Work Function Data Research Articles

Band gap opening of metallic single-walled carbon nanotubes via noncovalent symmetry breaking

Covalent bonding interactions determine the energy-momentum (E-k) dispersion (band structure) of solid-state materials. Here, we show that noncovalent interactions can modulate the E-k dispersion near the Fermi level of a low-dimensional nanoscale conductor. We demonstrate that low energy band gaps may be opened in metallic carbon nanotubes through polymer wrapping of the nanotube surface at fixed helical periodicity. Electronic spectral, chiro-optic, potentiometric, electronic device, and work function data corroborate that the magnitude of band gap opening depends on the nature of the polymer electronic structure. Polymer dewrapping reverses the conducting-to-semiconducting phase transition, restoring the native metallic carbon nanotube electronic structure. These results address a long-standing challenge to develop carbon nanotube electronic structures that are not realized through disruption of π conjugation, and establish a roadmap for designing and tuning specialized semiconductors that feature band gaps on the order of a few hundred meV.

An extensive theoretical quantification of secondary electron emission from silicon

Though intensive experimental studies have been carried out on electron emission properties in past decades, the reliable data from accurate experimental measurements for clean and smooth surfaces are still quite limited. In this work, we have performed a comprehensive Monte Carlo simulation of electron emission yields, the secondary electron yield (SEY, δ), backscattered electron coefficient (BSC, η) and total electron yield (TEY, σ=δ+η), from silicon and the uncertainty quantification of the theoretical calculation results for the first time. The simulations were made on total 17280 scattering models. The considered uncertainty factors in the physical modelling that can influence the calculated yields include work function data, optical energy loss function data, dielectric function models for electron inelastic scattering and the scattering potentials for electron elastic scattering. The calculation results show that δ is significantly influenced by the work function while the dielectric function model has a slight effect. Elastic scattering potential (or elastic scattering cross section) and energy loss function affects to a certain extent both δ and η. Our simulated η data agree with most of the experimental measurements while the simulated mean δ data are well below most of the experimental data. At a 75% level of confidence, the experimental σ data measured by Goto et al. lie within the uncertainty range of our simulation results. This work has provided sufficient ground for the necessity of building a theoretical database of electron emission yield considering the fact that most of the previous experimental data are not reliable due to the presence of surface contamination during the measurements.

Effective Work Functions of the Elements

Effective Work Functions of the Elements

Density Functional Theory Study of Influence of Oxide Thickness and Surface Alloying on Cl Migration within α-Al2O3

Insertion and migration of chlorine atoms (Cl) in α-Al2O3 were studied by density functional theory calculations focusing on the influence of oxide thickness and doping by Mg, Cu and Si. Work function, electronic band gap, and insertion energy were calculated to explore thermodynamics of Cl migration. Partial density of state (PDOS) calculations revealed the role of dopants in the electronic character of metal–O and Al–Cl bonds. Work function data showed the effect of Cl insertion into the oxide film on the corrosion resistance. Cl can locate at an O vacancy () with a large exothermic insertion energy, independent of oxide thickness, but can only locate at superficial Al vacancy () exothermically. The energy barrier for Cl migration via neighboring increases with oxide thickness, and is 2 ∼2.5 eV for thicker oxides. Cl insertion causes a work function reduction exceeding 2 eV, implying a decreased corrosion resistance. The inhibition of Cl migration by Si-doping can be explained by a more intense hybridization peak of Si–O over Al–O in the PDOS profile, while the reduction of energy barrier by Mg–/Cu-doping probably is due to the deviation of metal-s state from the Fermi energy, thus facilitating Cl movement within the oxide.

Self-powered and temperature-tunable infrared-visible photodetector based on a VO2/Si heterojunction

In this work, a type-III heterojunction based on a pulsed-laser-deposited vanadium dioxide (VO2) and p-type silicon (p-Si) substrate is realized. The device shows a large self-powered and room-temperature photoresponse to IR (950 nm), green (515 nm) and blue (456 nm) LEDs. A short-circuit current (I sc) of ∼3 µA and an open-circuit voltage (V oc) of ∼−120 mV are observed under IR LED illumination. The work function data in literature along with the sign of V oc measurement is used to sketch the energy band diagram of the heterojunction. The temperature-dependent I sc properties of the junction, contrary to conventional photodetectors, show an initial rise and then a sharp transition from maximum (3.5 μA) to almost zero near 337 K, corresponding to a metal–insulator phase transition, paving the way for photodetectors with temperature-tunable photoresponsivity.

