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Electrochemical Characteristics of 5,10,15,20 -Tetrakis-Octadecyloxymethylphenyl-Porphyrin-Zn(II) Langmuir-Blodgett(LB) Films

by
Ja-Ryong Kooa, Don-Soo Choib, Bong-Ok Kimc, Sung-Min Kimc, No-Gil Parkc, Mi-Yong Kwakc,
Jae-Hoon Shimc, & Young Kwan Kim*d

 

aDept. of Electrical, Information & Control Eng., Hongik Univ., Seoul 121-791, KOREA
bResearch Institute of Science & Technology, Hongik Univ., Seoul 121-791, KOREA
cCenter for Organic Materials & Information Devices, Hongik Univ., Seoul 121-791, KOREA
dDept. of Chemical Eng., Hongik Univ., Seoul 121-791, KOREA
*Corresponding auther : email : kimyk@wow.hongik.ac.kr


This paper is based upon a presentation made at the
Ninth Foresight Conference on Molecular Nanotechnology.


Abstract

In the study, we have investigated the possibility of redox-active organic monolayers as molecular-scale information storage systems. The memory elements are porphyrin molecule; information is stored in the oxidation state of these molecule. While both porphyrin anions and cations can be formed electrochemically, in this study we have used the cation state due to its greater chemical stability. Monolayer films of 5,10,15,20-Tetrakis-Octadecyloxymethylphenyl-Porphyrin-Zn(II) (C120H184O4N4Zn, molecular weight=1744.754) were prepared by Langmuir-Blodgett(LB) method and characterized by using UV/vis Absorption Spectroscopy, Cyclic Voltammetry(C-V), and Current-Voltage measurement(I-V). A limiting area per molecule was about 135 Å2 and relatively low collapse pressure of 45 mN/m was shown. Because the limiting areas for face-on and edge-on orientations of porphyrin ring arranged in a monolayer are known to be about 160 Å2 and 70 Å2, relatively. Therefore this value implies that porphyrin rings are tilted away from the air-water interface. Starting points of oxidation/ reduction potentials were measured to be +0.59 V and -1.98 V, respectively. The C-V measurement showed that the ionization potential was 5.39 eV and the electron affinity 2.82 eV; the band gap of oxidation/reduction potential was 2.57 eV.  The I-V characteristics of the solid state device can be understood in terms of the molecular properties observed in C-V. Current flow in both the solid-state devices and solutions is reversible at positive potentials and irreversible at negative ones. This correspondence between the solution and solid-state results suggests that the fundamental molecular electronic properties are retained in the solid-state devices. If this is the case, the forward bias current flow should be determined by the HOMO state, whereas the reverse bias current should be determined by LUMO state. Further details of the electrical properties of Porphyrin-Zn(II) derivative films will be discussed.

 

Introduction

Molecules that might be suitable for use in molecular electronic devices have recently been the subject of much attention.[1],[2] Current approaches are based on the development of molecular-scale switches, which can be used in both logic and memory circuits. Because the basic paradigm for electronic information storage is the retention of charge in a capacitor, the most straightforward approach to molecular scale memory will store charge at the molecular level. One example of this type of approach employs quantum-dot cellular automata(QCA).[3] More fundamental approach will utilize the oxidation states of individual molecules to store charge. This approach has an advantage that multiple oxidation states within one molecule can be addressed to access > 1 bit.[4]

The study on organic ultrathin films has recently attracted greast interest due to the potential technological applications of these materials, in molecular photoelectronic devices, medical applications, and other appliance.[5][6][7][8] One of the main problems related with the fabrication of these organic films is controlling the orientation and aggregation of molecules. This problem is particularly important in the case of porphyrins due to the high tendency of these molecules to form different type of aggregates.

In this study, we have investigated the possibility of redox-active organic monolayers as molecular-scale information storage systems. The memory elements are porphyrin molecules; information is stored in the oxidation states of these molecules. While both porphyrin anions and cations can be formed electrochemically, we have selected to employ the cations due to their greater chemical stability. The basic fabrication process of porphyrin LB films was explained and the UV/vis absortion spectra, cyclic voltammetry, and current-voltage(I-V) characteristics of porphyrin LB films were obtained and discussed.

 

Experimental

Materials & ¥>- A isotherm

In this study, 5,10,15,20-Tetrakis-Octadecyloxymethylphenyl-Porphyrin-Zn(II) (Zn-Porphyrin)(C120H184O4N4Zn, molecular weight = 1744.754) molecules, one of the porphyrin derivatives, were used as a material for a memory device. The molecular structure of a Zn-porphyrin derivative is shown in Fig. 1. The ¥>- A isotherm of the Zn-porphyrin Langmuir film was measured using a Kuhn-type LB trough (NIMA 611), where purified water(18.3 MΩ·cm) was used as the subphase. Chloroform (CH3Cl) was used as a solvent for Zn-porphyrin with concentration of 1 × 10-4mol/l.

