Solution versus Vapor Growth of Dipolar Layers on Activated Oxide

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Solution versus Vapor Growth of Dipolar Layers on Activated Oxide Substrates F. Nu¨esch,* M. Carrara, and L. Zuppiroli Laboratoire d’optoe´ lectronique des mate´ riaux mole´ culaires, Institut des mate´ riaux, EPFL, CH-1015 Lausanne, Switzerland Received December 7, 2002. In Final Form: April 7, 2003 Monolayer adsorption of benzoic acid derivatives on activated oxide surfaces from solution and from the vapor phase are investigated with the view of modifying the work function of a conductive substrate. Para-substituted benzoic acids with a wide range of electrical dipoles allow adsorption to be followed by measuring the surface potential of the grafted substrates using the Kelvin probe technique. From the linear correlation between adsorbed molecular dipole and work function, it is possible to obtain the dipole contribution of the anchoring group. The vapor growth method has striking advantages over the monolayer growth from solution: it is extremely fast and avoids numerous problems related to solution processing such as intercalated solvent molecules in the dipole layer or chemical reactivity between the solvent and the oxide surface.

Introduction Dipole layers play a significant role at interfaces, especially when charge-transfer processes occur. In electrochemistry, the role played by a double ionic layer at the electrode immersed in the electrolyte is to produce chemical potential shifts across the electrode interface to allow interfacial redox processes. Similarly, dipole layers adsorbed on solid substrates can be used to induce important shifts of the electrode work function. This is of particular interest in organic optoelectronic devices, where a good match between electrode work function and frontier molecular orbital is desired.1 The study and growth of monolayers on solid substrates has a long history. The first study on the deposition of multilayers of a long-chain carboxylic acid onto a solid substrate was carried out by Blodgett.2 Since then, amphiphilic molecules have been used to form selfassembled monolayers (SAMs). Alkanethiols on gold surfaces and silanes on oxidized silicon are the most widely studied systems.3 Other systems such as acidic compounds on metal oxide surfaces have also been investigated.4,5 Mineral acids and bases have been used to produce dipolar layers on oxides.6 In most cases, the growth of self-assembled monolayers is based on solution processes. Although very cheap, solution growth has major technological drawbacks: its slowness and the use of organic solvents. Alternatively, SAMs have been grown from the vapor phase, mainly to explore the growth mechanism and structure in situ under clean conditions. There are much fewer structural studies * To whom correspondence should be addressed. E-mail: [email protected]. (1) (a) Zuppiroli, L.; Si-Ahmed, L.; Kamaras, K.; Nu¨esch, F.; Bussac, M. N.; Ades, D.; Siove, A.; Moons, E.; Gra¨tzel, M. Eur. Phys. J. B 1999, 11, 505. (b) Cui, J.; Huang, Q. L.; Wang, Q. W.; Marks, T. J. Langmuir 2001, 17, 2051. (c) Hatton, R. A.; Day, S. R.; Chesters, M. A.; Willis, M. R. Thin Solid Films 2001, 394, 292. (d) Vilan, A.; Shanzer, A.; Cahen, D. Nature 2000, 404, 166. (2) Blodgett, K. J. Am. Chem. Soc. 1935, 43, 1007. (3) Schreiber, F. Prog. Surf. Sci. 2000, 65, 151. (4) (a) Onishi, H.; Aruga, T.; Egawa, C.; Iwasawa, Y. Surf. Sci. 1988, 193, 33. (b) Guo, Q.; Cocks, I.; Williams, E. M. J. Chem. Phys. 1997, 106, 2924. (c) Guo, Q.; Cocks, I.; Williams, E. M. Surf. Sci. 1997, 393, 1. (5) Ullman, A. Chem. Rev. 1996, 96, 1533. (6) Nu¨esch, F.; Rothberg, L. J.; Forsythe, E. W.; Le, Q. T.; Gao, Y. Appl. Phys. Lett. 1999, 74, 880.

