992 Langmuir 1987, 3,992-1000

with copper electrodes. The resistance of each sample was measured with a Keitley 178 Model digital multimeter. the vapor depositions were carried out with a Veeco Model VS-90 metal evamrator. the values of thickness and resistance are summarized

metal films using either KBr pellets or Fluorolube yielded only evidence for VGH (2980 cm-') and va (1740 cm-') showing the same shape as the acetone standard.

in Table IV.

(0.0236 M) was tested with the following solvents: acetone, ethanol, THF, benzene, toluene, and The films were completely insoluble after 24 h of contact with stirring at 25 o c .

Acknowledgment. The support of the Office of Naval

Matthew T. Franklin for helpful discussions and Larry L. Seib for we want to thank Dr. IIeana Nieves for her assistance in

Solubility Studies. The solubility Of the Pd-acetone film Research is with gratitude. we also thank

with the SEM-TEM experiments* _ _ _.

Infrared Studies. Infrared spectra were recorded on a Perkin-Elmer PE-1330 infrared spectrometer. IR studies of the

obtaining spectra and Thomas J. Groshens for assistance with the mass spectrometer.

Sensitized Hole Injection from Dioctadecylindolocarbocyanine and

Dioctadecyloxacarbocyanine Monolayers into Anthracene and Perylene Single Crystals

M. Van der Auweraer,* B. Verschuere, G. Biesmans, and F. C. De Schryver Chemistry Department, KU Leuven, 3030 Leuven, Belgium

F. Willig Fritz Haber Imtitut, 1000 Berlin 33, Germany

Received February 4,1987. In Final Form: May 5, 1987

Langmuir-Blodgett monolayers of w( 2-anthry1)alkanoic acids were deposited on anthracene and perylene single crystals. This assembly was covered by a mixed monolayer of arachidic acid and dioctadecyi- substituted carbocyanines. Excitation of these dyes leads to sensitized hole currents. Already at field strengths of 5 X lo3 V cm-', the escape of the injected hole is much faster than the recombination with the reduced dye. The dependence of the rate of the charge generation on the applied electric field leads to an increase of sensitized hole current. Thii effect is more important when this charge generation process is less exothermic. The influence of the chain length of the w-(2-anthryl)alkanoic acid on the sensitized hole current permitted the determination of the height of the barrier for the tunneling process, which equals 0.5 f 0.1 eV.

Introduction Sensitized hole injection from adsorbed dyes into organic

single crystals has been investigated by several At low dye coverage the photocurrent saturates4 at field strengths of about 5 X lo4 V cm-l. Under these experi- mental circumstances the escape of the injected holes from the solution interface is much faster than eventual re- combination processes leading to a quantum yield near unity for the SHC. A field strength of 5 X lo4 V cm-l means that for a crystal with a thickness of cm a voltage of 50 V has to be applied. This differs considerably from the voltage that has to be applied to obtain sensi- tized-hole or electron currents in inorganic semiconduc-

(1) Steketee, J. W.; De Jonghe, J. Proc. K . Ned. Akad. Wet., Ser. B: Phys. Sci. 1963,66, 761.

(2) (a) Gerischer, H.; Wfig , F. Top. Curr. Chem. 1976, 61, 31. (b) Gerischer, H.; Spitler, M. T.; Willig, F. In Proceedings of the Third International Symposium on Electrode Processes; Bruckenstein, S., Ed.; The Electrochemical Society: Princeton, NJ, 1979; p 115.

(3) (a) Groff, R. P.; Suna, A.; Avakian, P.; Merrifield, R. E. Phys. Reu. E 1974,9,2655. (b) Nickel, B. Ber. Bunsenges. Phys. Chem. 1971, 75, 795-800.

(4) (a) MMer, N.; Papier, G.; Charll, K.-P.; Willig, F. Ber. Bunsenges. Phys. Chem. 1979,83,130. (b) Papier, G.; Charll, K.-P.; Willig, F. Ber. Bunsenges. Phys. Chem. 1982,86,670.

tor^.^^^ To obtain efficient charge separation at the in- terface between an organic crystal and water a t lower ap- plied fields one has to increase the rate of the escape process or decrease the rate of the recombination process. We prove that both could be realized by depositing a monolayer of 7-(2-anthryl)heptanoic acid (2A7) on top of the organic insulator crystals. At field strengths of less than 2.5 X lo3 V cm-I a monolayer of dioctadecylindolo- carbocyanine6 was able to inject holes with an overall quantum yield of 0.28 f 0.08 in anthracene or perylene single crystals. When the dioctadecylindolocarbocyanine was replaced by a dioctadecyloxacarbocyanine the quan- tum yield dropped' to values between 0.05 f 0.01 and 0.004 f 0.001 a t low (<2%) and high (>20%) dye coverages, although the features of the current-voltage plot did not

(5) (a) Spitler, M.; Ltibke, M.; Gerischer, H. Ber Bunsenges. Phys. Chem. 1979,83,663. (b) Kirsch-Demesmaeker, A.; Nasielski, J.; Leem- poel, P. Electrochim. Acta 1978,23,605. (c) hang, Y.; Ponte Goncalves, A. M.; Negus, D. K. J. Phys. Chem. 1983, 87, 1. (d) Watanabe, T.; Fujishima, A.; Tatauoki, D.; Honda, K. Bull. Chem. SOC. Jpn. 1976,49, 8.

(6) (a) Willig, F.; CharlB, K.-P.; Van der Auweraer, M.; Bitterling, K. Mol. Cryst. Lip. Cryst. 1986,137,329. (b) Van der Auweraer, M.; Willig, F.; CharlB, K.-P. Chem. Phys. Lett. 1986,128, 2140.

(7) Van der Auweraer, M.; Willig, F. Isr. J. Chem. 1985, 25, 274.

0743-7463/81/2403-0992$01.50/0 0 1987 American Chemical Society

Sensitized Hole Injection

a '18'37

Langmuir, Vol. 3, No. 6, 1987 993

I oA 16 6 A

Figure 1. Hole-injecting system with mixed dye monolayer and a 2A7 monolayer deposited on the surface of a perylene crystal, This system is covered with two more arachidic acid monolayers for protective reasons. (a) With the indolocarbocyanine as in- jecting dye; (b) with the oxacarbocyanine as injecting dye.

change in a dramatic way. In this contribution we want to report in a more detailed way on the effect on the current-voltage plot when the chain length of the anthr- yl-substituted fatty acid, the nature of the dye, and the crystal are changed.

