Corrected electrostatic model for dipoles adsorbed on a metal surface

Maschhoff, Brian L.; Cowin, James P.

doi:10.1063/1.468241

Cited by 85 publications

(84 citation statements)

References 25 publications

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“…Similar adsorption behavior as seen in Figure 2 was previously discussed for CD 3 Cl. 8 On the basis of the Helmholtz equation (4Φ ) 4Πn i µ, where n i is the number of EC molecules in each of the first three layers (i ) 1, 2, 3) that orient chlorine down (n 1 , n 3 ) or ethyl down (n 2 ) and µ is the (fixed) dipole moment of an adsorbed EC molecule) one can extract the fraction of a monolayer of EC molecules that orient in either orientation.…”

Section: Work Function Change Measurements (∆φ)supporting

confidence: 85%

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Interaction of Ethyl Chloride with Amorphous Solid Water Thin Film on Ru(001) and O/Ru(001) Surfaces

Ayoub

Asscher

2009

J. Phys. Chem. A

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The adsorption of ethyl chloride (EC) on clean and oxygen covered Ru(001) surfaces and its interaction with coadsorbed amorphous solid water (ASW) is reported based on temperature-programmed desorption (TPD) and work function change (4Φ) measurements. Adsorption of EC is characterized by a decrease of 2.1 eV in work function at monolayer coverage. Flipped adsorption geometry on average (ethyl facing the surface) is found in the second layer with a 0.3 eV increase in the work function. On ruthenium substrates the orientation of EC molecules could be controlled chlorine down or up by varying the oxygen coverage, as indicated by work function change measurements. ∆P-TPD from clean Ru(001) surface reveals a complete dissociation of the EC molecules on the clean Ru(001) surface at coverages up to 0.1 ( 0.05 ML. At higher coverage it leads to two distinct TPD peaks: at T )175 and 120-130 K, for the monolayer and multilayer coverage, respectively. Compression of preadsorbed 0.3 ML EC molecules into small islands on the surface occurs when ASW layers at coverage in the range 0.5-6 bilayers (BL) are adsorbed on top of the EC covered surface, associated with a shift in peak desorption from 190 K down to 125 K. A Caging process of EC molecules within the layers of ASW takes place by adsorbing more than 6 BL until a fully caged system has developed above 20 ASW BL. A reversed TPD peak shift of the trapped EC molecules occurs from its compressed phase at 125 K up to 165 K, the onset for ASW transition to crystalline ice and desorption. The detachment process leads to partial flipping over the geometry of the EC at the second layer, as determined by 4Φ measurement. The distance of the lifted and caged EC molecules from the Ru surface has been determined to reside 1.0 ( 0.2 nm above the solid surface, based on the thermal dissociation and hydrogen uptake measurements of sandwiched EC molecules between a variable thickness ASW film and a fixed 10 BL layer, on top of clean Ru(001). We conclude that the caged EC molecules are trapped in a micelle-like geometry with the ethyl groups pointing inward. This conclusion is based on photodecomposition studies of the caged EC molecules at 193 nm, as a function of EC initial coverage.

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Section: Work Function Change Measurements (∆φ)supporting

confidence: 85%

“…[1][2][3][4] This is partially due to their importance in the pesticide industry as well as their unique photochemistry and its damaging impact on the ozone layer. 5 The first experimental demonstration of a molecular cage in amorphous solid water (ASW) was that of N 2 6 more than a decade ago.…”

Section: Introductionmentioning

confidence: 99%

Interaction of Ethyl Chloride with Amorphous Solid Water Thin Film on Ru(001) and O/Ru(001) Surfaces

Ayoub

Asscher

2009

J. Phys. Chem. A

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“…[ 19 ] To understand the driving force of the work function modifi cation, attraction by image charges in the substrate, the electrostatics of the system, the substrate and the WML should be taken into account. [ 38 ] When the WML is physisorbed on an insulator, the only electrostatic interaction is that between the charged constituents of the molecules of the WML itself. The small difference between the relative dielectric constants of the WML and the insulator results only in minor image force interaction.…”

