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This paper is available on the web at http://www.foresight.org/Conferences/MNT05/Papers/Nicolini4/index.html
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For example, to investigate the electrical properties of dielectric organic materials, structures composed by a bottom metal electrode, an intermediate film, and a top metal electrode are usually created. Often, the organic layer was formed by Langmuir-Blodgett (LB) technique [4, 5], but a poor deposition of LB films on metal base electrodes was observed yielding the authors to propose more complicated structures [6,7]. The use of aluminium to form both contact elctrodes was proposed by many authors but recent works demonstrated that, in reality, the characterizations may be affected by the oxide layers rather than by the film itself and by short-circuits between the upper and the lower metal contacts, usually created when the upper metal is deposited [8].
In this work, we formed Metal/Insulator/Metal (M/I/M) sandwich structures, transferring monolayers of barium behenate onto the pre-treated surface of a glass or sapphire substrate, using evaporated aluminium for both contacts. We believe that the problem of pinhole influence by the use of a procedure consisting in a substrate pre-treatment, proposed by ref. 9 for other purposes have been solved by us. A high dose electron beam irradation of a first deposited monolayer before the further film deposition always results in the cross-linking of the molecules; thus a very strong homogeneous monolayer has been obtained. From one side, it makes more difficult the penetration of aluminium up to the bottom electrode under evaporation, from the other side, a smooth and highly hydrophobic surface is formed. We performed an AC characterization of the above mentioned structures, optimizing the junction and contact fabrication. Investigating the DC conduction properties of the samples, we propose also a transport mechanism inside the organic dielectric, dependent upon the magnitude of the applied bias and upon the number of monolayers, corresponding to the literature data which were obtained with more complicated solutions.
Among the organic compounds that behaves like organic metals, we investigated charge-transfer salts and conducting polymers. Surfactant compounds containing the bis(ethylene-dithio)tetrathiafulvalene (BEDT-TTF) appear to be very promising for production of highly conducting LB films [10,11]. However, the procedures of functional element preparation are usually complicated and the obtained results, in our experience, are poorly reproducible. Thus, for a real application, the technology must be simplified as much as possible and the reproducibility has to be the first feature. For this reason, we investigated the realization of layered nanoarchitectures via directed assembly of anionic and cationic molecules, as reviewed by ref. 12. In order to highlight the attractive features of this technique (no dedicated and sensitive equipment is needed and the adsorption is carried out from aqueous solutions), we investigated the electrical properties of polypyrrole deposited on glass slides and interdigitated electrodes. Polypyrrole is one of the most important electronic materials among the heterocyclic conducting polymers [13] and one of the most extensively studied conductive polymers. Polypyrrole films synthetized in para-toluene sulpnonate (PTS) electrolyte have been found to have excellent electrical conductivity [14].
When polypyrrole films are in conducting forms, the anions which are affiliated with the cationically charged polymer chains, have been found to be 10-35% by weight. The anions such as p-toluene sulphonate are poorly nucleophylic and permit the formation of good quality films. The level of oxidation of pyrrole is 0.25-0.3 per pyrrole unit, corresponding to one anion for every 3-4 units [15,16]. The recent past has shown the preparation of in situ-self assembling layer by layer film of polypyrrole [12]. So, experiments have been carried out with this polyanion by varying the deposition techniques and the deposition parameters. Further, the time-dependent electrical properties of various doped polypyrrole films have been investigated and the morphology of the samples have been checked with STM and AFM techniques.
After the deposition of all the organic layers, a 1000 Å-thick top aluminium electrode was then evaporated, with a careful attention to avoid heating damages of the samples; in particular, the contact was formed in several steps, evaporating small quantities of metal at each step. The entire procedure is depicted in Figure 1. The final configuration of the M/I/M structure is shown in Figure 2; the pad surfaces utilized for measurements are of the order of 0.002 cm2.
Spreading solvent benzene Concentration of compound 0.33 mg ml-1 (behenic acid) Surface pressure 33 mN m-1 Speed of dipping 5 mm min-1 Rate of compression 100 mm2 min-1 Subphase 10-4 (barium acetate in deionized water) Temperature 20 °C (approximately) pH 7
Figure 1. Schematic representation of the M/I/M structure described in the text.
