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

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Surface Plasmon Excitations and Emission Lights in Organic Films 259

The dispersion property of SP represents a relation between the

wavenumber k

of SP in the x-direction as shown in Fig. 14 and

the angular frequency

ω of the wavelength λ. k

and ω are given by the

following equations:

= n

ksin

= n

(2π/λ)sin

(4)

ω = kc = (2π/λ)c (5)

where n

is the refractive index of the prism, k (= 2π/λ) is the

wavenumber of light in air and c is the light velocity in air. For the ATR

Fig. 16. Relation between emission intensities and separations of MC LB layers from metals.

(a) (b)

Fig. 17. Spectra of emission lights (a) and dispersion properties (b). Emission lights were

measured for Ag/C20(2 layers)/MC(16 layers) sample at various emission angles. The

dispersion properties were calculated from ATR and emission light properties.

600 700

Wavelength [nm]

Photon count [a.u.]

θθ

=50º

600 700

Wavelength [nm]

Photon count [a.u.]

θθ

=50º

Al Ag

Number of Layers

Normalized light intensity

1L 5L 9L 13L

Al Ag

Number of Layers

Normalized light intensity

1L 5L 9L 13L

K. Kato 260

properties,

is the resonant angle of SP and λ is the wavelength of the

incident light. For the emission light measurement,

is the angle of

the peak intensity of emission spectrum and

λ is the peak wavelength.

Figure 17(b) shows the dispersion properties. Closed and open circles

indicate the properties obtained from the ATR and the emission light

measurements, respectively. The dispersion property of the emission

lights agreed well with that of the ATR measurements. It was thought

that multiple SPs were simultaneously excited by the reverse irradiation

and the emission light was generated due to the dispersion property of SP

in the Kretschmann ATR configuration of the prism/Ag/C20/MC LB

film. Similar emission lights through the prism are reported for the

Ag/Rhodamine-B (RB) LB film by the resonant excitations of SPs in the

conventional ATR method and the calculated dispersion properties also

coincide with one of the ATR measurements.

Figure 18 shows the emission light for luminescent molecular films

due to SP excitation. The SP emission lights involve the following

processes. The excitation of SPs directly occurs by the excited MC

molecules and/or through the surface roughness, and then the light

emission is coupled with the excited SPs through the ATR prism, where

the SP emission is influenced largely by the film properties with the MC

molecules and/or by the metal surface roughness. It is tentatively

estimated that small emissions are due to non-radiative energy transfer or

charge transfer

from the excited MC molecules to the metal thin film in

the small separations and due to smaller induced charges on the metal

thin film in the large separations. The electromagnetic modes localized at

Fig. 18. Emission light for luminescent molecular films due to SP excitation.

Evanescent wave

Emission light

Surface plasmon

Incident light

Molecule (excited)

Metal

Prism

Near-field Interaction

Evanescent wave

Emission light

Surface plasmon

Incident light

Molecule (excited)

Metal

Prism

Near-field Interaction

Surface Plasmon Excitations and Emission Lights in Organic Films 261

the surface are induced by dipoles with electromagnetic waves and have

suggested that the dipole-surface interaction will be very important for

device applications utilizing near-field optics. It is thought that the

phenomenon is very useful for nanostructured device applications.

4.2. SP Emission Lights due to Molecular Luminescence

and Interaction

SP emission lights have been investigated for rhodamine-B (RB)

LB films. RB is a photosensitizing organic dye showing strong

photoluminescence. Figure 19 shows the emission lights as a function

of emission angle for the prism/Ag/C20/RB LB films with various

separations between the Ag and the RB LB films under the reverse

irradiation with two polarized laser beams.

The SP emission lights in

Figs. 19(a) and 19(b) strongly depend upon the thickness of the

separations and show the maxima with the separation of 8 layers. The

properties depending upon the separation were similar to that of the

photoluminescence. The emission peaks by the p-polarized laser at 0°

with the electric fields parallel to the observation plane were much larger

than those by the s-polarized one at 90° and the peaks by the s-polarized

beam were approximately 15% of those by the p-polarized one.

(a) (b)

Fig. 19. Emission light for the RB LB film using p-polarized light (a) and s-polarized

light (b) at 488.0 nm under reverse irradiation.

