Article Large Molecular Cluster Formation from Liquid Materials and Its Application to ToF-SIMS
Kousuke Moritani * , Shogo Nagata, Atsushi Tanaka, Kosuke Goto and Norio Inui
Graduate School of Engineering, University of Hyogo, 2167 Shosha, Himeji, Hyogo 671-2201, Japan; [email protected] (S.N.); [email protected] (A.T.); [email protected] (K.G.); [email protected] (N.I.) * Correspondence: [email protected]; Tel.: +81-79-267-4921
Abstract: Since molecular cluster ion beams are expected to have various chemical effects, they are promising candidates for improving the secondary ion yield of Tof-SIMS. However, in order to clarify the effect and its mechanism, it is necessary to generate molecular cluster ion beams with various chemical properties and systematically examine it. In this study, we have established a method to stably form various molecular cluster ion beams from relatively small amounts of liquid materials for a long time by the bubbling method. Furthermore, we applied the cluster ion beams of water, methanol, methane, and benzene to the primary beam of SIMS and compared the molecular ion yields of aspartic acid. The effect of enhancing the yields of [M+H]+ ion of aspartic acid was found to be the largest for the water cluster and small for the methane and benzene clusters. These results indicate that the chemical effect contributes to the desorption/ionization process of organic molecules by the molecular cluster ion beam. Keywords: SIMS; cluster ion beam; desorption/ionization; water cluster; molecular cluster; mixed Citation: Moritani, K.; Nagata, S.; cluster Tanaka, A.; Goto, K.; Inui, N. Large Molecular Cluster Formation from Liquid Materials and Its Application to ToF-SIMS. Quantum Beam Sci. 2021, 1. Introduction 5, 10. https://doi.org/10.3390/ ToF-SIMS has become a unique surface analysis method that can detect high-mass qubs5020010 molecular ions since the large cluster ions such as argon (Ar) cluster ion beam [1] was commercialized as the primary ion of SIMS. The advantages of large cluster ion beams are Academic Editor: Klaus-Dieter Liss less damage to the organic sample during sputtering [2] and the ability to detect higher mass secondary ions [3]. Now it is expected as a powerful tool for chemical imaging of Received: 12 March 2021 Accepted: 27 April 2021 biological samples and organic materials in medical and biological fields. However, in Published: 29 April 2021 order to detect and image the distribution of small amounts of biopolymers in a sample with high sensitivity and high resolution, it is necessary to further increase the yield of
Publisher’s Note: MDPI stays neutral secondary ions. Therefore, improving the secondary ion yield is still an important issue with regard to jurisdictional claims in for SIMS. published maps and institutional affil- For the improvement of the secondary ion yield, some researchers have tried to iations. use molecular clusters [4–7] instead of inert Ar cluster or clusters mixed with highly reactive molecules [8,9] in expectation of chemical effects. Among these challenges, a water cluster [5–7] is the most successful case. It has been reported that the molecular ion yields enhanced by several tens to 1000 fold or more for some molecules. However, in order to apply the chemical imaging using SIMS to the fields of medical science and biology, Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. further increase of the molecular ion yield is required. Furthermore, the mechanism of This article is an open access article the chemical effects for desorption and ionization of organic molecules by the primary ion distributed under the terms and beams is not well understood, and then no effective method for improving the ionization conditions of the Creative Commons rate has been established yet. The reaction dynamics at the moment of cluster collision is Attribution (CC BY) license (https:// difficult to observe directly, because there are multi-body non-equilibrium processes that creativecommons.org/licenses/by/ occur in a very short time. However, systematically examining the increase or decrease in 4.0/).
Quantum Beam Sci. 2021, 5, 10. https://doi.org/10.3390/qubs5020010 https://www.mdpi.com/journal/qubs Quantum Beam Sci. 2021, 5, 10 2 of 13
secondary ion yield by colliding various molecular clusters which have different chemical properties will provide important information for elucidating the ionization mechanism. In this study, we have established a method to stably form various molecular cluster ion beams for a long time from a relatively small amount of liquid material by adopting the bubbling method. This paper describes the formation of the cluster ion beams of water, methanol, and benzene. In addition, we applied these cluster ion beams to the primary beam of ToF-SIMS and compared the molecular ion yields of aspartic acid. The chemical effect of the beam will depend on the combination of primary ions and target molecules. However, the dynamics of desorption/ionization of organic molecules by the collision of molecular clusters is still not clear, and thus it is difficult to predict their effects. The techniques for the stable formation of various types of molecular cluster ion beams enable the systematic study of the chemical effects of primary ion beams, opening the door for exploring new SIMS projectiles.