Experimental Section 7-(2-Anthryl)heptanoic acid (2A7), 8-(2-anthryl)octanoic acid

(2A8), 9-(2-anthryl)nonanoic acid (2A9), and 10-(2-anthryl)de- canoic acid (2A10) were prepared according to Kaplun et ala8 They were purified by column chromatography on silica using chloroform as eluent followed by recrystallization from chlorofoq. Dioctadecyloxacadmcyanine perchlorate was synthesized according to Sondermanne and purified by successive recrystallization from acetic acid and 1-propanol or by column chromatography on silica gel using a mixture of chloroform and ethyl acetate as eluent. Dioctadecylindolocarbocyanine perchlorate was prepared and purified according to ref 10.

The sample preparation and the experimental setup are de- scribed in ref 7 (Figure l). The oxacarbocyanine and indolo- carbocyanine were excited at respectively 495 and 610 nm, the maxima of the action6*' spectrum of the sensitized photocurrent. To obtain absorption and emission spectra, mixed monolayers of the dye and arachidic acid were deposited on glaas slides cleaned and treated with dichlorodimethylsilane, as indicated in the literature." Absorption spectra were recorded on a Perkin-Elmer Lamda-5 spectrophotometer. Fluorescence spectra were recorded on a Spex Fluorolog. The spectra were recorded at an angle of 90°, and the slide was positioned to make an angle of 45O with

(8) Kaplun, A. P.; Basharuli, V. A.; Shchukina, L. G.; Svheta, V. I. Bioog. Khim. 1979,5, 1826.

(9) Sondermann, J. Justus Liebigs Ann. Chem. 1971, 749, 183. (10) Van der Auweraer, M.; Vandenzegel, M.; Boens, N.; De Schryver,

F. C.; Willig, F. J. Phys. Chem. 1986,90,1169. (11) M6bius, D.; Kuhn, H. In Techniques of Chemistry; Weissberger,

A., Rossiter, B., Eds.; Wiley: New York, 1972; Vol. 1, Part IIIB, p 577.

0 0 5 10 15 2 0 2 5

F[lO' V cm-l]

Figure 2. Plot of the relative quantum yield of the SHC vs. the applied field strength for an assembly corresponding to the structure of Figure la (perylene crystal/2A7 monolayer/mixed indolocarbocyanine-arachidic acid monolayer). All quantum yields are normalized at 2.5 X lo4 V cm-'. 0, Y = 0.02; 0 , Y = 0.41.

1 5

c m 03 m rp

0

D O 0

n 2 0 2 5 0 0 5 10 1 5

F [ lo5 vc"]

Figure 3. Plot of the relative quantum yield of the SHC vs. the applied field strength for an assembly corresponding to the structure of Figure la (perylene crystal/2A7 monolayer/mixed indolocarbocyanine-arachidic acid monolayer). All quantum yields are normalized at 1.0 X lo6 V cm-l. 0, Y = 0.02; 0, Y = 0.41.

the incoming light to minimize reflection. To eliminate photo- oxidation of the dye a sample compartment was constructed in which it was possible to execute fluorescence decay measurements under reduced (1 Torr) pressure.

For the experiments performed with different chain lengths, a circular Langmuir trough12 was used. 2A8,2A9, and 2A10 were deposited at a pH of 4.7 and at a surface pressure of 15 dyn cm-', as the pressure-area isotherms of 2A8,2A9, and 2A10 are identical with that of 2A12.13

Results 1. Hole Injection from Dioctadecylindolocarbo-

cyanine. The monolayer assembly mounted on the pe- rylene crystal is shown in Figure l. Upon excitation of the indolocarbocyanine as sensitized hole current (SHC) with an action spectrum correspondin$ to the absorption spectrum of a mixed indolocarbocyanine/arachidic acid monolayer is observed. Figures 2 and 3 give the depen- dence of the SHC of the applied electric field. The SHC is normalized at 2.5 X lo4 and 1.0 X lo6 V cm-l, respec- tively, in Figures 2 and 3. After an initial rise the SHC saturates at 3 X lo3 V cm-'. When the applied field is increased to 2.5 X lo6 V cm-l a further increase of 30% of the SHC is observed.

With an anthracene crystal instead of a perylene crystal identical6 results are obtained. At a field strength of 1.0

~ ~~~

(12) Fromherz, P. Rev. Sci. Instrum. 1975,46, 1380-1385. (13) Bieamans, G.; Verbeek, G.; Verschuere, B.; Van der Auweraer, M.;

De Schryver, F. C., unpublished results.

994 Langmuir, Vol. 3, No. 6, 1987 Van der Auweraer et al.

0 C

P

- 2 C G 1 1 I ' 1 1 ' 1 1 1 ' 1 I l l , , ' " 3 - 3 0 2 0 C 9 C

In L I

Figure 4. Influence of the dye coverage on the quantum yield of the SHC for the structure shown in Figure 1 with indolo- carbocyanine as injecting dye at lo6 V cm-': 0, anthracene; O , perylene.

I

a 1 I I I I 1 I 1 I

0 05 10 1 5 2 0 2 5 F [ I O L V c m ' ]

Figure 5. Plot of the relative quantum yield of the SHC vs. the applied field strength for an assembly corresponding to the structure of Figure l b (perylene crystal/2A7 monolayer/mixed oxacarbocyanine-arachidic acid monolayer). All quantum yields are normalized as 2.5 X lo4 V cm-'. 0, Y = 0.02; A, Y = 0.09; V, Y = 0.17; 0, Y = 0.4.

X 105 V cm-' the quantum yield equals 0.28 f 0.08. Figure 4 shows the dependence of the quantum yield of the SHC (QSHC) on Y, the fraction of the area of the mixed mono- layer occupied by the dye. Contrary to what was observed for the oxacarbocyanine anthracene7 crystals, @sHC does not change appreciably when the fraction of the area of the mixed monolayer occupied by the dye (Y) is changed.

2. Hole Injection from Dioctadecyloxacarbo- cyanine. The behavior of the monolayer assembly shown in Figure l b mounted on an anthracene crystal is described elsewhere.' After a steep initial rise the SHC levels off a t 5 X 103 V an-'. Upon a further increase of the applied field to 2.5 X lo5 V cm-l the SHC increases considerably faster than that observed for the indolocarbocyanine (55% be- tween 105 and 2.5 X 105 V cm-l). In analogy to the results observed for the indolocarbocyanine, the shape of the current-voltage plot does not depend upon the amount of dye in the monolayer, given by Y, the coverage. However, the overall quantum yield at 105 V cm-' decreases from 0.05 f 0.01 at low coverages to 0.004 f 0.001 at high coverages. This was considered to be due to the formation of dye dimers and energy transfer to those dimer^.^ Due to this efficient energy transfer even at a coverage of 1/20 nearly all excitation energy is harvested by the dimers, and fur- ther hole injection occurs from the dimers. Therefore, a further increase of the coverage will not lead to a change in the injection mechanism. Neglect of this energy-transfer process leads to the conclusion that hole injection occurs with the same efficiency from monomers and dimers.14

1 5

0 05 IO 15 2 0 2 5

F C I O ~

Figure 6. Plot of the relative quantum yield of the SHC vs. the applied field strength for an assembly corresponding to the structure of Figure l b (perylene crystal/2A7 monolayer/mixed oxacarbocyanine-arachidic acid monolayer). All quantum yields are normalized at 1.0 x lo6 V cm-'. 0. Y = 0.02: A. Y = 0.09: V, , , Y = 0.17; 0, Y = 0.4.

