Atomic Force Microscopy Setup Research Articles

Integration of piezoelectric and electrothermal actuators for high-resolution Atomic Force Microscopy

The development of ultrasensitive silicon-based microcantilevers has significantly improved the imaging resolution of the Atomic Force Microscope (AFM). However, most of these microcantilevers require one or more external actuators and sensors for imaging, thus increasing the footprint and operational complexity of the instrument. Here we propose a novel active microcantilever that does not require any external actuator apart from the sample positioner to perform high-resolution AFM imaging. The proposed microcantilever is equipped with two different on-chip actuators: a piezoelectric actuator for oscillating the probe at a high frequency and an electrothermal actuator for providing large-range Z-motion. An on-chip differential piezoelectric sensor measures probe oscillation during imaging. To demonstrate the proof of concept, the microcantilever was microfabricated and integrated with an in-house developed AFM setup comprising of sample positioner and drive electronics. Finally, 3 different calibration gratings were imaged with a closed-loop bandwidth of 150Hz.

A Next-Generation qPlus-Sensor-Based AFM Setup: Resolving Archaeal S-Layer Protein Structures in Air and Liquid.

Surface-layer (S-layer) proteins form the outermost envelope in many bacteria and most archaea and arrange in two-dimensional quasicrystalline structures via self-assembly. We investigated S-layer proteins extracted from the archaeon Pyrobaculum aerophilium with a qPlus sensor-based atomic force microscope (AFM) in both liquid and ambient conditions and compared it to transmission electron microscopy (TEM) images under vacuum conditions. For AFM scanning, a next-generation liquid cell and a new protocol for creating long and sharp sapphire tips was introduced. Initial AFM images showed only layers of residual detergent molecules (sodium dodecyl sulfate, SDS), which are used to isolate the S-layer proteins from the cells. SDS was not visible in the TEM images, requiring more thorough sample preparation for AFM measurements. These improvements allowed us to resolve the crystallike structure of the S-layer samples with frequency-modulation AFM in both air and liquid.

Near-Surface Nanomechanics of Medical-Grade PEEK Measured by Atomic Force Microscopy.

Detecting subtle changes of surface stiffness at spatial scales and forces relevant to biological processes is crucial for the characterization of biopolymer systems in view of chemical and/or physical surface modification aimed at improving bioactivity and/or mechanical strength. Here, a standard atomic force microscopy setup is operated in nanoindentation mode to quantitatively mapping the near-surface elasticity of semicrystalline polyether ether ketone (PEEK) at room temperature. Remarkably, two localized distributions of moduli at about 0.6 and 0.9 GPa are observed below the plastic threshold of the polymer, at indentation loads in the range of 120-450 nN. This finding is ascribed to the localization of the amorphous and crystalline phases on the free surface of the polymer, detected at an unprecedented level of detail. Our study provides insights to quantitatively characterize complex biopolymer systems on the nanoscale and to guide the optimal design of micro- and nanostructures for advanced biomedical applications.

Kondo nanomechanical dissipation in the driven Anderson impurity model

The cyclic sudden switch of a magnetic impurity from a Kondo to a non-Kondo state and back was recently proposed to involve an important dissipation of the order of several ${k}_{B}{T}_{K}$ per cycle. The possibility to reveal this and other electronic processes through nanomechanical dissipation by, e.g., ultrasensitive atomic force microscope (AFM) tools would represent an unusual and interesting form of spectroscopy. Here, we explore the dependence on the switching time of the expected dissipation, a quantity whose magnitude is physically expected to drop from maximum to zero between sudden and slow switching, respectively. As an application of a recently established matrix-product-state-based time-dependent variational algorithm, we study the magnetic-field-induced Kondo switching in an Anderson model of the magnetic impurity. We find, quite reasonably, that dissipation requires switching within the Kondo timescale $\ensuremath{\hbar}{({k}_{B}{T}_{K})}^{\ensuremath{-}1}$ or faster. While such a fast switching seems problematic for current AFM setups, the challenge remains open for future means to detect this dissipation by time-dependent magnetic fields, an electrostatic impurity level shift, or hybridization switching. The technical aspects revealed by this approach will be of interest for future nonequilibrium calculations.