2. Experimental Details of the Ar cluster ion beam and ToF-SIMS apparatus are given elsewhere [4,10]. Briefly, the apparatus consisted of three parts: a cluster-source chamber, an ionization and cluster-size-selection chamber, and a sample chamber. All parts were differentially pumped. The base pressure of the sample chamber was 8 × 10−8 Pa. Before measuring mass spectra of ions detected under surface bombardment with cluster ions at the sample chamber, the target surfaces were etched sufficiently with the Ar cluster ion beam to remove surface contaminants. A brass conical nozzle with a 0.1 mm diameter aperture and 60 mm expansion and cooling zone was attached in the source chamber. The neutral cluster is generated by adiabatic expansion from the nozzle. The clusters were ionized by electron impact from a tungsten filament at an energy of 150 eV and then, extracted at an acceleration voltage of 5 kV. We selected the required cluster size (n) by employing a time-of-flight (ToF) technique using two ion deflectors. Here, the cluster size indicates the number of cluster constituent particles. The cluster size resolution (n/∆n) of more than ten was achieved by shortening the ion deflectors’ pulse width. The methods of the Ar [10] and methane and Kr [4] cluster beam generation is described in our previous paper. Briefly, methane gas (10% concentration) was seeded in Ar gas (90% concentration) for cooling, and the mixed gas is ejected from the nozzle. In this study, cluster ion beams of water, methanol, and benzene were generated and used as a primary ion beam of ToF-SIMS. These molecules do not expand adiabatically from the nozzle due to the low vapor pressure usually. The techniques for forming clusters from liquid materials in a vacuum could be divided into three major types: bubbling, baking, and electrospray method. The baking method requires a relatively large volume of the liquid heating reservoir. The electrospray method requires a different nozzle setup from the Ar cluster formation, such as capillaries. Thus, we adopted the bubbling method for generating the water, methanol, and benzene clusters in this study. The 100 mL of liquids (water (distilled water, Wako) and D2O (purity: 99.9%, Wako), methanol (purity: 99.8%, Wako), and benzene (purity: 99.5%, Wako)) were sealed in a stainless steel reservoir in the gas line and kept at room temperature. The liquid was bubbled and vaporized by the Ar carrier gas, and the gas mixture was adiabatically expanded from the nozzle to form clusters. The flow rate of the Ar carrier gas was controlled by a mass flow controller. The chemical compositions of the beams were estimated from the change in the component of the background gas in the sample chamber when the beam was on and off by the deflector. Since the sensitivity of QMS varies depending on the molecule due to the ionization rate, we have estimated the Ar mixing ratio per charge (CAr) of the cluster ion beams from the equation: CAr = (∆PAr-mix/Imix)/(∆PAr-Ar/IAr) × 100. (1)
Here, Imix and IAr are the ion beam currents of the cluster ion beam generated from the mixed gas and generated from the pure Ar gas. The ion beam currents were measured Quantum Beam Sci. 2021, 5, x FOR PEER REVIEW 3 of 13
CAr = (ΔPAr-mix/Imix)/(ΔPAr-Ar/IAr) × 100. (1) Quantum Beam Sci. 2021, 5, 10 3 of 13 Here, Imix and IAr are the ion beam currents of the cluster ion beam generated from the mixed gas and generated from the pure Ar gas. The ion beam currents were measured using a Faraday cup placed at the sample position. ΔPAr-mix and ΔPAr-Ar are the Ar partial pressureusing changes a Faraday when cup each placed beam at theenters sample into position.the sample∆P chamber,Ar-mix and which∆PAr-Ar wereare measured the Ar partial by thepressure quadrupole changes mass when spectrometer each beam (Q entersMS) equipped into the sample with the chamber, sample which chamber. were measured by the quadrupole mass spectrometer (QMS) equipped with the sample chamber. As a target sample for SIMS measurements, thin films of aspartic acid (Asp; C4H7NO4, molecularAs weight; a target 133.10) sample were for SIMSused. measurements,The Asp film was thin prepared films of asparticby dropping acid (Asp;20 µL C of4H an7NO 4, molecular weight; 133.10) were used. The Asp film was prepared by dropping 20 µL Asp aqueous solution (1 g/L) onto a Si substrate and freeze-drying. The film thickness of an Asp aqueous solution (1 g/L) onto a Si substrate and freeze-drying. The film measured with a stylus-type profiler (DekTak-3, Veeco) was 1–4 µm. Samples were not thickness measured with a stylus-type profiler (DekTak-3, Veeco) was 1–4 µm. Samples prepared under controlled conditions such as in a clean room, so it is necessary to consider were not prepared under controlled conditions such as in a clean room, so it is necessary to the contamination of the laboratory air and container materials. In this experiment, typical consider the contamination of the laboratory air and container materials. In this experiment, contaminants such as Na and polydimethylsiloxane (PDMS) were detected by SIMS. typical contaminants such as Na and polydimethylsiloxane (PDMS) were detected by SIMS. Therefore, in order to remove these contaminants, the experiments were performed after Therefore, in order to remove these contaminants, the experiments were performed after irradiating Ar cluster ions with 1.0 × 1013 ions/cm2 at an accelerating voltage of 5 kV and irradiating Ar cluster ions with 1.0 × 1013 ions/cm2 at an accelerating voltage of 5 kV and sputtering the sample surface before SIMS measurement. Consequently, almost all the sputtering the sample surface before SIMS measurement. Consequently, almost all the contaminants by PDMS have been removed by cluster sputtering, but it was difficult to contaminants by PDMS have been removed by cluster sputtering, but it was difficult to remove Na contamination. remove Na contamination.
3. Results3. Results and Discussion and Discussion FigureFigure 1 shows1 shows the background the background intensity intensity of the of mass the mass peaks peaks at m/z at ofm/z 18of and 18 40 and in 40 the in the samplesample chamber chamber when when the Ar the cluster Ar cluster ion beam ion beam entered entered (on) (on)and was and wasshut shutoff (off) off (off)by the by the deflector.deflector. The flow The flowrate of rate Ar of gas Ar was gas was260 sccm. 260 sccm. Only Only the partial the partial pressure pressure of Ar of (m/z Ar (=m/z 40)= 40) changeschanges with withthe beam the beam on and on off. and off.
