4086
J. Phys. Chem. 1995, 99, 4086-4090
Vibration-Induced Order-Disorder Transitions in a Langmuir-Blodgett Film As Investigated by Vibrational Sum-Frequency Generation Spectroscopy Y. Goto, N. Akamatsu, K. Domen, and C. Hirose" Research Laboratory of Resources Utilization, Tokyo Institute of Technology, 4259 Nagatsuta, Midori-ku, Yokohama 226, Japan Received: February 23, 1994; In Final Fonn: August 23, 1994@
It was observed by vibrational sum-frequency generation (VSFG) spectroscopy that the excitation of CH stretching bands by IR pulses induced the destruction of the in-plane orientation of cadmium arachidate molecules on a nine-layer Langmuir-Blodgett (L-B) film. The resonant excitation of the antisymmetric CH stretch band, va(CH)c~*, of methylene groups by an infrared beam with an energy exceeding 100 pJ/pulse gave basically the same result as was obtained by thermally heating the sample to above 110 "C; the in-plane orientation of the alkyl frame was destroyed, but the terminal methyl groups stayed pointing outward. However, irradiation by pulses with lower energy gave results which varied significantly according to the vibrational mode excited.
Experimental Section
Introduction The occurrence of two-dimensional order in the orientation of molecules on the surface of Langmuir-Blodgett (L-B) films has been known from investigations by, for example, infrared (IR) spectro~copy,l-~recently developed vibrational sumfrequency generation (VSFG) s p e c t r o s ~ o p yand , ~ ~atomic ~ force microscopy (AFM).l0J1 As for multilayer L-B films of cadmium arachidate (CdA), the presence of the orientational order of alkyl tail groups was revealed by IR and Raman s p e c t r o ~ c o p yand , ~ ~the ~ ~occurrence ~ of the preferential inclination of terminal methyl groups on the surface was shown by the VSFG ~ t u d y . ~Thermal ,~ destruction of such orientational order was investigated by IR spectroscopy, DSC (differential scanning calorimetry),12and Raman spectro~copy,'~ and it was shown that the order-disorder transition in L-B films of CdA takes place in three steps. The first step starts within a temperature range of 45-65 "C and was ascribed to the onset of chain tilting near voids and vacancies. The second step starting at 90 "C was pictured as the onset of the conformational change of alkyl chains to introduce t-g and/or g-t-g' sequences. The final step occurred at 110 "C, which is the melting point of the film as determined by DSC,12 and was postulated as the breakup of the head group lattice. It was noted, however, that there was no indication of an annealing effect, that is, the increase of the orientational order at a premelting temperature. In this paper, we report on the investigation of a phase transition induced by the irradiation by IR pulses exciting specific CH stretching bands of methyl or methylene groups of CdA on a nine-layer L-B film. VSFG spectroscopy was used to probe the transition. The preeminent feature of the VSFG spectrum, that is the elimination of intensity change with the azimuthal angle of the sample plate closely resembled the feature observed on thermally heating the sample. It was shown for the first time that methyl groups kept sticking out and retained their tilt angle from the surface normal distributed around 4547". The "melting" of CdA did not randomize the head-to-tail orientation. A slight improvement of the orientational order was indicated when the pulse energy of the beam resonating with the methylene va(CH) band was slightly below the value needed for the destruction of two-dimensional order.
* Author to whom correspondence should be sent. @Abstractpublished in Advance ACS Abstracts, March 1, 1995.
