Two color, low intensity photocurrent feedback for local photocurrent spectroscopy

In this work, we introduce a two color, low intensity photocurrent feedback method for photocurrent spectroscopy utilizing an atomic force microscope (AFM). In most applications, measurements with weak optical excitations are not feasible with an AFM because the powerful AFM feedback laser severely disturbs the measurements. Therefore, we have developed a feedback system based on the pressure dependent Schottky barrier height at the tip-sample interface. The versatility of the new feedback system is demonstrated by recording high resolution photocurrent spectra on GaAsInAs heterostructures.     

Local raster scanning for high-speed imaging of biopolymers in atomic force microscopy

A novel algorithm is described and illustrated for high speed imaging of biopolymers and other stringlike samples using atomic force microscopy. The method uses the measurements in real-time to steer the tip of the instrument to localize the scanning area over the sample of interest. Depending on the sample, the scan time can be reduced by an order of magnitude or more while maintaining image resolution. Images are generated by interpolating the non-raster data using a modified Kriging algorithm. The method is demonstrated using physical simulations that include actuator and cantilever dynamics, nonlinear tip-sample interactions, and measurement noise as well as through scanning experiments in which a two-axis nanopositioning stage is steered by the algorithm using simulated height data.     

Quantitative temperature measurement of an electrically heated carbon nanotube using the null-point method

Previously, we introduced the double scan technique, which enables quantitative temperature profiling with a scanning thermal microscope (SThM) without distortion arising from heat transfer through the air. However, if the tip-sample thermal conductance is disturbed due to the extremely small size of the sample, such as carbon nanotubes, or an abrupt change in the topography, then quantitative measurement becomes difficult even with the double scan technique. Here, we developed the null-point method by which one can quantitatively measure the temperature of a sample without disturbances arising from the tip-sample thermal conductance, based on the principle of the double scan technique. We first checked the effectiveness and accuracy of the null-point method using 5 μm and 400 nm wide aluminum lines. Then, we quantitatively measured the temperature of electrically heated multiwall carbon nanotubes using the null-point method. Since the null-point method has an extremely high spatial resolution of SThM and is free from disturbance due to the tip-sample thermal contact resistance, and distortion due to heat transfer through the air, the method is expected to be widely applicable for the thermal characterization of many nanomaterials and nanodevices.     

Optimization of adhesion mode atomic force microscopy resolves individual molecules in topography and adhesion.

The force sensor of an atomic force microscope (AFM) is sensitive enough to measure single molecular binding strengths by means of a force-distance curve. In order to combine high-force sensitivity with the spatial resolution of an AFM in topography mode, adhesion mode has been developed. Since this mode generates a force-distance curve for every pixel of an image, the measurement speed in liquid is limited by the viscous drag of the cantilever. We have equipped our adhesion mode AFM with a cantilever that has a low viscous drag in order to reach pixel frequencies of 65 Hz. Optimized filtering techniques combined with an auto-zero circuitry that reduces the drift in the deflection signal, limited high- and low-frequency fluctuations in the height signal to 0.3 nm. This reduction of the height noise, in combination with a thermally stabilized AFM, allowed the visualization of individual molecules on mica with an image quality comparable to tapping mode. The lateral resolution in both the topography and the simultaneously recorded adhesion image are only limited by the size of the tip. Hardware and software position feedback systems allows individual molecules to be followed in time during more than 30 min with scan sizes down to 60 x 60 nm2.     

A large-sample atomic force microscope observing in both air and liquid

A large-sample atomic force microscope (AFM) that allows high resolution observation in both air and liquid has been developed. With a unique beam tracking method, laser beam is capable of reflecting off the same spot on the AFM cantilever throughout raster scan over the entire scan area, either operating in air or in liquid environment. Incorporating the stand-alone AFM probe unit with an automated large sample stage, wide-scan-range imaging can be realized with high resolution and slight distortion. In addition, an image stitching method is utilized to build a broad merged image with range up to millimeters while keeping nanometer order resolution. By using a large-volume liquid bath, large and massive sample can be observed in liquid with this AFM system. Several typical experiments have been carried out to demonstrate the imaging ability and stability of this AFM. Topographic structures of gold pattern on a glass substrate are scanned at two different places on the same specimen surface. The porosity of a sheet of filter paper is then characterized in both air and water. Finally, larger-area AFM image of anodic aluminum oxide template in oxalic acid is on spot obtained by merging several individually scanned images together. Experiments show that this AFM system can offer high resolution and wide range AFM images even for large samples with remarkable capabilities in various environments.     

