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Highly ordered thin films prepared with octabutoxy copper phthalocyanine complexes
Ultramicroscopy 97 (2003) 271–278
Highly ordered thin films prepared with octabutoxy copper phthalocyanine complexes Kelly Stevensona, Naoko Miyashitaa, Joanne Smiejab, Ursula Mazura,* a
Department of Chemistry and Materials Science Program, Washington State University, Pullman, WA 99164-4630, USA b Department of Chemistry, Gonzaga University, Spokane, WA 99258, USA Received 11 September 2002; received in revised form 31 October 2002
Abstract Langmuir–Blodgett (LB) films of copper (II) 1,4,8,11,15,18,22,25-octabutoxyphthalocyanine, nCuPc(OBu)8, (nonperipheral substitution) and copper (II) 2,3,9,10,16,17,23,24-octabutoxyphthalocyanine, pCuPc(OBu)8, (peripheral substitution), were fabricated and characterized by optical spectroscopy and scanning probe microscopy. The LB films were transferred onto hydrophilic substrates by vertical dipping. Although they posses relatively short polar substituents both compounds form smooth, uniform, dense, and highly stable LB monolayers composed of linear arrays of cofacial oligomers. The long range discotic assemblies of LB and spun cast films of pCuPc(OBu)8 and nCuPc(OBu)8 posses physical and chemical properties favorable for molecular electronic device application. r 2003 Elsevier Science B.V. All rights reserved.
1. Introduction Of past, present, and future interest as components in molecular electronic devices are the phthalocyanine-like molecules. Because of their thermal, chemical, and physical stability (in excess of 500 C), and useful photophysical, redox, and catalytic properties, they are used in many advanced technological applications [1]. They are employed in laser printers [2], in optical data storage systems [3], and in electronic devices such as WORM drives [4]. They are also of continuing interest as active elements in photovoltaic cells [5], fuel cells [6], and chemical sensors [7].
*Corresponding author. Fax: +1-509-335-8867. E-mail address: [email protected] (U. Mazur).
The exploitation of the desirable optical and electrical or electrochemical properties of phthalocyanines (Pc), relies on the precise control over the molecular packing and ordering in the solid phase. Unsubstituted Pc self-organize into a variety of crystalline polymorphs and have been used to prepare ordered thin films by vapor deposition [1,8–10]. In addition, many Pc derivatives with flexible side-chains make excellent candidates for Langmuir–Blodgett (LB) film fabrication and also films generated by spin coating, and self-assembly [1,11–15]. Our interest in Pc materials is in employing them as model systems for studying electron transport processes by orbital-mediated tunneling spectroscopy (OMTS) both in the scanning tunneling microscopy (STM) and the metal–insulator– metal diode (M–I–M) environments. We have
0304-3991/03/$ - see front matter r 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0304-3991(03)00052-4
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investigated vapor deposited monolayer, and submonolayer films of several unsubstituted metal phthalocyanines (MPc) on Au(1 1 1) substrates and incorporated into tunnel diodes [16–21]. Our studies produced tunneling spectra that show a direct correlation between orbital mediated electron transfer processes and the expected HOMO and LUMO energies in single molecules (STM) and in collections of 109 molecules (M–I–M) [16–21]. We are now extending our efforts to the study of single LB layers of substituted Pc in which the macrocyclic rings are oriented in linear cofacially stacked domains. Electrochemical properties of linear cofacial arrays of Pc polymers are known to be strongly affected by the ring–ring spacing, twist angle, and position relative to the neighboring rings [1,13,22–25]. Functionalized and cofacially arranged Pc suitable for study either in the STM or M–I–M’ environment, need to have relatively short R groups in order to facilitate the tunneling processes. Many of the excellent candidates reported in the literature, however, proved unsuitable for our purposes because of their long and bulky R groups [1,11–15,26,27]. Two molecules that show great promise for our OMTS studies are the non-peripherally substituted copper (II) 1,4,8, 11,15,18,22,25-octabutoxyphthalocyanine, nCuPc(OBu)8 and the peripherally substituted copper (II) 2,3,9,10,16,17,23,24-octabutoxyphthalocyanine, pCuPc(OBu)8 [27]. In this article we report on the fabrication and structure of LB and spin-cast monolayer films of these complexes.
general synthetic method [28]. All solvents used were either reagent or spectroscopic grade and were acquired either from Aldrich Chemical Co. (see footnote 1) or J.T. Baker.2 Corning3 microscope slides were cleaned in a HNO3/H2O2 bath, and stored in high purity water until usage. Mica strips 1 4 cm (catalog 54) from purchased from Ted Pella.4 2.2. Film preparation Langmuir-Blodgett monolayers were prepared using the KSV 50005 LB double trough system equipped with two Whilhelmy balances. Each balance was mounted at the midpoint between two moving barriers. Films were spread on a MilliPore Milli Q-purified water subphase typically kept at approximately 20 C. Aliquots (200–700 ml) of 2 10 4 M solutions of the complexes in chloroform were employed. The Pc layer was allowed to stabilize for several minutes and was then compressed at a barrier speed of 20 mm/min until the desired surface pressure was reached. Mono- and multi-LB layers were transferred via Z-type deposition onto freshly cleaved mica substrates or pre-cleaned microscope slides. The dipping speed was 20 mm/min and the transfer ratios ranged between 0.95 and 1.05. Spin-cast films on mica and glass slides were made with the same solutions used for the LB film preparation. The substrates were exposed to the individual solutions for 15–30 s and then spun dry for 60 s to remove the solvent. 2.3. UV-vis
2. Experimental 2.1. Materials CuPc and the non-peripheral CuPc(OBu)8 were obtained from Aldrich Chemical Co.1 The parent complex was twice sublimed and the nCuPc(OBu)8 was recrystallized from tetrahydrofuran and ethanol prior to usage. The peripheral CuPc(OBu)8 analogue was prepared using a previously reported 1 Aldrich Chemical Co. Inc., 1001 West Saint Paul Avenue Milwaukee, WI 53233, USA. Tel.: 1-800-558-9260.
