S5-1

Fabrication and Control of Nanoparticle-based Optical Materials
by High-flux Negative-ion Implantation
大電流負イオン注入によるナノ粒子・光学材料の生成と制御
N. Kishimoto, N. Umeda*O.A. Plaksin** and Y. Takeda
岸本直樹，梅田直樹*，O.A. Plaksin**，武田良彦
Nanomaterials Laboratory, National Institute for Materials Science,
3-13 Sakura, Tsukuba, Ibaraki 305-0003, Japan
物質･材料研究機構ナノマテリアル研究所，〒305-0003 つくば市桜3-13*VBL, Yokohama National University, 79-5 Tokiwadai, Hodogaya-ku, Yokohama 240-0067, Japan
横浜国立大学VBL, 〒240-0067 横浜市保土ヶ谷区常盤台79-5**
SSC RF - A. I. Leypunsky Institute of Physics and Power Engineering, Obninsk, Russia

A high-flux negative-ion implanter, with a plasma-sputter-type ion source, has been applied to create
nanoparticle-based nonlinear optical materials. Usage of negative ions alleviates surface charging of
insulators and provides merits of not only efficient atom injection but also kinetic precipitation control bywidely changing ion flux. Negative metal ions of 60 keV were implanted into insulators, up to 100 µA/cm2,
fixing a total fluence of 3.0×1016 ions/cm2. Spherical nanocrystals formed within a narrow region near theprojectile range. The high-flux implantation is subjected to enhanced diffusion and atomic release ofimplants. At an optimum flux, the high flux may cause self-organized 2D distribution of nanoparticles.

1. Introduction
High-flux ions have been demanded primarily toexpedite ion implantation in semiconductor industry
or to develop high-power technologies, such as
fusion reactors and pulsed power devices. Anotherdemand of high flux has emerged for fabrication ofmetal nanoparticles in insulators, since the required
fluence (e.g., 1-10 at%) is much larger than that for
shallow-impurity doping into semiconductors (e.g.,10-1-100 at.ppm). Although ion implantation
methods have a superb merit to introduce metal
atoms immiscible in insulators, surface charging forinsulating substrates is of concern, especially forlow energy/high flux.
Extensive work for insulators has been carriedout with positive ions to modify optical properties[1-3], especially to obtain ultrafast nonlinear opticalsusceptibility associated with metal nanoparticles.
However, the surface voltage may reach theacceleration voltage or the breakdown voltage ofinsulators, though radiation-induced conductivity
practically dissipates the surface charge to someextent. Occasionally, usage of positive ions gave
extremely large flux dependence in opticalproperties [1], where surface charging may play asignificant role.
Contrarily, usage of negative ions suppresses
surface charging down to several volts [4],balancing with secondary electron emission fromthe substrate surface. Up to now, negative-ion
implantation has been applied for powder- andpolymeric materials. We have applied high-flux

negative-ion implantation to insulators andfabricated nanoparticles in insulators [5, 6].
In this paper, we describe the high-flux
negative-ion implantation technique and reportkinetic features of nanoparticle fabrication
processes induced by high flux.
2. High-flux Negative-ion Implanter
Negative ions are generated by a Cs-assisted
heavy-ion source of plasma-sputter type, with acusp magnetic field [7], as is illustrated in Fig. 1.
Sm-Co Magnet
LaB6
Filament

Target

Xe+
Xe+

Water

Xe+
PlasmaCsNegativeIon
Extraction
Electrode

Cs Reservoire

Fig.1. Negative heavy-ion source of plasma-sputter typewith multi-cusp magnetic fields.

Thermal electrons from a LaB6 filament ignite a Xe
plasma, which is confined by the cusp magnetic
field and powerfully sputters the solid target
electrically biased at 200-800V. Concurrently, Cs
coverage on the target surface enhances negative

ion production. The concave target contributes toincrease the beam brightness.
The intense
negative ions are extracted from the plasma by two
extraction electrodes, being accelerated up to 60
keV. The ion source provides us with awell-oriented and intense negative beam. Althoughspace-charge effects tend to become significant
with increasing the flux to a mA-level, the total Cufluxt of 3 mA was attained at the specimen position,
after an efficient transport system including avariable permanent quadrupole-magnet triplet.

3.2. Nanoparticle self-organization by high flux
High flux enhances not only spontaneous
precipitation but also atomic rearrangement ofnanoparticles along the depth, depending onmaterial species. The a-SiO2 shows significantatomic rearrangement depending on ion flux, while
spinel MgO.2.4(Al2O3) has less precipitation
tendency and more stable atomic distribution.

3. Material Kinetics by High-Flux Negative Ions
3.1 Beam-surface interactions with negative Ions
Employment of negative ions excludes
surface-charge-related
instabilityin
ion
implantation and we, therefore, can extract material
kinetics induced by high flux.
Fig. 3 Cross-sectional TEM images of a-SiO2 implantedwith 60 keV Cu- at 45 µA/cm2 to 3 × 1016 ions/cm2.

Fig. 2. Flux-dependence of optical absorbance atsurface plasmon for Cu-implanted a-SiO2. Data after
Magruder et al [1] and Hosono et al [2,3] are also given
for comparison.

Optical absorption of Cu--implanted a-SiO2 isgreatly dependent on ion flux. Fig. 2 shows
absorbance of a surface-plasmon peak at 2.2 eV.
The absorbance has a maximum around 10 µA/cm2,
where the surface-plasmon peak becomesprominent. The convex variation results from theCu retention and implies that beam-surface
interactions play a significant role and that theoptimum flux exists for modification. The result ofnegative ions can be compared to the data of
positive Cu ions by Magruder et al [1] and Hosono
et al [2,3]. The difference means that negativeions are twice as effective as the positive ions andthat repulsion of surface charging is alleviated byuse of the negative ions. The rapid increase ofabsorbance is caused by a decrease in surface
repulsion effects.
On the other hand, the absorbance decreases
beyond the peak flux, where the Cu retention
decreases. It is due to sublimation/sputtering
processes enhanced by high flux.

Fig 3 shows a cross-sectional TEM image ofan a-SiO2 implanted with 60 keV Cu- at a criticalhigh flux. Right before losing implants at the higher
fluxes, spherical nanoparticles are spontaneouslyaligned at a certain depth, as a 2D distribution.
The 2D nanocrystals are fabricated by high flux
without
post-implantation
annealing.
Thenanostructure formation is regarded asa
self-organization induced by high flux. The depth
position is much shallower than the projected range,indicating significant atomic rearrangement.
The dynamic in-beam processes are likely caused
by electronic excitation at high fluxes and areapplicable for controlling nanoparticle structures
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