Modulation of the electrical conductance of phthalocyanine films by circularly polarized light

Thin Solid Films, 102 (1983)231-244 ELECTRONICSAND OPTICS

231

M O D U L A T I O N OF THE ELECTRICAL C O N D U C T A N C E OF PHTHALOCYANINE FILMS BY CIRCULARLY POLARIZED L I G H T T. W. BARRETT Surface Chemistry Branch Code 6170, Chemistry Division, Naval Research Laboratory, Washington, DC 20375 (U.S.A.)

(ReceivedOctober 28, 1982;acceptedJanuary 14, 1983)

The modulation of the electrical conductance by circularly polarized light of phthalocyanine films sublimed onto an interdigital electrode surface (metal/semiconductor/metal structure) is reported. The influence of bound oxygen in creating the p-type semiconductor behavior of the phthalocyanines has been investigated. The Fermi level of the phthalocyanine electrode system may be changed by circularly polarized light acting as an effective magnetic field in an inverse Faraday effect.

1. INTRODUCTION The phthalocyanines (Pcs) (Fig. 1) are well known as gas adsorbers and useful as ambient vapor sensorst-S, although their extreme sensitivity has seemed to make them unreliable for such use 6. A new method, reported here, involves circularly polarized light control of the Fermi level of a Pc electrode system and determines the sensitivity range. Such control indicates the possibility of reliable sensing devices. The circularly-polarized-light-induced paramagnetic switching of the Pcs in an inverse Faraday effect also indicates a possible use in future computer technology. Light interaction with the Pcs in an oxygen or electron-accepting atmosphere is possible from two points of view. On the one hand, magnetic circular dichroism studiesT-t 2 of the metal Pcs indicate resonance interaction with the metal in the red (long wavelength) end of the spectrum. On the other hand, light interaction with 02 adsorbed to the Pc ring occurs over most of the visible spectrum. In the present study, the two effects were dissociated by studying both metal-substituted and metal-free Pcs. The interaction of incident light with oxygen adsorbed to Pcs is of importance as this electron-accepting oxygen molecule appears to be crucial in increasing the conductivity of Pc films by the formation of a p-type Pc semiconductor material t3'14. The photoconductive effects in Pcs are also the result of adsorbed oxygen impurities 15. However, this photoexcitation process is due to adsorbed oxygen with singlet and singlet-triplet excitations ~4 and involves a slower diffusionrelated process. The singlet occurs only in metal-free Pc, and both the singlet and the singlet-triplet in metal-substituted Pc. Both types of excitations are dependent ElsevierSequoia/Printedin The Netherlands

232

N

(a)

T . W . BARRETT

14

N

N

M

N

(b)

Fig. 1. (a) H2-Pc and (b) metal-substituted Pc.

either on adsorbed or on deeply trapped oxygen 14. The reversible thousandfold increase in conductivity which occurs on exposure to an oxygen atmosphere 16 is due to this slower diffusion-related process. 2. EXPERIMENTAL PROCEDURE

2.1. Sample preparation C u - P c and metal-free Pc compounds (Aldrich Chemical Company) were purified by soxhlet extraction with tetrahydrofuran followed by vacuum sublimation. Thin Pc films were deposited on an interdigital microelectrode array by vacuum sublimation (less than 10-z Torr) at 340 °C. The films were easily visible to the eye and reasonably transparent to visible light. Under an optical microscope, the electrodes were clearly visible and the film had a uniform appearance with a slight graininess in texture. This was smoothed by gentle bulling with a clean soft tissue. 2.2. Microelectrode preparation All conductivity measurements were performed using an interdigital microelectrode array. The electrodes were microfabricated on a polished quartz disk which was 1 in in diameter. The array consisted of 50 pairs of gold "fingers" (2000/~ of gold on 200/~ of chromium). Each finger was 25 gm wide. The space between fingers was 25 gm and the overlap distance was 7250 gm. Pc films (both copper substituted and metal free) were deposited onto the microelectrodes by sublimation at a temperature of 340 °C and a vacuum pressure of less than 10-2 Torr. The film thickness was not measured but was estimated to be less than 0.5 lain on the basis of its optical density. The resistance of a clean dry microelectrode array was greater than 3 x 10 ~1 fL 2.3. Measurement technique The microelectrode array was clamped to an aluminum block and sealed inside a 21 glass flask containing Drierite. Electrical measurements were performed using a system interfaced to an Apple II Plus computer. The measurement system consisted of a selectable-gain current-to-voltage converter, analog-to-digital (AD) converter and digital-to-analog (DA) converter. The bias voltage was applied to the microelectrode from the DA converter under computer control. The resulting electrode current was converted into a voltage, digitized by the AD converter and stored in the Apple II Plus. Thus resistance versus time data could be obtained by operating at a fixed bias voltage and sampling the device current at periodic

