Jialin Shi

Experimental study and modeling of atomic-scale friction in zigzag and armchair lattice orientations of MoS2

Abstract Physical properties of two-dimensional materials, such as graphene, black phosphorus, molybdenum disulfide (MoS2) and tungsten disulfide, exhibit significant dependence on their lattice orientations, especially for zigzag and armchair lattice orientations. Understanding of the atomic probe motion on surfaces with different orientations helps in the study of anisotropic materials. Unfortunately, there is no comprehensive model that can describe the probe motion mechanism. In this paper, we report a tribological study of MoS2 in zigzag and armchair orientations. We observed a characteristic power spectrum and friction force values. To explain our results, we developed a modified, two-dimensional, stick-slip Tomlinson model that allows simulation of the probe motion on MoS2 surfaces by combining the motion in the Mo layer and S layer. Our model fits well with the experimental data and provides a theoretical basis for tribological studies of two-dimensional materials.

Performance Investigation of Multilayer MoS2 Thin-Film Transistors Fabricated via Mask-free Optically Induced Electrodeposition

Transition metal dichalcogenides, particularly MoS2, have recently received enormous interest in explorations of the physics and technology of nanodevice applications because of their excellent optical and electronic properties. Although monolayer MoS2 has been extensively investigated for various possible applications, its difficulty of fabrication renders it less appealing than multilayer MoS2. Moreover, multilayer MoS2, with its inherent high electronic/photonic state densities, has higher output driving capabilities and can better satisfy the ever-increasing demand for versatile devices. Here, we present multilayer MoS2 back-gate thin-film transistors (TFTs) that can achieve a relatively low subthreshold swing of 0.75 V/decade and a high mobility of 41 cm2·V-1·s-1, which exceeds the typical mobility value of state-of-the-art amorphous silicon-based TFTs by a factor of 80. Ag and Au electrode-based MoS2 TFTs were fabricated by a convenient and rapid process. Then we performed a detailed analysis of the impacts of metal contacts and MoS2 film thickness on electronic performance. Our findings show that smoother metal contacts exhibit better electronic characteristics and that MoS2 film thickness should be controlled within a reasonable range of 30-40 nm to obtain the best mobility values, thereby providing valuable insights regarding performance enhancement for MoS2 TFTs. Additionally, to overcome the limitations of the conventional fabrication method, we employed a novel approach known as optically induced electrodeposition (OIE), which allows the flexible and precise patterning of metal films and enables rapid and mask-free device fabrication, for TFT fabrication.

A rapid and automated relocation method of an AFM probe for high-resolution imaging.

The atomic force microscope (AFM) is one of the most powerful tools for high-resolution imaging and high-precision positioning for nanomanipulation. The selection of the scanning area of the AFM depends on the use of the optical microscope. However, the resolution of an optical microscope is generally no larger than 200 nm owing to wavelength limitations of visible light. Taking into consideration the two determinants of relocation-relative angular rotation and positional offset between the AFM probe and nano target-it is therefore extremely challenging to precisely relocate the AFM probe to the initial scan/manipulation area for the same nano target after the AFM probe has been replaced, or after the sample has been moved. In this paper, we investigate a rapid automated relocation method for the nano target of an AFM using a coordinate transformation. The relocation process is both simple and rapid; moreover, multiple nano targets can be relocated by only identifying a pair of reference points. It possesses a centimeter-scale location range and nano-scale precision. The main advantages of this method are that it overcomes the limitations associated with the resolution of optical microscopes, and that it is label-free on the target areas, which means that it does not require the use of special artificial markers on the target sample areas. Relocation experiments using nanospheres, DNA, SWCNTs, and nano patterns amply demonstrate the practicality and efficiency of the proposed method, which provides technical support for mass nanomanipulation and detection based on AFM for multiple nano targets that are widely distributed in a large area.

Moving trajectory analysis and simulation in atomic friction for zigzag and armchair lattice orientation of MoS2

A friction modeling in zigzag and armchair lattice orientation of MoS2 has been demonstrated in this paper. Combing the assumption on the moving trajectories of the probe in both lattice orientations with two-dimension Tomlinson model, simulation of relationship between friction and orientation was smoothly performed with Matlab software. The lateral friction Microscopy(LFM) based experiment was conducted to verify the theoretical analysis. The consistency between simulating and realistic result reveals the validity of the entire modeling method, which is critically important for the fundamental study of the relationship between friction force and lattice orientation of MoS2 as well as its future application in real-time orientation detecting .

