Work Function Of Metal Surfaces Research Articles

Effective Descriptor for Screening Single-Molecule Conductance Switches.

The adsorption-type molecular switch exhibits bistable states with an equivalently long lifetime at the organic/inorganic interface, promising reliable switching behavior and superior assembly ability in the electronic circuits at the molecular scale. However, the number of reported adsorption-type molecular switches is currently less than 10, and exploring these molecular switches poses a formidable challenge due to the intricate interplay occurring at the interface. To address this challenge, we have developed a model enabling the identification of diverse molecular switches on metal surfaces based on easily accessible physical characteristics. These characteristics primarily include the metal valency electron concentration, the work function of metal surfaces, and the electronegativity difference of molecules. Using this model, we identified 56 new molecular switches. Employing the gradient descent algorithm and statistical linear discriminant analysis, we constructed an explicit descriptor that establishes a relationship between the interfacial structure and chemical environment and the stability of molecular switches. The model's accuracy was validated through density functional theory calculations, achieving a 90% accuracy for aromatic molecular switches. The conductive switching behaviors were further confirmed by nonequilibrium Green's function transport calculations.

Formula omitted]-[formula omitted] junction from localized doping of metal-WSe2 contact with N2H4, NH4 and BF4 molecules: A first-principles investigation

Using the density-functional theory, we investigate the possibility of obtaining a p–n junction in transition metal–semiconductor junctions. The systems consist in metal surfaces, M = Ti(0001), Co(0001), Pd(111) or Pt(111) in proximity to the semiconductor WSe2 monolayer. The latter is doped with hydrazine (N2H4), ammonium (NH4), or tetrafuoroborate (BF4). We determine for the M-WSe2 systems the binding energy, work function of metal surfaces, the Schottky barrier energy with and without molecular doping and the spin polarization. Results show a strong binding between the metal and WSe2 which implies a stable M-WSe2 contacts. The Spin-polarization in Ti-WSe2 (35%), Co-WSe2 (37,5%), Pd-WSe2 (13%) and Pt-WSe2 (-0.2%) suggest a significant metal-to-WSe2 spin injection in M-WSe2. Localization of hydrazine or ammonium on the WSe2 in M-WSe2 (M = Pd, Pt) changes its conductivity from p to n-type. The strong binding, the finite spin-injection, the change in conductivity and the reduction in Shottky barrier, raise the potential for a molecule-based p–n junction in metal-WSe2 field-effect transistor.

Linear Correlation between Water Adsorption Energies and Volta Potential Differences for Metal/water Interfaces

Potential of zero charge (PZC) is an important reference for understanding the interface charge and structure at a given potential, and its difference from the work function of metal surface (ΦM) is defined as the Volta potential difference (ΔΦ). In this work, we model 11 metal/water interfaces with ab initio molecular dynamics. Interestingly, we find ΔΦ is linearly correlated with the adsorption energy of water (Eads) on the metal surface. It is revealed that the size of Eads directly determines the coverage of chemisorbed water on the metal surface and accordingly affects the interface potential change caused by electron redistribution (ΔΦel). Moreover, ΔΦ is dominated by the electronic component ΔΦel with little orientational dipole contributing, which explains the linear correlation between ΔΦ and Eads. Finally, it is expected that this correlation can be helpful for effectively estimating the ΔΦel and PZC of other metal surfaces in the future work.

The Swiss‐Army‐Knife Self‐Assembled Monolayer: Improving Electron Injection, Stability, and Wettability of Metal Electrodes with a One‐Minute Process

A novel Self‐assembled Monolayer (SAM) forming molecule bisjulolidyldisulfide (9,9'‐disulfanediylbis(2,3,6,7‐tetrahydro‐1H,5H‐pyrido[3,2,1‐ij]quinoline)) is demonstrated which lowers the work function of metal surfaces by ≈1.2 eV and can be deposited in a 1 min process. Bisjulolidyldisulfide exists in a stable disulfide configuration prior to surface exposure and can therefore be stored, handled, and processed in ambient conditions. SAM from bisjulolidyldisulfide are deposited on metal surfaces (Au and Ag), including inkjet printed Ag on polyethylene terephthalate substrates, investigated by photoelectron and infrared spectroscopy, and used as electrodes in n‐type organic field effect transistor (OFET). Treatment of electrodes in OFET devices with with bisjulolidyldisulfide‐SAMs reduces the contact resistance by two orders of magnitude and improves shelf life with respect to pristine metal electrodes. The presented treatment also increases the surfaces wettability and thereby facilitates solution processing of a subsequent layer. These beneficial properties for device performance, processing, and stability, combined with ease of preparation and handling, render this SAM‐forming molecule an excellent candidate for the high‐throughput production of flexible electronic devices.