of adsorbates on metal-oxide surfaces. For example, the adsorption of carboxylic acids on well-characterized TiO2 surfaces has been reported.4,7 In this work, we investigate two different methods to grow dipolar monolayers of benzoic acid derivatives on oxide surfaces that are relevant to optoelectronic devices. In particular, correlation between work function and adsorbed dipole is investigated. Both solution and vapor growth are explored under clean conditions and compared. The work function shifts induced by the adsorbed dipoles are measured by the Kelvin probe technique inside the glovebox, when solution growth is used, or in vacuo, when the monolayers are grown from the vapor phase. Experimental Section Chemicals and Substrates. p-Aminobenzoic acid (AmBA), p-anisobenzoic acid (ABA), p-benzoic acid (BA), p-bromobenzoic acid (BBA), p-cyanobenzoic acid (CBA), and p-nitrobenzoic acid (NBA) were supplied by Aldrich. All benzoic acid derivatives used in this work are presented in Chart 1 together with the dipole moment of the corresponding substituted benzene (without carboxylic acid). Negative dipole moments apply when the negative pole of the dipole is oriented toward the carboxylic acid group. The carboxylic acid functional group was chosen as the anchoring group for indium-tin oxide (ITO) and aluminum surfaces. Conducting glass ITO substrates (AFC, 20 Ω/square) were cleaned using a standard procedure8 before being exposed to argon or oxygen plasma. Aluminum substrates were obtained by vapor deposition of a 100 nm thin aluminum film on cleaned glass substrates. The metal substrates were also exposed to oxygen and argon plasma, respectively. Work functions were measured by Kelvin probe after argon plasma treatment and ranged from 4.8 to 4.9 eV for ITO and from 4.1 to 4.2 eV for aluminum. After oxygen plasma treatment, the work functions ranged from 5.8 to 5.9 eV for ITO and from 4.5 to 4.7 eV for aluminum. Kelvin Probe Measurements. The Kelvin probe technique is a convenient method to measure the surface potential of a (7) (a) O’Regan, B.; Moser, J.; Anderson, M.; Gra¨tzel, M. J. Am. Chem. Soc. 1994, 94, 8720. (b) Meyer, T. J.; Meyer, G. J.; Pfennig, B. W.; Schoonover, J. R.; Timpson, C. J.; Wall, J. F.; Kobusch, C.; Chen, X.; Peek, B. M.; Wall, C. G.; Ou, W.; Erickson, B. W.; Bignozzi, C. A. Inorg. Chem. 1994, 33, 3952. (8) Nu¨esch, F.; Rotzinger, F.; Si-Ahmed, L.; Zuppiroli, L. Chem. Phys. Lett. 1998, 288, 861.

10.1021/la026962w CCC: $25.00 © 2003 American Chemical Society Published on Web 05/10/2003

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Chart 1. Para-Substituted Benzoic Acid Derivatives Used in This Worka

a The carboxylic acid is used as an anchoring group for the adsorption on oxide substrates such as ITO or aluminum with a native oxide layer. The dipole moments of the corresponding substituted benzenes are taken from the literature (ref 11) and are indicated in brackets. The minus sign applies when the negative pole of the dipole is oriented toward the carboxylic acid group.

