Iwamoto M., Kwon Y.-S., Lee T. (Eds.) Nanoscale Interface for Organic Electronics

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Charge Transport and Electric Conduction in Viologen SAMs 179

In addition, in the viologen derivatives that were self-assembled to

an Au substrate for 24 h under the concentration of 1 mM, the self-

assembled monolayers were verified using STM. The STM measurement

was performed at room temperature, and the scanning in the

measurement was processed by a constant current mode. The HOPG

surface was observed before scanning the sample surface in order to

verify the normal state of the probe. The reason that the HOPG was

selected in this process is that it is possible to observe the surface as an

atomic level of resolution not only for a high vacuum condition but also

in air. The normal state of the probe was verified by investigating the

surface of well-arranged graphite single crystal using a 10 nm × 10 nm

scan size. For verifying the fact that the STM is accurately operated

while it has an atomic level of resolution, this study observed the

surfaces of the highly-oriented pyrolytic graphite (HOPG), which has a

crystallized surface, and Si wafers. Because the HOPG is stable in air

where carbon atoms form certain lattices, it has been largely used to

calibrate x and y axes for measuring STM images.

2.2.2. Cyclic Voltammetry Test and Charge Transport Measurement

This study performed a cyclic voltammetry test using the QCM self-

assembled by using viologen in which the self-assembly was not

Fig. 3. Liquid cell assembly of quartz crystal for liquid analysis.

N.-S. Lee et al. 180

implemented by chemical adsorption but washing physically precipitated

molecules using a solvent.

The cyclic voltammetry test was applied using the Potentiostat

(PerkinElmer, USA) and QCA 922 (Seiko EG&G, Japan), and the results

of the test was processed using a computer connected by using GPIB.

The QCM in which viologen was self-assembled to a 0.196 cm

electrode was used as a working electrode in all tests. In addition, for

configuring a cell with three electrodes, a 1.5 × 4.5 cm

Pt plate and an

Ag/AgCl (MW-4130, BAS) plate were used as counter and reference

electrodes, respectively. Then, the oxidation and reduction reactions

were measured using this three electrodes cell. The test equipments and

measuring process are presented in Fig. 4.

This study performed the cyclic voltammetry test in three different

electrolytes, such as 0.1 M NaClO

, Na

, and Na

, in order to

verify the influence of the negative ion on the oxidation and reduction of

the viologen in which the voltage ranges were configured by 0 to -1 V.

All cyclic voltammetry tests were repeated by 10 times, and the

electrolyte used in these tests was 18.3 MΩ ultra-pure water, which was

applied after fully purifying it in Ar gas for 30 min.

Fig. 4. Schematic diagram of electrochemical detection sensing system used.

Charge Transport and Electric Conduction in Viologen SAMs 181

2.2.3. Analysis of the Characteristics of Electric Conduction

STM is a microscope that can observe individual atoms in a real space. It

has a simple principle in its operation in which a specific voltage level is

applied to the space between a conductive sample and a metal probe and

represents tunnel current as the space is approached up to 1 nm. The

current corresponds to the change in the distance between two materials

and is very sensitive to the change. The change is represented as an

exponential function. As the change in the distance shows 0.1 nm, the

tunnel current is to be changed by the scale of one order.

For producing a video of atoms, the probe is inserted to the surface of

the sample using a piezoelement while the tunnel current is maintained

as a constant level. If the change in voltages that is applied to the

piezoelement is detected and visualized, the surface structure of the

sample can be observed as an atomic scale.

For observing the characteristics of electric conduction in self-

assembled monolayers, the surface image and voltage-current

characteristics were measured using the system by Veeco as shown in

Fig. 5.

Because the tunnel current ranged by 1 ~ 10 nA can be detected and

controlled as a higher level than 1%, the resolution for the vertical

direction is to be determined by about 0.01 nm. Also, in the resolution

within the face, the sharper end of the probe shows more higher

(a) (b)

Fig. 5. Principle of STM system and the image of STM tip.

N.-S. Lee et al. 182

resolution levels than other cases. Based on a model calculation under

the conditions that the radius of curvature and distance in the end of the

metal probe are determined by R and Z, respectively, the resolution

within the face can be determined as Eq. (1):

1/2

[0.2( )] .

R Z+ (1)

In the case of the end of the probe that has one atom, the values of

R and resolution within the face are determined by 0.1 nm and 0.3 nm,

respectively.

