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Langmuir 1996, 12, 6518-6520
Electron Attenuation Length in Rhodamine B-Arachidic Acid Langmuir-Blodgett Films by Laser Two-Photon Ionization Joon Myong Song, Takanori Inoue, and Teiichiro Ogawa* Department of Molecular Science and Technology, Kyushu University, Kasuga-shi, Fukuoka 816, Japan Received June 10, 1996X The photoelectron attenuation through layers of arachidic acid in a Langmuir-Blodgett film has been investigated by measuring the photocurrent. The photoelectrons were generated by the two-photon ionization of rhodamine B on a NESA glass substrate. The electron attenuation length was determined to be 4.2 nm for photoelectrons with a kinetic energy of 0.5-1.1 eV. The value did not depend on the applied electric field of 1.25-3.75 kV/cm.
1. Introduction Electron transfer is an important primary step during photochemical processes in solution and on a surface. It is a key parameter in determining the depth profile of solute concentration in the surface region. The attenuation length (AL) of photoelectrons with a high kinetic energy has been determined by many researchers mainly on a metal surface and its overlayer using X-ray photoelectron spectroscopy1-5 and Auger electron spectroscopy.6 However, few have measured the AL of photoelectrons with a kinetic energy of 1 eV or less. The typical AL of photoelectrons of 500-1500 eV in a long hydrocarbon chain is in the range 2-6 nm,1-5 as determined by photoelectron spectroscopy. There are a few determinations of AL for low kinetic energy photoelectrons. The AL in a condensed D2O solid made by an overlayer technique was determined to be 0.87 nm for photoelectrons with 0.1-0.5 eV of kinetic energy.7 The AL in a long-chain hydrocarbon was determined to be 1.0 nm for 0.1 eV photoelectrons and 3.0 nm for 1.5 eV photoelectrons.8 The AL in a long-chain hydrocarbon evaporated film was also reported to be 2-5 nm for 0.1-5 eV photoelectrons.9 In all these cases photoelectrons were provided by photoelectron emission from a metal plate substrate. Laser multiphoton ionization (a stepwise resonant process) is an efficient photoionization technique10 and has been applied to highly sensitive analysis on a metal surface11,12 and water surface13,14 in ambient air. In the former, photoelectrons were created not from the metal * Author to whom correspondence should be addressed. X Abstract published in Advance ACS Abstracts, December 1, 1996. (1) Brundle, C. R.; Hopster, H.; Swalen, J. D. J. Chem. Phys. 1979, 70, 5190. (2) Bain, C. D.; Whitesides, G. M. J. Phys. Chem. 1989, 93, 1670. (3) Laibinis, P. E.; Bain, C. D.; Whitesides, G. M. J. Phys. Chem. 1991, 95, 7017. (4) Schreck, M.; Schmeisser, D.; Gopel, W.; Schier, H.; Habermeier, H. U.; Roth, S.; Dulog, L. Thin Solid Films 1989, 175, 95. (5) Cave, N. G.; Cayless, R. A.; Hazell, L. B.; Kinloch, A. J. Langmuir 1990, 6, 529. (6) Palmberg, P. W. Anal. Chem. 1973, 45, 549A. (7) Jo, S. K.; White, J. M. J. Chem. Phys. 1991, 94, 5761. (8) Monjushiro, H.; Watanabe, I. Anal. Sci. 1995, 11, 797. (9) Cartier, E.; Pfluger, P. Appl. Phys. A 1987, 44, 43. (10) Ogawa, T. In Handbook of Advance Materials Testing Cheremisinoff, N. P., Cheremisinoff, P. N., Eds.; Marcel Dekker Pub.: New York, 1995. (11) Ogawa, T.; Yasuda, T.; Kawazumi, H. Anal. Chem. 1992, 64, 2615. (12) Kawazumi, H.; Yasuda, T.; Ogawa, T. Anal. Chim. Acta 1993, 283, 111.
