NANO LETTERS
Photoactive Nanowires in Fullerene−Ferrocene Dyad Polyelectrolyte Multilayers
2002 Vol. 2, No. 7 775-780
Dirk M. Guldi,*,† Chuping Luo,† Dmitry Koktysh,‡ Nicholas A. Kotov,*,‡ Tatiana Da Ros,§ Susanna Bosi,§ and Maurizio Prato*,§ Radiation Laboratory, UniVersity of Notre Dame, Notre Dame, Indiana 46556, Chemistry Department, Oklahoma State UniVersity, Stillwater, OK, 74078, and Dipartimento di Scienze Farmaceutiche, UniVersita` di Trieste, Piazzale Europa, 1, 34127 Trieste, Italy Received April 25, 2002; Revised Manuscript Received May 22, 2002
ABSTRACT Layer-by-layer (LBL) films of a fulleropyrrolidinium−androstane−ferrocene dyad revealed a surprisingly high level of intermolecular organization. In particular, in every adsorption layer, the films self-assemble into linear nanowire superstructures. Given the inherent intramolecular charge transfer performance of the dyads, such a film organization facilitates the charge diffusion from the photoactive centers to the electrodes, due to extensive interpenetration of the layers, which leads to multiple contacts between the fullerene−ferrocene nanowires.
One of the key challenges in layered nanostructured materials is the tailoring of materials by adequately chosen assembly methods, which control/modify the electronic structures of the layers enhancing desired functionalities.This task requires the understanding and control over a complex interplay of intermolecular interactions of nanoscale units. Among other methods, layer-by-layer assembled (LBL) films1 directed by electrostatic and van der Waals interactions,2 offer convenient ways for processing materials and molecular engineering of thin films into organized layered assemblies.3 The control over both the layer sequence and composition of the sandwich-like structures can be utilized for fine-tuning the electronic properties of nanostructures.4 Despite some limitations imposed by the interpenetration of the adjacent layers, this method constitutes a potent alternative to traditional thin film techniques in fabricating photoactive molecular devices due to its simplicity and universality combined with high quality of LBL films. This has been demonstrated by the groups of M. Rubner and M. Durstock.5 LBL multilayers of fullerene-based donor-acceptor ensembles revealed promising characteristics for energy conversion, high photoactivity, and environmental stability. Importantly, photocurrents evolving from the efficient gen* Corresponding authors. Guldi: Fax 1-219-631-8068; Tel 1-219-6317441; E-mail
[email protected]. Kotov: Fax 1-405-744-6007; Tel 1-405744-3991; E-mail
[email protected]. Prato: Fax 39 040 52572; Tel 39 040 558 7883; E-mail
[email protected]. † University of Notre Dame. ‡ Oklahoma State University. § Universita ` di Trieste. 10.1021/nl025594d CCC: $22.00 Published on Web 06/06/2002
© 2002 American Chemical Society
eration of charge-separated states increase with number of layers deposited.6 In this work, a ferrocene moiety was probed as the electron donor, connected via an androstane bridge, providing a rigid, all-σ-bonded framework, to a fulleropyrrolidinium ion (Scheme 1). In addition to avoiding extensive modification of the C60 core leading to higher electron-transfer energy barrier,7 the rigidity of the linear skeleton gives a higher degree of structural organization of the 2D film. Analogously to previously investigated ferrocene-fullerene dyads, compounds 1 and 2 give rise to photoinduced electron transfer from the ferrocene donor to the fullerene acceptor,14b with a very favorable ratio between charge-separation and charge-recombination rates.8 The positively charged pyrrolidinium group of 1 is a key requisite to facilitate the controlled modification of photoactive semiconductor electrodes, such as ITO. At the same time, effective charge separation at the molecular level poses the question of how this efficiency can be translated into the charge separation in the macroscale, and, in turn, into photopotential. The solution to this problem involves nanoscale organization of the photoactive thin films. As a general approach, we intended to utilize thin films with intrinsic gradient of electron potential in the perpendicular direction to the electrode. Composites with controlled property gradient were recently obtained for LBL assemblies.3a,c,9 This technique is very convenient because the composition of the film can be easily changed from layer to layer in accord with the desired evolution of properties. However, in the course of this
Scheme 1
Figure 1. AFM images of 1/PSS/PDDA film after the first (a,c) and second (b) adsorption cycles adsorbed from DMSO solutions (a,b) and water (c). EFM image of 1/PSS/PDDA film (d). Applied voltage 5 V.
