Conference-ALC ’ 15-Crystal Structure and Accelerated Ion-Irradiation Effect of Water Clusters

We obtained HEED patterns for water cluster beams, which showed the cubic structure that is characteristic of ice. The intensity of the (111), (220), and (311) diffraction peaks increased with an increase in the vapor pressure. As an extension of the water cluster studies, a methane hydrate cluster was generated by a gas bubbling method. In addition, the fundamental phenomenon of water cluster ion irradiation was studied using photoluminescence measurements. The cluster ion beam-induced luminescence was observed, and the luminescence intensity increased with an increase in the acceleration voltage. This indicated that the kinetic energy was converted to thermal energy and the cluster temperatures as well as impact area could be very high. Furthermore, polymer substrates such as poly(methyl methacrylate) (PMMA), polyethylene terephthalate (PET), and polycarbonate (PC) were also subjected to irradiation by the water cluster ion beams, and the sputtered depth increased with an increase in the acceleration voltage. In particular, the sputtering yield of the PMMA surface was 206 molecules per ion at an acceleration voltage of 9 kV. Compared to the PET and PC surfaces, extremely high sputtering yield was obtained owing to the chemical modification of the PMMA surface by water cluster ion irradiation. [DOI: 10.1380/ejssnt.2016.144]


I. INTRODUCTION
Liquid water exhibits different properties depending on the way water molecules assemble.Experimental and theoretical studies have been carried out to investigate clusters of water molecules that form the microscopic structure of liquid water [1][2][3][4].Two states are proposed for the structure of liquid water.One of them is the molecular state, which is amorphous and has relatively low density.The other state is the cluster state, which has a three-dimensional structure consisting of hydrogen bonding bridge networks between single water molecules.The properties of liquid water at the cluster state are not well known as they are influenced by both the liquid density as well as the network structure [5][6][7].In addition, it is hard to distinguish between the molecular and cluster states because water in liquid form is unstable.
A water cluster is defined as an aggregation of water molecules that occurs as an isolated particle with a diameter of a few nanometers.Studies on other isolated particles, e.g.aerosol particles, have revealed the microscopic aspects of nucleation phenomena [8,9].The formation of water clusters has been studied based on nucleation growth, and these clusters reveal the initialformation state of liquid water.This area has attracted much interest because the physical and chemical properties of the clusters are different from those of the bulk state [10,11].Furthermore, a water cluster is the polyatomic cluster forming the multiple structures of atoms, and radicals such as the hydroxyl radical can be found in the structure.In general, these radicals play an important role in the chemical modification of materials.
The water cluster ion beam process has several advantages for surface treatment.One of these is that equivalently low-energy and high-current ion beams can be obtained using cluster ion beams.Another advantage is that high energy density deposition and the collective motions of cluster atoms are available [12,13].As thousands of molecules impact the target at almost the same time, many-body interactions between the clusters and target atoms are induced during dense energy deposition.As a result, it is possible to achieve extremely high temperatures on the impact area by accelerating the cluster ion beams [14,15].In addition, distinctive irradiation effects, which are not achievable by a conventional ion beam process, could be obtained with water cluster ion beams.For example, it is difficult to induce sputtering on a polymer substrate using monomer ion beams.This is because the highly energetic ions cause the bonds between the monomer molecules to break, thus resulting in polymerization.On the other hand, the water cluster ion irradiation is expected to induce sputtering in the polymer substrate to give monomer molecules via thermal evaporation, which then results in high sputtering yields.
In this paper, the atomic level characterizations for the structure of water clusters, as well as elucidation of the irradiation effects of water cluster ions, are performed in order to shed light on the unique properties of water cluster ions.The high-energy electron diffraction (HEED) patterns for water cluster beams are obtained, and the crystal structure of a water cluster is analyzed.As an extension of the water cluster study, methane hydrate clusters are generated.Furthermore, the luminescence induced by the cluster ion beam is measured using highly sensitive photomultipliers to investigate the distinctive irradiation effects of the water cluster ions.

