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Article

The Mutual Influence of Elemental S and Cs on the Ni(100) Surface at Room and Elevated Temperatures

by
Aris Chris Papageorgopoulos
,
Dimitrios Vlachos
* and
Mattheos Kamaratos
Department of Physics, University of Ioannina, P.O. Box 1186, GR-451 Ioannina, Greece
*
Author to whom correspondence should be addressed.
Surfaces 2025, 8(3), 68; https://doi.org/10.3390/surfaces8030068
Submission received: 10 August 2025 / Revised: 4 September 2025 / Accepted: 6 September 2025 / Published: 12 September 2025
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Abstract

The behavior of S and Cs during the alternate adsorption of each adsorbate on the Ni(100) surface is studied at room and elevated temperatures by means of low-energy electron diffraction (LEED), Auger electron spectroscopy (AES), thermal desorption spectroscopy (TDS) and work function (WF) measurements. For Cs deposition on the S-covered Ni(100) surface, the presence of sulfur increases the binding energy and the maximum amount of adsorbed cesium, as happens with other alkalis too. The first Cs overlayer is disordered, while the second strongly interacts with S with a tendency toward a CsxSy surface compound formation. This interaction causes the gradual demetallization of the Cs overlayer with the increasing S coverage in the underlayer. When the CsxSy stoicheometry is complete, however, subsequent Cs deposition forms an independent rather metallic overlayer. When the sulfated covers the surface, S(0.5ML)/Ni(100) is preheated to 1100 K, the S-Ni bond strengthens and S-Cs interaction correspondingly weakens to a degree that the S underlayer retains a periodic structure on the Ni substrate. This behavior indicates that the preheated S/Ni(100) surface is passivated to a degree against Cs with reduced mobility of sulfur adatoms. Differently, when S is adsorbed on the Cs-covered Ni(100) surface at room temperature, sulfur adatoms diffuse underneath the Cs overlayer to interact with the nickel substrate and form the same structural phases as on a clean surface. During that process, the sticking coefficient of sulfur remains constant regardless of the amount of pre-deposited cesium. The presence of Cs, however, increases the amount of S that can be deposited on the Ni substrate, probably in favor of the CsxSy compound formation, which demetallizes the surface independent of the sequence of adsorption.

Graphical Abstract

1. Introduction

Sulfur (S) is an electronegative element which can drastically modify the surface properties of materials in different ways by forming buffer layers, controlling texture oxides’ layers, improving lattice mismatch and chemical inertness between superconducting YBCO layers and substrates, etc. [1,2,3]. Also, in heterogeneous catalysis, S can affect the activity and selectivity of catalysts in a rather controversial way, either showing a poisoning behavior [4,5,6,7], improving the catalytic activity under some circumstances [8] or even better, acting as a catalyst promoter or selectivity modifier [9]. More recently, cobalt and nickel sulfides have endowed superior electocatalytic performance in electrocatalysts for alkaline hydrogen evolution reactions and hydrogen resources [10,11]. In addition, sulfur adsorption often passivates surfaces by improving the surface and interface quality of heterojunctions [12], increasing the performance of photodetectors [13] and the efficiency of solar cells [14].
On the other hand, alkalis, due to their electropositive nature, can drastically reduce the work function (WF) of the surfaces. The WF is a very prominent physical property which can crucially affect the catalytic [15,16,17,18], photoelectric [19,20] and thermionic properties [19,21] of materials. Low-WF interfaces are especially desirable for optimizing the opto-electronic properties of semiconducting devices such as diodes and transistors [20]. One efficient way to produce such interfaces is to adsorb low-WF elements such as alkalis or alkaline earth metals (barium and calcium) on surfaces. Among the existing alkalis, cesium (Cs) is the most electropositive element, with the lowest WF of all the metals (2.0–2.5 eV) [22]. Therefore, since the 70s, Cs has been applied as an adsorbate on surfaces to decrease WF down to even 1.0–1.6 eV (at submonolayer regime) [23,24,25]. However, the impurities of electronegative elements such as oxygen or sulfur can modify the surface properties of the cesium overlayer, such as WF, thus affecting the catalytic and thermionic efficiency of the formed interface. In the particular case of nickel substrate, extensive reports over the last decades show a serious poisoning effect of S in catalytic efficiency [26,27,28,29,30,31,32]. Moreover, the interaction of the adsorbed S and alkalis from the gas phase on nickel-based catalysts can influence the catalytic activity for producing valuable fuels and chemicals from biomass gasification [33,34]. Unfortunately, the exact atomistic mechanism of S and alkali interaction on such types of catalytic surfaces is not accurately known. For this reason, the detailed study of the mutual influence of elemental S and adsorbed alkali and how the relative atomic concentrations of the two elements affect the dynamics, the kinetics and finally, the activity of an alkali-modified surface is of particular interest. Based on that motivation, this contribution reports a detailed and systematic experimental investigation of S and Cs coadsorption on the Ni(100) surface. In particular, two coadsorbing processes are followed: (1) the adsorption of Cs on an S-covered Ni(100) surface at room temperature (RΤ) and with heat treatment, and (2) the adsorption of S on a Cs-covered Ni(100) surface. The surface analysis techniques of low-energy electron diffraction (LEED), Auger electron spectroscopy (AES), thermal desorption spectroscopy (TDS) and relative work function (WF) measurements are used to acquire data.

