Experimental Evidence of Molecular Cooperative Effect in a Mixed

Oct 5, 2010 - The Institute of Chemistry and the Center for Nanoscience and Nanotechnology, The Hebrew University of Jerusalem, Jerusalem 91904, Israe...
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J. Phys. Chem. C 2010, 114, 20531–20538

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Experimental Evidence of Molecular Cooperative Effect in a Mixed Parallel and Antiparallel Dipole Monolayer† Ruthy Sfez,‡,§ Naama Peor,‡ and Shlomo Yitzchaik*,‡ The Institute of Chemistry and the Center for Nanoscience and Nanotechnology, The Hebrew UniVersity of Jerusalem, Jerusalem 91904, Israel, and Jerusalem College of Engineering, Jerusalem 91035, Israel ReceiVed: May 31, 2010; ReVised Manuscript ReceiVed: September 13, 2010

Self-assembled monolayers (SAMs) of polar and polarizable organic molecules are widely used to tune semiconductors’ electronic properties for various applications. In the case of the dipoles’ arrangement in a dense and ordered SAM, intermolecular interaction between neighboring dipoles arises, inducing a change in the electrostatic properties of the polar SAM. These intermolecular long-range dipole interactions give rise to a molecular cooperative effect (MCE) through the layer, thus influencing the magnitude of the net surface dipole and suppressing the substrate contribution to dipole formation. Molecular engineering of the desired MCE could be a useful tool in various molecular electronics derived applications. In this work, we propose an experimental design to tailor the magnitude of the MCE through an organic monolayer. We constructed a mixed dipole monolayer containing parallel and antiparallel randomly organized dipoles. The creation of a mixed dipole monolayer enables controlling the MCE through the layer by giving rise to a smaller normal component to the surface coefficient and larger parallel component to the surface dipole coefficient. A deeper understanding of the MCE can be obtained by comparing the experimental and calculated values of such mixed dipole monolayers. The experimental values were obtained from contact potential difference measurements, and the calculated values were extracted by using the modified Helmholtz equation, based on the relative dipole contribution introduced in this work. This comparison enables analyzing the limit of the expected MCE, based on the evaluated surface dipole density along with its individual longitudinal molecular dipole. Introduction 1

The pioneering work of Ratner and Aviram has paved the way for a new conceptual field, molecular electronics, both theoretically and experimentally. First studies, inspired from the molecular rectification concept, dealt with molecules containing electron-rich (donor) and electron-poor (acceptor) units separated by an aliphatic spacer (A-σ-D). Along with other interesting research directions evolving in this field,2 the work on selfassembled monolayers (SAMs)3 containing π-conjugated polarizable molecules of the A-π-D type has led to outstanding nonlinear optical performances.4 These types of SAMs have also proved highly useful in molecular electronics as novel gate dielectric materials5 and in organic transistor applications.6 Moreover, it is well documented that polar SAMs can tune the electronic properties of their interfaces with semiconductors or metals by shifting the surface potential,7-10 carrier density,11,12 electron affinity (EA),13-15 and work function.15-17 The ability to tune and control electronic properties of semiconductors, by assembling organic molecules with various functionalities, attracted much attention, mainly, due to the synthetically tunable electronic properties of organic molecules. Indeed, it was already shown that organic SAMs can tune the electronic properties of interfaces by modifying the existing surface dipole through the addition of an external dipole layer.18-21 The change in the dipole of the adsorbed molecules is considered to be a major †

Part of the “Mark A. Ratner Festschrift”. * To whom correspondence should be addressed. E-mail: [email protected]. Phone: 972-2-658-6971. Fax: 972-2-658-5319. ‡ The Hebrew University of Jerusalem. § Jerusalem College of Engineering.

controlling factor in novel chemical and biological moleculebased sensing electronic and optoelectronic devices.22,23 Because of the fact that Si/SiO2 is the most widespread interface for electronic devices, there is a great motivation to explore the influence of organic monolayers on the electronic properties of this substrate. Systematic studies that were conducted on SAMs containing Si/SiO2 substrates18,19,24-26 or devices27,28 indicated amendment of the electronic properties of the semiconductor even through a few nanometers of an oxide layer. It is clear that the hybrid consisting of a Si substrate, native or thermal oxide, and adsorbed organic monolayer has high importance and potential in both device application and fundamental science. It is widely appreciated that the work function, the minimum energy required for an electron to escape into vacuum from the Fermi level, of a substrate depends on the conditions of the surface, both morphological and chemical. The work function of a semiconductor is determined by three factors: (i) the electron affinity (EA), the energy needed to bring an electron from vacuum just outside the semiconductor to the vacuum level at the surface; (ii) the band bending (BB), the electrical potential difference between the surface and the semiconductor bulk; and (iii) the energy difference between the Fermi level and conductive band in the bulk. To be able to separate the different parameters contributing to the work function, we use relative parameters by subtracting the reference value of the bare Si from the modified Si after each reaction step.17,19 The use of relative parameters enables eliminating the unknown difference between the conductive band and the Fermi level. The separation of ∆BB (sample-reference) and ∆EA parameters were achieved by illumination of the sample until photosaturation caused band

