Work Function Changes Research Articles

Comparison of thermionic filament and carbon nanotube field emitter-based electron ionization sources in cycloidal coded aperture mass analyzers

This work compares the coded aperture imaging performance of thermionic filament and carbon nanotube (CNT) field emitter-based electron sources in cycloidal-coded aperture mass spectrometers. The use of spatially coded apertures in mass spectrometry enables miniaturization by improving throughput without sacrificing resolution. CNT-based electron ionization sources for mass spectrometers provide several potential benefits over conventional thermionic emitters, including low voltage and low power consumption, room temperature operation, long lifetime, and ability to emit electrons in a pulsed mode. However, spatiotemporal variation in electron emission from CNTs is a major disadvantage. In this study, electron emission stability and spatiotemporal stability of the coded aperture image were compared for coded aperture cycloidal mass analyzers with either a CNT-based ion source or a thermionic filament-based ion source. We found that the thermionic filament-based ion source produced a significantly more stable coded aperture image than the CNT based ion source. The aperture image fluctuations in the CNT-based source are likely a result of adsorption and desorption of molecules on the CNT surface that cause local work function changes and induce spatiotemporal variation in electron emission and subsequent ion generation.

Laser-directed synthesis of strain-induced crumpled MoS2 structure for enhanced triboelectrification toward haptic sensors

Two-dimensional (2D) transition metal dichalcogenide (TMDC) nanomaterials are currently regarded as next-generation electronic materials for future flexible, transparent, and wearable electronics. Due to the lack of compatible synthesis and study, however, the characteristic influences of 2D TMDC nanomaterials have been little investigated in the field of triboelectric nanogenerator (TENG) devices that are currently one of the main technologies for mechanical energy harvesting. In this report, we demonstrate a fast, non-vacuum, wafer-scale, and patternable synthesis method for 2D MoS 2 using pulsed laser-directed thermolysis. The laser-based synthesis technique that we have developed can apply internal stress to MoS 2 crystal by adjusting its morphological structure, so that a surface-crumpled MoS 2 TENG device generates ~40% more power than a flat MoS 2 one. Compared to other MoS 2 -based TENG devices, it shows high-performance energy harvesting (up to ~25 V and ~1.2 μA) without assistance from other materials, even when the counterpart triboelectric surface has a slightly different triboelectric series. This enhanced triboelectrification is attribute to work function change as well as enlarged surface roughness. Finally, the direct-synthesized MoS 2 patterns are utilized to fabricate a self-powered flexible haptic sensor array. The technique we propose here is intended to stimulate further investigation of the triboelectric effects and applications of 2D TMDC nanomaterials. • Fast, non-vacuum, large-scale and patternable synthesis of 2D MoS 2 is developed by laser-directed thermolysis. • Surface morphology of MoS 2 is intentionally controlled by tuning laser synthesis mechanism. • Surface-crumpled MoS 2 shows high-performance triboelectric energy harvesting signals. • Improved triboelectrification is attributable to work function change, surface area enlargement, and secondary effects. • Self-powered flexible haptic sensor array is fabricated using directly patterned MoS 2 .

Chemoresistive CO2 Gas Sensors Based On La2O2CO3: Sensing Mechanism Insights Provided by Operando Characterization.

Our previous studies demonstrated that rare-earth oxycarbonates Ln2O2CO3 (Ln = La, Nd, and Sm) and rare-earth oxides Ln2O3 (Ln = Nd, Sm, Gd, Dy, Er, and Yb) are sensitive to CO2 and that hexagonal La2O2CO3 is the best among them in terms of sensitivity, stability, and selectivity. In this study, we have conducted a comprehensive operando characterization on a hexagonal La2O2CO3 based sensor for the basic understanding of the sensing mechanism. This was done by performing under actual operating conditions simultaneous DC resistance and work function changes measurements, AC impedance spectroscopy measurements, and simultaneous DC resistance and DRIFT spectroscopy measurements. The results demonstrate that the double Schottky barriers at grain-grain boundaries are dominant contribution to sensor resistance; there is a competitive adsorption between carbonate species and hydroxyl groups, which depends on both CO2 concentration and humidity and leads to the change in height of the Schottky barriers. Finally, we propose a reaction model stating that there are three types of adsorbates, -CO32-, -OH-, and -O2-, and the relative concentration of which is controlled by a reaction with ambient humidity and CO2. This model is able to consistently explain all our experimental findings.

