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Dissociation Dynamics of Molecular Ions in High DC Electric Field Ivan Blum, Lorenzo Rigutti, François Vurpillot, Angela Vella, Aurore Gaillard, and Bernard Deconihout J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.6b01791 • Publication Date (Web): 02 May 2016 Downloaded from http://pubs.acs.org on May 7, 2016

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The Journal of Physical Chemistry

Dissociation Dynamics of Molecular Ions in High DC Electric Field

Ivan Blum*, Lorenzo Rigutti, François Vurpillot, Angela Vella, Aurore Gaillard and Bernard Deconihout Groupe de Physique des Matériaux, UMR 6634 CNRS, University and INSA of Rouen, Normandie University, 76800 St. Etienne du Rouvray, France

1 ACS Paragon Plus Environment


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Abstract In atom probe, molecular ions can be field evaporated from the analyzed material and, then, can dissociate under the very intense electric field close to the field emitter. In this work, field evaporation of ZnO reveals the emission of Zn2O22+ ions and their dissociation into ZnO+ ions. It is shown that the repulsion between the produced ZnO+ ions is large enough to have a measurable effect both on the ions trajectories and times of flight. Comparison with numerical simulations of the ions trajectories gives information on the lifetime of the parent ions, the energy released by the dissociation and repulsion and also the dissociation direction. This study not only opens the way to a new method to obtain information on the behavior of molecular ions in high electric fields by using atom probe, but also opens up the interesting perspective to apply this technique to a wide class of materials and molecules.


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1. Introduction When a high electric field is applied to a molecule, its orbitals can change in a continuous way. This causes chemical reaction pathways to be changed continuously as well. Based on this phenomenon, H.J. Kreuzer introduced the concept of “continuous Periodic Table” in high field, which leads to new field-induced chemistry1. It has applications like, for example, fieldassisted nanopatterning or bias-induced scanning probe nanolithography, where the field generated by a sharp needle is used to transfer certain species to a nearby substrate. 2. In this domain, high electric fields are increasingly used to confine a variety of chemical reactions and/or decompose molecules that lead to either a locally controlled deposition or to the growth of material on a surface 3–5. However, in order to optimize such control on materials and their processing, a good understanding of high-field chemistry is necessary. High-field chemistry can be studied using field ion microscopy coupled with the atom probe technique, as demonstrated by Block and his group in the 1990s 6. The high electric field generated at the apex of a nanometric needle was used to study dynamic reaction phenomena on the surface of field emitters. Catalytic reactions such as CO oxidation, or NO reduction with H2 or water formation from O2+H2 were studied using field ion microscopy coupled with atom probe

7,8

. However, little study

has yet been done on the reaction of molecules evaporated from solid field-emitter materials under high electric field. Tsong et al. used pulsed-laser atom probe to study the dissociation of RhHe2+ ions, formed by reaction of surface atoms with the gas of the chamber

9,10

. They

showed that this technique could be used to measure molecule lifetimes of less than a picosecond9, and also inferred from the results that the rotation of the molecule could play a key role in its stability10. In this paper we show that atom probe can be used as a unique technique to study the highfield molecular dissociation pathway of complex molecular ions, with no addition of external 3 ACS Paragon Plus Environment


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gas, and that additional parameters of the dissociation can be measured. In atom probe tomography11–13, the sample is shaped like a sharp needle which is held at a high voltage. Thanks to its small radius of curvature (between 50 and 100 nm), the electric field is enhanced at the tip surface up to several tens of V/nm, a field that is almost high enough to enable field ionization of surface atoms. In the case of materials having a high conductivity, an additional voltage pulse is applied to the tip or to an electrode facing the tip to trigger field ionization. In the case of materials having low conductivity a laser pulse can be used 14,15. Surface atoms are then ionized and desorbed in a process known as field evaporation. They are then accelerated to a two-dimensional position sensitive detector (Figure 1a). The times of flight of the field evaporated ions enable their elemental identification, and their impact positions enable 3D reconstruction of the evaporated volume16,17. This technique has proven to be able to analyze a wide range of materials in addition to metals, including semiconductors 18, insulators

19

or

even geological 20 and in some cases biological materials 21. These materials also have a high tendency to produce molecular ions and these ions sometimes dissociate in the high electric field surrounding the tip9,22. The dissociation products are then subjected to coulomb repulsion 23,24, which is the effect that is generally put to use in coulomb explosion imaging 25. In the atom probe geometry, one could suppose that such repulsion should have complex effects both on the ions trajectories and kinetic energies as schematically depicted in Figure 1a and b. In this work, we study the field evaporation of ZnO, and observe the field evaporation of Zn2O22+ molecular ions and their dissociation into ZnO+ ions. We observe that the coulomb repulsion between the dissociation products does have a significant effect both on their trajectories and times of flight. We use this information to estimate the lifetime of the parent ion, i.e. the dissociation distance Ld (Figure 1c). Comparison with numerical simulations also gives information about the dissociation angle αd, i.e. the orientation of the dissociation axis 4 ACS Paragon Plus Environment

