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Designing Molecular Architecture to Control Diffusion and Adsorption on Insulating Surfaces Matthew Watkins, Thomas Trevethan,* Maria L. Sushko, and Alexander L. Shluger Department of Physics and Astronomy, London Centre for Nanotechnology, Materials Simulation Laboratory, UniVersity College London, Gower Street, London WC1E 6BT, United Kingdom ReceiVed: September 24, 2007; In Final Form: December 24, 2007
We present the results of calculations that have been performed to simulate the adsorption and diffusion of several model molecules, consisting of two carboxylic acid binding groups connected to a molecular backbone, on the TiO2 (110) rutile surface in order to investigate the effect of molecular structure on their surface mobility. The calculations were performed using a set of interatomic potentials that have been specifically developed to correctly reproduce the molecule-surface interaction for this system, along with established potentials for the isolated surface and intramolecular interactions. These potentials were tested through a comparison of adsorption energies and diffusion barriers of prototype molecules. We show that the rigidity of the molecular structure can significantly affect both the adsorption energy and the energy barriers for diffusion on the surface. As a result of the simulations we suggest a rigid molecular structure that will maximize the diffusion barrier. Calculations such as these will enable the design of molecules in order to tailor their diffusive properties for specific applications.
Introduction Understanding the structure and dynamics of large organic molecules adsorbed on surfaces is important in many areas of surface science and nanotechnology. In particular, it will be essential to control both the adsorption geometry and the diffusion of large isolated molecules on insulating surfaces in order to create working single molecule devices and to develop architectures for molecular electronics.1,2 With the continued development and success of scanning probe microscopy methods, it is now possible to both image and manipulate individual atoms and molecules on surfaces with a high degree of precision,3,4 and the engineering of single molecule devices now seems possible.5,6 In applications where an individual molecule would be placed on a sample surface and then utilized,7,8 the major question that arises is how to secure the molecule so that it is bound and immobile over the duration of the experiment and at experimental temperatures. It is therefore desirable to develop schemes for designing and/or selecting molecules and molecular structures that will have a low mobility (higher diffusion barrier) on surfaces as the prototypes for single molecule devices. Schemes which could then develop into techniques for the ‘bottom up’ design of molecular devices. In addition it would also be highly attractive if individual molecules could not only be designed to be placed in a stable configuration on a surface but also designed to be easily manipulated in a controlled way, for example using a scanning probe technique.9 In many applications, and specifically in the development of single molecule electronic and computational devices, it is a requirement that the molecule is decoupled electronically from the substrate.1,2 In this case, it is not appropriate for the entire molecule to react or interact strongly with the surface, and the molecule should be bound to the surface via one or more * To whom correspondence should be addressed. E-mail: t.trevethan@ ucl.ac.uk.
‘anchoring groups’. These groups essentially consist of distinct parts of the molecule (or groups attached to the main, functional part of the molecule) that will interact or react strongly with the surface at specific points in order to prevent diffusion of the entire molecular structure. The nature and strength of the interaction of a particular anchoring group with the surface will be a major factor in the diffusion and adsorption properties of the molecule. In this paper we demonstrate that, if the molecule has more than one anchoring group, the way these groups are connected together through the molecular structure also has a critical influence on the adsorption and diffusion properties of the molecule as a whole. By varying the flexibility and mechanical properties of how the binding groups are attached to the “backbone” of the molecule (which does not interact strongly with the surface) by changing the molecular structure, the diffusion of the molecule can be drastically altered. The structure of the molecule and how the binding groups are attached to the backbone can be controlled when synthesising molecules for specific applications, and a well designed molecule will have far more suitable properties for investigation and potential application for a device on an insulating surface. To investigate the adsorption and diffusion of these type of molecules on insulating surfaces, we employ a model system that consists of organic molecules with two binding groups, adsorbed on an ionic oxide surface. Previous studies, and the nature of molecules successfully observed on insulators, predominantly alcohols, carbonyls and organic acids, suggest that polar anchoring groups are almost a prerequisite for successfully binding a molecule to an ionic surface. Particular attention has been paid to carboxylic acid groups that are both strongly polar and have the potential to form two bonds to the surface per functional group (see for instance refs 10-17). In this paper we examine the behavior of five related model organic molecules with carboxylic binding groups on the TiO2 rutile (110) surface. These molecules are shown in Figure 1.