Anomalous vibrational behavior of two dimensional tellurium: Layer thickness and temperature dependent Raman spectroscopic study

A few layer/multilayer two-dimensional (2D) Tellurium (Te) nanoflakes were mechanically exfoliated and their structural and vibrational properties were probed. Layer thickness reduction was confirmed using optical microscopy, atomic force microscopy and kelvin probe force microscopy. Raman spectroscopy measurements revealed anomalous shift in the frequencies of their intrachain vibrational modes, where, after an initial increasing trend (blue shift) with thickness reduction from bulk, they changed to decreasing trend (red shift) on further layer thinning. Such unusual vibrational behaviour of 2D Te, exhibiting reversal from blue shift to red shift, is attributed mainly to the deformations occurring in the Te chains on layer thinning, followed by modifications in the local structures and bondlengths of the corresponding Te atoms. The deformation under relaxed conditions are also confirmed from first principle calculations. Red shift can also be correlated to lesser hole concentration for thinner flakes, as revealed by work function changes measured on the nanoflakes surfaces. Interestingly, in spite of such anomalous behavior, the difference in frequencies of both modes showed initial increase with thickness and saturation beyond a certain thickness, as generally observed in other 2D materials. Temperature dependent Raman spectroscopic measurements further confirmed anomalous behaviour for thinner flakes.

Nanotechnological structure for observation of current induced contact potential difference and creation of effective Cooper pair mass-Spectroscopy

Changes of the electron work-function of a superconductor proportional to the square of the current density Δϕ=−βj2 are known as Bernoulli effect in superconductors or current induced Contact Potential Difference (CPD). The temperature dependent coefficient β(T; m*) is parametrized by the effective mass of Cooper pairs m*. In such a way the study of the Bernoulli effect leads to creation of Cooper pair mass-spectroscopy. In this paper a short review on the Bernoulli effect in superconductors is given and a proposed experimental set-up for its measurement is described in detail. The experiment requires standard electronic equipment and can be implemented in every laboratory related to physics of superconductivity. This experimental set-up for observation of current induced CPD requires nano-technological hybrid superconductor structures with atomically clean interfaces and measurement of nano-volt signals with capacitive coupling to the sample. As the quality of the superconductor surface is crucial for the proposed experiment the research is better to start with crystals used for angle-resolved photo-emission spectroscopy (ARPES); all those samples have excellent quality guarantying success even for the first experiment.

Analysis of Oxygen and Nitrogen Redistribution at Interfaces of HfO2 with Laminate TiN/TiAl/TiN Electrodes

The physicochemical nature of the change in the effective work function (EWF) of aluminum doped TiN (using TiN/TiAl/TiN laminate structure) was studied by X-ray photoelectron spectroscopy with high...

Electrical and optical characterizations of erbium doped MPS/PANI heterojunctions

Macroporous silicon (MPS) films were yielded by electrochemical anodization of p-type crystalline silicon in HF:DMF. Following, a Schottky structure was obtained by depositing polyaniline (PANI), which was doped with different contents of erbium (Er) using cyclic voltammetry method. The structural analysis showed the formation of pores with diameters ranging between 0.25 and 1.20 μm and length around 15 μm and, after PANI deposition, the diameter became about 17% lower. The chemical characterization by Energy dispersive X-ray spectroscopy (EDS) and Rutherford Backscattering Spectroscopy (RBS) pointed out that the erbium is primarily incorporated at polyaniline surface, but it spreads through the polymer layer and achieves the MPS/PANI interface. According to the Attenuated Total Reflectance Fourier Transform Infrared (ATR-FTIR) analysis, the presence of Er gave rise to the formation of additional functional groups besides that corresponding to polyaniline and silicon oxide phases. The optical characterization within the UV–vis region revealed a modified optical bandgap of PANI as a function of the Er doping concentration. The results of the electrical measurements pointed out to a lowering of the space charge region width in the silicon as well as a change of the PANI workfunction as a function of the Er doping. On the other hand, the current flow through the MPS/PANI Schottky junction was lowered by the increase of the Er doping and by the tunneling mechanism through a thin silicon oxide (≈1–2 nm) grown on MPS during PANI deposition in aqueous solution.

