Article pubs.acs.org/JPCC
NiAl(110) Surface as a Template for Growing Transition Metal Linear Atomic Chains: A DFT Investigation Miguel A. San-Miguel,† Edgard P. M. Amorim,‡ and E. Z. da Silva*,§ †
Institute of Chemistry, University of Campinas, Unicamp, Campinas, SP 13083-970, Brazil Departamento de Física, Universidade do Estado de Santa Catarina, Joinville, SC 89219-710, Brazil § Institute of Physics “Gleb Wataghin”, University of Campinas, Unicamp, Campinas, SP 13083-859, Brazil ‡
ABSTRACT: First-principles calculations based on periodic density functional theory (DFT) have been used to investigate structural, energetic, and electronic properties of different transition metal atoms (Pd, Pt, Cu, Ag, and Au) on the NiAl(110) surface for coverages ranging from 0.25 monolayer up to completing full coverage, with special emphasis on the different possible depositions to form linear atomic chains (LAC). The analysis of the energetic contributions and electronic structure reveals that metal atoms are greatly favored to be aligned along the [001] direction to form LACs. The calculated negative work function changes are interpreted taking into account both the electronegativity and the polarizability of the deposited metal adatoms. This work function change decreases particularly for LACs along the [001] direction and, intriguingly, vanishes for Pt, suggesting an electronic behavior similar to the corresponding free-standing LAC.
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INTRODUCTION The ability provided by the new experimental tools that manipulate atoms such as scanning tunneling or atomic force microscopy (STM or AFM) has opened new and important avenues of research such as the adsorption of impurities, dimers, trimers, and longer nanowires on atomic surfaces. Besides the great interest of studying experimentally zero- and one-dimensional physics provided by these systems, these studies can be important for technology since the insertion of atomic chains may provide new and interesting devices at the atomic scale and their realization is happening now or in the near future. Therefore, the study of physics of these lowdimensional systems, such as linear atomic chains (LACs) and supported nanowires is of great importance. The function of this novel system goes beyond the bottom-up engineering of atomic level interconnects and allows the use of atomic chains as template bases for production of more complicated and higher-order structures for other applications. Another important line of research is the opportunities presented by these structures from their chemical point of view. The catalytic properties of materials depend strongly on their sizes, and hence clusters1 and chains2 can be employed as catalysts of various reactions. Therefore, studies such as the present research can provide new platforms for catalytic studies. The study of metal nanowires (NWs) evolved from the use of experimental techniques such as mechanically controllable breaking junctions (MCBJs),3 high-resolution transmission electron microscopy (HRTEM),4 and also scanning tunneling or atomic force microscopy (STM or AFM)5 that were fundamental to allow the production and study of free-standing © 2015 American Chemical Society
linear atomic chain (LAC) nanowires. Computer simulations were also used to understand some of these results.6 The investigation of these nearly one-dimensional systems suggested the possibility to exploit them in engineering new devices. Unfortunately, it is hard to perform controlled experiments using suspended LACs, and therefore the use of substrates as supports was suggested. Thus, the manipulation of atoms on surfaces was achieved by Nilius et al. in a series of seminal papers where the adsorption of Au atoms, dimers, trimers, and longer one-dimensional chains on NiAl(110) were studied.7,8 With STM they imaged these Au NW structures. They were able to assemble a chain of Au atoms on a NiAl(110) surface using a STM tip. They studied catalytic properties of Pd,9 Au, and Ag10 on NiAl. They also studied the alloying of Au chains with Pd.11−13 The great stability of the NiAl(110) surface makes it an ideal template in which atoms can be deposited to form interesting supported nanostructures. The Au LAC NWs studied by Nilius’s group are a beautiful example. Besides being good templates for metal chain studies, NiAl alloys are very important industrially since they are lightweight materials used in bimetallic catalysts14 and also in high-temperature applications such as jet turbine engines where they are used with thermal barrier coating formed by alloying them with metals such as Pt. Pt substitution forming NiAl-Pt alloys was studied since Pt improves oxidation resistance in NiAl.15 They Received: September 26, 2014 Revised: January 11, 2015 Published: January 13, 2015 2456
DOI: 10.1021/jp5097635 J. Phys. Chem. C 2015, 119, 2456−2461
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The Journal of Physical Chemistry C find that Pt prefers the Ni sites. Previous studies have shown that the NiAl(110) surface is the closest packed, stoichiometric with equal amounts of Ni and Al in alternating rows16 and was shown by DFT studies to be the most stable surface.17 Recently we performed a systematic study of the adsorption of group 10 (Cu, Ag, and Au) and group 11 (Pd and Pt) transition metal (TM) monomers on the NiAl(110) surface.18 The aim of the present work is to investigate the effect of increasing the surface coverage with the metal atoms adsorbed on the most favorable site from 0.25 monolayer (ML) up to 1.0 ML (full coverage). Here, the surface coverage is defined as the ratio of adatoms/surface sites where surface sites are discussed further in the Results section. In particular, coverages of 0.5 ML correspond to the formation of linear atomic chains (LACs) along different directions. We characterize them from the energetic and electronic points of view, and we report on the role of the substrate as a template to favor the building up of the LACs along the [001] direction.
