Implementation and Optimization of Large Gas Cluster Laser Post

Article pubs.acs.org/JPCC

Implementation and Optimization of Large Gas Cluster Laser PostIonization Secondary Neutral Mass Spectrometry for Molecular Analysis Andreas Pelster,† Martin Körsgen,† Marcel Heeger,† and Heinrich F. Arlinghaus*,† †

Physikalisches Institut, University of Münster, Wilhelm-Klemm-Strasse 10, 48149 Münster, Germany ABSTRACT: In this study, we describe for the first time a new combined technique for molecular surface analysis using large argon gas clusters as primary ions for laser postionization secondary neutral mass spectrometry (Laser-SNMS). This new technique was investigated on polymer polystyrene (PS) surfaces using pulsed Ar2000+ ions with 20 keV energy and a 157 nm laser beam for postionization. To optimize the ion yield, time-of-flight (ToF) distributions of characteristic sputtered neutral PS fragments were determined. The data could be fitted well with Maxwell−Boltzmann distributions. The data show that the maxima of the individual ToF distributions of the sputtered neutrals move to higher delay time values with increasing mass, which directly correlates to a decrease of velocity of the sputtered neutrals with increasing mass. The delay time dependence on mass could be fitted with a quadratic curve, thus indicating a constant mean kinetic energy for the sputtered molecules. The ion yields obtained with large gas cluster Laser-SNMS have been compared to ion yields obtained with large gas cluster ToF secondary ion mass spectrometry (ToF-SIMS). The data clearly show that the yields determined with Laser-SNMS are typically between one and four orders of magnitude higher than those obtained with ToF-SIMS. Thus, a significant enhancement of sensitivity and efficiency for detecting PS fragments can be achieved by using large gas cluster Laser-SNMS.

1. INTRODUCTION Time-of-flight secondary ion mass spectrometry (ToF-SIMS) and laser postionization secondary neutral mass spectrometry (Laser-SNMS) are well-established techniques for characterizing the chemical composition of surfaces. Both techniques rely on the detection of particles sputtered from a surface by ion bombardment. In the case of ToF-SIMS, the sputtered secondary ions can be directly detected, while in the case of Laser-SNMS, the sputtered neutrals have to be postionized with a pulsed laser beam prior to detection. In general, primary ions (PI) used for sample bombardment are either monatomic or small polyatomic ions. They cause high fragmentation of the surface molecules. To reduce molecular fragmentation, large gas cluster ions have become more and more important for sputtering. Several publications have shown that, for organic depth profiling, a significant improvement can be achieved by using large gas cluster ions for bombardment. In particular, the use of large argon cluster ions for sputtering enables depth profiling of organic materials without significant degradation of sample material as a function of depth.1−5 In addition, large gas cluster ions have the advantage of increasing the sputter yield for organic compounds. This increase depends on the energyper-atom ratio.6 Kayser et al.7 showed that the secondary ion yields observed for large argon gas cluster ions are similar to those obtained for PI such as small bismuth clusters. Researchers are looking for new ways to further enhance the ion yield for large cluster ion © 2017 American Chemical Society

bombardment. Since the intrinsic ionization probability depends on both the surface composition and the species used for bombarding the sample, investigators started to use different gas mixtures for cluster production, such as water,8,9 to enhance the ionization probability. Another possibility to enhance the ion yield for primary cluster ions is to laserpostionize sputtered neutrals, which in most cases represent the majority of the sputtered particles. In this case, the sputtering and ionization processes are decoupled, and the ionization probability mainly depends on laser wavelength and power density. In general, the ionization energy of organic molecules lies between 7 and 14 eV.10 It was found that a wavelength of 157 nm (photon energy of 7.9 eV) is well suited for postionizing sputtered molecules with high ionization probability and low fragmentation rate, as compared to higher wavelengths, e.g., 193 nm.11,12 In this study, we describe for the first time a new combined technique using large argon gas clusters as PI for Laser-SNMS (gas-cluster Laser-SNMS). We show results obtained from polystyrene (PS) samples using pulsed Ar2000+ ions with 20 keV energy and a 157 nm laser beam for postionization. To optimize the ion yield, defined by the number of detected ions divided by the number of PI, the time-of-flight (ToF) Received: May 9, 2017 Revised: June 22, 2017 Published: July 10, 2017 15266

