Conventional Contact Mode Research Articles

Control of interaction force in constant-height contact mode atomic force microscopy

Contact mode is a versatile and widely used technique for imaging samples using the Atomic Force Microscope (AFM). When contact mode imaging is performed in constant-height mode, it enables linear and faster response but leads to uncontrolled tip-sample forces. Here, a control strategy based on magnetic actuation is proposed to achieve high-bandwidth control of the tip-sample forces in constant-height contact mode AFM. A magnetic particle attached to the AFM probe is actuated by an external solenoid and employed for force regulation. A quasi-static model has been proposed and employed to develop the control strategy. Likewise, the contact natural-frequency, which decides the limit of achievable speed, has been shown to be significantly higher relative to the free probe and to be relatively insensitive to the particle size. Subsequently, a setup is developed to validate the control strategy and demonstrate reduction of tip-sample force variation by over a factor of 12 compared to conventional constant-height mode operation. Likewise, in comparison with conventional contact mode AFM, an improvement of linearity by over a factor of 9 and improvement in response speed by a factor of 100 have been demonstrated while imaging hard samples. The system has been shown to image topography at speeds of 2.44 frames per second while regulating the interaction force. Finally, the stiffness of a sample has also been characterized using the developed system and simultaneous estimation of topography has also been demonstrated. They are shown to agree well with theoretical expectations.

In-Situ AFM Imaging of Solid Electrolyte Interphase on Silicon Negative Electrodes with an Artificial Surface Layer

Silicon is regarded as the most promising negative electrode material for next-generation lithium-ion batteries because of the high theoretical capacity. We have developed a Si powder with a flake shape (Si LeafPower®, Si-LP, OIKE & Co., Ltd.) that is 100 nm thick with a 3-5 μm lateral dimension.[1-3] The Si-LP electrode demonstrates a superior cycle performances because the unique flake structure resists pulverization during charge and discharge cycling. For further improvement of cycle performances, inhibition of reductive decomposition of electrolytes on the Si negative electrode is necessary. We tried to suppress the electrolyte decomposition by an artificial surface coating on Si thin films and investigated formation of solid electrolyte interphase (SEI) by in-situ atomic force microscopy (AFM) coupled with a potential sweep. An amorphous Si thin film with the thickness of 100 nm was deposited by RF magnetron sputtering on a mirror polished Cu substrate. After that, LiF layer was sequentially deposited on the Si film electrode by the RF sputtering. AFM observations coupled with cyclic voltammetry were performed in a liquid immersion sample stage equipped with electrochemical connectors. The Si thin film as a working electrode and Li wires as a counter and a reference electrodes were immersed in the electrolyte of 1 M LiTFSI/EC+DEC (1:1 by vol.). AFM scan was conducted with a conventional contact mode at a room temperature in an Ar-filled glovebox. The potential was managed with a potentiostat/galvanostat and the sweep rate was 0.5 mV s-1. In the first cycle of the cyclic voltammetry for the Si film electrode without LiF, a small cathodic current began to flow ~2 V, and a current peak arose around 0.9 V. This current peak disappeared in the second cycle, which indicated an irreversible reductive decomposition reaction of the electrolyte and formation of SEI on the Si electrode. In the AFM image coupled with the potential sweep, surface deposits emerged from ~0.9 V corresponding to the potential of the electrolyte decomposition. On the other hand, a reductive current peak and surface deposits caused by the electrolyte decomposition were not observed in the Si film electrode with LiF coating. Furthermore, the Coulombic efficiencies of the Si film electrodes improved by the LiF-coating. These results indicated that the LiF layer on the Si electrode prevented the electrolyte decomposition, and thus LiF is a very important component for SEI layer. Acknowledgments: This work was supported by ALCA-SPRING (JST, Japan) and KAKENHI (JSPS, Japan) Grant Number 16H04649.

Adaptive-scanning, near-minimum-deformation atomic force microscope imaging of soft sample in liquid: Live mammalian cell example

In this paper, an adaptive-scanning mode (ASM) of atomic force microscope (AFM) with near-minimum sample deformation is proposed for imaging live biological samples in liquid. Conventional contact mode (CM) imaging of live cells is rather slow (scan rate < 0.2 Hz), and as the imaging speed increases, significant deformation of the soft and highly corrugated cell membrane is induced. Such a low speed CM imaging of live biological samples is not only time consuming, but also incapable of capturing dynamic biological evolutions occurring in seconds to minutes. The proposed ASM approach aims to address these issues through two synergetic efforts integrated together. First, an adaptive-scanning technique is proposed to optimally adjust the lateral scanning speed to accommodate the sample topography variation and the probe-sample interaction force, so that the scanning-caused sample deformation is maintained below the threshold value while the overall imaging time is minimized. Secondly, a data-driven iterative feedforward control is integrated to the vertical feedback loop along with a gradient-based optimization of the deflection set-point to substantially improve the tracking of the sample topography while maintaining the vertical sample deformation around the minimal. The ASM technique is experimentally validated through imaging live human prostate cancer cells on AFM. The experimental results demonstrate that compared to the conventional CM imaging, the imaging speed is increased over eight times without loss of tracking the topography details of the live cell membrane, and the probe-sample interaction force is substantially reduced.

