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Communications

2d[] =

DF Hz 1X832rgacm3

(1)

Scanning electron micrographs were obtained with a Hitachi S-900 instrument at an acceleration voltage of 25 kV. The samples were coated with 20 thick Pt by use of an ion coater (Hitachi E-1030 ion sputter, 10 mA/ 10 Pa) under argon atmosphere to prevent charge up. FTIR spectra and Xray photoelectron spectra were obtained using a Nicolet 710 FTIR spectrometer and a Perkin Elmer PHI 5300 ESCA system, respectively. n D(+)-Maltose, D(+)-glucose, titanium butoxide (Ti(O Bu)4), and zirconium propoxide (Zr(OnPr)4) were purchased from Kishida Chem., Japan, and aluminum butoxide (Al(OnBu)3) and niobium butoxide (Nb(OnBu)5) were obtained from Wako Pure Chem. 4,4¢,4²,4¢²-(21H,23H-Porphine5,10,15,20-tetrayl)tetrakis(benzoic acid) (TCPP) was purchased from Aldrich. Received: September 17, 1997 Final version: January 12, 1998 ± [1] a) B. M. Novak, Adv. Mater. 1993, 5, 422. b) C. Sanchez, F. Ribot, New J. Chem. 1994, 18, 1007. c) H. Krug, H. Schmidt, New J. Chem. 1994, 18, 1125. [2] D. O'Hare, New J. Chem. 1994, 18, 989. [3] G. Decher, J. D. Hong, Ber. Bunsenges. Phys. Chem. 1991, 95, 1430. [4] a) E. R. Kleinfeld, G. S. Ferguson, Science 1994, 265, 370. b) G. S. Ferguson, E. R. Kleinfeld, Adv. Mater. 1995, 7, 414. c) Y. Lvov, K. Ariga, I. Ichinose, T. Kunitake, Langmuir 1996, 12, 3038. d) S. W. Keller, H.N. Kim, T. E. Mallouk, J. Am. Chem. Soc. 1994, 116, 8817. [5] a) A. Krozer, S.-A. Nordin, B. Kasemo, J. Colloid Interface Sci. 1995, 176, 479. b) K. Ariga, Y. Lvov, M. Onda, I. Ichinose, T. Kunitake, Chem. Lett. 1997, 125. [6] a) D. Ingersoll, P. J. Kulesza, L. R. Faulkner, J. Electrochem. Soc. 1994, 141, 140. b) M. Sano, Y. Lvov, T. Kunitake, Annu. Rev. Mater. Sci. 1996, 26, 153. [7] D. Li, M. A. Ratner, T. J. Marks, J. Am. Chem. Soc. 1990, 112, 7389. [8] I. Ichinose, H. Senzu, T. Kunitake, Chem. Lett. 1996, 831. [9] E. R. Kleinfeld, G. S. Ferguson, Mater. Res. Soc. Symp. Proc. 1994, 351, 419. [10] I. Ichinose, H. Senzu, T. Kunitake, Chem. Mater. 1997, 9, 1296. [11] Sowa Science Corp., Japan, Polymer Catalog 1997. [12] Handbook of X-Ray Photoelectron Spectroscopy, Perkin Elmer 1978. [13] I. Ichinose, N. Kimizuka, T. Kunitake, J. Phys. Chem. 1995, 99, 3736. [14] C. Roger, M. J. Hampden-Smith, J. Mater. Chem. 1992, 2, 1111. [15] G. Sauerbrey, Z. Phys. 1959, 155, 206. [16] Y. Lvov, K. Ariga, I. Ichinose, T. Kunitake, J. Am. Chem. Soc. 1995, 117, 6117.

rameter a, for example, the refractive index. The contrast is defined as in Equation 1, and at the same time is the function of many variable quantities (the activator concentration, the growth conditions, etc.). K= amax þ amin amax amin (1)

In Situ Preparation of Bulk Crystals with Regularly Doped Structures
By Vladimir N. Kurlov* and Svetlana V. Belenko A current problem in the field of optoelectronics is the development of new materials that can combine in themselves several different functions simultaneously. One way to solve this problem is the growth of bulk crystals with regularly doped structures. In particular, the presence of spatial resonance structures (periodic structures of variable composition) in crystals of Al2O3:Ti3+ sharply reduces the threshold of the laser oscillation and makes it almost independent of the outside cavity characteristics.[1] The characteristics of the laser medium are also influenced by the contrast of the spatial resonance structures in terms of a pa± [* ] Dr. V. N. Kurlov, S. V. Belenko Institute of Solid State Physics, Russian Academy of Sciences 142432 Chernogolovka, Moscow District (Russia)