MODULATION OF ELECTRICAL CONDUCTANCE OF PC FILMS 6328

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233 6328

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2050000(

770

(a) 13600000(

478s

(b) 18800000(

o

770 4880

1 17000000(

]2750000(

(C) 20500000

0

~7o 5145

].2125E~10

1a4o 4765

9.2E~09

]950000(

(e)

(d)

1340

(f)

1340

Fig. 2. Light-induced modulation of (a)-(e) Cu-Pc and (f) H2-Pc conductivity (on the abscissa the total length is 2500 s or 100 s point- * ; the ordinate is the resistance in ohms; increasing conductivity is in the downward direction) where the polarization changes in the incident lasing line were made by cycling in the following order: (a) first linearly and then right circularly polarized incident light etc. (bias, 0.1 V; incident light wavelength and power, 3. = 6328 A. and 5 mW); (b) first linearly and then right circularly polarized incident light etc. (bias, 0.1 V; incident light wavelength and power, 3. = 6328 A and 5 mW); (c) first left and then right circularly polarized incident light etc. (bias, 0.05 V; incident light wavelength and power, 3. = 4765 A and 15 mW); (d) first left and then right circularly polarized incident light etc. (bias, 0.1 V; incident light wavelength and power, 2 = 4880A and 50mW); (e) first right circularly polarized, then linearly and then left circularly polarized incident light etc. (bias, 0.2 V; incident light wavelength and power, 3. = 5145 .~ and 9 mW); (f) first left and then right circularly polarized incident light etc. (bias, 0.5 V; incident light wavelength and power, 2 = 4765 A. and 15 mW).

234

T.W. BARRETT

intervals. Current-voltage ( I - V ) curves could be obtained by scanning the DAgenerated bias voltage and measuring the resulting device current. The incident radiation was obtained from an argon laser (for the 4765, 4880 and 5145 A wavelengths) and an H e - N e laser (for the 6328 ~ wavelength). The laser beam was passed through a quarter-wave plate set to 0 ° for linear polarization or to + 45 ° for right or left circular polarization. The light intensity was measured at all polarizations and found to be the same. 3. RESULTS (1) The resistance of thin Pc films can be modulated by the polarization (linear versus circular, or right circular versus left circular) in the presence of an electrode

acceptor such as oxygen (Figs. 2 and 3). (2) The effect is more pronounced in the metal-free Pc than in the C u - P c film (i.e. about 16~o modulation compared with about 5~o) (Fig. 2). (3) The I - V curves are slightly non-linear and pass through the origin indicating some small deviation from ohmic behavior (Fig. 4). The slight hysteresis shown can be attributed to the time for oxygen readsorption. (4) The I - V curves can exhibit major changes upon irradiation with circularly polarized light (Fig. 4(f)). (5) 02, or an electron acceptor, appears to be necessary for the effect, since the effect is not observed after lengthy (1-2 h) exposure to dry N2, helium or argon atmospheres. (6) The modulation effect could not be observed with pulsed (7 ns pulse, 10 s - 1) laser excitation, although the photovoltaic effect was still obtained. As (i) the modulation effect is expected to be fast (less than 7 ns), (ii) the photoconductive effect is a slower diffusion process and (iii) the data collection system integrates over 100 ms during which the modulation effect lasts for at most 7 ns and the photovoltaic effect lasts considerably longer, this difference between the two effects is to be expected. 6328

