pubs.acs.org/Langmuir © 2010 American Chemical Society
Layer-by-Layer Electrostatic-Assembly: Magnetic-Field Assisted Ordering of Organic Molecules Sukumar Dey and Amlan J. Pal* Department of Solid State Physics, Indian Association for the Cultivation of Science, Jadavpur, Kolkata 700032, India Received July 28, 2010. Revised Manuscript Received September 21, 2010 During the layer-by-layer (LbL) electrostatic assembly process, we orient organic molecules (nickel phthalocyanine) by an external magnetic field. Orientation of the magnetic moment of the molecules in a monolayer is immobilized by depositing a monolayer of a suitable polycation. Due to the orientation of magnetic moments, the electrostatic adsorption process becomes enhanced in subsequent layers. By cycling the deposition sequence layer after layer, we have achieved highly ordered and closely packed LbL films of the molecules with their magnetic moments oriented perpendicular to the substrate. Nonmagnetic copper phthalocyanine expectedly showed neither a magnetic-field assisted alignment nor an enhanced adsorption in LbL film deposition process.
Introduction Layer-by-layer (LbL) electrostatic assembly has been a unique method to grow ultrathin films of polyions.1-4 The method has been extended to organic molecules and inorganic quantum dots.5-14 The LbL method relies on surface charge reversal during adsorption of anionic and cationic layers in sequence. The morphology of such films has been found to depend on the pH of ionic species in solutions and functional groups attached to molecules/polyions. Since LbL layers are being used for a range of thin-film based applications,15-18 the morphology of the films has become increasingly important. In many cases, it has a large impact on the efficiency or output of devices. Efforts have been made to control the morphology of the films or the LbL growth process. Since the materials used here are ionic *Corresponding author. E-mail:
[email protected]. Telephone: þ91-3324734971. Fax: þ91-33-24732805.
(1) Decher, G. Science 1997, 277, 1232–1237. (2) Ferreira, M.; Rubner, M. F. Macromolecules 1995, 28, 7107–7114. (3) Ariga, K.; Lvov, Y.; Kunitake, T. J. Am. Chem. Soc. 1997, 119, 2224–2231. (4) Schoeler, B.; Kumaraswamy, G.; Caruso, F. Macromolecules 2002, 35, 889– 897. (5) Kotov, N. A.; Dekany, I.; Fendler, J. H. J. Phys. Chem. 1995, 99, 13065– 13069. (6) Sarathy, K. V.; Thomas, P. J.; Kulkarni, G. U.; Rao, C. N. R. J. Phys. Chem. B 1999, 103, 399–401. (7) Clark, S. L.; Handy, E. S.; Rubner, M. F.; Hammond, P. T. Adv. Mater. 1999, 11, 1031–1035. (8) Mamedov, A. A.; Kotov, N. A. Langmuir 2000, 16, 5530–5533. (9) Das, S.; Pal, A. J. Langmuir 2002, 18, 458–461. (10) Schneider, G.; Decher, G. Nano Lett. 2004, 4, 1833–1839. (11) Suda, M.; Miyazaki, Y.; Hagiwara, Y.; Sato, O.; Shiratori, S.; Einaga, Y. Chem. Lett. 2005, 34, 1028–1029. (12) Lee, D.; Rubner, M. F.; Cohen, R. E. Nano Lett. 2006, 6, 2305–2312. (13) Baba, A.; Locklin, J.; Xu, R. S.; Advincula, R. J. Phys. Chem. B 2006, 110, 42–45. (14) Doherty, W. J.; Friedlein, R.; Salaneck, W. R. J. Phys. Chem. C 2007, 111, 2724–2729. (15) Wu, A.; Yoo, D.; Lee, J. K.; Rubner, M. F. J. Am. Chem. Soc. 1999, 121, 4883–4891. (16) Cassagneau, T.; Mallouk, T. E.; Fendler, J. H. J. Am. Chem. Soc. 1998, 120, 7848–7859. (17) He, J. A.; Mosurkal, R.; Samuelson, L. A.; Li, L.; Kumar, J. Langmuir 2003, 19, 2169–2174. (18) Ariga, K.; Hill, J. P.; Ji, Q. M. Phys. Chem. Chem. Phys. 2007, 9, 2319–2340. (19) Gao, M. Y.; Sun, J. Q.; Dulkeith, E.; Gaponik, N.; Lemmer, U.; Feldmann, J. Langmuir 2002, 18, 4098–4102. (20) Wang, Y.; Wang, X. J.; Guo, Y.; Cui, Z. C.; Lin, Q.; Yu, W. Z.; Liu, L. Y.; Xu, L.; Zhang, D. M.; Yang, B. Langmuir 2004, 20, 8952–8954.
