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Charge Transport and Electric Conduction in Viologen SAMs 189

In this study, it was possible to verify the energy gap of viologen

molecules through the dI/dV-V characteristics. The differential

conductance (dI/dV-V) corresponds to the state density in molecular

systems. As the lock-in is detected for reducing the influence of noises, a

stable spectrum will be obtained. Because the dI/dV-V represents an

empty state that does not include electrons as a positive bias voltage and

a filled state that includes electrons as a negative bias voltage, the

difference in the edge region between positive and negative biases is to

be determined as the energy gap. It was also possible to verify the

decrease in tunneling currents due to the increase in tunnel barrier

heights caused by the decrease in the hole injection barrier height and the

increase in energy gaps according to the increase in the lengths of

viologen molecules.

6. Conclusion

This study observed the change in the structure of viologen molecules in

a nanoscale and analyzed the electric conduction characteristics. Also,

this study analyzed the forming of the monolayers of viologen molecules

using QCM and the morphology of self-assembled viologen monolayers

Fig. 10. dI/dV-V characteristics of the viologen derivatives.

N.-S. Lee et al. 190

using STM. In addition, the cyclic voltammetry method was used to

analyze the characteristics of the charge transport caused by the

oxidation and reduction of self-assembled viologen monolayers. The

characteristics of the electric conduction was analyzed using STM.

The results of this study can be summarized as follows:

First, in the results of the analysis of the self-assembly process of

viologen molecules using QCM, the amounts of the forming of VC

and HSC

SH were 436 ng and 217 ng, respectively. These values

agreed with that of the values calculated in each molecular structure and

weight. After completing the self-assembly of viologen molecules on

an Au(111) substrate for 24 h, this study investigated the forming of

viologen monolayers in which the heights of the molecules were

observed by using STM and an ellipsometer. In the results of the analysis

of the morphology of the viologen molecules, which were formed on the

Au(111) substrate using STM, the heights of viologen derivatives,

SH, VC

SH, and HSC

SH were 0.13 nm, 0.24 nm, 0.41 nm, and

0.45 nm, respectively. The results showed the same as the values

measured by using the ellipsometer.

Second, in the results of the simultaneous analyses of the I-V

characteristics and changes in the resonance frequencies of QCM

through cyclic voltammetry tests, it was verified that there were

reversible oxidation and reduction reactions. Also, it was possible to

verify the current characteristics according to the transport of

electrolytes.

Third, in the results of the I-V characteristics of self-assembled

viologen monolayers using STM at a nanolevel, it was investigated that

the I-V characteristics according to the change in the lengths of viologen

molecules represented a decrease in currents from 292 nA to 40.6 nA in

the positive bias region of +1.65 V. In addition, in the conduction

mechanism at a high electric field ranged by ~1 V, it was verified that the

Fowler- Nordheim tunneling dominated the mechanism.

Acknowledgment

This research was supported by Strategic Technology under the Ministry

of Knowledge Economy (MKE).

Charge Transport and Electric Conduction in Viologen SAMs 191

References

1. A. Aviram, M. A. Ratner, Chem. Phys. Lett., Vol. 29, p. 277 (1974).

2. A. Ulman, “An Introduction Organic Films”, Academic Press, Boston, MA, 1991.

3. M. C. Petty, “An Introduction to Molecular Electronics”, Edward Arnold, London,

1996, p. 131.

4. A. Ulman, Chem. Rev., Vol. 96, p. 1533 (1996).

5. R. C. Young, T. J. Meyer, D. G. Whitten, J. Am. Chem. Soc., Vol. 97, p. 4781

(1975).

6. M. D. Ward, J. R. White, A. J. Bard, J. Am. Chem. Soc., Vol. 105, p. 27 (1983).

7. L. Michaelis, E. S. Hill, J. Am. Chem. Soc., Vol. 55, p. 1481 (1933).

8. L. A. Summers, “The Bipyridinium Herbicides”, Academic Press, New York, 1980.

9. J. H. Fendler, Phys. Chem., Vol. 89, p. 2730 (1985).

10. T. J. Meyer, Acc. Chem. Res., Vol. 22, No. 5, p. 163 (1989).

11. G. S. Ostrom, D. A. Buttry, J. Phys. Chem., Vol. 99, p. 15236 (1995).

12. S. Cosnier, C. Innocent, Y. Jouanneau, Anal. Chem., Vol. 66, p. 3198 (1994).

13. G. Inzelt, “Charge Transport in Polymer-Modified Electrodes: In Electroanalytical

Chemistry”, Vol. 18, Marcel Dekker, New York, 1994.

