Semiconductor Interfaces - The Journal of

Article pubs.acs.org/JPCC

Surface Effects on Catechol/Semiconductor Interfaces Arrigo Calzolari,*,† Alice Ruini,†,¶ and Alessandra Catellani†,§ †

CNR-NANO, Istituto Nanoscienze, Centro S3 I-41125 Modena, Italy Istituto Nanoscienze, Centro S3 I-41125 Modena, Italy ¶ Dipartimento di Fisica, Università di Modena e Reggio Emilia, I-41125 Modena, Italy § CNR-IMEM, Parco Area delle Scienze, 37A, I-43100 Parma, Italy ‡

S Supporting Information *

ABSTRACT: We present a density functional investigation of the surface effects on the catechol sensitization of selected hexagonal semiconductors (SiC, GaN, InN, CdS, CdSe). The atomic relaxation, the ionicity, and the reactivity, which characterize the selected substrates, are found to crucially influence both the bonding geometry and the electronic level alignment at the interface. Our results indicate that surface effects must be considered in order to obtain a correct picture of the optoelectronic response of the system. Our findings pave the way to the fundamental understanding and future design of hybrid catecholate materials for optoelectronic and biomedical applications.

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near-IR region,10−13 photosensitive glasses,14 and light emitters.15 In the context of hybrid nanostructures, the catechol unit (1,2-dihydroxybenzene) has emerged as a very attractive ligand for the functionalization of a wide range of substrates (including those mentioned above) by providing effective, stable, and versatile surface modification, also in wet conditions.16−20 As a matter of fact, catechol is a prototype sensitizer for solar cells,21−23 a model system for the photocatalytic oxidation of organic pollutants,24,25 and a radical scavenger in biomolecular systems.26,27 Catechol is a common functional group that efficiently links more complex polymers28,29 or (bio)molecules30 for biomedical applications, as well as it is a noninnocent ligand in the formation of valence-tautomeric compounds for spintronics.31 Indeed, its favorable anchoring mechanism is via bidentate chelation, typically leading to ring coordination complexes at the interface,32−34 and the details of the interface electronic structure crucially affect the functionality and efficiency of the resulting device. In the realization of controlled molecule/semiconductor interfaces for nanotechnology, one open challenge is the identification of the nanocrystal surface effects on both structural and electronic properties of the resulting interface. From the theoretical side, a microscopic understanding of the mechanisms that regulate the molecular adsorption and formation of specific bandlike properties is still missing. In fact, although a large effort has been dedicated to the comprehension of the quantum confinement on the optoelectronic properties of low-dimensional nanostructures, very little is known about the effects of the outermost atomic

INTRODUCTION Organic functionalization is a well-established strategy in nanoscience and nanotechnology: heterointerfaces between organic and inorganic matter offer the possibility of hybrids to perform some specific functions better than either purely inorganic or purely organic systems.1,2 Inorganic semiconductor nanostructures (dots, wires, hyperbranched rods, etc.) are some of the most promising systems to be exploited for electronic and optoelectronic nanoscale applications: they provide a direct path for charge transport, high carrier mobilities, solution processability, thermal and ambient stability, and a high electron affinity necessary for charge injection from the complementary organic donor material. Semiconductor compounds with hexagonal symmetry, such as metal oxides, calchogenides, and nitrides, are especially suitable to build efficient hybrid nanostructures; their noncentrosymmetric crystal structure also provides interesting polarization and piezoelectric properties, with promising perspectives for next-generation self-powered nanodevices.3 In particular, wide-band-gap semiconductors, such as ZnO, SiC, and GaN,4 have excellent properties, such as high robustness, biocompatibility, nontoxicity, and chemical stability, under physiological conditions. Their surface modification5 through molecular adsorption results in new materials with enhanced properties (e.g., sensitization, band alignment at the interface), particularly suitable for light harvesting in solar cells,6 optoelectronic switches and logic devices,7 and for nanomedicine and therapeutics.8,9 The molecular coating of chalcogenide nanocrystals (e.g., CdS, CdSe, CdTe, ZnS) combines the easy processability of organic materials with the higher electron affinity of the inorganic compounds. The resulting hybrid materials are largely used for the realization of optical sensors and hybrid solar cells working in the visible and © 2012 American Chemical Society