Nanoscale friction characteristics of layered-structure materials in dry and wet environments

Bulk layered materials, such as graphite and molybdenum disulfide, have long been used as solid lubricants in various industrial applications. The weak interlayer van der Waals interactions in these materials generate a low shear slip-plane, which reduces the interfacial friction. The cumulative trends toward device miniaturization have increased the need for basic knowledge of the nanoscale friction of contact-mode devices containing layered materials. Further, the decomposition and degradation of bulk layered solids subjected to shear forces are detrimental to their lubricating characteristics. Layered-structure materials, such as graphene, hexagonal boron nitride, and MXenes consisting of single or few atomic layers, behave as a new class of lubricious substances when deposited at a sliding interface. The exceptional mechanical strength, thermal conductivity, electronic properties, large theoretical specific area, and chemical inertness of these materials make them ideal antifriction materials for continuous sliding interfaces, especially when operated at elevated temperatures. These properties hold great promise for widespread applications both in dry environments, such as solid film lubrication for micro/nano-electromechanical systems, nanocomposite materials, space lubrication, and optical devices, as well as in wet environments, such as desalination membranes, lubricant additives, and nanofluidic transporters. However, accurate and reliable prediction of the frictional behavior of layered-structure materials is challenging due to the complex physicochemical transformations encountered under tribostress. The presence of a liquid in the vicinity of a surface in wet-environment applications further complicates the lubrication behavior of layered-structure materials. Furthermore, insight into the origins of interfacial friction and adhesion due to localized contact interactions can be accomplished by atomic-level experimental techniques and computational methods, such as atomic force microscope (AFM) in combination with molecular dynamics (MD) and density functional theory (DFT). The AFM setup mimics asperity-asperity contact at the atomic level and can measure the friction force of layered-structure materials, whereas MD and DFT can provide insight into the chemomechanical transformations commencing at hidden interfaces, which cannot be detected by experimental methods. The objective of this review article is threefold. First, the relationship between friction and potential energy surface is examined for different layered-structure material systems, and the parameters that mainly affect the energy corrugation are interpreted in the context of reported results. Second, the atomic-scale friction mechanisms of layered-structure materials in dry or vacuum environments are discussed in light of experimental and theoretical findings, focusing on the most crucial frictional energy dissipation mechanisms. Third, the complex mechanisms affecting the nanosccale friction of layered-structure materials incorporated in liquid media are introduced for ionic, polar, and non-polar solutions.

A cantilever-based, ultrahigh-vacuum, low-temperature scanning probe instrument for multidimensional scanning force microscopy.

Cantilever-based atomic force microscopy (AFM) performed under ambient conditions has become an important tool to characterize new material systems as well as devices. Current instruments permit robust scanning over large areas, atomic-scale lateral resolution, and the characterization of various sample properties using multifrequency and multimodal AFM operation modes. Research of new quantum materials and devices, however, often requires low temperatures and ultrahigh vacuum (UHV) conditions and, more specifically, AFM instrumentation providing atomic resolution. For this, AFM instrumentation based on a tuning fork force sensor became increasingly popular. In comparison to microfabricated cantilevers, the more macroscopic tuning forks, however, lack sensitivity, which limits the measurement bandwidth. Moreover, multimodal and multifrequency techniques, such as those available in cantilever-based AFM carried out under ambient conditions, are challenging to implement. In this article, we describe a cantilever-based low-temperature UHV AFM setup that allows one to transfer the versatile AFM techniques developed for ambient conditions to UHV and low-temperature conditions. We demonstrate that such a cantilever-based AFM offers experimental flexibility by permitting multimodal or multifrequency operations with superior force derivative sensitivities and bandwidths. Our instrument has a sub-picometer gap stability and can simultaneously map not only vertical and lateral forces with atomic-scale resolution, but also perform rapid overview scans with the tip kept at larger tip–sample distances for robust imaging.

Electron dynamics of tip-tunable oxygen species on TiO2 surface

The redox states of oxygen species on the surface of TiO2 can be altered by electron tunneling by varying the applied bias voltage of an atomic force microscope tip. However, tunneling is stochastic in nature and typically requires ultra-low temperatures to obtain statistically significant data. Here, we use a highly sensitive fast atomic force microscopy setup to study redox transitions of oxygen atoms on a TiO2 surface, in the form of reactive oxygen species and single-atom quantum dots, at 78 K. The fast and highly sensitive nature of our experimental setup enables a statistically necessary amount of data to be collected without having to resort to ultra-low temperatures. This enabled us to study multiple dots and provide insight into the electronic structure and correlation between the oxygen species, which are inaccessible by standard atomic force microscopy. We show that single-atom quantum dots exist in two charge states with drastically different conductance, with one being conducting and the other non-conducting.

