Faculty Profiles - AOKI Takaaki

Papers - AOKI Takaaki

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Matrix-free high-resolution imaging mass spectrometry with high-energy ion projectiles Reviewed

Yoshihiko Nakata, Yoshiro Honda, Satoshi Ninomiya, Toshio Seki, Takaaki Aoki, Jiro Matsuo

JOURNAL OF MASS SPECTROMETRY Vol. 44 ( 1 ) page： 128 - 136 2009.1

Language：English Publishing type：Research paper (scientific journal) Publisher：JOHN WILEY & SONS LTD

The importance of imaging mass spectrometry (MS) for visualizing the spatial distribution of molecular species in biological tissues and cells is growing. We have developed a new system for imaging MS using MeV ion beams, termed MeV-secondary ion mass spectrometry (MeV-SIMS) here, and demonstrated more than 1000-fold increase in molecular ion yield from a peptide sample (1154 Da), compared to keV ion irradiation. This significant enhancement of the molecular ion yield is attributed to electronic excitation induced in the near-surface region by the impact of high energy ions. In addition, the secondary ion efficiency for biologically important compounds (> 1 kDa) increased to more than 10(10) cm(-2), demonstrating that the current technique could, in principle, achieve micrometer lateral resolution. In addition to MeV-SIMS, peptide compounds were also analyzed with cluster-SIMS and the results indicated that in the former method the molecular ion yields increased substantially compared to the latter. To assess the capability of MeV-SIMS to acquire heavy-ion images, we have prepared a micropatterned peptide surface and successfully obtained mass spectrometric imaging of the deprotonated peptides (m/z 1153) without any matrix enhancement. The results obtained in this study indicate that the MeV-SIMS technique can be a powerful tool for high-resolution imaging in the mass range from 100 to over 1000 Da. Copyright (c) 2008 John Wiley & Sons, Ltd.

DOI： 10.1002/jms.1482

A Processing Technique for Cell Surfaces Using Gas Cluster Ions for Imaging Mass Spectrometry International journal

YAMADA Hideaki, ICHIKI Kazuya, NAKATA Yoshihiko, NINOMIYA Satoshi, SEKI Toshio, AOKI Takaaki, MATSUO Jiro

Journal of the Mass Spectrometry Society of Japan Vol. 57 ( 3 ) page： 117 - 121 2009

Language：English Publishing type：Research paper (scientific journal) Publisher：The Mass Spectrometry Society of Japan

We developed a processing technique for biological samples, that is, animal cells, for imaging mass spectrometry using gas cluster ion beams. Secondary ion mass spectrometry has been investigated for cellular imaging with high spatial resolution. Nanoscale processing techniques are needed for cellular level imaging, for example, for removing contamination from cell surfaces and exposing the insides of cells. In this study, we compared the mass spectrum of the unetched cell sample to that of the cell irradiated with an Ar cluster ion beam. The contamination was removed from the sample surface and the signals of intracellular components were detected. The results indicate that the cells were etched with low damage using the gas cluster ion beam. Finally, we compared the ion image of an unetched cell to that of an etched cell and demonstrated that the technique of cluster ion irradiation could be applied to the processing of samples for cellular level imaging mass spectrometry.

DOI： 10.5702/massspec.57.117

Other Link： https://jlc.jst.go.jp/DN/JALC/00332526476?from=CiNii

Stress measurement of carbon cluster implanted layers with in-plane diffraction technique Reviewed

Jiro Matsuo, Kazuya Ichiki, Masaki Hada, Satoshi Ninomiya, Toshio Seki, Takaaki Aoki, Tsutomu Nagayama, Masayasu Tanjyo

Extended Abstracts of the 9th International Workshop on Junction Technology, IWJT 2009 page： 84 - 85 2009

Language：English Publishing type：Research paper (international conference proceedings)

Cluster ion implantation has a great potential for semiconductor manufacturing, such as ultra shallow junction formation and strain engineering of channel. Stress in Si implanted with carbon cluster ions (C7H 7) was much higher than that in Si implanted with carbon monomer ions. Multiple collisions of atoms occurred during cluster ion implantation, and a fully amorphized Si layer can be formed. The stress in the implanted layer strongly depends on the annealing condition. There are two different directions for the stress. One is parallel to the surface, which is usually measured with conventional X-ray diffraction. The other is perpendicular to the surface, which can be measured with in-plane X-ray diffraction technique. The stress perpendicular to the surface was about 50% of the stress parallel to the surface even after flash lamp annealing. ©2009 IEEE.