Received: July 18, 2012 Published: August 7, 2012 17158

dx.doi.org/10.1021/jp307117h | J. Phys. Chem. C 2012, 116, 17158−17163

The Journal of Physical Chemistry C

Article

reconstruction, the surface stress/strain, and of the dangling bonds of the surfaces on the hybrid interfaces and on their properties. Because of the large range of applications and the structural simplicity, the catechol unit and nonpolar surfaces of hexagonal semiconductors represent an inimitable opportunity for a fundamental study of controlled prototypical models, well representative of the complex systems realized in the experimental setups. We here present a comprehensive ab initio study of the atomic and electronic structure modifications in the catechol functionalization of several wurtzite substrates presenting different ionicities, reactivities, and band properties. We disregard the fine details of the semiconductor nanoparticle shape and we consider flat periodic (101̅0) surfaces, which are among the most frequently exposed faces in hexagonal nanostructures.35 If, on one hand, this simplification does not take into account some characteristic features of the nanostructures (such as edges or kinks), it provides, on the other hand, a more general identification of the surface effects on molecular adsorption, not restricted to the unique details of a specific nanosystem. This realizes a simplified, but controlled, model system that allows for a more fundamental investigation of the mechanisms that rule the formation of the catechol/ semiconductor interfaces. The effects due to a specific atomic rearrangement in the nanostructure could be easily included as a further refinement. First, we demonstrate how the geometrical and electrostatic modifications in the substrate surfaces affects the catechol adsorption and the bonding path. We then show that a correct picture of the electronic level alignment is recovered only if surface effects are fully taken into account, which is especially crucial for the most reactive substrates. Our findings indicate that the usual schematic model to characterize the hybrid interface based on the simple electronic diagrams of the isolated subsystems36 is inappropriate in all cases where surface states induce high surface reactivity. The rationale we get after this study represents a step forward in improving the catecholic chemistry and in the realization of hybrid interfaces for optoelectronic and biomedical applications.

Figure 1. Schematic top (a) and side (b) views of adsorbed catechol on wurtzite (1010̅ ) surfaces. Black, gray, red, and white circles identify the atomic species at the interface, namely, surface anions (A) and cations (C), and molecular oxygen (O) and hydrogen (H). A = (C, N, O, S, Se) and C = (Si, Ga, In, Zn, Cd) for SiC, GaN, InN, ZnO, CdS, and CdSe, respectively.

of the relaxed (101̅0) surface. Electronic properties of (1 × 1) clean surfaces are also reported in the Supporting Information. Each structure is optimized until forces on single atoms are smaller than 0.03 eV/Å The well-known gap underestimation of DFT would induce a semimetallic behavior in the case of InN. In this case, we corrected the InN band gap and improved the energy lineup of molecular states at the interface, including an ad hoc Hubbardlike potential on both 4d orbitals of In (UIn = 6 eV) and 2p orbitals of N (UN = 3.5 eV), as previously proposed by Terentjevs and co-workers.40