10.1021/jp104965s  2010 American Chemical Society Published on Web 10/05/2010

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Sfez et al. SCHEME 2: Creation of Mixed Antiparallel (Step a) and Parallel (Steps b and c) Dipoles Containing Monolayers

SCHEME 1: Cooperative Effect through a Mixed Organic Layera

a (a) Molecular cooperative effect of small dipoles pointing away from the surface. (b) Molecular cooperative effect of large dipoles pointing towards the surface.

flattening. In this way, the only contribution to the work function under illumination is the EA parameter alone, and the ∆EA parameter can be determined experimentally. Whereas the ∆BB parameter is usually related to the surface states and the electron traps on the surface, the ∆EA parameter is related to the net surface dipole of the layer and can be derived theoretically by the Helmholtz equation (eq 1). The change in the EA parameter, which can be measured experimentally by the contact potential difference (CPD) technique or derived theoretically by the Helmholtz equation is a good estimation of the net surface dipole. It is important to notice that, while using the CPD technique, only the longitudinal net surface dipole can be measured. As was shown before, two major parameters contribute to the net surface dipole: the vectorial molecular dipole of the organic molecules and the density of the layer.17,30 These parameters are manifested in the Helmholtz equation

∆V )

Nµ cos(θ) εrε0

(1)

where N is the dipole density (molecules/m2), µ is the molecular dipole moment (1 D ) 3.34 × 10-30 C · m), θ is the average tilt angle of the dipole with respect to the surface normal, εr is the relative dielectric constant of the media, and ε0 is the permittivity of vacuum; hence, ∆V is expressed in units of volts. From the Helmholtz equation, it is now clear that not only the intramolecular dipole of the individual molecules comprising the monolayer is important but also the dipole density. It was shown both experimentally and theoretically that the density of the layer influences greatly the electronic properties of the substrate.17,31 In previous studies, it was shown that indirect characteristics of the monolayer, such as order and depolarization, could be derived from the Helmholtz equation,17 by comparing experimental values of the change in the electron affinity parameter (∆EA) obtained by CPD measurements and the calculated values obtained from the Helmholtz equation. Reduction of the free energy is the driving force for all systems, including 2D organized molecular dipoles containing layers on semiconductors. When the polar layer is not dense and the dipoles are located far from each other, each dipole can be considered as an individual local dipole,29 leading to a net surface dipole enhancement, by creating a mirror image in the substrate (Scheme 1a, large dipoles, and Scheme 1b, small dipoles). However, with a highly ordered and dense polar monolayer, a dipole-dipole interaction through the layer occurs, giving rise to a molecular cooperative effect (MCE) and depolarization behavior, which suppress the substrate contribution to the net surface dipole. This MCE was already described both theoretically28,30,31 and experimentally17 in the case of a single molecular species of dipoles. Scheme 1 represents an

extreme case in which the mirror image in the substrate does not exist (Scheme 1a, small dipoles, and Scheme 1b, large dipoles). The first case (Scheme 1a) consists of a mixed monolayer in which the small dipoles are close to each other, and therefore, the MCE is created between them, decreasing the longitudinal (and measurable) net surface dipole. However, the larger opposite dipoles are far apart, and their longitudinal dipole can be regarded as an individual intramolecular dipole. The second case (Scheme 1b) consists of a dense arrangement of large opposite dipoles, in which the MCE is obtained for the large dipoles due to intermolecular interaction between nearby dipoles, but the small dipoles can be regarded as individual ones. The creation of a mixed dipole monolayer enables controlling the MCE through the layer by giving rise to a smaller perpendicular normal to the surface coefficient and a larger parallel to the surface dipole coefficient. As was discussed before,27-33 in the case where the distance between neighboring molecules is smaller than the length of the longitudinal molecular dipole (axis), a molecular cooperative effect is created through the layer. Because SAMs of organic molecules are widely used in devices, the MCE should be taken into account when the layer is dense and ordered. In this paper, we propose a way to experimentally tailor a controllable surface depolarization, by engineering the surface density of the molecules comprising a mixed antiparallel and parallel dipole monolayer. When a layer is composed from one sort of polarizable molecules in a 2D dense and uniform arrangement, the measured net surface dipole diverges from the calculated net surface dipole toward lower absolute values. This difference corresponds to interdipole interactions within the layer, giving rise to the MCE. To engineer a desired MCE, a random distribution of parallel or antiparallel dipoles with a controllable density was introduced. In that way, the overall MCE within the mixed layer is composed from both dipole contributions, depending on each dipole density. At first, a molecular cooperative effect occurs due to the dense arrangement of the first dipole. Upon alteration of the first dipole arrangement by increasing the density of the second one, simultaneous changes of the MCE through the mixed layer occur, giving rise to a net surface dipole, composed from the MCE of the first dipole, along with the growth of the MCE of the second dipole (Scheme 2). Additionally, we propose a modified weighted Helmholtz equation (eq 2) to estimate more accurately the calculated change