40 50 60 70

0.01

0.02

12layers

Emission light intensity [arb.units]

λ =488.0 [nm] RB C20=1 5

Emission angle [deg]

0layers

4layers

8layers

40 50 60 70

0.001

0.002

0.003

Emission light intensity [arb.units]

λ =488.0 [nm] RB C20=1 5

Emission angle [deg]

12layers

8layers

4layers

0layers

K. Kato 262

Since orientations of the RB dyes were thought to be almost uniform

in the LB film plane, it is tentatively estimated that some RB dyes having

the long axis parallel to the polarized plane of the laser were mainly

excited by the reverse irradiation, and these excited dyes induce

anisotropic SPs which propagate along the Ag surface, which can be

observed mainly in the observation plane of the half-cylindrical prism.

Figure 20 shows the SP emission due to p- and s- polarized laser beams

in the reverse irradiation. Anisotropic molecules are excited mainly by

the polarized laser beam with the electric fields parallel to the molecular

transition moment. The emission phenomenon includes conversions from

three- to two-dimensional optical waves and those from two to three-

dimensional ones. It is thought that the SP emission properties depending

upon the very short separation of the RB from the Ag are very useful for

conversion between three- and two-dimensional optical waves.

Figure 21 shows the emission light curves as functions of the

emission angle

for the MC LB film (16 layers) and the MC/CV hetero-

LB film (16 layers) due to the reverse irradiation of p- or s-polarized

laser beams at 488.0 nm.

For the MC and MC/CV hetero-LB films, the

peak angles did not depend on the polarization direction of the excitation

light. However, the emission light intensity dependence on polarization

direction differed between the films. The SP emission light intensity was

high for the p-polarized excitation light of the MC LB film, as shown in

Fig. 21(a). For the emission light curves of the MC/CV hetero-LB film,

the light intensity did not depend on the polarization direction of the

Fig. 20. SP emission due to p- and s- polarized laser beams in the reverse irradiation.

s-pol.

p-pol.

SPSP

Emission light

Prism

Incident lightIncident light

LB film

Excited Molecule

S-polarized laser P-polarized laser

s-pol.

p-pol.

SPSP

Emission light

Prism

Incident lightIncident light

LB film

Excited MoleculeExcited Molecule

S-polarized laser P-polarized laser

Surface Plasmon Excitations and Emission Lights in Organic Films 263

excitation light. The spectra of SP emission lights strongly depend on

Figure 22 shows the emission light due to SP excitations via energy

transfer between molecules.

4.3. Application of SP Emission Lights to Organic Devices

Figure 23 shows the emission lights measured at 488.0 nm from the

prism/Ag thin film/ Aluminum phthalocyanine chloride (AlClPc) thin

film structure.

The intensity of the emission light is normalized as the

(a) (b)

Fig. 21. Emission light intensity curves of MC LB film (a) and MC/CV hetero-LB films

(b) observed using p- and s-polarized excitation lights.

Fig. 22. Emission light due to SP excitations via energy transfer between molecules.

Incident light

Energy Transfer

Near-field Interaction

Molecule 2

Metal

Prism

Molecule 1

Evanescent wave

Emission light

Surface plasmon

Incident light

Energy Transfer

Near-field Interaction

Molecule 2

Metal

Prism

Molecule 1

Evanescent wave

Emission light

Surface plasmon

20 40 60 80

Emission angle

[deg.]

Light intensity [a.u.]

Ag/C20(2L)/MC(16L)

p–pol.excitation

s–pol.excitation

40 60

p–pol.excitation

s–pol.excitation

Emission angle

[deg.]

Light intensity[a.u.]

Ag/C20(2L)/(MC:2L/CV:2L)4L

K. Kato 264

peak intensity of the Ag thin film is unity. The peak angles of the

emission light curves correspond to resonant angles of ATR curves,

which suggests that the emitted lights were due to SP excitations. The

intensity of SP emission light through the prism strongly depends on the

nanostructures such as surface roughness of the organic and metal thin

films in the Kretschmann configuration. Figures 24(a) and 24(b) show

the AFM images of the as-deposited AlClPc thin film on Ag thin

film and the film treated with ethanol vapor for 10 min, respectively.

