Adsorbate-induced Work Function Research Articles

Can We Predict Interface Dipoles Based on Molecular Properties?

We apply high-throughput density functional theory calculations and symbolic regression to hybrid inorganic/organic interfaces with the intent to extract physically meaningful correlations between the adsorption-induced work function modifications and the properties of the constituents. We separately investigate two cases: (1) hypothetical, free-standing self-assembled monolayers with a large intrinsic dipole moment and (2) metal–organic interfaces with a large charge-transfer-induced dipole. For the former, we find, without notable prior assumptions, the Topping model, as expected from the literature. For the latter, highly accurate correlations are found, which are, however, clearly unphysical.

The Potential of X-ray Photoelectron Spectroscopy for Determining Interface Dipoles of Self-Assembled Monolayers

In the current manuscript we assess to what extent X-ray photoelectron spectroscopy (XPS) is a suitable tool for probing the dipoles formed at interfaces between self-assembled monolayers and metal substrates. To that aim, we perform dispersion-corrected, slab-type band-structure calculations on a number of biphenyl-based systems bonded to an Au(111) surface via different docking groups. In addition to changing the docking chemistry (and the associated interface dipoles), the impacts of polar tail group substituents and varying dipole densities are also investigated. We find that for densely packed monolayers the shifts of the peak positions of the simulated XP spectra are a direct measure for the interface dipoles. In the absence of polar tail group substituents they also directly correlate with adsorption-induced work function changes. At reduced dipole densities this correlation deteriorates, as work function measurements probe the difference between the Fermi level of the substrate and the electrostatic energy far above the interface, while core level shifts are determined by the local electrostatic energy in the region of the atom from which the photoelectron is excited.

Toward Efficient Toxic-Gas Detectors: Exploring Molecular Interactions of Sarin and Dimethyl Methylphosphonate with Metal-Centered Phthalocyanine Structures.

The rapid and reliable detection of lethal agents such as sarin is of increasing importance. Here, density-functional theory (DFT) is used to compare the interaction of sarin with single-metal-centered phthalocyanine (MPc) and MPc layer structures to a benign model system, i.e., the adsorption of dimethyl methylphosphonate (DMMP). The calculations show that sarin and DMMP behave nearly identical to the various MPcs studied. Among NiPc, CuPc, CoPc, and zinc phthalocyanine (ZnPc), we find the interaction of both sarin and DMMP to be the strongest with ZnPc, both in terms of interaction energy and adsorption-induced work function changes. ZnPc is thus proposed as a promising sensor for sarin detection. Using X-ray photoelectron spectroscopy, the theoretically predicted charge transfer from DMMP to ZnPc is confirmed and identified as a key component in the sensing mechanism.

Orientation-Dependent Work-Function Modification Using Substituted Pyrene-Based Acceptors.

The adsorption of molecular acceptors is a viable method for tuning the work function of metal electrodes. This, in turn, enables adjusting charge injection barriers between the electrode and organic semiconductors. Here, we demonstrate the potential of pyrene-tetraone (PyT) and its derivatives dibromopyrene-tetraone (Br-PyT) and dinitropyrene-tetraone (NO2-PyT) for modifying the electronic properties of Au(111) and Ag(111) surfaces. The systems are investigated by complementary theoretical and experimental approaches, including photoelectron spectroscopy, the X-ray standing wave technique, and density functional theory simulations. For some of the investigated interfaces the trends expected for Fermi-level pinning are observed, i.e., an increase of the metal work function along with increasing molecular electron affinity and the same work function for Au and Ag with monolayer acceptor coverage. Substantial deviations are, however, found for Br-PyT/Ag(111) and NO2-PyT/Ag(111), where in the latter case an adsorption-induced work function increase of as much as 1.6 eV is observed. This behavior is explained as arising from a face-on to edge-on reorientation of molecules in the monolayer. Our calculations show that for an edge-on orientation much larger work-function changes can be expected despite the prevalence of Fermi-level pinning. This is primarily ascribed to a change of the electron affinity of the adsorbate layer that results from a change of the molecular orientation. This work provides a comprehensive understanding of how changing the molecular electron affinity as well as the adsorbate structure impacts the electronic properties of electrodes.

Low pressure hydrogen sensing based on carbon nanotube field emission: Mechanism of atomic adsorption induced work function effects

The mechanism of the innovative adsorption based field emission hydrogen sensing technique is investigated from experimental and first-principles investigations. Hydrogen pressure from 10−3 to 10−7 Pa range could be detected. High carbon nanotube (CNT) crystallinity plays key role for the reduction of the effective work function from the dissociated hydrogen adsorption, leading to the field emission current increase. For CNT emitters with high dangling-bond (DB) type of defects/structures, the dissociated H adsorption occurs preferentially on the DB site with negligible work function variation. Joule heating & temperature rising is a core factor to assist the dissociated hydrogen adsorption with enhanced sensing effect. For work function sensitive CNT devices, operations under hydrogen & high temperature conditions may produce performance stability issue.

