Electronic density of state in metal/polyimide Langmuir–Blodgett filminterface and its temperature dependenceEiji Itoh and Mitsumasa Iwamoto Citation: J. Appl. Phys. 81, 1790 (1997); doi: 10.1063/1.364035 View online: http://dx.doi.org/10.1063/1.364035 View Table of Contents: http://jap.aip.org/resource/1/JAPIAU/v81/i4 Published by the American Institute of Physics. Additional information on J. Appl. Phys.Journal Homepage: http://jap.aip.org/ Journal Information: http://jap.aip.org/about/about_the_journal Top downloads: http://jap.aip.org/features/most_downloaded Information for Authors: http://jap.aip.org/authors

Downloaded 03 May 2013 to 142.150.190.39. This article is copyrighted as indicated in the abstract. Reuse of AIP content is subject to the terms at: http://jap.aip.org/about/rights_and_permissions

Electronic density of state in metal/polyimide Langmuir–Blodgett filminterface and its temperature dependence

Eiji Itoh and Mitsumasa Iwamotoa)Department of Physical Electronics, Tokyo Institute of Technology, 2-12-1 O-okayama, Meguro-ku,Tokyo 152, Japan

~Received 25 April 1996; accepted for publication 16 November 1996!

Surface potentials of heat-treated polyimide~PI! Langmuir–Blodgett~LB! films deposited on Au,Cr, and Al electrodes were measured in a dark vacuum vessel at various temperatures as a functionof the number of deposited layers. The potential depended on the thickness of PI LB films and thework function of base metal electrode. The spatial charge distribution in PI LB films on variouselectrodes was determined from the relationship between the surface potential and the number ofdeposited layers. Based on this result, distribution of the density of electronic state in PI LB filmswas determined. It was experimentally shown that the electrostatic phenomena in PI LB films at themetal/film interface were explained taking account of surface states which exist within the range of;1 nm from the interface and molecular–ion states which exist in the entire range. Further, it wasfound that distribution of electronic density in states of polyimides was broadened with theincrement of temperature. ©1997 American Institute of Physics.@S0021-8979~97!06904-1#

toec

ymlehmnth

inltengtuo

roe,lldol

ck

olaurree

in-rgeralodeu-

o-weofs.tro-lly,byted

gs.

tron-Rest

aslar

plethe-an-

a

I. INTRODUCTION

Electrostatic phenomena occurring at the metal/insulainterface are fundamentally interesting to the fields of eltronics and electrical engineering.1–7 Over the last few de-cades, the charge exchange mechanism at the metal/polinterface has been one of the most important subjects of etrostatics. In order to clarify the charge exchange mecnism, many investigations have been done for metal/polysystems, and as a result many models have been preseKruppet al.proposed the surface states model assumingpolymers act electronically as semiconductors.4 Duke andFabish proposed the molecular–ion–state model assumthat polymers have a tendency to polarize into electron doand accepting states,5,6 and Gibsonet al. proposed the loca~intrinsic! models assuming that charge exchange is demined by the number of insulator states which can exchacharges with the metals.7 However, these are not sufficienbecause recent progress in the fields of so-called molecelectronics and high-technology demands the informationthe interfacial phenomena on the nanometer scale.

In order to gain a better understanding of the electstatic phenomena occurring at the metal/polymer interfacis essential to use ultrathin films whose thickness is smathan the thickness of the electrostatic double layer formethe interface and to gain information on the distributionthe electronic density of state~D.O.S.! as well as the spatiacharge distribution in films. In our previous studies,8,9 weused polyimide~PI! Langmuir–Blodgett~LB! films with amonolayer thickness of 0.4 nm which is less than the thiness of the electrostatic double layer,10 and investigated theelectrostatic potential across heat-treated PI LB filmsvarious metals as a function of the number of depositeders by means of the conventional surface potential measment. It was found that electronic charges were transfefrom metal to PI LB films at the metal/PI LB film interfac

a!Electronic mail: iwamoto@pe.titech.ac.jp

1790 J. Appl. Phys. 81 (4), 15 February 1997 0021-8979/9

Downloaded 03 May 2013 to 142.150.190.39. This article is copyrighted as indicated in the abstract

r-

erc-a-erted.at

ngor

r-e

larn

-iteratf

-

ny-re-d

until thermodynamic equilibrium was established at theterface. It was also found that the electronic space chalayer was formed at the interface within the range of sevenm from the interface and it depended on base electrmaterials. Further, we preliminarily determined the distribtion of electronic density of states in PI LB films.11

