Original Papers

1. Tetsu Yonezawa,* Hiroki Tsukamoto, and Masaki Matsubara
   “Low-temperature nanoredox two-step sintering of gelatin nanoskin-stabilized submicrometer-sized copper fine particles for preparing highly conductive layers”
   RSC Advances, 5(75), 61290-61297 (2015).
   DOI: 10.1039/C5RA06599B (Published (web): 7 July 2015)

   
   Abstract: A conductive paste was prepared with submicrometer-sized copper fine particles that were coated by a gelatin nanoskin. The average particle size was ca. 130 nm. After drying, an oxidation-reduction two-step sintering process was proposed for these submicrometer-sized copper fine particles to obtain low-resistivity layers at a very low temperature. The optimized conditions gave a low resistivity of 8.2 × 10^-6 Ω cm, even when the particles were covered by a common polymer. Our results suggest that the formation of the small copper oxide prominences on the surface of the copper fine particles could be reduced at low temperatures, forming connected submicrometer-sized metallic copper particles.

2. Xu Lu, Yohei Ishida, Tomobumi Mishina, Thanh Mai Nguyen, and Tetsu Yonezawa
   “Enhanced Terahertz Emission from Cu_xO/Metal Thin Film Deposited on Columnar-Structured Porous Silicon”
   Bulletin of Chemical Society of Japan, in press.
   DOI: 10.1246/bcsj.20150173 (Published (web): 13 July 2015)
   Abstract: We propose a novel method to enhance the THz emission from Cu_xO/metal nano thin film by using columnar-structured micro-sized porous silicon (MS-PSi) as the substrate. THz emission was improved several times higher than that from the planar substrate due to the decreased optical reflectance and enlarged surface area.
3. Xu Lu, Yohei Ishida, Mai Thanh Nguyen, and Tetsu Yonezawa*
   ”Synthesis and fluorescence properties of nanoisland-structured SiO_x/Cu_xO composite”
   Journal of Materials Chemistry C, in press.
   DOI: 10.1039/C5TC01685A (Published (web): 7 July 2015)

   
   Abstract: A nano-island structured SiO_x (x ≤ 2)/Cu_xO (x ≤ 2) composite was prepared from a Cu deposited Si wafer by electrochemical anodization in a hydrofluoric acid (HF) and ferric nitrate (Fe(NO₃)₃) electrolyte. The surface composition of the nano-island consisted of Cu, Cu oxides, Si nanoparticles and Si oxides. The distribution of Cu element on the surface was uniform. Cracks and native Cu oxides in the deposited Cu layer are two main factors for the formation of the nano-island structured surface. Different from a single red photoluminescence (PL) band emitted from the electrochemical anodized bare Si wafer, an uncommon dual-visible-band PL at red and blue ranges respectively with comparable intensity was observed. The red PL quenching in Cu ions doped nanostructured Si did not happen due to the different distribution areas of Cu⁺ and red PL centre.The origin of the red PL was due to oxygen defects in the band gap of Si nanoparticles/Si oxides; the blue PL was the consequence of the transitions between 3d₁₀ ↔ 3d₉4s₁ of Cu⁺ ions and the intra d → d band transition of Cu²⁺ ions in the interstitial vacancies of Si oxides.  Compared to the electrochemical anodized bare Si sample with a single PL band, the nano-island structured SiO_x/Cu_xO composite emitting a dual-PL-band is more promissing for making white light emitter.

4. Yohei Ishida, Wataru Doshin, Hiroki Tsukamoto, and Tetsu Yonezawa*
   “Black TiO₂ Nanoparticles by a Microwave-induced Plasma over Titanium Complex Aqueous Solution”
   Chemistry Letters, in press.
   DOI: 10.1246/cl.150531 (Published (web): 4 July 2015)
   Abstract: We herein report a novel synthesis of black TiO₂ nanoparticles by using a microwave-induced plasma over Ti complex aqueous solution. The resultant TiO₂ nanoparticles showed very strong reflectance over the whole visible light region. Detailed HAADF-STEM analysis of the obtained particles revealed that this plasma method enables to dope W ions inside the TiO₂ crystal structure.
5. Xu Lu, Yohei Ishida, and Tetsu Yonezawa*
   “Synthesis and fluorescence property of columnar porous silicon: the influence of Cu-coating on photoluminescence behaviours from hydrofluoric-acid-treated aged columnar porous silicon”
   New Journal of Chemistry, in press.
   DOI: 10.1039/C5NJ00909J (Published (web): 2 June 2015)