MCE in a Mixed Dipole Monolayer in electron affinity of Si/SiO2 substrates modified by a mixed dipole monolayer. This approach enables associating a more realistic picture of a mixed dipole monolayer to the observed experimental change in electron affinity (∆EA) of the Si substrate. Assuming additive dipoles, a summation of the various dipoles and their relative density contributions were taken into account.

J. Phys. Chem. C, Vol. 114, No. 48, 2010 20533 SCHEME 3: Synthetic Routes for Assembling Mixed Dipole Monolayers

Materials and Methods General. Substrate Cleaning. Quartz (Chemglass), glass slides (Knittel Glaser), and n-Si〈100〉 (Virginia Semiconductors) substrates were cleaned in aqueous detergent, rinsed copiously with triple distilled water (TDW), and immersed in hot (90 °C) piranha solution for 60 min (3:7 by volume of 30% H2O2 (MOS) and conc. H2SO4 (MOS “BAK-ANAL” REAG); caution, strong oxidizing solution, handle with care). The substrates were then rinsed with TDW and further cleaned with H2O/H2O2/NH3 (5: 1:0.25) solution while sonicating for 15 min at 60 °C. After subsequent washing with TDW, the substrates were immersed in pure acetone and finally dried under a stream of nitrogen. Chemicals. The coupling agents (4-chloromethyl)phenyltrichlorosilane (BzCl-TCS), (3-bromopropyl)trichlorosilane (PrBrTCS), and (3-chloropropyl)trichlorosilane (PrCl-TCS) were purchased from Gelest and vacuum-distilled prior to use. The hexane (95% n-hexane ULTRA RESI Analyzed) was purchased from J. T. Baker and distilled on sodium under a nitrogen atmosphere; acetonitrile was distilled on CaH2; n-propanol was used after passing through an alumina column; and 2-propanol, ethanol, acetone, and methanol were purchased from Aldrich and used as received. SAM Preparation. General Synthetic Procedure for Silylated SAMs. Freshly cleaned and activated Si/SiO2 (silicon’s native oxide) or quartz substrates were immersed in a 1% (v/v) coupling agent/hexane solution for 20 min under inert conditions in a Schlenk line system. Upon completion of the reaction, the substrates were washed three times with dry hexane under inert conditions, sonicated for 1 min in acetone in order to remove any excess of the coupling agent, and allowed to dry in an oven at 110 °C for about 15 min for polysiloxane formation by a condensation reaction. Monolayer Functionalization with 4-[4-(N,N-Dimethylaminophenyl)azo]pyridinium Halide (MAP+) Chromophore. The silylated surfaces with terminated halide groups were modified by two different ways: In (a) solution derived assembly, an antiparallel mixed dipole monolayer was built by dipping freshly prepared BzCl- or PrBr-modified substrates in ∼30 mM MAP solution in dry acetonitrile at 70 °C (Scheme 3a,b, step i). Upon completion of the reaction, the substrates were washed and sonicated for 5 min in 2-propanol and finally dried under a nitrogen flow. To reach a parallel mixed dipole monolayer, the mixed antiparallel dipole samples were immersed in neat Py at 40 °C overnight (Scheme 3b, step ii). In the (b) solid-state derived (topotactic) assembly, the silylated substrates were spincoated at 4000 rpm with a saturated solution of MAP in MeOH and heated at 110 °C/30 Torr for 13 min. The samples were washed with 2-propanol to remove unreacted precursor and allowed to cool to room temperature. To get parallel dipole monolayers with different concentrations of the anchored MAP+ chromophore, a pyridine precursor was used. The MAP solution was diluted with different ratios of pyridine, which serves as a competitive precursor for an in situ SN2 reaction, and then spincoated under the same conditions (Scheme 3b, step iii). The surface density of the obtained chromophoric assembly was

(a) Synthetic route for assembling an antiparallel mixed dipole monolayer from solution or solid-state assembly on a PrBr-modified silicon substrate. (b) Synthetic approaches for an antiparallel mixed dipole monolayer assembly (step i) and a parallel mixed dipole monolayer solution derived assembly (step ii) and topotactic assembly (step iii). The interfacial siloxane network in the surface reaction products is omitted for clarity.