FigureFigure 1. The 1. backgroundThe background intensity intensity of the of m/z the =m/z 18 and= 18 40 and mass 40 masspeaks peaks in the in sample the sample chamber chamber when when the Arthe cluster Ar cluster ion beams ion beams enter enter(on) an (on)d are and shut are shutoff (off) off by (off) the by deflector. the deflector.
FigureFigure 2 shows2 shows when when the water the water cluster cluster ion ionbeams beams are on are and on andoff. off.Ar flow Ar flow rates rates are are 180 (a)180 and (a) and280 (b) 280 sccm. (b) sccm. In Figure In Figure 2a, H2a,2O H increases2O increases and anddecreases decreases when when the beam the beam is is turnedturned on and on off, and but off, Ar but does Ar doesnot change, not change, which which indicates indicates that pure that pure water water clusters clusters were were generatedgenerated despite despite using using mixed mixed gas. gas.On the On other the other hand, hand, in Figure in Figure 2b, both2b, both H2O H and2O andAr Ar increaseincrease and decrease and decrease simultaneously simultaneously with with the beam the beam on and on off. and This off. Thisindicates indicates that water that water and Arand are Ar included are included in the in cluster the cluster ion beam ion beam at a athigher a higher Ar flow Ar flow rate. rate.The TheAr content Ar content of of each eachcluster cluster was wascalculated calculated according according to Equa to Equationtion (1). (1). The The ArAr content content was was 0% 0% when when the the Ar Ar flowflow rate rate was was below below 180 180 sccm. sccm. As As the the Ar Ar flow flow rate rate increased increased further further above above 190 190 sccm, sccm, the the ArAr content content increased, increased, reaching reaching 76% 76% at 280 sccm. We performedperformed thethe samesame experimentexperiment with with methanolmethanol andand calculatedcalculated thethe ArAr contentcontent inin thethe methanolmethanol clustercluster ionion beams.beams. Figure3 shows the Ar and Ne flow rate dependence of the Ar content in the cluster beam (a) and ion beam current (b) for water and methanol cluster generated using Ar or Ne carrier gas. In the case of the water cluster, pure water cluster beams are generated below the Ar flow rate of 180 sccm, and mixed cluster beams of water and Ar are generated above 190 sccm. In the case of methanol, pure methanol cluster beams are formed below Quantum Beam Sci. 2021, 5, 10 4 of 13
220 sccm. As shown in these figures, the content of Ar in the cluster beams increases with increasing the Ar flow rate. The beam current decreases once Ar begins to mix into the cluster and then increases again. Although the adiabatic expansion of gas flow should generate a cluster, a theory to predict cluster formation in gas jets has yet to be established. However, an empirical scaling parameter Γ* can describe the condition to produce a cluster [11,12]. The average cluster size
Figure 2. FigureThe background 2. The background intensity of intensity the m/z = of 18 the andm/z 40= mass 18 and peaks 40 massin the peaks sample in chamber the sample when chamber the water when cluster ion Figure 2. The background intensity of the m/z = 18 and 40 mass peaks in the sample chamber when the water cluster ion beams enterthe water(on) and cluster are shut ion beamsoff (off) enter by the (on) deflector. and are Ar shut flow off rates (off) are by the180 deflector.(a) and 280 Ar (b flow) sccm. rates Data are in 180 (a) ( aare) taken beams enterfrom (on) Ref. and[15]. are shut off (off) by the deflector. Ar flow rates are 180 (a) and 280 (b) sccm. Data in (a) are taken b a from Ref. [15]. and 280 ( ) sccm. Data in ( ) are taken from Ref. [15].