- 2

::~: - 5

0 3 0

0 0 0 0 1

' 8 0 -6

1 ! l 1 l / l l / 1 l I - 6 - 5 -L -3 - 2 - I 0

I n Y

Figure 7. Influence of the dye coverage on the quantum y i d of the SHC for the structure shown in Figure 1 with indolo- carbocyanine as injecting dye at lo6 V cm-': 0, anthracene; 0, perylene.

The shape of the figures of the SHC vs. the applied field for the combination of a perylene crystal and the oxa- carbocyanine are given in Figures 5 and 6. The SHC is normalized at 2.5 X lo4 and 1.0 X lo5 V cm-' in Figures 5 and 6, respectively.

As in Figures 2 and 3, a steep initial rise is followed by a slow increase of the SHC. The increase of the SHC at higher field strengths is much shallower than that observed for the oxacarbocyanine and the anthracene crystal and approaches that observed for the indolocarbocyanine. Contrary to what is observed for the anthracene crystals, the quantum yield of the SHC is independent of the coverage Y (Figure 7).

3. Influence of the Tunneling Distance. In the systems considered in the previous sections, excitation of the dye leads to hole injection in the 2A7 monolayer, which on its term injects a hole in the anthracene or perylene crystal. To determine which process limits the quantum yield, the distance over which the primary hole injection occurs has been changed systematically by replacing, in the structure shown in Figure 1, the 2A7 monolayer by a 2A8, a 2A9, and a 2A10 monolayer. In this way the dis- tance for the first electron transfer increases from 10.0 A (edge to edge or the length of the fully stretched alkane chain extended with a carboxylate oxygen) to respectively 11.25, 12.50, and 13.75 A. The quantum yield of the SHC for a mixed (1/100) monolayer of the oxacarbocyanine and arachidic acid into an anthracene crystal decreases from

~ ~~~

(14) Killesreiter, H.; Bassler, N. Ber . Bunsenges. Phys. Chem. 1978, 82, 503.

Sensitized

I

4 I

C -

Hole Injection

- 4 0

- 5 0

- 6 0

- 7 0

- 8 0 100 110 120 1 3 0 1 4 0 150

d ( I " A 1

Figure 8. Plot of In @ vs. d'(in A). The slope and the intercept amount respectively to 0.73 * 0.07 A-l and 3.4 h 0.8. The cor- relation coefficient amounts to 0.97.

Scheme I

k+ R**O h * - p m r.0

A+ngem R-

I R %

D - R ;p

X

3% for the 2A7 monolayer to 0.1% for the 2A10 monolayer (Figure 8). At this mixing ratio excitation at 500 nm will only lead to the population of the excited state of dye monomers. The features of the plots of the SHC vs. the applied field resemble those observed for 2A7.

Discussion 1. Kinetic Model. Taking into account the e ~ i s t i n g ~ - ~

model for sensitized hole injection into organic crystals, the photophysical processes that occur after the absorption of a photon by the dye monolayer in the assembly of Figure 1 can be represented by Scheme I, where D and D* are the ground state and excited state of the dye, A+,,, and Atngem are the hole injected in the 2A7 monolayer at the same and a different lateral position as the reduced dye, h+ is the hole in the first molecular layer of the crystal, hv is ab- sorption of light by the dye, kf is intramolecular (radiative and radiationless) decay of the excited dye, KO, is injection of a hole by the excited dye in the 2A7 monolayer, k, is transfer of an electron from the HOMO of the reduced parent dye to the hole in the 2A7 monolayer (this process yields a singlet excited dye), k+ is injection of a hole by the 2A7 monolayer into the crystal, k- is injection of a hole by the crystal into the 2A7 monolayer, k,, is escape of the injected hole into the crystal bulk, kreC is recombination between a hole in the 2A7 monolayer and the electron in the reduced dye (recombination with the parent dye is called geminal), k1 is lateral movement away from the startin position of the injected hole in the 2A7 monolayer,

monolayer and the electron of a molecule or ion present in the solution (alien recombination).

For all the systems investigated, the shape of the cur- rent-voltage plots was not influenced by the amount of dye in the injecting monolayer. This means that nonge- minate recombination at reduced dye monomers or dye dimers is not an important process. This process should become more important the higher the amount of dye in the monolayer. Therefore, at higher amounts of dye in the monolayer the initial saturation should be obtained only

and It,,, s is recombination between a hole in the 2A7

Langmuir, Vol. 3, No. 6, 1987 995

at higher values of the rate constant for the escape out of the Coulombic potential well a t the crystal surface and thus at higher field strengths. Since the current-voltage plots retain their saturation behavior when the amount of dye is increased in the mixed monolayers, we can neglect nongeminate recombination at reduced dye monomers and dimers in this system. Alien recombination at impurities or physical defects in the monolayer assembly acting as a trap cannot, however, be excluded.

The increase of the rate of the escape process (cf. infra) leads to a considerable decrease of the residence time of the hole in the 2A7 monolayer compared to that on the first crystal plane in the case of an adsorbed dye. This leads further to a decrease of the probability that a hole that left its starting position by two-dimensional diffusion4 parallel to the crystal surface will return to its starting position.

Scheme I leads to the following differential equations:

d[A+gemI /dt = -(k+ + k!:: + K,, + k-J[A+,,,] + ko,[D*] + k-[h+] (2)

d[A+ngeml /dt = -(k+ + &.tm + k&)[A+ngeml + k-l[A+gemI + k-[h+l (3)

(4)

where Iabs is the absorbed light intensity in ein&ein/s, S is the area of the crystal in cm2, q is the elementary charge, and [D*], [Afgem], and [h+] are the concentrations of D*, A+ em, and h+ in mol cm-2.

finder stationary conditions of light excitation, the SHC (i) will be given by the number of injected holes escaping per second into the crystal bulk:

d[h+l/dt = -(kea, + k-)[h+l + k+([A+geml + [A'ngeml)

i = qS[h+Ikesc/N~~ (5)

where q = 1.6 X

for the injected current:

C and NAv is Avogadro's number. Solving eq 1-4 under the conditions of stationarity yields

qSkoxR+(~ t + k-1 + kit) (kf + kox)(R+ + k W + + k-1 + R E ) i = (6)

where .. k+ = ktkesc/(k- + kesc) (7)

