- Email: [email protected]
Molecular alignment in organic thin films
Synthetic Metals 109 Ž2000. 43–46 www.elsevier.comrlocatersynmet
Molecular alignment in organic thin films Toshihiro Ehara, Hidekazu Hirose, Hiroyuki Kobayashi, Masahiro Kotani
)
Faculty of Science, Gakushuin UniÕersity, Mejiro, Tokyo 171-8588, Japan Received 26 June 1999; received in revised form 22 July 1999; accepted 10 September 1999
Abstract Molecules can be oriented when they are adsorbed onto a solid surface with an appropriate surface structure. Examples are shown which illustrate a unidirectional alignment. 4-Dimethylamino-4X-nitrostilbene ŽDMANS. exhibits an oriented adsorption when evaporated onto a cleaved face of a glycine single crystal. The same compound exhibits also a high degree of orientational order when an evaporated film of the compound is mechanically brushed in a direction. The mechanism of the molecular alignment is discussed based on the results of polarized optical absorption, optical second-harmonic generation, X-ray diffraction and scanning force microscopy. q 2000 Elsevier Science S.A. All rights reserved. X
Keywords: Molecular alignment; Organic thin films; 4-Dimethylamino-4 -nitrostilbene
1. Introduction Epitaxial growth is a powerful method to control structures of thin films. Intensive research is being made to realize epitaxially grown films of metals and semiconductors. GaAs may be a most well known example. Regularity of periodic structures is a subject of most concern and is studied mainly by diffractional methods. In the case of molecular thin films an additional characteristic is the orientation. Unlike atoms, which are spherical entities and have no orientational freedom, molecules can exhibit many different behaviours depending on their relative orientations in a condensed phase. Liquid crystals may be the best examples which illustrate the importance of the orientational order. There are a number of reports on the epitaxial growth of organic films on single crystal substrates, such as metals, graphite, alkali halides and molybdenum disulfide w1–4x. In most cases, however, single crystal domains of an epitaxially grown film are small and have to be verified by electron microscopy w5x, or by scanning probe microscopy w6x. There are two reasons for this. The molecules adsorbed are in many cases much larger than the unit cells of these substrates and, accordingly, a molecule experiences many shallow energy minima when it is translated along the surface. Also, the symmetry of such a substrate surface is )
Corresponding author. Fax: [email protected]
q81-3-5992-1029;
e-m ail:
high and there exists a fourfold symmetry, or sixfold symmetry, and, accordingly, a stable orientation of a molecule can also be stable if it is rotated by 908, or 608, depending on the symmetry of the surface. As a result, adsorbates tend to form a patch of single crystal domains, which are related by a rotation, of 60 degrees, for example w7x. A substrate with a lower symmetry is desirable if a macroscopic orientation along the surface is to be realized, which is important for the generation of optical secondharmonics, for instance. The surface of an organic solid can also serve as a template upon which a compound can be adsorbed with a preferred orientation w8x. Liquid crystals can be aligned on polyimide substrate which has been treated by ‘‘rubbing’’. The molecular mechanism of the rubbing is, however, not well understood w9x. Enzymes, which combine only molecules of a particular shape with a so-called key-andthe-keyhole specificity, may be considered another example. Most organic crystals, however, have high vapour pressure and their surfaces can be etched when a crystal is kept in a vacuum for some time and hence such crystals have been regarded as not suitable for a study in an ultrahigh vacuum. Here, we propose that a glycine crystal can be a good substrate for orienting organic molecules. In spite of its small molecular weight, glycine has a high melting point. Actually, it decomposes when it is heated to its melting point of 510 K. This anomalously high thermal stability is due to its zwitterionic lattice. The glycine molecules are
0379-6779r00r$ - see front matter q 2000 Elsevier Science S.A. All rights reserved. PII: S 0 3 7 9 - 6 7 7 9 Ž 9 9 . 0 0 1 9 6 - 4
44
T. Ehara et al.r Synthetic Metals 109 (2000) 43–46
present as H 3 Nq–CH 2 –COOy and the large lattice energy comes essentially from ionic bonding w10x. Because of the tight bonding the vapour pressure of this compound is sufficiently low and it was possible to bring this material into an UHV chamber without deteriorating the vacuum. We have studied orientational order that can be realized when polar molecules such as 4-dimethyamino-4X-nitrostilbene ŽDMANS. is evaporated onto a cleaved face of a glycine single crystal. Orientational order can be verified by polarized absorption. The same compound exhibits also a high degree of orientational order when a film of the compound, evaporated on a glass substrate, is mechanically brushed in a direction. The mechanism of the molecular alignment is discussed based on the results of polarized optical absorption, optical second-harmonic generation, X-ray diffraction and scanning force microscopy.