Sample Preparation. Arachidic acid as received from Kanto Chemicals Inc. (purity, 99%) was spread from a chloroform solution onto an aqueous subphase of purified water which contained 4 x m0l-L-l CdCl2 and KHC03 to maintain a pH value of 6.84. Nine monolayers of the cadmium salt of arachidic acid were transferred onto the surfaces of quartz or CaF2 plates at a pressure of 17 m"-'.The film on the CaF2 substrate was used for the FTIR measurement. The pull-out speed was 4 mmmin-' for the first layer and between 10 and 15 mmmin-' for subsequent layers. The multilayer assembly consisted of nine Y-deposited layers with the f i s t layer in the head-on configuration, Le., the head carbonyl groups were attached to the substrate (see Figure 1). Spectroscopy. A schematic layout of the VSFG measurement is shown in Figure 2. The output beam of a mode-locked Nd:YAG laser operating at 10 Hz was used for producing both the visible and infrared pulse beams of the SFG experiment. The visible beam, abbreviated as vis beam hereafter, of 532 nm wavelength had a pulse duration time of 35 ps and a pulse energy of about 2 mT. The beam was focused onto a sample plate at an incident angle of 40" and to a beam diameter of about 0.5 mm. The frequency-tunable infrared beam, abbreviated as IR beam hereafter, was produced by optical parametric oscillatiodamplification (OPO) by lithium niobate (LiNbO3) crystals. The frequency was scanned from 2700 to 3000 cm-' in which region four CH stretch bands are located, Le., the symmetric and degenerate methyl bands, abbreviated as Y,(CH)C, and Yd(CH)CH3bands, respectively, and the symmetric and antisymmetric methylene bands, abbreviated as Y,(CH)CH~ and va(CH)c~,bands, respectively, are located at 2880, 2956, 2850, and 2920 cm-', respectively. The IR beam irradiated the sample at a incident angle of 45" and with a beam diameter of about 0.6 mm. The beam energy was typically 130, 180, 103, and 150 pJlpulse at 2880, 2956, 2850, and 2920 cm-', respectively, but occasional jumps by about +100% to -50% were seen on the oscilloscope trace monitoring the IR pulse. The jumps typically occurred on 1 out of 20 pulses, although the rate varied day to day or even time to time, and we found no formula to suppress it. Si films, KRS5 plates, and Ge plates were used to attenuate the IR power when necessary.
0022-3654/95/2099-4086$09.00/0 0 1995 American Chemical Society
J. Phys. Chem., Vol. 99, No. 12, 1995 4087
Order-Disorder Transitions in an L-B Film
Results and Discussion
Withdrawal direction 4 5"
'3-+
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Figure 1. Orientation of cadmium arachidate molecules on a freshly prepared multilayer Y-type L-B film.
The experiment was carried out using the vis and IR beams in p-polarization and detecting the p-polarized component of the SFG signal. The signal was detected by a photomultiplier tube (PMT) after elimination of stray light by a monochromator. A BOXCAR integrator was used to accumulate from 300 to 1000 pulses of PMT output, and the SFG signals were divided by the intensity of the IR pulses.
M.L. YAG 3 5 p s lOHz
1064 nm
532 nm
The linear absorption spectrum of CdA on a nine-layer L-B film, observed by transmission FTIR spectroscopy, is shown in Figure 3a, and the plots of VSFG signal intensities, observed by scanning the frequency of the IR beam, are shown in Figure 3 parts b-d. The VSFG spectra shown are the results obtained by placing the sample plate so that its direction of withdrawal from the liquid trough is at a right angle to the propagation direction of the light beams. Figure 3b is the result obtained when the frequency of the IR beam skipped the region of the Y~(CH)CH~ band to avoid the excitation of the band while Figure 3 parts c and d are the plots observed when the IR frequency was scanned continuously from higher to lower and lower to higher frequency, respectively. It was noted that the CH stretch bands of methylene groups, which dominate the FTIR spectrum, did not show up on the VSFG spectrum and that the intensity of the VSFG signal of either the Y,(CH)CH~ or the v,-~(CH)CH~ bands decreased after the IR beam excited the v,(CH)CH*band. The pre-irradiation by 1000 pulses at Y,(CH)CH*eliminated the stated decrease of signal intensity and thus the features shown by Figure 3 parts c and d. The pre-irradiation also led to the elimination of the change of signal intensity by the azimuthal angle. Figure 4 shows the IR-induced change of azimuthal angle-dependent features of the signal intensity. The observation was made by rotating the sample plate around the surface normal, and the results obtained by keeping the IR frequency fixed at the peaks of the v,(CH)CH, and the v~(CH)CH~ bands are shown in Figure 4 parts a and b, respectively. The azimuthal angle @ is defined as the angle between the substrate-projected direction of the SFG beam's propagation and the direction of sample withdrawal from the liquid trough (see inset of Figure 4a). The open and filled circles in Figure 4 represent respectively the results obtained before and after the irradiation by lo00 IR pulses tuned at the Va(CH)CH2 band. As has been reported b e f ~ r e ,the ~ , ~features shown by the open circles are due to the in-plane orientation of methyl groups and one sees from the disappearance of the @-dependent features that the in-plane orientation of the methyl groups was destroyed almost completely by the irradiation. No VSFG signal was observed at either the Ya(CH)CH2 or the v,(CH)CH*bands indicating that most of the alkyl tails retained their all-trans conformational sequence^.^,^ The conformational change as suggested in previous worksloJ1 was not significant in our experiment. We compare in Figure 5 the VSFG spectrum observed after the irradiation of 10 000 IR pulses of 150 pJ energy tuned at
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Goto et al.
