The emission properties of organic field ion emitters: A combined FIM and FI-MS investigation

The emission properties of tungsten emitters activated with benzonitrile, such as are used in analytical organic field ion mass spectrometry, have been investigated using n-heptane as test gas. It was discovered that these emission properties are influenced principally by the morphological structure and electronic state of the surfaces of the microneedles; the bulk structure of these organic agglomerates has practically no effect on their ionization behaviour.Field ion microscope investigation of emitters activated at high temperature (HT) revealed a similar surface structure to that of pyrolytic graphite; emitters activated at room temperature (RT) showed less order. The emission regions of highest electric field strength on the tips of the needles are projecting points, ledges, and in the case of HT-emitters, structure planes.A decrease in the mean emission field strength results from the formation of solid deposits on the needle surfaces. A field corrosion of the needle tips occurs in the presence of high pressures of water and leads also to an irreversible decrease in the mean emission field strength. The effect of physically adsorbed layers on the emission properties of the emitter surface is discussed in terms of the influences of the electric field penetration on the electronic properties of the needles.     

High field microscopy of nickel on tungsten

Field emission and field ion microscopy have been used to study the properties of nickel layers adsorbed on tungsten, and the growth of nickel crystallites. The first monolayer of nickel has a maximum density of 0.97 ± 0.05 × 10¹⁹ atoms m^?2 and results in an increase in the work function which can be attributed to the formation of dipoles of moment μ₀ = 1.70 ± 0.08 × 10^?30Cm at zero coverage and polarizability $\text{α = 7.3 ± 0.05}\text{A}\text{?}{}^3$. Nickel desorbs from the tungsten surface with activation energy 4.22 ± 0.01 eV and second layer atoms desorb with activation energy 3.2 ± 0.02 eV. Surface diffusion of second and higher layers over clean tungsten layer is believed to proceed by the “unrolling carpet” mechanism, with activation energy 0.93 ± 0.03 eV in close agreement with measurements of surface self-diffusion of nickel. Nickel does not dissolve appreciably in single-crystal tungsten and we confirm that atomic disordering at the nickel-tungsten interface is confined within a few angstroms of the interface. Well-ordered crystallites can be grown from a central nuclear structure which develops on (110)W. Combination of field ion and field emission techniques indicate that the crystallites adopt the expected growth form, having surfaces comprising large low-index faces, and also serve to confirm that field emission images alone cannot be relied upon to give an indication of crystallite shape. Crystallites invariably form upon an adsorbed layer which is at least one atom thick but may be thicker depending upon conditions of growth. The growth of crystals in situ offers the possibility of generating well-ordered low-index planes of large area which are suitable for further study, but it has yet to be confirmed that they behave as surface planes of bulk nickel.     

Study of mercury surface diffusion on tantalum with and without an electric field by means of a field emission microscope

The surface diffusion of mercury atoms on tantalum substrate with and without high electric field was studied by means of a field emission microscope (Müller's). The activation energy during surface migration Q_m of mercury atoms with and without an electric field F on tantalum substrate depending on the thickness of the adsorbate was measured. It is shown that the electron density distribution at coverage θ < 0.65 with adsorbate is due to a dipole momentum P. At θ > 0.65 the slope of the curves of Q^F_m = ?(θ) is explained with the appearance of the effect of polarization. The energy of desorption Q_d as a function of the thickness of the adsorbed layer in the temperature range 100–300 K was measured also.     

Effects of X-ray radiation on hydrogenated amorphous silicon thin-film transistors and characteristics of tungsten shield layer

Hydrogenated amorphous silicon (a-Si:H) thin-film transistors (TFTs) which have tungsten shield layer for AMOLED were fabricated. E-beam evaporation is useful method to prepare cathode layer above organic layers with high throughput and no damage to the organic layers but TFTs are damaged by X-ray radiation generated during the e-beam process. When the thickness of the tungsten layer was increased to 2000 Å, it was found that the characteristics of TFTs, such as threshold voltage, field effect mobility and sub-threshold slope, were slightly varied by X-ray radiation.     

The high temperature and field treatment of the tungsten tip in a field emission microscope and the adsorption of oxygen and hydrogen

The adsorption of hydrogen and oxygen on the clean surface of tungsten obtained by thermal desorption of impurities from the tip in FEM and on the surface of a tungsten tip formed at a high temperature under the influence of a high voltage was studied. It could be deduced from the experiments that at 78K even the adsorption heat of oxygen is not sufficient for the surface layers of tungsten to be rearranged. The influence of temperature and an electric field on the emission pattern of the tip and its shape was also studied.     

