Density Functional Study of the Structures and Electronic Properties of

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Density Functional Study of the Structures and Electronic Properties of Nitrogen-Doped Nin Clusters, n = 1
10 A. Chikhaoui,† K. Haddab,† S. Bouarab,*,† and A. Vega‡ †

Laboratoire de Physique et Chimie Quantique, Universit
e Mouloud Mammeri de Tizi-Ouzou, B.P. No. 17 RP, 15000 Tizi-Ouzou, Algeria ‡
Departamento de Física Te
orica, At
omica y Optica, Universidad de Valladolid, Prado de la Magdalena s/n, E-47011 Valladolid, Spain ABSTRACT: We present a first-principles study of the equilibrium geometries, electronic structure, and related properties (binding energies, ionization potentials, electron affinities, and magnetic moments) of free-standing Nin (n = 1
10) clusters doped with one impurity of N. Calculations have been performed in the framework of the density functional theory, as implemented in the SIESTA code within the generalized gradient approximation to exchange and correlation. We show that, in contrast to the molecular adsorption of N2, the adsorption of a single N atom can dramatically change the structure of the host Nin cluster, examples of which are Ni5N, Ni7N, and Ni10N, and that noticeable structure relaxations take place otherwise. Doping with a nitrogen impurity increases the binding energy as well as the ionization potential (except for Ni6N), which proves that N-doping works in favor of stabilizing the Ni clusters. We also find that the magnetic moments decrease in most cases upon N-doping despite the fact that the average Ni
Ni distance increases. The HUMO
LUMO gap for one spin channel strongly changes as a function of size upon N-doping, in contrast with the HUMO
LUMO gap for the other spin channel. This might have important implication in electronic transport properties through these molecular contacts anchored to source and drain electrodes.

’ INTRODUCTION Atomic clusters have attracted a lot of attention and are still a matter of intense research because of their unique combination of molecular and condensed matter physics. Of particular interest are the binary clusters composed of transition metals and nonmetal elements with potential applications in many fields.1 Unfortunately, some fundamental issues remain largely unsolved, particularly when the clusters are produced in the freestanding environment. One of the most important challenges in this context is the direct determination of the geometrical structure from experiments. Geometrical information of freestanding transition metal clusters can be obtained from indirect measurements such as chemical probe experiments.2 Such a technique has provided information on the structure of small Ni clusters.3
10 For example, analyzing the adsorption of N2 on small Nin (3e n e 15) clusters allows to infer their structural shape.4 Thus, when it was assumed that N2 binds to every exposed nickel atom, that the binding energies decrease with increasing the Ni
Ni coordination, and that atoms with four or less coordination can bind two nitrogen molecules, plausible geometries were proposed for these clusters. From the theoretical point of view, several studies have been performed to analyze the possible geometries of pure Ni clusters and their related electronic structure.11
26 As regards mixed nitrogen
nickel clusters, to the best of our knowledge, only one DFT study has been devoted to N2 adsorption around small Nin clusters (n = 2
4).27 It was found that the adsorption sites in small clusters are similar to those on bulk surfaces r 2011 American Chemical Society

(the 3-fold hollow sites) and that, despite the expansion of the Ni
Ni distance upon nitrogen adsorption, the geometry of the pure Ni clusters is essentially preserved. The only qualitative change caused by the N2 adsorption is the drop of the magnetic moment of the cluster upon N2 saturation. The fact that the structures of pure Ni clusters are preserved after the molecular adsorption of nitrogen is consistent with the fact that the valence electrons of N are involved in the formation of the molecular bond of N2 so that bonding of N2 with Ni is weak. Therefore, in this case it is possible to extract conclusions on the geometrical shape of the pure Ni host cluster after adsorption of N2. However, in the case that dissociative adsorption occurs instead of molecular adsorption, like in some catalytic reactions, the host structure can be affected to a large extent. The purpose of the present work is to perform detailed density functional calculations of the equilibrium geometries and the spin-polarized electronic structure of Ni clusters containing a single nitrogen impurity. We address how N-doping affects the host structure, how the resulting binding energy is as compared with the pure clusters, and how electronic properties such as the ionization potential, electron affinity, dissociation energy and magnetic moment, are modified. The technological interest of N-doped nickel clusters lies mainly in the fact that nitrides of transition metals display, like their oxide counterparts, a rich Received: August 16, 2011 Revised: October 18, 2011 Published: October 25, 2011 13997

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variety of relevant properties,28 in particular high melting points, ultra hardness and corrosion resistance, with great impact even the field of superconductivity. In the present work, we also show that doping with N can be a way to modify the electronic transport properties through these molecular contacts. In the next section we give the details of the computational method. The results are discussed in section III, and our main conclusions are summarized at the end.