Using Microbubble as Contrast Agent for High-Energy X-Ray In-line Phase Contrast Imaging: Demonstration and Comparison Study.

The ability of microbubbles to benefit the imaging quality of high-energy in-line phase contrast as compared with conventional low-energy contact mode radiography was investigated. The study was conducted by comparing in-line phase contrast imaging with conventional contact-mode projection imaging under the same dose delivered to a phantom. A custom-designed phantom was employed to simulate a segment of human blood vessel injected with microbubble suspensions. The microbubbles were suspended in deionized water to obtain different volume concentrations. The area contrast-to-noise ratio (CNR) values corresponding to both imaging methods were measured for different microbubble volume concentrations. The phase contrast images were processed by phase-attenuation duality phase retrieval to preserve the imaging quality. Comparison of the resultant CNR values indicates that the microbubble suspension images deliver a higher CNR than the water-only image, with monotonically increasing trends between the CNR values and microbubble concentrations. Compared to low-energy conventional images of the microbubble suspensions, high-energy in-line phase contrast CNRs are lower at high concentrations and are comparable, even better than, at low concentrations. This result suggests that 1) the performance of copolymer-shell microbubble employed in this study as x-ray contrast agent is constrained by the detective quantum efficiency of the system and the attenuation properties of the shell materials, 2) the phase-attenuation duality phase retrieval method has the potential to preserve image quality for areas with low concentration of microbubbles, and 3) the selection of microbubble products as a phase contrast agent may follow criteria of minimizing the impact of absorption attenuation properties of the shells and maximizing the difference factor of electron densities.

Insight into mechanics of AFM tip-based nanomachining: bending of cantilevers and machined grooves

Atomic force microscope (AFM) tip-based nanomachining is currently the object of intense research investigations. Values of the load applied to the tip at the free end of the AFM cantilever probe used for nanomachining are always large enough to induce plastic deformation on the specimen surface contrary to the small load values used for the conventional contact mode AFM imaging. This study describes an important phenomenon specific for AFM nanomachining in the forward direction: under certain processing conditions, the deformed shape of the cantilever probe may change from a convex to a concave orientation. The phenomenon can principally change the depth and width of grooves machined, e.g. the grooves machined on a single crystal copper specimen may increase by 50% on average following such a change in the deformed shape of the cantilever. It is argued that this phenomenon can take place even when the AFM-based tool is operated in the so-called force-controlled mode. The study involves the refined theoretical analysis of cantilever probe bending, the analysis of experimental signals monitored during the backward and forward AFM tip-based machining and the inspection of the topography of produced grooves.

Low dose high energy x-ray in-line phase sensitive imaging prototype: Investigation of optimal geometric conditions and design parameters.

The objective of this study was to investigate the optimization of a high energy in-line phase sensitive x-ray imaging prototype under different geometric and operating conditions for mammography application. A phase retrieval algorithm based on phase attenuation duality (PAD) was applied to the phase contrast images acquired by the prototype. Imaging performance was investigated at four magnification values of 1.67, 2, 2.5 and 3 using an acrylic edge, an American College of Radiology (ACR) mammography phantom and contrast detail (CD) phantom with tube potentials of 100, 120 and 140 kVp. The ACR and CD images were acquired at the same mean glandular dose (MGD) of 1.29 mGy with a computed radiography (CR) detector of 43.75 μm pixel pitch at a fixed source to image distance (SID) of 170 cm. The x-ray tube focal spot size was kept constant as 7 μm while a 2.5 mm thick aluminum (Al) filter was used for beam hardening. The performance of phase contrast and phase retrieved images were compared with computer simulations based on the relative phase contrast factor (RPF) at high x-ray energies. The imaging results showed that the x-ray tube operated at 100 kVp under the magnification of 2.5 exhibits superior imaging performance which is in accordance to the computer simulations. As compared to the phase contrast images, the phase retrieved images of the ACR and CD phantoms demonstrated improved imaging contrast and target discrimination. We compared the CD phantom images acquired in conventional contact mode with and without the anti-scatter grid using the same prototype at 1.295 mGy and 2.59 mGy using 40 kVp, a 25 μm rhodium (Rh) filter. At the same radiation dose, the phase sensitive images provided improved detection capabilities for both the large and small discs, while compared to the double dose image acquired in conventional mode, the observer study also indicated that the phase sensitive images provided improved detection capabilities for the large discs. This study therefore validates the potential of using high energy phase contrast x-ray imaging to improve lesion detection and reduce radiation dose for clinical applications such as mammography.