II. EXPERIMENTAL
The experimental apparatus is described in detail in the literatures [16,17].Pure water was placed in a metal box and heated to 150 • C by a wire heater attached around the source.The typical source temperature was 134 • C, which corresponded to a vapor pressure of 0.3 MPa.Water vapor was ejected through a nozzle into a vacuum region, and the clusters were produced by adiabatic expansion.The cluster size was measured by the time-of-flight (TOF) method.The peak size of water clusters increased with an increase in vapor pressure, reaching approximately 3000 molecules at a pressure of 0.3 MPa [18].
The generated clusters entered the ionizer after passing through a skimmer and a collimator.In the ionizer, the neutral clusters were ionized by electron bombardment.The electron voltage for ionization (V e ) was adjusted to be between 0 and 300 V, and the electron current for ionization (I e ) was adjusted to be between 0 and 300 mA.The cluster ions were extracted by applying an extraction voltage, and the extracted ions were separated according to size using a retardation potential.Cluster ions with sizes larger than 100 water molecules were accelerated towards a substrate on a substrate holder.The acceleration voltage (V a ) was adjusted to be between 0 and 10 kV.The substrates used were Si(100), SiO 2 , Au, Cu, and polymer surfaces.The ion current during the cluster ion irradiation on the substrate was measured using a Faraday cup.The background pressure around the substrate was 1.33×10 −5 Pa, which was attained using a turbo-molecular pump.

A. Structure of water clusters
The HEED patterns of water cluster beams were obtained using an electron gun (ULVAC-PHI Corp.: Type of MB1000) installed in the cluster beam system.The electron beam emitted from the electron gun was focused by a condenser lens, and it crossed the neutral cluster beam at a point 40 cm from the exit of skimmer.The primary electron beam was stopped by a beam trap made of a graphite rod with 11 mm diameter and 45 mm length.The energy and current of the electron beam were adjusted to 10 kV and 20 µA, respectively.The diffraction pattern for the cluster beam was recorded on a fluorescent screen.A digital camera (Cannon: EOS 5D Mark II) was used to observe the pattern on the screen.The relative aperture of the camera was set at F4, and the camera sensitivity was ISO1600.The exposure time of the camera was adjusted to 8 min based on the stable and clear observation.
Figure 1  most the same at different vapor pressures.The peak intensity increases with an increase in the vapor pressure.The average distance between the adjacent (111) planes is 0.333 nm and the lattice constant is estimated to be 0.578 nm.This indicates Debye-Scherrer rings corresponding to an ice structure (Type I c ), although the lattice constant should be calibrated by taking the camera length into account.Thus, the crystal state of the water cluster is different from the amorphous state of liquid water.In addition, the water cluster might consist of a networked structure of hydrogen bonds.
It should be noted that the electron diffraction system that is developed can also be used to observe diffraction patterns for other clusters if the cluster beam flux is larger than 1.0×10 21 molecules/m 2 /s.For example, the diffraction pattern for CO 2 clusters held together by van der Waals forces indicated that it has the FCC structure corresponding to dry ice.Similarly, Ar clusters bonded by van der Waals forces exhibited the FCC structure, although the diffraction pattern was weaker.
The inclusion of gas molecules into water cluster cages with specific cluster sizes could be performed by introducing the gas into liquid water to induce cluster formation.Gas hydrates, also known as clathrate hydrates, are crystalline compounds consisting of both water and gas molecules like methane [19,20].Gas hydrates exhibit many features that are useful for a number of applications such as flow assurance, safety, energy recovery, gas storage or transportation, and climate change mitigation [21,22].A vapor of waters bubbling with methane gas was ejected through a nozzle into a vacuum region, and mixed beams of water clusters and methane-hydrate clusters were produced by adiabatic expansion.We measured the mass spectra for residual gases after the impact of methanehydrate cluster ions on the Cu surface.The acceleration and retardation voltage were 6 kV and 14 V, respectively, and the minimum size of the cluster was approximately 100 molecules.Several peaks appeared at mass numbers gas, indicating that the oxygen peak was negligibly small.Furthermore, it should be noted that the residual gaseous water molecules were dissociated by electron bombardment during the ionization process in the mass spectrometer.The mass analysis showed that the peak intensities for H atom, OH radical, and H 2 O molecule were weak prior to cluster ion irradiation.
Figure 2 shows the changes in the CH 4 /H 2 O peak ratio relative to the CH 4 flow rate.The CH 4 peak intensity was normalized using the H 2 O peak intensity.The CH 4 peak intensity rises as the CH 4 flow rate is increased.This is attributed to the irradiation by CH 4 molecules, which are contained in the hydrate clusters.For larger clusters, the CH 4 molecule could be attached to the surface of the water cluster.In addition, the cluster state has a threedimensional structure of hydrogen bond networks, and the relatively smaller methane hydrate clusters (cluster size: 20 water molecules) might possibly include CH 4 gas molecules.

