Work Function Change Measurements Research Articles

An n- to p-type conductivity transition induced by oxygen adsorption on α-Fe2O3

The simultaneous measurements of conductance and work function changes induced by gaseous ambient have been performed on α-Fe2O3 thick film polycrystalline samples kept at 280 °C and exposed to different gaseous atmospheres. The switching from n- to p-type conductivity on α-Fe2O3 is shown to have an electronic origin, which is due to the oxygen adsorption and formation of a surface inversion layer and, therefore, to the inversion of the surface conduction type. The modeling of the n–p switching is described in terms of conductance dependence on the band bending induced by gaseous ambient.

Co-adsorption and reactions of Na and H 2O on graphite

Experimental data for the co-adsorption of sodium and water on graphite (0001) at low temperature (95 K) and the subsequent thermal transformations has been obtained by thermal desorption spectroscopy (TDS), work function change (Δ φ) measurements, and high-resolution electron energy loss spectroscopy (HREELS). Water molecules co-adsorbed in the low Na coverage regime do not dissociate, but are strongly perturbed. The interaction results in the formation of hydrated ion shells. The estimated number of water molecules in the core (inner) shell is four. A series of thermal desorption peaks, which are related to the degree of stabilization by the H 2O–Na complexes formed, emerge with increasing water coverage. Up to 20 ± 3 water molecules can be affected per co-adsorbed Na-atom. The Na–H 2O interaction increases the water desorption energy by as much as 100 ± 20 meV/water molecule. Thermal interaction leads to the formation of sodium hydroxide species at ∼200 K, which subsequently decomposes to yield simultaneous desorption of Na and H 2O at ∼565 K.

A p- to n-transition on α-Fe 2O 3-based thick film sensors studied by conductance and work function change measurements

Transition from p- to n-type response, induced by the change in the gas concentration and the operating temperature, was observed for α-Fe 2O 3-based semiconducting sensors. This phenomenon is due to the formation of an inversion layer at the surface and therefore to the inversion of the type of mobile carrier at the surface. Different gas atmospheres leads to the different contribution of surface electrons and holes in the overall conductivity, which leads for the predominating either p- or n-type response.

Photoinduced Fragmentation of Multilayer CH3Br on Cu/Ru(001)

The broadband UV (230−420 nm) photoinduced chemistry of CH3Br adsorbed on Cu(2ML)/Ru(001) in the coverage range of 1−50 ML was studied by monitoring the desorption products (Δp-TPD mode) in combination with post irradiation work function change measurements before and during surface heating (Δφ-ΤPD mode). Δφ measurements enabled us to follow multilayer restructure and desorption of parent molecules and photochemical reaction products in the temperature range of 80−700 K. Methyl radicals accumulated on the surface are the precursor for the thermal formation of methane and ethylene at 450 K. Dehydrogenation of the methyl group is the rate-limiting step of the surface reaction resulting in the formation of these molecules. Based on work function change measurements, an estimate of the adsorbed methyl dipole moment is μ0 = 0.48 D. Dissociative electron attachment (DEA) driven CH3Br dissociation produced CH3 and CH2 fragments within the parent molecules multilayer matrix. At the multilayer coverage range, Δφ increases by up to 1.1 eV after 10 min UV irradiation. Model calculations qualitatively describe the post irradiation work function changes induced by the embedded photofragments (mostly Br- ions) inside the CH3Br dielectric film. Comparison of the Δφ-TPD spectra on Cu(2ML)/Ru(001) to those on clean Ru(001) indicates that the nature of the molecule−surface interaction and the structure of the first few layers strongly influence the resulting photochemistry at layer thickness up to at least 20 ML.

Initial stages of Yb/Si(1 0 0) interface growth: 2 × 3 and 2 × 6 reconstructions

The initial stages of Yb/Si(100) interface growth have been studied by scanning tunneling microscopy, low-energy electron diffraction, Auger electron spectroscopy (AES), thermal desorption spectroscopy, and work-function change measurements. It is shown that a two-dimensional (2D) adsorbed layer of Yb forms at low coverage (<0.5–0.6 monolayers (ML)), followed by three-dimensional (3D) growth of Yb silicide phase at higher coverages. The two surface reconstructions of Yb/Si(100), 2×3 and 2×6 phases, are identified in submonolayer regime. The submonolayer adsorbed phases are found to exhibit higher thermal stability compared to the silicide one. The formation of adsorbed phases results in a large decrease of work-function (by up to 1.8eV), which suggests that the reconstructions are terminated by Yb and a charge transfer occurs from top Yb atoms to underlying Si substrate. Moreover, the structural model of the 2×3 reconstruction is proposed and the features of the 2×6 phase are interpreted.