In this paper, for further understanding of the electrstatic phenomena occurring at the metal/film interface,examined the distribution of electronic density of statemetal/polyimide LB films interface at various temperatureWe then presented a model used for explaining the elecstatic phenomena at the metal/PI LB film interface. Finawe explained our experimental electrostatic phenomenausing a simple calculation based on the model presenhere.

II. EXPERIMENT

A. Polyimide LB films

Two kinds of polyimide LB films~denoted as PI, andUPLEX-R!, whose chemical structures are shown in Fi1~a! and 1~b!, were used in this study.8,9 The monolayerthickness of these films was 0.4 nm. PI has a large elecaffinity and a strong tendency to accept electrons. UPLEXhas a smaller electron affinity, whose energy level of lowunoccupied molecular orbital~LUMO! is higher than that ofPI.12 In contrast, the difference in the ionization energywell as in the energy level of highest occupied molecuorbital ~HOMO! between UPLEX-R and PI is very small.

B. Surface potential measurement

Figure 2 shows the electrode configuration of the samused for investigating the electrostatic phenomena atelectrode/PI LB film interface. Polyimide LB films were deposited onto Au, Cr and Al base-electrodes in the same mner as described in our previous papers.8,9 Four glass slidescoated with PI LB films as shown in Fig. 2 were placed in

7/81(4)/1790/8/$10.00 © 1997 American Institute of Physics

. Reuse of AIP content is subject to the terms at: http://jap.aip.org/about/rights_and_permissions

lsn

ofll

vin

umoth

tia

ceehahC/

-Altem-s theed ain-odeLBueeI toin-orken-nti-ones-thedo-eandumchtheesres.er ofsatu-tu-andthan

ncytruc-atu-ofan-

lec-PIce

in

vacuum vessel at the same time, and the surface potentiathese LB films were measured by means of the conventioKelvin probe method~TRek Model 320B!. Humidity andphotoillumination directly give effects on the magnitudethe surface potential of polyimide LB films. Therefore, asamples were heat-treated for more than 1 h at atemperatureof 150 °C in a vacuum of the order of 1026 Torr before thesurface potential measurement for the purpose of remowater molecules adsorbed on the surface of samples andcess charges generated inside of the samples by photoillnation. Subsequently, all samples were cooled down to rotemperature in the period of about half a day. After that,surface potential of polyimide LB films~at position Q indi-cated in Fig. 2! was measured with reference to the potenof the clean base metal electrodes~at position P in Fig. 2! onthe base electrode.8,9 Temperature dependence of the surfapotential was measured at an interval of 25 °C betw2100 and 150 °C on keeping the temperature for more t1 h at each temperature before measurements. Here, theing or cooling rate of the samples was smaller than 0.05 °

FIG. 1. Chemical structure of polyimide molecule used in this study:~a! PIand ~b! UPLEX-R.

FIG. 2. Electrode configuration of the samples used in this study.

J. Appl. Phys., Vol. 81, No. 4, 15 February 1997

Downloaded 03 May 2013 to 142.150.190.39. This article is copyrighted as indicated in the abstract

ofal

gex-i-me

l

enneat-s.