LB films of the CuPc(OBu)8 complexes were deposited on glass slides employing vertical dipping. CuPc was vapor deposited on glass slides in a HV deposition chamber at o6 10 7 Torr. Film 2 KSV Instruments USA, P.O. Box 192 Monroe, CT 06468, USA. Tel.: +1-800-280 6216. 3 J.T. Baker, 222 Red School Lane, Phillipsburg NJ 08865, USA. Tel.: +1-800-582-2537. 4 Ted Pella, Inc., P.O. Box 492477, Redding, CA 96049-2477, USA. Tel.: +1-800-237-3526. 5 GBC Scientific Equipment, 3930 Ventura Drive, Arlington Heights, IL 60004, USA. Tel.: +1-800-445-1902.
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Fig. 1. Surface pressure-area isotherms (ca. 20 C, pH 7) for pCuPc(OBu)8, thick line and for nCuPc(OBu)8, thin line.
thickness was monitored with a quartz crystal microbalance. Film deposition rate was 0.01 nm/s and a CuPc density of 1.6 g/cm3 was assumed. A clean glass slide was used as a reference. Electronic spectra of Pc compounds were recorded with a Cintra 406 UV-visible spectrometer from GBC Scientific. Data was collected at 150 nm/s and a slit opening of 2 nm.
were acquired in tapping mode with etched silicon tips having a diameter of 5 10 nm. The specified cantilever length was 127 mm and a force constant of the order of 50 N/m. The setpoint varied between 1.85 and 2.75 V and the resonance frequency of the cantilever was 260–340 kHz. The standard DI software was used for data acquisition.
2.4. AFM Monolayers of CuPc(OBu)8 LB films were prepared on mica substrates. The specimen was then mounted on a metal disk with a double stick tape and placed on the top of the scanner tube. The samples were always set in such a way that the fast scan direction and the dipping direction were the same. Spin-doped MPc samples were also prepared on mica. A precut mica substrate was exposed to 2 10 4 M solutions of CuPc(OB)8 complexes in chloroform solutions for 10–15 s and spun dry for 30 s. The AFM used for topographic imaging of thin films was Nanoscope IIIA multimode microscope from Digital Instruments, DI.7 All micrographs 6 VWR, P.O. Box 5229, Buffalo Grove, IL 60089-5229, USA. Tel.: +1-800-727-4368. 7 Digital Instruments. 112 Robin Hill Road, Santa Barbara, CA 93117, USA. Tel.: +1-800-873-9750.
3. Results and discussion Langmuir films of pCuPc(OBu)8 on water exhibit two distinct phase transitions (Fig. 1). Armstrong et al. also observed two sharply defined phase transitions in the LB isotherms of a peripherally substituted Pc derivative, 2,3,9,10, 16,17,23,24-octakis(2-benzyloethoxy) phthalocyanato copper (II), CuPc(OC2OBz)8 [13,14]. They associated these two transitions with formation of a monolayer at the lower surface pressure (first phase transition) and a bilayer of the complex at the second phase transition. The very stable and rigid CuPc(OC2OBz)8 monolayer was determined to consist of coherent rigid Pc columns. The driving forces for this strong structural stability are both the Pc–Pc macrocycle interactions and the associations between the terminal benzyl groups. The bilayer structures on the subphase
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were found to consist of tightly folded bilayer sheets. ( 2/molecule In Fig. 1, we assign an area of 96 A at the first phase transition for pCuPc(OBu)8 (20 mN/m), where the Pc macrocycles are cofacially stacked into columnar aggregates at a small angle to the normal of the subphase surface. The observed molecular area for the second phase ( 2 and is associated with the transition is 52 A formation of a MPc bilayer of ordered aggregates. The compression of nCuPc(OBu)8 films produced well-defined isotherms with an apparent single-phase transition (Fig. 1). Thus, for the nonperipheral derivative the molecular area at an estimated first phase transition at 20 mN/m is ( 2/molecule. Cook and coworkers reported a 82 A ( 2/molecule for the same molecule meavalue 97 A sured at a higher surface pressure of 30 mN/m [27]. LB monolayers of pCuPc(OBu)8 and nCuPc (OBu)8 were deposited onto freshly cleaved mica and imaged by tapping mode AFM. A surface pressure between 12 and 18 mN/m with barrier speed and the substrate withdrawal rate of 20 mm/ min were determined to be the optimum parameters for pCuPc(OBu)8 film deposition, yielding a reproducible transfer ratio of 1.0702. Film compression–relaxation studies showed no hysteresis in the surface pressure vs. area expansion curves if the film was manipulated in the pressure region of the first phase transition. The film quality was not compromised by a series of sequential compression–relaxation cycles. However, if the
(A)
