Minimum Work Function Research Articles

Effect of initial stages of oxidation of W(111) and Mo(112) on the adsorption of Dy, Nd and Li

Changes in the work function and adsorption energy of ultrathin layers of Dy, Nd and Li deposited on preoxidized Mo(112) and W(111) faces have been investigated using the method of CPD and adsorption isobars. It has been found that the preadsorbed oxygen brings about a lowering of the work function minimum for the co-adsorption system and increasing of the area occupied by the metallic adsorbate, that is relevant to the work function minimum. Discussed are the results obtained for different values of initial oxygen coverage.

VALENCE-BAND PHOTOEMISSION STUDY OF (Ba, Cs) on W(110)

We present the first valence-band photoemission study of Ba and Cs on an atomically clean W(110) surface. The cleanness of the tungsten substrate can be justified from both high surface 4f core emission and sharp surface states in the valence-band spectra. Upon Ba (Cs) adsorption, the work function is lowered to 2.02 (1.76) eV, corresponding to one third of a monolayer. Further, the two symmetric high-lying surface states move little in energy but quench rapidly in intensity and diminish at the work-function minimum. On the other hand, the low-lying surface state at 1.41 eV shifts about 215 meV toward higher binding energy and reduces its emission at thicker coverage. The bonding of the Ba d orbital with the W d-like surface states is not found, in contrast to the theoretical calculation and angle-resolved inverse photoemission study of Ba on W(100) . From the coverage-dependent interfacial electronic structure and the parallel behaviors manifest in both Ba on W and Cs on W , the polarized-covalent model for the alkali-metal–substrate interactions is firmly reinforced.

Vibrational and electronic excitations of [formula omitted

The vibrational and electronic excitations of Na atoms chemisorbed on Mo(100) have been investigated with high-resolution electron energy loss spectroscopy (HREELS). An intense loss mode at 19 meV is assigned to perpendicular vibrations of the Na atoms. While the frequency of this mode does not shift with coverage within ± 1 meV, the intensity is strongly coverage dependent, becoming too weak to be observed above the work function minimum (∼0.5 ML). The absence of a frequency shift is contrary to the expectations of the Gurney model as well as more recent theoretical calculations. Work-function changes, the coverage dependence of the dynamic charge and low coverage electronic losses are all accounted for by the phenomenological Newns-Anderson model.

Novel GaAs photocathodes with alkali antimonide intermediate layers and caesium-oxygen adlayers

Novel reflection mode and transmission mode GaAs (grown by metal organic chemical vapour deposition) photocathodes with the structures (GaAs-Na 3Sb-Cs)-O-Cs, (GaAs-K 3Sb-Cs)-O-Cs, (GaAs-Cs 3Sb-Cs)-O-Cs and (GaAs-Na 2KSb-Cs)-O-Cs were fabricated. Forreflection mode (GaAs-Na 3Sb-Cs)-O-Cs, (GaAs-K 3Sb-Cs)-O-Cs, (GaAs-Cs 3Sb-Cs)-O-Cs, (GaAs-Na 2KSb-Cs)-O-Cs, and transmission mode (GaAs-Na 2KSb-Cs)-O-Cs compound photocathodes, the measured maximum photosensitivities were respectively 100, 200, 60, 450, and 600 μA lm −1, the measured minimum work functions were respectively 1.35,1.4,1.25,1.05 and 1.3 eV, the decay rates of photosensitivity were respectively about 12, 10, 15, 9 and 8 times slower than that of conventional GaAs photocathodes. In this paper, surface atomic models and energy band diagrams of the new compound GaAs photocathodes are analysed briefly.

Photoemission and high resolution electron energy loss spectroscopy study of CO/K/Cu(110)

Adsorption of CO on the Cu(110) surface modified with a low (near the work function minimum) potassium precoverage has been investigated by use of low energy electron diffraction, photoelectron spectroscopy based on synchrotron radiation, work function measurements (ΔΦ), and high resolution electron energy loss spectroscopy (HREELS). During potassium adsorption at 118 K the binding energy (BE) of the K 3p peak shifts −0.3 eV. For a thick potassium overlayer both a surface and a bulk K 3p peak, with a 0.9 eV difference in BE, are observed. The work function results at ΘK∼0.5 ML show after saturation with CO an increase of 1.11 eV. An electrostatic interaction energy between CO and K equal to 0.23 eV is determined. This energy indicates a moderate attractive interaction between CO and K. A CO overlayer on K/Cu(110) shows the 4σ satellite, the 4σ orbital, and the 5σ/1π joint feature. No observable change in the BE separation between the 4σ and the 5σ/1π band (3.1 eV) was found, thereby indicating a negligible CO–CO interaction. From the 4σ to the 5σ/1π intensity ratio (∼2) a weakening of the CO–metal binding with increasing CO coverage is deduced. In the HREEL spectrum, an intense feature at 1731 cm−1 besides the fundamental stretching frequency at 2030 cm−1 implies a substantial potassium induced weakening of the C–O bond.

