Atomic Force Microscopy and Micro-Raman Imaging of Mixed

Instituto de Fı´sica de Sa˜o Carlos, Universidade de Sa˜o Paulo, Sa˜o Carlos, SP, Brazil,. Materials and Surface Science Group, University of Win...
0 downloads 0 Views 365KB Size
Download PDF
Langmuir 2002, 18, 3561-3566

3561

Atomic Force Microscopy and Micro-Raman Imaging of Mixed Langmuir-Blodgett Films of Ytterbium Bisphthalocyanine and Stearic Acid L. Gaffo,† C. J. L. Constantino,‡ W. C. Moreira,§ R. F. Aroca,‡ and O. N. Oliveira, Jr.*,† Instituto de Fı´sica de Sa˜ o Carlos, Universidade de Sa˜ o Paulo, Sa˜ o Carlos, SP, Brazil, Materials and Surface Science Group, University of Windsor, Windsor, Ontario N9B 3P4, Canada, and Departamento de Quı´mica, Universidade Federal de Sa˜ o Carlos, Sa˜ o Carlos, SP, Brazil Received August 2, 2001. In Final Form: January 29, 2002 The degree of mixing and surface coverage in Langmuir-Blodgett (LB) films of ytterbium bisphthalocyanine (YbPc2) mixed with stearic acid (SA) has been probed using atomic force microscopy (AFM) and micro-Raman imaging. The morphologies extracted for LB films at the nanometer (AFM) and micrometer (micro-Raman) scales are consistently in good agreement. The results show that a 5-layer mixed LB film with 75/25% YbPc2/SA (relative content by weight) displays smaller and more homogeneous distribution of aggregates compared with a 5-layer mixed film of 25/75% YbPc2/SA. Raman microscopy of neat LB films of ytterbium bisphthalocyanine (YbPc2) and mixed YbPc2/stearic acid LB films deposited on glass and silver islands is reported. Resonance Raman scattering (RRS), surface-enhanced resonance Raman scattering (SERRS), and surface-enhanced Raman scattering (SERS) were obtained using laser lines at 633 and 780 nm. The enhanced (SERRS) and unenhanced (RRS) resonance Herzberg-Teller spectra of neat YbPc2 LB films and mixed LB films excited with the 633 nm line are identical.

I. Introduction The fabrication of novel supramolecular structures with the ultimate goal of molecular control is the groundwork for the emerging nanoscience and nanotechnology that has brought an increasing demand for the development of methods to assemble and characterize such structures. Assembling supramolecular structures has normally been achieved with the well-established Langmuir-Blodgett (LB)1,2 and self-assembly3,4 techniques. Characterization at the nanometric level is now possible with surface probe microscopy methods, among which atomic force microscopy (AFM)5,6 plays a prominent role. AFM has been used extensively in the investigation of LB and other thin films,5,6 providing unique information.7 However, AFM provides morphological images, not chemical information. Chemical imaging where the presence and/or orientation of specific molecular groups is visualized has been made possible in recent years through Raman imaging, where nanometer resolution may be obtained with scanning nearfield optical microscopy (SNOM).8 Raman microscopy with * To whom correspondence should be addressed. Tel: +55 16 2739825. Fax: +55 16 2715365. E-mail: [email protected]. † Universidade de Sa ˜ o Paulo. ‡ University of Windsor. § Universidade Federal de Sa ˜ o Carlos. (1) Gan, L.; Liang, B.; Lu, Z. Supramol. Sci. 1998, 5, 583. (2) Burghard, M.; Fischer, C. M.; Schmelzer, M.; Roth, S.; Haisch, P.; Hanack, M. Synth. Met. 1994, 67, 193. (3) Cooper, T. M.; Campbell, A. L.; Crane, R. L. Langmuir 1995, 11, 2713. (4) Zhang, X.; Shen, J. Adv. Mater. 1999, 11, 1139. (5) Constantino, C. J. L.; Dhanabalan, A.; Cotta, M. A.; Pereira-daSilva, M. A.; Curvelo, A. A. S.; Oliveira, O. N., Jr. Holzforschung 2000, 54 (1), 55. (6) Bourgoin, J. P.; Palacin, S. Langmuir 1998, 14, 3967. (7) Qian, P.; Nanjo, H.; Sanada, N.; Yokoyama, T.; Itabashi, O.; Hayashi, H.; Miyashita, T.; Suzuki, T. M. Thin Solid Films 1999, 349 (1-2), 250. (8) Deckert, V.; Zeisel, D.; Zenobi, R.; Vo-Dinh, T. Anal. Chem. 1998, 70, 2646.