Nanoscale characterization of CH3-terminated Self-Assembled Monolayer on copper by advanced scanning probe microscopy techniques

In this study, we used Self-Assembled Monolayer (SAM) with CH3 end-group molecules to protect copper surfaces from oxidation and investigated at nanometer scale the integrity and temperature stability of the protective film. The films were characterized by dynamic Chemical Force Microscopy (dCFM), Torsional Resonance Tunneling Atomic Force Microscopy (TR-TUNA) and Attenuated Total Reflection Fourier Transform Infrared Spectroscopy (ATR-FTIR).We observed that temperature stress degraded local properties of our SAM films significantly, when compared to unstressed films. After temperature stress at 100°C, tunneling current increased and hydrophobicity decreased substantially. In combination with the ATR-FTIR results we assigned local high current spots and local hydrophobic variations to cuprous oxide (Cu2O). After temperature stress at 150°C, the measurements indicate a decomposition of the SAM film and a further oxidation of the copper surface. In addition, the results show that dynamic dCFM and TR-TUNA are appropriate tools to characterize SAM films structurally, chemically and electrically. Most important, in contrast to conventional contact mode Atomic Force Microscopy techniques, we did not observe any damage to the SAM film by dCFM and TR-TUNA measurements.

A surface topography-independent friction measurement technique using torsionalresonance mode in an AFM

The recently introduced torsional resonance mode (TR mode) technique has been appliedfor friction force measurements at the micro/nanoscale. In this technique, an atomic forcemicroscope (AFM) with a vibrating cantilever tip in torsional mode is used, andthe contact torsional vibration amplitude of the tip motion (TR amplitude) ismonitored at a contact resonance frequency under constant normal load. In thispaper, the TR mode friction force images are compared to the friction force imagesacquired by conventional contact mode friction force microscopy on three samples:a self-assembled monolayer (SAM) with two phase structure, a silicon ruler, and a metalevaporated (ME) tape. The results from those samples show that the TR mode frictionforce images are much less affected by the surface topography and are almostindependent of the scanning direction of the tip, whereas this is not the case forthe contact mode friction force measurements. Mechanisms responsible for theseobservations are also discussed in the paper. In order to predict coefficient of frictionas a function of the change in the TR amplitude, an energy balance model isproposed.

Local surface cleaning and cluster assembly using contact mode atomic force microscopy

Conventional contact mode atomic force microscopy (AFM) has been used for local surface cleaning and cluster alignment. By using the AFM tip to sweep and push in contact mode, we have demonstrated that Cu clusters, prepared by vacuum evaporation onto Dow Cyclotene 3022 polymer and subsequent exposure to atmosphere, can easily be moved by the AFM tip, and assembled at the outer edge of the scanned region to form a line of clusters. We have found that the force applied by the tip plays an important role in the ease of cluster motion. Cyclotene surface treatment that enhances cluster adhesion hinders this ability, and may be used as a method of nanofabrication.

Mapping surface elastic properties of stiff and compliant materials on the nanoscale using ultrasonic force microscopy

The increasing production of nano-devices and nano-composite materials has prompted the development of new instruments to probe smaller and smaller volumes. Regarding mechanical properties in particular, modified atomic force microscopes using force modulation at frequencies below the cantilever resonance have been successfully employed to investigate relatively compliant materials such as bio-materials and polymers but have shown limitations to highly stiff materials. The alternative approach of ultrasonic force microscopy (UFM) uses sample vibration at frequencies far above the cantilever primary resonance, exploiting the inertial stiffness of an atomic force microscopy cantilever and detection of ultrasonic vibration via nonlinearity of the tip-surface force interaction. In this paper we demonstrate that UFM can discriminate elastic properties of materials ranging from quite stiff to relatively compliant with a lateral resolution of a few nanometres and with high sensitivity to the elastic modulus. Furthermore a phenomenon of ultrasonically induced friction reduction permits imaging of fragile samples otherwise swept away in conventional contact mode atomic force microscopes. The possible influence of adhesive properties also has been analysed and criteria for distinguishing elastic and adhesive contributions have been established. We also explore another promising application of UFM for detection of nanoscale subsurface delamination.

Stiction analysis of magnetic tapes

This study investigates the differences between two MP tapes exhibiting different stiction behavior. Surface roughness and microscale friction of the tapes were measured using an Atomic Force/Friction Force Microscope (AFM/FFM). Numerical modeling was used to predict the contact area of the tape surfaces which were verified by actual measurement. Hardness and elastic modulus of the tapes were determined using a nanoindenter. Bending stiffness of the tapes was also measured. We found that the tape exhibiting lower stiction showed lower contact area primarily due to higher kurtosis of the surface in addition to higher elastic modulus, hardness and bending stiffness than the tape exhibiting higher stiction. Based on this study, tapes with surfaces exhibiting high kurtosis are desirable for good stiction behavior. Higher values of elastic modulus and bending stiffness are also favorable for low stiction. We also found that roughness measurements made with the AFM yielded better lateral resolution in tapping mode as compared to the conventional contact mode.