On the HSAB based estimate of charge transfer between adsorbates and metal surfaces

The applicability of the HSAB based electron charge transfer parameter, Δ N, is analyzed for molecular and atomic adsorbates on metal surfaces by means of explicit DFT calculations. For molecular adsorbates Δ N gives reasonable trends of charge transfer if work function is used for electronegativity of metal surface. For this reason, calculated work functions of low Miller index surfaces for 11 different metals are reported. As for reactive atomic adsorbates, e.g., N, O, and Cl, the charge transfer is proportional to the adatom valence times the electronegativity difference between the metal surface and the adatom, where the electronegativity of metal is represented by a linear combination of atomic Mulliken electronegativity and the work function of metal surface. It is further shown that the adatom-metal bond strength is linearly proportional to the metal-to-adatom charge transfer thus making the Δ N parameter a useful indicator to anticipate the corresponding adsorption energy trends.

On the consistent use of electrophilicity index and HSAB-based electron transfer and its associated change of energy parameters

The electrophilicity index is often used in an inconsistent manner with other HSAB (Hard and Soft Acids and Bases) based parameters that depend on absolute hardness, such as, electron transfer (Δ N) and its associated change of energy (Δ E). We further comment on the use of Δ N parameter in the quantum chemical studies of corrosion inhibitors, where this parameter is used to discuss the interaction between an inhibitor molecule and metal surface. It is argued that the work function of metal surface is an appropriate measure of its electronegativity and should be used together with its vanishing absolute hardness to estimate the Δ N.

Tailoring the Work Function of Gold Surface by Controlling Coverage and Disorder of Polar Molecular Monolayers

We show here that the work function of Au can be controlled not only by the magnitude and direction of adsorbed polar molecules but also by the molecular coverage and (dis)order within the molecular patterns. Molecular monolayers composed of two types of molecules with opposite dipoles were deposited on Au films containing the changes of the work function due to variation of molecular coverage and disorder were monitored. The results indicate that the requirements on molecular layers covering Au surfaces are significantly relaxed and that partial, disordered monolayers can be used to tailor the work function of metal surfaces.

Determination of the Work Function by the Density Functional Theory

Theoretical methods to determine the work function of metal surfaces using the density functional theory are reviewed. Evaluation of the Fermi energy relative to the vacuum level is usually done by utilizing rapid convergence of a local scalar potential obtained by subtracting the exchange-correlation potential from the total effective potential. We checked dependence on thickness deff of the vacuum layer, which is 1/deff, both for the Fermi energy and the vacuum level in finite slab models. Agreement between the theory and the experiments is shown for some typical metals.

PEEM study of work function changes in Cu, Au and Pd metal surfaces with surface acoustic wave propagation

The effects of surface acoustic wave (SAW) on the work function of Cu, Au and Pd metal surfaces with different surface structures were studied by photoelectron emission microscopy (PEEM). SAW propagation produced bright PEEM images for Cu, Au and Pd metal surfaces consisting of high-index planes and step sites, whereas it yielded dark images for the metals exposing low-index planes, indicating that the SAW enhanced photoemission from rough metal surfaces containing coordinatively-unsaturated metal atoms and lowered that from densely packed smooth metal surfaces. Changes in the PEEM images with SAW-on and SAW-off were reversible and were associated with decreases and increases in the work function of the metal surfaces, respectively. The SAW caused periodic and vertical lattice displacement, and it was demonstrated that large lattice displacement was responsible for work function changes from coincidence between the patterns of photoemission and lattice displacement. A mechanism for work function changes is proposed on the basis of effects on the spatial structures and electronic properties of metal surfaces.