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RESULTS AND DISCUSSION Structural Properties. We considered the adsorption of catechol dye on nonpolar surfaces of a set of hexagonal wurtzite semiconductor compounds, including SiC, nitrides (GaN, InN), and calchogenides (CdS and CdSe), and we compared the results with the paradigmatic case of the catechol/ZnO interface for completeness.34 For all systems, the optimized clean (101̅0) surface exhibits a strong reorganization of the outermost layer, which relaxes, forming ordered rows of tilted dimers along the [121̅ 0] direction (see Figure 1). The vertical buckling of surface dimers is mainly related to the original dimension of the crystal lattice (see Table S1, Supporting Information) and to the ionicity level of the anion−cation bond. Similar to the case of the ZnO substrate,34 we prepared the starting configurations, setting the catechol at ∼3.2 Å from the surface, with the phenyl ring perpendicular to the exposed dimers. The resulting optimized geometries are reported in Figure 2, and the structural results are summarized in Table 1. In the final configurations, catechol adsorbs, bridging two consecutive surface dimers, exposing the oxygen atoms toward the cation species. The molecule−substrate interaction is always attractive, giving a net energy gain to the overall system upon molecular adsorption. The absolute value of the adsorption energies (Eads) varies from −4.27 eV/molecule (SiC) to −0.11 eV/molecule (CdSe), being indicative of different coupling mechanisms. In the case of adsorption on SiC, during the relaxation path, the two −OH terminations of catechol first rotate, pointing toward the next-neighbor dimers along the [0001]̅ direction, then release both H protons41 that are promptly captured by frontal carbon atoms, making two C−H bonds. In the final configuration, the molecule is fully deprotonated and forms two tight Si−O bonds with the surface. This makes the surface dimers highly inequivalent: two dimers are involved in bonding with catechol oxygens (labeled O, in Table 1), two dimers in binding the catechol hydrogens (labeled H), while the

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COMPUTATIONAL DETAILS We performed first-principles total-energy-and-forces simulations by using the PWscf code, included in the Quantum ESPRESSO suite.37 The electronic structure is described by the density functional theory (DFT) within the PBE38 generalized gradient approximation to the exchange-correlation functional. Single-particle wave functions (charges) are expanded on a plane-wave basis set up to a kinetic energy cutoff of 28 Ry (280 Ry). Ionic potentials are described by ab initio ultrasoft pseudopotentials of the Vanderbilt’s type;39 d shells of cation metals (e.g., Zn3d, Ga3d, In4d, Cd4d) are explicitly included as valence electrons. A regular (4 × 4) grid of k-points is used to sample the 2D Brillouin zone of the interfaces. All systems are simulated using periodically repeated supercells. The unit cells have a (2 × 3) lateral periodicity and contain six bilayers of wurtzite semiconductor (namely, SiC, GaN, InN, CdS, CdSe) exposing the nonpolar (101̅0) surface. One catechol molecule is symmetrically adsorbed on each surface. Slab replicas are separated by ∼12 Å of vacuum (see Figure 1). The lattice parameters of semiconductor surfaces are obtained from the optimization of the corresponding bulk crystals and are listed in Table S1 in the Supporting Information, along with the most relevant structural parameters 17159



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pointing outward with respect to the O−O atoms), without rotation or protonation of the lateral hydroxyl groups (Figure 2). The interplay between the final molecular adsorbed geometry (i.e., protonated/deprotonated configuration) and the hexagonal surface would seem related to the lattice parameters of the substrate: a shorter distance between surface dimers would make the deprotonation process more probable. However, the comparison with the ZnO substrate (Figure 2)34 highlights a more complex scenario. Indeed, ZnO lattice parameters are intermediate between GaN and InN; thus, one could expect adsorption geometries similar to those of nitride systems. The number of bonds with the surface (two chemical and two hydrogen bonds) is maintained, but only a proton release takes place (see ref 34 for further details). This is related to the presence of oxygens on both catechol termination and ZnO surface dimers, which makes the chemical environment more symmetric than the nitride case. Thus, the catecholic chemistry at hexagonal surfaces depends on two main aspects of the substrate: one is structural and related to the mutual distances of neighbor surface dimers, and one is electrostatic and related to the acidity of the surface. To demonstrate this statement, we considered the adsorption of catechol on elongated SiC and on compressed CdSe surfaces. In the case of SiC, we separately increased the lattice parameters a0 and c (i.e., affecting the interdimer distance and the dimer length, respectively) by 2, 6, and 10% with respect to the minimum energy ones (see Table S1, Supporting Information). This corresponds to a progressive interdimer separation along the [12̅10] and [0001̅] directions, respectively (see Figure 1). We first optimized the atomic structure of the strained clean surfaces; then we let the molecule adsorb, adopting the same preparation setup described above. The results are summarized in Figure S3 in the Supporting Information. Independent from the a 0 elongation, the adsorbed geometries are similar to Figure 2a, where a double deprotonation occurred, indicating that the interdimer distance along the dimer row is not an issue for this