III. RESULTS AND DISCUSSIONS

A. Surface potential of metal/polyimide LB filminterface

Figures 3~a!, 3~b! and 3~c! shows the relationship between the potential across PI LB films on Au, Cr andelectrodes and the number of deposited layers at variousperatures. The surface potentials gradually decreased anumber of deposited layers increased, and then reachconstant saturated potential at 20–50 layers. This resultdicated that PI films acquired electrons from base electrand that the electrostatic layer was formed at the metal/PIfilm interface with a thickness of the order of nm. The valof surface potential in PI LB films shifts negatively with thincrease of temperature, indicating that the tendency of Paccept electrons became stronger as the temperaturecreased. Figure 4 shows the relationship between the wfunction of metals and the saturation value of surface pottial of PI LB films. A linear relationship is observed betweethem. A work function of Au evaporated electrode was esmated to be 4.75 eV from the ultraviolet photo-emissispectroscopy. Work functions of Cr, Al electrodes weretimated using Au electrode as the reference by means ofcontact potential method.8,9,13The difference in the saturatepotential of PI LB films deposited on various electrodes cincided with the difference in the work function of theselectrodes, in a manner similar to that reported in Refs. 910. We therefore concluded that thermodynamic equilibriwas established at the metal/PI LB film interface at eatemperature. Figure 5 shows the relationship betweenpotential across UPLEX-R LB films on Au and Al electrodand the number of deposited layers at various temperatuThe surface potentials gradually decreased as the numbdeposited layers increased, and then reached a constantration value. It should be noted here that the potential sarated when the number of deposited layers was 10–30,the magnitude of saturated surface potential was smallerthat of PI @see Fig. 3~a!, Fig. 5~a!, Fig. 3~c!, and Fig. 5~b!#.These results indicate that UPLEX-R has a smaller tendeto accept electrons than does PI, as we expected. It is instive here to note that the linear relationship between the sration value of surface potential and the work functionmetal was again observed at various temperatures in a mner similar to that shown in Fig. 5~not shown here!.

B. Determination of the distribution of electronicdensity of state

Since excess charges are displaced from metal etrodes, electric flux diverging from the excess charges inLB films falls on the metal electrodes. Therefore the surfapotential built across PI LB films is given by14

VS5E0

D xr~x!

e0e rdx. ~1!

Here,e0 is dielectric permittivity of a vacuum,e r ~53! is therelative dielectric constant of PI,10 D is the film thickness,andx is the distance from metal electrode.r(x) is the spatialcharge density atx5x. In the same manner as describedprevious papers,9,10 differentiating surface potentialVs with

1791E. Itoh and M. Iwamoto

. Reuse of AIP content is subject to the terms at: http://jap.aip.org/about/rights_and_permissions

the

FIG. 3. Relationship between the surface potential of PI LB films andnumber of deposited layers at various temperatures on~a! Au, ~b! Cr, and~c!Al electrode.

l

t tth

s.

rino

LB

respect to the film thicknessD gives a quantity proportionato the spatial distribution of chargesr(D); that is,r(D) isobtained by the change in the surface potentialDVswith theincrement of thickness of one-layerDD as follows:

r~D !5e0e rD

•

DVs

DD. ~2!

It should be noted here that the spatial charge densityr(D)is formed due to the displacement of excess charges ametal/film interface. In the calculation, we assumed thatdisplaced electrons are located at positionx5(n2 1

2)•DD(n51,2,3,...,N), where DD is the monolayerthickness of 0.4 nm andn is the number of deposited layerFigure 6 shows an example of ther(D) in PI LB films at25 °C, calculated using Fig. 3 with Eq.~2!. The charge den-sity decreases steeply with the increment of the numbelayers. Most of the excess charges exist in PI LB films witha distance of 4 nm from the electrodes. About 1% to 10%

1792 J. Appl. Phys., Vol. 81, No. 4, 15 February 1997

Downloaded 03 May 2013 to 142.150.190.39. This article is copyrighted as indicated in the abstract

hee

of

fFIG. 4. Relationship between saturation value of surface potential of PIfilms and the work function of electrode materials.

E. Itoh and M. Iwamoto

. Reuse of AIP content is subject to the terms at: http://jap.aip.org/about/rights_and_permissions

deis-yta

thLBidPIollenrmte

s

ftot is,-

tates

e

-Isityonept-perdo-iss aafterEq.c-eV

ms

ted

monomer units of PI accept electrons from metal electroin this region, where the density of the PI molecule unitabout 331027 m23. This result indicates that very high density of electronic surface states, which have a tendencaccept electrons, exist within a range of 4 nm from meelectrodes.