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with respect to the electric field upon dissociation (Figure 1d) and the dissociation energy Ed, or the energy dissipated by the dissociation and subsequent coulomb repulsion (Figure 1e). Such information can be precious not only for the optimization of field-induced materials deposition and field ion evaporation, but also for the understanding of high-field chemistry. Moreover, the possibility to couple the effect of the DC field with laser pulses might be of interest for experiments similar to laser assisted coulomb explosion imaging 26,27. Last, we also use experimental results to discuss the possible effects of this phenomenon on the quality of atom probe data, both in terms of spatial and mass resolution.

Figure 1. Schematics of the dissociation of a molecular ion and subsequent repulsion between the dissociation products. (a) shows the ions trajectories between the sample and the detector (not to scale). The field evaporated ions and their dissociation products are accelerated toward a two dimensional detector. Both impact position and time-of-flight are recorded. (b) shows the ion trajectories that can be expected before and after dissociation as a result of ion/ion and ion/tip coulomb interactions. The positions of the ions at regular time intervals highlight their differences in kinetic energy. The effect of coulomb repulsion is amplified for clarity. The



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different parameters of the dissociation that are studied in this work, are (c) the dissociation distance Ld, (d) the dissociation angle αd and (e) the dissociation energy Ed.

2. Experimental Methods 2.1 Sample preparation. For each sample, a micrometer-sized piece of material is extracted from the original flat substrate using a focused ion beam based lift-out method 28. Prior to lift-out, a platinum protective layer is deposited in situ on its surface using electron-beam and then ion-beam induced deposition. This piece of material is then mounted on the top of a macroscopic sized tungsten needle. It is then milled down to the shape of a field-emission tip with a radius of curvature of 50 to 100 nm by focused ion beam milling with 30 keV Ga ions. The damage produced by the 30 keV beam is removed using a 2 keV beam, making amorphisation and implantation effects negligible in the framework of the present study29.

2.2 Atom probe tomography. The analysis is performed on a Laser-Assisted Wide-Angle Tomographic Atom Probe (Lawatap) instrument using UV (343 nm) laser pulsing with an energy of 58 nJ, a laser spot size of approximately 20 µm FWHM, a pulse rate of 100 kHz and a base temperature of 80 K. In this instrument ions fly over a straight path, which makes the analysis of the flight of ions and dissociation products straightforwardly interpretable. The detector is a multi-channel plate coupled to an advanced delay line, with extremely high sensitivity to multiple events, i.e. the detection of multiple ion impacts on the detector after a unique pulse. The detection efficiency 6 ACS Paragon Plus Environment

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of multiple events decreases only if two impacts are both closer than 1.5 mm on the detector and closer than 1.5 ns in time. For ZnO+ ions in the experiment used in this work, this corresponds to a difference in measured mass-to-charge-state ratio of 0.2 amu. The detector yield, i.e. the probability to detect any single ion reaching the detector, is equal to 65%. More detailed features of the detector, especially its performance for multiple hit discrimination, are reported in the references 30,31, while further details about the instrument are available in reference 14. During the experiment reported in this work, the voltage applied to the sample is regulated to maintain a detection rate of 0.01 at/pulse. The processes that are studied in this work could also depend on the electric field present at the specimen surface during the analysis. However, using the same conditions of analysis on a different instrument might result in a different electric field, because the latter also depends on the exact laser spot size, laser wavelength and alignment, and sample size. The electric field cannot be measured directly, but can be estimated using the relative abundances of the charge states of a same ions species32. In this experiment, the ratio of Zn2+/Zn+ is equal to 0.32, which corresponds to a field of 22 V/nm In these conditions, the measured Zn:O atomic ratio is equal to 69:31 based on the elements present in every detected molecule.

2.3 Numerical calculation methods. The ions trajectories before and after dissociation are calculated by solving Newton’s equation in the electric field surrounding the tip. As a first approximation, the specimen is assumed to have a paraboloïdal shape. The electric potential around a three dimensional semiinfinite tip with a paraboloïdal shape with cylindrical symmetry can be expressed in parabolic coordinates 33: ⁄ ⁄

=

(1)



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Where V0 is the electrostatic potential at the tip surface, ξ is the distance to the tip in parabolic coordinates, ξ0 is the position at which the potential is equal to zero, which is the position of the two dimensional detector in this case, and ξt is the position of the tip surface. The parabolic coordinates ξ and η can be defined in cylindrical coordinates as:

= + + =

(2)

(3)

