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Transport Phenomena Investigation
in Nanomaterials
Towards Molecular Devices

by
M. Adami, P. Faraci, M.K. Ram, and C. Nicolini

Polo Nazionale Bioelettronica, Marciana (LI), Italy;
Fondazione Elba, Rome, Italy;
Institute of Biophysics, University of Genova, Italy.

Corresponding Address:
Claudio Nicolini, Fondazione EL.B.A.,
Via Medaglie d'Oro 305 - 00136 Rome, Italy
ph: +39 6 35410728, fax: +39 6 35451637,
email: info@fondazione-elba.org

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.

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Abstract

A detailed study of several organic-based structures is reported in this paper. Organic materials with different electrical properties have been characterized, such as fatty acids salts used as dielectric materials, charge transfer salts and conductive polymers used as organic metals. Different technologies have been utilized in obtaining the samples suitable for investigations, as Langmuir-Blodgett deposition technique, electrochemical deposition and layer-by layer adsorption (or polyelectrolyte self assembling).

Aluminium/ Langmuir-Blodgett film of fatty acids salts/Aluminium sandwich structures have been constructed; the sample preparation procedure has been optimized to minimize problems related to short circuits between upper and lower electrodes by means of chemical and physical treatments of the substrate surfaces. The results show that the measured characteristics are related only to the properties of the films and wich are in good agreement to the theoretical predictions. Starting from these results, the behaviour of more attractive materials have been investigated: conducting polymers (such as polypyrroles and polyanilines) and metalloproteins (such as redox enzymes and cytochrome P450) have been studied in order to highlight their capabilities to develop hybrid structures and devices. Charge transfer salts and polypyrrole layers deposited on interdigitate electrodes have been characterized in order to highlight their electrical properties. The multilayer formation was controlled by Quartz Crystal Microbalance technique and the resulting morphology has been investigated by Scanning Tunneling and Atomic Force Microscopy.

1. Introduction

In the last years, many efforts were performed to investigate the electronic properties of organic materials and their possible applications [1, 2]. Bioelectronics and molecular electronics are two different aspects of this attempt, together with the technologies suitable to organize the biomaterials, preserving their activity and functionality [3].

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.

2. Experimental Procedures

2.1 Preparation of dielectric structures

The MIM structures were formed on glass or sapphire slides. Before metallization, substrates were cleaned and heated at 80 ╔C with sulphuric acid and then rinsed with deionized water. A metal pattern consisting of a 1000 ┼-thick aluminium strips was then deposited by shadow evaporation under vacuum (10-5 torr), with substrates heated at approximately 200 ░C to improve adhesion. After evaporation, samples were kept at atmospheric pressure for about 12 hours to obtain a stable aluminium oxide layer. The barium behenate layers were deposited with a standard LB system and all deposition parameters are reported in Table 1. At each dipping step a bilayer was added to the structure, forming samples with different number of monolayers (from 2 to 40). Transfer ratios were always equal to 1.0 ▒ 0.1. The first monolayer was irradiated by an electron beam at 1kV, with a dose of about 10-3 Ccm-2, causing a cross-linking between the molecules, i.e. forming an uniform, mechanically strong, hydrophobic covering. According to ref. 14, the thickness of barium behenate monolayer decreases in a range between 5 ┼ and 10 ┼.

Table 1. LB Deposition parameters

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 

After the deposition of all the organic layers, a 1000 ┼-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.

Figura 1

[Full size Figure 1: 13K, 684 x 800 pixels]

Figure 1. Schematic representation of the M/I/M structure described in the text.

 

Figura 2

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.

2.2 Preparation of conducting structures

The electrochemical deposition and in situ self-assembling layer by layer absorption techniques were used for obtaining the conducting samples. First of all, the substrates were treated with the following procedure to create charged surfaces. They were cleaned with a 7:3 concentrated sulfuric acid/hydrogen peroxide solution and a 1:1:5 ammonium hydroxide /hydrogen peroxide/ water solution for 1 h; in this way, a hydrophilic surface was obtained.

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.

2.3 Characterization of conducting structures

The samples were tested with an home-built nanogravimetric apparatus, in order to check the relationship between deposition time and an increment of the multilayer thickness. A different number of layers was transferred onto 10 MHz AT-cut quartz crystal microbalances with gold electrodes (Nuova Mistral, Italy) and the decreasing frequency of the polypyrrole multilayers and of the alternating PSS/polypyrrole structures were analyzed. In Figure 4 a typical result is presented, showing a linear dependence of the frequency shift versus the deposition time, as expected. Figure 5 represents the same result in the case of alternating deposition of PSS/polypyrrole layers (a deposition step consists of two parts, the deposition of PSS during 10 min. and the deposition of polypyrrole during 5 min.). Also in this case, a linear relationship has been obtained.