2. Experimental Part

The experiments were carried out in an ultra-high vacuum (UHV) chamber with base pressure of the order 10−10 mbar. The chamber was equipped with a Varian cylindrical mirror analyzer (CMA) for AES measurements, a quadropole mass spectrometer (QMS) by Hiden Analytical in correlation with a constant heating rate sample heater of ~20 K/s for TDS measurements, a Kelvin probe for relative WF measurements, an Omicron LEED optics device for observing the surface structure and an appropriate a Varian apparatus for Ar+ ion sputtering. All the AES measurements were recorded in the first derivative mode and thus, the intensity of the Auger electron transition line (AETL) in each case was measured from the Auger peak to peak height (Ap-pH).
The Ni(100) sample was cleaned by Ar+ ion bombardment (2 keV, 4 μA) and subsequent annealing at 1000 K. This cycle was repeated several times until the Auger peak heights of the main impurities (carbon, sulfur and oxygen) were under the detection limit. The sample heating was performed inductively by passing an electric current through a heating tape of tantalum (Ta), uniformly pressed and fixed between the back side of the crystal and a metallic case of Ta, where the sample was firmly fixed inside. The temperature of the sample was measured by a Cr-Al thermocouple spot welded to the back side of the sample metallic case and calibrated with an infrared pyrometer in the range of 900 to 1300 K. The whole heating system with the crystal was mounted at the edge of a rotatable manipulator. In this manner, the sample could be located in front of each experimental technique and evaporation source each time.
Cesium was evaporated from a commercial dispenser source (SAES getters), while elemental sulfur was deposited by thermal dissociation of molybdenum disulfide (MoS2) single crystal flakes mounted on a tungsten (W) heating filament and enclosed in a home-made Ta dispenser. In the past, most S adsorption studies have taken place with exposure of the substrate surfaces to hydrogen sulfide (H2S) gas. Το remove the H2 from the surface, however, the substrates should be heated to temperatures between 400 K and 1300 K [35,36,37,38,39]. Coadsorption studies of S and alkali metals, therefore, cannot take place at RΤ using H2S as a sulfur source. Differently, with our method of MoS2 thermal dissociation, a reliable evaporation source of S atoms with a controllable flux is created based on the current flowing through the W filament in the dispenser. Actually, we applied this method in earlier studies for successful coadsorption of sulfur with alkalis on the Ni(100) surface at RT [40,41,42,43]. The coverages of both Cs and S were calculated by correlating LEED and AES measurements. The surface atomic density of 1 ML of each adsorbate is considered equal to that of the outermost layer of the Ni(100) single crystal, 1.6 × 1015 atoms/cm2. Based on the combined AES and LEED measurements, the coverage of one dose (1 D) of S deposition time was estimated at ~0.038 ML, while that of Cs was ~0.020 ML.
Apart from the Cs adsorption on a clean sulfated Ni(100) surface at RT, we tried adsorption on the preheated S(0.5 ML)/Ni(100) surface at 1100 K to investigate if the preheating process changes the adsorption properties of the sulfated nickel. We choose this temperature since annealing at higher temperatures causes substantial desorption of S and defects in the nickel crystal [44].

3. Results and Discussion

Before commencing with the analysis of the data pertinent to the coadsorption of S and Cs on Ni(100) obtained in the current work, it is useful to recall from the literature the behavior of each element when adsorbed separately and not only on the clean nickel.
Concerning the adsorption of Cs on the Ni(100) surface at 300 K, it was first studied in an early work [24]. We briefly mention that as Cs deposits on a surface, a uniform layer with a hexagonal close-packed (hcp) structure is completed at a cesium coverage of ΘCs = 0.29 ML. In addition, measurements of the WF change follow the characteristic WF curve of alkali metals deposited on metal surfaces, where the WF initially drastically decreases down to a minimum value Φmin = 1.6 eV (at ΘCs = 0.14 ML) and subsequently increases to a maximum constant value Φmax = 2.0 eV, which coincides with the formation of the hcp surface structure and the metallization of the surface. No more than one physical layer of Cs is formed on the nickel surface at RT.
With regards to the adsorption of elemental S on the clean Ni(100) surface at RT, it is well-known that the p(2 × 2) super-lattice is at sulfur coverage Θs = 0.25 ML, and thereafter, the c(2 × 2) is at Θs = 0.5 ML [44,45,46]. The latter structure coincides with the completion of the first physical sulfur layer on the surface. For both symmetrical structures, it seems that S adatoms preferably occupy fourfold-hollow adsorption sites [45,46,47,48,49]. At increasing coverage, the sulfur growth occurs in a layer-by-layer mode, where the second layer is observed to be disordered. During S deposition on Ni(100) and the formation of the p(2 × 2) phase, the WF increases by ΔΦ = 0.17 eV, while when the phase is transformed into the c(2 × 2) one, a final ΔΦ = 0.25 eV is recorded [44]. This sequential WF increase probably reflects the change in the surface electronic structure between the two phases [46,49].
Relating to coadsorption experiments of Cs and S on the Ni(100) surface, some results have been previously reported [41]. To describe them briefly, the presence of S on the Ni(100) surface at RT increases the maximum coverage of Cs from one single layer to at least two atomic layers, indicating a layer-by-layer growth mode. Furthermore, the initial sticking coefficient of Cs remains unaffected by S presence. Deposited Cs at coverage ΘCs > 0.5 ML on both p(2 × 2) and c(2 × 2) structures of the S-covered Ni(l00) surface at RΤ is randomly distributed on top of the sulfur layer, causing the disappearance of its symmetrical structures. This effect indicates a rearrangement of the S underlayer to a disordered configuration, probably due to a strong Cs-S interaction. This interaction seems to increase the binding energy of Cs on the surface, spoiling the metallization and the well-ordered hpc form of the alkali overlayer observed in the absence of sulfur [24]. Nevertheless, based on TDS measurements, there are no indications for desorbing Cs-S chemical compounds [41].
In the current work, we followed two different coadsorption procedures for studying the Cs and S interaction on the Ni(100) surface: (1) the cesiation of the sulfur-covered surface and (2) the sulfation of the cesiated surface. With this framework, we reproduced and verified some earlier reported results and also collected new data revealing more details of the Cs and S mutual influence on the nickel surface, especially at elevated temperatures. The results and discussion of each coadsorption procedure are as follows.