157000000

273000000

6328

S 147000000

251000000

1340

1340

(a) (b) Fig. 3. Light-inducedmodulation of Pc conductivity(ordinate, resistance in ohms; abscissa, voltage) (incident light wavelength and power, A = 6328,/~ and 5 mW): (a) Cu-Pc with first linearly and then circularly polarized light incident etc.; (b) Cu-Pc with first linearly and then circularlypolarized light incident etc.

MODULATION OF ELECTRICAL CONDUCTANCE OF

/

8.5E-Og

-8.5E-O~

(a)

-2.

6328

J

235

FILMS

/

9E-Og

.5

=/

(b)

8328

J

-9E-O

5148

1.25E-08

Pc

-2.5 5145

1.25E-08

/ LCP

S

- I , 25E-0

(C)

- I • 2SF'-OIB

2.5

0

- 2.5

¢

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4880

(d)

i

(e)

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0

2.5

(f)

0

2.5 4765

3.SE-,O

j - I . 25E-08[

- 2.5

RCP

f

1 -2.5

Fig. 4. I - V relations (ordinate, resistance in ohms; abscissa, voltage) for (a) Cu-Pc (incident light wavelength and power, ~. = 6328 .~ and 5 mW; linearly polarized incident light), (b) Cu-Pc (incident light wavelength and power, g = 6328 A, and 5 mW; circularly polarized incident light), (c), (d) Cu-Pc (incident light wavelength and power, ). = 5145 A and 9 mW), (e) Cu-Pc (incident light wavelength and power, = 4880 A and 50 mW) and (f) H2-Pc (incident light wavelength and power, 1 = 4765 A and 15 roW): RCP, right circularly polarized incident light; LCP, left circularly polarized incident light. (7) Circularly p o l a r i z e d light can either increase the electrical c o n d u c t i v i t y or decrease the electrical c o n d u c t i v i t y of the s a m e P c film, d e p e n d i n g o n the m a g n i t u d e of the F e r m i 1¢v¢1 w h i c h is set by b o t h the a m b i e n t a t m o s p h e r e and the bias v o l t a g e

236

T.W. BARRETT

of the metal/semiconductor/metal (MSM) structure. This result agrees with previous reports17 21. (8) The interaction of light and Pc film shows a chiral preference. Left circularly polarized light has an antagonistic action to that of right circularly polarized light (see Fig. 2(e)). (9) These results may be interpreted within the context of modern semiconductor theory. 4. DISCUSSION

The experiments reported here constitute an investigation into the I - V relations of a symmetrical MSM structure, the semiconductor being the Pc with adsorbed oxygen. The photovoltaic conversion is either anodic or cathodic 17 20, depending on whether the potentials applied to the MSM device are positive or negative with respect to the flat-band voltage VFB21. This flat-band voltage VFB is defined as qNDW 2 vF.

-

- -

(1)

2es

where ND is the ionized impurity density (or density of holes in the Pc created by adsorbed Oz), q is the charge, W = W1 + I412 where WI and I411 are the depletion widths in the p layer for the forward- and reverse-biased barriers respectively, and es is the (Pc) semiconductor permittivity. For experiments reported VFB ~ 0.2-2.0 V. If the applied voltage equals the flat-band voltage, then the Fermi energy level is equidistant between q~pn, and qq~Bp, where ¢PBnand ~PBpare the Schottky barrier heights on the n (metal, gold in the present case) and the p (semiconductor, Pc in the present case) layers respectively. Thus the bias of an MSM structure influences whether incident light enhances or decreases the conductivity (see result (7)). Another influence is that of adsorbed gases. For example, the hole and electron densities are -EF+Ev"

p = Nv exp ,

kT [-Ec+Ev'~

n = ~Vcexp~

~-7~

/I

(2)

)