14. R. H. Terrill, J. E. Hutchison, R. W. Murray, J. Phys. Chem. B, Vol. 101, p. 1535

(1997).

15. H. Muramatsu, J. M. Dick, E. Tamiya, I. Karube, Anal. Chem., Vol. 59, p. 2760

(1987).

16. G. Binnig, C. F. Quate, Ch. Gerber, Phys. Rev. Lett., Vol. 56, No. 9, p. 930 (1986).

17. J. Y. Ock, H. K. Shin, D. J. Qian, J. Miyake, Y. S. Kwon, Jpn. J. Appl. Phys.,

Vol. 43, No. 4B, p. 2376 (2004).

18. D. Y. Lee, A. K. M. Kafi, S. H. Park, Y. S. Kwon, J. Nanosci. Nanotechnol., Vol. 6,

p. 3657 (2006).

19. M. A. Reed, J. Chen, A. M. Rawlett, D. W. Price, J. M. Tour, Appl. Phys. Lett.,

Vol. 78, p. 3735 (2001).

20. J. M. Beebe, B. S. Kim, J. W. Gadzuk, C. D. Frisbie, J. G. Kushmerick, Phys. Rev.

Lett., Vol. 97, p. 026801 (2006).

21. Y. Noguchi, T. Nagase, R. Ueda, T. Kamikado, T. Kubota, S. Mashiko, Jpn. J. Appl.

Phys., Vol. 46, No. 4B, p. 2683 (2007).

22. T. Ishida, W. Mizutani, Y. Aya, H. Ogiso, S. Sasaki, H. Tokumoto, J. Phys. Chem.

B, Vol. 106, p. 5886 (2002).

23. C. Arena, B. Kleinsorge, J. Robertson, W. I. Milne, M. E. Welland, J. Appl. Phys.,

Vol. 85, p. 1609 (1999).

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193

CHAPTER 10

TIME-AVERAGED DEUTERIUM NMR STUDIES OF THE

DYNAMIC PROPERTIES FOR A LOW MOLAR

MASS NEMATIC

Akihiko Sugimura

and Geoffrey R. Luckhurst

Department of Information Systems Engineering, Osaka Sangyo University,

3-1-1 Nakagaito, Daito, Osaka 574-8530, Japan

E-mail: sugimura@ise.osaka-sandai.ac.jp

School of Chemistry, University of Southampton, Highfield, Southampton,

SO17 1BJ, United Kingdom

E-mail: G.R.Luckhurst@soton.ac.uk

There have been many investigations of the alignment of nematic

liquid crystals by either a magnetic and/or an electric field. The basic

features of these hydrodynamic processes have been characterized

for the systems in their equilibrium and non-equilibrium states. To

complement the experiments theoretical models, based on continuum

theory, have been developed which successfully describe the static and

dynamic phenomena. This technique has proved to be especially

important for the investigation of liquid crystals. In this chapter, we

describe some of our studies of the field-induced oscillatory behaviour

of the nematic director for low molar mass nematics using deuterium

NMR but with a sinusoidal electric as well as a static magnetic field to

align the director. The director orientations have been observed from

the time-resolved NMR experiments but we have found that they are

also available, more directly, from the time-averaged NMR spectrum.

The maximum and minimum angles made by the director with the

magnetic field were determined, as a function of frequency, from the

NMR spectrum averaged over many thousand cycles of the oscillations.

These NMR experiments can give dynamic as well as static

information concerning the nematic phase and could be used to provide

a new route to certain material properties of the nematic.

A. Sugimura and G. R. Luckhurst 194

1. Introduction

Nuclear magnetic resonance (NMR) spectroscopy is widely used in the

study of liquid crystals. Deuterium NMR has proved to be especially

important for the investigation of liquid crystals because the spectra of

specifically or fully deuteriated materials are rather simple compared to

the corresponding proton NMR spectra.

