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Preparation and Characterization of Quantum-Sized PbS Grown in Amphiphilic Oligomer Langmuir-Blodgett Monolayers Lin Song Li, Lianhua Qu, Lijun Wang, Ran Lu, Xiaogang Peng,† Yingying Zhao, and Tie Jin Li* Department of Chemistry, Jilin University, Changchun, 130023, People’s Republic of China Received May 20, 1997X Three kinds of amphiphilic oligomers, poly(maleic acid) with octadecanol ester (PMAO) with 2.0:1, 3.7:1, and 6.0:1 between the carboxylic groups and the hydrocarbon chains, were synthesized and have been used as a matrix to form and grown the quantum-sized PbS. Pure Y-type PbPMAO Langmuir-Blodgett (LB) films were prepared at a dipping speed of 4.0 cm/min and surface pressure of 18 mN/m. The results from Fourier tranform IR spectroscopy and X-ray diffraction measurements showed that an ordered structure was obtained from the PbPMAO LB films. The quantum-sized PbS particles were prepared by exposure of PbPMAO LB films to H2S and nanoparticulate PbS monolayers were formed without destroying the layered structure. The result from structured UV-vis absorption spectra shows that the PbS particles within the LB matrix are relatively monodispersed. It was found that the PbS within PMAO LB films indicated a larger blue shift of the optical absorption edge with the decrease of the ratio between the carboxylic groups and the hydrocarbon chains. Meanwhile, it can be deduced that the size of the aggregate of the sulfides within PMAO LB films is smaller than that formed in stearic acid LB films. The observed absorption peaks showed similarity with the monomolecular PbS.
Introduction Quantum-sized (Q-sized) semiconductor materials have been an active area of research due to their unique electrooptical and redox properties.1-3 It is possible to prepare monodispersed nanoparticles with quantum sizes of 1-10 nm from a variety of chemical methods, including reversed micelles,4 Nafion,5 vesicles,6 zeolite,7 self-assembled monolayers,8 air-water monolayers,3,9 and Langmuir-Blodgett (LB) films.10-13 The preparation of a large number of IIVI, II-V, IV-VI, and III-V materials in the form of quantum dots has been reported,14-16 and also much research has focused on the preparation, characterization, and photophysics of the quantum dot and quantum well system among the different nanoparticles.17,18 The key to any synthetic investigations of them must be careful * To whom correspondence should be addressed. † Present address: Department of Chemistry, University of California, Berkeley, CA 94720. X Abstract published in Advance ACS Abstracts, October 1, 1997. (1) Alivisatos, A. P. J. Phys. Chem. 1996, 100, 13226. (2) Henglein, A. Ber. Bunsenges. Phys. Chem. 1995, 99, 903. (3) Fendler, J. H.; Meldrum, F. C. Adv. Mater. 1995, 7, 607. (4) Dutta, P. K.; Jakupa, M. ; Reddy, K. S.; Salvatl, L. Nature 1995, 374, 44. (5) Miyoshi, H.; Yamachika, M.; Yoneyama, H.; Mori, H. J. Chem. Soc., Faraday Trans. 1990, 86, 815. (6) Tricot, Y. M.; Fendler, J. H. J. Phys. Chem. 1986, 90, 3369. (7) Stucky, G. D.; MacDougall, J. E. Science 1990, 247, 669. (8) Colvin, V. L.; Goldstein, A. N.; Alivisatos, A. P. J. Am. Chem. Soc. 1992, 114, 5221. (9) Yang, J.; Fendler, J. H. J. Phys. Chem. 1995, 99, 5505. (10) Smotkin, E. S.; Lee, C.; Bard, A. J.; Campion, A.; Fox, M. A.; Mallouk, T. E.; Webber, S. E.; White, J. M. Chem. Phys. Lett. 1988, 152, 265. (11) Peng, X.; Zhang, Y.; Yang, J.; Zou, B.; Xiao, L. Z.; Li, T. J. J. Phys. Chem. 1992, 96, 3412. (12) Kang, Y. S.; Risbud, S.; Rabolt, J.; Stroeve. P. Langmuir 1996, 12, 4345. (13) Nabok, A. V.; Richardson, T.; Davis, F.; Stirling, C. J. M. Langmuir 1997, 13, 3198. (14) Murray, C. B.; Norris, D. J.; Bawendi, M. G. J. Am. Chem. Soc. 1993, 115, 8706. (15) Moon, K.; Shea, K. J. J. Phys. Chem. 1994, 98, 3027. (16) Salata, O. V.; Dobson, P. J.; Hull, P. J.; Hutchison, J. L. Adv. Mater. 1994, 6, 772. (17) Kortan, A. R.; Hull, R.; Opila, R. L.; Bawendi, M. G.; Steigergerwald, M. L.; Carroll, P. J.; Brus, L. E. J. Am. Chem. Soc. 1990, 112, 1327. (18) Mews, A.; Eychmu¨ller, A.; Giersig, M.; Schooss, D.; Weller, H. J. Phys. Chem. 1994, 98, 934.
