Organic Nanofibers from Chloride-Functionalized p-Quaterphenylenes

The islands are usually situated at the end of fibers and form star-shaped entities .... within the needles; the double-headed arrow represents the tr...
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CRYSTAL GROWTH & DESIGN

Organic Nanofibers from Chloride-Functionalized p-Quaterphenylenes Manuela Schiek, Arne

Lu¨tzen,†

2007 VOL. 7, NO. 2 229-233

and Katharina Al-Shamery

Institute of Pure and Applied Chemistry, UniVersita¨t Oldenburg, P.O. Box 2503, D-26111 Oldenburg, Germany

Frank Balzer‡ Institut fu¨r Physik/ASP, Humboldt-UniVersita¨t zu Berlin, Newtonstrasse 15, D-12489 Berlin, Germany

Horst-Gu¨nter Rubahn* Mads Clausen Institute, NanoSYD, UniVersity of Southern Denmark, Dybbolsgade 2 - Alsion, DK 6400 Sønderborg, Denmark ReceiVed April 4, 2006; ReVised Manuscript ReceiVed NoVember 30, 2006

ABSTRACT: The generation of long, mutually oriented nanofibers of functionalized phenylene oligomers is reported for the case of symmetrically 4,4′′′-functionalized p-quaterphenylene molecules with chloride groups. Two domains of up to 30 µm long, oriented fibers with nanoscopic widths and heights grow along muscovite mica 〈110〉 directions. Their mutual alignment increases with increasing substrate temperature during deposition. The photoluminescence spectrum around 380 nm shows well-resolved excitonic transitions and its overall intensity depends on fiber morphology. Introduction The growth of needle-like nanoaggregates from p-phenylene, from R-thiophene and from thiophene/phenylene oligomers on metal and dielectric surfaces has attracted significant interest within recent years.1-6 These nanoaggregates show promise of becoming new building blocks for electronic and optoelectronic devices like organic light-emitting diodes (OLEDs) or organic field-effect transistors (OFETs).7-9 Waveguiding10 as well as organic nanolasers11,12 are other potential applications, if the fibers possess sufficient widths and heights. Previous growth studies using various types of rodlike conjugated organic molecules deposited on different dielectric substrates revealed a unique combination of molecules and substrate that allows one to grow straight, mutually parallel aligned nanofibers with lengths of tens to hundreds of micrometers. This combination consists of muscovite mica as the growth substrate and p-phenylene oligomers, especially p-hexaphenylene (p-6P) as the molecular building blocks. p-Phenylenes with shorter molecular chains such as p-quaterphenylene (p-4P) result in shorter and less oriented nanoaggregates on a mica substrate.13 Phenylene nanofibers grow on muscovite mica via dipoleassisted self-assembly.2 From coverage-dependent AFM images of p-6P on mica, from low-energy electron diffraction,13 and from thermal desorption spectroscopy,14 it has been concluded that a wetting layer of lying molecules forms first, followed by three-dimensional clusters. After reaching a critical number density and/or a critical size, these clusters aggregate into long fibers.14,15 The nanofiber morphology can be tailored by modifying the growth conditions such as substrate temperature, deposition rate, amount of deposited material, substrate rough* To whom correspondence should be addressed. E-mail: [email protected]. † Current address: Kekule ´ -Institute of Organic Chemistry and Biochemistry, University of Bonn, Gerhard-Domagk-Strasse 1, D-53121 Bonn, Germany. ‡ Current address: Institute of Pure and Applied Chemistry, Universita ¨t Oldenburg, P.O. Box 2503, D-26111 Oldenburg, Germany.