When thermodynamic equilibrium is established atinterface, it is expected that the surface Fermi level of PIfilms and the Fermi level of metals are brought in to coincas shown in Fig. 7~a!. Therefore, the electronic states ofwhose electronic energy is higher than the Fermi levelmetal can donate electrons to metal if the states are fiwith electrons before electrification, whereas the electrostates of PI, whose electronic energy is lower than the Felevel of metal can accept electrons from metal if the sta

FIG. 5. Relationship between the surface potential of UPLEX-R LB filand the number of deposited layers at various temperatures on~a! Au and~b! Al electrode.

J. Appl. Phys., Vol. 81, No. 4, 15 February 1997

Downloaded 03 May 2013 to 142.150.190.39. This article is copyrighted as indicated in the abstract

s

tol

e

e

fdicis

are empty. Therefore, the spatial charge densityr(x) is writ-ten as

r~x!5E2`

1`

2e•nA~E,x,T! f ~E2eVS!dE

1E2`

1`

e•nD~E,x,T!~12 f ~E2eVS!!dE. ~3!

Here, f (E) is a Fermi–Dirac distribution function defined a

f ~E!51

11exp$~E1Wm!/kT%, ~4!

ande is electron charge. In Eqs.~3! and~4!, E represents thedepth of the energy measured from the vacuum level~V.L.!~see Fig. 7!, whereE 5 0. nA and nD are the densities ostates~D.O.S.! in PI, which work to accept electrons anddonate electrons due to electrification, respectively. Thathe electron accepting statesnA are empty before electrification, whereas the electron donating statesnD are filled withelectrons. It should be noted here that these electronic sof PI depend on energyE, positionx, and temperatureT @seeFig. 7~b!#. Wm is the work function of metal electrode. ThFermi level of the metal electrode is located at2Wm eVfrom the vacuum level atx50. VS is the electrostatic potential at positionx in PI LB film. As shown in Figs. 3 and 6, PLB films charged negatively, and the spatial charge denwas very high at the interface even for the films depositedAu electrodes. We therefore concluded that electron accing states distribute into the energy level, which is deethan the Fermi level of Au electrode, whereas electronnating states distribute in the range of energy level whichdeep enough with respect to the Fermi level of Au and aresult they cannot donate electrons to metal electrodesPI and Au are brought into contact. Thus the 2nd term in~3! may be neglected in the following discussion. The eletronic states are distributed with over several hundred min electron energy, much larger than the thermal energykT.

FIG. 6. Charge distribution in PI LB films at room temperature calculausing data in Fig. 3 and Eq.~2!.

1793E. Itoh and M. Iwamoto

. Reuse of AIP content is subject to the terms at: http://jap.aip.org/about/rights_and_permissions

nhasesiutia

e-adn

-

ria

y-

om9 ifges

e

fly,p,

--

d.ing

to

e ofeareBsur-

oft

o

Therefore,f (E) is approximately written as a step functiovarying around the Fermi level. It should be noted here tthe energyE0 just corresponds to the Fermi level of the bametal electrode measured from the V. L. in the film at potion x after contact with the PI and base electrode. Becaof the energy level shift due to the electrostatic potenVS , E0 at positionx is given by2Wm1eVS ~see Fig. 7!.Thus, Eq.~3! becomes approximately

r~x!.E2`

E02enA~E,x,T!dE. ~5!

The termur(x)/eu @ [ G(E0 ,x,T)# represents the sum of thelectron accepting statesnA which are occupied with electrons after electrification. It is instructive here to note thnA does not depend on the work function of base electrowhereas the magnitude ofE0 depends on the work functioof base electrode and the electrostatic potentialVs built dueto the electrification. Therefore,E0 can be altered by choosing the base metal electrode material andG(E0 ,x,T) is ob-tained experimentally for the corresponding base mate

FIG. 7. Electrification model at the metal/PI LB films interface:~a! Spatialchange of energy diagram due to the electrification.~b! Schematic illustra-tion of distribution of electronic density of state and the sum of densitystateG(E0 ,x,T).