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in nature, the electric field has been a natural choice to do so.19,20 the LbL deposition process of polyions and suitably capped quantum dots has been found to be greatly affected by an electric field. For ferromagnetic quantum dots, magnetic fields could orient their magnetic dipoles in a monolayer that in turn supplemented electrostatic force of attraction for the subsequent layer and augmented LbL deposition process.21 In most of the LbL films deposited with magnetic materials, such as magnetic colloids, core-shell nanoparticles, nanocomposites, etc., their magnetic properties have so far been studied.22-24 With magnetic organic molecules, it can be intriguing to orient the molecules by aligning their magnetic moments in LbL films. A monolayer of molecules with their magnetic moments oriented at a particular direction can be used for molecular spintronics and organic data storage applications. In this article, we have taken a derivative of nickel(II) phthalocyanine, namely, the tetrasulfonic acid tetrasodium salt of nickel(II) phthalocyanine (NiPc) in forming magnetic-field assisted LbL films. A monolayer of a polycation, in sequence with the aligned and anionic NiPc monolayer, froze the magnetic alignment of NiPc molecules in the monolayer and at the end yielded closely packed LbL films, where magnetic moments of the molecules are aligned perpendicular to the substrate.
Materials and Methods LbL Film Deposition. Tetrasulfonic acid tetrasodium salt of nickel(II) phthalocyanine (NiPc) and copper(II) phthalocyanine (CuPc), and a polycation, namely poly(allylamine hydrochroride) (PAH), were purchased from Aldrich. PAH had an average molecular weight of 70 000 g/mol. LbL films of NiPc and PAH were deposited by the electrostatic adsorption process. Films were grown on precleaned quartz substrates and on silicon wafers. To obtain LbL films, PAH solution, 5 10-3 M based on its repeat (21) Dey, S.; Mohanta, K.; Pal, A. J. Langmuir 2010, 26, 9627–9631. (22) Jaiswal, A.; Colins, J.; Agricole, B.; Delhaes, P.; Ravaine, S. J. Colloid Interface Sci. 2003, 261, 330–335. (23) Aliev, F. G.; Correa-Duarte, M. A.; Mamedov, A.; Ostrander, J. W.; Giersig, M.; Liz-Marzan, L. M.; Kotov, N. A. Adv. Mater. 1999, 11, 1006–1010. (24) Hong, X.; Li, J.; Wang, M. J.; Xu, J. J.; Guo, W.; Li, J. H.; Bai, Y. B.; Li, T. J. Chem. Mater. 2004, 16, 4022–4027.