Growth and field emission of tungsten oxide nanotip arrays on ITO glass substrate

High-density and uniformly aligned tungsten oxide nanotip arrays have been deposited by a conventional thermal evaporation on ITO glass substrates without any catalysts or additives. The temperature of substrate was 450-500 °C. It was shown that the tungsten oxide nanotips are single-crystal grown along [0 1 0] direction. For commercial applications, field emission of the tungsten oxide nanotip arrays was characterized in a poor vacuum at room temperature. The field emission behaviors are in agreement with Fowler-Nordheim theory. The turn-on field is 2.8 V μm⁻¹ as d is 0.3 mm. The excellent field emission performances indicated that the tungsten oxide nanotip arrays grown by the present approach are a good candidate for application in vacuum microelectronic devices.     

Femtosecond electron dynamics on solid surfaces probed by low energy ion scattering and stimulated desorption of secondary ions

The cations emission from condensed matter surfaces has been investigated on the basis of localization and delocalization of valence hole(s) in the femtosecond timescale. The yield of scattered H⁺ (E₀=100 eV), though negligibly small from the Pt(1 1 1) substrate, increases markedly when Ar is adsorbed on it, indicating the localization of a valence (H⁺ 1s) hole on the physisorbed Ar layer. However, the yield of H⁺ scattered from a thick H₂O layer is considerably small relative to that from Ar and CO layers. The delocalized nature of a valence hole in water ice is caused by some covalency in hydrogen bonds. Hydrated protons, H⁺(H₂O)_n, n=1,2,…,10, are emitted efficiently in electron stimulated desorption from water molecules adsorbed on the Ar layer; the ion yields are highest at the initial adsorption stage and decay steeply with increasing coverage. Coulombic repulsion between the hydrated protons confined in physisorbed nanoclusters is responsible for the explosive ion emission.     

Adsorption and decomposition of ammonia on W(100); XPS and UPS studies

The adsorption and decomposition of ammonia on a clean and c(2 × 2)-N ordered W(100) surface has been studied by photoemission spectroscopy (XPS and UPS). At 120 K molecularly adsorbed ammonia was identified by N(1s) core level emission at 400.9 eV and the valence emissions at 7.6 and 11.7 eV. By heating the sample stepwise the N(1s) core level shifted to lower binding energy. In the valence region, the corresponding spectral changes were obtained, where the dependence of the peak intensity on photon energy was observed. These observations were interpreted to demonstrate that adsorbed ammonia dissociates its hydrogen successively to form NH_x(a) and finally to atomic nitrogen. On the other hand, ammonia was molecularly adsorbed on a c(2 × 2)-N ordered surface even at temperatures as high as 300 K, although the spectra at 400 K or above were very similar to those under a steady state flow condition, where the tungsten surface was mostly covered by atomic nitrogen. At higher ammonia pressure up to about 100 Pa thicker nitride layers were formed at 700 K, which were characterized by the N(1s) core level at 397.3 eV and a broad emission around 6 eV in the valence level.     

Field electron emission changes by face specific adsorption of aluminum oxide and corresponding decomposition products on tungsten {110} and {100} planes

After deposition of aluminum oxide on a tungsten field emission microscope (FEM) tip and stepwise heating, three stages of emission changes were observed on {100}. Stages I and II cause work function decreases of 0.28 and 0.07 eV, respectively. Stage III is characterized by a large increase (Δ??+3 eV). The changes are discussed in terms of interaction of decomposition products (oxygen and aluminum) and adsorption of aluminum oxide. On {110} only a single aluminum oxide layer growth, which results in a work function decrease to ?=4.69 eV, is observed. The field electron emission from this layer was measured between 1400°K and room temperature. The experimental values were compared with those determined from Christov's unified theory of field and thermionic emission. The {110} layer values coincide with those obtained earlier from an aluminum oxide covered tungsten {112}.     

Direct identification of atomic binding sites on a crystal

The atomic resolution of the field ion microscope, in conjunction with its ability to remove and identify individual atomic layers, allowed us to map unambiguously the unit cell of the (111) plane of tungsten and to determine directly the location of single tungsten atoms adsorbed on this plane. Adatoms have been observed to occupy two binding sites only. The predominant site corresponds to a normal lattice position. The second site is of similar symmetry, in that the adatom sits between three first layer atoms; however, at this position the adatom is located above an atom in the second rather than the third lattice layer. The former site is favored energetically, but only by ≈ 0.5 eV. All observations have been made at high fields, however, it is shown from studies of migration and other effects that the binding sites identified in the field ion microscope are typical of a normal, field free environment.     