compression of the film advances to the second phase change, considerable hysteresis is observed in the isotherm curve and the bilayer film which was then formed, did not relax to its precompressed state. This was also found to be true in the case of CuPc(OC2OBz)8 [13,14]. A surface pressure of 15–20 mN/m and barrier compression and substrate dipping speeds of 20 mm/min furnished the best quality nCuPc(OBu)8 monolayers. The transfer ratios varied from 0.89 to 1.05. Unlike the monolayer of pCuPc(OBu)8, the nCuPc(OBu)8 Langmuir film did not survive multiple compression–relaxation cycles. The film of the non-peripheral complex did show small hysteresis in the surface pressure vs. area expansion curve for a single compression–relaxation cycle. Cook also observed significant hysteresis in the compression and relaxation isotherms plots of non-peripherally substituted octaalkoxy Pc monolayers and reported transfer ratios of 0.50–0.75 for those complexes with Z and Y-type film deposition employed [27]. Fig. 2 displays AFM images of monolayers of pCuPc(OBu)8 after a vertical transfer onto mica. The films were air-dried but not annealed. The vertical axis (slow scan direction) of the images correspond to the direction of the film withdrawal from the LB through. The films were smooth, dense, and essentially pinhole free. Film thickness was uniform and averaged 1.2 nm over areas of many m2. The monolayer height was determined by making cross-sectional measurements through
(B)
Fig. 2. Tapping mode AFM images of a pCuPc(OBu)8 LB monolayer on hydrophilic mica substrate obtained in an ambient environment. Image A represents a 3.5 3.5 mm area while image B that of 500 500 nm section. In B, strands of Pc columns can be observed with inter-column distances of about 0.2 nm. The film height in both micrographs is 1.2 nm.
K. Stevenson et al. / Ultramicroscopy 97 (2003) 271–278
the rare perforations in the film. The measured film height is consistent with the Pc molecules orienting at an angle relative to the substrate normal [12–15]. Close-up AFM images of the film (Fig. 2B) exhibit the expected long range columnar packing extending for distances of tens of nanometers. The observed molecular ordering was generally disrupted by defects in the substrate. AFM images of LB monolayer and bilayer films of peripherally substituted CuPc(OC2OBz)8 deposited via vertical and horizontal methods onto silica showed similar characteristics [15]. The intercolumn spacing in the pCuPc(OBu)8 film was about 2 nm which suggests that the butoxy groups on the Pc ring are not fully extended (molecular modeling predicts they will stretch over 2.7 nm in the fully extended configuration) but are either contracted or interdigitated with adjacent aggregates. It is surprising that pCuPc(OBu)8, a complex with relatively short ring substituents and none of the ancillary terminal group interactions present in CuPc(OC2OBz)8 gives rise to such longrange ordered and stable columnar films. The non-peripheral complex also formed stable nearly pinhole free films on mica substrates but not with the same reproducibility as pCuPc(OBu)8, Fig. 3A. The AFM images of the nCuPc(OBu)8 monolayer (1.5 nm thick) appear grainy and show less organization than the pCuPc(OBu)8. This result is to be expected for films made from non-peripherally derivitized Pc. Cook notes that
(A)
275
non-peripherally substituted Pc produce discotic mesophase LB layers in which there is disorder within the column aggregates [12,15]. The bilayer films of nCuPc(OBu)8 appear to be highly structured as shown in Fig. 3B. Many of the Pc fibers we imaged were more than 5 mm long and measured 20 nm across, on average. The height of the bilayer was about 3.0 nm. The fibers, therefore, consist of bundles of individual cofacial strands one-molecule in thickness. The morphological images of spin-cast films (from chloroform solutions) made from the octabutoxy complexes are depicted in Fig. 4. Both pCuPc(OBu)8 and nCuPc(OBu)8 produced similarly uniform net-like films on mica. Varying the concentration of the material in solution, the evaporation rate of the solvent, the exposure time of the surface to the solution, and the rate of substrate spin can control the thickness and density of this network. Using Pc concentrations of 10 6 M, 10 s contact with mica, and spinning dry for 60 s at 2000 rpm produced films one molecular layer thick. The higher magnification inset, in Fig. 4B, shows formation of columnarlike assemblies with structural characteristics similar to those prepared by LB deposition. Thicker, 3 nm spin-cast films of pCuPc(OBu)8 with larger surface area were obtained from more concentrated solutions, Fig. 5. This very large surface to volume ratio may make these cast films especially useful for sensor applications. There is
(B)
Fig. 3. Tapping mode AFM images of nCuPc(OBu)8 monolayers on hydrophilic mica substrate acquired in air. Image A represents a 3.5 3.5 mm area. Image B displays a part of a bilayer of the complex on a 1500 1500 nm scale. The fibers visible in the image appear as twisted strands each about 20 nm wide and microns in length. The film thickness of each layer is about 1.5 nm.
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(A)
(B)
Fig. 4. AFM tapping mode images of pCuPc(OBu)8 and nCuPc(OBu)8 films spin-coated on mica, A and B, respectively. The mica substrates were exposed to 10 6 M chloroform solutions of the compounds for 15 s and spun dry for 1 min. Both images are 3.5 3.5 mm in size. Films in graphs A and B are 1.3 and 1.57 nm thick, respectively. The inset in graph A is a 1.5 1.5 mm scan of the pCuPc(OBu)8 and the fibers are 1.3 nm in thickness. The fiber bundles are 10 nm in diameter.