achieved by both synthetic routes was determined by UV-vis analysis, as was shown before.17 Instrumentation. UV-vis spectra were acquired on a Shimadzu UV-3101PC spectrophotometer using quartz and glass substrates. CPD measurements were conducted using a Au grid (Kelvin probe S, DeltaPhi Besocke, Ju¨lich, Germany) that vibrates by a piezoelectric crystal. To measure the VCPD value, a semiconductor sample, with an InGa Ohmic back contact, is placed parallel to the grid, thus creating a closely spaced parallel plate capacitor. The entire experimental setup was placed in a home-built Faraday cage in an inert atmosphere. Upon electrical connection, equilibrium is reached and then the VCPD value is equal to the difference in the work function of the semiconductor surface and the vibrating Au grid. The vibrating capacitor leads to a time-dependent variation in capacitance, which induces an ac current flow through the circuit. By tuning the external dc bias, the compensating VCPD value is determined when the current flow is nullified. Therefore, the change in the semiconductor’s work function can be determined. As was mentioned earlier, the separation of the different parameters contributing to the work function (i.e., BB, EA) was obtained by illumination. Upon illumination of the semiconductor, electron-hole pairs are generated close to the surface, leading to a decrease in the BB. This reduction of BB upon illumination will continue until the bands become almost flat (photosaturation condition). A

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quartz tungsten halogen lamp, (intensity ) 130 mW/cm2), which was assumed to lead to complete flattening of the bands, was used. The BB was determined by comparing the CPD value in the dark with the value under intense illumination, where the bands are nearly flat. The EA value was determined by subtracting the BB value from the work function due to the fact that the Si substrate is n-doped. We used relative parameters (∆EA, ∆BB) by subtracting the reference value of the bare Si substrate from that of the modified Si value, in order to eliminate the unknown value of the difference between the Fermi level and the conducting band. The CPD measurements were taken after the few minutes needed for the signal to stabilize. To avoid the molecules’ decomposition during photosaturation measurements, we used an optical filter that blocks wavelengths adsorbed by the molecules (Schott, RG-780). Results and Discussion Construction of various submonolayer surface number densities of a polarizable chromophore via an in situ SN2 reaction is exemplified in Scheme 3. Freshly prepared silylated Si substrates with either propylbromide (PrBr) or benzyl chloride (BzCl) functionalization (Scheme 3a,b, respectively) were used for the chromophore grafting, quaternization step. The chromophoric precursor was chosen to be 4-[4-(N,N-dimethylamino phenyl)azo]pyridine (MAP), due to the fact that, upon quaternization reaction, it becomes a polarizable molecule with a high calculated dipole moment (of about -10 D) that can be a good candidate to induce electronic changes in the Si substrate. Moreover, its density could easily be deduced by UV-vis absorbance spectroscopy at λmax ) 550 nm. Full coverage of the substrate was obtained by coupling small complementary dipoles to the remaining sites. A pyridinium moiety was chosen as a parallel complementary small dipole (-1.2 D), and the coupling agent moiety of BzCl or PrBr was chosen as the antiparallel small dipole (+0.8 and +1.6 D, respectively). All small dipoles have the same molecular footprint (25 Å2/ molecule), thus having the same surface density at full coverage (ca. 4 × 1014 molecules/cm2). The variable parameters for the small complementary dipoles are, therefore, intramolecular dipole magnitude and molecular polarizability parameters. The mixed dipole monolayers consisted in an arrangement of the MAP+ chromophore and the complementary dipoles, either a parallel or an antiparallel ensemble of dipoles. It is important to notice that the BzCl moiety has an aromatic spacer, whereas the PrBr moiety has an aliphatic one. The influence of the spacer on the electronic properties of Si was already shown,17 introducing the ring shielding effect, vide infra. In the case of the antiparallel dipole monolayer, the starting point was a dense and organized coupling agent monolayer, with a footprint of 25 Å2/molecule, corresponding to a distance of about 5 Å between neighboring molecules, for both coupling agents. Chromophoric assembly from solution was conducted on both coupling agent monolayers (see Scheme 3) and was monitored by UV-vis absorption spectroscopy in order to determine the chromophore density on the substrate, as shown before.17 Briefly, to obtain different surface coverage of the MAP+ chromophore from solution (Scheme 3b, step i), various reaction times were examined.17 The monitoring was done by UV-vis absorption spectra measurement at λmax ) 550 nm, until a full chromophore coverage (ca. 2 × 1014 molecules/cm2) was achieved, which corresponds to a molecular footprint area of about 50 Å2/molecule. The substrates were extracted from the reaction mixtures after different amounts of time and characterized by CPD and spectroscopy. For the BzCl moiety, another

