Mechanism Of Field Evaporation Research Articles

Correlating laser energy with compositional and atomic-level information of oxides in atom probe tomography

Atom probe tomography (APT) is a 3D analysis technique that offers unique chemical accuracy and sensitivity with sub-nanometer spatial resolution. There is an increasing interest in the application of APT to complex oxides materials, giving new insight into the relation between local variations in chemical composition and emergent physical properties. However, in contrast to the field of metallurgy, where APT is routinely applied to study materials at the atomic level, complex oxides and their specific field evaporation mechanisms are much less explored. Here, we perform APT measurements on the hexagonal manganite ErMnO3 and systematically study the effect of different experimental parameters on the measured composition and structure. We demonstrate that both the mass resolving power (MRP) and compositional accuracy can be improved by increasing the charge-state ratio (CSR) working at low laser energy (< 5 pJ) for a given fixed detection rate. Furthermore, we observe a substantial preferential retention of Er atoms, which is suppressed at higher CSRs. We explain our findings based on fundamental field evaporation concepts, expanding the knowledge about the impact of key experimental parameters and the field evaporation process in complex oxides in general.

Understanding the Underlying Field Evaporation Mechanism of Pure Water Tips in High Electrical Fields.

We investigated the field evaporation process of frozen water in atom probe tomography (APT) by density functional simulations. In previous experiments, a strong tailing effect was observed for peaks caused by the molecular structure (H2O)nH+, in contrast to other peaks. In purely field-induced and thermally assisted evaporation simulations, we found that chains of protonated water molecules were pulled out of the dielectric surface by up to 6 Å, which are stable over a wide range of field strengths. Therefore, the resulting water clusters experience only part of the acceleration after evaporation compared to molecules evaporating directly from the surface and, thus, exhibit an energy deficit, which explains the tailing effect. Our simulations provide new insight into the complex evaporation behavior of water in high electrical fields and reveal possibilities for adapting the existing reconstruction algorithms.

Correlative transmission electron microscopy and atom probe tomography on field evaporation mechanism of a bulk LaAlO3 oxide

Laser-assisted atom probe tomography (APT) has commonly been performed to determine the composition of non-conductive oxide materials at the atomic scale. However, its field evaporation mechanism is still insufficiently understood due to the complexity of laser-matter interaction on the surface of oxide tips. To understand the physical mechanism underlying laser-assisted field evaporation in a bulk oxide LaAlO3 (LAO), we conducted an interrupted experiment that combines transmission electron microscopy with APT. This correlated technique visualized the shape evolution of LAO tips during laser-assisted APT. We demonstrated that the evaporation field strength of the tips depends on the APT experimental parameters including applied laser energy and base temperature. An increase in the laser energy led to a reduction in the field evaporation strength of the tips and the formation of apex asymmetries on the tips, thereby considerably influencing the mass resolution of the technique. Moreover, an increase in the base temperature caused a slight decrease in the evaporation strength but did not significantly affect the mass-resolving ability of APT. The results discussed in this work suggest that it is the employed laser energy that primarily affects APT's resolving capability and the field evaporation strength of oxide materials. Our interrupted technique provides fundamental insight into the field evaporation sequences of LAO oxide tips under laser illumination, which can be feasibly applied to the study of various metallic and oxide materials.

Observation of DC field‐evaporated ion species extracted from transition metal nitride thin film deposited on tungsten‐tip

The ion species extracted from transition metal nitride thin films were investigated in order to understand the field evaporation mechanism of nitrogen in atom probe analysis. Nitrides of group IV transition metal, ie, titanium (Ti), zirconium (Zr), and hafnium (Hf) nitrides, were chosen for analysis, owing to their good electrical conductivities. The samples were prepared by sputtering deposition of nitride thin film on a tungsten needle. Measurements were performed at 3 different direct current voltages, and for each voltage, we observed different ion species. For TiN and ZrN, both atomic metal ions and molecular ions were detected and TiN and ZrN tended to evaporate in the form of triple‐charged molecular ions. For ZrN, Zr2+, Zr3+, ZrN3+, and (ZrN)23+ were observed at lower direct current voltages. For a higher tip voltage, N+ ions were detected in addition to these ions. These results suggest that the evaporation field of nitrogen is higher than those of Zr3+ and (ZrN)23+. In the analysis of an HfN tip, no ions could be detected. These results can be explained in terms of the differences between evaporation fields that were roughly estimated from the work functions and the bond energies of the analyzed nitrides.

Understanding of Capping Effects on the Tip Shape Evolution and on the Atom Probe Data of Bulk LaAlO3 Using Transmission Electron Microscopy.