4088 J. Phys. Chem., Vol. 99, No. 12, 1995
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Figure 4. Change in the angle-dependent features of the SFG signal by irradiation at the Y,(CH)CH,band. The signal intensity at the vs(CH)CH~ and Y~(CH)CH~ bands is plotted in (a) and (b), respectively, against the azimuthal angle q5 of the sample (see inset for definition). The open and filed circles represent the results obtained before and after the irradiation by lo00 pulses of an IR beam tuned at the v,(CH)CH,band. The IR pulses had 30 ps duration time and 150 pJ/ pulse energy.
Wavenumber / cm-1 FTIR and VSFG spectra observed on the nine-layer L-B films of CdA. (a) Transmission FTIR spectrum of CdA on a CaF$
Figure 3.
substrate, (b) VSFG spectrum observed when the scan of the IR beam skipped the region of the v,(CH)CH,band, (c) VSFG spectrum observed by scanning the IR beam without the skip from higher to lower frequency, and (d) VSFG spectrum obtained by scanning the IR beam without the skip from lower to higher frequency. In the VSFG experiment, vis, IR, and SFG beams were all p-polarized and their mutually common planes of incidence were perpendicular to the withdrawal direction of the sample plate. the va(CH2) band (Figure 5a) with the spectrum observed at a temperature of 125 "C (Figure 5b). The SFG signal should disappear altogether when the molecular orientation is fully isotropic, and the present result proves that the tilt angle of methyl groups was not random even after the irradiation. Both traces did not change by the rotation of the sample plate and by the direction of the frequency scan. These features closely resemble the spectrum of the one-layer L-B film on which the surface orientation of CdA molecules was found to be isotropic
and the tilt angle was 37" f 25" (Figure 5 ~ ) The . ~ relative intensities of the Y,(CH)CH~ peak with respect to the vd(CH)cH3 peak in Figure 5 parts a and b differ from that in Figure 5c. A computer s i m ~ l a t i o revealed n ~ ~ ~ ~that ~ ~the ~ feature in Figure 5 parts a and b conforms with the spectrum calculated for the methyl tops having a tilt angle of 45-47" instead of 37", and we conclude that both the thermal melting and the heating by laser irradiation of the sample caused the alkyl chains to have their tilt angle increased from 0" to around 20". The somewhat broader feature of Figure 5b might indicate the presence of the VSFG signal from the Y(CH)CH* bands at the low frequency wing. To s d a r i z e , the surface orientation of CdA molecules has been destroyed by the pulsed excitation of the Ya(CH)CH, band and also by thermal heating of substrate to 110 "C, but carbonyl heads stayed pointing toward the substrate surface. Compared in Figure 6 are the results obtained by exciting different vibrational bands. In the experiment, the frequency of the exciting IR beam was tuned to the Y,(CH)CH~, vd(CH)cHs, Y,(CH)CH*,or Y,(CH)CH,bands and the peak intensity of the VSFG signal of the vd(CH)CH3band was measured as a function of the number of irradiated pulses. The IR pulses had 130, 180, 103, and 150 pJ/pulse energy at v~(CH)CH~, vd(CH)cH3,
J. Phys. Chem., Vol. 99, No. 12, 1995 4089
Order-Disorder Transitions in an L-B Film
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squares, filled circles, and unfilled circles, respectively. The probed signal is the vd(CH)CH3.and the azimuthal angle 4 was set at 0". The pulse repetition rate was 10 Hz, and the pulse energy was 130, 180, 103, and 150 pJ at the peak frequencies of the Y~(CH)CH~, Y~(CH)CH,, Y,(CH)CH,, and Y,(CH)CH~ bands, respectively.