micrometer resolution can be used to create point-to-point maps (lines, areas, or volumes in a sample) using a motorized microscope stage with the acquisition of the Raman spectrum at each point.9 Global images where a wide field of light is detected from the scattering from a single vibrational fundamental can also be achieved using liquid crystal tunable filters.10,11 LB films of ytterbium bisphthalocyanine have been the subjects of a variety of studies describing their electrochromic and conducting properties.12,13 The structure of the YbPc2 is shown in the inset of Figure 1. Monolayer transfer is more efficient when YbPc2 is mixed with a filmforming material such as stearic acid (SA), and thereby producing films with distinct relative contents of YbPc2 is of practical value. In this work, we present the morphological and chemical properties as provided by AFM and the chemical Raman imaging, which permit us to discuss the level of heterogeneity or phase separation in the mixed LB films. II. Experimental Section Ytterbium bisphthalocyanine was synthesized following the method given by Kirin et al.14 A mixture of ytterbium acetate and phthalonitrile (1:8 mole ratio) was heated at 300 °C for 2 h. The reaction mixture was cooled, and the product obtained was purified by column chromatography and characterized by UVvis and Fourier transform infrared (FTIR) spectroscopy. LB (9) Hayward, I. P.; Baldwin, K. J.; Hunter, D. M.; Batchelder, D. N.; Pitt, G. D. Diamond Relat. Mater. 1995, 4, 617. (10) Williams, K. P. J.; Pitt, G. D.; Smith, B. J. E.; Whitley, A.; Batchelder, D. N.; Hayward, I. P. J. Raman Spectrosc. 1994, 25, 131. (11) Yang, X. M.; Tryk, D. A.; Ajito, K.; Hashimoto, K.; Fujishima, A. Langmuir 1996, 12, 5525. (12) Petty, M.; Lovett, D. R.; O’Connor, J. M.; Silver, J. Thin Solid Films 1989, 179, 387. (13) Petty, M.; Lovett, D. R.; Townsend, P.; O’Connor, J. M.; Silver, J. J. Phys. D: Appl. Phys. 1989, 22, 1604. (14) Kirin, I. S.; Moskalev, P. N.; Maskashev, Y. A. Russ. J. Inorg. Chem. 1967, 12 (3), 369.

10.1021/la011234e CCC: $22.00 © 2002 American Chemical Society Published on Web 04/05/2002

3562

Langmuir, Vol. 18, No. 9, 2002

Gaffo et al.

