Second harmonic generation and dielectric study of the fine and coarse grain PMN-35PT ceramics

We investigated the grain size effect on the high temperature behaviour of PMN-35PT ceramics around the tetragonal–cubic phase transition using second harmonic generation (SHG) and dielectric spectroscopy. Our data revealed coexistence of the first-order ferroelectric phase transition and relaxor behaviour in the coarse grain (grain size ～4 µm) µ_PMN-35PT. The phase transition is hindered and relaxor features prevail in the fine grain (grain size ～150 nm) n_PMN-35PT ceramics. Effect of the grain-size reduction on the SHG and dielectric response could be attributed to stabilisation of intermediate domain configurations between the frozen polar nanoclusters and large ferroelectric domains and to the enhanced stability of polar nanoclusters in the fine grain ceramics.     

Magnetic Phase Transitions in Nanoclusters and Nanostructures

New phenomena – the first order magnetic phase transitions were observed in nanoclusters and nanostructures. For isolated ferrihydrite nanoclusters (d ～ 1–2 nm) in porous materials, for α-,γ-Fe₂O₃ nanoclusters (d ～ 20–50 nm) and for composites of nanostructured metallic Eu with additives of α-, γ-Fe₂O₃ nanoclusters and adamantane the critical temperatures (T _C, T _N) and magnetic cluster critical sizes (R _cr) were determined by means of thermodynamic models and Mössbauer spectroscopy. The first order magnetic phase transitions (jump-like) proceed by such a way when magnetization and magnetic order disappear by jump without superparamagnetic relaxation. According to thermodynamic model predictions the cluster and interface defects were suggested to play the main role in magnetic behavior. Thus, for the defective α-, γ-Fe₂O₃ nanoclusters, at R ≤ R _cr, the presence of the first order (jump-like) magnetic phase transition was described in terms of magnetic critical size of cluster. The action of high pressure (up to 2 GPa) with shear (120–240°) was effective for defect generation and nanostructure formation. For nanosystems including iron oxide nanoclusters, adamantane and metallic europium and subjected to shear stress under high pressure loading the critical value of defect density was estimated by the study of the character of magnetic phase transition. First-to-second-order (nanostructured metallic Eu) and second-to-first-order (α-, γ-ferric oxide nanoclusters) changes of the character of magnetic phase transition were shown to accompany by the variation of critical temperatures compared to the corresponding bulk values.     

Binding of electron states in multilayer strained Ge/Si heterostructures with type-II quantum dots

Mechanical strains in a multilayer Ge/Si(001) heterostructure with vertically aligned Ge nanoclusters (quantum dots) are calculated using an interatomic potential based on the Keating valence-force-field model. It is found that the nonuniform spatial elastic strain distribution in this medium gives rise to a three-dimensional potential well for electrons in the strained Si layers near Ge nanoclusters. The depth of the potential well reaches 100 meV, and its spatial dimensions are determined by the diameter of the Ge nanoclusters. For a structure consisting of four Ge islands 23 nm in diameter arranged one above another, the electron binding energies in this well and the spatial electron density distribution are determined. The ground state has an s-like symmetry and is characterized by an electron binding energy of ～95 and ～60 meV for the elemental composition of Ge in the nanoclusters c = 1 and c = 0.7, respectively. The existence of bound electron states in the conduction band of strained Si must lead to a relaxation of the selection rules that determine the low efficiency of the radiative recombination in indirect-gap semiconductors. This explains the high value of the oscillator strength observed for the interband transitions in multilayer Ge/Si(001) structures with vertical correlation of the arrangement of Ge nanoclusters.     