Where z is the height with respect to the tip apex and ρ is the radial distance. The molecular ion is field evaporated from the tip apex and its trajectory is calculated by solving Newton’s equation iteratively. After a defined time-of-flight td, dissociation is simulated by replacing the parent ion by the two dissociation products separated by a distance d and forming an angle αd with the tip axis. We define the dissociation angle αd as the angle formed by the direction of the electric field and the direction on the axis along which the heavier daughter ion (when applicable) is projected immediately after the dissociation. d is calculated so that the electrostatic potential energy of the two ions corresponds to the dissociation energy, which is an input parameter. Then the trajectories of the two dissociation products are calculated taking into account only the electrostatic interactions between the two ions, and between each ion and the tip. The ions time-of-flight to the detector, t1 and t2 are then calculated as well as the ion impact positions on the detector x1 and x2. Then, the corresponding measured ion mass-tocharge-state ratios M’1 and M’2 are calculated as in a regular atom probe analysis, using equation (4):

= = 2

(4)


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Where m is the ion mass, n is the charge state, t is the ion time-of-flight, L is the distance from the tip to the impact position on the detector and e is the elementary charge. As it will be shown later, these calculations are used to fit the experimental data and extract values for the dissociation distance and energy from the fitting procedure. The effect of the molecule lifetime td and dissociation energy Ed on the results can be found in Supporting Information.

3. Results 3.1 Correlation histogram The ZnO specimen was prepared from a bulk ZnO sample doped with 2 at% Al and 2 at% Ga. Multiple hits were then analyzed using the so-called correlation histogram 22. Such a histogram is computed by taking into account all the possible pairs of ions that can be built from each multiple event (e.g.. a triple event containing ions A, B and C, would yield the pairs AB, BC and AC) and by plotting them in the two dimensional mass-to-charge state spectrum. A fraction of the correlation histogram corresponding to the analysis of the ZnO sample is displayed in Figure 2a. The order of ion arrival on the detector was not taken into account, and a symmetry axis was added on the diagonal of the histogram. The mass spectrum for all events in this mass range, including single events, is displayed at the bottom of the histogram (Figure 2b). It shows 3 groups of peaks that correspond mainly to the different isotopes of Zn+, Zn2O2+ and ZnO+. Additional minor peaks in the first and third group of peaks can be explained by the presence of 0.4 ion % of Zn22+ and 1 ion % of Zn2O22+ respectively. In the correlation histogram, 9 different groups of peaks can be observed, corresponding to all the possible combinations of two ions that can be made using Zn+, Zn2O2+ and ZnO+, each peak corresponding to a combination of different isotopes or isotopologues. Lines or curves with a negative slope can be observed next to the peaks 9 ACS Paragon Plus Environment


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corresponding to the pairs ZnO+/Zn+ and ZnO+/ZnO+ (arrows 1 and 2 respectively). Similar lines were already observed in different materials 22,34–36. These dissociation tracks correspond to pairs of ions produced by the dissociation of molecular ions at various distances from the tip surface: if the molecular ion dissociates close to the tip surface, the times of flight of the daughter ions will yield their correct mass-to-charge-state ratios. Conversely, if the molecular ion dissociates at the end of the field acceleration, the times of flight of the daughter ions will yield mass-to-charge-state ratios closer to the one of the parent ion. In the correlation histogram, this translates into counts located closer to the symmetry axis. In the case of the lines close to the ZnO+/Zn+ pairs, the dissociation tracks are short and start from the ZnO+/Zn+ peaks and point toward the mass-to-charge-state ratio of Zn2O2+ on the symmetry axis of the histogram, which is consistent with an effect of the dissociation of Zn2O2+ ions into ZnO+ and Zn+ ions close to the tip surface.


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Figure 2. Detail of an analysis of ZnO. (a) Correlation histogram and (b) corresponding part of the mass spectrum for all ions including single events. Regions corresponding to pairs of ions of Zn2O2+ / Zn2O2+ and ZnO+ / ZnO+ in the correlation histogram are circled in red. The red arrows indicate lines having a negative slope. Arrow 1 is the dissociation track corresponding to dissociation of Zn2O2+ into Zn+ and ZnO+. The color scale represents the logarithm normalized to unity of the number of counts. Arrow 2 highlights the elongated peak shapes observed for ZnO+ / ZnO+ pairs of ions.

