Langmuir 1991, 7, 1473-1477
1473
Studies of Molecular Alignments of Monolayers Deposited by a Chemical Adsorption Technique K. Ogawa,**tN. Mino,? K. Nakajima,t Y. Azuma,j and T. Ohmural Central Research Laboratories, Matsushita Electric Industrial Co., Ltd., and Matsushita Technoresearch, Inc., Yagumo-Nakamachi 3-15,Moriguchi, Osaka 570,J a p a n Received July 13,1990.I n Final Form: January 16,1991 Molecular alignments of monolayers of w-nonadecenyltrichlorosilane(V-NTS) deposited on SiOz/Al/ glass or SiOl/ITO/glass substrates by a chemical adsorption (CA) technique in a nonaqueous solvent containing V-NTS has been investigated by using IR spectroscopy, electron spectroscopy for chemical analysis (ESCA), and a technique to evaluate the degree of alignment of liquid crystals along the monolayers. By use of both ESCA and the technique, an alignment angle of the molecules in the monolayers deposited by the CA technique can be determined. Although the tilt angle of the V-NTS molecules in the monolayer deposited at room temperature was not determined, that at 80 OC was about 32'. The molecular density or the molecular alignment of the monolayers can be controlled by the temperature of the CA condition. The evaluation technique using the alignment of liquid crystals is useful to investigate the molecular alignment in the monolayers. By use of the CA technique, highly ordered monolayers as an alignment film for the liquid crystals may also be obtained easily without rubbing. 1, Introduction A characteristic of the Langmuir-Blodgett (LB) technique1*2is that it is possible to prepare the films having desired molecular arrangement by building up monolayers under properly selected surface pressure. Much of the recent research efforts have been invested in the optimization of the LB technique in devising ways for improving the stability of the resulting films, and considerable progress has been achieved in developing techniques to control the microstructure of LB films. Although the LB technique is powerful as a tool for handling molecular entities, it has some inherent drawbacks They are uncontrollable for occurrence of domains, uncontrollable for building up three-dimensional molecular structures, and difficult to increase building up efficiency. Thus the LB technique has a certain limitation when the technique is applied to more sophisticated systems such as three-dimensional molecular structures, in which self-assembly or self-organization of molecules may be strongly required. For this purpose, fundamental studies on chemical adsorption (CAI were already carried out by Sagiv3 and his c o - ~ o r k e r s . ~The - ~ CA technique takes advantage of the possibility of obtaining oriented compact monolayers onto a planer solid surface. Although in the LB technique, molecular arrangements can be determined easily by a surface pressure-area curve, the molecular arrangement of monolayers deposited by the CA technique is not determined as clearly as in the LB technique. The present study has been carried out to investigate molecular arrangements in the monolayers deposited by the CA technique using IR spectroscopy, electron spectroscopy for chemical analysis (ESCA), and a technique to evaluate the degree of alignment of liquid crystals along a chemically adsorbed monolayer (CA monolayer means the monolayer attached covalently to the surface of a solid + Matsushita Electric Industrial Co., Ltd. t
Matsushita Technoresearch, Inc.
(1) Blodgett, K. B. J. Am. Chem. SOC.1936,57, 1007. (2) Blodgett, K. B.; Langmuir, I. Phys. Rev. 1937,51,964. 1980, 102,92. (3) Sagiv, J. J. Am. Chem. SOC. (4) Polymeropouloe, E. E.; Sagiv, J. J. Chem. Phys. 1978, 69, 1836. (5) Netzer, L.; Sagiv, J. J . Am. Chem. Soc. 1983,105,674. (6) Netzer, L.; Iscovici, R.; Sagiv, J. Thin Solid Films 1983, 100, 67. (7) Netzer, L.; Iscovici, R.; Sagiv, J. Thin Solid Films 1983,99, 235.
0743-7463f 91/2407-1473$02.5O/O
substrate). The fact that liquid crystals are aligned along the monolayer deposited by the LB technique is also known and the LB technique is studied to prepare a rubbing-free alignment film for a liquid crystal cell.*v9 We wish toreport the evaluation results on the molecular alignment angles in the monolayers deposited by the CA technique.