DOI： 10.1109/IWJT.2009.5166225

A fragment-free ionization technique for organic mass spectrometry with large Ar cluster ions Reviewed

Satoshi Ninomiya, Yoshihiko Nakata, Yoshiro Honda, Kazuya Ichiki, Toshio Seki, Takaaki Aoki, Jiro Matsuo

APPLIED SURFACE SCIENCE Vol. 255 ( 4 ) page： 1588 - 1590 2008.12

Language：English Publishing type：Research paper (scientific journal) Publisher：ELSEVIER SCIENCE BV

In this report, we propose to use the incidence of cluster ions that are much larger than molecular ions as a fragment-free ionization technique for organic secondary ion mass spectrometry. Secondary ions were measured for amino acid and peptide targets bombarded with 3, 8 and 13 keV large Ar cluster ions. The relative yields of the fragment ions decreased drastically with increasing incident cluster size. Fragment-free ionization of the molecule was accomplished when large cluster ions with optimized size and energy were incident on a biomolecular sample. (C) 2008 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.apsusc.2008.05.004

MD simulation study of the sputtering process by high-energy gas cluster impact Reviewed

Takaaki Aoki, Toshio Seki, Satoshi Ninomiya, Jiro Matsuo

APPLIED SURFACE SCIENCE Vol. 255 ( 4 ) page： 944 - 947 2008.12

Language：English Publishing type：Research paper (scientific journal) Publisher：ELSEVIER SCIENCE BV

We performed molecular dynamics (MD) simulations to study the characteristic sputtering process with large cluster ion impact. The statistical properties of incident Ar and sputtered Si atoms were examined using 100 different MD simulations with Ar(1000) cluster impacting on a Si(0 0 1) target at a total acceleration energy of 50 keV. The results show that the kinetic energy distribution of Ar atoms after impact obeys the high-temperature Boltzmann distribution due to thermalization through high-density multiple collisions on the target. On the other hand, the kinetic energy distribution of sputtered target atoms demonstrates a hybrid model of thermalization and collision-cascade desorption processes. (C) 2008 Elsevier B. V. All rights reserved.

DOI： 10.1016/j.apsusc.2008.05.008

Low Damage Etching and SIMS Depth Profiling with Large Ar Cluster Ions Reviewed International journal

Takaaki Aoki

Transactions of the Materials Research Society of Japan Vol. 33 ( 4 ) 2008.12

High-Speed Nano-Processing with Cluster Ion Beams Reviewed International journal

Takaaki Aoki

Transactions of the Materials Research Society of Japan Vol. 33 ( 4 ) 2008.12

High sputtering yields of organic compounds by large gas cluster ions Reviewed

K. Ichiki, S. Ninomiya, Y. Nakata, Y. Honda, T. Seki, T. Aoki, J. Matsuo

APPLIED SURFACE SCIENCE Vol. 255 ( 4 ) page： 1148 - 1150 2008.12

Language：English Publishing type：Research paper (scientific journal) Publisher：ELSEVIER SCIENCE BV

We measured the sputtering yield, surface roughness and surface damage of thin leucine films bombarded with Ar cluster ions and examined the usefulness of large gas cluster ions for the depth pro. ling of organic compounds. Ar cluster ion beams with a mean size of 2000 atoms/cluster and energies from 5 to 30 keV were used. Sputtering yields increased linearly with incident ion energy and were extremely high compared to inorganic materials. Surface damage was investigated by measuring positive secondary ions emitted from the leucine film before and after cluster ion irradiation. After irradiation the leucine surface became smoother. The yield ratio of protonated leucine ions to other fragment ions kept constant before and after Ar cluster ion irradiation. These results indicate that large gas cluster ions are useful for depth pro. ling of organic compounds. (c) 2008 Published by Elsevier B.V.

DOI： 10.1016/j.apsusc.2008.05.032

Secondary ion emission from Si bombarded with large Ar cluster ions under UHV conditions Reviewed

Satoshi Ninomiya, Kazuya Ichiki, Yoshihiko Nakata, Yoshiro Honda, Toshio Seki, Takaaki Aoki, Jiro Matsuo

APPLIED SURFACE SCIENCE Vol. 255 ( 4 ) page： 880 - 882 2008.12

Language：English Publishing type：Research paper (scientific journal) Publisher：ELSEVIER SCIENCE BV