Mechanisms of TiN Effective Workfunction Tuning at Interfaces with HfO2 and SiO2

By use of a combination of electrical measurements and internal photoemission interface barrier characterization, the effective workfunction (EWF) changes of nm-thin TiN layer deposited on top of o...

Double Beneficial Role of Fluorinated Fullerene Dopants on Organic Thin-Film Transistors: Structural Stability and Improved Performance.

The present work assesses improved carrier injection in organic field-effect transistors by contact doping and provides fundamental insight into the multiple impacts that the dopant/semiconductor interface details have on the long-term and thermal stability of devices. We investigate donor [1]benzothieno[3,2-b]-[1]benzothiophene (BTBT) derivatives with one and two octyl side chains attached to the core, therefore constituting asymmetric (BTBT-C8) and symmetric (C8-BTBT-C8) molecules, respectively. Our results reveal that films formed out of the asymmetric BTBT-C8 expose the same alkyl-terminated surface as the C8-BTBT-C8 films do. In both cases, the consequence of depositing fluorinated fullerene (C60F48) as a molecular p-dopant is the formation of C60F48 crystalline islands decorating the step edges of the underlying semiconductor film surface. We demonstrate that local work function changes along with a peculiar nanomorphology lead to the double beneficial effect of lowering the contact resistance and providing long-term and enhanced thermal stability of the devices.

Determination of the potential of zero charge of Pt/CO electrodes using an impinging jet system

In this manuscript, the potential of zero charge (pzc) of platinum single-crystal electrodes has been determined using the impinging jet system and recording the transient associated with the interphase formation. Initially, the Pt(111), Pt(110), and Pt(100) electrodes covered with CO were used. To demonstrate the validity of the procedure, the values obtained for the pzc of these electrodes were compared with those previously reported in the literature and also with those expected from work function changes. An excellent agreement has been found. After that, the values for the pzc of the stepped surfaces having (111) terraces and (110) steps were determined. For these surfaces, the pzc initially diminishes as the step density increases, in accordance with the expected diminution of the work function due to the Smoluchowski effect on the step site. However, for the shorter terraces, the pzc value increases with the step density, probably because of step dipole-dipole coupling due to the presence of a CO layer on the electrode.

Alkali Doping Leads to Charge-Transfer Salt Formation in a Two-Dimensional Metal–Organic Framework

Efficient charge transfer across metal–organic interfaces is a key physical process in modern organic electronics devices, and characterization of the energy level alignment at the interface is crucial to enable a rational device design. We show that the insertion of alkali atoms can significantly change the structure and electronic properties of a metal–organic interface. Coadsorption of tetracyanoquinodimethane (TCNQ) and potassium on a Ag(111) surface leads to the formation of a two-dimensional charge transfer salt, with properties quite different from those of the two-dimensional Ag adatom TCNQ metal–organic framework formed in the absence of K doping. We establish a highly accurate structural model by combination of quantitative X-ray standing wave measurements, scanning tunnelling microscopy, and density-functional theory (DFT) calculations. Full agreement between the experimental data and the computational prediction of the structure is only achieved by inclusion of a charge-transfer-scaled dispersion correction in the DFT, which correctly accounts for the effects of strong charge transfer on the atomic polarizability of potassium. The commensurate surface layer formed by TCNQ and K is dominated by strong charge transfer and ionic bonding and is accompanied by a structural and electronic decoupling from the underlying metal substrate. The consequence is a significant change in energy level alignment and work function compared to TCNQ on Ag(111). Possible implications of charge-transfer salt formation at metal–organic interfaces for organic thin-film devices are discussed.