Two challenges exist in laser-assisted atom probe tomography (APT). First, a drastic decline in mass-resolving power is caused, not only by laser-induced thermal effects on the APT tips of bulk oxide materials, but also the associated asymmetric evaporation behavior; second, the field evaporation mechanisms of bulk oxide tips under laser illumination are still unclear due to the complex relations between laser pulse and oxide materials. In this study, both phenomena were investigated by depositing Ni- and Co-capping layers onto the bulk LaAlO3 tips, and using stepwise APT analysis with transmission electron microscopy (TEM) observation of the tip shapes. By employing the metallic capping, the heating at the surface of the oxide tips during APT analysis became more symmetrical, thereby enabling a high mass-resolving power in the mass spectrum. In addition, the stepwise microscopy technique visualized tip shape evolution during APT analysis, thereby accounting for evaporation sequences at the tip surface. The combination of "capping" and "stepwise APT with TEM," is applicable to any nonconductors; it provides a direct observation of tip shape evolution, allows determination of the field evaporation strength of oxides, and facilitates understanding of the effects of ultrafast laser illumination on an oxide tip.

Impact of Extreme Electrical Fields on Charge Density Distributions in Alloys

The purpose of this work is to identify the field evaporation mechanism associated with charge density distribution under extreme fields, linking atom probe tomography (APT) experiments with density functional theory (DFT) modeling. DFT is used to model a materials surface bonding, which affects the evaporation field of the surface atoms under high electric fields. We show how the evaporation field of atoms is related to the charge density by comparing the directionality and localization of the electrons with the evaporation of single ions versus dimers. This evaporation mechanism is important for the reconstruction of APT data, which is partially dependent on the input evaporation fields of the atoms. In $L1_{2}-Al_{3}Sc$, $Al-Al$ surface atoms are more likely to evaporate as dimers than $Al-Sc$ surface atoms. We find that this is due to $Al-Al$ having a shared charge density, while $Al-Sc$ has an increased density localized around the $Sc$ atom. Further, the role of subsurface layers on the evaporation physics of the surface atoms as a function of charge density is considered. Beyond the practical considerations of improving reconstruction of APT data, this work provides an approach for design of surface chemistry for extreme environments.

Energetics and dynamics of laser-assisted field evaporation: Time-dependent density functional theory simulations

High positive electrostatic (dc) field can break the bonds in molecules and strip away atoms from the solid surfaces. The dynamics of this field evaporation under laser pulse is studied for a ${\mathrm{H}}_{2}$ molecule and a ${\mathrm{Si}}_{4}$ cluster using time-dependent density functional theory combined with molecular dynamics. This allows us to monitor the position and charge state of the evaporated atom in real time. Our simulations demonstrate that the critical dc field for the evaporation is lower if the molecule/cluster is illuminated by a laser pulse. The behavior of the evaporation threshold as a function of laser intensity and dc field is in qualitative agreement with experiments and provides important insights into the mechanisms of laser-assisted field evaporation. Additionally, the laser-assisted field evaporation is found to be sensitive to the laser energy according to the photoabsorption spectra that demonstrate a pronounced redshift in the lower energy region at higher dc field values.

Modeling Atom Probe Tomography: A review

Improving both the precision and the accuracy of Atom Probe Tomography reconstruction requires a correct understanding of the imaging process. In this aim, numerical modeling approaches have been developed for 15 years. The injected ingredients of these modeling tools are related to the basic physic of the field evaporation mechanism. The interplay between the sample nature and structure of the analyzed sample and the reconstructed image artefacts have pushed to gradually improve and make the model more and more sophisticated. This paper reviews the evolution of the modeling approach in Atom Probe Tomography and presents some future potential directions in order to improve the method.

Do dielectric nanostructures turn metallic in high-electric dc fields?

Three-dimensional dielectric nanostructures have been analyzed using field ion microscopy (FIM) to study the electric dc field penetration inside these structures. The field is proved to be screened within a few nanometers as theoretically calculated taking into account the high-field impact ionization process. Moreover, the strong dc field of the order of 0.1 V/Å at the surface inside a dielectric nanostructure modifies its band structure leading to a strong band gap shrinkage and thus to a strong metal-like optical absorption near the surface. This metal-like behavior was theoretically predicted using first-principle calculations and experimentally proved using laser-assisted atom probe tomography (APT). This work opens up interesting perspectives for the study of the performance of all field-effect nanodevices, such as nanotransistor or super capacitor, and for the understanding of the physical mechanisms of field evaporation of dielectric nanotips in APT.