ness, and free surface energy.16,17 It is also possible to control the morphology and the luminescence of the nanofibers by chemically functionalizing the organic molecules. However, it is rather difficult to modify p-phenylene oligomers because of their low solubility, which even decreases with increasing chain length of the molecule. It is therefore reasonable to try functionalization of the shorter phenylenes, e.g., quaterphenylenes. In the present work the synthesis has been carried out under standard Suzuki cross-coupling conditions shown schematically in Figure 1.18 A quaterphenylene functionalized with methoxy groups (MOP4) has been investigated before.19,20 In this paper we present first results for a quaterphenylene functionalized symmetrically with chloride groups (CLP4). Experimental Section The organic molecule deposition has been performed in high vacuum (base pressure 5 × 10-8 mbar) via sublimation from a home-built Knudsen cell. The substrate temperature TS was controlled during growth, and the deposition rate of 0.1-0.2 Å/s was monitored by a water-cooled quartz microbalance (Inficon XTC/2). Before deposition the molecules were kept in high vacuum for at least 8 h and heated just below the sublimation temperature for half an hour to outgas and to remove residual organic solvents and water. The pressure during deposition increased to about 2 × 10-7 mbar. As the growth substrate, freshly cleaved muscovite mica (grade V-4 from Structure Probe, Inc.) was used. After immediate transfer into the high vacuum chamber, the mica substrate was heated for at least 2 h at TS J 400 K for outgassing purposes. The samples were investigated under ambient air conditions via fluorescence microscopy and atomic force microscopy, AFM (JPK NanoWizard in intermittent contact mode).

Results and Discussion The deposition of p-6P, MOP4, and CLP4 on muscovite mica at elevated substrate temperatures leads to the formation of mutually aligned nanofibers. In a fluorescence microscope the fibers from all three molecules emit polarized blue light after excitation with UV light from a high-pressure mercury lamp (λexc ≈ 365 nm), with the plane of polarization almost

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230 Crystal Growth & Design, Vol. 7, No. 2, 2007

Figure 1. Simplified schematic for the synthesis of symmetrically functionalized quaterphenylenes.

Figure 2. Fluorescence microscopy images, (85 × 85) µm2, of p-6P (a), MOP4 (b), and CLP4 aggregates (c), (d) on muscovite mica. Growth temperatures TS ) 440 K (a), 330 K (b), 330 K (c), and 370 K (d).

Figure 3. Fluorescence spectra of p-4P (dashed line), CLP4 (dark solid line), MOP4 (dotted black line), and p-6P (gray solid line). The (0-1) transitions are marked for all four spectra by vertical lines at the top axis.

perpendicular to the needle directions. As for the unsubstituted phenylenes,21,22 the lowest energy transition dipole for MOP4 and CLP4 is expected to be oriented along the long molecular axis. In all cases fluorescence can be observed under normal incidence excitation, which indicates lying molecules on the surface as light emitters, Figure 2. The exact wavelength of the luminescence within the blue region is tunable by changing the functional group as demonstrated in Figure 3. The spectra were obtained by recording the fluorescence with a fiber optic spectrometer (Ocean Optics S2000, 5 nm resolution) after irradiating the samples on mica with a Nd:YAG laser at λexc ) 355 nm and with a He:Cd laser at λexc ) 325 nm (CLP4), respectively. All spectra show wellresolved excitonic transitions. Considering the most intensive (0-1) transition, the CLP4 spectrum is blue-shifted compared to p-6P by 0.3 eV because of the shorter basis unit, and blueshifted compared to p-4P by 0.02 eV due to the electron-pulling chloride groups. The spectrum of MOP4 is red-shifted compared to p-4P by 0.11 eV because of the electron pushing methoxy groups.23 Figures 4a-4c show AFM images of nominally 5 nm thick films of CLP4 deposited at substrate temperatures between 310 K (a) and 370 K (c). Here, the reported thickness refers to the

Schiek et al.