Simultaneous Measurement of Multiple Mechanical Properties of Single Cells Using AFM by Indentation and Vibration

Objective: The mechanical properties of cells, which are the main characteristics determining their physical performance and physiological functions, have been actively studied in the fields of cytobiology and biomedical engineering and for the development of medicines. In this study, an indentation-vibration-based method is proposed to simultaneously measure the mechanical properties of cells in situ, including cellular mass (m), elasticity (k), and viscosity (c).Methods: The proposed measurement method is implemented based on the principle of forced vibration stimulated by simple harmonic force using an atomic force microscope (AFM) system integrated with a piezoelectric transducer as the substrate vibrator. The corresponding theoretical model containing the three mechanical properties is derived and used to perform simulations and calculations. Living and fixed human embryonic kidney 293 (HEK 293) cells were subjected to indentation and vibration to measure and compare their mechanical parameters and verify the proposed approach. Results: The results that the fixed sample cells are more viscous and elastic than the living sample cells and the measured mechanical properties of cell are consistent within, but not outside of the central region of the cell, are in accordance with the previous studies.Conclusion: This work provides an approach to simultaneous measurement of the multiple mechanical properties of single cells using an integrated AFM system based on the principle force vibration and thickness-corrected Hertz model. Significance: This study should contribute to progress in biomedical engineering, cytobiology, medicine, early diagnosis, specific therapy and cell-powered robots.

Hardness determination at nanoscale by ultrasonic vibration-assisted AFM

Mechanical properties at nanoscale are crucial fators in the applications such as nanoscale interconnects and active components in electronic, optoelectronic, and electromechanical devices. Determination of the hardness of nanostructures, especially nano-thin-film, with regime from several to hundreds nanometers is a challenge. In this study, we proposed an ultrasonic vibration (USV)-assisted atomic force microscopy (AFM) method to measure the hardness of bulk materials and nano-thin-film with the thickness of 25 and hundreds nanometers. The hardness properties of material can be detected by the cantilever phase response in the USV machining process. By recording the depth-phase data, the hardness information can be extracted through fitting the data by the mathematical model. The theoretical analysis and experimental results valid the proposed method.

A design of phase-closed-loop nanomachining control based ultrasonic vibration-assisted AFM

This paper proposed a phase-closed-loop nanomachining control method to realize the directly control of machining depth based on ultrasonic vibration-assisted AFM. By using applied force to control the machining depth, conventional AFM machining approaches unable to machining a nanostructure with specified machined depth. With the proposed method, the vibration phase of micro-cantilever has a specific relationship with machining depth. Therefore, the nano-grooves with desired depth can be machined by using phase value as feedback of PID control. In this paper, the theoretical analysis and simulation are carried out, and the experiments of phase-closed-loop control method are conducted. The experimental results verify the primary feasibility of the proposed method. The present method also demonstrates the potential on the fabrication of three-dimension nanostructures and nanoelectronic device.

AFM measurement of the mechanical properties of single adherent cells based on vibration

Cellular mechanical properties as the main physical performance characteristics have been actively studied in the past years for the study of cytobiology and the development of medicine. In this study, by combining Hertz model, a novel strategy is proposed to simultaneously measure the cellular mechanical properties including cellular mass, elasticity and viscosity, based on the principle of forced vibration stimulated by simple harmonic force, with piezoelectric transducer (PZT) as vibrator and Atomic Force Microscope (AFM) as detector. The corresponding theoretical model was derived and the simulation was realized based on the proposed model. The experiments of indentations and vibrations with myoblasts and myotubes were implemented to calculate the three mechanical parameters of cells according to the proposed strategy. The results validated the proposed approach. This work would be useful for the development of cytology, medicine, previously diagnose, specific therapy and so on.

Real-time detecting and tracking nanoscale feeble vibrations based SF-AM AFM

Nanoscale vibration, a critical nanomechanical property of cell membranes/walls, is a crucial aspect of cell physiology. However, limitations of current nanoscale vibration detecting methods remain the major obstacle for scientific study and cell vibration experiments. Due to the absence of effective method of feeble nanoscale vibration detecting, most sorts of quantitative and dynamic cell vibrations cannot be observed. Therefore, a real-time tracking detection method is vital for the study of cell physiology. In this paper, a real-time tracking detection of nanoscale vibrations based on sweep frequency (SF) - amplitude modulation (AM) method using cantilever sweep frequency as a carrier frequency was proposed. Furthermore, the process of tip-sample vibration coupling is analyzed by using the idea of amplitude modulation model. The nanoscle vibration detecting experiments were carried out on a piezoceramic disc, which can mimic cell vibrations. The experiment results show that the SF-AM AFM real-time vibration tracking and detecting approach can accurately detect and track feeble sample vibration within few nanometers amplitude.