Near-Fermi level surface states induced by Cs adsorption on the Si(111)7 × 7 surface

The electronic structure of the Si(111)7 × 7 surface modified due to Cs-adsorption has been studied by means of a new method which uses the p- and s-polarized light excitation of threshold photoemission spectra. We found that the mechanism of interaction of polarized light with surface states differs from that with bulk states. Photoemission from the Cs-induced band was observed with the use of p-polarized light in the absence of bulk photoemission. Based on this phenomenon, the fundamental parameters of the Cs-band and the energy position of the Fermi level relative to both the vacuum and valence band maximum were obtained. The final pinning position of the Fermi level is found to be equal to 0.18 eV at a Cs coverage corresponding to the work function minimum. Metallization occurs due to Cs-coverage before the Fermi-level pinning.

A photoemission investigation of the adsorption of potassium on perfect and defective TiO 2(110) surfaces

Potassium adsorption on stoichiometric and defective rutile TiO 2(110) surfaces at room temperature has been studied by means of angle-resolved photoemission spectroscopy (ARUPS and ARXPS) from the valence states and core levels. Potassium adsorption on the stoichiometric surface creates reduced Ti ions (“Ti 3+”) with lowered Ti2p core-level binding energy and 3d-like band-gap states centered at 0.9 eV binding energy, arising from K-to-substrate charge transfer. The Ti 3+ 3d intensity versus coverage, which is a measure of the transferred charge, shows a behavior similar to that of the work function, with a very pronounced maximum at the coverage of the weak work-function minimum. In contrast, the Ti 3+ 2p signal, which is a measure of the number of reduced Ti ions, increases continuously with K coverage. The results are discussed in terms of a recently proposed “ionic-to-neutral transition” of the K—substrate bond. On the defective surface, K adsorption results in a decrease of the initial concentration of “Ti 3+-V o” surface defects, probably caused by K-induced oxygen diffusion from the bulk to the surface.

Reconstruction and disordering on (110) fcc metal surfaces

Due to the large number of possible defects available to disorder the fcc metal (110) 2x1 missing row reconstruction, there is a particularly rich variety of possible disordered phases. The five distinct disordering scenarios that have been predicted for this reconstruction depend on the relative energy cost to generate the different defects that can form on this surface. Recent high resolution LEED measurements on the clean and alkali covered Pd(110) surfaces have explored a few of these phases. For clean Pd(110), a surface consisting of semi-ordered islands has been demonstrated to exist up to 950 K. The periodic island structure can be predicted by a simple model that includes step-step interactions and strain relaxation in the islands. Subsequent alkali adsorption destroys the island structure and forms the 2x1 missing row reconstruction. Diffraction results will be presented as a function of alkali coverage that show that the Pd(110)+K 2x1→1x1 transition is an example of deconstruction to the “even” flat phase. The existence of this intermediate phase has important implications to the growth of the missing row reconstruction and the range of the alkali-substrate interaction. In addition to these observations, higher potassium coverage data demonstrates that the Pd(110)+K work function minimum is correlated with a surface roughening transition.

Cesium on molybdenum (110)

Surface structure, work function changes and plasma losses have been studied for the adsorption of Cs on a Mo(110) surface. At 77 K, up to the density of 2.2 × 10 14 cm −2, Cs is in a two-dimensional liquid state. At higher coverages the adlayer crystallizes, forming domains rotated 19° with respect to each other. The domains possess a hexagonal structure and undergo a rotation of 90° at a coverage of 3 × 10 14 cm −2. A full monolayer has a close-packed hexagonal array of adatoms with a density of 5.1 × 10 14 cm −2. The onset of condensation in the second layer results in the immediate formation of two-dimensional islands with a close-packed hexagonal structure of 4 × 10 14 cm −2 density. The work function passes through a minimum (1.42 eV) at a coverage of 3 × 10 14 cm −2. After completion of the first layer the work function continues to grow during Cs condensation in the second layer and levels up to 2.15 eV. The EEL spectrum for clean Mo(110) possesses a set of peaks which are identified with the surface and volume plasmons. The first feature to appear in the energy plasma loss spectrum due to Cs deposition is the 2.2 eV loss. It appears at the Cs adatom density corresponding to a work function minimum. This signal is interpreted as due to the surface plasmon loss in Cs. The Cs bulk plasmon loss at 4.6 eV appears at the beginning of the second layer.