3.1. Cesium on S-Covered Ni(100)

We first carried out detailed LEED observations at RT for Cs deposition on clean and S-covered Ni(100) surfaces. The results are summarized in Table 1. The first row (top one) demonstrates the increasing Cs coverage in ML (ΘCs), while the first column (left hand one) demonstrates the S coverage in ML (Θs). Every row describes the surface structural changes observed when ΘCs increases on a certain S-covered Ni(100) surface at RT. On the other hand, every column describes the surface structural changes when ΘS increases on a certain Cs-covered Ni(100) surface at RT. The order of the structural changes caused by Cs deposition on surface is denoted by the horizontal arrows along the rows, while the corresponding order of the structural changes caused by S deposition is denoted by the vertical arrows along the columns. Note that LEED changes are presented for different S-covered nickel surfaces in the absence of Cs (ΘCs = 0, second column), while LEED changes are also presented for different Cs-covered nickel surfaces in the absence of S (ΘS = 0, second row). In general, Cs deposition takes place on the S-covered nickel surfaces at RT in all cases; that is why this temperature appears in brackets in the boxes of the first column. In one case only, Cs deposits at RT but on a preheated S(0.5 ML)/Ni(100) surface at 1100 K (see the bottom row in Table 1). Also, Table 1 depicts the background of the LEED patterns. In fact, the higher the background intensity in the LEED patterns, the darker the shading boxes demonstrating the phases.
Cesium deposition on the clean Ni(100) surface produces the hcp phase at 0.3 ML as is expected by forming a complete single physical layer [24,41]. On the sulfated p(2 × 2):S/Ni(100) surface at Θs = 0.25 ML, Cs adsorption initially creates a superimposed hcp LEED pattern with the p(2 × 2) one, while as the deposition proceeds and for ΘCs ≥ 0.5 ML, the pattern is transformed into a 1 × 1 with a significant background increase. These LEED observations indicate that Cs adatoms from the second developing layer interact mutually with the underlying symmetrically pre-adsorbed S atoms, as well as with the hcp coordinated first-layer Cs adatoms. This interaction spoils the coexisting sulfur and cesium structural phases, rearranging the mixed overlayer into the initial 1 × 1 substrate symmetry. A similar effect is noticed for Cs adsorption on the c(2 × 2):S/Ni(100) surface at Θs = 0.5 ML. In that case, however, no superimposed hcp pattern forms with the c(2 × 2) one, probably due to the much fewer existing available free symmetrical adsorption sites.
On the other hand, the situation appears to be different for cesium deposition on the c(2 × 2):S/Ni(100) preheated at 1100 K for 30 s. In that case, Cs adsorption initially causes a change from a c(2 × 2) pattern into a p(2 × 2) one with a background increase (see the heavy shadowed box in the bottom row in Table 1). This change becomes obvious above 0.3 ML of Cs (the completion of the first physical layer) and ceases at ~0.5 ML. Further Cs deposition changes the p(2 × 2) pattern to a 3 × 3 one. As we noticed above, the pattern’s changes are obvious after the completion of the first Cs layer (ΘCs > 0.3 ML). Therefore, it is very likely that Cs adatoms from the second layer interact with part of the S underlying atoms by initially removing them from the c(2 × 2) phase, thus transforming the sulfur underlayer into the less dense p(2 × 2) one. More Cs adsorption on the surface (ΘCs > 0.5 ML) generates continued interaction with the underlayered S atoms in the p(2 × 2) structure, transforming it into a new 3 × 3 structural phase. As a result, the coverage of the sulfur underlayer, bound directly to the Ni(100) substrate, reduces further to about 0.11 ML. In addition, the appearing higher background designates the amorphous state of the Cs overlayer above the 3 × 3 sulfur phase. The LEED observations presented above in Table 1 predicate that the annealing of the c(2 × 2) sulfated nickel surface at 1100 K affects the dynamics and kinetics of a significant part of the adsorbed S atoms by reducing their mobility and establishing a stronger chemisorption state at the same time. In other words, the mobility of the S atoms on the preheated surface is confined since only a part of the participating c(2 × 2) phase S atoms are mobilizing and becoming disorganized by the second layer’s Cs atoms.
The annealing effect is shown in Figure 1, where the Ap-pH of Cs (563 eV) AETL for ΘCs = 0.3 ML on the clean Ni(100) surface. In addition, ΘCs = 0.6 ML on S-covered Ni(100) surfaces for ΘS ≤ 0.5 ML and ΘS > 0.5 ML is recorded, after annealing up to 1100 K with temperature increments of 100 K. All the shown Ap-pH measurements were performed after the sample cooled down to temperatures near RT. Cesium on the clean Ni(100) surface is thermally removed in two stages according to the lowest curve of Figure 1, most likely corresponding to the two binding states of Cs on the surface, one of low and one of high adsorption energy. This is also verified by the TDS measurements described later. For Θs ≤ 0.5 ML, most of Cs is removed from the surface before 800 K, while the same two-stage desorption pattern is observed in the corresponding Ap-pH curve. The Cs was removed from the substrate up to 800 K, which corresponds to the ΘCs = 0.6 ML curve, including the extra amount of alkali, which the presence of S has allowed to adsorb onto the Ni(100) surface. This amount of desorbed Cs at temperatures less than 800 K belongs to the low-adsorption energy state. It is interesting to note that the amount of Cs left on the surface when Θs ≤ 0.5 ML after heating to 800 K is about the same as that on the clean Ni(100) surface. Bear in mind that 0.6 ML of Cs corresponds to approximately two physical layers [41]. This means that prior to 800 K, the top layer is removed along with the first layer’s loosely bound Cs atoms. The removal of this initial portion of Cs is, however, delayed to higher temperatures with increasing amounts of pre-deposited S. Specifically, when Θs > 0.5 ML and sulfur starts to form a second layer [44], the corresponding curve in Figure 1 shows that most of the Cs (~0.6 ML) remains on the surface until 1000 Κ and is subsequently completely removed at 1100 K. This effect can be ascribed to a relatively strong interaction between the Cs adatoms and the S ones of the second sulfur layer, supporting that the binding energy of cesium depends on the pre-deposited quantity of sulfur and that the maximum amount of that energy results when a particular stoichiometry between the two adsorbates is attained. This, in turn, may suggest a kind of cesium sulfide compound CsxSy formation corresponding to the delay in its removal from the surface. The developing Cs-S interaction is consistent with the deorganization of the c(2 × 2) sulfur structure (Table 1).
Figure 2 compares the heat-influenced Auger electron signal intensity of Cs, S and Ni at the higher adsorbate coverage range. Specifically, it shows the variation in the Ap-pH of Cs (563 eV), S (151 eV) and Ni (61 eV) AETLs after annealing of the Cs(0.6 ML)/S(1 ML)/Ni(100) surface at different temperatures in increments of 100 K. All the shown AES measurements were carried out when the sample’s temperature was restored back to RT. The Cs (563 eV) Ap-pH exhibits almost the same pattern described in Figure 1, with a small decrease up to 900 K. Above this temperature, its rapid decrease indicates a drastic desorption of Cs, which is integrated at 1100 K, leaving the surface clean of Cs. On the other hand, the S (151 eV) Ap-pH increases with temperature up to 700 Κ and thereafter remains nearly constant up to 900 K. Post-annealing, therefore, most likely favors a Cs-S interaction and intermixing which brings S atoms into the Cs overlayer and thus closer to the surface. As cesium starts to desorb above 900 K, sulfur interacts exclusively with nickel and possibly partially diffuses into the metal substrate bulk. This argument is in line with the final dramatic increase in the Ni (61 eV) Ap-pH signal accompanied by the slight decrease in the S (151 eV) one for temperatures higher than 900 K. It is also in agreement with the earlier finding that before the desorption temperature at about 1200 K, S interacts strongly with Ni and an inter-diffusion between the two elements takes place, always leaving a sharp c(2 × 2) phase [44]. In that manner, in our experiments, there was always about 0.5 ML of S remaining on the surface because the predicted second desorption state above 1500 K was never achieved [50]. Based on the above discussion, it is likely that the substrate annealing up to 1100 K causes the dissolution of the formed CsxSy compound on the surface.