用巯基乙酸作稳定剂制备CdSe纳米晶的光学性质

以巯基乙酸为稳定剂制备了CdSe纳米晶，通过尺寸选择沉淀得到2nm到3nm之间不同尺寸的纳米晶，利用室温光吸收，光致发光（PL）和光致发光激发（PLE）谱来研究了CdSe纳米团簇的光学性质。紫外－可见吸收谱给了具有清晰激光特征的尖锐吸收边，这表明样品的尺寸分布很窄。光致发光研究表明，样品有两个发射带，一个具有较高能量位于吸收边，来自电子－空穴对从最低激发态能级弛豫后的辐射复合，另一个低能发射带归属于基质与纳米晶界面存在的俘获中心。PLE谱中有2个吸收带，分别是S－S和P－P跃迁。最后还给出了不同激发能量下的发光特性。     

小尺寸铝纳米团簇的相变行为

采用分子动力学方法模拟了半径从0.3–1.3 nm变化的小尺寸铝纳米团簇的熔化、凝固行为. 基于势能-温度曲线、热容-温度曲线分析, 获得了熔点、凝固点与尺寸的依变关系, 并利用表面能理论、小尺寸效应开展了现象分析.研究表明, 铝团簇原子数小于80时, 熔点和凝固点的尺寸依赖性出现无规律的异常变化; 而大于该原子数, 熔、凝固点则随着团簇尺寸的减小而单调下降; 当原子数为27时, 团簇熔点高于块材熔点近40 K. 同时, 铝纳米团簇呈现出凝固滞后现象, 即凝固点低于熔点. 关键词： 纳米团簇 熔点 凝固点 分子动力学     

大尺寸硫醇配体保护金团簇的激发态动力学研究

纳米金团簇作为桥梁连接了金纳米粒子和单个金原子,对于揭示表面等离子激元共振和金属键的来源具有重要意义。有机配体保护的纳米金团簇为理解金纳米粒子从金属性质到非金属性质的转变提供了理想的研究对象,而处于转变区域的金团簇的激发态动力学还尚待研究。在本文中,我们总结了表现出分子性质,并且尺寸较大（大于100 个金原子）的纳米金团簇的激发态动力学,同时将其与表现出金属性质的金纳米粒子的激发态动力学进行比较。本文通过对处于转变区域的金团簇的电子和振动动力学的描述,进一步讨论了其电子结构。对大尺寸纳米金团簇激发态弛豫机理的深入理解,将有助于理解金属纳米团簇和纳米粒子的光学性质,从而进一步推动这一类功能材料的设计和应用。     

Atomic-force microscopy and Raman scattering studies of laser-induced structural disordering on the <Emphasis Type="Italic">p</Emphasis>-CdTe surface

Atomic-force spectroscopy and Raman scattering are applied to study structural modification of the p-CdTe (111) surface under exposure to pulsed multimode ruby laser radiation of nanosecond duration and a power density below the material-melting and destruction thresholds. It is shown that irradiation of crystals produces nanometer quasi-ordered surface nanoclusters 1.0–11.3 nm in height with a lateral size of 41–336 nm. Possible mechanisms of their formation are discussed.     

A study of many-body phenomena in metal nanoclusters (Au, Cu) close to their transition to the nonmetallic state

The results of a study of many-body phenomena in gold and copper nanoclusters are presented. The measured conductivity as a function of nanocluster height h was found to have a minimum at h ≈ 0.6 nm. Conductivity was local in character at nanocluster sizes l ≤ l _c ≈ 2.5 nm. Changes in core hole screening and an anomalous increase in the Anderson singularity index α in gold and copper nanoclusters could be caused by changes in permittivity from metallic (? → ∞) to nonmetallic (? ∝ l ²). The many-body phenomenon characteristics observed in the X-ray photoelectron and tunnel spectra of gold and copper nanoclusters as the size of the nanoclusters changed led us to suggest changes in the band structure of the nanoclusters and, therefore, their possible transition from the metallic to nonmetallic state.     

Morphological and optical properties of soft-landed supported nanoclusters: effect of rapid thermal annealing

We report the formation of large islands of bi-modal lateral size distributions having one peak at lateral size ～100 nm (height ～70 nm) and another at ～160 nm (height ～110 nm) by soft-landing of size-selected copper nanoclusters (3 nm in diameter) at room temperature (26 °C). Si(100) wafer containing native oxide is used as substrate. Transmission electron microscopy (TEM), scanning electron microscopy (SEM) and atomic force microscopy (AFM) are employed to study the morphological aspects and for optical aspect cathodoluminescence measurement is used. Rapid thermal annealing (RTA) (200 °C, dry N₂, 120 s) induced effects on the morphology shows that individual islands are morphologically stable. A careful qualitative study of the optical property using cathodoluminescence in a SEM before and after the thermal treatment, using RTA, reveals very low diffusion of the cluster material into the substrate.     