Figure 3. Ar/Ne flow rate dependence of Ar/Ne concentration per charge (a) and the beam current (b) for the water and the methanol cluster ion beams. Data for water clusters are taken from Ref. [15]. Figure 3. Ar/Ne flowFigure rate 3. dependenceAr/Ne flow of rate Ar/Ne dependence concentration of Ar/Ne per concentrationcharge (a) and per the charge beam (currenta) and the(b) beamfor the current water and the methanol cluster(b) forion the beams. water Data and for theIn water methanol order clusters to clustergenerate are iontaken a beams. higher-purity from Data Ref. for[15]. waterwater clusters cluster, are it should taken from be better Ref. [15 to]. use Ne as a carrier gas. However, the advantage of using Ar gas is that Ar is inexpensive and the Figure4 showsInmixing order the toratio typical generate can size be adjusteda distribution higher-purity by changing of water,water the methanol,cl uster,flow rate. it should and Therefore, benzene be better Ar clusters was to useduse Ne as carrieras a generatedcarrier at an gas Argas. flowin However, this rate study. of 180the The advantage sccm. advantage The distributionof of using the bubbl Ar curvesgasing methodis that are normalizedAr is thatis inexpensive a cluster at the beam and can the be peak top.mixing The ngenerated meansratio can the stablybe number adjusted for ofa long constituentby changing time with molecules the a smalflowl rate. ofamount a cluster.Therefore, of liquid The Ardistribution material. was used We as have carrier con- of n was obtainedgas in firmedthis by study. converting that 100The mL advantage the of m/zdistilleddistribution of water the bubblcan measured stabingly methodgenerate using isthe that water ToF a method.cluster cluster ionbeam As beam can totally be shown ingenerated this figure,for far stably nmore= 570–2350for than a longseveral for time water,tens with hours 990–3450 a smalat thel forAramount flow methanol, rate of liquid of and180 sccm.material. 1100–3900 We for have con- benzene withfirmed a full that width Figure100 mL at 4 halfofshows distilled maximum the typical water (FWHM), sizecan stabdistribution respectively.ly generate of water, the The water methanol, cluster cluster size and ion can benzene beam be totally clusters adjustedfor by changingfar generatedmore than the temperatureat several an Ar tensflow of hoursrate the of liquidat 180 the sccm. reservoirAr flow The rate distribution and of gas 180 line. sccm. curves Heating are abovenormalized at the 388 K increasedFigurepeak cluster top.4 shows size The and nthe means beam typical the current. size number distribution On of the constitu other of hand,entwater, molecules the methanol, liquid of a material andcluster. benzene wasThe distribution clusters consumedgenerated in a shortof n atwas time. an obtained Ar In flow this by study,rate converting of the180 gas sccm. the line m/z Th was distributione distribution kept atroom measured curves temperature, usingare normalized the andToF method. at the As shown in this figure, n = 570–2350 for water, 990–3450 for methanol, and 1100–3900 for clusters werepeak stably top. The formed n means for an the extended number period.of constituent molecules of a cluster. The distribution benzene with a full width at half maximum (FWHM), respectively. The cluster size can be of n was obtained by converting the m/z distribution measured using the ToF method. As adjusted by changing the temperature of the liquid reservoir and gas line. Heating above shown in this figure, n = 570–2350 for water, 990–3450 for methanol, and 1100–3900 for benzene with a full width at half maximum (FWHM), respectively. The cluster size can be adjusted by changing the temperature of the liquid reservoir and gas line. Heating above
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388 K increased cluster size and beam current. On the other hand, the liquid material was Quantum Beam Sci. 2021, 5, 10 consumed in a short time. In this study, the gas line was kept at room temperature, 6and of 13 clusters were stably formed for an extended period.
FigureFigure 4. 4. SizeSize (n ()n )distribution distribution of of water, water, methanol, methanol, and and be benzenenzene cluster cluster ion ion beams beams at at an an Ar Ar flow flow rate rateof 180of 180 sccm sccm before before size size selection. selection.
FigureFigure 5 shows thethe massmass spectra spectra of of dissociative dissociative cluster cluster ions ions scattered scattered from from a silver a silver (Ag) sample under bombardment with the size-selected water (D2O) cluster2 ions at an incident (Ag) sample◦ under bombardment with the size-selected water (D O) cluster ions at an incidentangle of angle 45 at of the 45°n =at 900 the(energy n = 900 per(energy molecule; per molecule;E/n = 5.6 E eV)./n = A 5.6 series eV).of A peaksseriesassociated of peaks associatedwith the decomposed with the decomposed water cluster water ions cluster can be ions observed can be inobserved the mass in spectra. the mass A spectra. periodic + series of the mass peaks [(D O)n+D] indicates that the clusters decompose during the A periodic series of the mass peaks2 [(D2O)n+D]+ indicates that the clusters decompose dur- ingsurface the surface collisions, collisions, however, however, the molecules the molecules themselves themselves do not do dissociate. not dissociate. At higher At higherE/n , Ethe/n, the clusters clusters were were broken broken down down into into smaller smaller clusters, clusters, but but the the molecules molecules themselves themselves did not dissociate even at the E/n = 10.0 eV. The detected multimer ions of D2O molecules did not dissociate even at the E/n = 10.0 eV. The detected multimer ions of D2O molecules are attached with a deuterium atom. Since it is difficult to remove hydrogen (H2) from are attached with a deuterium atom. Since it is difficult to remove hydrogen (H2) from residual gas in a vacuum, the main component of the residual gas is H2 in our sample residual gas in a vacuum, the main component of the residual gas is H2 in our sample chamber. Thus, it is considered that a certain amount of hydrogen atom (H) is adsorbed chamber. Thus, it is considered that a+ certain amount of hydrogen atom (H) is adsorbed on the Ag surface. The [(D2O)n+D] indicate that the protons supply from inside of the on the Ag surface. The [(D2O)n+D]+ indicate that the protons supply from inside of the cluster, not from the target surface, in the ionization reaction caused by cluster collision cluster, not from the target surface, in the ionization reaction caused by cluster collision and decomposition. As E/n increased, a shift towards smaller ions was observed in the and decomposition. As E/n increased, a shift towards smaller ions was observed in the size distribution of the dissociated components of the cluster. The dissociation rate of the size distribution of the dissociated components of the cluster. The dissociation rate of the cluster ions is related to the impulsive stress caused by the collision of the cluster ions with cluster ions is related to the impulsive stress caused by the collision of the cluster ions the surface [16,17]. with the surface [16,17].
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Figure 5. Mass spectra of Figureions detected 5. Mass from spectra Ag ofsample ions detected under bombardment from Ag sample with under the bombardmentwater cluster (D with2O)900 the+ ion water at clusteran + ◦ incident angle of 45°. (D2O)900 ion at an incident angle of 45 .