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(a) Spectrum observed after the sample was irradiated by 10 000 IR pulses tuned at the Y.(CH)CH,band, (b) spectrum observed after heating the sample to 125 "C, (c) spectrum observed on the monolayer sample on which the terminal methyl groups were tilted by 37" and had no two-dimensional anisotropy. Y,(CH)CH*, and Y,(CH)CH~, respectively. It is seen that the SFG signal was not affected by the excitation of methyl v(CH) bands but the excitation of methylene v(CH) bands led to a significant decrease of signal intensity, and the irradiation of 1000 pulses virtually finished the induced change. We next examined the dependence of effect of pulse energy through the excitation of the Y~(CH)CH* band. In Figure 7a the relative intensities obtained by the irradiation of 100, 500, and 1000 IR pulses with 63, 75, 100, and 150 pJ pulse energy each are plotted. Figure 7b is the result obtained by thermally heating the sample. The signal intensity showed a sharp decrease when the pulse energy increased from 75 to 100 pJ (Figure 7a) and when the substrate temperature rose from 100 to 110 "C (Figure 7b). We now combine the results shown in Figures 4-7 to assess the phenomena that took place on the excitation of CH stretching bands. The first feature to note is that the induced disorder is not due to the accumulated energy but basically the phenomenon that terminates before the excitation by the next pulse, although there exists some dependence on the number of irradiated pulses.
The feature is explained by postulating that the energy absorbed through the vibrational excitation of the Y~(CH)CH~ or Y,(CH)CH~ modes was transformed into heat and dissipated by thermal diffusion to the surroundings within 0.1 s. Actually, when we estimate the values of the thermal conductivity, the specific heat capacity, and the density of the sample film to be 0.2 Wm-'.K-', 2.0 J.g-'*K-', and 1.2 x lo6 respectively, a simple model of thermal diffusion gives a value of about 0.1 ns as the characteristic time for the diffusion of heat from the top layer to the substrate surface at 20 nm distance and a value of 0.9 s for the diffusion from the center of the circular plate with a 0.3 mm radius to its edge. The relaxation time of vibrational excitation is of the order of 10-100 ps in condensed media.16 The pulsed temperature jump, which terminates within 1 ns due to the thermal diffusion to the substrate, presumably caused the primary feature, and the subsequent 1000 pulses or 1.7 min were needed to stabilize the transverse distribution of temperature. As mentioned earlier, investigations by IR12 and Raman13 spectroscopies showed that the head group lattices of CdA molecules on L-B films break up on heating the sample to 110 "C. The features shown in Figure 7a suggest that the energy absorbed by the band from the IR pulses with 100 pJ energy caused the local temperature to jump to 110 "C, that is, the temperature jumped by 90 "C from room temperature. This value is in agreement with the one estimated from the absorbance of spectral lines (see below). The irradiation by the IR pulses with 63,75, and 150 pJ, which are the values that led to the results shown in Figure 6a, then caused the temperature to jump to 77, 88, and 155 "C, respectively. When one assumes that the surface area occupied by a CdA molecule was 0.22 nm2and thus 4.1 x 1013 molecules were present inside the irradiated area, the absorbed energy needed for the temperature jump of 90 "C is derived as 4.8 pJ by using the above-assumed value of the heat capacity. Alternately, we estimate from the difference in the FTIR spectra of 21- and 9-layer films that a 9-layer film absorbs 6.1
Goto et al.
4090 J. Phys. Chem., Vol. 99, No. 12, 1995 after 100 pulses :O after 500 pulses : A
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the irradiation at the Y,(CH)CH~ band by 100 pulses of 63 pJ energy. Although the feature was persistent at the three conditions of this experiment, the magnitude of the change was less than one-half of the estimated error bars. We thus do not go into the details of the phenomenon. Finally, it is pointed out that the changes of orientational order described in this paper were all irreversible for at least several weeks.
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Investigation by VSFG spectroscopy has been made of the effect induced by resonant excitation of CH stretch bands of methyl and methylene groups of CdA molecules in a nine-layer L-B film. The results revealed that the excitation of CH vibrational modes by IR pulses induced the disordering of the in-plane orientation of terminal methyl groups on nine-layer L-B films. The phenomenon was strongly dependent on the pulse energy of the IR beam, and the primary feature closely resembled the one observed for a thermally heated sample. The irradiation of 1000 pulses or about 100 s was needed to attain a stabilized state. The irradiation by IR pulses tuned at the Y(CH)C, band and with a pulse energy of 100 pJ was found to cause the local temperature to jump to greater than 110 "C,the melting point of the layer. The resultant layer retained the alkyl tails of CdA molecules standing outward from the substrate surface although their two-dimensional orientation on the surface was destroyed. The irradiation by the pulses having an energy less than 100pJ caused a slight disordering too as was the case observed below the melting point temperature in the thermal heating experiment. The improvement of orientational order was indicated, however, when the band was excited by 63 pJl pulse radiation.