Surface functionalization of coal and quartz with aniline: A study on work function and frictional charge

The present study deals with the surface functionalization of coal and quartz using aniline with an intention to achieve a higher differential charge facilitating their separation in an electric field. The Fourier Transform Infrared (FT-IR) spectroscopic studies confirm the adsorption of aniline on the coal and quartz surface after chemical conditioning. The surface work function of both coal and quartz are found to decrease after being treated with aniline. However, the work function of coal decreases to a greater extent (3.464%) in comparison to quartz which decreases by 1.016%. Thereby, the net work function difference between the coal and quartz particles increases by 129% (from 0.089 eV to 0.204 eV) after chemical treatment. The work function of the copper tribo-charging medium falls between the work function values of both the original and the aniline treated coal and quartz samples. Therefore, the charge density of the treated coal particles increases on contact with copper while there is a minor reduction in charge of the treated quartz. The charge acquisition data corroborates well with the surface work function data. Density Functional Theory (DFT) based simulation has been carried out to validate the trend of the decrease in work function after the treatment with aniline. The DFT calculations are found to be in good agreement with the experimentally measured change in the surface work function data of both coal and quartz sample after chemical treatment.

Evaluation of multi-layered graphene nano-platelet composite coatings for corrosion control part II – Cathodic delamination kinetics

In-situ Scanning Kelvin probe (SKP) measurements are used to follow the corrosion-driven cathodic delamination kinetics of model coatings comprising graphene nano-platelets (GNP) dispersed in polyvinylbutyral (PVB) adherent to iron and zinc (galvanised steel). To reduce delamination rates by >90% (relative to unpigmented PVB) a GNP volume fraction of 0.056 is required on iron but only 0.028 on zinc. On this basis, together with work function and O2 permeability data it is proposed that the GNP acts principally to slow through-coating oxygen transport on iron; whereas on zinc a galvanic couple forms between zinc and GNP, displacing cathodic oxygen reduction.

Mixed Aliphatic Self-Assembled Monolayers with Embedded Polar Group

We studied structure, molecular orientation, morphology, and electrostatic properties of mixed self-assembled monolayers (SAMs) comprised of alkanethiolates (ATs) which are modified by a dipolar ester group embedded into the alkyl backbone at two different orientations, namely, with the dipole directed upward and downward from the substrate. The packing density and molecular orientation in these SAMs were found to be similar to those of the “parent” single-component monolayers. Applying X-ray photoelectron spectroscopy (XPS) as a morphology tool, we could estimate that the mixed SAMs represent homogeneous intermolecular mixtures of both components, down to the molecular level, excluding existence of “hot spots” for charge injection. The analysis of the C 1s XPS spectra and the work function data suggests that the composition of the mixed SAMs fully mimicked the mixing ratio of both components in the solutions from which these SAMs were prepared, which suggests a minor role of the dipole–dipole interaction...

Surface Energy and Work Function Control of AlOx/Al Surfaces by Fluorinated Benzylphosphonic Acids.

The performance of organic electronic devices can be significantly improved by modifying metal electrodes with organic monolayers, which alter the physical and chemical nature of the interface between conductor and semiconductor. In this paper we examine a series of 12 phosphonic acid compounds deposited on the native oxide layer of aluminum (AlOx/Al), an electrode material with widespread applications in organic electronics. This series includes dodecylphosphonic acid as a reference and 11 benzylphosphonic acids, seven of which are fluorinated, including five newly synthesized derivatives. The monolayers are experimentally characterized by contact angle goniometry and by X-ray photoemission spectroscopy (XPS), and work function data obtained by low-intensity XPS are correlated with molecular dipoles obtained from DFT calculations. We find that monolayers are formed with molecular areas ranging from 17.7 to 42.9 Å(2)/molecule, and, by the choice of appropriate terminal groups, the surface energy can be tuned from 23.5 mJ/m(2) to 70.5 mJ/m(2). Depending on the number and position of fluorine substituents on the aromatic rings, a variation in the work function of AlOx/Al substrates over a range of 0.91 eV is achieved, and a renormalization procedure based on molecular density yields a surprising agreement of work function changes with interface dipoles as expected from Helmholtz' equation. The ability to adjust energetics and adhesion at organic semiconductor/AlOx interfaces has immediate applications in devices such as OLEDs, OTFTs, organic solar cells, and printed organic circuits.

