强流辐照钛合金表面特征的原子力显微研究

利用原子力显微术的轻敲模式(TM-AFM),并采用形貌与相位同时成像技术对强流脉冲离子束(IPIB)辐照前后试样表面进行了系统标征,得到了试样表面的高度像及相位像的衬度.分析结果表明:在高流强密度、多次脉冲条件下,IPIB辐照可使试样表面变得光滑化,从相位像中可以定性分析出辐照后表面硬度也得到一定程度的提高.     

原子力显微镜高次谐波幅度对样品弹性性质表征的研究

轻敲模式下原子力显微镜微悬臂探针在接近其基态共振频率的外加驱动下振荡, 其末端针尖周期性靠近、远离样品, 产生于针尖与样品非线性相互作用过程中的高次谐波信号包含更多的待测样品表面纳米力学特性等方面的信息. 通过理论分析、计算, 系统地研究了针尖与样品接触时间受样品弹性模量的影响, 以及高次谐波幅度与接触时间的关系, 获得了通过高次谐波幅度区分待测样品表面弹性性质差异的规律. 并在自制的高次谐波成像实验装置上, 得到了与理论预期一致的实验结果. 关键词： 轻敲模式原子力显微镜 接触时间 高次谐波幅度 弹性模量     

原子力显微镜在轻敲模式下的动力学模型

基于Hamaker假设、Lennard-Jones势能定律及经典弹性理论建立了一种新型的球体与平面黏着接触的弹性模型，该模型显示黏着力在原子力显微镜(AFM)针尖趋近和撤离样品表面，即加载和卸载的两个过程中存在黏着滞后现象，表明了AFM在轻敲工作模式中存在能量耗散.同时，根据所建的黏着接触弹性模型，建立了AFM在轻敲工作模式下的动力学模型，研究了AFM在轻敲工作模式下的振动幅度、相位差及耗散功率随针尖与样品表面间距的变化规律，仿真结果与现有的实验结果相一致.     

原子力显微镜探针悬臂几何结构变化对高次谐波信息增强的研究

轻敲模式原子力显微镜高次谐波信号包含待测样品表面纳米力学特性等方面的信息, 但是传统原子力显微镜的高次谐波信号非常微弱. 里兹法证明在探针悬臂的特定位置打孔可以实现探针的内共振从而增强高次谐波信号强度. 本文通过有限元仿真计算获得探针第一共振频、第二共振频及其比值随着孔的尺寸和位置变化的规律. 在实验上通过聚焦离子束在探针悬臂上打孔使其第二共振频约为第一共振频的6倍, 提高了第6次谐波信号的信噪比, 并在实验室研制的高次谐波成像实验装置上获得了6次谐波图像. 关键词： 轻敲模式原子力显微镜 探针悬臂几何结构 高次谐波 聚焦离子束加工     

同步辐射硬x射线衍射增强成像新进展

对于相位衬度成像的实验方法研究及其在医学、材料等领域的应用研究已成为国际上研究的热点.衍射增强成像方法(DEI)作为其中的一种方法通过测量相位衬度的一阶导数而成像.在北京同步辐射装置(BSRF)4W1A束线上，对衍射增强成像方法进行了研究，并对一系列生物医学样品及材料样品成像，得到了非常清晰的相位衬度图像. 关键词： 衍射增强成像 同步辐射 硬x射线     

同轴X射线相位衬度计算机X射线断层摄影术研究

基于北京同步辐射装置(BSRF)开展了同轴X射线相位衬度计算机X射线断层摄影术(CT)研究.利用北京同步辐射的14 keV单色X射线作为光源,以高分辨能力的X射线胶片作为探测器,分别开展吸收衬度和同轴相位衬度成像的比较研究以及相位衬度计算机X射线断层摄影术研究.相位衬度计算机X射线断层摄影术重建采用Bronnikov提出的算法.结果显示,与传统的吸收衬度图像相比,相位衬度图像具有更好的衬度和更高的空间分辨力;实验获得人工样品和蝗虫的相位衬度计算机X射线断层摄影术重建图像.重建图像中可见样品的一些结构细节.实验结果表明,相位衬度X射线成像更适合于研究弱吸收或吸收差异很小的材料;利用北京同步辐射开展同轴X射线相位衬度计算机X射线断层摄影术研究是可行的.     

