Dynamics of tip-substrate interactions in atomic force microscopy

Dynamical interactions between a scanning tip and a silicon substrate are investigated using molecular dynamics simulations of both the constant-height and constant-force scan modes. Localized temporary and permanent modifications of the substrate occur, depending on tip-substrate-strate separation and scan geometry. Implications for resolving structural and force characteristics in scanning tip spectroscopies, employing atomically sharp as well as large ordered of disordered tips are discussed.     

Linear and nonlinear approaches towards amplitude modulation atomic force microscopy

Frequency response behavior of microcantilever is analytically and experimentally investigated in amplitude modulation Atomic Force Microscopy (AFM). AFM microcantilever probe is modeled as a continuous beam, and tip-sample interaction force is considered to include both attractive and repulsive force regimes. The developed model is compared with the linear lumped-parameters model that has been extensively used in the literature so far. Experimental measurements are also provided for the frequency response of a typical microcantilever-sample system to demonstrate the advantages of the developed model over the linear formulation. The results indicate that the nonlinear continuous model is more accurate, particularly in the estimation of the saturated amplitude value and frequency zone in which the tip-sample contact happens.     

High-speed atomic force microscopy with phase-detection

In order to improve the scanning speed of tapping mode AFM, we have studied the phase-detection mode AFM with a high frequency (1.5 MHz) cantilever. The phase shifts versus tip-sample distance with different types of samples including polymer, semiconductor, and graphite were measured and the interaction forces were analyzed. It was found that the phase shift in repulsive region is nearly linear as a function of distance, which can be used for feedback control in general, except that some blunt tips cause reversed polarity of phase shift due to excessive energy dissipation. High-speed image with scan rate of 100 Hz was obtained which were controlled with phase shift as a feedback signal.     

Studies of the logical state of integrated microcircuits by atomic force microscopy

The logical state of the memory cells in a nondestructive microcontroller was determined and analyzed by atomic-force microscopy. An effective optimal recording procedure of the electric potential on the microchip surface was created.     

Probing bacterial interactions: integrated approaches combining atomic force microscopy, electron microscopy and biophysical techniques

Recent developments in the application of Atomic Force Microscopy (AFM) and other biophysical techniques for the study of bacterial interactions and adhesion are discussed in the light of established biological and microscopic approaches. Whereas molecular-biological techniques combined with electron microscopy allow the identification and localization of surface constituents mediating bacterial interactions, with AFM it has become possible to actually measure the forces involved in bacterial interactions. Combined with the flexibility of AFM in probing various types of physical interactions, such as electrostatic interactions, specific ligand-receptor interactions and the elastic forces of deformation and extension of bacterial surface polymers and cell wall, this provides prospects for the elucidation of the biophysical mechanism of bacterial interaction. However, because of the biochemical and a biophysical complexity of the bacterial cell wall, integrated approaches combining AFM with electron microscopy and biophysical techniques are needed to elucidate the mechanism by which a bacterium interacts with a host or material surface. The literature on electron microscopy of the bacterial cell wall is reviewed, with particular emphasis on the staining of specific classes of cell-wall constituents. The application of AFM in the analysis of bacterial surfaces is discussed, including AFM operating modes, sample preparation methods and results obtained on various strains. For various bacterial strains, the integration of EM and AFM data is discussed. Various biophysical aspects of the analysis of bacterial surface structure and interactions are discussed, including the theory of colloidal interactions and Bell's theory of cell-to-cell adhesion. An overview is given of biophysical techniques used in the analysis of the properties of bacterial surfaces and bacterial surface constituents and their integration with AFM. Finally, we discuss recent progress in the understanding of the role of bacterial interactions in medicine within the framework of the techniques and concepts discussed in the paper.     

Equilibrium capillary forces with atomic force microscopy

We present measurements of equilibrium forces resulting from capillary condensation. The results give access to the ultralow interfacial tensions between the capillary bridge and the coexisting bulk phase. We demonstrate this with solutions of associative polymers and an aqueous mixture of gelatin and dextran, with interfacial tensions around 10 microN/m. The equilibrium nature of the capillary forces is attributed to the combination of a low interfacial tension and a microscopic confinement geometry, based on nucleation and growth arguments.     

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.     

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.     

Polymer tips for application of atomic force microscopy

A hydrophobic polymer tip for atomic force microscopy has been fabricated by two-photon adsorbed photopolymerization techniques and has been applied for the high resolution imaging of a hydrophilic metal surface. Using optimized photopolymerization conditions, we have succeeded in fabricating sub-100-nm sized polymer tips. This fabricating resolution of two-photon adsorbed photopolymerization is also confirmed by other supporting experiments. The imaging results show that the capillary-force-induced image distortion can be successfully removed by applying a pure hydrophobic polymer tip with a lateral resolution better than 5 nm, which is difficult to achieve with a commercial tip without any environmental control.     

