Structure and electronic properties of CH3- and CF3-terminated alkanethiol monolayers on Au(): a scanning tunneling microscopy, surface X-ray and helium scattering study

Pflaum, Jens; Bracco, G.; Schreiber, Frank; Colorado, Ramon; Shmakova, Olga E.; Lee, T. R.; Scoles, G.; Kahn, Antoine

doi:10.1016/s0039-6028(01)01495-9

Cited by 89 publications

(172 citation statements)

References 30 publications

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“…The dipole at the PTCDA/Ag interface corresponds to electron transfer from the metal to interface molecules, which gives rise to the filled gap states at −0.6 and −1.8 eV on the 4Å UPS spectrum. The broad feature around −0.6 eV on the 4Å PTCDA/Au STS spectrum corresponds to the Au surface state, which is not eliminated by the deposition of organic molecules [9].…”

Section: Transport Gapmentioning

confidence: 98%

Electronic polarization at surfaces and thin films of organic molecular crystals: PTCDA

Tsiper

Soos

Gao

et al. 2002

Chemical Physics Letters

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268

166

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The electronic polarization energies, P = P+ + P−, of a PTCDA (perylenetetracarboxylic acid dianhydride) cation and anion in a crystalline thin film on a metallic substrate are computed and compared with measurements of the PTCDA transport gap on gold and silver. Both experiments and theory show that P is 500 meV larger in a PTCDA monolayer than in 50Å films. Electronic polarization in systems with surfaces and interfaces are obtained self-consistently in terms of charge redistribution within molecules. I. TRANSPORT GAPThe electronic structure of organic molecular crystals is strikingly different from the conventional inorganic semiconductors, such as Si, in that the electronic polarization of the dielectric medium by charge carriers constitutes a major effect, with energy scale greater than transfer integrals or temperature [1,2]. The transport gap E t for creating a separated electron-hole pair has a substantial (1-2 eV) polarization energy contribution [3] and exceeds the optical gap by ∼ 1 eV. Limited overlap rationalizes the modest mobilities of organic molecular solids. Devices such as light-emitting diodes, thin film transistors, or photovoltaic cells require charge transport and are consequently based on thin films, quite often deposited on metallic substrates [4,5]. Organic electronics relies heavily on controlling films with monolayer precision, on forming structures with several thin films, and on characterizing the interfaces. The positions of transport states and mechanisms for charge injection are among the outstanding issues for exploiting organic devices. We focus here on the electronic polarization of crystalline thin films near surfaces and interfaces. We find that electronic polarization, and hence E t , in a prototypical organic crystal is significantly different at a free surface, near a metal-organic interface, in thin organic layers, and in the bulk.Weak intermolecular forces characterize organic molecular crystals, whose electronic and vibrational spectra are readily related to gas-phase transitions [1,2]. Due to small transfer integrals, charge carriers are molecular ions embedded in the lattice of neutral molecules. The transport gap E t in the crystal is derived from the charge gap for electron transfer in the gas phase, I(g) − A(g), which is the difference between the ionization potential and the electron affinity. But crystals have electrostatic interactions even in the limit of no overlap, and charge carriers are surrounded by self-consistent polarization clouds. In contrast to polaronic effects, electronic polarization is instantaneous and directly affects the positions of energy levels. Formation of polarization clouds is associated with stabilization energy P + for cations (the "holes") and P − for anions (the "electrons"). The combination P = P + + P − occurs in E t = I(g) − A(g) − P . Since Coulomb interactions are long-ranged, polarization clouds extend over many lattice constants and P depends on the proximity to surfaces and interfaces.

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Section: Transport Gapmentioning

confidence: 98%

Electronic polarization at surfaces and thin films of organic molecular crystals: PTCDA

Tsiper

Soos

Gao

et al. 2002

Chemical Physics Letters

Self Cite

268

166

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“…The configuration for the CH 3 -terminated case is similar and it is not displayed here. In the experiments from Pflaum et al 19 CH 3 -terminated decanethiols are tilted at an angle of ϳ32°from the vertical direction, while for CF 3 -terminated the tilting angle is ϳ39°. 19 Our simulation cells however have the molecules placed perpendicular to the surface.…”

Section: B Transport Properties Of the Molecules On Gold Surfacesmentioning

confidence: 97%

“…The molecules used in the experiments of Pflaum et al 19 are both methyl-terminated and fluorine-terminated alkanethiols. A range of different alkane chain lengths was used in that study, but most of the data were collected for decanethiols ͓see Figs.…”

Section: A Electronic Structure Of the Moleculesmentioning

confidence: 99%

“…In contrast the TH approach necessitates only either a single electronic structure calculation or two electronic structure calculations plus a convolution integral if the structure of the tip is considered. 18 An example of experiments where the tip-to-molecule interactions are believed to be important is that performed by Pflaum et al 19 in which a monolayer of alkanethiol molecules is deposited on a gold surface, and the transport properties are then probed by using a STM setup. The obtained zero-bias conductance is low, being on the order of 10 −7 G 0 at zero bias ͑the quantum conductance G 0 is defined as 2e 2 / h, with e as the electron charge and h as the Planck constant͒.…”

Section: Introductionmentioning

confidence: 99%

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Simulating STM transport in alkanes from first principles

2009

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Simulations of scanning tunneling microscopy ͑STM͒ measurements for molecules on surfaces are traditionally based on a perturbative approach, most typically employing the Tersoff-Hamann method. This assumes that the STM tip is far from the sample so that the two do not interact with each other. However, when the tip gets close to the molecule to perform measurements, its electrostatic interplay with the substrate may generate nontrivial potential distribution, charge transfer, and forces, all of which may alter the electronic and physical structure of the molecule. These effects are investigated with the ab initio quantum transport code SMEAGOL, combining the nonequilibrium Green's-function formalism with density functional theory. In particular, we investigate alkanethiol molecules terminated with either CH 3 or CF 3 end-groups on gold surfaces for which recent experimental data are available. We discuss the effects connected to the interaction between the STM tip and the molecule, as well as the asymmetric charge transfer between the molecule and the electrodes.

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“…The two phases are nearly isoenergetic, † In the literature the 3 × 2 √ 3 structure is sometimes denoted as the c(4 × 2) structure. and indeed scanning tunneling microscopy studies have found them to coexist in separate spatial domains [110]. A persisting controversy surrounds the precise adsorption site of the sulfur headgroup on the Au(111) surface.…”

Section: Sam Induced Surface Reconstructionsmentioning

confidence: 99%

Of interfaces and spin transport across nanoscale layers

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Structure and electronic properties of CH3- and CF3-terminated alkanethiol monolayers on Au(): a scanning tunneling microscopy, surface X-ray and helium scattering study

Cited by 89 publications

References 30 publications

Electronic polarization at surfaces and thin films of organic molecular crystals: PTCDA

Electronic polarization at surfaces and thin films of organic molecular crystals: PTCDA

Simulating STM transport in alkanes from first principles

Of interfaces and spin transport across nanoscale layers

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