Investigation of O-18 enriched hematite (α-Fe2O3) by laser assisted atom probe tomography

International Journal of Mass Spectrometry 335 (2013) 57–60

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Investigation of O-18 enriched hematite (␣-Fe2 O3 ) by laser assisted atom probe tomography Mukesh Bachhav a,b,c,d , Frédéric Danoix a,b,c,d,∗ , Béatrice Hannoyer a,b,c,d , Jean Marc Bassat e , Raphaële Danoix a,b,c,d a

Normandie Univ, France UR, Groupe de Physique des Matériaux - GPM, 76 801 Saint Etienne du Rouvray, France c CNRS UMR 6634, 76 801 Saint Etienne du Rouvray, France d INSARouen, 76 801 Saint Etienne du Rouvray, France e CNRS, Université de Bordeaux, ICMCB, 87 Avenue du Dr. A. Schweitzer, 33 608 Pessac, 6 France b

a r t i c l e

i n f o

Article history: Received 10 March 2012 Received in revised form 17 October 2012 Accepted 17 October 2012 Available online 28 November 2012 Keywords: Laser assisted atom probe tomography Mass spectrometry Iron oxides Isotopic measurements

a b s t r a c t Quantitative composition measurements of metal-oxides by atom probe tomography (APT) face the problem of the nature of the oxygen peak at 16 Da. Due to a possible overlap between singly charged monomer (O+ ) and doubly charged dimer (O2 2+ ), the contribution of this peak to the total amount of oxygen in the analyzed samples is still matter of debate. In order to study this issue, laser assisted APT was used to analyze hematite (␣-Fe2 O3 ) samples containing large amounts of oxygen 18. Peak identification reveals that only singly charged monomers (O+ ) are detected, and that oxygen in detected water molecules does not originate from the oxide samples. With this information, composition of hematite measured by APT is found to be close, but not equal to the expected stoichiometry. © 2012 Elsevier B.V. All rights reserved.

1. Introduction Time-of-flight mass spectrometry provides a value for the massto-charge (m/n) ratio of detected particles, which can be used to derive information related to the composition of the specimen. Quantitative analysis of materials by mass spectrometry requires a proper assignment of the different peaks of the mass spectrum to the correct elemental or molecular ions. This issue becomes critical when there is overlapping between two or more peaks. As a mass spectrometry technique, atom probe tomography (APT) faces this issue of peak identification. APT relies on the field evaporation of atoms located at the surface of a specimen prepared as a sharply pointed needle. Ions produced are accelerated toward a detector, and chemically identified by time of flight mass spectrometry [1–4]. Ions produced are initially singly charged, but due to the very high electric field conditions, they might be subject to subsequent ionization. This process is known as post ionization, and the ratio of different states of charge is directly proportional to the electric field at the specimen surface [5]. The detected particles may be elemental or molecular ions. These latter may result from strongly bounded

∗ Corresponding author. Tel.: +33 2 32 95 50 43. E-mail address: [email protected] (F. Danoix). 1387-3806/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.ijms.2012.10.012

atoms within the evaporated specimens, which are not dissociated during field evaporation, or from surface recombination during field evaporation. They are commonly observed when analyzing samples containing light elements, such as carbon, nitrogen and oxygen [6–10], or poorly conducting materials or organics [11,12]. One of the consequences is that singly charged monomers and doubly charge dimers can coexist in APT mass spectra, leading to imprecision in composition data. APT analyses of metal-oxides have been very limited until the last decade [13–15], but became progressively routinely conducted, thanks to the development of ultra-fast laser assisted field evaporation [16–18]. Analysis of many metal oxides, such as Al2 O3 [19–21], MgO [22,23], SiO2 [21,24,25], FeO [26], Nb2 O5 [27], CeO2 [28], ZnO [29,30] has since been reported. In the case of oxides, the issue of peak identification is critical, due to the possible overlap between singly charged oxygen monomer (O+ ) and doubly charged oxygen dimer (O2 2+ ), since both have the same mass over charge (m/n) ratio, equal to 16 Da. This point is critical in terms of quantitative composition determination of the analyzed oxide as an O2 2+ molecular ion contributes to two oxygen atoms whereas an O+ ion accounts only for one single oxygen atom. Some authors assume that the peak at 16 Da is only composed of O2 2+ ions [24,29], whereas some other assume it to be O+ ions [19,21,28,30,31]. In both cases, the assignment is solely based on the argument that it leads to a correct stoichiometry. It should be noted that the

