Determination Of Work Function Research Articles

Work Function-Guided Electrocatalyst Design.

The development of high-performance electrocatalysts for energy conversion reactions is crucial for advancing global energy sustainability. The design of catalysts based on their electronic properties (e.g., work function) has gained significant attention recently. Although numerous reviews on electrocatalysis have been provided, no such reports on work function-guided electrocatalyst design are available. Herein, a comprehensive summary of the latest advancements in work function-guided electrocatalyst design for diverse electrochemical energy applications is provided. This includes the development of work function-based catalytic activity descriptors, and the design of both monolithic and heterostructural catalysts. The measurement of work function is first discussed and the applications of work function-based catalytic activity descriptors for various reactions are fully analyzed. Subsequently, the work function-regulated material-electrolyte interfacial electron transfer (IET) is employed for monolithic catalyst design, and methods for regulating the work function and optimizing the catalytic performance of catalysts are discussed. In addition, key strategies for tuning the work function-governed material-material IET in heterostructural catalyst design are examined. Finally, perspectives on work function determination, work function-based activity descriptors, and catalyst design are put forward to guide future research. This work paves the way to the work function-guided rational design of efficient electrocatalysts for sustainable energy applications.

Determination of Work Function for p- and n-Type 4H-SiC Single Crystals via Scanning Kelvin Probe Force Microscopy

Silicon carbide (SiC) is a promising platform for fabricating high-voltage, high-frequency and high-temperature electronic devices such as metal oxide semiconductor field effect transistors in which many junctions or interfaces are involved. The work function (WF) plays an essential role in these devices. However, studies of the effect of conductive type and polar surfaces on the WF of SiC are limited. Here, we report the measurement of WFs of Si- and C-terminated polar surfaces for both p-type and n-type conductive 4H-SiC single crystals by scanning Kelvin probe microscopy (SKPFM). The results show that p-type SiC exhibits a higher WF than n-type SiC. The WF of a C-terminated polar surface is higher than that of a Si-terminated polar surface, which is further confirmed by first-principles calculations. By revealing this long-standing knowledge gap, our work facilitates the fabrication and development of SiC-based electronic devices, which have tremendous potential applications in electric vehicles, photovoltaics, and so on. This work also shows that SKPFM is a good method for identifying polar surfaces of SiC and other polar materials nondestructively, quickly and conveniently.

Optical measurement of the work function and the field reduction factor of metallic needle tips.

Quintessential parameters for needle tip-based electron sources are the work function, the tip apex radius, and the field reduction factor. They determine the static emission properties and strongly influence laser-triggered photoemission experiments at these needle tips. We present a simple method based on photoemission with two different commonly available continuous-wave laser diodes to determine both parameters in situ. We demonstrate our technique at tungsten needle tips. In a first application, use the method to in situ monitor changes of the emitter caused by illumination with strong femtosecond laser pulses. After illumination, we observe an increase in the work function caused by laser-induced changes to the apex of the tip. These changes are reversible upon field evaporation and are accompanied by a change in the spatial electron emission distribution. We believe that this simple in situ work function determination technique is applicable to any metal and in many experimental settings.

Precise Determination of Energy Levels, Work Function and Explanation of Near-Ambient Superconductivity of Solids By Quantum Quantitative Model

Precise Determination of Energy Levels, Work Function and Explanation of Near-Ambient Superconductivity of Solids By Quantum Quantitative Model

A unified secondary electron cut-off presentation and common mistakes in photoelectron spectroscopy

Photoelectron spectroscopy is a powerful surface analysis technique that can differentiate different bonding environments and directly determine the absolute work function of a sample. Despite its ever-easier accessibility—or perhaps precisely because of it—some common mistakes or bad habits are often found in the literature when it comes to the evaluation or presentation of photoelectron spectroscopy data. Here we address some of these issues and give suggestions for best practice, i.e., a proper presentation of the secondary electron cut-off used for work function determination, correct binding energy referencing and some tips for appropriate peak fitting, as well as valuable literature references to more detailed tutorials. Finally, we present a concise step-by-step guide on how to conduct a complete x-ray photoelectron spectroscopy analysis of an unknown sample.