Investigation of low temperature effects on work function based CO2 gas sensing of nanoparticulate CuO films

The gas sensing behavior of semiconducting nanoparticulate copper oxide (CuO‐NPs) towards CO2 with respect to different conditions affecting the signal response is investigated with the help of adsorption induced work function changes. Work function measurements have been carried out with the help of Kelvin probe. The analysis of cross‐sensitivities to humidity is conducted in the low temperature range between 25°C and 110°C and it is shown that for CuO‐NPs, optimization of work function based CO2 sensing is possible by using a combined effect of humidity and temperature. Furthermore, the temperature induced effects during CO2 sensing are explained using considerations of kinetics and thermodynamics of the process. We show that moderate temperatures of about 65°C exert a positive influence on the kinetics of chemical processes occurring on the CuO‐NPs surface in the presence of water and CO2 as well as counterbalance the negative impact of high humidity dependence on CO2 gas sensing. However starting from 65°C, CO2 response decreases owing to a negative effect of temperature on the thermodynamics of the sensing reaction. Compared to other metal oxide based materials for CO2 sensing, CuO‐NPs show promising results and enables the prospect towards the development of new CO2 gas sensor operable at low temperatures.

In-situ probing of thermal desorption of vapor molecules on a nanowire via work function variance

Nanowire sensors based on variations of their electrical properties show great potential for real-time, in situ monitoring of molecular adsorption and desorption. Although the molecular adsorption-induced change in the electronic work function is very sensitive, it does not have any specificity. However, the temperature dependency of the adsorption-induced work function variation can provide limited selectivity based on the desorption temperature. In this study, we report the in situ probing of molecular desorption by monitoring the work function variations of a single Pt nanowire as a function of temperature. The work function of a clean Pt nanowire shows a significant variation due to vapor adsorption at room temperature. Increasing the temperature of the nanowire results in a variation of the work function due to molecular desorption. Experimentally measured differential work function as a function of temperature shows desorption peaks at 36 and 44 °C for methanol and ethanol molecules respectively. Adsorption-induced variation of the Pt nanowire work function was further confirmed using ultraviolet photoelectron spectroscopy before and after exposure to methanol vapor. These results show that the molecular adsorption/desorption-induced variation of the work function and its temperature dependency can be used for developing nanoscale electro-calorimetric sensors.

Density functional theory study of mercury adsorption on metal surfaces

Density functional theory (DFT) calculations are used to characterize the interaction of mercury with copper, nickel, palladium, platinum, silver, and gold surfaces. Mercury binds relatively strongly to all the metal surfaces studied, with binding energies up to $\ensuremath{\sim}1\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$ for Pt and Pd. DFT calculations underestimate the energy of adsorption with respect to available experimental data. Plane-wave DFT results using the local density approximation and the Perdew-Wang 1991 and Perdew-Burke-Ernzerhof parametrizations of the generalized gradient approximation indicate that binding of mercury at hollow sites is preferred over binding at top or bridge sites. The interaction with mercury in order of increasing reactivity over the six metals studied is $\mathrm{Ag}<\mathrm{Au}<\mathrm{Cu}<\mathrm{Ni}<\mathrm{Pt}<\mathrm{Pd}$. Binding is stronger on the (001) faces of the metal surfaces, where mercury is situated in fourfold hollow sites as opposed to the threefold hollow sites on (111) faces. In general, mercury adsorption leads to decreases in the work function; adsorbate-induced work function changes are particularly dramatic on Pt.