FigureFigure6 shows6 shows the the mass mass spectra spectra of ionsof ions scattered scattered from from the Ag the surface Ag surface under under bombard- bom- mentbardment with purewith waterpure water (H2O) (H (a)2O) and (a) water/Ar and water/Ar mixed mixed (b and (b c) and cluster c) cluster ions. Theions. selected The se- clusterlected cluster mass is mass 45,000 is 45,000 Da, so Da,n = so 2500 n = 2500 and Eand/n E=/n 2.0 = 2.0 eV eV for for the the pure pure H 2HO2O cluster. cluster. The The verticalvertical axis axis is is the the ion ion intensity, intensity, representing representing the the detected detected ion ion count count divided divided by the by incident the inci- iondent current. ion current. Since theSince E/ nthewas E/n lower was thanlower the than case the of case Figure of5 Figure, the degree 5, the of degree dissociation of dissoci- of theation water of the cluster water is cluster small. Figureis small.6b,c Figure shows 6b the,c shows case of the a mixedcase of cluster a mixed of cluster water andof water Ar. Theand Ar Ar. mixing The Ar ratio mixing is 46 ratio and 76%,is 46 and respectively. 76%, respectively. The energy The per energy projectile per massprojectile (E/m mass) is 0.11(E/m eV/amu) is 0.11 eV/amu for all cases for all of Figurecases of6a–c. Figure 6a–c. Here,Here, wewe consider consider that that the the H H2O/Ar2O/Ar mixedmixed clustercluster beamsbeams areare mainly mainly composed composed of of ArArn(Hn(H22O)O)n’n’ clusters. For the mixed cluster (b,(b, c),c), thethe scatteredscattered fragmentsfragments of of water water molecules molecules areare different different from from those those for for the the H H2O2O cluster cluster (a). (a). The The notable notable difference difference is is that that as as the the mixing mixing ratio of Ar increases, the proportion of the smaller water cluster (H O+, [(H O) +H]+, ratio of Ar increases, the proportion of the smaller water cluster 3(H3O+, [(H2 2O)2 2+H]+, [(H O) +H]+) increases. Besides, fragments such as [H O+OH]+ are detected in Figure6b,c. [(H2 2O)33+H]+) increases. Besides, fragments such as [H22O+OH]+ are detected in Figure 6b,c. These fragments are produced with the breakage of H O intramolecular bonds. In these These fragments are produced with the breakage of H2 2O intramolecular bonds. In these 2 experiments,experiments, the the current current density density of of the the beams beams are are about about a fewa few nA/cm nA/cm,2, so so the the possibilities possibilities ofof two two clusters clusters colliding colliding at at the the same same site site on on the the surface surface simultaneously simultaneously are are negligible. negligible. The change in the dissociative scattering spectrum due to the Ar mixture ratio differ- The change in the dissociative scattering spectrum due to the Ar mixture ratio differ- ence can be interpreted consistently by the change in the impact stress when the mixed ence can be interpreted consistently by the change in the impact stress when the mixed clusters collide. The dissociation rate in the dissociation scattering spectrum depends on clusters collide. The dissociation rate in the dissociation scattering spectrum depends on the impact stress acting on the contact region [16] The impact stress should be smaller for the impact stress acting on the contact region [16] The impact stress should be smaller for the water clusters than for the Ar clusters because a part of the collision energy is consumed the water clusters than for the Ar clusters because a part of the collision energy is con- to excite rotation and vibration inside the water molecule. For the Arn(H2O)n’ cluster, Ar sumed to excite rotation and vibration inside the water molecule. For the Arn(H2O)n’ clus- atoms, which do not have the degree of freedom of vibration and rotation, attack H O ter, Ar atoms, which do not have the degree of freedom of vibration and rotation, attack2 molecules at the collision time, so the dissociation of the cluster and the molecule may have H2O molecules at the collision time, so the dissociation of the cluster and the molecule been promoted. may have been promoted. It is difficult to exclude the possibility that the mixed cluster beam contains Arn It is difficult to exclude the+ possibilit+ y that the mixed cluster beam contains Arn and and (H2O)n. The peaks of Ar2 and Ar3 are also detected in Figure6b,c. However, we (H2O)n. The peaks of Ar2+ and Ar3+ are also detected in Figure 6b,c.+ However,+ we can con-+ can consider the composition of the cluster from the ratio of Ar2 /Ar3 . When Ar1500 sider the composition of the cluster from the ratio of Ar2+/Ar+ 3+. When+ Ar+1500+ (60,000+ Da) is (60,000 Da) is scattered on the Ag surface, the ratio of Ar2 to Ar3 (Ar2 /Ar3 ) is about + + + + 0.47scattered at a collision on the energyAg surface, of 5 keV the [ 16ratio]. Since of Ar the2 to dissociation Ar3 (Ar2 /Ar rate3 ) increases is about with0.47 theat a incidentcollision energy of 5 keV [16]. Since the dissociation+ rate+ increases with the incident velocity of the velocity of the cluster, the values of Ar2 /Ar3 should be larger than 0.47 when assuming cluster,+ the values+ of Ar2+/Ar3+ should be larger than 0.47 when+ assuming that Ar2+ and that Ar2 and Ar3 in Figure6b,c are all generated from Ar1125 (45,000 Da). In Figure6b,c, Ar3+ in Figure 6b,c+ are +all generated from Ar1125+ (45,000 Da). In Figure 6b,c, the values of the values of Ar2 /Ar3 are 0.26 and 0.43, respectively, indicating that they are mainly generated from Arn(H2O)n’.