2. Experimental Glycine single crystals, with a typical size of 1 = 1 = 2 cm3, were grown from aqueous solution by slow evapora-
tion of the solvent. The temperature during the crystal growth was controlled within 0.1 K. The crystal was cleaved and the orientation of the crystal axes was examined by X-ray diffraction. DMANS was purchased from Tokyo Chemical and was used after recrystallization from carbon tetrachloride solution. Evaporated films were prepared with a conventional vacuum evaporation system, or with an ultrahigh vacuum system. The pressure during the evaporation was 10y6 Torr in the former case and 10y9 Torr in the latter case. In brushing experiments an evaporated film was brushed a few times in one direction with a cloth or laboratory tissue paper. UV–visible absorption spectrum was measured with an arrangement constructed in the laboratory w11x. It was a single beam spectrophotometer capable of optical density measurements with a high precision. The high stability in the measurement of the optical density was realized by a combination of luminous optics, a mechanically stable construction and an extensive averaging. In favorable cases, an optical density could be determined to the precision of 10y5 , which allowed us to measure the absorption spectrum of an organic film as thin as 1r30 of a monolayer.
Fig. 1. Molecular arrangement in a cleaved Ž010. face of a glycine single crystal. A two-dimensional network of hydrogen bonds extends through the plane. Unit cell dimensions are: a s 510 pm, c s 545 pm, b s 111.68. The structure appears polar, but the layer underneath has an opposite polarity, so that the crystal as a whole is centrosymmetric. The special feature of a glycine crystal is that it is composed essentially of a stack of double layers, two neighboring layers being strongly bonded by hydrogen bonds, while there is only a weak van der Waals type interaction between the pair of double layers. As a result, the surface exhibits always a definite polarity.
T. Ehara et al.r Synthetic Metals 109 (2000) 43–46
45
Fig. 4. Polarized absorption of a thin film of DMANS on glass substrate. Ža. As deposited. Žb. After brushing, observed with light polarized parallel to the brushing direction. Žc. After brushing, observed with light polarized normal to the brushing direction. Absorption becomes intense, and strongly polarized, by unidirectional brushing. Fig. 2. Polarized absorption of a thin film of DMANS, evaporated onto a cleaved Ž010. face of a glycine single crystal. The temperature of the substrate was 263 K during the evaporation. A large contrast in polarized absorption indicates that DMANS molecules are well oriented parallel to the c-axis of the glycine crystal.
Second-harmonic generation was measured with transmission, either with a Q-switched Nd:YAG laser as the light source. The second-harmonic signal at 532 nm was detected with a photomultiplier and the shot-to-shot fluctuation was averaged with a personal computer w12x. X-ray diffraction was measured with a commercial diffractometer ŽRigaku. with a molybdenum target. Atomic force microscopy images were taken with a commercial AFM unit ŽNanoscope III, Digital Instruments., operated with tapping mode. 3. Results and discussion 3.1. Glycine single crystal as an orienting substrate
bonds. The crystal structure is constructed by stacking this two-dimensional network. The crystal has a symmetry of P2 1rn, and the unit cell dimensions are: a s 510 pm, c s 545 pm, b s 111.68 and Z s 4. The structure shown in Fig. 1 appears polar, but the layer underneath has an opposite polarity, so that the crystal as a whole is centrosymmetric. The special feature of a glycine crystal is that it is composed essentially of a stack of double layers. There are fairly strong hydrogen bonds within the double layer, while only a weak van der Waals type interaction operates between the double layers. As a result, the surface layers can be removed only in pairs and the surface exhibits always polarity of either sign. Fig. 2 shows the polarized UV–VIS absorption spectrum of a film of DMANS evaporated onto a cleaved Ž010. face of a glycine single crystal. Glycine is transparent in this spectral region and hence the absorption is due to adsorbed DMANS. The absorption band in the visible is due to an intramolecular CT transition and its transition
Fig. 1 shows the arrangement of the molecules in Ž010. surface, expected from the bulk structure w10x. The molecules form a tight network of intermolecular hydrogen
Fig. 3. X-ray diffraction pattern of an evaporated film of DMANS on a glycine crystal, measured with u –2 u scan, which explores the scattering vector normal to the surface. Note that only Ž h00. reflections have been observed. The intense peak at 2 u s11.58 corresponds to a spacing of 767 pm, which is close to the length of a DMANS molecule, suggesting that the molecules are standing against the substrate surface. The diffraction lines disappear when the film is brushed.