10.1021/jp077680d CCC: $40.75 © 2008 American Chemical Society Published on Web 02/23/2008
Designing Molecular Architecture
Figure 1. Molecules considered in this study.
They consist of “backbones” of one or three phenyl rings with two carboxylic acid groups attached to the backbones by saturated hydrocarbon chains of varying length. By tuning the length of the aliphatic hydrocarbon chains we can adjust the flexibility of the molecule and the freedom of the binding groups to move independently. From consideration of the first four molecules we suggest a molecule, an anthracene derivative, that would have a significantly larger barrier for diffusion, and would be suitable for study as an independent entity on the surface of rutile (110) using scanning probe methods. The rutile (110) surface is an attractive model system to investigate general issues of molecular adsorption and diffusion for several reasons: it is extremely well studied,18 isolated organic species have been observed on this substrate,19,20 and from a theoretical point of view its structure reduces the problem to that of 1D diffusion. To see why the 1D diffusion occurs consider the binding of a carboxylic acid group to the rutile surface: calculations show that it binds preferentially in a bidentate configuration to two surface titanium ions in the surface (Figure 2). The rutile surface is strongly anisotropic: along the directions there are rows of Titanium ions, and diffusion can occur quite readily. However, moving along the direction rows of raised bridging O ions are encountered, which present a virtually insurmountable barrier at room temperature and below (i.e., greater that 1.5 eV from atomistic calculations). A possible complication for studying these acidic species on this surface is that they may de-protonate on adsorption. In fact the weight of experimental19-22 and theoretical data16,23-25 strongly suggest that this is the case. Therefore, in this study we consider two diffusion scenarios: both the molecularly adsorbed species and their respective fully de-protonated carboxylate anions. The situation could be even more complex with protons staying closely associated with their conjugate base,16 and in this case their diffusion could be strongly correlated and differ from that of either isolated species. We do not consider this situation here as it is not the main focus of this contribution. The plan of the remainder of the paper is as follows: in the next section the computational methods we employ are described along with the details of the system. In the following section, the results of the calculations are described in detail. Then in the finial section, the results are discussed and a conclusion is given. Table 1 provides a summary of the adsorption energies and diffusion barriers calculated in this paper. Methods The calculations presented here employed an atomistic model that includes the relevant interactions for the molecule adsorbed
J. Phys. Chem. C, Vol. 112, No. 11, 2008 4227 on the rutile surface. Intramolecular interactions were described using the AMBER force-field,26 the rutile surface itself by the force-field of Bandura et al.27 and a force-field specifically constructed for the interaction of organic molecules with the rutile (110) structure28 provided the final element. The construction of this organic-rutile force-field was achieved by fitting pair potentials of the Buckingham form to potential energy surfaces obtained from ab initio calculations at the HartreeFock 6-31G* level of theory and the procedure is fully described in ref 28. Mulliken charges from the same calculations were used to model electrostatic interactions. The use of the HartreeFock method here was shown to reproduce adsorption curves using other density functional theory (DFT) methods. A particular point in this method of constructing the surfacemolecule force-field is the assumption that the interaction of the molecule as whole with the surface can be built up from summing the interactions of individual functional groups. In this way interactions for carboxylic acid groups, saturated hydrocarbon chains and phenyl groups with the surface were developed. To test this assumption we performed a comparison of the adsorption energies and diffusion barriers of several molecules, calculated using the derived interatomic potentials and full ab initio method, which is described below. The classical atomistic calculations presented here were carried out using an 8 × 4 supercell of rutile, nine atomic layers deep. Two-dimensional periodic boundary conditions, with cell