of 5,10,15,20 -Tetrakis-Octadecyloxymethylphenyl-Porphyrin-Zn(II) Langmuir-Blodgett(LB)
Ja-Ryong Kooa, Don-Soo Choib,
Bong-Ok Kimc, Sung-Min Kimc, No-Gil Parkc,
Jae-Hoon Shimc, & Young Kwan Kim*d
aDept. of Electrical, Information & Control
Eng., Hongik Univ., Seoul 121-791, KOREA
of Science & Technology, Hongik Univ., Seoul 121-791, KOREA
cCenter for Organic Materials & Information
Devices, Hongik Univ., Seoul 121-791, KOREA
Eng., Hongik Univ., Seoul 121-791, KOREA
: email : email@example.com
This paper is based upon a presentation made at the
Ninth Foresight Conference on Molecular Nanotechnology.
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.
Molecules that might be suitable for use in molecular
electronic devices have recently been the subject of much attention.,
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). 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.
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. 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.
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
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
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.
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),
Results and Discussion
¥>- A isotherm, film deposition and device
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, 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
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.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.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¥ìA/cm2, this NDR is < -25.3M¥Øcm2, and peak-to-valley
ratio(PVR) is 1.33:1. This value is similar to data of Tour group.
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.
Fig. 8. Band diagram of the Zn-porphyrin
device (a) equilibrium(V=0), (b) positive bias(V>0), (c) negative bias(V<0).
The following conclusions have been obtained from
the experiments with the Zn -porphyrin LB films :
- The proper surface pressure for film deposition
was found to be about 25mN/m from the ¥>-A isotherm of the Zn-porphyrin
- In the cyclic voltammetry experiment, the
IP, and EA of the Zn-porphyrin derivative were 5.39 and 2.82 eV, respectively.
- From I-V characteristics, NDR behavior was
observed at room temperature.
- From cyclic voltammogram, the band diagram
of the Zn-porphyrin derivative was obtained. In this results, the resonant
tunneling phenomena of electrons were explained.
This work was fully supported by Tera-level Nano Devices(Project NO : 2000-003-04).
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