Anal. Chem. 1989, 6 1 , 386-390
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Absorption Spectrometry of Bound Monolayers on Integrated Optical Structures Dennis A. Stephens and Paul W. Bohn*
Department of Chemistry, University of Illinois at Urbana-Champaign, 1209 West California Street, Urbana, Illinois 61801
Integrated optlcal Waveguide structures were utlllzed In enhanced path length absorptlon measurements of "layers deposited on a 150-pm glass Waveguide surface. Enhancements on the order of lo3 were obtalned. A ray optics approach was utked to measure the thickness of a physrsorbed metal-free phthalocyanlneto be 14 A, whlch was In excellent agreement wlth a conventlonal absorbance measurement, recorded at the peak of the Q band, which ylelded 17 A. Linear dkhrolsm experlments lndlcate that the macrocycle Is lying flat on the wavegulde surface. A p-nltrobenzolc acid chemlsorbed monolayer on a 150-pm wavegulde was also studled. A surface density of 1.5 X 10" cm-l was measured, which agreed well wlth literature values. Llnear dlchrolsm experiments also lndlcated that the electron transltlon dlpoie moment was preferentially orlented paratlel to the plane of the wavegulde surface. An average orlentatlon of 62' f 6' wlth respect to the surface normal was calculated, again in concurrence wlth published values.
INTRODUCTION Monolayer film assemblies have gained a prominent position in modern technology. Heightened interest in these structures has been fueled by the drive toward miniaturization of form and function and by their ability to modify the chemical, optical, electronic, or mechanical properties of a surface. As a result, monolayer structures have found applications in such diverse areas such as microlithography (2), preparative monolayer chemistry (2), monolayer catalysis (3), electrical insulators (4,and biomimetic membranes (5,6). As in bulk chemical systems, however, the ability to manipulate the chemistry purposefully demands a thorough understanding of the relationship between structure and function in the monolayer. Gaining this understanding, though, presents a formidable challenge to the experimentalist due to the special nature of these ultrathin films. There are several techniques that can be used to probe these ultrathin film systems, perhaps the most useful of which are optical spetroscopic approaches. Over the past few years there has been a great deal of activity investigating how vibrational spectroscopy can be performed with the surface enhanced Raman scattering effect (7, 8) and with the reflection-absorption (9,10) and attenuated total internal reflection (11-14) infrared techniques. Although at present these experiments are limited to a select few surfaces, for these materials the surface vibrational spectroscopies are powerful tools for the investigation of surface molecular structure. In addition there have recently been successful efforts to remove the materials restrictions for surface Raman scattering (15, 16). Surface plasmon (17,18) experiments have also been utilized to study monolayers on metallic surfaces. However, these techniques are only applicable to monolayers that are bound to metallic surfaces and, as a result, have limited applicability. Second harmonic generation (SHG) is particularly well suited to study interfaces between two centrosymmetric media (19, 20). 0003-2700/89/0361-0386$01.50/0
Because the SHG process is not electric-dipole-allowed in centrosymmetric media, and the symmetry is naturally broken at the interface, the large majority of the SHG signal comes from within a few atomic layers of the interface. The drawback is that the SHG signal is related to the hyperpolarizability of interfacial molecules. As a result the experiment can provide structural information only to the extent that the details of the hyperpolarizability tensor are known. The application of UV-visible absorption spectrometry to monolayer systems has been limited to cases in which the monolayer was highly absorbing. Multilayers of more weakly absorbing molecules are required to generate observable signals (21). In both cases, however, the techniques are limited in scope. Consider for example a typical strong absorber with a solid-state optical absorption coefficient of lo4 cm-'. A 5-a monolayer would generate an absorbance of only 5 x below the detection limit of standard absorption spectrometers. Classical approaches to increasing the measured absorbance involve either chemically transforming the chromophore or increasing the path length. The former approach, which is satisfactory in bulk chemical analyses, is undesirable in monolayer films, because the properties of the system being probed are altered, obviating the possibility of obtaining structural information and relating it to function. The current work focuses on utilizing an integrated optical sampling geometry in which the sample of interest is bound to the surface of a ca. 150 pm thick symmetric-slab dielectric optical waveguide. This sample geometry offers the advantage of path length enhancement, because the radiation propagates parallel, rather than perpendicular, to the plane of the film. The waveguide also allows one to control the polarization of the incident beam, making it easy to perform linear dichroism experiments in order to probe the orientation of the electronic transition moment of the molecule. Both physisorbed metal-free phthalocyanine and chemisorbed p-nitrobenzoic acid (PNBA) monolayer systems deposited on 150-pm glass waveguides were investigated. THEORY The attenuation as radiation propagates through the waveguide is observed to have an exponential decay that is described by I = I, exp(-aTx) (1) where I is the intensity of the radiation at a given distance, x , from the coupling element, I , is the intensity coupled into the waveguide, and aT is the total loss coefficient, which chracterizes the attenuation suffered as radiation propagates in the guide. The total loss can be expressed as the sum of the following three contributions (22): aT = av, a,, LYA (2) a,, and assrepresent scattering from the bulk and surface respectively, while cyA is due to absorption. The absorption can have contributions from both the waveguide and monolayer overlayer being probed. There are a variety of experimental approaches for determining the total loss coefficient of the waveguide (22-33). The key requirement is that the