The lines observed on the ZnO+/ZnO+ peaks have a different interpretation, however. Figure 3a, displays a close-up view of this region of the histogram. The lines do have a negative slope like dissociation tracks. However, they extend on both sides of each peak, whereas dissociation tracks should only be located between the main peak and the symmetry axis of the histogram. As it will be shown, this result is an effect of coulomb repulsion between ions produced by the dissociation of a molecule. The fact that the line has a negative slope indicates that the times-of-flight of the two ions are anti-correlated. This can actually be explained by the coulomb repulsion between dissociation products. When a molecule dissociates into two fragments, the electrostatic repulsion between the dissociation products will cause them to accelerate in opposite directions. If αd is close to 0° or 180°, one fragment will be accelerated toward the detector, and the other one will be accelerated toward the tip. Essentially, the first ion will gain kinetic energy and the second ion will lose kinetic energy. Because of the conservation of momentum, the times-of-flight of the dissociation products and the calculated mass-to-charge state ratios will eventually be anti-correlated. This effect on the peak shape observed on the correlation histogram is visible on the corresponding mass spectra (Figure 3b). The peak shape is the sum of a sharp and a broad contribution (pointed by arrow 1 and 2 respectively). The width of the broad contribution corresponding to the width



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of the diagonal lines observed on each peak on the correlation histogram. This indicates that a fraction of the ZnO+ ions are produced by the dissociation of molecular ions, while the remaining ZnO+ ions are field evaporated without being subjected to ion / ion interactions. A similar broadening can also be observed for triple events. A possible explanation is that it corresponds to correlated evaporation of ZnO+ and Zn2O22+ and the dissociation of the latter.

Figure 3. Detail of an analysis of ZnO focusing on ZnO+ / ZnO+ pairs of ions. (a) Correlation histogram showing only the peaks corresponding to pairs of ions ZnO+ / ZnO+. The circled regions 1 (green), 2 (red) and 3 (black) correspond to the pairs of ions having a difference in mass of 4, 2 or 0 amu respectively. The color scale represents the logarithm of the number of counts normalized to unity. (b) is the corresponding mass spectrum for the events of different multiplicities. The peaks are composed of a sharp and a broad contribution (arrow 1 and 2


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respectively). The dashed line is a guide to the eye that highlights the broad part of the peak. Only the ions represented in the correlation histogram are used to calculate the multiplicity and are represented in the mass spectrum.

3.2 Correlation in impact time and spacing This effect of the electrostatic repulsion between the dissociation products should depend greatly on the direction in which the coulomb repulsion is applied, which can be assumed to be related to the dissociation angle. Coulomb repulsion in a direction perpendicular to the trajectory of the parent ion should affect only the trajectories of the daughter ions and not their time-of-flight, and eventually increase the distance between their impacts on the position-sensitive detector, which is what was already observed recently23,24. In order to observe both effects, the different pairs of ions are represented in another histogram, with the distance between the two impacts on the detector on the abscissa and the difference in their calculated masses on the ordinate (Figure 4a, c and e). The pairs of ions corresponding to regions of interest (ROI) 1, 2 and 3 in Figure 3a are each represented in separate histograms.



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Figure 4. Two-dimensional correlation histograms showing the difference in mass-to-chargestate ratio of the different pairs of ZnO+ / ZnO+ as a function of the distance between the corresponding impacts on the detector. (a), (c) and (e) correspond to experimental data for ROI 1, 2 and 3 respectively together with the best fit obtained using numerical simulations for the different possible dissociation angles (red dashed lines). (b), (d) and (f) are Monte-Carlo type simulations using a rectangular distribution of molecule lifetime between 0.3 and 3 ps and an arcsinus distribution of dissociation angles and a dissociation energy of 3.2 eV. They correspond to the dissociation of XZn64Zn16O2 with X equal to 68, 66 and 64 respectively. The color scale represents the number of counts normalized to unity. The simulated data points corresponding to a dissociation angle of αd = 1.8, 90 and 180° are indicated by symbols in (a), (c) and (e), 1.8° corresponding to the configuration with the heavy isotopologue ahead of the light one. The configuration with αd = 0° is not shown because it creates a singularity (see Supporting Information for details). Regions 1 and 2 in (d) highlight the low and high density regions of the simulated histogram respectively.


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In the case of ions located in ROI 3 (Figure 4e), two different lines can be observed. One line, close to the abscissa, is horizontal and corresponds to pairs of ions producing impacts that are between ~2 and ~7 mm apart on the detector but are all detected with a very small difference in time-of-flight and hence a very small difference in calculated mass-to-charge-state ratio. These events most likely correspond to correlated evaporation 37, i.e. atoms that are field evaporated separately, after the same laser pulse and from the same region on the tip. The other line follows a negative slope. This negative slope indicates an anti-correlation between the difference in time-of-flight for each ion pair and the distance between the impacts of the two ions on the detector. This is consistent with the expected effects of electrostatic repulsion of the dissociation products after dissociation. Molecules which dissociate in a direction parallel to the electric field will produce impacts on the same position on the detector but with different times of flight and thus will yield a dot lying on the ordinate of the correlation histogram. On the contrary, molecules which dissociate perpendicular to the electric field will produce impacts that will be located farther apart but that will be almost simultaneous and thus will yield a dot lying on the abscissae of the correlation histogram. The molecules which dissociate in an intermediate direction will produce a combination of the two effects and will yield a dot lying on a line joining these two points. The resulting dot will lie on a different point of the line depending on the dissociation direction. In the histogram corresponding to ROI 2 (Figure 4c), a horizontal line is also observed and can also be explained by correlated evaporation. Here, however, the line is located at a difference in mass-to-charge-state ratio of 2, which is simply due to the actual difference in mass for pairs of ions taken from region 2 in Figure 3a. Two diagonal lines can now be observed on both sides of the horizontal line. For both lines, we can observe that when the impact positions are the farthest apart, i.e. when the molecule dissociates perpendicular to its trajectory, both ions are detected with the correct difference in mass-to-charge-state ratio that actually matches their real difference in mass of 2



