Silicon-Containing Multidecker Organometallic Complexes and

Letter pubs.acs.org/JPCL

Silicon-Containing Multidecker Organometallic Complexes and Nanowires: A Density Functional Theory Study Guiling Zhang,†,‡ Rulong Zhou,‡,§ Yi Gao,‡ and Xiao Cheng Zeng*,‡ †

College of Chemical and Environmental Engineering, Harbin University of Science and Technology, Harbin, 150080, China Department of Chemistry, University of NebraskaLincoln, Lincoln, Nebraska 68588, United States § School of Science and Engineering of Materials, Hefei University of Technology, Hefei, Anhui, 230009, China ‡

S Supporting Information *

ABSTRACT: Silicon-containing multidecker organometallic complexes and nanowires, Crn(SiBz)m and Vn(SiBz)m (SiBz = 1,3,5-silicon-substituted benzene), are predicted to possess novel electronic, magnetic, and electron transport properties by using density functional theory and nonequilibrium Green’s function calculations. The multidecker complexes and nanowires are stabilized by the strong Cr−Si and V−Si interaction. It is found that the Crn(Bz)m nanowire exhibits an antiferromagnetic ground state, while the Vn(SiBz)m nanowire exhibits properties of a diluted magnetic semiconductor, contrary to the known quasi-half-metallic properties of the carbon analogue, Vn(Bz)m complexes and nanowires. Between two metal electrodes, the finite-size V3(SiBz)4 complex not only possesses higher conductivity than Cr3(SiBz)4 but also exhibits a distinctive feature of negative differential resistance (NDR). In general, the minority spin channel of V3(SiBz)4 is the main transport channel at a relatively low voltage bias (1.6 V), both the majority and minority spin channels of V3(SiBz)4 become the main channel for electron transport. For the [V(SiBz)]∞ nanowire, however, the majority spin channel becomes the main channel for electron transport. SECTION: Molecular Structure, Quantum Chemistry, General Theory

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complexes are still challenging to make largely because silicon cannot provide a highly delocalized π-bonding environment like carbon, a key factor to stabilize the transition-metal−ligand coordination. Recently, Sergeeva and Boldyrev29 investigated the multidecker complexes and nanowires built on Si5H5− and Mg2+ using ab initio methods. They showed that the Si5H5− anion itself is nonplanar due to the second-order Jahn−Teller (SOJT) effect; however, the multidecker environment makes it planar, while the SOJT effect is suppressed. Mohan and Datta30 predicted a highly stable sandwich complex of the type η6Cr(Si3C3H6)2 via density functional theory (DFT) calculation. They find that the 1,3,5-Si-substituted benzene ligand Si3C3H6 (SiBz) exhibits perfect planarity. These findings motivate us to investigate the stability of silicon-containing multidecker organometallic complexes and nanowires. Two prototype silicon-containing multidecker metallocene complexes, Crn(SiBz)m and Vn(SiBz)m, are focused in this study as Cr atoms are typically antiferromagnetically coupled in the benzene complexes Crn(Bz)m,17,18,25 while V atoms are ferromagnetically coupled in the benzene complexes Vn(Bz)m.12,13,17,18,23−28 First, we examine the stability of the

ultidecker organometallic sandwich clusters and nanowires have attracted growing interest over the past decade owing to their potential applications in molecular magnetic and electronic devices.1−12 Among the multidecker systems under active investigation, the cyclopentadienyl complexes Mn(Cp)m (M = Fe and V, Cp = C5H5)3−6 and their benzene analogues Mn(Bz)m (M = Sc−Cu, Bz = C6H6)7−12 have received the most attention. Many of these multidecker complexes not only have been synthesized in the laboratory but also have been imaged or detected using scanning tunneling microscopy (STM),4 UV−vis and IR,7 electron paramagnetic resonance (EPR),7 time-of-flight mass spectroscopies,3,8−12 and photoionization spectroscopies.9−11 Moreover, based on theoretical calculations, novel properties of the complexes, including electronic,13−20 chemical bonding,13−20 vibrational,16,21,22 electron transport,5,6,23 and magnetic5,6,13,17,23−28 properties, have been studied as well. In many cases, results of theoretical calculations are fully consistent with the experimental measurements. Silicon-containing multidecker complexes and nanowires are likely more compatible with current nanoelectronic devices as they are mostly silicon based as well. Although a variety of silicon or silicon-containing nanowires have been fabricated in the laboratory, to our knowledge, the silicon-containing multidecker organometallic nanowires have not been reported in the literature. Such silicon-containing nanowires or © 2011 American Chemical Society

