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Structure and electronic properties of palladium chains supported by NiAl(1 1 0): A first-principles study
Computational Materials Science 124 (2016) 398–402
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Computational Materials Science journal homepage: www.elsevier.com/locate/commatsci
Structure and electronic properties of palladium chains supported by NiAl(1 1 0): A first-principles study Ping Wu a,b, Naiqiang Yin a, Wenjing Cheng a, Peng Li a, Min Huang b,⇑ a b
School of Physics and Electrical Information, Shangqiu Normal University, Shangqiu 476000, China State Key Laboratory of Magnetic Resonance and Atomic and Molecular Physics, Wuhan Institute of Physics and Mathematics, Chinese Academy of Sciences, Wuhan 430071, China
a r t i c l e
i n f o
Article history: Received 9 July 2016 Received in revised form 5 August 2016 Accepted 7 August 2016
Keywords: Pd chains First-principles study Band of resonance states Density of states (DOS)
a b s t r a c t In this work, the geometry, stability and electronic structures of atomic palladium chains supported on the NiAl(1 1 0) surface have been investigated by using the first-principles calculations. Palladium monomer strongly adsorbed at NiANi and AlAAl bridge sites bonding with the two most neighboring Ni atoms. There is competitive mechanism between interactions of PdAPd and interactions between Pd and NiAl substrate for Pd ad-chains. We also observe the interactions of resonance states give rises to a band of resonance states, which is mainly related to the overlap of 5sp orbital of adjacent Pd atoms. Finally, we demonstrated that the atomic spacing in Pd ad-chains and adsorption sites had significant influences to the bandwidth of resonance band. These results may have potential applications in the fields of nanoelectronics and nanoscale catalysts. Ó 2016 Elsevier B.V. All rights reserved.
1. Introduction In recent decades, metal nanostructures have been widely studied due to the extensive applications in the fields of nanoelectronics and magnetic storage devices as well as nanocatalysis and gas sensors [1–7]. Compared with the properties of bulk metal, the metal nanostructures including metal clusters (such as Ag, Au, Pt and Pd) and wires possess extraordinary sizes-dependent mechanical, electronic, magnetic, optical properties, which became key issues attracting wide attentions of researchers. For instance, theoretical calculations and experiments studies suggested that Pd clusters exhibit remarkable catalytic activities in oxidation reaction [8] and synthesis of vinyl acetate and methanol [9], which is due to the large surface area-to-volume ratio. Besides, some Pd clusters with certain sizes possess magnetic moment [10,11], which is absent in the bulk materials. Previous study reported that MoS2 nanosheet decorated with gold nanoparticles (AuNPs) can be used to modify glass carbon electrode of electrochemical glucose biosensor, which accelerates the electron transfer from electrode to the immobilized enzyme [12]. The physical and chemical properties of metal nanostructures can be modulated by tailoring or assembling the geometries. Recently, fabrication and characterization of clusters and ordered chains of heavy transition metals on various substrates has been
⇑ Corresponding author. E-mail address: [email protected] (M. Huang). http://dx.doi.org/10.1016/j.commatsci.2016.08.008 0927-0256/Ó 2016 Elsevier B.V. All rights reserved.
studied by experimental and theoretical studies [13–17]. Wang et al. [13] reported that the structure and bonding of Pd clusters are modulated by the Moire pattern of the graphene/Ru(0 0 0 1), allowing a resolution of their role in field of nanocatalysis in future. The adsorption of Pd clusters supported on both carbon nanotubes and graphite have been comparatively studied by DFT [14], which show that curved nanotube surface seem to be more suitable to support palladium clusters with respect to the flat graphite surface due to the stronger PdAC bonding. It’s known that the substrate plays an important role to act as the template in the growth and assembling process of metal nanostructures. For instance, surface defects such as oxygen vacancies impact the nucleation, growth processes and diffusion of Pd clusters supported on MgO, Al2O3 and CeO2 surface [15–17]. Interestingly, the surface reconstruction and terrace of metal substrates may be conducive to the preparation of the monoatomic metal chains. Using spin-polarized scanning tunneling microscopy (STM), Matthias and co-workers demonstrated the occurrence of an atomic-scale spin spiral in finite individual bi-atomic Fe chains on the 5 1-Ir(0 0 1) surface [18]. It’s known that bare metallic NiAl(1 1 0) surface with one dimensional troughs is a simple structure template for fabrication of artificial metal chains with negligible effect on the electronic properties of chains, indicating that the metal chains may exhibit true metallic electronic properties in one-dimension [19]. Welldefined Au and Pd mono-atomic chains have been constructed on NiAl(1 1 0) surface based on the atom manipulation of STM [20,21]. It has been revealed that the overlap between adjacent Au 6sp
