Liquid Crystal (8CB) Molecular Adsorption on Lithium Niobate Z-Cut

Article pubs.acs.org/JPCC

Liquid Crystal (8CB) Molecular Adsorption on Lithium Niobate Z‑Cut Surfaces C. Braun,* S. Sanna, and W. G. Schmidt Lehrstuhl für Theoretische Physik, Universität Paderborn, 33095 Paderborn, Germany ABSTRACT: Density functional theory is used to study the adsorption of 4′n-octyl-4-cyanobiphenyl (8CB) liquid crystal molecules on the lithium niobate (0001) and (0001̅) surfaces. The interface energy and configuration are found to depend strongly on the surface polarization and the molecular coverage. A flat adsorption configuration is most favored in the case of positive Z-cuts as well as for single, isolated molecules on both the positive and the negative Zcut. With increasing coverage on negative Z-cuts, however, an upright-bonding configuration is preferred.

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charges were found to enable artificial photosynthesis12 and to drive photocatalytic dye decolorization.13 The integration of ferroelectric thin films within liquid environments is investigated in the context of lab-on-chip device designs, for example, for localizing, sensing, or activating charged biomolecules.14 High surface electric fields due to pyroelectricity were found to efficiently pole electro-optic polymers15 and to lead to the reversible fragmentation and self-assembling of nematic liquid crystals (LCs).16 LCs are highly interesting adsorbates in this context as their optical properties are easily manipulated by electrical or magnetic fields. For many years, LCs have been employed for various applications in optics and optoelectronics17 such as displays,18 adaptive lenses,19 or subterahertz switches.20 The detailed understanding of their interaction with strongly polar and switchable substrates is thus of obvious interest. First experimental results indicate a strong polarization dependence for the LC adsorption on LN16 but do not give any detailed information on the structure of the LC−LN interface. In order to explore that interface microscopically, we calculate here the adsorption of 4′-n-octyl-4-cyanobiphenyl (8CB; cf. Figure 1b) on the LN negative and positive Z-cut surfaces from firstprinciples. 8CB is a suitable model system for such investigations because of (i) its comparatively simple and compact structure and (ii) its large dipole moment of about 5 D, resulting from the terminating cyano group.21

INTRODUCTION The investigation of lithium niobate (LiNbO3, LN; cf. Figure 1a) has a long history1 and is fuelled by its distinctive

Figure 1. (a) LN bulk structure with O, Li, and Nb atoms indicated by red, dark, and light balls, respectively. (b) 8CB molecule from two different perspectives with H, C, and N atoms indicated by cyan, yellow, and gray balls, respectively. The angular degrees of freedom probed for the gas-phase molecule are indicated on the right-hand side.

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pyroelectric, piezoelectric, elctro-optical, and photoelastic attributes that make it one of the most common electro-optical materials nowadays.2−4 More recently, the LN surface properties came into focus. This concerns, in particular, the strongly polar (0001̅) and (0001) surfaces, commonly referred to as negative and positive Z-cuts, respectively.5−7 For example, it is possible to grow group-III nitrides with spatially varied, absolute polarity control on LN substrates.8,9 The dipole orientation can be switched at the nanoscale, bearing a great potential for domain-specific surface chemistry as a route toward the fabrication of nanoscale devices.10,11 LN surface © 2015 American Chemical Society

METHODOLOGY Density functional theory (DFT) calculations are performed using the Vienna ab initio simulation package (VASP 5.3.5).22 The electron−ion interaction is modeled with the projector augmented wave (PAW) approach,23 and the electron− electron exchange and correlation (XC) effects are described Received: January 28, 2015 Revised: April 16, 2015 Published: April 16, 2015 9342

DOI: 10.1021/acs.jpcc.5b00894 J. Phys. Chem. C 2015, 119, 9342−9346

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The Journal of Physical Chemistry C

point mesh are used to sample the Brillouin zones of 3 × 2 and 2 × 2 surface unit cells, respectively.