Effects of dislocation on electron work function of metal surface

The great sensitivity of the electron work function (EWF) to surface condition has attracted increasing interest from surface scientists and tribologists. However, the effect of deformation on the EWF is unclear; this results in difficulty in applying the EWF in order to properly characterise tribological behaviour of materials. In this study, a simple electrostatic action model is proposed to investigate the effect of a dislocation on the EWF of a one-dimensional lattice. Using the model, one may illustrate how a dislocation influences the EWF. It has been demonstrated that the dislocation decreases the EWF and that both tensile and compressive plastic deformations can decrease the EWF. However, the situation is different if the deformation is in the elastic range.

Electronic Structures of Porphyrins and their Interfaces with Metals Studied by UV Photoemission

The electronic structures of porphyrins and their interfaces with metals were investigated by UV photoemission spectroscopy (UPS). The UPS spectral features of porphine, 5,10,15,20-tetraphenylporphynatozinc(II) (ZnTPP), 5,10,15,20-tetra(4-pyridyl)porphyrin (H2T(4-Py)P) and 5,10,15,20-tetraphenylporphyrin (H2TPP) could be assigned by comparison with MOPAC PM3 calculations. The electronic structures of the porphyrins can be regarded as the superposition of those of porphine and the substituents. The UPS results for ZnTPP/metal (Mg, Al, Ag, Au) interfaces indicated that the energies of the levels of ZnTPP relative to the Fermi level of substrate metals could be expressed as linear functions of the work function of metals with a shift of the vacuum level at interface. The slope of the linear functions was about unity. This indicates that the energy levels of ZnTPP are fixed to the vacuum level of substrate metal with constant interfacial dipole. The level positions of ZnTPP at the interface exhibited drastic change by exposure to oxygen. This change could be explained by the change of the work function of metal surface upon oxidization. For H2T(4-Py)P and H2TPP/metal (Mg, Ag, Au) interfaces, similar linearity was observed between the energy levels of porphyrin and the work function of metal with the slope of much smaller than unity. This deviation of the slope from unity might be explained by the existence of interface state. These findings require the modification of the traditional concept of a common vacuum level at organi/metal interface. The relation between the observed electronic structures and the conduction type of the porphyrins are also discussed.

Chemical potentials and workfunctions of metal surfaces in the theory of an inhomogeneous electron gas

The selfconsistent equations of Kohn and Sham (1965) are applied to metal surfaces. It is shown that at T=0 the highest energy of an occupied state epsilon F is equal to the chemical potential, and that the workfunction is equal to the difference between the vacuum level of the electrostatic potential and epsilon F.

Variation in the electron work function of metal surfaces in the presence of a surface-active medium

Variation in the electron work function of metal surfaces in the presence of a surface-active medium

A Directional Analyzer for Ion and Electron Velocity Distributions

The two basic difficulties usually encountered in multigrid retarding-potential analyzers have been identified and effectively overcome. The frequently experienced difficulty with such probes caused by the continually changing work function of metal surfaces due to a more or less evanescent sorption of ions has been eliminated by coating the nickel surfaces with a bonded graphite dispersion that leaves the surface microscopically rough and that is peculiarly free of transient sorbed films. The very divergent electric field near the wires of grids on which retarding potentials are applied defocuses the charged particle streams and interferes with the proper application of retarding potentials unless the fields near such grids are kept small. Double grids are used where necessary in order to insure minimum field penetration from the interior of the analyzer to the medium under study, and from one region to the next inside the analyzer. The device is versatile in that it can be used as a classical analyzer or as a filter-type analyzer. The amount of lens-effect defocusing of the charged particles is different for the two modes and, for most applications, the filter mode is preferable. The performance of the device is experimentally evaluated.

Electrostatic Energy Analyzer for Studying Gas-Focused Electron Beams and Their Background Media

An analyzer is described which is especially suitable for the study of electrostatically self-focusing electron beams and their associated background media, and, in general, for media which by their nature are very sensitive to direct insertion of probes or other diagnostic devices. The basic difficulties with devices of this nature, namely, (a) the continually changing work functions of metal surfaces due to sorbed films and (b) the lens effect defocusing due to nonuniformities of the potential in the neighborhood of grid wires, have been identified and effectively overcome. The analyzer is employed flush with the boundary of the medium under study and it incorporates a shielding electrode so that the disturbance of the medium is insignificant. When the dimensions of the instrument are scaled so that the space charge effects are absent, it can be utilized successfully to detect energy spreads as small as a few tenths of an electron volt.