RIKEN KEIKI CO., LTD1 Photo-electron spectrometer in air Model AC-2 The counting mechanism of the...

RIKEN KEIKI CO., LTD 1

Photo-electron spectrometer in airModel AC-2

The counting mechanism of the photoelectron

and

The application to studies of

The Organic Light Emitting Diode (OLED)


5. Conclusion

4. The application to the OLED The measurements of the IPs and WFs of the Organic materials.

3. The data analysis What do the photoelectron spectrums mean?

2. The mechanism of counting photoelectrons How does the open counter detect the electrons in the air?

1. The outline of AC-2 applications, features

Contents





5. Conclusion


Contents





5. Conclusion


1. Outline ofPhoto-Electron Spectrometer in Air (PESA)

Light source part Measuring part Personal computer

More than 170sets are used in Japan and world market.

MODEL AC-2

120cm45cm

36cm

Mainly applications •Material research of the OLED .•The quality check of an ITO cleaning.•The surface research of an MgO film for the PDP.

Latest applications •Organic transistor•Organic solar battery•Catalyst of fuel cell


The features of AC-2

•Measurements can be done in the air. Usually a photoelectron spectrometer needs a vacuum.

Because it is very difficult to detect and to count electrons in air.

•Easy operation & short measuring time.

•The work function and ionization potential can be

measured in the air in very high resolution.

•Measurement of contamination or film thickness on

a sample surface of 1mono layer-20nm.


Contents


2. The mechanism of counting photoelectrons We employed the open counter for the detector of AC-2.

3. The data and the analysis What do the photoelectron spectrums mean?


5. Conclusion


UV Light

Sample

HV

Pulse counter

Vacuum chamber

A photoelectron is detected as an electric pulse.

Conventional methods for detecting electrons

1pA (10-12A )

6.24x106cps (62 million/s)

Photoelectrons are detected as quite small amount of

electric current.

Sample

UV light

Collector

e

e

Picoammeter Channeltron

e

A pulse of 10 million electrons

The electrons strike the channel walls and produce additional electrons.

This method is not sensitive enough for photoelectron spectroscopy.

This device works only in a vacuum.


UV Light

Quenching GridSuppresser Grid　

Anode Suppresser circuit

Quenching circuit

High Voltage Supply

Scaling circuit and Rate meter

Preamplifier

Sample holder

Sample

O2

e

The open counter

The open counter is employed the photoelectron spectrometer in air. The open counter is unique counter which can detect and count photoelectrons in the air.

e

Display

1 count

Reference: H.Kirihata, and M.Uda; Rev. Sci. Instrum. 52 (1981) 68.

Open counter was invented in 1979 by Uda and Kirihata.


Anode

Quenching GridSuppresser Grid

Structure

The open counter is composed of an anode, a quenching grid, suppresser grid and electric circuits.

Suppresser circuit

Quenching circuit

High Voltage Supply


Preamplifier

Display

0 count


14mm

80mm




Quenching circuit

High Voltage Supply


Preamplifier

Sample holder

Sample

The test sample is mounted on the sample stage which is kept at earth potential (0V).

Suppresser circuit

Quenching circuit

High Voltage Supply


Preamplifier

Display

0 count

Structure Reference: H.Kirihata, and M.Uda; Rev. Sci. Instrum. 52 (1981) 68.

15mm




Quenching circuit

High Voltage Supply


Preamplifier

Sample holder

Sample

+80V

+100V

+2900V

Suppresser Grid is kept at +80V, Quenching Grid is +100V, Anode is +2900V.

Display

0 count

Structure Reference: H.Kirihata, and M.Uda; Rev. Sci. Instrum. 52 (1981) 68.


UV Light



Quenching circuit

High Voltage Supply


Preamplifier

Sample holder

Sample

+80V

+100V

+2900V

Measurement

Monochromatized UV photons are used to excite photoelectrons from the sample surface.

Display

0 count



UV Light



Quenching circuit

High Voltage Supply


Preamplifier

Sample holder

Sample

+80V

+100V

+2900V

Electrone

If the energy of an UV photon(=h) becomes bigger than a work function of a sample, a photoelectron is emitted from the sample surface.

Display

0 count

Measurement Reference: H.Kirihata, and M.Uda; Rev. Sci. Instrum. 52 (1981) 68.


UV Light



Quenching circuit

High Voltage Supply


Preamplifier

Sample holder

Sample

+80V

+100V

+2900V

Electrone

The electron is accelerated by a weak electric field applied between the sample (0V) and the suppresser grid kept at +80V.

