Article pubs.acs.org/JPCC
DNA-Templating Mass Production of Gold Trimer Rings for Optical Metamaterials Ryoko Watanabe-Tamaki,† Atsushi Ishikawa,† Takuo Tanaka,*,†,‡ Tamotsu Zako,§ and Mizuo Maeda§ †
Metamaterials Laboratory, Advanced Science Institute, RIKEN, Wako, Saitama 351-0198, Japan Research Institute for Electronic Science, Hokkaido University, Sapporo, Hokkaido 001-0020, Japan § Bioengineering Laboratory, Advanced Science Institute, RIKEN, Wako, Saitama 351-0198, Japan ‡
S Supporting Information *
ABSTRACT: Mass production of trimer rings consisting of 20 nm gold nanoparticles by using DNA template is demonstrated. Three kinds of DNA-monoconjugated gold nanoparticles are programmed to self-assemble into a trimer ring structure with nanoscale gaps through simple hybridization process. Self-assembled gold trimer rings are immobilized on a centimeter-scale quartz substrate to investigate their optical properties by the far-field transmission spectroscopy. Quantitative characterization of the gold trimer rings by atomic force microscope measurements reveals that 43% of gold nanoparticles keep forming the ring configuration on the substrate after the immobilizing process. In the far-field transmission spectroscopy, the trimer ring sample clearly exhibits two absorption dips in the visible spectrum, while monomer one has a single dip. The experimental results are in good agreement with the corresponding numerical simulations, proving that the unique spectral feature for the trimer ring arises from the hybridization of plasmon resonances of gold nanoparticles. The plasmonic responses of gold nanoparticle assemblies can be entirely controlled by designing DNA templates and thus may open up a novel approach for the realization of large-scale optical metamaterials.
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INTRODUCTION Plasmonic properties of gold nanoparticles (NPs) have been intensively studied because their strongly enhanced optical field can afford a wide variety of applications in optics, electronics, and sensing.1−6 Metamaterial is another fast-growing field where the gold NPs play an important role to create anomalous optical dispersions. Plasmonic hybridization in engineered assemblies of gold NPs gives rise to magnetic resonances, which in turn modulate the effective permeability of the system.7−12 Our recent theoretical investigations also predict that metal NPs in a ring arrangement exhibits negative effective permeabilities in the visible light region13,14 for the realization of negative refractive index material, superlens, and cloaking.15−20 However, for practical applications of these physical phenomena, a large-scale metamaterial consisting of a huge number of plasmonic resonators is essentially required. There have been a number of reports on the realization of optical metamaterials by using electron beam lithography nanofabrication, but their total sizes are mostly limited to micrometer scale.19,21 Direct laser writing has been recently introduced to realize a large number of metallic nanostructures in three-dimensional (3D) volume.18,22,23 Although 3D metamaterials operating at infrared frequencies have been demonstrated, their time-consuming processes remains a major challenge. The bottom-up approach, on the other hand, allows for the productivity and usually offers fine nanostructures smaller than © 2012 American Chemical Society
one-tenth of the wavelength of the visible light. Among the bottom-up techniques, the self-assembly of DNAs enable us to fabricate 1D, 2D,24−27 and 3D28−30 nanostrucutres with high fidelity owing to the programmable hybridization between the complementary bases.31 DNAs with thiol moiety easily bind with gold through a covalent bond. Therefore, thiolated singlestranded DNAs and their complementary DNAs perform as templates for arranging gold NPs in a particular design. This technique offers high structural controllability in nanoscale called DNA template.32−35 Because the chemical reactions through the synthesis and hybridization of DNAs are fully spontaneous processes, DNA-templating for fabrication of metallic nanostructures is truly mass-productive. Here, we demonstrate mass production of gold NP trimer rings by using DNA templates. Simple hybridization process of programmed DNA templates enables to cover a centimeter-scale substrate by self-assembled gold trimer rings. By using the far-field transmission spectroscopy with the incident angle of 0°−70°, we also characterize their optical responses in the visible light region. Polarization and incident angle dependences on the transmission spectra are in good agreement with the corresponding numerical simulations, confirming the hybridization of plasmonic resonances in the gold trimer ring. Since Received: March 12, 2012 Revised: June 13, 2012 Published: July 6, 2012 15028