S0743-7463(97)00522-2 CCC: $14.00
control of the quantum dot size and, even more important, control of the size distribution. Q-sized nanoparticles in LB film systems have been widely studied because the LB monolayer transfer method provides a technique for precise control of the assembly and growth of large numbers of superimposed layers which are used for the assembly and synthesis of quantum-sized nanoparticle layers.19-21 We have synthesized Q-sized PbS monolayers consisting of a two-dimensional domain or linear form of PbS in the polar planes of stearic acid (SA) LB films by chemical reactions.20 PbS, as a semiconductor with a very small band gap, has an absorption threshold that can change from the near-infrared region, which is characteristic of bulk PbS to the visible region in quantum size.22 The Q-particle of PbS was widely investigated by many groups and was found to usually act as sensitizers for nanoporous widebandgap semiconductors, such as the sensitized porous TiO2 electrode.23,24 The band gap of Q-particle PbS is widen to ∼2 eV from the 0.41 eV bulk value of PbS. In this paper, we investigate the PbS within PMAO LB films with different X/Y ratios in detail. These kinds of amphiphilic oligomers (poly(maleic acid) with octadecanol ester, PMAO) and their corresponding salts which can be used for the ordered synthesis and assembly of the inorganic nanoparticles are reported. As an oligomer, PMAO should be better than common fatty acids in thermal and mechanical stability in LB films. Their molecular structure is shown below. From the molecular structure, it can be seen that the ratio of carboxylic groups to hydrocarbon chains is always larger than 1. Our previous work only reported that the ratio is 2.0 (PMAO1); this value is twice as large as that (19) Zylberajch, C.; Ruaudel-Teixier, A.; Barraud, A. Thin Solid Films 1989, 179, 9. (20) Peng, X.; Guan, S.; Chai, X.; Jiang, Y.; Li, T. J. J. Phys. Chem. 1992, 96, 3170. (21) Geddes, N. J.; Urquhart, R. S.; Furlong, D. N.; Lawrence, C. R.; Tanaka, K.; Okahata Y. J. Phys. Chem. 1993, 97, 13767. (22) Gallardo, S.; Gutie´rrez, M.; Henglein, A.; Janata, E. Ber. Bunsenges. Phys. Chem. 1989, 93, 1080. (23) Vogel, R.; Hoyer, P.; Weller, H. J. Phys. Chem. 1994, 98, 3183. (24) Hoyer, P.; Ko¨nenkamp, R. Appl. Phys. Lett. 1995, 66, 349.
