Communication: Identification of the molecule–metal bonding geometries of molecular nanowires

The Journal of Chemical Physics

2012

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It is widely believed that when a molecule with thiol (S-H) end groups bridges a pair of gold electrodes, the S atoms bond to the gold and the thiol H atoms detach from the molecule. However, little is known regarding the details of this process, its time scale, and whether molecules with and without thiol hydrogen atoms can coexist in molecular junctions. Here we explore theoretically how inelastic tunneling spectroscopy (IETS) can shed light on these issues. We present calculations of the geometries, low bias conductances and IETS of propanedithiol and propanedithiolate molecular junctions with gold electrodes. We show that IETS can distinguish between junctions with molecules having no, one or two thiol hydrogen atoms. We find that in most cases the single-molecule junctions in the IETS experiment of Hihath et al. [Nano Lett. 8, 1673(2008] had no thiol H atoms, but that a molecule with a single thiol H atom may have bridged their junction occasionally. We also consider the evolution of the IETS spectrum as a gold STM tip approaches the intact S-H group at the end of a molecule bound at its other end to a second electrode. We predict the frequency of a vibrational mode of the thiol H atom to increase by a factor ∼ 2 as the gap between the tip and molecule narrows. Therefore, IETS should be able to track the approach of the tip towards the thiol group of the molecule and detect the detachment of the thiol H atom from the molecule when it occurs.

Section: Introductionmentioning

confidence: 99%

Inelastic tunneling spectroscopy of gold-thiol and gold-thiolate interfaces in molecular junctions: The role of hydrogen

Demir

The Journal of Chemical Physics

2012

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“…Electronic mail: asaffarz@sfu.ca of fullerene dimers, C 120 in which the two C 60 molecules are connected by a cyclic C 4 unit, is the simplest one and has the lowest energy as well as interesting physical and chemical properties [22], we have chosen this isomer of the C 60 dimer in our calculation. We note that, the electronic transport properties of a pure C 60 dimer [23], N and B doped C 60 dimer [24], and Li@C 60 dimer [25] have been reported theoretically, however, encapsulation of magnetic atoms in fullerene dimers has not been addressed in the previous theories.We consider an endohedral fullerene Co@C 60 dimer contacted via single gold atoms (as in some experimentally realized single-molecule electronic devices [26,27]) by semi-infinite non-magnetic electrodes, as shown in Fig. 1.…”

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confidence: 99%

“…We consider an endohedral fullerene Co@C 60 dimer contacted via single gold atoms (as in some experimentally realized single-molecule electronic devices [26,27]) by semi-infinite non-magnetic electrodes, as shown in Fig. 1.…”

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confidence: 99%

“…For Au atoms in the electrodes we use 6s, 6p x , 6p y , 6p z , 5d x 2 −y 2 , 5d xy , 5d 3z 2 −r 2 , 5d zx , 5d zy atomic valence orbitals. Due to the nine valence orbitals for each Au atom, we model the electrodes, as in previous work [27,[31][32][33][34]43], by nine semi-infinite one-dimensional chains of orthogonal atomic orbitals, with one orbital per site and periodicity a between orbital sites. The one-dimensional chains are decoupled from each other, i.e., there is no hopping between orbitals in different chains.…”

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confidence: 99%

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Voltage-controlled spin injection with an endohedral fullerene Co@C60 dimer

