A Thin Liquid Film and Its Effects in an Atomic Force Microscopy Measurement

Recently, it has been observed that a liquid film spreading on a sample surface will significantly distort atomic force microscopy （AFM） measurements. In order to elaborate on the effect, we establish an equation governing the deformation of liquid film under its interaction with the AFM tip and substrate. A key issue is the critical liquid bump height yoc, at which the liquid film jumps to contact the AFM tip. It is found that there are three distinct regimes in the variation of yoc with film thickness H, depending on Hamaker constants of tip, sample and liquid. Noticeably, there is a characteristic thickness H^＊ physically defining what a thin film is; namely, once the film thickness H is the same order as H^＊, the effect of film thickness should be taken into account. The value of H^＊ is dependent on Hamaker constants and liquid surface tension as well as tip radius.     

LOCAL ELECTROCHEMICAL DEPOSITION OF METAL ISLANDS MECHANICALLY INDUCED WITH THE TIP OF AN ATOMIC FORCE MICROSCOPE

The investigation of electrochemical processes on the nanometer scale is of great scientific as well as technological interest. Here we study the electrodeposition of copper on a polycrystalline gold surface, and demonstrate that copper deposition can be locally induced by mechanical activation with the tip of an atomic force microscope (AFM). Whereas at higher values of the deposition voltage (>100mV), a solid copper film can grow on the gold surface without tip activation, at lower voltages (approx. 30-60mV), copper deposition only occurs at the position where the surface is activated by the AFM tip due to scanning in mechanical contact with the sample. With this mechano-electrochemical "writing" process, which can be performed at ambient conditions, the controlled local deposition of metallic islands is possible, at applied force loads of the order of 10nN. Both the size-dependence of the locally induced structures on the deposition time and the reversibility of the local deposition process are studied. Depending on the deposition parameters, individual copper islands between 50nm and 200nm in size were deposited at predefined locations on the gold surface. The investigations open perspectives for the controlled mechano-electrochemical writing of more complex nanostructures with the AFM tip.     

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.     

Theoretical Study on the Capillary Force between an Atomic Force Microscope Tip and a Nanoparticle

Considering that capillary force is one of the most important forces between nanoparticles and atomic force microscope （AFM） tips in ambient atmosphere, we develop an analytic approach on the capillary force between an AFM tip and a nanoparticle. The results show that the capillary forces are considerably affected by the geometry of the AFM tip, the humidity of the environment, the vertical distance between the AFM tip and the nanoparticle, as well as the contact angles of the meniscus with an AFM tip and a nanoparticle. It is found that the sharper the AFM tip, the smaller the capillary force. The analyses and results are expected to be helpful for the quantitative imaging and manipulating of nanoparticles by AFMs.     

Mechanical Properties of Breast Cancer Cell Membrane Studied with Optical Tweezers

Membrane tethers are extracted from breast cancer cells using a force generated by an optical trap. It is experimentally obtained that the radius of tether is about 0.1μm and the static tether force is about 8.5 pN. Calculations based on the experimental measurements give a bending modulus for the tether of 1.35 x 10^-19 N.m and a surface membrane tension of 6.76 x 10^-6 N/m in the breast cancer cell. The treatment with cytochalasin D results in the decreasing bending modulus and decreasing apparent surface tension. When the membrane protein caveolin is over-expressed, similar cases occur in bending modulus and apparent surface tension. In addition, the viscous resistance coefficient of the membrane is calculated to be 1.15pN.s/μm according to the dynamic tether forces obtained under different pulling velocities.     

