Constant Height Mode Research Articles

Temperature Effect on the Electrochemical Current Response during Scanning Electrochemical Microscopy of Living Cells.

Scanning electrochemical microscopy (SECM) is being used increasingly to monitor electrochemical processes at the interface of living cells and electrodes. This allows the detection and quantification of biomarkers that further the understanding of various diseases. Rapid SECM experiments are often carried out without monitoring the analyte solution temperature or are performed at room temperature. The reported research demonstrates that temperature control is crucial during SECM imaging of living cells to obtain reliable data. In this study, a SECM-integrated thermostatic ring on the sample stage enabled imaging of living biological cells in a constant height mode at various temperatures. Two-dimensional line scans were conducted while scanning single Adenocarcinoma Cervical cancer (HeLa) cells. Numerical modeling was carried out to evaluate the effect of the temperature on the electrochemical current response of living cells to compare the apparent heterogeneous rate constant (k0), representing cellular reaction kinetics. This study reveals that even slight temperature variations of approximately 2 °C affect the reaction kinetics of single living cells, altering the measured current during SECM.

Scanning near-field optical microscopy measurements and simulations of regularly arranged silver nanoparticles

Silver nanoparticles on a glass substrate are experimentally investigated by aperture scanning near-field optical microscopy (a-SNOM). To understand the experimental results, finite-element-method simulations are performed building a theoretical model of the a-SNOM geometry. We systematically vary parameters like aperture size, aluminum-coating thickness, tip cone angle, and tip-surface distance and discuss their influence on the near-field enhancement. All these investigations are performed comparatively for constant-height and constant-gap scanning modes. In the end, we establish a reliable and stable optical model for simulating a-SNOM measurements, which is capable of reproducing trends observed in experimental data.

Direct etching of nano/microscale patterns with both few-layer graphene and high-depth graphite structures by the raster STM electric lithography in the ambient conditions.

The development of raster STM electric lithography has enabled the etching of nano/microscale patterns on both few-layer graphene (FLG) and high-depth graphite structures on the bulk HOPG substrates under ambient conditions. This approach utilises a nanoscale probe tip as a machining tool to directly fabricate conductive sample surfaces without the need for resists or masks. Compared to conventional nano/micro machining methods, the capability of ultraaccurate fabrication of nanoscale patterns using this technique is unmatched. The resulting FLG structures exhibit ultrasmooth flat bottoms and uniformly controlled depths ranging from 0.34 to 3.0nm (less than 10 layers). This work represents a significant advancement as it demonstrates the perfect etching of FLG structures in designated nano/microscale regions using raster STM electric lithography in the constant current mode, which reaches the limitation of top-down manufacturing techniques. Additionally, raster STM electric lithography in the constant height mode can directly etch high-depth structures (up to ∼100nm). The geometric shape and number of layers of the etched graphene structures determined by either local anodic oxidation (LAO) or the electric discharge (ED) mechanism. The LAO mechanism results in less debris and smoother edges compared to the ED mechanism, which is caused by the random electrical discharge between the tip and the sample. The well-controlled raster STM electric lithography technique is believed to be a promising and facile approach for constructing nano/microscale graphene-based devices.

Normalization procedure for obtaining the local density of states from high-bias scanning tunneling spectroscopy

Differential conductance spectroscopy performed in the high bias regime---in which the applied voltage exceeds the sample work function---is a suboptimal measure of the local density of states due to the effects of the changing tunnel barrier. Additionally, the large applied voltage oftentimes makes constant-height measurement experimentally impractical, lending constant-current spectroscopy an advantageous edge; but the differential conductance in that case is even further removed from the local density of states due to the changing tip height. Here, we present a normalization scheme for extracting the local density of states from high bias scanning tunneling spectroscopy, obtained in either constant-current or constant-height mode. We extend this model to account for the effects of the in-plane momentum of the probed states to the overall current. We demonstrate the validity of the proposed scheme by applying it to laterally confined field-emission resonances, which appear as peak-shaped spectroscopic features with a well-defined in-plane momentum.

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.

Stitched open-loop measurements with a focal-distance-modulated confocal sensor

Abstract The article presents measurements with a high-speed focal-distance modulated fibre-coupled confocal sensor integrated into a nano measuring machine (NMM-1). Combined with an appropriate signal processing, this axial beam modulating sensor yields a linear characteristic curve within a range of about 600 nm. This characteristic curve enables scans in closed-loop constant distance mode, where the controller of the NMM-1 keeps the distance between the measured surface and the sensor constant. Therefore, only one lateral scan is necessary to measure a given topography. Nevertheless, it is also possible to conduct open-loop constant height measurements on discrete heights. The axial offset between the heights needs to be only slightly less than the measuring range of the sensor, leading to a great reduction of the number of needed lateral scans to measure a given topography compared to a conventional confocal sensor, where the axial offset for a similar optical system is typically about 50 nm. To combine the scans taken at different heights, it is possible to stitch them using overlapping measuring points taken at two adjacent heights. Within this article, measurements are conducted on a roughness standard in closed-loop constant distance and in open-loop constant height mode, where the latter are evaluated with and without the application of the stitching algorithm.