Mg deposition on Ag(1 0 0): temperature evolution of the structural and electronic properties

Structural and electronic properties of ultrathin Mg films on Ag(100) have been investigated by means of metastable de-excitation spectroscopy (MDS), work function change (Δφ), electron diffraction and helium reflectivity (RHe) measurements. At room temperature Mg grows on Ag(100) following the Stranski–Krastanov growth mode. Successive annealing of a 10ML thick Mg film leads to its almost complete desorption at about 390K and to the formation of a Mg–Ag alloy. RHe and electron diffraction measurements indicate that for annealing temperatures between 480 and 570K, the alloy orders with a c(2×2) periodicity. Further annealing at higher temperatures restores the initial Ag(100) surface.

CF 3 I on a silicon surface: Adsorption, temperature-programmed desorption, and electron-stimulated desorption

CF 3 I adsorption on a silicon surface and the effect of low-energy electron bombardment of a CF3I-covered silicon surface are relevant to plasma etching. Dissociative chemisorption of CF3I on Si(100) surface is observed at 370 K. Uptake measurements corroborated by work-function change measurements and temperature-programmed desorption (TPD) gives a sticking probability of at least 0.34±0.05. Molecular orbital calculations yield an adsorption energy greater than 3 eV for dissociative chemisorption of CF3X (X=F, Cl, Br, and I) on Si(100) (modeled by Si9H12) with X transferred to a silicon atom. We conclude that the variation in the sticking probability across the CF3X family is a consequence of the activation energy barrier for C–X bond cleavage. In TPD, SiF4 desorbs at 370 and 840 K, SiF3 radical at 770 K, and atomic iodine at 790 K. The parent CF3I does not desorb. Electron-stimulated desorption (ESD) yields F+, F− and a trace of I+. The threshold for the appearance of F+ is 20 eV and for F− is 29 eV. Dissociative ionization and dipolar dissociation are possible mechanisms for ESD of F+. Dipolar dissociation and harpooning are possible mechanisms for ESD of F−. There is evidence that iodine on the surface quenches the electronically excited states that lead to desorption of F−.

Study of the azidoacetic acid adsorption in Ag (1 1 1)-C

The interaction of the azidoacetic acid, N 3CH 2COOH, with an Ag (1 1 1) crystal modified by surface carbon has been observed. XPS measurements of C (1 s), O (1 s), N (1 s) and Ag 3d 3/2,5/2 as a function of the exposure have been used to follow the predominant species rising with the azide adsorption. Dissociative chemisorption occurs at room temperature leading to the formation of molecular nitrogen followed by NCH 2COOH, CH x , NCH 2CHO, CO, CO 2 and CN. The interaction is coverage dependent. The adsorption occurs preferentially around the carbon sites located on the silver surface. The surface decomposition can be related to studies developed for the same molecule in the gas phase. Measurements of work function changes are consistent with the XPS results and this combination is a promising approach to study adsorption.

Adsorption, decomposition, and stabilization of 1,2-dibromoethane on Cu(111)

The molecular adsorption and thermal reaction of 1,2-dibromoethane (DBE) on Cu(111) has been studied using ultraviolet photoelectron spectroscopy, work function change (ΔΦ) measurements, low energy electron diffraction, and line-of-sight temperature programmed desorption. At 100 K, DBE adsorbs molecularly, exhibiting a decrease in work function of 0.86 eV at the completion of the monolayer. Layer-by-layer growth is observed up to two molecular layers, with further adsorption leading to the growth of tower crystallites. The first layer of DBE undergoes thermal decomposition at 125 K to give gaseous ethene and chemisorbed bromine in a (√3×√3)R30° structure, which increases the work function of the surface by 0.58 eV. Chemisorbed bromine from partial decomposition of the first layer is found to stabilize the remaining DBE molecules in the decomposing layer, thereby arresting complete decomposition until a higher temperature. Subsequent decomposition of the stabilized DBE leads to ethene emission at 145 and 160 K for initial DBE coverages in the submonolayer and multilayer regime, respectively.

Uptake of C 6 cyclic hydrocarbons on Pt

Adsorption of benzene and cyclohexane was studied by Auger electron spectroscopy and work function change measurements on polycrystalline Pt, and the results were compared with earlier studies with C 1–C 3 hydrocarbons. It was found that the adsorption of the cycles is not hindered, the adlayer hardly interacts with hydrogen, and that benzene and cyclohexane form similar adspecies.