lengths of 23.9 × 26.2 Å were used. Transition barriers were calculated using a constrained minimization method, where the x coordinate of a single hydroxyl oxygen atom bound to the Rutile surface was constrained; we also performed the minimization constraining the carbon atom of the carboxylic acid groups or a carbonyl oxygen and observed no significant difference in the resulting energy barriers. The diffusion of the acid should ideally be described by a better reaction coordinate than the x position of a single atom. To check that the constrained minimization did lead to genuine transition states the highest energy configurations along the paths were then optimized to a transition state using an eigen-following algorithm. The energies of the fully optimized TS were always within 0.1 eV of the highest point on the constrained optimization path. When considering charged systems (i.e., de-protonated acids) a neutralizing background charge was used, in the form of a proton adsorbed on a bridging oxygen on the opposite side of the surface slab. All of the atomistic calculations performed in this study were carried out using the general utility lattice program (GULP) simulation code.29 Results Adsorption and Diffusion of Benzoic Acid. We first consider the motion of benzoic acid (Figure 1a), which can be considered as a single anchoring group, along the Ti rows of the rutile (110) surface. The molecule adsorbs perpendicularly in a bidentate manner with a binding energy of 1.6 eV, each oxygen of the acid group binding to a bridging Ti ion. Diffusion along the surface Ti row can occur with an energy barrier of 0.55 eV between adjacent sites, where the transition state consists of the two carboxylic acid oxygen atoms bridging a single Ti ion. The motion of the benzyl carboxylate ion along the (001) directions is quantitatively similar, and leads to an identical barrier of 0.55 eV. To test the accuracy of the atomistic model we also calculated the adsorption energy and barriers for the benzoic acid and benzyl carboxylate molecules using the fully ab initio Hartree-
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Figure 2. Top: Adsorption geometry of molecule (d) viewed along (001). Motion along (-110) directions are energetically prohibitive due to the raised bridging O rows (center and extreme left and right of figure). The diffusion mechanisms considered in this paper are those of the molecule toward and away from the viewpoint. Bottom: Schematic view of the three different adsorption configurations. Left and right correspond to dII and the center to dI.
TABLE 1: Diffusion Barrier (ED) and Adsorption Energies (EA) of the Five Molecules Calculated Using the Atomistic Model, in eVa molecule
EA
ED
(a) (b) (c) (d) (e)
1.60 (1.65) 2.35 (2.34) 3.15 3.65 2.01
0.55 (0.52) 0.43 (0.39) 0.61 0.51 0.87
ED deprotonated 0.55 (0.59) 0.22 0.53 0.51
a Also given are the diffusion barriers for each of the molecules in the de-protonated state. The energies from full Hartree-Fock calculations (where these were performed) are shown in brackets for molecules (a) and (b).
Fock method. These calculations (the details of which are the same as in ref 28) were performed using the 6-31G* basis set for the molecule and surface oxygen atoms and the lanl2dz basis set for the titanium atoms, within a finite cluster model employing the Gaussian 98 code.30 The Hartree-Fock method is employed to increase the efficiency of the calculations, and was shown to reproduce the adsorption curves of organic molecules calculated using DFT methds (GGA) and quantum chemistry methods (B3LYP) to a high degree of accuracy, which is discussed in detail in ref 28. With the benzoic acid adsorbed on the surface in the same configuration as described above and fully relaxed, the binding energy is 1.6 eV, which is in very good agreement with the classical model. The barrier for diffusion along the Ti row was estimated by performing a constrained minimization of points along the reaction coordinate. Here the maximum for the barrier is 0.52 eV, which is very close to the 0.55 eV calculated using the classical model. The barrier for the diffusion of the benzyl carboxylate ion was also estimated using the Hartree-Fock method and a constrained minimization. The maximum of the barrier is now higher at 0.59 eV, but it still compares well with the result from the interatomic potentials of 0.55 eV. In the Hartree-Fock calculation of the barrier, there is some redistribution of charge along the molecule at the transition state mainly due to elongation of the CdO bonds of the carboxylic groups.