Published on Web 10/07/2010
DOI: 10.1021/la102996t
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units, was used as a cationic bath. A solution of NiPc in deionized water (resistivity = 18.2 MΩ 3 cm) with a concentration of 5 10-3 M was used as an anionic bath. To deposit LbL films, quartz or Si substrates were first dipped in the NiPc solution (anionic bath) for 15 min to adsorb a monolayer of the dye. To remove electrostatically unbound moieties on the surface, the films were thoroughly washed by dipping the substrates in three separate deionized water baths. The substrates were then dipped in a PAH solution (cationic bath) for 15 min followed by the same rinsing protocol in a separate set of water baths. This has resulted in one bilayer of LbL film of NiPc. The dipping sequence was repeated in cycle to obtain a desired number of bilayers of NiPc LbL film. The same procedure was followed to deposit LbL films of CuPc. LbL Assembly with “Magnetic-Moments Oriented”. Apart from the conventional LbL deposition process, we have aimed to form LbL films with NiPc’s magnetic moment oriented perpendicular to the substrate. This additionally meant LbL deposition with an assistance from oriented magnetic moments of the molecules. To orient the magnetic moments of NiPc in LbL films, we first adsorbed a monolayer of NiPc followed by the usual three rinses. A magnetic field of 320 mT was then applied perpendicular to the film, which was kept in a suspended position in an empty beaker. After 2 min, a 5 mM aqueous solution of PAH was poured in the beaker so that the film became completely immersed in PAH solution. Electrostatic adsorption of PAH occurred and was allowed to continue for 15 min. The magnetic field was then switched off and the film was removed from the polycation solution. The rinsing protocol completed adsorption of a bilayer of LbL deposition process with NiPc’s “magnetic moments oriented”. The dipping sequence was repeated in sequence to obtain a desired number of LbL layers. The same procedure was followed to deposit LbL films of CuPc as a control experiment to find out if alignment of CuPc was achievable. Characterization of the Films. In all the cases, the electronic absorption spectrum of a thin film was recorded after adsorption of every monolayer of a dye followed by a monolayer of PAH. From atomic force microscopy (AFM) of a scratch on the film, the depth profile and hence the thickness of each of the films were estimated. The AFM topographies also gave roughness of the surface of the films. Orientation of magnetic moments in the films was Recorded by Magnetic Force Microscopy (MFM) images. The MFM images were recorded with a magnetized tip that recognized magnetic phases of the films. The images were scanned in a lift mode (100 nm). While MFM studies were carried out with Veeco CPII, AFM images were recorded with Nanosurf EasyScan2 in ambient condition. Films were deposited also on indium tin oxide (ITO) coated glass substrates. Aluminum as the top electrode was grown via thermal evaporation under vacuum to form sandwiched device structures. Current-voltage (I-V) characteristics and impedance spectroscopy of the devices were recorded with a Keithley 6517 Electrometer and a Solartron 1260 Impedance Analyzer, respectively.
Results and Discussion We have deposited LbL films of NiPc with PAH in sequence. Electronic absorption spectrum of the film was recorded after deposition of every cationic layer to follow the growth of LbL films (part a of Figure 1). The spectra show the signature of NiPc in the films. Part c of Figure 1 sums up the spectra as a plot of absorbance of NiPc as a function of the number of deposited bilayers. Absorbance of NiPc increases linearly with the number of LbL bilayers evidencing regular film formation layer after 17140 DOI: 10.1021/la102996t
Figure 1. Electronic absorption spectra of different bilayers of NiPc LbL films with magnetic moments of the molecules (a) unoriented and (b) oriented. (c) Absorbance at 616 nm versus number of deposited-bilayers for the two cases. The inset of (c) shows the chemical structure of NiPc.