sample.9 This technique uses a vibrating reference electrode which is placed as close as possible to the surface of interest without contacting it. Due to the work function difference between the sample and the reference electrode, the capacitor formed by the two electrodes is being charged. The vibrating electrode modulates the charging current, which can be measured by a lock-in amplifier. By application of an external compensating voltage, which is equal to the work function difference between reference and sample, the modulated current is zeroed. In this work, we used a vibrating gold electrode together with a controller (DeltaPhi). The Kelvin probe was mounted in a Faraday cage inside the glovebox or inside a vacuum chamber. Apparatus for Vapor Growth and Characterization. A cylindrical vacuum chamber with a base pressure of 10-7 Torr was used both for monolayer fabrication and for surface potential measurements. The sample holder on the top of the chamber can be connected to a radio frequency (rf) power source (Advanced Energy X-600) to allow plasma surface treatment of the substrates. Argon or oxygen gas is provided by two separate gas lines, and the gas pressure during the plasma is adjusted to 0.1 Torr. The plasma treatment is applied for 4 min at a power of 10 W. Then, the surface potential is measured using a calibrated Kelvin probe (DeltaPhi) which can be retracted in a side chamber while not measuring. After 5 min, treated surfaces are exposed to the vapor of benzoic acid derivatives. Both the substrate as well as the effusion cell are at room temperature. The surface potential is again measured to obtain the work function shift induced by the grafting process. In some cases, the desorption kinetics were measured by monitoring the surface potential as a function of time. To investigate thin films with a nominal thickness of several monolayers, the molecules were vapor deposited from a resistive boat at a rate of 0.1 Å‚s-1 at a pressure of 5 × 10-7 Torr. The surface potential of thin films was measured layer by layer. Monolayers Grown from Solution. To guarantee clean conditions, the monolayer growth from solution was carried out in a glovebox filled with argon according to a previous procedure.8 First, the clean substrates were exposed to argon plasma at a pressure of 10-1 Torr and a power of 10 W. The plasma cleaning chamber is connected to the glovebox such that the treated substrates can be transferred directly to the grafting solution without exposure to the ambient atmosphere. Solute concentrations ranged from 10-5 to 10-4 M in tetrahydrofurane (THF, Carlo Erba, 99.8%) or tetrahydrothiophene (THT, Fluka, 97%). After a grafting time of 15 h, the samples were rinsed in pure THF and THT, respectively, to remove excess molecules from the grafted surface. These chemically modified substrates were used for Kelvin probe measurements in the glovebox. To measure adsorption isotherms, monolayers were also grown on ITO and aluminum powders. The powders were supplied by CERAC and have surface densities of 1.25 m2‚g-1 and 7 m2‚g-1 for ITO and aluminum, respectively. Solutions with a given amount of powder comprising six different solute concentrations were stirred for 24 h and filtered. The concentration of adsorbed molecules was derived from UV absorption measurements of the filtered solution using a CARY 05 spectrophotometer. The maximum surface density of adsorbed benzoic acid derivatives (9) Evens, S. D. In Characterization of Organic Thin Films; Butterworth-Heinemann: Sotenham, 1995; p 113.

Figure 1. Adsorption isotherm at room temperature, plotting the adsorbed p-anisobenzoic acid concentration cads versus the initial concentration cinit. THF solutions of 20 mL with a given benzoic acid concentration cinit were used, each containing 0.1 g of aluminum powder with a specific surface of 0.7 m2/g. cads was inferred from UV absorption of the filtered solution after a stirring time of 12 h. and the equilibrium constant for the adsorption reaction were obtained from a fit to the data using the Langmuir model.

Results and Discussion In solution, it is relatively easy to study the thermodynamical parameters of the adsorption reaction. The adsorption reaction is investigated on fine powders of indium-tin oxide and aluminum, which are electrode materials that are used in optoelectronic devices. A typical curve obtained for the adsorption isotherm on aluminum and ITO powders is shown in Figure 1. For a fixed concentration of powder, the concentration cads of adsorbed molecules shows saturation behavior as the initial benzoic acid concentration is increased. This indicates monolayer coverage. To estimate the molecular surface concentration Γ0 at saturation and the equilibrium constant Kads, we fitted the data to the Langmuir equation

ceq ceq 1 ) + cads K Γ0 Γ0 ads

(1)

where ceq is the benzoic acid solute concentration and cads is the concentration of adsorbed molecules at equilibrium. The values are reported in Table 1. From the surface concentration, we can infer a mean molecular area of 30 Å2, which indicates rather dense monolayer coverage. If the equilibrium constant Kads is measured as a function of temperature, the standard enthalpy of adsorption can