Broadband nanodielectric spectroscopy by means of amplitude modulation electrostatic force microscopy (AM-EFM)

In this work we present a new AFM based approach to measure the local dielectric response of polymer films at the nanoscale by means of Amplitude Modulation Electrostatic Force Microscopy (AM-EFM). The proposed experimental method is based on the measurement of the tip-sample force via the detection of the second harmonic component of the photosensor signal by means of a lock-in amplifier. This approach allows reaching unprecedented broad frequency range (2-3×10⁴ Hz) without restrictions on the sample environment. The method was tested on different poly(vinyl acetate) (PVAc) films at several temperatures. Simple analytical models for describing the electric tip-sample interaction semi-quantitatively account for the dependence of the measured local dielectric response on samples with different thicknesses and at several tip-sample distances.     

A compact CCD‐monitored atomic force microscope with optical vision and improved performances

A novel CCD‐monitored atomic force microscope (AFM) with optical vision and improved performances has been developed. Compact optical paths are specifically devised for both tip‐sample microscopic monitoring and cantilever's deflection detecting with minimized volume and optimal light‐amplifying ratio. The ingeniously designed AFM probe with such optical paths enables quick and safe tip‐sample approaching, convenient and effective tip‐sample positioning, and high quality image scanning. An image stitching method is also developed to build a wider‐range AFM image under monitoring. Experiments show that this AFM system can offer real‐time optical vision for tip‐sample monitoring with wide visual field and/or high lateral optical resolution by simply switching the objective; meanwhile, it has the elegant performances of nanometer resolution, high stability, and high scan speed. Furthermore, it is capable of conducting wider‐range image measurement while keeping nanometer resolution. Microsc. Res. Tech. 76:931–935, 2013. © 2013 Wiley Periodicals, Inc.     

Effects of higher oscillation modes on TM-AFM measurements

The finite element method and molecular dynamics simulations are used for modeling the AFM microcantilever dynamics and the tip-sample interaction forces, respectively. Molecular dynamics simulations are conducted to calculate the tip-sample force data as a function of tip height at different lateral positions of the tip with respect to the sample. The results demonstrate that in the presence of nonlinear interaction forces, higher eigenmodes of the microcantilever are excited and play a significant role in the tip and sample elastic deformations. Using comparisons between the results of FEM and lumped models, how some aspects of the system behavior can be hidden when the point-mass model is used is illustrated.     

基于 LSTM 的矩形纳米光栅 AFM 图像复原方法

原子力显微镜(AFM)利用探针与待测物之间的交互作用力进行成像,通过获取矩形纳米光栅计量标准器具的高分辨率成像得到相关的几何量参数并进行标定,实现从标准计量器具到工作计量器具的量值传递。在AFM扫描过程中,由于针尖的影响作用,使得扫描所获图像是探针和样品共同作用的结果,而不是样品形貌的真实描述。针对这一现象,本文提出了一种基于长短期记忆网络(LSTM)的AFM图像复原方法,该方法对通过膨胀法获得的仿真图像各扫描行进行训练,进而获得适用于矩形纳米光栅AFM图像复原模型。实验结果表明,针对线宽20 nm,高40 nm的矩形纳米光栅,经过该方法复原后光栅线宽的相对误差为7.40%,相较于传统的复原方法进一步提高了测量准确度。     

Dual resonance excitation system for the contact mode of atomic force microscopy

We propose an improved system that enables simultaneous excitation and measurements of at least two resonance frequency spectra of a vibrating atomic force microscopy (AFM) cantilever. With the dual resonance excitation system it is not only possible to excite the cantilever vibrations in different frequency ranges but also to control the excitation amplitude for the individual modes. This system can be used to excite the resonance frequencies of a cantilever that is either free of the tip-sample interactions or engaged in contact with the sample surface. The atomic force acoustic microscopy and principally similar methods utilize resonance frequencies of the AFM cantilever vibrating while in contact with the sample surface to determine its local elastic modulus. As such calculation demands values of at least two resonance frequencies, two or three subsequent measurements of the contact resonance spectra are necessary. Our approach shortens the measurement time by a factor of two and limits the influence of the AFM tip wear on the values of the tip-sample contact stiffness. In addition, it allows for in situ observation of processes transpiring within the AFM tip or the sample during non-elastic interaction, such as tip fracture.     

Real-time displacement measurements with a Fabry-Perot cavity and a diode laser

We present the basic operating principles of a traceable measurement system suitable for use with atomic force microscopes (AFMs) and nanometer-resolution displacement sensors. Our method is based upon a tunable external-cavity diode laser system which is servo-locked via a phase-modulated heterodyne locking technique to a Fabry-Perot interferometer cavity. We discuss mechanical considerations for the use of this cavity as a displacement metrology system and we describe methods for making real-time (sub 10 ms sampling period) measurements of the optical heterodyne signals. Our interferometer system produces a root-mean-squared (RMS) displacement measurement resolution of 20 pm. Two applications of the system are described. First, the system was used to examine known optical mixing errors in a heterodyne Michelson interferometer. Second, the Fabry-Perot interferometer was integrated into the Z axis of a commercial AFM scanning stage and used to produce interferometer-based images of a 17 nm step height specimen. We also demonstrate atomic resolution interferometer-based images of a 0.3 nm silicon single atomic step-terrace specimen.     