Si was bombarded with size-selected 40 keV Ar cluster ions and positive secondary ions were measured using the time-of-flight technique under high and ultra-high vacuum (HV and UHV respectively) conditions. Si(+) ions were main species detected under the incidence of 40 keV Ar cluster ions, and the yields of Si cluster ions such as Si(4)(+) were also extremely high under both conditions. On the other hand, oxidized secondary ions such as SiO(+) were detected with high intensity only under the HV condition. The yield ratios of oxidized ions decreased in UHV to less than 1% of their values in HV. The effect of residual gas pressure on Si cluster ion yields is relatively low compared to oxidized ions, and the UHV condition is required to obtain accurate secondary ion yields. (c) 2008 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.apsusc.2008.05.063

SIMS Analysis of Biological Mixtures with Fast Heavy Ion Irradiation Reviewed International journal

Takaaki Aoki

Transactions of the Materials Research Society of Japan Vol. 33 ( 4 ) 2008.12

What size of cluster is most appropriate for SIMS? Reviewed

Jiro Matsuo, Satoshi Ninomiya, Yoshihiko Nakata, Yoshiro Honda, Kazuya Ichiki, Toshio Seki, Takaaki Aoki

APPLIED SURFACE SCIENCE Vol. 255 ( 4 ) page： 1235 - 1238 2008.12

Language：English Publishing type：Research paper (scientific journal) Publisher：ELSEVIER SCIENCE BV

Secondary ion emission with large gas cluster ion is reviewed from the point of view of secondary ion mass spectroscopy (SIMS). We have proposed to use large cluster ions to realize fragment-free ionization for SIMS analysis for various organic materials, such as amino acids and peptides. When large cluster ions with optimized size and energy were incident on biomolecular samples, the relative yields of the fragment ions decreased drastically with the velocity of incident cluster ions. Molecular depth pro. ling capability is also demonstrated by using large gas cluster ions as the primary ion for SIMS. (C) 2008 Elsevier B. V. All rights reserved.

DOI： 10.1016/j.apsusc.2008.05.057

Yield enhancement of molecular ions with MeV ion-induced electronic excitation Reviewed

Y. Nakata, Y. Honda, S. Ninomiya, T. Seki, T. Aoki, J. Matsuo

APPLIED SURFACE SCIENCE Vol. 255 ( 4 ) page： 1591 - 1594 2008.12

Language：English Publishing type：Research paper (scientific journal) Publisher：ELSEVIER SCIENCE BV

We performed secondary ion mass spectrometry for arginine thin films in the incident energy range from 10 keV to 2 MeV, in which electronic energy loss in the surface region increases with energy and exceeds the nuclear energy loss. The yield of protonated arginine increased with incident energy, and at 2 MeV it was approximately two orders of magnitude larger than at 10 keV, where the nuclear collision process is dominant. It was found that the molecular ion desorption is enhanced by the electronic excitation induced in the near-surface region. In addition, the yield ratios of the fragment ions to the protonated arginine decreased with incident energy, which indicates that the nuclear energy deposition significantly affects molecular fragmentation. These results demonstrate a soft desorption and ionization of biomolecules by the MeV ion-induced electronic excitation. (C) 2008 Elsevier B. V. All rights reserved.

DOI： 10.1016/j.apsusc.2008.05.108

Carbon nanotubes from a divided catalyst: the carbon transmission method Reviewed

Takeshi Hikata, Kazuhiko Hayashi, Tomoyuki Mizukoshi, Yoshiaki Sakurai, Itsuo Ishigami, Takaaki Aoki, Toshio Seki, Jiro Matsuo

APPLIED PHYSICS EXPRESS Vol. 1 ( 3 ) page： 0340021 - 0340023 2008.3

Language：English Publishing type：Research paper (scientific journal) Publisher：IOP PUBLISHING LTD

We have fabricated carbon nanotubes (CNTs) using a catalyzed growth technique that we call the carbon transmission method (CTM). We could control independently functions for carbon source gas supply and CNT growth by using a foil barrier (Ag) penetrated by pure Fe fibers. CNTs grew on one end of the Fe fibers, formed from diffused carbon that originates in carbon source gas at the other end of the fibers. Long CNTs with length over 100 mu m were obtained on the end of the Fe fibers in the carbon transmitted side. (C) 2008 The Japan Society of Applied Physics.