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Table 1. Molecular Surface Density Γ0 and Equilibrium Constant Kads of the Adsorption Reaction in Solution Obtained by a Least-Squares Fit to Equation 1a Γ0 specific Kads ∆G0ads substrate molecule (1018 m-2) area (Å2) (mol-1) (kJ‚mol-1) ITO

Al

AmBA ABA BA BBA CBA NBA AmBA ABA BA BBA CBA NBA

2.3 2.0 2.3 1.7 3.0 1.3 2.8 2.7 4.6 3.2 2.2 3.8

43 50 43 58 33 76 35 37 21 31 45 26

17 000 10 000 18 000 15 000 19 000 16 000 27 000 26 000 10 000 15 000 18 000 23 000

-23.9 -22.6 -24.0 -23.6 -24.2 -23.7 -25.0 -24.9 -22.6 -23.6 -24.0 -24.6

a The corresponding specific areas and the standard free energy changes ∆G0ads ) -RT ln(Kads) are also listed. Langmuir adsorption isotherms were measured at T ) 295 K in THF solution using fine ITO or aluminum powders as adsorbing surfaces. The abbreviations used to designate the molecules correspond to the molecular structures given in Chart 1.

be obtained from the Van’t Hoff equation.10 For benzoic acid, we obtain binding enthalpies of -0.06 and -0.11 eV for ITO and Al powders, respectively. This binding energy is typical of hydrogen bonds or ionic binding. It is however too small to be attributed to covalent bonding. We used the same solutions to adsorb benzoic acid monolayers on solid aluminum and ITO substrates. Figure 2 shows the surface potential shifts ∆V as a function of the dipole moment µbody of the para-substituted benzene taken from the literature.11 ∆V correlates in a linear fashion to the dipole moment µbody. By using the measured surface density at saturation Γ0 as obtained from the Langmuir isotherm, the adsorbed dipole moment µads can be calculated using

µads )

∆V0 Γ0

(2)

where ∆V is the surface potential shift due to the monolayer and 0 is the vacuum permittivity. The relative permittivity  is taken to be 5.3 according to previous work.1a Table 2 shows the surface potential shifts ∆V due to the grafting of benzoic acid derivatives as well as the measured work function Φ of the derivatized ITO and aluminum electrodes. On ITO, the surface dipole corresponds to the molecular dipole in the gas phase. Indeed, if we add the carboxylic acid dipole moment of -1.7 D12 to that of the functionalized benzene µbody, we can rationalize the surface dipole values µads. On the aluminum substrates, however, the adsorbed dipole values are shifted to higher values. In a previous work, we have attributed this effect to the protonation of the aluminum surface by the carboxylic acid.13 This effect is greatly diminished when using THT instead of THF as the grafting solvent. If the monolayer growth occurs from THT, the work function change is similar to the one measured on ITO (Figure 2). Obviously there is a huge solvent effect in the grafting process on aluminum substrates. Possibly the aluminum (10) Atkins, P. W. In Physical Chemistry, 4th ed.; Oxford University Press: Oxford, 1990; p 219. (11) Weast, R. C. In CRC Handbook of Chemistry and Physics, 57th ed.; CRC Press: Cleveland, 1976; E 63. (12) Exner, O. In Dipole Moment in Organic Chemistry; Georg Theme Publishers: Stuttgart, 1975. (13) Carrara, M.; Nu¨esch, F.; Zuppiroli, L. Synth. Met. 2001, 121, 1633.

