Sputtering Yields for Mixtures of Organic Materials Using Argon Gas

Sep 30, 2015 - The pure layers were each prepared as single deposits of 100 nm, but the mixed layers were made using 50 cycles of small depositions of...
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Sputtering Yields for Mixtures of Organic Materials Using Argon Gas Cluster Ions M. P. Seah,* R. Havelund, A. G. Shard, and I. S. Gilmore Analytical Science Division, National Physical Laboratory, Teddington, Middlesex TW11 0LW, United Kingdom S Supporting Information *

ABSTRACT: The sputtering yield volumes of binary mixtures of Irganox 1010 with either Irganox 1098 or Fmoc-pentafluoro-Lphenylalanine (FMOC) have been measured for 5 keV Ar2000+ ions incident at 45° to the surface normal. The sputtering yields are determined from the doses to sputter through various compositions of 100 nm thick, intimately mixed, layers. Because of matrix effects, the profiles for secondary ions are distorted, and profile shifts in depth of 15 nm are observed leading to errors above 20% in the deduced sputtering yield. Secondary ions are selected to avoid this. The sputtering yield volumes for the mixtures are shown to be lower than those deduced from a linear interpolation from the pure materials. This is shown to be consistent with a simple model involving the changing energy absorbed for the sputtering of intimate mixtures. Evidence to support this comes from the secondary ion data for pairs of the different molecules. Both binary mixtures behave similarly, but matrix effects are stronger for the Irganox 1010/FMOC system.

1. INTRODUCTION The introduction of argon gas cluster ions1 for the sputterdepth profiling and imaging of organic materials by secondary ion mass spectrometry (SIMS)2−6 has created a powerful new capability for 3D imaging of organic devices. For example, Aizawa et al.7 used time-of-flight SIMS (ToF-SIMS) to study the interfacial differences between the electron transport layer and the molecular emitting layer of OLEDs fabricated using novel solution processing and traditional evaporation processing. Bailey et al.8 used 3D SIMS imaging to study polymer multilayer reflective coatings fabricated by spin-coating alternating layers of polystyrene (PS) and poly(vinylpyrrolidone) (PVP). They showed the ability to identify buried defects and to generate profiles for layers up to 15 μm thickness, which is of practical relevance to many industrial sectors such as light management films. The unique capability for label-free 3D imaging with high sensitivity is of particular relevance in the pharmaceutical industry for the study of drug uptake at the cellular and subcellular level. Passarelli et al.9 have recently demonstrated the ability to show in three dimensions the disposition of the drug amiodarone in a single macrophage cell. In all of these studies there is a desire to move toward more quantitative depth location, and this requires an improved understanding of the sputtering yield of mixed materials. Without such understanding, it is not clear how to relate the measured depth in terms of time or dose for each voxel in a set of stacked SIMS images (the 3D image) to their depths in terms of distance. Over the past few years there has been a focused effort to characterize and model the sputtering process and the ion Published 2015 by the American Chemical Society

yields. In the period before argon gas clusters were used, many cluster ions species were tried,10 and while some had particular advantages in specific experiments, they lacked the universal applicability that would lead to wider characterization and use. A general analytical description of the argon gas cluster ion sputtering yields for both organic and inorganic materials has been developed by Seah.11 This shows how the yields change with both the energy, E, and cluster size, n, of the sputtering particles. Importantly, it also shows how the relative yield for organic materials, while 100 times those of inorganic materials for E/n values above 40, increases to a massive 104 times when E/n has reduced to unity. Thus, the relative rates of sputtering of different types of material and, indeed, of different organic materials, change greatly with the value of E/n for the sputtering ion used in any depth profile. The values of E and n are important in depth profiling since, as shown in the early work of Niehuis et al.,12 lower energies and higher cluster sizes significantly improve the measured depth resolution. Changing the values of E and n also affects the secondary ion yields when the analytical beam is the argon gas cluster beam rather than a second liquid metal analytical beam. Reducing the value of E/n reduces the fraction of low mass fragments in the spectrum by up to 1000 times.13 Recently Cristaudo et al.14 have measured the sputtering yields of two polymers, PS and poly(methyl methacrylate) (PMMA) with molecular weights, M, from 1000 to 300 000 Da. Received: July 13, 2015 Revised: September 11, 2015 Published: September 30, 2015 13433