磨料水射流切割碳纤维复合材料的表面粗糙度试验

采用超高压磨料水射流技术对碳纤维复合材料进行切割试验,借助μscan激光共聚焦显微镜重构样品切口的三维表面,测得样品切口表面粗糙度;研究了扫描分辨率对表面粗糙度测量的影响,以及切割速度、样品厚度对样品切口表面粗糙度的影响规律。试验结果表明：扫描分辨率对表面粗糙度的测量无明显的影响;当切割深度较小（0~0.6 mm）时,即在切口入口处,表面粗糙度随切割深度的增大而减小,当切割深度较大（大于0.6 mm）时,表面粗糙度随切割深度的增大而增大;当样品厚度一定时,随着切割速度的增大,切口最大表面粗糙度在整体趋势上是增大的,而样品厚度的大小对表面粗糙度的影响并无明显的规律。     

Analysis of simulated scanning of atomic-scale silicon surface by atomic force microscopy

This study constructs a contact-mode atomic force microscopy (AFM) simulation measurement model with constant force mode to simulate and analyze the outline scanning measurement by AFM. The simulation method is that when the probe passes the surface of sample, the action force of the atom of sample received by the atom of the probe can be calculated by using Morse potential. Through calculation, the equivalent force on the cantilever of probe can be acquired. By using the deflection angle equation for the cantilever of probe developed and inferred by this study, the deflection angle of receiving action force can be calculated. On the measurement point, as the deflection angle reaches a fixed deflection angle, the scan height of this simulation model can be acquired. By scanning in the right order, the scan curve of the simulation model can be obtained. By using this simulation measurement model, this study simulates and analyzes the scanning of atomic-scale surface outline. Meanwhile, focusing on the tip radii of different probes, the concept of sensitivity analysis is employed to investigate the effects of the tip radius of probe on the atomic-scale surface outline. As a result, it is found from the simulation on the atomic-scale surface that within the simulation scope of this study, when the tip radius of probe is greater than 12 nm, the effects of single atom on the scan curve of AFM can be better decreased or eliminated.     

Mapping the tip-sample interactions on DPPC and DNA by dynamic force spectroscopy under ambient conditions

Self-assembled monolayers of DPPC and DNA adsorbed on mica are examined by dynamic force spectroscopy under ambient conditions. By a systematic recording of the frequency shift caused by the tip-sample interaction we determine the corresponding tip-sample potential and force curves. In difference to the conventional measurement of force-vs.-distance curves this technique allows the continuous measurement of tip-sample forces without instabilities caused by a jump-to-contact. Due to the systematic mapping of the tip-sample interaction we are able to compute contour maps of the tip-surface potential of the DPPC films and to extract local properties like contact stiffness and adhesion force. The lateral resolution capabilities of the introduced spectroscopy method are examined by the example of lambda phage DNA.     

Abrasive flow machining (AFM) finishing of conformal cooling channels created by selective laser melting (SLM)

Conformal cooling channels are widely adopted in the mold industry because of rapid and uniform cooling during injection molding. These complicated cooling channel geometries become feasible via selective laser melting (SLM) technology. However, the SLM fabricated internal channel surface shows high surface roughness of about 10 μm Ra. This rough surface can cause stress concentration, reducing the fatigue life of the mold. Therefore, the objective of this study is to investigate the surface finish of the SLM fabricated conformal channels by abrasive flow machining (AFM), which is widely used in the surface finishing of internal channels. To fulfill this objective, a combination of single/multiple and straight/helical channels for conformal cooling channel geometries are employed. Seven different types of conformal cooling channels (ø3mm) inside the bar are fabricated using SLM. The bar is put in the AFM fixture, and the internal channels are polished by flowing AFM media (ULV50%-54) through the channel at the same extrusion pressure of 80 bars for ten cycles. Fourteen bars (seven before AFM and seven after AFM) are machined to have the internal channel surfaces exposed for surface roughness measurement. Surface topographies of the exposed surfaces of seven types of internal channels are obtained using focus variation microscopy. The areal roughness parameters, such as arithmetical mean height (Sa) on the internal channel surfaces before and after AFM. By comparing SLM as-built conformal channel surfaces with AFM finished ones, AFM is shown to be effective in improving all SLM conformal cooling channels’ arithmetical mean height, Sa. Areal roughness parameters, such as developed interfacial area ratio (Sdr), root mean square gradient (Sdq), reduced peak height (Spk), reduced valley height (Svk), and skewness (Ssk), on those internal surfaces, were found to be sensitive to surface finishing by AFM.     