DOI： 10.1143/APEX.1.034002

Low damage smoothing of magnetic materials using off-nonnal gas cluster ion beam irradiation Reviewed

S. Kakuta, S. Sasaki, K. Furusawa, T. Seki, T. Aoki, J. Matsuo

SURFACE & COATINGS TECHNOLOGY Vol. 201 ( 19-20 ) page： 8632 - 8636 2007.8

Language：English Publishing type：Research paper (scientific journal) Publisher：ELSEVIER SCIENCE SA

A damage-free smoothing process for magnetic materials using a gas cluster ion beam (GCIB) has been studied. As the areal density of hard disk drives (HDDs) has increased, the flying height of the magnetic recording head above the disk has been decreasing. In order to further reduce the flying height and improve the sensitivity of sensors, a damage-free smoothing technology for magnetic materials is imperative. A GCIB process has been proposed as a novel smoothing technique for various materials. With this process it is expected that the damage will be low, since the gas clusters are aggregations of a few to thousands of atoms or molecules. In this paper, oblique GCIB irradiation has been studied in order to achieve low damage smoothing of PtMn and NiFe thin films. The surface morphology after GCIB irradiation was observed by scanning electron microscopy (SEM) and atomic force microscopy (AFM). A smooth surface with an average roughness of less than 1 nm was obtained by using oblique GCIB irradiation. Irradiation damage, such as fluctuations in the ratio of the material components and oxidation of the surface, were investigated by secondary ion mass spectrometry (SIMS). The fluctuations were suppressed by using oblique GCIB irradiation with incident angles larger than 80 degrees. Consequently, irradiation from a GCIB at grazing incidence can be applied to achieve low-damage smoothing of magnetic materials. (C) 2007 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.surfcoat.2006.03.064

Molecular dynamics study of glancing angle gas cluster irradiation on irregular-structured surfaces Reviewed

Takaaki Aoki, Jiro Matsuo

NUCLEAR INSTRUMENTS & METHODS IN PHYSICS RESEARCH SECTION B-BEAM INTERACTIONS WITH MATERIALS AND ATOMS Vol. 261 ( 1-2 ) page： 639 - 642 2007.8

Language：English Publishing type：Research paper (scientific journal) Publisher：ELSEVIER SCIENCE BV

Surface modification processes resulting from glancing angle gas cluster ion irradiation were studied by molecular dynamics (MD) simulations. Large Ar-2000 gas clusters accelerated with 20 keV were irradiated onto Si(100) substrates to which were attached small Si block-shaped (size of 4096, 16384 or 65536 atoms) structures. The MD simulations revealed that when an Ar cluster is irradiated at 80 degrees of incident angle, the cluster slides on the substrate surface without damage and impacts on the wall of the block structures causing multiple collisions, and dynamic deformation of the blocks. Especially, in the case of the impact on the small surface block (Si-4096), a large part of the block was sputtered as a large chunk, and the target substrate was smoothed with only one cluster impact. The evaluation of the damage distribution showed that silicon interstitials, caused by the multiple collision mechanism, reside within 15 angstrom from the surface, which suggests that low-damage and high-performance surface modification processing would be realized by utilizing a glancing angle gas cluster ion beam irradiation technique. (c) 2007 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.nimb.2007.04.005

Molecular dynamics study of monomer and dimer emission processes with high energy gas cluster ion impact Reviewed

Takaaki Aoki, Toshio Seki, Satoshi Ninomiya, Jiro Matsuo

SURFACE & COATINGS TECHNOLOGY Vol. 201 ( 19-20 ) page： 8427 - 8430 2007.8

Language：English Publishing type：Research paper (scientific journal) Publisher：ELSEVIER SCIENCE SA

The mechanism of the sputtering process by energetic large cluster ion impacts was studied using molecular dynamics (MD) simulations. MD simulations of Ar-1000 clusters accelerated with 50 keV total energy impacting on a Si(100) target were performed with 50 trials at different impact points in order to improve the statistical reliance. From these MD simulations, a sputtering yield of about 20 Si atoms/impact was obtained. Si clusters such as Si-2, Si-3, etc. were also found in the sputtered particles, and the sputtering yields for each cluster size obey the power-decay rule with a decay index of 2.3-3. The investigation of trajectories of sputtered Si-1 and Si-2 showed that initial coordinates of sputtered particles residing within a region corresponds to a crater-shaped hole caused by cluster impact. As for the sputtering of dimers, it is found that, not only the emission of a preformed dimer (i.e. a pair of target atoms initially bound before the cluster impact), but the emission of dynamically formed dimers through multiple collisions is also probable event for cluster impact. (C) 2007 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.surfcoat.2006.09.325

Molecular dynamics simulations of surface smoothing and sputtering process with glancing-angle gas cluster ion beams Reviewed

Takaaki Aoki, Jiro Matsuo

NUCLEAR INSTRUMENTS & METHODS IN PHYSICS RESEARCH SECTION B-BEAM INTERACTIONS WITH MATERIALS AND ATOMS Vol. 257 ( 1-2 SPEC. ISS. ) page： 645 - 648 2007.4

Language：English Publishing type：Research paper (scientific journal) Publisher：ELSEVIER SCIENCE BV