Impact of the photoelectric threshold sensitivity on the work function determination—Revealing ultra-low work functions of caesiated surfaces

The exploitation of the photoelectric effect is a prominent method for the in situ measurement of the absolute work function of a surface. In the case of metallic surfaces, the Fowler theory is routinely applied for fitting or extrapolating the measured photoelectric yield data to determine the work function value. However, for the reliable application of the Fowler method, attention must be paid to the experimental sensitivity to the photoelectric behavior close to the threshold, which is mainly determined by the signal-to-noise ratio for photocurrent detection and the available photon energies used for irradiation. This is illustrated by means of applying a photoelectric work function measurement system during a Cs coating process of a metal surface, where insufficiently low photon energies or an unfavorable noise level can lead to a severe overestimation of the work function of the order of 1 eV. By a sufficient enhancement of the photoelectric sensitivity, it is now unveiled that ultra-low surface work functions of 1.25 ± 0.10 eV can be generated via caesiation of metallic surfaces (here molybdenum and stainless steel) under vacuum conditions of 10−6–10−5 mbar, which is most probably the result of the formation of an oxidized Cs adlayer.

Influence of Charge Transport Layers on Open-Circuit Voltage and Hysteresis in Perovskite Solar Cells

Summary Perovskite materials have experienced an impressive improvement in photovoltaic performance due to their unique combination of optoelectronic properties. Their remarkable progression, facilitated by the use of different device architectures, compositional engineering, and processing methodologies, contrasts with the lack of understanding of the materials properties and interface phenomena. Here we directly target the interplay between the charge-transporting layers (CTLs) and open-circuit potential ( V OC ) in the operation mechanism of the state-of-the-art CH 3 NH 3 PbI 3 solar cells. Our results suggest that the V OC is controlled by the splitting of quasi-Fermi levels and recombination inside the perovskite, rather than being governed by any internal electric field established by the difference in the CTL work functions. In addition, we provide novel insights into the hysteretic origin in perovskite solar cells, identifying the nature of the contacts as a critical factor in defining the charge accumulation at its interface, leading to either ionic, electronic, or mixed ionic-electronic accumulation.

An elementary theory for determination of precise and reliable work function via photon energy and photoelectric yield

An elementary theory of using the definition of work function in photoelectric effect to determine precise and reliable work function by contributing frequency to optical-electric yield is investigated in this paper. On account of the theory of Fermi-Dirac distribution as well as the definition of work function proposed by Einstein in photoelectric effect, a representative mathematical approach is applied to analyze the frequency-based optical-electric yield in quantitatively terms. Supplementing applications to In14 cluster and three kinds of metals, the simulations agree well with the observed spectra (photoelectric yield-frequency). At the same time, the threshold frequency of light-dependent the work function is theoretically explained successfully via one equation so that the work function can be predicted precisely and reliably. The consequences provide that the formalism pursed in this paper, which is straight forward and practical, could play a significant role in studying metal cluster spectroscopy. The experimental results in this paper may hopefully promote overall analysis on theoretical basis. They may further analyze the usability of bulk-derived models to cluster photo-ionization activity, and the conversion of molecular or atomic form into surface photoemission form.

Study of the modes of adsorption and electronic structure of hydrogen peroxide and ethanol over TiO2 rutile (110) surface within the context of water splitting

While photocatalytic water splitting over many materials is favourable thermodynamically the kinetic of the reaction is very slow. One of the proposed reasons linked to the slow oxidation reaction rate is H2O2 formation as a reaction intermediate. Using Density Functional Theory (DFT) H2O2 is investigated on TiO2 rutile (110) surface to determine its most stable adsorption modes: molecular, (H)O(H)O − (a), partially dissociated, (H)OO − (a), and fully dissociated (a) − OO − (a). We then compare H2O2 interaction to that of a fast hole scavenger molecule, ethanol. Geometry, electronic structure, charge density difference and work function determination of both adsorbates are presented and compared using DFT with different functionals (PBE, PBE-D, PBE-U, and HSE + D). H2O2 is found to be strongly adsorbed on TiO2 rutile (110) surface with adsorption energies reaching 0.95 eV, comparable to that of ethanol (0.89 eV); using GGA PBE. The negative changes in the work function upon adsorption were found to be highest for molecular adsorption ( − 1.23 eV) and lowest for the fully dissociated mode ( − 0.54 eV) of H2O2. This may indicate that electrons flow from the surface to the adsorbate in order to make O(s)-H partially offset the overall magnitude of the oxygen lone pair interaction (of H2O2) with Ti4+ cations. Examination of the electronic structure through density of states (DOS) at the PBE level of computation, indicates that the H2O2 highest occupied molecular orbital (HOMO) level is not overlapping with oxygen atoms of TiO2 surface at any of its adsorption modes and at any of the computation methods. Some overlap is seen using the HSE + D computational method. On the other hand the dissociated mode of ethanol (ethoxides) does overlap with all computational methods used. The high adsorption energy and the absence of overlapping of the HOMO level of H2O2 with TiO2 rutile (110) surface may explain why water splitting is slow.