Diffusion of Two-Legged Molecules. (a) Flexible Legs. Benzoic acid serves as a structural motif in the more complex cases considered later. From the nature of its binding we can expect that the larger molecules will attempt to position any carboxylic groups in a similar local geometry to that of the carboxylic group in benzoic acid. The relatively deep potential well for the bidentate bonded acid group also suggests that we can characterize the adsorption geometries of more complex molecules by the position of the anchoring acid groups on the Ti rows. The lower portion of Figure 2 shows three possible adsorption geometries of molecule (d), where the configurations are shown by the positions of the four carboxylic/carboxylate oxygen ions: in this case the two groups are on neighboring Ti rows and the states consist of (i) the left-hand carboxylate group leading the right by one lattice constant in , (ii) the two groups having the same position in , and (iii) the right leg leading. The succession of configurations from left to right also correspond to the movement of the center of mass of the entire molecule downward . The configurations with the different legs leading are mirror images of each other, we label them dII. Figure 3 shows the energy barrier for the motion of molecule (d) corresponding to moving from dII - dI - dII. It can be seen that the two configurations are virtually identical in energy (dE ∼ 30 meV) as the legs are long enough and flexible enough to not introduce any strain into either configuration. The barrier for the motion of each leg is similar to that observed for benzoic acid. Further minimum energy configurations of the molecule with the legs stretched even wider apart in the x direction (dIII) are higher in energy, by ∼0.5 eV, and are not considered further as they would be inaccessible at room temperature. The geometry of the diffusion path agrees very well with that calculated using periodic density functional calculations for formic acid in ref 24. The transition state shows a significant rotation of the moving carboxylic acid group around the z axis, which allows the hydrogen of the hydroxyl group to interact more strongly with the bridging oxygen row. After the transition state the system becomes monodentate, with the hydroxyl group oxygen rising a significant distance above the Ti plane (∼2.6 Å), again in excellent agreement with ref 24. To see the effect of the entire molecule moving along the surface
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Figure 5. Adsorption configuration of molecule (b) viewed along y (top) and z (bottom).
Figure 3. Adiabatic potential energy surface for the motion of one leg of molecule (d) as function of the position of the leading hydroxyl oxygen of one of the legs. The molecule starts from configuration dII, with one of the legs leading the other by a lattice constant in the (001) direction. The molecule moves to configuration dI, then back to dII, but with the opposite leg leading, and finally to dIII, where the legs are separated by two lattice constants.
Figure 6. Adsorption configuration of molecule (e) viewed along y (top) and z (bottom).
Figure 4. Adiabatic potential energy surface for the motion of one leg of molecule (c) as a function of the leading hydroxyl oxygen of one of the legs.
in a single transition, we artificially constrained both legs to move simultaneously along the surface in the x direction. Here the diffusion barrier is approximately doubled, showing that in this case, when we force the molecule’s legs to move together, i.e., the molecule stays in configuration dII and moves to an equivalent configuration shifted by one lattice constant along , the observed diffusion barrier is approximately the sum of the barriers for each individual leg. The diffusion barrier for
the de-protonated ion was almost indistinguishable from that of the protonated molecule. (b) Rigid Legs. To investigate the effect of the flexibility and length of the legs and how this affects the independent motion we then consider molecules (b) and (c), which have shorter hydrocarbon chains linking them to the phenyl ring. The behavior of molecule (c) is qualitatively similar to (d), however, the shorter legs induce strain into the stretched configurations of the molecule (analogues of dII), which are now 0.13 eV higher in energy than the lowest energy configuration (dI analogue). This will introduce asymmetry into the diffusion process of the molecule with the motion from the strained configuration, cII, back to cI having a lower energy barrier (Figure 4). Molecule (b) behaves in a totally different manner to the molecules previously considered. Here the carboxylic groups are directly bound to the phenyl ring, which has a major effect on its adsorption: the molecule is unable to accommodate the phenyl group in a position over the bridging oxygen row, so the molecule is forced to bind via two monodentate carboxylic