DOI: 10.1021/acs.jpcc.7b04424 J. Phys. Chem. C 2017, 121, 15266−15271

Article

The Journal of Physical Chemistry C distribution of the characteristic sputtered neutral PS fragments was determined. From these data, the ion yields of characteristic polystyrene fragments were determined and compared with those obtained with ToF-SIMS.

2. EXPERIMENTAL SECTION Instrument. Large gas cluster Laser-SNMS was implemented using a combined ToF-SIMS/Laser-SNMS instrument, which is comparable to a ToF-SIMS 5 instrument (ION-TOF GmbH, Germany). The instrument is equipped with a gas cluster ion beam (GCIB, ION-TOF GmbH) and an excimer laser system (wavelength 157 nm) LPF 220 (Lambda Physik AG (Coherent Inc., USA)). The gas cluster ions are produced by adiabatic expansion of a high-pressure gas (∼30 bar) into a vacuum chamber (∼0.01 mbar) through a nozzle, followed by electron impact ionization. The resulting DC cluster ion beam is manipulated by magnetic and electrostatic fields. The source is equipped with two electrostatic deflectors, which are generally used for measuring the cluster size distribution. The two deflectors work as a ToF mass filter for the DC cluster ion beam. Hereby, the first deflector generates a short pulse (pulse width in the μs range) which contains all cluster sizes, while after a specific delay a pulse on the second deflector separates out clusters with a specific mass. At constant kinetic energy, cluster ions of different masses drift apart on their path between the first deflector and the second deflector. This deflector configuration was used to generate short PI pulses with specific cluster sizes for gas cluster Laser-SNMS. To maximize the Laser-SNMS yield, the laser power density as well as the timing between PI and laser and extraction field pulses, respectively, have to be optimized. The laser power density can be varied with a dielectric attenuator. The postionized particles are accelerated into a reflectrontype ToF−MS. The detection system consists of a 10 keV postacceleration system, a microchannel plate, a scintillator, and a photomultiplier. For signal registration, an 8-bit digitizer is used to digitize the analogous signals. To facilitate a direct comparison between the analogous Laser-SNMS signal and the number of detected ions, the analogous Laser-SNMS signal must be converted into counts using a calibration factor. This factor was determined by measuring the ion signals obtained from standard samples with the time-to-digital converter and the 8-bit digitizer. Conditions. The first large gas cluster Laser-SNMS experiments were carried out using Ar2000+ cluster ions with an energy of 20 keV for sputtering and a 157 nm excimer laser system for postionization. A laser power density of approximated 5 × 106 W/cm2 was used. Recently, it was shown that this laser power density is suitable for efficiently postionizing sputtered PS fragments.13 To characterize the GCIB, the cluster size distribution of the DC cluster beam was measured using the ToF mass filter system described above, measuring the current as a function of selected cluster masses (Figure 1). By using the same setup, an Ar cluster size of 2000 ± 60 atoms was selected for the analysis beam (see Figure 1 selected cluster for the analysis beam). The PI pulse width ranges from a few hundred nanoseconds up to one microsecond. A primary motivation for performing laser postionization is to improve measurement efficiency (i.e., signal/material consumed). Improvements in efficiency are required for measurements that involve low concentrations, high spatial resolution or studies of monolayers. When sputtering organic materials with monatomic or small polyatomic ions, damaged

Figure 1. Gas cluster size distribution of the DC Ar2000+ cluster beam (20 keV) and the selected pulsed cluster ion beam distribution for the analysis (normalized intensities).

