© Copyright 2001 by the American Chemical Society
VOLUME 105, NUMBER 15, APRIL 19, 2001
LETTERS Adsorption of Polar Molecules on a Molecular Surface Geoffrey R. Hutchison, Mark A. Ratner,* and Tobin J. Marks* Department of Chemistry and Materials Research Center, Northwestern UniVersity, EVanston, Illinois 60208-3113
R. Naaman* Department of Chemical Physics, Weizmann Institute of Science, RehoVot 76100, Israel ReceiVed: September 14, 2000; In Final Form: February 22, 2001
Calculations are presented on the electrostatic potential modification arising from molecular adsorbates on a molecular monolayer. Analysis at the ab initio level indicates that the overall dipole moment of the bimolecular monolayer-adsorbate complex formed on physisorption is dominated by the dipole moments of the individual molecules, with a small correction due to charge transfer. The induced dipole moment is greatest with more polarizable molecular monolayer constituents such as p-nitroaniline. Additionally, numerical simulation shows that the average electric field across the adsorbing surface increases linearly with the number of adsorbed analyte molecules and exhibits an inverse dependence on the thickness of the monolayer. This thickness dependence indicates a long-range effect of the dipole layer resulting from the two-dimensional nature of the system.
Introduction The electronic properties of organic materials are currently of great interest for a wide variety of applications. Such properties are often very important at organic/inorganic interfaces.1,2 For example, it is possible to alter the electronic properties of a semiconductor surface by adsorption of organic molecules.3-8 Indeed, experimental systems exist that are highly sensitive to changes in surface potentials induced by such adsorption processes.3,5-8 Such adsorption phenomena present the possibility of highly sensitive molecular sensors. For example, field-effect transistorlike structures, in which the semiconducting channel between the source and drain is modified by a chemisorbed molecular monolayer, allow detection of bound analyte molecules via changes induced in the electrostatic potential within the channel (Figure 1). As a specific example, a polar analyte molecule such as water influences the potential of the monolayer, in turn
Figure 1. Diagram of a semiconductor device with attached molecular monolayer and adsorbed analyte molecule.
affecting the surface potential of the underlying semiconductor. This is similar to effects used for chemically sensitive fieldeffect transistor-based “CHEMFET” sensors.9-12 The purpose of this contribution is to demonstrate computationally that adsorption of polar molecules such as water or ammonia on a molecular monolayer surface can induce large changes in the electrostatic potential beneath the monolayer. It will be seen that this effect is largely due to the dipole moment
10.1021/jp003282y CCC: $20.00 © 2001 American Chemical Society Published on Web 03/27/2001
2882 J. Phys. Chem. B, Vol. 105, No. 15, 2001 of the polar analyte itself. To this end, we treat simple model systems offering both polar and nonpolar surfaces at the ab initio level and consider how the results would scale over a macroscopic surface. Model The system we first consider consists of bifunctional organic molecules (R,ω-chloroalkanols) having varying extended molecular lengths, as shown below. The chlorine end of the molecule is considered as adsorbed onto the semiconductor substrate, with the hydroxyl group exposed as the outer surface of the monolayer. The single molecule represents a first-order approximation to the extended molecular surface. Calculations on optimized geometries were performed for this Lewis base molecule and the molecule with one water molecule positioned near the hydroxyl group chain end.
As the electrostatic potential induced by the molecules is dictated by the dipole moments, it is important to realize that with odd or even chain lengths, the individual bond dipole moments of the carbon-chlorine bond and the oxygenhydrogen bond will be oriented either roughly parallel for evennumbered chain lengths or at an angle for odd-numbered chain lengths. Since the dipole moments of the molecules will be dictated primarily by these two large bond dipoles, when comparing dipoles as a function of increasing chain lengths, one should compare dipole moments of odd or even-number chain lengths.