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aqueous solution of 8-methoxypsoralen (8-MOP) (SigmaAldrich) was added into the incubated solution. A subsequent incubation under dark for 1 h encouraged intercalation of 8MOP between two hybridizing DNA strands. The solution was irradiated by 365 nm UV light (CL-1000, UVP, LLC) at irradiation dose of 9 J/cm2 so that the cross-linking reaction took place between DNAs and resulted in a durable ring structure. Finally, the solution underwent AGE to separate the trimer from monomer, dimer, and other oligomers (Figure S4). The AGE conditions were the same as those for the building blocks. The trimer was then redispersed into a 0.5 TBE solution at the concentration of 20 mM NaCl and 5 mM BSPP. The colloid solutions of monomer was also obtained as byproducts and used as a reference sample. Immobilization of Gold NPs on Quartz Substrates. Quartz substrates (z-cut, 0°) with a thickness of 0.15 mm were cut into 5 mm × 12 mm or 11 mm × 15 mm and rinsed in acetone under ultrasonication. The both surfaces of the substrates were exposed to UV light by using a 172 nm excimer lamp (USHIO Inc.) under a nitrogen gas atmosphere to obtain hydroxyl groups on the surfaces. The substrates were then immersed into 1 vol % (3-aminopropyl)trimethoxysilane (APTMS) (Sigma-Aldrich) toluene solution for 15 min. Excessively deposited APTMS on the surfaces was rinsed away with water under ultrasonication, leaving the covalently bonded APTMS molecules that provide positive charges. The APTMS-modified quartz substrates were immersed into the gold NP colloid solutions. The gold NPs surrounded by DNA and BSPP molecules with negative charges adsorbed on the positively charged quartz surfaces by the electrostatic force. The typical concentration of gold NP colloid solution was 0.35 nM, and the immersion time was 1 h. The gold NP colloid solution was gradually diluted with 0.5 TBE buffer, and the TBE buffer was finally replaced with water before taking the quartz substrates out of the solution. The substrates were dried in a nitrogen stream. Measurements. Scanning transmission electron microscope (STEM) and scanning electron microscope (SEM) images were taken with an S-5200HV (Hitachi HighTechnologies Co.). A droplet of the DNA-templated gold trimer ring colloid solution was cast on a Cu grid with an elastic carbon film and dried to be a specimen for STEM. Gold NPs were immobilized on APTMS-modified Si wafer with 300 nm oxide layer for SEM observations. Tapping mode atomic force microscope (AFM) observations were performed with an SPA400/SPI3800N (SII NanoTechnology Inc.) with NCH probes (NanoWorld Inc.). Transmission spectra were collected using a UV−vis−NIR spectrophotometer (UV-3600, Shimadzu Co.). A Glan-Taylor polarizer was located in front of specimens to provide a polarized light. The sample stage was a rotation stage and the incident angle varied from 0° to 70°. Simulations. The simulations were performed by using the finite-element method (FEM) software package, COMSOL, with the dielectric constant of quartz = 2.25 and the empirical value for gold.37 The total calculation volume was (x, y, z) = (200 nm, 200 nm, 2000 nm) with the Floquet periodic boundary conditions in the y-axis. The gold NP was placed on the surface of the quartz substrate with the thickness of 1000 nm. The polarized light with the incident angle of 0° and 70° was lunched toward the positive z direction. Calculated transmittance was normalized by the corresponding result without the gold NP to compensate the effect of Fresnel reflection at the air−substrate interface.
the plasmonic responses of gold NP assemblies can be designed by synthesizing DNA templates, our approach may pave the way toward the realization of large-scale optical metamaterials.