Fig. 5. A 2 2 mm image of spin coated pCuPc(OBu)8 film on mica prepared with 10 4 M chloroform solution. The substrate was exposed to the dissolved complex for 30 s and spun dry for 60 s. This high surface area rough bilayer film has an average thickness of 2.5–3.0 nm.
very little fundamental work on the structural order within spin-coated films for comparison. Spin coating of asymmetric Pc derivatives containing both polar and non-polar side chains produced films with multilayer structure analogous to those obtained from the LB technique [1,12]. These films were largely amorphous and contained crystalline domains. Substantial ordering was introduced into
Fig. 6. UV-visible spectra obtained from both parent CuPc sublimed onto a glass substrate and pCuPc(OBu)8 and nCuPc(OBu)8 LB films deposited on microscope slides. The LB multilayers were prepared employing Z-type deposition with a transfer ratio of 0.9270.05 for pCuPc(OBu)8, and a transfer ratio of 0.8770.07 for nCuPc(OBu)8. The surface pressures used in fabrication the samples were consisted with the formation of monolayer films for each complex.
the multilayers by post-deposition annealing as determined by X-ray diffraction. The UV-visible spectrum of 10 LB layers of pCuPc(OBu)8 shown in Fig. 6 is almost identical to the spectrum of 3 nm thick vapor deposited film of the unsubstituted parent compound, CuPc, in terms of the position and the characteristic splitting of the Q-band, 624 and 690 nm. The
K. Stevenson et al. / Ultramicroscopy 97 (2003) 271–278
CuPc polycrystalline film evaporated under vacuum conditions is the a-type [29]. In this form the Pc rings are stacked like a deck of cards and ( apart [9]. This molecular stacking is spaced 3.76 A assisted by the Cu metal in one macrocycle interacting with aza nitrogens on the adjacent Pc ring [9]. Because of the close similarity of the absorbance spectra of the CuPc film and pCuPc(OBu)8 LB monolayer the molecular orientation in these films is expected to be the essentially same. The peripheral butoxy substituents appear to have very little effect on the optical spectra of the CuPc complex. The absorption spectrum of the nCuPc(OBu)8 LB layers, on the other hand, displays a significant shift of the Q bands to the red. These results are consistent with reports that functionalities in the peripheral positions bring about only minor perturbation in the Q-band energy, whereas spectra of Pc derivatives with alkoxy groups located closest to the porphyrazine ring, exhibit significant bathochromic shifts of the p-p* transition [1,11,26,30].
4. Conclusions Both the peripherally and non-peripherally symmetrically substituted CuPc(OBu)8 complexes produce high quality well ordered LB films. The columnar aggregation of these films is consistent with the position of the polar side-chains on the Pc macrocycle. The quality of the monolayers and their long-range ordering is unexpected for Pc with such relatively short ring substituents and no assisting terminal group interactions. Spin-coated films of the butoxy Pc derivatives produced films with aggregate structure analogous to those obtained from the LB technique. Absorption spectra of LB films of pCuPc(OBu)8 and nCuPc(OBu)8 are consistent with literature UV-visible data on octaalkoxy substituted Pc complexes. We are presently working on obtaining STM of our LB films both air-dried and annealed to obtain better-resolved structural information. We have good evidence that the octabutoxy Pc LB and spin-doped films are suitable candidates for incorporation into M–I–M junctions [31].
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Acknowledgements This work was supported by the Petroleum Research Fund (ACS-PRF#32317-AC6,5) and we gratefully acknowledge their assistance. We also thank Tammy Oshiro for help with obtaining AFM images of the spin-cast films.
References [1] N.B. McKeown, Phthalocyanine Materials: Synthesis, Structure, and Function, Cambridge University Press, Cambridge UK, 1998. [2] P. Gregory, J. Porh. Phthal. 4 (2000) 432. [3] R. Ao, L. Kilmmert, D. Haarer, Adv. Mater. 7 (1995) 495. [4] Q. Chen, D. Gu, F. Gan, L. Xu, M. Li, Appl. Surf. Sci. 93 (1996) 151. . [5] D. Wohrle, D. Meissner, Adv. Mater. 3 (1991) 129. [6] A.B.P. Lever, M.R. Hempstead, C.C. Leznoff, W. Lui, M. Melnik, W.A. Nevin, P. Seymour, Pure Appl. Chem. 58 (1986) 1467. [7] C. Hamann, M. Hietschold, A. Marwa, M. Mueller, M. Starke, R. Kilper, Top. Mol. Organ. Eng. 7 (1991) 129. [8] M.K. Engel, Report Kawamura Institute of Chemical Research, 1997, pp. 11–54. [9] J. Simons, J.A. Andre, Molecular Semiconductors, Springer, New York, 1985. [10] N.B. McKeown, Chem. Ind. (1999) 92. [11] M.J. Cooke, A.J. Dunn, M.F. Daniel, R.C.O. Hart, R.M. Richardson, S.J. Foster, J. Chem. Soc. Perkin Trans. I (1988) 2453. [12] M.J. Cook, Pure Appl. Chem. 71 (1999) 2145. [13] P.E. Smolenyak, E.J. Osburn, S.Y. Chen, L.K. Chau, D.F. O’Brian, N.R. Armstrong, Langmuir 13 (1997) 6568. + [14] P.E. Smolenyak, R. Peterson, K. Nebesny, M. Torker, D.F. O’Brian, N.R. Armstrong, J. Amer. Chem. Soc. 121 (1999) 8628. [15] M. V!elez, S. Viera, I. Chambrier, M.J. Cook, Langmuir 14 (1998) 4227. [16] U. Mazur, K.W. Hipps, J. Phys. Chem. 98 (1994) 8169. [17] X. Lu, K.W. Hipps, X.D. Wang, U. Mazur, J. Amer. Chem. Soc. 118 (1996) 7197. [18] X. Lu, K.W. Hipps, J. Phys. Chem. B 101 (1997) 5391. [19] D. Barlow, K.W. Hipps, J. Phys. Chem. B 104 (2000) 5993. [20] L. Scudiero, D.E. Barlow, K.W. Hipps, J. Phys. Chem. B 104 (2000) 11899. [21] L. Scudiero, D.E. Barlow, U. Mazur, K.W. Hipps, J. Amer. Chem. Soc. 123 (2001) 4073. [22] N.B. McKeown, J. Mater. Chem. 10 (2000) 1979. [23] B. Simic-Glavinski, A.A. Tanaka, M.E. Kenny, Y. Yeager, J. Electroanal. Chem. 229 (1987) 285. ! [24] H. Rengel, P. Gattinger, R. Silerov! a (Beck), D. Naher, J. Phys. Chem. B 103 (1999) 6822.