Time-frequency modeling of atomic force microscopy

Atomic force microscopy is modeled in the time-frequency phase space. In this phase space it is equivalent to a succession of temporal lenses and free spaces which includes a temporal fractional Fourier device. Then, the Wigner transform and its second order moments are introduced to model the atomic force microscopy as a detector of ultrafast electrical signals.     

Theory of multifrequency atomic force microscopy

We develop a theory that explains the origin of the high force sensitivity observed in multifrequency force microscopy experiments. The ability of the microscope to extract complementary information on the surface properties is increased by the simultaneous excitation of several flexural cantilever modes. The force sensitivity in multifrequency operation is about 0.2 pN. The analytical model identifies the virial and the energy dissipated by the tip-surface forces as the parameters responsible for the material contrast. The agreement obtained among the theory, experiments and numerical simulations validates the model.     

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。     

Adaptive semi-empirical model for non-contact atomic force microscopy

Non-contact atomic force microscope is a powerful tool to investigate the surface topography with atomic resolution. Here we propose a new approach to estimate the interaction between its tips and samples, which combines a semi-empirical model with density functional theory (DFT) calculations. The generated frequency shift images are consistent with the experiment for mapping organic molecules using CuCO, Cu, CuCl, and CuO_x tips. This approach achieves accuracy close to DFT calculation with much lower computational cost.     

Quantitative atomic force microscopy with carbon monoxide terminated tips

Noncontact atomic force microscopy (AFM) has recently progressed tremendously in achieving atomic resolution imaging through the use of small oscillation amplitudes and well-defined modification of the tip apex. In particular, it has been shown that picking up simple inorganic molecules (such as CO) by the AFM tip leads to a well-defined tip apex and to enhanced image resolution. Here, we use the same approach to study the three-dimensional intermolecular interaction potential between two molecules and focus on the implications of using molecule-modified AFM tips for microscopy and force spectroscopy experiments. The flexibility of the CO at the tip apex complicates the measurement of the intermolecular interaction energy between two CO molecules. Our work establishes the physical limits of measuring intermolecular interactions with scanning probes.     

General theory of amplitude-modulation atomic force microscopy

We present a general analytical theory that enables one to determine accurately the unknown tip-sample interactions from the experimental measurement of the amplitude and phase of the oscillating tip in amplitude-modulation atomic force microscopy (AM-AFM). We apply the method to the known Lennard-Jones-type forces and find excellent agreement with the reconstructed results. AM-AFM, widely used in air and liquid, is now not only an imaging tool but also a quantitative force measurement tool.     

Sub-Angstrom oscillation amplitude non-contact atomic force microscopy for lateral force gradient measurement

We report the first results from novel sub-Angstrom oscillation amplitude non-contact atomic force microscopy developed for lateral force gradient measurements. Quantitative lateral force gradients between a tungsten tip and Si(1 1 1)-(7 × 7) surface can be measured using this microscope. Simultaneous lateral force gradient and scanning tunnelling microscope images of single and multi atomic steps are obtained. In our measurement, tunnel current is used as feedback. The lateral stiffness contrast has been observed to be 2.5 N/m at single atomic step, in contrast to 13 N/m at multi atomic step on Si(1 1 1) surface. We also carried out a series of lateral stiffness-distance spectroscopy. We observed lateral stiffness-distance curves exhibit sharp increase in the stiffness as the sample is approached towards the surface. We usually observed positive stiffness and sometimes going into slightly negative region.     

Lead zirconate titanate cantilever for noncontact atomic force microscopy

Noncontact atomic force microscopy with frequency modulation detection is a promising technique for surface observation with true atomic resolution. The piezoelectric material itself can be an actuator and sensor of the oscillating probe simultaneously, without the need for additional electro-mechanical transducers or other measurement systems. A vertical resolution of 0.01 nm rms has been achieved using a microfabricated cantilever with lead zirconate titanate thin film in noncontact mode frequency modulation detection. The cantilever also has a sharpened pyramidal stylus with a radius of about 10 nm for noncontact atomic force microscopy.     

Study of MgO tunnel barriers with conducting atomic force microscopy

The local topographic and conducting properties of ultra-thin MgO films with polycrystalline Ru and amorphous CoFeB as lower electrodes were investigated. The local conductance and topography of the ultra-thin MgO films (from 0.5 to 1.0 nm) were simultaneously measured with a modified conducting atomic force microscope (CAFM). The imaging force was carefully chosen in order to avoid structural damages in the insulating layers. The promising results include the decrease of the density of hotspots with large conductance with the thickness of MgO and show that an insulating barrier is obtained at 1 nm thickness.