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Quantum Beam Sci. 2021, 5, 10 8 of 13 Ar2+/Ar3+ are 0.26 and 0.43, respectively, indicating that they are mainly generated from Arn(H2O)n’.
+ Figure 6. Mass spectra of ions detected from Ag sample under bombardment with the water cluster (H2O)2500 ion (a) and Figure 6. Mass spectra of ions detected from Ag sample under bombardment with the water cluster (H2O)2500+ ion (a) and water/Ar mixed cluster ions (b,c). The selected mass of the clusters are 45,000 Da and E/m is 0.11 eV/amu. water/Ar mixed cluster ions (b,c). The selected mass of the clusters are 45,000 Da and E/m is 0.11 eV/amu. + In the mass spectrum for the mixed clusters, the peak intensities of the [(H2O)n+H] In the mass spectrum for the mixed+ clusters, the peak intensities of the [(H2O)n+H]+ ions are higher than those of Arn , even though Ar content of 76% in a cluster. This + ionsmay are be higher due tothan the those difference of Arn in, even ionization though energiesAr content of of Ar 76% and in water a cluster. molecules. This may The beionization due to the energydifference of Ar in clusterionization is ~15 energi eV [es13 ],of but Ar ifand the water water molecules. molecules The were ionization ionized by energythe protonation of Ar cluster due is ~15 to recombination eV [13], but if ofthe hydrogen water molecules bonds in were a cluster, ionized it wouldby the ionizeproto- at nationlower due energies to recombination than Ar. Thus, of hydrogen the water bonds molecules in a cluster, may have it would been preferentially ionize at lower ionized en- ergiesduring than the Ar. collision. Thus, the water molecules may have been preferentially ionized during the collision.To investigate the irradiation effect of molecular cluster ions on the desorption and ionizationTo investigate of organic the irradiation materials, weeffect measured of molecular the SIMS cluster spectrum ions on using the desorption the various and types ionizationof cluster of ions organic as primary materials, ions. we We measured used thin the films SIMS of spectrum the Asp molecule using the (M) various as the types target of samplecluster ions and comparedas primary the ions. proton-attached We used thin films molecular of the ion Asp [M+H] molecule+ intensity (M) as when the target Ar, Kr, samplewater, and methanol, compared methane, the proton-attached and benzene molecular were used ion as [M+H] the primary+ intensity ion. when The Ar, selected Kr, water,cluster methanol, size n is methane, 1500 for and all beams. benzene Figure were7 used shows as the primary relative intensitiesion. The selected of the cluster [M+H] + sizeion n is from 1500 Asp for thinall beams. films obtainedFigure 7 shows withthe the variousrelative primaryintensities ion of beamthe [M+H] irradiation.+ ion from The Aspvertical thin films axis obtained is normalized with bythe thevarious intensity primary for theion Arbeam cluster irradiation. irradiation. The vertical A remarkable axis is normalizedenhancement by of the molecular intensity ionfor intensitythe Ar cluster was observedirradiation. for A the remarkable water and enhancement the methanol + of clustermolecular ion ion beams intensity compared was observed to the Ar. for For the thesewater beams, and the peaks methanol of [M+(H cluster2O) ionn+H] beamsand + + + + compared[M+(CH 3toOH) then +H]Ar. Forwere these detected beams, in additionpeaks of to [M+(H [M+H]2O),n respectively+H] and [M+(CH [15]. Such3OH) behaviorn+H] wereof moleculardetected in ions addition has been to reported[M+H]+, respectively in the previous [15]. study Such of behavior water cluster of molecular ion beam ions [5, 6] hasand been neutral reported SO2 incluster the previous beam [18 study]. On of the water other cluster hand, ion under beam the [5,6] methane and neutral and benzene SO2 + clustercluster beam ion [18]. bombardments, On the other no hand, progression under the of themethane [M+H] andpeaks benzene with cluster the CH ion4 or bom- C6H6 + bardments,attachment no and progression also no enhancement of the [M+H] of+ peaks [M+H] withwere the CH observed4 or C6H in6 theattachment spectrum. and also no enhancement of [M+H]+ were observed in the spectrum.
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14
12 11.58
10
8
6 4.69 4 3.66 Relative intensity (arb. unit) (arb. intensity Relative 1.87 2 1 0.87 0.47 0.13 0 Ar Kr Benzene Methane Methanol Water Water/Ar (46%) Water/Ar (76%)
Primary ion beams
++ 1500++ 1500++ FigureFigure 7. 7. RelativeRelative intensities intensities of molecular ionion forfor asparticaspartic acid acid ([M+H] ([M+H],,m/z m/z= = 134134 ) ) obtained obtained with with Ar Ar1500 ,, KrKr1500 , ,benzene benzene (C6H6)1500+, methane+ (CH4)1500+, methanol+ (CH3OH)1500+, water+ (H2O)1500+, and+ water (H2O)/Ar mixed clusters primary ion (C6H6)1500 , methane (CH4)1500 , methanol (CH3OH)1500 , water (H2O)1500 , and water (H2O)/Ar mixed clusters primary beams. The acceleration voltage of the clusters is 5 kV. The selective mass of the mixed cluster is 27,000 Da, which is the ion beams. The acceleration voltage of the clusters is 5 kV. The selective mass of the mixed cluster is 27,000 Da, which is the + same as the (H2O)1500 cluster+ at m/z. Each intensity is normalized by the intensity for the Ar cluster irradiation. same as the (H2O)1500 cluster at m/z. Each intensity is normalized by the intensity for the Ar cluster irradiation.