Figure 2. Adsorption configurations of isolated catechol molecules on wurtzite (1010̅ ) surfaces: (a) SiC, (b) GaN, (c) ZnO, (d) InN, (e) CdS, and (f) CdSe. Dashed lines identify molecule−substrate H bonds.

remaining two are not involved in the molecular binding. In particular, the formation of Si−O bonds inverts the vertical buckling (dz) of the corresponding dimers, while the C−H bonds increase the original value, indicating a charge transfer reorganization at the interface. Very similar is the case of nitride substrates (GaN and InN), which display the formation of two cation−O bonds, a double deprotonation, and the consequent formation of two N−H bonds at the surface. In the present case, the different polarity between catechol oxygen and surface nitrogen gives rise to the formation of two molecule−substrate H bonds, not observed in the case of SiC. On the contrary, adsorption on Cd-based surfaces presents different features: catechol is still sitting in the proximity of two Cd surface cations, but at a slightly larger distance than in the previous cases. The corresponding Cd−O coupling is weaker, as confirmed by the absence of the surface dimer relaxation and the lower adsorption energy. Catechol maintains its original symmetric geometry (i.e., with both H's

Table 1. Adsorption Energy and Structural Parameters for Catechol-Functionalized (101̅0) Surfacesb Eads

dAC

dz

dCO

dOH

dAH

dOO

α

nH+

SiC

−4.27

2.43(S)

1.10(S)

2.85

86.2

2

−3.07

1.88

2.01(S)

1.04(S)

2.91

80.3

2

ZnOa

−2.25

2.04

1.62(S) 1.60(M)

1.03(S) 1.03(M)

2.88

75.1

1

InN

−3.87

2.16

2.15(S)

1.04(S)

2.88

90.7

2

CdS

−0.18

3.14

0.98(M)

2.76

90.3

0

CdSe

−0.11

0.18(N) −0.13(0) 0.23(H) 0.25(N) −0.06(O) 0.23(H) 0.34(N) −0.01(O) 0.27(H) 0.28(N) 0.04(0) 0.19(H) 0.73(N) 0.68(O) 0.81(N) 0.79(O)

1.65

GaN

1.74(N) 1.82(O) 1.85(H) 1.83(N) 1.89(O) 1.90(H) 1.88(N) 1.91(O) 1.95(H) 2.08(N) 2.13(O) 2.14(H) 2.43(N) 2.44(O) 2.54(N) 2.55(O)

3.15

0.99(M)

2.75

90.0

0

a

From ref 34. bEnergies (Eads) are expressed in eV per molecule, distances (dX) in Å, and molecular slope (α) in degrees. Subscript labels refer to atoms as in Figure 1. Labels in parentheses, (N), (O), and (H), discriminate the surface dimers that are not involved in bonding with the molecule (N), and dimers involved in bonding with catechol oxygen (O) and hydrogen (H), respectively. nH+ is the number of protons released by catechol and catched by the surface; protons attached to molecular oxygens (surface anions) are labeled with M (S), respectively. 17160



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Figure 3. (a) Total and projected DOS for catechol on the SiC(101̅0) surface. Zero energy refers to the Fermi level. Inset: HOMO−LUMO (blue lines) energy alignment of catechol with respect to the SiC band gap (shaded area). All energies are aligned to the vacuum level. (b) Isosurface plots of selected single-particle states, corresponding to arrows in panel a. Subscripts N, O, and H refer to surface dimers not involved in bonding with the molecule (N), and dimers involved in bonding with catechol oxygen (O) and hydrogen (H), respectively. Superscripts v and c refer to valence and conduction bands, respectively.