Figure 2. Correlation between the surface potential shifts ∆V induced by adsorbed monolayers of benzoic acid derivatives and the literature values of the dipole moment µbody of the corresponding substituted benzene without the carboxylic group. The three experimental sets are labeled in the graph. They correspond to argon plasma treated aluminum and ITO films that are grafted in either a THF or THT solution. The straight lines are linear fits to the respective data points. The same abbreviations as in Chart 1 were used to identify the molecules in the graph. Table 2. Work Functions Φ and Surface Potential Shifts ∆V Obtained by Grafting Monomolecular Layers of Benzoic Acid Derivatives on Argon Plasma Treated ITO and Aluminum Substratesa substrate

molecule

Φ (eV)

∆V (V)

µads (D)

µbody (D)

ITO

AmBA ABA BA BBA CBA NBA AmBA ABA BA BBA CBA NBA

4.08 4.28 4.38 4.86 4.92 4.98 4.02 4.18 4.28 4.90 5.13 5.32

-0.72 -0.52 -0.42 0.06 0.12 0.18 -0.18 -0.02 0.08 0.70 0.93 1.12

-4.4 -3.7 -2.6 0.5 0.6 2.0 -0.9 -0.1 0.4 3.1 6.0 4.2

-1.5 -1.2 0.0 1.6 4.1 4.2 -1.5 -1.2 0.0 1.6 4.1 4.2

Al

a µ ads was calculated by formula 2 taking the surface densities Γ0 from Table 1 and using a permittivity  of 5.3 (ref 1a). For comparison, the literature values (ref 11) of the electrical dipole moments µbody of the para-substituted benzenes (without carboxylic acid) are also included. The abbreviations used to designate the molecules correspond to the molecular structures given in Chart 1.

surface undergoes a strong oxidation in THF which does not occur in THT. To avoid the solvent problem, we have developed a method to graft the monolayers from the vapor phase using a high-vacuum chamber comprising an effusion cell filled with the benzoic acid derivative at room temperature. Prior to grafting, the aluminum or ITO substrates were exposed to low-power argon or oxygen plasma as described in the Experimental Section. It is known for the case of argon that the plasma induces radicals on the surface, thereby activating the surface.14 Grafting of the molecules was achieved by exposing the activated surfaces for a few seconds up to 20 s to the vapor of the benzoic acid derivative. Longer exposure times did not induce any (14) (a) Pereira, G. J.; da Silva, L. P.; Tan, I. H.; Gouvea, D. J. Mater. Chem. 2000, 10, 259. (b) Grundmeier, G.; Stratmann, M. Appl. Surf. Sci. 1999, 141, 43.

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further changes to the work function, indicating that the activated substrates reach their maximum surface coverage within a short time. Figure 3 shows the surface potential shifts measured in vacuo using the Kelvin probe technique. Clearly the shifts are much more important than in the solution case. Also, within experimental precision, the shifts do not depend on the substrate surface but they do depend slightly on the plasma treatment. These shifts correspond to a huge variation in the work functions of both ITO and aluminum. On ITO, work functions as high as 7.5 eV can be obtained using oxygen plasma treated substrates grafted with NBA. The lowest ITO work function of 3.8 eV was obtained on an argon plasma treated substrate grafted with AmBA. Similarly, the highest aluminum work function of 6.3 eV was obtained on oxygen plasma treated aluminum grafted with NBA, while the lowest aluminum work function on argon plasma treated substrates grafted with AmBA was found to be 3.1 eV. The linear dependence of the surface potential shift on the dipole moment µbody of the para-substituent is striking. Also, the shifts are slightly more important in the oxygen plasma case as compared to the argon plasma treatment. This can be attributed to the chemical action of oxygen removing adsorbed hydrocarbons more efficiently and allowing thereby a higher surface coverage Γ0. Let us first estimate the rate of benzoic acid molecules hitting the activated substrate surface. The base pressure in the chamber during exposure remained at 5 × 10-7 Torr such that the mean free path of the molecules exceeds the chamber dimensions. Therefore the rate of escape from the cell containing the benzoic acid derivatives can be treated by effusion. Assuming that every molecule that is impinging on the activated surface remains attached, we can estimate the minimal dose Dmin required for monolayer growth using the kinetic theory of ideal gases:15