(3)

where Nv and Nc are the density of states in the valence band and the conduction band respectively. The conductivity at a p-n junction is (4)

a = q(l~pp + I~.n)

where/~p is the mobility of the holes and #n is the mobility of the electrons, or o" = a /~pNvexp~

~-~

) + #.Nc exp -- k T

Thus a variation in Ev influences the conductivity. On gas adsorption, the mobilities/% and/~, are not expected to change, in agreement with an earlier analysis 22'z3. Furthermore, competition with the 02

MODULATION OF ELECTRICAL CONDUCTANCE OF Pc FILMS

237

molecule for adsorption sites on the Pc ring and on the metal in metal-Pc, and on the ring only in metal-free Pc, will not affect the density of states but rather the number of occupied levels, i.e. the hole density. Thus gas adsorption on Pcs is expected to result in the modification p = Nv exp

( - E F + E v +~ ) kT

(6)

where ( is the gas-adsorption-dependent change in the valence band energy. A consequent decrease in conductivity is then expected using eqn. (4). The conductivity of the Cu-Pc MSM structure could be modulated by constant-wavelength lasers provided that the bias to ground was approximately equal to VFB.The modulations obtained varied from about 2 k~ to about 6 Mf~ or from about l~o to about 5~o. Both linearly-circularly and left-right circularly polarized light changes induced conductivity modulations. The conductivity could be increased or decreased by a specific polarization setting depending on whether the bias voltage was above or below VFB. The conductivity of the metal-free Pc MSM device was modified to a greater extent (corresponding to about 1500 Mf~ or about 16~o) by a variation in polarized light than was that of the Cu-Pc MSM device (see Fig. 3). The photoconductivity and the polarized light modulation of the conductivity could be abolished by application of N2, helium and argon, indicating that 02 was required for both effects. The two effects could be dissociated by use of a pulsed laser with a pulse time of 7 ns and a cycle time of 0.10 s. While the photoconductive effect still remained, the polarized light modulation was not detected by the computerized integrating resistance counter, indicating that the modulation effect is of very short time constant in decay after light removal. Figures 2(a) and 2(b) demonstrate the effect of linearly, as compared with right circularly, polarized light on the resistance of Cu-Pc MSM structures. In the instances shown, circularly polarized light increased the resistance of the MSM structure. However, depending on the magnitude of the bias voltage with respect to VFB,circularly polarized light can result in either an increase (Fig. 3(a)) or a decrease (Fig. 3(b)) in resistivity. Figures 2(c) and 2(d) demonstrate the effect of left, as compared with right, circularly polarized light on the resistivity of Cu-Pc MSM structures, and Fig. 2(e) demonstrates the effects of right circularly, linear and left circularly polarized light. At the beginning of the record, linearly polarized light decreases the resistance more than does right circularly polarized light. Progressively through the record, however, the relative effect of linearly polarized light in diminishing the resistance becomes less than that of right circularly polarized light. This variability indicates vapor changes in ( (eqn. (6)). Figure 2(e) demonstrates the effect of left, as compared with right, circularly polarized light on the resistivity of metal-free Pc MSM structures. Whereas the resistance ofthe Cu-Pc MSM structures studied could be modulated by up to about 5~ of the initial resistance after photoconduction was elicited, the resistivity of the H 2 - P c MSM structures, of comparable semiconductor material thickness, could be modulated by up to about 16~o. This result indicates that circularly polarized light removal of singlet excitons (creating hole carriers in the case of H2-Pc ) requires less

238

T . W . BARRETT

CONTACT

CONTACT

I

2

o-

%

SEMICONDUCTOR

(a)

aX W

=

(b)

.-It

L

(c)

1

l

Iq~Dp

q(,IbBp+Vbi-V I)