Characterization of the interfaces between Au(hkl) single crystal basal plane electrodes and [Emmim][Tf2N] ionic liquid

The interface between Au(hkl) basal planes and the ionic liquid 1-Ethyl-2,3-dimethyl imidazolium bis(trifluoromethyl)sulfonil imide was investigated by using both cyclic voltammetry and laser-induced temperature jump. Cyclic voltammetry showed characteristic features, revealing surface sensitive processes at the interfaces Au(hkl)/[Emmim][Tf2N]. From laser-induced heating the potential of maximum entropy (pme) is determined. Pme is close to the potential of zero charge (pzc) and, therefore, the technique provides relevant interfacial information. The following order for the pme values has been found: Au(111)>Au(100)>Au(110). This order correlates well with work function data and values of pzc in aqueous solutions.

Modified Hydroxyapatite Structure and Properties: Modeling and Synchrotron Data Analysis of Modified Hydroxyapatite Structure

First principle modeling and calculations of hydroxyapatite both native and surface modified and having various defects (OH vacancies, H inter-nodes) were performed. Local Density Approximation method used with calculations of Density of States allows us to analyze the experimental obtained work function data. Molecular modeling was confirmed by photo-electron measurements up to 6.5 eV and photoluminescence experimental data from synchrotron DESY up to 30 eV. Brief analysis of the influence of heating, microwave radiation, hydrogenation, and synchrotron radiation on hydroxyapatite surface is presented in this work. New data on the structure of modified hydroxyapatite are obtained.

Interface dipoles of organic molecules on Ag(111) in hybrid density-functional theory

We investigate the molecular acceptors 3,4,9,10-perylene-tetracarboxylic acid dianhydride (PTCDA), 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F4TCNQ) and 4,5,9,10-pyrenetetraone (PYTON) on Ag(111) using density-functional theory (DFT). For two groups of the Heyd–Scuseria–Ernzerhof (HSE(α, ω)) family of exchange-correlation functionals (ω = 0 and 0.2 Å) we study the isolated components as well as the combined systems as a function of the amount of exact-exchange (α). We find that hybrid functionals favour electron transfer to the adsorbate. Comparing with experimental work function data, for α ≈ 0.25 we report a notable but small improvement over (semi) local functionals for the interface dipole. Although Kohn–Sham eigenvalues are only approximate representations of ionization energies, incidentally, at this value also the density of states agrees well with the photoelectron spectra. However, increasing α to values for which the energy of the lowest unoccupied molecular orbital matches the experimental electron affinity in the gas phase worsens both the interface dipole and the density of states. Our results imply that semi-local DFT calculations may often be adequate for conjugated organic molecules on metal surfaces and that the much more computationally demanding hybrid functionals yield only small improvements.

Interactions between glycine and amorphous solid water nanoscale films

The interactions of glycine (Gly) with amorphous solid water (ASW) nanolayers (≤100ML), vapor-deposited on single crystalline AlOx surfaces at 100K, have been investigated by near-edge X-ray absorption fine structure spectroscopy (NEXAFS) at the oxygen K-edge, temperature-programmed thermal desorption (TPD), X-ray photoelectron spectroscopy (XPS), and temperature-dependent work function measurements. Gly-on-ASW, ASW-on-Gly, and Gly on top of ASW-on-Gly ultrathin films have been fabricated. In contrast to the uniform ASW films grown directly on the hydrophilic AlOx, water molecules adsorb on the hydrophobic Gly films in the form of 3D ASW clusters. This leads to significant differences in the NEXAFS and work function data obtained from ASW-on-AlOx and ASW-on-Gly films, respectively. Furthermore, these structural differences influence the chemical state of Gly molecules (neutral vs. zwitterionic) adsorbed on top of ASW films. N1s XPS measurements revealed an increased amount of neutral Gly molecules in the film top-deposited on the ASW-on-Gly structure in comparison to the neutral Gly in the films directly condensed on AlOx or grown on the ASW substrate. H2O TPD spectra demonstrate that the crystallization and desorption processes of ASW are affected in a different way by the Gly layers, top-deposited on to ASW-on-AlOx and ASW-on-Gly films. At the same time, Gly adlayers sink into the ASW film during crystallization/desorption of the latter and land softly on the alumina surface in the form of zwitterionic clusters.