Figure 1. Electronic absorption spectra for YbPc2 in chloroform solution and monolayer LB films on glass and silver islands. The absorbance scale is related to the solution spectrum and is arbitrary for Ag islands and LB films. The molecular structure of YbPc2 is also shown. experiments were carried out with a KSV-5000 LB instrument placed in a class 10 000 clean room and with a Lauda Langmuir film balance equipped with an electronically controlled dipping device, Lauda Film lift FL-1. Ultrapure water (18.2 MΩ cm) at room temperature was used as the subphase in both cases. Langmuir films of YbPc2 and mixed 25/75% and 75/25% YbPc2/ SA were fabricated by spreading chloroform solutions (concentration, 0.1 mg mL-1) onto the pure water subphase. YbPc2 and mixed YbPc2/SA LB films were fabricated by transferring the Langmuir monolayers from the water subphase to Corning 7059 glass slides half-coated with 6 nm mass thickness silver island films (evaporation at 10-6 Torr with the glass heated at 200 °C), with a transfer ratio of ca. 1. Mixed 5-layer LB films of YbPc2/SA were fabricated by transferring the Langmuir films from the water subphase to mica substrates with a transfer ratio of ca. 1, with Y-type deposition. The resonance Raman scattering (RRS), surface-enhanced resonance Raman scattering (SERRS), and surface-enhanced Raman scattering (SERS) spectra and RRS area mapping were obtained for LB films with a Renishaw Research Raman microscope system RM2000 equipped with a computer-controlled three-axis encoded (XYZ) motorized stage (minimum step of 0.1 µm) and with a Leica microscope (DMLM series). Two laser lines were used for the excitation of the inelastic light scattering, 633 and 780 nm. The power at the sample was varied from 25 µW to a maximum of 2 mW in both cases. The spectrograph is equipped with a 1200 g/mm grating with additional angle-tuned band-pass filter optics. The Raman spectra were recorded at room temperature with ca. 4 cm-1 resolution. The maps reported here were recorded by using the 633 nm laser line and a 50× microscope objective to focus the laser beam onto a spot of ca. 1.0 µm2 and collecting spectra point-by-point for an area of 42 × 42 µm2 with a step of 3 µm. The integrated areas for the Raman peaks at 680 and 1524 cm-1 were used to create the area mapping images. The baseline correction was applied to avoid any background contribution. Data acquisition and analysis were carried out using the WIRE software for Windows and Galactic Industries’ GRAMS/ 32 C software including the 3D package. The UV-visible spectra of thin solid films and solutions were obtained using the Varian Cary 50 Scan UV-vis spectrometer. AFM measurements were obtained with an atomic force microscope from Digital Instruments, Nanoscope IIIA, using the tapping mode and a frequency of 300 kHz.

homogeneity in mixed films of YbPc2 and SA at the nanometer and micrometer scales; that is accomplished by combining AFM and Raman imaging techniques. The relative proportions of mixed YbPc2/SA LB films were chosen to compare the homogeneity of the films using a small and a large amount of stearic acid. The monolayer characteristics and the nature of interaction between the components have already been investigated for mixed films of SA and YbPc2.15 A study using FTIR data on the deposited mixed 50/50% YbPc2/SA LB films indicated no interaction between the components at the molecular level. In addition, the mixed monolayers are stable and amenable to transfer as Y-type LB films with a unity transfer ratio.15 Surface pressure isotherms with various relative concentrations of SA and YbPc2 were studied. The average area per molecule increased linearly with the concentration of YbPc2 which occupies a larger area than stearic acid. For all relative concentrations, Y-type LB films could be deposited with a transfer ratio of approximately 1. 3.1. Electronic Absorption Spectra. The electronic absorption spectra for YbPc2 in solution and a neat LB monolayer film on glass and on silver islands as well as the spectra for the silver plasmon are shown in Figure 1. The absorption at 650 nm (1.9 eV) is the phthalocyanine Q-band due to transitions from the split π (a2u) orbitals to the upper π* (eg*) orbitals. The band at 460 nm (2.7 eV) corresponds to transitions from the π (eg) levels to the half-occupied π (a2u) orbital, while the B (Soret) band that appears in the ultraviolet region (320 nm, 3.8 eV) is attributed to the transitions between π (b2u) and π* (eg*) levels.16 The plasmon absorption of silver coated with the LB film in Figure 1 shows the effect of changing the dielectric constant of the medium, as air was replaced by an organic layer.19 The 633 nm laser line is in resonance with the electronic absorption spectrum of the YbPc2 LB film, a prerequisite for the RRS effect. Both 633 and 780 nm laser lines are in resonance with the broad silver plasmon, a necessary condition to achieve surfaceenhanced spectra (SERRS and SERS, respectively, in this

III. Results and Discussion

(15) Gaffo, L.; Gonc¸ alves, D.; Dhanabalan, A.; Moreira, W. C.; Oliveira, O. N., Jr. Synth. Met. 2001, 124, 351. (16) Mendonca, C. R.; Gaffo, L.; Misoguti, L.; Moreira, W. C.; Oliveira, O. N., Jr.; Zilio, S. C. Chem. Phys. Lett. 2000, 323 (3-4), 300.