molecules accumulate on the sample surface, so the primary ion dose density (PIDD) must be kept below the static limit, i.e., a sample surface damage less than 1%. With monatomic or small polyatomic ions, the static limit is typically defined as a PIDD of less than 1012 cm−2.14 Large argon cluster sputtering, in contrast, leaves little or no measurable damaged material on the surface and so the static limit has often been ignored. Nevertheless, material is consumed by the sputtering process, and so the static limit remains relevant to the efficiency of the measurements. The equivalent PIDD for large argon clusters must be estimated because there is as yet no generally accepted static limit for large argon cluster sputtering. As stated above, the static SIMS condition for argon cluster bombardment is given by areahit ≤ 1% areaanalysis where areahit is the area of removed particles after PI bombardment, and areaanalysis is the entire area used for analysis. The areahit is a multiple of each bombarded area by the number of PI (nPI) with the estimated bombarded area for a single bombardment area hit,single , which is comparable to the disappearance cross section: areahit = nPI ·areahit , single With the definition of the PIDD given as PIDD = nPI/ areaanalysis, the static SIMS condition can be described as follows: PIDD ≤ 1%·1/areahit , single

The area areahit,single can be estimated as the surface area of the sputter crater produced by the PI. The volume of the sputter crater (i.e., the volume sputter yield Ysp) produced by a PI at an angle perpendicular to the surface can be described as a hemisphere. However, if the surface is bombarded at an angle φ of 45°, the depth d of the crater will be reduced and the direction of PI propagation will be extended compared to the size of the hemisphere. Thus, the sputter yield Ysp can be estimated as the volume of half an ellipsoid (Vhe) with a depth smaller than the radius of the hemisphere: 2 Ysp = Vhe = areahit , single·d 3 15267

DOI: 10.1021/acs.jpcc.7b04424 J. Phys. Chem. C 2017, 121, 15266−15271

Article

The Journal of Physical Chemistry C By using the universal equation for the argon gas cluster volume sputter yield Ysp by Seah,6 the maximum PIDD for static conditions for primary argon gas cluster ions can be estimated by

PIDD ≤

2 ⎛ 0.01· 3 d⎜1 + ⎝

q− 1⎞

( AE·n ) E q B ·( A·n ) n

⎟

⎠

where q, B, and A are fitting parameters, n is the cluster size, and E is the energy of the cluster. For PS, Seah6 used the following fitting parameters: q = 3.4, B = 0.0111 nm3, and A = 2.36. When one is bombarding the surface at an angle φ of 0° to surface normal, the depth d can be estimated by the radius of the hemisphere rh. If the change of penetration depth in the direction of PI propagation by variation of the angle φ is negligible, the depth d can be written as

d = rh cos φ The radius of the hemisphere rh can be directly calculated by the known sputter yield using the Seah equation,6,15 which leads to a depth d of approximately 2.2 nm for a single Ar2000+ bombardment with an energy of 20 keV under an angle of 45°. This depth d is within the range of crater depths observed in argon cluster bombardment.16,17 Using these values results in a maximum PIDD of 1.6 × 1010 cm−2 for analyzing PS with Ar2000+ (20 keV). This is significantly lower than the static limit values used for monatomic or small polyatomic ions. However, Delcorte et al.18 showed that Seah’s equation for volume sputter yield is only valid for large PS molecules. For smaller PS molecules, the volume sputter yield per cluster size (Ysp/n) increases in an approximately linear fashion with energy-per-cluster size (E/n). For the sample used in our study (molecular weight Mn ∼ 1020 u), the sputter yield is a factor of ∼2.4 larger18 than the volume sputter yield determined by Seah. Thus, the depth d of the crater increases to approximately 3 nm and the maximum PIDD for analyzing PS with Ar2000+ (20 keV) under static conditions decreases to a PIDD ≤ 0.9 × 1010 cm−2. These estimations were used to set up the experiments. To stay below the static limit for the determination of the ToF distribution of neutral PS fragments, a PIDD of only 2.5 × 108 cm−2 for a single scan was used, resulting in a total PIDD of 6.2 × 109 cm−2 for the entire analysis. To determine the ion yields, for Laser-SNMS, a total PIDD of 6.3 × 108 cm−2 was used, and for ToF-SIMS, a PIDD of 1.25 × 1010 cm−2, which is close to the static limit. The ToF-SIMS and Laser-SNMS data were evaluated using software from ION-TOF GmbH, Germany.