Magnetic Phase Transitions in Nanosystems: Role of Size Effects,Intercluster Interactions and Defects

Shear stress under high pressure loaded nanoclusters of α, γ-ferric oxides and metallic europium showed first- and second-order magnetic phase transitions. For nanoclusters of α, γ-ferric oxides and metallic europium 57Fe and 151Eu, Mossbauer spectroscopy revealed a considerable change in the Curie (Neel) points. The thermodynamic model predicts first-to-second-order (Eu) and second-to-first-order (Fe oxides) changes in the character of magnetic phase transitions and a decrease or increase of critical Curie (Neel) points. The model defines the critical cluster size and concentration of defects in nanosystems responsible for the change in magnetic properties. For nanoclusters of Fe oxides and metallic Eu, the critical (and maximum) concentration of defects corresponds to a cluster size 20–50nm.     

Magnetic properties of monodisperse nanomagnetite

The monodisperse magnetite nanoclusters 13–15 nm in size were synthesized and studied. The nanoclusters were stabilized in colloid solution by means of surface-active-substance (SAS)—oleic acid in the presence, paraffin—docosane and separated from each other by paraffin layers at the distance approximately ∼3 nm. The first order magnetic phase transition was observed in magnetite, when magnetization of a cluster disappears by jump at some critical temperature similar to that of the Curie or Neel temperatures typical of the bulk material. The observed effect is explained by the influence of surface-induced defects in the nanomagnetite. The developed surface of a cluster generates defects and causes surface tension. The model is proposed, and transition from a paramagnetic to magnetic ordered (superparamagnetic) state of a cluster is found experimentally by applying an external magnetic field which is more than some critical one. Nanomagnetite itself exists in a special “oxidized condition”, which elementary cell is the same as in normal magnetite, but it contains only trivalent cations in a B-sublattice.     

Ag-Cu离子注入玻璃后不同气氛退火的光吸收研究

采用MEVVA源（metal vapor vacuum arc ion source）引出的强束流脉冲Ag,Cu离子先后注入到SiO₂玻璃，x射线光电子能谱仪（XPS）分析显示Ag，Cu大多仍为金属态，有部分氧化态Cu存在.透射电镜观察分析和光学吸收谱都表明在衬底中形成了纳米合金颗粒.结合有效媒质理论，得到模拟的光学吸收谱，与实验结果基本符合，较好地验证了以上结论.样品退火后颗粒发生分解，分解的颗粒在氧化气氛下被氧化，且有部分向样品表面蒸发；在还原气氛下氧化态元素被还原并成核生长.故 关键词： 离子注入 纳米颗粒 退火 光学吸收率     

On the size dependence of the heats of melting of metal nanoclusters

The size dependence of the heats of melting of transition metal nanoclusters (Ni, Au, Cu and Ag) is investigated using two alternative methods of computer simulation (isothermal molecular dynamics and Monte Carlo). Au and Cu nanoclusters are subjected to more detailed study over a wide range of their sizes. The heats of melting are shown to fall along with nanocluster size and to tend in some approximation to a linear dependence on inverse particle radii.     

飞秒脉冲激光辐照下硅表面光致荧光特性

采用钛宝石飞秒脉冲激光对单晶硅在空气中进行辐照,研究了硅表面在不同扫描速度和能量密度下的光致荧光特性。光致荧光谱（PL）测量表明,在样品没有退火处理的情况下,激光扫描区域观察到橙色荧光峰（603 nm）和红色荧光带（680 nm附近）。扫描电子显微镜（SEM）测量显示,在飞秒脉冲激光辐照硅样品的过程中硅表面沉积了大量的纳米颗粒。利用傅里叶变换红外光谱仪（FT-IR）检测到了低值氧化物SiOx（x＜2）的存在,并且结合能谱仪（EDS）检测结果发现氧元素在光致发光中起着重要作用。研究表明：603 nm处橙色荧光峰来自微构造硅表面低值氧化物SiOx,680 nm附近红色荧光带来自量子限制效应。同时样品表面硅纳米颗粒的尺寸和氧元素的浓度分别决定了红色荧光带和橙色荧光的强度,通过调节飞秒激光脉冲的扫描速度和能量密度,可以有效地控制样品的荧光强度。     

