Details of a Researcher - Isobe, Tetsuhiko

Research Projects of Competitive Funds, etc. - Isobe, Tetsuhiko

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Pioneering Liquid Phase Synthesis of Innovative Carbon Quantum Dot Fluorescent Materials

2024.04

2027.03

Grants-in-Aid for Scientific Research, Grant-in-Aid for Scientific Research (C), Research grant, Principal investigator

蛍光ピークが単一であり、蛍光ピーク幅が狭く、蛍光ピーク波長が調節可能である蛍光特性を持ち合わせた、環境負荷の少ない炭素を主成分とするカーボン量子ドット（CQD）蛍光材料を作製することを追究する。研究代表者のグループのこれまでの無機化合物量子ドットの液相合成での知見と技術を活かして、高沸点の非水溶媒を使用し、電子供与性官能基をもつ炭素原料分子をビルディングブロックとして組み立てる液相合成法を構築する。sp2ドメインを形成し、HOMO－LUMO間のπ電子遷移により励起・蛍光を起こさせ、周囲環境の極性により励起・蛍光エネルギーを制御できる革新的CQD蛍光材料を開拓する。

Development of advanced liquid-phase synthesis of carbon dots with controlled photoluminescence properties

2019.06

2022.03

Keio University, Grants-in-Aid for Scientific Research, Isobe Tetsuhiko, Challenging Research (Exploratory), Research grant, Principal investigator

Recently, quantum dots (QDs), which emit light in the band gap and whose emission color can be tuned by controlling particle size through the quantum size effect, have emerged. CdSe, InP, and CsPbBr3 have been developed as QD materials; however, they have problems in terms of toxicity and stability. Against this background, light-emitting materials with high environmental friendliness are required; therefore, we focused on the preparation of luminescent carbon dots (CDs) from abundant carbon sources. In this work, we pioneered an advanced liquid-phase method to synthesize CDs by polymerizing aromatic compound materials in non-aqueous solvents. Furthermore, the effects of surface modification and surrounding environment on the luminescence properties were investigated to control them.

Improvement of properties of solar cells by quantum dots with the function of UV - visible wavelength conversion

2018.04

2021.03

MEXT,JSPS, Grant-in-Aid for Scientific Research, ISOBE Tetsuhiko, Grant-in-Aid for Scientific Research (B), Research grant, Principal investigator

An ultraviolet light absorber is used in a photovoltaic module to protect its parts from ultraviolet light. As a result, in photovoltaic modules, the spectral sensitivity of wavelengths below 400 nm is reduced. If a phosphor converting ultraviolet light into visible light is used instead of the ultraviolet light absorber, it is possible to protect the photovoltaic module from ultraviolet light and at the same time to improve the spectral sensitivity of the photovoltaic module in ultraviolet range. In this research, we have developed core / shell type CuInS2/ZnS and CuGaS2/ZnS quantum dot phosphors that convert ultraviolet light to visible light and have high transparency to visible light with high spectral sensitivity of photovoltaic modules. We investigated the improvement of photoelectric conversion efficiency of the photovoltaic module by wavelength conversion of these quantum dots.

Preparation, characterization, and application of near-UV -> red conversion fluorescent nanosheets and their composite films

2015.04

2018.03

MEXT,JSPS, Grant-in-Aid for Scientific Research, ISOBE Tetsuhiko, Grant-in-Aid for Scientific Research (C), Research grant, Principal investigator

Y2O3:Bi3+,Eu3+ nanosheets, which emit red light under near-UV irradiation, were obtained by calcining hydroxide precursor nanosheets synthesized through a hydrothermal method. An aqueous dispersion of positively charged Y2O3:Bi3+,Eu3+ nanosheets with polyethyleneimine adsorbed to the surface was prepared for their deposition. Fluorescent nanosheets were electrophoretically deposited on a transparent conductive substrate under a constant voltage. Next, polyvinylpyrrolidone was coated on the film. The effects of the PVP-coated film samples on the properties of a commercial monocrystalline silicon photovoltaic module were evaluated.

Preparation and characterization of biocompatible composite beads with fluorescent nanoparticles

2011.04

2014.03

Keio University, ISOBE Tetsuhiko, Grant-in-Aid for Scientific Research (B), Research grant, Principal investigator

Preparation of chitosan beads by cross-linking method through electrostatic interaction with tripolyphosphate ions, combined with emulsification method using SPG membrane was studied. Next, hybridizations between green-emitting YAG:Ce3+ nanoparticles and chitosan beads, and between red-emitting Eu3+-doped apatite nanoparticles and chitosan beads were studied. Hybridizations between Eu3+-doped apatite nanoparticles and silica beads, and between YAG:Ce3+ nanoparticles and alginate beads were also studied.