 

Fig. 1. Molecular structure of a Zn-porphyrin derivative.

 

Film deposition

In the Langmuir trough experiments, 400µl of a solution was carefully spread onto the subphase by a gas-tight syringe. After the solvent was evaporated (ca. 10 min), the floating film was continuously compressed at a speed of 25cm2/min. Surface pressure was simultaneously monitored by a Wilhelmy Balance while getting an isotherm of the sample. The film was then transferred onto the substrate by a usual vertical dipping method at the surface pressure of 25mN/m and at a speed of 5mm/min during the upstroke. Microscope-slide glass and quartz were used as substrates for I-V measurement and UV/vis absortion spectroscopic measurement, respectively. The Zn-porphyrin LB film was formed with Y-type.

 Measurement

The UV/vis absorption spectra of Zn-porphyrin LB films were measured with an HP 8452 Diode Array Spectrophotometer. The current-voltage(I-V) characteristics of the film along the direction vertical to the substrate were measured using Keithley 238 Electrometer. Voltage was applied from -2 to 2 V in an interval of 100mV/s. Aluminium top and bottom electrodes for electrical measurement was vacuum-deposited at a pressure of 10-5 Torr. The thickness of top and bottom electrodes was 1,000Å and 1,500Å, respectively.The device structure is shown in Fig. 2.

Fig. 2. Top view of the device structure prepared in this study.

 

For the electrochemical measurement of the Zn-porphyrin derivative, El Electroanalysis(PIMACS Co.) equipment was used, where Bu4NClO4 and acetonitrile were used as an electrolyte and a solvent, respectively. Counter, reference, and working electrodes were each used with Pt wire, Ag/Ag+(0.1M AgNO3), and ITO/Al.

 

Results and Discussion

¥>- A isotherm, film deposition and device characteristics

Fig. 3. shows typical ¥>-A isotherm characteristics of 5,10,15,20-Tetrakis- Octadecyloxymethylphenyl-Porphyrin-Zn(II) derivative. The limiting area per molecule was about 135Å2 and relatively low collapse pressure of 45mN/m was shown. Because the limiting area for the face-on and edge-on orientation of porphyrin rings arranged in a monolayer is known to be about 160 Å2 and 70 Å2, respectively[9], this value implies that porphyrin rings are tilited away from the air-water interface. Target surface pressure for film deposition was from 15 to 35 mN/m. In this study, the surface pressure of 25 mN/m was chosen for film deposition. The film deposition ratio for pulling out the substrate through the floating film onto water was always in the range from 1.00 ± to 0.1.

 

Fig. 3 .¥>-A isotherm characteristics of the Langmuir film of the Zn-porphyrin.

 

UV/vis absorption spectrum

The absorption spectrum of the Zn-porphyrin derivative in monolayer LB films deposited on quartz glass is shown in Fig. 4. The absorption peaks of the LB films occurred at 444nm(Soret band), and 502, 555nm(Q bands).

 

Fig. 4. UV/vis absorption spectrum of the LB films of the Zn-porphyrin derivative.

 

Cyclic voltammetric measurement

The cyclic voltammogram of the Zn-porphyrin derivative is shown in Fig. 5. The electrochemical analysis of Zn-porphyrin was done by using the three-step analytic method. First, the film of the Zn-porphyrin derivative was formed on the working electrode(ITO, Al electrode) by sping-coating. Second, this working electrode was put in the electrochemical cell, and the oxidation/reduction potential of the Ag/Ag+ reference electrode was measured and analyzed with cyclic voltammetry. Third, the potential conversion of the reference electrode was carried out by criterion materials, and the ionization potentials(IP), electron affinity(EA), and band gap of the Zn-porphyrin sample were finally confirmed. In this potential conversion, ferrocene and AlQ3 were used as criterion materials. The potential conversion constant of SCE reference electrode vs. the Ag/Ag+ reference electrode was  + 0.31 V, and the potential conversion constant of the SCE reference electrode vs. the ionization potential, the electron affinity of the electrochemical potential, was 4.8. The scan rate of cyclic voltammetry was 100 mV/sec. Fig. 5. shows that the onset point of the oxidation/reduction potential appeared at +0.59 V and -1.98 V, respectively. It was also found that IP and EA were 5.39 eV and 2.82 eV, The electrochemical band gap of the oxidation/reduction potential was 2.57 eV and 482 nm. Because oxidation reaction by electron donating and removing was symmetric, Zn-porphyrin has a reversible characteristic, which means that this material is very stable.