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orbitals is responsible for the electronic resonance states of Au chains supported by NiAl(1 1 0) surface [22,23]. Several firstprinciples studies on artificial monoatomic chains on NiAl(1 1 0) surface revealed the atomic configurations of adatoms result in the different degree of interactions with NiAl substrate and give rises to diverse electronic behaviors [24–26]. In order to explore the relationships between geometry and electronic behaviors of metal atomic chains and thus better construct artificial chains on NiAl(1 1 0) surface, it is important to explain the effects of interactions between chains and substrate as well as interatomic interactions on the electronic properties. For the fabrication and applications of low dimensional nanostructures, it is indispensable to probe evolution of the electronic properties with chains length increasing. However, there are few theoretical studies on the geometry and electronic structure of Pd chains supported by NiAl(1 1 0) surface, which may supply the basis for interpreting the experiment results. In this paper we focus our investigations on the adsorption geometry, stability and electronic structure of Pd chains supported by NiAl(1 1 0) surface. In order to get insight into the nucleation and initial growth of chains, we investigated the adsorption configurations and charge transfer between Pd and NiAl substrate. Based on the DFT calculations, we detailedly study the influences of the competitive mechanism between the adsorbate-adsorbate interactions and adsorbate-substrate interactions on the stability and electronic properties of Pd chains, and discuss the influence of atomic spacing on the electronic behaviors of infinite Pd chains. 2. Computational details All calculations were carried out within the frame of density functional theory (DFT) as implemented in the VASP code [27]. The generalized gradient (GGA) corrected by Perdew-BurkeErnzerhof (PBE) [28] is adopted for the exchange and correlation functional, and the projector augmented plane wave (PAW) was used to describe the ion-electron interactions [29]. A kinetic energy cutoff of 400 eV is used, all the structures were fully relaxed until total energies of the system were smaller than 1.0 105 eV. The Monkhorst-Pack meshes [30] were used to sample the supercells. The lattice parameter of 2.89 Å is used for bulk NiAl, which is good agreement with experimental results [31]. To simulate the Palladium chains supported by NiAl(1 1 0) surface, we used supercell slabs containing four layers of metal alloy with the periodicity of 1 4 and 4 1 to simulate the adsorption of infinite Pd chains along [0 0 1] and [1 1 0] directions, respectively. We used meshes of 9 4 1 and 4 9 1 k-points to sample Brillouin zone integrations, respectively. The thick vacuum layer of 15 Å is adopted to eliminate the interactions between adjacent supercells. In all calculations, the bottom most NiAl layer was fixed and the rest of atoms were fully relaxed. 3. Results and discussions 3.1. Geometry and properties of clean NiAl(1 1 0) surface It was reported that the surface energy of the NiAl(1 1 0) is 1.65 eV, which is the most stable surface for the NiAl crystal with CsCl structure [32]. After relaxation, the pristine NiAl(1 1 0) surface was found to be rumpled with 0.16 Å outward displacement of the Al rows with respect to the Ni rows as shown in Fig. 1b, which is consistent with the previous STM results and theoretical calculations [21,26]. There are troughs with an interatomic spacing of 2.98 Å along the [1 1 0] direction as shown in Fig. 1a, which is usually used as the common support for the formation of monoatomic chains [22]. Our previous DFT study has verified that the
(b) side view
(a) top view Ni Al
2.98Å
0.16Å
[110]
[110]
[1-10] Ni-Ni bridge site [001]
[00-1]
[1-10]
Al-Al bridge site
Fig. 1. The top view (a) and side view (b) of bare NiAl(1 1 0) surface after relaxation. The blue and gray balls represent Al and Ni atoms, respectively. The adsorption sites of NiANi bridge and AlAAl bridge are labeled by square and triangle, respectively. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
total density of states (DOS) of substrate are primarily contributed by the d states of Ni from 4.0 eV to Fermi level, and present a depletion in the range of 0–2.20 eV, whereas the DOS are ascribed to states with sp characters at higher energies [33]. 3.2. Adsorption of Pd monomer on the NiAl(1 1 0) surface We first investigated the adsorption properties of single Pd adatom on the NiAl(1 1 0) surface. Based on the experiments results by STM, here we considered two high-symmetric adsorption sites for Pd monomer: NiANi bridge and AlAAl bridge sites, as shown in Fig. 1a. The adsorption energy is defined as Pd=NiAl EAds ¼ Eatom þ ENiAl , where EPd=NiAl and ENiAl are the total tot tot Etot tot tot energies of NiAl(1 1 0) supercell slab with and without adsorbate, is the total energy of an isolated Pd atom in respectively. Eatom tot the same spuercell. The calculated results show strong preference of Pd atoms for NiANi bridge sites and AlAAl bridge sites with very similar adsorption energies of 3.98 eV and 3.96 eV, which is consistent with the experiment phenomenon observed by STM [20]. However, it has been reported that single Au or Ag prefers to decorate at NiANi sites of NiAl(1 1 0) surface, and different adsorption behaviors can be explained by the different atomic configuration of metal elements [24]. In order to analyze the character of bonds and give a direct insight into the charge transfer between Pd monmer and substrate, the charge density difference for the two considered adsorption configurations were calculated, which is defined as