RgF = kg? + k-,,k,/(k, + kox) (8)

Unless the concentration of alien recombination centers is very high, kg, will always be considerably smaller than k1, which is already estimated close to lo9 s-l in the case of an adsorbed dye? This leads to the following simpli- fication for eq 6:

QSkox(R+ + k-$+ (kf + kox)(R+ + It-1 + Rg:)(R+ + hi , )

1 . = (9)

The quantum yield of the SHC is then given by

where Q = @CT@ESC (10)

996 Langmuir, Vol. 3, No. 6, 1987 Van der Auweraer et al.

Oris t

- 0 2 3 '\ \

50 'CIC 53 Z C 3 253

D S T A N C E n A

Figure 9. Shape of the image potential barrier for hole injection into anthracene: -, 0.0 V cm-'; -., 5 X lo3 V cm-'; - - -, 5 x IO4 V cm-'; -.-, 5 x lo6 V cm-'.

b ! a i 10

Figure 10. Energy levels for electron tunneling (in eV); the energy of the electron in a vacuum is uses as zero: (a) between the LUMO of an excited donor and that of 2,2'-bipyridyl; (b) between the HOMO of 2A7 and that of an excited dye.

aCT and amC are respectively the quantum efficiency for the hole injection from the dye into the 2A7 monolayer and from the 2A7 monolayer into the crystal bulk.

2. Escape Process. The injected charge is bound to the surface of the crystal by an image potential caused by the difference in dielectric constant of the water and the crystal bulk. The combination of this image potential and the applied electric field leads to eq 13 for the electrical potential15 of the injected hole:

-e2 V(X) = ~ - eFx

16xc0q

where eo is the permittivity of the vacuum (8.85 X C2/J.m, c, is the dielectric constant of the crystal, and x is the distance from the crystal surface (adsorbed dye) or from the polar side of the monolayer. Figure 9 shows V(x) as a function of x in the absence of an applied field and for an applied field of 5 X lo3, 5 X lo4, and 5 X lo5 V cm-'. Increasing the applied field leads to a decrease of the height and the width of the barrier over which the escape of the injected hole must occur. The height of this barrier is given by

According to Willig,l6 k,,, can be calculated by D exp[V(x,)/kTI

Liomexp[V(z)/kTl dx kesc = (15)

where L is the thickness of one layer of the crystal (c' lattice parameter for anthracene) and xo is the distance from the surface of the crystal where the injected hole is thermalized (first layer).

While x o amounts to 5 A for an adsorbed dye, it will increase to respectively 12.50, 13.75, 15, and 16.25 A for a charge at the center of the anthracene in the 2A7,2A8, 2A9, and 2A10 monolayers. This will lead to a decrease of the barrier height (at 5 X lo3 V cm-l) from 0.217 eV to respectively 0.086,0.078,0.071, and 0.066 eV. For a charge in the first crystal plane the barrier height will be reduced even more. This decrease of the image charge barrier will (according to eq 13,14, and 15) lead to values of k,,, that are, e.g., at an applied field strength of lo4 V cm-', 3 orders of magnitude larger than those calculated17 or experi- mentally obtainedls for an adsorbed dye (xo = 5 A).

Depending upon the relative magnitude of k- and k,,,, k, will approach k+ or k&+/k-. Equation 15 suggests that k,, is strongly field dependent. Taking into account the effect of the image potential, it will also be possible to estimate the order of magnitude of k+. Considering the nearly epitaxial packing13J9 of the anthracene chromo- phores in the monolayer and in the crystal, this rate will be close to the hopping time s ) ~ ~ , ~ ~ between neigh- boring anthracenes in the c'direction when abstraction is made for the effect of the image potential. Even if a potential difference of 0.15 V due to an electric field exists between the first monolayer and the crystal, k+ will still be equal to 1O1O s-l. For a 2A7 monolayer on a perylene crystal this will also be the case; as k+ describes here a process that is exothermic for 0.4 eV2J5J6 its value will be at least of the same order of magnitude as in the case of anthracene. This value is much larger than or k L , processes that involve electron tunneling over a (edge to edge) distance of 10.0-13.7 A. This means that as soon as k,,, becomes larger than k-, aESC approaches unity. When the ionization potential of the crystal is higher than that of the monolayer, as would be the case for naphtha- lene or phenantrene crystals, k+ can become considerably smaller, and these conclusions will no longer be valid.

At lower field strengths where k, is smaller than k-, eq 1 2 can be approached by

with k::: = k+k,,,/k- (17)

k+/k- = exp[-AV(xo)/kT] (18) where AV(xo) is the difference in electrochemical potential of the injected hole between the 2A7 monolayer and the first layer of the crystal.

As the ionization potential of anthracene and 2A7 are identical, the ratio k+/k- will only be determined by the

(15) (a) Landau, L. D.; Lifschitz, E. M. In Elektrodynamik der Con- tinua; Akademie Verlag: Berlin, 1967. (b) Skinner, S. M. J. Appl. Phys. 1965, 26, 498.

(16) (a) Willig, F. In Advances in Electrochemistry and Electrochem- ical Engineering; Gerischer, H., Tobias, C. W., Eds.; Wiley-Interscience: New York, 1982; Vol. 12, pp 1-106. (b) Willig, F.; Scherer, G. Chem. Phys. Lett. 1978, 53, 128.

(17) Eichhorn, M.; Willig, F.; CharlC, K.-P.; Bitterling, K. J . Chem. Phys. 1982, 76, 4848.

(18) CharlC, K.-P.; Willig, F. Chem. Phys. Lett. 1978, 57, 253. (19) Durfee, W.; Willig, F.; Storck, W.; von Frieling, M. J. Am. Chem.

SOC. 1987, 109, 1297. (20) Karl, N. Festkorperprobleme 1974, 14, 261.

Sensitized Hole Injection

difference of the image potential between the center of the anthracene in the monolayer and the center of the first layer of the anthracene crystal and can be expected to have a value between 0.22 for 2A7 and 0.36 for 2A10. In this case k$! will be 3-5 times smaller than k,,,.

For a 2A7 monolayer on a perylene crystal, the difference of the ionization potential between 2A7 and perylene (0.45 eV) has to be taken into account. This would lead to values of k+/k- between 3 X lo7 and 2 X lo7 for 2A7 and 2A10, respectively. For the perylene crystals k,$! will be (2-3) X lo7 times larger than k,,, although it can never become larger than k+. For perylene, k::! can be expected to be up to 7 orders of magnitude larger than that for anthracene at field strengths where k,,, is smaller than k-. Only, in this case values of aESC smaller than 1 are possible. Nevertheless, a t field strengths larger than 5000 V cm-' the field dependence of the quantum yield of the SHC is very small for both anthracene and perylene. This indi- cates that the quantum yield of the escape process is identical and equal to 1. This is reasonable, as k, is much larger than that focan adsorbed dye (cf. supra), while there is no reason why kf:? or k:, should be larger than those in the case of an adsorbed dye. aESC will be considerably larger than that for an adsorbed dye at the same strength of an applied field.