Reliable Work Function Determination of Multicomponent Surfaces and Interfaces: The Role of Electrostatic Potentials in Ultraviolet Photoelectron Spectroscopy

AbstractUltraviolet photoelectron spectroscopy (UPS) is a key technique to determine the work function (Φ) of surfaces by measuring the secondary‐electron cut‐off (SECO). However, the interpretation of SECO spectra as obtained by UPS is not straightforward for multicomponent surfaces, and it is not comprehensively understood to what extent the length scale of inhomogeneity impacts the SECO. Here, this study unravels the physics governing the energy distribution of the SECO by experimentally and theoretically determining the electrostatic landscape above surfaces with defined patterns of Φ. For such samples, the measured SECO spectra exhibit actually two cut‐offs, one representing the high Φ surface component and the other one corresponding to an area‐averaged Φ value. By combining Kelvin probe force microscopy and electrostatic modeling, it is quantitatively demonstrated that the electrostatic potential of the high Φ areas leads to an additional energy barrier for the electrons emitted from the low Φ areas. Theoretical predictions of the induced energy barrier dependence on the Φ‐pattern length scale and sample bias are further experimentally verified. These findings establish a solid base for reliable SECO interpretation of heterogeneous surfaces and improved reliability of interfacial energy‐level diagrams from UPS experiments.

Determination of work function in the individual carbon nanotubes using electrostatic force microscopy

In this letter we report the determination of electron work function (WF) in the individual undoped, as-grown and thermally treated nitrogen-doped carbon nanotubes (CNTs) with the use of the electrostatic force microscopy (EFM). The determination of WF is based on the measurement of the contact potential difference between CNTs and the probe tip. Obtained mean values of WF for a large number of individual CNTs are in quantitative agreement with the value of WF obtained by X-ray photoelectron spectroscopy at the CNTs layers. The ability to determine WF in individual CNTs is due to the sensitivity of the EFM signal to the interaction of CNTs with the tip apex.

Support Vector Machines Approach for Estimating Work Function of Semiconductors: Addressing the Limitation of Metallic Plasma Model

Experimental determination of work function of metals is not only a difficult task but also characterized with certain degree of inaccuracy. The success of metallic plasma model (MPM) remains incomplete due to its inability to correctly estimate work functions of elemental semiconductors (Silicon and Germanium). Support vector machine (SVM) with accuracy of over 98% in the work function estimation is hereby proposed to address the limitations of MPM. The estimated work functions obtained using SVM approach were compared with the universally accepted experimental work functions as well as work functions obtained from MPM and other theoretical model. Work functions obtained using SVM approach agree well with the experimental values.

More precise determination of work function based on Fermi–Dirac distribution and Fowler formula

More precise numerical method to simulate current–voltage of metal at fixed temperature is presented in this paper. The new algorithm for the simulation has been developed via Fermi–Dirac distribution step by step. These calculated characteristics are shown to remain in excellent agreement with the experimental ones, taken for a range of different metals, which strong supports the validity of the model. It is also shown that based on the Fowler formula, higher precise work function can be determined.

Growth and surface properties of epitaxial SnO2

The surface potentials of SnO2 films grown epitaxially by magnetron sputtering on TiO2 and Al2O3 substrates with (110), (001), (101), and (100) SnO2 surface orientations are determined using in situ photoelectron spectroscopy. Epitaxial growth is verified using X-ray diffraction and low energy electron diffraction. The emphasis lies on the determination of work functions and ionization potentials of epitaxial SnO2 surfaces. SnO2 films prepared under chemically reducing conditions exhibit work functions φ of 4.25–4.48 eV and ionization potentials IP of 7.54–8.11 eV. It is furthermore demonstrated that a subsequent annealing in oxygen alters the surface dipole, visible through a large increase of ionization potential. This is due to a change of the surface termination from a reduced to a stoichiometric surface which exhibits Sn in the +IV oxidation state. The observed increase of IP varies from 0.48 eV for the (110) SnO2 surface to 1.05 eV for the (101) SnO2 surface.

Determination of effective work function of Pr0.7Ca0.3MnO3 and Pt films on ZrOx using terraced-oxide method

We have determined the effective work function (Φm,eff) of thin-film sputter-deposited Pr0.7Ca0.3MnO3 (PCMO) on atomic-layer-deposited ZrOx. Pt/PCMO/ZrOx/SiO2/Si and Pt/ZrOx/SiO2/Si metal-oxide-semiconductor (MOS) capacitors were fabricated. The Φm,eff were extracted from capacitance-voltage curves of the MOS capacitors by using flat-band voltage versus effective-oxide-thickness plots. PCMO was crystallized using a high-temperature anneal step and crystallinity was confirmed by x-ray diffraction and transmission electron microscopy. Φm,eff of polycrystalline PCMO and Pt on ZrOx was determined to be 5.43 eV and 5.40 eV, respectively. The significantly higher Φm,eff of PCMO is attributed to the presence of oxygen-rich PCMO(002)/ZrOx interfaces as indicated by our density-functional-theory calculations.

