Yongda Yan

Top-Down Nanomechanical Machining of Three-Dimensional Nanostructures by Atomic Force Microscopy

Aclosed-loop nanoscale precision stagewas integrated with an atomic force microscope (AFM) to mechanically fabricate 3D nanostructures according to predetermined designs, such as 3D human face nanostructures, nanoline arrays of sine-wave and triangular nanostructures, and nanodot arrays of sine-shaped, hemispheric, and concave/convex nanopatterns in a controllable and reproducible fashion. The surface roughness (Ra) of the machined nanostructures is 4 nm. The time consumed to fabricate a single 3D human face nanostructure is less than 10min. Such nanomechanical machining techniques should find more applications in manufacturing nanoscale lens arrays, nanograting, nanomolds, or other nanoscale components with complex 3D geometries. Amajor goal of nanotechnology is the fabrication of active nanostructures and nanodevices with functionalities that cannot be realized at the macro-/microscale. Recently, bottom-up processes have been widely used to fabricate 3D nanostructures in a rational, controllable, and high-throughput manner. For example, biological macromolecule self-assembly has evolved into a very powerful approach to constructing nanostructures by design with massive-parallel fabrication ability and nanometer precision. On the other hand, developing top-down machining techniques capable of fabricating nanostructures has been left behind. One of the grand challenges in nanotechnology is to machine 3D nanostructures in a controllable and reproducible fashion. This requires the combination of two functionalities: manipulating and machin-

Deformation and fracture behaviors of microporous polymer separators for lithium ion batteries

The functionality and reliability of the separator are crucial to the abuse tolerance of a battery since the separator serves as the physical barrier to prevent any contact (short circuit) between the positive and negative electrodes. Therefore, understanding the mechanical behavior, especially the deformation and fracture behaviors of the separator are of great importance for battery design and manufacturing. Here we report the deformation behaviors of five commercially available microporous polymer separators investigated by conventional tensile testing coupled with in situ tensile testing under an atomic force microscope. Morphological models were developed to elucidate the tensile deformation mechanisms. For anisotropic separators (Celgard 2325 and 2400) made by the dry process, material direction dictates the significant diversity in overall mechanical integrity of the separator: they have limited mechanical properties when stretched in the transverse direction (TD), whereas they are rather robust when pulled in the machine direction (MD). The anisotropy of these separators is a result of the distinct deformation mechanism of the stacked lamellae in the separator. Separators manufactured by the wet technique (Toray V20CFD and V20EHD, Teijin Lielsort) behaved more biaxially – all mechanical properties were nearly identical in both MD and TD. Moreover, in order to evaluate the fracture properties of these separators, the essential work of fracture (EWF) approach was adopted. The EWF results show that the fracture properties for the dry processed separators also present orientation dependence. When stretching in the MD, the MD-oriented slit-like pores serve as crack tip blunters to inhibit the propagation of cracks whereas the TD-oriented pores exactly facilitate the crack propagation by linking up the pores with the crack tip when stretching in the TD. The same toughening mechanism (tip blunting) was also found in the case of wet processed separators.

Effect of cantilever deformation and tip-sample contact area on AFM nanoscratching

The present work investigates the effect of cantilever deformation and tip-sample contact area on the performance of atomic force microscopy (AFM)-based scratching tests. Nanoscratching tests are carried out using a pyramidal diamond tip on aluminum alloy surfaces. Three typical AFM cantilever deformation states are illustrated, and the actual normal loads applied on the surface of the sample are obtained using a new method. A theoretical model for AFM-based nanoscratching using a three-sided pyramidal tip is also established to calculate the effect of the tip-sample contact area for different scratching directions. The corresponding theoretical normal loads can be obtained with this model. Experimental and theoretical results are compared for the normal loads at an expected machined depth, with different scratching directions. The contact length between the chip and the rake face of the tip is found to be the key factor leading to an increase in the tip-sample contact area. This results in an actual norm...

Fabrication of microstructures on the surface of a micro/hollow target ball by AFM

An AFM-based mechanical scratching technique is employed to fabricate microstructures with a depth of several nanometers on the surface of a micro (the diameter is 0.1–0.5 mm) thin wall and a hollow (the thickness of the wall is 0.8–1.2 µm) glass target ball. Based on analysis of the materials removal mechanism on the hollow target ball surface by an AFM diamond tip, effects of the normal load, fixed conditions on the machining process are studied. Using the AFM-based nanomachining system which is integrated with a high-precision stage, triangular and circular microstructures are fabricated on the target ball surface. Moreover, square taper holes are fabricated by this technique which solves the problems of fabrication of micro inflation holes in inertial confinement fusion (ICF) experiments. It indicates that the AFM-based mechanical machining approach has potential applications in the fields of machining a curved surface and real three-dimensional microstructures.

Controlled nanodot fabrication by rippling polycarbonate surface using an AFM diamond tip

The single scratching test of polymer polycarbonate (PC) sample surface using an atomic force microscope (AFM) diamond tip for fabricating ripple patterns has been studied with the focus on the evaluation of the effect of the tip scratching angle on the pattern formation. The experimental results indicated that the different oriented ripples can be easily machined by controlling the scratching angles of the AFM. And, the effects of the normal load and the feed on the ripples formation and their periods were also studied. Based on the ripple pattern formation, we firstly proposed a two-step scratching method to fabricate controllable and oriented complex three-dimensional (3D) nanodot arrays. These typical ripple formations can be described via a stick-slip and crack formation process.

