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     

Nanolithographic patterning of thin metal films with a scanning probe microscope

Using an atomic force microscope (AFM) operating in air, we locally modify thin films of e-beam-deposited Cr and Ti by applying voltage pulses between the AFM tip and the sample, which is positively biased with respect to the tip. The modifications consist in anodization and/or mechanical deformation and reach the metal/substrate interface. Metallic gates can thus be fabricated without pattern transfer.     

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.     

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.     

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.     

Study on the nano machining process with a vibrating AFM tip on the polymer surface

The polymer has been proved to be nano machined by a vibrating tip in tapping mode of Atomic Force Microscope (AFM). The force between the tip and the surface is an important factor which determines success of the machining process. Controlling this force with high accuracy is the foundation of nanomachining in AFM tapping mode. To achieve a deeper understanding on this process, the tip is modeled as a driving oscillator with damping. Factors affecting the nano machining process are studied. The Hertz elastic contact theory is used to calculate the maximum contact pressure applied by the tip which is employed as a criterion to judge the deformation state of the sample. The simulation results show that: The driven amplitude can be used as a main parameter of controlling the machined depth. Sharper tips and harder cantilevers should be used for successful nanomachining with the vibrating tip. Under the same conditions, a larger tip radius will not only result in the machining error, but also lead to failure of the nanomachining process. The higher driving frequency will lead to a larger tapping force. However it cannot be used as a parameter to control the machined depth because of its narrow variation range. But it is a main error source for the nanomachining process in AFM tapping mode. Moreover, a larger Young's modulus polymer sample will induce a smaller machined depth, a larger maximum contact pressure and a bigger tapping force.     

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.     

GaSb/GaAs复合应力缓冲层上自组装InAs量子点的生长

研究了GaSb/GaAs复合应力缓冲层上自组装生长的InAs量子点.在2ML GaSb/1ML GaAs复合应力缓冲层上获得了高密度的、沿［100］方向择优分布量子点.随着复合应力缓冲层中GaAs层厚度的不同,量子点的密度可以在1.2×10¹⁰cm^-2和8×10¹⁰cm^-2进行调控.适当增加GaAs层的厚度至5ML,量子点的发光波长红移了约25nm,室温下PL光谱波长接近1300nm. 关键词： 自组装量子点 分子束外延 Ⅲ-Ⅴ族化合物半导体     

Magnetic force microscopy signal of flux line above a semi-infinite type II-superconductor

We have examined a single flux line in the semi-infinite type-II superconductor. The stray magnetic field of the flux line has been calculated. We have found that the vertical force exerted on a magnetic force microscopy (MFM) tip from the flux line is measurable by currently existing MFM. Two types of magnetic tips were taken into consideration, solid and thin film tips. For example, with a Cobalt film of the thickness of 100 nm and 30 nm on a tip, we found a vertical force of 4*10^–10 N and 1.5*10^–10 N, respectively. The lateral force exerted on a tip by the flux line was also calculated. The lateral force must be small enough to prevent the flux line from becoming depinned.     

Hard disk magnetic domain nano-spatial resolution imaging by using a near-field scanning microwave microscope with an AFM probe tip

A near-field scanning microwave microscope (NSMM) incorporating an atomic force microscope (AFM) probe tip was used for the direct imaging of magnetic domains of a hard disk under an external magnetic field. We directly imaged the magnetic domain changes by measuring the change of reflection coefficient S₁₁ of the NSMM at an operating frequency near 4.4 GHz. Comparison was made to the magnetic force microscope (MFM) image. Using the AFM probe tip coupled to the tuning fork distance control system enabled nano-spatial resolution. The NSMM incorporating an AFM tip offers a reliable means for quantitative measurement of magnetic domains with nano-scale resolution and high sensitivity.     

AFM detection of the mechanical resonances of coiled carbon nanotubes

We introduce a method for atomic force microscopy (AFM)-based detection of mechanical resonances in helix-shaped multi-walled carbon nanotubes. After deposition on an oxidized silicon substrate, the three-dimensional structure of suspended nanotubes, which bridges an artificially created step on the surface, can be visualized using AFM operating in the non-contact mode. The suspended coiled nanotubes are resonantly excited, in situ, at the fundamental frequency by an ultrasonic transducer connected to the substrate. When the AFM tip is positioned above the coiled nanotube, the cantilever is unable to follow the fast nanotube oscillations. Nevertheless, an oscillation amplitude-dependent signal is generated due to the non-linear force-to-distance dependence. Measurement of the mechanical resonances of the helix-shaped carbon nanotubes can be used to quantitatively determine their elastic properties. Assuming that a coiled nanotube can be modeled as a suspended helix-shaped uniformly thin elastic beam, the obtained resonance frequency is consistent with a Young's modulus of 0.17ǂ.05 TPa.     

Imitation of non-contact mode while scanning in the presence of an electric double layer?

Non-contact atomic force microscopy (NCAFM) minimizes the physical interaction between the AFM tip and the surface of interest. Several recent studies have reported observation of single atom defects using this technique. The repulsive force is presumably the primary interatomic force (cf. our paper on pseudo-non-contact mode in this issue) responsible for the reported atomic resolution in these studies. The combination of these factors, minimal tip–sample deformation and repulsive force interaction, are responsible for the observation of the single atom defects. In the present study, we show that similar resolution can be achieved utilizing the same two factors but which employs scanning in a surfactant. The method decreases the tip–sample interaction by eliminating the attractive forces between the tip and sample. The surfactant solution induces an electrical double-layer (EDL) on the surface of the tip and sample. This EDL creates additional repulsion that is distributed over a large area, and hence does not contribute noticeably to the image contrast during scanning. However, it does compensate for the high pressures normally experienced by the tip in the absence of surfactant. In addition, the presence of the EDL enhances tip stability during the image scan. This method has been tested on surfaces of such minerals as mica, chlorite, and anhydrite.     

脂质体结构特性的原子力显微镜研究

用原子力显微镜(AFM)研究了1,2二油酸甘油3磷酸1甘油(DOPG)脂质体胞囊的形态和脂双层膜结构.报道了AFM探针与吸附在氧化硅膜上脂质体的相互作用结果.实验结果表明,在液晶态的DOPG中,AFM图像是一些球形或椭球形颗粒.这些球形或椭球形颗粒与液晶态的DOPG脂质体的结构特性有关.当AFM的探针与脂质体表面相互作用力超过某临界值时,脂质体胞囊破裂,变成脂双层结构.从图上可以看到,第二层的DOPG膜吸附在第一层上,膜的厚度约为5nm. 关键词： 原子力显微镜 脂质体 纳米结构     

Shear modulation force microscopy study of near surface glass transition temperatures

We report results of glass transition (T(g)) measurements for polymer thin films using atomic force microscopy (AFM). The AFM mode, shear modulation force microscopy (SMFM), involves measuring the temperature-dependent shear force on a tip modulated parallel to the sample surface. Using this method we have measured the surface T(g) of thin (17-500 nm) polymer films and found that T(g) is independent of film thickness (t>17 nm), strength of substrate interactions, or even presence of substrate.