2. Experimental Procedures 2.1. Preparation of Monolayers. Materials. The w-nonadecenyltrichlorosilane (V-NTS)was synthesized and was characterized by NMR and IR spectroscopies and mass spectrometry (MS). In the NMR spectra in CC4, bands typical of long-chain trichlorosilane with n carbon atoms and terminal ethylenic double bonds were observed at a chemical shift (6) of 5.55 (C=CH-), 4.85(HzC=C-),and 1.95ppm (=C-CHr). In IR spectra, strong absorption bands were observed at 2920 (CH2vu), 2845 (CH2 u,), and 915 cm-1 (C=C-CH-) and weak bands at 3080 (H&= v ) and 1640 cm-' (C-C u). The molecular weight was confirmed to be 398 by mass spectrometry. Partial migration of the terminal double bond occurred during the final distillation of V-NTS, as indicated by the appearance of nonterminal vinyl bands at 6 = 5.00 ppm (multiple)in the NMR spectrum and 995 cm-' (weak) in the IR spectrum. The purity of V-NTS as measured by gas chromatography was 89.4%. The synthetic methods will be reported elsewhere. o-Tricosenoic acid (o-TSA CHyCH(CH2)&OOH), purchased from Wako Pure Chemical Industries, Ltd.,was used without further purification for building up LB film as a reference sample. Substrates having an "inert" hydrophilic surface for the Fourier transform IR (FTIR) measurement were prepared by sputtering Al first and sputtering Si02 second on a four-sided glass plate (SiOz/Al/glass), and for ESCA or the evaluation of molecular alignmentof a liquid crystal, substrates were prepared by sputtering IT0 (indium tin oxide) first and sputtering Si02 second on a four-sided glass plate (SiOp/ITO/glass). Chemical Adsorption Steps. By use of the CA technique already recommended by Netzer et al.? monolayers were deposited by chemical adsorption on solid substrates at different temperatures. To obtain the CA monolayer, the substrate was immersed in a solution for the CA (CA solution) under a nitrogen gas atmosphere produced by vaporization from liquid nitrogen and passage through a 0.2-pm filter. The conditions of the CA (8) Ikeno, H.;Oh-saki, A.; Nitta, M.; Ozaki, N.; Yokoyama, Y.; Nakayama, K.; Kobayaahi, S . Jpn. J . Appl. Phys. 1988,27, L475. (9) Niehikawa, Y.; Morikawa, A.; Takiguchi, Y.; Kanemoto, A.; Kakimob, M.; Imai, Y. Jpn. J. Appl. Phys. 1988,27, L1163.
0 1991 American Chemical Society
1474 Langmuir, Vol. 7, No. 7, 1991
Ogawa et al.
processes were 18, 23 (room temperature (RT)), 45, and 80 "C for 30 min, and the concentration of V-NTS was 7 X lO-3mol/L in a solvent mixture of 80% n-hexadecane, 12% CCL, and 8% CHCl3. All solvents were purchased from Aldrich Chemical Co. and used without purification. The substrates were washed by chloroform and then by water, which was purified by ultrafiltration of deionized water in order to stabilize the deposited monolayers. The surface of the substrates covered with chemically adsorbed V-NTS monolayers was very hydrophobic. The CA monolayers were further heated at various temperatures in a nitrogen atmosphere or under a vacuum of lo+ Torr. The LB films of w-TSA also were built up on a SiOz/Al/glass or SiOz/ITO/glass substrate by using a Joyce-Loebl Trough IV in a clean room of class 100 for the reference. The built up conditions are noted with a dot on a surface pressure-area curve in Figure 1. 