project, we found an unexpected structural organization of the individual LBL bilayers of C60 derivatives, which also leads to very efficient charge transport in the macroscale. We believe that this effect relating the nanoscale morphology/ ordering of the light adsorbing species and photovoltaic activity of the composite material is essential for the molecular design of electrooptical thin films from different fullerene species and therefore transcends the boundaries of photovoltaics, albeit the details of the self-assembly mechanism are yet to be uncovered. In the initial phase of the substrate modification, a monolayer of poly-(diallyl-dimethylammonium) (PDDA) polycations is adsorbed on the substrate surfacesseither quartz-slides, silicon wafers, or ITO electrodes. The resulting hydrophilic and positively charged surfaces promote the deposition of poly-(sodium-4-styrenesulfonato) (PSS) polyanions, bearing sulfonic acid functionalities, to yield PDDA/ PSS templates. When the surface is sufficiently overlaid with negative charges from PSS, the target compounds 1, 2, and 3 can be deposited onto the substrates by immersing them into their DMSO solutions. With the objective to visualize the morphology of each layer, and, in turn, to ensure the quality of the modified electrode surfaces, AFM images were taken after successful PDDA-, PSS- and (1, 2, and 3) coatings. Representative topographic and phase contrast images of Si wafer (Figure S1a,b), PDDA on Si (PDDA 0.5%, pH ) 3, t ) 10 min; Figure S1c,d), PSS-PDDA on Si (Figure S1e,f), and PSSPDDA on Si after rinsing with DMSO (Figure S1g,h) reveal fairly flat surfaces with no observable regularities. Rinsing with DMSO, at its very best, changes the topography of the PSS-PDDA layer in terms of making it rougher (i.e., 776
compare Figure S1e with Figure S1g). It is important that no particular lateral ordering was observed in all of these adsorption layers. The morphology of PDDA/PSS/1 is significantly different from that of PDDA and PDDA/PSS: After adsorption of dyad 1, linear aggregates appear, which are typically 10 nm in height and 1-3 micron in length (Figure 1a). Importantly, the morphology of the film was also reproduced in the subsequently deposited LBL layers, as exemplified by the AFM image of the second LBL layer in Figure 1b. It should be noted that the wire-like nanostructures are quite robust, showing no significant deterioration during the imaging and also when the photochemical experiments (vide infra) were performed. Surprisingly, the rod-like superstructures retained their mutual orientation, remaining oriented approximately parallel to each other and parallel to the preceding layer. The replacement of the DMSO media with water results in disappearance of the rods (Figure 1c). The external magnetic field results in alteration of the adsorption pattern, making the individual aggregates substantially longer, however, blurring them in the same time (Figure 2). The similarly prepared films from fulleropyrrolidiniumferrocene dyad 2, connected by a flexible oligoethylene glycol bridge, and those from a fulleropyrrolidinium salt 36 did not reveal any appreciable intralayer self-organization (Figure 3a). The topographies of their surface have round, fine-grained structures emanating into a continuous uniform film. Comparison of Figures 1 and 3 leads to the conclusion that the rigidity of the fulleropyrrolidinium-ferrocene dyad (1) determines the self-organization in the form of onedimensional aggregates. We attribute their formation to Nano Lett., Vol. 2, No. 7, 2002
Figure 2. AFM images in different magnification of dyad 1 film adsorbed from DMSO on PDDA/PSS polyelectrolyte layer in magnetic field 3.6 kG oriented parallel to the silicon wafer substrate.
Figure 3. AFM image of 2/PSS/PDDA (a) and EFM image of 2/PSS/PDDA (b) assemblies. The fullerenes were dissolved in DMSO similarly to dyad 1. All other film preparation parameters were identical to that used in Figure 1 (a,b).