(d) Fig. 5. (a) MSM structure with a uniformlydoped p-type semiconductor;(b) charge distributionunder low bias; (c) fielddistribution;(d) energyband diagram.(AfterSze et al. 3'*)

photons than does removal of the singlet-triplet excitons (creating hole carriers in the case of Cu-Pc). A long wavelength photoconductive response is a property of Cu-Pc, but not of H2-Pc, indicating the preponderance of singlet-triplet adsorption in Cu-Pc due to spin-orbit coupling. Two excitons derived from singlet-triplet adsorption would be needed to create free carriers, while only one singlet exciton is needed 14"24"25. Circularly polarized light removal of those excitons created by adsorbed or bound oxygen requires more photons in the case of the metal triplet excitons than in the case of the Pc ring singlet excitons. Figures 4(a) and 4(b) demonstrate non-linear resistance-voltage relations for Cu-Pc MSM structures with incident linearly (Fig. 4(a)) and circularly (Fig. 4(b)) polarized light. The increasing applied voltage passes through a flat-band voltage in each case, going from VaT < V < liFe through VRT < VvB < V and returning, where VRT is the reach-through voltage. Beyond reach-through, the current increases exponentially with applied voltage. Figures 4(c) and 4(d) demonstrate the reverse effects of right and left circularly polarized light, indicating that light effects are relative to the flat-band voltage VFB, which is influenced by ambient vapors, i.e. by (. Linear changes in the resistance-voltage characteristics elicited by right versus

MODULATION OF ELECTRICAL CONDUCTANCE OF PC FILMS

239

left circularly polarized light are demonstrated in Figs. 4(e) and 4(f). As incident circularly polarized light is expected to deplete the hole density (eqn. (2)), this linear relation indicates that the density of states is modified by circularly polarized light interaction, and eqn. (6) becomes p = ( N v - ~/) exp

(-EF+Ev+') kT

(7)

where ~ is the circularly-polarized-light-dependent change in the density of states. This result is in agreement with recent evidence that the applied field and current density influence the position of the Fermi level 26.

0

ol

W

X

(a)

--[

....

E

f

~//EF

•---.~d p

q ((~ Bp +Vbi-Vl )

(c)

(d)

Im X

(e)

ot (f)

.

' v

i_

,

r - L /

-

-~qe,. :

:

! q~e.! I ~

/////~q~.o /////

i, " / / / . ' / / / ~ /

"

E~

w

~

~X

Er

~Jp

q(~Bp÷Vbi-V,i) (g)

~ X

(b)

E

I

w

(h)

Fig. 6. (a), (b), (e), (f) Field distribution and (c), (d), (g), (h) energy band diagrams: (a), (c) at reach-through; (b), (d) at flat-band conditions; (e), (g) effects of voltage bias and vapors on the flat-band conditions; (f), (h) effect of circularly polarized light on the flat-band conditions. (After Sze e t al. 34)

240

T.W. BARRETT

Figures 3(a) and 3(b) indicate the vapor-dependent changes in (, while circularly polarized light is at the same time producing changes in t/. Interestingly, ( is increasing in Fig. 3(a) and decreasing in Fig. 3(b). The circularly-polarized-light-induced modulation of the resistance of MSM structures is due to the removal of the electron-accepting oxygen molecule. Drago and Corden 27 have proposed a spin pairing model of oxygen binding in transition metal systems, the key feature of which is that the metals have one or more unpaired electrons in d orbitals with enough energy to spin pair with the electron in the oxygen binding orbitals. In the case of oxygen adsorption to the Pc ring, the same mechanism could apply. The adsorbed oxygen imparts optical activity to the Pc ring and the left v e r s u s right circularly polarized light resistance effect differences indicate the presence of circular dichroism. An inverse Faraday effect 28-3°, in which circularly polarized light acting as an effective magnetic field induces paramagnetism in the irradiated sample, is a possible mechanism mediating the removal of the oxygen from the sample. This inverse Faraday effect involves the inducing of magnetization in a nonabsorbing gas, solution or material through which a polarized beam of arbitrary

ib, r (c)

]

(d)

]

L

Fig. 7. Energy band diagrams corresponding to Figs. 5(c),6(a), 6(b),6(e) and 6(f): (a) before reach-through, (b) at reach-through and (c) at flat-band conditions (linear I - V response; the current comprises the reverse saturation current, the generation-recombinationcurrent and the surface leakage current) and (d) beyond reach-through (exponential I-Vresponse; the current is dominated by the thermionically emitted component).