Preparation and characterization of composites consisting of nanoparticles with dual functions of fluorescence and magnetism

2008

2010

Keio University, ISOBE Tetsuhiko, Grant-in-Aid for Scientific Research (B), Research grant, Principal investigator

Preparation of transparent nanocrystal phosphors and their application for wavelength conversion optical devices

2005

2006

Keio University, ISOBE Tetsuhiko, Grant-in-Aid for Scientific Research (B), Research grant, Principal investigator

Chemically and thermally stable LaPO_4 doped with Ce^<3+> and Tb^<3+> exhibits green emission corresponding to the f-f transition of Tb^<3+> through energy transfer from Ce^<3+> to Tb^<3+> during exciting the d-f transition of Ce^<3+>. We discuss the influence of solvents and surface modifiers of the transparent LaPO_4 : Ce, Tb colloidal solutions on their crystal fields and photoluminescence properties. The transparent LaPO_4 : Ce, Tb colloidal solution was prepared from acetates and trimethyl phosphate in the mixed solvent of water and methanol by autoclave treatment below 150℃, followed by dialysis. According to the observation by transmission electron microscope and the particle size measurement by dynamic light scattering, the particles were less than 10nm in diameter. We found out the method for preparing transparent colloidal solutions of LaPO_4 : Ce, Tb with surface modification, e.g., by the addition of sodium polyphosphate, lauryl phosphate and hydrophilic phosphate modifiers, or by redispersing modified nanoparticles in n-hexane and dimethyl sulfoxide. We verified surface passivation, i.e., capping surface anion vacancies by surface modification based on the quantitative analysis of photoluminescence internal quantum efficiency and the decay curve measurement. On the other hand, we attempted to produce nanoparticle-dispersed resin sheets from the colloidal solution of 〜5nm ZnS : Mn nanoparticles. We also investigated other phosphors with different luminescence colors and the same excitation region as that of ZnS : Mn.

Characterization of hybrid nanoparticle phosphors synthesized in reverse micelle nano-reactors

2002

2004

Keio University, ISOBE Tetsuhiko, Grant-in-Aid for Scientific Research (C), Research grant, Principal investigator

ZnS:Mn^<2+> nanocrystals of 3nm in diameter were coated with silica to form 20±4nm core/shell particles, as observed by transmission electron spectroscopy. For ZnS:Mn^<2+> nanocrystals modified with silica, the intensity of photoluminescence (PL) due to d-d transition of Mn^<2+> increases with increasing the TEOS amount and hence the thickness of silica shell. This is attributed to the passivation of surface defects and quantum confinement effect. On the contrary, the PL intensity of ZnS:Mn^<2+> nanocrystals modified by both lauryl phosphate (HLP) and silica decreases with increasing the TEOS amount. ^<31>P CP/MAS NMR spectroscopy reveals that chemical interaction between phosphate groups and ZnS:Mn^<2+> is interrupted by silica shell. These results suggest that surface modification of HLP around ZnS:Mn^<2+> nanocrystals is more effective than the formation of silica shell from the viewpoint of PL enhancement.
Aging of the of ZnS:Mn^<2+>/SiO_2 core/shell colloidal solution for 14 days increases the PL intensity of the obtained powder sample by a factor of three, as compared with that for 1 day. This is possibly attributed to the better quantum confinement effect and the more homogeneous incorporation of Mn^<2+> to ZnS by aging. The transparency of the ZnS:Mn^<2+>/SiO_2 core/shell nano-phosphor-dispersed sheet was higher than that of ZnS:Mn^<2+> bulk-phosphor- dispersed sheet. This is attributed to the markedly lower Rayleigh scattering of visible light for the former. The Mie scattering occurs for the bulk-phosphor-dispersed sheet because of the micrometer particle size. The PL intensity of the nano-phosphor-dispersed sheet increased with increasing the phosphor's content, while the PL intensity of the bulk-phosphor-dispersed sheet decreased with increasing the phosphor's content. This is also explained by the high transparency and the negligible backscattering loss of the orange emission for the nano-phosphor's sheet.