In Figure 3, the WF change ΔΦ is shown as a function of Cs deposition on the clean surface and the Ni(100) surface pre-covered with different amounts of sulfur. For Cs on clean Ni(100), the WF curve shows the very well-known characteristic curve observed with cesium on several surfaces [23,24,25,51,52,53]. According to the electrostatic image model [54,55,56], the initially drastic and almost linear WF decrease is due to the initial dipole moment of the Cs adatoms “pointing outward” from the surface, because of their electropositive nature. Therefore, the initially isolated chemisorbed Cs adatoms on the nickel surface are expected to be in an ionic state, while as the coverage increases, the interaction between the dipoles induces depolarization effects, first leading the WF to a minimum and finally leveling it off to a higher value (see plateau in Figure 3). At that final stage of adsorption, the interaction between the Cs adatoms prevails over that with the substrate, resulting in the metallization of cesium overlayer.
The shape of the WF curve described above, however, is gradually modified as the coverage of the pre-deposited S increases. The significant noticed modifications are the following: (a) The WF minimum Φmin shifts to lower ΘCs and its value increases. (b) The value of the final WF Φmax (maximum value at the formed plateaus in curves in Figure 3) also shifts to lower ΘCs and increases. (c) Above Θs = 0.5 ML, the WF curve does not show any minimum, but simply levels off to a final plateau. (d) For much higher Cs coverages (ΘCs > 1 ML) and Θs = 1 ML, the WF decreases to a minimum value for ΘCs = 3.0 ML and subsequently increases at ΘCs = 4.0 ML. A similar WF minimum, however, is not recorded for higher pre-deposited S quantity (Θs = 1.5 ML). By considering the WF of the clean Ni(100) equal to 5.2 eV [22,57], we present in Table 2 some numerical data for the WF based on the above reported modifications. In addition, for more clarity, Figure 4 depicts some of these data with regards to Φmin and Φmax as a function of the Cs coverage on different sulfured Ni(100) surfaces (Figure 4a) and the difference of Φmax − Φmin as a function of the S coverage (Figure 4b).
Let us now discuss these modifications of the WF curves induced by the pre-deposited S quantity. At first, the shift of the Φmin as well as the Φmax to lower Cs coverages as the S coverage increases on the surface (a and b modifications) was also observed during Cs deposition on pre-deposited hydrogen (H)-covered W(100) and W(110) surfaces [23,58] and potassium (Κ) deposition on H-covered Si(100) surfaces [59]. In those works, both of these shifts were attributed to the existence of the H interlayer and thus the longer interatomic distance between the alkali adatoms and the substrate, resulting in a larger initial dipole moment po of the alkali adsorbate according to Wimmer et al.’s polarization model [60]. In addition, the rather inert H interlayer, apart from the po increase, inhibits the electron transfer from the alkali overlayer to the substrate, thus increasing the electron density and resulting in an earlier metallization (lower alkali coverages) with higher Φmax, an effect consistent with Muscat’s and Batra’s theory [61]. Under the same consideration, the slope of the initial almost linear WF decrease observed in Figure 3 is proportional to po of Cs adatoms on surface, and it is evident that pre-deposited sulfur for coverages up to Θs = 0.5 ML does not change the slope, at least within the experimental error. This means that S does not increase the po of Cs as H does for Cs and K [23,58,59]. The reason for that may be the relatively strong Cs-S interaction which reorganizes the array of S atoms, reforming the sulfur interlayer from p(2 × 2) and c(2 × 2) into an amorphous state (see Table 1) with Cs and S intermixing. This behavior is quite different than that of H and possibly happens because of the higher electronegativity of S (2.58 against 2.20 in Pauling scale). In that point of view, there is not a rather stable S interlayer between the Cs and nickel substrate. Also, the displacement of the S atoms into the developing Cs overlayer appears to prevent the metallization of Cs; that is why the Φmin becomes shallower, less evident (i.e., smaller value of Φmax l − Φmin) and of higher value as Θs increases (see Figure 3 and Figure 4b). In addition, more S atoms on the surface lead to fewer available adsorption sites for Cs on bare nickel substrate. This adsorption blocking effect also contributes to the shift of Φmin to higher values because of fewer Cs-Ni dipoles. For Θs > 0.5 ML, the absence of Φmin with the plateau formation at Φmax for just 0.2 ML of Cs (c modification) declares a non-metallic cesium overlayer due to the abundance of S atoms to interact with the Cs ones. The metallization does not occur even for the second Cs layer (ΘCs = 0.6 ML). This is consistent with the data in Figure 2, where no substantial Cs desorption occurs before 900 K, indicating a high energy binding state in contrast to that of the metallic state related to the desorption of Cs between 300 and 800 K for Θs < 0.5 ML. A similar behavior connecting the absence of Φmin together with the plateau formation in the WF curve and the non-metallization of the alkali adsorbate is well-known for alkalis’ adsorption on oxidized substrates [24,62,63]. It is concluded, therefore, that as the amount of the pre-deposited S increases, the bonding of Cs adatoms with the substrate remains ionic, thus preventing the final metallization of the Cs overlayer, as observed on the clean Ni(100). In other words, the oxide-like bonding of Cs to sulfated Ni(100) is antagonistic to the final metallic bonding of Cs on clean Ni. This is consistent with Φmax = 2.13 eV of Cs on the clean Ni(100), which approaches the 2.14 eV of the metallic cesium [64]. The more pre-deposited S, however, the more the value of Φmax declines (Table 2 and Figure 4a).
In addition, a closer inspection of Figure 3 leads us to note that at high Cs coverage (ΘCs = 4 ML), on both heavily sulfured 1.0 and 1.5 ML nickel surfaces, the WF of the surface decreases and becomes almost equal to that of the Cs overlayer on clean nickel (d modification). This observation supports the final metallization of the cesiated surface independent of the pre-adsorbed sulfur. In that view, it seems that with the increasing ΘCs and when the compound-like bonding of Cs and S is completed stoichiometrically, the subsequently deposited cesium behaves as an independent Cs overlayer on the CsxSy compound-like underlayer. This produces the characteristic WF curve behavior with an initial decrease followed by an increase in the WF marking the metallization of the Cs overlayer after a certain number of Cs layers are formed. Thus, it appears that when ΘCs exceeds a few MLs on the S/Ni(100) surface, depending on the quantity of S, an ionic to metallic state transition takes place.
Some further thoughts with regard to the Cs-S interaction and the forming CsxSy compound-like overlayer concern the thermodynamically stable chemical compound of cesium sulfide Cs2S with the molar enthalpy of formation, ΔHο = −359.8 kJ/mol [65]. Indeed, as ΘCs approaches 2 ML on the S (1 ML)/Ni(100) surface, the atomic stoichiometry of Cs2S is likely achieved. Any extra Cs deposition behaves like that on a typical surface, with the WF curve being characterized by Φmin and final Φmax formation. On the S(1.5 ML)/Ni(100) surface, the stoichiometry of Cs2S is achieved for ΘCs = 3 ML. However, on that surface, no clear Φmin is formed, perhaps due to the rather extensive three-dimensional cesium sulfide growth and the much higher demanded Cs quantity for the final metallization, which was not realized in this study. In fact, based on Figure 4b, the absence of Φmin seems to happen at much lower ΘS (~0.7 ML), which means that the metallization prevention due to Cs-S interaction takes place even for an atomic ratio of Cs/S lower than 2. Nevertheless, unfortunately, it is not possible to detect the chemical state of the adsorbates for sure based only on the WF results. In that view, the Cs2S stoichiometry is only a proposed one based on the nominated Cs and S coverages in certain adsorption conditions. In contrast, whenever the coverage ratio ΘCs/ΘS is less than 2, the interacting compound of CsxSy is not of a specific atomic ratio.