TheThe energy energy per per projectile projectile mass mass (i.e., (i.e., substantia substantiallylly velocity) velocity) is is known known to to be be one one of of the the physicalphysical parameters parameters affecting thethe sputteringsputtering processprocess and and thus thus the the sputtering sputtering yields yields [19 [19––21]. 21].The The molecular molecular clusters clusters used used in this in experimentthis experiment have have the same the same cluster cluster size and size total and energy total energy(5 keV), (5 but keV), have but different have different masses duemasses to the du differente to the different masses of masses the constituent of the constituent molecules, molecules,and therefore and different thereforeE/m. differentBefore E/m. discussing Before the discussing chemical the effects chemical caused effects by the caused molecules by theof themolecules cluster, of we the consider cluster, thewe co effectsnsider of the cluster effects mass of clusterm. Comparing mass m. Comparing the E/m [eV/amu] the E/m [eV/amu]values for values each for projectile, each projectile, Ar (0.083) Ar and(0.083) methanol and methanol (0.10), Kr(0.10), (0.40) Kr and(0.40) benzene and benzene (0.43), (0.43),and methane and methane (0.21) (0.21) and water and water (0.19) (0.19) are very are close,very close, respectively. respectively. Furthermore, Furthermore, water wa- and terwater/Ar and water/Ar mixed mixed clusters clusters have have equal equalE/m Evalues./m values. Comparing Comparing the the relative relative intensities intensities of of[M+H] [M+H]+ +when when using using similar similarE E//m projectilesprojectiles shows that E/mm cannotcannot consistently consistently explain explain thethe signal signal enhancement enhancement due due to to the the molecular molecular clusters clusters in inFigure Figure 7.7 For. For instance, instance, the the E/mE /ofm methaneof methane and andwater water clusters clusters are about are aboutthe same, the bu same,t the signal but the intensities signal intensities are very different. are very (Relativedifferent. intensity (Relative is intensity0.87 for methane is 0.87 forbut methane 11.6 for water.) but 11.6 For for benzene water.) and For krypton, benzene andthe Ekrypton,/m is about the theE/ samem is about (slightly the smaller same (slightly for Kr), smallerbut the signal for Kr), intensity but the is signal much intensity lower for is benzenemuch lower (0.13) for than benzene for Kr(0.47). (0.13) than Even for if Kr(0.47). E/m is larger Even ifthanE/m Ar,is largerthe relative than Ar, intensity the relative in- creasesintensity like increases water and like methanol water and or methanol decrease ors like decreases methane. like Furthermore, methane. Furthermore, when compar- when ingcomparing water and water water/Ar and water/Ar clusters, clusters,the signal the intensity signal intensity decreased decreased as the Ar as themixture Ar mixture ratio increases,ratio increases, even though even though the E/m the is Ethe/m same.is the We same. cannot We find cannot a consistent find a consistent tendency tendency due to Edue/m in to theE/ signalm in the intensity signal betw intensityeen these between data. theseThe effect data. of The E/m effect seems of toE /bem significantlyseems to be smallsignificantly compared small to the compared chemical to effect, the chemical at least effect,in the atenergy least and in the mass energy range and in massFigure range 7. in FigureA possible7. factor to explain signal enhancement in Figure 7 is the polarity of mole- cules. Molecular dynamics simulations by P. Schneider [22] suggest that polar clusters
Quantum Beam Sci. 2021, 5, 10 10 of 13
A possible factor to explain signal enhancement in Figure7 is the polarity of molecules. Molecular dynamics simulations by P. Schneider [22] suggest that polar clusters increase Quantum Beam Sci. 2021, 5, x FOR PEERthe REVIEW probability of spatter by attracting internal target molecules. This10 of mechanism13 may
be an essential point to unravel the signal enhancement by molecular cluster projectiles. However, since the secondary ion yield is not comparable to the sputtering yield, including increaseneutral the particles, probability it of is spatter unclear by whetherattracting internal the effect target can molecules. explain This all the mechanism changes in secondary mayion be yield an essential in Figure point7. to unravel the signal enhancement by molecular cluster projec- tiles. However,In terms since of ionization the secondary probability, ion yield is the not strongly comparable polar to the water sputtering and methanol yield, molecules including neutral particles, it is unclear whether the effect can explain all the changes in may promote protonation to the similarly polar molecule like Asp. The mechanism, secondary ion yield in Figure 7. however,In termsdoes of ionization not seem probability, to be simple. the strongly Figure polar8 shows water theand SIMS methanol spectra molecules for an Asp sample + maymeasured promote protonation using (D2O) to 1500the similarlyas the pola primaryr molecule ion. like The Asp. protonated The mechanism, molecular how- ion of Asp is ever,detected, does not but seem the to attached be simple. protons Figure 8 areshows mainly the SIMS H instead spectra for of D.an Asp Protonated sample ions may be + measuredformed using by reaction (D2O)1500 with as the water primary dissolved ion. The in protonated the sample molecular [23]. However, ion of Asp the is Dde-2O cluster beam tected,was irradiatedbut the attached below protons the are static mainly limit H in instead this of experiment, D. Protonated and ions wemay have be formed confirmed that