At lower values of the applied field, where a current drop occurs, it is possible that in this system k::: becomes indeed smaller than k!:?; however, the same remarks that were made for the perylene crystal can also be made here.

That the escape process occurs with an efficiency near unity is confirmed by the dependence of the SHC of the chain length on the monolayer intermediate between the anthracene crystal and the mixed dye layer. On the basis of eq 13-16, an increase of this chain lepgth would lead to an increase of xo and therefore of k,. It!,", on the other hand, corresponds to the transport of a hole through this aliphatic moiety and would therefore rather decrease when the length of this chain increases. Both would lead to an increase of aESC when the length of the aliphatic chain is increased. However, the inverse is observed.

The drop of the SHC at still lower field strengths is due to other effects. At those low field strengths the potential of only a few volts is applied (a field strength of lo3 V cm-l over a crystal of 10 pm corresponds to a potential differ- ence of 1 V). Our setup, however, measures the total applied potential, which is not only used to apply an electric field over the anthracene crystal but also to provide an overvoltage for an electrochemical reaction (oxidation of water or chloride) at the backside of a crystal and at both platinum electrodes and to apply a voltage difference over the entrance of the electrometer. Therefore, at low values of the applied electric field the potential difference really applied over the crystal can be considerably smaller than the one that is measured. Furthermore, in these circumstances space charge limitationz1 of the SHC will be possible, especially in the presence of traps.22

3. Charge-Generation Step. In spite of the experi- mental evidence for the saturation of the escape process at low field strengths, the overall quantum yield is, espe- cially for the oxacarbocyanine, considerably lower than unity. Considering Scheme I, this must be due to a low quantum yield aCT. Furthermore, when the applied field strength is increased after the initial saturation of the SHC

Langmuir, Vol. 3, No. 6, 1987 997

a further increase, which is more inportant for the oxa- than for the indolocarbocyanine is observed. The latter observation could be explained by a dependence of the extinction of the monolayer on the applied electric field. However, even at the highest applied field strengths (5 X lo6 V cm-') this leads to relative changes of the extinction that are smaller than 1% when the parameters for Rho- damine B,23a dioctadecyloxacarbocyanine," or a mero- cyanine23b are used. This effect can never explain changes of the SHC that amount 20-50%.

The dependence of @cT on the applied field can be ex- plained by using the existing theory describing electron- transfer processes at an electrode.24 In a first approxi- mation kf will be independent of the applied field while k,, will be given by eq 20. The free energy of an elec- tron-transfer process in the presence of an applied field is given by

(19) where AGO and AGO(0) are the free enthalpy change for the electron-transfer process in the presence and absence of the applied field (in eV), F is the applied electric field (directed from the monolayer to the crystal bulk, in V cm-9, and d is the distance over which the electron transfer takes place (in cm). This allows one to write for k,, (using eq 19)

AGO = AG"(0) - Fdq

k,, = k,,(O) exp(PdFq/kT) (20) This yields for @cT

(21) (a) Wright, G. T. Solid-state Electron. 1965,2, 165. (b) Prock, A.; Das, K. N.; Melman, P. J. Chem. Phys. 1983, 79, 4069.

(22) (a) Helfrich, W. In Physics and Chemistry of the Organic Solid State; Labes, M., Fox, D., Weissberger, A., Eds.; Interscience: New York, 1967; Vol. 3, p 1. (b) Shilinsh, E. A. Phys. Status Solidi A 1970,3,817.

where @cT and @cT(O) are the quantum yield for the electron-transfer process in the presence and absence of the applied field, is the transfer coefficient, and k,, and k,,(O) are It,, in the presence and absence of the applied electric field.

When PdFqlkT is smaller than unity, k,, will be given by

(22) When @cT is much smaller than unity, which is the case

for the oxacarbocyanine, then the following relationship holds:

@CT = @cT(O) exp(PdFq/kT) (23) As long as /3dFq/kT is also much smaller than unity, aCT will be given by

@CT = @cT(O)P + PFdq/kTl (24) For an electron-transfer process where eq 25 is satisfied, P can be given in a first appro xi ma ti or^^^,^^ by eq 26:

X > AGo(0) > -A (25)

k,, = k,,(0)(1 + PdFq/kT)

8 = dAG*/dAGO(O) = 1/2(1 + AGo(0)/X) (26)

where X is the reorganization energy for the electron- transfer process (in eV).

The values of P determined by using eq 24 for the dif- ferent combinations of dyes and crystals are given in Table I. AGo(0) can be calculated from the energy levels of the

(23) (a) Schmidt, S. Ph.D. Thesis, Technishe Universitat Berlin, 1973. (b) Bucher, H.; Wiegand, J.; Snavely, B. B.; Beck, K. H.; Kuhn, H. Chem. Phys. Lett. 1969, 3, 508.

(24) Bockris, J. OM.; Khan, S. U. M. In Quantum Electrochemistry; Plenum: New York, 1979.

(25) (a) Marcus, R. A. Ann. Rev. Phys. Chem. 1964, 13, 155. (b) Brunschwig, B. R.; Logan, J.; Newton, M. D.; Sutin, N. J. Am. Chem. SOC. 1980,102, 5798. (c) Sutin, N. Acc. Chem. Res. 1968, 1, 225.

(26) (a) Kurz, J. M. Chem. Phys. Lett. 1978, 57, 243. (b) Levine, R. D. J. Phys. Chem. 1979,83, 159.

998 Langmuir, Vol. 3, No. 6, 1987 Van der Auweraer et al.

Table I. Parameters of the Charge-Injection Process

Assuming Injection in the 2A7 Layer oxacarbocyanine-anthracene -1.04 2.45 1.41 0.01 0.51 0.49 0.05 f 0.01 bis[ oxacarbocyanine]-anthracene 2.15 0.45 0.004 f 0.001 oxacarbocyanine-perylene -1.04 2.45 1.41 0.01 0.51 0.27 0.07 f 0.02 bis[ oxacarbocyanine]-perylene 2.15 0.27 0.06 f 0.02 indolocarbocyanine-anthracene 0.02 1.90 1.92 -0.50 0.23 0.21 0.28 f 0.08 indolocarbocyanine-perylene 0.02 1.90 1.92 -0.50 0.23 0.21 0.32 f 0.05

