Maximizing the notional area in single tip field emitters.

One of the critical aspects in advancing high-brightness field emitter devices is determining the conditions under which single-tip emitters should be constructed to optimize their emission area. Recent experiments have explored varying the axis ratio ξ of the cap of a single-tip emitter, ranging from an oblate semi-spheroid to a prolate shape, mounted on a nearly cylindrical conducting body. In this work, we present a strategy, based on high-accuracy computer simulations using the finite element technique, to maximize the emission area of those single-tip emitters. Importantly, our findings indicate that the notional emission area achieves its maximum when the emitter's cap is adjusted to an oblate semi-spheroid with a characteristic axis ratio ξC≈0.85. We do a comparison of notional emission area as a function of ξ for an ellipsoidal emitter on a post and compare these results from other emitter configurations, which are feasible to fabricate.

Investigation of Field Emission Properties of Carbon Nanotube Arrays of Different Morphologies.

This article presents, for the first time, a comparative analysis of the emission characteristics of large-area field-effect cathodes (LAFE) based on carbon nanotubes (CNTs) of various morphologies according to key parameters using a unique computerized technique. The work presents a description of a technology for creating various CNT arrays and their comprehensive structure characterization. All CNT arrays synthesized by the catalytic PECVD method on a silicon substrate showed a high degree of chemical purity under the presented technological conditions. In some cases, nanoisland films of Fe were used as a catalyst; in others, thin films of NiO were used, which were deposited on a silicon wafer by chemical vapor deposition (CVD) and atomic layer deposition (ALD), respectively. As a result of these studies, it turned out that an array with a thick CNT coating has good resistance to the action of strong electric fields, fairly good uniformity of distribution of emission centers, a fairly high selection current (2.88 mA/cm2 at 4.53 V/µm), and compliance with the normal current mode according to the "orthodox" test, which makes the morphology of such structures the most promising for further technological optimization of CNT-based cathodes for various practical applications.

Deconstruction of activity-dependent covalent modification of heme in human neutrophil myeloperoxidase by multistage mass spectrometry (MS(4)).

Myeloperoxidase (MPO) is known to be inactivated and covalently modified by treatment with hydrogen peroxide and agents similar to 3-(2-ethoxypropyl)-2-thioxo-2,3-dihydro-1H-purin-6(9H)-one (1), a 254.08 Da derivative of 2-thioxanthine. Peptide mapping by liquid chromatography and mass spectrometry detected modification by 1 in a labile peptide-heme-peptide fragment of the enzyme, accompanied by a mass increase of 252.08 Da. The loss of two hydrogen atoms was consistent with mechanism-based oxidative coupling. Multistage mass spectrometry (MS(4)) of the modified fragment in an ion trap/Orbitrap spectrometer demonstrated that 1 was coupled directly to heme. Use of a 10 amu window delivered the full isotopic envelope of each precursor ion to collision-induced dissociation, preserving definitive isotopic profiles for iron-containing fragments through successive steps of multistage mass spectrometry. Iron isotope signatures and accurate mass measurements supported the structural assignments. Crystallographic analysis confirmed linkage between the methyl substituent of the heme pyrrole D ring and the sulfur atom of 1. The final orientation of 1 perpendicular to the plane of the heme ring suggested a mechanism consisting of two consecutive one-electron oxidations of 1 by MPO. Multistage mass spectrometry using stage-specific collision energies permits stepwise deconstruction of modifications of heme enzymes containing covalent links between the heme group and the polypeptide chain.

Molecular modelling and computational studies of peptide diphenylalanine nanotubes, containing waters: structural and interactions analysis.