S0743-7463(96)00572-0 CCC: $12.00
substrate but from a small amount of photoabsorbing molecules placed on it. The mechanism of photoelectron drift in ambient air was discussed elsewhere.11,13 Photoelectrons can be generated on any surface by using the laser two-photon ionization technique. Thus, the present technique offers a new method for the determination of the electron attenuation length. In the present investigation, we have determined AL in the rhodamine B-arachidic acid LB film on NESA glass using the advantage of the laser two-photon ionization. The dependence of AL on the applied voltage has also been measured. 2. Experimental Section 1. Materials. Rhodamine B with a long hydrocarbon chain (C60H95ClN2O7; [9-(o-carboxyphenyl)-6-(N-ethyl-N-octadecylamino)-3H-xanthen-3-ylidene]-N-ethyl-N-octadecylammonium perchlorate) was of special grade and was obtained from Nippon Kankoh-Shikiso Kenkyusho Co. Arachidic acid (CH3(CH2)18COOH), methanol, and toluene were of special grade and were obtained from Kishida Chemicals. These were all used as received. Water was deionized and purified using a Millipore Milli-Q system, and its pH was adjusted to 6.3. 2. LB Film Preparation. Y-type LB films were prepared using a dipping method. The substrate was sonicated in toluene and then in methanol for about 3 h. The LB trough (Kyouwa Scientific) was set in a closed housing to avoid any dust and filled with clean water of pH 6.3. The first layer was prepared by spreading a benzene solution of a 1:10 mixture of rhodamine B and arachidic acid over the water subphase in such a way that a mixed monolayer was created on it after waiting 30 min for evaporation of the benzene. The monolayer was then compressed to 30 mN/m at a rate of 20 mm/min. The first layer was prepared by dipping the substrate into the water subphase at a rate of 8 mm/min and then by withdrawing it. The second layer was prepared by dipping the substrate into the arachidic acid monolayer on the water subphase and withdrawing the substrate after removing the monolayer on the water. The three-layer sample was prepared by withdrawing the substrate without removing the monolayer on the water. One, two, or three layers of arachidic acid were placed on the mixed monolayer in this way. A schematic diagram of the LB film thus produced is shown in Figure 1a. 3. Laser Two-Photon Ionization. A schematic diagram of the experimental apparatus is shown in Figure 1b.13,14 The third harmonic (355 nm) of an Nd:YAG laser (Spectra Physics GCR11, 6 ns, 10 Hz) irradiated the LB film on the glass substrate without focusing. A mesh electrode was placed 8 mm above the substrate and was positively biased by a high-voltage power supply unit (Hamamatsu, 0.5-3 kV). The photoionization (13) Inoue, T.; Masuda, K.; Nakashima, K.; Ogawa, T. Anal. Chem. 1994, 66, 1012. (14) Chen, H.; Inoue, T.; Ogawa, T. Anal. Chem. 1994, 66, 4150.
© 1996 American Chemical Society
Rhodamine B-Arachidic Acid Langmuir-Blodgett Film
Langmuir, Vol. 12, No. 26, 1996 6519
Figure 2. Dependence of photoionization charge on the laser pulse energy. The slope of the line was 2. The concentration of rhodamine B on the glass was 1.04 × 10-9 mol/cm2. Figure 1. Schematic diagram of the LB film and the experimental apparatus. (a) The number of monolayers in a sample is indicated as 1, 2, and 3. The first layer is the rhodamine B-arachidic acid mixed monolayer (mol ratio 1:10), where a solid circle indicates a rhodamine B moiety and an open circle indicates an arachidic acid moiety. (b) The laser beam irradiated the LB film without focusing. The photoionization current was measured in ambient air. current from the substrate was amplified by a current amplifier (Keithley 427, time resolution 10 µs) and recorded on a digital storagescope (Iwatsu DSC411). The photoionization current was integrated over 32 laser pulses, and four such measurements were averaged to obtain the photoionization charge using a microcomputer (NEC9801RX). The details of experimental procedures were described elsewhere.13,14
3. Results and Discussion 1. Effect of Laser Pulse Energy. The present measurements were carried out in ambient air, while most of the previous measurements were carried out in high vacuum, because they were based on photoelectron spectroscopy. Thus the present method is simpler. In the present system, the photoelectron source should be the rhodamine B ring on the NESA glass, and the long hydrocarbon chains of rhodamine B and arachidic acid should be the only obstacle against photoelectron transmission. There was no photoionization signal without rhodamine B, and photoionization of the NESA glass substrate and arachidic acid was negligible. The photoionization signal of rhodamine B, which was spin-coated on the glass substrate, was proportional to the square of the laser pulse energy, as shown in Figure 2. This finding confirmed that rhodamine B could be photoionized in a two-photon process. Although a good quadratic proportionality held up to 3 mJ/pulse, the laser pulse energy was kept at 0.215 mJ/ pulse for further measurements in order to avoid any photodesorption and/or photodecomposition during a series of experiments. If the laser pulse energy was much larger than 0.215 mJ/pulse or if the laser beam was tightly focused, the signal gradually decayed probably because of photodesorption and/or photodecomposition of the LB film. 2. Photoionization Threshold. Rhodamine B has absorption bands at 355 nm (3.49 eV) and at around 550 nm and is resonant to a two-photon process at 355 nm and at 532 nm (2.33 eV). However, no photoionization signal was observed by irradiation at 532 nm (the second harmonic of the Nd:YAG laser). Wavelength dependence of the photoionization charge was measured using a nanosecond Ti-sapphire laser
Figure 3. Dependence of photoionization charge on the number of monolayers. The three symbols at each number of monolayers indicate results of three independent measurements. The solid line represents the best fit. The laser pulse energy was 0.215 mJ/pulse.