Laser-assisted field evaporation from insulators triggered by photoinduced hole accumulation

We analyze the mechanism of laser-assisted field evaporation from insulators triggered by photoinduced hole accumulation at the tip apex. The effects of hole accumulation on the electric flux line and the activation barrier for field evaporation are analyzed by the boundary element method with a mesoscopic model and atomistic ab initio calculations. The ab initio calculations indicate that a certain amount of hole accumulation reduces the activation barrier for field evaporation substantially. We propose the following scenario for laser-assisted field evaporation from insulators: 1) a laser pulse creates holes in the valence band of oxide through electronic excitation, 2) the holes accumulate at the tip apex due to the external direct-current electric field, 3) the positive charge at the apex reduces the activation barrier for cation evaporation, 4) the cation evaporates from the apex, and 5) the next laser pulse creates holes again and triggers subsequent field evaporations.

Ultrafast laser-triggered field ion emission from semiconductor tips

We study experimentally and theoretically the controlled field evaporation of single atoms from a semiconductor surface by ultrafast laser-assisted atom probe tomography. The conventional physical mechanisms of field evaporation cannot explain the experimental results recently reported for such materials. A new model is presented in which the positive dc field leads to band bending with a high density of laser-generated holes near the surface of the sample. The laser energy absorption by these holes and the subsequent energy transfer to the lattice considerably increase the tip temperature. We show that this heating plays an important role in the field ion emission process. In addition, experiments are carried out for germanium and silicon tips to check the role of the dc field in the absorption processes, as well as the heating of the tip and the following evaporation. Good agreement between the predictions of our model and the experimental data is found.

Evidence of lateral heat transfer during laser assisted atom probe tomography analysis of large band gap materials

Investigation of wüstite (FeO), iron (Fe), and silicon (Si) specimens by infrared laser assisted three dimensional atom probe tomography shows evidence of confined surface absorption of the laser energy as the mechanism of laser assisted field evaporation for high band gap materials. The absorption region is shown to be located on the laser illumination side, close to the specimen apex. A simple model based on thermal diffusivity and spatial extent of the absorption region is derived and applied to the experimental results. The values of thermal diffusivity thus obtained are in good agreement with the published data.

Field evaporation mechanism of bulk oxides under ultra fast laser illumination

The controlled field evaporation of single atoms from an oxide surface assisted by ultra fast laser pulses has recently been demonstrated. When UV light is used, a photoionization mechanism was proposed. However, experimental results observed when the laser intensity and wavelength are changed cannot be explained by this mechanism. Instead, a thermal assisted evaporation mechanism characterized by two evaporation times is proposed. The fast and slow evaporation rates are associated to two cooling processes inside the tip sample. Experiments are carried out on TiO2 and MgO field emitter tips to check the dependence of the evaporation process on structural properties of the oxide. A good agreement between the predictions of our model and the experimental data is found.

Influence of Tip Temperature on Field Evaporation in Atom Probe

Recently, laser pulses on a three-dimensional atom probe have been used to trigger the field evaporation. The advantages of laser-pulse atom probes are high mass resolution and application to higher resistivity materials such as semiconductors. Most recent studies using laser pulses have indicated that the field evaporation of atoms occurred by a thermal pulsing mechanism. In this study, we analyzed the metal specimens, tungsten, nickel and aluminum, by using the hand-made 3DAP in our laboratory, and verified the mechanism of field evaporation by laser pulses. From the results, the spatial resolution might be extremely better at lower tip temperature. The difference of mass resolution between the laser-irradiated and shadow sides was observed and this might resulted from the difference of cooling duration in both sides. In the reconstruction calculation, the difference of curvature radii in the laser-irradiated and shadow sides was compensated and the spatial resolution was evaluated. [DOI: 10.1380/ejssnt.2011.375]

Study of the field evaporation mechanism of laser-assisted atom probe

Study of the field evaporation mechanism of laser-assisted atom probe

Mechanism of laser assisted field evaporation from insulating oxides

To explain the recent successful three-dimensional atom probe (3DAP) analyses of insulating oxides by laser assisted field evaporation, we investigated the mechanism of the laser-induced field evaporation of oxides by ab initio calculations. The calculated potential energy surfaces (PESs) for the ground and excited states indicated that the activation barrier height for field evaporation is substantially reduced by the accumulation of holes near the tip apex. This would make the direct electronic excitation possible to promote field evaporation along with thermal excitation. These theoretical calculations are supported by experimental observations.

