Au; (b) rl-, Au; (c) r+, Ag; (d) rl-, Ag. The size of the data symbols is an estimate of the random errors in data acquisition and curve fitting at the la level. solution, washed with absolute ethanol, and blown dry with nitrogen. All samples were autophobic; i.e. the adsorptionsolvent retracted from the surface upon removal of the sample from the adsorption solution. Sum-frequencyspectra of the samples were acquired after adsorption periods ranging from 12 h to 3 days. There were no observable differences between the spectra of samples immersed for different periods of time. Spectra were obtained on duplicate samples in a random order.
Results and Discussion Sum-frequency spectra of mixed monolayers of octadecanethiol (ODT) and deuterated octadecanethiol (ODTd37) on gold a n d silver are shown in Figure 2. The spectra of monolayers on gold are dominated by t h e three methyl stretching modes: the symmetric stretching modes, r+ at 2876 cm-I and r+FR at 2936 cm-' (this second peak arising from a Fermi resonance with an overtone of a bending mode), and t h e in-plane asymmetric stretch r,- at 2964 cm-'.
To model t h e interval 2930-2980
cm-I accurately,
1566 Langmuir, Vol. 7, No.8, 1991
Letters
the composition of the monolayer is linear in each case (R2 = 0.97 to 0.99). Close examination of Figure 3 reveals a small, but statistically significant, curvature in some of the graphs. This curvature is probably an experimental artifact due to a breakdown in our assumption that the monolayer composition is identical with our calculated solution composition. Qualitatively, we can assign the slight preference for incorporation of ODT in preference to ODTd37 to two factors. First, the sample of ODT was purer than that of ODT-d37. Second, there may be a kinetic or thermodynamic preferencefor adsorption of perprotio over perdeutero amphiphiles.ls Studies by Allara and coworkerszzof the adsorption of deuterated and nondeuterated carboxylic acids on AgzO showed a measurable preference for adsorption of the nondeuterated acid.
it is necessary to add a small negative-going peak at 2953 cm-l.18 The inclusion of this peak in the fitting program did not significantly alter the calculated intensities of the major peaks at higher and lower frequency. The spectrum of ODT-d37 on gold is flat and entirely featureless, confirming our assumption that X ( ~ ) N Ris independent of frequency over this spectral range. Sum-frequencyspectra of monolayers on silver appear markedlydifferent from those on gold due to the different value of c (- -14 on Ag, +u/2 on Au). As with monolayers on gold, three methyl stretches dominatethe spectra. Weaker features near 2850 and 2910 cm-l (probably due to methylene modes) are evident when one attempts to fit these spectra. These weak features are difficult to model accurately but, fortunately, do not materially influence the calculated intensities of the three principal methyl modes.l0 The spectrum of ODT& on silver contains a weak feature at -2940 cm-l arising from the in-plane C-H stretch20 of the CDzH impurity (estimated at -10%) in the ODT-&. The signal to noise in the spectrum of ODTd37 on gold was not sufficiently high to identify an impurity at this level. These spectra were modeled according to eq 7 and the corresponding values of BY/XNR were determined for each mode. In Figure ~,B,/xNR is plotted againstthe percentage fraction of ODT in the adsorption solution, for the r+ and ra- modes.z1 The relationship between the intensity of the sum-frequency peaks (as measured by Br/xm)and (18) Thin featam could pomibly arise fromthe outof-plane asymmetric stretch of the methyl groups. We cau estimate the dependenceof ~ ( 2 ) for the b- mode, on the Euler angles, 0 and $, of the terminal methyl group. If the CSaxin of the methyl group, in a staggered conformation, is tilted by an angle 0 from the surface normal toward the 8-carbon,and the methyl group is then rotated by an angle $ about the CSaxis,x(2)zzz a sin20 sin*$ (0.90- 0 - 1.24 sin 0 COB $). Since$ and 0 are both believed tobeleesthanr/2 (refs),thetwotermsareofoppmitesign,Consequently x(2)z,zwould be small, with the sign of ~ ( 2 dependent ) ~ ~ ~ on the details of the orientationalditribution. (19) Ifthe weak methylenefeaturea areignored,the symmetricstretch at 2876 cm-* can be fit well with a slightly smaller value of c -r/12. (20) MacPhail, R. A,; Straw, H.L.;Snyder, R. G.;Elliger, C. A. J. Phye. Chem. 1984,88,334-341.
-
Conclusion The magnitude, B,, of the resonant nonlinear susceptibility determined from sum-frequencyspectra is directly proportional to the number density of molecules at the interface. This linear relationship is in direct agreement with theory. The presence of a nonresonant background results in complex peak shapes but does not prevent the analysis of sum-frequencyspectra. In fact, XNR provides an internal reference that obviatesthe need for an external reference, such as a quartz plate, in comparing spectra obtained from different samples or from different laboratories. ~
Acknowledgment. We thank the SERC and Unilever Researchfor support,H. Osbournfor technical assistance, and G. M. Whitesidesand P. E. Laibinis for the gift of the deuterated thiol. C.D.B. is grateful to the Royal Society for a Research Fellowship.
~
(21) The intensity of the r + p mode ~ is known to be very sensitive to environment(ref 6) and ale0 overlapsthe weak impuritypeak from ODTdm. (22) AUara, D. L. Paper presented at the 94th Colloid and Surface Science Symposium, Lehigh, 1990.