1-5

The quadrupolar splitting for

deuterons observed in the liquid crystal phase is related, primarily, to the

second rank orientational order parameter for the C-D bond direction

and so deuterium NMR has been widely used for the study of the

orientational order of liquid crystals as well as their phase transitions.

The quadrupolar splitting is also determined by the angle between the

director and the magnetic field. In consequence, deuterium NMR

spectroscopy has proved to be a powerful method with which to

investigate the director orientation and its distribution, as well as the

director dynamics in nematic

6-25

and smectic liquid crystals.

26-29

In more

recent years deuterium NMR spectroscopy, combined with continuum

theory, has been applied successfully to investigate the static director

distribution in thin nematic liquid crystal cells, with different film

thicknesses and different surface anchoring strengths, subject to both

magnetic and electric fields.

11-13

Deuterium NMR spectroscopy has also

been employed to investigate the dynamic director alignment process in a

thin nematic film following the application or removal of an electric

field.

14-25

This technique has the added advantage that the presence of the

magnetic field of the spectrometer ensures that during the electric

field-induced alignment the director rotates as a monodomain,

which

facilitates the analysis of the results, except for the case when the

angle made by the magnetic field with electric field is close to a right

angle.

The rotational viscosity coefficient,

, for the director is one of the

central quantities in determining the response times of liquid crystal

display devices based on nematics.

A particularly apposite method with

which to determine

is to generate a non-equilibrium state by applying

a field at some orientation to the director and then to monitor this as the

system returns to equilibrium.

For example, for the alignment of a

nematic with a positive diamagnetic anisotropy,

∆

, by a magnetic field,

Time-Averaged Deuterium NMR Studies of the Dynamic Properties 195

the director orientation,

, with respect to the field, is predicted

to vary

with time according to

tan ( ) tan exp ,

θ θ

 

= −

 

 

(1)

in the absence of other external constraints. Here,

is the angle made by

the director with the field at time zero and the magnetic relaxation time is

given by

1 0

γ µ

∆

(2)

where

is the magnetic constant and B is the magnetic flux density.

One powerful method by which the director orientation can be monitored

is deuterium NMR spectroscopy and in experiments using this technique

the magnetic field causing the director alignment is provided by the

spectrometer.

Such experiments have been extended by using an electric

field both to create the original non-equilibrium state of the system and

to drive the director alignment.

The dynamics of the changes in the

director orientation are given, within the framework of the Leslie-

Ericksen hydrodynamic theory, by the torque-balance equation

2 2

sin2 sin2( ).

2 2

B E

θ χ ε ε

γ θ α θ

∆ ∆

= − + −

(3)

Here

is the angle between the magnetic and electric fields,

∆

is the

dielectric anisotropy and

is the vacuum permittivity. This equation

ignores the elastic terms which is justified because in our experiments

the director remains uniformly aligned; in addition, the inertial term is

ignored. The director orientation varies according to Eq. (1) when the

electric field is switched off; when the electric field is switched on and

provided

∆

is positive the orientation now changes according to

tan( ( ) ) tan( )exp .

θ θ θ θ

∞ ∞

 

− = − −

 

 

(4)

Here,

∞

is the limiting value of the director orientation as t goes to

infinity and the composite relaxation time,

, is given by

2 1 1 2 1/2

( 2 cos2 ) ,

M M E E

τ τ τ τ α τ

− − − − −

= + + (5)

A. Sugimura and G. R. Luckhurst 196

where

is the relaxation time for the pure electric field-induced rotation

of the director

ε ε

∆

(6)

The experiments combining both electric and magnetic fields have the

advantage that the ratio of anisotropic susceptibilities,

ε χ

∆ ∆

ɶ ɶ

, is readily

measured and so if

∆

is available independently then the rotational

viscosity can be determined from the relaxation times.

5,14

In the experimental investigations it is usual to employ a high

frequency electric field, typically several kHz, to ensure that disruption

of the director orientation by conductive motion of ions is eliminated.