Figure 4. (80 × 80) µm2 AFM images of 5 nm CLP4 on muscovite mica. Substrate temperatures during deposition were TS ) (a) 310 K, (b) 330 K, and (c) 370 K. The height scale for all images is 180 nm.

readout of the quartz microbalance. The number density N of the aggregates decreases with increasing substrate temperature from N ) 3.3 × 107 cm-2 at TS ) 310 K to N ) 1.2 × 106 cm-2 at TS ) 370 K, Figure 5a, whereas the mean length of individual aggregates increases from about 1 to 9 µm, with individual needles up to 30 µm in length, Figures 5b and 5c. For substrate temperatures close to room temperature aggregates consist of straight needles and up to 500 nm tall islands of µm2 area. The islands are usually situated at the end of fibers and form star-shaped entities, Figures 4b and 7b. The mean height of the needles increases with substrate temperature from 80 to 150 nm, but the number of star-shaped entities decreases with temperature, until from TS ≈ 350 K on mostly needles with homogeneous heights form, Figure 4c. For somewhat higher substrate temperatures of TS J 400 K no CLP4 nanofibers at all are observed. Needles from p-6P and MOP4 exhibit heights and widths comparable to those reported here. However, MOP4 and p-6P needles are usually significantly longer than the ones from CLP4: lengths up to several hundred micrometers have been obtained, Figures 2a and 2b, whereas for CLP4 needles the maximum length is about 30 µm, Figure 4c. For constant deposition temperature but increasing film thickness one has to distinguish between the low- and hightemperature case. For both cases the number density of the aggregates increases strongly for the first 5 nm, beginning to saturate afterward, Figure 5a. For temperatures close to room temperature, the dominating effect is a strong increase in height of the tall islands reaching heights up to 400 nm for 9 nm film thickness. Fibers increase in length only slightly with a mean length below 2 µm, Figure 5b. At higher substrate temperatures, where these islands are diminished, the increase of the mean length of the fibers with increasing nominal CLP4 thickness is still marginal, Figure 5c. Between 2 and 9 nm thickness newly deposited material mainly increases the number density and the height of the needles. Obviously, an appropriate substrate temperature is more efficient in producing long fibers than the overall film thickness. Corresponding AFM images for increasing CLP4 thickness from 2 to 9 nm are displayed in Figures 6a-6c. As a function of substrate temperature TS during deposition an Arrhenius type of behavior for the aggregate number density N has been observed, N ∝ exp(EN/kTS) with an activation energy EN ≈ 0.56 ( 0.1 eV, cf. the linear regressions in Figure 5a. For homogeneous nucleation such a behavior has been predicted in the literature, with the activation energy EN being a function of characteristic energies like the energies for adsorption and diffusion and of the critical nucleus size of the system.24 Similar to the case of p-6P, the needles are presumably not the initial aggregates formed. Three-dimensional clusters between the needles have been observed on AFM images for the CLP4 samples grown at elevated substrate temperatures, Figure 7a. Needles probably grow by agglomeration of such clusters. However, for a reasonably homogeneous size distribution of

Organic Nanofibers from p-Quaterphenylenes

Crystal Growth & Design, Vol. 7, No. 2, 2007 231

Figure 5. (a) Number density N of CLP4 aggregates on mica as a function of the reciprocal substrate temperature 1/TS, for samples with thicknesses of 2 nm (circles), 5 nm (triangles), and 9 nm (squares). Straight lines depict linear regressions for the different sample thicknesses. The insert shows the thickness dependence of N for TS ) 310 K. The length distributions of needles for deposition at room temperature (b) and at TS ) 370 K (c) demonstrate a strong increase of needle length with TS. The black bars correspond to a nominal film thickness of 2 nm and the gray bars to 9 nm CLP4.

Figure 6. (80 × 80) µm2 AFM images of 2 nm (a), 5 nm (b), and 9 nm (c) CLP4 on muscovite mica, deposited at a substrate temperature of TS ) 350 K. The height scale for all images is 150 nm.