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amu. When the molecule dissociates in a different direction, however, two different cases are observed. After dissociation, the two ions are accelerated by the field of the tip, which results in a shorter time of flight for the lighter ion. The times-of-flight are also affected by coulomb repulsion. As shown in Fig. 5, in the first case, if the light isotopologue is ahead of the heavy one, (corresponding to the situation with the angle of dissociation equal to αd~180°, Figure 5a), the difference in time of flight between the two ions will be increased, producing the line on top of the horizontal line. Conversely, in the second case, if the light isotope is behind the heavy one (αd~0°, Figure 5b), coulomb repulsion will reduce the difference in time of flight, producing the line below the horizontal line. In the histogram corresponding to ROI 1 (Figure 4a), similar features can be observed, but centered on a difference in mass-to-charge-state ratio of 4 amu.

Figure 5. Schematic of the effect of the dissociation angle on the difference in time of flight in the case of dissociation into different isotopologues of ZnO+ in the high field region. (a) shows the case with the light isotopologue ahead of the heavy one, and (b) shows the opposite situation. The light and heavy ions are represented by the empty and full circles respectively. The ion positions are represented at the moment of dissociation (t=0) and after coulomb repulsion (t=∆t).

For comparison, the same type of histogram is plotted for the pairs of ions Zn2O2+ / Zn2O2+ (Figure 6a) which, contrary to ZnO+/ZnO+ did not present the elongated peak shape (Figure 2a). All the pairs corresponding to the different isotopologues combinations are now plotted 16 ACS Paragon Plus Environment

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in a single histogram. In this case, only horizontal lines can be observed, corresponding to the different combinations of isotopologues, and no correlation between the difference in time-offlight and spacing on the detector can be observed. This indicates that there is no significant sign of molecular dissociation and that these events likely correspond to correlated evaporation. This is consistent with the fact that the peaks are not elongated in the m/c correlation histogram (Figure 2a). This also indicates that ions subjected to correlated evaporation are not close enough during their acceleration to interact electrostatically, although they are field evaporated at similar times and location on the tip 37. Because their trajectories are not affected by the fact that they originate from correlated evaporation, their impact position on the detector should reflect their original position on the tip with the same accuracy as single events. Thus, it is possible to determine the distance between their evaporation sites using a conventional reconstruction algorithm 38. Figure 6b reports the distribution of the probability to observe correlated evaporation for two given ions as a function of their original separation distance on the tip surface. This distribution is given by:

! =

"# #$

∙

&

(5)

,() ' )* +#

Where N is the number of detected pairs, and r is the distance between the two field evaporation sites. The probability distributions were calculated for different pairs of ions including Zn2O2+ / Zn2O2+ pairs. The distributions for the different pairs of ions are very similar, which would not necessarily happen for dissociation of molecular ions, but which can be expected for correlated evaporation 24.



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Figure 6. Distribution analysis of multiple events. (a) two-dimensional histogram showing the difference in mass-to-charge-state ratio of the different pairs of Zn2O2+ / Zn2O2+ as a function of the distance between the corresponding impacts on the position sensitive detector. The color scale represents the number of counts normalized to unity. (b) distribution of the probability to observe a given pair of ions in a multiple event as a function of the distance between the two atoms on the surface of the tip.

4. Discussion 4.1 Molecule stability These different results evidence that pairs of ZnO+ ions are produced by the dissociation of molecular ions. The detection of ZnO+ ions was not significantly correlated with other ions, indicating that the dissociation only produces ZnO+ ions, and, thus likely corresponds to the reaction: Zn2O22+ ZnO++ ZnO+

(6)

Different information about this dissociation can be extracted from the results. First, the data contains information about the lifetime of the parent ion. It was shown already that 18 ACS Paragon Plus Environment

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dissociation tracks can give information about the relative position of the parent ion in the field region upon dissociation 22,36. Dissociation tracks can be observed only if the dissociation products have a different mass-to-charge-state ratio, i.e. a mass-to-charge-state ratio that is different from the one of the parent ion. In the case of the dissociation of Zn2O22+, even though dissociation forms two ions that are chemically identical, small differences in mass-to-charge-state ratios can still exist because of the difference in masses of the different isotopologues of ZnO. In Figure 3a, the hits corresponding to the combination of isotopes 64

