Confinement of the field electron emission to atomic sites on ultra sharp tips

The spatially controlled field assisted etching method for sharpening metallic tips, in a field ion microscope (FIM), is used to study the evolution of the field emission when the tip apex radius is decreased below 1 nm. Unlike the conventional image formation in a field emission microscope (FEM), we demonstrate that at this scale the field emission is rather confined to atomic sites. A single atom apex fabricated at the end of such tips exhibits an outstanding brightness compared to other atomic tips. The measurements have been repeated for two double atom tips, with different atom-atom separations, and images of atomic field emission localization have also been obtained. We have found that the field emission intensity alternates between adjacent atoms when the applied voltage is gradually increased beyond a threshold value.     

Focused ion beam preparation of atom probe specimens containing a single crystallographically well-defined grain boundary

Needle-shaped atom probe specimens containing a single grain boundary were produced using the focused ion beam (FIB) of a two-beam FIB/SEM (scanning electron microscope) system. The presented specimen preparation approach allows the unprecedented study of a grain boundary which is well characterised in its crystallographic orientation by means of the field ion microscope (FIM) and the tomographic atom probe (TAP). The analysis of such specimens allows in particular the determination of solute excess atoms at this specific grain boundary and hence the investigation of the segregation behaviour. The crucial preparation steps are discussed in detail in the present study for the Sigma 19 a {331} 110 grain boundary of a 40 at.ppm-Bi doped Cu bi-crystal. Transmission electron microscope (TEM) images and TAP analyses of the atom probe tips demonstrate unambiguously the presence of the selectively prepared grain boundary in the apex region of some of the specimens.     

STEM observation of tungsten tips sharpened by field-assisted oxygen etching

Tungsten tips oriented toward the <111> direction were fabricated by field-assisted O₂ etching, and observed by means of scanning transmission electron microscopy (STEM) and field ion microscopy (FIM). The radii of curvature of the tip apexes were sharpened from 16–20 nm to less than 2 nm. The O₂ etching is considered to start from the O₂ imaging region depending on the field distribution around the tip apex and shank. We estimated the effect of field distribution derived from a shank shape and applied bias voltage. The results showed that the tip with a cylindrical shank before the O₂ etching became sharper than tips having an initial paraboloidal shape, with respective cone angles (defined in Fig. 3) of 58° and 80°. The field emission (FE) patterns of these etched tips became a single spot derived from the W(111) plane, and their opening angles (defined by the full width at half maximum) were 14.4° and 7.8°, respectively.     

Silver whiskers and epitaxial layers on tungsten tips as fem specimens; Their growth and behaviour on annealing and oxidation

In order to investigate whether field-emission microscopy (FEM) can be applied to the examination of adsorption on low-melting metals, a study has been made of the preparation and the stabilization of silver surfaces that are suitable as field electron emitters. Single silver whiskers, showing negligibly distorted FEM patterns, could be grown at 873 K by in-situ deposition of silver vapour onto the apex of thermally cleaned blunt tungsten tips. Single-crystal silver layers could be grown at 296 K by the same technique on field-evaporated sharp tungsten tips. The nucleation and growth of the layers take place more uniformly on these crystallographically perfect surfaces than on thermally cleaned tips. At elevated temperatures (673 K) considerable diffusion of silver from the apex of the tip to the shank was noticed when the latter had been thoroughly cleaned by previous heating. The occurrence of this effect was avoided by applying the field-evaporation step in the tip-preparation procedure after the tip had been strongly oxidized ; thus, the oxidized tungsten bordering the cleaned apex acts as a silver diffusion barrier. The silver surfaces obtained have a high degree of cleanliness. Adsorbed oxygen can easily be removed by heating to 673 K ; hardly any blunting results from this treatment. However, when a silver layer has been strongly oxidized, complete field desorption of silver reveals oxidation also of the tungsten at the tungsten-silver interface. Application of positive voltages to a crystalline silver layer on a tungsten tip at 77 K in ultra-high vacuum deforms the layer extensively before actual field evaporation of silver is achieved.     