Sfez et al. synthetic approach was conducted, using topotactic self-assembly. This method consists of spin-coating of a few nanometers thick MAP chromophore precursor, followed by heating that leads to chromophore grafting to the surface while the excess precursor is sublimed away.4 In this method also, the footprint was determined to be 50 Å2/molecule at full chromophoric coverage. The topotactic self-assembly method was conducted also on the PrBr-modified substrate, but only for full MAP+ coverage, which was determined to be identical to the one obtained on the BzCl moiety. It must be noted that, in both approaches, for full chromophore coverage, there is still 50% of the coupling agent sites that remains unreacted, due to a major difference in the molecular cross section dimensions of the coupling agent and chromophore (Schemes 2 and 3). Consequently, all the unreacted sites still have the original coupling agent dipole (+0.8 D for BzCl and +1.6 D for PrBr) that points out of the surface. In that way, a mixed antiparallel dipole containing monolayer is obtained (see also Scheme 3b, step i). The mixed parallel dipole baring monolayer was built only on the BzCl coupling agent moiety, using two complementary strategies: The first synthetic approach involves immersion of the antiparallel mixed monolayers (Scheme 3b, step i), with various MAP+ coverages in neat pyridine (Py) (Scheme 3b, step ii). The second synthetic approach consists of topotactic assembly of various ratios of MAP/Py (precursors) on a BzClmodified substrate (Scheme 3b, step iii). In this approach, a competition between the Py and MAP chromophore gives rise to a homogeneous dipole distribution on the surface. On the basis of XPS analysis conducted on a similar SN2 quaternization of Py+ molecules, a 100% substitution of coupling agent sites by Py+ was observed. Thus, it can be assumed that the small Py+ molecules have a molecular footprint of 25 Å2/molecule, similar to the coupling agent molecular footprint, but with dipoles in the opposite direction. In this way, all the sites that are not occupied by the MAP+ bulky chromophore are taken by Py+ and a mixed parallel dipoles containing monolayer with various chromophore densities is obtained. The mixed parallel dipole monolayers obtained from both synthetic approaches gave similar results in CPD and UV-vis spectral absorption measurements, indicating a complete reaction of the coupling agent moiety with Py, leading to a full coverage of Py+ containing monolayer. Figure 1 presents experimental values of ∆EA for full MAP+ surface coverage created in both methods (i.e., from solution and topotactic assembly) on BzCl and PrBr moieties. It can be seen that similar results are obtained for both chromophore assembly methods, with the same coupling-agent moiety, for full coverage. Moreover, the influence of the organic spacer on the electronic properties of the surface was compared by using two types of coupling agents, aliphatic and aromatic. As was shown before,17 a ring shielding effect is observed for the aromatic spacer when a halogen end group is attached to an aromatic spacer, such as in the case of the BzCl moiety, thus diminishing the measured ∆EA parameter. Without this shielding effect for the aliphatic spacer, with the same halogen end group, a larger change in the ∆EA parameter is observed (Figure 1, part a). This ring shielding effect is kept even when the next chromophoric layer is assembled. As can be seen, very similar results are obtained for the aliphatic (PrBr coupling agent) and aromatic (BzCl coupling agent), in both topotactic and solution derived assemblies (Figure 1, parts b and c). In both methods, the ring shielding effect can be estimated to have a value of about 200 mV.

MCE in a Mixed Dipole Monolayer

J. Phys. Chem. C, Vol. 114, No. 48, 2010 20535 sumption is based on the fact that the substrate is a Si/SiO2 substrate, with a native insulating oxide, which diminishes the screening and enables assuming dipoles addition

(

∆Vnet ) R

Figure 1. Change in EA as a function of surface functionality: (a) aromatic ring effect in BzCl (gray, aromatic spacer) and PrCl (black, aliphatic spacer) coupling agent monolayers, (b) aromatic ring effect preservation after buildup of 100% MAP+ coverage from solution, and (c) aromatic ring effect preservation following buildup of 100% MAP+ coverage by the topotactic self-assembly strategy.