Theoretical Study of Interfacial and Electronic Properties of Transition Metal Dichalcogenides and Organic Molecules Based van der Waals Heterostructures

AbstractHeterostructures built from 2D materials and organic semiconductors offer a unique platform for addressing many fundamental physics and construction of functional devices. Interfaces play a crucial role in tailoring the heterostructure properties. Here, density functional theory computations are performed to explore the interfacial properties of heterostructures made of group VI transition metal dichalcogenides (TMD) and organic molecules such as perylene tetracarboxylic dianhydride (PTCDA) and pentacene. First principle calculations predict that the organic pentacene layer exhibits covalent interfacing with MoSe2 and WSe2, while the interface of other studied TMD/organic heterostructures form van der Waals (vdW) interfaces. Owing to the different molecular geometry of PTCDA and pentacene in their respective heterostructures, the work function can be modulated of the order of 1.0 eV in comparison with pure monolayer MX2 in MX2/pentacene (M = Mo, W; X = S, Se) heterostructures, while the change of work function in MX2/PTCDA (M = Mo, W; X = S, Se) is negligible (order of 0.1 eV) in comparison with pure monolayer MX2. This study will be helpful to design high‐performance optoelectronic devices based on TMDs and organic semiconductors.

Electrochemically Enhancing Nanostructured Fe-Oxide/Co3O4 for the Reverse Water Gas Shift Reaction

The abundant amount of CO2 in the atmosphere is a valuable resource that when managed correctly can replace energy from the fossil fuel industry and provide a carbon-neutral solution that will reduce the impacts of the climate and ecological crisis[1]. This is achieved by reducing CO2 with hydrogen (H2) through the reverse water gas shift (RWGS) reaction to produced carbon monoxide (CO). CO then acts as a source molecule in the Fischer-Tropsch synthesis to manufacture hydrocarbon fuels. Due to the high stability of the CO2 molecule, significant thermal energy is required to undertake the reaction. Iron-oxide (FeOx) has been shown to be an active and thermally stable catalysts towards the RWGS reaction. FeOx (iron-oxide) nanowires are fabricated through the polyol reduction method and have been characterized through scanning transmission electron microscopy (STEM) analysis[2]. Following the wet deposition method, the Fe nanowires are finely dispersed on cobalt-oxide (Co3O4) to act as a support and semi-conductor. The dispersion of Fe on Co3O4 enhances the electronic properties of the catalyst through the metal-support interaction (MSI) effect[3]. The MSI entails the back spillover of promoting oxygen (Oδ-) ionic species from Co3O4 to FeOx by an increase in temperature, altering the work function of FeOx and allowing to cycle oxygen from the breaking of CO2. Through the Electrochemical Promotion of Catalysis (EPOC) phenomenon the catalytic activity can be enhanced by altering the binding energy of the reactant and intermediate species on the catalytic surface[4]. The MSI and EPOC phenomena have been shown to be functionally equivalent in terms of the change in work function. The difference between them is that in EPOC the movement of ions can be controlled through the application of an electrical current or potential difference, while the MSI effect is controlled by a thermal input. EPOC entails the use of solid electrolytes, for instance yttria-stabilized zirconia (YSZ) and barium zirconate yttrium-doped (BZY) which are oxygen and proton conductors, respectively. The catalyst-working electrodes (i.e. FeOx nanowires) are deposited on one side of the electrolyte and on the opposite side, inert gold counter and reference electrodes. Through the application of a potential difference between the electrodes, promoting species migrate through the three-phase (solid-gas-catalyst) boundary in order to interact with the exposed-catalyst surface. In the case of BZY, when a potential difference is applied between the counter and working electrode, H+ are removed from the surface through the three-phase boundary (tpb) towards BZY, referred to as positive polarization. When the polarization is reversed from the working to counter electrode, H+ migrate from BZY through the tpb towards the catalytic surface. Utilizing YSZ, results in the opposite migration with O2- ions. Regardless of the type of solid electrolyte used the catalyst is oxidized under positive polarization and reduced under negative polarization. The use of the Co3O4 semiconductor allows to combine both the MSI and EPOC effect at 350°C, by finely dispersing the Fe nanowires on Co3O4 to expose active sites and lowering the rate of sintering while being conductive to close the circuit[5]. As opposed to using other supports that do not display fully conductive properties, this approach provides an electronic path for the promoters to follow. Results have shown the working-catalyst FeOx/Co3O4 on YSZ and BZY to be highly selective to CO formation under oxidizing (3CO2:H2) and reducing (CO2:7H2) conditions, with a superior activity experienced under rich reducing conditions. Furthermore, alteration in work function from the EPOC effect, has led to a promotional response for both YSZ and BZY. Combining the MSI and EPOC effect allows for high RWGS activity, establishing a promising solution in utilizing CO2 as a resource while using transition metals. Additionally, the EPOC effect will be elucidated through Fourier-Transform Infrared (FTIR) spectroscopy to provide insight on the promoting mechanism occurring during polarization. 1. Porosoff, M. D., Yan, B. & Chen, J. G. Catalytic reduction of CO2 by H2 for synthesis of CO, methanol and hydrocarbons: challenges and opportunities. Energy Environ. Sci. 2, 303 (2015).1. Baranova, E. A., Bock, C., Ilin, D., Wang, D. & MacDougall, B. Infrared spectroscopy on size-controlled synthesized Pt-based nano-catalysts. Surf. Sci. 600, 3502–3511 (2006).2. Panaritis, C., Edake, M., Couillard, M., Einakchi, R. & Baranova, E. A. Insight towards the role of ceria-based supports for reverse water gas shift reaction over RuFe nanoparticles. J. CO2 Util. 26, 350–358 (2018).3. Vayenas, C. G., Bebelis, S., Pliangos, C., Brosda, S. & Tsiplakides, D. Electrochemical Activation of Catalysis. (Springer US, 2001).4. Zagoraios, D. et al. Electrochemical promotion of methane oxidation over nanodispersed Pd/Co3O4 catalysts. Catal. Today (2019). doi:10.1016/j.cattod.2019.02.030

