Field Ion Imaging Research Articles

Reflections on the Spatial Performance of Atom Probe Tomography in the Analysis of Atomic Neighborhoods.

Atom probe tomography (APT) is often introduced as providing “atomic-scale” mapping of the composition of materials and as such is often exploited to analyze atomic neighborhoods within a material. Yet quantifying the actual spatial performance of the technique in a general case remains challenging, as it depends on the material system being investigated as well as on the specimen's geometry. Here, by using comparisons with field-ion microscopy experiments, field-ion imaging and field evaporation simulations, we provide the basis for a critical reflection on the spatial performance of APT in the analysis of pure metals, low alloyed systems and concentrated solid solutions (i.e., akin to high-entropy alloys). The spatial resolution imposes strong limitations on the possible interpretation of measured atomic neighborhoods, and directional neighborhood analyses restricted to the depth are expected to be more robust. We hope this work gets the community to reflect on its practices, in the same way, it got us to reflect on our work.

Field-ion imaging of nano-objects at far-subangstrom resolution

Field-ion imaging of nano-objects at far-subangstrom resolution

How the World First Saw Atoms: An Outline of How Field Ion Imaging Works

How the World First Saw Atoms: An Outline of How Field Ion Imaging Works

A new approach to preparing tips for atom probe field ion microscopy from powder materials

A new approach to preparing tips for atom probe field ion microscopy from powder materials is described. It is especially useful for particle sizes exceeding >0.050 mm. Particles resulting from mechanical alloying of Cu and Ag, and from cryomilling of conventional Ni powders have been prepared this way for field ion imaging and atom probe composition analysis. In both cases, useful information complementing results from transmission electron microscopy and X-ray diffraction was obtained.

Remote atom probe field ion microscopy

Each of the normal stages of operation of the atom probe, i.e., specimen loading, field ion imaging, selection of the feature of interest for analysis, and atom probe analysis, have been examined to determine the feasibility of operating an atom probe over the Internet. Experiments have been performed that have demonstrated the feasibility of remotely monitoring and controlling a conventional atom probe field ion microscope and a three-dimensional atom probe.

Performance of a microsphere plate electron multiplier in APFIM applications

The microsphere plate (MSP) is a recently introduced electron multiplier, that is comparable in principle of operation to the well-known microchannel plate. The performance of a MSP detector has been evaluated for field ion imaging and single atom detection in time-of-flight atom probe mass spectrometry. The image performance of a MSP is not sufficient for obtaining high-quality field ion images, due to the lack of spatial resolution as well as the granularity observed in the images. Gain, pulse height resolution, and temporal resolution obtained from one double thickness MSP are sufficient for use in a time-of-flight single atom sensitivity detector. The detection efficiency was estimated to be ∼42% for ions in the energy range between 10 and 40 keV. The operation of the MSP detector was successfully demonstrated in a typical atom probe application.

Focused ion-beam specimen preparation for atom probe field-ion microscopy characterization of multilayer film structures

Focused ion-beam milling with a sub-10 nm diameter beam of gallium ions has been used to fabricate field-ion specimens from a multilayer film nanostructure containing 100 repetitions of a bilayer deposited directly onto a planar substrate. Successful field-ion specimen preparation has allowed the observation of these layers on the atomic scale by both field-ion imaging and atom probe compositional analysis.

Three-dimensional atom probe field-ion microscopy observation of Cu/Co multilayer film structures

Focused ion-beam milling has been used to fabricate field-ion specimens from a multilayer film structure containing 100 repetitions of a (Cu2 nm/Co2 nm) bilayer deposited directly onto a planar substrate. The as-deposited films showed a magnetoresistance ratio of ∼5% over a 250 Oe range at room temperature, and a coercivity of ∼60 Oe. The magnetic data suggest that the films are coupled ferromagnetically. Successful field-ion specimen preparation has allowed the observation of these layers by field-ion imaging and three-dimensional atom probe compositional analysis. Examination of the multilayer images reveals that, in some regions, the layers are nonparallel, but the interfaces are chemically quite sharp, with a diffuse interface region of ∼3 atomic layers. In addition, in some areas adjacent cobalt layers appear to be in contact. The fact that the layers are wavy suggests that the ferromagnetic coupling may be a result of Néel “orange peel” type magnetostatic coupling between adjacent cobalt layers. The relatively high coercivity may be a result of the poor layer planarity leading to a high number of domain wall pinning sites.