Dq ¼ qPd=NiAl qPd qNiAl ; where qPd=NiAl is the charge density of total system, qNiAl and qPd are the charge densities of bare NiAl substrate and the isolated Pd adatom, respectively. Above calculations were done within the same relaxed geometry as that of the Pd-adsorbed NiAl(1 1 0) system. Fig. 2 shows charge density differences for Pd monomer adsorbed at the NiANi (or AlAAl) bridge sites of NiAl(1 1 0) surface. For both considered cases, it is clearly seen that the charge densities are reduced at the center of Pd atom, while strong electron density accumulation presents between Pd adatom and the surrounding Al and Ni atoms of host, indicating that strong charge transfer between the Pd and NiAl substrate. It can be explained by the strong hybridizations between Pd adatom and neighboring Ni atoms of the NiAl substrate with Pd atom bridge-bonded to two Ni atoms, which is consistent with the previous results [34]. The bond lengths of PdANi are 2.49 Å and 2.73 Å in two considered cases, which are smaller than PdAPd bond length of 2.75 Å in bulk Pd [35]. It is not surprising since the atomic radius of Ni (1.25 Å) is smaller than that of Pd (1.37 Å). It is also found that Pd adatom adsorbed at AlAAl site has similar strong binding characteristics. The results explained
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the intra-chain formation energy can describe the strength of the adsorbate–adsorbate atomic interactions. The binding energy EB defined as the discrepancy between the values of adsorption energy and formation energy: chain EB ¼ Echain ads Eform : And EB reflects the additional attractive interactions of Pd ad-chain with the NiAl substrate. Table 1 summaries the structural parameters and calculated energies for infinite Pd adchains along X and Y directions. The calculated results show that
the adsorption energies (Echain ads ) of Pd chains adsorbed at NiANi bridges along X direction and Y direction of NiAl(1 1 0) surface are 3.50 eV and 3.54 eV, respectively. Echain of XAl and YAl adads chains are 3.66 eV and 3.84 eV, respectively. It’s also observed that the formation energy of XNi ad-chains is 0.67 eV higher than that of
(a) Pd on Ni-Ni bridge site
(b) Pd on Al-Al bridge site
Fig. 2. (a) and (b) Plots the charge density difference for Pd adatom adsorbed at AlAAl bridge and NiANi bridge sites of NiAl(1 1 0) surface, respectively. Yellow and blue regions represent charge accumulation and charge loss, respectively. The isosurface value is 1.5 103 eV/Å3. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
the facts of the similar adsorption stability for two adsorption sites of NiANi and AlAAl bridges to some extent. It has been observed the charge redistributions mainly involve Pd atom and the neighboring Ni atoms of top layer of NiAl substrate for Pd adsorbed at NiANi bridge sites, while the contributions of Al atoms are very weakly modified under Pd adsorption, as shown in Fig. 2a. Upon the adsorption of Pd at AlAAl bridge sites as shown in Fig. 2b, the charge redistributions mainly involve Ni atoms of both top layer and the second layer of NiAl substrate. Besides, it was assumed that the slight difference from each other may be affected by the difference of surrounding atomic environments of Pd adatom.
3.3. Geometries and relative stabilities for Pd chains adsorbed on NiAl (1 1 0) surface We have considered infinite Pd chains along [0 0 1] and [1 1 0] directions of NiAl(1 1 0) surface, which are referred to as the X and Y chains, respectively. According to the different adsorption sites, the XNi(Al) and YNi(Al) chains represent the infinite Pd chains adsorbed at NiANi or AlAAl bridges along the X and Y directions, respectively. After fully optimization, the atomic chains are well ordered and do not present structural distortion. The spacing between Pd atoms for the X and Y chains are 2.89 Å and 4.10 Å, which is modulated by the NiANi (or AlAAl) periodicity along X and Y directions, respectively. Our previous study reported that the competitions between the adsorption of Pd adatom on the NiAl substrate and the interactions among the Pd atoms play important roles for the growth of palladium clusters on NiAl(1 1 0) [33]. In order to further confirm this competition mechanism, here we compare the adsorption energy (Echain ads ) of the chain on the surface (Echain form )
of an isolated chain with the same and the formation energy geometry. According to Ref. [24], the adsorption energy are defined atom Pd=NiAl as following: Echain þ ENiAl . The larger the adsorpads ¼ Etot tot Etot tion energy is, the more stable the system of adsorbate-substrate.
While Echain form is the energy difference between the isolated chain and the total energies of isolated Pd atoms, which described as atom chain Echain is the formation energy Echain form ¼ Etot tot : In the formula, Etot of a floating Pd chain with the same geometry as Pd chains supported by NiAl(1 1 0). Within the definition, it is reasonable that
YNi ad-chains, and Echain form of XAl ad-chains is 0.66 eV higher than that of YAl ad-chains, which indicated that PdAPd interactions play much more important roles in the formation of Pd ad-chains supported by NiAl(1 1 0). For all considered Pd ad-chains with the binding energies larger than 2.69 eV, we think the higher adsorption stabilities may be related to strong interactions between Pd adatoms and NiAl substrate. As listed in Table 1, it is clearly observed that the interactions between Pd and NiAl substrate for XNi ad-chain is weaker than those of YNi ad-chain, which is consistent with the values of dPdANiAl for X and Y ad-chains, 1.75 Å and 1.67 Å, respectively. However for the Pd ad-chains absorbed at AlAAl bridges, the distance between Pd and NiAl substrate are both closed to 1.75 Å, which played little influence on the adsorption energies. Furthermore, by comparing the formation energies with the binding energies, we found that the interactions between Pd chains with NiAl substrate are much stronger than the interactions of PdAPd in all considered adsorption configurations. Above results revealed the existence of competition mechanism between PdAPd interactions and Pd-substrate interactions in the formation process of Pd ad-chains supported by NiAl(1 1 0). 