2.2. IR Measurements. The FTIR spectrophotometer (FTIR-4000, Shimadzu Co., Ltd.; resolution, 2 em-') was used to evaluate the molecular arrangement or density of the monolayers. The reflection IR spectra of the monolayers were taken on SiOz/Al/glass substrates with a multiple external reflection (MER) attachment. The angle of incidence was 73", and the number of external reflections was 7 for MER measurements. Seven thousand interferograms had to be accumulated to obtain an IR spectrum of the monolayers. All spectra reported here were results of subtraction of the curves measured with the clean SiOz/Al/glass plates from those measured with the same plates of the respective deposited monolayers, and assignments of the absorption bands were made in reference to the reports by Hayashi and UmemuralO and Kimura et al." 2.3. Measurement of ESCA Spectra. The ESCA spectra of the V-NTS CA monolayers or the w-TSA LB monolayers deposited on SiOz/ITO/glass substrates were measured at different temperatures between RT and 200 O C and 7 X lo4 Torr by VG Scientific ESCALAB-5. The energy resolution was adjusted at 1.07 eV (Ag 3da/z), and the conditions of the Mg anode were 5 mA and 10 kV. The thickness (d) of the CA films was calculated with eqs la, Ib, and 2 Z, = z,'%e(i
- exp(-d/Acb))
zSi= zSiltd(l- e) + Is;% exp(-ci/Asib) Zc/Zcutd - a(1- exp(-d/hcb)) 1 - Zsi/Zs;"
(1- exp(-d/Xsib))
(la) (lb) (2)
where ZC and Zsi are intensities of C 1s and Si 2p on the ESCA spectra, respectively. ZcBtdand ZsiUtdare ZC and Zsi obtained on standard samples, respectively, which were the w-TSA multilayer of 21 layers built up by the LB technique on a Si substrate covered with Si02 and a clean Si substrate covered with SiOz. AC and Xsi are the escape depths for C 1sand Si 2p, respectively.The parameter a is a correction coefficient for ZcUtd, b is a correction coefficient for A, and 8 is the degree of coverage. The calculation procedures of the thickness are as follows: First, by use of eqs la and lb, XC and Xsi were determined to be 26 and 33 A, respectively, by measuring the intensities of Z, and Zsi on the w-TSA monolayers built up by the LB technique (LB monolayer), respectively, when the thickness (d) used was assumed to be 30.4 A, which had been determined by X-ray diffraction analysis on the w-TSA multilayer of 25 layers, and parameters a, b, and 0 all equal to 1. On the other hand, the film density ( p ) of the w-TSA LB monolayer was calculated to be 1.3 X 109 kg/m3 with Xc obtained on the w-TSA LB monolayer by using eq 3, as recommended by Roberts et a1.12 49 0.llJP5(nm) (3) 10-3p~2 1 0 - ~ ~ where E is the electron kinetic energy in eV. Equation 3 A=-
+
(10) Hayashi, S.; Umemura, J. J. Chem. Phys. 1975, 63, 1732.
(11) Kimura,
F.; Umemura, J.; Takenaka, T. Langmuir 1986, 2, 96. Buchanan, D. N. E.;
(12) Roberta, R. F.; Allara, D. L.; Pryde, C. A.; Hobbins, N. D. Surf. Interface Anal. 1980, 2, 5.
75 Subphase ; 20T,pH7,Ca2*1.3x164moL/L
-E \
50I
UJ
2
w
1
Deposition Speed; Icm/min
-TSA.