(Continuum, TS-60). Due to a poor signal-to-noise ratio, it was difficult to determine a definite wavelength where the two-photon ionization charge disappeared. However, photoionization charge was observed at and below 380 nm (3.26 eV), and no charge was observed at and above 420 nm (2.95 eV). Thus, the photoionization threshold of rhodamine B on the NESA glass should lie between 6.5 and 5.9 eV. The kinetic energy of photoelectrons produced by two-photon ionization at 355 nm should then be between 0.5 and 1.1 eV. The ionization potential of rhodamine B in the crystal was reported to be 5.1 eV,15 which is substantially smaller than the estimated value in the present study; a molecule in a crystal and on a NESA glass may be different. 3. Electron Attenuation Length. The dependence of photoionization charge on the number of layers of arachidic acid is shown in Figure 3. In this experiment, the one-layer sample layer, which was expressed as 1, was a 1:10 rhodamine B-arachidic acid film. Two- and three-layer samples, which were expressed as 2 and 3, had one and two layers of pure arachidic acid film. The data taken at more than four layers were too noisy due to the weak signal intensity and were excluded from the following analysis. The measurements were repeated on three LB films made independently for each number of layers, and the deviation of each value from the average was small, as shown in Figure 3. The solid line in the figure represents the best fit to the data based on a leastsquares calculation. (15) Gerischer, H.; Ranke, W. Z. Naturforsch. 1969, 24a, 463.
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Song et al.
Table 1. Electron Attenuation Length in Rhodamine B-Arachidic Acid LB Film
coverage 93% coverage 99% extrapolated value to R ) 0
observed value (nm)
after patch correction (nm)
5.90 ( 0.49 4.46 ( 0.54
4.55 4.28 4.22
Photoelectrons were created at the rhodamine ring on the NESA glass. Some of them lose their kinetic energy during interaction with the hydrocarbon chain and recombine with cations. The others can escape from the film and are detected as photocurrents. The attenuation of photoelectrons can be expressed as a function of the film thickness (t).1,3
I0 ) Ii exp(-t/λ)
(1)
I0 and It are the photoelectron intensities initially created by photoionization and at a film thickness of t, and λ is the electron attenuation length, where the initial photoelectron intensity (I0) is reduced to 1/e. λ can be determined from the slope of Figure 3. The thickness of a hydrocarbon chain was set at 2.68 nm, as Brundle et al.1 suggested. If we take it to be 2.5 nm, as Schreck et al.4 suggested, value of λ should decrease by 7%. The dependence of It versus t gave a straight line, and this finding indicates that any effect1 due to surface roughness or surface contamination was negligible. 4. Correction. An LB film may consist of patches and may have many small holes in it.1 For the determination of λ, small holes may give a serious deviation, because photoelectrons can pass through them without any interaction with the hydrocarbon chain. The present technique is, however, less susceptible to holes than a conventional method, because photoelectrons are produced at a rhodamine ring on which a hydrocarbon chain is attached. The observed photoionization charge may consist of two components: one is photoelectrons arriving through the hydrocarbon chain, and the other is those arriving through a hole in the film without attenuation. In this case the measured photoionization charge (Iph) may be expressed by the following equation:1
Iph ) RIR + (1 - R)It
(2)
where R is the fraction of the hole in the whole area on the surface (R ) 0 represents no hole), IR is the photoionization charge through holes, and It is that through the hydrocarbon chain of thickness t. We prepared two kinds of LB films by controlling the surface pressure of the arachidic acid monolayer to 30 and 22 mN. Their surface coverage was 99% and 93%, respectively: the coverage was determined as a ratio of the decrease of the water surface area to the surface area of the substrate. R of the former was set to 0.01 and that of the latter to 0.07. λ was determined to be 4.5 nm for R ) 0.01 and 5.9 nm for R ) 0.07. The final value of λ was determined to be 4.2 nm by extrapolation to R ) 0. The results are summarized in Table 1.
Figure 4. Dependence on the electric field. (a) Photoionization charge from the three different samples. (b) Attenuation length. It is almost independent of the electric field.
The present value of λ for a hydrocarbon chain is approximately equal to that obtained by Cartier and Pfluger,9 but is slightly larger than that obtained by Monjushiro and Watanabe.8 This may be due to a difference between an LB film and a self-assembled monolayer film. 5. Effect of Applied Voltage. The electron attenuation length may depend on the applied voltage, because electrons may be attracted from a deeper region at a higher applied voltage. A decrease of λ at a low kinetic energy was reported.8 If the applied voltage has an effect on the kinetic energy of photoelectrons, λ may depend on the applied voltage. Photoionization charge slightly depended on the applied voltage, as shown in Figure 4a. The slope in Figure 4a did not depend on the number of layers. Thus, the electron attenuation length changed only slightly versus the applied voltage, as shown in Figure 4b. The applied voltage had only a minor effect on the electron attenuation length in hydrocarbon films. We have shown that the laser two-photon ionization technique is another useful technique for a determination of the electron attenuation length. Acknowledgment. The authors wish to thank prof. Kazuhiko Seki of Nagoya University for his helpful comments. The present research was partially supported by a Grant-in-Aid for Scientific Research on Priority Areas (No. 07228253). LA960572Q