3.4. Electronic structures of Pd chains on NiAl(1 1 0) surface In previous studies, Ho et al. have constructed well-defined Pd and Au atomic chains on metallic NiAl(1 1 0) surface by STM [20,21], and the dI/dV images revealed that delocalization gives rise to the electronic density oscillations. Motivated by the experimental results and in order to better understand the nature of resonance states, we calculated the electronic structures of Pd chains supported by NiAl(1 1 0). In order to quantificationally analyze the influence of atomic distances and adsorption sites on resonance states of Pd chains, we calculated the electronic structures of infinite Pd chains along X and Y directions of NiAl(1 1 0). The total and projected densities of states (PDOS) are shown in Fig. 3 for Pd-chains adsorbed at NiANi or AlAAl bridge sites along both X and Y directions of NiAl(1 1 0) surface, respectively. Compared with DOS of isolated Pd atom (here not shown), the most prominent feature is the appearance of the resonance states when Pd monomer adsorbed at NiANi or AlAAl bridge sites, which was Table 1 Calculated distance between Pd chains and NiAl substrate (dPdANiAl), PdAPd distance chain (dPdAPd), formation energy and adsorption energies of chain Echain form and Eads , and binding energy EB for the most stable configurations of Pd ad-chains along X and Y directions. Adsorption site
Direction
dPdANiAl (Å)
dPdAPd (Å)
Echain ads (eV)
Echain form (eV)
EB (eV)
NiANi bridge
X Y
1.75 1.67
2.89 4.10
3.50 3.54
0.81 0.14
2.69 3.40
AlAAl bridge
X Y
1.75 1.76
2.89 4.10
3.66 3.84
0.81 0.15
2.85 3.69
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P. Wu et al. / Computational Materials Science 124 (2016) 398–402 0.6
0.4
TDOS (States/eV)
0.5
XNi chain
(a) 1.50 eV
0.3 0.2 0.1 0.0 0.5
Ni_d Pd_s Pd_p Pd_d
0.4 0.3
PDOS (States/eV)
PDOS (States/eV)
TDOS (States/eV)
0.6
0.2
XAl chain
(c)
0.5
1.22 eV
0.4 0.3 0.2 0.1 0.0 0.5
Ni_d Pd_s Pd_p Pd_d
0.4 0.3 0.2 0.1
0.1 0.0 0.5
1.0
1.5
2.0
2.5
3.0
0.0 0.5
3.5
1.0
1.5
0.80 eV 0.4 0.3 0.2
(d) TDOS (States/eV)
TDOS (States/eV)
YNi chain
(b)
0.1
Ni_d Pd_s Pd_p Pd_d
0.4 0.3
PDOS (States/eV)
PDOS (States/eV)
2.5
0.8
0.6 0.5
2.0
3.0
3.5
E-E f (eV)
E-E f (eV)
0.2 0.1
YAl chain
0.60 eV
0.6 0.4 0.2 0.0 0.5
Ni_d Pd_s Pd_p Pd_d
0.4 0.3 0.2 0.1
0.0 0.5
1.0
1.5
2.0
2.5
3.0
3.5
0.0 0.5
E-E f (eV)
1.0
1.5
2.0
2.5
3.0
3.5
E-E f (eV)
Fig. 3. (a) and (b) The total and projected density of states for Pd and surrounding Ni atom for XNi and YNi chains, repectively. (c) and (d) The total and projected density of states for Pd and surrounding Ni atom for XAl and YAl chains, respectively.
(a)
(b)
Fig. 4. (a) and (b) The charge density difference for Pd ad-chains along XNi and YNi directions of NiAl(1 1 0) surface, respectively. The top and bottom planes correspond respectively with the top and side views. Yellow regions represent charge accumulation, and blue regions show charge loss. The iso-surface value is 1.5 103 e/Å3. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
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caused by the strong hybridizations between sp states of Pd and d states of surrounding Ni atoms. With increasing the length of chains, it’s shown that the interactions among resonance states eventually give rises to a remarkable band of the resonance states for Pd ad-chains, indicating that the direct overlap of the neighboring Pd 5sp orbital. As shown in Fig. 3, the bandwidths are 1.50 and 0.80 eV for XNi and YNi ad-chains, while these of XAl and YAl are about 1.22 and 1.00 eV, respectively. It’s clear that the bandwidth of the XNi (XAl) ad-chains is larger than that of YNi (YAl) adchains, which results from the stronger PdAPd interactions with shorter bond length. Besides, the bandwidth of Pd ad-chain adsorbed at NiANi bridge sits is larger than that of ad-chain adsorbed at AlAAl bridge sites, which results from the stronger hybridizations between Pd and nearest neighboring Ni. In conclusion, the band of response states of Pd ad-chains was modulated by the PdAPd bond length and adsorption sites. In order to analyze the character of bonds and give a direct insight into the charge transfer between Pd chains and NiAl substrate, take the cases of Pd chains adsorbed at NiANi bridge sites as an example, we calculated the charge density difference as shown in Fig. 4. Compared with calculated results obtained for a single Pd atom interacting with the NiAl in the most stable configuration, the charge transfer and the nature of bonding between Pd chains and NiAl substrate have little variations, perhaps because of the weaker interactions between neighboring Pd adatoms. However, it has been observed that the complexity of electronic structure increases for the case of the Pd chain along X direction that caused by direct overlap of atomic orbital between neighboring Pd adatoms. This finding explains the high formation energy of Pd chain along X direction than that of Y ad-chains to some extent. 4. Conclusions The geometry and electronic structures of Pd ad-chains supported by NiAl(1 1 0) were investigated by DFT calculations. The Pd monomer strongly bonded to two neighboring Ni atoms, and reflected the strong hybridizations between Pd and NiAl substrate to some extent, which nearly does not depend on the adsorbed sites. Furthermore, it has been observed that the stability of Pd ad-chains is high with large adsorption energies for considered NiANi and AlAAl bridge sites. In the case of infinite Pd chains the adsorbate-substrate interaction is stronger than adsorbateadsorbate interaction. Interestingly, for infinite Pd ad-chains, the interaction of resonance states which caused by Pd atom and NiAl substrate give rises to a band of resonance states, which are the results of hybridizations between 5sp of Pd and electronic states of substrate combined with direct orbital overlap of adjacent Pd atoms. We concluded that bandwidth may be impacted by PdAPd bond length combined with adsorption sites.