a2
8 25-
L
z 0
,
Figure 1. Surface pressure-area curve when the w-TSA monolayers or multilayer5 were built-up. The 0 indicates the building up condition. represents a least-squares fit of all available experimental determinations of A. For the V-NTS CA films, the film density ( p ) was thought to be smaller than that of the w-TSA LB monolayer, because a molecular formula of V-NTS having a long hydrocarbon chain is similar to that of w-TSA and the molecular density of the U-TSALB monolayer was thought to be at maximum density from the surface pressure-area curve, as shown in Figure 1.Thus Icetd was thought to be smaller than that obtained on the LB monolayer and hc and Xsi to be larger than that obtained on the LB monolayer. Accordingly, the parameters a, b, and 8 and the thickness were selected to satisfy eqs la, lb, and 2 in the region of a I1, b L 1,B I 1, and d = 5-25 A. 2.4. Evaluation of the Molecular Alignments with a Liquid Crystal. In order to clarify the tilt angle of the V-NTS molecules in the monolayers deposited by the CA technique, pretilt angles (which was measured relative to the normal to the surface of the substrate) of a nematic liquid crystal (ZLI-1565; Merck Co., Ltd., the molecular formula is shown on the right side of Figure 7a) were investigated by measuring the capacitance changes of the liquid crystal cells by rotating a magnetic field of 10 kG. The liquid crystal cells were constructed with a 16 wm gap with two glass plates, the inner surfaces of which were covered with monolayers of V-NTS deposited by the CA technique on SiOz/ITO/glass. The area of the IT0 electrode was 1 em2. As the effect of the magnetic field is applicable only for a liquid crystal having a dipole moment, the liquid crystal aligns along the magnetic field when the magnetic field is applied. Thus, by measurement of the capacitance of the liquid crystal cell without a magnetic field and capacitances with a rotating magnetic field, i.e., by finding the angle of the magnetic field when the capacitance measured with the rotating magnetic field is equal to that without the magnetic field, the pretilt angle of the liquid crystal,which is assumed to correspondto the alignment angle of the CA monolayer, can be determined.
-
3. Results and Discussion 3.1. IR Spectra of Monolayers Deposited by the CA or LB Technique. An MER IR spectrum of the V-NTS monolayer deposited by the CA technique at RT (A) was measured for investigating the quality of the monolayers in comparison with the o-TSA monolayer built up by the LB technique at RT (B)and the V-NTS film coated by spin coating technique at RT (C), as shown in Figure 2. In Figure 2, the absorption bands due to the CH2 stretching and scissoring vibrations (v, 2930 cm-l, vB 2860 cm-l, and 6 1470 cm-') and those due to the C = C stretching vibration (v 1640 cm-') were observed on all spectra.
--
Langmuir, Vol. 7, No. 7, 1991 1475
Molecular Alignments of Monolayers
10)
I
e -
I UJ-TSA by LB
m m a I
V-NTS by spin-coating
I
I
3500
2000
1000
1200
900
Wavenumber lcm-'1
I 1500
2500 1500
3000
3500
500
Wavenumber l c m - ' I
Figure 2. MER IR spectra of the V-NTS monolayer deposited by the CA technique (A), the w-TSA monolayer built-up by the LB technique (B),and a thin film of V-NTS coated by a spinner (0. The absorption bands due to the CH2 stretching vibration on the CA monolayer (A) and the LB monolayer (B) split clearly, but not those on the coated film (C), as shown in Figure 2. The intensity ratios of MER absorption bands due to the CH2 stretching vibrations to that due to the C=C stretching vibration or the CH2 scissoring vibration on the CA monolayer and the LB monolayer were much larger than those on the coated film (C),and the spectra on the CA monolayer and the LB monolayer were similar to each other. Although tilt angles of the V-NTS molecules cannot be determined only by these data, the molecular density and the degree of molecular alignments in the V-NTS CA monolayers may be similar to that in the w-TSA monolayer built up by the LB technique.