Fig. 5. Optical second-harmonic generation measured with 1.06 mm pulse from a Nd:YAG laser as the incident light, observed at 532 nm. The incident light hits the sample at normal incidence and the second-harmonic output has been measured in transmission, with the polarization parallel to that of the incident light, while the sample is rotated around the surface normal. The angles indicate the angle between the electric field of the incident light and the direction of the brushing. The fact that a secondharmonic signal can be observed with normal incidence indicates that the film has a C 1 symmetry around the normal of the surface.
46
T. Ehara et al.r Synthetic Metals 109 (2000) 43–46
moment is parallel to the molecular long axis. Fig. 2 shows clearly that DMANS molecules are aligned with their long axes parallel to c-axis of the glycine crystal. A high degree of orientational order is obvious. Only a preliminary analysis has been made of the crystal structure of DMANS, but there is a close matching with the crystal lattice of glycine. The crystal symmetry is P2 1rn. The lattice parameters of a DMANS crystal is: a s 6822 pm, b s 754 pm, c s 600 pm, b s 1168 and Z s 12. If the structure in the ac plane is superposed on that of Ž010. face of glycine, both structures closely match. A DMANS molecule occupies the same area as three glycine molecules. If the positive –NŽCH 3 . 2 group of a DMANS molecule lies on top of the negative -COy 2 group of a glycine molecule, the negative –NO 2 group of DMANS molecule falls on the positively charged –NHq 3 group of the third glycine molecule. The matching is, however, not perfect. These results suggest that DMANS molecules are lying with their molecular long axes parallel to the c axis in the cleaved face of a glycine crystal, while the arrangement of DMANS molecules may differ somewhat, compared to that in the crystal. 3.2. Mechanical brushing When the substrate is a glass plate, on the other hand, the film appears pale yellow. The very weak absorption is due to the fact that the molecules stand with their molecular long axes almost normal to the surface. X-ray diffraction, with u –2 u scan, which explores the scattering vector normal to the surface, shows exclusively Ž h00. reflections ŽFig. 3.. This indicates that the film is crystalline and that the molecules are essentially standing against the glass substrate. Polarization analysis with the second-harmonic generation indicates that the film has an axial symmetry. When the film is mechanically brushed in one direction, the film turns dark brown. The optical absorption not only becomes strong, but also becomes strongly polarized along the direction, indicating that the molecules are now aligned along the brushing direction ŽFig. 4.. A strong SHG can be observed at normal incidence, which indicates that molecules are oriented along the direction of the brushing ŽFig. 5.. What makes this extremely efficient alignment possible? Since a mechanical brushing provides only an occasional point contact w13x and since we do not touch every molecule by brushing, a nearly 100% alignment is not self-evident and a mechanism must be sought which explains the ‘‘amplification’’ of the alignment of the molecules in the brushed region. In the case of polymers or hygroscopic amorphous films, a shear flow-induced alignment could be the mechanism w9x, However, this cannot
Fig. 6. An image, taken with scanning force microscopy, of a scratched area of an evaportated film of DMANS on a glass plate. The area is 5 mm=10 mm. The lower part shows the debris made by the scratching. In the upper part, which has been brushed, small crystallites, of a typical size of 100 nm=200 nm, are seen to be aligned along the direction of the brushing.
apply to a crystalline film, such as of DMANS. An image of a brushed region taken with a scanning force microscope shows that small crystals, typically 200 nm long and 100 nm wide, are aligned along the brushing direction ŽFig. 6.. This could account for an efficient alignment of a large number of molecules by a macroscopic method such as mechanical brushing.