’ COMPUTATIONAL METHODS Our calculations were based on the density functional theory, as implemented in the SIESTA code29,30 with the spin polarized generalized gradient approximation (GGA) for exchange correlation potential as parametrized by Perdew, Burke, and Ernzerhof.31 This method is based on norm-conserving pseudopotentials and linear combinations of atomic orbitals as basis sets for which we used triple-ζ with double polarization functions for both nickel and nitrogen. The triple-ζ orbitals are defined in the split-valence spirit, and the polarization functions are obtained perturbatively by applying a small electric field to the free atoms.32 In order to limit the range of the pseudoatomic basis orbitals defined by limiting the orbital confinement energy32 they are slightly excited by an energy shift of 6.10
5 Ry for both Ni and N. The core interactions were described using nonlocal normconserving Trouiller
Martins pseudopotentials33 factorized in the Kleinman-Bylander form34 including nonlinear core corrections. The ionic pseudopotentials were generated from the valence configurations 3d84s2 for Ni and 2s22p3 for N. The pseudopotential radii for s, p, and d orbitals were fixed to 2.05 au for Ni and 1.14 au for N. Partial-core correction of 0.7 au for nonlinear exchange correlation35 was included in the Ni pseudopotential. The clusters were located in a periodic simple cubic supercell with a lattice size of 20 Å, large enough to avoid any interaction between the cluster and its images. We used only the Γ point to sample the Brillouin zone. The numerical integration in the finite real space grid was performed within an energy cutoff of 200 Ry whereas the total energy was converged to 10
4. Most of the inputs used for the structural optimization of Ni clusters were taken from previous works.8,11
26 The geometries of NinN clusters were obtained starting from those tested for the pure nickel clusters (stable or not) and considering the possible adsorption sites for N. The geometries were then relaxed using a conjugate gradient method36 until the interatomic forces were smaller than 0.002 eV/Å. The relative stability of different isomers was further checked by performing calculations in different spin states to be sure of the total spin of the putative ground state. In order to test the accuracy of our theoretical approach, we benchmarked our calculations for N2 and Ni2 dimers as well as for N2 adsorbed on the Ni2 dimer, against other density functional calculations available in the literature.27 The ground state of Ni2 was found to be ferromagnetic with bond length and binding energy of 2.16 Å and 2.28 eV, respectively. This bond length is consistent with earlier theoretical studies that provided values between 2.01 and 2.20 Å17,18,20
22,24
26,37 as well as with experimental ones (values between 2.15 and 2.20 Å).38
40 Our dimer binding energy (2.28 eV) enters also the range of the values calculated (1.86 - 6.30 eV) elsewhere17,18,20
22,25,26,37 and it is consistent with the available experimental value (2.07 eV).38

Figure 1. Geometrical parameters (in Å) and relative total energies (ΔE) for on-top (O), bridge (B), and transverse (T) adsorption of N2 molecules around Ni2 dimer. The optimized parameters obtained in ref 27 are also added in parentheses for comparison.

For N2, we obtain bond length and binding energy of 1.14 Å and 9.79 eV, respectively, in keep with earlier ab initio calculations (1.12 and 10.0 eV)27 and with the corresponding experimental values of 1.10 Å and 9.76 eV.41 As for the adsorption of N2 on the Ni2 dimer, it can be done on top (O), bridge (B), or transverse (T) positions, as shown in Figure 1, where we provide our optimized structural data together with the values reported in ref 27 (obtained within the local density approximation (LDA)). Overall, the agreement is reasonable, assuming the typical differences between GGA and LDA. We note that N2 is adsorbed in O sites like in bulk surfaces.42