Theoretical modeling
Figure 3 shows a schematic illustration of the impact of polyatomic cluster ions.The figure is modified based on a previous one [23], which represented the sputtering phenomena of solid surfaces by the impact of cluster ions.The diameter of a cluster comprising several thousand molecules is a few nanometers and these clusters penetrate the solid surface to a depth of only a few nanometers.As a result, multiple collisions occur between cluster molecules and surface atoms.Furthermore, the incident energy of a colliding cluster ion is transferred to the surface of the material.This dense energy deposition results in an enhancement of physical and chemical sputtering as well as cluster temperature.The interaction of the cluster ion with the surface atoms is in a non-equilibrium and non-steady state, which is different from a quasi-static process.In addition, the cluster temperature represents the collective property of many particles in the interacting region.It is highly correlated to the incident energy per molecule or radical in a water cluster, which is an important variable for the analysis of physical and chemical sputtering.
The rate of chemical reaction (η) of multiple reaction paths by water cluster ion irradiation is described as follows [24]: where N d is the number density of water molecules, h is Planck's constant, k is the Boltzmann constant, T is the temperature of the water clusters after impact, Q j (j = 1, 2, • • •, n) is the activation energy for the j-th reaction path, and n is the number of each reaction path.
The equation indicates that the rate of chemical reactions such as oxidation and hydration increases with increasing temperature.

Cluster ion beam induced luminescence
Light was emitted from Si(100) substrates irradiated by water cluster ion beams.The luminescence was monitored in situ outside the vacuum chamber through an optical lens and light guide.The light intensity was measured using a photon counter (Hamamatsu H8259), which exhibited a sensitive response in the 185 to 850 nm wavelength range.The total optical efficiency was approximately 0.15% [17].The TTL signals from the photon counter were counted using a terminal board (National Instruments, BNC-2121), which was connected to a computer.
Figure 4 shows the dependence of the light intensity on the beam current density.The acceleration voltage (V a ) was 9 kV, and the electron voltage for ionization (V e ) was 200 V.The ion current was adjusted by changing the electron current for ionization (I e ), and it was between 0 and 250 nA/cm 2 .As shown in the figure, the light intensity increases with increasing ion current, and it is approximately 1300 cps at the ion current density of 250 nA/cm 2 .A minimum count for the photon counting system is approximately 100 cps, and a very weak luminescence can be http://www.sssj.org/ejssnt(J-Stage: http://www.jstage.jst.go.jp/browse/ejssnt/) detected.Furthermore, the increase of the light intensity with respect to beam current intensities over 120 nA/cm 2 is large.This might be due to the decrease in the cluster size with increasing the electron current for ionization.
Figure 5 shows the relationship between the light intensity and acceleration voltage for the water cluster ions.The ion current density was 76 nA/cm 2 .The measured wavelength was between 185 and 850 nm.As shown in the figure, luminescence is observed, when the acceleration voltage (V a ) is larger than 3 kV.The intensity increases with increasing acceleration voltage.The peak size of water cluster ions is approximately 3000 molecules and the average incident energy is a few electron volts per molecule.If the accelerating energy (eV a ) is converted to thermal energy (kT ), the cluster temperature (T ) could be extremely high.In this case, e is the electric charge.More detailed experiments on the Ar cluster ion irradiation on the Si target indicated that the cluster temperature was approximately 15000 K at V a = 9 kV.
There are several origins of photon emissions from clus-  The acceleration voltage was adjusted to be 6 kV and 9 kV.In this case, several photon counters were used in order to measure the emission angle dependence of the light intensity [25], and the intensity was different from the values shown in Fig. 5.As illustrated, the number of photons emitted is approximately 100 cps/nA/cm 2 at an acceleration voltage of 9 kV for Ar cluster ion irradiation and the intensity is similar for various surfaces that were irradiated.The electron excitation caused by thermal energy transfer could be enhanced.The photon emission could be ascribed to electron relaxation from excited levels to the ground level.On the other hand, in the case of water cluster ion irradiation, the intensity is approximately 60 cps/nA/cm 2 at an acceleration voltage of 9 kV for the Au surface.The intensity is five times that for Si and SiO 2 surfaces.Photon emission from the Au substrate is interesting and the emission mechanism is currently under investigation.As for Si(100) and SiO 2 surfaces irradiated by water cluster ion beams, oxidation occurred and the intensity was small when compared to that for Ar cluster ion irradiation.With regards to porous Si and SiO 2 , ion beam-induced luminescence was observed.Previous reports ascribed the origin of the emission to some defect centers in the SiO 2 /Si interface and SiO 2 surfaces [26,27].Therefore, the radiative defects decrease as the extent of oxidation by water cluster ion irradiation increases, and the luminescence intensity decreases.