in generalized gradient approximation (GGA), using the PW91 functional.24 The accurate modeling of (in particular, loosely) bonded adsorbate systems is a major challenge for DFT because (semi)local XC functionals do not properly describe the long-range vdW interactions.25−29 In order to account approximately for dispersion interactions, we use a semiempirical, so-called DFT-D scheme30−33 based on the London dispersion formula.34 Specifically, we use the D2 parameters suggested by Grimme.33 Test calculations using DFT-D335 showed only very minor deviations. While this approach is certainly a simple approximation that restricts the predictive power of the calculations with respect to the absolute magnitude of the adsorption energy, it is expected to be sufficiently accurate to allow for valid conclusions concerning energy differences and energetic trends.36 The LN substrate is modeled with 10 trilayers stacked along the z direction. Thereby, the atoms of the uppermost four trilayers are allowed to relax, while the atoms of the bottom six trilayers are kept frozen at ideal bulk positions (cf. Figure 2).

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RESULTS AND DISCUSSION The determination of the structurally relaxed 8CB ground state is the starting point of our calculations. Thereby, we calculated the molecular energy for various geometries that were obtained by relaxing starting configurations characterized by different twist angles between the two phenyl groups and the cyano group, as depicted on the right-hand side of Figure 1. The most stable configuration found here features a slight twist of φ1 = 30° between the two phenyl groups. The angles φ2 and φ3 have been calculated to be 73 and 146°, respectively. This molecular structure was then used as the starting configuration for the study of the molecular adsorption on LN. Next, the potential energy surface (PES) for an upright absorption of 8CB on both the positive and negative LN Z-cut was calculated. Because the interaction between the molecular octyl group and the substrate is found to be very weak, on the order of 0.1 eV, we focus on the configurations where the 8CB molecule bonds with the cyano group to the LN surface. A lateral mesh of 48 equidistant points was used to map the adsorption energy. The nitrogen lateral position was fixed, while all other molecule and substrate atom degrees of freedom were freely relaxed. Due to the complexity of the molecule as well as the surface, the PES energies obtained in this way do not necessarily correspond to the actual adsorption energies but rather give an indication for finding the molecules’ most favorable adsorption sites. The results are shown in Figure 3. In

Figure 2. Material slabs used for modeling the LN surface with a frozen bulk region and a region corresponding to the positive and negative Z-cut.

Hexagonal 2 × 2 and orthorhombic 3 × 2 unit cells are used to describe the translational symmetry of the surface for upright and flat-lying admolecules, respectively. The adstructures considered here thus contain up to nearly 700 atoms. The LN atomic surface structure is chosen in accordance with the unreconstructed equillibrium geometries determined earlier, 5,6,37 that is, the positive and negative Z-cut are characterized by −Nb−O3−Li2 and −Li−O terminations, respectively. The LN materials slabs are separated along the z direction by more than 40 Å of vacuum, which is sufficient to decouple the two slab sides even if additional molecules are adsorbed. The influence of dipole corrections that prevent electrostatic fields in the vacuum region is found to be small, on the order of 30 meV for the adsorption energies. Therefore, dipole corrections are neglected for the larger 3 × 2 unit cells. Gas-phase molecule calculations are performed using an orthorhombic 20 × 20 × 30 Å3 unit cell. A force convergence criterion of 20 meV/Å is used throughout the calculations. The electron wave functions are expanded here into plane waves up to a kinetic energy of 410 eV. The Γ point and a 2 × 2 × 1 k-

Figure 3. Calculated PES for 8CB positioned upright on (a) the positive and (b) the negative LN Z-cut. Zero energy corresponds to the desorbed 8CB molecule.