Dependence of Field Evaporation Voltage on Polarization Angle of Femtosecond Laser in 3D Atom Probe

Recently, laser pulses on a three-dimensional atom probe have been used to induce field evaporation. The advantages of laser-pulse atom probes are application to higher resistivity materials such as semiconductors and high mass resolution. Early studies of laser-pulse atom probes were performed with laser pulses of nanosecond duration. All evidence from these early studies and most recent studies using sub-picosecond laser pulses have indicated that the field evaporation of atoms by laser pulses occurred by a thermal pulsing mechanism. On the other hand, some recent experiments with sub-picosecond laser pulses have resulted in the proposal of athermal mechanisms (e.g., optical rectification) of field evaporation. Thus, the mechanism of field evaporation at the apex of a needle specimen by sub-picosecond laser pulses has not yet been established. We report the dependence of field evaporation voltage on the polarization angle of femtosecond laser pulses for metal specimens. [DOI: 10.1380/ejssnt.2009.70]

Emission characteristics of Au60Be40 and Au62Si23Be15 liquid metal ion sources

The angular current densities, relative abundance, and total energy distributions (TEDs) of the various ion species from the binary Au60Be40 and ternary Au62Si23Be15 liquid metal ion sources (LMISs) have been measured. The results support a postionization mechanism for the formation of the various monoatomic M+2 species, while the M+ species are formed by a combination of field evaporation, field ionization, and charge exchange mechanisms. All ion species show an increase in the full width at half maximum of the TEDs with total emission current, mass, and charge. The dominant monoatomic species at 5μA total emission current are Au+ (64%), Au+2 (15%), Be+2 (10%), and Be+ (3.2%) for the binary LMIS and Au+ (63%), Au+2 (13%), Si+2 (3.2%), Be+2 (2.1%), and Be+ (0.71%) for the ternary LMIS. The predicted beam sizes for Au+2 and Be+2 for the binary LMIS at a beam current of 5pA in a typical focusing column are 15 and 8nm, respectively.

Vacuum electrospray ionization study of the ionic liquid, [Emim][Im

An analysis is presented of positive and negative vacuum electrospray currents from the ionic liquid, [Emim][Im], wetted on a sharp tungsten needle. The source is operated in a cone–jet configuration established with a low extraction voltage of approximately ±1 kV. Emission current and mass flow are measured as a function of the emitter angle with respect to the instrument axis. Mass spectra of field evaporated ions and mass-resolved energy distributions of emitted charges are recorded as a function of emitter angle. The measurements at both source polarities indicate that the cone–jet emits a mixture of ions and charged droplets. While the charged droplets are emitted with a narrow angular distribution centered on axis, the angular distributions of ions are significantly broader than those of the droplets and centered off axis. Pure ion emission is observed at emission angles larger than ∼15°. The main mass spectral peaks are from X ±{[Emim][Im]} n ions (X = Emim or Im for positive and negative polarities, respectively) with n = 0, 1, and 2. Quantum chemical calculations of the n = 0 and 1 ions and the isolated ion pairs are presented, yielding thermochemical information on the observed ionic species. The present results demonstrate that the cone–jet emits droplets from the tip of the jet, and provide direct evidence that ions are produced in an Iribarne–Thomson field-evaporation mechanism at the transition region between the Taylor cone and jet. Electrochemical modification of the cone–jet emitter properties are observed when maintaining the emitter at a positive polarity for a prolonged (minutes to hours) period. New ions emerge consisting of clusters containing a neutralized Im −. The associated current suppression can be mitigated through polarity alternation at 1 Hz. Measurements are also presented with the tungsten needle heated to 373 K, thereby reducing the liquid viscosity and increasing the flow rate. The average mass of the droplets is observed to increase with temperature while the mass distributions of the ions are not found to change noticeably.

Fabrication and Investigation of Nanostructures on Transition Metal Dichalcogenide Surfaces Using a Scanning Tunneling Microscope

Nanometer-scale holes have been fabricated on the surfaces of the semiconducting transition metal dichalcogenides (TMDCs) molybdenum ditelluride (MoTe2) and molybdenum disulfide (MoS2) by applying voltage pulses from the tip of a scanning tunneling microscope (STM) operating in ultrahigh vacuum (UHV). It was found that the tip geometry (tip shape and sharpness) influences the formation and structure of the atomic-scale nanostructures. Threshold voltage ranges for the surface modification of MoTe2 (3.0 +/- 0.3 V) and MoS2 (3.4 +/- 0.3 V) were determined. Negative sample voltage pulses applied to a p-type MoTe2 surface produced much larger and deeper nanometer-scale holes when compared with those produced by positive voltage pulses. The existence of threshold voltages and the pulse polarity dependence of nanostructure fabrication suggests that an electric field evaporation mechanism is applicable. Support for this mechanism was obtained by nanostructuring metallic TMDC NbSe2, where both the produced features and the threshold voltages (3.0 +/- 0.3 V) were similar for both positive and negative voltage pulses.