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Structure and electronic properties of palladium chains supported by NiAl(1 1 0): A first-principles study Ping Wu a,b, Naiqiang Yin a, Wenjing Cheng a, Peng Li a, Min Huang b,⇑ a b
School of Physics and Electrical Information, Shangqiu Normal University, Shangqiu 476000, China State Key Laboratory of Magnetic Resonance and Atomic and Molecular Physics, Wuhan Institute of Physics and Mathematics, Chinese Academy of Sciences, Wuhan 430071, China
a r t i c l e
i n f o
Article history: Received 9 July 2016 Received in revised form 5 August 2016 Accepted 7 August 2016
Keywords: Pd chains First-principles study Band of resonance states Density of states (DOS)
a b s t r a c t In this work, the geometry, stability and electronic structures of atomic palladium chains supported on the NiAl(1 1 0) surface have been investigated by using the first-principles calculations. Palladium monomer strongly adsorbed at NiANi and AlAAl bridge sites bonding with the two most neighboring Ni atoms. There is competitive mechanism between interactions of PdAPd and interactions between Pd and NiAl substrate for Pd ad-chains. We also observe the interactions of resonance states give rises to a band of resonance states, which is mainly related to the overlap of 5sp orbital of adjacent Pd atoms. Finally, we demonstrated that the atomic spacing in Pd ad-chains and adsorption sites had significant influences to the bandwidth of resonance band. These results may have potential applications in the fields of nanoelectronics and nanoscale catalysts. Ó 2016 Elsevier B.V. All rights reserved.
1. Introduction In recent decades, metal nanostructures have been widely studied due to the extensive applications in the fields of nanoelectronics and magnetic storage devices as well as nanocatalysis and gas sensors [1–7]. Compared with the properties of bulk metal, the metal nanostructures including metal clusters (such as Ag, Au, Pt and Pd) and wires possess extraordinary sizes-dependent mechanical, electronic, magnetic, optical properties, which became key issues attracting wide attentions of researchers. For instance, theoretical calculations and experiments studies suggested that Pd clusters exhibit remarkable catalytic activities in oxidation reaction [8] and synthesis of vinyl acetate and methanol [9], which is due to the large surface area-to-volume ratio. Besides, some Pd clusters with certain sizes possess magnetic moment [10,11], which is absent in the bulk materials. Previous study reported that MoS2 nanosheet decorated with gold nanoparticles (AuNPs) can be used to modify glass carbon electrode of electrochemical glucose biosensor, which accelerates the electron transfer from electrode to the immobilized enzyme [12]. The physical and chemical properties of metal nanostructures can be modulated by tailoring or assembling the geometries. Recently, fabrication and characterization of clusters and ordered chains of heavy transition metals on various substrates has been
⇑ Corresponding author. E-mail address: [email protected] (M. Huang). http://dx.doi.org/10.1016/j.commatsci.2016.08.008 0927-0256/Ó 2016 Elsevier B.V. All rights reserved.
studied by experimental and theoretical studies [13–17]. Wang et al. [13] reported that the structure and bonding of Pd clusters are modulated by the Moire pattern of the graphene/Ru(0 0 0 1), allowing a resolution of their role in field of nanocatalysis in future. The adsorption of Pd clusters supported on both carbon nanotubes and graphite have been comparatively studied by DFT [14], which show that curved nanotube surface seem to be more suitable to support palladium clusters with respect to the flat graphite surface due to the stronger PdAC bonding. It’s known that the substrate plays an important role to act as the template in the growth and assembling process of metal nanostructures. For instance, surface defects such as oxygen vacancies impact the nucleation, growth processes and diffusion of Pd clusters supported on MgO, Al2O3 and CeO2 surface [15–17]. Interestingly, the surface reconstruction and terrace of metal substrates may be conducive to the preparation of the monoatomic metal chains. Using spin-polarized scanning tunneling microscopy (STM), Matthias and co-workers demonstrated the occurrence of an atomic-scale spin spiral in finite individual bi-atomic Fe chains on the 5 1-Ir(0 0 1) surface [18]. It’s known that bare metallic NiAl(1 1 0) surface with one dimensional troughs is a simple structure template for fabrication of artificial metal chains with negligible effect on the electronic properties of chains, indicating that the metal chains may exhibit true metallic electronic properties in one-dimension [19]. Welldefined Au and Pd mono-atomic chains have been constructed on NiAl(1 1 0) surface based on the atom manipulation of STM [20,21]. It has been revealed that the overlap between adjacent Au 6sp
