ZnO growth on Si with low-temperature CdO and ZnO buffer layers by molecular-beam epitaxy

Low-temperature (LT) buffer-layer techniques were employed to improve the crystalline quality of ZnO films grown by molecular-beam epitaxy (MBE). Photoluminescence (PL) spectra show that CdO, as a hetero-buffer layer with a rock-salt structure, does not improve the quality of ZnO film grown on top. However, by using ZnO as a homo-buffer layer, the crystalline quality can be greatly enhanced, as indicated by PL, atomic force microscopy (AFM), x-ray diffraction (XRD), and Raman scattering. Moreover, the buffer layer grown at 450°C is found to be the best template to further improve the quality of top ZnO film. The mechanisms behind this result are the strong interactions between point defects and threading dislocations in the ZnO buffer layer.     

Acquisition of high-precision images for non-contact atomic force microscopy

The atomic force microscope (AFM) system has evolved into a useful tool for direct measurements of intermolecular forces with atomic-resolution characterization that can be employed in a broad spectrum of applications. The distance between cantilever tip and sample surface in non-contact AFM is a time-varying parameter even for a fixed sample height, and typically difficult to identify. A remedy to this problem is to directly identify the sample height in order to generate high-precision atomic-resolution images. For this, the microcantilever (which forms the basis for the operation of AFM) is modeled as a single mode approximation and the interaction between the sample and cantilever is derived from a van der Waals potential. Since in most practical applications only the microcantilever deflection is accessible, we will use merely this measurement to identify the sample height. In most non-contact AFMs, cantilevers with high-quality factors are employed essentially for acquiring high-resolution images. However, due to high-quality factor, the settling time is relatively large and the required time to achieve a periodic motion is long. As a result, identification methods based on amplitude and phase measurements cannot be efficiently utilized. The proposed method overcomes this shortfall by using a small fraction of the transient motion for parameter identification, so the scanning speed can be increased significantly. Furthermore, for acquiring atomic-scale images of atomically flat samples, the need for feedback loop to achieve setpoint amplitude is basically eliminated. On the other hand, for acquiring atomic-scale images of highly uneven samples, a simple PI controller is designed to track the desired constant sample height. Simulation results are provided to demonstrate the feasibility of the approach for both sample height identification and tracking the desired sample height.     

Nanoparticle detachment: a possible link between macro- and nanotribology?

Although particle detachment is a common phenomenon associated with most tribological processes, it seldom occurs that each piece of elemental debris can be considered as the result of a single event. Such an association has been revealed by the systematic study of a specific system, where a pin of graphite is made to rub against thoroughly polished steel. While the discontinuous nature of the transfer film allows a quantitative assessment of the volume of transfer h _e to be made by 3D optical-profilometry, the linear dependence of the rate of particle detachment dh _e/dn (n=number of rubbing cycles) with the logarithm of sliding speed v strongly suggests the existence of a particular type of stick–slip, where each stick may lead to the detachment of a debris particle. The variations in size of these debris with environment as revealed by AFM, further suggest that the global rate of particle detachment is of the form: dh _e/dn=Nx _i , where N is the number of stick–slip events per rubbing cycle, x the proportion of stick events leading to a cohesive rupture, and _i the mean volume of an elemental particle. While this relation is apparently supported by most experimental results, its actual validation can only be made by experiments at the level of single (nanoscale) asperities, carried out under well-controlled experimental conditions.     

二维有限元网格全自动生成方法——AFM法

研究任意二维平面多连接区域内点生成及ＡＦＭ三角化算法，引进形状因子使形成的三角形网格形状达到近优。实例及特性表明所提出的算法可靠、有效且易实现二维ＣＡＤ／ＦＥＭ一体化。     

Unusual crystallization of polyethylene at melt/atomically flat interface: Lamellar thickening growth under normal pressure

The morphology of polyethylene (PE) crystallized at the melt/atomically flat substrate interface was studied using atomic force microscopy (AFM). Our attention is concentrated on isothermal crystallization of PE on HOPG and MoS₂ substrates at high temperatures up to 135 °C. By quenching after different times of crystallization, it was possible to “freeze” the lamellar morphology at various stages of its development at a given supercooling. After detachment of the PE sample from the substrate, individual lamellae (even 150 nm thick) and stacks of the edge-on lamellae after different stages of growth were observed. The similarity of the individual lamellae with those grown from the hexagonal phase under high pressure (characteristic tapered edges), allows to conclude that at the interface, even under normal pressure, the crystallization proceeds according to the mechanism of lamellar thickening growth.     

Calibration of AFM cantilever stiffness: a microfabricated array of reflective springs

Calibration of the spring constant of atomic force microscope (AFM) cantilevers is necessary for the measurement of nanonewton and piconewton forces, which are critical to analytical applications of AFM in the analysis of polymer surfaces, biological structures and organic molecules.