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ANALYTICAL CHEMISTRY, VOL. 61, NO. 5, MARCH 1, 1989 387
and
e[(i + n324)sin2 e - ns12] (1- n312)[(1+ n3l2) sin2 e - n312]
4n2,d cos dell = Figure 1. Schematic diagram of how the waveguide samples were
covered with monolayer, top view. The shaded region indicates the region that was covered with the analyte monolayer. Either coupling position could be accessed simply by translating the sample. absorption of the monolayer must be separated from the other attenuation processes contributing to the total loss. Experimental separation of the various contributions was first accomplished by using measurements at a variety of wavelengths and eigenmodes (22). The current experiments utilize a variation of an approach first proposed by Swalen et al. (34) employing the sample geometry diagrammed in Figure 1. Half of the waveguide is covered with the sample monolayer, while the other half is used as a blank region. The total loss coefficients are determined for each region separately, and the attenuation due to the absorption of the monolayer is simply attributed to the difference between the two values. This approach is valid, if there is no spatial variation in the intrinsic waveguide loss a t different physical locations within the waveguide. Once the attenuation due to the overlayer is known, the treatment of the data can be perpetuated along two different lines: calculation of the overlayer thickness, or calculation of the surface coverage. Both treatments are based upon the ray optics model of the optical waveguide. Calculation of the overlayer thickness is based on Harrick's treatment of a total internal reflection at an interface covered with a very thin film, and it requires knowledge of the refractive index of the overlayer (35). This approach characterizes the absorption of electromagnetic radiation polarized parallel and perpendicular to the plane of incidence by a phenomenological parameter, the effective thickness, de. The effective thickness is given as a constant times the geometric thickness and takes into account the angle of incidence and the electric field intensity at the interface. When the films are much thinner than a penetration depth of the electric field, as is the case for these samples, the field can be assumed to be constant over the f i i thickness, and the effective thickness is then given by de = n21E02d/cos8
(3)
where 0 is the angle that the incident beam makes with respect to the surface normal, and Eo is the incident electric field. The monolayer thickness is given by d, and nij = ni/njis the ratio of refractive indices where n2 and nl are the refractive indices of the monolayer and waveguide, respectively. The electric field amplitudes for the transverse electric and transverse magnetic polarizations respectively are given by
(4) and
n3 is the refractive of the superstrate which; in this application, is air. It is interesting to note that the electric fields are controlled more nearly by media 1 and 3 than by media 1and 2. The effective path length for a single total internal reflection can be written as follows by substituting eq 4 and 5 into eq 3: 4nzld cos 8
(7)
Thus, if the incident angle of the radiation and the waveguide thickness are known, the absorption a t a single total internal reflection can be ascertained. A measured value for the optical absorption coefficient can then be used to calculate de, or dell, and the overlayer thickness back-calculated, if the effective index (nz)is known, by use of either eq 6 or 7. These equations are valid only if the sample is weakly absorbing (36). If this is not the case, the complex portion of the index of refraction must be included in the derivation of de, and dell(37). Alternatively de can be obtained from a Taylor series expansion of the Fresnel equation in the absorption index (38). This approach is valid for highly absorbing samples when enough terms are included in the series expansion. The other approach involves calculation of a surface coverage. This is accomplished by calculating an effective concentration from the experimentally measured value of de, (or de,,)and the molar extinction coefficient e. The coverage can then be calculated by using an estimated thickness. This can be approximated by assuming that the molecule is a linear rod with an average orientation of 4 with respect to the surface normal. The length of the linear rod, L, can be approximated by the geometric summingof atomic radii, and the estimated thickness is simply L sin ( r / 2 - 4). This treatment can also work in reverse in that it allows one to calculate an average molecular orientation from the experimental value of de, if an independent measurement of surface coverage exists.