基于快速原子力显微镜的正弦驱动信号设计

随着人们对微观世界的探知需求,原子力显微镜（AFM）的扫描速度与扫描范围愈发限制其在纳米级领域的应用。各种提高AFM扫描速度的手段应运而生。通过分析扫描速度过快对图像的影响以及对正弦驱动可行性的考察,利用Filed Programmable Gate Array（FPGA）为核心,结合基于PCI04控制系统的AFM的快速扫描的特点,设计正弦波驱动信号。设计中采用查表的方式以及嵌入式nios Ⅱ处理器,实现正弦信号的输出与串口通讯。从而将AFM扫描探针的驱动信号由传统的三角波变为正弦波以提高其扫描速度。     

Investigation of laser and process parameters for Selective Laser Erosion

The process of Selective Laser Erosion (SLE) was investigated to study the effects of different process and laser parameters on the process outputs such as surface quality and erosion rate. The SLE process is a direct method to remove material in a layer-by-layer fashion due to high energy densities provided by the laser beam. In addition to its direct use as a subtractive manufacturing method, SLE may be used in combination with layer-additive techniques such as Selective Laser Melting (SLM). Such combination mainly makes sense when both processes can be performed with the same laser. However, one of the major problems involved in SLE process is the high number of the laser and process parameters (laser power, pulse frequency, scan speed, scan spacing, ambient atmosphere, etc.) and the complexity of the relations between them which has not yet been investigated completely.This paper presents an overview of the laser erosion process with nano-second Nd:YAG laser pulses and the results of several single-factor experiments that were carried out to determine the influence of the major parameters on the depth of erosion per layer and surface roughness. Additionally, the relations between the parameters are studied to investigate the interactions between them. The results from single-factor experiments showed that some relations were highly governed by the power intensity of the laser beam and also that cross interactions between the parameters play an important role on the output characteristics. The paper explains how multiple parameters (spot size, pulse frequency, scan speed, scan spacing) can be combined to define two indirectly controlled geometrical parameters, namely the scan and pulse overlap factors. Those two parameters allow calculating the number of hits of the laser beam on a same location on the workpiece possible which is the first step in physical modeling the topography of the surface left behind.     

Control of an equipment for fabricating periodic nanostructure

This study presents the control for an equipment that is designed for fabricating periodic nanostructures. This equipment can generate the patterns required for nanostructure production using direct writing laser lithography. The equipment incorporates a direct writing laser lithography instrument, a linear motor-driven long-stroke stage (X, Y), a piezoelectric-driven two degrees of freedom (2-DOF) nano-stage (Y, θ_z), a 3-DOF laser interferometer measurement system, and a system control unit. The working stage of this equipment is combined by a long-stroke stage and a nano-stage; therefore, it can provide long-stroke and high-precision positioning. The feedback signal for this stage is obtained using a 3-DOF laser interferometer measurement system. Integral sliding-mode controllers are used to control the linear motor-driven stage and PID controllers are used to control the piezo-stage for precision positioning. This paper presents the design of the controllers and the control results. Experimental results show that satisfactory writing results can be obtained at a 100 mm/s scan speed.     

Note: Integration of trapped ion mobility spectrometry with mass spectrometry

The integration of a trapped ion mobility spectrometer (TIMS) with a mass spectrometer (MS) for complementary fast, gas-phase mobility separation prior to mass analysis (TIMS-MS) is described. The ion transmission and mobility separation are discussed as a function of the ion source condition, bath gas velocity, analysis scan speed, RF ion confinement, and downstream ion optical conditions. TIMS mobility resolution depends on the analysis scan speed and the bath gas velocity, with the unique advantage that the IMS separation can be easily tuned from high speed (~25 ms) for rapid analysis to slower scans for higher mobility resolution (R > 80).     

基于原子力显微镜的线宽粗糙度测量

给出采用原子力显微镜(Atomic force microscope,AFM)测量线宽粗糙度(Line width roughness,LWR)的分析步骤。分析线宽和LWR及其偏差随刻线横截面位置的高度变化的关系,线宽及其偏差和LWR及其偏差随刻线横截面位置的高度值增加而减小。分别采用四种边缘提取算子提取了碳纳米管针尖AFM测量的刻线顶部线宽边缘,计算了刻线顶部线宽和LWR,顶部线宽和LWR测量结果对边缘提取算子不敏感。结合被测单晶硅台阶的顶表面和底表面加工方法,提出采用各扫描线轮廓高度相等的方法校正AFM压电驱动器的z向非线性。比较了采用普通氮化硅探针针尖、超尖针尖以及碳纳米管针尖AFM测量名义线宽为1 000 nm刻线LWR的结果,显示采用三种针尖的LWR测量结果存在差异,但考虑到AFM分辨率,可认为测量结果基本相同。因此,为更精确描述刻线边缘,必须提高AFM分辨率。