MODULATION OF ELECTRICAL CONDUCTANCE OF

PC FILMS

241

ellipticity is passed. A resonance inverse Faraday effect may be obtained as the incident light energy approaches the negative value of the zero-field splitting energy of magnetic sublevels, quite independent of electronic dipole transitions which are also involved. In the present study the spin resonance of the 02 molecule, being broad, permitted the use of incident light of widely varying wavelengths in the visible. The induced paramagnetism would populate states split in zero field and hence remove electrons from the oxygen spin pairing required by the Drago and Corden 27 model, returning them to the Pc ring or the metal. The holes would be removed. Certainly such an inverse Faraday effect exists in the porphyrins 31-33, a species of molecules closely related to the Pcs.

P CARRIERS n

E

<

C

F E t F- . . . . . . . fE v-

. . . . . .

I I ~

. . . . . . .

;~ . . . . .

~ .

.

.

.

.

.

.

_ ~ .

.

.

.

.

P ~,E V

.

. . . . . . .

. . . . . . . .

I

Ek EF

EC >

n or p

n or p

Fig. 8. Effects of bias voltage and vapors on the carrier concentrations: flat-band conditions.

, at reach-through; - - - , at

The complicated interactions of light and the MSM system are shown schematically in Figs. 5-10. Figure 5 shows the charge distribution under low bias, the field distribution and the energy band diagram for an MSM structure. Figure 6 illustrates the result of obtaining reach-through and the fiat-band condition. The effects of vapors and voltage bias reported here and by others 17-21 are indicated and the separate effect of circularly polarized light on the fiat-band condition is illustrated. The slight non-linearity in the I - V curves shown in Fig. 4 is attributed to the

242

T . W . BARRETT

p CARRIERS

n(

E

C

E' F .

.

.

.

.

.

.

.

~

.

.

.

.

.

.

.

.

.

.

:

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'PC

I,E V E

[ . . . . . . . .

~" .

.

.

.

.

.

.

.

.

F

E w

F

E norp

i)

m

n

C

norp

O

~

Fig. 9. Effect of circularly polarized light on the carrier concentrations before ( circularly polarized light was applied (p majority carriers).

Ec

) and after ( - - - )

n

E F- . . . . . . . .

~ .............

'I'E

,14)

p

"bE

'tO ..........

,I, . . . . . . . . . . .

V

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EC norp

norp m

r

(a)

Ec EF

CE

. . . . . . . . .

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'tO . . . . . . . . . .

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i

(h) Fig. 10. Contrary effects of circularly polarized light: (a) p majority carriers (the effect of circularly polarized light is to decrease the conductivity); (b) n majority carriers (the effect of circularly polarized light is to increase the conductivity).

MODULATION OF ELECTRICAL CONDUCTANCE OF Pc FILMS

243

energy bands at the four conditions shown in Figs. 7(a)-7(d). During measurement of the I - V curves, the conditions (a)-(d) will be swept through. Figure 8 illustrates the effects of bias voltage and ambient vapors on carrier concentrations at reach-through and at the flat-band condition. The effect of circularly polarized light is illustrated in Fig. 9 and the contrary effect of circularly polarized light, shown in Fig. 3, is illustrated in Fig. 10 and attributed to the possibility of either p or n majority carriers in the MSM structure. 5. CONCLUSION

The electrical behavior of Pc MSM structures can be controlled by adjustments to the Fermi level achievable in three different ways: (1) by gas adsorption which displaces an electron-accepting p-semiconductor-creating species of molecule such as 02; (2) by changes in the bias voltage which determines the state of the device with respect to reach-through and flat-band conditions; (3) by conductance modulation through circularly polarized light irradiation which removes the electron-accepting adsorbed species of molecule, resulting in fewer p states. REFERENCES 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15