process. On the contrary, increasing the c distance, we get the detachment of 2, 1, and 0 protons, respectively, meaning that the distance between the catechol oxygen and the surface anion is one of the order parameters for the molecule dissociation. We note, however, that, in the case of a larger elongation of c (10%), the molecular hydroxyl groups rotate, pointing H toward the next C atoms, confirming the capability of C attracting protons. This does not happen in the case of the CdSe surface. We contracted the crystalline parameters a0 and c by 10% (Figure S3, Supporting Information), thus reproducing lattice parameters closer to InN. In both cases, catechol maintains its initial symmetrical configuration as in Figure 2f, without any rotation of the hydroxyl groups toward the Se anions, which does not easily accept protons. This confirms that, besides the structural effects, the adsorption chemistry of catechol (weak acid) is strongly related to the acidity of the surface. These two aspects of the substrate (structure and acidity) are expected to be crucial and enhanced in nanostructures, where the high surface-to-bulk ratio gives rise to highly frustrated bonds and a high level of strain at the surface (typically much larger than in the corresponding bulk and surfaces), which affects the reactivity of the nanocrystal and its interplay with the functionalizing molecular dyes. Moreover, since semiconductor nanostructures are usually operating in solvents and humid environments at a controlled pH, it must be considered that the external variation of the acid−base equilibrium of the substrate can modify the catecholic adsorption chemistry. Electronic Properties. In the standard description of the band alignment problem between a molecular dye and a semiconducting substrate, the only parameters that are usually taken into account are the energy position of frontier orbitals (e.g., HOMO and LUMO) and the absolute value of the substrate gap.42 This approach was profitably used to a fast estimate of interface band alignment in systems based on TiO219,32,36,43 and ZnO,34,44 which are wide-band-gap semiconductors, almost without surface states in the gap. However, though this is the necessary prerequisite to discriminate between straddling (type-I) and staggered (type-II) interfaces, this analysis completely neglects the complexity of the electronic structure of the substrate. This is particularly true for nanostructures, where the effects of the quantum confinement and surface defects combine with the intrinsic surface state distribution, typical of most semiconductors. Here, we

show how these features modify the electronic structure of the interface and, in some cases, the final band alignment.45 All the semiconductors we considered in this work present intrinsic surface states in the main gap,46 one fully occupied (labeled SSv) and one fully unoccupied (labeled SSc), as shown in the surface band structure plots, reported in the Supporting Information. Each surface state is associated with the exposed buckled dimers; in particular, SSv involves surface anions and SSc the surface cations. Moving from SiC to CdSe, we observe a progressive displacement of both surface states from the center of the gap toward the edges of the valence and conduction bands, respectively, indicating a progressive reduction of the surface reactivity. We assume the catechol/SiC as the reference case. The simple band alignment between the molecular states (blue lines) and surface gap (shaded area) is displayed in the inset of Figure 3. If we discard any surface effect, the catechol HOMO state (Hm, blue line) lies in the SiC gap, just above the bulk valence band top (VBT), and the LUMO is well inside the SiC conduction band, far from the conduction band minimum (CBM). This would correspond to a type-II interface and suggest a large voltage drop (Vg = CBM − Hm), which is a critical parameter for optoelectronic devices. However, the inclusion of surface states dispersion (black boxes) remarkably reduces the effective gap and may alter the interaction with the adsorbed moiety; that is, it may change the optoelectronic response of the interface. Indeed, the explicit calculation of the entire system attests a much more complex scenario, hardly derivable from the simplified model based on isolated subsystems, as presented above. Figure 3 shows the total (black line) and projected density of states (DOS), for catechol on the SiC(101̅0) surface. The complex adsorption path, which leads to the formation of O−Si and H−C bonds at the interface, breaks the surface degeneracy of the surface states that now split, dramatically reducing the effective gap of the overall system to ∼0.4 eV. More in detail, below the Fermi level (fully occupied states), we can recognize valence surface states that are not affected by the presence of the molecule, as in the clean surface (SSvN) or that are involved in the formation of the Si−O bonds (SSvO), as well as the HOMO-derived states of the catechol (Hm). The corresponding single-particle orbitals are plotted in Figure 3b. In a similar way, just above the Fermi level (virtual states), we distinguish surface states that are hardly affected by the molecule (SSNc) and others that are localized on H−C 17161