Reactivity of Rutile with Oxygen and Hydrogen and Related Charge Transfer

The present work reports the effect of the gas phase including oxygen and an argon–hydrogen mixture on the surface electrical properties of TiO2 (rutile) at 1073 and 298 K. The effect of the gas-phase composition on the electrical properties was monitored using work function measurements. It was found that oxidation of rutile at 1073 and 298 K in oxygen results in oxygen incorporation and oxygen chemisorption, respectively. In both cases, oxidation leads to a work function increase. On the other hand, exposure of rutile at 298 K to a gas phase involving argon and hydrogen results in the formation of the H4TiO4 surface structure, leading to a work function decrease. The obtained work function data, along with the data on surface photovoltage spectroscopy (reported elsewhere), indicate that the reactivity and photoreactivity of rutile with oxygen are closely related to its defect disorder and the related semiconducting properties.

Metal-induced gap states modeling of metal-Ge contacts with and without a silicon nitride ultrathin interfacial layer

Metal-induced gap states (MIGS) modeling is used to elucidate the lack of Fermi level pinning at metal-insulator−Ge interfaces. Energy band diagram assessment reveals the existence of two dipoles at the metal-insulator and the insulator−semiconductor interface. The metal−insulator dipole modulates the metal-insulator interface electron barrier and the voltage drop across the insulator but does not affect the barrier to electron transport across the metal-insulator−Ge interface. Rather, this electron transport barrier is established by the metal-semiconductor work function difference and the insulator−semiconductor dipole. Thus, the lack of Fermi level pinning at a metal-insulator−Ge interface is attributed to the fact that the electron transport barrier does not depend upon MIGS screening. A quantitative formulation of this metal-insulator−semiconductor interface MIGS-based model confirms the lack of Fermi level pinning. Furthermore, it indicates that care must be taken when assessing experimental barrier height- work function data since the slope parameter should only be evaluated for the range of metal work function in which the semiconductor is in depletion. This range of work function for which the semiconductor is in depletion is quite limited for the case of a narrow bandgap semiconductor, such as Ge.

The Ionic Work Function and its Role in Estimating Absolute Electrode Potentials

The experimental determination of the ionic work function is briefly described. Data for the proton, alkali metal ions, and halide ions in water, originally published by Randles (Randles, J. E. B. Trans Faraday Soc. 1956, 52, 1573) are recalculated on the basis of up-to-date thermodynamic tables. These calculations are extended to data for the same ions in four nonaqueous solvents, namely, methanol, ethanol, acetonitrile, and dimethyl sulfoxide. The ionic work function data are compared with estimates of the absolute Gibbs energy of solvation obtained by an extrathermodynamic route for the same ions. The work function data for the proton are used to estimate the absolute potential of the standard hydrogen electrode in each solvent. The results obtained here are compared with those published earlier by Trasatti (Trasatti, S. Electrochim. Acta 1987, 32, 843) and more recently by Kelly et al. (Kelly, C. P.; Cramer, C. J.; Truhlar, D. G. J. Phys. Chem. B 2006, 110, 16066. Kelly, C. P.; Cramer, C. J.; Truhlar, D. G. J. Phys. Chem. B 2007, 111, 408). A comparison of the ionic work function with the absolute Gibbs solvation energy permits an estimation of the surface potential of the solvent. The results show that the surface potential of water is small and positive whereas the surface potential of the nonaqueous solvents considered is negative. The sign of the surface potential is consistent with the known structure of each solvent.