’ RESULTS AND DISCUSSIONS Structural Properties. The putative lowest-energy structures and some low-lying isomers of NinN (n = 2
10) clusters are shown in Figure 2. For the sake of comparison, we have also plotted the ground-state geometries of the corresponding pure Ni clusters. The optimized geometries of other isomers can be obtained from the authors upon request. Before going into the results of the N-doped clusters, we note that pure Ni clusters have been studied by several authors in the last two decades, either by means of semiempirical methods,11
16,18,19,21,23,25 or within the density functional theory.17,20,22,24,26 Since different approaches, at the DFT level, give rise to different putative ground state structures for several sizes (although, in general, all these structures are low-lying isomers when not found as the ground state), it is worth to compare pertinent structural-dependent electronic properties with the experimental data. An indicative of the plausibility of our putative ground-state structural and electronic configuration for the pure Ni clusters can be obtained through the comparison of the resulting ionization potentials and electron affinities with those experimentally measured. In the same spirit, such a comparison was also made for iron clusters more than ten year ago by part of the authors in the framework of a tight-binding model.44 As we will see in the next section, a fine overall agreement is reached which give us confidence in our theoretical approach and the results obtained for the NinN clusters. The putative lowest-energy structure of Ni2N is found to be an isosceles triangle having C2v symmetry, with Ni
N and Ni
Ni bond lengths of 1.75 and 2.27 Å, respectively. This arrangement maximizes the number of bonds. The second and third isomers 13998

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Figure 2. Putative ground-state structures of Nin clusters with their symmetry (column 1). The structures on the right side show the lowest-energy isomers together with their symmetry (column 2) and some of the low-energy isomers for NinN clusters (n = 1
10). Gray and dark spheres represent Ni and N atoms respectively.

are the linear configurations Ni
N
Ni and N
Ni
Ni, being energetically well above the ground state (ΔE = 0.79 and 2.43 eV, respectively). We note that pure Ni3 has also a triangular structure but with D3h symmetry instead. The putative ground-state structure of Ni3N is a threedimensional regular tetrahedron (C3v) with Ni
Ni and Ni
N bond lengths of 2.32 and 1.78 Å, respectively. In this case, the N atom is located in a hollow site of 3-fold symmetry. The triangular structure of the host Ni3 cluster is preserved upon

N adsorption which produce only local relaxations. The planar kite-like and the linear configurations of Ni3N are the other low-lying structures found at higher energy (0.78 and 2.03 eV, respectively). For Ni4N, the ground-state geometry is a trigonal bipyramid configuration of Cs symmetry, with a similar adsorption site as in Ni3N, that is a 3-fold hollow site which maximizes the number of bonds with Ni. Again, the structure of the host Ni4 cluster is essentially preserved (apart from local relaxations). 13999

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The Journal of Physical Chemistry A The second more stable structure within 0.10 eV is also a trigonal bipyramid but with a C3v symmetry. The two next lowlying isomers, quasidegenerated, are the butterfly structure and the square pyramid with higher energies, 0.42 and 0.43 eV, respectively. The fourth isomer (ΔE = 0.89 eV) is a planar geometry with D4h symmetry. The putative ground-state structure of Ni5N is a capped trigonal bipyramid (Cs) obtained by adding the N atom over one of the 3-fold hollow sites of the Ni5 bipyramid configuration. Therefore, in this case, the N adsorption modifies the square pyramid structure of the host Ni5 cluster. N adsorption on the 3-fold hollow site of this square pyramid host leads to the next low-lying isomer with ΔE = 0.41 eV, whereas in the second lowlying isomer (0.66 eV), the N atom is adsorbed on the 4-fold hollow site of the square bipyramid. The third isomer found at higher energy (0.71 eV) corresponds to Ni4 tetrahedron subcluster where Ni and N atoms occupy a bridge and 3-fold hollow sites respectively. The fourth one (0.95 eV) is a planar-like Ni cluster over which the N atom is capped on 3-fold hollow site. The putative lowest-energy structure of Ni6N is a pentagonal pyramid of C2v symmetry, where one site of the pentagonal ring is occupied by the N atom. This structure can be interpreted as a N atom adsorbed on a bridge site of the square ring of the square bipyramid structure of the pure Ni6 host. Thus, the structure of the host cluster is essentially preserved, but with noticeable distortion of the side of the pentagonal ring whose Ni atoms are bonded with N. We note that this is the first case where N do not sit in a 3-fold hollow site, although the next isomer corresponds to this situation and is very close in energy to the groundstate (0.07 eV). The next two isomers enter in a relatively small energy window 0.21
0.27 eV, whereas the fourth one, which corresponds to the bicapped square pyramid host, is found at higher energy (0.49 eV). The putative ground state of Ni7N is a pentagonal bipyramid of Ni7 in which the N atom is adsorbed on one of the available 3-fold hollow sites. This corresponds to a Cs symmetry. Therefore, N-doping in this case modifies the capped square bipyramid structure of the host Ni7 cluster. However, N adsorption on different 3-fold hollow sites of this capped square pyramid host, leads to the next three low-lying isomers that enter in a narrow energy window of 0.11
0.20 eV. The fourth isomer with ΔE = 0.22 eV is the Ni6N pentagonal pyramid capped by a Ni atom. The putative ground-state structure of Ni8N is a capped bisdisphenoid configuration with Cs symmetry. It corresponds to N atom adsorbed on a 3-fold hollow site of the structure of the host Ni8 cluster. This host structure is a square bipyramid capped with two additional Ni atoms located at adjacent hollow sites. The N atom sits on the triangular face formed by these two extra Ni atoms and one Ni of the bipyramid. The next isomer, nearly degenerated (ΔE = 0.17 eV), is a bicapped octahedron Ni8 subcluster with C2v symmetry in which N is adsorbed on 4-fold hollow site formed by the two capped Ni atoms and two Ni atoms of the octahedron base. Other variants of bicapped square bipyramid (0.22 eV) as well as a single capped pentagonal bipyramids (0.20 and 0.23 eV), always with N in hollow sites but with different symmetries (C1 and Cs), complete the set of low-lying isomers. For Ni9N, the putative ground-state corresponds to the tricapped trigonal prism arrangement of the host Ni9 cluster in which the N atom is adsorbed on a 3-fold hollow site formed by two Ni atoms of the prism and one of the capping Ni atoms.

