Role of Photoexcitation and Field Ionization in the ... - ACS Publications

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Role of Photoexcitation and Field Ionization in the Measurement of Accurate Oxide Stoichiometry by Laser-Assisted Atom Probe Tomography A. Devaraj,*,†,§ R. Colby,†,§ W. P. Hess,‡ D. E. Perea,† and S. Thevuthasan† †

Environmental Molecular Sciences Laboratory and ‡Physical Sciences Division, Pacific Northwest National Laboratory, P.O Box 999, Richland, Washington 99352, United States ABSTRACT: The addition of pulsed lasers to atom probe tomography (APT) extends its high spatial and mass resolution capability to nonconducting materials, such as oxides. For a prototypical metal oxide, MgO, the measured stoichiometry depends strongly on the laser pulse energy and applied voltage. Very low laser energies (0.02 pJ) and high electric fields yield optimal stoichiometric accuracy. Correlated APT and aberration-corrected transmission electron microscopy (TEM) are used to establish the high density of corner and terrace sites on MgO sample surfaces before and after APT. For MgO, long-lifetime photoexcited holes localized at oxygen corner sites can assist in the creation of oxygen neutrals that may spontaneously desorb either as atomic O or as molecular O2. The observed trends are best explained by the relative field-dependent ionization of photodesorbed O or O2 neutrals. These results emphasize the importance of considering electronic excitations in APT analysis of oxide materials. SECTION: Spectroscopy, Photochemistry, and Excited States

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through pump−probe experiments on cleaved single-crystalline MgO and nanocrystalline MgO powders subjected to different wavelengths of laser irradiation.34−37 Calculations indicate that desorption of neutral Mg and O atoms preferentially occurs at corner sites on the MgO surface under laser illumination.38−40 Sub-band-gap optical absorption at low-coordinated MgO sites, such as at corners and steps, has been reported.41−44 In a typical needle-shaped atom probe sample, it might be expected that such low-coordinated surface features would be exposed during field evaporation. The creation and field ionization of neutral Mg and O species with differing ionization potentials from such low-coordinated surface sites would lead to field- and laser-dependent detection of their respective ionic species. In this study, it will be experimentally demonstrated that the laser pulse energy and applied electric field strength do have profound effects on the apparent stoichiometry of the oxide as detected by the atom probe. The validity of including laserinduced desorption (excited-state) mechanisms to a system incorporating electric-field-induced evaporation is considered to help explain these results. A focused ion beam based lift-out procedure45 was used to prepare needle-shaped APT tips from single-crystal MgO substrates obtained from MTI Corporation. Multiple specimens were fabricated with a final tip diameter of 60−80 nm and a half shank angle of ∼10°. Specimens were also prepared on a fivepronged Omniprobe half-grid (Ted Pella) to facilitate coupled

etal-oxide-based materials are ubiquitous across a range of crucial technologies and applications, such as catalysis1−4 and energy storage,5−7 often where an effective optimization of properties relies on understanding atomic-scale structure and composition. Atom probe tomography (APT) is a uniquely powerful tool for obtaining subnanometer spatial resolution and part-per-million level elemental sensitivity in three dimensions.8,9 Starting with the pioneering work by Kellogg and Tsong in 1980, pulsed laser APT has extended the range of materials amenable to APT analysis to include semiconductors and insulators in addition to metals and alloys.8,10−19 Improvements in the laser performance and ongoing developments in the understanding of field evaporation for different materials under laser excitation have aided an explosive growth of research interest in this field. However, the mechanisms governing laser-assisted field evaporation of dielectric oxides remain relatively unexplored, particularly in the context of APT. MgO serves as an appropriate model oxide, being an important material for several applications, including tunnel barriers in magnetic tunnel junctions,20 as a support material for catalysts,21−26 and as a matrix for optical memory and switching devices with embedded metal nanoparticles.27−29 Furthermore, crystalline MgO has a simple rock salt cubic structure and a nominal 1:1 stoichiometry. Recent studies using laser-assisted field evaporation of insulating metal oxide materials, such as MgO, have attributed the evaporation to either a thermal mechanism or an electronic excitation (photoexcitation).30−33 In addition to the more limited APT studies, there is a substantial literature exploring laser−oxide interactions in MgO in the absence of a field, © 2013 American Chemical Society