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2. EXPERIMENTAL SECTION The sample materials were generated as described by Shard et al.23 for a VAMAS Interlaboratory Study25 to improve the interlaboratory consistency in SIMS depth profiling of organic systems. Briefly, Irganox 1010 (pentaerythritol tetrakis(3-(3,5di-tert-butyl-4-hydroxyphenyl)propionate), C73H108O12, Mr = 1177.6) and Irganox 1098 (N,N′-hexane-1,6-diylbis(3-(3,5-ditert-butyl-4-hydroxyphenyl)propionamide), C40H64N2O4, Mr = 636.96) from CIBA-Geigy and FMOC (fluorenylmethyloxycarbonyl-L-pentafluorophenylalanine, C24H16F5NO4, Mr = 477.4) from Sigma-Aldrich were each sublimed in a Qbox 450 (Mantis Deposition Ltd. Thame, U.K.) with relevant monitoring, shuttering, and sample rotation to create the structures shown in Figure 1. The evaporators were controlled

This mass range covers those of large organic molecules like Irganox 1010 to those of polymers with a high number of repeat units. These data are shown to conform with the universal sputtering yield equation of Seah11 with a change in the single parameter A that is linearly related15 to M−1/3. That work effectively links the behaviors of large molecules and polymers. The angle-dependence of the sputtering yield for argon gas clusters was measured for PS by Rading et al.16 This showed a broad angular maximum in the yield for ion incidence angles around 45−60°. Later, Seah et al.17 developed a generic description for these data and also extensive data for Irganox 1010. Again, the ratio E/n was important with an effect again through the parameter, A. As E/n fell to low values, the reduction in yield at 0°, relative to that at 45°, was greatly increased. The above shows that there is now an extensive description of the sputter yield behavior for organic materials sputtered by argon gas clusters. The analogous behavior for elements sputtered by monatomic argon ions evolved over a much longer time frame, and reasonably accurate yields could be predicted using Seah et al.’s18 relations with angular dependence given by Yamamura et al.19 and by Seah.20 The difficult problem of the sputtering yield of compounds by monatomic argon was studied by Seah and Nunney21 who showed that the energy dependence could be accurately described and that the absolute values were good except for materials with a very high heat of formation. The fact that these were stoichiometric compounds and not simple mixtures was not critical. It was just that data for stoichiometric compounds and also their heats of formation were available. It is this last aspect of mixtures that we study here since there are currently limited data for, and no predictions of, the sputtering yields of mixtures of organic materials using argon gas clusters. Fleischmann et al.22 give data for the measured sputtering yield of mixtures of two materials used in the photoactive layer of some organic photovoltaic devices [P3HT (C10H14S)n and PC60BM (C72H14O2)]. A nonlinear dependence of the sputtering yield with the composition was seen for this combination, but no analytical description or explanation of the behavior was given. There are many parallels between the sputtering behaviors of monatomic argon ions impacting elemental solids and of cluster argon ions impacting organic materials. There are also many differences, particularly in that the impacting cluster deposits its energy mostly at the surface whereas the monatomic ion penetrates more deeply. In the present work, we focus on binary mixtures of the organic molecules Irganox 1010 and either Irganox 1098 or FMOC. All these materials have been studied extensively as pure compounds and in discrete layers.13,23−25 Recently, mixtures of these materials became available,25 and a batch of samples which contained five different compositions as discrete layers was generated for the purpose of comparing the matrix effect in different SIMS instruments in a VAMAS interlaboratory comparison. These samples enable a preliminary investigation of how sputtering yields change in mixed organic materials. This is important in establishing the depth scale in three-dimensional reconstructions of molecular distributions of real samples. Technological and biological samples are invariably mixtures of different materials, and therefore, a means of assessing the sputtering yield as the composition changes both vertically and laterally is required.