Bacterial attachment to oxygen-functionalized graphenic surfaces.

In this work we have investigated the effect of oxygen plasma treatment of graphenic surfaces and the introduction of functional groups on changes in work function, wettability, surface free energy and bacterial adhesion. The plasma parameters were adjusted (generator power: <60W, exposure time: <20min) to limit the modifications to the surface without changing the bulk structure. The parent and modified graphenic surfaces were thoroughly characterized by μRaman spectroscopy, thermogravimetry, scanning electron microscopy, contact angle, X-ray photoelectron spectroscopy, work function and microbiological tests. It was found that even the short time of plasma modification results in a significant increase in work function, surface free energy and hydrophilicity. The changes in surface chemistry stimulate also substantial changes in bacterial adhesion. The strong relationship between work function and adhesion of bacteria was observed for all the investigated strains (Staphylococcus epidermidis, Staphylococcus aureus, Pseudomonas aeruginosa, Escherichia coli) whereas the bacterial colonization trend correlates with the bacterial zeta potential. The bacteria-graphenic surface interaction is discussed in terms of total interaction energy. The results point out the work function lowering of the graphenic biomaterial surface as an effective strategy for the infection risk limitation.

Graphene as interface modifier in ITO and ITO-Cr electrodes

We explore graphene as interface modifier for electrodes in optoelectronic organic devices by measuring the electrical properties of ITO/graphene and ITO/Cr/graphene. For this purpose, exfoliated graphene (EG) was electrochemically synthesized and deposited by spray-pyrolysis. The built-in voltage (Vbi) values were 450 mV for the ITO/CuPc/Al reference, 750 mV for ITO/Cr/graphene/CuPc/Al and 1000 mV for ITO/graphene/CuPc/Al device structures. From these results, we estimate the work functions as 3.20 eV, 3.45 eV and 4.75 eV for ITO/EG, ITO/Cr/EG and ITO. To understand how the work function changes, we carried out first-principles calculations based on density-functional theory (DFT) where Cr work function (~4.2 eV) is not modified by the deposition of pristine graphene; however there is a substantial increase (from 4.2 eV to 5.2 eV), upon deposition of graphene oxide (GO), resulting from a complete transfer of O atoms from the GO sheet to the Cr surface forming a thin layer of chromium oxide.

