Atomic Force Microscopy and Micro-Raman Imaging of Mixed Langmuir−Blodgett Films of Ytterbium Bisphthalocyanine and Stearic Acid

Abstract

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.

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“…Figure 3C presents a similar result, but with the Raman spectra collected along an area (instead of a line) of 50 µm x 50 µm with a step of 5 µm leading to a total of 121 spectra (one of these spectra is shown at the top right corner in Figure 3C). Again, the brighter spots indicate a stronger Raman signal for the band at 1515 cm -1 assigned to CdN pyrrole stretching, 25,26 which is shown in the spectrum at the top right corner. Overall, the FePc PVD and LB films seem to be the more homogeneous at the micrometer scale while the LbL would be the less homogeneous one accordingly to the optical images.…”

“…The bands at 729 cm -1 are attributed to C-H wagging (FePc macrocycle out-of-plane), while the bands at 1117 and 1331 cm -1 are assigned, respectively, to C-H in-plane deformation and CdC or CdN stretching (pyrrol), also in the plane of the FePc macrocycle. 25,26,31,32 The C-H wagging is relatively intense in all the spectra (KBr pellet and PVD films), and dominates the FTIR-reflection spectrum. This difference in the relative intensity for the transmission and reflection modes reveals anisotropy in the molecular organization of FePc in the PVD films.…”

“…These lines are the 2-dimensional Raman mappings where the brighter spots indicate stronger Raman signals for the band at 750 cm -1 assigned to C-H wagging. 25,26 All the 101 Raman spectra collected along that line are given at the top right corner forming the 3-dimensional Raman mappings. Figure 3C presents a similar result, but with the Raman spectra collected along an area (instead of a line) of 50 µm x 50 µm with a step of 5 µm leading to a total of 121 spectra (one of these spectra is shown at the top right corner in Figure 3C).…”

“…Furthermore, the red shift in the maximum absorption for LB films in relation to PVD films suggests distinct molecular organizations and/or molecular aggregate formation (J and H types), although the molecule is the same. 25,26,32,34 Film Morphology at Micrometer and Nanometer Scales. The morphology at micrometer and nanometer scales of LbL, PVD, and LB films was investigated through micro-Raman spectroscopy and AFM.…”

“…There is now a surge on their use for devices, for example, in photocopiers, solar cells, electrochromic displays, fuel cells, light-emitting diodes, transistors, optical limiters, in photodynamic therapy, [6][7][8][9][10][11][12][13][14][15][16][17][18] and sensors. 15,[19][20][21] In several of the possible applications, one important requirement is to produce materials in the form of thin films, which has been carried out for phthalocyanines with various methods, including Langmuir-Blodgett (LB), [22][23][24][25][26] electrostatic layer-bylayer (LbL), [27][28][29][30] physical vapor deposition (PVD), 31,32 and casting. 33 For sensing, in particular, it has been shown that film morphology and surface properties are relevant for the sensor performance.…”