Zn16O+ and 66Zn16O+ form a peak at the coordinates (80:82). If dissociation happened at a

wide range of distances between the tip and the detector a short dissociation track should be observed between (80:82) and (81:81), 81 being the mass-to-charge-state ratio of the corresponding parent ion. Likewise, if dissociation happened in the field free region, a peak should be visible at coordinates (81:81). No peak or end of a dissociation track can be observed at this location, indicating that the molecule preferentially dissociates in the high field region, close to the tip surface. In addition, it was shown that non-dissociated Zn2O22+ ions can also be identified in the mass spectrum, which indicates that not every Zn2O22+ ions dissociate in the high field region. Thus a fraction of them is stable enough once leaving the high field region in order to reach the detector without dissociating. By integrating the number of dissociation events in Figure 3, and taking into account the effect of the limited detection efficiency of 65%, one can show that 47% of the field evaporated Zn2O22+ ions do dissociate, while the remaining ions reach the detector unaffected. 4.2 Dissociation parameters quantification Another information that can be extracted is the lifetime of the parent ion and the dissociation energy, i.e. the energy dissipated by the coulomb repulsion. The higher this energy the more the dissociation products will exhibit a spread in time-of-flight and/or in spacing between their impacts on the detector. In order to extract both the lifetime of the parent ion and the 19 ACS Paragon Plus Environment


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dissociation energy, the experimental results were compared to simulations of the ions trajectories, taking into account only the ion/ion and ion/tip electrostatic interactions. More details about the simulations are given in the methods section. In order to simulate events appearing in ROI 1, 2 and 3, the simulation was performed for the dissociation of X

Zn64Zn16O2 with X equal to 68, 66 and 64 respectively. The results are plotted in Figure 4a, c

and e, (red dashed lines) and are compared to the experiment. The calculation was repeated for the different possible dissociation angles, producing lines in the two-dimensional histogram. The lifetime of the parent ion and the dissociation energy were adjusted in order to give the best fit of the experimental results, according to visual observation. The best fit for the three ROIs was obtained for a molecule lifetime of 0.80.2 0.1 ps and a dissociation energy of 3.2.4 0.& eV. These parameters give a reasonable fit that reproduces the main features corresponding to the events due to dissociation. It was also shown that both parameters have different effects on the simulation results, indicating that only one couple of parameters could fit the experimental data. A more detailed analysis of the effect of the different parameters is given in Supporting Information. The uncertainties on the values extracted from the fit are estimated empirically for each parameter. This is done by increasing (or decreasing) the value of that parameter while adjusting the other one to obtain the best fit possible. The boundaries of the uncertainties are considered to be reached when the value of the first parameter can not be increased (or decreased) further without yielding a fit for the three different isotopologues that is not acceptable. Assuming a parabolic potential, this first result of a lifetime of 0.8 ps corresponds to a flight length of 7.5 nm. At this distance from the tip surface, the potential at the moment of dissociation is still as high as 97 % of the tip potential, which is consistent with the fact that dissociation tracks, if any, are too short to be observed. The positions on the lines corresponding to 1.8, 90 and 180° are shown by specific symbols, 1.8° corresponding to the configuration with the heaviest ZnO isotopologue ahead of the light one. These results


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confirm that the two diagonal lines in figure 4a and c correspond to the two possible isotopologues configurations. As explained in Supporting Information, the configuration where α= 0° is not calculated because it creates a singularity. This simple simulation allows to measure a dissociation energy and a first value for the lifetime of the parent ion. It might be possible, however, to extract more information from such experimental data. First, experimental data about dissociation of molecular ions that present dissociation tracks indicate that molecular ions do not have a unique lifetime, but tend to exhibit a wide distribution of lifetimes 22,36. In our experimental data, even though the lifetime of parent ions is too short for dissociation tracks to be observed, it can be expected to also follow a distribution that is relatively wide compared to the average lifetime, and that could be extracted from the present experimental data. And second, as it was shown, the data also contains information about the dissociation angle. The distribution of events along the curved lines (Figure 4a, c and e) should be correlated with the dissociation angle. In order to verify if such information can be extracted from the experimental data, a second simulation method is developed. In this Monte Carlo type simulation, series of ion trajectories are calculated using the previous method, except by using specific random distributions for the lifetime of the parent ion and the dissociation angle. As a first simple approximation, the dissociation angles were assumed to be independent of the direction of the electric field and randomly oriented in three dimensions. This means that the probability distribution of the dissociation angle follows an arcsinus distribution. The molecules lifetime were given a rectangular distribution ranging from 0.3 to 3 ps, which is the range reproducing the main features of the experimental data. The dissociation energy, however, was always equal to 3.2 eV, as given by the first fit of the experimental data. The simulation was run 20,000 times for each isotopologue combination and the results are plotted in two-dimensional histograms (Figure 4b, d and f). The result given by the simulations 21 ACS Paragon Plus Environment


