Jianqiang Qian

A homemade atomic force microscope based on a quartz tuning fork for undergraduate instruction

Atomic force microscopes are a key tool in nanotechnology that overcome the limitations of optical microscopes and provide imaging capabilities with nanoscale resolution. We have developed an atomic force microscope that uses an inexpensive quartz tuning fork as a micro cantilever. Because of its ease of operation and its open structure, it can be easily customized by students. Due to its low costs, it is possible that every student in the course has access to one setup, allowing all students to obtain deep insights into nanotechnology and to understand the principles of atomic force microscopy.

Contributed Review: Application of voice coil motors in high-precision positioning stages with large travel ranges.

Recent interest in high-precision positioning stages with large travel ranges has sparked renewed attention to the development of voice coil motors (VCMs). Due to their large output force, VCMs can actuate more complicated flexure structures, eliminate rail friction, and improve positioning speed. The VCM structure is both compact and flexible; hence, it is convenient to design VCMs for a variety of stage structures. Furthermore, VCMs combined with other actuators are able to achieve large travel ranges with high precision. In this paper, we summarize the principles and control methods of a typical VCM, and we analyze its properties, including thrust force, acceleration, and response time. We then present recent research on high-precision VCM positioning stages with large travel ranges.

A High-Q AFM Sensor Using a Balanced Trolling Quartz Tuning Fork in the Liquid

A quartz tuning fork (QTF) has been widely used as a force sensor of the frequency modulation atomic force microscope due to its ultrahigh stiffness, high quality factor and self-sensing nature. However, due to the bulky structure and exposed surface electrode arrangement, its application is limited, especially in liquid imaging of in situ biological samples, ionic liquids, electrochemical reaction, etc. Although the complication can be resolved by coating insulating materials on the QTF surface and then immersing the whole QTF into the liquid, it would result in a sharp drop of the quality factor, which will reduce the sensitivity of the QTF. To solve the problem, a novel method, called the balanced trolling quartz tuning fork (BT-QTF), is introduced here. In this method, two same probes are glued on both prongs of the QTF separately while only one probe immersed in the liquid. With the method, the hydrodynamic interaction can be reduced, thus the BT-QTF can retain a high quality factor and constant resonance frequency. The stable small vibration of the BT-QTF can be achieved in the liquid. Initially, a theoretical model is presented to analyze the sensing performance of the BT-QTF in the liquid. Then, the sensing performance analysis experiments of the BT-QTF have been performed. At last, the proposed method is applied to atomic force microscope imaging different samples in the liquid, which proves its feasibility.

Time-frequency analysis of the tip motion in liquids using the wavelet transform in dynamic atomic force microscopy

The tip motion of the dynamic atomic force microscope in liquids shows complex transient behaviors when using a low stiffness cantilever. The second flexural mode of the cantilever is momentarily excited. Multiple impacts between the tip and the sample might occur in one oscillation cycle. However, the commonly used Fourier transform method cannot provide time-related information about these transient features. To overcome this limitation, we apply the wavelet transform to perform the time-frequency analysis of the tip motion in liquids. The momentary excitation of the second mode and the phenomenon of multiple impacts are clearly shown in the time-frequency plane of the wavelet scalogram. The instantaneous frequencies and magnitudes of the second mode are extracted by the wavelet ridge analysis, which can provide quantitative estimations of the tip motion in the second mode. Moreover, the relations of the maximum instantaneous magnitude (MIM) to the amplitude setpoint and the Youngs modulus of the sample surface are investigated. The results suggest that the MIM can be used to characterize the nanomechanical property of the sample surface at high amplitude setpoints.

Intelligent tuning method of PID parameters based on iterative learning control for atomic force microscopy

Proportional-integral-derivative (PID) parameters play a vital role in the imaging process of an atomic force microscope (AFM). Traditional parameter tuning methods require a lot of manpower and it is difficult to set PID parameters in unattended working environments. In this manuscript, an intelligent tuning method of PID parameters based on iterative learning control is proposed to self-adjust PID parameters of the AFM according to the sample topography. This method gets enough information about the output signals of PID controller and tracking error, which will be used to calculate the proper PID parameters, by repeated line scanning until convergence before normal scanning to learn the topography. Subsequently, the appropriate PID parameters are obtained by fitting method and then applied to the normal scanning process. The feasibility of the method is demonstrated by the convergence analysis. Simulations and experimental results indicate that the proposed method can intelligently tune PID parameters of the AFM for imaging different topographies and thus achieve good tracking performance.

Real-time scan speed control of the atomic force microscopy for reducing imaging time based on sample topography

Here, a novel method, real-time scan speed control for raster scan amplitude modulation atomic force microscopes (AM-AFMs), is proposed. In general, the imaging rate is set to a fixed value before the experiment, which is determined by the feedback control calculations on each imaging point. Many efforts have been made to increase the AFM imaging rate, including using the cantilever with high eigenfrequency, employing new scan methods, and optimizing other mechanical components. The proposed real-time control method adjusts the scan speed linearly according to the error of every imaging point, which is mainly determined by the sample topography. Through setting residence time on each imaging point reasonably, the performance of AM-AFMs can be fully exploited while the scanner vibration is avoided when scan speed changes. Experiments and simulations are performed to demonstrate this control algorithm. This method would increase the imaging rate for samples with strongly fluctuant topography up to about 3 times without sacrificing any image quality, especially in large-scale and high-resolution imaging, in the meanwhile, it reduces the professional requirements for AM-AFM operators. Since the control strategy employs a linear algorithm to calculate the scanning speed based on the error signal, the proposed method avoids the frequent switching of the scanning speed between the high speed and the low speed. And it is easier to implement because there is no need to modify the original hardware of the AFM for its application.