We mentioned in the Introduction that one primary aim of this work is to investigate the degree of mixing and

Films of Ytterbium Bisphthalocyanine/Stearic Acid

Langmuir, Vol. 18, No. 9, 2002 3563

Table 1. Characteristic Molecular Vibrations in cm-1 from RRS (5 Layers on Mica and 1 Layer on Glass) and SERRS (1 Layer on Ag) Spectra Recorded Using the 633 nm Laser Line for Mixed YbPc2/SA LB Filmsa RRS/633 nm 5-layer LB (mica) (75% Yb) 478 (6) 25 559 (8) 0.5 575 (8) 21 678 (7) 100 740 (8) 52 769 (9) 13 779 (8) 16 816 (7) 27 839 (19) 2 940 (6) 11 1008 (7) 3 1030 (8) 3 1102 (7) 23 1144 (10) 14 1158 (29) 1 1198 (16) 10 1216 (13) 24 1258 (9) 2 1299 (11) 15 1331 (8) 21 1346 (11) 10 1362 (14) 5 1415 (23) 8 1423 (8) 17 1451 (8) 3 1528 (32) 37 a

SERRS/633 nm monolayer LB (Ag island) (25% Yb)

RRS/633 nm monolayer LB (glass) (25% Yb)

SERRS/633 nm monolayer LB (Ag island) (75% Yb)

479 (7) 16

480 (6) 24

479 (8) 16

576 (8) 24 680 (7) 100 740 (12) 36

577 (8) 21 681 (7) 100 741 (8) 45

776 (15) 13 816 (8) 19 841 (12) 2 941 (5) 3 1007 (13) 1 1030 (2) 1 1102 (6) 12 1141 (12) 11

777 (16) 13 818 (7) 25

576 (8) 22 680 (7) 100 740 (8) 56 768 (8) 16 778 (8) 20 816 (8) 27 838 (18) 3 940 (6) 14 1007 (6) 3 1030 (4) 2 1101 (7) 26 1142 (10) 22

1197 (20) 7 1216 (13) 13 1258 (8) 4 1300 (9) 9 1331 (9) 17 1344 (12) 8

1203 (20) 8 1217 (10) 26

1424 (12) 14 1451 (13) 4 1525 (33) 26

942 (6) 11 1103 (8) 23 1145 (11) 11

1301 (9) 15 1332 (8) 20 1347 (11) 7 1363 (11) 6 1422 (15) 19 1529 (32) 25

1201 (36) 11 1216 (11) 22 1257 (15) 1 1300 (9) 17 1330 (9) 26 1344 (11) 13 1361 (12) 6 1421 (14) 24 1450 (6) 3 1523 (31) 43

assignments pyrrole ring deformation C-C-C bending benzene radial Pc breathing C-H wag Pc ring benzene breathing C-C-C bend C-H wag C-H bend C-H bend C-H bend pyrrole C-H bend C-H bend C-H bend C-H bend C-H bend pyrrole stretch isoindole stretch isoindole stretch isoindole stretch isoindole stretch CdN pyrrole stretch

The relative intensities are indicated in italics, and the fwhm is given in parentheses in cm-1.

Figure 3. RRS spectra for mixed 25/75% and 75/25% 5-layer YbPc2/SA LB films on mica recorded with the 633 nm laser line.

Figure 2. Top: RRS spectrum for a 1-layer YbPc2 LB film on glass and SERRS spectrum for a 1-layer YbPc2 LB film on Ag islands recorded with the 633 nm laser line. Bottom: SERS spectrum for a 1-layer YbPc2 LB film on Ag islands recorded with the 780 nm laser line.

case).17 The LB monolayer film absorption on glass and on silver islands is red shifted with respect to the solution spectra. Following Kasha’s point-dipole model, the red shift can be attributed to a head to tail arrangement of the dipoles, which results in J-type aggregates.18 3.2. RRS, SERRS, and SERS Spectra. The RRS and SERRS spectra recorded with the 633 nm line (top) and the SERS spectrum recorded using the 780 nm laser line (17) Moskovits, M. Rev. Mod. Phys. 1985, 57, 783. (18) Photochemistry in Organized & Constrained Media; Ramamurthy, V., Ed.; VCH Publishers: New York, 1991; p 691.