Surface modification processes with glancing-angle large gas cluster irradiation were studied by molecular dynamics simulations. The MD simulation of an Ar-2000 gas cluster impacting on Si(1 0 0) substrate with small Si-4096 block structure attached on the surface was performed. The MD result has shown that, when the Ar cluster impacts on the target at 20 keV of total acceleration energy and at 80 degrees of incident angle, the incident cluster slides on the target surface without creating surface damage and most of the constituent atoms in the cluster collide with the sidewall of the surface block. Up to 10 keV of kinetic energy of the incident cluster was transferred to the target irregular structure, which results in the sputtering or stretching of the surface block and the target surface is smoothed. This characteristic surface modification effect was observed when the surface block is located farther than 200 angstrom from the point where the incident cluster contacts with the target surface. (c) 2007 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.nimb.2007.01.048

Size effect in cluster collision on solid surfaces Reviewed

Jiro Matsuo, Satoshi Ninomiya, Yoshihiko Nakata, Kazuya Ichiki, Takaaki Aoki, Toshio Seki

NUCLEAR INSTRUMENTS & METHODS IN PHYSICS RESEARCH SECTION B-BEAM INTERACTIONS WITH MATERIALS AND ATOMS Vol. 257 ( 1-2 SPEC. ISS. ) page： 627 - 631 2007.4

Language：English Publishing type：Research paper (scientific journal) Publisher：ELSEVIER SCIENCE BV

New surface modification processes have been demonstrated using gas cluster ion irradiations. Multiple collision and high energy density collision of cluster ions are responsible for "non-linear phenomena", which play an important role in the surface modification process. Because of the unique interaction between cluster ions and surface atoms, atomistic mechanisms of cluster ion bombardment must be understood for the further developments of this technology. Cluster size is a unique parameter for cluster ions. One of the fundamental questions in this surface modification technique is the cluster size effect. It is important to use appropriate cluster size in each process. Size dependence of sputtering yields and secondary ion yields with large Ar cluster (N > 300) have been measured. (c) 2007 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.nimb.2007.01.164

Measurements of secondary ions emitted from organic compounds bombarded with large gas cluster ions Reviewed

Satoshi Ninomiya, Yoshihiko Nakata, Kazuya Ichiki, Toshio Seki, Takaaki Aoki, Jiro Matsuo

NUCLEAR INSTRUMENTS & METHODS IN PHYSICS RESEARCH SECTION B-BEAM INTERACTIONS WITH MATERIALS AND ATOMS Vol. 256 ( 1 ) page： 493 - 496 2007.3

Language：English Publishing type：Research paper (scientific journal) Publisher：ELSEVIER SCIENCE BV

We propose to use cluster ions that are much larger than the molecular ions as primary ions for organic secondary ion mass spectrometry. Incident Ar cluster ion beams with energies from 10 to 20 keV and a mean size of about 1000 atoms/cluster were used. Secondary ions were measured for a thin arginine film target (200 nm) bombarded with large Ar cluster ions using a time-of-flight technique. Molecular ions of arginine and characteristic fragment ions were detected with high sensitivity. When large Ar cluster ions such as Ar-1500 were incident on the arginine target, molecular ions of arginine were detected with little fragment ions. This indicates that large cluster ions can ionize arginine molecules without damaging them. (c) 2006 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.nimb.2006.12.047

Surface oxidation of Si assisted by irradiation with large gas cluster ion beam in an oxygen atmosphere Reviewed

K. Ichiki, S. Ninomiya, T. Seki, T. Aoki, J. Matsuo

NUCLEAR INSTRUMENTS & METHODS IN PHYSICS RESEARCH SECTION B-BEAM INTERACTIONS WITH MATERIALS AND ATOMS Vol. 256 ( 1 ) page： 350 - 353 2007.3

Language：English Publishing type：Research paper (scientific journal) Publisher：ELSEVIER SCIENCE BV

Surface oxidation of Si assisted by Ar cluster impact with a current density of a few mu A/cm(2) under O(2) atmosphere was investigated. Thin-film formation by cluster ion beam presents a number of advantages such as atomic-scale surface smoothing, high density and precise stoichiometry. We used an Ar cluster ion beam with 20 keV and a mean size of 1000 atoms per cluster and measured the emission yields of Si(+) and SiO(+) after Ar cluster ion irradiation in O(2) atmosphere using a quadrupole mass spectrometer to investigate the dependence of Si surface oxidation on oxygen partial pressure. It was found that the Si surface was oxidized by Ar cluster ion irradiation in O(2) atmosphere. (c) 2006 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.nimb.2006.12.026

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