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orbitals is responsible for the electronic resonance states of Au chains supported by NiAl(1 1 0) surface [22,23]. Several firstprinciples studies on artificial monoatomic chains on NiAl(1 1 0) surface revealed the atomic configurations of adatoms result in the different degree of interactions with NiAl substrate and give rises to diverse electronic behaviors [24–26]. In order to explore the relationships between geometry and electronic behaviors of metal atomic chains and thus better construct artificial chains on NiAl(1 1 0) surface, it is important to explain the effects of interactions between chains and substrate as well as interatomic interactions on the electronic properties. For the fabrication and applications of low dimensional nanostructures, it is indispensable to probe evolution of the electronic properties with chains length increasing. However, there are few theoretical studies on the geometry and electronic structure of Pd chains supported by NiAl(1 1 0) surface, which may supply the basis for interpreting the experiment results. In this paper we focus our investigations on the adsorption geometry, stability and electronic structure of Pd chains supported by NiAl(1 1 0) surface. In order to get insight into the nucleation and initial growth of chains, we investigated the adsorption configurations and charge transfer between Pd and NiAl substrate. Based on the DFT calculations, we detailedly study the influences of the competitive mechanism between the adsorbate-adsorbate interactions and adsorbate-substrate interactions on the stability and electronic properties of Pd chains, and discuss the influence of atomic spacing on the electronic behaviors of infinite Pd chains. 2. Computational details All calculations were carried out within the frame of density functional theory (DFT) as implemented in the VASP code [27]. The generalized gradient (GGA) corrected by Perdew-BurkeErnzerhof (PBE) [28] is adopted for the exchange and correlation functional, and the projector augmented plane wave (PAW) was used to describe the ion-electron interactions [29]. A kinetic energy cutoff of 400 eV is used, all the structures were fully relaxed until total energies of the system were smaller than 1.0 105 eV. The Monkhorst-Pack meshes [30] were used to sample the supercells. The lattice parameter of 2.89 Å is used for bulk NiAl, which is good agreement with experimental results [31]. To simulate the Palladium chains supported by NiAl(1 1 0) surface, we used supercell slabs containing four layers of metal alloy with the periodicity of 1 4 and 4 1 to simulate the adsorption of infinite Pd chains along [0 0 1] and [1 1 0] directions, respectively. We used meshes of 9 4 1 and 4 9 1 k-points to sample Brillouin zone integrations, respectively. The thick vacuum layer of 15 Å is adopted to eliminate the interactions between adjacent supercells. In all calculations, the bottom most NiAl layer was fixed and the rest of atoms were fully relaxed. 3. Results and discussions 3.1. Geometry and properties of clean NiAl(1 1 0) surface It was reported that the surface energy of the NiAl(1 1 0) is 1.65 eV, which is the most stable surface for the NiAl crystal with CsCl structure [32]. After relaxation, the pristine NiAl(1 1 0) surface was found to be rumpled with 0.16 Å outward displacement of the Al rows with respect to the Ni rows as shown in Fig. 1b, which is consistent with the previous STM results and theoretical calculations [21,26]. There are troughs with an interatomic spacing of 2.98 Å along the [1 1 0] direction as shown in Fig. 1a, which is usually used as the common support for the formation of monoatomic chains [22]. Our previous DFT study has verified that the
(b) side view
(a) top view Ni Al
2.98Å
0.16Å
[110]
[110]
[1-10] Ni-Ni bridge site [001]
[00-1]
[1-10]
Al-Al bridge site
Fig. 1. The top view (a) and side view (b) of bare NiAl(1 1 0) surface after relaxation. The blue and gray balls represent Al and Ni atoms, respectively. The adsorption sites of NiANi bridge and AlAAl bridge are labeled by square and triangle, respectively. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
total density of states (DOS) of substrate are primarily contributed by the d states of Ni from 4.0 eV to Fermi level, and present a depletion in the range of 0–2.20 eV, whereas the DOS are ascribed to states with sp characters at higher energies [33]. 3.2. Adsorption of Pd monomer on the NiAl(1 1 0) surface We first investigated the adsorption properties of single Pd adatom on the NiAl(1 1 0) surface. Based on the experiments results by STM, here we considered two high-symmetric adsorption sites for Pd monomer: NiANi bridge and AlAAl bridge sites, as shown in Fig. 1a. The adsorption energy is defined as Pd=NiAl EAds ¼ Eatom þ ENiAl , where EPd=NiAl and ENiAl are the total tot tot Etot tot tot energies of NiAl(1 1 0) supercell slab with and without adsorbate, is the total energy of an isolated Pd atom in respectively. Eatom tot the same spuercell. The calculated results show strong preference of Pd atoms for NiANi bridge sites and AlAAl bridge sites with very similar adsorption energies of 3.98 eV and 3.96 eV, which is consistent with the experiment phenomenon observed by STM [20]. However, it has been reported that single Au or Ag prefers to decorate at NiANi sites of NiAl(1 1 0) surface, and different adsorption behaviors can be explained by the different atomic configuration of metal elements [24]. In order to analyze the character of bonds and give a direct insight into the charge transfer between Pd monmer and substrate, the charge density difference for the two considered adsorption configurations were calculated, which is defined as
Dq ¼ qPd=NiAl qPd qNiAl ; where qPd=NiAl is the charge density of total system, qNiAl and qPd are the charge densities of bare NiAl substrate and the isolated Pd adatom, respectively. Above calculations were done within the same relaxed geometry as that of the Pd-adsorbed NiAl(1 1 0) system. Fig. 2 shows charge density differences for Pd monomer adsorbed at the NiANi (or AlAAl) bridge sites of NiAl(1 1 0) surface. For both considered cases, it is clearly seen that the charge densities are reduced at the center of Pd atom, while strong electron density accumulation presents between Pd adatom and the surrounding Al and Ni atoms of host, indicating that strong charge transfer between the Pd and NiAl substrate. It can be explained by the strong hybridizations between Pd adatom and neighboring Ni atoms of the NiAl substrate with Pd atom bridge-bonded to two Ni atoms, which is consistent with the previous results [34]. The bond lengths of PdANi are 2.49 Å and 2.73 Å in two considered cases, which are smaller than PdAPd bond length of 2.75 Å in bulk Pd [35]. It is not surprising since the atomic radius of Ni (1.25 Å) is smaller than that of Pd (1.37 Å). It is also found that Pd adatom adsorbed at AlAAl site has similar strong binding characteristics. The results explained