Figure 1. (a) Schematic of the structure of the multilayer samples with their nominal thicknesses, after Shard et al.25 The samples are created by alternate evaporation of Irganox 1010 (black) and either Irganox 1098 or FMOC (white), respectively. The chemical structure of Irganox 1010 is in part b, that for FMOC is in part c, and that for Irganox 1098 in part d. Reproduced with permission from ref 25. Copyright 2015 American Chemical Society.

by the outputs of quartz crystal oscillators (QCOs) to deposit layers of defined thicknesses. The QCOs were, in turn, calibrated to relate their outputs to the thicknesses of each material deposited on the wafer substrates by ellipsometry using an M2000DI spectroscopic ellipsometer (Woollam, NE). The pure layers were each prepared as single deposits of 100 nm, but the mixed layers were made using 50 cycles of small depositions of one material followed by deposition of the other, with the pair of deposits being 2 nm thick. Since the molecular sizes are around 1 nm, this generates intimate mixing. Compositions nominally of 20%, 50%, and 80% by volume were prepared. These are called volume fractions and are prepared, for example, by mixing 20% by volume of Irganox 1010 and 80% by volume of Irganox 1098 with the total volume assumed unchanged as a result of mixing. The thicknesses used in the sputter yield calculations were evaluated from the 13434

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characteristic of Irganox 1010 and three characteristic of Irganox 1098. The secondary ion fragment compositions are given in Table 1. The intensities have been normalized by dividing by the average Irganox 1098 plateau intensity in layer 7 and the average Irganox 1010 plateau intensity in layer 8, allowing for the delta layers. It is clear that the Bi3+ ion current has slowly increased during the profile, and this is fitted to an exponential that asymptotes to unity. The intensities are rescaled to the new unity value. Each of the secondary ions is unique to one of the two materials. The renormalized intensities, plotted as a function of the volume fraction of Irganox 1010, ϕ1010, are shown in Figure 3. On this plot are also

calibrated monitoring QCO and showed a typical divergence of 1 nm from the nominal 100 nm. The monitoring QCO gives the thicknesses of each of the mixed layers assuming that the total volume is unchanged as a result of mixing. If the volume reduces 1%, the true thicknesses are 1% lower, and the sputtering yield, while being of the same mass, has a volume that is 1% lower. Volume reductions do occur for soluble systems but are generally less than 1%.26 Measurements using X-ray photoelectron spectroscopy (XPS) confirmed each composition of the mixed layers 4, 5, and 6.25 In the present work, these materials were all depth profiled by SIMS using 5 keV Ar2000+ gas cluster primary ions in an ION-TOF SIMS IV instrument (ION-TOF GmbH) with the incident ions at 45° to the surface normal and the cluster size distribution selected with a width of ∼30%. Negative secondary ions were measured using 25 keV Bi3+ ions, also at 45° incidence angle, but in an azimuth at 90° to the argon gas cluster sputtering beam. The sputtering beam was rastered over an area of 500 μm by 500 μm, and the analysis, at a low Bi3+ dose, was in a central zone of 200 μm by 200 μm. Electron flood charge compensation was used, and the spectra were dead time corrected. Secondary ion spectra for the three materials and two mixtures at 50:50 composition are available in the Supporting Information.

3. RESULTS AND DISCUSSION 3.1. Irganox 1010/Irganox 1098 Sample. Figure 2 shows the depth profile using three negative secondary ions

Figure 3. Normalized intensities for secondary ions shown in Figure 2, with the same color coding, as a function of the volume fraction of Irganox 1010 in the Irganox 1010/Irganox 1098 mixture. The smooth curves are eqs 1 and 2 with relevant α and β values fitted.

the fitted curves for the intensities resulting from both enhancement and suppression of the ion yields of one material by the proximity of the other. It is important to check this since enhancement of the yield on sputtering from one material to the other will shift the profile into the latter material and vice versa, so that the dose to the “interface” is incorrect. So, for a layer of A between layers of B, enhancement of the signal for A, or suppression of that for B, makes the layer of A look wider, and its deduced sputtering yield will be too low. The ions with yields closest to the straight lines from 1 to 0 or 0 to 1, and which have the least matrix effect, will give the least error from this effect. The equation describing the enhancement of the intensity of a secondary ion from A arising from a content of B, ϕB, is given by Shard et al.23 Here, we modify the equation to give

Figure 2. Depth profile of the Irganox 1010/Irganox 1098 structure using the normalized intensities of the m/z = 26 (red), 42 (orange), and 635 (purple) negative secondary ions characteristic of Irganox 1098 and the 59 (dark green), 227 (blue), and 1176 (light green) secondary ions characteristic of Irganox 1010. The thin line shows a fit to the maxima as an asymptotically growing exponential assumed to arise from a small drift in the Bi3+ analytical beam current.