Chemical erosion and sputtering
PMMA has attracted interest as an organic glass and has been used in various kinds of devices such as microreactors.PET and PC contain the -O-(C=O)-radical in their monomers.Both have been used as bottle materials and also as organic glasses.The sputtered depths of polymer surfaces by water cluster ion irradiation were measured using a DEKTAK-3173933 step profiler manufactured by Veeco Instruments.
Figure 7 shows the sputtered depth for PMMA, PET, and PC surfaces irradiated at different acceleration voltages by water cluster ion beams.Thin sheets of these polymers are commercially available.The sputtered depth increases with an increase in the acceleration voltage.By taking the sputtered depth and ion dose into account, the sputtering yield was calculated by first estimating the density of PMMA, PET, and PC to be 1.19, 1.38, and 1.20 g/cm 3 , respectively.The sputtering yield of PMMA, PET, and PC was found to be 206, 34.5, and 17.7 molecules per ion, respectively, at an acceleration voltage of 9 kV.In this case, molecules mean monomer molecules consisting of polymer.The sputtering yield of polymer substrate surfaces is much higher than that using Ar monomer ion irradiation.In the case of the monomer ion irradiation, the sputtered depth was negligibly small, even though all the bonds in the polymer substrate were broken easily by the irradiation of high-energy monomer ion beams.In the case of the water cluster ion irradiation, however, the temperature of the polymer surface is high, and the surface molecules are evaporated via the dissociation of weak bonds.As a result, sputtered particles are ejected as monomer units.The high rate of sputtering of polymer surfaces is achieved by momentum transfer of the incident energy of water cluster ion irradiation.It should be noted that the high rate of sputtering of PMMA could be achieved by Ar cluster ion irradiation [28].
In particular, the high sputtering yield of PMMA substrates when compared to PET and PC is ascribed to the chemical erosion of the substrate surface.Chemical erosion is the process initiated by chemical reactions between incident atoms and surface atoms [29].After chemical re-actions, newly formed species with a lower surface binding energy are more easily sputtered such that the sputtering yield increases.On the other hand, newly formed compounds have a higher binding energy than the corresponding unreacted surfaces and, as a consequence, the sputtering yield decreases.PMMA is a polymer with a repeating unit of CH 3 CCH 2 COOCH 3 .Erosion occurs when the CH 3 radical in COOCH 3 exchanges with an H atom of the water cluster, or when a OCH 3 radical exchanges with a OH radical [18].As a result, the PMMA surface changes to poly(methacrylic acid), which has a melting point below room temperature and is able to dissolve in water.The impact of the water cluster ions on the altered surface enhances the ejection of methacrylic acid molecules in the monomer state from the surface.Thus, the high sputtering yield of PMMA surface arises from the chemical reaction at high temperatures, which were achieved by water cluster ion irradiation.

IV. CONCLUSION
Both the characterization of the structure of water clusters and the study of the irradiation effects of water cluster ions were performed at the atomic level.HEED patterns for the water cluster beams could be observed more clearly by adjusting the beam intensity.The intensity of (111), (220), and (311) diffraction peaks increased with an increase in vapor pressure.By analyzing the diffraction peaks, the water cluster was found to adopt the cubic structure corresponding to ice.As an extension of the water cluster study, a methane hydrate cluster was generated by a gas bubbling method.
In addition, the fundamental phenomenon of water cluster ion irradiation was studied using photoluminescence measurements.Cluster ion beam-induced luminescence was observed and its intensity increased with increase in the acceleration voltage.This indicated that the accelerating energy could be converted to thermal energy, and the temperature of the cluster and impact area were high.
The chemical erosion and sputtering of polymer substrates were also investigated.The sputtered depth for PMMA, PET, and PC substrates irradiated by the water cluster ions increased with an increase in the acceleration voltage.In particular, the sputtering yield of PMMA surfaces was 206 molecules per ion at an acceleration voltage of 9 kV.This was higher than that of PET and PC substrates, because the chemical erosion and sputtering caused by OH radicals were enhanced at an equivalently high temperature around the impact area.

FIG. 1 .
FIG. 1. Diffraction image (a) and diffraction patterns (b) of water clusters generated at different vapor pressures.
FIG. 2. Changes in the CH4/H2O peak ratio relative to the CH4 flow rate.

FIG. 3 .
FIG. 3. Schematic illustration of the impact of polyatomic cluster ions on solid surfaces.
FIG. 4. Relationship between the light intensity and beam current density for the water cluster ions.

FIG. 6 .
FIG. 6. Luminescence intensity for (a) water cluster and (b) Ar cluster ion irradiation on Si, SiO2 and Au surfaces.The acceleration voltage was adjusted at 6 kV and 9 kV.
ters and substrates.The luminescence induced by cluster ion irradiation on various substrates was measured.Figure 6 shows the light intensity for (a) water cluster, and (b) Ar cluster ion irradiation on Si(100), SiO 2 , and Au surfaces.

FIG. 7 .
FIG. 7. Sputtered depth for PMMA, PET, and PC surfaces irradiated at different acceleration voltages by water cluster ion beams.