In general, information about the kinetics and dynamics of the adsorbates on surfaces can be extracted by TDS measurements. Figure 5 shows a series of the Cs(133 amu) QMS signal after Cs deposition on a S(0.5 ML)/Ni(100) surface preheated at 1100 Κ as compared to that of a Cs(0.3 ML)/Ni(100) surface, shown with a dotted line (curve 9). The insert at the upper right depicts some TDS spectra of the Cs(133 amu) QMS signal after the deposition of 0.3 ML of Cs on a clean S-covered Ni(100) surface without preheating. The data of this insert have been previously reported [41], but it is useful to present them here for the purposes of comparison with the TDS data of the main portion of Figure 5. As the insert depicts, for a constant Θcs ≈ 0.3 ML, Cs TDS spectra shift to higher temperatures with increasing S coverage. Of special interest is spectrum 3 of the insert, representing Θcs = 0.3 ML on Θs = 0.5 ML with a c(2 × 2) structure, which corresponds to the completion of the first physical sulfur layer [44], since this spectrum corresponds directly to the S coverage utilized in the measurements represented by the spectra of the main part of Figure 5. In the insert, where the sulfated surfaces were not preheated, we notice that Cs desorbs from the surface at higher temperatures as ΘS increases. This means that S increases the binding energy of Cs on the surface due to the Cs-S interaction. Particularly, the Cs TDS peak of greatest intensity appears at about 1060 K for ΘS ≥ 0.5 ML (curve 4 in the insert), clearly suggesting that sulfur energetically shifts the bigger part of the absorbed Cs atoms into a much higher binding energy state as the stoichiometric ratio S/Cs increases. As we have already postulated, this is probably due to the surface CsxSy sulfide compound formation. This effect is in agreement with the discussed results of Figure 1. The same effect of alkali sulfide compound formation was observed for other alkalis’ adsorption too, such as sodium (Na) and potassium (K) on the sulfur-covered Ni(100) surface [40,42]. It is also noteworthy that the areas of all the Cs(0.3 ML) QMS curves presented in the insert are almost of the same area, supporting the fact that the sticking coefficient of Cs on the sulfated nickel remains constant and equal to that on the clean Ni(100) surface. In fact, as it has been previously shown for Cs and other alkalis’ adsorption on clean surfaces, the sticking coefficient presumably approaches unity [42,66,67,68]. After the complete desorption of Cs near to 1060 K, most of the S amount remains on the surface, and the c(2 × 2) or p(2 × 2) structures reappear. This implies that the first layer of S stays intact on the nickel surface after the annealing. For this reason, the S TDS peak for ΘS = 1.0 ML recorded at about 1140 K (dashed line of insert) is due to the desorption of the second sulfur layer leaving the first one on the surface with the c(2 × 2) symmetry. This result is consistent with our assumption that possibly, the formed surface cesium sulfide should be thermally decomposed on the surface with only Cs-desorbing species and S remaining on the surface. In that direction, we need to point out that no desorbing cesium sulfide compound was detected by the QMS.
It is useful to compare the spectra of the main portion of Figure 5 with those in the insert, as well as with the LEED observations in the last two rows in Table 1. The comparison actually shows that the behavior of Cs on the preheated at 1100 K, S(0.5 ML)/Ni(100) surface with a c(2 × 2) symmetry is surprisingly different to that on the unheated one. As was mentioned above, heating the S-covered Ni(100) surface with Θs > 0.5 ML at ~1200 K will remove all but the first physical sulfur layer (0.5 ML), preserving the c(2 × 2) phase. When Cs is added, however, the heat-induced interactions between adsorbates and substrate are revealed by the changes in the TDS spectra of Cs, as compared to those on unheated S-covered Ni(100) surfaces (insert of Figure 5). As seen in Figure 5, Cs (133 amu) QMS signal is represented by three TDS peaks, designated as β1, β2 and β3, respectively, depending on Cs coverage. In particular, the highest temperature β1 peak corresponds to the stronger binding energy of Cs of the first 0.1 ML. As shown in Table 1, the LEED pattern remains a clear c(2 × 2) when peak β1 dominates in the TDS spectrum. As ΘCs increases, β1 moves to lower temperatures, while for ΘCs > 0.1 ML, a new peak β2 appears at ~755 K, which develops and maximizes at 0.5 ML of Cs. At the same time, for ΘCs > 0.3 ML, a third peak β3 appears at the even lower desorption temperature of ~555 K and grows for up to ΘCs = 0.6 ML. Assuming a first-order desorption of Cs atoms from the surface, the desorption energy can be calculated by applying Readheads’ equation [69]:
E = R T ln ( ν T κ ) − 3.64 ,
where R = 8.617 × 10−5 eV∙atom−1∙K−1 is the gas constant, T is the desorption temperature of the TDS peak, ν is a frequency factor ~1013 s−1 and κ is the heating rate of the sample ~20 K/s. Using this equation, we report in Table 3 the desorption energies of the recorded Cs TDS peaks β1, β2 and β3, which are generally related with all different cesium adsorption states.
The desorption energy of β2 being lower than that of β1 designates a lower binding energy of Cs than the initial one, which rules within a coverage range of 0.1 ML < ΘCs < 0.3 ML. This new β2 adsorption state is probably due to a lower degree of ionicity in the Cs adatoms as ΘCs increases. In addition, the observed c(2 × 2) surface structure suggests that Cs interacts mostly with Ni at that stage of absorption. As ΘCs increases further (ΘCs > 0.3 ML) and the second Cs physical layer starts to grow, the third adsorption state β3 of much lower binding energy develops on the surface and approaches the desorption state of the metallic state depicted with a vertical arrow and dashed line (see curve 9 in the main Figure 5). Indeed, it is well-known that the first physical layer of Cs on the clean Ni(100) surface is in a metallic state [41].
An explanation of the dynamics of Cs adsorption on the heat-treated c(2 × 2)-S/Ni(100) surface as compared to the respective unheated surface is called for at this point. From the above-described TDS results and the combined LEED observations, it is concluded that the preheating process provides the activation energy for a stronger S-Ni interaction, thus strengthening the bonding between the two elements. This results in the subsequent weakening of the Cs-S interaction, but not enough to allow the metallization of the first Cs layer. It is evident that even near the completion of the second Cs overlayer (ΘCs = 0.6 ML), the β3 adsorption state (curve 8) has not attained the desorption energy of the metallic state. This non-metallic Cs overlayer, however, exhibits an increased interaction with part of the sulfur underlayer by changing the c(2 × 2) atomic symmetry into p(2 × 2) (curve 7) and 3 × 3 (curve 8), as mentioned in the LEED analysis regarding the last row of Table 1. In that view, probably, the gradual attraction and the induced mobility of a portion of the S atoms underneath by the Cs ones of the second cesium layer prevents the metallization of the latter. The weakening of the Cs-S interaction induced by the preheating process at 1100 K is evident from the lower desorption temperatures (proportional with the binding energies) of the β2 and β3 peaks of curve 5 in Figure 5 compared to those of curve 3 in the insert (both curves for ΘCs = 0.3 ML). Moreover, the second Cs layer (ΘCs ≥ 0.5 ML) on the unheated surface interacts strongly with the S underlayer in such a manner as to result in a disordered mixed Cs-S overlayer, giving the 1 × 1 LEED patterns shown in the second and third rows of Table 1. In contrast, the second Cs layer on the heat-treated c(2 × 2) − S(0.5 ML)/Ni(100) surface does not cause any disordering of the S underlayer. Most likely, the deposited Cs atoms of the first layer transfer some electric charge to the Ni substrate, thus causing a weakening of the S-Ni bond. This allows for the Cs of the second layer, also in a rather ionic state, to attract a portion of the S atoms by breaking some of the S-Ni bonds as well, thus changing the c(2 × 2) structure to a less dense p(2 × 2) where ΘS ≈ 0.25 ML. As the density of the Cs overlayer increases further, the portion of the attracted S atoms also increases, and the sulfur structure changes to 3 × 3 (see last row of Table 1) with ΘS ≈ 0.11 ML. In that manner, the Cs-S interaction prevents the metallization of the second Cs layer, the desorption of which gives the β3 peak in Figure 5. It therefore appears that the S in the preheated c(2 × 2) structure on the Ni(100) surface decreases the interaction of even the highly active electropositive alkali elements such as Cs. The clear evidence for that is the low intensity of the high-energy β1 adsorption state compared to those of β2 and β3. In other words, in our case, a heated single layer of S acts as a passivation element on the Ni(100) surface, as it happens on Ge(100) [70], GaSb(100) [71], InAs(001) [72] and other surfaces too [12].