Saffarzadeh

Applied Physics Letters

2013

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Spin-dependent transport through an endohedral fullerene Co@C60 dimer with gold electrodes is explored theoretically using density functional and extended Hückel theory. Density of states spin polarizations up to 95%, due to spin-splitting of Co 3d orbitals, are found by varying the gate and/or bias voltage. The current-voltage characteristics and strong (up to 100%) spin polarization of the current indicate that the device can be utilized for highly efficient spin injection into nonmagnetic conductors. This finding opens the way to the realization of electrostatically tuned spintronic nano devices less than 2 nanometers in size, without ferromagnetic electrodes.Carbon-based nanostructures such as fullerenes, carbon nanotubes (CNTs), and graphene, are promising candidates for spintronic applications because of their weak spin-orbit coupling and hyperfine interaction which lead to long spin coherence lengths [1,2]. In particular, the fullerene C 60 molecule is an interesting carbon nanostructure that can be used as a molecular bridge in magnetic tunnel junctions due to its remarkable structural stability and electronic properties [3,4].One way to generate a spin-polarized charge current is to encapsulate magnetic atoms or magnetic nanowires in fullerenes or CNTs [5][6][7][8][9][10][11][12][13][14][15][16][17][18][19][20]. For example, ab initio calculations showed that Co atoms in CNTs can provide strong spin polarization and considerable magnetic moments [5]. The electrical and magnetic properties of C 60 -Co nanocomposites have also been studied [7,11,12] and tunnel magnetoresistance ratios up to about 30% at low bias voltages were reported, indicating significant spin polarizations [7]. Recent density functional theory (DFT) studies of encapsulated Co atoms in C 60 molecules [18] found hybridization between Co and C 60 orbitals and the most stable structure of Co@C 60 to have the Co atom on top of a hexagonal face. Lu et al. [19] showed that, in the most stable structure of Gd@C 60 , the Gd ion is over a hexagonal ring of the C 60 molecule and the Gd atomic orbitals to hybridize with the C 60 molecular orbitals. Moreover, strong hybridization between the Gd 5d and 6s and carbon orbitals in Gd@C 60 has been observed, both theoretically and experimentally [20].Magnetic atoms encapsulated in carbon nanocages, like the C 60 molecule, are effectively protected against environmental effects such as oxidization, stabilizing the encapsulated magnetic atoms for potential applications for spin injection in nanoscale devices. Accordingly, one can propose magnetic nano junctions to produce high spin-polarized currents with long spin coherence lengths [21]. In this paper we explore theoretically the possibility of such molecular magnetic junctions in which the carbon nanocage is a fullerene dimer. Since, among various types * Author to whom correspondence should be addressed. Electronic mail: asaffarz@sfu.ca of fullerene dimers, C 120 in which the two C 60 molecules are connected by a cyclic C 4 unit, is the simplest one a...

“…Therefore, the spin-valve effect that we predict here does not depend critically on the gold nano-cluster size. The non-magnetic leads are modeled as a small number of semi-infinite one-dimensional ideal channels that represent macroscopic electron reservoirs (as in previous studies of electron and spin transport through single molecules with gold contacts 9,10,[21][22][23] ) to simulate moderately weak coupling between the nano-device and electrodes.The geometries of our nano-devices are shown in Fig. 1(a) and (b).…”

mentioning

confidence: 99%

Switching of a quantum dot spin valve by single molecule magnets

Renani

2013

Phys. Rev. B

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We explore theoretically the spin transport in nanostructures consisting of a gold quantum dot bridging nonmagnetic electrodes and two Mn12-Ph single molecule magnets (SMMs) that are thiol-bonded to the dot but are not in direct contact with the electrodes. We find that reversal of the magnetic moment of either SMM by the application of a magnetic field leads to a large change in the resistance of the dot, i.e., a strong spin valve effect. We show that this phenomenon arises from a novel physical principle: The spin-dependent molecular orbitals that extend over the dot and both SMMs change drastically when the magnetic moment of either SMM is reversed, resulting in a large change in the conduction via those orbitals. The same physics may also be responsible for the spin valve phenomena discovered recently in carbon nanotube quantum dots with rare earth SMMs by Urdampilleta, Klyatskaya, Cleuziou, Ruben and Wernsdorfer [Nature Mater. 10, 502506 (2011)]. PACS numbers: 85.75.Bb,75.50.Xx, 85.35.Be Molecular spintronics 1 combines two active fields of study: molecular electronics and spintronics. The ultimate goal of this field is to control the electronic spin and charge on the molecular scale where quantum effects emerge. In the search for new molecular spintronic nano devices, single molecule magnets (SMMs) 2 appear to be natural candidates. 3 A SMM is a molecular-size nano-magnet that exhibits quantum behaviour such as quantum tunneling of the magnetization (QTM) 2 , Berry phase interference 4 , the Kondo effect 5 , and magnetoresistance phenomena. 3,6 Spin valve-like effects have also been predicted in SMMs with magnetic electrodes. 7,8 In a recent advance, experimental detection of spin valve-like behavior has been reported 6 in a system with non-magnetic electrodes and SMMs coupled to a (non-magnetic) carbon nanotube quantum dot. It was suggested 6 that this behavior was due to the magnetic moments of individual SMMs bound to the dot reversing at different values of the applied magnetic field as the field was swept. It was proposed 6 that this would result in changes in the resistance of the dot due to modulation of the spin transport through the dot, i.e., a spin valve effect. However, to our knowledge, no relevant quantitative theory has as yet been reported. Thus the physics responsible for the observed novel behavior 6 has not been definitively identified.Here we explore spin valve phenomena in devices with a pair of SMMs bound to a non-magnetic quantum dot bridging non-magnetic electrodes theoretically for the first time. Our results reveal that spin valve functionality in such devices can arise from current carrying electronic resonant states that extend over the whole device, including the quantum dot and both SMMs that are bound to it. When the magnetic moment of one of the SMMs is reversed, these states, being spin-dependent, are strongly modified. Consequently the resistance of the device changes. Thus the system exhibits a spin valve effect. Thus our findings demonstrate a novel principle for spin...