Received: January 3, 2013 Accepted: March 6, 2013 Published: March 6, 2013 993

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work in α-Fe2O3 has demonstrated the absence of O2+2 and the presence of only singly charged monomers at 16 amu; 52 however, at present, it remains unclear that this result is universal for all oxides. The range of this uncertainty can be assessed by quantifying the stoichiometry, for example, for the 16 amu peak, assigned entirely as either O+1 or as O2+2. The Mg and O stoichiometry in the MgO oxide is found to be dependent on the pulsed laser energy. To obtain quantitatively comparable stoichiometries from the mass-tocharge state ratio spectra obtained at different energies, a fixed range width of 0.15 amu, centered at the standard isotopic peak of each, was used consistently for analyzing all of the elemental and molecular peaks, with the exception of the 16 and 32 amu peaks. For 16 and 32 amu, asymmetric range windows of 15.92−16.145 and 31.914−32.214 amu, respectively, were used to accurately account for the larger tails associated with these two peaks. Prior to the quantification of composition, the uniform background was subtracted, and the overlap of the O2+1 and Mg2O+2 peaks at 32 amu was deconvolved using the natural abundance of other nonoverlapping Mg2O+2 isotopic peaks. The oxygen concentration obtained as a function of laser energy for a fixed 0.005 detected ions/pulse evaporation rate is shown in Figure 2a, including concentrations obtained by assigning the 16 amu peak as either O+1 or O2+2 as upper and lower limits. The estimated standard deviation of all APT measurements included herein is less than 0.2% and thus safely ignored with respect to the overall trends being considered. As the laser energy is decreased from 80 pJ, the O concentration is shown to increase. The O concentration closest to the expected stoichiometric Mg/O ratio (i.e., 50% O) is obtained at the lowest pulse energy of 0.02 pJ, with the 16 amu peak assigned as O2+2. Under the field evaporation conditions used, magnesium is found to occur as both Mg+2 and Mg+1 ions. The Mg+2/Mg+1 ratio can be observed to increase with a reduction in laser pulse energy, increasing sharply for energies below ∼5 pJ (Figure 2b). The 16 amu oxygen peak to O2+1 peak ratio (O16/O2+1 ratio) was also observed to increase with decreasing laser energy (Figure 2c). For Mg, the low-temperature evaporation field estimated using the image-hump model predicts a lowest evaporation field for +1 and +2 charge states, of 21 and 25 V/nm, respectively.53 The observation of an increase in the Mg+2/Mg+1 charge state ratio with decreasing laser pulse energies can be attributed to the higher electric field needed to maintain a constant evaporation rate. The trend given in Figure 2b matches remarkably well with the change in the theoretically calculated relative abundance of charge states for pure Mg metal as a function of applied electric field by Kingham.54 Our observations are also consistent with field evaporation experiments that investigated the variation in abundance of different charge states of Be and Cu when the specimen temperature was increased from 21 to 300K.55 In those studies, a higher preponderance of +2 charge states of Cu and Be was shown to result at lower temperatures, which was attributed to the higher electric fields required to maintain a constant evaporation rate. The similarity of the Mg charge state ratio to the field/laser energy trend between pulsed laser APT results of MgO and the well-established literature on field ionization of metals is remarkable but unappreciated in the literature concerning APT analysis of oxides. The observation of an increasingly accurate stoichiometry measurement with decreasing laser energies can be compared to past studies of laser interactions with nanoscale MgO

APT and transmission electron microscopy (TEM) measurements on the same specimen.46−49 The APT analysis was carried out using a CAMECA 4000X HR LEAP (local electrode atom probe) system equipped with a 355 nm picosecondpulsed UV laser. Pulsed laser energies ranged from 0.02 to 80 pJ at a 30 K sample base temperature and a 200 kHz pulse repetition rate, which is sufficient to avoid masking any higher mass-to-charge ratio ions. The LEAP 4000XHR provides a focused laser spot size of approximately 5 μm in diameter at the sample, can use very low laser pulse energies down to 0.02 pJ, and is shown to achieve high mass resolution of up to 1200 m/ ΔM at fwhm.50 Atom probe data reconstruction and analysis was performed using Cameca IVAS software. Atom probe data is typically collected at a fixed evaporation rate, such that the applied electric field strength is inversely coupled to the laser energy. Lowering the laser energy during a run, for instance, would require the electric field strength to be increased to maintain the evaporation rate. TEM analysis of the APT samples before and after APT analysis was conducted with an aberration-corrected FEI Titan 80-300 instrument operated at 300 kV. The aberration-corrected high-resolution TEM (HRTEM) images of MgO samples before APT analysis show pronounced steps along (001) planes of MgO (Figure 1a), indicating a high