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[25] A.B. Anderson, T.L. Gordon, M.E. Kenney, J. Amer. Chem. Soc. 107 (1995) 192. [26] A.N. Cammidge, I. Chambrier, M.J. Cook, A.D. Garland, M.J. Heeney, K. Welford, J. Porh. Phthal. 1 (1997) 77. [27] M.J. Cook, A.J. Dunn, M.F. Daniel, R.C.O. Hart, R.M. Richardson, S.J. Foster, Thin Solid Films 159 (1988) 395.
[28] S. Foley, J. Chem. Soc. Perkin Trans. 2 (1997) 1725. [29] J.J. Hoagland, J. Dowdy, K.WW. Hipps, J. Phys. Chem. 95 (1991) 2246. . [30] D. Wohrle, V. Schmidt, J. Chem. Soc. Dalton Trans. (1988) 549. [31] K. Stevenson, N. Miyashita, J. Smieja, U. Mazur, to be published.
Highly ordered thin films prepared with octabutoxy copper phthalocyanine complexes Kelly Stevensona, Naoko Miyashitaa, Joanne Smiejab, Ursula Mazura,* a
Department of Chemistry and Materials Science Program, Washington State University, Pullman, WA 99164-4630, USA b Department of Chemistry, Gonzaga University, Spokane, WA 99258, USA Received 11 September 2002; received in revised form 31 October 2002
Abstract Langmuir–Blodgett (LB) films of copper (II) 1,4,8,11,15,18,22,25-octabutoxyphthalocyanine, nCuPc(OBu)8, (nonperipheral substitution) and copper (II) 2,3,9,10,16,17,23,24-octabutoxyphthalocyanine, pCuPc(OBu)8, (peripheral substitution), were fabricated and characterized by optical spectroscopy and scanning probe microscopy. The LB films were transferred onto hydrophilic substrates by vertical dipping. Although they posses relatively short polar substituents both compounds form smooth, uniform, dense, and highly stable LB monolayers composed of linear arrays of cofacial oligomers. The long range discotic assemblies of LB and spun cast films of pCuPc(OBu)8 and nCuPc(OBu)8 posses physical and chemical properties favorable for molecular electronic device application. r 2003 Elsevier Science B.V. All rights reserved.
1. Introduction Of past, present, and future interest as components in molecular electronic devices are the phthalocyanine-like molecules. Because of their thermal, chemical, and physical stability (in excess of 500 C), and useful photophysical, redox, and catalytic properties, they are used in many advanced technological applications [1]. They are employed in laser printers [2], in optical data storage systems [3], and in electronic devices such as WORM drives [4]. They are also of continuing interest as active elements in photovoltaic cells [5], fuel cells [6], and chemical sensors [7].
*Corresponding author. Fax: +1-509-335-8867. E-mail address: [email protected] (U. Mazur).
The exploitation of the desirable optical and electrical or electrochemical properties of phthalocyanines (Pc), relies on the precise control over the molecular packing and ordering in the solid phase. Unsubstituted Pc self-organize into a variety of crystalline polymorphs and have been used to prepare ordered thin films by vapor deposition [1,8–10]. In addition, many Pc derivatives with flexible side-chains make excellent candidates for Langmuir–Blodgett (LB) film fabrication and also films generated by spin coating, and self-assembly [1,11–15]. Our interest in Pc materials is in employing them as model systems for studying electron transport processes by orbital-mediated tunneling spectroscopy (OMTS) both in the scanning tunneling microscopy (STM) and the metal–insulator– metal diode (M–I–M) environments. We have
0304-3991/03/$ - see front matter r 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0304-3991(03)00052-4
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investigated vapor deposited monolayer, and submonolayer films of several unsubstituted metal phthalocyanines (MPc) on Au(1 1 1) substrates and incorporated into tunnel diodes [16–21]. Our studies produced tunneling spectra that show a direct correlation between orbital mediated electron transfer processes and the expected HOMO and LUMO energies in single molecules (STM) and in collections of 109 molecules (M–I–M) [16–21]. We are now extending our efforts to the study of single LB layers of substituted Pc in which the macrocyclic rings are oriented in linear cofacially stacked domains. Electrochemical properties of linear cofacial arrays of Pc polymers are known to be strongly affected by the ring–ring spacing, twist angle, and position relative to the neighboring rings [1,13,22–25]. Functionalized and cofacially arranged Pc suitable for study either in the STM or M–I–M’ environment, need to have relatively short R groups in order to facilitate the tunneling processes. Many of the excellent candidates reported in the literature, however, proved unsuitable for our purposes because of their long and bulky R groups [1,11–15,26,27]. Two molecules that show great promise for our OMTS studies are the non-peripherally substituted copper (II) 1,4,8, 11,15,18,22,25-octabutoxyphthalocyanine, nCuPc(OBu)8 and the peripherally substituted copper (II) 2,3,9,10,16,17,23,24-octabutoxyphthalocyanine, pCuPc(OBu)8 [27]. In this article we report on the fabrication and structure of LB and spin-cast monolayer films of these complexes.