Investigation of field electron microscope tips bombarded with microparticles at limited field emission currents

Mo, Ta, W, and Re field electron microscope (FEM) tips, bombarded with microparticles at limited field emission currents (400 MΩ resistor in series with the FEM tip) were investigated by means of FEM, transmission electron microscopy (TEM), and scanning electron microscopy (SEM). Two groups of tips could be distinguished: One group had a slight tip radius increase to a maximum of 2.5 μm and microcraters were formed along the tip shank. The other group had no detectable tip radius change; however, there was microcrater formation near the tip apex area. FEM patterns showed surface contamination clearly. Heating such contaminated tips and tips where phosphorescent material (Zn:Cd)S had been deposited in less than monolayer concentration by evaporation from a heating coil, resulted in similar sequences of FEM patterns. A new type of microcrater with smooth crater lips could be detected in both groups. These results support the combined microparticle-field emission mechanism [1], which was proposed earlier to be responsible for melting cap and microcrater formation. Excerpts were presented at the 19th Field Emission Symposium, Urbana, Illinois (USA), August 1972.     

Catalyst-free MOCVD growth of aligned ZnO nanotip arrays on silicon substrate with controlled tip shape

Quasi-aligned zinc oxide nanotip arrays have been grown by MOCVD without using catalyst. The tip shape was controlled systematically by varying the gas flow rate, demonstrating a technique for growing tip arrays of ZnO on silicon. This technology can be large-scale on the wafer level and it has the potential to be directly integrated with clean room silicon technology. The diameter of these ZnO nanowires or nanotips could be controlled by the varying of source flow rate, providing a simple but unique way of fabricating ZnO nanotip arrays for application in field emission and nanogenerators.     

Silicon nanotips formed by self-assembled Au nanoparticle mask

The Au nanoparticle monolayer is formed by self-assembly technology on the Si substrates terminated with different functional groups. Silicon nanotips were fabricated by a self-assembled gold colloidal particle monolayer as an etch mask. The silicon nanotips with high density and uniformity in height and shape were obtained using reactive ion etching (RIE). The Si nanotips on the surface of the 3-aminopropyltrimethoxysilane (APTMS)-treated Si substrate are less-ordered array and uniformity than 3-mercaptopropyltrimethoxysilane (MPTMS)-treated Si substrate at the same etching conditions. The ordered array and uniformity of Si nanotips on the APTMS-modified Si substrate was improved through heat-treatment. This result is implied the different functional groups on the Si surfaces could affect the formation of the Si nanostructures during RIE process. The uniformly nanotip pattern with height of >20 nm is obtained on the etched nanoparticle-coated Si substrate. This method can be applied to patterning a wide variety of thin film materials into tip arrays.     

Compatibility of field emitter studies of oscillating surface reactions with single crystal measurements: catalytic CO oxidation on Pt

Kinetic oscillations in catalytic CO oxidation on Pt have been studied on large (millimeter size) single crystal planes of Pt as well as on a Pt field emitter tip that exposes different crystal facets of nanometer size. In order to examine the compatibility of results from the two types of experiments, the regions of different dynamical behavior (bifurcation diagram) have been mapped out in p_CO,T-parameter space using a field electron microscope (FEM) and a field ion microscope (FIM). The comparison with the results of single crystal measurements shows that in the case of applied electrostatic fields less than 5 V nm⁻¹ (FEM), the field-induced effects are negligible, but they are significant for fields exceeding 12 V nm⁻¹ (FIM). The field-induced shift of the bifurcation diagram toward lower p_CO values, observed with FIM, is explained in terms of a field-modified interaction of CO and O₂ with Pt studied here with field ion appearance energy spectroscopy. With coadsorbed lithium (submonolayer coverage), the existence range for rate oscillations is shifted toward higher p_CO values. This shift is attributed to a redistribution of the electron density at the surface induced by alkali metal co-adsorption.     

Fabrication of <1 1 0> oriented tungsten nano-tips by field-assisted water etching

We report the field-assisted H₂O etching that enabled us to fabricate nano-tips from polycrystalline <1 1 0> oriented tungsten wires at room temperature. We optimized the sharpening procedure in order to obtain field emissions (FEs) with high collimation. The typical tip apex was composed of a large base and a nano-protrusion with a radius of curvature less than 3.5 Å. The narrowest opening angle (full width at half maximum) of the FE was 4.3° at 150 pA. We prepared two types of tips using two different applied bias voltages during the H₂O etching. The electron microscope images and the analysis of Fowler-Nordheim (FN) plots revealed that the sizes of the individual bases depended on the fixed bias voltages during the H₂O etching and affected their FE properties. In addition, we could confirm that the FE current from the nano-tip was more stable than that of the normal tip.     