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Figure 4. Total and projected DOS for catechol on the (1010̅ ) surface of (a) GaN, (b) InN, (c) CdS, and (d) CdSe. Zero energy refers to the Fermi level. Insets: HOMO−LUMO (blue lines) energy alignment of catechol with respect to the substrate band gap (shaded area). All energies are aligned to the vacuum level. Labels refer to Figure 3.

terminations (SSH). The presence of localized states in the gap may strongly influence the optoelectronic response of these materials, that is, by changing the light-absorption edge or inserting localized states that trap charge carriers and/or favor exciton splitting. The presence of surface states due to a specific nanostructure termination is expected to further enhance this effect. This example is paradigmatic of the surface effect on the electronic structure of the interface. To confirm this statement, we consider the coadsorption of catechol and water on SiC(101̅0) (see Figure S4, Supporting Information). Catechol adsorbs similar to the previous case. Water molecules dissociate, in agreement with previous theoretical works,47,48 with OH− fragments forming dative bonds with surface Si atoms, and H+ forming bonds with surface carbons. The overlayer self-arranges in order to optimize the formation of intramolecular H bonds. At one monolayer coverage (i.e., one H2O per surface dimer), all surface states are saturated and involved in chemical bonds either with catechol or with water. This corresponds to a complete depletion of the gap region in the density of states (Figure S4b, Supporting Information), confirming that the presence/absence of surface states cannot be disregarded a priori. The comparison with the ZnO substrate is reported in the Supporting Information for completeness: the observed differences in the interface band alignment can be traced back to the different surface acidities of the two substrates (see the Supporting Information). Similar argumentations hold for the other substrates we are considering in this work, especially for nitride compounds. GaN is a wide-band-gap semiconductor, whose surface states permeate the fundamental gap. Thus, similar to the SiC case, the adsorption of catechol promotes splitting of surface states and the partial filling of the pristine gap (Figure 4a). The same surface state redistribution takes place in the catechol/InN interface (Figure 4b), even though the final picture is slightly

different, being that the molecular HOMO drifted into the conduction band. This is simply due to the very low band gap of the original InN material, which makes the Hm state almost resonant with both the valence and the conduction bands. In the CdS and CdSe substrates, the surface states SSv and SSc are resonant with the valence and conduction bands, respectively, that is, less reactive than before. In fact, as in the ZnO case, the total density of states (Figure 4c,d) looks very similar to the simple band alignment obtained from the isolated subunits, with the Hm states lying in the gap, close to (CdS) or resonant with (CdSe) the VBT. This confirms thatat least for the present class of hexagonal materialsthe latter approach is not intrinsically wrong, but sometimes so oversimplified to be practically useless.

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CONCLUSIONS We studied the adsorption of the catechol unit on a set of planar surfaces of hexagonal semiconductors, which we assumed as simplified, but controlled, models for the description of the hybrid interfaces between organic molecules and inorganic nanostructures. We investigated the effects of the intrinsic geometrical and electronic properties of the substrates in the catechol adsorption process, inferring the effects of the surface strain and acidity in the realistic nanoscale systems. We finally identified a mechanism that clearly relates the presence of surface electronic states to the band alignment of the resulting interface, which is the base for a rational engineering of selected optoelectronic properties for selected applications.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information includes structural and electronic characterization of clean semiconductor substrates (section S1) and the molecular adsorption modification upon SiC and CdSe surface strain (section S2). Structural and electronic properties 17162



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of catechol on the hydroxylated OH:SiC(1010̅ ) surface are reported in section S3. This material is available free of charge via the Internet at http://pubs.acs.org.