Assuming Direct Injection into the Crystal 1.41 0.06 0.54 0.31 0.05 f 0.01 oxacarbocyanine-anthracene -1.04 2.45

bis[ oxacarbocyaninel-anthracene 2.15 0.29 0.004 f 0.001 oxacarbocyanine-per ylene -1.04 2.45 1.41 -0.41 0.24 0.17 0.07 f 0.02 bis[oxacarbocyanine]-perylene 2.15 0.17 0.06 f 0.02 indolocarbocyanine-anthracene 0.02 1.90 1.92 -0.45 0.25 0.13 0.28 f 0.08 indolocarbocyanine-per ylene 0.02 1.90 1.92 -0.92' -0.01 0.13 0.32 f 0.05

EDID-' hu" ED.IDP AGo(o)a P&db &ptl PCTd

"In eV. *Assuming X = 0.9 eV. CFor these values of AGO eq 26 gives values of /3 that are too low. dQuantum efficiency a t lo5 V cm-'.

dyes and the anthracene monolayer. The oxidation po- tential of an anthracene and a perylene molecule in the first layer of a crystal in contact with water amounts to respectively 1.33 and 0.86 V vs. the NHE.2J6J6 These values have to be corrected for changes of the image po- tential (cf. supra) when the first layer of an anthracene or a perylene crystal is covered by a 2A7 monolayer; they then equal respectively 1.47 and 1.01 f 0.05 V. Considering that the ionization potential of 2-alkylanthracenes equals that of anthracene and that the polarizability of a monolayer is close to that of an anthracene crystal, a value of 1.42 f 0.05 V can be assumed for the oxidation potential of a 2A7 monolayer. The reduction potential of the oxa- and in- dolocarbocyanines in acetonitrile amounts2' respectively to -1.04 and 0.02 V vs. the NHE. From the fluorescence maximum of a diluted (1/100) monolayer of the oxa-, a concentrated (1/5) monolayer of the oxa-, and a monolayer (1/10) of the indolocarbocyanine a singlet energy of re- spectively 2.45, 2.15, and 1.9 eV was determined for these species. Knowing the reduction potential of the excited dye and the oxidation potential of the 2A7 monolayer or the first molecular layer of the crystal, it is possible for one to calculate AGO (0) for the charge-generation process, considering hole injection in the 2A7 layer and in the first layer of the crystal. For the oxacarbocyanine dye and anthracene or perylene, AGo(0) is close to 0; therefore, 0 should be close to 0.5. For anthracene this is indeed ob- served, but for perylene 0 is considerably smaller (Table I). When direct hole injection in the perylene crystal is considered, the hole injection becomes 0.42 eV more exo- thermic, which leads (according to eq 25) to a smaller value of 0. The effect of the more negative AG"(0) is larger than that of the increase of the width of the tunneling barrier; this is not surprising, as the anthracene chromophore will only be a small barrier for the electron-tunneling process. For the indolocarbocyanine the hole injection in the 2A7 monolayer is more exothermic (rt0.5 eV) than for the ox- acarbocyanine, which corresponds to the smaller value of 0 observed for anthracene and perylene crystals and in- dolocarbocyanine. For this dye the hole injection into the 2A7 is already sufficiently exothermic, and the improved thermodynamics for direct hole injection into the perylene crystal will be compensated by the increased28 barrier width. This hypothesis also explains why aCT identical

(27) (a) Gilman, J. B. Photogr. Sci. Eng. 1974, 18, 475. (b) Seefeld, K.-P.; Mobius, D.; Kuhn, H. Helv. Chim. Acta 1977,60, 2608. (c) Large, F. In Photographic Sensitioity; Cox, R. J., Ed.; Proc. Symp. Photogr. Sci., Cambridge, 1972; Academic: London, 1974; p 241.

(28) (a) Bucher, H.; Kuhn, H.; Mann, B.; Mobius, D.; Von Szentpay, L.; Tillmann, P. Photogr. Sci. Eng. 1967, 11, 233. (b) Mann, B.; Kuhn, H. J . Appl. Phys. 1972, 42, 4398.

for hole injection from monomers and dimers of the oxa- carbocyanine in a perylene crystal: although ED.iD- is probably less favorable for the dimer, this will increase AG* less in the case of exothermic hole injection in the perylene crystal than in the case of slightly endothermic hole in- jection in the 2A7 monolayer.

When the observed values of p and the calculated values of AG"(0) are introduced into eq 26 a value of 0.9 f 0.1 eV is found for A. The values of the reorganization energy here ae considerably larger than those proposed by Wil- ligBbPc (0.3 eV) for adsorbed dyes but still smaller than the value obtained by Miller for electron transfer between two aromatic groups in MTHF.29a Using the value of 0.9 eV for X and neglecting the effect of the barrier width, we find that k,, will be a factor of lo3 times larger for injection from the oxacarbocyanine into a perylene crystal than into a 2A7 monolayer. For the indolocarbocyanine this factor will be less than 5.

Also, the other rate constants (e.g., k , and k-) and the ratio k, /k- will exhibit a dependence on the applied field. For an anthracene crystal covered by 2A7 the values of 0 will e q ~ a 1 ~ ~ ~ ~ ~ ~ ~ ~ respectively 0.5, -0.5, and 1. For the pe- rylene crystal the p values will be close to respectively 0, -1.0, and 1.0. If the rate of these steps would determine the overall quantum yield, the values of p should be in- dependent of the dye.

4. Influence of the Barrier Width. To test the as- sumption tht the low overall quantum yields of the SHC were due to a low value of @cT rather than to incomplete escape of the injected carriers, 2A7 was replaced by 2- anthrylalkanoic acids with a longer chain. When the distance over which the electron transfer occurs with a rate constant k,, increases, this process becomes slower. This shifts the competition between k,, and kf in favor of kf and leads to a decrease of the overall quantum yield. The width of the barrier is determined by the edge to edge distance (d? of the .rr-systems. Since the relationship be- tween k,, and d ' is given by eq 27,28r30 In 9 = In aCT is plotted vs. d':

k,, = A,, exp(-ad') (27)

a = 2h-1(2em(a)1/2 (28)

where A,, is the preexponential factor for the tunneling process in s-l, e is the electron charge (1.6 X 10-19C), m is the electron mass (9.3 X kg), (a is the tunneling barrier

(29) (a) Miller, J. R.; Calcaterra, L. T.; Closs, G. L. J. Am. Chem. SOC. 1984,106,3047. (b) Willig, F.; CharlB, K.-P. Faraday Discuss. Chem. SOC. 1982, 74, 141. (c) Willig, F.; Muller, N.; CharlB, K.-P. Electrochim. Acta 1979, 24, 463.

(30) Chow, C. K. J. Appl. Phys. 1965, 36, 559.

Sensitized Hole Injection

in eV, d is the edge to edge tunneling distance in A, and f i is Planck's constant divided by 27r (1.05 X Jas).

