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エレクトロニクス材料の性能限界を突破するスピントロニクス材料の一つとして、その表面状態が特異なことで知られている鉄シリサイド(FeSi)が注目されている。我々はFeSiの表面・界面での電子スピン状態を陽電子測定にて観測した所、FeSi中に多くの空孔欠陥があることを見出した。従来、FeSiでのスピンダイナミクスの機構として、局所クーロン相互作用等の様々な仮説が提唱されているが、未だその起源は明確になっていない。我々は結晶中の空孔欠陥の影響が少なく無いと考え、空孔の詳細な温度特性を測定するとともに、空孔、表面、界面等の状態を第一原理バンド計算にて解析した。 FeSiバルク中の空孔欠陥の解析にはFeSiの基本単位格子(8原子)の(2✕2✕2)サイズモデル(64原子)を用いた。DFT計算にはABINITコードを用いたPAW法を使用した。空孔欠陥としてはFe単空孔、Si単空孔、Fe+Si複空孔、Fe+Fe複空孔、Fe+Fe+Fe複空孔を仮定し、各空孔が生成した場合の生成エネルギー及び電子・陽電子状態を評価した。, 第59回アイソトープ・放射線研究発表会

The effective addition of surface oxygen groups, which are active sites for redox reactions, on carbon clothes as electrodes by electron beam irradiation in normal air which contains environmental humidity, dry air, or nitrogen atmosphere was carried out. The irradiation introduced 20 at% oxygen at the carbon surface as determined by X-ray photoelectron spectroscopy and the phenol-type hydroxyl group, the carboxylic group, etc., were detected by temperature-programmed desorption. Single-cell measurements indicated the current density at 1.3 V-IR-corrected of the irradiated electrode in normal air was 28% higher than that of the as-received electrode. Since double-layer capacitance between the as-received carbon cloth and irradiated carbon cloth in normal air was similar, the improvement of current density is attributed to the increase of surface oxygen groups. In addition, the radiation in both normal air and dry air improved electrochemical activity similarly. This result suggests the radiation-chemical reaction in this study is dominated by the oxidation reaction with ozone or nitrogen oxides (NOx), while in the meantime, the contribution of the hydroxyl radical from water is considered to be negligible.

表⾯ポジトロニウム(Ps)三光⼦消滅強度の変化から最表⾯の電⼦スピン偏極状態を測定する装置の開発を進めている。分光⼿法の⾼度化として、放出ポジトロニウムのエネルギー分解と、角度分解測定を目指している。エネルギー分解では、磁性体表⾯にのみ存在する電⼦のうち、フェルミ準位付近のみのスピン偏極率の測定に成功した。現在、放出ポジトロニウムの角度分解測定の準備を進めている。, 日本物理学会第77回年次大会

数keVのエネルギーを持つ陽電子ビームを物質に打ち込むと、拡散して表面に戻ってきた陽電子が最表面電子とポジトロニウム(Ps)を形成して消滅することがある。この時、陽電子ビームがスピン偏極していれば、オルソポジトロニウムの三光子消滅強度の変化から最表面の電子スピン偏極状態を知ることができる。Psの仕事関数が負になる物質では、放出されたPsの最速成分がフェルミ面近傍の電子スピン状態を反映するため、物質最表面において電気伝導に寄与する電子のみのスピン偏極状態を得られる。我々はそのような測定が可能なスピン偏極Ps飛行時間測定(SP-PsTOF)装置を開発した。本装置で測定されたニッケル薄膜表面のSP-PsTOFスペクトルは、フェルミエネルギー(～0eV)近傍でスピン偏極率が負に傾く現象を捉えているが、第一原理計算で求まるスピン偏極率分布とは異なって見える。これはエネルギー分解能と引き換えに計測効率を高めたことに起因しており、装置分解能でコンボシューションを施した計算結果は実験結果をよく反映する。現在の測定ジオメトリではフェルミ準位付近(0eV付近)のエネルギー広がりは1.5eV程度となり、ハーフメタルなど実用材用の評価で望まれるエネルギー領域(フェルミ準位下0.1～0.5eV)を精度良く観測することが難しい。そこで計数率低下を抑えつつ分解能を向上させるべく、検出器の追加や測定系の最適化などの改良を進めた。その結果、改良後のスペクトルには、それまで低分解能のために訛されていたフェルミ面下0.5～1.5 eV付近の強度上昇を伴う構造が現れ、モンテカルロ計算との比較から分解能は0.5eVに向上したと考えられる。今後はハーフメタル表面のスピン偏極率測定などの表面スピン研究に本装置を活用していく予定である。, 京都大学複合原子力科学研究所専門研究会「陽電子科学とその理工学への応用」