than two other Al atoms. All initial configurations considered here started with the monomers only on these sites. Thus, six structures were studied: one monomer in the 2 × 2 supercell corresponds to 0.25 ML coverage; two metal atoms is equivalent to 0.5 ML, and there are three different possibilities to align them as indicated in Figure 1 (cases ab, ac, and ad); three metal atoms correspond to 0.75 ML; and finally, four atoms complete one monolayer (1.0 ML).
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METHODS Planewave DFT calculations were performed using the VASP code.19−21 The valence electrons are described with a plane wave basis set, and the effect of the inner cores on the valence electron density is taken into account using the projector augmented wave (PAW) method.22,23 A kinetic energy cutoff of 300 eV for the plane wave expansion was used, which was adequate to obtain total energies converged to at least 1 meV/atom. The Brillouin zone was sampled using a 7 × 9 × 1 k-point mesh generated by the Monkhorst−Pack scheme,24 with the 0.1 eV smearing25 of the occupied states. Spin polarization was also included in all calculations. The generalized gradient approximation (GGA) in the Perdew−Burke−Ernzerhof (PBE) formulation was used for the electron exchange and correlation contribution to the total energy.26,27 The conjugated gradient (CG) energy minimization method was used to obtain relaxed systems. Atoms were fully relaxed until the Hellmann−Feynman forces converged to less than 0.005 eV/Å per atom. The NiAl(110) surface was modeled using a (2 × 2 × 5) slab with a vacuum distance of 12 Å to avoid the interaction between the surface and its images and also to guarantee meaningful calculations of the work function. A dipole correction was added in the surface normal direction.28,29 Work functions (Φ) were calculated using the following equation Φ = V ( +∞) − E F
Figure 1. Top view of a (2 × 2) supercell of NiAl(110). The arrows ad, ab, and ac indicate the representative directions discussed in the text.
After the geometry optimization process, the stability of these structures has been analyzed by estimating three different significant quantities that have been defined as follows: (i) binding energy, Eb Eb =
Etot − Esurf − Nat × Eat Nat
(2)
(ii) metal−substrate interaction energy, EM−S EM − S =
Etot − Esurf − Eadats Nat
(3)
(iii) metal−metal interaction energy, EM−M EM − M =
(1)
Eadats − Nat × Eat Nat
(4)
where Nat is the number of metal atoms deposited on the surface; Eat is the energy of the isolated atom in the ground state electronic configuration; Etot is the total energy of the relaxed adsorption system; Esurf is the energy of the relaxed surface slab; and Eadats is the energy of the free-standing adatoms at the geometry of the adsorption system. Thus, Eb is the total binding energy per atom for each type of adatom; EM−S is the interaction energy between the adatoms and the substrate; and EM−M is the energy of adsorbate− adsorbate interactions. Certainly, it is obvious that
where V(+∞) and EF are electrostatic potential in the middle of the vacuum region of the supercell and the Fermi energy level, respectively.
