Evaluation of the contact resonance frequencies in atomic force microscopy as a method for surface characterisation (invited)

The combination of ultrasound with atomic force microscopy (AFM) opens the high lateral resolution of scanning probe techniques in the nanometer range to ultrasonics. One possible method is to observe the resonance frequencies of the AFM sensors under different tip-sample interaction conditions. AFM sensors can be regarded as small flexible beams. Their lowest flexural and torsional resonance frequencies are usually found to be in a range between several kHz and several MHz depending on their exact geometrical shape. When the sensor tip is in a repulsive elastic contact with a sample surface, the local indentation modulus can be determined by the contact resonance technique. Contact resonances in the ultrasonic frequency range can also be used to improve the image contrast in other dynamic techniques as, for example, in the so-called piezo-mode. Here, an alternating electric field is applied between a conducting cantilever and a piezoelectric sample. Via the inverse piezoelectric effect, the sample surface is set into vibration. This excitation is localised around the contact area formed by the sensor tip and the sample surface. We show applications of the contact resonance technique to piezoelectric ceramics.     

Quantitative measurement of local elasticity of SiOx film by atomic force acoustic microscopy

<正>In this paper the elastic properties of SiO_x film are investigated quantitatively for local fixed point and qualitatively for overall area by atomic force acoustic microscopy(AFAM) in which the sample is vibrated at the ultrasonic frequency while the sample surface is touched and scanned with the tip contacting the sample respectively for fixed point and continuous measurements.The SiO_x films on the silicon wafers are prepared by the plasma enhanced chemical vapour deposition(PECVD).The local contact stiffness of the tip-SiO_x film is calculated from the contact resonance spectrum measured with the atomic force acoustic microscopy.Using the reference approach,indentation modulus of SiO_x film for fixed point is obtained.The images of cantilever amplitude are also visualized and analysed when the SiO_x surface is excited at a fixed frequency.The results show that the acoustic amplitude images can reflect the elastic properties of the sample.     

试样激励下轻敲模式原子力声显微镜的非线性动力学特性

利用Euler-Bernoulli梁理论和DMT针尖-样品作用力模型建立了试样激励下轻敲模式原子力声显微镜(AFAM)系统的动力学方程,并应用非线性动力学分析方法对AFAM微悬臂梁的振动特性进行研究。通过合理改变超声激励幅值、超声激励频率和针尖-样品初始间距等模型参数模拟得到微悬臂梁的超谐波、次谐波、准周期和混沌振动现象,采用时间序列、频谱、相空间、Poincare截面和Lyapunov指数等方法对不同非线性振动特性进行表征。通过分析不同模型参数条件下微悬臂梁针尖-样品作用力特性,探索了微悬臂梁不同非线性振动现象的产生机制。此外,研究了AFAM微悬臂梁运动的分岔特性,发现当超声激励幅值和针尖-样品初始间隙连续变化时,周期、准周期和混沌运动交替出现。研究结果对AFAM系统非线性动力学行为分析和混沌振动控制提供了理论参考。      

Analyses of vibration responses on nanoscale processing in a liquid using tapping-mode atomic force microscopy

An analytical solution of the vibration responses of biological specimens using atomic force microscopy (AFM), which often requires operation in a liquid, is developed. In this study, the modal superposition method is employed to analyze the vibration responses of AFM cantilevers in tapping mode (TM) operated in a liquid and in air. The hydrodynamic force exerted by the fluid on AFM cantilevers is approximated by additional mass and hydrodynamic damping. The tip–sample interaction forces were transformed into axial, distributed transversal, and bending loading, and then applied to the end region of the AFM through the tip holder. The effects of transverse stress and bending stress were adopted to solve the dynamic model. With this model, a number of simulations were carried out to investigate the relationship between the transient responses of the cantilever in a liquid and the parameters considered in nanoscale processing. The simulations show that the vibration of AFM cantilevers in a liquid has dramatically different dynamic characteristics from these of that in air. The liquid reduces the magnitude of the transversal response and reduces the cantilever resonances. Moreover, the magnitudes of response become larger with increasing intermolecular distances and smaller with decreasing tip length. The cantilever vibration amplitudes significantly depend on the damping constant and the mass proportionality constant.     