Received: November 17, 2011 Accepted: December 24, 2011 Published: December 25, 2011 151

dx.doi.org/10.1021/jz201514h | J. Phys. Chem.Lett. 2012, 3, 151−156

The Journal of Physical Chemistry Letters

Letter

the structural optimization. In the NBO analysis, the strength of the bond interaction can be evaluated based on the magnitude of stabilization energy, E(2), defined as36,37

finite-size clusters Crn(SiBz)n+1 and Vn(SiBz)n+1 (n = 1−3). Here, DFT calculations are performed based on the B3PW91/ 6-31+G* level of theory (for Si, C, H) and the B3PW91/ Lanl2dz level (for Cr and V), both implemented in Gaussian09.31−33 Full structural optimization is performed for each cluster without any symmetry constraint and with consideration of different spin multiplicities. Breaking the symmetry of the wave function34,35 is considered when the lowspin states are incompatible with the single determinant of symmetry-adapted orbitals. Frequency calculation is undertaken to confirm that the optimized structures are true local minima. Two distinctive initial structures for both Crn(SiBz)n+1 and Vn(SiBz)n+1 (n = 1−3) are examined: the cis and the trans conformations (Figure 1). In the cis structure, the silicon atoms

E(2) = − 2

(⟨φD|F |φA *⟩)2 εA * − εD

where F is the Fock operator; φD is an electron-donating orbital; and φA is a neighboring electron-accepting orbital, and εA* and εD are the NBO energies of the acceptor and donor orbitals, respectively. Higher stabilization energy indicates that the two orbitals interact more strongly. The stabilization energies, E(2), of inter-ring Si···Si pairs are 11.06 and 7.89 kcal/ mol for Cr(SiBz)2 and V(SiBz)2, respectively, much greater than those of C···C pairs ( 3.0 eV), compared to that of the β spin channel (Δβ < 3.0 eV). The difference in the HOMO−LUMO gap likely leads to different conductivity. Next, we investigate electron transport properties of the finite-size Cr3(SiBz)4 and V3(SiBz)4 clusters, using the TRANSIESTA program implemented in the SIESTA 3.0 package.41 To this end, the clusters are coupled to two opposing metal electrodes of Co(111) (Figure 2(a)). The Perdew−Burke−Ernzehof (PBE) functional42 within the generalized gradient approximation (GGA) and the single-ζ plus polarization (SZP) basis set are selected. A real-space grid with an equivalent energy cutoff of 200 Ry is adopted to expand the electron density for numerical integration. The core corrections are included for generating the corresponding pseudopotentials. Nonrelativistic and relativistic norm-conserving pseudopotentials generated in the Troullier−Martins scheme43 are used for H, C, Si and V, Cr, Co, respectively.

Figure 2. (a) Two-probe systems for electron transport calculations of the finite-size Cr3(SiBz)4 and V3(SiBz)4 clusters and (b) calculated I− V curves of the Cr3(SiBz)4 and V3(SiBz)4. For comparison, the I−V curves for the Cr3(Bz)4 and V3(Bz)4 analogues are also given.

