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.     

X射线相位成像的最佳衬度

根据Zernike相衬观察法的基本思想和Fresnel–Kirchhoff衍射理论,采用傅里叶变换和卷积处理,考虑到临床实际,利用振幅近似和相位缓变条件,将X射线经过生物组织产生的相位改变转换成强度分布.在所导出的理论公式的基础上,作出相位衬度和吸收衬度与物平面到像平面距离的关系曲线.分析得到X射线相位衬度成像的最佳物像距离,并通过微聚焦X射线成像装置对蟑螂成像的实验加以验证,其结果表明理论所预言的成像最佳位置符合度较高.     

优化X射线同轴相位衬度成像的理论研究

采用同轴相位衬度成像方法,基于PGW理论获得的频域相位衬度传递函数,对于理想光源和多色X射线源,用计算机模拟分析了如何选择物像距离和空间频率以获取最佳相位衬度像。实际应用中要综合考虑具体成像实验装置对于物像距离的可调范围以及成像分辨率的要求。对于具有一定谱宽的X射线源,应选择谱宽尽量小的源,以获得含信息量丰富、分辨率高的图像。     

X射线相位衬度成像

文章使用形象、生动、通俗、易懂的语言,介绍X射线相位衬度成像的基本概念,物理思想和方法,其中包括X射线的基本性质、光的波粒二象性、同步辐射X射线光源和常规非相干X射线光源的相位相干性,以及X射线相位衬度成像方法、三维成像的基本原理和相位衬度成像的最新进展,将抽象的相位、相位一阶导数和相位二阶导数概念与形象的光波阵面平移、倾斜和弯曲等形变联系在一起,着重介绍相位衬度成像发展中的创新思想,力图使读者能分享人类文明在这个学科发展中积累的精神财富.     

X射线相位衬度成像

文章使用形象、生动、通俗、易懂的语言,介绍X射线相位衬度成像的基本概念,物理思想和方法,其中包括X射线的基本性质、光的波粒二象性、同步辐射X射线光源和常规非相干X射线光源的相位相干性,以及X射线相位衬度成像方法、三维成像的基本原理和相位衬度成像的最新进展,将抽象的相位、相位一阶导数和相位二阶导数概念与形象的光波阵面平移、倾斜和弯曲等形变联系在一起,着重介绍相位衬度成像发展中的创新思想,力图使读者能分享人类文明在这个学科发展中积累的精神财富.     

原子力显微术研究进展

原子力显微术是微纳米尺度实空间形貌成像与结构表征的关键技术之一。近些年,原子力显微术衍生发展出了一系列令人瞩目的功能化探测模式和新技术。文章从以下两个方面论述了原子力显微术的前沿进展：(1)原子力显微术的功能化探测模式及其在微纳米尺度物性研究与测量以及微纳加工等领域的应用;(2)原子力显微术自身在更高精度、更高分辨率、更快速度、更多功能等方面的进展及在基础和应用研究领域中的应用。文章还展望了原子力显微术的下一步发展方向和正在不断扩展的研究领域。     

高聚物薄膜表面堆起结构的变温原子力显微术研究

利用原子力显微镜研究了不同温度（室温至58℃）及扫描速率（1—20μm/s）对高聚物PtBuA薄膜（厚度为133nm）表面上形成的突起结构和周期状堆起结构的影响.发现较快的扫描速率或较低的温度易产生随机的团状突起，较慢的扫描速率或较高的温度则易形成周期状突起结构（周期宽度约为100nm），且温度和扫描速率对周期状堆起结构的影响定性满足高聚物的时温等效关系.此外,当温度低于高聚物的玻璃化转变温度Tg时，表面粗糙度随扫描次数的增加而增加；高于Tg时，表面粗糙度基本不变.实验表明堆起结构的形成需要高聚物有较大的 关键词： 高聚物 原子力显微术 纳米技术     

同步辐射相位衬度成像医学应用初探

近年来,相位衬度成像技术医学领域应用的发展引人瞩目.同步辐射相位衬度成像无须使用对比剂就能显示传统X射线无法显示的肌腱、软骨、韧带、脂肪、血管及神经等软组织.文章作者应用包括类同轴相衬成像和衍射增强成像在内的相位衬度成像技术进行了肝胆、肺脏、肾脏、胃肠道、心脏、血管、骨关节组织与肿瘤成像,证实了同步辐射相位衬度成像较传统X射线成像图像清晰,分辩率明显提高,特别适用于软组织、血管等的成像.     

