Adsorption Of Residual Gas Molecules Research Articles

Carbon-coated electron field emitters

The properties of field electron emitters made of iridium and rhenium, whose surface is coated with carbon by decomposition of benzene molecules on the surface, are studied. The stability of the emission characteristics and the stability of the obtained emitters to the adsorption of residual gas molecules and ion bombardment are shown. The coating is obtained either in the form of a monoatomic graphene film on the flat faces of the emitter, or in the form of graphite growths. The field electron emission is localized on the edges and corners of the faceted graphite outgrowth. After adsorption of alkaline and alkaline earth metals onto the carbon coating, an abnormally large increase in emissions from flat faces and localization of emissions on graphene-coated faces occurs. The emission amplification occurs in the presence of alkali metal atoms, both on the surface and only under the graphene film. Keywords: electron emission, carbon, iridium, rhenium, alkali metals, barium, intercalation.

Полевые эмиттеры электронов с углеродным покрытием

The properties of field electron emitters made of iridium and rhenium, whose surface is coated with carbon by decomposition of benzene molecules on the surface, are studied. The stability of the emission characteristics and the stability of the obtained emitters to the adsorption of residual gas molecules and ion bombardment are shown. The coating is obtained either in the form of a monoatomic graphene film on the flat faces of the emitter, or in the form of graphite growths. The field electron emission is localized on the edges and corners of the faceted graphite outgrowth. After adsorption of alkaline and alkaline earth metals onto the carbon coating, an abnormally large increase in emissions from flat faces and localization of emissions on graphene-coated faces occurs. The emission amplification occurs in the presence of alkali metal atoms, both on the surface and only under the graphene film.

Impact of residual gas on the optoelectronic properties of Cs-sensitized In0.53Ga0.47As (0 0 1) surface

Near-infrared InxGa1-xAs photocathode with better optoelectronic properties is a good candidate for low-light-level (LLL) night-vision system. However, the residual gases in the ultra-high vacuum (UHV) system inevitably affects the stability and photo-emission performance of LLL photoelectric devices such as their quantum efficiency and life-time. In this study, the first-principles calculations were used to investigate the adsorption effect of five different residual gas species, including H2, CH4, CO, H2O and CO2 on Cs-sensitized In0.53Ga0.47As (001) β2 (2 × 4) surface. The study results indicate that CO2 gas molecule is the most easily attached to the Cs-sensitized surface. The adsorption of residual gases leads to the formation of a new dipole pointing from inner Cs atoms to gas molecules. It makes the charge center of the adsorbates escape from the surface, which weakens the interaction between the inner Cs atoms and the clean surface. This results in the increase of the surface work function and degradation of the performance of photoelectric devices. Also, the adsorption of residual gas molecules influences the absorption and reflection coefficients of Cs-sensitized In0.53Ga0.47As (001) β2 (2 × 4) surface.

Adsorption of residual gas molecules on (10–10) surfaces of pristine and Zn-doped GaAs nanowires

Using first-principles calculations, the effects of residual gas molecules (H2O, CO, CO2, H2 and N2) adsorption on the photoelectric properties of pristine and Zn-doped GaAs nanowire surfaces are investigated. Total energy calculations show that p-type doping surface is beneficial to reduce the damage of residual gases to cathodes and improve the stability of GaAs nanowire photocathodes. After adsorption of gas molecules, the electrons are transferred from surface to adsorbates, leading to a dipole moment pointing from surface to residual gas molecules, which obstructs the escape of electrons and increases the work function of photocathodes. Through Zn doping, the charge transfer between gas molecules and nanowire surface is reduced and the force of dipole moment induced by gas molecules is weakened. Besides, the conduction energy bands shift toward higher energy region and the band gap increased after adsorption of residual gas molecules. Moreover, residual gas adsorption will weaken the absorption characteristic of GaAs nanowire photocathodes.