   
   Abstract: A column-shaped porous silicon (PSi) sample was successfully prepared by electrochemical anodization in a hydrofluoric acid (HF) containing electrolyte. PSi exhibited an unstable red photoluminescence (PL). The PL arose from the surface oxygen defects under quantum confinement effect and degraded as the exposure time in air increased. After hydrofluoric acid treatment and 2-min Cu sputtering coating, the PL-degraded aged column-shaped PSi showed a dual PL bands at blue and yellow range. Cu-coated fresh PSi without aging and Cu-coated aged PSi without HF treatment did not show that result. The PL intensity of these two bands was improved compared to that of the fresh column-shaped PSi. These two emissions also showed a relatively good stability as determined from the same intensity after air treatment for three months. The improvement of PL can be attributed to the surface unstable SiO that combines with the sputtered Cu, forming new Cu related defect sites and surface nanostructures. The method in this study does not require any heating treatment, because SiO is very unstable, particularly in a high-moisture environment.

6. Yohei Ishida, Taiki Sumi, and Tetsu Yonezawa*
   “Sputtering Synthesis and Optical Investigation of Octadecanethiol–Protected Fluorescent Au Nanoparticles”
   New Journal of Chemistry, in press.
   DOI: 10.1039/C5NJ01011J (Published (web): 29 May 2015)

   
   Abstract: Here we report for the first time the synthesis of octadecanethiol–capped gold nanoparticles (Au NPs) by sputtering of Au over a liquid matrix (silicon–oil). Au NPs prepared in silicone–oil showed plasmon absorption; however, those prepared in the presence of 1–octadecanethiol did not show plasmon absorption but fluoresced in the near IR region.

7. Yohei Ishida, Ryota Nakabayashi, Masaki Matsubara, and Tetsu Yonezawa*
   “Silver sputtering into a liquid matrix containing mercaptans: the systematic size control of silver nanoparticles in single nanometer-orders”
   New Journal of Chemistry, 39(6), 4227-4230 (2015).
   DOI: 10.1039/C5NJ00294J (Published (web)：27 April 2015)

   
   Abstract: We herein propose a novel methodology to synthesize silver nanoparticles with precisely controlled diameters in single nano-meter orders by sputtering into a liquid matrix (polyethylene glycol) containing mercaptans (11-mercaptoundecanoic acid) as an environmental friendly preparation without reductants.

8. Yingqiong Yong, Tetsu Yonezawa,* Masaki Matsubara, and Hiroki Tsukamoto
   “Mechanism of Alkylamine-Stabilized Copper Fine Particles towards Improving Electrical Conductivity of Copper Films at Low Sintering Temperature”
   Journal of Materials Chemistry C, 3(23), 5890-5895 (2015).
   DOI: 10.1039/C5TC00745C (Published (web) : 09 April 2015)

   
   Abstract: To facilitate sintering of copper particles at low temperatures and achieve excellent electrical conductivity of copper films, reducing particle size is a common method. However, strong reducing agents which are usually used for reducing the size of particles limit their application. Here we report a novel approach to obtain highly conductive copper films. Firstly, copper fine particles were prepared by a one-pot reduction reaction utilizing D-isoascorbic acid as a mild reductant. Secondly, tight connection facilitating the sintering of particles was formed by generating convex surfaces, nanorods or nanoparticles during the oxidation procedure. The mechanism of the oxidative preheating process and its effects on the conductivity were clarified. High conductivity of copper films at low temperatures can be achieved due to the critical role of the oxidative preheating process.

9. Yohei Ishida, Thatchanont Jirasupangkul, and Tetsu Yonezawa*
   “One-Pot Preparation of Cationic Charged Pt Nanoparticles Through an Autocatalytic Hydrolysis of Acetylthiocholine”
   New Journal of Chemistry, 39(6), 4214-4217 (2015).
   DOI: 10.1039/C5NJ00420A (Published (web) : 08 April 2015)

   
   Abstract: We propose a novel synthetic approach for cationic charged platinum nanoparticles via the autocatalytic hydrolysis of acetylthiocholine. This method can be extended to the platinum group of metals.