3.2. Sulfur on Cs-Covered Ni(100) Surface

As Table 1 describes, deposition of Cs on clean Ni(100) forms an hcp structure at saturation coverage ΘCs = 0.3 ML. Adsorption of S on this structure causes a superposition of the hcp with a p(2 × 2) at ΘS = 0.25 ML. With further S adsorption, the LEED pattern changes to a diffused c(2 × 2) at ΘS = 0.5 ML, while the hcp structure disappears. The disappearance of the hcp structure of Cs indicates that S adatoms disorganize the uniform ordered cesium layer by displacing the Cs atoms. However, the appearing self-organized p(2 × 2) and c(2 × 2) two-dimensional nanostructures of S look the same as those on the clean Ni (100) (see the second column in Table 1). This means that the S atoms diffuse under the Cs layer and become bound directly to the nickel substrate for at least up to 0.5 ML. It is notable, therefore, that the sulfur diffusion takes place at RT, declaring a strong tendency of S to interact with Ni.
Figure 6 shows the Ap-pH of S (151 eV), Ni (61 eV) and Cs (47 eV) AETLs as a function of S deposition on the Cs(0.3 ML)/Ni(100) surface. For comparison reasons, we also show the evolution of the S (151 eV) Ap-pH on clean Ni(100) with a dashed line and the corresponding appearing LEED patterns. The initial slope of the S (151 eV) AETL curve on cesiated Ni(100) is lower than that of the clean substrate surface. This is attributed to the masking effect due to the submergence of the initially deposited S atoms under the Cs layer. With increasing adsorption of S, however, the slope of the sulfur Ap-pH curve increases too, but for coverages above 0.25 ML of S (~7.5 D), the slope of the curve decreases gradually. On the other hand, the Cs (47 eV) Ap-pH does not substantially change versus S coverage. In contrast, the intensity of the Ni (61 eV) Ap-pH decreases substantially during S deposition. The above-described behavior of the three elements’ Ap-pHs supports that adsorbed S atoms are not relaxed on top of the Cs layer but immediately diffuse in to be bound directly to the Ni substrate under and in between the pre-deposited Cs atoms, thus giving the same sulfur structures, p(2 × 2) and c(2 × 2), as in the case of S adsorption on clean Ni(100). This sulfur’s atom arrangement in combination with the almost double and a half times bigger atomic size of Cs is expected to eliminate the masking effect, keeping the intensity of the Cs (47 eV) Ap-pH almost constant.
A comparison between S adsorption on the clean and the cesiated nickel surface can be made using the Auger signals’ ratio, too. Figure 7 shows the Ap-pH ratio of the S (151 eV) and Ni (61 eV) AETLs during S adsorption on the clean and the Cs(0.3 ML)/Ni(100) surface. The dependence of the aforementioned ratio versus the S concentration on the two surfaces is almost identical, showing the same linear increase at around 15 D, where the first layer of S is expected to complete. This indicates that the sticking coefficient of S on clean and Cs-covered Ni(100) is the same and independent of the presence of Cs. Furthermore, the TDS measurements of the Cs (133 amu) QMS signal after S deposition on the Cs-covered Ni(100) surface are very similar to those of Cs on S-covered Ni(100) (see Figure 5) and are not shown here for this reason. In that view, it seems that the surface dynamics of the two elements are independent of the adsorption sequence.
It is also interesting to record the work function change ΔΦ during S adsorption on cesiated nickel surfaces. Figure 8 shows ΔΦ as a function of S deposition on the clean and Cs-covered Ni(100) surface at varying Cs coverages. As is evident from this figure, S adsorption causes an increase in the cesiated nickel WF, where its final magnitude depends on ΘCs. More specifically, at relatively low ΘCs (≤0.15 ML) where less than half the Cs layer is formed, S adatoms reside mostly on bare areas of the nickel substrate between the pre-deposited Cs atoms. This causes an initial slow WF increase, as it happens on the clean surface, which is reasonable because of the electronegative nature of sulfur. As ΘS increases further, S adatoms start to interact ionically with the Cs ones, possibly forming “pointing inward” electric dipoles, thus increasing the WF. The more pre-deposited Cs on the surface, the bigger the WF increase ΔΦ* that is recorded, where ΔΦ* = Φfinal − Φinitial, with Φfinal being the final WF after 30 D of S deposition (corresponding to ΘS = 1 ML) and Φinitial the initial WF of the cesiated nickel surface each time. Both Φinitial and Φfinal values can be extracted for each Cs coverage from Figure 8, bearing in mind that the WF of the clean Ni(100) surface is taken as equal to 5.2 eV. For lucidity, Figure 9 illustrates ΔΦ* as a function of ΘCs. A linear relationship holds for ΘCs ≤ 0.15 ML, reflecting the proportionality between the numbers of interacting S and Cs atoms. The situation, however, changes at ΘCs = 0.3 ML, where the WF curve in Figure 8 shows a different behavior. In fact, the WF initially decreases and subsequently slowly increases, forming a plateau at Φfinal. This behavior is consistent with an adsorption model where at the beginning stages of sulfur deposition, the S atoms adsorb underneath the Cs layer, while at higher ΘS, they reside in between the Cs atoms by displacing them, inducing a disordering of the hcp structure of cesium in agreement with LEED observations. On balance, the above discussion supports the argument that the S adatoms, due to their relatively small atomic size and high reactivity, are initially burrowing under the Cs overlayer, reacting with nickel and later on with Cs. It seems that the interaction between Cs and S results in a CsxSy compound-like, independent of the sequence of Cs and S adsorption.