Interaction of cesium and barium on partially oxygen covered Nb(110)

We present results of investigations of the adsorption and desorption of Cs and Ba on clean and oxygenated Nb(110) surfaces. The experiments were conducted under ultrahigh vacuum conditions using the techniques of line-of-sight thermal desorption mass spectrometry, Auger electron spectroscopy, and retarding potential work function measurement. The room temperature work function was found to increase from 4.62 eV for clean Nb(110) to 5.63 eV for Nb(110) with a saturated chemisorbed layer of oxygen. For a 4.8 L (1 L=1 Langmuir=1×10−6 Torr s) oxygen exposure, corresponding to about 70% of saturated oxygen coverage, minimum work functions of 2.21 and 1.42 eV were found with the adsorption of Ba and Cs, respectively. The minimum work function tends to increase with further adsorption of oxygen. The adsorption of Cs on the Ba/Nb(110) surface yielded a minimum work function of 1.47 eV. The results of the thermal desorption studies showed an initial (low coverages) binding energy of 3.72 eV for Ba on Nb(110) and 2.42 eV for Cs on Nb(110). We also present desorption energy data for Cs on the Ba/Nb(110) surface.

The electronic structure of CsI adlayers on W(110)

The electronic structure of thin CsI adlayers deposited on W(110) at room temperature is studied by ion impact electron spectroscopy (IIES) with He + and He 2+ ions, metastable impact electron spectroscopy (MIES) with He ∗ atoms, ultraviolet photoelectron spectroscopy (UPS), work function measurements, and to some extent with electron energy loss spectroscopy (EELS). The transition from the eletronic structure of single adsorbed molecules to that of the bulk structure could be studied by following the emergence of bulk properties, in particular the bandgap, as a function of the CsI exposure. The experimental results are consistent with the following conclusions: The first step in the layer growth is molecular adsorption of CsI perpendicular to the surface with the iodine end of the molecule pointing toward the substrate accompanied by decrease of the surface work function. The next step, starting with the occurrence of a pronounced work function minimum, is the formation of two-dimensional islands formed by CsI molecules. The results are consistent with lateral growth of the islands until the substrate is largely covered by CsI. The electronic structure of the adlayer cannot be distinguished anymore from that of CsI bulk material by means of electron spectroscopic techniques at exposures which are more than three times that of the occurrence of the work function minimum. The production of vacancies in the I − (5p) and Cs + (5p) orbitals under the influence of the ionizing radiation is studied as a function of the CsI exposure. Their relevance for the desorption process induced by electronic transitions (DIET) is discussed.

Structural and vibrational properties of alkali-metal overlayers on metals

Structural and vibrational properties of adsorbed alkali-metal layers on a semi-infinite jellium surface are studied over a wide coverage range. Weakening of the adatom-substrate bond with increasing coverage leads to outward relaxation of the overlayer and decrease in the adatom-substrate stretching frequency. The dynamical charge associated with the adatom-substrate stretching vibration decreases much faster with increasing coverage than the static adatom dipole moment and changes its sign near the work-function minimum.

Quantum size effect for Na films on Si(100)

Abstract Photoemission spectra, recorded at low photon energies (ℏ ω eV ) for very thin Na films evaporated onto a cooled Si(100) crystal, show peaks ascribed to quantum well type electronic states and resonances appearing due to the confinement of the alkali valence electrons to the overlayer. The work function, eΦ , shows a pronounced minimum in the Na monolayer coverage range. At around the coverage range required to obtain the minimum work function there is an intensity onset in the photoemission spectra at the Fermi edge indicating a metal character for the surface region.

Quantum size effect for Na films on Si(100)

Photoemission spectra, recorded at low photon energies (ℏ ω < 6.0 eV) for very thin Na films evaporated onto a cooled Si(100) crystal, show peaks ascribed to quantum well type electronic states and resonances appearing due to the confinement of the alkali valence electrons to the overlayer. The work function, eΦ, shows a pronounced minimum in the Na monolayer coverage range. At around the coverage range required to obtain the minimum work function there is an intensity onset in the photoemission spectra at the Fermi edge indicating a metal character for the surface region.