Positronium is spontaneously emitted from solids, when the formation potential or the work function is negative. So far, positronium emission from solids have been investigated mainly with metals and insulators. The number research on semiconductors is rather limited. It is intriguing to study positronium emission from semiconductors, because semiconductor has intermediate features between metal and insulator, for instance, the band gap and free carrier density. In this presentation, we will report our research about positronium emission from Si, SiC and GaN(0001) surfaces mainly through the positronium time-of-flight spectroscopy. The theoretical positronium formation potentials were also obtained by first-principles calculation. The positronium energy spectra for Al(111) and W(001) rise around the calculated positronium formation potentials. This means that positrons pick up electrons from the top of the occupied state (the Fermi level) as assumed in the calculation of formation potential. Contrarily, the positronium energy spectra for SiC(0001) and Si(111) rise significantly above the calculated positronium formation potentials. Moreover, the spectrum shapes are different from those for Al(111) and W(001). In the calculation of formation potential, the electrons at the top of the valence band are assumed to be picked up by positrons. Therefore, the above experimental results imply that electrons located sufficiently above the valence band top are picked up by positrons. The maximum energies observed for SiC(0001) and Si(111) are separated from their positronium formation potentials approximately by the band gap energies (3.2 eV for SiC, 1.1 eV for Si). Thus, we propose that the conduction electrons also participate in the positronium formation in SiC and Si. In the presentation, we also report the results on GaN and more thorough analyses of positronium energy spectra based on the Monte Carlo simulation., 12.5th International Workshop on Positron and Positronium Chemistry (PPC 12.5)

固体表面からのポジトロニウム放出の研究では、主に金属や絶縁体が調べられて来たが、半導体に関する研究例は極めて少ない。半導体には、励起型（間接・直接励起）、バンドギャップ（100meV～10eV）、伝導型（n型、p型、半絶縁性）、そして、強いスピン軌道相互作用の有無によって様々な違いがある。半導体には、金属や絶縁体と同じように、負のポジトロニウム仕事関数を持つものが多くある。これらのことから、半導体からのポジトロニウム放出過程を調べることは、意味のあることと言える。そこで、我々は、2017年頃から、半導体からのポジトロニウム放出過程の研究を開始した。これまで、間接半導体であるシリコン（Si）と炭化ケイ素（SiC）からのポジトロニウム放出についての研究結果を報告してきた。今回は、直接励起型でワイドギャップを持つ窒化ガリウム（GaN）について得られた結果を報告する。, 第58回アイソトープ・放射線研究発表会

電子と陽電子との束縛系であるポジトロニウムが生成されるのは物質表面の電子密度が低い領域に限られるため、ポジトロニウム分光法を用いる事で表面第一層でのスピン状態を直接観測する事が出来る。我々はポジトロニウム分光測定を行うと共に、第一原理バンド計算法を用いて分光スペクトルの解析を行っている。本講演では主にポジトロニウムの表面からの放出角制限をした場合におけるk空間選択について述べる。, 第58回アイソトープ・放射線研究発表会