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METHODS All the reagents used in this study are commercially available. We performed no further purification before use. High-purity water (Milli-Q water) was used throughout the experiments. Incubations of all the solutions were carried out at room temperature. The preparation of the gold trimer ring was conducted following our previous paper.36 The experimental procedures are briefly described below. DNA Template for Gold Trimer Ring. All the DNAs were purchased from Operon Biotechnologies. The DNA template used in this study was designed so that three kinds of building blocks, each of which includes a DNA-monoconjugated gold NP, spontaneously organize a ring structure. The DNA sequences and the structure of the DNA template are illustrated in Figures S1 and S2. A gold NP, “Ligand DNA” (a, b, c), “Template DNA” (ab, bc, ca), and “Supporting DNA” (sp) are the components of the building blocks. In the case of building block 1, a binds to a gold NP via a covalent bond between gold and sulfur. ca hybridizes with a and sp. The set of (b, ab, sp) and (c, bc, sp) constitute building block 2 and 3, respectively. Finally, building blocks 1, 2, and 3 hybridize each other and form the gold trimer ring. The detailed structure of the gold trimer ring is shown in Figure S2. Preparation of the Building Blocks. The DNAs were diluted to several μM by using 0.5 Tris-Borate-EDTA (TBE) (NIPPON GENE CO., Ltd.) buffer. The concentrations of the DNA solutions were estimated based on UV−vis absorbance at 260 nm. Bis(p-sulfonatophenyl)phenylphosphine dihydrate dipotassium salt (BSPP) (Shigma-Aldrich) was added into gold NP colloid solution (British BioCell International) at a concentration of 0.9 mM to stabilize the gold NPs. The diameter of gold NP is 20 nm, and the concentration of the gold NP is 7.0 × 1011 particles/mL, whose UV−vis absorbance is 1.0 at 520 nm, according to the specification of the gold NP colloid solution. The concentration of gold NP is converted from 7.0 × 1011 particles/mL to 1.16 nM, supposing that a gold NP is a molecule. The stabilized gold NP colloid solution was concentrated to several μM by centrifugation. The solutions of concentrated gold NP, Ligand DNA, Template DNA, and Supporting DNA were mixed at the molar ratio of 1:1:2:4. Finally, NaCl was added to be 30 mM in the mixed solution, followed by incubation for a day. After the incubation, the building block, i.e., the DNA-monoconjugated gold NP, was separated by agarose gel electrophoresis (AGE) (Figure S3). The AGE was performed using 2% (w/v) agrose gel in 0.5 TBE buffer. The band where the gel contained the building block was cut off. Diffusion of the building block out of the gel took several hours in a 0.5 TBE buffer solution. The building block redispersed in a 0.5 TBE buffer solution was stabilized with NaCl and BSPP at the concentration of 20 and 5 mM, respectively. The building blocks 1, 2, and 3 were prepared separately. Preparation of the Gold Trimer Rings. The concentrations of the 0.5 TBE solutions of the building blocks were estimated based on UV−vis absorbance at 520 nm. The 0.5 TBE solutions of the building blocks 1, 2, and 3 were mixed at the equimolar ratio and concentrated by centrifugation to several μM. The concentrated solution was incubated overnight. After the incubation, excess amount of the saturated 15029
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RESULTS AND DISCUSSION Nanostructures of the DNA-Templated Gold Trimer Rings. The schematic illustration of the DNA-templating process is shown in Figure 1. Three different kinds of DNA-
of the gel. In the case of the trimer ring, the constituting all NPs had the similar size just about 20 nm and the rounded shape (Figure 2a), while the linear trimer was composed of triangular and rounded NPs (Figure 2b). Since the triangular NPs were larger in size compared with the rounded one, the DNA templates failed to hybridize between the NPs at both ends. The higher uniformity of the shape and size of the rounded gold NPs may improve the yield of the ring structure of trimer. Since the shape and the size of each gold NP should also have a significant influence on spectra of the trimers,40 we also carried out the cross-sectional and top-view SEM observations to estimate the averaged shape and the feature size of the gold NPs (Figure 3). As for the cross section, the average height and
Figure 1. Schematic outline of the DNA-templating process for mass production of gold trimer ring. Ligand DNA covalently binds with gold NP and then hybridizes with Template DNA and Supporting DNA to form building blocks. They finally assemble into a trimer ring structure through hybridization. Figure 3. (a) Cross-sectional and (b) top-view SEM images of gold NPs on a silicon wafer. The red cross lines in (a) and (b) were used to estimate the height (h), width (w), and length (l1, l2) of the gold NPs.