By use of eq 11 and 21, a value of 25 f 10 can be ob- tained for A,,/kf (assuming that k, is much smaller than kf). Assuming a value of 4 X lo8 s-l for kf, A,, equals (1.0 f 0.5) X 1O'O s-l.

The experimental value of a is considerably larger than that determined by Whitten for the tunneling of electrons from the excited state of a stilbene chromophore31 to an electron acceptor.

From the experimental data a value of 0.49 f 0.1 eV can be obtained for cp with eq 28. This agrees to the barrier obtained by Kuhn3, for a system where the electrons have to tunnel from a donor (dichlorobenzimidazolocarbo- cyanine) to an excited acceptor (diazanaphthimidazolo- carbocyanine). The barrier obtained here is larger than the one obtained by Kuhn for the tunneling of electrons from an excited donor (pyrenyl chromophore, oxacyanine) to a bipyridinium saltM acting as a ground-state acceptor. Kuhn et al. assumed that this was due to a difference in ionization potential of the orbitals involved in the tun- neling process.

Using a value of -4.5 eV for the abso1ute.electrode po- tential of the NHE3S and 2.3 eV for the of the conduction band of the aliphatic chain, one calculates a value of 1.35 eV for the barrier for tunneling from the singlet excited state of the oxacyanine to a bi- p y r i d i n i ~ m ~ ~ ~ ~ ~ ~ ~ ~ salt, 3.56 eV for the tunneling of an electron from anthracene to the singlet excited state of the oxacarbocyanine, and 3.36 eV for the system considered by K ~ h n . ~ ~ ~ * ~ ~ ~ The barriers calculated for the latter sys- tems are close to those found for tunneling in glasses be- tween molecules and radical ions in the ground state or between metals separated by "inert" monolayers of alka- noic acids. This discrepancy between the experimental value of the barrier and the theoretical one, observed for tunneling through fatty acid monolayers, can be explained by using several models.32b*35*36 A first explanation lies in a tunneling time (t,) that is small compared to the relax- ation time of the nuclei ( t J . In that case, the width of the energy levels involved in the tunneling process is deter- mined by tL1 rather than by t;l. In this framework a is given by

a = h-l(2emcp)lI2 (29)

With eq 29 and a = 0.73 f 0.07, a value of 2 f 0.4 eV is obtained for the barrier height, which is closer to the theoretical values.

with the HOMO% and the LUMO of the fatty acid chain, depending upon the energy of the levels between which electron tunneling takes place. In that case the matrix element H D A describing the interaction between the HOMO of anthra- cene and that of the dye can be calculated by using an

Another explanation lies in the

Langmuir, Vol. 3, No. 6, 1987 999

extended Huckel type c a l ~ u l a t i o n ~ ~ - ~ ~

HDA (P/A)" (30)

H D A ~ ( P / W " (31) where 0 is the exchange integral between two neighboring C-C a-bonds and A is the energy difference between the HOMO or the LUMO of the chain and the HOMO of anthracene or the oxacarbocyanine.

I t has to be remarked that the probability of a-assisted electron transfer also depends exponentially on the dis- tance between donor and acceptor. When a is the com- ponent of the length of a single C-C bond perpendicular to the plane of the monolayer, eq 31 can be rewritten as

This yields for a, a s ~ u m i n g ~ ~ i ~ ~ , ~ that t, is similar or equal to h/HDA and larger than t,, the following expression:

(33) The observed value of 0.73 f 0.07 A-1 for a yields a value

of 1.58 f 0.1 for PIA. This means that cDCT or k, decreases by a factor of 0.40 when the alkyl chain is extended by an extra CH, group (an extra a-bond). These values can be compared to those obtained by Miller39* for electron transfer (0.34) or by Paddon-Row and V e r h o e ~ e n ~ ~ ~ ~ ~ (0.27 for charge separation and 0.44 for charge recombination) in rigid bichromophoric systems. Padd0n-Row3~" obtains a value of 0.43 for P I A from the extinction coefficient of the charge-transfer absorption band (0.43) in analogous molecules. As the component of the o-bond parallel to the line linking the edge atom of both chromophores is dif- ferent for the systems considered in this contribution and the molecules considered by Miller and V e r h o e ~ e n , ~ ~ ~ different values of a will be found when eq 27 is used. As the energy level of the valence band of an arachidic acid monolayer is about 10 V below the vacuum level,40 the observed value of /3/A can be obtained with reasonable values37c of 0. A the electron affinity of solid benzene equals 1-1.5 eV,4l a value of 2.3 eV below the vacuum level (as considered by Kuhn et al.28932) is probably too much for the electron affinity of the aliphatic chains, which will be closer to values between -0.3 and 0.20 eV.42 This suggests that for the same value of 0 the interaction with the LUMO of the chain will be smaller than with the HOMO.

HDA' exp[-d@/d In @/A)] (32)

a = @/a) In @/A)

Conclusions The kinetics of the photosensitized hole injection into

an anthracene single crystal are modified profoundly when a 2A7 monolayer is deposited between the anthracene crystal and the dye. The reduction of the image potential a t the position where the injected hole is t h e r m a l i ~ e d ~ ~ leads to a 100-fold acceleration of the escape of the injected hole into the crystal bulk. As this effect is magnified by a probable decrease of the recombination rate, the escape

(31) Mooney, W. F.; Whitten, D. G. J. Am. Cheni. SOC. 1986,108,5712. (32) (a) Kuhn, H. In Light-Induced Charge Separation in Biology and

Chemwtry; Gerischer, H., Katz, J. J., Eds.; Dahlem Konferenzen: Berlin, 1979; pp 151-169. (b) Kuhn, H. In Modern Trends of Colloid Science in Chemistry and Biology; Birkhauser Verlag: Basel, 1985; pp 97-125. (c) Marcus, R. A,; Sutin, N. Biochim. Biophys. Acta 1985, 811, 265.

(33) MLibius, D. Ber. Bunsenges. Phys. Chem. 1978, 82, 848. (34) Noyes, M. J. Am. Chem. SOC. 1962, 84, 513. (35) Kuhn, H. Isr. J. Chem. 1979, 18, 375. (36) Mc Connell, H. M. J. Chem. Phys. 1971, 35, 508. (37) (a) Heitele, M. E.; Michel Beyerle, E. J. Am. Chem. SOC. 1985,

107,8286. (b) Lmson, S. Faraday Discuss. Chem. SOC. 1982,74,390. (c) Beratan, D. N.; Hopfield, J. 3. J. Am. Chem. Soc. 1984,106, 1584. (d) Mc Lendon, G.; G u m , T.; Mc Guirre, M.; Simolo, K.; Strauch, S.; Taylor, K. Coord. Chem. Reu. 1985,64, 113.