Letters TABLE 1: Calculated Dipole Moments for r,ω -Chloroalkanols in the 6-31*G Basis Using the RHF Method chain length (# of carbon atoms)
dipole moment (D)
dipole moment with H2O (D)
difference (D)
2 3 4 5 6 7 8 9
2.306 3.639 2.481 3.696 2.527 3.717 2.553 3.728
5.255 5.795 5.367 5.878 5.349 5.969 5.377 5.631
2.949 2.156 2.886 2.182 2.822 2.252 2.823 1.904
TABLE 2: Calculated Dipole Moments for r,ω-Chloroalkanols in the 6-311**G++ Basis Using the RHF Method chain length (# of carbon atoms)
dipole moment (D)
dipole moment with H2O (D)
difference (D)
2 3 4 5 6 7 8 9
2.224 3.687 2.405 3.709 2.461 3.740 2.496 3.742
5.040 5.827 5.011 5.870 5.255 5.909 5.274 5.855
2.816 2.139 2.606 2.162 2.793 2.169 2.777 2.113
Results All ab initio calculations were carried out using version 3.5 of the Jaguar software package.13 These calculations were performed at the RHF level using ultrafine cutoffs and with pseudospectral methods turned off. The optimized geometries indicate that the water molecule is oriented with the oxygen atom positioned for interaction with the hydrogen atom of the alcohol hydroxyl group and with the water O-H bonds pointing away from the hydrocarbon chain along the chain axis. The calculated water oxygen-alcohol hydrogen atom distance averages 2.025 Å, indicating the hydrogen bonding suggested above. The calculated distance between the nearest (-CH2OH) carbon atom and the water oxygen atom is 3.6-3.7 Å, which compares well with experimental measurements made on methanol-water and ethanolwater mixtures.14,15 Both geometry optimization and dipole moment calculations were performed in the 6-31*G basis set and in the larger 6-311**G++ basis set. The results do not differ significantly. Additionally, the ab initio results were repeated for several chain lengths using local MP2 calculations on all atoms in the larger 6-311**G++ basis set. These calculations yielded somewhat lower calculated dipole moments; however, the trends are again similar. Even with the local MP2 corrections, the calculated dipole moments are (as expected from reference 16) significantly larger than the experimental gas-phase values, 1.854 D for water and 1.78 D for 2-chloroethanol,17 versus the computed values of 2.116 D for water and 1.942 D for the anti conformation of 2-chloroethanol in the 6-311**G++ basis with local MP2 corrections (2.196 D and 2.224 D, respectively, without local MP2 corrections). Computational results for the RHF method are summarized in Tables 1 and 2.
Figure 2. Invariance of calculated R,ω-chloroalkanol dipole moments in the 6-31*G basis set for increasing Cl(CH2)nOH chain length (n). Note the very pronounced odd-even alternation, produced by the relative geometric orientation of the two large local dipole moments.
The present results indicate that there is a large increase in total molecular dipole moment on going from the free chloroalkanol to the molecule-water complex, as shown in Figure 2. There is not, however, a noticeable dependence of calculated dipole moment on alkyl chain length. Furthermore, the calculated Mulliken charges after complex formation are not significantly different along the alkyl chain and only slightly different in the bound water molecule, while significant changes are observed in the calculated charges for the hydroxyl group. These results indicate that the σ-bond structure of the alkyl chain is not significantly polarized in these systems. Thus, the changes in electronic structure accompanying water complex formation are largely localized at the terminus of the molecule and at the hydroxyl group in particular. Despite the localized changes in electronic structure and alterations in geometry of the alkanol-water complex, the effect of the bound water molecule is quite substantial. The overall dipole moment of the alkanol-water molecule complex is calculated to be greater than 5.0 D in both basis sets, sometimes