3. Analysis of Self-Assembled Monolayers Using STM

STM has a resolution that is able to observe atoms in air. In the case of

the distance between a conductive sample and a metal probe that is

approached by about 0.01 nm while a specific voltage is applied to this

distance, there is a quantum physical tunneling effect that shows some

currents in which electrons pass through an energy barrier by applying a

proper voltage between these two sides even though these two

conductors are separated. If the distance is changed by 0.1 nm, the tunnel

current will be varied by a scale of one order.

The obtaining of the images of atoms or molecules using STM, it is

possible to observe the surface structure of the sample up to the scale of

atom or molecule based on the image processing using the detection of

the change in tunneling currents during the injection through moving the

probe onto the surface of the sample using the piezoelement while the

tunneling current is maintained as a constant level.

This study observed the surface images of the viologen derivatives,

which were self-assembled to the surface of an Au(111) substrate, and

presented a possibility of the application of the derivatives as molecular

elements using STM.

In this study, the Au(111) substrate used as a lower electrode was

fabricated using a thermal evaporation system in which the fabrication

was performed by the thermal evaporation with 100 nm thickness Au

on the prebaked mica at 320°C for 2 h under maintaining the vacuum

of 6.5

× 10

-7

Torr. The viologen derivatives were self-assembled on the

Charge Transport and Electric Conduction in Viologen SAMs 183

fabricated Au(111) substrate at the concentration of 1 mM for 24 h. The

fabricated samples after completing the self-assembly were washed using

ethanol and stored these samples in a desiccator after drying them using

gas.

The STM measurement was performed at room temperature using

the UHV-STM (UNISOKU, USM-1200) in which a probe made by

using a Pt-Ir(80:20) alloy was used. Also, the voltages applied to the

space between the STM tip and the sample were ranged from -2.5 V to

+2.5 V. The scanning was applied by a constant current mode at the

tunneling region of ~0.20 nA, and the change in tunneling currents was

verified using STM/STS.

Figures 6(a)-6(d) show the data analyzed by using the ChemDraw

for viologen. In addition, an ellipsometer was used to measure the

thicknesses of viologen molecules, which were self-assembled on the

Au(111) substrate, VC

SH, VC

SH, HSC

SH, and HSC

SH.

In the results of the measurement using the ellipsometer, the heights of

SH, VC

SH, HSC

SH, and HSC

SH were 0.33 nm,

0.74 nm, 0.84 nm, and 0.87 nm, respectively. Although there were some

little differences between these values and the results analyzed by using

STM, the results of these two cases showed similar figures within a

specific error range because the heights obtained by using STM were

determined as the convolution of the electric conductivity and physical

height of samples and that caused some differences in heights.

Fig. 6. Chemical Structure of viologen derivatives. (a) VC

SH, (b) VC

SH, (c)

HSC

SH, and (d) HSC

SH.

N.-S. Lee et al. 184

The STM is able to measure the spectra measurement of tunnel

spectrum (STS). It measures the change in tunnel currents by varying

bias voltages while feedbacks are halted under the application of set-

point bias voltages after moving the probe to the region that is to be

measured. Figure 7 illustrates the I-V characteristics of the viologen

monolayers measured by using STS. The I-V characteristics were

observed by fixing the STM tip at a specific point after measuring four

different viologen monolayers.

4. Cyclic Voltammetry and Charge Transport

4.1. Oxidation and Reduction According to Changes in Electrolytes

The previous results reported the relationship between the cyclic

voltammetry curve and the peak current versus injection speed in a 0.1 M

NaClO

solution for VC

SH. The peaks in oxidation and reduction were

presented at -0.45 V and -0.51 V, respectively, in which the currents in

the oxidation and reduction showed the same levels. Also, it can be

seen that the oxidation and reduction of the positive radicals, which

were caused by the one electron reduction presented by V

, were

Fig. 7. I-V characteristic curves of viologen derivatives (VC

SH, VC

SH, HSC

SH,

and HSC

SH).

Charge Transport and Electric Conduction in Viologen SAMs 185

reversibly generated as it is expected that the peak currents of the

oxidation and reduction showed the same levels, and the relationship

between the peak current and the injection speed was determined as a

linear way.

Although the oxidation and reduction peaks were generated

at a certain constant voltage level regardless of the type of positive ion

due to the reaction of V

in viologen monolayers, the scales of

peak currents decreased by the order of SO

, ClO

, and PO

. It is

considered that there are some differences in the amount of transported

charges caused by the limited molar conductivity according to the

mobility in each ion.