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the intra-chain formation energy can describe the strength of the adsorbate–adsorbate atomic interactions. The binding energy EB defined as the discrepancy between the values of adsorption energy and formation energy: chain EB ¼ Echain ads Eform : And EB reflects the additional attractive interactions of Pd ad-chain with the NiAl substrate. Table 1 summaries the structural parameters and calculated energies for infinite Pd adchains along X and Y directions. The calculated results show that
the adsorption energies (Echain ads ) of Pd chains adsorbed at NiANi bridges along X direction and Y direction of NiAl(1 1 0) surface are 3.50 eV and 3.54 eV, respectively. Echain of XAl and YAl adads chains are 3.66 eV and 3.84 eV, respectively. It’s also observed that the formation energy of XNi ad-chains is 0.67 eV higher than that of
(a) Pd on Ni-Ni bridge site
(b) Pd on Al-Al bridge site
Fig. 2. (a) and (b) Plots the charge density difference for Pd adatom adsorbed at AlAAl bridge and NiANi bridge sites of NiAl(1 1 0) surface, respectively. Yellow and blue regions represent charge accumulation and charge loss, respectively. The isosurface value is 1.5 103 eV/Å3. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
the facts of the similar adsorption stability for two adsorption sites of NiANi and AlAAl bridges to some extent. It has been observed the charge redistributions mainly involve Pd atom and the neighboring Ni atoms of top layer of NiAl substrate for Pd adsorbed at NiANi bridge sites, while the contributions of Al atoms are very weakly modified under Pd adsorption, as shown in Fig. 2a. Upon the adsorption of Pd at AlAAl bridge sites as shown in Fig. 2b, the charge redistributions mainly involve Ni atoms of both top layer and the second layer of NiAl substrate. Besides, it was assumed that the slight difference from each other may be affected by the difference of surrounding atomic environments of Pd adatom.
3.3. Geometries and relative stabilities for Pd chains adsorbed on NiAl (1 1 0) surface We have considered infinite Pd chains along [0 0 1] and [1 1 0] directions of NiAl(1 1 0) surface, which are referred to as the X and Y chains, respectively. According to the different adsorption sites, the XNi(Al) and YNi(Al) chains represent the infinite Pd chains adsorbed at NiANi or AlAAl bridges along the X and Y directions, respectively. After fully optimization, the atomic chains are well ordered and do not present structural distortion. The spacing between Pd atoms for the X and Y chains are 2.89 Å and 4.10 Å, which is modulated by the NiANi (or AlAAl) periodicity along X and Y directions, respectively. Our previous study reported that the competitions between the adsorption of Pd adatom on the NiAl substrate and the interactions among the Pd atoms play important roles for the growth of palladium clusters on NiAl(1 1 0) [33]. In order to further confirm this competition mechanism, here we compare the adsorption energy (Echain ads ) of the chain on the surface (Echain form )
of an isolated chain with the same and the formation energy geometry. According to Ref. [24], the adsorption energy are defined atom Pd=NiAl as following: Echain þ ENiAl . The larger the adsorpads ¼ Etot tot Etot tion energy is, the more stable the system of adsorbate-substrate.
While Echain form is the energy difference between the isolated chain and the total energies of isolated Pd atoms, which described as atom chain Echain is the formation energy Echain form ¼ Etot tot : In the formula, Etot of a floating Pd chain with the same geometry as Pd chains supported by NiAl(1 1 0). Within the definition, it is reasonable that
YNi ad-chains, and Echain form of XAl ad-chains is 0.66 eV higher than that of YAl ad-chains, which indicated that PdAPd interactions play much more important roles in the formation of Pd ad-chains supported by NiAl(1 1 0). For all considered Pd ad-chains with the binding energies larger than 2.69 eV, we think the higher adsorption stabilities may be related to strong interactions between Pd adatoms and NiAl substrate. As listed in Table 1, it is clearly observed that the interactions between Pd and NiAl substrate for XNi ad-chain is weaker than those of YNi ad-chain, which is consistent with the values of dPdANiAl for X and Y ad-chains, 1.75 Å and 1.67 Å, respectively. However for the Pd ad-chains absorbed at AlAAl bridges, the distance between Pd and NiAl substrate are both closed to 1.75 Å, which played little influence on the adsorption energies. Furthermore, by comparing the formation energies with the binding energies, we found that the interactions between Pd chains with NiAl substrate are much stronger than the interactions of PdAPd in all considered adsorption configurations. Above results revealed the existence of competition mechanism between PdAPd interactions and Pd-substrate interactions in the formation process of Pd ad-chains supported by NiAl(1 1 0). 