monoconjugated gold NPs were prepared separately as building blocks. The building block was composed of a gold NP with 20 nm in diameter, “Ligand DNA”, “Template DNA”, and “Supporting DNA”. Ligand DNA binds with a gold NP via an Au−S bond. Ligand DNA hybridized with Template DNA, which is a linker for assembling. Template DNA hybridized with Supporting DNA, which enables the purification of the DNA-monoconjugated gold NP as the building block. Our DNA-template preparation was previously reported in a communication.36 A set of these three building blocks selfassembled into a ring structure through hybridization between the complementary DNA strands of Ligand and Template DNAs. After the incubation for hybridization, not only trimers but also monomers, dimers, and other origomers formed as byproducts due to incomplete hybridization of the DNAtemplates. AGE separated monomer, dimer, and trimer dominantly by their weight. We collected the monomers as a reference sample besides the trimers. Before the AGE separation, photo-cross-linking of DNA templates using 8MOP36,38,39 was applied to increase the stability of the ring structure. An STEM clearly visualized the nanostructure of gold NPs after DNA-templating as shown in Figure 2. The individual NPs
width (h and w in Figure 3a) of 137 gold NPs were 15.6 and 18.5 nm, respectively. In the top view, the average diameter of 174 gold NPs was estimated to be 18.5 nm by measuring two lines bisect at right angles (l1 and l2 in Figure 3b). The width in the cross-sectional images and the diameter in the top-view images are almost consistent, indicating that the gold NPs have the average diameter of ∼18.5 nm in the plane. In contrast, the height estimated by the cross-sectional images was ∼3 nm smaller than the diameter in the plane. Thus, the gold NPs are characterized to have an oblate spheroidal shape as an averaged shape with a 15.6 nm polar diameter (dz) and a 18.5 nm equatorial diameter (dx and dy). Quantitative Characterization of the Gold Trimer Rings on Quartz Substrates. Immobilization of the gold NPs after the DNA-templating via electrostatic force successfully resulted in well-dispersed gold NPs on a centimeter-scale quartz substrate. Positive charges of the amino group of the functionalized quartz surface and negative charges of the DNA template and the stabilizing molecules attracted each other. Figure 4 shows tapping mode AFM images of the gold NPs on quartz substrates prepared by using the colloid solution of trimer. From the height image (Figure 4a), it is confirmed that the gold NPs were adsorbed on the quartz surface without stacking in the height direction. Although the constituting gold NPs were visualized as if they were tightly attached as shown in the enlarged images in
Figure 2. STEM images of a gold (a) ring and (b) linear trimer obtained after AEG separation.
of trimers are completely discrete each other keeping a nanogap of about 1−2 nm. When dried, the NPs gathered because of capillary force, resulting in the shorter distance compared with the extended length of the DNA template. The nanogaps are not vacant but filled with the stabilizing molecules and the DNA templates surrounding the gold NPs. The AGE currently employed was not able to separate trimer rings from linear trimers; thus, both structures were observed in the trimer band
Figure 4. AFM (a) height and (b) phase images of gold trimers on quartz substrates. The insets are the enlarged images. The trimers are indicated by the dashed white circles in (a). The height range is 25 nm. 15030
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modes increases. The increase of plasmon modes provides many resonances. Thus, the plasmonic resonant absorption dip of the metal nanostructure becomes broader. The plasmonic properties of trimers of metal particles were discussed to have two resonances corresponding to the bonding and antibonding modes of plasmon coupling based on in-plane symmetry adapted coordinates.41,42 Sheikholeslami et al. calculated the extinction cross section of isosceles trimers that break symmetry. Because of the interplay of electric and magnetic modes, an isosceles trimer has two resonances even with the orthogonal incidence.43 Actually, we cannot neglect the asymmetric properties of the DNAtemplated gold trimer ring because the self-assembly is imperfect at each structure. Additionally, the chemically synthesized gold NPs have the dispersion in shape and size, leading symmetry breaking in the nanogaps of trimers. Therefore, the DNA-templated gold trimer ring may have a magnetic dipole to effect on the resonant wavelength. Plasmonic Responses of Trimers of Gold NPs for Polarized Oblique Incidence. To further understand the spectra of the gold NP trimers, we carried out transmission measurements using polarized oblique incidence. Figures 6a and 6b show the transmission spectra of the trimer sample measured for transverse magnetic (TM) and transverse electric (TE) polarizations. In the case of TM wave at the incident angle of 0° (Figure 6a), the spectrum had two dips at 531 and 591 nm. As the angle increased to 70°, the dip at the longer wavelength disappeared, while the dip at the shorter wavelength was strengthened and blue-shifted to 518 nm. This change is reasonably understood as follows. At the incident angle of 0°, the electric field of TM wave induced in-plane plasmon resonances of the