Acknowledgements This work has been made in collaboration wit Y. Shimizu, M. Kojima, and M. Fujii. The authors wish to thank Toyo Corporation for technical support in operating the scanning force microscopy.
References w1x T.J. Schuerlein, A. Schmidt, P.A. Lee, K.W. Nebesny, N.R. Armstrong, Jpn. J. Appl. Phys. 34 Ž1995. 3837. w2x S.R. Forrest, Chem. Rev. 97 Ž1997. 1793. w3x E. Umbach, K. Gloeckler, M. Sokolowski, Surf. Sci. 402 Ž1998. 20. w4x N. Karl, Ch. Guenther, Cryst. Res. Technol. 34 Ž1999. 243. w5x T. Kobayashi, S. Isoda, J. Mater. Chem. 3 Ž1993. 1. w6x J.-Y. Grand, T. Kunstmann, D. Hoffmann, A. Haas, M. Dietsche, J. Seifritz, R. Moeller, Surf. Sci. 366 Ž1996. 403. w7x A. Soukopp, K. Gloekler, P. Kraft, S. Schmitt, M. Sokolowski, E. Haedicke, Phys. Rev. B 58 Ž1998. 13882. w8x H. Kobayashi, T. Ehara, M. Kotani, J. Chem. Soc. Faraday Trans. 90 Ž1994. 3685. w9x J.M. Geary, J.W. Goodby, A.R. Kmetz, J.S. Patel, J. Appl. Phys. 62 Ž1987. 4100. w10x G. Albrecht, R.B. Corey, J. Am. Chem. Soc. 61 Ž1939. 1087. w11x K. Yamasaki, O. Okada, K. Inami, K. Oka, M. Kotani, H. Yamada, J. Phys. Chem. 101 Ž1997. 13. w12x Y. Shimizu, M. Kotani, Opt. Commun. 74 Ž1989. 190. w13x J. Lowell, A.C. Rose-Innes, Adv. Phys. 29 Ž1980. 947.
Molecular alignment in organic thin films Toshihiro Ehara, Hidekazu Hirose, Hiroyuki Kobayashi, Masahiro Kotani
)
Faculty of Science, Gakushuin UniÕersity, Mejiro, Tokyo 171-8588, Japan Received 26 June 1999; received in revised form 22 July 1999; accepted 10 September 1999
Abstract Molecules can be oriented when they are adsorbed onto a solid surface with an appropriate surface structure. Examples are shown which illustrate a unidirectional alignment. 4-Dimethylamino-4X-nitrostilbene ŽDMANS. exhibits an oriented adsorption when evaporated onto a cleaved face of a glycine single crystal. The same compound exhibits also a high degree of orientational order when an evaporated film of the compound is mechanically brushed in a direction. The mechanism of the molecular alignment is discussed based on the results of polarized optical absorption, optical second-harmonic generation, X-ray diffraction and scanning force microscopy. q 2000 Elsevier Science S.A. All rights reserved. X
Keywords: Molecular alignment; Organic thin films; 4-Dimethylamino-4 -nitrostilbene
1. Introduction Epitaxial growth is a powerful method to control structures of thin films. Intensive research is being made to realize epitaxially grown films of metals and semiconductors. GaAs may be a most well known example. Regularity of periodic structures is a subject of most concern and is studied mainly by diffractional methods. In the case of molecular thin films an additional characteristic is the orientation. Unlike atoms, which are spherical entities and have no orientational freedom, molecules can exhibit many different behaviours depending on their relative orientations in a condensed phase. Liquid crystals may be the best examples which illustrate the importance of the orientational order. There are a number of reports on the epitaxial growth of organic films on single crystal substrates, such as metals, graphite, alkali halides and molybdenum disulfide w1–4x. In most cases, however, single crystal domains of an epitaxially grown film are small and have to be verified by electron microscopy w5x, or by scanning probe microscopy w6x. There are two reasons for this. The molecules adsorbed are in many cases much larger than the unit cells of these substrates and, accordingly, a molecule experiences many shallow energy minima when it is translated along the surface. Also, the symmetry of such a substrate surface is )
Corresponding author. Fax: [email protected]
q81-3-5992-1029;
e-m ail:
high and there exists a fourfold symmetry, or sixfold symmetry, and, accordingly, a stable orientation of a molecule can also be stable if it is rotated by 908, or 608, depending on the symmetry of the surface. As a result, adsorbates tend to form a patch of single crystal domains, which are related by a rotation, of 60 degrees, for example w7x. A substrate with a lower symmetry is desirable if a macroscopic orientation along the surface is to be realized, which is important for the generation of optical secondharmonics, for instance. The surface of an organic solid can also serve as a template upon which a compound can be adsorbed with a preferred orientation w8x. Liquid crystals can be aligned on polyimide substrate which has been treated by ‘‘rubbing’’. The molecular mechanism of the rubbing is, however, not well understood w9x. Enzymes, which combine only molecules of a particular shape with a so-called key-andthe-keyhole specificity, may be considered another example. Most organic crystals, however, have high vapour pressure and their surfaces can be etched when a crystal is kept in a vacuum for some time and hence such crystals have been regarded as not suitable for a study in an ultrahigh vacuum. Here, we propose that a glycine crystal can be a good substrate for orienting organic molecules. In spite of its small molecular weight, glycine has a high melting point. Actually, it decomposes when it is heated to its melting point of 510 K. This anomalously high thermal stability is due to its zwitterionic lattice. The glycine molecules are
0379-6779r00r$ - see front matter q 2000 Elsevier Science S.A. All rights reserved. PII: S 0 3 7 9 - 6 7 7 9 Ž 9 9 . 0 0 1 9 6 - 4
44
T. Ehara et al.r Synthetic Metals 109 (2000) 43–46
present as H 3 Nq–CH 2 –COOy and the large lattice energy comes essentially from ionic bonding w10x. Because of the tight bonding the vapour pressure of this compound is sufficiently low and it was possible to bring this material into an UHV chamber without deteriorating the vacuum. We have studied orientational order that can be realized when polar molecules such as 4-dimethyamino-4X-nitrostilbene ŽDMANS. is evaporated onto a cleaved face of a glycine single crystal. Orientational order can be verified by polarized absorption. The same compound exhibits also a high degree of orientational order when a film of the compound, evaporated on a glass substrate, is mechanically brushed in a direction. The mechanism of the molecular alignment is discussed based on the results of polarized optical absorption, optical second-harmonic generation, X-ray diffraction and scanning force microscopy.
2. Experimental Glycine single crystals, with a typical size of 1 = 1 = 2 cm3, were grown from aqueous solution by slow evapora-
tion of the solvent. The temperature during the crystal growth was controlled within 0.1 K. The crystal was cleaved and the orientation of the crystal axes was examined by X-ray diffraction. DMANS was purchased from Tokyo Chemical and was used after recrystallization from carbon tetrachloride solution. Evaporated films were prepared with a conventional vacuum evaporation system, or with an ultrahigh vacuum system. The pressure during the evaporation was 10y6 Torr in the former case and 10y9 Torr in the latter case. In brushing experiments an evaporated film was brushed a few times in one direction with a cloth or laboratory tissue paper. UV–visible absorption spectrum was measured with an arrangement constructed in the laboratory w11x. It was a single beam spectrophotometer capable of optical density measurements with a high precision. The high stability in the measurement of the optical density was realized by a combination of luminous optics, a mechanically stable construction and an extensive averaging. In favorable cases, an optical density could be determined to the precision of 10y5 , which allowed us to measure the absorption spectrum of an organic film as thin as 1r30 of a monolayer.
Fig. 1. Molecular arrangement in a cleaved Ž010. face of a glycine single crystal. A two-dimensional network of hydrogen bonds extends through the plane. Unit cell dimensions are: a s 510 pm, c s 545 pm, b s 111.68. The structure appears polar, but the layer underneath has an opposite polarity, so that the crystal as a whole is centrosymmetric. The special feature of a glycine crystal is that it is composed essentially of a stack of double layers, two neighboring layers being strongly bonded by hydrogen bonds, while there is only a weak van der Waals type interaction between the pair of double layers. As a result, the surface exhibits always a definite polarity.