Figure 2. Laser-SNMS spectrum (top) and ToF-SIMS spectrum (bottom) of PS (intensity in arbitrary units).

characteristic fragment ions of PS, such as C3H3+, C4H3+, C4H9+, C6H5+, C7H7+, C8H7+, C8H9+, C9H7+, C9H9+, C13H9+, C15H9+, and C15H13+, which are well-known for ToF-SIMS19 and for Laser-SNMS13 using bismuth as PI, are observed in both spectra. To maximize the Ar cluster Laser-SNMS yield, the ToF distributions of characteristic PS ions with different masses were investigated by measuring the signal intensity as a function of delay time between the PI pulse and the laser pulse using a fixed distance between the surface and the laser beam. This distribution also yields information on the sputtering process, especially the dependence of the velocity/kinetic energy distribution on the mass of the sputtered neutral fragments. For Laser-SNMS using monatomic or small polyatomic PI, it was shown that the ToF distribution can be typically fitted with a Maxwell−Boltzmann distribution.20 However, in comparison to Bi PI bombardment, where very short pulses down to the low ns range can be used to measure the ToF distribution of sputtered particles, it is impractical to use such short pulses for Ar cluster Laser-SNMS (pulse length typically in the range of a few hundred nanoseconds up to one microsecond), because the resulting number of PI would be too low to obtain reasonable signal intensities. Thus, the resulting ToF distribution produced by long Ar cluster ion pulses has to be simulated by summing up ToF distributions produced by consecutive short Ar cluster ion pulses. An example is shown in Figure 3, where 11 individual ToF distributions (colored lines) were summed up, resulting in the blue dashed-line ToF distribution. A similar procedure was used to evaluate the measured ToF distributions. The experimental results were fitted by summing up the following Maxwell−Boltzmann function:

3. RESULTS AND DISCUSSION A layer of PS from Sigma-Aldrich (polystyrene analytical standard 1,000) was produced by spin-coating a PS-solution (PS solved in toluene with a concentration of 10 mg/mL) onto a clean silicon wafer. For the identification of characteristic ions, the polymer PS were analyzed with Laser-SNMS and ToFSIMS. Spectra of PS are depicted in Figure 2. The Laser-SNMS spectrum was obtained using the 157 nm excimer laser with a laser power density of approximated 5 × 106 W/cm2 for postionization after scanning Ar2000+ (20 keV) PI over a 200 × 200 μm2 area with a PIDD of 6.3 × 108 cm−2. The ToF-SIMS spectrum was obtained scanning Ar2000+ (20 keV) PI over a 200 × 200 μm2 area with a PIDD of 1.25 × 1010 cm−2. Similar

dN /dr(t ) = c0, MB/t 3·exp( −c1, MB/t 2) 15268

DOI: 10.1021/acs.jpcc.7b04424 J. Phys. Chem. C 2017, 121, 15266−15271

Article

The Journal of Physical Chemistry C

Figure 3. Calculated ToF distribution: individual ToF distribution (colored lines) and the sum of these ToF distributions (blue dashed line).