Letters
J. Phys. Chem. B, Vol. 105, No. 15, 2001 2883
TABLE 3: Calculated Dipole Moments for Straight-Chain Alkanes in the 6-31*G Basis with an Adsorbed H2O Molecule Using the RHF Method chain length (# of carbon atoms)
dipole moment with H2O (D)
induced dipole with H2O (D)
2 3 4 5 6 7
2.406 2.487 2.433 2.498 2.443 2.503
0.207 0.222 0.234 0.233 0.244 0.237
TABLE 4: Calculated Dipole Moments for Straight-Chain Alkanes in the 6-31*G Basis with an Adsorbed NH3 Molecule Using the RHF Method chain length (# of carbon atoms)
dipole moment with NH3 (D)
induced dipole with NH3 (D)
2 3 4 5 6 7
2.230 2.262 2.264 2.281 2.234 2.384
0.310 0.342 0.344 0.314 0.314 0.397
closer to 5.8-5.9 D (Tables 1 and 2). Much of the increase is simply due to the added dipole moment of the water molecule itself, which is calculated to be 2.199 and 2.196 D in the 6-31*G and 6-311**G++ basis sets, respectively. At a position 10 Å away from the chlorine end of the molecule, along the molecular axis, this increase in dipole moment corresponds to a increase of approximately 1.69 × 10-2 V/Å in the magnitude of the electric field. At this position, this would correspond to a change in the semiconductor channel carrier distribution given by the Boltzmann relationship, e-∆/kT, of a factor of 25.6. We next consider the case of nonpolar surfaces. Here it is found that modification of the dipolar field is observed even in calculations on purely aliphatic monolayers. However, the geometry optimization finds several local energetic minima structures for the molecule-water complex rather than one fixed minimum. Thus, dipole moments were calculated for a series of saturated hydrocarbon chain lengths with either a complexed water or ammonia molecule. Again, a single hydrocarbon molecule represents a first-order approximation to the larger surface. The results are summarized in Tables 3 and 4, with an estimate of the induced dipole moment as the difference between the dipole moment of the molecule-water or moleculeammonia complex and the sum of the dipole moments separately. For reference, the calculated dipole moment of the NH3 molecule is 1.920 D in the 6-31G* basis and 1.787 D in the 6-311**G++ basis. As with the R,ω-chloroalkanols, the larger 6-311**G++ basis set was also used with and without MP2 corrections. The results reveal a variety of different geometries and, thus, different calculated dipole moments; this trend holds true in the larger basis set as well. Importantly, as in the case of the chloroalkanols, the calculations with larger basis sets agree qualitatively with those in the smaller basis, and the dipole moments do not vary with increasing chain length. Again, this indicates that the electronic effects of the complexed polar molecule are largely localized at the end of the alkyl chain and that the σ bonds along the chain are not significantly perturbed. In general, the dipole moment induced by a water molecule on a straight-chain alkane is found to be -0.20 D (Table 3). This is approximately 3 times greater than the dipole moment calculated for an odd chain length straight-chain hydrocarbon itself (0.0667D in the 6-31*G basis set) and is thus an experimentally measurable change. However, the value is still
Figure 3. Optimized hydrogen-bonded molecular structures of a complexed water molecule and p-nitrophenol (left) or p-nitroaniline (right). Calculations on these geometries reveal large degrees of chargetransfer and a significant increase in overall dipole moment of the complex.