Relativistic density functional study of FCC americium and the (111) surface

Full-potential all-electron density-functional calculations with mixed basis augmented plane wave plus local orbital--linearized-augmented plane wave have been carried out to investigate the electronic and geometric properties of bulk FCC Am and quantum size effects in the surface energies and the work functions of the (111) ultrathin films up to seven layers at six levels of approximations, namely, non-spin-polarized--no-spin-orbit coupling, non-spin-polarized--spin-orbit coupling, spin-polarized--no-spin-orbit coupling, spin-polarized--spin-orbit coupling, antiferromagnetic--no-spin-orbit coupling, and antiferromagnetic--spin-orbit (AFM-SO) coupling. The antiferromagnetic state including spin-orbit coupling is found to be the ground state of FCC Am with the $5f$ electrons primarily localized. Our calculations show that both spin-polarization and spin-orbit coupling effects play important roles in determining the equilibrium lattice constant, the bulk modulus, as well as the localized feature of $5f$ electrons for FCC Am. Our calculated equilibrium lattice constant of $9.21\phantom{\rule{0.3em}{0ex}}\mathrm{a.u.}$ and a bulk modulus of $25.64\phantom{\rule{0.3em}{0ex}}\mathrm{GPa}$ at the AFM-SO level are in good agreement with the experimental values of $9.26\phantom{\rule{0.3em}{0ex}}\mathrm{a.u.}$ and $29.4\phantom{\rule{0.3em}{0ex}}\mathrm{GPa}$, respectively. For the films, surface energy rapidly converges and the semi-infinite surface energy is predicted to be $0.81\phantom{\rule{0.3em}{0ex}}\mathrm{J}∕{\mathrm{m}}^{2}$ at the ground state. The work function of the FCC Am(111) surface at the AFM-SO ground state is predicted to be $3.06\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$, and the work function shows some oscillations when the number of layers is less than five, while it becomes relatively stable when the number of layers is greater than five. This suggests that a five-layer film may be required for any future adsorption investigation that requires, for example, an accurate prediction of one-electron properties and adsorbate-induced work function shift.

N O 2 detection by adsorption induced work function changes in In2O3 thin films

A potentiometric sensor platform has been used for NO2 detection at room temperature by measuring the adsorption induced surface work function changes in In2O3 thin films deposited on Si using a solution based process. The highly resistive films were unsuitable for amperometric detection of NO2; however, significant work function changes of ∼30mV were measured for ∼50s of exposure to 70ppm NO2, and detection down to 600ppb (parts per 109) was possible. A model for the transient response was developed, which satisfactorily fits the experimental data. Acceleration of the desorption transient under ultraviolet illumination has also been investigated.

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...

A fully-relativistic full-potential-linearized-augmented-plane-wave study of the (1 1 1) surface of δ-Pu

Full-potential linearized-augmented-plane-wave calculations indicate that the antiferromagnetic state including spin–orbit coupling effect is the ground state of bulk δ-Pu with a lattice constant of 8.66 a.u. and a bulk modulus of 32.8 GPa. It is found that spin-polarization and spin–orbit coupling effects play competing roles in the localization to delocalization behavior of 5f electrons. The optimized lattice constants of δ-Pu bulk are used to calculate the electronic structure properties of δ-Pu(1 1 1) films up to seven layers at six theoretical levels, namely non-spin-polarized-no-spin–orbit-coupling (NSP-NSO), non-spin-polarized-spin–orbit-coupling (NSP-SO), spin-polarized-no-spin–orbit-coupling (SP-NSO), spin-polarized-spin–orbit-coupling (SP-SO), antiferromagnetic-no-spin–orbit-coupling (AFM-NSO), and antiferromagnetic-spin–orbit-coupling (AFM-SO). For the δ-Pu(1 1 1) films also, AFM-SO is found to be the ground state. For the films, surface energy rapidly converges and the semi-infinite surface energy is predicted to be 1.16 J/m 2. On the other hand, the magnetic moments show an oscillating behavior, gradually approaching the bulk value of zero with increase in the number of layers. It is also predicted that the work function of δ-Pu(1 1 1) films at the AFM-SO ground state is approximately 3.41 eV, and the work function shows some oscillations when the number of layers is less than five, while it becomes relatively stable when the number of layers is greater than five. This suggests that a 3-layer film might be sufficient for computations of, for example, adsorption energies while a 5-layer film may be necessary for precise computations of, for example, adsorbate-induced work function shifts. The calculated results are compared with other experimental and theoretical results in the literature and the agreements between them are excellent, given the complexity of the physical systems and different computational formalisms.

Energy Level Alignment at Metal−Octaethylporphyrin Interfaces

We studied the electronic structure of copper-octaethylporphyrin (CuEOP) adsorbed on three metal surfaces--Ag(001), Ag(111), and Cu(111)--by means of ultraviolet photoelectron spectroscopy (UPS). The adsorption-induced work function shifts saturate roughly beyond two monolayers. The saturation values are substrate dependent, negative, and range from -1.30 to -0.85 eV. This shift is larger than that for tetraphenylporphyrins. The two highest occupied molecular orbitals (HOMO and HOMO-1) of the organic are clearly resolved in the UPS spectra. The origin of the negative work function shift is discussed.

Vibrating capacitor method in the development of semiconductor gas sensors

Adsorption usually results in work function shifts on catalytically active surfaces such as semiconductor gas sensors. The purpose of the present article is to summarise the capabilities of the vibrating capacitor from the simplest adsorption-induced work function tests to the scanning, vibrating, capacitor-yielded olfactory pictures and other chemical pictures. After a brief history and review of theoretical bases, the latest results will be discussed in detail. Olfactory pictures from semiconductor surfaces give a new chance to improve the selectivity of gas analysis. Chemical pictures from thin SnO 2 layers produced by atomic layer epitaxy reveal the inhomogeneities of the technology. CPD maps taken from Pd nanolayer (activator)-covered surfaces help to find the best layer-depositing parameters for the activation process of the thin semiconductor gas sensor films.