逆ミセル微小反応場を利用した表面修飾ナノ粒子の合成と発光特性

2000

2001

Keio University, Grant-in-Aid for Encouragement of Young Scientists (A), Research grant, Principal investigator

本研究では、界面活性剤によって形成される逆ミセルを微小反応場と利用してZnS:Mnナノクリスタルを合成し、さらにリン酸系界面活性剤PhosHを添加したときのMn^<2+>による発光の強度変化を調べた。ヘプタン/AOT/H_2O系逆ミセル溶液を調製し、同逆ミセル溶液80cm^3に1MZn(CH_3COO)_2水溶液0.545cm^3および0.1MMn(CH_3COO)_2水溶液0.05cm^3を混合した。この溶液を、所定のS/(Zn+Mn)モル比になるようにNa_2Sを含む逆ミセル溶液30cm^3に混合し、ZnS:Mn^<2+>ナノ粒子が分散した透明なサスペンションを合成した。このとき、すべてのサンプルで[H_2O]/[AOT]が9.64となるように調製した。さらに[PhosH]/[AOT]=0.0943になるようにサスペンションにPhosHを加えた。ZnS:Mn^<2+>ナノ粒子が分散したサスペンションのUV-Vis吸収スペクトルはS/(Zn+Mn)モル比の減少とともにブルーシフトした。これはZnSのバンドギャップ(Eg)がS/(Zn+Mn)モル比の減少に伴い増加することを意味する。吸収端から求めたEgを用いて、有効質量近似法によって求めた光学的粒子直径は、S/(Zn+Mn)モル比が1.6から0.6に減少するに伴って3.3nmから2.8nmへ減少した。この原因としてS/(Zn+Mn)モル比の減少に伴う逆ミセル内のアニオン濃度およびpHの減少、PhosH添加によるpHの減少および逆ミセル同士の衝突頻度の減少が考えられる。ZnS:Mn^<2+>ナノ粒子が分散したサスペンションは、どの試料もMn^<2+>のd-d遷移によるオレンジ色(波長580nm)発光を示す。この発光に対する励起スペクトルはS/(Zn+Mn)モル比の減少に伴い光学的粒子直径と相関してブルーシフトした。これより、S/(Zn+Mn)モル比を操作することによって粒子直径およびMn^<2+>の発光に最適な励起波長を制御できることが明らかにされた。ZnS:Mn^<2+>ナノ粒子が分散したサスペンションにPhosHを添加することで発光量子効率は増大した。PhosH添加による発光量子効率の増大量はx=1.2のところで最大であった。^<31>P-NMRのケミカルシフトも類似の傾向が見られることから、PhosHとZnS:Mn^<2+>との相互作用が発光効率の増加に重要な役割をすることが明らかにされた。

CHANGE IN THE MAGNETIC PROPERTIES OF BY NON-ISOTROPIC MECHANICAL STRESS

1997

1999

Keio University, SENNA Mamoru, YOSHIOKA Naoki, INOUE Hidenari, ISOBE Tetsuhiko, Grant-in-Aid for Scientific Research (B), Research grant, Principal investigator

We have concentrated ourselves on the molecular strain of Fe (II) complex compounds throughout this project. While we started from anomalous magnetic properties of such strained complexes, larger portion is devoted to the ligand exchange and solid-state synthesis of those complexes. All the solid-state reactions in the latter category are the consequences of molecular strain. Main results are correspondingly divided into two parts, I.e,
(A) Change in the magnetic properties due to molecular strain, and
(B) Ligand exchange reaction as a consequence of disproportionation of metal-ligand coordination bonds.
The category (A) is attributed to the change from the low spin (ィイD11ィエD1AィイD21gィエD2) to high spin (ィイD15ィエD15TィイD22gィエD2) state. The spin crossover temperature lowers while a metastabe high spin state at temperatures as low as 10K appears simultaneously. These are explained by the change in the distribution of electron energy states induced by the molecular strain and related loss of ligand field symmetry.
We then proceeded to the chemical consequences of loss of the ligand field symmetry. Disproportionation of the coordination bonds between Fe (II) and the ligands inevitably results in the attack of any chemical species, as counter ions, as solvents or as an independently dissolved organic molecules, which are capable to form coordination bonds with Fe (II). Examples of ligand exchange in its narrow definition are: formation of [Fe(bpy)(phen) ィイD22ィエD2](NCS) ィイD22ィエD2, [Fe(bpy)ィイD22ィエD2(phen)] (NCS) ィイD22ィエD2, [Fe(phen)ィイD23ィエD2](NCS) ィイD22ィエD2 from a mixture of [Fe(bpy) ィイD23ィエD2](NCS)ィイD22ィエD2(bpy=2,2'-bipyridine) and phen (1,10-phenanthroline) by simple milling in a planetary mill.
We further succeeded direct solid-state synthesis of [Fe(phen)ィイD23ィエD2] ClィイD22ィエD2 by milling a mixture of FeClィイD22ィエD2・4HィイD22ィエD2O and phen. We confirmed the disproportionation of coordination bonds between Fe (II) and four HィイD22ィエD2O molecules as 1igands by different spectroscopies. Finally, we concluded that the deletion of the weakest HィイD22ィエD2O is immediately followed by the new coordination bonding with phen. By the same token, preliminary milling of FeClィイD22ィエD2・4HィイD22ィエD2O alone turned out to accelerate rate of [Fe(phen) ィイD23ィエD2] ClィイD22ィエD2 formation.