Biaxial atomically resolved force microscopy based on a qPlus sensor operated simultaneously in the first flexural and length extensional modes.

Frequency-modulation atomic force microscopy (AFM) with a qPlus sensor allows one to atomically resolve surfaces in a variety of environments ranging from low-temperature in ultra-high vacuum to ambient and liquid conditions. Typically, the tip is driven to oscillate vertically, giving a measure of the vertical force component. However, for many systems, the lateral force component provides valuable information about the sample. Measuring lateral and vertical force components simultaneously by oscillating vertically and laterally has so far only been demonstrated with relatively soft silicon cantilevers and optical detection. Here, we show that the qPlus sensor can be used in a biaxial mode with electrical detection by making use of the first flexural mode and the length extensional mode. We describe the necessary electrode configuration as well as the electrical detection circuit and compare the length extensional mode to the needle sensor. Finally, we show atomic resolution in ambient conditions of a mica surface and in ultra-high vacuum of a silicon surface. In addition to this, we show how any qPlus AFM setup can be modified to work as a biaxial sensor, allowing two independent force components to be recorded.

Atomic Force Microscopy for Live-Cell and Hydrogel Measurement.

Atomic force microscopy (AFM) has emerged as a popular method for determining the mechanical properties of cells, their components, and biomaterials. Here, we describe AFM setup and application to obtain stiffness measurements from single indentations for hydrogels and myofibroblasts.

Perovskite Oxides Tunable by Electromechanical and Electrothermal Couplings

Perovskite oxide thin films is an attractive material group due to their unique multi-functionalities. To broaden the impact of perovskite oxides to practical electronic device applications, their electrical properties will need to be tunable by external means. In this work, we show that electrical conductivity of perovskite STO (SrTiO3) thin films can be largely tuned by mechanical stress (electromechanical coupling) and thermal annealing (electrothermal coupling). The conductive atomic force microscope setup was carefully calibrated to enable precise measurements of electrical response upon application of varying forces, resulting in a high electromechanical coupling sensitivity of up to ~1,000 Sm-1MPa-1. The thermal annealing study also suggests that electrical conductivity of STO is strongly affected by the ambient temperature due to the varying amount of oxygen vacancies.

Direct Measurement of Lateral Molecular Diffusivity on the Surface of Supersaturated Amorphous Solid Dispersions by Atomic Force Microscopy.

Quantifying molecular surface diffusivity is of broad interest in many different fields of science and technology. In this study, the method of surface grating decay is utilized to investigate the surface diffusion of practical relevant amorphous solid dispersions of indomethacin and the polymeric excipient Soluplus (a polyvinyl caprolactam-polyvinyl acetate-polyethylene glycol graft copolymer) at various polymer concentrations (1-20% w/w). The study confirms that measuring surface diffusivity below the system's glass transition temperature is possible with a simplified atomic force microscopy setup. Results highlight a striking polymer influence on the surface diffusivity of drug molecules at low polymer concentrations and a turnover point to a polymer dominated diffusion at around three percent (w/w) polymer concentration. The surface diffusion measurements further correlate well with the observed increase in physical stability of the system as measured by X-ray powder diffraction. These findings are of vital interest in both the applied use and fundamental understanding of amorphous solid dispersions.

Effect of Relative Humidity on the Viscoelasticity of Thin Organic Films Studied by Contact Thermal Noise AFM.

Material scientists are in need of experimental techniques that facilitate a quantitative mechanical characterization of mesoscale materials and, therefore, their rational design. An example is that of thin organic films, as their performance often relates to their ability to withstand use without damage. The mechanical characterization of thin films has benefited from the emergence of the atomic force microscope (AFM). In this regard, it is of relevance that most soft materials are not elastic but viscoelastic instead. While most AFM operation modes and analysis procedures are suitable for elasticity studies, the use of AFM for quantitative viscoelastic characterizations is still a challenge. This is now an emerging topic due to recent developments in contact resonance AFM. The aim of this work was to further explore the potential of this technique by investigating its sensitivity to viscoelastic changes induced by environmental parameters, specifically humidity. Here, we show that by means of this experimental approach, it was possible to quantitatively monitor the influence of humidity on the viscoelasticity of two different thin and hydrophobic polyurethane coatings representative of those typically used to protect materials from processes like weathering and wear. The technique was sensitive even to the transition between the antiplasticizing and plasticizing effects of ambient humidity. Moreover, we showed that this was possible without the need of externally exciting the AFM cantilever or the sample, i.e., just by monitoring the Brownian motion of cantilevers, which significantly facilitates the implementation of this technique in any AFM setup.