Themean roughnesses R

evaluated from the AFM measurement were

1.6 nm and 9.7 nm for the as-deposited and ethanol-vapor-treated AlClPc

thin films, respectively. The surface roughness of the AlClPc thin

film was found to increase markedly with ethanol-vapor treatment. The

increase of 8.1 nm in the surface roughness of the film due to the

ethanol-vapor treatment is extremely large against the thickness of about

10 nm for the as-deposited film. Therefore, the result of the SP emission

lights in Fig. 23 was considered to be caused by the large surface

roughness of the AlClPc thin film due to the treatment with ethanol

vapor. The SP emission light due to molecular luminescence is also

applied to the sensing devices using poly(vinyl alcohol) films with

fluorescent microsphers.

Fig. 23. SP emission lights measured at 488.0 nm from the prism/Ag thin film/aluminum

phthalocyanine chloride (AlClPc) thin film structure. a: as-deposited, b: ethanol-vapor-

treated, c: H

O-vapor-treated, d: H

O-vapor-treated after ethanol-vapor-treated.

40 50 60

Emission angle [deg.]

Normalized Light intensity

40 50 60

Emission angle [deg.]

Normalized Light intensity

Normalized light intensity

Emission angle [deg.]

Surface Plasmon Excitations and Emission Lights in Organic Films 265

The ATR and SP emission lights have been investigated in the

prism/Ag/MgF

/organic dye/MgF

/Ag structure. Poly(2-methoxy-5-

(20-ethyl-hexyloxy)-1,4-phenylenevinylene) (MEH-PPV), which is a

photoluminescent dye, was used as the organic dye. Figure 25(a) shows

the sample configuration.

35,36

The emission light properties for the

(a) (b)

Fig. 24. AFM images of AlClPc thin films on Ag thin films before and after the treatment

with ethanol vapor. (a) and (b) show the results for the asdeposited and ethanol-vapor-

treated AlClPc thin films, respectively.

(a) (b)

Fig. 25. Sample configuration of ATR and SP emission light measurements for the

Ag/MgF

/MEH-PPV/MgF

/Ag structure (a) and the emission light properties for the

Ag/MgF

/MEH-PPV/MgF

/Ag sample measured at 488.0 nm by the usual and reverse

irradiations (b).

reflected light

(ATR signal)

MgF

p-polarized incident light

cover glass

half-cylindrical prism

MEH-PPV

p-polarized incident light

for reverse irradiation method

emission light

0.2

0.4

0.6

Emission angle [deg.]

Emission light intensi

ty [a.u.]

usual (θ

= 0°)

reverse

usual (θ

= 72.0°)

K. Kato 266

prism/Ag(25 nm)/MgF

(100 nm)/MEH-PPV(40 nm)/ MgF

(60 nm)/

Ag(40 nm) structure are shown in Fig. 25(b).

The emission light

measured at the incident angle

of 72.0° of SP excitation had the largest

intensity. The emission angle corresponded to the resonant angle of the

ATR curve measured at around the photoluminescence wavelength of

the MEH-PPV dye. It was found that the emission light intensity and the

spectra strongly depended on the emission angles and the emission light

was also related to the photoluminescence of the organic dye. The SP

emission lights can be controlled by the inserted dye layer and are useful

to device applications.

4.4. Electrochemical SP Excitations and Emission Lights

SP excitation and the emission light properties are investigated in the

doped and dedoped states of regioregular poly(3-hexylthiophene-2,5diyl)

(P3HT) thin films.

Electrochemical experiments were performed in a

conventional three-electrode cell with the Au/SFL11 glass substrate as

the working electrode, a platinum wire as counter electrode, and an

Ag/Ag

nonaqueous reference electrode in acetonitrile solution with 0.1 M

tetrabutylammonium hexafluorophosphate (TBAPF

) as a supporting

electrolyte. The experimental setup for the electrochemical ATR and

emission light measurements utilizing the SP excitations is shown in

Fig. 26.

A Kretschmann configuration of highly refractive prism

(SFL11)/Cr/Au/P3HT thin film/acetonitrile solution containing an

electrolyte (TBAPF

) structure was used for the measurements. SPs were

excited at the metal–dielectric interface, upon total internal reflection of

polarized light from a He–Ne laser with the wavelength of 632.8 nm. The

optical/electrochemical processes at the Au thin film were detected by

monitoring the reflectivity as a function of the incident angle. The SP

emission light was obtained by the irradiation of Ar

laser beam with

thewavelength of 488.0 nm. The P3HT thin filmwas luminous upon the

light irradiation, and excited SP emission light was measured. A sharp-

cut filter that eliminates incident light was used for the measurement of

SP emission light intensity. The SCF enables the observation of only

SP emission light due to excited fluorescent organic molecules.