Electronic Properties and Scanning Tunneling Microscopy Simulation of MoS2 Nanosheets by Using Density Functional Theory

The ab-initio Density Functional Theory (DFT) approach is used to study the electronic properties of bulk and layered MoS2 nanosheets. For the layered structures mono, bi, tri, tetra and penta layered structure is used. The direct to indirect transition of bandgap is observed as the number of layers is increasing. This transition of bandgap is attributed to the van der Waals interlayer interaction between two layers of MoS2 nanosheets. The indirect bandgap in the bulk MoS2 is found to be 0.94 eV, whereas for a single layered nanosheet is found to be direct bandgap with the value of 1.83 eV. To confirm the surface termination and understand the surface morphology of MoS2 the scanning tunneling microscopy (STM) simulation is performed in constant height mode. It is found that the detection of surface atoms via STM depends on the tip atom of the STM.
 Dhaka Univ. J. Sci. 69(1): 53-57, 2021 (January)

Single frequency vertical piezoresponse force microscopy

Piezoresponse force microscopy (PFM) uses a cantilever arm to track the electromechanical motion of the electric dipole moment to visualize the ferroelectric domain structure, which provides an important insight into the physics of switchable electric polarization—especially for memory devices and integrated microelectronics. Here, I provide a tutorial on single frequency vertical PFM, the most basic mode of PFM. I will start with the basic components of atomic force microscopy (AFM), including tip, cantilever, X–Y stage, Z actuator, and lock-in amplifier. Contact mode AFM will be briefly explained and discussed, where you can find two modes: constant deflection and constant height modes. Single-frequency vertical PFM splits the frequency domain of tip vibration into low and high frequencies and uses a low-pass filter to nullify any motion caused by topography (constant deflection). In contrast, the lock-in amplifier will pinpoint the vibration induced by the vertical piezoelectric strain along the sample’s surface normal (constant height). This tutorial will provide an overall and detailed step by step instruction to conduct PFM imaging and piezoresponse hysteresis loop measurement using atomic force microscopy and a lock-in amplifier and teach how to interpret the PFM images and the piezoresponse hysteresis loops for various applications.

Design and Fabrication of a High-Speed Atomic Force Microscope Scan-Head

A high-speed atomic force microscope (HS-AFM) requires a specialized set of hardware and software and therefore improving video-rate HS-AFMs for general applications is an ongoing process. To improve the imaging rate of an AFM, all components have to be carefully redesigned since the slowest component determines the overall bandwidth of the instrument. In this work, we present a design of a compact HS-AFM scan-head featuring minimal loading on the Z-scanner. Using a custom-programmed controller and a high-speed lateral scanner, we demonstrate its working by obtaining topographic images of Blu-ray disk data tracks in contact- and tapping-modes. Images acquired using a contact-mode cantilever with a natural frequency of 60 kHz in constant deflection mode show good tracking of topography at 400 Hz. In constant height mode, tracking of topography is demonstrated at rates up to 1.9 kHz for the scan size of with pixels.

Bond-level imaging of organic molecules using Q-controlled amplitude modulation atomic force microscopy

The bond imaging atomic force microscopy (AFM) technique has become an invaluable tool for studying organic molecules on surfaces. The key feature of this technique is the functionalization of the AFM-tip with a single CO molecule, which improves the lateral resolution and allows us to visualize the chemical structure of adsorbed organic molecules. Such experiments are usually performed at low temperatures in an ultrahigh vacuum environment in the frequency modulation (FM) mode. Here, we use the Q-controlled amplitude modulation (AM) mode for imaging organic molecules with CO-functionalized tips in constant-height mode. By reducing the effective quality factor (Qeff) of the sensor from about 20 000–30 000 to 1500–4000, we are able to image molecules with atomic resolution. Detailed instructions for determining the optimum Qeff and oscillation amplitude are given. To compare the phase and frequency shift images of the Q-controlled AM and the FM mode, we define an effective signal-to-noise ratio (SNR) that relates the observed contrast between the bonds and centers of imaged carbon rings to the noise in the respective image regions. This effective SNR is systematically analyzed in different regions of the imaged molecule for different oscillation amplitudes and average tip-substrate distances. By using appropriate imaging parameters, an increased effective SNR is achieved in the Q-controlled AM mode (on the order of 30–60%). This advantage over the conventional FM mode might, e.g., be used for increasing the experimental throughput.