Pressure Measurement Using Field Electron Emission Phenomena

Adsorption of residual gas molecules damped the emission current of a W (310) field electron emission (FE) emitter. The damping speed was linearly proportional to the pressure gauge readings at pressure ranging from ∼10 -8 Pa to ∼10 -9 Pa, and the proportionality constant was employed to measure pressure in the 10 -10 Pa range. A time plot of FE current revealed the existence of an “initial stable region” after the flash heating of W(310) FE, during which the FE current damps very slowly. The presence of non-hydrogen gas removed this region from the plot, supplying a means of qualitatively analysing the gas species.

Using Photoelectron Spectroscopy and Quantum Mechanics to Determine d-Band Energies of Metals for Catalytic Applications

The valence band structures (VBS) of eight transition metals (Fe, Co, Ni, Cu, Pd, Ag, Pt, Au) were investigated by photoelectron spectroscopy (PES) using He I, He II, and monochromatized Al Kα excitation. The influence of final states, photoionization cross-section, and adsorption of residual gas molecules in an ultrahigh vacuum environment are discussed in terms of their impact on the VBS. We find that VBSs recorded with monochromatized Al Kα radiation are most closely comparable to the ground state density of states (DOS) derived from quantum mechanics calculations. We use the Al Kα-excited PES measurements to correct the energy scale of the calculated ground-state DOS to approximate the “true” ground-state d-band structure. Finally, we use this data to test the d-band center model commonly used to predict the electronic-property/catalytic-activity relationship of metals. We find that a simple continuous dependence of activity on d-band center position is not supported by our results (both experimentall...

Time variation of the work function of field emitter tip surface and the development of adsorption of residual gas molecules studied by sawtoothlike emission current method

The sawtoothlike emission current (STEC) method and its theory are described, and this method is shown to make possible the simultaneous measurement of the time variation of the work function φ(t) and its derivative dφ/dt. Application of the STEC method to the field emission (FE) current from a tungsten emitter tip in an ultra-high vacuum of 2×10−8 Pa shows that the time variation of φ(t) and dφ/dt can be divided into four stages. In addition, it gives the effective tip radius αr0=1.1 μm, resulting in α=6.9 for the observed tip radius r0=0.16 μm. The observed time variation of φ(t) and dφ/dt is analyzed from the viewpoint of the development of the adsorption of residual gas molecules on the emitter tip surface, and each stage is successfully identified as follows: The first stage is the formation stage of adsorption nuclei and clusters, the second stage is the free growth stage of adsorption islands, the third stage is their coalescence growth stage, and the fourth stage is their Langmuir growth stage. The mode of adsorption development is similar to the mode of crystal growth taking place in three-dimensional space and on surface.

Field emission microscopy of adsorption and desorption of residual gas molecules on a carbon nanotube tip

Field emission of electrons from multiwall carbon nanotubes has been investigated by field emission microscopy (FEM) in ultra-high vacuum. A carbon nanotube, at the end of which at least six pentagons exist to make a closed cap, gives an FEM pattern consisting of bright pentagonal rings if the nanotube surface is clean. Adsorption of residual gas molecules is observed as bright spots in the FEM pattern, giving rise to an abrupt increase in the emission current. Adsorbed molecules seem to reside preferentially on the pentagonal sites where the strong electric field is concentrated. A heat cleaning of the emitter at about 1300 K allows the molecules to desorb, and the nanotube emitter recovers its original clean surface. It has been revealed that the adsorption and desorption of gas molecules are responsible for stepwise fluctuation of the emission current.

A Novel Approach for True Work Function Determination of Electron-Emissive Materials by Combined Kelvin Probe and Photoelectric Effect Measurements

ABSTRACTFor the development of new electron-emissive materials knowledge of the work function Φ and changes in Φ is of particular interest. Among the various methods, the ultra-high vacuum (UHV) compatible scanning Kelvin Probe has been proven to be a superior technique to measure work function changes due to e.g. UHV cleaning processes, chemical contamination, thermal processing etc. with high accuracy (<1meV).The Kelvin Probe measures local work function differences between a conducting sample and a reference tip in a non-contact, truly non-invasive way over a wide temperature range. However, it is an inherently relative technique and does not provide an absolute work function if the work function of the tip (Φtip) is unknown.Here, we present a novel approach to measure Φtip with the Kelvin Probe via the photoelectric effect, where a Gd foil is used as the photoelectron source. This method thus provides the true work function of the sample surface with an accuracy of approx. 50meV. We demonstrate the application of the technique by in situ work function measurements on evaporated layers of the low work function material LaB6 on a Re substrate and follow the changes in Φ of LaB6 due to the surface adsorption of residual gas molecules. Thus, the extended Kelvin Probe method provides an excellent tool to characterise and monitor the stability of low work function surfaces.