Field-emission barrier height of an alloy tip: Possible effect of surface segregation

We have investigated the influence of surface segregation on the field-emission characteristics of alloy emitters. Our tip material is a Ni-0.8 at. % Au alloy where gold segregates to the surface. Emission barrier height was obtained from the slope of the Fowler–Nordheim plot assuming the field proportionality constant estimated from the best image voltage in the field-ion imaging. Upon annealing the alloy tip at 700 K, the barrier height is found to decrease by ∼0.5 eV. As no lowering of the barrier height was observed in an annealed Ni tip, the observed reduction of the barrier height is attributable to the surface enrichment of gold due to its partial segregation. Our findings thus support the claim by Yoshihara and Yoshitake [J. Vac. Sci. Technol. A 13, 2407 (1995)] that the surface segregation can be used as a method for improving the emission characteristics of field emitters.

Molecular adsorption states of O 2 on W(111) at low temperatures (down to 5 K)

Adsorption of O 2 on W(111) at temperatures down to 5 K is studied using molecular beam (MB) and thermodesorption spectroscopy (TD) methods. The low-temperature TD spectra for the molecularly adsorbed O 2 on W(111) contain three peaks at 25, 27 and 60 K corresponding respectively to multilayer condensation, physisorption and weak molecular chemisorption. The role of the substrate structure for the O 2 chemisorption is discussed on the basis of present results for W(111) and earlier data for different metal surfaces. Field-effect on the O 2W(111) interaction is studied using field ion appearance energy spectroscopy (FIAES) of O + 2 ions emitted from the apex plane of a [111]-oriented W field emitter tip. Applying a thermionic cycle, field-modified binding energies for O 2 W(111) are derived from the O + 2 appearance energies measured at 79 K. FIAES data reveals a field-adsorbed O 2 layer as the origin of O + 2 field ions emitted from W(111) for field ion imaging.

Weakly chemisorbed CO layer on Rh(100) as detected by reverse flash and field ion appearance energy measurements

Using the reverse flash (RF) method, the sticking coefficient and binding energy of CO molecules on Rh(100) are obtained for CO surface densities up to 1.2 × 10 15 cm −2. The presence of weakly bound CO molecules with a binding energy of 0.34 eV is detected at n > 1.0 × 10 15 cm −2. Mass-to-charge-resolved retarding potential analysis of CO + emitted from Rh(100) is performed and absolute values of CO + field ion appearance energy obtained. By applying a thermionic cycle field-dependent values of the binding energy of CO Rh(100) at high coverages are obtained. A comparison of field-free (RF) results and field ion appearance energy spectroscopy (FIAES) data reveals the weakly bound (field-stabilized) CO layer as the origin of CO + field ions emitted from Rh(100) for field ion imaging.

Field-ion imaging old and new

This paper is a historical review with two aims. It records some lighthearted personal recollections of early days in the development of field-ion microscopy in Cambridge University in the early 1960s. And it presents a short systematic account of the results and theoretical ideas about field-ion image formation that emerged from this period. Experimental results are included that, although discussed informally or presented to scientific meetings in the 1960s and 1970s, have not previously been published. The theory emphasised gas-kinetic aspects of image formation, and much of it has survived the test of time. Some relevant recent work is noted.