EXPERIMENTAL SECTION The basic waveguide spectrometer is similar to that employed by Swalen and cc-workers (34) and has been previously described elsewhere (39).There are, however, some minor variations to the basic setup. Two different collection fibers were utilized for the experiments. A 1-mm polymer fiber (Hewlett-Packard HFBR3500) was used for the visible experiments. This fiber had excellent transmission characteristics in the wavelength region where the loss coefficientswere obtained (A 817 nm), along with having minimal fiber fluorescence, which would interfere with the measurement. A 1-mm silica fiber with a soft polysiloxane cladding (General Fiber Optics) was utilized for the UV experiments. The total loss coefficients, aT, were determined by monitoring the intensity of fluorescence from the glass waveguide as a function of distance from the coupling spot. For the visible experiments, fluorescence centered at 817 nm was monitored, while fluorescence at 534 nm was monitored for the UV experiments. Conventional absorbance measurements were recorded with a Hewlett-Packard 8450A diode array spectrometer using 1-cm quartz cells (J & S Scientific). The waveguides were BK-7 glass cover slips (Corning) that were 24 mm X 60 mm X 150 pm. The cover slips were cleaned by using an oxidizing etch procedure. It consisted of flit dipping the slides into three successive hot (-170 OC) concentrated H2S04baths for 3 min each, followed by rinsing with distilled, deionized HzO. The slides were then dipped into a fresh 5050 solution of 30% H202:concentratedNHIOH (-50 "C) for 3 min. The slides were then rinsed and stored in distilled, deionized H20. The waveguides were dried with 0.22 wm filtered N2just prior to monolayer deposition. A 45O-45O-9Oo LaSF-9 leaded glass prism (Precision Optical) was utilized as the coupling element. The waveguide samples were suspended between two 1 X 1 in. standard microscope slides to give enough rigidity to support the clamping of the coupling element to the surface of the waveguide. It was important to pay careful attention to the mechanical support assembly in order to avoid introducing inhomogeneous strain in the thin glass waveguide. Such strain was immediately evident by the curved beam path in the guide. Metal-free phthalocyanine (Aldrich) was utilized as the physisorbed sample. The porphyrin was deposited on the waveguide
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Table I. Comparison of Loss Coefficients at Different Positions and Wavelengths A,,
change in coupling position
C Y T ( ~dB/cm ),
95% CI,” dB/cm
( Y T ( ~dB/cm ),
95% CI, dB/cm
1.81 4.27
0.39 1.42
i0.06 f0.10
0.49 1.32
*os2
W(1)
i0.06
aq1)
514.5 363.8 a
conclusion based on null hypothesis test = aT(2) = aT(1)
Confidence interval.
Table 11. Monolayer Absorption Data 95% CI,”
polarization
aT(samp),dB/cm
dB/cm
aT(blank),dB/cm
dB/cm
phthalocyanine
TE TM TE TM
1.57 0.83 2.58 1.65
*0.13 i0.14 f0.22 i0.41
0.68 0.79 1.79 1.37
10.11 *0,12 *0,10 f0.32
PNBA a
95% CI,
analyte
conclusion based on null hypothesis test ablauk
< ample
abknk
= %ample
ablauk
< aenmple
abbt
= ample
Confidence interval.
by using sublimation in a bell jar (NRC 3117) equipped with a digital thickness monitor (Sloan). The phthalocyanine was deposited by using a ceramic boat that sat in a tungsten boat (Balzers). A current of 100 A was necessary for sublimation, and a gradual preheating of the boat, before deposition (- 10 A/10 min), was required in order to prevent macroscopic particles of the porphyrin from being transported to the sample. Samples were prepared having mass thicknesses in the range 10-100 A. A spatial mask was utilized in order to coat the sample as depicted in Figure 1. The chemisorbed samples were prepared by first heating the waveguide samples to 150 “C to drive off any excess water that was bound to the surface. The samples were then modified by partially submerging them in a 1mM solution of PNBA (Aldrich) in absolute ethanol for 2 min. The waveguides were dipped in such a fashion that only a portion of the guide was modified, as diagramed in Figure 1. Samples were then removed from solution, rinsed thoroughly with ethanol, and dried with 0.22 Nm filtered dried NB. Previous work has established that this procedure produces a close-packed film exactly 1 monolayer thick.