The electronic configurations of 3p63d34s2, 3p63d44s2, and 3d74s2 are used for referring to the valence state of V, Cr, and Co, respectively. The computed I−V curves for the Cr3(SiBz)4 and V3(SiBz)4 complexes are plotted in Figure 2(b). For comparison, the I−V curves of the Cr3(Bz)4 and V3(Bz)4 carbon analogues, using the same computational method, are also displayed in Figure 2(b). Clearly, the V3(SiBz)4 shows the highest conductivity than the other three complexes and exhibits a distinct NDR feature near 1.6 V. To gain more insight into this striking NDR feature, we compute the transmission spectra T(E) and the density of states (DOS) of the V3(SiBz)4 at the bias of 0.8, 1.6, and 1.8 V, respectively (Figure 3). By comparing the transmission peaks and the DOS distributions around the Fermi level Ef, it can be seen that the carriers mainly transport through the minority spin (β) channel at the low bias of 0.8 V, owing to the narrower band gap Δβ compared to Δα. When the bias rises to 1.6 V, the transmission and DOS peaks of α spin and β spin states are lowered altogether, leading to a drop of the current. It is noteworthy that the majority spin (α) channel can be also opened if the bias increases to 1.8 V. This means that the V3(SiBz)4 cluster can carry out electron transport via both the α and β channels under a higher bias. As such, the current rises rapidly at higher biases. Furthermore, to understand the effect of substitution of certain carbon atoms by silicon atoms on the electron transport, T(E) and DOS of the benzene complex V3(Bz)4 are also computed using the same set of biases (0.8, 1.6, and 1.8 V, respectively) and are presented in the 153



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the [Cr(SiBz)]∞ and [V(SiBz)]∞, respectively. The Monkhost− Pack mesh49 sampling with 1 × 1 × 21 k-points in a string Brillouin zone (x, y, z directions, respectively) is used. The out-of-plane distortion of the SiBz rings is also observed in the [Cr(SiBz)]∞ and [V(SiBz)]∞ nanowires upon geometric optimization (Figure 1). The average C−SiBz distance is 3.320 Å, appreciably shorter than that of V−SiBz (3.504 Å), consistent with results of finite-size clusters. Computed electronic band structures of the [Cr(SiBz)]∞ and [V(SiBz)]∞ nanowires are shown in Figure 4, and their total density of

Figure 4. Computed band structures of the infinite [Cr(SiBz)]∞ and [V(SiBz)]∞ nanowires. Five top valence bands in the majority band structure are highlighted in color, and so are the corresponding five empty bands in the minority band.

states and projected density of states (PDOS) are displayed in Figure 5. The [Cr(SiBz)]∞ exhibits semiconducting features with a band gap of ∼0.6 eV. For [V(SiBz)]∞, the majority band structure exhibits five additional occupied valence bands, whose corresponding empty bands in the minority channel are pushed upward to the bottom of the conduction band (color lines in Figure 4). The majority spin channel exhibits a very small band gap of 0.18 eV, and the Fermi level lies just above the top of the valence band, suggesting a narrow-gap semiconducting feature. The minority spin channel opens up a larger 0.86 eV band gap, and the Fermi level lies in the middle of the gap. Hence, for the [V(SiBz)]∞ nanowire, the electron transport would be mainly through the α channel rather than the β channel. This can also be understood from the length-dependent HOMO−LUMO gap as shown in Figure S1 (Supporting Information). Note that with the increase of the cluster length the magnitude of the band gap of the β channel Δβ becomes closer to the Δα. For example, the difference between Δα and Δβ is 1.39 eV for V(SiBz)2, while it reduces to 0.80 eV for V3(SiBz)4. Assuming that the decrease of Δα and Δβ for Vn(SiBz)n+1 correlates linearly with the chain length n, the Δβ would become higher than the Δα if the length of the complex is longer than n = 7 (Figure S2, Supporting Information). As such, one expects that the α channel should serve as the main channel for electron transport in the case of an infinite nanowire. The difference in band fillings between the minority and the majority spin channel results in a net 4.45 μB magnetic moment per supercell for [V(SiBz)]∞, that is, a magnetic moment of 0.89 μB per unit cell, very close to the value of 0.8024 and 0.88 μB25 per unit cell for [V(Bz)]∞ from previous DFT calculations. More interestingly, the [V(SiBz)]∞ nanowire shows features of diluted magnetic semiconductor. This behavior is in stark contrast to the quasi-half-metallic behavior of the [V(Bz)]∞ for

Figure 3. (a) Transmission spectra and (b) DOS of the V3(SiBz)4 cluster at a bias of 0.8, 1.6, and 1.8 V, respectively.