Dmin ) psattmin ) 2Γ0xmkBT

(3)

where psat is the saturating pressure of the benzoic acid derivatives at temperature T, tmin is the minimum time for obtaining a monolayer, m is the molecular mass, and kB is the Boltzmann constant. To estimate the minimum exposure time tmin and dose Dmin, we assume a monolayer coverage of Γ0 ) 4 × 1018 m-2 (corresponding to a specific surface area of 25 Å2) and use the saturated pressure of benzoic acid derivatives at 298 K from the literature.16 The result is given in Table 3. The minimal deposition times range from 2 to 8 ms. These values are considerably shorter than the exposure times used in our experiment. For an ideally sticking surface, we would therefore not expect to observe any time (15) Taking the mean quadratic velocity 〈vx2〉 ) kBT/m along the direction of deposition x of benzoic acid molecules with molecular mass m at temperature T ) 297 K, the rate of escape Z per unit time and per unit surface is expressed as Z ) 1/2N〈vx2〉1/2 where N ) p/kT is the number of molecules per unit volume at pressure p. Assuming that all molecules leaving the effusion cell hit the activated substrate surface and remain attached to it, we can write the time-dependent surface density Γ of adsorbed molecules as Γ(t) ) pt/[2(mkBT)1/2], where p is the partial pressure of benzoic acid molecules at temperature T and t is the time of substrate exposure. The minimal exposure time tmin is obtained for Γ ) Γ0. The partial pressure p inside the effusion cell is taken as the saturation pressure psat of benzoic acid derivatives at room temperature. (16) (a) Ribeiro da Silva, M. A. V.; Matos, M. A. R.; Monte, M. J. S.; Hillesheim, D. M.; Marques, C. P. O.; Vieira, N. F. T. G. J. Chem. Thermodyn. 1999, 31 1429. (b) Monte, M. J. S.; Hillesheim, D. M. J. Chem. Thermodyn. 1999, 31 1443. (c) Monte, M. J. S.; Hillesheim, D. M. J. Chem. Thermodyn. 2001, 33, 745.

Figure 3. Correlation between the surface potential shifts ∆V induced by a series of benzoic acid derivatives and the literature values of the dipole moment µbody of the corresponding substituted benzene without the carboxylic group. The four different experimental sets are indicated in the legend, comprising argon plasma treated aluminum and ITO samples as well as oxygen plasma treated aluminum and ITO surfaces. The straight lines are linear fits to the respective data points. The same abbreviations as in Chart 1 were used to identify the molecules in the graph. Table 3. Minimum Exposure Times tmin and Doses Dmin (in langmuir ) 10-6 Torr‚s) at Room Temperature Estimated from Equation 3 at T ) 295 Ka molecule

psat at 298 K (×10-4, Torr)

tmin (ms)

Dmin (langmuir)

AmBA ABA BA BBA CBA NBA

7.54 2.40 7.55 3.67 4.20 7.56

2.7 8.0 2.3 6.0 4.5 2.7

2.0 1.9 1.7 2.2 1.9 2.2

a The saturated pressure values at room temperature are taken from the literature (ref 16). Abbreviations used to designate the molecules correspond to the molecular structures given in Chart 1.

dependence in the time range of seconds, which corroborates our observations. After several days, desorption of the dipole layer can be observed. By re-exposing the substrates to the vapor, the work function shift is restored, although after a much longer time than observed on the activated substrate (Figure 4). This indicates that surface activation is necessary to achieve monolayer coverage in less than 1 s. From a heated boat inside the chamber, we have evaporated layers of up to 20 Å of benzoic acids onto the substrates. Figure 5a,b shows the surface potential shifts upon vapor deposition of thin films of NBA and AMBA onto grafted aluminum and ITO substrates. The addition of thin films of benzoic acid derivatives does not change the surface potential of the grafted substrates, which means that the surface potential shift is coming from the very first surface-adsorbed monolayer. Furthermore, desorption of the dipole layer is strongly inhibited by depositing a 20 Å thick overlayer. On the basis of the above discussion, we can assume monolayer coverage of our substrates. As mentioned above, there is a striking linear dependence of the surface potential shift on the molecular dipole moment µbody of the substituted benzene (Figure 3). Moreover, the potential shifts are astonishingly similar on both ITO and aluminum and depend only slightly on the surface treatment. If we