IA /y = φA + φBα[1 − exp(−βφA )] I A∞/yA

(1)

where IA is the characteristic ion intensity from A in the mixture and I∞ A is its intensity from pure A. y is the yield volume for the Table 1. Secondary Ion Fragments from the Pure Materials and Their Formulae Irganox 1010

Irganox 1098

FMOC

m/z

fragment

m/z

fragment

m/z

fragment

13 41 59 231 277 1176

CH− C2OH− C2O2H3− C16OH23− C17O3H25− (one leg −CH2−) C73O12H107− (M1010 − H)−

26 42 635

CN− CNO− C40N2O4H63− (M1098 − H)−

236 476 952 953

C15NO2H10− C24NO4F5H15− (MFMOC − H)− (2MFMOC − 2H)− (2MFMOC − H)−

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mixture, and yA is that for pure A. Since the signals are generated by 25 keV Bi3+ ions, the y yield volumes are for that primary ion. It is assumed that IA is zero in material B and the volume fractions ϕA and ϕB sum to unity. This equation arises from a kinetic model of charge transfer in which the total transfer is integrated over the time of the ion−solid interaction27 where the parameters α and β are both positive. Equation 1 differs from that by Shard et al.23 by the inclusion of the sputtering yields for the analytical ion. By symmetry, Shard et al.23 gives an equation for the suppression of signal which becomes IA /y = φA {1 + α[1 − exp( −βφB)]} I A∞/yA

functions with their centers, depth resolutions, and heights as fitting parameters. In Figure 3, the signals for m/z = 26, 42, and 59 Da all show minimal matrix enhancement or suppression and here, also, have minimal relative shift. The positions of the interfaces are thus established on the time or dose scale and are referenced to the average positions for masses 26 and 59 Da. The signal for m/z = 635 Da, a secondary ion for Irganox 1098, shows enhancement, whereas that for m/z = 1176 Da, a secondary ion for Irganox 1010, shows suppression, and both of these lead to positive slopes in Figure 4. The secondary ion, m/z = 277 Da, for Irganox 1010, shows enhancement and gives a negative slope in Figure 4. From Figure 2, we see that 2.3 s of sputtering removes ∼1 nm so that these shifts reach some 4 nm which, uncorrected, would each cause an error of 4%. This generally doubles for the sputtering yield since two interfaces are involved so that one material is increased 8% and one is reduced 8% leading to an overall error of 16%. The sputtering yields deduced from these layers, for a fragment with low matrix effect for each material as a function of the composition, are shown in Figure 5. Here we have

(2)

i.e., eq 1 with ϕA and ϕB partly reversed since the charge transfer is in the reverse direction. Equation 2, here, differs from the equivalent equation by Shard et al.23 by including the yields and changing the sign of α so that, if α is positive, we use the enhancement equation and, if α is negative, we use, or have used, this suppression equation. We do not yet know the ratio y/yA. We shall discuss that more later, but we initially assume that ratio to be unity. With that assumption, these two equations clearly describe the data in Figure 3 very closely and so allow the shifts in the profiles to be readily calculated. A further difference from the equivalent equation by Shard et al.23 is that IA is 0 in B. If IA had been nonzero, some fragment intensity would clearly come from B when A is zero, and then, there are two identical mass fragments but sourced from both A and B so that IA then needs to be treated as a sum of two equations with, possibly, different α and β values. In addition to the above shifts is a shift that arises from the probing depth of the Bi3+ analysis ion.28 This shifts the apparent interface by an amount that depends on the Bi3+ energy and the species analyzed, and on whether the species analyzed is in the material before or after the interface. If the effect is consistent between the two materials in the profile, there is no correction to be made to the doses to remove each layer except the first. In the present work, the dose for the first layer is not used. Figure 4 shows the shifts of the interfaces as a function of the change in ϕ1010 that occurs between each layer. To obtain these shifts, the data in Figure 2, after correction for the asymptotic increase in the Bi3+ current, are fitted to integral Gaussian