Maxwell−Boltzmann distributions resulting from the summation of 63 single ToF distributions with an entire PI pulse of approximately 500 ns (td = 8 ns). The fit functions clearly show that the measured ToF distribution can be well described by a Maxwell−Boltzmann distribution. This is also indicated by the coefficient of determination (R2) for all fits (R2 = 0.990 for C3H3+, R2 = 0.993 for C7H7+, and R2 = 0.951 for C15H9+). The higher intensity scattering observed for C15H9+ is due to a lower detection yield. Additionally, Figure 4 shows directly that the ToF distributions of the sputtered PS fragments with different masses drift apart. If the mass and drift path are constant, the ToF distribution represents the kinetic energy distribution of the sputtered neutrals. Thus, sputtered neutrals of the same mass with high velocity, and therefore, high kinetic energy will be detected at shorter delay times than sputtered neutrals with low velocity and, therefore, low kinetic energy. The shift of the maxima to a higher delay time tTOF for increasing masses indicates that for argon cluster bombardment, the velocity of the sputtered neutrals is dependent on their masses. The velocity of the sputtered neutrals can be described by the most probable velocity vmax, which is given by the maximum of the Maxwell− Boltzmann distribution for velocity. With vmax being proportional to tTOF,max, it corresponds to the maximum of the ToF distribution tTOF,max. In order to determine the dependence of mass on vmax, the masses of sputtered neutrals as a function of the maximum of their ToF distributions tTOF,max are shown in Figure 5. Assuming a Maxwell−Boltzmann distribution, the averaged kinetic energy Ekin ∼ kBT, where kB is the Boltzmann constant and T the absolute temperature, is independent of the mass of the sputtered neutrals, thus, the arrival time of the maximum of the ToF distribution for each individual mass depends on m ∼ tToF,max2. Hence, the data points were fitted with a quadratic curve. This curve shows a good correlation with R2 = 0.970, indicating a constant averaged kinetic energy Ekin for the sputtered molecules. Using a linear curve for fitting resulted in a smaller R2 (R2 = 0.949), indicating a better fit to a quadratic trend of the data. Since the maximum occurs at a different time for each mass, an optimal delay time tTOF has to be selected, which makes it possible to detect both small sputtered molecules with higher velocities and larger sputtered molecules with lower velocities. From Figure 5, it was estimated that a suitable delay time tTOF for the analysis of PS is in the range 1.10−1.15 μs, which

where c0, MB = 2/π ·(m /kBT )3/2 r 2 , c1,MB = mr2/2kBT, and r is the constant drift path between sample surface and ionization volume, for n different starting times for the sputtered particles with a delay time of td between the starting times. This results in the following function: k

In(t ) =

∑ c0 n=0

⎛ ⎞ 1 1 ⎟ exp⎜ −c1 3 2 (t − t0 − [ntd]) ⎝ (t − t0 − [ntd]) ⎠

+ y0

where y0 represents an offset due to background signals such as residual gas and noise signals, t0 represents the offset due to the unknown starting time of the sputtered particles, and ntd represents each new starting time of the sputtered particles used for calculating the ToF distribution. In Figure 4, the ToF distributions of the sputtered postionized PS neutral fragments C3H3+, C7H7+, and C15H9+

Figure 4. ToF distribution of the characteristic PS ions C3H3+, C7H7+, and C15H9+.

are depicted by plotting their normalized intensities In as a function of delay time tTOF. tTOF is described by the delay time t between the PI and laser pulses minus an offset ti, which represents the unknown starting point of sputtered ions (tTOF = t − ti, ti = const). Since the ToF distributions resembled Maxwell−Boltzmann distributions, they were fitted by summed 15269

DOI: 10.1021/acs.jpcc.7b04424 J. Phys. Chem. C 2017, 121, 15266−15271

Article

The Journal of Physical Chemistry C

higher than the ion yields of ToF-SIMS. In general, with increasing ion mass, the difference of ion yields between ToFSIMS and Laser-SNMS decreases. Still, for all PS fragments with the exception of C15H13+, Laser-SNMS yields are more than 1 order of magnitude higher than ToF-SIMS yields. Thus, a high yield enhancement with a correspondingly high increase in efficiency and sensitivity can be achieved by postionizing PS fragments sputtered from a PS surface using large argon clusters as PI.