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     

Theory of higher harmonics imaging in tapping-mode atomic force microscopy

The periodic impact force induced by tip-sample contact in tapping mode atomic force microscope (AFM) gives rise to non-harmonic response of a micro-cantilever. These non-harmonic signals contain the full characteristics of tip-sample interaction. A complete theoretical model describing the dynamical behaviour of tip--sample system was developed in this paper. An analytic formula was introduced to describe the relationship between time-varying tip--sample impact force and tip motion. The theoretical analysis and numerical results both show that the time-varying tip--sample impact force can be reconstructed by recording tip motion. This allows for the reconstruction of the characteristics of the tip--sample force, like contact time and maximum contact force. It can also explain the ability of AFM higher harmonics imaging in mapping stiffness and surface energy variations.     

Measurement of the evanescent field using noncontact mode atomic force microscope

Using the noncontact mode atomic force microscope (AFM) with frequency modulation detection method, force gradient acting on the AFM tip induced by the evanescent field was measured in a high vacuum. Exponential distance dependence of the force gradient by the evanescent field was successfully measured for the first time. Decay lengths of the force gradient were estimated to be 40±3 nm and 43±3 nm for Ar and He-Ne lasers, respectively, and independent of wavelength within the experimental error. The minimum detectable force was estimated to be about 0.1 pN. There was a tendency for the measured decay length to become shorter at a distance less than z=10 nm in many cases. The force gradient induced by the evanescent field inp-polarization was larger than that ins-polarization. This paper was originally presented at the first Asia-Pacific Workshop on Near Field Optics, which was held on August 17 and 18, 1996 at Seoul Education and Culture Center, Seoul, Korea, organized by the Condensed Matter Research Institute, Seoul National University.     

Investigation of nuclear reaction products by atomic force microscopy

The use of Atomic Force Microscope (AFM, from Corporate Head, Santa Clara, California, USA) opened a new way to study latent nuclear tracks. In our experiments we used plastic track detectors of the type CR-39 (Columbia Resin No. 39) Impinging ions with energy above a threshold of 180 keV can alter the molecular structure forming latent tracks. Since nuclear latent tracks have diameters in the range of 10 to 1000 nm, they can be visualized by AFM with a slight chemical etching (6 min in 6 n NaOH solution at 70 °C). These tracks are significant for the energy, momentum and the mass of the incoming particles. In our study, passive CR-39 detectors were irradiated by secondary particles produced bombarding ¹⁰³Rh by ¹⁶O and ¹²C in a wide range of energy (1 MeV/amu to 33 MeV/amu) at the MP Tandem generator of the Laboratorio Nazionale del Sud in Catania, Italy. The experiment was carried out in order to identify the secondary particles and to determine their density and the spatial distribution.     

How to measure energy dissipation in dynamic mode atomic force microscopy

When studying a mechanical system like an atomic force microscope (AFM) in dynamic mode it is intuitive and instructive to analyse the forces involved in tip–sample interaction. A different but complementary approach is based on analysing the energy that is dissipated when the tip periodically interacts with the sample surface. This method does not require solving the differential equation of motion for the oscillating cantilever, but is based entirely on the analysis of the energy flow in and out of the dynamic system. Therefore the problem of finding a realistic model to describe the tip–sample interaction in terms of non-linear force–distance dependencies and damping effects is omitted. Instead, it is possible to determine the energy dissipated by the tip–sample interaction directly by measuring such quantities as oscillation amplitude, frequency, phase shift and drive amplitude. The method proved to be important when interpreting phase data obtained in tapping mode, but is also applicable to a variety of scanning probe microscopes operating in different dynamic modes. Additional electronics were designed to allow a direct mapping of local energy dissipation while scanning a sample surface. By applying this technique to the cross-section of a polymer blend a material specific contrast was observed.     