3.2. IR Spectra of the CA Monolayers Depending on the CA Temperature. The MER IR spectra of the V-NTS monolayers deposited by the CA technique a t different adsorption temperatures in the CA solution a t a constant concentration of V-NTS (7 X mol/L) and constant deposition time (30 min) were measured, as shown in Figure 3a. The absorption bands due to the CH2 stretching and scissoring vibrations (vu, v,, and a), those due to the C=C stretching vibration (v), and those due to the Si-C-, which is connected to the hydrocarbon chain in the V-NTS, stretching vibration (v 1080 cm-l) were observed clearly in all spectra. The change of relative intensities of the bands (CH2 ,v CH2 u,, C-C v, and Si-C- v) on the MER IR spectra also are plotted as a function of the deposition temperature as shown in Figure 3b. The intensities of the two bands due to the C=C (v) and the Si-C- (v) increased with increasing the deposition temperature and reached a constant value over 45 "C. The intensity of the bands due to CH2 (v, and v,) increased again a t 80 "C. This indicates that thedensity of the monolayer deposited a t R T was a little smaller than that a t 80 "C, and as the intensity due to the C=C and the Si-C- bands did not change a t 80 "C, the hydrocarbon chains of V-NTS molecules had tilted to the surface of the substrate when the temperature was a t 80 "C. The intensity of the Si-C bond is linked with the molecular density, because it does not depend on the molecular alignment angle of the CA monolayers. Thus the molecular densities in the CA films increased in the deposition temperature order 45 OC,80 OC,and RT. On the other hand, as an annealing effect for the mo-
o CH2 uas
.-%
2 9 3 0 cm-'
2
lo
c)
&Si-C
6
u
1080
c C
20
40
60
80
Temperature I'C)
Figure 3. (a) Change of the MER IR spectra of the V-NTS monolayers deposited for 30 min at a V-NTS concentration of 7 x 10-9 mol/L and different deposition temperatures: (A) 18; (B) 23; (C)45; (D) 80 O C . (b) Relative intensity plots of the MER IR bands due to the stretchingvibrations of CHI v, and v,, C=C V , and Si-C- v. o CH2 uas 293Ocm-' O C H ~U S 2850 1640 XC=C v ASi-C u 1080
r,
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.-
c)
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-
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Ogawa et al.
1476 Langmuir, Vol. 7, No. 7, 1991
(01
V - N T S , R T depo.
, A
19' 0
I
200
100 Temperature
I
("C)
V-NTS
, 8OOC depo.
Figure 5. Thickness change of the V-NTS monolayers deposited at RT and 80 "C with increasing thermal heating temperature. The error was f l A. process, with little change by thermal annealing between R T and 200 "C. 3.3. Molecular Tilt Angles Calculated by the ESCA Data, In order to determine the tilt angle of molecules in the V-NTS monolayers, ESCA spectra of C 1s and Si 2p bands on the monolayers of V-NTS deposited a t R T and 80 "C were measured a t different temperatures of thermal heating (RT-200 "C). The thickness as deposited was calculated by eqs la, 1b, and 2 as 20 f 1and 22 f 1A for the monolayer deposited a t R T and 80 OC, when a = 0.74, b = 1.15, and 8 = 0.93 for the monolayer deposited a t RT and a = 0.68, b = 1.2, and B = 0.93 for the monolayer deposited a t 80 "C, respectively. Since the length of the hydrocarbon chain of the V-NTS molecule is 25.0 A, if it is assumed that the hydrocarbon chain is in the all-trans zigzag conformation and aligned along the substrate, the molecular alignment angles for 20 A of the monolayer deposited a t R T and 22 A a t 80 "C were calculated relative to the normal to the surface of the substrate to be about 37" and about 29", respectively. The IR data show that the molecules in the CA film are highly oriented along the substrate, and the molecular arrangement in the CA film is similar to that of the LB film. Thus, all-trans conformation is reasonable. It is well-known that if the molecular arrangement was not so good (Le., not in the all-trans conformation), the IRspectra on the CA film should be similar to that on the spincoated film, as shown in Figure 2. The densities ( p ) of the monolayers were also calculated as recommended by Roberts et al.12 Those were 1.1X lo3 and 1.2 x 10s kg/m3 for the monolayer deposited a t RT and 80 OC, respectively. The difference was also supported by the IR spectra in Figure 3a. In order t o elucidate the molecular rearrangement in the CA monolayers with increasing temperature, the thickness changes calculated with ESCA data on both V-NTS monolayers deposited a t R T or 80 "C were plotted further as a function of heating temperature, as shown in Figure 5. The thickness deposited a t RT changed from 20 to 24 A with increasing the thermal heating temperature between R T and 200 "C,but that deposited a t 80 "C increased from 22 to 24 A with increasing the thermal heating temperature between R T and 100 "C and leveled off a t a constant value of 24 A over 100 "C. This indicates
m
: O
90
I81
Magnetic Field Angle ldeg.)