4. CONCLUSION A new combined technique using large argon gas clusters as primary ions for Laser-SNMS was used to analyze PS surfaces. The surfaces were analyzed under static conditions using pulsed Ar2000+ ions with 20 keV energy. The sputtered neutral fragments were postionized with a 157 nm laser beam. To optimize the ion yield, ToF distributions of characteristic sputtered neutral PS fragments were determined. The data could be fitted well with Maxwell−Boltzmann distributions. The observed drift of the maxima of the individual ToF distributions as a function of delay time indicates a direct correlation between the velocity of the sputtered neutrals and mass. The delay time dependence on mass fitted a quadratic curve, indicating a constant mean kinetic energy for the sputtered molecules. A comparison between the PS ion yields obtained with large gas cluster Laser-SNMS and large gas cluster ToF-SIMS showed a significant enhancement in efficiency and sensitivity (up to 4 orders of magnitude) for Laser-SNMS. It can be concluded that combining large gas cluster PI bombardment, which causes less degradation of organic material, with laser postionization of sputtered neutrals, which enhances yields, results in a powerful tool for the analysis of a wide range of organic samples.

Figure 5. Mass of the sputtered neutrals as a function of the maximum of the ToF distribution tTOF,max. Data points determined from the ToF distribution shown in Figure 4 are depicted as stars. The data were fitted with a quadratic curve (R2 = 0.970).

corresponds to the maximum of the ToF distribution of C7H7+ and includes the intersections of the ToF distributions of the lower selected characteristic ion signal (C3H3+) with the higher selected characteristic ion signal (C15H9+) (see Figure 4). On the basis of these results, the ion yields of characteristic PS ions were determined for Laser-SNMS and ToF-SIMS. Since, in general, the majority of the sputtered particles are neutrals, an enhancement of the ion yield should be expected when including the sputtered neutrals in the analysis. Figure 6 depicts the ion yields of characteristic PS fragment ions for both Laser-SNMS and ToF-SIMS. For ToF-SIMS

■

AUTHOR INFORMATION

Corresponding Author

*(H.F.A.) E-mail: [email protected]. ORCID

Andreas Pelster: 0000-0003-2021-3997 Notes

The authors declare no competing financial interest.

■ ■

ACKNOWLEDGMENTS The authors thank Rudolf Möllers from ION-TOF GmbH (Germany) for helpful discussions. REFERENCES

(1) Ninomiya, S.; Ichiki, K.; Yamada, H.; Nakata, Y.; Seki, T.; Aoki, T.; Matsuo, J. Precise and fast secondary ion mass spectrometry depth profiling of polymer materials with large Ar cluster ion beams. Rapid Commun. Mass Spectrom. 2009, 23, 1601−1606. (2) Niehuis, E.; Möllers, R.; Rading, D.; Cramer, H.-G.; Kersting, R. Analysis of organic multilayers and 3D structures using Ar cluster ions. Surf. Interface Anal. 2013, 45, 158−162. (3) Rading, D.; Moellers, R.; Cramer, H.-G.; Niehuis, E. Dual beam depth profiling of polymer materials: comparison of C60 and Ar cluster ion beams for sputtering. Surf. Interface Anal. 2013, 45, 171−174. (4) Körsgen, M.; Pelster, A.; Dreisewerd, K.; Arlinghaus, H. F. 3D ToF-SIMS Analysis of Peptide Incorporation into MALDI Matrix Crystals with Sub-micrometer Resolution. J. Am. Soc. Mass Spectrom. 2016, 27, 277−284. (5) Pelster, A.; Tyler, B. J.; Körsgen, M.; Kassenböhmer, R.; Peterson, R. E.; Stöver, M.; Unger, W. E. S.; Arlinghaus, H. F. Improved 3D-

Figure 6. Laser-SNMS and ToF-SIMS ion yields of selected PS fragment ions.

analysis, a PIDD of 1.25 × 1010 cm−2 was used. For LaserSNMS analysis, a PIDD of 6.3 × 108 cm−2 was used in conjunction with a laser power density of approximated 5 × 106 W/cm2, suitable for postionizing sputtered PS fragments.13 The data clearly shows that the yields determined with Laser-SNMS are much higher than the yields determined with ToF-SIMS. For smaller fragment ions such as C3H3+ and C4H3+, the ion yields of Laser-SNMS are more than 4 orders of magnitude 15270