Guidelines for the achievement of true atomic resolution with noncontact atomic force microscopy

We investigated the conditions to achieve true atomic resolution with an atomic force microscope under noncontact mode (NC-AFM). At first, we derived the equation of vertical resolution as a function of the signal-to-noise ratio and the decay length of frequency shift by assuming an exponential tip-to-sample distance dependence of frequency shift. Next, by assuming a single atom probe, we derived the equation of lateral resolution as a function of the vertical resolution and the tip-to-sample distance, from which we clarified the guidelines to achieve true atomic resolution with NC-AFM. At last, we made clear the attainable decay length of frequency shift both for the van der Waals potential and the electrostatic (Coulomb) potential.     

Contact dynamics of tapping mode atomic force microscopy

A comprehensive model on the dynamics of a tilted tapping mode atomic force microscopy (AFM) is presented, which includes the multimodal analysis, mode coupling mechanisms, adhesion, contact and friction forces induced by the tilting angle. A displacement criterion of contact/impact is proposed to eliminate the assumptions of the previous models such as infinite stiffness of sample or zero impact velocity, which makes the model presented here suitable for more general AFM application scenario, especially for the soft sample case. The AFM tip mass, tip–sample damping, contact forces and intermittent contact can all induce the higher modes participation into the system motion. One degree of freedom or one mode study on the AFM contact dynamics of tapping mode is shown to be inaccurate. The Hertz and Derjaguin–Muller–Toporov models are used for the comparison study of the non-adhesive and adhesive contacts. The intermittent contact and the contact forces are the two major sources of the system nonlinearity. The rich dynamic responses of the system and its sensitivity to the initial conditions are demonstrated by presenting various subharmonic and nonperiodic motions.     

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.     

Material anisotropy revealed by phase contrast in intermittent contact atomic force microscopy

Phase contrast in intermittent-contact atomic force microscopy (AFM) reveals in-plane structural and mechanical properties of polymer monolayers. This is surprising, because measurements of nanoscale in-plane properties typically require contact mode microscopies. Our measurements are possible because the tip oscillates not just perpendicular but also parallel to the sample surface along the long axis of the cantilever. This lateral tip displacement is virtually universal in AFM, implying that any oscillating-tip AFM technique is sensitive to in-plane material properties.     

qPlus型原子力显微镜技术

扫描探针显微镜主要包括扫描隧道显微镜和原子力显微镜,其利用尖锐的针尖逐点扫描样品,可在原子和分子尺度上获取表面的形貌和丰富的物性,改变了人们对物质的研究范式和基础认知。近年来,qPlus型高品质因子力传感器的出现将扫描探针显微镜的分辨率和灵敏度推向了一个新的水平,为化学结构、电荷态、电子态、自旋态等多自由度的精密探测和操控提供了前所未有的机会。文章首先简要介绍原子力显微镜的发展历史和基本工作原理,然后重点描述qPlus型原子力显微镜技术的优势及其在单原子、单分子和低维材料体系中的应用,最后展望该技术的未来发展趋势和潜在应用。     

铁磁共振磁交换力显微镜

原子间的自旋相互作用力对原子级别磁性纳米构造体的表面磁性质的理解是极为重要的. 磁交换力显微镜是测量表面自旋作用力的重要手段, 但它的缺点一是需要加外部强磁场, 二是不能分离表面形貌和自旋信息, 这就导致材料表面受外部磁场的影响, 而且表面形貌和自旋信息之间相互影响, 使自旋间的相互作用力的检测和成像研究受到限制.为了解决上述问题, 利用原子力显微镜, 并采用微波照射的方法, 根据铁磁共振原理, 分别独立提取磁性材料表面形貌和自旋信息, 称之为铁磁共振磁交换力显微镜, 理论和实验结果表明此方法可以有效地分离两种信息. 铁磁共振磁交换力显微镜可以促进对原子级磁性材料机能的理解以及磁性相关科学领域的进步, 特别是对自旋电子元件的发展有很大的促进作用, 是新世纪高度信息化社会不可缺少的测量技术. 关键词： 原子力显微镜 磁交换力显微镜 自旋 铁磁共振