layer. Such a film has been labeled as the “magnetic-moments unoriented” case, depicting regular LbL films of NiPc. We also monitored the growth of LbL films of NiPc with their “magnetic moment oriented”. This also was done by recording the electronic absorption spectrum of the film after deposition of every bilayer (part b of Figure 1). Here, as compared to “magnetic-moment unoriented” films, the absorbance corresponding to NiPc increased at a higher rate as the LbL deposition process progressed. Part c of Figure 1 shows that the slope of the absorbance versus layer number plot was more in the “magneticmoments oriented” case than the “magnetic-moments unoriented” films. That is, NiPc molecules in “magnetic-moments oriented” LbL films, due to their oriented magnetic moments, enhanced electrostatic adsorption of NiPc molecules in the subsequent monolayer(s). In the following, we will analyze LbL deposition process of NiPc molecules, which were aligned with a magnetic field. NiPc molecules have a magnetic moment due to their 3d8 electrons. Upon electrostatic adsorption through one or more of its -SO3moiety(ies), the molecules initially remained unoriented in the film. At this stage, when a magnetic field was applied perpendicular to the substrate, magnetic moments of the molecules become oriented along the field. Since the film was immersed in PAH solution at this stage, the remaining -SO3- groups of oriented NiPc molecules now bind to -NH2þ moieties of PAH. The adsorption process of PAH hence freezes the alignment of NiPc molecules in LbL films. We will now present results from several support experiments to validate formation of LbL films with magnetic moments of Langmuir 2010, 26(22), 17139–17142
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Figure 3. Magnetic force microscopy images (amplitude-lift mode scan) of a 15-bilayer LbL film of NiPc (a) without orienting magnetic moments of the molecules and (b) orienting magnetic moments with a 320 mT magnetic field. The area of each MFM images was 600 nm 600 nm.
Figure 2. Absorbance at 609 nm versus number of deposited bilayers for CuPc LbL films without and with a magnetic field during PAH adsorption of LbL film deposition. The inset of the figure shows the chemical structure of CuPc.
NiPc being aligned. First of all, the results were reproducible and independent of substrates on which LbL films were deposited. Other (substituted) metal phthalocyanines with cobalt, iron, or manganese that have different magnetic dipole moments and hence would have responded differently in magnetic-field assisted LbL deposition process, were not commercially available. We therefore varied the magnetic field that is expected to be a parameter for the alignment of NiPc molecules in LbL films. We have observed that a critical magnetic field is required to observe an augmentation in the LbL deposition process. That is, if a low magnetic field (95 mT) was applied to the film during the deposition process, electronic absorption corresponding to NiPc increased with the number of bilayers at the same rate as when a magnetic field was not applied. In another control experiment, we have chosen CuPc to form LbL films with a 320 mT magnetic field. In this case, the electronic absorbance increased monotonically with the number of deposited layers (Figure 2). In contrast to the NiPc case, magnetic field did not accelerate LbL assembly process of CuPc. That is, the plots of absorbance versus number of bilayers are invariant to application of a magnetic field (Figure 2). Here, CuPc, due to 3d10 electrons of copper, is diamagnetic and hence did not respond to the magnetic field in LbL films. Magnetic force microscopy (MFM) of LbL films is another support experiment. In Figure 3, we present MFM images of LbL films of NiPc with “magnetic moments unoriented” and “magnetic moments oriented”. The latter image clearly shows oriented magnetic domains evidencing aligned magnetic moments of NiPc in LbL films. Since the magnetic moment of NiPc is perpendicular to its molecular plane, NiPc molecules remain parallel to the substrate in the “magnetic-moments oriented” case. Polarized electronic absorption studies of NiPc films with their magnetic moments oriented perpendicular to the substrate hence did not show any difference. Here we used linearly polarized light; we compared electronic absorption spectra with the polarization of incident beam being vertical and horizontal. There was no difference between the two spectra, implying that the aligned NiPc molecules remained parallel to the substrate in LbL films. We then looked at the origin of increased absorbance in NiPc LbL films deposited by aligning the magnetic moments of the molecules (part b of Figure 1). An increase in absorbance could be Langmuir 2010, 26(22), 17139–17142
Figure 4. (a) Typical AFM image of a scratched 20-bilayer LbL film along with (b) the depth profile of the scratch. The area of the AFM image was 58.3 μm 58.3 μm. (c) Film thickness versus number of bilayers of NiPc with and without their magnetic moments oriented.