Figure 5. Sputtering yields of the mixtures of Irganox 1010 and Irganox 1098, normalized to the sputtering yield of Irganox 1010, as a function of the volume fraction of Irganox 1010 together with the fits of eq 7 for the m/z = 26 (red) and 59 (green) negative secondary ions characteristic of Irganox 1098 and Irganox 1010, respectively. The curves show the fits for eq 7. The average scatter of the data points around the fits is 0.017.

ratioed these data to the yields for Irganox 1010 to evaluate the changes arising from the compositional variation. This removes the, typically few percent, errors associated with the measurement of dose. The smooth curves are the predictions for the model described below. Here, unlike the monatomic sputtering case, we consider sputtered volumes rather than numbers of atoms and assume that the yield simply depends on local aspects of the bonding. It will not be a major effect since the intermolecular bonding does not have such a strong effect as the interatomic bonding in the sputtering of elements where the emitted particles are mainly single atoms. Had we used Irganox 1010 fragments with enhancement or Irganox 1010 fragments with suppression, the Irganox 1098 and 1010 yields would have been around 3% higher and 5% lower, respectively, leading to a total change of 8% across Figure 5. For the opposite cases, these changes have the reverse sign. Selecting fragments with low matrix factors has improved the data by some 8%. In the early study of argon gas cluster sputtering yields, Ichiki et al.29 established that the argon gas cluster yields were approximately inversely proportional to the bonding energy, U,

Figure 4. Shifts in the depth profile for the secondary ions of Figures 2 and 3 as a function of the change in volume composition. The results for the m/z = 26, 42, and 59 secondary ions show no significant shifts at the scale shown. 13436

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from the heat of sublimation, for elemental samples. The universal equation for the yield volume, Y, derived by Seah,11 mentioned earlier, is B(E /An)q Y = n 1 + (E /An)q − 1

(3) 2

where B is a volume coefficient between 0.004 nm and 0.02 nm3, q is a power ∼2−3, and A is an energy which, for organic materials, ranges from 0.8 to 5 eV, the latter being for strongly bonded organics. At high E/n values, approximately

E (4) A The parameter A for organics is an analogue of U, the interatomic bond energy, in elemental solids. If we consider a mixture of two molecules A and B, then it is this mixture that is being sputtered, and typical sputtered volumes may contain the fragments of hundreds of molecules of both species. The effective overall value of U for this mixture may be taken to arise for the sum of the relevant fractions: Y=B

U = φA U A* + φBUB*

Figure 6. Change in the normalized intensities of secondary ions involving both Irganox 1010 and Irganox 1098, blue M1010 + M1098 − 3H− (1810), red M1010 + M1098 − 2H− (1811), and green M1010 + M1098 − H− (1812), as a function of composition. The black data show the calculation for the normalized average for the sum of product intensities for (M1010 − pH)− and (M1098 − qH)− with p + q = 1, 2, or 3 and (black, dashed) the populations from eq 8.

(5)

Here, UA* and UB* are the effective average binding energy of the fragments of A and B allowing for the proximity of B and A, respectively. Thus, we may write U A* = UA(1 + ΩA φB) and UB* = UB(1 + Ω BφA )

mixing and a strong interaction. The dashed line is what would be predicted if there were no enhancement or suppression of intensities of the joined molecules and is asymmetrical since the two molecules have greatly different sizes. The probability that the molecules are adjacent is just proportional to xAxB where x is a molecular, rather than volume, fraction. If we assume that the molecular size is proportional to the molecular weight, then φA φBR xAx B = (φA R + φB)2 (8)