Analysis of the induction of the myelin basic protein binding to the plasma membrane phospholipid monolayer

Myelin basic protein(MBP) is an essential structure involved in the generation of central nervous system(CNS)myelin.Myelin shape has been described as liquid crystal structure of biological membrane.The interactions of MBP with monolayers of different lipid compositions are responsible for the multi-lamellar structure and stability of myelin.In this paper,we have designed MBP-incorporated model lipid monolayers and studied the phase behavior of MBP adsorbed on the plasma membrane at the air/water interface by thermodynamic method and atomic force microscopy(AFM).By analyzing the pressure–area(π–A) and pressure–time(π–T) isotherms,univariate linear regression equation was obtained.In addition,the elastic modulus,surface pressure increase,maximal insertion pressure,and synergy factor of monolayers were detected.These parameters can be used to modulate the monolayers binding of protein,and the results show that MBP has the strongest affinity for 1,2-dipalmitoyl-sn-glycero-3-phosphoserine(DPPS) monolayer,followed by DPPC/DPPS mixed and1,2-dipalmitoyl-sn-glycero-3-phospho-choline(DPPC) monolayers via electrostatic and hydrophobic interactions.AFM images of DPPS and DPPC/DPPS mixed monolayers in the presence of MBP(5 n M) show a phase separation texture at the surface pressure of 20 m N/m and the incorporation of MBP put into the DPPC monolayers has exerted a significant effect on the domain structure.MBP is not an integral membrane protein but,due to its positive charge,interacts with the lipid head groups and stabilizes the membranes.The interaction between MBP and phospholipid membrane to determine the nervous system of the disease has a good biophysical significance and medical value.     

FABRICATION AND APPLICATION OF NEAR-FIELD OPTICAL FIBRE PROBE

In this paper, the fabrication of a large cone angle near-field optical fibre probe, using the two-step chemical etching method and bent probe, is introduced, and the controlling parameters of the coated Cr－Al film at the probe tip are presented. The scanning electron microscopy images display that the tip diameter of the uncoated large cone angle fibre probe obtained is less than 50nm, the cone angle over 90°, and the diameter of light aperture at the coated probe tip is less than 100nm. The measured results of the optical transmission efficiency for various probe tips show that the uncoated straight optical fibre probe, film-coated straight probe and film-coated bent probe are 3×10^-1, 2×10^-3, and 1×10^-4 times that of the flat fibre probe, respectively. In addition, the force images and near-field optical images of a standard sample are acquired using a large cone angle and film-coated bent probe.     

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.     

Effect of proton and laser irradiation on the elastic and inelastic properties of a V-Ti-Cr alloy

The behavior of Young’s modulus E and the decrement of ultrasonic vibrations δ in a V-4Ti-4Cr alloy is studied during proton (8-MeV protons, dose rate 10⁴ Gy/s) or IR laser (YAG: Nd³⁺ laser, wavelength 1.06 μm, intensity up to 10² W/cm²) irradiation. Measurements are performed using the method of a composite piezoelectric oscillator (longitudinal 100-kHz resonance vibrations). The sizes of the irradiated surface regions of a sample in the proton and laser experiments are the same in order to provide the same thermal conditions in the sample-quartz transducer system. The amplitude, time, and temperature dependences of E and δ are measured before and after preliminary plastic deformation, as well as before, during, and after irradiation of a sample. The process of postdeformation aging (the kinetics of recovery of internal friction after deformation) during proton irradiation is shown to differ substantially from that during laser irradiation. The specific features detected can be explained by the more intense evolution of the defect structure during proton irradiation. Analysis shows that radiation annealing is related to the ionizing component of proton irradiation, which excites the electronic subsystem of the metallic alloy and, thus, creates hot electrons and plasmons. The electron excitations relax at lattice defects (dislocations) and increase the dislocation mobility; this results in a relatively rapid decrease in the dislocation density and in a more significant (as compared to the laser irradiation) decrease in the level of internal stresses in the material. __________ Translated from Fizika Tverdogo Tela, Vol. 46, No. 8, 2004, pp. 1409–1415. Original Russian Text Copyright ? 2004 by Kardashev, Plaksin, Stepanov, Chernov.     

双力程核子力的讨论

The binding energies of nuclei H³. He³, He⁴, the low energy n-p scattering length and effective range are calculated by using the standard variational methods. A two-range central Yukawa potential is considered in the first two sections. The longer range corresponds to the π meson mass. The smaller range corresponds either to the heavier meson or to the higher order field interactions. No repulsive core appears, when the force parameters are chosen to fit the low energy scattering and deuteron data. The calculated binding energies of nuclei H³, He³ and He⁴ are too high. This result is in agreement with most of the previous calculations. Tensor force of the Schwinger mixed type is considered in the third and fourth sections. The force parameters are chosen to fit the low energy two body data. They are not uniquely determined and are given for a set of possible D-percentage values. The adding of the tensor force reduces considerably the calculated binding energies of nuclei H³, He³, He⁴. But still, the calculated values increase too fast with the mass number. It does not fit the triton and helium binding energies simultaneously. The possibility of adding many body forces is discussed at the end of the paper.     