Figure 1. (a) High-resolution TEM image of the MgO sample before APT analysis indicates single-crystalline MgO with a high density of step-like (001) faceting. (b) An average MgO mass-to-charge state ratio spectrum obtained at 20 pJ laser pulse energy showing peaks corresponding to elemental and complex ion species.

density of low-coordinated surface sites. APT analysis of a similar sample was performed using a 20 pJ laser pulse energy, obtaining 8 million ions. Primarily, Mg+2, Mg+1, O+/O2+2, O2+1, MgO+1, MgO+2, Mg2O+2, and MgO2+1 ions were observed in the mass spectra (Figure 1b). At higher laser energies (60 and 80 pJ), some minor Mg2O2+2 was also observed. There is an inherent uncertainty in the overall assessment of the stoichiometry; for example, a peak at 16 amu can consist of either O+1, O2+2, or a combination thereof. This uncertainty can introduce variation in the calculated stoichiometry.51 Recent 994

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Figure 3. HRTEM images of MgO samples after APT for 0.02 (a) and 60 pJ (b) laser pulse energies. These samples evidence a general decrease in uniformity as compared to samples before APT analysis but still maintain significant (001) faceted surfaces. Image (b) is spliced together from two TEM images. The increased prominence of thickness fringes in relation to Figure 1a can be attributed to the larger tip diameter in samples examined after laser induced APT.

probe sample both before and after APT analysis. It has been shown that upon laser irradiation, holes are generated at the MgO tip apex, through observation of magnification changes during field ion microscopy.30,58 These holes are known to be localized on the oxygen corners of the MgO lattice by electron paramagnetic resonance (EPR) measurements.41,59 The lifetimes of these holes have been shown to be quite long, ranging from minutes to hours.41 These holes are known to induce formation of oxygen and magnesium neutrals following UV photoexcitation.8,40 Formation and desorption of oxygen and magnesium neutrals could reasonably be expected to occur during UV laser irradiation during APT analysis of MgO. Neutral atoms and molecules formed by laser excitation can then be field-ionized with an increasing ionization probability with increasing applied field. Neutral species that are not fieldionized would go undetected, resulting in an apparent loss of those elements. Field ionization of Mg is efficient, at relatively low fields, due to its low ionization potential (7.66235 eV).56 In contrast, atomic and molecular oxygen have much higher ionization potentials (13.61805 and 12.0697 eV, respectively) and are much less likely to field ionize in comparison to Mg. Desorbed neutral O atoms or O2 molecules that do not field ionize do not contribute to the APT signal. One could then attribute the increasing detection of oxygen, with decreasing laser energy, to the greater evaporation field and subsequent higher efficiency of field ionization of O and O2. To confirm this hypothesis, APT analysis was repeated for 1, 20, and 80 pJ laser energies with varying fixed ion evaporation rates of 0.005, 0.015, and 0.03 detected ions/pulse. For fixed laser energy, the applied electric field on the sample tip surface is determined by the target evaporation rate. As expected, the measured O stoichiometry is relatively closer to 50:50 for

Figure 2. (a) Measured MgO stoichiometry as a function of laser pulse energy from 0.02 to 80 pJ. To account for the potential ambiguity in peak assignment, plots are included for the peak at 16 amu assigned as pure O+1 (dotted line) or O2+2 (solid line). The oxygen yield can be observed to increase sharply below a 5 pJ laser pulse energy. (b) Mg+2/ Mg+1 charge state ratio. (c) O16/O2+1 peak ratio as a function of laser pulse energy from 0.02 to 80 pJ. The Mg+2/Mg+1 charge state ratio and O16/O2+1 ratios can be observed to increase sharply when the laser pulse energy drops below 5 pJ.