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RESULTS Surface Structure and Energetics. In this study, we have considered the effect of increasing the surface coverage on the NiAl(110) surface. We have used a 2 × 2 surface model in which up to four metal atoms can be adsorbed to complete a monolayer. There are seven possible adsorption sites where an atomic adsorbate can be allocated, and they have been described precisely in a previous publication.18 The most favorable site is the 4-fold coordinated site where the monomer bonds four surface atoms: two Ni atoms at shorter distances
E b = EM − S + EM − M
(5)
This energetic partition aids the understanding of the chemical bonding. The binding energy itself does not clearly show the differences that occur in these processes for all the different atoms studied. The partitioning of Eb as EM−S and 2457
DOI: 10.1021/jp5097635 J. Phys. Chem. C 2015, 119, 2456−2461
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The Journal of Physical Chemistry C EM−M, however, sheds light on the different behavior that these metal atoms have, in particular for coverages associated with the formation of linear atomic chains, as will be discussed. Figure 2
and electronically from the point of view of the chemical bonding. The other two energy contributions (EM−S and EM−M) aim to understand this result. The energy values for the metal− substrate interactions (EM−S) are significantly similar in all cases for 0.25 and 0.5 ML (ad case), but these values increase for ab and more noticeably for the ac case indicating that the interaction with the support diminishes. On the other hand, the metal−metal interaction energies (EM−M) are negligible at the lowest coverage (0.25 ML) suggesting that the adatoms are not interacting. Similarly, when increasing the coverage at 0.5 ML (ad case) the interactions are still insignificant, but a slight stabilization is reached at the ab system. However, the stabilization for the ac case is quite remarkable for all metals but more significant for Pt. For abcd coverage, the EM−M is similar to the ac case demonstrating that full coverage is equivalent to two parallel LACs in the ac direction, indicating also that there is negligible lateral interaction between these chains. The case of 0.75 ML is a cross chain configuration between two chains in the ac and ab directions. Since the ab case behaves almost as a noninteracting atoms, the EM−M value for the abc case is therefore an intermediate value between the ac and the ab values. Therefore, for the ac case, it is clear that the larger the metal−metal interaction stabilization is, the smaller the metal− substrate interaction stands. This energetic observation is also reflected in the fact that the LAC atoms shift upward in the ac systems by at least 0.1 Å with respect to the other directions and more noticeably for Pt and Au. On the other hand, the metal−metal interaction energy plots (EM−M in the bottom graph of Figure 2) show that in this situation the stabilization is quite noticeable, whereas the contribution corresponding to the interaction with the support (EM−S in the middle graph of Figure 2) becomes less negative (i.e., unfavorable). It is worth noticing that this stabilization effect is also more notorious for Pt followed by Au. This result is quite relevant indicating that the NiAl(110) substrate acts as a framework in which the growth of metal LACs is strongly favored in a particular direction (i.e., ac case). This will have important consequences for the formation of supported LACs in particular for the case of Pt. Electronic Structure. In order to further understand these energetic considerations we have analyzed the electronic structure. First, we have computed the work function changes after depositing the metal adsorbates on the NiAl(110) surface. In general, the work function change of the substrate upon adsorption depends on the electronegativity of the adsorbed atom. If the adsorbate is more electronegative than the substrate, it is expected to withdraw electrons from the surface, and then the work function change is positive. Conversely, when the adsorbate is less electronegative, a negative workfunction change arises from a donation of charge from the adsorbate to the surface. However, this general trend is not always kept, and different exceptions have been reported in the literature.32−35 In particular, the systems considered here do not obey that common rule, and the interpretation must be done taking into account both the electronegativities and the polarizabilities of the adatoms.18 The variation of work function upon adsorption for the different coverages is displayed in Figure 3. For all cases, the work function decreases when depositing the metal atoms at any coverage despite that they are more electronegative than the surface atoms. When the coverage increases from 0.25 to
Figure 2. Energetic contributions associated with the adsorption of metal atoms on NiAl(110) at different coverages (a: 0.25 ML; ad, ab, ac: 0.5 ML; abc: 0.75 ML; abcd: 1 ML): (a) the total binding energy per atom (Eb), (b) the interaction energy between the adatom and the substrate (EM−S), and (c) the adatom−adatom interaction energy (EM−M).