(bottom) of the LB YbPc2 monolayer are presented in Figure 2. The RRS and SERRS spectra are in the same scale, but RRS was recorded using 100% of the laser power, 10 s of collecting time, and 1 accumulation, while SERRS was collected using 10% of the laser power, 1 s of collecting time, and 1 accumulation. An average enhancement factor (EF) greater than 300 is estimated by the intensity ratio between the SERRS/RRS signals. The wavenumbers are the same for RRS and SERRS, which indicates the absence of a strong metal-molecule interaction. Therefore, the enhancement in SERRS must be attributed to the electromagnetic mechanism where the contribution of the chemical effect to the overall signal intensity is negligible.19 EF for SERS cannot be estimated since the RS for the film on glass could not be detected even with long collecting times and several accumulations. The SERS in Figure 2 was recorded using 100% of the laser power, 1 s of collecting (19) Weitz, D. A.; Garoff, S.; Nitzan, A. J. Chem. Phys. 1983, 78, 5324.

3564

Langmuir, Vol. 18, No. 9, 2002

Gaffo et al.

Figure 4. RRS area maps for mixed 25/75% (top) and 75/25% (bottom) YbPc2/SA 5-layer LB films on mica recorded with the 633 nm laser line. The images show all of the spectra recorded at 42 × 42 µm2 with a step of 3 µm.

time, and 5 accumulations. The RRS and SERRS results in Figure 2 are consistent with the YbPc2 results presented by Aroca et al.20-22 The RRS spectra recorded with the 633 nm laser line for 5-layer LB films deposited on mica from mixtures of 25/75% and 75/25% YbPc2/SA are presented in Figure 3. Table 1 contains the characteristic molecular vibrations in cm-1 for the spectra obtained: RRS and SERRS spectra for the mixed YbPc2/SA LB films. The relative intensities are indicated in italic type, and the numbers given in parentheses are the full width at halfmaximum (fwhm) in cm-1 obtained using the curve fit programs in GRAMS/32 with mixed Gaussian-Lorentzian band shapes. The assignments are listed in the fifth column. Some qualitative conclusions can be drawn from the data in Table 1 and Figures 2 and 3. The spectra recorded with the 633 nm line for LB films from neat YbPc2 and YbPc2 mixed with SA (25%/75%) have identical wavenumbers, with the strongest band at 680 cm-1. The latter support the conclusion that SA is not interacting with YbPc2. The surface-enhanced signals obtained with silver islands also showed that the wavenumbers are not altered, with the strongest band at 680 cm-1. When the results for 1-layer YbPc2 LB films are compared, using SERRS with the 633 nm laser line and SERS with the 780 nm laser (20) Tarcha, P. J.; DeSaja-Gonzalez, J.; Rodriguez-Llorente, S.; Aroca, R. Appl. Spectrosc. 1999, 53, 43. (21) Aroca, R.; Clavijo, R. E.; Jennings, C. A.; Kovacs, G. J.; Duff, J. M.; Loutfy, R. O. Spectrochim. Acta 1989, 45A, 957. (22) Clavijo, R. E.; Battisti, D.; Aroca, R.; Kovacs, G. J.; Jennings, C. A. Langmuir 1992, 8, 113.