4.2. Characteristics of Interfacial Charge Transport Caused by the

Change in Mass

The previous results reported the change in the resonance frequencies

of QCM measured in the cyclic voltammetry test of VC

SH and

HSC

SH. As results, the change in resonance frequencies occurred

by oxidation and reduction reactions simultaneously and that can be

presented as a change in mass using the Sauerbrey equation. According

to the reduction reaction in two stages, the change in resonance

frequencies increased by two stages, and the decrease in resonance

frequencies occurred by two stages according to the oxidation. It shows a

process that reassociates the ClO

ion, which is associated to the N

ion

that is an electrochemical active place of the viologen monolayers, is

separated by its reduction according to the oxidation. The entire changes

in the resonance frequencies of VC

SH and HSC

SH were 18.1 Hz

and 8.4 Hz, respectively, and that can be converted by the change in

masses as 19.36 ng and 8.98 ng, respectively. The numbers of associated

and dissociated ions obtained from the mass of ClO

ion for VC

SH and

HSC

SH were 2.33 × 10

and 1.08 × 10

. It can be considered that

the results were affected by the electrical double layer that formed the

confronted layers of charges, which were located at both the electrode

and solution in which the Au electrode where the viologen monolayers

were unself-assembled represented electricity at the boundary between

the metal surface and the solution, because the viologen monolayers

were not self-assembled on the surface of the Au electrode of the QCM.

N.-S. Lee et al. 186

In the case of the 0.1 M Na

electrolyte solution, the changes in

the resonance frequencies were 19.3 Hz and 9.5 Hz and that can be

converted by the change in masses as 20.65 ng and 19.0 ng. Also, the

numbers of associated and dissociated Cl

ions obtained from the change

in masses were 2.49

× 10

and 1.22 × 10

, respectively. In the case of

the 0.1 M Na

electrolyte solution, the changes in the resonance

frequencies were 16.0 Hz and 7.1 Hz and that can be converted by

the change in masses as 17.16 ng and 7.60 ng. Also, the numbers of

associated and dissociated Cl

ions obtained from the change in masses

were 2.06

× 10

, 9.1 × 10

, respectively.

5. Electric Conductive Characteristics of Viologen Derivatives

In general, the charge insertion at the interface between metal and

organic materials can be explained by the thermionic emission

(dominated in a low bias level) and Fowler-Nordheim tunneling

(dominated in a high bias level). That is, as a strong electric field is

applied to the interface between metals and conductors, the barrier is

decreased due to the Schottky effect and that shows a decrease in the

width, which is forward to the same electric field in a Fermi level. Then,

tunneling occurs at this position.

If the energy of the electron that

is approached to the interface within metal shows the distribution

of Fermi-Dirac, the tunnel current, I, can be obtained using Eqs. (2)

and (3):

exp

I V

−

 

 

 

(2)

1/ 2 3/2

8 (2 )

= (3)

where m is electron effective mass, and h is Plank constant. The I-V

relationship in Eqs. (2) and (3) shows that it is possible to obtain a line,

which has a negative value as log(I/V

) -1/V. Figure 8 shows the

Fowler-Nordheim plot obtained from the I-V characteristics shown in

Fig. 7 for each viologen molecule, VC

SH, VC

SH, HSC

SH, and

Charge Transport and Electric Conduction in Viologen SAMs 187

HSC

SH. A linear relation that has a negative gradient in a high

voltage region determined by ~1 V was obtained. Then, it is verified that

the Fowler-Nordheim tunneling dominated the conduction mechanism in

a high electric field region for the viologen molecules, VC

SH, VC

SH,

HSC

SH, and HSC

SH.

20,21

Figure 9 shows the resistance values for the lengths of molecules at

1.63 ~ 1.67 V positive bias regions in the viologen derivatives measured

in self-assembled monolayers. In the results, it was considered that the

resistance values increased according to the increase in the lengths of

molecules and that caused decreases in tunneling currents.

(a) (b)

Fig. 8. Fowler-Nordheim tunneling plot. (a) VC

SH, (b) VC

SH, (c) HSC

SH, and

(d) HSC

SH, respectively.

N.-S. Lee et al. 188

In addition, based on the measured I-V and resistance characteristics,

the barrier height (φ) determined by damping coefficient (β) was

calculated using Eqs. (4) and (5)

exp( )

R R d

(4)

1/2

(2 ) .

= Φ (5)

Figure 10 illustrates the differential conductance (dI/dV-V) for each

molecule in VC

SH, VC

SH, HSC

SH, and HSC

SH. In

the characteristics of dI/dV-V, there is symmetric or asymmetric

characteristic according to the lengths of the molecules of viologen

derivatives, i.e., the scale of the conductivity in molecules, because

the dI/dV-V represents such symmetric or asymmetric characteristic

according to the lengths of viologen molecules and positions of the

STM tip.

Fig. 9. Resistance versus molecular length plots of the viologen derivatives.