The work is devoted to computer studies of the structural and physical properties of such self-organizing structures as peptide nanotubes (PNT) based on diphenylalanine (FF) dipeptide with different initial isomers of the left (L-FF) and right (D-FF) chiralities of these dipeptides. The structures under study are considered both with empty anhydrous and with internal cavities filled with water molecules. Molecular models of both chiralities are investigated using quantum-chemical DFT and semi-empirical methods, which are in consistent with the known experimental data. To study the effect of nano-sized clusters of water molecules embedded in the inner hydrophilic cavity on the properties of nanotubes (including the changes in their dipole moments and polarizations), as well as the changes in the structure and properties of water clusters themselves (their own dipole moments and polarizations), the surfaces of internal cavities of nanotubes and outer surfaces of water cluster structures for both types of chirality are analyzed. A specially developed method of visual differential analysis of structural features of (bio)macromolecular structures is applied for these studies. The results obtained of a number of physical properties (interacting energies, dipole moments, polarization values) are given for various cases and analyzed in comparison with the known data. These data are necessary for analyzing the interactions of water molecules with hydrophilic parts of nanotube molecules based on FF, such as COO- and NH3 + , since they determine many properties of the structures under study. The data obtained are useful for further analysis of the possible adhesion and capture of medical molecular components by active layers of FF-based PNT, which can be designed for creating capsules for targeted delivery of pharmaceuticals and drugs on their basis.

Field emission: calculations supporting a new methodology of comparing theory with experiment.

This paper provides a demonstration-of-concept of a new methodology for comparing field electron emission (FE) theory and experiment. It uses the parameter κ in the mathematical equation I m = CV m κ exp[-B/V m] (where B and C are weakly varying or constants) that is taken to describe how measured current I m depends on measured voltage V m for electronically ideal FE systems (i.e. systems that (i) have constant configuration during voltage application and (ii) have I m(V m) given by the emission physics alone). Experimental parameter values (κ m) are used to compare two alternative FE theories, for which allowable (but different) κ ranges have been established. At present, contributions to the 'total theoretical κ' made by voltage dependence of notional emission area are not well known: simulations reported here provide data about four commonly investigated emitter shapes. The methodology is then applied to compare 1928/1929 Fowler-Nordheim (FN) FE theory and 1956 Murphy-Good (MG) FE theory. It is theoretically certain that the 1956 theory is 'better physics' than the 1928/1929 theory. As in previous attempts to reach known correct theoretical conclusions by experimentally based argument, the new methodology tends to favour MG FE theory, but is formally indecisive at this stage. Further progress needs better methods of establishing error limits and of measuring κ m.

Properties of blade-like field emitters.

Blade-Like Field Emitters (BFE), as defined here, are emitters expanded in one direction, forming a sharp emitting edge instead of a sharp tip. These structures have four main advantages compared to their needle counterparts, i.e., they are mechanically firmer, are better electrical and thermal conductors, and provide a larger emission area. We focus on the optimization of the last of these. We evaluate the emission properties of three types of BFEs, which we short-named hSoC-blade, HCP-blade and Elli-blade. Each is built from the expansion of a hemisphere-on-a-cone (hSoC), hemisphere-on-a-cylindrical-post (HCP) and an ellipsoidal (Elli) emitter, respectively. The characteristics of the field enhancement factor, the local electrostatic field distribution on each blades' edges and their notional area (An) of emission as a function of the expansion length are described. Finally, we point out how to improve the edge of the HCP-blade to obtain the optimal profile, which yield the largest An.

Influence of NiO ALD Coatings on the Field Emission Characteristic of CNT Arrays.

The paper presents a study of a large-area field emitter based on a composite of vertically aligned carbon nanotubes covered with a continuous and conformal layer of nickel oxide by the atomic layer deposition method. The arrays of carbon nanotubes were grown by direct current plasma-enhanced chemical vapor deposition on a pure Si substrate using a nickel oxide catalyst which was also deposited by atomic layer deposition. The emission characteristics of an array of pure vertically oriented carbon nanotubes with a structure identical in morphology, covered with a layer of thin nickel oxide, are compared using the data from a unique computerized field emission projector. The deposition of an oxide coating favorably affected the emission current fluctuations, reducing them from 40% to 15% for a pristine carbon nanotube and carbon nanotube/nickel oxide, respectively. However, the 7.5 nm nickel oxide layer coating leads to an increase in the turn-on field from 6.2 to 9.7 V/µm.