Figure 2 shows experimental values of ∆EA, obtained by CPD measurements of the various mixed monolayers, demonstrating a collective dipole change of the surface. Figure 2a shows parallel and antiparallel mixed dipole monolayers built on the BzCl moiety, whereas Figure 2b shows antiparallel dipole containing monolayers on both coupling agent moieties. In Figure 2a, both mixed monolayers were assembled on the BzCl dense and organized moiety. It is noteworthy that, for the same MAP+ chromophore coverage, two mixed monolayers were built, one with antiparallel (BzCl coupling agent monolayer and chromophore moiety) and the other one with a parallel (Py+ and chromophore moiety) mixed dipole monolayer. The change in the EA parameter of the substrate as a function of MAP+ chromophoric percentage coverage is presented for both antiparallel (Figure 2a) and parallel (Figure 2a, curve b) mixed dipole monolayers obtained from solution (Scheme 3b, step i for antiparallel and step ii for parallel dipoles). Upon decreasing the chromophore density, a major difference between the two cases is observed (Figure 2a), which can be explained by the growing densities of the complementary dipoles. For the case of the antiparallel mixed monolayer, there is a positive change in the measured ∆EA parameter due to the increasing density of the positive BzCl dipole, which points out of the surface, thus creating an antiparallel dipole assembly. On the other hand, while introducing parallel Py+ dipoles to the monolayer, a much lower value of ∆EA was measured. This fact can be explained by the introduction of the Py+ negative dipoles, which point toward the surface, altering the coupling agents’ sites into opposite dipoles. Indeed, as expected, the addition of small Py+ dipoles pointing toward the surface instead of coupling agent dipoles that point away from the surface gives rise to a bigger change in the ∆EA parameter. Figure 2b shows the antiparallel mixed dipole monolayer, built on the PrBr moiety or BzCl moiety. From this comparison, a clear influence of the ring shielding effect can be observed. This effect can account for the less stiff slope obtained for the aromatic coupling agent comparing the aliphatic one. These experimental results can be explained with the weighted Helmholtz equation (eq 2), which takes into account the relative contributions of the two dipoles comprising the monolayer, weighted by their respective densities, in the approximation of additive dipoles. This as-

)

(

N1µ1 N2µ2 cos(θ1) + (1 - R) cos(θ2) εr1ε0 εr2ε0

)

(2)

where R represents the relative occupied sites of the first dipole (i.e., the coupling agent dipole or the Py+ dipole), N1 and N2. are the number densities of the two dipoles (N1 value for full coverage of the coupling agent or Py monolayer is 4 × 1014 molecules/cm2, which corresponds to a footprint of 25 Å2/ molecule, and for an N2 value of 2 × 1014 molecules/cm2, which corresponds to a footprint of about 50 Å2/molecule for a full coverage of MAP + chromophore monolayer), εr1 and εr2 are the relative dielectric constants of the media (for the coupling agents, 3.6, and for the pyridinium derivatives, 4.5), µ is the molecular dipole moment of the first (+0.8, +1.6, and -1.2 D for BzCl and PrBr coupling agents and Py+, respectively) and second (MAP+ dipoles, -10.6 D) dipoles comprising the layer, and θ is the average tilt angle of the two dipoles with respect to the surface normal (20 and 42°, respectively). All the values are based on experimental results obtained and described elsewhere.17 The molecular dipole, µ, was obtained from MOPAC, after minimization (AM1 method on the trimethoxysilanes derivatives as “free” molecules), which gave the dipole value in debyes. While using eq 1 to describe various densities of MAP+ submonolayers, with only the chromophore’s parameters, a large deviation from the experimental curve was obtained. Many reasons can account for this deviation, and one of them can be the unreacted sites of the complementary dipoles, which are taken into account in eq 2. Equation 2 enables linking a more realistic picture of a mixed dipole monolayer to the observed experimental change in electron affinity (∆EA), and a better fit is observed, though there are still numerous parameters that cause deviation from the calculated curve, as will be disscussed below. Figures 3 and 4 show a comparison between measured and calculated ∆EA values for both mixed monolayer types. Figure 3 describes the change in the net surface dipole (∆EA) of both the experimental and the calculated results, for both coupling agents, as was described earlier. The experimental values were determined by CPD measurements, and the net surface dipole of the mixed-monolayer calculated values were obtained by the weighted Helmholtz equation (eq 2). Both net surface dipoles are represented as a function of the relative coverage percentage of the chromophoric monolayer, with respect to the coupling agent twice denser monolayer. This means that the zero point of the x axis, that is, 0% chromophore coverage, corresponds to full and only coupling agent coverage. On the other hand, a 100% chromophore coverage corresponds only to 50% of the coupling agents overall sites due to their smaller molecular footprints. In this way, a mixed dipole monolayer is formed, comprising the unreacted coupling agent sites with dipoles pointing away from the surface, whereas the other half belongs to the MAP+ chromophoric layer containing opposite dipole sites, pointing toward the surface. Every point in the graphs, calculated (using eq 2) and experimental (measured by CPD), corresponds to different coverage percentages of the chromophoric monolayer. From this comparison, an estimation of the MCE in the layer can be derived, due to the fact that it is not considered in the equation, among other factors.

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Figure 2. Chromophore coverage dependence of the change in the EA parameter (vs bare Si/SiO2 substrate) for (a) antiparallel (BzCl moiety) and parallel (pyridinium moiety) complementary dipoles and for (b) various antiparallel mixed dipole derived monolayers built with PrBr and BzCl moieties.