© 1997 American Chemical Society
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COOH [(
CH2
)x
( CH2 )y]N C
N ~ 20, X >Y
O
O C18H37
X:Y = 2.0:1.0, PAMO1; X:Y = 3.7:1.0, PAMO2; X:Y = 6.0:1.0, PAMO3
in common fatty acids. The PbS or CdS within PMAO LB films showed a larger blue shift of the optical absorption edge than that formed within the stearic acid LB films.25 We can control efficiently the sizes of inorganic nanoparticles by controlling the ratio between carboxylic groups and hydrocarbon chains. Here, as a first step toward potential use for vertical quantum dot superlattices, we attempt to develop them into electro-optical devices. Experimental Section The synthesis of the poly(maleic anhydride) (PMAN) used in this study has been described elsewhere.26 The average molecular weight of PMAN was determined using a osmometer. And then the PMAOs with different X:Y ratios were synthesized by the reaction of poly(maleic anhydride) with octadecanol in nonaqueous solution. The ratio of carboxylic groups to hydrocarbon chains of the resulting products was determined by nonaqueous neutralization titration. Lead chloride was analytical grade and was recrystallized prior to use. Water, with a conductivity of 18 MΩ cm-1, was used to prepare the monolayer subphase. The pH value of the subphase was 5.8. Monolayers were spread from chloroform solutions (concentration, 1 × 10-4 mol/L) onto the subphase containing PbCl2 at a concentration of about 1 × 10-4 mol/L. The surface pressure-area isotherms were recorded to about 20 °C with a compression speed of 7 cm2/ min on a LB trough with a multicompartmental round trough from Mayer-Fein technic (Germany). The Y-type multilayer films (17 layers) were built up by the vertical dipping method at 20 ( 1 °C under a constant pressure. The surface pressure for transfer and the dipping speed were chosen by a detailed experimental condition. In a vacuum system, PbSt2 LB films were exposed to H2S gas for 150 h at the pressure of 1 Torr (133.3 Pa). The substrate for the multilayers was a thoroughly cleaned hydrophilic CaF2 when UV-vis measurements were made, whereas silicon wafers were used for small angle X-ray diffraction measurement. The samples were dried in air prior to their characterization. X-ray diffraction patterns were obtained with the diffraction vector perpendicular to the plane of the films using a Rigaku D/max rA X-ray diffractometer (Cu KR, 0.154 18 nm; angular resolution, 0.02°). The period of repetition of the periodic structure (in this case the bilayer thickness) was calculated directly from the position of the Bragg reflections. Fourier transform infrared (FTIR) and UV-visible spectra were taken with a Nicolet 5PC FTIR spectrometer and a Shimadzu UV-365 spectrometer, respectively. Results and Discussion Surface Pressure vs Surface Area Isotherms. Figure 1 shows the surface pressure (π) vs surface area (A) isotherms of PMAO and PbPMAO. The isotherms were reproducible and showed little dependence on temperature fluctuations. The area of per side chain of PMAO extrapolated to π ) 0 is approximately 0.28 nm2 on pure water and approximately 0.23 nm2 on PbCl2 (25) Peng, X.; Lu, R.; Zhao, Y.; Qu, L.; Chen, H.; Li, T. J. J. Phys. Chem. 1994, 98, 7052. (26) Zhao, Y.; Gao, M.; Ren, Y.; Dong, Z.; Li, T. J. Thin Solid Films 1993, 210/211, 610.
solution. From PMAO1 to PMAO3, the points of collapse pressure are approximately 40-47 mN/m on pure water (Figure 1), whereas for their corresponding salts the collapse pressure of PbPMAO1 is 50 mN/m and the point of collapse pressure of PbPMAO3 sharply decreases to 25 mN/m. From the area-time isobaric curve of the PMAO monolayer on pure water surface under different surface pressures, no relaxation can be detected at 18-48 mN/m, showing that a stable solid-state monolayer can be obtained. Transfer Characteristics of PbPMAO LB Films. Monolayer can always be transferred to solid substrate to form multilayer thin films with well-defined thickness and structure. The usual vertical transfer process requires that the film be stable at a reasonable surface pressure to ensure accurate transfer ratios and ordered depositions. Owing to the pure Y-type films with head-to-head and tail-to-tail structures, the films deposited on the substrates exhibit reasonable stability at the desired transfer pressure. The transfer ratios were recorded during the depositions of PbPMAO multilayers at