4230 J. Phys. Chem. C, Vol. 112, No. 11, 2008 acid groups along the same row. The most favorable adsorption configuration is shown in Figure 5. The rigidity of the system prevents the carboxylic acid groups from binding in a bidentate manner to the surface, instead a single bond is formed by each carbonyl group to Ti atoms in the row, and a hydrogen bond is formed between the hydroxyl groups and oxygen’s in the bridging row. As a consequence of the hindered adsorption the binding energy of the molecule, 2.35 eV, is significantly reduced when compared to molecule (d). As well as affecting the manner of adsorption to the surface, the rigidity of the molecule means that the two anchoring groups do not have independence of motion; there are no other intermediate accessible minima. The molecule has to move as a single unit, with both O-Ti bonds to the surface breaking at the same time. As a consequence the diffusion barrier for molecule (b) is very similar to that observed for molecule (d) despite binding in a less efficient manner to the surface and having significantly smaller adsorption energy. The adsorption energy and barrier for this molecule in the above configuration were also calculated using the HartreeFock method, as described for benzoic acid above, as a further test to the interatomic potentials. The adsorption energy was equal to 2.34 eV and the energy barrier for diffusion to be 0.39 eV, which again agree very well with the results of the interatomic potentials: a binding energy of 2.35 eV and a barrier of 0.43 eV. This molecule, with its unusual bonding to the surface is the only one considered that shows a noticeable difference in diffusion behavior in its de-protonated form. As each anchoring group is now bound to the surface more weakly via a single oxygen atom, the binding of the carboxyl proton to surface bridging atoms becomes more significant. The proton-bridging oxygen interaction increases the diffusion barrier for the protonated molecule, the deprotonated form being extremely mobile on the surface with a barrier to diffusion of only 0.22 eV. The final molecule (e) has been proposed, in view of the behavior of the other molecules considered, to maximize its diffusion barrier along the Ti row. A rigid molecular backbone (anthracene) is selected that allows two carboxylic acid groups to bind in their most favorable configuration to the surface, i.e., the molecule is selected such that the four oxygen atoms of the molecule are commensurate with four Ti atoms along one (001) orientated row (see Figure 6). This molecule is given as an example of a molecule whose rigid structure significantly restricts its motion but is still commensurate with the surface. As was expected from the behavior of molecule (b), which is similarly rigid, the diffusion path requires the simultaneous breaking of the bonds formed by both anchoring groups, and there are no accessible minima for the individual movement of the legs. The barrier to diffusion is therefore slightly less than double that for benzoic acid at 0.87 eV. This molecule would have a room-temperature residence time at a given surface site suitable for probing with a scanning probe technique or other slow measurements over a laboratory time-scale. Discussion and Conclusions In this paper we have considered the adsorption and diffusion of several related model organic molecules on the rutile (110) surface. After revisiting the diffusion of benzoic acid, which binds to the surface via a single anchoring group, we consider a family of molecules that form bonds to the surface with two separate connections. The diffusion and adsorption of benzoic acid is relatively straightforward and provides a benchmark for the behavior of a carboxylic acid groups that can bind in an
Watkins et al. optimal way to the rutile surface. The acid group binds in a bidentate manner with adsorption energy of ∼1.5-2.0 eV and diffusion barrier along (001) directions of ∼ 0.5 eV. The situation becomes more complex when molecules make multiple connections to the surface: here we only consider the simplest extension, with two separate anchoring groups. The binding groups cannot, in general, be considered independently, but the body (or backbone) of the molecule acts as a spring holding them at a given distance, the strength of the spring depending on the rigidity of the body of the molecule and the length and flexibility of the legs to which the anchoring groups are attached. The more complex behavior of these systems offers the potential to tune their behavior by adjusting the previously alluded to spring, via the structure of the molecule. The sequence of molecules considered (b-d) show that the diffusive properties of the system can be adjusted significantly if the molecule’s structure is well adapted to the surface considered. Here, we consider fully oxidized rutile, where