Display

0 count



Sample surface

O2

N2

N2

N2

N2

N2

N2

N2

N2

N2

N2

N2

N2

N2

N2

O2e

N2

N2N2

O2

N2

N2

O2-ion

The electron becomes attached to an oxygen molecule to form O2-

ion in the air during drift to the counter.

Form the O2- ion



UV Light



Quenching circuit

High Voltage Supply


Preamplifier

Sample holder

Sample

+80V

+100V

+2900V

O2

e

When an O2- ion reaches the inner cylinder of the open counter, the ion is accelerated again by a

strong electric field applied between the quenching grid ( +100V) and the anode kept at +2900V.

Display

0 count



Anode surface

O2

O2

O2

N2

N2

N2

N2

N2

N2

N2

N2

N2

N2

N2

N2

N2N2

N2

N2

N2

N2

O2

N2N2 N2

O2

Electric Field

N2

N2

N2N2 O2

e

O2-ion

O2

e

The electron avalanche

When the O2- ion arrives near the anode, the electron is detached from the O2

- ion and then the electron is accelerated again to the anode.



Anode surface

O2

O2

O2

N2

N2

N2

N2

N2

N2

N2

N2

N2

N2

N2

N2

N2N2

N2

N2

N2

N2

O2

N2N2 N2

O2

Electric Field

N2

N2

N2N2 O2

e

O2-ion

O2

e+

ee+

+

++

+

++

++

+

ee e e ee e e e ee e

The electron avalanche

This causes an electron avalanche, which produces many electrons and positive ions around the anode wire, where only the electrons are collected on the anode.



UV Light



Quenching circuit

High Voltage Supply


Preamplifier

Sample holder

Sample

+80V

+100V

+2900V

The electron avalanche makes a small electric pulse on anode. This pulse is detected and counted as one electron.

Display

0 countDisplay

1 count



Quenching Grid

UV Light

Suppresser Grid　


Quenching circuit

High Voltage Supply


Preamplifier

Sample holder

Sample

+2900V

Quenching Grid

+400V

When the quenching circuit detects the electric pulse, +400V is applied in place of +100V to the quenching grid.

Display

1 count



Quenching Grid

UV Light

Suppresser Grid　


Quenching circuit

High Voltage Supply


Preamplifier

Sample holder

Sample

+2900V

Quenching Grid

+400V

This reduction of the electric field around the anode causes the electron avalanche to be quenched.

Display

1 count



Suppresser Grid　UV Light

Quenching Grid


Quenching circuit

High Voltage Supply


Preamplifier

Sample holder

Sample

Suppresser Grid　

-30V

+400V

+2900V

On the other hand, -30V is applied in place of +80V to the suppressor grid.

Display

1 count




Quenching Grid


Quenching circuit

High Voltage Supply


Preamplifier

Sample holder

Sample

Suppresser Grid　

-30V

+400V

+2900V

Ion

This voltage switch prevents leaving of the positive ions to the sample surface, and entering of the next O2

- ions into the counter during quenching.

Display

1 count




Quenching Grid


Quenching circuit

High Voltage Supply


Preamplifier

Sample holder

Sample

Suppresser Grid　

-30V

+400V

+2900V

3ms

Such voltages applied to the quenching grid and suppressor grid are kept for 3msec i.e. the quenching time.

Display

1 count




Quenching Grid


Quenching circuit

High Voltage Supply


Preamplifier

Sample holder

Sample

Suppresser Grid　

-30V

+400V

+2900V

3ms

All positive ions produced around the anode wire arrive and neutralize at the quenching grid or suppressor grid during quenching time.

Display

1 count




Quenching Grid


Quenching circuit

High Voltage Supply


Preamplifier

Sample holder

Sample

Suppresser Grid　

-30V

+400V

+2900V

3ms

If the next electron has emitted from the sample surface, the suppresser grid prevents entering of the electron during quenching time.

Display

1 count



UV Light



Quenching circuit

High Voltage Supply


Preamplifier

Sample holder

Sample

+80V

+100V

+2900V

After the quenching time (3ms), the initial voltages (+100V and +80V, respectively) are restored and the quenching procedure is over.