T. Ehara et al.r Synthetic Metals 109 (2000) 43–46
45
Fig. 4. Polarized absorption of a thin film of DMANS on glass substrate. Ža. As deposited. Žb. After brushing, observed with light polarized parallel to the brushing direction. Žc. After brushing, observed with light polarized normal to the brushing direction. Absorption becomes intense, and strongly polarized, by unidirectional brushing. Fig. 2. Polarized absorption of a thin film of DMANS, evaporated onto a cleaved Ž010. face of a glycine single crystal. The temperature of the substrate was 263 K during the evaporation. A large contrast in polarized absorption indicates that DMANS molecules are well oriented parallel to the c-axis of the glycine crystal.
Second-harmonic generation was measured with transmission, either with a Q-switched Nd:YAG laser as the light source. The second-harmonic signal at 532 nm was detected with a photomultiplier and the shot-to-shot fluctuation was averaged with a personal computer w12x. X-ray diffraction was measured with a commercial diffractometer ŽRigaku. with a molybdenum target. Atomic force microscopy images were taken with a commercial AFM unit ŽNanoscope III, Digital Instruments., operated with tapping mode. 3. Results and discussion 3.1. Glycine single crystal as an orienting substrate
bonds. The crystal structure is constructed by stacking this two-dimensional network. The crystal has a symmetry of P2 1rn, and the unit cell dimensions are: a s 510 pm, c s 545 pm, b s 111.68 and Z s 4. The structure shown in Fig. 1 appears polar, but the layer underneath has an opposite polarity, so that the crystal as a whole is centrosymmetric. The special feature of a glycine crystal is that it is composed essentially of a stack of double layers. There are fairly strong hydrogen bonds within the double layer, while only a weak van der Waals type interaction operates between the double layers. As a result, the surface layers can be removed only in pairs and the surface exhibits always polarity of either sign. Fig. 2 shows the polarized UV–VIS absorption spectrum of a film of DMANS evaporated onto a cleaved Ž010. face of a glycine single crystal. Glycine is transparent in this spectral region and hence the absorption is due to adsorbed DMANS. The absorption band in the visible is due to an intramolecular CT transition and its transition
Fig. 1 shows the arrangement of the molecules in Ž010. surface, expected from the bulk structure w10x. The molecules form a tight network of intermolecular hydrogen
Fig. 3. X-ray diffraction pattern of an evaporated film of DMANS on a glycine crystal, measured with u –2 u scan, which explores the scattering vector normal to the surface. Note that only Ž h00. reflections have been observed. The intense peak at 2 u s11.58 corresponds to a spacing of 767 pm, which is close to the length of a DMANS molecule, suggesting that the molecules are standing against the substrate surface. The diffraction lines disappear when the film is brushed.
Fig. 5. Optical second-harmonic generation measured with 1.06 mm pulse from a Nd:YAG laser as the incident light, observed at 532 nm. The incident light hits the sample at normal incidence and the second-harmonic output has been measured in transmission, with the polarization parallel to that of the incident light, while the sample is rotated around the surface normal. The angles indicate the angle between the electric field of the incident light and the direction of the brushing. The fact that a secondharmonic signal can be observed with normal incidence indicates that the film has a C 1 symmetry around the normal of the surface.