Multiple-fibre interferometry setup for probe sample interaction measurements in atomic force microscopy

Atomic force microscopy (AFM) often relies on the assumption that cantilever bending can be described by simple beam theory and that the displacement of the tip can be evaluated from the cantilever angle. Some more advanced metrological instruments use free-space or fibre interferometers for measuring the position of the cantilever apex directly, thereby simplifying the metrology traceability chain. The next logical development, covering measurements of both the cantilever apex position and its deformation due to lateral forces acting during different AFM measurement regimes, is presented in this paper. It is based on using a set of closely packed fibre interferometers that can be used to determine localised bending of the cantilever at different positions along the cantilever. This can be used for detection of cantilever deformation beyond classical beam theory, and can yield both better understanding of sources of uncertainty in individual AFM force–distance measurements and more accurate scanning in constant height mode in high-speed AFM applications.

Bottom-tracking: the possibilities and physical meaning of keeping the bottom of the frequency shift in atomic force microscopy

In ultrahigh vacuum atomic force microscopy, the frequency of an oscillator holding the scanning tip takes a typical Lennard–Jones like curve with a local minima just before contact. We demonstrate here the application of a control scheme to keep this local minima, or the bottom of the frequency shift curve (FC) as the working point for imaging in UHV, and discuss its physical meaning and possible applications. Tip-sample distance modulation and Lock-in detection were used to obtain the gradient of the FC, where null control of the gradient signal resulted in implementing the designated control. Atomic resolution was confirmed for Si(111) and solder. The histogram of minimum frequency shift on the apex of atomic features on solder showed two to four peaks, implying the ability of the method to map characteristic differences of the depth of the FC per site. The method is an alternative to the existing constant-frequency-shift mode and constant-height mode, with the possibility to access chemical information on-the-fly.

A beetle-type, variable-temperature scanning tunneling microscope for video-rate imaging

A setup is described that combines video-rate imaging with temperature variability for a beetle-type scanning tunneling microscope (STM). The STM is operated in the constant height mode. Sinusoidal driving voltages are used for the scan lines; the resulting image distortions are corrected by a straightforward mathematical operation. A phase shifter is used to compensate the time lag between the driving voltage and the tip motion. A preamplifier with a bandwidth of 300 kHz and an rms noise of ∼0.1 nA has been developed. Software has been written for data acquisition rates of up to 2 MHz and for saving movies of more than 3700 STM images of 200 × 200 pixels. Frame rates between 10 and 50 frames s−1 have been achieved, where the highest line rates are above the lowest resonance frequency of the STM. The accessible temperature range of 50–500 K is not impaired by the video mode.

Contrast inversion of O adatom on rutile TiO2(1 1 0)-(1 × 1) surface by atomic force microscopy imaging

Oxygen adatom on rutile TiO2(1 1 0)-(1 × 1) surface was imaged by atomic force microscopy (AFM) with atomic resolution. The atomic contrast inversion of oxygen adatom is dependent on tip-sample distance and can be obtained in both constant frequency shift and constant height mode. The origin of contrast inversion was analyzed, combining the effect of adsorbed species, feedback loop and tip apex structure. Experimental frequency shift versus distance curves around O adatom show that there is a turning-point in the constant frequency shift mode and a cross-point in the constant height mode. The repulsive force is thought to contribute to the contrast inversion of oxygen adatom. The present results show the origin of contrast inversion of oxygen adatom on rutile TiO2(1 1 0)-(1 × 1) surface and illustrate the importance of tip-height for the atomic contrast inversion of the surface adsorbate on the metal oxide surface.

WiFi-controlled portable atomic force microscope.

This article proposes to develop a WiFi-controlled portable atomic force microscope (AFM). The AFM consists of a horizontal probe, controlling circuits, digital to analog (D/A) and analog to digital (A/D) interfaces, a microcomputer (Raspberry Pi, RPi), and a laptop. The proposed AFM uses a pocket-size power supply to drive the controlling circuits, the D/A and A/D interfaces, as well as the RPi that constructs network hotspots and generates scanning signals. With special design and integration of the whole system, both of the AFM probe and electronic controlling system are portable. At a distance of 50 m from the proposed AFM, experiments in the constant height mode and the constant force mode are conducted to evaluate its performance. The results show that this WiFi-controlled AFM has a maximum scan range of 3.6 × 3.6 μm2 with nanometer order resolution. Meanwhile, it achieves satisfactory image contrast, stability, and repeatability. Compared with conventional AFMs, the AFM proposed in this paper no longer relies on commercial AC mains supply or high-voltage DC power supply, and realizes WiFi-controlled AFM scanning and imaging in 50 m or farther without wire or network cable connection to a laptop or a desktop computer. Given credits to these features, WiFi-controlled AFMs are expected to own a wider range of application, especially in isolated environments, outdoor researches, or even fieldwork investigations.