Display

1 count



UV Light



Quenching circuit

High Voltage Supply


Preamplifier

Sample holder

Sample

O2

e

Now the counter is ready to count the next electron.

e

Display

1 countDisplay

2 count



The calculation of counts

Electron counts may be lost during the quenching time. The counting rates of electrons are estimated based on the counting rates of counter pulses by calculation.

NO

1-tNO

N =

NO: Counting rate of counter pulses

N : Counting rate of photo electronst :Quenching time



Measuring part

open counter

Light source part

grating monochromator

deuterium lamp

e

: UV light : photoelectrone

sample

sample stage

Structure and functions of AC-2

open counter controller

personal computer

optical fiber

deuterium lamp


open counter

3.4eV 6.2eV photodiode


Measuring part

open counter

Light source part


deuterium lamp

: UV light : photoelectrone

sample stage

Structure and functions of AC-2

open counter controller

personal computer

optical fiber

deuterium lamp


open counter

3.4eV 6.2eV photodiode


Contents





5. Conclusion


Yield : counting rate after calibtation

counting rate of photoelectrons (E) / intensity of UV-ray (E) x intensity of UV-ray (5.9eV)

(Yie

ld[c

ps]

)1/2

Incident Photon Energy [eV]

Photoelectron Spectrum

Photoemission Threshold Energy[eV]

Metal : the relationship between the photon energy and yield1/2 looks like a linear line. Semiconductor : yield1/3 gives a linear line.


The relationship between the threshold energy and the energy diagrams

Conduction Band Valence Band Fermi Level

The energy level diagrams of metals, semiconductors and general materials

Energy

Vacuum level

General materialMetal Semiconductor

Lowest unoccupied molecular orbital (LUMO) Highest occupied molecular orbital (HOMO)




Energy

Vacuum level



A work function is an energy difference between a vacuum level and a Fermi level. On the other hand, the ionization potential is an energy difference between a vacuum level and highest occupied molecular orbital.

Ionization potential



Work function Work function Work function




Energy

Vacuum level



A UV photon excites an electron from the occupied states to the higher energy states than the vacuum level. And this electron can be emitted from the sample surface. The electron is called the photoelectron.

Photoelectrone eeUV photon




Energy

Vacuum level



Therefore, the photoemission threshold energy is the ionization potential.




PhotoelectronUV photon e ee




Energy

Vacuum level





The metal is special material. Because, the energy of highest occupied molecular orbital and the Fermi level are same. Therefore the photoemission threshold energy of a metal is the work function.


Metal

Work function

PhotoelectronUV photon e ee

We can estimate the work functions or ionization potentials of the materials from the photoemission threshold energy.


The shape of a photoelectron spectrum

Potential energy

DOS

Reference: M.Uda, Y.Nakagawa, T.Yamamoto, M.Kawasaki, A.Nakamura, T.Saito, and K.Hirose ”Successive change in work function of Al exposed to air”, J. Electron. Spectrosc. and Related Phenom. 88 (1998) 767.

Energy level diagramE

nergy

Vacuum level

unoccupied molecular orbital occupied molecular orbital


Potential energy

Energy level diagram

DOS

the relationship between an energy level diagram and the photoelectron spectrum

Reference: M.Uda, Y.Nakagawa, T.Yamamoto, M.Kawasaki, A.Nakamura, T.Saito, and K.Hirose ”Successive change in work function of Al exposed to air”, J. Electron. Spectrosc. and Related Phenom. 88 (1998) 767.

Photoemission yield (Y)

Photoelectron spectrum

Photon energy

0

dYdE=

The DOS is estimated by the differential of the photoelectron spectrum with respect to the photon energy E.


WFf> WFs

The difference of a photoelectron spectrum caused by the thickness of a surface film

Contaminationor

Oxide film(0-20nm)

Substrate

Incident photon (E)

WFf

WFs

-

--

-

--

--

The cross section of the sample surface covered with a thin film

WFf> E > WFs


WFf> WFs


Contaminationor

Oxide film(0-20nm)

Substrate

Incident photon (E)

WFf

WFs

-

--

-

--

--

When photoelectrons pass through a surface film, some electrons collide with a molecule forming the surface film, and the photoelectrons are scattered.So some of photoelectrons can not escape from the sample surface.

WFf> E > WFs


WFf> WFs


Contaminationor

Oxide film(0-20nm)

Substrate

Incident photon (E)

WFf

WFs

-

--

-

--

--

So, when the surface film is thick, many photoelectrons can not be emitted from the sample surface.