150
Temperature / "c Figure 7. Signal intensity of the Y~(CH)CH, band at 4 = 0" as a function of pulse energy (a) and substrate temperature (b). The IR pulse was band in obtaining the tuned at the peak frequencies of the Y,(CH)CH~ result shown in (a), and the substrate was thermally heated in obtaining the result shown in (b).
ic 0.4% of the incident IR power at the Ya(CH)CHZ band. At
the incident angle of 45", 12% of the incoming beam's power is reflected at the surface and thus the irradiation by the IR beam with 100 pJ pulse energy results in the absorption of 5.4 pJ. This value is in fair agreement with the above-derived value of 4.8 pJ. We also estimate from the relative intensity of the FTIR spectrum that the amount of energy absorbed by the Y,(CH)CH,band from a IR pulse with 103 pJ energy is nearly the same as that absorbed by the Y,(CH)CH~ band from the IR pulse with an energy of 63 pJ. Thus the temperature jump should have been 57 "C in the experiment which gave plots given b y filled circles in Figure 6. We then expect the behavior of the SFG signal resulting from the excitation of the Y~(CH)CH~ band by 103 pJ pulses (filled circles in Figure 6 ) to be similar to that obtained from the excitation of the Y~(CH)CH~ band by 63 pJ pulses. However, it turned out that the result obtained by the excitation of the Y,(CH)CH,band differed from that obtained by the excitation of the Y,(CH)CH,band although the signal intensity in both cases decreased to some extent as was observed below the melting point temperature of 110 "C on the thermally heated sample. An apparently unique feature that was not observed on the thermally heated sample is seen in Figure 7a; the intensity of the SFG signal increased slightly by
Acknowledgment. We thank Dr. Y. Kawabata, Dr. T. Nakamura, and Ms. R. Asami of National Institute of Materials and Chemical Research for their assistance and expertise in preparing the sample. This work was supported by the Grantin-Aid for Scientific Research (A) by the Ministry of Education, Science and Culture of Japan, Grant No. 04403004. References and Notes (1) Koyama, Y.; Yanagishita, M.; Toda, S.; Matsuo, T. J. Solid Interface Sci. 1987, 61, 438. (2) Davies, G. H.; Yanvood, J. Spectrochim. Acta 1987, 43, 1619. (3) Jones, C. A.; Petty, M. C.; Roberts, G. G.; Davies, G.; Yarwood, J.; Ratcliff, N. M.; Barton, J. W. Thin Solid Films 1987, 155, 187. (4) Kimura, F.; Umemura, J.; Takenaka, T. Lungmuir 1986, 2, 96. (5) Chollet, P.-A.; Messier, J. Thin Solid Films 1983, 99, 197. (6) Amdt, T.; Bubeck, C. Thin Solid Films 1988, 159, 443. (7) Kuramoto, N.; Ozaki, Y. Thin Solid Films 1992, 209, 264. (8) Akamatsu, N.; Domen, K.; Hirose, C.; Onishi, T.; Shimizu, H.; Matsutani, K. Chem. Phys. Left. 1991, 209, 264. (9) Akamatsu, N.; Domen, K.; Hirose, C. J . Phys. Chem. 1993, 97, 10070. (10) Schwartz, D. K.; Viswanathan, R.; Zasadzinski, J. A. N. Langmuir 1993, 9, 1384 and references cited therein. (1 1) Schwartz, D. K.; Viswanathan, R.; Garnaes, J.; Zasadzinski, J. A. N. J . Am. Chem. SOC. 1993, 115, 7374 and references cited therein. (12) Naselli, C.; Rabolt, J. F.; Swalen, J. D. J . Chem. Phys. 1985, 82, 2136. (13) Rabe, J. P.; Swalen, J. D.; Rabolt, J. F. J . Chem. Phys. 1987, 86, 1601. (14) Hirose, C.; Akamatsu, N.; Domen, K. J . Chem. Phys. 1992, 96, 997. (15) Hirose, C.; Yamamoto, H.; Akamatsu, N.; Domen, K. J . Phys. Chem. 1993, 97, 10064. (16) Bakker, H . J.; Planken, P. C. M.; Lagendijk, A. J. Chem. Phys. 1991, 94, 6007.
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