3.4. Electronic structures of Pd chains on NiAl(1 1 0) surface In previous studies, Ho et al. have constructed well-defined Pd and Au atomic chains on metallic NiAl(1 1 0) surface by STM [20,21], and the dI/dV images revealed that delocalization gives rise to the electronic density oscillations. Motivated by the experimental results and in order to better understand the nature of resonance states, we calculated the electronic structures of Pd chains supported by NiAl(1 1 0). In order to quantificationally analyze the influence of atomic distances and adsorption sites on resonance states of Pd chains, we calculated the electronic structures of infinite Pd chains along X and Y directions of NiAl(1 1 0). The total and projected densities of states (PDOS) are shown in Fig. 3 for Pd-chains adsorbed at NiANi or AlAAl bridge sites along both X and Y directions of NiAl(1 1 0) surface, respectively. Compared with DOS of isolated Pd atom (here not shown), the most prominent feature is the appearance of the resonance states when Pd monomer adsorbed at NiANi or AlAAl bridge sites, which was Table 1 Calculated distance between Pd chains and NiAl substrate (dPdANiAl), PdAPd distance chain (dPdAPd), formation energy and adsorption energies of chain Echain form and Eads , and binding energy EB for the most stable configurations of Pd ad-chains along X and Y directions. Adsorption site
Direction
dPdANiAl (Å)
dPdAPd (Å)
Echain ads (eV)
Echain form (eV)
EB (eV)
NiANi bridge
X Y
1.75 1.67
2.89 4.10
3.50 3.54
0.81 0.14
2.69 3.40
AlAAl bridge
X Y
1.75 1.76
2.89 4.10
3.66 3.84
0.81 0.15
2.85 3.69
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0.4
TDOS (States/eV)
0.5
XNi chain
(a) 1.50 eV
0.3 0.2 0.1 0.0 0.5
Ni_d Pd_s Pd_p Pd_d
0.4 0.3
PDOS (States/eV)
PDOS (States/eV)
TDOS (States/eV)
0.6
0.2
XAl chain
(c)
0.5
1.22 eV
0.4 0.3 0.2 0.1 0.0 0.5
Ni_d Pd_s Pd_p Pd_d
0.4 0.3 0.2 0.1
0.1 0.0 0.5
1.0
1.5
2.0
2.5
3.0
0.0 0.5
3.5
1.0
1.5
0.80 eV 0.4 0.3 0.2
(d) TDOS (States/eV)
TDOS (States/eV)
YNi chain
(b)
0.1
Ni_d Pd_s Pd_p Pd_d
0.4 0.3
PDOS (States/eV)
PDOS (States/eV)
2.5
0.8
0.6 0.5
2.0
3.0
3.5
E-E f (eV)
E-E f (eV)
0.2 0.1
YAl chain
0.60 eV
0.6 0.4 0.2 0.0 0.5
Ni_d Pd_s Pd_p Pd_d
0.4 0.3 0.2 0.1
0.0 0.5
1.0
1.5
2.0
2.5
3.0
3.5
0.0 0.5
E-E f (eV)
1.0
1.5
2.0
2.5
3.0
3.5
E-E f (eV)
Fig. 3. (a) and (b) The total and projected density of states for Pd and surrounding Ni atom for XNi and YNi chains, repectively. (c) and (d) The total and projected density of states for Pd and surrounding Ni atom for XAl and YAl chains, respectively.
(a)
(b)
Fig. 4. (a) and (b) The charge density difference for Pd ad-chains along XNi and YNi directions of NiAl(1 1 0) surface, respectively. The top and bottom planes correspond respectively with the top and side views. Yellow regions represent charge accumulation, and blue regions show charge loss. The iso-surface value is 1.5 103 e/Å3. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
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caused by the strong hybridizations between sp states of Pd and d states of surrounding Ni atoms. With increasing the length of chains, it’s shown that the interactions among resonance states eventually give rises to a remarkable band of the resonance states for Pd ad-chains, indicating that the direct overlap of the neighboring Pd 5sp orbital. As shown in Fig. 3, the bandwidths are 1.50 and 0.80 eV for XNi and YNi ad-chains, while these of XAl and YAl are about 1.22 and 1.00 eV, respectively. It’s clear that the bandwidth of the XNi (XAl) ad-chains is larger than that of YNi (YAl) adchains, which results from the stronger PdAPd interactions with shorter bond length. Besides, the bandwidth of Pd ad-chain adsorbed at NiANi bridge sits is larger than that of ad-chain adsorbed at AlAAl bridge sites, which results from the stronger hybridizations between Pd and nearest neighboring Ni. In conclusion, the band of response states of Pd ad-chains was modulated by the PdAPd bond length and adsorption sites. In order to analyze the character of bonds and give a direct insight into the charge transfer between Pd chains and NiAl substrate, take the cases of Pd chains adsorbed at NiANi bridge sites as an example, we calculated the charge density difference as shown in Fig. 4. Compared with calculated results obtained for a single Pd atom interacting with the NiAl in the most stable configuration, the charge transfer and the nature of bonding between Pd chains and NiAl substrate have little variations, perhaps because of the weaker interactions between neighboring Pd adatoms. However, it has been observed that the complexity of electronic structure increases for the case of the Pd chain along X direction that caused by direct overlap of atomic orbital between neighboring Pd adatoms. This finding explains the high formation energy of Pd chain along X direction than that of Y ad-chains to some extent. 4. Conclusions The geometry and electronic structures of Pd ad-chains supported by NiAl(1 1 0) were investigated by DFT calculations. The Pd monomer strongly bonded to two neighboring Ni atoms, and reflected the strong hybridizations between Pd and NiAl substrate to some extent, which nearly does not depend on the adsorbed sites. Furthermore, it has been observed that the stability of Pd ad-chains is high with large adsorption energies for considered NiANi and AlAAl bridge sites. In the case of infinite Pd chains the adsorbate-substrate interaction is stronger than adsorbateadsorbate interaction. Interestingly, for infinite Pd ad-chains, the interaction of resonance states which caused by Pd atom and NiAl substrate give rises to a band of resonance states, which are the results of hybridizations between 5sp of Pd and electronic states of substrate combined with direct orbital overlap of adjacent Pd atoms. We concluded that bandwidth may be impacted by PdAPd bond length combined with adsorption sites.
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