A Leveling Method Based on Current Feedback Mode of Scanning Electrochemical Microscopy

Substrate leveling is an essential but neglected instrumental technique of scanning electrochemical microscopy (SECM). In this technical note, we provide an effective substrate leveling method based on the current feedback mode of SECM. By using an air-bearing rotary stage as the supporter of an electrolytic cell, the current feedback presents a periodic waveform signal, which can be used to characterize the levelness of the substrate. Tuning the adjusting screws of the tilt stage, substrate leveling can be completed in minutes by observing the decreased current amplitude. The obtained high-quality SECM feedback curves and images prove that this leveling technique is valuable in not only SECM studies but also electrochemical machining.

Mechanisms of anisotropic friction in nanotwinned Cu revealed by atomistic simulations

The nature of nanocrystalline materials determines that their deformation at the grain level relies on the orientation of individual grains. In this work, we investigate the anisotropic response of nanotwinned Cu to frictional contacts during nanoscratching by means of molecular dynamics simulations. Nanotwinned Cu samples containing embedded twin boundaries parallel, inclined and perpendicular to scratching surfaces are adopted to address the effects of crystallographic orientation and inclination angle of aligned twin boundaries cutting the scratching surface. The transition in deformation mechanisms, the evolution of friction coefficients and the friction-induced microstructural changes are analyzed in detail and are related to the loading conditions and the twinned microstructures of the materials. Furthermore, the effect of twin spacing on the frictional behavior of Cu samples is studied. Our simulation results show that the crystallographic orientation strongly influences the frictional response in different ways for samples with different twin spacing, because the dominant deformation mode varies upon scratching regions of different orientations. A critical inclination angle of 26.6. gives the lowest yield strength and the highest friction coefficient, at which the plasticity is dominated by twin boundary migration and detwinning. It is demonstrated that the anisotropic frictional response of nanotwinned Cu originates from the heterogeneous localized deformation, which is strongly influenced by crystallographic orientation, twin boundary orientation and loading condition.

Fabrication of nanochannels with ladder nanostructure at the bottom using AFM nanoscratching method

This letter presents a novel atomic force microscopy (AFM)-based nanomanufacturing method combining the tip scanning with the high-precision stage movement to fabricate nanochannels with ladder nanostructure at the bottom by continuous scanning with a fixed scan size. Different structures can be obtained according to the matching relation of the tip feeding velocity and the precision stage moving velocity. This relationship was first studied in detail to achieve nanochannels with different ladder nanostructures at the bottom. Machining experiments were then performed to fabricate nanochannels on an aluminum alloy surface to demonstrate the capability of this AFM-based fabrication method presented in this study. Results show that the feed value and the tip orientation in the removing action play important roles in this method which has a significant effect on the machined surfaces. Finally, the capacity of this method to fabricate a large-scale nanochannel was also demonstrated. This method has the potential to advance the existing AFM tip-based nanomanufacturing technique of the formation these complex structures by increasing the removal speed, simplifying the processing procedure and achieving the large-scale nanofabrication.

Electrochemical mechanical micromachining based on confined etchant layer technique

The confined etchant layertechnique (CELT) has been proved an effective electrochemical microfabrication method since its first publication at Faraday Discussions in 1992. Recently, we have developed CELT as an electrochemical mechanical micromachining (ECMM) method by replacing the cutting tool used in conventional mechanical machining with an electrode, which can perform lathing, planing and polishing. Through the coupling between the electrochemically induced chemical etching processes and mechanical motion, ECMM can also obtain a regular surface in one step. Taking advantage of CELT, machining tolerance and surface roughness can reach micro- or nano-meter scale.

Effects of Atomic Force Microscope Silicon Tip Geometry on Large-Scale Nanomechanical Modification of the Polymer Surface

The shape of an atomic force microscope (AFM) silicon tip has a significant effect on the mechanical modification of the polymer surface, especially for a longer sliding distance of from several to several hundreds of millimeters. In this work, a pyramidal silicon tip was used to cut into the polymethyl methacrylate (PMMA) surface, forming nanogrooves with a linear sliding distance of about 80 mm and wear box structures with a total tip sliding distance of 1,024 mm. The effects of the tip edges and the tip radius on the form of the wear debris chips, wear depth, and debris transferred to the tip were investigated. The experimental results showed that four sides of the tip influenced the morphology of the removed material. Adhesion appeared to play a role in the tip wear mechanism by successive removal of SiO2 layers during transfer of adhered PMMA from the tip to the surface. The tip radius generally increased with sliding distance. Simultaneously, adhesion of the removed materials to the tip induced a larger tip radius and a sharper tip was revealed as dropping off of the materials during the test from time to time. Thus, with the same normal load the worn tip may induce failure of the machining process. The results presented in this study provide insight into long-term nanoscratch/wear and nanomechanical machining of glassy polymer surfaces with a silicon AFM tip.