10. Taiki Sumi, Shingo Motono, Yohei Ishida, Naoto Shirahata, and Tetsu Yonezawa*
    “Formation and Optical Properties of Fluorescent Gold Nanoparticles Obtained by Matrix Sputtering Method with Volatile Mercaptan Molecules in the Vacuum Chamber and Consideration of Their Structures”
    Langmuir, 31(14), 4323-4329 (2015).
    DOI: 10.1021/acs.langmuir.5b00294 (Published (web):March 15, 2015)

    
    Abstract: This paper proposes a novel methodology to synthesize highly fluorescent gold nanoparticles (NPs) with a maximum quantum yield of 16%, in the near-infrared (IR) region. This work discusses the results of using our (previously developed) matrix sputtering method to introduce mercaptan molecules, α-thioglycerol, inside the vacuum sputtering chamber, during the synthesis of metal NPs. The evaporation of α-thioglycerol inside the chamber enables to coordinate to the “nucleation stage” very small gold nanoclusters in the gas phase, thus retaining their photophysical characteristics. As observed through transmission electron microscopy, the size of the Au NPs obtained with the addition of α-thioglycerol varied from approximately 2–3 nm to approximately 5 nm. Plasmon absorption varied with the size of the resultant nanoparticles. Thus, plasmon absorption was observed at 2.4 eV in the larger NPs. However, it was not observed, and instead a new peak was found at approximately 3.4 eV, in the smaller NPs that resulted from the introduction of α-thioglycerol. The Au NPs stabilized by the α-thioglycerol fluoresced at approximately 1.8 eV, and the maximum wavelength shifted toward the red, in accordance with the size of the NPs. A maximum fluorescent quantum yield of 16% was realized under the optimum conditions, and this value is extremely high compared to values previously reported on gold NPs and clusters (generally ∼1%). To our knowledge, however, Au NPs of size >2 nm usually do not show strong fluorescence. By comparison with results reported in previous literature, it was concluded that these highly fluorescent Au NPs consist of gold–mercaptan complexes. The novel method presented in this paper therefore opens a new door for the effective control of size, photophysical characteristics, and structure of metal NPs. It is hoped that this research contributes significantly to the science in this field.

11. Chaorui Xue, Tetsu Yonezawa,* Mai Thanh Nguyen, and Xu Lu
    “Cladding Layer on Well-defined Double-Wall TiO₂ Nanotubes”
    Langmuir, 31(4), 1575-1580 (2015).
    DOI: 10.1021/la504670p　(Published (web):January 10, 2015)

    
    Abstract: Highly ordered double-wall TiO₂ nanotube arrays were obtained by a two-step anodization method in a fluoride-containing glycerol based electrolyte. The low water and fluoride content and high viscosity of the electrolyte support a partly undissolved fluoride-rich layer, and its hydrolyzed products remain on the tube walls. The double-wall structure and a cladding layer originating from the fluoride-rich layer were clearly observed after annealing. The morphology and crystal structure of the cladding layer were investigated. The study of the cladding layer gives a fundamental insight into the wall structure design of the anodic TiO₂ nanotubes in the glycerol-based electrolyte.

Book Chapters and Review Papers

1. 【Japanese】石田洋平
   「自分の人生の成功を測るものさしは何か」
   CCIサロン　この人、紹介
   化学と工業、68(2), 172 (2015)
2. 【Japanese】北海道大学　工学研究院　材料科学部門　先進材料ハイブリッド工学研究室
   触媒年鑑　第６編　研究活動総覧 (2015).

Report

1. 【Japanese】米澤 徹・石内俊一
   「レーザーイオン化を促進する無機ナノ材料の設計」
   平成26年度　物質・デバイス領域共同研究拠点　成果報告書　2014275 (2015)
2. 【Japanese】米澤 徹・能木雅也・菅沼克昭
   「省エネルギーナノデバイスのための銅ベース導電配線ナノ基材の開発と構造解析」
   平成26年度　物質・デバイス領域共同研究拠点　成果報告書　2013A17　(2015)