 

Fig. 5. Cyclic voltammogram of the Zn-porphyrin derivative.

 

Current-voltage(I-V) characteristics[10][11]

The current-voltage characteristics of the devices made from Zn-porphyrin monolayers are shown in Fig. 6. These devices, initially probed at forward bias (positive) voltages, show a remarkably nonlinear current increase with the applied voltage (Fig. 6, "1st scan"). Subsequent scans yield slightly lower current levels from 0.1 V  to 0.8 V. A reverse bias scan (Fig. 6. inset) is characterized by a reduced current level. Here, we have the basis for a singly configurable molecular switch which is initially "closed"(positive bias) and then, after negative biasing, "open." The configurability of this junction was recently exploited to make logic gates from the parallel array of molecular-based devices[1].In the inset of Fig. 6, the I-V curves show small hysteresis. According to the Aviram-Ratner model, the rectification mechanism depends on a match(resonant tunneling) between the Fermi levels of metal electrodes and the HOMO and LUMO levels of molecule sandwiched between the metal layers, followed or preceded by inelastic tunneling within the molecule.[12]Generally speaking, the use of metal electrodes with different work functions may also hinder the observation of molecular rectification, or at least complicate the metal/LB film/metal junctions. It is well known that higher currents are always observed in metal-insulator -metal junctions when a positive bias is applied on the electrode with lower work function . This consideration leads us to use Al for both electrodes.

Fig. 6. I-V characteristics of the Zn-porphyrin derivative(at positive bias).

 

Fig. 7. shows the current-voltage(I-V) characteristics of the Al/Al2O3/LB film/ Al junctions from 0 V to 1.5 V at 293K. The same data as Fig. 6. are plotted. The negative differential resistance(NDR) was observed. The peak current density was > 49.5¥A/cm2, this NDR is < -25.3M¥cm2, and peak-to-valley ratio(PVR) is 1.33:1. This value is similar to data of Tour group.[13]

 

Fig. 7. I-V characteristics of a Al / Zn-porphyrin / Al device at 293K. PVR is 1.33:1.

 

Resonant tunneling through molecular states

The I-V characteristics of the solid state device can be understood in terms of the molecular properties observed in the solution. Current flow in both the solid-state device and the solution is reversible at positive and irreversible at negative potentials. This correspondence between the solution and solid-state results suggests that the fundamental molecular electronic properties are retained in the solid-state devices. If this is the case, then the forward bias current flow should be determined by HOMO state (Fig. 8b), whereas the reverse bias current should be determined by LUMO states(Fig. 8c).

 

                                                      (a)                                                                                                             (b)

                                                                                                   

                                                                                                                                                                    (c)

Fig. 8. Band diagram of the Zn-porphyrin device (a) equilibrium(V=0), (b) positive bias(V>0), (c) negative bias(V<0).

 

Conclusions

The following conclusions have been obtained from the experiments with the Zn -porphyrin LB films :

  1. The proper surface pressure for film deposition was found to be about 25mN/m from the ¥>-A isotherm of the Zn-porphyrin Langmuir film.
  2. In the cyclic voltammetry experiment, the IP, and EA of the Zn-porphyrin derivative were 5.39 and 2.82 eV, respectively.
  3. From I-V characteristics, NDR behavior was observed at room temperature.
  4. From cyclic voltammogram, the band diagram of the Zn-porphyrin derivative was obtained. In this results, the resonant tunneling phenomena of electrons were explained.

 

Acknowledgment

This work was fully supported by Tera-level Nano Devices(Project NO : 2000-003-04).

 

References

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  2. Chen, J.; Reed, M.A.; Rawlett, A.M.; Tour, J.M. (1999) Science, 286, November 19, pages 1550-1551, Large On-Off Ratio and Negative Differential Resistance in a Molecular Electronic Device
  3. Lent, C.S. (2000) Science, 288, June 2, page 1597-1599, Bypassing the Transistor Paradigm
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  9. Qian, X.; Tai, A.; Sun, X.; Xiao, S.; Wu, H.; Ju, Z.; Wei, Y. (1996) Thin Solid Films, 285, page 432-435, Molecular packing in LB films of a new porphyrin investigated by atomic force microscopy
  10. Wong, E.W.; Collier, C.P.; Behloradsky, M.; Raymo, F.M.; Stoddart, J.F.; Heath, J.M. (2000) J. Am, Chem, Soc., 122, page 5831-5840, Fabrication and Transport Properties of Single-Molecule-Thick Electrochemical Junctions
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