trimer, and their hybridization properties resulted in the unique spectrum. At large incident angles, on the other hand, the plasmon resonances along the height direction (dz) of the trimer was dominant; thus, the spectrum became simple and similar to that of the individual gold NP. For TE wave (Figure 6b), the spectral shape was almost the same as that of the TM wave at the incident angle of 0°, indicating the isotropic property of the sample. This result is reasonable under the consideration of the random orientation caused by the self-assembling process. However, different from the TM wave case, no dip shift was observed for TE wave even when the incident angle increased. This is because the electric field of TE wave always lies onto the plane of trimer, providing the constant electromagnetic interaction with the trimer at any incident angle. To analyze the experimental results, we carried out a set of the FEM simulations for transmission spectra of the trimer rings. In the simulation model, three gold oblate spheroids with the averaged size obtained from Figure 3 were located on a quartz substrate so as to form a ring with three equivalent nanogaps. Considering that the self-assembled trimers contained the dispersion in nanogaps, trimer models with three different nanogaps of 2, 0.5, and 0 nm were calculated. Figures 6c and 6d show the simulated transmission spectra of the trimers for TM and TE polarizations. For TM wave at the incident angle of 0° (Figure 6c), the spectra of the models with 2 and 0.5 nm nanogaps had one dip at 521.4 and 542.1 nm, respectively. In the case of 0 nm nanogap, on the other hand, the spectrum had two dips at 536.3 and 778.7 nm. The new dip at 778.7 nm is due to the connection of gold NPs. Since no dip between 700 and 800 nm was observed in the experimental result (Figure 6a), the DNA-templated trimers had nanogaps
Figures 4a and 4b, the boundaries between the individual gold NPs of an assembly in both height and phase AFM images distinguished among dimer, trimer, and others. By counting the number of the gold NP assemblies and their constituting NPs, the existence probability of trimer was estimated. Existence probability is defined as the percentage of the number of NPs forming the objective assembly out of the total number of NPs counted in the AFM images. Although the trimer colloid solution obtained after AGE separation was supposed to contain only trimers, the existence probability of trimer was 65% out of 272 NPs. There were 43% of trimer rings and 22% of linear trimers. As for the rest, 8%, 12%, and 15% were monomer, dimer, and others, respectively. The major cause for the decrease of existence probability is that the adsorption− desorption process dynamically destroys the bonding of DNA template. Obviously, the strength of bonding between complementary DNAs of DNA templates influences the stability of the trimer ring structure. As a fact, the existence probability of trimer ring was as low as 20% without photocross-linking because of the weakness of the bonding between the DNA templates.36 While the improvement of the existence probability is an issue of this process, the obtained existence probability, 43%, is high enough to expect a far-field transmission spectrum with the properties of the trimer ring as the dominant assembly structure. Note that we prepared the centimeter-scale quartz substrates with DNA-templated gold trimer rings, demonstrating a prominent feature of selfassembly nanofabrication. Plasmonic Responses of Monomers and Trimers of Gold NPs for Nonpolarized Normal Incidence. Figure 5
Figure 5. Transmission spectra of gold monomers (red line) and trimers (dark blue line) on quartz substrates measured for nonpolarized normal incidence. The spectra are vertically shifted for clarity.
shows transmission spectra of gold NPs immobilized on quartz substrates. The monomer sample exhibited a monomodal absorption with the dip position of 522 nm. Considering that this dip position is almost consistent with the absorption dip of the gold NP colloid solution used for the preparation, the gold NP monomers are assumed to have no serious aggregation. The trimer sample, on the other hand, exhibited a broader absorption dip having two dip maxima at 532 and 596 nm. The trend of the spectral change was that the dips were redshifted and broadened as the gold NP structure changed from monomer to trimer. Generally, as the size of metal nanostructure increases, the plasmonic resonant wavelength increases. In addition, as the nanostructure becomes complicated, the number of the possible local surface plasmon 15031
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Figure 6. Measured transmission spectra of gold trimers on a quartz substrate measured for (a) TM and (b) TE polarizations. The incident angle changes at 0° (yellow and light blue lines) and 70° (orange and blue lines) for each polarization. Corresponding simulated transmission spectra of gold trimer rings for (c) TM and (d) TE polarizations with different timer’s gaps of 2, 0.5, and 0 nm. The insets show the simulation model to define the incident polarization and the spectra are vertically shifted for clarity. The incident angle dependence of the absorption dip positions for (e) TM and (f) TE polarizations. Triangle is for the experimental results, and circle and cross are for the simulation ones with the gaps of 2 and 0.5 nm, respectively.
