Nano-scale Control and Detection of Electric Dipoles in Organic Molecules
K. Matsushige*, H. Yamada, H. Tanaka, T. Horiuchi, and X. Q. Chen
Graduate School of Engineering, Kyoto University, Kyoto, 606-01 Japan
This is a draft paper for a talk at the Fifth
Foresight Conference on Molecular Nanotechnology. The final version has been submitted for publication in the special Conference issue of Nanotechnology.
This paper is available on the web at http://www.foresight.org/Conferences/MNT05/Papers/Matusige/index.html
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The nanoscopic ferroelectric domains could be formed in P(VDF/TrFE) thin films by applying electric pulses with a conductive atomic force microscope (AFM), and detected by using piezoelectric response, revealing that the directions of electric dipoles in organic molecules can be controlled in nano-scale. By changing the polarity of the applied pulses, temporally stable binary information could be "written" in this films. Moreover, the possibilities of the molecular manipulation and the creation of high-density molecular memory devices utilizing such the electric interaction between the polar molecules and the scanning probe microscopy (SPM) tips are discussed.
In recent information technology, it is desired to store a large capacity of digital data in a small size media. Although there exist various types of memory devices such as CD (compact disc) and MO (magneto-optical) disc and so on, the drastic increase in their capacity can not be expected because of the fundamental limitations as far as the laser lights with the wavelength of several hundreds nm are used. Even the memory devices utilized Si-based semiconductor technique, the similar size limitations exist due to the lithography method, which use lights, too. Therefore, one must seek and employ quite different materials and novel write/read methods.
Molecules are recognized to contain themselves lots of information in the spatial conformation and electrical characteristics. Among them, an electric dipole may be one of the possible information units, if the direction of the dipoles can be controlled in a nanometer scale. Although ferroelectric organic thin films have gained much attraction over the last several years because of its technological applications such as piezo- and pyro-electric devices, the ferroelectric nature has net been utilized to form nanometer scale memories due to the lack of the manipulation method for such the dipole orientation.
While, SPM has been demonstrated as a powerful tool not only for imaging surface structures but for material modifications on nanometer scale including atomic and molecular manipulations [1-2]. By using inorganic ferroelectric thin films, it was reported that nanometer-scale polarization domains can be created by a voltage applied between a metal coated AFM tip and a conductive substrate of the film [3-5] . One of the important parameters which determine the size of the domain is the sample thickness because the electric interaction area of the film is increased as the thickness becomes large, and the ferroelectric properties in such inorganic thin films often disappear due to some defects of the structures / composition at the interface when the film thickness is on the order of 10 nanometer or less. In contrast, ferroelectric organic films are expected to keep the ferroelectricity even in ultra thin films owing to its weak interaction such as van der Waals force with the substrate. Guthner et. al first demonstrated the formation and the visualization of the micrometer-scale domains in ferroelectric polymers films by SPM . Also, the present authors successfully observed the D-E hysteresis loop in the nano-scale area .
In this study, we used Au-coated AFM conductive tips and a lock-in amplifier measuring system to investigate local properties of organic ferroelectric films and to demonstrate formation of local polarization domains as small as 30 nm. The domains exhibited the polarization reversal as well as the stable piezo response. Finally, the possibility for the creation of the extremely-high density molecular memory and the novel molecular nano-devices using the ferroelectric molecules and SPM technique is discussed.
The sample used in this experiments is copolymer of vinylidene fluoride and trifluoroethylene with a molar ratio of 73/27. As shown in figure1, this polymer has electric dipoles perpendicular to the molecular axis originated from the large difference in the electron affinity of H and F atoms. The copolymer samples were deposited on Pt substrate by a spin-coating method. The Pt substrate was sputtered on silicon dioxide. The sample was thermally annealed at 130 °C for 1 hour after the deposition to increase its crystallinity.
The polarization alignment and switching were conducted by applying electric pulses to the film sample by using an Au-coated conductive AFM tip, as shown in figure 2. The write and erase procedure was done by changing the polarity of the applied electric pulses. While, the ferroelectric domains was detected by utilizing piezo response as schematically shown in figure 3. When we locate the tip on a polarization domain with applying the oscillating weak electric field, piezoelectric vibration is caused and the tip in contact with the film is consequently vibrated. The vibration is detected using the optical lever method and then the signal is demodulated by lock-in 9/30/97amplifier so that both in-phase and quadrature phase signals, i.e., both amplitude and phase information can be obtained. In this setup the oscillation frequency used must be located between the bandwidth of the feedback loop and the resonant frequency of the cantilever. When the tip is scanned, it follows the topography by feedback electronics but does not follow the oscillation. Then both topographic image and piezoelectric response can be simultaneously taken. From the detected phase information, we can determine the polarity of the domains, too.