How to select the best condition for operating a positive ion source of molecular beam surface ionization typea)

In easily attainable high vacua (Pr≊20–0.2 μTorr), the efficiency (β+) for producing ions (M+) from diatomic molecules (MX) incident on a polycrystalline surface of W, Mo, or Re was measured as a function of surface temperature (T≊800–2300 K). Theoretical analysis of β+ elucidated the temperature dependence of the effective work function (φ+) for the ionization. Namely, (1) above ∼2000 K, each surface is kept virtually clean, having φ+≊5.2, 4.9, and 5.4 eV for W, Mo, and Re, respectively. (2) At ∼1400–1200 K, the range of which depends upon Pr, φ+ is increased up to ∼7 eV by adsorption of residual gas molecules (RGM), thereby yielding β+=1 for even Tl and the current density of ∼10 μA cm−2 for the flux of 1014 molecules cm−2 s−1. (3) Below ∼1200 K, however, φ+ is much decreased by coadsorption of MX and RGM. A theoretical diagram of T vs φ+ is devised to predict the best condition for β+=1. The best condition (φ+, T, and Pr) to make β+ as large as possible may readily be selected according to the diagram and the present data on φ+.

How to select the best condition for operating a positive ion source of molecular beam surface ionization type (abstract)a)

In easily attainable high vacua (Pr≊20–0.2 μTorr), the efficiency (β+) for producing ions (M+) from diatomic molecules (MX) incident on a polycrystalline surface of W, Mo, or Re was measured as a function of surface temperature (T≊800–2300 K). Theoretical analysis of β+ elucidated the temperature dependence of the effective work function (φ+) for the ionization. Namely, (1) above ∼2000 K, each surface is kept virtually clean, having φ+≊5.2, 4.9, and 5.4 eV for W, Mo, and Re, respectively. (2) At ∼1400–1200 K, the range of which depends upon Pr, φ+ is increased up to ∼7 eV by adsorption of residual gas molecules (RGM), thereby yielding β+=1 for even Tl and the current density of ∼10 μA cm−2 for the flux of 1014 molecules cm−2 s−1. (3) Below ∼1200 K, however, φ+ is much decreased by coadsorption of MX and RGM. A theoretical diagram of T vs φ+ is devised to predict the best condition for β+=1. The best condition (φ+, T, and Pr) to make β+ as large as possible may readily be selected according to the diagram and the present data on φ+.

Time variation of the effective work function for thermal positive ion production from lithium atoms incident upon a rhenium surface in a high vacuum

In order to study the adsorption effect of both Li atoms and residual gas molecules (RGM) upon the effective work function, φ +, for producing Li + ions from Li atoms impinging with a constant flux (∼7×10 13 atoms cm −2 s −1) upon a polycrystalline rhenium surface heated to various temperatures at the residual gas pressure of ∼2×10 −7 torr, φ + was measured as a function of the elapsed time after flashing the surface at ∼2200 K. The patterns of the time variation thus investigated were quite different from those observed previously by the incidence of LiI molecules under nearly the same conditions. This difference gives additional evidence to support our recent conclusions, viz. (1) completely dissociative adsorption of LiI molecules on the surface above ∼1250 K has little effect upon φ + and hence (2) the increase in φ + observed with the LiI/Re system is caused mainly by adsorption of RGM, but (3) the adsorption effect of RGM is thoroughly suppressed by incidence of Li atoms and, consequently, (4) φ + in the Li/Re system above ∼1200 K is kept nearly constant at ∼5.4 eV as if the surface were essentially clean, even below ∼1750 K.