16 17 18

19 20 21 22 23 24 25 26 27 28 29

A.B.P. Lever, Adv. Inorg. Chem. Radiochem., 7 (1965) 27. P. Bergveld, N. F. DeRooij and J. N. Zemel, Nature (London), 273 0978) 438. J.N. Zemel, Sensors and Actuators, 1 (1981) 31. J.N. Zemel, B. Keramati, C. W. Spivak and A. D'Amico, Sensors and Actuators, I (1981) 427. G.F. Gutman and L. E, Lyons, Organic Semiconductors, Wiley, New York, 1967. A.W. Barendsz, C. A. van Beest and P. P. M. M. Wittgen, unpublished, 1982. B.R. Hollebone and M. J. Stillman, Chem. Phys. Left., 29 (1974) 284. M.J. Stillman and A. J. Thompson, J. Chem. Soc., Faraday Trans. I1, 70 (1974) 790. M.J. Stillman and A. J. Thompson, J. Chem. Soc., Faraday Trans. II, 70 (1974) 805. B.R. Hollebone and M. J. Stillman, J. Chem. Soc., Faraday Trans. II, 74 (1978) 2107. K.A. Martin and A. J. Stillman, Can. J. Chem. Commun., 57 (1979) 1111. C.H. Langford, B. R. Hollebone and T. Vandernoot, Adv. Chem. Set. 184 0980) 139. S.E. Harrison and K. H. Ludwig, J. Chem. Phys., 45 (1966) 343. S.E. Harrison, J. Chem. Phys., 50 (1969)4739. G. Tollin, D. R. Kearns and M. Calvin, J. Chem. Phys., 32 (1960) 1013. D. R. Kearns, G. Tollin and M. Calvin, J. Chem. Phys., 32 (1960) 1020. D. R. Kearns and M. Calvin, J. Am. Chem. Soc., 83 (1961) 2110; J. Chem. Phys., 34 (1961) 2022. J.M. Assour and S. E. Harrison, J. Phys. Chem., 68 (1964) 872. H. Tachikawa and L. R. Faulkner, J. Am. Chem. Soc., 100 (1978) 4379. F.-R. Fan and L. R. Faulkner, J. Chem. Phys., 69 (1978) 3334. F.-R. Fan and L. R. Faulkner, J. Chem. Phys., 69 (1978) 3341. F.-R. Fan and L. R. Faulkner, J. Am. Chem. Soc., 101 (1979) 4779. C.D. Jaeger, F.-R. Fan and A. J. Bard, J. Am. Chem. Soc., 102 (1980) 2592. B. Rosenberg, J. Chem. Phys., 36 (1962) 816. T.N. Misra, B. Rosenberg and R. Switzer, J. Chem. Phys., 48 (1968) 2096. P. Day and R. J. P. Williams, J. Chem. Phys., 37(1962) 563. P. Day and R. J. P. Williams, J. Chem, Phys., 42 (1965) 4049. W. Mycielski, B. Zi6tkowska and A. Lipifiski, Thin Solid Films, 91 (1982) 335. R.S. Drago and B. B. Corden, Acc. Chem. Res., 13 (1980) 353. P.S. Pershan, Phys. Rev., 130(1963)919. P.S. Pershan, M. Gouterman and R. L. Fulton, Mol. Phys., 10 (1966) 397. J. P. van der Ziel, P. S. Pershan and L. D. Malmstrom, Phys. Rev. Lett., 15 (1965) 190.

244

30 31 32 33 34

T . W . BARRETT

P.W. Atkins and M. H. Miller, Mol. Phys., 15 (1968) 503. T.W. Barrett, Chem. Phys. Lett., 78 (1981) 125. T, W. Barrett, in F. L. Carter (ed.), Proc. Molecular Electronic Devices Workshop, Memo. Rep. 4662, 1981, pp. 274-290 (U.S. Naval Research Laboratory). T, W. Barrett, Chem. Commun., l (1982) 9, Commun. 789. S.M. Sze, D. J. Coleman and A. Loya, Solid-State Electron., 14 ( 1971) 1209.