Aluminum-enhanced sharpening of silicon nanocones

Here we report on the fabrication of high-density aligned Si nanotip arrays by chemical vapor deposition followed by dry oxidation and an etching treatment. The dry oxidation investigations indicated that Al-catalyst particles located at the tip of Si nanocones enhance their sharpening. This oxidation behavior is quite different from that of Au-catalyzed Si nanowires and is more favorable to form very sharp Si nanotips. Field emission from an individual Si nanotip showed good field-emission characteristic with a high emission current density of 1×10⁴ A/cm² because of its sharp tip geography, suggesting their potential application for field emitters. Our work provides an effective approach to fabricate high-density Si nanotip arrays, which overcomes some problems in the conventional fabrication approaches, such as high cost, poor controllability, and complicated process.     

Dynamics of ferroelectric domain formation in an atomic force microscope

The dynamics of ferroelectric domain formation in a non-homogeneous electric field of an atomic force microscope (AFM) tip are considered. Contributions of the apex and the conical part of the tip into the field are taken into account. It is supposed that this process passes through the same stages as the corresponding process in homogeneous external fields. However, the character of these stages may differ significantly. We consider all the stages of this process: formation of a nucleus, its growth, and the equilibrium domain parameters. Quantitative analysis is carried out for barium titanate. It is shown that the activation energy of nucleation strongly decreases with the applied voltage, and it is too small to limit the rate of the process even under low voltages. Dynamic equations for the time dependence of the domain length and radius are constructed and solved. Comparison of the calculated domain sizes with those observed in experiment is carried out. Calculated results obtained using different models for the field of the tip are compared.     

Mechanical vertical manipulation of selected single atoms by soft nanoindentation using near contact atomic force microscopy

A near contact atomic force microscope operated at low-temperature is used for vertical manipulation of selected single atoms from the Si(111)-(7 x 7) surface. The strong repulsive short-range chemical force interaction between the closest atoms of both tip apex and surface during a soft nanoindentation leads to the removal of a selected silicon atom from its equilibrium position at the surface without additional perturbation of the (7 x 7) unit cell. Deposition of a single atom on a created vacancy at the surface is achieved as well. These manipulation processes are purely mechanical, since neither bias voltage nor voltage pulse is applied between probe and sample. Differences in the mechanical response of the two nonequivalent adatoms of the Si(111)-(7 x 7) with the load applied is also detected.     

EHD sprayings induced by the pulsed voltage superimposed to a bias voltage

Effects of pulsed voltage superimposed on dc bias voltage and meniscus height on electrohydrodynamic (EHD) spraying were investigated. Results show that greater pulsed voltages were associated with jet formation while a dripping mode was apt to appear with a lower pulsed voltage. This is because that with increasing pulsed voltages the energy gain per unit area of the liquid and the tangential electric stress at the meniscus lateral were increased more quickly than the normal electric stress at the apex of the meniscus. Additionally, the increment of meniscus height led to an unchanged tangential electric stress at the meniscus lateral, but a more quickly increased energy gain per unit area of the liquid than the normal electric stress at the apex of the meniscus. For the same pulsed voltage, spraying in Dripping I mode was produced from menisci of smaller heights due to the intensive normal electric stress. A much greater meniscus height, on the other hand, led to spraying in Dripping II mode when the pulsed voltage was insufficiently great. These various modes were determined by contributions of the tangential electric stress, the normal electric stress and the meniscus height.     

Experimental evidence for intra-atomic noncollinear magnetism at thin film probe tips

The energy-dependent spin-density orientation (SDO) at the apex of thin magnetic film tips is studied by spin-polarized scanning tunneling spectroscopy (SP-STS) at different bias voltages. At most energies the SDO is collinear with the tip magnetization resulting in a domain or domain-wall contrast in SP-STS images of out-of-plane magnetized samples measured with Gd or Fe coated tips, respectively. For some bias voltages, however, the SDO of the tip is found to be almost perpendicular to its magnetization. This result is explained in terms of intra-atomic noncollinear magnetism.     

Plasmon–polariton assisted formation of nanotip arrays on surfaces of bulk aluminum upon femtosecond laser irradiation

Two new methods for the formation of nanotip arrays under the action of single or double femtosecond laser pulses on a surface of bulk aluminum are presented. It is shown that a key role in the formation of nanotips is played by the excitation of surface electromagnetic waves and their mutual interference, and by their interference with the exciting electromagnetic field of the laser pulse.     