扫描探针声显微镜轻敲模式中相位像的反差机理

根据扫描探针声显微镜(SPAM)轻敲工作模式中探针作周期振动的特点,以及在探针与试样相接触过程中激振力和悬臂探针自由振动的阻尼力很小的假设下,解析求解了探针与试样的碰撞运动方程,得到了最大压痕深度和碰撞时间与探针半径、等效杨氏模量以及界面吸附能等之间的关系式,较直观地说明了SPAM中轻敲模式的相位像反差机理:信号的相位与试样微区的性质、探针振幅、设置点以及试样形貌等有关。并定量预计了纳米金刚石像中的相位差值72.59°,与实验测量平均值73.2°±8.2°一致。同时,合理地解释了实验得到的光学薄膜试样相位像的反差。这些表明SPAM轻敲模式的相位成像是一种纳米分辨率测量材料表面物理性质的成像技术。      

近场光学显微技术

本文在介绍近场光学显微镜原理的基础上，对近场光学显微技术进行了一定深度的探讨，并着重研究了纳米级探针的制作和纳米级样品与探针间距的控制这两个近场光学显微技术中的关键问题，说明了近场光学显微探针的工作方式，阐述了近场光学成像的衬度类型，介绍了近场光学显微技术在多个领域的应用。在参考大量国内外最新研究成果的基础上，提出了一些个人的见解。     

原子力显微镜扫描成像DNA分子

采用Mg2+处理DNA、APTES或戊二醛修饰云母表面、DNA拉直方法制备了λ-DNA及DNA-组蛋白复合物样品.室温下原子力显微镜以轻敲模式在空气中扫描样品成像.实验结果表明:AFM扫描成像的效果与样品的制备方法有关,同时也受操作因素影响.     

衍射增强成像原理

研究了衍射增强成像过程中X射线与样品和晶体的相互作用,重点分析了小角散射对衍射增强成像的影响,为衍射增强成像方程补充了小角散射噪声项,建立了更普遍的衍射增强成像方程.根据新的衍射增强成像方程,推导出峰位像和腰位像的吸收衬度,消光衬度和折射衬度的数学表达,并讨论了两种合成像(表观吸收像和折射像)的衬度问题. 关键词： 相位衬度成像 同步辐射 晶体衍射 X射线照相术     

Theoretical and experimental study of phase optimization of tapping mode atomic force microscope

Phase image in tapping-mode atomic force microscope (TM-AFM) results from various dissipations in a microcantilever system. The phases mainly reflect the tip-sample contact dissipations which allow the nanoscale characteristics to be distinguished from each other. In this work, two factors affecting the phase and phase contrast are analyzed. It is concluded from the theoretical and experimental results that the phases and phase contrasts in the TM-AFM are related to the excitation frequency and energy dissipation of the system. For a two-component blend, it is theoretically and experimentally proven that there exists an optimal excitation frequency for maximizing the phase contrast. Therefore, selecting the optimal excitation frequency can potentially improve the phase contrast results. In addition, only the key dissipation between the tip and sample is found to accurately reflect the sample properties. Meanwhile, the background dissipation can potentially reduce the contrasts of the phase images and even mask or distort the effective information in the phase images. In order to address the aforementioned issues, a self-excited method is adopted in this study in order to eliminate the effects of the background dissipation on the phases. Subsequently, the real phase information of the samples is successfully obtained. It is shown in this study that the eliminating of the background dissipation can effectively improve the phase contrast results and the real phase information of the samples is accurately reflected. These results are of great significance in optimizing the phases of two-component samples and multi-component samples in atomic force microscope.     

Study on topographic images of Sn/Si(1 1 1)-(√3 × √3)R30° surface by non-contact AFM

Various contrast of topographic images depending on a state of a tip apex on Sn/Si(1 1 1)-(√3 × √3)R30° surface was investigated using a low temperature non-contact AFM. With the type A tip, the image of the ring-type Sn, composed of six Sn atoms surrounding substitutional Si defect, was observed when the frequency shift (∣Δf∣) was small (the tip-sample distance, Z_tip-sample, was long), while the ring-type Sn was not observed and all the Sn atoms have the same contrast when ∣Δf∣ was large (Z_tip-sample was short). On the other hand, with the type B tip, modified from the type A tip by the tip-sample contact, the image of the ring-type Sn atoms was not observed regardless of variation of Δf. It is the first experimental result on the low temperature NC-AFM observation in the Sn/Si(1 1 1) system, which depends on short-range chemical bonding force or electrostatic force acting between the tip and the sample surface. In addition, the substitutional Si defects on the surface were seen as a dim spot or were not seen, also depending on the tip state.     