line, it can be seen that the strongest bands differ in the two spectra. For the 780 nm laser line, the most intense band is at 742 cm-1, while for the 633 nm line it is at 680 cm-1 as shown in Figure 2. This may be explained by the fact that the latter band is attributed to the totally symmetric breathing mode of the Pc macrocycle and the signal is further enhanced with the 633 nm line because it is in resonance. A comparison of the data in Table 1 shows that essentially the same results are obtained for the LB films with 25 and 75% of YbPc2, which should be expected, as the components do not interact. The intensities of several bands assigned to YbPc2 groups are higher for the film with a higher amount of YbPc2. Since there is no interaction between Ag-Pc, the wavenumbers are the same for RRS and SERRS, the only difference being the signal enhancement for SERRS. Also, the SERRS for a monolayer on a silver island is similar to the RRS for a 5-layer LB film. This indicates that the organization of the layers is not significantly affected when increasing the number of deposited layers from 1 to 5. 3.3. AFM and RRS Area Mapping Images. Figure 4 shows all of the RRS spectra collected for the area mapping (area of 42 × 42 µm2 and step of 3 µm) using the mixed 25/75% and 75/25% YbPc2 5-layer LB films on mica. The mixed LB film with 75% of YbPc2 presents more homogeneous and on average stronger relative intensities for the Raman activity for all spots on the micrometer scale. Figure 5 confirms this behavior with Raman images

Films of Ytterbium Bisphthalocyanine/Stearic Acid

Langmuir, Vol. 18, No. 9, 2002 3565

Figure 5. RRS area maps using the integrated area for the peak at 680 cm-1 for mixed 75/25% and 25/75% YbPc2/SA 5-layer LB films on mica recorded with the 633 nm laser line. The data were extracted from the RRS area maps presented in Figure 4.

Figure 6. AFM image for a mixed 25/75% YbPc2/SA 5-layer LB film on mica recorded with the tapping mode (top) and transversal section analysis (bottom). Also shown is the height profile.

using the integrated area at the 680 cm-1 peak for both mixed LB films. The mixed LB film with 25% of YbPc2 contains large aggregates (bright spots). Since the bright spots represent higher Raman activity, the absence of contrast in color for the film with 75% of YbPc2 indicates the uniform distribution of YbPc2 on the mica surface. It must be concluded that the 75% YbPc2 film presents aggregates homogeneously distributed on the surface on the micrometer scale. AFM images in Figures 6 and 7, from the mixed 25/75% and 75/25% YbPc2 5-layer LB films on mica, respectively, indicate that the behavior found on the micrometer scale can be extrapolated to the nanometer scale. Comparison of films transferred onto distinct substrates, that is, mica and Ag, is possible because YbPc2 does not interact with either mica or Ag. The similarity in behavior at the two scales is unlikely to be due to the substrate morphology, since mica has a flat surface while Ag forms nanoparticles. These figures show that the mixed LB film with 25% of YbPc2 has aggregates that are fewer in number but larger in

Figure 7. AFM image for a mixed 75/25% YbPc2/SA 5-layer LB film on mica recorded with the tapping mode (top) and transversal section analysis (bottom). Also shown is the height profile.

size (diameter and height). These aggregates seem to be of YbPc2 since put together they are able to cover a larger area for the mixed LB film with 75% of YbPc2. That films with 75% of YbPc2 are more homogeneous was confirmed with the height profiles shown in Figures 6 and 7 and with the root-mean-square (rms) roughnesses, which were 0.85 nm for 75% of YbPc2 and 1.58 nm for the LB film with 25% of YbPc2. Therefore, even though SA enhances the transferability of YbPc2 monolayers, the transfer ratios were identical for the mixtures with different concentrations and the AFM and Raman data indicate that the most uniform films are obtained with smaller amounts of SA. IV. Conclusions The use of Raman imaging and AFM for LB films of mixed SA and YbPc2 indicated that the level of heterogeneity at the nanometer scale observed with AFM is also

3566

Langmuir, Vol. 18, No. 9, 2002

apparent in the micrometer scale Raman images. Phase separation, as indicated by the formation of larger aggregates, is more apparent in the 25% YbPc2 film. Further characterization of the LB films using RRS, SERS, and SERRS confirms that there is no interaction between stearic acid and YbPc2 at the molecular level. The

Gaffo et al.

enhancement in SER(R)S on metal islands is due to the electromagnetic mechanism. Acknowledgment. This work was supported by FAPESP, CNPq (Brazil), and NSERC of Canada. LA011234E