Additional evidence for the work function increase due to adsorption of residual gases and for its decrease due to co-adsorption of the gases and alkali metal halide molecules during positive surface ionization in high vacua

Under the various conditions of sample beam flux ( N ⋍ 4×10 12-1×10 14 molecules cm −2 s −1) and of residual gas pressure ( P r ⋍ 0.2–2 μtorr), the emission current of lithium ions produced from lithium iodide molecules impinging upon a heated polycrystalline rhenium surface was measured as a function of surface temperature ( T ⋍ 1000–2100 K) or elapsed time (0–10 min) after flashing the surface. The experimental data thus obtained were analyzed by our theory on dissociative positive surface ionization to determine the change in effective work function (ϕ +) for the ionization. A boundary temperature ( T A) below which ϕ + increased gradually with a decrease in T changed from ≈1750 to ≈1900 K with an increase in P r but depended little upon N, suggesting that completely dissociative adsorption of LiI had little effect upon ϕ +. On the other hand, another boundary temperature ( T B) at which ϕ + began to decrease steeply with a lowering temperature shifted from 1265 to 1180 K according to a decrease in N, but T B was no longer dependent upon P r. Below T B, a rapid accumulation of sample molecules on the surface greatly suppressed the work function increase due to adsorption of residual gas molecules (RGM). These results verify our previous conclusions that the increase in ϕ + below T A is caused mainly by adsorption of RGM and also that the decrease in ϕ + below T B is caused by co-adsorption of sample molecules and RGM.

A simple method for analyzing the causes of a change in effective work function for thermal positive ionization

The effective work function (φ +) for producing a positive ion on a polycrystalline rhenium filament heated at a residual gas pressure of 2×10 −7 torr was previously found to increase by up to ≈1.5 eV at surface temperatures of ≈1000–2200 K. In order to analyze the causes of this increase, the emission current of Li + produced from LiI was measured as a function of both the elapsed time after flashing the filament with the sample beam shutter closed and the delay time before opening the shutter. When the sample beam flux ( N) was as low as 6.6×10 12 molecules cm −2 s −1, the increase was caused by adsorption of residual gas molecules (RGM). When N was increased ten times at as low a temperature as 1160 K, on the other hand, the increase due to the adsorption was suppressed by incidence of LiI, and hence θ + was a complex function of coadsorption of RGM and LiI.

Thermal positive ion production from KCl impinging upon Re: Comparison between theory and experiment

On the basis of our theory on thermal ionization, the emission current of K+ ions produced from KCl molecules incident upon a polycrystalline rhenium surface is calculated as a function of surface temperature in the range 950–2100 K. Theoretical results thus obtained are in good agreement with experimental data. This agreement gives an additional evidence for our previous conclusions that below ∼1800 K the work function of the surface is increased from 5.5 up to ∼7 eV mainly by adsorption of residual gas molecules at ∼2×10−7 Torr and also that the dissociation of incident sample molecules is greatly promoted by the work function increase.

Temperature dependence and time variation of thermal positive ion production through dissociation of lithium chloride molecules impinging on a heated rhenium surface

Production of Li + ions from LiCl molecules impinging with a constant flux (∼ 7× 10 13-∼4×10 11 molecules cm −2 s −1) on a polycrystalline rhenium surface heated in a residual gas pressure of about 2×10 −7 torr was studied as a function of either surface temperature ( T ⋍ 1000–2200 K) or elapsed time ( t ⋍ 0–10 min) after flashing at T = 2160 K. The ionization efficiency (β +) was virtually unity only in the middle temperature region (∼1300-∼1500 K). The emission current of Li + at constant T in the region increased gradually with t up to a saturated value corresponding to β + = 1. These results are analyzed by our previous theory on molecular beam surface ionization, leading to the conclusions that the effective work function for positive ion production on the surface increases from about 5.6 eV up to ∼ 7 eV as T decreases from ∼ 2000 K to ambient temperature, that this increase is caused mainly by the adsorption of residual gas molecules, and that the dissociation of LiCl molecules as well as the ionization of Li atoms is promoted by the increase, just as predicted by the theory.