1794 J. Appl. Phys., Vol. 81, No. 4, 15 February 1997

Downloaded 03 May 2013 to 142.150.190.39. This article is copyrighted as indicated in the abstract

t

-sel

te,

l.

G(E0 ,x,T) becomes zero at the position whereE0 coincideswith the Fermi level of PI (EPI). The spatial charge densitof r(x) at x5x will be positive as a result of the displacement of electrons from PI to metal ifE0 is lower thanEPI ,whereas it will be negative ifE0 is higher thanEPI . There-fore it is probable that the polarity of charge densityr(x)changes along with the increment of the distance frelectrode/PI interface x as we described in Refs. 8 andthe distribution of the electronic density of states chanalong with the increment ofx.

DifferentiatingG(E0 ,x,T) with respect to energy fromEq. ~5!, D.O.S. in PI LB films is given as:11

nA~E,x,T!5DG~E0 ,x,T!

DE0. ~6!

Figure 8~b! shows the relationship betweenG(E0 ,x,T)and the depth of energyE0 at room temperature@25 °C, ob-tained from Fig. 6 with Eq.~5!#. Curves 1–8 represent threlationship betweenG(E0 ,x,T) andE0 at positionsx50.2,0.6, 1.0, 1.4, 1.8, 2.2, 2.6, and 3.0 nm, respectively. Brieeach curve was plotted as follows: In the first steG(E0 ,x,T)5ur(x)/eu and the energy levelE0 for Au-, Cr-,and Al-electrodes atx50.2 nm were obtained from the relationship betweenr(x) and the surface potential at the number of deposited layer of one at a temperature of 25 °C~seeFigs. 3 and 6!. In the second step, the obtained value ofE0

andG(E0 ,x,T) for each metal in the 1st step was plotteThus, curve 1 gives the distribution of electronic acceptstates of the 1st layer~position atx50.2 nm!. Similarly, weplotted curves 2–8 for the number of layers corresponding2–8. In Fig. 8~b!, it was found thatG(E0 ,x,T) decreasessteeply as the distance from metal increases in the rangenergy24.4 through24.9 eV. This result indicates that thelectron accepting states located at deep energy levelsconfined within the region of 1–2 nm from the metal/PI Lfilm interface. These states probably correspond to theface states proposed by Kruppet al.4 It is interesting here tonote thatG(E0 ,x,T) steeply increases with the incrementthe energyE in Fig. 8~b!. If D.O.S. is constant with respec

f

FIG. 8. Relationship betweenG(E0 ,x,T) for PI LB films and the energyE0 of PI at ~a! 150 °C,~b! 25 °C, and~c! 2100 °C.

E. Itoh and M. Iwamoto

. Reuse of AIP content is subject to the terms at: http://jap.aip.org/about/rights_and_permissions

siorn-

yehar

8hol-a-

inainniabhthwi

oeneedf

o

en

al i

he

velntface

on-t ther-teshere a

c.n-t the

pting

-t to

efryf 1-tatesting

sof

to energy, the linear relationship betweenG(E0 ,x,T) andthe energyE will be obtained, while if molecular–ion statedistribute, e.g., with a Gaussian profile due to the fluctuatof the energy level of LUMO~lowest unoccupied moleculaorbital! of PI, G(E0 ,x,T) steeply increases around the eergy E near LUMO of PI.5,6 Figures 8~a! and ~c! show therelationships betweenG(E0 ,x,T) and the depth of energE0 at 150 °C and2100 °C, which is obtained in the sammanner as mentioned above. It was found from Fig. 8 tG(E0 ,x,T) increases at lower energy levels as the tempeture increases.