スピン偏極ポジトロニウム飛行時間測定(SP-PsTOF)法は、物質最表面において電気伝導に寄与する電子のみのスピン偏極状態を得られる新しい表面解析手法である。我々はこれまで培ってきたスピン偏極陽電子ビーム技術を活用し、SP-PsTOF測定装置の開発を進めている。22Na線源により生成したスピン偏極陽電子ビームを静電偏向して横偏極化し、試料に打ち込む。このとき放出される二次電子の検出時刻と、Ps消滅ガンマ線の検出時刻との時間差からPs放出エネルギーを算出する。このPsエネルギースペクトルの中で最速の成分がフェルミ面直下に存在する電子を用いて生成されたPsの成分となる。電磁石(0.05 T)によりスピン平行/反平行の2通りで試料を磁化し、それらのTOFスペクトルの差分からSP-PsTOFスペクトルを得て、さらにスピン偏極率へと変換する。本装置を用いて得られたニッケル薄膜表面のSP-PsTOFスペクトルは、フェルミエネルギー近傍で負の偏極率を示した。さらに、第一原理計算によって得られた電子状態密度の差分強度を、装置のエネルギー分解能でコンボリューションしたものは、実験値をよく再現した。現在、計数率との兼ね合いを調整しながらエネルギー分解能の向上を図っている。同時に、磁性材料表面の電子スピン偏極率測定を順次進めて行く予定である。, 京都大学複合原子力科学研究所専門研究会 「陽電子科学とその理工学への応用」

Positronium emission from wurtzite GaN(0001) and AlN(0001) surfaces was observed by positronium time-of-flight spectroscopy. The positronium energy spectra contained two positronium components distinguished by their energies. Through detailed analyses based on Monte Carlo simulations, these two components were attributed to positronium formed from valence and conduction electrons. The obtained results augment the previous arguments regarding the contribution of conduction electrons to positronium emission from 4H SiC(0001) and Si(111) surfaces.

The energy spectrum of positronium atoms generated at_solid surface reflects the electron density of states (DOS) associated solely with the first surface layer.Using spin-polarized positrons, the spindependent surface DOS can be studied.For this purpose, we have developed_spin-polarized positronium time-of-flight spectroscopy apparatus based on_22 Na positron source and an electrostatic beam transportation system, which enables the sampling of topmost surface electrons around the Γ point and near the Fermi level.We applied this technique to nonmagnetic Pt(111) and W(001), ferromagnetic Ni (111), Co(0001) and graphene on them, Co2FeGa0 .5Ge0 .5 (CFGG) and Co2MnSi (CMS).The results showed that the electrons of Ni(111) and Co(0001) surfaces have characteristic negative spin polarizations, while these spin polarizations vanished upon graphene deposition, suggesting that the spin polarizations of graphene on Ni(111) and Co(0001) were mainly induced at the Dirac points that were out of range in the present measurement.The CFGG and CMS surfaces also exhibited only weak spin polarizations suggesting that the half-metallicity expected for these bulk states was not maintained at the surfaces.

Positronium formation at 4H SiC(0001) surfaces were investigated upon the removal of natural oxide layers by hydrofluoric acid etching and heat treatment at 1000 K in ultra-high vacuum. Two types of positronium were observed in the positronium time-of-flight (PsTOF) measurements irrespective of conduction type and surface polarity. One type formed the major part of the PsTOF spectrum with a maximum energy of 4.7 ± 0.3 eV. This energy exceeded the theoretical value calculated with valence electrons. The PsTOF spectrum shape was different from those of metal surfaces, suggesting that the surface state electrons or conduction electrons need to be considered as the positronium source. Another positronium appeared at 1000 K in the tail of the PsTOF spectrum with a maximum energy of 0.2–0.5 eV. This thermally-assisted athermal positronium may be formed via the surface state positrons and electrons.

We report surface charge separation analysis of rutile with a Cr concentration gradient produced by ion implantation with the appropriate annealing process. For comparison, TiO 2 films with uniformly distributed Cr were prepared by pulse laser deposition using Cr/TiO 2 multi-targets. Cr depth profiles were obtained by secondary ion mass spectrometry and X-ray photoelectron spectroscopy was used to determine surface Cr concentration for both films. Photo-induced transient charge separation measurements were applied to demonstrate the inherent property without disturbing spontaneously formed surface band bending. As the results, Cr implanted rutile exhibited remarkable charge separation performance, even in the visible light. The charge separation was superior to that of uniformly Cr distributed rutile and of epitaxial anatase films.