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reproduces well the experimental results given in Figure 4a, c and e. On the left part of the histogram (region 1 in Fig 4d), the different events are spread apart and exhibit a low density. But for larger distances between impacts (region 2 in Fig 4d), the events are localized on a thinner line and exhibit a higher density. The presence of this thin high density region can be understood when studying the effect of lifetime of the parent ion on the simulations results (Supporting Information). Even though this thin line observed in the experimental data can be reproduced with a simulation using a single lifetime for the parent ion (Figure 4a, c and e) it is more likely that it is the result of a wide distribution of lifetimes similar to what is used in the Monte Carlo simulations. Nevertheless, this simulation reproduces well the main features of the experimental data and thus, does not provide any evidence of preferential dissociation angle. 4.3 Effect on atom probe data quality From this data, it is also clear that dissociation of molecular ions can have an effect on data quality for applications of atom probe tomography. Coulomb repulsion between two ions adds a large uncertainty on the atoms final positions in the reconstruction 23,24. According to the experimental data presented in Figure 4a, c and e, the distance between the impacts of dissociation products on the detector can be as large as 9 mm for this experiment, which, after three-dimensional reconstruction, corresponds to a distance of 6 nm on the tip surface whereas the atoms are known to originate from the same position. We also show that coulomb repulsion can have a significant effect on the time-of-flight of the dissociation products, which eventually affects the mass resolution. In the case of the dissociation of Zn2O22+, the dissociation induces a peak broadening of about 1 amu (Figure 3b). However, in the present results, dissociation of Zn2O22+ into ZnO+ ions affects only a small number of events (only 1.7 % of all evaporated ions in this analysis). Similar small values were found in other studies where dissociation events were quantified 22,34. Hence, the small effect on mass resolution 22 ACS Paragon Plus Environment

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together with the small number of affected events should have little effect on composition measurements. The measured Zn:O ratio, however, is equal to 69:31 in the conditions that were used in this experiment, which is far from stoichiometry. This error in composition measurement is due to non-optimized conditions of analysis 39. It can not be attributed to a lower detection efficiency of ZnO+ ions because of the limited detection efficiency for multiple events 31. Indeed, reducing the detection efficiency for ZnO+ ions would cause Zn and O atoms to be lost in equal amount. It is also interesting to notice that the coulomb repulsion between the dissociation products increases either their difference in time-of-flight or distance in impact position, in such a way that all impacts stay far from the dead-time effect region of the detector (less than 1.5 ns time of flight and 1.5 mm spacing in the case of this instrument 30,31). This dead-time effect region can be seen in the lower corner of Figure 6a and Figure 4c and e, where the density of impacts decreases to zero. The same might not occur, however, for the dissociation of other species or analyses using other instruments. Other effects have been discussed in past literature to explain such errors in composition measurements for compound semiconductors, such as field evaporation between pulses 40,41, desorption of neutral atoms or molecules 36,40 or dissociation of molecular ions into neutral atoms 22,34,36. Future studies of dissociation of molecular ions in high DC field might be of particular interest to gain better understanding of the latter. 5. Conclusions A ZnO sample was analyzed using atom probe. We observe the field evaporation of Zn2O22+ molecules and their dissociation into ZnO+ ions in the high field region in the vicinity of the tip surface. Different observations are made concerning this dissociation:

•

We observe that the coulomb repulsion between the resulting ZnO+ ions has a significant effect both on their trajectories and times of flight.



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•

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Using this data, we show that the Zn2O22+ ions that do dissociate have a lifetime between 0.3 and 3 ps, and that the energy dissipated by subsequent dissociation and coulomb repulsion is equal to 3.2.4 0.& eV. Comparison with simulations of the ions trajectories does not evidence any correlation between the dissociation angle and the direction of the electric field.

•

We also study the effect of coulomb repulsion between dissociation products on the quality of atom probe data. The resulting peak broadening in the mass spectrum does not exceed 1 amu for this material and thus, is not problematic for peak indexation. However, the spatial resolution for events resulting from the dissociation of molecular ions can be worsened by several nanometers and should be taken into account if the number of dissociating molecules is significant.

It is the first time that molecules dissociation angle, lifetime and dissociation energy can be estimated at the same time using an atom probe microscope. These different results show that atom probe can be used as a new and performing technique to obtain information about the behavior of molecular ions in high electric fields. Such information gives new insights about the dissociation of molecules under high field and their stability, and could be used to study and optimize field-assisted nanopattering or high field chemistry nanolithography. Indeed, such high fields, tens of V/nm, would be difficult to obtain without the field enhancement caused by the nanometric size of the tips used in atom probe tomography or in scanning probe techniques. In addition, the possibility to couple the effect of the DC field with laser pulses might be of interest for experiments similar to laser assisted coulomb explosion imaging 26,27. Associated content Supporting information. Simulations of the ions trajectories showing the effect of the different simulation parameters: figure and discussion. Author Information 24 ACS Paragon Plus Environment

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Corresponding Author *E-mail: [email protected] Phone: +33 (0)2 32 95 51 49

Notes The authors declare no competing financial interests

Acknowledgements This work was funded through project EMC3 Labex AQuRATE. The authors would also like to acknowledge financial support from CAMECA.