it has been shown that there is little charge transfer between the adsorbed species and the surface. The hydrocarbon body of the molecules considered here have a negligible interaction with the surface and their influence is merely to provide steric constraints on the motion of the carboxylic anchoring groups. In this case, we are therefore essentially optimizing geometric and mechanical parameters. The parameters probed in this study are as follows: the rigidity of the molecule, by controlling the length of the flexible legs joining the anchoring group to the aromatic body and, when the molecule is rigid, the distance between the anchoring groups. These results can be summarized in general as follows: 1. If the legs are long and flexible, all groups can bind optimally to the surface, maximizing the adsorption energy of the molecule. However, due to their flexibility the legs can move independently and the energy barrier for diffusion of the molecule will be little changed from a species anchoring with only a single group. This would be a good type of molecule to deposit if the aim was to achieve self-assembly of the molecules at pre-patterned anchoring sites, or trapping at locations like step edges, as they would be mobile but adsorb strongly to the substrate. 2. If the molecule is rigid then two effects may occur: The anchoring groups need to move in concert, as there is no intermediate minima, causing the diffusion barrier to be approximately the sum of the barriers of the legs. The molecule may have to compromise the binding of the anchoring groups to the surface as the energy cost of the strain of the anchoring groups to obtain good contact with the surface is high, as observed for molecule (b) in this study. A rigid molecule with all anchoring groups commensurate with the binding sites on the surface will maximize both adsorption and diffusion energies, as shown by molecule (e). Such molecules would be optimal for studying in isolation on insulating surfaces due to their potentially high site residence time. This situation would also be optimal for manipulation by scanning probe techniques, given that the diffusion barrier was not prohibitively high, as pushing any part of the molecule would lead to collective motion of the entire molecule. By altering the distance between the anchoring groups, through the structure of the molecular backbone, it should be possible to tune the diffusion and adsorption energies. In this study, the adsorption and diffusion of molecules on the perfect and defect-free rutile surface was considered. In a real experiment, the surface is likely to contain both oxygen vacancies and hydroxide defects, and these may have a
Designing Molecular Architecture significant effect on both the adsorption and diffusion of organic species. This may need to taken into account when comparing with experimental data, especially when the reduced titania surface is used, however this is beyond the scope of the present study. In summary, using a model system that effectively reduces the problem to one-dimensional motion, we have shown that the structure of a large molecule can have a significant effect on its adsorption and diffusion on a surface. Performing the same type of calculations for other systems where there are more degrees of freedom for the binding groups on the surface, or significantly more binding groups, would be significantly more complex. However, calculations such as these could help provide new insights into controlling the configuration and dynamics of molecule on surfaces. Acknowledgment. This work was supported by the EU FP6 project “PICO-inside”. We would also like to thank A. Gourdon for advice and valuable discussions. References and Notes (1) Joachim, C.; Gimzewski, J. K.; Aviram, A. Nature 2000, 408 (6812), 541-548. (2) Fiurasek, J.; Cerf, N. J.; Duchemin, I.; Joachim, C. Physica E 2004, 24 (3-4), 161-172. (3) Eigler, D. M.; Lutz, C. P.; Rudge, W. E. Nature 1991, 352 (6336), 600-603. (4) Sugimoto, Y.; Abe, M.; Hirayama, S.; Oyabu, N.; Custance, O.; Morita, S. Nat. Mater. 2005, 4 (2), 156-U36. (5) Heinrich, A. J.; Lutz, C. P.; Gupta, J. A.; Eigler, D. M. Science 2002, 298 (5597), 1381-1387. (6) Liljeroth, P. J.; Repp, J.; Meyer, G. Science 2007, 317, 12031206. (7) Chiaravalloti, F.; Gross, L.; Rieder, K. H.; Stojkovic, S. M.; Gourdon, A.; Joachim, C.; Moresco, F. Nat. Mater. 2007, 6 (1), 30-33. (8) Otero, R.; Hummelink, F.; Sato, F.; Legoas, S. B.; Thostrup, P.; Laegsgaard, E.; Stensgaard, I.; Galvao, D. S.; Besenbacher, F. Nat. Mater. 2004, 3 (11), 779-782. (9) Wolkow, R. A. Ann. ReV. Phys. Chem. 1999, 50, 413-441. (10) Daniels, B. G.; Lindsay, R.; Thornton, G. Surf. ReV. Lett. 2001, 8 (1-2), 95-120. (11) Thornton, G. Phys. ReV. B 2004. (12) Uetsuka, H.; Henderson, M. A.; Sasahara, A.; Onishi, H. J. Phys. Chem. B 2004, 108 (36), 13706-13710.
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