Figure 7. (a) (45 × 45) µm2 AFM image of CLP4 fibers growing on several mica terraces. The heights of the terraces in units of mica elementary steps are given at the bottom of the image together with gray lines marking terrace boundaries. The muscovite mica crystalline directions [100] and 〈110〉 (white arrows) have been obtained via the “Schlagfigur’’-technique.29 Even fibers crossing a 14 nm tall step in the (20 × 20) µm2 AFM image (b) do not change their growth directions. In between needles in (a) small, three-dimensional clusters are still present. The fibers were grown at TS ) 360 K (a) and at TS ) 330 K (b).

the needles the needle density should still somewhat reflect the distribution of the primary clusters. For the formation of, e.g., three-dimensional PTCDA islands on Ni a value for the activation energy of EN ) 0.37 eV has been reported,25 for PTCDA islands on Ag(111) 1.13 ( 0.3 eV,26 for p-6P islands on GaAs 0.90 ( 0.04 eV,28 and for sexithiophene islands on mica 0.36 ( 0.04 eV,28 all energies being on the order of a few hundred millielectronvolts. The value of EN will be of importance for future Monte Carlo simulations of the growth process.

From earlier studies it is known that the nanofibers from p-6P grow strictly parallel to each other. Two domains of needles exist with a rotational angle of 60° in between, each domain up to square centimeters in size. Needles grow along muscovite mica 〈110〉, never along [100]; the 〈110〉 directions correspond to the two groove directions on the muscovite basal surface which result from the two stacking directions of the 2M1 polytype.30,31 Needles grow along the grooves, the molecules being arranged approximately perpendicular to them. For CLP4 two domains with 60° between form, too, Figure 7a, but within each domain up to three different needle orientations exist, Figure 8. Similar to the other phenylenes the mean needle direction is along mica 〈110〉. The growth direction of the aggregates is coupled to the two different cleavage planes of the 2M1 muscovite polytype, i.e., to the stacking of the underlying two tetrahedral muscovite sheets, Figure 7a. At steps with an even multiple of the minimal mica step height of 1 nm, no change in growth direction occurs. At steps with an odd multiple of elementary steps the orientation changes by 60° because of a change in the direction of the grooves and the electric dipole fields on the mica surface.32 Similar electric field induced alignment of phthalocyanines and of anthraquinone on muscovite was reported some time ago.33,34 The mutual alignment of the CLP4 fibers strongly depends on the growth temperature, Figures 8a-8c. At relatively low surface temperature of TS ) 310 K the needles exhibit three orientations within one domain. Needles grow along 〈110〉 and along two directions about ∆φ ) (11° off 〈110〉, the “offneedles’’. For increased substrate temperature during deposition the number of off-needles decreases, until almost all needles form along 〈110〉 at TS J 350 K. The overall fluorescence intensity for normal incidence observation is different for the 〈110〉 aggregates and for the offaggregates. In Figure 2 fluorescence microscope images for two samples with the same nominal CLP4 thickness have already been presented, grown at two different substrate temperatures. Needles exactly along 〈110〉 emit 3-4 times less fluorescence per unit length than the off-direction fibers. However, the polarization dependence of the emitted light after excitation under normal incidence and under normal observation shows almost no difference for the two needle types. Both for the 〈110〉

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Figure 8. For three different substrate temperatures, TS ) 310 K (a), 330 K (b), and 350 K (c), distributions of needle orientations obtained from AFM and fluorescence microscope images are shown. The vertical lines at ∆φ ) 0° indicate the 〈110〉 mica orientation. Within the experimental error of (3° the main needle direction is along muscovite 〈110〉, with two additional directions at ∆φ ) (11° at low deposition temperatures. The solid lines are Gaussian fits to the experimental distributions (fwhm ) 6°).