It was also found thatG(E0 ,x,T) changes abruptly withrespect to energy at each temperature as shown in Figs.~a!,8~b! and 8~c!. The shift of the energy level at whicG(E0 ,x,T) becomes larger than 1% of the density of mecule unit i.e., 331025 m23 is due to the change in temperture much larger than thermal energykT. Therefore it wasconcluded that the distribution of the electron acceptstates is broadened with the increase of temperature probbecause the fluctuation of electronic energy levels in PIcreases with the increment of temperature. This broadeis analogous to the broadening of the linewidth of thesorption spectra with the increment of temperature, whicseen in many organic films. Thus, it is probable thattendency of PI to accept electrons becomes strongerincrement of temperature.

From Fig. 8 it was estimated that the energy levelLUMO of PI LB films was located at an energy betwe23.5 eV and24 eV. In order to confirm this estimation, wdetermined the LUMO and HOMO levels of PI to be locatat about23.8 and26.3 eV from vacuum level by means ophoto-emission measurement13 and cyclic voltammetry.These values were found to be within the energy range ofestimation.

We also investigated the relationship betweG(E0 ,x,T) and the energyE0 for the UPLEX-R whoseLUMO and HOMO levels are located at about23.2 and26.2 eV, respectively. As shown in Fig. 9, we found thG(E0 ,x,T) decreases steeply as the distance from meta

FIG. 9. Relationship betweenG(E0 ,x,T) for UPLEX-R LB films and theenergyE0 of UPLEX-R at ~a! 150 °C,~b! 25 °C, and~c! 2100 °C.

J. Appl. Phys., Vol. 81, No. 4, 15 February 1997

Downloaded 03 May 2013 to 142.150.190.39. This article is copyrighted as indicated in the abstract

n

ta-

gbly-ng-iseth

f

ur

tn-

creases, in a manner similar to that seen in Fig. 8. Tchange inG(E0 ,x,T) with respect to energyE is small incomparison with that of PI, possibly because the energy leof LUMO of UPLEX-R is located at an energy higher tha24 through25 eV ~see Fig. 9!. Therefore, we expected thaelectrons were dominantly accepted by the electronic surstates in the case of UPLEX-R.

Based on the aforementioned discussion, we may cclude that surface states and molecular–ion states exist ametal/PI LB film interface. In the region close to the inteface, i.e., within 1 nm from the interface, the surface stacontribute dominantly. In contrast, in the region where tdistance is greater than 1 nm, molecular–ion states amain contributer to the electronic charge exchange.

C. Calculation of the surface potential

In order to further clarify the model presented in SeIII B, we theoretically calculated the surface potential cosidering both surface states and molecular–ion states asame time, and compared the results with Fig. 8~b!. As tosurface states, we assumed the density of electron accestatesnAS and electron donating surface statesnDSwhich areexpressed as

nAS5NS exp~2aSx!u~E2E1! @m23•eV21#, ~7!

nDS5NS exp~2aSx!u~2E1E1! @m23•eV21#, ~8!

where

u~x!50; x,0

51; x>0. ~9!

That is, nAS and nDS decrease exponentially with the distancex from electrode, and they are constant with respecenergy in the regionE.E1 or E,E1 . In the calculation,looking at Fig. 8~b!, we choseNs52.531026 m23 eV21,aS543109 m21, andE1525.6 eV, where the value of thevolume surface state densityNS was estimated from a slopof curve 1, and aS was estimated from the shift oG(E0 ,x,T) with respect tox. These values suggest that vehigh density of electronic state exists within the region onm from metal/polymeric insulator film interface. In contrast, as to molecular–ion states, we assumed that these sconsist of electron accepting states and electron donastates, where electron accepting statesnAM have a Gaussianprofile with a standard deviation ofsA around the energylevel of LUMO ([ELUMO) and electron donating statenDM have a Gaussian profile with a standard deviationsD around the energy level of HOMO ([EHOMO). nAM andnDM are expressed as

nAM~E!5NPI

A2psA

expS 2~E2ELUMO!2

2sA2 D @m23

•eV21#,

~10!

and

nDM~E!5NPI

A2psD

expS 2~E2EHOMO!2

2sD2 D

@m23•eV21#

. ~11!