The Au coated tips and cantilevers had a spring constant of 0.75 N/m and a resonant frequency of 88 kHz, and the modulation signal applied to the tip had a frequency of 70 kHz and a voltage of 0.5 Vrms.
Results and Discussion
Figure 4 shows topographic and piezoelectric response images of the ferroelectric copolymer film deposited on Pt substrate, observed before and after application of the electric pulse of -6 V with a duration of 1 second. As shown clearly in these images, the bright spot with high piezoelectric activity was created at the portion where the electric pulse was imposed, although the corresponding topographic image taken simultaneously does not show any change in its feature. Thus, this fact suggests that the application of pulses can actually form locally the ferroelectric domains as small as about 30 nm in diameter. Here, it should be mentioned that the size of the domains are governed mainly by the degree of the concentration of electric field around the AFM tip, and so much smaller domains can be expected to be formed if the AFM tips with more sharper points are employed.
Next, the temporal stability of the polarization domains formed by the application of electric pulses was examined. Figure 5 show a series of such piezoelectric response images observed at a fixed region. As the time passed after the domain formation, the piezoelectric response became gradually lower and attained at the level of about 80 % of the value at the initial stage after 10 hours. The stability may depend on various factors such as surrounding atmosphere, temperature, molecular mobility of the ferroelectric polymers used, how frequently the piezoelectric measurements which give mechanical and electrical vibrations are done on the sample, and so on. Thus, although much detailed investigations are necessary, it may be said that the ferroelectric local domains can exist keeping their piezoelectric activities for sufficiently long period time.
As revealed above, it became clear that the ferroelectric domain with the sizes of several tens nm in diameter can be formed locally at the desired points. In addition, when the polarity of the applied electric pulses is reversed, the piezoelectric responses are also reversed, forming the ferroelectric domains with an opposite polarization direction. By utilizing these evidences and imposing electric pulses with positive and negative polarities during the scanning of the AFM tip, one can write the binary information in series, as depicted in figure 6.
Figure 7 shows the piezoelectric response image of the sample, for which the electric pulses were applied repeatedly at approximately 100 nm separation. The lower figure shows the line profiles across each formed domains, revealing the polarity difference in the piezoelectric responses. These evidences suggest that the novel molecular memory devices with nm scale information units and so extremely high storage density can be created based on the write/read method described here.
Finally, we discuss briefly the future possibility on the nano-scale polar devices utilizing the combination of the organic ferroelectric molecules and SPM technique. In the organic ferroelectric materials as this case of (VDF/TrFE) copolymer the electric dipoles exist with the molecular structures and the ferroelectric domain can be formed by controlling the molecular orientation. If we can develop the conductive AFM tips with much sharper heads and employ the molecular units which are composed of smaller crystalline units of oligomers, and distributed separately within thin films or individually on substrates, we will be able to change the molecular direction and rotate the molecular units in nano-scale level by using the electric force, and/or to control the molecular orientation to create some nanoscopic piezo-, pyro-, and ferroelectric devices, as schematically shown in figure 8 (a) and (b). Furthermore, as shown in figure 8 (c), an attractive and repulsive forces acting between the molecular dipole units and AFM tips can be utilized to transport the each molecular units to any desired positions, as seen in usual macroscopic world for magnets. This kinds of nanoscopic operation using the electric interaction will open new field of science, technology, novel devices, and molecular electronics.
It was revealed that nanoscopic ferroelectric domains can be formed in (VDF/TrFE) copolymer thin films by applying electric pulses with conductive AFM tips, and detected by using piezoelectric response. The formed ferroelectric domains were found to be rather stable and the binary information can be "written" by changing the polarity of the applied electric pulses. Moreover, the possibilities of the molecular manipulation and the creation of high-density molecular memory devices utilizing the electric interaction between the polar molecules and the SPM tips are discussed.
This work was partly supported from a Grant-in-Aid from the Ministry of Education, Science, Sports and Culture, and KU-VBL (Kyoto University-Venture Business Laboratory) project.
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- Figure 1. Molecular structure of (VDF/TrFE) copolymer and electric dipoles.
- Figure 2. Formation of locally polarized domains and switching by application of electric pulses to ferroelectric films.
- Figure 3. Detection of ferroelectric domains by using piezoelectric vibration and a lock-in amplifier.
- Figure 4. Piezoelectric response and topographic images of the ferroelectric copolymer film observed before and after the application of electric pulse.
- Figure 5. Temporal changes in the piezoelectric response images and amplitude.
- Figure 6. Binary information inputs by alternating the polarity of the applied electric pulses.
- Figure 7. Piezoelectric response image and line profiles across five points where electric pulses with positive or negative polarity were applied.
- Figure 8. Molecular axis rotation (a), controlled orientation(b), and transportation operations by utilizing electric interaction between the polar molecular units and SPM tips.
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