Steady-state and transient behavior in dynamic atomic force microscopy

We discuss the influence of external forces on the motion of the tip in dynamic atomic force microscopy (AFM). First, a compact solution for the steady-state problem is derived employing a Fourier approach. Founding on this solution, we present an analytical framework to describe the transient behavior of the tip after perturbations of tip–sample forces and the excitation signal. The static and transient solutions are then combined to obtain the baseband response of the tip, i.e., the deflection signal demodulated with respect to the excitation. The baseband response generalizes the amplitude and phase response of the tip, and we use it to find explicit formulas describing the amplitude and phase modulation following the influence of external forces on the tip. Finally, we apply our results to obtain an accurate dynamic model of the amplitude controller and phase-locked loop driving the cantilever in a frequency modulated AFM setup. A special emphasis is put on discussing the tip response in environments of high damping, such as ambient or liquid.

MicroMegascope

Atomic force microscopy (AFM) allows us to reconstruct the topography of surfaces with resolution in the nanometer range. The exceptional resolution attainable with the AFM makes this instrument a key tool in nanoscience and technology. The core of a standard AFM set-up relies on the detection of the change of the mechanical motion of a micro-oscillator when approaching the sample to image. This is despite the fact that AFM is nowadays a very common instrument for both fundamental and applied research. The fabrication of the micrometric scale mechanical oscillator is still a very complicated and expensive task requiring dedicated platforms. Being able to perform AFM with a macroscopic oscillator would make the instrument more versatile and accessible for an even larger spectrum of applications and audience. Here, we present atomic force imaging with a centimetric oscillator, an aluminum tuning fork of centimeter size as a sensor on which an accelerometer is glued on one prong to measure the oscillations. We show that it is possible to perform topographic images of nanometric resolution with a gram tuning fork. In addition to the stunning sensitivity, we show the high versatility of such an oscillator by imaging both in air and liquid. The set-up proposed here can be extended to numerous experiments where the probe has to be heavy and/or very complex, and so too the environment.

Imaging in Biologically-Relevant Environments with AFM Using Stiff qPlus Sensors

High-resolution imaging of soft biological samples with atomic force microscopy (AFM) is challenging because they must be imaged with small forces to prevent deformation. Typically, AFM of those samples is performed with soft silicon cantilevers (k ≈ 0.1–10 N/m) and optical detection in a liquid environment. We set up a new microscope that uses a stiff qPlus sensor (k ≥ 1 kN/m). Several complex biologically-relevant solutions are non-transparent, and even change their optical properties over time, such as the cell culture medium we used. While this would be problematic for AFM setups with optical detection, it is no problem for our qPlus setup which uses electrical detection. The high stiffness of the qPlus sensor allows us to use small amplitudes in frequency-modulation mode and obtain high Q factors even in liquid. The samples are immersed in solution in a liquid cell and long tips are used, with only the tip apex submerged. We discuss the noise terms and compare the minimal detectable signal to that of soft cantilevers. Atomic resolution of muscovite mica was achieved in various liquids: H2O, Tris buffer and a cell culture medium. We show images of lipid membranes in which the individual head groups are resolved.

Highly Conductive Thin Uniform Gold-Coated DNA Nanowires.

Over the past decades, DNA, the carrier of genetic information, has been used by researchers as a structural template material. Watson-Crick base pairing enables the formation of complex 2D and 3D structures from DNA through self-assembly. Various methods have been developed to functionalize these structures for numerous utilities. Metallization of DNA has attracted much attention as a means of forming conductive nanostructures. Nevertheless, most of the metallized DNA wires reported so far suffer from irregularity and lack of end-to-end electrical connectivity. An effective technique for formation of thin gold-coated DNA wires that overcomes these drawbacks is developed and presented here. A conductive atomic force microscopy setup, which is suitable for measuring tens to thousands of nanometer long molecules and wires, is used to characterize these DNA-based nanowires. The wires reported here are the narrowest gold-coated DNA wires that display long-range conductivity. The measurements presented show that the conductivity is limited by defects, and that thicker gold coating reduces the number of defects and increases the conductive length. This preparation method enables the formation of molecular wires with dimensions and uniformity that are much more suitable for DNA-based molecular electronics.