Jellium sphere model in overlayer-substrate systems

It was found in calculations that the jellium/slab model reduces the ability of the adsorbed atoms to attract the valence electrons. As a result, the calculated minimum of the work function is too low compared with the experiment. In this paper, as an improvement, we proposed a jellium sphere/slab model, in which the positive charge background is described by discrete jellium sphere. Test calculations show that this treatment lifts the work function minimum by 0.4 eV and 0.8 eV for Cs/W(001) and Cs/Ir(001) system, respectively, resulting in better agreement with experiments

Adsorption of potassium on Cu(100) and coadsorption with oxygen

Abstract Evidence is found for a critical coverage of potassium, θc, at which the mode of coadsorption of oxygen suddenly changes. This change occurs precisely at the coverage required for the work function minimum. In the high coverage region there are linear variations in work function; these point to the nucleation of a dense adsorbed potassium phase at θc. The uptake of oxygen for high K coverages occurs effectively in two stages which can be related to the presence of adsorption sites which place negatively charged oxygen predominantly first below and then above the potassium layer.

Adsorption of potassium on Cu(100) and coadsorption with oxygen: A study by AES, work function and secondary emission changes

Evidence is found for a critical coverage of potassium, θ c, at which the mode of coadsorption of oxygen suddenly changes. This change occurs precisely at the coverage required for the work function minimum. In the high coverage region there are linear variations in work function; these point to the nucleation of a dense adsorbed potassium phase at θ c. The uptake of oxygen for high K coverages occurs effectively in two stages which can be related to the presence of adsorption sites which place negatively charged oxygen predominantly first below and then above the potassium layer.

Negative electron affinity silicon heterojunction photocathodes with alkali antimonide intermediate layers

In this paper, some new types of p-Si(100) photocathodes with n-K3Sb, n-Na3Sb, and p-Cs3Sb intermediate layers are fabricated. They are the (Si-K3Sb-Cs)-O-Cs and (Si-Na3Sb-Cs)-O-Cs negative electron affinity heterojunction photocathodes, and the (Si-Cs3Sb-Cs)-O-Cs positive electron affinity photocathode. For cathodes with n-K3Sb, n-Na3Sb, and p-Cs3Sb intermediate layers, the maximum photoemission sensitivities attainable are, respectively, 1050, 950, and 150 μA/lm, and the measured minimum work functions of these cathodes are, respectively, 0.9, 1.0, and 0.85 eV. The photoemission stability of the Si photocathode with an alkali antimonide intermediate layer is better than that of the conventional Si-O-Cs and GaAs-O-Cs cathodes. The stability of the cathode is also related to the states of the cesium and oxygen during the activation process and the light illumination condition.

The interaction of alkali atoms with oxygen on W(110) as studied by UPS and metastable impact electron spectroscopy: II. K and Cs

UPS and metastable impact electron spectroscopy (MIES) were employed to study the interaction between oxygen and K (Cs) in the coverage range up to one monolayer on W(110) (in units of the first complete adlayer at room temperature) at room temperature. For small alkali coverages below 0.5 ML the oxygen molecules are dissociated at the reactive W(110) surface and adsorbed atomically on two different adsorption sites for all oxygen exposures. In the coverage range between 0.6 and 1 ML efficient charge transfer from the alkali s Orbitals to the impinging oxygen takes place as long as the s orbital is filled. Molecules are then incorporated into the alkali adlayer as O q− 2 ( q ⩽ 1) ions. Beyond the work function minimum (induced by the oxygen coadsorption) most of the adsorbed oxygen is dissociated; the oxygen atoms are then bound at the same two adsorption sites as for the oxidation below 0.5 ML alkali precoverage. The results are compared with previous results for the oxygenation of Li and Na layers on W(110) and for the oxidation of Cs films. A coadsorption diagram is constructed which allows the prediction of the oxidation stage of the surface as a function of the exposures to K (Cs) and oxygen.

Electronic excitations in thin alkali-metal layers adsorbed on metal surfaces.

The dynamical linear-response properties of realistic Na and K layers adsorbed on a semi-infinite jellium substrate corresponding to the electron density of Al are studied with the aim of elucidating the nature of the adlayer electronic excitations and their variation with coverage. The ground-state properties are described by a first-principles method and the dynamical response in the long-wavelength limit is treated within the time-dependent density-functional approach. At coverages near the work-function minimum, the adsorbate-induced excitations are dominated by intra-atomic excitations between adatom resonant states. Nevertheless, these atomiclike transitions do not lead to any spectral features in the electron-energy-loss function because of the strong hybridization between adatom and substrate states. Instead, as a result of surface screening processes and matrix element effects, a broad loss peak appears near the threshold for emission. This mechanism explains the frequently observed correlation between the coverage dependence of the work function and that of the energy loss induced by the alkali-metal adlayer. As the coverage is increased to one monolayer, the threshold mechanism is replaced by collective excitations consisting of heavily broadened volume plasmons and multipole surface plasmons of the alkali-metal adsorbate. At double-layer coverage, these two modes become very sharp and can clearly be resolved. These collective excitations show only small influence due to the lattice structure of the alkali-metal adlayer.