True atomic resolution under ambient conditions obtained by atomic force microscopy in the contact mode

2 ) has been investigated by contact-mode atomic force microscopy (AFM) in air. Both the terraces and the monolayer step itself were reproducibly imaged at atomic resolution in the repulsive-force regime at forces between tip apex and sample of the order of 10^-9 N. Several kinks were also imaged at atomic resolution. Details of the atomic registry of subsequent Se-Nb-Se sandwich layers as well as the arrangement of the individual atoms at the kink sites were resolved. The results are in perfect quantitative agreement with the lattice structure known from X-ray analysis and indicate that true atom-by-atom lateral resolution of microscopic defects is feasible by AFM in the contact mode and under ambient conditions. Published online: 10 February 1999     

Distance dependence of noncontact-AFM image contrast on Si(111) × –Ag structure

Atomic resolution imaging of the Si(111) × R30°–Ag surface was investigated using a noncontact atomic force microscopy (NC-AFM) in ultrahigh vacuum. NC-AFM images showed three types of contrasts depending on the distance between an AFM tip and a sample surface. When the tip–sample distance was about 1–3 Å, the images showed the honeycomb arrangement with weak contrast. When the tip–sample distance was about 0–0.5 Å, the images showed the periodic structure composed of three bright spots with relatively strong contrast. On the other hand, the contrasts of images measured at the distance of 0.5–1 Å seemed to be composed of the above-mentioned two types of contrasts. By comparing the site of bright spots in the AFM images with honeycomb-chained trimer (HCT) model, we suggested the following models: when the tip is far from the sample surface, tip–sample interaction force contributing to imaging is dominated by physical bonding interaction such as Coulomb force and/or van der Waals (vdW) force between the tip apex Si atoms and Ag trimer on the sample surface. On the other hand, just before the contact, tip–sample interaction force contributing to imaging is dominated by chemical bonding such as the force due to hybridization between the dangling bond out of the tip apex Si atom and the orbit of Si–Ag covalent bond on the sample surface.     

Influence of the inhomogeneous field at the tip on quantitative piezoresponse force microscopy

Ferroelectric domain imaging with piezoresponse force microscopy (PFM) relies on the converse piezoelectric effect: a voltage applied to the sample leads to electromechanical deformations. In the case of PFM one electrode is realized by a tip, therefore generating a strongly inhomogeneous electric field distribution inside the sample which reaches values up to 10⁸ V/m directly underneath the apex of the tip. Although often assumed, this high electric field does not lead to an enhancement of the electromechanical deformation of the sample. On the contrary, internal clamping of the material reduces the deformation as compared to the theoretically expected value which depends only on the voltage applied to the tip, thus being independent of the exact field distribution. PACS 77.80.Dj; 68.37.Ps; 77.84.-s     

原子力显微镜探针耦合变形下的微观扫描力研究

原子力显微镜(AFM)的微探针系统是典型的微机械构件，它在接触扫描过程处于耦合变形状态.采用数值模拟方法探究恒力模式下探针耦合变形对微观扫描力信号、微观形貌信号的影响.研究表明，AFM的恒力模式扫描中，法向扫描力并不是恒定大小，与轴向扫描力存在耦合作用，在粗糙峰峰值增加阶段，二力均增加；在粗糙峰峰值减小阶段，二力均减小；该耦合作用随形貌坡度、针尖长度等增加而加强.微观形貌的测试信号和横向扫描侧向力信号受探针耦合变形影响较小，但侧向力与形貌斜率密切相关，且其极值点与形貌极值点存在位置偏差，这些结果均与原子力 关键词： 原子力显微镜 探针悬臂梁 耦合变形 扫描力     