Water adsorption structures on flat and stepped Ru(0001) surfaces

Water adsorption and desorption (H2O and D2O) were studied on vicinal Ru(0001) surfaces with nominal (101̄l) (l=8, 12, and 22) orientations employing low-energy electron diffraction (LEED), thermal desorption spectroscopy, and work-function change measurements (ΔΦ). LEED measurements and theoretical calculations from clean Ru(101̄l) confirmed the existence of terraces with average regular spacing and steps of one atomic height. Due to the hcp structure of Ru the terrace width and the atomic arrangement of the step alternate from one terrace to the next. This is confirmed by LEED measurements and calculations showing a triple-spot splitting of the reflexes. For water adsorption, due to the presence of steps, new desorption states with higher desorption temperature (220–350 K) are observed, which are missing on the flat Ru(0001) surface. LEED experiments demonstrate that the ordered water overlayers found on Ru(0001) are absent on the stepped surfaces. The isotope effect for D2O (the so-called A1 desorption state found for H2O at 212 K is missing for D2O) is also observed on the stepped surfaces indicating that this effect is due to a short-range interaction rather than a longer-range interaction in the water bilayer.

The Adsorption and Decomposition of C2H4 on Ru(001): A Combined TPR and Work Function Change Study

Work function change (ΔΦ) measurements during adsorption and surface heating in a ΔΦ−temperature programmed reaction (TPR) mode combined with TPD are demonstrated to provide new information on the interaction and chemistry of ethylene on Ru(001). Rearrangement of second layer ethylene molecules has been observed between 82 and 120 K. This is a competing process with molecular desorption, interpreted as a result of migration of second layer molecules toward the surface. Our results are consistent with and support previous studies that suggested the formation of a surface intermediate (η2(C,C)CHCH2) during ethylene dehydrogenation to ethylidine. Employing a derivative mode with respect to temperature −d(ΔΦ)/dT, we find an early onset for ethylidine decomposition near 265 K. ΔΦ−TPR measurements in the range 560−720 K reveal three distinct CH decomposition peaks, reflecting different activation energies for the decomposition reaction sites. The dipole moment of an adsorbed CH has been determined to be μ = 0.43 D, suggesting a rather polarized Ru−CH complex; its structure is independent of the adsorption site. Finally, carbide polymerization to form graphite has been detected above Ts = 560 K for the first time using work function change measurements. Good agreement was found between the contribution to ΔΦ by the graphite layer formed on the Ru(001) surface and ab initio calculations performed previously on this system.

Adsorbed states of K on the diamond (100)(2×1) surface

Abstract The adsorbed states of K on the C(100)(2×1) surface have been studied by electron energy loss spectroscopy (EELS), work function change (Δφ) measurement and thermal desorption spectroscopy (TDS). In the region where the K coverage is less than one-half of a monolayer (θ K ≤0.5), a loss is observed from ∼1.2 eV (θ K =0.2) to 1.0 eV (θ K =0.5); the work function decreases upon K adsorption until reaching a shallow minimum of Δφ=−3.35 eV at θ K =∼0.5; and a desorption peak (β) is observed from ∼825 K (θ K =0.05) to 525 K (θ K =0.5). These results indicate that the K–substrate bond is highly polarized; the 1.2 eV loss is attributed to the electronic transition from the bonding to antibonding states formed at the K–substrate interface. In the region between θ K =0.5 and 1, two losses are observed at 0.7 and 1.4 eV (θ K =0.6); there is only a small increase of the work function; and a desorption peak (α) is observed in addition to the β peak. These results indicate that the K regains its electron and becomes, essentially, neutral. The 1.4-eV loss is ascribed to the transition from the 4s to 4p states of K. The origin of the 0.7-eV loss is discussed. The Δφ and TDS results are analyzed by the depolarization model.

Characterization of Sc–O/W(100) Surface as Schottky Emitter: Work Function Change for Activation Processing

To elucidate the electron emissivity of the Sc–O/W(100) Schottky emitter, the Sc–O/W(100) surface, prepared by scandium deposition onto the W(100) surface followed by oxygen exposure and heating at an operating temperature of ∼1400 K, was studied by ion scattering spectroscopy (ISS), Auger electron spectroscopy (AES) and measurement of the relative work function change. The results have revealed that heating of the oxygen adsorbed Sc/W(100) surface leads to the diffusion of the oxygen atoms into the substrate to locate very probably beneath the topmost scandium atoms. This relocation of the oxygen atoms forms an electric double layer, resulting in a significant decrease of the work function. The topmost surface composition of the Sc–O/W(100) system observed by ISS did not change during cooling the sample from ∼1400 K to room temperature under the present conditions. The results of the present work strongly suggest that the surface properties of the Sc–O/W(100) system are very similar to those of the Zr–O/W(100) system at the operating temperature.