Figure 2. Drawing of a typical sample (not in scale) incorporating the M/I/M structure of Figure 1. Several electrodes are first evaporated on a substrate, i.e. glass or sapphire; the dielectric film is then deposited with the metodology described in the paper; a single top electrode is finally evaporated onto the film.
The electrochemical deposition of polypyrrole was carried in a cell consisting of three electrodes, where glass indium-tin-oxide plate served as working electrode (sheet resistivity 20 Ohmcm), platinum as counter and AgAgCl as reference electrode. The anodic deposition of polypyrrole was carried out from 0.1 M pyrrole monomer in an aqueous solution of 0.1 M sodium salt of paratoluene sulphonic acid. The polypyrrole films was obtained at a current density of 0.1 mAcm-2. Various thicknesses of the polypyrrole films were obtained by varying the time deposition.
In the layer-by-layer adsorption, to covalently anchor charges onto the surface, the cleaned slides were immersed for 10 minutes each into the following solvents: methanol (HPLC grade), 1:1 methanol toluene, and toluene (analytical grade). Then they were exposed to 5% vol. solution of (N-2-aminiethyl-3-aminopropyl)trimethoxysilane (TMS) in toluene for 12 hours. After silanization, the slides were dipped in boiling toluene for 1 h. The substrates were then dipped for 10 minutes in toluene, 1:1 methanol/toluene and methanol and then thoroughly washed with deionized water. This procedure produces a surface with covalently anchored amine group (positively charged surface).
In addition, the sample was treated with sulphonated polystyrene (PSS) in order to obtain a negatively charged surface, suitable for the formation of conducting salts of polypyrrole (in situ, by simultaneous polymerization of the monomer and oxidation of the polymer). The supporting solution was 10-3 M (PSS 90% sodium styrene 4-sulfonate, Mw = 70,000) in deionized water, pH = 1. The active solution of polypyrrole was made by dissolving an oxidizing agent, ferric chloride, in deionized water, with pH = 1 by addition of HCl. The paratoluene sulphonic acid (PTS) was dissolved in the above solution followed by the addition of pyrrole monomer. The utilized concentrations for the fabrication of the polypyrrole films were 0.006 M FeCl3, 0.026 M PTS and 0.02 M pyrrole. After the addition of pyrrole monomer the solution was stirred for 15 minutes and filtered. The protonated substrates were dipped for 10 minutes in PSS solution (to obtain one layer) and rinsed several times before dipping in the pyrrole solution. A schematic drawing of the procedure is sketched in Figure 3.
Figure 3. Schematic drawing (not in scale) of a typical polymer-based conducting structure.A single layer of polypyrrole, obtained in five minutes, depends on the surface chemistry of the substrates. The alternating dipping sequence was repeated to buid alternate structures. Alternatively, the substrate was also immersed in polypyrrole dipping solution for several intervals, rinsed and dried.
Figure 4. Relationship between frequency variation and time deposition of polypyrrole samples deposited onto 10 MHz quartzes.
Figure 5. Relationship between frequency variation and deposition steps of PSS/polypyrrole samples deposited on 10 MHz quartzes.In addition, the conducting samples surfaces were investigated by a home-built Atomic Force Microscope working in air and by a Scanning Tunneling Microscope (AsseZ-MDT, Italy). Figure 6 shows examples of this type of investigation: the left images refer to STM analysis while the right ones refer to AFM characterization. In part (a) the morphology of a typical sample obtained by layer-by-layer technique has been reported: the surface is organized in spherical objects (grains) of about 300 Å in diameter. The image sizes are 0.5 x 0.5 µm.
In part (b) the organization of a sample obtained by electrochemical deposition has been shown: in this case the surface presents a cluster morphology. The aggregates present fine structures inside, like to those obtained in the above pictures. The STM image has a size of 0.5 x 0.5 µm, while the AFM image has a size of 2.2 x 2.2 µm.
| A |  |
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| B |  |
 |
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Figure 6. Morphological investigation by Scanning Probe Microscopes to characterize the surfacial morphology obtained by layer by layer and electrochemical techniques. The left images refer to STM analysis while the right ones refer to AFM characterization.
In part (a) the morphology of a typical sample obtained by layer-by-layer technique has been reported: the surface is organized in spherical objects (grains) of about 300 Å in diameter. The image sizes are 0.5 x 0.5 µm.