(38) Miller, J. R.; Beitz, J. V. J. Chem. Phys. 1981, 74, 6746.

(39) (a) Miller, J., Proceedings of the NATO Workshop On Electron and Energy Transfer in Supramolecular Species, Anacapri, Italy, 1987, to be published. (b) Hush, N. S.; Paddon-Row, M. N.; Cotsaris, E.; Hoevering, H.; Verhoeven, J.; Heppener, M. Chem. Phys. Lett. 1985,117, 8. (c) Verhoeven, J., personal communication.

(40) (a) Jahresbericht Synchrotronstrahlungslabo HASYLSAB, Deutache Electron Synchrotron DESY, 1984, pp 99-100. (b) Watanabe, K.; Nakayama, T.; Mottl, J. J. Quant. Spectrosc. Radiat. Transfer 1962, 2, 369.

(41) (a) Tuttle, T. L.; Weissman, S. I. J. Am. Chem. SOC. 1958, 80, 5342. (b) Gerson, F.; Ohya-Nishiguchi, H.; Wydler, C. Angew. Chem. 1976, 18, 617.

(42) Holvrod. R. A.: Russell. R. L. J . Phvs. Chem. 1974. 78. 2128. (43) (a) Smejtek, P.i Silver, M.; Dy, K. S:J. Chem. Phys. 1973, 59,

1374. (b) Holyrod, R. A.; Dietrich, K. B.; Schwarz, H. A. J. Chem. Phys. 1972, 76, 3794.

1000 Langmuir 1987, 3, 1000-1004

process saturates a t very low (<3 X lo3 V cm-l) field strengths. The low overall quantum yields can be ex- plained by competition between monomolecular decay of the excited dye and hole injection. Replacing the oxa- by the indolocarbocyanine, which makes the injection process more exothermic (0.50 eV), leads to a higher yield for the hole injection. The dependence of the SHC on the applied electric field a t high field strengths is due to a change of the free enthalpy change of this hole injection. Contrary to other systems, where a s e ~ o n d - o r d e r ~ ~ dependence of the rate of a photoinduced electron-transfer process of an applied electric field is observed, electron transfer is a vectorial process in the systems considered here and al- ready exhibits, therefore, first-order dependence on the applied field.

As the SHC depends upon the length of the alkyl chain of the w-(2-anthryl)alkanoic acid deposited between the

dye and the crystal, an attempt can be made to calculate the height of the barrier for through-space tunneling and medium-assisted tunneling. For the system considered here the discrepancy between the barriers that are calcu- lated on the basis of energy levels and those obtained from the distance dependence of the tunneling efficiency would in this case become larger when it is assumed that the electron tunneling is assisted by the LUMO of the aliphatic chain. The agreement for hole tunneling assisted by the HOMO of aliphatic chain would, however, remain.

Acknowledgment. We are indebted to the NFWO (M.V.) and the IWONL (B.V.) for continuous support and to the Alexander von Humboldt Stiftung for a fellowship for M.V. We thank Joachim Lehnert for the growing of the anthracene single crystals.

2A7, 73693-26-2; 2A8, 110015-64-0; 2A9, 82793-56-4; 2A10, 110015-65-1; dioctadecylindolocarbocyanine

Registry No.

perchlorate, 99708-02-8; dioctadecyloxacarbocyanine perchlorate, 34215-57-1; anthracene, 120-12-7; Perylene, 198-55-0; arachidic (44) Popovic, 2. D.; Kovacs, G. J.; Vincett, P. S. Chem. P h p . Lett.

1985, 116,405. acid, 506-30-9.

Ar+ Plasma Etching of Palmitic Acid Multilayers: Differential Erosion Rates of Exposed and Protected Layers

William W. Newcomb, Troy A. Johnston, and Jay C. Brown* Department of Microbiology, University of Virginia Medical Center,

Charlottesville, Virginia 22908

Received February 24, 1987. I n Final Form: May 5, 1987

The degree of spatial resolution obtainable by ion etching was examined by analyzing palmitic acid multilayers, of the type originally described by Langmuir and Blodgett, after erosion in a low-energy (0.5-keV) Ar' plasma. Palmitate multilayers were prepared on aluminum foil supporta in such a way that a reference, radioactively labeled bilayer was either exposed on top of the overall multilayer or covered by one or more nonradioactive bilayers. Etching of such multilayers revealed that the rate of loss (by sputtering) of the reference bilayer depended critically on the number of protecting, nonradioactive layers. More protection was afforded by four than by two bilayers and more by two than by one. Similar experiments involving multilayers containing a reference (radioactive) monolayer revealed significant protection when the reference layer was covered by a single palmitic acid monolayer. Chemical analysis after etching of exposed and covered bilayers demonstrated that a single bilayer was able to protect a reference layer from significant covalent chemical damage by the Ar+ beam. Together the results indicate that plasma etching techniques may be employed to resolve structures in biological materials if they are separated by distances comparable to one to two palmitic acid monolayers (2.2-4.4 nm).

Introduction Over the past 15 years, low-energy ion or plasma etching

has emerged as a method with considerable potential for use in the structural analysis of biological materials. Etching of whole cells and tissues, for instance, has enabled one to observe the nucleus and other organelles a t high resolution in the scanning electron microscope.lq2 Ar+ etching of human adenovirus 2 has revealed ultrastructural aspects of the virion core3 while biochemical analyses of etched T4 and X have clarified the arrangement of DNA in the bacteriophage head.- Further development of ion

(1) Tanaka, K.; Iino, A.; Naguro, T. Arch. Histol. Jpn. 1976,39, 165. (2) Fujita, T.; Nagatani, T.; Hattori, A. Arch. Histol. Jpn. 1974, 36,

(3) Newcomb, W.; Boring, J.; Brown, J. J. Virol. 1984, 51, 52. (4) Black, L.; Newcomb, W.; Boring, J.; Brown, J. Proc. Nutl. Acud.

195.

Sci. U.S.A. 1985, 82, 7960.

0743-7463/87/2403-1000$01.50/0

etching methods, however, particularly for virus-sized and smaller objects will require that one have accurate infor- mation about the degree of spatial resolution obtainable during the etching (sputtering) process. One needs to know the thickness of the layer of material that contributes to ejected species and/or is chemically altered immediately upon initiation of the plasma bombardment. Put differ- ently, one can ask how deeply beneath the surface an atom or molecule must lie to be temporarily protected from sputtering or from chemical damage by the ion plasma. The thicker the protective layer required, the less spatial resolution is theoretically obtainable.

We have recently devised a strategy, involving use of fatty acid multilayers, for directly measuring the effect of

(5) Brown, J.; Newcomb, W. J . Virol. 1986, 60, 564. (6) Bendet, I.; Rizk, N. Biophys. J. 1976, 16, 357.

0 1987 American Chemical Society