due to formation of multiple layers during each dipping stroke or a higher compactness in the film. As such, the electrostatic assembly process forbids formation of a multiple-layer during adsorption of a layer. To verify that a monolayer was indeed formed during each dipping in the magnetic-field assisted LbL films, we have measured the thickness of the films deposited with and without a magnetic field. We have measured the thickness for different numbers of deposited bilayers. In practice, we recorded the surface profile of an intentional scratch on the film (part a of Figure 4). From the depth profile of the scratch, we estimated the thickness of the film (part b of Figure 4). Thickness versus number of bilayers is then plotted for the two types of films (part c Figure 4). The results show that the thickness of NiPc films was not higher when magnetic moments of the molecules became aligned perpendicular to the substrate. Increased absorbance in the “magnetic-moments oriented” films must hence be due to higher compactness of the films. That is, the magnetic force of attraction, on top of the electrostatic one, has led to the formation of a compact monolayer during subsequence dipping in the NiPc DOI: 10.1021/la102996t
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compared to the usual LbL films. To measure the conductivity, we have recorded I-V characteristics of sandwiched devices with films on ITO glass and aluminum as the top electrode. Since the thickness of the films was the same in the two cases and we kept the area of the devices (overlap of ITO and aluminum strips) also the same, conductivity of the films could be compared from the current values at any voltage. We have observed a higher current in devices based on “magneticmoments oriented” LbL films (part a of Figure 5). Here highly ordered close-packing of NiPc molecules in films has led to efficient intermolecular hopping transport and correspondingly a higher current and conductivity in “magnetic-moments oriented” LbL films. We have recorded complex impedance of the devices as a function of ac test frequency under zero and at different dc biases, such as, 0.5 and 1.0 V. Here, the diameter of semicircular plots of imaginary versus real component of complex impedance (Cole-Cole plots) is a measure of dc resistance of the devices. Part b of Figure 5 shows that the diameter of the semicircular plot was shorter and hence the dc resistance of the devices was lower in films where magnetic moments of the molecules were aligned as compared to that with “magneticmoments unoriented” films. The results support formation of highly ordered LbL films of NiPc molecules with their molecular planes closely packed and magnetic moments oriented perpendicular to the substrate enabling increased electrical conduction process.
Conclusions
Figure 5. (a) Current-voltage characteristics and (b) Cole-Cole plots at three different dc voltages of a 10-bilayer LbL film of NiPc without orienting magnetic moments of the molecules and orienting magnetic moments with a 320 mT magnetic field during LbL deposition. While continuous lines represent films where magnetic moments of the molecules were oriented, broken lines represent “magnetic moments unoriented” cases.
bath. With the molecules’ magnetic moments oriented or aligned perpendicular to the plane of NiPc molecule and the substrate, this would mean highly ordered close-packing of NiPc molecules in the films similar to ultrathin crystalline flakes. This is supported by a decrease in surface roughness from 3.0 to 2.5 nm for a 20-bilayer films, as obtained from AFM images, of “magnetic-moments oriented” LbL films as compared to “magnetic-moments unoriented” ones. Close-packing of the molecules in the films has resulted in a higher conductivity in the “magnetic-moments oriented” case as
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In conclusion, while depositing LbL films of NiPc we have aligned magnetic moments of NiPc molecules perpendicular to the substrate. Electronic absorption spectroscopy shows that due to the orientation of magnetic moments, the LbL adsorption process has been enhanced in subsequent layers. Here, the magnetic force of attraction has supplemented the electrostatic adsorption process in LbL film deposition. Diamagnetic CuPc did not exhibit such an alignment in LbL deposition. By repeating the deposition sequence, we have achieved LbL films, where the NiPc molecules are aligned and closely packed with their magnetic moments oriented perpendicular to the substrate. Close-packing of NiPc molecules in the films has been supported by the results from conductivity and impedance measurements of sandwiched devices based on the LbL films. Acknowledgment. We acknowledge fruitful discussion with Dr. Subham Majumdar. The work was financially supported by Nano Mission project SR/NM/NS-55/2009 and Ramanna Fellowship SR/S2/RFCMP-01/2009. S.D. acknowledges CSIR Fellowship Nos. 9/080(0647)/2009-EMR-I (Roll No. 507031).
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