Surface charge separation behavior of photo-generated carriers in highly oriented TiO2 anatase and rutile films was investigated using a technique in which the transient surface charge is observed by laser pulse irradiation without metal contacts and an externally applied field. According to the measurements, the quantum efficiency of photo-generated holes transported toward the surface was determined as a function of incident laser energy. The photo-generated holes in anatase can be transported toward the surface for irradiation at the photon energy of its bandgap. The holes transported toward the rutile surface, however, were generated close to the surface for irradiation at the photon energy much higher than its bandgap.

Surface charge separation efficiencies of uniformly Cr-distributed rutile grown by pulse laser deposition (PLD) using Cr/TiO 2 multi-targets and rutile with Cr concentration gradient produced by ion implantation with the appropriate post-annealing process were characterized by photo-induced transient charge separation (PITCS) measurement. Cr depth profiles were obtained by secondary ion mass spectrometry (SIMS) and X-ray photoelectron spectroscopy (XPS) was used to determine that both surface Cr concentrations were the same. PITCS can be measured without direct contacts and without an externally applied field, and demonstrate the inherent property of charge separation without disturbing spontaneously formed surface band bending. There was a large charge separation for implanted rutile with Cr gradient, even in the visible light region (∼520 nm), which was more remarkable than that of uniform rutile and epitaxial anatase films.

Titanium dioxide (TiO 2 ) films on glass substrates were prepared by pulsed laser deposition (PLD) technique. The dependence of crystal structure, morphology and photocatalytic activity on oxygen partial pressure were investigated using X-ray diffraction (XRD), atomic force microscope (AFM) and UV–VIS spectrophotometer. By increasing oxygen partial pressure, the surface morphology changed from smooth to rough, and the crystal structure changed from rutile to anatase at fixed substrate temperature of 773 K. The change of the surface morphology and the formation of anatase structure by variation of oxygen partial pressure resulted in an increase of photocatalytic activity.

Epitaxial titanium dioxide thin films with anatase and rutile structures have been deposited by pulsed laser deposition (ArF excimer laser) under a controlled O 2 atmosphere. The anatase TiO 2 (0 0 1) films have been successfully prepared on LaAlO 3 (0 0 1), LSAT (0 0 1), SrTiO 3 (0 0 1) and YSZ (0 0 1) substrates. Also, high-quality epitaxial rutile TiO 2 (1 0 0) films were grown on α -Al 2 O 3 (0 0 0 1) substrates. The substrate temperature during the deposition was in the range from 349°C to 608°C under 6×10 −3 Torr O 2 gas pressure. The quality of films and crystallographic relationships were assessed by X-ray diffraction, X-ray pole figures and Rutherford backscattering spectroscopy (RBS)/channeling. The optical properties were characterized in the ultraviolet-visible region by optical absorption measurement. The optical band gaps for anatase and rutile TiO 2 epitaxial films were evaluated to be 3.22 and 3.11 eV, respectively.

Implantation with 100 keV C + ions in a transmission electron microscope (TEM) was performed in copper at temperatures from 570 to 970 K to reveal the agglomeration process of implanted species immiscible in the substrates. Phase and strain contrast features with diameters of ∼5 nm appeared in Cu TEM samples at a fluence of 1 × 10 17 C/cm 2 , at which carbon concentration and radiation damage were roughly evaluated as 10 at% and four displacements per carbon atom, respectively. The features were determined as carbon onions due to their spherical morphology and concentric graphene shells with the interlayer distance of bulk graphite. The strain contrast around onions indicated that they were embedded in the substrate enhancing isotropic strain. We have further derived evidence on the location of onions, such as the interaction of onions with dislocations and with the sample edge, the latter of which was observed due to irradiation-enhanced evaporation of Cu. On the other hand, carbon nanocapsules (concentric graphitic spheres with cavities), typically observed in implanted polycrystalline Cu substrates, were formed through the graphitization at substrate grain boundaries and following emission of the encapsulated copper particles.