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Figure 3. Average Ni
Ni bond distance in Nin and NinN clusters (n = 2
10) as function of cluster size.

Thus, again here the structure of the host cluster is essentially preserved, but for this cluster size this is particularly interesting. We note first that the growth sequence from the pure Ni5 until the pure Ni8 is based in the formation of the square bipyramid and subsequent capping by additional Ni atoms. This sequence is broken in Ni9, although it strongly competes with the tricapped trigonal prism. This is illustrated by the fact the next low-lying isomers of Ni9N with ΔE = 0.05 to 0.19 eV are all based tricapped square bipyramids with N occupying, as usual, different 3-fold hollow sites. The growth sequence is recovered in the pure Ni10 cluster whose structure is a tetracapped square bipyramid. However, the adsorption of the N atom to form Ni10N modifies the host structure which transforms into a tricapped pentagonal bipyramid in which the N atom is adsorbed on one of the hollow sites of the bipyramid (the whole cluster with Cs symmetry). The second isomer corresponds to a capped trigonal prism with ΔE = 0.01 eV, whereas the next three low-lying isomers, entering an energy window of 0.15
0.22 eV, are capped pentagonal bipyramid and capped square bipyramid structures for the Ni host in which the N atom is adsorbed on 3-fold hollow sites. In particular, the trigonal-prism-based isomer is nearly degenerated. We remind the reader that this was the structural motif of the pure Ni9. Besides, the pentagonal-bipyramid-based isomer is close in energy and we note that this is the structural motif of pure Ni11 for which the growth sequence based in the square bipyramid is again broken. Summarizing the main general trends, in contrast to the molecular adsorption of N2,27 N doping (or adsorption of a single N atom) can dramatically change the structure of the host Nin cluster, examples of which are Ni5N, Ni7N, and Ni10N. The N atom tends to be adsorbed on a triangular face, forming three bonds with Ni. Although the host structural symmetry is essentially preserved in many cases, noticeableFigure 3 structure relaxations take place nevertheless. As shown in Figure 3, the adsorption of N tends to increase the average Ni
Ni bond distance in NinN as compared to the pure Nin clusters. The average interatomic Ni
Ni distance in pure nickel clusters increases up to n = 4 and then it keeps nearly constant (it slightly rises monotonically from n = 4 to 7). It stabilizes around a value of 2.35 Å for this size range. The situation is somewhat different in the N-doped clusters, where the average Ni
Ni distance oscillates between n = 2 and 6 and then it stabilizes around 2.41 Å . 14000

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Figure 4. Size dependence of the binding energy per atom for the putative ground state structures of Nin and NinN clusters (n = 1,10).