Figure 4. Relationship between frequency variation and time deposition of polypyrrole samples deposited onto 10 MHz quartzes.

 

Figura 5

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 ┼ in diameter. The image sizes are 0.5 x 0.5 Ám.

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 Ám, while the AFM image has a size of 2.2 x 2.2 Ám.

A Figure 6 a
B Figure 6 b ____________ Figure 6 b

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 ┼ in diameter. The image sizes are 0.5 x 0.5 Ám.
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 Ám, while the AFM image has a size of 2.2 x 2.2 Ám.

2.4 Electrical measurements

For the electrical characterization, glass substrates and substrates with 50 pairs of chromium interdigitates electrodes have been used (each pair is spaced to 50 Ám and each track is 50 Ám in width, 5 mm in length and 40 nm in height).

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.

3. Results and discussion

3.1 Electrical behaviour of M/I/M structures

According to our AC measurements, between 20 Hz and 10 KHz, the capacitance of the Al/LB fatty acid salts/Al is almost independent on frequency, while for higher values it decreases, according to the following expression:

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

[Full size Figure 7: 19K, 664 x 752 pixels]

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 a,b

[Full size Figure 8: 6K, 664 x 676 pixels]

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.

3.2 Electrical conductivity on polypyrrole samples

We performed first the electrical measurements of doped polypyrrole films on glass substrates, simply by contacting two different points of the sample. Then, in order to not introduce some problems related to the contacts, we deposited the same samples on interdigited electrodes with the perspective that the interdigited chromium electrode metals do not diffuse in polypyrrole films and the interdigited electrodes have fixed size and equal separation from one strip to another. We carried out the electrical measurements on such type of interdigited electrodes depositing, with different procedures, various numbers of layers. First of all, we checked the behavior of the current versus voltage (I/V) curve. Figure 9 (part A) shows the I/V curve of a polypyrrole sample obtained via electrochemical deposition: the I/V curves do not show an ohmic behavior and the conductivity varies with time. Part B shows the same characterization performed on a sample deposited with layer-by-layer absorption: the ohmic behavior and the conductivity values are retained for 30 days time period. The conductivity of the samples ranges between 10-1-10-2 S/cm according to their thickness. The thickness values have been estimated via AFM by imaging samples with ad-hoc deposited steps of organic material.

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.

4. Conclusions

The paper deals with the investigation of the properties of organic compounds in order to highlight the possibility of their use in the development of real organic devices. The stability in time, the reproducibility of the procedure for deposition and the simple deposition technique are, in our opinion, the key points to focus for testing any practical application of these materials. So our approach was focused to investigate well known materials and technologies keeping in mind the final goal to realize a real device.

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.

Acknowledgments

The authors would like to thank Dr. M. Sartore for his important help in providing the software programs interconnecting the PC and all the measuring instruments, Dr. M. Salerno for AFM characterizations and Mr. R. Galletti for software support.

References

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[2] Stoldt M, B”uerle P, Schweizer H and Umbach E 1992 Nanostructures based on molecular materials (Weinheim: VCH) p 295

[3] Nicolini C 1996 Molecular Bioelectronics (Singapore and New York: World Publishing C.)

[4] Tieke B 1990 Adv. Mater. 2 (5) 222

[5] Petty M C 1991 J. Biomed. Eng. 13 209

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[8] Procarione W L and Kauffman J W 1974 Chem. Phys. Lipids 12 251

[9] Feigin L A, Lvov Yu M and Troitsky V I 1989 Sov. Sci. Rev. A Phys. 11 285

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[12] Decher G 1996 Comprehensive Supramoleculars Chemistry Vol. 9 "Templating, Self-Assembly and Self-Organisation" (Oxford: Pergamon Press) p 507

[13] Fukuyama M, Kudoh Y and Yoshimura S 1993 Mol. Cryst. Liq.Cryst. 224 61

[14] Batz P, Schmei■er D and Gopel W 1990 Solid State Commun. 74 461

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[18] Bockris J O'M and Reddy A K 1973 Modern Electrochemistry (New York: plenum Rosetta Edition) 387



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