Kelvin probe force microscopy of nanocrystalline TiO2 photoelectrodes

Dye-sensitized solar cells (DSCs) provide a promising third-generation photovoltaic concept based on the spectral sensitization of a wide-bandgap metal oxide. Although the nanocrystalline TiO2 photoelectrode of a DSC consists of sintered nanoparticles, there are few studies on the nanoscale properties. We focus on the microscopic work function and surface photovoltage (SPV) determination of TiO2 photoelectrodes using Kelvin probe force microscopy in combination with a tunable illumination system. A comparison of the surface potentials for TiO2 photoelectrodes sensitized with two different dyes, i.e., the standard dye N719 and a copper(I) bis(imine) complex, reveals an inverse orientation of the surface dipole. A higher surface potential was determined for an N719 photoelectrode. The surface potential increase due to the surface dipole correlates with a higher DSC performance. Concluding from this, microscopic surface potential variations, attributed to the complex nanostructure of the photoelectrode, influence the DSC performance. For both bare and sensitized TiO2 photoelectrodes, the measurements reveal microscopic inhomogeneities of more than 100 mV in the work function and show recombination time differences at different locations. The bandgap of 3.2 eV, determined by SPV spectroscopy, remained constant throughout the TiO2 layer. The effect of the built-in potential on the DSC performance at the TiO2/SnO2:F interface, investigated on a nanometer scale by KPFM measurements under visible light illumination, has not been resolved so far.

Alkali Halide Work Function Determination by Knudsen Effusion Mass Spectrometry

A new approach to determine the work function for ionic crystals by Knudsen Effusion Mass Spectrometry was suggested. The method was successfully tested on the alkali halide metals MX (M = Na, K, Rb, Cs; X = F, Cl, Br, I).

Thermionic emission properties and the work function determination of arrays of conical carbon nanotubes

Thermionic emission properties of conical carbon nanotubes (CCNTs) grown on platinum wires and planar graphite foils were investigated. The work function (Φ) values extracted from the thermionic emission data range from 4.1 to 4.7eV. The range of Φ values is attributed to the morphological characteristics, such as tip radius, aspect ratio, density, and wall structure of CCNTs. The observed lower values for Φ are significantly smaller than that of multi-walled carbon nanotubes (MWNTs). The reduced Φ values are attributed to field penetration effect as a result of the local field enhancement from these structures having high aspect ratio and an excellent field enhancement factor. The high amplification of the external field at the apex of the nanostructures is capable of reducing both the barrier height and the width, in turn contributing to the improved emission current at lower temperatures. The ultraviolet photoemission spectroscopy data of CCNTs grown on Pt wires are in reasonable agreement with the thermionic emission data. The conical carbon nanotubes may be potential candidates for thermionic cathodes with superior performance over conventional cathodes.

Determination of Work Function of Graphene under a Metal Electrode and Its Role in Contact Resistance

Although the work function of graphene under a given metal electrode is critical information for the realization of high-performance graphene-based electronic devices, relatively little relevant research has been carried out to date. In this work, the work function values of graphene under various metals are accurately measured for the first time through a detailed analysis of the capacitance-voltage (C-V) characteristics of a metal-graphene-oxide-semiconductor (MGOS) capacitor structure. In contrast to the high work function of exposed graphene of 4.89-5.16 eV, the work function of graphene under a metal electrode varies depending on the metal species. With a Cr/Au or Ni contact, the work function of graphene is pinned to that of the contacted metal, whereas with a Pd or Au contact the work function assumes a value of ∼4.62 eV regardless of the work function of the contact metal. A study of the gate voltage dependence on the contact resistance shows that the latter case provides lower contact resistance.

Determination of graphene work function and graphene-insulator-semiconductor band alignment by internal photoemission spectroscopy

We determined the band alignment of a graphene-insulator-semiconductor structure using internal photoemission spectroscopy. From the flatband voltage and Dirac voltage, we infer a 4.6× 1011cm−2 negative extrinsic charge present on the graphene surface. Also, we extract the graphene work function to be 4.56 eV, in excellent agreement with theoretical and experimental values in literature. Electron and hole injection from heavily doped p-type silicon (Si) are both observed. The barrier height from the top of the valence band of Si to the bottom of the conduction band of silicon dioxide (SiO2) is found to be 4.3 eV. The small optical absorption in graphene makes it a good transparent contact to enable the direct observation of hole injection from Si to graphene. The barrier height for holes escaping from the bottom of Si conduction band to the top of SiO2 valence band is found to be 4.6 eV.