In part (b) the organization of a sample obtained by electrochemical deposition has been shown: in this case the surface presents a cluster morphology. The aggregates present fine structures inside, like to those obtained in the above pictures. The STM image has a size of 0.5 x 0.5 µm, while the AFM image has a size of 2.2 x 2.2 µm.
The electrodes were contacted by tungsten tip probes connected to a micromanipulator control (PH 100, Karl Suss, Germany). The probes were connected to the measuring instrumentation by short shielded cables.
For the M/I/M structure, AC measurements were performed with a precision LCR meter (HP 4284A, Hewlett Packard, USA). The study was directed to understand the behaviour of these structures varying the number of bilayers, the applied bias potential and the frequency of the applied AC signal (the amplitude of the AC signal applied across the structures was usually equal to 150 mV).
The DC experiments were performed using a Keithley electrometer (model 6517). Normally, fatty acids salts behave like good insulators, with dielectric strenghts greater than 106 V cm-1. However, like in real dielectric materials, according to the temperature and bias potential, a transport phenomenon can be obtained.
Both the instruments were driven by a PC through a GPIB interface, with a home-made software (this solution allowed a fast data acquisition and an easy programming of the instruments).
For the conducting structures, the current/voltage characteristics were obtained by biasing the structure with a 16-bit resolution I/O board (National Instruments, mod. PCI MIO 16XE10) and measured by the Keithley electrometer.
Data from literature confirm this dependence on frequency, which could depend either on permanent or induced dipoles or on hopping electrons in the dielectric material.
Conductance measurements can be modeled by the following expression:
In Figure 7 this type of behaviour is shown. We obtained also a linear dependence of 1/C versus the number of layers demostrating the reproducibility of a monolayer capacitance and hence for one monolayer to the next. For the interfacial layer a capacitance of the order of 800 pF was obtained; if we assume that this layer has an average dielectric constant = 4.5 (native oxide), its thickness is of the order of few nanometers (as reported in refs. 6).
Figure 7. AC behaviour of the realized M/I/M structures. A detailed comparison of this dielectric phenomena characterization with similar ones published by other groups shows a good agreement; differences within the data are of the order of 5% (that is, of the same order as the accuracy in producing the electrodes and measuring their areas).In addition, to identify the conduction processes through the LB layers, the shape of their I/V characteristics was compared with that expected from theoretical considerations of the conduction processes through inorganic insulating films [17].
It was found that the I/V characteristics obey the I exp (Vn) relationship, with n depending on the insulating layer thickness (N): in Figure 8, plots of ln (I) versus Vn are shown. Figure 8(a) refers to a bilayer (n 0.5) while Figure 8 (b) refers to 8 monolayers of barium behenate (high applied field, n 1). Two transport phenomena can explain our experimental observations: if n 0.5, the conduction process arises mainly from the injection of carriers from the electrodes over the potential barrier formed at the metal-insulator interface (Schottky mechanism); if the film thickness increases, the conduction due to the Schottky mechanism can be negligible and charges are excited out of traps in the insulating film. The DC conduction in thicker multilayers structures is essentially determined by defects in the LB layers and obey to the Poole's law, based on the classical calculation of ionic conductivity [18]
Figure 8. Dependence of ln (I) versus Vn of samples consisting of 2 (part A) and 8 (part B) monolayers of barium behenate. Different transport mechanisms for the two samples and in general for samples of different thickness drive the carriers in the insulating layers, as proved in the text.
| A |  |
| B |  |
Figure 9. Part (A) shows the dependence of I versus V for a polypyrrole sample deposited via electrochemical technique. The sample shows a non-ohmic behavior and unstability in time. Part (B) shows the dependence of I versus V of a polypyrrole sample deposited via layer by layer technique. The ohmic behavior and the stability are mantained during a month of testing.The I/V characteristics versus number of layers and the effect of dopants on the electrical conductivity on these films are actually under study; preliminary results show a gradual increment in the current magnitude according to the increment in the number of layers, probably linked to an increase in the charge defects in the conducting films. In addition, the doping procedure can introduce some unstability in I/V characteristic, and the magnitude of the current seems to reach a saturation level.
A new series of experiments are currently being performed to incorporate monolayers of biological molecules such as redox enzymes in order to investigate the electrical properties of hybrid structures and devices.
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(New York: plenum Rosetta Edition) 387
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