powders. Bulk MgO has a band gap of 7.7 eV; however, subband-gap absorption near 5.7 and 4.6 eV has been reported for MgO nanopowders, attributed to the presence of lowcoordinated step and corner sites, respectively.41 Lowcoordinated corners and terraces are similarly observed on APT specimens (Figure 1a). Theoretical calculations have reinforced the experimentally observed correlation of energy of surface states with excitation energies.57 Sequential excitations with 4.66 eV nanosecond laser irradiation are thought to cause desorption of atoms from MgO corner sites.36 Furthermore, sub-band-gap surface absorption features are expected to be strongly Stark-shifted under the high electric field present in the atom probe, which could further broaden the absorption to include the 3.5 eV photon energy of the UV laser. The conspicuous (001) faceting observed by TEM before APT analysis was also observed after evaporation at multiple laser energies (Figure 3). Significant (001) faceting after evaporation at 0.02 (Figure 3a) and 60 pJ (Figure 3b) is evident, although the regularity of the faceting does appear less uniform after APT than that before. Nevertheless, this indicates a significant presence of corner and step sites on the atom 995

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in the sample, unless the electric field strength is sufficient to field ionize the free oxygen atoms and molecules. The results presented here support the contention that electronic excited states play a significant role in laser-assisted APT analysis of metal oxide materials. Our preliminary APT experiments on Al2O3 and La0.8Sr0.2MnO3 suggest similar trends of increasing yield of oxygen with lower laser energy or increased electric field. Therefore, similar mechanisms are expected to play a fundamental role not only in MgO and other simple binary oxides but also during laser-assisted APT analysis of more complex oxides.

higher evaporation rates or, equivalently, higher electric field strengths for each laser pulse energy (Figure 4). These results

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected].

Figure 4. The percent oxygen concentration yield as a function of evaporation rate. The peak at 16 amu was considered entirely as O2+2 for oxygen concentration estimations. The effect of the electric field strength on the measured MgO stoichiometry is decoupled from the effects of the laser energy by comparing different evaporation rates at laser pulse energies of 1, 20, and 80 pJ. For each laser energy, the oxygen concentration clearly increasesand approaches a stoichiometric 50%as the evaporation rate and, hence, the applied field strength are increased.

Author Contributions §

A.D. and R.C. contributed equally to this work.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The research described here was performed using EMSL, a national scientific user facility sponsored by the Department of Energy’s Office of Biological and Environmental Research, and is part of the Chemical Imaging Initiative conducted under the Laboratory Directed Research and Development Program at Pacific Northwest National Laboratory (PNNL). EMSL is located at PNNL, a multiprogram national laboratory operated by Battelle Memorial Institute under Contract No. DE-AC0576RL01830 for the U.S. Department of Energy. R.C. would like to acknowledge the EMSL William R. Wiley postdoctoral fellowship. We would also like to thank G. D. W. Smith, T. F. Kelly, D. J. Larson, and D. K. Schreiber for useful discussions.

clearly demonstrate that the stoichiometric accuracy of APT can be improved for MgO by operating with lower laser energies and higher electric fields. The stoichiometric discrepancy observed at higher laser energies can be attributed to desorption of neutral oxygen atoms that fail to field ionize and therefore go undetected during APT analysis. The higher fields used to maintain a high evaporation probability also result in a higher observed Mg+2/Mg+1 charge state ratio when using low laser pulse energies for analysis. Laser-induced charge transfer, or surface ionization, will strongly perturb the ionic bonding of the MgO sample tip surface. Removal of one or two electrons from an oxygen anion site can greatly weaken the ionic bonding in the nearby surface and subsurface region. This weakened region could then be much more susceptible to field evaporation. In particular, larger molecular clusters could be more easily removed and detected by APT. We indeed observe a larger fraction of high-mass clusters from MgO when performing APT using higher laser pulse energies, supporting this argument. These principles are readily extended to ionic metal oxides in general such that we would expect more accurate stoichiometric measurements to be made under high field and low laser pulse energy conditions. The alternative theories for the field and laser pulse energy dependence of the stoichiometry, such as uncorrelated detection of multiple ion hit events60 and preferential cation/ anion ionization under a standing field, do not provide a plausible explanation for the achievement of accurate stoichiometry in the limit of lower laser energies. These results highlight the need to consider fundamental electronic interactions of the laser with a sample being analyzed by APT and the potential for photoinduced changes to a sample surface during evaporation. Photoexcitation mechanisms operative in the absence of a field can be expected to play a role during APT analysis of MgO where both laser irradiation and high fields are present. In the case of MgO, long-lifetime photoexcited holes localized at oxygen corner sites can assist in the creation of oxygen neutrals that may spontaneously desorb either as atomic O or as molecular O2. These undetected neutral species would lead to an underestimation of the oxygen

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The Journal of Physical Chemistry Letters

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dx.doi.org/10.1021/jz400015h | J. Phys. Chem. Lett. 2013, 4, 993−998