Bond-Level Imaging of the 3D Conformation of Adsorbed Organic Molecules Using Atomic Force Microscopy with Simultaneous Tunneling Feedback.

The chemical structure and orientation of molecules on surfaces can be visualized using low temperature atomic force microscopy with CO-functionalized tips. Conventionally, this is done in constant-height mode by measuring the frequency shift of the oscillating force sensor. However, this method is unsuitable for analyzing 3D objects. We are using the tunneling current to track the topography while simultaneously obtaining submolecular resolution from the frequency shift signal. Thereby, the conformation of 3D molecules and the adsorption sites on the atomic lattice can be reliably determined.

Improving the atomic-resolution AFM imaging of monolayer MoS2 for worn tips: a molecular dynamics study

In this paper, molecular dynamics simulations were performed to investigate the atomic-scale imaging of a typical two-dimensional monolayer: molybdenum disulfide (MoS2). With a constant height mode, force-displacement curves of tips along a straight-line scanning path indicate resonant waveforms and the periodicities are capable of characterizing lattice parameters of monolayer MoS2. Some characteristic sites in the force-displacement curves arise only for tips with a radius of less than 3 Å under 1.47 Å tip-sample distance, which correlated to the atoms next to the scanning path. Such characteristic sites can be reproduced by using tips with a radius of 7 Å via increasing the tip-sample distance. The underlying mechanism was illustrated by the number of interactive atoms between the tip and the sample. The work may shed light on the atomic force microscope (AFM) operation when one expects to achieve high-resolution AFM imaging of 2D materials with a worn AFM tip.

Impacts of Forced Convection Generated via High Precision Stirring on Scanning Electrochemical Microscopy Experiments in Feedback Mode

AbstractIn this study, the effects of forced convection on scanning electrochemical microscopy (SECM) experiments in feedback mode using ferrocenemethanol as redox mediator are presented. Forced convection, which enhances the mass transfer inside the system, was generated via an electrical high precision stirrer integrated into the SECM setup. A thin‐film interdigitated array electrode serving as model substrate was investigated with probe scan curves in z‐direction and SECM imaging in constant height mode utilizing ultramicroelectrodes (UME) with diameters (dprobe) of 25 μm and 12.5 μm. It was found that forced convection increased the overall current during SECM imaging without distorting distinctive features of the imaged structure when working with a 25 μm UME at substrate‐to‐tip distances of 14 μm and 11 μm. Furthermore, the electrochemical contrast was improved under hydrodynamic conditions for a substrate‐to‐tip distance of 11 μm and scan rates of 5 μm s−1, 10 μm s−1, 20 μm s−1 and 40 μm s−1. When further decreasing the gap between the UME and the substrate to 9 μm almost no effects of the forced convection were observed. Consequently, for a 25 μm UME, forced convection led to higher currents and improved performance during SECM experiments in feedback mode at substrate‐to‐tip distances of 14 μm and 11 μm, whereas no effects were observed for a 12.5 μm UME at a distance of 8 μm.

Hopping mode SECM imaging of redox activity in ionic liquid with glass-coated inlaid platinum nanoelectrodes prepared using a heating coil puller

Major difficulties in nanoscale scanning electrochemical microscopy (SECM), resulting from the short tip-to-sample distance, are tilt misalignment and vertical thermal drift. This renders constant height mode nanoscale electrochemical imaging experimentally problematic. Nanoscale SECM also requires application of reliable nanoprobes. Although SECM nanotips are commercially available, they are rather disposable, fragile and their shelf-life is limited. Many SECM laboratories employ self-prepared nanoprobes, fabricated using advanced apparatus. In this paper, we employ a hopping mode approach to nanoscale electrochemical imaging, which resolves the mentioned common difficulties. The applied platinum nanoelectrodes, insulated with borosilicate glass, were prepared using affordable equipment and materials. They were applied for nanoscale mapping of redox mediator regeneration at a model interdigitated microsensor, composed of Pt microband electrodes, in a solution of ferrocene in viscous hydrophobic room temperature ionic liquid (RTIL) – 1-butyl-3-methylimidazoliumbis(trifluoro-methylsulfonyl)imide (C4mim N(Tf)2). The presented nanosensors were also characterized with cyclic voltammetry in aqueous electrolyte containing hexamineruthenium (III) chloride as redox mediator.

Positioning and control of scanning electrochemical microscopy / Skenuojančiojo elektrocheminio mikroskopo pozicionavimas ir valdymas

Positioning problems of scanning electrochemical microscopy (SECM) are very important by means of living cells damaging by moving ultramicroelectrode (UME). The working principle and operating modes of SECM are introduced. Investigation of redox activity of living cells are outlined. Problems, which arise in investigations of living cells by constant distance and constant height modes are discussed. Technical challenges and advances in application of SECM in living cell investigations are provided.