presents the energetic contributions Eb (top graph), EM−S (middle graph), and EM−M (bottom graph). It can be seen that for all metals the binding energies Eb at the lowest coverage (0.25 ML) are similar to the ab and ad cases at 0.5 ML. This fact might infer that the lateral interactions are negligible. However, there is a significant stabilization at the same coverage (0.5 ML) for the ac case. It is worth pointing out that the nearest distance between neighboring adatoms for the different coverages is 5.82 Å (a), 5.04 Å (ad), 4.11 Å (ab), and 2.91 Å (ac). This means that only in the ac case the interatomic distance is similar to the interatomic distances in bulk systems and also comparable to those distances in typical metallic LACs30 in vacuum as can be seen in Table 1. Therefore, this Table 1. Experimental31 and DFT Calculated (GGA-PBE) Metal−Metal Distances (in Å) in Bulk fcc and in Suspended Linear Atomic Chains Calculated with the GGA-PBE Functional30 bulk - DFT bulk - exptl chain
Cu
Ag
Au
Pd
Pt
2.57 2.556 2.33
2.95 2.889 2.68
2.96 2.883 2.61
2.79 2.750 2.46
2.81 2.772 2.35
substrate can be employed as support to create LACs with enhanced conductance using STM techniques to manipulate the atoms in specific directions as suggested previously.7 For the other cases, the distance between adatoms is large, and metal−metal (M−M) interactions should not be expected. However, we have investigated this aspect both energetically 2458
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The planar averaged charge density difference plots depicted in Figure 4 were calculated in slices perpendicular to the surface normal using the following equation Δρz = ρz M/NiAl(110) − ρz NiAl(110) − ρz M
(6)
where ρzM/NiAl(110), ρzNiAl(110), and ρzM are the planar averaged density along the z direction (perpendicular to the surface plane) of the surface with adsorbed metal atoms, the clean surface, and the isolated metal atoms, respectively, all at their positions in the adsorption system. The large positive feature in the plots corresponds to a large charge accumulation between the metal chain and the surface. There are also two depletion regions located around the topmost surface atoms and above the center of mass of the metal chain. On the basis of the fact that all metals considered here are more electronegative than the surface atoms, it is expected for some charge transfer to occur from the surface to the metal chain. The loss charge of the surface is reflected in the first depletion region, and this charge is accumulated in the largest positive region below the center of mass of the chain. This peak is also increased by the chain contribution (revealed in the main second depletion region) to the covalent interaction. For some cases, there is also a second positive region above the center of mass of the ad atoms. The origin of these charge distributions can be better understood from the differences of electron density, which have been plotted in Figure 5. Positive values signify accumulation of
Figure 3. Work function changes (ΔΦ) upon the adsorption of metal atoms on the NiAl(110) for different coverages.
0.5 ML for the ab and ad cases, the work function decreases as expected; however, interestingly there is a significant increase for the ac systems. For Pt, there is no work function change. These results reflect the fact that the interactions between metal atoms along the [001] direction (ac case) weaken the interactions with the substrate, as observed in the previous section, and also the substrate electronic structure is less altered than expected for a coverage of 0.5 ML. This result might bring important implications, particularly for Pt LACs. Potentially it means that the electronic structure of a supported Pt LAC on the NiAl(110) substrate would be similar to the corresponding free-standing LAC case.
Figure 5. Cuts of electron density differences through planes parallel to the surface including centers of mass of the adatoms (Cu, Ag, Au, Pd, and Pt) on NiAl(110) for three coverages: ad (top panel), ab (middle panel), and ac (bottom panel). Accumulation of electron density is shown in red−yellow and depletion in blue (electron density differences given in e·Bohr−3).
charge upon adsorption, and negative values indicate locations where charge density is lost upon adatom adsorption. At the ad coverage the electron density accumulates closer to the adatoms as shown by the red regions, which are not intersected, therefore indicating the absence of chemical bonding between the adatoms as was also observed from the energetic contributions. For the other coverages, it becomes apparent that the charge accumulation regions are connected showing that the chemical interaction between the adatoms is significant for both ab and ac coverages. The blue regions corresponding to loss of charge density reveal that for ad coverage the bonding is strongly ionic with the beneath surface Al atom. The magnitude of the charge transfer is controlled by the electronegativity of the deposited metal. In fact, it can be seen that qualitatively the most intense blue agrees well with the electronegativity series: Pt > Pd > Au > Ag > Cu.
Figure 4. Planar averaged charge density difference plots for different metals (Cu, Ag, Au, Pd, and Pt) on NiAl(110) for three coverages: ad (black), ab (red), and ac (green). 2459
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Figure 6. Projected local density of states (LDOS) for each adsorption system at three coverages: ad (top panel), ab (middle panel), and ac (bottom panel).
with the Ni surface atoms, the d band is already broad at the longest adatom−adatom distances.