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Langmuir, Vol. 19, No. 12, 2003 4875

the adsorbed carboxylic acid group. Thus we obtain a dipole moment of -1.1 D for the adsorbed carboxylic acid, which is almost independent of the surface treatment. This value is smaller than the gas-phase dipole moment of -1.7 D,11 which is probably due to the displacement of the carboxylic acid proton involved in the surface binding. It has been pointed out in the literature17 that dielectric screening can be rationalized by introducing macroscopic dielectric constants for the different parts of the adsorbed layer. Here we call COOH and body the dielectric constants for the attachment group and the molecular body, respectively. In this view, formula 2 becomes

µCOOH µbody ∆V0 + ) COOH body Γ0 Figure 4. Dependence of the surface potential upon desorption and readsorption of NBA grafted from the vapor phase onto argon plasma cleaned aluminum (full squares) and ITO (empty squares) substrates. Desorption occurs at negative times, while readsorption onto the passivated substrates occurs at positive times in the graph.

(4)

If we fit this relation to the data corresponding to the argon plasma treated substrates and take a specific molecular surface area of 25 Å2, we obtain values of 3.9 and 3.8 for COOH and body, respectively. This value is considerably smaller than the one generally used in the case of solution-grown monolayers. It might reflect the fact that intercalated polar solvent molecules are absent in the monolayer growth from vapor, while the latter are able to screen the adsorbed dipoles in the case of solution growth. Another reason might come from differences in the adsorption densities and dipole orientations in the two cases. In summary, we have shown that monolayers of benzoic acid derivatives on ITO and aluminum substrates with native surface oxide can be deposited both from solution and from the vapor phase. Conventional solution processing yields derivatized substrates with important surface potential shifts which depend to a large extent on the solvent used in the self-assembly. Vapor deposition on activated substrates yields much larger shifts, which can be reproducibly obtained. From the significantly lower shifts observed in the solution process, we infer that solvent molecules coadsorb on the surface and might even screen the surface dipoles. Conclusions

Figure 5. Dependence of the surface potential shift on the vapor deposition of thin films of nitrobenzoic acid (a) and dimethylamino-benzoic acid (b) onto grafted aluminum (full squares) and ITO (empty squares) substrates. The surface potential shifts were measured layer by layer using a Kelvin probe in vacuo. Before deposition of the thin organic layers, the substrates were activated using an argon plasma and grafted from the vapor phase using nitrobenzoic and dimethylaminobenzoic acid, respectively. The potential shift coming from the first monolayer is indicated in the graph at zero thickness.

look at the straight lines in Figure 3 obtained by a linear fit to each of the four experimental series (i.e., argon plasma surface treatment on aluminum and ITO as well as oxygen plasma surface treatment on aluminum and ITO), we notice that they all cross the abscissa at a value of 1.1 D. At this point µbody would be opposite and equal in absolute value to the electrical dipole moment µCOOH of

By activating oxide substrates such as ITO or aluminum films with native oxide, it is possible to obtain extremely fast vapor growth of dipole layers that exhibit much higher surface potential shifts than dipole layers grown from solution. Oxide work functions can be tuned by more than 3 eV using appropriate surface activation and grafting. The vapor growth method is of considerable technological interest in the fabrication of organic optoelectronic devices where monolayers play a crucial role in wetting and charge-transfer processes. Acknowledgment. We thank the Swiss National Science Foundation for financial support under Contract Number 20-67929.02. We are grateful to Dr. D. Berner for helpful discussions. LA026962W (17) (a) Oliveira, O. N., Jr.; Taylor, D. M.; Lewis, T. J.; Salvagno, S.; Stirling, C. J. M. J. Chem. Soc., Faraday Trans. 1 1989, 85, 1009. (b) Demchak, R. J.; Fort, T. J. Colloid Interface Sci. 1974, 46, 191.