general synthetic method [28]. All solvents used were either reagent or spectroscopic grade and were acquired either from Aldrich Chemical Co. (see footnote 1) or J.T. Baker.2 Corning3 microscope slides were cleaned in a HNO3/H2O2 bath, and stored in high purity water until usage. Mica strips 1 4 cm (catalog 54) from purchased from Ted Pella.4 2.2. Film preparation Langmuir-Blodgett monolayers were prepared using the KSV 50005 LB double trough system equipped with two Whilhelmy balances. Each balance was mounted at the midpoint between two moving barriers. Films were spread on a MilliPore Milli Q-purified water subphase typically kept at approximately 20 C. Aliquots (200–700 ml) of 2 10 4 M solutions of the complexes in chloroform were employed. The Pc layer was allowed to stabilize for several minutes and was then compressed at a barrier speed of 20 mm/min until the desired surface pressure was reached. Mono- and multi-LB layers were transferred via Z-type deposition onto freshly cleaved mica substrates or pre-cleaned microscope slides. The dipping speed was 20 mm/min and the transfer ratios ranged between 0.95 and 1.05. Spin-cast films on mica and glass slides were made with the same solutions used for the LB film preparation. The substrates were exposed to the individual solutions for 15–30 s and then spun dry for 60 s to remove the solvent. 2.3. UV-vis
2. Experimental 2.1. Materials CuPc and the non-peripheral CuPc(OBu)8 were obtained from Aldrich Chemical Co.1 The parent complex was twice sublimed and the nCuPc(OBu)8 was recrystallized from tetrahydrofuran and ethanol prior to usage. The peripheral CuPc(OBu)8 analogue was prepared using a previously reported 1 Aldrich Chemical Co. Inc., 1001 West Saint Paul Avenue Milwaukee, WI 53233, USA. Tel.: 1-800-558-9260.
LB films of the CuPc(OBu)8 complexes were deposited on glass slides employing vertical dipping. CuPc was vapor deposited on glass slides in a HV deposition chamber at o6 10 7 Torr. Film 2 KSV Instruments USA, P.O. Box 192 Monroe, CT 06468, USA. Tel.: +1-800-280 6216. 3 J.T. Baker, 222 Red School Lane, Phillipsburg NJ 08865, USA. Tel.: +1-800-582-2537. 4 Ted Pella, Inc., P.O. Box 492477, Redding, CA 96049-2477, USA. Tel.: +1-800-237-3526. 5 GBC Scientific Equipment, 3930 Ventura Drive, Arlington Heights, IL 60004, USA. Tel.: +1-800-445-1902.
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Fig. 1. Surface pressure-area isotherms (ca. 20 C, pH 7) for pCuPc(OBu)8, thick line and for nCuPc(OBu)8, thin line.
thickness was monitored with a quartz crystal microbalance. Film deposition rate was 0.01 nm/s and a CuPc density of 1.6 g/cm3 was assumed. A clean glass slide was used as a reference. Electronic spectra of Pc compounds were recorded with a Cintra 406 UV-visible spectrometer from GBC Scientific. Data was collected at 150 nm/s and a slit opening of 2 nm.
were acquired in tapping mode with etched silicon tips having a diameter of 5 10 nm. The specified cantilever length was 127 mm and a force constant of the order of 50 N/m. The setpoint varied between 1.85 and 2.75 V and the resonance frequency of the cantilever was 260–340 kHz. The standard DI software was used for data acquisition.
2.4. AFM Monolayers of CuPc(OBu)8 LB films were prepared on mica substrates. The specimen was then mounted on a metal disk with a double stick tape and placed on the top of the scanner tube. The samples were always set in such a way that the fast scan direction and the dipping direction were the same. Spin-doped MPc samples were also prepared on mica. A precut mica substrate was exposed to 2 10 4 M solutions of CuPc(OB)8 complexes in chloroform solutions for 10–15 s and spun dry for 30 s. The AFM used for topographic imaging of thin films was Nanoscope IIIA multimode microscope from Digital Instruments, DI.7 All micrographs 6 VWR, P.O. Box 5229, Buffalo Grove, IL 60089-5229, USA. Tel.: +1-800-727-4368. 7 Digital Instruments. 112 Robin Hill Road, Santa Barbara, CA 93117, USA. Tel.: +1-800-873-9750.