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possible coexistence of both O+ and O2 2+ is never considered. The reason for such pragmatic assumptions is that it is impossible to discriminate between O+ and O2 2+ by atom probe tomography as they have exactly the same mass over charge ratio. An alternative option would be to use the existence of different natural isotopes of oxygen. Indeed, in the case of elements having more than one isotope, the presence of doubly charged dimmer can potentially be detected from differences in the distribution of peaks in the mass spectrum, due to the combination of the various isotopes, as discussed, for example, in [6]. For illustration purpose, one may consider the simplest case  of an element E with only two isotopes m E and m E with respec  tive masses m and m , and a relative abundance of m E equal to p (0 < p < 1). If the peak at m Da was m E+ , then only two peaks, with respective abundances (1-p) and p should be detected at m and m Da. Conversely, if the peak was m E2 2+ , then three contributions with respective proportions (1-p)2 , 2(1-p)p and p2 should be observed at m, (m + m )/2 and m Da. The presence of the intermediate peak at (m + m )/2 Da is thus indicative of the presence of E2 2+ . Its relative abundance will be maximum for p = 0.5, when 50% of the total E signal would arise at (m + m )/2 Da. In the particular case of APT, this approach was shown to be valid as long as the natural abundance of at least a second isotope is larger than about 1%. For example, it has been used successfully to discriminate between C+ and C2 2+ at 12 Da in Fe–C alloys, as 13 C natural abundance is 1.11% [6]. The same authors concluded that this approach could not be used in the Fe–N system, as the natural abundance of 15 N is only 0.37%. In this case, they used specimens artificially enriched in 15 N to solve the ambiguity. In the case of oxygen, the natural abundance of the second most abundant isotope, 18 O, is only 0.20%, making it impossible to discriminate between O+ and O2 2+ . The situation is even more complicated, as the mass over charge ratio of doubly charged 16 O18 O and 18 O18 O ions is 17 and 18 Da, respectively, superimposing with the HO+ and H2 O+ peaks (the origin of which will be discussed later). An alternative way to solve this ongoing debate would be the use of Kinetic Energy discriminator for APT as proposed by Kelly [32]. This set-up would be able to measure directly the energy of incoming ions, making it possible to distinguish between singly and doubly charged ions. But to date no atom probe equipped with this feature is available. As the purpose of this work is to investigate the possible O2 2+ /O+ overlap in APT mass spectra at 16 Da, it was thus decided to work on specimens artificially enriched with 18 O, in such a way that 16 O18 O peak investigation becomes experimentally possible.

2. Experimental 2.1. Materials In the case of natural oxygen, as previously mentioned, the amount of the second most important stable isotope, i.e., 18 O, is only 0.20%. Thus, to use this isotope combination procedure, it is necessary to artificially increase its importance in the specimen. To achieve this goal, it was decided to re oxidize wüstite (FeO) in a 18 O atmosphere to produce hematite (␣-Fe2 O3 ). Ideally, the 18 O uptake during this operation should lead to an isotopic ratio 16 O/18 O of 2/1, leading to an average relative proportion of 18 O equal to 0.33, almost ideal for the aimed study. The procedure was to anneal a commercial FeO powder at 850 ◦ C for 2 h in nearly pure (97%) 18 O (from CEA, Euriso – top company) atmosphere (total pressure in the device p = 210 mbar). For comparison purposes, hematite with the natural oxygen isotopic ratio was also produced by annealing the same FeO powder in the same conditions (T, pO2 ). Controlled