Changes in the structure of nanoclusters of nickel and copper under the action of temperature

Molecular dynamics with tight-binding potential was used to study the processes of thermal impact on Ni nanoclusters with radii of up to 0.8 nm. The performed simulation demonstrated that under the action of heating, there is a structural transition from the initial face-centered cubic phase to a icosahedral modification. It was observed that upon an increase of cluster size, the transition temperature shifted to the melting point of the cluster. The obtained relationships were tested on the copper nanoparticles of similar size. It was concluded that the size of a metal cluster of 200–250 atoms is probably the limit below which it is impossible to maintain the initial face-centered cubic modification under actual conditions of exploitation.     

Temperature-induced phase transition in quartz nanocrystals dispersed in pseudotachylite

The size and concentration of α-quartz nanocrystals dispersed in samples of pseudotachylite and the internal stresses in these nanocrystals have been determined using infrared spectroscopy in the temperature range 300–800 K. Pseudotachylite is a product of intense crushing of granite that undergoes in the Earth’s crust faults. It has been found that the size of the nanocrystals is ～20 nm and does not depend on temperature. As the temperature increases, their concentration decreases monotonically and tends to zero at ～650 K. This process is paralleled by a growth of the concentration of β-quartz nanocrystals. The α-quartz nanocrystal concentration regains its initial level with decreasing temperature. Thus, the α → β phase transition in quartz nanocrystals in pseudotachylite starts at temperatures lower by ～500 K than that in the bulk of the macrocrystal (846 K), and is stretched by ～350 K. At room temperature, the unit cell of nanocrystals is compressed by surface tension forces. These forces retard the α → β phase transition. The thermal expansion coefficient of nanocrystals is larger than that of macrocrystals, which entails a decrease of compression and a monotonic decrease of the concentration of α-quartz nanocrystals with increasing temperature.     

新疆哈密绿松石的矿物学和光谱学特征研究

最近在新疆哈密发现了可规模开采的宝石级绿松石矿床。采用X射线粉晶衍射仪、激光剥蚀电感耦合等离子体质谱仪、扫描电子显微镜、傅里叶变换红外光谱仪、拉曼光谱仪、紫外-可见光谱仪等测试方法,对该地绿松石的化学成分、矿物组成、表面微形貌、红外吸收光谱、拉曼散射光谱、紫外-可见吸收光谱等矿物学和光谱学特征进行了系统对比研究。新疆哈密绿松石的主要化学成分以富Cr (1 617 ppm),V (435 ppm),Ti (428 ppm),贫Ba (99.9 ppm)为特征,随着Fe₂O₃/CuO比值的递减,绿松石的色调由绿变蓝。由磷酸根、羟基和结晶水引起的特征峰出现在该地绿松石的拉曼光谱和红外吸收光谱,其中磷酸根的振动峰位于1 000~1 200和420~650 cm-1,羟基的振动峰出现在3 400~3 600 cm-1,而结晶水引起的振动峰位于3 000~3 300 cm-1。此外,该地绿松石的紫外-可见吸收光谱显示,在600~700和430 nm处分别有由Cu2+和Fe3+引起的吸收峰,这两处的峰强与绿松石的蓝绿色调之间的关系,和新疆哈密绿松石成分中Fe₂O₃/CuO的比值与颜色之间的关系对应一致。     

Iron/iron oxide core-shell nanoclusters for biomedical applications

Biocompatible magnetic nanoparticles have been found promising in several biomedical applications for tagging, imaging, sensing and separation in recent years. Most magnetic particles or beads currently used in biomedical applications are based on ferromagnetic iron oxides with very low specific magnetic moments of about 20–30 emu/g. Here we report a new approach to synthesize monodispersed core-shell nanostructured clusters with high specific magnetic moments above 200 emu/g. Iron nanoclusters with monodispersive size of diameters from 2 nm to 100 nm are produced by our newly developed nanocluster source and go to a deposition chamber, where a chemical reaction starts, and the nanoclusters are coated with iron oxides. HRTEM Images show the coatings are very uniform and stable. The core-shell nanoclusters are superparamagnetic at room temperature for sizes less than 15 nm, and then become ferromagnetic when the cluster size increases. The specific magnetic moment of core-shell nanoclusters is size dependent, and increases rapidly from about 80 emu/g at the cluster size of around 3 nm to over 200 emu/g up to the size of 100 nm. The use of high magnetic moment nanoclusters for biomedical applications could dramatically enhance the contrast for MRI, reduce the concentration of magnetic particle needs for cell separation, or make drug delivery possible with much lower magnetic field gradients     