Supporting Information (Figure S1). It can be found that charge carriers may no longer transport easily through either the α channel or the β channel within the bias range considered, hence the lack of NDR behavior in the V3(Bz)4 system. The striking NDR behavior of the V3(SiBz)4 complex may find some applications in molecular electronics. In addition to finite-size clusters, we have also investigated electronic structures of the infinite nanowires [Cr(SiBz)]∞ and [V(SiBz)]∞ in cis structures. Considering the out-of-plane distortion of SiBz planes, we have used a relatively larger supercell containing five repeating units of Cr(SiBz) or V(SiBz) along the axial direction (z direction) (Figure 1). The supercell dimensions in the x and y directions are set to be 30 Å, large enough to avoid direct wire−wire interactions. Computations are performed using the PW91/GGA functional,44 implemented in the VASP software package.45,46 The electron−ion interaction is represented by projector augmented wave (PAW) potentials.47,48 The plane-wave basis set cutoff is 400 eV. Spin unpolarized and spin polarized calculations are carried out for 154



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Figure 5. Computed density of states (DOS) and the projected density of states (PDOS) of the infinite (a) [Cr(SiBz)]∞ and (b) [V(SiBz)]∞ nanowires.

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which the majority valence bands are completely filled, while two minority bands are partially occupied and cross the Fermi level.12,13,17,18,23−28 The net magnetic moment of the [V(SiBz)]∞ nanowire is mainly attributed to the 3d orbitals of V (Table 1), which can be also understood from the PDOS (Figure 5) where a large contribution from the V 3d component and small contribution from C/Si 2pz component can be seen near the Ef. Lastly, to confirm the on-site correlation effects among 3d electrons of the Cr or V, we have also performed DFT calculation using the LDA+U scheme50 with the PAW method as implemented in the VASP package. The parameter U−J is set to be 3 for Cr and V atoms. A comparison of results from the two methods is shown in Table 1. The inclusion of a Hubbard U term yields nearly consistent results on magnetic features of [V(SiBz)]∞ and AFM features of [Cr(SiBz)]∞. In conclusion, we have investigated electronic, magnetic, and electron transport properties of the Crn(SiBz)m and Vn(SiBz)m complexes and nanowires using DFT and nonequilibrium Green’s function methods. The Crn(Bz)m exhibits features of an antiferromagnetic semiconductor, while Vn(SiBz)m shows features of a diluted magnetic semiconductor. The magnetic moment per [V(SiBz)] unit is 0.89 μB. The V3(SiBz)4 cluster gives rise to higher conductivity than the Cr3(SiBz) cluster, and the I−V curve of V3(SiBz)4 exhibits a distinct NDR behavior. For Vn(SiBz)m, the electron transport properties are dependent on the applied voltage bias and the length of the cluster. The β spin channel of V3(SiBz)4 serves as the main transport channel at a relatively low bias (1.6 V), the α spin channel can also contribute significantly to the conductivity that can lead to a rapid increase of the current. For the infinite [V(SiBz)]∞ nanowire, the electron transport is mainly through the α spin channel. These novel features of silicon-containing multidecker organometallic complexes and nanowires may have implication to nanoelectronic design.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected].

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ACKNOWLEDGMENTS

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REFERENCES

XCZ is supported by grants from the NSF (EPS-1010674) and ARL (W911NF1020099) and by the University of Nebraska’s Holland Computing Center. GLZ thanks the NSF of China (51073048), the NSF of Hei Long Jiang Province of China (B201102), the SF for leaders in academe of Harbin City of China (2010RFJGG016), and the SF for elitists of Harbin University of Science and Technology for the financial support.

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ASSOCIATED CONTENT

S Supporting Information *

Calculated total energies for different isomers at various spin multiplicities, the stabilization energies, E(2), for Cr(SiBz)2 and V(SiBz)2, the transmission spectra and DOS distributions of the V3(Bz)4 cluster at a bias of 0.8, 1.6, and 1.8 V, and the dependence of the Δα and Δβ on the chain length n of the Vn(SiBz)n+1 clusters are collected. This material is available free of charge via the Internet at http://pubs.acs.org. 155



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Silicon-Containing Multidecker Organometallic Complexes and

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