Filled and empty states of alkanethiol monolayer on Au (111): Fermi level asymmetry and implications for electron transport

Qi, Yabing; Yaffe, Omer; Tirosh, Einat; Vilan, Ayelet; Cahen, David; Kahn, Antoine

doi:10.1016/j.cplett.2011.06.050

Cited by 48 publications

(72 citation statements)

References 39 publications

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“…In some cases, the sign of the majority carrier can be assessed experimentally (25,26). Often, the frontier orbital energies are estimated from experimental data or theoretical calculations, and the Fermi level of the contacts are taken from literature or experimental work function data (e.g., UPS, Kelvin probe).…”

Section: Resultsmentioning

confidence: 99%

“…Electronic structure measurements via UPS in ultrahigh vacuum have been used to correlate molecular junction conductance with energy level alignment (4,26). When substrates are modified, the observed UPS work function is sensitive to a local vacuum level shift induced by the (new) chemical composition of the surface including any dipoles present (11,28).…”

Section: Resultsmentioning

confidence: 99%

See 1 more Smart Citation

Charge transport in molecular electronic junctions: Compression of the molecular tunnel barrier in the strong coupling regime

Sayed

Fereiro²,

Yan³

et al. 2012

Proc. Natl. Acad. Sci. U.S.A.

142

253

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Molecular junctions are essentially modified electrodes familiar to electrochemists where the electrolyte is replaced by a conducting "contact." It is generally hypothesized that changing molecular structure will alter system energy levels leading to a change in the transport barrier. Here, we show the conductance of seven different aromatic molecules covalently bonded to carbon implies a modest range (<0.5 eV) in the observed transport barrier despite widely different free molecule HOMO energies (>2 eV range). These results are explained by considering the effect of bonding the molecule to the substrate. Upon bonding, electronic inductive effects modulate the energy levels of the system resulting in compression of the tunneling barrier. Modification of the molecule with donating or withdrawing groups modulate the molecular orbital energies and the contact energy level resulting in a leveling effect that compresses the tunneling barrier into a range much smaller than expected. Whereas the value of the tunneling barrier can be varied by using a different class of molecules (alkanes), using only aromatic structures results in a similar equilibrium value for the tunnel barrier for different structures resulting from partial charge transfer between the molecular layer and the substrate. Thus, the system does not obey the Schottky-Mott limit, and the interaction between the molecular layer and the substrate acts to influence the energy level alignment. These results indicate that the entire system must be considered to determine the impact of a variety of electronic factors that act to determine the tunnel barrier.energy alignment | molecular electronics | electronic coupling | charge transport | Fermi-level pinning T he conductance of electrical charge through and across molecular entities is the basis of molecular and organic electronics (1, 2). Understanding, controlling, and designing electronic circuits using organic molecules as components is a major goal of molecular electronics (3); however, it has been a challenge to identify all of the factors that govern the conductance of a molecular junction. Rather than being a simple property of the molecule itself, many circumstances contribute to the measured electronic properties of the junction. Some of the important features include the nature of the molecule-contact bonding (4), the properties of the contact materials (5, 6), the orientation of the molecules relative to the contacts (7), and the structure of the molecule (5,8,9). Although there is no general consensus on exactly how each of these features affects the conductance of the junction, it is generally agreed that the alignment of the molecular and contact energy levels is an important factor (10-13). The offset between the substrate Fermi energy (E f ) and the molecular orbital closest in energy to E f is often used to estimate charge transport barriers in the context of tunneling or charge injection models; however, it is increasingly clear that the situation is complex and that there is no simple meth...

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Section: Resultsmentioning

confidence: 99%

Section: Resultsmentioning

confidence: 99%

Charge transport in molecular electronic junctions: Compression of the molecular tunnel barrier in the strong coupling regime

Sayed

Fereiro²,

Yan³

et al. 2012

Proc. Natl. Acad. Sci. U.S.A.

142

253

View full text Add to dashboard Cite

show abstract

“…Photoemission spectroscopy measurements of alkanethiol monolayers, with = 18, on Au have determined the HOMO-LUMO gap to E H−L = 7.85 eV. 24 We assume that this corresponds to the HOMO-LUMO gap of the backbone of the chain. Weak signatures of what could be binding group and contact states contributed to the photoemission in the gap.…”

Section: Explaining the Experimental Trendsmentioning

confidence: 99%

Understanding the length dependence of molecular junction thermopower

Karlström

Strange

Solomon

2014

The Journal of Chemical Physics

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Thermopower of molecular junctions is sensitive to details in the junction and may increase, decrease, or saturate with increasing chain length, depending on the system. Using McConnell's theory for exponentially suppressed transport together with a simple and easily interpretable tight binding model, we show how these different behaviors depend on the molecular backbone and its binding to the contacts. We distinguish between resonances from binding groups or undercoordinated electrode atoms, and those from the periodic backbone. It is demonstrated that while the former gives a length-independent contribution to the thermopower, possibly changing its sign, the latter determines its length dependence. This means that the question of which orbitals from the periodic chain that dominate the transport should not be inferred from the sign of the thermopower but from its length dependence. We find that the same molecular backbone can, in principle, show four qualitatively different thermopower trends depending on the binding group: It can be positive or negative for short chains, and it can either increase or decrease with length.

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“…33,36,37 While these methods provide information about junction structure and carrier sign, they do not directly probe system energy levels. Other measurements that do provide knowledge regarding energy levels (e.g., ultraviolet photoelectron spectroscopy 38,39 ) are not readily carried out on completed devices.…”

Section: ■ Introductionmentioning

confidence: 99%

Internal Photoemission in Molecular Junctions: Parameters for Interfacial Barrier Determinations

et al. 2015

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Filled and empty states of alkanethiol monolayer on Au (111): Fermi level asymmetry and implications for electron transport

Cited by 48 publications

References 39 publications

Charge transport in molecular electronic junctions: Compression of the molecular tunnel barrier in the strong coupling regime

Charge transport in molecular electronic junctions: Compression of the molecular tunnel barrier in the strong coupling regime

Understanding the length dependence of molecular junction thermopower

Internal Photoemission in Molecular Junctions: Parameters for Interfacial Barrier Determinations

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