Figure 6. Capacitance change of the liquid crystal cells obtained by using the V-NTS monolayer deposited at RT (a) or 80 O C (b) for the liquid crystal alignmentfilm as a function of the magnetic field angles. In parts a and b, 0 represents the data calculated with increasing the magnetic field angle and X represents the data calculated with decreasing the angle. that the thickness change was caused by decrease of the hydrocarbon chain tilt angle during heating in both CA monolayers. 3.4. Molecular Alignment Evaluated with Liquid Crystals. Typical capacitance changes are plotted as a function of angle of the applied magnetic field to the liquid crystal cells with the monolayers deposited a t R T and 80 "C, as shown in Figure 6. The capacitance change of the cell with the monolayers deposited a t R T showed a hysteresis between increasing and decreasing angles of the magnetic field as shown in Figure 6a. On the other hand, the capacitance change of the cell with the monolayers deposited a t 80 "C did not show any hysteresis a t all, and the pretilt angle of 32" for the liquid crystal was exactly determined relative to the normal to the surface of the substrate, as shown in Figure 6b. In the CA monolayer deposited a t RT, molecular arrangement might be random, thus the alignment force should be weak and the hysteresis appeared. Taking into account the results by ESCA and IR spectra, the molecular alignments of the monolayers and aligning liquid crystal on deposited monolayers are shown schematically in Figure 7. The liquid crystal did not align along the hydrocarbon chain of the V-NTS molecules in the monolayer deposited at RT, as shown in Figure 7b, but did align along the hydrocarbon chain of the molecules in the monolayer deposited a t 80 "C, as shown in Figure 7c. Here, the
Molecular Alignments of Monolayers (0)
Figure 7. Schematic view of the aligning of the liquid crystal molecules and of the arranged V-NTSmonolayers deposited at RT (b)and 80 "C (c), where the V-NTSmolecule and the liquid crystal molecule were drawn schematically as shown in part a. V-NTS molecule adsorbed on Si02 and the liquid crystal molecule used in these experiments is shown Schematically in Figure 7a. Accordingly, the tilt angle of molecules of the monolayer on the substrate deposited a t RT cannot be determined, but that a t 80 "C can be determined to be about 32" by this method. Taking into account the error of the ESCA data, this value can be also supported by the result of ESCA, because the angle obtained by ESCA was 29".
Langmuir, Vol. 7, No. 7, 1991 1477 4. Conclusions The molecular arrangement of the V-NTS monolayers deposited by the CA technique has been investigated by using IR spectra, ESCA, and alignment of a liquid crystal along the monolayers. The tilt angle of the molecules in the monolayers deposited a t room temperature was not determined, but that of molecules deposited a t 80 "C was determined to be 32". Molecular density or molecular alignment of the monolayers a t the deposition was controlled by the CA conditions, such as the CA temperature. The molecular arrangement of the monolayer deposited a t R T changed more than that a t 80 "C by thermal annealing below 200 "C after the deposition. This can be explained by the difference in the density of the monolayer. We concluded that the difference of alignment of the liquid crystal does not depend on the annealing effect a t the deposition temperature of 80 "C but depends on the molecular arrangement due to the molecular density in the CA films, and the tilt angle of the molecules in the CA film decreases (i.e., molecules in the film rise up) with increasing temperature. This indicates that the molecular density is important to obtain high-grade alignment films for liquid crystals. With the CA technique, highly aligned monolayers can be easily obtained without rubbing; the CA technique may be useful for the preparation of organic ultrathin films such as a rubbing-free alignment film for liquid crystals. The influence of some substituents a t the surface of the monolayers deposited by the CA technique on the molecular arrangement and on the alignment of liquid crystals is being studied a t present. Acknowledgment. The authors wish to thank Director Dr. T. Nitta of Matsushita Electric Industrial Co., Ltd., Central Research Laboratories, and Professor J. Sagiv of The Weizmann Institute of Science for their helpful comments. ITO,50926-11-9;SiOz, Registry No. V-NTS, 125282-19-1; 7631-86-9; Al, 7429-90-5.