3. Results and discussion Langmuir films of pCuPc(OBu)8 on water exhibit two distinct phase transitions (Fig. 1). Armstrong et al. also observed two sharply defined phase transitions in the LB isotherms of a peripherally substituted Pc derivative, 2,3,9,10, 16,17,23,24-octakis(2-benzyloethoxy) phthalocyanato copper (II), CuPc(OC2OBz)8 [13,14]. They associated these two transitions with formation of a monolayer at the lower surface pressure (first phase transition) and a bilayer of the complex at the second phase transition. The very stable and rigid CuPc(OC2OBz)8 monolayer was determined to consist of coherent rigid Pc columns. The driving forces for this strong structural stability are both the Pc–Pc macrocycle interactions and the associations between the terminal benzyl groups. The bilayer structures on the subphase
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were found to consist of tightly folded bilayer sheets. ( 2/molecule In Fig. 1, we assign an area of 96 A at the first phase transition for pCuPc(OBu)8 (20 mN/m), where the Pc macrocycles are cofacially stacked into columnar aggregates at a small angle to the normal of the subphase surface. The observed molecular area for the second phase ( 2 and is associated with the transition is 52 A formation of a MPc bilayer of ordered aggregates. The compression of nCuPc(OBu)8 films produced well-defined isotherms with an apparent single-phase transition (Fig. 1). Thus, for the nonperipheral derivative the molecular area at an estimated first phase transition at 20 mN/m is ( 2/molecule. Cook and coworkers reported a 82 A ( 2/molecule for the same molecule meavalue 97 A sured at a higher surface pressure of 30 mN/m [27]. LB monolayers of pCuPc(OBu)8 and nCuPc (OBu)8 were deposited onto freshly cleaved mica and imaged by tapping mode AFM. A surface pressure between 12 and 18 mN/m with barrier speed and the substrate withdrawal rate of 20 mm/ min were determined to be the optimum parameters for pCuPc(OBu)8 film deposition, yielding a reproducible transfer ratio of 1.0702. Film compression–relaxation studies showed no hysteresis in the surface pressure vs. area expansion curves if the film was manipulated in the pressure region of the first phase transition. The film quality was not compromised by a series of sequential compression–relaxation cycles. However, if the
(A)
compression of the film advances to the second phase change, considerable hysteresis is observed in the isotherm curve and the bilayer film which was then formed, did not relax to its precompressed state. This was also found to be true in the case of CuPc(OC2OBz)8 [13,14]. A surface pressure of 15–20 mN/m and barrier compression and substrate dipping speeds of 20 mm/min furnished the best quality nCuPc(OBu)8 monolayers. The transfer ratios varied from 0.89 to 1.05. Unlike the monolayer of pCuPc(OBu)8, the nCuPc(OBu)8 Langmuir film did not survive multiple compression–relaxation cycles. The film of the non-peripheral complex did show small hysteresis in the surface pressure vs. area expansion curve for a single compression–relaxation cycle. Cook also observed significant hysteresis in the compression and relaxation isotherms plots of non-peripherally substituted octaalkoxy Pc monolayers and reported transfer ratios of 0.50–0.75 for those complexes with Z and Y-type film deposition employed [27]. Fig. 2 displays AFM images of monolayers of pCuPc(OBu)8 after a vertical transfer onto mica. The films were air-dried but not annealed. The vertical axis (slow scan direction) of the images correspond to the direction of the film withdrawal from the LB through. The films were smooth, dense, and essentially pinhole free. Film thickness was uniform and averaged 1.2 nm over areas of many m2. The monolayer height was determined by making cross-sectional measurements through
(B)
Fig. 2. Tapping mode AFM images of a pCuPc(OBu)8 LB monolayer on hydrophilic mica substrate obtained in an ambient environment. Image A represents a 3.5 3.5 mm area while image B that of 500 500 nm section. In B, strands of Pc columns can be observed with inter-column distances of about 0.2 nm. The film height in both micrographs is 1.2 nm.
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the rare perforations in the film. The measured film height is consistent with the Pc molecules orienting at an angle relative to the substrate normal [12–15]. Close-up AFM images of the film (Fig. 2B) exhibit the expected long range columnar packing extending for distances of tens of nanometers. The observed molecular ordering was generally disrupted by defects in the substrate. AFM images of LB monolayer and bilayer films of peripherally substituted CuPc(OC2OBz)8 deposited via vertical and horizontal methods onto silica showed similar characteristics [15]. The intercolumn spacing in the pCuPc(OBu)8 film was about 2 nm which suggests that the butoxy groups on the Pc ring are not fully extended (molecular modeling predicts they will stretch over 2.7 nm in the fully extended configuration) but are either contracted or interdigitated with adjacent aggregates. It is surprising that pCuPc(OBu)8, a complex with relatively short ring substituents and none of the ancillary terminal group interactions present in CuPc(OC2OBz)8 gives rise to such longrange ordered and stable columnar films. The non-peripheral complex also formed stable nearly pinhole free films on mica substrates but not with the same reproducibility as pCuPc(OBu)8, Fig. 3A. The AFM images of the nCuPc(OBu)8 monolayer (1.5 nm thick) appear grainy and show less organization than the pCuPc(OBu)8. This result is to be expected for films made from non-peripherally derivitized Pc. Cook notes that
(A)
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non-peripherally substituted Pc produce discotic mesophase LB layers in which there is disorder within the column aggregates [12,15]. The bilayer films of nCuPc(OBu)8 appear to be highly structured as shown in Fig. 3B. Many of the Pc fibers we imaged were more than 5 mm long and measured 20 nm across, on average. The height of the bilayer was about 3.0 nm. The fibers, therefore, consist of bundles of individual cofacial strands one-molecule in thickness. The morphological images of spin-cast films (from chloroform solutions) made from the octabutoxy complexes are depicted in Fig. 4. Both pCuPc(OBu)8 and nCuPc(OBu)8 produced similarly uniform net-like films on mica. Varying the concentration of the material in solution, the evaporation rate of the solvent, the exposure time of the surface to the solution, and the rate of substrate spin can control the thickness and density of this network. Using Pc concentrations of 10 6 M, 10 s contact with mica, and spinning dry for 60 s at 2000 rpm produced films one molecular layer thick. The higher magnification inset, in Fig. 4B, shows formation of columnarlike assemblies with structural characteristics similar to those prepared by LB deposition. Thicker, 3 nm spin-cast films of pCuPc(OBu)8 with larger surface area were obtained from more concentrated solutions, Fig. 5. This very large surface to volume ratio may make these cast films especially useful for sensor applications. There is
(B)
Fig. 3. Tapping mode AFM images of nCuPc(OBu)8 monolayers on hydrophilic mica substrate acquired in air. Image A represents a 3.5 3.5 mm area. Image B displays a part of a bilayer of the complex on a 1500 1500 nm scale. The fibers visible in the image appear as twisted strands each about 20 nm wide and microns in length. The film thickness of each layer is about 1.5 nm.