A photomultiplier tube was used for detecting the emission light.

Surface Plasmon Excitations and Emission Lights in Organic Films 267

A constant potential was applied to the P3HT thin film and the change of

SP emission light properties was observed upon doping and dedoping of

the P3HT thin film. The SP emission light is observed due to the

molecular luminescence of P3HT.

The electrochemical ATR properties of the P3HT thin film applied at

constant potentials are shown in Fig. 27(a) for the doped-state P3HT thin

film.

It is found that the dip of the ATR curve becomes deeper with

increase of the applied potential and the resonant angle changes with the

doping of the P3HT thin film. From the ATR properties, the dielectric

constants of the P3HT films are found to change remarkably with the

doping. On the contrary, the ATR curve hardly changes with the

dedoping. These ATR properties correspond to the optical absorption

properties well.

Figure 27(b) shows the electrochemical SP emission light properties

of the P3HT thin film applied at constant potentials.

The SP emission

light in the undoped state is observed at the emission angle of around

52°. The intensity of the SP emission light decreases in the doped state at

0.2 V. At 0.4 V, the SP emission light is hardly observed. The emission

light properties correspond to the resonant conditions of SPs in the

Kretschmann configuration, and it is considered that multiple SPs are

induced due to the excitation of P3HT molecules by the incident light

and that the SPs are converted into emission light through near-field

coupling. It is considered that the SP emission light excited by the

(a) (b)

Fig. 26. Experimental setup for the electrochemical measurements of ATR (a) and SP

emission lights (b).

counter electrode

organic thin film

cover glass

incident light

detector

highly refractive prism

electrolysis cell

reference electrode

metal thin film

potentiostat

electrolyte solution

counter electrode

organic thin film

cover glass

incident light

detector

highly refractive prism

electrolysis cell

reference electrode

metal thin film

potentiostat

electrolyte solution

reference electrode

incident light

detector

sharp-cut filter

organic thin film

metal thin film

potentiostat

counter electrode

cover glass

highly refractive prism

Teflon cell

electrolyte solution

reference electrode

incident light

detector

sharp-cut filter

organic thin film

metal thin film

potentiostat

counter electrode

cover glass

highly refractive prism

Teflon cell

electrolyte solution

K. Kato 268

molecular luminescence decreased since the molecular luminescence of

P3HT became weaker at the doped state. On the other hand, since a

strong molecular luminescence of P3HT is obtained in the dedoped state,

it is considered that the SP emission light in the dedoped state increased

as can be seen in Fig. 27(b). The reversible change of the SP emission

light was observed by the doping and the dedoping. The SP emission

light excited by molecular luminescence can be controlled by the control

of doping–dedoping state.

4.5. Grating Coupling SP Excitations and SP Emission Lights

Figure 28 shows the configuration for the diffraction grating SP

excitation. The SP dispersion branches at the interface between the metal

grating and air. These branches are obtained by the following equation:

π 2π

ε (ω) sin

sp px m

k k G m

= + = +

(6)

where

Λ is the diffraction grating pitch, λ is the wavelength, m is the

diffraction orders, and

(ω) is the wavelength-dependent dielectric

(a) (b)

Fig. 27. Electrochemical ATR and SP emission light properties of the doped-state P3HT

thin film applied at constant potentials. (a) ATR properties, (b) SP emission light

properties.

40 60

0.002

0.004

0.006

Emission angle [deg.]

λ = 488.0 nm with SCF

initial state

(undoped state)

doped state (0.2V)

dedoped state (–0.4V)

doped state (0.4V)

Emission light intensity [arb. units]

50 60

0.2

0.4

0.6

0.8

Reflectivity

Incident angle [deg.]

= 632.8 nm

0.2V

0.3V

0.4V

0.6V

0.8V

0.9V

initial state (undoped state)

50 60

0.2

0.4

0.6

0.8

Reflectivity

Incident angle [deg.]

= 632.8 nm

0.2V

0.3V

0.4V

0.6V

0.8V

0.9V

initial state (undoped state)