the case of the positive Z-cut, the molecular nitrogen prefers the bonding to uppermost substrate lithium atoms. There is a moderate corrugation of the PES corresponding to about 0.36 eV, indicating a limited molecular mobility on the surface. While the PES corrugation calculated for the negative Z-cut is far stronger, about 1 eV, the diffusion barriers are of similar height as those for the positive Z-cut. This is because in the case of the negative Z-cut, the attachment of the molecular nitrogen to both the surface Li and O atoms is favorable. The most favorable adsorption sites determined on the PESs are used in the following to generate starting configurations that were structurally completely relaxed. The thus-obtained minimum-energy structures for upright-adsorbing 8CB on the positive and negative Z-cut surface are shown in Figure 4a and b together with the calculated adsorption-induced charge 9343


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The Journal of Physical Chemistry C

small adsorption energy, where, moreover, more than 0.1 eV is due to dispersion interaction. Still, there is some adsorptioninduced charge accumulation between the molecular nitrogen and the surface lithium, as can be seen in Figure 4a. Considerably stronger charge accumulation, shorter N−Li bonds of 1.96 Å, and a larger adsorption energy of more than 1 eV characterize the upright-bonding configuration of 8CB on the negative Z-cut (cf. Figure 4b and Table 1). While the contribution of dispersion interaction to the adsorption energy of upright-standing molecules is minor, on the order of 0.1 eV, much stronger contributions can be expected for the situation that the molecules adsorb in a flatlying configuration. In fact, rotating the molecule toward a flat configuration increases monotonously the magnitude of the adsorption energy. This is shown in Figure 5 for the case of the

Figure 5. Calculated energy variation depending on the angle between the molecular axis and the surface (see inset) for the adsorption of 8CB on the positive LN Z-cut.

positive Z-cut. Similarly, the adsorption energy increases in the case of the negative Z-cut when the molecule approaches a horizontal bonding configuration. The final, rather flat bonding geometries are different for the two substrate polarizations, and there is clearly a polarization-specific contribution to the substrate−molecule bonding that can be seen by the differences in the charge redistribution in Figure 4c and d. On the negative surface, there is covalent bond formation between the molecular cyano as well as, to a much weaker extent, the phenyl group with the substrate. The surface O atoms cause some repulsive interaction that leads to molecular deformations. The molecular charge redistribution upon attachment to the positive Z-cut is smaller than that for the negative Z-cut, however involving again mainly the molecular cyano group. The molecule−surface interaction is dominated by dispersion forces that account for more than two-thirds of the total interaction energy. The van der Waals interaction masks the polarization-specific bonding characteristics and leads finally to rather similar adsorption energies of 2.9 and 2.8 eV for flat molecules on the positive and negative Z-cut, respectively (cf. Table 1). In order to put the adsorption energies presented in Table 1 in perspective, it is helpful to compare them with the bulk cohesive energy of 8CB. Accordingly, calculations for 8CB in its crystalline bulk form, using a monoclinic four-molecule unit cell (192 atoms, cf. refs 38 and 39), were performed. Using the same methodology as that for the surface calculations described above, a cohesive energy of 1.45 eV per 8CB molecule was obtained. Comparing this with the molecular adsorption

Figure 4. Calculated adsorption-induced charge redistribution (in e/ Å3) for an upright-standing/flat-lying 8CB molecule on the positive (a/c) and the negative LN Z-cut (b/d), respectively.

redistribution. The corresponding energies are compiled in Table 1. Both for positive and negative Z-cut surfaces, uprightTable 1. Calculated 8CB adsorption energy on LN Z-Cut Surfaces adsorption configuration

Eads/eV

upright@pos. Z-cut upright@neg. Z-cut flat@pos. Z-cut flat@neg. Z-cut

0.312 1.076 2.893 2.793

adsorbing molecules prefer bonding to the surface Li. The adsorption energies, however, depend on the substrate polarization. For the positive surface, an adsorption energy of about 0.3 eV is obtained, along with a N−Li bond length of 2.26 Å. This bond length is considerable larger than the sum of the Li and N covalent radii, 1.98 Å. This is consistent with the 9344