RESULTS AND DISCUSSION Ray Optics Treatment. Initial experiments focused on checking the legitimacy of treating these cover-slipwaveguides with the relatively simple ray optics approach. This was accomplished by simply monitoring the intensity of Raman scattering from the asymmetric stretching Si-0 band (Av 1000 cm-’) as the coupling angle was varied. The results are shown in Figure 2. It is clear that the closely spaced peaks are resonant eigenmodes of the waveguide. The modes however are so closely spaced that they form a continuum. Although the efficiency changes with angle, as shown in the figure, radiation is free to propagate at any arbitrary angle. The overall decrease in the intensity as the incoupling angle is increased is a result of an overall decrease in coupling efficiency, which is probably due to a slight translation of the coupling spot. These data suggest that indeed these waveguides are sufficiently thick that the ray optics treatment is valid. In fact they have characteristics of both the discrete integrated optical waveguide and a continuous total internal reflection element. Blank Loss Measurements. The assumption that the nonmonolayer absorption contributions to the total loss are spatially invariant was checked by measuring the total loss at different coupling spots on an unmodified waveguide. Values from different physical locations were then compared. A null hypothesis that the losses at all locations were identical was tested in conjunction with a Student’s t distribution table. Typical results are summarized in Table I for both visible and ultraviolet excitation. The measured loss in the visible is quite small, as evidenced by the relatively large confidence interval (--f20% of the value a t 95% confidence level). The blank
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‘000
900
/
I
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,
400
‘0
1
I
1
‘2
‘it
‘6
I U C O U W N C ANGLE
20
-
Figure 2. Intensity of the Si-0 asymmetric stretching band (A2 1000 cm-’) as a function of incoupling angle, with respect to the incoupling prism’s hypotenuse. The peaks represent resonant coupling into eigenmodes of the waveguide.
loss is larger for UV excitation as a result of the increase in the elastic scattering contribution due to surface roughness and refractive index inhomogeneities within the film. In both instances the null hypothesis test indicated that the variation in the loss was statistically insignificant at the 95% confidence level, leading to the conclusion that the contribution to the total loss from the glass waveguide is spatially invariant. As a result, the approach of measuring the loss in the blank and sample regions and attributing the difference to the monolayer absorbance is indeed legitimate. Physisorbed Samples. The absorption spectrum of a 1300-A film of phthalocyanine that was deposited on glass is shown in Figure 3. The Soret band centered at 330 nm has a* transitions (40). been attributed to two degenerate a The Q band centered at 620 nm has also been attributed to aa r* transition of the phthalocyanine macrocycle (41). The low-energy shoulder of Q band has been variously explained as a second-order a T* transition, as an exciton peak (42),as a vibration interval (43),and as a surface state (44). The shape of the Q band in Figure 3 is consistent with the published spectrum of the a-polymorphic crystalline form (40). It is clear from the spectrum that at X = 514.5 nm the absorption is weak, which indicates that eq 6 and 7 are valid for the analysis of the data. The phthalocyanine data are summarized in the top half of Table 11. It can be seen that there is a significant increase in the total attenuation upon going from blank to sample region for the transverse electric (TE)polarization, while there
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I . 80 I . 60 1. 40
1.20
1.0
0. 80 0.60
0. 40 0.20
0.0 0
R
a m
0
3
0 m
8
0
R
P
8
0 m
E?
a .t
a
8
WAVELENGTH (nm)
Figure 3. Absorption spectrum of a 1300 A thick film of metal-free phthalocyanine on a glass waveguide.
-Oa50 -1.0
.
I
1
WAVELENGTH (nn)
Flgure 4. Absorption spectrum of 1 m M solution of PNBA In absolute ethanol.
is no significant difference for the transverse magnetic (TM) polarization. This is consistent with the notion that the electric transition dipole moment is preferentially oriented parallel to the plane of the surface of the waveguide, since this is the plane in which the electric vector of TE radiation is contained. This supports the notion that the macrocycle of the molecule is lying flat on the waveguide surface. The T E data was used in conjunction with the measured value of the optical absorption coefficient for the solid phthalocyanine of (~(514.5 nm) = 9120 cm-', a refractive index n2(514.5nm) = 1.42 (45), and eq 6 to calculate a film thickness of 14 f 2 A (95% confidence interval). This compares to a conventional absorption measurement made at the peak of the Q band, which yields a thickness of 17 A for the same sample. The agreement between the two techniques is excellent, considering that the absolute discrepancy is less than a monolayer. The small difference between the two mea-
surements can be attributed to two factors. First, the diode array spectrometer samples a different area than the waveguide experiment, and the discrepancy may simply be due to an actual difference in the mean thickness of the phthalocyanine film. Another possibility is that elastic scattering centers in the waveguide may partially depolarize the incident beam, converting some of the incident T E radiation into nonabsorbed TM light. Thus, a portion of the incident radiation would not be absorbed due to the orientation of the electric dipole transition moment, and the measured absorption would be lowered. A better measurement of the TM polarized absorbance would have been possible by operating closer to the critical angle. However, at these smaller angles various optical interference problems prevented good measurements from being obtained. It is anticipated that this problem can be overcome by switching to integral grating couplers.