(6)

where UAΩA and UBΩB are the increases in the effective average binding energies for A and B molecules arising from the proximity of the other molecule, respectively. Using eq 4, and assuming that BA = BB, the yield of the mixture, Y, is YAUA/U; hence, if we put the ratio YB/YA = Θ, and ΩB + ΘΩA = K, we get Y Θ = YA 1 + φA (Θ − 1) + φA φBK

where R is the ratio of molecular weights, MB/MA. This result, shown by the dashed line, is very close to the sum of the product intensities for the separate molecular ions and shows that the enhancements and suppressions shown in Figure 3 have largely neutralized each other. In the above, we have made the assumption that y/yA is unity. It is very difficult to measure the yields for Bi3+ ions since, after sufficient sputtering to make a measurement, Bi is implanted and alters the sputtering yield of the sample. However, the above discussion and the data on ion yields13 all indicate that y Y = yA YA (9)

(7)

With material A for Irganox 1010 and B for Irganox 1098, eq 7 is used in Figure 5 with Θ26 = 0.72, K26 = 0.23, Θ59 = 0.72, and K59 = 0.24. Of course, both Θ and K should not depend on the secondary ion used, and the closeness of these values confirms this. Thus, the volume yield of Irganox 1098 is 72% of that for Irganox 1010 for 5 keV Ar2000+ ions at 45° incidence angle. The parameter, K, shows a significant energy in the heat of mixing of these two molecules. If ΩB and ΩA were zero, K would be zero, and the sputtering yields would almost follow a straight line between the values for the two pure materials. The positive value of K implies that these materials are mixed through associative interactions and not merely through entropy. This association should also become apparent through a significant contribution of Irganox 1010 and Irganox 1098 molecules combined as secondary ions in the emitted spectra. The normalized intensities of relevant ions are shown in Figure 6. These ions are not present in the pure material, of course, but are relatively strong in the mixtures. The results show the fragments (M1010 + M1098 − rH)− with r = 1 (green), r = 2 (red), and r = 3 (blue). The black results are the products of the individual secondary ion intensities for (M1010 − pH)− and (M1098 − qH)− summed over the many values of p and q to give each value of (M1010 + M1098 − rH)− ion and then normalized to unity at the midpoint. The sums of the product intensities for r = 1, 2, and 3 are all similar and fall between the measured data for r = 1 and 3. It is clear that there is intimate

We have just measured the values of Y/YA for the argon cluster ions, and this can now be fed back into the refitting of eqs 1 and 2 for Figure 3. The result has no visible change from Figure 3 except that the values of α and β change slightly, and the rootmean-square fit of the ordinate measurements improves from 0.011 to 0.009. 3.2. Irganox 1010/FMOC Sample. Figure 7 shows the depth profile for the second sample as a complement to Figure 2. The structure is again as in Figure 1. Here, however, the Bi3+ current has remained more constant through the profile. The profile for mass 13 Da has a constant background removed, without which it would change from 0.5 to 1.5 on this plot. The intensities for several secondary ions as a function of the steady state composition from layers 2 to 7 are shown in Figure 8, together with the descriptions of eqs 1 and 2. As can be seen, there is strong enhancement and suppression of many secondary ion intensities, but a few strong ions may still be 13437

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Figure 9. Sputtering yields of the mixtures of Irganox 1010 and FMOC, normalized to the sputtering yield of Irganox 1010, as a function of the volume fraction of Irganox 1010 together with the fits of eq 7 for the CH− and (2MFMOC − H)− negative secondary ions at m/z = 13 and 953. The curves show the fits for eq 7. The average scatter of the data points around the fits is 0.065.

Figure 7. Depth profile of the Irganox 1010/FMOC structure using the normalized intensities for the m/z = 476 (purple) and 953 (orange) secondary ions characteristic of FMOC and the 13 (light green), 41 (dark blue), 59 (black), 231 (green), and 1176 (light blue) secondary ions characteristic of Irganox 1010.

