Xiaotang Hu

Cylindrical coordinate machining of optical freeform surfaces

The cylindrical coordinate machining method (CCM) is systematically studied in generating optical freeform surfaces, in which the feature points are fitted to typical Non-Uniform Rational B-Splines (NURBS). The given points have the mapping coordinates in the variable space using the point inversion technique, while the other points have their NURBS coordinates due to the interpolation technique. The derivation and mathematical features are obtained using the fitting formula. The compensation and optimized values for tool geometry are studied using a proposed sectional curve method for fabricating designed surfaces. Typical freeform surfaces fabricated by the CCM method are presented.

Ultra-precision machining of sinusoidal surfaces using the cylindrical coordinate method

Micro- and ultra-precision machining is an effective approach to achieving a nanometric surface finish, which is important for the sinusoidal surface as the calibrator of multi-axis precision machines. A cylindrical coordinate micromachining method is studied for the ultra-precision cutting of sinusoidal surfaces in this paper. The three-axis (X, Z, C) turning machine is used, whose NC path is generated in a spiral curve considering the workpiece surface and tool geometry. The avoiding interference technique is proposed, including the optimal selection of tool geometry and simulation of the cutting process. The tool geometry is optimized by using a sectional curve method. The cutting simulation is implemented using a constrained B-spline path fitting and a pipe model of the cutting face. With the method developed, sinusoidal surfaces with a nanometric finish of 5.54 nm in Ra are achieved effectively.

Fabrication of micro DOE using micro tools shaped with focused ion beam

A novel method is proposed to fabricate micro Diffractive Optical Elements (DOE) using micro cutting tools shaped with focused ion beam (FIB) milling. Micro tools with nanometric cutting edges and complicated shapes are fabricated by controlling the tool facets orientation relative to the FIB. The tool edge radius of less than 30 nm is achieved for the nano removal of the work materials. Semi-circular micro tools and DOE-shaped micro tools are developed to fabricate micro-DOE and sinusoidal modulation templates. Experiments show that the proposed method can be a high efficient way in fabricating micro-DOE with nanoscale surface finishes.

Force spectroscopy studies on protein–ligand interactions: A single protein mechanics perspective

Protein–ligand interactions are ubiquitous and play important roles in almost every biological process. The direct elucidation of the thermodynamic, structural and functional consequences of protein–ligand interactions is thus of critical importance to decipher the mechanism underlying these biological processes. A toolbox containing a variety of powerful techniques has been developed to quantitatively study protein–ligand interactions in vitro as well as in living systems. The development of atomic force microscopy‐based single molecule force spectroscopy techniques has expanded this toolbox and made it possible to directly probe the mechanical consequence of ligand binding on proteins. Many recent experiments have revealed how ligand binding affects the mechanical stability and mechanical unfolding dynamics of proteins, and provided mechanistic understanding on these effects. The enhancement effect of mechanical stability by ligand binding has been used to help tune the mechanical stability of proteins in a rational manner and develop novel functional binding assays for protein–ligand interactions. Single molecule force spectroscopy studies have started to shed new lights on the structural and functional consequence of ligand binding on proteins that bear force under their biological settings.

Optimization of tool positions locally based on the BCELTP for 5-axis machining of free-form surfaces

The Basic Curvature Equations of Locally Tool Positioning (BCELTP) are an accurate description of the relationships between the second order approximations of the cutter surface, the tool envelope surface and the designed surface, which was proposed in our previous paper [Gong Hu, Cao Li-Xin, Liu Jian. Second order approximation of tool envelope surface for 5-axis machining with single point contact. Computer-Aided Design 2008;40:604-15]. Based on them, for a given tool path with single cutter contact point, a new local optimization method of tool positions is presented to maximize the machining strip width by minimizing the relative normal curvature between the tool envelope surface and the designed surface. Since the BCELTP are accurate analytical expressions, the proposed optimization method of tool positions is accurate and effective in computation. Furthermore, another new optimization method of tool positions based on a dual-parameter envelope is subsequently proposed. The most interesting point is that it will result in the same results as the method based on the BCELTP. It also proves the correctness of the method based on the BCELTP from a different angle. Finally, several examples are given to prove its effectiveness and accuracy.