DOI: 10.1021/acs.jpcc.7b04424 J. Phys. Chem. C 2017, 121, 15266−15271

Article

The Journal of Physical Chemistry C imaging of a sirolimus/probucol eluting stent coating using laser postionization secondary neutral mass spectrometry and time-of-flight secondary ion mass spectrometry. Biointerphases 2016, 11, 041001. (6) Seah, M. P. Universal Equation for Argon Gas Cluster Sputtering Yields. J. Phys. Chem. C 2013, 117, 12622−12632. (7) Kayser, S.; Rading, D.; Moellers, R.; Kollmer, F.; Niehuis, E. Surface spectrometry using large argon clusters. Surf. Interface Anal. 2013, 45, 131−133. (8) Sheraz née Rabbani, S.; Berrueta Razo, I.; Kohn, T.; Lockyer, N. P.; Vickerman, J. C. Enhancing Ion Yields in Time-of-Flight-Secondary Ion Mass Spectrometry: A Comparative Study of Argon and Water Cluster Primary Beams. Anal. Chem. 2015, 87, 2367−2374. (9) Alnajeebi, A. M.; Vickerman, J. C.; Lockyer, N. P. Matrix effects in biological SIMS using cluster ion beams of different chemical composition. Biointerphases 2016, 11, 02A317. (10) Hesse, M.; Meier, H.; Zeeh, B. Spectroscopic Methods in Organic Chemistry. Thieme: Stuttgart, 2008. (11) Galla, S.; Pelster, A.; Draude, F.; Arlinghaus, H. F. Laser-SNMS investigations on pyrene using Ga+, Bi1+, Bi3+, Bi5+ as primary ions and different laser wavelengths and laser power densities for photoionization. Surf. Interface Anal. 2013, 45, 31−34. (12) Tyler, B. J.; Dambach, S.; Galla, S.; Peterson, R. E.; Arlinghaus, H. F. Investigation of the Utility of Laser-Secondary Neutral Mass Spectrometry for the Detection of Polyaromatic Hydrocarbons in Individual Atmospheric Aerosol Particles. Anal. Chem. 2012, 84, 76− 82. (13) Pelster, A.; Körsgen, M.; Kurosawa, T.; Morita, H.; Arlinghaus, H. F. ToF-SIMS and Laser-SNMS Imaging of Heterogeneous Topographically Complex Polymer Systems. Anal. Chem. 2016, 88 (19), 9638−9646. (14) Vickermann, J. C.; Briggs, D. ToF-SIMS: Surface Analysis by Mass Spectrometry. IM Publications: Chichester, U.K., 2001. (15) Seah, M. P.; Spencer, S. J.; Shard, A. G. Angle Dependence of Argon Gas Cluster Sputtering Yields for Organic Materials. J. Phys. Chem. B 2015, 119, 3297−3303. (16) Yamada, I.; Matsuo, J.; Toyoda, N.; Kirkpatrick, A. Materials processing by gas cluster ion beams. Mater. Sci. Eng., R 2001, 34, 231− 295. (17) Houzumi, S.; Mochiji, K.; Toyoda, N.; Yamada, I. Scanning Tunneling Microscopy Observation of Graphite Surfaces Irradiated with Size-Selected Ar Cluster Ion Beams. Jpn. J. Appl. Phys. 2005, 44 (8), 6252−6254. (18) Delcorte, A.; Cristaudo, V.; Lebec, V.; Czerwinski, B. Sputtering of polymers by keV clusters: Microscopic views of the molecular dynamics. Int. J. Mass Spectrom. 2014, 370, 29−38. (19) Briggs, D. Analysis of Polymer Surfaces by SIMS 2-Fingerprint Spectra from Simple Polymer Films. Surf. Interface Anal. 1982, 4, 151− 155. (20) Schnieders, A.; Möllers, R.; Benninghoven, A. Molecular secondary particle emission from molecular overlayers under 10 keV Ar+ primary ion bombardment. Surf. Sci. 2001, 471, 170−184.

15271

DOI: 10.1021/acs.jpcc.7b04424 J. Phys. Chem. C 2017, 121, 15266−15271