1795E. Itoh and M. Iwamoto

. Reuse of AIP content is subject to the terms at: http://jap.aip.org/about/rights_and_permissions

hecero

e

u-

an

a

tinanin

w-iothe

anf

hedh

Thlablu

d

tesl-id-t to

thee-po-acersu-rlyithtalughof

toPI

of

dbe-

apre-ex-rub-

d in

ingos-°C.spec-

We choseNPI5631027 m23, sA50.265 eV,sD50.23 eV,ELUMO523.8 eV, andEHOMO526.3 eV in the calculation.The value ofNPI was chosen to be twice the density of tmolecular unit of PI because one electronic state may actwo electrons. In the calculation, to obtain the total electaccepting statesnA and electron donating statesnD , weadded the surface states and molecular–ion states atenergyE. We then re-calculatednA andnD and Fermi levelof PI ([EPI) self-consistently to satisfy the condition of netrality of charges at each positionx. Here,EPI was defined asthe energy level at which the probability of occupancy ofelectronic state becomes 0.5. Work functionWm of Au, Crand Al electrodes was assumed to be 4.75 eV, 4.45 eV4.02 eV, on the basis of our previous study.9

Figures 10~a!–10~d! show the distributions of D.O.S. aposition x50.2, 0.6, 1.0 and 3.0 nm. Solid lines plottedFig. 10 indicate the density of electron accepting states,the broken lines indicate the density of electron donatstates. The dashed line indicates the position ofEPI . It isinstructive here to note thatEPI changes along withx, be-cause of the spatial change of D.O.S. in PI LB film. Hoever, the change in the distribution is very small in the regwhere the number of layers is greater than 8. That is,distribution of D.O.S. becomes a bulky one, i.e., independof x, whenx is larger than 3 nm.

As shown in Fig. 10, the electron accepting statesthe electron donating states near the surface Fermi level osteeply decrease withx, whereas these states nearELUMO orEHOMO change little. Based on this result, we explain treason why theG0(E,x,T) near Fermi level of PI decreasesteeply with respect to positionx and steeply changed witrespect to energy nearELUMO ~see Fig. 8!. At the presentstage, we are not certain of the origin of surface states.effects of the energy shift and/or the broadening of molecuorbital induced as a result of the contact with metal probamake a significant contribution to the creation of these sface states, as discussed by Anderson and Newns.15,16 Theincrease in the number of surface states will lead to the

FIG. 10. Distribution of the density of state at~a! x50.2, ~b! x50.6, ~c!x51.0, and~d! x53.0 nm.~Solid and broken lines are the electron acceptstates and donating states, respectively.!

1796 J. Appl. Phys., Vol. 81, No. 4, 15 February 1997

Downloaded 03 May 2013 to 142.150.190.39. This article is copyrighted as indicated in the abstract

ptn

ach

nd

dg

nent

dPI

eryr-

e-

crease in the number of molecular–ion states if both staoriginate from the molecular orbital of constituent PI moecule. In the calculation, we did not take effect into conseration. Because the sum of surface states with respecenergy was much smaller than the number ofNPI , e.g., thesum of the number of surface states atx50.2 nm ~in therange of several electron volts! is less than 10% ofNPI .

We then calculated the surface potential by usingD.O.S. obtained in Fig. 10 taking into account the rdistribution of excess charge caused by layer-by-layer desition. Figure 11 shows the relationship between the surfpotential of PI LB films and the number of deposited layeon Au, Cr and Al electrodes at 25 °C. In Fig. 11, the calclated potential curves plotted with broken lines are in faigood agreement with the experimental results plotted wsolid lines. It should be noted here that both experimenand calculated potentials reached a constant value, althothere was still a difference between the calculated valuethe Fermi level of PI (EPI) and the energy level ofE0. Thatis, the Fermi level of PI and metal was not brought incoincidence. This means that the surface Fermi level ofobtained in our experiment was not equal toEPI . As weplotted in Fig. 10, D.O.S. nearEPI becomes very small whenthe number of layers is greater than 8, and the sumnA(E,x,T) below the energyE0 becomes very small~lessthan 0.001% ofNPI). Therefore, the potential curve reachea constant value, although there was still a differencetween the energy level ofEPI andE0.