Enhanced field ionization of water adsorbed on a carbon monoxide-covered, platinum field emitter tip

The influence of carbon monoxide, adsorbed on a platinum field emitter tip, on field ionization of adsorbed water was examined. Ramped field desorption (RFD) measurements of water ionization were performed at 108 K for water layer thicknesses up to 80 ML on a clean or CO-saturated tip surface. In RFD the applied field is ramped linearly in time until water ionization is detected, giving the onset field of ionization. Water ionization yields hydrated hydroxide ions and protons; the hydroxide ions remain within the water layer on the tip, while the hydrated protons are emitted into vacuum. At a low water coverage of 1.5 ML, the CO adlayer substantially reduced the onset field of ionization (that is, facilitated ionization) of water by 40%, from a value of 0.43 V/Å for water on clean Pt to 0.26 V/Å for water on CO-covered Pt. The extent of the reduction gradually diminished with thicker coverages of water and was absent at coverages of 20 ML or greater. The characteristic decay length of the field enhancement was 4.7 ± 1 ML. The results were analyzed with the charge exchange model of ionization kinetics and changes in dipole moments of water adsorbed without and with CO. The analysis reveals that a change in water structure (dipole moment) caused by CO is an important contributor in field enhancement and that the dipole moment for hydrated hydroxide ion in an ice-like layer must be greater than that for bulk ice-like water. The significance of these results with respect to electrochemical oxidation of CO is discussed.     

Structure and composition of tungsten silicide thermal-field microprotrusions

The thermal-field microprotrusions that grow on the surface of a tungsten tip coated with silicon when the tip is heated in an electric field are investigated by a suite of field emission methods, including electron field emission, ion desorption microscopy, and the atomic-probe method. For Si coatings more than a few monolayers thick, microprotrusions are observed to grow in the field desorption regime when the tip is heated to temperatures T=1100–1200 K in an electric field with initial intensity F=5.7–8.6×10⁷ V/cm. The field at which they evaporate is 1.2–1.8×10⁸ V/cm. The set of moving spots (i.e., microprotrusions) forms rings whose collapse signals the dissolution of the thermal-field growths on the developed faces. The most interesting structures are the sharp microprotrusions that grow on the central facet of a {110} tungsten tip under certain conditions. Atomic-probe analysis of their composition reveals that they consist of tungsten trisilicide WSi₃ with a monolayer surface skin whose composition is close to WSi₂. The intense growth of these formations on an initially smooth closepacked {110} face of tungsten is evidence that reconstruction of the latter takes place under the influence of the strong field and the interaction with silicon. Zh. Tekh. Fiz. 67, 102–109 (September 1997)     

Strong amplification of an electric field and electro-optical response in ultrathin heterostructures ferroelectric-linear dielectric

An electroded heterostructure consisting of a dye layer sandwiched between two polymer ferroelectric layers is discussed. The dye layer plays a role of the probe of the electric field measured by an electroabsorption technique. Using this new method the electric field in ferroelectric and dielectric layers can be measured separately. When an a.c. voltage is applied to the heterostructure, the electric field in the dye layer increases 2.2 times (up to 0.55 GV/m), whereas the field in the ferroelectric decreases 2 times with respect to the average field in the entire structure. Moreover, the dye layer sandwiched between the ferroelectric layers may stand without breakdown the fields 5–7 times higher than a neat reference dye layer confined between metal electrodes. Therefore, the performance of electro-optical, electromechanical and other field controlled devices may be improved considerably when their functional materials are placed between ferroelectrics layers.     

Emission photoélectrique d'une pointe EN W OU W-Ba Soumise à un champ électrique

The photoelectric emission from a pure or barium-coated single crystal tungsten tip submitted to a high electric field is studied under illumination by light of energy smaller than the work fonction of the substrate. The results are discussed according to the range of the electric field applied. It is found that the average values of the work function for barium coated tips as obtained by photofield emission are different from those obtained by field emission. The photofield emission and field emission patterns are compared.     