As a model material we have used sapphire doped with the active laser ions Ti3+. As layer materials crystals grown by the Verneuil method have been used that contain no more than 10±4 wt.-% Ti in undoped layer material and up to 0.5 wt.-% in doped layer material. The doping contents in the grown crystals were measured using the X-ray microanalyzer ªCamebaxº. The modulation-doped structures in the grown crystals were observed using the scanning electron microscope DSN-960 (from Opton), the image contrast depending on the contents of the luminescent impurity Ti3+ in the matrix±activator pair. In the first variant of in situ production of bulk crystals with regularly doped structures, sapphire crystals doped by Ti ions were grown by the EFG (edge-defined film-fed growth) method[2] in the shape of ribbons, rods, and tubes. To obtain the periodic structures in situ we used a periodic change of the pulling rate. Change of the crystallization rate leads to a periodic disturbance of conditions at the crystal±liquid interface and hence to periodic capture of the impurity by the crystal. The periodic change of the pulling rate was achieved in two different ways. Either the rate was periodically increased and decreased or the pulling mechanism was periodically switched on and off. The periods of the periodic structures produced in this way ranged from 5 to 100 mm. The periodic structure in a longitudinal section of a sapphire rod is shown in Figure 1. This approach also permits us to change the period of the spatially doped structures during the process of growth and to go from a periodically doped structure to a homogeneously doped structure. The contrast in the stripes produced in the structures is not strong because it is limited by the redistribution of the impurity in front of the crystallization front in only a small volume of meltïin the meniscus. The difference in the

Fig. 1. The periodic structure in the longitudinal section of a sapphire rod grown by the EFG method. The structure was prepared by changing the pulling rate. 0935-9648/98/0705-0539 $ 17.50+.50/0

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Communications

concentrations of the impurity within one period is not more than a factor of ten. The second variant of in situ production of bulk crystals with regularly doped structures consists in the simultaneous use of two or more melts with different impurity contents. The dies are placed at some distance from the rotation axis, the menisci are small elements of the melt form. This technique is called the GES (growth from an element of shape) method.[3] The crystal grows layer-by-layer and the period is determined by the ratio V/o, where V is the pulling rate and o is the rotation frequency. Shaped crystals in the form of rods and tubes have been grown at a pulling rate of 3± 25 mm/h and at a rotation rate of 0.5±20 rpm. Grown crystals have included the regularly doped structures Al2O3± Al2O3:Ti3+ with period 5±100 mm (Fig. 2a). The impurity

concentration in undoped parts of crystal was less than 10±4 wt.-% and in doped parts of crystal was up to 0.2 wt.-%. The doped and undoped layers constitute spirals with the angle of the spiral line slope given by a = tan±1(V/2pRo), where R is the distance from the rotation axis of the seed holder to the axes of the dies (when the dies are located at equal distance from the axis of rotation of the seed holder). The volume of the transition region between layers is defined mainly by the width of partial melting of the earliercrystallized layer at the moment of contact of this layer and the meniscus. The width of the partially melted zone and the character of the impurity distribution in the transition region depend on the thermal conditions in the crystallization zone, the frequency of rotation, and the pulling rate of the crystal. In particular, high contrast structures with small period were produced under conditions close to supercooling on the crystallization front, that is, with a small height of meniscus. Application of the GES method lets us greatly develop the possibilities of production in situ of various types of spatially periodic structures: l A change of period during the growth process can be realized by variation of the relation V/o (Fig. 2b). l Application of this method permits the required ratio of layer heights within a period to be predetermined and this ratio to be varied during the growth process (Fig. 2c). Change of doped±undoped layer width ratio can be realized by two ways, either by variation of the top surface level of one of the dies or by variation of the relative location of the dies in the case when the top surface levels are constant. l The transition from a periodic structure to a uniformly doped or undoped crystal can be achieved by stopping the feed of melt to one of the dies during the growth process. l Use of the dies with different areas of work surface, different distances of the dies from the axis of rotation, and/or combinations of the arrangement of the dies makes it possible to obtain various types of doping structures in crystals. Both techniques make it possible to control not only the contrast, period, and ratio of heights of doped and undoped regions in one period, but also the character of dopant distribution within the period (Fig. 3). The two techniques of in situ preparation of the modulated structures described here can be used universally in the growth of crystals of other compositions from the melt. The structures in bulk crystals outlined above amount to a new class of materials. There is reason to believe that these structures will find applications in the development and fabrication of various devices.
Received: May 14, 1997 ± [1] Physics and Spectroscopy of Laser Crystals (Ed: A. A. Kaminsky), Nauka, Moscow 1986. [2] H. E. LaBelle, J. Cryst. Growth 1980, 50,8. [3] P. I. Antonov, Yu. G. Nosov, S. P. Nikanorov, Bull. Acad. Sci. USSR, Phys. Ser. 1985, 49, 2295. 0935-9648/98/0705-0540 $ 17.50+.50/0 Adv. Mater. 1998, 10, No. 7