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(A)
(B)
Fig. 4. AFM tapping mode images of pCuPc(OBu)8 and nCuPc(OBu)8 films spin-coated on mica, A and B, respectively. The mica substrates were exposed to 10 6 M chloroform solutions of the compounds for 15 s and spun dry for 1 min. Both images are 3.5 3.5 mm in size. Films in graphs A and B are 1.3 and 1.57 nm thick, respectively. The inset in graph A is a 1.5 1.5 mm scan of the pCuPc(OBu)8 and the fibers are 1.3 nm in thickness. The fiber bundles are 10 nm in diameter.
Fig. 5. A 2 2 mm image of spin coated pCuPc(OBu)8 film on mica prepared with 10 4 M chloroform solution. The substrate was exposed to the dissolved complex for 30 s and spun dry for 60 s. This high surface area rough bilayer film has an average thickness of 2.5–3.0 nm.
very little fundamental work on the structural order within spin-coated films for comparison. Spin coating of asymmetric Pc derivatives containing both polar and non-polar side chains produced films with multilayer structure analogous to those obtained from the LB technique [1,12]. These films were largely amorphous and contained crystalline domains. Substantial ordering was introduced into
Fig. 6. UV-visible spectra obtained from both parent CuPc sublimed onto a glass substrate and pCuPc(OBu)8 and nCuPc(OBu)8 LB films deposited on microscope slides. The LB multilayers were prepared employing Z-type deposition with a transfer ratio of 0.9270.05 for pCuPc(OBu)8, and a transfer ratio of 0.8770.07 for nCuPc(OBu)8. The surface pressures used in fabrication the samples were consisted with the formation of monolayer films for each complex.
the multilayers by post-deposition annealing as determined by X-ray diffraction. The UV-visible spectrum of 10 LB layers of pCuPc(OBu)8 shown in Fig. 6 is almost identical to the spectrum of 3 nm thick vapor deposited film of the unsubstituted parent compound, CuPc, in terms of the position and the characteristic splitting of the Q-band, 624 and 690 nm. The
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CuPc polycrystalline film evaporated under vacuum conditions is the a-type [29]. In this form the Pc rings are stacked like a deck of cards and ( apart [9]. This molecular stacking is spaced 3.76 A assisted by the Cu metal in one macrocycle interacting with aza nitrogens on the adjacent Pc ring [9]. Because of the close similarity of the absorbance spectra of the CuPc film and pCuPc(OBu)8 LB monolayer the molecular orientation in these films is expected to be the essentially same. The peripheral butoxy substituents appear to have very little effect on the optical spectra of the CuPc complex. The absorption spectrum of the nCuPc(OBu)8 LB layers, on the other hand, displays a significant shift of the Q bands to the red. These results are consistent with reports that functionalities in the peripheral positions bring about only minor perturbation in the Q-band energy, whereas spectra of Pc derivatives with alkoxy groups located closest to the porphyrazine ring, exhibit significant bathochromic shifts of the p-p* transition [1,11,26,30].
4. Conclusions Both the peripherally and non-peripherally symmetrically substituted CuPc(OBu)8 complexes produce high quality well ordered LB films. The columnar aggregation of these films is consistent with the position of the polar side-chains on the Pc macrocycle. The quality of the monolayers and their long-range ordering is unexpected for Pc with such relatively short ring substituents and no assisting terminal group interactions. Spin-coated films of the butoxy Pc derivatives produced films with aggregate structure analogous to those obtained from the LB technique. Absorption spectra of LB films of pCuPc(OBu)8 and nCuPc(OBu)8 are consistent with literature UV-visible data on octaalkoxy substituted Pc complexes. We are presently working on obtaining STM of our LB films both air-dried and annealed to obtain better-resolved structural information. We have good evidence that the octabutoxy Pc LB and spin-doped films are suitable candidates for incorporation into M–I–M junctions [31].
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Acknowledgements This work was supported by the Petroleum Research Fund (ACS-PRF#32317-AC6,5) and we gratefully acknowledge their assistance. We also thank Tammy Oshiro for help with obtaining AFM images of the spin-cast films.
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