strong noncovalent intermolecular interactions between the molecules of dyad 1 based on hydrophobic attraction of C60 heads to each other augmented by π-π interactions of the electronic clouds. Molecules with aromatic rings and dipolar structure were shown to produce one-dimensional superstructures known as J-aggregates.10 In adsorption layers, the J-aggregates display surprisingly similar morphology to the one obtained in the LBL films of dyad 1 (Figure 1).11 The self-assembly of fullerene derivatives in the linear nanostructures when adsorbed to polyelectrolytes in LBL films can be considered a natural consequence of the structural anisotropy, whose influence becomes particularly critical for rigid compounds. The origin of the mutual orientation of the individual rods with respect to each other is not entirely understood. We considered several possibilities that could lead to the alignment of the fullerene aggregates with respect to each other: (a) the effect of the crystalline silicon substrate; (b) intrinsic alignment of the polyelectrolyte macromolecules; (c) adsorption of preassembled bunches of crystallites. Considering this problem, neither topographic nor phase contrast images of the “bare” silicon surface revealed any striped patterns, resembling that of LBL films of dyad 1 (Figure S1a,b). We believe that in LBL multilayers any influence of the silicon substrates used for AFM is unlikely because of the fairly chaotic PDDA/PSS layer preceding the assembly of 1 and its substantial thickness, especially that of the PDDA coverage ∼5 nm (see Figure 4). Nano Lett., Vol. 2, No. 7, 2002
Figure 4. (a) UV-vis absorption spectra of a quartz substrate covered with one through twenty one monolayer films of dyad 1. (b) Plots of absorbance at 265 and 330 nm as a function of monolayers. (c) Ellipsometric thickness of consecutive adsorption layers of dyad 1 assembled on a silicon wafer.
The role of anisotropy introduced by the dipping direction, i.e., intrinsic alignment of the polyelectrolyte chains caused by the liquid flow, was studied in several experiments. We found neither topographic (Figures S1c,e) nor phase image (Figure S1d,f) evidence indicating the presence of a linear pattern in the PDDA and PDDA/PSS adsorption layers. Thus, in the framework of possible origins of the mutual orientation of the linear aggregates, only the adsorption of the preassembled bunches of fullerene rods remain a viable hypothesis. The tendency of fullerenes to self-assemble, which is well-known for positively charged fullerene derivatives,12 adds credibility to it. However, the effects of the assembly media (Figure S2) and external magnetic field (Figure 2) on the chain morphology do not correlate with it very well, at least within the limits of the current understanding of intermolecular interactions. It can be noted that the change of the assembly media from DMSO to water inhibits the formation of the fullerene rods, therefore films with regular grain morphology are 777
Figure 5. Photoaction spectrum of one self-assembled monolayer of dyad 1 (solid), dyad 2 (dotted) and 3 (dashed) on ITO under deoxygenated conditions.
obtained (not shown). We carried out the LBL adsorption of dyads 1 from a mixture of water and DMSO, which still allowed the rod self-assembly, and yet enabled the observations of the initial stages of the aggregate formation. The images of dyad 1 films made at such conditions (Figure S2) demonstrate the characteristic pattern of aggregate growth rather than a patchy morphology that should be expected from the adsorption of preassembled bunches of rod-like crystallites. Interestingly, the rods retain approximate parallelism to each other during the growth, which indicates that the rigid dyad 1 has intrinsic tendency to form linear patterns, when in contact with polyelectrolyte-coated substrate. The effect of the magnetic field gives further support for this conclusion (Figure 2). We observe blurring of the features regardless of the orientation of the substrate in respect to magnetic field. This is consistent with the influence of the external field on the intermolecular interactions of fullerenes dyads rather than with the alignment of the preexisting rods in respect to the magnetic field, which should have produced a better ordered film. Therefore, we must admit that the genesis of the peculiar morphology of the LBL multilayers of fullerene dyads 1, in addition to the tendency of bipolar rigid molecules to form rod-like aggregates, remains an open question. It is likely to involve a fairly complex interplay of short- and long-distance intermolecular forces between the fullerene derivative and the polyelectrolyte. The EFM images of the LBL films of fullerene derivatives show that the dyad aggregates possess higher polarizability than the surrounding matrix, while retaining the general topographic features of the rods (Figure 1d). This is indicative of the efficient charge transport in the aggregates, which always increases following the growth of polarizability, and substantiates their characterization as nanowires. Note that in the synthesis of the dyads, we avoided the excessive modification5a of C60 to facilitate electrical transport between them. Interestingly, the density of the nanowires observed in EFM is somewhat lower than that seen in AFM images, which is likely to be due to defects in packing of some of the aggregates, impeding the charge transport. Importantly, 778