WFf> E > WFs


The relationship between the slope and film thickness

Large slope

Thin contamination film-

--

-

--

- -

(Yie

ld[c

ps]

)n


-

--

-

--

- -Thick contamination film

(Yie

ld[c

ps]

)n

Small slope

WFf WFs

WFf

WFs

WFf

WFs

- -

-

--

-

--

- -


Contents





5. Conclusion


Glass SubstrateITO transparence anode

Organic layers

Metal cathode

The applications to the OLEDs

+

-

OLED device

Photon

WF

Ionization Potential

HOMO

ITO

Vacuum Level

Organic

LUMO

Fermi Level

Energy diagram of ITO and Organic Layer

HOMO

BarrierFermi Level

Hole

+


Work functions of several metals

In Air 1)[eV] In UHV2) [eV]Fe 4.35 4.50Ni 4.25 5.15Cu 4.45 4.65Al 3.60 4.20Zn 3.80 -Au 4.78 5.101) M. Uda ; Jpn. J.Appl.Phys. 24,284 (1985)

2) D.E. Eastman ; Phys.Rev. B2, 1 (1970)


Work functions or Ionization potentials of EL materials

AC-2 [eV] UPS 1) [eV]

Alq3 5.84 5.8

-NPD 5.50 5.4

CuPc 4.99 5.2

1) I.G.Hill and A.Kahn, J.Appl.Phys. 86,4515 (1999)


4.7

4.9

5.1

5.3

5.5

0 10 20 30 40 50 60duration time after treatment (min)

Wor

k f

un

ctio

n (

eV)

before cleaningUV-ozone 10min

UV-ozone 20min

UV-ozone 60min

UV-ozone 0min (boiling in IPA )

The temporal change in the work function of ITO treated with UV-ozone

Reference: Y. Nakajima, T. Wakimoto, T. Tuji, T. Watanabe, M.Uda, The 10th International Workshop on Inorganic and Organic Electroluminescence (2000.12.4), Hamamatu, Japan.


Change in WF of Al by Cl2 Contamination

Al in air : 4.05eV

Al exposed mixed gas 20sec

(Cl21.34ppm + air) : 4.33eV

(Yie

ld[c

ps]

)0.5



5. Conclusion

1. We can detect and count photoelectrons in the air by the open counter.

2. We can measure the work function or the ionization potential of the OLED materials easily.

3. We can estimate the amount of the contamination on the ITO surface from 1mono-layer to 20nm in the thick.

4. AC-2 is the de facto standard equipment of the work function measurement on the OLED development.


References

[1] H.Kirihata, and M.Uda “Externally quenched air counter for low-energy electron emission measurements”, Rev. Sci. Instrum. 52 (1981) 68.

[2] M.Uda “Open counter for low energy electron detection”, Jpn. J.Appl. Phys. 24 (1985) 284.

[3] T. Noguch, S. Nagashima and M. Uda “An electron counting mechanism for the open counter operated in air” , Nucl. Instr. Meth. A342 (1994) 521.

[4] S. Nagashima, T. Tsunekawa, N, Shiroguchi, H. Zenba, M. Uda “Double cylindrical open counter of pocket size”, Nucl. Instr. Meth. A373 (1996) 148.

[5] A. Koyama, M. Kawai, H. Zenba, Y. Nakajima, A. Yoneda and M. Uda “Electron counting by a double cylindrical open counter in mixtures of N2 and inert gases of various concentrations” , Nucl. Instr. and Meth. in Phys. Res. A422 (1999) 309.


References

[6] M.Uda, Y.Nakagawa, T.Yamamoto, M.Kawasaki, A.Nakamura, T.Saito, and K.Hirose ”Successive change in work function of Al exposed to air”, J. Electron. Spectrosc. and Related Phenom. 88 (1998) 767.

[7] Y. Nakajima, M. Hoshino, D. Yamashita and M. Uda “ Near Edge Structures of Tetraphenylporphyrins Measured by PESA and Calculated with DV-Xα” , Adv. Quantum Chem. 42 (2003) 399.

[8] Y. Nakajima, T. Wakimoto, T. Tuji, T. Watanabe, M.Uda “Measurements of the work function of ITO Using an electron spectroscopy in air and a contact potential difference method” ,The 10th International Workshop on Inorganic and Organic Electroluminescence (2000.12.4), Hamamatu, Japan.

RIKEN KEIKI CO., LTD1 Photo-electron spectrometer in air Model AC-2 The counting mechanism of the...

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