The time-and spatially-resolved properties of laser ablated carbon, boron and silicon particles were measured by LAPXAS (Laser Plasma Soft X-ray Absorption Spectroscopy). The maximum speed of positively charged ions is higher than those of neutral atoms and negatively charged ions. The spatial distributions of the laser-ablated particles in the localized rare gas environment were measured. In helium gas environment, by the helium cloud generated on the top of ablation plume depressed the ablation plume. There is no formation of silicon clusters till 15μs after laser ablation in the argon gas environment.

We have performed (1) time-resolved soft X-ray absorption measurements on a time scale up to 15 μs after pulsed-laser ablation of silicon (Si) in an ambient Ar gas with a transient pressure of about 10 Torr and (2) investigation of the correlation between 1.6-eV photoluminescence (PL) from Si nanocluster-based films and the surface oxidation. No soft X-ray absorption lines corresponding to Si clusters were observed, indicating that it is likely that after 15 μs, significant clustering takes place and then Si nanoclusters can grow. From experiments involving hydrogen termination and hydrogen atom treatment in addition to natural oxidation, it is found that the 1.6-eV PL originates from the interface of a Si nanocluster core and a surface oxide layer and occurs without hydrogen atoms.

Time- and space-resolved characteristics of laser-ablated carbon particles were studied by x-ray absorption spectroscopy. Absorption peaks from the 1s→2p transition of neutral carbon atom ( C 0), C −, C +, and C 2+ ions were observed, An absorption peak from 1s→π* and a broad absorption maximum from the 1s→σ* transition of small clusters were also observed. Neutral carbon, C −, C +, and small carbon clusters were emitted from the sample long after laser irradiation. Spatial distributions of laser-ablated carbon particles in the localized helium-gas environment were measured. In the helium-gas environment, the ablation plume was depressed by the helium cloud generated on the top of the ablation plume.

We have performed x-ray absorption measurements of laser-ablated Si particles in vacuum and in Hegas environment. It is found that the spatial distributions of laser-ablated particles are very different between the two cases. This suggests that a higher-density region of helium gas is formed at the top of the plume of the ablated particles, and free expansion of particles is restrained by this helium cloud. Moreover, the spectrum obtained at 260 ns indicates that it takes more than 260 ns to form Si clusters. It is also demonstrated that Al particles can be sputtered in vacuum by laser-ablated Si particles if an Al plate is placed in front of the plume.

We have studied dynamical mechanism of laser ablation using combined time- and spatially-resolved measurements of laser plasma soft x-ray absorption spectroscopy and visible light emission imaging/spectroscopy. The obtained results indicate that there are no significant formation of silicon clustres and no growth of silicon nanoclusters till 2.55 ms after laser ablation. We have fabricated nanocluster-based silicon films by laser ablation in ambient gases and investigated photoluminescence at room temperature from the films. It is found that the growth of silicon nanocluster-based films with visible light emission can be well controlled by means of novel laser ablation methods.

We have performed time-and-space resolved X-ray absorption spectroscopy with a time resolution of 10 ns to study laser-ablated Si particles in a time scale ranging from 0 ns to 120 ns. Neutral and charged particles produced by laser ablation are observed through X-ray absorption spectra. Assignments of transitions from 2s and 2p initial states to higher Rydberg states of Si atom and ions are achieved, and we experimentally determine the LI \kern-1ptI,I \kern-1ptI \kern-1ptI absorption edges of neutral Si atom ( Si0) and Si ions such as Si+, Si2+, Si3+ and Si4+. The main ablated particles are found to be Si atom and Si ions in the initial stage of 0 ns to 120 ns. The relative amounts depend strongly on times and laser energy densities. We find that the spatial distributions of particles produced by laser ablation are changed with supersonic helium gas bombardment, but no cluster formation takes place. This suggests that a higher-density region of helium gas is formed at the top of the plume of ablated particles, and free expansion of particles is restrained by this helium cloud, and that it takes more than 120 ns to form Si clusters.