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Chemisorbed Samples. The absorption spetrum of a 1.06 mM solution of PNBA in ethanol is shown in Figure 4. The absorption at the UV laser wavelength of 363.8 nm was measured to yield an absorption cross section for PNBA of ~(363.8)= 4.80 X cm2. This value can be used in conjunction with the film density and molecular weight to calculate a solid absorption coefficient of a(363.8 nm) = 2786 cm-I. The results of the experiment are summarized in the bottom half of Table 11. It is clear that there is a significant T E absorption, while T M polarization does not exhibit any appreciable absorption. Again, this is consistent with the notion that the electric transition dipole moment is preferentially orientated parallel to the plane of the waveguide surface. This agrees with published second harmonic data on this same system, which indicate that the hyperpolarizability ellipsoid of the molecule is oriented at an angle of 70' f 3" with respect to the surface normal (46). Since the ,@ ,, tensor component dominates, and since the electronic transition moment lies along the same axis, it is reasonable to predict that linear dichroism and SHG measurements should give similar measurements of molecular orientation. The difference between the blank and sample absorbance for the T E case in Table I1 was attributed to the absorption of the chemisorbed monolayer. This difference was used with the measured optical absorption coefficient and absorption cross section to calculate an effective concentration of 5.82 x loz1 molecules ~ m - ~In. order to calculate the surface coverage of the PNBA, an estimate of the thickness of the monolayer was needed. The length of the PNBA molecule was approximated by first estimating the length of the individual bonds in the molecule by summing atomic radii. Geometric addition of the bond lengths gave a total length of 7.3 A. The thickness of the monolayer was simply calculated as 7.3 A sin (90" - 70") = 2.5 A. This allowed the calculation of the surface coverage from the effective concentration, which was determined to be 1.5 X 1014cm-2,in excellent agreement with Shen's value of 2 X 1014cm-2 determined with SHG (46). Alternatively, the experimentally determined concentration could be used in conjunction with the published surface density to calculate an average orientation of 62" f 6O with respect to the surface normal, justifying our prediction that electronic absorption and SHG measurements should yield the same orientation for the PNBA molecule. Several points about these measurements and their implications for surface structural determinations should be emphasized. First, large increases in the absorbances are observed compared to what would be observed in a traditional perpendicular geometry. For the chemisorbed PNBA the enhancement was calculated to be ca. 3.3 x lo3. Thus a monolayer of a reasonably weak absorber, like PNBA at a nonabsorbing wavelength, can easily be measured. Applying the IUPAC definition for detection limit, and the statistical data for PNBA, we estimate than an absorber with a molar absorptivity of ca. 100 M-' cm-' would yield a detectable signal for ca. half a monolayer. Of course if the path length were increased, or if the absorber were stronger, the detection limit would be correspondingly lower. We might also speculate, since the effective interaction per unit length scales as cot 8 / t , where 8 is the internal waveguiding angle and t is the waveguide thickness, that decreasing the thickness of the waveguide, thus increasing the effective interaction per unit length, would improve the detection limit. Using the same numbers, we calculate for a 1wm thick glass waveguide a detection limit of 0.003 monolayer. Of course, since inorganic waveguides of this thickness are typically composed of sputtered materials,
which have high scattering losses, the experimental problem of producing glass waveguides with small scattering efficiencies would have to be addressed. In addition to the sensitivity of the monolayer absorption experiment, polarized absorbance information can be used to obtain surface orientations of the electronic transition moments of adsorbed molecules. If the surface coverage is known from an independent measurement, as it is for PNBA, then the T E measurement alone is sufficient for calculation of the tilt angle. On the other hand, if the statistics of measuring TM absorption coefficients can be improved, then linear dichroism measurements will permit the orientation to be obtained without reference to a surface coverage.
ACKNOWLEDGMENT This work was supported by the National Science Foundation through Grant NSF/CHE 800992.
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RECEIVED for review August 25, 1988. Accepted December 5, 1988.