AFM tip-based nanomachining with increased cutting speed at the tool-workpiece interface

This paper reports a study towards enhancing the throughput of the Atomic Force Microscope (AFM) tip-based nanomachining process by increasing the cutting speed at the interface between the tool and the workpiece. A modified AFM set-up was implemented, which combined the fast reciprocating motions of a piezoelectric actuator, on which the workpiece was mounted, and the linear displacement of the AFM stage, which defined the length of produced grooves. The influence of the feed, the feed direction and the cutting speed on the machined depth and on the chip formation was studied in detail when machining poly(methyl methacrylate). A theoretical cutting speed over 5m/min could be achieved with this set-up when the frequency of the piezoelectric actuator reciprocating motions was 40kHz. This is significantly better than the state of the art for AFM-based nanomachining, which is currently less than 1m/min.

Feasibility study and cell design for performing local electrochemical impedance spectroscopy measurements using an atomic force microscopy set-up

This paper studies the feasibility of performing Local Electrochemical Impedance Spectroscopy (LEIS) and Atomic Force Microscopy (AFM) within a single set-up, using a hybrid probe. The geometry of the set-up along with the extremely small size of the probe used is expected to result in distorted impedances and very low potentials, respectively. Numerical simulations using a multi-ion transport and reaction model (MITReM), show that the distortions can be mathematically corrected for and that our electronic equipment is capable of measuring the minute potentials.

True non-contact atomic force microscopy imaging of heterogeneous biological samples in liquids: topography and material contrast.

The present work analyses how the tip-sample interaction signals critically determine the operation of an Atomic Force Microscope (AFM) set-up immersed in liquid. On heterogeneous samples, the conservative tip-sample interaction may vary significantly from point to point - in particular from attractive to repulsive - rendering correct feedback very challenging. Lipid membranes prepared on a mica substrate are analyzed as reference samples which are locally heterogeneous (material contrast). The AFM set-up is operated dynamically at low oscillation amplitude and all available experimental data signals - the normal force, as well as the amplitude and frequency - are recorded simultaneously. From the analysis of how the dissipation (oscillation amplitude) and the conservative interaction (normal force and resonance frequency) vary with the tip-sample distance we conclude that dissipation is the only appropriate feedback source for stable and correct topographic imaging. The normal force and phase then carry information about the sample composition ("chemical contrast"). Dynamic AFM allows imaging in a non-contact regime where essentially no forces are applied, rendering dynamic AFM a truly non-invasive technique.

Investigation of geometrical effects in the carbon allotropes manipulation based on AFM: multiscale approach

Carbon allotropes are used as nanocarriers for drug and cell delivery. To obtain an accurate result in the nanoscale, it is important to use a precise model. In this paper, a multiscale approach is presented to investigate the manipulation process of carbon allotropes based on atomic force microscopy (AFM). For this purpose, the AFM setup is separated into two parts with different sizes as macro field (MF) and nano field (NF). Using Kirchhoff’s plate model, the cantilever (the main part of MF) is modeled. The molecular dynamics method is applied to model the NF part, and then the MF and NF are coupled with the multiscale algorithm. With this model, by considering the effect of size and shape, the manipulation of carbon allotropes is carried out. The manipulations of armchair CNTs and fullerenes are performed to study the diameter changing effects. The result shows that the manipulation and friction force increases by increasing the diameter. The result of the indentation depth for the armchair CNTs indicates that decreasing the diameter causes the indentation depth to reduce. Moreover, the manipulations of four kinds of carbon allotropes with the same number of atoms have been studied to investigate the geometrical effects. The shapes of these nanoparticles change from sphere to cylinder. The results illustrate that the manipulation and the friction force decrease as the nanoparticle shape varies from sphere to cylinder. The Von-Mises results demonstrate that by changing the nanoparticle shape from the spherical to the cylindrical form, the stress increases, although the manipulation force reduces.