Figure 5. Second order differences of energy for Nin and NinN clusters in their putative ground state as function of cluster size n.

Electronic Properties. The binding energies (BEs) per atom of these clusters, defined as BE(NinN) = [
E(NinN) + nE(Ni) + E(N)]/(n + 1), are plotted in Figure 4. BEs increase with cluster size rather monotonically for both NinN and Nin clusters. However BEs of NinN clusters are larger than those of pure Nin, indicating that N-doping enhances the stability of Ni clusters. The BE of NinN (Nin) clusters increases substantially up to n = 3 (n = 6) and then moderately for larger sizes. The sharp increase of BE up to Ni3N reflects the strong tendency of N to form a 3-fold bonding with the Ni host. This feature is reflected in other quantities as we will see later. The tendency of BEs of pure and doped clusters to converge to each other as increasing cluster size reflects the fact that a single N atom is averaged with an increasing number of Ni atoms. Indeed, for very large clusters, it is expected that the effects of N adsorption are local, that it is adsorbed preferentially in 3-fold hollow sites (like in the (111) surface of a fcc substrate) with concomitant local relaxations and keeping unaffected a large region of the host cluster. We note that the general behavior of the binding energy of pure Ni clusters as function of their size (Figure 4) is consistent with other theoretical results.22,26 The second difference in energy, defined as Δ2E(n) = E(n + 1) + E(n
1)
2E(n), is a sensitive quantity that reflects the relative stability of a cluster of a given size with respect to its neighboring sizes.45 Figure 5 shows Δ2E(n) for the Nin and NinN clusters as a

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Figure 6. Fragmentation energies of NinN clusters via the loss a Ni (ΔNi) or N atom (ΔN), as function of cluster size n.

function of size n. For N-doped clusters, the most remarkable feature is the noticeable peak at n = 3 indicating the high stability of Ni3N for which the completion of the 3-fold bonding of N with Ni takes place. This is not possible in Ni2N (bridge adsorption) while in Ni4N the additional Ni atom weakens the bonding. Slight oscillations of Δ2E(n) are obtained for n > 4. There is also a small peak of stability at n = 8, also reflected, as we will see later, in the fragmentation energy, ionization potential and hardness. For pure Ni clusters, Δ2E(n) displays a small maximum for n = 4 and sharp maxima for n = 6 and 9, indicating the high stability of the corresponding clusters. n = 6 corresponds to the completion of the octahedron structure, whereas for n = 9 a transition of the growth pattern is found (the capped octahedrons transform into a capped triangular prism for this size). The relative stability of the clusters should also be reflected in their fragmentation energies. Let us consider two single-atom fragmentation channels of NinN, involving one Ni atom or one N atom. These quantities, plotted in Figure 6, are defined in terms of total energies as follows: ΔEN ½Nin N ¼ E½Nin  þ E½N
E½Nin N

ð1Þ

ΔENi ½Nin N ¼ E½Nin
1 N þ E½Ni
E½Nin N

ð2Þ

One can infer from Figure 6 that ΔEN is higher than ΔENi, to conclude that extracting the N atom from NinN requires more energy than extracting the less bonded Ni atom of the cluster. In other words, it is easier to dissociate Ni than N atom from the NinN system; in general, the N
Ni bonding is stronger than the Ni
Ni one. We also find that the largest fragmentation energy corresponds to the extraction of the N atom from Ni3N which correlates with the high relative stability of this cluster, as reflected in the peak obtained in the second difference of energy (Figure 5). It also confirms the strong N
Ni bonding in this cluster. We also find a small increase of the fragmentation energy for Ni8N. Its relative stability was already reflected in the second difference of energy (Figure 5). Another peculiarity is that Ni4N is relatively unstable in regard to Ni dissociation, since the required energy for this process is the lowest one. This could be explained from the evolution of the Ni
Ni(N) bond lengths in NinN clusters for n = 3
5. The average distance RNi
Ni increases by 6% as going from Ni3N to Ni4N and then decreases by 2% from Ni4N to Ni5N (Figure 3), making the Ni atoms less bonded in Ni4N than in its more compact neighboring clusters. 14001

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Figure 8. Total magnetic moments of NinN and Nin as a function of cluster size n. The experimental values for Nin clusters are also included for the sake of comparison.