Figure 3. Comparison of the calculated and experimental change in EA (vs bare Si/SiO2 substrate) values for the antiparallel mixed-dipole monolayer, obtained by the weighted Helmholtz equation and by CPD measurements, respectively. Variable chromophore densities grafted on (a) PrBr- and (b) BzCl-containing SAMs.

The sign of the slopes features of the graphs are of interest. The negative slope is explained by the introduction of a new large molecular dipole pointing toward the surface, to the system, which has an opposite direction comparing the coupling agent small and positive dipole. Upon the growing density of the negative signed dipole, a decrease in the ∆EA parameter is observed. Second, the curves’ crossing point for both cases happens at a certain surface coverage, which corresponds to about 40% chromophore coverage. These features can be interpretated in terms of MCE formation through the monolayer. For 2D dense and well-organized polarizable monolayers, an MCE through the layer is formed, leading to a decrease in the measured perpendicular surface dipole. As was mentioned above, the MCE is mostly observed when the distance between two molecules in the layer is smaller than the length of the molecular longitudinal axis. It can be seen (Figure 3) that, at the curves’ edges, the difference between the calculated and experimental values is more pronounced. This can be explained by a formation of MCE (i.e., depolarization) through the layer, first, for the coupling agent, and later for the chromophore. Starting with a very dense BzCl or PrBr coupling agent, a monolayer with dipoles pointing away from the surface is formed, and the experimental net

Figure 4. Comparison of the calculated and experimental change in EA values (vs bare Si/SiO2 substrate) for the parallel mixed-dipole monolayer, comparing the weighted Helmholtz equation and CPD measurements, respectively.

MCE in a Mixed Dipole Monolayer surface dipole is smaller than the calculated one. In this case, the distance between two coupling agent molecules is about 5 Å, as derived from the coupling agents’ footprint, which is smaller than the length of the perpendicular molecular axis (which is about 6 Å). Upon introduction of a covalently attached opposite dipole instead of the coupler’s dipole, a larger distance between adjacent coupling agent molecules is obtained, resulting in a smaller MCE through the layer. Hence, the experimental values are much closer to the calculated values. Following the crossover point, the MAP+ chromophore molecules are close enough to create an MCE between neighboring chromophores. This effect is expressed in a positive slope of the experimental curve, indicating that the MCE arises from adjacent chromophoric sites with a dipole pointing toward the surface. The crossover point represents the lack of depolarization, meaning that the mixed dipoles are arranged in a minimal electrostatic repulsion. This point corresponds to a density of about a 40% relative chromophoric layer coverage, which leads to a surface area per molecule that corresponds to about 65 Å2/molecule, giving rise to a distance of about 8.3 Å between adjacent chromophore ocupied sites, as supported by ellipsometric measurments. As was discussed earlier, it can be shown that, when the distance between two adjacent MAP+ sites is smaller than 8 Å, an MCE through the chromophoric layer should be obtained, as can be seen in both cases (Figure 3). The MCE created between the MAP+ dipoles will grow upon growing density, thus diverging from the calculated estimated curve. Figure 4 shows a comparison between calculated and measured ∆EA parameters for parallel dipoles containing monolayers. First of all, it is interesting to notice that the crossing point of the calculated and experimental curves occurs at about 10% of chromophoric coverage, whereas in the case of antiparallel dipoles, the crossing point was at a 40% chromophore coverage (Figure 3). This observation could be explained by the fact that, in the parallel mixed dipoles case, the MCE is created through the Py+ and MAP+ moieties, which are parallel dipoles; thus, enhancement of the MCE through the mixed layer is obtained. Indeed, it can be seen that, at 0% MAP+ coverage, no MCE is observed at all for the Py+ monolayer; however, upon very low MAP+ coverage, the experimental curve diverges strongly from the calculated one, suggesting a beginning of an MCE through the MAP+-Py+ mixed monolayer. Upon growing densities of the MAP+ chromophore, an increase of the MCE is observed, giving rise to a decrease in the absolute value of the measured ∆EA parameter, as expected. It can also be seen that the two curves coincide at a full coverage of Py+. The immediate interpretation is that there is no MCE through the Py+ moiety. The reason for that is the fact that the Py+ molecule possesses a small polarizability and has a very low molecular dipole. Usually, depolarization should be taken into account for molecules with high molecular dipoles (∼5 D) or polarizabilty. In the case of the coupling agents (Figure 3), depolarization was obtained despite the low molecular dipole, persumably due to the presence of the polarizable halogen terminal group. However, it is evident that, upon creation of a pure Py+-based monolayer (see Figure 4, triangles), almost the same result is obtained for the calculated and experimental values, suggesting that the MCE is negligeble in this case. On the other hand, upon growing MAP+ densities, the curves diverge from one another due to a growing MCE through the chromophoric layer with growth in the large molecular-dipole density. At maximal MAP+ coverage, a difference of about 400 mV is found between the two curves, suggesting that a major MCE is created through the mixed-parallel dipole monolayer.