Reactivity of Titanium Dioxide with Oxygen at Room Temperature and the Related Charge Transfer

The measurements of electron work function were applied for in situ monitoring of the charge transfer during oxidation and reduction for well-defined titanium dioxide, TiO 2, at room temperature. The TiO 2 specimen was initially standardized at 1173 K in the gas phase of controlled oxygen activity, at p(O 2) = 10 Pa, and then cooled down in the same gas phase. The work function changes were monitored during oxidation at room temperature at p(O 2) = 75 kPa and subsequent reduction at p(O 2) = 10 Pa. It is shown that oxidation of TiO 2 at room temperature results in fast oxygen chemisorption, involving initially the formation of singly ionized molecular oxygen species, followed by the formation of singly ionized atomic oxygen species, and subsequent slow oxygen incorporation. Although all these processes lead to work function increase, the components of the work function changes related to the individual processes may be distinguished based on different kinetics. The obtained work function data indicate that oxidation results in rapid surface coverage with singly ionized molecular oxygen species, which are subsequently dissociated leading to the formation of singly ionized atomic species. The related chemisorption equilibria are established within 2 and 5 h, respectively. Oxygen incorporation leads to slow work function changes, which achieve a maximum within 100 h. The determined work function data were assessed by using a theoretical model that describes the electrical effects related to different mechanisms of TiO 2 oxidation. The work function data indicate that oxygen incorporation leads to structural changes of the outermost surface layer resulting, in consequence, in a change of the external work function component. Reimposition of the initial gas phase, p(O 2) = 10 Pa, leads to partial desorption of weakly adsorbed molecular species formed during oxidation.

Interaction of Cobalt(II) Tetraarylporphyrins with a Ag(111) Surface Studied with Photoelectron Spectroscopy

The interaction of cobalt(II) tetraphenylporphyrin (CoTPP) and cobalt(II) tetrakis-(3,5-di-tert-butylphenyl)porphyrin (CoTTBPP) with a Ag(111) surface has been investigated with photoelectron spectroscopy (XPS/UPS). It is demonstrated that these adsorbed metal complexes are excellent model systems for studying the electronic interaction between a coordinated metal ion and a metal surface. The photoelectron spectra and work function data provide evidence that the electronic interaction between the cobalt ion and the silver surface results in a transfer of electron density from the surface to the ion. The presence of an additional electronic state located ∼1 eV above the singly occupied molecular orbital (SOMO) of the metalloporphyrins is consistent with a molecular orbital (MO) model of the Co−Ag interaction as is the fact that the energetic position of this state depends on the distance between the Co ion and the Ag surface. The adsorbate-induced work function changes for the saturated monolayers amount t...

Variation of the potential of zero charge for a silver monolayer deposited onto various noble metal single crystal surfaces

The potential of zero charge (PZC) of a pseudomorphic silver monolayer electrodeposited onto Au(1 1 1), Pt(1 1 1), Pd(1 1 1), Ir(1 1 1) and Rh(1 1 1) has been determined in diluted perchlorate solution by measuring capacitance–potential curves. The PZC of these Ag overlayers varies in the potential range between −0.8 V and −0.45 V versus SCE depending on the nature of the substrate material and thus reveals significant shifts compared to −0.7 V versus SCE for a massive Ag(1 1 1) electrode. This bulk value for the PZC has also been obtained for two monolayer thick Ag overlayers, demonstrating that mainly the first monolayers are electronically altered by an underlying foreign substrate. In addition, the PZC of thicker deposits has always been found to be identical to the PZC of Ag(1 1 1), showing an epitaxial growth of Ag on the electrodes under study. While no dependence on the lattice constants of the substrates for the PZC of the pseudomorphic Ag monolayers was found, it has been observed that calculated shifts in the d-band centre show the same systematic variation. Work function data for Ag monolayers on Au(1 1 1) and Pt(1 1 1) are in good agreement with the measured PZC values. Consequences of altered double-layer properties on electrocatalytic reactions are briefly addressed.

Atomic Structure and Bonding of Water Overlayer on Cu(110): The Borderline for Intact and Dissociative Adsorption

By carefully comparing the calculated and measured work function data, energetics, and vibrational spectroscopy, we determine explicitly the water structure in c(2 x 2) periodicity on Cu(110) to be an intact water overlayer with a majority component of H-down bilayer (95%) in low temperature experiments. Water dissociation is accessible by heating or ultraviolet illumination, resulting in a sensitive change in electron density at the surface and could therefore be monitored by work function measurement.