Appearance energy spectroscopy of CO+ field ions emitted from W(111)

Field ion appearance energy spectroscopy of CO+ field ions emitted from a few surface sites of the W(111) plane was performed. Using a newly developed retarding potential technique, absolute values for the CO+ appearance energies were derived from mass-to-charge resolved retarding potential analyses of field ions of the carbon monoxide supplied as an imaging gas in the field ion microscope. From analysis of the CO+ field ion appearance energies, field ionization of CO on the W(111) surface was found to proceed by field desorption. Applying a thermionic cycle, field-dependent values of the binding energy for CO on W(111) at high coverages were obtained. A comparison of the results for CO/W(111) with data for the field-free binding energy of CO on rough W planes, measured independently, is made. The mechanism of field ion imaging of metal surfaces with CO+ ions and the field dependent chemisorption of CO on different metal surfaces is discussed on the basis of experimental results and theoretical calculations.

Atom probe field ion microscope studies of palladium silicide on silicon

Palladium silicide coated silicon specimens have been formed by thermally evaporating palladium onto clean field evaporated n-type silicon (100) tips. The thin palladium overlayer is reacted to form palladium silicide by thermal annealing at 675 K. The tips were studied by field ion imaging, pulsed laser atom probe and field emission current-voltage characterisation. These analyses were carried out in a modified atom probe field ion microscope which allows all these techniques to be employed without breaking vacuum. The stoichiometry of the 2–3 nm thick silicide layer was close to that expected for Pd 2Si. All analyses showed a thin silicon rich layer on top of the palladium silicide. Fowler-Nordheim plots of the field emission characteristics of silicide coated tips were obtained as the layer was removed by field evaporation. Analysis of the slopes and intercepts of these plots indicates that the silicide coated tips have a rougher surface than the silicon tips. This was confirmed by field ion microscope and field emission microscope images which showed localised electron emission from the sharpest areas of the tip. PLAP analysis of a tip exposed to air for 4 days showed an oxidised silicon rich surface layer which was 2–4 monolayers thick.

Field emission and atom probe field ion microscope studies of palladium-silicide-coated silicon emitters

Palladium-silicide-coated field emitters were prepared by palladium thermal evaporation onto silicon tips, followed by annealing at 675 K. Pulsed laser atom probe analysis showed a clean near stoichiometric, Pd2Si layer with a thin silicon-enriched surface. In spite of the higher work function of palladium silicide compared to silicon, enhanced emission was obtained from the silicide-coated tips. Field ion and field emission imaging of the tips showed that this enhanced emission was due to the roughness of the silicide layer. Pulsed laser atom probe analysis of a palladium silicide tip exposed to air for four days showed that the silicon-rich surface layer was converted to an oxidized layer 2–4 monolayers thick. The field emission characteristics of silicide tips exposed to air were similar to those observed previously in oxidized silicon tips. After an initial current burst which corresponded to the disruption of the oxide layer, intense emission was observed from sharp areas around the circumference of the tip.

Field ion appearance energy spectroscopy of CO + originated from Rh(111) and Au(111) surface step sites

Field ionization of CO at step sites of Rh(111) and Au(111) was studied with field ion microscopy and mass-to-charge resolved field ion energy spectroscopy. Using a newly developed retarding potential technique, absolute values for CO + field ion appearance energies were derived from measurements of the high energy onset of ion retardation curves and from in situ measurements of the work function of the retarding electrode. From analyses of the CO + field ion appearance energies, field ionization of CO on Rh and Au surfaces was found to proceed by field desorption. Applying a thermionic cycle, field-dependent values of the binding energy for CO on Rh(111) and Au(111) were derived. For comparison, CO field ionization was also studied at a passivated Rh surface. The mechanism of field ion imaging of metal surfaces with CO + ions and the field-dependent chemisorption of CO on different metal surfaces is discussed on the basis of experimental results and theoretical calculations.