J. Phys. Chem. C, Vol. 114, No. 48, 2010 20537 In the case of the mixed antiparallel dipoles, a much smaller difference between the curves (of about 150 mV) was observed, for maximal MAP+ coverage (see Figure 3). These results suggest a magnification of the MCE within a mixed parallel dipole monolayer through the MAP+-Py+ dipole-dipole repulsion, comparing mixed antiparallel ones, as could have been expected. Conclusions We have shown a way to control and tune the degree of a desired MCE through an organic monolayer, by random introduction of parallel and antiparallel dipoles on a polar and polarizable chromophoric monolayer. The mixed chromophoric monolayer was constructed via two synthetic routes: from solution and by topotactic self-assembly. Both synthetic methods gave very similar results as was observed by the measured UV-vis absorption spectroscopy and ∆EA parameter, indicating similar chromophore densities. An aromatic ring shielding effect was observed and preserved even on the chromophoric monolayers derived from solution or topotactic SAMs. The MCE was observed to grow in the case of the parallel dipole monolayer, with growing chromophore density, and decreased with chromophore density for the antiparallel mixed dipole monolayer. For the case of the antiparallel dipoles, a specific surface composition can be designed in which no MCE at all is observed. At this point, the calculated and experimental curves coincide. To eliminate or control the degree of MCE through a layer, the molecular footprint and the dipole’s longitudinal axis should be considered. The molecular cooperative effect obtained for this mixed dipole monolayer was based on both dipole contributions, weighted by their densities. It is clear that the weighted Helmholtz equation should be modified and include interaction terms in order to describe more accurately the experimental data. Acknowledgment. This work was supported by the EC through contract FP6-029192 for Future & Emerging Technologies. We gratefully thank Prof. D. Cahen, Prof. U. Peskin, and Prof. B. G. Sfez for valuable discussions. References and Notes (1) Aviram, A.; Ratner, M. A. Chem. Phys. Lett. 1974, 29, 277. (2) Ratner, M. A.; Davis, B.; Kemp, M.; Mujica, V.; Roitberg, A.; Yaliraki, S. Molecular Electronics; Aviram, A., Ratner, M. A., Eds.; New York Academy of Sciences: New York, 1998; Vol. 852. (3) Ulman, A. An Introduction to Ultrathin Organic Films from Langmuir-Blodgett to Self-Assembly; Academic Press: San Diego, CA, 1991. (4) Yitzchaik, S.; Marks, J. T. Acc. Chem. Res. 1996, 29, 197. (5) DiBenedetto, S. A.; Facchetti, A.; Ratner, M. A.; Marks, T. J. J. Am. Chem. Soc. 2009, 131, 7158. (6) DiBenedetto, S. A.; Facchetti, A.; Ratner, M. A.; Marks, T. J. AdV. Mater. 2009, 21, 1407. (7) Miramond, C.; Vuillaume, D. J. Appl. Phys. 2004, 96, 1529. (8) Ray, S. G.; Cohen, H.; Naaman, R.; Liu, H.; Waldeck, D. H. J. Phys. Chem. B 2005, 109, 14064. (9) Haick, H.; Ambrico, M.; Ligonzo, T.; Tung, R. T.; Cahen, D. J. Am. Chem. Soc. 2006, 128, 6854. (10) Scott, A.; Janes, D. B.; Risko, C.; Ratner, M. A. Appl. Phys. Lett. 2007, 91, 033508. (11) Kobayashi, S.; Nishikawa, T.; Takenobu, T.; Mori, S.; Shimoda, T.; Mitani, T.; Shimotani, H.; Yoshimoto, N.; Ogawa, S.; Iwasa, A. Nat. Mater. 2004, 3, 317. (12) Huang, C.; Katz, H. E.; West, J. E. Langmuir 2007, 23, 13223. (13) Cohen, R.; Kronik, L.; Shanzer, A.; Cahen, D.; Liu, A.; Rosenwaks, Y.; Lorenz, J. K.; Ellis, A. B. J. Am. Chem. Soc. 1999, 121, 10545. (14) Lenfant, S.; Guerin, D.; Tran Van, F.; Chevrot, C.; Palacin, S.; Bourgoin, J. P.; Bouloussa, O.; Rondelez, F.; Vuillaume, D. J. Phys. Chem. B 2006, 110, 13947. (15) de Boer, B.; Handipour, A.; Mandoc, M. M.; van Woundenbergh, T.; Blom, P. W. M. AdV. Mater. 2005, 17, 621.

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