Nanoscale oxide structures induced by dynamic electric field on Si with AFM

Abstract Nanoscale oxide structures are very important for the study of functional nanodevices. Local oxidation induced by electric field with scanning probe microscopy is a promising method. Some oxide lines and dots on Si surface were fabricated using conductive atomic force microscope in this paper. Nanoscale oxide lines induced by dc voltages exhibit the characteristic of single peak, but the hollow structures with higher aspect ratio are observed under the effect of square wave voltages. We present the hollow structures result from the finite diffuse speed and concentration of oxygen ion, and the higher aspect ratio results from the effect of dynamic electric field in the conductor–nonconductor–semiconductor junction formed in the course of fabrication.

Characterization of static and dynamic microstructures by microscopic interferometry based on a Fourier transform method

The surface profile and dynamic characteristics are some of the important specifications that influence the performance and stability of a MEMS device. Microscopic interferometry is, up to now, the most widely used technique for surface profiles of microstructures, and is also capable of measuring out-of-plane motion of microstructures with stroboscopic illumination. In this paper, a stroboscopic Mirau microscopic interferometer system was developed by integrating some commercially available components and instruments. In order to obtain a lower acquisition time and improve the measurement accuracy and stability of interference phases, an improved Fourier transform method (FTM) for fringe pattern analysis is described. It is necessary to process two interferograms with different phase shifting to achieve reliable phase demodulation in the measurement of surface profile. Out-of-plane motion can be calculated from one interferogram sequence, in which only one interferogram per motion phase is collected. Experimental studies of the measurement of a microcantilever surface profile and out-of-plane motion are described, and the measurement results are compared with that of a five-step phase-shifting method. It is demonstrated that stroboscopic microscopic interferometry based on a FTM can be used to determine the static and dynamic characteristics of microstructures.

Reversible Unfolding and Folding of the Metalloprotein Ferredoxin Revealed by Single-Molecule Atomic Force Microscopy

Plant type [2Fe-2S] ferredoxins function primarily as electron transfer proteins in photosynthesis. Studying the unfolding-folding of ferredoxins in vitro is challenging, because the unfolding of ferredoxin is often irreversible due to the loss or disintegration of the iron-sulfur cluster. Additionally, the in vivo folding of holo-ferredoxin requires ferredoxin biogenesis proteins. Here, we employed atomic force microscopy-based single-molecule force microscopy and protein engineering techniques to directly study the mechanical unfolding and refolding of a plant type [2Fe-2S] ferredoxin from cyanobacteria Anabaena. Our results indicate that upon stretching, ferredoxin unfolds in a three-state mechanism. The first step is the unfolding of the protein sequence that is outside and not sequestered by the [2Fe-2S] center, and the second one relates to the force-induced rupture of the [2Fe-2S] metal center and subsequent unraveling of the protein structure shielded by the [2Fe-2S] center. During repeated stretching and relaxation of a single polyprotein, we observed that the completely unfolded ferredoxin can refold to its native holo-form with a fully reconstituted [2Fe-2S] center. These results demonstrate that the unfolding-refolding of individual ferredoxin is reversible at the single-molecule level, enabling new avenues of studying both folding-unfolding mechanisms, as well as the reactivity of the metal center of metalloproteins in vitro.

Analysis of electric field-induced fabrication on Au and Ti with an STM in air

Abstract Studies of the modification mechanism of Au and Ti surfaces by electric fields applied with a scanning tunneling microscope (STM) are carried out at atmospheric pressure and room temperature. For pulses of 1–100xa0μs, nanodots can only be formed on Ti under conditions where the current shows substantial fluctuations, indicating tip–sample contact. If the field is applied gradually, Au is not affected and pits are formed on Ti. We interpret pit formation as a result of field-induced oxidation. Moreover, experiments show that the relative humidity plays an important part in nanostructure formation on Ti. Our results can be explained through a combination of direct contact and oxidation induced by electric field.

Improving the scanning speed of atomic force microscopy at the scanning range of several tens of micrometers

The atomic force microscope (AFM) is a powerful instrument which can measure the surface of samples at the nanoscale. The resonance of the scanner in xy directions, and the feedback control in the z direction are two major sources of image distortion at high scan speed. In order to improve the scanning speed of the AFM, a low-cost and easy method, which includes sinusoidal scans in the fast scan direction, and an intelligent fuzzy controller in the z direction, is proposed in this paper. The use of a single-frequency driving signal in the fast scan direction allows the scanner to move at a higher speed without exciting its mechanical resonance. The intelligent fuzzy controller automatically selects appropriate PI parameters through the analysis of the tracking errors, thus improving the dynamic tracking performance of the z scanner. The development and functioning of the sinusoidal fast scans and the intelligent fuzzy controller are demonstrated, as well as how this approach significantly achieves faster scans and a higher resolution AFM imaging.