4. Conclusions

In this work, we studied the behavior of S and Cs during deposition of (1) Cs on S-covered Ni(100) and (2) S on Cs-covered Ni(100) surfaces at RT. The annealing effect for this coadsorption system was also investigated. In agreement with the literature, separate adsorption experiments show that cesium on clean Ni(100) forms a complete dense and metallic layer at 0.3 ML with hcp structure, while on the other hand, sulfur on the same clean substrate forms a p(2 × 2) structure at 0.25 ML and a c(2 × 2) one at 0.5 ML, marking the completion of the first physical layer. Coadsorption experiments show that the presence of S increases the binding energy and the maximum coverage of Cs on nickel at RT, as happens with other alkalis too [40,42]. The first Cs layer on both p(2 × 2) and c(2 × 2) sulfur structures at RT is disordered, while the second layer strongly interacts with S, causing a disordering of the sulfur underlayer in a tendency to form a surface CsxSy compound-like. This Cs-S interaction causes a gradual demetallization of the Cs overlayer. When this compound is completed stoicheometrically, subsequent Cs deposition forms an independent metallic overlayer.
By heating the c(2 × 2):S/Ni(100) surface to 1100 K, the subsequently deposited Cs (ΘCs ≥ 0.5 ML) alters the c(2 × 2) structure to a p(2 × 2), and then to a 3 × 3. This situation is much different to the complete sulfur disorganization happening without the preheating process. Most likely, the mobility of a significant part of the S adsorbed atoms is strongly confined at the preheated c(2 × 2) phase. In addition, the binding energy of the Cs overlayer is smaller than that on an unheated S-covered Ni(100) surface. The above results support the assertion that the preheating process of the c(2 × 2)-S/Ni(100) surface passivates the substrate against cesium to a degree.
Adsorbed elemental S on Cs-covered Ni(100) at RT initially diffuses under the Cs layer directly bound to the substrate, initially forming the p(2 × 2) phase, as on the clean Ni(100). Continued adsorption of S, however, results in the c(2 × 2) phase, causing a disordering of the cesium hcp structure and the demetallization of the surface. The sticking coefficient of S remains the same, independent of the pre-deposited Cs amount. The presence of Cs increases the amount of S that can be deposited on the Ni surface, as it happens for Cs adsorption on the sulfated nickel. This probably leads to a more extensive CsxSy compound formation, which seems to be independent of the sequence of the two elements’ adsorption on the nickel surface.

Author Contributions

Conceptualization, M.K.; methodology, A.C.P. and M.K.; software, A.C.P. and D.V.; validation, D.V. and M.K.; formal analysis, A.C.P.; investigation, A.C.P. and M.K.; data curation, A.C.P. and D.V.; writing—original draft preparation, A.C.P.; writing—review and editing, A.C.P. and D.V.; visualization, M.K.; supervision, M.K. All authors have read and agreed to the published version of the manuscript.

Funding

This work received no external financial support.

Acknowledgments

All the authors specially thank “Elsevier” for giving the license to reproduce the inset in Figure 5, which has been published in reference [41].

Conflicts of Interest

The authors declare no conflicts of interest.