Dipole–dipole interactions among CH3Cl molecules on Ru(001): Correlation between work function change and thermal desorption studies

Work function change measurements (ΔΦ) combined with temperature programmed desorption (TPD) were employed to study layer growth mechanism and the CH3Cl dipole–dipole interactions on Ru(001), over the temperature range of 97 K–230 K. The activation energy for desorption (Ea) and the molecular dipole moment (μ) both decrease from 55.9 kJ/mol and 2.44 D, at the zero coverage limit, to 38.6 kJ/mol and 1.27 D, at one monolayer. This coverage dependence originates from strong dipolar lateral repulsion among neighbor CH3Cl molecules. Using a model introduced by Maschhoff and Cowin (MC) [J. Chem. Phys. 101, 8138 (1994)], the isolated adsorbed molecule’s dipole moment, μ0 (2.35 D) and polarizability α(8.1×10−24 cm3), were extracted from TPD data. These values agree very well with μ0 (2.12 D) and α(9.2×10−24 cm3) obtained from work function change measurements by employing the same MC model. The ability to simulate both TPD and work function change data over a wide coverage range within the framework of a single electrostatic model has been demonstrated. It enabled better understanding of fine details of surface dipolar interactions.

Dissociation of water deposited on W field emitter tip

The results of field emission microscopy measurements of work function changes while heating the water covered W tip are presented. At temperatures up to 280 K non-dissociated H 2O particles are present on the tip. Warming the tip at 300 K caused these molecules to dissociate to OH and H. OH groups dissociated to oxygen and hydrogen at temperatures up to 350 K.

CO adsorption on the c(8×2)-Mn/Cu(100) phase

Resumen Adsorption of carbon monoxide on the c(8×2) phase of Mn/Cu(100) at 100 K was studied by means of UPS, work function change measurements, and LEED. Two stages of adsorption can be separated. The first type of molecularly adsorbed CO does not change the substrate structure. No adsorbate-induced superstructure can be observed, and the work function is increased by 0.9 eV. Further adsorption beyond a critical exposure leads to a destruction of the ordered Mn film, and the CO–metal interaction as revealed by UPS is changed accompanied by a decrease of the work function by 0.16 eV. It is not possible to re-establish the ordered film structure by annealing. No dissociation of CO occurs below 200 K.

Interaction of oxygen with Ni 3Al(111) at 300 K and 1000 K

We have investigated the oxidation of Ni 3Al(111) at 300 K and 1000 K with STM and work function change measurements. Oxygen exposure at 300 K initially increases the work function, indicating oxygen chemisorption. Exposures above 5 L lead, however, to a rapid decrease of the work function, indicative of oxygen incorporation. This process is accompanied by a strong surface roughening which leads to a disordered surface. Annealing this surface at 1000 K results in the formation of “strawberry”-shaped oxide islands on the substrate terraces. Adsorption of oxygen at 1000 K produces a well-ordered oxide phase that gives rise to a Moiré pattern with a periodicity of 2.5 nm. This pattern disappears for bias voltages below ∼3 V, accompanied by a strong decrease in the apparent height of the oxide layer. This effect can be explained in terms of the band gap of the alumina layer.

Ethene adsorption on Pt 3Cu(111)

The adsorption of ethene on the Pt 3Cu(111) surface at 95 K has been studied by high-resolution electron energy loss spectroscopy (HREELS), temperature-programmed desorption (TPD), low-energy electron diffraction (LEED), and work-function change measurements. Both the TPD and HREELS measurements reveal the existence of two different ethene species on the surface. These species can be identified as a weakly bonded π-complex and a stronger bonded di-σ-complex. No ethene-induced superstructure has been found with LEED. The TPD experiments indicate that ethene is not dehydrogenated/decomposed when the surface is heated above the desorption temperature.

Surface Chemistry of CH3Br and Methyl Modified by Copper Deposition on Ru(001)

The chemistry of methyl bromide on Cu/Ru(001) has been studied utilizing work function change (Δφ) and temperature-programmed desorption (TPD) measurements. The remarkable modification in the methyl fragments dehydrogenation at the completion of a single copper layer and the significant difference in reactivity of the Cu(2 ML)/Ru(001) or Cu(111) surfaces are the focus of this study. A decrease in work function at the completion of 1 ML CH3Br of 2.15 ± 0.02 eV and 1.33 ± 0.05 eV was measured, respectively, for Ru(001) and Cu(2 ML)/Ru(001) held at 82 K. Methyl bromide does not dissociate upon adsorption on clean or the copper-covered surfaces, and it is bound with the bromine down. Copper modifies the reactivity of the Ru substrate, gradually decreasing the dissociated fraction of CH3Br from 0.55 of the initial one monolayer on clean Ru(001) to 0.06 on Cu(2 ML)/Ru(001), probably because of defects in the copper layer. The methyl fragment dehydrogenation rate slows as the copper coverage increases. At a narr...