For ab coverage, the bonding is also dominated by the interaction between the adatom and the surface Al underneath. However, the proximity between two adatoms along the direction [11̅ 0] enhances the lateral interactions, and the accumulation of charge density between the adatoms is appreciable. The ac case presents new and interesting features. It can be seen that the interaction between the adatoms and the surface is enhanced by the presence of surface Ni atoms that contribute to the bonding with d orbitals. In fact, the sp interaction with the Al atoms practically becomes negligible, and the bonding is strongly covalent dominated by localized d interactions between the adatoms and the Ni surface atoms and also by lateral d−d interactions between adatoms which are closer (2.91 Å) at this coverage. Particularly, for ac coverage, the interaction between Pt atoms is more efficient as can be seen from Figure 5, which shows clearly the origin of the second positive region in the planar charge density difference plots in Figure 4. The projected local density of states (LDOS) for the adatoms and the nearest bonding Ni are plotted in Figure 6. In all cases d bands dominate the density of states. The group 11 metals have the d states fully occupied, and they are well below the Fermi level, particularly Ag and Au. Conversely, for Pd and Pt the d band extends up to ≈2 eV with appreciable unoccupied states. Evidence for covalent contribution to the bonding is the degree of mixing between electronic states of the adatom and the surface Ni atoms. It is evident that this mixing is more effective for the group 10 metals and also for Cu, in which both d bands overlap extensively. Differently, the mixing for Ag and Au is very weak, and this is reflected in the plots of charge density differences (Figure 5) where only for these metals in the ac system the charge accumulation under the adatoms does not form a continuous region. In fact, the d band for Ag and Au in the systems with larger interatomic adatom distances (ab and ad) shows narrow peaks corresponding to localized d orbitals. However, for the ac case, those peaks become broader and split due to the lateral interactions between the adatoms. This effect can be observed also for the other metals; although caused by the simultaneous interaction
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CONCLUSIONS In this paper we have used periodic DFT calculations to investigate the changes in the electronic properties of the NiAl(110) surface, caused by adsorption of different transition metals (Pd, Pt, Cu, Ag, and Au) as a function of coverage from 0.25 to 1.0 ML. Particularly, the 0.5 ML coverage results in three possible geometrical configurations in which the adatom−adatom distances are significantly different, and one of them becomes especially relevant since that distance is close to the existing one in free-standing metal LACs. Thus, the NiAl(110) surface can be used as a support to build up stable linear atomic chains (LACs) provided STM or alternative techniques allow for manipulating the atoms in specific directions. The estimate of the energy of both adatom−surface and lateral adatom−adatom interactions has demonstrated that the interactions between adatoms are significantly enhanced along the [001] direction, whereas the interaction with the substrate is decreased with respect to the other cases. Additionally, the analysis of the electronic structure from the calculation of the work function changes, and charge density differences and the projected local density of states have allowed us to shed light onto the nature of the chemical bonding. Thus, it has been proved that metal adatoms form chemical bonding with the substrate by means of ionic interactions with the Al surface atoms and covalent contributions with the surface Ni atoms. On increasing the coverage from 0.25 ML, the interactions between adatoms become significant particularly when adatoms form LACs along the [001] direction because the metal−metal interatomic distance turns out to be similar to that presented in freestanding LACs and the bonding between the adatoms develops quite effectively by means of d−d interactions. A final interesting result was the special behavior of Pt LAC in the ac direction, and this NW although supported by the NiAl(110) substrate shows free-standing properties. We suggest 2460
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the realization of experimental measurements to provide new observations allowing the validation of these predictions.
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AUTHOR INFORMATION
Corresponding Author
*E-mail: zacarias@ifi.unicamp.br. Phone: +55-19-3521-5491. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This research had the financial support from FAPESP (Process2010/16970-0), CNPq, CAPES, and FAEPEX-UNICAMP. MASM acknowledges FAPESP for a Visiting Professor grant (2013/02032-7) during the stay at the Institute of Physics “Gleb Wataghin” of Unicamp. The calculations were performed using IFGW-UNICAMP computer facilities and the National Center for High Performance Computing in São Paulo (CENAPAD-SP).
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