A density functional theory study of adsorbate-induced work function change and binding energy: Olefins on Ag(111)

The change of work function of the Ag (111) surface induced by the physisorption of ethylene and its derivatives, vinyl chloride and butadiene, is examined with density functional theory (DFT) calculations employing a generalized gradient approximation for the exchange correlation functional. It is found that the calculations can generate optimized adsorption structures in agreement with experiment, although as expected the calculation does not compute the small binding energies accurately. This DFT approach, however, predicts the work function change induced by adsorption reasonably well. Since there appears an empirical correlation between the measured adsorption energy and the calculated work function change for the studied olefins that can be justified by molecular orbital interactions, the work function changes computed within DFT may be used as a relative calibration for binding energies in physisorbed systems.

The role of an oxygen adsorbate on the secondary emission properties of low energy ion-bombarded magnesium

The absolute secondary electron and anion emission probabilities for low energy (<500 eV) O−, N2+, Ar+, Ne+ and He+ impacting an oxygen-adsorbed Mg(0001) single crystal surface have been measured as a function of oxygen exposure and of incident ion impact energy. Secondary anion emission was observed throughout the range of adsorbate coverage studied, from that of a “clean” metal (defined by a demonstrated adsorbate-induced work function shift) to that rendering an insulative metal oxide, and was determined to comprise mostly O−. Both electron and anion emission were observed to be largely unaltered during the metal–metal oxide transition. Secondary anion and electron kinetic energy spectra were also measured; no significant dependence on oxygen exposure was observed. The relevance of secondary emission mechanisms that are driven by the ionization potential of the incident ion ranges from limited to none in our experiment in the case of incident O−; we suggest that decay of a collisionally-excited MgO surface state may account, in part, for secondary emission related to the presence of adsorbed oxygen.

Chemical imaging by direct methods

Chemical imaging is a very old principle of analysis. It has been used for many millions of years in biological tasting and smelling systems. Artificial versions of chemical imaging are traditional paper chromatography and the pH indicator strip used in chemical analysis, which directly result in real (visible) chemical images. In a recent article the indirect and direct chemical imaging methods are introduced, especially the evaluation methods for gas sensitive surfaces by direct pixelizing of the surface and interface potential changes. The scanned light pulse technique (SLPT) and the scanning vibrating capacitor are very effective tools for the chemical mapping of the surfaces. These methods are sensitive for the interface or surface potential shifts (adsorption induced work function shifts) respectively, that depend on the composition of adsorbed layers. Advantages and disadvantages, some technical limits, theory, practice and some results are discussed, too.

On the specific features of work function coverage dependence for Na adatoms on the Cs substrate

Model based on the simple modification of the Anderson–Newns Hamiltonian is proposed for the description of the adsorption induced work function coverage dependence Δφ( θ). It is shown that the change of Δφ( θ) sign for submonolayer coverages ( θ<1 ML) can be attributed to the adsorption bond length alteration.

Molecular beam studies of sticking of oxygen on the Rh(111) surface

The sticking coefficients of oxygen on the Rh(111) surface have been measured using O2 molecular beams seeded with different rare gases to achieve beam energies in the range Ei=25–600 meV. With increasing O2 incident beam energy, the adsorption evolves continuously from a molecular precursor-mediated process at low energies to a direct adsorption process described by Langmuir kinetics at high energies. The analysis of the dependence of the initial sticking coefficient on the incident kinetic energy and the angle of incidence as well as on the surface temperature provide information on important parameters of the potential energy surface. Further insight into the nature of the adsorbed species could be obtained from the UPS measurements of the adsorption induced work function changes and from helium atom diffraction measurements of structural modifications as a function of the kinetic energy angle of incidence of the oxygen molecules, and the surface temperature. The “simple washboard” model accounts for the trapping well of the molecular precursor and is able to describe many of the observed features in the precursor region.

Charge transfer dynamics of low energy collisions of Li + with alkali-covered Cu(001)

We present measurements of branching ratios to different final states of the scattered particles for 400 eV Li + ions incident on alkali-covered Cu(001). In particular, we have measured the charge state fractions and the relative yield of excited states in the scattered flux as a function of the adsorbate-induced work function shift. The results of a resonant charge transfer theory which includes excited states, negative ions, and level crossings are compared to the measurements. Qualitative agreement is achieved with the observed work function dependence of both the charge state fractions and the relative yield of Li(2p). In particular, the theory predicts that the peak observed in the Li(2p) yield is due to a competition between the Li(2p) state and the Li −(2s 2) state.