We have developed a compact and easy-to-use reference standard for this calibration. The new artifact consists of an array of 12 dual spiral-cantilever springs, each supporting a mirrored polycrystalline silicon disc of 160 μm in diameter. These devices were fabricated by a three-layer polysilicon surface micromachining method, including a reflective layer of gold on chromium. We call such an array a Microfabricated Array of Reference Springs (MARS). These devices have a number of advantages. Cantilever calibration using this device is straightforward and rapid. The devices have very small inertia, and are therefore resistant to shock and vibration. This means they need no careful treatment except reasonably clean laboratory conditions.

The array spans the range of spring constant from around 0.16 to 11 N/m important in AFM, allowing almost all contact-mode AFM cantilevers to be calibrated easily and rapidly. Each device incorporates its own discrete gold mirror to improve reflectivity. The incorporation of a gold mirror both simplifies calibration of the devices themselves (via Doppler velocimetry) and allows interferometric calibration of the AFM z-axis using the apparent periodicity in the force–distance curve before contact. Therefore, from a single force–distance curve, taking about one second to acquire, one can calibrate the cantilever spring constant and, optionally, the z-axis scale. These are all the data one needs to make accurate and reliable force measurements.     

Structural diversity of sphingomyelin microdomains

In cells plasma membrane, sphingomyelin (SM) plays a key role in the formation of a category of lipid microdomains enriched in cholesterol (Chl) often referred to as rafts. Atomic force microscopy (AFM) was used to analyze the mesoscopic topography of enriched SM microdomains in supported bilayers made of SM/dioleoylphosphatidylcholine (SM/DOPC) and SM/palmitoyl-oleoyl-phosphatidylcholine (SM/POPC) equimolar mixtures, in buffer, at room temperature. Gel–fluid phase separation occurs in both SM/DOPC and SM/POPC bilayers. The gel phase SM-enriched microdomains adopt a variety of size, shape and mesoscopic structure, from homogeneous flat domains of a few hundreds of nanometer in diameter to domains of several micrometers made of closely packed globular structures. Gel–gel phase separation in SM domains is also observed which gives rise to different structures for the diunsaturated and the mixed-saturated PC species. These differences could also extend to the interactions with Chl. This suggests that studies on rafts formation commonly performed using SM/DOPC mixture as a model should also include the physiologically more relevant POPC species.     

Water and toluene barrier properties of a polyamide 12 modified by a surface treatment using cold plasma

CF₄ and CO₂ plasma treatments have been used to modify the barrier properties of a polyamide 12 (PA12) towards permeant molecules, which present opposing characteristics: water and toluene. The surface modifications were observed by atomic force microscopy, X-ray photoelectron spectroscopy and surface Gibbs function measurements. Both treatments lead to different surfaces; one is rather hydrophobic (with CF₄) whereas the other is more hydrophilic (with CO₂. The effect of this modification on permeametric properties has been investigated by liquid water and liquid toluene permeation measurements. Our results show opposite effects of the two treatments. CF₄ plasma treatment leads to a reduction of water and toluene permeability. With CO₂ plasma treatment, in terms of permeation, two different behaviours were observed, an increase and a decrease of permeancy for water and toluene respectively. These results are in full agreement with those obtained for the surface characterization, and confirms change in the polymeric surface affinity for the permeant leading to a variation of the materials permeancy.     

The mechanism of adhesion and printability of plasma processed PET films

Of the several techniques available for the surface modification, plasma processing has proved to be very appropriate. The low temperature plasma is a soft radiation source and it affects the material only over a few hundred Å deep, the bulk properties remaining unaffected. Plasma surface treatment also offers the advantage of greater chemical flexibility. PET films are widely used for packaging and electrical insulation. The studies of adhesion and printability properties are important. In the present study PET films are treated in air plasma for different time of treatment. The improvement in adhesion is studied by measuring T-peel and Lap shear strength. In addition, printability of plasma treated PET films is studied by cross test method. It has been found that printability increases considerably for plasma treatment of short duration. Therefore it is interesting to study the surface composition and morphology by contact angle measurement, ESCA and AFM. Surface energy and surface roughness can be directly correlated to the improvement in above-mentioned surface related properties. It has been found that the surface oxidation occurs containing polar functional groups such as C-O, COO. A correlation of all such observations from different techniques gives a comprehensive picture of the structure and surface composition of plasma treated PET films.     

单晶硅片纳米磨粒磨损的AFM模拟

当前的CMP磨损模型中,大多数主要是基于理论分析以及CMP中宏观影响因素所建立,并且都欠缺微观试验的证明.本文提出使用原子力显微镜（AFM）作为CMP中单个纳米磨粒模拟的方法,来验证模拟实验中载荷与扫描速率对微观磨损的影响,并依据此模拟试验的结果探讨了CMP磨损机理.