dipping speeds of 0.7-4.0 cm/ min and surface pressures of 9-30 mN/m. As the surface pressure is 9 mN/m and the dipping speed is 0.7 cm/min, the transfer ratios are only in the range of 0-0.65 for the downward strokes, although the upward strokes are nearly 1.0. However, the transfer ratios can be noticeably changed by increasing the surface pressure and the dipping speed. If the dipping speeds are increased to 4.0 cm/min and the surface pressure to 18 mN/m, the transfer ratios exceed 0.95, which are in the range of 0.98-1.02 for both the upward and downward strokes. Pure Y-type PbPMAO LB films can be well prepared. X-ray Diffraction of PbPMAO1 LB Films before and after the Formation of PbS. As a powerful method for studying order structure of materials, X-ray diffraction measurement is widely used to measure many kinds of LB films. Such as, it was always used to detect the change in the long spacing on the LB films prior and subsequent to the treatment with H2S.10,27,28 In this part, we choose the X-ray diffraction data of PbPMAO1 LB films as an example to discuss in detail. At a surface pressure of 18 mN/m and dipping speed of 4.0 cm/min, PbPMAO1 can be transferred with an orientation perpendicular to the film plane, indicating a 2θ value of 1.55° (d ) 5.70 nm) (Figure 2a). According to the chain length of a PbPMAO1 molecule, the repeating unit in the film is bilayer, which conforms to the features of Y-typedeposited LB films and also coincides with the d-value of other metal behenate LB films.29,30 It is concluded that the long spacing of PbPMAO1 multilayer of ca. 5.7 nm indicates the vertical orientation of the chain axis to the film surface. In the X-ray diffraction patterns of LB films after the reaction with H2S gas, there is no new phase diffraction peak unit. The long spacing of PbPMAO1 determined by the X-ray diffraction measurement was changed to 5.8 nm (2θ ) 1.523°) (Figure 2b). The measurements show that exposure of the LB films of PbPMAO to H2S gas results in an increase in thickness of about 0.1 nm per layer. This value increases little compared with the value before the reaction, revealing that there is no marked change in the thickness of the LB (27) Luo, X.; Zhang, Z.; Liang, Y. Langmuir 1994, 10, 3213. (28) Pan, Z.; Liu, J.; Peng, X.; Li, T. J.; Wu, Z.; Zhu, M. Langmuir 1996, 12, 851. (29) Fromhera, P. Oelschla¨gel, U.; Wilke, W. Thin Solid Films 1988, 159, 421. (30) Erokhin, V. V.; Lvov, Yu. M.; Mogilevsky, L. Tu.; Zozulin, A. N.; Ilyin, E. G. Thin Solid Films 1989, 178, 433.
Preparation of Q-PbS Grown in Oligomer LB Film
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Figure 1. π-A isotherms of (a) PMAO1, (b) PMAO2, and (c) PMAO3. Solid lines are on the surface of pure water; dashed lines are on the surface of the PbCl2 solution. The isotherms were all recorded at 20 ( 1 °C and the compression speed used was 7 cm2/min.
Figure 2. X-ray diffraction pattern of 17-layer PbPMAO1 LB films (a) before and (b) after exposure to H2S. The surface pressure is 18 mN/m; the dipping speed is 4.0 cm/min.
film. The PbS/PMAO LB film still retains the well-ordered periodic structure, indicating that the formation of PbS in a LB matrix has a two-dimensional distribution; i.e., PbS generated by the reaction forms monolayers in the polar planes of PMAO LB films. Otherwise, the formation of three-dimensional microcrystallites or clusters of PbS will destroy the well-ordered arrays of hydrocarbon chains of LB films. This is the same as the result of our previous report on the PbSt2 LB film prior and subsequent to the treatment with H2S.20 FTIR Spectra of PbPMAO LB Films before and after Reaction with H2S. Compared with X-ray diffraction, Fourier-transformed infrared spectroscopy (FTIR), either in the transmission mode or with grazing incidence reflection (GIR), has been the most widely used experimental tool for the characterization and determination of LB film structure. GIR is especially useful in determining the molecular orientation in the film structures because it senses only the vibration component perpendicular to the substrate surface.31-33 (31) Naselli, C.; Rabolt, J. F.; Swalen, J. D. J. Chem. Phys. 1985, 82, 2136. (32) Urquhart, R. S.; Hoffmann, C. L.; Furlong, D. N.; Geddes, N. J.; Rabolt, J. F.; Franz, G. J. Phys. Chem. 1995, 99, 15987.
Figure 3. FTIR transmission spectra of PMAO powder in KBr pellet: (a) PMAO1; (b) PMAO2; (c) PMAO3.