X-ray and AFM studies of polydisperse TiO2 (anatase) particles

Titanium dioxide (TiO₂) materials of a high chemical purity, as-prepared by the thermal hydrolysis, as well as subsequently modified by adsorption of different metal cations (Fe³⁺, Co²⁺, Cu²⁺), have been investigated by the X-ray diffraction, X-ray fluorescence and AFM microscopy methods. All TiO₂ powders have a fine-dispersated anatase structure and consist of grown together nanocrystallites of ∼8-17 nm. TiO₂ particles, usually ranging from 100 to 600 nm, show the ability to form large agglomerates, up to 2 μm in size. Contrary to the pure anatase, metal-modified TiO₂ particles possess a positive charge on their surface and can be lifted away by the AFM tip from the substrate surface during the scanning. This effect is mostly pronounced for the Fe-modified TiO₂ sample, where particles up to 250 nm are removed. The possible interaction mechanisms between different TiO₂ particles and the silicon tip are discussed. The electrostatic force has been found to play an essential role in the sample-tip interaction processes, and its value depends on the type of metal cation used.     

Atomic-force microscope observation and molecular-scale flattening of the single-crystal surface of 4-dimethylamino-N-methyl-4-stilbazolium tosylate (DAST)

The surface of an organic electro-optic crystal tends to be covered with a degenerate rough layer, which may cause light scattering or unfavorable transmission of light. We demonstrate a novel method of removing this layer and flattening the (001) surface of a 4-dimethylamino- N -methyl-4-stilbazolium tosylate (DAST) crystal on a molecular scale by applying suitable force on the tip of an atomic-force microscope (AFM). When the loading force on the AFM tip is kept near 10 nN, the DAST molecules can be removed layer by layer. This method produced a large, flat terrace of 250,000 nm(2) , and the molecular-scale flatness of this area was confirmed by AFM observation.     

Tip-surface interactions at redox responsive poly(ferrocenylsilane) (PFS) interface by AFM-based force spectroscopy

Poly(ferrocenylsilanes) (PFS) belong to the class of redox responsive organometallic polymers. Atomic force microscopy (AFM)-based single molecule force spectroscopy (SMFS) was used earlier to study single chain PFS response and redox energy driven single chain PFS molecular motors. Here we present further AFM investigations of force interactions between tip and a grafted PFS surface under potential control in electrochemical redox cycles. Typical tip-Au interaction is considered as reference in the force measurements. First the electrostatic component in the diffused double layer (DL) in NaClO₄ electrolyte environment was considered for a “grafted to” PFS, which dominated the interplay between the tip and sample surface. The DL forces can also hinder the physisorption of PFS chain onto the tip when the voltage was applied at −0.1 V. On the other hand, if the tip contacted the PFS surface prior to the electrochemical process, physisorption of PFS chains governed the overall interaction regardless of subsequently applied surface potential. In addition, prolonged contact time, t_c, may also contribute to the stability of tip-PFS bridging and detection of electrostatic forces between the tip-PFS interface. The results showed that tip-substrate interaction forces without PFS grafts have negligibly small force contributions under similar, electrochemically controlled, conditions used in single PFS chain based molecular motors.     

Structural and optical investigation of nonpolar α-plane GaN grown by metalben organic chemical vapour deposition on r-plane sapphire by neutron irradiation

Nonpolar (1120) α-plane GaN films are grown by metal-organic chemical vapour deposition (MOCVD) on r-plane (1102) sapphire. The samples are irradiated with neutrons under a dose of 1 × 10¹⁵ cm^-2. The surface morphology, the crystal defects and the optical properties of the samples before and after irradiation are analysed using atomic force microscopy (AFM), high resolution X-ray diffraction (HRXRD) and photoluminescence (PL). The AFM result shows deteriorated sample surface after the irradiation. Careful fitting of the XRD rocking curve is carried out to obtain the Lorentzian weight fraction. Broadening due to Lorentzian type is more obvious in the as-grown sample compared with that of the irradiated sample, indicating that more point defects appear in the irradiated sample. The variations of line width and intensity of the PL band edge emission peak are consistent with the XRD results. The activation energy decreases from 82.5 meV to 29.9 meV after irradiation by neutron.