Sub-100 nm IR spectromicroscopy of living cells

We have performed IR spectromicroscopy of cells immersed in liquid water, with a lateral resolution better than 100 nm. Here, we use the motion of an atomic force microscope tip, probing the local transient deformation induced by an IR pulsed laser tuned at a sample absorbing wavelength. By Fourier analysis of the vibration of the cantilever tip, we can discriminate frequencies that are characteristic of the object, thus eliminating the influence of the water absorption. This opens the door of chemical imaging of living species in vivo, with spatial resolution of the order of the size of cell components.     

Adhesion hysteresis in dynamic atomic force microscopy

The effects of adhesion hysteresis in the dynamic‐dissipation curves measured in amplitude‐modulation atomic force microscopy are discussed. Hysteresis in the interaction forces is shown to modify the dynamics of the cantilever leading to different power dissipation curves in the repulsive and attractive regimes. Experimental results together with numerical simulations show that power dissipation, as measured in force microscopy, is not always proportional to the energy dissipated in the tip–sample interaction process. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

High resolution atomic force microscopic imaging of the Si(111)-(7 x 7) surface: contribution of short-range force to the images

Observation of the rest-atom layer of the Si(111)-(7 x 7) surface is performed by atomic force microscopy. By detecting the force due to the single chemical covalent bond formed between the tip and the sample surface, individual atoms on the layer were clearly resolved. Unprecedented high spatial resolution was achieved by setting the detection force at a small value and by reducing background forces due to the long-range interactions with the small oscillation amplitude of the cantilever and sharp probe tip.     

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.     

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     

Semiconductor acousto-electric potential detection using a force microscope

Acousto-electric force microscopy (AEFM), developed as a technique for high-resolution mapping of carrier characteristics in solids is described. This technique is based on an acousto-electric voltage (AEV) detection scheme where AEV pulses are obtained from the cantilever deflection caused by the electrostatic force acting between the cantilever tip and the sample surface. AEFM appears to be feasible, but an improved AEV detection scheme is required.     

Simultaneous imaging of tunneling current and damping energy by noncontact-AFM in ultra-high vacuum

We have simultaneously observed the images of tunneling current and damping energy with the nc-AFM (noncontact atomic force microscopy) image of Si(111)7ǻ. When inverted contrast is observed in the constant frequency shift nc-AFM image, the current image is not inverted, and vice versa. On the other hand, the damping images show a contrast similar to that of the nc-AFM images; the damping decreases at a narrow separation between the tip and the sample. This possibly indicates that the damping decreases as the attractive interaction increases under a constant oscillation amplitude of the cantilever. To keep the oscillation amplitude constant under an attractive interaction between a tip and a sample, the total energy of the cantilever oscillation is reduced and the oscillation velocity of the cantilever decreases. An explanation is proposed that the change in energy dissipation occurs in the cantilever oscillation, depending on the oscillation velocity of the cantilever, and the value is estimated from a simple model.     

Electrical Measurement Techniques in Atomic Force Microscopy

A conductive tip in an atomic force microscope (AFM) has extended the capability from conventional topographic imaging to electrical surface characterization. The conductive tip acts as a voltage electrode to provide stimuli and monitor electrical surface properties. In this review article, we have organized the AFM electrical techniques based on whether the electrical properties are monitored at the cantilever tip or across the sample. Furthermore, the techniques are organized based on probe detection signal. A number of acronyms are used in the literature, and the more commonly used ones are identified. The principle of each technique is described, and representative applications are presented. A better understanding of the spectrum of techniques should serve as the driver to expand the application of electrical techniques to study interdisciplinary phenomena at the nanoscale.     

Physical properties of dynamic force microscopies in contact and noncontact operation

In the field of Scanning Force Microscopy several dynamical contact and noncontact modes have been introduced increasing the range of detectable surface and interface properties, and allowing to detect material properties such as elasticity and mass density on the nanometer scale. A detailed understanding of tip/surface interactions and the dynamic processes involved is required to understand the origin of a material contrast using these techniques. Here a general method to solve the equation of motion of a vibrating SFM cantilever/tip system in an external force field is presented. Contact modes as well as intermittent contact modes are discussed using a single set of equations describing the cantilever/tip motion, and by varying the size of amplitudes of the vibrating cantilever/tip system. To quantitatively describe the oscillation behavior of the SFM cantilever at large amplitudes the computer simulations are based on the MYD/BHW model providing a realistic contact model with respect to the contact area, the size of the contact forces as well as the transition from repulsive to attractive forces. The results are compared with the experiment and with different approaches based on analytical and numerical models.     