Fig. 2. The periodic structures Al2O3±Al2O3:Ti grown by the GES method: a) structure with constant period; b) structure with variable period; c) structure with different ratio of width of doped and undoped regions in one period.

3+

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Fig. 3. Variously shaped distribution of the cathodoluminescence intensity corresponding to the content of the luminescence impurity. The arrows show the pulling direction.

Electrosynthesis of Highly Electroactive Tetrathiafulvalene-Derivatized Polythiophenes
By Laurent Huchet, Said Akoudad, and Jean Roncali* The concept of combining tetrathiafulvalene (TTF) based molecular conductors with linear p-conjugated systems offers exciting potentialities to develop original organic conducting materials.[1] Whereas incorporation of TTF into processable polymeric matrixes has been known for some time,[2,3] the association of TTF with a p-conjugated polymer structure is a relatively recent development.[4±8] From the viewpoint of TTF chemistry, such a combination may contribute to increase the dimensionality

± [*] Prof. J. Roncali, Dr. L. Huchet, Dr. S. Akoudad Ingünierie Molüculaire et Matüriaux Organiques CNRS UMR 6501, Universitü d'Angers 2 Blvd Lavoisier, F-49045 Angers Cedex (France)

and perhaps the processability of TTF-based conductors. On the other hand, the strong propensity of TTF to selfassemble into regular p-stacks, when covalently attached to a p-conjugated backbone, may be an interesting tool for indirectly controlling the long-range order in the conjugated chain. Furthermore, the grafting of TTF groups may lead to a significant increase of the charge storage capacity of the resulting substituted conjugated polymer. At a more basic level, and provided that properly organized materials can indeed be synthesized, the coexistence of two different charge-transport mechanisms, namely aromaticity transfer in mixed-valence TTF stacks[9] and polaron/bipolaron conduction in the conjugated chain,[10] may open exciting prospects of materials with hybrid conduction. The synthesis of TTF-derivatized conjugated polymers by electropolymerization of tailored precursors has been attempted by several groups and in each case polythiophene (PT) was chosen as the conducting polymeric backbone.[4±8] However, until now, only a thiophene monomer with the TTF group linked at the 3-position of thiophene through an oxadecyl spacer has been unequivocally electropolymerized.[5] Attempts to electrolymerize this monomer in acetonitrile were unsuccessful and only application of recurrent potential scans to nitrobenzene electrolytic solutions led to the electrodeposition of a dark blue polymer film. As illustrated by these many unsuccessful attempts and difficulties, the design of an appropriate precursor for TTFderivatized PT remains a challenging task that requires the simultaneous control of many structural variables such as solubility, steric effects, and the relative position of TTF moieties in the polymer in order to ensure a proper stacking arrangement allowing mixed-valence interactions and hence possible intra-stack conduction. Finally, future development of materials showing hybrid conduction requires that both components become conductive in a common potential range, which will necessitate a precise tuning of the oxidation potential of both the attached TTF group and the supporting p-conjugated backbone. One of the major obstacles to the efficient electropolymerization of TTF-derivatized thiophene monomers lies in the very large difference between the first oxidation potential of TTF (0.40 V / SCE)[3] and that of thiophene (2.07 V).[11] An important consequence of this difference is that at the potential needed to produce the thiophene cation radical, the main fraction of the current is consumed by the oxidation of the TTF groups. Furthermore, a large part of the thiophene cation radicals can be scavenged by neutral TTF groups. In order to solve this problem we report here preliminary results on the synthesis and electropolymerization of new TTF-derivatized bithiophenic precursors (1 and 2). We show that the much lower oxidation potential of bithiophene (BT) compared to thiophene allows the facile electropolymerization of these new precursors into highly elec0935-9648/98/0705-0541 $ 17.50+.50/0

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