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Figure 1. The Ap-pH of Cs (563 eV) AETL for ΘCs = 0.3 ML on the clean Ni(100) surface and ΘCs = 0.6 ML on S-covered Ni(100) surfaces for ΘS ≤ 0.5 ML and ΘS > 0.5 ML, as a function of temperature of the sample post-annealing. The Auger electron signal was recorded when the sample’s temperature was restored to close RT.
Figure 1. The Ap-pH of Cs (563 eV) AETL for ΘCs = 0.3 ML on the clean Ni(100) surface and ΘCs = 0.6 ML on S-covered Ni(100) surfaces for ΘS ≤ 0.5 ML and ΘS > 0.5 ML, as a function of temperature of the sample post-annealing. The Auger electron signal was recorded when the sample’s temperature was restored to close RT.
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Figure 2. The Ap-pH of Cs (563 eV), S (151 eV) and Ni (61 eV) AETLs of Cs (0.6 ML) on the S(1 ML)-covered Ni(100) surface, as a function of temperature. All the Ap-pH signals were recorded when the temperature of the sample was restored to near RT.
Figure 2. The Ap-pH of Cs (563 eV), S (151 eV) and Ni (61 eV) AETLs of Cs (0.6 ML) on the S(1 ML)-covered Ni(100) surface, as a function of temperature. All the Ap-pH signals were recorded when the temperature of the sample was restored to near RT.
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Figure 3. The WF change ΔΦ at RT as a function of Cs deposition on the clean Ni(100) surface pre-covered with different amounts of sulfur.
Figure 3. The WF change ΔΦ at RT as a function of Cs deposition on the clean Ni(100) surface pre-covered with different amounts of sulfur.
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Figure 4. (a) The WF values of Φmin and Φmax as a function of Cs coverage on different S pre-covered Ni(100) surfaces. (b) The difference Φmax − Φmin, as a function of S coverage. The data have been extracted from Table 2.
Figure 4. (a) The WF values of Φmin and Φmax as a function of Cs coverage on different S pre-covered Ni(100) surfaces. (b) The difference Φmax − Φmin, as a function of S coverage. The data have been extracted from Table 2.
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Figure 5. A series of Cs(133 amu) TDS spectra after Cs deposition on an S(0.5 ML)/Ni(100) surface preheated at 1100 K, as compared to that of 0.3 ML of Cs on clean Ni(100) (shown with dotted line). The insert at the upper right depicts some TDS spectra of Cs(133 amu) after deposition of 0.3 ML of Cs on clean and S-covered Ni(100) surfaces for comparison reasons [41].
Figure 5. A series of Cs(133 amu) TDS spectra after Cs deposition on an S(0.5 ML)/Ni(100) surface preheated at 1100 K, as compared to that of 0.3 ML of Cs on clean Ni(100) (shown with dotted line). The insert at the upper right depicts some TDS spectra of Cs(133 amu) after deposition of 0.3 ML of Cs on clean and S-covered Ni(100) surfaces for comparison reasons [41].
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Figure 6. The Ap-pH of the of S (151 eV), Ni (61 eV) and Cs (47 eV) AETLs as a function of sulfur deposition on the Cs(0.3 ML)/Ni(100) surface. The evolution of the S (151 eV) Ap-pH on the clean Ni(100) surface is also shown with the dotted line, as well as the appearing LEED patterns.
Figure 6. The Ap-pH of the of S (151 eV), Ni (61 eV) and Cs (47 eV) AETLs as a function of sulfur deposition on the Cs(0.3 ML)/Ni(100) surface. The evolution of the S (151 eV) Ap-pH on the clean Ni(100) surface is also shown with the dotted line, as well as the appearing LEED patterns.
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Figure 7. The Ap-pH ratio of the of S (151 eV)/Ni (61 eV) AETLs during S deposition on the clean and the Cs(0.3 ML)/Ni(100) surface.
Figure 7. The Ap-pH ratio of the of S (151 eV)/Ni (61 eV) AETLs during S deposition on the clean and the Cs(0.3 ML)/Ni(100) surface.
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Figure 8. The WF changes ΔΦ at RT for S adsorption on Cs-covered Ni(100) surfaces with varying Cs coverage.
Figure 8. The WF changes ΔΦ at RT for S adsorption on Cs-covered Ni(100) surfaces with varying Cs coverage.
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Figure 9. The WF change ΔΦ* at RT for S (1 ML) adsorption on Cs-covered Ni(100) surfaces with varying Cs coverage. See the text for definition of ΔΦ*.
Figure 9. The WF change ΔΦ* at RT for S (1 ML) adsorption on Cs-covered Ni(100) surfaces with varying Cs coverage. See the text for definition of ΔΦ*.
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Table 1. LEED observations are shown as a function of S and Cs coverages on the Ni(100) surface at RT and Cs coverage on the preheated c(2 × 2):S/Ni(100) surface at 1100 K. The sequential phase changes caused by the Cs coverage increase are shown by the horizontal arrows, while those caused by S coverage increase are shown by the vertical ones. In addition, the heavier the shadow of the boxes in the table, the higher the background in the appearing LEED pattern. For example, in the case of Cs adsorption on the preheated c(2 × 2):S/Ni(100) at 1100 K, the p(2 × 2) pattern appears with a higher background than that of the c(2 × 2) one (see text for details).
Table 1. LEED observations are shown as a function of S and Cs coverages on the Ni(100) surface at RT and Cs coverage on the preheated c(2 × 2):S/Ni(100) surface at 1100 K. The sequential phase changes caused by the Cs coverage increase are shown by the horizontal arrows, while those caused by S coverage increase are shown by the vertical ones. In addition, the heavier the shadow of the boxes in the table, the higher the background in the appearing LEED pattern. For example, in the case of Cs adsorption on the preheated c(2 × 2):S/Ni(100) at 1100 K, the p(2 × 2) pattern appears with a higher background than that of the c(2 × 2) one (see text for details).
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Table 2. The work function values of Φmin (eV) and Φmax (eV) at the corresponding cesium coverages for Cs deposition on the clean and S-covered Ni(100) surfaces. The difference of Φmax − Φmin is also shown wherever it is applied.
Table 2. The work function values of Φmin (eV) and Φmax (eV) at the corresponding cesium coverages for Cs deposition on the clean and S-covered Ni(100) surfaces. The difference of Φmax − Φmin is also shown wherever it is applied.
ΘCs (ML)
(±0.004 ML)		ΘS (ML)Φmin
(±0.02 eV)		Φmax
(±0.02 eV)		Φmax − Φmin
(±0.04 eV)
0.18001.56-0.57
0.323-2.13
0.1580.251.78 -0.39
0.251-2.17
0.1410.401.92 -0.34
0.201-2.26
0.1220.502.24-0.13
0.182-2.37
0.2001.00-2.57-
0.2001.50-2.57-
2.0001.00-2.19-
3.0001.70--
4.000-2.21-
2.0001.50-2.48-
3.000-2.10-
4.000-2.06-
Table 3. The calculated desorption energies of Cs for the three different desorption states β1, β2 and β3, as a function of the cesium coverages on the S(0.5 ML)/Ni(100) preheated surface at 1100 K. The data are extracted from Figure 5.
Table 3. The calculated desorption energies of Cs for the three different desorption states β1, β2 and β3, as a function of the cesium coverages on the S(0.5 ML)/Ni(100) preheated surface at 1100 K. The data are extracted from Figure 5.
Cs TDS PeakCesium Coverage ΘCs (ML)Temperature (K)
(±10 K)		Desorption Energy (eV·atom−1) (±0.03 eV)
β10.0510202.66
0.109102.36
0.158752.27
β20.15−0.507551.95
0.607451.92
β30.405851.50
0.50−0.605551.42
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Papageorgopoulos, A.C.; Vlachos, D.; Kamaratos, M. The Mutual Influence of Elemental S and Cs on the Ni(100) Surface at Room and Elevated Temperatures. Surfaces 2025, 8, 68. https://doi.org/10.3390/surfaces8030068

AMA Style

Papageorgopoulos AC, Vlachos D, Kamaratos M. The Mutual Influence of Elemental S and Cs on the Ni(100) Surface at Room and Elevated Temperatures. Surfaces. 2025; 8(3):68. https://doi.org/10.3390/surfaces8030068

Chicago/Turabian Style

Papageorgopoulos, Aris Chris, Dimitrios Vlachos, and Mattheos Kamaratos. 2025. "The Mutual Influence of Elemental S and Cs on the Ni(100) Surface at Room and Elevated Temperatures" Surfaces 8, no. 3: 68. https://doi.org/10.3390/surfaces8030068

APA Style

Papageorgopoulos, A. C., Vlachos, D., & Kamaratos, M. (2025). The Mutual Influence of Elemental S and Cs on the Ni(100) Surface at Room and Elevated Temperatures. Surfaces, 8(3), 68. https://doi.org/10.3390/surfaces8030068

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