ϕA = 0.2 does not come from the fitting of the profiles that define the sputtering doses. It may arise from extra terms, omitted from eq 6, or from specific molecular interactions. Had we used m/z = 476 Da with high enhancement for FMOC or m/z = 1176 Da with strong suppression for Irganox 1010, the Irganox 1010 yields increase and the FMOC decrease with a total change twice as strong as for the Irganox 1010/ Irganox 1098 sample, i.e., over 20%. Analysis of the molecular pairs, M1010 + MFMOC − 3H− (1651), M1010 + MFMOC − 2H− (1652), and M1010 + MFMOC − H− (1653), as a function of composition, shows results similar to that in Figure 6. Again, these intensities are relatively strong in the mixtures but absent in the pure materials. This shows that the molecules are attracted to each other as assumed for eq 7. Of course, if the molecules were repelled from each other (Ω significantly negative), the intensities of the combined molecules would be very weak or undetected as the mixed region would separate into small volumes of each component as single phases. The average yield volume would then be given by a linear interpolation between the values for each of the pure materials.

Figure 8. Normalized negative secondary ion intensities as a function of the volume fraction of Irganox 1010 in the Irganox 1010/FMOC mixture. The smooth curves are eqs 1 and 2 with relevant α and β values fitted. The curves are for the following m/z negative ion fragments: FMOC, 476 (purple), 236 (red), 953 (light green), 952 (dark green); and Irganox 1010, 13 (brown), 41 (purple), 231 (black), 59 (light blue), 1176 (dark blue).

found that are close to linear at all compositions. Here, we use the ion at m/z = 953.3 Da, characteristic of FMOC, and 13 Da, here with its offset, to be characteristic of Irganox 1010. Of course, CH− appears from both Irganox 1010 and FMOC but is weaker for the latter. Ions appearing from both constituents are usually avoided, but here there appears to be a minimal matrix effect from both molecules, and so CH− is useful. As before, the shifts in depth arising from low matrix effect ions are very small. Figure 9 shows the resulting sputtering yields for the low enhancement/suppression fragment ions m/z = 953.3 Da, characteristic of FMOC, and 13 Da, here, characteristic of Irganox 1010, normalized to the average value for Irganox 1010. Also shown are the fits for eq 7 with material A = Irganox 1010 and B = FMOC. Here, Θ13 = 1.31, K13 = 0.59 and Θ953 = 1.32, K953 = 0.52, again showing little variation with the secondary ion used providing it has a small matrix effect. The yield of FMOC is 1.3 times that for Irganox 1010 for 5 keV Ar2000+ primary ions. As before, the yield for the mixtures is reduced below that for a linear interpolation, implying an evolution of energy in mixing. This energy seems greater than that for the Irganox 1010/Irganox 1098 mixture. The sputtering yield results for other secondary ions characteristic of the two materials gave similar results, and so the deviation of the data at

4. CONCLUSIONS The sputtering yields of mixtures of Irganox 1010 with either Irganox 1098 or FMOC have been measured for 5 keV Ar2000+ sputtering ions incident at 45°. The yields conform to a new model for mixtures that form a single phase solid solution. The sputtering yield volume for pure Irganox 1098 is 0.72 that of Irganox 1010 whereas that for FMOC is 1.3 times that of Irganox 1010. The sputtering yield values for mixtures are below that calculated from a linear interpolation from the two materials, as predicted by the model. This reduction is, for these two binary systems, always in the range 0−15%. To determine the yields, secondary ions with the minimum enhancement or suppression of the ion yield by the colocating molecule should be chosen. The use of ions showing significant nonlinearity with composition, arising from ion yield enhancement or suppression, may lead to mislocation of each interface by up to 10 nm, directly leading to errors in the determined sputtering yields. 13438

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcb.5b06713. Negative secondary ion spectra from pure materials and 50:50 mixed layers (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone +442089436634. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors would like to thank S. J. Spencer and S. A. Smith for the preparation of the two samples used in this study and the referees for helpful comments. This work forms part of the 3D nanoSIMS project in the Strategic Capability Programme of the National Measurement System of the UK Department of Business, Innovation and Skills and with funding by the European Metrology Programme for Innovation and Research (EMPIR) project 3DMetChemIT.

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DOI: 10.1021/acs.jpcb.5b06713 J. Phys. Chem. B 2015, 119, 13433−13439