the flexible dyads do not form a film with continuous conductivity (Figure 3b), while nanowires made of rigid dyads form a network of interconnected percolation paths, which should significantly improve the lateral charge transfer. It is important to note that the charge transport along the normal to the substrate should significantly increase as well because the individual nanowires from adjacent LBL films should interpenetrate extensively into each other. A transparent film of PDDA/PSS/1 on quartz, as shown in Figure 4a, is distinguished by a maximum at 226 nm and a set of broad shoulders at 260/330 nm, in excellent agreement with those transitions seen in solution. For comparison an ortho-dichlorobenzene solution of dyad 1 exhibits transitions at 218, 257, and 330 nm, which rise exclusively from the fullerene moiety. The ferrocene donor, on the other hand, lacks significant transitions in this wavelength region, since its 440 nm absorption maximum has a comparatively low extinction coefficient. In fact, identical features were noted for PDDA/PSS/2 and PDDA/ PSS/3, although the latter lacks the ferrocene donor. Despite the qualitative resemblance, the overall broadening of the spectrum is probably due to the strong interaction occurring between individual molecules in the densely packed films (vide infra).13 LBL assembly of 1 and PSS allows for the stacking of bilayer units to any desired thickness, while retaining a high degree of transparency and uniformity. After each deposition cycle, the films were probed by absorption spectroscopy. The fullerene characteristics of dyad 1 were clearly represented in all absorption spectra (see Figure 4a) and gave rise to a linear growth as a function of the coating sequence as typical for LBL. The linearity is exemplified in Figure 4b by plotting two of the major fullerene transitions (at 265 and 330 nm) as a function of PSS/1 layers. In a complementary ellipsometric assay, the linearity, seen in the absorption spectra, was further corroborated. Figure 4c depicts the absolute thickness of 1, 3, 5, and 7 layers of PSS/1 on a silicon wafer covered with PDDA. This relationship reasons clearly the regularity of the arrangement. From these experiments a deposition cycle increment of 45 Å is extrapolated for each LBL layer; that is, PSS/1. We should consider that the typical thickness of a single PSS layer, under the chosen deposition conditions, is in the 5-10 Å range and that the molecular dimensions of dyad 1 (distance between the outermost carbon atom of Fc and the outermost carbon atom of C60) is ca. 19.5 Å. Thus, 45 Å corresponds to the adsorption of two monolayers of dyad 1.14 Associative forces such as formation of van der Waals interactions, as they typically govern the connection between individual fullerene and also ferrocene entities, are then believed to be the driving force for the 1-dimensional controlled build-up of another monolayer on this hydrophobic surface. Notably, although π-stacking and hydrophobic-hydrophobic interactions are likely to occur, it is limited to just two monolayers. This is implicit from the following observation. Variation of the dipping time, such as doubling or halving, did not bring about discernible changes in the film thickness and/or film absorbance. Our structural conclusion is, after all, Nano Lett., Vol. 2, No. 7, 2002
essential: It implies that after completion of the LBL deposition the positive charges of the pyrrolidinium group create a hydrophilic surface, which serves as a template for placing a PSS layer. Although a similar linear thickness versus monolayer dependence was established for dyad 2, appreciably smaller increments (∼35 Å) were derived for each PSS/2 layer. This supports the critical notion that the flexible oligoethylene glycol bridge gives room to a tighter packing of the sandwiched C60-Fc layers. The actual, compact structure, in which C60 and Fc adopt an ordered conformation, is given by an alternative organization principle, namely, attractive π-π association. This is seen, for example, to prevail in condensed media, where it has a crucial contention on the electron-transfer dynamics, since donor and acceptor are packed together tightly. In the LBL films, this aspect becomes even more consequential (vide infra). The photoaction spectra of assembled layers of dyad 1 on ITO track the absorption spectrum of the fulleropyrrolidinium moiety with a maximum around 375 nm (Figure 5), thereby indicating that the incorporated dyad retains the basic reactivity of the photosensitizer. The photocurrent of monolayer coverage of 1 under deoxygenated conditions reveals the