An atom probe study of grain boundary and matrix chemistry in microalloyed NiAl

The atom probe field ion microscope (APFIM) has been used to perform a quantitative study of the grain boundary and matrix chemistry in NiAl microalloyed with either boron, carbon or beryllium. Boron was observed to segregate to grain boundaries whereas carbon and beryllium did not. Atom probe measurements of the matrix chemistry revealed severe depletion of the solute in the boron- and carbon-doped alloys. Field ion imaging, atom probe analyses and transmission electron microscopy (TEM) of the matrix revealed ultrafine MB 2- and MC-type precipitates ranging in size between 2 and 20 nm in diameter in the boron- and carbon-doped NiAl. The number densities of these precipitates were estimated by atom probe analyses to range between 1 × 10 20 m −3 and 1 × 10 23 m −3 for precipitates of 20 and 2 nm diameter, respectively. Atom probe analyses of the matrix in beryllium-doped NiAl revealed that almost all the beryllium was in solid solution. This was consistent with the fact that ultrafine beryllide precipitates were not detected in the atom probe. The enormous increase in yield stress in the boron- and carbon-doped alloys is predominantly due to a precipitation hardening effect. The relatively small increase in yield stress in beryllium-doped NiAl is due to a weak substitutional solid solution hardening effect.

Atom probe analysis and field emission studies of silicon

Single ungridded silicon tips have been studied by field emission current–voltage characterization, field emission imaging, field ion imaging, and pulsed laser atom probe (PLAP) analysis. Changes in field emission characteristics were observed as a thin contaminant layer (oxygen, hydrogen, carbon, and carbon monoxide) formed on the surface of clean field evaporated silicon tips. Removal of the contaminant layer by pulsed laser-assisted field desorption restored the original field emission characteristics. Clean silicon tips that were oxidized in air began to emit at half the voltage required for unoxidized tips. As the voltage was increased, a sudden irreversible increase in field emission current was observed. PLAP analysis and field emission imaging showed that this effect was caused by the disruption of the oxide layer shortly after the onset of field emission. After the oxide disruption, field emission occurred from regions of sharp local geometry around the circumference of the tip. Removal of these regions by field evaporation or PLAP produced field emission characteristics very similar to those obtained from clean silicon tips. This explains a long-standing problem in understanding the ‘‘switch-on’’ behavior of silicon emitters.

Comparative studies on field ionization at surface sites of Rh, Ag and Au—differences in local electric field enhancement

Field ion imaging conditions were studied for clean Rh, and for Rh epitaxially covered with Ag and Au, respectively. Significant, metal-specific decreases of the image voltages have been observed. This is caused by different enhancements of local fields in the vicinity of protruding Rh, Ag and Au atoms due to their different electronic structures. The interpretation is based on self-consistent calculations of local electrostatic field distributions in the vicinity of single atoms adsorbed on a metal surface. For different facets of a surface, a close correlation between local field distributions and the degree of localization of the field ionization process is establihed from the measured full width at half maximum (FWHM) of field ion energy distributions and is discussed in association with possible field ionization mechanisms.

New aspects of field adsorption and accommodation in field ion imaging

Field ionization (FI) experiments characterized by field ion imaging and by field ion appearance spectroscopy have been performed of Ne on Rh(012) at emitter temperatures of 79 K, and slightly above at 107 K, as well as of Ne with admixed Ar on Rh(111) at 79 K, and also of CO on Rh(111) in the temperature range of 79–300 K. The results obtained for rare gas FI give evidence of weakly bound image gas atoms on the interior sites of crystal facets playing an active role in imaging gas accommodation. These atoms are as necessary for localized FI as arc the earlier suggested actively involved field-adsorbed atoms on the imaged surface step sites of the emitter. FI of CO was found to proceed by field desorption. From analyses of the appearance energies of CO +, arguments have been derived that the earlier model of localized FI, at which at the instant of FI a touching of imaging gas at the sites of strong field adsorption was assumed, is of minor importance and should be replaced in favour of a collision-induced exchange process prior to FI being sufficient and necessary for highest contrast field ion images.