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Figure 1. Schematics of the dissociation of a molecular ion and subsequent repulsion between the dissociation products. (a) shows the ions trajectories between the sample and the detector (not to scale). The field evaporated ions and their dissociation products are accelerated toward a two dimensional detector. Both impact position and time-of-flight are recorded. (b) shows the ion trajectories that can be expected before and after dissociation as a result of ion/ion and ion/tip coulomb interactions. The positions of the ions at regular time intervals highlight their differences in kinetic energy. The effect of coulomb repulsion is amplified for clarity. The different parameters of the dissociation that are studied in this work, are (c) the dissociation distance Ld, (d) the dissociation angle αd and (e) the dissociation energy Ed. 250x137mm (96 x 96 DPI)

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Figure 2. Detail of an analysis of ZnO. (a) Correlation histogram and (b) corresponding part of the mass spectrum for all ions including single events. Regions corresponding to pairs of ions of Zn2O2+ / Zn2O2+ and ZnO+ / ZnO+ in the correlation histogram are circled in red. The red arrows indicate lines having a negative slope. Arrow 1 is the dissociation track corresponding to dissociation of Zn2O2+ into Zn+ and ZnO+. The color scale represents the logarithm normalized to unity of the number of counts. Arrow 2 highlights the elongated peak shapes observed for ZnO+ / ZnO+ pairs of ions. 138x204mm (96 x 96 DPI)


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Figure 3. Detail of an analysis of ZnO focusing on ZnO+ / ZnO+ pairs of ions. (a) Correlation histogram showing only the peaks corresponding to pairs of ions ZnO+ / ZnO+. The circled regions 1 (green), 2 (red) and 3 (black) correspond to the pairs of ions having a difference in mass of 4, 2 or 0 amu respectively. The color scale represents the logarithm of the number of counts normalized to unity. (b) is the corresponding mass spectrum for the events of different multiplicities. The peaks are composed of a sharp and a broad contribution (arrow 1 and 2 respectively). The dashed line is a guide to the eye that highlights the broad part of the peak. Only the ions represented in the correlation histogram are used to calculate the multiplicity and are represented in the mass spectrum. 136x209mm (96 x 96 DPI)



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Figure 4. Two-dimensional correlation histograms showing the difference in mass-to-charge-state ratio of the different pairs of ZnO+ / ZnO+ as a function of the distance between the corresponding impacts on the detector. (a), (c) and (e) correspond to experimental data for ROI 1, 2 and 3 respectively together with the best fit obtained using numerical simulations for the different possible dissociation angles (red dashed lines). (b), (d) and (f) are Monte-Carlo type simulations using a rectangular distribution of molecule lifetime between 0.3 and 3 ps and an arcsinus distribution of dissociation angles and a dissociation energy of 3.2 eV. They correspond to the dissociation of XZn64Zn16O2 with X equal to 68, 66 and 64 respectively. The color scale represents the number of counts normalized to unity. The simulated data points corresponding to a dissociation angle of αd = 1.8, 90 and 180° are indicated by symbols in (a), (c) and (e), 1.8° corresponding to the configuration with the heavy isotopologue ahead of the light one. The configuration with αd = 0° is not shown because it creates a singularity (see Supporting Information for details). Regions 1 and 2 in (d) highlight the low and high density regions of the simulated histogram respectively. 166x181mm (96 x 96 DPI)


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Figure 5. Schematic of the effect of the dissociation angle on the difference in time of flight in the case of dissociation into different isotopologues of ZnO+ in the high field region. (a) shows the case with the light isotopologue ahead of the heavy one, and (b) shows the opposite situation. The light and heavy ions are represented by the empty and full circles respectively. The ion positions are represented at the moment of dissociation (t=0) and after coulomb repulsion (t=∆t). 129x82mm (96 x 96 DPI)



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Figure 6. Distribution analysis of multiple events. (a) two-dimensional histogram showing the difference in mass-to-charge-state ratio of the different pairs of Zn2O2+ / Zn2O2+ as a function of the distance between the corresponding impacts on the position sensitive detector. The color scale represents the number of counts normalized to unity. (b) distribution of the probability to observe a given pair of ions in a multiple event as a function of the distance between the two atoms on the surface of the tip. 222x83mm (96 x 96 DPI)


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Dissociation Dynamics of Molecular Ions in High dc Electric Field - The

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