heating and cooling of wüstite sample was conducted to ensure complete oxidation of FeO to Fe2 O3 . For convenience, hematite containing natural proportions of oxygen isotopes (i.e., mostly 16 O) will be referred as Hematite16, whereas the one enriched with 18 O will be referred as Hematite1618. 2.2. Analyses Characterizations by X-ray diffraction (XRD), Raman and Mössbauer spectroscopies were performed on both samples in order to ensure hematite formation. XRD analysis was carried out with a Brucker D8 equipment using the Co-K␣ radiation ( = 1.78897 nm) and Mössbauer measurements in transmission mode at 300 K using a constant acceleration spectrometer (Wissel) with a 57 Co(Rh) source, with isomer shift values referred to ␣-Fe. Raman spectra of the samples were measured at room temperature on a Raman spectrometer (Aramis, Horriba Jobin Yvon) in confocal configuration using a He-Ne laser (632 nm). Since hematite is a poor conducting material, APT analyzes were carried out on laser assisted Cameca LAWATAP system [16], using green laser ( = 515 nm, laser energy 20 nJ/pulse) with a base temperature of 80 K. APT specimens of both samples were prepared according to the procedure described in a previous paper [33]. It involves gluing a 5–8 ␮m diameter powder grain at the apex of a tungsten (W) pre sharpened tip. Ion milling in scanning electron microscope (Zeiss 1540 SEM – Ga+ ions, 2–30 kV) equipped with an Orsay Physics focused ion beam (FIB) column was used to shape the grain into requested sharply pointed needle shaped tip with an end radius of less than 100 nm. 3. Results and discussion 3.1. Characterization of hematite Fig. 1(a) shows XRD pattern of both samples. The absence of any FeO peak combined with the indexation of all peaks as ␣Fe2 O3 (JCPDS No. 033-0664) confirms the complete transformation of wüstite to hematite. Mössbauer spectra of both samples, shown in Fig. 1(b), consist of a sextet pattern, with ı = 0.37 and 0.37 mm/s, ε = −0.18 and −0.19 mm/s, B = 51.1 and 51.2 T for Hematite 16 and Hematite 16–18, respectively, again in perfect agreement with literature data for hematite [34]. Fig. 1(c) shows the Raman spectra of Hematite16 (black) and Hematite 16–18 (red). The position of the various peaks of Hematite16 sample can be unambiguously indexed as ␣-Fe2 O3 . The negative shift of the Raman peaks for the Hematite16-18 sample is due to an increase in the reduced mass of the constituent of the sample, indicating that 18 O was substituted for 16 O [35]. A precise quantification of 18 O (using the effective vibration masses for each individual mode) was not conducted in this study. As a first conclusion, these results indicate that FeO was fully transformed to ␣-Fe2 O3 , and that 18 O was successfully introduced in the Hematite16-18 sample. 3.2. APT results Typical mass spectra obtained on Hematite16 (in black) and Hematite16-18 (in red) samples are shown in Fig. 2. The large majority of detected ions are Fe1+ , Fe2+ , FeO+ , FeO2+ , O2 2+ /O+ , O2 1+ , FeO2 1+ , Fe2 O1+ and FeO3 1+ . Gallium (Ga+ ) ions were not detected in these analyses, indicating that there is minimal Ga implantation, if any, during the FIB milling process of sample preparation. The main information provided by the mass spectrum of Hematite16-18 is the detection of a significant peak at 18 Da, giving direct confirmation of the presence of 18 O in the analyzed specimen. As previously mentioned, the presence, or absence, of the (16 O18 O)2+ peak at

M. Bachhav et al. / International Journal of Mass Spectrometry 335 (2013) 57–60

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Fig. 2. Mass spectrum obtained in APT for Hematite16 and Hematite16-18 samples. Blue label refers to common species observed in both specimens and red label refers to species found only in Hematite16-18 samples. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of the article.)

the ‘water related’ peaks. Indeed, the origin of the HO+ , H2 O+ , H3 O+ , and (H2 O)2 H+ peaks appearing at 17, 18, 19 and 37 Da, respectively, in the mass spectrum shown in Fig. 3 is still a matter of debate. It could result from the ionization at the specimen surface of residual water molecules in the vacuum chamber, or to the combination of residual hydrogen with oxygen originating from the specimen itself [36]. Consideration, or not, of these peaks (which can reach up to 2% of the ions) in the determination of oxygen level is an important issue when dealing with quantitativity of APT analyses. Indeed, in both cases, the origin of oxygen is different, contamination in the first case, from the specimen in the second. Careful analysis of Hematite16-18 peaks elucidates the origin of the involved oxygen ions. In the hypothesis of hydrogen reacting with oxygen from the specimen to form H2 O+ , two peaks corresponding to H2 16 O+ (18 Da) and H2 18 O+ (20 Da) should be observed. Similarly, the peak at 37 Da ((H2 16 O)2 H+ ) should be accompanied by a peak at 41 Da ((H2 18 O)2 H+ ). But the absence of such peaks in the mass spectrum of Hematite16-18 (Fig. 3, highlighted with blue circles) overrules the hypothesis of a specimen origin of oxygen. It thus clearly indicates that these ions arise from residual molecules in the vacuum chamber, or alternatively from pre-adsorbed molecules, present before APT analysis. As a consequence, these peaks, and the corresponding

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Fig. 1. (a) X-ray diffraction (XRD) of Hematite16-18 samples. (b) Mössbauer spectroscopy of Hematite16 and Hematite16-18 samples. (c) Raman spectra for Hematite16 and Hematite16-18 sample. Shifts are indicated by blue arrows.