3 times smaller than the induced dipole moments calculated for R,ω-chloroalkanols, which are approximately 0.60 D. Calculations were next performed on an optimized grid of four parallel ethane molecules, representing a larger portion of an aliphatic surface. The grid nuclei were frozen to model the molecular monolayer surface, and geometry optimizations were performed using the 6-31*G basis set for adsorbed H2O and NH3 molecules. In both cases, the adsorbate molecules are located in the optimized structure at a position approximately in the center of the grid and several angstroms above the alkyl chains with the oxygen and nitrogen atoms oriented downward. After subtracting the intrinsic dipole moments of the adsorbed molecules, we found the calculated induced dipole moments to be 0.103 D for adsorbed water and 0.023 D for ammonia. This suggests that some charge transfer is occurring; however, the overall effect is very smalls a few percent of the dipole moment of the molecules themselves. On the other hand, the previous approximations of a molecular surface in terms of an isolated molecule still appears to hold. Thus, the great majority of the dipole moment of the adsorbate-molecular monolayer assemblies is due to the adsorbed polar molecules themselves. The charge transfer induced by adsorbing a polar molecule on an organic surface is likely to be a general effect. One would naturally assume that monolayers composed of highly polarizable molecules would exhibit much larger induced dipole moments, hence, larger changes in electrostatic potential and, consequently, enhanced sensor response. Indeed, we find that two very simple conjugated donor-acceptor molecules exhibit far larger induced dipole moments. Figure 3 presents the optimized geometries for a water molecule bound to pnitroaniline and p-nitrophenol. In the case of p-nitroaniline, the largest dipole moment is calculated for the geometry with the water molecule bound to the nitro group through two hydrogen-bonds, which is the computed minimum energy structure (Figure 3). In the 6-31*G basis set, an induced dipole moment of 1.098D is calculated, roughly twice the induced dipole moment found for the chloroalkanols. In the case of p-nitrophenol, the largest dipole moment is calculated for the structure with the water molecule positioned with the molecular plane perpendicular to the aromatic plane, wedged between a phenylene hydrogen atom and the hydroxyl group as shown in Figure 3. In the 6-31*G basis set, this structure exhibits an induced dipole moment of 1.337 Ds even higher than that for p-nitroaniline. The above models illustrate on a molecular level the effect of Lewis base complexation and the modifications of the dipolar field for a given molecular monolayer surface and adsorbate molecule. A practical sensor, however, will require the effect to be induced across a macroscopic area of the active device and through a thickness on the order of nanometers. It is important therefore to estimate the response curve for a surface of this sort, in terms of the perturbation of the field across the
2884 J. Phys. Chem. B, Vol. 105, No. 15, 2001
Letters the electrostatic potential at the monolayer, even in the case of completely aliphatic monolayers. This change, however, is greater for more polarizable monolayer systems such as those having a hydroxyl group or a conjugated π-system. The magnitude of the change in electrostatic potential is of value in modeling quantitative response in FET-based molecular sensors.9-12 These fields can also modulate the injection properties of organic-metal interfaces, important in organic LEDs and organic FETs.1,2 Since the effect diminishes inversely with distance from the semiconducting channel, the sensor should contain as thin a monolayer as is practical.
Figure 4. Falloff of the electric field generated per water molecule adsorbed averaged across a simulated 50 nm × 50 nm monolayer surface (having variable thickness), consisting of closely packed R,ωchloroalkanol molecules. The dotted line is a best fit to a (1/thickness) relationship.