and the each constituting NP was completely discrete. As the incident angle increased to 70°, the dip at around 530 nm was blue-shifted for all the simulation models, and this trend also agrees with the experimental results. Similar to the TM wave case, the spectra of the models with 2 and 0.5 nm nanogaps had one dip for TE wave at the incident angle of 0° (Figure 6d). On the other hand, the spectrum of the model with 0 nm nanogap had three dips at 523.2, 571.0, and 708.7 nm. The new dip at 708.7 nm is again due to the connection of gold NPs. As the nanogap decreased from 0.5 to 0 nm, the dip at 548.1 nm split into two dips at 523.2 and 571 nm, corresponding to the antibonding and bonding plasmon coupling of trimer, respectively.41,42 The broadening of the absorption dip of the trimer with 0.5 nm nanogap may indicate that the 0.5 nm nanogap starts to allow the antibonding and bonding plasmon coupling observed in the model with 0 nm nanogap. The dip positions for the measured and simulated results are summarized in Figures 6e and 6f as a function of the incident angle. For TM wave (Figure 6e) at the incident angle of 0°, the measured dip position sits in between the simulated ones with 0.5 and 2 nm nanogaps. As a result, the averaged gap distance of the DNA-templated trimers is estimated to be within 0.5 and 2 nm. At the incident angle of 70°, on the other hand, the dip positions were almost identical. This is because the electric field of TM wave at large incident angles can induce the plasmon resonances only along the height direction (dz) of the gold NP. The difference between the dip positions of the measured and simulated spectra was 6.5 nm. This difference is mainly caused by the discrepancy between the surrounding refractive index of the gold NPs in the experiment and simulation. Therefore, the red-shift caused by the stabilizing molecule and the DNA
templates surrounding the gold NPs is estimated to be about 6.5 nm. Finally, we discuss the incident angle dependence of the dip positions for TE wave shown in Figure 6f. TE wave at large incident angles may induce two possible plasmonic resonances of the trimer ring: electric and magnetic ones. The electric resonance, which arises from the in-plane plasmons in the trimer, is always observed for TE wave at any incident angle. On the other hand, the magnetic resonance, which is associated with a cyclic oscillation of electrons around the trimer ring, can be excited by a magnetic flux perpendicular to the plane of the trimer. Since this magnetic mode has a different resonant wavelength from that of the electric one, one could observe a dip shift when the incident angle changed. However, such a behavior was not observed in Figure 6f for either the experimental or simulation results due to weak magnetic interaction with the trimer rings. Mühlig et al. demonstrated magnetic responses in the visible range by a large core−shell structure consisting of gold NPs.44 Therefore, we believe that a DNA-templated gold NP ring with a larger diameter realizes much stronger magnetic resonances in the visible light region.
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CONCLUSIONS In this report, we have demonstrated the DNA-templated gold trimer ring that is pronounced among the metal NP assemblies aiming the practical application to the optical metamaterials. Our DNA-templating process relays only on self-assembly through simple hybridization of DNAs and thus should be compatible to mass production. Since DNA templates are also conventionally designed and synthesized, a wide variety of gold NP assemblies can be promptly prepared with different numbers and sizes of gold NPs. Hence, one can reach large15032
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scale metamaterials following further investigations of optical functions of gold NP rings.
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ASSOCIATED CONTENT
S Supporting Information *
Sequences and the detailed structure of the DNA template; the AGE results for the building blocks and the gold trimer rings. This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This research was partly supported by Grant-in-Aid for Scientific Research on Innovative Areas (22109006) and Grant-in-Aid for Challenging Exploratory Research (22651045). R. Watanabe-Tamaki thanks Research Fellowships of the Japan Society for the Promotion of Science for Young Scientists. We thank the RIKEN Integrated Cluster of Clusters (RICC) at RIKEN for the computer resources used for the calculation.
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REFERENCES
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