分层掺B和吸附H2O碳纳米管的结构稳定性及电子场发射性能

运用基于第一性原理的密度泛函理论,系统研究了处于外电场中分层掺B并吸附不同数目H₂O碳纳米管体系的结构稳定性和电子场发射性能. 研究表明:第3层掺B并吸附5个H₂O的B₃CNT+5H₂O体系结构最稳定,管帽处Mulliken电荷最密集,尤其与单独掺B的B₃CNT和单独吸附H₂O的B₃CNT+5H₂O相比,其Fermi能级处态密度分别     

Work function of the adsorption system of potassium on tungsten

The work functions φ_hklof tungsten single crystal planes as functions of the surface densities N_hkl of the adsorbed potassium have been measured by means of the field emission method. Sealed-off field emission tubes with a Faraday collector and rotatable emitter were used. Another, special tube was made in order to determine the surface density of potassium. The unequilibrated adlayers on {110}, {112}, {100} and {111} tungsten single crystal planes have been investigated. For all the planes investigated the φ_hkl(N_hkl) dependence exhibited a distinct minimum. An attempt has been made to compare the experimental results with the theoretical models suggested recently.     

氮掺杂及水分子吸附碳纳米管电子场发射第一性原理研究

对闭口碳纳米管(CNT)顶端分层掺氮及吸附不同数目水分子体系,运用第一性原理研究了有电场存在时的电子场发射性能.结果表明：掺氮并吸附水分子的CNT结构稳定;外电场愈强、水分子数愈多,体系态密度(DOS)向低能端移动幅度愈大且最高分子占据轨道(HOMO)/最低分子空轨道(LUMO)能隙愈小.吸附能,DOS/LDOS,HOMO/LUMO及其能隙分析一致表明,第三层氮掺杂CNT吸附不同数目水分子体系的场发射性能最佳. 关键词： 氮掺杂 水吸附 密度泛函理论 电子场发射     

Study of adsorption of titanium on tungsten and rhenium by field electron emission

The deposition and layer growth of titanium, evaporated on to tungsten and rhenium field emitter tips has been studied, and field emission average work-functions measured at varying titanium layer thicknesses from less than a monolayer to several monolayers. Titanium evaporated on to a clean tungsten tip at 293 K in the submonolayer region tended to collect preferentially on atomically rough planes. With further deposition, a “pseudo-clean” emission pattern was obtained at about a monolayer coverage when the titanium atoms were concluded to be in epitaxial registry with the BCC substrate; at about this coverage the work-function for the system passed through a minimum. Further titanium deposition in excess of three monolayers resulted in a progressive roughening of the surface due to microcrystallinity: brief heating (~ 1 min, 1300 K) of such a surface resulted in a well ordered layer identified as ß-titanium (BCC) presumably epitaxed to the tungsten. This epitaxed layer remained indefinitely stable at 293 K.     

Competitive adsorption of polymers on metal nanoparticles

We investigate the effect of system properties and adsorption sequence on competitive adsorption of poly(methyl methacrylate) (PMMA) and polystyrene (PS) on narrowly polydispersed cobalt (Co) nanoparticles (D ∼ 27 nm). The adsorbed layer composition is studied using thermo-gravimetric analysis (TGA). We find that adsorbed layers of PS are completely displaced by PMMA when the solvent is a common good solvent. An adsorbed layer of only PMMA is also obtained through competitive adsorption from a common good solvent. However, in a selective solvent that is poor for PS, sequential adsorption leads to the formation of mixed layers.     

Optimization of microcavity OLED by varying the thickness of multi-layered mirror

We optimized the emission efficiency from a microcavity OLEDs consisting of widely used organic materials, N,N′-di(naphthalene-1-yl)-N,N′-diphenylbenzidine (NPB) as a hole transport layer and tris (8-hydroxyquinoline) (Alq₃) as emitting and electron transporting layer. LiF/Al was used as a cathode, while metallic Ag was used as an anode material. A LiF/NPB bi-layer or NPB layer on top of the cathode was considered to alter the optical properties of the top mirror. The electroluminescence emission spectra, electric field distribution inside the device, carrier density, recombination rate and exciton density were calculated as a function of the position of the emission layer. The results show that for optimal capping layers thicknesses, light output is enhanced as a result of the increase in both the reflectance and transmittance of the top mirror. Once the optimum structure has been determined, the microcavity OLED devices were fabricated and characterized. The experimental results have been compared to the simulations and the influence of the thickness of the mirror layers, emission region width and position on the performance of microcavity OLEDs was discussed.