surface per adsorbed molecule and in terms of the falloff of the effect over thicker monolayers. Since the effects discussed above appear to be general, we will use for our model surface a monolayer of R,ω-chloroalkanol molecules and assume that an adsorbed water molecule will increase the total molecular dipole moment by 2.80 D. Since the dominant sensor effect of the adsorbed species is to modify the potential at the channel boundary, the calculated change in electronic potential will be considered at the surface of the FET device itself. The monolayer is treated as a square grid of closely packed chloroalkanol molecules at separations of the diameter of methane (5.15 Å, from van der Waals constants).17 The average electric field experienced across the monolayer surface was modeled using both analytic integration based on one water molecule per chloroalkanol molecule and numeric simulation calculating the field as water molecules “adsorbed” randomly across the grid sites.18 The average field increases linearly with the number of added water molecules. Furthermore, the field per water molecule can be fit to a 1/r relationship, where r is the thickness of the chloroalkanol monolayer, as shown in Figure 4. The “long-range” effect obtained here results from the twodimensional nature of the system and indicates that the dipole field can be effective over a much longer range than that expected based on the single-molecule description. Here we treat the semiconducting channel as the layer of atoms directly attached to the chemisorbed organic monolayer, and the channel is assumed to have negligible thickness. Therefore, a sensor could be rendered more sensitive by decreasing the thickness of the organic molecular layer. Conclusions We have shown that adsorption of a polar molecular analyte onto a molecular monolayer can induce a significant change in
Acknowledgment. We thank the NSF/MRSEC program for partial support through the Northwestern MRSEC (NSF DMR96324732). We are also grateful to the DOD/MURI program (N00014-95-1-1319) for partial support. G.H. thanks M. Lane and J. Ireland of the Northwestern Department of Electrical and Computer Engineering for helpful discussions about organic FET devices. References and Notes (1) Ishii, H.; Sugiyama, K.; Ito, E.; Seki, K. AdV. Mater. 1999, 11, 605-625. (2) Yaliraki, S. N.; Roitberg, A. E.; Gonzalez, C.; Mujica, V.; Ratner, M. A. J. Chem. Phys. 1999, 111, 6997-7002. (3) (a) Gartsman, K.; Cahen, D.; Kadyshevitch, A.; Libman, J.; Moav, T.; Naaman, R.; Shanzer, A.; Umansky, V.; Vilan, A. Chem. Phys. Lett. 1998, 283, 301-306. (b) Wu, D. G.; Ashkenasy, G.; Shvarts, D.; Ussyshkin, R. V.; Naaman, R.; Shanzer, A.; Cahen, D. Angew. Chem., Int. Ed. Engl. 2000, 39, 4496. (4) Wu, D. G.; Cahen, D.; Graf, P.; Naaman, R.; Nitzan, A.; Shvarts, D. Chem. Eur. J., in press. (5) Vilan, A.; Shanzer, A.; Cahen, D. Nature 2000, 404, 166-168. (6) Cohen, R.; Kronik, L.; Shanzer, A.; Cahen, D.; Liu, A.; Rosenwaks, Y.; Lorenz, J. K.; Ellis, A. B. J. Am. Chem. Soc. 1999, 121, 10545-10553. (7) Kru¨ger, J.; Bach, U.; Gra¨tzel, M. AdV. Mater. 2000, 12, 447-451. (8) Campbell, I. H.; Kress, J. D.; Martin, R. L.; Smith, D. L.; Barashkov, N. N.; Ferraris, J. P. Appl. Phys. Lett. 1997, 71, 3528-3530. (9) Reinhoudt, D. N. Sens. Actuators, B 1995, 24-25, 197-200. (10) Reinhoudt, D. N. Sens. Actuators, B 1992, 6, 179-185. (11) Domansky´, K.; Baldwin, D. L.; Grate, J. W.; Hall, T. B.; Li, J.; Josowicz, M.; Janata, J. Anal. Chem. 1998, 70, 473-481. (12) Cobben, P. L. H. M.; Egberink, R. J. M.; Bomer, J. G.; Begveld, P.; Verboom, W.; Reinhoudt, D. N. J. Am. Chem. Soc. 1992, 114, 1057310582. (13) Jaguar 3.5; Schro¨dinger, Inc.: Portland, Oregon, 1998. (14) Nishi, N.; Takahashi, S.; Matsumoto, M.; Tanaka, A.; Muraya, K.; Takamuku, T.; Yamaguchi, T. J. Phys. Chem. 1995, 99, 462-468. (15) Soper, A. K.; Finney, J. L. Phys. ReV. Lett. 1993, 71, 4346-4349. (16) Hehre, W. J.; Radom, L.; Schleyer, P. v. R.; Pople, J. A. Ab Initio Molecular Orbital Theory; Wiley: New York, 1986. (17) Lide, D. R., Ed. In Handbook of Chemistry and Physics; CRC Press: Boca Raton, FL, 1995. (18) The field can be calculated simply by taking the sum of the dipole moments of each molecule in the grid interacting with a water molecule at a fixed position. Analytically, the sum is taken to infinity, and numerically, the sum is taken averaging the converged central region, one-ninth of the total grid over a large number of random distributions of water molecules. The 50 × 50 nm surface used here is thus sufficient to represent an infinite surface.