Section: Proposed Mechanismmentioning

confidence: 99%

Origin of Work Function Modification by Ionic and Amine‐Based Interface Layers

Reenen

Kouijzer

Janssen

et al. 2014

Adv Materials Inter

130

138

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can evolve into a successful thin fi lm photovoltaic technology. To this end, further improvements in the sunlight-toelectricity conversion effi ciency are still needed. To do so, not only the optoelectronic, active layer should be considered, but also the connection between this layer and the electrodes. This connection is facilitated by so-called work function modifi cation layers (WMLs) that serve to align the transport levels in the organic semiconductor and the conductor by modifi cation of the work function. Materials that are often used are poly(3,4-ethylenedioxythiophene) poly(styrenesulfonate) (PEDOT:PSS) for improving hole collection and injection in combination with an indium tin oxide (ITO) electrode and LiF for the electron contact in combination with an Al metal electrode. A new generation of WMLs fi rst introduced by Cao et al. is based on polyelectrolytes or tertiary aliphatic amines, such as poly [(9,9-bis(3′-(N,N-dimethylamino)propyl)-2,7-fluorene)-alt -2,7-(9,9-dioctylfluorene)] (PFN) and the corresponding ethyl ammonium bromide. [ 1 ] Although these materials improve carrier injection or extraction in different organic diode devices, [1][2][3][4][5][6][7] the mechanism behind this improvement is not completely clear. This ambiguity hinders design, selection and optimization of new WMLs.Mechanisms that are generally considered to result in electric fi elds and concomitant work function shifts at metal-organic interfaces are doping, charge transfer, and dipole formation. [8][9][10] Regarding the work function modifi cation of amine side-groups, Lindell et al. [ 11 ] found by photoelectron spectroscopy and density functional calculations that the electron donor para-phenylenediamine chemisorbs onto atomically clean Ni in vacuum. [ 12 ] Due to partial electron transfer from the amine unit, a work function reduction of 1.55 eV is achieved, resulting in a less deep Fermi level. This explanation is not likely to hold for the technologically more relevant procedure on which we shall focus, being the deposition of WMLs from solution on a metal at atmospheric pressure: the metal is not atomically clean, which likely inhibits chemisorption. In other work by Zhou et al. [ 13 ] it is shown that the work function reduction, at atmospheric pressure, by the amine groups in poly(ethylenimine ethoxylated) (PEIE), is generally the same on different conductors. Work function modifi cation by polyelectrolytes and tertiary aliphatic amines is found to be due to the formation of a net dipole at the electrode interface, induced by interaction with its own image dipole in the electrode. In polyelectrolytes differences in size and side groups between the moving ions lead to differences in approach distance towards the surface. These differences determine magnitude and direction of the resulting dipole. In tertiary aliphatic amines the lone pairs of electrons are anticipated to shift towards their image when close to the interface rather than the nitrogen nuclei, which are sterically hindered by the alkyl side chains. ...

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“…5 However, we note that the effective polarizability α of a single impurity may be significantly increased by the presence of a nearby conducting surface. 41 In Fig. 4 we consider a structure that consists of a dielectric material of finite thickness L (we choose HfO 2 with ǫ 1 = 22) and a semi-infinite layer of SiO 2 (either H → ∞ with ǫ 2 = 3.9, or H = 0 with ǫ 3 = 3.9) with graphene placed right on their boundary at z g = 0.…”

Section: Resultsmentioning

confidence: 99%

“…Assuming n imp to be small enough, we may neglect mutual depolarization among the impurities and sim- ply write E ⊥ = 4πen/ǫ 1 , with E ⊥ being positive (negative) for electron (hole) doping of graphene. 41 The two samples were fitted in Ref. 5 by assuming that the impurities reside in graphene (d = 0) and are uncorrelated, and the optimal linear symmetric fits were found with n imp = 2.2 × 10 11 cm −2 for K17 and with n imp = 4×10 11 cm −2 for K12.…”

Section: Resultsmentioning

confidence: 99%

Effects of the structure of charged impurities and dielectric environment on conductivity of graphene

Aničić

Mišković

2013

Phys. Rev. B

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We investigate the conductivity of doped single-layer graphene in the semiclassical Boltzmann limit, as well as the conductivity minimum in neutral graphene within the self-consistent transport theory, pointing up the effects due to both the structure of charged impurities near graphene and the structure of the surrounding dielectrics. Using the hard-disk model for a two-dimensional (2D) distribution of impurities allows us to investigate structures with large packing fractions, which are shown to give rise to both strong increase in the slope of conductivity at low charge carrier densities in graphene and a strongly sub-linear behavior of the conductivity at high charge carrier densities when the correlation distance between the impurities is large. On the other hand, we find that a super-linear dependence of the conductivity on charge carrier density in heavily doped graphene may arise from increasing the distance of impurities from graphene or allowing their clustering into disk-like islands, whereas the existence of an electric dipole polarizability of impurities may give rise to an electron-hole asymmetry in the conductivity. Using the electrostatic Green's function for a three-layer structure of dielectrics, we show that finite thickness of a dielectric layer in the top gating configuration, as well as the existence of non-zero air gap(s) between graphene and the nearby dielectric(s) exert strong influences on the conductivity and its minimum. While a decrease in the dielectric thickness is shown to increase the conductivity in doped graphene and even gives rise to finite conductivity in neutral graphene for a 2D distribution of impurities, we find that an increase in the dielectric thickness gives rise to a super-linear behavior of the conductivity when impurities are homogeneously distributed throughout the dielectric. Moreover, the dependence of graphene's mobility on its charge carrier density is surprisingly strongly affected, quantitatively and qualitatively, by the graphene-dielectric gap(s) when combined with the precise position of a 2D distribution of charged impurities. Finally, we show that the conductivity minimum in neutral graphene is increased by increasing the correlation distance between the impurities, reduced by increasing the graphene-dielectric gap, and increased by decreasing the dielectric thickness in a top-gated configuration, even though the corresponding residual charge carrier density is reduced by decreasing the dielectric thickness.

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Corrected electrostatic model for dipoles adsorbed on a metal surface

Cited by 85 publications

References 25 publications

Interaction of Ethyl Chloride with Amorphous Solid Water Thin Film on Ru(001) and O/Ru(001) Surfaces

Interaction of Ethyl Chloride with Amorphous Solid Water Thin Film on Ru(001) and O/Ru(001) Surfaces

Origin of Work Function Modification by Ionic and Amine‐Based Interface Layers

Effects of the structure of charged impurities and dielectric environment on conductivity of graphene

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