Size effects on phonon localization and Raman enhancement in silicon nanotips

Silicon nanotip arrays exhibit a wide variety of interesting optical and electronic properties associated with their dimensionality. We here investigate the effect of size‐induced changes on phonon localization and explain the enhanced Raman response. The occurrence of normally forbidden transitions in the photoluminescence spectra provides evidence for the predicted localization effect. Spatially resolved Raman spectroscopy reveals a continuous change of the silicon Raman peak position and peak width along the nanotip that is attributed to a smooth change between bulk properties at the base to size‐induced phonon confinement in the apex of the nanotip. This approach allows to exclude heating effects that normally overwhelm the phonon confinement signature. The Raman spectra are in excellent agreement with the spatial correlation model and the extracted correlation length is comparable to the tip dimensions. The observed phonon confinement coincides with an enhancement of the Raman scattering efficiency at the tip apex and results in a 40‐fold increase of the sample's Raman intensity compared with bulk silicon. These results provide a step toward the integration of Si based optoelectronic devices. Copyright © 2012 John Wiley & Sons, Ltd.     

Carbon fractals grown from carbon nanotips by plasma-enhanced hot filament chemical vapor deposition

Carbon nanomaterials with different structures were prepared in a custom-designed plasma-enhanced hot filament chemical vapor deposition system using methane, hydrogen and nitrogen. They were investigated by scanning electron microscopy (SEM) and micro-Raman spectroscopy. The SEM images show that the smooth carbon nanotips are formed under a high bias current and the carbon fractals can grow from the tips of the carbon nanotips under a low bias current. The results of micro-Raman spectroscopy indicate that the graphitization of the carbon nanomaterials was improved by ion bombardment. Combined the ion bombardment, electric field enhancement and electron emission mechanisms, the formation model of the carbon fractals was suggested.     

Superfocussing in a metal-coated tetrahedral tip by dimensional reduction of surface-to edge-plasmon modes

Metal-coated dielectric tetrahedral tips (T-tip) have long been considered to be interesting structures for the confinement of light to nanoscopic dimensions, and in particular as probes for scanning near-field optical microscopy. Numerical investigations using the Finite-Difference Time-Domain (FDTD) method are used to explore the operation of a T-tip in extraction mode. A dipole source in close proximity to the apex excites the tip, revealing the field evolution in the tip, the resulting edge and face modes on the metal-coated surfaces, and the coupling from these modes into highly directional radiation into the dielectric interior of the tip. These results are the starting point for illumination-mode numerical investigations by a Volume Integral equation method, which compute the field distribution that develops in a T-tip when a Gaussian beam is incident into the tip, and which show that a highly confined electric field is produced at the apex of the tip. The process of light confinement can be considered as a superfocussing effect, because the intensity of the tightly confined light spot is significantly higher than that of the focussed yet much wider incident beam. The mechanism of superfocussing can be considered as a dimensional reduction of surface plasmon modes, where an edge plasmon is the most important link between the waveguide-modes inside the tip and the confined near field at the apex.     

Localized multiphoton emission of femtosecond electron pulses from metal nanotips

Intense multiphoton electron emission is observed from sharp (approximately 20 nm radius) metallic tips illuminated with weak 100-pJ, 7-fs light pulses. Local field enhancement, evidenced by concurrent nonlinear light generation, confines the emission to the tip apex. Electrons are emitted from a highly excited nonequilibrium carrier distribution, resulting in a marked change of the absolute electron flux and its dependence on optical power with the tip bias voltage. The strong optical nonlinearity of the electron emission allows us to image the local optical field near a metallic nanostructure with a spatial resolution of a few tens of nanometers in a novel tip-enhanced electron emission microscope.     

Magnetic nanotips: the realisation of an atomic-scale beam-splitter

We present the first realisation of ferromagnetic Fe nanotips. Studies of the field emitted beams of electrons and metallic ions above and under the Curie temperature T_c were made. We have observed a reversible splitting of the electron beams when the temperature crossed through T_c and multiple spot patterns for the emitted metallic ion beams. These phenomena were not observed with Fe microtips or non-magnetic nanotips and thus were correlated to a strong magnetic interaction at the atomic scale apex of the nanotips. The Fe nanotips constitute an atomic scale beam-splitter.