Effect of tip mass on frequency response and sensitivity of AFM cantilever in liquid

The effect of tip mass on the frequency response and sensitivity of atomic force microscope (AFM) cantilever in the liquid environment is investigated. For this purpose, using Euler–Bernoulli beam theory and considering tip mass and hydrodynamic functions in a liquid environment, an expression for the resonance frequencies of AFM cantilever in liquid is derived. Then, based on this expression, the effect of the surface contact stiffness on the flexural mode of a rectangular AFM cantilever in fluid is investigated and compared with the case where the AFM cantilever operates in the air. The results show that in contrast with an air environment, the tip mass has no significant impact on the resonance frequency and sensitivity of the AFM cantilever in the liquid. Hence, analysis of AFM behaviour in liquid environment by neglecting the tip mass is logical.     

Primary information sensors for AFM based on quasiparticle flows

A new class of experimental techniques is presented which allows the behavior of elementary collective excitations (quasiparticles) in solids to be studied at the mesoscopic scale. New experimental equipment is being constructed in the classical scheme of an atomic force microscope in which the sensor of primary information is a cantilever. The defining feature of the proposed sensor based on a cantilever is the addition of a generator and detector of quasiparticles to its design. The generator is located either on the tip of the cantilever or in close proximity to the needle on the cantilever, so that the flow of quasiparticles emitted by the generator propagates along the needle of the cantilever to the point where the needle tip touches the surface. The detector is located in a similar way. The measured quantity is the reflection coefficient of the flux of quasiparticles from the interface between the cantilever needle tip and the surface being scanned.     

Atomic structure of alkali halide surfaces

The atomic structure of surfaces of alkali halide crystals has been revealed by means of high-resolution dynamic force microscopy. True atomic resolution is demonstrated both on steps surrounding islands or pits, and on a chemically mixed crystal. We have directly observed the enhanced interaction at low-coordinated sites by force microscopy. The growth of NaCl films on metal surfaces and radiation damage in a KBr surface is discussed based on force microscopy results. The damping of the tip oscillation in dynamic force microscopy might provide insight into dissipation processes on the atomic scale. Finally, we present atomically resolved images of wear debris found after scratching a KBr surface. PACS 68.37.-d; 68.37.Ps; 75.55.Fv     

Lateral force microscopy in low normal force limit

We have studied frictional force between SiN tip and Si surface by using lateral force microscopy. The cantilever we have used has very low stiffness of 0.006 N/m, and the normal force acting on the surface was much lower than the attractive force such as van der Waals force. In this low normal force limit, it was found that the frictional force did not depend on the normal force. We suggest a calibration method to estimate the attractive force from the lateral force data in this limit. The estimated attractive force between Si sample and SiN tip with radius of 10 nm was 0.4 nN in flat region and 0.65 nN at the corner of a rectangular hole.     

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     

Off resonance ac mode force spectroscopy and imaging with an atomic force microscope

Using an off resonance ac technique in ultrahigh vacuum we have directly measured the force-gradient interaction characteristics of a gold tip and sample and demonstrated a new atomic force microscope imaging mode with the tip located very close to the surface. The method involves the application of a small sinusoidal oscillating force to the tip via a magnetic field created by a conducting coil which interacts with a magnetic particle glued on the backside of the cantilever. By measuring the change in amplitude during the approach and retraction of the sample we have a continuous and accurate measure of the force gradient. The interaction potential is thus found without the need for complex analysis as is necessary in the case of the commonly used technique of measuring frequency shifts.     

SFM/SNOM结合的扫描探测显微镜

采用光纤探针的扫描近场光学显微镜 (SNOM)存在某些弱点 ,如探针特别脆 ,不易贴近样品表面扫描 ,探针的转输效率低等。近年来发展了将SFM /SNOM结合起来的扫描探测显微镜。利用微加工工艺技术 ,将小孔集成在悬臂探针中 ,使探针既能批量制备 ,又具有很好的重复性。探针悬臂在垂直于样品表面方向上的弹性常数较小 ,针尖不易损坏。在接触模式中利用这种SFM /SNOM组合探针可将样品的形貌像、摩擦力和光学透射像等信息同时记录下来。对于综合研究样品表面的介观性质十分有利。