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(28) Fan, X.; Lin, L.; Dalsin, J. L.; Messersmith, P. B. J. Am. Chem. Soc. 2005, 127, 15843−15847. (29) Villa, G.; Povie, G.; Reaud, P. J. Am. Chem. Soc. 2011, 133, 5913−5920. (30) Hurst, S. J.; Fry, H. C.; Gosztola, D. J.; Rajh, T. J. Phys. Chem. C 2011, 115, 620−630. (31) Hendrickson, D. N.; Pierpont, C. G. In Spin Crossover in Transition Metal Compounds II; Gutlich, P., Goodwin, H., Eds.; Topics in Current Chemistry; Springer: Berlin, 2004; Vol. 234, pp 786−786. (32) Janković, I. A.; Šaponjić, Z. V.; Č omor, M. I.; Nedeljković, J. M. J. Phys. Chem. C 2009, 113, 12645−12652. (33) Savic, T.; Jankovíc, I. A.; Šaponjíc, Z. V.; Comor, M. I.; Veljkovic, D. Z.; Zaric, S. D.; Nedeljkovic, J. M. Nanoscale 2012, 4, 1612−1619. (34) Calzolari, A.; Ruini, A.; Catellani, A. J. Am. Chem. Soc. 2011, 133, 5893−5899. (35) Notably, the (101̅0) surfaces of hexagonal semiconductors are generally known to be very stable and well-ordered, with a small amount of surface defects, also in nanostructures (e.g., wires, rods). This further validates the choice of defect-free periodic 2D surfaces, as representative models for catechol/semiconductor interfaces. (36) Grätzel, M. Nature (London) 2001, 414, 338−344. (37) Giannozzi, P.; Baroni, S.; Bonini, N.; Calandra, M.; Car, R.; Cavazzoni, C.; Ceresoli, D.; Chiarotti, G. L.; Cococcioni, M.; Dabo, I.; Dal Corso, A.; de Gironcoli, S.; Fabris, S.; Fratesi, G.; Gebauer, R.; Gerstmann, U.; Gougoussis, C.; Kokalj, A.; Lazzeri, M.; Martin-Samos, L.; Marzari, N.; Mauri, F.; Mazzarello, R.; Paolini, S.; Pasquarello, A.; Paulatto, L.; Sbraccia, C.; Scandolo, S.; Sclauzero, G.; Seitsonen, A. P.; Smogunov, A.; Umari, P.; Wentzcovitch, R. M. J. Phys.: Condens. Matter 2009, 21, 395502. See also www.quantum-espresso.org. (38) Perdew, J. P.; Burke, K.; Ernzerhof, M. Phys. Rev. Lett. 1996, 77, 3865−3868. (39) Vanderbilt, D. Phys. Rev. B 1990, 41, R7892−R7895. (40) Terentjevs, A.; Catellani, A.; Prendergast, D.; Cicero, G. Phys. Rev. B 2010, 82, 165307. (41) Bouchoux, G.; Defaye, D.; McMahon, T.; Likholyot, A.; Mó, O.; Yánez, M. Chem.Eur. J. 2002, 8, 2900−2909. (42) Van de Walle, C. G.; Neugebauer, J. Nature (London) 2003, 423, 626−628. (43) Nazeeruddin, M. K.; De Angelis, F.; Fantacci, S.; Selloni, A.; Viscardi, G.; Liska, P.; Ito, S.; Takeru, B.; Grätzel, M. J. Am. Chem. Soc. 2005, 127, 16835−16847. (44) Rangan, S.; Theisen, J.-P.; Bersch, E.; Bartynski, R. Appl. Surf. Sci. 2010, 256, 4829−4833. (45) We consider the qualitative modification of band alignment in the proposed interfaces, as a significant key test of the effect of surface states on the electronic properties of the overall system. The quantitative evaluation of the band offset, which suffers in our approach of the well-known limitation of DFT, goes beyond the aim of the present work. (46) Vogel, D.; Krüger, P.; Pollmann, J. Phys. Rev. B 1996, 54, 5495− 5511. (47) Cicero, G.; Grossman, J. C.; Catellani, A.; Galli, G. J. Am. Chem. Soc. 2005, 127, 6830−6835. (48) Catellani, A.; Calzolari, A. J. Phys. Chem. C 2012, 116, 886−892.

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work is partially funded by FP7-ITN:Nanowiring (Grant No. 265073). Computational resources were provided at CINECA by project IscraC_ACID_sph and IscraC_SHOCK.

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REFERENCES

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