Ab initio calculations of optical properties of silver clusters: cross-over from molecular to nanoscale behavior

Electronic and optical properties of silver clusters were calculated using two different ab initio approaches: (1) based on all-electron full-potential linearized-augmented plane-wave method and (2) local basis function pseudopotential approach. Agreement is found between the two methods for small and intermediate sized clusters for which the former method is limited due to its all-electron formulation. The latter, due to non-periodic boundary conditions, is the more natural approach to simulate small clusters. The effect of cluster size is then explored using the local basis function approach. We find that as the cluster size increases, the electronic structure undergoes a transition from molecular behavior to nanoparticle behavior at a cluster size of 140 atoms (diameter ~1.7 nm). Above this cluster size the step-like electronic structure, evident as several features in the imaginary part of the polarizability of all clusters smaller than Ag₁₄₇, gives way to a dominant plasmon peak localized at wavelengths 350 nm ≤ λ ≤ 600 nm. It is, thus, at this length-scale that the conduction electrons’ collective oscillations that are responsible for plasmonic resonances begin to dominate the opto-electronic properties of silver nanoclusters.     

深紫外波段苯的差分光学吸收光谱DOAS定量方法研究

取代基通过取代苯环上的H原子形成不同苯系物（苯、甲苯、二甲苯等）,其共有结构苯环上的不固定π键电子受到激发,使得苯系物在紫外波段240～280 nm具有明显的特征吸收结构,鉴于此大气中的苯及相关的苯系物可以通过差分光学吸收光谱（DOAS）方法来进行定量,但采用该波段测量需要考虑以下问题：首先是氧气（O₂）的吸收干扰问题,苯(C₆H₆)在该波段的吸收截面与O₂在243～287 nm Herzzberg带相互重叠,且O₂的特征光谱结构随O₂的浓度不同而变化, 导致O₂的吸收光学密度与O₂的浓度不成线性关系。其次,苯系物结构上的相似性使其在紫外波段的特征吸收结构差别较小并且相互重叠,从而对C₆H₆的拟合产生干扰。此外,除了O₂和苯系物以外,还有臭氧（O₃）、二氧化硫（SO₂）等干扰。C₆H₆在195～208 nm的深紫外波段具有较大的吸收截面（2.417×10-17 cm2·molecule-1）,为240～260 nm处截面大小（2.6×10-18 cm2·molecule-1）的9倍左右,针对C₆H₆在深紫外195～208 nm波段的吸收特征,开展便携式DOAS定量方法研究,采用该波段进行C₆H₆的光谱定量分析并应用到实际的外场观测。通过建立C₆H₆与干扰气体SO₂,氨(NH₃),二硫化碳(CS₂)和一氧化氮(NO)的差分吸收截面的二维相关性矩阵,获取C₆H₆光谱定量的最优反演波段。通过开展实验室条件下C₆H₆,SO₂和NH₃不同浓度配比的混气实验对195～208 nm波段反演C₆H₆的效果进行评估。实验结果显示,采用195～208 nm波段进行光谱反演的探测限为17.6 μg·m-3,光谱反演浓度与理论浓度的相对测量误差均小于5%且RSD（相对标准偏差）小于3%,同时与240～260 nm波段反演结果进行对比,相对误差小于5%。在外场实际情况下,利用便携式DOAS系统获取190~300 nm的大气测量光谱,通过DOAS方法解析并结合GPS信息,获得了某化工园区C₆H₆的污染浓度分布,实验结果表明采用195～208 nm深紫外波段同样能适用于对C₆H₆的光谱定量,与240～260 nm波段反演结果进行对比,二者的相关性达到了0.98且相对误差小于10%。