Figure 7. Calculated adiabatic ionization potential (IP) (a), adiabatic electron affinity (EA) (b), and hardness (c) of NinN and Nin as function of cluster size n. The experimental values of IP and EA for pure Nin clusters are also included for the sake of comparison.

The adiabatic ionization potential (IP), the adiabatic electron affinity (EA), and the difference IP-EA, known as hardness, of the NinN clusters are plotted in Figure 7. We also include those obtained for the pure Nin clusters as well as experimental data reported for them.46,47 These quantities can be obtained experimentally by means of photoelectron spectroscopy. The hardness is used as a reactivity index, or in other words, as an index of the stability of the system. As long as the values of IP and EA are calculated as the difference of the total energy of the neutral and charged clusters (cation and anion, respectively, for IP and EA), these properties are well-defined within the DFT. Here, we show the adiabatic IPs and EAs, which means to use the optimized geometries also for the charged clusters. We calculated also the vertical IPs and EAs (not shown), that is using for the charged cluster the optimized geometry of its neutral counterpart. The results did not change significantly as compared with the adiabatic values, which means that changes in the structure upon the extraction or addition or one electron are negligible here. The calculated IPs for Nin clusters follow the experimental data46 rather well as a function of cluster size. Although the quantitative agreement between theory and experiment is not totally perfect, especially for Ni1, Ni5, and Ni7 (deviations of about ∼6%), we reach an improvement of the calculated IPs as compared to previous calculations,22,48
55 which gives further support to our putative ground states structures. Unfortunately, experimental data concerning IP and EA of N-doped clusters are not available so far. However, one can bring out some trends concerning the effect of N atom on IP of NinN by comparing their calculated values with those of the pure Ni clusters. From Figure 7a one can see that N-doping produces in general a slight increase of the IP (an exception being n = 6), a fact that is

consistent with the higher binding energy of the N-doped clusters as compared with the pure ones. We also find that the IPs decrease as a function of size in a more monotonic fashion for the N-doped Ni clusters than for the pure ones. The Mulliken population analysis for neutral and ionized clusters allows one to determine the orbital-, spin-, and atomic-site character of the electron extracted in the ionization process. Doing this for the very stable Ni3N cluster (Figure 5), we find that the extracted electron has s character and belongs to the Ni majority spin states. For large clusters one can expect a different situation due to the more significant s
d hybridization; however, the extracted electron in Ni10N cluster also has a s-up Ni character. For this cluster size the IPs are already stabilized. Our calculated EAs of Nin clusters agree also rather well with the experimental data, except for n = 1, 6, and 10 for which the difference in the absolute values amounts to ∼0.33 eV. For pure Ni clusters the EA increases quite linearly with size, whereas for N-doped clusters two marked characteristics comes out (see Figure 7b). For Ni3N, and to a lesser extent for Ni8N, a drop of the EAs is obtained. This correlates with the relative stability of those clusters already observed through the second difference in the energy (Figure 5). In view of the above facts, the hardness of N-doped clusters (Figure 7c) also displays a relative increase for Ni3Ni and Ni8N which seem to be the less reactive N-doped cluster overall. The Mulliken population analysis for neutral and anionic Ni3N clusters shows that the extra electron has 100% Ni-s character and belongs to the minority spin states. We discuss now the magnetic behavior of the Ni-doped clusters as compared with that of the pure Ni clusters. The total magnetic moment as a function of cluster size is depicted in Figure 8. We also show experimental data available for pure Ni clusters larger than four atoms.56 A number of calculations of the magnetic moments for pure Ni clusters, using different methods and approximations, have been carried out in the past.17,19,22,51,54,57
60 The quantitative agreement between theory and experiments was generally poor. Although our calculated magnetic moments of Nin compare with the experimental values better than previous calculations for many sizes, especially for n = 5, 7, and 9, there exist still discrepancies with the measured values so that only the qualitative trend is captured. From Figure 8 one can see that N-doping reduces the total magnetic moment of the clusters with more than 3 Ni atoms, and inversely for smaller clusters. The total moment 14002

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Figure 9. Total density of states projected on the 8 Ni atoms (black curve) and on the N atom (red curve) of the putative ground-state Ni8N (a), as compared to the total DOS of the host Ni8 (b). The vertical dotted line indicates the Fermi energy. A Gaussian smearing (0.1 eV) of the states has been used in the plot.