カルボン酸と複合化したマンガンドープ硫化亜鉛微粒子の発光特性を支配する因子

1997

1998

Keio University, Grant-in-Aid for Encouragement of Young Scientists (A), Research grant, Principal investigator

逆ミセルは有機溶媒中における界面活性剤会合コロイドであり、ミセル内部に水を可溶化することでナノサイズのウォーターブールを形成できる。このウォーターブール中に様々な水溶性の反応物質を可溶化することによって、ナノサイズ粒子を合成できる。本年度の研究では、逆ミセルを用いて、Mn^<2+>にZnSをドープしたZnS:Mnナノクリスタル蛍光体を合成する手法を確立し、さらに、粒子の凝集状態が光学特性に与える影響を検討した。
溶媒としてヘプタン中に界面活性剤ビス(2-エチルヘキシル)スルホこはく酸ナトリウム(AOT)と水を加え、ミセル溶液を作成した。一方に、酢酸亜鉛水溶液と酢酸マンガン水溶液を加え、もう一方には硫化ナトリウム水溶液を加え、2種類のミセル溶液を作成し、両者を混合することによって、ZnS:Mnナノクリスタルを合成した。
光吸収スペクトルから、合成したZnS:MnのバンドギャップE_gは4.13eVであり、バルクの値3.58eVよりも大きかった。この差から、effectivemasstheoryに基づくと粒子半径は1.5nmである。粒子半径はポーア半径3nmよりも小さいので、量子サイズ効果によってE_gが増大したことが確認された。発光スペクトルには、2.14eVにMn^<2+>のd-d遷移による発光が見られた。励起スペクトルのピーク位置は4.01eVであり、バルクのE_gよりもブルーシフトした。一方、動的光散乱法(DLS)から求めた粒子半径は11±3nmであり、上記の光測定から求めた値を上回る。これは次のように解釈できる。逆ミセル溶液中で、ー次粒子同士は凝集しているが、AOTによる立体障害によって直接接触していない。したがって、粒子間で電子やホールの移動は起こらず、量子サイズ効果が現れる。今後は、逆ミセル剤にカルポン酸系の添加物を使用して発光強度を増幅させることを計画している。

ダブルイオン注入法による強磁性ナノ微粒子分散型シリカガラスの製造と特性評価

1996

Keio University, Grant-in-Aid for Encouragement of Young Scientists (A), Research grant, Principal investigator

イオン注入を利用して異なる注入イオン同士を選択的に反応できれば、マトリックス中に化合物が分散したニューコンポジットを合成できる。本研究では窒化鉄の磁気的性質を利用して、シリカガラス中にNとFeを連続してイオン注入した場合に、両者が選択的に反応するかどうかを検討した。シリカガラス(Corning 7940-excimer grade)に、室温、4μA・cm^<-2>の条件でイオン注入を行った。NとFeの深さ方向の分布が一致するように、TRIM89によってあらかじめシミュレーションし、N^+とFe^+注入エネマギ-をそれぞれ52keVと160keVに決定した。Fe注入量は6×10^<16>cm^<-2>で一定とし、N注入量を0から1.2×10^<17>cm^<-2>まで変化させた。Nの次にFeを注入した試料はN+Fe、その逆に注入した試料をFe+Nと記述する。強磁性共鳴(FMR)スペクトルの測定の結果、共鳴磁場H_s(θ)は、磁場の方向がイオン注入面の法線方向と水平である(θ=0°)方が垂直である(θ=90°)よりもおおきかった。化学組成N/Feに対してFMR吸収強度および角度依存性から計算した磁化を調べたところ、N/Fe〜0.2付近で極大を示すことが明らかにされた。このとき、両者の値は試料Fe+Nの方が試料N+Feよりも小さかった。窒化鉄の磁化はN/Fe組成比に依存して変化し、N/Fe〜0.125で極大を示す。したがって、FMRの結果は注入したNとFeの間に少なからず反応が起こることを示唆する。さらに、^<57>Feを用いて作製した試料の内部転換電子メスバウアー共鳴や上記の注入量を5倍にした試料のX線回折の結果から、この推論が正しいことが裏付けられた。

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