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The Journal of Physical Chemistry C energies of nearly 3 eV for flat-lying molecules on the LN Z-cut, one sees that in the low-coverage case, both on the positive and on the negative surface, a molecular thin film of flat molecules corresponds to the ground state. Also, for increasing coverage, molecules oriented parallel to the surface still correspond to the lowest-energy configuration on the positive Z-cut because the surface energy gained from parallel packing (∼3 eV) is nearly twice as large as that due to molecular condensation (∼1.5 eV). The surface should thus act as a condensation nucleus that favors molecular ordering parallel to the LN cut. The situation is different for the negative Z-cut, however. Here, a comparatively large binding energy of more than 1 eV is released when 8CB bonds upright. Together with the higher packing density that can be realized for the upright adsorption configuration, the total interface energy gain for perpendicular stacking is larger than that for parallel attachment. In case of the negative Z-cut, an increase of coverage is therefore expected to be accompanied by a molecular reordering, at least if the interface reaches the thermodynamic ground state. This expectation is corroborated by additional calculations for the adsorption of one complete bilayer 8CB in a bonding configuration parallel to the surface. We calculated average molecular adsorption energies of about 1.5 and 1.2 eV on the positive and negative Z-cut, respectively. In that case of the positive surface, the adsorption energy is thus considerably larger than that for single upright-standing molecules, and molecular ordering parallel to the LN cut is favorable. This is in contrast to the negative surface, where similar adsorption energies are obtained for a closed bilayer of flat-lying 8CB and single upright-bonded molecules. Due to the higher packing density that can be achieved for the latter configuration, a molecular reordering may occur. There are experimental data available for the interaction of LC 4-(trans-4′-hexylcyclohexyl)-isothiocyanatobenzene (6CHBT) with differently poled LN Z-cuts.16,40 In these (mesoscopic) studies, phenomena were observed that are compatible with the pronounced polarization as well as configuration-dependent adsorption energies calculated here. Fragmentation and coalescence of LC drops and migration to different sample regions in response to the surface electric field were reported. However, these findings cannot directly be compared with the present results because (i) isothiocyanate bonded to the phenyl ring serves as the terminal group for 6CHBT rather than a cyano group, as in the present case, and (ii) the substrates in refs 16 and 40 were functionalized with polydimethylsiloxane (PDMS) polymer rather than being atomically clean, as assumed here. Still, the present calculations are useful in the context of the experiments discussed above. While the authors of refs 16 and 40 discuss their findings exclusively in terms of the surface electric fields due to the pyroelectric effect, the present results show that the chemical bond formation between the adsorbates and ferroelectrics depends strongly on the surface polarization and certainly needs to be taken into account as well for a complete understanding of the LC−LN interface.

does a strong covalent adsorbate−substrate bonding occur. Dispersion forces contribute substantially to the molecular adsorption and lead to the favoring of a flat-bonding configuration for single, isolated molecules. In the case of the negative Z-cut, however, the interface energy can be lowered for increasing coverage by a molecular reordering toward an upright-bonding configuration. The interface morphology as well as the interface energy will thus strongly depend on the substrate polarization.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank Eva Rauls and Hazem Aldahhak for helpful discussions. Financial support by the Deutsche Forschungsgemeinschaft DFG (SFB-TRR142, SCHM 1361/21) as well grants of high-performance computer time from the Paderborn Center for Parallel Computing (PC2) and the High Performance Computing Center in Stuttgart (HLRS) are gratefully acknowledged.

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CONCLUSIONS DFT calculations for a series of adsorption configurations of 8CB molecules on the positive and negative LN(0001) surface have been performed. A complete mapping of the PES of the cyano group interaction with the LN surface shows that the interaction with substrate Li (positive Z-cut) and Li as well as O (negative Z-cut) is preferred. However, only in the latter case 9345


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Liquid Crystal (8CB) Molecular Adsorption on Lithium Niobate Z-Cut

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