However, in the torque-balance equation (see Eq. (3)), the electric field is

taken to be constant. Clearly the periodic variation in the square of the

electric field will influence the director dynamics, thus as the electric

field increases so the relaxation time, given by Eq. (5), will decrease

caused by a reduction in

and as the field decreases so

will increase

caused by an increase in

. In addition, the direction of the director

alignment will be reversed when the electric field starts to decrease;

however, the precise moment when this occurs will depend on the

frequency of the electric field in comparison with the composite

relaxation rate,

−

. The importance of these variations to the validity of

the analysis, based on the assumption that the magnitude of the electric

field is constant, will also depend on the frequency of the electric field

and the composite relaxation rate for the director. Here, we consider this

question both theoretically and experimentally. As we shall see such

considerations have lead us to a novel variation in the NMR method used

to study field-induced director dynamics and hence determine certain

nematic properties.

The layout of this chapter is the following. In the next section we give

the theoretical background to the dynamic alignment of the nematic

director subject to a static magnetic field and a sinusoidal electric field;

from this we calculate the time dependence of the director orientation, in

particular as a function of frequency. In Sec. 3 we describe the NMR

experiments that have been performed. The results are presented in the

following section where their significance is discussed and comparison is

Time-Averaged Deuterium NMR Studies of the Dynamic Properties 197

made with an analogous experiment in which the nematic is spun about

an axis orthogonal to the magnetic field. The outcomes of this study are

given in Sec. 5 together with our conclusions.

2. Theory

With a sinusoidal electric field the E

term in the torque-balance equation

must be replaced by

2 2 2

sin 2 ,

E E ft

(7)

where

f is the frequency of the sinusoidal field. In addition, the dielectric

anisotropy will also depend on

f but for the frequency range and low

molar mass materials with which we are concerned it is a good

approximation to ignore this dependence.

The torque-balance equation

can then be written in terms of generalised or scaled coordinates as

2 * *

sin 2 sin 2 sin 2( ) .

2 2

f t

θ ρ

θ π α θ

= − + −

(8)

The generalised time coordinate is given by

(9)

where the magnetic relaxation time,

, is given by Eq. (2), the

generalised frequency is

−

(10)

and the parameter

is the ratio of electric to magnetic anisotropic

energies

0 0

ρ µ ε

 

∆

 

∆

 

(11)

It depends, therefore, on the material property,

ε χ

∆ ∆

ɶ ɶ

, and the

experimental parameter,

/B. For a given value of

, the angle between

the two fields, the time dependence of the director orientation will

be determined by

and the scaled frequency,

To obtain this

A. Sugimura and G. R. Luckhurst 198

dependence we have solved the generalised torque-balance equation

numerically using a finite difference method with the time step chosen

according to the procedure described by Killian and Hess.

The results of these calculations are shown in Fig. 1 for

equal to

44.7°, the angle used in the majority of our experiments, and which

removes the degeneracy in the alignment pathway for the director found

for

= 90°,

3,23

that is the director moves uniformly and the angle made

with the aligning field decreases. The ratio

was given the value of

3.47 which is typical of 4-pentyl-4

′-cyanobiphenyl (5CB) and the field

strengths used in our experiments. The director was taken to be parallel

to the magnetic field prior to the application of the electric field. The

highest scaled frequency used in the calculations was 15.4 and to place

this in context we note that when

is of the order of ms the frequency

corresponding to this value of

is about 15 kHz. As we can see, for

this value of

the angle

increases from zero and then levels off,

apart from an extremely weak, barely discernible, high frequency

oscillation superimposed on this curve. The time dependence of the

director orientation is indistinguishable from that calculated from Eq. (4)

with

set equal to zero and the limiting director orientation,

∞

given,

in terms of the variable,

2 1/2

2 cos2

cos2 .

(4 4 cos2 )

ρ α

ρ α ρ

∞

+ +

(12)

(t*)/

f*=0.154

f*=1.54

f*=15.4

0 2500 5000 7500 10000

Fig. 1.

The scaled time dependence of the angle made by the director with the magnetic

field,

(t*), calculated with

= 44.7° and

= 3.47 for f* equal to 15.4, 1.54 and 0.154.