no by[M+D]+ reaction with species water are dissolved detected in inthe the sample samples [23]. However, after D Othe irradiation D2O cluster bybeam SIMS was measurements irradiated below the static limit in this experiment, and we have2 confirmed that no [M+D]+ using Ar cluster projectiles. In addition to it, we performed SIMS measurements using species are detected in the samples after D2O irradiation by SIMS measurements using Ar −5 clusterAr clusters projectiles. projectile In addition in a to D it,2O we atmosphere performed SIMS (7.3 measurements× 10 Pa) and using confirmed Ar clusters that no [M+D]+ + projectilespecies in were a D2O detected. atmosphere [M+D](7.3 × 10−5 speciesPa) and confirmed may not that have no [M+D]+ been caused species were by D 2O dissolved detected.in the [M+D] sample+ species from may the not beam, have butbeen rathercaused by by D the2O dissolved transient in the mixing sample and from H/D exchange the beam, but rather by the transient mixing and H/D exchange reaction between H2O reaction between H2O contained in the sample, the target molecule, and the D2O cluster containedduring in the the cluster sample, collision.the target molecule, We have and also the D observed2O cluster during the enhancement the cluster colli- of intensities of sion. We have also observed the enhancement of intensities of fragment ions without pro- fragment ions without protons attachment [15]. In order to elucidate the mechanism of tons attachment [15]. In order to elucidate the mechanism of desorption/ionization of or- ganicdesorption/ionization molecules by the molecular of organic cluster molecules collision, byit is the necessary molecular to measure cluster collision,both the it is necessary sputteringto measure rate bothand the the secondary sputtering ion yield, rate andwhich the is secondarya future subject. ion yield, which is a future subject.
+ Figure 8. SIMS spectra of an Asp (m/z = 133 for M) film by the bombardment of (D2O)1500 at an acceleration voltage of 5 kV. The ion beam fluence is 2.0 × 1011 ions/cm2. Quantum Beam Sci. 2021, 5, 10 11 of 13
4. Conclusions We generated cluster ion beams of water, methanol, and benzene using the bubbling method and irradiated these molecular cluster ion beams to the silver and Asp thin film samples. The mass spectrum of the scattered ions and the secondary ions emitted from these samples were measured. The mass spectrum of the scattered ions provided information on the nature of the molecular clusters and the reaction upon collision with the surface. First, the water clusters decompose when they collide onto the surface, but the molecules themselves do not dissociate even at E/m = 0.56 eV (E/n = 10.0 eV). However, when mixed with Ar, the water molecules are decomposed even at E/m = 0.11 eV. This may be because Ar atoms attack water molecules as the clusters collide onto the surface and collapse. The dissociated ions of the water cluster are protonated, and the protons are supplied from inside the cluster. On the other hand, the SIMS spectrum of the Asp sample shows that the protons are mainly + supplied to Asp from outside the cluster. Experiments using (D2O)n may provide a key to elucidating the dynamics of organic molecular protonation by molecular cluster irradiation. From a result of the SIMS measurement of Asp molecules, remarkable enhancement of [M+H]+ ion intensity was observed in the water and the methanol clusters. On the other hand, no enhancement was observed for methane and benzene clusters. At this time, the detailed mechanism of secondary ion formation is still unknown, but systematic studies using various types of molecular cluster ions with different chemical properties may reveal the basic mechanism of secondary ion formation.
Author Contributions: K.M. wrote the manuscript. K.M., S.N., A.T. and K.G. performed the experi- ments and the data analysis. K.M. supervised the research. N.I. reviewed and edited the manuscript. All authors have read and agreed to the published version of the manuscript. Funding: This work was supported by JSPS KAKENHI Grant Number JP16K05821. Data Availability Statement: The data presented in this study are available on request from the corresponding author. Acknowledgments: The authors would like to thank Masanori Kanai for the assistance with the experiments. The authors also appreciate Noriaki Toyoda for his help in measuring film thickness of the target samples. Conflicts of Interest: The authors declare no conflict of interest.
Appendix A We measured the change in the background pressure of water in the irradiation chamber during beam irradiation using Ar and He as the carrier gas, respectively. As a result, the amount of water entering the irradiation chamber was almost the same at a carrier gas flow rate of 260 sccm, suggesting that the He carrier gas also takes in enough water. Besides, the size distribution of water clusters was measured when the carrier gas was a mixture of Ar and He. Figure A1 shows the size distribution of water clusters generated using Ar and Ar/He carrier gases. The size distribution of the water cluster shifts to the smaller side by mixing He with the Ar carrier gas. Furthermore, when the Ar/He mixing ratio was varied, the cluster size became smaller as the He mixing ratio increased. These results indicate that He is inhibiting the clustering of water molecules. He, which has a mass close to that of hydrogen, may excite the vibrations of hydrogen atoms by collisions, and, as a result, the internal energy of water molecules is not sufficiently reduced. QuantumQuantum Beam Beam Sci. Sci.2021 2021, ,5 5,, 10 x FOR PEER REVIEW 1212 ofof 1313
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