In this work, ilmenite structure FeTiO3 thin films on α-Al2O3 (0001) substrates were prepared by Nd:YAG pulsed laser deposition. Different deposition conditions with different temperatures, and different partial gas pressures of different gases were used. It was found that an epitaxial and stoichiometric FeTiO3 single-crystal film can be prepared at 450°C in several different ambients, including pure argon, vacuum and Ar+5%O2 at different partial pressures respectively. However, the best film was ablated using 5% oxygen and argon gas mixed ambient of 0.01 mbar at 450°C. With the temperature lower than 300°C or higher than 500°C, we could not obtain crystalline or even single-phase FeTiO3 film. X-ray diffraction and Rutherford backscattering spectrometry with channeling were used to characterize the films.

Compressed C60 powder, a C60 pellet, has been irradiated by second harmonics of Nd:YAG/glass pulsed laser light. From the x‐ray photoelectron spectroscopic measurement, it was suggested that some reorganization occurred on the surface of the ablated C60 pellet. A periodic structure was discovered on the laser ablated C60 pellet and the spacing of that periodic structure was in good agreement with the prediction based on the surface scattering model.

Time‐resolved X‐ray absorption spectroscopy using laser plasma has been developed to measure the time evolution of the electronic and geometrical structures of laser ablated particles and their spatial distribution. This method is demonstrated to have potential for pulsed‐laser induced, fast‐phase changes like laser annealing and laser ablation. Laser‐ablated particles of silicon and carbon have been investigated by this method to clarify dynamics of ablated particles. We show the obtained results of laser ablation of Si and C, the first experimental determination of Si–L2,3 edges of Si atom and ions, and absorption lines suggesting formation of Si clusters and negative C ions.

A time-resolved x-ray absorption spectroscopy apparatus on a laboratory scale is being prepared which utilizes a laser plasma as an x-ray source. One of the major aims imposed on this apparatus is to observe x-ray absorption or reflection spectra of various materials in an energy region from 100 eV up to 3 keV. For this purpose, two sets of optical systems are provided, i.e., a toroidal mirror and grazing incidence flat-field grating for the lower-energy region (100-1000 eV), and a toroidal mirror and curved KAP crystal for the higher-energy region (1-3 keV). Detection of x-rays is performed with MCPs for both energy regions. Preliminary experiments on the generation of x-rays have revealed that the intensity of x-rays in the energy region below 300 eV is as high as 10(16) photons/pulse in 2(pi) steradian.© (1991) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

We propose a new simple method for characterizing photocatalytic activity by measuring photo-generated transient charge separation at the surface of semiconductor photocatalysts. In this method, the charge separation generated by a pulse dye laser is obtained as a function of the incident laser energy. Using this method, the photocatalytic activity and the type of surface reaction (reduction or oxidation) in titanium dioxide films were rapidly determined.

We have investigated the dynamic behavior of laser-ablated Si particles by means of time-resolved X-ray absorption spectroscopy. Laser-ablated Si particles are found to induce sputtering of an Al plate which is located near the Si target. Moreover, it is suggested that Si cluster formation is enhanced by collisions between laser-ablated Si particles and Si particles backscattered by a Si wafer located near the Si target. We tentatively assign unidentified X-ray absorption peaks appearing near 120 eV to small Si clusters.

A time-resolved X-ray absorption spectroscopy apparatus in the laboratory scale has been set-up which utilizes laser plasmas as a soft X-ray source. One of the major objectives using this apparatus is to observe X-ray absorption fine structures (XAFS) of various materials during materials processing in an energy region between 100eV and 3keV. For this purpose, we have provided two sets of optical and detecting systems, i.e., a toroidal mirror, grazing incidence flat-field grating and MCP for lower energy region (100-1000eV), and a toroidal mirror, curved KAP crystal and MCP for higher energy region (1-3keV). The most intense X-rays are generated from Au plasma and the intensity reaches as high as 1016 photons/pulse in 2π steradian in the energy region below 300eV. In this case, the conversion efficiency of the laser energy to X-rays is 4.6%. The energy resolution of the spectrometer ΔE/E has been found to be 5×10-3. We show some examples of XAFS measurements on carbon during laser ablation.