Figure 9. (a) For the two needle typess〈110〉 needles and off-needless the angle βMol of needle orientation relative to the angle, at which the polarized fluoresce of the needle is maximal, gives two different distributions of angles for the mean transition dipole. The insert displays a model for the orientation of the molecules (dotted lines) within the needles; the double-headed arrow represents the transition dipole of the molecules and thus the polarization maximum of the emitted light. (b) A typical (5 × 5) µm2 AFM image of two fibers growing along the 〈110〉 muscovite mica direction (arrows) and a single fiber along the off-direction. Whereas the 〈110〉 needles show facets and a rather singlecrystalline shape, the off-needle looks rather ill-defined. The heights of the needles are between 60 and 120 nm.

needles as well as for the off-needles the angular distribution of fluorescence can be fit by Malus law and shows a maximum in intensity for light polarized perpendicular to 〈110〉 (4°, i.e., perpendicular to the needle direction for the 〈110〉 needles (βMol ) 90 o) and at βMol ) 77° ( 5° for the off-needles, Figure 9. The solubility of the p-phenylenes is low, complicating the growth of single crystals for a structure analysis. Assuming a similar herringbone packing of the molecules for CLP4 as for a quaterphenylene functionalized by fluor atoms,35 the easiest growing axis of the aggregates (i.e., the long needle axis) is approximately perpendicular to the long molecular axis and along the layers of parallel molecules. A similar behavior has been observed for many rodlike molecules like, e.g., pphenylenes,36,13 thiophenes,37 and thiophene/phenylene cooligomers.7 Different angles βMol can easily be explained by different contact faces of the bulklike needles with the substrate, cf. the insert in Figure 9a. For needles from lying molecules with the (100) contact plane (or similar planes with just different tilt angles of the molecules with respect to the substrate) a value of βMol ) 90° would result, and for lying molecules with the (110) contact plane βMol ) 76°, i.e., the observed values. For p-6P these two needle types have indeed been observed.4,38 An AFM image of an off-needle and of two 〈110〉 needles, Figure 9b, discloses that the two fiber types have quite different morphologies. The 〈110〉 needles are straight and show clear facets, whereas the off-needles have a rugged morphology. Heights and widths, however, are similar or even lower than

those for the straight fibers. The reason for the higher luminescence efficiency therefore is not due to more material within the needle, but might result from a different crystal structure with the same lateral orientation of the molecules with respect to the substrate. This would, e.g., imply that the molecules are more parallel to the substrate for the rugged than for the straight fibers. Another possible explanation are different waveguiding and light scattering properties for the two needle types. The 〈110〉 fibers from the high surface temperature case exhibit brightly fluorescent spots within the fibers and at the ends, Figure 2d. The bright spots within a fiber originate from breaks, which can be clearly seen by atomic force microscopy. In the case of p-6P it has been demonstrated that the fibers act as waveguides1,39 and the light is scattered at breaks within the fiber. According to this the 〈110〉 needles from CLP4 are believed to show pronounced waveguiding phenomena. The generated fluorescence light upon UV excitation can be kept inside the well-shaped fiber and waveguided to breaks and ends before it is scattered. Therefore, only spots appear very bright whereas the main body of the fibers seem to be less fluorescent. The off-needles may not function as waveguides due to inhomogeneity of the fiber’s shape. Generated fluorescence light can be scattered at every uneven point; light is not kept within the fiber. So rugged off-needles exhibit a homogeneously fluorescence intensity along the whole fiber. To clarify this point, electron diffraction on single aggregates has to be performed in the future. Conclusions In this paper we have shown that nanofibers from chloridefunctionalized p-quaterphenylenes can be grown on muscovite mica. The growth is similar to that of the well-known phexaphenylene, but the orientation of the functionalized molecules is more strongly influenced by the growth temperature as compared to nonfunctionalized hexaphenylene. The present observations together with that reported previously on methoxyfunctionalized nanofibers open up a new field of on-demand tailored nanofibers with widely adjustable electronic and optical properties. As demonstrated in the present article, fibers grown on the basis of functionalized molecules exhibit a luminescence which is tunable by modification of the functional groups through the blue spectral region. By the same token, electric or nonlinear optical properties are expected to be largely variable. Acknowledgment. M.S., A.L., and K.S. thank the German research foundation DFG for financial support. H.G.R. thanks the Danish research foundations SNF (21-03-0469) and STVF

Organic Nanofibers from p-Quaterphenylenes

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