Figure 3 shows the infrared transmission spectrum of PMAO powder with different ratios between carboxylic groups and hydrocarbon chains (2:1, 3.7:1, and 6:1) in KBr. It has been reported that the CdO stretching vibration band is located at about 1745 cm-1 in the free -COOH (no hydrogen bonding) at 1720-1730 cm-1 when the -COOH groups form a sideways dimmer structure and at approximately 1700 cm-1 if the -COOH groups form ring dimmer structures.25 Figure 3 shows that the v(CdO) stretching vibration band (1723 cm-1) is located in the sideways hydrogen bonding region. The reason is that the adjacent -COOH groups within the individual molecules are only separated by no more than 0.3 nm and the distance of each individual oligomer molecule separation exceed 0.5 nm.25 Therefore, the hydrophilic part of each molecule of oligomer that aggregates by forming the (33) Yang, J.; Peng, X.; Zhang, Y.; Wang, H.; Li, T. J. J. Phys. Chem. 1993, 97, 4484.
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Figure 5. GIR spectra of 17-layer PbPMAO LB films: (a) PbPMAO2; (b) PbPMAO3.
Figure 4. FTIR transmission spectra of 17-layer PbPMAO LB films: (a) PbPMAO1; (b) PbPMAO2; (c) PbPMAO3. Table 1. Band Assignments for PbPMAO Monolayers E| bc
E⊥bc
PbPMAO2
PbPMAO3
PbPMAO2
PbPMAO3
assignmenta
2959 (m) 2920 (vs)
2959 (m) 2925 (vs)
2850 (vs) 1718 (m) 1541 (s) 1468 (m) 1396 (s) 1181 (m)
2850 (vs) 1723 (m) 1541 (s) 1466 (m) 1395 (s) 1179 (m)
2958 (m) 2921 (vs) 2865 (m) 2851 (vs) 1731 (m) 1572 (s) 1466 (w) 1408 (s) 1190 (w)
2959 (m) 2925 (vs) 2870 (m) 2854 (vs) 1731 (m) 1568 (s) 1466 (w) 1413 (s) 1190 (m)
νa(CH3) νa(CH2) νs(CH3) νs(CH2) ν(CdO) νa(COO-) δ(CH2) νs(COO-) ν(C-O)
a v, stretch; δ, bend. b vs, very strong; s, strong; m, medium; w, weak. c E| the electric vector of incidence is parallel to the substrate plane. E⊥ the electric vector of incidence is perpendicular to the substrate plane.
sideways hydrogen bonding should be limited within itself. The size of aggregates should be in the nanometer range because the average degree of the polymerization is approximately 20. The relative absorption intensity of vs(CH2) for PMAO1 is higher than that of v(CdO) and the change for PMAO2 is very little, whereas for PMAO3 absorption intensity of the v(CdO) groups is stronger than that of vs(CH2) groups. The number of -COOH groups is twice as large as the number of hydrocarbon chains for PMAO1 (X:Y ) 2:1). In addition, in the case of PMAO3, the number of -COOH groups is six times than that of hydrocarbon chains. Absorption intensity can give good proof to determine the molecular structure of the PMAO series. This also suggests that the number of metal ions bound to one oligomer molecule apparently increases. The FTIR spectra of PbPMAO salts are shown in Figure 4. There are two sharp peaks nearly at 1540 and 1400 cm-1 which can be assigned to the asymmetric va(COO-) and symmetric vs(COO-) stretching vibrations, respectively. The detailed assignments of the infrared frequencies are given in Table 1. The aggregate manner of the oligomer molecule is not changed because the positions of the -COO- asymmetric and symmetric stretching vibration bands are not changed at the different ratios between the carboxylic groups and the hydrocarbon chains for
PMAO molecules. The peaks at 1465 cm-1 can be assigned to CH2 scissoring vibration which appears in the form of a single peak. It is interesting that the asymmetric va(COO-) stretching vibration is higher in frequency than the usual value of 1510 cm-1 in the PbSt LB films, but the symmetric vs(COO-) stretching vibration is moving to lower frequency, which in the PbSt LB films is located at 1421 cm-1.20 The ∆ values [va(COO-) - vs(COO-)] for PbPMAO LB films are greater than those of PbSt LB films. They range from 100 to 200 cm-1. It follows that hydrogen bonding of the PMAO molecules is replaced by metal ion (Pb2+) bridges, which form the bridging complex structures and are close to the ionic values. However, the ring dimmer hydrogen-bonding structure of -COOH groups in the LB films of fatty acids is replaced by the metal ion to form the chelating (bidentate) complexes, which exhibit ∆ values (