Elastic,piezoelectric and dielectric properties of ZnO and CdS single crystals in a wide range of temperatures

A complete set of elastic, piezoelectric and dielectric constants of ZnO and CdS at room temperature was determined by the method of resonance-antiresonance. Elastic constants s^E₁₁, s^E₁₂, s^E₅₅, c^D₃₃, c^D₅₅, coefficients of electromechanical couplingk₃₁, k₁₅, k_t and dielectric constants ε^T₁₁, ε^T₃₃ of ZnO single crystals were determined in the temperature range 4.2–800 K. Elastic constants s^E₁₁, s^E₁₂, s^D₃₃, s^E₅₅, s^D₃₃, s^D₅₅, coefficients of electromechanical coupling k₃₁, k₃₃, k₁₅, k_t and dielectric constants ε^T₁₁, ε^T₃₃ of CdS single crystals were determined in the temperature range 4.2–300 K.     

Growth morphology of nanoscale sputter-deposited Au films on amorphous soft polymeric substrates

The growth of a room-temperature sputter-deposited thin Au film on two soft polymeric substrates, polystyrene (PS) and poly(methyl methacrylate) (PMMA), from nucleation to formation of a continuous film is investigated by means of atomic force microscopy. In particular, we studied the surface morphology evolution of the film as a function of the deposition time observing an initial Au three-dimensional island-type growth. Then the Au film morphology evolves, with increasing deposition time, from hemispherical islands to partially coalesced worm-like island structures, to percolation, and finally to a continuous and rough film. The overall Au morphology evolution is discussed in the framework of the interrupted coalescence model, allowing us to evaluate the island critical radius for the partial coalescence R _c=8.7±0.9 nm for Au on PS and R _c=7.6±0.8 nm for Au on PMMA. Furthermore, the application of the kinetic freezing model allows us to evaluate the room-temperature surface diffusion coefficient D _s≈1.8×10⁻¹⁸ m²/s for Au on PS and D _s≈1.1×10⁻¹⁸ m²/s for Au on PMMA. The application of the Vincent model allows us, also, to evaluate the critical coverage (at which the percolation occurs) P _c=61% for Au on PS and P _c=56% for Au on PMMA. Finally, the dynamic scaling theory of a growing interface was applied to characterize the kinetic roughening of the Au film on both PMMA and PS. Such analyses allow us to evaluate the dynamic scaling, growth, and roughness exponents z=3.8±0.4, β=0.28±0.03, α=1.06±0.05 for the growth of Au on PS and z=4.3±0.3, β=0.23±0.03, α=1.03±0.05 for the growth of Au on PMMA, in agreement with a non-equilibrium but conservative and linear growth process in which the surface diffusion phenomenon plays a key role.     

Effect of flux-dependent Friedel oscillations upon the effective transmission of an interacting nano-system

We consider a nano-system connected to measurement probes via non interacting leads. When the electrons interact inside the nano-system, the coefficient |t_s(E_F)|² describing its effective transmission at the Fermi energy E_F ceases to be local. This effect of electron-electron interactions upon |t_s(E_F)|² is studied using a one dimensional model of spinless fermions and the Hartree-Fock approximation. The non locality of |t_s(E_F)|² is due to the coupling between the Hartree and Fock corrections inside the nano-system and the scatterers outside the nano-system via long range Friedel oscillations. Using this phenomenon, one can vary |t_s(E_F)|² by an Aharonov-Bohm flux threading a ring which is attached to one lead at a distance L_c from the nano-system. For small distances L_c, the variation of the quantum conductance induced by this non local effect can exceed 0.1 (e²/h).     

Damage spreading in a driven lattice gas model

We studied damage spreading in a Driven Lattice Gas (DLG) model as a function of the temperature T$T$, the magnitude of the external driving field E$E$, and the lattice size. The DLG model undergoes an order–disorder second-order phase transition at the critical temperature _Tc(E)$T_c\left(E\right)$, such that the ordered phase is characterized by high-density strips running along the direction of the applied field; while in the disordered phase one has a lattice-gas-like behavior. It is found that the damage always spreads for all the investigated temperatures and reaches a saturation value _Dsat$D_{sat}$ that depends only on T$T$. _Dsat$D_{sat}$ increases for T<_Tc(E=∞)$T<T_c(E=\infty )$, decreases for T>_Tc(E=∞)$T>T_c(E=\infty )$ and is free of finite-size effects. This behavior can be explained as due to the existence of interfaces between the high-density strips and the lattice-gas-like phase whose roughness depends on T$T$. Also, we investigated damage spreading for a range of finite fields as a function of T$T$, finding a behavior similar to that of the case with E=∞$E=\infty$.     