In contrast, for organic insulators with a wide energy gsuch as polyethylene and polytetrafluoroethylene it wasported that there is no relationship between the chargechange phenomena observed in the insulators withoutbing and the work function of metal.1,17 It is possible toexplain these phenomena on the basis of the model usethe calculation under the following assumptions:~i! LUMO

FIG. 11. Relationship between the surface potential of PI LB films depited on Au-, Cr-, Al-electrode and the number of deposited layers at 25~Solid and broken lines are the experimental and calculated curves, retively.!

E. Itoh and M. Iwamoto

. Reuse of AIP content is subject to the terms at: http://jap.aip.org/about/rights_and_permissions

el oati. Iul

Perlede

r-n-tiaio

thx-te

tesis

esusees

,

oc.h-

level of the film locates at higher energy than PI,~ii ! sA andsD are very small, and~iii ! the number of electronic surfacstates is very small, i.e., the D.O.S. near the Fermi levemetal base-electrode becomes very small. With thesesumptions, it is provable that the calculated surface potendoes not depend on the work function of metal electrodeother words, the metal/film interface acts as an ideal instor.

IV. CONCLUSION

We investigated the electrostatic potential built acrossLB films deposited on various electrodes at various temptures, and then determined the relationship between the etronic density of state and the depth of energy. We concluthe following:

~1! The density of electronic states in PI LB films is detemined as a function of the distance from metal/film iterface from the relationship between surface potenand the number of deposited layers deposited on varmetal electrodes with different work function.

~2! Surface states and molecular–ion states exist atmetal/PI LB films interface. For electronic charge echange at the metal/PI LB film interface, surface stacontribute dominantly within the region of 1 nm from

J. Appl. Phys., Vol. 81, No. 4, 15 February 1997

Downloaded 03 May 2013 to 142.150.190.39. This article is copyrighted as indicated in the abstract

fs-alna-

Ia-c-d

lus

e

s

the metal/PI interface, whereas molecular–ion stacontribute dominantly in the region where the distancelarger than 1 nm.

~3! The tendency to accept electrons of PI LB films becomstronger with the increment of the temperature becaof the broadening of the distribution of electronic statin PI LB films with the increase in temperature.

1J. Lowell and A. C. Rose-Innes, Adv. Phys.29, 947 ~1980!.2D. K. Davies, J. Phys. D2, 1533~1969!.3L. H. Lee, J. Electrostat.32, 1 ~1994!.4H. Krupp, Inst. Phys. Conf. Ser.11, 1 ~1971!.5C. B. Duke and T. J. Fabish, Phys. Rev. Lett.37, 1075~1976!.6T. J. Fabish and C. B. Duke, J. Appl. Phys.48, 4256~1977!.7H. W. Gibson, Polymer25, 3 ~1984!.8E. Itoh, A. Fukuda, and M. Iwamoto, J. Electrostat.33, 147 ~1994!.9M. Iwamoto, A. Fukuda, and E. Itoh, J. Appl. Phys.75, 1607~1994!.10M. Suzuki, M. Kakimoto, T. Konishi, Y. Imai, M. Iwamoto, and T. HinoChem. Lett. 395~1986!.

11E. Itoh and M. Iwamoto, Appl. Phys. Lett.68, 2714~1996!.12A. Viehbeck, M. J. Goldberg, and C. A. Kovac, Proc. Electrochem. S88, 17 ~1988!; Proc. Symp. Polym. Mater. Electron. Packag. High Tecnol. Appl., 1987.

13H. Kirihata and M. Uda, Rev. Sci. Instrum.52, ~Jan. 1981!.14G. M. Sessler, J. E. West, and D. A. Berkley, Phys. Rev. Lett.38, 368

~1977!.15P. W. Anderson, Phys. Rev.124, 41 ~1961!.16D. M. Newns, Phys. Rev.178, 1123~1986!.17J. Lowell, J. Phys. D9, 1571~1976!.

1797E. Itoh and M. Iwamoto

. Reuse of AIP content is subject to the terms at: http://jap.aip.org/about/rights_and_permissions