Figure 10. HOMO
LUMO gap for spin-up and spin-down electrons of NinN (a) and Ni8 (b) as function of cluster size n.

keeps constant (3 μB) for clusters up to 6 Ni atoms, to start their increasing up to 7 μB, a value that is maintained for clusters larger than n = 8. On the contrary, for the pure Ni clusters the total moment reaches 8 μB and keeps constant for clusters larger than 6 atoms after a monotonous increase up to this size. The general decrease of the magnetic moment upon N adsorption has to do with the hybridization between the sp electrons of N and the delectrons of Ni. The hybridization effects can be analyzed by comparing the density of states (DOS) [Figure 9] of Ni8 with that of Ni8N cluster where the average Ni
Ni distance is increased by ∼2.5% but the geometry of the host Ni8 cluster is essentially preserved

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(Figure 2). The nitrogen sp bonding states for spin-up and spindown are rather localized at low energy. Significant rearrangement around the Fermi level takes place upon N-doping. In particular, the gap for majority sates closes due to the hybridization with N. The spin polarization on the nitrogen atom is relatively small (0.06 μB). The exchange splitting concomitant with the magnetic character of these clusters, leads to a different HOMO
LUMO gap for spin-up and spin-down states. Although a unique HOMO
LUMO gap has to be considered in most cases for low-energy electron excitations, the HOMO
LUMO gap of each spin channel is a key quantity for spin-dependent electronic transport at low voltages when the clusters form a contact between a source and a drain electrodes. In Figure 10a, we plot as a function of cluster size, the spin-up, and spindown HOMO
LUMO gaps of NinN in their putative lowestenergy structures. For n = 3 we obtain a relative increase of the gaps which again reflects the stability of this cluster. The figure shows that, except for n = 3, both gaps generally decrease with cluster size, a trend that illustrates very well the tendency of these small finite systems, which are insulators, to become metallic as the molecular states transform into bands when the number of atoms is large enough. For n e 4, the N-doped clusters have relatively large HOMO
LUMO gaps for spinup states (∼1.00 to 1.60 eV) whereas, except for n = 3, spindown states have small gap (∼0.09 to 0.45 eV). The situation is somewhat different for the Ni-host clusters (Figure 10b) where the largest HOMO
LUMO gaps for spin-up states are obtained for sizes larger than n = 5; Ni6 displays the largest one (2.49 eV) which reflects the high stability of this cluster (Figure 5). The gaps corresponding to spin-down states are relatively small and do not vary significatively with the size. Therefore, the HUMO-LUMO gap for one spin channel strongly changes as a function of size upon N-doping, in contrast with the HUMO
LUMO gap for the other spin channel. This might have important implications in electronic transport properties through these molecular contacts anchored to source and drain electrodes.

’ CONCLUSIONS We have performed first-principles DFT calculations to determine the putative ground state structures and related electronic properties of Nin clusters doped with a single N impurity. In contrast to the molecular adsorption of N2, the adsorption of a single N atom can dramatically change the structure of the host Nin cluster, examples of which are Ni5N, Ni7N and Ni10N. The N atom tends to be adsorbed on a triangular face, forming three bonds with Ni. When structural symmetry is essentially preserved, noticeable structure relaxations take place nevertheless. The binding energies of NinN clusters are larger than those of the host Nin clusters, indicating that N-doping enhances the stability. This correlates with the general increase of the ionization potentials. The hardness of NinN clusters displays a relative increase for Ni3N and Ni8N which seem to be the less reactive N-doped clusters overall. These two clusters are also expected to be the most abundant ones as compared with neighboring sizes in view of the peaks obtained for them in the second energy difference. N-doping reduces the magnetic moment of the clusters with more than 3 Ni atoms despite the fact that the average Ni
Ni distance increases. This has to 14003

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The Journal of Physical Chemistry A do with the hybridization between the sp electrons of N and the d electrons of Ni. Finally, the HUMO
-LUMO gap for one spin channel strongly changes as a function of size upon N-doping, in contrast whith the HUMO
LUMO gap for the other spin channel. This might have important implications in electronic transport properties through these molecular contacts anchored to source and drain electrodes.

’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected]; [email protected].

’ ACKNOWLEDGMENT We acknowledge the financial support from the Algerian Project CNEPRU D00520080026, from the Spanish Ministry of Science and Innovation in conjunction with the European Regional Development Fund (FIS 2008-02490) and from the Junta de Castilla y Le
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