A method to separate conservative and magnetically-induced electric fields in calculations for MRI and MRS in electrically-small samples

This work presents a method to separately analyze the conservative electric fields (E_c, primarily originating with the scalar electric potential in the coil winding), and the magnetically-induced electric fields (E_i, caused by the time-varying magnetic field B1) within samples that are much smaller than one wavelength at the frequency of interest. The method consists of first using a numerical simulation method to calculate the total electric field (E_t) and conduction currents (J), then calculating E_i based on J, and finally calculating E_c by subtracting E_i from E_t. The method was applied to calculate electric fields for a small cylindrical sample in a solenoid at 600 MHz. When a non-conductive sample was modeled, calculated values of E_i and E_c were at least in rough agreement with very simple analytical approximations. When the sample was given dielectric and/or conductive properties, E_c was seen to decrease, but still remained much larger than E_i. When a recently-published approach to reduce heating by placing a passive conductor in the shape of a slotted cylinder between the coil and sample was modeled, reduced E_c and improved B1 homogeneity within the sample resulted, in agreement with the published results.     

An optically detected electron-paramagnetic-resonance study of the luminescent triplet state of fresnoite,Ba2TiSi2O8

The luminescent center of the fresnoite crystal has been studied by electron paramagnetic resonance with optical detection at temperatures between 0.5 and 10 K. The results prove that the strong blue-green luminescence originates from a spin triplet state that is centered on the Ti ion. The C₄ symmetry of the Ti site is lost on excitation through a static Jahn-Teller effect, and eight sets of equivalent, strongly inhomogeneously broadened optically detected EPR signals appear. The signals are described by a spin hamiltonian withS=1, |D| ≈ 70 GHz, |E|=3.02 GHz, g_zz=1.93, and g_xx=g_yy ≡ g₃=2.0023. The lifetimes of the three sublevels aret _x =3.4,t _y=30,t _z=23 ms; the dominant radiative decay is from τ_x and polarized perpendicular to thec axis.     

Differential Geometry from Differential Equations

We first show how, from the general 3rd order ODE of the form , one can construct a natural Lorentzian conformal metric on the four-dimensional space . When the function satisfies a special differential condition the conformal metric possesses a conformal Killing field, , which in turn, allows the conformal metric to be mapped into a three dimensional Lorentzian metric on the space ) or equivalently, on the space of solutions of the original differential equation. This construction is then generalized to the pair of differential equations, z _ss =S(z,z _s ,z _t ,z _st ,s,t) and z _tt =T(z,z _s ,z _t ,z _st ,s,t), with z _s and z _t the derivatives of z with respect to s and t. In this case, from S and T, one can again, in a natural manner, construct a Lorentzian conformal metric on the six dimensional space (z,z _s ,z _t ,z _st ,s,t). When the S and T satisfy differential conditions analogous to those of the 3^rd order ode, the 6-space then possesses a pair of conformal Killing fields, and which allows, via the mapping to the four-space of (z,z _s ,z _t ,z _st ) and a choice of conformal factor, the construction of a four-dimensional Lorentzian metric. In fact all four-dimensional Lorentzian metrics can be constructed in this manner. This construction, with further conditions on S and T, thus includes all (local) solutions of the Einstein equations. Received: 10 October 2000 / Accepted: 26 June 2001     

氧气氛中p-GaN/Ni/Au电极在相同温度不同合金时间下的欧姆接触形成机制和扩散行为

用卢瑟福背散射/沟道技术研究了p-GaN上的Ni/Au电极在氧气氛下相同合金温度(500℃)不同合金时间后的微结构演化,以揭示欧姆接触的形成机制.利用背散射随机谱和RUMP模拟程序研究了电极金属之间的互扩散,用沟道谱探测了电极金属中的氧分布.结合不同合金时间下比接触电阻ρ_c的变化,发现随着合金时间的延长比接触电阻持续降低,在合金时间60 s后降低的速度减慢, Au扩散到GaN的表面,在p-GaN上形成外延结构,O向电极内部扩散反应生成NiO对降低ρ_{关键词： GaN 卢瑟福背散射/沟道 欧姆接触}