Tunable adsorption behavior of small molecule on GeP monolayer by applied strain and electric field

The sensing properties of GeP monolayer to small gas molecule (CO, CO2, O2, H2O, NH3, NO and NO2) have been investigated by performing first-principle calculations. Based on calculated adsorption distance, adsorption energy and charge transfer, we find that GeP monolayer is the most sensitive to NO2 and NO molecules, and all gas molecules act as acceptors to gain electrons from GeP monolayer. The adsorption energy of NO2 is smaller than that of NO molecule, which implies that NO2 is more easily desorbed from GeP. And different work function changes also indicate the possibility to identify NO2 and NO in an oxygen-free environment. Moreover, we apply uniaxial and biaxial strains to GeP monolayer and find that a small biaxial compressive strain can effectively enhance the sensitivity of work function type sensor based GeP to NO2 molecule without increasing recovery time. We also discuss the effect of external vertical electric field on the adsorption performance of NO2 molecule. The negative electric field can significantly increase the adsorption strength, charge transfer and work function change of NO2-GeP system. Our results indicate that GeP monolayer is a potential candidate for NO2 gas sensor.

Sensitivity Assessment of RingFET Architecture for the Detection of Gas Molecules: Numerical Investigation

TCAD-based numerical investigation of RingFET has been presented in this paper for the detection of gas molecules. Sensitivity of the device has been assessed by calculating the change in off-state current with the change in gate work-function (which changes due to the presence of gas molecule). The behavior of the device has also been compared with the equivalent bulk MOSFET and it was observed that RingFET architecture shows 23% higher sensitivity against gas molecules than equivalent bulk MOSFET. Also, the degradation in device sensitivity with applied drain bias is higher in bulk MOSFET. Optimization in the sensitivity parameter has been done by using different channel materials like GaN, InGaN, and GaAs. Compared to GaAs and silicon, GaN-based RingFET is more sensitive towards gas molecules in the sub-threshold region (due to the wider band gap) besides having lower drain current than GaAs and silicon-based RingFET.

Directional Change of Interfacial Electric Field by Carbon Insertion in Heterojunction System TiO2/WO3.

Z-scheme transfer is an ideal photocatalytic system with stronger redox ability, but its design and construction still lack understanding. Herein, the work function difference and the band bending are found to be the determining factors for the construction of the Z-scheme transfer mechanism of photoexcited charges in TiO2/WO3. The control of work function and band bending achieved by carbon insertion results from the hybridization of orbitals and redistribution of electron density, as demonstrated by ultraviolet photoelectron spectroscopy and photocatalytic analysis. The heterojunction system, TiO2/WO3, with controlled work function and band bending, shows 2 times faster •OH radical formation rate (0.011 μmol min-1) compared to the undisturbed system. First-principles calculation reveals that the changes in work function and band bending result in an interfacial electric field, which shifts the charge transfer mechanism from type II to Z-scheme. This work proves that the design of work function and band bending allows reconstructing charge transfer mechanism by forming the interfacial electric field in heterojunction systems.

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.

Wide-range work function tuning in gold surfaces modified with fluorobenzenethiols toward application to organic thin-film transistors

Surface properties of Au electrodes modified by benzenethiol derivatives with a fluorine atom(s) have been methodically researched based on measurements of the work function and the contact angles. Benzenethiol derivatives with a fluorine atom(s) at ortho, meta, and/or para position were used for modification in this work. The measured work function was in a relatively wide range between 4.24–6.02 eV. The work function change from a bare Au surface was explained on the principle of dipole moments obtained by quantum chemical calculation. The water contact angle was found to vary between 64.8° and 97.7°. Furthermore, the surface tension was calculated from the measured contact angles of water and ethylene glycol. The calculated surface tension was reviewed from the perspective of the position of the substitute in the benzenethiol derivative. In addition, organic thin-film transistors (TFTs) with drain and source electrodes modified with 2-fluorobenzenethiol (2-FBT), 3-fluorobenzenethiol (3-FBT) or pentafluorobenzenethiol (PFBT) were characterized as other evaluations of the modified Au surface. The contact resistance in the TFT increased in the order of PFBT, 3-FBT and 2-FBT. The increase of the contact resistance was consistent with the decrease in the work function.