three- and ninety-fold enhancement relative to the earlier investigated fulleropyrrolidinium salt 3,6 and dyad 2, respectively. This performance improvement can be attributed to the charge conductance along the fullerene nanowires enhancing the efficiency of the energy conversion. The better performance in dyad 1 relative to dyad 2 is further corroborated by EFM images revealing substantial lateral conductivity of the linear aggregates from dyads 1. On the other hand, the lower efficiency of dyad 2 can be rationalized by the compact film structure and the lack of lateral long-distance conductivity as indicated by very small dot-like areas of high polarizability in EFM of dyad 2 films (Figure 3b). Moreover, despite a rapidly occurring charge separation,15 charge recombination in the film will be virtually instantaneously, due to close proximity of the donor and acceptor and, most importantly, the lack of nanoscale ordering stimulating the long-distance charge separation, such as nanowires. An interesting feature of the dyad 1 films, which also stems from their internal ordering, is that in oxygenated systems (i.e., air- and O2-saturated), dyad 1 gives rise to an additional photocurrent, whose magnitude depends on the oxygen concentration: factor of 1.3 (air-saturated) and 1.7 (O2saturated). This increase is related to the favored electron transfer from the photolytically generated fullerene π-radical anion to molecular oxygen. The resulting O2•-, which is persistent, acts as an electron carrier transferring the charges to the electrode. A comparison with the LBL films of 3, for which the photocurrent decreases by ∼50% in O2-saturated solution, yields a net 7-fold improvement for the electrode made of 1.16 In fact, photoexcited fullerenes (*C60), including fulleropyrrolidinium salt 3, react rapidly with O2 to form the reactive oxygen species, singlet oxygen (1∆g), in 100% yield. However, the latter is expected to fail in contributing constructively to the overall photocurrent generation. In other Nano Lett., Vol. 2, No. 7, 2002
words, any fraction of *C60 that deactivates via the energy transfer channel is wasted for the critical formation of charges. In principle, addition of an alternate electron acceptor (methyl viologen (MV2+)) leads to a qualitatively similar picture. While in dyad 1 an increase in photocurrent is seen, 3 shows a significant drop in intensity. In fact, PDDA/PSS/1 and PDDA/PSS/3 systems were probed under variable MV2+ acceptor concentration, in the range between 1 and 50 mM. In dyad 1 the photocurrent increases linearly with an increase of the concentration before becoming constant around 50 mM. Under these conditions the response corresponds to an amplification of 66%, which is rather moderate, relative to the oxygen case. A reasonable explanation is the small driving force for an interfacial electron transfer between Fc•+C60•- and MV2+ to yield MV•+. The deactivation of photoexcited C60 (3), on the other hand, is responsible for the diminishing photocurrent. Instead of forming the single reduced MV•+, which as a stable electron carrier could eventually enhance the photocurrent, and the oxidized C60•+, as free, separated entities, rapid charge recombination is likely to occur. This is a typical phenomenon when the highly unstable C60•+ is involved. In summary, the structural design of the fullerene derivative resulted in a significant improvement of the performance of layer-by-layer assembled photovoltaic thin films. First, the combination of both donor and acceptor moieties in close proximity leads to the efficient photoinduced charge separation. Second, the rigidity of the dyad brings about the nanowire superstructure of the individual adsorption layers, which promotes the charge transport in the film. Such selforganization of the films has not been seen for any LBL system yet and is relevant for both electrical and optical multilayer devices. These findings highlight both the fundamental importance of the structural control of LBL films and the morphological features that can lead to high light conversion efficiency. Acknowledgment. This work was supported by MURST (cofin prot. MM03198284), by C.N.R. through the program “Materiali Innovativi (legge 95/95)”, the Office of Basic Energy Sciences of the U.S. Department of Energy, and the NSF-NATO. This is contribution NDRL-4331 from the Notre Dame Radiation Laboratory. N.A.K. thanks NSF CAREER (CHE-9876265), NSF Biophotonics Initiative (BES-0119483), AFOSR (F49620-99-C-0072), OCAST (AR99(2)-026), and NSF/NATO (DGE-9902637) for the financial support of this research. Supporting Information Available: S1: Atomic force microscopy images in height and phase modes of cleaned silicon wafer, PDDA and PDDA/PSS assemblies on it. S2: Topography AFM image of dyad 1 assembled from a 1:1 v/v mixture of DMSO and water on PDDA/PSS polyelectrolyte bilayer. S3: Experimental section. This material is available free of charge via the Internet at http://pubs.acs.org. 779
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