46
T. Ehara et al.r Synthetic Metals 109 (2000) 43–46
moment is parallel to the molecular long axis. Fig. 2 shows clearly that DMANS molecules are aligned with their long axes parallel to c-axis of the glycine crystal. A high degree of orientational order is obvious. Only a preliminary analysis has been made of the crystal structure of DMANS, but there is a close matching with the crystal lattice of glycine. The crystal symmetry is P2 1rn. The lattice parameters of a DMANS crystal is: a s 6822 pm, b s 754 pm, c s 600 pm, b s 1168 and Z s 12. If the structure in the ac plane is superposed on that of Ž010. face of glycine, both structures closely match. A DMANS molecule occupies the same area as three glycine molecules. If the positive –NŽCH 3 . 2 group of a DMANS molecule lies on top of the negative -COy 2 group of a glycine molecule, the negative –NO 2 group of DMANS molecule falls on the positively charged –NHq 3 group of the third glycine molecule. The matching is, however, not perfect. These results suggest that DMANS molecules are lying with their molecular long axes parallel to the c axis in the cleaved face of a glycine crystal, while the arrangement of DMANS molecules may differ somewhat, compared to that in the crystal. 3.2. Mechanical brushing When the substrate is a glass plate, on the other hand, the film appears pale yellow. The very weak absorption is due to the fact that the molecules stand with their molecular long axes almost normal to the surface. X-ray diffraction, with u –2 u scan, which explores the scattering vector normal to the surface, shows exclusively Ž h00. reflections ŽFig. 3.. This indicates that the film is crystalline and that the molecules are essentially standing against the glass substrate. Polarization analysis with the second-harmonic generation indicates that the film has an axial symmetry. When the film is mechanically brushed in one direction, the film turns dark brown. The optical absorption not only becomes strong, but also becomes strongly polarized along the direction, indicating that the molecules are now aligned along the brushing direction ŽFig. 4.. A strong SHG can be observed at normal incidence, which indicates that molecules are oriented along the direction of the brushing ŽFig. 5.. What makes this extremely efficient alignment possible? Since a mechanical brushing provides only an occasional point contact w13x and since we do not touch every molecule by brushing, a nearly 100% alignment is not self-evident and a mechanism must be sought which explains the ‘‘amplification’’ of the alignment of the molecules in the brushed region. In the case of polymers or hygroscopic amorphous films, a shear flow-induced alignment could be the mechanism w9x, However, this cannot
Fig. 6. An image, taken with scanning force microscopy, of a scratched area of an evaportated film of DMANS on a glass plate. The area is 5 mm=10 mm. The lower part shows the debris made by the scratching. In the upper part, which has been brushed, small crystallites, of a typical size of 100 nm=200 nm, are seen to be aligned along the direction of the brushing.
apply to a crystalline film, such as of DMANS. An image of a brushed region taken with a scanning force microscope shows that small crystals, typically 200 nm long and 100 nm wide, are aligned along the brushing direction ŽFig. 6.. This could account for an efficient alignment of a large number of molecules by a macroscopic method such as mechanical brushing.
Acknowledgements This work has been made in collaboration wit Y. Shimizu, M. Kojima, and M. Fujii. The authors wish to thank Toyo Corporation for technical support in operating the scanning force microscopy.
References w1x T.J. Schuerlein, A. Schmidt, P.A. Lee, K.W. Nebesny, N.R. Armstrong, Jpn. J. Appl. Phys. 34 Ž1995. 3837. w2x S.R. Forrest, Chem. Rev. 97 Ž1997. 1793. w3x E. Umbach, K. Gloeckler, M. Sokolowski, Surf. Sci. 402 Ž1998. 20. w4x N. Karl, Ch. Guenther, Cryst. Res. Technol. 34 Ž1999. 243. w5x T. Kobayashi, S. Isoda, J. Mater. Chem. 3 Ž1993. 1. w6x J.-Y. Grand, T. Kunstmann, D. Hoffmann, A. Haas, M. Dietsche, J. Seifritz, R. Moeller, Surf. Sci. 366 Ž1996. 403. w7x A. Soukopp, K. Gloekler, P. Kraft, S. Schmitt, M. Sokolowski, E. Haedicke, Phys. Rev. B 58 Ž1998. 13882. w8x H. Kobayashi, T. Ehara, M. Kotani, J. Chem. Soc. Faraday Trans. 90 Ž1994. 3685. w9x J.M. Geary, J.W. Goodby, A.R. Kmetz, J.S. Patel, J. Appl. Phys. 62 Ž1987. 4100. w10x G. Albrecht, R.B. Corey, J. Am. Chem. Soc. 61 Ž1939. 1087. w11x K. Yamasaki, O. Okada, K. Inami, K. Oka, M. Kotani, H. Yamada, J. Phys. Chem. 101 Ž1997. 13. w12x Y. Shimizu, M. Kotani, Opt. Commun. 74 Ž1989. 190. w13x J. Lowell, A.C. Rose-Innes, Adv. Phys. 29 Ž1980. 947.