17 Da is the key argument to discriminate between O+ and O2 2+ at 16 Da. On the Hematite16-18 spectrum, the absence of any significant peak at 17 Da clearly rules out the existence of doubly charged oxygen dimmers, O2 2+ . As a consequence, it can be concluded that the peak at 16 Da is only related to singly charged monomers, i.e., O+ ions. Another important result that can be derived from the analysis of Hematite16-18 is the possibility to investigate the origin of

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oxygen ions, should not be accounted for in the determination of the oxide composition. Considering the previous results, the measured composition for Hematite16 is 42.5 ± 0.7 at.% Fe – 57.5 ± 0.7 at.% O. The composition of Hematite16-18 is 42.9 ± 0.8 at.% Fe – 57.1 ± 0.8 at.% O, similar to Hematite16 within experimental errors [37]. The measured Fe/O ratio is therefore 0.74 ± 0.02, which indicates an oxygen deficit with respect to the expected stoichiometric value of 0.67. Similar nonstoichiometry derived from APT data was reported by Marquis et al. [19] for Al2 O3 . If this deficit were intrinsic, it would lead to about 30% of iron as FeII (Fe2+ state). At this level, a significant contribution of FeII (having different magnetic parameters) should be observed on the Mossbauer spectrum, which is obviously not the case (Fig. 1(b)). This non stoichiometry must therefore be regarded as an experimental artifact. As discussed, the overlap of O2 2+ and O+ peaks cannot be considered as a possible source of oxygen loss in the case of ␣-Fe2 O3 . Other possible reasons explaining the loss of oxygen during APT analyses are simultaneous oxygen ion impacts on the detection system, and the dissociation of molecular ions during their journey to the detector. Work on these aspects, based on the procedure developed by Saxey [38] is currently in progress to understand the measured oxygen deficit. 4. Conclusions This work demonstrates that in laser assisted APT, the oxygen peak appearing at 16 Da in the mass spectra of hematite is only due to O+ ions, not to O2 2+ , and that oxygen ions detected as ‘water related’ molecular ions do not originate from the specimen. Hematite composition measurements, based on the previous conclusions, show an oxygen deficit. This deficit is clearly an experimental artifact which cannot be explained in terms of O+ /O2 2+ peaks overlap. Further work is therefore necessary to achieve quantitative atom probe analysis of oxides, at least for in the case of ␣-Fe2 O3. Acknowledgement M.B. acknowledges funding from Region Haute Normandie. References [1] M.K. Miller, A. Cerezo, M.G. Hetherington, G.D.W. Smith, Atom Probe Field Ion Microscopy, Oxford university press, Oxford, 1996. [2] T.F. Kelly, M.K. Miller, Invited review article: atom probe tomography, Review of Scientific Instruments 78 (2007) 031101–031120. [3] D. Blavette, A. Bostel, J.M. Sarrau, B. Deconihout, A. Menand, An atom probe for three-dimensional tomography, Nature 363 (1993) 432–435. [4] B. Deconihout, A. Bostel, P. Bas, S. Chambreland, L. Letellier, F. Danoix, D. Blavette, Investigation of some selected metallurgical problems with the tomographic atom probe, Applied Surface Science 76–77 (1994) 145–154. [5] D.R. Kingham, The post-ionization of field evaporated ions: a theoretical explanation of multiple charge states, Surface Science 116 (1982) 273–301. [6] W. Sha, L. Chang, G.D.W. Smith, C. Liu, E.J. Mittemeijer, Some aspects of atomprobe analysis of Fe–C and Fe–N systems, Surface Science 266 (1992) 416–423. [7] O. Nishikawa, Y. Ohtani, K. Maeda, M. Watanabe, K. Tanaka, Development of the scanning atom probe and atomic level analysis, Materials Characterization 44 (2000) 29–57. [8] H. Leitner, K. Stiller, H.O. Andren, F. Danoix, Conventional and tomographic atom probe investigations of secondary-hardening carbides, Surface and Interface Analysis 36 (2004) 540–545. [9] P. Jessner, R. Danoix, B. Hannoyer, F. Danoix, Investigations of the nitrided subsurface layers of an Fe–Cr-model alloy, Ultramicroscopy 109 (2009) 530–534. [10] F. Danoix, T. Epicier, F. Vurpillot, D. Blavette, Atomic-scale imaging and analysis of single layer GP zones in a model steel, Journal of Materials Science (2011), http://dx.doi.org/10.1007/s10853-011-6008-4. [11] B. Gault, W.R. Yang, K.R. Ratinac, R.K. Zheng, F. Braet, S.P. Ringer, Atom probe microscopy of self-assembled monolayers: preliminary results, LANGMUIR 26 (2010) 5291–5294.

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